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

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

Afshin Pendashteh

^a

, Mohammad Safi Rahmanifar

^b

, Mir Fazlollah Mousavi

^a,^⇑

aDepartment of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran

bFaculty of Basic Sciences, Shahed University, Tehran, Iran

a r t i c l e i n f o

Article history:

Received 30 June 2013

Received in revised form 14 August 2013 Accepted 19 August 2013

Available online 27 August 2013

Keywords:

Copper oxide Copper hydroxynitrate Nanostructures Ultrasound Pseudo-capacitance

a b s t r a c t

Various morphologies of copper oxide (CuO) nanostructures have been synthesized by controlling the reaction 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, ultrasound exposure time, and annealing temperature on the composition and morphology of the product(s) has been investigated. The prepared samples have been characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDAX), and thermogravimetric analysis (TGA). It has been found that Cu2(OH)3NO3nanoplatelets are achieved in mild conditions which can be then converted to various morphologies of CuO nanostructures by either using high concentrations of OH (formation of nanorods), prolonging sonication irradiation (nanoparticles), or thermal treatment (nanospheres). Application of the prepared CuO nanostructures was evaluated as supercapacitive material in 1 M Na₂SO₄solution using cyclic voltammetry (CV) in different potential scan rates ranging from 5 to 100 mV s ¹. The specific capacitance has been calculated using CV curves. It has been found that the pseudocapacitor performance of CuO can be tuned via employing morphologically controlled samples. Accordingly, the prolonged sonicated sample (nanoparticles) showed the high specific capacitance of 158 F.g ¹.

1. Introduction

It is now well known that the properties of materials can be tuned by manipulating their morphology and size; a fact that has resulted in enormous efforts and researches during the last two decades and is the main motivation for rapid growth in nanotech- nology[1–3]. Having unique properties arising from the nanoscale dimensions, fabrication of well-defined nanostructured materials with controlled morphologies and sizes is of great importance for the development in this field. Among all of the morphologies, one-dimensional (1D) nanomaterials including nanowires, nano- tubes, nanorods, nanobelts, and nanoribbons have been extensively studied because of their substantial importance and many potential applications[4]. Moreover, two- (2D) and three-dimensional (3D) nanostructured materials have also shown potential applications in various fields of technology such as sensors[5,6]

and energy storage devices[7]. As a result, preparing such structures with controlled morphology and particle size in large scales is of highly interested. Many techniques have been developed and applied to fabricate nanomaterials with different structures

using sol–gel reactions[8], thermal decomposition[9], electrodeposition[10,11], microwave or ultrasonic irradiation[12–14], and etc. Among all of the used physicochemical techniques, sonochemical reactions, utilizing unique phenomena induced by ultrasonic cavitation, have shown to be very promising in the preparation of nanostructured materials[15,16]. The technique stemmed from acoustic cavitation; the formation, growth, and implosive collapse of bubbles in a liquid which causes intense local heating (5000°C), pressure (1800 atm), and cooling rates greater than 10¹⁰K s ¹. Moreover, the ultrasound waves induce the formation of radicals which results in enhanced reaction rates at ambient temperatures.

It has also been shown that much smaller nanoparticles and higher surface area can be achieved through intense conditions[14].

In recent years, 2D solids have attracted increasing interest due to possible applications in various fields, which mainly originates from the capacity to intercalate ionic or neutral species in the interlayer region. Such layered structures consist of various kinds of materials such as titanates, birnessite-type manganese oxide, graphite, hydrotalcite, and some hydroxy salts of transition metal ions [17]. Layered inorganic–organic hybrid materials with the general composition of M2(OH)3X have shown interesting properties and to be promising materials for the improvement of mechanical and/or thermal stability in nanocomposites preparation[18].

http://dx.doi.org/10.1016/j.ultsonch.2013.08.009

⇑ Corresponding author. Tel.: +98 21 82883474; fax: +98 21 82883455.

E-mail addresses: [email protected], [email protected] (M.F. Mousavi).

