Probing the electronic structure and band gap evolution of titanium oxide clusters (TiO2)n- (n=1-10) using photoelectron spectroscopy

TiO2 is a wide-band-gap semiconductor, and it is an important material for photocatalysis. Here we report an experimental investigation of the electronic structure of (TiO2)(n) clusters and how their band gap evolves as a function of size using anion photoelectron spectroscopy (PES). PES spectra of (TiO2)(n)(-) clusters for n = 1-10 have been obtained at 193 nm (6.424 eV) and 157 nm (7.866 eV). The high photon energy at 157 nm allows the band gap of the TiO2 clusters to be clearly revealed up to n = 10. The band gap is observed to be strongly size-dependent for n < 7, but it rapidly approaches the bulk limit at n = 7 and remains constant up to n = 10. All PES features are observed to be very broad, suggesting large geometry changes between the anions and the neutral clusters due to the localized nature of the extra electron in the anions. The measured electron affinities and the energy gaps are compared with available theoretical calculations. The extra electron in the (TiO2)(n)(-) clusters for n > 1 appears to be localized in a tricoordinated Ti atom, creating a single Ti3+ site and making these clusters ideal molecular models for mechanistic understanding of TiO2 surface defects and photocatalytic properties.