Reduced graphene oxide-TiO2 nanocomposite as a promising visible-light-active photocatalyst for the conversion of carbon dioxide
© Tan et al.; licensee Springer. 2013
Received: 21 August 2013
Accepted: 11 October 2013
Published: 6 November 2013
Photocatalytic reduction of carbon dioxide (CO2) into hydrocarbon fuels such as methane is an attractive strategy for simultaneously harvesting solar energy and capturing this major greenhouse gas. Incessant research interest has been devoted to preparing graphene-based semiconductor nanocomposites as photocatalysts for a variety of applications. In this work, reduced graphene oxide (rGO)-TiO2 hybrid nanocrystals were fabricated through a novel and simple solvothermal synthetic route. Anatase TiO2 particles with an average diameter of 12 nm were uniformly dispersed on the rGO sheet. Slow hydrolysis reaction was successfully attained through the use of ethylene glycol and acetic acid mixed solvents coupled with an additional cooling step. The prepared rGO-TiO2 nanocomposites exhibited superior photocatalytic activity (0.135 μmol gcat−1 h−1) in the reduction of CO2 over graphite oxide and pure anatase. The intimate contact between TiO2 and rGO was proposed to accelerate the transfer of photogenerated electrons on TiO2 to rGO, leading to an effective charge anti-recombination and thus enhancing the photocatalytic activity. Furthermore, our photocatalysts were found to be active even under the irradiation of low-power energy-saving light bulbs, which renders the entire process economically and practically feasible.
Global warming caused by large-scale emission of carbon dioxide (CO2) in the atmosphere and the depletion of fossil fuels are two critical issues to be addressed in the near future . Great effort has been made to reduce CO2 emissions. Technologies involving carbon capture and geological sequestration have accelerated in the past decade . Unfortunately, most of the associated processes require extraneous energy input, which may result in the net growth of CO2 emission. Furthermore, there are many uncertainties with the long-term underground storage of CO2. In this regard, the photocatalytic reduction of CO2 to produce hydrocarbon fuels such as methane (CH4) is deemed as an attractive and viable approach in reducing CO2 emissions and resolving the energy crisis [3, 4]. Many types of semiconductor photocatalysts, such as TiO2, ZrO2, CdS , and combinations thereof  have been widely studied for this purpose.
By far the most researched photocatalytic material is anatase TiO2 because of its long-term thermodynamic stability, strong oxidizing power, low cost, and relative nontoxicity [9, 10]. However, the rapid recombination of electrons and holes is one of the main reasons for the low photocatalytic efficiency of TiO2. Moreover, its wide band gap of 3.2 eV confines its application to the ultraviolet (UV) region, which makes up only a small fraction (≈5%) of the total solar spectrum reaching the earth's surface . In order to utilize irradiation from sunlight or from artificial room light sources, the development of visible-light-active TiO2 is necessary.
In the past few years, carbon-based TiO2 photocatalysts have attracted cosmic interest for improved photocatalytic performance [12, 13]. Graphene, in particular, has been regarded as an extremely attractive component for the preparation of composite materials [14, 15]. In addition to its large theoretical specific surface area, graphene has an extensive two-dimensional π-π conjugation structure, which endows it with excellent conductivity of electrons . Carriers in pristine graphene sheets have been reported to behave as massless Dirac fermions . When combining TiO2 nanocrystals with graphene, excited electrons of TiO2 could transfer from the conduction band (CB) to graphene via a percolation mechanism . The heterojunction formed at the interface (termed Schottky barrier) separates the photoinduced electron–hole pairs, thus suppressing charge recombination . The enhancement of photocatalytic activity of graphene-based semiconductor–metal composites was first demonstrated by Kamat and co-workers in 2010 . Following that, Zhang et al. , Shen et al. , and Zhou et al.  carried out one-step hydrothermal methods to prepare graphene-TiO2 hybrid materials and showed that the composites exhibited enhanced photoactivity towards organic degradation over bare TiO2. Fan et al.  fabricated P25-graphene composites by three different preparation methods, i.e., UV-assisted photocatalytic reduction, hydrazine reduction, and hydrothermal method, all of which possessed significantly improved photocatalytic performance for H2 evolution from methanol aqueous solution as compared to pure P25. To the best of our knowledge, the study on the use of graphene-TiO2 composites on the photoreduction of CO2 is still in its infancy. This leads to our great interest in studying the role of graphene in the composite towards the photoreduction of CO2 into CH4 gas under visible light irradiation.
