Variable-range-hopping conduction and the Poole-Frenkel effect in a copper polyaniline vermiculite intercalation compound

Abstract

The physical properties of the copper polyaniline vermiculite intercalation compound [Cu(PANI)-VIC] have been studied by x-ray diffraction, electrical resistivity, and dc-magnetic-susceptibility measurements. Aniline is intercalated into the interlamellar space of Cu-VIC, where the ${\mathrm{Cu}}^{2+}$ ions act as oxidation centers for the polymerization of aniline, forming an insulator of polyaniline. A conductor is achieved by protonation via exposure to HCl gas. Cu(PANI)-VIC exhibits the following electrical properties: (i) ln(\ensuremath{\rho})\ensuremath{\approxeq}1/${\mathit{T}}^{\mathrm{\ensuremath{\mu}}}$, where \ensuremath{\mu}\ensuremath{\approxeq}1/3, suggesting a two-dimensional variable-range-hopping conduction mechanism in the range of 50 K\ensuremath{\le}T\ensuremath{\le}120 K; (ii) the Poole-Frenkel effect is observed in the range of 50 K\ensuremath{\le}T\ensuremath{\le}90 K, since, in an electric field F, ln(\ensuremath{\sigma}) is proportional to ${\mathit{F}}^{1/2}$. The dc magnetic susceptibility obeys the Curie-Weiss law above 150 K, with a Curie-Weiss temperature of FTHETA=12 K. The polymeric unit cell in Cu(PANI)-VIC contains one ${\mathrm{Cu}}^{2+}$ ion per two-ring protonated polyaniline, which may orient along the diagonal in the in-plane rectangular structure (a\ifmmode\times\else\texttimes\fi{}b) with a=5.33 A\r{} and b=9.26 A\r{}. The dc-magnetic-susceptibility data show that 56% of ${\mathrm{Cu}}^{2+}$ ions are linked with protonated polyaniline. The in-plane structure of Cu(PANI)-VIC is formed of two types of commensurate structures having (a\ifmmode\times\else\texttimes\fi{}b) and (3a\ifmmode\times\else\texttimes\fi{}b) unit cells, where u is the number ${\mathrm{Cu}}^{2+}$ ions per unit cell of (a\ifmmode\times\else\texttimes\fi{}b). The u=1-type structure contains the conductive polyaniline, while the u=4/3-type structure does not.