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 enhanced guidelines photocatalytic that could activity be applied are proposed to foster andthe development employed [18 of–25 efficient]. photodegradation systems.

Scheme 1. Schematic illustration of operational factors affecting the photodegradation of organic dyes over semiconductor photocatalysts.

This comprehensive review discusses the pseudo kinetics and mechanisms for photodegradation reactions. The operational factors affecting the photodegradation of either cationic or anionic dye molecules, as well as the charge-carrier properties of the photocatalysts, are also fully explored. Finally, we outline earlier works to reveal the key reactive species accounting for the photodegradation of CatalystsCatalysts2019 2019, 9, ,9 430, x FOR PEER REVIEW 3 ofof 3232 Catalysts 2019, 9, x FOR PEER REVIEW 3 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 3 of 32 CatalystsScheme 2019, 9 1., x FORSchematic PEER REVIEW illustration of operational factors affecting the photodegradation of organic3 of 32 differentScheme dyes, 1. providing Schematic helpful illustration guidelines of operational that could factors be applied affecting to the foster photodegradation the development of organic of efficient Schemedyes over 1. semiconductor Schematic illustration photocatalysts. of operational factors affecting the photodegradation of organic photodegradationdyesScheme over 1. semiconductor Schematic systems. illustration photocatalysts. of operational factors affecting the photodegradation of organic dyes over semiconductor photocatalysts. 2. Classificationdyes over semiconductor of organic dyesphotocatalysts. 2.2. ClassificationClassification of of Organic organic Dyesdyes 2. Classification of organic dyes 2. ClassificationBasically, the of chemicalorganic dyes structure of dye molecules determines their color and properties. Basically,Basically, the the chemical chemical structure structure of dye of molecules dye mole determinescules determines their color their and color properties. and properties. Therefore, Therefore,Basically, they the can chemicalbe classified structure according of dyeto thei moler chemicalcules determines structure (functionaltheir color groups), and properties. color, or theyTherefore, canBasically, be they classified thecan chemicalbe according classified structure to according their chemicalof dyeto thei mole structurer chemicalcules (functionaldetermines structure (functionalgroups),their color color, groups), and or aspectsproperties. color, of or Therefore,aspects of usagethey can [26]. be Inclassified the textile according industry, to commotheir chemicalnly used structure dyes include (functional acid, groups),basic, direct, color, azo, or usageaspectsTherefore, [26 of]. usagethey In the can [26]. textile be In classified industry,the textile according commonly industry, to commothei usedr chemical dyesnly used include structure dyes acid, include (functional basic, acid, direct, basic,groups), azo, direct, naphtha, color, azo, or aspectsnaphtha, of reactive, usage [26]. mordant, In the vat, textile disperse, industry, and commosulfur dyesnly used [27], withdyes azoinclude dyes acid, being basic, the most direct, used azo, at reactive,naphtha,aspects mordant,of reactive, usage [26]. vat,mordant, disperse,In the vat, textile anddisperse, industry, sulfur and dyes commosulfur [27], dyes withnly used[27], azo dyeswithdyes beingazoinclude dyes the acid,being most basic, usedthe most atdirect, present. used azo, at naphtha,present. Toreactive, study mordant, their properties vat, disperse, with andregard sulfur to dyesphotodegradation [27], with azo reacdyestions, being dyes the most are usuallyused at Topresent.naphtha, study their Toreactive, study properties mordant,their withproperties vat, regard disperse, towith photodegradation andregard sulfur to photodegradationdyes reactions, [27], with dyes azo reac aredyes usuallytions, being dyes the classified mostare usuallyused using at present.classified To using study their their molecular properties charge with upon regard dissociation to photodegradation in aqueous-based reactions, applications. dyes are Tableusually 1 theirclassifiedpresent. molecular To using study charge their their uponmolecular properties dissociation charge with in upon aqueous-basedregard dissociation to photodegradation applications. in aqueous-based Table reac1tions, presents applications. dyes the are chemical Tableusually 1 classifiedpresents theusing chemical their molecular properties charge of several upon dissociationrepresentative in aqueous-baseddyes that are applications.frequently usedTable in1 propertiespresentsclassified the ofusing several chemical their representative molecular properties charge dyesof several thatupon are dissociationrepr frequentlyesentative usedin aqueous-baseddyes in photodegradation that are applications.frequently applications. usedTable in 1 presentsphotodegradation the chemical applications. properties Acco ofrding several to the repr chemicalesentative structure, dyes thatthey areare dividedfrequently into usedcationic in Accordingphotodegradationpresents tothe the chemical chemical applications. properties structure, Acco they ofrding areseveral dividedto the repr chemical intoesentative cationic structure, anddyes anionic theythat areare dyes. dividedfrequently The cationic into usedcationic dyes, in photodegradationand anionic dyes. Theapplications. cationic dyes, Acco includingrding to the methylene chemical blue structure, (MB), rhodaminethey are divided B (RhB), into malachite cationic includingandphotodegradation anionic methylene dyes. Theapplications. blue cationic (MB), dyes, rhodamineAcco includingrding to B (RhB),the methylene chemical malachite blue structure, green(MB), (MG),rhodaminethey are rhodamine divided B (RhB), into 6G malachite (Rh6G),cationic andgreen anionic (MG), dyes. rhodamine The cationic 6G (Rh6G), dyes, including crystal violetmethylene (CV), blue and (MB), safranin rhodamine O (SO), B (RhB),contain malachite cationic crystalgreenand anionic violet(MG), dyes. (CV),rhodamine The and cationic safranin 6G (Rh6G), dyes, O (SO), including crystal contain violetmethylene cationic (CV), functional blue and (MB), safranin groups rhodamine O that (SO), can B (RhB),contain dissociate malachite cationic into greenfunctional (MG), groups rhodamine that can 6G dissociate (Rh6G), intocrystal positively violet charged(CV), and ions safranin [28] in anO (SO),aqueous contain solution. cationic The positivelyfunctionalgreen (MG), charged groups rhodamine ionsthat [can28 ]6G indissociate an(Rh6G), aqueous intocrystal solution.positively violet The charged(CV), most and commonions safranin [28] cationicin anO aqueous(SO), functional contain solution. group cationic The is functionalmost common groups cationic that canfunctional dissociate group into is positively the onium charged group ionsand, [28]thus, in most an aqueous of the cations solution. are The N+ themostfunctional onium common groupgroups cationic and, that thus, canfunctional dissociate most ofgroup the into cations is positively the on areium N charged+ groupions. Onand,ions the [28]thus, other in most an hand, aqueous of the the cations anionicsolution. are dyes The N+ mostions. commonOn the other cationic hand, functional the anionic group dyes is thecover on direct,ium group acid, and, and thus,reactive most dyes of the[29], cations such asare acid N+ coverions.most direct,Oncommon the acid, other cationic and hand, reactive functional the anionic dyes group [29 dyes], suchis coverthe as on acid direct,ium orange group acid, 7 and, (AO7),and thus,reactive eosin most Ydyes (EY),of the[29], methyl cations such orange asare acid N+ ions.orange On 7 the(AO7), other eosin hand, Y (EY),the anionic methyl dyesorange cover (MO), direct, acid acid, red 14and (AR14), reactive alizarin dyes [29],red Ssuch (ARS), as acidrose (MO),orangeions. acidOn 7 (AO7),the red other 14 (AR14),eosin hand, Y alizarin(EY), the anionicmethyl red S (ARS),dyesorange cover rose (MO), bengaldirect, acid (RB), acid,red and 14and (AR14), phenol reactive redalizarin dyes (PR). [29],red All S anionicsuch (ARS), as dyes roseacid orangebengal (RB),7 (AO7), and eosinphenol Y red(EY), (PR). methyl All anionicorange (MO),dyes contain acid red anionic 14 (AR14), functional alizarin groups, red S e.g., (ARS), sulfonic rose containbengalorange anionic (RB),7 (AO7), and functional eosinphenol Y groups,red(EY), (PR). methyl e.g., All sulfonic anionicorange ordyes(MO), carboxylic contain acid red acidanionic 14 groups (AR14), functional [5]. alizarin These groups, functionalred Se.g., (ARS), sulfonic groups rose bengalor carboxylic (RB), and acid phenol groups red [5]. (PR). These All functional anionic dyes groups contain are water-soluble anionic functional and can groups, effectively e.g., sulfonicinteract areorbengal water-solublecarboxylic (RB), andacid andphenol groups can red e[5].ffectively (PR).These All functional interact anionic with dyesgroups photocatalysts contain are water-soluble anionic with functional hydrophilic and can groups, effectively surfaces. e.g., sulfonic Hence,interact orwith carboxylic photocatalysts acid groups with hydrophilic[5]. These functional surfaces. grHencoupse, arecationic water-soluble and anionic and dyes can effectivelyare also known interact as cationicwithor carboxylic photocatalysts and anionic acid groups dyeswith are hydrophilic[5]. also These known functional surfaces. as basic grHenc andoupse, acidic arecationic water-soluble dyes, and respectively. anionic and dyes can effectivelyare also known interact as withbasic photocatalystsand acidic dyes, with respectively. hydrophilic surfaces. Hence, cationic and anionic dyes are also known as basicwith photocatalystsand acidic dyes, with respectively. hydrophilic surfaces. Hence, cationic and anionic dyes are also known as basic and acidic dyes, respectively. Table 1. Chemical properties of representative cationic and anionic dyes. MW—molecular weight. basic andTable acidic 1. Chemical dyes, respectively. properties of representa tive cationic and anionic dyes. MW—molecular weight. Table 1. Chemical properties of representative cationic and anionic dyes. MW—molecular weight. TableCationic 1. Chemical Dyes properties Abbreviation of representa MtiveW cationic and anionicStructure dyes. MW—molecularλmax weight.(nm) CationicTable dyes 1. Chemical Abbreviation properties of representa MWtive cationic and anionicStructure dyes. MW—molecular weight.𝛌𝒎𝒂𝒙 (nm) Cationic dyes Abbreviation MW Structure 𝛌𝒎𝒂𝒙 (nm) Cationic dyes Abbreviation MW Structure 𝛌𝒎𝒂𝒙 (nm) Cationic dyes Abbreviation MW Structure 𝛌 (nm) MethyleneMethylene blue blue MB MB 799.81 799.81 664𝒎𝒂𝒙664 Methylene blue MB 799.81 664 Methylene blue MB 799.81 664 Methylene blue MB 799.81 664

Rhodamine B RhB 479.02 553 RhodamineRhodamine B B RhB RhB 479.02 479.02 553 553 Rhodamine B RhB 479.02 553 Rhodamine B RhB 479.02 553

Rhodamine 6G Rh6G 479.02 534 Rhodamine 6G Rh6G 479.02 534 RhodamineRhodamine 6G 6G Rh6G Rh6G 479.02 479.02 534 534 Rhodamine 6G Rh6G 479.02 534

Malachite green MG 364.91 614 Malachite green MG 364.91 614 Malachite green MG 364.91 614 MalachiteMalachite green green MG MG 364.91 364.91 614 614

Crystal violet CV 407.98 573 Crystal violet CV 407.98 573 Crystal violet CV 407.98 573 Crystal violet CV 407.98 573

Catalysts 2019, 9, x FOR PEER REVIEW 3 of 32

Scheme 1. Schematic illustration of operational factors affecting the photodegradation of organic dyes over semiconductor photocatalysts.

2. Classification of organic dyes Basically, the chemical structure of dye molecules determines their color and properties. Therefore, they can be classified according to their chemical structure (functional groups), color, or aspects of usage [26]. In the textile industry, commonly used dyes include acid, basic, direct, azo, naphtha, reactive, mordant, vat, disperse, and sulfur dyes [27], with azo dyes being the most used at present. To study their properties with regard to photodegradation reactions, dyes are usually classified using their molecular charge upon dissociation in aqueous-based applications. Table 1 presents the chemical properties of several representative dyes that are frequently used in photodegradation applications. According to the chemical structure, they are divided into cationic and anionic dyes. The cationic dyes, including methylene blue (MB), rhodamine B (RhB), malachite green (MG), rhodamine 6G (Rh6G), crystal violet (CV), and safranin O (SO), contain cationic functional groups that can dissociate into positively charged ions [28] in an aqueous solution. The most common cationic functional group is the onium group and, thus, most of the cations are N+ ions. On the other hand, the anionic dyes cover direct, acid, and reactive dyes [29], such as acid orange 7 (AO7), eosin Y (EY), methyl orange (MO), acid red 14 (AR14), alizarin red S (ARS), rose bengal (RB), and phenol red (PR). All anionic dyes contain anionic functional groups, e.g., sulfonic or carboxylic acid groups [5]. These functional groups are water-soluble and can effectively interact with photocatalysts with hydrophilic surfaces. Hence, cationic and anionic dyes are also known as basic and acidic dyes, respectively.

Table 1. Chemical properties of representative cationic and anionic dyes. MW—molecular weight.

