Article Photocatalytic behavior of water-based styrene-acrylic coatings containing TiO

16 Pages·2017·2.96 MB·English

coatings
Article
Photocatalytic Behavior of Water-Based
Styrene-Acrylic Coatings Containing TiO Sensitized
2
with Metal-Phthalocyanine Tetracarboxylic Acids
ValentinRaditoiu1,AlinaRaditoiu1,*,MonicaFlorentinaRaduly1,VioricaAmariutei1,
IoanaCatalinaGifu1,2andMihaiAnastasescu2
1 NationalResearchandDevelopmentInstituteforChemistryandPetrochemistry—ICECHIM,202Splaiul
Independentei,060021Bucharest,Romania;[email protected](V.R.);[email protected](M.F.R.);
[email protected](V.A.);[email protected](I.C.G.)
2 InstituteofPhysicalChemistry“IlieMurgulescu”,202SplaiulIndependentei,060021Bucharest,Romania;
[email protected]
* Correspondence:[email protected];Tel.:[phone]
AcademicEditor:AlessandroLavacchi
Received:6November2017;Accepted:8December2017;Published:12December2017
Abstract: Thestudypresentstheresultsregardingthephotocatalyticbehaviorofsomewater-based
styrene-acrylic coatings containing TiO nanoparticles sensitized with metal-phthalocyanine
2
tetracarboxylic acids. Coating materials have been studied in terms of color characteristics,
photocatalytic behavior, and resistance to self-degradation depending on the structure of
phthalocyanine sensitizers. Coatings that were exposed to Xenon light showed degradation of
the organic sensitizer rather than of the binder. Photocatalytic tests using methylene blue as a
standardcontaminantindicatedthatthecoatingcontainingTiO nanoparticlessensitizedwithFe(III)
2
phthalocyanine tetracarboxylic acids showed the highest efficiency both in ultraviolet or visible
light. Inthiscase,theUVlightinducedaphotodegradationratethatwasgreatlyincreasedofabout
fifty times comparatively with that induced by LED light and was determined by two different
mechanisms,butsidereactionslikemethyleneblueandsensitizerselfdestructionarepossibleto
occur simultaneously. Photocatalytic materials of this type are suitable to be used as decorative
coatingsespeciallyforindoorapplications.
Keywords: photocatalysts;titaniumdioxide;phthalocyanines;waterbornecoatings;LEDlight
1. Introduction
Photocatalysisrepresentsanactivefieldofresearchduringthelastyears. Manyeffortshavebeen
madeinordertofindnewtypesofphotocatalyticmaterials,whichareabletopromotegeneration
ofspeciesthatreactwithdifferenttypesoforganiccompoundsandtoefficientlyremovepollutants
fromaqueousmediaorfromsolidsurfaces[1–3]. Besidesclassicalsemiconductingoxides,namely
TiO and ZnO, only CeO has proved a convenient behavior for this type of applications, despite
2 2
therapidrecombinationofphoto-generatedelectronsandholes. However,TiO remainsthemost
2
importantphotocatalystduetoitsconvenientsemiconductingproperties(bandgap), outstanding
chemicalresistanceandavailability[4,5].
PhotocatalyticpropertiesofTiO wereintensivelyinvestigatedinrelationshipwithcrystallinity,
2
shapeandsizeoftheparticles,thepresenceofdefects,ordopingcompounds[6–8]. Elucidationof
photocatalytic mechanisms for different types of materials, substrates, organic pollutants, and
irradiation wavelengths leads to a rapid development of highly efficient photocatalytic materials
basedontitania. Unfortunately,photocatalyticactivityoftitaniamaterialsismaximumwhenthey
Coatings2017,7,229;doi:10.3390/coatings7120229 www.mdpi.com/journal/coatings
Coatings2017,7,229 2of17
areexposedtoUVradiationbecauseelectron-holepairsareefficientlygeneratedduringirradiation
withlighthavingawavelengtharound380nm. Moreover,thereareseveralmajordrawbacksoftitania
photocatalyst,whichareduetotherapidelectron-holerecombinationandthewidebandgapofthe
materials,whichlimitslightabsorptiontotheUVregion[9,10]. Inaddition,UVradiationrepresents
lessthan5%ofsolarenergydistributionandthiswasthemainreasonoftheeffortsmadetoproduce
photocatalyststhatusevisiblelight,whichrepresentsaround43%ofsolarenergy[11].
