photocatalysis
Sonochemical synthesis of TiO 2nanoparticles on graphene for use as photocatalyst
Jingjing Guo a ,Shenmin Zhu a ,?,Zhixin Chen b ,Yao Li a ,Ziyong Yu a ,Qinglei Liu a ,Jingbo Li a ,Chuanliang Feng a ,Di Zhang a ,?
a State Key Laboratory of Metal Matrix Composites,Shanghai Jiao Tong University,800Dongchuan Road,Shanghai 200240,PR China b
Faculty of Engineering,University of Wollongong,Wollongong,NSW 2522,Australia
a r t i c l e i n f o Article history:
Received 29November 2010
Received in revised form 14March 2011Accepted 17March 2011
Available online 8April 2011Keywords:Graphene
Ultrasonication Photocatalysis Nanocomposite
a b s t r a c t
Using ultrasonication we succeed in a controlled incorporation of TiO 2nanoparticles on the graphene lay-ers homogeneously in a few hours.The average size of the TiO 2nanoparticles was controlled at around 4–5nm on the sheets without using any surfactant,which is attributed to the pyrolysis and condensation of the dissolved TiCl 4into TiO 2by ultrasonic waves.The photocatalytic activity of the resultant graphene–TiO 2composites containing 25wt.%TiO 2is better than that of commercial pure TiO 2.This is partly due to the extremely small size of the TiO 2nanoparticles and partly due to the graphene–TiO 2composite struc-ture consisting of homogeneous dispersion of crystalline TiO 2nanoparticles on the graphene sheets.As the graphene in the composites has a very good contact with the TiO 2nanoparticles it enhances the photo-electron conversion of TiO 2by reducing the recombination of photo-generated electron–hole pairs.
ó2011Elsevier B.V.All rights reserved.
1.Introduction
Graphene (GR),a ?at monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice.It has recently at-tracted a great deal of attention for potential applications in many ?elds,such as nanoelectronics,fuel-cell technology,supercapaci-tors and catalysts [1–5].Several approaches,including microme-chanical exfoliation of graphite [6,7],chemical vapor deposition [8,9],and solution-based chemical reduction [10–12]have been developed to produce single-layered graphene.Unfortunately,the yield of single-layer graphene sheets from various production methods is quite low [10,12–16].An even more serious problem is that single-layer sheets of graphene are not stable in solution and tend to aggregate back to graphite gradually because of the strong Johannes Diderik van der Waals interactions [17].
One possible technique of preventing aggregation and harness-ing the unique properties of single-layer graphene would be to incorporate graphene sheets into composite materials.Many metal or metal oxide nanoparticles,such as Au,Ag,Pt,Pd,TiO 2,SnO 2,and MnO 2have been deposited on graphene sheets [18–22].These me-tal or metal oxide nanoparticles not only prevent the aggregation of graphene sheets into graphite but also combine with the special two-dimensional (2D)graphene,giving rise to some unique elec-tronic,optical and catalytical properties which may be used in applications,such as biologic sensing,photocatalysis,optoelec-tronic and electrochemical devices [23].Approaches,such as elec-
trochemical methods,chemical vapor deposition methods [8],ion-exchange methods,intercalation [20],hydrothermal reduction methods [24],sol–gel methods [25],and inner modi?cation fol-lowed by in situ reduction methods,have been developed to incor-porate nanoparticles inside graphene sheets [21].To be effective as spacers,the nanoparticles have to adhere to the surface of the graphene as uniformly as possible.Therefore,the control of the for-mation and distribution of nanoparticles on the graphene layers is critical.
