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

Stephen Rhatigan,Michael Nolan

doi:10.3389/fchem.2019.00067

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 Mn4O6 nanoclusters modifying the TiO2 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. MnOx-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 TiO2 and other TiO2-based photocatalyst materials are presented throughout.

Highlights

Photocatalysts are semiconductor materials which absorb photons of energies in excess of the bandgap to produce electron-hole pairs
The red-shift in light absorption was attributed to cluster-derived states above the valence band maximum (VBM) which were identified by X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) simulations (Jin et al, 2011; Nolan, 2011a; Tada et al, 2014)
We investigate the impact of surface hydroxylation on the reduction of the heterostructures via oxygen vacancy formation and apply a model for photoexcitation to examine the associated energetics and charge localization

Summary

INTRODUCTION

Photocatalysts are semiconductor materials which absorb photons of energies in excess of the bandgap to produce electron-hole pairs. The red-shift in light absorption was attributed to cluster-derived states above the valence band maximum (VBM) which were identified by X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) simulations (Jin et al, 2011; Nolan, 2011a; Tada et al, 2014) These studies, and the subsequent development of similar systems, (Jin et al, 2012; Boppana et al, 2013; Iwaszuk et al, 2013; Bhachu et al, 2014; Fronzi et al, 2016a) mean that a multitude of nanocluster-surface composites can be investigated. Dissociation is favored for the reduced systems; this is an important step in the water oxidation reaction

METHODOLOGY

RESULTS

CONCLUSIONS