Ultrasonics Sonochemistry 21 (2014) 643–652

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Ultrasonics Sonochemistry

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Morphologically controlled preparation of CuO nanostructures underultrasound irradiation and their evaluation as pseudocapacitor materials

1350-4177/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ultsonch.2013.08.009

⇑ Corresponding author. Tel.: +98 21 82883474; fax: +98 21 82883455.E-mail addresses: mfmousavi@yahoo.com, mousavim@modares.ac.ir

(M.F. Mousavi).

Afshin Pendashteh a, Mohammad Safi Rahmanifar b, Mir Fazlollah Mousavi a,⇑a Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iranb Faculty of Basic Sciences, Shahed University, Tehran, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 June 2013Received in revised form 14 August 2013Accepted 19 August 2013Available online 27 August 2013

Keywords:Copper oxideCopper hydroxynitrateNanostructuresUltrasoundPseudo-capacitance

Various morphologies of copper oxide (CuO) nanostructures have been synthesized by controlling thereaction parameters in a sonochemical assisted method without using any templates or surfactants.The effect of reaction parameters including molar ratio of the reactants, reaction temperature, ultrasoundexposure time, and annealing temperature on the composition and morphology of the product(s) hasbeen investigated. The prepared samples have been characterized by X-ray diffraction (XRD), field emis-sion scanning electron microscopy (FESEM), energy dispersive X-ray (EDAX), and thermogravimetricanalysis (TGA). It has been found that Cu2(OH)3NO3 nanoplatelets are achieved in mild conditions whichcan be then converted to various morphologies of CuO nanostructures by either using high concentra-tions of OH� (formation of nanorods), prolonging sonication irradiation (nanoparticles), or thermal treat-ment (nanospheres). Application of the prepared CuO nanostructures was evaluated as supercapacitivematerial in 1 M Na2SO4 solution using cyclic voltammetry (CV) in different potential scan rates rangingfrom 5 to 100 mV s�1. The specific capacitance has been calculated using CV curves. It has been found thatthe pseudocapacitor performance of CuO can be tuned via employing morphologically controlled sam-ples. Accordingly, the prolonged sonicated sample (nanoparticles) showed the high specific capacitanceof 158 F.g�1.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

It is now well known that the properties of materials can betuned by manipulating their morphology and size; a fact that hasresulted in enormous efforts and researches during the last twodecades and is the main motivation for rapid growth in nanotech-nology [1–3]. Having unique properties arising from the nanoscaledimensions, fabrication of well-defined nanostructured materialswith controlled morphologies and sizes is of great importance forthe development in this field. Among all of the morphologies,one-dimensional (1D) nanomaterials including nanowires, nano-tubes, nanorods, nanobelts, and nanoribbons have been exten-sively studied because of their substantial importance and manypotential applications [4]. Moreover, two- (2D) and three-dimen-sional (3D) nanostructured materials have also shown potentialapplications in various fields of technology such as sensors [5,6]and energy storage devices [7]. As a result, preparing such struc-tures with controlled morphology and particle size in large scalesis of highly interested. Many techniques have been developedand applied to fabricate nanomaterials with different structures

using sol–gel reactions [8], thermal decomposition [9], electrode-position [10,11], microwave or ultrasonic irradiation [12–14], andetc. Among all of the used physicochemical techniques, sonochem-ical reactions, utilizing unique phenomena induced by ultrasoniccavitation, have shown to be very promising in the preparationof nanostructured materials [15,16]. The technique stemmed fromacoustic cavitation; the formation, growth, and implosive collapseof bubbles in a liquid which causes intense local heating (5000 �C),pressure (1800 atm), and cooling rates greater than 1010 K s�1.Moreover, the ultrasound waves induce the formation of radicalswhich results in enhanced reaction rates at ambient temperatures.It has also been shown that much smaller nanoparticles and highersurface area can be achieved through intense conditions [14].

In recent years, 2D solids have attracted increasing interest dueto possible applications in various fields, which mainly originatesfrom the capacity to intercalate ionic or neutral species in theinterlayer region. Such layered structures consist of various kindsof materials such as titanates, birnessite-type manganese oxide,graphite, hydrotalcite, and some hydroxy salts of transition metalions [17]. Layered inorganic–organic hybrid materials with thegeneral composition of M2(OH)3X have shown interestingproperties and to be promising materials for the improvement ofmechanical and/or thermal stability in nanocomposites prepara-tion [18].

