rsc advances · 2019. 5. 28. · benignity, and low cost.3 ruthenium oxide has been widely studied...

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aThin Film Physics Laboratory, Depart

Kolhapur-416004 (M.S.), India. E-mail: l_

2609233; Tel: +91 231 2609225bTechnische Universität Chemnitz, Institut

Chemnitz, Germany

Cite this: RSC Adv., 2013, 3, 24099

Received 27th June 2013Accepted 6th September 2013

DOI: 10.1039/c3ra43254h

www.rsc.org/advances

This journal is ª The Royal Society of

Porous CuO nanosheet clusters prepared by a surfactantassisted hydrothermal method for high performancesupercapacitors

Girish S. Gund,a Deepak P. Dubal,b Dattatray S. Dhawale,a Sujata S. Shindea

and Chandrakant D. Lokhande*a

This investigation demonstrates the surfactant assisted fabrication of nanosheet clusters of caddice clew,

yarn ball and cabbage slash-like microstructures of copper oxide (CuO) in thin film form directly grown

onto a stainless steel substrate using a binder free hydrothermal approach. The impact of organic

surfactants such as Triton X-100 (TRX) and polyvinyl alcohol (PVA) on the structural, morphological,

surface area and electrochemical properties of CuO is investigated. The X-ray diffraction study reveals

the structure-directing ability of the organic surfactants and confirms the nanocrystalline nature of the

CuO thin films. Additionally, these CuO microstructures show excellent surface properties like uniform

surface morphology, good surface area and a uniform pore size distribution. The electrochemical tests

manifest a high specific capacitance of 535 F g�1 at a scan rate of 5 mV s�1 with 90% capacitive

retention after 1000 cycles and low dissolution and charge transfer resistance of the yarn ball-like

structured CuO thin film. This approach renders a plain picture of the process–structure–property

relationship in thin film synthesis and provides significant schemes to boost the performance of

supercapacitor electrodes.

1. Introduction

The demand for efficient, clean and sustainable energy sourceswith new systems associated with energy conversion andstorage is going to rise because of the rapid development of theglobal economy, increasing environmental pollution, and thedepletion of fossil fuels. The best energy storage and poweroutput devices for digital communications, load cranes, hybridelectronic vehicles and renewable energy systems, which arelow-cost and environmentally friendly, have been provided byelectrochemical capacitors called ultra capacitors or super-capacitors.1 Supercapacitors have drawn major interest, mostlydue to their high capacitance, long life cycle, and the way thatthey have bridged the gap between conventional capacitors andbatteries/fuel cells by offering higher energy densities thanconventional capacitors and higher power densities thanbatteries/fuel cells.2 The range of well-organized electrodematerials for supercapacitor electrodes spans high surface areacarbon materials, polymers and transition metal oxides. Amongthese potential materials, a signicant attempt has been paid toboost the power and energy densities of transition metal oxides

ment of Physics, Shivaji University,

[email protected]; Fax: +91 231

für Chemie, AG Elektrochemie, D-09107

Chemistry 2013

because of their high specic capacitance, environmentalbenignity, and low cost.3 Ruthenium oxide has been widelystudied for active electrode applications due to its high theo-retical capacitance (�2000 F g�1) in a wider applied potentialwindow of �1.4 V; however the expensive and toxic nature ofruthenium makes it inadequate for commercialization. Amongthe other transition metal oxides, copper oxide (CuO) has beenstudied as an electrode material for supercapacitors as it has agood storage capacity in addition to its environmental friendlynature, low cost of the raw material, and natural abundance.4

The complex hierarchical nanostructures of metal oxidesgreatly inuence the properties and the overall functionality fortheir proposed application.5 However, it is possible to tune thestructure, size and shape of the nanostructure of metal oxides,which is an important goal in state-of-the-art materialsyntheses. Therefore, research on metal oxide nanostructuresshould be concentrated to develop various methods; conver-sional templating and template-free routes, surfactant effects,and the study of the fundamentals of growth kinetics, structuralcontrol, morphology, and dimensionality, and most impor-tantly low-cost and large-scale production, along with thepossibility of patterned growth and self-organization.6 Of theseways, we used the surfactant effect to control the shape ofcomplex hierarchical nanosheets and their microstructuresusing a low-cost chemical method. The shape control of nano-structures is most important for tuning their properties and theoverall functionality for their proposed application.7 Earlier,

