Journal of Experimental Nanoscience

ISSN: 1745-8080 (Print) 1745-8099 (Online) Journal homepage: http://www.tandfonline.com/loi/tjen20

Electrochemical bulk synthesis and characterisation of hexagonal-shaped CuO nanoparticles K.G. Chandrappa & T.V. Venkatesha To cite this article: K.G. Chandrappa & T.V. Venkatesha (2013) Electrochemical bulk synthesis and characterisation of hexagonal-shaped CuO nanoparticles, Journal of Experimental Nanoscience, 8:4, 516-532, DOI: 10.1080/17458080.2011.597440 To link to this article: http://dx.doi.org/10.1080/17458080.2011.597440

Published online: 18 Jul 2012.

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Date: 24 January 2017, At: 18:52

Journal of Experimental Nanoscience, 2013 Vol. 8, No. 4, 516–532, http://dx.doi.org/10.1080/17458080.2011.597440

Electrochemical bulk synthesis and characterisation of hexagonal-shaped CuO nanoparticles K.G. Chandrappa and T.V. Venkatesha* Department of P.G. Studies and Research in Chemistry, School of Chemical Sciences, Jnana Sahyadri campus, Kuvempu University, Shankaraghatta 577451, Karnataka, India (Received 11 February 2011; final version received 8 June 2011) A simple and efficient two-step hybrid electrochemical–thermal route was developed for the bulk synthesis of CuO nanoparticles using aqueous sodium nitrate electrolyte and Cu electrodes in an undivided cell under galvanostatic mode at room temperature. The influence of electrolyte concentration on the synthesis of CuO nanoparticles was studied at 1.0 A/dm2 current density. Electrochemically generated precursor was calcined for an hour at different levels of temperature in the range 200–900 C. The calcined samples were characterised by XRD, TG-DTA, XPS, SEM/ EDAX, TEM, FT-IR and UV–Vis spectral methods. The crystallite sizes were estimated and the thermal behaviour of as-prepared compound was examined. Rietveld refinement of X-ray data shows results matching the monoclinic structure with the space group of C2/c (no. 15). The TEM result revealed that the particle sizes were in the order of 30–50 nm diameter and 120–200 nm length. The blue shift was noticed in UV–Vis absorption spectra. All samples of CuO exhibited randomly oriented hexagonal morphology. Keywords: electrochemical; copper electrodes; nanoparticle; X-ray diffraction; CuO

1. Introduction Nanoparticles and nanostructured materials have received a significant interest in the past two decades. Among these, metal oxide nanostructures are promising materials and have attracted much attention because of their prominent properties in diverse fields of optics, optoelectronics, catalysis, biosensors and so on. The oxides of transition metals like titanium, iron, cobalt, nickel, copper, zinc, zirconium, tungsten, tin and lead have received considerable attention over the past few years due to their distinguished performance and potential applications such as microelectronics, photocatalysis, magnetic devices, powder metallurgy, solar energy transformation, semiconductors, varistors, gas sensors, and mechanical, electrical and optical switching devices [1–6]. Among these, the copper oxide (CuO) exhibits large configurations of nanostructures like nanotubes [7], nanorods [8], nanowhiskers [9], nanofibrils, nanoribbons, nanorings [10], nanowires, nanoplatelets, nanoleaflets, nanospindles [11], nanosheets [12], nanoneedles [13], nanoleaves [14], nanohoneycombs, nanoflowers [15], nanofibers [16], nanobelts, nanodendrites, nanoellipsoids, dendelions and pricky microspheres [12,17–20]. CuO is a p-type semiconductor with a band gap of 1.4 eV and possesses large surface area, higher catalytic activity and good chemical stability, with photoconductive and

