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One-pot synthesis of vanadium dioxide nanoflowers on graphene oxide
Author’s Accepted Manuscript One-pot synthesis of vanadium nanoflowers on graphene oxide
dioxide
Xiao Juan Kang, Jun Ming Zhang, Xiao Wen Sun, Feng Rui Zhang, Yu Xin Zhang www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)00213-3 http://dx.doi.org/10.1016/j.ceramint.2016.01.170 CERI12146
To appear in: Ceramics International Received date: 4 December 2015 Revised date: 11 January 2016 Accepted date: 19 January 2016 Cite this article as: Xiao Juan Kang, Jun Ming Zhang, Xiao Wen Sun, Feng Rui Zhang and Yu Xin Zhang, One-pot synthesis of vanadium dioxide nanoflowers on graphene oxide, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.01.170 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-pot synthesis of vanadium dioxide nanoflowers on graphene oxide Xiao Juan Kanga, Jun Ming Zhanga, Xiao Wen Suna, Feng Rui Zhang a, Yu Xin Zhanga, b* a
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P.R. China
b
National Key Laboratory of Fundamental Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, P.R. China
*Corresponding author, Tel.: +86 23 65104131; Fax: +86 23 65104131 E-mail address: [email protected] (Dr. Y.X. Zhang)
Abstract A novel graphene oxide (GO)-based nanostructure is synthesized by a one-pot hydrothermal reaction. The nano-hybrid composite materials are based on 2-dimensional (2D) GO sheets and 3-dimensional (3D) VO2 flowers, which are composed of ultrathin 2-dimensional (2D) nanobelts. Interestingly, the 3D VO2 flowers can be well controlled by fine-tuning the preparative parameters (e.g., temperature, reaction time). Such self-assembled 3D flower morphology possesses a higher surface area and a better contact with the graphene matrix, which could be an effective route to extend to other metal oxides. Keywords: Graphene Oxide; Nanocomposites; Vanadium oxide; Synthesis
1. Introduction Supercapacitors (SCs) [1, 2] have attracted significant attention due to their high power density, extraordinary pulse charge/discharge characteristics, super-high cycling life and safe operation ability. According to the working mechanisms, SCs are widely divided into two kinds: double layer supercapacitors and pseudocapacitors. Based on transition metal oxides, the pseudocapacitors exhibit much higher specific capacitance than those based on carbonaceous materials and conducting polymers as they can provide a variety of oxidation states which enjoy significant efficiency in redox charge
1
transfer. To date, numerous researchers have spending great efforts in searching for inexpensive transition metal oxides with good capacitive characteristics, such as NiO [3, 4], Co3O4 [5], MnO2 [6-9], Fe3O4 [10], RuO2 [11], V2O5 [12, 13], and CuO [14, 15]. Among various transition-metal oxides, VOx has been widely investigated as a high-potential candidate material due to their low cost, abundant resources, layered structure, high energy density, and wide potential window arising from its multivalent oxidation states. VO x-based materials have achieved remarkable benchmark properties in various fields, such as lithium-ion batteries, gas sensors, field-effect transistors and supercapacitors. However, transition metal oxides as well as porous carbon materials are suffering from some inverse properties, such as high resistivity, poor cyclic performance, low electronic conductivity and large volume change during the charge-discharge process. As a result, the energy storage in supercapacitors is currently an order of magnitude lower than that of batteries, which limits their adoption to considerate applications that require high cycle life and power density. Therefore, graphene/metal oxides are expected to provide a new solution to improve the performance of supercapacitor electrodes [16]. As a matter of fact, the performance of supercapacitor electrodes can be significantly improved by combining carbon materials with transition metal oxides. Recently, a readily low-temperature hydrothermal process which can fabricate graphene-decorated VO2 nanoflowers (GVNFs) rapidly without any harmful oxidizing or reducing chemical agents and surfactants has been reported. The generated product has shown incredible performance during characterizations, while most proportion of the reported compounds are net-like [17], needle-like [18] or bar-like [19, 20] with a relatively uniform morphology, the synthesized outcomes which are pretty long in third dimension may fall
2
behind when compared to an even elegant nanostructure. Herein, a novel morphology with an inconceivable flower-like feature has been fabricated with vanadium dioxide decorated on graphene oxide through a facile hydrothermal method. Furthermore, a novel formation mechanism of VO2 nano flowers was proposed by comparing the mutual distinction when modifying the ratio of precursors.
