Introduction

Nanomaterials, especially metal oxides such as TiO2, ZnO, SnO2, Fe2O3, PdO, WO3, etc., have received a considerable attention over the last few years due to their distinguished performance and potential applications in microelectronics, photocatalysis, magnetic devices, and powder metallurgy (Andres et al. 1996; Kamath 2002; Thomas 1988; Perenboom et al. 1981). Among these oxides, ZnO exhibits the most diverse and abundant configurations of nanostructures known to date, such as nanocombs, seamless nanorings, aligned nanopropellers, nanohelixes/nanosprings, nanobows, nanobelts (NB), nanowires (NW), and nanocages (Wang 2004). ZnO is a versatile semiconductor material and possesses several unique advantages such as high-specific surface area, nontoxicity, chemical stability, electrochemical activity, and high electron communication features. They have many applications in solar cells, luminescent, electrical and acoustic devices, chemical sensors, catalysis, electronics, gas sensor devices, optoelectronics, transducers, and biomedical science because of their biosafety (Wang et al. 2006; Vayssieres 2003). Along with the deepening knowledge of performance of nanocrystalline ZnO, its field of application also enlarges continuously. Aside from being a typical catalytic material, the nanocrystalline ZnO is also a kind of representative electrochemistry and photochemical semiconductor (Yan 2003; Hengxiang et al. 2004; Xitang et al. 1999; Caijun 2003).

The nanocrystalline ZnO has become a leading trade mark of cosmetics because of its ability to protect from ultraviolet ray and its ability to kill and restrain bacterium. Compared with the traditional bacterium-killing material, other characteristics of the nanocrystalline ZnO are stable performance, harmless for human body and steady appearance and color (Deng et al. 2001). In addition, the nanocrystalline ZnO used in ceramics manufacturing, microwave absorption, electronic parts manufacturing and has extensive application prospect (Lian and Wei 2002; Baolong et al. 1995; Yifeng et al. 2004; Liqiang et al. 2002; Qingfeng et al. 2004; Weizhong et al. 2005).

Large number of methods for the synthesis of ZnO are reported in the research literature, they are precipitation, microemulsion, nonmicroemulsion, ultrasonic radiation precipitation (Sihai and Xinmin 1999; Wenliang et al. 2001; Jiaqiang et al. 1998; Shiwen et al. 2002; Yongjun 2004; Xu et al. 2004; Wan et al. 2003), microwave irradiation (Faal Hamedani and Farzaneh 2006), precipitation/mechanical milling (Deng et al. 2001), solution combustion, microwave-assisted solvothermal (Rao et al. 2005) and sol–gel method (Vafaee and Ghamsari 2007; Ho Jung et al. 2004; Srinivasan and Kumar 2006). Different physical methods such as pulse laser deposition (Nakata et al. 2002; Yoo et al. 2002), vapor phase transparent process (Chen et al. 2004), chemical vapor deposition (Li et al. 2004), and vapor transparent deposition have been developed for the preparation of nano-ZnO. Recently, a new method of electrochemical route has been successfully used to synthesize CuO nanocrystals (Gao-Qing et al. 2007).

In recent years, an electrochemical method as aroused a considerable interest in the synthesis of nanosized metal oxide powder and films because of its simplicity, low temperature operation process, and viability of commercial production. Because of the multifunctional application of ZnO thin films, many methods which give ZnO films on substrate have been developed and their films are characterized for their distinguished properties. Many workers established that oriental crystallization of an electrodeposited ZnO is strongly dependent on electrolyte composition and contrast to substrate controlled orientation by epitaxy (Prasad et al. 2008).

Numerous applications have made the ZnO as wonder materials for material scientist and the quantity of ZnO used in different application is also increasing. Hence, its production is ever increasing and a suitable method for preparing ZnO possessing less operating cost, working at ambient temperature, less time with narrow size range and better properties is a challenge for scientists. The procedure used or developed in the existing methods for generating either ZnO films or its powder are complex with too many conditions which are to be satisfied in producing good amount of ZnO.

In the present study, we focus on the bulk synthesis of ZnO nanoparticles by a hybrid electrochemical–thermal route without using any templates or surfactants. The effects on electrolyte, current density and reaction media on the shape, size, and chemical composition of the generated product were investigated. It has been found that this is a simple and feasible electrochemical route for the synthesis of ZnO nanoparticles on commercial scale. The effect of electrolyte concentration and current density on the yield is investigated. The sample was dried and calcined at different temperature, and these samples were subjected to record IR absorption, UV–Vis absorption spectra, and its thermograms. Further the synthesized ZnO sample was characterized by recording its SEM, TEM images, and powder XRD patterns. Finally the optimum conditions for generation of ZnO nanoparticles were proposed.