Cationic dyes Abbreviation MW Structure 𝛌𝒎𝒂𝒙 (nm)

Methylene blue MB 799.81 664

Rhodamine B RhB 479.02 553

Rhodamine 6G Rh6G 479.02 534

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Malachite green MG 364.91Table 1. Cont. 614

Cationic Dyes Abbreviation MW Structure λmax (nm)

CrystalCrystal violet violet CV CV 407.98 407.98 573 573

Catalysts 2019, 9, x FOR PEER REVIEW 4 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 32

SafraninSafranin O O SO SO 350.85 350.85 520 520 Safranin O SO 350.85 520 Safranin O SO 350.85 520 Safranin O SO 350.85 520 Safranin O SO 350.85 520 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 32

Auramine O AO 303.83 420 AuramineAuramine O OSafranin O AO AO SO 303.83 303.83 350.85 520420 420 Auramine O AO 303.83 420 Auramine O AO 303.83 420

Auramine O AO 303.83 420

Auramine O AO 303.83 420

Victoria blue B VBB 506.08 614 Victoria blue B VBB 506.08 614 VictoriaVictoria blue blueB B VBB VBB 506.08 506.08 614 614 Victoria blue B VBB 506.08 614 Victoria blue B VBB 506.08 614 Victoria blue B VBB 506.08 614

Anionic dyes Abbreviation Mw Structure 𝛌𝒎𝒂𝒙 (nm) Anionic dyes Abbreviation Mw Structure 𝛌𝒎𝒂𝒙 (nm) Anionic dyes Abbreviation Mw Structure 𝛌𝒎𝒂𝒙 (nm) AnionicAnionic dyes Dyes Abbreviation Abbreviation Mw Mw Structure Structure λmax 𝛌(nm)𝒎𝒂𝒙 (nm) Anionic dyes Abbreviation Mw Structure 𝛌𝒎𝒂𝒙 (nm) MethylAnionic orange dyes Abbreviation MO 327.33 Mw Structure 𝛌𝒎𝒂𝒙464 (nm) Methyl orange MO 327.33 464 MethylMethyl orange orange Methyl orange MO MO MO 327.33 327.33 327.33 464464 464 Methyl orange MO 327.33 464 Methyl orange MO 327.33 464

Eosin Y EY 691.85 518 Eosin Y Eosin Y EY EY 691.85 691.85 518 518 Eosin Y EY 691.85 518 EosinEosin Y Y EY EY 691.85 691.85 518 518 Eosin Y EY 691.85 518

Acid orange 7 AO7 350.32 484 Acid orange 7 AO7 350.32 484 Acid orange 7 AO7 350.32 484 Acid orange 7 AO7 350.32 484 Acid orange 7 AO7 350.32 484 Acid orangeAcid orange 7 Acid 7 red 14 AO7 AO7 AR14 350.32 350.32 502.43 515484 484

Acid red 14 AR14 502.43 515 Acid red 14 AR14 502.43 515 Acid red 14 Alizarin red AR14 S ARS 502.43 240.21 426 515 Acid red 14 AR14 502.43 515 Acid red 14 AR14 502.43 515

Alizarin red S ARS 240.21 426 Alizarin red S ARS 240.21 426 Alizarin red S Rose bengal ARS RB 240.21 973.67 550 426 Alizarin red S ARS 240.21 426 Alizarin red S ARS 240.21 426

Rose bengal Phenol red RB PR 973.67 354.38 560 550 Rose bengal RB 973.67 550 Rose bengal RB 973.67 550 Rose bengal RB 973.67 550 Rose bengal RB 973.67 550

Phenol red PR 354.38 560 Phenol red PR 354.38 560 Phenol red PR 354.38 560 Phenol red PR 354.38 560

Phenol red PR 354.38 560

Catalysts 2019, 9, x FOR PEER REVIEW 4 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 32

Safranin O SO 350.85 520 Safranin O SO 350.85 520 Safranin O SO 350.85 520 Safranin O SO 350.85 520

Auramine O AO 303.83 420 Auramine O AO 303.83 420 Auramine O AO 303.83 420 Auramine O AO 303.83 420

Victoria blue B VBB 506.08 614 Victoria blue B VBB 506.08 614 Victoria blue B VBB 506.08 614 Victoria blue B VBB 506.08 614

Anionic dyes Abbreviation Mw Structure 𝛌 (nm) 𝒎𝒂𝒙 Anionic dyes Abbreviation Mw Structure 𝛌𝒎𝒂𝒙 (nm) Anionic dyes Abbreviation Mw Structure 𝛌 (nm) Anionic dyes Abbreviation Mw Structure 𝛌𝒎𝒂𝒙 (nm) Methyl orange MO 327.33 𝒎𝒂𝒙464 Methyl orange MO 327.33 464 Methyl orange MO 327.33 464 Methyl orange MO 327.33 464

Eosin Y EY 691.85 518 Eosin Y EY 691.85 518 Eosin Y EY 691.85 518 CatalystsEosin2019, Y9, 430 EY 691.85 5185 of 32

Table 1. Cont. Acid orange 7 AO7 350.32 484 Acid orange 7 AO7 350.32 484 Acid Anionicorange 7 Dyes Abbreviation AO7 350.32 Mw Structure λmax (nm)484 Acid orange 7 AO7 350.32 484

AcidAcid red red14 14 AR14 AR14 502.43 502.43 515 515 Acid red 14 AR14 502.43 515 Acid red 14 AR14 502.43 515 Acid red 14 AR14 502.43 515

Alizarin red S ARS 240.21 426 AlizarinAlizarin red S red S ARS ARS 240.21 240.21 426 426 Alizarin red S ARS 240.21 426 Alizarin red S ARS 240.21 426

Rose bengal RB 973.67 550 Rose bengal RB 973.67 550 Rose Rosebengal bengal RB RB 973.67 973.67 550 550 Rose bengal RB 973.67 550

Phenol red PR 354.38 560 Phenol red PR 354.38 560 Phenol red PR 354.38 560 PhenolPhenol red red PR PR 354.38 354.38 560 560 Catalysts 2019, 9, x FOR PEER REVIEW 5 of 32

Catalysts 2019, 9, x FOR PEER REVIEW 5 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 5 of 32

Congo red CR 696.67 497 Congo redCongo red CR CR 696.67 696.67 497497 Congo red CR 696.67 497

Acid violet 7 AV7 566.47 522 AcidAcid violet violetAcid 7 7 violet 7 AV7 AV7 AV7 566.47 566.47 566.47 522522 522

Reactive black 5 RB5 991.82 602 Reactive black 5 RB5 991.82 602 ReactiveReactive black black 5 5 RB5 RB5 991.82 991.82 602 602

3. Kinetics study for photodegradation reactions 3. Kinetics study for photodegradation reactions The kinetics for photodegradation reactions are examined based on the dye concentration changeThe bykinetics measuring for photodegradation the characteristic reactionsabsorbance are peakexamined at different based onirradiation the dye times.concentration All the 3. Kineticschangecommon study byfor dyes measuring phot haveodegradation their the specificcharacteristic characteristicreactions absorbance absorptions peak at in different the visible irradiation range (400–700 times. Allnm), the as commonshown in dyesTable have 1. Herein, their specificthe efficiency characteristic of photo absorptionsdegradation in(also the known visible as range the decolorizing (400–700 nm), ratio) as The kineticsshownis determined in forTable photodegradationusing 1. Herein, the following the efficiency equation: reactions of photo aredegradation examined (also basedknown ason the the decolorizing dye concentration ratio) change byis measuringdetermined using the thecharacteristic following equation: absorbance peak at different irradiation times. All the 𝐶 𝐶 Degradation Efficiency % = × 100, common dyes have their specific characteristic absorptions𝐶 in𝐶 the visible range (400–700 (1)nm), as Degradation Efficiency % = 𝐶 × 100, (1) shown in Table 1. Herein, the efficiency of photodegradation 𝐶(also known as the decolorizing ratio) where C0 and C are the solution concentration at t = 0 and after some irradiation time. However, the is determinedwhereabsorption using C0 and peakthe C arefollowing shift the or solution excitation equation: concentration of the dye moat tlecules = 0 and may after cause some the irradiation inaccurate time. estimation However, of thethe absorptionphotodegradation peak shift efficiency, or excitation which of is the furt dyeher modiscussedlecules inmay Sections cause 3.1the and inaccurate 3.2. estimation of the 𝐶 𝐶 photodegradation efficiency,Degradation which is Efficienc further discussedy % = in Sections× 3.1 100, and 3.2. (1) 3.1. Absorption peak shift of dye molecules 𝐶 3.1. Absorption peak shift of dye molecules where C0 and CDuring are the the solution photodegradation concentration reaction, at ta =redshift 0 and orafter blueshift some canirradiation be sometimes time. seen However, in the the absorptioncharacteristic peakDuring shift the orabsorption photodegradationexcitation of dyeof the molecules, reaction, dye mo aposslecules redshiftibly maycaused or blueshift cause by the thecan aggregation inaccuratebe sometimes of estimationorganic seen in dyes the of the characteristic absorption of dye molecules, possibly caused by the aggregation of organic dyes photodegradation[13,30]. Furthermore, efficiency, thewhich decomposition is further processdiscussed of dye in Sectionsmolecules 3.1could and also 3.2. cause an absorption [13,30].peak shift. Furthermore, These spectral the decomposition shifts bring difficulty process in of determining dye molecules the couldconcentration also cause of anthe absorption remaining peakdyes shift.from theThese absorbance spectral shiftsof the bring characteristic difficulty peak in determining. Special care the is, concentrationtherefore, necessary of the inremaining order to 3.1. Absorption peak shift of dye molecules dyesrepresent from accuratethe absorbance photodegradation of the characteristic efficiency. peak As. Specialshown incare Figure is, therefore, 1a, a typical necessary example in order can beto represent accurate photodegradation efficiency. As shown in Figure 1a, a typical example can be Duringfound the forphotodegradation RhB photodegradation reaction, on Ag3PO a 4redshift nanoparticles or blueshift [31]. The blueshift can be of sometimes the absorption seen band in the found for RhB photodegradation on Ag3PO4 nanoparticles [31]. The blueshift of the absorption band characteristicfrom absorption 554 to 530 ofnm dye was molecules,caused by theposs de-ethyibly causedlation of byRhB the molecule aggregations. The generationof organic of dyes fromintermediates 554 to 530can nmbe furtherwas caused observed by theby steadyde-ethy-statelation photoluminescence of RhB molecule s.(PL) The spectroscopy generation ofin [13,30]. Furthermore, the decomposition process of dye molecules could also cause an absorption intermediatesFigure 1b. The can emission be further at 575 observed nm decreased, by steady while-state a new photoluminescence emission appeared (PL) after spectroscopy 7 min of light in peak shift.Figure illumination,These 1b. spectral The confirming emission shifts at bringthe 575 formation nm difficulty decreased, of inthe while determiningde-ethylated a new emission intermediate.the concentration appeared Figure after 71cof min theshows of remaining light the dyes from illumination,de-ethylationthe absorbance confirmingprocess of theof RhB thecharacteristic molecules.formation Uponof peak the ligh .de-ethylated Specialt illumination, care intermediate. is, the therefore, four ethyl Figure groupsnecessary 1c ofshows RhB in ordercanthe to represent accuratede-ethylationbe sequentially photodegradation process removed of RhB until molecules. it isefficiency. totally Upon conver As lighted shownt illumination,into rhodamine in Figure the without four 1a, ethyla anytypical groups ethyl examplegroups. of RhB Thiscan can be be sequentially removed until it is totally converted into rhodamine without any ethyl groups. This 3 4 found for RhB photodegradation on Ag PO nanoparticles [31]. The blueshift of the absorption band from 554 to 530 nm was caused by the de-ethylation of RhB molecules. The generation of intermediates can be further observed by steady-state photoluminescence (PL) spectroscopy in Figure 1b. The emission at 575 nm decreased, while a new emission appeared after 7 min of light illumination, confirming the formation of the de-ethylated intermediate. Figure 1c shows the de-ethylation process of RhB molecules. Upon light illumination, the four ethyl groups of RhB can be sequentially removed until it is totally converted into rhodamine without any ethyl groups. This

Catalysts 2019, 9, 430 6 of 32

3. Kinetics Study for Photodegradation Reactions The kinetics for photodegradation reactions are examined based on the dye concentration change by measuring the characteristic absorbance peak at different irradiation times. All the common dyes have their specific characteristic absorptions in the visible range (400–700 nm), as shown in Table1. Herein, the efficiency of photodegradation (also known as the decolorizing ratio) is determined using the following equation: C C Degradation Efficiency (%) = 0 − 100, (1) C0 × where C0 and C are the solution concentration at t = 0 and after some irradiation time. However, the absorption peak shift or excitation of the dye molecules may cause the inaccurate estimation of the photodegradation efficiency, which is further discussed in Sections 3.1 and 3.2.

3.1. Absorption Peak Shift of Dye Molecules During the photodegradation reaction, a redshift or blueshift can be sometimes seen in the characteristic absorption of dye molecules, possibly caused by the aggregation of organic dyes [13,30]. Furthermore, the decomposition process of dye molecules could also cause an absorption peak shift. These spectral shifts bring difficulty in determining the concentration of the remaining dyes from the absorbance of the characteristic peak. Special care is, therefore, necessary in order to represent accurate photodegradation efficiency. As shown in Figure1a, a typical example can be found for RhB photodegradation on Ag3PO4 nanoparticles [31]. The blueshift of the absorption band from 554 to 530 nm was caused by the de-ethylation of RhB molecules. The generation of intermediates can be further observed by steady-state photoluminescence (PL) spectroscopy in Figure1b. The emission at 575 nm decreased, while a new emission appeared after 7 min of light illumination, confirming the formation of the de-ethylated intermediate. Figure1c shows the de-ethylation process of RhB molecules. Upon light illumination, the four ethyl groups of RhB can be sequentially removed until it is totally converted into rhodamine without any ethyl groups. This process causes a large blueshift in absorption from 553 nm to 498 nm [32,33], as shown in Figure2d. The further decomposition of rhodamine with its conjugated ring structure causes a further decrease of the absorption peak without a corresponding peak shift [34]. In the presence of benzoquinone (BQ) as an O2 scavenger (see · − Section 5.1), the peak intensity decreases without shifting, while the peak shows a blueshift in the presence of 2-propanol (IPA) as an OH scavenger, further confirming that the blueshift is caused by · the attack of active oxygen species on the N-ethyl groups. Moreover, it was found that the formation of N-de-methylated MG products through the attack of active oxygen species accounts for the observed absorption blueshift of the MG absorption during the MG photodegradation over TiO2 [35]. Catalysts 2019, 9, x FOR PEER REVIEW 6 of 32 process causes a large blueshift in absorption from 553 nm to 498 nm [32,33], as shown in Figure 2d. The further decomposition of rhodamine with its conjugated ring structure causes a further decrease of the absorption peak without a corresponding peak shift [34]. In the presence of benzoquinone (BQ) as an ·O2− scavenger (see Section 5.1), the peak intensity decreases without shifting, while the peak shows a blueshift in the presence of 2-propanol (IPA) as an ·OH scavenger, further confirming that the blueshift is caused by the attack of active oxygen species on the N-ethyl groups. Moreover, it was found that the formation of N-de-methylated MG products through the attack of active oxygen speciesCatalysts 2019accounts, 9, 430 for the observed absorption blueshift of the MG absorption during the 7MG of 32 photodegradation over TiO2 [35].

