Activation of Water on MnOx-Nanocluster-Modified Rutile (110) and Anatase (101) TiO2 and the Role of Cation Reduction

Abstract

Surface modification of titania surfaces with dispersed metal oxide nanoclusters has the potential to enhance photocatalytic activity. These modifications can induce visible light absorption and suppress charge carrier recombination which are vital in improving the efficiency. We have studied heterostructures of Mn 4 O 6 nanoclusters modifying the TiO 2 rutile (110) and anatase (101) surfaces using density functional theory (DFT) corrected for on-site Coulomb interactions (DFT + U). Such studies typically focus on the pristine surface, free of the point defects and surface hydroxyls present in real surfaces. In our study we have considered partial hydroxylation of the rutile and anatase surfaces and the role of cation reduction, via oxygen vacancy formation, and how this impacts on a variety of properties governing the photocatalytic performance such as nanocluster adsorption, light absorption, charge separation, and reducibility. Our results indicate that the modifiers adsorb strongly at the surface and that modification extends light absorption into the visible range. MnO x -modified titania can show an off-stoichiometric ground state, through oxygen vacancy formation and cation reduction spontaneously, and both modified rutile and anatase are highly reducible with moderate energy costs. Manganese ions are therefore present in a mixture of oxidation states. Photoexcited electrons and holes localize at cluster metal and oxygen sites, respectively. The interaction of water at the modified surfaces depends on the stoichiometry and spontaneous dissociation to surface bound hydroxyls is favored in the presence of oxygen vacancies and reduced metal cations. Comparisons with bare TiO 2 and other TiO 2 -based photocatalyst materials are presented throughout.

Modification of TiO2 with hBN: high temperature anatase phase stabilisation and photocatalytic degradation of 1,4-dioxane

“…In the current computational setup, DFT+U is implemented to yield solutions in which charges are localized, rather than delocalized over the entire system, as in standard DFT. Despite these shortcomings, inferences regarding the impact of the modification may be drawn by comparing the energies computed in the photoexcitation model across the different systems and, in particular, with reference to the unmodified anatase (101) surface [91][92][93].…”

“…For these ribbon models, the adsorption energy is computed as: To model photoexcitation, a triplet electronic state was imposed on the system; this forces an electron from the filled valence band (VB) to the empty conduction band (CB), leaving a hole in the VB. More details of this model are presented in the supporting information and in [90][91][92][93][94]. Charge localization is assessed via analysis of computed Bader charges [95], spin magnetizations and excess spin density plots.…”

Modification of TiO2 with Metal Chalcogenide Nanoclusters for Hydrogen Evolution

“…The Perdew-Wang (PW91) approximation to the exchange correlation functional is used. [40] In our earlier studies of nanocluster modified TiO2, [33,41,42] this functional provides a consistent description of these materials. [32,43] Figure S1 in the Supplementary Material) and then adsorbed in different configurations at the rutile (110) surface.…”

“…The negative adsorption energies indicate that the modifiersurface interaction is favourable and the magnitudes of these energies suggest that the nanoclusters are strongly bound at the surface. [32,33,41,42,53] For Sn4S4-r110, shown in From these data, we can see that the Ti-S bonds are shorter than the Ti-Se bonds. This is expected as Se has a larger ionic radius than S. [8,10] Nanocluster metal-S bonds are also shorter than metal-Se bonds, both in the gas-phase and after adsorption at the rutile TiO2 surface.…”

“…This forces an electron from the filled valence band to the empty conduction band, leaving a hole in the valence band. This computational approach has been used in previous work [32,33,41,42] and is described in detail in the Supplementary Material. We use this model to assess the impact of modification on the optical band gap, the stability and spatial separation of charges in the excited state.…”