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Transition metal oxide hierarchical nanotubes for energy applications
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2016 Nanotechnology 27 02LT01 (http://iopscience.iop.org/0957-4484/27/2/02LT01) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 139.184.14.159 This content was downloaded on 10/12/2015 at 10:49
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Nanotechnology Nanotechnology 27 (2016) 02LT01 (5pp)
doi:10.1088/0957-4484/27/2/02LT01
Letter
Transition metal oxide hierarchical nanotubes for energy applications Wei Wei1, Yongcheng Wang1, Hao Wu1, Abdullah M Al-Enizi2, Lijuan Zhang1 and Gengfeng Zheng1 1
Laboratory of Advanced Materials, Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, People’s Republic of China 2 Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia E-mail: [email protected] Received 29 October 2015, revised 8 November 2015 Accepted for publication 13 November 2015 Published 2 December 2015 Abstract
We report a general synthetic method for transition metal oxide (TMO) hierarchical nanotube (HNT) structures by a solution-phase cation exchange method from Cu2O nanowire templates. This method leads to the formation of hollow, tubular backbones with secondary, thin nanostructures on the tube surface, which substantially increases the surface reactive sites for electrolyte contacts and electrochemical reactions. As proofs-of-concept, several representative first-row TMO HNTs have been synthesized, including CoOx, NiOx, MnOx, ZnOx and FeOx, with specific surface areas much larger than nanotubes or nanoparticles of corresponding materials. An example of the potential energy storage applications of CoOx HNTs as supercapacitors is also demonstrated. S Online supplementary data available from stacks.iop.org/NANO/27/02LT01/mmedia Keywords: transition metal oxide, hierarchical, nanotube, etching, supercapacitor (Some figures may appear in colour only in the online journal) 1. Introduction
hierarchically tubular TiO2 with ultrathin nanosheets by etching Cu nanowires for lithium-ion batteries with high capacity, good rate performance and cycling stability [16]. Li et al used the hierarchical tubular ZnO aggregated by nanosheets as sensitive gas sensors to ethanol [17]. In this work, we report a simultaneous etching-regrowth method of synthesizing TMO hierarchical nanotubes (HNTs), using Cu2O nanowires as sacrificial templates (figure 1). Cu+ in Cu2O nanowires are easily substituted at room temperature via a solution-based etching process [18], during which hydroxide anions are controllably released near the original Cu2O nanowire surface. These OH− further react with other transition metal cations to form corresponding hydroxides (table S1 in the supporting information), which are subsequently transformed into oxides under thermal treatment. The Cu2O etching and the TMO nanotube regrowth steps take place simultaneously and near the original Cu2O nanowire
Rational design and tailored synthesis of transition metal oxide (TMO) nanostructures enable many architectures and functional building blocks [1–3], with promising energy applications in catalysis [4, 5], photoconversion and electrochemical energy storage [6–9]. In particular, one-dimensional (1D) nanotubes provide efficient charge and ion transfer that is beneficial for electrochemical signal production. However, the specific surface areas of the relatively smooth nanotube morphologies are generally low compared to particulate [10], porous [11] or 2D structures [12]. The capability of constructing 1D nanotube-based heterostructures with high surface area can offer advantages of enhanced interface charge transfer and electrochemical reactions [13, 14], as well as additional benefits such as improved stability as lithium-ion battery electrodes [15]. For example, Lou et al fabricated 0957-4484/16/02LT01+05$33.00
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© 2016 IOP Publishing Ltd Printed in the UK
Nanotechnology 27 (2016) 02LT01
with DI water and then ethanol for several times, followed by drying at 50 °C for 24 h. The CoOx (as well as other TMO) hierarchical tubular structures (HTSs) were synthesized by the cation-exchanging method. Typically, 10 mg of Cu2O nanowires and 3.4 mg of CoCl2·6H2O were mixed into 20 ml of DI water and ethanol (1:1, v/v) to form a yellow suspension. The mixture was stirred for 30 min, and then 8 ml of 1 M Na2S2O3·5H2O was dropped into the solution. After that, the mixture was further stirred for another 1 h, during which the yellow solution turned to light green gradually. Then the precipitate was collected by centrifugation, washed by DI water and ethanol for several times to form hydroxide precipitate. The precipitate was dried in a vacuum oven at 60 °C to get the final oxide product. For other TMOs, the required reagent amounts were listed in table S1. The chemical reaction equation may be described as: 2 -2 x + OH-, Cu2 O + xS2 O3 2 - + H2 O ⎡⎣ Cu2 ( S2 O3)x ⎤⎦
(1 ) -
M y+ + yOH M (OH)y ,
(2 )
S2 O3 2 - + H2 O HS2 O3 2 - + OH-,
(3 )
where, M represents the metal element (e.g. Co, Mn, Zn, Ni, and Fe). Figure 1. Schematic of the reaction process for synthesizing
2.2. Preparation of working electrodes
transition metal oxide HNTs. The section view shows the electrochemical process of these HNTs for energy storage applications.
