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One-step in situ synthesis of graphene–TiO2 nanorod hybrid composites with enhanced photocatalytic activity
Materials Research Bulletin 61 (2014) 280–286
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
One-step in situ synthesis of graphene–TiO2 nanorod hybrid composites with enhanced photocatalytic activity Mingxuan Sun *, Weibin Li, Shanfu Sun, Jia He, Qiang Zhang, Yuying Shi School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 July 2014 Received in revised form 24 September 2014 Accepted 12 October 2014 Available online 14 October 2014
Chemically bonded graphene/TiO2 nanorod hybrid composites with superior dispersity were synthesized by a one-step in situ hydrothermal method using graphene oxide (GO) and TiO2 (P25) as the starting materials. The as-prepared samples were characterized by XRD, XPS, TEM, FE-SEM, EDX, Raman, N2 adsorption, and UV–vis DRS techniques. Enhanced light absorption and a red shift of absorption edge were observed for the composites in the ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS). Their effective photocatalytic activity was evaluated by the photodegradation of methylene blue under visible light irradiation. An enhancement of photocatalytic performance was observed over graphene/TiO2 nanorod hybrid composite photocatalysts, as 3.7 times larger than that of pristine TiO2 nanorods. This work demonstrated that the synthesis of TiO2 nanorods and simultaneous conversion of GO to graphene “without using reducing agents” had shown to be a rapid, direct and clean approach to fabricate chemically bonded graphene/TiO2 nanorod hybrid composites with enhanced photocatalytic performance. ã 2014 Published by Elsevier Ltd.
Keywords: A. Composites A. Nanostructures B. Chemical synthesis D. Catalytic properties
1. Introduction Titanium dioxide (TiO2) has been widely investigated for their potential application in addressing environmental-related issues due to its low cost, high stability, and nontoxicity [1–3]. However, pure TiO2 suffers from fast recombination of photogenerated electron–holes and activation only under ultraviolet (UV) light irradiation. The intrinsic properties of TiO2 greatly reduce its utilization efficiency of solar light and hamper its full potential use. Thus, many researchers have focused on the modification of TiO2. To date, numerous methods have been developed to promote the visible light photocatalytic performance of TiO2. Doping TiO2 with metal [4,5] and nonmetal [6–8] or coupling TiO2 with semiconductors such as graphene oxide [9], CdS [10], Fe2O3 [11], and WO3 [12] has been widely investigated. Particularly, the composites of TiO2 and carbonaceous materials (activated carbon, carbon quantum dots, carbon nanotubes, graphene, etc.) are recently considered as potential photocatalysts [13–15]. Graphene has attracted much attention and possessed many potential applications, such as solar cells, photocatalysts and many others, due to its unique electronic properties, flexible structure, large theoretical specific surface area, and high transparency
* Corresponding author. Tel.: +86 21 67791474; fax: +86 21 67791201. E-mail address: [email protected] (M. Sun). http://dx.doi.org/10.1016/j.materresbull.2014.10.040 0025-5408/ ã 2014 Published by Elsevier Ltd.
[16–18]. Moreover, the surface properties of graphene could be adjusted with functional groups such as carboxyl, hydroxyl, epoxy or other organic groups via chemical modification. Owing to the excellent performance, graphene has been regarded as an extremely attractive component for the preparation of composite materials. In these years, graphene or GO has been incorporated into various semiconductor materials (such as TiO2, ZnO, and CdS) to obtain excellent photocatalysts [19–21]. Recently, the coupling of graphene and TiO2 is one of the active research areas and abundant demonstration of the photocatalytic enhancement has been reported [22–25]. The interface combination is quite important for graphene/TiO2 nanorod hybrid composites. To date, many strategies have been developed for fabricating graphene/TiO2 hybrid composites with high performance, including ultrasonic assist [26], hydrothermal [27,28], solvothermal [29,30] and so on. However, these approaches have generally produced graphene/TiO2 hybrid materials with weak interaction between graphene and TiO2 or no well-defined nanostructure of TiO2. In fact, the formation of chemical bonds between graphene and TiO2, and the nanostructure of TiO2 particle are crucial for the performance of graphene/TiO2 composites. Therefore, the synthesis of well-defined graphene/TiO2 composites with superior interactions is desirable for practical applications. One-dimensional (1D) nanostructured materials have attracted considerable attention due to their unusual electronic, optical, mechanical properties, and potential applications. In our previous
M. Sun et al. / Materials Research Bulletin 61 (2014) 280–286
Intensity(a.u.)
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work, we demonstrated the sensitizing effect of graphene oxide on the photoelectrochemical and photocatalytic properties of the TiO2 nanotube arrays under visible light [14]. In this work, we synthesized graphene and TiO2 nanorod hybrid composites using a one-step hydrothermal method. Enhanced visible light absorption and photocatalytic performance were demonstrated. Compared with previous strategies, our method presents a combination of advantages, such as an environmental friendly procedure, a low cost and a facile process.
