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ITO Heterojunction Photoanode with an Amorphous and Porous NiOOH Catalyst formed by NiCl2 activation for Water Oxidation
Accepted Manuscript Title: Efficient Si/SiOx /ITO Heterojunction Photoanode with an Amorphous and Porous NiOOH Catalyst formed by NiCl2 activation for Water Oxidation Authors: Sanghwa Yoon, Jae-Hong Lim, Bongyoung Yoo PII: DOI: Reference:
S0013-4686(17)30629-1 http://dx.doi.org/doi:10.1016/j.electacta.2017.03.146 EA 29180
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
8-11-2016 15-3-2017 19-3-2017
Please cite this article as: Sanghwa Yoon, Jae-Hong Lim, Bongyoung Yoo, Efficient Si/SiOx/ITO Heterojunction Photoanode with an Amorphous and Porous NiOOH Catalyst formed by NiCl2 activation for Water Oxidation, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.03.146 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Efficient Si/SiOx/ITO Heterojunction Photoanode with an Amorphous and Porous NiOOH Catalyst formed by NiCl2 activation for Water Oxidation
Sanghwa Yoona, Jae-Hong Lim*b and Bongyoung Yoo*a a
Department of Materials Engineering, Hanyang University, Ansan-si, Gyeonggi-do 426791, Republic of Korea b
Electrochemistry Research Group, Materials Processing Division, Korea Institute of Materials Science, Changwon-si, Gyeongnam 641-831, Republic of Korea
* Co-corresponding authors: Bongyoung Yoo, TEL: +82-31-400-5229, FAX: +82-31417-3701, E-mail: [email protected], Jae-Hong Lim, TEL: +82-55-280-3523, Email: [email protected]
Graphical abstract
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Highlights
Photoanode of Si/ITO heterojunction with highly active NiOOH catalyst layer.
Spray deposited NiO-Cl-OH catalyst layer transformed to NiOOH by activation.
Amorphous, porous NiOOH contained more active sites for O2 evolution.
Photoanode had high photovoltage, photocurrent density and long lifetime in 1 M KOH.
Abstract Solar-driven water splitting with silicon photoelectrodes exhibiting high solar-to-fuel conversion efficiency is a promising way for producing hydrogen fuel in the future. In this study, a heterojunction photoanode was fabricated by the deposition of a thin indium tin oxide (ITO) layer on n-type silicon/native SiOx. A NiCl2-containing precursor was sprayed on the top of the photoanode, affording a NiO-Cl-OH catalyst; this NiO-Cl-OH catalyst was then activated to form an amorphous and porous NiOOH (a-NiOOH) catalyst, which exhibited enhanced performance. The fabricated Si/SiOx/ITO/a-NiOOH photoanode exhibited a low photocurrent onset potential of ~0.98 V vs. RHE, a high saturation photocurrent density of 36.98 mA/cm2, a photocurrent density of 27.4 mA/cm2 at the standard oxidation potential of water, and a photovoltage as high as 545 mV under a solar illumination of 100 mW/cm2. The photocurrent marginally decreased after 30 h. These results suggested that such heterojunctions can replace homogeneous p–n junctions formed from Si doping for high photovoltage generation. In addition, porous a-NiOOH can improve the electrocatalytic performance of Si-based photoanodes.
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Keywords Silicon–silicon oxide–indium tin oxide heterojunction, Nickel chloride activation, Spray deposition, Active nickel-based catalyst
1. Introduction Clean fuel production has become increasingly important with the increase of the global energy demand and the depletion of fossil fuels, as well as the greenhouse gas effect caused by CO2 and CH4.[1-3] In recent years, fuel cells using hydrogen and oxygen as clean energy sources have attracted immense interest as they are free of carbon and exhibit high energy density, as well as are readily portable.[4-7] Among the possible methods for harvesting hydrogen and oxygen, water splitting by photoelectrochemical cells (PECs) using sunlight is a sustainable and eco-friendly method.[1-3, 8-13] However, PECs suffer from several crucial issues, such as the requirement for efficient catalysts and suitable semiconductor materials for capturing the solar spectrum, the generation of high photovoltage by homo- or heterojunctions, band alignment between the photoelectrode and electrolyte, and their stability in aqueous solutions.[1, 14-16] Photoelectrodes based on metal oxides, such as TiO2, WO3, and Fe2O3, are stable in acidic or alkaline solutions. However, these materials exhibit low photoelectrochemical activity in the visible wavelength region. Silicon, which exhibits a direct band gap of 1.1 eV, is an attractive candidate for photoelectrode materials in PEC cells, because its absorption spans a considerable wavelength range, and it is widely applied in photovoltaic devices. Unfortunately, Si also suffers from several limitations, such as corrosion, as well as generation of high overpotential without a catalyst in alkaline solutions.[8, 15, 17-20] Recently, Si photoanodes fabricated via various approaches have been demonstrated to
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be efficient for stable water oxidation. Generally, commercial p–n junctions obtained by p+- or n+-doping are utilized to achieve high photovoltage in Si photoanodes with a narrow band gap.[16, 19, 21] Metal–insulator–semiconductor photoanodes can be formed by the deposition of a metal with a high work function on Si/SiOx.[18] Nevertheless, metal contacts are needed for electrochemical activity, which in turn decrease the light absorption of the photoanodes. Another alternative involves the use of junctions using a transparent conducting oxide (TCO), which can provide high optical transmission and high electrical conductivity for collecting the photogenerated carriers. A heterojunction with TCO and a Si absorber exhibits high photovoltage while simultaneously protecting Si from the electrolyte.[22] In 1980, an indium tin oxide (ITO)–Si solar cell with an efficiency of 11.5% has been reported, demonstrating potential for high-performance Sibased PEC cells without a p–n homojunction.[23] Since this report, a few other studies on ITO-modified Si-based PECs have been reported.[20, 22] Besides high photovoltage, efficient, stable oxygen evolution reaction (OER) is also crucial for enhancing PEC performance. During water splitting, the transfer of at least four electrons is required for OER; hence, OER is slow as compared to the simple, rapid hydrogen evolution reaction (HER).[24, 25] For addressing this issue and achieving efficient water splitting, extensive studies have focused attention on the development of novel OER electrocatalysts exhibiting low overpotential and high activity.[26, 27] Furthermore, for ensuring that the Si absorber receives sunlight, optically transparent electrocatalysts are preferable for minimizing the loss of light. In this regard, transition metal oxides and hydroxides, such as IrOx, CoOx, Co(OH)2, NiOx, and Ni(OH)2, are excellent candidate electrocatalysts.[19-22, 24, 28-31] Among these electrocatalysts, Nibased catalysts are not only transparent but also resistant to corrosion in alkaline solutions. 4
Recently, NiOx has been deposited on Si-based PECs by sputtering, which is a vacuumbased method.[16, 21, 32] In this study, a novel Si/SiOx/ITO/NiO-Cl-OH photoanode exhibiting high photovoltage and efficient OER was designed and prepared. The native photoanode with n-Si was covered by an amorphous porous NiOOH photocatalyst, formed by the electrochemical transformation of the sprayed NiO-Cl-OH precursor. The resulting Si/SiOx/ITO/NiOOH heterojunction exhibited a high photovoltage of 545 mV, a high photocurrent density of 27.4 mA/cm2 at 1.23 V vs. RHE, and long-term stability in alkaline solutions.
