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Graphitic carbon-coated Ni metal core nanoparticles are synthesized using controlled thermal treatment of Ni-hydroxide in an aniline–formaldehyde copolymer matrix under a N2 atmosphere. The structural and microstructural studies substantiate the formation of a graphitic shell on the metallic nickel core. The 850 °C heat-treated core–shell structured material is ferromagnetic in nature with high saturation magnetization. The sample with the highest value of magnetic moment is impregnated in a rubber matrix to prepare composite samples with core–shell nanomaterials of different weight fractions. The sample with 70 wt% Ni/graphitic carbon core–shell powder in the elastomeric rubber matrix showed the optimal microwave absorption at a fairly low thickness of ∼1 mm. This is attributed to the optimized combination of the effective impedance matching and interfacial polarization losses. Further increase in the Ni/graphitic carbon filler material resulted in reduced microwave absorption due to the greater mismatch of impedance as compared to the desired free space impedance of ∼377 Ω. Thus, the optimized composite material is of great potential for microwave absorption in the Ku band.

Here, an ‘in situ’ co-precipitation reaction method has been reported for the preparation of CoFe2O4–RGO (CF–RGO) nanocomposites. To the best of our knowledge, this is the first time a simple synthetic method is reported for the preparation of CoFe2O4–RGO nanocomposites where a hydrothermal technique was not used. The novelty of this technique lies in its simplicity, cost-effectiveness, and the capability of large scale production of CoFe2O4–RGO nanocomposites. The synthesized CoFe2O4–RGO nanocomposites possess excellent microwave absorbing properties as well as high photocatalytic activity towards the degradation of various dyes under visible light irradiation. 85CF–15RGO (85 wt% CF and 15 wt% RGO) showed excellent microwave absorption properties with a Reflection Loss (RL) of −31.31 dB (∼99.94% absorption) at 9.05 GHz with an 8.2–10.92 GHz effective band width range. To the best of our knowledge 85CF–15RGO nanocomposite exhibited comparable and even superior microwave absorption properties in the X-band region than most of the ferrite based composites. 75CF–25RGO (75 wt% CF and 25 wt% RGO) acted as a very good magnetically separable photocatalyst for the degradation of various synthetic dyes (such as methyl orange, methylene blue, rhodamine B and a mixture of these dyes) under visible light irradiation emitted from a 100 W reading lamp. Moreover, CoFe2O4–RGO catalyst also showed easy magnetic separation with high reusability. The photocatalytic activity of 75CF–25RGO was found to be comparable and in some cases better than the various reported RGO–ferrite composites. The simple method of preparation and multifunctional character make CF–RGO nanocomposites attractive materials for application in the area of photocatalysis as well as microwave absorption.

A Ni0.8Zn0.2Fe2O4 reduced graphene oxide nanocomposite has been synthesized by a simple ‘in situ co-precipitation’ technique. This composite exhibited an ability to act as an excellent magnetically separable catalyst towards the degradation of various dyes as well as a toxic herbicide (trifluralin). It also demonstrated very good microwave absorption properties.

CoFe2O4 nanoparticles, synthesized via a co-precipitation method at 120 °C, exhibited excellent microwave absorption properties, with minimum reflection loss of −55 dB (∼99.99%) at 9.25 GHz. To the best of our knowledge, these synthesized CoFe2O4 nanoparticles show the highest minimum reflection loss in comparison with the reported CoFe2O4 based materials.

This study deals with the exploration of CoFe2O4 supported Ag nanoparticles as a catalyst for the decolorization of various dyes (such as 4-nitrophenol, Congo red, rhodamine B) and dye mixtures by employing a reduction reaction with excess NaBH4 in an aqueous medium. Nanocatalysts with 10 wt% Ag loading (10Ag@CoFe2O4) exhibited very high catalytic activity and dye solutions were found to be decolorized within 4 to 6 minutes. A simple method for the preparation of a catalyst (10Ag@CoFe2O4) was reported, which exhibited very high catalytic efficiency towards the decolorization of dyes with different chemical structures as well as demonstrating its magnetic properties for easy separation from reaction mixtures and its reusability.

The development of a simple yet novel aqueous solution based ‘one-pot’ method has been reported for the preparation of nanocomposites composed of soft ferrite (Mn0.2Ni0.4Zn0.4Fe2O4) and hard ferrite (BaFe12O19) phases. A physical mixing method has also been employed to prepare nanocomposites having the same compositions. The effects of synthetic methodologies on the microstructures of the nanocomposites as well as their magnetic and microwave absorption properties have been evaluated. The crystal structures and microstructures of these composites have been investigated using X-ray diffraction, transmission electron microscopy and scanning electron microscopy. In the nanocomposites prepared by both methods, the presence of nanocrystalline Mn0.2Ni0.4Zn0.4Fe2O4 and BaFe12O19 phases were detected. However, nanocomposites prepared by the one-pot method possessed better homogeneous distribution of hard and soft ferrite phases than the nanocomposites prepared by the physical mixing method. Very good spring exchange coupling interaction between the hard and soft ferrite phases was observed for the nanocomposites prepared by the one-pot method and these composites exhibited magnetically single phase behaviour. The spring exchange coupling interaction enhanced the magnetic properties (high saturation magnetization and coercivity) and microwave absorption properties of the nanocomposites prepared by the one-pot method, in comparison with the nanocomposites prepared by the physical mixing method as well as pure Mn0.2Ni0.4Zn0.4Fe2O4 and BaFe12O19 nanoparticles. The minimum reflection loss of the composites was found to be ∼−25 dB (i.e. >99% absorption) at 8.2 GHz with an absorber thickness of 3.5 mm.