Impact of Silicon Resistivity on the Performance of Silicon Photoanode for Efficient Water Oxidation Reaction

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1 Supporting Information Impact of Silicon Resistivity on the Performance of Silicon Photoanode for Efficient Water Oxidation Reaction Qian Cai, Wenting Hong, Chuanyong Jian, Jing Li, Wei Liu* Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, , China Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, , China Supplementary Materials S1. Preparation of NiFe/TiO 2 /n-si photoanode Three types of single side polished P-doped (100) n-type Si wafer with different resistivity (0.01 Ω cm -1,0. 5 Ω cm -1,3.5 Ω cm -1 ) were employed for photoanode fabrication. Heavily boron-doped (100) p-type silicon wafers (ρ= Ω cm, thickness µm) were used as conductive silicon substrates to study water oxidation in the dark. For NiFe/TiO 2 /n-si photoanode fabrication, Si wafer is dipped into buffered HF for 2 minutes to remove native oxide completely. Subsequently, the Si wafer is cleaned by deionized water and is loaded into the ALD chamber for TiO 2 deposition. TiO 2 layers were deposited by Oxford Instruments FlexAL ALD on the fresh etched Si substrates. The ALD were conducted at 200 C with the Tetrakis(dimethylamido)titanium (TDMAT) as Ti precursor and water vapor as oxygen source. N 2 was used as carrier gas, each source was heated to a line temperature of 80 C. The pulse time of TDMAT and water was 5 s and 0.7 s, respectively. The equilibrium purge pressure was approximately 600 mtorr. The deposition rate was maintained 0.6 Å/cycle, 25 cycles of ALD was performed make the thickness of TiO 2 up to 2 nm. NiFe alloy with various Ni/Fe weight ratios are deposited by electron beam evaporation in the 1

2 vacuum of Torr. The deposition rate (0.2 Å/second) and thickness (deposition set point) of NiFe alloy are monitored by resonating quartz crystal sensor. Finally, Ti/Au (20 nm/100 nm) films are deposited onto the backside of Si substrate by e-beam evaporation to form an Ohmic contact. S2. UV-Vis absorption spectra measurement The UV-Vis absorption spectra of Si substrates and the Si-based photoanodes were measured using a Lambda950 UV-Visible Spectrophotometer (Perkin Elmer). Figure S1. (a) UV-Vis spectra of the Si substrate with different resistivity. (b) The comparison UV-Vis spectra of Si substrate and NiFe/TiO 2 /n-si photoanode. S3. Photoelectrochemical characterization methods All electrochemical measurements are performed on a CHI 660e electrochemical workstation with a standard three-electrode cell. The As-prepared Ni x Fe (1-x) /TiO 2 /Si films are used directly as the working electrode without further treatments. Pt wire is used as the counter electrode. The Ag/AgCl electrode filled with 3.5 M KCl serves as the reference electrode. All measured potentials are converted to the RHE reference using the following equation: E RHE = E Ag/AgCl V *pH, where E RHE is the potential referred to a reversible hydrogen electrode (RHE), and E Ag/AgCl is the measured potential against Ag/AgCl reference electrode. Xe lamp (300 W, PLS-SXE) with an AM 1.5 filter is the light source with an intensity of 100 mw cm -2 2

3 which is simulated as the 1 sun AM 1.5 illumination. All the photoelectrochemical tests are conducted in 1 M electrolyte at room temperature and under the light intensity of 100 mw cm -2. Before data is recorded, the electrodes are pre-scanned for 200 cycles by cyclic voltammogram (CV) in the range of -0.2 V ~0.6 V versus Ag/AgCl reference electrode until a stable CV curve is observed. Then, the polarization curves are recorded by linear sweep voltammetry (LSV) at a scan rate of 50 mv s -1 in the range of -0.2 V ~0.6 V versus Ag/AgCl reference electrode. The onset potentials and overpotentials of OER are obtained from LSV curves. CV curves for electrochemical active surface area (ESCA) are collected in the range of 0.02 V ~ 0.1 V versus Ag/AgCl reference electrode with scan rate from 20 mv s -1 to 100 mv s -1. All data are collected without ir compensation. Electrochemical impedance spectroscopy is performed in 1 M KOM electrolyte. AC voltage is 5 mv amplitude with the frequency range from 0.1 Hz to Hz. The initial potential is around their corresponding onset potential. The data is fitted to a model that consisted of a parallel resistor, and a capacitor with a fixed constant phase element at the Si interface arranged electrically in series with another resistor and capacitor in parallel at the NiFe/electrolyte interface. S 3.1. Transient photocurrents for different Ni 80 Fe 20 /TiO 2 /n-si photoanodes Figure S2. Transient photocurrents recorded at constant bias of 1.4 V versus RHE under chopped simulated irradiation (AM 1.5G) (green: 0.01 Ω cm -1 ; red: 0.5 Ω cm -1 ; blue: 3.5 Ω cm -1 ). Enlarged images from Figure 2(c) 3