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2. Experimental details
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2.1. Synthesis of graphene/TiO2 nanorod photocatalysts All chemicals were used without further purification. Deionized water was used to prepare all the solution. Graphene oxide was synthesized by Hummers method [31], and 2.5 mg/mL GO aqueous solution was obtained by ultrasound irradiation of graphite oxide in water. Graphene/TiO2 nanorod composites were obtained using P25 and GO alkaline aqueous solution as the starting materials via a one step hydrothermal method. In a typical procedure, 4 mL of GO solution was transferred to 66 mL of deionized water followed by the complete dissolution of 28 g NaOH in it. Then, 1 g P25 was gradually added and stirred for another 0.5 h. The obtained suspension was placed into a Teflon-sealed autoclave of 100 mL
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Fig. 1. XRD patterns of P25 (a), TiO2 nanorods, (b) and 10 wt% graphene/TiO2 nanorods composites. (c) Inset XRD patterns showing the 20–30 degrees region.
capacity and maintained at 200 C for 24 h. After the autoclave was naturally cooled to room temperature, the products were sequentially washed with 0.1 M HCl aqueous solution, deionized water and absolute ethanol for several times and dried at 60 C for
Fig. 2. The TEM (A and B) and HR-TEM (C and D) images of 10 wt% graphene/TiO2 nanorods composites.
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10 h. Finally, soft fibrous powders with French gray color were obtained. 2.2. Characterizations X-ray diffraction (XRD) was carried out on a Bruker D/ 8 advanced diffractometer using Cu Ka radiation with a scan rate of 2 min1. The morphology and microstructure of graphene/ TiO2 nanorod composites were characterized using transmission electron microscope (TEM: Tecnai F20) and field emission scanning electron microscopy (FE-SEM, Philips XL30). A RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Al/Mg-K radiation was used to measure X-ray photoelectron spectroscopy (XPS) at pass energy of 93.9 eV, the binding energies were calibrated based on the containment carbon (C1s 284.6 eV). A Dilor LABRAM-1B microspectrometer with 632.8 nm laser excitation was used to record the Raman spectra of the samples. N2 adsorption was used to measure the Brunauer–Emmett–Teller (B–E–T) surface area at 195.8 C over a relative pressure ranging from 0.05 to 0.35. The UV–vis diffuse reflectance spectra (UV–vis DRS) of the samples were recorded using a U-4100 spectrophotometer in a region of 200–800 nm. 2.3. Photocatalytic measurements The photocatalytic activity of the graphene/TiO2 nanorod composites were evaluated by the rate of photodegradation of methylene blue (MB, C16H18N3S) under visible light irradiation at room temperature. Typically, 15 ml of MB aqueous solution (5 mg/L) and 7.5 mg photocatalyst powders were mixed together in an annular quartz reactor. The mixed solution was firstly kept stirring in the dark for 2 h to achieve adsorption/desorption equilibrium of MB. A 500 W Xenon lamp (CHF-XM35, Trusttech Co., Ltd., Beijing)
with an optical filter to cut off wavelength below 420 nm was used to provide visible light with intensity of 185 mW cm2. For measurements under visible light irradiation, the solution was subjected to the irradiation of visible light under continuous stirring. Once visible light illumination began, samples were tested at 20 min intervals for 100 min. The UV–vis adsorption spectrum of MB solution was collected with an UV–vis spectrophotometer (TECHCOMP UV 2300 (Shanghai)) in the wavelength range of 400–800 nm. The relative concentration of MB in the solution was determined by the absorbance at 664 nm. 3. Results and discussion 3.1. Sample characterizations XRD was employed to study the crystal phase of the asprepared samples with results as shown in Fig. 1. The starting materials (P25) was composed of anatase and rutile phase (curve a in Fig. 1), with the rutile content of 12.6%. The as-prepared approach in this study could produce TiO2 nanorods with only anatase crystal phase, which was demonstrated by the diffraction peaks at 25.3 (1 0 1), 37.9 (0 0 4), 48.1 (2 0 0), 54 (1 0 5, 2 11), 62.7 (2 0 4), 69.1 (11 6, 2 2 0), and 75 (2 1 5) (curves b and c in Fig. 1). Interestingly, there was no graphene peak in graphene/TiO2 nanorods. The disappearance of diffraction peak (u = 24.5 ) belonging to graphene was possibly due to overlapped with (1 0 1) TiO2 peak and low content of graphene in hybrid materials [32]. The diffraction peaks for the (1 0 1) plane of composites was slightly higher shift and broader than that of TiO2 nanorods (Fig. 1 inset), suggesting that the lattice structure of TiO2 was distorted by the interaction with graphene [33]. Scherer formula was applied to the anatase TiO2. The average crystal sizes were calculated to be around 43.5, 15.6, 11.3 nm for P25, TiO2
Fig. 3. FE-SEM of P25 (A), TiO2 nanorods (B) and 10 wt% graphene/TiO2 nanorods composites (C and D).
