• Abhilash

Articles written in Bulletin of Materials Science

• Microbial synthesis of iron-based nanomaterials—A review

Nanoparticles are the materials having dimensions of the order of 100 nm or less. They exhibit a high surface/volume ratio leading to different properties far different from those of the bulk materials. The development of uniform nanoparticles has been intensively pursued because of their technological and fundamental scientific importance. A number of chemical methods are available and are extensively used, but these are often energy intensive and employ toxic chemicals. An alternative approach for the synthesis of uniform nanoparticles is the biological route that occurs at ambient temperature, pressure and at neutral pH. The main aim of this review is to enlist and compare various methods of synthesis of iron-based nanoparticles with emphasis on the biological method. Biologically induced and controlled mineralization mechanisms are the two modes through which the micro-organisms synthesize iron oxide nanoparticles. In biologically induced mineralization (BIM) mode, the environmental factors like pH, pO2, pCO2, redox potential, temperature etc govern the synthesis of iron oxide nanoparticles. In contrast, biologically controlled mineralization (BCM) process initiates the micro-organism itself to control the synthesis. BIM can be observed in the Fe(III) reducing bacterial species of Shewanella, Geobacter, Thermoanaerobacter, and sulphate reducing bacterial species of Archaeoglobus fulgidus, Desulfuromonas acetoxidans, whereas BCM mode can be observed in the magnetotactic bacteria (MTB) like Magnetospirillum magnetotacticum, M. gryphiswaldense and sulphate-reducing magnetic bacteria (Desulfovibrio magneticus). Magnetite crystals formed by Fe(III)-reducing bacteria are epicellular, poorly crystalline, irregular in shapes, having a size range of 10–50 nm super-paramagnetic particles, with a saturation magnetization value ranging from 75–77 emu/g and are not aligned in chains. Magnetite crystals produced by MTB have uniform species-specific morphologies and sizes, which are mostly unknown from inorganic systems. The unusual characteristics of magnetosome particles have attracted a great interdisciplinary interest and inspired numerous ideas for their biotechnological applications. The nanoparticles synthesized through biological method are uniform with size ranging from 5 to 100 nm, which can potentially be used for various applications.

• Microwave dielectrics: solid solution, ordering and microwave dielectric properties of $(1−x)$Ba(Mg$_{1/3}$Nb$_{2/3}$)O$_3$−$x$Ba(Mg$_{1/8}$Nb$_{3/4}$)O3 ceramics

The effect of Ba(Mg$_{1/8}$Nb$_{3/4}$)O$_3$ phase on structure and dielectric properties of Ba(Mg$_{1/3}$Nb$_{2/3})O$_3$was studied by synthesizing$(1−x)$Ba(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$−$x$Ba(Mg$_{1/8}$Nb$_{3/4}$)O$_3$($x = 0,$0.005, 0.01 and 0.02) ceramics. Superlattice reflections due to 1:2 ordering appear as low as 1000$^{\circ}$C. Ba(Mg$_{1/3}$Nb$_2/3}$)O$_3$forms solid solution with Ba(Mg$_{1/8}$Nb$_{3/4}$)O$_3$for all ‘$$x’ values studied until 1350$^{\circ}$C. Ordering as confirmed by powder X-ray diffraction pattern, Raman study andHRTEM. Ceramic pucks can be sintered to density$>$92% of theoretical density. Temperature and frequency-stable dielectricconstant and nearly zero dielectric loss ($\tan \delta$) were observed at low frequencies (20 MHz). The sintered samples exhibit dielectric constant ($\epsilon_\tau$) between 30 and 32, high quality factor between 37000 and 74000 GHz and temperature coefficient of resonant frequency ($\tau_f$) between 21 and 24 ppm$^{\circ}$C$^{−1}$. • A study on strain and density in graphene-induced Bi$_2$O$_3$thin film An eccentric sol–gel method was used to synthesize the graphene oxide, reduced graphene oxide, Bi$_2$O$_3$and graphene-doped Bi$_2$O$_3$. The samples were subjected to X-ray diffraction for the structural analysis. The structural parameters of all the samples were estimated and various strains like lattice strain, RMS strain, etc. and densities like X-ray density, dislocation and screw density were also calculated. Modified Scherrer method and W–H plot method are also used to estimate the average crystallite size and the intrinsic strain of all the synthesized materials. • A short investigation on LiMn$_2$O$_4$wrapped with MWCNT as composite cathode for lithium-ion batteries The need for large-scale batteries impels the development of high-performance cathode material for advanced lithium-ion batteries (LIBs). The existing cathode materials such as LiCoO$_2$, LiNiO$_2$and LiMnO$_2$were rated as potentially viable cathodes for commercial applications. Among these, LiMn$_2$O$_4$and its composites was considered a sound cathode material for high-performance LIBs. In this study, the multi-walled carbon nanotube (MWCNT)-wrapped spinel LiMn$_2$O$_4$nanocathode was synthesized via simple sol–gel method and characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Raman, Impedance and galvanostatic charge/discharge analyses to study their structural, morphological, optical and electrochemical properties, respectively. XRD results reveal that the pure and MWCNT-embedded LiMn$_2$O$_4$nanocathode exhibited similar cubic structure with space group of Fd3m. The as-fabricated MWCNT/LiMn$_2$O$_4$battery showed the excellent reversible capacity (114 mAh g$^{–1}$) with higher coulombic efficiency after multiple cycles. Herein, simple wrapping methodology was adopted to overcome the drawbacks of the pure spinel. Incorporated MWCNT uniformly entwined in the LiMn$_2$O$_4$and lead to prevent the volume expansion, and pulverization in surface of the active LiMn$_2$O$_4$particles, which confirmed from post FESEM analysis and their results are discussed. Interestingly, MWCNT addition showed that the enriched electrochemical properties in LiMn$_2$O$_4\$ nanoparticles are able to hold as a potential cathode for high voltage LIBs.

• # Bulletin of Materials Science

Volume 44, 2021
All articles
Continuous Article Publishing mode

• # Dr Shanti Swarup Bhatnagar for Science and Technology

Posted on October 12, 2020

Prof. Subi Jacob George — Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru
Chemical Sciences 2020