2. Experimental 2.1 Fabrication of the Si/SiOx/ITO photoanode with the NiO-Cl-OH catalyst First, an ITO film (10% SnO2, 90% In2O3 (99.99%)) was deposited on an n-Si wafer with native SiOx by sputtering at 50 W and 150 C at a deposition rate of 0.6 Å/s. Second, the prepared Si/SiOx/ITO was sprayed with the NiO-Cl-OH precursor (10 mM NiCl26H2O in 50 mL of ethanol), followed by heating on a hot plate at 360 C for 30 min. Next, the sample was immersed in deionized (DI) water for 5 min for removing residual NiCl2xH2O. The thickness of the NiO-Cl-OH catalyst layer was controlled by the repetition of the above-described deposition–heating–immersion (DHI) process. For investigating the effects of chloride and hydroxide in the NiO-Cl-OH catalyst, the asdeposited catalyst was annealed at 450 C for 2 h in air for conversion to NiO.
2.2 Material characterization The morphology of the NiO-Cl-OH catalyst was examined by field-emission scanning electron microscopy (FE-SEM; S-4700, Hitachi) and transmission electron microscopy 5
(TEM; JEM-2100F, JEOL) equipped with energy-dispersive spectroscopy (EDS). A focused ion beam (FIB; NOVA 600 Nanolab, FEI) was used to prepare cross-sectional specimens of the Si-based photoanode. X-ray diffraction (XRD; D/MAX-2500/PC, Rigaku) was employed for analyzing crystal structures. X-ray photoelectron spectroscopy (XPS; R3000, VG Scienta) was employed to investigate the binding energies of Ni, Cl, and O. Atomic force microscopy (AFM; XE-100, Park Systems) images were recorded with a scan size of 7 μm × 7 μm. The root-mean-square (rms) roughness of the sample surface was calculated from the AFM data using XEI 1.7.6 software. The transmittance of the ITO films was recorded on a UV–vis spectrophotometer (Lambda 750, PerkinElmer).
2.3 PEC characterization The PEC characteristics of the Si-based photoanodes with the NiO-Cl-OH catalyst were measured using 1 M KOH (pH = 14) as the electrolyte under a light illumination of 100 mW/cm2 (Xenon lamp, Newport). For electrical contact, all sample photoanodes were scratched on the back side and attached to a copper tape with a Ga–In eutectic metal (75.5:24.5 w/w, Aldrich). Then, the photoanodes were sealed with an epoxy resin (Torr Seal, Varian) for protection from the electrolyte, except for the area (0.3 cm × 0.3 cm) to be irradiated. A platinum wire and a Ag/AgCl (KCl saturated) electrode were used as the counter and reference electrodes in a three-electrode system, respectively. Linear sweep voltammetry (LSV; VersaSTAT 3, Princeton Applied Research) was performed under light and dark conditions at voltages ranging from 0.7 V to 1 V vs. Ag/AgCl and at a scan rate of 2 mV/s. The measured potential (V) was converted to the reversible hydrogen electrode (RHE) reference according to the Nernst equation: 6
VRHE = Vo + 0.059 pH + 0.197 V. Here, Vo is the potential measured against the Ag/AgCl reference electrode. Chronoamperometric (CA) measurements were carried out under continuous light illumination at 1.23 V vs. RHE, and the open-circuit potential (OCP) was monitored to measure the photovoltage of the photoanodes with alternating light and dark periods. Before measuring the PEC properties, the NiO-Cl-OH catalyst was chemically activated by the application of 1.23 V vs. RHE for 2 h.