*Corresponding author. Email: [email protected] © 2013 Taylor & Francis



Journal of Experimental Nanoscience   517

photochemical properties. Also, it is employed for the design and fabrication of nanosensors, nanolasers, switches and transistors [21,22] and is good selective solar absorber since it possesses high solar absorbency and low thermal emittance [23]. A CuO is widely used as a pigment in ceramics, as dispersed nanoparticles in nanofluids to increase thermal conductivity [24], as catalyst in the abatement of NOx [25], superconductor [26], gas sensors [27], magnetic storage media [28], lithium-ion electrode materials [29], solar cells [30], field transistors and biosensors, etc. [31,32]. Furthermore, CuO exhibits high mechanical strength, high-temperature durability, giant magnetoresistance and piezoelectricity. Apart from being an excellent catalytic material for CO oxidation, it has been widely used in indoor air cleaning and automotive exhaust treatment [33]. Due to large applications of CuO, its synthesis in bulk quantity became an interesting field for many researchers, especially material scientists. A number of methods have been reported in the literature to synthesise CuO nanoparticles; they are sol–gel, one-step solid-state reaction, sonochemical, hydrothermal, solvothermal, template, solid–liquid phase arc discharge, gas phase, microwave-assisted, double-jet precipitation, self-catalytic growth, electrochemical, thermal decomposition, microwave irradiation, wet-chemical, precipitation-stripping, combustion, alkoxide-based, chemical-vapour deposition (CVD), laser vapourisation, exfoliation, simple hydrolysis, laser ablation methods and so on. In early 1950s, Nabarro and Jackson [9] prepared CuO nanowhiskers, Wen et al. [10] prepared CuO nanoribbon by solution solid reaction and Liu et al. [15] reported the CuO honeycombs and flower-like assemblies by hydrothermal process. Also, Lu et al. [11] reported the CuO nanoplatelets, leaflets and nanowires by two-step reactions, Hsieh et al. [16] have prepared CuO nanofibers by self-catalytic growth mechanism and Zhang et al. [19] prepared nanodendrite-like CuO by hydrothermal route. In all these methods, the excess organic solvents, severe conditions of reaction, high operating temperature and expensive equipments are used. The procedure used or developed in the existing methods for generating either CuO films or its powder is complex with too many operating conditions. Many methods takes more time and do not give the expected yield. To meet the demand for CuO, a suitable electrochemical method operating at low cost, working at ambient temperature and giving more yield with narrow size distribution has to be developed. In recent years, an electrochemical route has aroused a considerable interest in the synthesis of metal oxide nanopowder and films because of its simplicity, low-temperature operation and viability of commercial production. Recently, the ZnO nanoparticles have been bulk synthesised by electrochemical method [34]. Also, CuO nanoparticles are synthesised using Cu as sacrificial anode and steel as cathode [35,36] and Cu as anode and cathode [37] in different electrolytes. In this study, we focus on the bulk synthesis of CuO nanoparticles by a hybrid electrochemical–thermal route without using any templates or surfactants. The effects of electrolyte, current density and reaction media on the shape, size and chemical composition of the generated product were investigated. The sample was dried and calcined at different temperatures. The IR spectra, UV–Vis absorption spectra and thermograms of these samples were recorded. The scanning electron microscope (SEM), transmission electron microscopy (TEM) images and powder X-ray diffraction (XRD) patterns were taken to characterise CuO nanoparticles. The optimum conditions for the generation of CuO nanoparticles were proposed. 2. Experimental procedure 2.1. Synthesis of CuO nanoparticles High-purity Cu metal (99.99%) with dimension 4  4  1.2 cm3 and AR grade (NaNO3) sodium nitrate (99.80%) purchased from S-D Fine, Mumbai were used as such. In this procedure, three concentrations of NaNo3, 30, 60 and 120 mM, were prepared in Millipore water

518   K.G. Chandrappa and T.V. Venkatesha

Figure 1. Schematic diagram of electrolysis process.

(specific resistance, 15 M cm, Millipore Elix 3 Water Purification System, France). In all the cases, pH of the electrolyte was 6.5 and in each experiment, 300 mL of electrolyte was taken in a rectangular undivided cell. Both the anode and cathode were copper plates and placed inside the electrolyte. The copper plate surfaces were activated by immersing in dilute HNO3 (5%) for 30 s followed by washing with Millipore water. The electrolysis process was carried out under galvanostatic condition with constant stirring at 600 rpm. During the electrolysis, the constant current was drawn from a DC-regulated power supply (Model PS 618 potentiostat/galvanostat 302/2 A supplied by Chemlink, Mumbai). The schematic diagram of electrolysis process is shown in Figure 1. An electrolysis process was carried out for an hour, when the reaction begins in the anodic region, and there was a formation of brownish black particle in the solution. The pH of the solution rises during electrolysis, reaches a maximum value of 11 and remains constant even after prolonged electrolysis. The particles were filtered and isolated from the solution. The obtained particles were calcined at different temperatures from as low as 50� C to a maximum of 900� C for 1 h. 2.2. Structural characterisation The XRD patterns (PANalytical X’pert pro powder diffractometer, Ka-Cu ¼ 1.5418 A˚, scanning rate of 2� /min with a 0.02� step size in the 2 range 10� –90� working at 30 mA and 40 kV) were recorded for all the samples and were used for phase analysis and crystalline size determination. For Rietveld refinement analysis [38], data were collected at a scan rate of 1� /min with a 0.02� step size for 2 from 10� to 90� . The data were refined using the FullProf Suite-2000 version. The average crystallite sizes were calculated using the Debye–Scherrer equation [39]. D¼

