Nickel Coated Silicon Photocathode for Water Splitting in Alkaline Electrolytes

Transcription

1 Nano Research DOI /s Nano Res 1 Nickel Coated Silicon Photocathode for Water Splitting in Alkaline Electrolytes Ju Feng 1, Ming Gong 1, Michael J. Kenney 1, Justin Z. Wu 1, Bo Zhang 1, Yanguang Li 2 and Hongjie Dai 1 ( ) Nano Res., Just Accepted Manuscript DOI: /s on November Tsinghua University Press 2014 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer-review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 20 nm Pd P-Si 15 nm Ti TABLE OF CONTENTS (TOC) Authors are required to submit a graphic entry for the Table of Contents (TOC) in conjunction with the manuscript title. This graphic should capture the readers attention and give readers a visual impression of the essence of the paper. Labels, formulae, or numbers within the graphic must be legible at publication size. Tables or spectra are not acceptable. Color graphics are highly encouraged. The resolution of the figure should be at least 600 dpi. The size should be at least 50 mm 80 mm with a rectangular shape (ideally, the ratio of height to width should be less than 1 and larger than 5/8). One to two sentences should be written below the figure to summarize the paper. To create the TOC, please insert your image in the template box below. Fonts, size, and spaces should not be changed. Nickel Coated Silicon Photocathode for Water Splitting in Alkaline Electrolytes Ju Feng 1, Ming Gong 1, Michael J. Kenney 1, Justin Z. Wu 1, Bo Zhang 1, Yanguang Li 2 and Hongjie Dai* 1 1 Department of Chemistry, Stanford University, Stanford, California 94305, USA. 2 Institute of Functional Nano & Soft Materials, Soochow University, Suzhou , China. These authors contributed equally. 5 nm Ni We designed and fabricated a simple p-type Si based photocathode with high activity and good stability in potassium borate buffer solutions. Ni acts as both a protecting layer and hydrogen evolution reaction (HER) catalyst, while the low work function of Ti is necessary to afford a high photovoltage. Current density(ma/cm 2 ) KBi dark current Pt NPs on Ni/Ti/p-Si Potential(V vs RHE) Current density(ma/cm 2 ) Ni/Ti/p-Si in KBi dark light light after12h Potential (V vs RHE)

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4 Nano Research DOI (automatically inserted by the publisher) Research Article Nickel Coated Silicon Photocathode for Water Splitting in Alkaline Electrolytes Ju Feng 1, Ming Gong 1, Michael J. Kenney 1, Justin Z. Wu 1, Bo Zhang 1, Yanguang Li 2 Dai 1 ( ) 1 Department of Chemistry, Stanford University, Stanford, California 94305, USA. 2 Institute of Functional Nano & Soft Materials, Soochow University, Suzhou , China. and Hongjie Received: day month year Revised: day month year Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014 KEYWORDS Photoelectrochemical water splitting, Silicon photocathode, Nickel ABSTRACT Photoelectrochemical (PEC) water splitting is a promising approach to harvest and store solar energy.[1] Silicon is widely investigated for PEC photo-electrodes due to its suitable band gap (1.12 ev) matching the solar spectrum.[2] Here we investigated employing nickel both as a catalyst and protecting layer for p-type silicon photocathode for photoelectrochemical hydrogen evolution in basic electrolytes for the first time. The silicon photocathode was made by depositing 15 nm Ti on a p-type silicon wafer followed by 5 nm Ni. The photocathode afforded an onset potential of ~ 0.3 V vs. RHE in alkaline solution (1 M KOH). The stability of the Ni/Ti/p-Si photocathode showed a 100 mv decay over 12 h in KOH, but the stability was significantly improved when the photocathode was operated in potassium borate buffer solution (ph 9.5). The electrode surface was found intact after 12 h of continuous operation at a constant current density of 10 ma/cm 2 in potassium borate buffer, suggesting better protection of Ni in borate buffers for Si based photocathodes. Instructions for using the template Introduction Much effort has been put forth in the search for renewable and environmentally friendly energy sources due to the growing demand for energy supplies and the decreasing fossil fuel reserves. Solar energy is among the most promising candidates to serve as an energy source at the lowest cost of environment.[3] Since sunlight is intermittent, a major challenge is to harvest and store solar energy efficiently and cost-effectively.[4] Photoelectrolysis of water using semiconductors as both the light absorbers and energy convertors is becoming increasingly attractive and is considered by many to be "holy grail" of solar energy conversion and storage.[5] In principle, it is possible to complete overall water splitting with a single semiconductor but it has been challenging to find a material that is stable under both cathodic and anodic condition and has a suitable band alignment for water splitting. In addition, calculations have shown that a band gap greater than 3 ev is needed for a single Address correspondence to Hongjie Dai, hdai1@stanford.edu

