Supercritical Production of Nanoparticles - Part I: The SSEC Process - Part II: Characterization of Nanopartic

Abstract

Nanomaterials and nanoparticles are gaining a lot of public and research attention. Today it is

clear that particle properties change drastically when going from bulk materials to nanosized

particles and a number of physiochemical properties are uniquely size dependent and change

radically with the crystallite size. Therefore special interest is being devoted to investigating

these changes by developing new synthesis and characterizing methods.

Wet chemical and gas phase syntheses are among the number of synthesis techniques that

have been developed for nanoparticle formation. The sol-gel technique is the most broadly

applied wet chemical process and it can be used for the production of nanosized materials in

the formof particles or coatings for a wide range of materials. However, conventional sol-gel

techniques have a number of drawbacks. The process maintains long reaction times and

requires post treatment processing. For example, drying and calcination are required to obtain

a crystalline material which represents a considerable increase in energy consumption. In

addition to high energy usage, the post heat treatment reduces the specific surface area by up

to 80%due to sintering and particle growth.

The work presented in this thesis addresses the problems related to the conventional sol-gel

techniques by using supercritical CO2 as the reaction media. Supercritical fluids exhibit gas

like mass transfer properties and liquid like densities which are both particularly attractive to

the sol-gel process. Furthermore, these attractive properties can be fine tuned by controlling

the pressure and temperature. Thereby can the kinetics of the sol-gel processes often be

enhanced by more than an order of magnitude through utilizing supercritical fluids as the

solvent compared to traditional alcohols.

A key result of this thesis is the "Supercritical Seed Enhance Crystallization" (SSEC) process

for synthesizing homogeneous nanocrystals. During the creation of this thesis, the SSEC

process was fully developed, two patent applications were filed, and the SSEC methodology

was thoroughly investigated. SSEC is a modified sol-gel process which takes place in the

proximity of a seeding material in a supercritical environment. The seeding material is

introduced to enable the production and collection of nanosized crystalline particles. This

material acts as a seed or a catalyst as well as a reservoir for collecting the formed

nanoparticles.

Investigation included the influence of various synthesis parameters; scale up froma 24 ml to

a 100 ml reaction vessel and thorough analysis of the nano crystalline materials produced by

utilizing some of the most advanced characterization methods available. Analysis during the

testing phase of this thesis included real time in-situ wide and small-angle X-ray scattering

(WAXS/SAXS) and X-ray photoelectron spectroscopy (XPS). In several cases during this

project it was necessary to develop new standards and preparation methods to measure

specific properties. For example, a new method was designed and developed to define and

measure the absolute crystallinity of a material. Furthermore, methods on how to establish a

reliable size distribution frome.g. WAXS and SAXS data of a nanomaterial were developed.

A wide range of nano-sized crystalline and partly crystalline materials were synthesized by

the SEEC process including TiO2, AlOOH, GeO2, SiO2, and ZrO2. The crystallite sizes were

in the range 5 - 15 nm with corresponding particle sizes and the crystallization temperatures

were lowered by 100 - 250 ºC compared to the traditional sol-gel process.

Real time in-situ simultaneous SAXS and WAXS characterization of the SSEC process

obtained at the Advanced Photon Source (APS) at Argonne National Laboratory in Chicago

showed that the supercritical nanoparticle formation is a four step mechanism. First an

induction period where a gelation takes place, which ends when the first crystalline material is

x

detected and a latent period continues until a sudden change in the system is observed. A

rapid precipitation of nanocrystalline materials then occurs. After the rapid precipitation

period, a slow growth period starts. The simultaneous WAXS and SAXS showed that the

SSEC process was indeed a desupersaturation or precipitation of nanocrystalline material

from a gel and not a recrystallization of amorphous material. The in-situ investigation also

showed that the induction period in the SSEC process is mainly determined by the degree of

supersaturation and the heating rate. The latent period was influenced by supersaturation and

the final temperature and the time for the precipitation of nanocrystalline material was

dominated by the final temperature. Crystalline TiO2 on the anatase phase was synthesized at

an unprecedented low temperature. The in-situ study revealed that for the TiO2 system a

general crystallization temperature, where the first traces of crystalline material was observed,

was determined to 87 ± 5 ºC. The in-situ study also showed that the process time going from

the precursors to a final solid powder was obtainable within 30 min or less. The process time

was shown to be influenced by especially the heating rate, final temperature, and

supersaturation.