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Xian et al. Nanoscale Research Letters 2014, 27NANO EXPRESSOpen AccessPhotocatalytic reduction synthesis ofSrTiO3-graphene nanocomposites and theirenhanced photocatalytic activityTao Xian1,2, Hua Yang1,2*, Lijing Di2, Jinyuan Ma1, Haimin Zhang2 and Jianfeng Dai1,2AbstractSrTiO3-graphene nanocomposites were prepared via photocatalytic reduction of graphene oxide by UVlight-irradiated SrTiO3 nanoparticles. Fourier transformed infrared spectroscopy analysis indicates that grapheneoxide is reduced into graphene. Transmission electron microscope observation shows that SrTiO3 nanoparticles arewell assembled onto graphene sheets. The photocatalytic activity of as-prepared SrTiO3-graphene composites wasevaluated by the degradation of acid orange 7 (AO7) under a 254-nm UV irradiation, revealing that the compositesexhibit significantly enhanced photocatalytic activity compared to the bare SrTiO3 nanoparticles. This can beexplained by the fact that photogenerated electrons are captured by graphene, leading to an increased separationand availability of electrons and holes for the photocatalytic reaction. Hydroxyl (·OH) radicals were detected by thephotoluminescence technique using terephthalic acid as a probe molecule and were found to be produced overthe irradiated SrTiO3 nanoparticles and SrTiO3-graphene composites; especially, an enhanced yield is observed forthe latter. The influence of ethanol, KI, and N2 on the photocatalytic efficiency was also investigated. Based on theexperimental results, ·OH, h , and H2O2 are suggested to be the main active species in the photocatalyticdegradation of AO7 by SrTiO3-graphene composites.Keywords: SrTiO3-graphene nanocomposites; Photocatalysis; Photocatalytic mechanismPACS: 61.46. w; 78.67.Bf; 78.66.SqBackgroundSemiconductor photocatalysts have attracted considerableattention over the past decades due to their potentialapplications in solar energy conversion and environmentalpurification [1,2]. Among them, SrTiO3, a well-knowncubic perovskite-type multimetallic oxide with a bandgapenergy (Eg) of approximately 3.2 eV, is proved to be apromising photocatalyst for water splitting and degradation of organic pollutants [3-6]. Furthermore, the photocatalytic activity of SrTiO3 can be tailored or enhanced bydoping with metalloid elements, decoration with noblemetals, and composite with other semiconductors [7-10].It is generally accepted that the basic principle of semiconductor photocatalysis involves the photogeneration* Correspondence: hyang@lut.cn1State Key Laboratory of Advanced Processing and Recycling of Non-ferrousMetals, Lanzhou University of Technology, Lanzhou 730050, People’sRepublic of China2School of Science, Lanzhou University of Technology, Lanzhou 730050,People’s Republic of Chinaof electron–hole (e -h ) pairs, migration of the photogenerated carriers to the photocatalyst surface, redoxreaction of the carriers with other chemical species toproduce active species (such as · OH, ·O2, and H2O2),and attack of the active species on pollutants leading totheir degradation. In these processes, the high recombination rate of the photogenerated carries greatly limits thephotocatalytic activity of catalysts. Therefore, the effectiveseparation of photogenerated electron–hole pairs is veryimportant in improving the photocatalytic efficiency.Graphene, being a two-dimensional (2D) sheet ofsp2-hybridized carbon atoms, possesses unique propertiesincluding high electrical conductivity, electron mobility,thermal conductivity, mechanical strength, and chemicalstability [11-13]. On account of its outstanding properties,graphene has been frequently used as an ideal support tointegrate with a large number of functional nanomaterialsto form nanocomposites with improved performances inthe fields of photocatalysts [14-21], supercapacitors [22], 2014 Xian et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative CommonsAttribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproductionin any medium, provided the original work is properly credited.

