Abstract
Dispersions of graphene oxide nanoflakes in water can be manipulated into photonic crystals that reflect the full spectrum of visible colour. Normally, dissolving nanometre-thin graphene oxide crystals in water produces brownish solutions. Jang-Kun Song from Sungkyunkwan University in South Korea and co-workers used extensive oxidation and cleaning procedures in combination with bottom-up and top-down control strategies to coax unexpected optical activity from the irregularly shaped nanoflakes. The team's synthetic procedure produced a strong electric double layer on graphene oxide surfaces that induced the particles to stack into periodic, light-reflecting structures similar to cholesteric liquid crystals. Adding salt ions to the watery dispersion allowed the photonic structures, and hence the colour reflections, to be adjusted at molecular scales. Alternatively, the researchers could ‘paint’ glass slides by using shear forces produced by stirring to direct colour output.
Highlights
Graphene oxide (GO) flakes, that is, 1-nm-thick disk-like particles, can be well-dispersed in water to form a stable colloid with spontaneous nematic assembly and non-linear anisotropic rheology.[1,2,3] Nematic GO dispersions subjected to illumination typically exhibit a collective reflection of a brownish color from many GO particles, owing to the intrinsic absorption band of GO.[4]
From a practical point of view, photonic crystals potentially allow the use of GO in various optical applications, as long as an easy method is provided for manipulation.[16]
The second-order color reflection was recovered, and the color purity of the first-order samples improved (Figure 1c) when the ions were removed; this removal was achieved by centrifuging the degraded samples and replacing the supernatant water with pure distilled water.[19,20]. This indicates that the electrostatic repulsion between GO particles, owing to their electric double layer (EDL), extends 40.5 μm in the natural sample, which is free of polymer additives and consists of unmodified functional groups
Summary
Graphene oxide (GO) flakes, that is, 1-nm-thick disk-like particles, can be well-dispersed in water to form a stable colloid with spontaneous nematic assembly and non-linear anisotropic rheology.[1,2,3] Nematic GO dispersions subjected to illumination typically exhibit a collective reflection of a brownish color from many GO particles, owing to the intrinsic absorption band of GO.[4]. Finding ways to manipulate the reflection color and to achieve large-scale uniformity are, highly desirable for practical advances
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