Assomephotocatalyticmaterialsaresuitabletobeusedforindoorapplications,manyefforts
aremadetodevelopnewphotocatalyst,whichcouldbeusedonthetunnelsandtheunderground
parking’swallsthatareexposedtopollution,especiallyduetotheenginesemissions. Theeffectcould
beenhancedbyconcomitantusageofnewecologicalfriendlysourcesofartificiallight,suchasLED’s
thatprovideanintenselevelofilluminationandshifttherangeofmaximumphotocatalyticefficiency
in the visible light domain. Therefore, among research related to optimize nanoparticle’s shape,
size,andmorphology[12–15]agreatdealofattentionwaspaidtothemodificationoftitania-based
photocatalyst with metal and non-metal atoms to shift the optimum range of action to the visible
domainofthespectrum[16],buttherecombinationofphoto-generatedspeciesremainsanimportant
challenge. Anotherstrategyisrelatedtothedevelopmentofheterojunctionsbetweensemiconducting
oxidesanddifferentphotosensitizingmaterials[17–19].
Phthalocyanines can be one of the most efficient and stable class of photosensitizers for TiO
2
photocatalystsoperatingundervisiblelightirradiation,assomestudieshavealreadyreported[20–29],but
theanchoringgroupsthatcouldbindthemonthenanoparticlessurface[30]decisivelydeterminetheoverall
efficiency.Studiesrelatedtothestabilityofseveralcouplingfunctionalities,suchascarboxyl,phosphate,
sulfonate,acetyl,silyl,hydroxyl,oraminorevealthatcarboxylgroupsexhibitaverygoodstabilityduring
photocatalyticprocesses[31]. Moreover,asithadalreadybeenfound,photocatalyticcoatingsthatwere
basedonorganicbinderscanbeobtainedifanappropriatefillerisusedinordertoreducethephotoactivity
ofthesemiconductorandconsequentlytoretardtheself-destructionofthecoating[32,33].
Theobjectiveofourworkwastoobtainandtoevaluatevisiblelightresponsivecoatingsthatwere
capableofeffectivephotocatalysisinweaklightorintheabsenceofUVradiation,especiallyforindoor
applicationswhereLEDsareusedaslightsources. Thestudyshouldalsodeterminetheexistenceofa
possibledeactivationeffectcausedbytheembeddingofphotocatalystsinthefilmbuildingmaterial
bypreventingthecontactwiththecontaminant. Furthermore, itwasimportanttoelucidateifthe
self-degradation of the coating organic components caused by the interaction with photocatalysts
occursduringthedegradationprocesses,andwhichisthelevelofself-degradation.
2. MaterialsandMethods
2.1. Materials
Parmetol A26 and Parmetol DF12 were purchased from Brenntag BV Nederland (Dordrecht,
TheNetherlands). DowChemicalCo. (Midland,MI,USA)providedLatexDL450,DowanolDPnB
andCellosizeHEC.OMYACARB5TNwasobtainedfromOMYAInt. AG(Oftringen,Switzerland),
whileAeroxideP25wasprovidedbyEvonikInd. (Essen,Germany)BYK034waspurchasedfrom
BYK-ChemieGmbH(Wesel,Germany)andADDAPTDISP500wasobtainedfromADDAPTChemicals
BV(Helmond,TheNetherlands). Isopropylalcoholandaqueousammoniasolution(25wt%)were
purchasedfromNovachim(Bucharest,Romania),wereoflaboratoryreagentgradeandwereused
asreceived. De-ionizedwaterwasusedinexperiments, whileweobtainedmetal-phthalocyanine
tetracarboxylicacids(MPcTC,whereM=Ni,Al,Co,Zn,Fe(III),Cu,Mn)inlaboratoryfollowinga
well-knownmethod[34]andweusedthemassensitizersforTiO .
2
2.2. SurfaceModificationofTiO withMetal-PhthalocyanineTetracarboxylicAcids
2
Amixtureof1.9gTiO powder(P25)and0.05gMPcTCwasaddedto50mLisopropylalcohol
2
containing0.2mLammoniaaqueoussolution(25wt%)undervigorousstirringuntilafinesuspension
Coatings2017,7,229 3of17
isobtained. Thereactionmasswasheatedunderreflux5handtitaniumdioxidenanoparticlesthat
weremodifiedwithMPcTC’swereseparatedbycentrifugationat4000rpm,washedwithde-ionized
water,filteredanddriedinanoven,at120◦C,for10h.
