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Three-dimensional porous graphene-Co3O4 nanocomposites for high performance photocatalysts
Applied Surface Science 357 (2015) 439–444
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Three-dimensional porous graphene-Co3 O4 nanocomposites for high performance photocatalysts Zeng Bin a,∗ , Long Hui b a b
College of Mechanical Engineering, Hunan University of Arts and Science, Changde 415000, People’s Republic of China Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
a r t i c l e
i n f o
Article history: Received 6 May 2015 Received in revised form 27 July 2015 Accepted 5 September 2015 Available online 8 September 2015 Keywords: Graphene Freeze-drying Photocatalysis
a b s t r a c t Novel three-dimensional porous graphene-Co3 O4 nanocomposites were synthesized by freeze-drying methods. Scanning and transmission electron microscopy revealed that the graphene formed a threedimensional porous structure with Co3 O4 nanoparticles decorated surfaces. The as-obtained product showed high photocatalytic efficiency and could be easily separated from the reaction medium by magnetic decantation. This nanocomposite may be expected to have potential in water purification applications. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Environmental problems are considered a severe threat to sustainable human development. To address these problems, photocatalysis has been recognized as a “green” and energy saving technology for effective removal of environmental contaminants [1]. However, a major competing factor for photocatalytic reactions is rapid recombination of photogenerated electron–hole pairs, which limits efficiency [2]. Considerable efforts have been made to inhibit electron–hole pair recombination by coupling semiconductor photocatalysts to materials with high electronic conductivity, such as graphene (GR) [3], carbon nanotubes (CNTs) [4], and fullerenes [5]. It has been shown that assembling semiconductor photocatalysts (e.g. TiO2 , ZnO, CuO) on these materials can enhance their efficiencies in photocatalytic reactions [6–8]. Three-dimensional (3D) porous graphene has recently received considerable attention as the latest member of the carbon allotrope family [9]. This material combines the electronic properties of graphene with the features of a well-defined porous structure. Such 3D porous graphene materials show promise for applications in various fields. Zhong et al. have demonstrated a simple high-performance supercapacitor based on 3D graphene aerogels that showed a high specific capacitance, good rate capability and enhanced energy density [10]. Huang et al. synthesized graphene foams with controlled pore size and reported the highest total pore
∗ Corresponding author. E-mail address: [email protected] (Z. Bin). http://dx.doi.org/10.1016/j.apsusc.2015.09.051 0169-4332/© 2015 Elsevier B.V. All rights reserved.
volume value among graphene materials [11]. In these cases, the 3D porous graphene provided rich macroporosity and a multidimensional electron transport pathway. These features strongly suggest that assembling nanomaterials on 3D porous graphenes may lead to high performance photocatalysts. Compared with the conventional photocatalysts such as ZnO and TiO2 , Co3 O4 is not only a visible light photocatalyst with band gap energy of 2.1 eV, but also possess excellent separation property. However, to the best of our knowledge, there have been no reports on the use of 3D porous graphene loaded with nanoparticles to improve photocatalytic performance. The aim of this study was to explore and prepare 3D porous graphene loaded with Co3 O4 nanoparticles (Co3 O4 -G) and test the material’s performance as a photocatalyst. The graphene hybrids were synthesized and displayed am interconnected porous framework of graphene sheets with a uniform distribution of Co3 O4 nanoparticles. Compared with previous related work using graphene and photocatalysts composites, these systems exhibited high photocatalytic performance and the catalyst could be easily recovered from the reaction medium. 2. Materials and methods 2.1. Preparation of GO Graphite oxide (GO) was prepared using Hummers method [12]. In a typical synthesis, 3 g of graphite powder and 3 g of sodium nitrate were dispersed into 150 mL of H2 SO4 with cooling using an ice bath. KMnO4 (9 g) was slowly added, and the mixture
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was transferred to a 40 ◦ C water bath and stirred for 2 h to form a thick paste. Deionized water 150 mL was then slowly added to the mixture and the temperature was increased to 98 ◦ C. After 20 min, 30 mL of a 35% H2 O2 solution was added to the mixture. The mixture was stirred for another 10 min and then diluted with 1000 mL of deionized water. The solution was then filtered and the filtrate was washed with deionized water until the washings reached pH 7.
2.2. Synthesis of Co3 O4 -G In a typical synthesis, the as-prepared GO was added to Co(NO3 )2 solution (50 mL, 20 g L−1 ) in the desired weight ratio. After sonication for 30 min the blended suspension was frozen into a cube in a refrigerator (−18 ◦ C), and then freeze-dried with a condenser temperature of −20 ◦ C and an internal pressure of <20 Pa, to obtain a purple powder. This powder was then calcinated at 600 ◦ C for 1 h under an argon atmosphere in a tube furnace to thermally decompose Co(NO3 )2 , and reduce GO to graphene. Finally, the materials were allowed to cool to room temperature. The weight ratios of GO to Co(NO3 )2 were 0%, 1%, 2%, 3% and the as-prepared samples were labeled as Co3 O4 -G0, Co3 O4 -G1, Co3 O4 -G2, Co3 O4 G3, respectively.
