Graphene-based hollow TiO2 composites with enhanced photocatalytic activity for removal of pollutants

Journal of Physics and Chemistry of Solids 86 (2015) 82–89

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Graphene-based hollow TiO2 composites with enhanced photocatalytic activity for removal of pollutants Lixin Zhang a,n, Jia Zhang a, Hongfang Jiu b, Changhui Ni a, Xia Zhang a, Meiling Xu a a b

School of Chemical and Environment Engineering, North University of China, Taiyuan 030051, People's Republic of China School of Science, North University of China, Taiyuan 030051, People's Republic of China

art ic l e i nf o

a b s t r a c t

Article history: Received 4 February 2015 Received in revised form 18 June 2015 Accepted 25 June 2015 Available online 2 July 2015

Catalytically active graphene-based hollow TiO2 composites(TiO2/RGO) were successfully synthesized via the solvothermal method. Hollow TiO2 microspheres are uniformly dispersed on RGO. X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) were used for the characterization of prepared photocatalysts. The mass of GO was optimized in the photocatalytic removal of rhodamine B (RhB) as a model dye pollutants. The results showed that graphene-based hollow TiO2 composites exhibit a significantly enhanced photocatalytic activity in degradation of RhB under either UV or visible light irradiation. The formation of the graphene-based hollow TiO2 composites and the photocatalytic mechanisms under UV and visible light were also discussed. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Hollow RGO/TiO2 Solvothermal Photocatalytic Degradation

1. Introduction In recent decades, semiconductor photocatalysis has attracted extensive attention because it is one of the most advanced technologies for treating the energy crisis and the environmental pollution [1–3]. Among various semiconductor materials, titanium dioxide has been the most widely investigated material due to its strong oxidizing power, nontoxicity, high chemical stability, and photostability [4–6]. However, the two disadvantages of TiO2 severely limit its applications in photocatalysis. Firstly, the high recombination rate of electron–hole pairs generated from photocatalytic reactions results in the low quantum efficiency. On the other hand, TiO2 with a large bandgap energy about 3.2 eV, only can absorb ultraviolet (UV) light. To solve these problems, the coupling TiO2 with the functional materials is an effective method to enhance the photocatalytic performance. Among them, graphene/TiO2 composite has attracted much attention due to the outstanding physical and chemical properties [7–9]. Graphene, single atomic layer, two-dimensional graphitic carbon material, is suitable for utilize as the photocatalyst support material due to its unique properties, such as excellent electrical conductivity, high chemical stability, large specific surface area, and unique electronic and mechanical properties, flexible structure [10–12]. Because of its remarkable properties, there is interest in using graphene for various applications such as catalytic n

Corresponding author. Fax: þ86 351 3922271. E-mail address: [email protected] (L. Zhang).

http://dx.doi.org/10.1016/j.jpcs.2015.06.018 0022-3697/& 2015 Elsevier Ltd. All rights reserved.

activity, photovoltaics, supercapacitors, energy storage nano electronics, lithium-ion batteries [13,14]. Recently, a few works have shown that the introduction of graphene can improve the chargeseparation rate of titanium dioxide [15]. For instance, Shen et al. reported TiO2/RGO composites having been prepared by the ionic liquid-assisted one-step hydrothermal method showed excellent photocatalytic properties [16]. Sher Shah et al. applied TiO2–RGO nanocomposites that exhibited an enhanced photocatalytic activity for the degradation of RhB in aqueous solution [17]. Zhang et al. studied the photocatalytic activity by graphene encapsulated mesoporous hollow TiO2 nanospheres as photocatalyst [18]. Yan et al. used RGO/TiO2 composites as photocatalyst for the degradation of MB solution [14]. Owing to an effective relevance between TiO2 and GO, the prepared samples exhibit high photocatalytic properties. With the rapid development of nanotechnology, the nanoparticles with novel constructions and excellent properties have attracted increasing attention [19]. Among these structures, hollow nanospheres have attracted relatively great attention and been widely used in many important areas such as photocatalytic, sensors, energy storage, drug delivery and Lithium-ion batteries due to their special physical and chemical properties [20–22]. In this work, catalytically active graphene-based hollow TiO2 composites (TiO2/graphene composites with hollow structure) (HTGC) were synthesized through an effective and green solvothermal approach in the water/ethanol solvent without adding additional coupling agent. The typical synthetic process involves the preparation of hollow titanium dioxide using carbon

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microsphere templates and deposition of the TiO2 on GO sheets. Hollow TiO2 nanoparticles were well dispersed on RGO sheets and tightly in contact. In addition, its application in photocatalysis (Rhodamine B, as a model) is explored. The as-synthesized TiO2/graphene composites exhibited superior photocatalytic properties compared to hollow TiO2 nanoparticles. The formation of the HTGC and the degradation of RhB mechanisms were also discussed.