2.3. CharacterizationofTiO SensitizedwithMetal-PhthalocyanineTetracarboxylicAcids
2
Diffuse reflectance UV-Vis spectra of powders were recorded on a Jasco V570 spectrometer
(JASCO Int. Co., Ltd., Tokyo, Japan) equipped with a Jasco ILN-472 integrating sphere (150 mm),
usingSpectralonasareference. TheopticalbandgapwascalculatedusingJascoVWGA-589—band
gap measurement software (Ver. 1.03C, JASCO Int. Co., Ltd., Tokyo, Japan). Diffuse reflectance
Fourier-transforminfrared(DRIFT)spectrawasobtainedonthephotocatalyticpowdersmixedwithKBr
byusinganEasiDiffaccessoryfromPikeTechn.Inc.(Fitchburg,WI,USA)attachedtoaJascoFT-IR6300
instrument(JASCOInt. Co.,Ltd.,Tokyo,Japan),intherange400–4000cm−1 (30scansataresolution
of 4 cm−1). X-ray diffraction (XRD) analysis was performed on powders using a Rigaku SmartLab
X-raydiffractometer(Rigaku,Tokyo,Japan)withverticalgoniometer,inparallelbeamgeometry,atroom
temperature,andusingaCuKαradiation(λ=1.54056Å).Themorphologyofthenanoparticleswas
examinedbytransmissionelectronmicroscopy(TEM)onaJEOL200CXScanning-Transmission(JEOL,
Tokyo,Japan)Electronmicroscopeatanacceleratingvoltageof200kV.Nitrogensorptionisothermswere
recordedonaQuantachromeNova2200eautomatedgasadsorptionsystem(QuantachromeInstruments,
BoyntonBeach,FL,USA),attheliquidnitrogentemperature(−196◦C).Allofthesampleswereoutgassed
at300◦Cfor3hundervacuumpriortonitrogenadsorption.BETsurfaceareawascalculatedaccording
toBrunauer-Emmett-Tellerequation,totalporevolumewasestimatedfromtheamountofgasadsorbed
andthemicroporessurfaceareaandvolumeweredeterminedusingt-plotanalysis.
2.4. PreparationofStyrene-AcrylicCoatings
ThepaintformulationwasprovidedbyChimcolor(Bucharest,Romania),andisshowninTable1.
Inatypicalprocedure,thepreservatives,defoamingagent,coalescentanddispersantweremixedwith
waterusingadoublesuctiondiscLaboratoryDissolver(ATPEngineeringB.V.,Almere,TheNetherlands).
AfterthepHadjustmentto9withtheaqueousammoniasolution,thebinderwasslowlyaddedand
the mixture was homogenized. Finally, the thickener was added to the mixture and a stirring at a
rateof1500rpmwasmaintained30min. Inordertoobtainphotocatalyticcoatings,TiO sensitized
2
withphthalocyaninesandthestyrene-acrylicpaintpreviouslyobtainedwerehomogenizedusingan
Automatic Pigment Muller (J. Engelsmann AG, LudwigshafenamRhein,Germany). Coatings with
a thickness of 50 ± 1 µm were applied, using an Elcometer 3570 film applicator (Elcometer Ltd.,
Manchester, UK), on glass slides, previously degreased and cleaned in Piranha solution. The films
weredriedatroomtemperature(25±2◦C)for48handusedassuchforphotocatalyticexperiments
andcoatingscharacterization. ThicknessofthecoatingswasdeterminedwithaTROTECBB25layer
thickness-measuringdevice(TKLGmbH,Heinsberg,Germany).
Table1.Theformulationofwaterbornecoatings.
