rutile TiO2 nanorod ternary composites with enhanced photocatalytic activity

Accepted Manuscript Synthesis of Cu2O/graphene/rutile TiO2 nanorod ternary composites with enhanced photocatalytic activity Mingxuan Sun, Yalin Fang, Ying Wang, Shanfu Sun, Jia He, Zhi Yan PII:

S0925-8388(15)30698-8

DOI:

10.1016/j.jallcom.2015.08.002

Reference:

JALCOM 34996

To appear in:

Journal of Alloys and Compounds

Received Date: 29 March 2015 Revised Date:

31 July 2015

Accepted Date: 1 August 2015

Please cite this article as: M. Sun, Y. Fang, Y. Wang, S. Sun, J. He, Z. Yan, Synthesis of Cu2O/ graphene/rutile TiO2 nanorod ternary composites with enhanced photocatalytic activity, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.08.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Synthesis of Cu2O/graphene/rutile TiO2 nanorod ternary composites with enhanced photocatalytic activity Mingxuan Sun*, Yalin Fang, Ying Wang, Shanfu Sun, Jia He, Zhi Yan

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Shanghai 201620, China

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School of Materials Engineering, Shanghai University of Engineering Science,

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*Corresponding author: Mingxuan Sun

School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620 (China)

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E-mail: [email protected]; [email protected] Tel.: +86 21 67791474

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Fax: +86 21 67791201

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ACCEPTED MANUSCRIPT Abstract: A ternary composite of Cu2O, graphene and rutile TiO2 nanorods was prepared using Cu(CH3COO)2·H2O, graphene oxide and TiCl4 as the starting materials and its enhanced photocatalytic performance was demonstrated. Graphene/TiO2 nanorod

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composites (CG) were obtained by a simple hydrothermal method and then, Cu2O was coupled onto the surface of graphene/rutile TiO2 to form Cu2O/graphene/rutile TiO2 nanorod (CGT) composites via a chemical bath deposition process. The

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as-prepared sample was characterized by X-ray diffraction (XRD), emission field scanning electron microscope (FE-SEM), transmission electron microscopy (TEM),

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specific surface area analyzer (BET), Raman spectroscopy and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). It is found that the introduction of graphene and Cu2O has little effect on the morphology of TiO2 nanorods with average dimensions of 140 nm (length)×30 nm ( diameter)(L/D ratio

5). A red shift of light

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absorption edge and more absorption in the visible light region were observed for the resulted ternary samples compared with TiO2 and graphene/TiO2 composites. The photocatalytic activity was evaluated by the photodegradation of methylene blue

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under visible light irradiation, which showed 2.8 times corresponding enhancement of

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the degradation efficiency for the ternary composites compared with TiO2. This work provides a new strategy to improve the visible light response of TiO2 and facilitate its application in environmental remediation.

Keywords: Cu2O/graphene/TiO2; Ternary nanocomposites; Synergistic effects; Visible light; Photocatalytic activity

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ACCEPTED MANUSCRIPT 1. Introduction In past few years, photocatalytic decomposition of organic pollutants in waste water or air using semiconductors has attracted much attention [1,2]. Titanium dioxide (TiO2) has been considered to be one of the most promising photocatalysts

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because of its chemical stability, non-toxicity, low cost and superior activity [3-6]. However, the large energy band gap of TiO2 (3.2 eV) indicates only UV light (about 4% of the solar energy) can activate this materials. Furthermore, the rapid recombination

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of photo-induced electrons and holes in TiO2 also hampered its full potential application. Thus, improving its utilization efficiency of solar energy has generated

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considerable scientific interest. Up to now, numerous strategies have been developed to promote the photocatalytic activities of TiO2, such as doping TiO2 with metal [7,8] and nonmetal [9-11] or coupling TiO2 with narrow band gap semiconductors [12,13]. Among these, modification of TiO2 with narrow bandgap semiconductors has been

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reported as one of the best options for achieving high efficient utilization of solar energy in photocatalysis [14-16].

