Since the first demonstration of photoelectrochemical water splitting on a TiO2 surface by Fujishima and Honda in 1972,1 metal oxide semiconductors have become one of the most studied class of materials for a photoelectrochemical water splitting cell (PEC). One of the most promising metal oxide candidates is bismuth vanadate (BiVO4), an n-type semiconductor with a ~2.5 eV band gap and a conduction band position close to H2/H+ redox potential. BiVO4 has relatively long minority carrier lifetime (40 ns)2 compared to other well-studied metal oxides such as TiO2, WO3, and Fe2O3. However, due to the low carrier mobility of BiVO4, the charge extraction depth of this material is greatly limited.3 A promising strategy to mitigate the short charge extraction depth of BiVO4 is nanostructuring, which minimizes photogenerated charge transport distance while maintaining high optical density. Recent studies on nanoporous,4 nanowires,5 and inverse opals6 architectures have proven the success of the nanostructuring strategy. However, the greatest challenge in working with these nanostructured electrodes is identifying the active sites and the photocurrent distribution across the surface of such complex architectures.
We develop a qualitative photocurrent mapping strategy on highly active W-doped BiVO4 electrodes by using the sintering of Au nanoparticles as a nanoscale-resolution oxidation reaction indicator. This simple strategy allows us to identify the active sites on the W-doped BiVO4 electrodes, which were morphologically-controlled by varying the relative humidity during the sol-gel fabrication process. The electrode synthesized at moderate relative humidity of ~30% has the highest photoelectrochemical activity because it contained the most evenly distributed active sites. This mapping strategy elucidates the correlation between the electrode morphologies and the photoelectrochemical activities. This understanding is essential for designing nanostructured metal oxide electrodes for photoelectrochemical water splitting.
(1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Chem. Rev. 2010, 110 (11), 6446-6473.
(2) Abdi, F. F.; Savenije, T. J.; May, M. M.; Dam, B.; van de Krol, R. J. Phys. Chem. Lett. 2013, 4 (16), 2752-2757.
(3) Abdi, F. F.; Dabirian, A.; Dam, B.; van de Krol, R. Phys. Chem. Chem. Phys. 2014, 16 (29), 15272-15277.
(4) Kim, T. W.; Choi, K. S. Science 2014, 343 (6174), 990-994.
(5) (a) Su, J.; Guo, L.; Bao, N.; Grimes, C. A. Nano Lett. 2011, 11 (5), 1928-1933; (b) Rao, P. M.; Cai, L.; Liu, C.; Cho, I. S.; Lee, C. H.; Weisse, J. M.; Yang, P.; Zheng, X. Nano Lett. 2014, 14 (2), 1099-1105; (c) Shi, X.; Choi, I. Y.; Zhang, K.; Kwon, J.; Kim, D. Y.; Lee, J. K.; Oh, S. H.; Kim, J. K.; Park, J. H. Nat. Commun. 2014, 5.
(6) Zhang, L. W.; Reisner, E.; Baumberg, J. J. Energy Environ. Sci. 2014, 7 (4), 1402-1408.
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