pulsed laser deposition for water splitting

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1 Supporting Information Tailoring multilayered BiVO 4 photoanodes by pulsed laser deposition for water splitting Sebastián Murcia-López, a* Cristian Fàbrega, a Damián Monllor-Satoca, a María D. Hernández-Alonso, b Germán Penelas-Pérez, b Alex Morata, a Juan R. Morante, a, c and Teresa Andreu a* a Department of Advanced Materials for Energy, Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre, 1, Sant Adrià de Besòs, Catalonia, Spain. b Repsol Technology Center, Carretera de Extremadura A-5, km 18, Móstoles, Madrid, Spain. c Department of Electronics, Universitat de Barcelona (UB), Martí i Franquès, 1, Barcelona, Catalonia, Spain. Corresponding Author smurcia@irec.cat tandreu@irec.cat Tel: (34) Present Addresses Present address: Department of Electronics, Universitat de Barcelona (UB), Martí i Franquès, 1, Barcelona, Catalonia, Spain. Present address: IQS School of Engineering, Universitat Ramon Llull, via Augusta, 390, Barcelona, Catalonia, Spain. S-1

2 Preliminar evaluation of synthesis parameters Figure S1. SEM micrograph, EDX and XRD analyses of a BiVO 4 film deposited at lower pressure with an O 2 /Ar mixture. Under poor O 2 conditions and low deposition pressure (200 mtorr, O 2 /Ar = 1/1, 400ºC), the BiVO 4 film is enriched in Bi (Bi:V ratio ~1.3). Additionally, tetragonal BiVO 4 (JCPDS ) appears as secondary crystalline phase, besides the monoclinic one. A more irregular morphology can be observed. S-2

3 200 Figure S2. SEM image and XRD analysis of a BiVO 4 film prepared at 200ºC. As seen in the figure, at lower deposition temperature (200ºC, 200 mtorr, O 2 ), the obtained film is amorphous with a very porous morphology and low particle size. As it is well-known, both O 2 pressure and temperature are determining parameters affecting the growth and the morphology in vapor deposition methods. As expected, lower temperatures lead to a growth zone with almost negligible surface diffusion in which porous structures are favored. 1 Figure S3. Picture of a larger BiVO 4 electrode (~10 cm 2 ) prepared by PLD under standard conditions (400ºC, 200 mtorr, O 2 ). S-3

4 j ph (ma cm -2 ) j V RHE = 0.03 ma cm -2 Light I (a. u.) Dark -0.1 SnO 2 SnO Ann. 500ºC RT SnO E vs. RHE (V) Figure S4. Cyclic voltammetry (AM 1.5G) of a BiVO 4 electrode prepared at room temperature and postannealed at 500ºC for crystallization. In the inset, the XRD variation with thermal treatment, in which monoclinic BiVO 4 peaks can be observed. S-4

5 Figure S5. SEM image (top), absorption coefficient and penetration depth (bottom) of a WO 3 /FTO photoanode prepared by PLD. The layer has a thickness of 3.5 µm, similar to that of the BiVO 4 /WO 3 /SnO 2 heterostructure shown in the manuscript. S-5

6 EDX analysis Semi-quantitative analysis of the BiVO 4 target: Element Weight% Atomic% V K Bi M Totals Figure S6. Elemental mapping and EDX spectrum of the BiVO 4 target. S-6

7 EDX analysis of BiVO 4 /SnO 2 Element Weight% Atomic% V K Bi M Totals Figure S7. EDX spectrum of the BiVO 4 /SnO 2 photoanode. Figure S8. 3D image and thickness measurement of the BiVO 4 /WO 3 /SnO 2 heterostructure. S-7

8 j ph (ma cm -2 ) EE illumination SE illumination E vs. RHE (V) Figure S9. Cyclic voltammetries obtained with a WO 3 /FTO photoanode prepared by PLD, with a ~3.5 μm thickness. The measurement was carried out under AM 1.5G illumination in acidic medium (0.1M H 2 SO 4 as electrolyte), given the stability limitations of WO 3 at neutral and basic conditions. The dotted line represents the dark current. S-8

9 Figure S10. Photocurrent values at 1.23 V RHE obtained from the cyclic voltammetry measurements at both EE and SE illumination conditions (AM 1.5G) with several BiVO 4 electrodes prepared under the same conditions and different number of pulses. S-9

10 Figure S11. Ratio of the photocurrent values under EE and SE illumination at 1.23 V RHE obtained from the cyclic voltammetry (AM 1.5G) with the three photoanodes, without and in presence of 0.2M H 2 O 2. S-10

