Introduction

Among the more well-known semiconductor materials, ZnO is a II–VI semiconducting ceramic material that possesses a wide band gap of Eg = 3.37 eV and a large exciton binding energy of 60 meV at room temperature (Huang et al. 2001). The importance of this material is reflected in its excellent chemical, mechanical, electrical, optical and optoelectrical properties. For example, it provides both good electrical conductivity (Nakahara et al. 2001) and high transmittance. ZnO has great applicability in the preparation of solar cells (Wang et al. 2001), gas sensors (Lin et al. 1998; Xu et al. 2000), transparent electrodes (Sahu et al. 2007), piezoelectric materials (Wang and Song 2006), and varistors (Koch et al. 1995). However, these are simply a few examples that illustrate its importance. For further indication of its significance, the reader directed to "Appendix".

Nanoparticles are also one of the key elements in nanotechnology. Therefore, in this study we attempt to evaluate the role of different concentration ratios of reactants on ZnO nanoparticles synthesized via the direct precipitation method (DPM). There are also some other parameters that are important in preparation of ZnO nanoparticles by this method and which impact the size and distribution of the nanoparticles, such as the reaction temperature or stirring. Understanding these parameters is important because the properties of these particles depend closely on their size and morphology (Dai et al. 2003).

Over the past twenty years, ZnO nanostructures with various morphologies have been synthesized using a large variety of methods. Such systems provide a rich family of morphologies in comparison to other materials and, as a review of this wide range of nanostructures, we have listed some of these morphologies in "Appendix" so as to present a pertinent background on the works carried out on this important semiconductor material.

Different synthesis methods for the preparation of ZnO nanostructures have been employed in the literature including sol–gel (Tokumoto et al. 2003; Hohenberger et al. 1992), micro-emulsion (Singhal et al. 1997), mechanochemical processing (Tsuzuki and McCormick 2001), vapour phase transport (Zhang et al. 2005), chemical vapour deposition (Wu and Liu 2002), hydrothermal processing (Zhang et al. 2004; Liu and Zeng 2003), spray pyrolysis and drying (Park and Kang 1997; Okuyama and Lenggoro 2003), supercritical-water processing (Viswanthan et al. 2003), thermal decomposition of organic precursor (Rataboul et al. 2002), radio frequency plasma synthesis (Sato et al. 2003), self-assembly (Koh et al. 2004), sonochemical or microwave-assisted synthesis (Hu et al. 2004), homogenous precipitation (Kim et al. 2005) and direct precipitation (Wang and Gao 2003).

In the present study, as mentioned vide supra, ZnO nanoparticles were prepared by precipitation from zinc acetate dihydrate and sodium hydroxide. This method exhibits several advantages over the others listed, such as low cost, large-scale production, low-temperature, and the reduced need for equipment or catalyst assistance. ZnO nanoparticles were obtained and their sizes, morphologies, and structures characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy and transmission electron microscopy (TEM). The ratio of the concentration of zinc acetate dihydrate:sodium hydroxide precursor was varied from 1:1 to 1:4 and the size and morphology of the particles were found to depend strongly on this ratio.

The paper is organized as follows. Previously published works on the synthesis of ZnO nanoparticles were briefly reviewed above. In the section that immediately follows, the main conditions and key parameters of our experiments by the DPM, including the synthesis process, are presented using a schematic diagram. Results obtained by the change in the concentration ratio of the reactant raw materials used in the experiment, and its effect on controlling the size and morphology of the ZnO nanoparticles, are then discussed and show that these properties are size-dependent. Finally, we provide a summary of our findings and present some conclusions.

Material and experimental procedure

Reactant raw materials

Zinc acetate dihydrate (as a Zinc source), sodium hydroxide, ethanol, and distilled deionized water were used in the experiments described herein. Zinc acetate dihydrate and sodium hydroxide were used as the precursors for the formation of ZnO particles.

Synthesis of ZnO nanoparticles

The schematic diagram illustrating the synthesis procedures (DPM) for ZnO particles is presented in Fig. 1.

