The Structural and Optical Properties of Zinc-Oxide Nanomaterials Synthesized via Thermal Decomposition Method

The current study describes a novel, solvent-free, template solid-state synthesis of Zinc Oxide nanoparticles (NPs). The prepared sample was prepared by thermal decomposition route. The prepared sample was characterized by X-ray diffraction (XRD), Rietveld refinement, Field emissionscanningelectronmicroscopy (FESEM), Energy dispersive X-ray spectroscopy (EDX) and Ultra violet visible spectroscopy (UV-Vis). Singlephase crystalline structure of prepared NPs was confirmed using X-ray diffraction and the mean crystallite size of ZnO NPs was about 14nm. Rietveld refinement study,which yielded a good peak fitting between measured and calculated patterns. Chemical composition of the prepared sample was determined using EDX. The size and morphology were evaluated using FESEM which has almost spherical shapein the range of 6-35 nanometres. The optical properties of the as-prepared sample and their bandgap were determined using UV-Vis and it was calculated as 3.19 eV. These findings demonstrated that ethylene glycol (EG) inhibits nanoparticles for coagulate and results in the formation of homogeneous NPs with an optical bandgap of 3.19eV. This study represents an efficient thermal degradation synthesis process for the ZnO NPs with a broad range of potential applications.


Introduction
Nanotechnology is now widely recognized as a cutting edge technology with many applications in the chemical, medical, mechanical, and food processing industries.Generation of power, optics and environmental science are all applications of nanotechnology.In continuation, photocatalytic materials have received a lot of attention in recent years in waste water treatment and pollutant removal from water [1,2].Oxides of metals like ZnO, TiO 2 , Fe 2 O 3 , and Cu 2 O have drawn significant attention from the researchers due to their less cost, non-toxicity, non-complicated processing steps, and beneficial application in a variety of fields like opto-electronic and bio-logical labelling and catalyst [3][4][5].In particular, ZnO has received lots of attention due to its potential use in low cost dye sensitized solar cells (DSSC) [6].
Ntype semiconductor, like ZnO are the most prevalent photocatalytic materials.The numbers of electron created in the conductionband is more as compared to the number of holes created in the valence band (VB) because a free electron exists inside the ZnO semiconductor [7].However, the large bandgap of around 3.3eV and the high electron-hole recombination rate of ZnO are undesirable properties [8][9][10].Due to high surface reactivity of ZnO, many defects present due to nonstoichiometric oxygen form, creating zinc oxide more efficient photo catalyst than other metal oxides [11][12][13].As a result, ZnO nanoparticles will work as an efficient catalyst during the electron transfer phenomenon [14].
For the production ofZnO NPs, Sol-gel synthesis [15], Hydrothermal synthesis [16], Polyol refluxing synthesis [17], and Thermal decomposition synthesis [18] are some of the synthesis techniques used.The starting materialsis assorted with surfactant in the existance of solvents such as Ethanol and then after magnetically agitate for a time period of few hours in the sol-gel process.Zinc Oxide nanoparticles were prepared by drying and calcinating the resultant solution.The hydrothermal method uses a similar concept, but it also includes centrifugation of the solution.Porous ZnO nanoparticles have also been made using the polyol method.However, specialized equipment, complex techniques, and a strictly regulated synthesis environment are required for these methods.But, in the thermal decomposition method zinc oxide nanoparticles were produced by heating the grinded initial precursor at higher temperatures.
As a result, thermal decomposition method provides many advantages, including easy process of operation, high rate, lowercost, and have higher efficiency.In present investigation, zinc acetate dehydrate, a thermally degradable precursor was mixed with ethylene glycoland the solution was maintained at pH 9. The solution was calcinated at 500 °C for 3 hours.The annealed powder was then grinded to get a uniform particle size.

