Photoluminescence of urea- and urea/rhodamine B-capped TiO2 nanoparticles

• Urea- and urea/rhodamine B (RhB)-capped TiO2 nanoparticles preparation.
• Characterization of optical properties of urea- and urea/rhodamine B (RhB)-capped TiO2 nanoparticles.
• The recombination of electrons and holes is significantly reduced in the capped TiO2 nanoparticles, in comparison with TiO2.

Urea- and rhodamine B (RhB)-capped TiO2 nanoparticles (NPs) have been prepared by solvothermal synthesis and characterized by HRTEM, XRD, FTIR, XPS, optical absorption and photoemission. The urea and urea/RhB ligands are capped to the surface of the TiO2 NPs for the first time through carbamate bonding. The band gap of TiO2 is slightly reduced from 3.1 eV to 3.0 eV in the urea capped TiO2 NPs (TU) and 2.9 eV for the NPs capped with urea/RhB (TUR). The generation of new trapping states in TU and TUR at the conduction band edges (surface oxygen vacancies) has been confirmed by the Urbach law providing tail state energies of 180 meV and 270 meV, respectively. These tail states are considered to be responsible for the strong reduction of the photoluminescence at ≈400 nm and the increased emission at ≈600 nm in TU and TUR. The findings suggest that urea- and RhB-capped TiO2 NPs could have potential applications as photocatalysts, opto-electronic devices, sensors, biological labels and anti-bacterial agents.