Ultrasonication has proved to be an effective technique for gen-erating nanoparticles with attractive properties in a short period of reaction time [26–28].The enhanced chemical effect of ultrasound is due to acoustic cavitation phenomena:the rapid formation,growth,and the collapse of bubbles in liquid.The extremely high local temperature (>5000K),pressure (>20MPa)and very high cooling rates (>1010Ks à1)confer sonicated solutions unique prop-erties,and are able to reduce metal ions to metal or metal oxide nanoparticles [29,30].The major advantage of this method,apart from its fast quenching rate and operation at ambient conditions,is that it is a simple and energy ef?cient process.In our previous work [31],sonochemical method has been employed successfully for the synthesis of TiO 2butter?y wings from butter?y templates.TiO 2has attracted much attention on account of its high photo-conversion ef?ciency [32].TiO 2can produce photo-induced elec-tron–hole pairs under the irradiation of ultraviolet (UV)light.From the point view of photo-conversion ef?ciency,the photocatalytic properties of TiO 2can be further enhanced if the recombination of the photo-induced electron–hole pairs can be effectively suppressed.Graphene–TiO 2composites are thus promising photo-catalytic materials because graphene can act as an electron transfer
1350-4177/$-see front matter ó2011Elsevier B.V.All rights reserved.doi:10.1016/j.ultsonch.2011.03.021
Corresponding authors.Tel.:+862134202584;fax:+862134202749.
E-mail addresses:smzhu@https://www.sodocs.net/doc/eb13199474.html, (S.Zhu),zhangdi@https://www.sodocs.net/doc/eb13199474.html, (D.Zhang).
channel thus reduces the recombination of the photo-generated electron holes,and leads to improved photo-conversion ef?ciency [33,34].It was reported that a mixture of commercially pure TiO2 (P25)powder and graphene showed a relatively high photocata-lytic property[24].However,the particle size of P25was relative large and in micrometers and thus the photocatalytic property can be further improved if the size of TiO2is reduced to nanome-ters.In fact,TiO2particle size needs to be reduced down to nanometers in order to improve the photocatalytic activity for liquid-phase oxidation.
In this paper,we report a sonochemical method that embeds TiO2nanoparticles into graphene oxide(GO)nanosheets homoge-neously without functionalizing the surface with a surfactant. Graphene–TiO2composite(GR–TiO2)was obtained by chemical reduction of the graphene oxide–TiO2composite(GO–TiO2).The photocatalytic properties of the obtained GR–TiO2composites were investigated by measuring the photo-degradation of methy-lene blue under UV-light,which demonstrated a high photocata-lytic performance for the obtained composites.
2.Experimental
2.1.Preparation of GO
GO was prepared from natural graphite(crystalline,300mesh, Alfa Aesar)by a modi?ed Hummers method[35].The detailed pro-cessing is described as below:2g of graphite and1g of NaNO3 were mixed with46ml of H2SO4(98%)in a250ml wide necked bottle placed in an ice bath.Then,the mixture was stirred for 30min.While maintaining magnetic vigorous stirring,a certain amount of KMnO4(6g)was added to the suspension carefully. After that,the ice bath was replaced by an oil bath,and the mixture was stirred at15°C for2h.As the reaction progressed,the mixture gradually became pasty,and the color turned into light brownish. The next step was to increase the reaction temperature to40°C and keep for another1h.Then,92ml of H2O was slowly added to the pasty mixture with vigorous agitation.The reaction temper-ature was rapidly increased to98°C with effervescence,and the color changed to yellow.Finally,30ml of5%H2O2was added to the mixture and allowing the high-temperature reaction to go on for1h.For puri?cation,the mixture was rinsed and centrifugated with5%HCl and deionized water for several times.After?ltration and drying under vacuum at60°C,GO was obtained as a gray powder.
2.2.Preparation of GO–TiO2
GO–TiO2composite was synthesized by sonochemical reaction of TiCl4in the presence of the GO.The detailed process is described as follows:?rstly,TiO2precursors were prepared with the molar ratios of ethanol:H2O:TiCl4=35:11:1,then,0.25g of the GO was added and stirred for0.5h at ambient temperature,and?nally the suspension was sonicated at room temperature for3h using a high-intensity ultrasonic probe(Ti horn,20kHz,100W/cm2). The resulting composite was recovered by centrifugation and rins-ing with ethanol solvent several times,then dried under vacuum at 60°C.The obtained sample is called GO–TiO2.
2.3.Preparation of GR–TiO2
Chemical conversion of GO–TiO2to GR–TiO2was carried out by using a reduction method[36].In a typical reduction experiment, 0.7g of GO–TiO2powder was dispersed in180ml of deionized water and the mixture was sonicated for1h.Next,24ml of hydra-zine(85%)was added into the mixture under magnetic stirring and stirred for24h at50°C.GR–TiO2composite was obtained by?ltra-tion and drying under vacuum at60°C.After calcination at450°C for3h under nitrogen,crystallized GR–TiO2composite(GR–TiO2–T)was obtained.
