Hydrogen-free defects in hydrogenated black TiO2

Abstract

Black anatase TiO₂ has surprisingly enhanced solar energy harvesting efficiency and electrical conductivity, which makes it a promising material in a wide range of energy and environmental applications. Several experimental and theoretical studies have successfully revealed the mechanisms of band gap reduction by surface hydrogenation of anatase TiO₂. However, recent experimental evidence suggests the existence of bulk point defects that yield infrared (∼1.0 eV) photoabsorption and high conductivity of black anatase TiO₂. In the current study, using a combination of ab initio molecular dynamics simulations and electronic structure calculations, we successfully explain the physical properties, metallicity, and infrared/microwave absorption (i.e., black color) of highly reduced anatase TiO₂ crystal in a hydrogenated state with a newly found pair defect (Ti_i–V_O)⁴⁺. Hydrogen atoms in the bulk are unnecessary to understand the observed properties.