Materials
Analytical quality chemicals were used in the synthesis of ZnO nanoparticles.Zinc acetate dehydrate Zn(O 2 H 3 C 2 ) 2 (H 2 O) 2 was used as a starting agent, and ethylene glycol (EG) HOCH 2 CH 2 HO was introduced as a capping agent and the solution of ammonium hydroxide (NH 4 OH) was used to maintain the pH of the prepared mixture.All the chemicals were supplied from Merck Specialties Pvt. Ltd., Mumbai.

ZnO Nanoparticles Synthesis
As shown in the Figure 1, firstly a solution of 5gm Zinc Acetate Dehydrate and 15 ml EG were prepared.This mixture was heated in presence of air, in anoven at 80°C for 5 minutes and then some amount of ammonium hydroxide introduced in this solution for maintaining the pH value of the solution at around 9. After that, the mixture was moved to a crucible, which was then placed in muffle furnace for annealing process.Prepared solution was calcinated at 500°C for 3 hours.Finally, dried samples were obtained and crushed into a consistent powder.

Characterization
Proto AXRD, the X-Ray Diffractometer (XRD) with Cu radiation (wavelength = 1.54 nm) was used to analyze the structural properties of the prepared samples.The measurements were taken in the 2 angle range of 28° to 72°.Fullprof software was employed to refine the XRD data based on rietveld method.Energy Dispersive X-ray-Spectroscopy was used to detect the chemical composition (EDX).The optical bandgap of prepared ZnO sample was evaluated through a UV-Vis spectrophotometer (BioEra make) in the range 200-800 nm, and the surface morphology of the powder sample was investigated using field emission scanning electron microscope (FESEM-Supra55 Zeiss).The Zeta potential was investigated (with Nanoplus-3) the surface charges and the stability of the prepared NPs.

X-Ray Diffraction Analysis
Anisotropy and crystallized surface morphology were observed in the ZnO nanoparticles prepared by thermal decomposition.Figure 2, depicts the X-ray diffraction pattern of ZnO nanoparticles, which represents the angles 2θ values ranging from 28° to 72°.Peaks at 31.98°, 34.63°, 36.5°,47.69°, 56.87°, 63°, 66.59°, 36.18°and69.41° were assigned to crystallographic plane i.e., (100), (002), ( 101), (102), (110), ( 103), ( 200), (112), (201) respectively.These results show that the sample wellcrystallized in hexagonal wurtzite structure with narrow peaks and a high-intensity diffraction pattern.The XRD pattern has no additional peaks indicates that the sample is free of impurities and well crystallized.The crystallite size (D) of prepared ZnO nanoparticles were estimated using following Scherrer's formula: D= where the λ is wavelength of X-rays which was used; β is the broadening of the diffraction-peak measured at half of its maximum intensity; and θ is the Bragg diffraction angle.The mean crystallite size of ZnO NPs was about 14nm.

Rietveld Refinement fitting analysis
By using the Fullprof program, the Rietveld refinement technique was used to refine the XRD data, and the typical fitting output is shown in Figure 3.

Parameters Results
Crystal System Space group Atomic Density(gm/cm 3 )

Energy Dispersive X-ray Spectroscopy (EDXS)
Elemental composition and purity of surface of the prepared ZnO NPs can be confirmed via EDX analysis.Figure 4 demonstrates the availability of zinc and oxygen with high purity in the sample.As given in Table 2, the observed atomic percentages were 58.4% of zinc and 41.6% of oxygen.These energy peaks appear as per the concentration of the elements except the carbon and there is no more impurity witnessed in the limits of experimental data.
Table 2: Compositional analysis of as prepared ZnO NPs.

FESEM Analysis
Field Emission Scanning Electron Microscope images were used to evaluate the surface morphology of prepared ZnO NPs.Figure5(a).FESEM micrographs of ZnO powder 50,000 magnification.