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Among different members of this family, copper(II) hydrox-ynitrate, Cu2(OH)3NO3, known as gerhardtite in nature deservesspecial attention since its synthetic analog is being used invehicle airbags and is also considered as a precursor [19] inthe preparation of CuO, the well-known p-type semiconductorwith a narrow band gap (1.2 eV) with diverse applications inheterogeneous catalysis [20], gas sensors [21,22], field emission(FE) emitters, and recently in lithium ion batteries [8,23] andsupercapacitors [24–26]. As a result of this wide range ofapplications, CuO has received astonishingly more attentioncompared to other metal oxides and its nanostructures havebeen synthesized through different strategies including sono-chemical reactions [13].

Among all of the energy storage devices, supercapacitors whichare also known as electrochemical capacitors have received consid-erable amount of attention in recent years due to their high capac-itance and power characteristics [2,27]. The capacitance insupercapacitors can arise from electrical double layers (EDLC) orfrom faradaic redox reactions (pseudo-capacitance). Many transi-tion metal oxides including MnO2, V2O5, NiO, Co3O4, etc. have beenextensively studied as pseudo-capacitive materials. Recently, CuOhas attracted increasing attention due to its non-toxicity, low cost,and abundance [24,25,28].

Following our previous works on synthesis of nanostructuredmaterials [29,30] and investigating their applications in electro-chemical sensors [31,32] and energy storage devices [33–35], here-in, we report the synthesis of 2D Cu2(OH)3NO3 nanoplatelets andfurther conversion to 1D nanorods and 3D nanostructures of CuOunder ultrasound irradiation without the assistance of any tem-plates or surfactants. Different parameters during reaction includ-ing initial reactants ratio, reaction temperature, ultrasoundexposure time, and annealing temperature have been controlledto reach gerhardtite nanoplatelets or various morphologies ofCuO. Application of the prepared samples as pseudocapacitormaterials was also investigated.

Table 1The experimental parameters for the synthesized samples, and the correspondingobtained product(s) based on XRD.

Samples [NaOH]/ Sonication Reaction Annealing Product

2. Experimental

2.1. Synthesis

All chemicals were purchased from Merck and used withoutany purification. In a typical synthesis, 100 ml of 0.1 M Cu(NO3)2.3-H2O and 100 ml of NaOH (0.1–0.3 M) solutions were mixed andirradiated by 20 kHz ultrasonic waves (Misonix-3000, Max. poweroutput of 600 W, USA) at different exposure times (30–120 min).Ultrasonication was applied under constant amplitude (120 lm)in all experiments. In each experiment, the power of the sonicatorwas controlled according to consistency of the amplitude, solutionviscosity, and temperature. The reaction temperature wascontrolled with circulating water around the reaction vessel(60–90 �C). The greenish blue or dark brown products were centri-fuged, washed several times with doubly distilled water and driedat 60 �C overnight. The samples were then annealed at 100–300 �Cusing an electrical furnace (Paragon E10, USA).

[Cu(NO3)2] time (min) temp. (�C) temp. (�C) (s)

A 1 60 60 – GB 2 60 60 – G + TC 3 60 60 – TD 2 30 60 – G + TE 2 120 60 – TF 2 60 75 – G + TG 2 60 90 – G + TH 2 60 60 100 G + TI 2 60 60 200 G + TJ 2 60 60 300 T

G, Gerhardtite Cu2(OH)3NO3; T, Tenorite CuO.

2.2. Characterization

Structure of the synthesized samples were characterized byX-ray powder diffraction (XRD) using a Philips X’pert diffractom-eter equipped with Co Ka radiation (k = 1.789 Å) at 40 kV and30 mA with a step size of 0.04 �s�1. Semi-quantitative measure-ments of the samples were performed by X’pert HighScore Plussoftware to estimate synthesized different phases in each sam-ple. For further characterization of the samples, their morpholo-gies were investigated by a Philips field-emission scanning

electron microscopy (FE-SEM) at 15.0 kV. The Measurement soft-ware was used to estimate the average size of the obtained par-ticles from SEM micrographs. The prepared samples were furthercharacterized by energy dispersive (EDAX) analysis. Also, ther-mogravimetric analysis was performed with a thermal analysissystem (TA instruments Q50, USA), within a temperature rangeof 25–700 �C and a heat rate of 20 �C min�1 under Aratmosphere.

