The Influence of Calcination Temperature on Structural and Optical Properties of ZnO Nanoparticles via Simple Polymer Synthesis Route

A simple polymer synthesis was used to successfully synthesized Zinc Oxide Nanoparticles (ZnO NPs), and the influence of the different calcination temperature on the structural, and optical properties of the material was observed using several techniques. The formation of ZnO NPs was confirmed by FT−IR, EDX, XRD, FESEM and TEM images upon calcination from 500−750 °C. The FESEM images showed the ZNO NPs synthesized possessed a hexagonal shape and tended to become larger at higher calcination temperature. The XRD and FTIR revealed the precursor to be amorphous at room temperature but transform to a crystalline structure during the process of calcination. The crystalline and particle size increase as the temperature was increased. The crystalline size was between 24−49 nm for all samples calcined at 500−750 °C. The optical properties obtained by UV−vis reflectance spectrometer have further confirmed the formation of ZnO NPs. The band gap exhibits typical ZnO wide band gap, and the values decrease with an increase in calcination temperature.


Introduction
Properties of semiconductor material such Zinc Oxide (ZnO) have attracted significant attention recently from researchers and Scientist in various fields of human endeavor.ZnO exhibits a hexagonal wurtzite structure with a direct band gap of 3.37 eV.While in the visible region, it produces an optical transparency with a vast exciton binding energy of 60 meV.Nevertheless, ZnO is an n-type II-VI semiconductor and oxygen vacancies, zinc interstitials or complex defects are identified as principal donor centers [1,2].Due to the fundamental properties ZnO possessed, it becomes an attractive material for various application such include extensive panel application in optoelectronics like photodetector [3], light emitting diodes (LED), and laser devices [4].Others include biosensor [5], solar cells [6], antibacterial action [7], electroluminescent device and photocatalytic material [8].Moreover, due to the optical, chemical stability and low toxicity, the usage as a fluorescent label for bioimaging medical application has been expected [9].
Several procedures and methods have previously been reported on the synthesis of ZnO NPs with the aim of improving the chemical and physical behaviours.Such include microwave assisted method [10], thermal evaporation method [11], thermal decomposition method [12], spray pyrolysis [13], chemical vapour deposition [14], hydrothermal growth [15], sol gel technique [16], precipitation method.Insipid that, most of these methods have difficulties in applying for large scale production due to the complicated procedure, high temperature involved, a longer period of reaction, toxic chemical reagent and by product release to the immediate environment at the end of the experiments.
The present study synthesized ZnO NPs via a simple polymer synthesis route from an aqueous solution containing zinc acetate dihydrate, Poly (vinyl pyrrolidone) PVP and deionize water [18].The influence of calcination on the structural and optical properties of ZnO NPs have been studied and discussed in details.

Experimental 2.1. Materials
A soluble polymer PVP Poly (vinyl pyrrolidone) 2900 molecular weight was used as a stabilizer and capping agent which reduce agglomeration and stabilize the nanoparticle.Zinc acetate dihydrate reagent, [Zn(CH 3 COO)2•2H 2 O] (M w = 219.49g/mol) over 99 % purity which served as metallic precursor were purchased from sigma Aldrich and was used without further purification.Deionized water was used as a solvent.

Procedure
A solution of PVP was made by making 30 g of PVP to dissolve in 1000 ml of deionized water and continuously stirred using magnetic stirrer for 2 h until no precipitate formed.Later 0.45 g of Zn(CH 3 COO) 2 •2H 2 O was added and stirred continuously until a clear solution is obtained.The solution was poured into a clean glass petri dish and the water was allowed to be evaporated in an oven for 24 h at 80 °C.The resulting solid gel obtained after drying was ground in a sterilized mortar into powder form.The powder was placed into crucible boat and calcined in an electric box furnace between 500-750 °C for 3 h to decompose PVP and form the ZnO NPs.

Characterization
The synthesized ZnO NPs were characterized by several techniques such as FT-IR, EDX, XRD, TEM, FESEM and UV-vis.TGA analysis characterized the precursor before calcination.The bond formation and functional group of ZnO NPS have been studied by infrared spectra (FT-IR Perkin Elmer model 1650).The structural behaviour of the crystalline ZnO NPs was examined by X-ray diffraction (XRD Shimadzu model 6000 using Cukα (0.154 nm) as a radiation source to generate diffraction patterns from the sample at an ambient temperature in 2θ within the range of 20-80°.The morphological features were examined using Field electron scanning microscopy (FESEM) (JEOL JSM−7600F) equipped with EDX.The particle size and distribution were examined by Transmission electron microscopy (TEM) (JEOL TEM model 2010F UHR) with an accelerating voltage of 200 kN.The optical properties were determined by UV−vis spectrometer (Shimadzu model UV−3600) at room temperature in the range of 200−800 nm.