Xian et al. Nanoscale Research Letters 2014, 27field-emission emitters [23], and fuel cells [24]. Particularly,the combination of graphene with photocatalysts isdemonstrated to be an efficient way to promote theseparation of photogenerated electron–hole pairs andthen enhance their photocatalytic activity [14-21]. Inthese photocatalyst-graphene composites, photogeneratedelectrons can be readily captured by graphene which acts asan electron acceptor, leading to an increasing availability ofphotogenerated electrons and holes participating in thephotocatalytic reactions. But so far, the investigation concerning the photocatalytic performance of SrTiO3-graphenenanocomposites has been rarely reported.Up to now, semiconductor-graphene nanocompositeshave been generally prepared using graphene oxide asthe precursor, followed by its reduction to graphene. Toreduce the graphene oxide, several methods have beenemployed including chemical reduction using hydrazineor NaBH4 [14], high-temperature annealing reduction[15], hydrothermal reduction using supercritical water[16], green chemistry method [17], and photocatalyticreduction using semiconductors [18-21]. Among them,the photocatalytic reduction is an environment-friendly anda mild way for the synthesis of semiconductor-graphenecomposites. In this route, the solution containing thephotocatalyst and graphene oxide is irradiated with lightenergy greater than the Eg of the photocatalyst, duringwhich graphene oxide receives electrons from the excitedphotocatalyst and is thus reduced to graphene. During thephotocatalytic reduction process, photocatalyst nanoparticles are assembled onto graphene sheets to formphotocatalyst-graphene composites. Herein, we reportthe synthesis of SrTiO3-graphene nanocomposites viathe photocatalytic reduction method. The photocatalytic activity of the composites was evaluated by thedegradation of acid orange 7 (AO7) under ultraviolet(UV) light irradiation, and the photocatalytic mechanisminvolved was discussed.MethodsSrTiO3 nanoparticles were synthesized via a polyacrylamidegel route as described in the literature [25]. The grapheneoxide used in this research was purchased fromNan-Jing XF Nano Materials Tech Co. Ltd. (Nanjing,China). SrTiO3-graphene composites were prepared via aphotocatalytic reduction route. A certain amount of graphene oxide was dispersed in 50 mL distilled water,followed by ultrasonic treatment of the suspension for30 min. Then, 0.1 g SrTiO3 nanoparticles and 0.0125 g ammonium oxalate (AO) were added to the suspension undermagnetic stirring. After stirring for 10 min, the mixture waspurged with nitrogen and exposed to UV light irradiationfrom a 15-W low-pressure mercury lamp for 5 h undermild stirring. During the irradiation, the color of the mixture changed from brown to black, indicating the reductionPage 2 of 9of the graphene oxide. After that, the product was separated from the reaction solution by centrifugation at4,000 rpm for 10 min, washed several times with distilledwater and absolute ethanol, and then dried in a thermostatdrying oven at 60 C for 4 h to obtain SrTiO3-graphenecomposites. A series of samples were prepared by varyingthe weight fraction of graphene oxide from 2.5% to 10%.The photocatalytic activity of the samples was evaluatedby the degradation of AO7 under UV light irradiation of a15-W low-pressure mercury lamp (λ 254 nm). The initial AO7 concentration was 5 mg L 1 with a photocatalystloading of 0.5 g L 1. Prior to irradiation, the mixed solution was ultrasonically treated in the dark to make thephotocatalyst uniformly dispersed. The concentration ofAO7 after the photocatalytic degradation was determinedby measuring the absorbance of the solution at a fixedwavelength of 484 nm. Before the absorbance measurements, the reaction solution was centrifuged for 10 min at4,000 rpm to remove the photocatalyst. The degradationpercentage is defined as (C0 Ct) / C0 100%, where C0and Ct are the AO7 concentrations before and after irradiation, respectively. To investigate the photocatalytic stability of the SrTiO3-graphene composites, the recycling testsfor the degradation of AO7 using the composite werecarried