Current Applied Physics 13 (2013) 659e663 Contents lists available at SciVerse ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap Enhanced photocatalytic activity of grapheneeTiO2 composite under visible light irradiation N.R Khalid a, b, *, E Ahmed b, Zhanglian Hong b, **, L Sana a, M Ahmed a, b a b Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan State Key Laboratory of Silicon Materials and Department of Materials Science and, Engineering, Zhejiang University, Hangzhou 310027, China a r t i c l e i n f o a b s t r a c t Article history: Received July 2012 Received in revised form 22 October 2012 Accepted November 2012 Available online 21 November 2012 Novel grapheneeTiO2 (GReTiO2) composite photocatalysts were synthesized by hydrothermal method During the hydrothermal process, both the reduction of graphene oxide and loading of TiO2 nanoparticles on graphene were achieved The structure, surface morphology, chemical composition and optical properties of composites were studied using XRD, TEM, XPS, DRS and PL spectroscopy The absorption edge of TiO2 shifted to visible-light region with increasing amount of graphene in the composite samples The photocatalytic degradation of methyl orange (MO) was carried out using grapheneeTiO2 composite catalysts in order to study the photocatalytic efficiency The results showed that GReTiO2 composites can efficiently photodegrade MO, showing an enhanced photocatalytic activity over pure TiO2 under visible-light irradiation The enhanced photocatalytic activity of the composite catalysts might be attributed to great adsorptivity of dyes, extended light absorption range and efficient charge separation due to giant p-conjugation system and two-dimensional planar structure of graphene Ó 2012 Elsevier B.V All rights reserved Keywords: TiO2 Graphene Composite Extended light absorption Photocatalytic activity Introduction Semiconductor photocatalysis is an advanced technology in air purification, water disinfection and purification Titanium dioxide (TiO2) has been extensively used semiconductor in photocatalytic and photochemical processes due to its stability, low cost and nontoxicity [1,2] However, narrow light response range and low separation probability of the photoinduced electronehole pairs in TiO2 photocatalytic system limits its technological applications Therefore, various studies have been directed to shift the optical absorption of TiO2 from UV to the visible-light region and to increase the quantum efficiency of the TiO2 photocatalyst [3e7] Recently, graphene for improving photocatalytic properties has emerged as a high potential material due to its high surface area, transparency, conductivity and good interfacial contact with adsorbents [8e10] Furthermore, the surface properties of graphene could be adjusted via chemical modification, which * Corresponding author Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan Tel.: þ92 61 9210091; fax: þ92 61 9210098 ** Corresponding author Tel./fax: þ86 571 87951234 E-mail addresses: khalidbzu@gmail.com (N.R Khalid), hong_zhanglian@ zju.edu.cn (Z Hong) 1567-1739/$ e see front matter Ó 2012 Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.cap.2012.11.003 facilitates its use in composite materials [11,12] Thus, the combination of TiO2 and graphene is promising to simultaneously possess excellent adsorptivity, transparency and conductivity, which could facilitate the effective photodegradation of pollutants during the photocatalysis In a recent study, Williams et al [12] prepared TiO2egraphene nanocomposites by mixing ultrasonically TiO2 particles and graphene oxide (GO) colloids, followed by UV-assisted photocatalytic reduction of GO In another investigation, TiO2egraphene composite materials were prepared by self-assembly of TiO2 nanoparticles grown on graphene sheets by a one-step approach with the assistance of an anionic surfactant [13] Zhang et al [10] synthesized high performance P25-graphene composite photocatalyst for methyl blue degradation using hydrothermal method Sonophototcatalytic activity of graphene oxide based PteTiO2 composites for DBS degradation was investigated by Neppolian et al [14] N Farhangi et al [15] investigated Fe doped TiO2 nanowires on graphene sheets for photodegradation of 17b-estradiol (E2) Nevertheless, the understanding of