Precise Electronic Structures of Amorphous Solids: Unraveling the Color Origin and Photocatalysis of Black Titania

Abstract

Water splitting through efficient catalysts represents an ultimate solution for carbon neutrality within 40 years. To achieve this goal, amorphous photocatalysts are noted for their promising performances. Among them, the best known is black titania (amorphous TiO x , x ≤ 2). However, despite a large number of studies on black titania, its color origin, structure−property relationship, and photocatalytic mechanism remain a topic of hot debate, largely due to the difficulty to calculate its precise electronic structure. Here, using ab initio molecular dynamics simulations, we report the precise electronic structures of black titania and further reveal the generic evolution pattern of the electronic structures of covalent compounds upon amorphization and reduction. An interesting "disproportionation-like" process resulting in metal clusters of Ti is discovered in heavily reduced amorphous titania, accompanied by oxygen vacancies in different energy states. This study elucidates the workings of amorphous catalysts and offers practical guidance for enhancing their performances.

Predicting Electronic Structure of Realistic Amorphous Surfaces

“…It is difficult to run first principles calculations with the large number of atoms needed to define the amorphous structure, as well as, generally to generate atomistic structures that are representative of real materials. To address the latter, the “melt‐quench” method, [ 7–10,14,15 ] which involves using classical or ab initio dynamics simulations to heat a sample to a liquid phase, and then quenching the melt in order to lock in the amorphous configuration, has emerged as a convenient way to generate amorphous unit cells.…”

“…To achieve an amorphous surface, oftentimes a melt‐quench bulk structure is cleaved via optimization with a large vacuum placed in the direction orthogonal to the surface. [ 10,25–27 ] However, the suitability of this approach for generating amorphous surfaces has recently been called into question. [ 28,29 ] Specifically, this approach necessarily involves a high amount of bond breaking as the periodic boundary condition in the surface direction of the low energy quench is disrupted when creating the interface.…”

“…These studies have afforded insight ranging from the self‐trapping of holes and electrons [ 7–9 ] to defect and dopant effects. [ 10–13 ] . It is difficult to run first principles calculations with the large number of atoms needed to define the amorphous structure, as well as, generally to generate atomistic structures that are representative of real materials.…”