0739 nghiên cứu tổng hợp và biến tính vật liệu ms2 (mmo2 w) bằng g c3n4 làm chất xúc tác quang luận văn tốt nghiệp

OVERVIEWOFCURRENTPHOTOCATALYSTS

The first photocatalyst studied in 1972 by Fujishima and Honda [59] isTiO2acting as an anode for water splitting in a photochemical cell Five yearslater, in 1977, it was first used by Frank and Bard for the reduction of CNˉ inwaterwideningitsapplicationtophotodegradationofpollutantsintheenvironme nt[55] So far, TiO2has become themost widely investigatedphotocatalyst due to its unique photocatalytic efficiency, photo-stability, lowcost,nontoxicity,availability,thermalandchemicalstability.BesidesTiO2andthe other photocatalystZnOhaveexhibitedtheiradvancedphotocatalyticactivity for wastewater treatment as well [100] However, they all possess thesamedisadvantagethattheydonotworkunderthevisiblelightbecauseoftheirlargeband gap,e.g.anataseTiO2hasthebandgapof3.2eV,whichsignificantly prevents them from using solar energy for activation This is dueto the fact that in the solar spectrum ultraviolet light accounts for only 4-5%,meanwhile the visible light makes up to nearly 40% of the solar energy [135].To dealt with this vital deficiency many modifications have been employed tomake them become active in the visible light region with good efficiency suchasdoping [39], [52],[100],[130], [172],dy esensitization [9] ,[110],[112],

[203] heterogeneous coupling [87], [101], [189], [195], [203],etc In additiontothat,alotofefforthasbeenmadetodevelopnovelmaterialswhichthemselves are active under the visible light without any modification. ThematerialswhichfallintothiscategoryhavebeenwidelyinvestigatedincludingBi2W

O6[40], BiVO4[49], Bi24O31Br10[150], Ag3PO4[80], CaIn2O4[45], g-

C3N4[200],etc However, in order to improve the photocatalytic performancetheyhavebeenusuallycombinedwithothercomponents,forinstance,B iVO4/rGO[158],ZnSnO3/rGO[64],Ag3PO4/g-C3N4[73],Ag/AgBr/g-C3N4

[26], Ag/AgVO3/rGO [222] Doping has also been a technique for enhancingthephotocatalyticactivity,suchasO-g-C3N4[56],Bi-Ag3PO4[217],Co- BiVO4[226],Mo-BiVO4[30],B-Bi2WO6[58].

Among these visible-light-driven photocatalysts, MoS2-based and WS2- based,thetransition-metal-dichalcogenides-basedphotocatalysts,haveattracted attention of many scientists due to their appropriate bandgap forvisible-light harvesting and the other unique properties as you will see in thenext section, leading to a variety of applications such as hydrogen production,pollutionreduction,andphotosynthesis[74],[113],[121],[146].

Carbon nitride with graphite-like structure (g-C3N4), a metal- freeorganicsemiconductorhasalluredsignificantattentionasapotentialphoto catalyst due to its electronic structure with band gap of 2.7 eV andrelativelyhighchemicalstability[184].However,photocatalyticperforma nce of pure g-C3N4is limited because of its poor absorption in thevisible region, fast recombination of photo-generated charge carriers andlow specific surface area [214] In order to overcome these disadvantages,similar to above mentioned techniques, various methods have also beenapplied such as co-polymerization [86], [212] altering different precursors[213], [220] and non-metal doping [215] Therefore, coupling techniqueswithotherco- catalystsisapromisingwaytoenhancephotocatalyticefficiency[141].

Some reports showed that the presence of MS2plays a beneficial role inimprovement of light harvesting, electron transfer at interfaces, and chargecarrier separation in the composite materials with g-C3N4 These benefitsareexplainedwithproperbandgapedgepositionsandgoodlatticematc hing of MS2andg-C3N4[78],[181].

MS 2 -BASED(M=Mo,W)PHOTOCATALYSTS

StructuresofMS 2 (M=Mo,W)

MoS2and WS2are materials that belong to a family of transition metalchalcogenides(TMDs)withlayeredstructureinwhicheachunit(MS2)compr isesatransitionmetal(M=Mo,W)layersandwichedbetweentwosulfuratomiclayers.MoS2 structurerepresentativeisshown in Figure1.1.