Ultra Violet-Visible Spectroscopy Analysis
Room temperature absorption UV-Visible spectroscopy was used to evaluate the optical characteristics of the prepared Zinc oxide nanoparticles.Figure .6 represents absorption Vs wavelength plot of prepared ZnO nanoparticles in the spectral region of 300-800 nm.It is instructive to mention that the photocatalysts required radiation to be activated and the energy of that radiation is determined by the semiconductor band gap energy (E g ). Figure .6(a) illustrates that ZnO NPs exhibited high-intensity absorbance bands with an absorption edge at 346 nm, which is typical of the wurtzite phase of ZnO and have lesser value than recognized UV-Absorption line of bulk ZnO at around 388nm [20].These absorption bands could be attributed to intrinsic Band-Gap Absorption of zinc oxide caused by electron transit in the conduction bands from the valence bands.As observed, the absorption line has moved towards a shorter wavelength side, such decrement in wavelength is due to the strengthening of charge carrier quantum confinement as the size of the ZnO particles decreases [21,22].It is clear that the prepared sample has an absorption peak in the UV region, implying that the photocatalytic activity may be good under UV light irradiation.ZnO absorption peak was about 346 nm, which indicated no observable absorption in the visible region.The prepared ZnO NPs Optical-Band-Gap was calculated using the Tauc-Plot [23].Figure .6(b) represents the Tauc plot obtained for the prepared zinc oxide NPs [(αhυ) 2 vs. hυ] [24], where α is the absorbance coefficient and hυis energy of photon.The direct band-gap (E g ) of Zinc oxide NPs was calculated as 3.19 eV.Furthermore, the band-gap value has a considerable influence on photocatalytic performance.The absorption efficiency of ZnO NPs enhances in the UV region and may enlarge to the visible-light region when the energy Band-Gap (E g ) of the Zinc oxide NPs decreases.As per the result, the photocatalytic activity of Zinc oxide NPs in certain spectral areas will be enhanced [25,26].

Conclusion
In the present study, ZnO NPs were synthesized from a simple and less-toxic Thermal Decomposition method.The XRD analysis of the sample revealed that the prepared ZnO NPs have a single-phase crystalline structure and the mean crystallite size of the prepared NPs was 14 nm.According to the Rietveld refinement, the sample exhibits a hexagonal wurtzite structure, which corresponds to the space group P63mc.This refinement yielded geometric parameters that are quite near to the ICDD value.The FESEM results revealed spherical shape nanoparticles and have particles size ranges from 6-35 nm.EDX revealed a sharp zinc and oxygen peak intensity and have an atomic percentage of zinc and oxygen at 58.4 % and 41.6 % respectively.In UV-Vis.spectroscopy, the absorption of Zinc oxide nanoparticles was found to be around at 346 nm and has shown an optical band-gap of 3.19 eV.From the analysis of zeta potential, It was observed that the average surface charge of the prepared Nano-Particles was -15.13 mV with moderate stability.The findings of present investigation support for the development of inexpensive anodes used in the preparation of dye sensitized solar cells.

Figure 1 .
Figure 1.Scheme for making ZnO nanoparticles from zinc precursor.

Figure 3 .
Figure 3. Rietveld refinement analysis of X-ray diffraction patterns of ZnO nanoparticles.

Figure. 5
(a) and Figure.5(b),shows images of ZnO nanoparticles at different magnification.In these images, It is sharply Visible that the nanoparticles have spherical shapes and varying sizes in the nanometer scale.The ImageJ software was employed to measure the particle size.The mean Size of ZnO nanoparticles was found to be in the range of 6-35 nanometers.

3. 6 .
Zeta Potential Analysis (ζpotential)To carry out the information about the stability of colloidal solution and surface charge of the prepared sample was through the zeta potential characterization.The ζpotential investigated for the average surface charge of zinc oxide nanoparticles was about -15.13 mV as shown in the figure.7.The smaller size particles synthesized under the influence of surfactant maintained at a constant pH 9, leads to negative ζ-potential.As per the result, smaller ζpotential leads to repulsion between the particles and there were no aggregation found in the particles[27].Zeta potential of the particles was less negative which indicates the moderate stability of prepared ZnO nanoparticles.

Table 1 :
Various structural parameters obtained from Rietveld refinement for ZnO nanoparticles