3. Results and discussion 3.1 Photoelectrochemical properties of the Si/SiOx/ITO/NiO-Cl-OH photoanode As shown in Figure 1a, the NiO-Cl-OH catalyst was loaded on the Si/SiOx/ITO substrate by DHI. Immersion in water was crucial for creating a homogenous NiO-Cl-OH layer; this layer, serving as the precursor, was spread on the surface without the formation of any agglomerates (Figures S1a–S1c), which was not favorable for photoelectrocatalytic activity (Figure S1d). Figure 1b shows the cross-section image of the Si/SiOx/ITO photoanode with NiO-Cl-OH. A 2–3 nm thick SiOx layer was formed between the n-Si and ITO layers (thickness of each layer is 25 nm), and a 15–20 nm thick NiO-Cl-OH catalyst layer was obtained by 10 DHI cycles. SEM was employed to observe the morphology of the NiO-Cl-OH catalyst with several DHI cycles (Figure S2). After only 3 cycles, the catalyst layer was not uniform as particles were observed on the ITO, while after 6 cycles, a uniform layer was observed. The rms surface roughness increased from 4.483 to 17.64 nm (Figures S3). Moreover, EDS elemental mapping images clearly indicated the homogeneous distribution of Ni, Cl, and O in the NiO-Cl-OH catalyst (Figure 1c). 7
All as-deposited photoelectrodes were electrochemically activated by CA at 1.23 V vs. RHE for 2 h in 1 M KOH under light illumination. After CA, the LSV sweep was changed from 1.5 to 1.1 V vs. RHE at 10 mA/cm2. A minimal current was observed under dark conditions as n-type Si exhibited a small number of thermally induced holes (Figure 2a). Figure 2b shows the LSV curves of NiO-Cl-OH obtained after several DHI cycles, as well as the electrocatalytic behavior of a p+-Si/SiOx/ITO/NiO-Cl-OH electrode in a 1 M KOH solution. After 10 DHI cycles, the onset potentials of p+-Si/SiOx/ITO/NiO-Cl-OH and n-Si/SiOx/ITO/NiO-Cl-OH were ~1.6 and ~0.98 V vs. RHE, respectively. In the former, as p+-Si only served as a current collector similar to the metal, photogenerated voltage was not measured. On the other hand, after 10 DHI cycles, the onset potential of n-Si/SiOx/ITO/NiO-Cl-OH was less than the equilibrium potential for water oxidation by 250 mV, related to the contribution from the photogenerated electron–hole pairs. Although this value is less than an OCV of 740 mV for high-quality p–n junction Si solar cells (Figures 2c, S4a), the highest photovoltage (OCP) measured for n-Si/SiOx/ITO/NiOCl-OH (10 DHI cycles) was 545 mV; this value is comparable to those reported previously for Si PECs [33, 34]. On the other hand, between 3 and 10 DHI cycles, the photocurrent density at 1.23 V vs. RHE increased from 13.31 to 27.4 mA/cm2, related to the formation of active sites with high electrocatalytic activity. By contrast, at 20 DHI cycles, photocurrent density decreased to 20.55 mA/cm2 at 1.23 V vs. RHE (Figure 2c), related to the high resistance observed for a thicker NiO-Cl-OH layer. For the sample at 3 DHI cycles, the maximum saturation photocurrent density was observed at 36.97 mA/cm2 (Figure 2c). The average saturation photocurrent density for all photoanodes at different DHI cycles (red squares in Fig. 2c) was 35.1 mA/cm2. The saturation photocurrent density can increase with the minimal optical loss and efficient conversion 8
of solar energy to oxygen. The thin ITO film exhibiting ultrahigh transmittance and conductivity (Figures S5) and the NiO-Cl-OH catalyst exhibiting outstanding activity facilitated the penetration of light without optical loss and efficient OER, respectively. The 25 nm thick ITO layer was selected because it exhibited a high transmittance of nearly 90% in the visible region, a high carrier mobility of 32 cm2/(V·s), and a high conductivity of 4365 S/cm, all of which imply that it can transfer carriers from the Si absorber to the catalyst with a minimal IR drop and high optical absorption of Si. The electrical properties of a 12 nm thick ITO film could not be measured by Hall measurement owing to the formation of a non-continuous film. The Si/SiOx/ITO/NiO-ClOH photoanode fabricated from 10 DHI cycles maintained stable activity for 30 h in 1 M KOH without the rapid decrease of performance (Figure 2d).
3.2 Electronic structure of Si/SiOx/ITO/NiO-Cl-OH photoanode Figure 3a shows the processes occurring in the Si/SiOx/ITO/NiO-Cl-OH photoanode under light illumination. During the illumination of the photoanode, electron–hole pairs are generated within an area having a diameter less than the sum of the effective minority carrier diffusion length (Le) and depletion width (W), and holes tunnel through the native SiOx layer to the ITO film.[13] Next, these collected holes from the ITO film move to the active sites on the NiO-Cl-OH catalyst, where OER occurs. The heterojunction between n-Si and ITO, exhibiting a high work function, can generate large band bending and high photovoltage (Figure 3b). For confirming the roles of ITO and NiO-Cl-OH in Si-based PECs, the Si/SiOx/NiO-Cl-OH and Si/SiOx/ITO junctions, respectively, were constructed and tested. Photocurrent densities of both structures were remarkably less than that observed for Si/SiOx/ITO/NiO-Cl-OH (Figures 3c and 3d), with no oxygen evolution. 9
Only, oxygen bubbles were generated on the NiO-Cl-OH surface in the Si/SiOx/ITO/NiOCl-OH photoanode (Figure S6). These results revealed that ITO can form a heterojunction with n-Si for producing photovoltage, but it does not act as an electrocatalyst. The photoanode exhibited a photovoltage of 545 mV. However, a potential of 1.23 V (in practice, a potential of 1.5–1.8 V was required) was required to generate water splitting. The valence-band edge of Si (0.85 V vs NHE) is above the water oxidation potential (1.23 V vs NHE) at pH 0. Hence, theoretically, a 380 mV gap is present.[35] When water splitting is generated without the photoelectrode, a potential of 1.5–1.8 V is required. However, the photoanode exhibited a photovoltage of 545 mV (Figures 2b and S4). The theoretical open-circuit potential of a Si/SiOx/ITO solar cell (for Si resistivity: 1–10 Ω·cm, ITO sheet resistance: 30 Ω/□) ) is approximately 530–580 mV, which is similar to that of the photoanode structure in this study.[36] Hence, an external potential of approximately 1.0 V is applied. 3.3 Electrochemical activation of NiCl2 in the NiO-Cl-OH catalyst For investigating the electrochemical activation of the NiO-Cl-OH catalyst, as-deposited NiO-Cl-OH was annealed in air. After annealing, the broad peak of NiCl2 or Ni(OH)2 disappeared, while peaks related to the (111), (200), and (220) planes of NiO were retained (Figure 4a).[37] The examination of the cross section revealed a d-spacing of 0.58 nm for NiCl2 in the as-deposited NiO-Cl-OH. In contrast, the annealed NiO-Cl-OH sample was crystalline and contained porous NiO (Figure S7). XPS data indicated the existence of Ni2+, corresponding to NiCl2, Ni(OH)2, and NiO, lattice oxygen, and surface hydroxyl groups on the surface of as-deposited NiO-Cl-OH (Figures 4b and 4c).[38] In the annealed sample, the main Ni 2p peak shifted from 856.6 to 855.4 eV (Figure 4b),