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nanorod, and graphene/TiO2 nanorod, respectively. The decrease of the average crystal sizes for TiO2 nanorod with the introduction of graphene suggested that the existence of graphene could suppress the crystal growth of anatase TiO2. Fig. 2 illustrates TEM images of graphene/TiO2 nanorod composites. It is clearly observed from the TEM images (Fig. 2A and B) that TiO2 nanorods were anchored onto the surfaces of paper-like graphene, which proved that graphene/TiO2 nanorod hybrid composites had been achieved after the hydrothermal process. Graphene showed crumpled structure composed of layered graphene sheets. The high resolution TEM (HR-TEM) images of graphene/TiO2 nanorod composites were depicted in Fig. 2C and D, which further showed the interaction between graphene and TiO2 nanorods. The interaction was supposed to be chemical bonded style which could be confirmed by XPS results shown later. The stack structure of graphene was observed and the edge of graphite showed a characteristic inter-graphene spacing of 0.34 nm (Fig. 2D), demonstrating the presence of graphene in the obtained composites. The 1D structure of TiO2 nanorods, the transformation of GO to graphene and the strong interactions between TiO2 and graphene surface are expected to enhance the photocatalytic activity of the composites. The morphology of the samples was also characterized by FE-SEM (Fig. 3). As shown in Fig. 3A, spheres nanoparticles with grain size of 90 nm were observed for the starting materials of P25. After the as-prepared hydrothermal procedure, one dimensional (1D) nanorod structure was successfully obtained (Fig. 3B–D). There was little difference in morphologies of TiO2 nanorods and graphene/TiO2 nanorod composites, indicating that the introduction of graphene had little effect on the formation of nanorods. The shape and size of the nanorods were well uniform and a superior dispersity was observed. TiO2 nanorods exhibited average dimensions of 8000 nm(length) 200 nm(diameter)(L/D ratio = 40). The large aspect ratio of TiO2 nanorods synthesized by this procedure was the distinct advantage. The graphene/TiO2 nanorod composites were expected to present a larger specific surface area than that of TiO2 nanorod, which was further confirmed by the measurement of B–E–T surface areas over N2 adsorption. The values of B–E–T surface areas were 59, 65, and 74 m2 g1 for P25, TiO2 nanorods and graphene/TiO2 nanorod composites, respectively. It could be clearly concluded that graphene/TiO2 nanorod composites possessed the largest B–E–T surface areas, which was beneficial for the improvement of photocatalytic performance. Fig. 4 shows the energy dispersive spectroscopy (EDX) of the asprepared samples. The carbon content was 5 at% (atomic percent) and 33 at% for TiO2 nanorod and graphene/TiO2 nanorod composites, respectively. The carbon content of the composites was 6.6 times larger than that of TiO2 nanorods. The results confirmed the incorporation of graphene into the TiO2 nannorod during the hydrothermal process. The surface chemical compositions and valence states of the asprepared samples were further determined by XPS analysis (Fig. 5). Fig. 5A illustrates the XPS spectra of graphene/TiO2 nanorod composites, which exhibited the presence of C, Ti, and O elements. The peaks with binding energies of 458.5 eV and 464.2 eV were assigned to Ti 2p3/2 and Ti 2p1/2 for Ti (IV) of the surface titania, respectively, which confirmed the presence of TiO2 (Fig. 5B). Fig. 5C displays the deconvolution of C1s peak which has four fitting curves centered at 283.5 eV, 284.5 eV, 285.7 eV, and 287.9 eV. The curve at 283.5 eV was corresponding to Ti—C bond [34], and the curves at 284.5 eV, 285.7 eV, and 287.9 eV were assigned to the C—C/C¼C, C—O, and O—C¼O bonds, respectively [35]. These proved that the chemical bonds between graphene and TiO2 nanorods had been formed during the process in this study. The existence of graphene in the graphene/TiO2 nanorod composites was also detected in their Raman spectra. As shown
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Fig. 4. EDX spectrum of TiO2 nanorods (A) and 10 wt% graphene/TiO2 nanorod composites (B).
in Fig. 6, the characteristic D band and G band at 1330 cm1 and 1590 cm1 was observed for both GO and graphene/TiO2 nanorod composites. The D band provides information on sp3 defects in carbon, and the G band is a common feature for in plane vibrations of sp2 bonded carbons [36,37]. In addition, the intensity of D band to the G band usually indicates the order of defects in GO or graphene. The calculated ID/IG ratio of GO was 1.18, while the value for graphene/TiO2 nanorod was 1.09. For comparison with that of GO, a decreased ID/IG intensity ratio was observed for graphene in graphene/TiO2 nanorod composites in Raman spectroscopy. The results implied that the defects in graphene increased after the reduction of GO, suggesting that GO was reduced to graphene during the hydrothermal process. Fig. 7 shows the UV–vis diffusion reflectance spectra of graphene/TiO2 nanorod composites. Red shift and more absorption in the visible light region was also demonstrated for graphene/TiO2 nanorod composites with graphene content increasing (0–5 wt%) from the UV–vis diffuse reflectance spectroscopy. These results indicated that the narrowing of the band gap of TiO2 nanorods occurred with the graphene introduction. The absorption range of light played an important role in the photocatalysis, especially for
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mically bonded graphene/TiO2 nanorod formed through the interactions of Ti—O—C should be also attributed to the visible light absorption [38]. However, further increasing the graphene content to 10 wt%, a weaker absorbance than that of 5 wt% graphene was observed in the visible light region. The visible light photoactivity of graphene/TiO2 nanorod catalyst is expected to facilitate its use in practical environmental remediation.
