Insulator layer engineering towards stable Si photoanode for efficient

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Insulator layer engineering towards stable Si photoanode for efficient water oxidation Qian Cai, Wenting Hong, Chuanyong Jian, Jing Li, and Wei Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01398 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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Insulator layer engineering towards stable Si photoanode for efficient water oxidation Qian Cai, † Wenting Hong, † ‡ Chuanyong Jian, † Jing Li † and Wei Liu†* †

CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian

Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China ‡

University of Chinese Academy of Sciences, Beijing, 100049, China

ABSTRACT: The ultra-thin insulator layer in silicon metal-insulator-semiconductor (MIS) photoanode, plays the important roles in determining the performance of water oxidation. We report that insulator oxide electroforming phenomenon occurred during long-term OER testing under constant external voltage, is the primary reason for the degradation of silicon MIS photoanode. Compared with TiO2, ZrO2 exhibits high electro-reduce resistance, which minimizes the cation transport in the dielectric film, thereby significantly enhancing the stability of silicon MIS photoanodes. Silicon passivated with an ultra-thin ZrO2 film deposited by atomic layer deposition (ALD), exhibits high stability (>100 hours) in alkaline condition (PH=14). Combined with NiFe catalyst, NiFe/ZrO2/n-Si photoanode exhibits improved OER activity with a low onset potential (~0.96 V versus RHE), a high photocurrent density (26.6 mA·cm-2) at 1.23 V versus RHE, and a record high saturated current density of 36.4 mA·cm-2.

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KEYWORDS: Silicon photoanode, ZrO2 insulator layer, TiO2 insulator layer, electroforming, oxygen evolution reaction (OER) INTRODUCTION Photoelectrochemical (PEC) process has been widely employed to drive water splitting for the production of H2 fuel. 1,2 Photoanode, which can efficiently harvest solar energy to reduce the usage of electricity, is the key component in an efficient artificial photocatalyst system. Among various semiconductors, silicon (Si) is one of the most promising photoanode candidates due to its small band gap (~1.1 eV) for sufficient photon absorpotion.3-10 However, Si suffers from the severe corrosive in strong alkaline electrolytes.11-14 Passivating Si surface with stable materials, including transition metal, transition metal oxides, and noble metal silicide, 15-18 is critical for the application of Si photoanode in water oxidation. In a metal-insulator-semiconductor (MIS) photoanode (Figure 1a), the insulator layer not only serves as the corrosion resistant layer but also allows holes transporting efficiently from Si to catalyst.19 It has been widely accepted that hole tunneling through insulator layer is the major transport mechanism for Si MIS photoanode with several nanometers thick insulator layer.20 The insulator layer is very important for maintaining charge transport efficiency and stability of Si photoanode. Increasing the thickness of insulator layer can improve the stability of Si photoanode.21 Ji et al. reported a stable Si photoanode by creating a conducting filament in 30 nm SiO2 film using localized dielectric breakdown method to allow photogenerated carriers flowing to metal catalyst (Figure 1a).22 Hu et al. demonstrated that thick TiO2 conductive layer with high concentration of defects deposited could accelerate carrier transport in Si photoanode.23 However, thick insulator layer reduces light absorption, therefore lowering the saturation current of Si photoanode. Although on buried p+n silicon junction, insulator thickness dependence of OER performance can be minimized, the


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preparation of high-quality junctions requires additional complicated fabrication process.24 Therefore, it is desirable to boost the OER performance and stability of single Si junction photoanode using a stable and thin insulator to harvest solar energy ultimately. Si MIS photoanodes with different ultrathin insulator layers, such as 2 nm TiO2, 0.5 nm SiOx, and SiOx (1.5nm)/CoOx (2 nm), have been explored for water oxidation.8,9,25,26 However, the stability of Si MIS photoanodes with ultrathin insulator layer is generally less than 100 hours in a highly corrosive electrolyte (pH=14) (Table. S2, ESI). Although Ir (3 nm)/TiO2(2 nm)/n-Si photoanode shows excellent OER activity, its stability is limited (8 hours) in a highly corrosive electrolyte (pH=14).25 Si photoanode with 2 nm Ni/NiOx exhibits high OER activity and good stability (>80 hours) in aqueous borate buffer (pH = 9.5) solution. However, its performance degrades after 12 hours operation in a strong alkaline electrolyte (pH = 14).7 To date, it lacks the detailed study to reveal the degradation mechanism of Si MIS photoanode. It is well known that transition metal oxides (insulator layer) can change behavior under high electrical field through electroforming, which has been observed on transition metal oxide memristor.27-29 Si MIS photoanode has a similar structure with memristor. Therefore, electroforming phenomenon will occur in the ultrathin insulator layer (TiO2 as example) of Si MIS photoanode. The electroforming process in Si MIS is proposed in Figure 1c. At high electrical field, oxygen ion (O2−) generated from the electro-reduction of TiO2 will drift towards Si (Figure 1c).27 Oxygen ion will lose electron on the surface of Si, resulting in the formation of O2 and reduction of TiO2 to Ti atom on the surface of Si. The evaluation of O2 will create pinholes in TiO2 layer and accelerate the corrosion of Si. In addition, cation transport in the dielectric film will create conducting filament across the electrodes, which will influence the stability of Si photoanode.22 From the understating of electroforming process of metal oxide on Si photoanode, the efficient