RSC Adv., 2013, 3, 24099–24107 | 24099

https://doi.org/10.1039/c3ra43254hhttps://pubs.rsc.org/en/journals/journal/RAhttps://pubs.rsc.org/en/journals/journal/RA?issueid=RA003046

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some successful efforts have been made to synthesize CuO thinlms using microwave irradiation,8 chemical bath deposition,9

electrodeposition10 and solvothermal methods at high temper-atures or via complex and template-assisted methods togenerate bulk nanostructures of CuO.11 Subsequently, asurfactant assisted hydrothermal method was employed to tunethe nanostructures of CuO,12 TiO2,13 MnO2,14 ZnO15 etc. Thedirect preparation of thin lms using the hydrothermal methodis less investigated. Usually the hydrothermal method results ina powdered form and further, by using a binder and differentadditives, a powder paste is pressed onto a current collector inorder to make an electrode.

To address the above issues, herein, we demonstrate a novelsurfactant-assisted solution based hydrothermal electrodematerial design method which is additive-free and binderless,that is, caddice clew, yarn ball and cabbage slash complexnanosheet structured CuO active materials grown directly ontoa stainless steel substrate, which is generally employed for thesynthesis of bulk nanostructures. The shape, size and crystal-line nature of the CuO nanosheets can be changed easily byusing different surfactants and the subsequent effects on thespecic capacity for charge storage is examined. Moreover, inthe supercapacitors reported here an aqueous neutral Na2SO4electrolyte has been used in all electrochemical measurementswithin a wide potential window of 0.9 V (between �0.6 to +0.3 Vper SCE).

2. Experimental details

The CuO nanosheets were deposited through the hydrothermalmethod using 0.1 M copper sulphate (CuSO4) solution with0.1 M ammonium persulphate (APS) as an oxidizing agent indouble-distilled water. For morphology evolution of the CuOthin lms, 1 wt% of the non-ionic surfactants Triton X(TRX)-100 and polyvinyl alcohol (PVA) were added to the aboveprepared solutions, separately. The cleaned stainless steel (SS)strips were immersed in the above prepared three baths and thebaths were put into a hydrothermal autoclave (Equitron auto-clave-pad (port/mini)). The autoclave was maintained at 353 Ktemperature for 5 h and aerwards cooled to room temperature,the substrates were taken out of the bath and washed byrepeated rinsing in double distilled water and dried at ambientconditions. Further, these lms were annealed to 423 Ktemperature to remove any hydrous/water content. The lmsprepared with TRX and PVA surfactant are denoted as Cu-TRXand Cu-PVA, respectively, whereas without surfactants the lmis named as Cu-bare.

The crystallographic study of the CuO lms was carried outusing a Bruker AXS D8 Advance diffractometer with copperradiation (Ka of l ¼ 1.54 Å). The Fourier transform infra red(FTIR) spectra of the samples were collected using a ‘Perki-nElmer, FTIR Spectrum one’ unit. The surface morphology ofthe lm was visualized by scanning electron microscopy (SEM)(JEOL-JAPAN 6360). Raman spectra were measured using aJobin Yvon Horiba LABRAM-HR visible spectrometer with anargon-ion continuous-wave laser (488 nm) as the excitationsource. N2 adsorption–desorption was determined by

24100 | RSC Adv., 2013, 3, 24099–24107

Brunauer–Emmett–Teller (BET) measurements using an ASAP-2010 surface area analyzer. Cyclic voltammograms (CVs) andgalvanostatic charge–discharge (GCD) studies were carried outusing an Automatic Battery Cycler (WBCS3000). The electro-chemical impedance study was assessed using the electro-chemical workstation (ZIVE SP5) in the frequency range 100 kHzto 100 mHz with an AC amplitude of 10 mV. An electrochemicalcell constituted of a CuO lm as the working electrode, plat-inum as a counter electrode and a saturated calomel electrode(SCE) as a reference electrode.