Component Role(ActiveSubstance) Quantity(wt%)
Water medium 20.3
preservative(mixtureof5-Chloro-2-methyl-and
ParmetolA26 0.1
2-Methyl-2H-isothiazol-3-one)
ParmetolDF12 preservative(mixtureofisothiazolinones) 0.1
BYK034 defoamer(mixtureofparaffinbasedmineraloilsandsilicones) 0.2
ADDAPTDISP500 dispersant(ammoniumpolyacrylate) 0.4
Ammonia(5wt%)solution pHadjustment(ammonia) 0.1
LatexDL450 binder(styrene-acrylic) 18.7
DowanolDPnB coalescent(dipropyleneglycoln-butylether) 1
OMYACARB5TN filler(CaCO3) 48.5
CellosizeHEC thickener(hydroxyethylcellulose) 10.6
Coatings2017,7,229 4of17
2.5. CoatingsCharacterizationandEvaluationofPhotocatalyticPerformances
DiffusereflectanceUV-Visspectrawererecordedoncoatingsthatweredepositedonglassslides
usingthesameapparatusasinthecaseofphotocatalysts. ColormeasurementsinCIEL*a*b*system
were performed using Jasco VWTS-581 color analysis software (Ver. 2.00A, JASCOInt. Co.,Ltd.,
Tokyo,Japan)fora10◦standardobserverandunderilluminantD65.Colormodificationsarequantified
byitsattributes: hue(H*),chroma(C*),andlightness(L*). Thetotalcolordifferenceiscalculatedfrom
theequation:
(cid:113)
∆E∗ = (∆L∗)2+(∆a∗)2+(∆b∗)2 (1)
whereL*isthelightness,a*andb*arered-green,respectively,yellow-bluecolorcomponentsandthe
calculateddifferencesindicatehowmuchthetwostatesofasampledifferfromoneanother,whilethe
compositionisevaluatedusing:
(cid:113)
∆H∗ = (∆E∗)2−(∆L∗)2−(∆C∗)2 (2)
whereH*ishueandC*ischroma. ACAM200opticalcontactangleandsurfacetensiongoniometer
(KSVInstruments,Helsinki,Finland)wasusedtoassesscoatingswettability. Atomicforcemicroscopy
(AFM) was performed in noncontact mode on a Park Systems XE 100 microscope (Park Systems,
Suwon, Korea). Specular reflection of the coatings was measured with the Multi Gloss 268Plus
gloss-meterfromKonicaMinoltaInc. (Tokyo,Japan).
Coating’sphotocatalyticactivitywasevaluatedbyperformingphotodegradationexperiments
usingMethyleneBlue(MB)asamodelcontaminantandUV-VisspectroscopytodetectchangesofMB
concentrationduringphotochemicalreactions. MethyleneBluesolutionof1g/Linwaterwassprayed
onthesurfaceofthecoatingsthatweredepositedonglassslides,followedbydryingfor2hat120◦C
inanoven. Sampleswereirradiatedinahomemadechamberusingvisiblelightgeneratedbytwo
LEDprojectorsof100Weach(SuperBrightLEDsInc.,St. Louis,MO,USA),providingawarmwhite
light(correlatedcolortemperature2700–3000K).Theirradianceof30±1W/cm2wasmeasuredat
thesurfaceofthesampleswithaDeltaOHM-HD2302.0Light-meter(DeltaOHMSrl,Padova,Italy)
equippedwithLP471RADradiometricprobeforeffectiveirradiancemeasurementinthespectral
range400–1050nm. Forcomparisonpurposes,similarsampleswereirradiatedinanATLAS-Xenotest
150S+apparatus(2200W,AtlasMaterialtestingSolutions,MountProspect,IL,USA),accordingto
ISO105-B02[30]normalconditions(irradianceof42±2W/cm2intherange300–400nm). Duringthe
photocatalyticexperiments,theconcentrationofMBwasmeasuredfromvisiblediffusereflectance
spectraandthedegradationefficiency(%)wascalculatedfromthemostsignificantminimumofthe
reflectancespectrumusing:
R −R
DE= t 0 ×100 (3)
R
0
whereDEisdegradationefficiency(%),R representsthereflectanceatvariousintervalsofexposureto
t
light,andR theinitialreflectanceofthecoating.
0
Total organic carbon (TOC) was determined by using a Multi N/C 2100 TOC analyzer from
AnalyticJenaAG(Jena,Germany). Sampleswerepreparedbyextractionindistilledwaterofallthe
solublespecies(MB,sulfonicacidsandphenolsarewatersolublespecies)thatwerelocatedatthe
surfaceoftheinitialandLEDlightexposedphotocatalyticcoatings.
3. ResultsandDiscussion
3.1. CharacterizationofthePhotocatalysts
Inordertoobtainthedesiredeffectofincreasingthephotocatalyticactivityinthevisiblerangeof
thespectrum,itwasnecessarytoestablishtheoptimalconcentrationofsensitizerthatwasdeposited
ontotheP25surface. Asaresultoftheexperiments,a2.5wt%sensitizertoP25wasestablishedas
Coatings2017,7,229 5of17
optimal. Atthisconcentration,maximizationofvisibleradiationabsorptionisachieved,sothatin
theabsenceofUVradiationthephotocatalyticprocessescanbecarriedoutwithinacceptabletime
limits,attheusualloadingofthestudiedcoatingswithphotocatalysts,aswillbefurtherdemonstrated.