K  cos 

where D is the diameter of the crystallite size, K the shape factor (the typical value is 0.9),  the wavelength of the incident beam,  the broadening of the diffraction line measured in radians at half of its maximum intensity (FWHM, full width at half maximum intensity) and  is the Bragg’s angle. Morphology and compositional analyses were carried out in a SEM (Philips XL 30)



Journal of Experimental Nanoscience   519

fitted with an energy-dispersive X-ray analyzer (EDAX). TEM model (JEOL 2000 FX-II) study was carried out on the selected samples to confirm the size of CuO particles in nanometre scale. The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed in the temperature range 30–800 C at a heating rate of 5 C/min under nitrogen atmosphere using a SDTA-85 1e from Mettler Toledo. The X-ray photoelectron spectra (XPS) were recorded by Thermo-Scientific Multilab 2000 equipment employing Al-Ka X-rays at 150 W. XPS binding energies were accurate within 0.1 eV. Fourier transform infrared spectra (FT-IR) were obtained on KBr pellets at ambient temperature using a Bruker FT-IR spectrometer (TENSOR 27). The UV–Vis spectra were recorded by Elico SL 159 UV–Vis spectrophotometer. The nanoparticles were dispersed in ethanol and were sonicated prior to UV–Vis measurement for uniform dispersion. 3. Results and discussion 3.1. Influence of electrolyte The hybrid electrochemical–thermal method is superior to hydrothermal, sol–gel, co-precipitation and combustion methods. The hydrothermal method operates at high temperature and pressure; moreover, it takes longer time. The sol–gel and co-precipitation methods not only take a longer time but also produce particles of non-uniform size distribution. The combustion method gives a higher yield in a short time, but it requires a higher temperature. Also, the other methods like solvothermal, thermal evaporation, arc-discharge, ball milling, microwave irradiation, pechini, CVD and physical-vapour deposition operate at high temperature; excess organic solvents, severe conditions of reaction and expensive equipments are used. The procedure used or developed in the above (existing) methods for generating the nanoparticle powder is complex with too many operating conditions. In any metal oxide nanoparticle synthesis by electrochemical method, the choice of the electrolyte is quite important. A large number of electrolytes has been reported by many workers: Izaki and Omi [40] used the aqueous solution of soluble Zn salt and Li et al. [41] used aqueous ZnCl2 solution as the electrolyte for preparing ZnO films. Joseph and Kamath [42] used equal volumes of CuSO4 and lactic acid solution as an electrolyte for preparing Cu2O films, and also equal volumes of CuSO4 and disodium tartarate solution as electrolyte for preparing CuO films [43]. In these works, Cu2O and CuO, and ZnO films on cathode surface are obtained successfully as one-pot synthesis by adjusting chemical and electrochemical parameters such as pH, metal ion/ligand ratio, temperature and current density. The electrolytes contain metal salts as one of the ingredients and the obtained metal oxide film strongly adheres on the electrode surface. From this method, the nanosized thin films are generated rather than their powders. This limitation restricts the use of this method for the bulk preparation of nanomaterial powders. One of the solutions to this is the selection of soluble metal electrodes on electrolysis, they supply metal ions to the electrolyte which initially contains no metal salts. Some researchers have synthesised copper oxide nanoparticles by the electrochemical method; Yuan et al. [35] used copper plate as anode and stainless steel as cathode; the electrolyte was 200 mM NaNO3. Yang et al. [37] used copper slice electrodes in 2 M NaCl, 0.5 g/L NaOH and 0.04 g/L K2Cr2O7 to generate Cu2O at 70 C. Pourmortazavi et al. [36] obtained basic copper carbonate nanoparticles by electrosynthesis using NaHCO3. In this study, an attempt has been made to synthesise CuO nanoparticles by electrochemical–thermal method using the same metal electrodes (anode and cathode) and electrolyte without metal salt, at ambient temperature. In this study, two copper plates (4  4  1.2 cm3), one as anode and the other as cathode, were employed in 300 mL of electrolyte(s). The electrolyte(s) were NaHCO3 and NaNO3. In 30 mM NaHCO3, a colour which was a mixture of brown and green was

520   K.G. Chandrappa and T.V. Venkatesha observed during electrolysis. The green colour of the solution is due to Cu(OH)2 formed by the following reaction: Cu ! Cu2þ þ 2e� NaHCO3 ! NaOH þ CO2 "