Many different reports have described visible bandgap semiconductors for

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modification [12-17]. Alternatively, cuprous oxide (Cu2O), a p-type semiconductor

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with a direct band gap of about 2.0 eV, is a promising material for the conversion of solar energy into electrical or chemical energy [18-20]. Recently, Cu2O and TiO2 composites have been studied for improving the photocatalytic activity of TiO2. Cheng et al. [21] developed Cu2O decorated TiO2 via chemistry bath process and demonstrated that the hydrogen evolution rate over the as-prepared composites was one order of magnitude higher than that of commercial P25 TiO2. The Cu2O can serve as an electron-hole separation centre to promote H2 evolution. Wang et al. [13] prepared Cu2O/TiO2 p-n heterojunction photoelectrodes by an ultrasonication-assisted 3

ACCEPTED MANUSCRIPT sequential chemical bath deposition. They confirmed that the obtained composites possessed enhanced photocurrent, more effective photoconversion capability and superior photoelectrocatalytic activity in the degradation of Rhodamine B. The narrow bandgap Cu2O nanoparticles act as sensitizers to promote the charge transfer

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to TiO2, leading to efficient photogenerated charge carrier separation.

Previous reports have shown that graphene have certain beneficial effects on the photocatalytic activity of semiconductors [22], such as increasing the light absorption

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and charge transport. More recently, graphene have been widely studied to incorporate into semiconductor materials (such as TiO2 [23,24], ZnO [25], and so on)

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to obtain photocatalysts with superior catalytic performance. Graphene, a two-dimensional sp2-hybridized carbon material, possesses many excellent properties such as high electrical conductivity, flexible structure, large theoretical specific surface area, unique mechanical strength and superior transparency [26,27]. Due to

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the introduction of graphene, the band structure and the interfacial charge transfer of semiconductor were modified, leading to an improvement in their photocatalytic performance [28,29]. Lu et al. [30] synthesized graphene-TiO2 composites via a

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solvothermal process and reported excellent photocatalytic activity for the

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degradation of methylene orange under visible light. Sang et al. [31] prepared reduced graphene oxide/TiO2 nanobelt by an in situ photochemical reduction method. The as-prepared sample exhibited remarkable visible light driven photocatalytic activity in photodegradation of methylene orange and hydrogen production by water splitting. Our recent studies also revealed that TiO2nanorods modified with graphene showed high performance in photocatalytic degradation of organic pollutants [ 32 ]. Furthermore, we also demonstrated the sensitizing effect of graphene oxide on the photoelectron chemical and photocatalytic properties of the TiO2 nanotube arrays 4

ACCEPTED MANUSCRIPT under visible light [33]. The multivariate perturbation in a multi-element composite of TiO2 can provide synergistic effects to further enhance the photocatalytic activity of TiO2. Recently, there has been growing interests about the multiple modification methods, such as

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co-dopings [34,35], co-couplings [36-38] or doping-coupling [39] hybrid modification of TiO2. Aragaw et al. [34] confirmed the highest saturated photocurrent density of Sn and C codoped TiO2 nanowires and 60%, 94%, and 100% efficiency improvements of

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the maximum solar energy conversion efficiency compared to undoped, Sn doped, and C doped TiO2 nanowires. The improvement is attributed to the synergetic effects

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of Sn and C codopants which can lower recombination and enhance life time of photogenerated charge-separated carriers on the surface states. Wang et al. [37] prepared TiO2 loaded CuS and NiS by hydrothermal approach and proved that CuS and NiS can act as effective dual co-catalysts to enhance the photocatalytic H2

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production activity of TiO2. Luo et al. [40] synthesized Cu2O/N-TiO2 via a two-step route and demonstrated its extended absorption edge and markedly enhanced photodegradation efficiency of methyl orange solutions under both the visible and

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full-spectrum light irradiation in comparison with TiO2 and Cu2O/TiO2. It was

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observed that the trapping of electrons and the charge carrier lifetime enhancement of Cu2O/N-TiO2 are more effective than that of TiO2 and Cu2O/TiO2, which was attribute to the formation of p-n heterojunction and intraband deep localized states. It is worth noting that one-dimensional (1D) TiO2 nanorods are especially

appealing for applications in various areas due to their high surface area, efficient light harvesting, photoinduced charge separation and transport [41]. Besides, they can also avoid agglomeration in the photocatalysis compared with TiO2 anon-powder [42]. Furthermore, rutile TiO2 have also attracted considerable attention for photocatalysis 5

ACCEPTED MANUSCRIPT because of thermodynamically stable phase and an excellent combination of physical properties such as exceptional light-scattering efficiency, high refractive index, opacity, and chemical inertness [43,44]. Herein, we developed a feasible fabrication approach for a ternary nanocomposite of Cu2O, graphene and rutile TiO2naorods.