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way to enhance the stability of Si photoanode is to improve the electron reduction resistivity of metal oxide. Compared with TiO2, ZrO2 will be an alternative high-quality tunnelling insulator layer with high anti-corrosion property, high dielectric constant (25), wide band gap (5.8 eV), and broad spectrum transparent region. Besides, ZrO2 exhibits a high electron-reduction resistance which can reduce the electroforming in Si MIS photoanode. Using NiFe as catalyst, NiFe/ZrO2/n-Si photoanode exhibits a high stability for at least 100 hours without distinct degradation in strong alkaline electrolyte (PH=14). NiFe/ZrO2/n-Si photoanode exhibits remarkable OER activity with a low onset potential (~0.96 V versus RHE), a high photocurrent density (26.8 mA·cm-2) at 1.23 V versus RHE as well as a record high saturated current density of 37.4 mA·cm-2.

Figure 1. Representative energy band diagrams of MIS photoanodes for water oxidation. (a) MIS photoanodes with an ultrathin insulator layer by tunneling mechanism. (b) MIS photoanodes with a thick insulator layer by localized conduction mechanism. (c) Electroforming process between insulator layer and semiconductor under an applied voltage. RESULTS AND DISCUSSION


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Plasma-enhanced ALD (PE-ALD) method is used to deposit ZrO2 and TiO2 (~1-3nm) on Si surface. NiFe (2.0 nm) catalyst is subsequently deposited by electron beam evaporator on the surface of metal oxides as the active OER catalyst (Supplementary info. S1 and Figure S1, ESI). Compared with other low-cost Ni, Co, Co(OH)2 catalysts, NiFe catalyst exhibits high stability in strong alkaline electrolyte with remarkable OER activity.30-32 Figure 2a shows a cross-section high-resolution transmission-electron microscopy (HRTEM) image of NiFe/ZrO2/n-Si photoanode interface. The thickness of ZrO2 and NiFe are confirmed to be 2.1 nm and 2.08 nm, respectively. There is no distinct SiO2 layer between ZrO2 layer and Si substrate because Si is quickly loaded into the ALD chamber after the HF dip to remove the native oxide. In this work, we found that thin SiOx layer between Si and passivation layer can significantly reduce the OER performance of Si photoanode (Figure S2, ESI). Scanning transmission-electron microscopy energy-dispersive spectroscopy (STEM-EDS) mapping of NiFe/ZrO2/n-Si interface displayed in Figure 2b further depicts that Si photoanode has relatively well-defined layers of Si, ZrO2, and NiFe. The fabrication method of Si photoanodes is highly reproducible indicated by the geometry of NiFe/TiO2/n-Si photoanode (cross-section HRTEM image, Figure S3, ESI). To exclude the possible influence of NiFe catalyst preparation method on OER activity comparison between NiFe/TiO2/n-Si and NiFe/ZrO2/n-Si, atomic force microscopy (AFM) analysis, optical absorption spectra, and electrochemical active surface area (ECSA) measurements are conducted on Si photoanodes as shown in Supplementary information S4 (Figure S4-S6, ESI). The results demonstrate that NiFe catalyst exhibits the similar diameters, thickness, roughness, and the number of active sites for both two types of Si photoanodes. The optical absorption measurement of Si photoanode shows that the ultra-thin NiFe layer will effectively be optically “transparent”


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and will not significantly affect the light absorption of Si substrate (Supplementary information Figure S7, ESI).