ðon anodeÞ

ð1Þ

ðin electrolyte solutionÞ

ð2Þ

Cu2þ þ 2OH� ! CuðOHÞ2 CuðOHÞ2 þ 2OH� ! CuðOHÞ2� 4

ð green colourÞ ðsoluble complexÞ

CuðOHÞ2 ! CuO þ H2 O

ð3Þ ð4Þ ð5Þ

The brown colour in the solution indicated the formation of CuO. The total amount of Cu2þ ions formed at the anode is not used completely for the formation of CuO, since these ions participate in other reactions to form CuðOHÞ2� 4 (aq). Hence, the yield of CuO in NaHCO3 was around 30%. The next set of experiments was performed at 60 mM NaHCO3 yield with only 20% of CuO. But, upon use of NaNO3, the yield was raised to 80%. Hence, the use of NaNO3 is superior to that of NaHCO3. The NaNO3 strength was 30 mM and the total volume of electrolyte 300 mL. In this study, 4 � 4 � 1.2 cm3 copper plates were used as anode and cathode. The electrolysis was carried out under galvanostatic condition at 1.0 A/dm2 and the time of electrolysis was 1 h. The electrolyte solution changed gradually from a colourless one to a blue suspension and finally turned to a brownish black precipitate. The samples were separated by filtration through Whatmann filter paper (no. 41), washed three times with Millipore water and dried at 60� C. The dried samples were preserved in desiccators for further studies. The electrolysis parameters and process efficiencies are given in Table 1. This indicated the formation of CuO particles via the electrochemical route in the aqueous sodium nitrate bath. The possible reactions are: Cu ! Cu2þ þ 2e� at anode;

E� ¼ 0:34 V

� � � � NO� 3 þ H2 O þ 2e ! NO2 þ 2OH at cathode; E ¼ 0:01 V

ð6Þ ð7Þ

OH� ions are generated and these OH� ions combine with Cu2þ ions to give Cu(OH)2 Cu2þ þ 2OH� ! CuðOHÞ2

ð8Þ

CuðOHÞ2 ! CuO þ H2 O

ð9Þ

During electrolysis, the pH of the solution rises from 6.5 to 11 and remains at this value throughout the electrolysis. The brownish gas was observed at the cathode, indicating the � possible reduction of NO� 3 to NO2 . 3.2. XRD analysis The powder XRD patterns for the as-prepared and calcined compounds obtained from the electrolysis of 30 mM NaNO3 at 1.0 A/dm2 are shown in Figure 2. All the diffraction peaks in XRD pattern can be ascribed as monoclinic (tenorite) phase of CuO and close to the reported data [JCPDS 48-1548; a ¼ 4.688, b ¼ 3.422, c ¼ 5.131 A˚ and � ¼ 99.50� , SG: C2/c (no. 15)]. It can be seen from Figure 2(a) that the four small peaks appearing at 2� angles 16.4� , 43.5� , 72.3� and 73.6� correspond to characteristic peaks of copper hydroxide in accordance with standard

Journal of Experimental Nanoscience   521



Table 1. Electrolysis parameters and process efficiencies for the preparation of CuO nanoparticles. pH Concentration Current of electrolyte density (mM) (A/dm2) Before After 30 60 120

1.0 1.0 1.0

6.49 6.52 6.48

Cell voltage (V)

11.02 0.21–0.85 11.08 0.23–0.88 11.05 0.19–0.82

Copper Raw Efficiency weight sample CuO CuO of the loss weight weight weight process (gm) (gm) (theoretical) (observed) (%) 0.450 0.340 0.535

0.6456 0.4853 0.7654

0.5633 0.4256 0.6697

0.5390 0.4052 0.6391

95.68 95.20 95.43

Figure 2. Powder XRD patterns of CuO obtained from 30 mM NaNO3 at 1.0 A/dm2: as-prepared at 25 C (a) and calcined for 1 h at different temperatures of 200 C (b), 300 C (c), 500 C (d), 700 C (e) and 900 C (f).

JCPDS card no. 80-0656. Also, three small peaks appearing at 2 angles of 36.6 , 42.4 and 50.4 correspond to the characteristic peaks of cuprous oxide (Cu2O) [35]. The other main peaks appearing at 2 angles 32.6 , 35.6 , 38.9 , 48.9 , 53.7 , 58.4 , 61.7 , 66.3 , 68.1 , 72.5 and 75.2 correspond to pure monoclinic (tenorite) structure of CuO [JCPDS card no. 48-1548]. The XRD patterns for the calcined samples for 1 h at different temperatures (200–900 C) are shown in Figure 2(b)–(f). All diffracted lines match with the JCPDS card no. 48-1548. Figure 2(b)–(f) revealed that the broadening of the diffracted lines decreases with increasing calcination temperature from 200 C to 900 C. This inferred the higher crystallinity of CuO nanoparticles.

