One-step hydrothermal synthesis of anatase TiO2_reduced graphene

One-step hydrothermal synthesis of anatase TiO 2/reduced graphene oxide nanocomposites with enhanced photocatalyticactivityDayu Liang a ,Can Cui a ,⇑,Haihua Hu b ,Yaping Wang a ,Sheng Xu a ,Beili Ying a ,Peigang Li a ,Bingqing Lu a ,Honglei Shen aa Center for Optoelectronics Materials and Devices,Department of Physics,Zhejiang Sci-Tech University,Hangzhou 310018,China bZhejiang University City College,Hangzhou 310015,Chinaa r t i c l e i n f o Article history:Received 10April 2013Received in revised form 29July 2013Accepted 9August 2013Available online 20August 2013Keywords:TiO 2Reduced graphene oxide Hydrothermal Photocatalysisa b s t r a c tA facile and efficient one-step hydrothermal approach has been developed to synthesize chemically bonded TiO 2/reduced graphene oxide (RGO)nanocomposites with Ti(SO 4)2and graphene oxide (GO)as precursors and ethanol/water solvent as a reducing agent.This approach aims to realize a partial reduc-tion of GO and a growth of anatase TiO 2nanoparticles on RGO sheets simultaneously.The morphology and microstructure characterizations of TiO 2/RGO nanocomposites reveal that this method leads to close interfacial contact of TiO 2/RGO and efficient dispersion of TiO 2nanocrystals on the surface of RGO sheets.The TiO 2/RGO nanocomposites exhibit superior photocatalytic activity compared to bare TiO 2nanoparti-cles and a mechanical mixing TiO 2+RGO sample in the degradation of Rhodamine B.Based on the pho-toluminescence spectrum and photocurrent response measurements,the enhanced photocatalytic activity of TiO 2/RGO is mainly attributed to the excellent electron trapping and transportation properties of RGO.Ó2013Elsevier B.V.All rights reserved.1.IntroductionTiO 2has received much attention as a promising semiconductor photocatalytic material for uses in solar energy conversion and pollutant treatment,owning to its high redox potential,cheapness,nontoxicity and chemical stability [1,2].However,the rapid recom-bination of photogenerated electron–hole pairs in TiO 2nanoparti-cles results in a low quantum efficiency and poor photocatalytic activity [3].In the last decade,various strategies have been devel-oped to suppress the recombination of photogenerated electron–hole pairs in TiO 2.Coupling TiO 2with other semiconductor (ZnO,CdS,WO 3,and Bi 2WO 6)[4–7]or carbonaceous substance [8,9]to form heterojunction nanostructure is an efficient method to reduce the electron–hole pairs recombination rate of TiO 2.Graphene,a single layer of sp 2-bonded carbon atoms,is suitable for using as a photocatalyst support material due to its unique properties,such as excellent electrical conductivity,high theoreti-cal specific surface area,flexible structure,exceptional transpar-ency and adsorption activity [10,11].Recent reports indicate that hybridizing TiO 2nanoparticles with graphene can lead to efficient electron–hole separation and enhance the electronic and photocat-alytic properties of TiO 2in a large degree [12,13].However,it is dif-ficult to realize high yields of single-layer graphene sheets since many synthesis routes like mechanical exfoliation,epitaxial growth or chemical vapor deposition are limited by the high cost and particularly complex process [14,15].Therefore,RGO obtained from partial reduction of GO in solution has been widely used as an inexpensive substitute for graphene [16].Because of the residual oxygen-containing functional groups on the sheet surface,the hydrophilic RGO can form close interfacial contact with nanoparti-cles by chemical bonds and be able to suspend in both polar and non-polar solvents [17].In the past years,several methods,such as sol–gel method,hydrothermal treatment and UV or micro-wave-assisted reduction method,have been used