Supporting Information

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1 Supporting Information Enhanced Charge Injection and Collection of Nb-Doped TiO 2 /Gradient W-doped BiVO 4 Nanowires for Efficient Solar Water Splitting Zhangliu Tian, a,b Feng Shao, a Wei Zhao, a Peng Qin, a Jianqiao He, a,b and Fuqiang Huang a * a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai , P.R. China; b University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing , P.R. China. huangfq@mail.sic.ac.cn EXPERIMENTAL SECTION Synthesis: Arrays of Nb-doped TiO 2 nanowires (NWs) were grown on Fluorine-doped tin oxide (FTO) substrates (NSG, 14 Ω per square,wuhan Geao, China) by a hydrothermal method. In a typical synthesis, 1 ml Tetrabutyl titanate (TBOT, Aladdin, >99%) was slowly dripped into a mixture of 30 ml de-ionized water and 30 ml concentrated hydrochloric acid (HCl, 37 wt%) to form TBOT precursors, and mg NbCl 5 was dissolved into a mixture of 50 ml de-ionized water and 50 ml concentrated HCl to form NbCl 5 precursors. For Nbdoped TiO 2 NWs, NbCl 5 precursors were added to TBOT precursors at 0.5 mol% Nb/Ti ratio and the mixture was stirred intensively. After that, the FTO substrates were placed within a sealed Teflon reactor (50 ml), containing 30 ml the mixture obtained above. The mixture was kept in 180 C for 4 h. At last, the FTO substrates with as-synthesized NW samples were rinsed with DI water, dried in air, and then annealed in air at 500 C for 1 hour. S1

2 The BiVO 4 nanoparticles were deposited onto the TiO 2 NWs using spray pyrolysis. A solution containing 10 mm bismuth and 10 mm vanadium prepared by dissolving bismuth nitrate pentahydrate (Aladdin, 99%) and vanadyl acetylacetonate (Aladdin, 99%) in 2- methoxyethanol (Alfa Aesar, 99%) was sprayed 21 cycles onto the TiO 2 NWs at a temperature of 450. A pulsed deposition mode was used, with one spray cycle consisting of 15 s spray time followed by a delay of 60 s to allow solvent evaporation. The 1% W-doped BiVO 4 sample was prepared by spraying 21 cycles of the W-doped BiVO 4 precursor solution containing 1 at% ( W/(W+V) ) of W by added Tungsten(VI) isopropoxide (Alfa Aesar, 5% w/v in isopropanol) into the above BiVO 4 precursor. To deposit the gradient-doped W: BiVO 4, firstly, W-doped BiVO 4 precursor solutions containing 1 at% ( W/(W+V) ), 0.95 at% ( W/(W+V) ), 0.9 at% ( W/(W+V) ),, 0.1 at% ( W/(W+V) ), 0.05 at% ( W/(W+V) ) and 0 at% ( W/(W+V) ) were prepared. Then, the W- doped BiVO 4 precursor solutions were changed in step every cycle starting from containing 1 at% ( W/(W+V) ) to 0 at% to obtain gradient-doped W: BiVO 4. Co-Pi catalysts were deposited on the surface of photoanodes by photoassisted electrodepostion described previously. 1 Typically, a 0.1M KH 2 PO 4 solution was first prepared and titrated to ph 7 with 1 M KOH, and then Co(NO 3 ) 2 (0.5 mm, 99%, Aladdin ) was added in the solution to form the electrolyte. The potential of the working electrode was controlled by an electrochemical workstation (CHI660B, CH Instruments). A Pt wire and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The electrodeposition was performed at a constant voltage of 0.2 V RHE for 5 min Characterization of the Samples: To investigate the microstructure and composition of the samples, UV-Vis-NIR spectrometer (Hitachi U4100), field emission scanning electron microscopy (FE-SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEOL JEM- 2100F) with an energy dispersive spectrometer (Oxford), X-ray diffraction (XRD, Bruker D8 S2

