1 Introduction

Transparent conducting oxides (TCOs) have been widely used in various optoelectronic devices such as light emitting diodes (LEDs), solar cells, flat panels, and flexible displays [1, 2]. In recent years, various types of TCOs have been extensively researched. Among them, indium tin oxide (ITO) is the most widely used for these applications owing to its high transmittance in the visible spectrum and low resistivity (~1.0 × 10−4 Ω cm) [2]. However, the ITO possesses significant drawbacks such as its high cost, scarce resources, and poor chemical stability. Therefore, researchers have been exploring alternative materials for the TCO industry applications [3–6]. Researchers showed that the crystallinity and morphology as well as the porosity of the ZnO nanostructures have direct effect on its physical properties [7, 8]. Some ZnO structures have been successfully synthesized including nanorods [9], nanowires [10], nanoflowers [11], comb-like and dumbbells [12, 13]. Among all the synthesized structures the 3D nanostructures, micro/nanoflowers in particular, are believed to be the kind of structures that have the most enhanced properties, especially for photocatalysis [14].

Zinc oxide (ZnO) is one of the most attractive II–IV compound semiconductors because of its excellent electrical, optical and piezoelectric properties. It is widely applied in various fields such as transparent conducting electrodes in solar cells, plasma panel displays, surface acoustic wave devices, and chemical sensors. ZnO thin film can be deposited by various methods such as spray pyrolysis [7, 12], chemical vapor deposition (CVD) [8], sol–gel process [9], sputtering techniques [10], and pulsed laser deposition (PLD) [11]. Among these methods, the sputtering method shows great advantages e.g., high deposition rate, large area deposition, high density, strong adhesion, and good uniformity at low substrate temperature [12].

It is well recognized that substrate surface treatment prior to deposition has crucial effects on both deposition mechanism and film properties [13, 14]. Previous literature showed that substrate surface was often treated by cleaning, etching or mechanical polishing [15–18]. In addition, there are a few reports on the effect of ZnO thin films by using plasma to pretreat substrate surface prior to deposition. Li et al. [19] used RF plasma to clean substrate. Kwon et al. [20] improved the structural and electrical properties of Ga-doped ZnO films by pretreating polyimide substrate with high-density plasma. Zhang et al. [21] reported that textured ZnO:Al films were prepared by RF magnetron sputtering initiated by pretreatment of glass substrate with mixed argon and oxygen ions. The pretreatment of substrates, especially micro-pretreatment is an important and effective method to change micro-structural character of substrates, and consequently to influence the morphology, structure and optical and electrical properties of films significantly. For this reason, ZnO nanoparticles are promising candidates for various applications, such as nanogenerators, gas sensors, solar cells, photodetectors and photocatalysts [22–24].

In this paper we report the synthesis of ZnO nanopowder by sol–gel method for opto-electronic applications. The morphological, structural, optical and electrical properties were systematically studied to assess the intrinsic specificities of the elaborated material.

2 Experimental

2.1 Preparation of ZnO nanoparticles

The sample preparation processes have been reported by Omri et al. [25]. For the synthesis of the samples, we dissolved 2 g of zinc acetate dehydrate in 14 ml of methanol, under magnetic stirring for 2 h. Then, the resulting solution underwent rapid drying to obtain powder aerogels. Furthermore, we conducted drying in supercritical conditions of ethanol at 250 °C, with a heating rate of 45 °C/h.

As obtained powder was annealed at different temperatures such as at 250, 300, 400, and 500 °C for 2 h to obtained white powder. The annealed ZnO samples obtained were then characterized in detail in terms of their morphological, structural, optical and electrical properties using various analytical techniques.

2.2 Characterizations

The crystalline phases of the obtained nanopowders were identified by X-ray diffraction (XRD) using a Bruker D5005 powder X-ray diffractometer using a CuKα source. Crystallite sizes (D, in Å) were estimated from the Scherrer’s equation [26]:

$$D = \frac{{0.9\uplambda}}{{B\cos \theta_{B} }}$$
(1)

where λ is the X-ray wavelength (1.5418 Å), θ B is the maximum of the Bragg diffraction peak and B is the linewidth at half maximum (in radians).

Transmission electron microscopy (TEM, JEM-200CX) were used to study the morphology and particle size of the powders. The specimens for TEM were prepared by putting the as-grown products in EtOH and immersing them in an ultrasonic bath for 15 min, then dropping a few drops of the resulting suspension containing the synthesized materials onto TEM grid. For ac measurements, an Agilent 4294A impedance analyzer was used to collect impedance measurements over a wide frequency range (40 Hz–100 MHz). We employed a parallel mode to measure conductance G using an alternating signal with voltage amplitude of 50 mV. The optical absorbance of the powders was determined using Schimadzu UV-3101 PC spectrophotometer with integrating sphere in the wavelength range from 200 to 2000 nm.

