SrTiO3/Bi4Ti3O12 Nanoheterostructural Platelets Synthesized by Topotactic Epitaxy as Effective Noble-Metal-Free Photocatalysts for pH-Neutral Hydrogen Evolution

Low-temperature hydrothermal epitaxial growth and topochemical conversion (TC) reactions offer unexploited possibilities for the morphological engineering of heterostructural and non-equilibrium shape (photo)catalyst particles. The hydrothermal epitaxial growth of SrTiO3 on Bi4Ti3O12 platelets is studied as a new route for the formation of novel nanoheterostructural SrTiO3/Bi4Ti3O12 platelets at an intermediate stage or (100)-oriented mesocrystalline SrTiO3 nanoplatelets at the completed stage of the TC reaction. The Bi4Ti3O12 platelets act as a source of Ti(OH)62– species and, at the same time, as a substrate for the epitaxial growth of SrTiO3. The dissolution of the Bi4Ti3O12 platelets proceeds faster from the lateral direction, whereas the epitaxial growth of SrTiO3 occurs on both bismuth-oxide-terminated basal surface planes of the Bi4Ti3O12 platelets. In the progress of the TC reaction, the Bi4Ti3O12 platelet is replaced from the lateral ends toward the interior by SrTiO3, while Bi4Ti3O12 is preserved in the core of the heterostructural platelet. Without any support from noble-metal doping or cocatalysts, the SrTiO3/Bi4Ti3O12 platelets show stable and 15 times higher photocatalytic H2 production (1265 μmol·g–1·h–1; solar-to-hydrogen (STH) efficiency = 0.19%) than commercial SrTiO3 nanopowders (81 μmol·g–1·h–1; STH = 0.012%) in pH-neutral water/methanol solutions. A plausible Z scheme is proposed to describe the charge-transfer mechanism during the photocatalysis.


INTRODUCTION
Utilizing sunlight to drive chemical reactions over semiconductor photocatalysts represents a promising strategy to overcome the world's problems related to energy shortages and environmental pollution. The production of storable, green H 2 fuel from water-splitting reactions addresses these challenges. 1−6 At the moment, the process still suffers from toolow efficiencies to become economically viable. However, recent findings about the importance of heterojunctions, 3 mesocrystallinity, 6 type of exposed facets, 2 and preferential orientation 7 for improved photocatalytic performance increasingly promote an interest in the morphological engineering of functional nanostructures. In particular, the integration of two different functional materials with different band gaps and band-edge positions has attracted a great deal of scientific and technological attention. 3,8 Such heterojunction systems lead to an improvement of the photocatalytic efficiency by enhancing the photogenerated charge carriers' separation. 3 However, designing heterostructural photocatalysts in terms of achieving the target characteristics and boosted photocatalytic perform-ance remains challenging. The same is true for the creation of single-phase photocatalyst particles with morphologies that are different from the thermodynamic equilibrium crystal shape. The engineering of the functional characteristics of the particles based on an understanding of the nucleation and growth can ensure that rational morphological design prevails over serendipity.
Topochemical conversion (TC) reactions from Aurivillius perovskite platelets (Bi 4 Ti 3 O 12 and MBi 4 Ti 4 O 15 (M = Sr, Ba)) in molten salts (NaCl/KCl) were intensively studied for the preparation of MTiO 3 perovskites with non-equilibrium, platelet-shape crystallites. 9−12 However, slow ionic diffusion in the solid-state lattice at much lower hydrothermal synthesis temperatures (100−200°C) provides a better insight and understanding of the reactions at the interface compared to that in molten salt TC. 9,13 Hydrothermal TC reactions initiated by the epitaxial growth of a new phase on the precursor (template) particles enable the formation of heterostructures at an intermediate state of the transformation or after complete conversion with the formation of new-phase particles with a preserved morphology and having a crystallographic relationship with the parent phase. 13 −15 In the field of hydrothermal TC reactions, Kalyani et al. performed an indepth study of the epitaxial growth of SrTiO 3 on anatase (TiO 2 ) nanowires and their complete TC to SrTiO 3 mesocrystalline nanowires. 14 Exploring the TC reaction mechanisms for various template precursors will help us to engineer more complex heterostructures in the future.
In this study, we present an example of employing the TC reaction concept in the rational design of new heterostructural and mesocrystalline nanoparticles under hydrothermal conditions. We have studied the hydrothermal epitaxial growth of SrTiO 3 on Bi 4 Ti 3 O 12 template platelets with the intermediate formation of new nanoheterostructural SrTiO 3 /Bi 4 Ti 3 O 12 platelets and after the complete transformation formation of (100)-oriented SrTiO 3 mesocrystalline nanoplatelets.
