Apparent Semiconductor Type Reversal in Anatase TiO₂ Nanocrystalline Films

Abstract

The electronic properties of anatase and rutile TiO 2 polycrystalline particle films have been studied using surface photovoltage (SPV) and infrared (IR) spectroscopies. The films were prepared from aqueous suspensions using a range of particle sizes (8-400 nm) and were examined under different ambient conditions. The results show that all of the examined films exhibited the expected n-type semiconductor characteristics in dry nitrogen ambient. Films created from the 30 and 400 nm anatase particles exhibited the largest surface photovoltage and also a broad mid-IR absorption attributed to shallowly trapped electrons. The latter was absent from films made from rutile particles. When examined under as-prepared "wet" conditions, the SPV was reduced in magnitude and the IR signal was absent. Further, the films formed from 8 and 30 nm particles exhibited an apparent p-type behavior in their SPV spectra. Interestingly, small anatase particles are known to exhibit enhanced photocatalytic activity when compared to larger anatase particles. These correlations indicate that water, adsorbed on TiO 2 particles of nanodimensions, induce surface states and enable redistribution of photogenerated charge carriers, which is conducive to photocatalysis.Photocatalysis at semiconductor surfaces has been extensively investigated over recent decades. 1-3 Titanium dioxide has usually been the photocatalyst of choice and Degussa P25, which contains about 80% anatase and 20% rutile, has emerged as the benchmark material. 3 Photocatalysis studies have spanned well-characterized, single-crystal (usually rutile) surfaces under ultrahigh vacuum (UHV) conditions 4-6 to polycrystalline powders under catalysis operational conditions. 7-9 The latter have generally involved the catalyst in contact with an aqueous solution or water vapor. It is acknowledged that the presence of water is important for photocatalysis, 10-11 but its exact role in the process is poorly understood. 12-15 Numerous models 13,[16][17][18][19] have been proposed for the influence of adsorbed water on charge carrier trapping but consensus has still to be reached.Surface photovoltage (SPV) spectroscopy has been used to study both polycrystalline and single crystal (110) rutile TiO 2 . [20][21][22] For both types of material, it was reported that water modified the surface work function of rutile TiO 2 surfaces. There appear to have been no corresponding studies of anatase TiO 2 surfaces. It has been observed that adsorbed water influences a broad mid-infrared spectral absorption between 4000 and 700 cm -1 , which arises when TiO 2 polycrystalline films are irradiated with UV light 13,17,23,24 and that has been assigned to excitation of shallowly trapped electrons. 24 Panyatov and Yates 17 observed that this signal decreases when water is chemisorbed to the surface of a thermally reduced P25 TiO 2 film. They propose that the water depletes conduction band electrons during the formation of Ti-OH groups in the early stages of chemisorption. In this paper, we report the results...

Investigation of nanostructured TiO2 surface and interface electric fields with photoreflectance spectroscopy

“…Gradients in the ordering of near‐surface water layers alter the electric field at the H 2 O‐TiO 2 interface. This phenomenon manifests experimentally in surface photovoltage and infrared spectra for water‐decorated anatase TiO 2 . Warren et al note that adsorbed H 2 O species affect both the magnitude and direction of the H 2 O‐TiO 2 interface electric field via surface‐mediated charge carrier trapping.…”

Kelvin probe force microscopy of nanocrystalline TiO2 photoelectrodes

“…We conclude that the measured SPV of the sensitized TiO 2 is created by two different effects: a change of the surface dipole (after electron donation) and a charge carrier concentration gradient between the illuminated surface and the bulk due to different diffusion coefficients for electrons and holes (photo-Dember effect). The latter effect causes a potential drop forming an electric field in the z-direction across the TiO 2 layer [29,53–54]. The Dember photovoltage is caused by a non-uniform generation or recombination of charge carriers within the sample [18].…”

“…SPV spectroscopy is usually performed with a macroscopic vibrating capacitor and is hence limited by its poor lateral resolution [19–20]. Bare and dye-sensitized nanocrystalline (nc) TiO 2 have been investigated with such a macroscopic Kelvin probe (KP) revealing details about the electronic structure [21–23], trap states [24], the surface dipole [25], charge-carrier dynamics [26], and indicating changes upon chemical treatments [24,27–29]. KP studies have helped to select surface treatments that are beneficial for the DSC performance.…”

PbSe nanocrystals remain intrinsic after surface adsorption of hydrazine

“…The reversal of the conductivity type due to the creation of trapped surface states by etching has been previously reported in CdSe quantum dots 30 and TiO 2 nanocrystals. 31 There, the conductivity type could be switched from n-to p-type after mild etching with HCl. The underlying mechanism behind switching is the trapping of charge carriers in surface states leading to change in the type of the majority carrier.…”