Asaresult,itensuredtheavoidanceofdyeaggregationonP25surfaceandtheformationofsensitizer
islands, as well as ensuring an adequate intensity of the obtained colors so that the coatings also
provideadecorativeeffect.
TheKubelka-MunktransformedUV-VisreflectancespectrashowthepresenceoftwoQ-bands,
corresponding to π–π* transitions, frequently denoted as α-Q-band and β-Q-band, situated at
607–650nmand668–703nm, respectively. Thefirstbandisattributedtotheformationofdimeric
species,whilethelastoneisduetomonomers,aswasalreadyreportedforphthalocyaninederivatives
insolution[35]. Inthecaseofallthephotocatalysts,theabsorptionbandsofphthalocyaninesensitizers
are red shifted and broadened because of interactions with the solid host. It is obvious from the
spectrathatinthecaseofFe(III)PcTCsensitizedphotocatalystwasobtainedthelargestred-shifted
andbroadenedabsorptionspectrum,havingahighratiobetweenα-andβ-Q-bandintensities,ascan
beobservedinFigure1a.
Figure 1. (a) Kubelka-Munk transformed reflectance spectra and (b) diffuse reflectance
Fourier-transforminfrared(DRIFT)spectraofP25-Fe(III)PcTC.
ThisbehaviorcanbeexplainedbyastronginteractionbetweentheFe(III)PcTCmonomericspecies
andP25hostingmaterialduetothepresenceofcarboxylstronganchoringgroupsinthestructureofthe
sensitizer[31].Furthermore,theopticalbandgapthatwascalculatedfromthediffusereflectancespectra
showsthatphotophysicalpropertiesinthevisiblerangearesignificantlyimprovedinallthecasesof
photocatalystssensitizedwithMPcTCincomparisontoP25,ascanbeobservedinTable2.Theresults
demonstratetheeffectivenessofthephthalocyaninesensitizersbearingcarboxylgroupsthatwillbe
beneficialforthephotocatalyticpropertiesinthevisiblerange,inaccordancetootherreports[36].
Table2.Absorptiondataandbandgapofthephotocatalysts.
AbsorptionPeaks(nm) α/βQ-Band WidthofQ-Band EnergyBand
Photocatalyst
α-Q-Band β-Q-Band IntensityRatio AbsorptionPeaks(nm) Gap(eV)
P25 – – – – 3.38
P25-NiPcTC 607 668 1.25 150 3.34
P25-AlPcTC 637 694 1.01 183 3.17
P25-CoPcTC 610 670 1.2 165 3.32
P25-ZnPcTC 637 686 0.97 148 3.18
P25-Fe(III)PcTC 650 703 1.17 174 3.34
P25-CuPcTC 609 678 1.25 165 2.74
P25-MnPcTC 639 674 1.01 149 3.34
Coatings2017,7,229 6of17
TheDRIFTspectraofthephotocatalystsischaracterizedbythepresenceofabroadbandthatis
centeredat3330cm−1 thatcorrespondstothehydroxylgroupsinvolvedinhydrogenbondsanda
shoulderat3195cm−1,whichisduetothestretchingvibrationofC–Haromaticbondscorresponding
tothephthalocyaninesensitizer. Groupsthatareinvolvedinsuchinteractionscanbelongtocarboxyl
groupsorhydroxylgroupsontheTiO surfaceandtoadsorbedwatermolecules. Animportantpeakis
2
situatedat3690cm−1andcorrespondstothestretchingvibrationsofhydroxylisolatedgroupsonthe
titaniasurface. Anotherimportantbroadpeakthatiscenteredat1630cm−1isduetotheH Obending
2
vibrationscombinedwithtwootherbandsduetotheC=Ostretchingvibrationofthecarboxylgroups
situatedat1719cm−1. Nexttoit,the–C–N=stretchingvibrationlocatedat1664cm−1asashoulder,
demonstratethepresenceofthephthalocyaninesensitizeronthephotocatalystsurface,asitcanbe
observedfromFigure1b. Theabsorptionpeakat1380cm−1maybeassignedtothecarboxylateion,
whileat1335cm−1 islocatedthestretchingvibrationofC=Caromaticgroups. Othercharacteristic
peaks are located at 1094 and 713 cm−1, due to the aromatic –CH deformation while at 738 and
504cm−1aresituatedbandscorrespondingtotetrahedronandoctahedronTi–Osymmetricstretching
vibrations. DRIFTspectraclearlydemonstratethattheinteractionbetweenphthalocyaninesensitizers
andTiO isachemicalonethroughthecarboxylgroups.