One of the reasons for the selection of the SrTiO 3 /Bi 4 Ti 3 O 12 heterostructure was the interesting photocatalytic properties of the individual materials. 2,5,7,16 SrTiO 3 meets the thermodynamic criteria for an overall photocatalytic water-splitting reaction in terms of the appropriate band-edge positions and bandgap. 3,7,17 Several efficient H 2 -evolution photocatalysts based on SrTiO 3 were developed using various design strategies, aiming to improve the light-harvesting capabilities and the charge-carrier separation. 2,7,18−24 For example, Zhang et al. prepared (100)-oriented SrTiO 3 mesocrystalline superstructural platelets by hydrothermal topotactic epitaxy from a TiO 2 mesocrystalline precursor template that consisted of assembled anatase nanocrystals with a dominant exposure of [001] facets. 7 The authors proved that these (100)-oriented SrTiO 3 platelets exhibited 3 times higher photocatalytic efficiency for H 2 evolution compared to conventional disordered SrTiO 3 systems. The abovementioned study revives the interest in further photocatalytic investigations of SrTiO 3 nanostructures with controlled morphologies and orientations. Similar to SrTiO 3 , Bi 4 Ti 3 O 12 was also explored as a photocatalyst for H 2 generation. However, according to several reports, pure Bi 4 Ti 3 O 12 does not exhibit an outstanding H 2 evolution activity, 16,25 although modifications such as reduction (Bi 4 Ti 3 O 1 2 − x ) 1 29 who also used a combination of molten-saltsynthesized Bi 4 Ti 3 O 12 platelets and alkaline hydrothermal conditions for the growth of SrTiO 3 . However, their Bi 4 Ti 3 O 12 platelets were larger (side length: 10−15 μm, 1−2 μm in this study). The major difference was in the hydrothermal step where the titanium precursor (tetrabutyl titanate) was added for the formation of SrTiO 3 where the titanium was not proposed to originate from the dissolution of Bi 4 Ti 3 O 12 , as in our study. Accordingly, the reported morphology of the 10−15 μm Bi 4 Ti 3 O 12 platelets with submicrometer attachments 29 was completely different from the SrTiO 3 /Bi 4 Ti 3 O 12 heterostructural platelets described in this study. Therefore, we believe that the presented SrTiO 3 / Bi 4 Ti 3 O 12 composite nanostructures are unique and their functional properties are worth investigating. Our study is focused on a detailed microstructural examination of the platelets at different stages of TC in order to understand the SrTiO 3 /Bi 4 Ti 3 O 12 interface and gain a detailed insight into the mechanism of hydrothermal epitaxial growth and the TC reaction. The epitaxial growth of SrTiO 3 on a layered structure of Bi 4 Ti 3 O 12 is expected to be more complex and illustrative than the epitaxial growth on mono metal oxides. Moreover, the anisotropic shape of the primary Bi 4 Ti 3 O 12 platelets with different dissolution rates of the basal and lateral surfaces is an interesting characteristic of this system that also influences the morphological evolution. The described TC mechanism provides general guidelines for the morphological engineering of nanoheterostructures through hydrothermal epitaxial growth. The study emphasizes the key parameters that must be considered for the selection of a heterostructural system, which include the structural matching at the interface, the thermodynamic stability and solubility of the involved materials, and the supersaturation. A light-induced, good, and stable H 2 production rate from a pH-neutral solution established these novel, noble-metal-free SrTiO 3 /Bi 4 Ti 3 O 12 nanoheterostructural platelets as promising candidates in the field of photocatalytic H 2 evolution.
2.1.2. Synthesis of Bi 4 Ti 3 O 12 Template Platelets. Bi 4 Ti 3 O 12 platelets were synthesized in molten KCl/NaCl salt using Bi 2 O 3 (1.9453 g) and TiO 2 (0.5 g) nanopowders as the starting materials. To synthesize the Bi 4 Ti 3 O 12 platelets via the molten-salt route, the molar ratio of NaCl:KCl:Bi 2 O 3 :TiO 2 was optimized as 50:50:2:3. The synthesis was performed at 800°C with a holding time of 2 h with heating and cooling rates of 10°C/min. Details of the procedure and selected parameters are described elsewhere. 30 After the synthesis, the Bi 4 Ti 3 O 12 platelets were separated from the salt by washing with ultrapure water. To ensure the complete removal of any surface contamination, 12 the platelets were soaked in 2-M HNO 3 for a short time (5 min) and washed again with ultrapure water. The product particles were freeze-dried to obtain a powder with well-separated platelets.
2  12 platelets were admixed to the solution in an amount corresponding to the Sr:Ti molar ratio of 12:1. Suspensions were sonicated for 25 min followed by the addition of NaOH solutions. In the precursor suspension before the hydrothermal reaction, the concentrations of the SrCl 2 · 6H 2 O, Bi 4 Ti 3 O 12 , and NaOH platelets were 0.0388, 0.00107, and 6 M, respectively. The hydrothermal syntheses were performed by stirring at 200°C in a Berghof high-pressure reactor using a Teflon (PTFE) insert. The reaction time was varied from 1 to 15 h. After the hydrothermal synthesis, the product particles were separated from the reaction solution by centrifugation and washed several times with ultrapure water. The solid product was soaked in 1 M HNO 3 for 5 min to remove the side products, and afterward, the particles were again repeatedly washed with ultrapure water to completely remove any traces of acid. At the end, the particles were freeze-dried to obtain the final product.
2.2. Characterization of the Samples. X-ray powder diffraction was employed using a Bruker AXS D4 Endeavor with Cu Kα radiation (1.5406 Å) for the powder samples and for the platelets cast on the Si monocrystalline substrate. The weight ratio of SrTiO 3 :Bi 4 Ti 3 O 12 in the heterostructural platelet was estimated from the calibration curve, which was produced from XRD measurements of the mixtures of SrTiO 3 and Bi 4 Ti 3 O 12 platelets in various weight ratios. These XRD measurements were performed for preferentially oriented platelets cast on the Si monocrystalline substrate.