5 2 Nano Res. semiconductor to drive water splitting due to a series of large energy loss processes involved which limit the solar to hydrogen efficiency significantly.[6] An alternative approach is to use a combination of a photocathode and a photoanode to drive the overall water splitting. Silicon is known to have a suitable band gap that is well matched to the solar spectrum. Up to now, silicon is still the dominant material used in photovoltaic systems.[7] The success of silicon photovoltaic systems has motivated wide exploration of silicon photoanodes and photocathodes. A major challenge with silicon PEC devices is that silicon is easily oxidized and corroded under PEC conditions in solution.[8] Under acidic condition, p-type silicon photocathodes were reported to be stable for one or two hours.[8-12] As for long-term stability, however, the activity of p-type silicon photocathode would decay to less than 10% of its original value after 5-12 h operation.[13-14] Thick TiO2 films have recently been demonstrated to successfully protect underlying silicon electrode for 12 h (80 nm TiO2)[13] and 72 h (100 nm TiO2)[14] under hydrogen evolution condition in acidic electrolytes. Thus far, p-type silicon based photocathodes have only been explored under acidic conditions and not in basic solutions.[8-9, 14-19] It is desirable to pair high performance photocathodes and photoanodes for water splitting in basic solutions, since it would allow the utilization of low cost non-noble metal based oxygen evolution catalysts with high electrocatalytic activities (higher activity than in acids)[20] integrated into photoanodes. We recently reported a highly active and stable Ni/n-Si photoanode in basic electrolytes, using Ni as the Si protection layer and an electrocatalyst for oxygen evolution reaction (OER).[21] Here, we explored the possibility of employing Ni as a coating layer of p-type silicon photocathode alkaline solutions. Results and discussion We deposited 15 nm Ti via electron beam (e-beam) evaporation on a p-type silicon wafer followed by 5 nm Ni. Ohmic contact was made to the backside of the silicon wafer by electron beam deposition of 20 nm Pd (Figure 1(a)). Cyclic-voltammograms (CV) of the Ni/Ti/p-Si cathode in 1 M KOH under illumination from a 150 W Xe lamp (~ 225mW/cm 2 ) showed high activity for photoelectrochemical water reduction (Figure 1(c)). The Ni/Ti/p-Si photocathode gave an hydrogen evolution reaction (HER) onset potential of ~0.3 V vs. RHE with a saturation current of ma/cm 2 (Figure S1), while bare p-type silicon by itself, Ti/p-Si and Ni/p-Si all afforded much lower HER activity in terms of onset potential (Figure 1(c)). We investigated the stability of our Ni/Ti/p-Si photocathode in 1 M KOH. The electrode was continuously operated at a constant cathodic current density of 10 ma/cm 2 and CV curves were taken before and after 12 h of operation. As shown in Figure 2(a), there was about 100 mv photovoltage loss after 12 h from CV characterization. The loss of activity was also observed in the chronopotentiometry curve (Figure 2(b)). Figure 2(b) indicated that most of the activity decay occurred during the first 2-3 hours of operation. The Ni/Ti/p-Si electrode was characterized by scanning electron microscopy (SEM) and auger photoelectron spectroscopy (AES) chemical mapping after the 12 h stability test. SEM image (Figure 2(c)) showed that some plate-like structures were formed and AES mapping indicated that the surface was corroded with gradual loss of Ni (Figure 2(d)). Recently, we found that Ni coating on n-type silicon photoanodes afforded higher stability in potassium borate buffer than in KOH under photoelectrochemical anodization conditions.[21] This motivated us to test our Ni/Ti/p-Si photocathode in potassium borate buffer. The electrode afforded an HER onset potential in potassium borate buffer at 0.25 V vs. RHE (Figure 3(a)), which was about 50 mv worse than that in 1 M KOH. The Tafel slope in the potassium borate buffer electrolyte was also larger, mostly due to the higher impedance (note: all the electrochemical data in this work was recorded and presented without ir compensation). Though the photocathode activity was slightly lowered by using potassium borate buffer, we observed significantly improved stability. The electrode was operated at a constant current density of 10 ma/cm 2 for 12 h in potassium borate buffer and CV curves were taken before and after the long time operation. The photocatalytic activity was found to be retained after 12 h of continuous operation. (Figure 3(a, b)) Importantly, SEM (Figure 3(c)) showed that the electrode surface was intact after the photoelectrochemical measurement. AES mapping

