Silica Multilayers

As an example of the nanoparticle architecture, let us analyze a 45-nm silica assembly by alternate adsorption with PDDA [58-59]. In-situ QCM monitoring of alternate PDDA and SiO2 adsorption gave the kinetics of the assembly process. In the first step, PDDA was adsorbed onto a Ag-electrode. The QCM frequency decreased during the first 60 s, after which a slower change was observed as adsorption saturation set in. Then the resonator was immersed in pure water for washing. Next, the film was immersed in SiO2 dispersion and silica adsorption saturation occurred within several seconds. After subsequent water rinsing, the film was immersed again in a PDDA solution, and so on. Each growth step was reproducible, and the adsorption process reached 90% saturation in 10 s for SiO2 and 30 s for PDDA. The film assembly was not possible simply by the multiple immersion of the substrate in the silica solution. An alternation with an oppositely charged polyion was necessary. At every assembly step, the component monolayers were formed, as was recorded by QCM, SEM (Fig. 3).

The average density of SiO2/PDDA multilayers is (p) = 1.43 ± 0.05 g/cm3. SiO2/PDDA film volume composition is: 60% SiO2 + 10% polycation + 30% air-filled pores. These pores are formed by closely packed 45-nm SiO2 and have a typical dimension of 20 nm. The films have controlled pores, which can be varied by the selection of the nano-particle diameter. We estimated the diffusion limitation for surface coverage A{t) by adsorption from solution of particles with the diffusion coefficient D from A{t) = 2/'nCs/Dt. For t = 2 s, C = 10 mg/cm3, and assuming for 45-nm silica D = 1.1 x 0-7 cm2/s, A ~ 3 x 10-6 g/cm2 and the layer thickness: L = A(t)/(p) ~ 21 nm. This is reasonably close to the experimental silica monolayer thickness of 24.6 nm. Thus, 2 s corresponds roughly to the diffusion-limited time for the SiO2 monolayer adsorption; this time is the fastest nano-particle monolayer formation rate that we have achieved.

Figure 4. (a-b) SEM cross-sectional images of (MnO2/PDDA)9 and, on right, (12-nm diameter SnO2/PDDA)18 films. The films formed on a silver electrode.

Figure 4. (a-b) SEM cross-sectional images of (MnO2/PDDA)9 and, on right, (12-nm diameter SnO2/PDDA)18 films. The films formed on a silver electrode.

Figure 3. SEM image of multilayer containing 18 monolayers of 45-nm-diameter silica alternated with polycation PDDA.

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