A field-emission scanning electron microscope (FE-SEM, JSM 7600 F, JEOL, Japan) was used to observe the morphology of the particles. A nanoscale analysis of the platelets was performed using a 200 kV scanning transmission electron microscope (STEM, Jeol ARM 200 CF, JEOL, Japan) equipped with an energy-dispersive X-ray spectrometer (EDXS, Jeol Centurio 100). Samples of platelet-like particles for the STEM analyses were prepared using two approaches. For observations along the shorter zone axis of the platelets, the powdered sample was sonicated in absolute ethanol and a droplet of the suspension was applied to the lacey, carbon-coated copper grid. This resulted in a spontaneous alignment along the preferential orientation with the largest surface parallel to the carbon film substrate. The thickness of the platelets of up to 100 nm allowed STEM analyses without any further thinning. For edge-on observations of the platelet-like particles with a side length between 1 and 2 microns, the particles had to be thinned to electron transparency. This was accomplished by embedding the powders in epoxy resin and further mechanical and ion milling (Gatan PIPS Model 691, USA).
The Brunauer−Emmett−Teller (BET) surface areas of the powders were measured by nitrogen adsorption with a Micromeritics Gemini II 2370 nitrogen-adsorption apparatus (Norcross, GA).
Band-gap energies of the synthesized platelets were determined from their diffuse reflection spectra with BaSO 4 as a reference. The measurements in the ultraviolet and visible (UV−vis) spectral ranges were performed with an integrating sphere and a UV−vis spectrophotometer (Shimadzu UV-3600, Tokyo, Japan). Photoluminescence (PL) spectra of the samples were recorded using a Synergy H1 microplate reader with monochromator optics (Bio-Tek, U.S.A.) at an excitation wavelength of 320 nm.
2.2.1. Photocatalytic H 2 Evolution. The photocatalytic H 2 evolution measurements were carried out in a 50 mL quartz roundbottom flask at ambient temperature and atmospheric pressure using mixing to achieve the particle suspension. A commercial solar simulator equipped with a Xenon arc lamp (300 W, Newport) and an AM 1.5G filter was used as the light source. In a typical photocatalytic measurement, 20 mg of photocatalyst was suspended in 40 mL of aqueous solution containing 25 vol % methanol and the suspension was sonicated for 30 min to obtain a well-dispersed particle suspension. Before light irradiation, the quartz flask was sealed with a rubber septum and purged with a nitrogen flow for 40 min to remove the excess oxygen in the reaction mixture. Finally, the sealed quartz flask was placed under light irradiation. All the photocatalysts were subjected to 4 h of light irradiation, and the H 2 evolution was measured periodically every hour. The generated gas composition (1 mL) was analyzed with a gas chromatograph (GC, SRI-8610C) equipped with a thermal conductivity detector (TCD), and highpurity nitrogen was used as the carrier gas.

RESULTS AND DISCUSSION
The transformation of Bi 4 Ti 3 O 12 into SrTiO 3 under hydrothermal conditions is governed by the chemistry at the interface and the concentrations of the dissolved titanium and strontium species (supersaturation). In this particular TC reaction, the Bi 4 Ti 3 O 12 platelets act as a source of titanium and as the substrate for the epitaxial growth of SrTiO 3 . To control and direct the growth of SrTiO 3 on the surface of the Bi 4 Ti 3 O 12 platelets, the characteristics of the latter must be studied first.

Structural Studies of Bi 4 Ti 3 O 12 Template Platelets.
In the first step of our investigations, we characterized the Bi 4 Ti 3 O 12 platelets down to the atomic scale. Knowledge about the morphology, the termination of the Bi 4 Ti 3 O 12 crystallites, and the nature of the surface after applying different treatment procedures following the synthesis in molten salt (and after washing with water or acid) is important for selecting the optimum strategy for the treatment of the synthesized Bi 4 Ti 3 O 12 powders and gives fundamental knowledge for steering and understanding the heterogeneous nucleation of SrTiO 3 on Bi 4 Ti 3 O 12 templates. Orthorhombic Bi 4 Ti 3 O 12 platelets grown in NaCl/KCl molten salt at 800°C for 2 h are shown in Figure 1a. The crystallites have a typical tabular or platelet-like morphology with a side length of up to a few microns and a thickness of well below a micron. 30 The plateletlike morphology represents the equilibrium shape of Bi 4 Ti 3 O 12 reflecting its layered crystal structure. High-angle annular dark  Figure  1d), perhaps even across the whole crystallite since the presence of steps and terraces on the basal surface were never observed in the TEM. In contrast to the basal-plane surfaces, the lateral surfaces of the crystallites contain growth steps where the TiO 6 octahedra are exposed ( Figure 1e). HAADF-STEM images recorded at the edge of the crystallite also imply weaker bonding of the adatoms on the exposed surface of the lateral side of the platelet (see the inset in Figure 1e). These basic differences between the basal and lateral surfaces reflect the layer-by-layer growth mode and probably influence the Bi 4 Ti 3 O 12 dissolution rates in different crystallographic orientations. It is expected that the dissolution of the Bi 4 Ti 3 O 12 platelets will proceed faster from the lateral stepped surface. 31 6 2− ions are the predominant aqueous titanium species. [13][14][15]32,33 Considering that Sr(OH) 2 exhibits a high solubility in aqueous media at higher temperatures of 100°C ≤ T ≤ 300°C, the formation of SrTiO 3 under hydrothermal conditions is presented using the following equation: 13,15 (1) In our system, under alkaline conditions, the Sr 2+ ions from the dissolved SrCl 2 precipitate first as Sr(OH) 2 , which then dissolves at higher temperatures (100°C ≤ T ≤ 200°C). The Ti(OH) 6 2− species form presumably by the dissolution of Bi 4 Ti 3 O 12 in alkaline media according to eq 2.
