Journal of Nanomaterials
Volume 2016 (2016), Article ID 6012896, 5 pages
http://dx.doi.org/10.1155/2016/6012896

Research Article

Photocatalytic Reduction of CO₂ Using TiO₂-Graphene Nanocomposites

Jinghua Liu,¹ Yinghua Niu,2,3 Xiong He,¹ Jingyao Qi,⁴ and Xin Li1,4

¹Department of Chemistry, Harbin Institute of Technology, Harbin 150090, China
²School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China
³School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China
⁴State Key Lab of Urban Water Resources and Environment, Harbin Institute of Technology, Harbin 150090, China

Received 23 November 2015; Accepted 7 February 2016

Academic Editor: Godwin Ayoko

Copyright © 2016 Jinghua Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

TiO₂-graphene (TiO₂-RGO) nanocomposites were prepared via a simple chemical method by using graphene oxide (GO) and TiO₂ nanoparticles as starting materials. The morphologies and structural properties of the as-prepared composites were characterized by X-ray diffraction, Raman spectroscopy, N₂ adsorption-desorption measurements, and transmission electron microscopy. TiO₂-RGO nanocomposites exhibited great photocatalytic activity toward reduction of CO₂ into CH₄ (2.10 μmol g−1 h⁻¹) and CH₃OH (2.20 μmol g⁻¹ h⁻¹), which is attributed to the synergistic effect between TiO₂ and graphene.

1. Introduction

The past few decades have witnessed the problem of global warming, which is dominantly caused by carbon dioxide emissions from fossil fuel consumption. Therefore, seeking for any alternative to minimize CO₂ emission has attracted increasing attention all over the world. The traditional method used to solve this problem is carbon capture and storage, but in fact this problem has never been solved radically because CO₂ does not convert into other substances [1]. The photocatalytic reduction of CO₂ into hydrocarbon fuels was found to be a prospective way [2].

TiO₂ is one of the most promising semiconductor materials used in the photocatalysis due to its excellent stability, nontoxicity, and low price [3]. Graphene, one atomic sp²-bonded planar carbon nanosheet, is a nearly ideal 2D nanoscale carbon material [4]. Due to the unique structure and excellent electronic, thermal, and mechanical properties, graphene is expected to be catalyst supports [5, 6]. The combination of graphene with TiO₂ can create composites with both the outstanding characters and functions of two components and lead to some additional novel properties. Thus, graphene-based photocatalyst possesses numerous opportunities for the photocatalytic CO₂ reduction field [7].

In the previous researches, Wang et al. prepared TiO₂-RGO composite via a single-step aerosol approach, and they also investigated the role of synthesis temperature and TiO₂/GO ratio in CO₂ photoreduction [8]. Tu et al. developed a simultaneous reduction-hydrolysis technique to fabricate TiO₂-graphene hybrid nanosheets for the reduction of CO₂ into CH₄ and C₂H₆ [9]. Tan et al. reported a series of noble metal nanoparticles supported on reduced graphene oxide/TiO₂ via a solvothermal method, which exhibited enhanced photocatalytic activities toward the photoreduction of CO₂ into CH₄ gas [10]. Sim et al. reported rapid thermal reduced graphene oxide/Pt-TiO₂ nanotube arrays for enhanced photocatalytic reduction of CO₂ into CH₄ [11]. Kuai et al. achieved a CdS/reduced graphene oxide/TiO₂ core-shell nanostructure, which yielded greater improvements in the photoreduction of CO₂ [12]. Hasan et al. showed the photoelectrocatalytic reduction of CO₂ to CH₃OH and HCOOH using a Cu-RGO-TiO₂ photoelectrocatalyst [13]. Xing et al. coupled boron-doped graphene nanosheets with TiO₂ nanoparticles for enhancing CO₂ photoreduction [14]. Cheng et al. provided a photoelectrocatalytic technique for reduction of CO₂ into chemicals [15]. Liang et al. demonstrated greater improvements in the photoreduction of CO₂ by a graphene-TiO₂ nanocomposite with low graphene defect densities [16].

