室温合成介孔二氧化钛-main

Room temperature synthesis of mesoporous TiO 2nanostructures with high photocatalytic efficiencyDebashree Das,Atal Shivhare,Soumen Saha,Ashok K.Ganguli *Department of Chemistry,Indian Institute of Technology,Hauz Khas,New Delhi 110016,India1.IntroductionTiO 2is of tremendous interest due to the wide-ranging applications in the areas of environmental purification [1],gas sensors [2],dielectric ceramics [3],pigments [4],high efficiency dye-sensitized solar cells [5,6],cosmetics [7],inorganic mem-branes [8]and catalysis [9,10].Considerable efforts have been made toward the controlled synthesis of TiO 2nanomaterials with different morphologies varying from nanospheres [11,12],nano-structured thin films [13,14],nanotubes [15],nanowires [16],nanofibers [17],mesoporous structures [18]and others [19–22].Naturally occurring crystalline TiO 2has three common poly-morphs:rutile (tetragonal),anatase (tetragonal),and brookite (orthorhombic),along with some less common structures like TiO 2-II (columbite),TiO 2-III (baddeleyite),TiO 2-H (hollandite),TiO 2-R (ramsdellite)and TiO 2-(B)(monoclinic)[23–27]many of which can be stabilized under specific conditions.TiO 2nanos-tructures have been synthesized by different methods such as sol–gel,hydothermal/solvothermal and reverse-micellar methods [28–30].Among these methods,the use of w/o microemulsion offers a unique microenvironment in which monodispersed ultrafine nanoparticles with a narrow size distribution can be obtained [31–33].Among the earlier studies there are very few reports specifically on TiO 2nanospindles.Yang et al.reported the synthesis of size-tunable TiO 2(anatase)nanospindles through hydrothermaltreatment using tubular titanates in the presence of alkyl amines at 1808C and investigated the role of reaction solvent in controlling the size of the TiO 2nanospindles.TiO 2nanospindles obtained using hydrothermal method have been used to fabricate a double-layered photoanode for dye sensitized solar cells [34,35].However,in most of the studies,the reactions were carried out at high temperature ($1808C)and lead to spindles with large size and a broad size distribution [36–38].Here for the first time we report a room temperature synthesis of spindle-shaped TiO 2using a reverse micellar route.These anisotropic structures (spindles)have diameter of $6nm and length of 30nm and are monodispersed with high surface area and high degree of thermostability.Such size and shape tunable TiO 2nanospindles lead to efficient photocatalysis,as we have shown in this study taking the case of Rhodamine B as the organic pollutant.2.ExperimentalTitanium isopropoxide was purchased from Acros (Belgium)and used as a source of titanium for the preparation of TiO 2nanoparticles.A commercial form of TiO 2(Degussa P25)was used for the comparison of the photocatalytic activity.For the preparation of microemulsion CTAB (C 16H 33N(CH 3)3Br)was used as the surfactant (Sigma–Aldrich),n -butanol (Fischer scientific)as a co-surfactant and isooctane (Spectrochem,India)as the non-polar solvent were used.Rhodamine B was selected as the dye for studying the photocatalysis.Powder X-ray diffraction (PXRD)studies were carried out on a Bruker D8Advance Diffractometer using Ni-filtered Cu-K a radiation.The data was collected in the range (2u )of 10–708with a step size ofMaterials Research Bulletin 47(2012)3780–3785A R T I C L E I N F OArticle history:Received 19December 2011Received in revised form 27April 2012Accepted 11June 2012Available online 18June 2012Keywords:A.NanostructuresB.Chemical synthesisC.Electron microscopyD.Catalytic propertiesA B S T R A C TMesoporous TiO 2nanostructures of spindle shape (diameter $6nm and length of 30nm)have been obtained at room temperature by a simple microemulsion method.The nanospindles are highly monodisperse with high surface area and high degree of thermostability.These TiO 2nanostructures have a high surface area ($200m 2/g)and show nearly twice the photocatalytic efficiency than of commercially available TiO 2and comparable to the best titania-based photocatalyst (TiO 2doped with expensive metals like Pt and obtained at higher temperature).There is no decrease in the efficiency of the photocatalyst even after three cycles.ß2012Elsevier Ltd.All rights reserved.