Figure 2. (a) High-resolution transmission electron microscope (HRTEM) image of the crosssection of NiFe/ZrO2/n-Si photoanode. (b) Energy-dispersive spectroscopy (EDS) mapping of the cross-section of NiFe/ZrO2/n-Si interface. (c) Linear sweep voltammograms (LSVs) curves of NiFe/ZrO2/n-Si photoanodes and NiFe/TiO2/n-Si photoanodes. (d) Mott-Schottky curves of NiFe/TiO2 (2.0 nm)/n-Si and NiFe/ZrO2(2.0 nm)/n-Si photoanodes. (e) The transient photocurrent curves of NiFe/TiO2 (2.0 nm)/n-Si and NiFe/ZrO2 (2.0 nm)/n-Si photoanodes at an applied bias 1.23V versus RHE. (f) The correlations between ln D and time (t), the transient time constant (τ) is defined as the time when lnD = -1. Two different thicknesses of insulator layers (2.0 nm and 3.0 nm) are prepared to investigate the influence of ultrathin insulator layer (TiO2 and ZrO2) on the OER activity and stability of Si photoanodes. The linear sweep voltammograms (LSVs) curves of Si photoanodes are conducted


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in 1.0 M KOH electrolyte (pH=14) under the simulated air mass (AM) 1.5G illuminations (100 mW·cm-2) without IR compensation as shown in Figure 2c. The detailed comparisons of OER performance of Si-based photoanodes are extracted from LSV curves and summarized in Table. S1. It is noticed that NiFe/ZrO2(3.0 nm)/n-Si photoanode (red dotted line in Figure 2c) has an onset potential of 0.96 V versus reversible reference electrode (RHE). To date, this onset potential (0.96 V) is one of the smallest values for Si-based photoanodes without a buried nSi/p+-Si junction (Table S2, ESI). More importantly, the onset potential of Si photoanodes with ZrO2 insulator layer (red line in Figure 2c) is smaller than that of Si photoanodes with TiO2 insulator layer (blue line in Figure 2c) for an equivalent thickness. The onset potential of NiFe/ZrO2(2.0 nm)/n-Si photoanode (1.04 V) is about 140 mV smaller than that of NiFe/TiO2(2.0 nm)/n-Si photoanode (1.18 V). The onset potential and photovoltage of PEC photoanode are dominated by the flat band potential (Efb), which determines the built-in electric field (Figure S8, ESI).8,33,34 Hence, a good insulator layer should form a large barrier height with Si to the enhancement of charge carrier separation rate, eventually leading to the small onset potential and large photovoltage. The barrier height (φbh) of NiFe/ZrO2(2.0 nm)/n-Si photoanode and NiFe/TiO2(2.0 nm)/n-Si photoanode are calculated to be 1.32 eV and 1.25 eV, respectively (Figure 2d, Supplementary S7). Consequently, the large barrier height of NiFe/ZrO2(2.0 nm)/n-Si photoanode will lead to the improvement of the built-in electric field and photovoltage, eventually results in the small onset potential. Additionally, the photovoltage generated by NiFe/ZrO2(3.0 nm)/n-Si photoanode is estimated to be 560 mV (Figure S8 and Table S1, ESI), which is close to the best result achieved on single junction silicon photoanode. 26


