Copper nanocoils synthesized through solvothermal method

Recently helical nanostructures such as nanosprings and nanocoils have drawn great interests in nanotechnology, due to their unique morphologies and physical properties, and they may be potential building blocks in sorts of electromechanical, magnetic, photoelectronic and plasmonic devices at micro/nanoscales. In this report, multi-turns copper nanocoils were synthesized through a modified solvothermal method, in which the mixture of water and N-methyl-2-pyrrolidone (NMP) were selected as reaction medium and copolymer poly(1-vinylpyrrolidone-co-vinyl acetate) (PVP/VA 64E) as reductant. In the liquid solution, nanosprings could be formed from relaxed nanocoils and demonstrated high elasticity. These nanocoils and nanosprings are of single crystalline structure, with the characteristics wire diameters ranging from tens to a few hundreds of nanometers and the ring/coil diameters mostly ~10–35 microns. Their growth and deformation mechanisms were then investigated and discussed along with that of previously reported single-turn copper nanorings. This work could be of importance for researchers working on synthesis and applications of novel 1-D helical nanomaterials and their functional devices.


Suitable depositing and preserving of copper sample and destructive result in general washing process.
Figure. S1. Suitable and unsuitable treatments of copper samples. (a) Stock solutions (top layers) and NaCl solutions (bottom layers) before, in and after the suggested depositing treatment (in test tubes from left to right). Copper samples can be found on bottoms of latter test tubes (pointed by black arrows); (b) A strand of twisted copper nanowires and nanocoils formed in general washing and cleaning process.
General washing and cleaning operation using distilled water and various alcohols will introduce great disturbance and inevitably damage as-prepared nanostructures by intensely twisted them into "knitting wool", which is result of turbulent flow and is driven by density (of salts and polymer) gradients in mixed solutions. This universal effect did not affect normal nanowires seriously in most occasions, supersonic oscillating can unravel the "knitting wool" of nanowires almost nondestructively. However it may affect suspended nanocoils and nanorings seriously, and possibly damage them.
The hydrazine in preserving copper nanocoils: The corrosion of copper in moisture circumstance follows such reaction function: 2Cu + O 2 +H 2 O+CO 2 = Cu 2 (OH) 2 CO 3 (1) We believe that the effect of hydrazine in keeping the copper sample is not reducing Cu 2 (OH) 2 CO 3 to elemental Cu, but simply blocking the oxidation path from Cu to Cu 2+ in water before the formation Cu 2 (OH) 2 CO 3.
The choosing of hydrazine is based on results of series of control experiments. The hydrazine cannot totally prevent these copper nanostructures from corrosion, but it indeed greatly reduced the corrosion rate.
Without the protection of hydrazine, copper samples deposited on bottom of normal test-tube could be corroded within short period (such as a half day). However, with the protection of hydrazine, most of our copper nanocoils can still keep their circular shape in water even after 1-2 months, which is enough for most research purposes.
For SEM and TEM observations, the samples should be carefully washed with distilled water and the hydrazine shall be washed away. The dry state and vacuum environment, instead, can preserve the nanocoils from oxidation/corrosion, so in our SEM and TEM characterizations, interference of hydrazine can be essentially ruled out. Statistics analysis is based on measurements of ~100 nanocoils recorded in our micrographs, in which samples were dispersed on copper grids or glass substrates for SEM/TEM//optical imaging. We believe that in untreated stock solutions, more nanocoils can be found, because sample cleaning and transferring processes will inevitably damage many nanocoils. From data shown, ring diameters of nanocoils are mainly in the range of 10-35μm.

Diameter distribution of nanocoils
3. Evidences of end-closed nanocoils. Nanobelts with circular midpieces are another common bent structure in stock solution.
Their widths make a multi-turns structure rather difficult. These unclosed structures show stability to mild external disturbance, the bent midpieces and straight ends might suggest the different residual inner stresses.

Figure S5. Optical images of one bent nanowire/nanobelt with a non-uniform edge.
A transition stage from nanowires to nanobelts. Triangle plates along the nanowires reveal that the surfaces of some nanobelts should be {111} crystal faces.