For electrochemical characterization of CoOx HTSs in the three-electrode system, CoOx powder, carbon powder and polyvinylidene fluoride were mixed at the mass ratio of 8: 1: 1, then N-methyl pyrrolidone was added into the mixture until a uniform black slurry was achieved. After that, the slurry was coated on the current collector Ni foam (1×1 cm2), followed by drying at 80 °C in a vacuum oven. Before the Ni foam was used as the working electrode, it was pressed under a pressure of 10 MPa on the performing press, in order to ensure good contact of the active material with Ni foams.
surface, and hollow [19], tubular backbones with secondary, thin nanostructures are formed on the tube surface, which substantially increases the surface reactive sites for electrolyte contacts and electrochemical reactions. This facile and controllable method can allow for a variety of TMOs with uniform morphologies and large surface areas. As proofs-ofconcept, several representative first-row TMO HNTs have been synthesized, including CoOx, NiOx, MnOx, ZnOx and FeOx, with specific surface areas much larger than nanotubes or nanoparticles of corresponding materials. Furthermore, as an example for potential energy storage applications, supercapacitors made of the CoOx HNTs show a good capacitance of 450 F g−1 at a current density of 5 A g−1, with 85% capacitance retaining after 10 000 charge–discharge cycles.
2.3. Electrochemical characterization
The electrochemical tests were conducted under a threeelectrode cell at room temperature by a CHI 660D electrochemical workstation (CH Instrument Inc.) in 1 M KOH solution. A platinum wire and an Ag/AgCl were used as the counter and reference electrodes, respectively. The Ni foam coated with the samples was used as the working electrode.
2. Experiment
3. Results and discussion
2.1. Material synthesis
Cu2O nanowires were first synthesized using Fehling’s reaction via a screw dislocation-driven growth mechanism [20], during which Cu2+ was reduced to Cu+ by glucose. The tartrate anion serves as the ligands to bind Cu2+ and reduce its concentration, resulting in a low supersaturation level and subsequently the screw-dislocation growth of Cu2O NWs [21]. The obtained Cu2O nanowire templates have uniform
The Cu2O nanowires were synthesized by Fehling’s reaction. In brief, 125 mg of CuSO4·5H2O, 140 mg of NaOH, 20 mg of glucose, 460 mg of sodium tartrate and 800 ml de-ionized (DI) water were mixed at room temperature for 10 min. Then the solution was heated at 95 °C in an oven for 100 min. The product was then collected by centrifugation, and washed 2
Nanotechnology 27 (2016) 02LT01
hollow tubes with an average diameter of ∼200 nm, corresponding to the size and morphology of the original Cu2O nanowire templates. These HNTs are amorphous, confirmed by the absence of well-resolved XRD patterns. In addition, the tubular surfaces become much rougher, with secondary, thin nanostructures to form a hierarchical assembly. Close examinations reveal that these surface secondary nanostructures are ultrathin nanosheets for CoOx (figure 2(b)), MnOx (figure 2(d)), ZnOx (figure 2(f)) and FeOx (figure 2(j)), and small nanoparticles for NiOx (figure 2(h)). X-ray photoelectron spectroscopy was further carried out to investigate the composition, and reveals that the major oxidation state of the metal ions of the corresponding TMO nanotubes is +2 (figure S3). Moreover, these HNTs are entangled to form three-dimensional (3D) frameworks, enabling efficient charge and ion transfer for electrochemical