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On the other hand, the photocurrent density of NiFe/ZrO2(2.0 nm)/n-Si photoanode at the thermodynamic water oxidation potential (1.23 V versus RHE) reaches as high as 26.6 mA·cm-2, which is higher than most of Si photoanodes (Table S2, ESI). The saturation current density of 36.4 mA·cm-2 reaches one of the highest values achieved on Si photoanodes and is 83.3 % of the theoretical value (43.7 mA·cm-2) of crystalline Si.35 The high saturated current density can be attributed to the transparent properties of the ZrO2 layer and NiFe layer. All the above data indicate that ZrO2 insulator layer can enhance OER performance of Si photoanode compared with Si photoanode with TiO2 layer. The basic principle of designing a prosperous PEC system is to achieve a low charge recombination rate, therefore reducing the photo-generated carrier recombination during the period of charge transportation. As shown in Figure 2e, the transient photocurrent spectra of NiFe/TiO2(2.0 nm)/n-Si and NiFe/ZrO2(2.0 nm)/n-Si photoanode are measured with chopping light irradiation at 1.23 V versus RHE.36 The recombination peaks are typically observed during the transient state when the light is switched on/off. The weak spike current of NiFe/ZrO2(2.0 nm)/n-Si photoanode indicates an excellent interface quality with reduced charge recombination which can be quantitatively reflected by the transient time constant (τ). The value of τ for NiFe/TiO2(2.0 nm)/n-Si and NiFe/ZrO2(2.0 nm)/n-Si photoanodes (Figure 2f) are extracted to be around 0.19 s and 0.36 s, respectively (Supplementary S8 and Figure S10, ESI). Therefore, one can conclude that ZrO2 passivation layer on Si decreases the recombination of photo-generated electrons and holes at the interface of the photoanode and electrolyte. The improved charge transport and recombination properties of NiFe/ZrO2(2.0 nm)/n-Si photoanodes also can be confirmed by electrochemical impedance spectroscopy (EIS) spectra as shown in Supplementary Figure S11. The smaller charge transfer resistance (Rct) of NiFe/ZrO2(2.0 nm)/n-Si photoanodes


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results in the efficient separation of photogenerated electron-hole pairs, thereby leading to the fast-interfacial charge transfer rate.

Figure 3. The chronoamperometric curves of NiFe/TiO2/n-Si photoanode (green line) and NiFe/ZrO2/n-Si photoanode (red line) with the passivation layer thickness of 1.0 nm (a), 2.0 nm (b) and 3.0 nm (c) in 1.0 M KOH electrolyte. Except for the electrochemical activity of Si photoanode, the stability is the most important issue for Si-based photoanodes. The stability of Si photoanodes with TiO2 and ZrO2 passivation layers with the same thickness are measured 1.0 M KOH electrolyte at a constant external potential to obtain the current density around 15 mA·cm-2. For 1.0 nm passivation layer (Figure 3a), NiFe/ZrO2(1.0 nm)/n-Si photoanode could work continually over 100 hours with slow performance drop in strong alkaline electrolyte, while NiFe/TiO2(1.0 nm)/n-Si photoanode (green curve, Figure 3a) experiences a dramatic decrease of the OER activity only after 8 hours testing. NiFe/TiO2(1.0 nm)/n-Si photoanode lost almost 76.2 % of the initial current density after 40 hours testing. The durability test of Si photoanodes passivated by TiO2 and ZrO2 with the thickness of 2.0 nm and 3.0 nm is also conducted as shown in Figure 3b and 3c. NiFe/ZrO2(3.0 nm)/Si photoanode (red curve, Figure 3c) retains nearly 95 % OER activity over a 100 hours durability operation, while the current density of NiFe/TiO2 (3.0 nm)/n-Si photoanode (green


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curve, Figure 3b) drops nearly 50 % in 100 hours testing. Along with the increase of the thickness of passivation layer, the stability of photoanode is enhanced for both TiO2 and ZrO2 layer. NiFe/TiO2 (3.0 nm)/n-Si photoanode retains ~90.5 % activity after 24 hours testing (Figure S12) which is comparable to the previously reported Si photoanodes in strong alkaline electrolyte.7,8,15,29. It is also confirmed that ZrO2 layer presents better stability compared with the same thickness of TiO2 layer, which is according to the passivation layer of 2.0 nm and 3.0 nm (Figure 3a, b). The slight loss of OER activity of NiFe/ZrO2/n-Si photoanodes is partly from the dissolution/exfoliation of NiFe catalyst in strongly alkaline solution as confirmed by the Ni and Fe X-ray photoelectron spectroscopy (XPS) signal intensity decreasing after 100 hours operation (Figure S13, ESI). 8,30