2.3. Electrochemical studies

Electrochemical properties of CuO nanostructures have beeninvestigated on a PGSTAT30 electrochemical workstation (Autolabinstruments, The Netherlands). A bare glassy carbon electrode(GCE) was mirror-polished with 0.05 lm alumina slurry for5 min, sonicated in ethanol for 20 min, and then rinsed with dou-ble-distilled water. To prepare the working electrode, 9.0 mg ofthe sample and 1.0 mg charcoal active were dispersed ultrasoni-cally in 1.0 ml isopropanol for 30 min to form an ink, and then1.0 ll of this dispersion ink was drop casted onto the pretreatedGCE with a micropipette. After drying, 1.5 ll of the diluted Naf-ion� solution (0.5% wt.) was used to cover the coated GCE to forma uniform electrode layer for electrochemical measurements. Cyc-lic voltammetry measurements were performed in a three-elec-trode configuration at a scan rate range of 5–100 mV s�1, in 1 MNa2SO4 solution as the electrolyte. The coated GCE was used asthe working, a Pt wire as the counter, and SCE as the referenceelectrode. The working electrodes were conditioned in the elec-trolyte solution for 10 min before the electrochemical measure-ments. The reported current densities were normalized to theloaded mass of the active material on GCE. All electrochemicalmeasurements were conducted at room temperature and atmo-spheric pressure. The specific capacitance was evaluated basedon the area under the forward and backward scans from the CVmeasurements.

3. Results and discussion

The effects of chemical and physical parameters including reac-tants ratio, reaction temperature, ultrasound exposure time, andannealing temperature on the composition and morphology ofthe products were investigated. XRD technique was carried outto structurally investigate the synthesized samples. Table 1 sum-marizes the experimental conditions and corresponding obtainedproduct(s), based on XRD results. As it is seen, the compositionof the prepared product(s) can be controlled by controlling thereaction parameters and pure CuO is obtained in three differentconditions. In other cases, Cu2(OH)3NO3 is formed purely or in amixed state with CuO.

Fig. 1. XRD patterns and estimated phase values of the prepared samples in three different [NaOH]/[Cu(NO3)2] molar ratios 1, 2, and 3 in samples A–C, respectively.

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3.1. Effect of [NaOH]/[Cu(NO3)2] ratio

In order to investigate the effect of concentration ratio of thereactants on the composition and morphology of the obtainedproduct, the synthesis was conducted in three different [NaOH]/[Cu(NO3)2] molar ratios of 1, 2, and 3, at a constant exposure time(60 min) and temperature (60 �C).

Fig. 1 shows the XRD patterns of the prepared samples atdifferent molar ratios. As it is seen, in [NaOH]/[Cu(NO3)2] ratioof 1 (sample A), all of the diffracted peaks can be clearly in-dexed to monoclinic phase of Cu2(OH)3NO3 (Gerhardtite). Incomparison with the standard diffraction peaks (JCPDS CardNo. 75-1779), no other peak corresponding to impurity is ob-served. By increasing the [NaOH]/[Cu(NO3)2] ratio to 2 (sample

Fig. 2. FESEM micrographs of the samples prepared in three variou

B), it can be seen that the peak intensities (e.g. 001, 002, and200 peaks) of gerhardtite phase are significantly suppressedwhile new peaks have sprouted out which can be ascribed tothe tenorite phase. Finally in sample C, at [NaOH]/[Cu2(NO3)2]ratio of 3, the peaks belonging to Cu2(OH)3NO3 have completelyvanished and all of the obtained peaks can be well indexed tomonoclinic phase of CuO, with lattice parameters of a = 4.6853,b = 3.4257, and c = 5.1303 (JCPDS Card No. 45-0937). Moreover,no characteristic peaks assigned to the impurities such asCu(OH)2 or Cu2O were observed. So, it can be concluded thatthe molar ratio of the reactants can influence the compositionof the final product. Generally, in lower [NaOH]/[Cu(NO3)2] ra-tios gerhardtite, Cu2(OH)3NO3, is obtained based on the follow-ing reaction:

s [NaOH]/[Cu(NO3)2] molar ratios of (A) 1, (B) 2, and (C, C0) 3.