Fig. 1
figure 1

Schematic diagram showing the preparation of ZnO nanoparticles

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First, zinc acetate dihydrate (Merck, 99 %) and sodium hydroxide (Merck, 99 %) were each dissolved in distilled deionized water to form liquid media of the desired concentrations. The zinc acetate dihydrate solution was slowly added drop-wise to the sodium hydroxide solution under vigorous stirring at room temperature, forming a transparent white solution. In this way, different concentration ratios of aqueous solutions of zinc acetate dihydrate to sodium hydroxide were prepared. The ratio of the concentrations was varied from 1:1 to 1:4 (zinc acetate dihydrate:sodium hydroxide). These solutions were slowly reacted to produce ZnO particle precipitates. In the next step, the precipitates obtained upon the reaction between the zinc acetate dihydrate and sodium hydroxide solutions were collected by filtration and rinsed three times with distilled deionized water and ethanol. Following this, the washed precipitates were dried in an electric oven at 100 °C for 5 h until ZnO precursors formed. At the final step, the precursors were calcinated at 250 °C for 3 h to obtain ZnO nanopowder.

Results and discussion

X-ray diffraction

Analysis of the crystal structures of the ZnO nanoparticles prepared by the DPM was performed with an X-ray diffractometer (MMA, GBC, Australia) using Cu Kα radiation. The diffractograms, which are provided in Fig. 2, we recorded in the 2θ range of 20° to 80° with a step size of 0.05° and scan time of 1 s per step.

Fig. 2
figure 2

XRD patterns of ZnO nanopowder synthesized under different ratio of precursor concentration a 1:1, b 1:2, c 1:3, d 1:4

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The four distinct results (a–d) correspond to the different precipitator concentration ratios (zinc acetate dihydrate:sodium hydroxide) of 1:1, 1:2, 1:3, and 1:4, respectively. The prominent peaks labeled on the figure and which were observed at angles of 31.6°, 34.2°, 36.1°, 47.3°, 56.3°, 62.7°, 66.2°, 67.5° and 68.8° belong to the (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes, respectively. These peaks show very good agreement with the reported values of the Joint Committee on Powder Diffraction Standards data (JCPDS 36-1451) and confirm the formation of hexagonal wurtzite zinc oxide (Lepkova et al. 2007; Bhat 2008). No peaks that may correspond to impurities were detected, thus implying that a relatively high purity of ZnO nanoparticles was obtained. EDX spectroscopy was then used to provide elemental analysis and study the compositions of the particles, with the obtained spectrum displayed in Fig. 3.

Fig. 3
figure 3

EDX spectrum of the ZnO particles

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Figure 3 clearly shows that only Zn and O atoms are present, as evidenced by the determined spectrum. In other words, the product is composed 100 % (Wt and At) of ZnO. We cannot observe the O peaks that must be present from the ZnO because this analysis was carried out using an instrument that cannot depict elements lighter than Na. Figures 2 and 3 both confirm the formation of pure ZnO particles.

The average crystallite size of the obtained particles was measured from the broadening of the diffraction peaks and application of the Scherrer formula (West 1985).

$$ D = (0.89\lambda )/(\beta \cos \,\theta ) $$
(1)

Therein, λ is the wavelength of the Cu kα radiation (1.54 Å), β is the full width at half maximum of the peak, and θ is the Bragg’s angle obtained from the 2θ value corresponding to the maximum intensity peak in XRD pattern. The measured values varied between 20 and 36 nm for samples a–d, as described in Table 1.

Table 1 Crystallite sizes according to the major diffraction peaks observed in Fig. 2
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As the reactant raw material concentration ratios increases from 1:1 to 1:4, the intensity of the reflections peaks increases, as can be seen from Fig. 2. Therefore, of the samples studied, ZnO nanopowder at a concentration ratio of 1:4 has the most intense peaks and largest crystallite size of 36 nm.

