Supporting Information for: High Light Absorption and Charge Separation Efficiency at Low Applied Voltage from Sb-doped SnO 2 /BiVO 4 Core/Shell Nanorod-Array Photoanodes Lite Zhou 1,2, Chenqi Zhao 1,2, Binod Giri 1, Patrick Allen 1, Xiaowei Xu 1,2, Hrushikesh Joshi 1, Yangyang Fan 1,2, Lyubov V. Titova 3 and Pratap M. Rao 1,2 * 1 Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA 169, USA 2 Materials Science and Engineering Program, Worcester Polytechnic Institute, Worcester, MA 169, USA 3 Department of Physics, Worcester Polytechnic Institute, Worcester, MA 169, USA *Correspondence and requests for materials should be addressed to P. M. R. (email: pmrao@wpi.edu) 1
Figure S1. Optical image of FTO (left), FTO/Sb:SnO 2 NRAs (middle), FTO/Sb:SnO 2 NRAs/BiVO 4 (right). 2
Figure S2. (a) X-ray diffraction patterns of Sb:SnO2 NRAs (red), BiVO 4 film (purple), and Sb:SnO 2 /BiVO 4 core/shell NRAs (black), all on quartz substrates. Scanning electron microscopy top view images of (b) Sb:SnO 2 NRAs, and (c) BiVO 4 drop-casted onto the Sb:SnO 2 NRAs to form Sb:SnO 2 /BiVO 4 core/shell NRAs, all on quartz substrates. 3
Figure S3. Scanning electron microscopy cross-section images (and inset top-view images) of (a) 2 layers and (b) 4 layers of BiVO4 drop-casted onto Sb:SnO2 NRAs. Performance of 2 layers (red), 3layers (blue) and 4 layers (purple) of BiVO4 drop-casted onto Sb:SnO2 NRAs for photoelectrochemical sulfite oxidation, measured using a 3-electrode configuration in aqueous 4
phosphate buffer (ph 7) with 1M Na 2 SO 3, under back-side illumination. (c) Plots of current density (J sulfite ) versus potential under illumination (solid lines), and in the dark (dashed lines), (d) product of light absorption and charge separation efficiency (η abs η sep ) versus potential, (e) light harvesting efficiency (LHE) versus wavelength (and inset overall absorption efficiency - η abs - versus numbers of BiVO 4 layers) and (f) charge separation efficiency (η sep ) versus potential. 5
LHE (%) 1 8 6 Solar Simulator 4 2 AM 1.5 G 3 35 4 45 5 55 Wavelength (nm) 3 25 2 15 1 5 Spectral Irradiance (µw/cm 2 /nm) Figure S4. Light harvesting efficiency (LHE) of Sb:SnO 2 /BiVO 4 (3 layers) core/shell NRAs (blue), and spectral irradiance of Xe lamp solar simulator (red) and standard AM 1.5G solar spectrum (purple). 6
a LHE (%) 1 9 8 Sb:SnO 7 2 /BiVO 4 6 5 4 Sb:SnO 3 2 2 FTO 1 3 4 5 6 Wavelength (nm) b J sulfite (ma/cm 2 ).5.45.4.35.3.25.2.15.1.5 Sb:SnO2 on Sb:SnO2 off.2.4.6.8 1 Potential (V RHE ) Figure S5. (a) Light harvesting efficiency (LHE) of Sb:SnO 2 /BiVO 4 core/shell NRAs on FTOglass (blue), Sb:SnO2 NRAs on FTO-glass (red) and FTO-glass alone (purple); (b) Plots of current density (J sulfite ) versus potential for Sb:SnO 2 NRAs (without BiVO 4 coating) under illumination (blue) and in the dark (red), measured using a 3-electrode configuration in aqueous phosphate buffer (ph 7) with 1M Na 2 SO 3. 7
Figure S6. Mott-Schottky measurement (obtained at 1 khz frequency, 2 mv amplitude, 5s equilibration time) of planar, non-porous film of BiVO 4 in (a) aqueous phosphate buffer (ph 7); (b) aqueous phosphate buffer (ph 7) with the addition of 1M Na 2 SO 3. The presence of sulfite does not alter the surface band energies of BiVO 4. 8
Figure S7. Scanning electron microscopy cross-section image (and inset top-view image) of 3 layers of porous BiVO 4 directly drop-casted onto a planar FTO substrate. 9
4.5 Light off Jsulfite (ma/cm2) 4 3.5 3 2.5 2 1.5 1 Light on.5 1 Time (h) 2 Figure S8. Stability measurement of Sb:SnO2/BiVO4 core/shell NRA photoanode, measured using a 3-electrode configuration in aqueous phosphate buffer (ph 7) with 1M Na2SO3 at.6vrhe under simulated solar illumination. 1