Figure 1.1 MoS2structure in three dimensions with the distance between thetwo adjacentlayersof 6.5 Å[142].

TherearethreetypesofstructuresofTMDsdependingonthearrangement of the atoms, namely, hexagonal (H), tetragonal (T) and theirdistortedphase(T’) [34].TakingMoS2asarepresentativeofthetwomaterials,it has four following polytypes 1H, 1T 2H and 3R as with the drawings shownin Figure1.2.

More specifically, in the 1H phase, the basic unit of MoS2monolayer,the sulfur atoms are organized into two layers creating a sandwich structurehaving a layer of molybdenum atoms in the middle, and the S atoms in theupperlayerarelocateddirectlyabovethoseonthelowerlayers.Meanwh ile, for 1T-MoS2the Mo atoms located at the center positions of the octahedralinterstices of the S layers and the S atoms in the upper and lower planes areoffset fromeach other toformaunitcell[74].

Figure1.2.Four commonMoS2poly-types [12].

The next polytype of MoS2is 2H with two MoS2units in the unit cell.Each layer has the structure of 1H with a monatomic Mo plane between twomonatomic S planes in a manner described above Two layers, which areweakly coupled by van der Waals interaction, in the unit cell are arranged sothatMoatomsofonelayerarelocatedontopofSatomsintwoadjacentlayers[10].The3R MoS2alsohasthesametrigonalprismaticcoordinationasthe2HMoS2,however,therear ethreeMoS2unitsperunitcell.Offourpolytypes,1His the most stable, it is also the stable phase of 2D MoS2[202] In bulk MoS2,both 1T and 2H exist However, the latter phase is usually more stable and itsproperties is different from the former (1T is a metal, meanwhile, 2H is asemiconductor)

[193].AsbulkformMoS2(2H)isanindirectbandgapsemiconductor with a bandgap value of 1.3 eV, when reducing the samplethickness down to a few atomic layers or even to a 2H monolayer the bandgapis widened to 2.1 eV [61], however, still as an indirect form As mentionedabove,a2Hmonolayeriscomposedoftwolayersof1Hlinkedtogetherjustby weak interaction of van der Waals force, this allows that monolayer to befurther divided, leading to a three atomic layer sheet [11] The transformationalso converts the for a single 1H layer from indirect to direct bandgap with thecalculated value increasing to 2.3 eV [38] Similarly, the bandgap of WS2hasthe value of1.35 eV for bulk material as indirect [94] and approximate 2.0 eVfor monolayer as direct one [210] This optical property suggests that suchmaterials can strongly absorb in the visible region of the solar spectrum, and itis more appropriatewhenusedasacocatalyst.

MS 2 -basedcomposites

Similar to the other photocatalysts such as TiO2, MS2(M = Mo, W) hasbeen widely used in the form of composites to improve the photocatalyticactivitiesoftheindividualcomponents,especiallyinthefieldofphotocatalyticde gradation of organic contaminants The composites in which MS2(M = Mo,W) used as a cocatalyst have been recently developed such as MoS2/grapheneoxide[47],[96],[106],[224],[229];MoS2/g-

MoS2/TiO2[79],[165],[218];MoS2/BiOBr[43],[155],[196];WS2/WO3[44],

C3N4hasbeenconsideredasapromisingcandidateduetoits electronic structure with band gap of2.7 eV,in additiontolow-cost,abundanceofsource,non- toxicityandchemicalstability[83],[114],[211].ThecombinationofMoS2andg-

C3N4tocreateacompositeisfavourableduetothetwo facts, firstly, both of them are layered materials could result in intimatecontactsfacilitatingforthechargetransferbetweenthetwophases,andanothercomesf romtheproperbandedgesofthetwocomponents[181]toproducetypeIIofsemiconductor heterojunctions[185].Regardingtheproperbandedges,g-C3N4has the conduction band (CB) and valence band (VB) edges at -1.13 eVand+1.57eV,respectively,meanwhile,thecorrespondingvaluesofMoS2 as nanosheetsare-0.1and+2.0eV[61].Thisdifferenceinbandedgesallowstheelectrons to transfer from the CB of g-C3N4to that of MoS2and the holestransfer from the VB of MoS2to that of g-C3N4 As a result, in the formedcompositeMoS2/g-

C3N4bothelectronsandholesaremoremobilecomparedtotheindividualcomponents.Thisl eadstothereductioninrecombinationrateofphotoinducedchargecarriers,thereforeincreasi ngthephotocatalyticactivity.

