Low-temperature solvothermal synthesis of graphene–TiO2

nanocomposite and its photocatalytic activity for dye degradation

Yang Wang a,b, Zhen Li a,b,n, Yuan He a,b, Fei Li a,b, Xueqin Liu a,b, Jianbo Yang a,b

a Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR Chinab Faculty of Material Science and chemistry, China University of Geosciences, Wuhan 430074, PR China

a r t i c l e i n f o

Article history:Received 13 June 2014Accepted 12 July 2014Available online 19 July 2014

Keywords:Composite materialsChemical synthesisAFMOptical materials and properties

a b s t r a c t

Graphene/TiO2 (GR–TiO2) nanocomposite was synthesized via a low-temperature solvothermal routeusing graphite oxide (GO) as a precursor of graphene and TiCl3 as a single-source precursor of TiO2. Theformation of TiO2 nanoparticles were accompanied by the reduction of GO to graphene during asolvothermal reaction. The samples were characterized by X-ray diffraction (XRD), atom force micro-scopy (AFM), Fourier transform infrared (FT-IR) and UV–vis spectroscopy. Compared to neat TiO2

nanoparticles, the results reveal that the visible light photocatalytic activity of the composite isenhanced greatly on decomposition of degrade methyl orange (MeO) solution, which making it apromising candidate for widely potential application in the field of photocatalysis.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Graphene (GR), a monolayer of sp2-hybridized carbon atomspacked into a 2D honeycomb network [1], has attracted agreat deal of scientific interest since its discovery by Geim andco-workers [2]. Especially, graphene offer new opportunities inphotovoltaic conversion and photocatalysis by the hybrid struc-tures with a variety of nanomaterials, due to their excellent chargecarrier mobility, a large specific surface area, and good electricalconductivity [3–7].

As one of the most important semiconductor nanomaterials,titanium oxide (TiO2) has been widely investigated in the degra-dation of different pollutants for its nontoxicity, high stability, andinexpensiveness [8]. However, TiO2 with rapid recombination rateof photogenerated electron–hole pairs can only be excited by highenergy UV light irradiation, which greatly limits its practicalapplication [9]. In order to restrain electron–hole recombinationand increase the limited optical absorption of TiO2 under sunlight,research efforts have been focused on integrating TiO2 with othermaterials. Graphene, with large specific surface area and highmobility of charge carriers, is a good candidate to improve thephotocatalytic activity of TiO2 [10,11].

In this paper, we demonstrated a low-temperature solvother-mal approach assembling TiO2 nanoparticles onto graphene sheetsby employing GO and TiCl3 as starting materials. The precipitation

of TiO2 and the reduction of GO occurred simultaneously. Further-more, the photocatalytic property of the as-synthesized nanocom-posite was investigated.

2. Experimental

Preparation of GR–TiO2: All reagents used in the experimentswere analytic grade and used without further purification. GO wassynthesized by the Hummers method [12]. In a typical synthesis,50 mg of GO was dispersed in a solution of absolute ethanol(20 mL) and distilled water (10 mL) by ultrasonic treatment for30 min, and 10 mL of TiCl3 solution was added to the obtainedGO suspension. After stirring for 30 min at room temperature,the homogeneous solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained at 120 1C for 16 hfollowed by cooling to ambient temperature naturally. The result-ing black solid was harvested by centrifugation and rinsed exten-sively with distilled water to remove unreacted reactants andundecorated TiO2 nanoparticles. Finally, the products were driedin oven at 60 1C overnight before characterization. For comparison,TiO2 were synthesized via a similar protocol in the absence of GOduring the reaction.

Characterization: X-ray diffraction (XRD) were performed on aBruker D8-Focus X-ray diffractometer equipped with Cu Kα radia-tion (λ¼0.15406 nm). Atom force microscopy (AFM) images wereobtained with a Veeco Digital Instruments Nanoscope IIIa ScannedProbe microscope in the tapping mode and freshly cleaved micawas used as substrates. Fourier transform infrared (FT-IR) spectrawere recorded on a Nicolet 6700 spectrometer using the KBr pellet

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.07.0760167-577X/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author at: Engineering Research Center of Nano-Geomaterials ofMinistry of Education, China University of Geosciences, Wuhan 430074, PR China.Tel.: þ86 27 678 83737; fax: þ86 27 678 83732.

E-mail address: zhenli@cug.edu.cn (Z. Li).

Materials Letters 134 (2014) 115–118

method. Diffuse reflectance spectra (DRS) of powder samples werecarried out using a Shimadzu UV-2550 UV/vis spectrophotometerand BaSO4 was used as a reference.

Photocatalytic activity measurements: The photocatalytic activ-ity of GR–TiO2 and TiO2 was evaluated by photodegradation ofMeO under nature light irradiation. In a typical measurement, a30 mg portion of catalyst powder was dispersed into 50 mL ofMeO aqueous solution (10 mg/L). Prior to irradiation, the suspen-sion was magnetically stirred in the dark for 10 min to establishedadsorption-desorption equilibration. At given time intervals, 5 mLaliquots were sampled and centrifuged to remove the photocata-lyst completely. Then the solution was put into a quartz cell,and the absorption spectrums were measured with a UV-2401spectrophotometer.

