Ordered Nanoporous Particles

Encyclopedia of Nanoscience and Nanotechnology

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Ordered Nanoporous Particles Ferry Iskandar Japan Chemical Innovation Institute, Hiroshima, Japan

Mikrajuddin, Kikuo Okuyama Hiroshima University, Hiroshima, Japan

CONTENTS 1. Introduction 2. Synthesis 3. Characterization 4. Properties 5. Summary Glossary References

1. INTRODUCTION Porous materials are of scientific and technological interest because of their potential for applications in separation processes, catalysts, chromatography, the controlled release of drugs, low dielectric constant fillers, pigments, microelectronics, electro-optics, and other emerging nanotechnologies [1–4]. Based on the IUPAC (International Union of Pure and Applied Chemistry) definition, the pores of materials are classified according to size: pore sizes in the range of 2 nm and below are defined as micropores, those in the range of 2 nm to 50 nm, as mesopores, and those above 50 nm, as macropores [5]. A direct relationship exists between the distribution of sizes, shapes, and volumes of the void spaces in porous materials and their ability to perform the desired function in a particular application. Hence, a need exists to create uniformity within a pore size, shape, and volume, since such properties can lead to superior applications. For example, a material with uniform micropores could be used to separate molecules on the basis of their size by selectively adsorbing a small molecule from a mixture containing molecules that are too large to enter its pores [1]. In another example, the controllability of pore size as well as the morphology of a material are important in the development of ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

size-selective filters and selective optical cavities. The wavelength of the optical mode in a cavity is dependent on cavity size and the bandgap of optical crystals depends on the crystal periodicity, typically denoted as a photonic crystal [6]. In 1988, the first report of a crystalline microporous material, aluminophosphate (AlPO4 ; VPI-5), with uniform pores of 1.2 nm was reported [7]. Following the discovery of VPI-5, numerous extra-large pore materials were synthesized. In 1992, Mobil researchers disclosed the first family of highly ordered mesoporous molecular sieves M41S (pore size in the range 2–10 nm) where surfactants were used as the template or pore forming agents during a hydrothermal sol–gel synthesis [8, 9]. The use of appropriate starting materials and synthesis conditions permitted the preparation of different mesoporous silica oxides with ordered structures in hexagonal form (denoted as MCM-41), cubic form (denoted as MCM-48), and lamellar form (denoted as MCM-50). Several reviews dealing with microporous and mesoporous materials have appeared [1, 2]. In 1997, the first ordered macroporous materials produced via a template were reported [10]. A variety of macroporous ceramics, metals, semiconductors, and polymers with well-defined pore sizes in the submicrometer range have been successfully synthesized using self-assembled templates of a colloid as well as self-assembled templates of emulsion [10, 11]. Several reviews on the subject of macroporous materials have appeared in the literature [3, 4]. The above methods for preparing ordered porous materials have, thus far, resulted in irregular shapes and/or thin-film shapes. However, for practical applications, ordered porous materials must have the shape of a usable object. It would be desirable, for example, to produce ordered porous materials having a particular form. Reviews of previous methods used in the preparation of well-ordered porous particles are described here with the following main subjects: (1) synthesis, (2) characterization, and (3) properties. Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (259–270)

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2. SYNTHESIS

2.2. Biomimetic Templating

The preparation of ordered nanoporous particles was carried out in several ways as shown in Table 1. Surfactants, which function as templates for forming ordered porous materials, were typically used, particularly for the preparation of ordered microporous and mesoporous particles. A colloidal templating procedure was also used for the preparation of ordered macroporous particles. For the materials, silica was typically used in this process.

Tanev and Pinnavaia [14] condensed silica in the interlayer regions of multilamellar vesicles, a biomimetic templating approach, to form roughly spherical particles. The procedure is based on the hydrolysis and cross-linking of a neutral silicon alkoxide precursor in the interlayered regions of multilamellar vesicles formed from a neutral diamine bola-amphiphile. Unlike surfactant templating approaches, this method produces porous lamellar silicas with vesicular particle morphology. In a typical preparation, TEOS as the silica source was added to a 1,12-diaminododecane (DDAD) template in ethanol and deionized water. The reaction mixture was vigorously stirred at ambient temperature to give the templated lamellar product with a vesicular morphology. The crystalline product was recovered by filtration, washed with deionized water, and air-dried. The template was removed either by calcination in air or by solvent extraction. Complete cross-linking of the structure of the ethanol-extracted product was accomplished by subsequent calcination in air. The use of 1-alkylamine as a templating agent in the synthesis of mesoporous silica particles has also been reported by Kosuge and Singh [15]. In a typical preparation, TEOS was mixed with dilute aqueous HCl and stirred at a constant speed. To this solution, 1-alkylamine (designated by using carbon numbers such as C6 , C8 , C10 , or C12 ) was added and the solution was then stirred. The resulting mesoporous silica particles were collected by centrifugation, dried, and calcined. Appropriate control of the synthesis conditions led to the production of various morphologies of mesoporous silica particles.

2.1. Emulsion Templating Schacht et al. [12] reported on the preparation of the mesostructured and macroscopically structured ordered porous particles by interfacial reactions conducted in oil/water emulsions. The use of emulsion biphase chemistry offers the possibility of simultaneously controlling shape on the micrometer to centimeter scale and the ordered mesostructure at the molecular scale. The cetyltrimethylammonium bromide (CATB), or other related surfactants as the pore structure directing agent, was used as surfactant. The surfactant was dissolved in water and an aqueous solution of HCl was then added to the solution. To this solution a mixture of the auxiliary (typically mesitylene) and tetraethoxysilane (TEOS) was added slowly with stirring at room temperature, resulting in the formation of an emulsion. The morphology of the final product can be controlled by the simple tuning of the stirring speed in the synthesis mixture. A flat mesoporous silica film, mesoporous silica fibers, and hollow porous silica particles form can be produced by appropriate control of the stirring speed. Huo et al. [13] extended this general emulsion-based approach to the preparation of mesoporous particles with a diameter of 0.1–2.0 mm. A similar synthesis method was employed, using CTAB as a surfactant. NaOH, as an optional source of basic catalyst, was added to the CTAB solution. To this solution, tetrabutyl orthosilicate (TBOS) was added with stirring. The resulting product, a collection of hard spheres, was filtered, and air-dried at room temperature. They reported that the size of the silica spheres can be controlled by varying the reaction conditions and the volume.

