Loading Amorphous NiMoO4âxSx Nanosheet Cocatalyst to Improve

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Loading Amorphous NiMoO4-xSx Nanosheet Cocatalysts to Improve Performance of p-Silicon Wafer Photocathode Fangli Wu, Wei Tian, Fengren Cao, Linxing Meng, and Liang Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00017 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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ACS Applied Energy Materials

Loading Amorphous NiMoO4-xSx Nanosheet Cocatalysts to Improve Performance of p-Silicon Wafer Photocathode Fangli Wu, Wei Tian, Fengren Cao, Linxing Meng, Liang Li*

College of Physics Optoelectronics and Energy, Center for Energy Conversion Materials & Physics (CECMP), Jiangsu Key Laboratory of Thin Films, Soochow University, Suzhou 215006, P. R. China E-mail: [email protected]; [email protected]

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ABSTRACT: Considering its small band gap and appropriate conduction band level, Si is a suitable semiconductor for photocathode in photoelectrochemical (PEC) cell. Unfortunately, its PEC activity is far from desirable because of inferior stability, high recombination of photogenerated electron-hole pairs and sluggish hydrogen evolution kinetics at the Si/electrolyte interface. The introduction of low-cost and earth abundant non-noble metal cocatalysts onto Si photocathode is a promising route to improve its PEC performance. Herein, we fabricate efficient and stable photocathodes by integrating non-noble metal NiMoO4-xSx nanosheets with planar p-Si wafers, along with ultrathin TiO2 as protective and passivative film at the intermediate and outermost layer. An onset potential of 0.3 V (vs. RHE) and a photocurrent density of 5.1 mA cm-2 at -0.5 V (vs. RHE) are obtained for the optimal photocathode, representing significant enhancement compared to the pristine Si wafers. Moreover, long-term measurement demonstrates that the TiO2 layer can effectively protect the photocathode from degradation of electrolyte and photocorrosion.

KEYWORDS: Si, photoelectrochemical, NiMoO4-xSx, cocatalyst, atomic layer deposition


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INTRODUCTION Hydrogen fuel is regarded as one of the most attractive alternatives for solving the global energy crisis and achieving an environmental-friendly fuel economy, owing to its clean, storable, and sustainable properties. Solar-driven photoelectrochemical (PEC) water splitting is a promising approach to directly convert the solar energy into hydrogen fuel.1-5 In the past few decades, extensive research efforts have been paid on the design and fabrication of high performance photoelectrodes,6 as they are essential parts for achieving high PEC activity. A favorable photoelectrode should meet several requirements: small band-gap so as to absorb a large amount of sunlight, proper conduction/valence band position that straddles the water oxidation and reduction potentials, excellent charge separation and transfer properties, good stability and low cost.7-10 Unfortunately, there is no such a material that can satisfy all these requirements simultaneously. Silicon (Si) with a small band gap (~1.1 eV) is an earth-abundant and inexpensive semiconductor, which has been widely used as photoelectrode for its highly efficient light absorption and suitable energy band position.11-15 However, its PEC performance is still unsatisfactory up to date because of poor stability, high recombination of photogenerated electron-hole pairs and sluggish hydrogen evolution kinetics at the Si/electrolyte interface. The deposition of cocatalysts on the surface of Si has been proposed as a promising approach to accelerate the hydrogen evolution reaction. In such a structure, Si serves as light absorber and charge transport pathway, while cocatalysts promote the PEC process by accelerating the hydrogen and oxygen evolution (HER and OER) process and thus reducing electron-hole recombination. Pt and other noble metals have been demonstrated as efficient cocatalysts for water splitting,5, 16-18 while the high cost hinders their wide application. Alternatively, some ACS Paragon Plus Environment


inexpensive and nontoxic catalysts with analogous performance in HER/OER reaction, such as chalcogenides,19-21 metal oxides,22 phosphides,23 nitrides,24 carbides,25 and metal alloys,26, 27

have attracted much attention. Among these materials, metal oxides are relatively stable and

