Synthesis of Reduced Graphene Oxide Wrapped TiO 2 Ball Composites for Enhanced Photodegradation of Methylene Blue

Reduced graphene oxide wrapped TiO2 ball composite (RGO-TiO2) was synthesized via two step hydrothermal processes. SEM analysis indicated the formation of TiO2 balls with an average diameter of 1.5 μm and the RGO sheets were wrapped on the surface. XRD characterization indicated the formation of anatase phased TiO2 after the annealing treatment while UV-vis and FTIR spectroscopy confirmed that the graphene oxide in the resulting material was in the reduced form. The photocatalytic activity of the as-prepared RGO-TiO2 composite was determined by the photodegradation of methylene blue under UV illumination. The RGO-TiO2 composite showed much higher photocatalytic performance than pure TiO2 balls. More specifically, the degradation rate constant of using RGO-TiO2 composite was twice higher than pure TiO2 balls. The main reason of enhanced photocatalytic property might be the increased adsorption capacity, and the strong electron transfer ability of RGO sheets in the composites as well as the retarded charge recombination rate contributed by the energy level of the two materials. In addition, RGOTiO2 composites also exhibited an excellent reusability. We believe that this TiO2 based composite material can be effectively used as a highly active and stable photocatalyst to remove various pollutants.


Introduction
Semiconductors have been wildly used as photocatalysts to cope with worldwide water pollution problems.Among the many types of semiconductors, titanium dioxide (TiO2) has been received lots attention due to its high photocatalytic activity as well as the low cost and nontoxicity [1,2].However, the photocatalytic performance of TiO2 still restricted by the fast electro-hole pair recombination rate [3,4].In order to improve the photocatalyttic performance of TiO2, many materials have been studied to coupling with TiO2 for suppressing the charge recombination rate.Recently, studies showed the introduction of carbon materials can effectively decrease the charge recombination rate, thus enhance the photocatalytic performance of TiO2 [5,6].
Among the carbon materials, graphene, a monolayer two-dimensional graphitic carbon system, has attracted much attention since it was isolated in 2004 [7].The two-dimensional structure, large surface area, outstanding electronic and catalytic properties of graphene makes it become a suitable candidate for incorporating with TiO2.In the last few years, both graphene and reduced graphene oxide (RGO) were used for designing TiO2 based photocatalysts and showed enhanced photocatalytic activity.For example, Lei and co-workers [8] were developed a RGO/TiO2 composite by UV treatment of GO mixed with Degussa P25 TiO2.The prepared composite was used to photocatalytically reduce decabromodiphenyl ether, and the degradation rate was twice as much in comparison to pure TiO2.Gao et al. [9] synthesized a GO-TiO2 composite by a layer-by-layer method to sequentially fabricate TiO2 and GO on a polysulfone base membrane.The modified membrane was utilized to photodegradation of methylene blue in both batch and filtration experiments.The results showed the photodegradation kinetics of GO-TiO2 composite modified membrane has 60-80% higher than TiO2 modified membrane under UV illumination.
In the perspective, even though there are already a number of RGO-TiO2 composites have been developed and investigated, but most of them are mainly focused on nano-sized TiO2.It is well know that the nano-sized TiO2 is prone to aggregation and can hardly be separated from the aqueous system, which highly limits its practical applications.Thus, design a micro-sized TiO2-graphene composite is much suitable for water treatment purpose.Herein, we reported the preparation of RGO wrapped TiO2 ball composite by two step hydrothermal processes.The as-prepared RGO-TiO2 composite showed enhanced photocatalytic performance towards photodegradation of methylene blue (MB).

Synthesis of RGO-TiO2 composites.
The synthesis of TiO2 balls was based on a hydrothermal method reported by Liu et al. [10] with some modifications.Briefly, 1 mL titanium butoxide was added into 10 mL acetic acid and stirring for 0.5 h.The mixture then transfer to a 20 mL Teflon-lined stainless steel autoclave.The autoclave was heated to 140°C and maintained for 10 h in an oven and naturally cooled down to the room temperature.The sediment was collected and centrifuged few times (5000 rmp, 10 min) followed by a water wash to remove the excess acetic acid.The result solid was then heated to 300°C in a furnace for 3 h to form crystallised TiO2.
In order to synthesis of RGO-TiO2 composite, PDDA was invoked as binding material.In a typical synthesis process, 1 mL PDDA was added into a 5 mL dispersion contains 50 mg TiO2.The dispersion was kept stirring for 1 h to achieve PDDA functionalized TiO2.The excess PDDA was removed by three times centrifugation followed by water wash.Then, 1 mL GO (1 mg/mL) was added into a 5 mL dispersion contains 50 mg PDDA functionalized TiO2.The mixture was stirring for 1 h and transfer to a 20 mL Teflon-lined stainless steel autoclave.The autoclave was heated to 120°C and maintained for 2 h to reduce GO.The finial RGO-TiO2 composite was collected by centrifugations and dried overnight at 80°C.