out. After the first cycle, the photocatalyst wascollected by centrifugation, washed with water, anddried. The recovered photocatalyst was introduced tothe fresh AO7 solution for the next cycle of the photocatalysis experiment under the same conditions. Theprocess was repeated four times.Terephthalic acid (TA) was used as a probe molecule toexamine hydroxyl (·OH) radicals produced over the irradiated SrTiO3-graphene composites. It is expected that TAreacts with · OH to generate a highly fluorescent compound, 2-hydroxyterephthalic acid (TAOH). By measuringthe photoluminescence (PL) intensity of TAOH that ispronounced around 429 nm, the information about · OHcan be obtained. TA was dissolved in a NaOH solution(1.0 mmol L 1) to make a 0.25-mmol L 1 TA solution andthen to the solution was added 0.5 g L 1 SrTiO3-graphenecomposites. The mixed solution, after several minutes ofultrasound treatment in the dark, was illuminated under a15-W low-pressure mercury lamp. The reacted solutionwas centrifuged for 10 min at 4,000 rpm to remove thephotocatalyst and was then used for the PL measurementsthrough a fluorescence spectrophotometer with the excitation wavelength of 315 nm.The phase purity of the samples was examined by meansof X-ray powder diffraction (XRD) with Cu Kα radiation.Fourier transform infrared spectroscopy (FTIR) measurements were carried out on a Bruker IFS 66v/S spectrometer(Ettlingen, Germany). The morphology of the samples wasobserved by a field emission transmission electron microscope (TEM). The UV-visible diffuse reflectance spectra

Xian et al. Nanoscale Research Letters 2014, 27Page 3 of 9were measured using a UV-visible spectrophotometer withan integrating sphere attachment.Results and discussionFigure 1 schematically shows the photocatalytic reductionprocess of graphene oxide by UV light-irradiated SrTiO3nanoparticles. It is noted that the SrTiO3 particles have anisoelectric point at pH 8.5 [26]; that is, they bear a negative surface charge when pH 8.5 and a positive surfacecharge when pH 8.5. When the SrTiO3 particles areadded to the graphene oxide suspension, the pH value ofthe mixture is measured to be approximately 6.5, implyingthat the SrTiO3 particle surface is positively charged. Onthe other hand, the oxygen-containing functional groupsof graphene oxide (such as carboxylic acid -COOH andhydroxyl -OH) are deprotonated when it immersed inwater, which leads to negative charges created on grapheneoxide [27]. As a result, the SrTiO3 particles are expected tobe adsorbed onto the graphene oxide sheets through electrostatic interactions. Upon UV-light irradiation, electronsand holes are produced on the conduction band (CB) andvalence band (VB) of the SrTiO3 particles, respectively. Thephotogenerated holes are captured by ammonium oxalatethat is a hole scavenger [28], leaving behind the photogenerated electrons on the surface of the SrTiO3 particles.The electrons are injected from the SrTiO3 particles intothe graphene oxide and react with its oxygen-containingfunctional groups to reduce graphene oxide.Figure 2 shows the FTIR spectra of graphene oxide,SrTiO3 particles, and SrTiO3-graphene(10%) composites.In the spectrum of graphene oxide, the absorption peakFigure 2 FTIR spectra of graphene oxide, SrTiO3 particles, andSrTiO3-graphene(10%) composites.at 1,726 cm 1 is caused by the C O stretching vibration ofthe COOH group. The peak at 1,620 cm 1 is attributed tothe C C skeletal vibration of the graphene sheets. Theabsorption peak of O-H deformation vibrations in C-OHcan be seen at 1,396 cm 1. The absorption bands at around1,224 and 1,050 cm 1 are assigned to the C-O stretchingvibration. For the SrTiO3 particles, the broad absorptionbands at around 447 and 625 cm 1 correspond to TiO6octahedron bending and stretching vibration, respectively[29]. The absorption peak at around 1,630 cm 1 is due tothe bending vibration of H-O-H from the adsorbed H2O.In the spectrum of the SrTiO3-graphene composites, thecharacteristic peaks of SrTiO3 are detected. The absorptionFigure 1 Schematic illustration of the photocatalytic reduction process of graphene oxide by UV light-irradiated SrTiO3 nanoparticles.