offer a variety of matched energy levels to water oxidation/reduction potentials.28 Moreover, catalytic activity adjustable alloy-metal oxides can be easily synthesized. As for metal sulfides, although they generally suffer from photocorrosion, the good photocatalytic properties as well as visible light photoactivity enable their wide application in water splitting. For instance, Akihiko Kudo et al.29 demonstrated that the sulfide solid solution AgInS2-CuInS2-ZnS photocatalysts were highly active for H2 evolution in the presence of sulfur compounds as electron donors. The active S atoms on the exposed edge of sulfides could strongly bond with H+ ions in solution, enabling the effective reduction of H+ ions to H2. Approaches based on cationic and anionic substitution have been utilized to further optimize physicochemical properties to engineer the catalyst activities.30-32 Metal compounds with mixed anions such as spinel-type lithium cobalt oxide,33 NiCo2S4,34 MoS2/NiS2,35 phosphidized CoS236 and a series of binary monometallic phosphides and sulfides37 have demonstrated superior performance. This enhancement is also ascribed to the “dopant” effect of the low anions such as S2-, which exist in the surface layer of materials to modify the hydrogen adsorption energy on the cocatalysts. The introduction of low-cost and earth abundant non-noble metal cocatalysts, including MoS238, 39 and Ni/NiOx40 to the surface of Si have been developed. However, the fabrication of quaternary oxysulfides and their integration with Si is still a big challenge. On the other hand, although the PEC performance of Si wafer is lower than Si nano- and micro- wires because of


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its high reflectivity and poor durability, the utilization of Si wafer as photocathode can greatly simplify the fabrication procedure and raise the possibility of practical application. However, there is limited work to improve the PEC activity of Si wafers utilizing cocatalysts.41-43 Herein, we report the integration of non-noble metal NiMoO4-xSx nanosheets with TiO2 film coated planar p-Si wafers as efficient and robust photocathodes for PEC hydrogen generation. The effects of thickness (synthesis time) of NiMoO4-xSx nanosheets and TiO2 on the PEC activity are investigated. TiO2/NiMoO4-xSx nanosheets/TiO2/Si electrodes are achieved via a hydrothermal process to synthesize the NiMoO4-xSx nanosheets and a subsequent ALD (Atomic layer deposition) deposition step to prepare the TiO2 film. Such photocathodes possess an onset potential of 0.3 V (vs. RHE) and a photocurrent density of 5.1 mA cm-2 at -0.5 V (vs. RHE), exhibiting dramatic enhancement compared to the pristine Si wafers. Both the electrochemical impedance spectra and Mott-Schottky results prove that the loading of NiMoO4-xSx nanosheets and TiO2 can promote efficiently charge separation and transport, leading to the enhanced PEC activity.

EXPERIMENTAL SECTION Synthesis of TiO2/NiMoO4-xSx nanosheets/TiO2/Si. The p-type (100) Si wafers (1-10 Ω cm) were ultrasonically cleaned in acetone, ethanol, and deionized water for 20 min, respectively, and dried in vacuum at room temperature. The cleaned Si wafers were immediately immersed into a solution of 4.6 M HF to remove native SiOx on the surface. TiO2 film was then deposited on Si wafer by atomic layer deposition (ALD, Ensure NanoTech, Beijing, China) technique. Tetra (dimethylamino) titanium (TDMAT) and high-purity water



were used as titanium and oxygen precursors, respectively, which were injected into the reactor under a continuous flow of N2 at a rate of 20 sccm (standard cubic centimeter per minute). The titanium cylinder was heated to 75 °C and the ALD chamber was set at 150 °C. During the deposition, the water was pulsed for 0.02 s, and the chamber was purged for 30 s subsequently. Then a 0.4 s Ti precursor pulse was introduced followed by 20 s purge. The growth rate under this condition is 0.55 Å per cycle. The thickness of TiO2 film can be tuned by controlling the cycle numbers. The NiMoO4-xSx nanosheets were grown on Si wafers by a hydrothermal method. 2 mmol Ni (NO3)3·6H2O, 2 mmol NaMoO4·2H2O and 4 mmol thiurea were dispersed in 50 ml deionized water under stirring for 10 min. Then, 2 mmol hexamethylenamine (HMTA) were added into the solution to regulate it to be alkaline. The TiO2 coated Si wafer was put into stainless autoclaves with 10 ml of above aqueous solution and maintained at 120 °C for different time (20, 30, and 40 min). As the autoclave cooled down to room temperature naturally, the samples were rinsed with ethanol and water repeatedly and dried at 60 °C under vacuum. Finally, TiO2 film was deposited on the NiMoO4-xSx nanosheets again using ALD technique as depicted before to obtain TiO2/NiMoO4-xSx nanosheets/TiO2/Si sample. Materials characterization. The morphology of samples was characterized by scanning electron microscopy (FE-SEM, Hitachi SU8010) and atomic force microscope (AFM, Asylum Research MFP-3D-BIO). The microstructure was analyzed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (FEI Tecnai G2 F20 S-TWIN TMP). Energy dispersive X-ray spectroscopy (EDS) elemental mapping was conducted under the TEM with an annular dark-field (ADF) detector. The specimens for TEM were prepared by