applications. The Brunauer–Emmett–Teller (BET) surface areas of these HNTs were characterized by N2 sorption isotherms, and summarized and compared with other TMO nanostructures (table S2 in the supporting information). The CoOx HNT has the highest specific surface area of 357 m2 g−1 (figure S4(a)), compared to previously reported CoOx nanoparticles (243 m2 g−1) [22], nanowires (76 m2 g−1) [23], nanosheets (118 m2 g−1) [24] and nanotubes (41 m2 g−1) [25]. Similarly, the surface areas of the MnOx, ZnOx NiOx, and FeOx HNTs are 169, 36, 106, and 147.5 m2 g−1, which are much larger than most of the previous reports of corresponding materials (figures S4(b)–(e) and table S2 in the ESI). These high surface areas are attributed to the hierarchical assembly of thin nanosheets or small nanoparticles on the TMO tubular backbones, which are beneficial for exposing more surface active sites for electrolyte contact and electrochemical reactions. The potential energy application of these TMO–HNTs was then investigated, using the CoOx HNT-based supercapacitor energy storage as an example. Cycle voltammetry (CV) was first measured by a three-electrode system to investigate the electrochemical reactions. A pair of redox peaks with a symmetric feature is observed at different sweeping rates from 1 to 20 mV s−1 (figure 3(a)), indicating the reversible Faradaic redox reactions between Co–O/Co– O–OH [26, 27]. The galvanostatic charge–discharge curves at various current densities were further measured at the applied potential window between 0 and 0.5 V. The specific capacitance is calculated to 554, 520, 450, 304 and 184 F g−1 at current densities of 1, 2, 5, 10 and 20 A g−1, respectively (figure 3(b)). These excellent capacitances are attributed to the enhanced large surface area by the secondary thin nanosheet structures, fast charge transfer by the hollow nanotubes, and efficient ion diffusion through the entangled nanotube frameworks. The power densities and the energy densities are summarized in the Ragone plot (figure 3(c)). Finally, the long-time stability of the CoOx HNT-based supercapacitors was demonstrated by 10 000 charge–discharge cycles at 5 A g−1. The reversible capacitance can be retained at 380 F g−1, corresponding to ∼85% of its initial
Figure 2. (a), (c), (e), (g), (i) SEM, (b), (d), (f), (h), (j) TEM images
and (insets) photographs of CoOx, MnOx, ZnOx, NiOx and FeOx HNT structures, respectively.
1D morphologies and the lengths are over tens of microns (figure S1(a)). X-ray diffraction (XRD) pattern of the Cu2O nanowires show distinct diffraction peaks that are well indexed to the 110, 111, 200, 211, 220, 311 and 222 planes of Cu2O crystals (PDF#78-2076) (figure S1(b)). The Cu2O nanowire templates were then mixed with Na2S2O3, as an etchant, and other metal ion precursors. The solution color change was observed from several minutes to ∼1 h (figure S2), suggesting efficient etching of Cu2O nanowires by slightly alkaline S2 O3 2- and regrowth of new metal oxide structures. The newly formed TMO HNT structures are clearly exhibited by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (figures 2(a)–(j)). The backbones of these HNTs are
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Nanotechnology 27 (2016) 02LT01
