Ab-initio calculations of the Optical band-gap of TiO2 thin films

TL;DR

This study combines experimental measurements and ab-initio calculations to analyze how lattice modifications in TiO2 thin films influence their optical band-gap, aiming to enhance photocatalytic efficiency.

Contribution

It provides a detailed correlation between lattice constants and optical band-gap in TiO2 thin films, supported by ab-initio calculations, offering insights for band-gap engineering.

Findings

Optical band-gap increases in thin films compared to bulk TiO2.

Band-gap expansion correlates linearly with lattice constant changes.

Compression of lattice c and expansion of lattice a reduce the band-gap.

Abstract

Titanium dioxide has been extensively studied in recent decades for its important photocatalytic application in environmental purification. The search for a method to narrow the optical band-gap of TiO2 plays a key role for enhancing its photocatalytic application. The optical band gap of epitaxial rutile and anatase TiO2 thin films deposited by helicon magnetron sputtering on sapphire and on SrTiO3 substrates was correlated to the lattice constants estimated from HRTEM images and SAED. The optical band-gap of 3.03 eV for bulk-rutile increased for the thin films to 3.37 on sapphire. The band gap of 3.20 eV for bulk-anatase increases to 3.51 on SrTiO3. In order to interpret the optical band gap expansion for both phases, ab-initio calculations were performed using the Vienna ab-initio software. The calculations for rutile as well anatase show an almost linear increase of the band gap width with decreasing volume or increasing lattice constant a. The calculated band gap fits well with the experimental values. The conclusion from these calculations is, in order to achieve a smaller band-gap for both, rutile or anatase, the lattice constants c has to be compressed, and a has to be expanded.