Charge separation characteristics of a high-activity, mixed-phase titania photocatalyst (Degussa P25) are probed
by EPR spectroscopy. While previous proposals consider rutile as a passive electron sink hindering
recombination in anatase, this research details the critical and active role of rutile in TiO2 formulations. The
inactivity of pure-phase rutile is due in part to rapid rates of recombination. Yet, in mixed-phase TiO2, charges
produced on rutile by visible light are stabilized through electron transfer to lower energy anatase lattice
trapping sites. These results suggest that within mixed-phase titania (P25) there is a morphology of nanoclusters
containing atypically small rutile crystallites interwoven with anatase crystallites. The transition points between
these two phases allow for rapid electron transfer from rutile to anatase. Thus, rutile acts as an antenna to
extend the photoactivity into visible wavelengths and the structural arrangement of the similarly sized TiO2
crystallites creates catalytic “hot spots” at the rutile−anatase interface.
With its unique electronic and optical properties, graphene is proposed to functionalize and tailor titania photocatalysts for improved reactivity. The two major solution-based pathways for producing graphene, oxidation-reduction and solvent exfoliation, result in nanoplatelets with different defect densities. Herein, we show that nanocomposites based on the less defective solvent-exfoliated graphene exhibit a significantly larger enhancement in CO(2) photoreduction, especially under visible light. This counterintuitive result is attributed to their superior electrical mobility, which facilitates the diffusion of photoexcited electrons to reactive sites.
Electron-hole recombination limits the efficiency of TiO2 photocatalysis. We have investigated the efficacy with which anatase/carbon nanotube (CNT) composite materials reduce charge recombination and enhance reactivity. We synthesized nanostructured assemblies composed of different proportions of anatase (5 or 100 nm) and either single-or multi-walled CNTs. The composites were prepared using a simple low temperature process in which CNTs and anatase nanoparticles were dispersed in water, dehydrated at 80 degrees C, and dried at 104 degrees C. The structures of the various TiO2/CNT composites were characterized by scanning electron microscopy (SEM) and their function was tested by phenol oxidation. Charge recombination was compared by measuring the photoluminescence spectra of select composites. We found that a nanostructured composite assembled from the 100 nm anatase and single-walled CNTs (SWCNTs) exhibited enhanced and selective photocatalytic oxidation of phenol in comparison to both pure anatase and Degussa P25. A mechanism for the enhanced reactivity is proposed in which electrons are shuttled from TiO2 particles to the SWCNTs as a result of an optimal TiO2/ CNT arrangement that stabilizes charge separation and reduces charge recombination. In addition, the SWCNT assembly provides better catalyst-support (dispersal and connection) than multi-walled CNTs.
Using the electron paramagnetic resonance technique, we have elucidated the multiple roles of water and carbonates in the overall photocatalytic reduction of carbon dioxide to methane over titania nanoparticles. The formation of H atoms (reduction product) and (•)OH radicals (oxidation product) from water, and CO(3)(-) radical anions (oxidation product) from carbonates, was detected in CO(2)-saturated titania aqueous dispersion under UV illumination. Additionally, methoxyl, (•)OCH(3), and methyl, (•)CH(3), radicals were identified as reaction intermediates. The two-electron, one-proton reaction proposed as an initial step in the reduction of CO(2) on the surface of TiO(2) is supported by the results of first-principles calculations.
Accumulating evidence suggests that outdoor air pollution may have a significant impact on central nervous system (CNS) health and disease. To address this issue, the National Institute of Environmental Health Sciences/National Institute of Health convened a panel of research scientists that was assigned the task of identifying research gaps and priority goals essential for advancing this growing field and addressing an emerging human health concern. Here, we review recent findings that have established the effects of inhaled air pollutants in the brain, explore the potential mechanisms driving these phenomena, and discuss the recommended research priorities/approaches that were identified by the panel.
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