Nanoporous materials

Nanoporous materials

Scanning electron microscopic (SEM) picture of nanoporous alumina via electrochemical anodic oxidation resulting in well ordered parallel pores with pore diameters of appr. 50 nm and pore distances of appr. 100 nm.
Scanning electron microscopic (SEM) picture of nanoporous alumina via electrochemical anodic oxidation resulting in well ordered parallel pores with pore diameters of appr. 50 nm and pore distances of appr. 100 nm.

Nanoporous materials can have spongy structures like Vycor glass or cylindrically shaped, parallel pores with (1.) 2D hexagonal order (e.g. nanoporous silicon, silica or alumina) or (2.) 2D random distribution (e.g. ion track etched polycarbonate).
They are excellent samples to confine materials and investigate structure and dynamics in this situation.

An example are diblock-copolymers, that show self organised structures (microphase separation) in the bulk. This effect is due to demixing of both blocks while they are covalently bound together. This prevents them from macroscopic demixing and results in the above mentioned self organised structures on a nanometer length scale.

The confinement in nanoporous materials with diameters comparable to these structure sizes can have influence on the dynamics and results of the self organisation process and possibly show new structures.

We investigate structures on a nanometer lengthscale with small angle scattering (small angle x-ray scattering (SAXS) or small angle neutron scattering (SANS)). In the case of SAXS the sample electrons are forced to oscillate with the frequency of an incoming x-ray wave and emit a spherical wave. The spherical waves interfere with each other and the resulting interference pattern is collected as an intensity on a detector. The scattering curves are analysed with specially suited model functions. In the case of highly oriented parallel nanopores we use a goniometer like sample holder to align the samples precisely. We can measure the samples at different temperatures (between 400°C and -160°C) and even time dependent on a 2D detector.

Sketch of scattering geometry: an incoming plane wave with wave vector k0 = 2π/λ excites the samples electrons to perform dipole oscillations and emit spherical waves. If the scattering process is elastic |k’| = |k0|. The scattering vector q is defined as q := k’ – k0 resulting in q = |q| = 4πsin(q)/λ.

Scattering curve of a measurement of nanoporous alumina with fitted model function. From the varied parameter one gets the mean pore distance, the mean pore diameter and its polydispersity and some more important property values.