Conventional TiO(2)-based photocatalysts oxidize NO(x) to nitrate species, which do not spontaneously desorb and therefore deactivate the catalyst. We show that the selectivity of this reaction can be changed by creating a large concentration of oxygen vacancies in TiO(2) nanoparticles through thermal reduction in a reducing atmosphere. This results in the photoreduction of nitric oxide (NO) to N(2) and O(2), species which spontaneously desorb at room temperature. The activity of the photoreduction reaction can be greatly enhanced by doping the TiO(2) nanoparticles with Fe(3+), an acceptor-type dopant that stabilizes the oxygen vacancies. Moreover, the photoinduced reduction of Fe(3+) to Fe(2+) provides a recombination pathway that almost completely suppresses the formation of NO(2) and thus enhances the selectivity of the reaction for N(2) formation. Gas chromatography confirms that N(2) and O(2) are formed in a stoichiometric ratio, and the activity for NO decomposition is found to be limited by the concentration of oxygen vacancies. A series of internally consistent reaction equations are proposed that describe all experimentally observed features of the photocatalytic process. The observed influence of oxygen vacancies on the activity and selectivity of photoinduced reactions may lead to new routes toward the design of highly selective photocatalysts.

The incorporation of Fe as a dopant in anatase TiO 2 nanoparticles has been systematically investigated with the aim of changing the coordination geometry of Ti via the formation of oxygen vacancies. Although Fe 3+ ions are present in the solution during growth of the nanoparticles, a high-temperature heat treatment is found to be necessary to incorporate Fe 3+ as a substituent for Ti 4+ in the bulk of the TiO 2 nanoparticles. The Fe 3+ acceptors are found to be charge-compensated by oxygen vacancies, up to dopant concentrations as high as 10%. The surprisingly high solubility of Fe is attributed to the very similar radii of Ti 4+ and Fe 3+ and to the energetically favorable Coulomb attraction between the negatively charged Fe acceptor and the positively charged oxygen vacancies. A combined EXAFS/XANES study reveals that part of the Ti 4+ ions changes their coordination number from 6 to 4 at high oxygen vacancy concentrations. The deliberate use of oxygen vacancies to modify the coordination geometry of metal ions represents a new strategy that offers exciting possibilities to tune the selectivity of photocatalytically active metal oxide nanoparticles.

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In this work, novel CuGaS-ZnS p-n type semiconductor nanoheterostructures were synthesized by a solution route, and demonstrated experimentally to be a very promising visible light active photo-catalyst for water-splitting hydrogen production. The construction of CuGaS-ZnS heterostructures follows a multi-step strategy, employing CuS nanocrystals first as catalytic assistants for the hetero-growth of ZnS on their surfaces, and then as sacrificial seeds for the formation of CuGaS. Excitingly, attributed to the efficient charge separation introduced by the p-n heterojunctions, the hydrogen production ability of the CuGaS-ZnS nanoheterostructures under visible light irradiation is 15 times higher than that of the CuGaS component, and comparable to that of the CdS nanophase which is currently regarded as one of the most active visible photo-catalysts for hydrogen generation.