Publication date: 31st March 2013
The development of photochemical systems capable of mimicking the natural photosynthesis by driving useful chemical transformations has attracted significant interest motivated by the need to meet various environmental concerns and to secure the future supply of clean and sustainable energy. One of the most attractive approaches is solar-driven splitting of water into hydrogen and oxygen. Due to the complex chemistry involved in four-electron oxidation of water to dioxygen, the major challenge in photoelectrochemical water splitting is the development of cheap, efficient and stable photoanodes.
Recently, we have been developing photoanodes based on a novel class of visible-light photoactive inorganic/organic hybrid materials – TiO2 modified at the surface with polyheptazine (also known as “graphitic carbon nitride”). Importantly, polyheptazine exhibit high thermal and chemical stability in contrast to conventional organic dyes. The optical absorption edge of the TiO2-polyheptazine hybrids is red-shifted into the visible as compared to the bandgaps of both of the single components, which is due to the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO2 (acceptor). In other words, the direct optical charge transfer leads to generation of electrons with a relatively negative potential in the conduction band of TiO2, while the holes photogenerated in the polyheptazine layer can drive photooxidation of water, as evidenced by visible light-driven evolution of dioxygen on hybrid electrodes modified with iridium or cobalt oxide nanoparticles acting as oxygen evolution co-catalysts [1-4]. We have found that particularly the photoelectrochemical in-situ deposition of metal oxide co-catalysts is highly beneficial in terms of establishing a good coupling between the absorber and the co-catalyst. Our current attempts at improving the efficiency of kinetic charge separation in such hybrid photoanodes will be discussed.
[1] Bledowski, M.; Wang, L.; Ramakrishnan, A.; Khavryuchenko, O. V.; Khavryuchenko, V. D.; Ricci, P. C.; Strunk, J.; Cremer, T.; Kolbeck, C.; Beranek, R. Phys. Chem. Chem. Phys. 2011, 13, 21511. [2] Bledowski, M.; Wang, L.; Ramakrishnan, A.; König, D.; Ludwig, A.; Beranek, R. J. Electrochem. Soc. 2012, 159, H616. [3] Bledowski, M.; Wang, L.; Ramakrishnan, A.; Bétard, A.; Khavryuchenko, O. V.; Beranek, R. ChemPhysChem 2012, 13, 3018. [4] Bledowski, M.; Wang, L.; Ramakrishnan, A.; Beranek, R. J. Mater. Res. 2013, 28, 411.
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