2.3. Characterization Characterization of the samples was carried out by powder X-ray diffraction (XRD, D5000), X-ray photoelectron spectroscopy (XPS, K-Alpha 1063), Fourier transform infrared spectroscopy (FTIR, WQF-410), scanning electron microscopy (SEM, S4800), and transmission electron microscopy (TEM, JEM-2100F). Brunauer–Emmett–Teller (BET) specific surface areas and porosity of the samples were evaluated from nitrogen adsorption isotherms measured at −196 ◦ C using a gas adsorption apparatus (ASAP 2020, Micromeritics, USA).
2.4. Dye adsorption experiments A solution of methyl orange (MO) in a 500-mL glass container was stirred under ambient conditions. The calcinated product (20 mg) was mixed with 100 mL of MO solution (20 mg L−1 ) using a vortex mixer. Portions were removed from the mixture at predetermined time intervals, and then centrifuged at 5000 rpm for 10 min. The absorbance of the supernatant was recorded at 464 nm (UV-2550). The equilibrium concentration (Ce) of MO was calculated from a MO calibration curve. The equilibrium sorption capacity of the calcinated product was calculated as (C0 − Ce)/C0 , where C0 is the initial concentration of MO and Ce is the final concentration of MO.
Fig. 1. XRD patterns of GO, graphene and Co3 O4 -G.
3. Results and discussion The XRD patterns of graphene oxides, graphene and Co3 O4 -G nanocomposites are shown in Fig. 1. It can been seen that Co3 O4 -G nanocompaosites possess similar XRD patterns and the values of 37.2◦ , 45.1◦ can be attributed to the (3 1 1) and (4 0 0) lattice planes of Co3 O4 spinel (JCPDS NO42-1467). An evident diffraction peak at 24.98◦ is assigned to (0 0 2) plane of GR. No other impurity phases were detected. The FTIR spectra gave evidence for the existence of the hydrophilic groups on the surface of GO and Co3 O4 -G2. As shown in Fig. 2, characteristic bands of GO were observed at: 1074 cm−1 , alkoxy C–O; 1375 cm−1 , carboxyl O–H stretching; 1618 cm−1 , H–O–H stretching; and 1723 cm−1 , C O stretching (carboxyl or carbonyl groups). Compared with GO, Co3 O4 -G2 showed the similar functional groups in its spectrum but with much lower absorption intensity. This suggests partial removal of some of the oxygencontaining functional groups of GO and an effective reduction of GO to GR was achieved by the thermal treatment. In addition, the band at 557 cm−1 is attributed to Co–O stretching vibration. The porous nature of the product was investigated by nitrogen adsorption–desorption and Barrett–Joyner–Halenda (BJH) methods. The Co3 O4 -G2 sample showed a large specific surface area (18.8 m2 g−1 ), reflecting the mesoporous structure of the nanocomposites. A representative pore-size distribution curve for Co3 O4 -G2 is inset in Fig. 3, showing a pore size distribution of about 3.81 nm. Porous structures may be expected to interact with atoms, ions, molecules or even larger guest species, not only at the external surface, but also throughout the internal pore network.
2.5. Photocatalytic experiments Photodegradation reactions were conducted using a 500 W xenon arc lamp ( > 400 nm). In a typical reaction, 300 mL of aqueous MO solution (20 mg L−1 ), 2 mL of 30% H2 O2 and 20 mg of the calcinated product were mixed and magnetically stirred in the dark to achieve an adsorption equilibrium between MO and the catalyst before irradiation. The solution was placed 10 cm from the center of the lamp. Over the course of the experiment, 10 mL portions of the reaction mixture were withdrawn at predetermined time intervals and centrifuged. The absorbance of the centrifuged solution was then measured at ambient temperature using a UV–vis spectrophotometer.
Fig. 2. FTIR of GO and Co3 O4 -G2.
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Fig. 5. TG analysis of Co3 O4 -G1, Co3 O4 -G2 and Co3 O4 -G3.
Fig. 3. A typical nitrogen adsorption–desorption isotherm of Co3 O4 -G2 nanocomposites. Inset shows typical plot of the pore size distribution for Co3 O4 -G2.
