Urea-Based Synthesis of Zinc Oxide Nanostructures at Low Temperature

The preparation of nanometer-sized structures of zinc oxide (ZnO) from zinc acetate and urea as raw materials was performed using conventional water bath heating and a microwave hydrothermal (MH) method in an aqueous solution. The oxide formation is controlled by decomposition of the added urea in the sealed autoclave. The influence of urea and the synthesis method on the final product formation are discussed. Broadband photoluminescence (PL) behavior in visible-range spectra was observed with a maximum peak centered in the green region which was attributed to different defects and the structural changes involved with ZnO crystals which were produced during the nucleation process.

The use of polymers or surfactants [22,23] to prepare zinc oxide nanoparticles is advantageous due to a surface modification process which eliminates agglomeration during synthesis and controls the morphology and the shape of developed ZnO nanocrystals.However, repetitive washing and centrifugation is required with appropriate reagents such as absolute ethanol and distilled water.Therefore, directly controlling experimental factors to obtain nanoparticles with ideal morphologies is a significant objective which is essential for future device application [24].Furthermore, the MH method has commanded intensive interest due to simple manipulation, low cost, clean technology, and short synthesis time [25][26][27].
In this paper, the effect of the synthesis method on the formation of zinc oxide nanostructures in an aqueous solution was investigated.Thus, we prepared this oxide using conventional water bath heating and the MH method using urea as one of the reactants.The samples were characterized by field emission gun scanning electron microscopy (FE-SEM) and Raman spectroscopy.The formation of a hexagonal ZnO wurtzite phase was verified by X-ray diffraction (XRD) patterns.The morphology, growth mechanism, and PL properties were recorded.
The solution was then heated by two different methods: conventional water bath heating and the MH method.In the MH heating, the solution was transferred to a Teflonlined stainless steel autoclave, sealed, and placed in domestic microwave (2.45 GHz) which was maintained at 100 • C for 2 and 8 min.The pressure in the sealed autoclave was stabilized at 1.0 atm.The autoclave was cooled to room temperature naturally.A white product was separated by centrifugation, washed with deionized water and ethanol, and dried at 60 • C in air.

Characterization of Samples.
The powders obtained were structurally characterized by XRD using a Shimadzu XRD 6000 (Japan) equipped with CuK ∝ radiation (λ = 1.5406Å) in the 2θ range from 10 • to 80 • with 0.02 • /min scan increment.The morphology was characterized by FE-SEM (Supra 35-VP, Carl Zeiss, Germany).Raman spectra were recorded on a RFS/100/S Bruker FT-Raman spectrometer with a Nd:YAG laser providing an excitation light at 1064.0 nm and a spectral resolution of 4 cm −1 .The PL was measured with a Thermal Jarrel-Ash Monospec 27 monochromator and a Hamamatsu R446 photomultiplier.The 350.7 nm exciting wavelength of a krypton ion laser (Coherent Innova) was employed, and the nominal output power of the laser was maintained at 550 mW.All measurements were made at room temperature.