References [1] Yang X Y, Leonard A, Lemaire A, Tian G and Su B L 2011 Self-formation phenomenon to hierarchically structured porous materials: design, synthesis, formation mechanism and applications Chem. Commun. 47 2763–86 [2] Hwang J, Jo C, Hur K, Lim J, Kim S and Lee J 2014 Direct access to hierarchically porous inorganic oxide materials with three-dimensionally interconnected networks J. Am. Chem. Soc. 136 16066–72 [3] Shan Z W, Adesso G, Cabot A, Sherburne M P, Asif S A S, Warren O L, Chrzan D C, Minor A M and Alivisatos A P 2008 Ultrahigh stress and strain in hierarchically structured hollow nanoparticles Nat. Mater. 7 947–52 [4] Peng Z, Jia D S, Al-Enizi A M, Elzatahry A A and Zheng G F 2015 From water oxidation to reduction: homologous Ni–Co based nanowires as complementary water splitting electrocatalysts Adv. Energy. Mater. 5 1402031 [5] Chen S and Qiao S Z 2013 Hierarchically porous nitrogendoped graphene–NiCo2O4 hybrid paper as an advanced electrocatalytic water-splitting material ACS Nano 7 10190–6 [6] Wang Y C, Tang J, Peng Z, Wang Y H, Jia D S, Kong B, Elzatahry A A, Zhao D Y and Zheng G F 2014 Fully solarpowered photoelectrochemical conversion for simultaneous energy storage and chemical sensing Nano Lett. 14 3668–73 [7] Zheng F L, Li G R, Ou Y N, Wang Z L, Su C Y and Tong Y X 2010 Synthesis of hierarchical rippled Bi2O3 nanobelts for supercapacitor applications Chem. Commun. 46 5021–3 [8] Pang H C, Yang H B, Guo C X, Lu J L and Li C M 2012 Nanoparticle self-assembled hollow TiO2 spheres with well matching visible light scattering for high performance dyesensitized solar cells Chem. Commun. 48 8832–4 [9] Guan C, Zeng Z Y, Li X L, Cao X H, Fan Y, Xia X H, Pan G X, Zhang H and Fan H J 2014 Atomic-layerdeposition-assisted formation of carbon nanoflakes on metal oxides and energy storage application Small 10 300–7 [10] Wang H, Li H Y, Wang J S, Wu J S, Li D S, Liu M and Su P L 2014 Nitrogen-doped TiO2 nanoparticles better TiO2 nanotube array photo-anodes for dye sensitized solar cells Electrochim. Acta 137 744–50 [11] Cui Z T, Wang S G, Zhang Y H and Cao M H 2015 Highperformance lithium storage of Co3O4 achieved by constructing porous nanotube structure Electrochim. Acta 182 507–15 [12] Peng Y, Shang L, Bian T, Zhao Y F, Zhou C, Yu H J, Wu L Z, Tung C H and Zhang T R 2015 Flower-like CdSe ultrathin nanosheet assemblies for enhanced visible-light-driven photocatalytic H2 production Chem. Commun. 51 4677–80 [13] Zhao Z L, Wu H X, He H L, Xu X L and Jin Y D 2014 A High-performance binary Ni–Co hydroxide-based water oxidation electrode with three-dimensional coaxial nanotube array structure Adv. Funct. Mater. 24 4698–705 [14] Xu X, Liang J, Zhou H, Ding S J and Yu D M 2014 The preparation of hierarchical tubular structures comprised of NiO nanosheets with enhanced supercapacitive performance Rsc. Adv. 4 3181–7 [15] Wang J X, Zhang Q B, Li X H, Zhang B, Mai L Q and Zhang K L 2015 Smart construction of three-dimensional hierarchical tubular transition metal oxide core/shell heterostructures with high-capacity and long-cycle-life lithium storage Nano Energy 12 437–46 [16] Hu H, Yu L, Gao X H, Lin Z and Lou X W 2015 Hierarchical tubular structures constructed from ultrathin TiO2(B) nanosheets for highly reversible lithium storage Energy Environ. Sci. 8 1480–3 [17] Fan F Y, Tang P G, Wang Y Y, Feng Y J, Chen A F, Luo R X and Li D Q 2015 Facile synthesis and gas sensing
Figure 3. CV curves of the CoOx HNTs electrode at different scan rates; (b) the charge–discharge curves of the CoOx HNTs electrode at various current densities; (c) Ragone plot of the CoOx HNTs electrode; (d) cycling stability of the CoOx HNTs at the current density of 5 A g−1.
value, thus suggesting the potential of utilizing the CoOx HNTs as a promising candidate for supercapacitor applications.