Xian et al. Nanoscale Research Letters 2014, 27Page 4 of 9peak at 1,630 cm 1 is the overlay of the vibration peak ofH-O-H from H2O and C C skeletal vibration peak in thegraphene sheets. However, the absorption peaks of oxygencontaining functional groups, being characteristic forgraphene oxide, disappear. The results demonstratethat graphene oxide is completely reduced to grapheneduring the photocatalytic reduction process.Figure 3 shows the XRD patterns of the SrTiO3 particlesand the SrTiO3-graphene (10%) composites. It is seen thatall the diffraction peaks for the bare SrTiO3 particles andthe composites can be index to the cubic structure ofSrTiO3, and no traces of impurity phases are detected.This indicates that the SrTiO3 particles undergo nostructural change after the photocatalytic reduction ofgraphene oxide. In addition, no apparent diffractionpeaks of graphene in the composites are observed, whichis due to the low content and relatively weak diffractionintensity of the graphene.Figure 4a shows the TEM image of graphene oxide,indicating that it has a typical two-dimensional sheetstructure with crumpled feature. Figure 4b shows theTEM image of the SrTiO 3 particles, revealing that theparticles are nearly spherical in shape with an average sizeof about 55 nm. The TEM image of the SrTiO3-graphene(10%) composites is presented in Figure 4c, from whichone can see that the SrTiO3 particles are well assembledonto the graphene sheet.Figure 5a shows the UV-visible diffuse reflectance spectraof the SrTiO3 particles and SrTiO3-graphene composites.The composites display continuously enhanced light absorbance over the whole wavelength range with increasinggraphene content. This can be attributed to the strong lightabsorption of graphene in the UV-visible light region [30].Figure 5b shows the corresponding first derivative of thereflectance (R) with respect to wavelength λ (i.e., dR / dλ),where the peak wavelength is characterized to be theFigure 3 XRD patterns of the SrTiO3 particles andSrTiO3-graphene(10%) composites.Figure 4 TEM images of (a) graphene oxide, (b) SrTiO3particles, and (c) SrTiO3-graphene(10%) composites.

Xian et al. Nanoscale Research Letters 2014, 27Page 5 of 9Figure 6 Photocatalytic degradation of AO7 over SrTiO3particles and SrTiO3-graphene composites. This degradation is afunction of irradiation time, along with the blank experiment result.Figure 5 Diffuse reflectance spectra and corresponding firstderivative. (a) Diffuse reflectance spectra of the samples. (b)Corresponding first derivative of diffuse reflectance spectra.absorption edge of the samples. It is seen that theSrTiO3 particles and composites present two absorption peaks in the derivative spectra. The strong andsharp absorption edge at approximately 370 nm is suggested to be attributed to the electron transition fromvalence band to conduction band. In comparison tothe SrTiO3 particles, the SrTiO3-graphene compositesshow almost no shift in this absorption edge, indicatingthat the effect of graphene on the band structure ofSrTiO3 can be neglected. From this absorption edge, theEg of the samples is obtained to be approximately 3.35 eV.In addition, the relatively weak absorption edge at approximately 335 nm may be ascribed to the surface effects.The photocatalytic activity of the SrTiO3-graphenecomposites was evaluated by the degradation of AO7 underUV light irradiation. Figure 6 shows the photocatalytic degradation of AO7 over the SrTiO3-graphene composites as afunction of irradiation time (t). The blank experiment resultis also shown in Figure 6, from which one can see thatAO7 is hardly degraded under UV light irradiation withoutphotocatalysts, and its degradation percentage is less than8% after 6 h of exposure. After the 6-h irradiation in thepresence of SrTiO3 particles, about 51% of AO7 is observedto be degraded. When the SrTiO3 particles assembled onthe graphene sheets, the obtained samples exhibit higherphotocatalytic activity than the bare SrTiO3 particles. Inthese composites, the photocatalytic activity increases gradually with increasing graphene content and achieves thehighest value when the content of graphene reaches 7.5%,where the degradation of AO7 is about 88% after irradiation for 6 h. Further increase in graphene content leads tothe decrease of the photocatalytic activity.Figure 7 shows the PL spectra of the TA solution afterreacting for 6 h over the UV light-irradiated SrTiO3particles and SrTiO3-graphene(7.5%) composites. Theblank experiment result indicates almost no PL signal at429 nm after irradiation without photocatalyst. On irradiation in the presence of the SrTiO3 particles, the PL signalFigure 7 PL spectra of the TA solution after reacting for 6 hover the irradiated samples. The blank experiment result isalso shown.