2. Experimental details 2.1. Materials and reagents Tetrabutyl titanate (TBOT, 498.0%) and HCl (37%), Glucose (A. R), Rhodamine B, graphene oxide (GO, laboratory homemade), P25 deionized water, absolute ethanol (A.R). All materials were used as received. 2.2. Synthesis of graphite oxide (GO)

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2.4. Synthesis of HTGC Scheme 1 shows the schematic illustration of the synthesis process. Hollow TiO2/graphene nanocomposites (HTGC) were synthesized by the solvothermal method. GO was synthesized according to the modified Hummers method [23]. A certain amount of GO was dispersed in deionized water (20 mL) and ethanol (10 mL) under ultrasonic treatment for 1 h. The designed masses of GO were 0,10, 20, 30 and 40 mg, and the corresponding final products are denoted as Gx–TiO2, where x is 0, 5, 10, 15, and 20 wt%, respectively. Approximately 0.2 g of the hollow TiO2 nanoparticles was then added to this solution and stirred for another 1 h to get a homogeneous suspension. The suspension was placed in a 50 mL Teflon-lined autoclave and heated at 120 °C for 24 h for reducing GO to RGO and depositing hollow TiO2 nanoparticles on RGO [25]. After cooling, the resulting products were isolated by centrifugation at 8000 rpm for 10 min, washed with deionized water and ethanol several times and then dried in an oven at 50 °C for 12 h. 2.5. Characterization

Graphite oxide (GO) was synthesized according to the modified Hummers method. 3.00 g of graphite powder was put into a mixture of 12 mL of concentrated H2SO4, 2.50 g of K2S2O8, and 2.50 g of P2O5. The solution was heated to 80 °C and stirred for 4 h in an oil bath. The mixture was diluted with 500 mL of deionized water. The product was obtained by filtering the solution using 0.2 m Nylon film, washed with deionized water several times and dried under ambient condition. The oxidized graphite was added to the 120 mL of H2SO4 in an ice bath. Thereafter, the product was reoxidized by Hummers and Offeman methods to produce the graphite oxide [23]. Then, 15 g of KMnO4 was put slowly with controlling the temperature below 20 °C. Stirring was continued for 2 h at 35 °C. After that 250 mL of deionized water was added slowly with keeping the temperature below 50 °C. After 2 h, 2.8 L of deionized water and 50 mL of 30% H2O2 were added to the mixture. The mixture was centrifuged and washed with a total of 5 L of 10% HCl solution followed by 5 L of deionized water to remove the acid. The resulting solid was subjected to dialysis for a week. Finally, the product was dried under the vacuum at ambient temperature.

The phase characterization of the samples was done by a TD300 X-ray powder diffraction diffraction (XRD, Japan D/max-rB) with Cu Ka as the radiation source (λ ¼1.5408 Å ) through the 2θ range from 5° to 80°and operated at 40 Kv and 100 mA. Fouriertransform infrared (FTIR) spectra of the HTGC were analyzed using Nicoletis 10 (KBrdisk) (China). Transmission electron microscopy (TEM) images were conducted on a Japan JEOL JEM-1400 microscope. X-ray photoelectron spectroscopy (XPS) characterization was taken using Thermo ESCALAB 250 (America). The thermo gravimetric (TG) curves of HTGC were recorded on a China ZCT2000 thermo-gravimetric analysis. The optical properties of the samples were characterized by a UV-2600 ultraviolet/visible diffuse reflectance spectrophotometer (DRS), during which BaSO4 was employed as the internal reflectance standard. The photoluminescence spectra (PL) were investigated on a Hitachi fluorescence spectrometer (PL, Japan) at room temperature with a F-2500 fiuorescence spectrophotometer. Photochemical reactions instrument (PhchemIII) was purchased from NBET (China). The photocatalytic activities were determined on China UV-2300 UV– vis.

