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Tailored NiO/Ni cocatalysts on silicon for highly efficient water splitting photoanodes via pulsed electrodeposition Sol A Lee, Tae Hyung Lee, Changyeon Kim, Mi Gyoung Lee, MinJu Choi, Hoonkee Park, Seokhoon Choi, Jihun Oh, and Ho Won Jang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01999 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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ACS Catalysis

Tailored NiOx/Ni cocatalysts on silicon for highly efficient water splitting photoanodes via pulsed electrodeposition Sol A Lee, † Tae Hyung Lee, † Changyeon Kim, † Mi Gyoung Lee, † Min-Ju Choi, † Hoonkee Park, †

Seokhoon Choi, † Jihun Oh *,‡, and Ho Won Jang*,†

† Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea. ‡ Graduate School of EEWS (Energy, Environment, Water and Sustainability), Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea.

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Table of Contents

ABSTRACT Converting solar energy by photoelectrochemical water splitting has been regarded as a promising way to resolve the global energy crisis and alleviate environmental pollution. Silicon, which is earth-abundant and has a narrow band gap, is an attracting material for photoelectrochemical water splitting. However, Si-based photoelectrodes suffer from photocorrosion which leads to instability in electrolytes and high overpotential. Herein, we have fabricated a metal-insulator-semiconductor structure of NiOx/Ni/n-Si photoanodes for highly efficient water splitting. NiOx/Ni nanoparticles, which act as well-known oxygen evolution catalysts, are deposited on the surface of silicon by facile pulsed electrodeposition. Light absorption and catalytic activity are greatly affected by the coverage of Ni nanoparticles and the highly efficient NiOx/Ni catalyst structure is induced by simple annealing. The NiOx/Ni nanoparticles show highly enhanced charge separation and transport efficiency which are vital factors for photoelectrochemical water splitting, leading to ~100% faradaic efficiency and incident-photon-to-current efficiency. A low onset potential of 1.08 V versus a reversible hydrogen electrode for 1 mA/cm2 and a high photocurrent density of 14.7 mA/cm2 at 1.23 V are obtained.


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KEYWORDS Photoelectrochemical water splitting, Si photoanode, Oxygen evolution reaction, Cocatalyst, Pulsed electrodeposition

Photoelectrochemical (PEC) water splitting is a promising way to convert solar energy, an infinite energy source, into hydrogen, a renewable, carbon-free energy source with high energy density.1–6 A solar water splitting cell consists of a photocathode which reduces water to generate hydrogen and/or a photoanode which oxidizes water to form oxygen. For efficient water splitting, the photoelectrodes require semiconductor materials that are highly photoactive and durable. In particular, it is necessary to concentrate on the development of a photoanode because the oxygen evolution reaction at the surface of the photoanode involves a four electron transfer process, which is a bottleneck in water splitting.7–9 Silicon is a promising candidate for the photoanode because it has high carrier mobility and a small bandgap that can absorb a wide range of the solar spectrum, and high crystallinity and large area wafers are achievable at relatively low cost. However, silicon photoelectrodes have the problem of photocorrosion in the electrolyte due to the position of their thermodynamic oxidation potential which is more negative than that of water oxidation.10–14 A corrosion of the silicon surface induces surface defects, which trap electrons in surface states. This leads to charge recombination which is detrimental for photoelectrochemical water splitting. Therefore, studies have been carried out to prevent the surface from being photocorrosive using catalytic layers of metals and metal oxides.7,15–17 The surface protective layer not only facilitates the charge separation and transport of photogenerated carriers by using photoactive oxygen evolution catalysts but also protects the


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surface of silicon from the electrolyte by eliminating the surface states of Si. In particular, Ni, Co and NiOx catalysts have been studied for their high catalytic activity for the oxygen evolution reaction (OER).9,18 As a protective layer of silicon photoanode, Ni thin films deposited by an ebeam evaporator protected the silicon surface well, demonstrated by Kenney et al.19 Lewis’s group has reported deposition of NiOx by sputtering and CoOx by ALD, which showed highly stable photoanodes for 1700 h and 100 days, respectively.20,21 The interface between the silicon substrate and the protection layer is being intensively studied recently.22,23 However, deposition of protective layer by complicated high-vacuum processes, such as atomic layer deposition, evaporation, and sputtering, has the critical limit of high cost to application to commercialization. Therefore, recent studies have shown that electrodeposition, which is a facile solution process, can be an alternative to high-vacuum processes.24,25 Electrodeposition, which is a commercialized technology, has the advantage of not only significantly reducing synthesis time and costs but also easily controlling the reaction by changing the deposition voltage or current.24,26 Yu et al. has presented electrodeposited NiFe alloys on np+-Si, which showed high performance for the OER.27 Switzer’s group has reported inhomogeneous n-Si/SiOx/Co/CoOOH photoanodes showing large photovoltage.28 Zou’s group has demonstrated core-shell Ni@Ni(OH)2 particles for efficient oxygen evolution catalysts.17 In particular, these reports have shown the effects of inhomogeneous metal-insulator-semiconductor (MIS) structure, leading to high photovoltage independent of solution potential. Junction of Si and high work function metal can facilitate the charge separation and transport by band bending. Herein, we fabricated photoactive Ni particles on Si substrate by simply controlling the coverage by pulsed-electrodeposition and investigated the effect of post-annealing on the microstructure of Ni particles. The photoactivities of the fabricated photoanodes were affected by


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the amount of particles on the Si substrate and the presence of the thin NiOx shell. We conducted a qualitative investigation of the relationship between the coverage of the NiOx/Ni nanoparticles and the photoactivity of the nanoparticles. The core-shell structure of the Ni/NiOx nanoparticles with optimal conditions can enhance the charge separation of photogenerated carriers and yields ~100% quantum efficiency. The NiOx/Ni/n-Si photoanodes recorded a photocurrent of 14.7 mA/cm2 at 1.23 V vs. RHE without additional catalysts. An onset potential of 1.08 V vs. RHE and saturated current density of 31.7 mA/cm2 were recorded.