2. Experimental Section 2.1 Materials NaNO3 (≥99.0%), H2C2O4, H2SO4 (≥99.0%), KMnO4, H2O2 (30% aqueous solution), and HCl were purchased from Alfa Aesar. And graphite powder (SP-1, Bay carbon) was purchased from Aladdin company. All the chemicals were used in the synthesis without any further treatment.
2.2 Synthesis of graphene oxide (GO) Graphene oxide was prepared according to the modified Hummers’ method [21]. Briefly, graphite powders (2 g), NaNO3 powder (1 g) and concentrated H2SO4 (100 ml) were added to a Pyrex flask in an ice bath under mechanical stirring, followed by slow addition of KMnO4 (6.0 g) for 4 hours. Subsequently, ultrapure water (200 mL) was added slowly into the mixture, and the obtained dispersion was stirred for 15 min at 95 °C. Then an additional deionized water (200 mL) and H2O2 (10 mL) were added. Finally, HCl aqueous solution (20 ml) was transferred into the flask by pipette to terminate the reaction. The obtained dispersion was filtered and washed with deionized water through stirring and stewing for 4 days to remove part of the metal ions and residual acid. After that, the obtained GO suspension was further purified by centrifugation four times at 8500 rpm for 15 min each to remove un-exfoliated graphene oxide particles.
2.3 Synthesis of Graphene-decorated VO2 Nanoflowers (GVNFs) 3
In a typical hydrothermal procedure, NH4NO3 (30 mg) and H2C2O4 (0.832 g) were dissolved in ultrapure water and stirred for 10 min. After that, the dispersion solutions were dropped wisely into the graphene oxide aqueous solution under vigorous stirring for 15 min. And then, the obtained solution was transferred into a Teflon-lined stainless steel autoclave (50 ml) and sealed at 120 °C for 24 h in an oven. When the hydrothermal process was finished, the heated autoclave cooled down to room temperature naturally. The products were obtained by suction filtration and washed with distilled water for 5 times. Finally, the as-prepared sample was freeze-dried overnight. To obtain the comparison products, the other VO2 nanostructures were synthesized just by varying the ratio of NH4VO3 to 45 mg and 90 mg respectively.
2.4 Characterization The crystallographic information and chemical composition of as-prepared products were established by powder X-ray diffraction (XRD, D/max 1200, Cu Ka). The morphological investigations were carried out with focused ion beam scanning electron microscopy (FIB/SEM, Zeiss Auriga).