*Corresponding author.Tel.:+911126591511;fax:+911126854715.E-mail address:ashok@chemistry.iitd.ernet.in (A.K.Ganguli).Contents lists available at SciVerse ScienceDirectMaterials Research Bulletinj o u rn a l h om e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m a t r e s b u0025-5408/$–see front matter ß2012Elsevier Ltd.All rights reserved./10.1016/j.materresbull.2012.06.0210.028and step time of 1s.The crystallite size was deduced using X-rayline broadening using the Scherrer formula (d =0.9l /(B s ÀB r )cos u ),where l is the X-ray wavelength,B s indicates the full width at half maxima of the sample,while B r is that of a standard (Quartz).Diffuse-reflectance spectra (DR)spectra were recorded on Shimadzu UV-2450spectrophotometer where the baseline was fixed using a barium sulfate reference.Nitrogen adsorption–desorption isotherms were recorded at liquid nitrogen temperature (77K)on a Quadrasorb-1C (Quantachrome Corp.)equipment and the specific area was deter-mined by the Brunauer–Emmett–Teller (BET)method.The as-prepared TiO 2samples were degassed at 2008C for 12h prior to the surface area measurements.For the adsorption–desorption measure-ments the sample was degassed for 12h.The morphology of titanium dioxide nanospindles was observed by transmission electron micros-copy (TEM)on an FEI Technai G 220electron microscope operated at 200kV.TiO 2powders were dispersed in ethanol in an ultrasound bath for few minutes and few drops of the suspension was deposited on a carbon covered microgrid for the TEM studies.2.1.Synthesis of TiO 2nanospindlesMonophasic TiO 2nanospindles were obtained by the reverse micellar method using titanium isopropoxide as the source oftitanium.The synthesis of titania nanospindles was achieved using two microemulsions (I and II)(Fig.1).The optimized composition of the microemulsion consisted of CTAB as the surfactant (16.95wt%),1-butanol as the cosurfactant (13.91wt%),isooctane as the non-polar solvent (58.94wt%),and (10.2wt%)aqueous phase.In the first microemulsion (I)the aqueous phase contained 0.1M hydroxyl titanium acylate solution [39],and the second microemulsion (II)contained 0.1M NH 3solution.The ratio of water to surfactant was kept constant (W o =12).The two microemulsions I and II were mixed and stirred for 24h.The TiO 2nanoparticles precipitated were separated by centrifuging at 4000rpm for 10min and then washed with chloroform:methanol (1:1)mixture to remove the surfactant.The product was dried at 408C in air.Fig.2shows the formation process of TiO 2nanospindles.2.2.Measurement of photocatalytic activityThe photocatalytic activity of TiO 2powders under UV light was estimated by measuring the decomposition rate of Rhodamine B in an aqueous solution.The photocatalytic reactions were carried out in a quartz reactor with a water-circulating jacket.Experiments were also performed in the absence of photocatalyst andFig.1.Flow chart for the synthesis of TiO 2nanospindles.Fig.2.Formation process of TiO 2nanospindles.D.Das et al./Materials Research Bulletin 47(2012)3780–37853781illumination (blank tests).200ml of Rhodamine B solution at concentration of 15m mol/dm 3containing suspended photocata-lyst 80mg was stirred continuously for half an hour before irradiation to ensure the adsorption/desorption equilibrium during the photocatalytic process.After 30min of stirring in dark to attain the adsorption equilibrium,the UV lamp (125W high pressure Hg lamp)was positioned above the reactor and switched on.The photocatalytic reactions were carried out at room temperature.Aliquots of 1cm 3of the aqueous suspension were collected at regular time periods during irradiation and absence of light.These aliquots were centrifuged to remove the photocatalyst particles.Rhodamine B concentration was estimated using UV–vis spectro-photometer at its maximum absorption.We have also studied the effect of pH and the presence of O 2on the photocatalytic efficiency.3.Results and discussionMonophasic TiO 2nanospindles were obtained at room temper-ature by the reverse micellar method using titanium isopropoxide as the source of titanium.We have optimized the conditions to tune the particle size of TiO 2nanospindles.All the reflections in the XRD pattern could be indexed satisfactorily and indicates presence of anatase form of TiO 2nanoparticles (Fig.3).The broad nature of the XRD peaks indicatesa small crystallite size of the particle.The average crystallite size of the particles obtained from the X-ray line broadening studies [40]of the (101)reflection was found to be 25nm.From the diffuse reflectance spectra (Fig.4)the band gap was estimated to be 3.28eV (onset of the absorption edge).The specific surface area for the anatase type