RESULTS AND DISCUSSION

Figure 1. (a) Schematic illustration of pulsed electrodeposition of Ni nanoparticles on a silicon substrate for solar water splitting. (b) Pulsed electrodeposition of Ni nanoparticles. (c) X-ray diffraction data of Ni nanoparticles by changing annealing conditions. (d-g) FESEM images with Ni/n-Si with various number of deposition cycles: (d) 1 cycle, (e) 4 cycles, (f) 12 cycles and (g) 24 cycles. The schematic of the pulsed electrodeposition used in this study is shown in Figure 1(a). Ni2+ ions are reduced to be Ni metal on the silicon surface applied at a constant current of -0.8 mA/cm2 vs. SCE by the electrochemical method.29 The amplitude and duration of the pulse for synthesizing Ni nanoparticles used in this study by pulsed electrodeposition are shown in Figure 1(b).


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Considering that the electrochemical reaction occurs at the semiconductor-electrolyte surface, it is possible to fabricate more uniform films by pulsed electrodeposition rather than continuous electrodeposition since the ions can be replenished during the pulse off-time, which leads to decreasing the concentration gradient.24,26,30,31 Figure 1(c) shows the grazing incidence X-ray diffraction patterns of electrodeposited films by changing the annealing temperature. The asdeposited nanoparticles and nanoparticles annealed at 300 °C correspond to Ni phase XRD peaks. After annealing at 400 °C, it became a NiO phase.32,33 The field emission scanning electron microscopy (FESEM) images with different number of deposition cycles are shown in Figure 1(dg). The SEM images clearly indicate that coverage and size increased as the number of pulse cycles increased. For 1 and 4 cycle deposition, Ni nanoparticles are randomly dispersed whereas Ni nanoparticles begin to aggregate at 12 and 24 cycles deposition. Electrodeposited Ni is an attractive material which acts as both passivation layer and oxygen evolution catalysts.8,34,35 Nickel forms a heterojunction with the silicon which efficiently separate charges. Ni islands, which have high work function act as nanoemitter which effectively collect charges in MIS structure.16 Ni islands formed inhomogeneous junction, which can enlarge the Schottky barrier. However, excessive Ni nanoparticles in the MIS structure induced significant light absorption loss because of its nature of reflecting light. We compared the PEC performance with different coverage of Ni nanoparticles on n-type silicon which is denoted as x-Ni/n-Si, where x is the number of deposition cycles. (Figure S1). The linear sweep voltammetry (LSV) measurement clearly shows the impact of Ni coverage on the silicon surface. The saturated photocurrent density and photovoltage decreased when excessive coverage of Ni nanoparticles formed and the onset potential toward the anodic direction. The shift in onset potential can be explained by the fundamental junction characteristics which


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Figure 2. (a) Current-potential (J-V) curves of Ni/n-Si and NiOx/Ni/n-Si under AM 1.5G sunlight (100 mW/cm2) in 1 M NaOH aqueous solution. (b) Amperometric current density-time profile for Ni/n-Si and NiOx/Ni/n-Si. (c) Comparison of onset potential (V at 1 mA/cm2) versus number of deposition cycles. (d) Comparison of amperometric current density-time profiles with different number of deposition cycles. depend on the thickness of the Ni layer.16 To elucidate the effects of annealing in the photovoltage difference, Mott-Schottky analyses of Ni NPs with different annealing condition which were deposited on FTO were conducted (Figure S2). The negative slopes of the Mott-Schottky plot indicate that the synthesized nanoparticles show p-type behavior independent of annealing and the high capacitance of NiO coincides well with the previous reports.35–37 Compared to the flat band potential of Ni on FTO, NiOx/Ni NPs on FTO showed 0.7 V, which is 200 mV lower than Ni. This clearly shows that the shift of flat band potential leads to the difference in photovoltage, which is clearly illustrated when the coverage increases.


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In a high pH electrolyte, nickel hydroxide thin layer was formed during the OER. This layer is ion permeable, which affects the work function of core Ni nanoparticles, changing the Ni-Si Schottky barrier.34,38 We fabricated NiOx/Ni/n-Si photoanodes by simply annealing in the ambient atmosphere to form transparent NiOx layers which can influence the Schottky barrier of the MIS structure. Figure 2(a) shows the current density vs. potential (J-V) curves of the Ni/n-Si photoanode with/without annealing under 1 sun illumination. We define the onset potential in the J-V curve as the potential when the photocurrent density reaches 1 mA/cm2. The 4-Ni/n-Si photoanode without annealing showed an onset potential of 1.14 V, and the photocurrent density was measured at 1.23 V vs. RHE of 10.1 mA/cm2. The onset potential for 4-NiOx/Ni/n-Si photoanode with annealing was 1.08 V versus RHE which was lower than the photoanode without annealing. The effects of annealing were remarkable in the excessive deposition of Ni nanoparticles on the silicon substrate. The 24-Ni/n-Si photoanode without annealing showed the onset potential of 1.44 V and saturated current density of 12 mA/cm2. The annealed photoanodes with the same coverage showed the onset potential of 1.12 V, which was shifted to ~320 mV toward the cathodic direction and higher saturated photocurrent density of 22.1 mA/cm2. The amperometric photocurrent density-time profiles of Ni/n-Si and NiOx/Ni/n-Si photoanodes measured at 1.6 V vs. RHE are shown in Figure 2(b). All samples showed fast responses to the illumination. Figure 2(c) presents the onset potential (at 1 mA/cm2) of the photoanodes with various deposition cycles. Compared to the as-deposited Ni/n-Si photoanodes which depend on the amount of Ni nanoparticles, the annealed photoanodes showed almost constant values of onset potential. We observed the saturated current density at 1.8 V vs. RHE, as shown in Figure 2(d). In both photoanodes, the saturated current density decreased as the excessive Ni nanoparticles


