Synthesis of Micro- and Nanoparticles in Sub- and Supercritical Water: From the Laboratory to Larger Scales

Abstract

The use of micro- and nanoparticles is gaining more and more importance because of their wide range of uses and benefits based on their unique mechanical, physical, electrical, optical, electronic, and magnetic properties. In recent decades, supercritical fluid technologies have strongly emerged as an effective alternative to other numerous particle generation processes, mainly thanks to the peculiar properties exhibited by supercritical fluids. Carbon dioxide and water have so far been two of the most commonly used fluids for particle generation, the former being the fluid par excellence in this field, mainly, because it offers the possibility of precipitating thermolabile particles. Nevertheless, the use of high-pressure and -temperature water opens an innovative and very interesting field of study, especially with regards to the precipitation of particles that could hardly be precipitated when CO2 is used, such as metal particles with a considerable value in the market. This review describes an innovative method to obtain micro- and nanoparticles: hydrothermal synthesis by means of near and supercritical water. It also describes the differences between this method and other conventional procedures, the most currently active research centers, the types of particles synthesized, the techniques to evaluate the products obtained, the main operating parameters, the types of reactors, and amongst them, the most significant and the most frequently used, the scaling-up studies under progress, and the milestones to be reached in the coming years.

Effect of the use of inorganic- based photoluminescent materials in solar energy devices application

“…Dielectric constant (the property of a material to insulate) of the dispersing TiO2 host material was thought to be proportional to the resulted density electron holes pairs which determined the ensued ionic dissociation. A similar study by [12] reported that reduction in charge carriers density gave rise to instantaneous downturns of potential profiles. Imbalance between the dopant and perovskite species affected the parameters of interest resulting from the intensity of excitation energy [14].…”

“…This was attributed to the corresponding the quantity of charge per generated with time which is also referred to as charge rate density delivered to the external circuit. A study by [12] noted that fluorescent dopants mass ratios was proportional to the excited charge carriers density. Similar results reported by [13] indicated that dielectric constant of solvent material was proportional to the excited electron holes pairs density which determined the ensued ionic dissociation.…”

A Bibliometric Analysis and Review of Supercritical Fluids for the Synthesis of Nanomaterials

“…Supercritical fluids used for the synthesis of nanomaterials have been developed on the basis of supercritical research. A large number of studies have found that when the temperature and pressure approach the critical point of the fluid, (1) the density of the fluid increases sharply, which affects the solubility of salt, and is conducive to supersaturation and particle nucleation; (2) the viscosity decreases and the reaction rate increases; and (3) the decrease of the dielectric constant and increase of the ion product greatly affect the hydrolysis of metal salts, resulting in supersaturation and particle nucleation [ 15 ]. These changes are beneficial to the preparation of nanomaterials.…”

Supercritical hydrothermal synthesis of silver nanoparticles, composites, and their characterizations

“…By employing those unique properties of supercritical conditions, the supercritical hydrothermal synthesis was studied successfully for metal and metal oxide nanoparticles synthesis [2,3]. The hydrothermal technique is ideal for the processing of fine particles having high purity, controlled stoichiometry, narrow particle size distribution, uniformity, high crystallinity, high reactivity with ease of sintering, and so on, but it has a controlled micro-sized structure [4], and it takes long reaction time [5]. Therefore, after obtaining micro-sized particles, it's necessary to conduct thermal treatment and need additional chemicals such as organic solvents, reducing agents, or dispersants to get nano-sized materials [6].…”