Water adsorption on TiO2 surfaces probed by soft X-ray spectroscopies: bulk materials vs. isolated nanoparticles

Abstract

We describe an experimental method to probe the adsorption of water at the surface of isolated, substrate-free TiO2 nanoparticles (NPs) based on soft X-ray spectroscopy in the gas phase using synchrotron radiation. To understand the interfacial properties between water and TiO2 surface, a water shell was adsorbed at the surface of TiO2 NPs. We used two different ways to control the hydration level of the NPs: in the first scheme, initially solvated NPs were dried and in the second one, dry NPs generated thanks to a commercial aerosol generator were exposed to water vapor. XPS was used to identify the signature of the water layer shell on the surface of the free TiO2 NPs and made it possible to follow the evolution of their hydration state. The results obtained allow the establishment of a qualitative determination of isolated NPs’ surface states, as well as to unravel water adsorption mechanisms. This method appears to be a unique approach to investigate the interface between an isolated nano-object and a solvent over-layer, paving the way towards new investigation methods in heterogeneous catalysis on nanomaterials.

Water splitting and the band edge positions of TiO2

“…Surface oxygen vacancies dissociate water on both R(110) [67] and A(101) [68], and recent experimental [69] and theoretical [70] studies of the nonreduced anatase surface also find 1/4 of the adsorbed water to be dissociated. The majority of PES studies confirm the mixed dissociative and molecular adsorption mechanism of water on various titania surfaces [71][72][73] despite some claims to the opposite [74]. Electrolyte solutions always contain free H + and OH À ions (in about equal number at pH = 7), which will easily adsorb on the surface and, considering the high barrier between dissociated and molecular surface water [68,75], it appears reasonable to assume that large portions of the surface will contain H + adsorbed on O 2c and OH À on Ti 5c surface atoms (bridging OH + and terminal OH À groups, respectively), i.e., dissociated water.…”

Diffusion and reaction pathways of water near fully hydrated TiO2 surfaces from ab initio molecular dynamics

“…17 TiO 2 nanoparticles are small enough to penetrate the blood-brain barrier, 18 can aggregate in organisms and the environment, and yield a nanotoxic response by increasing the levels of intracellular reactive oxygen species, leading to apoptosis. 19 Considerable research efforts have been devoted to sorting out the molecular mechanisms behind water dissociation and diffusion on TiO 2 surfaces 20,21 and nanoparticles, 22,23 as well as adsorption of biological molecules on TiO 2 24-28 under ambient conditions. The TiO 2 water interface 29 can be probed with a range of experimental techniques.…”

Suppressing the Photocatalytic Activity of TiO2 Nanoparticles by Extremely Thin Al2O3 Films Grown by Gas-Phase Deposition at Ambient Conditions

“…The fingerprints of TiO 2 are featured by the peaks at binding energies (BE) of 529.4 eV (O 1 s , Figure 2a), BE = 463.9 eV and BE = 458.2 eV (Ti 2 p , Figure 2c). The deconvolution of O 1 s spectrum revealed the presence of a considerable amount of –OH groups (BE = 530.8 eV) and physisorbed H 2 O (BE = 532.3 eV) [76,77]. The physisorbed H 2 O can be removed by applying a heat treatment in air at 170 °C for 1 h, as indicated by Figure 2b.…”