3. Results and Discussion Fig.1 shows XRD patterns of as-prepared GO, NH4VO3 powders and graphene-decorated VO2 nanoflowers (GVNFs) composites. The GO pattern is predominated by a single strong peak at 9.8˚, corresponding to an interlayer distance of 9.0 Å (Fig. 1a) [20]. The expansion of the gaps relative to the graphite (d002=3.4 Å) is caused by oxidation of the graphene sheets and intercalation of water and oxygen functionalities. The characteristic peaks in the XRD patterns of pure NH4VO3 samples can be unambiguously indexed to the orthorhombic NH 4VO3 phase (JCPDS card no.01-070-0678) space group (a = 4.91 Å, b =11.78 Å, and c = 5.83 Å), without other impurity peaks were detected (Fig. 1b), confirming the fitness of precursors. Furthermore, the patterns of VO2 nanoflowers grown on GO 4
sheets are presented in Fig. 1c, which are indexed in the space group C2/m with standard lattice constants a=11.60 Å and c=6.15 Å for VO2 (B) with a monoclinic structure (JCPDS no. 31-1438). It is universally acknowledged that the ultrathin and flexible nature of GO sheets without aggregating can be clearly observed in the high magnification FESEM image (Fig. 2a and b). Subsequently, the GVNFs have been also synthesized for comparison. As seen from Fig. 2c and d, the as-prepared ultrasmall VO2 microspheres are flower-like and densely assembled from many uniform nanosized VO2 ribbons whose length are typically about 50 nm width and the nanoflowers’ diameter of 800 nm accounts for more than 60%. While the large VO2 nanoflowers growing on the GO sheets (Fig. 2e and f) are configured by vast quantity of big VO 2 nanoribbons. Clearly, randomly cross-linked GO sheets can be observed in a flexible state, forming a hierarchical 3D architecture. As a consequence, this interaction between VO2 and GO can alleviate the irreversible agglomeration of GO and also improve electrical conductivity of the composite. Noteworthy, the graphene network serves as an efficient electron pathway in the transmissive channel to provide better electrical contacts with VO 2 nanobelts for pseudocapacitance generation. Subsequently, the EDS mappings across the surface of the GVNFs showed the homogeneous distributions of V, O, and C elements, respectively, illustrated in Fig. 3. Accordingly, one can conclude that the VO2 has been uniformly inserted into GO interlayers and considerately amplifies the distances between adjacent GO sheets. The hydrothermal reaction of a mixture of colloidal dispersion of GO and ammonium vanadate (NH4VO3) in the presence of oxalic acid (H2C2O4) results in the formation of the desired hybrid. A GO@VO2 hybrid in which ultrathin nanobelt-built VO2 flowers are anchored on GO has been fabricated. During the formation of graphene hydrogels, oxalic acid has been used in the reaction to create the
5
necessary acidity and reductive atmosphere for the formation of VO2. Amid the atmosphere of oxalic acid, NH4VO3 is initially dissolved and permeated in the suspensions in the form of VO2+ ions. As VO2+ diffused around the circumference of GO sheets, they preferentially clustered on the defects of GO sheets. Hydrothermal reduction of ammonium vanadate in the presence of colloidal dispersion of GO results in simultaneous reduction of GO into reduced graphene oxide and formation of VO2. VO2 nucleus was first sprouted on the imperfections of graphene sheets, subsequently, when the radius reached the critical point which can ensure the steady growth of VO2 nucleus, the vast quantity of VO2+ ions continue to aggregate to promote the extension of the core. Consequently, VO2 nanoflowers are constructed from ultrathin nanobelts in a novel self-assembly approach. The relative reaction equation of the hydrothermal synthesis can be expressed as follows: NH 4VO3 2H e VO2 H 2O NH 4
(1)
The average size of nanoflowers is determined by the ammonium vanadate’s weight ratio as well as the nucleation process and the growth law. With the increasing of the concentration of NH4VO3, more VO2+ would be produced in the solution system which can promote the growth of the crystal nucleus and the radius of the nanoflowers. The size of the nanoflowers in the solution system with low concentration of NH4VO3 is typically smaller than that with high concentration of NH4VO3 because of the less VO2+ supporting for the growth of nucleus. As a consequence of kind of novel flower-like morphology with a high SSA and a high surface-to-bulk ratio, ions transparency amid this 3D network can be much more readily, resulting in better kinetic performance. It remains true that such kind of nanostructure can be a promising aspect in the subsequent investigation, i.e., molecular polymer coating will be surveyed to further improve its prosperity [22-24].
4. Conclusion 6
In summary, 3-dimensional composites based on a unique combination of graphene oxide nanosheets and VO2 nanoflowers have been successfully synthesized via a facile low-temperature hydrothermal method. It is worth noting that the size of GVNFs changed dramatically with the increasing of NH4VO3 precursor’s proportion from 1:1 to 2:1 in the suspension. There is no doubt that these V2O5 nanoflowers/GO composites with stylish performance can be very promising for high performance electrode materials in supercapacitors and other energy-storage devices.
