durable superhydrophobic coating material Preparation of sulfur hydrophobized plasmonic photocatalysttowards Journal of Materials Science & Technology

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ContentslistsavailableatScienceDirect

Journal of Materials Science & Technology

jou rn a l h o m e p a g e :w w w . j ms t . o r g

Research Article

Preparation of sulfur hydrophobized plasmonic photocatalyst towards durable superhydrophobic coating material

Emese Lantos

^a

, László Mérai

^a

, Ágota Deák

^a

, Juan Gómez- Pérez

^b

, Dániel Seb ˝ok

^a

, Imre Dékány

^a

, Zoltán Kónya

^b

, László Janovák

^a,∗

aUniversityofSzeged,InterdisciplinaryExcellenceCentre,DepartmentofPhysicalChemistryandMaterialsScience,H-6720,RerrichBélatér1,Szeged, Hungary

bUniversityofSzeged,InterdisciplinaryExcellenceCentre,DepartmentofAppliedandEnvironmentalChemistry,H-6720,RerrichBélatér1,Szeged,Hungary

a r t i c l e i n f o

Articlehistory:

Received6February2019

Receivedinrevisedform23March2019 Accepted15April2019

Availableonline16November2019

Keywords:

Nanocomposite Coatingmaterials Wetting

Lotus-typestructure Visiblelightphotocatalyst

a b s t r a c t

Thewidelyusedphotocatalyticself-cleaningcoatingmaterialsareoftenmadeofpolymersandpolymer basedcomposites,wherethephotocatalystimmobilizationoccurswithmacromolecules.However,these organicpolymersareoftenunstableunderexposuretoUVirradiationandeasilydegradedbyreactive radicalsproducedinthephotocatalyticreaction.Inordertosolvethisproblem,inthispaper,wepresent thefacilepreparationofamultifunctionalcoatingwithdualsuperhydrophobicandphotocatalyticprop- erties,wherethefixationandthehydrophobizationoftheplasmonicAg-TiO2photocatalystparticles withvisiblelightactivitywasperformedwithnon-watersolublesulfur,whichisacheapandeasily availablematerial.Theresultednovelnanocompositewithroughandnano-tructuredsurfaceroughness (1.25–2.45nmdeterminedbysmall-angleX-rayscattering)hassufficientlowsurfaceenergy(3.3mJ/m²) forsuperhydrophobic(=151.1^◦)properties.Moreover,incontrastoftheorganicandexpensivefluo- ropolymerbasedcomposites,thisnon-wettingnaturewasdurable,becausethemeasuredwashigher than150^◦duringthelong-termLED(␭max=405nm)lightirradiation.

Technology.

1. Introduction

Photocatalyst-basedfunctionalsurfacesareoneofthestate-of- the-artmaterialsthatarecommerciallyappliedinseveralﬁelds, including wastewater and air treatment or even food-packing.

After the discovery of the photo-induced superhydrophilicity of photocatalyst (e.g.,TiO₂) layers [1], commercially applicable photofunctionalmaterialswithself-cleaningpropertiesweresyn- thesizedusingTiO2 particlesandcompositethinﬁlms.Sincethe bandgapof TiO2 fallswithintheUV A range, it ispossibleto extendtheabsorptionspectrumofthesemiconductorphotocata- lystsbymodifyingthecatalystparticleswith,forexample,different metallicornon-metallicelements[2–4].Thefunctionalizedcata- lystscanbeexcitedbytheUV Aorvisible(VIS)lightduetothe surfaceplasmonresonanceeffect:themetalnanoparticlesabsorb electromagneticradiationatlowerenergywavelengths[4,5].

The incorporation of semiconductor photocatalyst particles in an appropriate polymer-ased support or binder material is expected to create antimicrobial and self-cleaning properties,

∗Correspondingauthor.

E-mailaddress:janovakl@chem.u-szeged.hu(L.Janovák).

whichwouldextenditsﬁeld ofapplication.Thestate-of-the-art materialsattheareaofphotocatalyst/polymercompositelayers arethefunctionalsurfaces,whicharegainingattentionfordifferent practicalﬁeldsofapplication,suchasfoodpackingmaterialswith antibacterialbehaviours [6]and self-cleaning superhydrophobic coating[7,8].

Sincetheirdiscovery,superhydrophobicsurfaces(≥150^◦,low contactanglehysteresis)areinfocusowingtotheirlowsurfacefree energyandbeneﬁcialself-cleaningnature.Besidestheoriginally hydrophobicnatureofthematerial,asurfacehastobetextured inboththemicro-,andnano-scaletopossesssuperhydrophobic characteristics[9].Therearenumerousreportedwaystoproduce morphologicallyproper surfaces,including lithographic,chemi- caletching,templatesandevenspray-coatingmethods,whichis cheap,scaleable,andeasilyimplementable.Inthiscase,particulate material(e.g.,aphotocatalyst)getssprayedonasubstrate,forming thedesiredsurﬁcialstructureswithadequatesurfaceroughness.

