Spatioselective Deposition of Passivating and Electrocatalytic Layers on Silicon Nanowire Arrays

Metal–silicon nanowire array photoelectrodes provide a promising architecture for water-splitting because they can afford high catalyst loading and decouple charge separation from the light absorption process. To further improve and understand these hybrid nanowire photoelectrodes, control of the catalyst amount and location within the wire array is required. Such a level of control is currently synthetically challenging to achieve. Here, we report the synthesis of cm2-sized hybrid silicon nanowire arrays with electrocatalytically active Ni–Mo and Pt patches placed at defined vertical locations within the individual nanowires. Our method is based on a modified three-dimensional electrochemical axial lithography (3DEAL), which combines metal-assisted chemical etching (MACE) to produce Si nanowires with spatially defined SiO2 protection layers to selectively cover and uncover specific areas within the nanowire arrays. This spatioselective SiO2 passivation yields nanowire arrays with well-defined exposed Si surfaces, with feature sizes down to 100 nm in the axial direction. Subsequent electrodeposition directs the growth of the metal catalysts at the exposed silicon surfaces. As a proof of concept, we report photoelectrocatalytic activity of the deposited catalysts for the hydrogen evolution reaction on p-type Si nanowire photocathodes. This demonstrates the functionality of these hybrid metal/Si nanowire arrays patterned via 3DEAL, which paves the way for investigations of the influence of three-dimensional geometrical parameters on the conversion efficiency of nanostructured photoelectrodes interfaced with metal catalysts.


INTRODUCTION
Silicon nano-and microwire arrays provide versatile architecture with outstanding and tunable optoelectronic properties that can combine light trapping, Mie resonances, waveguiding, and diffractive effects. 1−6 These properties have enabled their use in a range of applications such as sensing, solar conversion, photodetection, and synthesis of optically controlled biointerfaces. 7−25 In most cases, the semiconductor wires are interfaced with metal structures that act either as plasmonic materials, 10,11 electrical contacts, 26 or catalysts. 12−24 The ability to precisely locate the metal within the nanostructured substrates is necessary to optimize and control their biological, photocatalytic, and optoelectronic properties. 10−18,20−26 This is especially true for photo(electro)chemical systems that take advantage of the superior photonic properties of nanowire arrays. 12−19 In these systems, a number of factors can affect the local catalyst activity due to spatially inhomogeneous mass transport, charge recombination, light absorption/scattering, catalyst coverage, and defect distribution. 16,17,27,28 Recent studies have shown that the deleterious effects of such inhomogeneities can be mitigated by optimizing catalyst location at the semiconductor surface. 20, 21,27,29 However, such investigations are complex and synthetically challenging.
In the context of solar water-splitting, single-crystalline nanostructured silicon is the substrate of choice because of its abundance, defined and tunable electronic properties, appropriate band gap for solar light absorption, and proper conduction band edge energy for the hydrogen evolution reaction. Because of the slow reduction kinetic of protons, silicon photoelectrodes require the addition of catalytically active materials, which are usually based on noble metals, such as Pt. 30 Earth-abundant materials such as Ni, MoS 2 , and Ni− Mo are attractive alternatives to the traditional and expensive Pt catalyst. 20,22,30,31 Their relatively lower activity requires the addition of larger amounts of catalyst on the semiconductor surface. In the case of planar photocathodes, this can lead to large optical losses caused by light absorption in the metal catalyst layer and a decreased photocathode efficiency in a front-face illumination configuration. 20 The use of micro-and nanowire arrays can mitigate this issue by decoupling light absorption and charge separation to allow higher catalyst loading without compromising the light absorption process. 12 20 This demonstrates that the catalyst position within the Si wire arrays critically influences the conversion efficiencies. However, this effect remains largely unexplored because of the difficulty to precisely control its location within the array.
The spatioselective decoration of vertically aligned silicon nanowire (VA-SiNW) arrays with metals is conventionally performed using a polymeric membrane that selectively protects the bottom of the wires and directs the functionalization at the uncovered top parts of the Si wires. 12,20,32 This strategy provides no real control over the catalyst location in the axial direction (i.e., along the nanowire length). Templated electrochemical techniques have emerged as promising alternatives to synthesize and control heterostructures at the nanoscale. 33−46 For example, the on-wire lithography 38−41 and the coaxial lithography 42−44 have allowed the structuring of electrochemically grown nanowires in both axial and radial directions. More recently, the three-dimensional electrochemical axial lithography (3DEAL) has presented a flexible platform to interface materials around silicon nanowires with a controlled axial deposition. 46 The original 3DEAL yields silicon nanowires decorated with well-defined metal rings that are electrically insulated from each other by a thin SiO 2 layer, which impedes the charge transport. This complicates the direct use of 3DEAL to prepare Si−metal hybrid photoelectrodes.