2.3. Liquid-Phase Surfactant Templates 2.3.1. Preparation under Acidic Conditions Ozin and co-workers [16] reported on the preparation of mesoporous silicates particles using an extremely dilute aqueous acidic solution using cetyltrimethylammonium chloride (CTACl) as cationic surfactant template and TEOS as silica precursor. The prepared particles show the co-presence of various morphologies, for example, toroidal, disklike, spiral, and spheroidal shapes.

Table 1. Synthesis method for the fabrication of ordered nanoporous particles. Synthesis method

Template

Materials

Porous structure

Ref.

Oil–water interface emulsion templating Biomimetic templating Liquid-phase surfactant templating under acidic and basic condition Liquid-phase surfactant templating with modification of Stöber method Aerosol spray surfactant templating Aerosol spray colloidal templating Aerosol spray surfactant and colloidal templating Micromolding in inverted polymer opals Air–oil interface colloidal templating Double templating

Surfactants Diamines Surfactants

Silica Silica Silica

Mesoporous Micro- and mesoporous Mesoporous

[12, 13] [14] [16–18]

Surfactants

Silica, composite

Mesoporous

[19, 20, 22–25]

Surfactants Latex Surfactant and latex

Silica Silica Silica

Mesoporous Macroporous Meso- and macroporous

[26, 27] [30, 31] [32]

Surfactant and latex Latex Polystyrene latex, silica spheres

Silica Silica, titania Silica, titania

Mesoporous Macroporous Macroporous

[32] [34] [35]

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This synthesis method was extended by Qi et al. [17] to the synthesis of micrometer-sized mesoporous silica spheres by mixed cationic-nonionic surfactant templating. The mixed surfactants used were cetyltrimethylammonium bromide (CTAB) and decaethylene glycol monohexadecyl ether (C16 EO10 ). In a typical synthesis, CTAB and C16 EO10 were dissolved in HCl aqueous solution. To this solution, TEOS was added at room temperature with stirring. After the complete addition of TEOS, the mixture was allowed to age either without stirring or with additional stirring. The white precipitate was then recovered by filtration, washed with water, and dried. Finally, the product was calcined in air to remove the surfactants.

2.3.2. Preparation under Basic Conditions Chai et al. [18] reported on the preparation of mesoporous particles using a dilute aqueous basic medium. Silica particles with several morphologies were prepared. In this study, CTAB was used as a cationic surfactant, while TEOS served as the silica source. Aqueous sodium hydroxide or ammonia was used as the catalyst. The synthetic conditions, both stirring speed and surfactant concentration, were varied so as to control the morphologies and size of the prepared particles. Typically, a solution of CTAB, TEOS, NaOH, and water, in a known ratio, was mixed at different stirring speeds. The resulting product was filtered, washed, dried, and then subjected to calcination.

2.3.3. Modification of Stöber Method Unger and co-workers [19, 20] first reported the synthesis of spherical silica particles featuring a hexagonal (MCM-41) mesoporous structure, using a modification of the Stöber synthesis [21] by adding surfactants to the reaction mixture during particle formation. The synthesis was carried out in an alcohol-water-ammonia system. Porosity was created by adding two different types of pore structure directing agents to the starting solution: one was an n-alkyltrialkoxysilane which was covalently bonded to the silica framework, while the other was an n-alkylamine which acted as a nonionic template. Water hydrolyzes the tetraethoxysilane (TEOS) to silicic acid which further condenses to oligomers, and these primary particles agglomerate to the final silica spheres in sizes up to 1.5 mm. Ethanol acts as a cosolvent to produce a homogeneous solution, and ammonia acts as a morphological catalyst. The particle size can be adjusted by the ratio of water, alcohol, ammonia, and silane. After removal of the solvent, the product was calcined to remove the porogen and to form porous silica particles. In another paper [22], they extended this method to the preparation of submicrometer-sized solid core/mesoporous shell (SCMS) silica particles. The particles were nearly perfectly spherical in shape, with the pores being randomly distributed over the silica shell, whereas the core was composed of dense silica. The formation of the pore structure of the shell was studied as a function of the amount of n-octadecyltrimethoxysilane (C18 -TMS) as the porogen in the starting solution during the growth process. Yoon et al. [23] extended this approach to the production of mesoporous carbon hollow spheres. In this synthesis, silica (or aluminosilicate) spheres with SCMS structures

were used as template materials and an in-situ polymerized phenol-resin or poly(divinylbenzene) as carbon source. Aluminum was incorporated into the silicate framework via an impregnation method to generate strongly acidic catalytic sites for the polymerization of phenol and formaldehyde. Phenol and formaldehyde were incorporated into the mesopores of SCMS aluminosilicate, and further carbonized to obtain carbon/aluminosilicate nanocomposite. The dissolution of the aluminosilicate template using either NaOH or HF solutions generated HCMS carbon capsules. Schumacher et al. [24] reported a similar synthesis route for the formation of submicrometer- to micrometersized particles with a three-dimensional cubic structured (MCM-48) pore system. n-Hexadecyltrimethylammonium bromide was used as template. Aluminum-, chromium-, gallium-, niobium-, and vanadium-MCM-48 were also synthesized using this general procedure by using different metallic precursors (e.g., Al2 (SO4 )3 · 18 H2 O and V2 O5 ).

