Synergistic effect of Co + Mo codoping on the photocatalytic performance of titania thin films

摘要

Thin films of anatase codoped with 0.01 mol% Co and 0.01 mol% Mo up to 5.00 mol% Co and 5.00 mol% Mo were fabricated by spin coating on soda-lime-silica glass substrates. The films subsequently were calcined in air at 450 °C for 2 h. The mineralogical, chemical, morphological, topographical, optical, and photocatalytic properties of the films were investigated in order to assess the factors and mechanisms affecting the photocatalytic properties. The analytical techniques included glancing angle X-ray diffraction, X-ray photoelectron spectrometry, atomic force microscopy, UV-Vis spectrophotometry, and methylene blue photodegradation. A key determinant of the data was the solubility limit of the dopants, particularly Co. When this was exceeded (≥0.30 mol%), the formation of undetectable precipitates commenced blockage of the active sites, thereby reducing the photocatalytic performance. The initial precursor cation valences altered from Co~(2+) and Mo~(5+) to Co~(2+)Co~(3+) and Mo~(4+)/Mo~(5+) upon interstitial solid solution. The absence of charge-compensating Ti~(3+)/Ti~(4+) redox indicates that charge compensation for the codoping was by IVCT through the reaction Co~(2+) + Mo~(5+) ↔ Co~(3+) + Mo~(4+). While the three dominant factors affecting the band gap were crystallinity, grain size, and precipitation, the band gap did not dominate the photocatalytic performance or its kinetics. These were reduced by blockage of the active sites by dopant precipitates and, to a lesser extent, from contamination from the cations from the substrate. These phases were the dominant factor in decreasing the optical transmission and thus increasing the band gap through the absorption, reflection, and scattering mechanisms of these grains; the crystallinity and grain size were of secondary importance.