Fabrication of hydroxyl group modified monodispersed hybrid silica particles

This article appeared in a journal published by Elsevier.The attached copy is furnished to the author for internal non-commercial research and education use,including for instruction at the authors institutionand sharing with colleagues.Other uses,including reproduction and distribution,or selling or licensing copies,or posting to personal,institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle(e.g.in Word or Tex form)to their personal website orinstitutional repository.Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:/copyrightFabrication of hydroxyl group modified monodispersed hybrid silica particles and the h-SiO 2/TiO 2core/shell microspheres as high performance photocatalyst for dye degradationZhe-Ying Shen a ,Long-Yu Li a ,Yat Li b ,Chang-Chun Wang a ,⇑aKey Laboratory of Molecular Engineering of Polymers (Minister of Education),Department of Macromolecular Science and Laboratory of Advanced Materials,Fudan University,Shanghai 200433,People’s Republic of China bDepartment of Chemistry and Biochemistry,University of California,Santa Cruz,CA 95064,United Statesa r t i c l e i n f o Article history:Received 2September 2010Accepted 11October 2010Available online 16October 2010Keywords:Core/shellHybrid silica particles Anatase titania shell Click reaction Photocatalysta b s t r a c tThe monodisperse hybrid silica particles (h-SiO 2)were firstly prepared by a modified sol–gel process and the surface was modified in situ with double bonds,then abundant carboxyl moieties were introduced onto the surface of the silica core via thiol–ene click reaction.Afterward,the h-SiO 2/TiO 2core/shell micro-spheres were prepared by hydrolysis of titanium tetrabutoxide (TBOT)via sol–gel process in mixed eth-anol/acetonitrile solvent,in which the activity of TBOT could be easily controlled.The carboxyl groups on the surface of silica particles promote the formation of a dense and smooth titania layer under well con-trol,and the layer thickness of titania could be tuned from 12to 100nm.The well-defined h-SiO 2/TiO 2core/shell structures have been confirmed by electron microscopy and X-ray photoelectron spectroscopy studies.After calcination at 500°C for 2h,the amorphous TiO 2layer turned into anatase titania.These anatase titania-coated silica particles showed good photocatalytic performance in degradation of methyl orange aqueous solution under UV light.Ó2010Elsevier Inc.All rights reserved.1.IntroductionThe nanocrystalline titania particles as a heterogeneous photo-catalyst have attracted significant attention in the last few decades,due to their applications in water splitting [1],removing organic pollutants from environment [2–4]and eliminating bacteria and fungi in water [5].However,nanosized titania powders with high surface area are not thermally stable [6],and could easily lose their surface area at elevated temperature through phase transforma-tion and crystallite growth.For preparation of high performance photocatalyst,titania is usually coated onto inert supports,such as porous silica,carbon fibers,glass fibers and beads [3,4].Many ef-forts have been made to encapsulate nano-or sub-micron size sil-ica microspheres with titania so as to create more accessible photocatalytic sites,meanwhile,enhance the adsorption of organic toxin.Titania-shell composite particles are prepared by sol–gel pro-cesses [2–4],or coating of titania nanoparticles onto the silica or polymer microspheres through the layer-by-layer (LBL)technique [3,7].Although the LBL technique shows good control on the size and uniformity of the resulting particles,the LBL method is moretedious with long synthetic steps.Alternatively,a simple sol–gel method is more attractive.However,the high reactivity of titania precursors leads to several challenges in utilizing the sol–gel ap-proach for titania shell deposition,such as the ease aggregation of the coated microspheres,and the difficulty in the formation of regular shell-layer with controlled shell thickness of titania [2,3].Thus,to achieve a dense,uniform titania coating onto the surface of silica microspheres via sol–gel method remains challenging.Recently,Lee et al.