CuO and Co 3 O 4 Nanoparticles : Synthesis , Characterizations , and Raman Spectroscopy

Copper oxide and cobalt oxide (CuO, Co 3 O 4 ) nanocrystals (NCs) have been successfully prepared in a short time using microwave irradiation without any postannealing treatment. Both kinds of nanocrystals (NCs) have been prepared using copper nitrate and cobalt nitrate as the starting materials and distilled water as the solvent. The resulted powders of nanocrystals (NCs) were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) measurements. The obtained results confirm the presence of the both of oxides nanopowders produced during chemical precipitation using microwave irradiation. A strong emission under UV excitation is obtained from the prepared CuO and Co 3 O 4 nanoparticles.The results show that the nanoparticles have high dispersion and narrow size distribution. The line scans of atomic force microscopy (AFM) images of the nanocrystals (NCs) sprayed on GaAs substrates confirm the results of both X-ray diffraction and transmission electron microscopy. Furthermore, vibrational studies have been carried out using Raman spectroscopic technique. Specific Raman peaks have been observed in the CuO and Co 3 O 4 nanostructures, and the full width at half maximum (FWHM) of the peaks indicates a small particle size of the nanocrystals.


Introduction
Nanocrystalline semiconductor particles have drawn considerable interest in recent years because of their special properties such as a large surface to volume ratio, increased activity, special electronic properties, and unique optical properties as compared to those of the bulk materials [1,2].Moreover, it has been shown that colloidal NCs can be integrated in epitaxial grown layers, which allows for the realisation of real device implementations based on NCs [3].The oxides of transition metals represent an important class of semiconductors, which have applications in magnetic storage media, solar energy transformation, electronics, and catalysis [4].Among the oxides of transition metals, copper oxide nanoparticles are of special interest as nanofluids in heat transfer application due to their efficiency.For example it has been reported that 4% addition of CuO improves the thermal conductivity of water by 20% [5].CuO is a semiconducting compound with a narrow band gap and used for photoconductive and photothermal applications [6].
CuO has attracted much attention because it is the basis of several high-T c superconductors.CuO is known as ptype semiconductor, which makes it a promising material for gas sensors, magnetic storage media, solar energy transformation, electronics, semiconductors, varistors, and catalysis [7].The CuO exhibits high activity for photocatalysis of H 2 evolution reaction (HER) in oxalic acid solution under simulated sunlight irradiation [8].Co 3 O 4 NCs are a promising candidate as anode material for lithium secondary batteries because of its electrochemical capacity and high recharging rate [9].Hybrid material consisting of Co 3 O 4 nanocrystals grown on reduced graphene oxide is a high-performance bifunctional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) [10].Microwave irradiation has shown very rapid growth in its application to material science due to its unique reaction effects such as rapid volumetric heating and the consequent dramatic increase in reaction rates.Compared with the conventional methods, the microwave synthesis has the advantages of short reaction time, small particle size, narrow particle size 2 Journal of Nanomaterials distribution, and high purity [11,12].Raman spectroscopy is a powerful nondestructive technique that has been successfully used to study a wide range of materials.Raman spectroscopy is of particular relevance to investigate structural properties of nanosized materials because slight variations are easily detected [13].Advanced Raman spectroscopy methodologies, such as surface-enhanced Raman spectroscopy (SERS), [14] shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), [15] near-field scanning optical microscopy-Raman (SNOM-Raman) [16], and tip-enhanced Raman spectroscopy (TERS), have been developed for ultrasensitive detection, identification, and dynamic study of molecules (SERS and SHINERS) and material surfaces (TERS) [17,18].
In this paper we present a new rapid growth method to prepare different monoclinic metal oxide nanocrystals in about 10 min using microwave irradiation without any postannealing treatment.The product has good crystallinity, uniform morphology, and high purity as demonstrated by Xray diffraction, TEM, and Raman spectroscopy.