Scanning electron microscopy

The morphologies of the nanoparticles were analyzed using a scanning electron microscope (Philips) equipped with an energy-dispersive X-ray spectrometer. The EDX spectroscopy results were discussed in the previous section.

Figures 4a–d show SEM micrographs of the ZnO particles prepared at different ratios of reactant concentration.

Fig. 4
figure 4

SEM images of ZnO nanopowder synthesized under different ratios of precursor concentration a 1:1, b 1:2, c 1:3, d 1:4

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These images clearly show remarkably different morphologies for these ZnO particles. As can be seen from Fig. 4a, at a concentration of 1:1 the nanoparticles are spherical, whereas at a concentration of 1:2, they are cauliflower-like (Fig. 4b). By further increasing the concentration ratio, the clusters become irregularly shaped and non-uniform. As the concentration ratio was gradually increased, the average size of the nanoparticles increased from 18 to 36 nm. From the aforementioned results, it is therefore concluded that the sizes and morphologies of as-prepared ZnO powders depends markedly on the concentration ratio of the reactants. Furthermore, as the nanoparticles become smaller, the surface-to-volume ratio or nanoparticles aspect ratio increases and their chemical reactivity increases.

Transmission electron microscopy

The synthesized ZnO nanoparticles were further characterized using transmission electron microscopy (TEM) (CM10, Philips). Figure 5, which shows a typical TEM image obtained for the sample a, gives further insight into some noteworthy details of the formed particles.

Fig. 5
figure 5

TEM image of ZnO nanoparticles

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As can be seen from Fig. 5, the ZnO nanoparticles are approximately spherical in shape, confirming the findings obtained by SEM (see Fig. 4a). A histogram (Fig. 6) illustrates the distribution in nanoparticle size; with this figure indicating that a normal distribution of nanoparticles was obtained, centered at a diameter of 15–20 nm. Thus, our samples have a high surface area, making them appropriate for catalytic applications. The higher surface area is attributed to the formation of smaller particles. Photocatalytic activity of ZnO nanoparticles depends on the surface area. Due to the unique properties originating from their high surface area, ZnO nanoparticles and more generally, semiconductor nanoparticles are subjected to much research attentions. Also, synthesis method provides control over the surface area.

Fig. 6
figure 6

A histogram representing distribution of ZnO nanoparticles prepared at 1:1 concentration ratio

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Discrepancies in the estimation of nanoparticle size by the different techniques used in this work can be understood. We know that under optimal conditions the precision of the Debye–Scherrer in measuring crystallite size is in the order of ±10 % (Cullity 1978; Azaroff 1968). Therefore, the actual average size of the nanoparticles in this work, as determined by TEM, is 18 nm. However, the nanoparticle size estimated by XRD, which identifies the underlying lattice planes, is 20 nm. Therefore, this value for nanoparticle size is within an acceptable precision limit to the TEM result.

Conclusion

In this work, ZnO nanoparticles were synthesized using the direct precipitation method at room temperature. The advantages of this method are that a large quantity of ZnO nanoparticles can be synthesized with high purity and a low cost of production. The role of different concentration ratios of zinc acetate dihydrate to sodium hydroxide (1:1–1:4) was studied.

X-ray diffraction results show the formation of a hexagonal wurtzite zinc oxide structure with a high degree of crystallinity. By increasing the concentration ratio of the reactant raw materials from 1:1 to 1:4, the intensity of the reflection peaks increased and the average size of the as-prepared nanoparticles increased from 20 to 36 nm.

Scanning electron microscopy images show spherical particles, cauliflower-like particles, and irregular cluster morphologies. It can therefore be concluded that the size and morphologies of as-prepared ZnO powders markedly depends on the concentration ratio.

Transmission electron microscopy revealed the size distribution of the nanoparticles. The actual average size of nanoparticles obtained by TEM for a concentration ratio of 1:1 was 18 nm. The ZnO nanoparticles were approximately spherical, confirming the result obtain by SEM. The nanoparticle sizes estimated given XRD, SEM, and TEM are all in good agreement with each other.