4. Conclusions In summary, we have developed a general, simultaneous etching-regrowth method for synthesizing TMO HNTs structures. Several representative TMO structures have been obtained, showing interconnected 3D frameworks and significantly enhanced surface areas. As a proof-of-concept, the electrochemical energy storage using CoOx HNTs is demonstrated, showing the promising potential as supercapacitor electrodes. Our study suggests a general approach for realizing a variety of new TMO architectures for energy devices and applications.
Acknowledgments We thank the following funding agencies for supporting this work: the National Key Basic Research Program of China (2013CB934104), the Natural Science Foundation of China (21322311, 21473038, 21471034), the Science and Technology Commission of Shanghai Municipality (14JC1490500), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and the Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChem). The authors extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding the Prolific Research group (PRG1436-14). 4
Nanotechnology 27 (2016) 02LT01
[18]
[19] [20]
[21] [22]
[23] Rakhi R B, Chen W, Cha D Y and Alshareef H N 2012 Substrate dependent self-organization of mesoporous cobalt oxide nanowires with remarkable pseudocapacitance Nano Lett. 12 2559–67 [24] Yuan C Z, Yang L, Hou L R, Shen L F, Zhang X G and Lou X W 2012 Growth of ultrathin mesoporous Co3O4 nanosheet arrays on Ni foam for high-performance electrochemical capacitors Energy Environ. Sci. 5 7883–7 [25] Tong G X, Guan J G and Zhang Q J 2013 In situ generated gas bubble-directed self-assembly: synthesis, and peculiar magnetic and electrochemical properties of vertically aligned arrays of high-density Co3O4 nanotubes Adv. Funct. Mater. 23 2406–14 [26] Meng F L, Fang Z G, Li Z X, Xu W W, Wang M J, Liu Y P, Zhang J, Wang W R, Zhao D Y and Guo X H 2013 Porous Co3O4 materials prepared by solid-state thermolysis of a novel Co–MOF crystal and their superior energy storage performances for supercapacitors J. Mater. Chem. A 1 7235–41 [27] Naveen A N and Selladurai S 2014 Investigation on physiochemical properties of Mn substituted spinel cobalt oxide for supercapacitor applications Electrochim. Acta 125 404–14
properties of tubular hierarchical ZnO self-assembled by porous nanosheets Sensors Actuators B 215 231–40 Nai J W, Tian Y, Guan X and Guo L 2013 Pearson’s principle inspired generalized strategy for the fabrication of metal hydroxide and oxide nanocages J. Am. Chem. Soc. 135 16082–91 Park J, Zheng H M, Jun Y W and Alivisatos A P 2009 Heteroepitaxial anion exchange yields single-crystalline hollow nanoparticles J. Am. Chem. Soc. 131 13943–5 Wang Y C, Jiang K, Zhang H, Zhou T, Wang J W, Wei W, Yang Z Q, Sun X H, Cai W B and Zheng G F 2015 Bioinspired leaf-mimicking nanosheet/nanotube heterostructure as a highly efficient oxygen evolution catalyst Adv. Sci. 2 1500003 Hacialioglu S, Meng F and Jin S 2012 Facile and mild solution synthesis of Cu2O nanowires and nanotubes driven by screw dislocations Chem. Commun. 48 1174–6 Peng L, Feng Y Y, Bai Y J, Qiu H J and Wang Y 2015 Designed synthesis of hollow Co3O4 nanoparticles encapsulated in a thin carbon nanosheet array for high and reversible lithium storage J. Mater. Chem. A 3 8825–31
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Transition metal oxide hierarchical nanotubes for energy applications
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2016 Nanotechnology 27 02LT01 (http://iopscience.iop.org/0957-4484/27/2/02LT01) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 139.184.14.159 This content was downloaded on 10/12/2015 at 10:49
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 27 (2016) 02LT01 (5pp)
doi:10.1088/0957-4484/27/2/02LT01
Letter
Transition metal oxide hierarchical nanotubes for energy applications Wei Wei1, Yongcheng Wang1, Hao Wu1, Abdullah M Al-Enizi2, Lijuan Zhang1 and Gengfeng Zheng1 1
Laboratory of Advanced Materials, Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, People’s Republic of China 2 Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia E-mail: [email protected] Received 29 October 2015, revised 8 November 2015 Accepted for publication 13 November 2015 Published 2 December 2015 Abstract
