Rich Landscape of Colloidal Semiconductor–Metal Hybrid Nanostructures: Synthesis, Synergetic Characteristics, and Emerging Applications

Nanochemistry provides powerful synthetic tools allowing one to combine different materials on a single nanostructure, thus unfolding numerous possibilities to tailor their properties toward diverse functionalities. Herein, we review the progress in the field of semiconductor–metal hybrid nanoparticles (HNPs) focusing on metal–chalcogenides–metal combined systems. The fundamental principles of their synthesis are discussed, leading to a myriad of possible hybrid architectures including Janus zero-dimensional quantum dot-based systems and anisotropic quasi 1D nanorods and quasi-2D platelets. The properties of HNPs are described with particular focus on emergent synergetic characteristics. Of these, the light-induced charge-separation effect across the semiconductor–metal nanojunction is of particular interest as a basis for the utilization of HNPs in photocatalytic applications. The extensive studies on the charge-separation behavior and its dependence on the HNPs structural characteristics, environmental and chemical conditions, and light excitation regime are surveyed. Combining the advanced synthetic control with the charge-separation effect has led to demonstration of various applications of HNPs in different fields. A particular promise lies in their functionality as photocatalysts for a variety of uses, including solar-to-fuel conversion, as a new type of photoinitiator for photopolymerization and 3D printing, and in novel chemical and biomedical uses.


INTRODUCTION
Semiconductor−metal hybrid nanoparticles (HNPs) combine two disparate materials on a single nanosystem. 1 The motivation for such unconventional combination of materials arises, first, from the great quest of expanding the horizons of nanochemistry to form heterogeneous systems. This achievement has risen and continues to strive on the basis of the ongoing significant synthetic advances in forming, on the one hand, highly controlled colloidal semiconductor nanostructures as well as their metal counterparts, on the other hand. Second, such hybrid semiconductor−metal nanoparticles manifest a combination of the original properties of their constituents on a single nanosystem. Moreover, they reveal unique synergistic effects emerging from the nanoscale semiconductor−metal combination, thus providing an outcome greater than the simple sum of its parts. 2,3 The ongoing developments in synthesis of HNPs along with the in-depth understanding of their combined and synergetic physical and chemical properties enable their functionality in numerous promising applications. This includes utilization of HNPs in optical, electronic, biomedical, and environmental fields. 4−7 A notable direction is the application of HNPs as efficient photocatalysts for timely challenges including direct solar-to-fuel conversion, waste degradation, and water purification, as novel photopolymerization and photocuring agents, for antimicrobial functions, and in photodynamic therapies. These different aspects of HNPs are illustrated in Scheme 1. Joining semiconductor and metal segments into a single nanoparticle introduces several fundamental synthetic challenges. The synthesis design needs to overcome the competing homogeneous nucleation of either semiconductor or metal phase that would form separate nanosystems instead of a unified hybrid nanoparticle. Additionally, lattice mismatching can hinder the formation of stable semiconductor−metal junctions. Another factor is the competition with cation exchange reactions and metal diffusion that would generate a nanostructure with altered composition rather than the desired HNP. Last but not least, a basic challenge for the colloidal nanosystems is the tailoring of the surface coating needed both for colloidal chemical stability as well as for chemical and physical passivation, and in HNPs, both the semiconductor and metal nanosurfaces must be considered in tandem. This review presents, in section 2, the vastly rich synthetic approaches and strategies successfully addressing the above challenges and yielding numerous semiconductor−metal HNPs with different compositions and dimensionalities. The focus is primarily on semiconductor nanostructures from the well-developed family of metal−chalcogenides with diverse metals. The extensive investigation and in-depth understanding of metal−chalcogenides fundamental physical properties allowing one to decipher their energy band structure and electronic properties such as excited charge carriers dynamics set the ground for their implementation in hybrid nanosystems. Specifically, studies of metal−chalcogenide-based HNPs, such as CdS and CdSe/CdS in the forms of dots, rods, and platelets, in the context of their photocatalytic application are highly interesting due to their tunable band-gap energy covering the blue to visible to nearinfrared region and the lower recombination rates, giving them the ability to efficiently utilize the solar spectrum. Other related systems including metal−oxide nanostructures with metals, metal−metal combinations, etc., are covered in prior other reviews and are not addressed herein. 8−14 The successful synthesis of semiconductor−metal HNPs has enabled the ongoing study of their emergent properties. The semiconductor nanostructure properties that can be tuned by control of the size and shape via quantum confinement as well as by altering the composition can now be augmented and enhanced by the features of the combined metal nanosegment. This opens up novel possibilities, for example, contact metal points directly grown on the semiconductor nanostructure for facile electrical wiring and as chemical anchor points. Manipulating the semiconductor optical properties allows one to achieve absorption enhancement and emission enhancement or quenching via near-field effects and interactions induced by coupling to the metal. One of the most prominent properties arises from the combination of excitonic semiconductor features with the plasmonic metal response and their couplings that lead to light-induced charge separation from either side of the semiconductor−metal HNPs. This charge separation is affected by several key parameters. Among these parameters are structural effects such as the metal component size and type as was demonstrated in various reports on similar CdS-based HNPs with different metal sites including Au and Pt, all showing size-dependent charge-transfer dynamics. The light-induced charge-separation characteristic constitutes the first step that provides excited charge carriers on both the semiconductor and the metal segments, serving as the basis for their photocatalytic activity toward different redox reaction pathways. Section 3 surveys the emergent combined and synergetic properties of the HNPs including the fundamental basis of such behavior while emphasizing the recent findings.
The advanced synthetic control of the semiconductor−metal HNPs provides the ability to tailor their characteristics according to the requirements of specific photocatalytic applications. A shining example is the ability to match the semiconductor nanostructure gap with a particular illumination source while tailoring the metal segment for a specific chemical reaction. This opens the path for using HNPs as a photocatalyst for water splitting to generate hydrogen fuel from water while tailoring the absorption to the visible range of the solar spectrum. Fundamental studies of charge separation on model systems such as CdS−Au and CdS−Pt nanorods along with advanced investigations of favored working conditions such as excitation regimes and alkaline environments for enhanced photocatalytic performance have been conducted and can set a basis for the future utilization of such and related systems in addressing "real-world" challenges. Additionally, HNPs have been applied as a new type of photoinitiator termed "quantum photoinitiator" for photopolymerization reactions in general and in 3D printing scenarios in particular as was demonstrated by CdS−Au nanorods, which were shown to enable photopolymerization of polyacrylamide. Photodynamic therapies and bioimaging based on HNPs are also of interest in this respect. Remarkably, HNPs such as Au−Cu 2−x S and AuPt−CuS showed antitumor activity followed by a decrease in tumor volume under NIR irradiation in both in vitro and in vivo experiments. Section 4 discusses the emergent photocatalytic applications of chalcogenide-based HNPs.
While two decades of research have led us to an advanced stage in the synthesis of HNPs, understanding their properties, and developing their utilization as photocatalysts, there is a vast landscape of potential further developments and breakthroughs yet to be discovered. The outlook and perspective of such HNPs is presented in the closing section of the review, section 5. different materials, semiconductor and metal. Although physical deposition methods such as physical vapor deposition (PVD), pulsed laser deposition (PLD), and molecular beam epitaxy (MBE) have been reported and manifested in the literature, one of the most widely used controllable synthetic methods is chemical reduction deposition. 7,12 As illustrated in Scheme 2, from direct heterogeneous deposition toward core/shell structures through direct heterogeneous nucleation for heterodimer nanostructures formation along with selective growth mechanisms based on the anisotropic material characteristics with or without assistance of light-induced photodeposition, these synthetic mechanisms allow deposition of metals on semiconductors or their incorporation into them and growth of semiconductors on metal seeds. Thermal or chemical reduction is typically obtained in the presence of organic molecular reducing agents, commonly alkyl amines such as dodecylamine and oleylamine, at various temperature conditions from room temperature to hot batch syntheses according the type of metal ions and reducing agents. The amine electron's lone pair can reduce the nearby metal ions preferentially at the surface of the semiconductor nanocrystals (NCs), as will be elaborated in the next section (Scheme 2a). In addition, these reducing agents also take part in the colloidal stabilization of the formed HNPs. Formation of HNPs via photoreduction of metal ions is a remarkable manifestation of the HNPs synergetic properties. By exploiting the absorption and excitonic features of the semiconductor segment that lead to accumulation of free charges on the semiconductor surface, the metal ions can be reduced by it and form metal islands or segments in a controlled manner, including material composition and site-selective deposition (Scheme 2b). Additionally, formation of HNPs can be attained by partial transformation deposition (Scheme 2c). This strategy is based on the oxidation or sulfidation of the seed surface as an initial step to allow the deposition of different materials by introducing the desired organic precursors at the proper temperature conditions as will be discussed extensively in section 2.2.3.

Thermodynamic Aspects in Heterogeneous Deposition
Fundamentally, the underlying mechanism that promotes the formation of heterogeneous growth at the semiconductor− metal interface can be derived from the thermodynamic perspective of a similar macrosystem of thin film epitaxial growth over a crystal surface. The growth mode of a second material on a pre-existing substrate (first material) is formally dependent on the sign of the total Gibbs free surface energy function, ΔG S , that defines the heterogeneous deposition reaction and is given by 21,22 where γ 1 and γ 2 are the surface energies associated with the respective materials (the solid/solution interfacial energies in the case of colloidal nanostructures in a liquid medium) and γ 1,2 is the solid/solid interfacial energy. In a colloidal phase synthesis, the former two terms can be significantly influenced by adhesion of capping ligands, precursors, and solvent, while the latter depends on the bonding strength and degree of crystallographic compatibility of the concerned lattices. Hence, in the case of ΔG S > 0, which is a result of a lower energy surface of the secondary material (γ 2 < γ 1 ) and/or a good crystallographic matching with the substrate (γ 1,2 is small), a layer-bylayer deposition will probably take place (i.e., Frank�van der Merwe (FM) growth mode, Scheme 3, upper panel), resulting in a continuous and uniform coverage. When ΔG S > 0, due to higher energy surfaces of the secondary material (γ 2 > γ 1 ) and/or significant lattice mismatch (γ 1,2 is high), it will tend to deposit adopting the habit of a discontinuous island-like domain array (i.e., Volmer�Weber (VW) growth mode, Scheme 3, middle panel) to minimize of the overall interfacial area shared with the substrate. An additional route of heterogeneous growth is a combination of the two former mechanisms. First, a continuous film of the second material (up to several monolayers) can be deposited (wetting layer). The strain energy induced by the lattice mismatch between the two materials accumulates, and above a critical thickness, three-dimensional island growth will be favored to relieve the misfit strain, leading to an island-like formation (Stranski�Krastanov (SK) growth mode, Scheme 3, lower panel). These different deposition routes can be realized via specific synthetic approaches. One of the most common strategies for the synthesis of heterogeneous nanoparticles in general and HNPs in particular is "seeded growth" implemented in solution phase, which may be considered as a derivative approach of traditional vapor-phase heteroepitaxial deposition. According to classical nucleation theory, the energy barrier for further growth on an already formed particle via heterogeneous nucleation is substantially lower than the activation energy for the formation of a new particle via homogeneous nucleation, as can be described by the following relationship (2) where θ is the contact angle between the seed and the additional growth phase and the wetting function, f(θ), ranges between 0 (θ = 0°) in the case of heterogeneous nucleation and 1 (θ = 180°) for homogeneous nucleation. Note that the contact angle is influenced by the structural characteristics of both seed and deposited phases and the surface and interface tension at their boundaries.
In light of the above explanation, colloidal synthesis involving different organic stabilizers and other solution species, which affect the ultimate Gibbs free energy balance, along with kinetic factors such as solution supersaturation, various reactive precursor types, and different chemical diffusion conditions that may relieve the misfit strain provides a multitude of potential pathways to achieve an enormous diversity of HNPs with different morphologies and architectures.

Synthetic Methods
The metal−semiconductor nanojunction can be formed by two strategies. As illustrated in Scheme 4, one way is to use semiconductor NCs as seeds to promote metal reduction deposition on its surface, and the second is semiconductor segment growth on metal NPs, which serve as seeds. In the next section, we will first focus on the HNP nanostructures achieved by semiconductor-seeded methods such as thermal and photoinduced procedures. The following section will review the metal-seeded approach.

Chemical Reduction Deposition.
A general synthetic strategy for the formation of semiconductor metal HNPs in which metal salt precursors are reduced on the surface of presynthesized semiconductor nanoparticles was first demonstrated by Banin et al. for colloidal CdSe nanorods (NRs) with selectively deposited Au metal domains on their apexes. 1,23 Typically, deposition of Au domains on various semiconductor materials is performed in organic phase solution at room temperature where the metal precursors are dissolved in this nonpolar environment with the assistance of surfactant molecules such as didodecyldimethylammonium bromide (DDAB), while the reduction occurs in the presence of a reducing agent, for example, dodecylamine (DDA), which can act also as stabilizer for the nanoparticles. Utilization of this method was expanded to a variety of HNPs including different semiconductor materials and shapes. This includes 0D structures such as PbS−Au dots 24 and Cu 2 ZnSnS 4 (CZTS)− Au cubes. 25 Extensive work on synthetic Au decoration has been reported on quasi-1D structures including CdS−Au, 26−29 ZnSe−Au, 30 CdSe/CdS−Au, 31−34 CdTe−Au, 35 and CZTS− Au 36 nanorods and tetrapods. More recently, with the synthetic development of 2D nanoplatelet-like semiconductor structures (NPLs), their semiconductor−metal hybrid forms were generated also. This includes CdS−Au, 37,38 CdSe−Au, 39−41 and CdSe/CdS−Au. 42,43 Various HNPs with different morphologies and material compositions which were synthesized via this approach are shown in Figure 1.
Modification of this procedure while using oleic acid and oleylamine as stabilizing and reducing agents, respectively, along with higher reaction temperatures allowed for the deposition of additional metals on similar semiconductor components.  44 In addition, binary metal tips were also reported including PtNi and PtCo, where both metal precursors are present during the reduction reaction. A similar synthetic route was used to deposit Pt nanodomains on the apexes of heterostructure semiconductors such as CdSe/CdS, 45 ZnSe/ CdS, and ZnTe/CdS nanorods 46 and CuInS 2 nanoparticles. 47,48 Additionally, other metals were epitaxially deposited under similar conditions including Ni. 49 The reactivity of the semiconductor rod apexes was also exploited for the deposition of PdS. 50 Dong et al. reported on the formation of CdSe/CdS− Ag NRs by reducing Ag−trioctylphosphine (TOP) complexes with oleylamine, 51 thereby avoiding common cation exchange yielding CdSe/CdS−Ag 2 S heterostructures based on hard and soft (Lewis) acid and base theory. 52 In the presence of TOP, which is a soft base, complexes consisting of a soft acid like Ag ions would preferably form while hampering the driving force of a hard acid, Cd ions, to migrate outside of the crystal structure. Reduction by oleylamine was also used for deposition of presynthesized metal nanoparticles such as Pt or Pd, which were used as seeds on CZTS nanoparticles to form core/shell and heterodimer architectures. 53 Cobalt deposition on the tips of CdSe nanorods was first achieved by reduction of [Co(η 4 -C 8 H 12 )(η 3 -C 8 H 13 )] in toluene with lauric acid and hexadecylamine under H 2 atmosphere and elevated temperature. 54 Pyun et al. presented cobalt metal heterogeneous growth on a pre-existing Pt tip serving as a substrate to allow the growth of a second metal phase 55 while applying carboxylic acid-terminated polystyrene as a reducing agent used previously for cobalt deposition reactions. 56 A unique metal decoration of Ru and Rh metallic frames over Cu 2 S nanoparticles was achieved by temperature-controlled thermal reduction of Ru or Rh acetylacetonate (acac) with octadecylamine in octyl ether or diphenyl ether/dichlorobenzene solution, respectively. 57,58 Solvothermal formation of semiconductor−metal HNPs was achieved also in the absence of additional reducing agents and surfactants as reported by Cozzoli and co-workers. CdS−Co and CdSe/CdS−Co NRs were obtained at high temperature (240°C) in octadecene and Co 2 (CO) 8 as the metal precursor. 59 Although chemical reduction of metal precursors is mostly reported in organic solutions, its feasibility was proven also in aqueous environments. For example, CdSe−Pt nanorods and nanonets were obtained under different pH values of aqueous solutions. 60 Additionally, aqueous condensation of Pd II onto CdS NRs was reported by Shemesh et al. 50 As Au growth was utilized on 2D nanosystems, implementation of the described oleylamineassisted synthetic route was also conducted on NPLs. Typically, Pt or Pd metal domains were deposited on the edges of homogeneous or heterostructure semiconductor NPLs resulting in CdSe−Pt, 41 CdSe−Pd, 41 CdS−Pt, 37,61 and CdSe/CdS−Pt 43 ( Figure 3). Recently, a modified reducing procedure for Au deposition on nanoplatelets was presented by Mews et al. where CuS−Au hybrid NPLs were obtained in the presence of oxalic acid and oleylamine. 62