TiO2egraphene photocatalysis system is unclear Herein, we demonstrated a simple hydrothermal method to prepare TiO2egraphene (GReTiO2) composites The prepared GRe TiO2 composites showed extended light absorption range and higher photocatalytic activity for degradation of methyl orange under visible-light irradiation 660 N.R Khalid et al / Current Applied Physics 13 (2013) 659e663 Experimental (101) 2.1 Preparation of GReTiO2 composites 2.2 Sample characterization The crystal structure of composites was identified by powder X-ray diffraction (XRD) and patterns were collected from 10 to 80 in 2q with 0.02 steps/s using a Rigaku D/max-3B X-ray diffractometer with Cu Ka as radiation source (l ¼ 0.15406 nm) at 40 kV and 36 mA Chemical compositions of composites were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo-VG Scientific, ESCALAB250, monochromatic Al Ka X-ray source) All binding energies were calibrated by using the containment carbon (C1s ¼ 284.6 eV) Transmission electron microscopy (TEM) was carried out on a JEOL JEM-1200EX electron microscope instrument operated at 200 kV TEM samples were prepared by dispersing the final powder in ethanol, one drop of the dispersion was then dropped on the carbonecopper grid UVevis diffuse reflectance spectra (DRS) were measured in the range of 300e800 nm using a (HITACHI U-4100 UVeVis spectrometer) with an integrating sphere accessory The powders were pressed into pellets, and BaSO4 was used as a reference standard for correction of instrumental background Ther reflectance was converted to absorbance by KubelkaeMunk equation: F(R) f K/S ¼ (1 À R)2/2R, where K is the molar absorption coefficient, S is the scattering coefficient, and R is the diffuse reflectance The photoluminescence (PL) emission spectra were obtained using (HITACHI F-4500 Fluorescence spectrophotometer) The samples excitation was made at 380 nm at room temperature, and the emission spectra were collected between 400 and 700 nm (200) (211) (204) (220) (215) 05GR-TiO2 Intensity (a.u.) GO was synthesized from graphite powder (99.99% Alfa Aesar) by a modified Hummers method [16] and TiO2 nanoparticles were preparedbysolegelmethodaccordingtoourpreviousstudy[17].GRe TiO2 compositeswerepreparedbysimplehydrothermalmethodbased onZhang’swork[10].Briefly,20mgofGOwasdissolvedinasolutionof H2O(80mL)andethanol(40mL)mixtureusingultrasonictreatmentfor 2h,andthen200mgof(TiO2)wasaddedtotheobtainedGOsolutionand stirred for another h to get a homogeneous suspension The suspension was then placed in a 200 mL Teflon-sealed autoclave and maintainedat120 Cfor3htosimultaneouslyattainthereductionofGO and the deposition of TiO2 on graphene sheets Finally, the resulting composite was recovered by filtration, rinsed by deionized water several times, and dried at 70 C for 12 h Finally, the weight ratio of graphene to TiO2 was (1e10 wt.%) and the composites obtained were labeled as 01GReTiO2, 02GReTiO2, 05GReTiO2 and 10GReTiO2 respectively (004) 10GR-TiO2 02GR-TiO2 01GR-TiO2 TiO2 10 20 30 40 50 60 70 2θ (angle) Fig XRD patterns of TiO2, GReTiO2 composites with various graphene contents supernatant was taken out for absorption measurement The intensity of the main absorption peak (464 nm) of the methyl orange dye was considered as a measure of the residual MO dye concentration (C) and the initial concentration of dye was referred as (C0) 2.3 Photocatalytic activity measurement The photocatalytic activities of different composites were estimated by monitoring the degradation of methyl orange (MO) in a home-made apparatus with a halogenetungsten lamp (400 W) as the radiation source The visible-light (l ! 420 nm) used in the present study was obtained by the filter with cut-off wavelength of 420 nm Typically, for the photocatalytic experiment, 100 mg photocatalysts were suspended in 100 mL MO aqueous solution with a concentration of 10 mg LÀ1 in a beaker The suspension was magnetically stirred for 0.5 h to reach the adsorption/desorption equilibration without visible-light exposure Following this, the photocatalytic reaction was started by the exposure to the visible light The temperature of the suspension was kept at about 20 C by an external cooling jacket with recycled water After a setup exposure time, ml suspension was sampled, centrifuged, and the 80 Fig TEM images of (a) TiO2, (b) 