catalysts Review Mechanistic Insights into Photodegradation of Organic Dyes Using Heterostructure Photocatalysts Yi-Hsuan Chiu 1 , Tso-Fu Mark Chang 2,*, Chun-Yi Chen 2,*, Masato Sone 2 and Yung-Jung Hsu 1,3,* 1 Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan; [email protected] 2 Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan; [email protected] 3 Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan * Correspondence: [email protected] (T.-F.M.C.); [email protected] (C.-Y.C.); [email protected] (Y.-J.H.) Received: 12 April 2019; Accepted: 3 May 2019; Published: 9 May 2019 Abstract: Due to its low cost, environmentally friendly process, and lack of secondary contamination, the photodegradation of dyes is regarded as a promising technology for industrial wastewater treatment. This technology demonstrates the light-enhanced generation of charge carriers and reactive radicals that non-selectively degrade various organic dyes into water, CO2, and other organic compounds via direct photodegradation or a sensitization-mediated degradation process. The overall efficiency of the photocatalysis system is closely dependent upon operational parameters that govern the adsorption and photodegradation of dye molecules, including the initial dye concentration, pH of the solution, temperature of the reaction medium, and light intensity.Additionally, the charge-carrier properties of the photocatalyst strongly affect the generation of reactive species in the heterogeneous photodegradation and thereby dictate the photodegradation efficiency. Herein, this comprehensive review discusses the pseudo kinetics and mechanisms of the photodegradation reactions. The operational factors affecting the photodegradation of either cationic or anionic dye molecules, as well as the charge-carrier properties of the photocatalyst, are also fully explored. By further analyzing past works to clarify key active species for photodegradation reactions and optimal conditions, this review provides helpful guidelines that can be applied to foster the development of efficient photodegradation systems. Keywords: cationic dye; anionic dye; direct photoderadation; sensitization-mediated degradation; quantum yield 1. Introduction The widespread presence of organic dyes in industrial wastewaters from the textile, apparel, and paper industries results in significant environmental contamination. These dye-polluted effluents contain highly hazardous, carcinogenic, non-biodegradable, and colored pigments that can cause damage to humans [1,2]. Even at very low concentrations (below 1 ppm), dyes are clearly visible in water and seriously deteriorate aqueous environments [3–5]. Therefore, the removal of colored organic dyes from wastes is imperative and important. For conventional treatment on industrial wastewater, adsorption [6] and coagulation [7] are common methods used to remove the organic dyes. However, these processes cause secondary hazardous pollution because dyes are only changed from a liquid phase into a solid phase. Thus, further treatments are necessary to resolve the problem of secondary pollution [8,9]. Over the past few years, photocatalysis was regarded as a promising Catalysts 2019, 9, 430; doi:10.3390/catal9050430 www.mdpi.com/journal/catalysts Catalysts 2019, 9, x FOR PEER REVIEW 2 of 32 problem of secondary pollution [8,9]. Over the past few years, photocatalysis was regarded as a promising alternative treatment in the aspect of water purification [10]. Essentially, the photocatalytic reaction involves heterogeneous catalysis, where a light-absorbing catalyst is put in contact with the target reactants, in either a solution or gas phase. This heterogeneous approach was successfully employed as an effective tool for the degradation of various hazardous materials, including atmospheric and aquatic organic pollutants, and shows many advantages over traditional wastewater treatment techniques. For instance, the complete degradation of organic pollutants using Catalysts 2019, 9, 430 2 of 32 active photocatalysts can occur within a few hours at room temperature. In addition, organic pollutants can be entirely mineralized to relatively non-toxic products (CO2 and water) without the alternativeformation of treatment secondary in hazardous the aspect ofproducts water purification [11,12]. [10]. Essentially, the photocatalytic reaction involvesThe heterogeneoustypical mechanism catalysis, for the where photodegradation a light-absorbing of organic catalyst dyes is put is show in contactn in Scheme with the 1. targetUpon reactants,irradiation in with either incident a solution photons, or gas phase. electrons This heterogeneous are excited to approach the