Introduction

Nanocrystalline semiconductor particles have drawn considerable interest in recent years due to their interactive properties such as large surface-to-volume ratio and distinctive electronic and optical properties as compared to bulk materials (Huang et al. 2010; Son et al. 2009; Xu et al. 2007; Lim et al. 2012; Qi et al. 2007; Kidowaki et al. 2012). Cupric oxide (CuO) is a transition metal oxide. It has a monoclinic structure and a narrow bandgap of 1.2 eV (indirect) with p-type semiconductor material. It is also an attractive candidate for light-harvesting applications due to its band gap energy of 1.4 eV (Son et al. 2009; Xu et al. 2007; Lim et al. 2012). CuO compounds are technologically well-known materials that have their applications in areas (Qi et al. 2007) such as solar energy materials, electronic materials, gas sensor, magnetic media, optical devices, batteries and catalyst (Kidowaki et al. 2012; Lang et al. 2006; Teng et al. 2008; Stewart et al. 2004; Wang et al. 2007; Morales et al. 2005), and constructing junction devices such as p-n diode (Muhibbullah et al. 2003), as well as for photoconductive, photothermal, and photoelectrochemical applications (Chiang et al. 2012).

Attempts are made to prepare CuO nanoparticles using different methods such as spray pyrolysis (Chiang et al. 2012), electrochemical techniques (Chen et al. 2009), hydrothermal treatments (McAuleya et al. 2008), sonochemical method (Gandhi et al. 2010), and wet chemical methods (Mahapatra et al. 2008) with different morphologies. Among these methods, sonochemical preparation method is used to break the chemical bond of the solution compound. In general, formation, development, and implosive collapse of microcavities are the three steps involved in the production of nanoparticles through sonochemical method (Gandhi et al. 2010). Most inorganic materials such as Al2O3, ZnO, MnO2, SnO2, Y2O3, and clay particles may be more dispersed in sodium hexametaphosphate. Sodium polyphosphate (Calgon) is also a suitable dispersing agent (Greenwood 2003). These are the solvents used to readily dissolve in water and they are normally added at approximately 1 wt %.

The optical behavior of CuO nanomaterials is mainly assessed through UV–vis and photoluminescence techniques to explore the electronic transitions in semiconductors including band-edge or near-band-edge transitions. Only a few works based on UV–vis analysis are available on the absorption and band gap studies of CuO nanoparticles (Cho 2013; Lin et al. 2004; Kaur et al. 2006; Tauc et al. 1966). However, optical properties of CuO nanoparticles are not revealed extensively.

In this article, using a simple and low-cost approach, CuO nanoparticles were synthesized using sonochemical method and their optical properties were investigated using different characterization techniques.

Experimental

Materials

Analytic grade (AR) chemicals namely copper (II) nitrate [Cu(NO3)2], sodium hydroxide (NaOH), and sodium hexametaphosphate [Na(PO3)6] obtained from Merck, and deionized water, obtained in our laboratory, were used without any further purification.

Preparation of copper oxide nanoparticles

In a typical procedure, 50 ml NaOH solution was drop-wise added into 100 ml aqueous Cu(NO3)2·2H2O solution under sonication process. Meantime, the mixed solution was kept under sonication at a constant interval of 60 s and frequency of 25 kHz for 60 min and pH was maintained at 12. The Cu(OH2) solution thus formed was blackish brown. The precipitate was centrifuged at room temperature at 10,500 rpm and then washed with deionized water several times. Finally, the obtained solution was washed with ethanol to remove ionic impurities and then annealed at 673 K under vacuum for 4 h. Finally, black CuO nanoparticles were obtained.

Characterization

The phase identification was carried out using powder X-ray diffraction (XRD; X’Pert PRO PW-1830; Philips, Germany). The functional group present in the sample was recorded using Fourier transform infrared spectroscopy (FTIR; Spectrum 100; PerkinElmer). The microscopical analysis of the samples was carried out by a scanning electron microscope (SEM; JSM-200; JEOL, USA) equipped with energy-dispersive spectroscopy (EDS), and the powder sample was mixed with ethanol to analyze the sample through transmission electron microscopy (TEM; JEM-2100; JEOL) along with selected area electron diffraction (SAED) pattern. The CuO nanoparticles were dispersed in (NaPO3)6 solution at a ratio of 1:100 and then kept under ultrasonic irradiation (25 kHz) to obtain a homogeneous mixture solution, which was used to analyze particle size distribution by dynamic light scattering technique using a particle size analyzer (PSA; Nanophox, Germany) and optical properties were determined by recording the absorption spectra using a UV spectrometer (Lamda 20; PerkinElmer).