2.4.Preparation of graphene
Graphene was obtained by chemical conversion of GO according to the literature[36].In a typical reduction experiment,0.5g of GO powder was dispersed in150ml of deionized water and the mix-ture was sonicated for1h.Next,18ml of hydrazine(85%)was added under magnetic stirring,and the mixture was continuously stirred at50°C for24h.Finally,black graphene powder was obtained by?ltration and drying under vacuum at60°C.
2.5.Characterization
The synthesized samples were characterized by X-ray diffrac-tion(XRD)using a RigakuD/max2550VL/PC system operated at 40kV and40mA with Cu K a radiation(k=1.5406?),at a scan rate of5°minà1and a step size of0.050°in2h.Nitrogen adsorption measurements at77K were performed using an ASAP2020volu-metric adsorption analyzer,after the samples had been outgassed for8h in the degas port of the adsorption apparatus.Field-emis-sion scanning electron microscopy(FE-SEM)was performed on a JEOL JSM-6360LV?eld emission microscope at an accelerating voltage of15kV.Transmission electron microscopy(TEM)and en-ergy-dispersive X-ray measurements(EDX)were carried out on a JEOL2010microscope at200kV.TEM specimens were prepared by grinding the synthesized samples into powder with a mortar and pestle and the powder was dispersed in pure ethanol and picked up with holey carbon supporting?lms on copper grids.Fou-rier transform-infrared measurements(FT-IR)were recorded on KBr pellets with a PE Paragon1000spectrophotometer.Thermal gravimetric analysis(TGA)was conducted on a PE TGA-7instru-ment with a heating rate of20°C/min.X-ray photoelectron spectra (XPS)were collected on a physical electronics PHI5400using Mg K a radiation as X-ray source.All the spectra were corrected with the C1s(285.0eV)band.Diffuse re?ectance electronic spectra were measured with a Perkin–Elmer330spectrophotometer equipped with a60mm Hitachi integrating sphere accessory.
2.6.Photocatalytic experiments
30mg of catalyst(GR,TiO2,and GR–TiO2–T)was dispersed in a 40ml0.02g/L aqueous solution of methylene blue dye in a quartz test cell.The reaction system was kept saturated with oxygen by purging a slow stream of oxygen at ambient temperature.Before irradiation,the reactor was stirred constantly in the presence of oxygen in dark for30min,and then irradiated by four UV lamps (6W each)at room temperature.The photocatalytic activity was evaluated on the basis of decrease of the absorbance band of the methylene blue at665nm recorded at a regular time interval fol-lowing the UV illumination.The absorbance measurement of the reaction solution was taken after separating the TiO2from the reaction system by centrifugation.
3.Results and discussion
3.1.Characterizations
Wide-angle XRD patterns of the pure TiO2,GR,GR–TiO2and GR–TiO2–T are given in Fig.1.The very broad peak at2h=25.5°of GR–TiO2indicates that TiO2in the sample is largely of amor-phous in nature.After the calcination at450°C,this peak at
J.Guo et al./Ultrasonics Sonochemistry18(2011)1082–10901083
2h=25.5°becomes relatively narrower and resolved,indicating that nanocrystalline TiO2formed on the graphene.However,the peak is still too broad to be used to estimate the size of crystallites with Scherrer equation[37].Nevertheless the broad peak suggests that growth of the nanocrystalline TiO2on the graphene was very limited during the calcinations.
The loading of the TiO2nanoparticles on the graphene was determined by performing TGA in air by heating up from40to 900°C(Fig.2).A signi?cant mass loss was observed at tempera-tures from510to700°C for both GR and GR–TiO2–T,due to the destruction of the carbon skeleton(carbonyl/double bond)of graphene.The weight loss of GR–TiO2–T was stabilized at75%at
the graphene sheets in the GR–TiO2–T composite exhibited nano-scale textures,indicative of a much rougher surface(Fig.3b).At a high magni?cation(Fig.3d),we can clearly see the uniform disper-sion of TiO2nanoparticles on both the graphene surface and the interlayers.There is no agglomeration in the composite as shown in the SEM image of the GR–TiO2sheet(Fig.3f).Fig.3f also shows that the TiO2particle size was in nanometers which will be further characterized by using TEM.