In this paper, we present a simple solvothermal method to prepare reduced graphene oxide-TiO2 (rGO-TiO2) composites using graphene oxide (GO) and tetrabutyl titanate as starting materials. During the reaction, the deoxygenation of GO and the deposition of TiO2 nanoparticles on rGO occurred simultaneously. The photoactivity of the as-prepared rGO-TiO2 composite was studied by evaluating its performance in the photoreduction of CO2 under visible light illumination. In contrast to the most commonly employed high-power halogen and xenon lamps, we used 15-W energy-saving light bulbs to irradiate the photocatalyst under ambient condition. This renders the entire process practically feasible and economically viable. The rGO-TiO2 composite was shown to exhibit excellent photocatalytic activity as compared to graphite oxide and pure anatase.
Graphite powder, tetrabutyl titanate (TBT), acetic acid (HAc), and ethylene glycol (EG) were supplied by Sigma-Aldrich (St. Louis, MO, USA). All reagents were of analytical reagent grade and were used without further purification.
Synthesis of reduced graphene oxide-TiO2 composite
Graphite oxide was prepared from graphite powder by modified Hummers' method [23–25]. The detailed experimental procedure is given in Additional file 1. To obtain GO sheets, graphite oxide was dispersed into distilled water (0.5 g L−1) and ultrasonicated for 1 h at ambient condition. The solution was then chilled to ≈ 5°C in an ice bath. Meanwhile, a titanium precursor composed of 1.5 mL TBT, 7.21 mL EG, and 1.14 mL HAc was also chilled to ≈ 5°C in an ice bath. The mixture was then added dropwise into the chilled GO aqueous solution under vigorous stirring. Subsequently, the GO-TiO2 stock solution was transferred into a 200-mL Teflon-lined stainless steel autoclave and was heated at 180°C for 8 h. The greyish-black precipitate was harvested by centrifugation (5,000 rpm, 30 min) and was washed with ethanol several times to remove undecorated TiO2 particles, unreacted chemicals, and residual EG. Finally, the product was dried in an air oven at 60°C overnight before characterization.
Morphology observation was performed using an SU-8010 field emission scanning electron microscope (FESEM; Hitachi Ltd., Tokyo, Japan) equipped with an Oxford-Horiba Inca XMax50 energy-dispersive X-ray (EDX; Oxford Instruments Analytical, High Wycombe, England). High-resolution transmission electron microscopy (HRTEM) was conducted with a JEOL JEM-2100 F microscope (JEOL, Tokyo, Japan) operating at 200 kV. The X-ray powder diffraction data were obtained on a Bruker AXS (Madison, WI, USA) D8 Advance X-ray diffractometer with CuKα radiation (λ = 0.15406 nm) at a scan rate (2θ) of 0.02° s−1. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. The crystallite size measurements of anatase TiO2 were quantitatively calculated using Scherrer's equation (d = kλ/β cos θ) where d is the crystallite size, k is a constant (=0.9 assuming that the particles are spherical), β is the full width at half maximum (FWHM) intensity of the (101) peak in radians, and θ is Bragg's diffraction angle . Raman spectra were recorded at room temperature on a Renishaw inVia Raman microscope (Renishaw, Gloucestershire, UK). UV-visible absorption spectra for the samples were collected with an Agilent Cary-100 UV-visible spectroscope (Agilent Technologies, Santa Clara, CA, USA). A Nicolet iS10 Fourier transform infrared (FTIR) spectrometer (Thermo Scientific, Logan, UT, USA) was used to record the FTIR spectra of all samples.