We report a general synthetic method for transition metal oxide (TMO) hierarchical nanotube (HNT) structures by a solution-phase cation exchange method from Cu2O nanowire templates. This method leads to the formation of hollow, tubular backbones with secondary, thin nanostructures on the tube surface, which substantially increases the surface reactive sites for electrolyte contacts and electrochemical reactions. As proofs-of-concept, several representative first-row TMO HNTs have been synthesized, including CoOx, NiOx, MnOx, ZnOx and FeOx, with specific surface areas much larger than nanotubes or nanoparticles of corresponding materials. An example of the potential energy storage applications of CoOx HNTs as supercapacitors is also demonstrated. S Online supplementary data available from stacks.iop.org/NANO/27/02LT01/mmedia Keywords: transition metal oxide, hierarchical, nanotube, etching, supercapacitor (Some figures may appear in colour only in the online journal) 1. Introduction
hierarchically tubular TiO2 with ultrathin nanosheets by etching Cu nanowires for lithium-ion batteries with high capacity, good rate performance and cycling stability [16]. Li et al used the hierarchical tubular ZnO aggregated by nanosheets as sensitive gas sensors to ethanol [17]. In this work, we report a simultaneous etching-regrowth method of synthesizing TMO hierarchical nanotubes (HNTs), using Cu2O nanowires as sacrificial templates (figure 1). Cu+ in Cu2O nanowires are easily substituted at room temperature via a solution-based etching process [18], during which hydroxide anions are controllably released near the original Cu2O nanowire surface. These OH− further react with other transition metal cations to form corresponding hydroxides (table S1 in the supporting information), which are subsequently transformed into oxides under thermal treatment. The Cu2O etching and the TMO nanotube regrowth steps take place simultaneously and near the original Cu2O nanowire
Rational design and tailored synthesis of transition metal oxide (TMO) nanostructures enable many architectures and functional building blocks [1–3], with promising energy applications in catalysis [4, 5], photoconversion and electrochemical energy storage [6–9]. In particular, one-dimensional (1D) nanotubes provide efficient charge and ion transfer that is beneficial for electrochemical signal production. However, the specific surface areas of the relatively smooth nanotube morphologies are generally low compared to particulate [10], porous [11] or 2D structures [12]. The capability of constructing 1D nanotube-based heterostructures with high surface area can offer advantages of enhanced interface charge transfer and electrochemical reactions [13, 14], as well as additional benefits such as improved stability as lithium-ion battery electrodes [15]. For example, Lou et al fabricated 0957-4484/16/02LT01+05$33.00
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© 2016 IOP Publishing Ltd Printed in the UK
Nanotechnology 27 (2016) 02LT01
with DI water and then ethanol for several times, followed by drying at 50 °C for 24 h. The CoOx (as well as other TMO) hierarchical tubular structures (HTSs) were synthesized by the cation-exchanging method. Typically, 10 mg of Cu2O nanowires and 3.4 mg of CoCl2·6H2O were mixed into 20 ml of DI water and ethanol (1:1, v/v) to form a yellow suspension. The mixture was stirred for 30 min, and then 8 ml of 1 M Na2S2O3·5H2O was dropped into the solution. After that, the mixture was further stirred for another 1 h, during which the yellow solution turned to light green gradually. Then the precipitate was collected by centrifugation, washed by DI water and ethanol for several times to form hydroxide precipitate. The precipitate was dried in a vacuum oven at 60 °C to get the final oxide product. For other TMOs, the required reagent amounts were listed in table S1. The chemical reaction equation may be described as: 2 -2 x + OH-, Cu2 O + xS2 O3 2 - + H2 O ⎡⎣ Cu2 ( S2 O3)x ⎤⎦
(1 ) -
M y+ + yOH M (OH)y ,
(2 )
S2 O3 2 - + H2 O HS2 O3 2 - + OH-,
(3 )
where, M represents the metal element (e.g. Co, Mn, Zn, Ni, and Fe). Figure 1. Schematic of the reaction process for synthesizing
2.2. Preparation of working electrodes
transition metal oxide HNTs. The section view shows the electrochemical process of these HNTs for energy storage applications.