2.3.4. Mesoporous Nanocomposite Particles The preparation of monodisperse spherical mesoporous silica/gold nanocomposites consisting of mesoporous silica and a single gold particle approximately 60 nm in diameter using a liquid-phase self-assembly process was reported by Nooney et al. [25]. To prepare gold vitreophilic, a bifunctional ligand, mercaptopropyl trimethoxysilane (MPTS) was bound to the surface of the gold. Following this, an organic mesopores template and tetraethylorthosilicate (TEOS) were added using a Stöber method. The template used in this work was a quaternary ammonium salt, cetyltrimethylammonium bromide (CTAB). In an aqueous solution this surfactant forms an ordered hexagonal micellar array, which serves as a template for the polymerization of silicate, and the length of the carbon chain controls the radius of the mesopores. After 2 h of growth at room temperature, the sample was filtered and washed.

2.4. Aerosol Spray Method 2.4.1. Surfactant Templating Bruinsma et al. [26] first used an aerosol spray method (spray drying) to prepare mesoporous powders by the rapid evaporation of hydrolyzed silicon alkoxide-surfactant in an acidic alcohol/water mixture. During solvent drying, the silica and surfactant, cetyltrimethylammonium chloride, self-assemble to form a hexagonally ordered mesophase structure and all of the nonvolatile components (silica, polymer, and surfactant) are incorporated into the mesophase. Spray-dried particles, consisting of hollow particles with mesoporous shells, were produced. Lu and co-workers [27, 28] extended this method to the preparation of spherical well-ordered mesoporous particles via the self-assembly of surfactant templates. The synthesis process starts by using a homogeneous solution of soluble silica plus a surfactant prepared in an ethanol/water solvent. The solution was generated in the form of an aerosol dispersion within a tubular reactor (Fig. 1). In a continuous process, the aerosol particles are dried, heated, and collected on a filter. Preferential alcohol evaporation during drying enriches the particles in the surfactant, water, and silica,

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Ordered Nanoporous Particles

Droplet

Mesoporous particle Hexagonal/cubic structure

Atomizer Or

Vesicular structure

Reactor Solution reservoir

Figure 1. Schematic for the preparation of ordered mesoporous particles via a surfactant template using aerosol spray method.

inducing micelle formation and the successive co-assembly of silica–surfactant micellar species into liquid-crystalline mesophases. Typically, particles produced by this method are solid, with highly ordered hexagonal, cubic, or vesicular mesostructures. Rao et al. [29] extended this method to the preparation of mesoporous particles with diameters in the range of 5 to 10
m.

2.4.2. Colloidal Templating The production of spherical-shaped porous particles with a nanoscale ordering porosity by means of an aerosol spray method via a colloidal crystal template has been reported by Iskandar et al. [30, 31] The synthesis via colloid crystallization permits pore sizes to be controlled in the range of nanometers to micrometers. Figure 2 depicts the schematic for the preparation of such ordered porous particles. Colloidal silica nanoparticles were mixed with polystyrene (PS) latex, in a certain fraction in water, which was then atomized to generate droplets. The size of the silica particles varied from 5 nm to 25 nm and the PS latex from 42 nm to 178 nm. The reactor consisted of two heating zones: the first was used to evaporate the solvent in the droplet and the second was used to evaporate the PS latex particles, resulting in the formation of porous silica particles. This method is rapid and relatively simple. A large amount of porous particles can be produced in just several seconds. The production rate can be easily controlled by the appropriate control of the rate of flow of the nitrogen carrier gas. This method is an in-situ process. No post treatment is required to produce porous particles. Particle size can be controlled by altering the droplet size as well as the concentration of primary particles in the precursor, which represents additional advantages of this method.

µm size droplet

2.4.3. Combining of Surfactant and Colloidal Templating The synthesis of hierarchical porous silica nanoparticles with well-defined pore sizes (or cells) templated by combining colloidal particles with surfactants, or microemulsions, through an aerosol process has been reported by Fan et al. [32]. Adjustable meso- or macroporosity nanoporous silica particles could be produced using a soft microemulsion and solid polystyrene beads as templates. The synthesis process starts by using a homogeneous solution of polystyrene spheres/silica, polystyrene spheres/ surfactant/silica, or microemulsions/silica. The method for preparing the porous silica particles was similar to that used for the preparation described in Figures 1 and 2. After removal of surfactants and polymer spheres, the resulting materials exhibited a controlled meso- and macroporosity.

2.5. Micromolding in Inverted Polymer Opals The synthesis of silica spheres composed of the hexagonal symmetry form of mesoporous silica using micromolding in inverted polymer opals (MIPO) was reported by Yang et al. [33]. MIPO begins with the synthesis of a polystyrene micromold having the structure of an inverted opal (Fig. 3). This is achieved by the radical polymerization of styrene monomer within the void spaces of a silica opal that had been grown and sintered followed by removal of the silica opal by fluoride-based etching. The polystyrene micrometer-scale structure is well ordered and within each spherical void regular arrangements of smaller holes that originate from the necking points between silica spheres in the sintered fcc opal template can be observed. The polystyrene inverted opal is subsequently infiltrated with a silicatropic liquid crystal composed of a nonionic surfactant C12 H25 (OCH2 CH2 10 OH, water, hydrochloric acid, and TEOS, which slowly undergoes hydrolytic polycondensation to a monolithic periodic mesoporous silica. Removal of the polystyrene mold either by solvent extraction or by calcination in air leaves behind

Silica etching by HF

Infiltration of styrene and polymerization

Sub-micrometer particle

Drying at low temperature

Drying at high temperature

Two temperature zones reactor Silica nanoparticle Polystyrene latex (PSL) particle

Figure 2. Schematic for the preparation of ordered macroporous particles via a colloidal template using aerosol spray method.