a 7 6 J H2O (ma/cm 2 ) 5 4 3 2 Sb:SnO 2 /BiVO 4 /NiFeO x -Bi 1 Sb:SnO 2 /BiVO 4.2.4.6.8 1 1.2 Potential (V RHE ) b J H2O (ma/cm 2 ) 3.5 3 2.5 2 1.5 1.5 1 2 3 Time (min) c Amount of Oxygen Evoled (µmol) 14 12 1 8 6 4 2 O 2 Calculated O 2 1 2 3 Time (min) Figure S9. Performance of Sb:SnO 2 /BiVO 4 and Sb:SnO 2 /BiVO 4 /NiFeOx-Bi (in which NiFeOx- Bi is NiFe-(oxy)hydroxide/borate OER catalyst) for photoelectrochemical water oxidation, measured using a 3-electrode configuration in aqueous 1M potassium borate (ph 9) solution. (a) Plots of current density (J H2O ) versus potential for Sb:SnO 2 /BiVO 4 /NiFeO x -Bi NRAs under illumination (blue) and in the dark (blue dashed) and Sb:SnO 2 /BiVO 4 NRAs under illumination (red) and in the dark (red dashed). (b) Stability test of Sb:SnO 2 /BiVO 4 /NiFeOx-Bi at.6v RHE 11
under illumination. (c) Oxygen evolution detected by a phase fluorimeter probe (blue) and oxygen calculated from photocurrent (red). 12
a.6 Cs -2 (uf -2 ).4.2 plateau b -.2.3.8 1.3 1.8 Potential (V RHE ) 1 Depletion width (nm) 8 6 4 2.5 1 1.5 2 2.5 Potential (V RHE ) Figure S1. a) Mott-Schottky plot of planar, non-porous BiVO 4 over extended potential range, and b) depletion width of BiVO 4 versus potential. The Mott-Schottky plot is a plot of 1/C 2 vs V, where C is the capacitance, and V is the voltage. The equivalent circuit for the Mott-Schottky measurement is a resistor in series with a capacitor. The capacitor is taken to represent the capacitance across the semiconductor depletion layer. The capacitance of the electrical double layer at the semiconductor/electrolyte interface is assumed to be much larger, and can therefore be neglected. No additional circuit elements are needed to account for mass transport, etc. since these measurements are made at high frequency. 13
The capacitance C at frequency f is calculated from the imaginary part of the measured impedance Z as: 1 Im( Z) = 2πfC In the linear region at low potentials, the depletion width is smaller than the thickness of the semiconductor. When the depletion width equals or exceeds the thickness of the semiconductor at higher potentials, the capacitance plateaus,[1] which can be seen in the Mott-Schottky plot in Figure S1a. The capacitance of the semiconductor/liquid junction is given by the following equation 1 = 2 ( ) where C is the capacitance, A is the surface area of the planar semiconductor (2 cm 2 for our planar BiVO 4 ), ε = 8.8*1-12 F/m is the permittivity of free space, ε is the relative permittivity of the semiconductor, (68 for BiVO 4 [2]), e=1.6*1-19 C is the electron charge, k=1.38*1-23 (m 2 kg s -2 K -1 ) is the Boltzmann constant, T is temperature =3 K, and V FB is the flat band potential of the semiconductor. The concentration of electron donors (N D ) is calculated from the slope of the measured Mott-Schottky plot using the above equation. The value obtained is 1.7 1 18 /cm 3. Then the depletion width as a function of applied voltage, is calculated using the following equation, and is plotted in Figure S1b. = 2 This calculation shows that the depletion width becomes equal to the film thickness of ~8 nm for a potential of around 1.5 V, which matches the potential at which the MS plot plateaus. [1] Li, M.; Zhao, L.; Guo, L. Int. J. Hydrogen Energy 21, 35, (13), 7127-7133. [2] Zhong, D. K.; Choi, S.; Gamelin, D. R. J. Am. Chem. Soc. 211, 133, 1837-18377. 14
LHE (%) 1 9 8 7 6 5 4 3 2 1 Sb:SnO 2 /BiVO 4 WO 3 /BiVO 4 7% W-doped BiVO 4 BiVO 4 3 35 4 45 5 55 6 Wavelength (nm) Figure S11. Light harvesting efficiency (LHE) of planar Sb:SnO 2 /BiVO 4 (red), planar WO 3 /BiVO 4 (blue), planar 7% W-doped BiVO 4 (green) and planar undoped BiVO 4 (purple). 15
a Lateral Scattering Diffuse Transmission Diffuse Reflection Specular Transmission Specular Reflection Incident light Diffuse Reflection Diffuse Transmission Lateral Scattering b 7 6 Efficiency (%) 5 4 3 2 Reflection Scattering Specular Transmission 1 Diffuse Transmission 45 55 65 75 Wavelength (nm) Figure S12. (a) Schematic of transparency test; (b) Specular transmission efficiency (purple), reflection efficiency (red), scattering efficiency (black), diffuse transmission efficiency (green) of Sb:SnO 2 /BiVO 4 core/shell nanorod array photoanode under 45nm to 79nm irradiation. 16