Contents lists available atScienceDirect

Ultrasonics Sonochemistry

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / u l t s o n

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Among different members of this family, copper(II) hydroxynitrate, Cu2(OH)3NO3, known as gerhardtite in nature deserves special attention since its synthetic analog is being used in vehicle airbags and is also considered as a precursor [19] in the preparation of CuO, the well-known p-type semiconductor with a narrow band gap (1.2 eV) with diverse applications in heterogeneous catalysis [20], gas sensors [21,22], field emission (FE) emitters, and recently in lithium ion batteries [8,23] and supercapacitors [24–26]. As a result of this wide range of applications, CuO has received astonishingly more attention compared to other metal oxides and its nanostructures have been synthesized through different strategies including sonochemical reactions[13].

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

Following our previous works on synthesis of nanostructured materials[29,30] and investigating their applications in electrochemical sensors[31,32]and energy storage devices[33–35], here- in, we report the synthesis of 2D Cu2(OH)3NO3nanoplatelets and further conversion to 1D nanorods and 3D nanostructures of CuO under ultrasound irradiation without the assistance of any templates or surfactants. Different parameters during reaction including initial reactants ratio, reaction temperature, ultrasound exposure time, and annealing temperature have been controlled to reach gerhardtite nanoplatelets or various morphologies of CuO. Application of the prepared samples as pseudocapacitor materials was also investigated.

2. Experimental 2.1. Synthesis

All chemicals were purchased from Merck and used without any 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 and irradiated by 20 kHz ultrasonic waves (Misonix-3000, Max. power output of 600 W, USA) at different exposure times (30–120 min).

Ultrasonication was applied under constant amplitude (120

l

m) in all experiments. In each experiment, the power of the sonicator was controlled according to consistency of the amplitude, solution viscosity, and temperature. The reaction temperature was controlled 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 dried at 60°C overnight. The samples were then annealed at 100–300°C using an electrical furnace (Paragon E10, USA).

2.2. Characterization

Structure of the synthesized samples were characterized by X-ray powder diffraction (XRD) using a Philips X’pert diffractom- eter equipped with Co K

a

radiation (k= 1.789 Å) at 40 kV and 30 mA with a step size of 0.04°s ¹. Semi-quantitative measurements of the samples were performed by X’pert HighScore Plus software to estimate synthesized different phases in each sample. For further characterization of the samples, their morphologies were investigated by a Philips field-emission scanning

electron microscopy (FE-SEM) at 15.0 kV. The Measurement software was used to estimate the average size of the obtained particles from SEM micrographs. The prepared samples were further characterized by energy dispersive (EDAX) analysis. Also, thermogravimetric analysis was performed with a thermal analysis system (TA instruments Q50, USA), within a temperature range of 25–700°C and a heat rate of 20°C min ¹ under Ar atmosphere.

2.3. Electrochemical studies

Electrochemical properties of CuO nanostructures have been investigated on a PGSTAT30 electrochemical workstation (Autolab instruments, The Netherlands). A bare glassy carbon electrode (GCE) was mirror-polished with 0.05

l

m alumina slurry for 5 min, sonicated in ethanol for 20 min, and then rinsed with double-distilled water. To prepare the working electrode, 9.0 mg of the sample and 1.0 mg charcoal active were dispersed ultrasoni- cally in 1.0 ml isopropanol for 30 min to form an ink, and then 1.0

l

l of this dispersion ink was drop casted onto the pretreated GCE with a micropipette. After drying, 1.5

l

l of the diluted Naf- ion^Òsolution (0.5% wt.) was used to cover the coated GCE to form a uniform electrode layer for electrochemical measurements. Cyc- lic voltammetry measurements were performed in a three-electrode configuration at a scan rate range of 5–100 mV s ¹, in 1 M Na2SO4 solution as the electrolyte. The coated GCE was used as the working, a Pt wire as the counter, and SCE as the reference electrode. The working electrodes were conditioned in the electrolyte solution for 10 min before the electrochemical measurements. The reported current densities were normalized to the loaded mass of the active material on GCE. All electrochemical measurements were conducted at room temperature and atmo- spheric pressure. The specific capacitance was evaluated based on the area under the forward and backward scans from the CV measurements.

3. Results and discussion

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

Table 1

The experimental parameters for the synthesized samples, and the corresponding obtained product(s) based on XRD.

