Journal of Nanoparticle Research

, Volume 12, Issue 1, pp 143–150

Porous ZnO nanobelts: synthesis, mechanism, and morphological evolutions

Authors

  • Xia Cao
    • School of Materials Science and EngineeringJiangsu University of Science and Technology
    • School of Chemistry and Environmental EngineeringBeijing University of Aeronautics and Astronautics
    • School of Chemistry and Environmental EngineeringBeijing University of Aeronautics and Astronautics
  • Long Wang
    • Zhongshan Torch polytechnic
    • School of Chemistry and Environmental EngineeringBeijing University of Aeronautics and Astronautics
Research Paper

DOI: 10.1007/s11051-009-9588-z

Cite this article as:
Cao, X., Wang, N., Wang, L. et al. J Nanopart Res (2010) 12: 143. doi:10.1007/s11051-009-9588-z

Abstract

Porous ZnO nanobelts with rough surface and poly-crystalline nature have been developed from a facile wet chemical method. The as-prepared products were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), cold field emission scanning electron microscopy (CFE-SEM), and energy dispersive analysis of X-rays (EDAX). The ZnO nanobelts were synthesized with usually 5 to 6 nm in thickness, 10 to 40 nm in width, and about several micrometers in length. A PVP promoted self-assembly mechanism is believed to be responsible for the morphology shaping process of the ZnO nanostructures. This first wet chemical synthesis of such hierarchical structures without any hard templates implies a simple and inexpensive way to prepare transition metal superstructures on a large scale for modern chemical synthesis. Optical characterization by a confocal laser Raman were also carried out to explore their optical properties; the PL and Raman results showed both good agreement with the characters of our samples and potential for future applications such as sensors and other modern technologies.

Keywords

ZnONanowirePorousPVPSoft templatePLNanostructure

Introduction

The direct fabrication of controlled nanostructures is not only of scientific interest but also could produce real benefits such as for the many technological applications that derive from their peculiar and fascinating properties, superior to the corresponding bulk counterparts (Wang et al. 2007, Cao et al. 2008). Also, the shape, crystalline structure, and size of semiconductors are important elements in determining their physical and chemical properties (Han et al. 2000); thus, rational control over these elements has become a hot topic in recent material research fields (Chen et al. 2000, Mohamed et al. 1998).

As a wide band gap semiconductor oxide with a large excitation binding energy (60 meV), zinc oxide has become one of the most important functional materials in broad areas such as near-UV emission (Wong and Searson 1999), optical transparency and electric conductivity (Kim et al. 1994, Hunt 2001), and piezoelectricity (Roy and Basu 2002). Nanostructured ZnO, via quantum confinement, shape, size, and surface dependent effects hold promise for tuning the optical properties and for assembling the nanostructures in nano scale devices.

During the past several years, various methods have been developed for the synthesis of one-dimensional nano materials that have included vapor liquid–solid (VLS) (Hao et al. 2006, Wang et al. 2006), template-assisted, [11] colloidal micellar (Lu et al. 2003), and electrochemical processes (Tsai et al. 2006). Of these works, vast efforts were endeavored to the synthesis of solid wirelike nanostructured ZnO (Vayssieres et al. 2001, Hu et al. 2002, Wu and Liu 2002, Wang et al. 2002, Huang et al. 2001, Yang et al. 2002, Zhang et al. 2002). For example, Zhang et al have developed a simple and high-yield dry CVD method involving vapor-solid (VS) growth mechanism for large-scale fabrication of ZnO nanobelts at 700°C (Zhang et al. 2002). The as-synthesized products are pure, structurally uniform, single-crystalline ZnO nanobelts. Here, it is important to point out that though the strategy mentioned above is genius and interesting and has the advantages of high purity and quality, generally high temperature vapor deposition processes all share the limitations of high energy cost (high temperature) as well as low production. New synthetic strategies and a better understanding of the growth mechanisms by which the size and shape of nanostructures can be easily tailored have become key issues in material chemistry.

Porous nanowires possess higher surface-to-volume ratios, a key advance feature in practical applications such as catalysts and gas sensing in comparison with their solid wire counterpart. Meanwhile, aqueous solution processes have been noteworthy as a new nanofabrication technique of functional materials. In comparison, wet chemical methods such as microemulsion technique offer the advantages of simplicity and mild reaction conditions and are suited for large-scale preparation of 1-D ZnO nanostructures. But, to the best of our knowledge, reports on the wet chemical preparation of porous ZnO nanowires are quite rare; thermal evaporation and the noble metal catalyzed vapor-phase transport process are the major vapor methods to fabricate one-dimensional ZnO nanostructures with porous nature (Wang et al. 2004, Huang et al. 2001, Pan et al. 2001, Song et al. 2007).

