Publication date: 31st March 2013
Photocatalytic water splitting has attracted much attention as a means of renewable hydrogen production. Development of narrow band gap semiconductors is essential for efficient utilization of solar energy, because the major part of sunlight consists of visible light. Non-oxide semiconductors have band gaps narrow enough to absorb visible light while straddling the reduction and oxidation potentials of water, therefore being promising in solar water splitting.
Ta3N5 is one of the most interesting non-oxide photocatalysts because it has a band gap of 2.1 eV and enables hydrogen or oxygen evolution from water under visible light irradiation in the presence of a sacrificial electron donor (methanol) or acceptor (silver ion), respectively. Ta3N5 can be prepared by nitriding Ta2O5 powder under a high-temperature NH3 flow. There seems much room to improve the photocatalytic activity of Ta3N5 by controlling the morphologies and improving the crystallinity, since such simple nitridation would not allow for production of well-crystalline and morphologically-designed Ta3N5. In this presentation, recent progress in morphological control of crystalline Ta3N5 will be presented.
Ordered mesoporous Ta3N5 with crystalline thin-wall structures was obtained by nitriding amorphous mesoporous Ta2O5 after coating the pore wall with a silica layer by chemical vapor deposition of tetramethyl orthosilicate. Photocatalytic activity of mesoporous Ta3N5 was three times that of conventional bulk Ta3N5 for H2 evolution under visible light, presumably because of efficient charge transfer to the surface in the former’s thin crystallized pore wall.
A monodisperse SiO2/Ta3N5 core/shell photocatalyst was synthesized through a sol-gel process and subsequent thermal nitridation. It exhibited higher H2 evolution activity than bulk Ta3N5, owing to the larger surface area and lower defect density of the former. Pt and IrO2 or CoOxcocatalysts were loaded on the inner and outer surfaces of the Ta3N5 shell as electron and hole collectors, respectively, to assist charge separation. The resulting Ta3N5 showed higher water reduction and oxidation performances in the presence of sacrificial electron donors and acceptors, respectively.
Compared with conventional Ta3N5, Ta3N5 nitrided from AM salt-modified Ta2O5 had better crystallinity and smaller particles with smoother surfaces and demonstrated a six-fold improvement in photocatalytic activity for O2 evolution. Detailed characterization of the Na2CO3-modified Ta3N5 suggested partial dissolution of Ta2O5 and nucleation of NaTaO3 in the early stages of nitridation, which gave rise to the characteristic particle morphologies and improved the crystallinity of the nitridation products. Loading of an oxygen evolution catalyst improved the photocatalytic activity further.
Ta3N5 with various morphologies. (a) conventional, (b) ordered-mesoporous, (c) core/shell, and (d) Na2CO3-added Ta3N5.
Hisatomi, T.; Otani, M.; Nakajima, K; Teramura, K.; Kako, Y.; Lu, D.; Takata, T.; Kondo, J. N.; Domen, K. Preparation of Crystallized Mesoporous Ta3N5 Assisted by Chemical Vapor Deposition of Tetramethyl Orthosilicate, Chem. Mater. 2010, 22, 3854-3861. Wang, D.; Hisatomi, T.; Takata, T.; Pan, C.; Katayama, M.; Kubota, J.; Domen, K. Core/Shell Tantalum Nitride Photocatalyst Modified with Spatially-separated Cocatalysts for Efficient Water Splitting, submitted. Ma, S. S. K.: Hisatomi, T.; Maeda, K.; Moriya, Y.; Domen, K. Enhanced Water Oxidation on Ta3N5 Photocatalysts by Modification with Alkaline Metal Salts, J. Am. Chem. Soc. 2012, 134, 19993−19996.
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