IntroductionEnvironmental pollution is oneof the major global challenges faced by human beings. Semiconductorphotocatalysis is the most efficient method for decomposing organic pollutantsin aqueous media 1. Among varioussemiconductor photocatalysts that have been studied, TiO2has attracted much attention as a photocatalyst due to its desirable propertiessuch as lowcost, strong oxidizing power, nontoxicity, photostability, and chemicalinertness 2. However, practical application of TiO2 inphotocatalytic reactions is obstructedby two essential drawbacks: wide energy gap (3–3.
2 eV) that limits itsapplication to ultraviolet region and fast recombination of electron–holepairs, which are generated after photon absorption when the TiO2 isirradiated with energy equal to or higher than its band gap 3. One of the approachesapplied to solve these problems is to change the energy structure of TiO2,i.e. to extend optical absorption range from ultraviolet to visible region and decreasethe electron/hole recombination rate 4. Aneffective method for improved photocatalytic efficiency of TiO2 is coupling the TiO2 with wide band gap semiconductors suchas WO3, V2O5, SnO2, CdS, CdSe, etc.5. CdS has relatively low band gap energy (~2.3 eV) and its mixing with TiO2enhances the photocatalytic activity of TiO2/CdS system not onlybecause of promotion of visible light absorption, but it also features betterseparation of photogenerated electron–hole pairs 6-8.
The position of CdSconduction and valence band gap edges enables the injection of photoexcitedelectrons from conduction band of CdS into the low-lying conduction band of TiO2.On the other hand, the holes generated in CdS valence band cannot be transferredto valence band of TiO2 because CdS valence band is more cathodicthan that of TiO2. The recombination between photogenerated electronsand holes is suppressed as a result of the separation effect and overall photocatalyticactivity TiO2/CdS system is improved.
Hydrothermal method 9, liquid ion-exchange technique 10, sol–gelmethod 11, solvothermal method 12, etc. have been used to prepare TiO2/CdS photocatalysts. In thiswork, we applied high-voltage plasma electrolytic oxidation process 13,14 oftitanium in alkaline electrolyte containing CdS particles for the formation of TiO2/CdSphotocatalyst. Generally,PEO is considered a valuable pathway for the formation of mixed oxide coatings.High temperatures and pressures present inside of the micro-dischargingchannels cause the melting of the substrate material which reacts withelectrolyte (which is much cooler), thus solidifying and crystallizing quicklyupon being ejected from the micro-discharge channel. This process repeatsrandomly over the substrate surface, resulting in the formation of relativelyuniform oxide coating 14. In-situ incorporation of particles into the PEO coatingshas been explored as new strategies to provide the coatings with a wider rangeof compositions and functionalities 15.
CdS particles have negative zetapotential in alkaline media 16 which promotes their movement toward thetitanium anode. CdS particles have melting point around 1750 °C, hence locally high temperature inducedat the micro-discharging sites (~5000 °C) during PEO of titanium 17 should result in depositionof CdS particles on the surface of coatings.