Structure and Stability of Small TiO₂ Nanoparticles

Abstract

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.

TiO2 nanocluster modified-rutile TiO2 photocatalyst: a first principles investigation

“…It is important to point out that the cluster with the least negative adsorption energy has no change in the number of terminal oxygen atoms while in other clusters, with more favourable 105 adsorption energies, there is always a reduction in the number of terminal oxygen atoms, so that increasing the coordination of terminal oxygen atoms may play an important role in the stability of adsorbed clusters at the TiO 2 surface. [62][63][64][65] (which are also found in this work) and proposed in the rutile (011) (2x1) reconstructed surface [70][71][72][73] . Given that the free nanoclusters are stable with titanyl groups, it is not so surprising that the majority of the adsorbed cluster structures are also stable with titanyl groups and these 15 are found in both the QEq and DFT+U relaxations.…”

“…This reduction arises from the presence of the electronic states from the deposited cluster, which shifts the valence band to higher energy, means that we predict that 30 these hetereostructures will have improved photocatalytic activity, at least in terms of light absorption in the visible region of the solar spectrum. There are no obvious trends in the computed DFT+U energy gaps with the size of the nanocluster, which would be expected for nanoclusters of 35 TiO 2 of this size [62][63][64][65] . The shifts in the energy gaps determined from the DOS analysis are from simple Kohn-Sham energy eigenvalue differences, which do not have the same physical meaning as an excitation energy.…”

Effect of Size and Structure on the Ground-State and Excited-State Electronic Structure of TiO2 Nanoparticles

“…In other reported studies specifically searching for low energy (TiO 2 ) n clusters, those with sizes up to n = 16 are indeed found to not display the anatase crystal structure. 36,37,38,39 Rather, small to moderately large (TiO 2 ) n nanoclusters typically exhibit a significant number of non-bulk-like energetically low lying isomers. Since this implies that experimental measurements may provide information over an ensemble of particles rather than on the most stable structural ground state, it is necessary to investigate the properties of different isomers of TiO 2 nanoparticles.…”

Advances in computational studies of energy materials

“…Again, a hybrid genetic algorithm (Hamad et al 2005) has been used to predict the five most stable configurations of (M 2 O 3 ) n , for each value of M and n, where M = B, Al, Ga, In, Tl and Ce, and n = 1-8. The materials concerned span a wide range of physical and chemical properties and are utilized as catalysts, lasing and more generally light-emitting materials, elements of complex photoelectric devices, etc.…”