《复合材料 Composites》课程教学资源（学习资料）第二章增强体_Controllable growth of zinc oxide micro- and nanocrystals by oxidization of Zn–Cu alloy.pdf_大学文库

Availableonlineatwww.sciencedirect.com DIRECTo JOURNAL OF SOLID STATE CHEMISTRY ELSEVIER Journal of Solid State Chemistry 178(2005)819-824 Controllable growth of zinc oxide micro- and nanocrystals by oxidization of Zn-Cu alloy J. Ling, Cheng Chun, J. Zhang, Y. Huang, F.J. Shi, X. X Ding, Chi. Tang, S.R. Qi Department of Physics, Central China Normal Unicersity, Wuhan 430079, China Received 26 November 2004: received in revised form 4 January 2005: accepted 8 January 2005 Abstract A simple and fast, controllable gas vaporization of alloy method for producing various micro- and nanostructures of Zno in air atmosphere with a large yield was presented in this paper. The presence of Cu was proved to strongly affect the growth morphology of the synthesized Zno through the control of the partial pressure of zinc, which could be fulfilled by changing the composition of Zn and Cu for the used reactants. Given the simplicity of the procedure, and the economic advantages, the method described here is likely to be of interest in industrial-scale applications C 2005 Elsevier Inc. All rights reserved. PACS: 73.61.Tm Keywords: Nanostructures; Nanowire: Vapor-liquid-solid growth; Zinc oxide; Semiconducting materials 1. ntroduction films, and nanoclusters [2]. In addition, low cost, high chemical flexibility and low threshold intensity make Zinc oxide(Zno) has received great attention in ZnO nanostructures an ideal candidate for commercial recent years for its potential applications in photoni miniaturized laser light sources 3] material. It possesses a direct bandgap of 3. 37eV at Owing to these promising applications, one-dimen room temperature with a large exciton binding energy of sional Zno nanostructures such as nanowires [31 60mev. The strong exciton binding energy, which is nanobelts [4, 6], nanotubes [5], and nanodendrites [6] much larger than that of Gan (25 me V)and the thermal have been synthesized by a variety of methods energy at room temperature (26 meV) can ensure an However, most methods are relatively complicate and efficient exciton emission at room temperature under have low yield, not suitable for commercial production low excitation energy [1]. As a consequence, Zno is The traditional approaches to synthesize one-dimen recognized as a promising photonic material in blue-UV sional ZnO nanostructures are usually by means of a region. For example, as a UV laser it can allow reading vapor-phase transport process with the assistance of compact disks with much more information, greatly metal catalysts or a template-assisted growth [3-6 creasing the amount of data stored. Room-tempera Furthermore, considering that electrical and optical ture UV emission properties have recently been demon- properties of nanomaterials depend sensitively on both strated from ZnO epitaxial films, microcrystalline thin shape and size, it is important to obtain the expected hape and size in a controllable way. In this work, we describe a method to synthesize high-purity Zno micro- Corresponding author. Department of Physics, The Hong Kong and nanocrystals and nanowires in air by a controllable University of Science and Technology, Clear Watre Bay, Kowloon. Hong Kong. New Territories. China. Fax: +85223581652 gas vaporizing by alloy (CGVA)method, which only E-mailaddresses:nano(@phyccnu.edu.cn, includes a simple, fast process, and may be applied for hengchun(@phyccnu.edu.cn(C.Chun commercial production 0022-4596/- see front matter C 2005 Elsevier Inc. All rights reserved. doi:10.1016/jssc.2005.01006

Journal of Solid State Chemistry 178 (2005) 819–824 Controllable growth of zinc oxide micro- and nanocrystals by oxidization of Zn–Cu alloy J. Ling, Cheng Chun, J. Zhang, Y. Huang, F.J. Shi, X.X Ding, Chi. Tang, S.R. Qi Department of Physics, Central China Normal University, Wuhan 430079, China Received 26 November 2004; received in revised form 4 January 2005; accepted 8 January 2005 Abstract A simple and fast, controllable gas vaporization of alloy method for producing various micro- and nanostructures of ZnO in air atmosphere with a large yield was presented in this paper. The presence of Cu was proved to strongly affect the growth morphology of the synthesized ZnO through the control of the partial pressure of zinc, which could be fulfilled by changing the composition of Zn and Cu for the used reactants. Given the simplicity of the procedure, and the economic advantages, the method described here is likely to be of interest in industrial-scale applications. r 2005 Elsevier Inc. All rights reserved. PACS: 73.61.Tm Keywords: Nanostructures; Nanowire; Vapor–liquid–solid growth; Zinc oxide; Semiconducting materials 1. Introduction Zinc oxide (ZnO) has received great attention in recent years for its potential applications in photonic material. It possesses a direct bandgap of 3.37 eV at room temperature with a large exciton binding energy of 60 meV. The strong exciton binding energy, which is much larger than that of GaN (25 meV) and the thermal energy at room temperature (26 meV) can ensure an efficient exciton emission at room temperature under lowexcitation energy [1]. As a consequence, ZnO is recognized as a promising photonic material in blue-UV region. For example, as a UV laser it can allowreading compact disks with much more information, greatly increasing the amount of data stored. Room-temperature UV emission properties have recently been demonstrated from ZnO epitaxial films, microcrystalline thin films, and nanoclusters [2]. In addition, lowcost, high chemical flexibility and lowthreshold intensity make ZnO nanostructures an ideal candidate for commercial miniaturized laser light sources [3]. Owing to these promising applications, one-dimensional ZnO nanostructures such as nanowires [3], nanobelts [4,6], nanotubes [5], and nanodendrites [6] have been synthesized by a variety of methods. However, most methods are relatively complicate and have lowyield, not suitable for commercial production. The traditional approaches to synthesize one-dimensional ZnO nanostructures are usually by means of a vapor-phase transport process with the assistance of metal catalysts or a template-assisted growth [3–6]. Furthermore, considering that electrical and optical properties of nanomaterials depend sensitively on both shape and size, it is important to obtain the expected shape and size in a controllable way. In this work, we describe a method to synthesize high-purity ZnO microand nanocrystals and nanowires in air by a controllable gas vaporizing by alloy (CGVA) method, which only includes a simple, fast process, and may be applied for commercial production. ARTICLE IN PRESS www.elsevier.com/locate/jssc 0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2005.01.006 Corresponding author. Department of Physics, The Hong Kong University of Science and Technology, Clear Watre Bay, Kowloon, Hong Kong, NewTerritories, China. Fax: +852 2358 1652. E-mail addresses: nano@phy.ccnu.edu.cn, chengchun@phy.ccnu.edu.cn (C. Chun)

J. Ling et al./ Journal of Solid State Chemistry 178(2005)819-824 2. Experimental Zno micro- and nanocrystals were synthesized using chemical vapor transport and condensation system developed in our lab. In a typical synthesis processes, 5 of pure metal Zn(99.99%)or Cu-Zn(99.99%)mixed powders at certain weight ratio( Cu/Zn=2/8, 4/ 6, 5/5, 6/4, 7/3, and 9/1, respectively) were put in a small alumina boat to serve as the source material. The alumina boat was then pushed to the central of the A BA B alumina tube in air atmosphere at 1250C and held for several minutes. After the reaction, a large amount of white powder could also be collected at the alumina boat and the downstream area of this alumina tube. a Fig. I. XRD pattern of the as-synthesized dendrite Zno nanocrystals representative yield for the reactions of Cu/zn=6/4 (Cu/Zn=4/6 weight ratio). Indices of the peaks are specified above was~7.0%. the peaks(A) peaks of purity of zinc, (B)peaks of purity of zinc Zno nanowires were synthesized in the same manner nitride) by oxidizing a block of brass (41.51 wt% of Zn)with the weight of 46 g. The furnace was first heated to 1250C and held for 30 minutes under the protection of an argon flow of 80 standard cubic centimeters per minute. Cu/Zn= 4 6, Fig. 2d Cu/Zn= 5/5, and Fig. 2e Cu/ Followed let the melted brass exposed to the air Zn= 6/4). The diameter of the arms in the tetrapod atmosphere and reacted for several minutes. After the structure decreases with increasing Cu content. The thin reaction, the alumina tube was clogged up with a cotton- tetrapods generally have the needle-shaped arms(Fig like Zno product. The collected product in the tube 2d), while the thick ones have two kinds of uniform weighted 4.0 g and the yield was x8.7%. Small brass structures: special edge -like arms(Fig. 2c) and hexago- bean remained in the alumina boat nal cylinder arms(Fig. 2e and the inset), which are Morphology, structure and chemical composition of unique morphologies of Zno nanocrystals compared to the samples were examined using an X-ray diffraction that reported previously [6]. Furthermore, while a (XRD, D/max-rB, CuKo radiation), a scanning electron her Cu proportion was used, microwires microscopy (SEM, Hitachi X-650) equipped with an with a radiation flower (Fig. 