Synthesis of copper micro-rods with layered nano-structure by thermal decomposition of the coordination complex Cu(BTA)2
© Qu et al.; licensee Springer. 2015
Received: 10 December 2014
Accepted: 19 January 2015
Published: 5 February 2015
Porous metallic copper was successfully prepared by a simple thermal decomposition strategy. A coordination compound of Cu(BTA)2 with the morphology of micro-rod crystal was synthesized as the precursor. The precursor to copper transformation was performed and annealed at 600°C with the shape preserved. The copper micro-rods are assembled from unique thin lamellar layers, each with the thickness of approximately 200 nm and nano-pores of approximately 20 to 100 nm. This morphology is highly related to the crystal structure of the precursor. The mechanism of the morphology formation is proposed, which would be able to offer a guideline toward porous metals with controllable macro/micro/nano-structures by the precursor crystal growth and design.
Porous metallic materials have become a burgeoning field in both applied technology and basic scientific research, especially for their significant thermal or electron conductivity, catalysis properties, importance in interface engineering, energy industry, and biomedical applications [1-4]. During the past two decades, the synthesis methods of porous metals evolved with the development of nano-science and nano-technology. Sol-gel, dealloying, and soft-template methods are typical synthetic strategies [5-8]. Among the periodic table of elements, metals of silver, nickel, copper, palladium, ruthenium, titanium, and platinum have been intensively investigated for their porous foam. For example, Walsh et al. used dextran as a sacrificial template to fabricate silver sponges, Yamauchi and co-workers prepared mesoporous nickel by an electroless deposition method in the presence of lyotropic liquid crystals, and Kuroda et al. reported 2D hexagonally ordered mesoporous metals (Ru, Pt, or Pd) by dissolving silica replica [9-11]. In addition to these methods, the combustion technology is a general method to synthesize various porous metals and oxides [12-14]. Compared with porous noble metals [15-17], porous copper was less focused, possibly for its high reactivity of oxidization in ambient atmosphere [18-26].
Materials and equipment
All chemicals were purchased from commercial sources (Sigma-Aldrich, St. Louis, MO, USA) and were used without further purification. Single-crystal X-ray diffraction measurements were carried out on a Bruker SMART APEX CCD (Bruker AXS, Inc., Madison, WI, USA). Thermogravimetric analyses (TGA) were measured on a simultaneous SDT 2960 thermal analyzer with a heating rate of 20°C∙min−1 under N2 atmosphere. Powder X-ray diffraction (XRD) patterns were collected using a Bruker D8 ADVANCE X-ray diffractometer equipped with Cu-Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. Scanning electron microscopy (SEM) characterizations were performed on a Hitachi S-4800 SEM (Hitachi, Ltd, Chiyoda-ku, Japan), equipped with energy-dispersive X-ray spectroscopy (EDS). The transmission electron microscopy (TEM) images were obtained from JEOL JEM-2100 (JEOL Ltd., Akishima-shi, Japan) operating at 200 kV. The electronic semiconducting property f the samples was recorded by the semiconductor device analyzer B1500A from Agilent Technologies (Santa Clara, CA, USA). Elemental analysis is measured by Elementar vario MICRO (Hanau, Germany). The Fourier transform infrared (FTIR) spectrum was measured by a VECTOR 22 spectrometer with KBr pellets (Bruker AXS, Inc., Madison, WI, USA).
A mixture of CuCl2 · 2H2O (AR, 0.1 mmol), BTA (99%, 0.1 mmol), and NH3 · H2O (AR, 25 to 28 wt%, 1.0 ml) in CH3CN (AR, 5 ml) was sealed in a Teflon-lined stainless steel autoclave and heated to 100°C under autogenous pressure for 48 h. When cooled to room temperature, blue rodlike Cu(BTA)2 crystals were isolated. They were rinsed with CH3CN and dried in vacuum at 60°C overnight. The Cu(BTA)2 was decomposed and annealed in an Ar flow atmosphere at 600°C to yield the porous metallic copper.
Results and discussion
Single-crystal X-ray diffraction reveals that the crystal structure of the precursor belongs to the monoclinic space group C2/c (Figure 1). The mononuclear complex contains one four-coordinated Cu(II) cation and two BTA anions with the formula Cu(BTA)2. Each of the BTA ligands provides two donor N atoms to coordinate to the Cu(II) ions, exhibiting a distorted tetrahedron coordination geometry (Additional file 1: Figure S1). The Cu-N bond lengths for each ligand are 1.954 and 1.969 Å, and the dihedral angle of two BTA planes is 36.35°. The CCDC no. 1035211 contains the supplementary crystallographic data for the Cu(BTA)2 complex (Additional file 2).
The XRD measurement reveals that after the decomposition at 600°C, Cu(BTA)2 completely transformed into metallic copper. The diffraction peaks (Figure 2b) can be indexed to (111), (200), and (220) crystal planes of fcc copper (pdf #88-1326). The original morphology of the Cu(BTA)2 crystals was largely inherited by the metallic copper, showing a short rodlike shape, while at the same time, remarkable size shrinkage (up to approximately 90% volume shrinkage) is observed for the metallic copper compared with the Cu(BTA)2 crystal precursor.
In summary, we report a thermal decomposition method to prepare copper micro-rods with layered porous structure for the first time, by using a well-designed coordination compound of Cu(BTA)2 crystal as the precursor. The shape of the crystals was preserved for the copper product. This allows us to obtain copper with various morphologies by the growth of the precursor crystals of different sizes and shapes, without changing the molecule itself. Moreover, the layered nano-structure is highly related to the crystal parameters of the precursor. It could be expected that the same precursor crystallizes in different crystalline spaces; accordingly, the micro/nano-structure would be tuned.
This work was supported by the Major State Basic Research Development Program of China (Grant Nos. 2013CB922102 and 2011CB808704), the National Natural Science Foundation of China (Grant Nos. 91022031 and 21301089), Jiangsu Province Science Foundation for Youths (BK20130562), and the Natural Science Foundation of Jiangsu Province (BK20130054).
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