3. Results and discussion3.1 Film formation and reaction mechanism

Controlled precipitation through water vapor heating is calledthe hydrothermal method, and the deposition process is basedon the formation of a solid phase upon transformation of asupersaturated solution to the saturated state. This trans-formation consists of various steps such as nucleation, aggre-gation, coalescence and self-assembly of particles, where metalions are slowly oxidized to form a product in thin lm form. Inthe present case, CuSO4 is the source of Cu

2+ ions (metal ions)and APS acts as the oxidizing agent for the controlled oxidationof the Cu2+ ions for the deposition of the CuO thin lms.However, this control is improved through the addition of anon-ionic surfactant for evaluation of the surface morphology.The growth kinetics of a thin lm deposition process is an ion-by-ion growth mechanism, which comprises of ion-by-iondeposition at nucleation sites on the immersed substratesurfaces. Considering the mechanism of the development of theCuO lms, the possible chemical reaction in the solution bathcan be supposed as follows.

8CuSO4 + (NH4)2S2O8 + 4H2O/ 4Cu2O + 8H2SO4 + 2NSO4(1)

2Cu2O + O2 0 4CuO (2)

The clear transparent solution of CuSO4 with APS has pH�5 � 0.1. The stainless steel substrates are immersed in thesolution and heated at 353 K. As soon as the solution attains thetemperature, a precipitate appears in the solution. Duringthe controlled precipitation, as the ionic product of the super-saturated solution surpasses the solubility product, the selectivegrowth of the Cu2O nanosheet structures occurs on thesubstrate via heterogeneous nucleation according to reaction(1). While the homogeneous nucleation and further growthforms the nanosheet clusters in the solution phase,16 these areadsorbed on the substrate surface. Subsequently, the Cu2Ophase may be converted into CuO owing to heat treatmentaccording to reaction (2). The schematic growth model andformation of different microstructures of CuO with the assis-tance of the surfactants on the SS substrates are shown in Fig. 1.

3.2 Surface morphology

The SEM images (Fig. 2(a–f)) show the CuO complex hierar-chical nanosheet based microstructures with three distinctshapes (caddice clew, yarn ball, and cabbage slash with some

This journal is ª The Royal Society of Chemistry 2013

https://doi.org/10.1039/c3ra43254h

Fig. 1 Schematic presentation of the formation of porous nanosheet clusters ofCuO directly onto the stainless steel substrate through: (a) bare, (b) TRX, and (C)PVA organic surfactant assisted growth by hydrothermal route. The process of theformation of clusters consists of different steps such as nucleation, aggregation,coalescence and lastly growth by self-assembly, and modification throughsurfactant addition is demonstrated in the schematic presentation (a–c). In (b),molecules of the TRX-100 surfactant are presented by hexagonal red heads andblack tails. The molecules of PVA surfactant are demonstrated by spherical light-green heads and dark-green tails in (c).

Fig. 2 Scanning electron micrographs of CuO: (a and b) Cu-bare, (c and d)Cu-TRX, and (e and f) Cu-PVA samples at two different magnifications�5000 and�15 000.

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cracks and clusters) which can be constructed simply by theaddition of organic surfactants. Further, the compact anddensely packed caddice clew morphology (of Cu-bare) can betuned to yarn ball (Cu-TRX) with diameters of about 2–3 mm anda cabbage slash (Cu-PVA) structure with a diameter ranging