Synthesismethods

InordertoobtainMS2asmonoorfewlayersthetwofollowingstrategies,namely, “top- down” exfoliation and “bottom-up” synthesis methods have beenwidelyused[219].Thetworepresentativesfortheformerstrategy areme chanicalexfoliationandexfoliationmethod,meanwhile,hydrothermalmet hod,solvothermalmethodandchemicalvapordepositionrepresentingforthelatt erone[193].Likegraphenesheetssynthesis,MoS2single- layercanbeexfoliatedf r o m S i O2/Siw i t h t h e S c o t c h - t a p e m e t h o d [ 1 0 3 ] R e g a r d i n g t h e exfoliationm e t h o d w h i c h c o u l d b e c h e m i c a l e x f o l i a t i o n [ 6 5 ] , l i q u i d - p h a s e exfoliation

[69] and electrochemical exfoliation [117] Lithium ionintercalationisthemostcommonchemicalmethodforfabricationof1Tmetallic MoS2usingn-butyllithium inhexanereactingwithMoS2powderunderargonatmosphere,at65 o Covernighttofor mLixMoS2.Aftertheremovalofunreactedn- butyllithiumanditsorganicresidue,theembeddedMoS2wasdispersedi n d e i o n i z e d w a t e r , t h e n c e n t r i f u g e d t o g e t t h e s t a b l e M o S2nanosheets[ 6 5 ] S o n i c a t i o n , a l i q u i d - p h a s e e x f o l i a t i o n i s a l s o a n e f f e c t i v e methodforpreparationofMo

S2nanosheets[37].TheexfoliationmethodcanbeusedtofabricateWS2nanosh eetsaswell.Amongthementionedmethods,the hydrothermal method is the most common regarding economical aspect Inthismethod,anamount ofsulfursource,commonlyused suchasthioureaand thioacetamide, and a molybdenum salt, such as Na2MoO4, (NH4)6Mo7O24aremixed together in deionized water, then the resultant solution was heated in anoven at 200 o C for 24h after being transferred to a Teflon-lined stainless steelautoclave [36], [227] If an organic solvent is used instead of water, the otherconditions keep the same then the method called solvothermal method Somewidely used organic solvent for this task include N-N-dimethylformamide, 1-methyl-2-pyrrolidinoneandpolyethyleneglycol- 600.Inordertocreateahigh-quality atomic thin MoS2, chemical vapor deposition (CVD) has been broadlyemployed For example, MoS2could be synthesized by sulfurization of MoO3using this method [104] WS2nanosheets were also synthesized using thosemethodsappliedforMoS2asmentionedabove,suchasmechanicalexfoliation[134 ],[138],chemicalexfoliation[35],[115],liquid-phaseexfoliation[223], hydrothermalmethod[ 2 1 ] , [46]andCVDmethod [171],[204].

In contrast to the synthesis of MoS2as discussed in the previoussection, the synthesis of the MoS2/g-C3N4composite has been reported injust a few studies [109], [154], [170] Generally, the reported methodsconsist of three steps in which the two individual materials g-

C3N4andMoS2are produced separately by heating precursors such as urea, melamine,cyanamide for the former, and using a hydrothermal method for the latter.Then, the mixture of g-C3N4and MoS2is treated by ultrasound. However,both of the hydrothermal and ultrasound steps require high energy and theformeroccursathydrothermalconditions.Similarly,fewreportsonWS2/g-

C3N4as photocatalysts have been published [4], [78] For example, WS2/mpg-

(NH4)2WS4as WS2precursor in the presence of mpg-C3N4under theatmosphereo f t h e g a s m i x t u r e o f H2Sa n d H2[ 7 8 ] I n a n o t h e r s t u d y, g -

CS2gas [4] Therefore, a facile method to synthesize a large amountof the materials without compromising photocatalytic activity is still inneed.

