CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2546–2548
Communications Facile Fabrication of Porous CuS Nanotubes Using Well-Aligned [Cu(tu)]Cl · 1/2H2O Nanowire Precursors as Self-Sacrificial Templates Jianfei Mao,† Qian Shu,† Yaqiong Wen,‡ Hongyan Yuan,‡ Dan Xiao,*,†,‡ and Martin M. F. Choi*,§ College of Chemistry, and College of Chemical Engineering, Sichuan UniVersity, Chengdu, 610064, P.R. China, and Department of Chemistry, Hong Kong Baptist UniVersity, Kowloon Tong, Hong Kong SAR, P. R. China ReceiVed June 10, 2008; ReVised Manuscript ReceiVed April 29, 2009 嘷 w This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal.
ABSTRACT: We have demonstrated for the first time an extremely facile procedure for the preparation of porous CuS nanotubes with good morphological and high chemical purity using [Cu(tu)]Cl · 1/2H2O nanowire precursors as self-sacrificial templates at room temperature (thiourea ) tu). Well-organized [Cu(tu)]Cl · 1/2H2O nanowires with good morphological and high chemical purities were prepared by directly mixing an aqueous solution of CuCl2 with thiourea at room temperature. Well-aligned [Cu(tu)]Cl · 1/2H2O nanowires were self-assembled into dandelion- or thornlike architectures depending on the initial concentrations of thiourea and CuCl2. The influence of the architectures of the nanowire precursors on the morphology of the CuS nanotubes product is discussed. The successful fabrication of nanoarchitectures from nanowires via self-sacrificial templates provides an efficient strategy for controllable preparation of hollow nanostructures. One-dimensional (1-D) nanomaterials such as nanorods, nanowires, nanobelts, and nanotubes have been the subject of extensive research because of their salient special properties and potential applications as compared to their bulk and zero-dimensional (0-D) counterparts.1 In particular, since the discovery of carbon nanotubes,2 increasing interest has been placed on 1-D nanotubes possessing ultra length-diameter ratios and hollow structures; thus, a variety of tubular nanostructures have been prepared using various synthetic techniques.3 Among them, self-sacrificial template from self-assembled nanostructures4 is an ideal synthetic strategy to fabricate desired hollow nanostructures with an interior space. This strategy offers the advantages of benign fabrication conditions, lowcost, large-scale, and potential for industrial-scale production.5 Copper(II) sulfide, one of the important semiconductor transition metal chalcogenides, shows superconductivity at low temperatures and has potential applications in sensor, solar energy conversion, cathode material, catalysis, and nonlinear optical material.6 Hitherto, CuS nanomaterials with different morphologies such as nanowires,7 nanotubes,8 and nanoparticles have been successfully prepared via wet solution methods. For instance, Nicolini et al. directly synthesized CuS nanoparticles in the polymeric matrix using the carboxylic groups of the polymer as nucleation centers and H2S as sulfur sources.9 Small organic molecules slowly releasing S2- by * To whom correspondence should be addressed. E-mail:
[email protected] (D.X.),
[email protected] (M.M.F.C.). † College of Chemistry, Sichuan University. ‡ College of Chemical Engineering, Sichuan University. § Hong Kong Baptist University.
hydrolysis are especially useful in the controllable preparation of CuS nanomaterials for its special chemical properties and selfassembling functions.10 For example, Ying et al. fabricated hierarchical CuS nanoparticles employing CuCl2 and thioacetamide (TAA) as the starting materials through an ionic liquid-assisted route at room temperature.11 Armelao et al. synthesized CuS nanoparticles by reaction of thioacetic acid with water and copper carboxylates in the corresponding carboxylic acid solvent.12 Qian et al. selectively synthesized CuS nanotubes and hollow nanospheres through the precitation reaction of CuCl2, thiourea (tu), and NaHCO3 from rodlike thiourea-copper(II) complex precursors and thiourea-copper(II) solid spheres, respectively.8a These thought-provoking works inspired us to explore a facile method to prepare CuS nanotubes at low-cost and large-scale under ambient conditions. Herein, we report the large-scale and low-cost synthesis of uniform CuS tubular nanostructures at ambient conditions from dandelion- and thornlike architectures of [Cu(tu)]Cl · 1/2H2O nanowires. Ultralong and uniform porous CuS nanotubes composed of smallnanocrystalswereconvenientlyfabricatedusingthe[Cu(tu)]Cl · 1/ 2H2O precursors as self-sacrificed templates. The polycrystalline and porous CuS tubular nanostructures possess high surface-tovolume ratio and may exhibit more superior application performance. In brief, well-organized [Cu(tu)]Cl · 1/2H2O nanowires with good morphological and high chemical purities were first prepared by mixing an aqueous solution of CuCl2 with tu at room temperature. Figure 1 shows the scanning electron microscopy (SEM) images of the dandelion- and thornlike [Cu(tu)]Cl · 1/2H2O nanowires prepared from various concentrations of CuCl2 and tu. The
10.1021/cg8006052 CCC: $40.75 2009 American Chemical Society Published on Web 05/11/2009
Communications
Figure 1. (a) Dandelion-like architecture of [Cu(tu)]Cl · 1/2H2O nanowires prepared from 0.01 M tu and 0.005 M CuCl2. (b) Thorn-like architecture of [Cu(tu)]Cl · 1/2H2O nanowires prepared from 0.1 M tu and 0.05 M CuCl2. (a1) and (b1) are the low magnified images. The high magnified images (a2) and (b2) show the morphology of [Cu(tu)]Cl · 1/2H2O nanowires and are magnification of the areas marked in white rectangles in (a1) and (b1), respectively. The insets of (a2) and (b2) display the overall image of dandelion- and thornlike architectures.
