Understanding the Electronic Structure and Electron Transfer Kinetics of Titanium Dioxide Photoanodes and Analyzing Parameters Affecting Flatband Potentials in Metal Oxides
Rutile TiO2 shows promise for being used as a photoanode semiconductor in dye-sensitized photoelectrosynthesis cells (DSPECs), devices that use sunlight to drive the production of solar fuels. The addition of a TiO2 coating or shell onto a mesoporous nanocrystalline TiO2 photoanode substrate has been shown to improve device efficiency for water oxidation, yet little is known about why this change improves the performance of DSPECs. In this research, TiOx shells were deposited using atomic layer deposition onto rutile TiO2 nanorods and the electrochemical effects of the deposition were probed to help elucidate changes to the electronic structure induced by the shell. Rutile TiO2 was found to have a monoenergetic collection of deep trap states that is positive in potential to an exponential trap distribution in the band gap below the conduction band minimum. When increasing the TiOx shell thickness, the deep trap states of rutile TiO2 shifted to more positive potentials without changing the density of these states. In addition, the band gap of the material was found to decrease as shell thickness increased as quantified using diffuse reflectance spectroscopy. From photoelectrochemical impedance spectroscopy, the calculated rates of back-electron transfer were lower for samples with a TiOx shell compared to samples of rutile TiO2 without a shell.
Metal oxides like TiO2 are gaining a lot of attention for their applications in energy technologies. A useful parameter used in the application of metal oxides is the flatband potential, yet values for the flatband potential are widely variable in the literature. A meta-analysis of flatband potential values for TiO2, SnO2, and ZnO was performed to study what variables impact the flatband potential for n-type metal oxides. Flatband potential values shifted –59 mV/pH with the exception of ZnO thin film flatband potentials, which showed an apparent lack of dependence on solution pH likely. The flatband potentials for anatase TiO2 nanotubes were shifted ~ 0.4V positive of other anatase TiO2 morphologies. Without the nanotube data points, anatase TiO2 and rutile TiO2 did not have a significant difference in mean flatband potential values, in contrast to what is often assumed for these two crystalline phases. Flatband potentials for ZnO appeared to shift negatively with increasing cation concentration, though previous literature precedence with other metal oxides suggests that the flatband potential should not be affected by non-proton cations in aqueous solutions. The findings of these analyses demonstrate the need to recognize the sensitivity of flatband potentials to multiple factors and the spread of flatband potential values that exist even between similar nanomaterials.Bachelor of Scienc