6 Nano Res. 3 (Figure 3(d)) also revealed that the electrode surface was still completely covered by Ni. Consistently, AES spectrum (Figure 3(e)) showed uniform Ni signals on the surface with negligible Ni etching or any observable silicon signals present, indicating that the underlying silicon was still under protection by the Ni film. Ni played two roles in our Ni/Ti/p-Si electrode. First, Ni expedited HER kinetics that are known to be very sluggish on bare silicon [17]. Both bare p-type silicon and Ti coated p-type silicon showed more than 400 mv higher HER overpotential compared to Ni/Ti/p-Si (Figure 1(c)). The electrochemical HER activity of Ni deposited on conducting fluorine-doped tin oxide was characterized, showing an HER onset potential of about V vs. RHE (Figure S2) in 1M KOH and confirming the HER catalytic activity of Ni in basic solutions. Ni served as a much better protection layer to the underlying silicon in the borate electrolyte than in KOH. Even after PEC in 1 M KOH, although SEM and AES mapping revealed surface corrosion of Ni/Ti/p-Si after 12 h PEC operations for HER (Figure 2(c, d)), Ni did help to slow down the Si corrosion evidenced by the fact that when Ti/p-Si was prepared and measured under same PEC condition for only 3h(Figure S3). Ti was previously explored as a protection layer for p-type silicon photocathodes in acidic solutions[9] and found relatively stable during a short~ 1 h test. However, longer time test led to Ti corrosion[14]. Our result here revealed that Ti was not an effective protection layer in alkaline solutions either. In potassium borate buffer, Ni/Ti/p-Si was found to be much more stable through spectroscopic and microscopic surface characterization, which was consistent with our previous observation in the photoanode case.[21] Since Pt is known to be the best HER catalyst with zero overpotential, we deposited Pt nanoparticles on our Ni/Ti/p-Si photocathode to glean the difference between Pt and Ni for HER catalysis in our system. As shown in Figure 4, Pt did improve the onset potential of our p-type silicon photocathode by mV in both 1 M KOH and potassium borate buffer and also afforded steeper Tafel slopes. This result indicated that we could combine our current electrode design with better HER catalyst to further boost the performance of silicon photocathode in basic solutions. Though Ni film provide a certain protection to the underlying p-type silicon in both KOH and potassium borate buffer, we did notice that the protection effect of Ni in Ni/Ti/p-Si photocathode was poorer than its superior protection for n-type silicon in the photoanode. This observation was initially surprising to us since we expected that the reducing potentials involved might be a less corrosive condition than in the highly oxidizing photoanode case. To better understand the similarities and differences of Ni protection between our p-type silicon photocathode and n-type silicon photoanode, X-ray photoelectron spectroscopy (XPS) depth profiling experiments were carried out by slow Ar ion milling of a 5 nm Ni /15 nm Ti/p-Si sample after 5 h PEC experiment in KOH (Figure 5). Similar to the Ni/n-Si photoanode case[21], the Ni and O signal intensities peaked at the surface and decreased as depth increased (Figures 5a). High-resolution Ni spectra at different milling times (Figure 5b) also revealed that there was a thin oxidized Ni layer followed by mainly metallic Ni. However, we found that the oxidation state of surface oxidized Ni layer on our photocathode was mainly Ni (II), while a higher oxidized phase of Ni (III) was formed under OER conditions[21]. In our photoanode work, we found that adding lithium to basic solutions including 1 M KOH and 1 M potassium borate electrolytes greatly improved the electrode s stability by slowing down Ni corrosion[21]. However, the same Li ion stabilization effect was not observed for Ni under cathodic conditions with the Ni/Ti/p-Si photocathode. We propose that in the photoanode case the Ni film is overcharged to form a gamma phase nickel oxyhydroxide (γ-niooh) under OER conditions and lithium can hinder the formation of the less dense and stable γ-niooh by incorporation of Li into β-niooh[22-23], resulting in a more stable film. Such an effect is not present under HER conditions when the Ni species on the surface is in a low oxidation state with oxidation number of ~ 2. This suggested that the high stability of Ni against silicon photoanode corrosion was owed to the higher oxidation state of Ni, which was further stabilized by Li incorporation to impede γ-phase formation. Thus, the current results also helped to shed light into the high stability of Ni/n-Si photoanodes. Nano Research

