Reaction mechanism of thermally-induced electric conduction of poly(vinyl alcohol)-silver nitrate hybrid films

The combination of metal nanoparticles and polymers endows their hybrid composites with a number of unique features and opens a new way to fabricate novel functional materials. In this study, poly(vinyl alcohol) (PVA) composite films containing different concentrations of silver nitrate (AgNO3) were fabricated by a solution casting method, and the interactions between silver ions and PVA molecules were investigated. The coordination of silver ions with the hydroxyl and carbonyl oxygens of PVA was observed in the Fourier transform infrared spectra (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses. This special coordination structure resulted in significant weight loss of the hybrid film upon heating to 169-190 degrees C, in which a redox reaction between PVA and AgNO3 occurred. This reaction generated nano-Ag particles, which caused a simultaneous sharp decrease in the electrical resistance of the films. Online techniques including FTIR, TG-FTIR, and X-ray diffraction (XRD) were employed to study the reaction processes during treatment with heat. Experimental results showed that PVA acted as a reductant, reduced AgNO3 to NO, N2O and silver. The amount of Ag nanoparticles generated from the reduction process increased with incremental increases in temperature and AgNO3 content. When the doping content of AgNO3 was higher than 30 wt%, the electrical resistance of the PVA/AgNO3 hybrid films sharply dropped by 4 or 5 orders of magnitude in less than one minute with the increase of temperature from 170 degrees C to 195 degrees C, which is due to an interconnected conduction pathway built by the neck formation of the particles. To further study the conductive mechanism of PVA/AgNO3 hybrid films when subjected to the thermal treatment process, the evolution of Ag nanoparticle microstructures was monitored by scanning electron microscopy (SEM). It was found that conductive paths were constructed from the migration and aggregation of Ag nanoparticles, which accounted for the sharp decrease in electrical resistance. The temperature sensitive PVA/AgNO3 films could have broad applications in temperature sensors or temperature trigger devices.