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The photocatalytic activity of the as-prepared photocatalysts was evaluated by degradation of methylene blue under visible light irradiation (Fig. 8). The advancement of graphene/TiO2 nanorod composites in the photocatalysis should be first attributed to the enhanced absorptivity. As shown in Fig. 8A, after equilibrium in the dark for 120 min, most dye molecules remained in the solution with bare TiO2 nanorod as the catalyst, whereas a large amount of dye molecules were adsorbed on the surface of graphene/TiO2 nanorod composites. These results were attributed to the larger B–E–T surface areas of graphene/TiO2 nanorod composites compared with pristine TiO2 nanorods. Moreover, methylene blue could be absorbed by graphene, which was also beneficial to the enhanced photocatalytic activity of graphene/TiO2 nanorod composites [39]. In addition, in the case of visible light photodegradation, the photocatalytic activity was improved as the GO content was raised from 1 wt% to 5 wt% and then degraded by changing the GO content from 5 wt% to 10 wt%. The highest photocatalytic performance was observed over graphene/TiO2 nanorod composite with 5 wt% content of GO, as 3.7 times larger than that of bare TiO2 nanorods. The enhancement of the photodegradation performance could be attributed to the excellent
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the visible light photodegradation of contaminants. Thus, the enhanced visible light absorption and narrow band gap promised that graphene/TiO2 nanorod composites were more active than bare TiO2 nanorods under visible light. The large background absorption for graphene/TiO2 composites in the visible light range might be partly caused by the dark color of the samples. Moreover,
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Fig. 7. The UV–vis diffuse reflectance absorption spectra of graphene/TiO2 nanorod composites with various graphene contents: a. 0 wt%, b. 1 wt%, c. 5 wt%, d. 10 wt%.
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formation of Ti—C bond and Raman results demonstrated that GO was reduced to graphene in the as prepared composites. A red shift in the optical absorption edge and enhanced absorbance was confirmed for graphene/TiO2 nanorod composites compared with bare TiO2 nanorods. Higher photocatalytic activity of graphene/ TiO2 nanorods than that of bare TiO2 was demonstrated by the photodegradation of methylene blue under visible light irradiation. The enhancement was attributed to the improved adsorptivity of contaminant molecules, the extended light absorption, and more efficient charge transport and separation with the introduction of graphene into TiO2 nanorods. This novel technique for synthesizing graphene/TiO2 nanorods composites could be also used to prepare other graphene based composites for prompting their potential application in addressing various environmental issues. Acknowledgments This work was financially supported by Training Program for Young Teachers in Shanghai Colleges and Universities (No ZZgcd14010), 2013 Municipal Undergraduate Innovative Training Project of Shanghai (No cs1305007) and Startup Foundation of Shanghai University of Engineering Science (No 2014-22). The authors would also like to thank Professor Xiaoli Cui (Department of Materials Science, Fudan University) for the assistance in the characterization of materials. References Fig. 8. The photocatalytic degradation of MB under visible light illumination: (A) graphene/TiO2 nanorod composites with various graphene contents: a. 0 wt%, b. 1 wt% c. 5 wt% d. 10 wt%; (B) the photodecomposition ratio of MB over 5 wt% graphene/TiO2 nanorod composites used for once, twice, and three times.