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Figure 4. Cross-section HRTEM image (a) and STEM-EDS line profile (b) of NiFe/TiO2 (2.0 nm)/n-Si photoanode after 60 hours continuous OER operation in 1.0 M KOH electrolyte. Crosssection HRTEM image (c) and STEM-EDS line profile (d) of NiFe/ZrO2 (2.0 nm)/n-Si photoanode after 100 hours continuous OER operation in 1.0 M KOH electrolyte. To acquire the interface of Si photoanodes after long-term stability operation, the HRTEM and STEM-EDX line profile are performed. For NiFe/TiO2(2.0 nm)/n-Si photoanode after 60 hours OER operation, only one layer with the thickness of ~5.6 nm remains on the surface of Si, indicating that TiO2 totally interact with a catalyst under constant OER testing (Figure 4a). This phenomenon can be explained by TiO2 electroforming phenomenon and cautions transport under long-term OER process as described in Figure 1c and Figure 5. Si atom will migrate to the interface, and the reduced Ti atoms near the silicon will move to NiFe layer along the direction of the electric field. Along with the OER process, electroforming will lead to the physical deformation and breakage of the TiO2 layer. Eventually, the upper NiFe layer will collapse and mix with the reduced Ti atoms to form a Ti-Ni-Fe mixing layer. The element distribution of NiFe/TiO2(2.0 nm)/n-Si conforms the electroforming and cation transport in the dielectric (Figure 4b). After the formation of Ti-Ni-Fe mixing layer, the silicon is still protected by this mixed layer because there is no Si oxide formed after 60 hours operation. However, the OER performance of NiFe/TiO2 (2.0 nm)/n-Si photoanode is significantly decreased due to the damaged TiO2 layer by oxide electroforming under bias, thereby losing the capability as the electron filter for Si MIS device. In comparison, cross-section HRTEM image of NiFe/ZrO2(2.0 nm)/n-Si photoanode (Figure 4c) after 100 hours stability test displays that ZrO2 protection layer is extremely stable without obviously pinhole or defect at the Si/ZrO2 interface, which is consistent with its excellent long-


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term OER stability, indicating that the ZrO2 has high electron reduction resistant. Zr atoms generated from electro-reduction of ZrO2 react with Si atoms diffused from Si substrate to form a Zr-Si layer between Si and ZrO2 layer. This assumption can be confirmed by the elements distribution of Si, O and Zr obtained from STEM-EDX line profile (Figure 4d). Therefore, the high electro-reduce resistance of dielectric is critical for maintaining the stability of Si photoanode. Additionally, we also observe that the metal layer is thicker than the pristine NiFe film (Figure 2a), which can be ascribed to the formation of NiOOH or Fe2O3 on the surface of NiFe layer during the long-term OER periods. This assumption can be confirmed by the abundant surface O contents from STEM-EDX line profiling image (Figure 4d, brown line).

Figure 5. The schematic diagram of electroforming process for NiFe/TiO2/n-Si photoanode and NiFe/ZrO2 /n-Si photoanode under continuous OER testing. The chemical composition evaluation of Si photoanodes is studied by X-ray photoelectron spectroscopy (XPS) using Ar ion milling on the same Si photoanodes which have been studied by HRTEM (Figure 4). Figure 6a shows the normalized XPS depth profile of NiFe/TiO2(2.0


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nm)/n-Si photoanode after 60 hours OER operation in 1.0 M KOH electrolyte. After 10 s etching, we find that TiO2 layer mixes with NiFe layer and O content sharply, which is consistent with the HRTEM analysis shown in Figure 4a and 4b. The above phenomenon indicates that TiO2 layer is destroyed under continues OER operation, which will lead to the decreasing of the OER performance of NiFe/TiO2/n-Si photoanode.

Figure 6. X-ray photoelectron spectroscopy (XPS) depth profiling of NiFe/ZrO2(2.0 nm)/n-Si photoanode and NiFe/TiO2(2.0 nm)/n-Si photoanode after continuous OER operation in 1.0 M KOH under illumination. (a) Normalized elemental depth profile of NiFe/TiO2(2.0 nm)/n-Si photoanode. (b) XPS depth profiling for Ti 2p spectra for NiFe/TiO2(2.0 nm)/n-Si photoanode. (c) Normalized elemental depth profile of NiFe/ZrO2(2.0 nm)/n-Si photoanode. (d) XPS depth profiling of Zr 3d spectra for NiFe/ZrO2(2.0 nm)/n-Si photoanode.