10GReTiO2 composite N.R Khalid et al / Current Applied Physics 13 (2013) 659e663 which is consistent with the value of Tiþ4 in the TiO2 lattice [19] O1s core level spectrum (Fig 3c) shows main peak at 530.3 eV due to the metallic oxides TieO bond, which is consistent with binding energy of O2À in the TiO2 lattice and the peak appearing at 532.4 eV was ascribed to adsorbed OHÀ on the surface of TiO2 [20] In C1s core level spectrum (Fig 3d), the main peak was observed at 284.4 eV, which corresponds to the adventitious carbon adsorbed on the surface of sample and the peak at 286.1 eV corresponds to CeC bonds of carbonate species [15,20] UVevisible DRS spectra of GReTiO2 composites with different weight ratio of graphene are shown in Fig 4(a) It is obvious that a red shift to longer wavelength regions occurred for GReTiO2 composites Band gap energy (Eg) for allowed indirect transitions can be estimated using the relation Results and discussion XRD patterns of TiO2 and GReTiO2 composites with different weight ratio of graphene are shown in Fig The patterns clearly show peaks of anatase phase structure of TiO2, namely, the planes (101), (004), (200), (211), (204), (220), and (215) at 2q values of ca 25.38 , 37.82 , 48.18 , 54.4 , 62.92 , 69.92 , 74.9 respectively, which indicate that all values are in good agreement with (JCPDS21-1272) In XRD patterns of GReTiO2 samples, graphene peaks were not observed and structure of TiO2 was nearly unchanged in all composites, only a slight change in FWHM of (101) peak of anatase TiO2 was observed It may be due to the fact that the characteristic (002) peak at 25.9 of graphene [17,18] is weak and might overlaps with the (101) peak of anatase TiO2 (25.4 ) The loading of the TiO2 nanoparticles onto graphene sheets was characterized by TEM, and the typical images of TiO2 and 10GRe TiO2 composite are shown in Fig Image for composite sample confirms that graphene is solid support for TiO2 nanoparticles During the formation of nanocomposite, the TiO2 nanoparticles adheres to the functional groups on graphene oxide plane and graphene oxide is reduced to form GReTiO2 composite during the subsequent hydrothermal process Fig 3a shows the XPS survey spectra of 10GReTiO2 composite, which contains 52% O 1s, 27% Ti2p and 21% C1s In core level XPS spectrum of Ti2p (Fig 3b), the Ti2p3/2 and Ti2p1/2 peaks are located at binding energies of 459.1 eV and 464.9 eV respectively, a À Á ðahyÞ1=2 ¼ Bd hy À Eg where a is absorption coefficient; hn is incident photon energy; and Bd is the absorption constant Plots of (ahn)1/2 versus hn from the spectral data of Fig 4(a) is presented in Fig 4(b) The intercept of the tangent to the plot gives a good approximation of band gap energy for TiO2 [21] The band gap energies estimated from the above relation are 3.20 eV for TiO2, 3.16 eV for 01GReTiO2, 3.13 eV for 02GReTiO2, 3.04 eV for 05GReTiO2, and 3.0 for 10GReTiO2 composite, respectively The results obviously demonstrate the significant influence of graphene on the optical properties, in b 459.1 eV Ti2p Intensity (a.u.) O1s Intensity (a.u.) 661 Ti2p Ti2s C1s 464.9 eV Ti3p Ti3s 800 600 400 200 470 468 466 Binding energy (eV) c 464 462 460 458 456 Binding energy (eV) d O1s C1s 530.3 eV Intensity (a.u.) Intensity (a.u.) 284.4 eV 532.4 eV 536 534 532 530 Binding energy (eV) 528 526 286.1 eV 292 290 288 286 284 Binding energy (eV) Fig XPS survey and core level spectra of Ti2p, O1s and C1s of 10GReTiO2 composite sample 282 662 a N.R Khalid et al / Current Applied Physics 13 (2013) 659e663 1.6 TiO2 01GR-TiO2 10GR-TiO2 02GR-TiO2 05GR-TiO2 02GR-TiO2 0.8 05GR-TiO2 Intensity (a.u.) Absorbance 1.2 01GR-TiO2 TiO2 10GR-TiO2 0.4 0.0 300 400 500 600 700 800 Wavelength (nm) b 450 3.0 600 650 Fig Photoluminescence spectra of GReTiO2 composites with various graphene contents (excitation wavelength of 380 nm) 2.0 1/2 550 Wavelength (nm) 2.5 ( α hν ) 500 10GR-TiO2 The photocatalytic activities of TiO2 and GReTiO2 composites with different wt% of graphene were investigated by photodegradation of methyl orange (MO) under visible light (l ! 