Interfacial behavior of benzoic acid and phenylphosphonic acid on nanocrystalline TiO₂ surfaces

The interfacial behavior of self-assembled thin films of benzoic acid (BA) and phenylphosphonic acid (PPOA) anchored on TiO₂ surfaces was studied by using temperature-dependent diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. On the basis of the disappearance of the OH band from the infrared spectra at room temperature, BA and PPOA appear to adsorb onto TiO₂ surfaces through carboxylate and phosphonate groups, respectively. Above 420°C, DRIFT spectra indicated that both BA and PPOA desorb from TiO₂ surfaces; however, dissimilar desorption behavior could be inferred for BA and PPOA from their temperature-dependent spectral changes. The benzene ring modes of PPOA remained above 420°C, whereas those of BA disappeared. Density functional theory calculations showed that the adsorption of BA and PPOA on TiO₂ surfaces corresponded to bidentate bridging geometry on TiO₂ surfaces, and the adsorption of PPOA is stronger than that of BA. The monodentate structures with energy differences of 4.9 and 9.1 kcal mol^-1 from the most stable bidentate structures of BA and PPOA, respectively, from the DFT calculations appeared to be possible, particularly at the high temperatures above 420 °C, as indicated by the intensified OH bands. The geometry of PPOA was also estimated to be more upright standing than that of BA on TiO₂ surfaces, which may lead a rather straight detachment from the TiO₂ surfaces based on the presence of in-plane ring modes in the DRIFT spectra at the higher temperature.