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One-step green synthesis of graphene–ZnO nanocomposites
Materials Letters 98 (2013) 168–170
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
One-step green synthesis of graphene–ZnO nanocomposites Guixiang Du a,n, Xiaoxu Wang a, Lidong Zhang b, Yan Feng a, Yue Liu a a Tianjin Key Laboratory of Structure and Performance for Functional Molecules; Key Laboratory of Inorganic–Organic Hybrid Functional Material Chemistry, Ministry of Education; College of Chemistry, Tianjin Normal University, Tianjin 300387, China b School of Energy and Chemical Technology, Tianjin Bohai Vocational Technical College, Tianjin 300402, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2012 Accepted 9 February 2013 Available online 17 February 2013
Graphene–ZnO nanocomposites were successfully synthesized by a one-step hydrothermal approach, using glucose as a green reducing agent. The in-situ formed hexagonal ZnO particles were randomly decorated on the surfaces of graphene oxide sheets, which were simultaneously reduced directly to form the graphene sheets and the ZnO particles acted as spacers to keep the neighboring sheets separate. Photoluminescence spectra of graphene–ZnO nanocomposites displayed the fluorescence quenching property. Electrochemical tests showed that the graphene–ZnO composite electrodes enhanced the capacitive behavior with higher capacitance values (118 F/g) and better capacitive behavior as compared with pure graphene, which predicted their potential application in energy storage. & 2013 Elsevier B.V. All rights reserved.
Keywords: Graphene oxide Zinc oxide Carbon materials Nanocomposites
1. Introduction
2. Experimental
Graphene (GN), a single two-dimensional carbon sheet with a honeycomb lattice structure, exhibits excellent unique properties [1]. Recently, one new perspective is to utilize the carbon sheet as conductive carbon mat to anchor functional nanomaterials such as metal or metal oxides nanoparticles to form new nanocomposite hybrid materials with potential application in optoelectronics and energy conversion devices [2,3]. In the meantime, such an attachment of particles onto the GN maybe can avoid the restack of GN sheets during the reduction process and thus benefit improving their properties. ZnO, an important semiconductor material, has attracted much attention due to its promising applications [4]. Consequently, it prompts researchers to synthesize the graphene– ZnO (GN–ZnO) composites and explore their potential applications, and the new composites might possess unusual properties as compared with their individual counterparts [5,6]. About the synthesis of composites, the solution-based chemical reduction is most used due to the low-cost and bulk-scale production; however, the normally used reducing agents like hydrazine are highly toxic, and a green and simple method is more feasible [7–9]. Herein, a facile and green method was developed for the largescale synthesis of GN–ZnO composties, in which ZnO particles were decorated on the surfaces of GN sheets and inhibited the restack of the GN sheets. Fluorescence quenching property was investigated by the photoluminescence (PL) measurement. Electrochemical tests indicated that the composites had better capacitive behavior than graphene.
Synthesis of GN–ZnO composites: Graphite oxide (GO) was prepared by a modified Hummers method [10]. The aqueous solution of GO (0.5 mg mL 1) was first sonicated for 1 h, and then 0.13 mmol of Zn(NO3)2 6H2O aqueous solution and ethanolamine were added into the dispersion, followed by stirring for 1 h. Subsequently, 0.17 g of glucose was added to the mixture, which was then hydrothermally treated at 100 1C for 6 h. The obtained samples were washed and dried in a vacuum oven at 60 1C. Characterization: XRD analyses were conducted on a Bruker D8A X-ray diffractometer with a Cu Ka radiation (l ¼ 0.15418 nm). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were performed on a FEI Nova Nano SEM 230 and a FEI Tecnai G2 F20 microscopy, respectively. The room temperature PL spectra were recorded with a HORIBA JY FL-3 spectrophotometer. Galvanostatic charge/discharge (GCD) tests were conducted on a Land cell tester: A mixture of 85 wt% of GN–ZnO and 10 wt% of acetylene black and 5 wt% of polytetrafluoroethylene binder (Alfa Aesar) was pressed on a nickel foam (1 cm2). A two-electrode capacitor was assembled with two GN–ZnO electrodes separated by a polypropylene membrane using 6 M KOH aqueous solution as electrolyte. For comparison, GN obtained by a similar method was used as a reference.
n
Corresponding author. Tel./fax: þ 86 222 376 6516. E-mail address: [email protected] (G. Du).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.02.046
3. Results and discussion The XRD patterns of the GO, GN and GN–ZnO composites are shown in Fig. 1. The feature diffraction peak of GO (Fig. 1a) appearing at 12.1o is observed with an interlayer spacing of 0.73 nm due to the introduction of oxygen-containing groups on carbon nanosheets [11].