2
TheanalysisofXRDspectra,ascanbeseeninFigure2a,showsthatallofthediffractionpeaks
canbeindexedtotetragonalstructuresofanataseandrutile,accordingtoPDFCardNos.: 00-064-0863
and00-021-1276,respectively.
Figure2.(a)X-raydiffraction(XRD)pattern;(b)TEMimageand(c)nitrogenadsorption-desorption
isothermofP25-Fe(III)PcTC.P/P istheratiobetweentheequilibriumandthesaturationpressure
0
ofnitrogen.
Interplanardistances(d )werecalculatedusingBraggEquation:
hkl
d =λ/(2sinθ) (4)
hkl
whereλistheX-raywavelengthandθistheBraggangle. Thelatticeconstantsofthetetragonalphases,
calculatedbyRietveldrefinementsofthediffractiondata,werea=3.7885andc=9.5188Åforanatase
Coatings2017,7,229 7of17
anda=4.6025andc=2.9556Åforrutile. Themediumsizeoftheparticleswasaround17–20nm,and
itwascalculatedfromthemostintensepeaksofthespectrum,usingScherrerEquation:
D =0.94λ/(β cosθ) (5)
p 1/2
whereD istheaveragecrystallitesize,β isthefullwidthathalfmaximum,whilealloftheother
p 1/2
parametershavethesamesignification,aswaspreviouslystated.
ThecompositionofP25,determinedfromdiffractiondata,is88.8wt%anataseand11.2wt%
rutileremainessentiallyunchangedafterdopingwithphthalocyanines. ForFe(III)PcTC,themost
intense peaks were recorded at 5.51◦, 14.10◦, 26.84◦, and 42.65◦ 2θ. For the other phthalocyanine
sensitizers thediffraction peaksaresituated inthe same regions. However, ifwe will analyzethe
spectraofthephotocatalystswewillobservetheinfluenceofphtalocyaninedopingmaterialsonthe
structureofP25titaniaphotocatalyst. Additionally,ashiftofthepeakcorrespondingtorutile(110)to
higher2θanglestakesplaceforallthephotocatalysts,whichisverylikelycausedbytensilestress,
aswasalreadyobservedinothercases[37]. Theexplanationofthisbehaviormightberelatedtothe
existenceofstructuresimilaritiesbetweenphthalocyaninesensitizersandrutile,whichcanleadto
favorableinteractionsonthisfaceofthecrystalsthroughcarboxylgroups. Thereby,acharacteristic
peakofFe(III)PcTCwasfoundat26.842◦,andforrutile,(110)peakwasobservedat27.375◦ inP25
photocatalyst,whileinTiO -Fe(III)PcTCthepeakisshiftedto27.428◦ 2θ. Thisbehaviorisexplained
2
bythepreferenceofcarboxylgroupstobindonthetitaniasurfacethroughbidentatechelatingmode
andthepresenceoffoursuchchelatinggroupsinthestructureofphthalocyanines,togetherwiththe
existenceofamixtureofisomersleadtodifferentbindinggeometries. However,itisclearthatthe
structuralchangesthatarecausedbythistypeofinteractionsdidnotalterthecrystallinestructure,
composition,orsizeofP25photocatalyst. Evidenceinthissenseisrepresentedbythesimilarintensity
ofthepeaksandcrystallitesizeofP25nanoparticlesthatisnotaffected,ascanbeobservedfromfull
widthathalf-maximumofthepeaks—about0.51forallofthesamples.
TheanalysisofP25-MPcTCTEMimagesconfirmsthepresenceofphotocatalystnanoparticleswith
sizesaround20nmandawell-definedcrystallinestructurethatisnotalteredbecauseofdepositionof
thesensitizeronthesurfaceofP25nanoparticles. Moreover,allMPcTCthataredepositedontoP25
surfacearenotamorphousanddonotexhibitfilmogeniccharacter,andtherearenoagglomerated
particlesduetothesensitizercrystallizationandaggregation,ascanbeobservedinFigure2b.
Textural parameters define the behavior of the materials during photocatalytic processes.
