A Simple Up-Scalable Thermal Treatment Method for Synthesis of ZnO Nanoparticles

A simple thermal treatment method, utilizing only zinc nitrate, polyvinyl pyrrolidone (PVP), and deionized water, was used to synthesize ZnO nanoparticles, and their characteristics were investigated by various techniques. The TGA measurement demonstrated that the bulk of the capping agent PVP can be removed at temperatures higher than 500 °C and is consistent with the absence of the majority of PVP absorption peaks in the FT-IR spectra. The formation of almost pure ZnO nanoparticles was established by the presence of single absorption peak in the FT-IR spectra due to being only Zn–O bonds at calcination temperatures of 500 °C and above. The TEM images revealed that the nanoparticles have a spherical shape and the particle size increased from 60.1–83.1 nm with an increase in calcination temperatures from 500–600 °C. The XRD diffraction patterns indicated that the particles are of a wurzite lattice structure. The optical properties were determined by UV-Vis spectrophotometer, and it was found that the band gap of ZnO nanoparticles decreased from 3.249–3.239 eV with an increase in calcination temperature from 500–600 °C.


Introduction
ZnO, a II-IV semiconductor which is one of the favourable candidates among metal oxides in nanoelectronic science, has a wide bandgap of 3.37 eV at room temperature, high exciton binding energy (60 meV), and perfect stability under ambient conditions [1][2][3].Due to its unique and fascinating structural, thermal, optical, electronic, and magnetic properties, different particle sizes of ZnO have been widely fabricated for transistors, light-emitting devices, solar cells, piezoelectric devices, photocatalysts, photovoltaic, gas sensors, and field-emission devices as well as chemical and biological sensors [4][5][6][7].
To fulfil the demand of ZnO in diverse technological applications, extensive investigations for finding a suitable synthesis technique have been conducted to produce ZnO nanoparticles of appropriate morphology and size that could be useful for all kinds of applications, and yet has potential for scaling up and large-scale production.Various methods have been used to synthesize ZnO nanoparticles including the sol-gel technique [8,9], thermal decomposition process [10], sonochemical method [11], and laser ablation [12].However, these methods rely on complex handling procedures, long reaction times, and dangerous reagents and they produce toxic by-products, which make them unsuitable for large-scale production.The trend for large-scale production of nanoparticles is to use low cost precursors, simple processes, consume less energy, and minimize environmental impacts, requiring no further size selection procedures [13].Recently, a simple and potential up-scalable thermal treatment route has been developed in our laboratory for synthesis of various nanomaterials [14][15][16][17][18].The method offers advantages in its simplicity, and it is environmentally friendly as it neither uses toxic reagent nor produces toxic by-products.
In this article, we present the fabrication of pure ZnO nanoparticles using a thermal treatment method by dissolving metal precursor, zinc nitrate and a capping agent, polyvinyl pyrrolidone (PVP), in deionized water before drying and grinding with simultaneous sintering and calcination.The physical properties of the prepared ZnO nanoparticles were investigated using thermogravimetric analysis (TGA), Fourier transform infrared analysis (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-Visible spectroscopy.

Experimental Section
Zinc nitrate used as metal source with a purity exceeding 99% was purchased from Acros Organics (New Jersey, NJ, US).The polyvinyl pyrrolidone (PVP, MW = 10,000 g/mol) used as capping agent to reduce particle agglomeration was purchased from Sigma Aldrich (Darmstadt, Germany).All the chemicals were used without further purification.Deionized water was obtained from Di-Pack ® Millipore (Waltham, MA, US) and used as solvent.
In a typical synthesis procedure, 10 mM of Zn(NO3)2• 6H2O was introduced into 3.5 wt.% PVP and the solution was stirred for 4 h.The transparent pale yellow solution was dried in an oven at 80 °C for 24 h.The dried solid cake was ground for 20 min in a mortar to form powder and named as-prepared sample.Some as-prepared samples were calcined at different temperatures from 450-700 °C in a furnace for 3 h with the objective of removing PVP and leaving behind pure ZnO nanoparticles.
TGA analysis was performed using Mettler Toledo TGA/SDTA 851Eand (Im Langacher Greifensee, Switzerland) ran at 10 °C/min under atmospheric environment from room temperature to 700 °C.The FT-IR was performed using Perkin Elmer Spectrum 100 (Labexchange, Swabian Burladingen, Germany) in the range of 250-4000 cm −1 .The XRD measurements were accomplished by Philips PW3050/60 PAN analytical (Lelyweg1, Almelo, The Netherlands) using copper anode at Cu Kα wavelength of 1.54 Å for 2θ angles from 4°-80°.The TEM images were evaluated by LEO 912AB EFTEM (Munich, German).The ZnO nanoparticles were dispersed in acetone (95%) by ultrasonification and dried on copper grids for TEM measurement.The diffuse reflectance spectra were measured using a UV-vis spectrophotometer (Shimadzu model UV-3600, Kyoto, Japan) in the range of 200-800 nm at room temperature to evaluate the optical properties of ZnO nanoparticles.

Results and Discussion
The TGA analysis of as-prepared sample is depicted in Figure 1.The massive mass loss of almost 77% occurred from room temperature up to 500 °C due to trapped moisture below 100 °C and organic matter (PVP) below 460 °C of the sample.The TGA result was used to investigate the thermal stability of the sample during the process of heat treatment.The residue of the sample shown in the TGA spectrum is the ZnO nanoparticles are thermally almost stable starting at 500 °C and continuing up to the maximum temperature of 700 °C.The FT-IR spectra of the as-prepared sample and synthesized ZnO nanoparticles are displayed in Figure 2. As can be seen in the figure, there are several characteristic absorption bands appearing in the range of 250-4000 cm −1 due to covalent bonds of PVP [16,19,20].The peaks located at 3262 and 2948 cm −1 represent the vibration mode of O-H group of the moisture and the stretching mode of anti-symmetry CH2, respectively.The presence of hydroxyl at peaks of 1646 and 1426 cm −1 came from surface adsorption of surrounding water and residual product.It is ascribed as the O-H bending of water partly overlapping with stretching vibration of C=O asymmetrical and bending CH2 in α position of C=O.The peak located at 1372 cm −1 is described to be the stretching vibration of nitrate (NO3 − ) band; while the spectrum peak at 1280 cm −1 is the C-N stretching vibration band.The peaks from 1068 and 569 cm −1 are attributed as the stretching C-C rings of "breathing" of pyrrolidone.The peak at 368 cm −1 is assigned to be the stretching of Zn-O bond.We expect the same peaks of PVP are present below calcination temperatures of 400 °C because the mass loss of the sample is slightly changed as indicated by TGA curve.At calcinations temperature of 450 °C, most of the appeared peaks in the as-prepared sample were removed except the peaks at 3377, 1508, 834, and 389 cm −1 which are ascribed as the remaining small amount of PVP compounds, while the sharp peak at 389 cm −1 is typical of a Zn-O bond.When the calcination temperature was increased, to 500 °C, we observed unwanted trace peaks below 1000 cm −1 possibly due to metal nitride, and an intense peak of 389 cm −1 that come from the Zn-O stretching bond of pure ZnO nanoparticles.As shown in Figure 2, the formation of ZnO nanocrystals started at 500 °C and this is consistent with TGA result where the majority of PVP compounds can be removed from the residue at temperatures of 500 °C and beyond.Figure 3 demonstrates the XRD patterns of as-prepared sample and the ZnO nanoparticles calcined at 450, 500, and 600 °C.The broad spectrum of as-prepared sample shows that the sample is an amorphous.On the other hand, the multiple peaks of diffraction patterns for calcined samples showed the polycrystalline property of synthesized ZnO nanoparticles with a wurzite structure.The diffraction peaks are coincided with the (100), (002), ( 101), ( 102), ( 110), ( 103), ( 200), ( 112), ( 201), (004), and (201) crystalline plans of wurzite structure of ZnO (ICDD 00-036-1451).At higher values of calcination temperatures, we observed an enhancement of the crystallinity of the ZnO nanoparticles as shown by increasing the intensity of the peaks.This crystallinity enhancement with increasing calcination temperatures is stemmed from the increment of the crystalline volume to surface ratio, as evidenced by TEM images, which occurred due to particle size enlargement [21].The crystalline size (D) of ZnO nanocrystals can be determined using Debye-Scherrer formula [22]: where λ is the X-ray wavelength, θ is the Bragg's angle, and β is the full width of the diffraction line at half maximum intensity measured in radian.The crystallite sizes of ZnO nanocrystals thermal treated at different temperatures were obtained from preferred plane of (101) and are listed in Table 1.The average crystalline size increases with increases of calcination temperature.The d-spacing of lattice planes (dhkl) can be determined based on Bragg's formula [23]: and the lattice parameters in hexagonal (wurzite) structure can be calculated by the following formula [23]: 14 () The mean values of a and c as well as the c/a ratio are very close with the standard value (a = 3.250 Å and c = 5.207 Å; c/a = 1.6021).In Table 1, the crystal was contaminated with a trace of PVP at 450 °C as shown in the FTIR spectra, Figure 2.However, the crystallinity was not well established at this temperature and it could not be used to justify the crystalline structure at the calcination temperature.Therefore, at calcination temperatures of 500 and 600 °C in Table 1, the crystallinity of the particles were well developed and the lattice parameters are shown to be temperature dependent as the particle size increased with the calcination temperature.Figure 4 illustrates the TEM micrographs of ZnO nanoparticles calcined at different temperatures.It is obvious that the prepared ZnO nanoparticles are almost a spherical shape.The average size of the synthesized ZnO nanoparticles are 34.14, 60.17, and 83.15 nm at 450, 500, and 600 °C respectively, which shows that the average particle size increased with increasing calcination temperature.These results indicate that the attained particle size increased with increasing calcination temperature due to the fact that as the temperature rises, many neighbouring particles were prone to fuse together to form larger particles as their surfaces melted [24][25][26].At higher calcination temperatures above 600 °C, the average particle size increased to greater than 100 nm and they became microstructures.It is apparent that each particle of ZnO nanoparticles was composed of a number of nanocrystallites; thus, the size of particles actually is much larger than that of crystalline.The optical band gap values for all samples calcined at different temperatures were determined from reflectance spectra using the Kubelka-Munk equation [23]: where F(R∞) is the so-called remission parameter or Kubelka-Munk function, (hv) is the incident photon energy, A is a constant depending on the transition probability and the diffuse reflectance (R∞), (R∞) is the diffuse reflectance that is obtained from R∞ = Rsample/Rstandard.The values of (F(R∞)• hv) 2 vs. (hv) were plotted as illustrated in Figure 6.The straight line ranges of the plot were extended to the (hv) axis in order to determine the optical band gap values of the ZnO nanoparticles at different calcination temperatures.It was found that the optical band gap decreased with increasing calcination temperature from 3.249 eV at 450 °C to 3.239 eV at 600 °C as shown in Table 2.A decrease in the band gap with increasing calcination temperature is attributed to the increase in the particle size due to fusion of surface atoms which improved the metal crystallinity, as was evidenced by XRD analysis.It is believed that as the particle size increased, the number of atoms that form a particle also increases, which consequently renders the valence and conduction electrons more attractive to the ion core of the particles, and hence decreasing the band gap of the particles.