Templating and assembly of inorganic nanostructures by surfactants or biomolecules have been frequently reported in recent years (Lao et al. 2002, Che et al. 2004, Mao et al. 2004, Schaaff and Whetten 2000, Yin et al. 2001). In these cases, the existence of surfactants is beneficial for self-assembling in a more ordered and tight manner to form regular superstructures, where the macromolecules probably form both template and inter-particle bilayers inducing the nanoparticles forming and gluing together one dimensionally during the assembly process. In this paper, we report on the one-step wet chemical synthesis of ZnO nanoparticles and their in situ large-scale assembly onto nanowires using a mild solution method under the directing effect of PVP molecules, which makes the free spatial arrangement of nanoparticles possible based on polymer template. Porous ZnO nanobelts were also obtained by simply annealing the as-prepared products in open air. In addition, the unique optical properties of the novel nanostructures are also reported.

Experimental

In a typical synthesis, the belt-like ZnO porous nanowires modified by PVP have been synthesized as follows: 1.1 g of Zn(CH3COO)2·2H2O and 14 g PVP (Polyvidone Povidone, (C6H9NO)n) were added to 150 ml of DMF (N,N-dimethylformamide) under vigorous stir. Then, 20 ml of 2 mol/L NaOH ethanol solution were introduced under stirring. The mixtures were then transformed to a 200-mL autoclave and kept at 423 K for 24 h. The precipitate was harvested by centrifugation, then rinsed with ethanol and distilled water, and dried in air; part of the samples was annealed in air for 3 h at 823 K for further characterization.

Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and scanning electron microscope images were obtained by employing a JEOL JEM-2100F transmission electron microscope and a Hitachi S4800 cold field emission scanning electron microscope (CFE-SEM), which was coupled with a 9100 EDAX for surface analysis. PL spectra of the samples were collected using a LabRAM HR800 (HORIBA Jobin Yvon) confocal Raman spectrometer. The X-ray powder diffraction (XRD) pattern of the as-prepared products was collected by a Rigaku X-ray diffractometer (Rigaku Goniometer PMG-A2, CN2155D2, wavelength = 0.15147 nm) with Cu Kα radiation. TEM samples were prepared by dispersing the powder in alcohol by ultrasonic treatment, positioning a drop onto a porous carbon film supported on a copper grid, and then drying in air.

All the reagents used in the experiments were of analytical grade (purchased from Beijing Chemical Industrial Co.) and were used without further purification. Photoluminescence measurements were carried out both at room temperature excited by a He-Cd laser at 325 nm. The spectra were resolved by a mono chrometer SPEX 500 M and recorded by a photomultiplier PMT 943.

Results and discussion

Figure 1 shows the X-ray diffraction (XRD) pattern of the as-prepared products. All the reflection peaks of the products can be indexed as pure hexagonal ZnO with cell parameters a = 3.249 Å and c = 5.206 Å, which are in good agreement with the literature values (JCPDS card number 75–1526). The diffraction peaks are well broadened, and analysis of the (101) peak with the Debye-Scherr equation shows the average crystal domain size to be 11.2 nm, implying a growth mechanism of particle attachment. Thus, the as-prepared products have a pure single phase of hexagonal wurtzite structure.
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Fig. 1

XRD patterns of the as-prepared ultra thin ZnO porous nanowires

Figure 2a and b shows CFESEM images of the samples prepared before annealing. The products consist of relatively uniform nanowires mostly with average length of several micrometers and diameter of 20 to 50 nm. Detailed information on the as-prepared composite nanowires can be obtained in Fig. 2b. From the higher magnification SEM image, we can see that the diameters of these nanowires fall mostly into the range of 30 to 40 nm, mainly about 30 nm. The surface of these nanowires is rather smooth and each nanowire displays high regularity and yields well-ordered wire morphologies. The one-dimensional nanostructure is sufficiently stable that it was kept unchanged even after long-time rinse and ultrasonication within water and organic solvents. Besides, the wires are intended to entangle with each other, which makes some of them bundle-like and seem to have a larger diameter.
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Fig. 2