FigureFigure 1. 1. PhotodegradationPhotodegradation of of RhB RhB over over Ag Ag3PO3PO4 4nanoparticlesnanoparticles under under visible visible illumination illumination recorded recorded as as ((aa)) ultraviolet–visibleultraviolet–visible light light (UV–Vis) (UV–Vis) absorption absorption spectra spectra (inset shows(inset theshows correlation the correlation between absorbance between absorbancechanges of changes maxima absorptionof maxima absorption peak (blue line)peak and(blue the line) corresponding and the correspon wavelengthding wavelength shifts (red curve)), shifts (redand curve)), (b) steady-state and (b) PLsteady-state spectra. (reproduced PL spectra. with (reproduced permission with from permission [31]. Copyright from [31] Royal. Copyright Society of RoyalChemistry, Society 2017). of Chemistry, (c) Scheme 2017.) of the de-ethylation(c) Scheme of processthe de-ethylation of RhB molecules. process (dof) PhotodegradationRhB molecules. (d of) PhotodegradationRhB over CoFe2O 4of/BiOCl RhB over microflowers CoFe2O4/BiOCl under visiblemicroflowers irradiation under (i) withoutvisible irradiation and with scavengers(i) without ofand (ii ) withBQ andscavengers (iii) IPA. of (inset (ii) BQ shows and photographs(iii) IPA. (inset of shows color change photographs of dye moleculesof color change with photodegradationof dye molecules withtime; photodegradation reproduced with permissiontime; reproduced from [34 with]. Copyright permission Royal from Society [34] of. Copyright Chemistry, Royal 2015). Society of 3.2.Chemistry, Photobleaching 2015.) of Dye Molecules

3.2. PhotobleachingThe direct excitation of dye molecules of dye molecules may induce the formation of colorless and unstable transition forms, instead of complete mineralization, especially in the presence of dissolved oxygen, The direct excitation of dye molecules may induce the formation of colorless and unstable which also causes the inaccurate estimation of the photocatalytic activity. Taking the TiO /MB system transition forms, instead of complete mineralization, especially in the presence of dissolved2 oxygen, as an example [36], the photoexcited electrons within TiO2 can transform blue MB molecules into their which also causes the inaccurate estimation of the photocatalytic activity. Taking the TiO2/MB colorless leuco form (LMB) upon UV irradiation, resulting in the photobleaching of MB (step i), as system as an example [36], the photoexcited electrons within TiO2 can transform blue MB molecules shown in Figure2a. In an oxygen-free atmosphere (N 2) without UV irradiation, the bleached condition

persists (step ii), with the formation of the stable LMB. In contrast, recoloration takes place if the system is exposed to air (step iii), leading to back electron transfer from LMB to electron acceptors, and causing a reversion to the blue oxidized form of MB. As shown in Figure2b, the recovery process becomes faster in an oxygen atmosphere (step iv), revealing the recovery rate is proportional to the level of O2 [37]. A similar observation can be found in the carbon-doped TiO2/MB system [38]. As Figure2c shows, under UV irradiation, the excitation of TiO2 makes the photobleaching reaction (step 1) become dominant. On the other hand, visible light irradiation can drive self-catalyzed LMB oxidation to MB, Catalysts 2019, 9, x FOR PEER REVIEW 7 of 32 into their colorless leuco form (LMB) upon UV irradiation, resulting in the photobleaching of MB (step i), as shown in Figure 2a. In an oxygen-free atmosphere (N2) without UV irradiation, the bleached condition persists (step ii), with the formation of the stable LMB. In contrast, recoloration takes place if the system is exposed to air (step iii), leading to back electron transfer from LMB to electron acceptors, and causing a reversion to the blue oxidized form of MB. As shown in Figure 2b, the recovery process becomes faster in an oxygen atmosphere (step iv), revealing the recovery rate is proportional to the level of O2 [37]. A similar observation can be found in the carbon-doped TiO2/MB system [38]. As Figure 2c shows, under UV irradiation, the excitation of TiO2 makes the Catalysts 2019, 9, 430 8 of 32 photobleaching reaction (step 1) become dominant. On the other hand, visible light irradiation can drive self-catalyzed LMB oxidation to MB, thus dramatically enhancing the recoloration rate (step 3). Athus competing dramatically reaction enhancing (step 4) the usually recoloration exists ratedue (stepto the 3). visible A competing photoactivity reaction of (step carbon-doped 4) usually existsTiO2, whereasdue to the this visible reaction photoactivity is drastically of carbon-doped suppressed TiOin the2, whereaspresence this of reactionO2; thus, is the drastically oxidative suppressed LMB to MB in transitionthe presence is predominant of O2; thus, the under oxidative visible LMB irradiation to MB transition in O2 atmosphere. is predominant Thus, under the transformation visible irradiation of MBin O and2 atmosphere. LMB can be Thus, achieved the transformation and repeated of by MB changing and LMB the can irradiation be achieved from and UV repeated to visible by changing light, as shownthe irradiation in Figure from 2d. UVThis to photobleaching visible light, as phenom shown inenon Figure and2d. recovery This photobleaching process was also phenomenon found for RhB.and recovery The recoloration process wasof RhB also from found leuco for RhB RhB. (LRhB) The recoloration can be observed of RhB in from 55 min leuco of RhBvisible (LRhB) irradiation, can be andobserved the decoloration–recoloration in 55 min of visible irradiation, process and can the be decoloration–recoloration repeated under sequential process UV and can visible be repeated light irradiationsunder sequential (Figure UV 2e). and visible light irradiations (Figure2e).

Figure 2. (a()a )UV–Vis UV–Vis absorption absorption spectra spectra of ofthe the TiO TiO2/MB2/MB system system upon upon irradiation irradiation for 2.5 for min 2.5 ( mini) with (i) withUV-A UV-A light and light in and (ii) inN2, ( ii(iii)N) air,2,( iiiand) air, (iv) and O2 atmospheres. (iv) O2 atmospheres. (reproduced (reproduced with permission with permission from [36]. Copyrightfrom [36]. CopyrightRoyal Society Royal of Society Chemistry, of Chemistry, 2004.) 2004).(b) Change (b) Change of recovery of recovery rate rate with with the the O O22 contentcontent (reproduced with permission from [37] [37].. CopyrightCopyright American Chemical Society, 2005).2005.) (c) Schematic illustration forfor photoreversible photoreversible color color switching switching between between MB (blue) MB and(blue) its leucoand its form leuco (LMB, form colourless) (LMB,

colourless)on carbon-doped on carbon-doped TiO2 nanocrystals. TiO2 nanocrystals. Test of photoreversible Test of photoreversible color switch color switch of (d) of MB (d and) MB ( ande) RhB (e) RhBon carbon-doped on carbon-doped TiO2 TiOnanocrystals2 nanocrystals under under repeated repeated UV and UV visibleand visible irradiation. irradiation. (reproduced (reproduced with withpermission permission from from [38]. Copyright[38]. Copyright American American Chemical Chemical Society, Society, 2014). 2014.). 3.3. Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) Analysis 3.3. Chemical oxygen demand (COD) and total organic carbon (TOC) analysis To precisely evaluate the extent of decomposition of organic dye molecules during To precisely evaluate the extent of decomposition of organic dye molecules during photodegradation, COD and TOC analyses are usually utilized. COD is an indicative measure photodegradation, COD and TOC analyses are usually utilized. COD is an indicative measure of of oxygen amount that is consumed by oxidation reactions in the solution, which can be used to oxygen amount that is consumed by oxidation reactions in the solution, which can be used to deduce deduce the number of organic molecules in water. TOC on the other hand is an indicative measure of the number of organic molecules in water. TOC on the other hand is an indicative measure of carbon carbon amount in organic compounds, which can more accurately reflect the total amount of organic amount in organic compounds, which can more accurately reflect the total amount of organic compounds in the solution. As shown in Figure3, the photodegradation of bisphenol A (BPA) was compounds in the solution. As shown in Figure 3, the photodegradation of bisphenol A (BPA) was demonstrated using BiOI as photocatalysts [39]. The morphology of BiOI with two specific facets ((110) and (001)) is shown in Figure3a. Under visible irradiation, BiOI-110 exhibited better photodegradation efficiency than BiOI-001 did because it has a higher capability of adsorbing O2, thereby facilitating the generation of reactive radicals. The TOC value of the BiOI-110 system decreased to 5% of the initial TOC value, indicating almost complete mineralization of BPA. In contrast, the TOC value of the BiOI-001 system was reduced by only 44%. This observation is consistent with the change in the photodegradation results. Zhao et al. reported the photodegradation of anionic sulforhodamine B (SRB) over TiO2-Pt photocatalysts [40] under visible irradiation, in which TiO2-Pt (0.2 wt.%) showed the fastest photodegradation rate, capable of degrading SRB molecules within 130 min (Figure3d). Catalysts 2019, 9, x FOR PEER REVIEW 8 of 32 demonstrated using BiOI as photocatalysts [39]. The morphology of BiOI with two specific facets ((110) and (001)) is shown in Figure 3a. Under visible irradiation, BiOI-110 exhibited better photodegradation efficiency than BiOI-001 did because it has a higher capability of adsorbing O2, thereby facilitating the generation of reactive radicals. The TOC value of the BiOI-110 system decreased to 5% of the initial TOC value, indicating almost complete mineralization of BPA. In contrast, the TOC value of the BiOI-001 system was reduced by only 44%. This observation is consistent with the change in the photodegradation results. Zhao et al. reported the Catalysts 2019, 9, 430 9 of 32 photodegradation of anionic sulforhodamine B (SRB) over TiO2-Pt photocatalysts [40] under visible irradiation, in which TiO2-Pt (0.2 wt.%) showed the fastest photodegradation rate, capable of degradingTo confirm SRB the molecules mineralization within of 130 SRB, min Figure (Figure3e,f 3(d)). respectively To confirm show the the mineralization temporal changes of SRB, of Figure COD 3e,fand respectively TOC during show the SRBthe temporal photodegradation. changes of InCOD the and presence TOC during of TiO2 the/Pt, SRB approximately photodegradation. 64% of theIn thetotal presence COD was of reducedTiO2/Pt, afterapproximately 210 min of 64% irradiation. of the Conversely,total COD was the SRBreduced/TiO2 systemafter 210 required min of a irradiation.much longer Conversely, irradiation the time SRB/TiO (around2 system 480 min) required to attain a much a similar longer decrease. irradiation In both time cases, (around the COD 480 min)remained to attain constant a similar with decrease. further irradiation,In both cases, indicating the COD the remained total discoloration constant with of SRB further molecules. irradiation, This indicatingobservation the was total consistent discoloration with of the SRB changes molecules. in TOC. This TOC’s observation increase was in consistent the first hour with of the irradiation changes inis TOC. due to TOC’s photodesorption increase in the of the first dye hour or theof irradi formationation ofis intermediates.due to photodesorption After a gradual of the decreasedye or the to formationthe steady of state, intermediates. the TOC remained After a unchangedgradual dec becauserease to the the degraded steady fragmentsstate, the wereTOC notremained further unchangeddecomposed because with longer the degraded irradiation. fragments were not further decomposed with longer irradiation.

FigureFigure 3. 3. (a()a )TEM TEM images images of of (i ()i )BiOI-110 BiOI-110 and and (ii (ii) )BiOI-001. BiOI-001. (b ()b )Photodegradation Photodegradation of of BPA BPA and and (c (c) ) reductionreduction of of TOC TOC under under different different conditions: conditions: A—without A—without photocatalyst photocatalyst and and visible visible light, light, B—with B—with BiOI-001BiOI-001 in in the the dark, dark, C—with C—with BiOI-110 BiOI-110 in in the the da dark,rk, D—with D—with BiOI-001 BiOI-001 and and visible visible light, light, and and E—with E—with BiOI-110BiOI-110 and and visible visible light light (reproduced (reproduced with with permission permission from from [39] [39].. Copyright Copyright American American Chemical Chemical Society,Society, 2015).2015.). (d(d)) Photodegradation Photodegradation of SRBof SRB and and changes changes in (e) in COD (e) andCOD (f) TOCand under(f) TOC light under irradiation light irradiationover (i) TiO over2,(ii ()i TiO) TiO2-Pt2, (ii (0.1) TiO wt.%),2-Pt (0.1 (iii) wt.%), TiO2-Pt (iii (0.2) TiO wt.%),2-Pt and(0.2 (wt.%),iv) TiO and2-Pt ( (0.5iv) TiO wt.%)2-Pt (reproduced (0.5 wt.%) (reproducedwith permission with frompermission [40]. Copyright from [40] American. Copyright Chemical American Society, Chemical 2002). Society, 2002.). 3.4. Pseudo Kinetics 3.4. Pseudo kinetics To quantify the heterogeneous photodegradation activity, the Langmuir–Hinshelwood (L-H) To quantify the heterogeneous photodegradation activity, the Langmuir–Hinshelwood (L-H) model is usually considered, as shown in Equation (2). model is usually considered, as shown in Equation (2). 𝑑𝐶dC 𝑘𝐾𝐶kKC == , ,(2) (2) − 𝑑𝑡dt 1𝐾𝐶1 + KC wherewhere KK andand kk areare the the thermodynamic thermodynamic adsorption adsorption constant constant and and photodegradation photodegradation rate rate constant, constant, respectively.respectively. Because Because one one of of the the reactants reactants acts acts as as a a photocatalyst photocatalyst whose whose concentration concentration remains remains unchanged,unchanged, the reactionreaction kineticskinetics can can be be simplified, simplifie andd, and the termthe term “pseudo” “pseudo” is used is toused prefix to theprefix reaction the rate expression. At high concentrations of dye, the photocatalyst surfaces are fully covered, leading to the approximation of (1 + KC) to KC. A pseudo zero-order reaction is, thus, observed for saturation coverage on the surface of the photocatalyst [41], since the photodegradation rate is independent of the change in the dye concentration, as shown in Equation (3).