PHOTOCATALYTIC PROCESS, LIGHT SOURCES ANDASSESSMENTBENCHMARKS

Photocatalyticdegradationmechanism

Ingeneral,thesystemofheterogeneousphotocatalysisusingthesemiconduc tor material to absorb the suitable light to produce charge carriers,thenthesespecieswilltakepartintheredoxreactionsoccurringonthesurfaceof the material but the semiconductor is still unchanged after the process. Thiskindofmaterialiscalledaphotocatalyst.Thesuitablelightasmentionedaboveis the electromagnetic irradiation which has photon energy,hν, equal to orgreaterthanthebandgap(E g )oftheemployedphotocatalyst.Theexcitationofan electron (e - ) from the valence band (VB) to the conduction band (CB) willhappenoncethelightisabsorbed,leavingbehindaphotogeneratedhole(h + )attheVB asillustratedin Figure 1.3.

These photoinduced electrons and holes can recombine and give off theabsorbed energy as heat form (Equation 1.2) or migrate to the surface of thecatalysttoreactwiththeavailablemoleculesinthesystemsuchasoxygenandwatertofo rmreactiveradicalsincludingHO˙andO2˙ˉ(Equations1.3and1.4),respectively Subsequently, these powerful species along with holes couldcompletely photodegrade the organic pollutants presenting in the system tosimple molecules such as carbon dioxide and water through a number ofintermediatespecies(Eq.1.5).Thesereactionswerebroadlyproposedasfollows[17]:

Therecombinationphenomenonofe - andh + occursjustwithinpicoseconds after photogeneration if there is no scavenger in the system [17].However,eveninthepresenceofscavengers,therecombinationofthephotoin duced charges is not completely avoidable due to its high rate Thisleads to the dissipation of energy and thereby reducing the quantum efficiency[32].Thecausesforthisphenomenoncouldbeascribedtoimperfectionsinthecr ystalanditmayhappenbothonthesurfaceandinthebulkofthesemiconductor.Thus,th etechniquesthatareusedinprolongingthelifetimeofthep h o t o g e n e r a t e d e l e c t r o n s a n d h o l e s p l a y a c r u c i a l r o l e i n i m p r o v i n g t h e

Reactant adsorption onto the surface of photocatalyst (fast)

Reactant (R) transportation in the solution

Product (P) transportation in the solution

(slow) photocatalytic activity of the photocatalyst The normal methods which havebeen applied for this purpose are doping, co-catalyst addition, heterogeneouscoupling,etc This will be discussed in more detail in the below sub-sectionsofthischapter.

Reactionkinetics

Inthephotocatalyticprocessforphotodegradationoforganiccontaminant, a series of five consecutive steps includes the transportation ofreactant from bulk of the solution to adsorb onto surface of the catalyst, thenthe interfacial photoreaction and finally the desorption of the product to thebulk from the surface This five-step process is illustrated by a flowchart asshownbelow.

For consecutive reactions as shown in Figure 1.4, the overall photocatalyticreaction rate is controlled by the slowest stage This rate determining step inthe photocatalysis process would be the photoreaction taking place on thecatalyst’ssurfaceifthereactorisundercontinuouslystirredcondition,thentherateof thereaction(r) converting (R)into (P)canbedefinedas, r=-dC/dt=k.θ (1.6) where θ is the fraction of the reactant absorbed,kis the reaction rate and if theadsorption process obeys the Langmuir model then this quantity can be foundas follows, θ=KC/(1+KC) (1.7) whereCis theconcentrationofthereactantattheadsorption- desorptionequilibrium, andKis the adsorption coefficient of the reactant From Eq.(1.6)and (1.7),the reactionrate becomes, r =-dC/dt=kKC/(1+KC) (1.8)

Equation1.8expressesthekineticsofthephotocatalyticdegradationoforganiccompounds which follow the Langmuir-Hinshelwood model [60] In the caseof a low concentration of the reactant,C TBA > DMSO The resultalso indicated that TEOA scavenger exhibited much stronger inhibitionshowingthatholewasthedominantreactivespeciesinthephotodegra dationprocessofRhBoverMCN1catalystunderstudiedconditions.Thisimpor tantroleofholewassupportedbythemorepositive valencebandedgeofg-C3N4comparedtotheredoxpotentialofRhB(+1.57V vs +1.43 V) [201].The hole of MoS2also was able to oxidize RhBdirectlyduetoitsmorepositivepotential+2V[136].Meanwhile,thequitestrong adverse-effect caused by BQ and TBA scavengers revealed that theoxygenreactivespecies,namely,thesuperoxideradicalanionandhydroxylradical also play their important part in the overall photocatalysis.

