Figure 1A shows the commercial nanohorn, carried out using SEM analysis. This material is made into spherical or ellipsoidal clusters exhibiting a size distribution of 5–20 μm. The dimension of the NH aggregates appear to be larger by about two orders of magnitude compared to the values reported from other authors (∼100 nm) by means of TEM analysis. To date, we have found only one publication that reports a SEM image for pristine NH, Cioffi and coworkers. In the consulted literature, sample preparation comprises a sonication step to allow a homogenous dispersion. To avoid a possible alteration of the NH aggregates, in our experiments we limit the high-energy treatment by mixing all samples manually. In order to observe the effect of a polar solvent on the assembling character, we dispersed NH in distilled water and ethanol and stored up to 1 week.
In figure 1B, an aqueous dispersion, stored for 1 day, shows NH assembly into spherical aggregates with average sizes 5–15 μm. By keeping the dispersion for 1 week, the average size of the spherical aggregates is increased up to 10–30 μm (as seen in Fig. 1C). On the other hand, the solvent substitution with ethanol produces parallelepiped-like aggregates, as seen in figure 1D1 and E1. It is worth noting that the top side of the parallelepiped aggregate appears to be composed of a cluster of spherical aggregates possessing an average size in the range 5–10 μm (Fig. 1D3, E3). By contrast, the bulk of this parallelepiped-like aggregate turns into a layered structure, as observed in figures 1D2 and E2. We posit that the solvent influences the size of the spherical aggregates, which are reduced when the polarity of the solvent is changed. The morphology of nanohorns can be explained by a parallelism with surfactants. These classes of materials exhibit the formation of spherical aggregates, micelles, that spontaneously assemble, minimising the van der Waals interactions. Eventually, surfactants will form compact structures (liquid crystals). Similarly, nanohorns assemble into a complex and hierarchical structure that determines its packing assembly character.