Characterization
FTIR was performed using a Nicolet iS5 (Thermo Scientific, USA).UV-vis spectra were carried out in the wavelength range from 190 to 800 nm.Surface morphology of samples were analyzed by a field emission scanning electron microscope (FeSEM, ZEISS SUPRA 40VP combined with EDX, Germany).The crystal structure of samples was collected from 5° to 90° in 2θ by a XRD with Cu Kα radiation (D8-Advanced, Bruker, Germany).

Photocatalytic degradation of MB:
The photocatalytic activities of different samples were determined by degradation of MB in solution using a UV lamp as light source (100 W Oliphant Bros, Pty.Ltd, Australian).In a typically process, 20 mg photocatalysts were added into a quartz tube containing 50 mL MB solution (20 mg/L).Prior to the illumination, the suspension was magnetically stirred in the dark for 30 min to reach the adsorptiondesorption equilibrium.At given time intervals, 2 mL of suspension was sampled and centrifuged, the supernatant was collected for absorption analysis on a UV-vis spectrophotometer.The absorbance of MB at 664 nm was used for measuring the residual dye concentration.

Characterization of RGO-TiO2 composites
In order to confirm the reduction of GO during the hydrothermal treatment, the RGO-TiO2 composite was sonicated for 0.5 h and centrifugated to collect the free RGO dispersion.Fig 1a shows the UV-vis spectra of GO and RGO dispersions.It can be observed that the GO dispersion exhibits a characteristic absorption peak at 227 nm corresponds to the π→π* transition of the C═C bonds.The RGO dispersion displays two main absorption peaks at 200 nm and 259 nm, which corresponding to the absorption of free PDDA [11] and the excitation of π-plasmon of graphitic bond [12], receptivity.This red shift from 227 to 259 nm indicates the successful achievement of RGO through the hydrothermal reaction.
The reduction process was further evidenced by FTIR analysis.As shown in Fig 1b, the GO spectrum shows several peaks at 2946 cm -1 , 1734 cm -1 , 1398 cm -1 and 1047 cm -1 , which belong to the stretching vibration of CH2, C═O stretching of COOH groups, C━OH stretching vibrations and C━O vibrations from alkoxy groups, respectively.After hydrothermal reaction, these peaks are dramatically decreased or even vanished, indicating the successful reduction of RGO.The top-view SEM images of TiO2 ball and RGO-TiO2 composite are shown in Fig. 2a-c.The pure TiO2 ball shows a rough surface with approximately 1.5 μm in diameters (Fig. 2a).This rough surface morphology produces a larger surface area to absorb dye molecules even the size of the particle is in the micro range.After loading with RGO sheets, the composite resembles a hydrangea-like shape.Under higher magnification view, it can be clearly observed that the surface of TiO2 ball represent nano-ribbon "petals", which indicating the TiO2 was wrapped with RGO sheets.The crystalline phase of RGO-TiO2 composite was examined by XRD.A series of characteristic peaks is noted in the XRD pattern at 2 θ of 24.9°, 35.6°, 47.8°, 54.0°, 54.0°and 62.4° which related to the (101), (004), ( 200), (105), ( 211) and (204) planes of anatase TiO2.Moreover, the broad peak located abound 25.0° illustrates the presence of exfoliated RGO.

Photodegradation of MB
According to the literatures, the typical work functions of TiO2 and graphene are 4.40 eV and 4.42 eV, receptivity [13].Thus, under UV irradiation, the electrons produced from CB of TiO2 can be transferred RGO, which could effectively separation of electrons and hinder the charge recombination.In this case, the photocatlytic activities of the as-prepared samples were investigated by the photodegradation of MB under UV illumination.As shown in Fig. 3a, the MB itself could slightly decomposed under UV light.After 90 min illumination, 8% of MB was degraded.It was also noted that the RGO sheets showed a higher adsorption capability but feeble photocatalytic ability.Moreover, the photodegradation rate of TiO2 ball was outperformed by the RGO-TiO2 composite.For the quantitative measurement of the difference in photocatalytic performances, the first order degradation rate constant (Kapp) was calculated.As shown in Fig. 3b, the Kapp of RGO-TiO2 (0.04002/min) was found to be almost double than that of TiO2 (0.02036/min), implying the introduction of RGO can highly accelerate the photodegradation rate.

Conclusion
Using two step hydrothermal reactions, RGO wrapped TiO2 ball composite was synthesized.According to the UV-vis spectroscopy and FTIR results, the RGO was formed from GO during the second step of hydrothermal treatment.The photocatalytic activity tests demonstrated that the incorporation of RGO with TiO2 could highly enhance the photocatalytic performance towards degradation of MB under UV illumination.

Fig. 3 .
Fig. 3. (a): Comparison of MB photodegradation in the presence of different photocatalysts.(b): The rate constant of different photocatalysts in degradation of MB.