Adsorption of organic molecules on TiO2 surfaces is widely used in a number of technological applications, from heterogeneous catalysis, in particular photodegradation of organic pollutants, to dye-sensitized solar cells (DSSCs), where in most cases the dye molecules are grafted to the anatase TiO2 surface through a carboxylic group. In particular, organic/TiO2 systems can be of relevant importance in the modeling of electronic devices, in which the molecular layer is able to finely tune the electric properties, as well as of highly efficient heterogeneous catalysts. A key step is the understanding of the nature of the carbon-oxygen-titanium bonds on such surfaces, which is the specific aim of our combined IR and ab initio study of the adsorption of CH3COOH on TiO2. The experimental determination of the CH3COOH frequency shifts due to the absorption on the P25 (Degussa) TiO2 surface was performed by means of a step-wise procedure, consisting of a preliminary outgassing at 600°C of TiO2, in order to have a high dehydroxilation degree of the surface, followed by IR measurements at increasing CH3COOH pressure and subsequent desorption. Frequency calculations to be compared with experimental results were performed within a cluster approach using GAUSSIAN03 package. In order to make such calculations feasible, we decided to use an ONIOM approach where the model system, i.e., the small portion corresponding to CH3COOH plus the surface atoms, is treated at DFT level while the real system, comprising the bulk atoms, at MSINDO level. Once properly tested the ONIOM approach to characterize the interaction of TiO2 with CH3COOH, we computed the vibrational frequencies and compared them with the results of the IR experiments, providing some insight for the interpretation of the experimental complex vibrational pattern.
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