Molecules and heterostructures at TiO2 surface: the cases of H2O, CO2, and organic and inorganic sensitizers

Physico-Chemical Approaches to Investigate Surface Hydroxyls as Determinants of Molecular Initiating Events in Oxide Particle Toxicity

The study of molecular recognition patterns is crucial for understanding the interactions between inorganic (nano)particles and biomolecules. In this review we focus on hydroxyls (OH) exposed at the surface of oxide particles (OxPs) which can play a key role in molecular initiating events leading to OxPs toxicity. We discuss here the main analytical methods available to characterize surface OH from a quantitative and qualitative point of view, covering thermogravimetry, titration, ζ potential measurements, and spectroscopic approaches (NMR, XPS). The importance of modelling techniques (MD, DFT) for an atomistic description of the interactions between membranes/proteins and OxPs surfaces is also discussed. From this background, we distilled a new approach methodology (NAM) based on the combination of IR spectroscopy and bioanalytical assays to investigate the molecular interactions of OxPs with biomolecules and membranes. This NAM has been already successfully applied to SiO₂ particles to identify the OH patterns responsible for the OxPs' toxicity and can be conceivably extended to other surface-hydroxylated oxides.

Elucidating the role of adsorption during artificial photosynthesis: H2O and CO2 adsorption isotherms over TiO2 reveal thermal effects under UV illumination

Adsorption measurements of CO₂ and H₂O over TiO₂ surfaces in dark and under illumination were carried out to reveal the ensuing bottlenecks of the initial steps of the artificial photosynthesis reaction. When the adsorption isotherms of both CO₂ and H₂O were measured under illumination, the results were comparable to isotherms measured at higher temperatures in dark. This evidence is interpreted as the presence of hot spots, due to charge carrier recombination reactions. Differential heat of adsorption measurements revealed that H₂O adsorption on TiO₂ is stronger, and with a higher coverage than that of CO₂. Dissociation of water is an energetically uphill reaction, and the local hot spots due to charge carrier recombination in indirect bandgap semiconductors can enhance the reaction probability. At higher temperatures, higher reaction probabilities are expected and estimated by a thermodynamic analysis for water splitting reaction. The potential role of these hot spots during natural and artificial photosynthetic reactions is discussed.

Understanding the nature and location of hydroxyl groups on hydrated titania nanoparticles

TiO₂ nanoparticles (NPs) are intensively studied and widely used due to their huge potential in numerous applications involving their interaction with ultraviolet light (e.g., photocatalysis and sunscreens). Typically, these NPs are in water-containing environments and thus tend to be hydrated. As such, there is a growing need to better understand the physicochemical properties of hydrated TiO₂ NPs in order to improve their performance in photochemical applications (e.g., photocatalytic water splitting) and to minimise their environmental impact (e.g., potential biotoxicity). To help address the need for reliable and detailed data on how nano-titania interacts with water, we present a systematic experimental and theoretical study of surface hydroxyl (OH) groups on photoactive anatase TiO₂ NPs. Employing well-defined experimentally synthesised NPs and detailed realistic NP models, we obtain the measured and computed infrared spectra of the surface hydroxyls, respectively. By comparing the experimental and theoretical spectra we are able to identify the type and location of different OH groups in these NP systems. Specifically, our study allows us to provide unprecedented and detailed information about the coverage-dependent distribution of hydroxyl groups on the surface of experimental titania NPs, the degree of their H-bonding interactions and their associated assigned vibrational modes. Our work promises to lead to new routes for developing new and safe nanotechnologies based on hydrated TiO₂ NPs.

Morphology, Surface Structure and Water Adsorption Properties of TiO2 Nanoparticles: A Comparison of Different Commercial Samples

Water is a molecule always present in the reaction environment in photocatalytic and biomedical applications of TiO₂ and a better understanding of its interaction with the surface of TiO₂ nanoparticles is crucial to develop materials with improved performance. In this contribution, we first studied the nature and the surface structure of the exposed facets of three commercial TiO₂ samples (i.e., TiO₂ P25, SX001, and PC105) by electron microscopy and IR spectroscopy of adsorbed CO. The morphological information was then correlated with the water adsorption properties, investigated at the molecular level, moving from multilayers of adsorbed H₂O to the monolayer, combining medium- and near-IR spectroscopies. Finally, we assessed in a quantitative way the surface hydration state at different water equilibrium pressures by microgravimetric measurements.

Low Temperature Synthesis of Photocatalytic Mesoporous TiO2 Nanomaterials

We report the synthesis of mesoporous TiO2 nanostructures based on the decomposition of TiOSO4 in aqueous alkaline solution at room temperature, followed by mild thermal treatment (110 °C) in an oven and suitable to yield up to 40 g of product per batch. The duration of the thermal treatment was found to be crucial to control crystalline phase composition, specific surface area, surface chemistry and, accordingly, the photocatalytic properties of the obtained TiO2 nanocrystals. The thorough investigation of the prepared samples allowed us to explain the relationship between the structure of the obtained nanoparticles and their photocatalytic behavior, that was tested in a model reaction. In addition, the advantage of the mild treatment against a harsher calcination at 450 °C was illustrated. The proposed approach represents a facile and sustainable route to promptly access an effective photocatalyst, thus holding a significant promise for the development of solutions suitable to real technological application in environmental depollution.