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The instability of TiO2 can further be verified by high-resolution Ti XPS depth dependent spectra (Figure 6b) indicating that TiO2 layer only can be detected on the surface of Si photoanode, and there is no TiO2 near the silicon, proving the electroforming process of TiO2 under the bias. NiFe/ZrO2(2.0 nm)/n-Si photoanode after 100 hours OER testing in 1.0 M KOH electrolyte are also investigated by XPS spectra as shown in Figure 6c. Except Si signal appears at the bottom of ZrO2 layer, the distribution of Ni, Fe, Zr and O elements are very similar with the pristine NiFe/ZrO2(2.0 nm)/n-Si photoanode, indicating the high stability of ZrO2. After exposure to the KOH electrolyte, the surface of Ni and Fe are transferred into NiOOH/Ni(OH)2 and Fe2O3/FeOOH, respectively. This phenomenon can be confirmed by high-resolution Ni, Fe and O spectra at different etch times shown in Figure S14. Besides, it also observed that the underneath Ni element existed in NiOX state (Figure S14a) which can also contribute significantly to the stability of the photoanode. Additionally, the active sites of NiOOH/Ni(OH)2 and Fe2O3/FeOOH exist at the surface of photoanode make the surface O content is significantly enhanced as presented in Figure 6c (yellow line), which is consistent with the STEM-EDS linear profiling analysis (Figure 4d). After ~40s etching, the O element nearly disappeared, implying that Si is well protected by ZrO2 without oxidization after 100 hours OER operation. In addition, high-resolution Zr 3d XPS depth dependent spectra (Figure 6d) shows that Zr silicide only exists at the ZrO2/n-Si interface, indicating that Zr atoms reduced by electrical field react with the silicon cation transport toward to the ZrO2.37 Besides, XPS depth profile of the pristine sample further verify the formation of Zr silicide is ascribed to the electrical field (Figure S15, ESI). It is known that silicide has the excellent thermal stability and good conductivity. The silicide has been verified to enhance the electrochemical performance in Lithium-ion batteries by improving conductivity and providing


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facile paths for charge transfer at the electrode/electrolyte interface.38 The formed silicide layer could employ as another protection layer to prevent the Si diffusion. Hence, the silicide will not only have any negative influence upon the final OER stability but also improve the final OER performance. By comparing the XPS depth profile of NiFe/ZrO2/n-Si and NiFe/TiO2/n-Si photoanode after continues OER operation, it is further confirmed that ZrO2 layer is a more stable protective layer than TiO2 layer under the high electric field. CONCLUSIONS In this work, NiFe/ZrO2/n-Si and NiFe/TiO2/n-Si photoanodes are fabricated for OER process by a simple CMOS fabrication technology. We suggested that TiO2 electroforming will occur during long-term OER testing under constant external voltage, which will lead to the physical deformation and breakage of the TiO2 layer, thus resulting of the degradation of silicon MIS photoanode. The NiFe/ZrO2/n-Si photoanode was firstly reported for water oxidation, which shows the stability of oxygen evolution for at least 100 hours at a constant external bias, while NiFe/TiO2/n-Si photoanodes exhibit obvious degradation during the 60 hours OER operation. Besides, the results also demonstrated that ZrO2 insulator layer leads to the superior OER activity and enhanced stability compared with TiO2 layer for any equivalent insulator layer thickness, which can be ultimately used to protection silicon. Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes There are no conflicts to declare. Supporting Information. The detail experimental processes, AFM images, UV-Vis spectra, LSV data, ESCA data, HRTEM images, IPCE data, EIS data, XPS data and the calculation process of Efb and φbh are supplied in supporting information. ACKNOWLEDGMENT This work was supported by Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, the National Natural Science Foundation of China (No. 61674152), the Natural Science Foundation of Fujian Province of China (No. 2017J01130, 2018J05097). REFERENCES (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar water splitting cells. Chem. Rev. 2010, 110, 6446-6473. (2) Lewis, N. S. Research opportunities to advance solar energy utilization. Science 2016, 351, 1920. (3) Cai, Q.; Hong, W.; Jian, C.; Li, J.; Liu, W. Impact of silicon resistivity on the performance of silicon photoanode for efficient water oxidation reaction. ACS Catal. 2017, 7, 3277-3283. (4) Hong, W. T.; Cai, Q.; Ban, R. C.; He, X.; Jian, C. Y.; Li, J.; Li, J.; Liu, W. Highperformance silicon photoanode enhanced by gold nanoparticles for efficient water oxidation, ACS Appl. Mater. Inter. 2018, 10, 6262-6268.


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TOC ZrO2 layer will minimize the cation transport in the dielectric film due to its high electro-reduce resistance, thereby enhancing the stability of silicon MIS photoanodes.


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Insulator layer engineering towards stable Si photoanode for efficient

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