Results and Discussion
XRD patterns of samples obtained using conventional water bath heating and the MH method for 2 and 8 min are shown in Figure 1.The results revealed that all diffraction peaks can be indexed to the hexagonal ZnO structure which shows good agreement with data reported in the literature (JCPDS card number 36-1451).The strong and sharp peaks indicate that the zinc oxide powders are highly crystalline and structurally ordered at long range.These results show that the MH processing promotes the complete crystallization of ZnO samples at low temperatures and reduced processing time.No secondary phases were detected.
Five active Raman modes can be observed for ZnO samples: (i) at 437 cm −1 a narrow strong band has been assigned to one of the two E 2 modes involving mainly a Zn motion which is a band characteristic of the wurtzite phase [35]; (ii) bands at approximately 332 cm −1 and several common low-frequency features should be assigned to the second-order Raman spectrum arising from zone boundary phonons 3E 2H -E 2L ; (iii) at 530 cm −1 , a very weak band from the E 1 (LO) mode of ZnO associated with oxygen deficiency [36].Its intensity depends upon the crystallinity, preparation method and crystal orientation.Figure 2 shows Raman spectra of ZnO powders obtained.The asymmetric band at 378 cm −1 (A 1T mode) related to the structural order-disorder in the lattice [37] is covered by broad band characteristic of a Zn-O bond at 437 cm −1 .
Well-crystallized zinc compounds with different morphologies can be obtained by several synthesis methods by using urea (see Table 1).Precursors, the concentration of urea, the synthesis method and the reaction time are important factors influencing the structural evolution and the morphology of the products.The weak basicity of the solution gives rise to a zinc carbonate species product (Table 1).The use of MH crystallization facilitates the direct preparation of pure oxide powders in less time with desired particle sizes and shapes from the control parameters such as solution pH, reaction temperature, reaction time, solute concentration and the type of solvent [38][39][40].
Hydrolysis characteristics of urea are well known in H 2 O over 293-373 K at 1 bar [41].Urea is highly soluble in water, and its controlled hydrolysis in aqueous solutions can yield ammonia and carbon dioxide.In the crystal growth process, first ZnO tiny crystalline nuclei were formed, and nanoparticles of this oxide were precipitated by an increase in pH due to NH 4 + ions generated from of NH 3 which resulted from urea decomposition when the temperature rose.The NH 4 + ion formation is controlled by ammonia in water, and the hydrolysis of urea leads to a rise in the pH.The urea hydrolysis progresses slowly, and the basic solution undergoes supersaturation of the zinc hydroxide species [42].Thus, the formation of ZnO occurs by a nucleation process and the preferred growth direction of the crystal.
During the MH process, the urea is readily hydrolyzed; its hydrolysis is also accompanied by the formation of gas molecules and an increase in the pressure in the system which is expected to perturb nanocrystalline growth and thereby   results in morphological changes.This process may accelerate the reaction between the synthesis precursors which leads to anisotropic crystal growth and the crystallization of oxide under mild temperature conditions and reaction times.A schematic representation of ZnO nanostructure formation is shown in Figure 3.The morphology of ZnO powders was investigated using FE-SEM (see Figure 4).These images reveal that samples prepared without the MH process had an irregular shape and were not uniform in size, whereas spherical and uniform particles were observed for samples prepared using the MH method.The MH method contributes significantly to ZnO production with homogeneous shapes after short processing times.
Diffusivity in the medium and interface mobility could be enhanced using the MH process which contributes significantly to ZnO production with homogeneous shapes after short processing times.Nanosized structures were formed by this method, and different average particle distributions were obtained after treatment under hydrothermal conditions (see inset in Figures 4(b) and 4(c)).Average particle diameters were 90 nm and 85 nm for samples treated by the MH process for 2 and 8 min, respectively.
Disordered structures in solids cause degeneracy and destabilization in the localized states of the atoms which act as electron-hole pairs and support broadband PL phenomena.PL spectra recorded at room temperature are illustrated in Figure 5(a).Theoretical results verify that a symmetry breaking process in the structure of various semiconductors associated with order-disorder effects is a necessary condition for intermediate levels in the forbidden band gap [43][44][45].These structural changes can be related to the charge polarization in different ranges that are (at the very least) manifestations of quantum confinement when they occur at short and intermediate ranges independent of the particle size.The main reason for quantum confinement to occur is the formation of discrete levels in the band gap which is not possible with as a periodic crystal defect (dispersion interaction) [46].The formation of isolated energy levels (quantum confinement) and [ZnO 3 • V • o ] clusters leads to a substantial recombination between photoexcited electrons and holes during the excitation process.Probably the [ZnO 4 ] x − −[ZnO 3 • V x o ] clusters are activated during the excitation process which changes their symmetry in progressing from singlet or triplet states as demonstrated for the perovskite structure [47].These defects induce new energies in the band gap which can be attributed to zinc-oxygen vacancy centers.The structural and electronic reconstructions of all possible combinations of clusters in a crystal are essential to understand the cluster-to-cluster charge transfer process and even the PL phenomenon.
PL curves were decomposed into five components using the Gaussian method and the Peak Fit program: a red component (1.76 eV), a yellow component (1.95 eV), two green components (2.15 and 2.36 eV), and a blue component (2.61 eV).These emissions arise from a radiative recombination between electrons and holes trapped in the gap states.possess an optimum structural order-disorder degree for PL to occur.

Conclusions
The MH process affects the growth process of ZnO nanostructures from an aqueous solution of zinc acetate and urea which leads to the rapid and uniform growth of particles.The PL emission of semiconductors is an important property because it can provide information on defects and relaxation pathways of excited states depending upon the preparation techniques which can generate different structural defects.

Figure 1 :
Figure 1: XRD patterns of the ZnO sample obtained by (a) conventional water bath heating, (b) the MH method for 2 min, and (c) the MH method for 8 min.

Figure 2 : 4 Figure 3 :
Figure 2: Raman spectra of ZnO powders prepared by (a) conventional water bath heating, (b) the MH method for 2 min, and (c) the MH method for 8 min.

Figure 4 :
Figure 4: FE-SEM images of ZnO powders prepared using (a) conventional water bath heating, (b) the MH method for 2 min (average distribution of particles is inserted), and (c) the MH method for 8 min (average distribution of particles is inserted).

Figure 5 :
Figure 5: (a) PL spectra at room temperature of ZnO powders; deconvolution results of ZnO samples obtained by (b) conventional water bath heating, (c) the MH method for 2 min, and (d) the MH method for 8 min.

Table 1 :
Zinc species obtained by different synthesis method using urea as a precursor.

Table 2 :
Fitting parameters of five Gaussian peaks.