[8]and Meng et al.[9]reported a new class of hybrid silica microspheres covered with vinyl groups on the sur-face prepared by one-pot approach.These hybrid silica micro-spheres should allow modification of the silica surface with click chemistry without the need of silane coupling agents,which could address the low efficiency and incomplete surface modification.Click chemistry,which is now extensively used in design and prep-aration of complex functional molecules due to its quantitative,ra-pid reactions with inoffensive by-product (if any)that can be easily removed.More importantly,it offers rich opportunities in func-tionalization and stabilization of nanoparticles.For instance,cop-per(I)catalyzed Huisgen [3+2]dipolar cycloaddition between an alkyne and azide is readily to be utilized to prepare a variety of functional materials as a post functionalization strategy.Other reactions,such as Diels–Alder reactions,metal-free dipolar cyc-loadditions and thiol–ene(yne)chemistry are of importance as well.So far,the reaction between thiols and vinyl groups has been0021-9797/$-see front matter Ó2010Elsevier Inc.All rights reserved.doi:10.1016/j.jcis.2010.10.023⇑Corresponding author.Fax:+862151630302.E-mail address:ccwang@ (C.-C.Wang).mostly implemented for the synthesis of networks andfilms as highlighted by Hoyle et al.[10].To date,thiol–ene click reaction has attracted much research interest as it is easy to execute under facile reaction conditions, and also it is versatile in reactants.Campos et al.[11]reported dif-ferent types of polymer functionalization based on thermal and photochemical initiated thiol–ene click reaction.Goldmann and coworkers[12]demonstrated the surface modification of poly(divinylbenzene)with PNIPAM and PHEMA via thiol–ene and azide–alkyne reactions,respectively.Connal et al.[13]employed LBL assembly technique to alternately deposit PMA-thiol/PMA-ene and poly(vinylpyrrolidone)(PVP)onto silica particles.The resulting thiol–ene cross-linked PMA capsules can be widely ap-plied in drug delivery system.However,all the above-mentioned thio–ene click reactions were performed on the surface of organic substrates.The modification of inorganic microspheres by click reaction is rarely explored[14].Here we report,for thefirst time,the surface modification of the hybrid silica particles(h-SiO2)through click reaction between mer-captoacetic acid and the vinyl groups located on the surface of sil-ica particles.The reaction resulted in hybrid silica particles covered with carboxylic acid groups,these particles could be well dispersed in water.Besides,the hydroxyl groups on the surface of hybrid sil-ica particles could facilitate titania deposition via sol–gel process in a mixed ethanol/acetonitrile solvent,in which the activity of tita-nium tetrabutoxide(TBOT)could be easily controlled.The core size and shell thickness of the core/shell nanoparticles can be well manipulated.After calcination at500°C for2h,the amorphous titania turned into polycrystalline anatase titania.These silica core–titania shell microspheres showed significant photocatalytic activity in dye degradation under UV light irradiation.2.Materials and methods2.1.MaterialsVinyl triethoxy silane(VTES)and sodium dodecylbenzene sulfo-nate(SDBS)were purchased from Sinopharm Chemical Reagent Co. Ltd.Mercaptoacetic acid from ACORS and titanium tetrabutoxide (TBOT)from Shanghai Qiangsheng Chemical Reagent Co.Ltd.,were used as received.Azobisisbutyronitrile(AIBN)was purchased from Sinopharm Chemical Reagent Co.Ltd.and recrystallized in ethanol. Acetonitrile was purchased from Shanghai Qiangsheng Chemical Reagent Co.Ltd.and purified by distillation.Aqueous ammonia solution(28%),ethanol from Shanghai Qiangsheng Chemical Re-agent Co.Ltd.and deionized water were used as received.2.2.Synthesis of vinyl-functionalized hybrid silica microspheres and modification with mercaptoacetic acidVinyl-functionalized