TiO2 materials are extensively used in the heterogeneous photooxidation, they are stable, available and induce redox reactions. Various factors, including crystal structure, particle size, surface area, and hydroxyls affect the photocatalytic activity.
Heterogeneous systems based on anatase nanocrystallites, pure and metal-doped (Cu, Cr, Co, Fe), and benzophenone (BP) as an organic disperse medium were studied by phosphorescence (28.000-16.000 cm-1). The analysis revealed two spectral series, resulted from a simultaneous realization of two BP structural modifications, amorphous and crystalline phases. The disordered amorphous structure is formed in thin (< 0.05 – 0.1 μm) near-surface layers and results from destruction of BP long-range ordering due to the interactions of neighbouring BP molecules with active centers of the anatase surface.
Phosphorescence bands of amorphous BP are non-uniformly broadened due to non-equivalent local fields, which affect each emitting molecule and due to the significant amount of BP molecule configurations. At heating from 4.2 to 100 K the triplet excitons effectively migrate and low-energy levels are primarily occupied (a red shift). Further, thermal activation equalizes the energy loss (a blue shift). Doped metals induce additional interactions and polarizing effects on BP molecules, leading to different spectral changes in each heterogeneous system.
In the case of BP thin films, luminescence-quenching rate is greater than in the bulk. When the thickness of amorphous BP layer is comparable to the diffusion shift length of triplet excitations at the given temperature, the film surface starts to quench the triplet excitons. This process is accompanied with a decrease in the phosphorescence lifetime and quantum yield. Depending on the metal doped, each sample has a different relative amount of the amorphous BP and a characteristic temperature dependence of its phosphorescence. Thus, phosphorescence spectral analysis allows to estimate the thickness of amorphous BP layers near TiO2 particles and to characterize the power of BP interactions with the TiO2 surface. |