Degradation study of malachite green on chitosan films containing heterojunctions of melon/TiO₂ absorbing visible-light in solid-gas interfaces

Urea modified TiO₂ nanoparticles were synthesized at different TiO₂: urea ratios (10, 30 and 60 (%wt) named UTiO₂-10, 30 and 60) by sol-gel method and annealing at 400 °C during 1 h. Characterization revealed the presence of TiO₂ nanoparticles with visible light absorption (between 400 and 500 nm) probably due by the formation of thermolysis urea byproducts such as melon, which may induce a TiO₂/melon heterojunction. Then, modified nanoparticles were immobilized on chitosan films by adding 30% (70QS/30UTiO₂) and 50% 50QS/50UTiO₂) of UTiO₂ nanoparticles. Functionalized films were characterized by SEM-EDS, DRS and AFM. Photo-induced degradation of malachite green stains was evaluated on these films under different wavelengths. Upon UV-A + visible light irradiation, films showed good activity even slightly better than those containing Evonik P-25 (70QS/30P-25). Upon visible light between 450 and 800 nm, films evidenced a satisfactory performance but slower than under UV-A + visible light irradiation whereas the photobleaching activity of 70QS/30P-25 was very poor. When visible light irradiation with wavelengths higher than 590 nm was used, a slight photobleaching was also observed. DRS spectra taken at different irradiation times showed that dye underwent N-demethylated oxidative reactions either upon UV–vis or visible light irradiation. ATR-FTIR measurements revealed the generation of benzophenones after 6 h of irradiation probably suggesting the formation of singlet oxygen under visible light. 70QS/30UTiO₂-10 films showed a good photo-bleaching activity after 18 h of continuous UV-A + visible light irradiation.