Photoreduction Deposition.
Harnessing the photophysical properties of the semiconductor component allows the development of an additional important synthetic avenue to form HNPs via light-induced metal deposition. Following irradiation of the semiconductor, the absorbed photon generates the formation of an excited charge carrier within the semiconductor, namely, an electron−hole pair. These charge carriers, typically electrons, may accumulate at the defect site or preexisting metal domains through a suitable relaxation route. This is then followed by site-selective reduction of dissolved metal ions in the semiconductor surroundings on the surface of the semiconductor segment or at the metallic seeds. Commonly, besides the participating surfactants in the reaction that are accountable for reducing and stabilizing the nanoparticles, addition of a hole scavenger such as ethanol is required to efficiently exploit the excited electrons and avoid e−h recombination processes. This strategy holds an advantage of permitting site-selective and controllable deposition as will be described briefly next and discussed in detail in section 3.1.
The first demonstrations of photodeposition on metal− chalcogenide semiconductor NCs were presented for CdS−Pt and CdSe/CdS−Pt NRs 63 along with Au growth on similar semiconductor structures, allowing different metal deposition morphologies and architectures. 27,64,65 While photodeposition of Au domains contains similar metal precursors and surfactants, typically, AuCl 3 , DDAB, and DDA, respectively, Pt light- induced growth required different reactive precursors in comparison to the thermal deposition method. Pt domains were obtained in the presence of (1,5-cyclooctadiene)dimethylplatinum(II) as the platinum source and an excess of a tertiary amine (such as triethylamine and diisopropylethylamine). Combination of seeded growth principles and photodeposition allowed for size control and bimetallic deposition. Since the formed metal island serves as an electron sink for the photoexcited electrons, the reduction of the nearby metal ions takes place on the preexisting metal domain. Li et al. reported on Pd/Au alloyed metal-tipped CdSe/CdS NRs, as can be seen in Figure 4A. Following UV excitation of CdSe/CdS−Au HNPs in the presence of PdCl 2 and additional surfactants such as tetraoctylammonium bromide (TOAB) and DDA, an increase in the average diameter is observed due to Pd deposition. 66 Further examples of different metal and bimetals deposition were obtained via photodeposition with slightly modified procedures, mostly by introducing different surfactants and various semiconductor/metal ion ratios. CdS−Au NRs with tunable metal domain sizes were formed via light-induced Au growth over activated CdS nanorods with a small metal tip (thermally deposited under dark conditions). 28 Formation of CdSe/CdS−Pd along with combined deposition of Au and Pd as the core/shell and alloyed tipped HNPs was presented by Bar-Sadan and co-workers ( Figure 4B). 67 Au and the Pd/Au domains in both textures showed spherical structures, while Pd metal deposition resulted in a match-like morphology with a quasi-rectangular shape. Similarly, the compositions of Au and Pt with both core/shell and Pt-decorated Au domains were achieved via photodeposition on CdSe/CdS HNPs. 68 Aqueous photodeposition was also reported, achieving CdS−Pt, 69 CdSe/ CdS−Au, 70 and CdS−Ni nanorods 71 and nanoparticles 72,73 following phase transfer performed via ligand exchange of the native organic ligands on the surface of the semiconductor component. In the latter case of Ni deposition, metal growth on the semiconductor segment was obtained "in situ" during the photocatalytic reaction of hydrogen generation.
Other semiconductor structures were shown to allow photoreduction of metal islands including Ni over CdS nanosheets 74 and Au on CdSe/Cd 0.5 Zn 0.5 S core/shell NPLs. 39 Recently, toward adopting greener synthesis protocols, photodeposition in ionic liquids in which additional surfactants are redundant has been developed for Pt, Au, and Ag deposition on CdSe−CdS NRs, 75 achieving comparable results to the common amine-capped colloidal synthesis in organic medium. Another recent approach combines flow reaction synthesis with photodeposition to achieve controllable upscale of the HNPs syntheses. Continuous processes have been already utilized for different semiconductor and metal nanocrystals; nevertheless, Cohen et al. presented this kind of implementation together with light-induced reaction to achieve CdSe/CdS−Au, ZnSe− Au, and novel ZnSe−Pt NRs. 76 2.2.3. Direct Heterogeneous Deposition via Partial Transformation. Among the first suggestions for semiconductor−metal HNP formation was partial chemical transformation of the seed material, typically the metal component. During this procedure, the metallic surface is either oxidized or sulfidized to obtain a thin wetting layer of metal−oxide or metal−sulfide for a consecutive growth of the desired semiconductor phase. This strategy was exploited by Gu et al. to generate FePt−CdS heterodimer HNPs in a facile one-pot synthesis ( Figure 5A). 77 In this procedure, as illustrated in Figure 5A-c, first, an amorphous CdS shell is deposited over the FePt core bimetallic nanoparticles via sulfidation followed by metal ion and stabilizing ligand precursors addition at 100°C. Next, the segregation of the metallic core from the semiconductor shell to obtain the heterodimer took place following additional heating to 280°C in which the crystallization of the CdS component and a dewetting process took place. Similar control utilizing the reaction temperature was observed by Xu and co-workers upon introducing chalcogens following oxidation of Cd(acac) 2 to create a CdO shell over the FePt nanoparticles surface. A lower reaction temperature (258°C) resulted in core−shell structures, while higher temperatures (285−300°C) promoted heterodimers formation. 78 Further control over this synthetic approach was achieved through a moderate temperature ramp and different reaction times as established by Parak and co-workers. Moreover, this procedure was expanded to form additional classes of hybrids that comprise a FePt component and a II−VI or IV−VI semiconductor domain including CdS, ZnS, PbS, and CdSe. 79 The components of the semiconductor−metal HNPs can also be predetermined by the choice of the seed composition. Schaak et al. demonstrated the formation of Au−Cu 2 S HNPs by introducing sulfur precursors in oleylamine to AuCu-alloyed nanoparticles under constant air bubbling. 80 Similar approach of sulfidation in the presence of oleylamine at high temperature was demonstrated on PtCu-alloyed NPs, allowing the formation of Pt−CuS HNPs starting from either a Cu-seeded or a Ptseeded NPs. 81 The oxidative environment was necessary to activate the reaction by forming an oxide surface or intermediate. A general strategy for synthesizing binary and ternary HNPs based on sulfidation of the metallic seed was reported by Shi et al., allowing the formation of Au−PbS and Au−PbSe with dumbbell and tripod architectures. 82 An additional common pathway is the use of Ag deposition on metallic nanoparticles as a preliminary step toward semiconductor−metal HNPs generation. This approach was exploited to produce a crystalline semiconductor shell over a metallic Au core based on the Lewis acid−base reaction mechanism. 83 Sulfidation of the Ag layer created a Ag 2 S amorphous matrix that allowed the growth of the semiconductor lattice independently of the metallic core and thus circumventing the lattice mismatch between the two components, as illustrated in Figure 5B-a. Figure 5B-b presents the different synthetic stages of Au−CdS growth. The process starts by the synthesis of core metal NPs (i) through a soft Lewis metal deposition layer (ii) and formation of an amorphous structure (iii). The final step is a cation exchange reaction in the presence of a soft base (tributylphosphine) that gradually expels Ag ions from the shell (iv, v, and vi). The final composition of the semiconductor shell can be controlled by a cation exchange This strategy was generalized for different noble metals (Au, Pd, and Pt), and improved control of Ag growth on metallic seeds was gained by addition of an appropriate amount of S 2− ions. In this manner, sulfur ions assist to manipulate the reduction kinetics of Ag + ions, which results in the growth of Ag predominantly on one side of the preformed metallic nanocubes. 84 The metallic heterodimers were used as a platform for the formation of metal−CdS HNPs through successive steps of sulfidation and cation exchange as described above ( Figure 5C). Modification of this synthesis by Zhao et al. allowed for structural control and the formation of heterodimers instead of core−shell structures. An increase of the crystallinity of the Ag 2 X (X = S, Se, Te) shell as well as a higher reaction temperature of the cation exchange reaction led to a larger phase separation between Au and CdX to reduce the interfacial and grain boundary energies. 85 In addition, Zhao and co-workers demonstrated the formation of Ag− and Ag 2 S− (or Ag 2 Se−) CdS, ZnS, MnS, and CdSe hybrid nanorods via controlled sulfidation (or selenidation) of Ag NPs. 86 Different morphologies, architectures, sizes, and shapes are available depending on the surface coating, reaction temperature, metal type, precursors, and concentrations, as will be described in detail in the next section.

Synthetic Control over Structural Properties of Hybrid Semiconductor−Metal Nanoparticles
As was described in the former section and previously reviewed extensively in the literature, 2,7,14,87−89 several synthesis mechanisms have been developed for the growth of HNPs. Applying one or a sequential combination of these strategies allows for good control over the structural characteristics of the HNPs in terms of their morphology and composition, which ultimately leads to different chemical and physical properties. 90 This section focuses on controllable synthesis and structural characterization of different semiconductor−metal hybrid nanosystems. This includes site-selective deposition, tunable size and shape, and a variety of material compositions of both the semiconductor and the metal segments.
2.3.1. Site-Selective Deposition. Selective deposition of either components of the semiconductor−metal HNPs may be dictated by the morphology and crystal structure of the semiconductor or metal phase. The crystal morphology and surface capping induce different chemical reactivity for different facets of the semiconductor nanocrystal, which can lead to specific growth of the second-phase material on the more reactive facets of the nanocrystals of the first-phase material. The architecture of site deposition can be determined through various synthetic approaches. Both thermal deposition and photochemical reduction methods can be used, resulting in different deposition patterns dependent on surface coating, reaction temperature, metal type, precursors, and concentrations.
A first demonstration of site-selective deposition dictated by the anisotropically shaped crystal structure was reported by Mokari et al., presenting Au single-metal domain deposition on the apexes of CdSe and CdS nanorods and tetrapods. 1,23 In principle, the preferential growth on the tip sites, in this case, is originated from the enhanced reactivity of the (101) or (001) terminal facets of the semiconductor crystal leading to a lower free energy barrier for the heterogeneous nucleation on these sites. 91 Furthermore, a transition from two tips to a single tip was observed and ascribed to an electrochemical Ostwald ripening mechanism stabilizing the larger single tip preferably over two tips. 23 In addition, a controlled site-selective deposition where the metal precursor concentration acts as a knob for different site deposition resulted in the formation of single-tipped CdSe/ CdS−Au nanorods along with dumbbell-like structures and body decoration on the semiconductor surface with increased Au concentrations. 92 This control was attributed to the hierarchy of the semiconductor facets reactivities where the sulfur-rich nature of one apex promotes initial deposition of a single Au domain due to strong Au−S interactions. With the addition of metal precursors, deposition on the less reactive Cdrich apex occurred. At saturated concentration conditions, spontaneous growth over defect sites on the surface of the semiconductor structure was observed.
Similar dependence on the polarity of crystal facets was observed for PbS−Au HNPs. 24 The deposition of Au occurred favorably on PbS facets with the highest reactivity. In the reported cubic crystal system, the growth rates on common crystal planes as deduced formerly for various NCs can be ordered as [111] > [110] > [100]. 93,94 This suggests that Au would preferentially grow on the {111} facets. Because of the different polarities of the (111) and (111) facets, only four of the {111} facets were deposited and the PbS−Au 4 (4 metal domains in average) nanostructure was formed. At high Au precursor concentration, gold would saturate all of the {111} facets and PbS−Au 8 (eight metal domains in average) was formed. At an even higher Au precursor concentration, a gold-crowned cubicshaped heterogeneous nanostructure was achieved. This evolutionary metal deposition as the metal precursor concentration increases is illustrated in Figure 6A.
An additional example of the semiconductor NCs templating the Au site deposition is manifested by Au deposition on SnS nanocubes of different sizes. Reducing HAuCl 3 in the presence of small nanocubes (<25 nm) in aqueous solution resulted in decorated SnS−Au HNPs with sub-5 nm sized metal domains. However, a similar reduction reaction in the presence of large SnS nanocubes (>50 nm) formed large single-metal domain growth on the corner of the nanocubes and a heterodimer structure ( Figure 6B). 95 Thus, the different sizes of SnS promoted different reaction routes. The decorated form that is only observed for small sized SnS is governed by the minimum hydrophobic surface, which is easier for penetration into the aqueous phase, and random metal reduction is observed. However, larger sized SnS nanocubes have a larger hydrodynamic radius; therefore, its dispersity in aqueous media is poor. Hence, Au deposition is slow and thermodynamically dependent, resulting in selective deposition on favorable facets. In the case of SnS, the [021] polar axis of SnS allows zero lattice mismatch, which acts as the driving force for the specific attachment of the metallic Au on the S end sites, and this also leads to the epitaxy along the (131) plane of SnS with (111) of Au.
Another example of the morphology dependence was demonstrated for cone-like CdSe/CdS tetrapods, which exhibited increased selectivity toward single Au-tipped deposition as a result of the tapered arm structure, in comparison to regular tetrapods with cylinder-like arms. 34,96 This tendency for tip growth is ascribed to an enhanced intraparticle electrochemical Ostwald ripening process due to the presence of more surface atoms in the cone-like structure in comparison to the cylindrical morphology.
Other shapes were found to dictate different hybrid architectures. Core/shell structures of CdSe−Au dihexagonal pyramidal HNPs were formed following mild AuCl 3 reduction. 97 This shell growth is attributed to the lower ligand density on the surface of the pyramidal nanocrystal in comparison to rod structures. Hence, more surface is accessible to the metal precursors to be deposited and forms a continuous shell. Further addition of a strong reducing agent or e-beam irradiation was reported to transform the metal shell to multiple isolated metal domains on the nanocrystal surface, as shown in Figure 6C.
Other methods of controlling the site and the morphology of the metal deposition are extensively reported in the literature. Pradhan and co-workers exploited the lattice mismatch of Au with Bi 2 S 3 (ca. 2.81%) to form unique HNPs with Au nanoparticles positioned at the center of Bi 2 S 3 NRs ( Figure  6D). 98 The authors attributed this morphology formation to the ability of Au particles to catalyze and enhance the rate of the 1D agglomeration along the [001] direction of the Bi 2 S 3 nanoparticles which possess minimized lattice mismatch with the metal. Recently, Boldt et al. reported selective metal growth of different metals (Pt, Pd, Au) on semiconductor nanorod structures with enhanced selectivity for metal tellurides over the lighter chalcogenides. 99 CdSe/CdS NRs with CdTe tips showed specific metal growth over the CdTe segment with suppressed body decoration over the nanorod surface. This behavior was attributed to a trap-mediated nucleation mechanism, in which metals are rapidly deposited at electrondeficient surface traps located at the Te−Te bond at the nanocrystal surface. The reduction potential at these trap states is sufficient to reduce the cations Pt 2+ , Pd 2+ , and Au 3+ .
Another strategy to arrest defect growth and surface metal decoration is through mediation via effective ligand capping and suitable temperature control. 27,100 Single-tipped CdS−Au were achieved by lowering the reaction temperature down to 0°C. At these conditions, phase transition of the alkyl chains of the surface amine ligands (DDA) is taking place and a static phase is formed, preventing the diffusion of Au ions to the semiconductor surfaces. Similar results were obtained by replacing the surface ligand with longer alkyl chain amines, octadecylamine, already at 25°C, which go through phase transition at ∼32°C 101 ( Figure 7). In addition, a postsynthesis temperature treatment was reported to suppress the multiple Au islands growth along the CdSe nanorod. Via thermal annealing, intraparticle Ostwald ripening is induced, combining both atomic and cluster diffusion. 102 At high temperature under vacuum, the smaller Au islands migrated to the apexes of the CdSe nanorod, forming a larger single Au tip at that site. Moreover, the high-temperature treatment allowed one to overcome the energy barrier for the thermodynamically most stable configuration, resulting in a high-quality epitaxial interface between the semiconductor and the metal domains. Recently, this ripening effect of small metal islands migration to form a single large metal domain was achieved by the Langmuir− Blodgett process in which the air/diethylene glycol interface facilitated ligand exchange and ripening along the rod surface. 103 A phase transition in the behavior of the capping ligand at different temperatures was also demonstrated to affect the selective deposition of Ru and Rh over Cu 2 S seeds. 57,58,104 At a distinct temperature of 205°C, metal growth along the defined edges of the faceted nanocrystals was achieved ( Figure 7B-e). At this temperature, a change on the seed surface occurs, as was verified by differential scanning calorimetry analysis, which increased the surface reactivity toward heterogeneous metal nucleation. Metal deposition at lower or higher temperatures (190 and 210−220°C) resulted in homogeneous nucleation or metal netlike structures, respectively ( Figure 7).
Chemical reaction conditions such as anaerobic or aerobic environments have been used to manipulate the selective deposition of metals on semiconductor components in the case of CdS−Au NRs. While an inert atmosphere led to the growth of single-metal domains on the apexes of the rod structure, in the presence of air and humidity, dumbbell-like and multiple-defectsite Au growth was obtained. 26 When the metal reduction was performed under air, in the presence of dissolved oxygen, or with trace amounts of water in the solvent, the etching rate is enhanced or activated, contributing to an increase in defect site formation which in turn promotes Au ions reduction on it ( Figure 8).
Depending on the reaction conditions, such as different surfactants, the metal domains can be selectively deposited. Single or multiple Pt domains were selectively grown on CuInS 2 nanoparticles by using TOP or acetylacetonate as the metal coordinating ligand, respectively ( Figure 9). 47 On the basis of hard and soft Lewis acid and base theory, in the presence of TOP, which is a soft Lewis base, its Pt binding is seemingly stronger than the hard Lewis base, acetylacetonate. Consequently, this attenuated the Pt reactivity and allowed for   single-domain growth. Moreover, further limitation of the Pt reactivity was achieved by removing the additional reducing agent, 1,2-hexadecanediol, which was present in the multiplemetal island growth ( Figure 9B).
In a similar manner, CdSe−Au NPLs with metal deposition in different morphologies were formed depending on the reaction surfactant and reducing agents. Small Au domains growth (<5 nm) on the corners of the nanoplatelets was observed in the presence of DDA and DDAB in toluene. However, using oleylamine in 1-octadecene revealed a quasi-spherical growth at the shorter edges of the NPLs with an Au domain size of ∼5.0 nm. 41 The authors noted that a possible fusion of initial corner growth to form larger domains at the entire edges might occur given the close proximity of the corners. Moreover, deposition of different metals including Pt and Pd resulted in different site metal deposition. Pd in a quasi-rectangular morphology grew in plane on the NPLs shorter edges, while Pt domains were deposited at all edges around the NPLs where maximum crystal defects are present ( Figure 10A). It is assumed that in comparison to Au coalescence, Pt has lower mobility along the edges of the NPLs so that growth takes place at the nucleation sites only. Similar morphology control by the choice of the reducing agents was demonstrated by Manna and coworkers, synthesizing either CuS−Au core−shell or Janus hybrid NPLs via Au ion reduction in the presence of ascorbic acid or oleylamine/ascorbic acid surfactants, respectively. 105 Differences in the growth of noble metal domains may also be attributed to the different reactivities of the metal precursors present in the respective systems. In a sequential work by the same group, quaternary Au-and Pt-decorated CdSe/CdS core/ crown NPLs were presented with site-selective metal deposition. 43 In all cases, independent of the CdSe core and CdS crown size, Pt domains are found only at the edges of the NPLs ( Figure  10B). In this case, deposition of Au, was observed in different sites depending on the CdSe core and CdS crown dimensions. Small quasi-spherical Au domains surrounding the edges was seen on small CdSe core NPLs ( Figure 10B-a,d) and single or multiple larger Au islands (4−6.5 nm) on the surface of large CdSe core with small and large CdS crown NPLs ( Figure 10Bb,c,e).
Light-induced reduction is another pathway for tuning and selectively depositing metals on semiconductor components. A complementary strategy to thermal annealing and Ostwald ripening (described above) in forming one-side Au-tipped CdS or CdSe/CdS NRs was presented by Sonnichsen and co-workers and others 27,106 via UV light excitation of CdS or CdSe/CdS semiconductor nanorods in Au precursors and surfactant solution. In this method, the ripening mechanism was suppressed, and instead, light-induced metal growth on preillumination metal nuclei was suggested ( Figure 11). 65 Following spontaneous growth of small metal domains favorably on the rod apexes, upon UV excitation, the excited electrons created in the semiconductor migrate preferentially to one metal tip, reducing further Au 3+ ions on the seed and resulting in a single large metal domain at the expense of the initially formed sites. Metal growth was monitored via absorption measurements, allowing the calculation of metal domain volume along the reaction time ( Figure 11e). To neutralize the system, excited holes are scavenged by sacrificial hole scavengers in the solution such as ethanol. In a similar manner but a less controlled deposition, CZTS−Au nanorods were obtained. 36 While thermal metal growth formed a surface decoration of multiple small metal particles (∼2 nm), applying light-assisted metal deposition yielded two large metal sites (∼15 nm) per rod in average, yet, surface decoration by small metal domains was not entirely arrested.
Photoreduction deposition was also utilized to selectively deposit metal domains on specific sites based on the semiconductor heterostructure composition. Comparing Pt light-induced deposition on CdS nanorods and CdSe/CdSseeded nanorods revealed different metal decoration. 63 While Pt growth on CdS NRs showed heterogeneous growth on their surface with up to 6 islands per rod, mostly a single-metal domain was observed following photodeposition of Pt on CdSe/ CdS NRs. Moreover, the location of the Pt island is near the CdSe seed. This specific site deposition is ascribed to the localization of the formed excitons under illumination. This trend was also observed for CdSe/CdS−Au NRs where a large Au domain is deposited at the CdSe seed region under irradiation conditions ( Figure 12A). 64 Similarly, The preference of this site was attributed to the electronic profile of the heterostructure in which excited electrons and a hole are relaxing at the CdSe seed location and allows Au ion reduction at this site ( Figure 12A-a). The effect of lattice mismatch and therefore enhanced surface defects at the seed region was considered to be less dominant since ZnSe-seeded CdS nanorods do not show any preferential growth location, even though the seed region is shown to be more defective than in the case of CdSe-seeded rods. 107 Moreover, less effective selective deposition was observed for a thicker CdS shell around the CdSe seed due to a larger potential barrier, which further supports the suggested mechanism. Further extension of this selective deposition methodology was demonstrated in photodeposition of Pd nanoparticles on CdS 0.4 Se 0.6 nanorods which have CdSe-and CdS-rich domains on opposite apexes of the rod. A dependence on irradiation wavelength was observed, allowing for preferred growth at the CdSe-rich end upon irradiation in the red or on both CdSe-rich and CdS-rich regions upon blue irradiation ( Figure 12B). 108 This partial site selectivity was explained by the varying band gaps and absorption properties of the two different semiconductor regions (CdSe has a smaller gap than CdS) accompanied by trapping of the electrons on surface defects in that area.
The charge transfer of an excited charge carrier to already deposited metal domains upon light stimulation can be also exploited for selective deposition of additional different metal types on top of the former metal decoration as will be described in detail in section 3.2.

