Preparation of silane-coated TiO 2 nanoparticles in supercritical CO 2 Carlos A. García-González * , Julio Fraile, Ana López-Periago, Concepción Domingo * Instituto de Ciencia de Materiales de Barcelona, ICMAB-CSIC, Campus de la UAB s/n, E-08193 Bellaterra, Spain article info Article history: Received 31 March 2009 Accepted 16 June 2009 Available online 21 June 2009 Keywords: Silane Nanoparticles Supercritical carbon dioxide Wettability Self-assembled Monolayer abstract Nanometric inorganic pigments are widely used as fillers for hybrid composite materials. However, these nanometric powders are hydrophilic in nature and their surface must be functionalized before use. In this work, titanium dioxide (TiO 2 ) nanoparticles were coated using silane coupling agents with alkyl functionality. A supercritical carbon dioxide (scCO 2 ) method was used for surface silanization. Five alkylalkoxysilanes with different alkyl chain length and structure were studied: methyltrimethoxy, isobutyltriethoxy, octyltriethoxy, octyldimethylmethoxy and octadecyltrimethoxysilane. The microstruc- ture and thermal stability of deposited monolayers were characterized using thermogravimetric analysis, ATR–IR spectroscopy, transmission electron microscopy, wettability characterization and low-tempera- ture N 2 adsorption/desorption analysis. The use of scCO 2 as a solvent provided an effective approach to functionalize individual inorganic nanoparticles due to the enhanced diffusivity of the solution molecules in the aggregates interparticle voids. The trifunctional silanes employed here yielded surfaces with better thermal stabilities and greater hydrophobicities than the used monofunctional silane. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Inorganic particles act as fillers for soft materials, and are essen- tial components of many industrial composite products: the fillers market value in Europe in 2007 was about 2.3 billion Euros [1]. Fill- ers are basically divided into inactive and functional pigments. Inactive fillers are mainly used to reduce final material cost, while functional fillers modify some properties of the end product, such us density, shrinkage, expansion coefficient, conductivity, perme- ability and mechanical or thermal behavior. It is known that con- siderable improvements in the properties of soft materials can be attained by adding nanofillers to make composites. However, the dispersion of the inorganic nanopowder into the soft organic or polymeric phase is technically challenging, due to the different polarity of the constituents and the high surface area of the nano- metric phase. In this aspect, hydrophobic modification of the nanoparticulate material has been envisaged as a solution to in- crease the poor interfacial adhesion between inorganic and organic phases. Titanium dioxide (TiO 2 ) nanometric powder was chosen in this work as the pigment to be coated. Modified TiO 2 is widely em- ployed in the engineering of photocatalysts and dye-sensitized so- lar cells, as well as a filler in the paper, plastic, cosmetic and biomaterials industries [2–4]. Among the large variety of mole- cules used for the preparation of hydrophobic surfaces, organo- functional silanes are widely used due to the dual character of the molecules [5–8]. Nanoparticles coating with organosilanes can be performed either by in situ deposition during synthesis (e.g., dispersion and microemulsion processes) or by post-synthe- sis grafting (e.g., Langmuir–Blodgett and vapor or liquid self- assembly methods) [8–16]. Post-silanization in solution is the most common approach, since it is not restricted to the use of vol- atile silanes. However, nanoparticle aggregation is a drawback of- ten accompanying the reaction in liquid media or the drying step [17–24]. In the search for a more efficient coating process, the use of supercritical carbon dioxide (scCO 2 ) as a solvent has been envisaged as an alternative silanization method [18,21,25–30]. scCO 2 is a commonly used supercritical fluid with moderate critical pressure and temperature (P c = 7.4 MPa, T c = 304.2 K, respectively), being at the same time an excellent solvent for most alkoxysilanes [31]. Moreover, scCO 2 properties of gas-like diffusivity and viscos- ity and zero surface tension facilitate the complete wetting of the internal surface of the mesoporous aggregates formed by nanoparticles. In this work, a scCO 2 method was used for coating the surface of TiO 2 nanoparticles by silanization. The supercritical process was studied for several alkylalkoxysilane molecules having different lengths and configurations of the alkyl chains: methyltrimethoxy- silane (C 1 ), isobutyltriethoxysilane (C 4 ), octyltriethoxysilane (C 8 ), octyldimethylmethoxysilane ðC 8 C 2 1 Þ and octadecyltrimethoxysi- lane (C 18 ). Structure and properties of resulting monolayers were investigated using thermal analysis, ATR–IR spectroscopy, trans- mission electron microscopy, wettability studies and textural char- acterization performed by low-temperature N 2 adsorption. Firstly, the extension of the advantages encountered on the use of scCO 2 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.06.035 * Corresponding authors. Fax: +34 93 5805729. E-mail addresses: cgarcia@icmab.es (C.A. García-González), conchi@icmab.es (C. Domingo). Journal of Colloid and Interface Science 338 (2009) 491–499 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis