Study On The Synthesis And Photocatalytic Properties Of Mo/W-Sb/Bi-S Tridoped TiO₂

Recently, TiO₂has been regarded as one of the most attractive photocatalyst due to its peculiar andfascinating properties, such as thermal and chemical stability, broad functionality, high oxidative power,relatively high photocatalytic activity, non-toxicity and low cost and a wide range of interesting potentialapplications in degradation of various dye wastewater. However, most applications are limited because ofthe low utilization of the visible part of solar energy and the high recombination rate of photogeneratedelectrons and holes. Many studies show that proper ion doping is one of the most promising approach toimprove the TiO₂photocatalytic activity. Here, suitable metal ions Mo/W and Sb/Bi and non-metal ion Swere selected to modify TiO₂so as to improve the visible light utilization and reduce the electron-holerecombination rate.In the paper, thi-doped TiO₂photocatalyst Mo-Bi-S-TiO₂, Mo-Sb-S-TiO₂, W-Sb-S-TiO₂andW-Bi-S-TiO₂were synthesized by simple sol-gel method. As-prepared TiO₂materials were characterizedby XRD, TEM, SEM, EDX, N2adsorption and UV–vis DRS, and the photocatalytic activity wereevaluated by photodegradation of methylene blue in aqueous solution under visible light irradiation. Theeffect of the crystal structure, specific surface area, doping and other factors on the photocatalytic activityof TiO₂were discussed. The significant results are listed as follows:（1） The proper amount of metal doping can improve the photocatalytic activity of TiO₂. ForMo-Bi-S-TiO₂, the optimal mole ratio of Mo: Bi: Ti is0.06:0.0125:1; for Mo-Sb-S-TiO₂, Mo: Sb: Ti is0.06:0.02:1; for W-Sb-S-TiO₂, W: Sb: Ti is0.008:0.02:1and for W-Bi-S-TiO₂, W: Bi: Ti is0.008:0.01:1.（2） The characterization results of this four catalysts shown that their high photocatalytic activity wasclosely related with their large specific surface area, high crystallinity, small grain size, pore structure, andstrong absorption in the the visible region. Meanwhile, the non-metal ion S was doped into TiO₂can narrowits band gap, the metal ions Mo/W and Sb/Bi can restrain the recombination of electron-hole pairs. Thesynergy of the three elements made tri-doped TiO₂photocatalyst show high degradation ratio of methyleneblue solution in the visible light.（3） The effect of catalyst dosage, initial concentration and initial pH value on photocatalytic activitiesof as-prepared TiO₂samples. The results showed that, when the catalyst dosage was0.67g/L, the initial concentration of methylene blue was0.035g/L, and the initial pH value was9.0, the four tri-doped TiO₂catalysts reached the best degradation efficiency, which higher than the corresponding double-doped TiO₂,single doped TiO₂and pure TiO₂.（4） Compared the four catalysts, the order of the catalytic activity was W-Sb-S-TiO₂（99.03%）>W-Bi-S-TiO₂（98.66%）> Mo-Sb-S-TiO₂（98.26%）> Mo-Bi-S-TiO₂（97.54%）. It can be seen that, as far asthe family of Mo and W was concerned, doping W was better than that of Mo; doping Sb was also betterthan that of Bi alike. That’s maybe because the ionic radius of Sb and Bi is larger than that of Ti, whichexist with inserstitial way can reduce the grain size of TiO₂. Compared to Bi, doped with Sb make theformation of smaller grain size and larger specific surface area easier, which played a key role to improvethe photocatalytic activity. The ionic radius of Mo and W is smaller than Ti, which can replace the Ti ionsdoped into TiO₂lattice, and both have large redox potential which can effectively capture the photoinducedelectrons and holes, and enhance the photocatalytic activity. Compared to Mo, W showed higher ability inrestrain the recombination of electron and hole, thus the photocatalytic activity of W-Sb-S-TiO₂was thehighest.