addition of tu to CuCl2 solution causes the reduction of Cu2+ to Cu+, resulting in the formation of [Cu(tu)]Cl · 1/2H2O nanowires as illustrated in Scheme S1, Supporting Information.13 Figure S1, Supporting Information displays the diffraction patterns of the [Cu(tu)]Cl · 1/2H2O nanowires that are well indexed to the monoclinic phase of [Cu(tu)]Cl · 1/2H2O (JCPDS No. 53-0121). No peaks of impurity were observed. The IR spectrum also demonstrates the nanowires as [Cu(tu)]Cl · 1/ 2H2O structure (Figure S2, Supporting Information).10a Well-aligned [Cu(tu)]Cl · 1/2H2O nanowires were assembled into dandelion- or thornlike architectures depending on the initial concentrations of tu and CuCl2 (Figure 1). At low concentrations of 0.01 M tu and 0.005 M CuCl2, uniform smooth two-dimensional (2-D) dandelionlike architectures of well-aligned [Cu(tu)]Cl · 1/2H2O nanowires of about 100 ( 20 nm in diameter and 0.8 ( 0.3 mm in length were obtained as shown in Figure 1a and Figure S3, Supporting Information. When higher concentrations of 0.1 M tu and 0.05 M Cu2+ were mixed, thornlike architecture of [Cu(tu)]Cl · 1/2H2O nanowires of about 100 ( 30 nm in diameter and 200 ( 100 µm in length were obtained. The surfaces of both types of nanowires are smooth as shown in the SEM images. However, the morphology and size of these two self-assembling architectures are quite different. First, at lower concentration of reactants, the nanowires self-assemble into a well-dispersed flat 2-D dandelion-like architecture, while their higher concentration counterparts exhibit a threedimensional (3-D) thornlike architecture with closely stacking nanowires. In addition, these two kinds of nanowires differ greatly in length (0.8 versus 0.20 mm). Splitting growth mechanism is possibly responsible for the formation of the dandelion- and thornlike [Cu(tu)]Cl · 1/2H2O nanowires.14 In a low reactant concentration solution (0.01 M tu and 0.005 M CuCl2), nucleation happens slowly and sparsely, and the nanowires just simply grow longer after the nucleation. Thus, the nanowires exhibit a well-dispersed flat 2-D dandelion-like architecture. While at higher concentration (0.1 M tu and 0.05 M Cu2+), as there is a homogeneous nucleus burst at the nucleation state in a short period of time, a high density of defects may occur on these nanowires.15 On the other hand, after the dense primarily nucleation, the residual reactant concentration in the solution is still relative high. The defect sites may induce new nucleation in the vicinity of the nanowires. When the nanowires spread out fast to further react with reactants,16 heterogeneous nucleation may be promoted over homogeneous growth of the supersaturated crystal material and
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Figure 2. CuS nanotubes prepared from architectures of (a) dandelionand (b) thornlike [Cu(tu)]Cl · 1/2H2O nanowires under low (a1 and b1) and high (a2 and b2) magnifications. The high magnified images are the magnification of the areas marked by white rectangles.