Fig.4shows TEM images of TiO2loaded on the graphene layers before(GR–TiO2)and after the calcination at450°C for3h(GR–TiO2–T).For the as-synthesized GR–TiO2(Fig.4a),there are plenty of wrinkles(in circles and at the edge)on the clean sheet owing to the2D nature of the sheet.The edges of the sheets are folded and give the appearance of ribbons.High-magni?cation TEM micro-graphs(Fig.4b and c)reveal no discrete and well de?ned nanocrys-talline TiO2particles on the graphene sheets.However EDX(see Supporting Information,Fig.s1)shows the presence of Ti,O and C in the sample.The corresponding selected area electron diffrac-tion pattern(SAED)of the composite shows that diffraction spots of the graphite superimposed with the diffraction rings of TiO2 nanoparticles.After the calcination at450°C for3h,discrete and well de?ned TiO2fully crystallized particles were uniformly scat-tered on the sheets as shown in Fig.4d and the particle size is around4–5nm from the high resolution image(Fig.4f).This obser-vation is consistent with the wide angle XRD results presented ear-lier(Fig.1).The inset in Fig.4f is the SAED of GR–TiO2–T,and it can be seen that the sample has a clear sixfold pattern of the graphene and the diffraction rings of the TiO2nanoparticles.The existence of elements Ti,O and C was detected by EDX analysis(see Supporting Information Fig.s2).As demonstrated in Fig.4e,the measured lat-tice-fringe spacing of0.34nm in the ribbons corresponds to the (002)of the graphene sheets,where the crystal lattice of TiO2 nanoparticles is also resolved.From Fig.4f the measured lattice fringe spacing of0.355nm in GR–TiO2–T composites corresponds to the(101)of anatase TiO2(JCPDS21-1272).No agglomeration of the TiO2particles was observed in this sample,which is usually observed in samples prepared by other methods[20,24].
The TEM images shown in Fig.4con?rm that a uniform disper-sion of TiO2nanoparticles on the graphene sheets have been achieved under sonochemical reactions without the use of any sur-factant.Which means ultrasound is very effective in dispersing TiO2nanoparticles on graphene layers.
In order to understand the texture of the composites,nitrogen isothermal adsorption technique was employed to investigate the ultrasonic irradiation effect on the pore structure as well as to determine the location of the TiO2in graphene(Fig.5).The shape of the isotherm for GR–TiO2–T was similar to that of the parent GR,indicating the presence of open pores.It has a speci?c surface area of49m2gà1according to the BET(Brunauer,Emmett and Tell-er)analysis.The P/P0position of the in?ection points is related to the pore size in mesopore range,and the sharpness of these steps indicates the uniformity[38].As the relative pressure increases, both the isotherms of GR and GR–TiO2–T exhibit sharp in?ections in the P/P0in the range0.5–0.6,a characteristic of capillary con-densation within a uniform of interlayer spacing.Interestingly, the space layer distribution of the starting GR centered at2.6nm but disappeared when the TiO2was loaded,suggesting the possi-bility that titanium oxide formed?rst in this interlayer spacing. As a result,the space layer distribution of GR–TiO2–T is more uni-form than that of the pure GR from pore size distributions shown in Fig.5(inset).The metal oxide nanoparticles increased the dis-tance between the GR sheets to several nanometers,thereby mak-ing the both faces of the graphene accessible.
The light-absorbance property of the samples was studied with a UV–vis spectrophotometer and the measured UV–visible diffuse re?ectance electronic spectra are shown in Fig.6.The absorption 18(2011)1082–1090
bands of GR–TiO2–T appeared to be fairly different from that of P25.Because the appearance of TiO2is white whereas the GR–TiO2–T is black,the Kubelka–Munk theory could not apply to this sample with such a strong adsorption.Therefore,the quantitative consideration is void between TiO2and GR–TiO2–T.However,a broad and strong absorption in the visible light region was ob-served for GR–TiO2–T due to the presence of75wt.%GR in the GR–TiO2–T composite which acts as a‘‘dyade’’structure[39].This enhanced light-harvesting intensity of GR–TiO2–T could be possi-bly explained by the formation of the chemical bonding between TiO2and GR,i.e.a Ti–O–C bond which favors charge transfer upon light excitation[40].
