Transformation of nanomaterials in aqueous environment has significant impact on their behavior in engineered application and natural system. In this paper, UV irradiation induced transformation of TiO2 nanoparticles in aqueous solutions was demonstrated, and its effect on the aggregation and photocatalytic reactivity of TiO2 was investigated. UV irradiation of a TiO2 nanoparticle suspension accelerated nanoparticle aggregation that was dependent on the irradiation duration. The aggregation rate increased from <0.001 nm/s before irradiation to 0.027 nm/s after 50 h irradiation, resulting in aggregates with a hydrodynamic diameter of 623 nm. The isoelectric point of the suspension was lowered from 7.0 to 6.4 after irradiation, indicating less positive charges on the surface. ATR-FTIR spectra displayed successive growth of surface hydroxyl groups with UV irradiation which might be responsible for the change of surface charge and aggregation rate. UV irradiation also changed the photocatalytic degradation rate of Rhodamine B by TiO2, which initially increased with irradiation time, then decreased. Based on the photoluminescence decay and photocurrent collection data, the change was attributed to the variation in interparticle charge transfer kinetics. These results highlight the importance of light irradiation on the transformation and reactivity of TiO2 nanomaterials.

A graphene-based two-dimensional (2D) nanoplatform provides new opportunities for fabricating 2D heterojunction interfaces to fortify charge transfer in semiconductor assemblies. In this report, TiO2 nanosheet/graphene composite based 2D–2D heterojunctions were fabricated by a solvothermal process. Microscopic and spectroscopic characterization revealed a homogeneous sheetlike morphology with intimate interfacial contact between the TiO2 nanosheet and graphene due to chemical interactions. Compared with 0D–2D Degussa P25 (TiO2)/graphene and 1D–2D TiO2 nanotube/graphene composites, the 2D–2D TiO2 nanosheet/graphene hybrid demonstrated higher photocatalytic activity toward the degradation of rhodamine B and 2,4-dichlorophenol under UV irradiation. Radical trapping and ESR experiments revealed the enhanced generation of ·OH and O2•– in the 2D–2D heterojunction system. By analyzing TiO2 excited state deactivation lifetime, the interfacial electron transfer rates determined for 0D–2D, 1D–2D, and 2D–2D TiO2/graphene composites were 1.15 × 108 s–1, 3.47 × 108 s–1, and 1.06 × 109 s–1, respectively. It was therefore proposed that the fast charge separation in the TiO2 nanosheet/graphene photocatalyst promoted the generation of reactive oxygen species and enhanced the photodegradation reactions. The results underscore the key role of nanomaterial dimensionality in interfacial charge transfer processes.Keywords: 2D−2D heterojunction; graphene; interfacial charge transfer; photocatalysis; TiO2 nanosheet;