Fabrication and Characterization of Ytterbium and Transition Metal Cations Doped BiFeO3 Multiferroics

Abstract

This dissertation presents the effect of ytterbium (Yb), nickel (Ni) and cobalt (Co) doping on the structural, spectral, magnetic, electrical, dielectric and photocatalytic properties of BiFeO3 (BFO). The micro-emulsion synthesis route along with cetyl trimethyl ammonium bromide (CTAB) was used to synthesize BiFeO3 multiferroic. The doping was made to enhance various properties of BiFeO3. A pure phase sample of BFO was achieved with reaction conditions; a molar concentration ratio 1:1 between precursors and CTAB. The optimized annealing temperature was found at 900 ºC for 7 h. The crystallite size was found to be ~ 18 nm. The synthesized BFo has optical band gap ~ 2.6 eV and resistivity 1.005 x 109 Ωcm-1. In first series (BYCFO) (Bi1-xYbxCoyFe1-yO3) nanocrystalline ytterbium and cobalt were co-doped in BiFeO3. The doping concentrations were arranged in sequential increasing order as follows x = 0.00, 0.025, 0.05, 0.075, 0.1, 0.125 and y = 0.00, 0.05, 0.1, 0.15, 0.2, 0.25. Each series comprises on six samples. The X-ray diffraction (XRD) analysis indicated normal growth habitat Rhombohedral (R3c) phase. The inclusion of ytterbium and cobalt ions in BiFeO3 was observed by XRD. The incorporation of higher concentration of Yb3+ and Co3+ ions (x= 0.125, y= 0.25) revealed few extra peaks. The magnetic properties were measured by vibrating sample magnetometer (VSM). The inclusion of Yb and Co enhanced saturation magnetization (Ms) = 0.385 emu/g, remnant magnetization (Mr) = 0.13 emu/g and corecivity = 405 KOe which is higher as compared to the pure BiFeO3. The grain size was found in the range of 20- 24 nm. The dopants appreciably impeded the dielectric loss and dielectric parameters in the low frequency region and resonance peaks were appeared in the spectra at higher frequency (1.98 GHz) region. The optical band gap was found to be 1.5 eV which was lower than pure (2.15eV). The optimized photocatalytic performance of (Bi0.875Yb0.125Co0.25Fe0.75O3) was found 42 %. In second series (Bi1-xYbxNiyFe1-yO3), Yb and Ni were co-doped in BiFeO3. The XRD analysis indicated that the diffraction peak (006) at 2θ = 38.85° exhibited the normal Rhombohedral (R3c) structure. It was observed that diffraction peaks (204) and (110) at the diffraction angle (2θ) = 31.56 ° and 32.74 ° for concentrations of dopants x =0.10, y = 0.20. The spectra for further doping of dopants x=0.125 and y = 0.25 showed the disappearance of diffraction peak (006) and appearance of diffraction peak (111) at 2θ =27° which showed a phase transformation from Rhombohedral to orthorhombic phase. The measured grain size was 24 nm and UV/visible scan showed λmax = 400 nm with optical band gap of 1.25. The maximum resistivity was found ~ 1.4 x109 Ω- cm. The dielectric constant and dielectric loss disclosed a solid distribution at low frequencies and the leakage current density was suppressed with an increase in dopants cations. The photocatalytic performance of this series was higher as compared to the previous series (66 %) for 0.5 ppm of Congo red and time interval 51minutes. The band gap was enhanced to (1.25 eV). The ceramic was separated through a magnet bar and recycled after water washing. The comparison between two series was distinct. The particle size of BYCFO ranges from 20 to 24.37 nm but BYNFO series showed a particle size in a short range between 21 to 23.78 nm. The BYNFO series have maximum resistivity ranges 1.2x 109 Ω-cm. The BYCFO series resistivity 2.56 x 1010 Ωcm-1is 10 times higher than nickel doped series. The optical band gap showed a significant decrease for BYNFO series (1.25 eV) as compared to BYCFO (1.5 eV). The BYNFO nanocrystalline series showed a photo degradation efficiency of 66 % which was higher than the BYCFO nanocrystalline series. BYCFO showed maximum magnetization of 0.354 emu/g, corecivity 405 KOe, remanant magnetization ~ 0.115 emu/g for dopants concentration (x =0.125, y = 0.25).