Correlation between Particle Size, Strain and Band Gap of Iron Oxide Nanoparticles

Effect of heat treatment was observed on size, strain and band gap of iron oxide nanoparticles. Mixed phase (gamma-Fe2O3 and alpha-Fe2O3) of iron oxide nanoparticles were obtained with increasing annealing temperature from 100 degrees C - to 700 degrees C. Minimum particle size of 40 nm was calculated around 400 degrees C using X-ray diffraction. The relation between size, strain and band gap of iron oxide nanoparticles has been established with the help of UV-VIS spectroscopy. Sample annealed at 400 degrees C shows direct and indirect band gap at 1.75 eV and 1.67 eV respectively. The band gap & particle size of the nanoparticles are decreasing with increasing annealing temperature up to 400 degrees C. Thereafter, it increases with increase in annealing temperature. But for 40 nm particles the lattice strain is 5 times larger in comparison to strain in larger particle. Strain is larger in mixed phase (before 400 degrees C) than pure phase (after 400 degrees C). The present results indicate that the direct variation of band gap with particle size is due to annealing effect which changes the phase, strain and size of nanoparticles. (C) 2015 Elsevier Ltd. All rights reserved.