Preparation of Palladium/Silver-Coated Polyimide Nanotubes: Flexible, Electrically Conductive Fibers

A simple and practical method for coating palladium/silver nanoparticles on polyimide (PI) nanotubes is developed. The key steps involved in the process are silver ion exchange/reduction and displacement reactions between silver and palladium ions. With the addition of silver, the conductivity of the PI nanotubes is greatly enhanced. Further, the polyimide nanotubes with a dense, homogeneous coating of palladium nanoparticles remain flexible after heat treatment and show the possibility for use as highly efficient catalysts. The approach developed here is applicable for coating various noble metals on a wide range of polymer matrices, and can be used for obtaining polyimide nanotubes with metal loaded on both the inner and outer surface.

In three neck flasks, 7.4 g polyetherimide was added in 30 mL of NMP and stirred to get clear solution at 60℃ water baths.

Preparation of PAA/PEI Coaxial Nanofiber
Coaxial needle was used to synthesize electrospun coaxial nanofibers, 20 wt % of PEI/NMP solution was linked to inner needle while PAA/DMF to outer. Electrospun run for 10 h under 17.2~17.4 KV and 1094 r/min. A white film was obtained.

Preparation of PI Nanotube Coated by Palladium Nanoparticles
The homogeneously palladium nanoparticles coated nanofibers could not easily got by directly metallization. As we mentioned in the commucatuion, "direct metallization with solution containing palladium ions corrode the surface of PAA nanofibers, resulting in a decrease in the number of attachment sites. The high concentration solution containing palladium ions even destroyed the PAA nanofibers." Figure S3. Corrosion of the nanofiber with the method of direct metallization with solution containing palladium ions PAA/PEI film was immersed into 0.1 M AgNO3 solution for 5 min, cleaned by ultrasonic cleaning for 2 min. Afterwards, the film was immersed into 0.01 M DMAB solution for 5 min and cleaned for 2 min. The above steps were repeated until the film presented black color. Then the black film was immersed into 0.1 M PdCl2 solution at 30 °C for 5 h. Firstly, 20 mL NMP was used to remove the PEI inner layer. The PEI inner layer was dissolved after 2 days immersing.
Then, PAA was convert into PI by heat treatment. The temperature was 300℃ and heating rate was 4℃/min, retain the temperature for 2hours. Then slow-air cooling overnight. Figure S4 shows the reaction in the cyclization reaction. Figure S5. Preparation scheme of polyimide. The result correspond to the table is showed in Figure S6.

Characterizations
ATR-FTIR spectra were collected using the thermal fisher Nexus 670 IR spectrometer Surface morphology and elemental analysis of the nanofibers was performed on a Zeiss supra 55 field emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive spectrometer produced by Oxford Inc. X-ray diffraction (XRD) experiments were conducted on a Rigaku RINT2200V/PC X-ray diffractometer using CuKα radiation (λ=1.54178) at an accelerating voltage of 40 kV and current of 40 mA.

Catalytic Activity Measurement
Electrochemical measurements were carried out in a three electrode cell at room temperature, chronoamperometric (CA) curves of 3×10 −5 M uric acid + 0.1 M H2SO4 , vs. Hg/Hg2Cl2 reference.
The results of cyclic voltammetry (CV) experiments carried out on a pure polyimide film in uric acid (Figure 8(b)) show a maximum current density of −5.7 mA·cm −2 . Qualitatively similar behavior is observed with the silver-coated polyimide film but with an increased maximum current density of −9.8 mA·cm −2 . Further, within the addition of palladium, a new peak at 0.52 V in the negative sweep appears alongside the silver film current density maximum at 0.65 V, indicating the presence of new redox pathways. Consequently, we note that the silver and palladium nanoparticles play separate but important roles in improving the conductivity and electrocatalytic performance of the Pd/Ag polyimide nanotube films. These results identify our films as excellent organic-inorganic composite material with electrical conductivity and electrocatalytic properties.