A template-free CVD route to synthesize hierarchical porous ZnO films
Introduction
Zinc oxide is considered to be a promising material for high performance photonics applications because of its direct wide band gap (Eg ∼ 3.3 eV at 300 K) and large exciton binding energy (∼ 60 meV) [1], [2]. However, the properties of ZnO are strongly dependent on the crystallite size and morphology. This promotes a lot of work on different nano-structures of ZnO with a large variety of growth methods, such as nanodots, nanoparticles, nanorods, nanowires, nanotubes and nanofilms [3], [4], [5], [6], [7], [8], [9]. Porous ZnO film appears to be an especially appealing one, due to its large specific surface area, large pores volume, chemical and photochemical stability, shape selectivity, and rich surface chemistry [10]. There are several techniques to produce porous films, however, an inevitable challenge in the synthesis process is the using of template. How to synthesize a large area and uniform template, and how to remove the template from the substrate completely are still unsolved [11]. Although some groups have improved the growth method to fabricate nanoplates, nanosheets and aggregated nanoparticles without template, the uniform and well-crystallized porous films have not been obtained [12], [13], [14], [15]. In particular, hierarchical porous structures have rarely been reported.
In this work, we report the synthesis, structure characterization, growth mechanism and luminescence of hierarchical porous ZnO film grown by chemical vapor deposition method (CVD) without templates or catalysts. Structural characterization of the thin film was analyzed by X-ray diffraction patterns (XRD) and Scanning Electron Microscopy (SEM). Different from previous reported porous ZnO films that only contained one kind of pores, ours were found to contain both mesopores and macropores [16], [17]. Further investigation showed that the grain boundaries that existed in the nanowalls and connected the macropores played an essential role in the growth of this unique porous ZnO film. A unique strong near ultraviolet emission peak at 420 nm was observed in the PhotoLuminescence (PL) spectrum.
Section snippets
Experimental procedure
In our experiment, we chose silicon wafer as the substrate and cut it into desired dimensions (approximately 1 cm × 2.5 cm). Firstly, we dipped the substrate into acetone solution and cleaned it with ultrasonic washer for 20 min, then blow-dried it by a steam of nitrogen. After that, we cleaned the substrate in ethanol solution using the same method. Prior to the CVD process, a seed layer was prepared on the surface of the Si substrate. We put a drop of ethanol solution (contains 0.02 mol∙L−1
Analyst of hierarchical porous ZnO film
The crystalline structure of this ZnO film characterized by X-ray diffraction is shown in Fig. 1. The black plot is the XRD of the sample. All the diffraction peaks can be indexed as hexagonal wurtzite-type ZnO structure (compared with the standard PDF card, JCPDS no.75–0576, see the red line). No apparent impurity peak can be found in the figure indicating a pure crystal of ZnO. Compared with the standard X-ray diffraction data of bulk ZnO, the relative intensity of (002) peak is greatly
Conclusion
In conclusion, we have deposited hierarchical porous ZnO films on Si substrate using CVD method without catalyst or template. After characterizing by SEM, we found that the porous films contain both macropores and mesopores. The seed layer is considered to play an important role in the growth process, as it is found to provide preference area (the grain boundaries) for ZnO to nucleate and then grow to nanowalls. An appropriate weight ratio of ZnO powders to graphite powders is needed to
Acknowledgment
We gratefully acknowledge the financial support of National Natural Science Foundation of China (Nos. 61176079, 21373156 and 11404253) and Natural Science Foundation of Education Bureau of Shannxi Province, China (No. 2013JK0621).
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