Structure Matters More than Size: Tuning the Electronic Properties of (TiO$_{2})_{\mathrm{n}}$ Clusters

To design (TiO$_{2})_{\mathrm{n}}$ clusters with desired properties we implemented a suite of three genetic algorithms (GA) tailored to optimize for low total energy (EGA), high vertical electron affinity (VEA-GA), and low vertical ionization potential (VIP-GA). The property-based GAs are an extension of the cascade GA reported in [1]. Analysis of the structures found by the VEA-GA and the VIP-GA vs. the EGA reveals structure-property relations. A high VEA is correlated with the presence of several dangling-O atoms (typically 3-4), rather than the previously suggested tri-coordinated Ti atom [2]. A low VIP is correlated with low bond connectivity (typically 2) between two dangling-O atoms. We show that the electronic properties of (TiO$_{2})_{\mathrm{n}}$ clusters with n up to 20 correlate more strongly with the presence of these structural features than with size. We further suggest that the presence of dangling-O atoms on TiO$_{2}$ clusters or surfaces may be associated with enhanced catalytic activity and that these O atoms may serve as active sites. The process of optimization for a target property reveals the underlying structure-property relations and the structural features that may serve as active sites for catalysis. This generally applicable approach may provide valuable physical insight and design rules for better nanocatalysts.\\[4pt] [1] S. Bhattacharya \textit{et al.} PRL 111, 135501 (2013); N. J. Phys, in press (2014)\\[0pt] [2] N. Marom \textit{et al.} PRL 108, 106801 (2012).