Journal of Colloid and Interface Science 229, 6–11 (2000) doi:10.1006/jcis.2000.6973, available online at http://www.idealibrary.com on Three-Dimensional Arrays Formed by Monodisperse TiO 2 Coated on SiO 2 Spheres Miguel Holgado, ∗, † Adelaida Cintas, ∗ Marta Ibisate, ∗, † Carlos J. Serna, ∗ Ceferino L ´ opez, ∗, † and Francisco Meseguer ∗, † , 1 ∗ Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Cantoblanco, 28049 Madrid, Spain; and †Unidad Asociada UPV-CSIC, Edificio Institutos II, Avenida Los Naranjos s/n, 46022 Valencia, Spain Received August 13, 1999; accepted May 16, 2000 Uniform coatings of TiO 2 on monodisperse spheres of silica have been obtained. The resulting particle sizes are within the range 0.4 to 1 µm. The thickness of the coating can be controlled by adjusting nuclei sizes (silica sphere), reaction times, and water and surfactant concentrations. The TiO 2 /SiO 2 spheres were sed- imented to achieve structures with three-dimensional order. Three- dimensional ordered materials with higher refractive index were obtained by transforming the amorphous titania layerto rutile by thermal treatment with improved mechanical rigidity. The sphere composition, quality, and morphology have been assessed by differ- ent techniques. C  2000 Academic Press Key Words: uniform coating; TiO 2 /SiO 2 spheres; three-dimen- sional order;monodisperse spheres. INTRODUCTION Mixed SiO 2 -TiO 2 powders are of interest in the paint industry as pigments (1, 2). Also, this type of powder can have applications as sensors (3–5), as dielectric ceramic materials (6), and for catalyst and catalyst support (7). On the other hand, monodisperse nanospheres when sedimented on a pristine substrate can form three-dimensional (3D) arrays like artificial opal, finding interesting applications as photonic crystals (8–10). Other domains where three-dimensional arrays could have applications are in molecular separation technology, where uniform porous support could provide optimal flow and improved efficiencies (11). The goal of this work is to prepare materials composed by monodisperse nanospheres with higher refractive index con- trasts than those obtained with silica opals to improve the 3D photonic characteristics. In this way preparation of the photonic crystals achieved of hollow titania spheres has been developed (12). For this purpose, we will study the optimal experimental conditions for the formation of the uniform TiO 2 shells over silica spheres, avoiding the heterogeneous nucleation of titania particles. Also, these structures, in which the average dielectric 1 To whom correspondence should be addressed at Unidad Asociada UPV- CSIC, Avenida Los Naranjos s/n, Edificio Institutos II, 46022 Valencia, Spain. Fax: 00 34 96 387 98 49. E-mail: fmese@fis.upv.es. constant can be tuned to different wavelengths, can be of interest in quantum electronics and optical communications (13). Our aim in this work is to understand the formation of uniform TiO 2 coatings on silica to build three-dimensional arrays with high refractive index contrast, which opens the possibility of forming photonic crystals with improved properties. EXPERIMENTAL Materials Tetraethoxysilane (TEOS, 98% purity, Merck), double- distilled water with 18 M of resistivity (DDW), ammonium hydroxide (28% purity, Fluka), titanium (IV) ethoxide (TEOT, Aldrich), hydroxypropyl cellulose (HPC; Aldrich), and ethanol absolute (Merck) were used. Techniques Transmission electron microscopy (TEM) measurement were performed with a Philips microscope EM-300. Scanning elec- tron microscopy (SEM) measurement with a Zeiss DSM-960 microscope with EDS-Link ISIS was used to perform microanalysis by energy dispersive spectroscopy (EDS). Termogravimetry-differential thermal analysis (TG-DTA) with a Stanton Redcroft STA-780 apparatus in air and Raman spectra were registered with a Micro-Raman Dilor XY. The experimental process that was followed to reach three- dimensional structures composed of TiO 2 /SiO 2 particles is shown schematically in Fig. 1. First monosdisperse SiO 2 par- ticles are prepared and then they are used as nuclei to obtain uniform TiO 2 coatings (14). The new TiO 2 /SiO 2 particles were sedimented to form ordered tridimensional arrays, which were later heated to increase the refractive index and to gain rigidity. Part I: Synthesis of the SiO 2 Core To produce highly monodispersed SiO 2 spheres, synthesis was carried out following the well-known St¨ ober-Fink-Bohn method (15). Several SiO 2 colloids from 220 to 470 nm in diame- ter were obtained with the concentrations that appear in Table 1, by altering only the water concentration in the synthesis pro- cess. Figure 2A shows a TEM micrograph, which illustrates the degree of monodispersity obtained. 6 0021-9797/00 $35.00 Copyright C  2000 by Academic Press All rights of reproduction in any form reserved.