Typical FESEM images of the as-prepared ZnO samples. (a) and (b): images of composite ZnO nanowires before calcination. (c) and (d): images of the porous belt-like ZnO nanowires

Figure 2c and d shows the morphologies of samples after calcination. Sharply different from their composite counterparts, nanowires within the sample 2 were clearly of porous nature though still remaining in the wire-like structures after the removal of their organic components. A higher magnification image (Fig. 2d) indicates that the wire-like structures are totally composed of very little ZnO nanoparticles and the surfaces are quite rough. The lengths of these nanowires are kept constant, that is, several micrometers (Fig. 2c), but their diameters changed obviously and fell into the range of 10 to 40 nm (Fig. 2c and d). More interestingly, the wire-like nanostructures before annealing were transformed into belt-like morphologies with a width of about 10 to 40 nm and thickness of about 5 to 6 nm. These nanobelts surely are composed of loosely packed nanoparticles; indentations resulted from crack can be seen in some part of the belt-like nanowires. Generally, shrinkage of composite materials after calcination can be easily understood, and this can partly be responsible for the far less slender belt-like nanowires observed within sample after calcination.

Despite their ultra thin radius and especially rough surface, the belt-like ZnO nanowires after annealing can be managed to disperse into ethanol by long time untrasonication. As can be seen from Fig. 3a, the diameter of these wires is about 10 to 40 nm and length is of about 1000 nm, which is slightly different with the SEM results because of the untrasonication process. Close examination of the HRTEM images (Fig. 3b) testifies that the thickness of the nanobelt is about 6 nm, close to the average diameter of the building ZnO nanoparticles. In addition, it can also be seen that the nanobelts have a twining characteristic, which is clearly either inherited from their organic template or resulted from the calcination. High resolution TEM also enabled the viewing of lattice planes, confirming crystallinity of the particles within the nanobelts and permitted the accurate measurement of distance between neighboring planes. The measured distance between neighboring planes along the growth axis is 2.46 Å and 2.79 Å, matching the d value for hexagonal ZnO (101) and (100) planes.
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Fig. 3

Typical TEM and HRTEM images of the as-prepared ZnO samples

Superstructures that spontaneously form from nanoparticle assembly might provide materials with new properties. Since this is the first transition of ZnO composite from nanowire to nanobelts, the study of their physical properties should be relevant and interesting. Room-temperature photoluminescence spectrum (PL) of the as-grown sample shown in Fig. 4 was measured using a He-Cd laser (325 nm) as the excitation source. The sharp UV emission at 385 nm shows a full width at half maximum (FWHM) of about 16 nm, which should be attributed to the radioactive annihilation of excitons or be attributed to near band-edge transition according to previous reports (Zhao et al. 2003, Greene et al. 2006, Yao and Zeng 2007). On the other hand, the well-known stronger and broader emission situated in the yellow-green part of the visible spectrum could hardly be registered compared with the sharp and strong peak. In addition, the sharpness of the UV peak gives a powerful attestation that our sample shows a narrow size distribution, which is consistent with the SEM and TEM observations.
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Fig. 4

Room temperature photoluminescence spectra of the as-prepared belt-like ZnO porous nanowires

In order to analyze the formation mechanism, EDAX analysis as used for identifying the constituents of the as-prepared sample without annealing; “C,” “N,” “O,” and “Zn” are found in this sample. Coupled with previously reports, it is reasonable to attribute the nitrogen to the existence of PVP. In fact, similar results of ZnO and PVP composite with morphology of dumbbell were also obtained within a similar reaction environment (Chang et al. 2005). Thus, we can conclude that the sample before annealing was mainly made up of ZnO and PVP.

Concerning the formation of belt-like ZnO nanowires, the role of the solution must be taken in to consideration. It has been reported that part of DMF may undergo a hydrolysis process to produce positive ionic species such as NH2(CH3)2+ (Duff et al. 1993), which could facilitate stabilization of \( \left( {000\overline{1} } \right) \) surface in the initial nucleation. In addition, the carbonyl functional groups of PVP intend to coordinate with the Zn2+ ions of ZnO crystal surfaces and prevent the crystals from rapid oriented growth, especially long the [0001] direction, which is bounded with zinc cations. Thus, the existence of DMF is indispensable to the formation of ZnO/PVP nanowires.

