Trap State Photoluminescence of Nanocrystalline and Bulk TiO 2, : Implications for Carrier Transport
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1268-EE03-08
Trap State Photoluminescence of Nanocrystalline and Bulk TiO2: Implications for Carrier Transport Christopher C. Rich, Fritz J. Knorr, and Jeanne L. McHale Washington State University, Department of Chemistry, Box 644630, Pullman, WA 99164-4630
ABSTRACT The visible photoluminescence of nanocrystalline TiO2 is examined in the presence of surface binding agents and as a function of vacuum annealing in order to probe the molecular nature of surface defects. The photoluminesence (PL) of bulk crystals of anatase TiO2 from (101) and (001) planes is also reported in order to test the hypothesis that electron and hole traps are spatially isolated on different crystal planes. We find that a number of hole scavengers are capable of quenching the PL associated with trapped electrons, while the ability of oxygen to quench PL through electron scavenging varies with the nature of the sample. We conclude that hole scavengers exert their influence on the PL through reaction with valence band holes rather than with spatially isolated trapped holes. Scavenging of electrons by O2, on the other hand, depends on adsorption at oxygen vacancies and varies with TiO2 sample. INTRODUCTION Surface defects, such as oxygen vacancies, undercoordinated Ti atoms, and hydroxyl groups, strongly influence the performance of nanocrystalline TiO2 in photocatalysis and solar energy conversion.1,2 Information about the spatial and energetic distribution of these traps would be enormously useful in the quest to tailor the surface properties of nano-TiO2 for a particular application. For example, in dye-sensitized solar cells energetically shallow electron traps are critical to electron transport in nanoporous TiO2 films, but deeper traps slow transport and enhance recombination.3 Empirical surface modifications, such as treatment of nanocrystalline TiO2 films by TiCl4 to improve photocurrents of dye-sensitized solar cells (DSSCs), may exert their influence by healing surface traps and reducing recombination.4 In previous work, we investigated the photoluminescence (PL) of nano-TiO2 in the presence of electron and hole scavengers in order to separate the contributions of trapped electrons and trapped holes to the PL spectrum.5 In nanocrystalline anatase, recombination of mobile electrons with trapped holes (Type 1 PL) accounts for emission which peaks at a green wavelength (~530 nm), while the recombination of trapped electrons with valence band holes (Type 2 PL) results in broad photoluminescence that extends well into the red: − + htr+ → hν green Type 1: eCB + Type 2: hVB + etr− → hν red In the presence of air, O2 acting as an electron scavenger strongly quenches the visible PL of nanocrystalline TiO2. In P25, which is ~75% anatase and ~25% rutile, the red-emitting electron traps are absent or quenched, and only the PL associated with hole traps is ob-
served. TiCl4 treatment eliminates the PL from these hole traps but does not influence the emission associated with electron traps.6 We speculated that the electron and hole traps of anata
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