Herein, we present a simple and relatively general approach for synthesis of TiO₂-RGO nanocomposites using GO and TiO₂ nanoparticles as starting materials. It is important that the procedure for preparing TiO₂-RGO nanocomposites can be carried out under ambient conditions. The high photocatalytic activity of TiO₂-RGO nanocomposites is confirmed by photocatalytic conversion of CO₂ to reusable hydrocarbons (CH₄ and CH₃OH).

2. Experimental

2.1. Synthesis of TiO₂-RGO Nanocomposites

GO was prepared according to the modification of Hummers methods as described previously [17]. Typically, 0.05 g as-prepared GO dissolves in the 250 mL water in the 500 mL round-bottom flask, yielding an inhomogeneous yellow-brown dispersion. This dispersion was sonicated 20 min using an ultrasonic bath cleaner (Fisher Scientific KQ5200) until it became clear with no visible particulate matter. TiO₂ nanoparticles (0.10 g) were then added into the flask, and the mixture was stirred for 24 h at room temperature. Then hydrazine monohydrate (3.00 mL, 50%) was then added to the above as-prepared product (TiO₂-GO), and the suspension was heated in an oil bath at 100°C under a water-cooled condenser for 24 h, where the RGO sheets gradually precipitated out as a black solid and gradually form the TiO₂-RGO. This product (Cat-2) was isolated by filtration over a medium fritted glass funnel, washed copiously with water and methanol, and dried under vacuum at 60°C. Cat-3 was synthesized using a similar approach and only changed the amount of GO to 0.1 g. Pure TiO₂ was marked as Cat-1.

2.2. Materials Characterization

The samples were characterized by Raman spectrometer (JobinYvon HR 800). Transmission electron microscope (TEM) micrographs were obtained on a Tecnai G20 (Philip) electron microscope. Brunauer-Emmett-Teller (BET) surface area of the catalysts was analyzed by N₂ adsorption and desorption isotherms with a micromeritics ASAP 2020™ physisorption system. X-Ray diffraction (XRD) analysis was recorded on a XRD-6000 X-ray diffractometer (Shimadzu) with Cu Kα radiation ( Å).

2.3. Evaluation of Photocatalytic Activity

The photocatalytic activity of the prepared TiO₂-RGO nanocomposites was studied in a double-wall cylindrical quartz reactor at ambient condition with continuous gas flow. Briefly, 0.1 g catalyst was dispersed in 100 mL distilled water containing KOH (0.2 mol L⁻¹). Before the reaction, CO₂ was bubbled into the solution for 30 min to remove any excess air and achieve the adsorption-desorption equilibrium. High pressure mercury lamp was used as the light source (250 W). CH₄ was detected as the sole product in the outlet gas based on gas chromatograph analysis. Methanol was detected by ultraviolet visible spectrophotometer.

3. Results and Discussion

3.1. Characterization of TiO₂-RGO Nanocomposites

Figure 1 shows a schematic procedure for fabricating TiO₂-RGO nanocomposites. In this straightforward procedure, GO was firstly prepared from graphite powder, followed by chemical deposited TiO₂ nanoparticles onto GO sheet; the as-prepared materials were reduced through a hydrothermal reaction to obtain TiO₂-RGO nanocomposites.

Figure 1: Illustration of the preparation of TiO₂-RGO nanocomposites.

Figure 2 shows the TEM images of the as-formed TiO₂-RGO nanocomposites. As seen from Figure 2, many nanoparticles adhered to the graphene sheets with little agglomeration, which confirms the successful immobilizing TiO₂ nanoparticles on the graphene sheet. The reason for the effective assembly is that TiO₂ nanoparticles can interact with functional groups such as epoxides, hydroxyl, and carboxylic acids on the GO sheets.

Figure 2: TEM image of TiO₂-RGO nanocomposites.

XRD characterization was conducted to determine the crystal structural about GO and TiO₂-RGO, and the results are shown in Figure 3. The pure GO showed a sharp peak at °, corresponding to the GO (001) plane with an interlayer spacing of 0.83 nm. It can be found that the interlayer spacing of GO is larger than that of the natural graphite powder because of the introduction of oxygen-containing functional groups onto the surface of carbon sheet. As for TiO₂-RGO, the feature peak of the GO has disappeared and a very broad peak at °, corresponding to graphene (002) plane with an interlayer spacing of 0.40 nm, was observed, which confirms the reduction of GO to graphene in the hydrothermal environment. However, there was a small amount of residual oxygen-containing functional groups in the as-formed TiO₂-RGO nanocomposites due to the incomplete reduction of GO sheet to graphene [18]. The same phenomenon was observed in previous studies [19]. The reflection peaks at , 35.92, 37.76, 48.14, 53.94, 55.20, and 62.70° can be indexed to (101), (103), (004), (200), (105), (211), and (204) crystal planes of anatase TiO₂. The XRD analysis further confirms the successful preparation of TiO₂-RGO nanocomposites.