Removal of polystyrene by calcination in air or toluene dissolution

Infiltration and condensation of silicatropic liquid crystal

Figure 3. Schematic for the preparation of ordered mesoporous particles via a surfactant template using micromolding in inverted polymer opals. Reprinted with permission from [33], S. M. Yang et al., Adv. Mater. 12, 1940 (2000). © 2000, Wiley-VCH.

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Ordered Nanoporous Particles

a well-ordered opal replica in which the individual particles are composed of periodic mesoporous silica.

2.6. Air–Oil Interface Colloidal Templating Ordered macroporous particles of silica and titania fabricated by colloidal templating in aqueous droplets straddling an air–oil interface have been investigated by Yi et al. [34]. The procedures involve the initial preparation of spherical colloidal crystalline particles of polystyrene latex spheres followed by infusion with metal precursor solutions that form silica or titania in the interstices (Fig. 4). Finally, calcination is employed to decompose the polystyrene latex spheres, leaving macropores at their sites. The shape of the template can be controlled by the presence of added surfactant or by the action of an applied electric field. Specifically, spherical, concaved disklike, and ellipsoidal colloidal crystals were successfully prepared and used as templates in the fabrication of ordered macroporous particles. An electric field can be applied to the droplet-template colloidal crystallization cell with electrodes that are connected to an alternating current (ac) electric power supply. The suspension droplet of the monodisperse PS latex spheres straddles the air–oil interface, but most of the suspension droplet is immersed in the oil phase. If necessary, an ac electric field can be applied to deform the suspension droplets. During the evaporation of the drop-phase liquid (i.e., water in the present case), the PS latex spheres in the suspension droplets begin to order into a macrocrystalline structure. In the presence of an applied electric field, the suspension droplets are deformed into spheroids. A viscous silicone oil or a partially fluorine-substituted silicone oil was used as the continuous oil phase. The suspension droplets underwent shrinkage during incubation. Finally, the oil that had infiltrated into the macrocrystalline structure was extracted with hexane and drying gave supraparticle assemblies having various shapes. The prepared macrocrystalline particles were soaked in a solution of a metal alkoxide precursor, which penetrated the interstices between the PS latex spheres by capillary force.

The macrocrystalline spheres into which the precursor had fully infiltrated were removed and then exposed to air. The metal oxide precursor was then hydrolyzed by moisture in the air. It is especially noteworthy that before the hydrolysis reaction proceeded, the residual alkoxide precursor remaining on the surface of spherical colloidal assemblies must be removed. Otherwise, a thick skin is formed on the ordered macroporous sphere. Skin formation can be avoided by removing the precursor solution on the surface of colloidal assemblies in a moisture-free space or by treatment with n-propanol. Finally, the polystyrene latex spheres constituting the organic-inorganic composite microstructured particles were removed by calcination, leaving ordered spherical air voids at their sites in a matrix of titanium oxide or silicon oxide.

2.7. Double Templating Yi et al. [35] reported on a fabrication method for preparing an array of uniform micron-sized ceramic spheres with ordered macropores by double templating. The synthetic method for preparing the macroporous particles is a twostep template-assisted fabrication process, as illustrated in Figure 5. Micrometer-sized silica particles are initially assembled into a close-packed colloidal crystalline array and are then encapsulated by polymerizing a polymer in the interstices. The silica spheres are then removed by selective chemical etching, leaving behind micrometer-sized air cavities. The polymer latex particles are then injected into the spherical air cavities inside the polymer matrix. The polymeric particles assemble within the voids to form an ordered close-packed structure. Finally, an inorganic precursor is infiltrated into the interstices between the latex particles and gelled to capture the ordered structure. A key feature of the polymer template is that the macropores are interconnected with windows sufficiently large for small polystyrene latex with size of about 500 nm to pass through.

Polymer encapsulation

Optical microscopy Desiccant

Silica Colloidal Crystal Ordered macroporous polymer

Electrode

Suspension droplet Silicone oil

Figure 4. Schematic of the colloidal crystallization cell with a suspension droplet as a template. Reprinted with permission from [34], G.-R. Yi et al., Chem. Mater. 13, 2613 (2001). © 2001, American Chemical Society.

Remove polymeric Filling the polymeric particles matrix and particles inside big pores through windows by burn-out and infiltrating inorganic precursor

Figure 5. Schematic for the preparation of ordered mesoporous particles via a colloidal template by double templating method. Reprinted with permission from [35], G.-R. Yi et al., J. Am. Chem. Soc. 124, 13354 (2002). © 2002, American Chemical Society.

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3. CHARACTERIZATION Characterization of the morphology and porous structure of prepared particles usually involves the use of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and adsorption analysis. In this section, the characterization of the prepared particles will be described from the following points of view: (1) microscopy observation, (2) X-ray diffraction, and (3) adsorption analysis. Table 2 shows some data relative to the characterization of ordered nanoporous particles.

3.1. Microscopy Observation Using SEM and TEM observation techniques, the morphology, size distribution, and porous structure of the prepared particles can be observed. In the following subsections, the microscopy characterization of the prepared particles will be summarized according to the synthesis method used.

