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Reduced graphene oxide-TiO2 nanocomposite as a promising visible-light-active photocatalyst for the conversion of carbon dioxide

Lling-Lling Tan1, Wee-Jun Ong1, Siang-Piao Chai1* and Abdul Rahman Mohamed2

Author Affiliations

1 Low Carbon Economy (LCE) Group, Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, Selangor 46150, Malaysia

2 Low Carbon Economy (LCE) Group, School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, Nibong Tebal, Pulau Pinang 143000, Malaysia

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Nanoscale Research Letters 2013, 8:465  doi:10.1186/1556-276X-8-465


The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/8/1/465


Received:21 August 2013
Accepted:11 October 2013
Published:6 November 2013

© 2013 Tan et al.; licensee Springer.

This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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.

Keywords:
Titanium dioxide; Nanoparticle; Graphene; Composite; Photocatalyst; Carbon dioxide; Methane

Background

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 [1]. Great effort has been made to reduce CO2 emissions. Technologies involving carbon capture and geological sequestration have accelerated in the past decade [2]. 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[5], ZrO2[6], CdS [7], and combinations thereof [8] 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 [11]. 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 [16]. Carriers in pristine graphene sheets have been reported to behave as massless Dirac fermions [17]. When combining TiO2 nanocrystals with graphene, excited electrons of TiO2 could transfer from the conduction band (CB) to graphene via a percolation mechanism [18]. The heterojunction formed at the interface (termed Schottky barrier) separates the photoinduced electron–hole pairs, thus suppressing charge recombination [16]. The enhancement of photocatalytic activity of graphene-based semiconductor–metal composites was first demonstrated by Kamat and co-workers in 2010 [18]. Following that, Zhang et al. [19], Shen et al. [20], and Zhou et al. [21] 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. [22] 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.

Methods

Materials

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.

Additional file 1. Preparation of graphite oxide powder. Detailed experimental procedure with two accompanying figures.

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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 = /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 [26]. 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

Synthesis strategy

GO sheets possess a rich assortment of oxygen-containing groups, such as epoxide, hydroxyl, carbonyl, and carboxylic groups [27]. Therefore, titanium alkoxides, in this case TBT, can be readily grafted onto the surface of GO through chemical adsorption at the molecular level [28]. On the other hand, it is widely known that titanium alkoxides are extremely sensitive to water. Rapid decomposition of the titanium precursor would result in the agglomeration of TiO2 crystals as well as hinder the homogeneous growth of TiO2 onto GO. Hence, EG and HAc were introduced into the mixture to co-control the hydrolysis rate of TBT [29]. In addition, the mixtures were also prechilled in an ice bath to further reduce the hydrolysis rate. During the solvothermal treatment, GO was reduced to rGO, and TiO2 nanoparticles formed on the rGO surface simultaneously. The preparation strategy is illustrated in Figure 1.

thumbnailFigure 1. Procedure for the synthesis of rGO-TiO2 nanocomposites.

Characterization of reduced graphene oxide-TiO2 composites

The surface morphology and structure of the rGO-TiO2 nanocomposite were characterized using FESEM and HRTEM. From Figure 2a, b, it is observed that the surface of rGO sheets was packed densely with TiO2 nanoparticles, which displayed a good combination of rGO and TiO2. Despite that, the profile of a single TiO2 nanoparticle could be clearly distinguished, indicating that the aggregation of TiO2 was well prevented during the preparation process. The TiO2 particles were also found to exhibit a narrow size distribution with an average crystallite size of 12 nm. The corresponding HRTEM images (Figure 2c, d) clearly showed the lattice fringes of rGO, which were parallel to the edges of the TiO2 nanoparticles. The lattice spacing of TiO2 was measured to be ca. 0.35 nm, which corresponds to the (101) plane of anatase TiO2 (JCPDS no. 2101272). The rGO sheets were composed of a mixture of two to five layers based on HRTEM observations.

thumbnailFigure 2. Electron microscopy of the rGO-TiO2 composites. (a) FESEM image, (b, c) HRTEM images, and (d) enlarged HRTEM image of a selected rGO-TiO2 heterojunction.

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 [30]. 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.