The formation of the Ti–O–C bond was con?rmed by FT-IR spec-troscopy as shown in Fig.7.GO depicts a strong OH peak at 3389.4cmà1and other C–O functionalities such as COOH (1726.6cmà1)and COC/COH(1383.3–1055.3cmà1)are clearly vis-ible.The spectrum also shows a C@C peak at1617.5cmà1corre-sponding to the remaining sp2character.After the hydrazine reduction,the characteristic absorption peaks(at1726.6,1383.3–1055.3,and591.2cmà1)of GO disappeared due to the reduction of oxygenous groups from the GO.As for GR–TiO2–T,the broad absorptions at low frequencies(below1000cmà1)were ascribed to the vibration of Ti–O–Ti bond(671.3,683.9cmà1)and Ti–O–C bond(780.2cmà1)which was not observed in the spectrum of GR.This demonstrates that the TiO2nanoparticles were strong chemically bonded on the graphene sheets,which was facilitated by the sonochemical reaction.Further it was found that the peak of the stretching vibration C-OH at3419.7cmà1of GR shifted to a higher wave number of3443.8cmà1for GR–TiO2–T,which could be explained by the in?uence of the formation of Ti–O–C bond. Similar to that of GR,the skeletal vibration of the GR sheets at 1569.6cmà1was also clearly observed for GR–TiO2–T,illustrating the formation of GR–TiO2–T composites.Due to the Ti–O–C bond the TiO2nanoparticles in GR–TiO2–T did not grow much upon cal-cinations,i.e.the host graphene sheet prevented the nanocrystals from sintering.
Complicated titanium coordination states in GR–TiO2–T,were revealed from the XPS results.Fig.8a shows the Ti2p XPS spectra of GR–TiO2–T,GR and GO.As expected,there are two peaks,Ti2p3/2 centered at458.5eV and Ti2p1/2at464.2eV in the XPS spectrum of GR–TiO2–T,which were not detected in the spectra of GR and GO.Fig.8b shows the XPS spectra of GR–TiO2–T,GO and GR in the binding energy range between280and294eV(C1s).The two peaks at287and285eV of GO indicate a considerable degree of oxidation of the graphite.The binding energy of285eV is due to non-oxygenated ring C(284.3eV),and287eV is due to the C in C–O bonds(285.2eV),the carbonyl C(C@O,287.4eV)and carboxyl C (COOH,289.1eV)respectively[41].Both the C1s in the XPS spec-tra of GR and GR–TiO2–T exhibit only a single strong peak at 284.3eV which is attributed to elemental carbon,with a
weak Fig.3.FE-SEM images of GR(a,c,e)and GR–TiO2–T(b,d,f)in different magni?cations.
Fig.6.Diffuse re?ectance electronic spectra of pure TiO2and GR–TiO2–T.
XPS spectra of GO,GR and GR–TiO2–T,(b)the C1s XPS spectra of GO,GR and GR–TiO2–T,(c)the survey XPS spectra of GO,
electrostatic binding,or through charge-transfer
might become attached to the carboxyl
to TiO2takes place on the surface of the
However,the number of ionizable carboxyl
enough to complex the metal cations
oxide particles formed,resulting in less
particles.Generally,surface functionaliza-
be effective for homogeneous dispersion
SB12is generally used to control the size
prevent aggregation[42].Here in our pro-nanoparticles were formed on the surface of graph-
modi?cation or additional agent.The unique ultrasonication may have effects on the formation aggregation of the TiCl4on the surface of the GO sheets can be sig-
ni?cantly retarded by the collapse of small bubbles,resulting in a layer coating,a few nanometers thick,on the GO surface,which is consistent with the disappearance of layer spacing at2.6nm from the BET analysis.As a result,uniform dispersion of?ne TiO2 forms in situ without any surface functionalization.The one-step formation of the TiO2-graphene composites under ultrasonic irra-diation illustrates the simplicity and ef?ciency of the sonochemical approach as compared with the commonly used sol–gel’s proce-dure which is generally complex and very time-consuming.