Conclusions
ZnO nanoparticles have been prepared by a simple and potentially up-scalable thermal treatment method which is easy to perform at low cost, and is an environmentally friendly procedure.The removal of PVP from the as-prepared sample to create pure ZnO nanoparticles was achieved by polymer calcinations and material annealing at 500 °C and above.The particle size increased with increases in calcination temperature due to the fusion of surface atoms at higher temperatures which improved the metal crystallinity.The band gap decreased with increasing calcination temperature, which was attributed to the increase in particle size.This is as a result of stronger Coulomb forces acting on the valence and conduction electrons increasing the number of core ions of the particles.

Figure 1 .
Figure 1.TGA analysis of as-prepared sample.

Figure 2 .
Figure 2. FT-IR spectra of as-prepared sample and ZnO nanoparticles at different calcination temperatures.

Figure 3 .
Figure 3. X-ray diffraction spectra of as-prepared sample and ZnO nanoparticles at different calcination temperatures.

Figure 5 .
Figure 5. UV-visible diffuse reflectance spectra of ZnO nanoparticles at different calcination temperatures.

Table 1 .
Structural parameters of ZnO nanocrystals from XRD pattern for (101) plane.

Table 2 .
Average particle size and band gap energy of ZnO nanoparticles at different calcination temperatures.