At the nanoscale, zirconium dioxide may form in a number of different polymorphs, depending in part upon the size of the particles. Although considerable attention has been given to finding methods for controlling the phase of zirconium dioxide nanocrystals, the role of nanomorphology in affecting the size-dependent phase transition has been largely ignored. To address this issue, we have used a shape-dependent thermodynamic model to investigate the relationship between nanomorphology and phase stability. Our results provide the free energy of formation for tetragonal and monoclinic nanocrystals with a variety of shapes, and show that the transition size is strongly dependent on the prevalence of particular surface facets. From these results we suggest that variations in the thermochemical results reported in the literature may also be partially attributed to variations in nanocrystal shapes.

Developing new semiconductor materials for improving photocatalytic reactivity is important for solving the challenging environmental and energy problems we are facing today. This work focuses on increasing the quantum efficiency in titania photocatalysts for photocatalytic (oxidation of acetaldehyde) and photosynthetic (photosplitting of water) reactions by synthesizing pure phase rutile and anatase nanostructures with well defined morphologies and investigating their photocatalytic performance compared to a commercial titania photocatalyst (Degussa P25). Nanostructured anatase is dominated by {100} surfaces with a small amount of {101} surfaces, whereas the rutile nanoplates consist of nanorods dominated by {110} and {111} crystal surfaces. In accordance with the signals from electron spin resonance (ESR) spectra, both nanostructured anatase and rutile phases have high photocatalytic activity for photosplitting of water compared to P25 titania. The anatase phase shows a high activity for photocatalytic oxidation (PCO) of acetaldehyde whereas the rutile phase shows a lower activity. The results of these experiments basically agree with previously published works that reported that the oxidation and reduction sites on rutile particles are on the {011} and {110} faces respectively, and on {001} and {011} faces respectively for anatase particles. The results have important implications for enhancing the photocatalytic activity of titania for environmental remediation, increasing the quantum efficiency in photo-voltaic (PV) solar cells and other photo-assisted processes.

This paper describes a method for increasing the quantum efficiency in the photosplitting of water by integrating the Degussa P25 titania photocatalyst with a polar mineral tourmaline having surface electrical fields due to the pyroelectric effect. The amount of hydrogen produced from the photosplitting of water increased considerably with a tourmaline-integrated P25 titania photocatalyst compared to that with P25 titania in systems containing pure water as well as solutions of different pH values. It is proposed that the enhancement in the photocatalytic performance is due to a reduction in the Schottky barrier for electrons to migrate to the surface of the semiconductor. The surface electric fields of tourmaline lower the barrier by reducing the band bending in the space charge layer and increasing the chemical potential (E F) of the electrons in titania. The increase in the amount of hydrogen produced at a lower pH is attributed to the decrease in the overpotential of the hydrogen evolution reaction at lower pH values. In contrast to photocatalytic reduction, tourmaline has very little effect on titania for the photocatalytic oxidation of acetaldehyde. It is proposed that the surface electric fields present on tourmaline crystals reduce the band bending in the semiconductor and thus decrease the oxidation potential of the photogenerated holes. The results have important implications for enhancing the photocatalytic activity of titania for environmental remediation, increasing the quantum efficiency in photovoltaic (PV) solar cells, and other photoassisted processes.

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Titanium oxide nanotubes are prepared and treated with Au (Au/nanotube sample) and Pt (Pt/nanotube sample), and the photoactivity of these catalysts compared to a standard Degussa P25 photocatalyst is investigated. The samples were analyzed using X-ray diffraction, field emission gun scanning transmission electron microscopy (STEM). Both high-resolution TEM images and high-angle annular dark-field (HAAD) images were recorded for the specimens. Oxidation of acetaldehyde was used to test the efficiency of the catalysts. Nanotube samples showed better photoactivity than the standard P25, because the P25 titania deactivates quickly. Enhanced reactivity of the nanotube is related to surface charge polarity developed on outer and inner surfaces due to the difference in overlap of oxygen anions that resulted from curving of octahedral sheets. A tentative and qualitative surface polarity model is proposed for enhancing electron-hole pair separation. The inner surface benefits reduction; whereas, the outer surface benefits oxidation reactions. Both the metal identity and the size of the metal particles in the nanotubes affected the photocatalytic activity. Specifically, the addition of platinum increased the activity significantly, and increased the total yield. The addition of gold had lesser impact compared to the platinum. Formation of Pt large nanoparticles on the nanotube surfaces reduces the oxidation reactivity.