Acknowledgments The authors gratefully acknowledge the financial supports provided by National Natural Science Foundation of China (Grant no. 21576034), and National Training Program of Innovation and Entrepreneurship for Undergraduates (2015510611104).
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[15] Y. Li, Q. Wang, P. Liu, X. Yang, G. Du, Y. Liu, Facile synthesis and capacitive performance of Cu@Cu2O/graphene nanocomposites, Ceramics International 41 (3) (2015) 4248-4253. [16] Y. Huang, J. Liang, Y. Chen, An overview of the applications of graphene-based materials in supercapacitors, Small 8 (12) (2012) 1805-1834. [17] T. Qian, N. Xu, J. Zhou, T. Yang, X. Liu, X. Shen, J. Liang, C. Yan, Interconnected three-dimensional V2O5/polypyrrole network nanostructures for high performance solid-state supercapacitors, J. Mater. Chem. A 3 (2) (2015) 488-493. [18] S.D. Perera, M. Rudolph, R.G. Mariano, N. Nijem, J.P. Ferraris, Y.J. Chabal, K.J. Balkus, Manganese oxide nanorod–graphene/vanadium oxide nanowire–graphene binder-free paper electrodes for metal oxide hybrid supercapacitors, Nano Energy 2 (5) (2013) 966-975. [19] Y. Sun, S.-B. Yang, L.-P. Lv, I. Lieberwirth, L.-C. Zhang, C.-X. Ding, C.-H. Chen, A composite film of reduced graphene oxide modified vanadium oxide nanoribbons as a free standing cathode material for rechargeable lithium batteries, Journal of Power Sources 241
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[20] H. Wang, H. Yi, X. Chen, X. Wang, One-step strategy to three-dimensional graphene/VO2 nanobelt composite hydrogels for high performance supercapacitors, J. Mater. Chem. A 2 (4) (2014) 1165-1173. [21] M. Zhang, W. Yuan, B. Yao, C. Li, G. Shi, Solution-Processed PEDOT:PSS/Graphene Composites as the Electrocatalyst for Oxygen Reduction Reaction, ACS applied materials & interfaces 6 (5) (2014) 3587-3593. [22] Y.S. Lim, Y.P. Tan, H.N. Lim, N.M. Huang, W.T. Tan, M.A. Yarmo, C.-Y. Yin, Potentiostatically deposited polypyrrole/graphene decorated nano-manganese oxide ternary film for supercapacitors, Ceramics International 40 (3) (2014) 3855-3864. [23] K. Zhang, L.L. Zhang, X.S. Zhao, J. Wu, Graphene/Polyaniline Nanofiber Composites as Supercapacitor Electrodes, Chemistry of Materials 22 (4) (2010) 1392-1401. [24] P.A. Mini, A. Balakrishnan, S.V. Nair, K.R. Subramanian, Highly super capacitive electrodes made of graphene/poly(pyrrole), Chemical communications 47 (20) (2011) 5753-5755. 8
Figure Captions Fig. 1 XRD spectra of (a) initial graphene oxide (b)NH4VO3 (c) composites graphene-decorated VO2 nanoflowers (GVNFs) measured in the 2θ range 5° to 80°. Fig. 2 SEM images of (a) and (b) GO Sheets, (c) and (d) small VO2 Nanoflowers/GOs, (e) and (f) large VO2 Nanoflowers/GOs. Fig. 3 The EDS mappings across the surface of the GVNFs. Fig. 4 The synthesis procedure of as-prepared vanadium dioxide nano-flowers grown on graphene oxide.
