Direct Precipitation and Characterization of ZnO Nanoparticles

ZnOnanoparticles are prepared through hydrolysis and condensation of zinc acetate dihydrate by potassium hydroxide in alcoholic medium at low temperatures. Thermal gravimetric analysis (TGA) of the precursor is made in order to specify the temperature range over which the weight loss and thermal effect are significant. X-ray diffraction of the as-prepared specimens shows that the hexagonal (a = 3.2459 Å, c = 5.1999 Å) structure is the predominant crystallographic structure. According to Scherer’s formula, the average size of the nanoparticles is 22.4 ± 0.6 nm. The structural properties of the synthesized ZnO nanoparticles have been confirmedusing theTEMmicrographs.Theoptical energy gap of the ZnOnanoparticles, as obtained fromapplyingTauc’s equation, is equal to 3.52 eV, which is higher than that of the bulk material. Absorption peak of the as-prepared sample is 298 nm which is highly blue shifted as compared to the bulk (360 nm). Large optical energy gap and highly blue shifted absorption edge confirm that the prepared ZnO nanoparticle exhibits strong quantum confinement effect.


Introduction
Semiconductor zinc oxide (ZnO) nanoparticles have attracted much attention because of their interest in fundamental study and also their applied aspects such as in solar energy conversion, varistors, luminescence, photocatalysis, electrostatic dissipative coating, transparent UV protection films, and chemical sensors [1][2][3][4].Various methods such as thermal decomposition, chemical vapor deposition, sol gel, spray pyrolysis, and precipitation have been developed for the fabrication of nanosized ZnO particles with uniform morphology and size [5][6][7][8][9].Among these synthetic routes, precipitation approach compared with other traditional methods provides a facile way for low cost and large-scale production, which does not need expensive raw materials and complicated equipments [10].
ZnO nanoparticles have been synthesized by Han et al. [11] via precipitation-pyrolysis (P&P), where the precursor zinc hydroxide carbonate (Zn 5 (CO 3 ) 2 (OH) 6 ) was obtained.Their TEM results indicated that pyrolysis temperature is the predominant factor for controlling the mean size of nanoparticles, ranging from 8 nm to 80 nm.It was found that increasing the pyrolysis temperature enhances the mean size.Highly dispersed uniform ZnO particles of different sizes and shapes were prepared by slowly adding zinc salt and sodium hydroxide solutions in parallel into aqueous solutions of Arabic gum [12].Except for the very early stages, the precipitated solids consisted of a well-defined zinc oxide phase.The reaction temperature affected both the size of the precursors and their arrangement in the final particles.At ambient temperature the primary nanoparticles, approximately 10 nm in size, formed spherical aggregates, while at 600 ∘ C they were much larger (44 nm) and combined to form rather uniform hexagonal ZnO prisms.
In the present work, a direct precipitation method is employed to synthesize the nanosized ZnO particles using the raw materials.Characterization of the obtained particles has been made using X-ray diffractometry in addition to the particles morphology using the transmission electron microscope.To get more information about the synthesized particles, the optical properties of the resulted colloidal solution are investigated using UV-VIS double beam spectrophotometer.

Experimental Techniques
ZnO nanoparticles are prepared using the method which was reported previously by Pacholski et al. [13] through hydrolysis and condensation of zinc acetate dihydrate by potassium hydroxide in alcoholic medium at low temperatures.ZnO nanoparticles are settled at the bottom and the excess mother liquor was removed; then the precipitate was washed several times with methanol.The energy dispersive spectrometry (EDS) analysis for detecting the characteristic X-rays of the constituent elements is carried out using an X-Max 80 detector unit which was equipped with transmission electron microscope (TEM) JEM-1230.
Thermal gravimetric analysis (TGA) was carried out using a simultaneous DTA-TG apparatus (DTG-60H, Shimadzu Co., Japan).Approximately 20 mg of the sample is placed in a platinum crucible on the pan of the microbalance and heated from room temperature to 700 ∘ C, using Al 2 O 3 as inert material.Analysis was performed under nitrogen flow at heating rate of 10 ∘ C/min −1 .Powder XRD measurements were performed using the X-ray diffractometer (Shimadzu XD-3A) in the diffraction angle range 20 ≤ 2 ≤ 80 ∘ , with monochromatic CuK radiation ( = 1.5418Å) source.High-resolution transmission electron microscope (HRTEM, JEM 2100) was used with a high resolution pole piece operates at 200 kV accelerating voltage.It works under a vacuum of ∼10 −6 Pa.These conditions give a lattice resolution of 0.14 nm and a point to point resolution of 0.23 nm which help the instrument to be a perfect technique for imaging materials on the atomic scale.The optical properties of the nanoparticles in solutions are studied using UV-Visible spectrophotometer (Shimadzu, UV-2450) in the wavelengths range of 200-900 nm.