X-ray photoelectron spectroscopy (XPS) provided further information on the surface electronic state of the Co3 O4 -G2 products. The C 1s, O 1s, Co 2p core photoionization signals are clearly shown in the survey spectrum, in Fig. 4a. The Co 2p XPS spectra of the composites exhibited two peaks at 795.6 and 780.2 eV, corresponding to the Co 2p1/2 and Co 2p3/2 spin-orbit peaks of Co3 O4 (Fig. 4b). The presence of Co3 O4 was further confirmed from the O 1s XPS peak at 530.1 eV, which corresponds to oxygen species in Co3 O4 (Fig. 4c). The C1s spectra of Co3 O4 -G2 (Fig. 4d) could be deconvoluted into three peaks at 284.5, 286.6, and 288.2 eV, which are associated with C–C, C–O (epoxyl and hydroxyl), and C O (carbonyl) environments, respectively. The peak intensity of C–O in Co3 O4 -G2 was much lower, suggesting the successful removal of oxygen containing
functional groups. This change may also be expected to be favorable for improving the electronic conductivity of the material. Fig. 5 shows the TG analysis to check the graphene and Co3 O4 content in the composite. According to the TG curve, it was estimated that the mass percent of graphene components in the nanocomposites was calculated to be 5.2% for Co3 O4 -G1, 6.8% for Co3 O4 -G2, and 8.1% for Co3 O4 -G3, respectively. Fig. 6a shows that GO has a crumpled appearance with micrometer-sized domains. Fig. 6b–d shows the SEM and TEM images of the Co3 O4 -G2 nanocomposite. The as-prepared Co3 O4 G2 had a well-defined and interconnected three-dimensional (3D) porous network (Fig. 6b). The pore size of the network was in the submillimeter to micrometer scale. The pore walls likely consist of graphene sheets, with the overlapping of flexible graphene sheets resulting in the formation of cross-linking sites in the framework. From a close observation (Fig. 6c) we can clearly see that uniform nanoparticles are spread on the surface of graphene network. Fig. 6d shows a typical TEM of the nanocomposites with 30–50 nm nanoparticles interacting with GR, to give a homogeneous distribution of nanoparticles on the graphene scaffold. From HRTEM image of a single nanoparticle (Fig. 6e), the lattice fringes can be clearly
Fig. 4. (a) Survey spectra (b) typical Co 2p (c) O 1s (d) C1s region of XPS spectrum of Co3 O4 -G2.
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Fig. 6. SEM of (a) GO, (b) and (c) SEM, (d) TEM and (e) HRTEM of Co3 O4 -G2.
observed and the lattice spaces of 0.283 nm could be indexed as (0 0 2) plane of Co3 O4 . These novel structures serve not only as a substrate for the Co3 O4 nanoparticles but also provide channels for the diffusion of molecules. Fig. 7 displays an MO solution in adsorption–desorption equilibrium with Co3 O4 -G0, Co3 O4 -G1, Co3 O4 -G2 and Co3 O4 -G3, under dark conditions. The 3D porous graphene nanocomposites show excellent adsorption capacity for MO. This is likely to be beneficial for photocatalysis, because efficient adsorption can improve contact between MO and the photocatalytic nanoparticles, and accelerate reactions with photogenerated active species. Based on these experimental results, a schematic illustration of the formation of GR-Co3 O4 is presented in Scheme 1. GO formed stable suspensions in water, having a highly negative surface charge (Scheme 1a). When the Co(NO3 )2 solution was added to the GO suspension, Co2+ ions were electrostatically attracted to the GO surface (Scheme 1b) [13]. After freeze-drying these suspensions, freestanding flexible GO composites were formed. Folding and stacking of GO layers resulted in the formation of a three-dimensional porous graphene oxide hybrid structure with Co(NO3 )2 adsorbed
Fig. 7. The absorbance of MO by Co3 O4 -G0, Co3 O4 -G1, Co3 O4 -G2 and Co3 O4 -G3.
Z. Bin, L. Hui / Applied Surface Science 357 (2015) 439–444
Scheme 1. Proposed scheme of the fabricated processes of Co3 O4 -G.
