International audienceThe catalytic, electrochemical, and optical properties of gold clusters and small nanoparticles depend on the cluster size, on the nature of the ligand shell and on the solvent environment. The structural and electronic properties of the neutral and trication bare cluster Au-11 are investigated using density functional theory. We focus on the influence of the cluster charge, the solvent, and the nature and number of the ligands (thiol, thiolate, thiyl radical, phosphine, chloride, and chlorine) on the cluster electronic structure and its geometry, with special attention to the structural motifs present in the metallic core of the different conformers. Single bindings are systematically studied and a comparison with full ligand coverage is discussed. Pure thiyl and phosphine ligand shells as well as mixed thiyl/phosphine shells are considered. This study provides a better understanding of the ligand shell and cooperative effects that could be probed experimentally

Microwave‐assisted synthesis represents a valuable improvement in the domains of molecular and organic chemistry and was recently extended to inorganic and materials chemistry. A comparison of titanium dioxide nanoparticles synthesised in aqueous solution prepared in a microwave or a conventional oven is presented here. More precisely, three different protocols were used in order to determine the impact of the heating mode on the final product in terms of crystalline structure, particle size and morphology. Therefore, the resultant powders were analysed by Raman spectroscopy as well as X‐ray and electron diffraction and transmission electron microscopy. The results show that microwave treatment significantly reduces the heating time and generally produces smaller nanoparticles. The rutile/anatase/brookite phase distribution is also modified by the heating mode in certain protocols up to the formation of a pure anatase phase, for instance. The impact of microwaves on the solvent and on the inorganic precursors has been demonstrated. A photocatalytic test and time‐resolved microwave conductivity experiments were performed on rather similar samples prepared with the two heating modes in order to probe the improvement of the crystalline quality and its consequences on the photocatalytic activity of the TiO2 material.

International audienceThe richness of titanium dioxide sol–gel syntheses described in literature provided a set of four different morphologies of pure anatase nanoparticles to study the impact of the exposed surfaces on the photocatalytic efficiency of the corresponding material. The selection of the experimental parameters such as the temperature, the heating method or organic additives allowed the synthesis of pure anatase materials with significantly different shapes. A thorough microscopic study of these particles gave the exposed crystallographic faces. The photocatalytic activity of the different materials was estimated following the degradation of the rhodamine B dye under UV-light and significantly different behaviors were observed. In the applied photodegradation conditions, two samples were shown to be more efficient than the reference photocatalyst P25. The rationalization of these results was done through the study of the oxide surface properties, using FT-IR spectroscopy with pyridine as a surface probe and the EPR analysis of photogenerated radicals under UV light. The most efficient photocatalyst for rhodamine B degradation was found to be the morphology presenting the stronger acidic surface sites

The aim of this study was to provide new insight into the evaluation of the effect of the crystallinity, size, and morphology of TiO2-anatase nanoparticles on their acid–base properties at the solid–liquid interface. This was achieved through monitoring the evolution in the surface charge density with the solution acidity and point of zero net proton charge (PZNPC) for a set of anatase nanoparticles with a mean size in the 5–20 nm range and various shapes. Different anatase nanoparticles were obtained by a sol–gel synthesis approach using different precursors, pH conditions, and various inorganic or organic additives. The measured PZNPC values were found to vary more than one pH unit depending on the degree of crystallinity and presence of differently exposed surfaces. To discriminate slight differences in the surface reactivity at the solid–water interface, high-resolution titration curves of surface charge for each anatase sample were recorded, and a fine analysis by the titration derivative isotherm summation (TDIS) method and proton affinity distributions (PADs) was performed. They provided accurate data on the strength (pK position) of various local domains of proton adsorption with their relative surface contributions in relation with the particle morphology. The increase in the proportion of sites that are more present on the {101} faces or that of medium acid sites largely present on the {100} faces shifted the point of zero net proton charge to more acidic pH values for particles possessing a majority of such surfaces. In addition to these experiments, the relative acidity of the surface sites on the different surfaces of anatase was evaluated with the multisite complexation model (i.e., MUSIC model) applied to the theoretically optimized surfaces, and a comparison was drawn.

The photocatalytic properties of titanium dioxide depend not only on its electronic properties, but also on the material size and shape, which can increase interactions between the reactants and catalyst. Most studies to date show that reducing the particle size down to the nanoscale increases photocatalytic efficiency, as a result of a higher surface to volume ratio and because a larger proportion of the material is actually irradiated by light. We demonstrate that a multiscale shape design, which integrates surface roughness, particle shape, and 1D material processing and orientation, can favor photocatalytic properties in the solid–gas regime, especially mineralization (conversion into CO2), when the hierarchical 1D orientation of the material is combined with unidirectional gas flow. Several materials with hierarchical structure were prepared and characterized. They have been tested for the photocatalytic mineralization of gaseous acetone and compared with commercial catalysts. Our study reveals that a suitable combination of multiscale design and optimization of the material orientation and gas flow favors high mineralization.

Anatase nanoparticles with shape controlled bipyramidal morphology (TiO 2-A-bipy) exhibited mainly {101} facets were synthesized through the sol-gel method and then used for the photodegradation of three model pollutants-Rhodamine B, phenol and formic acidunder UV-A radiation exposure. These titania samples exhibit better photocatalytic efficiency than the commercial TiO 2-P25 reference for the dye degradation while this one demonstrates a higher activity for both phenol and formic acid. Moreover, supplementary washings of the particles significantly enhanced their photocatalytic efficiency in any case. To better understand these differences in term of photoactivity and the role of the TiO 2 surface according to the nature of the targeted organic pollutant, various characterization techniques such as XRD, TEM and N 2-sorption were used. Their surface properties were studied by FT-IR, TRMC and EPR. The presence of more acidic sites on TiO 2-A-bipy surface could explain the faster degradation of the dye molecule through surface-mediated reactions. On the other side, a better generation and separation dynamic of photogenerated charges for TiO 2-P25 could account for its higher photocatalytic efficiency for both formic acid and phenol degradation. This study shows that even if a quick test of dye degradation is mostly used in literature to confirm the efficiency of a photocatalyst, further investigation is often needed.

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