Xian et al. Nanoscale Research Letters 2014, 27centered around 429 nm is obviously detected, revealingthe generation of · OH radicals. When the SrTiO3-graphenecomposites are used as the photocatalyst, the PL signal becomes more intense, suggesting that the yield of the · OHradicals is enhanced over the irradiated composites.Generally, h , ·OH, ·O2, and H2O2 are thought to be themain active species responsible for the dye degradation[31]. It is known that ethanol is a scavenger for · OH, andKI is a scavenger for both · OH and h [32,33]. By investigating the effect of ethanol and KI on the photocatalyticefficiency of the composites toward the AO7 degradation,we can clarify the role of h and · OH in the photocatalysis. The role of · O2 and H2O2, which are derived from thereaction between dissolved O2 and photogenerated e , onthe dye degradation can be examined by investigating theeffect of N2 on the photocatalytic efficiency since the dissolved O2 can be removed from the solution by the N2purging procedure. Figure 8 shows the effect of N2 (bubbled at a rate of 0.1 L min 1), ethanol (10% by volume), andKI (2 10 3 mol L 1) on the degradation percentage ofAO7 after 6 h of photocatalysis. It is demonstrated thatwhen adding ethanol to the reaction solution, thephotocatalytic degradation of AO7 undergoes a substantial decrease, from approximately 88% under normalcondition to approximately 40% on addition of ethanol.This suggests that · OH radical is an important activespecies responsible for the dye degradation. Figure 7provides direct evidence showing the generation of · OHradicals over the irradiated SrTiO3-graphene composites. The addition of KI to the reaction solution resultsin a higher suppression of the photocatalytic efficiencycompared to the addition of ethanol, where only 16% ofAO7 is caused to be degraded, indicating that the photogenerated h also plays a role in the degradation of AO7.In addition, the photocatalytic efficiency decreases slightlyFigure 8 Effects of N2, ethanol, and KI on the degradationpercentage of AO7 over SrTiO3-graphene(7.5%) composites.The irradiation time is 6 h.Page 6 of 9under N2-purging condition, implying comparatively minorrole of · O2 and/or H2O2 for the dye degradation.In order to understand the photocatalytic mechanismof semiconductor-based photocatalysts, it is essential todetermine their energy-band potentials since the redoxability of photogenerated carriers is associated with energyband potentials of photocatalysts. The conduction bandand valence band potentials of SrTiO3 can be calculatedusing the following relation [34]:E CB ¼ X E e 0:5E gE VB ¼ E CB þ E g ;ð1Þwhere X is the absolute electronegativity of SrTiO3(defined as the arithmetic mean of the electron affinityand the first ionization of the constituent atoms) andestimated to be 5.34 eV according to the data reportedin the literature [35,36], Ee is the energy of free electrons onthe hydrogen scale (4.5 eV), and Eg is the bandgap energyof SrTiO3 (3.35 eV). The conduction band and valenceband potentials of SrTiO3 vs. normal hydrogen electrode(NHE) are therefore calculated to be ECB 0.84 V andEVB 2.51 V, respectively.Based on the obtained experimental results, a possiblephotocatalytic mechanism of SrTiO3-graphene composites toward the degradation of AO7 is schematicallyshown in Figure 9. When SrTiO3 is irradiated with lightof energy greater than its bandgap energy, electrons areexcited to the conduction band from the valence band,thus creating electron–hole pairs (Equation 2). Generally, most of the photogenerated electrons and holesrecombine rapidly, and only a few of them participatein redox reactions. It is noted that graphene, which is anexcellent electron acceptor and conductor, has a Fermilevel ( 0.08 V vs. NHE [37]) positive to the conductionband potential of SrTiO3 ( 0.84 V). When SrTiO3 particlesFigure 9 Schematic illustration of the photocatalyticmechanism of SrTiO3-graphene composites toward thedegradation of AO7.

