1 Introduction
Along with the high-speed economic development and rapid population growth, energy and environmental issues are becoming the focus of global attention. As a clean, carbon-free fuel, H2 is considered as an ideal energy carrier to replace fossil fuels. Solar-driven water splitting has been proven to be a promising route toward renewable hydrogen production [1–3], which attracts ever-increasing attention for the development of high-performance and cost-effective photoelectrochemical (PEC) devices for hydrogen evolution reaction (HER). In this regard, silicon (Si) is demonstrated as an ideal candidate for PEC HER applications with its narrow bandgap (Eg = 1.1 eV), high earth abundance, low cost, and large-scale production [4,5]. Although significant progress has been made since Si was used as a photovoltaic material, the PEC efficiency and chemical stability are largely limited by its surface corrosion, especially in aqueous electrolytes, as well as extremely sluggish HER kinetics [1,6,7].
An efficient strategy to suppress the corrosion of Si photoelectrode during PEC reaction is to deposit a protective layer on the surface of Si. A variety of materials, such as Al2O3, TiO2, NiOx, and SiOx, have been used [6–10], among which, TiO2 is a common material to protect Si electrodes due to its superior electronic conductivity, high corrosion resistance, and chemical stability in a broad range of pH [11–13].
Sluggish HER kinetics of Si, resulting from its high hydrogen adsorption Gibbs free energy (ΔGH), is another big issue of Si photoelectrodes [14]. Therefore, substantial research endeavors have been dedicated to designing PEC Si photocathodes with enhanced HER activity. It is reported that a passivation layer, e.g., a TiO2 layer, can help fix surface defects of Si-based photocathode to reduce surface carrier recombination and hence improve the HER [11,15]. In addition, decorating the surface of photocathode with a cocatalyst can accelerate the reaction between the photo-generated carriers and the electrolyte [16,17]. Numerous methods of designing Si-based photocatalysts for high-efficiency photocatalytic systems mainly focusing on loading precious metals on p-Si surfaces have been proposed [5,12,18–20]. However, the high cost and scarcity of noble metals pose tremendous obstacles to their further application, though these precious metals show excellent catalytic activity at a low overpotential. As a consequence, extensive efforts have been made to develop earth-abundant non-noble metal HER catalysts [13,21,22].
Graphene-like MoS2, a layered transitional metal sulfide with a direct band gap, has been extensively investigated as a cocatalyst for Si photocathodes owing to its unique optical, electrical, and chemical properties [14,16,23–25]. However, the active sites of 2H-MoS2 are mainly concentrated at its edges, while the basal plane (0001) of MoS2 is inert to HER [25,26]. Therefore, designing the morphology of MoS2 and tuning its size with more exposed active edges are effective strategies to boost its HER performance [25,27–29]. For example, Kibsgaard and coworkers successfully constructed a MoS2 with a 3D network structure and a highly ordered double-gyroid (DG) morphology by using porous silicon as the template. The DG meso-structured MoS2 possesses a fully contiguous large-area thin film with a greater fraction of catalytically active sites that are favorable to charge transport and diffusion [29]. Wang et al. demonstrated that the edge-terminated structure of ultrathin MoS2 nanoflakes exhibited an enhanced H2 production rate, because the ultra-small MoS2 flakes substantially increased the ratio of exposed active edges [28]. To sum up, engineering an active site-rich structure by tuning the molecular size and creating a mesoporous network structure by morphological modulation at nanoscale are of great significance for MoS2 to improve the surface properties and charge carrier transfer. Nevertheless, it is still challenging to prepare ordered mesoporous MoS2 with a maximum of active edge sites.
Furthermore, the method to construct cocatalyst/Si photoelectrode has a significant influence on its PEC performance. The cocatalyst should be bonded onto the surface with enough strength to prevent separation and failure during PEC reaction. Meanwhile, the photogenerated carriers are able to be well transferred from Si to the cocatalyst.
