Surface Modification of Rutile TiO₂ with Alkaline-Earth Oxide Nanoclusters for Enhanced Oxygen Evolution

Abstract

The oxygen (O2) evolution reaction (OER) is accepted as the bottleneck in the overall water splitting and has seen intense interest. In this work, we prepared rutile TiO2 modified with nanoclusters of alkaline-earth metal oxides for the OER. Photocatalytic OER was performed over rutile TiO2 surface-modified with alkaline-earth oxide nanoclusters, namely, CaO and MgO. The O2 evolution activity is notably enhanced for MgO-modified systems at low loadings and a combination of characterization and first-principles simulations allows interpretation of the role of the nanocluster modification in improving the photocatalytic performance of alkaline-earth-modified rutile TiO2. At such low loadings, the nanocluster modifiers would be small, and this facilitates a close correlation with theoretical models. Structural and surface characterizations of the modified systems indicate that the integrity of the rutile phase is maintained after modification. However, charge-carrier separation is strongly affected by the presence of surface nanoclusters. This improved performance is related to surface features such as higher ion dispersion and surface hydroxylation, which are also discussed with first-principles simulations. The modified systems are reducible so that Ti3+ ions will be present. Water dissociation is favorable at cluster and interfacial sites of the stoichiometric and reduced modified surfaces. Pathways to water oxidation at interfacial sites of reduced MgO-modified rutile TiO2 are identified, requiring an overpotential of 0.68 V. In contrast, CaO-modified systems required overpotentials in excess of 0.85 V for the reaction to proceed.

Evaluation of Photocatalysts for Water Splitting through Combined Analysis of Surface Coverage and Energy-Level Alignment

“…Figure ) and, on the other hand, by the size of their band gaps (>3.0 eV). Although active defect sites have been shown to promote water dissociation, ,, the amount of ionic adsorbates is apparently still insufficient to make the reaction viable at a satisfactory rate. , Compared to GaN and ZnO, the design of highly efficient catalysts based on TiO 2 for the overall water splitting reaction appears to be more complicated, as it would require not only a significant reduction of the band gap without deteriorating the good alignment of the energy levels, but also a substantial modification of the surface coverage. Nevertheless, nitrogen doping has been extensively deployed for TiO 2 with the motivation of simultaneously narrowing the band gap and inducing a higher surface coverage of ionic species. − In an alternative approach, one could turn to nanoparticles of TiO 2 , which possess smaller gaps and favor ionic adsorbates at their higher-energy surfaces. − Such surfaces have been demonstrated to be more reactive and to promote electron-transfer processes in dye-sensitized solar cells, thus deserving a more in-depth investigation for water splitting.…”

Modification of TiO2 with Metal Chalcogenide Nanoclusters for Hydrogen Evolution

“…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.…”

“…[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 adsorption energies are computed using:…”

“…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.…”

“…Surface modification in this way can be performed using atomic layer deposition, [27] incipient wetness impregnation, [28][29][30] or chemisorptioncalcination cycles [7,31] and permits modulation of the light absorption properties of the titania substrate, promotes separation and stability of photoexcited electrons and holes, and provides low coordinated active sites for catalytic reactions. We have studied surface-modified TiO2 for water oxidation [32] and CO2 activation. [33,34] In the context of hydrogen evolution, this strategy enables us to combine the desirable properties of TiO2 through modification with nanoclusters that display low-coordinated, active chalcogen sites, which promote the HER.…”