Indian Journal of Chemistry Vol. 53A, July 2014, pp. 820-823 Highly active ZnS loaded TiO 2 photocatalyst for mineralization of phenol red sodium salt under UV-A light P Dhatshanamurthi, B Subash, B Krishnakumar & M Shanthi* Photocatalysis Laboratory, Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India Email: shanthimsm@gmail.com Received 6 February 2014, revised and accepted 17 June 2014 ZnS loaded TiO 2 (ZnS-TiO 2 ) has been successfully synthesized by precipitation-cum-sol gel method. The photocatalytic activity of (ZnS-TiO 2 ) has been investigated for the degradation of phenol red sodium salt in aqueous solution using UV-A light. In presence of ZnS, there is a shift in the absorbance of TiO 2 in both UV and visible region. ZnS-TiO 2 is found to be more efficient than bare TiO 2 , TiO 2 -P25 and ZnS at pH 6.5 for mineralization of the dye under UV-A light. The effects of operational parameters such as the amount of photocatalyst and dye concentration on photomineralization of the dye have been analyzed. The mineralization has been confirmed by COD measurements. Mechanism of degradation by ZnS-TiO 2 is proposed. The catalyst is found to be reusable. Keywords: Photocatalysis, Dye degradation, Zinc sulphide, Phenol red sodium salt, Titania Photocatalytic degradation of various types of dyes in water by UV-light irradiated semiconductor oxides like TiO 2 or ZnO is considered a valuable technique for purifying and reusing aqueous effluents 1,2 . However, their large band gap (3.0-3.2 eV) limits their practical applications. Moreover, the lack of effective surface area and low transfer rate of charge carriers also hamper the photocatalytic activity. Thus, it is necessary to develop novel photocatalysts with high photocatalytic activity. Metal or metal ion doping traps the electron from the semiconductor and enhances the photocatalytic efficiency by decreasing the recombination of electron-hole pairs. 3-7 Several methods such as photochemical, 8-10 co-precipitation, 11,12 sol-gel, 13 hydrothermal, 14-18 solvothermal, 19 flame spray pyrolysis, 5 electrospinning, 20 and RF magnetron sputtering 21 have been reported for metal doping. TiO 2 and ZnO as photocatalyst have been widely used in the degradation of organic dyes due to stability of their chemical structure, non-toxicity, optical and electrical properties. Yet, both TiO 2 and ZnO have a fast recombination rate of electron and holes couple, and can only absorb the ultraviolet light because of the relatively wide band-gap. Hence, it is necessary to restrain the recombination of electron and hole, and increase their visible light absorption for increasing their catalytic activity. The combination of TiO 2 and ZnO has stimulated extensive interest and attention for the recombination of electron-hole pair. 22, 23 TiO 2 , a metal oxide often used as a catalyst in photochemistry, electrochemistry, green defense, and in the cordless industry. 24 Recently, transition metal sulfides, in particularly ZnS and CdS, have been intensively studied since their catalytic functions are comparable to those of TiO 2 . 24,25 . Many reports on photoconduction in TiO 2 /CdS, TiO 2 /PbS, TiO 2 /CdSe, Ag 2 S-ZnO, ZnO/CdS, etc., are available and it is suggested that under band gap excitation transporter can outflow from higher band gap material to lower one. 26-30 The photocatalytic activity of the ZnS-TiO 2 nanocomposites strongly depends not only on the structure of the support (TiO 2 nanotubes) but also on the specific interaction between ZnS and support. The form of the support is important for obtaining highly dispersed ZnS particles and photocatalysts with good performance. The presently synthesized ZnS-TiO 2 composites exhibited new optical properties, which were different from those of the bulk ZnS or TiO 2 . The UV-light photocatalytic activity of the ZnS/TiO 2 was tested by degradation of a Phenol Red Sodium Salt (PRSS), a kind of pH indicator that is very stable under UV-light irradiation. It was found that ZnS-TiO 2 is more efficient than the other commercial and prepared catalysts. Experimental The commercial dye, Phenol Red sodium salt (PRSS, 99%), obtained from Sigma-Aldrich was used as such. A gift sample of Degussa TiO 2 -P25 (particle size 30 nm, BET surface area 50 m 2 g -1 ) was obtained from Evonic (Germany). AnalaR grade titanium isopropoxide (Sigma Aldrich, 99.0%), 2-propanol (Spectrochem, 99.5%), zinc nitrate hexahydrate (Himedia, 98%), sodium sulfide (Fischer), Ag 2 SO 4 (SD Fine), HgSO 4 and FeSO 4 ·7H 2 O (Qualigens) were used as received. Doubly distilled water was used to prepare all experimental solutions.