2f, Cu/Zn= 7/3)and EDS, and transmission electron microscopy (TEM, chrysanthemum petal-like microcrystals (Fig. 2g, JEM-100C Cu/Zn= 9 High yield of ZnO nanowires can be produced by oxidation of melted brass with a commercial purity 3. Results and discussion (41.51 wt of Zn)using the same COva method Fig 3 shows the typical SEM and TEM images of these ZnO Fig. 1 shows a typical XRD pattern of the as- nanowires Determined from TEM images(Fig. 3b), the synthesized white powder sample(Cu/Zn= 4/6, weight diameter of the ZnO nanowires varied from 10 to ratio). The diffraction peaks can be indexed to 150nm. The observed variation in diameter may be hexagonal structure of Zno with cell constants of a related to the inhomogeneous sizes of the nuclei of th 3. 24 and c=5.19A. No diffraction peaks produced oxide nanowires [7]. The lengths of these nanowires from Cu and Cuo could be found from the pattern, distributed from 3 to 10 um. The nanosheets and though there are some weak peaks from small amount nanoscale tripods, as by-products, often appear in the of A: zinc that has not been oxidized and B: zinc nitride products(Fig 3a). XRD pattern confirms a hexagonal from the reaction of zinc and nitrogen in the air structure of ZnO(Fig 3c). Some weak peaks from small Further structural characterization of the products amount of zinc and zinc nitride are also detected, same was performed using TEM and SEM. Fig. 2 shows as those in Fig. I typical SEM images of the Zno micro- and nanocrys- Understanding the growth mechanism will be im tals. When pure Zn was used as reactant, uniform portant in order to control and design nanostruc sprout-like ZnO tetrapod microstructures were obtained tures. All experiments in this study were carried out in (Fig 2a). When Cu was added to the Zn with the weight air atmosphere. It is apparent that the synthesis ratio 2/8, bullet-like product with hexagonal cylinder of Zno nanostructures discussed here is based on microstructures was achieved (Fig 2b). However, when the oxidation of thermal evaporation of Zn powder more proportion of Cu was added, the ZnO w under the controlled conditions. The crystal growth may tetrapod micro- and nanostructure appeared(Fig. 2c be dominated mainly by the following two growth

2. Experimental ZnO micro- and nanocrystals were synthesized using a chemical vapor transport and condensation system developed in our lab. In a typical synthesis processes, 5 g of pure metal Zn (99.99%) or Cu–Zn (99.99%) mixed powders at certain weight ratio (Cu/Zn ¼ 2/8, 4/6, 5/5, 6/4, 7/3, and 9/1, respectively) were put in a small alumina boat to serve as the source material. The alumina boat was then pushed to the central of the alumina tube in air atmosphere at 1250 1C and held for several minutes. After the reaction, a large amount of white powder could also be collected at the alumina boat and the downstream area of this alumina tube. A representative yield for the reactions of Cu/Zn ¼ 6/4 was 7.0%. ZnO nanowires were synthesized in the same manner by oxidizing a block of brass (41.51 wt% of Zn) with the weight of 46 g. The furnace was first heated to 1250 1C and held for 30 minutes under the protection of an argon flowof 80 standard cubic centimeters per minute. Followed let the melted brass exposed to the air atmosphere and reacted for several minutes. After the reaction, the alumina tube was clogged up with a cottonlike ZnO product. The collected product in the tube weighted 4.0 g and the yield was 8.7%. Small brass bean remained in the alumina boat. Morphology, structure and chemical composition of the samples were examined using an X-ray diffraction (XRD, D/max-rB, CuKa radiation), a scanning electron microscopy (SEM, Hitachi X-650) equipped with an EDS, and transmission electron microscopy (TEM, JEM-100C). 3. Results and discussion Fig. 1 shows a typical XRD pattern of the assynthesized white powder sample (Cu/Zn ¼ 4/6, weight ratio). The diffraction peaks can be indexed to a hexagonal structure of ZnO with cell constants of a ¼ 3:24 and c ¼ 5:19 A( : No diffraction peaks produced from Cu and CuO could be found from the pattern, though there are some weak peaks from small amount of A: zinc that has not been oxidized and B: zinc nitride from the reaction of zinc and nitrogen in the air. Further structural characterization of the products was performed using TEM and SEM. Fig. 2 shows typical SEM images of the ZnO micro- and nanocrystals. When pure Zn was used as reactant, uniform sprout-like ZnO tetrapod microstructures were obtained (Fig. 2a). When Cu was added to the Zn with the weight ratio 2/8, bullet-like product with hexagonal cylinder microstructures was achieved (Fig. 2b). However, when more