The optical band gap (E g) was obtained from the absorption coefficient (α), which was calculated from the transmittance spectra of CuO nanoparticles. The value of α is given as follows (Khalil et al. 2011; Sumangala et al. 2005; Mishra and Mishra 2009; Hamad et al. 2014; Growth 2011):

$$\alpha = \frac{1}{d}\ln ({\raise0.5ex\hbox{$\scriptstyle 1$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle T$}})$$

where d is the film thickness, T the transmittance, and α the absorption edge. CuO is a wide band gap semiconductor material with direct band gap. The optical band gap (E g) is given by

$$(\alpha hv)^{2} = A(hv - E_{g} )$$

where α is the absorption edge and hν the photon energy. The refractive index (n) and extinction coefficient (k) of CuO nanoparticles were calculated from the following expressions (Sumangala et al. 2005):

$$n = \left( {\frac{1 + R}{1 - R}} \right) + \sqrt {\left( {\frac{4R}{{(1 - R)^{2} }} - k^{2} } \right)}$$
$$k = \frac{\alpha \lambda }{4\pi }$$

where R is reflectance, λ the wavelength, and k the extinction coefficient.

The Urbach tail of the particles can be determined from the following relation (Growth 2011):

$$\alpha = \alpha_{0} \exp \left( {\frac{hv}{{E_{U} }}} \right)$$

where α is absorption edge, α 0 a constant, hν the photon energy, and E U the Urbach energy.

Results and discussion

Analysis of structural and functional properties

The crystal phase of CuO nanoparticles is consistent with JCPDS card no 05-0661 of all diffraction peaks, as shown in Fig. 1a. The sharp observed planes (110), (−111), (111), (202), (020), (220), (−113), and (022) show the monoclinic phase of CuO nanoparticles. Further, it is evident that there are no peaks corresponding to Cu2O or Cu(OH)2 phases. The absence of impurities is also reflected from the XRD pattern. The average crystallite size of the CuO nanoparticles obtained from full-width at half maximum of the diffraction peak is 21 nm.

Fig. 1
figure 1

Characterisation of CuO nanoparticles a XRD Pattern and b FTIR Spectra

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Figure 1b shows the functional property of FTIR spectra. The obtained spectra show an absorbed band of approximately 640 cm−1, which shows the characteristic band of monoclinic phase of pure CuO similar to that obtained in an earlier report (Gandhi et al. 2010). The bands obtained at 1585 and 1644 cm−1 show the carbonyl C=O stretching bonds. Similarly, the C–H stretching bonds occur in the region of 3300–2800 cm−1 (Xu et al. 2007). Moreover, the broad band centered at 3413 and 3392 cm−1 is attributed to the stretching and bending vibrations of absorbed water and surface hydroxyls (Gandhi et al. 2010).

Figure 2 shows the SEM and TEM images of the sample. Figure 2a shows the SEM image of CuO nanoparticles, which consists entirely of flake-like morphology. The fine nanoparticles aggregate due to their high surface energy. EDS analysis of the flake-like CuO architecture suggests that the sample contains only the identified elements of Cu and O with the atomic and weight percentages as shown in the inset of Fig. 2b. It also confirms that the grown nanoparticles are composed of CuO without any impurities. TEM image suggests that CuO nanoparticles show relatively uniform diameter in the range of 30–60 nm (Fig. 2c). SAED pattern shows that the CuO nanoparticles comprise small nanocrystalline structures with different orientation of the single crystal diffraction pattern. Therefore, the CuO nanoparticles are crystalline in nature, which closely matches with the XRD pattern. The distribution of CuO nanoparticles is in the range between 35 and 125 nm. However, the average distribution (d50) of the nanoparticles is 57 nm (Fig. 2d).

