Beneficial Effects of Microwave-Assisted Heating versus Conventional Heating in Noble Metal Nanoparticle Synthesis

Abstract

An extensive comparative study of the effects of microwave versus conventional heating on the nucleation and growth of near-monodisperse Rh, Pd, and Pt nanoparticles has revealed distinct and preferential effects of the microwave heating method. A one-pot synthetic method has been investigated, which combines nucleation and growth in a single reaction via precise control over the precursor addition rate. Using this method, microwave-assisted heating enables the convenient preparation of polymer-capped nanoparticles with improved monodispersity, morphological control, and higher crystallinity, compared with samples heated conventionally under otherwise identical conditions. Extensive studies of Rh nanoparticle formation reveal fundamental differences during the nucleation phase that is directly dependent on the heating method; microwave irradiation was found to provide more uniform seeds for the subsequent growth of larger nanostructures of desired size and surface structure. Nanoparticle growth kinetics are also markedly different under microwave heating. While conventional heating generally yields particles with mixed morphologies, microwave synthesis consistently provides a majority of tetrahedral particles at intermediate sizes (5-7 nm) or larger cubes (8+ nm) upon further growth. High-resolution transmission electron microscopy indicates that Rh seeds and larger nanoparticles obtained from microwave-assisted synthesis are more highly crystalline and faceted versus their conventionally prepared counterparts. Microwave-prepared Rh nanoparticles also show approximately twice the catalytic activity of similar-sized conventionally prepared particles, as demonstrated in the vapor-phase hydrogenation of cyclohexene. Ligand exchange reactions to replace polymer capping agents with molecular stabilizing agents are also easily facilitated under microwave heating, due to the excitation of polar organic moieties; the ligand exchange proceeds with excellent retention of nanoparticle size and structure.

Synthesis of Pr3+-doped CaTiO3 using polymeric precursor and microwave-assisted hydrothermal methods: A comparative study

“…Dahal et al [25] investigated the effects of microwave heating, versus conventional heating, on the nucleation and growth of nearly monodispersed Rh, Pd, and Pt nanoparticles. They used a one-pot synthesis method that combined nucleation and growth in a single reaction system and provided precise control over the precursor addition rate.…”

Microwave-Assisted Synthesis of PdxAu100–x Alloy Nanoparticles: A Combined Experimental and Theoretical Assessment of Synthetic and Compositional Effects upon Catalytic Reactivity

“…15 On a more localized level, geometric ensemble effects (dependent on specific arrangements of discrete groups of atoms) are also known to be important in 13,[15][16][17][18] lower temperatures than is required using conventionally-heated (convective; CvH) methods. 37 Alloy…”

Facile Synthesis of Size-controlled Rh Nanoparticles via Microwave-assisted Alcohol Reduction and Their Catalysis of CO Oxidation

“…On the other hand, heating the alcohols with a conventional electric heater is slow and produces heterogeneous temperatures; thus, it takes a long time to prepare nanoparticles, and nanometals with a wide particle-size distribution are obtained. 14 Furthermore, because of differences in the equipment and physicochemical properties of the alcohol, the influence of the reduction ability of low-and high-boiling-point alcohols on the synthesis of nanoparticles has not been investigated under the same preparation conditions.…”