Theformationofthesespeciescouldbeattributedtothemorenegativepotentialof electron which generated from g-C3N4conduction band edge -1.13 Vcompared to the reduction potential of the redox pair O2/O2˙ˉ of -0.28 V[197].Furthermore,theOH˙radicalwasjustproducedfromO2˙ˉasdiscussed detailedly later, this resulted in decreasing its role in the wholeprocess. Meanwhile, the direct formation of this type of reactive speciesfrom H2O and hole was unfavorable energetically due to the too muchpositive potential of the pair OH˙/H2O However, the role of electrons inthe process was insignificant as seen in Figure 3.38, when there was apresence of DMSO, an electron scavenger, just a minor effect observed onthe photodegradation rateofRhBover thecatalyst.

From the exploration of the role of reactive species as discussedabove and the separation of charge in the composite as seen in

PL spectrain Figure 3.24 could help us to propose a photocatalytic mechanism asillustrated in Figure 3.38a The charge separation could be attributed to thelower conduction band position of MoS2in the composite leading to itsability to receive photoexcited electrons from g-C3N4[154], which favourstheseparationofelectron-holepairs.TheseparatedelectronsonthesurfaceofMoS2reduces present oxidants in the solution In the case of dissolvedoxygen this reduction step, which also occurs on the surface,can be givenas,

The formed superoxide radical anion continuously undergoes threemore steps to release hydroxyl radical OH˙ which are described as follows[22]:

Hydroxylradical isa strong oxidizing species thatdegrades theorganicpollutantsinthesolution.Meanwhile,theaccumulatedholesonthesurface of g-C3N4could directly oxidize the adsorbed dye molecule on thesurface, h++dye →intermediates→CO 2 +H 2 O+ othersimplemolecules (3.10)

Thisreaction(3.10)ofoxidizingthetargetmoleculemainlycontributed to the overall processas supported in the carrier trappingexperiments.

The Equations from 3.6 to 3.9 relating to oxygen and the role ofH2O2as discussed in the previous subsection indicate that how importantthe presence of oxidant on the surface is, any factor that introduces adecreaseinamountofthatmoleculeonthecatalystsurfacewouldleadtoanegative effect on the overal rate of photodegradation of pollutant Thiscould be explained using the model as shown in Figure 3.38b, in the caseof just few target molecules adsorbing on the catalyst surface, the reactionrateforthiswouldbelowduetothelowadsorbed- moleculenumber.Whenthat numberincreases,so doesthe rate because the oxidizing reagentmolecules still are enough for the photocatalytic process, however, if theadsorbed molecules continue to increase, this would reduce the space foroxidantmoleculestobeadsorbed,thenmightresultinadecreaseinoverallrate.Thi s modelseems tosatisfywiththeresulthasbeenobservedsofar.

The UV-Vis spectra of RhB solution after 120 min of illuminationtheoptimal conditions as indicatedabovewas shown inFigure3.39.

Conditions of process: irradiatedvolume:25 mL, initialRhBconcentration: 5.0 mg.L -1 , pH 3.0, MCN1 catalyst loading: 0.7 g.L -1 , 25 o C,under blue light.

Iti s o b v i o u s t h a t a f t e r 1 2 0 m i n o f i l l u m i n a t i o n t h e m a x i m u m wavelength was shifted from 553 nm to 497 nm corresponding to thetransformation of RhB to RhB 110 [126] with the structures shown inFigure3.40.

Rhodamine B,λ max U3nm Rhodamine110,λ max I7nm

Applications

Enrofloxacin (ENR) is a colorless antibiotic substance which containsboth amino and carboxylic groups This aspect is similar to that of RhBmolecule, which could guide us to guess the acidic aqueous solution will besuitable for the photodegradation of this antibiotic over MoS2/g-

C3N4undervisiblelight.Figure3.41showsthatthepH4wasfavourableforthephoto degradationofENR,an acidic medium,asexpected.

