Impact of Surface Hydroxylation on the Structural and Electronic Properties of 2–3.5 nm Diameter Crystalline and Amorphous Titania Nanoparticles

We investigate how surface hydroxylation affects the energetic stability and electronic properties of 2–3 nm diameter titania (TiO2) nanoparticles (NPs) using density functional theory based calculations. Specifically, crystalline anatase NPs with faceted morphologies are compared to quasi-spherical amorphous NPs in their anhydrous (TiO2)84 and hydroxylated (TiO2)84(H2O)m states. In the anhydrous case, amorphous NPs are more energetically stable than anatase NPs. Surface hydroxylation is energetically favorable for both of the NP types. However, at higher –OH coverages, the stabilization from hydroxylation is less for amorphous NPs relative to crystalline NPs, leading to a convergence in energetic stability. Eventually, for high hydroxylation, a crossover in energetic stability occurs, where hydroxylated anatase NPs become more stable than hydroxylated amorphous NPs. This stability crossover occurs at a lower degree of hydroxylation for (TiO2)84 NPs than previously predicted for smaller (TiO2)35 systems. Hydroxylation also affects electronic properties due to the cumulative effect of the bound OH groups, leading to a ligand-induced dipole effect. In addition to energetic stability convergence, we also find a convergence in electronic properties (e.g., band gaps and band edge energies) with increasing hydroxylation for both NP types. Our results further underscore how small amorphous TiO2 NPs can be tuned by surface functionalization (e.g., hydroxylation in aqueous environments) to behave as crystalline mimicking “crystalike” systems. These insights help provide a pathway for optimizing TiO2 NPs for photocatalytic applications, where controlling the electronic structure is crucial.