and Technology, Luoyu Road 1037, Wuhan, , P. R. China. *Corresponding author. ciac - Shanghai P. R.

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1 Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A. This journal is The Royal Society of Chemistry Supplementary Information For Journal of Materials Chemistry A Photoelectrochemical Cells for Unbiased Solar Water Splitting Xiaofan Zhang a, Bingyan Zhang a, Kun Cao a, Jérémie Brillet b, Jianyou Chen a, Mingkui Wang a and Yan Shen a a Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Luoyu Road, Wuhan,, P. R. China. Corresponding author. ciac - sheny@mail.hust.edu.cn b Shanghai Institute of Ceramics, Chinese Academy of Sciences, Dingxi Road, Shanghai P. R. China The power conversion efficiency (η) of the perovskite solar cell is calculated according to the following formula: ( ) η % = P P = ( FF J V )/ P out in sc oc in (S) where P in ( mw cm - herein) and P out are the incident light intensity and output power of the solar cell device, FF is the fill factor, and J sc and V oc stand for the short-circuit current density and open-circuit voltage of the solar cell, respectively. The overall solar-to-hydrogen (STH) efficiency of the PV/PEC cell is calculated with the following equation: STH. J η P PEC cell F = in % (S) where J PEC cell is the maximum photocurrent given by the photoelectrochemical cell, η F is the Faradic efficiency for the H evolution that can be calculated with the following equation: η F nh % = Q c (S)

2 (a) (b) (c) (d) nm μm μm μm Fig. S The SEM images of BiVO (a), and the cross-section images of (b) heterojunction film (c). (d) TEM image of, the inset is the selected area electron diffraction (SAED) pattern. The elemental compositions were collected by X-ray photoelectron spectroscopy (XPS, AXIS-ULTRA DLD- W, Shimdzu) under ultrahigh vacuum (< Torr) and using a monochromatic Al Kα X-ray source operating at W. The survey and high-resolution spectra were collected at fixed analyzer pass energies of and ev, respectively. The binding energy shifts were corrected using the Cs level at. ev as an internal standard, to compensate for the surface-charging effect during data analysis. Intensity / (a.u.) (a) Bi f C s V p Ti p O s Intensity / (a.u.) (b) Ti p / Ti p / Ti p Binding energy / ev Binding energy / ev (c) Bi f (d) O s Bi f / Intensity / (a.u.) Bi f / Intensity / (a.u.) O s V p / V p / Binding energy / ev Binding energy / ev Fig. S XPS spectra of composite film.

3 Fig. S exhibits both the characteristic Raman modes of the rutile and the monoclinic BiVO. The mode at cm - (B g ), cm - (E g ), cm - (A g ). and cm - (E g ) is the characteristic peak of the rutile. The strongest peak at cm - and the shoulder peak at cm - are from the anti-symmetric and symmetric stretching modes of the VO tetrahedral, respectively. The peaks at cm - and cm - belong to the bending modes of the VO tetrahedral. The remaining Raman peaks at cm - and cm - are attributed to the vibration of the crystal lattice. Intensity (a.u.) /BiVO BiVO Wavenumber / cm - Fig. S Raman spectrum for, BiVO NPs composite films. The phase purity and crystal structure of the obtained samples were examined by X-ray diffraction (XRD). The XRD patterns for the and BiVO NPs are displayed in Fig. Sa, corresponding to a pure tetragonal rutile and monoclinic BiVO phase, respectively. After deposition of the BiVO, only rutile and monoclinic BiVO phase are detected in the XRD pattern composite film, suggesting that the phase and composition of the is unchanged after deposition the BiVO NPs. Fig. Sb compares the UV-visible absorption of the samples. The optical band gap can be estimated by dropping a line from the maximum slope of the Tauc plot ( αh ν vsh ν ) to the axis (Fig. Sc). Therefore, the band tail was estimated to be. and. ev for the BiVO NPs and, respectively. Compared to the, photoanode exhibits an increase in light absorption intensity in the range from to nm. The broad absorption band