Figure 3.41.Photodegradation of 20 mL ENR of 5 ppm, catalyst loading:0.5g.L -1 ,underbluelight (0.2 A,3.0V)for2h,25 o Catdifferent pHs.

The optimal catalyst loading was also obtained from Figure 3.42 withthe value of 1 g/L Here, the similar trend also observed as those of RhB andMB.

Under the optimal conditions of solution pH and catalyst loading,thereactionrateofthephotodegradationofENRwithotherspecificconditionswasdetermine dasshowninFigure3.43withthevalueof0.007min -1 Thisrate constantwas10timeslowerthanthatinthecaseofRhBphotodegradationoverthe same catalyst and initial concentration, even under the used lamp powertwicelarger.

Figure3.42.Effect ofcatalystloadingsonphotodegradationof20mLENRof5ppm, under LEDblue light (0.2A,3.0V) for2h,25 o CatpH4.

3.0 V) at pH 4, 25 o C, initial EFA concentration 5 ppm, catalyst loading 1 g.L -

C3N4underblueLEDs.AsshowninFigure3.44thechangesofCODandconcentrationoftheENRsolutionafter4 hours of illumination are completely different A reduction in COD of thesolutionfor8hoursofirradiationwasalsopresentedtoexploremoreaboutthemineralizatio ndegree.

Figure 3.44 ENR conversion and COD reduction after 4 h of irradiationunder LED blue light (0.2 A, 3.0 V), pH 4, initial concentration

More specifically, while ENR in the solution was entirely degraded, itsCODreducedonlyaround20%forthefirst4hours.Thissignificantdifferenceindicated that ENR itself could easily be totally degraded but not direct tosimple molecules, instead it was partially oxidized to intermediates during thephotocatalyticprocess.ThispointwasalsosupportedbytheHPLCchromatogram of the ENR solution as shown in Figure 3.45 The peak at theretention time of 10.24 min which belongs to ENR completely disappearedafter a 4-hour period of illumination under LED blue light, this is consistentwiththeaboveobservationfromUV-

Vismeasurement.Alltheformedintermediates with high relative abundance have higher molecular massescompared to that of ENR, which was confirmed by the corresponding massspectra (see Appendix 2), implying that the ENR molecule under the givenconditionswaspartiallydecomposed.Thisresultalsoexplainedthelowreduc tionofCODincomparisonwithENRconversionasdiscussedpreviously.

Figure3.45.HPLC chromatogram of ENRsolutionafter(a)0h,(b)4hand

This was also brought up a new question, whether the formed intermediatesbecomemoretoxicthantheENRitselftotheenvironmentorhumanhea lth.

Toanswerthistoughbutnecessaryquestionatoxicitytestshouldbeconducted[180] This test belongs to the biochemistry area, nevertheless, that is stillneeded toinvestigateinthe future research.

The recovery of the used catalyst for recycling is very important whenemploying a pilot in practical application The methods have been widelyappliedsuchascatalystsedimentation[53],immobilization[149],usingme mbraneusing[127],etc.Thepilotinthisworkwasdesignedtousethefirstone due to its simplicity In order to apply this method efficiently, an increasein sedimentation rate plays an important role To achieve this high rate, therehave been two common methods, namely, adjusting pH of the suspension tothe point of zero charge of the catalyst or adding an appropriate electrolytesubstancetothesystem[53].

The first option was chosen with the result shown in Figure 3.46 It isobvious that at pH 3.5 the catalyst exhibited the fastest rate of settlement dueto that value of pH closest to the pHpzc(3.6) of the catalyst MCN1, resulting inaquickaggregationofunchargedparticles,thereforetheincreaseinthesedimenta tion rate.

Asshownintheabovefigure,thesedimentationoccurredfastwithinthefirst 80 minutes of the process, then kept unchanged for the next 40 minutes.Basedonthisobservationtherecoveryofthecatalystwhenusingthepilotafter1, 2 and 3 hours of sedimentation of the suspension at pH 3.5 was carried out,with resultshowninFigure3.47.