Herein, a commercial planar Si based hybrid photocathode with a TiO2 passivation layer and ordered mesoporous MoS2 cocatalyst is reported. The highly-ordered mesoporous MoS2 was successfully synthesized by using a facile and controllable template method, which had a superior electrocatalytic performance in HER. The TiO2 worked both as an effective protective layer against corrosion of Si electrode, passivation layer and as an efficient carrier transfer path. This meso-MoS2/TiO2/p-Si architecture was constructed using a post-necking strategy to ensure the structural stability of the hybrid photocathode. As a result, an excellent PEC performance was achieved, with an onset potential of+ 0.06 V (versus RHE, VRHE), a current density of − 1.8 mA/cm2 at 0 VRHE, and a Faradaic efficiency close to 100% in acidic media. Very recently, Li et al. [30] have chosen MoS2 as a cocatalyst to construct a Si/TiO2/MoS2 photoelectrode and obtained a photocurrent density of − 0.24 mA/cm2 at 0 VRHE, an onset potential of 0.42 VRHE, and a stability of more than 8 h in 0.5 mol/L H2SO4 solution. Compared with the work mentioned above, such superior HER efficiency and chemical stability suggested that the present paper provides a promising strategy for fabricating Si-based photocathodes by loading mesoporous cocatalysts.
2 Experimental section
2.1 Materials
The thiourea (CH4N2S, ≥ 99.0%), sodium molybdate dihydrate (Na2MoO4·2H2O,≥99.0% purity), citric acid monohydrate (C6H8O7·H2O, 99.5%), potassium hydroxide (KOH, 95%), hydrofluoric acid (HF,≥40.0%), acetone (C3H6O,≥99.5%), sulfuric acid (H2SO4,≥98%), titanium tetrachloride (TiCl4,≥98.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. The ethanol (C2H5OH,≥99.7%) and the hexane (CH3(CH2)4CH3,≥97.0%) were obtained from Shanghai Titan Scientific Co., Ltd. The methanol (CH4O,≥99.9%) and the potassium hexachloroplatinate (K2PtCl6,≥98%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. All reagents used in this study were employed without any further treatment. Ultra-pure Milli-Q water was used in the entire experimental process.
2.2 Fabrication of mesoporous and nanostructured MoS2 powders
The mesoporous MoS2 was synthesized using a nanocasting method with KIT-6 as the template. Typically, 500 mg KIT-6 powder was added into 40 mL CH3(CH2)4CH and stirred at 50°C for 2 h. The cubic Ia3d mesoporous silica, KIT-6, was prepared according to Refs. [13,31]. Then, the precursor solution of MoS2 (0.5 mL) was dispersed into the above solution and stirred for 4 h. The precursor solution of MoS2 was obtained by dissolving CH4N2S (160 mg) and Na2MoO4·2H2O (200 mg) in 1 mL Milli-Q water. After being dried under vacuum at 60°C for 10 h, the mixture was calcined at 600°C at a slow ramping rate of 2°C/min for 5 h under Ar flow. Subsequently, the as-obtained MoS2/KIT-6 was dispersed in 5% HF aqueous solution to remove KIT-6 template. Finally, the mesoporous MoS2 was obtained after washing by using Milli-Q water and ethanol several times, and then dried at 60°C overnight. The synthesis procedures are illustrated in Fig. 1. The meso-Pt was prepared by the same process by using K2PtCl6 as precursor.
For comparison, the nonporous nanostructured MoS2 was fabricated by using a hydrothermal process. The synthesis was conducted as follows: 25 mg of Na2MoO4·2H2O and 35 mg CH4N2S were dispersed in 35 mL Milli-Q water via 10-min ultrasonic treatment before 235 mg C6H8O7·H2O was added. After another 10 min of ultrasonic dispersion, the mixture was then transferred into a 50 mL Teflonlined stainless steel autoclave, sealed, and heated at 200°C for 21 h. After being naturally cooled down to room temperature, the precipitate was washed with Milli-Q water and ethanol thoroughly, then dried at 60°C overnight.
2.3 Fabrication of TiO2/p-Si
Czochralski (CZ) p-type (100) silicon wafers (150 mm× 150 mm× 0.65 mm, ρ = 5–15 Ω·cm) were used for this work. First, the Si chips with a size of 1 cm × 2 cm were ultrasonically cleaned in Milli-Q water, ethanol, and acetone for 30 min, respectively. The surface native oxide layer was then removed with 5% HF aqueous solution for 10 min, followed by atomic layer deposition process (ALD, Ultratech, Savannah 100, USA). TiO2 passivation layer with a thickness of 5 nm was deposited onto p-Si substrates at 150°C and a stream of N2 (20 sccm) using tetrakis-dimethylamino titanium (TDMAT, 0.1 s pulse) and H2O (0.015 s pulse) as the Ti and O precursors, respectively.