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2CuðNO3Þ2 þ 2NaOHþ 2H2O! Cu2ðOHÞ3NO3 þ 2NaNO3

þH3Oþ þ NO�3 ð1Þ

Scheme 1. Schematic representation of the structure of (a) CuðOHÞ4�6 octahedrons;(b) copper hydroxynitrate, Cu2(OH)3NO3.

while tenorite, CuO, is achieved at higher [NaOH]/[Cu(NO3)2] ratios.It has been also found that it is possible to obtain CuO nanostruc-tures by controlling the reaction conditions even in short exposureto ultrasound irradiation of 1 h and saving more energy in compar-ison with the previous reports [36,37].

The effect of various [NaOH]/[Cu(NO3)2] ratios on morphology ofthe synthesized samples were examined by FESEM, as shown inFig. 2. As it is seen in Fig. 2A, the synthesized sample in low[NaOH]/[Cu(NO3)2] ratio is comprised of stacked nanoplatelets ofCu2(OH)3NO3 with a typical thickness of about 50 nm (shown byred arrows) and different surface areas ranging from nanometerto a few sub-micrometers. By increasing the [NaOH]/[Cu(NO3)2]ratio to 2, the morphology of the product has changed to muchsmaller nanoparticles, while the platelets can still be seen in Fig. 2B.Further increasing the molar ratio to 3, 1D nanorods of CuO with uni-form morphology are obtained which are about 35 nm in diameterand about 275 nm in length, as shown in Fig. 2C and C0. Therefore, itcan be concluded that the [NaOH]/[Cu(NO3)2] ratio affects both thecomposition and the morphology of the obtained products.

In order to explain the growth mechanism of 1D nanorods ofCuO, ‘‘coordination polyhedra growth unit’’ model can be em-ployed. Based on this model, the cations exist as complexes withOH� ions as ligands in aqueous solution [38]. Growth unit is thecomplex whose coordination number is the same as the crystalformed. The coordination number of Cu2+ ion is considered six inaqueous media, and therefore the growth units in the NaOH solu-tion are expected to be CuðOHÞ4�6 octahedrons (Scheme 1a). Thebinding energies of the two axial OH� ligands are lower than thefour equatorial ones [39]. In lower ratios of [NaOH]/[Cu(NO3)2],divalent copper hydroxide is formed in which one of the axialOH� sites is occupied by nitrate ion and therefore the copper ionis coordinated to four hydroxide ions in the equatorial positionsand to one nitrate and one hydroxide ion in the more distant axialpositions. In this structure, the cations occupy the interstices ofoctahedral units bound by the edges [17,18] as shown inScheme 1b. However, in higher ratios of [NaOH]/[Cu(NO3)2],the two axial hydroxide in CuðOHÞ4�6 are easily replaced anddehydrated to form CuO nanoparticles. After the formation ofCuO nanoparticles, the excessive OH� ligands might form intercon-nected hydrogen bonds which accelerate the one-dimensional

Fig. 3. XRD patterns and estimated phase values of the prepared samples in differen

aggregation rate and subsequently resulting in nanorod structures(Fig. 2C and C0).

3.2. Effect of ultrasound exposure time

In order to investigate the effect of ultrasound exposure time onthe composition and morphology of the synthesized samples, theproducts were prepared at three different exposure times of 30,60, and 120 min in samples D, B, and E, respectively, at molar ratioof 2 and reaction temperature of 60 �C.

The XRD patterns of the obtained products and their corre-sponding semi-quantitative estimations have been depicted inFig. 3. As it is seen, in lower exposure times (e.g. 30 min, sampleD), the product is comprised of both gerhardtite and tenoritephases. However, by increasing the exposure time to 60 min(Fig. 1, sample B), the peaks corresponding to gerhardtite are sup-pressed while the ones related to tenorite are remarkably in-creased. Finally, at the exposure time of 120 min (sample E), allof the appeared peaks can be indexed to pure monoclinic CuO.Moreover, it can be seen that the width of the characteristic peaksbroadens with the increase in ultrasound exposure time, indicatingthe formation of smaller particles at longer exposures (i.e. FWHMof 0.65 and 0.50 for samples D and E, respectively for peak 202).

The effect of various exposure times to ultrasound irradiationon morphology of the prepared samples was investigated byFESEM and the obtained results are shown in Fig. 4. As it is seen,the products are composed of aggregated particles at exposuretimes of 30 min (Fig. 4D and D0). An increase in ultrasound expo-

t ultrasonic-exposure times of 30 and 120 min (samples D and E, respectively).