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onto the GO surface (Scheme 1c) [11]. The final thermal treatment decomposed Co(NO3 )2 to Co3 O4 nanoparticles and reduced GO to GR (Scheme 1d) [14]. As a demonstration of the application of Co3 O4 -G, its photocatalytic performance was evaluated using MO as a representative organic dyestuff. Fig. 8a shows the percentage of MO adsorbed by different catalysts in the presence of H2 O2 in the dark. The percentage of MO adsorbed by Co3 O4 -G0, Co3 O4 -G1, Co3 O4 -G2, and Co3 O4 -G3 was 3.2%, 14.2%, 15.3%, and 18.3%, respectively, indicating that the ability of the above mentioned catalysts to adsorb MO is limited. Fig. 8b shows the profile of the photocatalytic degradation efficiency of MO using different catalysts in the presence of H2 O2 under visible light. It can be seen that 95.3% of MO was degraded using Co3 O4 -G2, while 57.6%, 88.9%, and 78.3% MO degradation was achieved using Co3 O4 -G0, Co3 O4 -G1, and Co3 O4 -G3, respectively. The superior photocatalytic performance of Co3 O4 -G2 is attributed to two factors: (1) as compared to pure Co3 O4 , the larger specific surface area of Co3 O4 -G2 offers more reaction centers, which favors the enhanced photocatalytic activity [15]; (2) in the Co3 O4 -G system, graphene serves as an acceptor of electrons generated in the Co3 O4 semiconductor and effectively decreases the recombination probability of photoexcited electron–hole pairs. However, a further increase in the percentage of black graphene led to shielding of active sites on the catalyst surface and thus decreasing the intensity of light that reaches the catalyst surface. The magnetic properties of Co3 O4 -G2 at room temperature were characterized by a vibrating sample magnetometer. As shown in Fig. 8c, the magnetization hysteresis loops of Co3 O4 -G2 were in the form of S-shaped curves and the saturation magnetization reached 36.8 emug−1 . As
Fig. 8. Photocatalytic degradation efficiency of MO (a) different catalysts in the dark. (b) Different catalysts with H2 O2 under visible light. (c) Magnetic hysteresis loops of Co3 O4 -G2. (The inset shows the removal of MO with the help of magnet.) (d) Cycling runs in the photocatalytic degradation of MO in the presence of Co3 O4 -G2 with H2 O2 under visible light irradiation.
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a powder, Co3 O4 -G2 exhibited a magnetic attraction to an external magnet. The Co3 O4 -G2 samples dispersed in the MO solution could be completely separated from the reaction medium by applying an external magnetic field, leaving a colorless solution (inset in Fig. 8c). The stability of the photocatalyst was tested and after three cycles of the photodegradation of MO, Co3 O4 -G2 did exhibit any obvious loss of activity, which indicated its high stability during the photocatalytic reaction (Fig. 8d). The high photocatalytic performance may be attributed to the following. First, 3D porous graphene provides an excellent support matrix for photocatalytic particles. Second, the excellent electronic conductivity of graphene, imparted by its 2D planar p-conjugation structure, allows it to act as an effective electron conductor. This improves the transfer of photogenerated charges from the photocatalyst surface to the graphene, and promotes the transport of photogenerated charges on the photocatalyst surface, to increase the photocatalytic efficiency. Third, the strong adsorption properties of the hybrid material concentrates dye molecule near to the catalyst. This increases the chances of dye molecules reacting with photogenerated active species, leading to photocatalytic degradation. 4. Conclusions We successfully prepared 3D porous graphene loaded Co3 O4 nanoparticles. The graphene hybrids showed an interconnected porous framework of graphene sheets, with Co3 O4 nanoparticles finely dispersed in the porous graphene. Tests of photocatalytic performance showed that the Co3 O4 -G hybrid materials displayed excellent photocatalytic performance under visible light irradiation and exhibited super paramagnetic properties that allowed for separation of the catalyst from solution. The as-obtained Co3 O4 -G composites were non-hazardous to the environment and may present a practical solution for removing contamination from water. Acknowledgments This work was supported by the Construct Program of the Key Discipline in Hunan Province (XJF[2011] 76), Technology Plan