4 S3.2. Electrochemical cyclic voltammetry curves Figure S3. Electrochemical cyclic voltammetry curves of Ni 80 Fe 20 /TiO 2 /n-si photoanodes with various resistivity of Si substrate at different scanning rates (20, 40, 60, 80, 100 mv s -1 ). S3.3. LSV curves of TiO 2 /n-si (0.5) photoanodes without NiFe catalyst Figure S4. LSV curves of TiO 2 /n-si (0.5) photoanodes without NiFe catalyst. It is clearly observed that the TiO 2 /n-si (0.5) photoanode without NiFe catalyst shows negligible OER activity. 4

5 S4. The comparison of OER performance Table S1. OER catalytic parameters of recently-reported NiFe-based and Si-based photoanodes. Photoanodes η 0 (V) η 10 (V) J sc (ma cm -2 ) Electrol yte Resistivity (Ω cm -1 ) Light (mw cm -2 ) Photovot age (mv) Reference NiFe/TiO 2 /n-si M This work Ir/TiO 2 /SiO 2 /p + n-si -0.3 / 4.8 PBS Ref.9 Nat. Mater Ni/TiO 2 / np + -Si M Ref.5 Science 2014 CoO x / SiO x / np + Si / 17 1 M NaOH Ref.S1 J. Am. Chem. Soc Ni/SiOx/ n-si M Ref.11 Science 2013 NiRuO x / n-bsi M / Ref.S2 Nano Na 2 SO 4 Fe 2 O 3 /n-sinw M NaOH n-si SiO x TiO 2 I r M NaOH IrO x /Ir/p+-n-Si M H 2 SO 4 Fe- NiOp + n-si n-si/sio x /CoO x /NiO x M M n-si/nio x M Na 2 SO 4 n-si/sio x /CoO x M p+n-si/coox M NaOH FeCoW/Ni / 1 M Foam NiFe-LDH/ / 1M Co 0.85 Se/EG NiFe/Ni Foam / 1M NiFeO x /CFP / 0.1M Exfoliated / 1 M NiFe LDH NiFe / 1 M LDH/CNT Lett / Ref.6 Angew. Chem. Int. Ed Ref.10 Nat. Mater Ref.S3 J. Phys. Chem. Lett / Ref.S4 J. Phys. Chem. Lett Ref.S5 Energy Environ. Sci Ref.S6 Energy Environ. Sci Ref.S7 Energy Environ. Sci Ref.12 Nature Mater 2017 Ref.S8 Science.2016 Ref.S9 Energy Environ. Sci Ref.12 Nat. Comm Ref.S10 Nat. Comm Ref.32 Nat. Comm Ref.S11 J. Am. Chem. Soc

6 S5. Photovoltage calculation. The photovoltage was defined as the difference between the overpotential to drive 1 ma cm -2 through the illuminated n-si photoanode and the dark p + Si anode. Figure S5. LSV curves of Ni 80 Fe 20 /TiO 2 /n-si (0.5) anode measured under illumination (blue curve) and Ni 80 Fe 20 /TiO 2 /p + -Si anode measured in the dark. S6. Stability test. The electrochemical stability test is performed in 1 M electrolyte by holding the potential at 1.16 V vs. RHE. To eliminate the influence of electrolyte concentration, the electrolyte is replaced by fresh solution after 10 h continuous measurements. Figure S6. The stability test of Ni 80 Fe 20 /TiO 2 /n-si photoanode in 1M electrolyte. S7. X-ray photoelectron spectroscopic (XPS) XPS measurements are conducted using a ESCALAB 250Xi system (Thermo Fisher), which is equipped with a 100 W Al Kα source on a spot size of 100 µm at 45 incident angle. The 6

