Selection Criteria for Metal Precursors and Solvents for Targeted Synthesis of Metallic Nanostructures Via Kinetic Control in the Polyol Process

Abstract

Development of a technology for the synthesis of monometallic or multimetallic nanoparticles is exceptionally vital for the preparation of novel magnetic, optical. and catalytic functional materials. In this context, the polyol process is a safe and scalable method for preparation of metal nanoparticles with controlled sizes and shapes in large scales. However, there is no systematic investigation that discusses the criteria for the selection of metal salt and solvent type that determine the kinetics of reduction reaction that influences the morphology of the particles. Consequently, the design of metallic nanoparticles, which is controlled by the kinetics and thermodynamics of the reduction reaction, has become difficult. In this paper, the selection criterion for metal salt precursor is established based on the presumption that the ligand of the metal precursor promotes the formation of active species of the solvent, and the criterion for the selection of the solvent type is based on the highest occupied molecular orbital (HOMO) energy value estimated using molecular orbital theory. The results suggested that the dissociation constants of metal salt precursors and HOMO energy of the polyol solvent can be tuned to control the kinetics of the reduction reaction. The reduction potential of polyol depends on the number of carbon atoms and the location of hydroxyl ligands within the molecule. Among the polyols considered in this study, 1,4-butanediol had the highest reduction potential. The predictions have been experimentally verified by synthesizing metallic Co and Fe nanoparticles. The findings could be extended to other techniques such as thermal decomposition and alcohol reduction for the synthesis of noble metal-transition metal magnetic and catalytic nanoparticles with novel properties.

Strategy to Design-Synthesize Bimetallic Nanostructures Using the Alcohol Reduction Method

“…It has been reported that the reduction time can also be tuned by selecting the appropriate counterion of the metal salt. 25 For example, in the ethylene glycol system, considering the value of pK a , the order of easy reduction of metal salts is M(acac) 2 > M(OAc) 2 > MSO 4 > MCl 2 (M = Metal). In other words, the reduction of Cu acetate could proceed easily, while the Cu sulfate and chloride take a longer time to reduce.…”

“…In the last few decades, polyols have been utilized to synthesize bimetallic nanostructures. 23,24 Here, the reduction rate can be controlled by selecting the polyol type, metal salt, 25 temperature, and surfactant. However, the reducing power of polyols was still high, and manipulating the characteristics of bimetallic nanostructures such as the shape, composition, and elemental distribution has become difficult.…”

“…In most cases, strong reducing agents such as sodium borohydride , and hydrazine are being used, focusing on synthesizing small-sized particles that limit the control over the reduction reaction kinetics. In the last few decades, polyols have been utilized to synthesize bimetallic nanostructures. , Here, the reduction rate can be controlled by selecting the polyol type, metal salt, temperature, and surfactant. However, the reducing power of polyols was still high, and manipulating the characteristics of bimetallic nanostructures such as the shape, composition, and elemental distribution has become difficult.…”

Formation and selected catalytic properties of ruthenium, rhodium, osmium and iridium nanoparticles

“…For example, Kaneko et al analyzed the reducing potential of various diols and showed that their reducing potential strongly depends on the number of carbon atoms between the -OH groups: the smaller distance between the hydroxyl groups results in higher reducing ability. 46 The reactivity of polyols with different positions of the hydroxyl groups are almost similar as far as the number of carbon atoms between them is the same.…”

Preparation of Cobalt Nanoparticles

“…Thus, using the polyol process, these drawbacks can be avoided. From a mechanistic point of view, the polyol process is based on the dehydration and oxidation of the alcohol to aldehydes or ketones (depending on the nature of the polyol used), and this process promotes the reduction of Co ions to Co metal [251,252] . Thus, a proper choice of the polyol is important, having the reducing potential suitable to carry out the reduction of the cobalt precursor.…”