3 Results and discussion

3.1 Structural properties

Figure 1 report typical TEM images taken from the as-prepared samples by sol–gel method and subsequent drying in supercritical conditions. Very small particles having size in the nanometer range are observed. The crystallites present a prismatic-like shape with a narrow particle size distribution. The majority of ZnO particles have a size of about 38–54 nm. After thermal treatment at 500 °C (Fig. 1), TEM analysis highlighted an increase of the particle size for this sample, in according with the XRD results reported below.

Fig. 1
figure 1

Typical TEM photograph showing the general morphology of zinc oxide nanopowder for different annealing temperatures

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In other hand, with the temperature continuing to rise, the phenomenon of “nuclear-aggregation” caused by the rapid formation of crystal nucleus is obvious, which results in aggregation among the crystal nucleus. The rate of particle aggregation is a major factor that controls the morphology and structure (crystalline) of the final products.

Figure 2 shows X-ray diffraction spectra of the ZnO nanoparticles for different thermal treatment temperatures. XRD patterns indicate the formation of hexagonal wurtzite phase of ZnO [27], were matched well with space group P63mc (no. 186) (JCPDS No. 36-1451). The lattice constants calculated from the XRD patterns, which are very close to ZnO ones, i.e., a = 3.249 and c = 5.206 Å. These results are in a good agreement with those obtained by El Mir et al. [28]. The average grain size was calculated from Scherer’s formula (1) using the most intense peak (101). By increasing the annealing temperature the average of the grain sizes increase from 33 to 52 nm, which can be understood by considering the merging process induced from thermal annealing. The estimated values, obtained using XRD data, are in close agreement with those obtained from the TEM photograph. It is noteworthy to mention that, similar to the results estimated by using Scherrer’s equation, the grain size estimated from TEM data found to increase with increasing the annealing temperature. These results can be explained on the basis of increased extent of agglomeration of the ZnO particles with increasing annealing temperature which also results in the diminution of the surface area [29]. Further, with increasing the annealing temperature the oxygen contents were also found to decrease which may be attributed to diffusion out of oxygen from its lattices [Eq. (2)] at very high temperature resulting the formation of anion vacancies or defects [5].

$${\text{ZnO}}\mathop{\longrightarrow}\limits^{\Delta } = {\text{Zn}}^{2 + } + \frac{1}{2}{\text{O}}_{2} + 2{\text{e}}^{ - }$$
(2)
Fig. 2
figure 2

X-ray diffraction spectra of ZnO nanoparticles obtained at different annealing temperatures

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3.2 AC conductivity

The ac electrical conductivity of the ZnO nanoparticles for different annealing temperatures has been studied in the frequency range 40 Hz–2 MHz. The frequency dependence of the measured conductivity is shown in Fig. 3, results are very similar to those for many other amorphous semiconductors including semiconducting glasses [30–32]. In general, the frequency dependence of the total conductivity measured within a fixed frequency window can be expressed as [33]:

$$\sigma (\omega ,T) = \sigma_{dc} (T) + A(T)\omega^{s(\omega ,T)}$$
(3)

where A is a constant depending on the temperature and the power law exponent s has a value less than unity 1 (s = 0.5), \(\sigma_{dc} = 5\times10^{ - 9}\,\Omega ^{ - 1} {\text{cm}}^{ - 1}\), the values of frequency exponent (s) were determined from the slope of the linear parts in Fig. 3. The changing behaviour of the energy band gap with temperature can be explained on the basis of the density of states model proposed by Davis et al. [34]. The unsaturated bonds or defects, which are responsible for the presence of localized states in the band gap, could be reduced with the increase of the temperature, thereby producing a large number of saturated bonds.

Fig. 3
figure 3

ac conductivity versus frequency at 300 K for ZnO nanoparticles annealed at different temperatures

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3.3 Effect of annealing temperature on dielectric properties

The dielectric response in a solid material can be briefly described by expressing the relative dielectric constant as a complex quantity made up of a real component and an imaginary component,

$$\upvarepsilon^{*} =\upvarepsilon^{\prime } + j\upvarepsilon^{\prime \prime }$$
(4)

The first term ɛ′ is the real part of dielectric constant and describes the stored energy while the second term ɛ″ is the imaginary part of dielectric constant, which describes the dissipated energy. The dielectric constants ɛ′ and ɛ″ of the materials constant have been calculated by the relation:

$$\upvarepsilon^{\prime } = \frac{{C_{p} e}}{{A\varepsilon_{0} }}$$
(5)

where Cp is the capacitance in Farad (F), e is thickness of pellet, \(\varepsilon_{\begin{subarray}{l} 0 \\ \end{subarray} }\) is the permittivity of free space (8.854 × 10−12 F/m) and A is the cross sectional area of the flat surface of the pellet.

$$\upvarepsilon^{\prime \prime } =\upvarepsilon^{\prime } * \tan \delta^{\prime }$$
(6)

where tan δ is the dielectric loss tangent which is proportional to the loss of energy from the applied field into the sample (this energy is dissipated as heat) and therefore denoted as dielectric loss.

In order to study the effect of annealing temperature on dielectric properties, the effect of frequency on dielectric constants real for all samples is shown in Fig. 4. It is clear that all coatings with different annealing temperature exhibits that dielectric constant value ɛ′ of the samples show a decrease with increase in frequency.

