Tuning the Porosity of Supraparticles

Significance 

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.

Tuning the Porosity of Supraparticles - Advances in Engineering
Supraparticles are formed through evaporation of aqueous suspension droplets on superamphiphobic surfaces followed by calcination of the sacrificial polymer particles. The structures of the obtained porous supraparticles are finely adjustable by regulating droplet volume, dispersion concentration, choice of primary particles, and sacrificial particles.

About the author

Dr. Wendong Liu is currently a postdoc researcher at the Department of Physics at Interfaces, Max Planck Institute for Polymer Research, Mainz. He has joined the department in 2017 and is working on the field of droplet evaporation. He specializes in evaporating colloidal droplets on a superamphiphobic surface for investigating the particle segregation behavior and fabricating functional supraparticles.

Dr. Liu has received his bachelor’s degree in Chemistry and Biotechnology, Jilin University, China, in 2012. Then he started to do his Ph.D. thesis under the supervision of Prof. Bai Yang at Jilin University and defended his Ph.D. thesis about the fabrication of ordered micro/nanostructures for controllable adhesion in June 2017. Then he went to Mainz and joined the group of Prof. Butt as a postdoc. Up to date, he has published 32 research articles (including 8 first-author articles, H index: 11) and contributed 2 chapters for scientific books.

About the author

Dr. Michael Kappl is currently a group leader at the Department of Physics at Interfaces, Max Planck Institute for Polymer Research, Mainz. He studied physics from 1983-1990 at the University of Regensburg and the Technical University of Munich. From 1991 to 1996 he did his PhD thesis work in Prof. Ernst Bamberg’s group at the Max-Planck-Institute of Biophysics in Frankfurt, on transport kinetics of the Na+-Ca2+exchanger membrane protein. From 1997 to 1998 he did one year of postdoctoral research at the University of Mainz in the group of Prof. Butt. From 1998 – 2000 he was working as a consultant for Windows NT network solutions at the Pallas Soft AG, Regensburg. He rejoined the group of Prof. Butt in August 2000 at the University of Siegen.

Since 2002 he is group leader at the MPIP, working in the fields of surface forces, wetting and characterization of mechanical properties on the micro- and nanoscale. In addition he is heading the Focused Ion Beam Service Lab, providing service for nanostructuring, cross-sectioning of samples, preparation of TEM lamella, and 3D tomography.

About the author

Prof. Dr. Hans-Jürgen Butt is a director at the Max Planck Institute for Polymer Research, Mainz, Germany. He studied physics in Hamburg and Göttingen. He defended his Ph.D. thesis in 1989 at the Max Planck Institute for Biophysics. After one-year postdoc experience at the University of California, he moved back to MPIBP and habilitated in 1995. In 1996, he became an associate professor at the Johannes Gutenberg-University in Mainz. In 2000, he became a full professor at the University of Siegen for Physical Chemistry. In 2002, he joined MPIP as a director working on soft matter interfaces.

Reference

Wendong Liu, Michael Kappl, Hans-Jürgen Butt. Tuning the Porosity of Supraparticles. ACS Nano 2019; volume13, page 13949−13956.

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