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Their enhanced visible light response and improved photocatalytic activity for the degradation of methylene blue were demonstrated. To the best of our knowledge, little information has been reported on such ternary composite and its photodegradation

2. Experimental 2.1 Materials and reagents

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performance [45].

All chemicals were analytical grade and used without further purification. Sodium hydroxide (NaOH, 98.0%), cupric sulfate (Cu(CH3COO)2·H2O, 99.95%),

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glucose (C6H12O6·H2O, 99.9%) and titanium tetrachloride (TiCl4, 99%) were purchased from Aladdin industrial corporation. 2.2 Synthesis of graphene/TiO2 rutile nanorod composites:

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Graphene oxide (GO) was synthesized using Hummer’s method [46], and the

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GO aqueous solution was obtained by ultrasonic treatment of graphene oxide in deionized water. GT composites were fabricated by a simple hydrothermal reaction process. Typically, 7mL of 0.5mg/mLGO solution was transferred to 60mL of deionized water and stirred for 60min. Subsequently, 3mL of TiCl4 solution was dropped gradually to the above solution followed by stirring for another 40min. The obtained suspension was placed into a Teflon-sealed autoclave of 100mL capacity and maintained at 180oC for 6h. After centrifuging, the solid products were sequentially washed with deionized water and absolute ethanol for several times and dried at 60oC 6

ACCEPTED MANUSCRIPT for 2h. 2.3 Fabrication of Cu2O/graphene/TiO2 rutile nanorod composites The combination of Cu2O nanoparticles onto the surface of the GT were carried out via a chemical bath deposition process. In detail, the obtained GT composites (0.3

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g) were added into pre-prepared ethanol solution of Cu(CH3COO)2·H2O with a desire concentration (0.001, 0.0025, 0.005 and 0.01M) and dispersed by ultrasound irradiation for 1h to allow the adsorption of Cu ions onto the surface of the

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composites. Subsequently, the powders were collected by centrifugation and put into a NaOH solution (10 mL, 0.1M). Ultrasound irradiation was again applied to disperse

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the powders. The well-dispersed suspension was maintained at 60oC in water bath and an aqueous solution of glucose (10mL, 0.1M) was gradually added into the above suspension with continuous stirring for a reaction time of 10 min. Finally, the precipitate was rinsed twice with deionized water and ethanol and dried overnight at

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60oC to afford the ternary composites of CGT. The obtained sample was noted as CGT xM, where x represents the concentration of Cu(CH3COO)2·H2O ethanol solution.

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2.4 Characterizations of the resulted samples

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The Crystallinity and phase composition of the as-prepared samples were characterized by XRD (PANalytical X'Pert, Holland) using CuKα radiation operated at 40 kV and 40 mA. The morphologies and microstructures of the samples were obtained using TEM (FEI Tecnai F20, USA) and SEM (JEOL JSM-7000F, Japan). The UV-vis DRS were obtained by UV-vis spectrophotometer (Shimadzu UV 3600, Japan) in a region of 300-800 nm. Raman spectra were recorded with the excitation of a 325 nm He-Cd laser guided by a Raman spectrometer (Renishaw, UK).The specific surface area of the samples were determined by N2 adsorption-desorption analysis 7

ACCEPTED MANUSCRIPT conducted at 77K (Micromeritics ASAP 2020 V4.01, USA). 2.5 Photocatalytic measurements The photocatalytic activity of the samples was evaluated by the photodegradation of methylene blue (MB,C16H18N3S) under visible light irradiation. A 500 W Xenon

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lamp (CHF-XM35, Trusttech Co., Ltd., Beijing) was applied as light source in the photocatalysis. An optical filter was used to cut off wavelength below 420 nm to provide visible light with intensity 42.0 mW·cm-2. Generally, 7.5 mg of photocatalyst

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was added to 15 mL 5mg/LMB solution and then magnetically stirred in the dark for 3hto achieve adsorption/desorption equilibrium of MB. Once light illumination began,

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samples were tested at 20 min intervals for 80 min and the UV-vis adsorption spectrum of MB solution was collected by an UV-vis spectrophotometer (Shimadzu UV 1601-PC, Japan) in the wavelength range of 400-800 nm.