Ifhydrophobizedphotocatalystsareusedasroughnessenhancer particulate materials,it resultsin theformation ofbifunctional compositesurfaceswithdualphotocatalyticandextremewetting properties[10–13].

Whenpolymerimmobilizedphotoreactivehybridsurfacesare prepared,thephotodegradationofthematrixalsohastobetaken

https://doi.org/10.1016/j.jmst.2019.04.046

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160 E.Lantosetal./JournalofMaterialsScience&Technology41(2020)159–167

intoconsideration,sothereportedlyappliedhydrophobicorganic polymers(e.g.perfluorynatedpolyacrylates,orPTFE)maybeinap- propriateforlongtermuses.Becauseofthis,thedemandishighfor durablematrixmaterials.Tosatisfytheneedsforincreaseddura- bility,sulfuralsocanbeappliedasaninert,non-photodegradable andinsolubleinorganichydrophobizingagent.Therearevarious applicationofelementalsulfurandsulfurnanoparticlesnowadays suchasagrochemicalindustries[14]fungicidesinagriculturefields [15],modificationofcarbonnano-tubes[16],antibacterialmate- rial[17], andsynthesisofnanocompositesforlithium batteries [18].However,accordingtoourbestofknowledge,thisisthefirst demonstrationofutilizingsulfurnanoparticlesasinerthydropho- bizingagentforcreationofsuperhydrophobicandphotoreactive functionalsurface.

Thus, in this paper, we present a simple and cheap spray- coatingmethodtopreparenon-photodegradableVISlight-active andmicro/nanostructuredsuperhydrophobicsurfacesoninorganic sulfurbasis:theVISlight-activityowestothe10wt%plasmonicAg- TiO₂ photocataystcontent,whilethedurablesuperhydrophobic natureisattributed tosulfur-nanoparticles,prepared bya sim- pleprecipitationmethodinaqueousmedium.Thesebifunctional inorganiccompositelayerswithlong-termdurabilityshowedcon- siderable photocatalytic efﬁciency duringEtOH (g) degradation testsatsolid/gasinterface.

2. Materialsandmethods

2.1. PreparationofthesulfurhydropobizedAg-TiO2layers

The VIS light-active plasmonic Ag-TiO₂ photocatalyst with 0.5wt% surfacesilver nanoparticle contentwassynthesized via thedirectfunctionalizationofTiO2particles.Thesyntheticmethod andtheopticalcharacterizationofthecatalystaredetailedinour previouspublication[4,5].

Toobtainsulfurnanoparticles,0.03gofsulfurpowder(Sigma Aldrich) wasdissolved in 4ml of 10M aqueousNaOH-solution (pH=14.15)atatemperatureof80^◦C.Afterwards,theliquidwas stirredthendilutedwith1mlof10MNaOHsolution,then4.65ml of2.5MH₂SO₄solutionwasaddeddropwiseuntilthedispersion becameturbid(pH=13.95atprecipitation)duetotheformedsulfur nanoparticles(c≈0.09g/100ml).

Theresulting precipitatedsulfur wascentrifuged (8000rpm, 5min),thoroughlywashedwithdistilledwateranddriedovernight in a drying cabinetat 60^◦C. Themixing of theobtained sulfur nanoparticlesandthepreviouslysynthetizedAg-TiO2 photocata- lystoccurredinaqueousdispersion(c=0.01g/100ml).Thesulfur/

photocatalystratio(0,40,60,80,85,90and100wt%S-content)was systematicallychangedduringthepreparation.

To prepare 5cm×5cm photoreactive inorganic layers con- sisting of 10wt% Ag-TiO₂ photocatalyst and 90wt% sulfur nanoparticles,theaqueousdispersions wereevenly sprayedon glasssubstrates,usinganR180-typeairbrushspraygunatanoper- atingpressureof3bar.

2.2. Methodsofsamplecharacterization

Thesizedistributionandmorphologyofthepreparedsulfur,and theAg-TiO₂/Scompositenanoparticleswerecharacterizedduring highresolution transmissionelectronmicroscopy(HRTEM).The appliedFEITecnaiG220X-TWINmicroscope,isequippedwitha tungstencathode(200kVaccelerationvoltage).