Thermogravimetric Analysis (TGA-DTG)
The appropriate temperature to decompose PVP and other unwanted anions, as well as to obtain ZnO NPs was determined by thermogravimetric analysis measurement and its derivative (TGA−DTG).Fig. 1 indicates the weight-loss percentage as a function of the temperature of the dried sample before the calcination.The sample shows two distinct decomposition stages.The first was weight loss at 85 °C, which was attributed to the moisture already contained in the sample.The second phase of weight loss was observed at a temperature of 435 °C which indicates that most PVP has been decomposed and removed.There was no significant weight loss the moment the temperature reached 485 °C; this was due to complete decomposition of PVP thereby turning into a carbonaceous product such that only Pure ZnO NPs remains as final residue [20,21].

Phase and Elemental Composition Analysis
FT-IR Spectroscopy aid in analysing multi-component and gives the related information of the material phase composition and the nature of interaction existing in various kinds of polymers.

Fig. 2. FT-IR spectra of PVP and ZnO NPs calcined at various temperatures.
The organic and inorganic properties and behaviour of the sample before and after calcination within the range of 280−4000 cm −1 have been shown by the FT−IR spectra in Fig. 2.
The sample in the amorphous state (Fig. 2) before calcination exhibits C−N stretching vibration at 1278 cm −1 .At 1428 cm −1 , C−H bending vibrations originated from methylene group were observed.The vibration band found at 850 and 639 cm −1 was assigned to a vibrational band that have arisen due to C−C ring and C−N=O.The band at 1648 cm −1 was due to C=C stretching, and the two different bands at 2945 cm −1 and 3414 cm −1 observed are attributed to C−H stretching and N−H stretching vibration respectively [21][22][23].The sample was calcined at 485 °C for 3 h in order to verify whether PVP has been decomposed and entirely removed as indicated by TGA result, it was reveal from Fig. 3, the organic material from PVP remained at 485 °C thus there was need to increase the temperature to enable complete removal of PVP in the material.While the sample was calcined at 500 and 750 °C for 3 h, the broad absorption bands due to the organic material (PVP) has been totally decomposed and disappeared, only vibration spectra of pure ZnO NPs was observed.In Fig. 3, the spectra were 393, 387, 381, 377, 376 and 373 cm −1 band of pure ZnO NPs were observed which calcined at a different temperature of 500, 550, 600, 650,700 and 750 °C respectively [24].The slight shift in wave number observed was due to improvement in crystallinity with an increase in temperature which indicates that pure ZnO NPs has been achieved in polymer synthesis method [19].
The energy-dispersive X-ray spectroscopy (EDX) is an analytical technique often employ for the elemental analysis or chemical composition of a sample.It's the fundamental principle is such that each distinct element possesses a unique atomic structure allowing a unique set of peaks on its X-ray spectrum.The purpose of carrying out the EDX analysis in this study was to confirm the elemental composition of the constituent atoms and detect if there are any foreign impurity atoms.Fig. 3 shows the EDX spectrum of ZnO calcined at 500 °C.The corresponding peaks of Zn and O were observed in the sample which confirms the formation of ZnO and further confirmed the results obtained from XRD analysis.The peak of carbon (C) found in the spectrum was originated from the preparation process of the sample for the analysis as carbon tape material was used to hold the sample.In conclusion, the EDX analysis confirms the formation of ZnO and also proves that the polymer synthesis technique is useful, as there was no loss of element was observed in the process [19].

The Structural Analysis
The crystalline phase and structure of ZnO NPs were analyzed and identified by the use of XRD.The precursor that contain zinc acetate and PVP after being dried at 80 °C exhibits no diffraction peaks indicating that the sample was still amorphous at room temperature before calcination as shown in Fig. 4. The sample shows sharper and narrower peaks as the precursor was calcined at 500 °C for 3 h which reveals the formation of ZnO NPs.However, as the temperature was increased to 550, 600, 650, 700 and 750 °C respectively, the crystallinity of the ZnO NPs was further enhanced as shown in Fig. 4.This incidence was attributed to particles size enlargement as results of continued application of heat [25].Where D is the crystalline size measured in (nm), β is the full width of the diffraction at half of the maximum intensity measured in radians, θ is the Bragg's angle, and λ is X−ray wavelength of Cukα (0.154 nm) [27].Fig 5 shows the graphical representation of the influence of the calcination temperature on the crystalline and particle size respectively obtained from XRD and TEM images.The increase in the size of the material was as a result of the improvement in crystallinity, and the particle tends to fuse together to form a larger particle as the temperature is increased [25].

