FIRST-PRINCIPLES DENSITY FUNCTIONAL THEORY STUDY OF COBALT (HYDR)OXIDES AND TITANIUM DIOXIDE FOR ELECTROCHEMICAL OXYGEN EVOLUTION

Abstract

The spinel cobalt oxide Co3O4 is a magnetic semiconductor containing cobalt ions in Co2+ and Co3+ oxidation states. We have studied the electronic, magnetic and bonding properties of Co3O4 using density functional theory (DFT) at the Generalized Gradient Approximation (GGA), GGA+U, and PBE0 hybrid functional levels.

(110) is a frequently exposed surface in Co3O4 nanomaterials. We employed DFT+U to study the atomic structures, energetics, magnetic and electronic properties of the two possible terminations, A and B, of this surface. These calculations predict A as the stable termination in a wide range of oxygen chemical potentials, consistent with recent experimental observations. The Co3+ ions do not have a magnetic moment in the bulk, but become magnetic at the surface, which leads to surface magnetic orderings different from the one in the bulk. Surface electronic states are present in the lower half of the bulk band gap and cause partial metallization of both surface terminations. These states are responsible for the charge compensation mechanism stabilizing both polar terminations.

We also carried out DFT+U to study the interaction of water with the (110) surface of Co3O4, a widely used oxidation catalyst. Dissociative water adsorption is preferred from low coverage up to one monolayer on the A termination and up to one-half monolayer coverage on the B termination. On the latter, a mixed molecular and dissociated monolayer is more stable at full coverage. The computed structures are used to investigate the free energy changes during water oxidation on both surface terminations.

Using first-principles density functional theory (DFT) calculations we determine the relative Gibbs free energies of CoO, Co(OH)2, Co3O4, CoO(OH) and CoO2 in electrochemical environment. We find that CoO(OH) and CoO2 are the stable phases under oxidation conditions. These results, combined with surface structure studies of CoO(OH) (0001), show that a CoO2x- (x=0~0.5) layer is present when the surface is

exposed to solution under oxidation conditions. Study of the oxygen evolution reaction (OER) reveals however that natural surface of a CoO2x- layer has a high overpotential, due to the difficulty of first deprotonation to form a surface OH species. Taken previous study of CoO(OH) (011̅2) surface into consideration, the OER reactivity of CoO(OH) could come from surface step-edge and defects.

As a promising candidate electrode material for photoelectrochemical water splitting, TiO2 is perhaps the most studied oxide semiconductor in photocatalysis. Recent computational studies of the oxygen evolution reaction (OER) have shown that the first proton-coupled electron transfer is responsible for the high overpotential of the OER on TiO2 surfaces. Here, we report a study of the chemical dynamics of the first proton and electron transfers across the TiO2-water interface. Using a periodic model that includes an anatase slab and explicit water molecules, we sample the solvent configurations by ab-initio molecular dynamics and determine the energy profiles of the two electronic states involved in the electron transfer by the hybrid PBE0 functional. Our calculated energy profiles suggest that the first proton and electron transfers are sequential, with the electron transfer (ET) following the proton transfer (PT). The ET is facilitated by a shared-hole state, and there is no significant solvent reorganization barrier during the ET.