Experimental details

Synthesis of ZnO nanoparticles

High purity zinc metal plate (99.99%) with dimension 5 × 6 × 0.8 cm3 and AR grade (NaHCO3) sodium bicarbonate (99.5%) purchased from Sisco research laboratories, Mumbai were used as such. In this procedure, three concentrations, 30, 60, and 120 mmol of NaHCO3 were prepared in distilled water. In all case, the pH of the electrolyte was 8.5 and in each experiment 400 mL of electrolyte was used in a rectangular undivided cell having the dimension 5 × 6 × 0.8 cm3. The both anode and cathode were zinc plates and placed inside the electrolyte. The zinc plate surfaces were activated by immersing in dilute HCl (1 M) for 30 s and followed by washed with distilled water. The electrolysis process was carried out under galvanostatic conditions with constant stirring at 800 rpm. During the electrolysis the constant current was drawn from a DC-regulated power supply, model PS 618 potentiostat/galvanostat 302/2A supplied by Chem link Mumbai. The schematic diagram of electrolysis process is shown in Fig. 1. An electrolysis process was carried out for an hour. The particles were filtered and isolated from the solution. The resulting particles were calcined at different temperatures from as low as 60 °C to a maximum of 600 °C for 1 h.

Fig. 1
figure 1

Schematic diagram of electrolysis process

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Characterization

Powder X-ray diffraction (XRD) patterns were recorded for all the samples using PANalytical X’pert Pro powder diffractometer with Cu Kα radiation, (λCu = 1.5418 Å) working at 30 mA and 40 kV. For Rietveld refinement analysis, data were collected at a scan rate of 1°/min with a 0.02° step size for 2θ from 10° to 90°. The data were refined using the FullProf Suite-2000 version. Morphology and compositional analysis were carried out in a scanning electron microscope (SEM, Philips XL 30) fitted with an energy dispersive X-ray analyzer (EDX). Transmission electron microscopy (TEM, Model: JEOL 2000 FX-II) study was carried out on the selected samples to confirm the size of ZnO particles in nanometer range. 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. 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. The average crystallite sizes were calculated by using the Debye–Scherrer equation (Scherrer 1918).

$$ D = {\frac{K\lambda }{{\beta \,{ \cos }\,\theta }}}, $$

where D is the diameter of the crystallite size, K is the shape factor (the typical value is 0.9), λ is the wavelength of incident beam, β is the broadening of the diffraction line measured in radians at half of its maximum intensity (FWHM), and θ is the Bragg’s angle.

Results and discussion

Influence of electrolyte

Examining the literature for the preparation of ZnO nanoparticles revealed that all the reported methods were involved two or more steps. For example, the preparation of ZnO by hydrothermal or solution combustion methods involved precipitation of metal as metal hydroxide precipitate and converting it into metal oxides by controlled rate of heating. The metal oxides nanoparticle obtained by these methods depends mainly on the process adopted in the preparation of metal hydroxide and to lesser extent on its rate of heating. Also in few methods, the ZnO is prepared by heating Zn in O2 atmosphere. Alternately ZnO films and coatings are made by RF sputtering (Stolt et al. 1993), CVD (Bae et al. 2004) and atomic layer epitaxy (Sang and Konagai 1996). These techniques are energy and capital intensive, however, and require high-vacuum generation besides high-power laser/ion beam sources.