Xian et al. Nanoscale Research Letters 2014, 27Page 7 of 9are assembled onto graphene sheets, the photogeneratedelectrons can readily transfer from the conduction band ofSrTiO3 to graphene (Equation 3). Thus, the recombinationof electron–hole pairs can be effectively suppressed inthe composites, which leads to an increased availabilityof electrons and holes for the photocatalytic reactions.The Fermi level of graphene is positive to the redox potential of O2/·O2 ( 0.13 V vs. NHE) but negative to that ofO2/H2O2 ( 0.695 vs. NHE) [31,38]. This implies that thephotogenerated e which transferred onto the graphenecannot thermodynamically react with O2 to produce · O2,but can react with O2 and H to produce H2O2 (Equation 4).H2O2 is an active species that can cause dye degradation,and moreover, H2O2 can also participate in the reactionsas described in Equations 5 and 6 to form another activespecies · OH. The valence band potential of SrTiO3( 2.51 V) is positive to the redox potential of OH /·OH( 1.89 V vs. NHE) [39], indicating that the photogeneratedh can react with OH to produce · OH (Equation 7). As aconsequence, the active species · OH, h , and H2O2 worktogether to degrade AO7 (Equation 8). SrTiO3 þ hν SrTiO3 e þ hþð2Þe þ Graphene Graphene ðe Þð3Þ2 Graphene ðe Þ þ O2 þ 2Hþ 2 Grapheneþ H2 O2ð4ÞH2 O2 þ hν 2 OHð5ÞH2 O2 þ e OH þ OH ð6Þhþ þ OH OHð7Þhþ ; OH; or H2 O2 þ AO7 Degradation productsð8ÞFrom Figure 6, it is found that the photocatalytic activity of the composites is highly related to the content ofgraphene, which can be explained as follows. With raising the graphene content, the amount of SrTiO3 particles decorated on the surface of graphene is expected toincrease, thus providing more photogenerated carriersfor the photocatalytic reaction. When the graphene content in the composites reaches 7.5%, the SrTiO3 particlesare decorated sufficiently, consequently leading to theachievement of the highest photocatalytic activity. However, with further increasing graphene content above7.5%, the photocatalytic efficiency begins to exhibit a decreasing trend. The possible reason is that the excessivegraphene may shield the light and decrease the photonabsorption by the SrTiO3 particles, and moreover, theamount of available surface active sites tends to be reduced due to an increasing coverage of graphene ontothe surface of the SrTiO3 particles.Figure 10 Degradation percentage of AO7 after irradiation for6 h over SrTiO3-graphene(7.5%) composites during the fivephotocatalytic cycles.Besides the photocatalytic activity, the reusability ofphotocatalysts is another crucial factor for their practicalapplications. The stability of the SrTiO3-graphene(7.5%)composites is examined by the recycling photocatalyticexperiment, as shown in Figure 10. It reveals that thedegradation percentage of AO7 maintains 80% to 88%for five consecutive recycles. The tiny or negligible loseof the photocatalytic efficiency indicates the excellentphotocatalytic reusability of the as-prepared SrTiO3graphene composites. Figure 11 shows the XRD patternsof the composites before and after the recycle experiment,revealing no obvious crystal structure changes. Figure 12shows the TEM images of the composites before andafter the recycle experiment, from which one can seethat SrTiO3 particles are still well decorated on thegraphene sheets.Figure 11 XRD patterns of SrTiO3-graphene(7.5%) compositesbefore and after the photocatalytic experiment.