11 APCE measurements The absorbed photon-to-current efficiency (APCE) values were obtained from the incident photon-to-current efficiency (IPCE) corrected with the light harvesting efficiency (LHE), similar to the concept applied to solar cells. For the APCE calculation, the LHE was first calculated from the optical characterization. For that purpose, the absorptance (A) in the samples was obtained, from the measured transmittance (T) and reflectance (R). Then, LHE was estimated with the following equation: 2 LHE= A Finally, the APCE for each wavelength was determined by the equation: APCE= IPCE LHE The IPCE was estimated from chronoamperometric measurements under monochromatic light at a constant bias of 1.23 V vs RHE. S-11

12 Transient pulse analysis As it is well established, the decay in transient current measurements is related to recombination processes mediated by positive charge accumulation. In this sense, the area defined between the initial (j 0, defined at t = 0.5 s) and the steady state photocurrent values (j ss, defined at t = 80 s) is proportional to the accumulated charge. j 0 Q0 Q C j SS Figure S12. Scheme of a transient current pulse indicating the j 0 and j ss values, along with the two defined pulses for the calculation of the initial (Q 0 ) and the collected charges (Q c ). Therefore, if we define two pulses, one with the j 0 (blue dotted line in Figure S11) and other with the j ss (black continuous line in Figure S11), which actually is the value used in the calculation of IPCE and APCE, a ratio between the maximum (Q 0, determined by the j 0 value) and the collected charge (Q C ) could be obtained. This ratio, according to the previous statement, would be related to the efficiency in which generated charges are collected instead of been accumulated and recombine in a particular electrode. S-12

13 Mott-Schottky Measurements Mott-Schottky measurements were performed for the BiVO 4 and the BiVO 4 /SnO 2 samples in the dark. For this purpose, the same electrolyte was used (Na 2 SO 4 with sodium phosphate buffer solution). The polarization range and frequency were V RHE and 20 khz, respectively. The frequency was chosen as the appropriate for crystalline materials. 3 Figure S13. Mott-Schottky dark measurements and linear regressions for the BiVO 4 /SnO 2 and BiVO 4 photoanodes. A linear fitting was made in the linear region for each sample; then, the fitting was modeled with the Mott-Schottky equation 3 and thus the free carrier density could be calculated (N D ). 1 C 2 = 2 ε r ε 0 A 2 (E E en fb kt D e ) A ε r value of 55 was used for BiVO 4, according to literature. 4 After estimating the N D values for both samples, the length of the depletion layer (W) was also calculated according to the following equation: W = 2ε rε o (E E fb ) en D S-13

14 For this purpose, E fb was estimated to be close to the onset potential observed in the cyclovoltammetric measurements in presence of H 2 O 2 as hole scavenger. This assumption is considered proper, as H 2 O 2 is used to avoid the Fermi level pinning. 5 S-14

15 Figure S14. IPCE (%) measurements with the BiVO 4 /WO 3 /SnO 2 and BiVO 4 /SnO 2 under EE illumination. In the inset, the IPCE values under SE illumination, compared with a WO 3 photoanode (measured with 0.1M H 2 SO 4 electrolyte). S-15

16 References 1. Infortuna, A.; Harvey, A. S.; Gauckler, L. J. Microstructures of CGO and YSZ Thin Films by Pulsed Laser Deposition. Adv. Funct. Mater. 2008, 18, Sung, S. D.; Lim, I.; Kang, P.; Lee, C. M.; Lee, W. I. Design and Development of Highly Efficient PbS Quantum Dot-Sensitized Solar Cells Working in an Aqueous Polysulfide Electrolyte. Chem. Commun., 2013, 49, Chen, Z.; Dinh, H. N.; Miller, E. Photoelectrochemical Water Ssplitting. Standards, Experimental Methods, and Protocols. Springer Briefs in Energy, Springer, 2013, Rettie, A. J. E.; Lee, J. C.; Marshall, L. G.; Lin, J. F.; Capan, C.; Lindemuth, J.; McCloy, J. S.; Zhou, J.; Bard, A. J.; Mullins, C. B. Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO 4 Single Crystals: Intrinsic Behavior of a Complex Metal Oxide. J. Am. Chem. Soc., 2013, 135, Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Hamann, T.; Bisquert, J. Water Oxidation at Hematite Photoelectrodes: The Role of Surface States. J. Am. Chem. Soc., 2012, 134, S-16