Effect of doping and codoping with Mn and Fe on the photocatalytic performance of TiO2 thin films

摘要

TiO2 thin films with varying levels of Mn and Fe dopants (0.01 - 5.00 mol% metal basisfor each) were deposited on soda-lime-silica glass substrates by spin coating, followedby annealing in air at 450°C for 2 h. Both single cation doping and codoping werecarried out to investigate the effects of these additions on the photocatalyticperformance of TiO2 thin films. The mineralogical, morphological, topographical,optical and photocatalytic properties were assessed for the films using glancing angleX-ray diffraction (GAXRD), field emission scanning electron microscopy (FESEM),atomic force microscopy, UV-VIS spectroscopy and methylene blue degradation tests.GAXRD analysis showed that anatase was the major phase in essentially all the films;no characteristic peaks representing any oxide of Mn or Fe were seen and this isattributed to either the incorporation of the dopants in the crystalline structure of titaniaor the low dopant concentrations being below the level of detection. A slight decreasein the degree of crystallinity with increasing Mn and Fe codopant concentrations, asshown in the GAXRD patterns, and a shift in the anatase peak locations in the laserRaman spectra suggested that solid solution formation occurred. All of the depositedfilms were of a consistent thickness of ~250 nm, as determined by transmission electronmicroscopy (TEM) imaging. FESEM and AFM analyses showed that the films werecomprised of grains with sizes in the range of 20-30 nm. The doping appears to triggeragglomeration and coarse roughening of the films. UV-VIS spectrophotometry analysisrevealed that all of the films were transparent (~80%) in the visible region and theoptical indirect band gaps were ~3.4 eV.The photocatalytic performance as measured in terms of the degradation of methyleneblue (MB) solution (≤24 h), showed that the extent photodegradation was generallydegraded with increasing dopant concentrations. Only 0.01 mol% Fe-doped TiO2 filmappeared to possess superior photoactivity than the undoped TiO2. This indicates thatmost samples probably were overdoped. Hence, improvement in the performancethrough band gap reduction through doping is likely to require low doping levels inorder to minimise the number of electron/hole recombination centres and to minimiselattice distortion, both of which are deleterious to the performance.