It is expected that the precipitation of the SrTiO 3 on Bi 4 Ti 3 O 12 platelets (heterogeneous nucleation) also proceeds following eq 1. According to the theory of heterogeneous nucleation, the energy for the formation of a critical nucleus is proportional to the third power of the interfacial free energy and inversely proportional to the square of the supersaturation. 34 In other words, the energy barrier for the nucleation of SrTiO 3 on Bi 4 Ti 3 O 12 is lowered by the close structural match between the Bi 4 Ti 3 O 12 substrate and the precipitating SrTiO 3 phase and by the higher concentrations of Ti(OH) 6 2− and Sr 2+ ions (supersaturation  12 and SrTiO 3 is good. The supersaturation (eq 3) in our system is defined as the ratio between the product of the activities of aqueous species immediately before the SrTiO 3 formation and the solubility product K s , which is the reciprocal of the equilibrium constant of eq 1: 13 (Ti(OH) ) (Sr ) In our reaction system, the concentration of Ti(OH) 6 2− is a complex function of Bi 4 Ti 3 O 12 dissolution and SrTiO 3 precipitation. Therefore, possibilities for the direct control of the supersaturation in terms of Ti(OH) 6 2 are limited. In contrast, tailoring of the supersaturation with respect to the Sr 2+ ions is easily feasible with the initial amount of strontium salt. To ensure the supersaturation conditions for SrTiO 3 formation, the selected strontium concentration with respect to the whole titanium content was higher than that required by the SrTiO 3 stoichiometry. The optimal concentration of Bi 4 Ti 3 O 12 was determined during our preliminary experiments to be 0.00107 M. 30 This relatively low concentration was also selected to avoid the eventual precipitation of bismuth titanium compounds (e.g., Bi 12 TiO 20 ) that would compete with SrTiO 3 for the Ti(OH) 6 2− species. Before studying the Bi 4 Ti 3 O 12 -to-SrTiO 3 transformation, the stability of the initial Bi 4 Ti 3 O 12 template platelets at 200°C and in highly alkaline conditions (6 M NaOH) without the presence of Sr 2+ ions was verified. The solubility and dissolution rates of the Bi 4 Ti 3 O 12 platelets in the alkaline media should be moderate to prevent their complete dissolution and the disintegration of the substrate for epitaxial growth, as was observed in some other systems. 35 Figure S1, Supporting Information). Considering the low concentration of Bi 4 Ti 3 O 12 platelets (0.00107 M), their solubility in 6 M NaOH is relatively low. Nevertheless, under similar hydrothermal conditions in the presence of dissolved Sr 2+ ions, it is assumed that SrTiO 3 formation according to eq 1 is the driving force for Bi 4 Ti 3 O 12 dissolution. The transformation of the initial Bi 4 Ti 3 O 12 template particles to the SrTiO 3 platelets was studied for a system with a strontium content that is 12 times higher than required by the SrTiO 3 stoichiometry. The strontium concentration (0.0388 M) was 4 times larger than in our previous study. 30 With a higher supersaturation, we aim to decrease the energy barrier for the nucleation of SrTiO 3 and promote its growth over the whole basal-plane surfaces of Bi 4 Ti 3 O 12 platelets and consequently ensure that the SrTiO 3 / Bi 4 Ti 3 O 12 heterostructural and final SrTiO 3 particles maintain the platelet-like shape of the initial template. 13,34 3.3. Mechanistic Interpretation of the Bi 4 Ti 3 O 12 -to-SrTiO 3 TC Process. The progress of the hydrothermal TC reaction was first inspected by XRD. Figure 2 and Figure S2 (Supporting Information) show the XRD patterns of the acidwashed platelets (free of side products) after different reaction times. The XRD patterns of the initial Bi 4 Ti 3 O 12 platelets with a high (001) preferential orientation are also shown in Figure 2 and Figure S2 for comparison. The formation of the SrTiO 3 was already observed after 1 h of the hydrothermal reaction (6 M NaOH, Sr/Ti = 12). The amount of SrTiO 3 compared to Bi 4 Ti 3 O 12 increased with a prolongation of the reaction time. Only SrTiO 3 with a (100) preferential orientation and no Bi 4 Ti 3 O 12 were detected after 15 h (Figure 2 and Figure S2).
In the figures, SrTiO 3 and Bi 4 Ti 3 O 12 are labeled as STO and BIT, respectively.
