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Imaging Luminescent Traps on Single Anatase TiO2 Crystals: The Influence of Surface Capping on Photoluminescence and Charge Transport

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journal contribution
posted on 19.11.2015, 00:00 by Riley E. Rex, Fritz J. Knorr, Jeanne L. McHale
The spatial distribution of intra band gap traps in micrometer-sized single crystals of anatase TiO2 was explored using single-particle photoluminescence (PL) spectroscopy and imaging. The PL from microcrystals with well-defined {001} and {101} facets was imaged for the same particle before and after annealing to explore the influence of fluorine, used as a capping agent in the synthesis of the microparticles, on luminescent traps. Unannealed particles reveal weaker photoluminescence and distinctly different spatial distribution of PL emission compared to annealed particles. The results show that the capped particles have fewer surface defects as a result of passivation of surface electron traps associated with undercoordinated titanium. PL images suggest that the remaining surface defects in the fluorine-capped particles concentrate at edges and corners of the microcrystal, and that anisotropic carrier transport takes place via hopping between adjacent Ti atoms. Annealed particles reveal greater surface defect density and more isotropic, diffusional carrier transport than the as-synthesized particles. The results are interpreted in terms of the influence of fluorine-capping groups on the spatial distribution and occupancies of traps and different pathways for carrier transport. The results have implications for applications of TiO2 nanosheets in solar energy conversion and photocatalysis.