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Figure 3. (a) XPS spectra of Ni 2p for the as-deposited Ni/n-Si photoanodes and annealed NiOx/Ni/n-Si photoanodes. (b) XPS spectra of O 1s for the as-deposited Ni/n-Si photoanodes and annealed NiOx/Ni/n-Si photoanodes. deposited. However, the degree of decrease in current density was smaller than Ni/n-Si photoanodes. X-ray photoelectron spectroscopy (XPS) was conducted to investigate chemical valence status of the as-deposited Ni/n-Si and annealed photoanodes. Figure 3(a) shows the Ni 2p spectrum of Ni/nSi with and without annealing, which can be deconvoluted into six peaks including two spin-orbit doublets of 2p3/2 and 2p1/2 and two satellite peaks of Ni 2p3/2 and Ni 2p1/2. The detailed spectrum of Ni 2p for photoanodes without annealing shows three main peaks of Ni 2p3/2 at 851, 854.7 and 860.1 eV, respectively. The peak at 851 eV is assigned to Ni0 and the peak at 854.7 eV is indicative of Ni2+ ions or nickel hydroxide. The binding energy difference of the Ni 2p3/2 and Ni 2p1/2 was 17.3 eV which was assigned to be metallic Ni0 and 17.5 eV for Ni2+. The Ni 2p spectrum of annealed NiOx/Ni/n-Si shows the multiplet-splits of Ni2+ peaks. The peak centered at 871.2 eV and 852.7 eV & 854.7 eV depicts the two edge split of the Ni2+ peaks of Ni 2p1/2 and Ni 2p3/2. The broad peak at 879.2 eV and 859.7 eV can be ascribed to shakeup process in the NiO structure. The binding energy difference between Ni 2p3/2 and Ni 2p1/2 was 17.6 eV, which clearly indicates Ni2+. Figure 3(b) shows the O 1s spectrum of the annealed Ni/n-Si photoanodes, which can be


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deconvoluted into three peaks. The peak at 528.6 eV corresponds to Ni-O bonding. The asdeposited photoanodes show distinct peaks at 530.3 eV and 531.9 eV which correspond to the chemical bonding state of oxygen in NiO and surface –OH groups, respectively. The sample after annealing shows an increased peak at 528.6 eV, which corresponds to Ni-O bonding and binding energy peaks at 530.3 eV and 532 eV, indicating O2- and surface –OH group, respectively.17,18,39– 44

Both samples showed high -OH peaks in the O 1s spectrum, which indicate that the annealing

process slightly change the surface of Ni nanoparticles to form NiOx. Transmission electron microscopy (TEM) was used to identify the morphology, element distributions, and material configurations of the samples. As shown in Figure 4, Ni nanoparticles are randomly deposited on the silicon surface. To analyse the elements of the fabricated photoanode, energy dispersive spectroscopy (EDS) was conducted. EDS revealed the difference in the presence of oxygen between samples without annealing and annealed samples. As shown in Figure 4(a) and (d), the EDS data clearly indicate that both particles consist of Ni element, but only the annealed sample shows a faint oxygen element along particle shape, which demonstrates that a thin NiOx layer was formed. After annealing, nickel silicide could be formed at the interface between silicon and the NiOx/Ni particle since Ni has high diffusivity in silicon.45,46 High-resolution transmission electron microscopy (HR-TEM) was used to elucidate the crystal structure of the synthesized nanoparticles on the Si substrate. The HR-TEM images and electron diffraction patterns of the as-deposited nanoparticles indicate d-spacing of 0.208 nm and 0.18 nm, which correspond to the (111) and (200) planes of 4-Ni NPs, respectively. (Figure 4(b) and (c)) After annealing the Ni nanoparticles, d-spacing of 0.208 nm and 0.241 nm, which match the (200) and (111) planes, indicate a NiOx phase, showing that NiOx/Ni nanoparticles formed (Figure 4(e) and (f)). Considering the XRD data, as shown in Figure 1(c) and TEM results, it can be clarified


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Figure 4. TEM images of as-deposited Ni/n-Si and annealed NiOx/Ni/n-Si. (a,d) Crosssectional TEM images of Ni/n-Si and corresponding energy dispersive spectroscopy (EDS) image of Si, Ni, and O distribution on Ni/n-Si and NiOx/Ni/n-Si. (b,e) High magnification of Ni/n-Si. Further magnification of Ni NPs (orange square) and NiOx/Ni NPs (green square) are shown in (c,f). Inset shows (111) and (200) planes of Ni and NiOx respectively. that the annealed nanoparticles are composed of a Ni core and a thin NiOx shell. Photoelectrochemical performances of the NiOx/Ni/n-Si photoanodes with different coverage were investigated. To draw a comparison with water splitting silicon based photoanodes previously reported to date, current density at 1.23 V vs. RHE, onset potentials at 1 mA/cm2 and saturated current density values are summarized in Table S2. PEC measurements of NiOx/Ni/n-Si photoanodes were performed using a standard three electrode cell with a 1 M NaOH electrolyte under AM 1.5G illumination. Figure 5(a) shows linear sweep voltammetry curves of NiOx/Ni/nSi photoanodes with different number of deposition cycles. The generated photovoltage of photoanodes were calculated by comparing the onset potential of n-Si and metallic p++-Si. The