G. Du et al. / Materials Letters 98 (2013) 168–170
For GN (Fig. 1b), a weak and broad diffraction peak (24.31), corresponding to the interlayer spacing of 0.37 nm, and the peak located at around 12.11 disappear, confirming the reduction of GO and the exfoliation of the layered GNs [11]. The diffraction peaks of GN–ZnO composites can be ascribed to a hexagonal phase ZnO (JCPDS36– 1451), and no characteristic peaks assigned to GN or GO are found, indicating the effective exfoliation of the layered GN oxide [8,9]. From Fig. 2a and b, it can be clearly indicated that the surface and interlayer of the GN sheets are decorated by ZnO particles with an average size of 500 nm to form sandwich-like structures, avoiding the
169
restack of GN sheets. The hexagonal ZnO particles covered by the curled thin veil-like GN sheets can be obviously observed (Fig. 2c), which agrees with the XRD results. It also can be observed that the corrugated GN sheets are highly transparent (Fig. 2d), illustrating that the GN sheets are really quite thin. The open pore structure implies that the GN–ZnO composites may have potential application in energy storage. To explore the potential applications of the GN–ZnO composite, the PL emission spectra were measured. PL spectrum of pure
Intensity(a.u.)
(a) ZnO (b) GN-ZnO
(a) (b)
500
550
600
650
Wavelength (nm) Fig. 1. (a) XRD patterns of GO, (b) GN and (c) as-prepared GN–ZnO composites.
Fig. 3. (a) Room-temperature PL spectra of ZnO and (b) as-prepared GN–ZnO composites.
Fig. 2. (a–c) SEM images and (d) TEM image of as-prepared GN–ZnO composites.
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G. Du et al. / Materials Letters 98 (2013) 168–170
120
1.0
GN-ZnO
Current : 1---0.1A g-1 2---0.2A g-1 3---0.5A g-1 4---1A g-1 5---2A g-1
0.4 0.2
100
80
GN-ZnO
80 Retention(%)
0.6
100 Specific capacitance(F/g)
Potential (V)
0.8
60 40
60 40
20
20 5 4
0.0
0
2
3
1
0
20
0 0
200
400
600
800
1000
1200
0.0
0.5
40 60 Cycle Number
1.0
80
1.5
100
2.0
Current density(A/g)
Time (S)
Fig. 4. (a) Charge/discharge profiles, (b) specific capacitances of GN–ZnO composites at different current densities (0.1–2 A/g). (The inset of b) Cyclic performance of GN– ZnO composites at 0.2 A/g.
ZnO (Fig. 3a) exhibits a broad emission peak in the wavelength range of 450–650 nm, which was observed in the previous report [9]. The obviously decreased emission intensity of GN– ZnO composites (Fig. 3b), which is caused by the quenching of photoemission, represents an effective interfacial charge-transfer process [9]. Charge recombination in semiconductor particles is often a problem in solar cells based on mesoscopic semiconductor films, and the PL results imply that the composites have potential application in photoelectrochemical cell [9]. The electrochemical performance of the GN–ZnO composites was evaluated by GCD measurement at different current densities from 0 to 1 V. As seen in Fig. 4a, the discharge curves are linear in the total range of potential, showing good capacitive behavior [8]. The specific capacitance (Cspec) of the GN–ZnO composites is 118 F/g at a discharge current density of 0.1 A/g, which is higher than that of previously reported GN–ZnO composites [8,12,13] and that (84 F/g) of our synthesized GN sheets. The Cspec can be kept at 110 F/g, exhibiting a high capacitance retention (93%) when current density increases to 2 A/g (Fig. 4b). The cyclic performances of the GN–ZnO electrode (inset of Fig. 4b) exhibit a little capacitance decay during the tests for 100 cycles, which implies their excellent recycling capabilities.
4. Conclusions In summary, GN–ZnO composites were synthesized in a one-step hydrothermal process, using a green reducing agent. It indicates that ZnO particles are decorated evenly on the surface of GN sheets and inhibit the restack of the GN sheets. Electrochemical tests indicate that the composites enhance the capacitive behavior by comparison to graphene. PL analysis implies the potential optoelectronic application of the composites. This simple synthesis method can be easily adapted to the synthesis of other graphene-based hydrid materials.
Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 51102180, 21103124 and 21245001), Tianjin Science Technology Fund Project for High Education (No. 20110311) and Doctoral Program of Tianjin Normal University (No. 52X09006). References [1] Zhang WQ, Wang D, Kim S, Kim SY, Yakes MK, Laracuente AR, et al. Nanoscale tunable reduction of graphene oxide for graphene electronics. Science 2010;328:1373–6. [2] Huang X, Yin ZY, Wu SX, Qi XY, He QY, Zhang QC, et al. Graphene-based materials: synthesis, characterization, properties, and applications. Small 2011;7:1876–902. [3] Bai S, Shen XP. Graphene-inorganic nanocomposites. Rsc Adv 2012;2:64–98. [4] Hochbaum AI, Yang PD. Semiconducor nanowires for energy conversion. Chem Rev 2010;110:527–46. [5] Li BJ, Cao HQ. ZnO–graphene composite with enhanced performance for the removal of dye from water. J Mater Chem 2011;21:3346–9. [6] Zheng WT, Ho YM, Tian HW, Wen M, Qi JL, Li YA. Field emission from a composite of graphene sheets and ZnO nanowires. J Phys Chem C 2009;113:9164–8. [7] Wu JL, Shen XP, Jiang L, Wang K, Chen KM. Solvothermal synthesis and characterization of sandwich-like graphene/ZnO nanocomposites. Appl Surf Sci 2010;256:2826–30. [8] Wang J, Gao Z, Li ZS, Wang B, Yan YX, Liu Q, et al. Green synthesis of graphene nanosheets/ZnO composites and electrochemical properties. J Solid State Chem 2011;184:1421–7. [9] Zou WB, Zhu JW, Sun YX, Wang X. Depositing ZnO nanoparticles onto graphene in a polyol system. Mater Chem Phys 2011;125:617–20. [10] Zhang JL, Yang HJ, Shen GX, Cheng P, Zhang JY, Guo SW. Reduction of graphene oxide via L-ascorbic acid. Chem Commun 2010;46:1112–4. [11] Zhu CZ, Guo SJ, Fang YX, Dong SJ. Reducing sugar: new functional molecules for the green synthesis of graphene nanosheets. ACS Nano 2010;4:2429–37. [12] Zhang YP, Li HB, Pan LK, Lu T, Sun Z. Capacitive behavior of graphene–ZnO composite film for supercapacitors. J Electroanal Chem 2009;634:68–71. [13] Lu T, Zhang YP, Li HB, Pan LK, Li YL, Sun Z. Electrochemical behaviors of graphene–ZnO and graphene–SnO2 composite films for supercapacitors. Electrochim Acta 2010;55:4170–3.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
One-step green synthesis of graphene–ZnO nanocomposites Guixiang Du a,n, Xiaoxu Wang a, Lidong Zhang b, Yan Feng a, Yue Liu a a Tianjin Key Laboratory of Structure and Performance for Functional Molecules; Key Laboratory of Inorganic–Organic Hybrid Functional Material Chemistry, Ministry of Education; College of Chemistry, Tianjin Normal University, Tianjin 300387, China b School of Energy and Chemical Technology, Tianjin Bohai Vocational Technical College, Tianjin 300402, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2012 Accepted 9 February 2013 Available online 17 February 2013
Graphene–ZnO nanocomposites were successfully synthesized by a one-step hydrothermal approach, using glucose as a green reducing agent. The in-situ formed hexagonal ZnO particles were randomly decorated on the surfaces of graphene oxide sheets, which were simultaneously reduced directly to form the graphene sheets and the ZnO particles acted as spacers to keep the neighboring sheets separate. Photoluminescence spectra of graphene–ZnO nanocomposites displayed the fluorescence quenching property. Electrochemical tests showed that the graphene–ZnO composite electrodes enhanced the capacitive behavior with higher capacitance values (118 F/g) and better capacitive behavior as compared with pure graphene, which predicted their potential application in energy storage. & 2013 Elsevier B.V. All rights reserved.
Keywords: Graphene oxide Zinc oxide Carbon materials Nanocomposites
1. Introduction
2. Experimental
Graphene (GN), a single two-dimensional carbon sheet with a honeycomb lattice structure, exhibits excellent unique properties [1]. Recently, one new perspective is to utilize the carbon sheet as conductive carbon mat to anchor functional nanomaterials such as metal or metal oxides nanoparticles to form new nanocomposite hybrid materials with potential application in optoelectronics and energy conversion devices [2,3]. In the meantime, such an attachment of particles onto the GN maybe can avoid the restack of GN sheets during the reduction process and thus benefit improving their properties. ZnO, an important semiconductor material, has attracted much attention due to its promising applications [4]. Consequently, it prompts researchers to synthesize the graphene– ZnO (GN–ZnO) composites and explore their potential applications, and the new composites might possess unusual properties as compared with their individual counterparts [5,6]. About the synthesis of composites, the solution-based chemical reduction is most used due to the low-cost and bulk-scale production; however, the normally used reducing agents like hydrazine are highly toxic, and a green and simple method is more feasible [7–9]. Herein, a facile and green method was developed for the largescale synthesis of GN–ZnO composties, in which ZnO particles were decorated on the surfaces of GN sheets and inhibited the restack of the GN sheets. Fluorescence quenching property was investigated by the photoluminescence (PL) measurement. Electrochemical tests indicated that the composites had better capacitive behavior than graphene.