522   K.G. Chandrappa and T.V. Venkatesha Table 2. Crystallite sizes obtained by powder XRD patterns for samples calcined at different temperatures. Crystallite size (nm) Family of crystallographic planes {hkl} Temperature  {1 1 1} {2 0 2}  {0 2 0} {2 0 2} {1 1 3}  {3 1 1}  {2 2 0} {3 1 1} {2 2 2}  Average (� C) {1 1 0} {1 1 1} As-prepared 200 300 500 700 900

20 25 21 21 32 31

21 26 32 21 36 38

23 22 22 32 34 32

19 27 27 34 33 36

22 17 23 23 38 33

25 23 28 35 35 38

24 24 28 36 34 36

26 24 29 37 32 32

24 18 21 25 28 40

22 38 22 25 38 38

20 25 16 18 31 37

22 24 25 28 34 36

Note: The bold values signify the average crystallite sizes of each calcined temperature of CuO nanoparticles.

From the XRD data, the average crystallite sizes were found to be 22–36 nm (Table 2). No peaks related to other phases and impurities were found in the XRD pattern, implying the purity of CuO. Similarly, the electrolysis experiments were performed at different concentrations of NaNO3 (60 and 120 mM) with current density 1.0 A/dm2. The CuO thus prepared was subjected to XRD measurement and found similar XRD patterns (plots are not shown). The preferred orientations of CuO nanoparticles were estimated from the X-ray data according to the methodology developed by Berube and Esperance [44], where the texture coefficient (Tc) is calculated using the equation below: P Iðhkl Þ I0ðhkl Þ Tc ¼ P X Iðhkl Þ I0ðhkl Þ where I(hkl) is the diffraction line intensity of the (hkl ) reflection of CuO powder and I(hkl) the sum of the intensities of all the diffraction lines monitored. The I0 refers to the intensity of the reference CuO sample [JCPDS no. 48-1548]. Figure 3 shows the texture coefficient of CuO sample obtained from electrolyte concentration 30 mM at current density 1.0 A/dm2. It can be seen that  and (1 1 1) planes. the majority of the CuO crystallites are oriented parallel to the (1 1 1) The structural parameters for all the calcined samples were refined using Rietveld refinement method. All the CuO nanoparticles were crystallised in the monoclinic (tenorite) structure with space group of C2/c (no. 15). The refined structural parameters for CuO nanoparticles calcined at 700� C for 1 h are given in Table 3. The observed lattice parameters agree well with the reported data in the standard JCPDS card no. 48-1548. The observed, calculated and the difference XRD patterns are shown in Figure 4. There is a good agreement between the observed and calculated patterns. Further, there is no appreciable change in the lattice parameters of low-temperature calcined samples. 3.3. Thermal analysis For determining the proper calcined temperature, the thermal behaviour data (TG–DTA) of asprepared CuO powder were recorded and are shown in Figure 5. The as-prepared compound was taken in an alumina crucible and heated in nitrogen atmosphere at temperature scanning rate 5� /min in the temperature range 30–800� C. It can be seen that there are two pronounced mass loss steps in TG curve. The first mass loss step is gradual, in the range 30–180� C.

Journal of Experimental Nanoscience   523



Figure 3. Texture coefficient of CuO nanoparticles obtained from 30 mM NaNO3 at 1 A/dm2, calcined for 1 h at different temperatures.

Table 3. Rietveld refined structural parameters for CuO nanoparticles synthesised by electrochemical method.

Atoms Cu1 O1

Oxidation state

Wyckoff notations

x

y

z

Biso

Occupancy

2 �2

(4c) (4e)

0.2500 0.5000

0.2500 0.5000

0.0000 0.5000

0.050 0.050

1 1

Notes: Crystal system ¼ monoclinic; lattice parameters (A˚); a ¼ 4.6888(2), b ¼ 3.4239(1), c ¼ 5.1330(2),  ¼ 90.00,  ¼ 99.50(2) and  ¼ 90.00. Space group ¼ C2/c (no. 15); unit cell volume (A˚3) ¼ 81.27(5). Rfactors; Rp ¼ 3.57, Rwp ¼ 4.61, 2 ¼ 1.35, RBragg ¼ 2.92, RF ¼ 2.77.