The nature and size of phthalocyanine sensing molecules could change essentially photocatalysts
porosityortheirspecificsurfacearea. However,inthecaseofP25photocatalyststhatweresensitized
withphthalocyaninestheBET(Brunauer-Emmett-Teller)surfaceareaandtheporesizedonotsuffer
majorchanges,ascanbeobservedinTable3. Nitrogenadsoptionisothermsforallthesamplesare
similarandshowatypeIVhysteresisloop(Figure2c).
Table3.Texturalpropertiesofthephotocatalysts.
Photocatalyst BETSurfaceArea(m2/g) PoresVolume(BJH)(cm3/g) MicroporesDiameter(BJH)(nm)
P25 57 0.43 24
P25-NiPcTC 55 0.33 24
P25-AlPcTC 56 0.34 24
P25-CoPcTC 53 0.30 24
P25-ZnPcTC 50 0.31 34
P25-Fe(III)PcTC 54 0.23 31
P25-CuPcTC 54 0.38 33
P25-MnPcTC 53 0.44 34
Coatings2017,7,229 8of17
3.2. CoatingsProperties
Watercontactangleatthesurfaceofthecoatingsshowsaslighthydrophobiccharacterobtained
whenP25sensitizedwithphthalocyaninedyesisusedforphotocatalyticcoatings. Besidesthebinder,
only the organic dyestuff molecules, which are bound onto the surface of the P25 nanoparticles
and hinder the Ti–OH groups, are able to determine the hydrophobic nature of the coatings.
The hydrophobic character of the photocatalysts, and as a consequence the water contact angle
ofthecoatings,hasthehighestvalueofabout97◦ inthecaseofP25-ZnPcTCphotocatalyticcoating,
whilethelowestvaluewasmeasuredforP25-NiPcTCcoating,asshowninFigure3. Surprisingly,films
thatareexposedtoLEDlightshowasmallvariationofthewatercontactangleafter160hofexposure.
Forallofthesamples,itisquiteevidentthataslightincreaseofthevalueswasrecordedduringthe
exposuretovisiblelight. Thephenomenonisrelatedtoapossiblecontinuationofthecrosslinking
process,whichiscommontowaterbornecoatings. Asitisalreadyknown[38],thistypeofcoatings
ischaracterizedbyalongdryingtime. Therefore,onlyanadvancementofthecrosslinkingprocess
canexplainthehighervaluesofthewatercontactanglerecordedattheendoftheexposuretovisible
light. Wecanpresumethatthiscanbeareasonforanincreasedcontactareabetweenthecontaminant
andthephotocatalyticcoatings,whichwillfinallyleadtoahigherefficiencyofthephotodegradation
process. Moreover,thisimportantevidenceallowsforustoconcludethattheorganicbinderwasnot
affectedduringthephotocatalyticprocessthatwasdevelopedundervisiblelight. Conversely,the
contactangleofthecoatingsdecreasedramaticallyandsurfacesbecomehydrophilicandmaintain
theirhydrophiliccharacterattheendoftheexposuretoXenonlightgeneratedinaXenotestapparatus,
ascanbeobservedfromFigure3.
This is another important finding, which can signify a higher self-cleaning property of the
coatingsandtheimpossibilityofpollutantmoleculestoadheretothehydrophilicsubstrate. Inthis
way,pollutantscanberemovedbywashingfromthecoating’ssurface.
Figure 3. Water contact angle of photocatalytic coatings before and after exposure to LED and
Xenonlight.
However,thehydrophiliccharactercanbeevidenceofthebinderphotodegradation,andthis
wasthereasontoinvestigatethechangesthatwassufferedbythecoatingsduringtheexposureto
Xenonlight,usingAFMmeasurements. AsitisobservedfromFigure4inthecaseofLEDlighttreated
coatings,thereisanincreaseinaverageroughnessparameter(R )androotmeansquareroughness(R )
a q
fromtheinitialvaluesof49.021and62.286nmto65.436and82.928nm,respectively. Onthecontrary,
inthecaseofXenonlighttreatedsamples,thesameparametersdecreaseto35.481and46.570nm.
Surprisingly,thereisaslightroughingofthecoatingsurfaceduringLEDlightirradiation,whichis
probably due to aggregation of the photocatalyst nanoparticles in the organic binder based on their
incompatibility with the polymeric matrix that is further crosslinked. This phenomenon probably
Coatings2017,7,229 9of17
takesplacebythedisplacementofthephotocatalystfromthematrixaroundthelargerfillerparticles.
Thisdisplacementissustainedbythelargerdifferencesof140nmbetweenthemaximumvalleydepth
(Min.)oftheLEDlightexposedcoating(Figure4b)andoftheunexposedcoating(Figure4a),comparatively
withthedifferenceofonly50nmbetweenthemaximumpeakheight(Max.)ofthesamecoatings.