Fig. 4. FESEM micrographs of the samples prepared in various ultrasonic-exposure times of (D, D0) 30 and (E, E0) 120 min, with different magnifications.

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sure time to 60 min in sample B, results in formation of segregatedparticles with more clear boundaries (Fig. 2B). Under prolongedultrasonic irradiation (120 min), the smaller particles are formedwith distinct boundaries (Fig. 4E and E0). These images show the fi-nal monoclinic tenorite nanoparticles synthesized by irradiatingthe reaction solution for 120 min in different magnifications. Itcan be seen in the wide scene (Fig. 4E) that the sample is com-prised of almost uniform nanoparticles. Moving closer in Fig. 4E0,it is revealed that the nanoparticles are themselves comprised ofmuch smaller particles ranging from 15 to 50 nm.

Based on the obtained results, it can be concluded that theultrasound waves should mainly play two aspects of roles besidesdispersion: proceeding the conversion of Cu2(OH)3NO3 to CuO andinducing the formation of smaller particles. In fact, ultrasound irra-diation induces the cavitation near the solid surface followed bythe cavity collapse and the consequential shockwave and surfacedamage. These shockwaves can induce high-velocity inter-particlecollisions. The separated particles from the surface react more effi-ciently than the solid surface in the bulky state, due to the in-

Fig. 5. XRD pattern (A) and SEM micrographs (B) of the prepared sample in similar cond

creased surface area. In the course of our case, the hydrolysis ofcopper (II) nitrate occurs in the presence of sodium hydroxide toyield either Cu2(OH)3NO3 (in small ratios of [NaOH]/[Cu(NO3)2])or CuO (in large ratios of [NaOH]/[Cu(NO3)2]) nuclei. After the for-mation of the nuclei, the ultrasound irradiation leads to crystalgrowth into the obtained structures rapidly by Ostwald ripeningprocess. Moreover, the formation of hydroxide radicals or ions un-der ultrasound irradiation may facilitate the transition of CuðOHÞ4�6

octahedrons to CuðOHÞ2�4 ions and their consequent conversion toCuO nanoparticles, according as following:

CuðOHÞ4�6 !ÞÞÞÞ

CuðOHÞ2�4 þ 2OH� ! CuOþ 2OH� þH2O ð2Þ

In order to investigate the role of ultrasound irradiation on thecomposition, size and morphology of the products, the experimentwas carried out with the same condition as the sample B underconventional mechanical stirring (without ultrasonication).Fig. 5A shows the XRD pattern of this sample, revealing that the ob-tained product is composed of tenorite and gerhardtite phases

ition of sample B under conventional mechanical stirring (without ultrasonication).

648 A. Pendashteh et al. / Ultrasonics Sonochemistry 21 (2014) 643–652

(similar to sample B), but with lower crystallinity than the synthe-sized sample under ultrasonication. The SEM image of the obtainedproduct (Fig. 5B) shows that the obtained sample without sonica-tion is comprised of aggregated particles with no special shape,most of them above 100 nm in size. These observations confirmthe special roles of ultrasound irradiation on the morphology andsize of the particles.

3.3. Effect of reaction temperature

For investigating the effect of reaction temperature on the com-position and morphology of the products, the synthesis of sampleswas carried out at three different reaction temperatures of 60, 75,and 90 �C, in constant [NaOH]/[Cu(NO3)2] molar ratio of 2 andultrasonic exposure time of 60 min. It should be mentioned thatcolor of the solution is changed from greenish blue to dark brownupon increasing the temperature, which reveals increase in reac-tion rate by raising the reaction temperature.

XRD was employed for evaluating the effect of reaction temper-ature on the composition of the prepared samples at different tem-peratures (Fig. 6). According to the resulted spectra, at low reactiontemperature of 60 �C (Fig. 1), the sample B contains gerhardtite andtenorite phases with clear diffracted peaks corresponding to bothof them. Raising the temperature, in samples F (75 �C) and G(90 �C), although the peaks related to gerhardtite are obviouslysuppressed in terms of intensity, however, it can be seen that thereaction has not been completed in employed conditions. As itcan be observed, a very small impurity exists in the prepared sam-ple at 90 �C (sample G) which can be indexed to monoclinic ger-hardtite. This observation may be attributed to the fact thatintense cavitational collapse cannot be expected in high tempera-tures due to high vapor pressure of solvent and the main cavita-tional effects of ultrasound are not occurred.

