Tunable Giant Magnetoresistance Behavior and Electron Conductivity of Conjugated Naopolyaniline
Authors: Guo Zhanhu , Guo Jiang , He Qingliang , Wang Yiran , Yan Xingru 
Published: February 2020

Due to the widely applications of giant magnetoresistance (GMR) phenomenon in magnetic recording, biosensing, and biochip systems, and the unique physical and chemical properties of polyaniline (PANI), the MR behavior of conjugated conductive PANI nanoparticles is widely studied in recent years to widen its applications in new area. The nanostructure, doping level, temperature, magnetic field direction, and types of dopants effects on the MR behavior of PANI nanoparticles are systematically studied in this work. The temperature (from 50 to 290K) dependent electrical conductivity of PANI were controlled by adjusting the doping level (0.5, 1.0, and 2.0 M), dopants type (HCL, H2SO4, H3PO4 and PTSA), and nanostructure (nanofiber and nanosphere of PANI). The Mott variable range hopping (VRH) model was used to study the electron transport mechanism of PANI. And the quasi 2 and 3-dimensionals variable range hopping (VRH) electrical conduction mechanism are observed in PANI nanofibers and nanospheres, respectively. The positive and negative MR are observed in the synthesized PANI at room temperature, and analyzed by the wave-function shrinkage model and orbital magnetoconductivity theory (forward interference model), respectively. The scanning electron microscope (SEM) is used to characterize the surface morphology of PANI nanoparticles. The chemical structure of PANI nanoparticles is characterized by Fourier transform infrared (FT-IR) spectroscopy. The thermal stability of PANI is assessed by thermogravimetric analysis (TGA). X-ray diffraction (XRD) is used to study the crystal structure of PANI. The dielectric permittivity and optical property are also reported in this work.

Experimental

PANI nanofibers were prepared by an interfacial polymerization method with an aniline: ammonium persulfate (APS):PTSA(p-toluene sulfonic acid) ratio of 8:2:25. Briefly, aniline (3.2 mmol) was dissolved in chloroform (10 mL) as Solution 1, and Solution 2 was prepared by dissolving APS (0.8 mmol) in 1.0 M PTSA aqueous solution (10 mL). Then, Solution 2 was added into Solution 1 quickly and maintained still for 2 h of polymerization at room temperature.1 PANI nanospheres were synthesized by oxidation polymerization method. Briefly, the molar ratio was aniline: APS: PTSA = 6:3:5. For solution 1, PTSA (30 mmol) and APS (18 mmol) were dissolved in deionized water (200 mL) in a beaker, which was treated by sonication in the ice water bath for 1 hour mechanical stirring (300 rpm). Solution 2 was aniline (36 mmol) dissolved in deionized water (50 mL). Solution 2 was then added into solution 1, and the mixture was sonicated for an additional 1.5 hour mechanical stirring (300 rpm) in the ice water bath for polymerization of aniline. After the polymerization, the suspensions contains PANI nanoparticles were vacuum filtered and washed acetone to remove the oligomer, then washed with ammonia to remove the doped acid, then wash with different concentrations (0, 0.5, 1.0, and 2.0 M) of dopant acid (PTSA, HCL, H2SO4, and H3PO4). The final product was dry in the oven at 50 oC.

Results and Discussion

The FT-IR, XRD and SEM images indicates that the PANI (emeraldine salt form) nanofibers and nanospheres are successfully synthesized by interfacial and oxidation polymerization methods, respectively. The temperature dependent resistivity results indicate that morphology, dopant acid, doping level have effect on the electrical conductivity of PANI nanoparticles. The resistivity decreases with increasing the temperature, indicating the typical behavior of a semiconductor. The Mott variable range hopping (VRH) model was used to study the electron transport mechanism of PANI nanoparticles, and the quasi 2 and 3-dimensionals variable range hopping (VRH) electrical conduction mechanism are observed in PANI nanofibers and nanospheres, respectively. The PANI nanoparticles shows different MR behavior due to the difference of morphology, doping level, and dopant acid. The positive and negative MR are observed in the synthesized PANI at room temperature and analyzed by the wave-function shrinkage model and orbital magnetoconductivity theory (forward interference model), respectively. The localization length(a0
), average hopping length (R
hop), and density of states at the Fermi level N(EF
) are calculated. In a word, the MR and electrical conductivity of the PANI nanoparticles is tunable by controlling the morphology of PANI, doping level, and dopant acid, which has significant effect on the application of MR based recording system.

Acknowledgments

The financial supports from University of Tennessee Knoxville are kindly acknowledged.

Reference

1. X. Zhang, Q. He, H. Gu, H. A. Colorado, S. Wei and Z. Guo;
ACS Applied Materials & Interfaces
, 5(3) 898 (2013)

Figure 1