Figure4. Atomicforcemicroscopy(AFM)imagesofthephotocatalyticcoatingcontaining10wt%
P25-Fe(III)PcTC(a)beforeand(b)afterexposuretoLEDlightfor156h;and(c)toXenonlightfor72h.
Apparently,afterUVirradiationthereisasmoothingoftheinitialsurface,whichisprobablydue
tothecoatingleveling. Thisissustainedbythevaluesoftherootmeansquareroughnessofthesample
exposedtoXenonlightcomparativelywiththatunexposed. Particularly,itshouldbenotedthatin
thiscasethedifferencebetweenthemaximumvalleydepth(Min.) oftheXenonlightexposedsurface
(Figure4c)andoftheinitialsurface(Figure4a),hasthesamemagnitudeasthedifferencebetween
themaximumpeakhighofthesamesamples. Thisfindingsustainsthelevelingphenomenonand
excludesthepossibledegradationofthebinderunderUVradiation.
Gloss measurement is frequently used to identify coatings photodegradation [39], which is
usuallyaccompaniedbyareductionoftheglossvalues. Therefore,afterthestudyofsurfaceroughness
coatingswereanalyzedbymeasuringglossbeforeandafterexposuretoLEDlightandtoaXenonlamp.
Theresultsconfirmthatthesamplescanbeclassifiedaslow-glosscoatingsduetoahighercontent
ininorganiccomponents,whichdetermineadiffusereflectionoflight,diminishingscatteredlight,
andconsequently,thesamplesgloss. Thus,measurementsat85◦ indicatesmalldifferencesbetween
samplesbeforeandafterLEDilluminationorafterexposuretoXenonlamp,ascanbeobservedfrom
Coatings2017,7,229 10of17
Table4. Theresultssustainustoconcludethatthereisnoevidenceregardingimportantdegradation
ofthebinderduringexposuretodifferenttypeoflightandcoatingspresentgoodretentionofthe
initialgloss.
Table4.Glossofthephotocatalyticcoatings.
Gloss(GlossUnits)
Photocatalyst
Initial LEDLight XenonLight
– 21.2 19.4 9.8
P25 16.8 14.5 11.9
P25-NiPcTC 22.6 16.9 14.7
P25-AlPcTC 23.7 17.8 13.8
P25-CoPcTC 22.6 17.8 19.1
P25-ZnPcTC 19.6 16.6 18.7
P25-Fe(III)PcTC 17.3 10.8 14.3
P25-CuPcTC 20.2 18.7 18.5
P25-MnPcTC 18.1 16.8 11.6
As it was already shown [40], the binder photodegradation is more pronounced as the
photocatalystloadingsinthecoatingmaterialsbecomeshigher. Therefore,inordertoanalyzethe
possiblesurfacedegradationcolormeasurementswereperformedoncoatingscontaining10%w/w
photocatalyst. AccordingtotheresultsobtainedinthecaseofLEDlightillumination,wecanobserve
onlysmallvariationsofthecolorparametersforallphotocatalyticcoatings,ascanbeobservedfrom
Figure5. Thisisprobablyduetodehydrationandcrosslinkingprocesswhichcanleadtoadifferent
environment around the photocatalyst, indicating that the binder was not visibly affected during
exposuretoLEDlight.
Figure5.TheevolutionofCIEa*andb*colorparametersofthephotocatalyticcoatingsduringexposure
toLEDlight.
However,whenthesampleswereexposedupto72htoXenonlight,thetotalcolordifferences
againsttheinitialsamplesarenotableandareduealmostexclusivelytochangesofchroma(C),as
can be seen in Table 5. Therefore, it is essential to note that this type of coatings is more suitable
for indoor applications, in the absence of a suitable UV absorber or HALS in the coating material.
IndepthdataanalysisrevealsahigherstabilityofthebinderatUVradiationcomparativelywiththe
dyesusedastitaniasensitizers. Inallofthecasesstudiedbyus,afterexposuretoXenonlight,the
photocatalyticcoatingsbecomelighter(∆L>0)andduller(∆C<0). Thisbehaviorismainlydueto
thedyesphotodegradation,whileinthecaseofthecoatingcontainingonlyP25,duringexposureto
Xenonlight,theshadebecomesyellowish(∆b>0)andtotalcolordifferenceisperceptiblethrough