Study of chromium modified TiO ₂ nano catalyst under visible light irradiation

Visible light active photocatalysts were successfully prepared by incorporating chromium into anatase TiO ₂ at two Cr/Ti atomic ratios (0.03% and 0.11 %) by the use of a modified sol-gel process. Results show that the size of the chromium modified TiO ₂ particles is approximately 14-25 nm. As indicated by diffuse reflectance ultra violet/visible absorption spectra, heavier chromium dosage tends to result in greater absorption in both ultra violet and visible light. The simulation results from Cr K-edge X-ray absorption spectra suggest that Cr(0) and Cr(III), accounting for approximately 25% and 75% of total Cr, respectively, coexist in the TiO ₂ catalyst doped with 0.11% Cr. Cr dopant is suggested to be responsible for the phenomenon of enhanced light absorption in both ultra violet and visible regions. Further, Cr(0) can act as an electron remover because of the formation of the Schottky barrier between Cr(0) and TiO ₂, thus reducing the possibility of electron hole recombination. Photo-catalytic degradation of methylene blue under irradiation of blue light (with peak flux at 460 nm wavelength and a small flux near 367 nm) was considerably enhanced under appropriate reaction time (12 and 24 h) as small amount of Cr was doped into anatase titanium dioxide catalyst. After prolonged reaction time, Cr(0) was suggested to be poisoned and/or oxidized by SO ₄ ^2-, one of the final products of mineralizing methylene blue, thus loosing the capability of the electron hole separation.