SurfacemorphologyoftheAg-TiO2/Slayerswasexaminedwith ascanningelectronmicroscope(SEM,HitachiS-4700,secondary electrondetector,accelerationvoltage:10or20kV),whilethesur- ﬁcialcompositionwasstudiedusingaRöntecEDSdetector.

OpticalcharacterizationbydiffusereﬂectanceUV–VISspectra wererecordedviausingaCHEM2000UV–VIS(USB2000+UV-VIS, OceanOpticsInc.,Dunedin,FL)spectrophotometer,equippedwith anintegratedsphere.

Thesurfacefractaldimensions(DS)andsurfaceroughness(SR) ofthesampleswereinvestigatedbysmall-angleX-rayscattering (SAXS)technique.SAXScurveswererecordedwithaslitcollimated Kratkycompactsmall-anglesystem(KCEC/3Anton-PaarKG,Graz, Austria)equipped witha position-sensitive detector(PSD 50M fromM.BraunAG.Munich,Germany)containing1024channels, 54␮minwidth.CuK˛radiationwasgeneratedbyusingaPhilips PW1830X-raygeneratoroperatingat40kVand30mA.TheKratky camerawascalibratedusingsilverbehenatewithawell-deﬁned lamellarstructure(d=5.848nm).

Surface charge values of the photocatalyst and the sulfur suspensionsweremeasuredbymeansofaparticlechargedetec- tor (PCD-04 Particle Charge Detector; Mütek Analytic GmbH, Germany)withmanualtitration.Underatitrationprocessthepos- itivesurfacechargeoftheAg-TiO₂photocatalystsparticles(in0.01

%aqueousmedium,pH=5.0setbyHCl)willbecompensatedwith negativelychargedsulfurnanoparticles(in0.09%aqueousmedium, pH=5.0 setbyHCl)withconcomitantstreamingpotentialmea- surements.

Composition-dependent crystalline properties were studied usingaPhilipspowderX-raydiffractometer(PW1830generator, PW1820goniometer,CuK_˛:␭=0.1542nm,40–50kV,30–40mA, 2:2^◦–70^◦,25.0±0.5^◦C).

TheRamanspectrawererecordedwith1.5cm⁻¹interferometric resolutionapplyingaSenterra(Bruker)Ramanmicroscopeunder 50xmagniﬁcationwith2.50mWoutputpowerofthe␭=532nm He–Nelaserlightsourceand3000msintegrationtime.Thepre- sentedspectraarethecoadditionsof5singlespectrainallcases, recordedatdifferentspotsofthesample.

Theapparentstaticcontact anglesonAg-TiO2/Slayerswere measuredaccordingtothesessiledroptechniqueat25.0±0.5^◦C under atmospheric pressure, applying an EasyDrop drop shape analysissystem(KrüssGmbH,Hamburg,Germany)equippedwith DSA100software,aPeltiertemperaturechamberandasteelsyringe needleof0.5mmdiameter,andusingdistilledwaterasatestliquid.

Beside thestatic contact anglesthedynamic wetting ofthe compositeswasalsomeasuredbecauseaccordingtothetheoryof DrelichandChibowski[13,19]themeasuredadvancing(adv)and receding(rec)contactanglesaresuitablefortheestimationofthe totalapparentsurfacefreeenergy(stot)ofthelayer,knowingthe surfacetensionoftheprobeliquid,(l=72.1mN/minthecaseof distilledwaterat25^◦C)anditscontactanglehysteresis,whichis deﬁnedasthedifferencebetweentheadvandrec,asshownby Eq.(1):

s^tot= 1

1+cosadv

2

2+cosrec+cosadv

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TostudythephotocatalyticpropertiesoftheAg-TiO₂/Slayers, ethanol(EtOH)degradationtestswererununderblueLEDlight (GeneralElectric’s,Veresegyház,Hungary,␭=405nm)illumination atsolid/gasinterface,whilegaschromatographicmeasurements werecarriedout(ShimadzuGC-14B)toquantifytherateofEtOH degradation.Duringthemeasurements,5␮lofEtOH(abs.,Molac Chemicals,Hungary)wasinjectedintothereactionchambercon- taining25cm²(25.0±0.5^◦C)hybridlayers(5mg/cm²),ﬁxedata 5cmdistancefromthelightsource.Thec/c₀valuescalculatedfrom peakareasweredeterminedasthefunctionofilluminationtime, wherecistheconcentrationofEtOHattime(t)andc0istheinitial concentration(c₀=0.36mM).Theapparentreactionrateconstants (k,min⁻¹)werecalculatedforﬁrst-orderdecayasshownbythe equation:ln(c/c0)=−kt.