Xian et al. Nanoscale Research Letters 2014, 27Page 8 of 9SrTiO3 to graphene and, hence, increased availability ofelectrons and holes for the photocatalytic reaction. Theenhanced generation of · OH radicals is observed over theirradiated SrTiO3-graphene composites compared tothe bare SrTiO3 nanoparticles. The photocatalytic efficiency is slightly deceased by purging with N2 but issignificantly suppressed by the addition of ethanol andKI (especially for the latter). Based on the experimentalresults, ·OH, h , and H2O2 are suggested to be the mainactive species causing the dye degradation.AbbreviationsAO: ammonium oxalate; AO7: acid orange 7; CB: conduction band;e : photogeneration of electron; Eg: bandgap energy; FTIR: Fouriertransform infrared spectroscopy; h : photogeneration of hole; H2O2: hydrogenperoxide; NHE: normal hydrogen electrode; OH: hydroxyl radicals;PL: photoluminescence; TA: terephthalic acid; TAOH: 2-hydroxyterephthalicacid; TEM: transmission electron microscope; UV: ultraviolet; VB: valenceband; XRD: X-ray powder diffraction.Competing interestsThe authors declare that they have no competing interests.Authors’ contributionsHY and TX conceived the idea of experiments. TX, LD, JM, and HZ carriedout the preparation and characterization of the samples. HY, TX, and JDanalyzed and discussed the results of the experiments. TX drafted themanuscript. HY improved the manuscript. All authors read and approved thefinal manuscript.Authors’ informationHY is a professor and a Ph.D. degree holder specializing in the investigationof photocatalytic and nanometer materials. JD is a professor and a Ph.D.degree holder specializing in the investigation of nanometer materials. JMand HZ are instructors and M.Sc. degree holders specializing in the researchof nanometer materials. TX is a doctoral candidate major in the study ofphotocatalytic materials. LD is a graduate student major in the preparationof photocatalytic materials.AcknowledgementsThis work was supported by the National Natural Science Foundation ofChina (Grant No. 51262018) and the Hongliu Outstanding TalentsFoundation of Lanzhou University of Technology (Grant No. J201205).Received: 24 April 2014 Accepted: 20 June 2014Published: 29 June 2014Figure 12 TEM images of the SrTiO3-graphene(7.5%) compositesbefore (top) and after (bottom) the photocatalytic experiment.ConclusionsSrTiO3-graphene nanocomposites were prepared by irradiating the mixture solution of SrTiO3 nanoparticlesand graphene oxide sheets, during which grapheneoxide receives electrons from the excited SrTiO3 nanoparticles to be reduced to graphene, simultaneouslyleading to the assembly of SrTiO3 nanoparticles ontographene sheets. Compared to the bare SrTiO3 nanoparticles, the as-prepared SrTiO3-graphene compositesexhibit an enhanced photocatalytic activity for the degradation of AO7 under irradiation of UV light. This canbe attributed to the effective separation of photogeneratedelectron–hole pairs due to the electron transfer fromReferences1. Mills A, Davies RH, Worsley D: Water purification by semiconductorphotocatalysis. Chem Soc Rev 1993, 22:417–425.2. Hofmann MR, Martin ST, Choi W, Bahnemann DW: Environmentalapplications of semiconductor photocatalysis. Chem Rev 1995, 95:69–96.3. Zheng Z, Huang B, Qin X, Zhang X, Dai Y: Facile synthesis of SrTiO3 hollowmicrospheres built as assembly of nanocubes and their associatedphotocatalytic activity. J Colloid Interface Sci 2011, 358:68–72.4. Kato H, Kobayashi M, Hara M, Kakihana M: Fabrication of SrTiO3 exposingcharacteristic facets using molten salt flux and improvement of photocatalyticactivity for water splitting. Catal Sci Technol 2013, 3:1733–1738.5. da Silva LF, Avansi W, Andres J, Ribeiro C, Moreira ML, Longo E, MastelaroVR: Long-range and short-range structures of cube-like shape SrTiO3powders: microwave-assisted hydrothermal synthesis and photocatalyticactivity. Phys Chem Chem Phys 2013, 15:12386–12393.6. Kuang Q, Yang S: Template synthesis of single-crystal-like porous SrTiO3nanocube assemblies and their enhanced photocatalytic hydrogenevolution. ACS Appl Mat Interfaces 2013, 5:3683–3690.7. Cao T, Li Y, Wang C, Shao C, Liu Y: A facile in situ hydrothermal methodto SrTiO3/TiO2 nanofiber heterostructures with high photocatalyticactivity. Langmuir 2011, 27:2946–2952.

Xian et al. Nanoscale Research Letters 2014, .25.26.27.28.29.Puangpetch T, Chavadej S, Sreethawong T: Hydrogen production overAu-loaded mesoporous-assembled SrTiO3 nanocrystal photocatalyst:effects of molecular structure and chemical properties of holescavengers. Energy Convers Manage 2011, 52:2256–2261.Guoa J, Ouyang S, Li P, Zhang Y, Kako T, Ye J: A new heteroju

employed including chemical reduction using hydrazine or NaBH 4 [14], high-temperature annealing reduction [15], hydrothermal reduction using supercritical water [16], green chemistry method [17], and photocatalytic reduction using semiconductors [18-21]. Among them, the photocatalytic reduction is an environment-friendly and

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