3.1.1. Emulsion Templating In the preparation of ordered porous particles via oil–water interface emulsion templates, Schacht et al. [12] reported that the final product morphology can be controlled by the stirring speed used in the synthesis (see Section 2.1). Stirring is one mechanism for controlling emulsion properties through modification of long-range hydrodynamic forces and is crucial in the formation of the secondary morphology. Under low stirring speed, a fiber type morphology was observed. By increasing the stirring speed, more spherical particles were formed and the fiber morphology disappeared. The spherical particle size decreased with increasing stirring speed. However, the size distribution was broad and some of the particles were agglomerated. By carefully controlling the synthesis conditions, a relatively narrow

size distribution of spherical particles was obtained. From other SEM images, these particles were shown to be hollow and most were crushed after the organic phase had been removed. Silica particles in the form of fibers have a size of 50–1000
m in length, and in the hollow spherical form have a diameter of 10 to 500
m. Using a similar method, Huo et al. [13] reported that by combining different surfactants and basic catalysts, the prepared particles have a solid and spherical morphology, where the stirring speed influences the condition of the particles. A low stirring speed resulted in the formation of soft gel particles, a medium stirring speed give millimetersized particles, and high stirring speeds resulted in smaller particle sizes. The size of the silica particles was uniform and could be controlled in the range of 0.1 to 2
m. TEM images suggest that the particles exhibit a small pore size in the nanometer range (1–5 nm) and a preferentially ordered hexagonal form (MCM-41), while other areas show pores that appear to have a narrow size distribution but with a random orientation.

3.1.2. Biomimetic Templating In the preparation of ordered porous particles via diamine templating, elliptical well-ordered multilamellar regions near the vesicles of 300 to 800 nm were observed, as evidenced by TEM and SEM observations [14]. The multilamellar regions near the vesicle surface are populated by a dense inorganic phase. This structural feature is indicative of biomimetic nucleation and the growth of lamellar silica material, as the self-assembly process occurs in the interlayered regions of the multilamellar bola-amphiphile vesicles. Tanev and Pinnavaia postulated that the formation of vesicular materials occurs in a manner reminiscent of natural biomineralization processes.

Table 2. Characterization data of ordered nanoporous particles.

Preparation method Oil–water interface emulsion templating Biomimetic templating Liquid–phase surfactant templating under acidic/ basic condition Liquid–phase surfactant templating with modification of Stöber method Aerosol spray surfactant templating Aerosol spray colloidal templating Aerosol spray surfactant and colloidal templating Micromolding in inverted polymer opals Air–oil interface colloidal templating Double templating

Size [
m]

Pore size [nm]

Specific surface area [m2 /g]

Specific pore volume [cm3 /g]

Hexagonal

1–2000

1.4–10

∼1100

024 ∼ 061

[12, 13]

Vesicle Hexagonal

0.3–0.8 2–20

0.6, 1.2 25 ∼ 5

∼984 ∼1042

047 ∼ 056 0.70

[14] [16–18]

Spherical

Hexagonal, cubic

0.4–2.3

2.10–11.2

∼1600

049 ∼ 12

[19, 20, 22–25]

Spherical

Hexagonal, cubic, vesicular Hexagonal

0.05–0.5

1.8–9.2

∼1770

—

[26, 27]

0.3

42–178

—

—

[30, 31]

Morphology Fibers, spherical solid and hollow Spherical hollow Toroidal, disklike, spiral, spherical

Ordered type

Ref.

Spherical, doughnut-like Spherical

Hexagonal

0.1

4.0–100

∼480

0.53–0.88

[32]

Spherical

Hexagonal

028 ∼ 036

4.1

∼560

—

[32]

Spherical, concave disklike, ellipsoidal Spherical

Hexagonal

2000

190–275

—

—

[34]

Hexagonal

60

500

—

—

[35]

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Ordered Nanoporous Particles

In the preparation of ordered porous particles via diamine templating, various types of spherical morphologies of spirals, hollow and solid mesoporous silica with submicrometersized diameters were obtained [15]. By changing the mixing order of the reactants used in the synthesis, mesoporous solid particles of several tens of micrometers in size were obtained. The morphologies of the particles produced and their porous properties are strongly influenced by the alkyl chain length of the 1-alkylamine template and the concentration and volume of the acidic aqueous solution used in the synthesis mixture. A longer alkyl chain (C10 , C12 ) leads to production of a flaky, ultrathin lamina morphology, while a short alkyl chain (C6 , C8 ) produces a spherical morphology, C6 templating produces hard mesoporous silica spheres, and C8 templating, spiral or hollow spheres. With a very long alkyl chain (above C12 ) or a short alkyl chain (below C6 ), the amorphous qualities significantly increase, indicating instability or incomplete solubility of the 1-alkylamine.

3.1.3. Liquid-Phase Surfactant Templating under Acidic and Basic Conditions Mesoporous particles prepared by this method have spherical and curved morphologies including toroidal, disklike, spiral, and spheroidal shapes [16–18]. In the case of the use of a cationic surfactant under acidic conditions, SEM images reveal that the morphologies are extraordinary in terms of their diverse and remarkable shapes and impressive range of curvature [16]. However, for the use of cationic-nonionic surfactants, SEM images reveal the production of entirely spherical particles, although some agglomeration is evident. The size of these silica spheres ranges from 2 to 6
m. In the case where a cationic-nonionic surfactant is used, the exterior surface of the particles is very smooth, in contrast to the faceted and corrugated surfaces exhibited by the mesoporous silica bodies synthesized by cationic surfactant templating. TEM images of the basic morphologies clearly show the presence of hexagonally close-packed channels with a center-to-center spacing of ∼5 nm.

3.1.4. Liquid-Phase Surfactant Templating: Modification of Stöber Method The morphologies of particles prepared by the Stöber method using the CTAB surfactant as a template were almost spherical with a size range from 400 to 1100 nm. In the case where n-alkylamines were employed as templates, spherical particles in a wide range of sizes, from 0.5 to 2.3
m, were obtained. The particles are nearly completely dispersed although some agglomeration was visible.