dC = k. (3) − dt Catalysts 2019, 9, 430 10 of 32

1 Integrating the equation under the boundary conditions C = C0 at t = 0 and C = 2 C0 at t = t1/2 respectively yields C C = k t, (4) 0 − 0 C t = 0 . (5) 1/2 2k A plot of C C vs. irradiation time gives a slope equal to the zero-order rate constant (k ). 0 − 0 Additionally, the half-life (t1/2) is the time required to photodegrade half of the initial dye concentration, which is used to quantitatively compare the photodegradation reaction. Therefore, the zero-order t1/2 can be expressed by Equation (5), and it increases with the initial concentration of dye molecules. On the other hand, at low initial concentrations of dye molecules, i.e., (KC + 1)–1, a pseudo first-order rate expression is obtained [42], as shown in Equation (6). The equation is valid by assuming the driving force of degradation is constantly proportional to the dye concentration.

dC = k C, (6) − dt 1 where k1 corresponds to the first-order rate constant. Integrating the equation under the two boundary conditions yields C  ln 0 = k t, (7) C 1 ln 2 0.6932 t = = . (8) 1/2 k k

The linear region can be obtained from the plot of ln(C0/C) vs. irradiation time, in which the slope gives the rate constant of photodegradation. This model is the most common one used to represent the entire photodegradation process. Here, the half-life is derived from Equation (8). Obviously, the t1/2 of the first-order model is independent of the dye concentration. Conversely, Kumara et al. reported that the photodegradation of AO over ZnO photocatalysts followed second-order kinetics [43]. A second-order reaction in which a single reactant is involved is characterized by the chemical reaction (2 C products). → At equilibrium, the second-order kinetics depends on the amount of dye molecules adsorbed on the photocatalyst surface, which is calculated as follows:

dC = k C2. (9) − dt 2 Similarly, by integrating the equation under the two boundary conditions, the second-order rate constant (k2), as well as t1/2, can be obtained. The second-order t1/2 increases as the initial concentration is decreased. 1 1 = k2t; (10) C − C0 1 t1/2 = . (11) kC0

3.5. Quantum Yield of Photodegradation The photodegradation efficiency is difficult to directly compare with other reported values, since the photodegradation rate constant is affected by various operational effects (see Section4). In order to objectively estimate the efficiency of the photodegradation reaction, quantum yield (φ) is used and defined as the number of reacted charge carries for dye molecule decomposition per absorbed photon. The apparent quantum yield (AQY) on the other hand is calculated by dividing the number of charge carries participating in degrading dye molecules by the incident photons. Because the absorbed photons are a certain fraction of the incident photons, the φ value is basically higher than the AQY. Bora et al. estimated the AQY of MB photodegradation over Au nanoparticle-decorated ZnO nanorods Catalysts 2019, 9, 430 11 of 32

(Au–ZnO NRs) [44]. The morphology of Au–ZnO is shown in Figure4a. It was reported that the photodegradation of MB is initiated by accepting one electron to form semi-reduced MB, followed by further accepting one electron to produce LMB. As a result, two electrons are required for the complete degradation of MB molecules, and the AQY of MB photodegradation can be calculated as follows:

2 number o f MB molecules AQY(%) = × 100% number o f incident photons × (12) = 2nMBNAhc 100%, PlightSλt ×

1 where nMB (mol) stands for the amount of MB degraded during the irradiation period, NA (mol− ) is 2 2 Avogadro’s constant, Plight (W/m ) is the incident power density, S (m ) is the irradiation area, t (s) is irradiation time, h is Planck’s constant, and λ is the wavelength of incident light. As shown in Figure4b, all the AQY values of Au–ZnO from 300 to 600 nm were higher than those of bare ZnO NRs and MB only, which was ascribed to the enhanced charge separation at the Au–ZnO interface. The highest AQY value obtained from Au–ZnO exceeded 30% in the UV region, while the bare ZnO NRs showed fairly low photodegradation activity in the visible region and almost no activity at wavelength longer than 450 nm. On the other hand, Au–ZnO NRs showed a six-fold increase of AQY over bare ZnO NRs in the visible region, which can be attributed to the plasmonic effect of Au. In addition, the BiOBr/Bi2O3 heterostructures were demonstrated for photodegradation of an RhB/p-cresol mixture [45]. The SEM image showed that the composites comprised BiOBr platelets arranging in whorls perpendicular to the Bi2O3 surface (Figure4c). Under blue light-emitting diode (LED) light (430–470 nm) irradiation, the AQY value of p-cresol photodegradation in the mixture was substantially higher than the value of RhB due to the carrier transfer from RhB to the co-adsorbed p-cresol. Note that there is no photobleaching

Catalystsof RhB 2019 as the, 9, x RhB FOR isPEER unable REVIEW to absorb blue light (Figure4d). 11 of 32

FigureFigure 4. 4. ((aa)) SEM SEM image image of of Au–ZnO Au–ZnO and and ( (bb)) AQY AQY of of MB MB photodegradati photodegradationon on on MB MB only, only, ZnO ZnO NRs, NRs, andand Au–ZnO NRs inin thethe wavelength wavelength range range from from 300–600 300–600 nm nm (reproduced (reproduced with with permission permission from from [44]. [44]Copyright. Copyright Nature Nature Publishing Publishing Group, Group, 2016). 2016.). (c) SEM (c) SEM image image of BiOBr of BiOBr/Bi/Bi2O3 2compositesO3 composites and and (d) ( thed) thecorresponding corresponding AQY AQY of RhBof RhB (red) (red) and andp-cresol p-cresol (green) (green) photodegradation photodegradation under under blue, blue, green, green, and and red redlight-emitting light-emitting diode diode (LED) (LED) light light irradiation irradiation (reproduced (reproduced with with permission permission from from [45]. Copyright[45]. Copyright Royal RoyalSociety Society of Chemistry, of Chemistry, 2017). 2017.).

4. Factors influencing the photodegradation reaction A heterogeneous photocatalytic reaction is composed of two consecutive steps. Firstly, dye molecules interact with the photocatalyst and are adsorbed onto its surface; then, the photodegradation of the dye commences. The overall efficiency of a photocatalysis system is closely dependent on operational parameters that dictate the adsorption and photodegradation of dye molecules. Herein, possible factors affecting the photodegradation reactions are discussed in detail.

4.1. Interaction between dye molecules and photocatalysts Adsorption is the initial step of the photodegradation reaction prior to the decoloration of dye molecules, which is, thus, an important process for initiating the photodegradation reaction. Both the surface of the photocatalyst and the structure of the dye molecule affect their interactions. Two of the most important interactions between the dye molecules and the photocatalysts are direct bonding and electrostatic interactions. Chemical bonding between functional groups of the dyes and the surface sites of the photocatalysts is a strong interaction to anchor dyes on to photocatalysts [47,48]. For example, RhB with a carboxylic group was demonstrated to bond onto the surface hydroxyls of TiO2 via an esterification reaction [48]. On the other hand, dye adsorption via electrostatic interaction depends on the nature of dyes, surface properties of photocatalysts, and solution pH. Essentially, the dye adsorption is determined by the strength of the ionic interactions between photocatalysts and dye molecules. An aqueous solution containing salts has a certain value of ionic strength (I), which is defined as

1 1 𝐼= 2 𝐶𝑧 𝐶𝑧 ⋯𝐶𝑧 = 2 𝐶𝑧 , (14) where 𝑧 is the charge of the salts, and 𝐶 is molar concentration. Higher ionic strength of the solution is obtained as the concentration of salts increases. Chen et al. demonstrated the influence of ionic strength on the adsorption of PR dye by tuning the concentration of NaCl [49]. The results

Catalysts 2019, 9, 430 12 of 32

In addition, Shams-Ghahfarokhi et al. reported the calculation of the φ value from the first-order rate constant of photodegradation process [15,46] using the following equation:

k φ = 1 , (13) 2.303 I ε l × 0,λ × λ × 1 1 1 where k1 (s− ) stands for the photodegradation first-order rate constant, Io,λ (Einstein l− s− ) is the 1 1 incident intensity at wavelength λ, ελ (cm− M− ) represents the molar absorptivity at wavelength λ, and l states the cell path length (cm).

4. Factors Influencing the Photodegradation Reaction A heterogeneous photocatalytic reaction is composed of two consecutive steps. Firstly, dye molecules interact with the photocatalyst and are adsorbed onto its surface; then, the photodegradation of the dye commences. The overall efficiency of a photocatalysis system is closely dependent on operational parameters that dictate the adsorption and photodegradation of dye molecules. Herein, possible factors affecting the photodegradation reactions are discussed in detail.

4.1. Interaction between Dye Molecules and Photocatalysts Adsorption is the initial step of the photodegradation reaction prior to the decoloration of dye molecules, which is, thus, an important process for initiating the photodegradation reaction. Both the surface of the photocatalyst and the structure of the dye molecule affect their interactions. Two of the most important interactions between the dye molecules and the photocatalysts are direct bonding and electrostatic interactions. Chemical bonding between functional groups of the dyes and the surface sites of the photocatalysts is a strong interaction to anchor dyes on to photocatalysts [47,48]. For example, RhB with a carboxylic group was demonstrated to bond onto the surface hydroxyls of TiO2 via an esterification reaction [48]. On the other hand, dye adsorption via electrostatic interaction depends on the nature of dyes, surface properties of photocatalysts, and solution pH. Essentially, the dye adsorption is determined by the strength of the ionic interactions between photocatalysts and dye molecules. An aqueous solution containing salts has a certain value of ionic strength (I), which is defined as 1  1 X I = C z2 + C z2 + ... + C z2 = C z2, (14) 2 1 1 2 2 n n 2 i i where zi is the charge of the salts, and Ci is molar concentration. Higher ionic strength of the solution is obtained as the concentration of salts increases. Chen et al. demonstrated the influence of ionic strength on the adsorption of PR dye by tuning the concentration of NaCl [49]. The results showed a decreased adsorption capacity upon increasing the ionic strength because the adsorption of charged moieties competed with that of dye molecules or adsorbents in the solution. Similar results were also published [50–52]. Furthermore, the variation of pH value in the solution modifies the electrical double layer of the photocatalysts, which is composed of the charged surface of the photocatalyst and dye molecules in solution. Thus, a high adsorption capacity can be found when anionic or cationic dyes are respectively adsorbed on the photocatalyst surface at acidic or basic pH. The electrical double layer refers to the two layers between the photocatalyst and the bulk solution. Here, the region with dye molecules adsorbed onto the surface-charged photocatalyst is called the Stern layer. The surface-charged photocatalysts create an electrostatic field which affects the dye molecules in the solution, forming the first layer of the electrical double layer. The second diffusion layer is loosely associated with the photocatalyst and is composed of dye molecules that diffuse in the solution under the influence of electrostatic attraction. The electric potential at this plane is called the zeta potential, while the point of zero charge (pzc) forms where the zeta potential is 0. Depending on the solution pH, either positive or negative charges can be + formed on a surface, since H and OH− are the charge-determining ions for most surfaces. Catalysts 2019, 9, 430 13 of 32

Amphoteric characteristics were observed in many photocatalyst materials, for example, g-C3N4 [53], TiO2 [54], and most metal oxides. The formation of metal hydroxyl groups (M–OH) is attributed to the adsorption of H2O molecules and dissociation of OH− groups at surface metal sites. When the solution pH is higher than the pzc of photocatalysts, their surface is negatively charged; it is positively charged at pH value < pzc of the photocatalysts. The equilibrium of amphoteric metal hydroxides under acidic and alkali conditions can be considered as

M-OH + H+ M-OH H+ M H O+, (15) → − → − 2

M-OH + OH− M O− + H O. (16) → − 2 Therefore, at lower pH, protonation of the photocatalyst creates a surface with a positive charge, making the photocatalyst behave as a strong Lewis acid [55]. The anionic dye, with strongly ionized anionic groups, serves as a strong Lewis base and can be readily adsorbed on the positively charged photocatalyst surface, as shown in Figure5a. This adsorption process is not favorable for negatively charged photocatalysts because of electrostatic repulsion [56], giving rise to negligible adsorption and a subsequent low degradation rate when pH > pzc of the photocatalyst. Bourikas et al. reported that the adsorption of AO7 on the surface of TiO2 cannot occur at a pH value higher than 7 [57] due to the electrostatic repulsion arising from the negative sulfonic group of the azo dye. Similar results were also observed for other anionic dye systems [58–60]. On the other hand, cationic dyes preferred to adsorb on the negatively charged photocatalysts in alkaline media. Bubacz et al. observed an increased photodegradation rate of MB on anatase TiO2 with an increase in pH [61]. Fan et al. demonstrated that polyacrylonitrile fiber–hyperbranched polyethylenimine (PANF-g-HPEIs) activated with solutions at different pH may selectively adsorb cationic or anionic dyes [62]. Figure5b,c show that PANF-g-HPEIs pre-treated with solution at pH = 5 can selectively adsorb the anionic MO dye from an MB/MO mixture, while PANF-g-HPEIs can selectively adsorb the cationic MB molecules after being pre-treated with solutionCatalysts 2019 at, pH 9, x =FOR10. PEER REVIEW 13 of 32

Figure 5.5. ((aa)) Model Model of of the the absorption absorption of cationicof cationic and and anionic anionic dye moleculesdye molecules on the on photocatalyst the photocatalyst surface undersurface acidic under and acidic alkali and conditions. alkali conditions. Evolution Evolution of UV–visible of UV–visible absorption absorption spectra spectra of MO /ofMB MO/MB mixed solutionmixed solution in the presence in the ofpresence PANF-g-HPEIs of PANF-g-HPEIs pre-treated atpre-treated (b) pH = 5at and (b ()c )pH pH = 105 and (reproduced (c) pH with= 10 permission(reproduced from with [ 62permission]. Copyright from Elsevier [62]. Copyright Science Publishers, Elsevier Sc 2015).ience Publishers, 2015.).

