NANO LETTERS
Fully Accessible Gold Nanoparticles within Ordered Macroporous Solids
2002 Vol. 2, No. 5 471-473
Benito Rodrı´guez-Gonza´lez, Vero´nica Salgueirin˜o-Maceira, Florencio Garcı´a-Santamarı´a,† and Luis M. Liz-Marza´n* Departamento de Quı´mica Fı´sica, UniVersidade de Vigo, E-36200, Vigo, Spain, and Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco 28049, Madrid, Spain Received February 13, 2002
ABSTRACT A multi-step approach was used for the preparation of macroporous polymer films (or inverse opals), in which fully accessible metal nanoparticles are incorporated. The method consists of the synthesis of silica-coated metal nanoparticles; their sedimentation to grow ordered compacts; sintering to ensure physical contact between spheres; infiltration with an epoxy resin; and dissolution of silica with HF. The isolation of the nanoparticles is ensured because they are locked within a single hole, and the pore size can be controlled through the thickness of the silica shell.
Ordered macroporous solids, also known as inverse opals, are the subject of intense research, mainly because of their application as photonic crystals.1 One of the general procedures used for the synthesis of such arrays comprises the assembly of suitable templates (such as artificial opals) from monodisperse colloids, followed by the deposition of a different material within the voids, and final removal of the colloid templates.1 Since the introduction of this sort of materials by Velev et al.2 and by Imhof and Pine in 1997,3 a whole range of both templates and infiltrating materials have been used.4-11 The choice of the specific template and infiltrating material is generally motivated by the properties to be obtained. Further advances based on this technique have been achieved, such as the synthesis of ordered macroporous spheres,12 or the use of the uniform cavities within inverse opals as hosts for the synthesis of monodisperse colloids.13,14 This latter procedure is particularly interesting for the preparation of materials for which monodisperse large particles have been traditionally difficult to obtain. On the other hand, micro- and mesoporous solids are also extremely useful as hosts for the immobilization of catalytically active metal nanoparticles.15 The control of the size and composition of the attached nanoparticles is an issue of major importance because the catalytic activity is directly dependent upon them. Several methods have been recently proposed for the incorporation of preformed metal particles within gels,16-18 and polymers,19,20 but the use of larger pores of controlled size is desirable, so that larger molecules can freely diffuse through the system.21 * To whom correspondence should be addressed: Tel: +34 986 812298. Fax: +34 986812556. E-mail:
[email protected]. † Instituto de Ciencia de Materiales de Madrid. 10.1021/nl025526r CCC: $22.00 Published on Web 03/30/2002
© 2002 American Chemical Society
Figure 1. Schematic representation of the general synthetic method used for the preparation of a macroporous polymer containing individual Au nanoparticles: (a) synthesis of nanoparticles; (b) coating with thick silica shells; (c) opal formation through natural sedimentation; (d) infiltration with epoxy resin and dissolution of silica shells.
We present here a multistep synthetic procedure that allows the incorporation of preformed nanoparticles within ordered macroporous materials (inverse opals). In this work, the nanoparticles are made of gold, and the inverse opal is made of an epoxy resin. However, a wide range of materials are available for straightforward variations of the procedure (metal, semiconductor, or magnetic nanoparticles within polymer, dielectric, metal or semiconductor walls). The general procedure is outlined in Figure 1. The initial step (a) comprises the synthesis of the nanoparticles to be incorporated. In our case, citrate stabilized Au nanoparticles
Figure 2. Electron micrographs showing the subsequent steps during the formation of an ordered macroporous polymer containing gold nanoparticles. (a) citrate stabilized gold nanoparticles (15 nm); (b) the same Au particles coated with silica (total diameter: 225 nm); (c) sintered opal from Au@SiO2 spheres; (d) detail of epoxy resin inverse opal with incorporated Au nanoparticles.
were prepared using Turkevich’s method.22 Subsequently (b), a shell of silica is grown on each gold nanoparticle, by means of a method previously described,23 consisting of sodium silicate deposition in water and tetraethoxysilane (TEOS) hydrolysis and condensation in ethanol. The amount of added TEOS determines the final diameter of the particle. In step (c), the Au@SiO2 core-shell particles in water are allowed to slowly settle on a substrate, so that a solid sediment is formed. The so-formed artificial opal is then sintered to improve its mechanical stability and to ensure the contact between neighboring composite spheres.24 Finally (d), the opal is infiltrated with Epon epoxy-resin,25 which is subsequently polymerized in an oven at 70 °C for 24 h, and the silica shells are removed by dipping the composite opal in a suitable HF solution for some hours26 and then washing in distilled water. The real counterpart of the scheme in Figure 1 is shown in Figure 2. The preparation of Au nanoparticles through citrate reduction ensures monodispersity and control of particle size (a). Careful coating with silica leads to nearly perfect core-shells with very little variation in total particle size (225 ( 10 nm) (b), so that high-quality opals are formed (c). After infiltration and silica removal, a highly ordered macroporous polymer is obtained within which the Au nanoparticles still remain (d). Several features should be commented upon regarding the morphology of the final composite system, which are exemplified in Figure 3, where several TEM images of the same inverse opal with different magnifications are shown. First, it is clear that large ordered arrays of macropores are obtained, as was previously shown for a similar procedure starting from pure silica spheres.10 It is also clear that the Au nanoparticles are not ordered in the final product. This was expected because after removal of the silica shell, the metal cores are free to move within the spherical void, and stick to the walls (see Figure 1d). This is especially apparent in Figure 3c, where the dark gold dots are concentrated along the lines of the polymer walls, whereas the central part is 472
Figure 3. Transmission electron micrographs showing the ordered macroporous polymer containing gold nanoparticles at different magnifications.
basically empty. Also, the Au nanoparticles attach to the polymer walls and do not flow out of the inverse opal, even after thorough washing with water. In some of the micrographs, it seems that several metal nanoparticles are in mutual contact. There are several plausible explanations for this. It has been sometimes detected that a very low percentage (
