Electronic Structure of the Au Nanoparticle-TiO ₂ Heterojunction: Influence of Nanoparticle Size, Shape, Oxygen Vacancies, and Temperature

The electronic structure of the gold nanoparticle-titanium dioxide (AuNP-TiO₂) heterojunction plays a critical role for charge transfer and recombination dynamics that underpin its photocatalytic function. However, building a representative model to capture the key physics remains a significant challenge. Here, we investigate the influence of the AuNP size and shape, as well as oxygen vacancy (V_O) defects at the anatase-phase TiO₂ (101) surface and the temperature of the heterojunction, on its interfacial electronic properties. Using density functional theory (DFT), we compare the closed-shell Au₂₀ and open-shell Au₁₉ clusters interfaced with pristine and V_O defect TiO₂ surfaces. We find that the presence of a V_O defect transforms pure TiO₂ from a p-type to an n-type semiconductor, reversing the interfacial band bending from downward to upward. For the heterosystem, density of states (DOS) analysis shows that V_O minimally affects the open-shell Au₁₉-TiO₂ system, but it significantly alters the closed-shell Au₂₀-TiO₂ system, converting it from p-type to n-type at the Γ-point at 0 K. Furthermore, ab initio molecular dynamics (AIMD) simulations at 300 K reveal significant thermal fluctuations in AuNP positions relative to the TiO₂ surface. These fluctuations result in dynamic variations in the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in all systems studied: Au₁₉NP<i>-</i>Pristine TiO₂, Au₂₀NP<i>-</i>Pristine TiO₂, Au₁₉NP<i>-</i>V_O defect TiO₂, and Au₂₀NP<i>-</i>V_O defect TiO₂. Complementary Bader charge analysis performed at both 0 K and finite-temperature AIMD snapshots supports the emergence of an upward band bending and the formation of a Schottky barrier at the V_O containing heterojunctions. Notably, we find that at finite temperature, an Au atom can dynamically passivate the V_O, leading to a pronounced widening of the HOMO-LUMO gap in the AuNP-V_O defect TiO₂ heterojunctions. Our computational findings underscore the pivotal role of the V_O defect and thermal effects in modulating interfacial band alignment, electronic states, and HOMO<i>-</i>LUMO gaps, providing insights for designing Au-TiO₂ heterojunctions with tailored electronic properties.