Magnetic Rattle-Type Core−Shell Particles Containing Iron Compounds with Acid Tolerance by Dense Silica Tomohiko Okada,* ,† Shoya Ozono, † Masami Okamoto, † Yohei Takeda, † Hikari M. Minamisawa, § Tetsuji Haeiwa, ∥ Toshio Sakai, † and Shozi Mishima † † Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan § Technology Division, Faculty of Engineering, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan ∥ Department of Computer Science and Engineering, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan * S Supporting Information ABSTRACT: Magnetic rattle-type particles, comprising magnetite or metallic iron in nonporous dense hollow silica microspheres, were fabricated by using sol−gel reactions of alkylsilyl trichlorides around droplets of aqueous iron nitrate solution in a water-in-oil emulsion. After evaporation of water within the silica capsules to leave iron salts, calcination of the dried sample was conducted to transform into a hematite (α-Fe 2 O 3 ) core and porous hollow silica shell by losing alkyl groups of polyalkylsiloxane. Hydrogen gas penetrated through the silica shell and reduced hematite to magnetite (Fe 3 O 4 ) at 310 °C and metallic iron (α-Fe) at 450 and 500 °C. The reduction at 310 °C resulted in largest magnetization at 12 kOe among the present magnetic particles. The core magnetic compounds were enclosed by a dense silica shell, which was transformed from porous silica by annealing in nitrogen at 700 °C. Because the magnetic particles were encapsulated by the dense silica shell, the magnetism was shown even after immersion in 1 M HCl for a longer period. Acidity was successfully imparted on this magnetic capsule by anchoring sulfonic groups covalently for its use as magnetically collectable solid acid. 1. INTRODUCTION Encapsulation of magnetic nanoparticles in confined spaces has received much attention in many areas of interest, including separation, catalysis, transportation, and biomedical science. 1−6 It is necessary to prevent the nanoparticles agglomerating in order to maintain their nanomagnetic functionality. Thus, core−shell, or A@B particles (A, core; B, shell) have been prepared using various synthetic strategies, including self- assembled monolayers, layer-by-layer deposition, and sol−gel reactions. 6−9 The rattle-type hollow structure (i.e., nano- particles surrounded by interstitial hollow space) has also received increasing interest as useful nanoreactors (i.e., for drug delivery, 10−12 catalysis with molecular sieving, 13 and separation by magnetism 14 ). Chemically stable magnetism, where the magnetic particles are protected from oxidation and dissolution, is a prerequisite for reusable adsorbents, catalysts, and biomedical agents. Therefore, the stability of the nanoparticles has been investigated in various atmospheres and liquids. 15−24 Silica is a useful shell substance owing to its acid-tolerance 23 as well as its structural and morphological forms. A dense shell, which prevents the passage water, is necessary to protect core magnetic compound. The thickness and density of silica shell can be controlled by varying the amount of silica precursors and by degree of the polycondensation, respectively, to provide chemically stable magnetic nanoparticles. We have reported a rattle-type architecture by which a polyorganosiloxane shell is deposited at the interface of a water- in-oil (W/O) emulsion to encapsulate metallic cobalt through the sol−gel reactions of octyltrichlorosilane (OTCS) and methyltrichlorosilane (MTCS) around the droplets 25,26 of Co(NO 3 ) 2 aqueous solution. 27−29 In studies by which the interface of an emulsion (or a reverse micelle) has been used to obtain metal nanoparticles coated by shells, 30−34 deposition of a shell at the liquid−liquid interface has been recognized as a synthetic route that is free from the need to use a solid template (i.e., organic polymer spheres). In addition, shell deposition is a useful way to occlude magnets with varied compositions of the liquid droplets for versatile magnetic properties. Here, we report chemically stable rattle-type magnetite or metallic iron nanoparticles encapsulated by dense silica hollow microcapsules. After the water droplets containing iron(III) nitrate have been stabilized by hydrolyzed OTCS in isooctane, MTCS has been added to polymerize to a polyalkylsiloxane shell, which is the precursor of the silica shell. The cooperative sol−gel reactions are followed by vaporization of water from the resulting capsules to leave the Fe salt. Calcination, which loses the alkyl groups in the shell, reduction of α-Fe 2 O 3 by hydrogen gas, and final annealing in nitrogen resulted in magnetic particles enclosed by a dense silica (Scheme 1). Because of the dense silica shell, the magnetism persisted even after treatment with 1 M HCl without substantial erosion of the magnetic particles. Because iron is an abundant and less Received: February 10, 2014 Revised: May 5, 2014 Accepted: May 6, 2014 Published: May 6, 2014 Article pubs.acs.org/IECR © 2014 American Chemical Society 8759 dx.doi.org/10.1021/ie500588j | Ind. Eng. Chem. Res. 2014, 53, 8759−8765