2.3. Synthesis of hollow TiO2

2.6. Measurements of photocatalytic activities

For the synthesis of hollow TiO2, 0.4 ml TBOT and 2.0 ml HCl were dissolved in 40.0 ml absolute ethanol. After that, the assynthesized carbon nanoparticles were dispersed to result a suspension [24]. The suspension was transferred into a 50 ml autoclave maintained at 180 °C for 12 h. The resulting product was washed with water and ethanol several times and dried at room temperature. Final products were prepared after annealing the black precipitates at 600 °C for 3 h with a heating rate of 2.0 °C/ min.

The photocatalytic activities of the composite were evaluated by the photodegradation of rhodamine B solution. In a typical process, rhodamine B (10 mg/L, 50 ml) placed in a cylindrical quartz vessel, 0.03 g photocatalyst was added and stirred for 30 min under shade conditions to ensure the establishment of an adsorption/desorption equilibrium. 100 W high pressure mercury lamp as a light source surrounded with a water circulation facility at the outer wall through a quartz jacket was placed at a distance of 10 cm. After stirring for 30 min, the lights were illuminated.

Scheme 1. The schematic illustration for the formation process of HTGC.

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Within a specified time, approximately 5 ml of the suspension was taken out and centrifuged. In the visible light photocatalysis, a similar procedure was constructed with a 500 W xenon lamp and a cut off filter equipped as the light source. Concentrations of the samples were tested using UV–vis spectrophotometer.

3. Results and discussion The crystalline phases of hollow TiO2 nanospheres, HTGC, and GO were examined by XRD patterns, which are shown in Fig. 1. According to the results of the XRD patterns, a reflection peak of GO appeared at 2θ ¼10.6°, which corresponded to the earlier results and proved that GO was synthesized by the modified Hummers method successfully. It is clear to see that the diffraction peaks of hollow TiO2 nanospheres located at 25.3°, 37.2°, 48.0°, 53.7°, 55.0°, 63.0°, 69.0°, 70.2° and 75.0° can be respectively indexed to the (101), (004), (112), (200), (105), (211), (204), (116), (220), and (215) crystal planes of anatase TiO2 (JCPDS card 211272) [26]. The XRD patterns of the HTGC are similar to that of hollow TiO2, which suggests that solvothermal treatment during composite formation has no remarkable effect on the hollow TiO2. In addition, the characteristic peak of 10.6° almost disappeared after solvothermal treatment, indicating that most of the GO was reduced. Notably, no typical diffraction peaks of RGO were observed in the HTGC pattern because the main RGO diffraction peak at 25.0° was overlapped with the stronger peak of anatase TiO2 at 25.3° [27]. TEM images clearly demonstrated the structure and morphology of the samples. Fig. 2 illustrates the TEM images of as-synthesized carbon spheres, C–Ti(OH)4 composites, hollow TiO2 nanospheres, GO and HTGC respectively. The carbon microspheres template showed in Fig. 2a exhibit a quite regular spherical morphology and the size is approximately 500 nm. From Fig. 2b, we can found that C–Ti(OH)4 have the same size with C spheres and the rough surface, which indicate that coating of precursor may be loaded into the carbon sphere template due to the existence of a large number functional groups such as –OH and –COOH [28]. By calcining the synthesis of C–Ti(OH)4 composite, TiO2 microspheres were successfully obtained in Fig. 2c, a hollow structure can be observed from the contrast of lighter pale center and darker edges clearly. In particular, Fig. 2d shows that the average diameter of TiO2 is approximately 230 nm. The morphology of as-synthesized GO and HTGC can be observed from TEM images exhibited in Fig. 2. Clearly, the two dimensional, free-standing GO with thin stacked flakes and layered structure at the edge can be seen in Fig. 2e. It was not flat but crumpled. Fig. 2f displays the TEM of HTGC. It is apparent that TiO2

Fig. 1. XRD of hollow TiO2, HTGC and GO.