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Figure 5. Photoelectrochemical performance of NiOx/Ni/n-Si heterojunction photoanodes. (a) Linear sweep voltammograms of NiOx/Ni/n-Si photoanodes with different number of cycles. (b) Electrochemical impedance spectra of NiOx/Ni/n-Si photoanodes. The equivalent circuits are shown inset. (c) IPCE of NiOx/Ni/n-Si photoanodes with various number of deposition cycles at 1.5 V vs. RHE. (d) Chronoamperometry of 12-NiOx/Ni/n-Si photoanode measured at 1.7 V vs. RHE in 1.0 M K-borate electrolyte under 1 sun illumination. The asterisks indicate the moment when 1 M K-borate was added. Stability test measured in 1 M NaOH is shown inset. (e) Oxygen evolution from water by NiOx/Ni/n-Si photoanodes. (f) Current density vs. time and faradaic efficiency (FE) for O2 vs. time. reversible redox peak (Ni3+/Ni2+) was observed in all cases, which corresponds to the oxidation of Ni/NiOx to NiOOH during cyclic voltammetry in alkaline media (Figure S3). The onset potential was almost independent of the number of deposition cycles since the formation of NiOx thin shells interacts with the Ni core, affecting the Schottky barrier. The maximum photocurrent density was clearly coverage-dependent and decreased after the optimum amount of NiOx/Ni nanoparticles. As shown in Figure S4, dense NiOx/Ni layer reflects the light, leading to significant light absorption loss. After annealing at 300 °C, the onset potential slightly decreased to 1.08 V, and the photocurrent density at 1.23 V (vs. RHE) reached a value of 14.7 mA/cm2. In contrast, the NiOx/nSi photoanodes annealed at 400 °C exhibited almost no photocurrent at the oxygen evolution


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potential (Figure S5). Nickel oxide is an efficient oxygen evolution catalyst. However, partially deposited nickel oxide has high resistance, which hinders charge separation and transport. The NiOx/Ni/n-Si photoanodes showed highly enhanced catalytic performances regardless of the coverage of nanoparticles since the nickel and nickel oxide itself act as powerful oxygen evolution catalysts. Figure 5(b) shows the electrochemical impedance spectroscopy (EIS) measurements which were performed to elucidate the kinetics of charge transfer during the OER. The impedance spectra were recorded at an external bias near the onset potential of samples to minimize the possible complex elements under 1 sun illumination. The small semicircle in the Nyquist plots indicates a fast charge transfer kinetics at the interface. The equivalent circuits used to fit the data are shown inset. The equivalent circuit consists of Rs, which indicates series resistance, the constant phase element (CPE) for the electrolyte/electrode interface, and Rct, which is related to the charge transfer resistance at the interface. The charge-transfer resistances of various degrees of coverage are summarized in Table S1. The Rct of the 4-NiOx/Ni/n-Si was 788.7 Ω∙cm2, the lowest value, confirming that the optimum coverage of Ni nanoparticles enhanced hole transfer to the redox potential. Incident-photon-to-current conversion efficiency (IPCE) measurements were conducted from 300 nm to 800 nm at the potential of the saturated current density. As shown in Figure 5(c), 4NiOx/Ni/n-Si showed ~100% over the visible spectrum, which indicates that fabricated photoanodes exhibit high quantum efficiency. In other words, the fabricated silicon-based photoanodes show great capacity for charge separation and transfer efficiency at the interfaces of silicon, NiOx/Ni nanoparticles and electrolyte. We calculated the charge injection efficiency by measuring LSV curves in 1 M NaOH with and without 0.5 M Na2SO3. The charge injection efficiency is the efficiency related to holes at the electrode/electrolyte interface used to oxidize


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water. As shown in Figure S6, the charge injection efficiency was over 90%, which shows that holes accumulated at the interface were used to oxidize water effectively. To evaluate whether the electrodeposited NiOx/Ni cocatalysts perform a role in passivation, the stability of the 12NiOx/Ni/n-Si photoanode was evaluated at a constant potential of 1.7 V versus RHE in 1.0 M Kborate electrolyte under 1 sun illumination by chronoamperometric measurements. As shown in Figure 5(d), the PEC performance showed no considerable degradation after 80 hours of continuous operation in 1.0 M K-borate. The stability of the 12-NiOx/Ni/n-Si photoanode showed ~7 h in 1 M NaOH with almost the same current density as shown inset. After 7 h, the performance of photoanode gradually decreased. Similar results can be observed in 4-NiOx/Ni/n-Si which showed a decrease in performance of ~15% after a 100 h stability test, as shown in Figure S7. The stability test at 1 M NaOH showed over 10000 s for 4-NiOx/Ni/n-Si. Compared to the high pH electrolyte, the degradation of NiOx/Ni/n-Si photoanode was significantly attenuated in K-borate electrolyte. Furthermore, those results clearly indicated that increasing coverage leads to enhanced stability. Gas chromatography (GC) was used to calculate the faradaic efficiency. The gas generated from the electrode was confirmed to be O2 (Figure 5(e)). The faradaic efficiency was up to 100%; the points higher than 100% are due to the bubbles captured in the surface of photoanode, which hinder light absorption, leading to a temporary decrease of current density (Figure 5(f)). The turnover frequency was calculated to confirm the ability of Ni nanoparticles as efficient OER catalysts.47 The amount of O2 was calculated by GC, and the number of Ni active sites were determined by measuring CV curves of Ni2+/Ni3+ (Figure S8). The calculated turnover frequency was 1.117 x 105 h-1. The PEC properties of our NiOx/Ni/n-Si photoanodes showed high performance without buried p-n junction, high vacuum process, or doping reported to date for water splitting photoanodes (Table S2).