Synthesis of GN–ZnO composites: Graphite oxide (GO) was prepared by a modified Hummers method [10]. The aqueous solution of GO (0.5 mg mL 1) was first sonicated for 1 h, and then 0.13 mmol of Zn(NO3)2 6H2O aqueous solution and ethanolamine were added into the dispersion, followed by stirring for 1 h. Subsequently, 0.17 g of glucose was added to the mixture, which was then hydrothermally treated at 100 1C for 6 h. The obtained samples were washed and dried in a vacuum oven at 60 1C. Characterization: XRD analyses were conducted on a Bruker D8A X-ray diffractometer with a Cu Ka radiation (l ¼ 0.15418 nm). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were performed on a FEI Nova Nano SEM 230 and a FEI Tecnai G2 F20 microscopy, respectively. The room temperature PL spectra were recorded with a HORIBA JY FL-3 spectrophotometer. Galvanostatic charge/discharge (GCD) tests were conducted on a Land cell tester: A mixture of 85 wt% of GN–ZnO and 10 wt% of acetylene black and 5 wt% of polytetrafluoroethylene binder (Alfa Aesar) was pressed on a nickel foam (1 cm2). A two-electrode capacitor was assembled with two GN–ZnO electrodes separated by a polypropylene membrane using 6 M KOH aqueous solution as electrolyte. For comparison, GN obtained by a similar method was used as a reference.
n
Corresponding author. Tel./fax: þ 86 222 376 6516. E-mail address: [email protected] (G. Du).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.02.046
3. Results and discussion The XRD patterns of the GO, GN and GN–ZnO composites are shown in Fig. 1. The feature diffraction peak of GO (Fig. 1a) appearing at 12.1o is observed with an interlayer spacing of 0.73 nm due to the introduction of oxygen-containing groups on carbon nanosheets [11].
G. Du et al. / Materials Letters 98 (2013) 168–170
For GN (Fig. 1b), a weak and broad diffraction peak (24.31), corresponding to the interlayer spacing of 0.37 nm, and the peak located at around 12.11 disappear, confirming the reduction of GO and the exfoliation of the layered GNs [11]. The diffraction peaks of GN–ZnO composites can be ascribed to a hexagonal phase ZnO (JCPDS36– 1451), and no characteristic peaks assigned to GN or GO are found, indicating the effective exfoliation of the layered GN oxide [8,9]. From Fig. 2a and b, it can be clearly indicated that the surface and interlayer of the GN sheets are decorated by ZnO particles with an average size of 500 nm to form sandwich-like structures, avoiding the
169
restack of GN sheets. The hexagonal ZnO particles covered by the curled thin veil-like GN sheets can be obviously observed (Fig. 2c), which agrees with the XRD results. It also can be observed that the corrugated GN sheets are highly transparent (Fig. 2d), illustrating that the GN sheets are really quite thin. The open pore structure implies that the GN–ZnO composites may have potential application in energy storage. To explore the potential applications of the GN–ZnO composite, the PL emission spectra were measured. PL spectrum of pure
Intensity(a.u.)
(a) ZnO (b) GN-ZnO
(a) (b)
500
550
600
650
Wavelength (nm) Fig. 1. (a) XRD patterns of GO, (b) GN and (c) as-prepared GN–ZnO composites.
Fig. 3. (a) Room-temperature PL spectra of ZnO and (b) as-prepared GN–ZnO composites.
Fig. 2. (a–c) SEM images and (d) TEM image of as-prepared GN–ZnO composites.
170
G. Du et al. / Materials Letters 98 (2013) 168–170
120
1.0
GN-ZnO
Current : 1---0.1A g-1 2---0.2A g-1 3---0.5A g-1 4---1A g-1 5---2A g-1
0.4 0.2
100
80
GN-ZnO
80 Retention(%)
0.6
100 Specific capacitance(F/g)
Potential (V)
0.8
60 40
60 40
20
20 5 4
0.0
0
2
3
1
0
20
0 0
200
400
600
800
1000
1200
0.0
0.5
40 60 Cycle Number
1.0
80
1.5
100
2.0
Current density(A/g)
Time (S)
Fig. 4. (a) Charge/discharge profiles, (b) specific capacitances of GN–ZnO composites at different current densities (0.1–2 A/g). (The inset of b) Cyclic performance of GN– ZnO composites at 0.2 A/g.