2.4 Fabrication of photoelectrodes
The meso-MoS2/TiO2/p-Si was obtained by using the spin-coating method. After the surface of TiO2/p-Si was cleaned by a drop of methanol solution, three drops of meso-MoS2 diluted solution (2 mg meso-MoS2 powder being added in 100 μL C2H5OH and 100 μL CH3OH) were evenly spin-coated onto the surface of TiO2/p-Si at 3000 r/min for 60 s, followed by a drop of 0.01 mol/L TiCl4 methanol solution. After that, the meso-MoS2/TiO2/p-Si was dried at 60°C for 10 min, and then sintered at 350°C at a heating rate of 20°C/min for 10 min under Ar flow. An Ohmic contact was established by coating an InGa alloy in the backside of p-Si and then adhering a Cu wire with Ag paste. Afterwards, the entire backside and the edges of p-Si were sealed by epoxy, while an area of 1 cm2 on the front of p-Si was exposed for PEC tests. The nano-MoS2/TiO2/p-Si and meso-Pt/TiO2/p-Si were prepared by the same process.
2.5 Characterization
The morphologies and composition of prepared samples were characterized by field-emission scanning electron microscopy (FE-SEM, Tescan MIRA3), transmission electron microscopy (TEM, FEI Tecnai G2) and High-resolution TEM (HRTEM, TALOS F200X). The energy dispersive X-ray spectroscopy (EDS) mapping profiles were also conducted in the FE-SEM (Tescan MIRA3). The structural properties were detected using X-ray diffraction diffractometer (XRD, Rigaku D/max 2550) with Cu Kα radiation (λ = 1.5418 Å) operated at an acceleration voltage of 40 kV.
2.6 Electrochemical and photoelectrochemical measurements
For electrochemical measurements, a glassy carbon electrode (0.07 cm2) covered with catalyst was used as the working electrode. Typically, 2 mg of resultant product was dispersed in the mixture of 8 μL Nafion solution (5% (mass percentage), Dupont Corporation) and 200 μL water-ethanol solution (volume ration:1:1) via 60-min ultrasonic oscillation to acquire a homogeneous ink. Then, 6 μL of the ink was pipetted on a GC electrode and dried naturally overnight.
A three-electrode mode served as electrochemical and PEC measurements, with a multifunctional electrochemical analysis instrument (Bio-logic VMP3), using 0.5 mol/L H2SO4 and 1 mol/L KOH aqueous solution as the electrolyte. In the acid solution, a Pt wire and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. A graphite rod (d = 5 mm) and a Hg/HgO electrode served as the counter and reference electrodes in the alkaline electrolyte, respectively. Photoelectrochemical measurements were evaluated under a 300 W Xe lamp with an optical filter (λ>400 nm) as the visible light source (AM 1.5 G one sun illumination) at room temperature. The electrochemical impedance spectroscopy (EIS) was performed from 200 kHz to 0.1 Hz at a bias of –0.1 VRHE and a sinusoidal voltage of 5 mV.