7 4 Nano Res. The use of Ti under Ni was important to maximize photovoltage due to the low work function of Ti affording a high Schottky barrier to the p-type silicon valence band (Figure 1(b)) (The Schottky contact was confirmed by I-V curve in Figure S5(b)). The resulting larger ban bending (Figure 1(b)) favors the transportation of photo-generated energetic electrons to electrode surface, while holes going into the bulk p-si. Therefore, Ni/Ti/p-Si afforded a much more positive onset potential for hydrogen evolution than Ni/p-Si electrode without the intermediate Ti layer as shown in Figure 1(c). Our photocathode was able to provide us with a photovoltage reaching an onset potential of HER at 0.3 V vs. RHE (Figure 1(c)). Conclusions In summary, we designed and fabricated a simple p-type Si based photocathode with high activity and good stability in potassium borate buffer solutions. Ni acts as both a protecting layer and HER catalyst, while the low work function of Ti is necessary to afford a high photovoltage. This approach involving Ni or other cheap metal/metal oxides could be applied to other p-type semiconductors which are unstable in alkaline solution under HER conditions, or/and have very sluggish HER kinetics. Acknowledgements This work was supported by a grant from Stanford GCEP, Precourt Institute of Energy and by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award # DOE DE-SC (for the microscopy and spectroscopy characterization part of this work). M.J.K. acknowledges support from an NSF Graduate Fellowship. Electronic Supplementary Material: Supplementary material is available in the online version of this article at (automatically inserted by the publisher). References [1] Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chemical Reviews 2010, 110, [2] Hamann, T. W.; Lewis, N. S. Control of the Stability, Electron-Transfer Kinetics, and ph-dependent Energetics of Si/H2O Interfaces through Methyl Termination of Si(111) Surfaces. The Journal of Physical Chemistry B 2006, 110, [3] United States, D. o. E. O. o. S. U. S. D. o. E. O. o. S.; Technical, I., United States. Dept. of Energy. Office of Science ; Distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy, Washington, D.C.; Oak Ridge, Tenn., [4] Lewis, N. S. Toward Cost-Effective Solar Energy Use. Science 2007, 315, [5] Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Accounts of Chemical Research 1995, 28, [6] Sivula, K.; Gratzel, M., in Photoelectrochemical Water Splitting: Materials, Processes and Architectures, The Royal Society of Chemistry, 2013, pp [7] Singh, R. Why silicon is and will remain the dominant photovoltaic material. Journal of Nanophotonics 2009, 3, [8] Hou, Y.; Abrams, B. L.; Vesborg, P. C. K.; Björketun, M. E.; Herbst, K.; Bech, L.; Setti, A. M.; Damsgaard, C. D.; Pedersen, T.; Hansen, O.; Rossmeisl, J.; Dahl, S.; Nørskov, J. K.; Chorkendorff, I. Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution. Nat Mater 2011, 10, [9] Seger, B.; Laursen, A. B.; Vesborg, P. C. K.; Pedersen, T.; Hansen, O.; Dahl, S.; Chorkendorff, I. Hydrogen Production Using a Molybdenum Sulfide Catalyst on a Titanium-Protected n+p-silicon Photocathode. Angewandte Chemie International Edition 2012, 51, [10] Tran, P. D.; Pramana, S. S.; Kale, V. S.; Nguyen, M.; Chiam, S. Y.; Batabyal, S. K.; Wong, L. H.; Barber, J.; Loo, J. Novel Assembly of an MoS2 Electrocatalyst onto a Silicon Nanowire Array Electrode to Construct a Photocathode Composed of Elements Abundant on the Earth for Hydrogen Generation. Chemistry A European Journal 2012, 18, [11] Warren, E. L.; McKone, J. R.; Atwater, H. A.; Gray, H. B.; Lewis, N. S. Hydrogen-evolution