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Fig.1.TEM-micrographsofprecipitatedsulfurnanoparticlesandwettingofspray-coatedS-nanoparticlelayerswithdeionizedwaterasatestliquid(a),aqueousdispersion ofpreparedsulfurnanoparticlesuponprecipitationand24hlater(b),size-distributionofprecipitatedsulfurnanoparticles(c).

During the stability tests, photoreactive composite layers of 5mg/cm²speciﬁcmasswereilluminatedfor14daysbythesame blueLED-light(=405nm)placedata5cmdistancefromthesam- ples, underambientatmosphere. Thechanges in water contact angleofthelayerswasmeasureddailyduringtheexperiment.Poly- merbasedcompositethinﬁlmswith60wt%incorporatedAg-TiO2

particleswereusedasreference.

Similartopreviouswork[4],theadhesivetapetestwasusedto investigatethemechanicalpropertiesofthepreparedcomposite films.Briefly,thedouble-sidedadhesivefoamtape(1.6mmthick) wasadheredtothefilmsfirstly,anda100gweightwasrolledover thetapetoensureconsistency.Thetapewasthenpeeledbackat anangleofapproximately45^◦.

3. Resultsanddiscussion

3.1. Structuralcharacterizationofcompositenanoparticles

AccordingtotheWenzelandCassie-Baxtermodels[20],toreach thesuperhydrophobicextremewettingcharacter,theprovisionof hierarchicalroughnesstoanaturallyhydrophobicsurfaceiscrucial.

Toachievethisgoal,sulfurnanoparticleswithamaximumdiam- eterof40nmweresynthesizedbyapplyingasimpleprecipitation method.Hencetherearemanyreportedbottom-upwaystopre- paresulfurnanoparticles,includingthesulfurvapourdeposition oncoldwater[21]ortheWeimarn-[22],andRaffo-methods[23], onlythelastoneoffersparticleyieldshigherenough(∼600g/l)in aqueousmediatobeindustriallyproﬁtable[24].Ourversionofthe appliedRaffo-methodisbasedonthefollowingchemicalreactions [25]:

S+6NaOH =Na2S2O3+2Na2S+3H2O (2) Na₂S₂O₃+H₂SO₄=H₂S₂O₃+Na₂SO₄ (3) xH₂S₂O₃=yH₂O+zSO₂+H₂S_mO₆ (4) H₂S_mO₆=H₂S_m-_nO₆+S_n(n>5) (5) Upon the acidiﬁcation of S₂O₃²⁻-containing solutions with sulfuric acid, SO₂ and colloidal S forms. We obtainedS₂O₃²⁻- containing solution as a result of dissolving elemental S in pH=14.15NaOH-solution at 80^◦C, then S-nanoparticles witha maximum meandiameterof 40nm formedupon thefollowing additionof 2.5MH2SO4 solution,which is thecorestepof the

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Fig.2. Measuredstreamingpotentialvaluesofthe0.01%Ag-TiO2photocatalystsuspensionduringtheadditionofthe0.09%suspensionofthesulfurnanoparticlesatapH of5.00(a)andtherecordedRamanspectraofAg-TiO2photocatalyst,sulfurnanoparticles,andAg-TiO2/Scompositesamplewithsuperhydrophobicwettingproperties(b).

classicalRaffo-method.AccordingtoEqs.(2–5),thedissolutionof sulfurin suchmediaisa disproportionationreaction,whilethe reverseprecipitationreactionissynproportionation.Asourprocess requiresbothelementalandprocessedS(H2SO4)andoffersgood yieldsitsindustrialapplicationcouldbebeneﬁcialintheelimina- tionofresidualSasaby-productofthepetrolindustry.

Asshown in Fig. 1(a), the size distribution of theobtained sphericalsulfurnanoparticlesisrelativelypolydisperse.Themean diametersrange between1and 40nm,which is comparableto thesizeoftheappliedVISlight-activeplasmonicAg-TiO2photo- catalystparticles(d_prim.∼25nm)[4,5].Becausethehydrophobic natureofthesulfurparticles,thestabilityoftheobtainedaqueous suspensionwasverylow.