proportion of Cu was added, the ZnO with tetrapod micro- and nanostructure appeared (Fig. 2c Cu/Zn ¼ 4/6, Fig. 2d Cu/Zn ¼ 5/5, and Fig. 2e Cu/ Zn ¼ 6/4). The diameter of the arms in the tetrapod structure decreases with increasing Cu content. The thin tetrapods generally have the needle-shaped arms (Fig. 2d), while the thick ones have two kinds of uniform structures: special edge-like arms (Fig. 2c) and hexagonal cylinder arms (Fig. 2e and the inset), which are unique morphologies of ZnO nanocrystals compared to that reported previously [6]. Furthermore, while a further higher Cu proportion was used, microwires with a radiation flower (Fig. 2f, Cu/Zn ¼ 7/3) and chrysanthemum petal-like microcrystals (Fig. 2g, Cu/Zn ¼ 9/1) were obtained. High yield of ZnO nanowires can be produced by oxidation of melted brass with a commercial purity (41.51 wt% of Zn) using the same CGVA method. Fig. 3 shows the typical SEM and TEM images of these ZnO nanowires. Determined from TEM images (Fig. 3b), the diameter of the ZnO nanowires varied from 10 to 150 nm. The observed variation in diameter may be related to the inhomogeneous sizes of the nuclei of the oxide nanowires [7]. The lengths of these nanowires distributed from 3 to 10 um. The nanosheets and nanoscale tripods, as by-products, often appear in the products (Fig. 3a). XRD pattern confirms a hexagonal structure of ZnO (Fig. 3c). Some weak peaks from small amount of zinc and zinc nitride are also detected, same as those in Fig. 1. Understanding the growth mechanism will be important in order to control and design nanostructures. All experiments in this study were carried out in air atmosphere. It is apparent that the synthesis of ZnO nanostructures discussed here is based on the oxidation of thermal evaporation of Zn powder under the controlled conditions. The crystal growth may be dominated mainly by the following two growth ARTICLE IN PRESS Fig. 1. XRD pattern of the as-synthesized dendrite ZnO nanocrystals (Cu/Zn ¼ 4/6 weight ratio). Indices of the peaks are specified above the peaks ((A) peaks of purity of zinc, (B) peaks of purity of zinc nitride). 820 J. Ling et al. / Journal of Solid State Chemistry 178 (2005) 819–824

J. Ling et al. /Journal of Solid State Chemistry 178(2005)819-824 Xp nt of Cu/Zn= 6/4 and the experiment of brass 41.45 wt% of Zn)(as shown in Fig. 2e and Fig. 3b) though these two kinds of reactants have approximately the same proportion of Zn. We believe the difference of morphology of products is caused by the difference of alloy melting degree. Brass has higher alloy melting degree than the mixed powders of Cu/Zn and this leads to a difference of control in the releasing of metal gas and then the difference in the morphology of final products. Further detailed investigation is needed In addition, for the cases of Zn mixed with a large of diagram it is much difficult to form a liquid Cu-Zn alloy at this experimental temperature. We only collected 2m33 some radiation-flower-like microwires with a large diameter (Fig. 2f). This morphology often exists in the thesized in the 1 catalyst via a CVD method, which may be attributed to a catalyst-assisted VLS growth [101 We also collected some bubble-like compact crusts at the alumina boat after the reaction when Cu-Zn alloy was used as reactant. The compositional and morpho logical studies for the byproducts can help us to further understand the proposed CGVA method. XRD analysis indicates that the byproducts mainly consist of ZnO and little zn. Moreover. SEM examination shows a three- tiered structure of the compact oxide crust(Fig. 4a). The exterior of the compact crust was covered by a large amount of white Zno micropods with a uniform umm diameter about lum(Fig. 4b), possibly caused by the oxidation of little Zn vapor at the surface of brass. Fig 4c shows the morphology appearing in the middle layer f the compact crust, which consists of large Zno microcrystal particles with a regular shape. We believe that the growth of Zno nuclei from the oxidation of a liquid Zn layer at high temperature may result in these Zno microcrystals. Much Zn gas aggregates inside the 1500 compact oxide crust and forms the fungus-like mor- phology(Fig. 4d). Based on above experimental results it is reasonable to suppose an enrichment of Zn vapor yss % ahead So 5 o 6s the brass as reactan, we need to break the bubble-like content at the surface of brass and a slow releasing of Zn crust, which obstructs the supply of oxygen, appearing Fig3.