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Figure 6. (a) Schematic of inhomogeneous NiOx/Ni NPs on n-Si. WNPs and Wplanar denote the space charge region of NiOx/Ni particles and the planar depletion width, respectively. (b) Proposed approximate relation between photoactivity and NiOx/Ni coverage. We would like to elucidate the factors that affect the photoelectrochemical properties of the fabricated Si-based photoanode. In light of the MIS structure, inhomogeneous NiOx/Ni nanoparticles showed a high photovoltage, which can be explained by pinch-off effect. As shown in Figure 6(a), NiOx/Ni islands induce spatially varied barrier heights at the interface where contact in the low barrier region is smaller than carrier depletion, leading to anomalous barrier height.16,28,48 In addition, a simple annealing process which forms a thin NiOx layer would affect the increased photoelectrochemical performance. The NiOx thin layer and the core Ni interact with each other, resulting in a flat band potential change. NiOx/Ni nanoparticles with annealing showed higher photovoltage than Ni nanoparticles independent of the coverage. However, a high coverage of nanoparticles will block the light absorption in silicon, which we call the shading effect of metal nanoparticles, leading to a lower saturated photocurrent density.49 In particular, dense electrodeposition of Ni nanoparticles without annealing affects the junction properties between nSi and Ni nanoparticles, leading to anodic onset shifts which lead to lower photovoltages. This trade-off relation clearly explains the importance of annealing and the moderate coverage of NiOx/Ni nanoparticles to achieve high PEC performance. Therefore, in light of the above relation,


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the optimum coverage for high photoactivity of the Si-based photoanode can be derived as shown in Figure 6(b).

CONCLUSION We have demonstrated highly efficient NiOx/Ni/n-Si photoanodes by using the facile pulsedelectrodeposition. The fabricated photoanodes with optimized coverage and the annealing condition showed photocurrent density of 14.7 mA/cm2 at 1.23 V vs. RHE by the annealing process. The enhanced PEC performance is affected by minimized shading effect of Ni nanoparticles and the presence of NiOx which showed high catalytic activity leading to decrease overpotential of the water oxidation. NiOx/Ni structure exhibited ~500 mV photovoltage, which was affected by the coverage and shift of the flat band potential. The fabricated photoanodes showed up to 100% faradaic efficiency and IPCE, clearly indicating the high quantum efficiency. We have demonstrated that the island-shaped NiOx/Ni nanoparticles work well as a passivation layer of silicon photoanodes. Our systematic methods qualitatively identified the relationship between particle coverage and the PEC performance of the material, emphasizing the importance of coverage and heat treatment. We anticipate that our research can be applied to various heterojunction systems.

EXPERIMENTAL Synthesis A n-Si (100) wafer was cleaned with acetone and isopropanol alcohol by ultrasonication. Before electrodeposition, ohmic contact was prepared by scratching the back side of silicon using a glass cutter and applying an InGa alloy (Sigma Aldrich). Then, copper wire was attached by depositing


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ACS Catalysis

silver paste on the InGa alloy. After the silver paste dried, the Si surface was covered with an adhesive tape, except for active area (0.5 cm x 0.5 cm), to prevent contact with the electrolyte. nSi was soaked for 30 seconds in buffer oxide etchant (6:1, J. T. Baker) to remove residual silicon oxide layer and organic solvent from the surface. Then, Ni was deposited on the silicon surface by modified pulsed electrodeposition. Ni aqueous plating solution was prepared by dissolving 0.1 M nickel sulfate hydrate (NiSO4∙6H2O, Daejung) and 0.1 M boric acid (H3BO3, JUNSEI). Pulsed electrodeposition was conducted in a standard three electrode system with an encapsulated Si electrode as the working electrode, a Pt plate as a counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Deposition of Ni nanoparticles was performed by applying -0.2 mA vs. SCE on the silicon surface. After electrodeposition, prepared samples were rinsed with deionized water and adhesive tapes were detached. Samples were annealed at 300 °C for 1 h in the ambient atmosphere.

Characterization Grazing Incidence X-ray Diffraction (GIXRD, X-pert) analysis was conducted to confirm the crystalline phase of Ni and NiO. The morphology of the Ni nanoparticles was examined by field emission scanning electron microscopy (MERLIN Compact, JEISS) and transmission electron microscope (JEM-2100F, JEOL). Transmittance of Ni nanoparticles was determined by UV-Vis characterization. XPS analysis was carried out to confirm the surface bonding of Ni/n-Si and NiOx/Ni/n-Si photoanodes. To clarify the exact bonding form of Ni, the narrow Ni spectrum was analyzed using XPSPEAK 41 software.

PEC measurements


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PEC measurements (Ivium Technologies, Nstat) were carried out with a three electrode system using a Ag/AgCl (sat.) reference electrode and a Pt plate as a counter electrode in a 1 M sodium hydroxide electrolyte. A Xe arc lamp was used as a light source and the light intensity was calculated to 1 sun (100 mW/cm2, AM 1.5G) using a reference photodiode. For measuring PEC performance, the potential was swept toward the anodic direction. The incident-photon-to-current conversion efficiency was carried out using a monochromator (MonoRa150) and light source. The applied potential was 1.5 V vs RHE, considering the voltage at the saturated current density. Electrochemical impedance spectroscopy (EIS) was conducted by applying 0.05 V vs. Ag/AgCl near the onset potential. The EIS data were fit to the equivalent circuits which were discussed in the text, using the Z plot 2.x software. The sweeping frequency ranged from 10000 Hz to 0.1 Hz using alternating current with an amplitude of 10 mV. The measured potential versus Ag/AgCl was converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation. 𝐸

The 𝐸

=𝐸

/

+ 0.059 pH + 𝐸

is the converted potential versus RHE, 𝐸

/

/

= 0.198 V at 25 °C and 𝐸

/

is

the experimentally measured potential versus the Ag/AgCl reference. Mott-Schottky analysis was conducted at 50 kHz with an amplitude of 10 mV. The gas chromatography system (FID-GC, PerkinElmer, NARL8502 Model 4003) was used to calculate the faradaic efficiency. The LSV curves in 1.0 M NaOH with or without 0.5 M 𝑁𝑎 𝑆𝑂 which was used as hole scavenger were used to calculate the charge injection efficiency (Φinj) at the electrode/electrolyte interface: =𝐽