Experimental Technique
A microwave oven with 650 W power (Sanle general electric corp.Nanjing, China) was used.Powder XRD measurements were performed on a Shimadzu XD-3A X-ray diffractometer at the 2 range from 30 to 60, with monochromatized CuK radiation ( = 0.15418 nm).The scanning electron microscopy (SEM) images pictures were recorded on a JEOL-JEM 200CX scanning electron microscope.The transmission electron microscopy (TEM) images pictures were recorded on a JEOL-JEM 200CX transmission electron microscope, using an accelerating voltage of 80 kV.The samples used for TEM observations were prepared by dispersing some products in ethanol followed by ultrasonic vibration for 30 min, then placing a drop of the dispersion onto a copper grid coated with a layer of amorphous carbon.AFM scans (10 × 10 m) were performed to measure the morphology of the sample's surface in highly sensitive AFM needle non-contact mode.The samples used for AFM were prepared by dispersing some products in methanol and chloroform (50% to 50%) followed by ultrasonic vibration for 20 min, then sprayed only one drop of the dispersion onto a GaAs substrate.After a short time, the solution of methanol and chloroform is evaporated and only the oxides of CuO or Co 3 O 4 NCs remain on the GaAs substrate.To measure the absorbance, a solution of methanol and chloroform is used for CuO NCs, while for the Co 3 O 4 NCs distilled water is used.The absorbance of the NCs in solution was measured in a cuvette with 1 cm diameter.A Shimadzu UV-3100 photospectrometer was used to record the UV-Visible absorption spectra of the asprepared particles.-Raman-spectroscopy was used to study the vibrational properties of the nanoparticle.The Raman spectroscopy was done in a backscattering geometry.For excitation a stabilized DPSS-Laser with 532 nm was used.The spectral analysis was done with a holographic grating and an attached Andor Newton camera with a backlight CCD-chip.A spectral resolution of about 2.3 cm −1 was reached.In a typical procedure, 25 mL water solution of 0.2 M Cu(NO 3 ) 2 ⋅6H 2 O was mixed with 25 mL water solution containing 0.2 M CO(NH 2 ) 2 in a round-bottom flask.The vessel containing the solution was introduced into a microwave oven operating at a maximum power of 800 W for 20 min.The solution boils and undergoes dehydration followed by decomposition with the evolution of large amount of gases.After the solution reaches the point of spontaneous combustion, it begins burning and releases lots of heat, vaporizes all the solution instantly, and becomes a solid powder [19].A black fine powder of CuO NCs is extracted.After cooling to room temperature, the precipitate was centrifuged and washed with distilled water.The final products were collected for characterizations.The same procedure has been repeated for Co 3 O 4 NCs using Cobalt nitrate as a starting material.

Results and Discussion
Figure 1 shows the XRD pattern for CuO and Co 3 O 4 nanoparticles, respectively.The intensities and angular positions of the peaks are in good agreement with corresponding values [20,21].No peaks related to impurity are found in the XRD pattern.In order to understand the phase symmetry of the prepared CuO and Co 3 O 4 nanocrystals a systematic study on the XRD was performed.Sharp peaks were obtained at angles corresponding to the planes (110), ( 002), ( 111), (202), and (202).This indicates the monoclinic structure of CuO nanocrystals [22] which was found to be highly crystalline.The XRD pattern obtained from the product (Figure 1(b)) is identical to Co 3 O 4 NCs.The sharp peaks corresponding to the planes (220), (311), (400), (422), (511), and (440) indicate the monoclinic structure of Co 3 O 4 nanocrystals which was also found to be highly crystalline.The average size of the both nanocrystals is estimated according to the following Debye-Scherer formula [23]: where the constant  is taken to be 0.94,  is the wavelength of used X-ray used which is CuK radiation ( = 1.5406Å).  is the full width at half maximum of the diffraction peak corresponding to 2.Using (1), the calculated crystallite sizes are found to be in the range of 14 ± 1 nm and 14.5 ± 1.3 for CuO and Co 3 O 4 nanoparticles, respectively.The size and morphology of the product are analyzed by transmission electron microscopy (TEM).Figures 2(a   In some cases the measurement of the absorption spectra of prepared CuO NCs yields a small fluctuated band at 270 nm and the other band at 440 nm which is visible in the absorption spectra [12] as illustrated in Figure 4(a).The absorption bands for CuO NCs have been reported to be in the range of 500-600 nm [24][25][26].As shown in Figure 4   state of copper from zero to +2.The peak at 520 nm can be attributed to a narrow size distribution of the particles formed in the solution.Figure 4(b) illustrates the absorbance of Co 3 O 4 nanoparticles.As can be detected from the figure, the absorption shoulder is centered at 205 nm.This value of the absorption peak position is used to calculate the size of the NCs.A recent result given by [27] shows that the agglomeration state of nanoparticles can produce important modifications on the Raman properties of nanoparticles.An increase of the agglomeration state produces a red-shift and a broadening of the Raman modes similar to those found by nanoparticles size reduction or by temperature increases.Raman spectra of CuO and Co 3 O 4 nanoparticles are shown in Figure 5.It can be seen from Figure 5(a) that there are three Raman peaks at 282, 330, and 616 cm −1 .Table 2 lists the experimental results regarding the position and full-width at half maximum (FWHM) of the Raman peaks for as prepared CuO NCs.Our CuO NCs belong to the  6 2ℎ space group with two molecules per primitive cell.There are nine zone-center optical phonon modes with symmetries 4A u +5B u + A g +2B g ; only three A g + 2B g modes are Raman active.In comparison with the vibrational spectra of CuO powder [28,29] and single crystals [30] we can assign the peak at 282 cm −1 to the A g and the peaks at 330 and 616 cm −1 to the B g modes.We note that these wavenumbers are close to those reported in the literature (288, 330 and 621 cm −1 ) [31].They studied the Raman spectra of CuO nanocrystals with different grain sizes at room temperature and high temperatures up to 873 K.They reported that the intensity is related to the grain size.Stronger and sharper Raman peaks are observed which also shift to longer wavenumbers with increasing/decreasing grain size.Our results are in good agreement with the latter findings.3 presents the experimental results regarding the position and full-width at half maximum (FWHM) of the Raman peaks for as prepared Co 3 O 4 NCs.These five Raman active peaks predict the Co 3 O 4 spinel structure.The Raman mode at 684.5 cm −1 (A 1g ) is attributed to characteristics of the octahedral sites and the E g and F 2g modes are likely related to the combined vibrations of tetrahedral site and octahedral oxygen motions [32].

