Characteristics of microwaves on second generation nitrogen-doped TiO 2 nanoparticles and their effect on photoassisted processes

Abstract

Characteristics of 2.45-GHz microwave radiation were examined on two second generation nitrogen-doped TiO 2 nanomaterials prepared by annealing Degussa P-25 TiO 2 and Ishihara ST-01 TiO 2 at 400 °C and 500 °C, respectively, in air in the presence of urea, and for comparison on the undoped pristine samples. Band gaps of all four samples were determined by diffuse reflectance spectroscopy. Both the sizes and the BET specific surface areas of the TiO 2 particles were determined, together with dielectric constants and dielectric loss factors. Nitrogen doping caused the size to increase and surface area to decrease. Temperature–time profiles showed that the heating efficiency of the N-doped specimens by the microwaves was greater, particularly significant for the N-doped P25 sample, but rather small for the N-ST01 sample. The effect of microwaves on the surface optical phonons of the samples, with and without UV–vis irradiation, was examined by an in situ Raman spectroscopic technique; for the undoped P-25 and nitrogen-doped N-P25 TiO 2 systems the effect was negligible. By contrast, microwave irradiation of Ishihara ST-01 and nitrogen-doped N-ST01 TiO 2 samples showed significant changes in the 144-cm −1 optical phonons. Results infer a microwave thermal effect on the Ishihara ST-01 and N-ST01 specimens, whereas for the Degussa P-25 samples the microwaves also imparted a specific effect as the microwaves influenced the N-dopant sites in contrast to the ST-01 systems where the dopant sites were unaffected as evidenced by temperature–time profiles. The microwave-/photo-assisted degradation of 4-chlorophenol under various conditions of UV–vis irradiation and conventional heating, as opposed to microwave heating, confirms the specific microwave effect for the P-25 systems.