Metal
oxide photocatalysts are widely studied for applications in solar driven
environmental remediation, antimicrobial activity, hydrogen production and CO<sub>2</sub>
reduction to fuels. Common requirements for each technology include absorption
of visible light, reduced charge carrier recombination and the ability to
activate the initial molecule be it a pollutant, water or CO<sub>2</sub>. The
leading photocatalyst is some form of TiO<sub>2</sub>. A significant amount of
work has been undertaken to modifying TiO<sub>2</sub> to induce visible light
absorption. The structure and composition of the catalyst should facilitate separation
of electrons and holes and having active sites on the catalyst is important to promote
the initial adsorption and activation of molecules of interest. In this paper
we present a first principles density functional theory (DFT) study of the
modification of rutile TiO<sub>2</sub> (110) with nanoclusters of the alkaline
earth metal oxides (MgO, Ca, BaO) and we focus on the effect of surface
modification on the key catalyst properties. The modification of rutile TiO<sub>2</sub>
with CaO and BaO induces a predicted red shift in light absorption. In all
cases, photoexcited electrons and holes localise on oxygen in the nanocluster
and surface Ti sites, thus enhancing charge separation. The presence of these
non-bulk alkaline earth oxide nanoclusters provides highly active sites for
water and CO<sub>2</sub> adsorption. On MgO-rutile, water adsorbs molecularly
and overcomes a barrier of only 0.36 eV for dissociation whereby hydroxyls are
stabilised. On CaO- and BaO-modified rutile water adsorbs dissociatively. We
attribute this to the high lying O 2p states in the alkaline earth oxide
modifiers which are available to interact with water, as well as the non-bulk
like geometry around the active site. Upon adsorption of CO<sub>2</sub> the
preferred binding mode is as a tridentate carbonate-like species, as
characterised by geometry and vibrational modes. The carbonate is bound by up
to 4 eV. Thus these heterostructures can be interesting for CO<sub>2</sub>
capture, helping alleviate the problem of CO<sub>2</sub> emissions.
Structures and Stabilities of Alkaline Earth Metal Oxide Nanoclusters: A DFT Study