from 3–5 mm using the TRX-100 and PVA surfactants, respec-tively. Additionally, the width of the nanosheets of the threemicrostructures varies viz. 25–30 nm for the compact anddensely packed caddice clewmorphology, 15–25 nm for the yarnball and 30–40 nm for the cabbage slash structure. Such avariation in microstructures and width of nanosheets of CuOcan be attributed to the structure directing properties of theorganic surfactants, which inuences the dynamics of thereaction. The microstructure formation process for CuO startswith nucleation (heterogeneous reaction at the substratesurface to form the clusters of the molecules), aggregation(a basic groundwork of the nanostructure) followed by coales-cence/self-assembly into larger particles with a particular crystalorientation and morphology on the substrate surface. Accord-ingly, the lm grows to a certain thickness on the substratesurface by stacking of the particles to minimize the surfaceenergy. The different surfactants may provide a different degreeof aggregation and grain size, which leads to creation of thedifferent microstructures. Cracks are associated with theterminal thickness of the CuO thin lms. Subsequently,the presence of small nanosheet clusters of different sizesadsorbed on the surface may be due to the homogeneousreaction in the bath. In the solution bath, during nuclei growth;the different crystal planes of the nuclei have a different surfaceenergy. So, the planes with higher energy surfaces have a strongtendency to capture smaller particles in order to reduce theirsurface energy. Thus, in solution the coalesced small nano-sheets appear to grow predominantly in lateral directions toform nanosheet clusters of random shape by Ostwald's ripeningprocess.17 The schematic representation of the construction ofthe different CuO microstructures with different organicsurfactants is illustrated in Fig. 1(a–c). In the case of thesurfactants, the morphological evaluation of the porous natureand high surface area, as a consequence of constraints on therandom growth leading to a higher growth rate in a particulardirection, is conrmed by Fig. 2(a–f). This leads to theenhancement in electrochemical properties.18

3.3 Structural studies

For structural investigation, the bare and surfactant assistedcomplex hierarchical nanosheet structures on stainless steelsubstrates were characterized by XRD, with the results shown inFig. 3. All the diffraction patterns demonstrate the features ofCuO polycrystalline thin lms, specically (�111), (111), (�202)and (020) planes indexing the monoclinic structure of CuO(JCPDS no. 80-0076). The absence of other characteristic peakscorresponding to any impurities demonstrates that pure CuO isthe sole product formed. Moreover, the addition of surfactantscaused a signicant change in the intensity of the XRD patterns.The intensities of the (�111) and (111) planes increased aer theaddition of TRX-100 and PVA, although the average crystal sizeaer the addition of TRX-100 and PVA along the (�111) and (111)planes decreased from 132 (Cu-bare) to 120 (Cu-TRX) and 121(Cu-PVA) nm. This suggests that the organic surfactant inu-ences the crystallographic orientation of the CuO thin lms andacts as a structure directing agent. As a consequence,

RSC Adv., 2013, 3, 24099–24107 | 24101


Fig. 3 The XRD patterns of CuO: (a) Cu-bare, (b) Cu-TRX, and (c) Cu-PVAsamples.

Fig. 4 (A) Fourier transform infra red (FTIR) and (B) Raman spectra of CuO: (a)Cu-bare, (b) Cu-TRX, and (c) Cu-PVA samples.

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enhancement in the surface area of the material is obtained.The increase in surface area affects the electrochemical activesites, which may stimulate the electrochemical properties of thematerial produced.19 The peaks marked with an asterisk are dueto the contribution from the stainless steel substrate.

3.4. FTIR and Raman studies

Fig. 4(A) shows the FTIR spectra of the CuO powders of theCu-bare, Cu-TRX and Cu-PVA samples and displays the chem-ical information and major functional groups existing in thematerials. Subsequently, these spectra reveal the same peaks forall samples and no extra peaks are present owing to the use oforganic surfactants. This implies the purity of the materials andagrees with the results of the XRD studies. In addition to this,the high intensity and sharpness of the peaks suggests a largernumber of functional groups and the good quality of theCu-TRX material.20 The spectra exhibit strong absorption bandsat 1114 and 505 cm�1 with small satellite peaks at 1018 and597 cm�1, which represent the construction of a monoclinicCuO phase.21 The peaks at 597 and 505 cm�1 are assigned toCu–O stretching along the (�202) direction.22 The absorptionpeak at 1114 cm�1 is associated with the –OH bending vibra-tions with the copper atoms. Moreover, the small absorptionpeak at 1018 cm�1 is due to the vibrational mode of the surfaceadsorbed amine ions (NH4

+), which may be from the prelimi-nary APS. Thus, the formation of the CuO material free fromorganic surfactants and the quality of the material wereconrmed by the FTIR spectra, which is a leading necessity forsupercapacitor applications.