to synthesize TiO 2/RGO nanocomposites with improved electronic or photocata-lytic performance [18–20].However,it is almost inevitable that these synthesis processes are realized by mechanically mixing TiO 2nanoparticles with the suspension of GO and then reducing GO into RGO in an additional step.These two-step synthesis strat-egies always lead to a weak interfacial contact and poor dispersion of nanoparticles on RGO sheets.What is more,strong reducing agents such as hydrazine are involved in the reduction process of a conventional synthesis,which may destroy the 2-dimentional structure and reduce the electron properties of RGO seriously [21].Therefore,it is still urgent to develop facile,green and effi-cient ways to prepare high quality TiO 2/RGO nanocomposites in large scale.Herein,we report a facile one-step hydrothermal method to synthesize TiO 2/RGO nanocomposites with Ti(SO 4)2and GO0925-8388/$-see front matter Ó2013Elsevier B.V.All rights reserved./10.1016/j.jallcom.2013.08.062Corresponding author.Tel.:+86057186843832.E-mail address:cancui@ (C.Cui).solutions as starting materials in an ethanol/water solvent. Well-crystalline anatase TiO2nanoparticles and large RGO sheets were synthesized simultaneously during the hydrothermal pro-cess.TiO2nanoparticles were well dispersed on RGO sheets and tight interfacial contact was achieved by chemical bonds between TiO2and the residual oxygen-contained functional groups on the surface of RGO sheets.As a result,the TiO2/RGO nanocomposites exhibited superior photocatalytic activity compared to bare TiO2nanoparticles and the mechanical mixing TiO2+RGO.2.Experimental2.1.MaterialsPristine graphite and Rhodamie B(RhB)were purchased from Fisher Scientific (Hong Kong)Co.,Ltd.Ti(SO4)2was supplied by Shanghai Chemical Reagent Com-pany.All the reagents were at analytical grade and used without further purifica-tion.The deionized water was produced from a Millipore Milli-Q water purification system and used throughout the whole experiment.2.2.Synthesis of TiO2/RGO nanocompositesFig.1shows the schematic illustration of the synthesis process.The GO suspen-sion was obtained by the modified Hummers method as described elsewhere [22,23].TiO2/RGO nanocomposites were prepared using Ti(SO4)2and0.5mg/ml GO aqueous suspension as the starting materials.In a typical preparation process, 0.48g Ti(SO4)2was added into20ml of ethanol and then a certain amount of GO suspension was gradually dropped into the solution.Subsequently,deionized water was added to get a total volume of40ml.After stirring for1h,the obtained solution was transferred into a50ml Teflon-sealed autoclave and maintained at180°C for 9h in a furnace.Finally,the resulting products were isolated by centrifugation at 6000rpm for10min,washed with deionized water and ethanol several times and then dried in an oven at70°C for12h.We refer to the as-prepared TiO2/RGO nanocomposites with5ml,10ml,15ml,and20ml GO suspension in the starting solution as TiO2/RGO-1,TiO2/RGO-2,TiO2/RGO-3,TiO2/RGO-4,respectively.For comparison,pure TiO2nanoparticles and RGO were also synthesized under the same conditions in the absence of GO and Ti(SO4)2,respectively.A sample denoted as TiO2+RGO was prepared by mechanical mixing TiO2and RGO with RGO mass ratio similar to TiO2/RGO-4.2.3.CharacterizationsThe phase structure of the samples was characterized by X-ray diffraction (XRD)on a Bruker D8diffractometer using Cu K a(k=1.5406Å)radiation at 2.4.Photocatalytic activity measurementThe photocatalytic tests were carried out at room temperature under atmo-spheric pressure in a quartz reactor system.In a typical process,25mg of the pho-tocatalyst was added to50ml aqueous solution containing 4.17Â10À5M Rhodamine B(RhB)in a150ml quartz beaker.The suspension was stirred in the dark for30min to