UPS and UV spectroscopies combined to position the energy levels of TiO2 anatase and rutile nanopowders

Abstract

An accurate experimental determination of electronic structures in semi-conductor nanopowders is a challenging task. We propose here to combine UPS and UV-Vis spectroscopies in order to get the full description of the electronic band alignment of powder samples, TiO₂ rutile and anatase. For UPS measurements, two preparation methods, namely the dropping method and electrophoretic deposition, were used to prepare layers of titania powders on a conducting substrate, ITO or Ag. Both methods lead to comparable results, with a quantitative description of the energy levels from the valence band. Combining these results with the UV-Vis spectra of the same powders enables the determination of the absolute position of the valence band maximum and the conduction band minimum. Combined UPS-UV-Vis spectroscopy provides a better insight into the properties of a powdered material which can differ from single crystal model systems. It can also be used to predict the electronic transfer in mixed phase systems during photocatalytic processes.