Raman spectra of the CuO thin lms in the range of 150–900 cm�1 are shown in Fig. 4(B). The Raman spectroscopy studygives information about the structure and phase of the CuO.The slight shi towards higher wavenumber, and sharper andstronger characteristics of Raman peaks relates to the incre-ment in grain size. Fig. 4(B) shows three Raman peaks for all theporous nanosheet microstructures, at 299, 346 and 631 cm�1,with the second one being much weaker and the third broad,

24102 | RSC Adv., 2013, 3, 24099–24107

which correspond to a monoclinic structure with a CuO singlephase.23 The full width at half-maximum (FWHM) of the299 cm�1 peak for the caddice clew, yarn ball and cabbage slashmicrostructures changed from 14.1 to 14.5 and 16 cm�1,respectively, and conrmed the increment in grain sizes of theCu-TRX and Cu-PVA samples. The sharpening of the peakswithout shiing may be related to the structure directinggrowth of the nanostructures due to the surfactant addition.Thus, all these results support the consequences of XRD, FTIRand SEM, and conrm the monoclinic structure withCuO phase.

3.5 BET surface area

Specic surface area and pore-size distribution studies of theCuO samples with and without surfactants were performed byN2 adsorption and desorption, and the isotherms are shown inFig. 5(a–c) with the inset showing the corresponding BJH poresize distribution plots. Nitrogen isotherms of all CuO samplesare type IV and present H3 type hysteresis, typical for thepresence of aggregated plate like particles with slit shape pores,according to the BDDT classication.24 The BET specic surfaceareas of the Cu-bare, Cu-TRX, and Cu-PVA samples were



Fig. 5 Nitrogen adsorption–desorption isotherms with corresponding pore sizedistribution curves (inset) of CuO: (a) Cu-bare, (b) Cu-TRX, and (c) Cu-PVA samples.

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measured to be 75.40, 95.31, and 93.87 m2 g�1, respectively.These results considerably agree with the observations from theSEM and XRD studies, as the surface area of the Cu-TRX sampleis the highest. However, the pore size distribution of thenanosheet based caddice clew, yarn ball and cabbage slash-likemicrostructures indicate the mesoporous nature with the sizeranging from 2 to 20 nm (inset gures of Fig. 5(a–c)). Also, the


total pore volume increased for Cu-TRX (0.159 cm3 g�1) anddecreased for the Cu-PVA (0.155 cm3 g�1) and Cu-bare(0.133 cm3 g�1) samples, which is consistent with specicsurface area. The largest dimensions of the nanosheets andtheir compact packing for the cabbage slash microstructuresled to a smaller pore volume, whereas the yarn ball micro-structure showed the greatest surface area and pore volumeowing to the smaller dimensions of the nanosheets and theirmoderate packing, which implies the variation of the pore sizedistributions correlates with the structure and packing. Theyarn ball-like microstructure with good specic surface area andpore volume/radius (mesoporous) is valuable for energy storageapplications and provides more active sites for chemical reac-tions. The less specic surface area and small pore volume/radius of the caddice clew and cabbage slash-like microstruc-tures may limit the charge storage capacity of the CuOelectrodes in supercapacitor applications.

3.6 Supercapacitive studies

The nanosheets of CuO in caddice clew, yarn ball, and cabbageslash-like morphologies expose a nanocrystalline material withgood surface area and mesoporosity, so these nanosheets withdifferent microstructures provide a competent way for thetransportation of an electrolyte during the redox reactions of theFaradaic charge storage process. Moreover, the charge storagecapacities of the material change due to differences in theorientation of the structures.