For electrochemical characterization of CoOx HTSs in the three-electrode system, CoOx powder, carbon powder and polyvinylidene fluoride were mixed at the mass ratio of 8: 1: 1, then N-methyl pyrrolidone was added into the mixture until a uniform black slurry was achieved. After that, the slurry was coated on the current collector Ni foam (1×1 cm2), followed by drying at 80 °C in a vacuum oven. Before the Ni foam was used as the working electrode, it was pressed under a pressure of 10 MPa on the performing press, in order to ensure good contact of the active material with Ni foams.
surface, and hollow [19], tubular backbones with secondary, thin nanostructures are formed on the tube surface, which substantially increases the surface reactive sites for electrolyte contacts and electrochemical reactions. This facile and controllable method can allow for a variety of TMOs with uniform morphologies and large surface areas. As proofs-ofconcept, several representative first-row TMO HNTs have been synthesized, including CoOx, NiOx, MnOx, ZnOx and FeOx, with specific surface areas much larger than nanotubes or nanoparticles of corresponding materials. Furthermore, as an example for potential energy storage applications, supercapacitors made of the CoOx HNTs show a good capacitance of 450 F g−1 at a current density of 5 A g−1, with 85% capacitance retaining after 10 000 charge–discharge cycles.
2.3. Electrochemical characterization
The electrochemical tests were conducted under a threeelectrode cell at room temperature by a CHI 660D electrochemical workstation (CH Instrument Inc.) in 1 M KOH solution. A platinum wire and an Ag/AgCl were used as the counter and reference electrodes, respectively. The Ni foam coated with the samples was used as the working electrode.
2. Experiment
3. Results and discussion
2.1. Material synthesis
Cu2O nanowires were first synthesized using Fehling’s reaction via a screw dislocation-driven growth mechanism [20], during which Cu2+ was reduced to Cu+ by glucose. The tartrate anion serves as the ligands to bind Cu2+ and reduce its concentration, resulting in a low supersaturation level and subsequently the screw-dislocation growth of Cu2O NWs [21]. The obtained Cu2O nanowire templates have uniform
The Cu2O nanowires were synthesized by Fehling’s reaction. In brief, 125 mg of CuSO4·5H2O, 140 mg of NaOH, 20 mg of glucose, 460 mg of sodium tartrate and 800 ml de-ionized (DI) water were mixed at room temperature for 10 min. Then the solution was heated at 95 °C in an oven for 100 min. The product was then collected by centrifugation, and washed 2
Nanotechnology 27 (2016) 02LT01
hollow tubes with an average diameter of ∼200 nm, corresponding to the size and morphology of the original Cu2O nanowire templates. These HNTs are amorphous, confirmed by the absence of well-resolved XRD patterns. In addition, the tubular surfaces become much rougher, with secondary, thin nanostructures to form a hierarchical assembly. Close examinations reveal that these surface secondary nanostructures are ultrathin nanosheets for CoOx (figure 2(b)), MnOx (figure 2(d)), ZnOx (figure 2(f)) and FeOx (figure 2(j)), and small nanoparticles for NiOx (figure 2(h)). X-ray photoelectron spectroscopy was further carried out to investigate the composition, and reveals that the major oxidation state of the metal ions of the corresponding TMO nanotubes is +2 (figure S3). Moreover, these HNTs are entangled to form three-dimensional (3D) frameworks, enabling efficient charge and ion transfer for electrochemical applications. The Brunauer–Emmett–Teller (BET) surface areas of these HNTs were characterized by N2 sorption isotherms, and summarized and compared with other TMO nanostructures (table S2 in the supporting information). The CoOx HNT has the highest specific surface area of 357 m2 g−1 (figure S4(a)), compared to previously reported CoOx nanoparticles (243 m2 g−1) [22], nanowires (76 m2 g−1) [23], nanosheets (118 m2 g−1) [24] and nanotubes (41 m2 g−1) [25]. Similarly, the surface areas of the MnOx, ZnOx NiOx, and FeOx HNTs are 169, 36, 106, and 147.5 m2 g−1, which are much larger than most of the previous