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3. Results and discussion

Fig. 1 shows the XRD patterns of the as-prepared TiO2, GT and CGT. Bare TiO2 was composed of anatase and rutile phase (Fig.1a), with the anatase phase content of

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10.2%. The typical diffraction peaks at 27.4°, 36.1°, 39.2°, 41.2°, 44.0°, 54.3°, 56.6°,

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62.8°, 64.5°, 69.0° and 69.8° were ascribed to the (110), (101), (200), (111), (210), (211), (220), (002), (310), (301) and (112) planes of rutile phase, while the diffraction peaks at 25.3°, 37.8° and 48.1° can be indexed to the (101), (004) and (200) planes of anatase phase, respectively. Interestingly, only rutile crystal phase of TiO2 was found in binary composites of GT and ternary composites of CGT, which is possibly attributed to the introduction of graphene. As the reported reference, the combination of graphene in composites is beneficial to the formation of rutile TiO2 [47]. However, no obvious carbonous diffraction peak for graphene was detected in the XRD patterns 8

ACCEPTED MANUSCRIPT of GT and CGT (Fig.1b-1f), which was possibly related to the small amount of graphene in the obtained samples [48]. The existence of graphene in the composites could be confirmed by Raman, FE-SEM and TEM results shown later. In Fig. 1(c-f), the diffraction peaks at 36.4°, 42.3°, 61.3° and 73.5° was assigned to the (111), (200),

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(220) and (311) crystal planes of Cu2O, matching well with JCPDS card no. 00-005-0667. The high intensity of (111) diffraction peaks of the Cu2O phase indicate that {111} facets are dominated. No peaks corresponding to CuO or Cu are observed

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in the pattern. Furthermore, Scherrer formula was applied to the rutile (110) peak of TiO2 and (111) peak of Cu2O. The average crystal sizes were calculated to be around

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30.5 nm and 51.3 nm for rutile TiO2 and Cu2O in CGT, respectively.

Fig. 2 illustrates the FE-SEM images of TiO2, GT and CGT composites. As shown in Fig. 2A, smallish nanorods with average dimensions of 140 nm (length)×30 nm (diameter) were observed for the bare TiO2. The shape and size of the nanorods

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were well uniform with a superior dispersity. The fluffy structure with large surfaces provide growing points for graphene and Cu2O, favoring the formation of CGT composites. There was little difference in morphologies of TiO2 (Fig. 2A) and GT

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composites (Fig. 2B), implying that the introduction of graphene had little effect on

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the formation of smallish nanorods. Cubic Cu2O nanoparticles were observed with approximately size of 160 nm in Fig. 2C and 2D, indicating the presence of Cu2O in the ternary composites.

The morphology of the CGT composite was also characterized by TEM (Fig. 3).

As shown in Fig. 3A, the general microstructure of CGT is observed, which is consistent with the SEM results. The high resolution TEM (HR-TEM) images are depicted in Fig. 3B, 3C and 3D, which clearly showed the strong interaction of the components in CGT. Smallish nanorod structure is clearly displayed in Figure 3B. As 9

ACCEPTED MANUSCRIPT Figure 3C and 3D shown, the lattice fringes spacing of 0.35 nm and 0.24 nm can be ascribed to the (110) and (111) crystal face of rutile TiO2 and cubic Cu2O, respectively. It is also observed that graphene shows winkled structure composed of layered graphene sheets. The 1D structure of TiO2 nanorods and the strong

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interactions among the components of CGT are expected to enhance the photocatalytic activity of the composites.

Fig. 4 presents the typical Raman spectra of GT (curve a) and CGT (curve b).

