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Surface Traps of TiO2 Nanosheets and Nanoparticles as Illuminated by Spectroelectrochemical Photoluminescence

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journal contribution
posted on 31.07.2014 by Riley E. Rex, Fritz J. Knorr, Jeanne L. McHale
The solvent-dependent energies of surface trap states of TiO2 nanoparticles are examined by spectroelectrochemical photoluminescence using two different particle morphologies. Trap-state photoluminescence of nanocrystalline TiO2 in aqueous environment under Fermi level control reveals the pH-dependent redox Fermi levels of the surface Ti3+/4+ couple associated with 5-fold coordinated titanium. In aqueous environment, this trap-state distribution is populated at lower energy in TiO2 nanosheets rich in exposed (001) texture, compared to commercial anatase TiO2 nanoparticles with exposed (101) surfaces. Lower-energy traps appear to be partially passivated in the case of nanosheets in acetonitrile environment. Self-modeling curve resolution of the photoluminescence under Fermi level control reveals three spectral components in aqueous and acetonitrile environments: the red and green photoluminescence we have previously associated with electron and hole traps, respectively, and a third intermediate (yellow) component that may result from a separate distribution of electron traps. An apparent overvoltage, which is larger for nanoparticles than for nanosheets, is found for occupation of surface electron traps in aqueous environment. In contrast, electron traps in acetonitrile are occupied at potentials consistent with their energetic position within the band gap as determined by the photoluminescence spectrum. Our results reveal the solvent-dependent redox potential of electron traps and lend insight into the effects of contacting solvent on performance of nano-TiO2 in applications such as dye-sensitized solar cells.