3 Results and discussion
The morphologies of the meso-MoS2/TiO2/p-Si and pristine nano-MoS2 samples were characterized by FE-SEM, TEM and HRTEM. As demonstrated in Figs. 2(a), S1(a), and S1(d), the mesoporous MoS2 particles, MoS2 nanoparticles, and mesoporous Pt particles were uniformly distributed on the entire surface of the p-Si substrate. Moreover, obvious MoS2 nanoflowers can be observed, but they all aggregated together and formed MoS2 clusters (Fig. S1(c)). The low magnification TEM image (Fig. 2(b)) exhibits an ordered mesoporous structure composed of ultra-thin and ultra-small MoS2 sheets, which provides a large surface area and more active sites for HER. As displayed in Fig. 2(d) and Fig. S1(b), the cross-sectional high-resolution TEM (HRTEM) images clearly reveals the tandem structure of the hybrid photoelectrodes, which contains the MoS2 particles (meso-MoS2 or nano-MoS2 particles) anchored on an amorphous-structured TiO2 thin film and the TiO2 thin layer coated on the surface of the p-Si. The well-resolved lattice fringes with interplanar spaces of 0.194 nm and 0.63 nm can be indexed to the (220) plane of Si and the (002) plane of MoS2, respectively (Figs. 2(e) and 2(f)). The EDS analysis of the synthesized meso-MoS2/TiO2/p-Si (Figs. 2(g)–2(l)) confirms the homogeneous dispersion of Mo and S atoms, as well as Ti and O atoms, demonstrating the formation of meso-MoS2/TiO2/p-Si. The XRD patterns of meso-MoS2 and nano-MoS2 are depicted in Fig. 2(c). The peak positions at 2θ = 14.3°, 32.6°, 39.5°, 49.7°, 58.3°, and 60.1° can be indexed to (002), (100), (103), (105), (110), and (008) planes of the hexagonal MoS2 phase (JCPDS 37-1492), further demonstrating the successful preparation of MoS2. For meso-MoS2, the diffraction peak of (002) shifted slightly to the left, indicating an expanded interlayer, which is beneficial for enhancing the charge transfer rate. In addition, theoretical studies have indicated that increasing the interlayer spacing of MoS2 can reduce the Gibbs free energy of hydrogen adsorption (DGH) compared with the non-expanded pristine MoS2, which benefits the desorption of hydrogen from the surfaces of cocatalysts [32,33]. As for nano-MoS2, the diffraction peaks were relatively weaker than that of meso-MoS2 and the peaks at 2θ = 58.3°and 60.1° merged into one broad peak, revealing a low crystalline degree.
To provide more information about the enlarged surface area of meso-MoS2, the specific surface area and porous structure were characterized by nitrogen adsorption-desorption isotherms (Fig. 3). The isotherm curve belongs to type IV with a hysteresis loop and sharp changed adsorption/desorption lines, indicative of large cylindrical-like mesopores in a narrow range of size. The BET surface area of meso-MoS2 is 587.613 m2/g, which is much higher than that of nano-MoS2 (10.530 m2 /g). Moreover, the pore size distribution (PSD, inset of Fig. S2(a)) of meso-MoS2 measured by the Barrett-Joiner-Halenda method is in a relatively narrow distribution, which is mainly centered at 3.82 nm. Comparatively, the PSD of nano-MoS2 was dispersed and has an average pore size of 2.46 nm, less than that of meso-MoS2, as exemplified in Fig. S2(b) and the inset. The significant increase in the specific surface area and the formation of mesoporous structure verify the expected design again, which might be conducive to charge transfer and hydrogen diffusion, as well as increasing the exposure of active edges.
The PEC performance of the prepared photocathodes was first evaluated in 0.5 mol/L H2SO4 electrolyte under simulated AM 1.5 G one sun illumination (λ>400 nm), as manifested in Fig. 3(a). Compared with all the samples, meso-MoS2/TiO2/p-Si yields the highest photocurrent density of –1.8 mA/cm2 at 0 VRHE except meso-Pt/TiO2/p-Si (–2.1 mA/cm2) and the lowest onset potential of 0.06 VRHE (j = –1 mA/cm2). Besides, the saturation current density (34.4 mA/cm2 at 1.4 VRHE) of meso-MoS2/TiO2/p-Si even surpasses the value of meso-Pt/TiO2/p-Si at the same potential. These results suggest that the introduction of meso-MoS2 promotes the charge migration process, endowing the photoelectrode with an improved photocatalytic activity [34]. Note that the HER performance of nano-MoS2/TiO2/p-Si is worse than that of TiO2/p-Si, presumably because the nano-MoS2 dispersed on the surface of p-Si affects the light absorption and hinders the hydrogen evolution process of the composite photoelectrode. After loading the cocatalyst, no matter what kind of composite photoelectrode it is, there is a light blocking effect, including meso-Pt/TiO2/p-Si and meso-MoS2/TiO2/p-Si. The HER photoelectrocatalytic performance of the photoelectrodes are not only influenced by the light absorption of the silicon substrate, but also by the properties and structure of the cocatalyst themselves. However, meso-Pt/TiO2/p-Si and meso-MoS2/TiO2/p-Si still show excellent PEC performance under the effect of light blocking, indicating the advantages of mesoporous structure.
