This journal is © the Owner Societies 2018 Phys. Chem. Chem. Phys., 2018, 20, 17739--17750 | 17739 Cite this: Phys. Chem. Chem. Phys., 2018, 20, 17739 Solvent dependent morphology and 59 Co internal field NMR study of Co-aggregates synthesized by a wet chemical method Harish K. Choudhary, † a M. Manjunatha, † b R. Damle, b K. P. Ramesh c and B. Sahoo * a Different shapes of Co-aggregates were synthesized via reduction of a Co salt (CoCl 2 6H 2 O) by chemical precipitation using glycerol, ethylene glycol and ethanol as solvents. The effect of solvent on the morphology, fcc or hcp phase-content and the magnetic properties of the synthesized samples were investigated. The Co-aggregates synthesized using glycerol have a dense spherical shape and high saturation magnetization (M S ), whereas ethylene glycol leads to formation of flower-shaped spherical aggregates through loose packing of smaller plate-like particles which have a moderate M S value. When ethanol was used as a solvent, a dendritic (leaf like)-shape of the aggregates with the lowest M S value was obtained. The formation of the obtained morphology of the aggregates was explained based on the size of the solvent molecule, the viscosity of the solvent and the number of polar groups (–OH) present in the solvent molecules. The magnetic domain state and domain wall dynamics of all the Co-samples were investigated using 59 Co Internal Field Nuclear Magnetic Resonance (IFNMR) spectroscopy at RT and at 77 K. Through the IFNMR spectroscopy, the presence of gain boundaries, single domain particles and multi-domain particles/aggregates with domain walls associated with fcc and hcp phases were identified and quantified. We observed that the use of ethanol facilitates formation of a higher amount of hcp phase in the sample than the use of glycerol or ethylene glycol. 1. Introduction Ferromagnetic metal (Co, Fe, Ni) particles are very useful in various applications such as catalysis, 1–3 battery electrode materials, 4 ferrofluids/magnetorheological fluids, 5–7 biomedical imaging, 8 magnetic tape coatings 9,10 and EMI shielding. 11 The magnetic and optical properties of Co particles depend on the arrangement of atoms in the structure and the size of the particles. 12 Cobalt is known to exist in two different close packed structures, hexagonal close packed (hcp) and cubic close packed (ccp)/face centred cubic (fcc) structures. 13 The atomic stacking orders for the fcc and hcp phases are ABCABC... and ABABAB..., respectively. 14 Synthesis conditions like temperature, solvent viscosity and the chemical environment (pH) etc., play crucial roles in determining the stacking of cobalt atoms during synthesis of the sample. 15–19 A pure hcp stacking is observed when low temperature synthesis methods are employed, while the presence of both phases (fcc and hcp) is probable when the synthesis is performed at higher temperatures. 20,21 The (fcc/ hcp) phase-stability of cobalt depends on the size of the grains/ particles, the reducing atmosphere and the temperature during synthesis. 22–24 Dmitry et al. have put forth the idea that a low activation energy is needed for the formation/stabilization of the stacking faults. 25 They observed that both phases co-exist for the samples prepared using high temperature crystallisation techniques such as melting–crystallization and evaporation–condensation. 25 Based on previous reports it was suggested that a low-temperature solution chemistry method yields exclusively single fcc-phase cobalt. Shape controlled synthesis of Co particles is an interesting area of research. The effect of temperature along with different solvents gives rise to different shapes, sizes and crystallographic phases of Co particles/aggregates. It was observed that addition of surfactants can lead to different morphologies of the Co nanoparticles. 26–28 Chen et al. have demonstrated that chain-like cobalt micro-rods were prepared in a water and ethanol mixed solution heated at 150 1C for 20 h. 29 They have used sodium hypophosphite and cetyltrimethyl- ammonium bromide (CTAB) as reducing reagents and cross-linkers, to control the shape of Co nanoparticles. Wang et al. have synthe- sized cobalt assemblies such as spheres, and flowers with dendritic petals and sharp petals via a liquid-phase reduction method. 30 a Materials Research Centre, Indian Institute of Science, Bengaluru-560012, India. E-mail: bsahoo@iisc.ac.in; Tel: +91 8022932943 b Department of Physics, Bangalore University, Bengaluru-560056, India c Department of Physics, Indian Institute of Science, Bengaluru-560012, India † Authors contributed equally. Received 19th March 2018, Accepted 4th June 2018 DOI: 10.1039/c8cp01780h rsc.li/pccp PCCP PAPER Published on 04 June 2018. Downloaded by Indian Institute of Science on 10/5/2018 11:15:32 AM. View Article Online View Journal | View Issue