Morphology: Size and Shape Control.
Another aspect of structural control of the HNPs synthesis is tuning and tailoring the morphology of HNP structures along with their component's sizes. The size and shape of both the semiconductor and the metal components can affect the physical and chemical properties of such hybrid nanosystems. Early investigation of semiconductor−metal HNPs showed fair control of the hybrids morphology via temperature-dependence syntheses. CdS−FePt have been synthesized in core/shell and heterodimer structures depending on the reaction temperature. While Cd ions reduced on FePt-sulfidized nanoparticles at 100°C formed a core/shell structure, raising the solution temperature to 280°C converts CdS from its amorphous to a crystalline state, accompanied by a dewetting process, and results in heterodimers of FePt−CdS HNPs. 77 This method was also demonstrated for the growth of CdS, ZnS, PbS, and CdSe on the FePt nanoparticle's surface, achieving improved stability and control by increased surfactant concentration in the reaction and gradually raising the reaction temperature in a 5°C/min rate. 79 FePt−CdS/Se sponge-like nanostructures were obtained by first introducing chalcogen (sulfur or selenium) precursors, promoting the formation of nanowires (NWs) of chalcogens which serve to connect FePt nanoparticles. 78 Successive addition of Cd(acac) 2 allowed formation of the hybrid nanosponges.
Further control of the anisotropic nature of Janus HNPs was gained by taking advantage of chemical thermodynamicsdirected colloidal strain. Au−Ag core/shell nanoparticles were used as seeds for initial sulfidation toward Au−Ag 2 X (X = S, Se, Te). The degree of Ag 2 X crystallinity dictated the extent of component separation following cation exchange to form a family of gradually separated Janus HNPs including Au−CdS, Au−CdSe, and Au−CdTe ( Figure 13A-a). 85 In general, the higher crystallization of the Ag 2 X shell as well as the higher reaction temperature of the cation exchange reaction lead to larger phase separation between Au and CdX, reducing the interfacial and grain boundary energies ( Figure 13A-b). The controlled separation of Ag 2 X from Au seeds was also reported for Au/Ag NRs where selenization by different Se concentrations allowed the formation of Au−CdSe core/shell NRs and CdSe-tipped Au nanorod structures ( Figure 13B). 109 Patra et al. showed morphology control of Au−CdSe HNPs by varying the precursor concentrations and reaction temperatures. CdSe growth on the Au nanoparticle surface in shapes of nanoflowers, tetrapods, and core/shell was achieved ( Figure  14A). While the same Se concentrations were used in the nanoflower and tetrapod structures, the lower reaction temperature in the flower-shaped synthesis decreased the growth rate along the [0001] direction of the wurtzite crystal and allowed also growth in the ⟨2110⟩ direction as shown and illustrated in Figure 14A-e−g. 110 A Core/shell architecture was obtained using a lower chalcogenide precursor concentration in which insufficient monomer in the reaction system did not allow them to grow further to obtain nanoflowers.
Using a related approach, a different type of control over the HNPs morphology was demonstrated in the synthesis of Cu− Cu x S HNPs. This was achieved by sulfidation under two different environments, H 2 /argon and argon. While the former resulted in Janus formation, the latter produced predominantly hetero-oligomer HNPs. This morphology control is assigned to an outcome of an interplay between a kinetically controlled reaction in the presence of H 2 as a reducing agent and a thermodynamically controlled reaction governed by selfdiffusion of the ions. 111 Au−Cu 2−x Se HNPs with different morphologies were synthesized by adding different polymers as stabilizers during the selenization process. 112 Recently, a different approach of morphology control was achieved by changing the pH value of Au−CdSe HNPs aqueous mediated synthesis. The synthesis procedure included Ag deposition and selenization followed by Cd(NO 3 ) 2 cation exchange at 2.5, 4.5, 7.2, and 8.1 pH values. Janus nanospheres, heterodimers, symmetric double-headed nanoparticles, and multiheaded nanoparticles were formed, respectively ( Figure 14B). At higher pH values, both Ag deposition on the Au surface in the first step and Cd ion exchange in the second step are accelerated, promoting a thicker Ag shell and stronger Ag 2 S ripening, respectively, both contributing to additional selective growth sites. 113 In addition, the increased growth of the CdSe sites induces a plasmon energy band red shift due to an increase in the effective refractive index of the environment ( Figure 14B-e).
As the size and shape of the semiconductor part can be synthetically controlled, HNPs with different sizes of metal components can be obtained across extended regimes via diverse synthetic approaches. Deposition of small cluster size domain metal particles (up to ∼200 atoms) was achieved using laser ablation methods. 114 Larger sizes, from several to a few tens of nanometers, were achieved via controlling the reaction temperature and/or precursor concentration. The latter approach was demonstrated on different hybrid nanosystems in several studies including CdX−Au nanorods (X = S, Se, Te) where metal tip sizes reaching up to 40 nm in fast reactions times (<120 s) were achieved ( Figure 15A). 115 Similar control was reported for Au growth over the edges of covellite copper sulfide NPLs. Increased metal concentration and reaction times formed larger sized domains and a higher number of metal domains per particle as well ( Figure 15B). 62 Size control of Pt and Ni metal domains was demonstrated for CdS NRs, 116 CdSe, and CdSe/ CdS NRs 117 and NPLs, 41,43 depositing Pt nanoparticles from 0.7 to 3.5 nm and Ni domain in the range from 2.3 to 10.1 nm. Thermal control was also suggested to allow tuning of the metal tip size. By using oleylamine solution and gold−oleate complexes as precursors at a temperature range between 95 and 140°C, 118 the Au tip sizes ranged between 3 and 16 nm, whereas higher temperatures yielded larger metal tips in the hybrid nanoparticles.
An additional degree of control was achieved by consecutive two-step synthesis combining spontaneous chemical reduction and light-induced reduction deposition. Specifically, following spontaneous metal nucleation in dark conditions to form siteselective small metal islands on the apexes of the CdS NRs, light-induced metal deposition at low temperature (2−4°C) allowed the metal domain size to be controlled by varying the Au ions/ NRs molar ratio ( Figure 15C). 28 With increased metal domain sizes, stability and aggregation phenomena have been reported especially in organic media. A novel synthesis in an aqueous high dielectric environment was reported to achieve high stability and size control of Au metal tips on CdSe/CdS NRs. 70 Au metal domains close to 50 nm in size were obtained by photochemically reducing Au ions on already phase-transferred NRs in ethylene glycol/H 2 O solvent with polyvinylpyrrolidone (PVP) as the reducing agent. The choice of PVP also allowed further surface functionalization of the gold domain since it is only loosely covered by PVP surfactants.
In addition, control of the metal domain shape was achieved by selective deposition of faceted Pt metal tips on CdS nanorods. By successive metal deposition reactions, nucleation of nonfaceted Pt tips on CdS nanorods was followed by facet-selective Pt deposition in the presence of CO molecules that favor the (100) facet growth over the (111) facet due to stronger binding to the former. 119 This strategy resulted in cube-shaped Pt tips with well-defined (100) facets.
Control over the morphology of HNPs can also dictate their assembly in more complex formations such as colloidal networks, 120 gels, 38 and aerogels. 106,121,122 Recently, Bigall and co-workers showed the influence of metal decoration architecture on the structural properties of HNPs gels and aerogels. While random Au metal decoration on the surface of CdS NRs led to a network which was mainly connected between the NRs (rod to rod connections), Au-tipped CdS NRs result in gold only in the connection points between the NRs. 106 Similar structural effects were reported for CdS NPLs decorated with Pt NPs. 121 Hybrid NPL network gelation reveals a homogeneous distribution of the metal in the cryoaerogel, in contrast to accumulation of Pt−NPs in close vicinity in the case of mixing and cogelation of NPLs and Pt NPs. Such differences in the metal distributions are expected to have a significant impact on the optical and chemical properties of HNP-based aerogels.
2.3.3. Material Composition Control. As described above, hybridization of the two different materials, semiconductor and metal, provided new synergistic properties. In addition, combination of more than one type of semiconductor or metal component can expand the chemical and physical characteristics of such HNPs. The influence of such combinations was previously presented in bimetallic nanoparticle systems, where changes in a variety of properties were observed including electric field enhancement effects, modification of surface plasmon properties, magnetic functionality, and, most notably, enhanced catalytic activity. 123  used as in the former examples or chemical reduction in the presence of both metal precursors on the semiconductor surface or apexes was used as demonstrated for CdS NRs tipped with PtNi and PtCo alloys. 44 A different mechanism of exploiting the favorable heterogeneous growth on a pre-existing metal domain serving as a substrate, over homogeneous nucleation of freestanding metal nanoparticles in solution, was reported by Pyun et al. Activation of small Pt islands at the apexes of such NRs allowed the formation of a Co shell or a hollow cobalt oxide shell via a Kirkendall effect in the presence of O 2 under high temperature. 55 This kind of synthetic control, similarly to the other forms of structural control, can be achieved by a light-induced route as well. Selective deposition of Pd on Au-tipped CdSe/CdS NRs was conducted under UV illumination. Exploiting the formed excitons to provide reductive charges at a specific site, the preexisting Au seed, due to charge transfer from the semiconductor segment to the metal domain, led to the reduction of Pd ions and formation of an alloyed AuPd tip which revealed interesting magnetic properties. 66 This strategy was adopted and utilized for the formation of several bimetal tip deposition in different hybrid nanosystems. Au-tipped CdSe/CdS NRs were used as template for photochemical reduction of other noble metals such as Pt 68 and Pd. 67 Controlling the order of metal precursors addition and depending on their reduction potential (Au possesses a higher reduction potential than Pt), different morphologies of the binary metal tip could be achieved, from core/shell through core and alloyed shell and formation of a single-metal core with small secondary metal islands deposition ( Figure 16A). Similarly, such morphology control was shown by Bar-Sadan and co-workers on CdSe/CdS−Ag/Pd NRs, where an Ag−Pd core/shell structure was obtained along with alloyed tips ( Figure 4B). 127 The tendency for Ag cation exchange to form Ag 2 S was avoided by slow addition of the reactants and an overall low loading (∼5% of the existing Cd atoms in the rods). In addition, the use of illumination for the photodeposition of the metals facilitates the reduction of Ag + to Ag 0 . Other hybrid nanosystems syntheses were also reported to employ photochemical deposition including NiCo-decorated Zn 0.5 Cd 0.5 S nanoparticles 128 and PtPd on CdS nanoparticles 129 utilizing these structures for photocatalytic applications.
Along with the deposition of binary metal domains, disparate metals and metal chalcogenide types on different sites were also synthetically obtained. Mokari et al. exploited the nanorod architecture to synthesized PbS−CdS−Pt HNPs where the metal and metal chalcogenide domains are each deposited on the opposite apex of the rod structure ( Figure 16B). 130 By depositing first the Pt by solvothermal growth on one side due to the favored reactive facet, it serves as a blocking material, preventing further growth at this site and promoting secondary PbS growth on the opposite apex through thermal decomposition of Pb−bis(diethyldithiocarbamate). In a similar manner, asymmetrically tipped PdS−CdSe/CdS−Au NRs were obtained. 131 Following Au deposition by common spontaneous metal growth in the presence of DDA in toluene solution, cation exchange at 180°C was performed to achieve PdS on the other rod tip ( Figure 16C). A different synthetic approach was presented by Li and co-workers to form asymmetric Cu 1.94 S−Zn x Cd 1−x S−Pt HNPs. 132 In this case, Cu 1.94 S nanospheres were initially prepared as starting materials followed by epitaxial growth of Zn x Cd 1−x S. Pt decoration at the surface of the Zn x Cd 1−x S component was performed via photochemical deposition in aqueous solution. Combination of separate domains of Ru and Pt on CdSe/CdS NRs was also obtained by presynthesis of CdSe−Ru hybrid dimers and their use as seeds for epitaxial growth of a CdS rod-like shell. 133 Controlled and gradual displacement of the dimer surface ligands was found to stabilize them and minimize detachment. Lastly, Pt thermal deposition of small domains mostly at the rod tips was conducted ( Figure 16D). Sequential metal deposition on CdSe NRs of Pt, again by thermal deposition and thereafter Au growth via dropwise addition of Au ions in DDAB and DDA toluene solution, resulted in nanodumbbells with heterometal tips, Pt−CdSe−Au. 134 Note that the inverse sequence of the precursor addition did not form the asymmetrical architecture because an irregular deposition of Au occurred at the CdSe nanorods. These two metal types were also deposited separately on MoS 2 −CdS nanostructures (CdS nanospheres on MoS 2 sheets). 135 Au loading was formed through reducing HAuCl 4 by sodium citrate followed by photochemical deposition of Pt, which allowed control of their size and density.
Another example of synthetic control via sequential metal deposition was demonstrated by binary metal deposition of Au and Pt on CdSe/CdS core/crown NPLs. 43 Au growth, followed by Pt deposition, formed a Pt−Au alloy or core/shell type morphology. Surprisingly, Au deposition on already decorated  Along with the variety of deposited metal component combinations, heterostructures of the semiconductor components have also been realized in the formation of HNPs. As was mentioned throughout the current and previous sections, different combinations of metal chalcogenides have been reported. These kinds of heterostructures reveal a different band alignment and therefore different electronic profile such as type I, type II, and quasi type II. The final energy band alignment of the HNPs with the specific metal deposition can influence and dictate the performance of the HNPs in its designated application, where different applications require different energy band design. The effect of the material combination on the HNPs characteristics and on their utilization in various applications will be discussed in the following sections.

SYNERGETIC PROPERTIES OF HNPS: WHOLE IS GREATER THAN THE SUM OF THE PARTS
HNPs hold unique properties which arise from the combination of two disparate materials with distinct different physical and electronic characteristics. The hybrid properties are either combined attributes of each of the segments that present both elements properties at the same single system or often a synergistic combination manifesting new properties that are a consequence of the semiconductor−metal interface. Notably, the resulting properties of the combined materials depend not only on their type but also on the size and morphology of the HNPs and the materials interface.

