Graphene oxide (GO) nanocolloids-sheets with lateral dimension smaller than 100 nm-were synthesized by chemical exfoliation of graphite nanofibers, in which the graphene planes are coin-stacked along the length of the nanofibers. Since the upper size limit is predetermined by the diameter of the nanofiber precursor, the size distribution of the GO nanosheets is much more uniform than that of common GO synthesized from graphite powders. The size can be further tuned by the oxidation time. Compared to the micrometer-sized, regular GO sheets, nano GO has very similar spectroscopic characteristics and chemical properties but very different solution properties, such as surface activity and colloidal stability. Due to higher charge density originating from their higher edge-to-area ratios, aqueous GO nanocolloids are significantly more stable. Dispersions of GO nanocolloids can sustain high-speed centrifugation and remain stable even after chemical reduction, which would result in aggregates for regular GO. Therefore, nano GO can act as a better dispersing agent for insoluble materials (e.g., carbon nanotubes) in water, creating a more stable colloidal dispersion.

Oxidative etching of graphene flakes was observed to initiate from edges and the occasional defect sites in the basal plane, leading to reduced lateral size and a small number of etch pits. In contrast, etching of highly defective graphene oxide and its reduced form resulted in rapid homogeneous fracturing of the sheets into smaller pieces. On the basis of these observations, a slow and more controllable etching route was designed to produce nanoporous reduced graphene oxide sheets by hydrothermal steaming at 200 °C. The degree of etching and the concomitant porosity can be conveniently tuned by etching time. In contrast to nonporous reduced graphene oxide annealed at the same temperature, the steamed nanoporous graphene oxide exhibited nearly 2 orders of magnitude increase in the sensitivity and improved recovery time when used as chemiresistor sensor platform for NO(2) detection. The results underscore the efficacy of the highly distributed nanoporous network in the low temperature steam etched GO.

Colloidal assembly is an attractive means to control material properties via hierarchy of particle composition, size, ordering, and macroscopic form. However, despite well-established methods for assembling colloidal crystals as films and patterns on substrates, and within microscale confinements such as droplets or microwells, it has not been possible to build freeform colloidal crystal structures. Direct-write colloidal assembly, a process combining the bottom-up principle of colloidal self-assembly with the versatility of direct-write 3D printing, is introduced in the present study. By this method, centimeter-scale, free-standing colloidal structures are built from a variety of materials. A scaling law that governs the rate of assembly is derived; macroscale structural color is tailored via the size and crystalline ordering of polystyrene particles, and several freestanding structures are built from silica and gold particles. Owing to the diversity of colloidal building blocks and the means to control their interactions, direct-write colloidal assembly could therefore enable novel composites, photonics, electronics, and other materials and devices.

Figure 4. Optical properties of direct-write colloidal crystals. Photographs of a) small-grain and b) large-grain colloidal crystal imaged under ring lighting around the objective lens. c) Schematic depicting the characterization of optical properties by illuminating the colloidal crystal with collimated light at an angle normal to the crystal and observing the projection of diffracted colors on a hemispherical screen. Photograph of colors projected from the d) smallgrain and g) large-grain colloidal crystal. Colors from the e) small-grain and h) large-grain samples linearly mapped onto azimuthal angle φ and polar angle θ. f) Plot of radiant intensity versus θ for each of the RGB channels, obtained by averaging over all values of φ. i) Plot of radiant intensity versus azimuthal angle φ derived from the blue channel of (h), ranging φ = 0°-60° averaged over the sixfold symmetry regions, at θ = 52°. The solid blue line is the radiant intensity and the shaded region represents standard deviation. The highest peak is from the largest grain, A; the second highest peak is from the second-largest grain, B; and the lowest radiant intensity, denoted by the solid black line, is an estimate of the amount of light from various other small grains, C. The dashed line represents the background illumination of the ping pong ball, measured in a noncolored region. j) Optical image of the illuminated region, with A, B, and C grain structures identified. k) SEM image analysis confirms that the largest grain A and the second-largest grain B are relatively orientated at 19.5°. www.advancedsciencenews.com

To integrate the exceptional properties of nanoscale building blocks into macroscopic devices, manufacturing methods that achieve control of nanoparticle (NP) assembly up to macroscale dimensions have to be developed. [1][2][3][4][5][6][7][8] In this regard, structural hierarchy emerges as a powerful strategy for creating functional materials, involving a precise control of composition and shape across several length scales. [1,[9][10][11][12][13] NPs can be assembled into larger structures by exploiting and further controlling their inherent intermolecular and surface forces. [3,14] Thus, by taking advantage of the short-range forces, colloidal self-assembly appears as a successful bottom-up approach for the development of new materials and their further integration into novel devices. Moreover, by specifically designing the nanobuilding blocks, the macroscopic behavior of the resulting engineered materials can be