420 nm) and results are shown in Fig 6(a) A control experiment study of MO degradation without catalyst under the same condition was also conducted The result indicates that the photolysis can be ignored as the corresponding degradation is about 0.4% after exposure for h to the visible light Photocatalytic degradation of MO follows roughly the pseudo-first-order reaction kinetics at low dye concentrations [24]: 05GR-TiO2 02GR-TiO2 1.5 01GR-TiO2 TiO2 1.0 0.5 0.0 1.5 2.0 2.5 3.0 3.5 4.0 Band gap (eV) Fig (a) UVevisible DRS spectra and (b) plots of (ahn)1/2 versus photon energy (hn) of pure TiO2 and GReTiO2 composites with various graphene loadings which increasing graphene amount increased the light absorption of TiO2, similar to the results of TiO2eCNT and C-doped TiO2 [10,22] The photoluminescence emission spectra is a useful investigation to study the efficiency of charge carrier trapping, immigration, transfer and to understand the fate of electronehole pairs in the field of photocatalysis over solid semiconductors [20,23] It is well-known that the PL emission is the result of the recombination of excited electrons and holes either directly (bandeband) or indirectly (via a band gap state), the lower PL intensity may show the lower recombination rate of electrons and holes and higher separation efficiency under light irradiation Fig shows the photoluminscence spectra of TiO2 and GReTiO2 composites The PL intensity of pure TiO2 is significantly higher than the other composite samples, which is showing the higher recombination of electrons and holes Moreover, the emission intensity is decreased with increasing graphene loadings and is found lowest for 10GRe TiO2 composite catalyst Therefore, it is concluded that presence of graphene in the composite samples might be effective to enhance the separation efficiency of electronehole pairs during the photocatalysis lnðC0 =CÞ ¼ k  t where k is the apparent first-order rate constant, used as the basic kinetic parameter for different photocatalysts; C0 is the initial concentration of MO in aqueous solution; and C is the residual concentration of MO at time t The apparent rate constant values were deduced from the linear fitting of ln(C0/C) vs irradiation time as shown in Fig 6(b) The results show that the apparent rate constant is remarkably enhanced by increasing the amount of graphene in the composite samples The apparent rate constant (k ¼ 5.66  10À3 minÀ1) for 10GReTiO2 composite catalyst is much higher than that of pure TiO2 (k ¼ 0.9  10À3 minÀ1) Therefore, we attribute the significantly enhanced photocatalytic performance of graphene based composites to the result of strong coupling between TiO2 and graphene Firstly, the graphene has been reported to be a competitive candidate as an acceptor material due to its p-conjugation structure, thus in GReTiO2 system, the excited electrons of TiO2 could transfer from the conduction band to graphene Thus graphene in the composite serves as an acceptor material of photogenerated electrons of TiO2 and inhibits the electronehole recombination, which results in more charge carriers to form reactive species and promotes the degradation of dye Secondly, graphene has unexpectedly excellent conductivity due to its two dimensional planar structure Therefore, both the electron accepting and transporting properties of graphene in the composite catalyst could contribute to the suppression of charge recombination, and thereby a higher degradation rate in the photocatalysis was achieved [10,15,22] N.R Khalid et al / Current Applied Physics 13 (2013) 659e663 a 663 pure TiO2 under visible-light irradiation for MO degradation The enhanced photocatalytic activity of the composite catalyst might be attributed to giant two-dimensional planar structure of graphene and possibility of more pep interaction between composite and organic compound 1.0 0.9 0.8 0.7 Acknowledgments C/C0 0.6 This work was supported partially by Natural Science Foundation of China (No 51072180), China Postdoctoral Science Foundation (No 20110491764), the Fundamental Research Funds for the Central Universities (No 2009QNA4005), and the State Key Laboratory of Silicon Materials (SKL2009-14) at Zhejiang University N R Khalid thanks to Higher Education Commission of Pakistan for IRSIP scholarship 0.5 MO (without catalyst) 0.4 TiO 0.3 01GR-TiO 0.2 02GR-TiO 05GR-TiO 0.1 10GR-TiO 0.0 20 40 60 80 100 120 140 160 180 200 Time (min) b 0.0 -0.2 ln (C/C0) -0.4 -0.6 -0.8 TiO -1.0 (k = 0.00090) 01GR-TiO (k = 0.00184) 02GR-TiO 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