Size-Dependent Electronic Structure of Titanium-Oxo-Alkoxides: Exploring Quantum Confinement at the Smallest Sizes

Abstract

The electronic structures of titanium-(oxo-)alkoxides with 1-18 Ti atoms are studied by UV spectroscopy, revealing size effects. These precise molecular systems act as the smallest titanium-oxygen systems for comparison with bulk and nanoparticulate TiO2. The results show that at these ultrasmall sizes, with Ti/O cores of less than 1.5 nm, the energy gap is larger than bulk TiO2 and increases significantly in the smallest systems. Electrochemical studies supported by DFT calculations show that the greatest changes in electronic structure occur when moving from a complex with a single Ti atom to a dimeric species with two Ti sites. Constructive interaction of d-orbitals, combined with changes in coordination geometry, result in significantly different Ti-based LUMO energies in monomers, dimers and larger clusters, whilst the O-based HOMO energy level remains similar throughout. Size appears to be the dominating factor for LUMO position and energy gap despite the different connectivities, shapes and surface chemistries in these molecular clusters, which differ from bulk TiO2. These results link materials chemistry with small inorganic molecules and show that (quantum) size effects remain applicable to the smallest systems. This understanding is important for the rational design of semiconductor materials with optimised properties.