Article pubs.acs.org/Langmuir
Near- and Supercritical Alcohols as Solvents and Surface Modifiers for the Continuous Synthesis of Cerium Oxide Nanoparticles Cédric Slostowski,†,‡ Samuel Marre,†,‡ Odile Babot,§ Thierry Toupance,§ and Cyril Aymonier*,†,‡ †
CNRS, ICMCB, UPR9048, F-33600 Pessac, France Univ. Bordeaux, ICMCB, UPR9048, F-33600 Pessac, France § University of Bordeaux, Institut des Sciences Moléculaires, UMR 5255 CNRS, C2M team, 351 cours de la Libération, 33405 Talence, France ‡
ABSTRACT: 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, nearcritical (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 CeO2 nanocrystals (NCs) size, morphology, and surface properties was investigated. The crystallite size of the CeO2 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 CeO2 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 CeO2 NCs. It also opens new route to CeO2 functionalization using supercritical alcohol derivatives.
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zirconia),16 new strategies have been adopted in CeO2 supercritical synthesis. Indeed, organic-ligand-assisted synthesis in supercritical solvents has emerged in order to produce colloidal CeO2 NCs with tailor-made surface crystal planes.17,18 Furthermore, it has been obviously demonstrated that the use of surfactants during their synthesis allows for tuning CeO2 NCs shape, size, and/or dispersion ability.19,20 Typically, fatty acids (higher decomposition temperature and better dispersion) with carboxylic function are used, since carboxylic groups have proved to easily graft over CeO2 surface.18,21 Such surfactants will preferentially graft on {001} planes, leading to nanocube morphologies with (001) surfaces exposed. If the surfactant is in excess, it will graft on both {001} and {111} planes, leading to truncated octahedron enclosed by {111} and {200} planes. The use of such surfactants induces an issue for continuous syntheses. Indeed fatty acids are hardly soluble in water due to their long carbon chain. To address this limitation, two strategies have been developed. First, syntheses have been conducted in batch mode. Due to the low dielectric constant of scH2O,22 the organic ligand become miscible with the supercritical aqueous solution, resulting in a single phase reaction.23 The second strategy has been either to use water-
INTRODUCTION Cerium oxide particles synthesis has been widely studied over the past 30 years for many applications such as catalysts or polishing materials.1−4 The physical properties being of key importance for such applications, researches have been focused on synthesizing monodispersed nanostructured materials with uniform shape and size. Among the CeO2 synthesis approaches, supercritical (sc) and near-critical (nc) ways have been extensively developed over the past decade. In particular, supercritical fluids (SCFs) exhibit unique properties, which can be continuously tuned as a function of temperature and pressure.5 These fluids act as unique media to control the formation of nanocrystals (NCs) and their associated technologies are easily scalable.6−8 Initially, scH2O was used by Adschiri and co-workers, who proposed the first supercritical synthesis of cerium oxide based on their previous works on metal oxide particles.9−11 Synthesis of crystalline cerium oxide particles has been then reported in nc- or sc-water, usually in a continuous process in which a precursor flow is mixed with a preheated scH2O flow into a reactor.12−14 Recently, in situ synchrotron powder X-ray diffraction measurements have been performed to follow and better understand the nucleation and growth of CeO2 NCs synthesized in scH2O.15 Since Sayle et al. predicted in their theoretical study that the (100) surface should be more reactive than (110) or (111) for the CeO2/YSZ(110) system (where YSZ is yttria-stabilized © 2012 American Chemical Society
Received: August 12, 2012 Revised: October 4, 2012 Published: November 5, 2012 16656
dx.doi.org/10.1021/la303265t | Langmuir 2012, 28, 16656−16663
Langmuir
Article
experiment. The appropriate flow rate, Q (m3·s−1), has been calculated for each alcohol using the following equation:
soluble surfactants, such as hexanedioic acid,24 limiting the choice of ligands or to use other solvent systems. In this purpose, interest has been focused on alcohols, in particular, methanol, as described by Veriansyah et al.25,26 In their studies, they suggested that methanol could act as a methylation, methoxylation, and/or hydroxylation agent, according to their characterization of the CeO2 NCs surface in Fourier transform infrared (FTIR) spectroscopy. To date, only water and methanol have been studied as solvents for continuous supercritical synthesis of cerium oxide NCs. Currently, studies on surface modification of cerium oxides are limited by the choice of surfactants and their associated solvent systems. Although mechanisms in supercritical water are well-known,27 alcohol-mediated syntheses remain poorly studied. The mechanisms and influence of alcohols in nc- or sccontinuous synthesis of CeO2 NCs had not yet been clarified. These mechanisms could open a new path to obtain functionalized cerium oxide NCs using the solvent also as surfactant or surface modifier. In this work, the influence of seven alcohols (methanol, ethanol, propan-1-ol, butan-1-ol, pentan-1-ol, hexan-1-ol, and propan-2-ol) was investigated in the continuous synthesis of cerium oxide NCs at 300 °C and 24.5 MPa. NCs surface modifications due to alcohol interactions during cerium oxide synthesis have been identified by FTIR and TGA-MS analyses, and functionalization mechanisms are proposed based on these observations.
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Q=
Vreactorρr τρi
(1)
where τ is the residence time, Vreactor is the volume of the reactor, and ρi and ρr are the solvent densities at p = 24.5 MPa and T = 25 °C or T = 300 °C, respectively. There are little data available in the literature concerning densities of alcohols in the high pressure and high temperature conditions; therefore, for some alcohols, these values were extrapolated from experimental results found in the literature.29−34 The densities are displayed in Table 1. Reference syntheses have also been carried out in nc-water (T = 300 °C, p = 24.5 MPa, and τ = 45 s) and sc-water (T = 400 °C, p = 24.5 MPa, and τ = 10 and 45 s), in order to compare with results obtained in alcohols. Characterization Techniques. The XRD patterns were recorded on a PANalytical X’Pert MPD powder diffractometer (θ−θ Bragg− Brentano geometry using Cu Kα1,α2 (λ1 = 1.54060 Å, λ2 = 1.54441 Å) radiation, equipped with a secondary monochromator and a X’Celerator detector, in the range of 8−120°, in continuous scan mode at 3.5 × 10−3 °·s−1. The powder was ground and sieved at 50 μm before being subjected to XRD. The TEM images were obtained using a Hitachi H7650 transmission electron miscroscope operated at 90 kV. Samples were prepared by depositing drops of toluene or ethanol dispersed samples on copper−carbon grids. The FTIR measurements were carried out using a Brüker Equinox 55 spectrophotometer. The spectra were recorded over 32 scans in the range of 400−4000 cm−1, with a resolution of 4 cm−1, using KBr powder technique. The TGA-MS studies have been conducted on a NETZSCH STA 409 apparatus coupled with a Thermostar Balzers Instruments quadrupole spectrometer. Thermogravimetry (TG) analyses were recorded in the 30−1000 °C temperature range with a heating rate of 5 °C·min−1 under an argon flow. The texture of the CeO2 nanocrystals was analyzed by nitrogen adsorption isotherm (77 K) measurements. Data collection was performed by the static volumetric method, using an ASAP2010 (Micromeritics) apparatus. Prior to each measurement, the samples were degassed at 150 °C in vacuo for a time interval high enough to reach a constant pressure (
