Journal of NanomaterialsVolume 2012 (2012), Article ID 427172, 7 pagesdoi:10.1155/2012/427172

Research Article

Urea-Based Synthesis of Zinc Oxide Nanostructures at Low Temperature

J. Z. Marinho,¹ F. C. Romeiro,¹ S. C. S. Lemos,¹ F. V. Motta,² C. S. Riccardi,³ M. S. Li,⁴ E. Longo,³ and R. C. Lima¹

¹Instituto de Química, Universidade Federal de Uberlândia, 38400-902 Uberlândia, MG, Brazil²Departamento de Engenharia de Materiais, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, RN, Brazil³Instituto de Química, Universidade Estadual Paulista, 14800-900 Araraquara, SP, Brazil⁴Instituto de Física de São Carlos, Universidade de São Paulo, 13566-590 São Carlos, SP, Brazil

Received 17 December 2011; Revised 29 February 2012; Accepted 15 March 2012

Academic Editor: Laécio Santos Cavalcante

Copyright © 2012 J. Z. Marinho et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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.

1. Introduction

Zinc oxide (ZnO) is a well-known semiconducting material with photoluminescent and electric conductivity which has a band gap value of 3.37 eV and an excitation energy band of 60 meV at room temperature [1, 2]. With these properties, ZnO has a wide area of application such as solar cells [3, 4], catalysis [5, 6], sensors [7], laser diodes [8], and varistors [5, 9]. Chemical and structural properties of ZnO particles are very important in these applications; different preparation methods for this oxide were used by various researchers such as a sol-gel process [10], homogeneous precipitation [5], thermal decomposition [11], microwave heating [12], a conventional hydrothermal method [13–17], a polymeric precursor method [10], and an MH-assisted method [18, 19]. The characteristics of the powders obtained for specific applications are determined by the crystal size, morphology, porosity, crystal type, and particle shape [6, 20, 21].

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 [25–27].

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.

2. Experimental

2.1. Synthesis

Zinc oxide nanostructures were obtained: zinc acetate (Zn(CH₃COO)₂2H₂O) (99%, Aldrich) and urea (CO(NH₂)₂) (99%, Synth) (1 : 1 stoichiometry) were dissolved in deionized water under constant agitation. A potassium hydroxide (KOH) (3.0 mol/L solution) was added until the pH reached 12 followed by stirring at room temperature for 15 min.

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 Teflon-lined 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.

2.2. Characterization of Samples

The powders obtained were structurally characterized by XRD using a Shimadzu XRD 6000 (Japan) equipped with CuKradiation ( Å) in the 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.

3. 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.

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.

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 E₂ 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 3-; (iii) at 530 cm⁻¹, a very weak band from the E₁ (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 ( 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.

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.

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 [38–40].

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

Synthesis method
Product 1
Temperature/time
Product 2
Morphology/size
Reference

Precipitation
Zn₅(OH)₆(CO₃)₂ + ZnO
4 h of synthesis
ZnO
Bipods/3.1–7.9 μm
[3]

Precipitation
[Zn(OH)₂(H₂O)₂]
1000°C/2 h
ZnO
Hexagonal plates/35–45 μm
[6]

Sol-gel
Zn₅(OH)₆(CO₃)₂
500°C–900°C/2 h
ZnO
Spherical particles/20 nm
[28]

Conventional hydrothermal
white powder
550°C/4 h
ZnO
Column-, rosette-fiber-like/0,5–10 μm
[29]

Microwave-induced combustion technique
ZnO
—
—
Flowers-like/2–5 μm
[30]

Refluxing route
ZnO
—
—
Rods-likes/30–40 nm (diameter) and 500–700 nm (length)
[31]

Conventional hydrothermal
Zn₅(OH)₆(CO₃)₂
500°C/1 h
ZnO
Spherical particles/25 nm
[32]

Conventional hydrothermal
Zn₄(OH)₆CO₃H₂O
400°C/2 h
ZnO
Flakes-like/0.65–1.5 mm
[33]

Conventional hydrothermal
Zn₅(OH)₆(CO₃)₂
600°C
ZnO
Flakes-like/10–20 nm
[11]

Solvothermal
ZnOCO₃ + ZnO
180°C/24 h
ZnO
Spherical particles/50–300 nm
[23]

Urea aqueous solution process
Zn₅(OH)₆(CO₃)₂
600°C/30 min
ZnO
Spherical chrysanthemums/2–6 μm
[34]

Conventional water bath heating
ZnO
—
—
Irregular nanoparticles
This work

Microwave-assisted hydrothermal
ZnO
—
—
Spheres-like/85 nm
This work

Hydrolysis characteristics of urea are well known in H₂O 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 ions generated from of NH₃ which resulted from urea decomposition when the temperature rose. The 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.

Figure 3: Schematic representation of ZnO formation.

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.

(a)

(b)

(c)

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).

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).

(a)

(b)

(c)

(d)

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.

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 [43–45]. 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 [] clusters leads to a substantial recombination between photoexcited electrons and holes during the excitation process. Probably the [ 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. Figures 5(b) to 5(d) illustrate decomposition data, and Table 2 lists the areas under each curve of the respective transitions. The percentage was obtained by dividing the area of each decomposed PL curve by the total PL area. Each color represents a different type of electronic transition and can be linked to a specific structural arrangement. The high PL intensity displayed by the sample obtained for 2 min under MH conditions seems to indicate that this material must possess an optimum structural order-disorder degree for PL to occur.

Table 2: Fitting parameters of five Gaussian peaks.

Peak center (eV)
1.76
1.95
2.15
2.36
2.61

Conventional water bath heating (area %)
11.1
29.0
34.5
19.0
6.4

MH for 2 min (area %)
10.1
26.1
31.2
24.1
8.5

MH for 8 min (area %)
9.2
29.9
33.5
20.8
6.5

4. 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.

Acknowledgments

The authors gratefully acknowledge the agencies FAPEMIG, CNPq, CAPES, and FAPESP.

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