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and the peak corresponding to NiCl2 disappeared from the Cl 2p region, related to the removal of chlorine and hydrogen during annealing (Figure 4d). As also shown in the TEM image in Figure 5a, the activated catalyst exhibited an amorphous and porous structure. In the XPS spectra (Figure 5b), the Ni2+ peak corresponding to NiO, NiCl2, and Ni(OH)2 disappeared, while the Ni3+ peak corresponding to NiOOH was observed at 856 eV in the Ni 2p region.[39] Lattice oxygen and Cl2 were not observed (Figures 5c and 5d). Hence, as-deposited NiO-Cl-OH is completely converted to amorphous NiOOH (a-NiOOH), while the NiO surface in annealed NiO-Cl-OH is changed to NiOOH only after CA at 1.23 V vs. RHE for 2 h. This result can be explained by the fact that NiCl2 undergoes chemical transformation into Ni(OH)2 under alkaline conditions.[40, 41] After the as-deposited NiO-Cl-OH catalyst was soaked in 1 M KOH for 12 h, Cl disappeared, while Ni was retained, as confirmed by EDS data (Figure S8). In addition, after immediately soaking the Si/SiOx/ITO/NiOCl-OH photoanode, its photocurrent density increased to 16 mA/cm2 without any activation time (Figure 5e). In this case, NiCl2 in the NiO-Cl-OH catalyst was already transformed to Ni(OH)2 during soaking, which was oxidized to NiOOH. However, the photocurrent density decreased to 8.3 mA/cm2 during oxygen evolution, possibly assumed to be caused by issues associated with adhesion on the ITO film. The photocurrent density of annealed NiO-Cl-OH (i.e., NiO) significantly increased after activation (Figure 5f). The peak at 1.2–1.4 V vs. RHE was believed to be related to the oxidation of Ni(OH)2 to NiOOH, indicative of the number of electrochemically active sites.[29, 42, 43] Hence, the large size for this oxidation peak corresponds to considerable charge transfer at the electrode surface. From electrochemical impedance spectroscopy measurements, activated Si/SiOx/ITO/NiO-Cl-OH exhibited a more rapid charge transfer 11
(i.e., smaller semicircle) as compared with that observed for the annealed photoanode (Figure S9), also related to the enhanced electrocatalytic activity on the surface.[10-12] All these results indicated that the activation of NiCl2 in the NiO-Cl-OH catalyst for the direct formation of NiOOH is induced by the number of electrochemically active sites and maintains the catalyst structure.
4. Conclusions A highly active, porous, and amorphous NiOOH catalyst was synthesized on a Sisputtered ITO photoanode by electrochemical activation via spraying a NiO-Cl-OH layer on the top of the photoanode. Activation decreased the resistance to charge transfer, related to the dramatic increase of electrochemically active sites. The chemical transformation of NiCl2 to Ni(OH)2 in the alkaline solution further accelerated the formation of a-NiOOH. High saturation photocurrent was achieved by the combination of a suitable Si/SiOx/ITO heterojunction, a thin ITO film with high solar absorption, and an active a-NiOOH catalyst with a porous structure. A high photovoltage of 545 mV and long-term stability for at least 30 h were observed for this photoanode in 1 M KOH under simulated light illumination. This study offers a guideline for the construction of stable, active semiconductor photoanodes with efficient OER catalysts under harsh conditions.
Acknowledgements This study was supported by New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) [grant numbers 20153030012190, 20153030013200] and the Basic Science Research
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Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning [grant number 2015R1A5A1037548].
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[31] Z. Zhao, H. Wu, H. He, X. Xu, Y. Jin, A high‐performance binary Ni–Co hydroxide‐based water oxidation electrode with three‐dimensional coaxial nanotube array structure, Adv. Funct. Mater. 24 (2014) 4698. [32] M.-J. Park, J.-Y. Jung, S.-M. Shin, J.-W. Song, Y.-H. Nam, D.-H. Kim, J.-H. Lee, Photoelectrochemical oxygen evolution improved by a thin Al2O3 interlayer in a NiOx/n-Si photoanode, Thin Solid Films 599 (2016) 54–58. [33] J. A. Switzer, The n‐silicon/thallium(III) oxide heterojunction photoelectrochemical solar cell, J. Electrochem. Soc. 133 (1986) 722. [34] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D. Dunlop, Solar cell efficiency tables, Prog. Photovoltaics Res. Appl. 23 (2015) 1. [35] A. G. Tamirat, J. Rick, A. A. Dubale, W. N. Su, B. J. Hwang, Using hematite for photoelectrochemical water splitting: a review of current progress and challenges, Nanoscale Horiz. 1 (2016) 243. [36] O. Malik, F. J. D. Hidalga-W, Zuniga-T, G. Ruiz-T, Efficient ITO-Si solar cells and power modules fabricated with a low temperature technology: Results and perspectives, J. Non-Cryst. Solids 354 (2008) 2472. [37] D.-J. Kang, K.-N. Kim, S.-G. J. Kim, Morphologies and properties of NiO particles prepared from NiSO4·6H2O and Ni(NO3)2·6H2O by spray pyrolysis, J. Mater. Sci. 40 (2005) 6283. [38] S. Giuffrida, G. G. Condorelli, L. L. Costanzo, G. Ventimiglia, R. L. Nigro, M. Favazza, E. Votrico, C. Bongiorno, I. L. Fragala, Nickel nanostructured materials from liquid phase photodeposition, J. Nanopart. Res. 9 (2007) 611. [39] M. A. Peck, M. A. Langell, Comparison of nanoscaled and bulk NiO structural and environmental characteristics by XRD, XAFS, and XPS, Chem. Mater. 24 (2012) 4483. [40] J.-Y. Park, K.-S. Ahn, Y.-C. Nah, H.-S. Shim, Y.-E. Sung, Electrochemical and electrochromic properties of Ni oxide thin films prepared by a sol–gel method, J. Sol-Gel Sci. Technol. 31 (2004) 323. [41] D. Dubal, V. Fulari, C. Lokhande, Effect of morphology on supercapacitive properties of chemically grown β-Ni(OH)2 thin films, Microporous Mesoporous Mater. 151 (2012) 511. [42] K. M. Young, T. W. Hamann, Enhanced photocatalytic water oxidation efficiency with Ni(OH) 2 catalysts deposited on α-Fe2O3 via ALD, Chem. Commun. 50 (2014) 8727. [43] C. Tang, H. S. Wang, H. F. Wang, Q. Zhang, G. L. Tian, J. Q. Nie, F. Wei, Spatially confined hybridization of nanometer‐sized NiFe hydroxides into nitrogen‐doped graphene frameworks leading to superior oxygen evolution reactivity, Adv. Mater. 27 (2015) 4516.
