A Multi-step Ion Exchange Approach for Fabrication of Porous BiVO 4 Nanorod Arrays on Transparent Conductive Substrate

Transcription

1 Supporting Information A Multi-step Ion Exchange Approach for Fabrication of Porous BiVO 4 Nanorod Arrays on Transparent Conductive Substrate Cong Liu, Jinzhan Su*, Jinglan Zhou and Liejin Guo International Research Centre for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi an Jiaotong University, Shaanxi , P. R. China. j.su@mail.xjtu.edu.cn EXPERIMENTAL SECTION Synthesis of films All the chemicals were used without further purification. Preparation of SnS x : 0.05 M tin tetrachloride (SnCl 4 ) and 0.1 M thioacetamide (TAA) were taken in 50 ml ethanol solvent. After dissolving them, the solution was transferred into a Teflon-lined autoclave (100 ml) with a fluorine-doped tin oxide (FTO) coated glass substrate inserted. Prior to the FTO substrates used for deposition were firstly cleaned with detergent solution, acetone solution, S1

2 deionized water and ethanol solution by ultrasonication for a total of 40 minutes and then dried in a nitrogen gas flow. The solvothermal reaction was performed at 80 for 4 hours. Preparation of BiS m : as-prepared SnS x film was placed on the bottom of a Teflon-lined autoclave containing a mixture of 0.04 M bismuth chloride, 2 ml concentrated hydrochloric acid and 50 ml deionized water. The hydrothermal reaction was performed at 100 for 6, 12, 24, 48 hours. Preparation of BiVO 4 : 1.2 ml of a dimethyl sulfoxide (DMSO) solution containing 0.2 M vanadyl acetylacetonate (VO(acac) 2 ) was placed on the as-prepared BiS m films and was heated in a muffle furnace at 450 for 2 hours in air to convert BiS m to BiVO 4. Excess V 2 O 5 present in the BiVO 4 films was removed by soaking them in 1 M NaOH solution for ca. 30 min with gentle stirring. The resulting pure BiVO 4 films were rinsed with DI water and dried at room temperature. Characterization of the films UV-vis absorption spectra was recorded using a UV-vis Spectrophotometer (Hitachi U-4100). The morphologies and structure of the films were examined by Scanning Electron Microscopy (SEM, JEOL JSM-7800FE) and transmission electron microscopy (TEM, FEI Tecnai G2 F30). X-ray diffraction (XRD) was performed using a PANalytical X pert MPD Pro Diffractometer with Cu-Kɑ (λ= nm) radiation in the scanning angle range 2 θ (10-80 ) to confirm the purity and crystallinity of the films. The chemical compositions were S2

3 determined by X-ray Photoelectron Spectroscopy (XPS, Axis Ultra DLD, Kratos) using monochromatic Al Kα radiation. All spectra in XPS were calibrated to C 1s (284.8 ev). Photo-electrochemical (PEC) measurements PEC measurements were taken with an electrochemical workstation of CHI 760D using a conventional three-electrode cell with a 0.5 M Na 2 SO 4 electrolyte solution. As-prepared films, a Pt plate and an Ag/AgCl electrode in saturated KCl solution were used as the working electrode, counter electrode and reference electrode, respectively. The light source using a 350 W Xe lamp solar simulator was adjusted to 100 mw/cm 2 through an AM 1.5 G filter. The applied bias was converted to a value against a reversible hydrogen electrode (RHE) using Equation S1 below. E = E + E + V ph (S1) 0 RHE Ag / AgCl Ag / AgCl ( E / = V vs. NHE at 25 ) 0 Ag AgCl CHARACTERIZATION S3

4 Figure S1. (a) UV-vis absorption spectra and the corresponding (b) Tauc plots of as-prepared films. Figure S2. XRD patterns of (a) SnS x and (b) BiS m heat-treated in N 2 at 450 for 2 h. Figure S3. XPS spectra of SnS x films. (a) Sn 3d electronic level and (b) S 3p electronic level S4

5 Figure S4. XPS spectra of BiS m films S5

6 Figure S6. SEM top view images of (a) BiS m films without hydrochloric acid in hydrothermal process and (b) a photograph of the samples. Inset shows a top view image at high magnification. (c) and (d) are SEM top view images of BiS m films treated for 12 h showing a radiate growth direction. S6

7 Figure S7. SEM top view images of BiS m films treated with (a) 6 h, (b) 12 h, (c) 24 h and (d) 48 h. Insets show top view images at high magnification. Figure S8. XRD patterns of BiS m films with different converting time. S7

