Electronic structure of aqueous-phase anatase titanium dioxide nanoparticles probed by liquid jet photoelectron spectroscopy
We report on the nature of water interactions with anatase TiO2 surfaces. TiO2 nanoparticles (NPs), 3, 6, 10, and 20 nm in diameter, dispersed in different aqueous solutions, were investigated by soft-X-ray photoemission spectroscopy from liquid microjets. One central aspect of this study is the characterization of the electronic structure and identification of the molecular species that exist at the NP–aqueous solution interface as a function of solution pH. Valence and core-level electron binding energies are determined by the respective non-resonant photoelectron spectra. In addition, we report resonant photoemission spectra at the Ti 2p and the O 1s edges, which considerably increases the detection sensitivity of the interfacial species. This also allows us to distinguish between titanium at the surface and inside the aqueous-phase NPs. Furthermore, from the Ti 2p resonant photoelectron spectra, we obtain the so-called partial electron yield X-ray absorption (PEY-XA) spectra, which help here to rule out an anatase-phase transformation or the occurrence of Ti3+ sites due to oxygen defects. However, a more direct spectral feature that allows us to distinguish between molecularly and dissociatively adsorbed water is provided by the actual O 1s resonant photoelectron spectra. This is then exploited to show that water adsorbs molecularly at low pH, and dissociative adsorption at the TiO2 NP (aq) surface is observed at basic pH. Based on our results, we propose a mechanism of the anatase TiO2–H2O interaction that explicitly accounts for the local solution chemical environment. Here, H2O and OH− adsorb at the Ti sites, and no oxygen defects exist.