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Figure captions: Figure 1. (a) Schematic of deposition–heating–immersion (DHI) for preparing the NiOCl-OH catalyst on the Si/SiOx/ITO photoanode, and (b) Cross-section and (c) EDS elemental mapping images of the as-deposited Si/SiOx/ITO/NiO-Cl-OH photoanode. Figure 2. (a) LSV curves of the Si/SiOx/ITO/NiO-Cl-OH photoanode before and after activation for 2 h at 1.23 V vs. RHE. (b) LSV curves of Si/SiOx/ITO/NiO-Cl-OH photoanodes at different DHI cycles. (c) Comparison of the photovoltage, photocurrent density (at standard oxygen evolution potential (Eo)), and saturation photocurrent density (at 2 V vs. RHE) of photoanodes at different DHI cycles. (d) Stability test of the photoanode for 30 h at 1.23 V vs. RHE. Figure 3. (a) Schematic side view and (b) energy band diagram of the Si/SiOx/ITO NiOCl-OH photoanode under illumination. LSV curves of the (c) Si/NiO-Cl-OH and (d) Si/SiOx/ITO structures. Figure 4. (a) XRD data and XPS spectra of the (b) Ni 2p, (c) O 1s, and (d) Cl 2p regions of the as-deposited and annealed Si/SiOx/ITO/NiO-Cl-OH photoanodes. Figure 5. (a) TEM image of the activated NiO-Cl-OH layer (inset: fast Fourier transformed image). XPS spectra of the (b) Ni 2p, (c) O 1s, and (d) Cl 2p regions of the as-deposited and activated Si/SiOx/ITO/NiO-Cl-OH photoanodes. (e) Current–time graph of the as-deposited Si/SiOx/ITO/NiO-Cl-OH photoanode and after soaking the photoanode in 1 M KOH for 12 h. (f) LSV curves of the annealed and activated Si/SiOx/ITO/NiO-Cl-OH photoanodes. Figure 1.
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S0013-4686(17)30629-1 http://dx.doi.org/doi:10.1016/j.electacta.2017.03.146 EA 29180
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
8-11-2016 15-3-2017 19-3-2017
Please cite this article as: Sanghwa Yoon, Jae-Hong Lim, Bongyoung Yoo, Efficient Si/SiOx/ITO Heterojunction Photoanode with an Amorphous and Porous NiOOH Catalyst formed by NiCl2 activation for Water Oxidation, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.03.146 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Efficient Si/SiOx/ITO Heterojunction Photoanode with an Amorphous and Porous NiOOH Catalyst formed by NiCl2 activation for Water Oxidation
Sanghwa Yoona, Jae-Hong Lim*b and Bongyoung Yoo*a a
Department of Materials Engineering, Hanyang University, Ansan-si, Gyeonggi-do 426791, Republic of Korea b
Electrochemistry Research Group, Materials Processing Division, Korea Institute of Materials Science, Changwon-si, Gyeongnam 641-831, Republic of Korea
* Co-corresponding authors: Bongyoung Yoo, TEL: +82-31-400-5229, FAX: +82-31417-3701, E-mail: [email protected], Jae-Hong Lim, TEL: +82-55-280-3523, Email: [email protected]
Graphical abstract
1
Highlights
Photoanode of Si/ITO heterojunction with highly active NiOOH catalyst layer.
Spray deposited NiO-Cl-OH catalyst layer transformed to NiOOH by activation.
Amorphous, porous NiOOH contained more active sites for O2 evolution.
Photoanode had high photovoltage, photocurrent density and long lifetime in 1 M KOH.
Abstract Solar-driven water splitting with silicon photoelectrodes exhibiting high solar-to-fuel conversion efficiency is a promising way for producing hydrogen fuel in the future. In this study, a heterojunction photoanode was fabricated by the deposition of a thin indium tin oxide (ITO) layer on n-type silicon/native SiOx. A NiCl2-containing precursor was sprayed on the top of the photoanode, affording a NiO-Cl-OH catalyst; this NiO-Cl-OH catalyst was then activated to form an amorphous and porous NiOOH (a-NiOOH) catalyst, which exhibited enhanced performance. The fabricated Si/SiOx/ITO/a-NiOOH photoanode exhibited a low photocurrent onset potential of ~0.98 V vs. RHE, a high saturation photocurrent density of 36.98 mA/cm2, a photocurrent density of 27.4 mA/cm2 at the standard oxidation potential of water, and a photovoltage as high as 545 mV under a solar illumination of 100 mW/cm2. The photocurrent marginally decreased after 30 h. These results suggested that such heterojunctions can replace homogeneous p–n junctions formed from Si doping for high photovoltage generation. In addition, porous a-NiOOH can improve the electrocatalytic performance of Si-based photoanodes.