Samples [NaOH]/

[Cu(NO3)2]

Sonication time (min)

Reaction temp. (°C)

Annealing temp. (°C)

Product (s)

A 1 60 60 – G

B 2 60 60 – G + T

C 3 60 60 – T

D 2 30 60 – G + T

E 2 120 60 – T

F 2 60 75 – G + T

G 2 60 90 – G + T

H 2 60 60 100 G + T

I 2 60 60 200 G + T

J 2 60 60 300 T

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

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

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

In order to investigate the effect of concentration ratio of the reactants on the composition and morphology of the obtained product, 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 at different molar ratios. As it is seen, in [NaOH]/[Cu(NO3)2] ratio of 1 (sample A), all of the diffracted peaks can be clearly indexed to monoclinic phase of Cu2(OH)3NO3 (Gerhardtite). In comparison with the standard diffraction peaks (JCPDS Card No. 75-1779), no other peak corresponding to impurity is observed. By increasing the [NaOH]/[Cu(NO3)2] ratio to 2 (sample

B), it can be seen that the peak intensities (e.g. 0 0 1, 0 0 2, and 2 0 0 peaks) of gerhardtite phase are significantly suppressed while new peaks have sprouted out which can be ascribed to the tenorite phase. Finally in sample C, at [NaOH]/[Cu2(NO3)2] ratio of 3, the peaks belonging to Cu2(OH)3NO3have completely vanished and all of the obtained peaks can be well indexed to monoclinic 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 as Cu(OH)2 or Cu2O were observed. So, it can be concluded that the molar ratio of the reactants can influence the composition of the final product. Generally, in lower [NaOH]/[Cu(NO3)2] ratios gerhardtite, Cu2(OH)3NO3, is obtained based on the following reaction:

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 samplesA–C, respectively.

Fig. 2.FESEM micrographs of the samples prepared in three various [NaOH]/[Cu(NO3)2] molar ratios of (A) 1, (B) 2, and (C, C⁰) 3.

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2CuðNO3Þ2þ2NaOHþ2H2O!Cu2ðOHÞ₃NO3þ2NaNO3

þH3O^þþNO₃ ð1Þ

while tenorite, CuO, is achieved at higher [NaOH]/[Cu(NO3)2] ratios.

It has been also found that it is possible to obtain CuO nanostructures by controlling the reaction conditions even in short exposure to ultrasound irradiation of 1 h and saving more energy in comparison with the previous reports[36,37].

The effect of various [NaOH]/[Cu(NO3)2] ratios on morphology of the synthesized samples were examined by FESEM, as shown in Fig. 2. As it is seen in Fig. 2A, the synthesized sample in low [NaOH]/[Cu(NO3)2] ratio is comprised of stacked nanoplatelets of Cu2(OH)3NO3with a typical thickness of about 50 nm (shown by red arrows) and different surface areas ranging from nanometer to a few sub-micrometers. By increasing the [NaOH]/[Cu(NO3)2] ratio to 2, the morphology of the product has changed to much smaller nanoparticles, while the platelets can still be seen inFig. 2B.

Further increasing the molar ratio to 3, 1D nanorods of CuO with uniform morphology are obtained which are about 35 nm in diameter and about 275 nm in length, as shown inFig. 2C and C⁰. Therefore, it can be concluded that the [NaOH]/[Cu(NO3)2] ratio affects both the composition and the morphology of the obtained products.

In order to explain the growth mechanism of 1D nanorods of CuO, ‘‘coordination polyhedra growth unit’’ model can be employed. Based on this model, the cations exist as complexes with OH ions as ligands in aqueous solution[38]. Growth unit is the complex whose coordination number is the same as the crystal formed. The coordination number of Cu²⁺ion is considered six in aqueous media, and therefore the growth units in the NaOH solution are expected to be CuðOHÞ⁴₆ octahedrons (Scheme 1a). The binding energies of the two axial OH ligands are lower than the four equatorial ones[39]. In lower ratios of [NaOH]/[Cu(NO3)2], divalent copper hydroxide is formed in which one of the axial OH sites is occupied by nitrate ion and therefore the copper ion is coordinated to four hydroxide ions in the equatorial positions and to one nitrate and one hydroxide ion in the more distant axial positions. In this structure, the cations occupy the interstices of octahedral units bound by the edges [17,18] as shown in Scheme 1b. However, in higher ratios of [NaOH]/[Cu(NO3)2], the two axial hydroxide in CuðOHÞ⁴₆ are easily replaced and dehydrated to form CuO nanoparticles. After the formation of CuO nanoparticles, the excessive OH ligands might form intercon- nected hydrogen bonds which accelerate the one-dimensional

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 on the composition and morphology of the synthesized samples, the products were prepared at three different exposure times of 30, 60, and 120 min in samplesD,B, andE, respectively, at molar ratio of 2 and reaction temperature of 60°C.