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As shown in Fig. 4 (a) and (b), the peaks at 241 cm-1, 441cm-1, and 607 cm-1 observed in Raman spectra of the samples are the characteristic Raman mode of the rutile TiO2

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phase [49], indicating the formation of rutile phase of TiO2 after the hydrothermal process. The existence of graphene was also demonstrated in their Raman spectra. The characteristic D band and G band of graphene at 1330 cm-1 and 1590 cm-1 are observed for GT and CGT. The D band is a common feature for in plane vibrations of

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sp3 defects in carbon, and the G band provides information on sp2 bonded of carbons [50,51]. The calculated ID/IG ratio of GO is 1.17 (as shown in the insert), while the values for GT and CGT were 1.01 and 1.05, respectively. Compared with GO, the

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reduced ratio of the D and G peak heights for the composites indicated that GO was reduced to graphene during the hydrothermal process. For the ternary CGT (curve b),

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the three additional faint peaks centered at 217 cm-1, 409 cm-1, and 519 cm-1 can be assigned to Cu2O phase, thus confirming the formation of Cu2O nanoparticles after the chemical bath deposition process. These further proved the successful fabrication of GT and CGT composites. The UV-vis diffuse reflectance absorption spectra of the resulted samples are displayed in Fig.5. For comparison with that of pure TiO2, the absorption edge of GT apparently shifted to longer wavelength with more absorption. A further red shift of 10

ACCEPTED MANUSCRIPT the absorption edge occurs with the introduction of Cu2O into GT composites. Also, the absorption intensity isstrengthened.These results indicate the modification of TiO2 with graphene and Cu2O is helpful to improve its visible light response. In addition, the absorption edges were 440 nm, 452 nm, 481 nm and 420 nm for CGT 0.01 M,

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0.005 M, 0.0025 M, and 0.001 M, respectively. It is obvious that the absorption edge shift to longer wavelength for CGT with the decrease of the Cu2O contents. The band

corresponding to indirect gap semiconductors: αhv= A(hv-Eg)2

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gap energy of the sample was calculated according to the following formula

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Where α, hv, Eg, and A are absorption coefficient (cm-1), energy of excition (eV), band-gap energy (eV), and a constant, respectively [52]. A plot of the square root of absorbance coefficient versus the energy of light affords the band gap as shown in insert. The intercept of the tangent to the X axis would give a good approximation of

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the band gap energy of the samples. The optical band gap energy (Eg) was 3.0 eV and 2.9eV for pure TiO2 and GT, respectively, which is in agreement with the literature values [24,53]. The obtained value for CGT (Cu2O, 0.0025M) was 2.5eV, which

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indicates that the modification of TiO2 with graphene and Cu2O results in the

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narrowing of the band gap of TiO2. However, further decreasing the Cu2O precursor concentrations to 0.001 M, it was observed that CGT 0.001 M presented a broader band gap (2.81 eV) than that of the CGT 0.0025 M, suggesting CGT 0.0025 M exhibited a superior visible light absorption. The unique light absorbance performance of the ternary composites in the visible light region is of great importance for its photocatalytic application since it can be activated even by visible light. The photocatalytic performance of the as-prepared photocatalysts was evaluated by degradation of methylene blue (MB) under visible light (λ>420 nm) irradiation 11

ACCEPTED MANUSCRIPT (Fig. 6 and 7). As shown in Fig. 6A, after equilibrium in the dark for 180min, most dye molecules remained in the solution with bare TiO2, whereas a large amount of dye molecules were adsorbed on the surface of GT and CGT, especially CGT 0.0025. The specific surface areas of the samples were measured using the BET method from

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N2 adsorption and desorption isotherm. The values for bare TiO2, GT and CGT composites are 64, 67 and 59 m2/g, respectively. Apparently, there were no significant changes in the BET specific areas among the three catalysts. Compared with bare

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TiO2, the enhanced adsorptivity of dye molecules for GT was largely driven by the π-π stacking between MB and aromatic regions of the graphene [54]. A further