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Keywords Silicon–silicon oxide–indium tin oxide heterojunction, Nickel chloride activation, Spray deposition, Active nickel-based catalyst
1. Introduction Clean fuel production has become increasingly important with the increase of the global energy demand and the depletion of fossil fuels, as well as the greenhouse gas effect caused by CO2 and CH4.[1-3] In recent years, fuel cells using hydrogen and oxygen as clean energy sources have attracted immense interest as they are free of carbon and exhibit high energy density, as well as are readily portable.[4-7] Among the possible methods for harvesting hydrogen and oxygen, water splitting by photoelectrochemical cells (PECs) using sunlight is a sustainable and eco-friendly method.[1-3, 8-13] However, PECs suffer from several crucial issues, such as the requirement for efficient catalysts and suitable semiconductor materials for capturing the solar spectrum, the generation of high photovoltage by homo- or heterojunctions, band alignment between the photoelectrode and electrolyte, and their stability in aqueous solutions.[1, 14-16] Photoelectrodes based on metal oxides, such as TiO2, WO3, and Fe2O3, are stable in acidic or alkaline solutions. However, these materials exhibit low photoelectrochemical activity in the visible wavelength region. Silicon, which exhibits a direct band gap of 1.1 eV, is an attractive candidate for photoelectrode materials in PEC cells, because its absorption spans a considerable wavelength range, and it is widely applied in photovoltaic devices. Unfortunately, Si also suffers from several limitations, such as corrosion, as well as generation of high overpotential without a catalyst in alkaline solutions.[8, 15, 17-20] Recently, Si photoanodes fabricated via various approaches have been demonstrated to
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be efficient for stable water oxidation. Generally, commercial p–n junctions obtained by p+- or n+-doping are utilized to achieve high photovoltage in Si photoanodes with a narrow band gap.[16, 19, 21] Metal–insulator–semiconductor photoanodes can be formed by the deposition of a metal with a high work function on Si/SiOx.[18] Nevertheless, metal contacts are needed for electrochemical activity, which in turn decrease the light absorption of the photoanodes. Another alternative involves the use of junctions using a transparent conducting oxide (TCO), which can provide high optical transmission and high electrical conductivity for collecting the photogenerated carriers. A heterojunction with TCO and a Si absorber exhibits high photovoltage while simultaneously protecting Si from the electrolyte.[22] In 1980, an indium tin oxide (ITO)–Si solar cell with an efficiency of 11.5% has been reported, demonstrating potential for high-performance Sibased PEC cells without a p–n homojunction.[23] Since this report, a few other studies on ITO-modified Si-based PECs have been reported.[20, 22] Besides high photovoltage, efficient, stable oxygen evolution reaction (OER) is also crucial for enhancing PEC performance. During water splitting, the transfer of at least four electrons is required for OER; hence, OER is slow as compared to the simple, rapid hydrogen evolution reaction (HER).[24, 25] For addressing this issue and achieving efficient water splitting, extensive studies have focused attention on the development of novel OER electrocatalysts exhibiting low overpotential and high activity.[26, 27] Furthermore, for ensuring that the Si absorber receives sunlight, optically transparent electrocatalysts are preferable for minimizing the loss of light. In this regard, transition metal oxides and hydroxides, such as IrOx, CoOx, Co(OH)2, NiOx, and Ni(OH)2, are excellent candidate electrocatalysts.[19-22, 24, 28-31] Among these electrocatalysts, Nibased catalysts are not only transparent but also resistant to corrosion in alkaline solutions. 4
Recently, NiOx has been deposited on Si-based PECs by sputtering, which is a vacuumbased method.[16, 21, 32] In this study, a novel Si/SiOx/ITO/NiO-Cl-OH photoanode exhibiting high photovoltage and efficient OER was designed and prepared. The native photoanode with n-Si was covered by an amorphous porous NiOOH photocatalyst, formed by the electrochemical transformation of the sprayed NiO-Cl-OH precursor. The resulting Si/SiOx/ITO/NiOOH heterojunction exhibited a high photovoltage of 545 mV, a high photocurrent density of 27.4 mA/cm2 at 1.23 V vs. RHE, and long-term stability in alkaline solutions.
2. Experimental 2.1 Fabrication of the Si/SiOx/ITO photoanode with the NiO-Cl-OH catalyst First, an ITO film (10% SnO2, 90% In2O3 (99.99%)) was deposited on an n-Si wafer with native SiOx by sputtering at 50 W and 150 C at a deposition rate of 0.6 Å/s. Second, the prepared Si/SiOx/ITO was sprayed with the NiO-Cl-OH precursor (10 mM NiCl26H2O in 50 mL of ethanol), followed by heating on a hot plate at 360 C for 30 min. Next, the sample was immersed in deionized (DI) water for 5 min for removing residual NiCl2xH2O. The thickness of the NiO-Cl-OH catalyst layer was controlled by the repetition of the above-described deposition–heating–immersion (DHI) process. For investigating the effects of chloride and hydroxide in the NiO-Cl-OH catalyst, the asdeposited catalyst was annealed at 450 C for 2 h in air for conversion to NiO.
2.2 Material characterization The morphology of the NiO-Cl-OH catalyst was examined by field-emission scanning electron microscopy (FE-SEM; S-4700, Hitachi) and transmission electron microscopy 5
(TEM; JEM-2100F, JEOL) equipped with energy-dispersive spectroscopy (EDS). A focused ion beam (FIB; NOVA 600 Nanolab, FEI) was used to prepare cross-sectional specimens of the Si-based photoanode. X-ray diffraction (XRD; D/MAX-2500/PC, Rigaku) was employed for analyzing crystal structures. X-ray photoelectron spectroscopy (XPS; R3000, VG Scienta) was employed to investigate the binding energies of Ni, Cl, and O. Atomic force microscopy (AFM; XE-100, Park Systems) images were recorded with a scan size of 7 μm × 7 μm. The root-mean-square (rms) roughness of the sample surface was calculated from the AFM data using XEI 1.7.6 software. The transmittance of the ITO films was recorded on a UV–vis spectrophotometer (Lambda 750, PerkinElmer).
2.3 PEC characterization The PEC characteristics of the Si-based photoanodes with the NiO-Cl-OH catalyst were measured using 1 M KOH (pH = 14) as the electrolyte under a light illumination of 100 mW/cm2 (Xenon lamp, Newport). For electrical contact, all sample photoanodes were scratched on the back side and attached to a copper tape with a Ga–In eutectic metal (75.5:24.5 w/w, Aldrich). Then, the photoanodes were sealed with an epoxy resin (Torr Seal, Varian) for protection from the electrolyte, except for the area (0.3 cm × 0.3 cm) to be irradiated. A platinum wire and a Ag/AgCl (KCl saturated) electrode were used as the counter and reference electrodes in a three-electrode system, respectively. Linear sweep voltammetry (LSV; VersaSTAT 3, Princeton Applied Research) was performed under light and dark conditions at voltages ranging from 0.7 V to 1 V vs. Ag/AgCl and at a scan rate of 2 mV/s. The measured potential (V) was converted to the reversible hydrogen electrode (RHE) reference according to the Nernst equation: 6
VRHE = Vo + 0.059 pH + 0.197 V. Here, Vo is the potential measured against the Ag/AgCl reference electrode. Chronoamperometric (CA) measurements were carried out under continuous light illumination at 1.23 V vs. RHE, and the open-circuit potential (OCP) was monitored to measure the photovoltage of the photoanodes with alternating light and dark periods. Before measuring the PEC properties, the NiO-Cl-OH catalyst was chemically activated by the application of 1.23 V vs. RHE for 2 h.