Photocatalytic CO2 reduction experiment
The photocatalytic experiment for the reduction of CO2 was conducted at ambient condition in a homemade, continuous gas flow reactor. A 15-W energy-saving daylight bulb (Philips, Amsterdam, Netherlands) was used as the visible light source. The catalyst powder was first fixed into a quartz reactor. Highly pure CO2 (99.99%) was bubbled through water (sacrificial reagent) to introduce a mixture of CO2 and water vapor into the photoreactor at ambient pressure. Prior to irradiation, CO2 was purged inside the reactor for 30 min to remove the oxygen and to ensure complete adsorption of gas molecules. The light source was then turned on to initiate photocatalytic reaction. The generated gases were collected at 1-h intervals and were analyzed by a gas chromatograph (GC), equipped with a flame ionization detector (FID) (Agilent, 7890A) to determine the yield of CH4. Control experiments were also carried out in the dark, and no product gases were detected for all tested catalysts. This indicates that light irradiation was indispensable for the photoreduction of CO2 to CH4.
Results and discussion
Characterization of reduced graphene oxide-TiO2 composites
It is known that few-layer rGO sheets have the tendency to aggregate back to the graphite structure due to strong van der Waals interaction . Therefore, the crystallization of TiO2 on the surface of rGO was particularly helpful in overcoming this interaction, which could ultimately alleviate the agglomeration and restacking of the graphene sheets. In addition, the intimate connection would allow the electrons to transfer easily from TiO2 to rGO sheets during the photoexcitation process, which could significantly increase the separation of photoinduced charges and enhance the photocatalytic activity.
Photocatalytic reduction of CO2 with H2O and mechanism
On the basis of our experimental data, it is proposed that the synergistic dyade structure of the rGO-TiO2 composite provided access to an optically active charge transfer transition. In other words, rGO and anatase TiO2 formed a joint electronic system. The enhancement in photocatalytic activity could be attributed to the combined effect of several concomitant factors. Firstly, the band gap narrowing of the rGO-TiO2 composite (3.2 eV → 2.90 eV) allowed an enhanced absorption of visible light. The CB of anatase TiO2 and the work function of rGO are −4.2 eV  and −4.42 eV , respectively. Such energy levels were beneficial for the photogenerated electrons to transfer from the TiO2 CB to the rGO, which could effectively separate the charge carriers and hinder electron–hole recombination. In the absence of rGO, most of these charges tend to recombine rapidly without undergoing any chemical reaction . This is primarily due to the adsorption kinetic of the CO2 molecules (10−8 to 10−3 s) on TiO2 being slower than the electron–hole recombination time (10−9 s) [47, 54].
In summary, a visible-light-active rGO-based TiO2 photocatalyst was developed by a facile, one-pot solvothermal method. To control the hydrolysis reaction rate of water-sensitive TBT, we employed EG and HAc mixed solvent coupled with an additional cooling step in our synthesis procedure. Anatase TiO2 nanoparticles with an average crystallite size of 12 nm were homogeneously anchored onto the rGO sheets with close interfacial contact. The activity of the rGO-TiO2 composite was tested by the photocatalytic reduction of CO2 under visible light irradiation. The composite displayed excellent photocatalytic activity, achieving a maximum CH4 product yield of 0.135 μmol gcat−1 h−1, which is 2.1- and 5.6-fold higher than that achieved by graphite oxide and pure anatase. The incorporation of rGO into the composite led to the reduction of band gap, rendering the rGO-TiO2 hybrid material sensitive to visible light irradiation (λ < 400 nm). In addition, the photoinduced electrons can easily migrate to the rGO moiety, leading to the efficient separation and prolonged recombination time of charge carriers. These contributions, together with increased reactant adsorption, are the primary factors in the enhancement of the rGO-TiO2 photoactivity. In contrast to the most commonly used high-power halogen and xenon arc lamps, we demonstrated that our photocatalysts were active even under the irradiation of low-power, energy-saving light bulbs. Interestingly, we have also found that graphite oxide was active in the photoconversion of CO2 into CH4 gas under visible light irradiation. Ongoing research is being carried out to develop more complex rGO-based semiconducting materials for the efficient conversion of CO2. We believe that our findings could open up a scalable and cost-effective approach to obtain robust materials for photocatalytic applications.
The work was funded by the Ministry of Higher Education (MOHE), Malaysia, under the Long-Term Research Grant Scheme (LRGS) (acc. no.: 2110226-113-00) and the Fundamental Research Grant Scheme (FRGS) (ref. no.: FRGS/1/2013/TK05/02/1MUSM).
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