Fig. 7 shows the SEM micrographs of the samples prepared atthree different reaction temperatures. The prepared sample at60 �C is comprised of non-uniform particles which the plateletscan still be seen with almost clear boundaries (Fig. 2B). It can beseen that smaller particles in size are obtained by increasing thetemperature. At 75 �C (Fig. 7F and F0) the particles range in sizefrom 20 to 85 nm with a higher order of aggregation. By further in-crease in reaction temperature, in 90 �C (Fig. 7G and G0), the sampleis composed of fine particles ranging from 15 to 40 nm, with much

Fig. 6. XRD patterns and estimated phase values of the prepared samples in diffe

higher order of aggregation. Hence, it can be seen that the mor-phology of the products are affected by the reaction temperature.However, it has been revealed that the oxidation process doesnot progress completely in the employed conditions, even in high-er temperatures up to 90 �C.

3.4. Effect of annealing temperature

To find the effect of annealing on the composition and morphol-ogy of the products, the sample B (prepared in [NaOH]/[Cu(NO3)2]molar ratio of 2, ultrasonic exposure time of 60 min, and in reac-tion temperature of 60 �C) was then annealed at three differenttemperatures of 100, 200, and 300 �C for 1 h (samples H–J, respec-tively, Table 1).

The obtained XRD patterns (Fig. 8) of the samples show that byannealing the sample in 100 �C in sample H, characteristic peakscorresponding to both gerhardtite and tenorite can be seen in sucha way that the gerhardtite is the major component. Further in-crease in annealing temperature, in 200 �C (sample I), the obtainedproduct is mainly comprised of tenorite phase. Finally, it can beseen that by annealing the sample in 300 �C (sample J), the productis purely transformed to the tenorite CuO.

Fig. 9 shows the SEM micrographs of the samples synthesized atvarious annealing temperatures. Annealing in 100 �C, accompaniedwith the transformation of a part of the gerhardtite phase into thetenorite phase, the morphology of the particles has been remark-ably changed from the platelets in sample B (Fig. 2B) to non-uni-form nanoparticles with broad size distribution (Fig. 9H and H0).Increasing the annealing temperature to 200 �C, the product iscomprised of aggregated small particles (Fig. 9I and I0). As it is seenin the Fig. 9J and J0, the sample prepared in 300 �C is consisting ofalmost uniform sphere-shaped nanoparticles with average size of45 nm. It can be concluded that the gerhardtite phase is completelyconverted to the tenorite in high temperature of 300 �C.

In order to further confirm the composition of the as-preparedproducts, EDAX analysis was performed. Fig. 10 shows typicalEDAX spectrum of sample J, revealing that the sample is only com-posed of Cu and O (existence of C and Au corresponds to samplepreparation for the analysis). Similar spectra were also obtainedfor samples C and E (not shown), showing that there are no impu-rities within the synthesized samples.

rent reaction temperatures of 75 and 90 �C in samples F and G, respectively.

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3.5. Thermogravimetric analysis

The composition of the samples was further studied by ther-mogravimetric analysis. As shown in Fig. 11, the CuO samplesshow insignificant mass loss in the analysis temperature range.However, the sample synthesized in higher [NaOH]/[Cu(NO3)2]ratio with nanorod morphology showed more decrease inweight, compared to nanosphere and nanoparticle-shaped CuOsamples. This may be attributed to some adsorbed hydroxylgroups in this sample due to more basicity of the synthesis

Fig. 7. FESEM micrographs of the samples prepared in various reaction tem

Fig. 8. XRD patterns and estimate phase values of the prepared samples in different

solution. On the other hand, Cu2(OH)3NO3 shows a dramaticmass loss of about 36.1% from 100 to 700 �C. Most of the weightloss was covered within a narrow temperature range of 230–290 �C. These results are in good agreement with the obtaineddata from the above investigated annealing effect, whichshowed that the gerhardtite is converted to tenorite phase athigh temperature of 300 �C according to the following reaction[40]:

Cu2ðOHÞ3NO3!D

2CuOðsÞ þH2OðgÞ þHNO3ðgÞ ð3Þ

peratures of 75 (F, F0) and 90 �C (G, G0), with different magnifications.

annealing temperatures ranging from 100 to 300 �C (samples H–J, respectively).

Fig. 9. FESEM micrographs of the samples prepared in various annealing temperatures of (H, H0) 100, (I, I0) 200, and (J, J0) 300 �C.