Raman spectroscopy has been accepted to be a powerful and nondestructive tool to characterize the quality of graphitic materials. The significant structural changes resulting from the solvothermal reaction process from GO to rGO were also reflected in the Raman spectra. Figure 3 (spectra a and b) shows the Raman measurements of graphite before and after the modified Hummers' method. There were two characteristic peaks in the spectrum of graphite: the D (disordered) peak centered at 1,347 cm−1 and the G (graphitic) peak at 1,582 cm−1. The D band is attributed to the disruption of the symmetrical hexagonal graphitic lattice as a result of edge defects, internal structural defects, and dangling bonds. On the other hand, the G band is due to the in-plane stretching motion of symmetric sp2 C-C bonds. A narrower G band indicates that fewer functional groups (i.e., non-C-C bonds) are present [31]. After the oxidation of graphite, the Raman spectrum of graphite oxide showed that the G band was broadened, while the intensity of the D band was increased significantly. These observations were ascribed to the substantial decrease in size of the in-plane sp2 domains, resulting from the introduction of oxygen-containing groups. In addition, the shift in the G band from 1,582 to 1,609 cm−1 was possibly due to the presence of isolated double bonds on the carbon network of graphite oxide [32]. It has been reported that isolated double bonds tend to resonate at higher frequencies as compared to the G band of graphite [33]. Figure 3 (spectrum c) shows the Raman spectrum of the rGO-TiO2 composite. The typical modes of anatase could be clearly observed, i.e., the Eg(1) peak (148 cm−1), B1g(1) peak (394 cm−1), Eg(2) peak (637 cm−1), and the A1g + B1g(2) modes centered at 512 cm−1, respectively [34]. The two characteristic peaks at about 1,328 and 1,602 cm−1 for the graphitized structures were also observed in the Raman spectrum of the rGO-TiO2 composite. The composite showed an increase in ID/IG ratio as compared to graphite oxide, indicating a decrease in the average size of the in-plane sp2 domains of C atoms in the composite, which is similar to that observed in chemically reduced GO [35].

thumbnailFigure 3. Raman spectra of (spectrum a) graphite powder, (spectrum b) graphite oxide, and (spectrum c) rGO-TiO2 composite.

Figure 4 shows the XRD patterns of graphite oxide and the rGO-TiO2 composite. The XRD pattern of graphite oxide (Figure 4, spectrum a) showed that the interlayer distance obtained from the characteristic (001) peak is ≈ 0.93 nm (2θ = 9.50°), which matches well with the values reported in literature [16,20,36]. This confirmed that most of the graphite powder was oxidized into graphite oxide by expanding the d spacing from 0.34 to 0.93 nm [20,37]. The large interlayer distance of graphite oxide could be attributed to the presence of oxygen-containing functional groups such as hydroxyl, carboxyl, carbonyl, and epoxide [38]. Figure 4 (spectrum b) shows the XRD patterns of the rGO-TiO2 composite. The peaks at 25.3°, 37.8°, 48°, 53.9°, 55.1°, 62.7°, 68.8°, 70.3°, and 75.0° can be indexed to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes of a pure tetragonal anatase phase (space group, I41/amd; JCPDS no. 21–1272) with lattice constants a = 3.78 Å and c = 9.50 Å [39,40]. Crystal facet (101) was the main crystal structure of the anatase TiO2 due to its maximum peak intensity. No rutile phase was detected due to the low reaction temperature employed in this work. The average crystal size of the TiO2 nanoparticles in the composite was calculated to be ca. 8.1 nm based on Scherrer's equation. No diffraction peaks from impurities and other phases could be detected, thus indicating that the product was pure and well crystallized. Notably, the typical diffraction peaks of graphene or GO were not found in the XRD pattern of the composite. A possible reason for this observation was that the most intense diffraction peak of graphene (2θ = 24.5°) [41] could be shielded by the main peak of anatase TiO2 at 25.3°.

thumbnailFigure 4. XRD spectra of (spectrum a) graphite oxide and (spectrum b) rGO-TiO2 composite.

