Beyond Janus Geometry: Characterization of Flow Fields Around Nonspherical Photocatalytic Microswimmers

Abstract

Catalytic microswimmers that move by a phoretic mechanism in response to a self-induced chemical gradient are often obtained by the design of spherical janus microparticles, which suffer from multi-step fabrication and low yields. Approaches such as irregular particle shapes, local excitation or intrinsic asymmetry are on the rise to facilitate manufacturing, but the effects on the generation of motion remain poorly understood. In this work, single crystalline BiVO4 microswimmers are presented that rely on a strict inherent asymmetry of charge-carrier distribution under illumination. The origin of the asymmetrical flow pattern is elucidated becauseof the high spatial resolution of measured flow fields around pinned BiVO4 colloids. As a result the flow from oxidative to reductive particle sides was confirmed. Distribution of oxidation and reduction reactions suggests a dominant self-electrophoretic motion mechanism with a source quadrupole as the origin of the induced flows. It is shown that the symmetry of the flow fields is broken by self-shadowing of the particles and synthetic surface defects that impact the photocatalytic activity of the microswimmers. The results demonstrate the complexity of symmetry breaking in nonspherical microswimmers and are leading the way towards understanding ofpropulsion mechanisms of phoretic colloids of various shapes.

Trendbericht Physikalische Chemie 2022: Aktive Materie und Licht

“…BiVO 4 , das als Kristall mit verschiedenen Facetten eine Option für intrinsische Asymmetrie hat, lässt sich in verschiedenen Morphologien herstellen: als gestapelte Quadrate, 19) Ovale, 20) oder monokristalline Bipyramiden (Abbildung 3, S. 74). 21) Die Ladungsverteilung auf den verschiedenen Facetten führt dazu, dass sich Oxidations-und Reduktionsreaktionen trennen. Dadurch bilden sich Gra-dienten und damit eine hinreichende Asymmetrie.…”

“…Dies zeigt sich in einer höheren Geschwindigkeit. Beobachtungen, wie sich diese Ladungsverteilung in Flussfeldern widerspiegelt, sind uns erst kürzlich gelungen, 21) und wir erwarten noch mehr Einblicke. Durch die intrinsische Asymmetrie entfallen die Herstellung von Monolagen und Bedampfung, was einen Vorteil für die Skalierbarkeit bedeutet.…”

A Path toward Inherently Asymmetric Micromotors

“…[23] Another elegant particle design relied on the asymmetric distribution of charge carriers in BiVO 4 upon irradiation. Heckel et al developed bismuth vanadate particles in squareshaped, [24,25] spheroidal, [26] and single-crystalline [27] geometries that exhibit motility and active assembly without any asymmetrization step because of their intrinsic crystal properties. This strategy is transferable only to materials with significantly different band structures of exposed crystal facets, for example, for TiO 2 semiconductors, conferring this type of asymmetry has not yet been achieved.…”

Microswimming by oxidation of dibenzylamine

“…Since the motion of microparticles is strongly influenced by factors like particle shape and size, a synthesis yielding more homogeneous monocrystals is desirable. As we recently described elsewhere, 35 the BiVO 4 hydrothermal synthesis can be tuned using NaCl and SDS as capping agents, resulting in truncated bipyramidal morphology, as shown in Fig. 1A.…”

“…As described previously, the facet-selective half reactions were able to propel active motion using traditional peroxide degradation, and we hypothesized that an organic solute should also be able to cause sufficiently strong gradients leading to propulsion. 35 Therein charge separation causes an electric field, which, in combination with the migrating H + ions and the gradient in imine concentration, causes fluid flows around the particle and an active motion as illustrated in Fig. 1C.…”