electronic conductivity and large specific surface area of graphene. The photogenerated electrons transport to the surface of the composites more easily, thus inhibiting the recombination between phoinduced electrons and holes. In addition, the enhanced and extended light response could also promote the photodegradation activity with the introduction of graphene into TiO2 nanorods. The cycle stability test for phtocatalytic activity of this composite material was also investigated under visible light irradiation. As shown in Fig. 8B, graphene/TiO2 nanorod composites could be reused one or two times without significant decrease in activity after being washed with ethanol and activated at 80 C for 1 h. The photodecomposition ratio of MB was about 71.9%, 70.2%, and 69.5% after 40 min over the composite materials used for once, twice and three times, respectively. The visible light photoactivity of the graphene/TiO2 nanorod catalyst is expected to facilitate its use in practical environmental remediation. 4. Conclusions In summary, graphene/TiO2 nanorod composites have been successfully synthesized via a one-step hydrothermal method. This method was environmentally friendly and featured the reduction of GO and formation of TiO2 nanorods at the same time. XRD proved that anatase phase of TiO2 was obtained and the introduction of graphene could suppress the crystal growth. The existence of graphene in TiO2 nanorods were confirmed by XPS, EDX, and Raman. Furthermore, XPS results indicated the
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One-step in situ synthesis of graphene–TiO2 nanorod hybrid composites with enhanced photocatalytic activity Mingxuan Sun *, Weibin Li, Shanfu Sun, Jia He, Qiang Zhang, Yuying Shi School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 July 2014 Received in revised form 24 September 2014 Accepted 12 October 2014 Available online 14 October 2014
Chemically bonded graphene/TiO2 nanorod hybrid composites with superior dispersity were synthesized by a one-step in situ hydrothermal method using graphene oxide (GO) and TiO2 (P25) as the starting materials. The as-prepared samples were characterized by XRD, XPS, TEM, FE-SEM, EDX, Raman, N2 adsorption, and UV–vis DRS techniques. Enhanced light absorption and a red shift of absorption edge were observed for the composites in the ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS). Their effective photocatalytic activity was evaluated by the photodegradation of methylene blue under visible light irradiation. An enhancement of photocatalytic performance was observed over graphene/TiO2 nanorod hybrid composite photocatalysts, as 3.7 times larger than that of pristine TiO2 nanorods. This work demonstrated that the synthesis of TiO2 nanorods and simultaneous conversion of GO to graphene “without using reducing agents” had shown to be a rapid, direct and clean approach to fabricate chemically bonded graphene/TiO2 nanorod hybrid composites with enhanced photocatalytic performance. ã 2014 Published by Elsevier Ltd.
Keywords: A. Composites A. Nanostructures B. Chemical synthesis D. Catalytic properties
1. Introduction Titanium dioxide (TiO2) has been widely investigated for their potential application in addressing environmental-related issues due to its low cost, high stability, and nontoxicity [1–3]. However, pure TiO2 suffers from fast recombination of photogenerated electron–holes and activation only under ultraviolet (UV) light irradiation. The intrinsic properties of TiO2 greatly reduce its utilization efficiency of solar light and hamper its full potential use. Thus, many researchers have focused on the modification of TiO2. To date, numerous methods have been developed to promote the visible light photocatalytic performance of TiO2. Doping TiO2 with metal [4,5] and nonmetal [6–8] or coupling TiO2 with semiconductors such as graphene oxide [9], CdS [10], Fe2O3 [11], and WO3 [12] has been widely investigated. Particularly, the composites of TiO2 and carbonaceous materials (activated carbon, carbon quantum dots, carbon nanotubes, graphene, etc.) are recently considered as potential photocatalysts [13–15]. Graphene has attracted much attention and possessed many potential applications, such as solar cells, photocatalysts and many others, due to its unique electronic properties, flexible structure, large theoretical specific surface area, and high transparency
* Corresponding author. Tel.: +86 21 67791474; fax: +86 21 67791201. E-mail address: [email protected] (M. Sun). http://dx.doi.org/10.1016/j.materresbull.2014.10.040 0025-5408/ ã 2014 Published by Elsevier Ltd.
[16–18]. Moreover, the surface properties of graphene could be adjusted with functional groups such as carboxyl, hydroxyl, epoxy or other organic groups via chemical modification. Owing to the excellent performance, graphene has been regarded as an extremely attractive component for the preparation of composite materials. In these years, graphene or GO has been incorporated into various semiconductor materials (such as TiO2, ZnO, and CdS) to obtain excellent photocatalysts [19–21]. Recently, the coupling of graphene and TiO2 is one of the active research areas and abundant demonstration of the photocatalytic enhancement has been reported [22–25]. The interface combination is quite important for graphene/TiO2 nanorod hybrid composites. To date, many strategies have been developed for fabricating graphene/TiO2 hybrid composites with high performance, including ultrasonic assist [26], hydrothermal [27,28], solvothermal [29,30] and so on. However, these approaches have generally produced graphene/TiO2 hybrid materials with weak interaction between graphene and TiO2 or no well-defined nanostructure of TiO2. In fact, the formation of chemical bonds between graphene and TiO2, and the nanostructure of TiO2 particle are crucial for the performance of graphene/TiO2 composites. Therefore, the synthesis of well-defined graphene/TiO2 composites with superior interactions is desirable for practical applications. One-dimensional (1D) nanostructured materials have attracted considerable attention due to their unusual electronic, optical, mechanical properties, and potential applications. In our previous
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work, we demonstrated the sensitizing effect of graphene oxide on the photoelectrochemical and photocatalytic properties of the TiO2 nanotube arrays under visible light [14]. In this work, we synthesized graphene and TiO2 nanorod hybrid composites using a one-step hydrothermal method. Enhanced visible light absorption and photocatalytic performance were demonstrated. Compared with previous strategies, our method presents a combination of advantages, such as an environmental friendly procedure, a low cost and a facile process.
