Spectroelectrochemical photoluminescence spectroscopy and imaging of reactive surface defects in titanium dioxide nanoparticles

Titanium dioxide (TiO2) nanoparticles have shown promise as a low-cost and effective material for a wide range of applications including batteries, photovoltaics, and photocatalysis. To optimize TiO2 nanoparticles in these applications, it is necessary to understand the nature of surface defect states which influence device efficiencies by their role in electron transport and surface reactivity. In this work, the solvent-dependent energies and spatial locations of surface defect states of TiO2 nanoparticles are investigated using spectroelectrochemical photoluminescence (SEPL) and single-particle photoluminescence spectroscopy and imaging. In the SEPL work, anatase nanoparticles, anatase nanosheets, and rutile nanowires are examined in aqueous and nonaqueous environments. Trap state photoluminescence of nanocrystalline TiO2 in aqueous environment under Fermi level control reveals the pH-dependent redox Fermi levels of the surface Ti 3+/4+ couple associated with five-fold coordinated titanium. In aqueous environment there is an overvoltage for occupying surface electron traps in rutile and anatase samples. For anatase, this overvoltage is larger on (101) nanoparticles than on (001) nanosheets. Electron traps in acetonitrile are occupied at potentials consistent with their energetic position within the band gap as determined by the photoluminescence spectrum. The anatase SEPL results lend insight into the effects of contacting solvent on performance of nano-TiO2 in applications such as dye--sensitized solar cells. The rutile SEPL results indicate that the near-IR PL of rutile nanowires, identical to that of bulk and conventional nanocrystalline rutile, arises from the radiative recombination of trapped electrons with valence band holes, and that valence band holes, rather than trapped holes, are more likely responsible for water oxidation. The single-particle photoluminescence experiments investigate microcrystals with well-defined {001} and {101} facets before and after annealing to remove the fluorine capping agent. The results reveal that the capping agent passivates surface electron traps associated with five-coordinated surface Ti sites and that anisotropic carrier transport takes place via hopping between adjacent Ti atoms. After annealing, the surface defect density is significantly increased and the carrier transport becomes more isotropic and diffusional. The results have implications for applications of nanostructured TiO 2 in solar energy and photocatalysis.