silica microspheres were synthesized according to the literature reported method[9].VTES3.8g and SDBS0.039g(0.3ÂCMC)(critical micelle concentration)were added into30ml water under vigorous stirring for1h,then0.5 ml aqueous ammonia solution(28%)was added,and the mixture was heated at50°C for48h.The resulting microspheres were sep-arated by centrifugation and washed several times with deionized water.0.05g AIBN and0.174g mercaptoacetic acid with a molar ratio of1:1(thiol:alkene)were added to a10ml ethanol solution con-taining0.1g vinyl-functionalized silica microspheres.After bub-bled with N2at45°C for30min,the solution was heated at 80°C for additional24h for complete reaction.Samples were also prepared with different amount of mercaptoacetic acid varied from molar ratio of0.5:1to5:1(thiol:alkene).2.3.Synthesis of h-SiO2/TiO2core/shell microsphereThe deposition of TiO2shell on carboxyl-modified h-SiO2parti-cles was performed by controlled hydrolysis of TBOT in the etha-nol/acetonitrile(3/1,V/V)mixed solvent in the presence of ammonia solution as catalyst.Typically,0.04g of carboxyl-modi-fied h-SiO2particles were dispersed in160ml of ethanol/acetoni-trile(3/1,V/V)and then mixed with1.2ml of25wt.%ammonia solution at room temperature.Finally,a TBOT solution(from 0.8ml to 2.0ml)in32ml of an ethanol/acetonitrile(3/1,V/V) was added to the above h-SiO2dispersion under stirring.After the hydrolysis reaction performed at room temperature for1h, the h-SiO2/TiO2core/shell microspheres were washed with aceto-nitrile using sonication for three times,and the particles were col-lected by centrifugation.The resulting particles were then dried in a vacuum oven at40°C till reaching a constant weight.To obtain anatase titania shell,the composite particles were calcined at 500°C for2h.2.4.Photocatalytic activity measurementThe photocatalytic activities of the calcined h-SiO2/TiO2micro-spheres were measured by the degradation of methyl orange in water.Twenty milligram h-SiO2/TiO2microspheres were added to100ml methyl orange aqueous solution(10mg/l).The mixture was stirred for1h in dark to ensure methyl orange absorbed by the catalyst before UV light exposure.An800W high-pressure Hg lamp with CuSO4solution as a cutofffilter was used as UV light source.The UV absorption spectra of the samples were collected by Lambda35Spectrometer(PerkinElmer,America).The concentra-tion of methyl orange was monitored on the basis of UV absorption peak centered at465nm.2.5.CharacterizationHigh Resolution Transmission Electron Microscopy(HR-TEM) images were taken on a JEM-2010(JEOL,Japan)TEM at an acceler-ating voltage of200kV.Samples were dispersed in acetonitrile and then transferred to a carbon coated copper grid.Energy dispersive X-ray spectroscopy was performed on a SUPERSCAN SSX-550scan-ning electron microscope(SEM)(Shimadz,Japan)at an accelerat-ing voltage of15kV.Samples dispersed in acetonitrile were transferred onto a glass slide at room temperature and sputter-coated with Au.Hydrodynamic diameter(Dh)measurements and Zeta-pH titration were conducted by dynamic light scattering (DLS)with a ZEN3600(Malvern,UK)Nano-ZS instrument con-nected to MPT-2(Malvern,UK)Automatic titrator.Infrared spectra were recorded on an Fourier transform infrared spectrometer(IR Prestige-21,Shimadz,Japan).The dried samples were mixed with KBr,and then compressed to a plate for measurement.Powder X-ray diffraction(XRD)patterns were collected on a X’Pert Pro(Panalytical,Netherlands)diffractometer with Cu K a radiation at k=0.154nm operating at40kV and40mA.3.Results and discussion3.1.Synthesis of vinyl-functionalized silica microspheres andmodification with mercaptoacetic acidThe detailed synthetic approach for the h-SiO2/TiO2core/shell microspheres is illustrated in Scheme1.The vinyl-functionalized silica particles were prepared by one-pot sol–gel process under dif-ferent SDBS surfactant concentrations.The size and the polydisper-sity index(PDI)of as-prepared hybrid particles were characterizedZ.