Hexagonal boron nitride (hBN) is a structural analog of graphene, with unique mechanical, thermal, and optical properties that make it desirable for a variety of applications. Production of stable dispersions of well-exfoliated hBN nanosheets, particularly in a nontoxic and inexpensive way, is an important step in the production of hBN macromaterials on an industrial scale. Here, we investigate the use of surfactants for exfoliating and dispersing hBN in aqueous solution. Dispersions in nine different surfactants and water were compared based on dispersion yield, quality, and stability. It was revealed that at low centrifugal force, large-molecular-weight nonionic surfactants disperse the most material. In contrast, when stronger centrifugation is applied, all surfactants produce similar dispersion yields, with dispersions in ionic surfactants containing significantly more exfoliated nanosheets and remaining stable over much longer periods of time. Finally, to demonstrate the scalability and effectiveness of these systems for making macroscopic materials, a dispersion of hBN in sodium dodecyl sulfate (SDS) was used to produce a transparent hBN film that can be deposited on glass and potentially used as an antibacterial or thermally resistant coating.

Boron nitride nanotubes (BNNTs) have attracted attention for their predicted extraordinary properties; yet, challenges in synthesis and processing have stifled progress on macroscopic materials. Recent advances have led to the production of highly pure BNNTs. Here we report that neat BNNTs dissolve in chlorosulfonic acid (CSA) and form birefringent liquid crystal domains at concentrations above 170 ppmw. These tactoidal domains merge into millimeter-sized regions upon light sonication in capillaries. Cryogenic electron microscopy directly shows nematic alignment of BNNTs in solution. BNNT liquid crystals can be processed into aligned films and extruded into neat BNNT fibers. This study of nematic liquid crystals of BNNTs demonstrates their ability to form macroscopic materials to be used in high-performance applications.

Conspectus Cryogenic-temperature transmission electron microscopy (cryo-TEM) of aqueous systems has become a widely used methodology, especially in the study of biological systems and synthetic aqueous systems, such as amphiphile and polymer solutions. Cryogenic-temperature scanning electron microscopy (cryo-SEM), while not as widely used as cryo-TEM, is also found in many laboratories of basic and applied research. The application of these methodologies, referred to collectively as cryogenic-temperature electron microscopy (cryo-EM) for direct nanostructural studies of nonaqueous liquid systems is much more limited, although such systems are important in basic research and are found in a very large spectrum of commercial applications. The study of nonaqueous liquid systems by cryo-EM poses many technical challenges. Specimen preparation under controlled conditions of air saturation around the specimen cannot be performed by the currently available commercial system, and the most effective cryogen, freezing ethane, cannot be used for most such liquid systems. Imaging is often complicated by low micrograph contrast and high sensitivity of the specimens to the electron beam. At the beginning of this Account, we describe the basic principles of cryo-EM, emphasizing factors that are essential for successful direct imaging by cryo-TEM and cryo-SEM. We discuss the peculiarities of nonaqueous liquid nanostructured systems when studied with these methodologies and how the technical difficulties in imaging nonaqueous systems, from oil-based to strong acid-based liquids, have been overcome, and the applicability of cryo-TEM and cryo-SEM has been expanded in recent years. Modern cryo-EM has been advanced by a number of instrumental developments, which we describe. In the TEM, these include improved electron field emission guns (FEGs) and microscope optics, the Volta phase plate to enhance image contrast by converting phase differences to amplitude differences without the loss of resolution by an objective lens strong underfocus, and highly sensitive image cameras that allow the recording of TEM images with minimal electron exposure. In the SEM, we take advantage of improved FEGs that allow imaging at a low (around 1 kV) electron acceleration voltage that is essential for high-resolution imaging and for avoiding specimen charging of uncoated nonconductive specimens, better optics, and a variety of sensitive detectors that have considerably improved resolution and, under the proper conditions, give excellent contrast even between elements quite close on the periodic table of the elements, such as the most important oxygen and carbon atoms. Finally we present and analyze several examples from our recent studies, which illustrate the issues presented above, including the remarkable progress made in recent years in this field and the strength and applicability of cryo-EM methodologies.

Methylcellulose (MC) is a commercially important, water-soluble polysaccharide. Many food applications exploit the thermoreversible gelation behavior of MC in aqueous media. The mechanism of MC gelation upon heating has been debated for decades; however, recent work has demonstrated that gelation is concurrent with the formation of ca. 15 nm diameter fibrils, which percolate into a network. The fibrillar network dictates the properties and mechanical behavior of the resulting hydrogel. The addition of salt to MC gels has also been an area of academic and commercial interest. It has been reported that MC solutions containing salts exhibit an increase or decrease in the gelation temperature, which generally follows the Hofmeister series. To build upon these investigations, we study the effect of salt on the MC fibril structure. We demonstrate the effect of salt (NaCl, NaI, NaBr, NaNO 3 , KCl, NH 4 Cl, LiCl, and CaCl 2 ) on the gelation and dissolution temperatures using rheology and cloud point measurements. From small-angle X-ray scattering (SAXS) and high contrast cryogenic transmission electron microscopy (cryo-TEM) we show that salty MC gels are also composed of fibrils. Fitting the SAXS curves to a semiflexible cylinder model, we demonstrate that the fibril diameter decreases monotonically with increasing salt molarity, largely independent of the salt anion or cation type.

Chlorosulfonic acid and oleum are ideal solvents for enabling the transformation of disordered carbon nanotubes (CNTs) into precise and highly functional morphologies. Currently, processing these solvents using extrusion techniques presents complications due to chemical compatibility, which constrain equipment and substrate material options. Here, we present a novel acid solvent system based on methanesulfonic or p -toluenesulfonic acids with low corrosivity, which form true solutions of CNTs at concentrations as high as 10 g/liter (≈0.7 volume %). The versatility of this solvent system is demonstrated by drop-in application to conventional manufacturing processes such as slot die coating, solution spinning continuous fibers, and 3D printing aerogels. Through continuous slot coating, we achieve state-of-the-art optoelectronic performance (83.6 %T and 14 ohm/sq) at industrially relevant production speeds. This work establishes practical and efficient means for scalable processing of CNT into advanced materials with properties suitable for a wide range of applications.