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.
Figure shows the representative TEM images of samples obtained as detailed in the experimental procedure (pyrrole 0.2960 g and CuOHCl 0.1995 g) at 150°C for various periods of time. These samples contain a large number of nanocables structures together with abundant globular formations. The high contrast of the nanocable cores suggested their metallic nature, which was confirmed by SAED of an isolated wire. This, together with powder X-ray diffraction of the bulk materials (Figure ) ruled out the possibility that the cores could be made of CuOHCl or copper oxides and confirmed the growth of face-centered cubic (fcc) copper. Two diffraction peaks of copper nanocables (Figure ) at 2θ
43.26°, 50.46° could be observed clearly from the XRD pattern, which could be indexed as (111) and (200) planes of fcc copper. It should also be remarked that the metallic copper core is single-crystalline in nature as showed by the SAED pattern. The SAED patterns of the marked region from Figure c are shown in bottom inset of that figure. The d
-spacing calculation was carried out using Equation 1.1, where d
is the d
-spacing, λ is the wavelength of the electron beam, L
is the camera length (in millimeters); the constant (λL
) is the camera constant which equals to 22.5 Å mm; and R
is radius of the diffraction pattern (in millimeters).
Figure 1 TEM image of Cu@PPy nanocables prepared by hydrothermal synthesis. (a-c) show different regions of the sample Cu@PPy nanocables that were prepared by hydrothermal synthesis of Py:CuOHCl, with molar ratio of 2:0.78 and period of 72 h at 150°C. (more ...)
Figure 2 X-ray diffraction patterns of the synthesized polypyrrole coated copper nanocables and copper chloride hydroxide powder. This figure shows the XRD patterns of the synthesized polypyrrole coated copper nanocables (top) and the copper chloride hydroxide (more ...)
Table shows the d-spacings calculated for the SAED pattern as shown in Figure c (inset). It can be seen that the d-spacings for copper fits well with the experimental data, which also agree with the XRD of Cu@PPy (Figure ).
Thed-spacings for the experimental data (calculated from SAED), copper (ICDD PDF No. 004–0836)
According to the calculation of electron diffraction spots in Figure c (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.
The Fourier transform infrared spectroscopy (FTIR) spectrum of the sample features the peaks characteristic of polypyrrole (Figure ). (peaks at 750–780 cm−1
and peak near 800 cm−1
assigned to N-H out of plane bending absorption). The band observed near 950 cm−1
is due to the C-H out of plane bending. The peaks near 1,035 cm−1
are due to the C
C stretching of aromatic compounds. The peak near 1,384 cm−1
corresponds to C-N stretching and C-C vibration. The prominent bands of polypyrrole structure are the aromatic ring vibrations at 1,571 cm−1
, C-N in plane deformation at 1,299 cm−1
and C-H in plane vibrations at 1,035 and 950 at 770 cm−1
]. The C
O band is observed at 1,675 cm−1
which can be assigned to the oxidized pyrrole rings in Cu@PPy. The broad band at 3,300 cm−1
is assigned to O-H stretching vibration. The frequency at 2,910 cm−1
refers to the stretching vibration of the C-H bond [17
]. Thus, we conclude that pyrrole was oxidized in situ
by CuOHCl, leading to the nanostructures formed by the reaction products, i.e., reduced metallic copper (single-crystalline) and oxidized polypyrrole. The remarkable finding is how this reaction under hydrothermal conditions leads to such a complex and well-ordered nanostructure through a simple synthesis.
FTIR-ATR spectra of Cu@PPy sample (I) and pure pyrrole sample (II).
The ultraviolet (UV)-vis spectra of the synthesized polypyrrole-coated copper nanocables at liquid phase (pH
6.4) (Figure ) show no absorption peaks corresponding to copper particles, which had a characteristic absorption peak at 580 nm [18
]. These results agree well with the experimental results obtained, since copper is always detected coated with polypyrrole. The Cu@PPy composites are normally settled down in the reaction vessel. Noteworthy, the UV–vis shows no peaks that could indicate the presence of Cu2+
-pyrrole complex, which in turn indicates that all the Cu2+
ions have reacted with pyrrole at the end of the preparation process.
The UV–vis absorption spectra of the synthesized Cu@PPy liquid phase.
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.
Concerning the size and shape of the Cu@PPy nanocables, Figure shows a few of them featuring Cu cores of 290
45 nm wide covered by shells of PPy approximately 130 nm wide. The length of these nanocables is difficult to asset in a statistically meaningful way. However, we have detected with TEM single nanocables as long as 85 μm (600 nm wide) and SEM images show even larger nanocables (Figure ).
SEM image of a sample obtained by hydrothermal synthesis of Py:CuOHCl. a: image of a sample obtained by hydrothermal synthesis of Py:CuOHCl with molar ratio of 2:0.78 and period of 72 h at 150°C, b: Lower magnification view of the sample.