The mass loss was 0.124 mg (2.59%), and this small value is attributed to the removal of surface adsorbed and/or crystalline water. The second step of mass loss appeared between 190� C and 270� C and indicated a major mass loss of 0.678 mg (14.55%), revealing the dehydration process of Cu(OH)2 to give CuO. Thus, 270� C is considered as an appropriate calcinations temperature for the formation of CuO from Cu(OH)2 present in the as-prepared compound. The loss of weight may be due to the following reaction: CuO � H2 O ! CuO þ H2 O

ðsurface adsorbed waterÞ

CuðOHÞ2 ! CuO þ H2 O

ð10Þ ð11Þ

The total weight loss in the entire thermal analysis study was 17.14%, indicating that the major amount formed might be CuO (brownish black colour; 82.86%) during electrolysis (Figure 2a).

524   K.G. Chandrappa and T.V. Venkatesha

Figure 4. (Colour online) Observed, calculated and the difference XRD patterns for CuO sample obtained from 30 mM NaNO3, 1.0 A/dm2, calcined at 700 C for 1 h.

Figure 5. TG–DTA analysis of as-prepared CuO nanoparticles obtained from 30 mM NaNO3 0 A/dm2.



Journal of Experimental Nanoscience   525

Figure 6. XPS of: (a) Cu 2p and (b) O 1s for CuO nanoparticles.

The DTA curves exhibited in two endothermic peaks correspond to two mass loss steps of TGA curve. The former mass loss at about 37� C corresponds to surface-adsorbed crystalline water and as well as other low molecular weight compounds. The major endothermic decomposition occurred around 245� C, which corresponds to decomposition of hydroxide to oxide material. The percent mass loss of the as-prepared compound considering both the experimental and theoretical values, is 17.14% which goes along with the XRD results; from this, we can predict that the as-prepared compound might be Cu(OH)2, Cu2O and CuO. The experimental mass loss is equal to the theoretical mass loss (17.14%). A plateau of the TGA curve at temperatures higher than 245� C is the final step of the transformation in to CuO. Also, it is clearly seen from the TGA curve that the as-prepared compound is completely converted to CuO after calcination at 270� C. Therefore, crystallised CuO begin to nucleate at about 350� C and CuO crystal growth is completed at about 660� C. The exothermic peak at 599� C is due to the direct crystallisation of nanocrystalline CuO from the amorphous component. It suggests that the complete crystallinity may be achieved after 700� C. 3.4. XPS analysis The phase purity, surface composition and chemical states of hexagonal-like CuO were further investigated by XPS spectra. Figure 6 shows the XPS spectra of CuO nanoparticles obtained from 30 mM NaNO3 at 1.0 A/dm2 and calcined at 500� C for 1 h. It can be seen that the whole region of XPS spectra contains characteristic peaks of Cu 2p and O 1s (Figure 6a and b). The Cu 2p signal could be deconvoluted into five peaks (Figure 6a). The binding energies at 933.7 and 953.6 eV are attributed to the Cu 2p3/2 and Cu 2p1/2 peaks, respectively, which are consistent with those observed in CuO bulk [45]. The satellite peaks are also produced by Cu2þ and this feature is located at higher binding energy by about 8.2 eV. As shown in Figure 6(b), the O 1s core-level spectrum is broad, and these peaks ranging from 526 to 536 eV are well-fitted by two components that have different energies. The peak observed at 529.9 eV is in agreement with O2� in CuO, while the peak at 531.8 eV is attributed to adsorbed oxygen on the surface of CuO. A higher density of oxygen atoms around will cause inner electrons of Cu atoms to be more tightened by the atomic nucleus, which is beneficial to the increase of binding energy on XPS spectrum. The peak area of Cu 2p and O 1s cores were measured and used to calculate the chemical composition of the sample. The binding energies reported here are with reference to C

526   K.G. Chandrappa and T.V. Venkatesha

Figure 7. Scanning electron micrographs of: as-prepared CuO obtained from 30 mM NaNO3 at 1.0 A/dm2 (a) and calcined at different temperatures of 200 C (b), 300 C (c), 500 C (d) and 700 C (e).

1 s at 284.5 eV and accurate within the range 0.1 eV. Thus, the XPS spectra demonstrate that the formed materials are composed of CuO nanoparticles.

