© Suarez-Guevara et al.; licensee Springer. 2012
Received: 6 July 2012
Accepted: 5 September 2012
Published: 25 September 2012
A simple hydrothermal redox reaction between microcrystalline CuOHCl and pyrrole leads to the isolation of striking nanostructures formed by polypyrrole-coated copper nanocables. These multicomponent cables that feature single-crystalline face-centered cubic Cu cores (ca. 300 nm wide and up to 200 μm long) are smoothly coated by conducting polypyrrole, which in addition to its functionality, offers protection against oxidation of the metal core.
Nanotubes and nanowires are two of the most striking objects being developed by nanotechnologists. Their synthesis and properties have been studied and, to a certain extent, mastered, in just the last two decades.
Many different types of materials have been fabricated in the form of nanowires from metals to oxides, other chalcogenides, and inorganic phases , as well as carbon or even polymeric materials . However, the fabrication of multicomponent or core-shell nanowires is a more complex task. This has not prevented the pioneering attempts to build heterostructured nanowires of increasing complexity, since the promises of these materials are high. Notable examples of these efforts are the growth of coaxial (p-i-n) silicon nanowires for solar cells and nanoelectronic power systems, [3, 4] the formation of atomic metal nanowires inside carbon nanotubes including Mo [5, 6] or Cu [7, 8], or the electrochemical growth of metals inside TiO2 nanotubes . In all of these cases, very valuable complex nanomaterials were produced which were nevertheless very elaborate to fabricate.
Along the path to complex materials, a double challenge remains: to prepare more and more complex materials and to do it through simpler and simpler methods. In great contrast with costly physical methods like chemical vapor deposition or molecular beam epitaxy, the bottom-up approach typically associated to the chemical methods provides many opportunities for the development of high-throughput synthesis of nanostructured materials. But as the complexity of the target materials increase, chemical methods tend to be limited by entropy. Fortunately, specific molecular interactions come to the rescue in the form of what we call self-organization or self-assembly.
In our group we have explored the development of complex hybrid materials through this type of approach . Specifically, we have recently carried out synthetic work of hybrids made of silver and biopolymers through matrix chemistry , as well as silver and conducting polymers through simple hydrothermal reactions [12, 13]. In this way we have recently reported the isolation of ordered Ag@PPy structures we dubbed as ‘nanosnakes’ . Silver or gold are noble metals that are very stable in their metallic form. On the other hand, cheaper but more reactive copper nanostructures are prone to oxidation in air/aqueous solutions. Protection of copper nanoparticles or nanostructures with polymers is an obvious approach to their stabilization. Furthermore, the use of multifunctional conducting polymers for this task could lead to novel materials with built-in multifunctionality. However, attempts to build this type of hybrid materials have been limited to the previous synthesis of Cu nanoparticles followed by coating with PPy which is prepared subsequently and limited to the coating of Cu nanoparticles [14, 15].
We report here the first synthesis of polypyrrole-coated copper nanowires (Cu@PPy), which, in great contrast with the mentioned silver nanosnakes, are formed not by attached nanoplates but by truly nanometric rods of metallic copper coated with polypyrrole forming wires of a few hundred nanometers in diameter and lengths up to a few hundred microns. Furthermore, these coated nanocables are prepared in a single reaction step where the simultaneous oxidation of pyrrole and reduction of copper precursor takes place.
Pyrrole (Sigma-Aldrich Corporation, St. Louis, MO, USA) was vacuum-distilled prior to its use. Greenish CuOHCl (J. T. Baker Chemical Company Phillipsburg, NJ, USA) was confirmed through powder X-ray diffraction and was used as received.
In a typical synthesis, 20 mL of deionized water and 0.2 g of CuOHCl were placed into a 25 mL screw-capped Pyrex bottle. The resulting suspension was stirred for 10 min. Then 0.3 g of pyrrole was added and the mixture was stirred for 10 more minutes. The bottle was then tightly closed with a Teflon screw-cap and heated at 150°C for different times. After reaction the resulting black suspensions were filtered through 0.8 μ cellulose acetate filters; the black solid was washed with deionized water and dried at 50°C.
Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEM-1210 by JEOL Ltd., Tokyo, Japan at 120 kV. Scanning electron microscope (SEM) images were taken with a Quanta 200 environmental scanning electron microscope using field-emission gun by FEI (FEI Company, Hillsboro, OR, USA). X-ray diffraction (XRD) analyses were performed with a Siemens D5000 diffractometer (Siemens AG, Munich, Germany) (λ = 1.54056). The FTIR spectra of all the samples were measured with a PerkinElmer model Spectrum One spectrometer (PerkinElmer Inc., Waltham, MA, USA) connecting with attenuated total reflectance accessory. UV-visible spectra were recorded using Cary 5 (Varian Medical Systems Inc., Palo Alto, CA, USA) UV–vis-NIR high-resolution optical spectrophotometer.
Results and discussion
The key conditions for getting these extraordinary self-assembled nanostructures include the use of hydrothermal conditions (under equilibrium at high temperatures) plus carrying out heterogeneous reactions between an oxidizing solid and a monomer solution prone to oxidative polymerization. Thus, in the present case we used solid Cu(OH)Cl to oxidize pyrrole in an aqueous solution at 150°C under hydrothermal autogenous pressure. The synthesis procedure is described in detail in the experimental section.
The d -spacings for the experimental data (calculated from SAED), copper (ICDD PDF No. 004–0836)
<1 1 1>
<2 0 0>
<2 2 0>
According to the calculation of electron diffraction spots in Figure 1c (inset), it was revealed that the crystal planes spacing of the inner core region were 0.21 nm, 0.187, and 0.125 which corresponded to the crystal plane distances of the main diffraction peaks of copper; the crystal planes are (111), (200), and (220), respectively.
It should be noted that PPy coats these nanocables very smoothly, but also, that excess PPy forms the conspicuous globular formations. The excess PPy in the form of nanospheres is consistent with the initial composition of the mixture 2:0.78 Py:Cu. We tried to adjust this ratio to optimize the volume fraction of nanocables but found that a 1:1 Py:Cu ratio would not lead to the formation of nanocables. The optimization of synthetic procedures and the isolation of pure nanocables will be the subject of future work.
The reaction of Copper(I) hydroxochloride with pyrrole under hydrothermal conditions leads to the reduction of the former to metallic Cu and the oxidative polymerization of pyrrole. Interestingly, under the conditions used, both solids grow together forming long PPy-coated nanocables, i.e., Cu@PPy nanocables featuring single-crystal fcc Cu cores. The fascinating possibilities for further work on these nanostructures are multiple. They could be used, for instance, as precursors for the fabrication of carbon-coated copper nanowires, which have been targeted as useful materials for a variety of applications . But in addition to that possibility, these nanocables would deserve their own attention. Since they are formed by a metallic-conducting core and a p-doped conducting polymer coating, the first intriguing question arises as to what special conductivity and electron-transfer properties these nanocables could have. Their quantum-size effects, the nanoelectrochemical and sensing performance of the materials, as well as the prospective research on synthesis of other related nanostructures by the same simple method, all will deserve and surely get due attention.
JSG is a Ph. D. student working under the supervision of PGR. OA is a post-doctorate researcher in the NEO-Energy group. He received his Ph. D. in Materials Science from Barcelona University last 2011. Currently, he is working on cathode nanomaterials for energy conversion and storage, nanoscale coating, and safety enhancement of the Li-ion batteries. PGR is a full research professor and group leader of the NEO-Energy group. He is also the vice-director of MATGAS Research Center and presently leading its research projects on energy-related materials and devices, Li batteries, supercapacitors, fuel cells, and solar energy.
Partial Funding for this research was made possible by a grant from the Spanish Ministry of Science and Innovation (MICINN) (MAT2011-28931). JSG gratefully acknowledges a JAE Predoc Fellowship from CSIC (Spain). We acknowledge the support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).
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