Morphology and Particle Size Distribution
The transmission electron microscopy (TEM) analysis was employed to study the influence of the calcination temperature ranging from 500−750 °C on the particle shape, size and particles distribution of the prepared ZnO NPs as shown in Fig. 6.The images were obtained using TEM (Model JOEL 2010F UHR version electron microscope) operating at an accelerating voltage of 200 kV.The distribution of the particles and the average size of the nanoparticles were analyzed with Image J software shown in Fig. 7.In this study, the particles ZnO range from 24 nm to 49 nm as the temperature varies from 500 to 750 °C is shown in Fig. 6 and Fig. 7.The TEM images seem to be hexagonal in shape at all calcination temperatures, and the smallest particle size was obtained at the temperature of 500 °C, and the particles size noticeably increases with the rise in calcination temperature.A lot of neighboring particles fused together to increase the particle size by the melting of their surfaces at a higher temperature that as the temperature increases [19,21].
The surface morphology of the ZnO nanoparticle was studied with FESEM.The observations were performed with the use of an FESEM JSM 7600F machine equipped with energy dispersive X−ray spectrometer (EDX).The particle as shown in Fig. 8 tends to exhibits a hexagonal shape with uniform distribution, as the calcination temperature was increased from 500 °C to higher values up to 750 °C.The ZnO NPs tends to fuse together and form a larger particle [25]; this is in good agreement with the TEM results obtained.

Optical Properties
The effect of calcination on the optical properties ZnO NPs was studied by UVvisible spectrophotometer.Fig. 9 shows the diffuse reflectance spectra.The diffuse reflectance technique has the tendency to collect the light flux after the interaction with the targeted sample.Then it compares the reflected fluxes that are coming from the targeted sample and the standard reflectance sample.The influence of calcination temperature on the diffused reflectance spectra measured between the ranges of 200-800 nm at normal room temperature for all the calcined samples was investigated and was deduce from the results in Fig. 9 that the observed spectra tend to shift gently to lower wavelength as the calcination temperature in increase; this is attributed to the improvement in crystal quality of the material as the calcination temperature got higher [29].

Band gap energy of ZnO NPs
The data information obtained from diffuse reflectance in Fig. 9 was used to evaluate the absorption coefficient from the Kubelka-Munk (KM) function defined as: Where "α" is given as the absorption coefficient, "S" is denoted as the scattering coefficient and F(R∞) is known as the Kubelka-Munk function.For the diffused reflectance spectra, the Kubelka-Munk function could be used instead of absorption coefficient "α" for the evaluation of the optical absorption edge energy.The plot of (F(R ∞ )E) 2 against E(eV) was linear near the edge for direct allowed transition (η = 1/2).Recall that E is the photon energy (hv).The intercept of the line on the abscissa (F(R ∞ )hv) 2 = zero) gives the optical band gap energy of the samples [29].
In this work, the band gaps energy were obtained to be 3.325, 3.319, 3.286, 3.275, 2.266 and 3.245 eV were obtained for the samples calcined at 500, 550, 600, 650, 700 and 750 °C as shown in Fig. 10 and Fig. 11 respectively.The values obtained correspond to the optical band gap values reported by other researchers [24,28].However, it happens that the values of the band gap are slightly decreasing from 3.325 eV 500 °C to 3.245 eV after calcination at 750 °C.This phenomenon is attributed to the improvement of crystalline of the sample due to continued application of heat as evidenced by XRD and TEM results.It was postulated that when the particles size increases, the atoms that form the particles will subsequently increase also, which as a result render the valence and the conduction electrons to be more to the ions core of the particles and thus decreasing the band gap energy of the particles [20].

Fig. 4 .
Fig. 4. XRD patterns of PVP at 30 °C before calcination and ZnO NPs calcined at various temperatures.

Fig. 5 .
Fig. 5.A graph showing the influence of calcination temperature on the ZnO NPs growth.

Fig. 10 .
Fig. 10.Band gap energy of ZnO nanoparticles a calcined at various temperatures.

Fig. 11 .
Fig. 11.A graph showing the effect of calcination temperature on the band gap energy.