Visible light carrier generation in co-doped epitaxial titanate films

TL;DR

This study demonstrates that co-doping SrTiO3 with La and Cr reduces its band gap, enhances visible light absorption, and improves photocatalytic activity, making it more suitable for solar energy applications.

Contribution

The paper reveals how La and Cr co-doping in SrTiO3 thin films effectively narrows the band gap and enhances visible light photocatalytic performance without introducing compensating defects.

Findings

Band gap reduced from 3.75 eV to 2.4-2.7 eV due to Cr doping.

Optically generated carriers persist longer than 2 ns.

Enhanced photocatalytic degradation of methylene blue under visible light.

Abstract

Perovskite titanates such as SrTiO$_{3}$ (STO) exhibit a wide range of important functional properties, including high electron mobility, ferroelectricity, and excellent photocatalytic performance. The wide optical band gap of titanates limits their use in these applications, however, making them ill-suited for integration into solar energy harvesting technologies. Our recent work has shown that by doping STO with equal concentrations of La and Cr we can enhance visible light absorption in epitaxial thin films while avoiding any compensating defects. In this work, we explore the optical properties of photoexcited carriers in these films. Using spectroscopic ellipsometry, we show that the Cr$^{3+}$ dopants, which produce electronic states immediately above the top of the O 2p valence band in STO reduce the direct band gap of the material from 3.75 eV to between 2.4 and 2.7 eV depending on doping levels. Transient reflectance spectroscopy measurements are in agreement with the observations from ellipsometry and confirm that optically generated carriers are present for longer than 2 ns. Finally, through photoelectrochemical methylene blue degradation measurements, we show that these co-doped films exhibit enhanced visible light photocatalysis when compared to pure STO.