giving rise to new nucleation on the primary branch crystal. Thus, many close stacked and branched nanowires form as shown in Figure 1b.17 Because of the dense nucleation and fast crystal growth, the reactants in solution are depleted faster; thus, the growth process is suspended resulting in shorter length of the branch crystals. In addition, as the direction of growth of the branches is less confined, the nanowires prefer orientations more evenly, resulting in thornlike 3-D structures. The growth of the thornlike nanowire architecture can be viewed in the accompanying video clip in MPG format. At concentrations below 10 mM tu and 5 mM Cu2+, crystals can hardly be observed as supersaturation is too low to overcome the free energy of nucleation. Nanowires prepared with different molar ratios of initial reactants were also investigated. Excess CuCl2 could disassemble the nanowires architectures, while excess tu could induce a slower growth rate. When the molar ratio of tu:Cu2+ was above 4, no nanowires were obtained. The as-prepared [Cu(tu)]Cl · 1/2H2O nanowire precursors were used to prepare the black porous CuS nanotubes in alkaline aqueous solution under ambient conditions. Figure 2 displays the SEM images of the CuS nanotubes prepared from dandelion- and thornlike [Cu(tu)]Cl · 1/2H2O nanowires. The reaction mechanisms are depicted in Scheme S2, Supporting Information.5a,13b,18 Figure S4, Supporting Information shows the X-ray diffraction (XRD) patterns of the as-synthesized black CuS nanotubes, which confirms the hexagonal phase crystalline structure (JCPDS No. 06-0464). No characteristic peaks of impurities such as Cu, Cu2S, Cu2O, and CuO were observed. The porous CuS nanotubes composed of numerous small dense nanoparticles are about 10 nm smaller in diameter than their precursors, as shown in Figure 2. The transmission electron micrograph (TEM) and selected area electron diffraction (SAED) pattern of the porous CuS nanotubes are shown in Figure S5, Supporting Information, clearly confirming the polycrystalline and porous hollow structure of CuS nanotubes. The [Cu(tu)]Cl · 1/2H2O nanowires acting as a self-sacrificial template provide both copper and sulfur sources to produce CuS nanotubes. The change of nanowires into nanotubes is attributed to the crystallization of CuS particles on the surface of precursor nanowires as depicted in Scheme 1, which is similar to the fabrication of micrometer-sized hierarchical tubular CuS with a larger diameter of 5-7 µm using an in situ formed Cu(I)-TAA complex as a self-sacrificial template.5a The etching or dissolution of [Cu(tu)]Cl · 1/2H2O by tu decomposition starts at the surface of the solid rectangular nanowires and continues toward the interior. Subsequently, CuS nanocrystals crystallized on the surface of the nanowires. The reaction rate is much faster on the corner than on the surface of the rectangle. In the subsequent growth process, the [Cu(tu)]Cl · 1/2H2O complex continues to be consumed. The limited
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Scheme 1. Growth Mechanisms of the Hierarchical Nanotubular Structures from Steps 1-3 Using (a) Dandelion- and (b) Thornlike [Cu(tu)]Cl · 1/2H2O Nanowires
solubility of CuS in the water solvent together with the strong interparticle interactions force the CuS nanocrystals to orderly align on the surface of nanowires.5a Thus, the smooth solid rectangular nanowires gradually convert into rough porous nanowires and finally to completely porous hollow nanotubes as shown in Figure 2.19 The architecture of the nanowire precursors can influence the morphology of CuS nanotubes. As shown in Figure 2a and Scheme 1a, when a better dispersed dandelion-like nanowire precursor is used, the CuS product is regular round porous nanotubes. Instead, although its thornlike counterpart also forms a porous nanotube structure, three or more of these tubes are fused together as a single pillar as depicted in Figure 2b and Scheme 1b due to the close packing architectural structure of the nanowires (Figure 1b). As such, the morphology of the template precursor can determine the morphology of the resulting CuS nanotubes products. In conclusion, we have demonstrated an extremely facile procedure for the preparation of ultralong porous CuS nanotubes with good morphological and high chemical purity using shapecontrolled architectures of [Cu(tu)]Cl · 1/2H2O nanowire precursors as self-sacrificial templates. The successful fabrication of nanoarchitectures from nanowires via self-sacrificial templates provides an efficient strategy for controllable preparation of hollow nanostructures. This method also provides new insights into the mineralization and biomimetic growth of inorganic solids at nanoand micrometer levels.
Acknowledgment. We express our sincere thanks to Mr. Juntao Liu, Mr. Maoxun Zeng, Mr. Yong Jin, and Prof. Fang Li for taking the SEM images, TEM image, XRD patterns, and IR spectrum, respectively. Financial support from the National Science Foundation of China (Nos. 20575042 and 20775050) and the Innovation Foundation of Sichuan University (No. 2006G006) is gratefully acknowledged. Supporting Information Available: Experimental and characterization data including XRD, IR, SEM, TEM, and SAED. This information is available free of charge via the Internet at http:// pubs.acs.org.
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CG8006052