Figure 3: XRD patterns of (a) GO and (b) TiO₂-RGO.

Raman spectroscopy was utilized to examine the information about structural changes during the fabrication process. As shown in Figure 4(a), the Raman spectra of RGO exhibited a D-band peak at 1362.6 cm⁻¹ and a G-band peak at 1574.7 cm⁻¹. In contrast, the G-band shifts by 6.2 cm⁻¹ to a high frequency at 1580.9 cm⁻¹ for TiO₂-RGO nanocomposites (Figure 4(b)), which suggests the occurrence of charge transfer between the GO sheet and TiO₂ nanoparticles. It is well known that the intensity ratio of D-band to G-band () is characteristic of the extent of disorder present within the material [20]. In our case, was 0.96 for TiO₂-RGO, and it increased compared to that of the RGO (), exhibiting a greater sp² character. Besides, the ratio of the intensities for TiO₂-RGO is slight increased, which confirms the presence of donor molecules on graphene.

Figure 4: Raman spectra of (a) RGO and (b) TiO₂-RGO.

To further investigate the specific surface area of TiO₂-RGO, the N₂ adsorption-desorption measurements were carried out at 77 K. Figure 5 shows that the nitrogen adsorption isotherm is a typical IV type curve. Additionally, the loop observed is ascribed as type H3 loops confirming the presence of a mesoporous structure. The specific BET surface area of the as-prepared TiO₂-RGO was measured to be 80.18 m² g⁻¹.

Figure 5: Typical nitrogen adsorption-desorption isotherm of TiO₂-RGO.

3.2. Photocatalytic Properties of TiO₂-RGO Composites under Ultraviolet Light Irradiation

To evaluate the photocatalytic activity of TiO₂-RGO samples, the photocatalytic CO₂ conversion in the presence of KOH solution was investigated. Figure 6 shows that CO₂ can be photoreduced to CH₃OH by using pure TiO₂ sample (Cat-1). With the addition of graphene amount of photocatalysts, CO₂ can be photoreduced to CH₃OH and CH₄. It is obvious that Cat-3 (2.20 μmol g⁻¹ h⁻¹ CH₃OH and 2.10 μmol g⁻¹ h⁻¹ CH₄) exhibits much higher activity than Cat-2 (2.0 μmol g⁻¹ h⁻¹ CH₃OH and 1.34 μmol g⁻¹ h⁻¹ CH₄). The CH₃OH yield with TiO₂ was 1.83 μmol g⁻¹ h⁻¹. The results demonstrated that the photocatalytic efficiency increased with the increases of graphene amount. With the graphene amount increased from 0 to 50%, the yields of CH₄ and CH₃OH increased simultaneously. It is attributed to the increasing separation efficiency of the photogenerated electrons, because the existence of graphene could offer more active adsorption sites and photocatalytic reaction centers [21]. Furthermore, the yield efficiency increased with the increase of dispersion degree.

Figure 6: Comparison of photocatalytic activity of samples Cat-1, Cat-2, and Cat-3.

4. Conclusions

In conclusion, we present a simple and relatively general approach for synthesis of TiO₂-RGO nanocomposites under ambient conditions by using GO and TiO₂ nanoparticles as starting materials. The results show that CO₂ can be photoreduced to CH₃OH and CH₄ by using TiO₂-RGO samples as photocatalysts. The yields of CH₄ and CH₃OH can reach 2.10 μmol g⁻¹ h⁻¹ and 2.20 μmol g⁻¹ h⁻¹. It is hoped that our current work could pave a way toward the fabrication of TiO₂-RGO hybrid materials and facilitate their significant applications in various fields.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

The authors are grateful for financial support from the National Natural Science Foundation of China (Grant no. 51178142).

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