3.5. SEM/EDAX and TEM analyses Scanning electron micrographs of as-prepared and calcined compounds of CuO nanoparticles are shown in Figure 7. The as-prepared compound resulting from the bath concentration 30 mM with current density 1.0 A/dm2 was calcined for 1 h at different temperatures 200 C, 300 C, 500 C and 700 C. Figure 7(a) shows the high magnification SEM image of the as-prepared CuO powder; it can be seen that the particles are agglomerated with urchin-like structure, whereas the calcined CuO at 200 C showed less randomly distributed small spherical-shaped particles (Figure 7b). The calcination at 300 C showed well-defined and randomly oriented hexagonallike CuO with somewhat compact structure (Figure 7c). Figure 7(d) corresponds to a lowmagnification SEM image of a CuO sample calcined for 1 h at 500 C, which shows that the particles are clear small-shaped hexagonal structures, whereas in the sample calcined at 700 C, the particles showed dense structure and crystals of hexagonal appearance (Figure 7e). The urchin-like structure in the as-prepared CuO converted to a hexagonal structure at 300 C. The as-prepared sample heated at 300 C for 1 h posseses uniformly oriented hexagonal structure due to the growth of CuO particles. As the heat treatment temperature increases, the particles tend to grow as expected. For instance, at 500 C and 700 C, the shape of the particle is hexagonal in nature with non-uniform grain distribution. The stoichiometry of the samples was examined by the EDAX spectrum, as shown in Figure 8. Only copper and oxygen signals have been detected, suggesting that the nanoparticles are indeed made up of Cu and O. In Figure 8, inset shows the ratio of copper and oxygen ion concentration. We did not see any other foreign ions in the final compound. But, in other methods like sol–gel and co-precipitation, there is some impurity present in the final compound.



Journal of Experimental Nanoscience   527

Figure 8. EDAX analysis of CuO nanoparticles obtained from 30 mM NaNO3 at 1.0 A/dm2 calcined at 500 C for 1 h.

This result indicates that the calcined CuO nanoparticles prepared by electrochemical–thermal method contain 100% of CuO. The TEM images of CuO nanoparticles obtained from the different concentrations, 30 and 60 mM, of NaNO3 at 1.0 A/dm2 calcined at 500 C for 1 h are given in Figure 9. After the heat treatment at 500 C for 1 h, the CuO particles were found in the range 30–50 nm diameter and 120–200 nm length (Figure 9a). It can be observed that CuO nanoparticles mainly present granules with hexagonal shape and are well-crystallised. The circles in Figure 9(a) and (b) emphasise that the particles have a clear hexagonal shape, and it was found that the average particle sizes ranged from 30 to 50 nm and these values are approximately in good agreement with the values obtained from XRD data by Debye–Scherrer equation. 3.6. FT-IR spectroscopy In Figure 10, we can see the FT-IR spectra of the: (a) as-prepared CuO obtained from the 30 mM NaNO3, 1.0 A/dm2 and calcined at different temperatures of (b) 300 C, (c) 500 C and (d) 700 C. Figure 10(a) indicates that the weak absorption peaks in the range 3100–3600 cm1 have become two broad peaks centred at 3317 and 3443 cm1, corresponding to the stretching vibration of hydrogen bond (O–H) of surface-adsorbed water, indicating higher amount of hydroxyl group. These vibrations are the evidence for the existence of water in the as-prepared compound. It is interesting to note the two strong bands at 1393 and 1504 cm1 corresponding to the symmetric and asymmetric stretching vibrations of NO2 groups in as-prepared compound, which indicates that its molecules were present on the surface of CuO nanoparticles. The weak bands at 820, 879 and 1048 cm1 appearing in the IR spectrum of the as-prepared compound indicate the presence of stretching vibrations of the intercalated –NO2 species and they disappear upon thermal treatment, as shown in Figure 10(b)–(d). The band near 755 cm1 confirms the presence of out-of-plane bending vibrations of the intercalated O–N species produced by the initial process of preparation. Furthermore, the strong absorption bands

528   K.G. Chandrappa and T.V. Venkatesha

Figure 9. TEM images of CuO nanoparticles obtained from: (a) 30 mM NaNO3 and (b) 60 mM NaNO3 at 1.0 A/dm2 calcined at 500 C for 1 h. Circles emphasise the hexagonal feature of the nanoparticles.