3.1.5. Aerosol Spray Surfactant Templating In the preparation of particles by aerosol spray surfactant templating, particles with ordered mesoporous structures and a spherical morphology were obtained. Different surfactant templates, that is, CTAB, Brij-58, Brij-56, or P123, exhibited different structural mesopores of the prepared particles. The use of the CTAB surfactant gave particles that exhibit a highly ordered hexagonal mesophase (Fig. 6a). Many of the particles adopted a polyhedral shape that is hexagonal in cross section. The use of nonionic surfactants (Brij-56/58) commonly resulted in vesicular mesostructures,

50 nm

50 nm

a

50 nm

c

50 nm

b

d

Figure 6. Representative TEM micrographs of mesostructured silica particles. (a) Faceted, calcined particles with hexagonal mesophases. The sol was prepared using 5 wt% CTAB as the surfactant template. (b) Calcined particles showing a cubic mesostructure. The sol was prepared using 4.2 wt% Brij-58. (c) Calcined particles showing a vesicular mesophase. The sol was prepared using 5% P123. (d) Uncalcined silica particles showing the “growth” of ordered vesicular domains from the liquid–vapor interface. The sol was prepared using 2.5% Brij-56. Reprinted with permission from [27], Y. Lu et al., Nature 398, 223 (1999). © 1999, Macmillan Magazines Ltd.

but cubic and hexagonal mesostructures could also be produced (Fig. 6b and d). The use of the P123 surfactant as the triblock copolymer template typically resulted in a vesicular mesophase (Fig. 6c). Evaporation during aerosol processing creates a radial gradient in surfactant concentration within each droplet that steepens with time and maintains a maximum concentration at the droplet surface. Starting with an initially homogeneous solution, the surfactant critical micelle concentration is exceeded first at the surface of the droplet, and, as evaporation proceeds, is progressively exceeded throughout the droplet. This surfactant enrichment induces a silica–surfactant self-assembly into the micelles and further organization into liquid-crystalline mesophases. The radial concentration gradient and the presence of a liquid–vapor interface causes the rapid inward growth of ordered silica– surfactant liquid-crystalline domains (Fig. 6d) rather than in the outward direction from a seed. Unlike films—which have flat liquid–vapor interfaces and show a progressive change in mesostructure (disordered → hexagonal → cubic → lamellar) with increasing surfactant concentration, particles prepared with comparable CTAB concentrations show only disordered or hexagonal mesophases. The reason for this is because the liquid– vapor interface serves as a nucleating surface for liquidcrystal growth; the high curvature imposed by this interface alters the generally observed relationship between the surfactant packing parameter and the resulting mesostructure.

266

Ordered Nanoporous Particles

Although the use of CTAB typically leads to the formation of lamellar mesophases in bulk and thin-film samples, it appears that this molecule cannot pack into a cone that is truncated by surfaces of high and opposite curvature, as is required to direct the vesicular mesostructure. Only surfactants containing ethylene oxide (EO) blocks consistently gave vesicular mesophases.

3.1.6. Aerosol Spray Colloidal Templating In the preparation of particles by aerosol spray colloidal templating, ordered macroporous particles were obtained. The resulting powders are almost spherical (Fig. 7a). Ordered pores with a close-packed hexagonal arrangement are clearly observed on their surfaces, indicating that a selforganization of the PSL particles occurred spontaneously during the evaporation process. The area of organization (arrangement) increased with an increase in powder particle size, which was obtained using a larger-sized droplet. TEM images reveal that the pore arrangement persists throughout the entire volume of the particles (Fig. 7b). The pore sizes are similar to the PS latex particle sizes. This suggests that the pore size can be easily controlled by appropriately altering the size of the PS latex particles (Fig. 7c and d). There is an optimum fraction of PS latex and primary silica particles to result in the formation of spherical particles with an organized pore arrangement.

3.1.7. Micromolding in Inverted Polymer Opals SEM images of prepared porous particles using this method showed that the morphology of particles was spherical (Fig. 8). The diameter of the product of mesoporous silica a)

b)

1000 nm 50 nm

c)

a

b

c

45 nm

Figure 8. Microscope images of mesoporous silica particles prepared by micromolding in inverted polymer opals. (a) SEM image of calcined mesoporous spheres, (b) a triplet coalescence defect, and (c) TEM image of a microtomed thin section. Reprinted with permission from [33], S. M. Yang et al., Adv. Mater. 12, 1940 (2000). © 2000, Wiley-VCH.

depends on the colloidal template (silica microspheres) size used. In the case where a silica microspheres size of 460 nm is used, the diameter of the air microspheres in the inverted mold is ca. 460 nm. The mesoporous silica opal showed that the size of the as-synthesized mesoporous silica microspheres is ca. 420 nm and this observed shrinkage is due to the condensation of silicate to silica (Fig. 8a). The mesoporous silica particles further shrink to ca. 390 nm after calcination because of further condensation polymerization of Si-OH groups on the surface of the mesoscale channels (Fig. 8c). In the case where a silica opal consisting of smaller ca. 360 nm microspheres is used, the diameter of the as-synthesized mesoporous silica microspheres is ca. 280 nm. The monodisperse mesoporous silica microspheres are packed in a regular fcc array. In addition, intriguing coalescence defect structures are observed, as depicted in Figure 8b. These structures are most pronounced when templating the mesoporous silica opal with smaller microspheres. TEM images of a thin microtomed section show hexagonally ordered mesoscale pores with a diameter ca. 5 nm.

d)

3.1.8. Air–Oil Interface Colloidal Templating

100 nm

100 nm

Figure 7. (a) Typical SEM image of ordered macroporous silica particles synthesized by templating spherical colloidal 178-nm PS particles using a spray drying method. (b) Typical TEM image of ordered macroporous silica particles. (c) SEM image at high magnification shows the hexagonal packing of macroporous particles. (d) SEM image of ordered macroporous silica particles synthesized by templating 79-nm PS particles. Reprinted with permission from [31], F. Iskandar et al., Nano Lett. 2, 389 (2002). © 2002, American Chemical Society.