Here, we present an electrochemical method of controlling catalyst position along the Si nanowires using a modified version of the 3DEAL. Our approach affords localized passivation of the Si surface, with feature lengths down to 100 nm in the axial direction, which we subsequently use to electrochemically grow Pt and Ni−Mo catalysts at specific locations along the nanowires (Figure 1).

RESULTS AND DISCUSSION
2.1. SiO 2 Patterning via 3DEAL. The p-type VA-SiNW arrays were synthesized via metal-assisted chemical etching (MACE; Figure 1). 47−50 In this process, a gold nanohole array produced via colloidal lithography is used to anisotropically etch silicon using a HF/H 2 O 2 solution (details in the Supporting Information). After the MACE process, this gold base layer is present at the bottom of the SiNW array ( Figures  1 and 2) and can be electrically connected to the working electrode of a potentiostat to electrochemically grow additional metal films on top (Figures 2b and S1). To ensure a homogeneous layered growth of the metal films at the bottom of the array, a sub-5 nm thin SiO 2 protective shell is chemically deposited onto the Si NWs before the electrodeposition step. This protection prevents metal nucleation at the top of the wires ( Figure S2). 46 The first metal film, made of gold, grows uniformly around the silicon nanowires from the bottom to the top ( Figure 2b). Nickel is subsequently grown on top of the gold layer ( Figures S2 and S3) to form a bilayered Au/Ni film embedded within the nanowire array. Selective etching of the gold film in the KI/I 2 solution yields a VA-SiNW array with a continuous nickel film of constant thickness and a defined distance from the bottom of the wires, as controlled by the thickness of the sacrificial gold film (Figures 2c and S4). The sol−gel deposition of SiO 2 generates a conformal SiO 2 film around the silicon substrate and the nickel film, as can be seen in Figure 2d,e. 46,51,52 Electron microscopy shows that the SiO 2 shell is slightly thicker above the nickel film, most likely due to mass-transport limitation caused by the continuous nickel film ( Figure S5). However, the wires are homogeneously passivated with SiO 2 above and below the nickel film as detailed below. The complete chemical dissolution of the nickel film with HNO 3 leads to a nanostructured SiO 2 layer coating the VA-SiNWs (Figures 2e and S6). This is similar to the reported acid-induced etching of gold and nickel nanowires through SiO 2 shells grown under the same conditions, suggesting that the SiO 2 layer exhibits some level of porosity. 51,52 The areas  (6) leads to silicon nanowire arrays that are patterned with a thick nanostructured SiO 2 layer. Thermal annealing and KOH etching (7) dissolve both the residual horizontal SiO 2 layer deposited on top of the nickel layer and the sub-5 nm SiO 2 shell, yielding SiO 2 passivated silicon nanowire arrays with exposed areas at defined axial positions. Spatioselective electrodeposition of metal particles (green, 8) can now be realized at these defined locations.
initially protected by the nickel film are now exposed but remain covered with a sub-5 nm thick SiO 2 shell, which was initially deposited prior to the electrodeposition of the metal films. The rest of the Si NWs remain passivated with a thick SiO 2 film (i.e., >80 nm). Energy-dispersive X-ray spectroscopy (EDX) shows that the dissolution of both Au and Ni films is complete across the whole cm 2 -substrates ( Figures S3, S4, and S6). Further thermal annealing (10 h at 500°C) was found to improve the homogeneity of the SiO 2 shells ( Figure S7), 53 yielding VA-SiNW arrays homogeneously patterned with insulating thick conformal SiO 2 shells and defined Si areas coated with a sub-5 nm SiO 2 layer (Figure 2e). The location and height of these unprotected areas are controlled by the thicknesses of the metal films deposited by electrochemical deposition after the MACE fabrication ( Figure S8), down to feature sizes of around 100 nm (Table S1 and Figure S8c).