Zhao etet al. investigatedal. investigated the photodegradation the photodegrada efficiencytion forefficiency RhB-sensitized for RhB-sensitized BiOCl nanostructures BiOCl atnanostructures pH 3.36 and pHat 11.08pH [633.36], asand shown pH in Figure11.08 6[63],. Note as that shown the self-photosensitization in Figure 6. Note ofthat cationic the RhBself-photosensitization adsorbed on negatively of cationic charged RhB BiOCl adsorbed nanostructures on negatively can degradecharged MOBiOCl and nanostructures RhB molecules can at pHdegrade 3.36 andMO pHand 11.08 RhB (seemolecules Section at 5.2 pH). 3.36 Both and RhB pH and 11 MO.08 (see display Section effi 5.2).cient Bo photodegradationth RhB and MO display within efficient photodegradation within 5 min of visible irradiation at pH 3.36. However, the photodegradation of both RhB and MO was suddenly depressed in an alkali solution with pH 11.08, indicating the pH of solution strongly affects the electrical double layer, even though RhB can adsorb onto BiOCl nanostructures at both pH 3.36 and pH 11.08. In an acidic solution with pH 3.36, the presence of a high concentration of H+ ions may push the cationic RhB molecules into the Stern layer, because of the electrostatic repulsion between H+ ions and cationic RhB molecules in the diffusion layer, thus improving the electron transfer from RhB to BiOCl and the following photodegradation reaction. On the other hand, the electrostatic attraction of the increased OH− ions to the cationic RhB molecules causes more RhB to stay in the diffusion layer in an alkali solution with pH 11.08, hindering the electron transfer from RhB to BiOCl and resulting in less efficient photodegradation.

Figure 6. Photodegradation of MO on RhB-sensitized BiOCl hierarchical nanostructures at (a) pH = 3.36 and (b) pH = 11.08 (reproduced with permission from [63]. Copyright Royal Society of Chemistry, 2016.).

Chen et al. further utilized the amphoteric properties of organosilica nanoparticles (OSNPs) to recover anionic PR molecules [49], based on the surface charge change of the OSNPs in acidic and

Catalysts 2019, 9, x FOR PEER REVIEW 13 of 32

Figure 5. (a) Model of the absorption of cationic and anionic dye molecules on the photocatalyst surface under acidic and alkali conditions. Evolution of UV–visible absorption spectra of MO/MB mixed solution in the presence of PANF-g-HPEIs pre-treated at (b) pH = 5 and (c) pH = 10 (reproduced with permission from [62]. Copyright Elsevier Science Publishers, 2015.).

Zhao et al. investigated the photodegradation efficiency for RhB-sensitized BiOCl nanostructures at pH 3.36 and pH 11.08 [63], as shown in Figure 6. Note that the self-photosensitization of cationic RhB adsorbed on negatively charged BiOCl nanostructures can Catalystsdegrade2019 MO, 9, and 430 RhB molecules at pH 3.36 and pH 11.08 (see Section 5.2). Both RhB and MO display14 of 32 efficient photodegradation within 5 min of visible irradiation at pH 3.36. However, the 5photodegradation min of visible irradiation of both RhB at pHand 3.36. MO was However, suddenly the depressed photodegradation in an alkali of bothsolution RhB with and pH MO 11.08, was suddenlyindicating depressed the pH of in solution an alkali strongly solution withaffects pH th 11.08,e electrical indicating double the pHlayer, of solutioneven though strongly RhB aff ectscan theadsorb electrical onto BiOCl double nanostructures layer, even though at both RhB pH can 3.36 adsorb and pH onto 11.08. BiOCl In an nanostructures acidic solution at with both pH pH 3.36, 3.36 andthe presence pH 11.08. of In a anhigh acidic concentration solution with of H pH+ ions 3.36, may the presencepush theof cationic a high RhB concentration molecules of into H+ theions Stern may pushlayer,the because cationic of RhB the moleculeselectrostatic into repulsion the Stern between layer, because H+ ions of the and electrostatic cationic RhB repulsion molecules between in the H+ ionsdiffusion and cationiclayer, RhBthus moleculesimproving in the dielectronffusion layer,transfer thus from improving RhB to the BiOCl electron and transfer the following from RhB tophotodegradation BiOCl and the following reaction. photodegradationOn the other hand, reaction. the electrostatic On the other attraction hand, of the the electrostatic increased attractionOH− ions to the cationic RhB molecules causes more RhB to stay in the diffusion layer in an alkali solution of the increased OH− ions to the cationic RhB molecules causes more RhB to stay in the diffusion layer inwith an alkalipH 11.08, solution hindering with pH the 11.08, electron hindering transfer the electronfrom RhB transfer to BiOCl from and RhB resulting to BiOCl in and less resulting efficient in lessphotodegradation. efficient photodegradation.

Figure 6.6. Photodegradation ofof MO MO on on RhB-sensitized RhB-sensitized BiOCl BiOCl hierarchical hierarchical nanostructures nanostructures at (a at) pH (a)= pH3.36 = and (b) pH = 11.08 (reproduced with permission from [63]. Copyright Royal Society of Chemistry, 2016). 3.36 and (b) pH = 11.08 (reproduced with permission from [63]. Copyright Royal Society of Chemistry, 2016.). Chen et al. further utilized the amphoteric properties of organosilica nanoparticles (OSNPs) to recover anionic PR molecules [49], based on the surface charge change of the OSNPs in acidic and Chen et al. further utilized the amphoteric properties of organosilica nanoparticles (OSNPs) to alkali solutions. The morphology of the OSNP is shown in Figure7a,b, which displays the apparent recover anionic PR molecules [49], based on the surface charge change of the OSNPs in acidic and color of OSNP changing from white to pink. Interestingly, by controlling the pH of the solvent, PR was desorbed from the surface of the OSNPs in an NaOH solution, and the color of the OSNP returned to white, due to the negatively charged surface of the OSNP in NaOH solution. As can be seen in Figure7c, the zeta potential of OSNPs returned to its initial value after PR desorption, which confirmed that OSNPs were stable during adsorption and desorption. Additionally, Figure7d shows that adsorption/desorption tests of anionic PR molecules can be repeated for 10 cycles, further pointing out the high stability of OSNPs. However, Kong et al. found a decline of the MB photodegradation reaction rates at high alkali pH [64], as shown in Figure8. Although electrostatic attraction occurs between the cationic MB molecules and negatively charged surface of Ta-doped ZnO at alkali pH, the otherwise Coulombic repulsion of the negatively charged Ta-doped ZnO surface against the OH− ions results in the breakage of hydroxylation of the ZnO surface. The Coulombic repulsion was reported for other cationic dyes (MB, RhB) at pH 9.5, which can reduce the number of OH radicals and thereby decrease the · photodegradation rate [65]. The steric structure of dyes also affects the adsorption process. Both MB and RhB are cationic dyes; however, research shows that MB exhibits a higher photodegradation rate than RhB, which can be attributed to the steric repulsion of the carboxylate anions in RhB which inhibits the extent of adsorption [65]. Catalysts 2019, 9, x FOR PEER REVIEW 14 of 32 alkali solutions. The morphology of the OSNP is shown in Figure 7a,b, which displays the apparent color of OSNP changing from white to pink. Interestingly, by controlling the pH of the solvent, PR was desorbed from the surface of the OSNPs in an NaOH solution, and the color of the OSNP returned to white, due to the negatively charged surface of the OSNP in NaOH solution. As can be seen in Figure 7c, the zeta potential of OSNPs returned to its initial value after PR desorption, which confirmed that OSNPs were stable during adsorption and desorption. Additionally, Figure 7d shows that adsorption/desorption tests of anionic PR molecules can be repeated for 10 cycles, further pointing out the high stability of OSNPs.

Catalysts 2019, 9, x FOR PEER REVIEW 14 of 32 alkali solutions. The morphology of the OSNP is shown in Figure 7a,b, which displays the apparent color of OSNP changing from white to pink. Interestingly, by controlling the pH of the solvent, PR was desorbed from the surface of the OSNPs in an NaOH solution, and the color of the OSNP returned to white, due to the negatively charged surface of the OSNP in NaOH solution. As can be seen in Figure 7c, the zeta potential of OSNPs returned to its initial value after PR desorption, which confirmed that OSNPs were stable during adsorption and desorption. Additionally, Figure 7d shows that adsorption/desorption tests of anionic PR molecules can be repeated for 10 cycles, further Catalysts 2019, 9, 430 15 of 32 pointing out the high stability of OSNPs.

Figure 7. (a) TEM image for OSNPs. (b) Color change of OSNP after adsorption (Ads.) and desorption (Des.) of phenol red (PR). (c) Zeta potential of original OSNP and after three cycles of dye adsorption (A)/desorption (D). (d) PR adsorption/desorption recycle test for OSNP (reproduced with permission from [49]. Copyright American Chemical Society, 2017.).

However, Kong et al. found a decline of the MB photodegradation reaction rates at high alkali pH [64], as shown in Figure 8. Although electrostatic attraction occurs between the cationic MB molecules and negatively charged surface of Ta-doped ZnO at alkali pH, the otherwise Coulombic repulsion of the negatively charged Ta-doped ZnO surface against the OH− ions results in the breakage of hydroxylation of the ZnO surface. The Coulombic repulsion was reported for other cationic dyes (MB, RhB) at pH 9.5, which can reduce the number of ·OH radicals and thereby Figure 7. a b decreaseFigure the 7. ( photodegradation()a) TEM TEM image image for OSNPs.for rateOSNPs. ([65].) Color (b The) Color change steric change of structure OSNP of afterOSNP of adsorption dyes after alsoadsorption (Ads.) affects and (Ads.) desorptionthe adsorption and (Des.) of phenol red (PR). (c) Zeta potential of original OSNP and after three cycles of dye adsorption process.desorption Both (Des.)MB and of phenol RhB are red cationic(PR). (c) Zetadyes; potential however, of original research OSNP shows and afterthat threeMB exhibitscycles of dyea higher (A)/desorption (D). (d) PR adsorption/desorption recycle test for OSNP (reproduced with permission photodegradationadsorption (A)/desorption rate than (D).RhB, (d )which PR adsorption/desorption can be attributed torecycle the sterictest for repulsion OSNP (reproduced of the carboxylate with from [49]. Copyright American Chemical Society, 2017). anionspermission in RhB whichfrom [49] inhibits. Copyright the extent American of adsorption Chemical Society, [65]. 2017.).

However, Kong et al. found a decline of the MB photodegradation reaction rates at high alkali pH [64], as shown in Figure 8. Although electrostatic attraction occurs between the cationic MB molecules and negatively charged surface of Ta-doped ZnO at alkali pH, the otherwise Coulombic repulsion of the negatively charged Ta-doped ZnO surface against the OH− ions results in the breakage of hydroxylation of the ZnO surface. The Coulombic repulsion was reported for other cationic dyes (MB, RhB) at pH 9.5, which can reduce the number of ·OH radicals and thereby decrease the photodegradation rate [65]. The steric structure of dyes also affects the adsorption process. Both MB and RhB are cationic dyes; however, research shows that MB exhibits a higher photodegradation rate than RhB, which can be attributed to the steric repulsion of the carboxylate anions in RhB which inhibits the extent of adsorption [65].

Figure 8. (a) TEM image and (b) pH effect on the photodegradation of MB over Ta-doped ZnO (reproduced with permission from [64]. Copyright Elsevier Science Publishers, 2010).

4.2. Operational Parameters

4.2.1. Initial Dye Concentration The initial concentration of organic dyes strongly affects the photodegradation reaction. Saquib et al. demonstrated the photodegradation of gentian violet (also known as CV) with varying concentrations from 0.18 mM to 0.5 mM [66]. The photodegradation rate and TOC increased with the concentration of gentian violet up to 0.25 mM and then declined, as shown in Figure9a. Kumar et al. reported that the degradation efficiency of VBB over TiO /polyaniline (PAni)/graphene oxide (GO) decreased with an 2 increase in initial dye concentration [67], as shown in Figure9b. This phenomenon was studied for other dye molecules, including the AR14/TiO2 [3] and other systems [13,14,68,69]. Figure9c shows the photodegradation of three different dye molecules (MG, MB, and RhB) over Fe3O4/reduced graphene oxide (rGO) photocatalysts [70]. The photodegradation efficiency also decreased with the increase in dye concentration. Catalysts 2019, 9, x FOR PEER REVIEW 15 of 32

Figure 8. (a) TEM image and (b) pH effect on the photodegradation of MB over Ta-doped ZnO (reproduced with permission from [64]. Copyright Elsevier Science Publishers, 2010.).

4.2. Operational parameters

4.2.1. Initial dye concentration The initial concentration of organic dyes strongly affects the photodegradation reaction. Saquib et al. demonstrated the photodegradation of gentian violet (also known as CV) with varying concentrations from 0.18 mM to 0.5 mM [66]. The photodegradation rate and TOC increased with the concentration of gentian violet up to 0.25 mM and then declined, as shown in Figure 9a. Kumar et al. reported that the degradation efficiency of VBB over TiO2/polyaniline (PAni)/graphene oxide (GO) decreased with an increase in initial dye concentration [67], as shown in Figure 9b. This phenomenon was studied for other dye molecules, including the AR14/TiO2 [3] and other systems [13,14,68,69]. Figure 9c shows the photodegradation of three different dye molecules (MG, MB, and RhB) over Fe3O4/reduced graphene oxide (rGO) photocatalysts [70]. The photodegradation efficiency Catalysts 2019, 9, 430 16 of 32 also decreased with the increase in dye concentration.