Foundation of China, Hunan (2014GK3094) and Project of Hunan Provincial Education Department (15B158). References [1] B. Zeng, X.H. Chen, X.T. Ning, Electrostatic-assembly three-dimensional CNTs/rGO implanted Cu2 O composite spheres and its photocatalytic properties, Appl. Surf. Sci. 276 (2013) 482–486. [2] L.W. Liwu Zhang, Y.F. Zhu, A review of controllable synthesis and enhancement of performances of bismuth tungstate visible-light-driven photocatalysts, Catal. Sci. Technol. 2 (2012) 694–706. [3] P.N. Zhu, A. Sreekumaran Nair, S.J. Peng, Facile fabrication of TiO2 -graphene composite with enhanced photovoltaic and photocatalytic properties by electrospinning, ACS Appl. Mater. Interfaces 4 (2012) 581–585. [4] J.C. Michael, L. Galit, W.H. Dennis, Synthesis of optically active ZnS-carbon nanotube nanocomposites in supercritical carbon dioxide via a single source diethyldithiocarbamate precursor, Ind. Eng. Chem. Res. 51 (2012) 11710–11716. [5] N. Serpone, A.V. Emeline, Semiconductor photocatalysis – past, present, and future outlook, J. Phys. Chem. Lett. 3 (2012) 673–677. [6] Md. Selim, A. Arif Sher Shah, Reum Park, Green synthesis of biphasic TiO2 – reduced graphene oxide nanocomposites with highly enhanced photocatalytic activity, ACS Appl. Mater. Interfaces 4 (2012) 3893–3901. [7] Q. Luo, X.Y. Yu, B.X. Lei, Reduced graphene oxide – hierarchical ZnO hollow sphere composites with enhanced photocurrent and photocatalytic activity, J. Phys. Chem. 116C (2012) 8111–8117. [8] B. Zeng, X.H. Chen, X.T. Ning, CNTs/rGO spheres decorated with Cux O (x = 1 or 2) nanoparticles and its photocatalytic performance, Catal. Commun. 43 (2014) 235–239. [9] V. Presser, M. Heon, Y. Gogotsi, Carbide-derived carbons – from porous networks to nanotubes and graphene, Adv. Funct. Mater. 21 (2011) 810–833. [10] S.W. Zhong, A. Winter, L. Chen, Adv. Mater. 24 (2012) 5130–5135. [11] X.D. Huang, K. Qian, J. Yang, et al., Functional nanoporous graphene foams with controlled pore sizes, Adv. Mater. 24 (2012) 4419–4423. [12] W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, Am. Chem. Soc. 80 (1958) 1339. [13] B. Zeng, X.H. Chen, Y.X. Luo, Graphene spheres loaded urchin-like Cux O (x = 1 or 2) for use as a high performance photocatalyst, Ceram. Int. 40 (2014) 5055–5059. [14] G. Qun, J.L. Cheng, B. Wang, et al., Needle-like Co3 O4 anchored on the graphene with enhanced electrochemical performance for aqueous supercapacitors, ACS Appl. Mater. Interfaces 6 (2014) 7626–7632. [15] B.G. Choi, S.J. Chang, Y.B. Lee, et al., 3D heterostructured architectures of Co3 O4 nanoparticles deposited on porous graphene surfaces for high performance of lithium ion batteries, Nanoscale 4 (2012) 5924–5930.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Three-dimensional porous graphene-Co3 O4 nanocomposites for high performance photocatalysts Zeng Bin a,∗ , Long Hui b a b
College of Mechanical Engineering, Hunan University of Arts and Science, Changde 415000, People’s Republic of China Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
a r t i c l e
i n f o
Article history: Received 6 May 2015 Received in revised form 27 July 2015 Accepted 5 September 2015 Available online 8 September 2015 Keywords: Graphene Freeze-drying Photocatalysis
a b s t r a c t Novel three-dimensional porous graphene-Co3 O4 nanocomposites were synthesized by freeze-drying methods. Scanning and transmission electron microscopy revealed that the graphene formed a threedimensional porous structure with Co3 O4 nanoparticles decorated surfaces. The as-obtained product showed high photocatalytic efficiency and could be easily separated from the reaction medium by magnetic decantation. This nanocomposite may be expected to have potential in water purification applications. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Environmental problems are considered a severe threat to sustainable human development. To address these problems, photocatalysis has been recognized as a “green” and energy saving technology for effective removal of environmental contaminants [1]. However, a major competing factor for photocatalytic reactions is rapid recombination of photogenerated electron–hole pairs, which limits efficiency [2]. Considerable efforts have been made to inhibit electron–hole pair recombination by coupling semiconductor photocatalysts to materials with high electronic conductivity, such as graphene (GR) [3], carbon nanotubes (CNTs) [4], and fullerenes [5]. It has been shown that assembling semiconductor photocatalysts (e.g. TiO2 , ZnO, CuO) on these materials can enhance their efficiencies in photocatalytic reactions [6–8]. Three-dimensional (3D) porous graphene has recently received considerable attention as the latest member of the carbon allotrope family [9]. This material combines the electronic properties of graphene with the features of a well-defined porous structure. Such 3D porous graphene materials show promise for applications in various fields. Zhong et al. have demonstrated a simple high-performance supercapacitor based on 3D graphene aerogels that showed a high specific capacitance, good rate capability and enhanced energy density [10]. Huang et al. synthesized graphene foams with controlled pore size and reported the highest total pore
∗ Corresponding author. E-mail address: [email protected] (Z. Bin). http://dx.doi.org/10.1016/j.apsusc.2015.09.051 0169-4332/© 2015 Elsevier B.V. All rights reserved.