hollow spheres are well dispersed on the RGO sheets. In addition, graphene sheets with crinkled textures are clearly found on the composite, and most of the hollow TiO2 spheres are arranged on the reduced graphite oxide after the solvothermal treatment because of interfacial interactions. Notably, the hollow TiO2 nanoparticles are not simply mixed with RGO; rather, they have been wrapped by the RGO sheets possibly. The size of hollow microspheres ranges from 200 to 400 nm with an average diameter of about 230 nm. In order to well understand the reduction of GO after solvothermal process and the interactions between RGO and TiO2, FTIR spectra were obtained. Fig. 3 shows the FTIR spectra of TiO2, GO and HTGC. The intensive broad bands between 400 and 800 cm  1 in the spectrum of TiO2 are clearly observed, which are related to the stretching vibration of Ti–O–Ti and Ti–O bonds [29]. The peaks of GO in the spectrum at 3400 cm  1 and 1618 cm  1 are assigned to hydroxyl (–OH) groups stretching vibration and the C¼ C skeletal vibration [30]. The other peaks at 1730, 1413, 1264 and 1085 cm  1 are ascribed to oxygen containing functional groups, i.e., carboxyl C–O, carboxyl C ¼O, alkoxy C–O and epoxy C– O stretching vibrations [5], respectively. In the FTIR spectrum of HTGC, most of the oxygen-containing functional groups have been decreased dramatically or even disappeared entirely, demonstrating that GO was significantly reduced by the hydrothermal treatment. It is worth noting that a small part of oxygen containing functional groups still exist,which promote the hybridizing of hollow TiO2 nanoparticles deposited on the RGO sheets [31]. To study the interaction between graphene and TiO2 in Gx –TiO2, the X-ray photoelectron spectroscopy (XPS) analysis was utilized. Fig. 4 shows the XPS spectra of C 1s region for samples. In Fig. 4a, the peak with a binding energy of 284.6 eV can be attributed to the C–C bonds, while the deconvoluted peaks centered at the binding energies of 286.9, 287.4, and 288.9 eV can be assigned to the C–O, C ¼O, and O¼C–OH functional groups, respectively [32]. C 1s spectra for the TiO2/RGO composite are shown in Fig. 4b. In the spectrum of TiO2/RGO, all peaks from oxygen-containing functional groups decreased dramatically in intensity or even disappeared entirely, indicating a significant reduction of GO by solvothermal treatment. In addition, an additional shoulder peak was found, which was usually assigned to the formation of a chemical bond between a carbon atom and a titanium atom in the lattice of TiO2, which resulted in formation of Ti–O–C bonds [33]. Fig. 5 shows the curves of thermal gravimetric analysis in air atmosphere with the rate of 20 °C/min for the C–TiO2 precursor and the graphene oxide/TiO2 composite. The C–TiO2 precursor exhibited two obvious steps of mass loss: generally, the weight loss occurring from 50 °C to 200 °C is assigned to the evaporation of free water. However, the main weight loss occurring from 200 °C to 450 °C, should be attributed to the removal of carbon spheres templates. As no obvious mass loss at temperatures higher than 450 °C was observed, it suggests that the carbon template was removed completely. As shown in Fig. 5b, with increasing temperature, a total weight loss of about 0.3% from adsorbed water vapor between room temperature and 100 °C and 2.3% from combustion of RGO in HTGC above 100 °C in air [27]. According to the TG curve, the weight content of graphene oxide in the composite was roughly evaluated to be 2.6% [34]. To clarify the function of RGO in the hollow TiO2 nanoparticles photocatalytic system, photoluminescence (PL) emission spectra have been carried out to demonstrate photogenerated charges transportation and recombination in HTGC photocatalyst. Fig. 6 shows the PL spectra of hollow TiO2 microspheres and HTGC with different graphene contents. All the samples show a broad emission peak under excitation at 300 nm. Obviously, the PL intensity of hollow TiO2 microspheres is much higher than that of TiO2/RGO samples. Therefore, the introduction of GO can effectively accept

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Fig. 2. TEM images of (a) carbon spheres, (b) C–TiO2, (c) hollow TiO2, (d) nanoparticle size of hollow TiO2, (e) GO, and (f) HTGC.

the photoexcited electrons to hinder the electron–hole recombination. The lowest PL intensity for G15–TiO2 implies the increase separation rate of photoinduced e þ –h  pairs, resulting the consistent with the photocatalytic activity of the sample [35]. The UVDRS spectra were measured to investigate the optical absorption property of TiO2/RGO nanocomposites. Fig. 7 shows the UV–vis diffuse reflectance spectra (DRS) of the HTGC with different graphene oxide contents, which were measured in the range of 100–800 nm at room temperature. As shown, HTGC enhanced the optical property in the whole visible region noticeably. The more introduction of RGO, the higher absorption capacity was acquired [36]. We can clearly see the hollow TiO2 photocatalyst shows a

fundamental absorption at about 390 nm. Compared with the hollow TiO2, a gradual red-shift to higher wavelength in the absorption edge of HTGC with the increasing of graphene oxide content have been observed, therefore indicating a narrowing of the band gap of hollow TiO2. This result revealed that the narrowing of the band gap of hollow TiO2 occurred with the graphene introduction. This narrowing can be attributed to the graphene itself which could absorb visible light [15,37] and the formation of Ti–O–C chemical bond in the as-prepared composites, which favors charge transfer upon light excitation [38,39]. As shown in Fig. 7b, the bandgap of hollow TiO2 is 3.2 eV which is similar to the reported Eg value of anatase TiO2 while the roughly estimated

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Fig. 3. FT-IR spectrum of resultant hollow TiO2, HTGC and GO.