2.2. Spatioselective Deposition of the Metal Catalysts. Next, the silicon substrate is used as a working electrode for the direct electrodeposition of the metal catalysts ( Figure  3). A KOH etching step is first used to remove the flat SiO 2 horizontal layers and the sub-5 nm SiO 2 shell in the patterned regions ( Figure 1). Remarkably, the horizontal SiO 2 layers dissolve much faster than the SiO 2 shells around the Si NWs. We expect that repeated pitting of these horizontal SiO 2 layers promotes the KOH solution infiltration within the interlayer region (i.e., where the nickel film was originally), dissolving SiO 2 from top and bottom, which accelerates the etching process. This is not possible for the SiO 2 shells covering the Si NWs, which can only be etched from one side, and thus dissolve slower. This KOH treatment also dissolves the sub-5 nm thick silicon oxide layer on the nonpassivated area (shown in dark blue in Figure 2e), which allows for the subsequent spatioselective electrodeposition of metals on the Si NWs (Figure 3e−l). As seen with SEM, scanning transmission electron microscopy (STEM), and EDX analysis (Figures 3a−  d and S9), the silicon surface of the unprotected areas is a bit rough, which is, in our experience, typical for Si wires prepared via MACE using a sputtered gold nanomesh as a mask, while the protected areas have a relatively smooth surface that originates from the successive sol−gel deposition steps. Longer KOH treatments etch into the exposed areas of the VA-SiNWs and can lead to ill-defined patterning ( Figure S10). As an example of catalytically active material, we electrochemically deposit platinum at the exposed areas of the Si NW arrays from a homemade hexachloroplatinic acid aqueous solution at a constant potential. Acidic plating solutions (5 mM H 2 PtCl 6 and 0.5 M Na 2 SO 4 , initial pH = 3) yield considerable parasitic deposition over the entire wire surface (i.e., even on the passivated SiO 2 shells; Figure S11). We tentatively attribute this failure of spatial localization to the generation of H 2 bubbles, which can electrochemically reduce the Pt ions and therefore induce heterogeneous nucleation of Pt nanoparticles at both Si and SiO 2 nanowire surfaces. The use of a buffered solution (15 mM H 2 PtCl 6 , 200 mM Na 2 HPO 4 , initial pH = 7), circumvents these problems and leads to the exclusive deposition of Pt on the predetermined unprotected regions of the nanowires (Figure 3i Figure 3 clearly demonstrates the controlled placement of different, catalytically active materials at controlled axial locations within the VA-SiNW arrays: The metal depositions occur selectively on the unprotected regions. In addition, the defined position of the catalyst in each single nanowire extends uniformly throughout the macroscopic substrate, as seen in the low-magnification SEM images ( Figure S13). We also report the functionalization of patterns at different locations along the nanowires (bottom, center, and top, Figures 3 and S14). This validates our experimental approach to site-specifically pattern VA-SiNW with insulating and electrocatalytic nanostructured layers.
2.3. Photoelectrochemical Tests. As a proof-of-principle experiment, we prepared photocathodes from the arrays and showed photoelectrochemical activity of the patterned hybrid VA-SiNW arrays synthesized via 3DEAL. The arrays were electrically connected to fluorine-doped tin oxide (FTO) glass   18 These results confirm the viability of 3DEAL to pattern VA-SiNW photoelectrodes with functional metal electrocatalysts.

CONCLUSIONS
To conclude, we report a simple electrochemical approach to pattern Si NW arrays with passivating and nanostructured electrocatalytic layers. The modified 3DEAL is based on the electrochemical deposition, sol−gel chemistry, and selective etching and is compatible with the patterning of densely packed VA-SiNW arrays with features down to 100 nm in the axial direction. The wire arrays selectively patterned with Ni− Mo nanoparticles show activity for the hydrogen evolution reaction at acidic pH. This demonstrates the potential of the technique to investigate the structure−property relationships linking the precise position of catalyst particles with the resulting (photo)electrochemical conversion performance of these complex structures. Such studies provide design rules to optimize and minimize catalyst loading within nanostructured electrodes. The process is flexible with respect to the active material decorating the silicon nanowires but could also be transferred to different semiconducting wire materials, therefore providing a flexible toolbox to design tunable and wellcontrolled hybrid nanowire arrays.