In addition, the effects of precipitator and capping agents should also be studied. First, it is interesting to find that the NaOH precursor concentration has a remarkable effect on the morphology of ZnO crystals, as shown in Fig. 5. When substituting NaOH precursor with NH3·H2O (equal molar ratio), nanoparticles with an average size less than 10 nm were formed under typical experimental condition. (Fig. 5a). When the molar ration between OH precursor and ZnO precursor (referred as M) as less than 1 or more than 14, no obvious precipitations could be obtained. In another case, when the value of M was about 12, rods with diameters of about 30 nm were obtained (Fig. 5b). This phenomenon may be attributed to the stronger inhibition effect of OH- precursor at higher concentration on the particle growth. In this case, the crystallization occurred more difficultly and the reaction rate was much slower. ZnO nanoparticles grew slower and formed crystallized nanorods instead of oriented particle assembly. In addition, ZnO particles at lower OH precursor concentration (M = 2) appeared a little irregular in shape and vague in outline (Fig. 5c). These results demonstrated that the growth mechanism must be closely related to reactions taken part by OH, which may serve to adjust the viscosity of the solution, the supply rate of zinc ions, and the mobility of precursor monomers. These are all important parameters in determining the final morphology of the products.
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Fig. 5

Products synthesized by changing the following experimental parameters: a) substituting NaOH precursor with NH3·H2O; b) M = 12; c) M = 2; d) M = 10; e) M = 6; f) M = 4. The scale bars of Fig. 5a, b, c, d, e, f are 300 nm, 500 nm, 300 nm, 500 nm, and 100 nm, respectively

For the shape control, PVP may also serve as a nonionic surfactant to form the micelles and/or vesicles together with the solvent DMF and a small amount of water (from starting chemical zinc oxylate) (Cölfen and Mann 2003). Furthermore, the adsorbed PVP will also reduce the surface reactivity of the primary ZnO nanocrystallites and prevent them from rapid growth and enlargement, including development into fractal crystal morphology, which has been commonly observed in ZnO crystals. Surely, the existence of PVP is beneficial for ZnO particle self-assembling in a more ordered and tight manner to form regular wire-like superstructures, where PVP molecules probably form both sphere-like template and interparticle bilayers, inducing the ZnO particles to form and glue together one dimensionally during the self-assembly process. In fact, such similar self-assemblies assisted by organic molecules such as macromolecules and surfactants as organic connectors to link inorganic building blocks to produce nanoparticle-based superstructures have been reported (He et al. 2006). Hence, with respect to the formation of ZnO belts self-assembled by ZnO particles in this work, the crystallinity, uniform shape, and size of ZnO particles may afford structure match and spatial proximity to realize effective self-assembly induced by PVP molecules. In addition, the reaction environment promotes the surface domains on neighboring nanoparticles to match up driven by dipole–dipole attraction, as reported previously (Chen et al. 2006). Meanwhile, the superstructure of nanoparticles can be assembled into belts to satisfy the geometric criterion of a low energy (Chen and Hsieh 2002, Puntes et al. 2001, Pearton et al. 2005). On the basis of the above-mentioned time dependent crystallinity and morphology evolution, a schematic illustration of the growth mechanism for the belt-like ZnO nanowires is listed in Fig. 6.
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Fig. 6

Schematic model of morphology evolution process: first self-assembly of the ZnO particles into 1D nanostructure under the direction of PVP molecules; then, the core-shell (PVP-ZnO) structures split off due to high temperature calcination when the PVP was burned off

Conclusions

In summary, a novel wet chemical method was developed to prepare belt-like ZnO nanowires with rough surface and polycrystalline nature assembled under the directing effects of PVP-based polymers. The novelty of this work is characterized by a one-pot procedure which combines formation of nanoparticle precursor, self-assembly, and morphology shaping under mild solution conditions. This simple soft assembly strategy represents an attractive path to large-scale assembly of other functional nanomaterials. Room temperature photoluminescence spectrum (PL) of the as-grown sample shows a strong UV band edge emission, which represents a good candidate for further studies of low-dimensional physics as well as for applications in various fields of nano scale science and technology.

Acknowledgements

Authors acknowledge the support from the China Postdoctoral Science Foundation (CPSF-2302172), the support from the National Natural Science Foundation of China (No. 20673009), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP-20060006005), and the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2006CB932300).

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© Springer Science+Business Media B.V. 2009