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Fig. 2
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Fig. 3
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Fig. 4
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dioxide
Xiao Juan Kang, Jun Ming Zhang, Xiao Wen Sun, Feng Rui Zhang, Yu Xin Zhang www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)00213-3 http://dx.doi.org/10.1016/j.ceramint.2016.01.170 CERI12146
To appear in: Ceramics International Received date: 4 December 2015 Revised date: 11 January 2016 Accepted date: 19 January 2016 Cite this article as: Xiao Juan Kang, Jun Ming Zhang, Xiao Wen Sun, Feng Rui Zhang and Yu Xin Zhang, One-pot synthesis of vanadium dioxide nanoflowers on graphene oxide, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.01.170 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-pot synthesis of vanadium dioxide nanoflowers on graphene oxide Xiao Juan Kanga, Jun Ming Zhanga, Xiao Wen Suna, Feng Rui Zhang a, Yu Xin Zhanga, b* a
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P.R. China
b
National Key Laboratory of Fundamental Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, P.R. China
*Corresponding author, Tel.: +86 23 65104131; Fax: +86 23 65104131 E-mail address: [email protected] (Dr. Y.X. Zhang)
Abstract A novel graphene oxide (GO)-based nanostructure is synthesized by a one-pot hydrothermal reaction. The nano-hybrid composite materials are based on 2-dimensional (2D) GO sheets and 3-dimensional (3D) VO2 flowers, which are composed of ultrathin 2-dimensional (2D) nanobelts. Interestingly, the 3D VO2 flowers can be well controlled by fine-tuning the preparative parameters (e.g., temperature, reaction time). Such self-assembled 3D flower morphology possesses a higher surface area and a better contact with the graphene matrix, which could be an effective route to extend to other metal oxides. Keywords: Graphene Oxide; Nanocomposites; Vanadium oxide; Synthesis
1. Introduction Supercapacitors (SCs) [1, 2] have attracted significant attention due to their high power density, extraordinary pulse charge/discharge characteristics, super-high cycling life and safe operation ability. According to the working mechanisms, SCs are widely divided into two kinds: double layer supercapacitors and pseudocapacitors. Based on transition metal oxides, the pseudocapacitors exhibit much higher specific capacitance than those based on carbonaceous materials and conducting polymers as they can provide a variety of oxidation states which enjoy significant efficiency in redox charge
1
transfer. To date, numerous researchers have spending great efforts in searching for inexpensive transition metal oxides with good capacitive characteristics, such as NiO [3, 4], Co3O4 [5], MnO2 [6-9], Fe3O4 [10], RuO2 [11], V2O5 [12, 13], and CuO [14, 15]. Among various transition-metal oxides, VOx has been widely investigated as a high-potential candidate material due to their low cost, abundant resources, layered structure, high energy density, and wide potential window arising from its multivalent oxidation states. VO x-based materials have achieved remarkable benchmark properties in various fields, such as lithium-ion batteries, gas sensors, field-effect transistors and supercapacitors. However, transition metal oxides as well as porous carbon materials are suffering from some inverse properties, such as high resistivity, poor cyclic performance, low electronic conductivity and large volume change during the charge-discharge process. As a result, the energy storage in supercapacitors is currently an order of magnitude lower than that of batteries, which limits their adoption to considerate applications that require high cycle life and power density. Therefore, graphene/metal oxides are expected to provide a new solution to improve the performance of supercapacitor electrodes [16]. As a matter of fact, the performance of supercapacitor electrodes can be significantly improved by combining carbon materials with transition metal oxides. Recently, a readily low-temperature hydrothermal process which can fabricate graphene-decorated VO2 nanoflowers (GVNFs) rapidly without any harmful oxidizing or reducing chemical agents and surfactants has been reported. The generated product has shown incredible performance during characterizations, while most proportion of the reported compounds are net-like [17], needle-like [18] or bar-like [19, 20] with a relatively uniform morphology, the synthesized outcomes which are pretty long in third dimension may fall
2
behind when compared to an even elegant nanostructure. Herein, a novel morphology with an inconceivable flower-like feature has been fabricated with vanadium dioxide decorated on graphene oxide through a facile hydrothermal method. Furthermore, a novel formation mechanism of VO2 nano flowers was proposed by comparing the mutual distinction when modifying the ratio of precursors.
2. Experimental Section 2.1 Materials NaNO3 (≥99.0%), H2C2O4, H2SO4 (≥99.0%), KMnO4, H2O2 (30% aqueous solution), and HCl were purchased from Alfa Aesar. And graphite powder (SP-1, Bay carbon) was purchased from Aladdin company. All the chemicals were used in the synthesis without any further treatment.