The side-products can carry valuable information about the TC mechanism. An insight into all the reactions accompanying the TC of Bi 4 Ti 3 O 12 to SrTiO 3 was obtained with an XRD analysis of the whole reaction product ( Figure S3, Supporting Information). The results revealed the formation of SrTiO 3 , SrCO 3 , and Bi 2 O 3 . SrCO 3 formed through a reaction of Sr(OH) 2 with carbonate impurities in the NaOH chemical and with atmospheric CO 2 . The formation of SrCO 3 also continued after the completed reaction and the opening of the autoclave when the alkaline suspension with excessive and unreacted Sr(OH) 2 is exposed to the atmosphere for a longer time. The formation of bismuth oxide, on the other hand, is a result of condensation of bismuth hydroxide Bi(OH) 3 , 37 which forms during the Bi 4 Ti 3 O 12 dissolution. No bismuth titanium compounds (e.g., Bi 12 TiO 20 ) were detected. This proves that the dissolved titanium is consumed for the crystallization of SrTiO 3 and not for the formation of bismuth titanium compounds (e.g., Bi 12 TiO 20 ). Single-phase SrTiO 3 was obtained after the dissolution of the side-products in 1 M HNO 3 (Figure 2 and Figure S2, Supporting information). A deeper insight into the process of the transformation from the initial Bi 4 Ti 3 O 12 platelets to SrTiO 3 was obtained by a microstructural investigation of the samples after the different times for the TC reaction. The partially and fully transformed platelets were examined by SEM and STEM from top and cross-sectional views. An SEM image of the powdered sample after 1 h of reaction is shown in Figure 3a. The initial morphology of the Bi 4 Ti 3 O 12 platelets is clearly preserved; however, the particles appear to have a core-rim structure. The XRD pattern of the platelets (cast from the isopropanol suspension of the platelets on the Si monocrystalline substrate) revealed the presence of Bi 4 Ti 3 O 12 and SrTiO 3 phases with preferential (001) and (100) orientations, respectively ( Figure  2).
The sample after 1 h of transformation was investigated in more detail using the HAADF-STEM. A typical particle is shown in Figure 3b, and here, the core-rim structure is even more evident. In the dark-field (DF) image, the core of the particles is much brighter, indicating a higher atomic density in the core region, whereas the rim is more electron-transparent   and Bi(OH) 3 (eq 2). The Ti(OH) 6 2− ions, exsoluted from Bi 4 Ti 3 O 12 , are consumed for the formation of SrTiO 3 according to eq 1, while Bi(OH) 3 , through the condensation reactions, results in Bi 2 O 3 , as already confirmed by the XRD (Figure S3, Supporting information) and visible in the STEM as dots with bright contrast (Figure 3e). During the epitaxial crystallization of the SrTiO 3 nanodomains, some amorphous bismuth oxide-rich inclusions remain captured between the SrTiO 3 nanocrystallites.
An additional insight into the transformation process is obtained from the analysis of partially transformed heterostructure platelets in the edge-on orientation (Figure 4). An SEM image of the platelets in this orientation after 1 h of the TC reaction (200°C, Sr/Ti = 12, 6 M NaOH) shows that a typical platelet contains a groove running along its edges, apparently splitting the platelet into two thinner parallel platelets (Figure 4a). A HAADF-STEM examination of the partially transformed platelets in an edge-on orientation reveals that the initial Bi 4 Ti 3 O 12 platelets actually start to separate into two parallel platelets aligned with the upper and lower basalplane surfaces of the Bi 4 Ti 3 O 12 platelet. The process starts at the edges and proceeds toward the interior of the platelet (Figure 4b,c: DF−BF pair of STEM figures). One of the particles thinned to electron transparency almost along the whole area (cross-section) was investigated in more detail in three different regionsin the central part and toward the edge of the partially transformed platelet (Figure 4d−h).  13 In this part of the crystal, the SrTiO 3 layer was thinned to electron transparency (the Bi 4 Ti 3 O 12 part was completely etched away in some areas) and one of the most interesting features of the SrTiO 3 platelets that form during the TC transformation from Bi 4 Ti 3 O 12 under hydrothermal conditions is revealed, i.e., the presence of an atomic bismuth-rich layer (Bi-rich layer), inside the SrTiO 3 platelet (Figure 4g). Similarly, the STEM image of the platelet close to the edge (Figure 4h) in the section of complete transformation to SrTiO 3 (rim region in Figure 3) revealed the formation of two parallel SrTiO 3 platelets that both contain an atomic Birich layer running along the middle part of both platelets. We believe that these Bi-rich layers correspond to the [Bi 2 O 2 ] 2+terminated top layers of the initial Bi 4 Ti 3 O 12 platelet.
