Proceedings of September Meeting 2016 (NFM16)
Publication date: 14th June 2016
Monoclinic bismuth vanadate (m-BiVO4) has attracted attention as a photoelectrode material for water splitting due to its suitable band gap energy of about 2.5 eV and remarkably high photocurrent densities of more than 4 mA/cm2 1. If such a material is to be used in practical water splitting applications, scale-up is one of the main challenges to be addressed 2. Specifically, a large-scale and cost-effective deposition technology for the preparation of efficient photoelectrodes is needed. Reactive sputtering is one of the techniques that can, in principle, meet these requirements. Our first attempts to sputter BiVO4 using metallic Bi and V targets with separate direct current power supplies illustrates that sputter deposition of photoactive BiVO4 is indeed viable, but the efficiencies still need to be improved3. One crucial factor is the stoichiometry of the films. To investigate this in more detail, photoactive BiVO4 thin films were prepared by direct current reactive magnetron sputtering from Bi and V targets with V-to-Bi ratios from 0.75 to 1.5. By changing the V-to-Bi ratio we observed that the highest photoactivity occurs under slightly V-rich deposition conditions (jph >1.5 mA/cm2). Time-resolved microwave conductivity (TRMC) measurements revealed a strong dependence of carrier mobility and lifetime on the V-to-Bi ratio. Moreover, the monoclinic BiVO4 phase became more prominent with increasing V-to-Bi ratio. Raman spectroscopy and chemical mapping by EDX showed that at a high V-to-Bi ratio a phase separation into BiVO4 and V2O5 occurs. The better crystallographic quality and the increased grain size at higher V-to-Bi ratios seem to be responsible for the improved electronic properties and hence increased photocurrents of the BiVO4 films.
References:
(1) Abdi, F. F.; Han, L.; Smets, A. H. M.; Zeman, M.; Dam, B.; Krol, R. v. d. Nat. Commun. 2013, 4, 2195.(2) Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F. Env. Sci. Techn. 2013, 6, 1983.(3) Gong, H.; Freudenberg, N.; Nie, M.; van de Krol, R.; Ellmer, K. AIP Advances 2016, 6, 045108.
Key words: BiVO4, water splitting, reactive magnetron sputtering, stoichiometry, grain size, charge carrier mobility, carrier lifetime, phase separation
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