7 binding energy scan range is ev in 1 ev steps. High-intensity excitation is provided by monochromatic Al Kα X-rays that is ev in energy with a 0.48 ev resolution at full width at half-maximum. XPS depth profiling is performed using an Ar + ion beam at 2 kv and 1 µa. Spectra are collected at intervals of 10 s during sputtering. The binding energy scale is calibrated to carbon line of ev. All XPS spectra are recorded with a resolution of 50 mev. Each data set is first corrected for the nonlinear emission background. Finally, the data is fitted with Gaussian function to find the deconvoluted peak positions. Figure S7. The Representative XPS survey spectrum of Ni 80 Fe 20 /TiO 2 /n-si(0.5) photoanode. The elements of Ni, Fe, C and O signals are detected from the survey spectrum. Figure S8. High-resolution XPS spectra of the Ni 80 Fe 20 /TiO 2 /n-si photoanode before and after OER tests. (a) High-resolution O 1s spectra. (b) XPS depth profiles spectra of O1s after OER test. Further investigating is taken on O 1s XPS spectrum shown in Fig. S4. The O 1s spectra for the pristine films display two predominant peaks while the tested film only presents a single peak. The peak centered at ev and ev are attributed to the metal oxide species (Ni-O, Fe-O) and metal hydroxide species (Ni-O-H, Fe-O-H), respectively. s12 This is consistent with previous analysis of Ni 2p spectra, the disappearance of a Ni-O species (

8 ev) after OER testing indicating a transformation from NiO x to NiOOH. The O 1s depth profile curves shown in Fig. S4b demonstrating the metal-oxide species is appeared beneath the surface, this changing of oxygen binding environments is corresponding with the depth profile spectra of Ni and Fe. The oxidized Ni and Fe species (NiOOH, FeOOH, Fe 2 O 3 ) on the composite surface are formed during the catalyst deposition and act as the OER active sites. References (S1) Yang, J.; Walczak, K.; Anzenberg, E.; Toma, F. M.; Yuan, G.; Beeman, J.; Schwartzberg, A.; Lin, Y. J.; Hettick, M.; Javey, A.; Ager, J. W.; Yano, J.; Frei, H.; W Ager, J. J. Am. Chem. Soc. 2014, 136, (S2) Sun, K.; Pang, X.; Shen, S.; Qian, X.; Cheung, J. S.; Wang, D. Nano Lett. 2013, 13, (S3) Mei, B.; Seger, B.; Pedersen, T.; Malizia, M.; Hansen, O.; Chorkendorff, I.; Vesborg, P. C. J. Phys. Chem. Lett. 2014, 5, (S4) 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. J. Phys. Chem. Lett. 2014, 5, (S5) 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. Energy Enviro. Sci. 2015, 8, (S6) Sun, K.; Park, N.; Sun, Z.; Zhou, J.; Wang, J.; Pang, X.; Paul, K. L. Energy Enviro. Sci. 2012, 5, (S7) Zhou, X.; Liu, R.; Sun, K.; Papadantonakis, K. M.; Brunschwig, B. S.; Lewis, N. S. Energy Enviro. Sci. 2016, 9, (S8) Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; García-Melchor, M.; de Arquer, F. P. G. Science 2016, 352, (S9) Hou, Y.; Lohe, M. R.; Zhang, J.; Liu, S.; Zhuang, X.; Feng, X. Energy Enviro. Sci. 2016, 9, (S10) Wang, H.; Lee, H. W.; Deng, Y.; Lu, Z.; Hsu, P. C.; Liu, Y.; Lin, D. C.; Cui, Y. Nat. Commun. 2015, 6, (S11) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. J. Am. Chem. Soc. 2013, 135, (S12) McIntyre, N. S.; Johnston, D. D.; Coatsworth, L. L.; Davidson, R. D.; Brown, J. R. Surf. Interface Anal. 1990, 15,