get an adsorption–desorption equilibrium.Then the suspension was exposed to a250W high pressure hydrargyric lamp under magnetic stirring. During illumination,a circulation of water through an external cooling coil was conducted to maintain the temperature of suspension at about25°C.Then samples of the suspension were obtained at5min intervals.The concentration of RhB in all the samples was estimated by measuring the absorbance at the maximum peak (554nm)with a UNICO UV-4802H spectrophotometer.2.5.Photocurrent response measurementThe TiO2,TiO2+RGO and TiO2/RGO-4electrodes for use in the photocurrent measurement were prepared by the doctor-blade method onfluorine-doped SnO2 (FTO)conductive glass.Briefly,0.1g of photocatalyst was dissolved in about1ml ethanol to form slurry.The slurry was coated onto FTO glass to form a uniformfilm, and then thefilm was heated in vacuum at400°C for2h.The transient photocur-rent characterization was carried out using a standard three-compartment cell,con-sisting of a working electrode,a platinum counter electrode,and an Ag/AgCl reference electrode.The electrolyte was0.5M Na2SO4aqueous solution degassed with N2and the illumination source was a150W halogen tungsten lamp.3.Results and discussionFig.2shows the XRD patterns of the samples.The XRD pattern of GO reveals a reflection peak at2h=10.7°,which corresponds to interlayer spacing of0.83nm.The XRD patterns of TiO2and TiO2/ RGO can be indexed to anatase TiO2(JCPDS21-1272).In contrast to other titanium precursors,Ti(SO4)2is more inclined to form well-crystalline TiO2nanoparticles during the low-temperature hydrothermal process and the conventional calcination treatment for TiO2crystallization is not mandatory in our method.In our experiment,the(002)peak of RGO at26.5°is not shown in the TiO2/RGO samples[24],which may be due to the disappearance of the layer-stacking regularity of RGO after intercalation by TiO2 nanoparticles[25].The average crystallite sizes of TiO2nanoparti-cles in all the samples were calculated to be around10nm with the Scherrer equation based on the full width at half maximum (FWHM)of(101)peak[26].The similar crystallite size of anatase TiO2nanoparticles in different samples indicates that the incorpo-Fig.1.The schematic illustration of the reduction of GO and in situ growth of TiOnanoparticles on RGO sheets via hydrothermal process.Fig.2.XRD patterns of GO,TiO2,and TiO2/RGO.D.Liang et al./Journal of Alloys and Compounds582(2014)236–240237the skeletal vibration of C@C.The other peaks at1730,1380,1220 and1058cmÀ1are assigned to stretching vibrations of oxygen-containing groups carboxyl C@O,carboxyl C A O,epoxy C A O and alkoxy C A O[28],respectively.The FTIR spectrum of TiO2/RGO-4 demonstrates that most of the oxygen-containing functional groups have been removed as the peaks associated with the oxy-gen-containing groups decrease dramatically,implying that GO was significantly reduced in the hydrothermal process.However, it is notable that a small fraction of oxygen-containing functional groups still exist and facilitate the hybridizing of TiO2nanoparti-cles on the RGO sheets.Fig.4shows the Raman spectra of the TiO2,GO and TiO2/RGO-4 samples.For the TiO2,four Raman peaks at around153cmÀ1, 400cmÀ1,528cmÀ1and636cmÀ1are associated with the E g,B1g, B1g+A1g and E g modes of anatase TiO2[29],respectively.The Ra-man spectrum of GO shows two characteristic peaks,namely,the D band at1355cmÀ1and the G band at1600cmÀ1.The D band is assigned to edge or in-plane sp3defects and disordered carbon, whereas the G band arises from the in-plane vibration of ordered sp2-bonded carbon atoms[30].All the above mentioned Raman peaks are observed in the spectrum of TiO2/RGO-4,whereas theintensity ratio of D bands and G bands(I D/I G)decrease for GO to1.004for TiO2/RGO-4.The