The HER photoelectrocatalytic activity was also investigated in the presence of a 1.0 mol/L KOH aqueous solution, as displayed in Fig. 3(b). Similar to the acidic solution, nano-MoS2/TiO2/p-Si shows a poor HER activity in alkaline media with an onset overpotential at 0.29 VRHE. In contrast, meso-MoS2/TiO2/p-Si requires only 0.14 VRHE, which is much smaller than that of TiO2/p-Si and nano-MoS2/TiO2/p-Si. Furthermore, the meso-MoS2/TiO2/p-Si composite photocathode shows a current density of –3.3 mA/cm2 at 0 VRHE, which is even higher than that of meso-Pt/TiO2/p-Si (–2.5 mA/cm2 at 0 VRHE).
Moreover, the H2 production from the meso-MoS2/TiO2/p-Si photocathode was collected to evaluate the Faradaic efficiencies at a bias of 0.1 VRHE. As displayed in Fig. 3(c), the H2 generation rate was 181.5 μmol/h in acidic electrolyte and 180 μmol/h in alkaline media, respectively; the corresponding Faradaic efficiencies were both close to 100% (97.4% in 0.5 mol/L H2SO4, 96.5% in 1.0 mol/L KOH), revealing that the generated photocurrent is indeed derived from HER. Compared with meso-MoS2/TiO2/p-Si, nano-MoS2/TiO2/p-Si photoelectrodes exhibit lower H2 production rates both in acidic (120 μmol/h) and alkaline solution (162 μmol/h) at an applied potential of –0.5 VRHE and the corresponding Faradaic efficiencies were 64.3% and 86.8%, respectively. Figure 3(f) shows the changes of IPCE values in different electrolytes from 350 nm to 975 nm. In acidic solution, the IPCE values of meso-MoS2/TiO2/p-Si photoelectrodes mostly surpass 57% in the range of 350–875 nm, and peak at 75% or so at 775 nm. In the alkaline solution, the majority of the IPCE value is above 60%, and reaches the maximum value of 73.5% at 825 nm.
The faster reaction kinetics is manifested by EIS under simulated AM 1.5 G one sun illumination (Figs. 3(d) and 3(e)). The Nyquist plots are fitted with the equivalent model, as shown in the insets of Figs. 3(d) and 3(e). Therein, Rs represents a series resistance of the whole synthetic circuit. Rct1-CPE1 simulates the resistances of the charge transfer process at the photoelectrode/electrolyte interface and the double layer of the nonideal capacitance. Rct2-CPE2 is closely related to the charge transfer resistances at the TiO2/Si and cocatalyst/TiO2 interfaces. Commonly, a smaller arc radius indicates a lower interface resistance. It should be noted that the arc radii of meso-MoS2/TiO2/p-Si photoelectrodes at low-frequency ranges are significantly smaller than those of nano-MoS2/TiO2/p-Si photocathodes in both electrolytes, suggesting that this unique mesoporous architecture reduces diffusion limitations of reactants and products (such as H+ and H2) and also effectively enhances the separation and transfer efficiency of photoelectrons during HER.
The PEC stability test of meso-MoS2/TiO2/p-Si was performed by a chronoamperometry test at a fixed applied potential of 0.1 VRHE in 1 mol/L KOH and –0.15 VRHE in –0.5 mol/L H2SO4, respectively. As illustrated in Fig. 4(a), the current density was maintained at (–6±0.2) mA/cm2 for more than 18 h in 1 mol/L KOH, and (–4±0.2) mA/cm2 for a duration of 30 h in 0.5 mol/L H2SO4, indicating that the as-prepared meso-MoS2/TiO2/p-Si photoelectrode possesses an excellent stability either in acidic media or alkaline media, which can be attributed to the TiO2 protective layer and the post-necking strategy. After 30 h of test in 0.5 mol/L H2SO4, as shown in Fig. 4(b), meso-MoS2 particles were still dispersed on the surface of p-Si, indicating that the catalyst can be stably anchored on the silicon surface under the protection of TiO2 layer. In addition, coarser particles were observed below the meso-MoS2 particles, which may be caused by the corrosion of the p-Si surface during the long-term testing. For comparison, the durability results of nano-MoS2/TiO2/p-Si photoelectrode in both two electrolytes were also collected, as shown in Fig. S4, the current density continued to increase within 40 h. After the test, it was observed that the surface of the photoelectrode was severely corroded. Therefore, the increased current density during the test was caused by the corrosion in the surface of the photocathode, indicating that the nano-MoS2/TiO2/p-Si photocathodes have a poor stability.
