Aloe vera Plant Extracted Green Synthesis, Structural and Opto-Magnetic Characterizations of Spinel CoxZn1-xAl2O4 Nano-Catalysts

Spinel CoxZn1-xAl2O4 (0 <= x <= 1) nano-catalysts were synthesized by a simple Aloe vera plant extracted green synthesis route. Powder XRD patterns and Rietveld analysis confirmed the formation of single phase, cubic spinet gahnite structure without other impurities. The lattice parameter increased from 8.089 to 8.125 angstrom with increasing Co2+ content. The average crystallite sizes were estimated using Scherrer's method, and it was found to be in the range of 15.72 nm to 26.53 nm. FT-IR spectra showed vibrational stretching frequencies corresponding to the spinet structure. HR-SEM and HR-TEM images showed the features of well particle shaped crystals with nano-sized grains. The elemental compositions of Co, Zn, Al and O were quantitatively obtained from EDX analysis. The band gap energy estimated using Kubelka-Munk method by UV-Visible DRS method, and the values are decreased with increasing the Co2+ content (4.12 eV to 3.67 eV), due to the formation of sub bands in between the energy gap. PL spectra showed emission bands in UV as well as in the visible regions for ZnAl2O4 and Co-doped ZnAl2O4, due to the defect centers acting as the trap levels. VSM measurements revealed that pure ZnAl2O4 has diamagnetic, while Co doped ZnAl2O4 samples (x = 0.2 to 0.8) have superparamagnetism, whereas the sample CoAl2O4 has ferromagnetic in nature. Catalytic oxidation of benzyl alcohol to benzaldehyde was found that the sample Co0.6Zn0.4Al2O4 showed 93.25% conversion with 99.56% selectivity, whereas for pure ZnAl2O4, the conversion was only 86.31% with 92.85% selectivity.