Nanocomposites and nanomaterials

Hydrothermally synthesized NiFe2O4 /rGO composites: 
evolution of structure, morphology,and electrical conductivity 
Hodlevska M.A., Kotsyubynsky V.O., Boychuk V.M., Budzulyak I.M, 
Rachiy B.I., Zapukhlyak R.I., Hodlevskyi M.A.

Vasyl Stefanyk Precarpathian National University.
57 Shevchenko Str., Ivano - Frankivsk, 76025, Ukraine.
E-mail: kotsuybynsky@gmail.com
The optimization of electrical conductivity and morphological characteristics of transition metals oxides by the forming of carbon-containing nanocompositions is a promising way to obtain high performance electrode materials of hybrid supercapacitors. In this work NiFe2O4 / reduced graphene oxide (rGO) systems were hydrothermally synthesized at 180◦C for 5 h. Colloidal graphene oxide (GO) has been obtained by the modified Tour method and used at next compatible synthesis. Mass ratios between NiFe2O4 and rGO components were 3:1 and 3:2 (N1 and N2 samples series, respectively) with additional calcination at temperatures of 200, 400, and 600◦C for 1 h. XRD, low temperature nitrogen adsorption porosimetry, XRF, Mossbauer and impedance spectroscopy methods were used. The oxide material without carbon component (N0 sample) prepared at the same conditions has a single phase of stoichiometry spinel structure (average particle size of 15 nm) while the both N1 and N2 composites contain admixture of α-Fe2O3. The growth of calcination temperature causes broadering and shifts of rGO diffraction peak (indicates the reduction process continuation) and the integral breadth increasing of spinel phase peaks (corresponds to material dispergation at GO reduction with gaseous products evaporation). The degree of inversion of N0 spinel is close to 1. Mossbauer spectra of initial N1 and N2 samples as well as N1-200 and N2-200 powders indicate the presence of multiple crystallographically non-equivalent positions of Fe+3 and also paramagnetic Fe3+ ions due superparamagnetic relaxation presence. The increasing of calcination temperature leads to inversion degree decreasing. All composite materials have a mesoporous structure with maximal values of BET specific surface area of 162 and 111 m2/g for N1-200 and N2-200 samples and smooth descending pore distribution in the range of 3-20 nm. The spectral analysis of specific conductivities changes of obtained materials for frequencies of 102-105 Hz for different temperatures in a range of 25-175оC allowed to calculate the activation energy of carriers interparticle hopping for N0 spinel (0.03-0.07 eV) and also the activation energy Ea of thermally activated conductivity of rGO (0.20-0.27 eV). Complex electrical conductivity spectra is observed for N1 and N2 composite materials with a demonstration of synergistic effects and Ea values < 0.05 eV. The observed increasing of electrical conductivity for N1 and N2 samples comparatively to pure spinel and rGO phases reaches 3-4 orders of magnitude that is a precondition the enhance the surface redox reactions rate.