Optical Properties
3.1.1. Absorbance. Conjugation of semiconductor and metal segments in a single nanocrystalline system may introduce new hybrid electronic states at the semiconductor−metal interface. Therefore, it is expected that the overall optical and electronic properties of such hybrid systems will be affected in comparison to their components. In HNPs, the coupling between excitons and plasmons becomes especially strong near resonance in which the exciton energy lies in the vicinity of the plasmon peak which results in broadening and a shift of the first exciton transition in the semiconductor spectra. 136 This optical alternation is highly pronounced in HNPs combining semiconductors where the excitonic band-gap transition and the plasmonic features of the metal segment are spectrally separated from each other.
Experimental observations of exciton−plasmon coupling were seen for Au−PbS core/shell HNPs 137 and CdS−Au NRs ( Figure 17). 26,29 In the latter hybrid nanosystem, the native CdS excitonic transitions as well as the Au surface plasmon resonance (SPR) modes are well separated and still discernible. Still, some changes are seen ( Figure 17B). A blue shift to higher energy accompanied by optical broadening was observed in the hybrid form in comparison to pristine CdS semiconductor NRs. Suppression of the excitonic features following Au growth was also observed and investigated by Zamkov et al. on similar CdS− Au HNPs with different metal domain sizes synthesized by a thermal reduction procedure. 118 This phenomenon was attributed to the tunneling of CdS excited charge carriers into Au domains. 138 This property is dependent on the size of the Au tips where for small-diameter Au tips, the delocalization of CdS electrons into the Au is limited to a few nanometers, which is insufficient to alter the character of quantum confinement in CdS. On the other hand, larger Au domains permit larger electronic delocalization into the metal, which in turn leads to the "washing out" of the excitonic transitions.
Along with the excitonic alternation, the plasmonic resonance in HNPs is red shifted compared to free-standing metal nanoparticles. This shift is ascribed to the change in the dielectric constant of the nearby surroundings of the metal component. In general, the plasma frequency of the bulk metal,  where n e is the electron concentration, m* is the effective mass of the electron, e is the charge of an electron, and ε 0 is the permittivity of vacuum. For a metal nanoparticle, the surface plasmon resonance frequency, ω SPR , can be expressed by the following relation according Mie theory 140 where ε m is the environment dielectric constant. Since the plasmonic resonance is inversely dependent on the dielectric constant of the environment, a coupled semiconductor domain with higher refractive index will result in a decrease in the plasmonic frequency and a spectral red shift. This shift is much more pronounced in core/shell HNPs structures due to the spherical symmetry as was demonstrated for PbS−Au core/shell and dumbbell-like nanoparticles. SPR shift to lower energies between 20 and 100 nm was measured, respectively ( Figure  17C). 82,137 The extent of the plasmonic shift can be tuned through changing the type and shell thickness of the semiconductor component. Such control was demonstrated for Au− Ag, Ag 2 S−Au, and CdS−Au core/shell NRs that show both transverse and longitudinal SPR signals. 141 A blue shift was observed upon growth of the metallic silver shell around the Au nanorods with respect to the Au rods SPR signal. For the CdS shell, a red shift is seen, which increased with shell thickness. A similar trend was recently reported for Au−CdS core/shell nanoparticles with different shell thickness from 3.6 to 14 nm, revealing a plasmon shift from 548 to 608 nm (center wavelength of the Au LSPR band), respectively ( Figure  17D). 142 For the case of matchstick-like HNPs such as Autipped CdS, the plasmonic shift was found to be more moderate, up to 10 nm, 26,29,143 as the surrounding environment of the gold tip is not homogeneous, where the tip is directly exposed to both the semiconductor domain on one side and the solvent on the other side. Additional manipulation of the plasmonic features of the metal component was reported by synthesizing the Cu 2 Se shell over Au NRs. In the absence of oxygen, the spectrum of the Au−Cu 2 Se core−shell HNPs exhibit metallic behavior, where the characteristic transverse and longitudinal plasmon bands of the AuNR cores are dominant. Under aerobic conditions, oxidation of the semiconductor shell resulted in vacancies Chemical Reviews pubs.acs.org/CR Review (Cu 2−x Se) that lead to diminishing of the longitudinal plasmon band of the Au due to the gradually overlapping and ultimately dominating transverse mode of the shell. 144 Measurements of extinction cross section as a function of wavelength of the CdS−Au HNP and a mixture of its individual components, CdS NRs, and Au nanoparticles showed a redshifted plasmon peak from 527 to 538 nm by about 50 meV ( Figure 18A). 143 This observed shift, as explained above, is related to the larger real part of the refractive index of the semiconductor compared to that of the solvent surrounding the metal tip. Comparison with discrete dipole approximation (DDA) simulations on HNP ( Figure 18A-b) with the same dimensions taking into account only electrodynamic interactions by describing the individual components by their independent dielectric function showed qualitative agreement with the experimental measurements. Other different HNP architectures including Au−CZTS 53,145 and Au−Bi 2 S 3 98 have presented a similar red shift trend yet with larger shifts, 60−100 and 40 nm, respectively. Such significant shift may suggest more efficient coupling of the metal plasmon with the semiconductor exciton. However, for HNPs where the optical features of the semiconductor and metal are in resonance such as CdSe−Au HNPs, the simplistic calculated electrodynamic treatment could not reproduce the observed absorption spectra. Typically, the absorption spectra of such HNPs exhibit very broad features with the tails extending to the low-energy region in their spectra. 1,146−149 The stronger coupling and interactions of both components may lead to a strong mixing of the electronic states that an electrodynamics approach solely cannot predict.
Additionally, upon excitation of free electrons by incident irradiation, the metal component can induce an intense electric field in its vicinity. 150 The proximity of the metal domain to the semiconductor component allows near-field effects that can lead to enhanced absorption in the semiconductor. 151 This field enhancement is maximized when these HNPs are irradiated at the corresponding plasmon resonance frequency. 152 The nearfield effect is expected to induce an enhancement in the oscillator strength of the dipole-forbidden transitions 153 Figure 18B). A gradient resonant electric field propagates along the nanorod long axis and allows quadrupole-induced transitions and even higher multipolar order transitions, which are forbidden under far-field selection rules. 154 This phenomenon was experimentally demonstrated on Au−PbS core/shell HNPs, where the absorbance cross section was enhanced by ∼28% compared to the measured absorbance of noninteracting separated Au and PbS nanocrystals. 137 Generally, the electric field enhancement strongly depends on the size and spacing between adjacent metal domain, as was reported for freestanding metal nanoparticles. 155,156 Recently, deposition of a Au nanochain (small distance between adjacent metal domains) on Zn 0.67 Cd 0.33 S NPs has shown enhanced formation of electron− hole pairs that led to improved photocatalytic properties in comparison to isolated Au metal domain decoration (3.5 times higher). 157 This was attributed to the strong plasmon coupling of the chain structure inducing highly intense and localized electromagnetic fields which in turn enhances the semiconductor segment absorption. As shown in Figure 18C, this enhancement was verified experimentally by Raman and UV− vis absorption measurements along with theoretical simulations using a three-dimensional finite difference time domain (FDTD) methodology.

Fluorescence Enhancement.
A well-known and highly applicable optical property of semiconductors is it fluorescence via spontaneous emission. When conjugating plasmonic metal and semiconductor nanomaterials into a single system, this radiative process can be either enhanced or quenched depending on the hybrid structural characteristics. These effects are strongly dependent on the distance between the metal and the fluorescent semiconductor NC, on the nature of the semiconductor−metal interface, and on the spectral overlap between the SPR and the emission spectra.
Fluorescence enhancement is typically associated with controlled separation between the two material components as was explored in molecule−metal nanoparticle interfaces. 158 The enhancement effect can be attributed to either of the two following origins: first, excitation enhancement, which relates to the increased excitations in the presence of an SPR electric field, also associated with the enhanced absorption discussed in the previous section and to the surface-enhanced Raman scattering (SERS) phenomena; 159 second, emission enhancement, which is affected by the coupling of the SPR field and the transition dipole moment of the emitting semiconductor NC, leading to an increased radiative rate. 160 The fluorescence enhancement is accompanied by shortened radiative lifetimes due to the inverse relation of lifetime and the radiative rate, which is proportional to the field enhancement factor. 161 One of the most prominent factors to control the fluorescence enhancement is the distance between the plasmonic metal and the emitting semiconductor components. 161−168 Demonstration of the distance effect on emission enhancement in semiconductor−metal nanoparticles was reported for CdSe/ ZnS core/shell nanoparticles deposited on Au colloids with a defined polyelectrolyte spacer allowing controlled distance between the semiconductor nanoparticles and the gold films. 162 Maximum enhancement by a factor of 5 was achieved for a 9-layer spacer (∼11 nm) due to a local enhanced electromagnetic field around the metal nanostructures ( Figure  19A). An additional example of controlling enhanced fluorescence by varying the distance between the two components was achieved by introducing different concentrations of Au nanoparticles to CdTe nanoparticles in aqueous solution, both being negatively charged. Their identical surface charges induce strong interparticle repulsion. By controlling the concentration and feed ratio of Au and CdTe, the interparticle distance between them can be tuned and hence the degree of fluorescence enhancement of the semiconductor NCs can be controlled. A 3-fold fluorescence enhancement of the Au−CdTe mixed solution was obtained compared to a CdTe NCs solution in the absence of Au NPs. 164 Colloidal Au/CdS and Au/ZnS core/shell HNPs linked with CdSe/CdS NPs were also used to study the distance dependence of fluorescence enhancement. A significant fluorescence enhancement was observed for 6−8 (CdS) and 8−10 nm (ZnS) shell thicknesses, with the yields dropping for thinner and thicker shells ( Figure 19B). 169 The authors assigned this effect mostly to the distance-dependent energy transfer rate with a minor contribution of spectral overlap of excitonic and plasmonic features of this hybrid assembly. A similar methodology of absorbed CdSe/ZnS semiconductor NPs on the surface of Ag/SiO 2 core/shell metal−semiconductor HNPs revealed a similar distance-dependent behavior, although in this case the effect was explained by excitation enhancement. 165 Additional parameters can influence the measure of fluorescence enhancement as was widely reported previously in the literature. The number of Au domains in the vicinity of emitting semiconductor nanostructures was found to be a key factor in this optical enhancement as was reported for CdTe NWs coaxially surrounded by Au NPs 161 as well as for ZnCdSeS−Au HNPs. 170 The collective character of an ensemble of interacting Au NPs promotes a strong electromagnetic field and therefore more efficient fluorescence enhancement compared to single or isolated Au domains.
Moreover, the size and shape of the plasmonic metal component can affect the degree of fluorescence enhancement. Royer et al. reported on a transition from a fluorescence quenching regime to enhancement behavior via the increased size of the Au metal NPs in an assembly with CdTe/CdS NCs while keeping a distinct distance of 3 nm between the two components. 171 Au NPs with diameters in the range of 60−160 nm were investigated. Au NPs larger than 130 nm in diameter yielded fluorescence enhancement, up to 260% for 160 nm diameter Au NPs compared to the reference sample. Smaller sized Au NPs (<130 nm) caused fluorescence quenching ( Figure 19C). The authors correlated this behavior with the SPR extinction, deducing that the extinction maximum must be red shifted with respect to the CdTe/CdS NC photoluminescence Chemical Reviews pubs.acs.org/CR Review wavelength to obtain fluorescence enhancement ( Figure 19Cc). The shape of the plasmonic metal was also shown to affect the fluorescence enhancement. 172 Using a rod-like structure of the plasmonic absorber induced stronger enhancement compared with spherical nanoparticles. Hybrid DNA-based assembly of CdSe/CdS with Au NPs and Au NRs showed a significant fluorescence enhancement of 15−75% depending on the different HNPS architectures in comparison to nonconjugated semiconductor NPs. 167 However, Au rod structures exhibit a greater magnitude of fluorescence enhancement (75% vs 48% for rods and spheres, respectively) due to the strong localization of the near field at the nanorod tips ( Figure 19D).
Along with the contribution of the metal component shape and size to the fluorescence properties, the effect of the morphology and architecture of the complete hybrid nanosystem has been investigated. Ramamurthy and co-workers reported on the improved up to 9-fold luminescence enhancement by Au body-decorated CdS NRs, while for the case of CdS−Au NPs, a 5-fold enhancement was observed. 173 Different metal sites deposition led to alternated photoluminescence. This effect was demonstrated by Au−AgCdSe HNPs with three different architectures. A single Au site at the apex of the AgCdSe segment in a microphone-like structure revealed the strong fluorescence in comparison to the negligible emission of the double Au sites on both apexes (dumbbell-like structure) and single site in the formation of a toothbrush-like structure. 174 Although quenching processes likely occurred in all nanostructures, the fluorescence enhancement of the mike-like architecture outcompetes these relaxation routes due to the overlap energies of the Au SPR and the band edge of CdSe exciton absorption which promote coupled field enhancement from the Au portion of the nanorods.
3.1.3. Fluorescence Quenching. As discussed above, the photophysical properties of HNPs are dictated by the unique interactions of the semiconductor−metal nanojunction. As was deduced for the case of fluorescence enhancement, the nanoscale separation between the two components of a hybrid nanosystem can also induce quenching effects as the distance between the semiconductor and the metal becomes smaller. In a   46 and CdTe−Au. 180 An example of the CdSe nanodumbbell quenching trend with increased metal deposition is provided in Figure 20A. Recently, with the synthetic development of 2D HNPs, hybrid structures such as nanosheets or nanoplatelets showed the same fluorescence quenching property as was demonstrated for CdS− Au, 61 CdSe−X (X = Au, Pd, Pt), 41 CdSe/CdS−Au, 42,43 and CdSe/CdS−Pt 43 NPLs and CdSe−Pt 181 and CdS−Ni 74 nanosheets ( Figure 20B). Although all studies report on the reduction in the fluorescence intensity upon metal deposition, there are differences regarding the charge carrier dynamics of this quenching process as will be further discussed in following sections.
As with fluorescence enhancement discussed above, quenching of HNPs emission has been shown to be size and shape dependent. An early example of the Au metal domain size effect on the fluorescence intensity was reported for ZnSe/CdS NRs where a sharp decrease in fluorescence intensity (up to 500-fold for a 4.5 nm diameter Au domain) with increasing Au NP size was observed. 1 Later, this trend was also reported for CdSe−Au HNPs with different sizes of the semiconductor as well as the metal components. 182 However, this monotonic behavior was shown to be further effected when taking into account different separating distances between the metal and the semiconductor segments. Time-resolved fluorescence measurements of HNPs with different sizes of Au NPs bound to CdSe/CdS/ZnS core/ shell/shell NPs with different lengths of spacer showed that for the smallest separations, the smallest Au NPs yield the fastest decay, while for larger separations, the largest AuNPs lead to the fastest decay ( Figure 20C). 183 Besides the size of the metal domain, Haldar et al. reported on additional shape effect comparing different sizes of Au NPs with Au nanorod structures conjugated to CdTe NCs. While quenching values by 47%, 65%, and 73% were measured for 2 ± 0.3, 9 ± 0.4, and 17 ± 0.2 nm Au NP, respectively, quenching by 86% was obtained for the Au NR conjugates. 180 Other studies have presented the influence of the number of either metal or Chemical Reviews pubs.acs.org/CR Review semiconductor sites on the fluorescence efficiency. Mattoussi and co-workers studied the effect of increased Au NPs to CdSe/ ZnS NPs ratio along with the semiconductor size and the separation distance between the components. For all semiconductor sizes and separating distances, an increasing ratio led to a higher quenching efficiency and faster decay lifetime. 184 The authors attributed the quenching mechanism to the strong energy transfer from the emitting NPs to the metal domains affecting mostly the nonradiative channel with negligible influence on the radiative channel. The effect of the inverse ratio of increased number of semiconductor NPs to a single plasmonic metal site was demonstrated by an Au nanorod coated with CdSe/ZnS NPs. 166 In this work, a shortened lifetime and increased quenching efficiency were obtained for decreasing semiconductor to metal ratios from 40% to 91% for from 62 to 10 CdSe/ZnS NPs for a single Au NR, respectively ( Figure 20D). These observations emphasize that the resonance between the exciton and the plasmon strongly influences their coupling and therefore their photophysical properties. Therefore, according to the above understandings regarding both fluorescence enhancement and quenching as were deduced from the extensive investigations of these optical properties of HNPs, several rules of thumb can be established for designing either enhanced or quenched emitting HNPs systems. While the size and shape of both the metal and the semiconductor components can contribute to the extent of desired optical property and enhance the overall fluorescence effect of both enhancement and quenching, the more pronounce parameters are the distance between the two segments and the spectral overlap. The first can be set by sorting the suitable synthetic pathway to achieve either sufficiently separated HNPs by inserting a separating layer in the form of SiO 2 or organic linkers (polymers or molecules) that will lead to fluorescence enhancement or deposing one material on the surface of the other, allowing charge transfer and therefore fluorescence quenching. The second parameter of spectral overlap can be tuned by the choice of materials to be conjugated efficiently to promote near-field effects that would enhance the absorption of the adjacent component and permit effective energy transfer.