4 from to nm could be attributed to the light absorption of BiVO, which remarkably enhances the light absorption ability. Intensity (a.u.) (a) () () () () () () () () () θ / degrees /BiVO BiVO Absorbance. (b).... BiVO /BiVO. Wavelength / nm (αhν) / (ev / cm -/ ) (c) BiVO Eg / ev Fig. S (a) XRD patterns of the, BiVO NPs composite film. Asterisks ( ) is corresponding to diffraction peaks arising from the FTO substrate. (b) UV-vis absorption spectrum for all prepared samples. (c) Tauc plots of and BiVO NPs for the band gap calculation. Table S. Comparison of various -BiVO photoanodes for PEC water splitting. Electrolyte solution Photocurrent density at. V vs. RHE (ma cm - ) Reference M NaOH. (visible light). M K PO (ph.). ( full solar spectrum). M PBS (ph.). ( full solar spectrum). (λ> nm) a This work a This photocurrent density is calculated by integrating the measured IPCE (Figure b) over standard AM. G spectrum within the region λ> nm.

5 Amount of evolved H (μmol) H e - / O time (h) Fig. S PEC water splitting of photoanode at. V vs. RHE (the theory potential of water decomposition) in three-electrode system under irradiation. Black line corresponds to the integration over time of the net photocurrent divided by. Blue and red circles correspond to the evolved O and H gas measured by gas chromatography during the experiment, respectively. The absorbed photo-to-current efficiency (APCE) was obtained by dividing the IPCE by light harvesting efficiency (LHE) at each wavelength using the formula with APCE=IPCE/LHE, and the LHE plots of the samples are shown in Fig. a. In order to explain the fact that more photogenerated holes with my structure can be collected than the mesoporous structure, the normalized APCE was obtained (Fig. S). APCE / % BiVO /BiVO Wavelength / nm Fig. S APCE plots of the, BiVO photoanodes.

6 .. /C F cm.... BiVO... /C F cm Potential / V vs. RHE Fig. S Mott-Schottky plots of the (black line) and BiVO NPs (blue line). The applied bias photon-to-current efficiency (ABPE) was calculated from the J-V curves obtained from a two-electrode system where V bias is the applied bias between WE and CE assuming % Faradaic efficiency using the following equation, ( bias ) Jp. V ABPE= % (S) P in where J p is the photocurrent density (ma cm - ), V bias is the applied bias between the working electrode and counter electrode (V), and P in is the inciden illumination power density (AM.G, mw/cm )... BiVO NPs /BiVO ABPE / % Potential / V Fig. S ABPE plots of all photoanodes obtained using a two-electrode system.

7 Amount of evolved H (μmol) +PVSC time /h Fig. S The Faradaic efficiency and quantity of detected hydrogen derived from the +PVSC device under irradiation. Black line corresponds to the integration of the net photocurrent divided by. Black circle corresponds to the H gas measured by gas chromatography during the experiment. Faradaic efficiency / % Electrochemical impedance spectroscopy measurement was further performed at open circuit potential under AM.G illumination. Fig. S presents the typical Nyquist plots for various photoanodes. An equivalent circuit model (the inset of Fig. S) was employed to analyze the EIS data, in which R s is the resistance of the electrolyte, C p is the capacitance phase element, and R ct represents the charge-transfer resistance at the photoanode/electrolyte interface where the water oxidation reaction occurred. Normally, a decrease of the charge transfer resistance indicates a fast interfacial charge transfer process. Here, heterojunction photoanode exhibits the smallest charge transfer resistance among the three samples, which indicates a more effective separation of photogenerated carriers and faster interfacial charge transfer occurring in photoanode. This result is consistent with the PEC measurements that sample shows the best PEC water oxidation performance.

8 BiVO /BiVO - Z / kω Z / kω Fig. S Nyquist EIS plots for all photoanodes at open circuit potential recorded under light illumination. The inset is the equivalent circuit model used for fitting the experimental data (The solid lines are the fitting results, the circles are the experimental results). Reference. M. Xie, X. Fu, L. Jing, P. Luan, Y. Feng, H. Fu, Adv. Energy Mater.,,.. S. Kimura, S. Moniz, A. Handoko, J. Tang, J. Mater. Chem. A,,, -.. X. Zhang, B. Zhang, D. Huang, H. Yuan, M. Wang and Y. Shen, Carbon,,, -.. T. Kim, K. Choi, Science,, -.. M. Wang, P. Chen, R. Baker, S. Zakeeruddin and M. Grätzel, ChemPhysChem,,, -.