Thesulfurnanoparticlesalsoshowsuperhydrophobiccharacter (=151.0^◦)astheyareevenlydispersedonplainsurfaces(Fig.1(b)), because the formed sulfur layer consists of hydrophobic non- water-solublenanoparticleswithroughsurfaceinthenanometer scale.TheﬁxationoftheplasmonicAg-TiO₂ photocatalystparti- clesonthesurfaceofthesulfurparticleswasexecutedthrough electrostaticinteractionbecausethepH-dependentsurfacecharge ofTiO₂andAg-TiO₂iswell-knownintheliteratureandithaspos- itivelychargedbelowpH=6(pointofzerocharge)[25],whilethe chargeofthesulfurnanoparticleswasslightlynegative(pointof zerochargeatpH=2[24]).Thus,thesurfaceﬁxationwascarried outatpH=5becauseat thisvaluethecomponentshave oppo- sitecharge and theobtainedcharge titrationcurve achievedat thispHispresentedonFig.2(a).Itcanbeseenthatthepositive chargeoftheAg-TiO₂ particleswascontinuouslydecreaseddur- ingtheadditionofthenegativelychargedsulfursuspension.The insertedphotosalsoshowthatatthechargecompensationpoint (streamingpotential=0)thestabilityoftheinitialAg-TiO₂andsul- fursuspensionwasdrasticallydecreasedbecauseofthelost(i.e., compensated)chargeofthecomponentsandtheformationofthe heterocoagulatedS/Ag-TiO₂system.

Fig.2(b)showstherecordedRamanspectraoforS,Ag-TiO₂,and Ag-TiO2/Scompositenanoparticles.HencetheP25TiO2useddur- ingthesynthesisoftheplasmonicphotocatalystconsistof75wt%

anataseand25wt%rutile,onlythecharacteristicpeaksofanatase at144cm⁻¹(Eg),397.5cm⁻¹(B1g),517.5cm^-1(B1g)and637.5cm^-1

(Eg)could bedetectedupon ␭=532nm laser-lightillumination [26].

Thepresence ofsilver isundetectableatthese experimental conditions,duetoitslowconcentration(0.5wt%).Inthecaseof sulfur samples,peaks(withthecorrespondingmolecularvibra- tionalmodes)at82.5cm⁻¹(B_2g),153cm⁻¹(Eg),187.5cm⁻¹(Eu), 219cm⁻¹(A1g),244.5cm⁻¹(A1g),435cm⁻¹(B2g)and472.5cm⁻¹ (A_1g and E_g)are characteristics tothe vibrational bondsof S₈- molecules[27,28], whilethepeak at51cm⁻¹ belongs totheag

lattice-vibrational mode. Fig. 2(b) also shows, that if we form superhydrophobiccompositefromtheinitialsulfur,andAg-TiO₂ nanoparticles, the intensity of sulfur peaks increases, while at 90wt% S-content, thecharacteristic peaks of rutile almostdis- appear, indicating the decreased surface concentration of the photocatalyst. However,no chemical interactionwas identiﬁed betweenthesulpurandtheAg-TiO₂particlesevidencedbyRaman measurements.

Fig.3showsthepowderX-raydiffractogramsoftheresulting composites:asthesulfur-contentincreases,thecharacteristicpeak oftheanatasestructureofTiO2at2=25.32^◦decreaseinintensity, whileotherpeaksappear,attributedtotheorthorhombic␣-sulfur (withhigherintensities:23.10^◦,25.88^◦,26.76^◦,27.76^◦,28.96^◦)[29], makingtheidentiﬁcationofthephotocatalystmoredifﬁcultdueto convolution.

Moreover,somepeaksarecharacteristicstomonoclinic␤-sulfur (27.74^◦,31.08^◦,37.84^◦)[30].Theobservationsindicatethecrys- talline nature of theprecipitated sulfur-nanoparticles,since no broadbandofamorphousphaseweredetected.Ascanbeseenin theinsertedpicturesofFig.3,thetwocomponentspowdersamples havehighdispersibility inaqueousmediaunder 90wt% sulfur- content,butthis waterdispersibilitydrasticallydropsathigher sulfurcontents.Thisindicatesanoptimallyhydrophobicnatureand lowsurfacefreeenergytobeappliedinsurfacehydrophobization.

Besidetheabovepresented X-raydiffractogramsresults,the compositeswerealsocharacterizedbysmall-angleX-rayscattering (SAXS) methodbecause thefractal plotof thesescattering curvesgivesreliableinformationaboutthequalityofthesurface roughness(SR)withaquantitativeinformationbasedonthesur- facefractal dimensions.The doublelogarithmic (fractal)plotof

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Fig.3. PowderX-raydiffractogramsandaqueousdispersionsofAg-TiO2,SandAg- TiO2/S.

thescatteringcurvesofAg-TiO2 andAg-TiO2/Spowdersamples with60wt%S-contentshowsthatthesampleshavearatherrough surface,indicatedbytheD_S=2.8(Fig.4(a)).

Furthermore,theh³I(h)vs.h²plotofthecurvesshowsapositive deviationfromtheasymptoticbehaviour(seeFig.4(a)),mainlyin thecaseoftheinitialAg-TiO₂samplewhichindicatesasigniﬁcant interfacialelectrondensityinhomogeneity,i.e.surfaceroughness.