Electrical Transport Properties
The semiconductor−metal interface also manifests unique electrical properties. Besides the electronic characteristics of each of the components on its own, synergetic features arise due to the conjugation of these two disparate materials. Understanding the synergistic effects at the semiconductor−metal nanojunction may lead to their utilization in various electrical applications including their use as electrical contacts and building blocks for nanoelectronic devices. Early study of the electronic synergistic effects was conducted on a single CdSe− Au nanodumbbell structure by cryogenic scanning tunneling microscopy and spectroscopy (STM and STS, respectively). 185 Since the derivative of the tunneling current to voltage is proportional to the local density of states (DOS), the method offers a unique view of the spatial dependence of the electronic levels. Therefore, by scanning along the nanorod structure, the changes in the local DOS can be visualized. As can be seen in Figure 21A, at the center of the rod, STS showed an energy gap value similar to that of a CdSe rod. On the metallic site, a modified Coulomb staircase structure, signifying single-electron charging events in the metal NP, was observed. At the semiconductor−metal interface, a subgap structure was A related electronic behavior was observed for PbS−Au nanodumbbells, while measurements at different locations along the length of a pristine PbS NR showed delocalization of both the VB and the CB. Partial delocalization of the conduction band along the nanodumbbell length was observed, whereas the VB was found to be localized to the semiconductor section. 186 This delocalized conduction band is attributed to the electronic coupling of the Au metallic contact with that of the PbS nanorod and the ability of charge-separation processes. This partial delocalization forms an n-type behavior that is manifested by the shift in the (dI/dV)/(I/V) spectrum to the negative bias ( Figure  21B). STM/STS studies performed on Cu 2 S−Ru nanocage structures also revealed a semiconductor gap along with in-gap states assigned to metal-induced gap states that developed at the semiconductor−metal interface ( Figure 21C), consistent with other reports. 187 The resulting in-gap states at the semiconductor−metal interface leads to an enhancement of the HNPs conductance, as was demonstrated for CdSe−Au nanodumbbells. I−V measurements of such HNPs compared to the bare semiconductor NR showed superior performance of the former attributed to a lower Schottky barrier in the hybrid structure (about 75% decrease), allowing an average 6-decade increase in conductivity near zero applied bias ( Figure 22A). 188 Charge-transport studies of CdSe−Au NR arrays and networks showed enhanced dark currents and photocurrents that are mediated by the morphology of the hybrid structures. 120,189,190 Selective Au deposition at the rod apexes resulted in a decrease of the conductivity at room temperature compared to random Au decoration on the rod surface. However, improvement at cryogenic temperature was observed. This phenomenon was assigned to the dominant thermionic emission across the nanosize Schottky barriers at room temperature, whereas at cryogenic temperature, the thermal activation of carriers becomes negligible and the charge transport is affected by charge tunneling and Coulomb blockade that corresponds to single-electron charging events. Similar results were obtained for CdSe−Pt HNPs. 191 Moreover, a metal domain size dependence was observed where for a 3 nm Pt site dark current was permitted due to increased thermionic and field emission processes, in contrast to small size Pt-decorated HNPs and pristine CdSe NPs that conduct only upon irradiation. Note that since the nanoparticles size is smaller than the depletion width, Schottky barrier formation models might not apply to the metal− semiconductor nanojuction.
Mahler et al. studied CdSe−Au NPLs reporting as well on conduction enhancement by an order of magnitude ( Figure  22B). 39 Yet, the authors withheld assigning this behavior to a reduction of the band-gap energy. Instead, they attributed these observations to tunneling events between metallic tips. Other transport mechanisms were reported for different HNPs depending on their materials combination. Hole conductivity was proposed for Au−PbS core−shell HNP thin films, where electrons are transferred to the metallic core. 137 In addition, a tunneling mechanism of charge carriers between the metallic cores was suggested to dominate the electron transport in FePt− Pb chalcogenide core/shell HNPs. 192 In spite of the existence of a relatively long distance between adjacent NPs, the tunneling is enabled by the low effective mass of electrons in the PbS shell as well as the contribution of electron-donating species as adsorbates, which lowers the energy barrier at the FePt−PbS interface.

Light-Induced Charge Separation
One of the most important and potentially applicable properties of HNPs is the formation of spatial charge separation following irradiation and light absorption. This characteristic arises from the conjoining of a semiconductor and a metal in a single HNP. The formed energy band alignment on both sides of the semiconductor−metal interface promotes excited charge carrier transfer across this interface.
3.3.1. Light-Induced Charge Separation Across the Semiconductor−Metal Nanointerface. Typically, the charge-separation process in HNPs includes the following steps. First, absorption of sufficient energy at the semiconductor segment (above the band-gap energy of the semiconductor) creates an excited electron−hole pair (i.e., exciton). Then, excited electrons relax and transfer to the metal domain, whereas the counter charge carriers (e.g., holes) are restricted to the semiconductor region. The described separation process competes with backward recombination processes, including electron−hole recombination and the loss of electrons in the metal, by recombination with the holes in the semiconductor component. The resulting spatial charge separation is the fundamental principle behind the photocatalytic characteristics of HNPs as will be extensively discussed later. This unique synergistic property of HNPs was extensively investigated in various HNPs architectures and morphologies. Generally, the efficiency and the dynamics of this process are dependent on the type, structure, and morphology of both components. Several reviews previously discussed and summarized the kinetics and dynamics of the different relaxation routes of excited charge carriers in HNPs, focusing on the effects of different structural and environmental parameters and their implications for different applications. 6,90,193−195 In this context, we will provide a brief overview on the dominant parameters that influence charge separation along with highlighting recent findings and understandings of different relaxation routes dynamics in HNPs.
Early evidence of charge separation was observed in the form of fluorescence quenching of emitting semiconductor NPs following metal deposition which was assigned to the relaxation of the excited electrons in the semiconductor through charge transfer to the metal domain. Hence, reduction of the fluorescence lifetime is obtained by forming an additional competitive nonradiative relaxation route (as was discussed in section 3.1.3). An additional primary observation showed that the charge transferred to the metal domain can either be accumulated as in the case of Au and Ag conjugated metals, leading to a Fermi level shift due to the electrons charging energy, or hold Ohmic-like behavior as was reported for Pt deposition. 17,147,196 A charging effect of the metal domain through charge transfer was demonstrated for CdSe−Au nanodumbbells in aqueous solutions accumulating up to tens of electrons per metal domain following irradiation. 177 Moreover, an in situ single-particle measurement of Au deposition on CdSe/CdS NRs was able to trace the charging effect of the metal domain with relaxed electrons from the semiconductor conduction band up to Fermi level equilibration with the latter via plasmonic shifts and increased optical intensities in their UV−vis spectra. 65 To unravel the excited charge carriers dynamics in the chargeseparation process, steady-state measurements and timeresolved fluorescence alone cannot differentiate between the Chemical Reviews pubs.acs.org/CR Review different relaxation routes of the different charge carriers (electrons, holes). Therefore, ultrafast transient absorption spectroscopy (TA) is commonly required. Indeed, in the past decade, excited charge carrier relaxation processes in HNPs have been extensively studied by combination of time-resolved fluorescence and TA measurements (refs 28, 29, 32, 46, 61, 74, 117, 134, 138, 147, 179, 181, 197−199). HNP intraparticle charge separation as well as interparticle hole removal were shown to be affected by the structural, surface, and chemical characteristics of the HNPs including their surrounding environment. For example, CdSe-based HNPs such as CdSe−Au NRs revealed hot and band-gap electron-transfer processes during the first 115 and 210 fs, respectively ( Figure  23A). 149 Similar observations of charge separation within <100 fs were reported for similar HNPs decorated with Au and Pt domains. 134,147,148,200 However, CdS-based HNPs including CdS−Pt, 179,198 CdS−Au, 28,138,197 and CdSe/CdS−Ni 117 showed ultrafast exciton bleach decays on time scales of at least 1 order of magnitude longer in the range from tens to several picoseconds. This difference can be explained as due to the spectral overlap of the plasmon resonance and the semiconductor exciton that exists in CdSe-based HNPs, whereas in CdS-based HNPs, these features are separated. 143 This explanation was also assigned to the slower dynamics of the charge-separation process observed for type II ZnSe/CdS−Pt or quasi-type II CdSe/CdS−Pt heterostructured HNPs (∼14 ps) than with the single-phase CdS−Pt HNPs (∼4 ps) ( Figure  23B). 201 Note that although trapping processes of the excited holes at surface defects are extremely fast (0.7 ps), 179 their transfer from the semiconductor to the surrounding media or to a sacrificial hole scavenger is slower than the analogous electron charge transfer (hundreds of picoseconds to nanoseconds time scale). 202−204 An additional structural effect was reported by Simon et al. comparing random decoration of multiple Pt metal clusters on CdS NRs with selective single-tipped deposition on same nanorods. The relaxation half-life of the excitonic bleach of the multiple-decorated HNPs was found to be much faster than the single-tipped form (4 vs 82 ps for random decoration and tip, respectively). 197 Moreover, the former architecture showed almost complete decay, implying an effective electron transfer to the metal domains showing 10% of the TA signal remaining after 75 ps, while significant TA signal (>20%) was measured along the entire time scale in the single-metal-tipped HNPs ( Figure  24A). These observations were attributed to the shortening of the relaxation path in the presence of multiple metal clusters. However, the recombination rate with the nearby holes increases, thereby canceling any benefit of the increased electron-transfer rate. These opposing trends were also demonstrated on CdS−Au NRs aerogel networks by spectroelectrochemical measurements. Efficient charge carrier separation has been found in the tipped−NR networks resulting in higher negative photocurrent efficiencies compared to the random Au-deposited NR networks where the charge recombination is more pronounced. 106 A similar behavior for the efficiency of the charge separation was shown to be dependent on the type of the metal component as well. The bleach recovery of CdSe−Au was reported to reach only 80% of the initial bleach signal, indicating remaining excited electrons at the CdSe rod due to suppressed charge transfer, in comparison to 100% bleach recovery in the CdSe−Pt system ( Figure  24B). 147 In addition, Choi et al. reported on improved electron-  Figure 24C). 134 However, the electron−hole charge recombination rate, at the metal− semiconductor interface, was found to be higher for the Pttipped NRs as well, which diminish this separation property faster.
Another key parameter that allows one to control the chargeseparation efficiency and dynamics is the size of both HNP components, the metallic segment and the semiconductor light absorber. TA measurements of matchstick-like hybrid structures such as CdS−Au 28 and CdS−Pt 116 with different metal size domains between 1.5 and 6.2 nm and 0.7 and 3 nm, respectively, showed monotonic increased electron charge-transfer rates with increased metal tip sizes ( Figure 25A and 25B). This behavior was explained by consideration of the increased density of states of the metal component with larger size using Fermi's golden rule, which hold steep dependence of the density of states on the tip volume (r 3 ), along with weaker contribution through the dependence of the Fermi energy and work function on the metal domain radius (r).
Kinetic measurements of charge separation in CdSe/CdS−Au HNPs with different Au size decoration showed that increasing the Au NP size from 1.25 to 2.5 nm caused the transfer time of hot electrons to decrease from 300 to 150 fs ( Figure 25C). 205 The charge-transfer rates of thermalized electrons from the conduction band to the free energy level at the metal domain were measured as 300 and 550 fs for large and small Au NPs, respectively, consistent with the previous reports ( Figure 25Cb). Interestingly, the charge-separation dynamics of CdSe/ CdS−Ni NRs with different metal domain sizes (2.3−10.1 nm) was found to be independent of the metal tip sizes. 117 Noteworthy, the authors reported the nonmonotonic behavior of the pre-exponential factor of the TA measurements multiexponential fit which is indicative of the relative weight of rod intrinsic relaxation versus charge-separation processes.
The size of the semiconductor component was also shown to influence the charge carrier dynamics. Jackel and co-workers reported on a variation of over 2 orders of magnitude in the transfer rates of photogenerated electrons in different sizes of spherical CdS−Pt HNPs in the range of 2.8−4.6 nm in particle diameter. In small NPs (2.8 nm), charge transfer to Pt sites has ps) ( Figure 26A). 206 This size-dependent electron-transfer rate is attributed to the tunable electron-transfer driving force, which is the result of the size-tunable band gap of the nanocrystals due to quantum confinement effects of the semiconductor NPs.
Since the conduction band edge shows an increasing energy offset with smaller NP size relative to the Pt sites, a larger driving force for electron transfer in the smaller nanocrystals is gained ( Figure 26A-e). A different aspect of the semiconductor size was observed in Pt-tipped CdSe NRs (both single and doubled tips) with different rod lengths. In this case, the rate constants for the electron transfer decrease with the rod lengths ( Figure 26B). 207 The origin of this trend, according to the authors, is a diffusioncontrolled regime in the one-dimensional nanorod system. The comparable transfer rates for 30 nm double-tipped NRs with 15 and 20 nm single-tipped NRs support this explanation ( Figure  26B-c). Surface ligands have long been recognized for their numerous roles and functionalities dictating the characteristics of semiconductor NPs and HNPs. Light-induced charge separation was also studied in this context. Comparison between the TA dynamics of CdS−Au NRs with different surface coatings reveals two substantial and related differences, as can be seen in Figure  27A. 32 First, the fastest charge-transfer dynamics is seen in the case of the polyethylenimine (PEI)-coated HNPs, slower with Lglutathione (GSH), and slowest with mercaptoundecanoic acid (MUA)-passivated HNPs with measured half-lives of 100 ps for PEI and 160 and 330 ps for GSH and MUA, respectively. A related second difference is that the decay amplitude over the measurement time scale in these experiments is also largest in the PEI-coated system. The trend of decay dynamics is attributed to an improved surface passivation leading to lower electron−hole trapping which promoted enhanced electron transfer. Moreover, this electron−hole trapping due to Coulombic interactions in the vicinity of multiple Pt sites deposited on CdS NRs promoted faster charge-transfer rates (0.4 ns) in comparison to the same HNPs in the presence of hole scavengers (8 ns). 198 Extracting the holes eliminates the Coulomb interactions, which leads to the loss of the electron wave function localization near the hole trap sites and therefore the transfer is delayed. Conversely, fast removal of holes populations in CdS−Ni HNPs by introducing high alkaline environment conditions resulted in faster electron transfer to the metal sites due to reduced Coulomb interactions which allowed increased overlap of the electron wave function with increased growth of the metal sites ( Figure 27B). 71 Structural characteristics of HNPs such as their components morphology can affect the excited charge carrier's relaxation mechanism as well as their dynamics. Two-dimensional HNPs (NPLs) compared to 0D and 1D HNPs (dots and NRs, respectively) are expected to promote faster charge transfer and a longer charge-separation distance due to more uniform quantum confinement and a large surface area, respectively. 193 Interestingly, while CdS−Pt NRs exhibit excitons relaxation solely through charge separation by charge transfer of electrons to the metal domain, in the case of CdSe−Pt NPLs, exciton quenching takes place mostly via fast diffusion to the semiconductor−metal interface followed by rapid energy transfer to Pt (∼87%). Only a small fraction of the excitons (∼13%) can undergo charge separation ( Figure 28A). 181 The key differences between CdSe NPLs and CdS NRs are that in the latter, exciton quenching through diffusion and energy transfer is slow compared to hole trapping, leading to a near unity yield of charge separation in their hybrid form. This difference can be attributed to an atomically precise thickness in the quantumconfined dimension in NPLs, which minimizes energy disorder and enables ultrafast exciton diffusion. However, a charge carriers relaxation study on CdS−Ni NPLs indicates excitons quenching via a charge-transfer pathway with a time constant of ∼165 ps ( Figure 28B). 74 This difference can be elucidated by the presence of a greater number of trap states in CdS in comparison to CdSe NPLs. 208 This may prevent ultrafast energy transfer and facilitates charge separation due to exciton trapping. This hypothesis was further supported by charge carrier dynamic measurements of CdS−Pt NPLs showing the growth of the charge-separation signal along with the recovery of the bleaching TA signal ( Figure 28C). 61

Plasmon-Induced Charge Separation.
Indeed, the majority of the charge-separation studies in HNPs have been focused on the photoinduced charge transfer from the semiconductor to the metal domain. However, combination of the unique properties of an excited plasmonic material, such as enhanced local fields near the metal nanostructures, broad spectral tunability, large absorption cross sections, and superior long-term stability, with excitonic sensitization through the semiconductor has interesting potential for SPR-driven hotelectron photochemistry applications. This promising route motivated the investigations of plasmon-induced hot charge carrier transfer across the interface of the heterostructured system. Within this process, hot carriers generated from plasmon decay can transfer into a nearby semiconductor through two mechanisms as illustrated in Figure 29: the conventional indirect plasmon-induced hot-electron transfer (PHET) 29,152,209,210 and the recently discovered direct plasmon-induced charge transfer (PICT). 148,211,212 In PHET, hot carriers are initially generated in the metal by plasmon decay and then undergo interfacial transfer to the acceptor component. PICT occurs when there is strong interdomain coupling and mixing of the metal and acceptor energy levels. In PICT, plasmon excitation is directly accompanied by a rapid charge-separation process that creates an electron in the acceptor region and a hole in the metal.
Indication of the PICT mechanism was demonstrated by TA measurements of the CdSe−Au NRs interband transition and mid-IR intraband absorption which showed corresponding decay rates following near-IR excitation at the metal plasmon absorption region, indicating the formation of semiconductor conduction band electrons ( Figure 29A-d). The estimated plasmon-induced hot-electron-transfer and charge-recombination times were found to be ∼20 fs and ∼1.45 ps, respectively, with a quantum efficiency that exceeds 24%. 148 By applying ultrafast spectroscopy with high temporal resolution on similar CdSe−Au NRs, an upper limit of 30 fs for electron transfer could be determined, and further fast back transfer of electrons to the metal domain within 200 fs time scale was observed ( Figure  29B). 149 Hence, for exploiting the electrons generated in the semiconductor via PICT, their extraction (by surface chemistry reactions) needs to be extremely rapid to compete with the fast back-transfer to the metal. In addition, complementary measurements of the charge transfer to the metal following visible-light excitation allowed one to present the complete relaxation pathways of excited charge carriers in this nanosystem, signifying that the PICT mechanism mostly generates band-gap electrons in CdSe since no hot-electron back-transfer to the gold at the relevant time scale was observed. Nevertheless, in the case of CdS−Au NRs, the Au plasmon band is weakly perturbed and the plasmon-induced hot-electron transfer occurs through the conventional PHET mechanism. Hence, the hotelectron-transfer rate is noticeably slower (97 fs) due to the rapid plasmon decay and the efficiency is reduced (∼2.75%) owing to the competition of hot-electron transfer with ultrafast relaxation. 29 However, Liu et al. presented an improved quantum efficiency of hot-electron transfer in CdS−Au NRs via control over the metal domain sizes. The quantum efficiency of this process increases from ∼1% to ∼18% as the particle size decreases from 5.5 to 1.6 nm ( Figure 29C). 213 This trend is attributed to a dual effect of enhanced surface damping contribution and decreased barrier height and possibly enhanced electronic coupling at the semiconductor−metal interface.
The effect of other structural parameters on the hot-electron transfer from the metal site to the semiconductor components was investigated. Weng and co-workers recently studied the effect of the semiconductor shell thickness in Au−CdS core/ shell HNPs with different thicknesses between 3.6 and 14 nm. A nonmonotonic trend of hot-electron-transfer efficiency was observed with maximum efficiency at 8.2 nm shell thickness. 142 This trend was assigned to two opposing factors: the initial hotelectron energy distribution in the Au core and the Schottky potential at the interface of the Au core and the CdS shell. Both factors are reduced with increased shell thickness, and therefore, an intermediate shell thickness can balance these trends. Energy band alignment was also reported to influence the electron− phonon relaxation processes. Varying the size and type of the semiconductor segment in semiconductor−metal dimers resulted with an acceleration of the electron−phonon scattering rate for HNPs owing to the well-aligned semiconductor conduction band and metal Fermi level. 214