Figure 3.46 Transmittance of 800-nm electromagnetic wave throughMoS2/g-C3N4suspension(0.7g.L -

Figure 3.47 Percentage of catalyst recovery after different sedimentationtimesofMoS2/g-C3N4catalyst (0.7g.L -1 )suspension at pH3.5.

The recovery reached nearly 80% after 2 hours of catalyst settlement,andalmostremainedunchangedfor1hourmore.Althoughthelossofcatal ystwas still high, however, this drawback could be acceptable due to the simpleand low-costprocess.

The photocatalytic activity of the recycled material was evaluated in thesameconditionsasforthefirst-usesample,theresultwasshowninFigure3.48.

Figure 3.48 Recycling test for the photocatalytic degradation of RhB overMCN1 sample Conditions of process: irradiated volume: 25 mL, initialRhB concentration: 5.0 mg.L -1 , pH 3.0, catalyst loading: 0.7 g.L -1 , 25 o C,under blue light.

From the recycling test, it could be concluded that the material after thefirstusestillexhibiteditsphotocatalyticactivityforthenextrunswithinsignificant decreases.

ToevaluatethethroughputofthepilotRhBsolutionwasusedassimulated wastewater with the following conditions: 30 L of 5 ppm RhBsolution at pH 3.5, MCN1 catalyst loading 0.7 g.L -1 , illuminated area 0.24 m 2 of 2 sets of blue LEDs (15 V, 5A), flow rate 8 L.min -1 These conditions alongwiththedistributionofwastewaterasathinlayerwhichwaspreviouslyprovengoodforth eexcitementofthecatalystwerealsotakenintoaccountinthepilotdesign Under these conditions, it took 120 h for stimulated wastewater beingcompletelytreated, therefore the calculatedpilotthroughputwas1.0L.h -1 m -2

Itisobviousthatthisvalueisproportionaltotheilluminatedarea,themoretheareaofthesolu tionbeingilluminated,thelargerthevolumeofthesolutioncanbe treated per hour That means the application of the pilot can be feasible iftheilluminatedareaislargeenoughtomeetaparticularrequirement.Actually,thepilotitse lfisunlikelytoapplydirectlytoaspecificsituation.Nevertheless,it becomes useful when putting it in a complete system in which the pilot isused as the last part before the treated water is discharged Thus, the pilot ismost suitable for wastewater containing substances that are not biodegradableat low concentration Instead of using electricity for the illumination, thesunlight can be applied effectively if the collector tilts a appropriate angledepending ontheposition where thepilotis beingplaced.

1 The heterojunction WS2/g-C3N4composites were successfully constructedviaafacilecalcinationdirectlyfromtheprecursorsoftungsticacidandthiourea in the solid state The experimental results indicates that the weight ratio ofWS2to g-C3N4in the composites affects their photocatalytic activities. Amongthe composites, 7WCN (synthesized from H2WO4and thiourea with the massratio 1:7) is the best material which could photodegrade 85.3% MB in 6 hoursunder visible light A synergistic effect of components in the heterojunction ofthe composites for enhancing photocatalytic performance was proposed. Inaddition,theMoS2/g-

C3N4c o m p o s i t e s w e r e s y n t h e s i z e d b y a s i m p l e method from sodium molybdate and thiourea in solid state, without the needfor hydrothermal-condition and ultrasound process steps as previous reports,therebya v o i d i n g t h e h i g h - p r e s s u r e a n d h i g h - e n e r g y c o n s u m p t i o n procedure.Thesynthesizedcompositeswereprove nt o b e e f f i c i e n t a n d activeinphotocatalysis,e s p e c i a l l y t h e M C N

1 ( s y n t h e s i z e d f r o m h e a t i n g the mixture of 0.06 gram MoS2and

N2gas)samplewiththeoptimumMoS2contentinwhichthei n t e r f a c i a l c h a r g e transferincreasedandthusreducedtheelectron- holer e c o m b i n a t i o n , improvingthephotocatalyticactivity.

2 The adsorption step plays a crucial role in the whole photocatalyticprocess,themorethetargetmoleculesadsorbonthephotocatalyst’ssurfa cethe faster they would be photodegraded However, too many adsorbedmolecules on the surface could lead to a negative effect on the overallphotodegradation rate due to the lack of oxidizers on the surface such asoxygen.