3.3.Photocatalytic measurements
The photocatalytic activities of GR–TiO2–T were measured by the photo-degradation of methylene blue as model reaction under UV-light(6400nm),and the results are shown in Fig.10.For a comparison,the photocatalytic activities of commercial TiO2and pure GR were also measured under the same condition.In order to exclude the in?uence of adsorption process,a time of60min was allowed to achieve adsorption equilibrium before the photo-catalytic reaction.Fig.10shows that the concentration of methy-lene blue decreases fast with irradiation time both for TiO2and GR–TiO2–T,but almost no concentration change was observed for the pure GR,illustrating that the photocatalytic reaction is related to the existence of active sites(here TiO2).The pseudo-?rst-order reaction was observed for the both TiO2and GR–TiO2–T(Fig.11). For GR–TiO2–T,the k value at0.0139is higher than that of P25 at0.0054;a high k value is well known to have a high photocata-lytic activity.Photocatalytic ef?ciency can be affected by three things:crystalline phase,surface area and hierarchical structures. It is also well known that the activity is proportional to the surface area accessible to the liquid.A large surface area could adsorb sig-ni?cant amounts of water thus hydroxyl groups which can react with photo excited holes and produce hydroxyl radicals which are powerful oxidants in degrading organics.In this investigation, the surface area of GR–TiO2–T was almost the same as that of TiO2but the former showed much higher photocatalytic activity than the latter.This may be explained by the crystalline TiO2nano-particles prepared by sonochemical and calcinations approach, which enhance the activity by facilitating the access to the reactive sites of TiO2.Further,the graphene in the composite can act as an electron transfer channel to reduce the recombination of the photo-generated electron holes,and lead to improved photo-con-version ef?ciency[33,34].It should be noted that the weight ratio of the TiO2in GR–TiO2–T composite is only25wt.%,so the real
9.The proposed TiO2nanoparticles formation mechanism on the graphene sheets by sonochemical method.
Fig.10.Photo-degradation of methylene blue solution under UV-light over GR,TiO
photocatalytic activity of the TiO 2nanoparticles should be even higher than commercial pure TiO 2.4.Conclusions
Using ultrasonication we succeed in controlled incorporation of TiO 2on the graphene layers homogeneously in a few hours.The average particle size of the TiO 2was controlled at around 4–5nm on the sheets without using any surfactant,which is attrib-uted to the pyrolysis and condensation of the dissolved TiCl 4into TiO 2by ultrasonic waves.The synthesis process is simple and ef?-cient.The photocatalytic performance of GR–TiO 2–T containing 25wt.%TiO 2is better than commercial pure TiO 2.The much im-proved photocatalytic activity of GR–TiO 2–T,prepared by sono-chemical method,is attributed to the graphene–TiO 2composite structure consisting of very small and homogeneous dispersion of crystal TiO 2on the graphene sheets.In this case the photocata-lytic activity of the TiO 2is enhanced not only by the ?ne size but also by the graphene which reduces the recombination of photo-generated electron–hole pairs.Thus,such integration of 2D supports with large surface areas,and the highly dispersed nanoparticles,can be an exciting material for use in future nano-technology.These results and other related data demonstrate that this sonochemical method may also be extended to synthesize other metal oxides on graphene sheets,with high performance in many potential applications,such as optical,electrical,catalysis,sensors,and energy conversion devices and so on.Acknowledgments
The authors gratefully acknowledge the ?nancial support of this research by the National Science Foundation of China (Nos.51072117,50772067),Shanghai Science and Technology Commit-tee (Nos.06PJ14063,07DJ14001),and Sino-French Project of MOST of China (No.2009DFA52410).We also thank SJTU Instrument Analysis Center for the measurements.Appendix A.Supplementary data
Supplementary data associated with this article can be found,in the online version,at doi:10.1016/j.ultsonch.2011.03.021.
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