Three-dimensional interconnected nanowire networks of ZnO

In this Letter, three-dimensional (3D) interconnected networks of ZnO nanowires and nanorods are synthesized by a high temperature solid–vapor deposition process. The nanorods and nanowires have diameters of 20–100 nm and they grow along the c-axis. Due to the diverse orientation of the nanowires grown from a polycrystalline substrate, the networks are formed by a sintering process of the crossed nanowires during growth. The thickness of the multilayer nano-network could be as thick as 30 lm. The sharp nanowire tips, the high degree of networking, and high surface area of these unique nanonetworks make them a potential candidate for field emission, ultra-sensitive gas sensing, catalysts and filtering. 2005 Elsevier B.V. All rights reserved. Wurtzite structured ZnO is of great importance due to its versatile applications in optoelectronic, photovoltaics and sensors [1,2]. Quasi-one-dimensional nanostructures of ZnO, such as nanowires, nanobelts and nanotubes have been a revalent research topics in nanotechnology for their unique properties and potential applications [3–11]. The non-central symmetric crystallographic structure and spontaneous surface polarization characteristics make ZnO one of the most exciting oxide nanostructures for investigating nanoscale physical and chemical phenomena [12]. Unique structural configurations such as nanojunction-arrays [13–15], piezoelectric nanobelts [16], nanosprings [17], nanorings [18,19], nanobows [20] etc., have been reported. In this Letter, we report a new nanostructure of ZnO: three-dimensional (3D) interconnected nanowire network. The structure and growth process of the nanonetworks will be presented. The high temperature welding and sintering effect on the formation of networks will be elaborated. The structure reported here could find potential applications in field emission, gas sensing, catalysis and filtering. The interconnected networks of ZnO nanowires were grown via a high temperature solid–vapor deposition 0009-2614/$ see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.04.024 * Corresponding author. Fax: +1 404 894 8008. E-mail address: [email protected] (Z.L. Wang). process [21]. The experimental set-up consists of a horizontal high temperature tube furnace 50 cm long, an alumina tube ( 75 cm in length), a rotary pump system and a gas controlling system. Commercial (Alfa Aesa) ZnO powder about 2 g was ground and then loaded in an alumina boat and positioned at the center of the alumina tube as the source material. The evaporation was conducted at 1400 C for 30–120 min under a constant pressure of 300 mbar throughout the heating, hightemperature synthesis and cooling processes. The Ar carrier gas flow rate was controlled at 50–100 sccm (standard cubic centimeters per minute) after the temperature had reached 800 C. The 3D networks of ZnO nanowires were grown on either a polycrystalline Al2O3 substrate or on a (110) Si wafer in a temperature zone of 700–800 C. Scanning electron microscopy (FE-SEM) (field emission LEO 1530 FEG at 5 & 10 kV) and transmission electron microscopy (TEM) (field emission TEM Hitachi HF-2000 at 200 kV), and energy-dispersive Xray spectroscopy (EDS) attached to the SEM and TEM, respectively were used to investigate the morphology, crystal structure and composition of the as-grown nanostructures. Fig. 1a is a typical low magnification SEM picture of the as-grown networks of ZnO nanowires and Fig. 1. (a) A typical low magnification SEM image of the as-grown networks of ZnO nanowires and nanorods consisting two types of morphologies, as indicated by area b (b,c) and c (d,e). (b,c) Enlarged SEM images of uniform networks of ZnO nanowires and nanorods. (d,e) Enlarged SEM images of clumps of nanowires showing the interconnected nanowires and nanorods. P.X. Gao et al. / Chemical Physics Letters 408 (2005) 174–178 175 ARTICLE IN PRESS nanorods. Two typical morphologies were found: one shown in the top of the figure is the area with distinct clumps of nanowires; the other in the middle area is a uniform layer of nanowires networks. The respectively enlarged SEM pictures of the two areas are described in Fig. 1b & c, and Fig. 1d & e. From Fig. 1b, the network feature is clearly displayed for which the mesh element is composed of nanowires with diameters around 100 nm. The further magnified picture in Fig. 1c, describes the structure of the networks with certain 3D void spaces confined by adjacent nanowires and nanorods. Fig. 1d and e depicted four clumps of radially grown Fig. 2. Three typical SEM images of the uniform networks of ZnO nanowire nanowires joining with each other, forming interconnected networks. Fig. 2 shows three typical SEM images of the uniform networks of ZnO nanowires respectively, from top view (Fig. 2a), 45 tilted view (Fig. 2b), and cross section view (Fig. 2c). The top view image indicates the interconnected and periodically spaced features of the network. The tilted view picture in Fig. 2b gives a description that these interconnected nanowires and nanorods have a tendency to align along a specific direction. The crosssectional image in Fig. 2c clearly proves that these network nanowires and nanorods are quasi-aligned normal s: (a) viewed from top; (b) viewed at 45 tilt; and (c) cross section view. Fig. 3. A series of cross-sectional SEM and TEM images of the 3D network structures of ZnO. (a) A cross-sectional SEM image showing the beginning layer of the network. (b) A comparatively dense network cross-sectional SEM image. (c) A TEM image of a broken network, three dotted circles depicted atleast three-layer interconnected network of ZnO nanowires. Fig. 4. (a,b) Interconnection types of ZnO nanowires in the nano-network. (c,d) Bright-field and dark-field TEM images of two nanowires interconnected with each other, indicating that the two nanowires are single crystals but they have no orientation relationship. The circle area is used for recording the selected area electron diffraction pattern (inset). 176 P.X. Gao et al. / Chemical Physics Letters 408 (2005) 174–178 ARTICLE IN PRESS