The shape of the prepared colloidal crystals was either spherical or nonspherical (oblate or prolate spheroids) and could be successfully controlled by applying an ac electric field or by adding a surfactant. Specifically, spherical, concaved disklike, and ellipsoidal colloidal crystal were successfully prepared and used as a template in the fabrication of ordered macroporous particles. The SEM images of the prepared macroporous particles showed that the pores were interconnected and ordered into a hexagonal arrangement. The sizes of the pores ranged from 190 to 275 nm, depending on the colloidal template size. All samples underwent shrinkage during calcination. Nevertheless, the spherical shapes of the colloidal crystals remained, as shown from a low-magnification image of an optical

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microscope (Fig. 9a). The SEM images reproduced in Figure 9b showed that calcined samples of titanium oxides were highly ordered in three dimensions over the entire range, resembling a cubic close packing of cages. However, the cubic close packing structure can clearly be seen from the two-dimensional Fourier transforms (FFTs) of the corresponding SEM images (see the insets in Fig. 9b). The resulting spot pattern indicates that the image plane is close to the (111) plane of an fcc lattice. The void spaces were interconnected in three dimensions through windows, the diameters of which typically exceeded 40 nm, and the wall thickness was about 40 nm (Fig. 9c).

A

B

C

3.1.9. Double Templating SEM images of mesoporous silica particles prepared by the double templating method are shown in Figure 10. Figure 10a shows several millimeter-sized polymer skeletons filled with small latex particles. After the large pores were filled with the polymer particles and the remaining water was removed by drying, the metal alkoxide precursor of silica or titania was infiltrated into the interstices formed between the latex particles (Fig. 10b), after the organic polymer matrix and latex particles were removed. Figure 10c shows that photonic balls with ordered spherical macropores were successfully produced. The size distribution of the photonic balls was determined by the size of the silica particles that were used in the original templating process. The inset of Figure 10c shows that the internal structure of each photonic ball is highly ordered.

3.2. X-Ray Diffraction X-ray diffraction techniques are frequently used to characterize the porous structure of ordered nanoporous particles. XRD patterns of hexagonal ordered mesoporous materials A

usually exhibit three or five reflections, or Bragg peaks, between 2 = 2 and 6 . Figure 11 shows XRD patterns of dried and calcined fibers prepared by an aerosol spray surfactant templating method [26]. The reflections are due to the ordered hexagonal array of pores and can be indexed assuming a hexagonal unit cell as (100), (110), (200), and (210) after calcination. The XRD pattern of lamellar mesoporous materials typically shows one intense peak between 2 = 2 and 6 , along with a broad weak shoulder [14]. However, samples with only one distinct reflection have also been reported to contain substantial amounts of hexagonal structure [16]. The XRD patterns of cubic structure mesoporous materials typically exhibit three or five Bragg peaks, between 2 = 2 and (1

B

Figure 10. SEM images show (A) shaped colloidal crystals in a polyurethane skeleton, (B) composite of shaped colloidal crystals and an infiltrated titanium alkoxide precursor solution, and (C) ordered macroporous titanium spheres by removal of the organic polymer phase of the previous composite. (Scale bars are 1
m). Reprinted with permission from [35], G.-R. Yi et al., J. Am. Chem. Soc. 124, 13354 (2002). © 2002, American Chemical Society.

Figure 9. (A) Optical microscope images of an ordered macroporous titania sphere synthesized by templating the spherical colloidal assembly formed from the 230-nm PS spheres. (B) Typical SEM image of an ordered macroporous titania sphere. The inset shows the fast Fourier transform of the SEM image. (C) SEM image at large magnification showing the interconnected cavity pores. Reprinted with permission from [34], G.-R. Yi et al., Chem. Mater. 13, 2613 (2001). © 2001, American Chemical Society.

1

2

3

4

(200)

B

(210)

(200)

(110)

(110)

(100)

C

5

6

A

7

8

× 3

9

10

2-Theta

Figure 11. XRD patterns of mesoporous silica fibers: (A) air-dried fibers; (B) calcined fibers. Reprinted with permission from [26], P. J. Bruinsma et al., Chem. Mater. 9, 2507 (1997). © 1997, American Chemical Society.

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Ordered Nanoporous Particles

3.3. Adsorption Analysis Adsorption analysis has been widely used to determine surface area and to characterize the pore-size distribution of ordered nanoporous particles. N2 , O2 , and Ar gases are typically used to characterize the porosity of mesoporous materials [36–38]. Figure 13 shows the adsorption-desorption isotherm of N2 for ordered nanoporous particles [13]. To determine the pore-size distribution, several methods based on geometrical considerations [39], thermodynamics [40], or a statistical thermodynamic approach have been used [41]. In addition, freezing point depression can be used, as well as nuclear magnetic resonance (NMR) [42]. A method for analyzing in the mesopores range is the Barret–Joyner–Halenda (BJH) method [43, 44] which is

ISOTHEREM PLOT + ads, des

*

300

200

100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

relative pressure, (P/Po)

Figure 13. Adsorption-desorption isotherm of nitrogen at 77 K for ordered mesoporous particles. Reprinted with permission from [13], Q. S. Huo et al., Chem. Mater. 9, 14 (1997). © 1997, American Chemical Society.

based on the Kelvin equation. Figure 14 show a sample of a BJH pore-size distribution plot from the adsorption branch of the sample shown in Figure 13 [13].

4. PROPERTIES 4.1. Pore Size and Surface Area Ordered nanoporous materials are used technically as adsorbents, catalysts and catalysts supports, and separation materials due to their high surface area and well-ordered pore size. The extremely high surface areas are conducive to high catalytic activity. From Table 2, it was found that mesoporous particles prepared using an aerosol spray surfactant template had the highest surface area, 1770 m2 /g [26]. Wellordered mesoporous materials also offer interesting potential for use in separation and adsorption [45].