Fig. 4. XPS spectra of the GO and HTGC. (a) C1s XPS core level of the GO and (b) C1s XPS core level of HTGC.

band gaps of G15–TiO2, G10–TiO2 and G5–TiO2 are 2.51 eV, 2.61 eV and 2.62 eV, respectively [40]. The photocatalytic degradation of rhodamine B (RhB) is selected as a model reaction to evaluate the photocatalytic activities of the hollow TiO2 and TiO2/RGO nanocomposites. Fig. 8a and b shows the degradation curves of RhB solution under UV and visible light, respectively, where C was the reaction concentrations of RhB at wavelength of 521 nm and C0 represents the initial concentrations. The photocatalytic degradation efficiency of RhB under UV light and visible light follows the order blankoP25ohollow TiO2 oG20– TiO2 oG5–TiO2 oG10–TiO2 oG15–TiO2, as shown in Fig. 8, which imply that the G15–TiO2 nanocomposite reveals to be much more efficient for the degradation of RhB. Clearly, the addition of GO in the G15–TiO2 nanocomposite should be at an appropriate amount. Further increasing the weight ratio of GO will lead to a significant

decrease. For comparison, the blank experiment without catalysts was also investigated. We found that the absence of the photocatalyst showing negligible degradation. The results show that the introduce of GO considerably enhanced photocatalytic activities both in the UV light and the visible light. In Fig. 8a, after 60 min of highpressure mercury lamp irradiation, nearly 95% of RhB was degraded with the HTGC, while Fig. 8b shows the HTGC degraded 75% of the RhB after Xenon lamp irradiation for 180 min. These performances are better than that of hollow TiO2 nanoparticles and P25 which degradation rate of RhB was about 50%. Clearly, the amount of incorporated GO has some effect on the photoactivity of the composite. All of the RGO/TiO2 composites exhibited a better photocatalytic performance. However, excess RGO might occupy active sites on the TiO2 surface and block subsequent reactions with water, dissolved O2, and dye molecules. Also, a large covering of RGO shielded some of the light arriving at TiO2, which lowered the quantum efficiency of the photocatalyst. The result concerning the effect of RGO content demonstrates that although the recombination of electron/hole can be retarded by graphene, also obstructed the light absorption [41]. So, G15–TiO2 composite present a better activity than other G–TiO2 composite. In addition, we investigated the kinetic process of RhB degradation. The degradation curves of the RhB dyestuff by UV and visible light are well fitted by a exponential curve, suggesting that a firstorder reaction model can be taken in consideration for describing the kinetic behavior.The degradation experiments follow identical firstorder kinetics with respect to the concentration of the dyestuff in the bulk solution (C):  dC/dt¼ kapp C. Integration of that equation (with the same restriction of C¼ C0 at t¼0, with C0 being the initial concentration in the bulk solution after dark adsorption) will lead to the expected relation: ln(C0/C)¼ kapp t, in which kapp is the apparent first-order rate constant and t is the reaction time. The curves of the photocatalytic degradation of RhB in Fig. 8c and d over the above catalysts showed that the above degradation reactions followed an equation: 1/C¼1/C0 þ Kapp  t, where Kapp represents reaction rate constant, C was reaction concentrations and C0 was initial concentrations. It can be seen from the Fig. 8c and d, the Kapp values of samples have a sequence as follows: blankoP25ohollow TiO2 oG20–TiO2 oG5–TiO2 oG10–TiO2 oG15–TiO2, which are corresponding to the photocatalytic activity in Fig. 8a and b. The kapp can be calculated from the slope of ln(C/C0) vs t plots. With the incremental RGO mass ratio, the kapp of TiO2/RGO nanocomposites increase from 0.0076 min  1 to 0.0249 min  1 in Fig. 8c. In Fig. 8d, the kapp increases from 0.0018 min  1 to 0.0083 min  1. A possible mechanism of the charge transfer, enhanced photocatalytic activity by HTGC under UV and visible light irradiation was put forward in Scheme. 2. The structure of TiO2/RGO nanocomposites can increase adsorption of RhB molecules and charge transfer along the RGO sheets effectively [33]. As shown in the graphical illustration in Scheme 2, during the photocatalysis, the mechanism involves two steps. This includes adsorption of the pollutant over the catalyst surface and photocatalytic degradation under illumination. Under irradiation, the TiO2 nanoparticles were excited, which lead to the creation of photogenerated holes (h þ ) in the valence band (VB) and electrons (e  ) in the conduction band (CB). The photo-induced electrons can transport rapidly and injected into RGO. It easily reacts with surface O2 to form superoxide ions, which can degrade RhB effectively due to its oxidation properties. Thereby avoiding recombination of h þ and e  during the photocatalytic process. At the same time, The h þ left in the VB of TiO2, which was the main oxidative species, will react with water to produce hydroxyl radicals, which could rapidly attack pollutants and decompose the organic dye, as well as transfer to the surface and directly oxidize the adsorbed dye molecules [42,43]. As illustrated in Scheme 2. The electron transfer between TiO2 and graphene nanosheets can be expressed as