Results and Discussion
After washing the synthesized material in methanol and drying with air, it shows a powdered form with white color.The purity of the as-prepared nanoscale ZnO particles was further confirmed by the energy dispersive X-ray spectroscopy (EDS).The EDS spectrum indicates the presence of only Zn and O elements in 1 : 1 atomic ratio in the analyzed ZnO sample.In order to understand the phase symmetry of the synthesized ZnO nanocrystals, a systematic study on the XRD was performed.The weight loss of the investigated material was analyzed.Figure 1 shows the thermogravimetric analysis (TGA) of the synthesized nanoparticles.In case of the as-prepared ZnO NPs, high rate of mass loss has occurred in the temperature range 30 ∘ C ≤  ≤ 100 ∘ C due to the evaporation of the water absorbed on the surface of the products.At higher temperatures, weight loss continues up to 500 ∘ C, which is not the case for the calcinated ZnO NPs.In the latter specimens, the mass loss ends at about 300 ∘ C which is due to the decomposition of Zn(OH) 2 .Comparison between these two TGA curves, shown in Figure 1, indicates the quality improvement of the ZnO NPs after calcination process at 400 ∘ C. Figure 2 shows the XRD pattern for the synthesized ZnO nanoparticles.No extra diffraction peaks corresponding to Zn, or other ZnO phases are detected, indicating that the pure ZnO nanoparticles are crystalline in nature.The peaks' intensity is sharp and narrow, confirming that the sample is of high quality with good crystallinity and fine grain size.XRD pattern of the as-prepared specimen shows that the hexagonal ( = 3.2459 Å,  = 5.1999 Å) structure is the predominant crystallographic structure according to the Cross-Ref PDF number 04-015-4060 card [14].Typical XRD pattern for hexagonal structure shows three strongest lines at 2 values equal to 31.76 ∘ , 34.58 ∘ , and 36.67 ∘ due to reflection from the crystallographic (100), (002), and (101) planes, respectively.Other reflection peaks have been summarized in Table 1.The average size of the nanocrystals is estimated according to the Debye-Scherer formula [15]: where the constant  is taken to be 0.94,  is the wavelength of the CuK radiation source, and  the full width at half maximum (FWHM-in radians) of the diffraction peak corresponding to the Bragg diffraction angle (2).Using the above equation, the average crystallite size is found to be in the range of 22.4 ± 0.6 nm.The strain induced in the obtained material due to crystal imperfection and distortion was calculated [16] using the formula: According to the above equation, plotting the values of  (in rad) versus 4 tan  should give a straight line, as shown in Figure 3. From the least square fit to the experimental points, the slope of the straight line gives directly the -value (8.15 × 10 −4 ) of the ZnO nanoparticles.In a previous work [17], the internal lattice strain value was found to be 14.6 × 10 −4 for the ZnO nanoparticles.A value which is higher than the present one indicates that the as-prepared ZnO nanoparticles have less imperfections than those reported in [17].These strain values confirm the specimen uniformity in all crystallographic directions, thus considering the isotropic nature of the crystal, where the material properties are independent of the direction along which they are measured.
The TEM morphology of the synthesized ZnO nanoparticles is shown in Figure 4.The mean size estimated from the TEM image is about 20 ± 0.5 nm and clearly indicates that the ZnO nanoparticles are crystalline with a wurtzite structure.This is in close agreement with the results calculated from powder XRD data using the Debye-Scherrer formula.
To characterize the optical properties of the obtained specimens, solutions of the synthesized nanoparticles have been made.While it is slightly soluble in methanol, it shows a stable colloid in mixture of methanol and chloroform [14].The spectral dependence of the absorbance is measured using UV-VIS double beam spectrophotometer in the wavelength range 300-2500 nm. Figure 5 shows the variations of the absorption coefficient with the energy of the incident photons.The absorption coefficient increases slowly with increasing the wavelength of the incident photons up to a certain value after which it increases very rapidly showing a peak at about 298 nm.It is well known that a narrow absorption peak means a good crystalline specimen.The absorption peak located at 298 nm in case of the obtained nanoparticles is highly blue shifted as compared with the bulk (360 nm) material.In the high absorption region ( ≥ 10 4 cm −1 ), the spectral dependence of the absorption coefficient can be described by Tauc relation: Taking the square of the above equation gives where  is a constant of proportionality and   is optical energy gap of the investigated solution.The experimental points fit with the above equation only if the direct electronic transitions are responsible for the photon absorption inside the nanoparticles.Intercept of the straight line with the photon energy axis at (ℎ]) 2 = 0, as shown in Figure 6, yields the optical energy gap (=3.52 eV), a value which is in good agreement with that (=3.53 eV) previously obtained [17], while it is slightly higher than that (3.4 eV) previously reported by Samuel et al. [18].Band gap energy increases with decreasing particle size due to quantum size effects.

Conclusions
The structural and optical properties of the synthesized ZnO nanoparticles have been confirmed using TEM, XRD, and UV-VIS spectroscopy.According to Scherer's formula, the average particle size of the sample is 22.4 ± 0.6 nm.The optical energy gap of the ZnO nanoparticles, as obtained  from applying Tauc's equation, is equal to 3.52 eV, which is higher than that of the bulk material.Absorption peak of the as-prepared sample is 298 nm which is highly blue shifted as compared to the bulk (360 nm).Large optical energy gap and highly blue shifted absorption edge confirm that the prepared ZnO nanoparticle exhibits strong quantum confinement effect.

Figure 3 :
Figure 3: Plot of the full width at half maximum ( in radians) versus 4 tan  of the synthesized ZnO nanoparticles.

Table 1 :
X-ray reflection planes of the synthesized ZnO NPs obtained at different diffraction angles (2) and the corresponding values of the maximum width at half maximum ().