Size and Shape Effects on Charge Recombination Dynamics of TiO₂ Nanoclusters

Investigation of charge carrier recombination dynamics is central to understanding and further enhancing the photocatalytic activity of water splitting and other photochemical reactions catalyzed by TiO₂. In this study, we carried out nonadiabatic molecular dynamics calculations combined with real-time time-dependent density functional theory to investigate the effects of size and shape on charge recombination in TiO₂ nanoparticles (NPs). Using the Wulff construction method, we considered both octahedral (10, 35, and 84 TiO₂ units) and cuboctahedral (29, 78, and 97 TiO₂ units) nanoclusters with size varying from 1 to 3 nm. Generally, the recombination rates decreased with increasing NP size. We rationalized the trend in terms of average transition energy, exciton binding energy (Δ<i>E</i>_ex), nonadiabatic coupling (NAC), and pure-dephasing time. The relaxation times increased with increasing NP size, as the NAC and Δ<i>E</i>_ex decreased. The cuboctahedral clusters showed smaller Δ<i>E</i>_ex compared to the octahedral clusters. For the octahedral clusters, the smaller NAC and shorter dephasing time contributed to longer relaxation, despite smaller transition energy, as the size increased. However, the influence of the NAC, transition energy, and dephasing time were intertwined for the cuboctahedral clusters. The smaller NAC of the 97-unit cluster rationalized its longer relaxation time compared to the 29-unit cluster, but the presence of a singly coordinated oxygen atom greatly reduced the transition energy, thus leading to a shorter relaxation time compared to the 78-unit cluster. Our results provide a detailed understanding on the effects of size and shape on the charge carrier dynamics in TiO₂ nanoclusters, separating these effects from other factors, such as presence of defects, dopants, and adsorbates that are hard to control precisely in experiments