titania nanos-pindles synthesized by the reverse micellar method was found to be 198m 2/g.This is the highest surface area found till now for TiO 2nanospindles.The N 2adsorption–desorption isotherms of titania nanospindles (Fig.5A)exhibit the type IV hysteresis indicative of cage like pores [41,42]and bimodal pore size distributions in the mesoporous region [43,44].Fig.5B shows the pore size distribution of the TiO 2nanospindles.From the BJH curve (Fig.5B)the average pore diameter was calculated to be 6.2nm.However,the pores obtained are not ordered.A sharp increase in nitrogen uptake at high relative pressures (P /Po >0.9)corresponds to the voids formed between the titania nanoparticles and is a measure of the textural porosity.The average size of the TiO 2nanospindles (Fig.6A)was observed to be 30nm along its major axis and 6nm diameter along its minor axis and the length-to-diameter ratio is 5:1.Fig.6D shows the HRTEM image of the TiO 2nanospindles and the latticespacing of 3.5A˚could be ascribed to the (101)planes of anatase.Keeping other experimental conditions fixed,the effect of the reaction time was investigated.The formation of TiO 2nanospin-dles was observed after carrying out the reaction for 24h (Fig.6A).When the reaction was carried out by stirring the microemulsions6050403020204105200004101I n t e n s i t y (a .u .)2-ThetaFig.3.Powder X-ray diffraction pattern of anatase TiO 2(anatase)synthesized by reverse micellar route at room temperature.A b s o r b a n c e (a .u .)Wavelength (nm)Fig.4.Diffuse-reflectance studies of TiO 2nanospindles.1.00.80.60.40.20.050100150200V o l u m e (c c /g )p/p o5040302010A d s o r p t i o n D v (l o g d )(C C /g )Pore Diameter(nm)Fig.5.(A)Nitrogen adsorption–desorption isotherms and (B)pore size distribution plot of TiO 2nanospindles synthesized by reverse micellar method.D.Das et al./Materials Research Bulletin 47(2012)3780–37853782for 12h,formation of spindle shape TiO 2nanoparticle could be observed with not so well-defined boundaries (Fig.6B).It has generally been observed that using a cationic surfactant like CTAB leads to anisotropic nanostructures [45].The formation ofanisotropic structure of TiO 2nanoparticles can be explained on the basis that the presence of negative surface charge over the titania particle followed by an assembly of cationic surfactant over the surface leading to the restricted growth of the nanoparticle along the ends which is more hydrophilic compared to the surface and hence resulted in the formation of anisotropic structures.The cooperative self-assembly of the titania precursor with surfactantmolecules followed by the basic hydrolysis lead to the formation of mesoporous TiO 2.The pores are formed by the removal of the intermediate assemblies of the surfactant molecules having a charge on the head group.In the present case we have used cationic surfactant (CTAB).The self-assembly of the cationic surfactant,(S +)and the anionic titania TiO 2ions,(I À)takes place by the S +I Àmechanism and is driven purely by electrostatic interactions [46].On removing the surfactant,pores are developed inside the particle.The particle size of TiO 2nanospindles was controlled by varying the W o (water/surfactant)parameter.At W o of 12,nanospindles of size 30nm were obtained.On increasing theFig.6.TEM micrographs of TiO 2nanospindles synthesized using reverse micellar method:(A)24h,W o 12(inset:TEM image of a single TiO 2nanospindle);(B)12h,W o 12;(C)24h,W o 14;and (D)HRTEM image of TiO 2nanospindles (24h),(E)calcined at 5008C.D.Das et al./Materials Research Bulletin 47(2012)3780–37853783value of W o to 14,it was observed that size of the TiO 2nanospindles increases from 30nm to 80nm (Fig.6C).This is attributed to the increase in flexibility of the surfactant film as a result of increase of aqueous content with W o .This leads to higher intermicellar exchange thereby resulting in increase in the size.Fig.6E shows the TEM image of the TiO 2nanospindles heated at 5008C indicating the thermostability and monodispersity.The shape and size of the particles remain unaffected on calcining the sample at 5008C as observed in the TEM image.The photocatalytic efficiency of the TiO 2nanospindles was examined under UV light by monitoring the degradation of Rhodamine B (Fig.7).The UV absorption maxima of Rhodamine B (l $546nm)were recorded at regular intervals.The adsorption of the dye on the catalyst surface is regarded as a crucial step for efficient photocatalysis and this is highly