8 Nano Res. 5 characteristics of Ni-Mo-coated, radial junction, n+p-silicon microwire array photocathodes. Energy & Environmental Science 2012, 5, [12] Esposito, D. V.; Levin, I.; Moffat, T. P.; Talin, A. A. H2 evolution at Si-based metal insulator semiconductor photoelectrodes enhanced by inversion channel charge collection and H spillover. Nat Mater 2013, 12, [13] Lin, Y.; Battaglia, C.; Boccard, M.; Hettick, M.; Yu, Z.; Ballif, C.; Ager, J. W.; Javey, A. Amorphous Si Thin Film Based Photocathodes with High Photovoltage for Efficient Hydrogen Production. Nano Letters 2013, 13, [14] Seger, B.; Pedersen, T.; Laursen, A. B.; Vesborg, P. C. K.; Hansen, O.; Chorkendorff, I. Using TiO2 as a Conductive Protective Layer for Photocathodic H2 Evolution. Journal of the American Chemical Society 2013, 135, [15] Boettcher, S. W.; Warren, E. L.; Putnam, M. C.; Santori, E. A.; Turner-Evans, D.; Kelzenberg, M. D.; Walter, M. G.; McKone, J. R.; Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S. Photoelectrochemical Hydrogen Evolution Using Si Microwire Arrays. Journal of the American Chemical Society 2011, 133, [16] Oh, J.; Deutsch, T. G.; Yuan, H.-C.; Branz, H. M. Nanoporous black silicon photocathode for H2 production by photoelectrochemical water splitting. Energy & Environmental Science 2011, 4, [17] Oh, I.; Kye, J.; Hwang, S. Enhanced Photoelectrochemical Hydrogen Production from Silicon Nanowire Array Photocathode. Nano Letters 2011, 12, [18] McKone, J. R.; Warren, E. L.; Bierman, M. J.; Boettcher, S. W.; Brunschwig, B. S.; Lewis, N. S.; Gray, H. B. Evaluation of Pt, Ni, and Ni-Mo electrocatalysts for hydrogen evolution on crystalline Si electrodes. Energy & Environmental Science 2011, 4, [19] Hou, Y.; Abrams, B. L.; Vesborg, P. C. K.; Björketun, M. E.; Herbst, K.; Bech, L.; Seger, B.; Pedersen, T.; Hansen, O.; Rossmeisl, J.; Dahl, S.; Nørskov, J. K.; Chorkendorff, I. Photoelectrocatalysis and electrocatalysis on silicon electrodes decorated with cubane-like clusters. Journal of Photonics for Energy 2012, 2, [20] Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A comprehensive review on PEM water electrolysis. International Journal of Hydrogen Energy 2013, 38, [21] Kenney, M. J.; Gong, M.; Li, Y.; Wu, J. Z.; Feng, J.; Lanza, M.; Dai, H. High-Performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation. Science 2013, 342, [22] Tuomi, D. The Forming Process in Nickel Positive Electrodes. Journal of The Electrochemical Society 1965, 112, [23] Oliva, P.; Leonardi, J.; Laurent, J. F.; Delmas, C.; Braconnier, J. J.; Figlarz, M.; Fievet, F.; Guibert, A. d. Review of the structure and the electrochemistry of nickel hydroxides and oxy-hydroxides. Journal of Power Sources 1982, 8, FIGURES. Nano Research

9 Current density(ma/cm 2 ) 20 nm Pd P-Si 15 nm Ti Potential (V vs RHE) 6 Nano Res. (a) (c) 0 5 nm Ni (b) Ni P-Si Ti electrode H + /H 2 solution (a) Current density (ma/cm 2 ) (c) nm Ni/15 nm Ti/ p-si 1M KOH dark light after 12 h Potential (V vs RHE) SEM after 12 h PEC in KOH (b) (d) M KOH 5 nm Ni/ 15nm Ti/ 10 ma/cm Time (h) Ni map dark current p-si 15 nm Ti /p-si 5 nm Ni /p-si 5 nm Ni /15 nm Ti /p-si 500 nm 400 nm Potential (V vs RHE) Figure 1: (a) Structure of Ni and Ti coated p-type silicon photocathode. (b) A band diagram for the Ni/Ti/p-Si photocathode. (c) Cyclic voltammograms (CV) of p-si, Ti/p-Si, Ni/p-Si, Ni/Ti/p-Si cathodes in 1 M KOH under illumination with a 150 W Xe lamp (~ 225mW/cm 2 ) and CV curve of Ni/Ti/p-Si in 1M KOH in the dark. Data in this work are all raw data without ir compensation applied. Figure 2: (a) Cyclic voltammograms of Ni/Ti/p-Si in 1M KOH (without ir compensation). CV data was taken before and after the test in (b). (b) Potential vs. time data under a constant current of 10 ma/cm 2 of Ni/Ti/p-Si photocathode under illumination in 1 M KOH for 12 h. (c) An SEM image of Ni/Ti/p-Si electrode after 12 h operation in KOH. (d) An Auger electron spectroscopic (AES) chemical mapping of Ni of Ni/Ti/p-Si electrode after 12 h operation in KOH.