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scratching samples from Si substrates. The element chemical valence status was measured by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi). The reflectance spectra were carried out on a UV-vis spectrophotometer (Shimadzu UV-3600). Photoelectrochemical measurements. Prior to PEC measurement, the indium-gallium alloy was pasted onto Si surface to provide Ohmic electrical contact and a copper wire was connected to the sample. The backside and edge of sample were sealed using epoxy and the copper wire was placed in a plastic tube for preventing electrical short to the electrolyte. The front side of sample was in contact with the electrolyte. The PEC measurements were performed in a 0.1 M NaH2PO4 (pH~5) aqueous solution with an Autolab electrochemical workstation (PGSTAT 302N) in a three-electrode configuration, where the as-prepared samples served as the working electrodes, a Pt mesh as the counter electrode and a saturated Ag/AgCl electrode as the reference electrode, respectively. During the measurements, N2 was bubbled to remove the dissolved oxygen in the electrolyte. The photocurrent density-potential (J-V) relation and -time (I-t) curves with light on/off cycles were conducted under AM 1.5G illumination (100 mW cm2) provided by a solar light simulator (Newport, model 94043A). The measured potential versus Ag/AgCl was converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation:44 ERHE = EAg/AgCl + 0.059pH + Eo Ag/AgCl where ERHE is the converted potential versus RHE, Eo

(1) Ag/AgCl=0.1976

V at 25 °C and

EAg/AgCl is the experimentally measured potential versus the Ag/AgCl reference. Incident-photo-to-current efficiency (IPCE) measurements were performed using a sunlight simulator (Newport, model 67001) with a monochromator (Newport, model 74125) and a Si



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detector (Newport, model 77330). IPCE was calculated based on the equation:45 IPCE= (1240J) / (λPlight)

(2)

where λ is the wavelength of the incident light, J and Plight are the photocurrent density and light power density at the wavelength λ, respectively. Electrochemical impedance spectra (EIS) were measured on the same work station at 0 V vs. RHE under light illumination with the frequency ranging from 0.1 to 100 kHz, and the amplitude is 5 mV.

RESULTS AND DISCUSSION The SEM images of TiO2/NiMoO4-xSx/TiO2/Si samples are shown in Figure 1a, b and c. The as-prepared TiO2/NiMoO4-xSx-30min/TiO2/Si sample presents rough surface with uniformly grown nanosheets, forming a dense layer with the thickness of about 200 nm (Figure 1c). The thickness of 2 nm ALD TiO2 film is too thin to be distinguished from SEM images (Figure S1), and thus it was measured using AFM (Figure S2). Meanwhile, we further investigated the relationship between morphology and reaction time for NiMoO4-xSx nanosheets. With increasing the growth time of nanosheets from 20 to 40 min, the nanosheet shape is preserved while the size becomes larger (Figure S3). The microstructure of NiMoO4-xSx nanosheets was explored using TEM. The low-magnification TEM image in Figure 1d reveals tightly aggregated NiMoO4-xSx nanosheets, which were scratched from TiO2/Si substrate. The regional high-resolution TEM image indicated in Figure 1e shows no obvious lattice fringe on the NiMoO4-xSx nanosheets, which could be concluded as evidences for the amorphous character. Furthermore, as shown in Figure 1f, the selected-area electron diffraction (SAED) pattern consists of several clear


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rings, confirming the amorphous structure of NiMoO4-xSx nanosheet. The ADF elemental mapping images in Figure S4 show that Ni, Mo, O and S elements uniformly distribute across the entire nanosheets and the atomic ratio is about 1:1:3:1.

