Supplementary Information

Transcription

1 Supplementary Information Supplementary Figures Supplementary Figure S1. Change in open circuit potential ( OCP) of 1% W-doped BiVO 4 photoanode upon illumination with different light intensities. Above an incident photon flux of ~10 21 s -1 m -2, OCP remains constant, indicating a maximum flattening of the energy bands of BiVO 4. The vertical dashed line indicates the intensity corresponding to AM1.5 sunlight. The thickness of the BiVO 4 film is ~200 nm.

2 a b c Supplementary Figure S2. Change in open circuit potential under dark (OFF) and AM1.5 illumination (ON) of 1% W-doped BiVO 4 (black), W:BiVO 4 homojunction (red), W:BiVO 4 reverse homojunction (green), and gradient-doped W:BiVO 4 (blue) (a). Distribution of the potential over a homogeneously-doped W:BiVO 4 film (b), and over a gradient-doped W:BiVO 4 film (c).

3 Supplementary Figure S3. Photographs of the 200 nm-thick BiVO 4 photoanode samples.

4 Supplementary Figure S4. Optical absorption of 200 nm-thick 1% W-doped BiVO 4 (black), W:BiVO 4 homojunction (red), W:BiVO 4 reverse homojunction (green), and gradient-doped W:BiVO 4 (blue).

5 Supplementary Figure S5. Scanning electron micrographs of 1% W-doped BiVO 4 (a), W:BiVO 4 homojunction (b), W:BiVO 4 reverse homojunction (c), and gradient-doped W:BiVO 4 (d).

6 Supplementary Figure S6. X-ray diffractogram (Bragg-Brentano geometry, Co Kα source) of 1% W-doped BiVO 4 (a), W:BiVO 4 homojunction (b), W:BiVO 4 reverse homojunction (c), and gradient-doped W:BiVO 4 (d). FTO substrate peaks are indicated by diamond markers.

7 Supplementary Figure S7. (a) Optical absorption of 200 nm-thick gradient-doped W:BiVO 4 on TEC-15 (black line) and the more highly textured Asahi UV conducting glass (red line). (b) Surface profile of TEC-15 (black line) and Asahi UV substrate (red line).

8 Supplementary Figure S8. The structure of (a) the 2-jn a-si and (b) the 1-jn a-si solar cell used in this article. The thickness of each layer is indicated (drawing is not to scale).

9 Supplementary Figure S9. The photograph of the hybrid photoelectrode device consisting of gradient-doped W:BiVO 4 photoanode and 2-jn a-si solar cell.

10 Supplementary Figure S10. (a) AM1.5 photocurrent vs voltage (J-V) curve of an uncatalyzed gradient-doped BiVO 4 photoanode in a two-electrode (black) and threeelectrode (red) measurement configuration. Both measurements were recorded simultaneously using a conventional three-electrode configuration, with an auxiliary high-impedance voltmeter (> 10 GΩ) connected to the working and counter electrodes to measure the two-electrode J-V curve. The horizontal dashed line indicates the current level used for the galvanostatic measurement shown in (b). This graph shows the difference between the two-electrode (black) and three-electrode (red) potential as a function of time when a constant current density of ~0.35 ma cm -2 (blue) is passed through the system. It takes ~18 minutes before the two-electrode potential approaches the three-electrode potential, which is the time it takes for the solution to become saturated with dissolved H 2 gas.

11 Supplementary Figure S11. AM1.5 spectrum and the measured fraction of the light that reaches the 2-jn a-si cell via transmission through the BiVO 4 photoanode as shown in Fig. 5.

12 Supplementary Figure S12. Short-circuit current vs time for the Co-Pi-coated FTO/2-jn a-si device under AM1.5 illumination.

13 Supplementary Figure S13. Two-electrode AM1.5 J-V curve of uncatalyzed gradientdoped W:BiVO 4 (black), Co-Pi catalyzed gradient-doped W:BiVO 4 (red), and Co-Pi on FTO substrate (green). Also shown are the J-V curves of the double-junction a-si solar cell (blue, solid) and that of the same solar cell placed behind the BiVO 4 photoanode (blue, dashed).

