How UV light lowers the conductivity of SrTiO3 by photochemical water splitting at elevated temperature.

Alexander Viernstein,Tobias M Huber,Matthäus Siebenhofer,Jürgen Fleig,Maximilian Morgenbesser,Markus Kubicek

doi:10.1039/d1ma00744k

Alexander Viernstein, Tobias M Huber + Show 4 more

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https://doi.org/10.1039/d1ma00744k

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Abstract

Nominally undoped SrTiO3 single crystals were illuminated by UV light at 350 °C in oxidizing as well as reducing atmospheres. In N2/O2 atmospheres, UV irradiation enhances the conductivity of SrTiO3 by several orders of magnitude. In dry H2 atmosphere UV exposure leads to the opposite conductivity effect, i.e., above band gap energy illumination surprisingly lowers the conductivity. This is discussed in the framework of a defect chemical model. We show that a shift in defect concentrations due to UV-driven oxygen incorporation from the gas phase into the oxide is the main cause of the measured conductivity changes. A model is introduced to illustrate the thermodynamic and kinetic drivers of the processes under UV irradiation. Noteably, in reducing H2/H2O atmospheres, the incorporation of oxygen into the investigated oxide under UV light takes place via water splitting. Owing to the predominant electron conduction of SrTiO3 in equilibrium with H2, oxygen incorporation upon UV and thus an increase of the oxygen chemical potential leads to a decrease of the majority electronic charge carrier, here electrons, which lowers the conductivity under UV irradiation.

Highlights

In recent years, the properties of the model perovskite SrTiO31–6 upon above band gap light exposure at elevated temperatures came into the focus of research
We show the effect of UV light on the conductivity of undoped SrTiO3 single crystals for four different gas atmospheres with very different oxygen partial pressures with special emphasis on measurements under reducing conductions
The in-plane conductivities of undoped SrTiO3 single crystals were deduced from the impedance spectra measured before, under, and after UV illumination

Summary

Introduction

The properties of the model perovskite SrTiO31–6 upon above band gap light exposure at elevated temperatures came into the focus of research. Several different phenomena were observed and described, such as UV-induced changes of the oxygen exchange kinetics,[7] light induced battery type voltages in solid oxide cells,[8] varying photovoltages in SrTiO3 based high temperature solar cells,[9] high temperature photochromism,[10] and conductivity variations under UV illumination.[8,11,12] Many of these observations can be explained by assuming a light driven oxygen incorporation into SrTiO3 and an increase of the oxygen chemical potential in the entire SrTiO3 bulk.[10] in literature the formation of oxygen vacancies as a consequence of UV illumination is described.[12] Even though different UV-driven defect chemical consequences are not necessarily in contradiction to each other, questions about the factors determining the defect chemical situation under UV irradiation arise from the mentioned findings. Experiments in a large pO2 range are of high interest

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