Ultralight, flexible, and fire-resistant carbon nanofiber aerogels from bacterial cellulose.

Carbon-based aerogels, composed of interconnected threedimensional (3D) networks, have attracted intensive attention because of their unique physical properties, such as low density, high electrical conductivity, porosity, and specific surface area. As a result, carbon-based aerogels are promising materials used as catalyst supports, artificial muscles, electrodes for supercapacitors, absorbents, and gas sensors. Especially, ultralight or flexible carbon-based aerogels have many potential applications. For example, ultralight nitrogen-doped graphene framework, used as an absorbent for organic liquids or the active electrode material, exhibits a high absorption capacity and specific capacitance; stretchable conductors, fabricated by infiltrating flexible graphene foam with elastic polymers, show high stability of electronic conductivity even under high stretching and bending strain. Traditionally, to fabricate carbon aerogels, resorcinol– formaldehyde organic aerogels were pyrolyzed in an inert atmosphere to form a highly cross-linked carbon structure. The carbon aerogels always have a high density (100–800 mgcm ) and tend to break under compression. Carbon nanotube (CNT) sponges, graphene foam, and CNT forests have been prepared through chemical vapor deposition (CVD). Meanwhile, CNTs and graphene can be employed as building blocks and assembled into macroscopic 3D architectures. However, the harmful and expensive precursors or complex equipments involved in these syntheses dramatically hamper the large-scale production of these carbon-based aerogels for industry application. Recently, we have developed a template-directed hydrothermal carbonization process for synthesis of carbonaceous nanofiber hydrogels/aerogels on macroscopic scale by using glucose as precursors. However, the use of expensive nanowire templates in this synthesis pushes us to explore a facile, economic, and environmentally friendly method to produce carbon-based nanostructured aerogels. Nowadays, there is a trend to produce carbon-based materials from biomass materials, as they are very cheap, easy to obtain, and nontoxic to humans, etc. Bacterial cellulose (BC), a typical biomass material, is composed of interconnected networks of cellulose nanofibers, and can be produced in large amounts in a microbial fermentation process. Recently, we reported a highly conductive and stretchable conductor, fabricated from BC, shows great electromechanical stability under stretching and bending strain. Herein, we report a facile route to produce ultralight, flexible, and fire-resistant carbon nanofiber (CNF) aerogels in large scale from BC pellicles. When used as absorbents, the CNFaerogels can absorb a wide range of organic solvents and oils with excellent recyclability and selectivity. The absorption capacity can reach up to 310 times the weight of the pristine CNF aerogels. Besides, the electrical conductivity of the CNF aerogel is highly sensitive to the compressive strain, thereby making it a potential pressure-sensing material. For fabricating the CNF aerogels, a piece of purified BC pellicle with the size of 320 240 12 mm was first cut into rectangular or cubic shape and then freeze-dried to form BC aerogels (see the Supporting Information). The dried BC aerogels were pyrolyzed at 700–1300 8C under argon atmosphere to generate black and ultralight CNF aerogels. After pyrolysis, the volume of obtained CNFaerogel is only 15% of that of the original BC aerogel. Meanwhile, the density decreases from 9–10 mgcm 3 for BC aerogels to 4–6 mgcm 3 for CNF aerogels, owing to evaporation of volatile species. The macroscopic sizes of the as-synthesized CNFaerogels are dependent on the sizes of the BC pellicles cut in the fabrication procedure. It is well-known that temperature has a great influence on pyrolysis products. To create ideal CNF aerogels, BC aerogels were pyrolyzed separately at different temperatures. Scanning electron microscopy (SEM) images show that BC aerogels exhibit a porous, interconnected, well-organized 3D network structure, which was formed through self-assembly in the bacteria culture process (Figure 1a). A high-magnification SEM image indicates that the nanofibers with a diameter of 20–80 nm are highly interconnected with large numbers of junctions (see the Supporting Information, Figure S1). After the pyrolysis treatment, the porous 3D structure of BC aerogels was maintained, and the diameter of the nanofibers decreased to 10–20 nm (Figure 1b, also see the Supporting [*] Z. Y. Wu, C. Li, Dr. H. W. Liang, Prof. Dr. J. F. Chen, Prof. Dr. S. H. Yu Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, the National Synchrotron Radiation Laboratory, University of Science and Technology of China Hefei, Anhui 230026 (P.R. China) E-mail: [email protected] Homepage: http://staff.ustc.edu.cn/~ yulab/