10 dn(e)/de Current density(ma/cm 2 ) Potential (V vs RHE) Nano Res. (a) Ni/Ti/p-Si in KBi dark light light after12h (b) KBi nm Ni/ 15nm Ti/ 10 ma/cm Potential (V vs RHE) (c) SEM after 12 h PEC in KBi (d) Ni map Time(h) 500 nm 400 nm (e) O Ni Ni / Ti / p-si after 12h in KBi Kinetic Energy (ev) Si Figure 3: (a) Cyclic voltammograms of a Ni/Ti/p-Si in potassium borate (KBi) (without ir compensation). CV data was taken before and after the operation in (b). (b) Potential vs. time data under constant current of 10 ma/cm 2 of Ni/Ti/p-Si photocathode in KBi for 12 h under illumination. (c) An SEM image of a Ni/Ti/p-Si electrode after 12 h operation in KBi. (d) An AES chemical mapping of Ni and (e) a spectrum of Ni/Ti/p-Si electrode after 12 h operation in KBi (O: 500 ev; Ni: ev; Si: 1600 ev). Figure 4: Cyclic voltammograms of Pt nanoparticle coated Ni/Ti/p-Si in 1 M KOH (a) and in KBi (b) (see SI for experimental details). Nano Research

11 Intensity (a. u.) Intensity ratio Nano Res. (a) Ni O Si Ti (b) Milling time (min) 3rd cycle 10th cycle Ni 2+ Ni Binding Energy (ev) Figure 5: X-ray photoelectron spectroscopy depth profiling of Ni/Ti/p-Si photoanode. (a) Elemental depth profile of 5 nm Ni /15 nm Ti /p-si photocathode after 5 h of continuous PEC operation (10 ma/cm 2 ) in 1M KOH under illumination. (b) Ni 2p spectra for the sample in (a) taken after two different ion-milling times into the Si substrate. 3rd cycle corresponds to 0.8 min milling time, and 10th cycle corresponds to 3.6 min

