Samuel Marre

Near- and Supercritical Alcohols as Solvents and Surface Modifiers for the Continuous Synthesis of Cerium Oxide Nanoparticles

Supercritical fluids offer fast and facile routes toward well-crystallized tailor-made cerium oxide nanoparticles. However, the use of surfactants to control morphology and surface properties remains essential. Therefore, although water, near-critical (nc) or supercritical (sc), is a solvent of choice, the poor water solubility of some surfactants could require other solvent systems such as alcohols, which could themselves behave as surface modifiers. In here, the influence of seven different alcohols, MeOH, EtOH, PrOH, iPrOH, ButOH, PentOH, and HexOH, in alcothermal conditions (300 °C, 24.5 MPa) over CeO(2) nanocrystals (NCs) size, morphology, and surface properties was investigated. The crystallite size of the CeO(2) nanocrystals can be tuned in the range 3-7 nm depending on the considered alcohol, and their surface has been modified by these solvents without the use of surfactants. Mechanisms are proposed for the interaction of primary and secondary alcohols with CeO(2) surface and its functionalization during the synthesis based on FTIR and TGA-MS studies. This study allows apprehending the role of alcohols during the synthesis and may lead to an informed choice of solvent as a function of the required size and surface properties of CeO(2) NCs. It also opens new route to CeO(2) functionalization using supercritical alcohol derivatives.

Teflon-Coated Silicon Microreactors: Impact on Segmented Liquid−Liquid Multiphase Flows

We describe fluoropolymer modification of silicon microreactors for control of wetting properties in chemical synthesis applications and characterize the impact of the coating on liquid-liquid multiphase flows of solvents and water. Annular flow of nitrogen gas and a Teflon AF (DuPont) dispersion enable controlled evaporation of fluoropolymer solvent, which in turn brings about three-dimensional polymer deposition on microchannel walls. Consequently, the wetting behavior is switched from hydrophilic to hydrophobic. Analysis of microreactors reveals that the polymer layer thickness increases down the length of the reactor from ∼1 to ∼13 μm with an average thickness of ∼7 μm. Similarly, we show that microreactor surfaces can be modified with poly(tetrafluoroethylene) (PTFE). These PTFE-coated microreactors are further characterized by measuring residence time distributions in segmented liquid-liquid multiphase flows, which display reduced axial dispersion for the coated microreactors. Applying particle image velocimetry, changes in segment shape and velocity fluctuations are observed resulting in reduced axial dispersion. Furthermore, the segment size distribution is narrowed for the hydrophobic microreactors, enabling further control of residence distributions for synthesis and screening applications.

In Situ IR Spectroscopy and Ab Initio Calculations To Study Polymer Swelling by Supercritical CO2

The CO(2) sorption and polymer swelling of hydroxytelechelic polybutadiene (HTPB) and poly(ethylene glycol) (PEG) have been investigated as a function of temperature and CO(2) pressure by combining in situ near-infrared spectroscopy with molecular modeling. The results reported here for the PEG-CO(2) system are in a very good agreement with literature data hence validating our experimental procedure. It has been found that CO(2) sorption and swelling effect is more important for PEG than for HTPB. For both polymers, an increase of temperature leads to a strong decrease of both the CO(2) sorption and swelling. In order to identify at a molecular level the nature and strength of intermolecular interaction occurring between CO(2) and the polymers, ab initio calculations have been performed on model structures, representative of the main functional group of the polymer, and their complex with CO(2). Trans-3-hexene (3-Hex), propyl methyl ether (PME) and methoxytrimethylsilane (MTMS) have been selected to mimic the functional groups of HTPB, PEG and polydimethyl siloxane (PDMS), respectively. The last system has been chosen since previous works on the swelling of PDMS by high pressure CO(2) have revealed the high ability of CO(2) to swell both uncrosslinked and crosslinked PDMS. The calculated stabilization energies of the MTMS-CO(2), PME-CO(2), and 3-Hex-CO(2) dimers indicate that CO(2) interacts specifically with the three moieties through a Lewis acid-Lewis base type of interaction with the energies displaying the following order: E(MTMS-CO(2)) = -3.59 > E(PME-CO(2)) = -3.43 > E(3-Hex-CO(2)) = -2.5 kcal/mol. Since the solubility of CO(2) in the corresponding homopolymers follows the same order, it is evidenced that the stronger the interaction between CO(2) and the polymer, the higher the CO(2) sorption. Therefore, even if one cannot exclude the influence of free volume and chain flexibility of the polymer, it appears that the solubility of CO(2) in the polymer is predominantly governed by the interaction between CO(2) and the polymer. Although the same trend is observed for the swelling of the polymer as a function of the CO(2) pressure, we have found that for a given value of CO(2) sorption, the swelling of the polymer depends on its nature, meaning that the swelling is not only governed by the CO(2)-polymer interaction but also by other intrinsic properties of the polymer.

Dripping to jetting transitions observed from supercritical fluid in liquid microcoflows

Dripping to jetting transition from liquid-liquid to liquid-gas systems has been widely studied in microfluidics. Recent developments have permitted the use of supercritical fluids at high pressure and various temperatures in microfluidic systems to develop sustainable processes. In this article, we use this strategy to demonstrate the possibility to generate stable droplets and reversible dripping to jetting transitions at microscale from supercritical carbon dioxide-liquid water microcoflows.

CeO2 Nanocrystals from Supercritical Alcohols: New Opportunities for Versatile Functionalizations?

