X-ray absorption spectroscopy (XAS) has been used to study the local Ti environment in titania xerogel samples containing nanoparticles of different sizes. The xerogels were prepared by hydrolysis of titanium isopropoxide followed by peptization with HNO3 and size control was achieved through calcination in air at different temperatures. An amorphous precipitate obtained by hydrolysis of titanium isopropoxide prior to peptization with HNO3 has also been studied. The X-ray absorption near edge structure (XANES) of the precipitate possesses a characteristic preedge that is dominated by a transition designated as A2 at 4970.7 eV, and assigned to five coordinate Ti. This A2 component is detected in all of the studied xerogels. Its intensity decreases as the surface-to-volume ratio of the titania particles decreases, suggesting that it is associated with surface layers of the anatase particles. Other changes observed in the XANES of the xerogels as a function of particle size include broadening of 1s → np transitions in the postedge region. EXAFS shows that the smallest particles contain Ti in distorted coordination with contracted Ti−O bonds and probably reduced coordination number.

Environment‐sensitive Bragg reflectors are built using functional mesoporous thin films as building blocks. Tuning of optical properties is achieved by changing the composition or porosity of the slabs or the introduction of planar defects. Sorption or capillary condensation of molecules into the pore system results in a 10–40 nm photonic bandgap (PBG) shift. Organic functions added to the pore surface change the response, permitting tailoring of the selectivity towards small‐size molecules.

Size-dependent variations in the electronic and structural properties of anatase and rutile nanoparticles have been compared. The anatase nanoparticles of the present study were prepared by hydrothermal ripening of an anatase sol and had diameters in the range 2−130 nm whereas the rutile nanoparticles were prepared by calcination of sol−gel derived rutile and had diameters in the range 3.6−60 nm. The hydrothermally ripened anatase nanoparticles had similar surface structures as deduced from the XANES as previously reported sol−gel anatase materials prepared through calcination (Luca et al., J. Phys. Chem. B 1998, 102, 10650). The optical band gap (E g) of the anatase nanoparticles as deduced from their electronic absorption spectra showed some variation with size but E g was not a smooth function of crystallite size, as would be dictated by the effective mass model for both types of anatase nanoparticles. In distinct contrast to the anatase nanoparticles, rutile nanoparticles showed a smooth size dependent variation in optical band gap in line with the dictates of the effective mass model. However, the XANES of the rutile nanoparticles was not dependent on size as it was for both the calcined and hydrothermally ripened anatase materials where the pre-edge XANES and EXAFS revealed a high concentration of distorted surface atoms with reduced coordination. The results suggest that sol−gel anatase nanoparticles consist of a core−shell structure in which the core is bulk-like and the shell interphase is less ordered with a high degree of Ti under-saturation. On the other hand, if such an interphase region was present at all in rutile nanoparticles, it was so thin as to avoid detection by XANES. The unique surface structure of anatase nanoparticles derived from sol−gel preparation methods is probably responsible for the lack of a clear quantum confinement effect.

The structures of tungsten trioxide, WO3, have been studied in fine temperature steps, from room temperature to 1000 °C, by means of very high-resolution neutron powder diffraction. It was confirmed that the sample used was single-phase monoclinic in space group P21/n at room temperature. In addition to this monoclinic structure, the structures observed were an orthorhombic structure in Pbcn from about 350 to 720 °C, another monoclinic structure in P21/c from about 720 to 800 °C, a tetragonal structure in space group P4/ncc from 800 to 900 °C, and above 900 °C a second tetragonal structure in P4/nmm. The transformation from the Pbcn orthorhombic to the P21/c monoclinic structure was certainly discontinuous, and indeed just above 720 °C two-phase mixtures were observed. The other transitions were continuous or nearly so, all three being apparently tricritical in nature. The sequence of phases, and nature (continuous or otherwise) of the transitions between them, can be well understood by reference to the results from a group theoretical analysis.

Novel mesoporous tungsten trioxide films with enhanced incident photon-to-current conversion efficiencies have been prepared by a sol−gel route from an aqueous precursor solution containing peroxopolytungstic acid (PPTA). For films heated in air at 500 °C, it was found that film texture depended in a precise and reproducible manner on adjustment of the pH of this precursor solution by addition of a small volume of a selected mineral acid. Mesoporous micrometer-thick transparent films were obtained from PPTA without pH adjustment while mesoporous semi-transparent films resulted when the pH was lowered. The transparent films had specific surface areas of 18 m2/g, average pore diameters of 7.3 nm, and average crystallite sizes of 30 nm. The semi-transparent films possessed specific surface areas of 30 m2/g, average pore diameters of 12.5 nm, and average crystallite diameters of 17 nm. In the case of the semi-transparent films, electron microscopy indicated that the fundamental crystallites formed part of larger 200−300 nm aggregates which were in turn interconnected to form an open micrometer-length scale porous network. The transparent films did not show this type of porous hierarchy with the absence of micrometer-scale porosity. Photoelectrochemical studies of the films indicated that the hierarchical semi-transparent films exhibited a considerably enhanced photo-response relative to transparent films due to increases in both the interface area and light scattering. After calcination of the semi-transparent films at 500 °C, anodic photocurrents up to an equivalent of 1.4 mA/cm2 under Air Mass 1.5 equivalent solar irradiation were measured. Our results suggest that film texture is a major factor in determining the performance of the films, and the method reported here provides a simple and convenient means for modulation of this texture.