3. Results and discussion 3.1 Photoelectrochemical properties of the Si/SiOx/ITO/NiO-Cl-OH photoanode As shown in Figure 1a, the NiO-Cl-OH catalyst was loaded on the Si/SiOx/ITO substrate by DHI. Immersion in water was crucial for creating a homogenous NiO-Cl-OH layer; this layer, serving as the precursor, was spread on the surface without the formation of any agglomerates (Figures S1a–S1c), which was not favorable for photoelectrocatalytic activity (Figure S1d). Figure 1b shows the cross-section image of the Si/SiOx/ITO photoanode with NiO-Cl-OH. A 2–3 nm thick SiOx layer was formed between the n-Si and ITO layers (thickness of each layer is 25 nm), and a 15–20 nm thick NiO-Cl-OH catalyst layer was obtained by 10 DHI cycles. SEM was employed to observe the morphology of the NiO-Cl-OH catalyst with several DHI cycles (Figure S2). After only 3 cycles, the catalyst layer was not uniform as particles were observed on the ITO, while after 6 cycles, a uniform layer was observed. The rms surface roughness increased from 4.483 to 17.64 nm (Figures S3). Moreover, EDS elemental mapping images clearly indicated the homogeneous distribution of Ni, Cl, and O in the NiO-Cl-OH catalyst (Figure 1c). 7
All as-deposited photoelectrodes were electrochemically activated by CA at 1.23 V vs. RHE for 2 h in 1 M KOH under light illumination. After CA, the LSV sweep was changed from 1.5 to 1.1 V vs. RHE at 10 mA/cm2. A minimal current was observed under dark conditions as n-type Si exhibited a small number of thermally induced holes (Figure 2a). Figure 2b shows the LSV curves of NiO-Cl-OH obtained after several DHI cycles, as well as the electrocatalytic behavior of a p+-Si/SiOx/ITO/NiO-Cl-OH electrode in a 1 M KOH solution. After 10 DHI cycles, the onset potentials of p+-Si/SiOx/ITO/NiO-Cl-OH and n-Si/SiOx/ITO/NiO-Cl-OH were ~1.6 and ~0.98 V vs. RHE, respectively. In the former, as p+-Si only served as a current collector similar to the metal, photogenerated voltage was not measured. On the other hand, after 10 DHI cycles, the onset potential of n-Si/SiOx/ITO/NiO-Cl-OH was less than the equilibrium potential for water oxidation by 250 mV, related to the contribution from the photogenerated electron–hole pairs. Although this value is less than an OCV of 740 mV for high-quality p–n junction Si solar cells (Figures 2c, S4a), the highest photovoltage (OCP) measured for n-Si/SiOx/ITO/NiOCl-OH (10 DHI cycles) was 545 mV; this value is comparable to those reported previously for Si PECs [33, 34]. On the other hand, between 3 and 10 DHI cycles, the photocurrent density at 1.23 V vs. RHE increased from 13.31 to 27.4 mA/cm2, related to the formation of active sites with high electrocatalytic activity. By contrast, at 20 DHI cycles, photocurrent density decreased to 20.55 mA/cm2 at 1.23 V vs. RHE (Figure 2c), related to the high resistance observed for a thicker NiO-Cl-OH layer. For the sample at 3 DHI cycles, the maximum saturation photocurrent density was observed at 36.97 mA/cm2 (Figure 2c). The average saturation photocurrent density for all photoanodes at different DHI cycles (red squares in Fig. 2c) was 35.1 mA/cm2. The saturation photocurrent density can increase with the minimal optical loss and efficient conversion 8
of solar energy to oxygen. The thin ITO film exhibiting ultrahigh transmittance and conductivity (Figures S5) and the NiO-Cl-OH catalyst exhibiting outstanding activity facilitated the penetration of light without optical loss and efficient OER, respectively. The 25 nm thick ITO layer was selected because it exhibited a high transmittance of nearly 90% in the visible region, a high carrier mobility of 32 cm2/(V·s), and a high conductivity of 4365 S/cm, all of which imply that it can transfer carriers from the Si absorber to the catalyst with a minimal IR drop and high optical absorption of Si. The electrical properties of a 12 nm thick ITO film could not be measured by Hall measurement owing to the formation of a non-continuous film. The Si/SiOx/ITO/NiO-ClOH photoanode fabricated from 10 DHI cycles maintained stable activity for 30 h in 1 M KOH without the rapid decrease of performance (Figure 2d).
3.2 Electronic structure of Si/SiOx/ITO/NiO-Cl-OH photoanode Figure 3a shows the processes occurring in the Si/SiOx/ITO/NiO-Cl-OH photoanode under light illumination. During the illumination of the photoanode, electron–hole pairs are generated within an area having a diameter less than the sum of the effective minority carrier diffusion length (Le) and depletion width (W), and holes tunnel through the native SiOx layer to the ITO film.[13] Next, these collected holes from the ITO film move to the active sites on the NiO-Cl-OH catalyst, where OER occurs. The heterojunction between n-Si and ITO, exhibiting a high work function, can generate large band bending and high photovoltage (Figure 3b). For confirming the roles of ITO and NiO-Cl-OH in Si-based PECs, the Si/SiOx/NiO-Cl-OH and Si/SiOx/ITO junctions, respectively, were constructed and tested. Photocurrent densities of both structures were remarkably less than that observed for Si/SiOx/ITO/NiO-Cl-OH (Figures 3c and 3d), with no oxygen evolution. 9
Only, oxygen bubbles were generated on the NiO-Cl-OH surface in the Si/SiOx/ITO/NiOCl-OH photoanode (Figure S6). These results revealed that ITO can form a heterojunction with n-Si for producing photovoltage, but it does not act as an electrocatalyst. The photoanode exhibited a photovoltage of 545 mV. However, a potential of 1.23 V (in practice, a potential of 1.5–1.8 V was required) was required to generate water splitting. The valence-band edge of Si (0.85 V vs NHE) is above the water oxidation potential (1.23 V vs NHE) at pH 0. Hence, theoretically, a 380 mV gap is present.[35] When water splitting is generated without the photoelectrode, a potential of 1.5–1.8 V is required. However, the photoanode exhibited a photovoltage of 545 mV (Figures 2b and S4). The theoretical open-circuit potential of a Si/SiOx/ITO solar cell (for Si resistivity: 1–10 Ω·cm, ITO sheet resistance: 30 Ω/□) ) is approximately 530–580 mV, which is similar to that of the photoanode structure in this study.[36] Hence, an external potential of approximately 1.0 V is applied. 3.3 Electrochemical activation of NiCl2 in the NiO-Cl-OH catalyst For investigating the electrochemical activation of the NiO-Cl-OH catalyst, as-deposited NiO-Cl-OH was annealed in air. After annealing, the broad peak of NiCl2 or Ni(OH)2 disappeared, while peaks related to the (111), (200), and (220) planes of NiO were retained (Figure 4a).[37] The examination of the cross section revealed a d-spacing of 0.58 nm for NiCl2 in the as-deposited NiO-Cl-OH. In contrast, the annealed NiO-Cl-OH sample was crystalline and contained porous NiO (Figure S7). XPS data indicated the existence of Ni2+, corresponding to NiCl2, Ni(OH)2, and NiO, lattice oxygen, and surface hydroxyl groups on the surface of as-deposited NiO-Cl-OH (Figures 4b and 4c).[38] In the annealed sample, the main Ni 2p peak shifted from 856.6 to 855.4 eV (Figure 4b),
