Abstract:
Deep eutectic solvents (DESs), grounded in sustainability principles, increasingly supplanted conventional, hazardous, and volatile reagents, capturing considerable attention from the research and industrial communities. This study introduced a DES-mediated methodology for producing morphology and size-controlled Ni nanoparticles (NPs), emphasizing their catalytic potential in the reduction of 4-nitrophenol. Three distinct DES formulations, namely choline chloride and ethylene glycol (ChCl/EG), choline chloride and diethylene glycol (ChCl/DEG), and choline chloride and urea (ChCl/urea), were employed as solvent matrices for the synthesis of Ni-ChCl/EG, Ni-ChCl/DEG, and Ni-ChCl/urea, respectively. Morphology and size-controlled Ni nanomaterials were synthesized through a simple chemical reduction method in the absence of any stabilizers or capping agents. The phase purity, structural composition, morphology, and surface functionality were characterized using X-ray Diffraction (XRD), Energy Dispersive X-ray (EDX), Field Emission Scanning Electron Microscope (FESEM) techniques, and Fourier Transform Infrared Spectroscopy (FTIR). XRD and EDX confirmed the formation of phase-pure Ni with a face-centered cubic (fcc) structure in Ni-ChCl/EG and Ni-ChCl/DEG, while Ni-ChCl/urea exhibited both hexagonal close-packed (hcp) and fcc structures due to the formation of a Ni/Ni(OH)2 nanocomposite. EDX also revealed the presence of a nitrogen-doped carbon matrix in Ni-ChCl/urea. FESEM images confirmed the formation of spike ball, nanoflower, and nanograin-like morphology for Ni-ChCl/EG, Ni-ChCl/DEG, and Ni-ChCl/urea, respectively. Among the synthesized materials, Ni-ChCl/urea exhibited the best catalytic performance in the hydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The kinetic rate constant of Ni-ChCl/urea (kapp = 0.1559 min-1) was approximately five-fold larger than that of Ni-ChCl/EG (kapp = 0.0369 min-1) and about three-fold larger than Ni-ChCl/DEG (kapp = 0.0593 min-1). The significant difference in catalytic activity was attributed to variations in morphology and size. The superior catalytic performance of Ni-ChCl/urea was due to its amorphous nature, which created surface defects that enriched surface activity, as well as its reduced size, which exposed a higher active surface area. Additionally, the presence of nitrogen-doped carbon in Ni-ChCl/urea improved its electrical properties, facilitating easier electron transport during the reduction of 4-NP.