The incorporation of the Bi-rich layer is also a consequence of the strong bonding between the termination layer of Bi 4 Ti 3 O 12 and growing SrTiO 3 . Layer-by-layer growth (Frank− van der Merwe mechanism 38 ), evident from our observations (Figure 3d), is also the result of strong bonding at the interface. The Bi-rich layer remains bonded to SrTiO 3 even after progressive dissolution of the remaining Bi 4 Ti 3 O 12 template. When the dissolution front of Bi 4 Ti 3 O 12 (inside the groove) reaches the Bi-rich layer, it remains attached to the epitaxial SrTiO 3 layer and the growth of the SrTiO 3 also proceeds from the inner side, and the Bi-rich layer becomes a coherent part of the newly formed SrTiO 3 , where it is usually observed to be approximately in the middle of each SrTiO 3 platelet (Figure 4h). It is obvious that the formation of two  (Figure 1e). When the solution becomes locally saturated with Sr 2+ and Ti(OH) 6 2− , nucleation of SrTiO 3 occurs in the areas with the lowest energy barrier. As noted earlier, the interfacial free energy for SrTiO 3 nucleation on the basal-plane surfaces of Bi 4 Ti 3 O 12 is low due to the close structural match at the interface and therefore, SrTiO 3 nucleation can immediately occur when saturation conditions are achieved. The areas close to the edges are subjected to higher concentrations of Ti(OH) 6 2− from the beginning of the reaction, and therefore, SrTiO 3 nucleation starts there (Figure 4i, step 2). Then, with the progressive dissolution of the Bi 4 Ti 3 O 12 , the growth of SrTiO 3 continues on both basal surfaces of the Bi 4 Ti 3 O 12 platelet. However, when the Bi 4 Ti 3 O 12 inside the groove completely dissolves to both the initially terminating Bi-rich layers, attached to the newly formed SrTiO 3 , the epitaxial growth of SrTiO 3 also proceeds on the inner side of these Bi-rich layers and they become coherently integrated into the SrTiO 3 platelets on both sides. In the end, the Bi-rich layers lie approximately in the middle of each SrTiO 3 platelet half (Figure 4i Figure 5. The general plate-like shape of the initial Bi 4 Ti 3 O 12 template particles is well preserved (Figure 5a); however, the integrity/ crystallinity of the SrTiO 3 platelets reflects the specifics of the recrystallization mechanism. The final platelets usually consist of two intergrown SrTiO 3 platelets, as shown by the STEM analysis of the sample after 15 h of hydrothermal treatment (Figure 5b). The presence of Bi 4 Ti 3 O 12 between the SrTiO 3 platelets is not observed, indicating that all the Bi 4 Ti 3 O 12 molecules dissolved and Ti(OH) 6 2− was used for the formation of SrTiO 3 . In the edge-on-oriented platelets, the two parallel Bi-rich atomic layers, which are a peculiarity of the studied hydrothermal TC reaction, were observed along the whole length of both SrTiO 3 platelet halves (Figure 5c).
From our calculation, a 60 nm-thin Bi 4 Ti 3 O 12 platelet would result in the formation of an approximately 42 nm-thin SrTiO 3 platelet ( Figure S5, Supporting Information) or two parallel 21 nm-thin platelets; however, in the process of Bi 4 Ti 3 O 12 dissolution, the smallest Bi 4 Ti 3 O 12 crystallites most probably dissolve and then these Ti(OH) 6 2− species are consumed for SrTiO 3 growth on the larger Bi 4 Ti 3 O 12 platelets. Therefore, the typical thickness of the final SrTiO 3 platelets is slightly larger and comparable to that of the initial Bi 4 Ti 3 O 12 platelets.
The crystallinity of the fully transformed SrTiO 3 platelets was analyzed in the top view (Figure 5d−f). A lowmagnification STEM image of a thinner SrTiO 3 platelet is shown in Figure 5d. The crystal is relatively dense at the edges where the transformation starts, and the porosity of the platelet increases toward the central region of the platelet. A highermagnification STEM image taken in the central part of the platelet with the FFT calculated from the whole area is shown in Figure 5e. It is clear that the matrix consists of epitaxially oriented nanocrystallites that formed (100)-oriented SrTiO 3 mesocrystalline platelets with some pores and nanosized inclusions with brighter contrast. The analysis showed that these are amorphous Bi-rich inclusions, which were trapped and overgrown by SrTiO 3 during the processes of Bi 4 Ti 3 O 12 dissolution and SrTiO 3 crystallization. The density of the amorphous Bi-rich inclusions appears to be higher in the central part of the SrTiO 3 platelets. The observed morphological development resulted in an interesting variation of the BET specific surface area through the progress of the TC reaction. Actually, the measured BET values of the SrTiO 3 /Bi 4 Ti 3 O 12 heterostructures increased in the first 6 h of the TC reaction, reaching a maximum at ∼20 m 2 ·g −1 , and then the BET values decreased and approached that of SrTiO 3 (∼10 m 2 ·g −1 ), which was still higher than the BET value of the initial Bi 4 Ti 3 O 12 (2−3 m 2 ·g −1 ) (Table S1, Supporting Information) . The high specific surface area of the SrTiO 3 /Bi 4 Ti 3 O 12 heterostructures is most probably related to the emerging groove and the high surface roughness of the growing SrTiO 3 layers. Smoothening of the surface of the SrTiO 3 platelets with the completion of the TC reaction is the reason for the lower specific surface area of the final SrTiO 3 platelets compared to that of the heterostructures.