influenced by the surface area.In order to determine the photostability of the dye under the above conditions,an experiment was performed in the absence of the photocatalyst.It was observed that the concentration of the dye in the reaction solution remains unchanged even after 3h of illumination.On comparing the photocatalytic activity of TiO 2nanospindles and commercially available TiO 2powder (Degussa P25),the rate of photodegradation was found to be significantly higher for the titania nanospindles discussed in this study.This is attributed to the high surface area of TiO 2nanospindles and mesoporous nature of the sample,which influences the adsorption of dye on the catalyst surface and its fast degradation.The rate of photocatalytic activity was significantly higher under basic condition pH (8.5)with continuous purging of oxygen during the irradiation.Absence of oxygen or lower pH (4.0)decreased the photocatalytic efficiency.The reason for this difference is the extent of adsorption of dye on the surface of catalyst.From the zeta potential measurements,the surface charge on TiO 2nanospindles at pH 8.5and pH 4.0was found to be À23.5mV and 11.2mV.Under basic conditions,since the catalyst surface is negatively charged,the dye can be easily adsorbed on the catalyst surface (Rhodamine B being a cationic dye)leading to enhanced photocatalysis.In acidic media,both Rhodamine B molecules and the TiO 2surface are positively charged.Because of the strong electrostatic repulsion due to the similarly charged surfaces,Rhodamine B molecules cannot adsorb easily on the TiO 2surface and hence the cationic dye is electrostatically destabilized resulting in lesser adsorption and lower degradation rate of dye [47].On comparing the photo-catalytic degradation of Rhodamine B by the TiO 2nanospindles and that of commercial TiO 2(Degussa P25),it was observed that the TiO 2nanospindles showed nearly twice the photocatalytic efficiency of Degussa P25TiO 2.We have employed the pseudo first order model to calculate the rate constant (K )of the photodegradation process using the equation,ln(C o /C )=Kt ,where C o and C are the concentration of Rhodamine B solution,respectively,at time 0and t .TiO 2nanospindles showed nearly twice the decomposition rate under UV irradiation with a pseudo first order rate constant of 0.48h À1compared to Degussa P25with a rate constant of 0.22h À1.This indicates that rate of photo-catalytic degradation of Rhodamine B by TiO 2nanospindles is higher than the photocatalytic efficiency of TiO 2(Degussa P25).Also,it is observed that the photocatalytic degradation of Rhodamine B by the TiO 2nanospindles is comparable to the best titania-based photocatalysts (N-doped and Pt-doped TiO 2)[48,49].Thus,we could achieve high efficiency by a simple room temperature process with no metal or other dopants.Further,to check the reusability of the TiO 2nanospindles,reactions were carried out on the same catalyst for five cycles (Fig.8).The degradation rate was almost same for nearly three consecutive cycles.4.ConclusionWe have described a simple room temperature method to obtain mesoporous TiO 2nanospindles (6nm diameter and 30nm length)with surface area of $200m 2/g (the highest reported so far).The specific nanostructured morphology has strong bearing on the high photocatalytic efficiency.These TiO 2nanospindles are highly monodisperse,and also show high degree of thermostability (till 5008C there is no change in the morphology and size).We believe this methodology can be extended to several other materials whose properties depend crucially on the surface area and morphology.AcknowledgementsAKG thanks Department of Science &Technology and Depart-ment of Information Technology,Govt.of India for financial support.DD thanks CSIR for a senior research fellowship.TheFig.8.Cycling studies of photodecomposition of TiO 2nanospindles under UV light (inset:photograph showing the degradation of rhodamine B using TiO 2nanospindles in 2h).140120100806040200.00.20.40.60.81.0C /C 0time(min)Fig.7.Photocatalytic degradation of Rhodamine B at 546nm as a function of irradiation time performed on TiO 2nanospindles:(a)blank experiment (absence of catalyst),(b)at pH 4,(C)commercial TiO 2(Degussa P25),(d)at pH 8.5,(e)at pH 8.5with O 2purging.D.Das et al./Materials Research Bulletin 47(2012)3780–37853784surface area measurements carried out in Dr.Eswaramoorthy’s laboratory,JNCASR,Bangalore is gratefully acknowledged. 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