FigureFigure 9. 9. EffectEffect of dye concentrationconcentration forfor ( a()a gentian) gentian violet violet photodegradation photodegradation on TiOon TiO2 (reproduced2 (reproduced with withpermission permission from from [66]. [66] Copyright. Copyright Elsevier Elsevier Science Scie Publishers,nce Publishers, 2003); 2003.); (b) VBB (b photodegradation) VBB photodegradation on TiO2 , onTiO TiO2/PAni,2, TiO and2/PAni, TiO2 /andPAni TiO/GO2/PAni/GO (reproduced (reproduced with permission with frompermission [67]. Copyright from [67] John. Copyright Wiley and John Sons, c Wiley2018); and ( ) MG,Sons, MB, 2018.); and RhB(c) MG, photodegradation MB, and RhB onphotodegradation Fe3O4/rGO. (reproduced on Fe3O4 with/rGO. permission (reproduced from with [70 ]. permissionCopyright from Royal [70] Society. Copyright of Chemistry, Royal 2016).Society Insets of Chemistry, of (b) and 2016.) (c) are Insets TEM imagesof (b) and of TiO (c) 2are/PAni TEM/GO and Fe O /rGO, respectively. images of3 TiO4 2/PAni/GO and Fe3O4/rGO, respectively. The adsorption of dye molecules on the photocatalyst surface affects its ability of photon absorption The adsorption of dye molecules on the photocatalyst surface affects its ability of photon and the subsequent generation of reactive radicals, posing a significant impact on the photodegradation absorption and the subsequent generation of reactive radicals, posing a significant impact on the rate. As the initial dye concentration increased, several monolayers of adsorbed dye formed, resulting photodegradation rate. As the initial dye concentration increased, several monolayers of adsorbed in more adsorbed dye molecules available for photodegradation. Until reaching the critical level, dye formed, resulting in more adsorbed dye molecules available for photodegradation. Until constant reaction rate is obtained because the surface is completely covered. The photodegradation reaching the critical level, constant reaction rate is obtained because the surface is completely rate is, however, decreased with further increases in dye concentration. A high amount of adsorbed covered. The photodegradation rate is, however, decreased with further increases in dye dye may have an inhibitive effect on the reactions between dye molecules and reactive radicals [71], concentration. A high amount of adsorbed dye may have an inhibitive effect on the reactions since the excessive dye concentration may hinder light penetration to the solution [72] and fewer between dye molecules and reactive radicals [71], since the excessive dye concentration may hinder photons can reach the photocatalyst surface. Therefore, the generation of charge carriers and reactive light penetration to the solution [72] and fewer photons can reach the photocatalyst surface. radicals is simultaneously reduced, resulting in a decrease of photodegradation efficiency.

4.2.2. Light Intensity It was shown that the photodegradation rate increases linearly with increasing light intensity at low light intensity. Figure 10a shows that the photodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D) increased as light intensity increased from 100 to 600 lx [73], with the efficiency proportional to the light intensity. In Figure 10b, the photodegradation of benzene on nitrogen-doped TiO2 was still enhanced by increasing light intensity, but the efficiency was nonlinearly increased [74]. With further increases to higher light intensity, the photodegradation rate became independent of light intensity. It is proposed that, at low light intensity, the separation of photoexcited electrons and holes competes with their recombination, thus impeding the generation of reactive radicals. The electron–hole generation becomes the predominant process as light intensity increases, resulting in a higher photodegradation rate. However, the total active sites for photodegradation remains constant and, therefore, the reaction rate shows a maximum value, even though the light intensity continues to increase [75]. Catalysts 2019, 9, x FOR PEER REVIEW 16 of 32

Therefore, the generation of charge carriers and reactive radicals is simultaneously reduced, resulting in a decrease of photodegradation efficiency.

4.2.2. Light intensity It was shown that the photodegradation rate increases linearly with increasing light intensity at low light intensity. Figure 10a shows that the photodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D) increased as light intensity increased from 100 to 600 lx [73], with the efficiency proportional to the light intensity. In Figure 10b, the photodegradation of benzene on nitrogen-doped TiO2 was still enhanced by increasing light intensity, but the efficiency was nonlinearly increased [74]. With further increases to higher light intensity, the photodegradation rate became independent of light intensity. It is proposed that, at low light intensity, the separation of photoexcited electrons and holes competes with their recombination, thus impeding the generation of reactive radicals. The electron–hole generation becomes the predominant process as light intensity increases, resulting in a higher photodegradation rate. However, the total active sites for photodegradation remains constant and, therefore, the reaction rate shows a maximum value, even though the light intensity continues toCatalysts increase2019 [75]., 9, 430 17 of 32

FigureFigure 10. 10. EffectEffect of oflight light intensity intensity on the on photodegradation the photodegradation of (a) of 2,4-D (a) 2,4-Dwith low with light low intensity light intensity (100– 600(100–600 lx) (reproduced lx) (reproduced with permission with permission from [73] from. Copyright [73]. Copyright Elsevier Elsevier Scienc Sciencee Publishers, Publishers, 2005.), 2005), and and(b) (b) benzene with high light intensity (36.7 104–75.1 104 lx) (reproduced with permission from [74]. benzene with high light intensity (36.7×10×4–75.1×10×4 lx) (reproduced with permission from [74]. CopyrightCopyright MDPI, MDPI, 2018.). 2018). 4.2.3. Reaction Temperature 4.2.3. Reaction temperature Not only do initial dye concentration and light intensity possess optimal conditions, but an optimal Not only do initial dye concentration and light intensity possess optimal conditions, but an temperature range also exists for photodegradation. Low temperature favors the adsorption of the optimal temperature range also exists for photodegradation. Low temperature favors the adsorption reactant, which is a spontaneous exothermic process, whereas the apparent activation energy increases of the reactant, which is a spontaneous exothermic process, whereas the apparent activation energy as the temperature decreases close to 0 ◦C. Low temperature also favors the adsorption of the final increases as the temperature decreases close to 0 °C. Low temperature also favors the adsorption of product, albeit while decreasing the number of active sites. Therefore, compared to photodegradation the final product, albeit while decreasing the number of active sites. Therefore, compared to and the adsorption of reactants, the slower desorption of product inhibits the reaction and serves as photodegradation and the adsorption of reactants, the slower desorption of product inhibits the the rate-limiting step under low reaction temperatures. In contrast, when the temperature increases up reaction and serves as the rate-limiting step under low reaction temperatures. In contrast, when the to the boiling point of the solvent (water for most of the cases), the exothermic adsorption of reactants temperature increases up to the boiling point of the solvent (water for most of the cases), the becomes disfavored, thus limiting the photodegradation reaction [76]. Charge-carrier recombination is exothermic adsorption of reactants becomes disfavored, thus limiting the photodegradation reaction also substantially promoted [77] as the reaction temperature exceeds 80 ◦C. At higher temperatures, [76]. Charge-carrier recombination is also substantially promoted [77] as the reaction temperature the enhanced kinetic energy of dye molecules might allow them escape from the the photocatalyst exceeds 80 °C. At higher temperatures, the enhanced kinetic energy of dye molecules might allow surface [78], leading to decreased photodegradation efficiency. Thus, the adsorption of dye molecules them escape from the the photocatalyst surface [78], leading to decreased photodegradation becomes the limiting step at high temperatures. As a result, reaction temperatures between 20 and efficiency. Thus, the adsorption of dye molecules becomes the limiting step at high temperatures. As 80 ◦C[15] are considered as the desired temperature for the effective photodegradation of dye molecules. a result, reaction temperatures between 20 and 80 °C [15] are considered as the desired temperature for4.3. the Intrinsic effective Properties photodegradation of Photocatalysts of dye molecules.

4.3. IntrinsicThe photodegradation properties of photocatalysts efficiency can be enhanced by increasing the photocatalyst amount, which is a feature of heterogeneous photocatalysis. The increased photocatalyst amount provides more active sites for the discoloration of dye solution. However, beyond a certain amount, the reaction solution turns into turbid and is subjected to limited light penetration to the photocatalyst surface, leading to the inhibition of the photodegradation reaction. In addition, the intrinsic properties of the photocatalyst, such as light absorption range, redox potential, charge separation efficiency, and stability, strongly affect the photodegradation activity. The fast charge recombination of a single-component material limits its photodegradation efficiency. Additionally, the single-component photocatalyst cannot simultaneously satisfy the requirement of large redox potential and wide light absorption. To improve the photocatalytic efficiency, a variety of studies were devoted to exploring new heterostructure systems and using them in the photodegradation reaction. Typical strategies include element doping, metal decoration, and semiconductor modification (type II and Z-scheme heterostructures), which can broaden the light absorption range to enhance the light utilization and inhibit charge recombination. Among the aforementioned factors, charge-carrier dynamics are crucial for determining the overall photocatalytic efficiency. It is, thus, of great importance to study the underlying charge-carrier dynamics in semiconductor photocatalysts. Time-resolved PL (TRPL) techniques were demonstrated to be a powerful tool to observe the charge transfer processes of semiconductor heterostructures. In this Catalysts 2019, 9, 430 18 of 32 section, the implication of charge-carrier dynamics in the photodegradation efficiency for the three most relevant heterostructure systems, i.e., metal–semiconductor, type II semiconductor–semiconductor, and Z-scheme semiconductor–metal–semiconductor heterostructures, is discussed.

4.3.1. Modification with Metals Au–CdS metal–semiconductor nanocrystals were demonstrated to photodegrade RhB molecules [79]. Using TRPL to study the photoexcited charge transfer kinetics, Figure 11a,b display the TRPL spectra for two Au–CdS nanocrystals with different shell thicknesses (14.0 nm and 18.6 nm). Compared with their CdS counterpart, obtained by etching Au cores, a fast decay was found in Au–CdS samples, indicating electron transfer from CdS to Au. This difference became more noticeable for Au–CdS with shell thickness increasing to 18.6 nm, suggesting much more pronounced electronic interaction between CdS and Au. By further analysis with biexponential kinetics, the interfacial charge transfer rate constant (ket) was estimated. In addition, the photodegradation rate constant (kRhB) changed with CdS thickness in the core–shell nanocrystals, as shown in Figure 11c. It was enhanced with increasing shell thickness due to the raised ratio of CdS to Au, which led to greater light absorption and, thus, generation of more charge carriers. The correlation among kRhB, ket, and CdS shell thickness is shown in Figure 11d. The change in kct with CdS shell thickness was consistent with the trend of kRhB, revealing that efficient charge separation can provide a hole-enriched CdS shell Catalystsfor the 2019 photodegradation, 9, x FOR PEER REVIEW reaction and further enhance the photodegradation efficiency. 18 of 32

FigureFigure 11. TRPLTRPL spectraspectra (dots) (dots) and and fitting fitting results results (solid curves)(solid curves) of Au–CdS of andAu–CdS pure CdSand nanocrystalspure CdS nanocrystalswith shell thicknesses with shell of thicknesses (a) 14.0 nm of and (a ()b 14.0) 18.6 nm nm and (inset (b) shows 18.6 nm the (inset corresponding shows the TEM corresponding images, with TEMscale images, bar of 20 with nm). scale (c) RhB bar photodegradationof 20 nm). (c) RhB under photodegradation visible irradiation under and visible (d) correlations irradiation of andket (andd) correlationskRhB for Au–CdS of ket and nanocrystals kRhB for Au–CdS with di nanocrystalsfferent shellthicknesses with different (reproduced shell thicknesses with permission (reproduced from with [79 ]. permissionCopyright from American [79]. ChemicalCopyright Society, American 2010). Chemical Society, 2010.).

Moreover,Moreover, the the metal metal content content and and composition composition were were tuned tuned in in a a metal metal (Ag, (Ag, Au, Au, Pd)-decorated Pd)-decorated ZnOZnO system system for for photodegradation photodegradation of of MB MB in in ethanol ethanol [80]. [80]. The The morp morphologyhology of of ZnO–Au ZnO–Au with with different different AuAu contents contents is isshown shown in inFigure Figure 12a. 12 Thea. TheAu content Au content is obviously is obviously increasing increasing and was and measured was measured to be 0.6to at.%, be 0.6 1.0 at.%, at.%, 1.0 1.3 at.%, at.%, 1.3 2.0 at.%,at.%, 2.0and at.%, 2.3 at.%. and As 2.3 shown at.%. Asin Figure shown 12b,c, in Figure an optimal 12b,c, Au an optimalcontent for Au thecontent photodegradation for the photodegradation reaction was reaction found, was found,as excess as excess metal metal loading loading for for metal–semiconductor metal–semiconductor heterostructuresheterostructures may may compromise compromise the the effectiveness effectiveness of of the the overall overall charge charge separation. separation. Figure Figure 12d 12d also also indicates that the photodegradation rate of ZnO–Pd was higher than that of ZnO–Au, while ZnO– Ag was the worst. This is due to the most positive Fermi level potential of Pd. Larger differences between the CB of ZnO and the Fermi level (EF) of the metal result in stronger driving forces for interfacial charge transfer, giving rise to the most efficient charge separation for ZnO–Pd and the most effective photodegradation of MB molecules.

Catalysts 2019, 9, 430 19 of 32 indicates that the photodegradation rate of ZnO–Pd was higher than that of ZnO–Au, while ZnO–Ag was the worst. This is due to the most positive Fermi level potential of Pd. Larger differences between the CB of ZnO and the Fermi level (EF) of the metal result in stronger driving forces for interfacial charge transfer, giving rise to the most efficient charge separation for ZnO–Pd and the most effective Catalystsphotodegradation 2019, 9, x FOR ofPEER MB REVIEW molecules. 19 of 32

Figure 12. SEM observations for (a1)) pure pure ZnO and (a2–a–a66)) ZnO–Au ZnO–Au nanocrystals nanocrystals with with increasing increasing Au content from 0.1 at.% to 2.3 at.%. ( (bb)) Photodegradation Photodegradation of of MB MB on on relevant relevant photocatalysts under UV

irradiation. ((c)) CorrelationsCorrelations of ofk etketand andk MBkMBfor for ZnO–Au ZnO–Au with with di ffdifferenterent Au Au content content and and for ( dfor) ZnO–metal (d) ZnO– metal(Ag, Au, (Ag, Pd) Au, with Pd) di withfferent different metal content metal (reproducedcontent (reproduced with permission with permission from [80]. from Copyright [80]. Copyright American AmericanChemical Society,Chemical 2016). Society, 2016.).

