A DENSITY FUNCTIONAL THEORY STUDY OF COPPER OXIDES NANOWIRES AND CLUSTERS ON ANATASE TiO2 (101) SURFACE FOR PHOTOCATALYTIC WATER SPLITTING

Abstract

Copper oxides deposited at titania surfaces have a beneficial effect on the photocatalytic activity of TiO2, particularly water splitting. However, the role of copper oxides and mechanisms of enhancement remains to be elucidated. In this work, possible nanostructures of copper oxide on TiO2(101) surface have been investigated by simulations based on density functional theory. Various stoichiometries, from Cu2O to CuO, and morphologies, from clusters to nanowires, have been considered. Nanowire structures were found to be consistently more stable than isolated clusters. In these structures, a Cu2O stoichiometry was found to be thermodynamically more stable than CuO at room temperature conditions, in contrast to what happens in bulk copper. Occupied Cu 3d and O 2p states were found to extend well into the band gap of titania, whereas the nature of the lowest-lying empty states depended on the stoichiometry: for Cu2O they consist mostly of Ti 3d orbitals, while in CuO unoccupied Cu 3d orbital at ∼0.8 eV above the Fermi level are present. Thus, both oxides reduce the band gap of the system with respect to pure titania, but only Cu2O is effective in separating photogenerated electrons and holes. This work also investigated oxidation of water on the most stable nanostructured Cu2O/TiO2 and CuO/TiO2 systems by density functional theory. Under photoelectrochemical conditions, the most stable structure has a CuO stoichiometry, and yields an overpotential of 0.66 V for water oxidation, with the active site being at the interface between copper oxide and titanium dioxide. A lower overpotential of 0.48 V is found on a metastable Cu2O nanostructure. These values are lower than the values for pure titania (1.39 V) and for copper oxides (0.77 V and 0.96 V). Moreover, in the case of the most stable structure, the active site is directly at the interface between CuO and TiO2. We therefore argue that a cooperative catalytic effect between cupric oxide and titania is at play in this system, beside the known increased photoabsorption coming from the reduced band gap. The nanostructures display a stronger adsorption of OH with respect to pristine titania, which is essential in lowering the overpotential, and switching the overpotential-determining step from hydroxyl formation to dehydrogenation of the adsorbed hydroxyl. These results provide insight into the role of copper oxides in the photocatalytic process. Moreover, the insight into the water oxidation reaction provides some guiding principles for the design of improved photocatalysts for solar water splitting.