Reactions of trimethylindium on TiO ₂ nanoparticles: Experimental and computational study

This article reports the results of an experimental and computational study on the reaction of trimethylindium, Iri(CH ₃) ₃, adsorbed on TiO ₂ nanoparticle films. Experimentally, Fourier transform infrared (FTIR) spectra have been measured by varying In(CH ₃) ₃ dosing pressure, UV irradiation time in the absence and presence of oxygen, and surface annealing temperature on both "clean" and HO-covered TiO ₂ nanoparticle films. Computationally, adsorption energies, molecular structures, and vibrational frequencies of possible adsorbates have been predicted by first-principles calculations based on the density functional theory (DFT) and the pseudopotential method. Three important reactions involving CH ₃ elimination, CH ₄ elimination, and CH ₃ migration from the adsorbed trimethylindium have been elucidated in detail. CH ₃ migration is the only exothermic process with the lowest reaction barrier. On the basis of experimental and computational results, the two sharpest peaks at 2979 and 2925 cm ^-1 detected in the dosage and UV irradiation experiments in the absence of oxygen, are attributable to the asymmetric and symmetric C-H vibrations of methyl groups in In(CH ₃) ₃(a) and its derivatives, (H ₃C) ₂In(a), H ₃CIn(a), and H ₃CO(a). In the UV irradiation experiment in the presence of oxygen, the methyl groups attached to the In atom were quickly oxidized to the methoxy with the C-H vibrations at 2925 and 2822 cm ^-1 and to the carboxyl group with vibrations at 2888 cm ^-1 (v _s(CH)), 1577 cm ^-1 (v _a(OCO)), 1380 cm ^-1 (δ(CH)), and 1355 cm ^-1 (v _s(OCO)). Finally, from the computed energies with vibrational analysis, the adsorbed structure of the carboxyl group was confirmed to involve two oxygen atoms doubly adsorbed on two surface Ti atoms.