reports of corresponding materials (figures S4(b)–(e) and table S2 in the ESI). These high surface areas are attributed to the hierarchical assembly of thin nanosheets or small nanoparticles on the TMO tubular backbones, which are beneficial for exposing more surface active sites for electrolyte contact and electrochemical reactions. The potential energy application of these TMO–HNTs was then investigated, using the CoOx HNT-based supercapacitor energy storage as an example. Cycle voltammetry (CV) was first measured by a three-electrode system to investigate the electrochemical reactions. A pair of redox peaks with a symmetric feature is observed at different sweeping rates from 1 to 20 mV s−1 (figure 3(a)), indicating the reversible Faradaic redox reactions between Co–O/Co– O–OH [26, 27]. The galvanostatic charge–discharge curves at various current densities were further measured at the applied potential window between 0 and 0.5 V. The specific capacitance is calculated to 554, 520, 450, 304 and 184 F g−1 at current densities of 1, 2, 5, 10 and 20 A g−1, respectively (figure 3(b)). These excellent capacitances are attributed to the enhanced large surface area by the secondary thin nanosheet structures, fast charge transfer by the hollow nanotubes, and efficient ion diffusion through the entangled nanotube frameworks. The power densities and the energy densities are summarized in the Ragone plot (figure 3(c)). Finally, the long-time stability of the CoOx HNT-based supercapacitors was demonstrated by 10 000 charge–discharge cycles at 5 A g−1. The reversible capacitance can be retained at 380 F g−1, corresponding to ∼85% of its initial
Figure 2. (a), (c), (e), (g), (i) SEM, (b), (d), (f), (h), (j) TEM images
and (insets) photographs of CoOx, MnOx, ZnOx, NiOx and FeOx HNT structures, respectively.
1D morphologies and the lengths are over tens of microns (figure S1(a)). X-ray diffraction (XRD) pattern of the Cu2O nanowires show distinct diffraction peaks that are well indexed to the 110, 111, 200, 211, 220, 311 and 222 planes of Cu2O crystals (PDF#78-2076) (figure S1(b)). The Cu2O nanowire templates were then mixed with Na2S2O3, as an etchant, and other metal ion precursors. The solution color change was observed from several minutes to ∼1 h (figure S2), suggesting efficient etching of Cu2O nanowires by slightly alkaline S2 O3 2- and regrowth of new metal oxide structures. The newly formed TMO HNT structures are clearly exhibited by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (figures 2(a)–(j)). The backbones of these HNTs are
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Figure 3. CV curves of the CoOx HNTs electrode at different scan rates; (b) the charge–discharge curves of the CoOx HNTs electrode at various current densities; (c) Ragone plot of the CoOx HNTs electrode; (d) cycling stability of the CoOx HNTs at the current density of 5 A g−1.
value, thus suggesting the potential of utilizing the CoOx HNTs as a promising candidate for supercapacitor applications.
4. Conclusions In summary, we have developed a general, simultaneous etching-regrowth method for synthesizing TMO HNTs structures. Several representative TMO structures have been obtained, showing interconnected 3D frameworks and significantly enhanced surface areas. As a proof-of-concept, the electrochemical energy storage using CoOx HNTs is demonstrated, showing the promising potential as supercapacitor electrodes. Our study suggests a general approach for realizing a variety of new TMO architectures for energy devices and applications.
Acknowledgments We thank the following funding agencies for supporting this work: the National Key Basic Research Program of China (2013CB934104), the Natural Science Foundation of China (21322311, 21473038, 21471034), the Science and Technology Commission of Shanghai Municipality (14JC1490500), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and the Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChem). The authors extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding the Prolific Research group (PRG1436-14). 4
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