The electrochemical preparation is a soft chemical technique which was used successively to generate ZnO film on substrate by many workers. The electrolyte used in these techniques involved the Zinc as ions or complexes in solution along with suitable surfactants. In the existing electrochemical methods for the synthesis of ZnO film, the electrolyte composition was associated with zinc compounds which serve as source for Zn ions. In these methods, the Zn2+ reduced at cathode (Zn2+ + 2e− → Zn). The simultaneous evolution of H2 at the vicinity of cathode raises the pH of the solution results in the formation of Zn(OH)2 which in turn decomposed to give ZnO through the reaction Zn(OH)2 → ZnO + H2O. Many metal oxide films of Cu2O, CuO, and ZnO are successfully obtained on cathode through electrochemical method as one-pot synthesis (Joseph and Kamath 2007) by adjusting chemical and electrochemical parameters such as pH, metal ion/ligand ratio, temperature, and current density. The working electrode used in generating ZnO films are either stainless steel or platinum (Helen Annal Therese and Kamath 2000). So far no report available in the literature using electrolyte without zinc salt in obtaining ZnO films strongly adhered on cathode. Moreover, ZnO films generated is of nanosize with few micrometer thickness. Hence, this limitation restricts the above method for the generation of ZnO powders on nanometer scale. Further the coverage of cathode by nonconducting ZnO film reduced the efficiency, thus longer time needed for preparing thick film. Also in all these methods, the auxiliary inert electrode used is platinum. However, there is a recent report, wherein Cu metal was used as a sacrificial anode for synthesis of CuO nanocrystals from NaNO3 electrolyte (Gao-Qing et al. 2007). The nanosized ZnO powder is in great demand, and bulk generation from the existing methods is not possible to achieve as they are limited to laboratory level.

The results of an electrochemical synthesis depend on the choice of a number of reaction parameters (e.g., electrode, electrolyte, temperature, composition of electrolyte solution, mode of electrolysis, and dimension or shape of the cell). Among them, the choice of electrolyte is of quite importance. Therefore, the influence of electrolyte like 30 mmol NaNO3, NaOH, and NaHCO3 on the electrochemical synthesis of ZnO was first investigated. The electrolysis, using zinc as anode and cathode was carried out in each of above electrolytes separately at 0.05, 0.1, 0.5, 1.0, and 1.5 A/dm2 for 1 h duration. It was observed that the electrolytes NaNO3 and NaOH produced negligible amount of precipitates where as in NaHCO3 electrolyte a large quantity of white precipitate was noticed. As the present work is aimed to large generation and hence the NaHCO3 was selected for the detailed investigation. The different samples of precipitates were generated under galvanostatic condition for each electrolyte (NaHCO3) concentrations of 30, 60, and 120 mmol of predetermined current densities such as 0.05, 0.1, 0.5, 1.0, and 1.5 A/dm2. The samples were separated by filtration through Whatman filter paper, washed three times with distilled water and dried at 333 K. The dried samples were preserved in a desiccator for further studies. The electrolysis parameter and process efficiency are given in Table 1.

Table 1 Electrolysis parameter and process efficiency for the preparation of ZnO nanoparticles
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XRD analysis

The powder XRD patterns for the as prepared and calcined compounds (300 °C for 1 h) obtained from the electrolysis of 30 mmol NaHCO3 at 0.5, 1, and 1.5 A/dm2 were shown in Fig. 2. It can be seen from Fig. 2a, the three main peaks appeared at 2θ of 13.1°, 33.1°, and 59.6° corresponds to characteristic peaks of zinc hydroxide and zinc carbonate, and is in accordance with standard JCPDS Card No. 36-1451, the similar observation was made by Zhaorigetu et al. (2006). Also the other small peaks appeared at 2θ of 21.9°, 24.1°, 28.1°, 31.0°, 36.0°, 54.2°, and 68.7° values corresponds to ZnO. The XRD patterns for the calcined samples at 300 °C for 1 h are shown in Fig. 2b–d, all the diffracted lines are matching with the JCPDS Card No. 36-1451. The typical indexed powder XRD pattern is given in Fig. 2d. Figure 2b–d reveals that the broadening of the diffracted lines decreases with increasing current density from 0.5, 1.0, and 1.5 A/dm2. This inferred that the crystallinity of ZnO nanoparticles increases. From the XRD data, the average crystallite sizes were found to be 18, 32, and 42 nm, respectively. Similarly, the electrolysis experiments were performed for the different electrolyte (NaHCO3) concentrations of 60 and 120 mmol, and varied the current densities from 0.05, 0.1, 0.5, 1.0, and 1.5 A/dm2. The similar observations were found in the powder XRD patterns.