3 Advance), X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA System) with Mg Ka X-ray (hv= ev) at 14 kv were employed. To characterize the PEC performance of the samples, a conventional three-electrode system was utilized to conduct electrochemical measurements with an electrochemical workstation (CHI660B, CH Instruments). The samples were used directly as the working electrode, with a Pt wire and an Ag/AgCl (KCl saturated) electrode as counter and reference electrodes respectively in 0.5 M potassium phosphate electrolyte buffered to ph 7. A set of linear sweeps and transient photocurrents responses were recorded under illumination. A 150W Xe lamp was used as the light source to simulate the sunlight irradiation. The light intensity was measured by a calibrated Si photodiode. Mott-Schottky plots were derived from impedancepotential tests conducted at a frequency of 1 khz in dark. Intensity-modulated photocurrent/photovoltage spectra (IMPS/IMVS) and IPCE were measured by the Zahner Zennium C-IMPS system. Theory of PEC photoanode: According to previous reports, 1-3 the photocurrent for water oxidation (J H2O ) is given the following equation (S1): J =J η η η (S1) where J max is determined by integrating the incident spectral irradiance over all wavelengths below the absorption edge of a photoanode. η abs represents light absorption efficiency, determined by integrating the product of the incident spectral irradiance and the light harvesting efficiency (LHE, the fraction of absorbed light) at each wavelength, and dividing by J max. η sep and represent charge separation efficiency and surface harge transfer efficiency, respectively. Due to that the oxidation of Na 2 SO 3 is very fast, the η trans is nearly equal to 100%, 4 the photocurrent for sulfite oxidation (J sulfite ) obtained was used to calculate η sep. Hence, the equation (S1) can be transformed as following equation (S2): J =J η η S3 (S2)

4 Therefore, the η trans can be calculated by divding J H2O by J sulfite, and the η sep can be obtained by divding J sulfite by J max η abs. The ABPE was calculated from the J V curves using the following equation (S3), ABPE=. 100 (S3) where J is the photocurrent density, V RHE is the applied potential and P in is the incident illumination power density (AM 1.5G, 100 mw cm 2 ). Figure S1. (a) Schematic of BiVO 4 coated on the side surface of Nb-doped TiO 2 NWs. (b) and (c) SEM images of N:T g-w:b host guest NWs. S4

5 Figure S2. (a) X-ray diffraction (XRD) patterns for N:T NWs before and after g-w:b deposition, (b) B, W:B and g-w:b coated on FTO substrates; and (c) B, W:B and g-w:b deposited on N:T NWs. S5

6 S6

7 Figure S3. (a)stem images and EDS mapping of 1 at% W-doped BiVO4 coated on N:T NWs. (b) HRTEM image of 1 at% W-doped BiVO4 coated on N:T NWs. Figure S4. (a) Optical absorption and (b) Tauc plots of B, W:B and g-w:b particles deposited on FTO substrate. The W concentrations doped in BiVO4 by the spray pyrolysis were too small to cause any significant change in the band gap. S7

8 Figure S5. (a) Optical absorption and (b) Tauc plot of N:T NWs coated on FTO substrate. Figure S6. Mott Schottky plots of N:T and W:B showing that W:B has more negative flat band than that of N:T. In addition to the UPS measurement, which was conducted dry ambient conditions, Mott Schottky plots were used to further estimate E C in an aqueous environment (test conditions). As shown in Figure S6, the flat band potential (V fb ) of W:B (0.02 V RHE ) is 0.14 V more negative than that of N:T (0.16 V RHE ). Under the reasonable assuming that the gap between V fb and E C is similar and small for W:B and N:T, the E C of W:B is more negative than that of N:T, in consistent with the analysis of the UPS. S8

9 Figure S7. Spectral irradiance of Xe lamp solar simulator (red) and standard AM 1.5G solar spectrum (purple). As the band edge of BiVO 4 is around 515 nm, the J max for BiVO 4 is 7.5 ma cm -2, 3 which is calculated by integrating the incident spectral irradiance of AM1.5 over all wavelengths below 515 nm. However, there are always differences between the incident spectral irradiance of AM1.5 and simulated solar illumination used in the experiment. Hence, the J max used during the experiments should be determined by integrating the incident spectral irradiance of simulated solar illumination, which gives 8.0 ma cm -2 in our study. Figure S8. Linear sweep voltammograms of different loadings of g-w:b partilces on N:T NWs for (a) sulfite oxidation and the corresponding (b) product of light absorption and charge S9