14 Supplementary Note 1 Definition of efficiency terms: IPCE, APCE and η sep The incident photon-to-current efficiency (IPCE, also called external quantum efficiency, or EQE) describes the amount of photocurrent collected per incident photon flux electrons cm s IPCE = EQE = (S1) -2-1 photons cm s The absorbed photon-to-current efficiency (APCE, also called the internal quantum efficiency, or IQE) is defined as the IPCE which is normalized to the amount of absorbed photons within the material. IPCE APCE = (S2) η abs where η abs is the number of electron-hole pairs generated per incident photon (absorptance). The APCE can also be defined as the product of the charge separation efficiency, η sep, and the catalytic efficiency, η cat: APCE = η η (S3) sep cat Here, the catalytic efficiency of the material is defined as the fraction of the photogenerated holes which, after arriving at the surface, successfully participate in the water oxidation reaction.

15 Supplementary Note 2 Band bending in the homojunction The amount of band bending at the interface of the homojunction is given by the following equation, D,W:BiVO4 E EF,W:BiVO E 4 F,BiVO ln N = = kt 4 N D,BiVO4 (S4) where EF,W:BiVO 4 and EF,BiVO 4 are the Fermi energy levels of W-doped and undoped BiVO 4, respectively, k is Boltzmann s constant, T is the temperature, and ND,W:BiVO 4 and ND,BiVO 4 are the donor densities of W-doped and undoped BiVO 4, respectively. Based on previously reported donor densities for undoped and W-doped BiVO 4, we estimate that the donor density in W-doped BiVO 4 is times higher than that in undoped BiVO 4. From equation S4, this leads to a total potential drop over the homojunction of ~ mv. The average thermal energy of the electrons (~kt) is sufficient to overcome a barrier of mv, which explains the somewhat modest decrease of the carrier separation for the reverse homojunction in Fig. 3.

16 Supplementary Note 3 Two-electrode and three-electrode J-V curves comparison The J-V curves of the BiVO 4 photoanode in the three-electrode measurement of Fig. 4 and the two-electrode measurement of Fig. 6a are different; Fig. 4 shows a more positive onset potential and lower photocurrents at potentials below ~0.7 V vs RHE. These differences arise from the different nature of the measurement. In a three-electrode measurement, the potential of the working electrode (WE) is measured with respect to that of a reference electrode (RE), which has a fixed potential. This is, however, not the case in a two-electrode measurement. In this case, the WE potential is measured against the potential of a platinum counter electrode (CE). The CE potential is not constant, but a function of the current density (overpotential) and the local ph and concentration of dissolved H 2 gas (Nernst potential). At low current densities, the equilibrium partial pressure of dissolved H 2 gas will (initially) be rather low, which causes the Nernst potential of the CE to be more positive than 0 V vs. RHE. Hence, (V WE -V CE ) is more negative than (V WE -V RE ) under otherwise identical conditions. This is illustrated in Supplementary Figure S10a for an uncatalyzed BiVO 4 photoanode. Since the potential difference between both curves is much larger than the typical overpotential observed for H 2 evolution on Pt (<0.1 V), most of it is attributed to a shift in the Nernst potential.to confirm this, a constant photocurrent density of ~0.35 ma cm -2 was passed through the sample while measuring the two-electrode and three-electrode potentials. As shown in Supplementary Figure S10b, the two-electrode potential slowly shifts towards the threeelectrode potential level. After ~18 minutes (without stirring) the difference between the curves no longer changes, indicating that the concentration of dissolved H 2 no longer

17 increases. The remaining ~50 mv difference is due to the overpotential for H 2 evolution at the platinum CE. This overpotential is also the main reason for the slightly higher photocurrents in Fig. 6 (two-electrode) compared to those in Fig. 4 (three-electrode) at potentials positive of 1 Volt. At these high current densities, the electrolyte solution quickly saturates with H 2 gas so that Nernst potential differences can be neglected.

18 Supplementary References 40. Sayama, K. et al. Photoelectrochemical decomposition of water into H 2 and O 2 on porous BiVO 4 thin-film electrodes under visible light and significant effect of Ag ion treatment, J. Phys. Chem. B 110, (2006). 41. Ye, H., Lee, J., Jang, J. S., & Bard, A. J. Rapid Screening of BiVO 4 -Based Photocatalysts by Scanning Electrochemical Microscopy (SECM) and Studies of Their Photoelectrochemical Properties, J. Phys. Chem. C 114, (2010). 42. Li, M. T., Zhao, L. A., & Guo, L. J. Preparation and photoelectrochemical study of BiVO 4 thin films deposited by ultrasonic spray pyrolysis, Int. J. Hydrogen Energy 35, (2010).