decreased I D/I Gspectrum of TiO2/RGO-4also confirms the partial reduction It is easy tofind platelets of RGO sheets among the TiO2nanopar-ticles.Figs.5(c)and(d)are the TEM images of TiO2/RGO-4in low and high magnification.Fig.5(c)shows that the crumpled RGO sheets are uniformly covered by TiO2nanocrystals.The crystallo-graphic spacing of0.352nm in Fig.5(d)matches well with the (101)crystallographic plane of anatase TiO2.The RGO sheets can be identified from the edges of TiO2nanoparticles in Fig.5(d). The SEM and TEM images coupled with the FTIR and Raman spec-tra offer sufficient evidence for the tight coupling of RGO sheets with anatase TiO2nanoparticles.The UVDRS spectra have been measured to investigate the opti-cal absorption property of TiO2/RGO nanocomposites(Fig.6).The bare TiO2exhibits the fundamental absorption at around390nm corresponding to the bandgap energy of3.2eV in the ultraviolet re-gion[31,32].Compared with that of the bare TiO2,a gradual red-shift to longer wavelengths is observed for the TiO2/RGO with increasing RGO mass ratio in the nanocomposites.The red-shift of the absorption onsets is ascribed to the formation of Ti A O A C bond[33],which causes the bandgap of the nanocomposites to be-come narrower.Accordingly,as shown in the inset of Fig.6,the col-or of the TiO2/RGO suspensions in ethanol changed from white to grey and then dark with increasing RGO mass ratio.The photocatalytic performance of TiO2/RGO nanocomposites was evaluated by degradation of RhB.The bare TiO2nanoparticles and mechanical mixing TiO2+RGO were used as reference sam-ples.Fig.7shows the photocatalytic degradation curves of RhB as a function of time.The photocatalytic performance of different photocatalysts follows the order of TiO2<TiO2/RGO-1<TiO2/ RGO-2<TiO2+RGO<TiO2/RGO-3<TiO2/RGO-4,indicating that the successful incorporation of RGO in the nanocomposites can en-hance the photocatalytic performance of TiO2efficiently.The pho-tocatalytic activity of TiO2+RGO is lower than that of TiO2/RGO-4 even though the two photocatalysts have similar RGO content.The photocatalytic degradation of RhB solution is a pseudo-first-order reaction and its kinetics can be expressed as:-ln(C/C0)=k appÂt, where k app is the apparent photocatalytic reaction rate constant, C0is the initial concentration of RhB before light irradiation and C is the concentration at irradiation time t[34].The k app can be cal-culated from the slope of-ln(C/C0)vs t plots and the values are listed in Fig.8.With the incremental RGO mass ratio,the k app of TiO2/RGO nanocomposites increase from0.0398minÀ1to 0.1472minÀ1.The highest degradation rate observed for TiO2/ RGO-4is about4.6times that of0.0264minÀ1for bare TiO2nano-particles.The mechanical mixing TiO2+RGO sample has a moder-ate k app of0.0973minÀ1.According to other reports of RGO modified TiO2photocatalysts[35],an optimal content of RGO in the nanocomposites was about5%and excess RGO would reduce the photocatalytic performance.PL spectra and photocurrent response measurements have been carried out to evaluate the photogenerated charges recombination and transportation in TiO2/RGO photocatalyst.Fig.9shows the PL spectra of TiO2/RGO nanocomposites with different RGO content. The PL peak at around390nm is ascribed to the bandgap recombi-nation of TiO2,while other luminescence peaks in the range from 425nm to500nm are related to transition from localized surface states to the valence band of TiO2[36].The gradual decrease in PL intensity at around390nm with increasing RGO mass ratio in TiO2/RGO implies the suppression of photogenerated electron recombination from conduction band to valence band in TiO2. The PL spectrum of TiO2+RGO was also measured,and its PL intensity at around390nm is also very weak and is similar to that of TiO2/RGO-2.The