3 The reaction system used MoS2/g-C3N4photocatalyst and lightemittingdiode(LED)wasproventohaveavalueofnewbenchmarkphotochemicals pace-time yield (PSTY) of 8.3x10 -3 day -1 kW -1 This value was more than100 times higher than that of the previous system (300 W Xe lamp, usedvolumeof50mL,andthesameinitialconcentrationofthetargetmolecule)also employed the same photocatalyst MoS2/g-C3N4over the same targetmolecule rhodamineB.

4 The designed photocatalytic pilot can operate automatically and applythe natural sedimentation for recycling photocatalyst, opening a new doortotransferthelab- scaleintovariouspracticalapplicationsincludingwastewater treatment under visible light.

1 Huu Ha Tran,Duy Huong Truong, Thanh Tam Truong, Thi Xuan DieuNguyen, Ying-Shi Jin, Sung Jin Kim, and Vien Vo, “A Facile SynthesisofWS2/g-

Chem.Soc.,vol.39,no.8,pp.965–971,2018.

2 D H Truong, V Vo, T Van Gerven, and M E Leblebici, “ A

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LC-MSofENR solutionafter0h,4hand8hofillumination a) 0h b) 4h

catalystintheconditions:irradiatedvolume:25mL,initialRhBconcentration

ppm, pH: 3.0, 25 o C, under blue light, and (b) MB degradation over 7WCNcatalystintheconditions:irradiatedvolume:90mL,i n i t i a l M B concentra tion:3 0 0 m g L -1 ,p H 6 4 , 2 5 o C,u n d e r 1 0 0 W i n c a n d e s c e n t l a m p

Therefore, at too high concentration of H2O2, the degradation reactionwasinhibited.TheeffectofH2O2itselfonthedegradationofRhBwasalsoco nducted for comparison It is clear that in the same conditions, however,without the catalyst then RhB concentration is still unchanged, this isconsistent with previousstudy[41].

This result shows the importance of the presence of an acceptorelectron or an oxidant to the photocatalytic process Therefore, in the caseofwithoutaddinganoxidizingreagent,thedissolvedoxygenintheaqueoussolution becomes vital for the photocatalytic reactions This observationsupportedtheconclusionthatwehavediscussedsofarabouttheadsorptionco mpetition of target molecules against the dissolved oxygen molecule ontothesurfaceofthephotocatalystleadingtoadecreaseinthephotodegradation rate.

Generally,thereactivespecieswhichparticipateinthesurfacereactiondu ringthephotocatalyticprocess,includinghole,superoxideradicalanion,hydrox ylradical,andelectron.Toexplorewhichofthemhadthemostcontributiontothephot ocatalyticprocess,thecorrespondingreactivespeciesscavengerswereemployedint hereactionsystemthatusedMoS2/g-C3N4asaphotocatalysttodegradeRhB asshowninFigure3.37.

Figure 3.37 Photodegradation of RhB over MCN1 catalyst in the presence ofdifferent trapping agents TEOA, BQ, TBA, and DMSO as hole, superoxideradicalanion,hydroxyl radical,electron scavengers,respectively.

ThechangesinRhBphotodegradationrateindicatedthatalltheusedscavengers hadnegativeeffectsonthephotocatalyticactivityofthecatalystto different extents in order of TEOA > BQ > TBA > DMSO The resultalso indicated that TEOA scavenger exhibited much stronger inhibitionshowingthatholewasthedominantreactivespeciesinthephotodegra dationprocessofRhBoverMCN1catalystunderstudiedconditions.Thisimpor tantroleofholewassupportedbythemorepositive valencebandedgeofg-C3N4comparedtotheredoxpotentialofRhB(+1.57V vs +1.43 V) [201].The hole of MoS2also was able to oxidize RhBdirectlyduetoitsmorepositivepotential+2V[136].Meanwhile,thequitestrong adverse-effect caused by BQ and TBA scavengers revealed that theoxygenreactivespecies,namely,thesuperoxideradicalanionandhydroxylradical also play their important part in the overall photocatalysis.