3.6.1 Cyclic voltammetry. The morphology dependentcyclic voltammogram (CV) tests on the CuO microstructures asan active material for a supercapacitor were executed atdifferent scan rates within�0.6 to +0.3 V per SCE in an aqueous1 M Na2SO4 electrolyte and are shown in Fig. 6(A–D). All CVcurves of the different microstructures with a strong redox peakdisplay reversible electron-transfer processes. In addition, theysuggest the electrochemical capacitance is mostly due to redoxreactions rather than the electric double layer capacitancehaving a regular rectangular CV shape. It should also be notedthat as the scan rate is increased, the shape of the CV curvechanges and the potential of the anodic and cathodic peaksshi in the more positive and negative directions, respectively,demonstrating a diffusion control process of the redox reac-tions. In this diffusion control process, the peak current densityor peak potential is proportional to the square root of the scanrate.25 The anodic and cathodic peaks in the curves are relatedto the oxidation–reduction reactions of Cu2+ 5 Cu1+ associatedwith the H+/Na+ ions. The two parallel mechanisms suggestedfor the charge storage in neutral electrolyte are (1) the interca-lation and deintercalation of the smaller H+ or bigger alkalimetal cations such as Na+ in the matrix of the material duringthe redox process and (2) the adsorption of the H+ and Na+ ionson the surface, rather than in the bulk of the sample,26 whichcan be represented as:

2CuO + A+ + e� 5 Cu2OOA (3)

(2CuO)surface + A+ + e� 5 (Cu2OOA)surface (4)

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Fig. 6 Cyclic voltammograms of CuO: (A) Cu-bare, (B) Cu-TRX, and (C) Cu-PVA samples at different scan rates. (D) Plots of specific capacitance versus potential scan ratefor CuO samples prepared using an organic surfactant: (a) bare, (b) TRX and (c) PVA.

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where, A can be either a H+ or Na+ ion. The aqueous Na2SO4 asan electrolyte has been fashioned recently and is reportedelsewhere.27 From the CV curves, it is observed that as the scanrate changes, the current separation between the anodic andcathodic peaks varies, which can be explained by an ion-exchange mechanism. The intercalation of the ions from thesolution to the active material depends upon the structure ofthe electrode material, a high surface area and porous structureleads to an enhancement in the utilization of the electrodematerial, and takes an adequate time for charging and the samefor deintercalation during discharging, which results in ahigher specic capacitance.28 The accurate specic capacitanceof different microstructures of CuO is calculated using thefollowing equation:

C ¼ 1mnðVc � VaÞ

ðVcVa

IðVÞdV (5)

where C is the specic capacitance (F g�1), n is the potential scanrate (mV s�1), Vc–Va is the potential range (�0.6 V to +0.3 V perSCE), I denotes the current density (mA cm�2) and m is thedeposited weight of the CuO material on the electrode per unitarea (here 1 cm2) dipped in electrolyte. The specic capacitancevalues for the Cu-bare, Cu-TRX, and Cu-PVA samples withpotential scan rates of 5–200 mV s�1 are observed to be 408–192 F g�1, 535–269 F g�1 and 470–230 F g�1, respectively(Fig. 6(D)). The higher specic capacitance of the Cu-TRXsamples for the yarn ball microstructure is a consequence of itshigh specic surface area and pore volume. The intercalation ofions from the solution to the surface of the active material of theelectrode requires enough time for charging and discharging.The intercalation–deintercalation of ions at slow scan rate takesa longer time and transfers more charges compared to higher

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scan rate. This leads to the highest utilization of the electrodematerial at slow scan rate and loss of specic capacitance athigher scan rate owing to depletion in utilization of electrodematerial.29

