Lyotropic Liquid Crystals from Colloidal Suspensions of Graphene Oxide

Abstract

Lyotropic liquid crystals from colloidal particles have been known for more than a century, but have attracted a revived interest over the last few years. This is due to the developments in nanoscience and nanotechnology, where the liquid crystal order can be exploited to orient and reorient the anisotropic colloids, thus enabling, increasing and switching the preferential properties of the nanoparticles. In particular, carbon-based colloids like carbon nanotubes and graphene/graphene-oxide have increasingly been studied with respect to their lyotropic liquid crystalline properties over the recent years. We critically review aspects of lyotropic graphene oxide liquid crystal with respect to properties and behavior which seem to be generally established, but also discuss those effects that are largely unfamiliar so far, or as of yet of controversial experimental or theoretical outcome. Crystals 2019, 9, 455 2 of 20 easy to handle, because graphene is largely insoluble in nearly all liquid solvents. This is very different for graphene-oxide (GO) which exhibits different functional groups at the edges and the plane of the graphene oxide sheets, and is readily soluble in many solvents, including water, which is a significant advantage for environmentally friendly applications (Figure 1b). At certain concentrations, roughly > 0.1-1 wt%, GO often exhibits very stable lyotropic nematic phases [11][12][13]. One of the interesting aspects of GO liquid crystals is the possibility of self-organized and self-ordered systems of graphene oxide. This order can then easily be manipulated by application of external stimuli, such as boundary conditions, mechanical shear, but possibly also electric and magnetic fields. After obtaining a certain directional order one could wash away or evaporate the solvent, leaving an ordered GO structure, which may even be reduced chemically or through heat application to produce reduced graphene oxide (rGO), which displays some of the originally desirable graphene properties. Lyotropic graphene oxide liquid crystals are thus of immense interest for nanotechnology and its applications. There have been several review articles on the properties of GO liquid crystals [14][15][16]. In this paper we want to give a short critical account of what is known, what is not yet known, and what are the controversial questions with regards to graphene oxide liquid crystals. Effects of flake size and solvent will be discussed, alignment, addition of salts and polymers, as well as electro-optic properties, and electric and magnetic field effects. Graphene and Graphene OxidePristine graphene is a two-dimensional hexagonal lattice of sp 2 hybridized carbon with a basis of two, giving rise to a honeycomb structure [10]. Graphene oxide is a very different material to graphene. Graphene is often obtained by mechanical exfoliation of graphite (the so-called Scotch Tape ® method) or grown by chemical vapor deposition. On the other hand, graphite that has undergone treatment to become graphite oxide readily exfoliates ...

Fine tuning the structural colours of photonic nanosheet suspensions by polymer doping

“…Apart from their many industrial applications, like the ubiquitous uses of clays, these colloidal suspensions have recently raised much interest from a fundamental perspective as versatile model systems to explore the liquid-crystalline behaviour expected for particles of such high aspect ratio (D/t  100 -1000). Apart from the nematic phase which has often been reported, [25][26][27][28][29][30][31][32][33][34][35][36][37][38] the lamellar (or smectic) phase also sometimes occurs (Figure 1). [39][40][41][42][43][44][45] Interestingly, for highly charged nanosheets dispersed in polar solvents, the lamellar phase may appear at very high dilution, for nanosheet weight fractions below  1 wt%.…”

Novel Trends in Lyotropic Liquid Crystals

“…The Kerr response times are at present longer than those of the well-discussed thermotropic Blue Phases, due to the higher viscosity of GO-LCs. So far, graphene oxide liquid crystals are by far the best studied of the lyotropic liquid crystals made from 2D materials, and their properties have been summarized in a number of review articles [108][109][110], although it should be mentioned that here, there are also still many open questions. One of these, which has, for example, been addressed via texture and dielectric studies, is the observation of mixtures of thermotropic nematics with GO [111].…”