𝐽 𝐽

ˣ𝛷 =𝐽

ˣ𝛷 ˣ𝛷


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ACS Catalysis

where 𝐽

is the observed photocurrent density and Jabs is the photocurrent density assuming that

absorbed photons are converted to current completely. The turnover frequency (TOF) was calculated by the produced moles of oxygen and the number of moles of the active sites; produced moles of oxygen was determined by gas chromatography and the number of moles of the active sites were calculated by the cyclic voltammogram of Ni. The turnover number (TON) and turnover frequency (TOF) are defined as follow: TON =

,

TOF =

ASSOCIATED CONTENT Supporting Information Additional information and figures. This material is available free of charge via the Internet at http://pubs.acs.org. Figures S1-S8; Linear sweep voltammetry, Mott-Schottky plots, Cyclic voltammetry, Transmittance, Table S1 and S2.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was supported by the Samsung Research Funding Center of Samsung Electronics. Sol A Lee acknowledges the Global Ph.D. Fellowship Program through the National Research Foundation of Korea funded by the Ministry of Education (2017H1A2A1044293). REFERENCES (1)

Matheu, R.; Moreno-Hernandez, I. A.; Sala, X.; Gray, H. B.; Brunschwig, B. S.; Llobet, A.; Lewis, N. S. Photoelectrochemical Behavior of a Molecular Ru-Based WaterOxidation Catalyst Bound to TiO2-Protected Si Photoanodes. J. Am. Chem. Soc. 2017, 139, 11345–11348.

(2)

Kuang, Y.; Jia, Q.; Ma, G.; Hisatomi, T.; Minegishi, T.; Nishiyama, H.; Nakabayashi, M.; Shibata, N.; Yamada, T.; Kudo, A.; Domen, K. Ultrastable Low-Bias Water Splitting Photoanodes via Photocorrosion Inhibition and In Situ Catalyst Regeneration. Nat. Energy 2017, 2, 16191.

(3)

Kwon, K. C.; Choi, S.; Lee, J.; Hong, K.; Sohn, W.; Andoshe, D. M.; Choi, K. S.; Kim, Y.; Han, S.; Kim, S. Y.; Jang, H. W. Drastically Enhanced Hydrogen Evolution Activity by 2D to 3D Structural Transition in Anion-Engineered Molybdenum Disulfide Thin Films for Efficient Si-Based Water Splitting Photocathodes. J. Mater. Chem. A 2017, 5, 15534–15542.

(4)

Li, S.; Zhang, P.; Xie, X.; Song, X.; Liu, J.; Zhao, L.; Chen, H.; Gao, L. Enhanced Photoelectrochemical Performance of Planar p-Silicon by APCVD Deposition of Surface Mesoporous Hematite Coating. Appl. Catal. B Environ. 2017, 200, 372–377.

(5)

Kuang, P. Y.; Su, Y. Z.; Chen, G. F.; Luo, Z.; Xing, S. Y.; Li, N.; Liu, Z. Q. G-C3N4 decorated ZnO Nanorod Arrays for Enhanced Photoelectrocatalytic Performance. Appl. Surf. Sci. 2015, 358, 296–303.

(6)

Phuan, Y. W.; Ibrahim, E.; Chong, M. N.; Zhu, T.; Lee, B. K.; Ocon, J. D.; Chan, E. S. In Situ Ni-Doping during Cathodic Electrodeposition of Hematite for Excellent Photoelectrochemical Performance of Nanostructured Nickel Oxide-Hematite p-n Junction Photoanode. Appl. Surf. Sci. 2017, 392, 144–152.

(7)

Mei, B.; Permyakova, A. A.; Frydendal, R.; Bae, D.; Pedersen, T.; Malacrida, P.; Hansen, O.; Stephens, I. E. L.; Vesborg, P. C. K.; Seger, B.; Chorkendorff, I. Iron-Treated NiO as a Highly Transparent p-Type Protection Layer for Efficient Si-Based Photoanodes. J. Phys. Chem. Lett. 2014, 5, 3456–3461.

(8)

Ding, C.; Shi, J.; Wang, Z.; Li, C. Photoelectrocatalytic Water Splitting: Significance of Cocatalysts, Electrolyte, and Interfaces. ACS Catal. 2017, 7, 675–688.


Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(9)

Park, M. J.; Jung, J. Y.; Shin, S. M.; Song, J. W.; Nam, Y. H.; Kim, D. H.; Lee, J. H. Photoelectrochemical Oxygen Evolution Improved by a Thin Al2O3 Interlayer in a NiOx/n-Si Photoanode. Thin Solid Films 2016, 599, 54–58.

