Nanocomposites and nanomaterials

Structural and Electrical Properties of 
Nickel-Iron Spinel / Reduced Graphene Oxide Nanocomposites

Boychuk V.M, Kotsyubynsky V.O., Bandura Kh.V., Fedorchenko S.V. 

Vasyl Stefanyk Precarpathian National University. 
57 Shevchenko Str., Ivano - Frankivsk, 76025, Ukraine.
E-mail: kotsuybynsky@gmail.com
Nickel-iron spinel (NIS) nanoparticles due to a good combination of electronic and magnetic properties are promising material varied novel technological applications– from spintronic and catalysis to electrochemistry and biomedicine. The range of ultrafine NIS practical using can significantly expanded using nanocomposite materials on the base of the NIS and carbon nanostructures. NIS / reduced graphene oxide (rGO) systems which combine the benefits of both functional materials are very attractive. In this case the study of synthesis conditions influence on the structural, morphological and electrical properties of NIS / rGO composite is crucial. Ultrafine NIS and NIS / rGO composite was hydrothermally synthesized (180 oC for 10 hours) using GO obtained by Hammers method. Structure, morphology, chemical, magnetic and electrical properties of the obtained materials were studied by XRD, SAXS, low temperature nitrogen absorption, XRF, Mossbauer and impedance spectroscopy. The reducing of average particles sizes from 26 nm for NIS to 8-9 for NIS / rGO was observed. The specific surface areas were 40 and 150 m2 / g for NIS and NIS / rGO, respectively. Normalized pair distance distribution function NIS / rGO presupposes the existence of particles with sizes of about 4, 8, 16 and 35 nm. The transitions between magneto - ordered, spin glass like and superparamagnetic states of ultrafine nickel ferrites were observed. Mixed spinel structures (Ni0.32Fe0.68)[Ni0.95Fe1.05]O4- and (Ni0.27Fe0.73)[Ni0.91Fe1.09]O4- were obtained for NIS and NIS / rGO cases. 
The frequency dependences (0.01-100 kHz) of the electrical conductivity () at the temperature of 20-200 oC were fitted by Johnsher equation. The observed decreasing of dc at the temperature ranges of 100-180 oC (for NIS) and 80-100 oC can be explained by effects of localization and delocalization of hopping charge carriers when eg electrons energy is less than electrostatic energy applied by grain boundaries. The formation of conductive channels happens for hopping of localized d-electrons through superexchange paths Fe3+ - O - Fe2+ and Ni2+ - O - Ni3+. The decreasing of power law exponent (n) at this temperature range corresponds to overlapping large polaron tunneling mechanism of conductivity when correlated barrier hopping mechanism is most probable for temperatures less than 80oC and higher than 100 oC. The shift of the temperature range of conductivity mechanism changes for NIS / rGO is caused by the presence of additional electronic trap centers and spin disorientation surface / core effects for ultrafine particles.