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2. Experimental details
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2.1. Synthesis of graphene/TiO2 nanorod photocatalysts All chemicals were used without further purification. Deionized water was used to prepare all the solution. Graphene oxide was synthesized by Hummers method [31], and 2.5 mg/mL GO aqueous solution was obtained by ultrasound irradiation of graphite oxide in water. Graphene/TiO2 nanorod composites were obtained using P25 and GO alkaline aqueous solution as the starting materials via a one step hydrothermal method. In a typical procedure, 4 mL of GO solution was transferred to 66 mL of deionized water followed by the complete dissolution of 28 g NaOH in it. Then, 1 g P25 was gradually added and stirred for another 0.5 h. The obtained suspension was placed into a Teflon-sealed autoclave of 100 mL
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Fig. 1. XRD patterns of P25 (a), TiO2 nanorods, (b) and 10 wt% graphene/TiO2 nanorods composites. (c) Inset XRD patterns showing the 20–30 degrees region.
capacity and maintained at 200 C for 24 h. After the autoclave was naturally cooled to room temperature, the products were sequentially washed with 0.1 M HCl aqueous solution, deionized water and absolute ethanol for several times and dried at 60 C for
Fig. 2. The TEM (A and B) and HR-TEM (C and D) images of 10 wt% graphene/TiO2 nanorods composites.
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10 h. Finally, soft fibrous powders with French gray color were obtained. 2.2. Characterizations X-ray diffraction (XRD) was carried out on a Bruker D/ 8 advanced diffractometer using Cu Ka radiation with a scan rate of 2 min1. The morphology and microstructure of graphene/ TiO2 nanorod composites were characterized using transmission electron microscope (TEM: Tecnai F20) and field emission scanning electron microscopy (FE-SEM, Philips XL30). A RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Al/Mg-K radiation was used to measure X-ray photoelectron spectroscopy (XPS) at pass energy of 93.9 eV, the binding energies were calibrated based on the containment carbon (C1s 284.6 eV). A Dilor LABRAM-1B microspectrometer with 632.8 nm laser excitation was used to record the Raman spectra of the samples. N2 adsorption was used to measure the Brunauer–Emmett–Teller (B–E–T) surface area at 195.8 C over a relative pressure ranging from 0.05 to 0.35. The UV–vis diffuse reflectance spectra (UV–vis DRS) of the samples were recorded using a U-4100 spectrophotometer in a region of 200–800 nm. 2.3. Photocatalytic measurements The photocatalytic activity of the graphene/TiO2 nanorod composites were evaluated by the rate of photodegradation of methylene blue (MB, C16H18N3S) under visible light irradiation at room temperature. Typically, 15 ml of MB aqueous solution (5 mg/L) and 7.5 mg photocatalyst powders were mixed together in an annular quartz reactor. The mixed solution was firstly kept stirring in the dark for 2 h to achieve adsorption/desorption equilibrium of MB. A 500 W Xenon lamp (CHF-XM35, Trusttech Co., Ltd., Beijing)
with an optical filter to cut off wavelength below 420 nm was used to provide visible light with intensity of 185 mW cm2. For measurements under visible light irradiation, the solution was subjected to the irradiation of visible light under continuous stirring. Once visible light illumination began, samples were tested at 20 min intervals for 100 min. The UV–vis adsorption spectrum of MB solution was collected with an UV–vis spectrophotometer (TECHCOMP UV 2300 (Shanghai)) in the wavelength range of 400–800 nm. The relative concentration of MB in the solution was determined by the absorbance at 664 nm. 3. Results and discussion 3.1. Sample characterizations XRD was employed to study the crystal phase of the asprepared samples with results as shown in Fig. 1. The starting materials (P25) was composed of anatase and rutile phase (curve a in Fig. 1), with the rutile content of 12.6%. The as-prepared approach in this study could produce TiO2 nanorods with only anatase crystal phase, which was demonstrated by the diffraction peaks at 25.3 (1 0 1), 37.9 (0 0 4), 48.1 (2 0 0), 54 (1 0 5, 2 11), 62.7 (2 0 4), 69.1 (11 6, 2 2 0), and 75 (2 1 5) (curves b and c in Fig. 1). Interestingly, there was no graphene peak in graphene/TiO2 nanorods. The disappearance of diffraction peak (u = 24.5 ) belonging to graphene was possibly due to overlapped with (1 0 1) TiO2 peak and low content of graphene in hybrid materials [32]. The diffraction peaks for the (1 0 1) plane of composites was slightly higher shift and broader than that of TiO2 nanorods (Fig. 1 inset), suggesting that the lattice structure of TiO2 was distorted by the interaction with graphene [33]. Scherer formula was applied to the anatase TiO2. The average crystal sizes were calculated to be around 43.5, 15.6, 11.3 nm for P25, TiO2
Fig. 3. FE-SEM of P25 (A), TiO2 nanorods (B) and 10 wt% graphene/TiO2 nanorods composites (C and D).