volume value among graphene materials [11]. In these cases, the 3D porous graphene provided rich macroporosity and a multidimensional electron transport pathway. These features strongly suggest that assembling nanomaterials on 3D porous graphenes may lead to high performance photocatalysts. Compared with the conventional photocatalysts such as ZnO and TiO2 , Co3 O4 is not only a visible light photocatalyst with band gap energy of 2.1 eV, but also possess excellent separation property. However, to the best of our knowledge, there have been no reports on the use of 3D porous graphene loaded with nanoparticles to improve photocatalytic performance. The aim of this study was to explore and prepare 3D porous graphene loaded with Co3 O4 nanoparticles (Co3 O4 -G) and test the material’s performance as a photocatalyst. The graphene hybrids were synthesized and displayed am interconnected porous framework of graphene sheets with a uniform distribution of Co3 O4 nanoparticles. Compared with previous related work using graphene and photocatalysts composites, these systems exhibited high photocatalytic performance and the catalyst could be easily recovered from the reaction medium. 2. Materials and methods 2.1. Preparation of GO Graphite oxide (GO) was prepared using Hummers method [12]. In a typical synthesis, 3 g of graphite powder and 3 g of sodium nitrate were dispersed into 150 mL of H2 SO4 with cooling using an ice bath. KMnO4 (9 g) was slowly added, and the mixture
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was transferred to a 40 ◦ C water bath and stirred for 2 h to form a thick paste. Deionized water 150 mL was then slowly added to the mixture and the temperature was increased to 98 ◦ C. After 20 min, 30 mL of a 35% H2 O2 solution was added to the mixture. The mixture was stirred for another 10 min and then diluted with 1000 mL of deionized water. The solution was then filtered and the filtrate was washed with deionized water until the washings reached pH 7.
2.2. Synthesis of Co3 O4 -G In a typical synthesis, the as-prepared GO was added to Co(NO3 )2 solution (50 mL, 20 g L−1 ) in the desired weight ratio. After sonication for 30 min the blended suspension was frozen into a cube in a refrigerator (−18 ◦ C), and then freeze-dried with a condenser temperature of −20 ◦ C and an internal pressure of <20 Pa, to obtain a purple powder. This powder was then calcinated at 600 ◦ C for 1 h under an argon atmosphere in a tube furnace to thermally decompose Co(NO3 )2 , and reduce GO to graphene. Finally, the materials were allowed to cool to room temperature. The weight ratios of GO to Co(NO3 )2 were 0%, 1%, 2%, 3% and the as-prepared samples were labeled as Co3 O4 -G0, Co3 O4 -G1, Co3 O4 -G2, Co3 O4 G3, respectively.
2.3. Characterization Characterization of the samples was carried out by powder X-ray diffraction (XRD, D5000), X-ray photoelectron spectroscopy (XPS, K-Alpha 1063), Fourier transform infrared spectroscopy (FTIR, WQF-410), scanning electron microscopy (SEM, S4800), and transmission electron microscopy (TEM, JEM-2100F). Brunauer–Emmett–Teller (BET) specific surface areas and porosity of the samples were evaluated from nitrogen adsorption isotherms measured at −196 ◦ C using a gas adsorption apparatus (ASAP 2020, Micromeritics, USA).
2.4. Dye adsorption experiments A solution of methyl orange (MO) in a 500-mL glass container was stirred under ambient conditions. The calcinated product (20 mg) was mixed with 100 mL of MO solution (20 mg L−1 ) using a vortex mixer. Portions were removed from the mixture at predetermined time intervals, and then centrifuged at 5000 rpm for 10 min. The absorbance of the supernatant was recorded at 464 nm (UV-2550). The equilibrium concentration (Ce) of MO was calculated from a MO calibration curve. The equilibrium sorption capacity of the calcinated product was calculated as (C0 − Ce)/C0 , where C0 is the initial concentration of MO and Ce is the final concentration of MO.
Fig. 1. XRD patterns of GO, graphene and Co3 O4 -G.
3. Results and discussion The XRD patterns of graphene oxides, graphene and Co3 O4 -G nanocomposites are shown in Fig. 1. It can been seen that Co3 O4 -G nanocompaosites possess similar XRD patterns and the values of 37.2◦ , 45.1◦ can be attributed to the (3 1 1) and (4 0 0) lattice planes of Co3 O4 spinel (JCPDS NO42-1467). An evident diffraction peak at 24.98◦ is assigned to (0 0 2) plane of GR. No other impurity phases were detected. The FTIR spectra gave evidence for the existence of the hydrophilic groups on the surface of GO and Co3 O4 -G2. As shown in Fig. 2, characteristic bands of GO were observed at: 1074 cm−1 , alkoxy C–O; 1375 cm−1 , carboxyl O–H stretching; 1618 cm−1 , H–O–H stretching; and 1723 cm−1 , C O stretching (carboxyl or carbonyl groups). Compared with GO, Co3 O4 -G2 showed the similar functional groups in its spectrum but with much lower absorption intensity. This suggests partial removal of some of the oxygencontaining functional groups of GO and an effective reduction of GO to GR was achieved by the thermal treatment. In addition, the band at 557 cm−1 is attributed to Co–O stretching vibration. The porous nature of the product was investigated by nitrogen adsorption–desorption and Barrett–Joyner–Halenda (BJH) methods. The Co3 O4 -G2 sample showed a large specific surface area (18.8 m2 g−1 ), reflecting the mesoporous structure of the nanocomposites. A representative pore-size distribution curve for Co3 O4 -G2 is inset in Fig. 3, showing a pore size distribution of about 3.81 nm. Porous structures may be expected to interact with atoms, ions, molecules or even larger guest species, not only at the external surface, but also throughout the internal pore network.