ThecalculatedvaluesareSR=2.45and1.25nminthecaseofAg- TiO₂and40%Ag-TiO₂/60%Ssamples,respectively(seeEq.E(6)in SupplementaryInformation):thesurfaceroughnessofthesam- plesdidnotdecreasedsigniﬁcantlyduringtheadditionofsulfur.

Becausethesurfaceroughnessisacrucialfactorofthesuperhy-

Fig.5.ApparentstaticwatercontactanglesonAg-TiO2/Scompositesurfacesasa functionofS-content.Thephotographillustratesthewater-repellentbehaviourof 90wt%S-containingcompositepowderﬁlmwithwaterdropcolouredwithmethy- lenebluedye.

drophobiccoatings,amoredetaileddescriptionofthecomposites structurecanbefoundinSupplementaryInformation(Figs.S1–S3).

3.2. Wettingandmorphologicalpropertiesandsurfacefree energydetermination

Theabovepresentedresultsindicatedthatthepresenceofthe sulfurnanoparticleschangedthewetting andstructuralproper- ties oftheAg-TiO₂,whichwasalsoevidenced bycontactangle measurements(Fig.5).

As thesulfur content increases, thecomposite-covered surfaces (spray-coated, with 1±0.5mg/cm² speciﬁc layer mass) changedtheirwettingcharacterfromsuperhydrophilic(≈0^◦)to superhydrophobic(>150^◦).Thissulfurinducedsuperhydrophobic characterofthecompositelayeralsocanbeseenontheinserted photographinFig.5.Thewaterdropscolouredwithmethyleneblue

Fig.4. I(h)vs.h(a)andh³I(h)vs.h²(b)plotsofthescatteringcurvesofAg-TiO2andAg-TiO2/Spowdersampleswith60wt%S-content.

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Fig.6.Evolutionofadvancingandrecedingcontactanglesonspray-coated90wt%

sulfur-hydrophobizedphotoreactiveAg-TiO2layers,aswellasthedeterminedtotal apparentsurfacefreeenergy(␥stot)valuesbeforeandafter180minblueLEDlight illumination(␭=405nm).

dyewererolleddownfromthesurfaceofthesuperhydrophobic compositeﬁlm.

Advancingandrecedingcontactangleswerealsomeasuredon thesuperhydrophobicﬁlmwith90wt%S-content,accordingtothe protocolofDrelich[13].ThedatawasanalysedaccordingtoEq.(1) ofChibowsky[19]toobtainthecorrespondingtotalsurfacefree energyvalues.

Fig.6showsalargecontactanglehysteresis(adv=146.8^◦±3.0^◦

→rec=30.1^◦±0.6^◦),indicatingtheWenzel-typeroughnesschar- acteristicsof the surface [19]. However, this superhydrophobic naturewithhighcontactangleandlowsurfaceenergywasdurable andresistanceforirradiation,becauseafterilluminatingthesam- plewithmax=405nmblueLED-lightfor180min,therecvalues showedamoderatedecrease(rec=19.4^◦±2.0^◦),whiletheadvval-

Fig.8.UV-VISdiffuse-reﬂectancespectraandphotographsofAg-TiO2,Snanopar- ticlesandAg-TiO2/ScompositeswiththecorrespondingS-content(giveninwt%).

uesremainedalmostthesame(148.2^◦±2.5^◦),andthecalculated total surfacefree-energyvalues of thesurfacesdecreased upon illuminationfrom3.3mJ/m²to2.3mJ/m².

As evidenced by SEM-images and EDX-measurements, the surfaceofthespray-coated layersof90wt%sulfur-hydrophized Ag-TiO₂photocatalysthascauliﬂower-likeaggregates(Fig.7)with relativelyhomogenousandevendistributionofthetwocompo- nents.The presence of both components onthe surface ofthe roughcompositeﬁlmisadvantageousbecausethesulfurnanopar- ticlesareresponsibleforthesuperhydrophobicnature,whilethe presenceoffreephotocatalystparticlesensuresthephotocatalytic activity.Thismeansthatphotocatalyticreactionscouldtakeplace alloverthepreparedsurfaces.

3.3. Opticalandphotocatalyticproperties

Theopticalpropertiesofthehydrophobizedphotocatalystsam- pleswereslightlychangeduponsulfur-addition,whichcanbeseen intheUV-VISdiffuse-reﬂectancespectraonFig.8.Theabsorption

Fig.7.SEMmicrographsandEDX-mappingofAg-TiO2/Ssuperhydrophobicsurfaceswith90wt%S-content.