ZnO (Fig. 3a) exhibits a broad emission peak in the wavelength range of 450–650 nm, which was observed in the previous report [9]. The obviously decreased emission intensity of GN– ZnO composites (Fig. 3b), which is caused by the quenching of photoemission, represents an effective interfacial charge-transfer process [9]. Charge recombination in semiconductor particles is often a problem in solar cells based on mesoscopic semiconductor films, and the PL results imply that the composites have potential application in photoelectrochemical cell [9]. The electrochemical performance of the GN–ZnO composites was evaluated by GCD measurement at different current densities from 0 to 1 V. As seen in Fig. 4a, the discharge curves are linear in the total range of potential, showing good capacitive behavior [8]. The specific capacitance (Cspec) of the GN–ZnO composites is 118 F/g at a discharge current density of 0.1 A/g, which is higher than that of previously reported GN–ZnO composites [8,12,13] and that (84 F/g) of our synthesized GN sheets. The Cspec can be kept at 110 F/g, exhibiting a high capacitance retention (93%) when current density increases to 2 A/g (Fig. 4b). The cyclic performances of the GN–ZnO electrode (inset of Fig. 4b) exhibit a little capacitance decay during the tests for 100 cycles, which implies their excellent recycling capabilities.
4. Conclusions In summary, GN–ZnO composites were synthesized in a one-step hydrothermal process, using a green reducing agent. It indicates that ZnO particles are decorated evenly on the surface of GN sheets and inhibit the restack of the GN sheets. Electrochemical tests indicate that the composites enhance the capacitive behavior by comparison to graphene. PL analysis implies the potential optoelectronic application of the composites. This simple synthesis method can be easily adapted to the synthesis of other graphene-based hydrid materials.
Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 51102180, 21103124 and 21245001), Tianjin Science Technology Fund Project for High Education (No. 20110311) and Doctoral Program of Tianjin Normal University (No. 52X09006). References [1] Zhang WQ, Wang D, Kim S, Kim SY, Yakes MK, Laracuente AR, et al. Nanoscale tunable reduction of graphene oxide for graphene electronics. Science 2010;328:1373–6. [2] Huang X, Yin ZY, Wu SX, Qi XY, He QY, Zhang QC, et al. Graphene-based materials: synthesis, characterization, properties, and applications. Small 2011;7:1876–902. [3] Bai S, Shen XP. Graphene-inorganic nanocomposites. Rsc Adv 2012;2:64–98. [4] Hochbaum AI, Yang PD. Semiconducor nanowires for energy conversion. Chem Rev 2010;110:527–46. [5] Li BJ, Cao HQ. ZnO–graphene composite with enhanced performance for the removal of dye from water. J Mater Chem 2011;21:3346–9. [6] Zheng WT, Ho YM, Tian HW, Wen M, Qi JL, Li YA. Field emission from a composite of graphene sheets and ZnO nanowires. J Phys Chem C 2009;113:9164–8. [7] Wu JL, Shen XP, Jiang L, Wang K, Chen KM. Solvothermal synthesis and characterization of sandwich-like graphene/ZnO nanocomposites. Appl Surf Sci 2010;256:2826–30. [8] Wang J, Gao Z, Li ZS, Wang B, Yan YX, Liu Q, et al. Green synthesis of graphene nanosheets/ZnO composites and electrochemical properties. J Solid State Chem 2011;184:1421–7. [9] Zou WB, Zhu JW, Sun YX, Wang X. Depositing ZnO nanoparticles onto graphene in a polyol system. Mater Chem Phys 2011;125:617–20. [10] Zhang JL, Yang HJ, Shen GX, Cheng P, Zhang JY, Guo SW. Reduction of graphene oxide via L-ascorbic acid. Chem Commun 2010;46:1112–4. [11] Zhu CZ, Guo SJ, Fang YX, Dong SJ. Reducing sugar: new functional molecules for the green synthesis of graphene nanosheets. ACS Nano 2010;4:2429–37. [12] Zhang YP, Li HB, Pan LK, Lu T, Sun Z. Capacitive behavior of graphene–ZnO composite film for supercapacitors. J Electroanal Chem 2009;634:68–71. [13] Lu T, Zhang YP, Li HB, Pan LK, Li YL, Sun Z. Electrochemical behaviors of graphene–ZnO and graphene–SnO2 composite films for supercapacitors. Electrochim Acta 2010;55:4170–3.