2.5. Photocatalytic experiments Photodegradation reactions were conducted using a 500 W xenon arc lamp ( > 400 nm). In a typical reaction, 300 mL of aqueous MO solution (20 mg L−1 ), 2 mL of 30% H2 O2 and 20 mg of the calcinated product were mixed and magnetically stirred in the dark to achieve an adsorption equilibrium between MO and the catalyst before irradiation. The solution was placed 10 cm from the center of the lamp. Over the course of the experiment, 10 mL portions of the reaction mixture were withdrawn at predetermined time intervals and centrifuged. The absorbance of the centrifuged solution was then measured at ambient temperature using a UV–vis spectrophotometer.
Fig. 2. FTIR of GO and Co3 O4 -G2.
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Fig. 5. TG analysis of Co3 O4 -G1, Co3 O4 -G2 and Co3 O4 -G3.
Fig. 3. A typical nitrogen adsorption–desorption isotherm of Co3 O4 -G2 nanocomposites. Inset shows typical plot of the pore size distribution for Co3 O4 -G2.
X-ray photoelectron spectroscopy (XPS) provided further information on the surface electronic state of the Co3 O4 -G2 products. The C 1s, O 1s, Co 2p core photoionization signals are clearly shown in the survey spectrum, in Fig. 4a. The Co 2p XPS spectra of the composites exhibited two peaks at 795.6 and 780.2 eV, corresponding to the Co 2p1/2 and Co 2p3/2 spin-orbit peaks of Co3 O4 (Fig. 4b). The presence of Co3 O4 was further confirmed from the O 1s XPS peak at 530.1 eV, which corresponds to oxygen species in Co3 O4 (Fig. 4c). The C1s spectra of Co3 O4 -G2 (Fig. 4d) could be deconvoluted into three peaks at 284.5, 286.6, and 288.2 eV, which are associated with C–C, C–O (epoxyl and hydroxyl), and C O (carbonyl) environments, respectively. The peak intensity of C–O in Co3 O4 -G2 was much lower, suggesting the successful removal of oxygen containing
functional groups. This change may also be expected to be favorable for improving the electronic conductivity of the material. Fig. 5 shows the TG analysis to check the graphene and Co3 O4 content in the composite. According to the TG curve, it was estimated that the mass percent of graphene components in the nanocomposites was calculated to be 5.2% for Co3 O4 -G1, 6.8% for Co3 O4 -G2, and 8.1% for Co3 O4 -G3, respectively. Fig. 6a shows that GO has a crumpled appearance with micrometer-sized domains. Fig. 6b–d shows the SEM and TEM images of the Co3 O4 -G2 nanocomposite. The as-prepared Co3 O4 G2 had a well-defined and interconnected three-dimensional (3D) porous network (Fig. 6b). The pore size of the network was in the submillimeter to micrometer scale. The pore walls likely consist of graphene sheets, with the overlapping of flexible graphene sheets resulting in the formation of cross-linking sites in the framework. From a close observation (Fig. 6c) we can clearly see that uniform nanoparticles are spread on the surface of graphene network. Fig. 6d shows a typical TEM of the nanocomposites with 30–50 nm nanoparticles interacting with GR, to give a homogeneous distribution of nanoparticles on the graphene scaffold. From HRTEM image of a single nanoparticle (Fig. 6e), the lattice fringes can be clearly
Fig. 4. (a) Survey spectra (b) typical Co 2p (c) O 1s (d) C1s region of XPS spectrum of Co3 O4 -G2.
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Fig. 6. SEM of (a) GO, (b) and (c) SEM, (d) TEM and (e) HRTEM of Co3 O4 -G2.
observed and the lattice spaces of 0.283 nm could be indexed as (0 0 2) plane of Co3 O4 . These novel structures serve not only as a substrate for the Co3 O4 nanoparticles but also provide channels for the diffusion of molecules. Fig. 7 displays an MO solution in adsorption–desorption equilibrium with Co3 O4 -G0, Co3 O4 -G1, Co3 O4 -G2 and Co3 O4 -G3, under dark conditions. The 3D porous graphene nanocomposites show excellent adsorption capacity for MO. This is likely to be beneficial for photocatalysis, because efficient adsorption can improve contact between MO and the photocatalytic nanoparticles, and accelerate reactions with photogenerated active species. Based on these experimental results, a schematic illustration of the formation of GR-Co3 O4 is presented in Scheme 1. GO formed stable suspensions in water, having a highly negative surface charge (Scheme 1a). When the Co(NO3 )2 solution was added to the GO suspension, Co2+ ions were electrostatically attracted to the GO surface (Scheme 1b) [13]. After freeze-drying these suspensions, freestanding flexible GO composites were formed. Folding and stacking of GO layers resulted in the formation of a three-dimensional porous graphene oxide hybrid structure with Co(NO3 )2 adsorbed
Fig. 7. The absorbance of MO by Co3 O4 -G0, Co3 O4 -G1, Co3 O4 -G2 and Co3 O4 -G3.
