The possibility of controlling the photocatalytic activity of TiO2 nanoparticles by tailoring their crystalline structure and morphology is a current topic of great interest. In this study, a broad variety of well-faceted particles with different phase compositions, sizes, and shapes have been obtained from concentrated TiOCl2 solutions by systematically changing temperature, pH, and duration of the hydrothermal treatment. The guide to select the suitable experimental conditions was provided by thermodynamic modeling based on available thermochemical data. By combining the results of TEM, HRTEM, XRD, density, and specific surface area measurements, a complete structural and morphological characterization of the particles was performed. Correlation between the photocatalytic activity in the UV photodegradation of phenol solutions and the particle size was established. Prismatic rutile particles with length/width ratio around 5 and breadth of 60-100 nm showed the highest activity. The surface chemistry of the particles was also investigated. Treatments that decrease the surface acidity, such as washing the powders with ammonia solution and/or calcining at 400 degrees C, have detrimental effect on photocatalytic activity. The overall results suggest correlation between particle morphology and photocatalytic activity and indicate that both electron-hole recombination and adsorption at the surface can be rate-controlling processes. The systematic approach presented in this study demonstrates that a substantial improvement of the photocatalytic activity of TiO2 can be achieved by a careful design of the particle morphology and the control of the surface chemistry.

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.

No abstract

Macroporous materials 1,2 based on metal oxides have become strategic for various applications like photonic crystals, 3À5 catalysts, 6À8 and sensor devices, 9À13 mainly due to their lightweight, their high surface area, and the well-defined porous architecture with a limited agglomerated morphology. 2 In sensing research the improvement in selectivity, reproducibility, and stability expected for the gas sensors based on macroporous metal oxides is still a challenge for pollutant detection. 14 Unfortunately, the delicate method needed to prepare macroporous materials still limits their wide employment in effective devices. 1,2 Macroporous metal oxides are typically synthesized by alkoxidebased solÀgel processes, employing two steps: 11,15À20 (i) the selfassembling of an ordered array of colloidal microspheres of polystyrene, polymethyl methacrylate, or silica, acting as template; (ii) the filling of interstices of the structure by metal alkoxide solution, followed by solÀgel hydrolysis and densification.The subsequent thermal treatment which removes the template leads to the macroporous structure called inverted opal, where the macropores are interconnected through holes resulting from the contact between the template spheres. 2 The application of this synthetic method becomes particularly tricky wherein highly reactive metal alkoxides are used as oxide precursors, 21,22 or if the oxide needs to be doped by sensitizing centers, 11 for example, by transition metal atoms. Moreover, when the preparation of films is required, such as in the case of semiconductor oxide-based gas sensors, the two-step approach appears unsuitable to produce metal oxide thin films having high reproducible morphology, homogeneously embedding dopants, like transition metal centers.To easily obtain macroporous metal oxides with high surface area and homogeneous dopant distribution, our group recently developed a novel one-step preparation of SnO 2 and Pt-doped SnO 2 as inverse opal thin films by dip-coating deposition of solÀgel precursors. 23 The procedure enables the self-assembled formation of the closely packed polystyrene (PS) microsphere array and the simultaneous infiltration of the precursors into the voids of the structure. Thus, the oxide solid skeleton around the spheres was obtained in one-step, unlike the conventional Received: October 22, 2010 ABSTRACT: Macroporous WO 3 films with inverted opal structure were synthesized by one-step procedure, which involves the self-assembly of the spherical templating agents and the simultaneous solÀgel condensation of the semiconductor alkoxide precursor. Transition metal doping, aimed to enhance the WO 3 electrical response, was carried out by including Cr(III) and Pt(IV) centers in the oxide matrix. It turned out that Cr remains as homogeneously dispersed Cr(III) centers inside the WO 3 host, while Pt undergoes reduction and aggregation to form nanoclusters located at the oxide surface. Upon interaction with NH 3 , the electrical conductivity of transition metal doped-WO 3 increase...