3.4. Photocatalytic Performance. To demonstrate the potential of the developed Bi 4 Ti 3 O 12, SrTiO 3 /Bi 4 Ti 3 O 12 and mesocrystalline SrTiO 3 platelets, the as-prepared materials were tested and assessed in terms of the photocatalytic activity for H 2 evolution in pH-neutral aqueous media (H 2 O/CH 3 OH = 75/25). The results were compared to those involving commercial SrTiO 3 nanopowders that were evaluated under the same conditions (Figure 6a). The Bi 4 Ti 3 O 12 platelets with the smallest specific surface area, approximately 2−3 m 2 ·g −1 (Supporting Information, Table S1), were found to exhibit the lowest H 2 evolution rate among the studied materials, only 7.5 μmol·g −1 ·h −1 . Mesocrystalline (100)-oriented SrTiO 3 platelets (65 μmol·g −1 ·h −1 ) and commercial nanocrystalline SrTiO 3 powders (81 μmol·g −1 ·h −1 ) show comparable photocatalytic activities, although the specific surface area of the platelets (10 m 2 ·g −1 ) was lower than that of the commercial nanopowder (24 m 2 ·g −1 ). An extraordinarily higher H 2 evolution rate (1265 μmol·g −1 ·h −1 ) was observed for the heterostructural SrTiO 3 / Bi 4 Ti 3 O 12 platelets (Figure 6a and Table S1, Supporting Information). In this study, the enhanced photocatalytic performance for H 2 evolution is only presented for the heterostructural SrTiO 3 /Bi 4 Ti 3 O 12 platelets with a SrTiO 3 / Bi 4 Ti 3 O 12 weight ratio of 60/40 and BET = 20 m 2 ·g −1 ( Figure  S6, Supporting information). We confirmed several times that the heterostructural platelets exhibiting a BET surface area of >15 m 2 ·g −1 typically show a considerably higher H 2 evolution rate than the pure SrTiO 3 platelets and the commercial SrTiO 3 nanopowders. The results support the important role of the heterojunction for an improvement of the photocatalytic efficiency. A systematic study of the mutual effect of the SrTiO 3 /Bi 4 Ti 3 O 12 ratio and the specific surface area on the photocatalytic H 2 evolution is beyond the scope of the present article, and the results of this research will be published in a forthcoming study. It is noteworthy that the relatively high H 2 evolution rate was determined for bare nanoheterostructural SrTiO 3 /Bi 4 Ti 3 O 12 platelets without any noble-metal doping or cocatalyst support. In terms of the H 2 production rate, our heterostructures demonstrate better performance than several other noble-metal-loaded photocatalysts (Table S2, Supporting Information). 19−22,39−41 Cycled measurements of the H 2 evolution revealed good repeatability and reusability of the nanoheterostructural SrTiO 3 /Bi 4 Ti 3 O 12 platelets ( Figure 6 and Figure S7). The stability of the H 2 evolution over the tested 24 h reaction time is similar to that reported for other SrTiO 3based photocatalysts ( Figure S7). 23,24 A band-structure analysis is an essential approach to provide a deep insight into the possible photocatalytic mechanism. The band-gap energy (E g ) of the constituents was calculated using the well-known Tauc method from the UV−vis diffuse reflectance spectra and Kubelka−Munk function (see calculation in the Supporting Information and Figure S8). 42 The obtained band gaps for SrTiO 3 and Bi 4 Ti 3 O 12 were 3.23 and 3.16 eV, respectively. The conduction band (E CB ) and valence band (E VB ) energies, another two important factors, are calculated using the empirical formulas (see calculation in the Supporting Information). 43 46,47 Namely, it is known that the experimental determination of the fundamental characteristics (e.g., E CB ) of nanostructural materials is associated with a high degree of uncertainty. 48 This is also true for the determination of E CB from the Mott−Schottky relationship, which is based on several assumptions and ideal conditions, which are not entirely fulfilled by nanostructures. 48,49 Finally, another important parameter, the Fermi energy (E F ), is determined according to the estimation that it lies at 0.3−0.1 eV below E CB for n-type semiconductors. These values are −0.51 and −0.11 eV for SrTiO 3 and Bi 4 Ti 3 O 12 , respectively ( Figure S9, Supporting Information), and also support the previous results. 50,51 The studied heterostructural SrTiO 3 / Bi 4 Ti 3 O 12 platelets can be categorized as staggered type II alignments, and for this current research, we found that the potential of water reduction (at pH 7) lies in between E CB / SrTiO 3 and E CB /Bi 4 Ti 3 O 12 . 5 Figure S8c (Supporting Information) shows the modified band gap of the heterojunction, and this supports the band rearrangements as well. Under simulated-light irradiation, the constituent elements absorb photons, and as a result, electron/hole pairs are generated (Figure 6b). It is already determined that E F /SrTiO 3 lies in a more negative position than E F /Bi 4 Ti 3 O 12 . During Fermi-level rearrangement, due to the higher Fermi energy of SrTiO 3 , the electrons tend to move from SrTiO 3 to Bi 4 Ti 3 O 12 . This phenomenon causes the SrTiO 3 and Bi 4 Ti 3 O 12 sites to be positively and negatively charged, respectively. As a result, a weak internal electric field is generated at the solid−solid interface. Therefore, the photo-generated electrons prefer to migrate from a CB of Bi 4 Ti 3 O 12 to a VB of SrTiO 3 via this lowresistance pathway. This prevents the electron/hole recombination, and this study supports the possible execution of a Zscheme transfer (Figure 6b). 