The promising properties of anatase TiO2 nanocrystals exposing specific surfaces have been investigated in depth both theoretically and experimentally. However, a clear assessment of the role of the crystal faces in photocatalytic processes is still under debate. In order to clarify this issue, we have comprehensively explored the properties of the photogenerated defects and in particular their dependence on the exposed crystal faces in shape-controlled anatase. Nanocrystals were synthesized by solvothermal reaction of titanium butoxide in the presence of oleic acid and oleylamine as morphology-directing agents, and their photocatalytic performances were evaluated in the phenol mineralization in aqueous media, using O2 as the oxidizing agent. The charge-trapping centers, Ti3+, O–, and O2 –, formed by UV irradiation of the catalyst were detected by electron spin resonance, and their abundance and reactivity were related to the exposed crystal faces and to the photoefficiency of the nanocrystals. In vacuum conditions, the concentration of trapped holes (O– centers) increases with increasing {001} surface area and photoactivity, while the amount of Ti3+ centers increases with the specific surface area of {101} facets, and the highest value occurs for the sample with the worst photooxidative efficacy. These results suggest that {001} surfaces can be considered essentially as oxidation sites with a key role in the photoxidation, while {101} surfaces provide reductive sites which do not directly assist the oxidative processes. Photoexcitation experiments in O2 atmosphere led to the formation of Ti4+–O2 – oxidant species mainly located on {101} faces, confirming the indirect contribution of these surfaces to the photooxidative processes. Although this work focuses on the properties of TiO2, we expect that the presented quantitative investigation may provide a new methodological tool for a more effective evaluation of the role of metal oxide crystal faces in photocatalytic processes.

A new, fast, one-pot synthesis of SnO 2 and Pt-doped SnO 2 inverted opal thin films, to be used as materials for gas sensing, was carried out. Films were built from crystalline cassiterite nanoparticles, uniform in size (∼5 nm), resulting in a well-organized hierarchical structure of macro-and mesopores. The noble metal was homogeneously dispersed into the sensing layer of the oxide and the doping centers were present as Pt(IV) and Pt(II) species, partially reduced to Pt(0) after the interaction with the reducing gas (CO). The values of the electrical sensitivity under CO/Air atmosphere demonstrated that the response of Pt-doped films is higher than that of bare SnO 2 films, and that the response of inverted opal films is higher compared to that of the sol-gel films. The regular array of cassiterite nanoparticles, strongly interconnected and ordered as close-packed hollow spheres, promotes the effective gas diffusion through the oxide layer and, along with the electron acceptor ability of Pt(IV) doping centers, significantly contributes to enhancing the electrical sensitivity. The conductance regime of the Pt-doped SnO 2 inverted opal film is indicative of a regular microstructure of SnO 2 nanoparticles.

Silica-natural rubber composites were prepared by in situ sol-gel synthesis of silica nanoparticles functionalized with alkylthiol or alkylpolysulfide. The functionalizing groups were linked to silica particles by hydrolysis and polycondensation of a mixture of tetraethoxysilane (TEOS) with a suitable amount of (3-mercaptopropyl) trimethoxysilane (TMSPM), bis (3-triethoxysilylpropyl) disulfide (TESPD) or bis (3-triethoxysilylpropyl) tetrasulfide (TESPT). Particles from TEOS are spherical, instead those from TESPD, TESPT and TMSPM have irregular anisotropic shapes. This is due to the presence of isotropic or anisotropic interactions among the particle base units. Silica particles synthesized in the presence of TMSPM can also undergo condensation of the alkylthiol chains with the silanol groups, thus giving rise to a strong preferential direction for the anisotropic shape. Predominant filler-filler interactions and easy self-assembly were detected in particles from TEOS while those from TESPD and TESPT showed high filler-rubber interactions. Both filler-rubber and strong filler-filler interactions are present in silica particles synthesized by TMSPM. The dynamic mechanical properties of the composites, tested with stress-strain measurements, show that the storage modulus increases by increasing the filler-filler interaction, and it is maximum when also the filler-rubber interaction occurs. Strong silica-rubber interaction favors the silica dispersion in rubber, while it makes the filler network less compact and lowers the storage modulus.