Charge Separation in Multiexciton Regime.
Most of the studies on excited charge carrier dynamics in general and specifically on charge separations have focused on a single exciton excitation regime. As was described in the previous sections, a single electron−hole pair is generated following light excitation. The excited-state relaxation routes and its dynamics were extensively reviewed in this manuscript and others. The ability of semiconductor NPs to generate and accommodate multiple excitons (MXs) through optical or electrical stimulation holds potential in utilizing HNPs in various applications that require multielectron reactions such as water splitting and CO 2 reduction. Typically, MXs in a semiconductor nanocrystal are generated by the absorption of multiple photons through high-intensity irradiation. 31,215 Generation via multiexciton generation processes in which the absorption of one high-energy photon creates two or more lower energy excitons was also demonstrated and studied. 216,217 MX decay dynamics in semiconductor NCs is governed by Auger recombination in which an exciton recombines nonradiatively and transfers its energy to another charge carrier which then rapidly dissipates it, resulting in the effective annihilation of one exciton. Interestingly, in HNPs, the multiexciton state can also dissipate through charge transfer from the semiconductor to the metal domain. Ben-Shahar et al. reported on a metal domain size effect in CdS−Au NRs governing MX dissociation. It was shown that  Figure 30A). 218 This competitive process can be influenced also by the material combination of HNPs. CdS−Pt NRs were reported to present slow Auger recombination rates due to trapped holes and spatially separated excitons varying between 2 ns and <1 ps for biexcitons and multiexcitons (>20 excitons), respectively ( Figure 30B). 219 Hence, under the MX state, the efficiency of multiple electron transfer to the metal site was set to ∼41% compared to 100% of the MX dissociation efficiency calculated for the biextonic state. Note that the differences between these two nanosystems can also originate from the different stabilizing ligands and solvent environment. Additionally, heterostructured semiconductor−metal HNPs such as CdSe/CdS−Pt NRs reveal fluence-dependent band-edge exciton dissociation. In this case, the first electron-transfer rate was measured with a time constant of 192 ps, while the second electron transfer is slowed by the trion Coulombic interaction down to 1700 ps. 220 The Coulomb barrier that arises following trion formation was estimated to increase by ∼60 meV, causing a slower transfer rate, and the transfer efficiency decreased by 1 order of magnitude.

Photocatalytic Properties
In addition to the former described properties that involve intraparticle characteristics, the photocatalytic ability of such semiconductor−metal HNPs stems from interparticle pro-cesses. The advantages of enhanced optical properties along with the synergistic light-induced charge separation form the basis for this unique feature. The efficient charge separation across the semiconductor−metal interface allowed the utilization of these charge carriers, electrons and holes, at the surface of each of the hybrid components for additional surface chemistry reactions.
Typically, the majority of such chemistry are redox reactions. Scheme 5 illustrates the available reduction and oxidation routes following light absorption by HNPs. For the excited electrons, typically transferred to the metal component, water reduction to hydrogen is possible, while under aerobic conditions, the electrons may reduce molecular oxygen to form hydrogen peroxide and/or superoxide radicals. Simultaneously, the excited holes, in some of the illustrated scenarios, are capable of oxidizing water molecules to form hydroxyl radicals. All of these routes have been utilized and demonstrated in various applicable fields including alternative energy generation, biomedical, environmental, and industrial applications. These will be discussed in detail in the next sections.
One of the most common methodologies to investigate and demonstrate the photocatalytic characteristics of HNPs is via organic dye photoreduction/oxidation. This photocatalytic process, which also bears relevance for various photocatalytic purification applications, can be mediated by either excited electrons or holes at both the semiconductor and the metal components. In each of the pathways, degradation of the organic dyes is occurring directly by charge carriers at the hybrid surface or through the formation of radicals (mainly in aerobic  Figure 31A). 177 In addition to in situ degradation measurements, charge retention of electrons on the metal tips was observed by using preirradiated experiments. Similar reports of photoinduced dye degradation by numerous different HNPs were reported using various dye molecules including MB, methyl orange (MO), rhodamine B (RhB), and rhodamine 6G (R6G). Cd−chalcogenide-based HNPs such as CdSe−Au NPs, 175 CdS−Au NRs, 221,222 CdSe/CdS−Pt NRs, 75,223 Au−CdS yolk/shell, 224 CdS−Au nanorings, 225 CdS−Au, 226 and CdSe/CdS−Au 42 NPLs were all used to explore the mechanism of the photocatalytic reaction and the HNPs structural and chemical effects on this specific property. A different mechanism was suggested by some of these reports describing a radical-mediated photocatalysis of dye molecules. Where following light absorption and charge carriers excitation, superoxide and hydroxyl radicals are generated by electron reduction of molecular oxygen and hole oxidation of H 2 O molecules, respectively. Consequently, these radicals can further degrade the dye molecules in their surroundings.
As was described for the other mentioned properties, the photocatalytic property is also influenced by the structural effect of the HNPs. In this context of dye degradation, it was reported that the increased shell thickness of Au−CdS core/shell NPs enhances its photocatalytic activity due to increased absorption by the larger semiconductor component that provides generation of more charge carriers ( Figure 31B). 227 The yolk/ shell architecture of Au−CdS was shown to be more efficient in photocatalytic R6G degradation in comparison to core/shell HNPs where R6G was almost completely degraded in the former form while the later achieved 68% bleaching. 224 The authors attributed this trend to several factors, among them the increase in hole trapping sites that can promote more hydroxyl radicals along with the larger surface area of the yolk structure. Other types of HNPs, such as Au−Bi 2 S 3 , 98 Au−SnS, 95 Au− CZTS, 25,36,53 Pt−CZTS, 25,53 Pd−CZTS, 53 and Cu 2−x S−CdS shells on Au NRs 228 also revealed an improved photocatalytic ability in comparison to their semiconductor NP component form, exemplified via organic dye degradation.
Moreover, this photocatalytic dye transformation has been applied to identify the spatial distribution of the catalytic activity on HNPs. Ha et al. has reported two distinct, incident energydependent charge-separation mechanisms that result in opposite energy flows and polarities on a single CdS−Au NR. By using Amplex red dye (nonfluorescent) that turns into resorufin (fluorescent) following light irradiation of HNPs in the presence of H 2 O 2 , the reaction turnover events can be monitored and localized its position by applying super-resolution mapping. 221 As seen in Figure 31C, under plasmon excitation (532 nm), the holes reactive sites are positioned at the gold tips on both ends of the HNPs while the electron reactive sites are located along the inner length of the CdS nanorods within a distance of a few tens of nanometers from the gold tips. This indicates that excited hot electrons are transferred from the metal domain to the semiconductor component at this energy excitation. However, under excitonic excitation (405 nm), an opposite polarity after photoinduced charge separation has been observed. In this case, the hole's reactive sites are distributed along the inside length of the CdS NR, while the electron's reactive sites are located at both ends; this map can be assigned to charge separation originating from excited electron transfer from the semiconductor to the metal. An additional means of probing charge separation and energy transfer across the semiconductor−metal nanojunction was demonstrated by measuring the quenching kinetics of ATTO dyes in the presence of excited CdS−Au NRs. 222 Similar trends were reported, showing more efficient catalytic performance under excitonic excitation in comparison to plasmon-induced photocatalysis.

HNPS AT WORK: EMERGING APPLICATIONS
Utilization of the above-described properties of HNPs is optimally manifested via photocatalytic applications, perhaps the most notable of which are alternative clean solar-to-fuel conversion in the form of hydrogen generation via photocatalytic water splitting and also toward photocatalytic CO 2 reduction. A complementary photocatalytic application route under aerobic conditions generating radicals and reactive oxygen species (ROS) was also realized in the fields of biomedicine along with environmental and industrial science. The advancement in synthesis control along with the ability to tune the optical and chemical properties of HNPs allows a judicious design of a predetermined HNPs that address the specific physical and chemical requirements of a given photocatalytic reaction.

Clean Energy−Photocatalytic Water Splitting
One of the most challenging photocatalytic reactions in the field of alternative energy harvesting is the water splitting reaction for hydrogen generation. 229−232 This reaction pathway opens the way for direct conversion of sustainable solar energy to chemical energy stored in chemical bonds of hydrogen as an alternative fuel. Utilization of hydrogen in fuel cells to generate electricity produces back water in a zero emissions cycle.
To perform complete water splitting, the equilibrated Fermi level of the electrons must be at least more negative than the  231 The actual band positions that are needed are in fact larger due to the condition to overcome the overpotential. Honda and Fujishima were the first to show water splitting by a photoelectrochemical reaction in the early 1970s by using a TiO 2 electrode under UV irradiation and a Pt counter electrode with applied bias to form a closed circuit. 233 Both wide-gap metal−oxide semiconductors and narrow-gap metal sulfide semiconductors can in principle address the minimal energetic requirements. However, the difference in the band-gap width determines the absorbance region. Hence, in the pursuit for increasing the solar spectral coverage, the use of narrower gap semiconductors which can absorb in the visible region accounting for about one-half of the solar spectrum is more favorable over the wide-gap common oxide semiconductor absorbing the UV range comprising less than 5% of the solar spectrum. 234 HNPs are considered promising photocatalysts for alternative energy harvesting in this form of hydrogen generation. 7,16,88,90,235 Their advantage over semiconductor NCs related to the ability to tune their photophysical and chemical properties through material combination and dimensions has raised great interest toward their utilization in such application. Yet, only a handful of reports have been able to demonstrate full water splitting HNPs as a single system photocatalyst which can effectively promote both reduction and oxidation of water. Therefore, the use of sacrificial agents such as sulfide/sulfite pair, alcohols, amines, and other electron-donating molecules is commonly imposed where the counter charge carrier cannot be  Figure 32A presents an illustration of photocatalytic hydrogen generation in the presence of sacrificial hole scavengers along with a typical CdS−Au HNPs band alignment diagram with additional recombination and relaxation routes of excited-state charge carriers. The significant dependence of the charge carrier's dynamics on both the HNPs structural properties and the chemicalmedium conditions, discussed in section 3.3, was found to directly affect the photocatalytic performance of the HNP nanosystems. These influencing factors, which are critical for progressing toward a viable water splitting HNP, are discussed in the following sections.

Structural Effects.
The ability to control the size and morphology of both HNP components along with their material combination and their architectures was addressed in previous sections. This allows a systematic and in-depth investigation of the influence of these structural parameters on the overall photocatalytic efficiency.

Semiconductor Component Structural Effects.
From the semiconductor component point of view, their size was shown to affect the hydrogen generation activity in different manners. Optical properties along with charge-separation and e−h recombination processes should be considered in this respect. For smaller nanoparticles, a larger driving force for electron transfer is observed due to the quantum confinementinduced increase in the energetic offset increasing between the electronic states of the semiconductor with respect to the metal, as shown for Pt-decorated CdS HNPs in various sizes ( Figure  26A) in which 2.8 nm HNPs displayed a H 2 generation quantum efficiency of 17.3%, much higher than the 11.4% for their 4.6 nm diameter counterpart ( Figure 32B). 206 Additionally, the distance that the charge carriers have to migrate to the active site decreases, and therefore, the probability of loss through the competing electron−hole recombination route decreases accordingly. 236 An opposite size effect was reported by Amirav et al. showing higher hydrogen production quantum yields for longer rod structures of heterostructured type II HNPs, CdSe/ CdS−Pt NRs. 237 This trend was explained as due to better spatial charge separation along the rod length (where holes are localized to the seed region) and reduced back-recombination from the metal.
Different considerations were demonstrated for CdSe−Pt NRs with single-and double-tipped structures. In this case, an optimal intermediate rod length of 15−20 nm was found to maximize the hydrogen-evolution rate ( Figure 32C). The two opposing factors setting this nonmonotonic behavior are the electron-transfer rate, which decreases with increased length (Figure 26B), and the absorption cross section, which increases with the rod length. 207 A similar nonmonotonic photocatalytic hydrogen generation activity was reported for Au−CdS core/ shell HNPs with optimal 8.2 nm shell thickness correlating with its charge-transfer dynamics as was discussed in section 3.3.2. 142 Additional size effects of the semiconductor component were demonstrated on CdSe/CdS−Pt core/crown NPLs with different core and crown sizes. Kinetic measurements of hydrogen generation showed higher photocatalytic water reduction rates for hybrid NPLs with both a large core and crown followed by a large core with small crown and small core with large crown with calculated apparent quantum efficiencies of 19.3%, 17.8%, and 13.2%, respectively ( Figure 32D). 43 Comparing the influence of morphology on the photocatalytic performance of different semiconductor components indicates that NPLs are favored over NRs and spherical HNPs structures. The vicinity of both hybrid components in 0D structure in comparison to the other 1D and 2D morphologies leads to inferior charge separation and faster charge recombination. 238 Quantum efficiencies of hybrid NPLs such as CdS− Ni 74 and CdS−Pt 61 were reported to exceed complementary NR structure efficiency values 71,197,223,239 (64% and 42% versus 3−27%). The advantage of the 2D structure over the 1D morphology is attributed to the elongated lifetime of the chargeseparated state in the 2D platelet. Applying the hole hopping model to simulate charge recombination indicates that many more random walk steps are required in the NPL case before the hole finds the recombination (Pt) site. 61 Another aspect of the semiconductor structure that can influence the photocatalytic performance is the electronic profile of the semiconductor. HNPs with a type II or quasi-type II heterostructured semiconductor component, such as CdSe/ CdS−Pt 237,240 and ZnSe/CdS−Pt 201 NRs, have been reported to exhibit enhanced photocatalytic hydrogen generation in comparison to HNPs with single-phase semiconductor components due to improved charge separation. A different strategy suggested by Li and co-workers includes the alloying of different elements in the semiconductor component, presented by Cu 1.94 S−Zn x Cd 1−x S−Pt HNPs. In this manner, two lightabsorbing regions are synthetically achieved with a tunable energy band gap as a function of Zn mole fraction that can modify the hybrid band alignment to attain a material combination that manifests more efficient charge transfer and photocatalytic activity ( Figure 32E). 132 Similar trends were demonstrated on Zn 1−x Cd x Se−Pt NRs with various compositions of the semiconductor segment. The optimal composition for efficient hydrogen generation was found to significantly depend on the presence of the Pt domain. 241 The interplay between the band alignment and the absorption capacity that dominated the pristine semiconductor photocatalytic activity was altered by charge transfer and the electron's mobility properties that accompanied the deposition of the Pt site.
The electronic profile can also determine the overall energy band alignment and therefore the actual overpotential for proton reduction. This was manifested by core−shell CdSe/CdS nickeldecorated HNPs in which the hydrogen generation rate increased with decreased core size and increased with the increasing shell thickness. 242 The former trend is attributed to the quantum confinement effect of raising the lowest conduction band level, while the latter originated from a large barrier for electron−hole recombination formed by the thicker shell. As important as the electronic profile is, in a comparative study of two types of HNPs with a type II band structure ZnSe/CdS−Pt and ZnTe/CdS−Pt NRs, the latter showed negligible photocatalytic activity. 46 The authors assigned this different behavior to a mismatch of the ZnTe conduction band compared with the highest occupied molecular orbital (HOMO) of the surface ligand (MUA), and thus, holes were not scavenged effectively in this case, unlike the behavior for the structure with a ZnSe core.
An additional approach for accelerating hole scavenging was suggested via surface etching of the rod shell in CdSe/CdS−Pt NRs, thereby increasing the hydrogen production rate by 3−4fold ( Figure 32F). 243 A different approach suggested improved charge transfer to the metal sites by passivating surface traps through postsynthesis addition of a CdS layer over CdSe−Pt nanodumbbells resulting in an increase of 6.5 times in the photocatalytic hydrogen generation rate compared to bare CdSe−Pt HNPs. 244 Chemical Reviews pubs.acs.org/CR Review Importantly, as part of the quest for an alternative nontoxic and earth-abundant material to replace the heavy-metal semiconductor in general and Cd-based HNPs in particular, HNPs such as ZnSe−Au, 30 ZnSe−Pt, 76 and CZTS−Au 25,36,53 NRs were synthesized and shown to achieve photocatalytic hydrogen evolution activity that exceeded their pristine semiconductor complementary NPs.