4.3.2. Modification Modification with semiconductors Semiconductors CdS–CdSnO type-II heterostructures were also employed to investigate the effect of CdSnO CdS–CdSnO33 type-II heterostructures were also employed to investigate the effect of CdSnO33 content forfor thethe photodegradation photodegradation of RhBof RhB [81 ].[81]. Figure Figure 13a shows13a shows the direct the contactdirect contact of CdS andof CdS CdSnO and3, and the content of CdSnO was precisely controlled. The surface-decorated CdSnO acts as an CdSnO3, and the content of 3CdSnO3 was precisely controlled. The surface-decorated CdSnO3 3 acts as aneffi efficientcient electron electron scavenger scavenger for for CdS CdS because because of of its its lower lower CB CB level level (+ (+0.90.9 V V vs. vs. NHE)NHE) than the CB level of CdS ( 0.5 V vs. NHE), giving rise to the fast PL decay of CdS–CdSnO3, as shown in the level of CdS (−−0.5 V vs. NHE), giving rise to the fast PL decay of CdS–CdSnO3, as shown in the TRPL analysisTRPL analysis in Figure in Figure 13b. 13Asb. a As result, a result, the thephotoe photoexcitedxcited electrons electrons of of CdS CdS nanowires nanowires preferentially transferred to CdSnO nanocrystals, leaving photoexcited holes in the CdS domain to react with RhB transferred to CdSnO3 nanocrystals, leaving photoexcited holes in the CdS domain to react with RhB molecules. As shown in Figure 13c, the photodegradation rate was enhanced with CdSnO content molecules. As shown in Figure 13c, the photodegradation rate was enhanced with CdSnO3 content increasing from 1.25 at.% to 2.5 at.%, and then depressed with further increases in CdSnO content. increasing from 1.25 at.% to 2.5 at.%, and then depressed with further increases in CdSnO3 content. This observation was in accordance with their interfacialinterfacial charge transfer kinetics, indicating that an excess amount of CdSnO would reduce the overall charge separation efficiency and result in the excess amount of CdSnO3 would reduce the overall charge separation efficiency and result in the depressed photocatalytic efficiency efficiency,, as shown in Figure 1313d.d. The amounts affecting affecting the subsequent photodegradation performance were also proposed in ZnSe–ZnO [82], In O –TiO –Pt [83], and photodegradation performance were also proposed in ZnSe–ZnO [82], In22O33–TiO2–Pt [83], and Cu O–rGO [84] heterostructures. These observations indicate that the interfacial charge transfer Cu2O–rGO [84] heterostructures. These observations indicate that the interfacial charge transfer kinetics of the photocatalysts play an important role role for the photodegradation of of dye dye molecules. molecules. Moreover, the composition and facet effects of heterostructure systems such as TiO –Au@Cu S [85] Moreover, the composition and facet effects of heterostructure systems such as TiO22–Au@Cu77S4 [85] and ZnS–Cu O[86] also influence the photodegradation efficiency. and ZnS–Cu2O [86] also influence the photodegradation efficiency.

Catalysts 2019, 9, 430 20 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 20 of 32

FigureFigure 13. 13. ((a1–a4a1–a4)) TEM TEM images images for for CdS–CdSnO CdS–CdSnO33 withwith increasing increasing CdSnO CdSnO3 3contentcontent from from 1.25 1.25 at.% at.% to to 10.010.0 at.%. at.%. ( (bb)) TRPL TRPL analysis. analysis. ( (cc)) RhB RhB photodegradation underunder visiblevisible irradiation irradiation and and ( d(d) correlations) correlations of

ofket ketand andkRhB kRhBfor for CdS–CdSnO CdS–CdSnO3 with3 with increasing increasing CdSnO CdSnO3 content3 content (reproduced (reproduced with with permission permission from from [81]. [81]Copyright. Copyright Elsevier Elsevier Science Scie Publishers,nce Publishers, 2010). 2010.).

4.3.3.4.3.3. Modification Modification with with metals Metals and and semiconductors Semiconductors Na Ti O –Au–Cu O Z-scheme heterostructure nanobelts (denoted as ST–Au–Cu O NBs) were Na2-x2-xTi3O3 7–Au–Cu7 2O2 Z-scheme heterostructure nanobelts (denoted as ST–Au–Cu2O2 NBs) were demonstrateddemonstrated to to photodegrade photodegrade MB MB molecules molecules [87]. [87]. Figure Figure 14a,b 14a,b display display the the successful successful decoration decoration of of Au nanoparticles on ST and the further growth of Cu O on Au surface. By decreasing the volume Au nanoparticles on ST and the further growth of Cu2O2 on Au surface. By decreasing the volume of of the Cu2+–citrate precursor, the shell thickness of the grown Cu O can be tuned from 1.4 nm and the Cu2+–citrate precursor, the shell thickness of the grown Cu2O can2 be tuned from 1.4 nm and 1.2 nm1.2 nmto 1.1 to 1.1nm, nm, resulting resulting in inthe the modulation modulation of ofinte interfacialrfacial charge charge transfer transfer dynamics dynamics and, and, thus, thus, the the enhancement of photodegradation efficiency. As shown in Figure 14c, the ST–Au–Cu O Z-scheme NBs enhancement of photodegradation efficiency. As shown in Figure 14c, the ST–Au–Cu2 2O Z-scheme showed higher photodegradation efficiency than ST–Au and ST–Cu O type-II NBs, illustrating the NBs showed higher photodegradation efficiency than ST–Au and ST–Cu2 2O type-II NBs, illustrating thesuperiority superiority of Z-scheme of Z-scheme heterostructures heterostructures in photodegradation in photodegradation applications. applications. Meanwhile, Meanwhile, the highest the photodegradation efficiency was achieved for ST–Au–Cu O prepared with 50 µL of Cu2+–citrate, highest photodegradation efficiency was achieved for ST–Au–Cu2 2O prepared with 50 µL of Cu2+– which can be attributed to the most efficient charge transfer dynamics at the Cu O thickness of 1.1 nm. citrate, which can be attributed to the most efficient charge transfer dynamics at2 the Cu2O thickness As Figure 14d presents, the electron-scavenging rate constant (k ) for ST–Au–Cu O was enhanced of 1.1 nm. As Figure 14d presents, the electron-scavenging rate constantes (kes) for ST–Au–Cu2 2O was with decrease in Cu O shell thickness, which can be ascribed to the quantum size effect of Cu O. Since enhanced with decrease2 in Cu2O shell thickness, which can be ascribed to the quantum size 2effect of decreasing Cu O shell simultaneously expanded the bandgap of Cu O, its CB and VB respectively Cu2O. Since decreasing2 Cu2O shell simultaneously expanded the bandgap2 of Cu2O, its CB and VB respectivelyshifted toward shifted higher toward and lower higher potential, and lower giving potential, a larger giving driving a forcelarger of driving interfacial force electron of interfacial transfer for ST–Au–Cu O and thereby improving the photodegradation efficiency. electron transfer2 for ST–Au–Cu2O and thereby improving the photodegradation efficiency.

Catalysts 2019, 9, x FOR PEER REVIEW 21 of 32 Catalysts 2019, 9, 430 21 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 21 of 32

Figure 14. TEM images of (a) ST–Au and (b) ST–Au–Cu2O NBs. (c) Photodegradation of MB on relevant photocatalysts under visible irradiation (inset shows the TEM image of ST–Cu2O NBs). (d) FigureFigure 14. TEM imagesimages of of (a ()a ST–Au) ST–Au and and (b) ST(b–) AuST–Cu–Au2–CuONBs2O .(NBsc) Photodegradation. (c) Photodegradation of MB onof relevantMB on Correlations of kes and kMB with the amount of Cu2+–citrate and the driving force (−ΔG) of interfacial relevantphotocatalysts photocatalysts under visible under irradiation visible irradi (insetation shows (inset the TEMshows image the TEM of ST–Cu image2O of NBs). ST–Cu (d)2O Correlations NBs). (d) 2+ electronof kes and transferk with for the ST–Au–Cu amount of Cu2O (reproduced–citrate and 2+thewith driving permission force ( ∆fromG) of interfacial[87]. Copyright electron Elsevier transfer CorrelationsMB of kes and kMB with the amount of Cu –citrate and the −driving force (−ΔG) of interfacial Science Publishers, 2015.). electronfor ST–Au–Cu transfer2O (reproducedfor ST–Au–Cu with2O permission(reproduced from with [87]. permission Copyright Elsevierfrom [87] Science. Copyright Publishers, Elsevier 2015). Science Publishers, 2015.). 5.5. Mechanism Mechanism for for photodegradation Photodegradation of of dye Dye 5. MechanismFigureFigure 1515 for displaysdisplays photodegradation the the redox redox potentials potentials of dye for thefor reactivethe reactive species species and the and band the structures band structures of common of photocatalysts, along with the lowest unoccupied molecular orbital (LUMO) and highest occupied commonFigure photocatalysts, 15 displays thealong redox with potentials the lowest for un thoccupiede reactive molecular species orbitaland the (LUMO) band structures and highest of molecular orbital (HOMO) levels of five representative dye molecules. Based on their band positions, commonoccupied photocatalysts,molecular orbital along (HOMO) with thelevels lowest of five un reoccupiedpresentative molecular dye molecules. orbital (LUMO) Based on and their highest band the generation of reactive species in semiconductor photocatalysts can directly degrade dye molecules occupiedpositions, molecular the generation orbital of (HOMO) reactive levels species of fivein semiconductor representative photocatalysts dye molecules. can Based directly on their degrade band (see Section 5.1). Alternatively, the self-photosensitization of dye may occur to improve the generation positions,dye molecules the generation (see Section of reactive5.1). Alternatively, species in semiconductor the self-photosensitization photocatalysts of can dye directly may occurdegrade to of reactive species when the CB of photocatalysts is more negative than the LUMO level of dye dyeimprove molecules the generation (see Section of reactive 5.1). Alternatively, species when thethe CBself-photosensitization of photocatalysts is moreof dye negative may occur than theto molecules (see Section 5.2), finally dissociating the dye molecules. improveLUMO level the generationof dye molecules of reactive (see Sectionspecies 5.when2), finally the CB dissociating of photocatalysts the dye is molecules. more negative than the LUMO level of dye molecules (see Section 5.2), finally dissociating the dye molecules.

FigureFigure 15. SchemeScheme for for the the band structures of common photocatalysts, and and potentials potentials of of the radical generation and HOMO and LUMO levels of five representative dye molecules. Figuregeneration 15. Scheme and HOMO for the and band LUMO structures levels of of five common representative photocatalysts, dye molecules. and potentials of the radical 5.1. Directgeneration Photodegradation and HOMO and Process LUMO levels of five representative dye molecules. 5.1. Direct photodegradation process In principle, the direct photodegradation of dye molecules involves the excitation of semiconductor 5.1. DirectIn principle, photodegradation the direct process photodegradation of dye molecules involves the excitation of photocatalysts under light irradiation, leading to the scavenging of photoexcited electrons by dissolved semiconductorIn principle, photocatalysts the direct underphotodegradation light irradiat ofion, dye leading molecules to the involvesscavenging the of excitationphotoexcited of semiconductor photocatalysts under light irradiation, leading to the scavenging of photoexcited

Catalysts 2019, 9, 430 22 of 32

O2 in the solution, as the CB of the photocatalyst is more negative than the reduction potential E(O / O ). The O - anion and H O are, thus, formed. H O can further transform into OH. 2 · 2− · 2 2 2 2 2 · Meanwhile, the photoexcited holes can oxidize the adsorbed water to generate OH, as the VB of · photocatalysts are more positive than the oxidation potential E(H O/ OH). These highly reactive 2 · O and OH can oxidize or degrade the adsorbed dye molecules. In addition, the photoexcited · 2− · electrons and holes which are essentially active may also attack the dye molecules to complete the photodegradation process. To clarify the major contributor in the photodegradation reaction, several studies investigated the change of the photodegradation rate in the presence of different scavengers, as listed in Table2. Pu et al. developed a Cu2O–rGO system (Figure 16a) and explored the photodegradation mechanism of MO, with TBA used as the OH scavenger [84]. Figure 16b shows various control experiments for the · photodegradation of MO. No obvious change on MO photodegradation was found with the addition of TBA, indicating that the photoexcited holes of Cu O and the subsequent OH radicals were minor 2 · factors. Furthermore, the photodegradation reaction was performed with purging using O2 and N2. The MO photodegradation was abated under an N2 purge, whereas the photodegradation rate was slightly enhanced under O2 purging, confirming that dissolved O2 in the solution played a crucial role for the MO photodegradation. Figure 16c shows the pathway for the MO photodegradation in the Cu2O/rGO system. Upon light irradiation, the photoexcited electrons transfer to the EF of rGO, and the generation of O occurs, degrading the MO molecules. As demonstrated by Zhao’s group, · 2− O is one of the main active species for MO photodegradation [88], which was also confirmed by · 2− the seriously depressed photodegradation of MO (to almost no activity) in the absence of O2. Li et al. also confirmed that photoexcited holes are a minor active species and the dissolved O2 dominates the MO photodegradation on g-C N photocatalyst [89] because the formation of O - is affected by direct 3 4 · 2 reduction of O The presence of O also determines the production of OH via multistep reduction 2. 2 · of O2.

Table 2. Common scavengers used for active species trapping experiments.

Type Sacrifice Reagent Abbreviation

AgNO3 - Electron scavenger CCl4 - K2Cr2O7 - KI - Ethylenediaminetetraacetic EDTA, EDTA-2Na acid Hole scavenger Tri-ethanolamine TEOA Ammonium oxalate AO Sodium oxalate (Na2C2O4)- Methanol - Ascorbic acid AA tert-Butyl alcohol TBA, t-BuOH OH scavenger · 2-Propanol IPA Benzoquinone BQ O scavenger Acrylamide AC · 2− Superoxide dismutase SOD Catalysts 2019, 9, x FOR PEER REVIEW 23 of 32

rate in the presence of AgNO3 is due to the decreased generation of ·O2− from photoexcited electrons. This outcome was also consistent with the g-C3N4 [91], ZnO/graphene [92], and C3N4–BiVO4 [93] systems for the photodegradation of MB. The aforementioned scavenger experiments revealed that Catalysts major2019, 9 contributors, 430 for each dye molecule might be different. Table 3 specifies the dominating active 23 of 32 species for other commonly reported dye molecules.