Figure 5 shows the FTIR spectra of graphite powder, graphite oxide, and the rGO-TiO2 composite. While no significant peaks were observed in raw graphite, graphite oxide was found to exhibit several characteristic absorption bands of oxygen-containing groups (Figure 5, spectrum b). The absorption peaks included 870 cm−1 for aromatic C-H deformation [42], 1,052 cm−1 for C-O stretching [21], 1,220 cm−1 for phenolic C-OH stretching [42], 1,625 cm−1 for the hydroxyl groups of molecular water [43], 1,729 cm−1 for C = O stretching [20], and a broad peak at 3,400 cm−1 for the O-H stretching vibrations of C-OH groups [44]. The small peaks at 2,854 and 2,921 cm−1 in the spectrum were attributed to the CH2 stretching vibration [45]. Figure 5 (spectrum c) shows the FTIR measurement for the rGO-TiO2 composite. It can be observed that the intensities of absorption bands of oxygen-containing functional groups such as C-O (1,052 cm−1) were dramatically reduced. The C-OH and carbonyl C = O bands at 1,200 and 1,729 cm−1, respectively, were also found to have disappeared for the rGO-TiO2 composite. However, it can be seen that the spectrum retains a broad absorption band centered at 3,400 cm−1, which was attributed to the residual O-H groups of rGO. These results implied that GO was not completely reduced to graphene through the solvothermal treatment but was instead partially reduced to rGO, which possessed residual oxygen-containing functional groups. Therefore, TiO2 could be susceptible to interactions with these functional groups in the nanocomposites [45]. The spectrum also showed strong absorption bands at 450 and 670 cm−1, indicating the presence of Ti-O-Ti bond in TiO2[46].

thumbnailFigure 5. FTIR spectra of (spectrum a) graphite powder, (spectrum b) graphite oxide, and (spectrum c) rGO-TiO2 composite.

UV-visible (UV–vis) spectroscopy has been proven to be an effective optical characterization technique to understand the electronic structure of semiconductors. Figure 6 (spectra a and b) shows the UV–vis diffuse reflectance spectra (DRS) of pure anatase and the rGO-TiO2 composite, respectively. Both samples displayed a typical absorption with an intense transition in the UV region of the spectra, which was assigned to the intrinsic band gap absorption of TiO2 resulting from the electron transitions from the valence band to the conduction band (O2p → Ti3d) [26]. In comparison with pure anatase, a substantial red shift to higher wavelength in the absorption edge of the rGO-TiO2 composite could be observed, therefore indicating a narrowing of band gap with the introduction of rGO. The optical band gaps of pure anatase and rGO-TiO2 were determined using a Tauc plot of the modified Kubelka-Munk (KM) function with a linear extrapolation (see inset of Figure 6). The approximated band gaps of pure anatase and rGO-TiO2 were 3.20 and 2.90 eV, respectively. This supported the qualitative observation of a red shift in the absorption edge of the composite as compared to pure anatase. The narrowing of band gap could be ascribed to the chemical bonding between TiO2 and the specific sites of carbon during the solvothermal treatment, which is analogous to the case of carbon nanotube (CNT)-TiO2 composite materials [47,48]. Pure anatase exhibited no absorption above its absorption edge, indicating that it was not photocatalytically responsive in the visible light region. In contrast, the introduction of rGO resulted in a continuous absorption band ranging from 400 to 800 nm, which was in agreement with the greyish-black color of the sample. The increased absorption intensity of light for the rGO-TiO2 composites suggested that they could exhibit an enhanced photocatalytic activity for a given reaction. This hypothesis was confirmed by its use in the photocatalytic reduction of CO2 under ambient condition.

thumbnailFigure 6. UV–vis diffuse reflectance spectra of (spectrum a) pure anatase and (spectrum b) rGO-TiO2 . Inset: plot of transformed KM function [F(R).hv]1/2 vs. hv for pure anatase and rGO-TiO2.

