Hydrogen constitutes an ideal green fuel but so far it is mainly obtained from fossil resources. Water splitting using TiO2 as a photocatalyst constitutes an alternative although its practical use under visible sunlight is hindered from the too large “band gap” of the known forms of this oxide. Suitable modifications have been proposed including doping and/or nanostructuring. Nevertheless, finding a practical solution without a deep knowledge of how the properties of TiO2 nanoparticles evolve with size, shape and morphology seems to be an insurmountable task. To make progress in this direction, a systematic study has been carried out regarding the relative stability and properties of differently shaped realistic (TiO2)n nanoparticles containing explicitly up to 1785 atoms or 595 formula units. Using all electron, relativistic, density functional theory based calculations with accurate numerical orbital centered basis sets, we investigated the relative stability of spherical and faceted nanoparticles as a function of size. A crossover between faceted and spherical morphology is found at ~100 TiO2 units where the faceted nanoparticles become more stable than the spherical ones. The present study provides compelling evidence that for stoichiometric nanoparticles of realistic size, a faceted morphology is preferred although spherical shaped ones tend to exhibit smaller optical gap. Therefore, the successful synthesis of spherical nanoparticles, often showing better photocatalytic activity, has to be attributed to the particular route used able to stabilize these energetically metastable structures. The present study suggests that controlling kinetic of particle growth rather than thermodynamic stability is the key towards TiO2 nanoparticles with better photocatalytic activity under sunlight.

Morphology dependence of stability and properties of stoichiometric TiO2 nanoparticles

2019 Spring National Meeting

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