1.IntroductionEnvironmental pollutionis one of the major global challenges faced by human beings. Semiconductorphotocatalysis is the most efficient method for decomposing organic pollutantsin aqueous media 1.
Amongvarious semiconductor 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 isobstructed by two essential drawbacks: wide energy gap (3–3.2 eV) that limitsits application 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.
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An effective method for improved photocatalytic efficiency of TiO2is 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 photogeneratedelectrons and holes is suppressed as a result of the separation effect and overallphotocatalytic activity TiO2/CdS system isimproved.Hydrothermal method 9, liquid ion-exchangetechnique 10, sol–gel method 11, solvothermal method12, etc. havebeen used to prepare TiO2/CdS photocatalysts. Inthis work, we applied high-voltage plasma electrolytic oxidation process13,14 of titanium in alkaline electrolyte containing CdS particles for the formationof 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 locallyhigh temperature induced at the micro-discharging sites (~5000 °C) during PEO of titanium 17 shouldresult in deposition of CdS particles on the surface of coatings.