Tailoring electronic and optical properties of TiO2: nanostructuring, doping and molecular-oxide interactions

TL;DR

This paper uses advanced ab initio computational methods to analyze and tailor the electronic and optical properties of TiO2 nanostructures, addressing band gap issues and doping effects relevant for photocatalysis and solar cells.

Contribution

It provides a comprehensive ab initio analysis of TiO2 phases and nanostructures, demonstrating how nanostructuring and doping can modify electronic properties for applications.

Findings

Quantum confinement widens the band gap in nanostructures.

Doping with boron or nitrogen introduces states that reduce the band gap.

Ab initio methods elucidate TiO2's role in dye-sensitized solar cells.

Abstract

Titanium dioxide is one of the most widely investigated oxides. This is due to its broad range of applications, from catalysis to photocatalysis to photovoltaics. Despite this large interest, many of its bulk properties have been sparsely investigated using either experimental techniques or ab initio theory. Further, some of TiO2's most important properties, such as its electronic band gap, the localized character of excitons, and the localized nature of states induced by oxygen vacancies, are still under debate. We present a unified description of the properties of rutile and anatase phases, obtained from ab initio state of the art methods, ranging from density functional theory (DFT) to many body perturbation theory (MBPT) derived techniques. In so doing, we show how advanced computational techniques can be used to quantitatively describe the structural, electronic, and optical properties of TiO2 nanostructures, an area of fundamental importance in applied research. Indeed, we address one of the main challenges to TiO2-photocatalysis, namely band gap narrowing, by showing how to combine nanostructural changes with doping. With this aim we compare TiO2's electronic properties for 0D clusters, 1D nanorods, 2D layers, and 3D bulks using different approximations within DFT and MBPT calculations. While quantum confinement effects lead to a widening of the energy gap, it has been shown that substitutional doping with boron or nitrogen gives rise to (meta-)stable structures and the introduction of dopant and mid-gap states which effectively reduce the band gap. Finally, we report how ab initio methods can be applied to understand the important role of TiO2 as electron-acceptor in dye-sensitized solar cells. This task is made more difficult by the hybrid organic-oxide structure of the involved systems.