The Measure of TiO2 Photocatalytic Efficiency and the Comparison of Different Photocatalytic Titania

Structure and Stability of Small TiO2 Nanoparticles

The effect of the nanostructure on the photochemistry of TiO2 is an active field of research owing to its applications in photocatalysis and photovoltaics. Despite this interest, little is known of the structure of small particles of this oxide with sizes at the nanometer length scale. Here we present a computational study that locates the global minima in the potential energy surface of Ti(n)O2n clusters with n = 1-15. The search procedure does not refer to any of the known TiO2 polymorphs, and is based on a novel combination of simulated annealing and Monte Carlo basin hopping simulations, together with genetic algorithm techniques, with the energy calculated by means of an interatomic potential. The application of several different methods increases our confidence of having located the global minimum. The stable structures are then refined by means of density functional theory calculations. The results from the two techniques are similar, although the methods based on interatomic potentials are unable to describe some subtle effects. The agreement is especially good for the larger particles, with n = 9-15. For these sizes the structures are compact, with a preference for a central octahedron and a surrounding layer of 4- and 5-fold coordinated Ti atoms, although there seems to be some energy penalty for particles containing the 5-fold coordinated metal atoms with square base pyramid geometry and dangling Ti=O bonds. The novel structures reported provide the basis for further computational studies of the effect of nanostructure on adsorption, photochemistry, and nucleation of this material.

Photon Flux and Wavelength Effects on the Selectivity and Product Yields of the Photocatalytic Air Oxidation of Neat Cyclohexane on TiO2Particles

Product selectivity and yields of cyclohexanol and cyclohexanone formation in the photocatalytic air oxidation of cyclohexane on TiO(2) particles were determined as a function of the irradiation wavelength (254 < or = lambda/nm < or = 366) and the photon flux (0.3 < or = I(0)/neinstein cm(-2) s(-1) < or = 5.0). Photonic efficiencies for total monooxygenated products (cyclohexanol + cyclohexanone) ranged from 10 to 25% depending on photon energy and fluency. The cyclohexanol-to-cyclohexanone ratio linearly increases with the incident photon flux at each wavelength and varies more than a magnitude order -- from 3 to 32%-at the same incident photon fluency -- 5 neinstein cm(-2) s(-1) -- by changing the irradiation wavelength from 366 to 303 nm. Experimental evidence indicates that both spectral and intensity effects emerge as a consequence of the change in the frequency of photon absorption per particle. A mechanism is proposed which accounts for the origin of the selectivity changes.

Mesoporous spherical aggregates of anatase nanocrystals with wormhole-like framework structures: their chemical fabrication, characterization, and photocatalytic performance

A facile and efficient approach for the fabrication of mesoporous spherical aggregates of anatase nanocrystals is reported in this paper. Cetyltrimethylammonium bromide was used as the structure-directing agent, and the interaction between cyclohexane microdroplets and the cetyltrimethylammonium bromide self-assemblies led to the assembly of 4-5-nm-sized anatase nanocrystals into spherical aggregates with mesoporous structures. The as-prepared anatase powders exhibited high photocatalytic activity and could be effectively used as the catalyst for the room-temperature photodegradation of a variety of organic dye pollutants in aqueous media including methyl orange, bromopyrogallol red, and methylene blue.