8 Figure S9. UV-vis absorption spectra of BiS m films with different converting time. S8

9 Figure S11. Photocurrent responses of SnS x (N 2 ), BiS m (N 2 ) and BiVO 4 films in a 0.5 M Na 2 SO 4 electrolyte solution under chopped illumination. For BiVO 4, the hole mobility is better than electron mobility 1-2. As shown in Figure S12a, the front illumination photo-induces the electron-hole pairs near the BiVO 4 /electrolyte interface. The long path length is detrimental to the electron transportation, resulting in low photocurrent (Figure S12c). For the back illumination, however, the photo-induced electron-hole pairs is near the FTO/BiVO 4 interface (Figure S12b). In this case, the transport of the holes becomes the main reason influencing the photocurrent of BiVO 4. The better hole mobility of BiVO 4 results in a good photocurrent performance for back illumination when compared to front illumination. S9

10 Figure S12. Schematic of (a) front and (b) back illumination for samples; (c) photocurrent of BiVO 4-24h with two types of illumination. To explore the influences of the residual SnO 2 on the photocurrent of BiVO 4, two hot-treated samples were prepared (Figure S13). The as-prepared SnS x direct annealed in air with a condition of 450 /2h became a white SnO 2 film (denoted as SnO 2, Figure S13a, c). When the VO(acac) 2 solution was dropped onto the as-prepared SnS x surface, the hot-treatment resulted in a light yellow SnO 2 film (denoted as SnO 2 with V, Figure S13a, c). Both types of SnO 2 films with band gaps of ev, different from FTO substrates, could absorb or scatter large portion of visible light (wavelength of smaller than nm, Figure S13a, b). However, there were only little photocurrents generated for the two types of SnO 2 films (Figure S13d). In the case of S10

11 FTO/SnO 2 /BiVO 4, as the SnO 2 interlayer has a significant thickness in comparison to the BiVO 4 layer, when the sample is back illuminated, most of the photons are absorbed or scattered in the SnO 2 and thus are not reach the BiVO 4, resulting in poor photocurrent responses. There are many studies reported the presence of SnO 2 in the FTO/SnO 2 /BiVO 4 films enhanced their photocurrent responses 1, 3-4. Here, we try to explain the different between this study and the reported studies. The SnO 2 thickness in the reported studies are much thinner compared to the BiVO 4 in their study. The thin thickness of SnO 2 has little influence on the Uv-vis absorbance which means that much portion of visible light in their study was absorbed or scattered by BiVO 4 (or other semiconductor) rather than SnO 2. However, in this study, the presence of the thicker SnO 2 absorbed or scattered visible light with a wavelength of smaller than nm (Inset of Figure S13b), leaving little part of the visible light (wavelength of nm to 500 nm) for BiVO 4. So, the main role of SnO 2 in the reported studies is to improve electrons collection at the internal SnO 2 /FTO interface. But in this study, the main role of SnO 2 is to reduce the absorbance of BiVO 4. Besides, the residual SnO 2 under BiVO 4 is different with the main composition of FTO substrates. The F-doped SnO 2 with an F element, as a conductor layer, improves the transmittance up 80% (Figure S13a, b). While the residual SnO 2 just with a transmittance below 50% (Figure S13a, b), especially for the wavelength smaller than 500 nm, is detrimental to the visible light absorbance for the samples. S11

12 Figure S13. Uv-vis of (a) transmittance and (b) absorption spectra, (c) XRD pattern and (d) photo-response for FTO, SnO 2 and SnO 2 with V; Insets are the corresponding photos and Tauc plots of the samples. REFERENCES (1) Liang, Y.; Tsubota, T.; Mooij, L. P. A.; van de Krol, R., Highly Improved Quantum Efficiencies for Thin Film BiVO 4 Photoanodes. J. Phys. Chem. C 2011, 115, (2) Abdi, F. F.; Savenije, T. J.; May, M. M.; Dam, B.; van de Krol, R., The Origin of Slow Carrier Transport in BiVO 4 Thin Film Photoanodes: A Time-Resolved Microwave Conductivity Study. J. Phys. Chem. Lett. 2013, 4, (3) Murcia-Lopez, S.; Fabrega, C.; Monllor-Satoca, D.; Hernandez-Alonso, M. D.; Penelas-Perez, G.; Morata, A.; Morante, J. R.; Andreu, T., Tailoring Multilayered S12

13 BiVO 4 Photoanodes by Pulsed Laser Deposition for Water Splitting. ACS Appl. Mater. Interfaces 2016, 8, (4) Saito, R.; Miseki, Y.; Sayama, K., Highly Efficient Photoelectrochemical Water Splitting Using a Thin Film Photoanode of BiVO 4 /SnO 2 /WO 3 Multi-Composite in a Carbonate Electrolyte. Chem. Commun. 2012, 48, S13