12 Nano Res. Electronic Supplementary Material Nickel Protected Silicon Photocathode for Water Splitting in Alkaline Electrolytes Type author names here. The font is "Helvetica 10". Please spell out first names and surnames. Do not include professional or official titles or academic degrees. Place an ( ) by the corresponding author(s). For example, First A. Firstauthor 1,2 ( ), Second B. Secondauthor 2,, and Third C. Thirdauthor 1 ( ) Ju Feng 1, Ming Gong 1, Michael J. Kenney 1, Justin Z. Wu 1, Bo Zhang 1, Yanguang Li 2 Dai 1 ( ) 1 Department of Chemistry, Stanford University, Stanford, California 94305, USA. and Hongjie 2 Institute of Functional Nano & Soft Materials, Soochow University, Suzhou , China. Supporting information to DOI /s12274-****-****-* (automatically inserted by the publisher) Experimental details Electrode preparation: Ni/Ti/p-Si photocathode: The p-type silicon was used as received. 15 nm Ti was deposited on an as received [100] p-type silicon wafer ( ohm cm) via e-beam evaporation followed by 5 nm Ni at a deposition rate of Å /s. Ni/p-Si and Ti/p-Si electrodes were also prepared by depositing15 nm Ti or 5 nm Ni on p-type silicon wafer. Pt NPs/Ni/Ti/p-Si photocathode: Pt nanoparticles (Pt NPs) synthesis: 0.1 ml 0.1 M H2PtCl6 and 0.25 ml 0.1 M sodium acetate were added into 10mL ethylene glycol (EG). The mixed solution was refluxed at 160 C in oil bath for 3h. The resulting Pt NPs were washed with ethanol for 3 times. Pt NPs were then dispersed into ethanol. 0.1mL Pt NPs solution (contain ~0.1mg Pt) was then dropped onto a Ni/Ti/p-Si photocathode. The electrode was dried at room temperature. Back contact: Ohmic contact of all the electrodes was made to the backside of the wafer by e-beam deposition of palladium (20 nm). Copper tape was used to contact the palladium on the backside for electrochemical experiments. The Ohmic contact between palladium and p-type silicon was confirmed by I-V curves (Figure S5(a)). Electrochemical characterization: Electrodes were analyzed in a homemade square cell with a circular 0.38 cm 2 aperture sealed by the electrode. A 150 W Xenon lamp from Newport Corporation was used as the light source. The power density of the light irradiating the sample was measured with a Thorlabs PMT50 power meter to be ~ 225mW/cm 2. The prepared electrode was also characterized under 1 Sun, that is 100 mw/cm 2, with an AM 1.5 filter involved (Figure S4). All data presented in main text was obtained under ~ 225mW/cm 2. We chose to run our test under this harsher condition to investigate the stability of our electrode. Electrochemical experiments were carried out in a three-electrode system controlled by a CHI 760D potentiostat. A standard calomel electrode (SCE) was used as reference electrode and a stainless steel electrode was used as counter electrode. SCE was converted to RHE using the following equation. E(RHE) = E(SCE) V *pH Nano Research

13 Nano Res. The electrolytes used were 1 M KOH and potassium borate buffer made by mixing 2 M boric acid and 1 M KOH aqueous solution. All CVs were taken at 100 mv/s. All electrochemical measurements were done without ir correction and all data presented in this work was not ir corrected either. Materials Characterization: Prior to any physical characterization, the samples were washed with water, toluene and ethanol to remove any organic contamination. Scanning electron microscopy (SEM): SEM images was taken by an FEI XL30 Sirion scanning electron microscope. Auger electron spectra and element mapping were taken by PHI 700 Scanning Auger Nanoprobe operating at 10 kv and 10 na. X-ray photoelectron spectroscopy (XPS): XPS spectra and depth profiles were collected on a PHI VersaProbe Scanning XPS Microprobe. The Ar milling was done at 5 kv and 1 μa with spot size of 1 mm 1 mm. Materials: Chemicals: Potassium hydroxide: >=85% KOH basis; supplier: SIGMA-ALDRICH Boric acid:acs reagent, >=99.5%; supplier: SIGMA-ALDRICH Chloroplatinic acid hexahydrate: ACS reagent, 37.50% Pt basis; supplier: SIGMA-ALDRICH Sodium acetate:>= 99.0 %; supplier: J.T. Baker Ethylene glycol:>= 99 %;supplier: SIGMA-ALDRICH Metal targets: Nickel pellets: 4N; supplier: ESPI Metals Titanium: 99.99s%; supplier: Plasmaterials, Inc. Palladium: 99.99%; supplier: Plasmaterials, Inc.

14 Current density(ma/cm 2 ) Nano Res. Supplementary Figures dark current 5 nm Ni /15 nm Ti /p-si V vs. RHE Potential (V vs RHE) Figure S1: Zoom-in of cyclic voltammograms (CV) of Ni/Ti/p-Si cathodes, which is shown in Figure 1(c) Nano Research

15 Current (ma) Nano Res M KOH KBi Potential(V vs RHE) Figure S2: Cyclic voltammograms of 5nm Ni/FTO in KBi and KOH.

16 Nano Res. (a) (b) 200µm 10µm Figure S3: SEM images Ti/p-Si electrode after 3 h operation in KOH showing significant etching. Nano Research

17 Nano Res. Figure S4: Cyclic voltammograms of 5nm Ni/ 15 nm Ti/ p-si in 1 M KOH under 1 Sun (100 mw/cm 2 ), with an AM 1.5 filter involved.

18 Current (ma) Current (ma) Nano Res. (a) Voltage (V) Voltage (V) Figure S5: I-V curves of (a) Pd/p-Si/Pd and (b) Pd/p-Si/Ti/Ni electrode. (b) Address correspondence to Hongjie Dai, hdai1@stanford.edu Nano Research