Figure 10. FT-IR spectra of: as-prepared CuO particles obtained from 30 mM NaNO3 1 A/dm2 (a) and calcined at different temperatures of 300 C (b), 500 C (c) and 700 C (d).

around 483, 517 and 584 cm1 were assigned to Cu–O stretching vibrations, indicating the presence of Cu–O structure in the as-prepared and calcined compounds (Figure 10a–d). On increasing calcination temperature, the adsorbed hydroxyl and –NO2 groups diminish and the characteristic peaks of CuO at 483, 517 and 584 cm1 increase. In Figure 10(b)–(d), the characteristic peak around 517 cm1 becomes stronger, indicating the formation of stretching mode of CuO. The metal–oxygen frequencies observed for the respective metal oxides are in accordance with literature values [46]. They quite agree with the measurement



Journal of Experimental Nanoscience   529

Figure 11. UV–Vis spectra of: as-prepared CuO obtained from the bath concentration at 30 mM NaNO3 at 1.0 A/dm2 (a), and calcined at different temperatures of 300� C (b), 500� C (c), 700� C (d) and 900� C (e).

results of TG–DTA, which indicate that the as-prepared compound was partly decomposed at 245� C. This indicates the presence of CuO nanoparticles in the as-prepared and calcined compounds.

3.7. UV–Visible spectroscopy The UV–Visible spectra of the as-prepared (generated from 30 mM NaNO3 at 1.0 A/dm2) and calcined compounds (at 300� C, 500� C, 700� C and 900� C for 1 h) of CuO are shown in Figure 11. For recording UV–Vis spectra, the sample of CuO solution was prepared by ultrasonically dispersing them in absolute ethanol. All the absorption peaks in Figure 11 corresponding to CuO sample calcined at different temperatures showed strong absorption in the wavelength range 221.4–223.8 nm. On increasing calcination temperature, the growth of CuO particles takes place, and the corresponding absorption band (max) increases. The band gaps (Eg) of CuO nanoparticles were calculated using Eg ¼ hc/, where h is the Plank’s constant, c the velocity of light and  the wavelength. The band gap values lie in the range 5.60–5.54 eV. Further, the XRD results at different temperatures suggest that the CuO crystallite size range was 22–36 nm. It implies that the calcination temperature influences the optical absorption a little. The other methods such as hydrothermal, sol–gel, co-precipitation and combustion require calcinations, excess of organic solvents, severe conditions of reaction, high operating temperature and expensive equipments. But, electrochemical–thermal method has advantages such as less electrical energy, ambient temperature, short time duration, low cost, high yield and ecofriendliness. In this context, the present electrochemical–thermal method is superior to other methods and it generates CuO nanoparticles of higher band gap exhibiting blue shift compared to bulk CuO (1.4 eV).

530   K.G. Chandrappa and T.V. Venkatesha 4. Conclusions In this study, the hexagonal-shaped CuO nanoparticles were successfully generated by electrochemical–thermal method using NaNO3 electrolyte without copper salts, templates or surfactants. The Cu2þ ions were generated at the sacrificial Cu electrode and were converted into CuO during electrolysis. The calcination temperature at 500� C resulting in the CuO crystallite size was around 30 nm. The crystallite size range of the generated CuO powder was 22–36 nm. TGA revealed that the as-prepared compound contains OH� and H2O up to 17.14% and remaining 82.86% of CuO. The SEM analysis shows that the particles morphology was a hexagonal structure. The EDAX spectrum showed that 100% CuO compound was present in the calcined particles. TEM images confirmed the hexagonal shape of CuO nanoparticles; they are well crystallised with nanosize 30–50 on nanometre scale. The FT-IR spectrum shows the existence of OH� and NO� 2 groups in uncalcined sample. The band gap was higher for synthesised CuO particles than their bulk counterparts. The yield of CuO is maximum for 1 h electrolysis in the concentration of 30 mM NaNO3 at 1.0 A/dm2. The process operated at room temperature and their insolubility in electrolyte makes the separation easier. The same electrolyte could be reused for the generation of CuO nanoparticles. But in the case of hydrothermal, sol–gel and co-precipitation methods, the separation of generated product is not easy. This electrochemical method could be effectively used to synthesise CuO nanoparticles on a large scale. Thus, the bulk synthesis of CuO nanoparticles by electrochemical method is a simple, fast, economical and eco-friendly one compared other methods. Acknowledgements The authors thank Kuvempu University, Karnataka, India for providing the lab facilities to bring about this study and Mrs. Manjula, Department of Physics, IISc, Bangalore for thermal analysis. Kodihalli G. Chandrappa and Thimmappa V. Venkatesha thank, respectively, the CSIR, New Delhi for SRF [sanction no. 09/908(0002) 2K9-EMR-I] and DST [no. S.R/S3/ME/014/2007], Government of India (GOI) for providing research grant.

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