Further insights in the conductivity behavior of nanocrystalline NiFe2O4

Control on the conductivity behaviors has been achieved in nanoparticles of NiFe2O4, synthesized by the hydrothermal route at a fixed temperature of 225 degrees C by varying the pH of the starting solution. Particles synthesized at pH 7 and 8 behave as n-type semiconductors, while those synthesized at pH 9 and above behave as p-type semiconductors. The observed conductivity behavior has been confirmed by gas sensing and thermo-emf studies, and the mechanism has been established by x-ray photoelectron spectroscopic studies. Complete physicochemical characterizations of their phase and morphology have been carried out by x-ray diffraction and transmission electron microscopy. The thermal and electrical characteristics, elemental composition, and magnetic properties have been evaluated by thermogravimetry and differential thermogravimetry, dc conductivity, ac impedance studies, atomic absorption spectroscopy, and vibrating sample magnetometry. Gas sensing studies reveal that the resistance across the samples synthesized at pH 7 and 8 decreases in presence of a reducing gas, while the same increases in case of the samples synthesized at pH 9 and above and these are phenomena exhibited typically by n-type and p-type semiconductors, respectively. Thermo-emf studies confirmed the observations of gas sensing studies with negative emf values for samples 7 and 8 and positive values for the samples 9 and 10. From XPS studies it is unambiguously established that excess Ni3+ ions in case of the higher pH samples results in Ni2+double left right arrow Ni3+ hopping and hence the observed p-type conductivity in case of samples 9 and 10. From all the above studies an attempt has been made to discuss the best possible explanation for the different conductivity behavior in these ferrite materials. (c) 2007 American Institute of Physics.