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Fig.9.CharacterizationofEtOHdecomposition(c0=0.36mM)onpureAg-TiO2andsulphurlayers,andonAg-TiO2/Scompositeﬁlm(90wt%S)underLEDlightillumination (=405nm)asthefunctionofilluminationtime.Thecompositelayerwasalsomeasuredunderdarkconditionforreference(a).EtOHconcentrationchangeonAg-TiO2/S ﬁlmafter0,24,48,72,96and168hcontinuousLEDlightirradiation(b).

wavelengthmaximagraduallyincreasefrom349to401nmasthe sulfurcontentisincreasedfrom0to100wt%,whiletheinitialpale browncolouroftheAg-TiO2graduallyturnedtoyellow.Theplas- monicabsorbancepeakataround450nmisduetothepresence ofAgNPsonTiO₂photocatalystparticlesasdemonstratedinour previouspapers[4,5].Inthisplasmonicphotocatalyst,visiblelight isharvestedbecausethesurfaceplasmonresonance,attributedto theAgNPs,whilethemetal–semiconductorinterfacetakespartin theseparationofthegeneratedholesandelectrons[4].

ItwasalsopresentedearlierthattheusedblueLEDlightsource with405nmemissionmaximumissufficientforexcitationofthe Ag-TiO₂catalyst[31,32].Inthisworkthephotocatalyticefficiency ofthesulfurhydrophobizedcompositephotocatalystlayerswere studiedusingthesameblueLEDlampandethanolasmodelvolatile organiccompound(Fig.9(a)).Asaresultof90minillumination,the pureAg-TiO2 photocatalystfilmdecomposedapproximatelythe wholeamount(c₀=0.36mM)oftheEtOH,whilethephotocatalytic effectivenessofthesulfurcontainingsuperhydrophobiclayerwere lower,butitwasshownsignificantphotocatalyticactivitycom- paredtothereferencesample(photolysiswithoutphotocatalyst andS/Ag-TiO₂ sampleindarkcondition).Thisdecreasedphoto- catalytic activityis becauseof thelower Ag-TiO2 photocatalyst content(only10%in90%non-photoreactivesulfur)butaccording tothewetting(Figs.5and6)andphotocatalyticresults(Fig.9), itcanbeclearly statedthatat acomposition of90%sulfur and 10wt%Ag-TiO2photocatalystthecompositelayerwasshownnot onlysuperhydrophobicbutalsophotocatalyticproperties.Inorder toprove thelong-term photoactivity of the composite sample, the EtOH concentration change was measured occasionally on S/Ag-TiO₂ filmafter0,24,48,72, 96and 168hcontinuousLED lightirradiation.TheobtainedresultsinFig.9(b)representthat theinitial(c0=0.36mM)concentrationofEtOHwasdecreasedto 0.26±0.015mM after30minadsorption timeonthesurface of 25cm²compositefilmwith5mg/cm²specificsurfacemass.After thisadsorptionequilibriumtheEtOHconcentrationwascontinu- ouslydecreasedduringtheLEDlightirradiationandthecalculated k’values (0.0090–0.0132 1/min)indicatethealmostunchanged photocatalyticactivityofthecompositeduringthestudiedtime interval.

Fig.10.Effectoflong-termUV-A(blueLED)irradiationonthewettingproperties oftheorganicpolymer-basedandinorganicsulfur-containingsuperhydrophobic compositecoatings.

In ourprevious work it was reported that this dual superhydrophobic and photocatalytic properties of the spray coated compositessurfacescouldbeveryusefulinvariousapplications, however,theundesiredself-photodegradationoftheearlierused organic(ﬂuoro)polymerbindercouldhindertheapplicationofthis coating materials [4,33]. In this work,the hydrophobization of theAg-TiO₂photocatalystparticleswasachievedbytheapplica- tionofinorganicsulfurinordertoavoiditsself-photodegradation.

Fig.10showsthatthemeasuredcontactanglevaluesofthesulfur hydrophobizedAgTiO₂wasunchanged(∼150^◦)duringthestudied timeinterval(14daysofcontinuousLEDlightirradiation).

Two polymer based composites were chosen as reference:

the Ag-TiO₂/ polyacrylate hybrid layer contains poly(ethyl- acrylate-co-methyl methacrylate) copolymer [4], while the

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polymer component of the Ag-TiO₂/fluoropolymer composite waspoly(perfluorodecylacrylate)fluoropolymer[31,32,34].Asa results,theinitialoftheAg-TiO₂/polyacrylatehybridlayerwas 85.2^◦±1.75^◦ which wasdrasticallychanged to0^◦ justafter two daysirradiation.The effectwasslowerinthecase of themore resistancefluoropolymerbasedbinderwithinitialsuperhydropho- bic(=151.1^◦±5.4^◦)properties.Thesedecreasingvaluesarethe resultsofthepreviouslyreportedpartialpolymerphotodegrada- tion:theilluminationchanges intheration ofcatalyst/polymer composition as the photocatalyst particles became uncoated [35]. This resulted in higher catalyst availability on the surface region. However, the sulfur hydrophobized photoreactive thin composite film shows durable hydrophobicity during the irradiation.