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and the peak corresponding to NiCl2 disappeared from the Cl 2p region, related to the removal of chlorine and hydrogen during annealing (Figure 4d). As also shown in the TEM image in Figure 5a, the activated catalyst exhibited an amorphous and porous structure. In the XPS spectra (Figure 5b), the Ni2+ peak corresponding to NiO, NiCl2, and Ni(OH)2 disappeared, while the Ni3+ peak corresponding to NiOOH was observed at 856 eV in the Ni 2p region.[39] Lattice oxygen and Cl2 were not observed (Figures 5c and 5d). Hence, as-deposited NiO-Cl-OH is completely converted to amorphous NiOOH (a-NiOOH), while the NiO surface in annealed NiO-Cl-OH is changed to NiOOH only after CA at 1.23 V vs. RHE for 2 h. This result can be explained by the fact that NiCl2 undergoes chemical transformation into Ni(OH)2 under alkaline conditions.[40, 41] After the as-deposited NiO-Cl-OH catalyst was soaked in 1 M KOH for 12 h, Cl disappeared, while Ni was retained, as confirmed by EDS data (Figure S8). In addition, after immediately soaking the Si/SiOx/ITO/NiOCl-OH photoanode, its photocurrent density increased to 16 mA/cm2 without any activation time (Figure 5e). In this case, NiCl2 in the NiO-Cl-OH catalyst was already transformed to Ni(OH)2 during soaking, which was oxidized to NiOOH. However, the photocurrent density decreased to 8.3 mA/cm2 during oxygen evolution, possibly assumed to be caused by issues associated with adhesion on the ITO film. The photocurrent density of annealed NiO-Cl-OH (i.e., NiO) significantly increased after activation (Figure 5f). The peak at 1.2–1.4 V vs. RHE was believed to be related to the oxidation of Ni(OH)2 to NiOOH, indicative of the number of electrochemically active sites.[29, 42, 43] Hence, the large size for this oxidation peak corresponds to considerable charge transfer at the electrode surface. From electrochemical impedance spectroscopy measurements, activated Si/SiOx/ITO/NiO-Cl-OH exhibited a more rapid charge transfer 11
(i.e., smaller semicircle) as compared with that observed for the annealed photoanode (Figure S9), also related to the enhanced electrocatalytic activity on the surface.[10-12] All these results indicated that the activation of NiCl2 in the NiO-Cl-OH catalyst for the direct formation of NiOOH is induced by the number of electrochemically active sites and maintains the catalyst structure.
4. Conclusions A highly active, porous, and amorphous NiOOH catalyst was synthesized on a Sisputtered ITO photoanode by electrochemical activation via spraying a NiO-Cl-OH layer on the top of the photoanode. Activation decreased the resistance to charge transfer, related to the dramatic increase of electrochemically active sites. The chemical transformation of NiCl2 to Ni(OH)2 in the alkaline solution further accelerated the formation of a-NiOOH. High saturation photocurrent was achieved by the combination of a suitable Si/SiOx/ITO heterojunction, a thin ITO film with high solar absorption, and an active a-NiOOH catalyst with a porous structure. A high photovoltage of 545 mV and long-term stability for at least 30 h were observed for this photoanode in 1 M KOH under simulated light illumination. This study offers a guideline for the construction of stable, active semiconductor photoanodes with efficient OER catalysts under harsh conditions.
Acknowledgements This study was supported by New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) [grant numbers 20153030012190, 20153030013200] and the Basic Science Research
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Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning [grant number 2015R1A5A1037548].
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Figure captions: Figure 1. (a) Schematic of deposition–heating–immersion (DHI) for preparing the NiOCl-OH catalyst on the Si/SiOx/ITO photoanode, and (b) Cross-section and (c) EDS elemental mapping images of the as-deposited Si/SiOx/ITO/NiO-Cl-OH photoanode. Figure 2. (a) LSV curves of the Si/SiOx/ITO/NiO-Cl-OH photoanode before and after activation for 2 h at 1.23 V vs. RHE. (b) LSV curves of Si/SiOx/ITO/NiO-Cl-OH photoanodes at different DHI cycles. (c) Comparison of the photovoltage, photocurrent density (at standard oxygen evolution potential (Eo)), and saturation photocurrent density (at 2 V vs. RHE) of photoanodes at different DHI cycles. (d) Stability test of the photoanode for 30 h at 1.23 V vs. RHE. Figure 3. (a) Schematic side view and (b) energy band diagram of the Si/SiOx/ITO NiOCl-OH photoanode under illumination. LSV curves of the (c) Si/NiO-Cl-OH and (d) Si/SiOx/ITO structures. Figure 4. (a) XRD data and XPS spectra of the (b) Ni 2p, (c) O 1s, and (d) Cl 2p regions of the as-deposited and annealed Si/SiOx/ITO/NiO-Cl-OH photoanodes. Figure 5. (a) TEM image of the activated NiO-Cl-OH layer (inset: fast Fourier transformed image). XPS spectra of the (b) Ni 2p, (c) O 1s, and (d) Cl 2p regions of the as-deposited and activated Si/SiOx/ITO/NiO-Cl-OH photoanodes. (e) Current–time graph of the as-deposited Si/SiOx/ITO/NiO-Cl-OH photoanode and after soaking the photoanode in 1 M KOH for 12 h. (f) LSV curves of the annealed and activated Si/SiOx/ITO/NiO-Cl-OH photoanodes. Figure 1.
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