Fig. 2
figure 2

XRD patterns of (a) ZnO precursor obtained from the bath concentration at 30 mM NaHCO3 at 0.5 A/dm2, and calcined at 300 °C for 1 h at different current densities of (b) 0.5, (c) 1.0, and (d) 1.5 A/dm2

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The preferred orientation of the ZnO nanoparticles was estimated from the X-ray data according to the methodology developed by Berube and Esperance (1989), where the texture coefficient (T c) is calculated by using the equation below:

$$ T_{\text{c}} = {\frac{{I_{{\left( {hkl} \right)}} }}{{\sum {I_{{\left( {hkl} \right)}} } }}} \times {\frac{{\sum {I_{{0\left( {hkl} \right)}} } }}{{I_{{0\left( {hkl} \right)}} }}}, $$

where I(hkl) is the diffraction line intensity of the (hkl) reflection of ZnO powder, ∑I(hkl) is the sum of the intensities of all the diffraction lines monitored. The Io refers to intensity of the reference ZnO sample. Figure 3 shows the texture coefficient of ZnO sample obtained from bath concentration of 30 mmol and at different current densities of 0.5, 1.0, and 1.5 A/dm2. It can be seen that the majority of the ZnO crystallites are oriented parallel to the (002) plane.

Fig. 3
figure 3

Texture coefficient of ZnO nanoparticles obtained from bath concentration of 30 mmol NaHCO3 at 0.5, 1.0, and 1.5 A/dm2 current densities and calcined at 300 °C for 1 h

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Further, the samples were generated from the electrolyte (NaHCO3) concentration of 30 mmol at 1 A/dm2 were calcined at different temperatures from 60 to 600 °C for 1 h. In Fig. 4, we shown the XRD patterns for the ZnO samples calcined at (a) 60, (b) 140, (c) 300, (d) 400, and (e) 600 °C. Figure 4a indicates that the diffracted lines are more broadened, less intensity and corresponds to the zinc hydroxide, zinc carbonate, and zinc oxide similar to Fig. 2a. On increasing calcination temperature of 140 °C, still there is some precursor of zinc hydroxide and zinc carbonate peaks appeared along with major ZnO peaks. Further on increasing calcination temperature from 300 to 600 °C, the characteristic ZnO peaks become sharper and the crystallinity increased (Zhaorigetu et al. 2006). The typical indexed powder XRD pattern for the sample calcined at 600 °C is given in Fig. 4e. Figure 4b–e reveals that the broadening of the diffracted lines decreases with increasing the calcination temperature from 140 to 600 °C. In the present investigation, the crystallite sizes vary from 22 to 75 nm. It was observed that the crystal size varied 22–40 nm from 60 to 300 °C, whereas from 300 to 400 °C the size range is 40–71 nm and after 400–600 °C the size varied from 71 to 75 nm. It is obvious that calcined temperature between 300 and 400 °C is more effective and enhanced the crystal size around 31 nm more, compare to the effect of other temperature ranges which showed smaller effect on crystallite size. It clearly indicates that an increase in calcination temperature brings about corresponding increase on crystallite sizes of ZnO nanoparticles, leading to sharper diffraction peaks.

Fig. 4
figure 4

XRD patterns of ZnO particles obtained from bath concentration of 30 mM NaHCO3 at 1 A/dm2, calcined for 1 h at (a) 60, (b) 140, (c) 250, (d) 400 and (e) 600 °C

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The structural parameters for all the calcined samples were refined using Rietveld refinement method. All the ZnO nanoparticles were crystallized in the hexagonal (Wurtzite) structure with space group of P63mc (No. 186). The refined structural parameters for the ZnO calcined at 600 °C for 1 h is given in Table 2. The observed lattice parameters agreed well with the reported in the standard JCPDS Card No. 36-1451. Observed, calculated, and the difference XRD patterns are given in Fig. 5; there is good agreement between the observed and calculated patterns. Further, we did not see any appreciable change in the lattice parameters on low-temperature calcined samples.

Table 2 Rietveld refined structural parameters for the ZnO calcined at 600 °C for 1 h
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Fig. 5
figure 5

Observed, calculated, and the difference X-ray diffraction patterns for the ZnO sample obtained from 30 mM NaHCO3, 1 A/dm2, calcined at 600 °C for 1 h

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Thermal analysis

The thermal behavior of precursor was studied with TG-DTA measurements. The precursor was heated in nitrogen atmosphere in alumina crucible at the temperature scanning rate of 5°/min. It can be seen from Fig. 6 that the precursor loses its weight in two steps. The first step is in the range of 125–148 °C, revealing the dehydration process of surface-adsorbed water molecules. The second step of weight loss appeared in 150–271 °C and indicated major weight loss of 2.14 mg (14.03%), suggesting that the precursor associated with decomposable zinc compounds like Zn(OH)2 and small amount of ZnCO3. The decomposition reaction may be represented as

$$ {\text{ZnO}} \cdot {\text{H}}_{2} {\text{O}} \to {\text{ZnO}} + {\text{H}}_{2} {\text{O}} $$
$$ {\text{Zn}}\left( {\text{OH}} \right)_{{ 2 }} \to {\text{ZnO}} + {\text{H}}_{ 2} {\text{O}} $$
$$ {\text{ZnCO}}_{3} \to {\text{ZnO}} + {\text{CO}}_{2} $$
Fig. 6
figure 6

TG-DTA analysis of as prepared ZnO particles obtained from 30 mM NaHCO3, 1 A/dm2

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Since the total weight loss in the entire thermal analysis study was 17.49% indicating the major amount of ZnO formation during electrolysis.