(a)SEM. (b)TEM micrograph and(c)typical XRD pattern for at the surface at initial reaction stage to meet with the he as-synthesized product of Zno nanowires(A: peaks of purity of latter growth of ZnO. However, in the latter reaction, zinc, B: peaks of purity of zinc nitride). the presence of Cu seems helpful to cease the formation of the crust According to the TEM observation(Fig. 5), we find ome un-developed ZnO multiply twinned structure [ll] realizes the control of Zn vapor pressure and then which explicitly shows that the following growth strongly confines the growth of the finally synthesized process: each grain within the Zno multiply twin Zno with various sizes and shapes by appropriately particles develops to a thick branch(Fig 5a)at the adjusting vaporization ratio during the reactions. step and then forms three thin nanowires(Fig. 5b) by Furthermore, experiments results show that there is a the incorporation of atoms at a whisker side surface and ig difference of morphology of products of the the diffusion of atoms along the lateral surface confined

vapor generation. However, here the presence of Cu realizes the control of Zn vapor pressure and then strongly confines the growth of the finally synthesized ZnO with various sizes and shapes by appropriately adjusting vaporization ratio during the reactions. Furthermore, experiments results showthat there is a big difference of morphology of products of the experiment of Cu/Zn ¼ 6/4 and the experiment of brass (41.45 wt% of Zn) (as shown in Fig. 2e and Fig. 3b), though these two kinds of reactants have approximately the same proportion of Zn. We believe the difference of morphology of products is caused by the difference of alloy melting degree. Brass has higher alloy melting degree than the mixed powders of Cu/Zn and this leads to a difference of control in the releasing of metal gas and then the difference in the morphology of final products. Further detailed investigation is needed. In addition, for the cases of Zn mixed with a large proportion of Cu, according to Cu–Zn binary phase diagram it is much difficult to form a liquid Cu-Zn alloy at this experimental temperature. We only collected some radiation-flower-like microwires with a large diameter (Fig. 2f). This morphology often exists in the microwires or nanowires synthesized in the presence of catalyst via a CVD method, which may be attributed to a catalyst-assisted VLS growth [10]. We also collected some bubble-like compact crusts at the alumina boat after the reaction when Cu–Zn alloy was used as reactant. The compositional and morphological studies for the byproducts can help us to further understand the proposed CGVA method. XRD analysis indicates that the byproducts mainly consist of ZnO and little Zn. Moreover, SEM examination shows a threetiered structure of the compact oxide crust (Fig. 4a). The exterior of the compact crust was covered by a large amount of white ZnO micropods with a uniform diameter about 1um (Fig. 4b), possibly caused by the oxidation of little Zn vapor at the surface of brass. Fig. 4c shows the morphology appearing in the middle layer of the compact crust, which consists of large ZnO microcrystal particles with a regular shape. We believe that the growth of ZnO nuclei from the oxidation of a liquid Zn layer at high temperature may result in these ZnO microcrystals. Much Zn gas aggregates inside the compact oxide crust and forms the fungus-like morphology (Fig. 4d). Based on above experimental results, it is reasonable to suppose an enrichment of Zn vapor content at the surface of brass and a slowreleasing of Zn vapor at the reaction temperature. Actually, when using the brass as reactant, we need to break the bubble-like crust, which obstructs the supply of oxygen, appearing at the surface at initial reaction stage to meet with the latter growth of ZnO. However, in the latter reaction, the presence of Cu seems helpful to cease the formation of the crust. According to the TEM observation (Fig. 5), we find some un-developed ZnO multiply twinned structure [11], which explicitly shows that the following growth process: each grain within the ZnO multiply twinned particles develops to a thick branch (Fig. 5a) at the first step and then forms three thin nanowires (Fig. 5b) by the incorporation of atoms at a whisker side surface and the diffusion of atoms along the lateral surface confined ARTICLE IN PRESS Fig. 3. (a) SEM, (b) TEM micrograph and (c) typical XRD pattern for the as-synthesized product of ZnO nanowires (A: peaks of purity of zinc, B: peaks of purity of zinc nitride). 822 J. Ling et al. / Journal of Solid State Chemistry 178 (2005) 819–824

《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_Controllable growth of zinc oxide micro- and nanocrystals by oxidization of Zn–Cu alloy

《复合材料 Composites》课程教学资源（学习资料）第二章增强体_Controllable growth of zinc oxide micro- and nanocrystals by oxidization of Zn–Cu alloy