Figure 1. (a, b) Top- and c) cross sectional- view SEM images of TiO2/NiMoO4-xSx-30 min/TiO2/Si wafers; (d) A typical TEM image of NiMoO4-xSx nanosheets scratched from the wafer, and (e, f) corresponding HRTEM image and SAED pattern. XPS was utilized to verify the chemical composition and states of related elements in the NiMoO4-xSx nanosheets. From the survey spectrum of the as-synthesized NiMoO4-xSx/TiO2/Si sample (Figure S5), Si, Ti, Ni, Mo, O and S signals can be clearly identified, along with a small quantity of C from the reference electrode. The high resolution XPS spectra of the Ni 2p, Mo 3d, O 1s and S 2p are displayed in Figure 2. The Ni 2p spectrum displays the main double peaks at 856.2 and 873.8 eV, with two satellite peaks at the higher binding energy side of 862.0 and 880.0 eV, confirming the state of Ni2+ in nanosheets.46 The main peaks located at 232.6 and 235.7 eV in the Mo 3d spectrum are assigned to Mo6+ state.47 Besides, the O 1s XPS peak is determined to be 530.7 eV, corresponding to O2- state.48 For the S 2p spectrum,



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the S 2p1/2 peak at 162.6 eV is fitted, which can be assigned to the terminal S2- ligands.49 Using the semi-quantitative analysis of XPS, the atomic ratio is further confirmed to be 1:1:3:1 for Ni : Mo : O : S. Based on the above analysis, we further confirm the successful synthesis of NiMoO4-xSx.

Figure 2. XPS spectrum of as-prepared NiMoO4-xSx nanosheets. (a) Ni 2p, (b) Mo 3d, (c) O 1s and (d) S 2p. To evaluate the light harvesting capability of samples, the UV-vis diffused reflection spectra

of

Si

wafer,

TiO2/Si,

NiMoO4-xSx/TiO2/Si

and

TiO2/NiMoO4-xSx/TiO2/Si

photocathodes were measured and shown in Figure 3. The pristine Si wafer shows the highest reflectivity due to its smooth surface. After depositing TiO2 thin films, the reflection displays a slight decrease. The loading of NiMoO4-xSx cocatalysts dramatically reduces the reflection


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in the measured spectral range from 300 to 900 nm, indicating enhanced light trapping ability of the NiMoO4-xSx/TiO2/Si photocathodes. The deposition of TiO2 on NiMoO4-xSx/TiO2/Si further suppresses the reflection, particularly in the ultraviolet light region. The improved light harvesting efficiency may contribute to the enhancement in photocurrent of PEC water splitting.

Figure

3. Reflectance spectra of pristine Si, TiO2/Si, NiMoO4-xSx/TiO2/Si and

TiO2/NiMoO4-xSx/TiO2/Si wafers. The PEC performance of the as-prepared photocathodes was investigated in 0.1 M NaH2PO4 solution using a three-electrode cell configuration. The J-V curves of photocathodes are shown in Figure 4. The dark currents are shown as dashed lines. Compared with the pristine Si wafer and TiO2/Si wafer, TiO2/NiMoO4-xSx/TiO2/Si exhibits a more positive onset potential and a larger photocurrent density (Figure 4a). Specially, for the pristine Si wafer, the photocurrent density is 0.43 mA cm-2 at -0.5 V (vs. RHE), while it is 0.79 mA cm-2 for the TiO2 (2 nm)/Si at the same potential, representing 2-fold enhancement than that of pure Si wafer. The onset potential also shifts from -0.04 V to 0.07 V (vs. RHE) after the deposition of TiO2 on Si wafer. For the TiO2/NiMoO4-xSx (30 min)/TiO2/Si electrode, the photocurrent



gradually increases without saturation upon increasing the bias potential and a high photocurrent density of 5.1 mA cm-2 is achieved at -0.5 V (vs. RHE). This photocurrent value is about 12 and 6.5 times higher than bare Si and TiO2/Si photoelectrode, respectively. Moreover, the onset voltage of the TiO2/NiMoO4-xSx/TiO2/Si electrode positively shifts largely from 0.07 V to 0.3 V (vs. RHE) compared to TiO2/Si, which suggests enhanced transfer of photogenerated carriers and thus reduced recombination of electrons with holes in valence band. To investigate the function of S elements, we also study the PEC performance of TiO2/NiMoO4/TiO2/Si wafer (Figure S6), which exhibits much smaller photocurrent density compared to TiO2/ NiMoO4-xSx/TiO2/Si and tends to reach saturation at the more negative applied potential. The result suggests that the incorporation of S element indeed promotes the PEC activity. The enhanced photocurrent and positively shifted onset potential demonstrate that the NiMoO4-xSx cocatalysts facilitate the PEC water splitting reaction on the Si surface. In order to investigate the effect of ALD TiO2 film on the HER activity of Si wafer electrode, we performed electrochemical measurement for the TiO2/Si wafer. The J-V curves of TiO2/Si electrodes with different thicknesses of TiO2 films (2, 5 and 10 nm) are shown in Figure 4b, revealing that the photocurrent decreases as the TiO2 thickness increases. This is because the photogenerated carriers have to travel longer distance through thicker TiO2 film to participate in the reaction at the photocathode/electrolyte interface, leading to more recombination and poor photoelectrical property. Figure 4c shows J-V curves collected from the TiO2/NiMoO4-xSx/TiO2/Si photocathodes as a function of NiMoO4-xSx reaction time in the dark and under light illumination. When the reaction time increases from 20 to 40 min, the