The DTA curves exhibited in two endothermic peaks correspond to two weight loss steps of TGA curve. The former weight loss at 138 °C corresponds to the decomposition stages possibly consists of the desorption of the hydroxide ions and the dehydration of surface adsorbed water trapped in the ZnO nanoparticles [Zn(OH)2 or ZnO · H2O] (Yuan-Qing et al. 2006). The latter weight loss at temperatures between 148 and 271 °C is a major weight loss, indicating the release of OH− and CO3 2− ions. The percent weight loss of the precursor is 17.49%, which can be proved that the molecular formula of the precursor is Zn(OH)2 or ZnO · H2O, because the experimental weight loss is quite close to the theoretical weight loss (18.48%). A plateau of the TGA curve at temperatures higher than 250 °C is the final step of the transformation into ZnO (Kakiuchi et al. 2006). The TGA and DTA curves have obviously endothermic peak around 255 °C, which illustrate that the decomposition of OH− and CO3 2− happen simultaneously (Zhaorigetu et al. 2006). Also, it is clearly shown from the TGA curve that the precursor completely converted to ZnO after calcination at 300 °C. Therefore, crystallized ZnO begins to nucleate at about 300 °C and ZnO crystal growth is completed at about 600 °C. The exothermic peak at 562 °C is due to direct crystallization of nanocrystalline ZnO from the amorphous component. It suggests that the complete crystallinity may be achieved after 700 °C, the similar observations was recorded by Liewhiran and Phanichphant (2006).

SEM/EDX and TEM analysis

Scanning electron micrographs of ZnO nanoparticles obtained from the bath concentration of (a) 60 mmol and (b) 120 mmol with 1 A/dm2 calcined at 300 °C for 1 h are shown in Fig. 7. Figure 7a reveals that when we used low concentration of the electrolyte in the bath, we observed the particles were well defined and randomly oriented spindle-like ZnO nanoparticles. When the higher concentration of the electrolyte (120 mmol) the particles were agglomerated, it can be seen from the Fig. 7b.

Fig. 7
figure 7

Scanning electron micrographs of ZnO obtained from a 60 mM and b 120 mM NaHCO3 at 1 A/dm2

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Figure 8 shows the SEM images of ZnO nanoparticles obtained at different current densities of (a) 0.05, (b) 0.1, (c) 0.5, and (d) 1.0 A/dm2 in a fixed electrolyte concentration of 30 mmol. The shapes of the ZnO nanoparticles obtained in all current densities are granular and all are identical and well dispersed.

Fig. 8
figure 8

Scanning electron micrographs of ZnO nanoparticles obtained from 30 mM NaHCO3 at a 0.05, b 0.1, c 0.5 and d 1.0 A/dm2 current density

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The stoichiometry of the samples was examined by the EDX spectrum as shown in Fig. 9. Only zinc and oxygen signals have been detected, suggesting that the nanoparticles are indeed made up of Zn and O. Fig. 9 inset shows the ratio of zinc and oxygen ion concentration. We did not see any other foreign ions in the final compound. This result indicates that calcined ZnO nanoparticles contain 100% of ZnO.

Fig. 9
figure 9

EDX analysis of ZnO nanoparticles obtained from 30 mM NaHCO3 at 1 A/dm2

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The TEM images of ZnO nanoparticles obtained from the different electrolyte concentration of (a) 30 and (b) 60 mmol at 1 A/dm2 calcined at 300 °C for 1 h are given in Fig. 10. After the heat treatment at 300 °C for 1 h, the ZnO particles were found in the range of 30–40 nm. It can be observed that ZnO nanoparticles mainly present granules with hexagonal shape and are well crystallized. The circles in Fig. 10a and b emphasize the particles having a clear hexagonal shape, and it was found that the average particle sizes ranging from 30 to 40 nm and these values are in good agreement with the values obtained from XRD data by Debye–Scherrer equation.

