Synthesis of Hollow Silica Nanoparticles

For the above-mentioned polymerization techniques, polyvinylpyrrolidone (PVP) is used as a steric stabilizer. This enables the direct coating of a silica shell upon the polymer particles without any further modifications, like charge inversion.^{42, 44} The prepared core-shell particles can be calcined afterward to remove the polymer core and to obtain silica hollow spheres. The different steps to get silica hollow nanospheres are schematically shown in Figure 3.4.

Figure 3.4. Schematic representation of the synthesis of silica hollow spheres. (1) Synthesis of the polystyrene template particles, stabilized by polyvinylpyrrolidone, (2) coating of the polymer particles with silica to obtain core-shell particles and (3) removal of the polystyrene core by calcining at temperatures above 500 °C.

The coating of the polystyrene template particles with a silica shell is achieved by a modified Stoeber condensation process.³⁵ The amount of precursor, used in the synthesis, allows the adjustment of the silica shell thickness. The experimental procedure is described in the following.

Hollow Silica Nanoparticle Synthesis – Experimental Procedure

For the core-shell particle synthesis, a certain amount of TEOS was added to 17.5 mL ethanol, 1.3 mL ammonium hydroxide solution, and 2.5 mL aqueous polystyrene dispersion (~10 wt.%). The TEOS volume was adjusted to obtain a certain shell thickness (see next section). After stirring at 500 rpm for ~18 h, the

particle dispersion was purified by several centrifugation steps. Subsequently, the polystyrene core was removed by calcining the particles at 500 °C for 12 h in air.

Hollow Silica Nanoparticle Synthesis – Results and Discussion

The facile synthesis method of the silica hollow nanoparticles enables the opportunity to change the particle morphology broadly. Parameters, which can be tuned are the diameter of the particles, the shell thickness, and the microstructure/porosity of the silica shell.

The size of the inner pore diameter can be adjusted by changing the size of the polystyrene template particles. A size series of particles between 185 and 990 nm is shown in Figure 3.5. Here, the shell thickness, which is given by the dark shaded ring surrounding the hollow core, is comparable for all capsules (~20 nm).

Figure 3.5.Transmission electron microscopy (TEM) images of silica hollow nanoparticles with a diameter of 185 – 990 nm and a comparable shell thickness of about 20 nm.

The size series in Figure 3.5 shows the impressively low polydispersity of the size and the shell thickness. The surface of the particles appears smooth.

Furthermore, no formation of aggregates/clusters or sinter necks between the

spheres is observed. The particles are rather individually redispersed in ethanol by ultrasonication and deposited on a carbon-coated TEM grid. For the larger particles, some buckling is observable. This can be traced back to the preparation of the TEM samples and the forces affecting the stability of the hollow spheres. The problem can be avoided by increasing the shell thickness. A shell thickness series with a size of 15 – 55 nm is shown in Figure 3.6. Here, the inner core diameter is comparable (~300 nm) for all particles.

Figure 3.6. Transmission electron microscopy (TEM) images of silica hollow nanoparticles with shell thicknesses between ~15 and 55 nm and a comparable inner core diameter of

~300 nm.

For the shell thickness series depicted in Figure 3.6 and two other series, the shell thickness versus the particle diameter is shown in Figure 3.7a. Impressively, it shows the broad range of shell thicknesses and particle diameters available by this synthesis method. Furthermore, it is possible to predict the shell thickness of the particles (t), knowing the diameter of the polystyrene template particle (d) and the volume of tetraethyl orthosilicate (VTEOS) added during the synthesis:

= 111.55 ∙ ( )

∙ + 7.36 = 35.51 ∙ ( )

+ 7.36 (3.1)

Figure 3.7b shows the plot to determine the shell thickness. Therefore, the volume of TEOS per particle surface area is plotted versus the shell thickness. It is evident that a good correlation exists between the different shell thickness series.

Furthermore, the fit of Equation (3.1) describes the correlation. Thus, it is easily possible to predict the shell thickness.

Figure 3.7. Shell thickness series of hollow silica nanoparticles for three different polystyrene template particles with diameters of 220, 320 and 414 nm. (a) Shell thickness versus particle diameter and (b) shell thickness versus volume TEOS per particle surface area. The black line represents the fit of Equation (3.1).

Another significant advantage of this synthesis method is the possibility to prepare large amounts of hollow silica nanoparticles. Figure 3.8 shows large scale approaches of silica hollow spheres with quantities of several grams.

Figure 3.8. Large-scale synthesis approaches of silica hollow spheres with quantities of several grams. The color of the particle powders differs according to the size of the particles. The size increases from left to right: 256 ± 5 nm, 319 ± 5 nm, 423± 8 nm.

The color of the particle powders in Figure 3.8 depends on the size of the hollow spheres and can be traced back to Mie scattering.¹⁰⁴ The size of the particles increases from left to right from 256 to 423 nm.

3.1.3 Colloidal Assembly

The assembly methods, used in this thesis, are introduced in the following. For the preparation of highly ordered colloidal particle assemblies (colloidal crystals), the evaporation-induced self-assembly process was applied.⁷³ This method is illustrated in Figure 3.9 and shows the fabrication of silica hollow sphere colloidal crystals.

Figure 3.9. Schematic illustration of the evaporation-induced self-assembly of the core-shell particles and the subsequent calcination step to obtain silica hollow sphere colloidal crystals. Reprinted from Ruckdeschel et al.¹⁰⁵ with permission from Wiley VCH.

The evaporation of the water occurs mainly near the air-water interface. This pushes the colloids towards the meniscus. After the formation of a few array layer, the solution flow will start to compensate the water loss caused by the evaporation in the top layers. This forces the colloids to move into the vacancies between the spheres, forming a close-packed structure. The assembled close-packed particle arrays possess a (111)-like plane parallel to the air-water interface.⁷³

For the preparation of HSNP colloidal crystals (Figure 3.9), core-shell particles are assembled and calcined afterward. Photographic images of these monoliths are shown in Figure 3.10. The color of the colloidal crystals is purple due to their comparable outer diameter of ~270 nm. From (a) to (c), the color is less pronounced

due to an increased scattering, originating from the increasing shell thickness (14, 27, and 42 nm).

Figure 3.10. Photographic image of colloidal crystals, consisting of silica hollow spheres.

The size of the particles is about 270 nm for all samples. However, the shell thickness increases from a to c (14, 27, and 42 nm). Reprinted from Ruckdeschel et al.¹⁰⁵ with permission from Wiley VCH.

A disadvantage of the described assembly process is the relatively long preparation time (~several days) due to the slow evaporation of the solvent. A faster process (~several minutes) to obtain colloidal arrays represent the vacuum filtration process (Figure 3.11). Due to the rapid assembly, the ordering of the particles is random, leading to colloidal glasses. Thus, this is a facile preparation method for random close-packed structures.

Figure 3.11. Schematic setup of the vacuum filtration system to prepare colloidal glasses.

Reprinted from Ruckdeschel et al.¹⁰⁵ with permission from Wiley VCH.

The difference in the ordering by using these two methods is highlighted by the SEM and optical microscopy images in Figure 3.12. Whereas the crystalline arrangement features Bragg colors in the optical microscopy image (a), the amorphous one shows only a diffusive scattering (b). This is validated by the corresponding SEM images (c, d).

Figure 3.12. Scanning electron microscopy images (a, b) and optical microscopy images (c, d) of hollow silica nanoparticle colloidal assemblies showing either a crystalline packing (a, c) or an amorphous ordering (b, d). Adapted from Ruckdeschel et al.¹⁰⁵ with permission from Wiley VCH.