Fig. 10. EDAX analysis of synthesized nanospheres of CuO (sample J).Fig. 11. Thermogravimetric analysis of the Cu2(OH)3NO3 and various morphologiesof CuO nanostructures.

650 A. Pendashteh et al. / Ultrasonics Sonochemistry 21 (2014) 643–652

Fig. 12. (a) Cyclic voltammograms of various morphologies of CuO nanostructuresin 1 M Na2SO4 at the scan rate of 5 mV s�1; (b) the specific capacitance versuspotential scan rates for three different morphologies of CuO nanostructures.

Scheme 2. Schematic illustration of the formation of Cu2(OH)3NO3 (Gerhardtite, G) nanoparameters.

A. Pendashteh et al. / Ultrasonics Sonochemistry 21 (2014) 643–652 651

3.6. Application

The applicability of the prepared samples evaluated as the ac-tive material in pseudocapacitors by employing cyclic voltamme-try measurements. Fig. 12a shows the cyclic voltammograms ofvarious morphologies of prepared CuO electrodes, recorded in1 M Na2SO4 electrolyte at the potential scan rate of 5 mV s�1. Vol-tammograms are the same in shape except in the current intensi-ties of the redox peaks corresponding to quasi-reversible transitionof Cu2+ to Cu+ and vice versa; the transition which is responsiblefor pseudocapacitance behavior of CuO. The specific capacitanceof the active material can be calculated from the charge transferredthrough the forward and backward scans, which is equal to thearea under the curves. The specific capacitance can then be calcu-lated using the following equation:

Csp ¼�I

mv ð4Þ

where Csp is the specific capacitance in F g�1, �I is the average currentin A, m is the mass of active material in grams, and m is the potentialscan rate in V s�1. Accordingly, the specific capacitance of 158, 110,and 92 F g�1 was obtained for nanoparticles, nanorods, and nano-spheres of CuO, respectively. The higher capacitance of the pro-longed sonicated sample (nanoparticles) may be attributed to themuch smaller particle sizes of this sample in comparison to the oth-ers. This results in more interfacial surface between the particlesand the electrolyte, leading to more fascinated reactions. In orderto further investigations, the cyclic voltammetry measurementswere also performed in various scan rates in the range of 5–100 mV s�1 (Fig. S1, Supplementary Information), and the specificcapacitances of the sample were calculated accordingly. The specificcapacitance has been plotted vs. different scan rates for theprepared morphologies and depicted in Fig. 12b. As it can be seen,the specific capacitance of all samples significantly decreases byincreasing the scan rate. This can be attributed to the fact that at

platelets and various morphologies of CuO (Tenorite, T) by controlling the reaction

652 A. Pendashteh et al. / Ultrasonics Sonochemistry 21 (2014) 643–652

low scan rate the electrode material can be fully utilized while athigher scan rates, some of the electrode active sites are not accessi-ble for redox processes. In fact, the contribution of pseudo-capaci-tance is decreased in higher potential scan rates. All these resultsreveal the strong effect of morphology of the samples on the pseu-do-capacitive performance of CuO material.

4. Conclusion

In conclusion, morphologically controlled preparation oftemplate/surfactant-free CuO was performed via conversion ofCu2(OH)3NO3 nanoplatelets, achieved in mild reaction conditions([NaOH]/[Cu(NO3)2] = 1; reaction temperature = 60 �C; and ultra-sound exposure time = 60 min). Accordingly, CuO nanorods, nano-particles, and nanospheres were synthesized by using highconcentrations of OH�([NaOH]/[Cu(NO3)2] = 3), prolonging sonica-tion irradiation (120 min), and thermal treatment (300 �C for 1 h),respectively (Scheme 2). The pseudocapacitive behavior of the pre-pared samples was evaluated through cyclic voltammetry mea-surements at different scan rates of 5–100 mV s�1. It has beenfound that the pseudocapacitive performance of CuO can be tunedby controlling the morphology of the samples. The sample pre-pared in prolonged sonication (CuO nanoparticles) showed thehighest specific capacitance of 158 F g�1 in 5 mV s�1 (110 and92 F g�1 for nanorods and nanospheres, respectively).

Acknowledgments

We gratefully acknowledge the Tarbiat Modares UniversityResearch Council for financial support of this work.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.ultsonch.2013. 08.009.

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