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enhanced adsorption ability of dye molecules was observed for CGT, which is attributed to a strong adsorption of dye molecules onto the surface of Cu2O [55,56]. Fig. 6C displays the normalized concentration changes (Ct/C0) of MB during the visible light driven photodegradation process, where C0 was the concentration of

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initial MB in solution, Ct was the concentration of remaining MB at each irradiated time interval. Under visible light illumination, ~21% and 35% of the initial MB were decomposed by TiO2 and GT after 100min, respectively. CGT exhibited even higher

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efficiency than that of GT. In addition, the photocatalytic efficiency of CGT increased

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from 27% to 39% as the concentration of Cu2O precursor changed from 0.01M to 0.0025M. and then it decreased from 39% to 26% by changing the concentration of Cu2O precursor from 0.0025 M to 0.001 M. The highest photocatalytic performance was observed over CGT 0.0025M, as about 2.8 times and 1.5 times larger than that of bare TiO2 nanoparticles and GT composites, respectively. The photocatalytic activity cycle stability test of CGT composites with 0.0025M concentration of Cu2O precursor was also investigated under visible light irradiation. As shown in Fig.7, the photodegradation performance of CGTs could be reused one 12

ACCEPTED MANUSCRIPT or two time without obvious decrease after being washed with distilled water and ethanol and dried at 80oC for 60 min. The photodecomposition ratio of MB was about 22.6%, 21.5% and 21.3% after 40 min over the composite materials used for once, twice and three times, respectively. The stable photocatalytic performance of CGT is

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expected to facilitate its use in practical environmental remediation.

The enhanced visible light absorption and photodegradation performance of CGT was associated with the synergistic effect of graphene and Cu2O on TiO2. The

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schematic mechanism of multivariate perturbation in CGT was presented in Fig. 8. Firstly, Cu2O is relatively narrow bandgap (2.0eV) semiconductor that can be

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activated by the visible light illumination [17]. In addition, both the conduction and the valence bands of Cu2O nanoparticles lie above those of TiO2. When TiO2 is hybridized with Cu2O, the photogenerated electrons can transfer from the conduction band of Cu2O to that of TiO2, thereby leading to the visible light response of TiO2 [57].

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Secondly, owing to the excellent electronic conductivity and large specific surface area, graphene can act as an electron acceptor and transporter [24]. When graphene is combined with TiO2, the photogenerated electrons can transport to the surface of the

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composites more easily, thus inhibiting the recombination between photoinduced

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electrons and holes [27]. More effective holes and electrons can induce more free radicals, thus promoting the photodecomposition reaction of methylene blue [58]. All of above is responsible for the improvement of the photocatalytic performance of TiO2. As such, it may stand out as a promising candidate for environmental applications in waste water treatment.

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ACCEPTED MANUSCRIPT 4. Conclusions In this work, a novel ternary composites of CGT was developed and characterized by XRD, FE-SEM, TEM, BET, Raman, UV-vis DRS and photocatalytic measurements. Enhanced visible light response and improved visible light driven

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photocatalytic performance were demonstrated for the resulted CGT by UV-vis DRS and photocatalytic degradation of methylene blue. The highest photocatalytic performance was observed over CGT, as 2.8 and 1.5 times larger than that of bare

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TiO2 and GT, respectively. Both Cu2O and graphene played an important role to improve the photocatalytic activity of TiO2. This work demonstrated that the

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modification of Cu2O on GT composites can further improve the photocatalytic performance of TiO2 and promote the application of TiO2 in photocatalytic field.

Acknowledgments

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This work was financially supported by Innovation Program of Shanghai Municipal Education Commission (15ZZ092), Training Program for Young Teachers in Shanghai Colleges and Universities (ZZgcd14010), Startup Foundation of Shanghai

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University of Engineering Science (No 2014-22) and Graduate Innovation Program of

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Shanghai University of Engineering Science (15KY0516).

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Figure Captions Fig. 1. XRD patterns of bare TiO2 (a), GT (b) and CGT 0.001M (c); 0.0025M (d); 0.005 M (e); 0.01 M (f) composites. A: anatase TiO2, R: rutile TiO2, B: Cu2O.

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Fig. 2. SEM images of the resulted TiO2 (A), GT (B), and CGT composites (C and D).