The XRD patterns of the obtained products and their corresponding semi-quantitative estimations have been depicted in Fig. 3. As it is seen, in lower exposure times (e.g. 30 min, sample D), the product is comprised of both gerhardtite and tenorite phases. However, by increasing the exposure time to 60 min (Fig. 1, sampleB), the peaks corresponding to gerhardtite are suppressed while the ones related to tenorite are remarkably in- creased. Finally, at the exposure time of 120 min (sampleE), all of the appeared peaks can be indexed to pure monoclinic CuO.

Moreover, it can be seen that the width of the characteristic peaks broadens with the increase in ultrasound exposure time, indicating the formation of smaller particles at longer exposures (i.e. FWHM of 0.65 and 0.50 for samplesDandE, respectively for peak 2 0 2).

The effect of various exposure times to ultrasound irradiation on morphology of the prepared samples was investigated by FESEM and the obtained results are shown inFig. 4. As it is seen, the products are composed of aggregated particles at exposure times of 30 min (Fig. 4D and D⁰). An increase in ultrasound expo- Scheme 1.Schematic representation of the structure of (a) CuðOHÞ⁴₆ octahedrons;

(b) copper hydroxynitrate, Cu2(OH)3NO3.

Fig. 3.XRD patterns and estimated phase values of the prepared samples in different ultrasonic-exposure times of 30 and 120 min (samplesDandE, respectively).

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sure time to 60 min in sampleB, results in formation of segregated particles with more clear boundaries (Fig. 2B). Under prolonged ultrasonic irradiation (120 min), the smaller particles are formed with distinct boundaries (Fig. 4E and E⁰). These images show the final monoclinic tenorite nanoparticles synthesized by irradiating the reaction solution for 120 min in different magnifications. It can be seen in the wide scene (Fig. 4E) that the sample is comprised of almost uniform nanoparticles. Moving closer inFig. 4E⁰, it is revealed that the nanoparticles are themselves comprised of much smaller particles ranging from 15 to 50 nm.

Based on the obtained results, it can be concluded that the ultrasound waves should mainly play two aspects of roles besides dispersion: proceeding the conversion of Cu2(OH)3NO3to CuO and inducing the formation of smaller particles. In fact, ultrasound irradiation induces the cavitation near the solid surface followed by the cavity collapse and the consequential shockwave and surface damage. These shockwaves can induce high-velocity inter-particle collisions. The separated particles from the surface react more effi- ciently than the solid surface in the bulky state, due to the in-

creased surface area. In the course of our case, the hydrolysis of copper (II) nitrate occurs in the presence of sodium hydroxide to yield 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 formation of the nuclei, the ultrasound irradiation leads to crystal growth into the obtained structures rapidly by Ostwald ripening process. Moreover, the formation of hydroxide radicals or ions under ultrasound irradiation may facilitate the transition of CuðOHÞ⁴₆ octahedrons to CuðOHÞ²₄ ions and their consequent conversion to CuO nanoparticles, according as following:

CuðOHÞ⁴₆ !^ÞÞÞÞCuðOHÞ²₄ þ2OH !CuOþ2OH þH2O ð2Þ In order to investigate the role of ultrasound irradiation on the composition, size and morphology of the products, the experiment was carried out with the same condition as the sampleBunder conventional mechanical stirring (without ultrasonication).

Fig. 5A shows the XRD pattern of this sample, revealing that the obtained product is composed of tenorite and gerhardtite phases 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.

Fig. 5.XRD pattern (A) and SEM micrographs (B) of the prepared sample in similar condition of sampleBunder conventional mechanical stirring (without ultrasonication).

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(similar to sampleB), but with lower crystallinity than the synthesized sample under ultrasonication. The SEM image of the obtained product (Fig. 5B) shows that the obtained sample without sonication is comprised of aggregated particles with no special shape, most of them above 100 nm in size. These observations confirm the special roles of ultrasound irradiation on the morphology and size of the particles.

