Under the right process conditions, nanoparticles can cluster together to form defined particular structures which are termed supraparticles. Supraparticles have potential applications, such as photonic crystals, drug carriers, or heterogeneous catalysts. Presently, a variety of fabrication approaches has been developed. Generally, most of the supraparticles are prepared in solution; for example, by kinetics/thermodynamics-controlled growth or template-based synthesis. A review of existing literature reveals that the approaches are mainly proposed with chemical dependence, particularly for size and composition engineering. These chemicals escalate the cost, consume much energy, and are not environmentally friendly. Therefore, developing simple strategies to reduce or completely avoid the use of solvent, emulsifiers, templates, or any other processing liquid is desirable. Fortunately, recent publications have introduced evaporation-induced self-assembly as a promising strategy for the fabrication of supraparticles. Ideally, such studies have demonstrated that supraparticles can be prepared by evaporation of sessile dispersion droplets on superhydrophobic surfaces. Superhydrophobic surfaces are subject to some limitations that can, however, be overcome with the use of superamphiphobic surfaces. The latter repels not only water but also nonpolar liquids, surfactant, or protein solutions.
In addition, the properties of supraparticles show great importance. Contemporary research has revealed that porosity is a critical parameter for supraparticles, in that is has a significant influence on their performance in practical applications. Porosity can be increased by allowing the dispersed particles to aggregate partially. However, it has been observed that during the fabrication process, the presenting compressive capillary forces act on the nanoparticle aggregates, leading to densification of loose agglomerates. To address this drawback, researchers from the Department of Physics at Interfaces at Max Planck Institute for Polymer Research, Mainz in Germany: Dr. Wendong Liu, Dr. Michael Kappl and Professor Hans-Jürgen Butt, developed a new alternative method to adjust the porosity of supraparticles. Their work is currently published in the research journal, ACS Nano.
Their goal was to introduce a method to increase the porosity of supraparticles and circumvent the limit set by capillary compression. To achieve this, the team prepared TiO2 supraparticles with hierarchical porosity by first constructing TiO2−polystyrene (PS) binary supraparticles on superamphiphobic surfaces and then removing the PS phase by calcination. Moreover, they related the porosity of superparticles to the volumetric ratio of TiO2 to PS.
The researchers reported that by regulating the concentration and droplet volume of the mixed particle dispersions, the size of the spherical TiO2 supraparticles could be well controlled. Remarkably, the three physicists were able to demonstrate that the hierarchical porous structures of the TiO2 supraparticles enhanced photocatalytic performance in degrading organic dye (Rhodamine B).
In summary, the study presented the fabrication of highly porous surpraparticles based on the evaporation of suspension droplets on superamphiphobic surfaces. By taking advantage of the liquid repellency of superamphiphobic surface, the contact line pinning was suppressed during the evaporation, leading to the formation of spherical multicomponent supraparticles that can be easily released from the surface. Overall, the increase of porosity of up to 92% resulted in enhanced photocatalytic activity while sufficient mechanical stability maintained. In a statement to Advances in Engineering, Dr. Wendong Liu, first author, highlighted that their approach for regulating the inner structure of supraparticles would allow optimizing them for specialized applications.
Wendong Liu, Michael Kappl, Hans-Jürgen Butt. Tuning the Porosity of Supraparticles. ACS Nano 2019; volume13, page 13949−13956.