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Fig. 5. TG curves of (a) C–TiO2 precursor and (b) graphene oxide/TiO2 composite.

Fig. 6. PL spectra of as-prepared hollow TiO2 and HTGC with different graphene oxide contents.

TiO2 þhv-e  (CB,TiO2)þ h þ (VB,TiO2) TiO2/RGO-RGO(e  ) þTiO2(h þ )

According to Scheme 2, the structure of TiO2/RGO nanocomposites can effectively increase adsorption of RhB molecules and charge transfer along the RGO sheets. The lower recombination rate and longer lifetimes of electrons and holes brought about the enhanced photocatalytic activity of the composite system. According to the above discussion, the reason that the enhanced photocatalytic performance of HTGC with the appropriate amount of GO could be attributed to several factors: 1. Graphene has been reported to be a competitive candidate for the acceptor material due to the unique 2D π-conjugation structure [44]; 2. RGO can suppress the recombination of photogenerated electrons and holes effectively and prolonged their lifetime, which serves as an acceptor of electrons; 3. complexes can achieve more solar spectrum, resulting in higher photocatalytic activity. This is consistent with the results of Fig. 7 (DRS); 4. the excellent adsorption performance of graphene material also contributes to the improvement of their photocatalytic properties; 5. hollow microspheres owned a void space encapsulated in an isolated shell, which have the good characters of low density, big specific surface area, an isolated void space with permeable shells [45].

RGO(e  )þ O2-RGO þ O2  TiO2 (h þ )þ H2O/OH  - TiO2 þOH

4. Conclusions

O2  þ RhB-degradation products

In summary, through the solvothermal method, unique graphene-based hollow TiO2 composites were prepared, which involves preparation of hollow TiO2 and loading of the hollow TiO2

 OH þRhB-degradation products

Fig. 7. (a) UV–vis diffuse reflectance spectra (DRS) of as-prepared samples and (b) estimated optical absorption edges.

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Fig. 8. Photodegradation of RhB aqueous solution under (a) UV light and (b) visible light; (c) corresponding  ln(C/C0) versus time plots of (a); (d) corresponding  ln(C/C0) versus time plots of (b).

Acknowledgments The authors appreciate the financial support from the Shanxi (20110321037-02) Provincial Science and Technology lan Foundation of China, Shanxi (2012081020) Provincial International Technological Cooperation of China, Shanxi (2015011016) Natural Science Foundation of China, and Taiyuan (110240) Bureau of Science and Technology Research Projects Foundation of China.

References Scheme 2. The possible photocatalytic mechanism of HTGC.

into GO sheet. The results showed that the HTGC are able to exhibit a high surface area, excellent structure, great optical properties. Their photocatalytic activities for the degradation of RhB were higher than that of hollow TiO2 photocatalyst under UV and visible light irradiation. The HTGC presents enormously high degradation of RhB by 75% within 3 h under the visible-light illumination whereas at the same time, using hollow TiO2 samples lead to 50% removal efficiency. Results showed that the incorporation of GO and hollow TiO2 could produce special e  and h þ transfer, which is able to facilitate the separation of the e  –h þ pairs and enhanced photocatalytic properties. A series of hollow structure materials with GO can be synthesized through the present method, which would show potential applications in photocatalysis.

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