2.2 Synthesis of graphene oxide (GO) Graphene oxide was prepared according to the modified Hummers’ method [21]. Briefly, graphite powders (2 g), NaNO3 powder (1 g) and concentrated H2SO4 (100 ml) were added to a Pyrex flask in an ice bath under mechanical stirring, followed by slow addition of KMnO4 (6.0 g) for 4 hours. Subsequently, ultrapure water (200 mL) was added slowly into the mixture, and the obtained dispersion was stirred for 15 min at 95 °C. Then an additional deionized water (200 mL) and H2O2 (10 mL) were added. Finally, HCl aqueous solution (20 ml) was transferred into the flask by pipette to terminate the reaction. The obtained dispersion was filtered and washed with deionized water through stirring and stewing for 4 days to remove part of the metal ions and residual acid. After that, the obtained GO suspension was further purified by centrifugation four times at 8500 rpm for 15 min each to remove un-exfoliated graphene oxide particles.
2.3 Synthesis of Graphene-decorated VO2 Nanoflowers (GVNFs) 3
In a typical hydrothermal procedure, NH4NO3 (30 mg) and H2C2O4 (0.832 g) were dissolved in ultrapure water and stirred for 10 min. After that, the dispersion solutions were dropped wisely into the graphene oxide aqueous solution under vigorous stirring for 15 min. And then, the obtained solution was transferred into a Teflon-lined stainless steel autoclave (50 ml) and sealed at 120 °C for 24 h in an oven. When the hydrothermal process was finished, the heated autoclave cooled down to room temperature naturally. The products were obtained by suction filtration and washed with distilled water for 5 times. Finally, the as-prepared sample was freeze-dried overnight. To obtain the comparison products, the other VO2 nanostructures were synthesized just by varying the ratio of NH4VO3 to 45 mg and 90 mg respectively.
2.4 Characterization The crystallographic information and chemical composition of as-prepared products were established by powder X-ray diffraction (XRD, D/max 1200, Cu Ka). The morphological investigations were carried out with focused ion beam scanning electron microscopy (FIB/SEM, Zeiss Auriga).
3. Results and Discussion Fig.1 shows XRD patterns of as-prepared GO, NH4VO3 powders and graphene-decorated VO2 nanoflowers (GVNFs) composites. The GO pattern is predominated by a single strong peak at 9.8˚, corresponding to an interlayer distance of 9.0 Å (Fig. 1a) [20]. The expansion of the gaps relative to the graphite (d002=3.4 Å) is caused by oxidation of the graphene sheets and intercalation of water and oxygen functionalities. The characteristic peaks in the XRD patterns of pure NH4VO3 samples can be unambiguously indexed to the orthorhombic NH 4VO3 phase (JCPDS card no.01-070-0678) space group (a = 4.91 Å, b =11.78 Å, and c = 5.83 Å), without other impurity peaks were detected (Fig. 1b), confirming the fitness of precursors. Furthermore, the patterns of VO2 nanoflowers grown on GO 4
sheets are presented in Fig. 1c, which are indexed in the space group C2/m with standard lattice constants a=11.60 Å and c=6.15 Å for VO2 (B) with a monoclinic structure (JCPDS no. 31-1438). It is universally acknowledged that the ultrathin and flexible nature of GO sheets without aggregating can be clearly observed in the high magnification FESEM image (Fig. 2a and b). Subsequently, the GVNFs have been also synthesized for comparison. As seen from Fig. 2c and d, the as-prepared ultrasmall VO2 microspheres are flower-like and densely assembled from many uniform nanosized VO2 ribbons whose length are typically about 50 nm width and the nanoflowers’ diameter of 800 nm accounts for more than 60%. While the large VO2 nanoflowers growing on the GO sheets (Fig. 2e and f) are configured by vast quantity of big VO 2 nanoribbons. Clearly, randomly cross-linked GO sheets can be observed in a flexible state, forming a hierarchical 3D architecture. As a consequence, this interaction between VO2 and GO can alleviate the irreversible agglomeration of GO and also improve electrical conductivity of the composite. Noteworthy, the graphene network serves as an efficient electron pathway in the