FigureFigure 16. (a) 16. SEM (a) andSEM TEM and TEM (inset) (inset) observations observations and and (b )( MOb) MO photodegradation photodegradation overover Cu22O–rGOO–rGO under differentunder experimental different experi conditions.mental conditions. (c) Schematic (c) Schematic illustration illustration of the of the band band structure structure and relevant relevant redox redox potentials for Cu2O–rGO (reproduced with permission from [84]. Copyright Elsevier Science potentials for Cu2O–rGO (reproduced with permission from [84]. Copyright Elsevier Science Publishers, Publishers, 2015.). (d) High-resolution (HR) TEM image of AgSiO/Ag2CO3. (e) MB photodegradation 2015). (d) High-resolution (HR) TEM image of AgSiO/Ag2CO3.(e) MB photodegradation under visible under visible irradiation in the presence of scavengers and (f) proposed mechanisms of f irradiationphotodegradation in the presence pathways of scavengers of MB andon AgSiO/Ag ( ) proposed2CO3 mechanisms(reproduced with of photodegradation permission from [90] pathways. of MB on AgSiOCopyright/Ag Nature2CO3 (reproduced Publishing Group, with permission2017.). from [90]. Copyright Nature Publishing Group, 2017).

Cao et al. proposedTable 3. Active that species the for photodegradation photodegradation of commonly of MB onreported AgSiO dyes/Ag molecules.2CO3 photocatalysts in water was dominated by O and photoexcited holes [90]. Figure 16d shows the morphology of Class Dye· 2− Active species Photocatalysts AgSiO/AgCationic2CO3 photocatalysts.dye RhB With addition H of+ IPA, the photodegradation g-C3N rate4 [89] slightly decreased, suggesting OH was not the main active speciesH+ (Figure 16e). Meanwhile,Ag/Ag the3PO addition4 [94] of EDTA-2Na · − + and N2 purging resulted in a significant decrease·O2 /H of photodegradationBiVO e4ffi/TiOciency,2 [95] indicating the ·O2−/H+ BiOI/C [96] important roles of O2− and photoexcited holes in the photodegradation process of MB. The possible · + − photodegradation pathway is displayedH (major), in Figure ·O2 (minor) 16f. This photodegradation CoFe2O4/BiO(Cl, Br, pathwayI) [34] can be MB ·O2−/H+ g-C3N4 [91] supported by adding AgNO3 into the reaction solution. The suppression of the photodegradation ·O2−/ H+ ZnO/graphene [92] rate in the presence of AgNO3 is due to the decreased generation of O2− from photoexcited electrons. ·O2− · C3N4-BiVO4 [93] This outcome was also consistent with the·O g-C2−/H3N+ 4 [91], ZnO/grapheneAgSiO/Ag [92], and2CO3 C[90]3N 4–BiVO4 [93] systems for the photodegradationCV of·O2 MB.− (major), The h aforementioned+/·OH (minor) scavenger BiOxCl experimentsy/BiOmIn [97] revealed that major contributors for each dye molecule·O2− (major), might h+/·OH be di (minor)fferent. Table3 specifies BiOxIy/GO the [98] dominating active species for other commonly reported·O2 dye−/h+ (major), molecules. ·OH (minor) BaFe2O4 [99] MG H+/·OH /·O2− CuFe2O4 [100] Table 3. Active species for photodegradation·OH/e−/H+ of commonly reportedFe3O4/TiO dyes2/CuO molecules. [101] ·OH/e−/H+ (major), ·O2− (minor) Ni-Bi2Se3 [102] Class Dye Active Species Photocatalysts Rh6G H+ (major) ·OH/·O2− (minor) Curcumin/Bi0.5Na0.5TiO3[103] + ·O2− H Zn/Yg-C3 N[104]4 [89 ] + + H− Ag/Ag3PO4 [94] H /·O2 + Quantum dot/Eu-metal organic RhB O2−/H BiVO4/TiO2 [95] · + framework [105] O2−/H BiOI/C[96] · − Anionic dye MO H+ (major), ·O2 O (minor) CoFe CuO2O-rGO/BiO(Cl, [84] Br, I) [34] · 2− 2 4 O /H+ g-C N [91] · 2− 3 4 O / H+ ZnO/graphene [92] MB · 2− O C N -BiVO [93] · 2− 3 4 4 Cationic dye O /H+ AgSiO/Ag CO [90] · 2− 2 3 + O2− (major), h / OH (minor) BiOxCly/BiOmIn [97] · + · CV O (major), h / OH (minor) BiOxIy/GO [98] · 2− · O /h+ (major), OH (minor) BaFe O [99] · 2− · 2 4 H+/ OH / O CuFe O [100] · · 2− 2 4 MG OH/e /H+ Fe O /TiO /CuO [101] · − 3 4 2 OH/e /H+ (major), O (minor) Ni-Bi Se [102] · − · 2− 2 3 H+ (major) OH/ O (minor) Curcumin/Bi Na TiO [103] · · 2− 0.5 0.5 3 Rh6G O Zn/Y[104] · 2− Quantum dot/Eu-metal organic H+/ O · 2− framework [105] Catalysts 2019, 9, 430 24 of 32

Table 3. Cont.

Class Dye Active Species Photocatalysts O Cu O-rGO [84] · 2− 2 MO 2,9,16,23-tetracarboxyl O phthalocyanine/amorphous · 2− TiO2 [88] O (major) H+ (minor) g-C N [89] · 2− 3 4 H+/ OH TiO [106] AO7 · 2 Ag/AgBr/SiO -coated Fe O Anionic dye h+ 2 3 4 [107]

OH ZrO2 [108] +· CR H / OH CuS-Bi2CuxW1 xO6 2x [109] · − − H+ (major), OH (minor) SnO [110] · 2 ARS OH/e /H+ ZnS/carbon quantum dots [111] · − AV7 O /H+/ OH CdS/Ta O [112] · 2− · 2 5 + RB5 H SrTiO3/CeO2 [113]

5.2. Sensitization-Mediated Degradation Process When the photon energy is not high enough to excite photocatalysts to generate reactive charge carriers and radicals, the photodegradation might occur via photosensitization process. Under visible light illumination, a dye molecule can be excited to its excited state (LUMO level), producing abundant excited electrons at the LUMO level. Provided that the LUMO of the dyes is more negative than the CB of photocatalysts, these photoexcited electrons can then transfer from the dye molecules to the photocatalysts, facilitating the generation of reactive species for commencing photodegradation. This process is known as the photosensitization pathway. The LUMO levels for five representative dye molecules in comparison with the CB levels of common photocatalysts are illustrated in Figure 15, from which one can tell whether or not the photosensitization can occur. Zhao et al. demonstrated the self-photosensitization process of RhB and MO over BiOCl hierarchical nanostructures [63], as shown in Figure 17a. Note that the as-synthesized BiOCl exhibits a negatively charged surface in the pH range from 2 to 11, suggesting the as-synthesized BiOCl can selectively adsorb cationic RhB in RhB/MO mixtures. As shown in Figure 17b,c, RhB showed almost 40% adsorption prior to the light irradiation, while MO showed a negligible adsorption. Upon visible light irradiation, although BiOCl nanostructures cannot be excited with visible light since the bandgap of BiOCl is approximately 3.3 eV, the RhB dye was completely degraded within 15 min, whereas no photodegradation was found for the MO/BiOCl system (Figure 17d). The RhB photodegradation was attributed to the self-photosensitization of RhB, in which the photoexcited electrons are injected from the LUMO level of RhB to the CB of the BiOCl nanostructures. The electrons on the BiOCl nanostructures can subsequently reduce O to O radicals, which further degrades the RhB molecules. The 2 · 2− self-photosensitization of MO should also be able to degrade the MO molecules; however, its poor adsorption capacity inhibits the electron transfer from the excited MO to the CB of BiOCl. Therefore, BiOCl nanostructures exhibited poor MO photodegradation efficiency. The self-photosensitization of the RhB/BiOCl system can be further applied to the photodegradation of MO dye molecules, as shown in Figure 17e. Compared to the extremely low MO photodegradation efficiency in the MO/BiOCl system, apparently, the MO photodegradation efficiency was significantly enhanced in the MO/RhB/BiOCl system, indicating that MO photodegradation is mediated by RhB via a photosensitization pathway. Moreover, RhB still demonstrated a higher photodegradation efficiency in the MO/RhB/BiOCl system. This photodegradation of dye molecules via the photosensitization process was reported in the cationic new fuchsin/graphene quantum dots [114], RhB/Zn-doped BiOBr [115], MB and MO/Eu3+-doped ZnO [116], and RhB/Nb2O5 [117] systems. Catalysts 2019, 9, 430 25 of 32 Catalysts 2019, 9, x FOR PEER REVIEW 25 of 32

FigureFigure 17. 17. (a(a) )SEM SEM image image of of BiOCl BiOCl and and the the results results of of ( (bb)) RhB RhB and and ( c) MO MO photodegradation photodegradation under under visiblevisible irradiation. irradiation. ( (dd)) Proposed Proposed mechanism mechanism for for the the enhanced enhanced MO MO photodegradation photodegradation on on RhB/BiOCl RhB/BiOCl underunder visible visible irradiation. irradiation. (e (e) )Photodegradation Photodegradation of of RhB RhB and and MO MO on on RhB/BiOCl RhB/BiOCl (reproduced (reproduced with with permissionpermission from from [63] [63].. Copyright Copyright Royal Society of Chemistry, 2016).2016.).

6.6. Summary Summary and and Outlook Outlook MostMost dyes dyes have have either either a a positive positive or or a a negative negative charge charge upon upon dissociation dissociation in in aqueous aqueous solutions, solutions, characteristiccharacteristic ofof cationiccationic and and anionic anionic dyes, dyes, respectively. respectively. The pHThe of pH the of solution the solution modifies modifies the electrical the electricaldouble layer double of the layer photocatalyst, of the photocatalyst, affecting the a interactionffecting the between interaction dye moleculesbetween dye and molecules photocatalysts, and photocatalysts,the charge transfer the for charge the self-photosensitization transfer for the self of-photosensitization dyes, and the subsequent of dyes, decoloration and the esubsequentfficiency, as decolorationhighlighted inefficiency, this review. as highlighted The optimization in this of revi otherew. operational The optimization factors (i.e.,of other initial operational dye concentration, factors (i.e.,light initial intensity, dye reactionconcentration, temperature) light intensity, and charge-carrier reaction temperature) properties ofand heterostructure charge-carrier photocatalysts properties of heterostructurecan establish the photocatalysts most efficient can photodegradation establish the most system. efficient In addition, photodegradation research on system. the reactive In addition, species, researchas well as on the the photosensitization reactive species, pathway as well ofas dyethe moleculesphotosensitization for photodegradation, pathway of dye is helpful molecules to clarify for photodegradation,the overall decoloration is helpful mechanism, to clarify which the overall is beneficial decoloration for further mechanism, application which to the is degradation beneficial for of furthermulticomponent application industrial to the degradation wastewaters. of With multic strongomponent progress industrial in photocatalytic wastewaters. techniques, With strong more progressin-depth characterizationin photocatalytic and techniques, modeling ofmore the photodegradation, in-depth characterization and even mineralizationand modeling processes of the photodegradation,for multicomponent and dyes even in practical mineralization applications processes will be possiblefor multicomponent in the near future. dyes Clearly,in practical many applicationsquestions still will remain be possible unanswered in the ornear are future. poorly Clearly, addressed, many which questions are briefly still remain outlined unanswered below. or are poorlyUnlike addressed, other heterogeneous which are briefly photocatalysis, outlined below. such as photoelectrochemical water splitting and CO2Unlikereduction other in heterogeneous which the solar-to-hydrogen photocatalysis, (STH)such as and photoelectrochemical AQY are introduced water to determinesplitting and the CO2 reduction in which the solar-to-hydrogen (STH) and AQY are introduced to determine the

Catalysts 2019, 9, 430 26 of 32 photocatalytic efficiency for comparison, a quantitative comparison with AQY is relatively unexplored for photodegradation reactions. The introduction of AQY into the photodegradation reaction provides helpful information to understand the efficiency of semiconductor photocatalysts. However, for visible-light-responsive photocatalysts, the feasible determination of photodegradation efficiency is especially challenging due to the relatively high photoabsorption coefficient of dye molecules, as well as the complicated mechanism of dye degradation [118]. Another major concern in reactions involving self-photosensitization of dye molecules is the interactions between photocatalysts and multiple types of dye molecules, which are not always clear. Limited studies described the underlying interactions and mechanisms. By using self-photosensitization of dye molecules, both cationic and anionic dyes can be simultaneously decomposed, which has the potential to degrade multicomponent dyes in real wastewater systems. However, for multicomponent dye systems, the quantification of efficiency for each individual dye molecule is difficult to determine with simple spectrophotometric methods especially when their absorption spectra overlap. The development of a new method to easily and reliably obtain quantitative detection of dye molecules is necessary for accurate estimation.

Author Contributions: Conceptualization, Y.-H.C.; investigation, Y.-H.C., T.-F.M.C., C.-Y.C., M.S., and Y.-J.H.; writing—original draft preparation, Y.-H.C.; writing—review and editing, Y.-J.H. Funding: This work was financially supported by the JST CREST of Japan (Grant Number JPMJCR1433), the Ministry of Science and Technology (MOST) of Taiwan (grant No. MOST 107-2113-M-009-004 and MOST108-3017-F-009-004) and the Center for Emergent Functional Matter Science of National Chiao Tung University from the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education in Taiwan. Conflicts of Interest: The authors declare no conflict of interest.

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