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Scheme 1. Proposed scheme of the fabricated processes of Co3 O4 -G.
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onto the GO surface (Scheme 1c) [11]. The final thermal treatment decomposed Co(NO3 )2 to Co3 O4 nanoparticles and reduced GO to GR (Scheme 1d) [14]. As a demonstration of the application of Co3 O4 -G, its photocatalytic performance was evaluated using MO as a representative organic dyestuff. Fig. 8a shows the percentage of MO adsorbed by different catalysts in the presence of H2 O2 in the dark. The percentage of MO adsorbed by Co3 O4 -G0, Co3 O4 -G1, Co3 O4 -G2, and Co3 O4 -G3 was 3.2%, 14.2%, 15.3%, and 18.3%, respectively, indicating that the ability of the above mentioned catalysts to adsorb MO is limited. Fig. 8b shows the profile of the photocatalytic degradation efficiency of MO using different catalysts in the presence of H2 O2 under visible light. It can be seen that 95.3% of MO was degraded using Co3 O4 -G2, while 57.6%, 88.9%, and 78.3% MO degradation was achieved using Co3 O4 -G0, Co3 O4 -G1, and Co3 O4 -G3, respectively. The superior photocatalytic performance of Co3 O4 -G2 is attributed to two factors: (1) as compared to pure Co3 O4 , the larger specific surface area of Co3 O4 -G2 offers more reaction centers, which favors the enhanced photocatalytic activity [15]; (2) in the Co3 O4 -G system, graphene serves as an acceptor of electrons generated in the Co3 O4 semiconductor and effectively decreases the recombination probability of photoexcited electron–hole pairs. However, a further increase in the percentage of black graphene led to shielding of active sites on the catalyst surface and thus decreasing the intensity of light that reaches the catalyst surface. The magnetic properties of Co3 O4 -G2 at room temperature were characterized by a vibrating sample magnetometer. As shown in Fig. 8c, the magnetization hysteresis loops of Co3 O4 -G2 were in the form of S-shaped curves and the saturation magnetization reached 36.8 emug−1 . As
Fig. 8. Photocatalytic degradation efficiency of MO (a) different catalysts in the dark. (b) Different catalysts with H2 O2 under visible light. (c) Magnetic hysteresis loops of Co3 O4 -G2. (The inset shows the removal of MO with the help of magnet.) (d) Cycling runs in the photocatalytic degradation of MO in the presence of Co3 O4 -G2 with H2 O2 under visible light irradiation.
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a powder, Co3 O4 -G2 exhibited a magnetic attraction to an external magnet. The Co3 O4 -G2 samples dispersed in the MO solution could be completely separated from the reaction medium by applying an external magnetic field, leaving a colorless solution (inset in Fig. 8c). The stability of the photocatalyst was tested and after three cycles of the photodegradation of MO, Co3 O4 -G2 did exhibit any obvious loss of activity, which indicated its high stability during the photocatalytic reaction (Fig. 8d). The high photocatalytic performance may be attributed to the following. First, 3D porous graphene provides an excellent support matrix for photocatalytic particles. Second, the excellent electronic conductivity of graphene, imparted by its 2D planar p-conjugation structure, allows it to act as an effective electron conductor. This improves the transfer of photogenerated charges from the photocatalyst surface to the graphene, and promotes the transport of photogenerated charges on the photocatalyst surface, to increase the photocatalytic efficiency. Third, the strong adsorption properties of the hybrid material concentrates dye molecule near to the catalyst. This increases the chances of dye molecules reacting with photogenerated active species, leading to photocatalytic degradation. 4. Conclusions We successfully prepared 3D porous graphene loaded Co3 O4 nanoparticles. The graphene hybrids showed an interconnected porous framework of graphene sheets, with Co3 O4 nanoparticles finely dispersed in the porous graphene. Tests of photocatalytic performance showed that the Co3 O4 -G hybrid materials displayed excellent photocatalytic performance under visible light irradiation and exhibited super paramagnetic properties that allowed for separation of the catalyst from solution. The as-obtained Co3 O4 -G composites were non-hazardous to the environment and may present a practical solution for removing contamination from water. Acknowledgments This work was supported by the Construct Program of the Key Discipline in Hunan Province (XJF[2011] 76), Technology Plan
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