transmissive channel to provide better electrical contacts with VO 2 nanobelts for pseudocapacitance generation. Subsequently, the EDS mappings across the surface of the GVNFs showed the homogeneous distributions of V, O, and C elements, respectively, illustrated in Fig. 3. Accordingly, one can conclude that the VO2 has been uniformly inserted into GO interlayers and considerately amplifies the distances between adjacent GO sheets. The hydrothermal reaction of a mixture of colloidal dispersion of GO and ammonium vanadate (NH4VO3) in the presence of oxalic acid (H2C2O4) results in the formation of the desired hybrid. A GO@VO2 hybrid in which ultrathin nanobelt-built VO2 flowers are anchored on GO has been fabricated. During the formation of graphene hydrogels, oxalic acid has been used in the reaction to create the
5
necessary acidity and reductive atmosphere for the formation of VO2. Amid the atmosphere of oxalic acid, NH4VO3 is initially dissolved and permeated in the suspensions in the form of VO2+ ions. As VO2+ diffused around the circumference of GO sheets, they preferentially clustered on the defects of GO sheets. Hydrothermal reduction of ammonium vanadate in the presence of colloidal dispersion of GO results in simultaneous reduction of GO into reduced graphene oxide and formation of VO2. VO2 nucleus was first sprouted on the imperfections of graphene sheets, subsequently, when the radius reached the critical point which can ensure the steady growth of VO2 nucleus, the vast quantity of VO2+ ions continue to aggregate to promote the extension of the core. Consequently, VO2 nanoflowers are constructed from ultrathin nanobelts in a novel self-assembly approach. The relative reaction equation of the hydrothermal synthesis can be expressed as follows: NH 4VO3 2H e VO2 H 2O NH 4
(1)
The average size of nanoflowers is determined by the ammonium vanadate’s weight ratio as well as the nucleation process and the growth law. With the increasing of the concentration of NH4VO3, more VO2+ would be produced in the solution system which can promote the growth of the crystal nucleus and the radius of the nanoflowers. The size of the nanoflowers in the solution system with low concentration of NH4VO3 is typically smaller than that with high concentration of NH4VO3 because of the less VO2+ supporting for the growth of nucleus. As a consequence of kind of novel flower-like morphology with a high SSA and a high surface-to-bulk ratio, ions transparency amid this 3D network can be much more readily, resulting in better kinetic performance. It remains true that such kind of nanostructure can be a promising aspect in the subsequent investigation, i.e., molecular polymer coating will be surveyed to further improve its prosperity [22-24].
4. Conclusion 6
In summary, 3-dimensional composites based on a unique combination of graphene oxide nanosheets and VO2 nanoflowers have been successfully synthesized via a facile low-temperature hydrothermal method. It is worth noting that the size of GVNFs changed dramatically with the increasing of NH4VO3 precursor’s proportion from 1:1 to 2:1 in the suspension. There is no doubt that these V2O5 nanoflowers/GO composites with stylish performance can be very promising for high performance electrode materials in supercapacitors and other energy-storage devices.
Acknowledgments The authors gratefully acknowledge the financial supports provided by National Natural Science Foundation of China (Grant no. 21576034), and National Training Program of Innovation and Entrepreneurship for Undergraduates (2015510611104).
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Figure Captions Fig. 1 XRD spectra of (a) initial graphene oxide (b)NH4VO3 (c) composites graphene-decorated VO2 nanoflowers (GVNFs) measured in the 2θ range 5° to 80°. Fig. 2 SEM images of (a) and (b) GO Sheets, (c) and (d) small VO2 Nanoflowers/GOs, (e) and (f) large VO2 Nanoflowers/GOs. Fig. 3 The EDS mappings across the surface of the GVNFs. Fig. 4 The synthesis procedure of as-prepared vanadium dioxide nano-flowers grown on graphene oxide.
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