Conclusion
Nanocrystalline CuO and Co 3 O 4 particles with a monoclinic structure have been prepared successfully by a novel method using microwave irradiation.The CuO and Co 3 O 4 NCs are soluble in a chloroform/methanol mixture, thus enabling the processing of these materials in solution.It is a simple and efficient method to produce CuO and Co 3 O 4 nanocrystals with regular shape, small size, narrow size distribution and high purity.From XRD, SEM, and TEM study, it is found that particles are spherical in shape with average size of 10-14 nm.According to the results of XRD, TEM, and optical properties, we can understand the characteristic features of the investigated metal oxide NCs.Raman scattering performed with CuO and Co 3 O 4 NCs confirm the NCs formation and estimated size distribution.We can foresee the upscaling of the process to form large quantities of nanosized particles, which have wide applications in various fields such as phonics, catalysis, and biosensors.
) and 3(b) show the 2D, 3D, and line scans extracted from AFM image of CuO and Co 3 O 4 nanoparticles, respectively, sprayed on GaAs substrate.From this image, the height of the particles can be determined.In (Figures 3(a) and 3(b)) single as well as accumulated particles are clearly visible.The height of particles which indicates the particle size is in good agreement with the X-ray and TEM investigations.The calculated size of CuO and Co 3 O 4 nanoparticles obtained from all characterization techniques are listed in Table 1.Figures 4(a) and 4(b) show the absorbance spectra recorded versus wavelength of CuO and Co 3 O 4 nanoparticles dispersions in respective solvents.

Figure 3 :Figure 4 :
Figure 3: (a) AFM image of as prepared CuO nanoparticles (A) 3D, (B) 2D, and (C) line profile of CuO.From (A) and (B), it is confirmed that CuO particles are uniformly distributed on GaAs substrate.In addition, line profile (C) reveals that the size of CuO is around 10 nm.(b) AFM image of as prepared Co 3 O 4 nanoparticles (A) 3D, (B) 2D and (C) line profile of Co 3 O 4 NCs.From (A) and (B), it is confirmed that Co 3 O 4 particles are uniformly distributed on GaAs substrate.In addition, line profile (C) reveals that the size of Co 3 O 4 is around 9 nm.

Figure 5 :
Figure 5: (a) Raman spectra of as prepared CuO nanoparticles.(b) Raman spectra of as prepared Co 3 O 4 nanoparticles.

Table 1 :
Comparison between the CuO and Co 3 O 4 nanoparticle sizes calculated from X-ray, TEM, and AFM.

Table 2 :
Positions and FWHM of Raman peaks for as-prepared by AFM technique.Figures3(a

Table 3 :
Positions and FWHM of Raman peaks for as prepared Co 3 O 4 NCs.