Band Edge Engineering in BiVO 4 /TiO 2 Heterostructure: Enhanced Photoelectrochemical Performance through Improved Charge Transfer

Transcription

1 Supporting Information Band Edge Engineering in BiVO 4 /TiO 2 Heterostructure: Enhanced Photoelectrochemical Performance through Improved Charge Transfer Aadesh P. Singh 1, Nisha Kodan 1, Bodh R. Mehta 1*, Alexander Held 2,3, Leonhard Mayrhofer 2,3, Michael Moseler 2,3 1 Thin Film Laboratory, Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi , India 2 Fraunhofer IWM, Wöhlerstr. 11, Freiburg, Germany 3 Freiburg Materials Research Center FMF, University of Freiburg, Stefan-Meier-Str. 21, Freiburg, Germany Corresponding Authors * aadshp1982@gmail.com (A.P.S.), brmehta@physics.iitd.ac.in (B.R.M.) (a) (b) 400 nm Figure S1: (a) Tilted view of FESEM images of pristine BiVO 4 and (b) BiVO4/TiO 2 deposited at 80W rf-power and annealed in air at 400 o C (Image is taken at 30 o take off angle and the approximate thickness as calculated from the cross-sectional view is 150 nm). Change in surface morphology with TiO 2 coating is noticeable in terms of smooth and dense surface. 1

2 Intensity (a.u.) The inset of Figure 3(b) shows heterostructure formation which is clearly visible (the determined thickness of TiO 2 from cross-sectional SEM is ~25 nm with average total thickness 175 nm, this average film thicknesses were provided from several measurements and used for all other calculations). BiVO 4 BiVO 4 /TiO 2 BiVO 4 /H:TiO (Degree) Figure S2: XRD pattern of BiVO 4, BiVO 4 /TiO 2 and BiVO 4 /H:TiO 2 heterostructures 2

3 Intensity (a.u.) BiVO 4 BiVO 4 /TiO 2 BiVO 4 /H:TiO Raman Shift (cm -1 ) Figure S3: Raman spectra of BiVO 4, BiVO 4 /TiO 2 and BiVO 4 /H:TiO 2 heterostructures 3

4 Intensity (a.u) Bi 4f 4f 7/2 4f 5/ ev ev ev ev Binding Energy (ev) Figure S4: Core level Bi4f spectra of BiVO 4 thin film 4

5 Intensity (a.u) V2p ev ev ev Binding Energy (ev) Figure S5: Core level V2p XPS spectra of BiVO 4 thin films 5

6 Intensity (a.u) (a) TiO 2 (b) H:TiO 2 Ti 2p 3/ ev (b) 465 ev Ti 2p 1/2 (a) (b) Binding Energy (ev) Figure S6. Ti2p XPS spectra of pristine TiO 2 and H:TiO 2 thin films incorporated in hydrogen atmosphere at 300 o C 6

7 Intensity (a.u) Valence Band Spectra TiO 2 H:TiO ev 1.62 ev Binding Energy (ev) Figure S7. Valence band XPS spectra of pristine TiO 2 and H:TiO 2 thin films incorporated in hydrogen atmosphere at 300 o C 7

8 Extinction Coefficient TiO 2 H:TiO Wavelength (nm) Figure S8. Ellipsometeric extinction coefficient of TiO 2 and H:TiO 2 thin films in the UVvisible NIR spectral region. 8

9 TiO 2 H:TiO 2 ( h ) (ev/nm) 1/ Photon Energy(eV) Figure S9. Dependence of (αhν) 2 on photon energy for pristine and hydrogen doped TiO 2 thin films. These Tauc's plots were drawn with the help of ellipsometeric data for TiO 2 and H:TiO 2 samples as given in Figure S8. 9

10 Current Density (ma/cm 2 ) Time (minute) Figure S10. Long-term photoanode stability test of the BiVO 4 /H:TiO 2 heterostructure under visible light illumination. In order to check the stability of the BiVO 4 /H:TiO 2 heterostructure photoelectrodes, chronoamperometric studies were performed. A bias potential of 0.5 V vs. Ag/AgCl in 0.5 M Na 2 SO 4 solution was applied to the photoelectrode and the photocurrent density is measured for 180 minutes. A stable photocurrent density was found. It is worth noting that the nominal decrease in photocurrent densities in this stability test measurements are achieved at 0.5 V vs Ag/AgCl applied biases. 10

11 Reduction charge collector CB CB H 2 /H 2 O - charge transport E f light absorption VB + + light absorption + BiVO 4 H:TiO 2 + VB 1.23 ev OH - H 2 O/O 2 4 Oxidation OH electrolyte Figure S11. Approximate energy band diagram of BiVO 4 /H:TiO 2 heterostructure photoelectrodes and scheme of the charge transfer mechanism in BiVO 4 /H:TiO 2 heterostructure photoanodes. 11

12 Figure S12. Calculated band alignment between BiVO 4 and TiO 2 for varying oxygen displacement d for displacing a single oxygen atom in a 2x2x2-fold repeated primitive cell containing 16 TiO 2 units. Disorder-free TiO 2 is represented by d = 0. VBM is the valence band maximum. 12

13 Figure S13. Absolute error of the conduction band edge (CBE) with respect to the charge neutrality level (CNL) for TiO 2 and BiVO 4 as a function of the number of k-points along one unit cell axis of the primitive cell. The reference calculation was performed using a 13x13x13 k-point sampling. 13

14 Figure S14. Same as Figure 1 in the main manuscript but without any scissor operators and with TiO 2 data calculated using HSE06 hybrid functional and the experimental structure for the d = 0 starting configuration. 14