Revealing the nature of trapping sites in nanocrystalline titanium dioxide by selective surface modification

Excess electrons in nanocrystalline TiO₂ were studied in bare and dopamine-capped TiO₂ nanoparticles by electron-beam pulse radiolysis. Reaction of hydrated electrons with dopamine-capped TiO₂ nanoparticles was found to be at the diffusion-controlled limit, k = 1 × 10¹¹ M^-1 s^-1, while the reaction with 1-hydroxy-1-methylethyl radicals, (CH₃)₂COH, was 2 orders of magnitude slower, k = 4 × 10⁸ M^-1 s^-1. The reactions result in injection of electrons into the conduction band of TiO₂ nanoparticles. Optical absorption spectra of injected excess electrons in dopamine-capped nanoparticles display monotonic featureless wavelength dependence up to 1800 nm. In contrast, bare particles have shown two preferential optical transitions with energies in the visible region (λ_max = 670 nm and λ_max = 900 nm). Flat band potential of dopamine-capped TiO₂ nanoparticles was shifted by 100 mV to more negative values. The strong coupling of dopamine to surface Ti atoms was also found to improve the separation of photogenerated charges. This was demonstrated by the enhanced efficiency of photogenerated electrons in reducing silver cations to metallic silver in systems linked via a dopamine bridge, compared to the same systems linked through carboxyl groups.