Metal Component Structural
Effects. The metal component, often named the cocatalyst in the context of the photocatalytic activity of HNPs, also plays an important role in the overall photocatalytic water reduction efficiency. As described in section 2.3, synthetic developments allowed the control of the HNPs morphology and architecture. Control over the cocatalyst size and material composition was achieved. Moreover, either selective deposition of the metal domain in the form of a single to several sites on the semiconductor segment or random multiple metal islands across the entire semiconductor surface (assigned to surface defect growth) may be compared. These different decorations along with different metal combinations were shown to have a significant impact on the photocatalytic hydrogen generation performance.
The size is a well-known essential parameter in many different properties of HNPs. The size effect of the metal cocatalyst domain on the hydrogen generation efficiency was addressed in various HNPs systems. HNPs with random metal decoration including CdSe−Pt tetrapods, 245 CZTS−Pt HNPs, 25 and CdS NRs decorated with multiple Pt clusters 114 (8−68 atoms) showed optimal hydrogen evolution at an intermediate metal size or metal loading ( Figure 33A−C). However, in this metal decoration form, the ability to isolate the size effect from other contributing effects and to control the actual size of the cocatalysts rather than the weight percentage loading of the metal is quite limited. This effect may be better addressed via single-metal deposition in various metal sizes with narrow size distributions as was demonstrated for CdS−Au NRs 28 and in following studies on CdSe/CdS−Ni 117 and CdS−Pt 116 nanorod structures. Systematic investigation of the cocatalyst effect in the former hybrid nanosystem revealed a nonmonotonic dependence in which an intermediate Au tip size provides the optimal hydrogen evolution rate. This behavior is in contrast to measured charge-transfer rates which increased with increased metal domain size as was described in section 3.3.1. This essential behavior was captured by a minimal kinetic model. This  Figure 33D and manifests the nonmonotonic dependence of the overall efficiency of photocatalytic hydrogen production on the Au tip size, consistent with the experimental measurements. 28 A similar nonmonotonic trend was reported in CdSe/CdS− Ni 117 and CdSe/CdS−Pd 127 NRs in which an optimal diameter of the Ni site of around 5.2 nm was observed within the range 2.3−10.1 nm ( Figure 33E) and Pd diameter site of 2.2 nm within the range of 1.5−4.5 nm. Interestingly, experimentally, in CdS− Pt NRs, increasing the metal site size from 0.7 to 3 nm led to an increase in the quantum efficiency of H 2 production by nearly 2 orders of magnitude, consistent with the previous studies ( Figure 33F). 116 The authors pointed out that in contrast to the earlier reports, the photocatalytic efficiency is predicted to increase with the increased Pt sizes according their kinetic calculations. Note that the morphology and the evolving facets reactivity changes upon increased metal domain, which were not included in that study, can also influence the photocatalytic performance. Nevertheless, these divergences can originate from the inherent differences of different metal types.
Interestingly, this trend of metal domain size effect was not preserved under nonlinear excitation conditions that promote MX generation. Under high-energy fluences, an advantage of larger tipped HNPs over small-tipped HNPs was revealed for photocatalytic water reduction and hydrogen generation. 218 This change in the size effect trend is due to the presence of an additional competing relaxation route of Auger recombination at this excitation regime which can be outcompeted by the faster electron charge transfer of large-tipped HNPs, while for a smaller metal domain, the Auger process dominates the relaxation resulting in less free charges at the metal domain available for the water reduction reaction.
The type of the cocatalyst was shown to have a substantial effect on both charge separation and photocatalytic activity. The photocatalytic efficiencies of HNPs with Au and Pt decorations, which are two of the most common noble metals that serve as cocatalysts in the HNP system, present a characteristic behavior that is derived from their charge-separation and -transfer processes on top of the well-known superior catalytic activity of Pt. 17,25,68,134,146 Photocatalytic activity measurements comparing CdSe−Pt and CdSe−Au NRs showed the higher photocatalytic efficiency of the Pt-decorated HNPs due to the efficient depletion of excited electrons. 147 This advantageous performance was assigned to their ohmic behavior, which enables the full extraction of electrons from the lowest excited states in the semiconductor component, while Au-tipped HNP's photocatalytic capacity may be restricted by Fermi-level equilibration and charge accumulation associated with gold metal NPs. 18 Note that although Pt is considered more effective toward water reduction, a higher efficiency of aerobic reduction processes such as H 2 O 2 formation by dissociation of the oxygen molecules was obtained by Au as a cocatalyst site. 246 An approach that aimed to exploit the two different behaviors of Au and Pt was manifested in the formation of asymmetric Au− CdSe−Pt nanodumbbells which presented higher hydrogen generation rates over Pt−CdSe−Pt and Au−CdSe−Au nanodumbbells. Combination of the slow electron−hole recombination behavior of the Au with the efficient water reduction reaction rate obtained by Pt decoration promoted the more effective nanosystem of the three ( Figure 34A). 134 A similar strategy of asymmetric HNPs was employed in the synthesis of Au−CdSe/CdS−PdS NRs. Again, the energy band alignment of this structure, in which energy band gaps of CdS and PdS are 2.4 and 1.6 eV, respectively, promotes efficient charge separation resulting in a hydrogen generation rate of over 2 orders of magnitude greater than the production achieved by CdSe/CdS and Au−CdSe/CdS. 131 Larger asymmetry was also reported as a favorable structure comparing the Au−CdS core/shell and heterodimers with different phase separation. 85 In addition, similar factors of Fermi-level shifting and enhanced electron− hole recombination have been reported to affect the photocatalytic activity of CdS and ZnS−CuInS 2 NRs with various Pd 4 S and PdO cocatalysts compared to Pt deposition. 50,247 An understanding of the different photocatalytic characteristics of various cocatalyst metal types led to their combination in order to maximize the hydrogen generation efficiency. Several studies have demonstrated the pivotal effect of the metal site composition. Kalisman et al. showed enhanced photocatalytic H 2 generation by a cocatalyst consisting of Au tips decorated by Pt islands deposited on the apex of CdSe/CdS NRs compared to Au/Pt core/shell-tipped NRs and single-phase Au-or Pt-based HNPs ( Figure 34B). 68 This trend was attributed to the formed Au/Pt interfaces that promote higher reactivity toward surface bonding interactions. A similar trend was reported for Pt− CdSe−Au and Au−CdSe−Au nanodumbbells where the latter was coated with Pt and a higher photocatalytic activity was measured compared to the original hybrid forms. 134 A significant improvement in the photocatalytic water reduction rate was also reported for Au/Pd alloy-tipped CdSe/CdS NRs compared to core/shell bimetal sites and single-phase metal deposition, showing an approximately 5-fold increase, which was attributed to the synergistic electronic effects of bimetal catalysts such as new surface rearrangement that decreases the absorbance strength and therefore promotes product dissociation. 67 Moreover, the photostability of such alloyed sites exceeded that of Pd-tipped NRs due to suppression of the cation exchange of Cd atoms in the rod structure with the presence of Au that serves as a barrier for Pd migration. 248 The advantage of bimetallic cocatalysts over a homogeneous metal cocatalyst was also reported for Zn 0.5 Cd 0.5 S decorated with Ni/ Co alloy NPs, 128 CdS decorated with several different noble metal−PdS cocatalysts, 249 and Pt−Pd hybrid NPs deposited on CdS NRs, 129 all exhibiting higher photocatalytic hydrogen generation rates compared to the single-metal phase-deposited nanosystems due to the lower Fermi energy of the alloyed tipped HNPs or the higher activity of the bimetal interface, respectively. Nevertheless, measurements of the photocatalytic efficiency of Ag/Pd core/shell and alloy cocatalysts deposited on CdSe/CdS NRs showed inferior efficiency compared to the monometallic Pd-tipped NRs. 127 In addition to the material features of the cocatalyst sites, the decoration architecture also has a critical impact on the photocatalytic hydrogen generation performance. The superior photocatalytic activity of a single cocatalyst domain selectively deposited on the semiconductor absorber was established through several studies in different HNP systems. In the case of CdSe−Pt NRs, a single tip revealed ∼50% higher hydrogen generation rates in comparison to dumbbell CdSe−Pt NRs. 146 CdS−Pt, 197 CdSe/CdS−Pt, 223 and ZnSe−Au 30 HNPs have shown a similar trend of reduced H 2 generation quantum efficiency for the case of random multiple cocatalyst sites on the semiconductor surface ( Figure 34C). Although excited charge carrier transfer in these HNPs was faster and more efficient, as was described in section 3.3.1, effectively, the vicinity of the trapped hole and the electron on the metal domain results in a faster recombination rate, outweighing the faster electron transfer and therefore dominating the kinetics of the overall photocatalytic process. Alternatively, it was suggested that the probability of harvesting two electrons following two sequential photon absorption incidents, which are required for photoreduction of water to hydrogen, is more likely to occur in the presence of a single cocatalyst site, whereas multiple sites randomly share the excited electrons, hence practically demanding more successive charge-transfer events. Interestingly, a recent report by Liu and co-workers presented the enhancement of photocatalytic hydrogen evolution by multiple plasmonic Au NP chain-like structures embedded in Zn 0.67 Cd 0.33 S NPs in comparison to Au metal domains deposited on the surface of the semiconductor component and even embedded separated Au NPs within the semiconductor ( Figure  34D). 157 This improvement is attributed to two main factors. One is the shorter distance between the semiconductor and the plasmonic Au that can induce more energy transfer. Second, the chain structure holds a short distance between adjacent NPs, which gives rise to highly intense and localized electromagnetic fields. Consequentially, the collective excitation of plasmonic metal facilitates much more plasmonic energy transfer from the metal to the semiconductor that eventually increases exciton formation and the following photocatalytic reaction.

Surface Effect on Photocatalytic Hydrogen Production.
In colloidal HNPs, due to their nanometric characteristics such as a large surface to volume ratio, the nature of the organic capping layer has a central influence on the photocatalytic performance via several optional mechanisms. Besides their well-known stabilizing function that allows their dispersion in both aqueous and organic media, the dynamics of the charge-separation process can be altered by different surface coatings via a direct influence on hole removal and charge transfer, as described in section 3.3.1. The specific dependence of the overall photocatalytic activity is derived from the type of organic molecules that are in the stabilizing layer and the nature of the chemical bond between the atoms at the HNP surface and the molecular ligands.

Chemical Reviews pubs.acs.org/CR Review
The effect of the surface capping ligand on the efficiency of hydrogen generation was investigated in various hybrid and semiconductor nanosystems. 250−253 The implications of this effect on semiconductor−metal HNPs were reported by Banin and co-workers on a single Au-tipped CdS NRs. Investigation of the photocatalytic activity and efficiency of HNPs with different types of surface capping ligands has revealed a significant surface effect. 32 Comparison of commonly used thiolated alkyl ligands with polymer encapsulation by amphiphilic or branched polymers revealed the superior photocatalytic performance by the polymer coating represented by PEI followed by the amphiphilic polymer coating in the form of poly(styrene-comaleic anhydride) which exhibited an intermediate quantum yield, while the thiolated alkyl ligand showed inferior activity, as presented in Figure 35A. Although several possible mechanisms can be assigned to the surface effect including colloidal stability or photostability that can be hampered in the presence of thiolbased ligands and alternatively the nature of the diffuse electric double layer that may attract or withdraw sacrificial hole scavenger agents, these mechanisms were shown to have minor contributions. Instead, the degree of surface traps passivation was found to dominate this effect. Improved passivation PEI led to reduced hole trapping and thus limited electron transfer to the metal domain due to electron−hole Coulomb interactions. The surface ligand effect was also reported for CdS−Pt HNPs showing a slightly different hydrogen evolution photocatalytic capacity between MUA and cysteine-capped HNPs. 254 However, for HNP-based photocatalysts where the photocatalytic reactions take place at the semiconductor segment via defect surface states, an inverse trend is expected, where better surface passivation caused inferior photocatalytic activity, as was reported for spherical CdS/CdS core/shell NPs. 255 It is worth noting that capping surface ligands can also dictate the photocatalytic reaction that can be executed by the HNPs. Demonstration of such orientation was obtained by sulfide treatment of Au-tipped CdS HNPs. 256 Post-treatment HNPs revealed negligible H 2 production compared to similar PEIcoated HNPs. Yet, oxidation processes on the semiconductor surface toward radicals (OH • and O 2 −• )) were significantly enhanced ( Figure 35B).

Chemical Sacrificial Effects.
The above sections discussed the photocatalytic activity and efficiency dependence by inherent HNP parameters; nevertheless, the hole removal process and excited charge-transfer routes and therefore the Chemical Reviews pubs.acs.org/CR Review overall photocatalytic performance can be affected by external parameters such as the sacrificial hole scavenger agents or the chemical conditions. Typically, these external factors are aimed to enhance the hole extraction process following light-induced charge separation given the consideration of this pathway as the bottleneck for an efficient photocatalytic hydrogen generation reaction. The effect of the type of the hole scavenger on photoinduced H 2 evolution was investigated by Berr et al. on Pt−CdS NRs showing improved photocatalytic efficiency as a function of the negativity of the hole scavenger redox potential. As presented in Figure 36A, a comparison of different electron-donating molecules including methanol, EDTA 4− , triethanolamine (TEA), and SO 3 2− showed increased hydrogen generation quantum efficiency in the presence of a hole scavenger within this order. 254 A more negative oxidation potential leads to a larger driving force for hole removal by sacrificial electron− excited hole recombination. Moreover, a low hole reduction rate was correlated with a loss of stability, attributed to photooxidation of the semiconductor rod itself that also hinders the photocatalytic performance. However, these thermodynamic considerations also should include the nature of the HNPs, structure, and material composition as was demonstrated by testing the photocatalytic water reduction activity of CdS−Pt and CdSe/CdS−Pt NRs with two different hole scavengers, methanol and sulfite. As shown in the former described report, the use of sulfite indeed allowed relatively higher efficiencies for both HNP systems in comparison to methanol. Still, comparing these different HNPs in the presence of each hole scavenger reveals an inverse photocatalytic behavior where when using methanol a higher quantum efficiency was observed for CdSe/ CdS−Pt, while a higher quantum efficiency with sulfite was achieved in CdS−Pt ( Figure 36B). 240 This trend was attributed to the interplay between the trapped holes at the HNP surface and the reductive nature of the electron-donating agents. For the strong reducing sulfite case, holes in CdSe/CdS-based HNPs are confined to the core and therefore are harder to access in comparison to surface traps that accumulate holes in the CdS structure. Using weak electron-donating agents such as methanol may not be energetically sufficient (small driving force) for reducing strongly bounded holes at the CdS surface compared to holes at the CdSe−CdS core−shell interface.
A different strategy for improving hole removal is by manipulating the reaction environmental conditions. Several reports demonstrated the influence of conducting the photocatalytic water reduction under extreme alkaline conditions (pH > 13). Simon et al. reported a significant 6-fold increase in H 2 generation rates by Ni-decorated CdS NRs at pH 13−14 in comparison with pH 11−12. At a pH of 14, a steep increase in the reaction rate appeared and an improvement in the photocatalytic function reached a 53% external and 71% internal quantum yield ( Figure 36C-a). 71 The underlying mechanism of this strategy is the formation and utilization of hydroxyl ions (OH − ) as an efficient redox shuttle with higher mobility and accessibility to the HNPs surface compared to other common hole scavenger agents, allowing faster hole reduction and formation of hydroxyl radical coupled with a high-rate catalytic oxidation reaction by the formed radicals of different electrondonating molecules in the solution such as methanol or TEA. Yet, as illustrated in Figure 36C-b, at pH < 14, reduction of holes is taking place mainly by relatively slow-rate direct oxidation of methanol to acetaldehydes. However, as illustrated in Figure  36C-b, above pH 14, the redox potential of hydroxyl (OH • / OH − ), which follows Nernstian dependence (59 mV per pH unit), crosses and exceeds the energy level of the semiconductor VB, which has a weaker pH dependence (33 mV per pH unit), therefore promoting the reduction of holes via fast and efficient hydroxyl shuttle agents which now dominate the photocatalytic process. Such pH dependence was observed for CdS−Pt NRs as well, showing a steady increase of H 2 evolution rates upon increasing the pH values from pH 12 (3.6 mmol h −1 g −1 ) to pH 13 (21.8 mmol h −1 g −1 ). 257 Similarly, CdS−Pt NPLs 61 and CdSe/CdS−Pt 239 NRs were also used to demonstrate the advantage of high alkaline conditions ( Figure 36D). Increasing the pH conditions even higher, up to pH 14.7 or 16 (for the former and latter cases, respectively), resulted in unity conversion of photons absorbed to hydrogen.

Toward Full Water Splitting.
The promising utilization of HNPs as photocatalysts for water reduction and H 2 production is clearly manifested throughout the advanced synthesis control and in-depth investigation of their photocatalytic properties that derived from their material combination and structure as was discussed in the former sections. Yet, to complete a full water splitting catalytic cycle that includes both H 2 and O 2 formation by water reduction and oxidation, respectively, a hybrid system that can exploit in a single excitonic cycle both charge carriers, electrons and holes, to promote these two catalytic pathways is desired. Generally, semiconductor−metal HNPs until now did not demonstrate efficient activity toward the second half-cell reaction of water oxidation. This is due to the inherent complexity of this multiple-charge carrier reaction that requires four hole charges to generate molecular oxygen from water along with its slow kinetics and high redox potential. As was addressed above, a long-lived charge-separated state ought to be to obtained to enable the four-hole oxidation process required to form the O− O bond of molecular oxygen and to effectively compete with intraparticle fast relaxation routes that lead to electron−hole recombination or surface oxidation.
A synthetic effort was made by Alivisatos and co-workers presenting of Ru−CdSe/CdS−Pt HNPs in which the Ru and Pt metal sites served as cocatalysts for the water oxidation and reduction reactions, respectively. 133 However, no actual photocatalytic measurements of water splitting were exemplified.
Recently, Stolarczyk and colleagues demonstrated a full water splitting photocatalytic cycle. The use of well-known CdS−Pt NRs that promote efficient charge separation was proven to effectively reduce water to generate hydrogen along with surface modification by a ruthenium-based complex that was shown to harvest excited holes to catalyze oxygen formation; both water reduction and oxidation were achieved and measured simultaneously (15.1 and 0.52 μmol h −1 , respectively) ( Figure  37). 258 The Ru complex that consisted of a derivative of the molecular oxidation catalyst Ru(tpy)(bpy)Cl 2 with dithiocarbamates anchors that attach to the HNP surface allowed a fast hole transfer rate within ∼300 fs. This time scale can successfully compete with the above-mentioned relaxation routes that typically hold a lifetime ranging from 0.7 to 1.0 ps. 259 Yet, the efficiency of the oxidation reaction achieved by this hybrid system was up to 0.27% and a molar ratio of H 2 to O 2 (∼20:1) that was much lower than the stoichiometric value of 2 leave plenty of room for further synthetic developments and fundamental investigations of these HNP-based photocatalysts for addressing the highly challenging full water splitting reaction. 4.1.5. HNP-Based Photoelectrochemical Cells. As discussed in the last section, full water splitting is rare in colloidal HNPs and typically requires a wide band gap due to the high over potential needed to drive the catalytic reactions. On the other hand, to maximize the absorption of the solar spectrum, small band-gap materials are favored. In addition, full water splitting in a colloidal solution on a single nanostructure leads to the formation of O 2 and H 2 in the same place, raising concerns about the possibility for their reaction and requiring separation processes, altogether challenging the applicability of this approach. Photoelectrochemical (PEC) water splitting has emerged as a path to address these problems. Here, H 2 and O 2 are formed on a separated photocathode and photoanode, respectively. As two different semiconductors may be used for water oxidation and reduction, the band gap of each can be smaller to maximize light absorption. The required overpotential for the chemical reaction is supplied by the electrical properties of the semiconductors such as the majority carriers; ptype semiconductors with high overpotential for water reduction are investigated as photocathode materials, while n-type semiconductors are used as photoanode materials due to the high overpotential for water oxidation. The overpotential to drive the catalytic reaction is further increased by applying external electrical bias, which is yet an additional attribute of the PEC approach. Thus far, HNP PEC cells have been demonstrated as photocathodes photoanodes and tandem cells, but the work in this area utilizing the precontrolled colloidal HNPs is still quite limited compared to the studies of their water splitting performance in suspensions.
Photocathodes made from gold-tipped HNPs have been reported to show superior photoelectrocatalytic performance for  the water reduction reaction; for example, Au−Cu 2−x Te in comparison to Cu 2−x Te with photocurrent densities being ∼10 times higher. 260 Au−CuInS showed enhancement of water splitting of four times in comparison to only CuInS nanostructures. 261 Photocurrent response in Au−CuGaS 2 over the pure material was also reported to be enhanced. 262 This can be attributed to a combination of the contributions of the gold catalytic ability and possibly to the plasmonic enhancement of light absorption.
Photoanodes with HNPs were also demonstrated. Ag−CdS NW showed a photocurrent density increase by about 4.7 times compared to that of the pure CdS NW photoanode, whereas the H 2 obtained for Ag−CdS NWs is 1.8 times higher than that for the CdS NW photoanode. As an example of overall water splitting, a tandem cell consisting of an undoped Au−CdS photoanode and a Cu-doped Au−CdS photocathode, respectively, modified with cocatalysts was fabricated and displayed stable H 2 and O 2 evolution as demonstrated in Figure 38. 263