3.3. Effect of reaction temperature

For investigating the effect of reaction temperature on the composition and morphology of the products, the synthesis of samples was carried out at three different reaction temperatures of 60, 75, and 90°C, in constant [NaOH]/[Cu(NO3)2] molar ratio of 2 and ultrasonic exposure time of 60 min. It should be mentioned that color of the solution is changed from greenish blue to dark brown upon increasing the temperature, which reveals increase in reaction rate by raising the reaction temperature.

XRD was employed for evaluating the effect of reaction temperature on the composition of the prepared samples at different temperatures (Fig. 6). According to the resulted spectra, at low reaction temperature of 60°C (Fig. 1), the sampleBcontains gerhardtite and tenorite phases with clear diffracted peaks corresponding to both of them. Raising the temperature, in samples F (75°C) and G (90°C), although the peaks related to gerhardtite are obviously suppressed in terms of intensity, however, it can be seen that the reaction has not been completed in employed conditions. As it can be observed, a very small impurity exists in the prepared sample at 90°C (sampleG) which can be indexed to monoclinic gerhardtite. This observation may be attributed to the fact that intense cavitational collapse cannot be expected in high temperatures due to high vapor pressure of solvent and the main cavitational effects of ultrasound are not occurred.

Fig. 7shows the SEM micrographs of the samples prepared at three different reaction temperatures. The prepared sample at 60°C is comprised of non-uniform particles which the platelets can still be seen with almost clear boundaries (Fig. 2B). It can be seen that smaller particles in size are obtained by increasing the temperature. At 75°C (Fig. 7F and F⁰) the particles range in size from 20 to 85 nm with a higher order of aggregation. By further increase in reaction temperature, in 90°C (Fig. 7G and G⁰), the sample is composed of fine particles ranging from 15 to 40 nm, with much

higher order of aggregation. Hence, it can be seen that the morphology of the products are affected by the reaction temperature.

However, it has been revealed that the oxidation process does not progress completely in the employed conditions, even in higher temperatures up to 90°C.

3.4. Effect of annealing temperature

To find the effect of annealing on the composition and morphology of the products, the sampleB(prepared in [NaOH]/[Cu(NO3)2] molar ratio of 2, ultrasonic exposure time of 60 min, and in reaction temperature of 60°C) was then annealed at three different temperatures of 100, 200, and 300°C for 1 h (samplesH–J, respectively,Table 1).

The obtained XRD patterns (Fig. 8) of the samples show that by annealing the sample in 100°C in sampleH, characteristic peaks corresponding to both gerhardtite and tenorite can be seen in such a way that the gerhardtite is the major component. Further increase in annealing temperature, in 200°C (sampleI), the obtained product is mainly comprised of tenorite phase. Finally, it can be seen that by annealing the sample in 300°C (sampleJ), the product is purely transformed to the tenorite CuO.

Fig. 9shows the SEM micrographs of the samples synthesized at various annealing temperatures. Annealing in 100°C, accompanied with the transformation of a part of the gerhardtite phase into the tenorite phase, the morphology of the particles has been remarkably changed from the platelets in sampleB(Fig. 2B) to non-uniform nanoparticles with broad size distribution (Fig. 9H and H⁰).

Increasing the annealing temperature to 200°C, the product is comprised of aggregated small particles (Fig. 9I and I⁰). As it is seen in theFig. 9J and J⁰, the sample prepared in 300°C is consisting of almost uniform sphere-shaped nanoparticles with average size of 45 nm. It can be concluded that the gerhardtite phase is completely converted to the tenorite in high temperature of 300°C.

In order to further confirm the composition of the as-prepared products, EDAX analysis was performed. Fig. 10 shows typical EDAX spectrum of sampleJ, revealing that the sample is only composed of Cu and O (existence of C and Au corresponds to sample preparation for the analysis). Similar spectra were also obtained for samplesCandE(not shown), showing that there are no impurities within the synthesized samples.

Fig. 6.XRD patterns and estimated phase values of the prepared samples in different reaction temperatures of 75 and 90°C in samplesFandG, respectively.

