Optical, electrical and magnetic properties of Co3O4 nanocrystallites obtained by thermal decomposition of sol–gel derived oxalates

Nanocrystallites of tricobalt tetraoxide (Co₃O₄) have been synthesized by sol–gel process using cobalt acetate tetrahydrate, oxalic acid as precursors and ethanol as a solvent. The process comprises of gel formation, drying at 80 °C for 24 h to obtain cobalt oxalate dihydrate (α-CoC₂O₄·2H₂O) followed by calcination at or above 400 °C for 2 h in air. These results combined with thermal analysis have been used to determine the scheme of oxide formation. The room temperature optical absorption spectra exhibits blue shift in both (i) ligand to metal (p(O²⁻) → e_g(Co³⁺), 3.12 eV), and (ii) metal to metal charge transfer transitions (a) t_2g(Co³⁺) → t₂(Co²⁺), 1.77 eV, (b) t₂(Co²⁺) → e_g(Co³⁺), 0.95 eV together with the d–d transitions (0.853 and 0.56 eV) within the Co²⁺ tetrahedra. The temperature dependent ac electrical and dielectric properties of these nanocrystals have been studied in the frequency range 100 Hz to 15 MHz. There are two regimes distinguishing different temperature dependences of the conductivity (70–100 K and 200–300 K). The ac conductivity in both the temperature regions is explained in terms of nearest neighbor hopping (NNH) mechanism of electrons. The carrier concentration measured from the capacitance (C)–voltage (V) measurements is found to be 1.05 × 10¹⁶ m⁻³. The temperature dependent dc magnetic susceptibility curves under zero field cooled (ZFC) and field cooled (FC) conditions exhibit irreversibilities whose blocking temperature (T_B) is centered at 35 K. The observed Néel temperature (T_N  25 K) is significantly lower than the bulk Co₃O₄ value (T_N = 40 K) possibly due to the associate finite size effects.