-Y.Shen et al./Journal of Colloid and Interface Science354(2011)196–201197by TEM and DLS,respectively.The results are summarized in Ta-ble1.The surfactant concentration had a great influence on the size and the uniformity of the silica particles.When the surfactant concentration of0.3ÂCMC was used,the size of silica particles is around121nm,and more importantly,the PDI is about0.05,indi-cating the as-prepared hybrid silica particles are very uniform.The particle size of the hybrid silica particles could be modulated by the surfactant concentration(Table1).To determine the existence of the vinyl groups on particle surface,we performed FTIR analysis on the particles.As shown in Fig.1a,the absorption peaks centered at966cmÀ1,1602cmÀ1are characteristic peaks of vinyl groups (A CH@CH2),while1008cmÀ1,1409cmÀ1are characteristic peaks of Si A C vibration,thus confirming the presence of Si A CH@CH2 groups.In addition,the peaks observed at around1133cmÀ1and 1049cmÀ1suggest the formation of polysiloxanes structure,which is consistent with the literature reported values[9].The surface of the silica particles was modified with different amount of mercaptoacetic acid through a click reaction between the thiol and alkene groups that was initiated by AIBN at50°C. After the surface modification,the appearance of A C@O (1710cmÀ1)and A COOH(3478cmÀ1)FTIR peaks support the suc-cessful coating of carboxyl groups on the particle surface(Fig.1). We observe the A C@O and A COOH peaks increase with thiol:alk-ene molar ratio from1:1to3:1,and then remained more or less the same with further increase of the molar ratio(Fig.1b–e).This data suggested the amount of mercaptoacetic acid less than molar ratio of1:1(thiol:alkene)was not sufficient to complete the sur-face modification,until the molar ratio reach3:1(thiol:alkene).We also carried out zeta potential measurement for pure hybrid silica particles and the core/shell microspheres with different de-grees of modification asa function of pH.As shown in Fig.2,pure silica particles showed an isoelectric point is at pH=3.4.After sur-face modification,the isoelectric point moves toward lower pH val-ues of2.3,1.9and1.6for the samples prepared with molar ratio of 1:1,3:1,5:1(thiol:alkene),respectively.The data confirmed the modification of carboxyl groups onto the silica surface,and the in-crease of the carboxyl groups onto the surface of silica particles as the molar ratio increase.Besides,a significant increase in aqueous dispersibility is another direct evidence that supports the presenceof carboxyl groups on the surface of hybrid silica particles[15].The carboxyl groups modified with the molar ratio of1:1on the surface of silica particles was determined to be100l mol/g determined by acid–base titration.3.2.Synthesis of h-SiO2/TiO2core/shell microspheresThe h-SiO2/TiO2core/shell microspheres were prepared by the controllable hydrolysis of TBOT that catalyzed by ammonium cat-ions in an ethanol/acetonitrile mixed solvent.It is expected that the carboxyl groups on the particle surface attracted the positively charged NHþ4,which further drew the negatively charged TiOÀspecies together,and thereby facilitate the coating of titania via sol–gel process[16].A series of experiments were conducted to investigate the effect of the TBOT feed on the morphology of theillustrates the synthetic approach for the preparation of monodisperse mercaptoacetic acid modified silica cores and anatase titaniasol–gel process.Table1A summary of the structural properties of the silica particles prepared with differentsurfactant concentrations.Sample code SDBS(ÂCMC)Particle size(nm)PDISilica-10.12180.11Silica-20.21870.15Silica-30.31210.05Silica-40.41160.04Silica-50.71240.12Note:VTES=3.8g,NH3ÁH2O=0.5ml.Z.