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3.5. Thermogravimetric analysis

The composition of the samples was further studied by thermogravimetric analysis. As shown in Fig. 11, the CuO samples show 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 in weight, compared to nanosphere and nanoparticle-shaped CuO samples. This may be attributed to some adsorbed hydroxyl groups in this sample due to more basicity of the synthesis

solution. On the other hand, Cu2(OH)3NO3 shows a dramatic mass loss of about 36.1% from 100 to 700°C. Most of the weight loss was covered within a narrow temperature range of 230–

290°C. These results are in good agreement with the obtained data from the above investigated annealing effect, which showed that the gerhardtite is converted to tenorite phase at high temperature of 300°C according to the following reaction [40]:

Cu2ðOHÞ3NO3!^D2CuOðsÞ þH2OðgÞ þHNO3ðgÞ ð3Þ

Fig. 7.FESEM micrographs of the samples prepared in various reaction temperatures of 75 (F, F⁰) and 90°C (G, G⁰), with different magnifications.

Fig. 8.XRD patterns and estimate phase values of the prepared samples in different annealing temperatures ranging from 100 to 300°C (samplesH–J, respectively).

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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 (sampleJ).

Fig. 11.Thermogravimetric analysis of the Cu2(OH)3NO3and various morphologies of CuO nanostructures.

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3.6. Application

The applicability of the prepared samples evaluated as the active material in pseudocapacitors by employing cyclic voltammetry measurements. Fig. 12a shows the cyclic voltammograms of various morphologies of prepared CuO electrodes, recorded in 1 M Na2SO4electrolyte at the potential scan rate of 5 mV s ¹. Vol- tammograms are the same in shape except in the current intensities of the redox peaks corresponding to quasi-reversible transition of Cu²⁺to Cu⁺and vice versa; the transition which is responsible for pseudocapacitance behavior of CuO. The specific capacitance of the active material can be calculated from the charge transferred through the forward and backward scans, which is equal to the area under the curves. The specific capacitance can then be calculated using the following equation:

Csp¼ I

m

v

^ð4Þ

whereCspis the specific capacitance in F g ¹,Iis the average current inA,mis the mass of active material in grams, and

m

is the potential scan rate in V s ¹. Accordingly, the specific capacitance of 158, 110, and 92 F g ¹was obtained for nanoparticles, nanorods, and nanospheres of CuO, respectively. The higher capacitance of the prolonged sonicated sample (nanoparticles) may be attributed to the much smaller particle sizes of this sample in comparison to the oth- ers. This results in more interfacial surface between the particles and the electrolyte, leading to more fascinated reactions. In order to further investigations, the cyclic voltammetry measurements were also performed in various scan rates in the range of 5–

100 mV s ¹(Fig. S1, Supplementary Information), and the specific capacitances of the sample were calculated accordingly. The specific capacitance has been plotted vs. different scan rates for the prepared morphologies and depicted inFig. 12b. As it can be seen, the specific capacitance of all samples significantly decreases by increasing the scan rate. This can be attributed to the fact that at Fig. 12.(a) Cyclic voltammograms of various morphologies of CuO nanostructures

in 1 M Na2SO4at the scan rate of 5 mV s¹; (b) the specific capacitance versus potential scan rates for three different morphologies of CuO nanostructures.

Scheme 2.Schematic illustration of the formation of Cu2(OH)3NO3(Gerhardtite, G) nanoplatelets and various morphologies of CuO (Tenorite, T) by controlling the reaction parameters.

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

4. Conclusion

In conclusion, morphologically controlled preparation of template/surfactant-free CuO was performed via conversion of Cu2(OH)3NO3nanoplatelets, achieved in mild reaction conditions ([NaOH]/[Cu(NO3)2] = 1; reaction temperature = 60°C; and ultrasound exposure time = 60 min). Accordingly, CuO nanorods, nanoparticles, and nanospheres were synthesized by using high concentrations of OH ([NaOH]/[Cu(NO3)2] = 3), prolonging sonication irradiation (120 min), and thermal treatment (300°C for 1 h), respectively (Scheme 2). The pseudocapacitive behavior of the prepared samples was evaluated through cyclic voltammetry measurements at different scan rates of 5–100 mV s ¹. It has been found that the pseudocapacitive performance of CuO can be tuned by controlling the morphology of the samples. The sample prepared in prolonged sonication (CuO nanoparticles) showed the highest specific capacitance of 158 F g ¹ in 5 mV s ¹ (110 and 92 F g ¹for nanorods and nanospheres, respectively).

Acknowledgments

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

Appendix A. Supplementary data

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

2013. 08.009.

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