Fig. 3. TEM (A) and HR-TEM (B, C and D) images of CGT composites.

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Fig. 4. Raman spectra of GT (a) and CGT composites (b); the inset shows the Raman spectra of graphene oxide.

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Fig. 5. The UV-vis diffuse reflectance absorption of TiO2 (a), GT (b) and CGT 0.001 M (c), 0.0025M (d); 0.005 M (e); 0.01 M (f) composites. The inset shows the corresponding (αhv)1/2 vs. hv curves of as-prepared samples.

Fig.6. The remaining methylene blue (MB) in solution equilibrated with bare TiO2 (a),

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GT (b) and CGT 0.001M (c),0.0025M (d); 0.005 M (e); 0.01 M (f) composites after stirring in the dark for 3 h (A); Photodegradation of MB under visible light: (a) bare

(B).

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TiO2, GT (b) and CGT0.001M (c),0.0025M (d); 0.005 M (e); 0.01 M (f) composites

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Fig.7. The photodecomposition ratio of MB over CGT composites used for once, twice, and three times.

Fig.8. Proposed schematic mechanism for the photocatalytic degradation of MB over CGT composite.

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B

B

f

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e

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R

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Intensity (a.u.)

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Fig. 1

R AR R A

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R R R

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20 25 30 35 40 45 50 55 60 65 70 75 80

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2 Theta (degree)

Fig. 1. XRD patterns of bare TiO2 (a), GT (b) and CGT 0.001M (c); 0.0025M (d);

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0.005M (e); 0.01M (f) composites. A: anatase TiO2, R: rutile TiO2, B: Cu2O.

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(B)

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(A)

(D)

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(C)

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D).

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Fig. 2. SEM images of the resulted TiO2 (A), GT (B), and CGT composites (C and

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ACCEPTED MANUSCRIPT Fig. 3

(B)

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(A)

(C)

(D)

TiO2 (110)

Cu2O (111)

Graphene

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TiO2 (110)

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0.24 nm

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0.35 nm

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0.35 nm

Fig. 3. TEM (A) and HR-TEM (B, C and D) images of CGT composites.

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Intensity (a.u.)

Intensity (a.u.)

Fig. 4

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600 800 10001200140016001800 Raman shift (cm-1)

b

a

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200 400 600 800 1000 1200 1400 1600 Raman shift (cm-1)

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Fig. 4. Raman spectra of GT (a) and CGT composites (b); The inset shows the Raman

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spectra of graphene oxide.

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f e d c

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3 4 hv(eV)

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400 500 600 700 Wavelength (nm)

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(αhv)1/2 (eV) 1/2

Absorbance (a.u.)

a

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Fig.5

6

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Fig. 5. The UV-vis diffuse reflectance absorption of TiO2 (a), GT (b) and CGT 0.001 M (c), 0.0025M (d); 0.005M (e); 0.01M (f) composites. The inset shows the

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corresponding (αhv)1/2 vs. hv curves of as-prepared samples.

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(A) Remaining Concentration Fraction of MB

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0.8 0.6 0.4

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(B) 1.0

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Ct/C0

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Fig.6. The remaining methylene blue (MB) in solution equilibrated with bare TiO2 (a), GT (b) and CGT 0.001M (c), 0.0025M (d); 0.005M (e); 0.01M (f) composites after stirring in the dark for 3h (A); Photodegradation of MB under visible light: (a) bare TiO2, GT (b) and CGT0.001M (c), 0.0025M (d); 0.005 M (e); 0.01 M (f) composites (B). 21

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Fig.7

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Degradation ratio (%)

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Fig. 7. The photodecomposition ratio of MB over CGT composites used for once,

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twice, and three times.

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Fig.8

Fig.8. Proposed schematic mechanism for the photocatalytic degradation of MB over CGT composite.

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Research highlight ►A ternary composite of Cu2O/graphene/rutile TiO2 nanorods were successfully

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fabricated. ►Red shift and more absorption in the visible light region were observed for the ternary composites.

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►Improved photocatalytic degradation was detected with the introduction of Cu2O

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and graphene.

►Both Cu2O and graphene played an important role to improve the photocatalytic

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activity of TiO2.