EXPERIMENTAL SECTION
Silicon nanowire arrays were synthesized via colloidal lithography and metal-assisted chemical etching (MACE) following the protocols we have previously developed. 6,46,49 Further details can be found in the Supporting Information. 4.1. Patterning of the SiO 2 Insulating Layers. The process starts with the electrodeposition of Au and Ni at the bottom of the Si wire array ( Figure S1). Before the electrode position, a thin protective sub-5 nm SiO 2 shell is deposited for 30 min around the wires via a sol−gel process 51 (more details in the Supporting Information) to avoid the parasitic growth of metal particles on the silicon wires ( Figure S2). 46 Electrochemical deposition was performed using Keithley Sourcemeter 2400 operated with a three-electrode configuration and a homemade electrochemical Teflon cell ( Figure  S1). A Pt mesh was used as the counter electrode, Ag/AgCl as the reference electrode, while the Au film at the bottom of the silicon wire arrays (remaining after MACE) was used as the working electrode, similar to the original 3DEAL. 46 After metal deposition, the gold film below the nickel is selectively etched in a KI/I 2 solution. The dissolution was monitored by the SEM EDX analysis ( Figure S4). At this stage of the synthesis, the wires are protected with a continuous flat nickel film, located at a fixed distance from the bottom of the wires. A SiO 2 shell is then deposited around the wires for 8.5 h (with the nickel film) using the same sol−gel process. Homogeneous and   Figure S6), which reveals a spatioselectively patterned silicon wire array with SiO 2 . The substrates are then annealed for 10 h at 500°C, which increases the homogeneity of the SiO 2 shell ( Figure S7). The location and height of the patterned area can be precisely controlled by the electrodeposition duration of Au and Ni ( Figures S8, S14 and Table S1). More details can be found in the Supporting Information. 4.2. Spatioselective Deposition of the Metal Catalysts. Prior to the spatioselective deposition of metal catalysts, the protective sub-5 nm SiO 2 shell is removed via a KOH etching step (Figures S9 and  S10). The metal catalyst electrodeposition was performed immediately after KOH etching to avoid the growth of the native SiO 2 layer. Ni−Mo and Pt were deposited using homemade aqueous solutions at a constant potential. Additional details can be found in the Supporting Information.
4.3. Preparation of the Photocathodes. The backside of the Si substrate was scratched with a glass cutter and a Ga−In eutectic mixture was added on top. The Si substrate was then placed on a glass slide coated with FTO (resistivity 6−9 Ω cm). The back, sides, and front of the photoelectrodes were covered with an electrically insulating epoxy. The photoelectrodes were photographed and the active surface area was measured with the open-source software ImageJ. The photoelectrode active surface areas were in the 0.3−0.8 cm 2 range.
4.4. Photoelectrochemical Measurements. Photoelectrochemical measurements were performed using a VersaSTAT 4 potentiostat in a three-electrode setup with a Pt wire counter electrode and a Ag/ AgCl (3M NaCl) reference electrode. A solar simulator composed of a 300 W Xe lamp (LotOriel) equipped with an AM1.5G filter with an intensity of ca. 120 mW cm −2 calibrated with a Si photodiode was used for light irradiation. The irradiated area was circular with a diameter of 8.5 mm. All measurements were performed in a front-face illumination configuration. The electrolyte, an aqueous 0.1 M H 2 SO 4 solution (pH = 1), was purged with Ar for 30 min prior to the measurement. The electrode potential was initially cycled several times in the dark between 0 and −1.6 V (vs Ag/AgCl) until the current value stabilized. The electrode potential was then cycled between −0.1 and −0.8 V (vs Ag/AgCl) under light illumination until the current value stabilized. Linear sweep voltammograms were acquired at a scan rate of 100 mV s −1 . An automatic optical shutter was used to observe the rapid changes in current induced by the incident light ( Figure S15). E Ag/AgCl , the electrode potential vs Ag/ AgCl was converted vs the potential of the reversible hydrogen electrode (RHE) and expressed as E RHE , defined by the following equation where E Ag/AgCl vs SHE 0 is the potential of the Ag/AgCl reference electrode vs SHE (standard hydrogen electrode), which is 0.209 V for an Ag/AgCl electrode stored in 3 M NaCl. Therefore, the overall shift in the potentials for the RHE was calculated using E(RHE) = E(Ag/ AgCl) + 0.268 V.
Materials and details of the experimental methods, tables listing the dimensions of the Si NW arrays, and additional SEM images are included in the Supporting Information (PDF) FTO, fluorine-doped tin oxide MACE, metal-assisted chemical etching RHE, reversible hydrogen electrode SEM, scanning electron microscopy SHE, standard hydrogen potential STEM, scanning transmission electron microscopy VA-SiNW, vertically aligned Si nanowires