(10) Yang, J.; Walczak, K.; Anzenberg, E.; Toma, F. M.; Yuan, G.; Beeman, J.; Schwartzberg, A.; Lin, Y.; Hettick, M.; Javey, A.; Ager, J. W.; Yano, J.; Frei, H.; Sharp, I. D. Efficient and Sustained Photoelectrochemical Water Oxidation by Cobalt Oxide/Silicon Photoanodes with Nanotextured Interfaces. J. Am. Chem. Soc. 2014, 136, 6191–6194. (11) Chakthranont, P.; Hellstern, T. R.; McEnaney, J. M.; Jaramillo, T. F. Design and Fabrication of a Precious Metal-Free Tandem Core–Shell p+n Si/W-Doped BiVO4 Photoanode for Unassisted Water Splitting. Adv. Energy Mater. 2017, 7, 1701515. (12) Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J. H.; Nocera, D. G. Wireless Solar Water Splitting Using Silicon-Based Semiconductors and EarthAbundant Catalysts. Science 2011, 334, 645–648. (13) Oh, J.; Deutsch, T. G.; Yuan, H.-C.; Branz, H. M. Nanoporous Black Silicon Photocathode for H2 Production by Photoelectrochemical Water Splitting. Energy Environ. Sci. 2011, 4, 1690-1694. (14) McDowell, M. T.; Lichterman, M. F.; Carim, A. I.; Liu, R.; Hu, S.; Brunschwig, B. S.; Lewis, N. S. The Influence of Structure and Processing on the Behavior of TiO2 Protective Layers for Stabilization of n-Si/TiO2/Ni Photoanodes for Water Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 15189–15199. (15) Strandwitz, N. C.; Comstock, D. J.; Grimm, R. L.; Nichols-Nielander, A. C.; Elam, J.; Lewis, N. S. Photoelectrochemical Behavior of n-Type Si(100) Electrodes Coated with Thin Films of Manganese Oxide Grown by Atomic Layer Deposition. J. Phys. Chem. C 2013, 117, 4931–4936. (16) Laskowski, F. A. L.; Nellist, M. R.; Venkatkarthick, R.; Boettcher, S. W. Junction Behavior of n-Si Photoanodes Protected by Thin Ni Elucidated from Dual Working Electrode Photoelectrochemistry. Energy Environ. Sci. 2017, 10, 570–579. (17) Xu, G.; Xu, Z.; Shi, Z.; Pei, L.; Yan, S.; Gu, Z.; Zou, Z. Silicon Photoanodes Partially Covered by Ni@Ni(OH)2 Core-Shell Particles for Photoelectrochemical Water Oxidation. ChemSusChem 2017, 10, 2897–2903. (18) Wu, F.; Liao, Q.; Cao, F.; Li, L.; Zhang, Y. Non-Noble Bimetallic NiMoO4 Nanosheets Integrated Si Photoanodes for Highly Efficient and Stable Solar Water Splitting. Nano Energy 2017, 34, 8–14. (19) Kenney, M. J.; Gong, M.; Li, Y.; Wu, J. Z.; Feng, J.; Lanza, M.; Dai, H. HighPerformance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation. Science 2013, 342, 836–840.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

(20) Zhou, X.; Liu, R.; Sun, K.; Friedrich, D.; McDowell, M. T.; Yang, F.; Omelchenko, S. T.; Saadi, F. H.; Nielander, A. C.; Yalamanchili, S.; Papadantonakis, K. M.; Brunschwig, B. S.; Lewis, N. S. Interface Engineering of the Photoelectrochemical Performance of NiOxide-Coated n-Si Photoanodes by Atomic-Layer Deposition of Ultrathin Films of Cobalt Oxide. Energy Environ. Sci. 2015, 8, 2644–2649. (21) Zhou, X.; Liu, R.; Sun, K.; Papadantonakis, K. M.; Brunschwig, B. S.; Lewis, N. S. 570 mV Photovoltage, Stabilized n-Si/CoOx Heterojunction Photoanodes Fabricated Using Atomic Layer Deposition. Energy Environ. Sci. 2016, 9, 892–897. (22) Shi, Y.; Han, T.; Gimbert-Suriñach, C.; Song, X.; Lanza, M.; Llobet, A. Substitution of Native Silicon Oxide by Titanium in Ni-Coated Silicon Photoanodes for Water Splitting Solar Cells. J. Mater. Chem. A 2017, 5, 1996–2003. (23) 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. (24) Park, I. J.; Kang, G.; Park, M. A.; Kim, J. S.; Seo, S. W.; Kim, D. H.; Zhu, K.; Park, T.; Kim, J. Y. Highly Efficient and Uniform 1 cm2 Perovskite Solar Cells with an Electrochemically Deposited NiOx Hole-Extraction Layer. ChemSusChem 2017, 10, 2660–2667. (25) Loget, G.; Fabre, B.; Fryars, S.; Mériadec, C.; Ababou-Girard, S. Dispersed Ni Nanoparticles Stabilize Silicon Photoanodes for Efficient and Inexpensive SunlightAssisted Water Oxidation. ACS Energy Lett. 2017, 2, 569–573. (26) Lee, M. G.; Kim, D. H.; Sohn, W.; Moon, C. W.; Park, H.; Lee, S.; Jang, H. W. Conformally Coated BiVO4 Nanodots on Porosity-Controlled WO3 Nanorods as Highly Efficient Type II Heterojunction Photoanodes for Water Oxidation. Nano Energy 2016, 28, 250–260. (27) Yu, X.; Yang, P.; Chen, S.; Zhang, M.; Shi, G. NiFe Alloy Protected Silicon Photoanode for Efficient Water Splitting. Adv. Energy Mater. 2017, 7, 1601805. (28) Hill, J. C.; Landers, A. T.; Switzer, J. A. An Electrodeposited Inhomogeneous MetalInsulator-Semiconductor Junction for Efficient Photoelectrochemical Water Oxidation. Nat. Mater. 2015, 14, 1150–1155. (29) Kang, D.; Kim, T. W.; Kubota, S. R.; Cardiel, A. C.; Cha, H. G.; Choi, K. S. Electrochemical Synthesis of Photoelectrodes and Catalysts for Use in Solar Water Splitting. Chem. Rev. 2015, 115, 12839–12887. (30) Wasekar, N. P.; Haridoss, P.; Seshadri, S. K.; Sundararajan, G. Influence of Mode of Electrodeposition, Current Density and Saccharin on the Microstructure and Hardness of Electrodeposited Nanocrystalline Nickel Coatings. Surf. Coatings Technol. 2016, 291, 130–140.


Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(31) Kim, W.; Weil, R. Pulse Plating Effects in Nickel Electrodeposition. Surf. Coatings Technol. 1989, 38, 289–298. (32) El-Sherik, A. M.; Erb, U. Synthesis of Bulk Nanocrystalline Nickel by Pulsed Electrodeposition. J. Mater. Sci. 1995, 30, 5743–5749. (33) Adhikari, S.; Madras, G. Role of Ni in Hetero-Architectured NiO/Ni Composites for Enhanced Catalytic Performance. Phys. Chem. Chem. Phys. 2017, 19, 13895–13908. (34) Han, T.; Shi, Y.; Song, X.; Mio, A.; Valenti, L.; Hui, F.; Privitera, S.; Lombardo, S.; Lanza, M. Ageing Mechanisms of Highly Active and Stable Nickel-Coated Silicon Photoanodes for Water Splitting. J. Mater. Chem. A 2016, 4, 8053–8060. (35) Varghese, B.; Reddy, M. V.; Yanwu, Z.; Lit, C.S.; Hoong, T. C.; Rao, G. V. Subba.; Chowdari, B. V. R.; Wee, A. T. S.; Lim, C. T.; Sow, C.H. Fabrication of NiO Nanowall Electrodes for High Performance Lithium Ion Battery. Chem. Mater. 2008, 20, 3360– 3367. (36) Sun, X.; Si, W.; Liu, X.; Deng, J.; Xi, L.; Liu, L.; Yan, C.; Schmidt, O. G. Multifunctional Ni/NiO Hybrid Nanomembranes as Anode Materials for High-Rate Li-Ion Batteries. Nano Energy 2014, 9, 168–175. (37) Huang, Y.; Huang, X.; Lian, J.; Xu, D.; Wang, L.; Zhang, X. Self-Assembly of Ultrathin Porous NiO Nanosheets/Graphene Hierarchical Structure for High-Capacity and HighRate Lithium Storage. J. Mater. Chem. 2012, 22, 2844-2847. (38) Lin, F.; Boettcher, S. W. Adaptive Semiconductor/Electrocatalyst Junctions in WaterSplitting Photoanodes. Nat. Mater. 2014, 13, 81–86. (39) Huang, W.; Ding, S.; Chen, Y.; Hao, W.; Lai, X.; Peng, J.; Tu, J.; Cao, Y.; Li, X. 3D NiO Hollow Sphere/Reduced Graphene Oxide Composite for High-Performance Glucose Biosensor. Sci. Rep. 2017, 7, 5220. (40) Guan, C.; Wang, Y.; Hu, Y.; Liu, J.; Ho, K. H.; Zhao, W.; Fan, Z.; Shen, Z.; Zhang, H.; Wang, J. Conformally Deposited NiO on a Hierarchical Carbon Support for High-Power and Durable Asymmetric Supercapacitors. J. Mater. Chem. A 2015, 3, 23283–23288. (41) Wu, D.; Wen, M.; Lin, X.; Wu, Q.; Gu, C.; Chen, H. A NiCo/NiO–CoOx Ultrathin Layered Catalyst with Strong Basic Sites for High-Performance H2 Generation from Hydrous Hydrazine. J. Mater. Chem. A 2016, 4, 6595–6602. (42) Ding, C.; Yan, D.; Zhao, Y.; Zhao, Y.; Zhou, H.; Li, J.; Jin, H. A Bubble-Template Approach for Assembling Ni–Co Oxide Hollow Microspheres with an Enhanced Electrochemical Performance as an Anode for Lithium Ion Batteries. Phys. Chem. Chem. Phys. 2016, 18, 25879–25886.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

(43) Kwon, U.; Kim, B. G.; Nguyen, D. C.; Park, J. H.; Ha, N. Y.; Kim, S. J.; Ko, S. H.; Lee, S.; Lee, D.; Park, H. J. Solution-Processible Crystalline NiO Nanoparticles for HighPerformance Planar Perovskite Photovoltaic Cells. Sci. Rep. 2016, 6, 30759. (44) Kuang, P.; Tong, T.; Fan, K.; Yu, J. In Situ Fabrication of Ni-Mo Bimetal Sulfide Hybrid as an Efficient Electrocatalyst for Hydrogen Evolution over a Wide pH Range. ACS Catal. 2017, 7, 6179–6187. (45) Lindroos, J.; Fenning, D. P.; Backlund, D. J.; Verlage, E.; Gorgulla, A.; Estreicher, S. K.; Savin, H.; Buonassisi, T. Nickel: A Very Fast Diffuser in Silicon. J. Appl. Phys. 2013, 113 204906. (46) Teodorescu, V.; Nistor, L.; Bender, H.; Steegen, A.; Lauwers, A.; Maex, K.; Van Landuyt, J. In Situ Transmission Electron Microscopy Study of Ni Silicide Phases Formed on (001) Si Active Lines. J. Appl. Phys. 2001, 90, 167–174. (47) Batchellor, A. S.; Boettcher, S. W. Pulse-Electrodeposited Ni-Fe (Oxy)Hydroxide Oxygen Evolution Electrocatalysts with High Geometric and Intrinsic Activities at Large Mass Loadings. ACS Catal. 2015, 5, 6680–6689. (48) Lewerenz, H. J. Operational Principles of Electrochemical Nanoemitter Solar Cells for Photovoltaic and Photoelectrocatalytic Applications. J. Electroanal. Chem. 2011, 662, 184–195. (49) Moon, C. W.; Lee, S. Y.; Sohn, W.; Andoshe, D. M.; Kim, D. H.; Hong, K.; Jang, H.W. Plasmonic Octahedral Gold Nanoparticles of Maximized Near Electromagnetic Fields for Enhancing Catalytic Hole Transfer in Solar Water Splitting. Part. Part. Syst. Charact. 2017, 34,1600340


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