Clean Energy�CO 2 Reduction
Water splitting is one way to form carbon-free fuel in the form of H 2 , while another clean energy source is the reduction of CO 2 , such that this greenhouse gas is sequestered while no new carbon is emitted to the atmosphere. The energy barrier of the first step in the reduction of CO 2 is very high, and therefore, even two-electron reduction to CO is challenging. If CH 4 and CH 3 OH are the main target products, the generation of CH 4 and CH 3 OH requires the transfer of eight and six electrons, respectively. Due to the high energy barrier and the multielectron reduction process, this reaction requires tight control on the chemistry of the catalytic metal to favor this reaction over other competitive reactions such as water reduction. A common strategy to address this requirement is by coupling the semiconductor to a metal complex to enhance reaction selectivity toward CO 2 . For example, CdS QDs with Ni-(cyclam), 264 Ni doping, 265 and Co 2 L complex 266 demonstrated selective photocatalytic CO 2 to CO conversion. Fe and Co complexes with CuInS 2 were also used to selectively reduce CO 2 to CO in water. 267−269 The metal atoms can also change the selectivity of the CO 2 reduction products. Under full solar spectrum irradiation, CO as the main product with 97.2% selectivity was observed on bare CdS, whereas the intentional introduction of an optimized amount of Ru metal enables CO 2 reduction to CH 4 with 97.6% selectivity. 270 Compared to water splitting, CO 2 reduction by metal tip deposition is so far less common for chalcogenide-based HNPs. For tip-decorated Cu 2 S−Pt, the CO formation rate was 3.02 μmol h −1 g −1 , 2 orders of magnitude higher than that for random decoration of Pt. Methane evolution was also observed for the tip-decorated sample, albeit with a much smaller formation rate, 0.13 μmol h −1 g −1 (Figure 39). In this system, both the bare CdS NRs and the NRs with randomly photodeposited Pt NPs did not lead to any gas evolution. 271 In another report, CdS−Pt was also proven useful for photocatalysis depending on the details of the growth method. In this work, when Pt nanoparticles were reduced onto CdS by ethylene glycol, the CO production rate was 2.99 μmol g −1 h −1 , compared to 0.12 μmol g −1 h −1 for CdS only and 0.18 μmol g −1 h −1 for 1 wt % Pt by photoreduction. 272 Visible-light photocatalysis using Cd−chalcogenide was also demonstrated by combining CdSe with Pt/TiO 2 . In this case, the photocatalytic conversion of CO 2 to methane and methanol was demonstrated in the presence of water. 273 This strategy was also reported for a PbS QD used to sensitize Cu/TiO 2 , where the highest total CO 2 conversion yield was 1.71 μmol g −1 h −1 (0.82 μmol g −1 h −1 CO, 0.58 μmol g −1 h −1 CH 4 , and 0.31 μmol g −1 h −1 C 2 H 6 ), which is >5 times the activity of only Cu/ TiO 2 . 274 Another example for tunable photocatalytic CO 2 reduction activity by CdS was shown for Au/MoS 2 -tipped CdS NWs. This study reported syngas generation with a H 2 /CO ratio ranging from 0.35 to 3.6 under visible-light irradiation by simply altering the Au particle size. 275 In this work, CO 2 reduction to methanol over noble metal-free hybrid semiconductor photocatalyst (Ni 2 P/CdS) under visible light was also reported. 276

Industrial Applications
The ability to utilize the exciton in HNP for photocatalytic reactions can be further extended to various additional applications. In this section, we show several examples of how these excited charge carriers can be exploited for industrially relevant processes. This includes the use of HNPs for photoinduced catalysis of selective organic reactions, and we also discuss their use as photoinitiators for polymerization. Last, we describe some examples of sensors, including chemical detectors and photodetectors made by HNPs.
4.3.1. Organic Transformations. As shown in the above sections for the reduction of H 2 O, CO 2 , and O 2 , the metal is crucial for the selectivity toward specific reactions. The use of HNPs for selective photocatalysis is also exemplified by using different metal tips to direct the conversion of molecules toward the desired chemical product utilizing a photocatalytic reaction route. Photochemical dehydrogenation and hydrogenolysis of benzyl alcohol was studied on CdS−Pt and CdS−Pd nanorods.
CdS−Pt favors dehydrogenation (H 2 ) over hydrogenolysis (toluene) with a ratio of 8:1, whereas CdS 0.4 Se 0.6 −Pd favors hydrogenolysis over dehydrogenation by 3:1. 277 The use of HNPs to catalyze organic chemical reactions was extended also to the near-infrared region, where the Cu 7 S 4 −Pd nanostructure demonstrated photocatalytic activity for Suzuki coupling reactions of iodobenzene with different reagents, selective oxidation of benzyl alcohol, and hydrogenation of nitrobenzene under 808, 980, and 1500 nm irradiation. 278 Selective reduction of nitroaromatic compounds was also enhanced by Au on CdS NWs. 279 The importance of the HNPs for selective hydrogenation reactions was also demonstrated in the selectivity of catalytic reaction products. Catalysis of the hydrogenation of cinnamaldehyde shows that while 3phenyl-1-propanol was the only product over Pt−Cu 2 S HNPs, CuPt−Cu 2 S HNPs exhibited a higher conversion rate and selectivity toward hydrocinnamaldehyde which was also enhanced compared to CuPt only. 280 4.3.2. Polymerization. The excitons in semiconductor NPs emerge as an alternative for organic photoinitiators for polymerization initiation, where the presence of holes can form radicals to initiate a polymerization chain reaction. 5 The mechanism is either by direct reduction/oxidation of the monomer itself or by some mediator such as amine or hydroxyl radicals. 281−283 The enhanced charge separation in HNPs and the formation of ROS can also be harnessed for this application. The effect of metal tip in the photopolymerization of polyacrylamide was studied in CdS−Au HNPs. The kinetics of the polymerization degree was followed by the FTIR signature of the acrylamide monomer double bond. CdS−Au demonstrated three times faster polymerization compared to CdS NRs without a metal tip ( Figure 40A). 284 The superior performance of CdS−Au is attributed to the electron−hole charge separation induced by the metal tip accompanied by its favored catalytic functionality toward reduction of O 2 , enhanced rate of electron removal, and enhanced hole removal to the monomer by hydroxyl radicals (see scheme in Figure 40A). This was supported by comparing the polymerization with the presence of the hole scavenger ethanol and sulfide, which led to a significant decrease in the polymerization rate. This observation is explained as the presence of these hole scavengers competes with the monomers on the radicals formed. 284 In a later work, hydroxyethyl acrylate was used as a monomer in performing the photopolymerization without solvent (solvent free) and in the absence of water. The suggested initiation mechanism in this case was by surfacemediated hole transfer from the semiconductor to the monomers, either directly to the double bond of the monomers or via the surface ligands, polyethylenimine ( Figure 40B). 285 The facile radical generation leading to photopolymerization, accompanied by the ability to tune the semiconductor absorption to the common blue wavelengths used in 3D printers, allowed the generation of a 3D-printed hydrogel structure utilizing such "quantum photoinitiators". Moreover, the enhanced two-photon absorption cross section akin to semiconductor nanorods allowed the HNPs to be used as photoinitiators for high-resolution 3D printing utilizing a twophoton absorption process. This was demonstrated both for water-based printing and for solvent-free formulation.

Sensing.
Combination of the optical and catalytic properties of the semiconductor and metal components was also suggested to be utilized for sensing applications. Photoelectrochemical detectors for Cu 2+ ions were studied by the Chemical Reviews pubs.acs.org/CR Review deposition of Au tips on CdS NPs. The signal of CdS−Au was enhanced three times upon Au deposition. The suggested mechanism of enhancement was explained by hot-electron transfer from gold to CdS which hinders the recombination of electron−hole pairs in the CdS NPs. Gas sensing was demonstrated on Au-decorated CdS NWs, where the response to ethanol was enhanced 5 times compared to that in nondecorated CdS NWs ( Figure 41). 286 This enhancement was attributed to the better catalytic oxidation of organic molecules in the presence of gold, consistent with the results of enhanced ROS formation by CdS−Au in solution. 246,287 In the area of light sensors, infrared photodetectors were prepared by the addition of Ag NPs to a film of PbS quantum dots, and the responsivity in the devices increased from 1.5 to 3.8 mA/W for 1% of Ag NPs. 288 Similar results were reported by the addition of Au NPs to PbS film. 289 An ultraviolet photodetector was prepared by layers of CdS and Au NPs. 290 The enhancements of the photodetection were attributed to the suppression of charge carrier recombination by the presence of Au NPs.

Photodynamic and Photothermal Therapies.
As mentioned above, excitation of metal−semiconductor HNPs by light in aerobic and aqueous environments results in ROS formation. They can be used for biomedical treatments of photodynamic therapy. This was demonstrated in vivo by the decreased volume of tumors and in vitro by the decreased cancer cell viability ( Figure 42A). Another way of treatment by local heating via the plasmonic response of a metal or a semiconductor in photothermal therapy, which requires a high absorption coefficient and the coupling of the semiconductor to plasmonic metal, can enhance the absorption cross section ( Figure 42C).
Utilization of HNPs for photothermal therapy was shown on Au−Cu 2−x S, demonstrating enhancement of the extinction coefficient by a factor of 2 compared to the mixture of Cu 2−x S and Au NPs in the relevant spectral regime. This factor was reported to yield an increase in the temperature achieved by Au−Cu 2−x S by up to 70°C at a high concentration of ∼0.1 mg/ mL. 291,292 This was also shown to correlate with the plasmon intensity of the gold NPs. 293 The photothermal effect of Au− Cu 2−x S showed antitumor activity in vitro, where only 10−20% of cervical cancer HeLa cells remained alive after NIR illumination for several minutes. 291,294 Murine breast cancer 4T1 cells were killed by laser irradiation for 24 h but not normal 3T3 cells. 292 Similar results were also reported for AuPt− CuS. 295 In an in vivo experiment, a decrease in tumor volume was demonstrated in the presence of Au−Cu 2−x S and AuPt− CuS with NIR irradiation. 292,293,295 The survival of the mice of control groups was around 40 days post-treatment, whereas the lifetime of the Au−Cu 2−x S-treated mice can be substantially prolonged, to 60 days post-treatment. 292 In photodynamic therapy, HNPs can contribute to the antitumor activity via the photocatalytic ROS formation. The ROS can be formed either by oxidation of water or by the reduction of O 2 as detailed in previous sections. It was described for the above Au/CuS and AuPt/CuS HNPs that resonant energy transfer from the excited plasmon in the metal core to excitons in the Cu 2−x S shell supplies holes for water oxidation and electrons for O 2 reduction. 293,295 Another way that Au metal on a semiconductor can contribute to the antitumor activity is via its catalytic activity. The catalytic efficiency of Au is known to appear in a small size regime of <3 nm of the Au. The cell viability of leukemia cells K-562 was studied in the presence of visible 1.6 nm Au-tipped CdSe/CdS nanorods and demonstrated enhancement in cell death compared to bare CdSe/CdS nanorods. 287 Cell viability test of similar CdSe/CdS−Au structures on 4T1 cells demonstrated almost complete cell death of 95% at a concentration of 0.1 mg mL −1 under hypoxia, while without the Au tip; less than 70% of the cells died. 296 In this small size of gold there is no plasmonic response, and this activity was attributed to the generation of ROS. Specifically, CdSe/CdS−Au demonstrated better photocatalytic activity toward the formation of H 2 O 2 and OH radicals and the consumption of oxygen compared to bare nanorods. 287,296 This functionality of HNPs as an antibacterial agent was also demonstrated by Au−Bi 2 S 3 core−shell structures under the illumination of NIR light, which showed superior antibacterial activities against both E. coli and S. aureus due to the synergistic photothermal and photodynamic killing. 297 4.4.2. Bioimaging. The enhanced absorption of the semiconductor induced by the metal plasmon was investigated as a contrasting agent in bioimaging applications. Surface- enhanced Raman spectroscopy (SERS) imaging was demonstrated on hybrid Au−Cu 2−x S. The characteristic Raman peaks of the rhodamine B dye molecule were enhanced by a factor on the order of 10 4 for trilayer core−shell NPs ( Figure 43A). 294 In these cases, the benefit from the semiconductor comes from the plasmonic rather than the band-gap properties. Nonplasmonic CdSe nanowires decorated with gold were also used to enhance the Raman signal of cresyl violet dye, but in this system, CdSe has only a structural role. 298 X-ray-computed tomography (CT) is another imaging technique studied in the context of HNP activity. High image contrast at the tumor sites could be observed after Au−Cu 2−x S or AuPt−CuS HNP injection, significantly larger than that of iopromide or iohexol ( Figure  43B). 291,292,295 The hybrids of Au−Cu 2−x S and Au−Cu 2−x Se were investigated also as contrast agents for photoacoustic (PA) imaging ( Figure 43C). 291,292,299 In another work, such HNPs were further combined with magnetic Fe 3 O 4 to form Fe 3 O 4 − Au−Cu 2−x S trimers for dual-heating agents based on photothermal and magnetic hyperthermia actuation. An intercalation protocol with radioactive 64 Cu ions on the Cu 2−x S domain reached high radiochemical yield and specific activity, making these HNP trimers suitable as carriers for 64 Cu in internal radiotherapy and traceable by positron emission tomography. 300 In the above examples of imaging techniques, there is no band-gap emission of the semiconductor in the HNPs and their contribution is the plasmon absorption or structural characteristic. This is due to the quenching of the emission when the metal is in direct contact with the semiconductor. Growth of an insulating layer between the semiconductor and the metal can prevent this quenching. In this way, the presence of the metal plasmon can contribute to the enhancement of the semi-conductor emission. Such structures were demonstrated as labels for immunoassay by the structures of Au and CdSe/CdS/ ZnS NPs embedded in SiO 2 . Bimodal labels with colorimetric and fluorescent readout were fabricated via a layered sequential assembly strategy. This resulted in amplified signals from the assemblies of individual single nanoparticles and allowed colorimetric and fluorescent detection of cystatin C (Cys C) after surface conjugation with antibodies. 301

OUTLOOK AND PERSPECTIVE
This review surveyed the remarkable accumulated research on semiconductor−metal HNPs, from their synthesis to properties to emergent photocatalytic applications. Yet, there is a prominent future for such HNPs that hinges on continued further research and innovation to meet the requirements for efficient photocatalytic reactions. Among the challenges, it is important to consider the introduction of novel synthetic approaches toward the formation of more complex HNP architectures. There is also a need to expand the HNP family to green and environmentally friendly semiconductor−metal systems. For example, the earth-abundant copper− and zinc− chalcogenide metal combination has not yet reached the full potential in their development owing to limited spectral coverage of the visible spectrum, the electronic properties, the charge extraction efficiency, or the chemical stability in aqueous solutions. III−V semiconductors as components in HNPs also hold promise due to their compliance with regulatory restrictions and their potential also as photocatalytic systems but have yet to be fully studied in these contexts. Recent progress in the synthesis of anisotropic shapes of InP nanocrystals and their surface control opens the way for development of new HNPs based on these materials. 302,303 Moreover, addressing the stability of HNPs in diverse demanding photocatalytic scenarios is also important. In this context, expanding the functionality while controlling the HNP surface coating while also utilizing inorganic ligands may be further addressed.
Open questions concerning the fundamental nature of the nanoscale semiconductor−metal interplay in HNPs remain and call for further experimental and theoretical investigations. With the development of plasmonic photocatalysis, HNPs could offer a unique platform for dual combined excitonic and plasmonic photocatalysis in a single nanosystems. Yet, following this avenue motivates the study of electronic structure and charge carrier dynamics and relaxation routes across the excitonic− plasmonic interface. Harnessing the plasmonic excitation route requires one to extract the charge carriers on an ultrafast time scale from either side of the HNPs, which can only be achieved via further multidisciplinary efforts combining synthesis, theory, and advanced experiments. An additional interesting avenue is to integrate HNPs as building blocks in new photocatalytic applications such as photoelectrochemical solar to fuel conversion schemes, which also constitutes a highly promising direction of further research.

Corresponding Authors
Yuval