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nanorod, and graphene/TiO2 nanorod, respectively. The decrease of the average crystal sizes for TiO2 nanorod with the introduction of graphene suggested that the existence of graphene could suppress the crystal growth of anatase TiO2. Fig. 2 illustrates TEM images of graphene/TiO2 nanorod composites. It is clearly observed from the TEM images (Fig. 2A and B) that TiO2 nanorods were anchored onto the surfaces of paper-like graphene, which proved that graphene/TiO2 nanorod hybrid composites had been achieved after the hydrothermal process. Graphene showed crumpled structure composed of layered graphene sheets. The high resolution TEM (HR-TEM) images of graphene/TiO2 nanorod composites were depicted in Fig. 2C and D, which further showed the interaction between graphene and TiO2 nanorods. The interaction was supposed to be chemical bonded style which could be confirmed by XPS results shown later. The stack structure of graphene was observed and the edge of graphite showed a characteristic inter-graphene spacing of 0.34 nm (Fig. 2D), demonstrating the presence of graphene in the obtained composites. The 1D structure of TiO2 nanorods, the transformation of GO to graphene and the strong interactions between TiO2 and graphene surface are expected to enhance the photocatalytic activity of the composites. The morphology of the samples was also characterized by FE-SEM (Fig. 3). As shown in Fig. 3A, spheres nanoparticles with grain size of 90 nm were observed for the starting materials of P25. After the as-prepared hydrothermal procedure, one dimensional (1D) nanorod structure was successfully obtained (Fig. 3B–D). There was little difference in morphologies of TiO2 nanorods and graphene/TiO2 nanorod composites, indicating that the introduction of graphene had little effect on the formation of nanorods. The shape and size of the nanorods were well uniform and a superior dispersity was observed. TiO2 nanorods exhibited average dimensions of 8000 nm(length) 200 nm(diameter)(L/D ratio = 40). The large aspect ratio of TiO2 nanorods synthesized by this procedure was the distinct advantage. The graphene/TiO2 nanorod composites were expected to present a larger specific surface area than that of TiO2 nanorod, which was further confirmed by the measurement of B–E–T surface areas over N2 adsorption. The values of B–E–T surface areas were 59, 65, and 74 m2 g1 for P25, TiO2 nanorods and graphene/TiO2 nanorod composites, respectively. It could be clearly concluded that graphene/TiO2 nanorod composites possessed the largest B–E–T surface areas, which was beneficial for the improvement of photocatalytic performance. Fig. 4 shows the energy dispersive spectroscopy (EDX) of the asprepared samples. The carbon content was 5 at% (atomic percent) and 33 at% for TiO2 nanorod and graphene/TiO2 nanorod composites, respectively. The carbon content of the composites was 6.6 times larger than that of TiO2 nanorods. The results confirmed the incorporation of graphene into the TiO2 nannorod during the hydrothermal process. The surface chemical compositions and valence states of the asprepared samples were further determined by XPS analysis (Fig. 5). Fig. 5A illustrates the XPS spectra of graphene/TiO2 nanorod composites, which exhibited the presence of C, Ti, and O elements. The peaks with binding energies of 458.5 eV and 464.2 eV were assigned to Ti 2p3/2 and Ti 2p1/2 for Ti (IV) of the surface titania, respectively, which confirmed the presence of TiO2 (Fig. 5B). Fig. 5C displays the deconvolution of C1s peak which has four fitting curves centered at 283.5 eV, 284.5 eV, 285.7 eV, and 287.9 eV. The curve at 283.5 eV was corresponding to Ti—C bond [34], and the curves at 284.5 eV, 285.7 eV, and 287.9 eV were assigned to the C—C/C¼C, C—O, and O—C¼O bonds, respectively [35]. These proved that the chemical bonds between graphene and TiO2 nanorods had been formed during the process in this study. The existence of graphene in the graphene/TiO2 nanorod composites was also detected in their Raman spectra. As shown
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Fig. 4. EDX spectrum of TiO2 nanorods (A) and 10 wt% graphene/TiO2 nanorod composites (B).
in Fig. 6, the characteristic D band and G band at 1330 cm1 and 1590 cm1 was observed for both GO and graphene/TiO2 nanorod composites. The D band provides information on sp3 defects in carbon, and the G band is a common feature for in plane vibrations of sp2 bonded carbons [36,37]. In addition, the intensity of D band to the G band usually indicates the order of defects in GO or graphene. The calculated ID/IG ratio of GO was 1.18, while the value for graphene/TiO2 nanorod was 1.09. For comparison with that of GO, a decreased ID/IG intensity ratio was observed for graphene in graphene/TiO2 nanorod composites in Raman spectroscopy. The results implied that the defects in graphene increased after the reduction of GO, suggesting that GO was reduced to graphene during the hydrothermal process. Fig. 7 shows the UV–vis diffusion reflectance spectra of graphene/TiO2 nanorod composites. Red shift and more absorption in the visible light region was also demonstrated for graphene/TiO2 nanorod composites with graphene content increasing (0–5 wt%) from the UV–vis diffuse reflectance spectroscopy. These results indicated that the narrowing of the band gap of TiO2 nanorods occurred with the graphene introduction. The absorption range of light played an important role in the photocatalysis, especially for
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mically bonded graphene/TiO2 nanorod formed through the interactions of Ti—O—C should be also attributed to the visible light absorption [38]. However, further increasing the graphene content to 10 wt%, a weaker absorbance than that of 5 wt% graphene was observed in the visible light region. The visible light photoactivity of graphene/TiO2 nanorod catalyst is expected to facilitate its use in practical environmental remediation.