Photocatalytic reduction of CO2 with H2O and mechanism

The photocatalytic performance of our rGO-TiO2 nanocomposite was measured by the photoreduction of CO2 under visible light irradiation using water vapor as a scavenger. Graphite oxide and pure anatase were separately tested under similar conditions. Control experiments indicated that no appreciable CH4 formation was detected in the absence of either light irradiation or photocatalyst, confirming that CH4 gas was produced by photocatalytic reactions. According to the procedure described in the ‘Methods’ section, the yield of CH4 gas (μmol gcat−1 h−1) was calculated and plotted in Figure 7 as a function of reaction time (h). The photocatalytic activity of CO2 reduction was found to follow the order rGO-TiO2 < graphite oxide < TiO2. Pure anatase TiO2 exhibited the lowest photocatalytic performance due to its limited photoresponse range under visible light irradiation. Graphite oxide showed an improvement in performance where it reached a maximum yield of 0.0628 μmol gcat−1 h−1 before leveling off after 2 h of testing. Yeh et al. [49] have demonstrated the use of graphite oxide as a photocatalyst for the steady evolution of H2 from water splitting. To the best of our knowledge, no paper has reported the use of graphite oxide in the conversion of CO2 into CH4 gas. This finding is interesting as it highlights the possibility of using inexpensive and abundant graphitic materials as photocatalysts to convert CO2 under solar illumination. Graphite oxide is the intermediate state between graphite and graphene [27]. It has been shown that its band gap is dependent on the number of oxygenated sites [49]. Also, the isolated sp2 clusters on graphite oxide with oxygen-containing functional groups such as C-OH and C-O-C would lead to the localization of electron–hole pairs on its basal plane [49,50]. These photoinduced charges would then migrate to the surface of graphite oxide and act as oxidizing and reducing sites, respectively, to react with the adsorbed reactants (in this case, CO2 and H2O vapor). Among all three samples, the rGO-TiO2 nanocomposite exhibited the highest photocatalytic performance towards CO2 reduction. The maximum CH4 product yield of 0.135 μmol gcat−1 h−1 was attained after 4 h of reaction. A slight decrease in yield can be observed at the third hour of reaction. This deviation is not uncommon in continuous gas-phase photocatalytic systems, and similar trends have been reported in literature [51,52]. The rGO-TiO2 nanocomposite was shown to exhibit an enhancement factor of 2.1 and 5.6 as compared to graphite oxide and pure anatase, respectively. It is interesting to note that the rGO-TiO2 composite was active even under the irradiation of low-power, energy-saving light bulbs. The use of high-intensity halogen and xenon arc lamps was not required for the photoexcitation process to take place.

thumbnailFigure 7. Time dependence on the photocatalytic formation rate of CH4. Over (curve a) pure anatase, (curve b) graphite oxide, and (curve c) rGO-TiO2 under visible light irradiation.

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 [53] and −4.42 eV [46], 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 [30]. 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 addition, the two-dimensional and planar π-conjugation structure of rGO endowed it with excellent conductivity of electron [16,55]. As we know, one photon can usually induce the transfer of only one electron in photochemical reactions. However, the photocatalytic reduction of CO2 required a multi-electron process to yield CH4.Therefore, in the rGO-TiO2 composite, rGO served as an electron collector and transporter to effectively separate the photogenerated electron–hole pairs. This in turn lengthened the lifetime of the charge carriers, which could be advantageous for overcoming this obstacle to improve the selective formation of CH4 gas. During the photocatalytic reaction, a large number of electrons would be produced due to the highly dispersed TiO2 nanoparticles over the rGO sheets (see Figure 2a,b). Furthermore, the large specific surface area of rGO also increased the adsorption of the CO2 molecules, thus favoring the formation of CH4. The mechanisms of photocatalytic enhancement over the rGO-TiO2 composite are depicted in Figure 8.

thumbnailFigure 8. Charge transfer and separation in the rGO-TiO2composite. Schematic illustrating the charge transfer and separation in the rGO-TiO2 composite for the photoreduction of CO2 under visible light irradiation with the introduction of a new energy level, EF*.

The photocatalytic conversion of CO2 to CH4 over the rGO-TiO2 composite can be understood using the energy band theory, which is based on the relative positions of CB, VB, and oxidation potentials. In general, the overall mechanism of the CO2 transformation process is a sequential combination of H2O oxidation and CO2 reduction. In the rGO-TiO2 composite, the TiO2 nanoparticles exhibited an intimate contact with the rGO sheet. The d orbital (CB) of TiO2 and the π orbital of rGO matched well in energy levels, thus resulting in a chemical bond interaction to form d-π electron orbital overlap [56]. The CB flatband potential of TiO2 is −0.5 V (vs. normal hydrogen electrode (NHE), pH = 7) [57], which is more negative than the reduction potential of CO2/CH4 (−0.24 V vs. NHE, pH = 7) [58] acts as a donor. This indicated that the photogenerated electrons and holes on the irradiated rGO-TiO2 composites can react with adsorbed CO2 and H2O to produce CH4 via an eight-electron reaction. The major reaction steps in the photocatalytic CO2 reduction process can be summarized by Equations 1, 2 and 3

Graphene TiO 2 hv graphene e / TiO 2 h VB + (1)

TiO 2 2 h VB + + H 2 O 2 H + + 1 2 O 2 E o = + 0.82 V (2)

CO 2 + 8 H + + 8 e CH 4 + 2 H 2 O E o = 0.24 V (3)

Conclusions

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.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

LLT and WJO conceived and designed the experimental strategy. LLT performed the experiments and prepared the manuscript. SPC and ARM supervised the whole work and revised the manuscript. All authors read and approved the final manuscript.

Acknowledgements

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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