The fast and controlled synthesis of surface-modified cerium oxide nanoparticles was carried out in supercritical {ethanol + alcohol derivative} mixtures. The newly found ability of supercritical alcohols to graft onto cerium oxide nanocrystals (CeO2 NCs) during their synthesis was exploited to control their surface chemistry via the addition of three aminoalcohols: ethanolamine, 3-amino-1-propanol and 6-amino-1-hexanol. Although the ethanol to aminoalcohol ratio was consistent (285:1), the successful grafting of these alcohol derivatives onto CeO2 NCs was identified based on Fourier transform infrared (FTIR) and thermogravimetric analysis-mass spectrometry (TGA-MS) measurements. Smaller crystallite size of CeO2 NCs synthesized in the presence of aminoalcohols, compared to those synthesized in supercritical ethanol alone, were also noticed and attributed to a possible intervention of amine groups helping the grafting of the alcohols, allowing one to stop the growth of the CeO2 NCs faster. The use of supercritical alcohol mixture-ethanol with hexanol, dodecanol, or octadecanol, with a 285:1 ratio-was also investigated. Such mixtures allow accessing a finer control in CeO2 NCs crystallite size compared to pure alcohols, according to calculation made from X-ray diffraction measurements. Finally, fluorescent molecules (fluorescein isothiocyanate) were grafted onto amine-modified CeO2 NCs. The powders displayed a fluorescent behavior under UV light, confirming the suitability and interest of CeO2 NCs surface modification by such technique.

Microfluidic Synthesis of Palladium Nanocrystals Assisted by Supercritical CO2: Tailored Surface Properties for Applications in Boron Chemistry

On the surface: A library of organic-inorganic hybrid palladium nanocrystals was synthesized using continuous supercritical microfluidic technology. The nanocatalysts show moderate to excellent activities towards C(Ar)-B and C(Ar)-C(Ar) bond-forming reactions, thus illustrating the relationship between surface properties and modulated catalytic activity.

High-Pressure/High- Temperature Microreactors for Nanostructure Synthesis

Chemically synthesized nanomaterials, such as nanocrystalline quantum dots, are being considered as the active element in many applications, including photovoltaics, displays, and biochem sensing. To realize the promise of these devices, it will be critical to have an efficient, reproducible synthesis technique of the nanostructures. Currently, nanoparticles are synthesized in a batch mode in small volumes, which is appropriate for studying the fundamental properties of nanosized structures and for developing proof of principle device structures. However, batch synthesis suffers from control of size, size distribution, and quality of the nanomaterial from batch to batch. Moreover, there is an inherent difficulty in scaling up to quantities more reasonable for device development and optimization. Continuous-flow reactors based on microfluidics (microreactors) integrated with heaters and fluid control elements offer a solution to these problems and additional advantages. We describe continuous synthesis of nanostructures in microfluidic systems consisting of multiple sub—millimeter-sized channels in which fluid flows continuously and chemical reactions take place. The small reaction volumes combined with the high heat and mass transfer rates enable reactions to be performed under more controlled conditions with higher yields than can typically be achieved with conventional reactors. Moreover, manipulation of reaction parameters, while the reaction proceeds, allows optimization of synthesis conditions. The ability to work at elevated temperatures and pressures while confining potentially toxic, high reactive starting materials will become important for the synthesis of novel nanostructured materials.

Fast and continuous processing of a new sub-micronic lanthanide-based metal–organic framework

Processing strategies for the synthesis of hybrid materials stand as relevant ways to modulate the particle size and morphology. We present herein the use of a continuous high temperature–high pressure (HT–HP) process for the synthesis of a new cerium based metal–organic framework (MOF). The HT–HP harsh thermodynamic synthesis conditions lead to MOF nanostructures exhibiting the same phase as for microparticles obtained under conventional batch solvothermal conditions but in exceptional much shorter residence times, opening avenues towards production scaling-up. The HT–HP process also tailors down the size of the particles, which still presents a major issue for most MOF applications.

High Yield Synthesis of Aspect Ratio Controlled Graphenic Materials from Anthracite Coal in Supercritical Fluids

This paper rationalizes the green and scalable synthesis of graphenic materials of different aspect ratios using anthracite coal as a single source material under different supercritical environments. Single layer, monodisperse graphene oxide quantum dots (GQDs) are obtained at high yield (55 wt %) from anthracite coal in supercritical water. The obtained GQDs are ∼3 nm in lateral size and display a high fluorescence quantum yield of 28%. They show high cell viability and are readily used for imaging cancer cells. In an analogous experiment, high aspect ratio graphenic materials with ribbon-like morphology (GRs) are synthesized from the same source material in supercritical ethanol at a yield of 6.4 wt %. A thin film of GRs with 68% transparency shows a surface resistance of 9.3 kΩ/sq. This is apparently the demonstration of anthracite coal as a source for electrically conductive graphenic materials.

Continuous synthesis of high quality CdSe quantum dots in supercritical fluids

We demonstrate in here a powerful scalable technology to synthesize continuously high quality CdSe quantum dots (QDs) in supercritical hexane. Using a low cost, highly thermally stable Cd-precursor, cadmium deoxycholate, the continuous synthesis is performed in 400 μm ID stainless steel capillaries resulting in CdSe QDs having sharp full-width-at-half-maxima (23 nm) and high photoluminescence quantum yields (45–55%). Transmission electron microscopy images show narrow particles sizes distribution (σ ≤ 5%) with well-defined crystal lattices. Using two different synthesis temperatures (250 °C and 310 °C), it was possible to obtain zinc blende and wurtzite crystal structures of CdSe QDs, respectively. This synthetic approach allows achieving substantial production rates up to 200 mg of QDs per hour depending on the targeted size, and could be easily scaled to gram per hour.