photocurrent responses of the TiO2,TiO2+RGO and TiO2/RGO-4electrodes under intermittent illumination(0V vs Ag/AgCl)are shown in Fig.10.The photocurrent response was reversible as the illumination was turned on and off.The photocur-rent density of TiO2/RGO-4electrode is6.97l A cmÀ2,which isFig.3.FTIR spectra of TiO2,GO,and TiO2/RGO-4samples.Fig.4.Raman spectra of TiO2,GO,and TiO2/RGO-4samples.Compounds582(2014)236–240Fig.5.SEM images of(a)TiO2,(b)TiO2/RGO-4,(c)TEM and(d)HRTEM images of TiO2/RGO-4.UV–vis diffuse reflectance spectra(UVDRS)of TiO2and TiO2/RGO,the inset the photos of the suspensions of TiO2and TiO2/RGO in ethanol.RhB photodegradation as a function of illumination time for TiO2,TiO2TiO2/RGO nanocomposites.Fig.8.The plots of-ln(C/C0)vs t of TiO2,TiO2+RGO and TiO2/RGO nanocomposites.Fig.9.PL spectra of TiO2,TiO2+RGO and TiO2/RGO nanocomposites.larger than that of the bare TiO 2and TiO 2+RGO electrodes.The weakened PL intensity and enhanced photocurrent in TiO 2/RGO nanocomposites confirmed the high efficiency of RGO on suppres-sion of photogenerated electron–hole recombination in TiO 2.Be-cause the Fermi level of RGO is more positive than the conduction band of TiO 2[37],the photogenerated electrons in TiO 2conduction band tend to inject into RGO,which act as accep-tors of photogenerated electrons.On the other hand,the high elec-trical conductivity of RGO enables these electrons to transport in the two dimensional p -conjugation structures of RGO sheets rather than recombination with holes that remain in the valence band of TiO 2[38].The relatively poor performance of mechanical mixing TiO 2+RGO compared to TiO 2/RGO-4is ascribed to the weak inter-facial contact and poor dispersion of nanoparticles on RGO sheets.So far,we have confirmed that the RGO sheets with excellent elec-tron trapping and transportation properties can suppress the recombination of photogenerated electron–hole pairs in TiO 2.Moreover,the well-crystalline anatase TiO 2with small crystallite size,the close interfacial contact,and the narrow bandgap in the composite structure are also beneficial to light absorption and electron transport,which ultimately contribute to the enhanced photocatalytic performance of TiO 2/RGO nanocomposites.4.ConclusionsWe have developed a facile and efficient one-step hydrothermal synthesis route to realize a partial reduction of GO and a growth of well-crystalline anatase TiO 2nanoparticles on RGO sheets simulta-neously.The microstructure characterizations indicate that the anatase TiO 2nanocrystals are well dispersed on RGO sheets with chemical bonds in the resulting TiO 2/RGO nanocomposites.The PL and photocurrent response measurements reveal that the TiO 2/RGO can efficiently restrain charges recombination due to the excellent electron trapping and transportation properties of RGO.As a result,the TiO 2/RGO nanocomposites exhibit superior photocatalytic activity compared to bare TiO 2nanoparticles and mechanical mixing TiO 2+RGO in degradation of RhB.The highest apparent photocatalytic reaction rate constant of TiO 2/RGO nano-composites is about 4.6times that of bare TiO 2nanoparticles.AcknowledgmentsThe authors are grateful to Dr.H.R.Sharma in the University of Liverpool for reading the manuscript.This work was supported by the National Natural Science Foundation of China (Nos.60806045and 11074220),Natural Science Foundation of Zhejiang Province (No.Y4100310),Qianjiang Talent Program of Zhejiang Province (2013R10060),Open Foundation of Zhejiang Provincial Top Key Academic Discipline of Applied Chemistry and Eco-Dyeing &Fin-ishing Engineering (No.YR2012008),Scientific Research Fund of Zhejiang Provincial Education Department (No.Y201223380),and 521Talents Project in Zhejiang Sci-Tech 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