Finallythe mechanical properties ofthe preparedS/Ag-TiO2

film were also investigated and compared to the fluoropolymer based superhydrophobic thin film. The adhesive tape test was used to get information about theadhesion of the layers [4]. The tape was applied on the coating surface and subse- quently was removed by a quick pull. As indicate in Fig. S4, minoramountofsuperhydrophobic layercouldberemovedby the tape from both surfaces. However, this mechanical dam- agecaused thatboththefluoropolymerandsulfur basedlayers werelosttheirsuperhydrophobicity(seeinserted contactangle values) after the peeling. This vulnerability of superhydrophobic surfaces is well known in the literature and it originated fromtheroughsurface andporousstructureof theselotus-like coatings[36,37]. However, in ourprevious paper we also pre- sentedthatthenatureinspiredself-healingabilityoftheartificial coatingscouldsolvethisproblem,ifwecanachievethatthedam- agedsurfaceprotrusionscanregenerateitself[32].Inthefuture, wetrytoimplementthisself-assembledmechanism(airhumid- itycatalysedsyneresisofthehydrophobicdodecyltrichlorosilane molecules) into the sulfur containing superhydrophobic layer in order to get thin film with quick superhydrophobic recov- ery.

4. Conclusions

To satisfy the needs for superhydrophobic coatings with increaseddurability,sulfuralsocanbeappliedasaninert,non- photodegradableand insolubleinorganichydrophobizingagent.

Inthispaperwe presentedasimple methodtoutilizesulfur in thearea of functional surfaces: bifunctional, superhydrophobic andphotocatalyticcoatingswerepreparedof90wt%(d≤40nm) sulfur-nanoparticles and 10wt% (d≈25nm) visible light-active Ag-TiO₂ photocatalyst. The surface fixation of sulfur on Ag- TiO₂ particles occurred through electrostatic interaction and althoughnochemical bondwasidentifiedbetweenthecompo- nents,theinitiallysuperhydrophylicphotocatalystnanoparticles (≈0.0^◦)becamesuperhydrophobic(=150.3^◦±1.0^◦)uponform- ingcomposite particles.AsevidencedbyEDX-SEM,theextreme wettingpropertiesareattributedtothemultiscalesurfacerough- ness: the spray-coated layers have cauliflower-like aggregates with relatively homogenous distribution of the two components.

Thephotocatalyticactivity ofthesulfur containingsuperhydrophobic layer waslower than that of the pureAg-TiO₂ ﬁlm (degraded 100% of c0=0.36M EtOH in 180min), still showing signiﬁcant photoreactivity (photocatalytc effectiveness: 68.9%) comparedtothereferencesample(photolysiswithoutphotocatalyst;effectiveness:26.4%).Thisdecreasedphotocatalyticactivity isbecausethelowerphotocatalystcontent(only10%)butaccord-

ingtothewettingandphotocatalyticresultsitcanbeclearlystated thatatoptimalcompositionthelayerwasshownnotonlysuper- hydrophobicbutalsophotocatalyticproperties.

As a main result, in contrast to the tested polymer-type hydrophobizingagents,sulfurnanoparticlesdidnotphotodegrade inthetimeintervalof14daysofcontinuousLED-lightillumination(=405nm),sothehydrophobizedphotocatalystlayerskept theirwettingproperties(≈150^◦).Inconclusion,astheexpensive ﬂuoropolymerbinderscanbereplacedwithsulfurinself-cleaning superhydrophobicsurfaces.

Acknowledgements

ThisworkwasﬁnanciallysupportedbytheHungarianScien- tiﬁcResearchFund(OTKA)K116323,PD116224andtheprojectof GINOP-2.3.2-15-2016-00013,theUNKP-18-4NewNationalExcel- lence Programof theMinistryofHuman Capacitiesand bythe JánosBolyaiResearch ScholarshipoftheHungarianAcademyof Sciences and the Ministry of Human Capacities, Hungary (No.

20391-3/2018/FEKUSTRAT).

AppendixA. Supplementarydata

Supplementarymaterialrelatedtothisarticlecanbefound,in theonline version,at doi:https://doi.org/10.1016/j.jmst.2019.04.

046.

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