Fig. 2
figure 2

Characterisation of CuO nanoparticles a SEM image, b EDS pattern, c TEM image and d particle size distribution

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Analysis of optical properties

The absorption edge is found so that the type of transition and bandgap value can be determined (Essic and Mather 1993). The absorption spectra were used to study the energy band and the type of electronic transitions. Absorption spectra of CuO nanoparticles are shown in Fig. 3a, which show a strong fundamental absorption edge approximately 219 nm due to direct transition of electrons. Optical absorption shows that the direct band gap compared to indirect band gap permits us to determine the crystallinity of a material. The functional relationship between αhγ and photon energy for CuO nanoparticles is presented in Fig. 3b. The E g value can be obtained by extrapolating the linear portion to the photon energy axis. If the direct band gap is higher than the indirect band gap, the materials will be crystalline (Radhakrishnan and Beena 2014). The calculated direct band gap value was 3.85 eV, which was higher than the bulk band gap value (3.5 eV). Here, only direct transition-related absorption was observed, and there was no indirect transition absorption peak (Fig. 3b). The observed increasing band gap could be ascribed to the presence of intragap states and quantum confinement effect.

Fig. 3
figure 3

Optical properties of CuO nanoparticles a UV absorption, b band gap, c absorption coefficient and d k & n values

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The variation of absorption coefficient of CuO nanoparticles as a function of photon energy is shown in Fig. 3c. The figure clearly shows that the absorption coefficient tends to decrease exponentially as the wavelength increases. This behavior is typical for many semiconductors and could be due to a variety of reasons, such as internal electric fields within the crystal, deformation of lattice due to strain caused by imperfection, and inelastic scattering of charge carriers by phonons (Moss et al. 1973; Honsi et al. 2006; Almqvist 1996). The highest absorption coefficient is observed in the UV region and is in the order of 106–107 from UV to visible region. Elongation of band gap in the form of band tail and absorption due to free carriers are observed from the absorption coefficient.

The k and n values against photon energy plot of CuO nanoparticles are shown in Fig. 3d. From the image, the refractive index of CuO nanoparticles is found to increase with an increase in photon energy, whereas it is found to have lesser value than that for the bulk CuO (n = 2.6). For instance at band gap region, the refractive index values range from 2.1 to 2.37. In general, refractive index of the semiconductor is a measure of its transparency to incident spectral radiation. The assessment of the refractive index of the optical material is notably important for applications in integrated optic devices (Xue et al. 2008). The extinction coefficient of CuO nanoparticles increases with increase in the photon energy. The observed extinction coefficient values are very low (0.42) in the absorption region, and it specifies the smoothness of the surface and homogeneity of the particles.

Optical conductivity of CuO nanoparticles with respect to the photon energy is shown in Fig. 4a, which describes the free charges (Madhup et al. 2010; Millis et al. 2005; Lupi et al. 2005). Optical conductivity increases with respect to the photon energy in the UV region and it in turn increases the free carriers. The optical conductivity dramatically increases in the higher energy region (UV absorption), which indicates the availability of free carriers of photon energy and hence, the maximum of optical conductivity is observed. However, an enormous decrease in the optical conductivity in the visible region is observed due to trapping of free carriers.

Fig. 4
figure 4

Optical properties of CuO nanoparticles: e optical conductivity and f Urbach energy

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The plot of ln (α) versus photon energy is linear, as shown in Fig. 4b. Urbach energy is obtained from the inverse of the slope (see the figure) and the calculated Urbach energy value of CuO nanoparticles is found to be 1.01 eV, which is higher than that of the other metal oxides. Basically, Urbach energy depends on the static and induced disorder, temperature, strong ionic bonds, and average photon energies (Gandhi et al. 2010). Urbach band tail calculation can be performed to confirm the change in band gap energy and the interband formation in CuO nanoparticles. The optical band structure and optical transitions are affected by the width of the localized states available in the band gap, which is called the Urbach tail. Therefore, the CuO nanoparticles can be used in semiconductor devices such as photoamplifier, photovoltaic cell, and photo detectors.

Conclusion

The CuO nanoparticles were successfully prepared via sonochemical method. The crystallite size of CuO was found to be 21 nm. Distribution of spherical CuO nanoparticles was observed from micrographical analysis. The average particle size was clearly visualized and measured 57 nm, respectively, through TEM and PSA studies. Optical properties of CuO nanoparticles were analyzed/clarified through UV absorption. The direct band gap of CuO nanoparticles was found to be large. Moreover, the absorption coefficient, optical conductivity, refractive index along with extinction coefficient and Urbach energy were calculated through UV absorption. The CuO nanoparticles could be used as an extensive semiconductor, optical devices, solar cell applications, and so on.