4.2. Thermal Properties

[100]

Thermal stability by annealing nanoporous silica particles has been investigated [24, 31]. By annealing particles below 750  C, the porous structure of silica particles (mesoporous

[100]

C

[100]

A

0.04

B (A) Hexagonal structure (d=32.5 Å, 5.0% CTAB) (B) Vesicular structure (d=64.9 Å, 4.2% CTAB) (C) Vesicular structure (d=92.0 Å, 5.0% CTAB)

Pore Volume

Intensity (arbitrary units)

400

Vol adsorbed, (cc/g STP)

6 . The reflections can be indexed assuming a cubic unit cell as (211), (220), (421), (332), and (431) [24]. Since the materials are not crystalline at the atomic level, no reflections at higher angles are observed. Moreover, these reflections would only be very weak in any case, owing to the strong decrease of the structure factor at high angles. It is not possible to quantify the purity of the materials by means of XRD. Samples with only one distinct reflection have also been found to contain substantial amounts of hexagonal materials. Using XRD patterns, the pore centerto-center diameter, or d-spacing, can be calculated. A small-angle X-ray scattering (SAXS) was also used to characterize the porous structure of the particles [27]. Figure 12 shows small-angle X-ray scattering (SAXS) patterns for silica particles prepared by aerosol spray surfactant templating method and different surfactants. The main peak is indexed as the [100] reflection of the hexagonal (A, prepared using CTAB surfactant) and vesicular mesophases (B and C).

0.03

0.02

0.01

[110] [200]

0.07

0.14

0.21

0.28

0.35

0.42

0.49

0.56

q (Å-1)

20

60

100

140

180

Pore Diameter (Å)

Figure 12. Small-angle X-ray scattering (SAXS) patterns of silica particles with (A) hexagonal or (B and C) vesicular mesophase. Reprinted with permission from [27], Y. Lu et al., Nature 398, 223 (1999). © 1999, Macmillan Magazines Ltd.

Figure 14. BJH pore-size distribution plot from adsorption branch. Reprinted with permission from [13], Q. S. Huo et al., Chem. Mater. 9, 14 (1997). © 1997, American Chemical Society.

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Ordered Nanoporous Particles

[24] and macroporous [31]) remains stable. At higher temperatures in the vicinity of 1000  C, the XRD patterns of materials show that several peaks disappear. From SEM images, by annealing at 1200  C, some silica nanoparticles collapse from the pore wall and give rise to smaller pore sizes inside the particles. The morphology of the powder becomes smoother and smaller, due to the sintering of silica, which occurs at this temperature. When the annealing temperature reaches 1500  C, the pores collapse and, finally, solid spherical silica particles are obtained. The difference in powder particle volume before and after annealing determines the porosity of the powder [31].

4.3. Optical Properties Chang and Okuyama [46] reported on the optical properties of ordered macroporous particles prepared by an aerosol spray colloidal surfactant method. Light scattering by solid and macroporous silica particles was investigated using a laser particle counter (LPC) coupled with a pulse height analyzer. The measured partial scattering cross section of solid and macroporous silica particles with same diameter showed significant differences. The effective relative indices of solid and macroporous silica particles were computed by the best fitting of the scattering intensity measurements. The result showed that the macroporous silica particles have a low effective refractive index, 1.147 (for solid silica particles, the effective refractive index was 1.46). By changing the porosity of the porous particles, the refractive index could be easily controlled.

5. SUMMARY Ordered nanoporous particles have been synthesized by several methods, in the liquid process as well as in the aerosol phase using a surfactant and/or colloidal templates. The morphology of prepared particles, the pore size, and the porous structure are typically influenced by the template types, the synthesis method, and the synthesis procedures. The use of a surfactant template led to the formation of micro- and/or mesoscaled structured porous particles. The use of a colloidal template gave particles with macroscale ordered pores. A high surface area and well-ordered nanoporous particles would be potentially useful as catalyst, adsorbent, separation materials, and low refractive index materials. Research in the preparation of particles that contain ordered nanopores (microporous, mesoporous, and macroporous) with a controlled morphology is in its infancy. It remains to be seen if these systems can be developed for other applications such as low dielectric constant, microelectronics, and electro-optics.

GLOSSARY Aerosol A substance composed of solid or liquid particles that are small but larger than most molecules in gas phase. Aerosol spray method A method for material processing by spraying the droplets containing precursor in gas phase. Biomimetic Biomimetic refers to human-made processes, substances, devices, or systems that imitate nature.

Colloid A substance composed of particles that are small but larger than most molecules. The particles in a colloid do not actually dissolve but remain suspended in a suitable liquid or solid. Among the colloids are polymers, such as rubber, plastics, and synthetic fibers. Emulsion A stable dispersion of one liquid in a second immiscible liquid, such as milk (oil dispersed in water). Macroporous materials Materials which have pore size above 50 nm. Mobil oil composite of matter (MCM) Silicate-surfactant composite structures that posses mesoscopic order which have attracted large interest since the pioneering work of the Mobil Oil group published in 1992. Mesoporous materials Materials which have pore size between 2 and 50 nm. Microporous materials Materials which have pore size below 2 nm. Scanning electron microscopy (SEM) The use of a stream of electrons controlled by electric or magnetic fields to obtain the profile materials surface. The examined object is scanned with a focused beam of accelerated electrons in a vacuum cannon. Under the influence of the bombardment the object emits various sorts of radiations that can be captured by fluorescence screen or film. Transmission electron microscopy (TEM) A fine electron beam passes through the specimen, which must therefore be sliced extremely thinly. The transmission electron may be thrown on a fluorescent screen or may be photographed to produce specimen image.

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