3.6.2 Galvanostatic charge–discharge study. The charge–discharge curves of the Cu-bare, Cu-TRX and Cu-PVA samples in1 M aqueous Na2SO4 solution at different current densities aregiven in Fig. 7(A–C). The charge–discharge curves are asym-metric in nature and reect the pseudocapacitive behavior ofthe CuO samples. The discharging curve indicates threedifferent divisions as follows: initial drop in the voltage relatedto the internal resistance of the electrode material, the linearvariation of potential with time corresponds to double layercapacitance owing to the charge separation at the electrode–electrolyte interface and nally slope variation of potential withtime representing the redox reaction between electrolyte andCuO electrode. The specic capacitance of the differentmorphologies is also evaluated from the galvanostatic dischargecurves derived from the equation as

Cs ¼ IDtmDV

(6)

where I (mA) is the discharge current for the applied timeduration Dt (s), DV (V) is the potential window, and m is theweight of the CuO microstructures. Subsequently, the compar-ison of the discharging curves of the different microstructuresof CuO reects a longer discharging time for the yarn ballstructure than the caddice clew and cabbage slash, signifyingthe yarn ball microstructure has a superior charge storageperformance among all the CuO microstructures. The values ofspecic capacitance for the various microstructures atdifferent current densities of 0.5, 1 and 2 mA cm�2 are: 289, 182



Fig. 7 Galvanostatic charge–discharge curves of CuO: (A) Cu-bare, (B) Cu-TRX, and (C) Cu-PVA samples at different current densities. (D) Plot of variation of specificcapacitance with current densities of (a) Cu-bare, (b) Cu-TRX and (c) Cu-PVA samples.

Fig. 8 Cyclic voltammogram (CV) curves of CuO (A) Cu-bare, (B) Cu-TRX, and (C)Cu-PVA samples at different cycles, (D) the plots of % capacitance retention withnumber of cycles and the inset figure demonstrates high magnified SEM imagesof the Cu-bare, Cu-TRX and Cu-PVA samples.

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and 166 F g�1 for caddice clew, 436, 250 and 232 F g�1 for yarnball, and 392, 213 and 187 F g�1 for cabbage slash (Fig. 7(D)).The higher surface area and pore radius of the yarn ballmicrostructure leads to its maximum specic capacitance.Furthermore, the yarn ball has a larger pore volume whichabsorbs the water molecules, works as a buffering reservoir toaccommodate ions for the redox reaction, provides conductingpathways through facilitating maximized contact and has fastdiffusion which causes an improvement in the kinetics of thereversible redox process for charge storage.30 The reduction ofspecic capacitance for the other microstructures is attributedto less surface area, a smaller pore radius and signicantsupport of the ion-exchange mechanism.

Prior to this, the charge–discharge proles of CuO werestudied by many researchers for obtaining higher super-capacitance. Shaikh et al.31 obtained the specic capacitance of136 F g�1 for CuO in a potential window of �0.4 to +0.7 V andshowed an analogous shape of curves in an acidic (1 M H2SO4)electrolyte. Zhang et al.32 reported a specic capacitance of130 F g�1 for CuO nanobelt structured electrodes in a potentialwindow of 0 to +2.5 V in a non-aqueous (1.0 M LiPF6/EC : DEC)electrolyte. The literature survey suggests that most of themeasurements are conducted in different potential windows.Moreover, in the present investigation, a moderate potentialwindow of 0.9 V (�0.6 V to +0.3 V per SCE) and the highly porousnanosheet microstructures led to the excellent electrochemicalperformance of CuO.

Long-term cycling stability is another important factor forevaluating the characteristics of a supercapacitor; therefore thecycling stability curves for the different microstructures wereexplored using CV measurements at a scan rate of 100 mV s�1

and illustrated in Fig. 8(A–C). The plots of specic capacitanceversus number of CV cycles for the Cu-bare, Cu-TRX and Cu-PVA


samples are shown in Fig. 8(D), along with inset gures of thehigh magnication SEM images of the different microstruc-tures. These data plots demonstrate that the specic capaci-tance decreases suddenly for the rst 100 cycles and then dropsmore slowly for all the microstructures. Fig. 8(D) manifests thevery similar capacitive retention for all the microstructures(caddice clew: 84%, yarn ball: 90% and cabbage slash: 87%).The signicantly higher capacitive retention of the Cu-TRXsample may be credited to the three dimensional networkstructure, which does not suffer pore and surface structuraldegradation through repeated charge discharge processes evenat higher scan rates by supplying a nanoporous architecture to

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Fig. 9 Nyquist plots of (a) Cu-bare, (b) Cu-TRX, and (c) Cu-PVA porous nano-sheets based CuO nanostructures electrodes.