10 separation efficiency (η abs η sep ) versus potential. An optimum is observed for spraying 21 cycles of the g-w:b. Figure S9. (a) Light harvesting efficiency (LHE) of different loadings of g-w:b particles on N:T NWs. (b) Integrated η abs from the LHE and η sep at 1.23V RHE versus spraying cycles. In order to optimize the g-w:b amount deposited on N:T NWs, the effect of g-w:b loading amount on η abs η sep was investigated via controlling the spraying cycles. In the process of the increase in loading amount, the change of η abs and η sep can be divided into three stages as shown in Fig. S9: i) the η sep keep high value and remain almost unchanged, and η abs increase rapidly during the increase in small loading amount; ii) η sep have a trend of decline or decrease slowly, and η abs increase slowly or have a trend of remaining unchanged during the increase in moderate loading amount; iii) η sep decrease rapidly, and η abs remain almost unchanged during the increase in large loading amount. Therefore, a maximum value of η abs η sep can be obtained in the process of the increase in loading amount. From J sulfite s for different spraying cycles (Fig. S8), it can be seen that an optimum loading amount exists, realized by spraying 21 cycles. S10

11 Figure S10. Schematic diagram illustrating the energy levels of conduction band and valence band of rutile WO 3 and SnO Table S1 η inj and η col determined for N:T B N:T W:B and N:T g-w:b photoanodes with different applied bias at the wavelength of 448 nm. Photoanodes LHE at 0.6 V RHE at 1.23V RHE (%) IPCE sulite (%) η col (%) η inj (%) IPCE sulite (%) η col (%) η inj (%) N:T B N:T W:B N:T g-w:b S11

12 Figure S11. Photocurrents of BiVO 4 particles on the FTO substrate for water oxidation (J H2O ). Figure S12. ABPE of TiO 2 -BiVO 4 NW heterostructure photoanodes. S12

13 Figure S13. J-V curve of N:T g-w:b Co-Pi photoanode for water oxidation measured at 1.23 V RHE for 4h. Figure S14. H 2 and O 2 evolution produced by N:T g-w:b Co-Pi photoanode at 1.23 V RHE and calculated H 2 and O 2 amount from the photocurrent assuming 100% Faradaic efficiency. References S13

14 1. Zhong, D. K.; Choi, S.; Gamelin, D. R., Near-Complete Suppression of Surface Recombination in Solar Photoelectrolysis by "Co-Pi" Catalyst-Modified W:BiVO4. Journal of the American Chemical Society 2011, 133 (45), Dotan, H.; Sivula, K.; Gratzel, M.; Rothschild, A.; Warren, S. C., Probing the photoelectrochemical properties of hematite (alpha-fe2o3) electrodes using hydrogen peroxide as a hole scavenger. Energy & Environmental Science 2011, 4 (3), Rao, P. M.; Cai, L. L.; Liu, C.; Cho, I. S.; Lee, C. H.; Weisse, J. M.; Yang, P. D.; Zheng, X. L., Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation. Nano letters 2014, 14 (2), Zhou, L. T.; Zhao, C. Q.; Giri, B.; Allen, P.; Xu, X. W.; Joshi, H.; Fan, Y. Y.; Titova, L. V.; Rao, P. M., High Light Absorption and Charge Separation Efficiency at Low Applied Voltage from Sb-Doped SnO2/BiVO4 Core/Shell Nanorod-Array Photoanodes. Nano letters 2016, 16 (6), Wu, W.; Changzhong, J.; Roy, V. A., Recent progress in magnetic iron oxidesemiconductor composite nanomaterials as promising photocatalysts. Nanoscale 2015, 7 (1), Saito, R.; Miseki, Y.; Sayama, K., Highly efficient photoelectrochemical water splitting using a thin film photoanode of BiVO4/SnO2/WO3 multi-composite in a carbonate electrolyte. Chemical communications 2012, 48 (32), Kumar, P. S.; Sundaramurthy, J.; Sundarrajan, S.; Babu, V. J.; Singh, G.; Allakhverdiev, S. I.; Ramakrishna, S., Hierarchical electrospun nanofibers for energy harvesting, production and environmental remediation. Energy Environ. Sci. 2014, 7 (10), S14