The physics of non-compensated co-doping as a route to band gap narrowing in oxide materials: electronic structure and ultra-fast charge transfer dynamic in N/Cr non-compensated co-doped TiO2 unveiled with soft-x-ray spectroscopies.

N. Mannella

University of Tennessee, Knoxville

Bandgap narrowing of oxide semiconductors has been recognized as the main avenue for enhancing their performance in photo-electrochemical solar energy conversion.  In a recent study, non-compensated n-p co-doping has been identified as an enabling concept to narrow the band gap of TiO2 with dramatically enhanced photoreactivity [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)].  Non-compensated n-p co-doping takes advantage of the strong tendency of the dopants with opposite charge states to bind together, which greatly enhances their thermodynamic and kinetic solubilities for dopant incorporation.  Furthermore, non-compensated n-p codoping enables tuning of the impurity band location and chemical potential by choosing different combinations and concentrations of the n- and p-type dopants.

In this talk, I will discuss the results of a series of x-ray spectroscopy studies carried out at synchrotron radiation facilities on N/Cr non-compensated co-doped TiO2 as a model system for addressing some of the key issues surrounding the physics of non-compensated co-doping, namely: 1) How the N and Cr dopants impact the electronic structure of TiO2, 2) The microscopic mechanisms associated with the band gap narrowing, 3) The degree of localization of the N/Cr induced states within the TiO2 band gap, and 4) The dynamics of the light induced electron transfer with chemical specificity. 

These results may be relevant towards a sound understanding of the microscopic mechanisms responsible for the band gap reduction and carriers mobility in TiO2, with the outcomes providing crucial feedback for the development of oxide semiconductors with dramatically enhanced photoreactivity.