Digestive ripening, a postsynthetic treatment of colloidal nanoparticles, is a versatile method to produce monodisperse nanoparticles and to prepare various bimetallic nanostructures. The mechanism of this process is largely unknown. Herein, we present a systematic study conducted using Au nanoparticles prepared by a solvated metal atom dispersion method to probe the mechanistic aspects of digestive ripening. In our study, experimental conditions such as concentration of capping agent, reaction time, and temperature, were found to influence the course of the digestive ripening process. Here it is shown that, during digestive ripening under reflux, nanoparticles within an optimum size window are conserved, and surface etching facilitated mass transfer resulted in monodisperse nanoparticles. Overall, digestive ripening can be considered as a kinetically controlled thermodynamic process.

No abstract

Co-digestive ripening is a versatile strategy for the preparation of binary metal nanoparticles such as core-shell, composites, and alloys. The mechanism of co-digestive ripening is hypothesized to involve inter-atom transfer facilitated by capping agents. In this work, co-digestive ripening of a metal (Au) and a semiconductor (CdS) system has been presented. This process involves co-refluxing of a physical mixture of Au and CdS colloids in presence of hexadecyl amine (HDA) in mesitylene solvent. Individual colloids of Au and CdS were prepared by solvated metal atom dispersion (SMAD) method. The growth control of nanoparticles using HDA resulted in the formation of monodisperse particles via a room temperature digestive ripening process. Au/CdS nanocomposite obtained via co-digestive ripening strategy was characterized by a matrix-like structure made up of CdS nanoparticles in which Au nanoparticles were embedded. CdS nanoparticles were found to establish an intimate surface contact with Au nanoparticles and the matrix of CdS surrounding Au was developed via aggregation during co-refluxing. Formation of the Au/CdS nanocomposite is proposed to involve a ligand replacement process wherein HDA molecules capping the Au surface are displaced by CdS owing to strong Au-CdS interaction. The Au/ CdS composite thus obtained showed superior photocatalytic activity compared to pure CdS nanoparticles for dye degradation reaction.[a] Dr.

The solvated metal atom dispersion (SMAD) method has been used for the synthesis of colloids of metal nanoparticles. It is a top-down approach involving condensation of metal atoms in low temperature solvent matrices in a SMAD reactor maintained at 77 K. Warming of the matrix results in a slurry of metal atoms that interact with one another to form particles that grow in size. The organic solvent solvates the particles and acts as a weak capping agent to halt/slow down the growth process to a certain extent. This as-prepared colloid consists of metal nanoparticles that are quite polydisperse. In a process termed as digestive ripening, addition of a capping agent to the as-prepared colloid which is polydisperse renders it highly monodisperse either under ambient or thermal conditions. In this, as yet not well-understood process, smaller particles grow and the larger ones diminish in size until the system attains uniformity in size and a dynamic equilibrium is established. Using the SMAD method in combination with digestive ripening process, highly monodisperse metal, core-shell, alloy, and composite nanoparticles have been synthesized. This article is a review of our contributions together with some literature reports on this methodology to realize various nanostructured materials.

No abstract