Fig. 10
figure 10

TEM images of ZnO nanoparticle obtained from a 30 mM and b 60 mM NaHCO3 at 1 A/dm2 current density. Circles emphasize the hexagonal feature of the nanoparticles

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FT-IR spectroscopy

In Fig. 11, we shown the FT-IR spectra of the (a) ZnO precursor obtained from the 30 mmol NaHCO3, 1 A/dm2 and calcined at 300 °C for different time (b) 1 h, (c) 2 h, (d) 3 h, and (e) 4 h. Figure 11a indicates that the absorption peak in the range of 3200–3600 cm−1 was observed. This was centered at 3340 cm−1 corresponds to the stretching vibration of intermolecular hydrogen bond (O–H) existing between the adsorbed water molecules and indicates the higher amount of hydroxyl group. The results were well matched with the observation done by Xianxi et al. (2002). The peaks at 1379 and 1511 cm−1 are attached to ZnCO3 and bending vibrations of Zn(OH)2, and the absorption band in 700–1100 cm−1 is due to the lattice vibration of CO3 2− (Yongning et al. 1996). On increasing calcination time, the adsorbed hydroxyl carbonate groups diminishes and the characteristic peak of ZnO at 420 and 486 cm−1 increases. In Fig. 11b–e, the characteristic peak around 420 cm−1 becomes stronger, indicating the formation of stretching mode of ZnO (Zhaorigetu et al. 2006). The metal–oxygen frequencies observed for the respective metal oxides are in accordance with literature values (Rao 1963). It quite agrees with the measurement results of TG-DTA that the precursor was partly decomposed at 150 °C.

Fig. 11
figure 11

FT-IR spectra of (a) as prepared ZnO precursor obtained at 30 mM NaHCO3 1 A/dm2 and calcined at 300 °C for different time (b) 1 h, (c) 2 h, (d) 3 h, and (e) 4 h

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UV–Vis spectroscopy

The UV–Vis spectra of ZnO nanoparticles obtained from 30 mmol NaHCO3 at 1 A/dm2 calcined at different temperature (a) 140, (b) 250, (c) 300, (d) 400, and (e) 600 °C were shown in Fig. 12. In Fig. 12a, we did not see any absorption band from 300 to 750 nm, due to the incomplete formation of ZnO phase. On increasing calcination temperature, the ZnO particles growth will takes place, the corresponding absorption band (λmax) increases from 367 to 388 nm. The band gap (E g) of ZnO nanoparticles were calculated by using E g = hc/λ, where h is plank’s constant, c is velocity of light, and λ is wavelength. The corresponding band gap was found to be 5.40–5.11 eV. Further, the XRD and TEM results suggest that the ZnO particles are in the range of 22–75 nm. The present electrochemical–thermal method for generating ZnO nanoparticles is in the blue region compared to the bulk ZnO (3.37 eV).

Fig. 12
figure 12

UV–Vis spectra of ZnO nanoparticles obtained from 30 mM NaHCO3 at 1 A/dm2, calcined at different temperature (a) 140, (b) 250, (c) 300, (d) 400, and (e) 600 °C

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Conclusions

In the present work, the nanosized ZnO particles were successfully generated by electrochemical–thermal method using NaHCO3 electrolyte without zinc salts, templates, or surfactants. The Zn2+ ions generated at the sacrificial Zn electrode and were converted into ZnO during electrolysis. The calcination temperature at 600 °C resulting larger size of 76 nm. The Rietveld refinement method showed similar lattice parameters for all ZnO samples calcined at different temperature. The size range of the generated ZnO powder was 22–75 nm. TGA revealed that the sample contains OH−, CO3 2−, H2O up to 17.5% and remaining 82.5% ZnO. The nanoparticles morphology was similar for all the samples obtained at different current densities and the size and shape do not depend on current density. The EDX spectrum showed that 100% ZnO compound in the calcined particles.TEM images confirmed the hexagonal shape of ZnO nanoparticles and are well crystallized in the nanosize of 30–40 on nanometer scale. The FT-IR spectrum shows the existence of OH−, CO3 2− groups in uncalcined sample. The band gap was higher for synthesized ZnO particles than their bulk counterparts. The yield of ZnO at 1 A/dm2 is maximum for 1 h electrolysis in all concentration of NaHCO3. The process operated at room temperature and their insolubility in electrolyte made the separation easier. The same electrolyte could be reused for generation of ZnO nanoparticles. The method could be effectively used to synthesize ZnO on large scale.