Solvent Mediated Assembly of Nanoparticles Confined in Mesoporous Alumina

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Kyle J. Alvine, Diego Pontoni, Oleg G. Shpyrko, Peter S. Pershan, David J. Cookson, Kyusoon Shin, Thomas P. Russell, Markus Brunnbauer, Francesco Stellacci, and Oleg Gang Phys. Rev. B 73, 125412 (2006).

Soft Matter Keywords

Nanoparticles, Small Angle X-Ray Scattering, Self Assembly, Capillary forces, Cohan Mechanism Capillary transition

Summary

The authors study the solvent mediated self assembly of 2nm gold colloidal nanoparticles within nanoporous alumina using in-situ small angle x-ray scattering (SAXS). They have been able to control the self assembly process reversibly by condensing and removing a solvent (toluene) from a vapor across a range of temperatures. A shift in the capillary transition of the pores is observed when nanoparticles are present. They observe that a cylindrical shell super structure of nanoparticles is present on the inside of the pores throughout the process, and the addition of the solvent increases the particle separation and decreases order. When the solvent is removed, the particles move closer together and become more ordered again. They also observe the temporary creation of isotropic clusters of particles on removal of the solvent, which they explain by the Cohan mechanism.

Experiment Details

The nanoporous alumina was prepared by acid etching an anodized membrane with phosphoric acid. The nanopores created were 30nm diameter and approximately 60nm apart from each other running perpendicular to the surface of the membrane. The membrane was then soaked in a solution of thiol stabilized gold nanoparticles, which were drawn into the pores by capillary forces.

Figure 1 - (Left) SEM image of the alumina nanopores. (Right) Bright-field TEM image of the nanoparticles on the wall of alumina pores. The nanoparticles are Au core with octane-thiol coating.

The sample was loaded into a hermetically sealed environmental chamber for in situ x-ray measurements. The amount of solvent (toluene) condensed into the membrane pores was controlled by changing the relative temperatures of the sample and solvent reservoir. The amount of solvent in the pores, as well as the structures of the nanoparticles in the pores were determined by the relative scattering angle of the x-ray data.


Figure 2 - Schematic of the experimental setup for in-situ x-ray experiments.

Results

The amount of solvent adsorbed into the pores was measured by changes in the xray diffraction signal. The behavior of the solvent being adsorped into the membrane is seen to be strongly hysteretic upon thermal cycling. In addition, the capillary transition for the solvent in the nanoparticle doped pores was shifted compared to pores without nanoparticles. The capillary transition was observed were modelled using the Kelvin equation.

<math>\Delta \mu_{adsorption} = \gamma / n R</math>

<math>\Delta \mu_{desorption} = 2 \gamma / n R</math>

where R is the cylindrical nanopore radius, γ is the surface tension of the solvent and n is the molar density of the solvent.

Figure 3 - Volume adsorption and desorption curves as a function of changing temperature. The behavior is strongly hysteretic. The vertical dashed lines show the predicted location of the capillary transition for empty nanopores.

Further analysis of the x-ray data shows that when the pores are dry, a nanoparticle monolayer is formed inside the pores. As solvent is added, the particle separation increases and ordering decreases. As the solvent is removed, isotropic clusters of particles are observed as the particle separation decreases.


Figure 4 shows an illustration of a possible mechanism for creation of isotropic clusters of particles upon desorption. It is thought that pores fill by cylindrical film growth and empty by hemispherical menisci from the ends. This is know as the Cohen mechanism. These menisci may drag particles along, creating clusters temporarily as the solvent is removed from the pores.


Figure 4 - Illustration of a possible mechanism for creation of isotropic clusters of particles upon desorption. Right, pores fill by cylindrical film growth and empty by hemispherical menisci from the ends. This is know as the Cohan mechanism. These menisci may drag particles along, creating clusters temporarily as the solvent is removed from the pores.

Conclusions and Soft Matter Discussion

Gold nanoparticles have demonstrated unique optical properties, and so are under investigation for use in information storage, plasmonics etc and so understanding the self-assembly of nanoparticles is important for both fundamental physics and future technological developments using these materials.

This paper may provide insights into nanotoxicology of nanoparticles in human cells. These plasmonic properties have also led to them being used by the [Halas Group|http://www.ece.rice.edu/~halas/] as novel chemotherapy agents. In order to assess the impact of the nanoparticles on the human subject, it is important to understand how they are absorbed into cells through the cell wall. The cell wall can be thought of as a porous medium which experiences different pressures of liquids.