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The photocatalytic activity of the as-prepared photocatalysts was evaluated by degradation of methylene blue under visible light irradiation (Fig. 8). The advancement of graphene/TiO2 nanorod composites in the photocatalysis should be first attributed to the enhanced absorptivity. As shown in Fig. 8A, after equilibrium in the dark for 120 min, most dye molecules remained in the solution with bare TiO2 nanorod as the catalyst, whereas a large amount of dye molecules were adsorbed on the surface of graphene/TiO2 nanorod composites. These results were attributed to the larger B–E–T surface areas of graphene/TiO2 nanorod composites compared with pristine TiO2 nanorods. Moreover, methylene blue could be absorbed by graphene, which was also beneficial to the enhanced photocatalytic activity of graphene/TiO2 nanorod composites [39]. In addition, in the case of visible light photodegradation, the photocatalytic activity was improved as the GO content was raised from 1 wt% to 5 wt% and then degraded by changing the GO content from 5 wt% to 10 wt%. The highest photocatalytic performance was observed over graphene/TiO2 nanorod composite with 5 wt% content of GO, as 3.7 times larger than that of bare TiO2 nanorods. The enhancement of the photodegradation performance could be attributed to the excellent
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the visible light photodegradation of contaminants. Thus, the enhanced visible light absorption and narrow band gap promised that graphene/TiO2 nanorod composites were more active than bare TiO2 nanorods under visible light. The large background absorption for graphene/TiO2 composites in the visible light range might be partly caused by the dark color of the samples. Moreover,
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Fig. 7. The UV–vis diffuse reflectance absorption spectra of graphene/TiO2 nanorod composites with various graphene contents: a. 0 wt%, b. 1 wt%, c. 5 wt%, d. 10 wt%.
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formation of Ti—C bond and Raman results demonstrated that GO was reduced to graphene in the as prepared composites. A red shift in the optical absorption edge and enhanced absorbance was confirmed for graphene/TiO2 nanorod composites compared with bare TiO2 nanorods. Higher photocatalytic activity of graphene/ TiO2 nanorods than that of bare TiO2 was demonstrated by the photodegradation of methylene blue under visible light irradiation. The enhancement was attributed to the improved adsorptivity of contaminant molecules, the extended light absorption, and more efficient charge transport and separation with the introduction of graphene into TiO2 nanorods. This novel technique for synthesizing graphene/TiO2 nanorods composites could be also used to prepare other graphene based composites for prompting their potential application in addressing various environmental issues. Acknowledgments This work was financially supported by Training Program for Young Teachers in Shanghai Colleges and Universities (No ZZgcd14010), 2013 Municipal Undergraduate Innovative Training Project of Shanghai (No cs1305007) and Startup Foundation of Shanghai University of Engineering Science (No 2014-22). The authors would also like to thank Professor Xiaoli Cui (Department of Materials Science, Fudan University) for the assistance in the characterization of materials. References Fig. 8. The photocatalytic degradation of MB under visible light illumination: (A) graphene/TiO2 nanorod composites with various graphene contents: a. 0 wt%, b. 1 wt% c. 5 wt% d. 10 wt%; (B) the photodecomposition ratio of MB over 5 wt% graphene/TiO2 nanorod composites used for once, twice, and three times.
electronic conductivity and large specific surface area of graphene. The photogenerated electrons transport to the surface of the composites more easily, thus inhibiting the recombination between phoinduced electrons and holes. In addition, the enhanced and extended light response could also promote the photodegradation activity with the introduction of graphene into TiO2 nanorods. The cycle stability test for phtocatalytic activity of this composite material was also investigated under visible light irradiation. As shown in Fig. 8B, graphene/TiO2 nanorod composites could be reused one or two times without significant decrease in activity after being washed with ethanol and activated at 80 C for 1 h. The photodecomposition ratio of MB was about 71.9%, 70.2%, and 69.5% after 40 min over the composite materials used for once, twice and three times, respectively. The visible light photoactivity of the graphene/TiO2 nanorod catalyst is expected to facilitate its use in practical environmental remediation. 4. Conclusions In summary, graphene/TiO2 nanorod composites have been successfully synthesized via a one-step hydrothermal method. This method was environmentally friendly and featured the reduction of GO and formation of TiO2 nanorods at the same time. XRD proved that anatase phase of TiO2 was obtained and the introduction of graphene could suppress the crystal growth. The existence of graphene in TiO2 nanorods were confirmed by XPS, EDX, and Raman. Furthermore, XPS results indicated the
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