conduction was successfully band employed(CB) of the as anphotocatalyst, effective tool while for the holes degradation are formed of various in the hazardous valance band materials, (VB). including The photoexcited atmospheric electrons and aquatic and organicholes can pollutants, either recombine and shows manyto generate advantages thermal over energy traditional or diffuse wastewater to the treatment photocatalyst techniques. surface For instance,reacting with the complete the adsorbed degradation molecule ofs. organic The reactive pollutants radical using species, active such photocatalysts as superoxide can occurradicals within (·O2− a) fewand hourshydroxyl at room radicals temperature. (·OH), Inare addition, further organic derived pollutants from the can photoexcited be entirely mineralized electrons toand relatively holes, non-toxicrespectively. products Moreover, (CO2 andthe photosensitization water) without the of formation dye molecules of secondary can provide hazardous photocatalysts products [11 with,12]. additionalThe typical electrons, mechanism which are for also the photodegradationcapable of generating of organic radicals dyes like is ·O shown2−. These in Scheme reactive1. species Upon irradiationcan quickly with and incident non-selectively photons, electrons decompose are excited organic to the pollutants. conduction The band whole (CB) ofphotodegradation the photocatalyst, whileprocess, holes from are the formed adsorption in the valance of dye band molecule (VB).s The on photoexcitedthe surface electronsof the photocatalyst and holes can to either the recombinedecomposition to generate of dye thermalmolecules energy by reactive or diffuse radicals to the photocatalyst, is affected by surface operational reacting parameters with the adsorbed such as molecules.the pH of solution, The reactive initialradical dye concentration, species, such reacti as superoxideon temperature, radicals and ( irradiationO ) and hydroxyl intensity radicals[13–17]. · 2− (ForOH), example, are further Neppolian derived et al. from reported the photoexcited that the degradation electrons of and reactive holes, yellow respectively. 17, reactive Moreover, red 2, and the · photosensitizationreactive blue 4 over of dyeDegussa molecules P-25 canfollowed provide pseudo photocatalysts first-order with kinetics additional [14], in electrons, which increasing which are alsoinitial capable dye concentration of generating depressed radicals like theO photodegra. These reactivedation speciesefficiency. can quicklyShahwan and et non-selectivelyal. performed · 2− decomposephotodegradation organic of pollutants. methyl blue The and whole methyl photodegradation orange [13], and process, found that from the the pH adsorption of solution of dyeand moleculessteric structure on the were surface highly of therelated photocatalyst to photocatal to theytic decomposition efficiency. In addition of dye molecules to these byoperational reactive radicals,parameters, is a fftheected band by operationalposition and parameters charge-carrier such asutilization the pH ofof solution, the photocatalysts initial dye concentration,also have an reactionimpact on temperature, the generation and of irradiationreactive radicals intensity and [the13– subsequent17]. For example, photodegradation Neppolian performance. et al. reported To thatimprove the degradationthe carrier of utilization reactive yellow and 17,thereby reactive achieve red 2, andefficient reactive reactive blue 4radical over Degussa generation, P-25 followedheterostructure pseudo photocatalysts first-order kinetics with [14enhanced], in which photocatalytic increasing initial activity dye are concentration proposed and depressed employed the photodegradation[18–25]. efficiency. Shahwan et al. performed photodegradation of methyl blue and methyl orangeThis [13 ],comprehensive and found that thereview pH of solutiondiscusses and stericthe structurepseudo werekinetics highly and related mechanisms to photocatalytic for ephotodegradationfficiency. In addition reactions. to these operationalThe operational parameters, factors the affecting band position the andphotodegradation charge-carrier utilizationof either ofcationic the photocatalysts or anionic dye also molecules, have an impactas well onas the generationcharge-carrier of reactiveproperties radicals of the and photocatalysts, the subsequent are photodegradationalso fully explored. performance. Finally, we outline To improve earlier the works carrier to utilization reveal the and key thereby reactive achieve species effi accountingcient reactive for radicalthe photodegradation generation, heterostructure of different dyes, photocatalysts providing with helpful