Because the proton reduction reaction on the cocatalyst surface is an important step for photoelectrocatalytic HER, the performance of surface reduction reaction has a great influence on the efficiency of HER. To understand the impact of cocatalyst morphology on the PEC performance of the composite photoelectrodes, the HER electrocatalytic activities of meso-MoS2 were evaluated in 0.5 mol/L H2SO4 and 1.0 mol/L KOH, respectively. For comparison, commercial Pt-C (20%), nano-MoS2, and bare GC were also investigated under the same conditions. As displayed in Figs. 5(a) and 5(b), Pt-C exhibits the lowest onset overpotentials (j = –1 mA/cm2) in both two electrolytes (–0.028 VRHE in 0.5 mol/L H2SO4 and –0.031 VRHE 1.0 mol/L KOH). Except for Pt-C, the meso-MoS2 electrocatalyst shows the lowest onset overpotentials with –0.22 VRHE in acidic electrolyte and –0.20 VRHE in alkaline solution, which are smaller than the corresponding values of nano-MoS2 (–0.24 VRHE in 0.5 mol/L H2SO4 and –0.24 VRHE in 1.0 mol/L KOH). A lower overpotential at the same current density for meso-MoS2 indicates a lower potential energy loss, resulting in a higher PEC rate for the meso-MoS2/TiO2/p-Si photocathode during proton reduction reaction. The results suggest that the enlarged surface area and ultra-small meso-MoS2 sheets maximizes the exposure active edge sites, which promotes the surface reduction reaction of the as-prepared photocathode. In addition, the unique mesoporous structure and larger pore size of meso-MoS2 than that of nano-MoS2 are more favorable for the diffusion of ions and hydrogen, thus increasing H2 production efficiency.
To identify the real active surface area of the cocatalyst, ECSA measurements were evaluated from the electrochemical double-layer capacitance (Cdl) by collecting the cyclic voltammetry (CV) results in a non-HER potential region (0.02–0.12 VRHE in KOH and 0.01–0.11 VRHE in 0.5 mol/L H2SO4) at different scan rates (Figs. 5(a) and 5(b), and (Fig. S3). The Cdl values could be obtained by plotting the ΔJ (Janodic– Jcathodic) at 0.1 VRHE against the scan rates, in which the slope is twice that of Cdl. The Cdl values of meso-MoS2 are 22.9 mF/cm2 in acidic media (Fig. 5(a)) and 9.2 mF/cm2 in alkaline solution (Fig. 5(b)) respectively, substantially larger than those of nano-MoS2 (17.5 mF/cm2 in 0.5 mol/L H2SO4, and 5.5 mF/cm2 in 1.0 mol/L KOH), and the corresponding ECSAs are 572.5 cm2, and 230.0 cm2, respectively, indicating that the unique mesoporous-structured MoS2 could provide a much larger effective surface area and active edge sites for surface reduction reaction of the composite photocathodes, as well as a rapid transport channel for ions and gas diffusion, thus contributing to an enhanced catalytic activity.
The above results demonstrate that the meso-MoS2 cocatalyst can greatly promote the PEC activity of the hybrid photocathodes and can be a promising alternative to noble metal based cocatalysts.
4 Conclusions
In summary, this paper has demonstrated a Si photocathode for enhanced PEC water splitting by using a highly-ordered mesoporous MoS2 structure as a novel cocatalyst of p-Si. The mesoporous structure synthesized by a facile and controllable method exposed abundant active sites, which greatly accelerated the HER kinetics. In addition, the formation of this mesoporous structure promoted the charge transfer and hydrogen evolution process. Moreover, a TiO2 protective layer was deposited on the p-Si substrate, and an additional TiO2 necking was introduced to strengthen the bonding between the MoS2 particles and the TiO2 layer. As a consequence, the prepared meso-MoS2/TiO2/p-Si hybrid photocathode exhibits an excellent PEC performance and durability in both acidic and alkaline solutions. These strategies provide helpful guidelines for engineering highly efficient catalysts and PEC photoelectrodes with a superior corrosion resistance.