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photocurrent density first increases and then decreases. The sample with the 30 min reaction exhibits the maximum photocurrent. Therefore, there is the optimal amount (reaction time) of NiMoO4-xSx nanosheets, because amorphous NiMoO4-xSx nanosheets possessing intrinsic defects would cause recombination although more NiMoO4-xSx nanosheets can provide increased reaction sites for higher catalytic activity of hydrogen evolution. Thus, the improvement of photocurrent is mainly attributed to the excellent catalytic properties of NiMoO4-xSx nanosheets besides the improved light harvesting capability. This is also verified by the following EIS and Mott-Schottky (M-S) results.

Figure 4. (a-c) Linear sweep J-V measurements under illumination: (a) Si, TiO2/Si and TiO2/NiMoO4-xSx/TiO2/Si wafers. (b) TiO2/Si wafer electrodes with different thicknesses of ALD TiO2 films. (c) TiO2/NiMoO4-xSx/TiO2/Si wafer electrodes with different reaction time of NiMoO4-xSx nanosheets. (d) I-t curves at an applied bias of 0 V vs. RHE.



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The I-t measurement was carried out at a bias potential of 0 V vs. RHE with several light on/off cycles to study the charge transfer characteristics of samples over time, as shown in Figure 4d. Upon illumination, the photocurrent rapidly reaches a steady-state value and returns to a negligible value once the light is switched off. The momentary photocurrent density (0.5 mA cm-2) of TiO2/NiMoO4-xSx/TiO2/Si photocathode is much larger than those of the Si wafer (0.03 mA cm-2) and TiO2/Si (0.06 mA cm-2), which is in agreement with J-V results. Furthermore, there are some distinguishable spikes in the I-t curves for the Si wafer and TiO2/Si electrode, while the TiO2/NiMoO4-xSx/TiO2/Si possesses no obvious spikes. The disappearance of spikes evidences the efficient charge transport ability of photoelectrodes decorated by NiMoO4-xSx nanosheets. In Figure S7, the long-term measurement over 1500 s demonstrates that the TiO2/NiMoO4-xSx/TiO2/Si has better stability as compared with NiMoO4-xSx/TiO2/Si. The inserted photographs in Figure S7 show the surface morphology of photocathodes

after

measuring

for

TiO2/NiMoO4-xSx/TiO2/Si

(Figure

S7a)

and

NiMoO4-xSx/TiO2/Si (Figure S7b), respectively, which indicate that the former maintains original color while the latter becomes pale compared with pristine samples. This suggests that the TiO2 film serves as a good anti-corrosion material, which can effectively protect NiMoO4-xSx/TiO2/Si from degradation of electrolyte and photocorrosion. This conclusion can also be verified by the comparison between Si wafer and TiO2/Si wafer electrode in Figure S8.


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Figure 5. IPCE spectra of Si, TiO2/Si and TiO2/NiMoO4-xSx/TiO2/Si wafers measured at an applied bias of -1 V vs. RHE. To characterize the photoactivity of electrodes at various wavelengths, the IPCE was measured at -1 V vs. RHE (Figure 5). In contrast to Si wafer and TiO2/Si, the IPCE of TiO2/NiMoO4-xSx/TiO2/Si is higher across the whole absorption range. The maximum value for TiO2/NiMoO4-xSx/TiO2/Si is about 60% at the range of 650-800 nm, while the peak value is only 30% and 40% for Si wafer and TiO2/Si, respectively. This result indicates that the TiO2/NiMoO4-xSx/TiO2/Si possesses more efficient separation and transport of photogenerated electron-hole pairs.

Figure 6. EIS spectra measured at an applied bias of 0V vs. RHE. (a) Nyquist plots under light illumination, and (b) corresponding Bode phase plots.