3. Results and discussion

The composition and structure of the samples were confirmedby XRD analysis. Fig. 1 exhibits the XRD patterns of the GO, GR–TiO2

and TiO2. The GO shows a (002) reflection peak at 2θ¼11.51corresponding with a interlayer spacing of �8.0 Å, which is largerthan individual graphene sheet. It mainly owned to the presenceof oxygen-containing functional groups attached on sheets and theroughness resulting from structural defects. As for GR–TiO2, theshift of (002) reflection peak to 2θ¼25.21 (�3.5 Å) confirms thereduction of GO. However, the interlayer spacing is slightly largerthan that of pristine graphite (3.34 Å), implying that the oxygen-containing functional groups are partially removed. The XRDpattern of GR–TiO2 indicates that the rutile phase of TiO2 (JCPDS21-1276) is formed. The broad diffraction peaks of TiO2 nano-particles suggest small crystal size with relatively low crystal-lization. In contrast, neat TiO2 matches well with the anatasephase (JCPDS 21-1272), which means that the phase transforma-tion of titania from anatase to rutile is favored in solvothermalcondition in the presence of GO [13].

Fig. 2 depicts AFM images of GO and GR–TiO2. The heightprofile diagram of the AFM image (Fig. 2b) shows that the heightof the GO platelet is about 1 nm, which is larger than the interlayerspacing (�0.8 nm) of GO measured by XRD. It can be attributed tothe presence of oxygen-containing functional groups attached onGO sheets and adsorbed H2O molecules. Compared to GO, thesurface of the GR–TiO2 is much rougher (Fig. 2c and d), whichmight be due to the TiO2 nanoparticles uniformly dispersing onthe graphene sheets. In addition, some immobilized TiO2 nano-particles with larger size were observed. It indicated that grapheneplays an important role in nucleation and crystal growth of TiO2.

FTIR spectra were employed to characterize the carbon species inthe prepared samples. Fig. 3 shows the FT-IR spectra of GO, GR–TiO2

and neat TiO2. As for GO, the characteristic peaks appearing at 1724,1620, 1402, 1224 and 1057 cm�1 can be assigned to carboxyl orcarbonyl CQO stretching, H–O–H bending band of the adsorbedH2O molecules, carboxyl O–H stretching, phenolic C–OH stretchingand alkoxy C–O stretching, respectively [14]. In the case of theGR–TiO2 composite, the typical absorption peaks of GO decreasedramatically in intensity or even disappear as compared withthose of the pure GO, indicating the reduction of GO. The broadabsorption below 1000 cm�1 is ascribed to the vibration ofTi–O–Ti bonds in TiO2 [15], corresponding to that in spectrumof TiO2.Fig. 1. XRD patterns of GO, GR–TiO2 and neat TiO2.

Fig. 2. AFM images of GO (a–d) and GR–TiO2 (c–d).

Y. Wang et al. / Materials Letters 134 (2014) 115–118116

Fig. 4a gives the UV–vis absorption spectra of TiO2 and GR–TiO2

composite. Obviously, the GR–TiO2 composite exhibits not only ared-shift in the absorption edge but also a strong absorption in thevisible light range. The plot of the transformed Kubelka–Munkfunction versus the energy of light is shown in Fig. 4b. The roughlyestimated band gaps of TiO2 and GR–TiO2 are 3.28 eV and 2.72 eVrespectively, which supports the qualitative observation of a red-shift in the absorption edge of GR–TiO2 [16]. It is evidenced that

the introduction of graphene can extend the light absorption rangeand improve the visible light utilization efficiency of TiO2. Fig. 4cshows the photoelectricity results of the TiO2 and GR–TiO2 using aphotocurrent test. It is observed that there is a fast and uniformphotocurrent responding to each switch-on and switch-off eventin both electrodes. Obviously, the photocurrent of the GR–TiO2 invisible light has been extremely improved as large as 9 times. Thephotocatalytic activities of as-prepared neat TiO2 and GR–TiO2

were measured by the photocatalytic degradation of MeO asmodel reaction under nature light irradiation, and the results areshown in Fig. 4d. It was clear that the GR–TiO2 showed moreexcellent photocatalytic activity in the photodegradation of MeOcompared to neat TiO2. For the GR–TiO2, about 90% of the initialdyes were degraded after 60 min nature light irradiation. Con-trastingly, about 80% of the MeO still remained after the same timeperiod for neat TiO2.

The improvement in the photodegradation of MeO is mainlyascribed to the following reasons: (1) graphene with π-conjugatedtwo-dimensional planar structure can increase MeO absorptionthrough π–π stacking [15]; (2) graphene with good optical trans-mittance can improve the visible light utilization efficiency ofTiO2; (3) graphene with high mobility of charge carrier can act asan electron transfer channel for improving the interfacial electrontransportation and reducing the recombination of the photogen-erated electron–hole pair, leading to the improved photocatalyticactivity [10,11,17]. The GR–TiO2 nanocomposite with high photo-catalytic activity is promising for further application in environ-mental remediation.Fig. 3. FTIR spectra of GO, GR–TiO2 and TiO2.

Fig. 4. (a) Diffuse reflectance UV–vis spectra of GR–TiO2 and TiO2. (b) Corresponding plot of transformed Kubelka–Munk function versus the energy of the light.(c) Photoelectrochemical responses of the neat TiO2 and GR–TiO2. (d) Photocatalytic degradation experiments of MeO by GR–TiO2 and neat TiO2 under nature lightirradiation.

Y. Wang et al. / Materials Letters 134 (2014) 115–118 117

4. Conclusion

This work has described a low-temperature solvothermalapproach to assemble GR–TiO2. It is expected that graphene playsan important role in crystal structure, crystal growth and absorp-tion range of TiO2 nanoparticles. The photocatalytic activity of thecomposite was also investigated. About 90% of MeO was degradedafter 60 min solar cell light irradiation. We believe that theprepared GR–TiO2 composite is efficient for the environmentalpurification of organic pollutant.

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