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compensate the strain exerted by fast insertion/de-insertion ofH+ and Na+ ions.

3.7 Electrochemical impendence spectroscopy study

Fig. 9 shows the Nyquist plots of the Cu-bare, Cu-TRX and Cu-PVA samples in the frequency range 100 kHz to 100 mHz, whichare recognized as a primary method to evaluate the fundamentalbehavior of electrode materials for supercapacitors. The Nyquistplot exhibits the frequency response of an electrochemical celland is a plot of the imaginary impedance component (Z

00im)

against the real impedance component (Z 0r) of the cell. Allimpedance spectra have a semicircular arc and straight line. TheX-intercept of the Nyquist plots corresponds to the equivalentseries resistance (Re), which consists of contributions fromelectronic and ionic resistances.33 The electronic resistance isrelated to the intrinsic resistance of the material whereas theinterfacial resistance corresponds to the inter-particle resistanceand resistance between particles and the current collector. Theionic resistance is associated with the electrolyte resistances inthe pores and the ionic (diffusion) resistance of the ions movingin the small pores. The values of Re for the Cu-bare, Cu-TRX andCu-PVA samples are 0.017, 0.013 and 0.021 U cm�2, respectively.The high-frequency semicircle arc in the Nyquist plots is relatedto the charge transfer resistance (Rct), which is a result of theFaradaic reactions and the double-layer capacitance (Cdl) at thecontact interface of the electrode and electrolyte solution. Rct canbe directly measured as the semicircular arc diameter. Themeasured values of Rct for the caddice clew, yarn ball andcabbage slash microstructures are 18.3, 2.7 and 13.4 U cm�2,respectively. These results show that the values of Rct and Re forthe Cu-TRX sample are smaller than those for the Cu-bare andCu-PVA samples, which is a consequence of the small nano-channels of the yarn ball microstructure owing to easier access(less resistance) for intercalation and deintercalation of charges.

4. Conclusions

A novel, additive free and binderless approach with the hydro-thermal method has been proposed for the synthesis of CuO

24106 | RSC Adv., 2013, 3, 24099–24107

thin lms. The surfactant-assisted nanosheet clusters aredirectly grown onto the stainless steel substrate. The nanosheetyarn ball structure exhibited the greatest surface area andmesoporous structure with highest specic pseudocapacitance,excellent rate capability, and good cycle stability. A high speciccapacitance of 535 F g�1 was obtained at a 5 mV s�1 scan ratewith 90% of the capacitive retention aer 1000 cycles. Theexcellent performance of the nanosheet yarn ball structured 3Dnano-network can be ascribed to the formation of nano-channels in the CuO structure, which permit easy contact andtransportation of the electrolyte for providing longer electronpathways. Thus, the easy and low cost hydrothermal fabricationof different microstructures of annealed CuO with excellentelectrochemical capacitive performance are proposed as acapable electrode material for supercapacitors.

Acknowledgements

Authors are grateful to the Council for Scientic and IndustrialResearch (CSIR), New Delhi (India) for nancial supportthrough the scheme no. 03(1165)/10/EMR-II. Authors are alsograteful to the Department of Science and Technology fornancial support through PURSE and FIST & University GrantCommission (UGC) through DSA-I scheme. Authors are alsothankful to UGC-DAE CSR, Indore centre for providing Ramanfacilities.

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RSC Adv., 2013, 3, 24099–24107 | 24107


Porous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitors

Porous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitorsPorous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitors

rsc advances · 2019. 5. 28. · benignity, and low cost.3 ruthenium oxide has been widely studied...

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