Fig. 4
figure 4

The real part ε′ of annealed coatings of versus frequency

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The decrease of ɛ′ with the increasing annealing temperature is due to the predominance of the species of vacancies like oxygen, grain boundary defects etc. In fact, The dielectric behavior of ZnO was controlled by grain boundary. Increasing annealing temperature will help grain growth and decrease in grain boundary [35], also it is ascribed to the decrease of polarization and associated relaxation induced by the decrease of ZnO content [36]. While the decrease in dielectric constant with frequency is natural because of the fact that any species contributing to polarizability is bound to show lagging behind the applied field at higher and higher frequencies [37]. Loss tangent or loss factor (tan δ) represents the energy dissipation in the dielectric system. It is considered to be caused by domain wall resonance. At higher frequencies the losses are found to be low since domain wall motion is inhibited and magnetization is forced to change rotation [38]. The variation of tan δ as function of frequency is shown in Fig. 5. It is obvious that the dielectric loss decreases with the increase in frequency for all the samples which exhibit dispersion behavior similar to the dielectric constant. The value of tan δ is large at lower frequencies, while becomes lower at higher frequencies [39].

Fig. 5
figure 5

The loss tangent tan δ of annealed coatings of versus frequency

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3.4 Optical properties

Absorption spectroscopy is powerful technique to explore the optical properties of nanoparticles. The absorption spectra of ZnO nanoparticles for different annealing temperatures in the UV and visible range are presented in Fig. 6. All spectra showed adsorption edges around 385 nm, which corresponded to the optical band gap of ZnO. The absorption peaks present a distinct blue shift of samples synthesized at low temperatures (250 and 300 °C). This blue shift in the absorption may be due to the decreasing of optical scattering caused by the densification of grains followed by grain growth and the reduction of grain boundary density [39, 40]. The optical band gap for ZnO nanoparticles for different annealing temperatures was evaluated using Eq. (7) [41]:

Fig. 6
figure 6

UV–Vis-IR absorption spectra of ZnO nanoparticles for different annealing temperatures. The inset showing the Eg spectra for different temperatures

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$$\left( {\upalpha{\text{h}}\upnu} \right)^{2} = {\text{A}}\left( {{\text{h}}\upnu - {\text{E}}_{\text{g}} } \right)$$
(7)

where A is a constant, hν the photon energy and Eg is the energy band gap. From Eq. (7), a Tauc plot can be drawn by reporting (αhν)2 versus hν. The point of the extrapolation of the linear part that meets the abscissa axis will give the value of the band gap energy (Eg) of the material (Fig. 6, inset). The Eg values are 3.34, 3.28, 3.24, and 3.21 eV for our samples treated at 250, 300, 400, and 500 °C, respectively. With increasing annealing temperature, the Eg of these samples decreases gradually. The energy band gap values obtained are in good agreement with other reported values [42].

The Eg is related to the grain size, carrier concentration, and stress state in material [43]. The maximum of Eg for the sample prepared at 250 °C may be related to the smallest grain size. Furthermore, with increasing annealing temperature, the decrease of Eg should be related to the increase of grain size, decrease of carrier concentration, the tensile stress and oxygen vacancies which lead to the decrease of the carrier concentration in the conduction band [44]. On the other hand, these results are consistent with those observed for the carrier concentration. It is found that there is a band gap narrowing of ZnO nanoparticles with increasing the annealing temperature, which is probably due to the Burstein–Moss effects [45].

Compared with other techniques, the sol–gel method have many advantages such as low cost, simple synthesis equipment, easy fabrication of large-area, easier adjustment of composition, being able to carry out doping at molecular level. Especially, are suitable for the fabrication of oxide nanoparticles [46, 47]. How to prepare high-quality ZnO nanoparticles by sol–gel method has become a researches subject for a comparison between the structural and optical properties of nanocrystalline ZnO nanoparticles such as crystallite size and band gap energy.

4 Conclusion

This paper reports the simple, rapid and high-yield synthesis of ZnO nanoparticles via sol–gel technique. From TEM analysis we found the sizes of nanoparticles vary between 38 and 54 nm. On the other hand, the XRD analysis results indicated that the synthesized ZnO powders had a wurtzite structure. Generally, zinc oxide (ZnO) has attracted much attention because of its novel optical and electrical properties. ZnO is an n-type semiconductor with a wide direct hand gap. The band gap of ZnO varies from 3.34 to 3.21 eV. These properties enable ZnO to have applications in for renewable energy. ZnO samples with different size, have already been prepared via synthesis sol–gel method. This method does not require harsh experimental conditions and complicated equipment. In addition, it can easily prepare ZnO products with controllable morphologies and high quality. Our results are believed to be fundamentally important for the future investigations on the optical properties of the ZnO nanoparticles. This ZnO nanopowder may have potential applications in visible optoelectronic devices, photoanodes of dye-sensitized solar cells and sensors. On the other hand, this work revealed a possibility of producing high-quality ZnO nanoparticles for futuristic optoelectronic devices by a simple low-cost process.