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...

Comparison of T-piece and concentric mixing systems for continuous flow synthesis of anatase nanoparticles in supercritical isopropanol/water

“…Furthermore, the sol-gel technique is time-consuming and the produced particles have a wide size distribution [4]. From in-situ synchrotron studies of supercritical sol-gel reactions, it has been established that the nanoparticle formation and growth phases progress as in the conventional sol-gel reactions but the synthesis time is reduced from hours to minutes [16,17]. The fast kinetics of the supercritical solvothermal reaction makes it possible to obtain fully crystalline products in a continuous synthesis [4].…”

In situ total X-ray scattering study of the formation mechanism and structural defects in anatase TiO2 nanoparticles under hydrothermal conditions

“…polymorphs, morphologies, particle sizes and defects, which all strongly affect the material properties. [14][15][16][17][18] However, PXRD analysis only applies to crystalline materials. Investigation of nanocrystal formation processes is highly challenging because it is difficult to obtain atomic scale information for very small particles or aggregates during the reaction.…”

High‐Pressure, High‐Temperature Formation of Phase‐Pure Monoclinic Zirconia Nanocrystals Studied by Time‐Resolved in situ Synchrotron X‐Ray Diffraction

“…Studies of reactions in supercritical fluids necessitate highly specialized equipment to withstand the extreme conditions, while still allowing X-ray penetration. Our group recently reported the first in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) study of supercritical fluid reactions using a batch reactor, [5] and novel designs for in situ continuous-flow supercritical reactors have also been described.[6] Studying the formation and growth of zirconia nanoparticles by time-resolved in situ high-pressure, hightemperature X-ray diffraction allows a detailed and simultaneous determination of phase and size development. Zirconia nanoparticles are of particular interest during nucleation and growth due to the size-dependent crystal-phase stability.…”