-Y.Shen et al./Journal of Colloid and Interface Science354(2011)196–201199of(a)mercaptoacetic acid modified silica microspheres;h-SiO2/TiO2core/shell microspheres before calcination with titania61nm(Sample B);(d)73nm(Sample C);(e)100nm(Sample D);and(f)calcined Sample B with shell thickness of59nm.FTIR spectra(Fig.6)revealed the compositive change of h-SiO2/TiO2structure after calcination.In Fig.6,the broad bands at500–900cmÀ1are corresponding to the Ti A O A Ti vibration.The peak at1641cmÀ1was attributed to O A H bending for Ti A OH,Si A OH and water,and the broad bands at2800–3800cmÀ1were assigned to O A H,N A H and C A H stretching.Significantly,we observed the N A H vibrational peak centered at1400cmÀ1,as an indication of the presence of NHþ4ions[16],which supports the proposed role of ammonium solution in the TiO2shell growth.After calcination at500°C for2h,the Ti A O A Ti vibration peak was enhanced signif-icantly,which might result from the removal of organic composi-tion and/or the formation of crystalline TiO2shell.The signals around1100cmÀ1corresponding to asymmetric vibration of Si A O A Si were also enhanced.More importantly,the disappearance of the N A H(1400cmÀ1)and the C A H(3180cmÀ1)peaks indicates the removal of N A H and C A H groups from organic component dur-ing calcination.The broad peak centered at3400cmÀ1can be attributed to the O A H vibration for Ti A OH and Si A OH groups. The remained A OH also known as surface hydroxylation are advantageous for photocatalytic activity of titania as it provides higher capacity for oxygen absorption[17–19].To determine the crystal structure,we performed XRD studies on both titania-coated silica particles before and after calcination at500°C for2h,the XRD patterns are shown in Fig.7.Before cal-cination,we did not observe any diffraction peaks from20°to80°2h,indicating the as-prepared titania shell was amorphous.In con-trast,the calcined structure exhibited multiple diffraction peaks, which can be indexed to the anatase phase(JCPDS File No.21-1272)of TiO2.The results confirmed the transformation of amor-phous to anatase phase of TiO2shell.3.3.Photocatalytic studyThe photocatalytic properties of h-SiO2/TiO2core/shell micro-spheres were evaluated through the degradation of methyl orange in water under UV light irradiation(Fig.8).All samples were cal-cined at500°C for2h,and equal weight of h-SiO2/TiO2micro-sphere catalysts were used in the measurement.There was almost no degradation of methyl orange without photoactive cata-lyst as expected.Meanwhile,all calcined h-SiO2/TiO2microspheres200Z.-Y.Shen et al./Journal of Colloid and Interface Science354(2011)196–201showed significant photocatalytic activity in the degradation of methyl orange.Sample B with the smallest shell thickness showed the best photocatalytic activity,which can be attributed to the lar-ger surface area compared to Samples C and D.Besides,such kind of monodispersed catalytic particles has additional advantages for easy separation and high recyclability.The capability of synthesiz-ing highly controllable anatase phase titania-coated silica micro-spheres opened up new opportunities in other applications such as photo-initiated polymerization and solar cells.4.ConclusionWe demonstrated the rational synthesis of h-SiO2/TiO2core/ shell microspheres by using carboxyl group modified silica particle as cores and titania as shell via the controllable hydrolysis of TBOT. The thickness of titania shell can be well tuned by the amounts of TBOT used in the hydrolyzation.The resulting core/shell h-SiO2/ TiO2microspheres were nearly monodisperse,where the titania shell is a dense and smooth amorphous layer.After calcination, the amorphous layer turned into anatase titania shell,which exhibited significant photocatalytic activity in degradation of methyl orange in water.The capability of synthesizing well-de-fined crystalline titania-coated silica particles opened up new opportunities in many applications including photo-catalysis and solar cells.AcknowledgmentsWe thank Prof.X.Y.Ni for the help in photocatalytic experi-ments.This work was supported by National Science Foundation of China(Grant Nos.20974023,21034003and51073040),National Science Foundation for Distinguished Young Scholars of China (50525310),the Cultivation Fund of the Key Scientific and Techni-cal Innovation Project,Ministry of Education of China(No.707023). 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