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J. Phys. Chem. C 2009, 113, 13780–13786
Preparation and Characterization of Surface Silanized TiO2 Nanoparticles under Compressed CO2: Reaction Kinetics Carlos A. Garcı´a-Gonza´lez,*,† Javier Saurina,‡ Jose´ A. Ayllo´n,§ and Concepcio´n Domingo*,† Instituto de Ciencia de Materiales de Barcelona, ICMAB-CSIC, Campus de la UAB s/n, E-08193 Bellaterra, Spain, Department of Analytical Chemistry, UniVersity of Barcelona, Diagonal 647, E-08028 Barcelona, Spain, and Departament de Quı´mica, UniVersitat Auto`noma de Barcelona, Campus UAB, E-08193 Bellaterra, Spain ReceiVed: April 1, 2009; ReVised Manuscript ReceiVed: May 4, 2009
The feasibility of the use of compressed carbon dioxide (CO2) as a solvent for the coating of titanium dioxide (TiO2) nanoparticles with octyltriethoxysilane was tested in this work. These studies were focused on investigating the influence of the pressure, temperature, and reaction time on the process kinetics as well as on the resulting materials properties. A fundamental kinetic study was performed for the more slow process occurring under near-critical conditions to obtain useful information about the silanization mechanism. Selfassembled silane monolayers were formed very fast (15 min) under supercritical CO2 with grafting densities of ca. 2.8-3 molecules per nm2. Obtained samples were characterized concerning coating thermal stability by thermogravimetric analysis, nanoparticles surface chemistry by 29Si nuclear magnetic resonance, textural characteristics by low-temperature N2 adsorption-desorption analysis, and dispersibility in oleophilic phases by laser scattering analysis. 1. Introduction Nanocomposites of the type polymer/inorganic filler hold a relevant position concerning the capacity of constructing nanoscopic assemblies with extraordinary mechanical, optical, and/ or electrical properties. The synergy established between the constituents of the hybrid nanocomposite is principally mediated by the interfacial interactions between the matrix and the filler.1-3 However, inorganic nanofillers often supply insufficient or no reinforcement to the properties of the organic matrix because there is little interaction at the interface between the dissimilar constituents of the composite. Inorganic phase segregation and nanoparticles’ agglomeration are usual observed phenomena.4 Coupling reagents are often used to improve the uniformity of the distribution of the nanofiller into the polymeric matrix.5 The interest of these additives arises from their capacity to upgrade composite components’ compatibility by providing a molecular bridge between phases. Silane coupling agents are common grafting agents used to promote interfacial adhesion between inorganic oxides and organic phases.4,6-9 For metal oxide substrates, the most usual approach relies on grafting organosilanes on the inorganic filler surface bearing hydroxyl groups. Commonly, trialkoxysilanes condense forming closely packed self-assembled monolayers (SAMs) with relatively high ordering.10,11 Deposition of SAMs of a wide variety of silane molecules on flat, polished, or particulate metal oxide surfaces have been extensively studied using either vapor- or liquid-phase reactions11-17 or supercritical methods.16-20 The use of anhydrous vapor-silane deposition routes is limited to volatile and thermally stable organosilanes. On the other hand, silane deposition from aqueous/alcohol liquid solutions often leads to the formation of low-quality SAMs due to the difficulty in * To whom correspondence should be addressed. E-mail:
[email protected] (C.A.G.-G.),
[email protected] (C.D.). Phone: 0034 93 5801853. † Instituto de Ciencia de Materiales de Barcelona. ‡ University of Barcelona. § Universitat Auto`noma de Barcelona.
controlling the amount of water in the solvent mixture.17,21 As an alternative, the supercritical carbon dioxide (scCO2) silanization method is an anhydrous route capable of preparing highly ordered and densely packed monolayers of a wide variety of silane precursors.16,18,20,22 The main objective of this study was the optimization of the scCO2 method used for the surface modification of nanoparticles with trialkoxysilanes (RSi(OR′)3). The titanium dioxide (TiO2)octyltriethoxysilane system was chosen as a model for analysis. Surface modification of TiO2 is of great interest in scientific fields such as medical implants,23 catalysis,24 and photocatalysis,25 as well as in the chemical industries of paper, plastics, and cosmetics where silanized TiO2 is used as a white filler.26,27 First, fundamental kinetic studies were performed to obtain useful information about the silanization mechanism. Next, the work was focused on analyzing the effects of the operating variables (pressure, temperature, and reaction time) on material properties. Characterization of the system with regard to infrared and Raman spectroscopies and thermogravimetric analysis has been done elsewhere.19 The system was herein further characterized concerning nanoparticles’ surface chemistry by 29Si nuclear magnetic resonance, surface area by low-temperature N2 adsorption, and dispersibility by laser scattering analysis. 2. Materials and Methods 2.1. Materials. Octyltriethoxysilane ((C8)Si(OEt)3) and octadecyltrimethoxysilane ((C18Si(OMe)3) from Fluka were the used organosilanes. Bare TiO2 (∼20 nm, TiO2 P25) supplied by Degussa was composed of anatase and rutile in a ratio 70: 30 wt %. CO2 (Carburos Meta´licos S.A.) was used as a solvent. 2.2. Equipment and Procedure. Supercritical silanization of TiO2 nanoparticles was performed using the setup depicted in Figure 1. A high-pressure autoclave (TharDesign, Re1) with a total volume of ∼115 ml running in the batch mode was used for silane deposition. CO2 was liquefied through a cooling unit (EX1) and compressed by a syringe pump (TharDesign SP240, P1). The reactor was charged with ∼0.5 g of TiO2 powder
10.1021/jp9029985 CCC: $40.75 2009 American Chemical Society Published on Web 07/13/2009
TiO2 Silanization under Compressed CO2
J. Phys. Chem. C, Vol. 113, No. 31, 2009 13781
Figure 1. High-pressure equipment designed for batch supercritical silanization (Re1) and variable volume view cell used for silane solubility estimations (Re2).
enclosed in a cylindrical cartridge made of 0.45 µm pore filter paper, which was suspended on the upper part of the autoclave. Liquid silane (∼0.8 ml) was added at the bottom of the reactor. The vessel was then heated at the chosen temperature (T) using resistances. The pressure was settled to the chosen value (P) in the control panel of the pump. The system was stirred at 300 rpm during the running time (t). At the end of each experiment, the system was depressurized by means of valve V4 at a CO2 flow rate of ∼1.2 gmin-1 and led to cool to room temperature. Recovered as-prepared samples were further washed with a continuous flow of scCO2 (∼5 gmin-1) at 22.5 MPa and 318 K during 30 min to remove the possible excess of deposited organosilane. Obtained samples were labeled as TiC8(P,T)t in accordance with silanization operating conditions for each experiment. Some solubility measurements were performed by coupling the compression section of the high-pressure equipment in Figure 1 to a variable volume view cell (Re2, Thar Instrument Supercritical phase Monitor 20). 2.3. Characterization. The quantification of the amount of deposited silane was performed using thermogravimetric analysis (TGA, PerkinElmer 7). To ascertain the surface chemistry of the silane coating, solid-state 29Si nuclear magnetic resonance (29Si NMR) spectra were recorded on an Avance AV400WB spectrometer with a cross-polarization magic-angle spinning head at a spinning rate of 8 kHz. Micrographs of the samples were recorded in a transmission electron microscope (TEM, JEOL JEM-1210). Textural characteristics of bare and silanized TiO2 were studied by low-temperature N2 adsorption-desorption analysis (ASAP 2000 Micromeritics). Prior to measurements, samples were dried under reduced pressure (
