In Situ High‐Energy Synchrotron Radiation Study of Sol–Gel Nanoparticle Formation in Supercritical Fluids

Abstract

Nanostructured materials have drawn much attention because of the dramatically different properties observed on going from bulk material to nanosized particles. Nanoparticles have properties that lie between the quantum effects of atoms and molecules and the bulk properties of materials. In the nanometer range (1-100 nm) the particle size affects structural characteristics (e.g., lattice symmetry, unit-cell dimension), electronic properties (e.g., band gap), and therefore also the physical (e.g., wetting, melting point) and chemical properties (e.g., catalytic effects) of a material. The applications of nanomaterials include lithium-based batteries, fuel cells, [1, 2] thin films, inorganic-organic hybrid materials, [3] sensors, piezoelectric devices, and catalysts. [4,5] In all production methods for nanomaterials, a key requirement is the ability to control nanoparticle size, shape, and crystallinity. Special attention has been devoted to metal oxides. These can adopt many different crystal structures and have metallic, semiconducting, or insulating properties.[5] Their chemical properties range from strong catalytic reactivity to chemical inertness and high-temperature stability, and the application of metal oxides is a multibillion-dollar industry.Several wet-chemistry synthesis routes are capable of producing nanomaterials, [6] but the sol-gel technique has become the standard method for fabricating metal oxides because of the possibility of obtaining high chemical homogeneity at low temperature and under mild chemical conditions. [5,7] The downside is the relatively long process time and the need for posttreatment (e.g., calcination), which make the process less attractive for industry. The solution appears to be synthesis in supercritical fluids, which provides unique control of chemistry and nanocrystal properties. [8][9][10][11][12][13][14][15][16][17] Use of supercritical fluids as solvents in sol-gel processes enhances the kinetics by more than an order of magnitude. Furthermore, supercritical fluids exhibit particularly attractive properties such as gaslike mass-transfer behavior, liquidlike densities, and changed dielectric properties. These properties can be fine-tuned by simple changes in pressure and temperature; for example, the solubility of a compound can be dramatically changed to cause very fast precipitation.To manipulate the properties of nanomaterials and synthesize new materials with unprecedented properties, the main challenge is to understand the nucleation, crystallization, and growth processes.[5] This requires development of analytical tools capable of following nanoparticle formation in real time. In situ measurements by dynamic light scattering (DLS) have been used to study particle growth and stabilization of primary particles by surfactants. [18] In situ synchrotron powder diffraction has become a widely used tool for following solid-state reactions and crystallization processes. [19][20][21] The crux of this technique is the high intensity of the synchrotron beam, which allow...

In Situ High‐Energy Synchrotron Radiation Study of Boehmite Formation, Growth, and Phase Transformation to Alumina in Sub‐ and Supercritical Water

“…The main conclusion of the study by Jensen et al [29] of the formation of TiO 2 in supercritical CO 2 was that the nucleation and growth in supercritical CO 2 follow the same pattern as the conventional sol-gel synthetic route, but on a much faster timescale. The general pattern for the formation of a solid in solution (or sol-gel reaction synthesis) consists of four periods: 1) The induction period, 2) the latent period, 3) the precipitation period, and 4) the slow growth period.…”

“…SAXS data contain information on particle size independent of the crystallinity of the particle. The first in situ SAXS/WAXS study of supercritical reactions was reported by Jensen et al, [29] who synthesized TiO 2 in supercritical CO 2 . The data revealed that nanoparticle formation follows the same progression as seen in conventional solvothermal reactions, but on a much faster timescale.…”

Oxide Nanomaterials: Synthetic Developments, Mechanistic Studies, and Technological Innovations

“…[236] The supercritical sol-gel process offers the considerable advantages of low reaction temperature (100 8C) and a fast time scale (ca. 50 min) compared to the corresponding conventional strategy.…”

One step hydrothermal synthesis of TiO2 with variable HCl concentration: Detailed characterization and photocatalytic activity in propene oxidation

“…On one hand, crystalline TiO2 seems to be more effective than amorphous titania to prevent recombination of electrons (e -) and holes (h + ) [17]. Maybe because of that, most references just deal with the characterization of crystalline TiO2, setting aside the analysis of the amorphous contribution, leaving unregarded the degree of crystallinity and the fact that amorphous titania could have some positive influence [17][18][19][20][21][22]. On the other hand, while in some works anatase is considered as the most photoactive TiO2 crystalline phase [23][24][25], other published papers state that materials containing different TiO2 polymorphs (mainly anatase with some rutile) are more active in oxidation processes [26][27][28][29].…”