52,53 For this experiment, the coupling of SrTiO 3 and Bi 4 Ti 3 O 12 greatly facilitates the photogenerated carrier transfer and separation of electron/hole pairs under light irradiation and, as a result, the H 2 evolution rate is enhanced significantly. Here, the holes at VB/Bi 4 Ti 3 O 12 were consumed by the hole-scavenger methanol. 54 PL spectroscopy with an excitation wavelength of 320 nm was used to evaluate the separation efficiencies of the photo-excited charge carriers in the studied photocatalyst platelets (Figure 7) In terms of band-gap energy, the SrTiO 3 /Bi 4 Ti 3 O 12 heterostructural platelets are, similar to SrTiO 3 , UV-active photocatalysts. Due to the small portion of UV light in the incident light spectra, the SrTiO 3 -based photocatalysts do not show a high solar-to-hydrogen (STH) efficiency. It has been reported that a modification of the SrTiO 3 by doping and/or cocatalyst deposition led to a variation of the STH from 0.037 to 0.65% (Table S3, Supporting Information). 22,24,56−59 The highest STH efficiency (0.65%) was reported by Domen and co-workers 24 for Al-doped SrTiO 3 loaded with Rh/Cr 2 O 3 and CoOOH cocatalysts. An STH greater than 1% was demonstrated for La-and Rh-codoped SrTiO 3 (H 2 evolution) combined with Mo-doped BiVO 4 (O 2 evolution) and Au in the Z-scheme-based photocatalysts. 56 In the current study, SrTiO 3 /Bi 4 Ti 3 O 12 heterostructural platelets without any noble-metal doping or cocatalyst loading exhibit an STH efficiency of 0.19%, which is moderate but comparable to several other reported STH values for noble-metal decorated SrTiO 3 photocatalysts (Table S3, Supporting Information). Considering that the SrTiO 3 /Bi 4 Ti 3 O 12 heterostructure was evaluated for the first time in terms of photocatalytic H 2 evolution, we believe that there is still room for improvement in its STH efficiency.

CONCLUSIONS
The epitaxial growth of SrTiO 3 on Bi 4 Ti 3 O 12 template platelets was studied under alkaline hydrothermal conditions at 200°C to illustrate the TC reaction for the formation of novel SrTiO 3 /Bi 4 Ti 3 O 12 heterostructural platelets and (100)-oriented SrTiO 3 mesocrystalline platelets. In the presented TC reaction, the Bi 4 Ti 3 O 12 platelets act as a source of dissolved Ti(OH) 6 2− species and also serve as a substrate for epitaxial growth of SrTiO 3 . The heterogeneously layered structure of the Bi 4 Ti 3 O 12 platelets with different dissolution rates of the basal and lateral surfaces results in an interesting morphological development and additionally offers a unique track and insight into the hydrothermal TC mechanism. Dissolution of the initial Bi 4 Ti 3 O 12 platelet from the lateral ends into the interior and the simultaneous epitaxial growth of SrTiO 3 on both bismuth-oxide-terminated basal-surface planes of the template platelet result in the formation of two parallel SrTiO 3 platelets separated by a groove that deepens with the progress of the TC reaction, whereas Bi 4 Ti 3 O 12 constitutes the core of the SrTiO 3 /Bi 4 Ti 3 O 12 heterostructural platelet. When the TC reaction is completed, the newly formed platelet-like particle consists of two parallel SrTiO 3 platelets, both of which have an incorporated monoatomic Bi-rich layer, the remains of the top layers of the parent Bi 4 Ti 3 O 12 platelet.
The intermediate heterostructural SrTiO 3 /Bi 4 Ti 3 O 12 and the final SrTiO 3 platelets develop approximately 5−10 times higher specific surfaces (10−20 m 2 ·g −1 ) than the initial Bi 4 Ti 3 O 12 platelets, mainly due to the newly formed groove and the high surface roughness of the growing SrTiO 3 . The photocatalytic activity for the H 2 evolution and the STH efficiency of the as-prepared SrTiO 3 /Bi 4 Ti 3 O 12 platelets free of noble-metal cocatalysts are reproducible, stable, and 18 times (1265 μmol·g −1 ·h −1 ; STH = 0.19%) higher than that of the (100)-oriented SrTiO 3 mesocrystalline platelets (65 μmol·g −1 · h −1 ; STH = 0.01%) and 15 times more than that of the commercial SrTiO 3 nanopowders (81 μmol·g −1 ·h −1 ; STH = 0.012%). The enhanced photocatalytic activity of the SrTiO 3 / Bi 4 Ti 3 O 12 heterostructural platelets is explained by the efficient transfer of the photogenerated carriers from Bi 4 Ti 3 O 12 to SrTiO 3 and separation of electron/hole pairs at the interface. The reduced recombination of photoinduced charge carriers in the SrTiO 3 /Bi 4 Ti 3 O 12 heterostructural platelets was confirmed by the decreased intensity of the photoluminescence.
The detailed insight into the mechanism of epitaxial growth for SrTiO 3 on Bi 4 Ti 3 O 12 expands the possibilities for using the hydrothermal TC reaction concept in the design of highly preferentially oriented heterostructures or mesocrystallites, involving other template particles and growing phases for the preparation of new efficient photocatalyst systems.