Structural investigations of synthetic ferrihydrite nanoparticles doped with Si

X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoacoustic/Fourier transform infrared spectroscopy (PA/FTIR) are employed to monitor the changes in the structural aspects of two-line ferrihydrite (FHYD) nanoparticles doped with silica viz. SixFe1−xOOH·nH2O for x=0, 0.02, 0.05, 0.10, 0.15, 0.20, 0.26, 0.40 and 0.50. In XRD, crystallinity decreases and d-spacings increase with increase in x. TEM studies show that the particle size increases systematically with increase in x, from 3.7 nm for x=0 to 5.7 nm for x=0.50. In PA/FTIR, two new bands appear, band F near 3700 cm−1 identified with the surface Si–O–H group and band A near 900 cm−1 identified with Si–O–Fe group, which shifts to higher wavenumbers with increase in x. These results are used to propose a model in which doped Si4+ ions do not displace Fe3+ ions but are chemisorbed on the FHYD surface making a shell of silica for higher doping. This model is consistent with the reported changes in the magnetic properties of FHYD with Si doping.