Preferred orientation of $n$-hexane crystallized in silicon nanochannels: A combined x-ray diffraction and sorption isotherm study

We present an x-ray diffraction study on $n$-hexane in tubular silicon channels of approximately 10 nm diameter both as a function of the filling fraction $f$ of the channels and as a function of temperature. Upon cooling, confined $n$-hexane crystallizes in a triclinic phase typical of the bulk crystalline state. However, the anisotropic spatial confinement leads to a preferred orientation of the confined crystallites, where the $⟨001⟩$ crystallographic direction coincides with the long axis of the channels. The magnitude of this preferred orientation increases with the filling fraction, which corroborates the assumption of a Bridgman-type crystallization process being responsible for the peculiar crystalline texture. This growth process predicts for a channel-like confinement an alignment of the fastest crystallization direction parallel to the long channel axis. It is expected to be increasingly effective with the length of solidifying liquid parcels and thus with increasing $f$. In fact, the fastest solidification front is expected to sweep over the full silicon nanochannel for $f=1$, in agreement with our observation of a practically perfect texture for entirely filled nanochannels.

Figure 1

(Color online) Orientation of the triclinic unit cell of $n$-hexane crystallized in tubular silicon nanochannels. Note, the crystalline (100) direction of $n$-hexane, which is the direction of fastest crystal growth, coincides with the long axis of the channel.

Figure 2

(Color online) Volumetric sorption isotherm of hexane 273 K.

Figure 3

(Color online) Diffraction patterns at selected temperatures with filling fraction 0.61 on cooling (upper panel) and heating (lower panel).

Figure 4

(Color online) Diffraction patterns at 100 K of all filling fractions $f$ (including the calculated and measured bulk pattern).

Figure 5

(Color online) Transition temperatures on cooling (squares)/heating (circles) of pore-confined hexane depending on the filling fraction $f$.

Figure 6

Texture parameter as a function of filling fraction as determined from a comparison with texture powder diffractometry.