Crystal-Face Dependence and Photoetching-Induced Increases of Dye-Sensitized Photocurrents at Single-Crystal Rutile TiO₂Surfaces

Abstract

Dye-sensitized photocurrents at (100)-, (001)-, and (110)-cut TiO(2) rutile surfaces were increased by photoetching of TiO(2), but the increasing ratio strongly depended on the cut crystal faces and the illumination intensity for the photoetching. For the (110)-cut surface, the photocurrent increase was moderately large and in proportion to the increase in the surface area of TiO(2) induced by the photoetching, irrespective of the illumination intensity for the photoetching. On the other hand, the photocurrent increases for the (001)- and (100)-cut surfaces, especially that for the (001)-cut surface, were prominent and largely exceeded the increases in the surface area. The results were explained by taking into account the following factors: (1) The (001)- and (100)-cut surfaces were thermodynamically unstable in contrast to the (110)-cut surface and had thicker inactive surface layers (or higher densities of surface defects), produced by surface reconstruction during heat treatment of TiO(2) at 550 degrees C in a hydrogen atmosphere for getting n-type semiconductivity. (2) Photoetching not only increased the surface area through formation of nanoholes and grooves at the surface but also effectively removed the thin inactive surface layers (or surface defects).

Comparison of the photoelectrochemical oxidation of methanol on rutile TiO2 (001) and (100) single crystal faces studied by intensity modulated photocurrent spectroscopy

“…A rather fast increase of the photocurrent to a saturation level is observed towards positive potentials for the (100) electrodes. The increase is considerably slower in the case of the (001) electrode, where a saturation level is still not reached at the highest potential of 1.5 V. This is mainly due to the formation of an inactive thin surface layer during H 2 treatment as previously reported 30 and supported here by scanning electron microscopy measurements (see Fig. 4a and b).…”

A surface science perspective on TiO2 photocatalysis

“…Small differences in the coupling of the ground state molecule to these surfaces were deemed less significant compared to the magnitude of the coupling of the excited electron with Ti 3d states in the surfaces. On surfaces pretreated in a H 2 atmosphere at 550°C prior to sensitization in solution, these authors found that the photocurrent generated on R TiO 2 (110) was over a factor of 10 greater than that on other surfaces [672]. They attributed this to generation of inactive (defective) regions on other surfaces as a result of surface reduction.…”

“…However, several groups have examined electron transfer between an excited dye and a single crystal TiO 2 surface [116,390,391,626,633,655,[671][672][673][674]. Schnadt et al [655] were one of the first groups to use the R TiO 2 (110) surface to study the dynamics of electron injection, albeit in their case from resonant core-hole excitation of adsorbed isonicotinic and biisonicotinic acids rather than via visible light absorption.…”

“…In other surface structure-sensitivity studies on single crystal R surfaces, Imanishi and coworkers examined the influence of R surface structure on dye sensitization [672,673]. On pristine surfaces, they found no significant difference in the per-molecule injection photocurrent from excitation of a benzothiazole merocyanine dye into the surfaces of R TiO 2 (100) and TiO 2 (110) despite the fact that NEXAFS data suggests the dye binds more strongly to the (110) surface [673].…”

“…In other words, the specific arrangements of cation and anion surface sites on TiO 2 is important in influencing both adsorbate properties and those of charge carriers Finally, studies discussed above illustrate the influence that surface structure plays in influencing photochemistry on R single crystal surfaces, but the effect that surface photochemical processes play in influencing surface structure is not wellunderstood. As mentioned above, Nakato's group [672,1653,1654,1675] has shown that R surfaces can be photoetched, but in the context of somewhat harsh electrochemical conditions. There are also examples in the literature of the electronic or electron paramagnetic signatures of photoinduced charge accumulation in R powders.…”