TiO 2 -polyheptazine hybrid photoanodes: Effect of cocatalysts and external bias on visible light-driven water splitting
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Photoanodes based on TiO2-polyheptazine (TiO2-PH) hybrids are, due to the energetics of photogenerated charges, very promising for solar water splitting in terms of possibly reduced need for external electric bias. Visible (k . 420 nm) light-driven photooxidation of water at TiO2-PH electrodes loaded with two different metal oxide cocatalysts was investigated. As compared with TiO2-PH photoanodes loaded with colloidal [iridium (IV) oxide] IrO2 deposited by colloidal deposition, photoelectrodes modified with CoOx oxygen-evolving cocatalyst (Co-Pi) deposited by photoassisted deposition precipitation method showed both higher photocurrents and more efficient oxygen evolution under prolonged irradiation. The minimum external electric bias needed to observe complete photooxidation of water to dioxygen at TiO2-PH photoanodes modified with Co-Pi was estimated to be ;0.6 V at pH 7. The key factor limiting the photoconversion efficiency at low bias potentials is the fast primary recombination of photogenerated charges.
I. INTRODUCTION
One of the most attractive strategies for direct conversion of sunlight energy into chemical energy is the use of photoelectrochemical cells in which simultaneous reduction and oxidation of water to hydrogen and oxygen takes place.1,2 Although impressive solar-to-hydrogen efficiencies reaching far beyond the 10% threshold required for commercial implementation have been demonstrated repeatedly,2–5 all these high-efficiency cells have been based on very expensive materials and cannot be easily scaled-up. The success of the photoelectrochemical water splitting approach therefore depends utterly on the development of novel photoelectrodes based on inexpensive and abundant materials. In this context it is also important to realize that it is particularly the water oxidation reaction that represents a major challenge in photoelectrochemical water splitting. This is because the dioxygen-evolving reaction is a very complex process requiring a protoncoupled transfer of four electrons from two water molecules.6 Accordingly, on most inorganic surfaces the kinetics of oxygen evolution is slow and associated with considerable overpotential.7,8 Furthermore, the relatively stable metal oxides typically utilized in watersplitting photoanodes either do not absorb visible (k . 400 nm) light (TiO2), or—if they do absorb in the visible (e.g., Fe2O3, WO3) light—they suffer from too positive conduction band edges, which translates into a requirement of a significant external electric bias applied a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.297 J. Mater. Res., Vol. 28, No. 3, Feb 14, 2013
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or the need for photocells with tandem configuration to allow for water reduction at the counter electrode.9–11 An interesting alternative is represented by hybrid photoelectrodes utilizing an electron collector with relatively negative conduction band edge (typically TiO2) sensitized by an organic compound (typically a dye) co
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