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The EIS is an operative method for appraising the interface charge transfer resistance and further comprehending the appreciable advancement of PEC water reduction behavior. We measured the EIS spectra of samples under light illumination at a bias potential of 0 V vs. RHE, as shown in Figure 6. For the Nyquist plot, a smaller arc radius generally represents faster charge transfer process and more effective charge separation. It is observed that TiO2/NiMoO4-xSx/TiO2/Si has the smallest arc radius, suggesting its low charge transfer resistance and superior charge transport performance (Figure 6a). Correspondingly, in the Bode phase plot (Figure 6b), the carrier lifetime is calculated as the reciprocal of frequency that corresponding to the highest phase value. The carrier lifetime is 3.5 ms, 2.5 s and 0.17 s for

Si

wafer,

TiO2/Si

and

TiO2/NiMoO4-xSx/TiO2/Si,

respectively.

Clearly,

the

TiO2/NiMoO4-xSx/TiO2/Si has prolonged electron lifetime compared to the pristine Si wafer. For TiO2/Si, although its frequency value is smaller than that for TiO2/NiMoO4-xSx/TiO2/Si, the larger charge transfer resistance results in inferior PEC performance.

Figure 7. M-S plots of Si, TiO2/Si and TiO2/NiMoO4-xSx/TiO2/Si wafers. M-S analysis is employed to determine the carrier density and flat band potential of the electrodes according to the following equation:50


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1/C2 = (2 / e0εε0NDA2) [(V-VFB)-κT/e0]

(3)

where C is the specific capacity, ε is the dielectric constant of NiMoO4-xSx or Si, ε0 is the electric permittivity of vacuum, ND is the carrier density, A is the area, V is the applied potential, VFB is the flat band potential, κ is the Boltzmann constant, T is the absolute temperature, and e0 is the electron charge. In Figure 7, all the samples exhibit negative slopes, indicating their p-type semiconducting feature. The flat band potentials are settled from the intercepts of 1/C2 vs. V by subtracting κT/e0 = 0.025V from the intercept. As depicted in M-S plots, the VFB values of Si wafer, TiO2/Si and TiO2/NiMoO4-xSx/TiO2/Si anodically shift sequentially, implying that NiMoO4-xSx effectively accelerates the water reduction kinetics of pristine Si. Benefiting from the improved transfer of photogenerated electrons from Si to the NiMoO4-xSx layer, the recombination of electrons with valence band holes in Si would be remarkably suppressed. Therefore, the photocurrent onset potential obviously displays positive shift, as evidenced in the J-V curves. On the other hand, the charge carrier density (ND) is inversely proportional to the 1/C2 as deduced from the M-S equation. It is observed that the ND of Si wafer is dramatically increased after the decoration of NiMoO4-xSx. These results prove that NiMoO4-xSx nanosheets effectively improve the carrier transfer properties of Si electrode, leading to the enhancement in PEC activity.

CONCLUSIONS In summary, we have successfully designed and synthesized NiMoO4-xSx nanosheets that greatly boost the PEC performance of pristine p-Si wafer. To overcome the lability of Si wafer, TiO2 film was deposited on the surface to act as a protective layer using ALD technique. The



TiO2 coated Si electrode demonstrated higher photocurrent density and more positive onset potential than Si wafer. Through further depositing NiMoO4-xSx nanosheets and thin ALD-TiO2 film on the TiO2/Si wafer, the resulting TiO2/NiMoO4-xSx/TiO2/Si photocathode delivered 12 times higher photocurrent density than pristine Si wafer, with the onset potential positively shifting 0.34 V. The greatly enhanced PEC performance was attributed to the improved light absorption, effective charge separation and transfer, reduced recombination and accelerated hydrogen evolution kinetics by the introduction of NiMoO4-xSx nanosheets and TiO2 thin film on Si wafer.


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ASSOCIATED CONTENT

Supporting Information Details of morphology and surface characterization, and long-term photoelectrochemical measurements.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research was supported by the National Natural Science Foundation of China (51772197, 51502184, 51422206), Key University Science Research Project of Jiangsu Province (17KJA430013), 1000 Youth Talents Plan, 333 High-level Talents Cultivation Project of Jiangsu Province, Six Talents Peak Project of Jiangsu Province, Postgraduate Research & Practice Innovation Program of Jiangsu Province, and Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).



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