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Li X., Zhu L., Kasuga T., et al. Frequency-tunable and absorption/transmission-switchable microwave absorber based on a chitin-nanofiber-derived elastic carbon aerogel. Chemical Engineering Journal 469, 144010 (2023); https://doi.org/10.1016/j.cej.2023.144010., The increasing use of microwaves in wireless communications has caused severe electromagnetic pollution. As the frequency range for wireless communication is expanding, it is highly desirable to develop a microwave absorber that can smartly and reversibly tune its absorption and transmission properties on demand to transmit required frequencies and absorb unwanted frequencies. Herein, an absorption-frequency-tunable and absorption/transmission-switchable microwave absorber is developed based on the controlled compression of a chitin-derived elastic carbon aerogel. The maximum absorption frequency is tuned from 10.4 to 11.0, 11.5, and 12.1 GHz by varying the compression strain from 0 to 20, 40, and 60%, respectively, while the maximum absorption intensity is maintained at approximately −40 dB. This frequency-tunable absorption is achieved by reducing the thickness of the carbon aerogel while retaining its moderate dielectric loss tangent. Further compression from 60 to 80% switches the carbon aerogel from being a microwave absorber to transmitter while causing impedance mismatch and changing its dielectric loss tangent from moderate to low levels. The frequency tunability and microwave absorption/transmission switching capability are reversible and repeatable for at least 60,000 cycles of compression and recovery. This study provides insights into the smart and reversible control of microwave absorbing properties and paves the way for multifunctional and robust absorbers.

Thanakorn Yeamsuksawat, Yoshitaka Morishita, Jun Shirahama, Yintong Huang, Takaaki Kasuga, Masaya Nogi, and Hirotaka Koga, Semicarbonized Subwavelength-Nanopore-Structured Nanocellulose Paper for Applications in Solar Thermal Heating, Chem. Mater. 2022, 34, 16, 7379–7388. ©2022 American Chemical Society., Recently, there has been remarkable progress in solar thermal heating by applying biomass-derived carbons, which can absorb and convert solar light into thermal energy. The design of subwavelength nanoporous and molecular structures of biomass-derived carbons is required for suppressed reflection and enhanced absorption of solar light. However, such designs are difficult because conventional biomass-derived carbons exhibit intrinsic microstructures and are prepared under specific carbonization conditions. In this study, a wood cellulose nanofiber-derived carbon is proposed to tailor both subwavelength nanoporous and molecular structures. Cellulose nanofibers are first constructed into a paper, denoted as "nanopaper", exhibiting subwavelength nanoporous structures by tailoring the pore spaces between cellulose nanofibers. The as-prepared nanopaper is then carbonized at various controlled temperatures to tailor the cellulose molecular structure, i.e., grow graphitic carbon domains. The graphitic carbon domains grown by semicarbonization at 500 °C adequately balance solar absorption and reflection, while the subwavelength nanoporous structures suppress solar reflection. Thus, the semicarbonized nanopaper with tailored nanoporous and molecular structures exhibits superior solar thermal heating to competitive nanocarbons, also affording thermoelectric power generation. This study can guide the structural and functional design of bionanocarbons for solar thermal heating.

Li, C.; Kasuga, T.; Uetani, K.; Koga, H.; Nogi, M. High-Speed Fabrication of Clear Transparent Cellulose Nanopaper by Applying Humidity-Controlled Multi-Stage Drying Method. Nanomaterials 2020, 10, 2194. https://doi.org/10.3390/nano10112194., As a renewable nanomaterial, transparent nanopaper is one of the promising materials for electronic devices. Although conventional evaporation drying method endows nanopaper with superior optical properties, the long fabrication time limits its widely use. In this work, we propose a multi-stage drying method to achieve high-speed fabrication of clear transparent nanopaper. Drying experiments reveal that nanopaper’s drying process can be separated into two periods. For the conventional single-stage evaporation drying, the drying condition is kept the same. In our newly proposed multi-stage drying, the relative humidity (RH), which is the key parameter for both drying time and haze, is set differently during these two periods. Applying this method in a humidity-controllable environmental chamber, the drying time can be shortened by 35% (from 11.7 h to 7.6 h) while maintaining the same haze level as that from single-stage drying. For a conventional humidity-uncontrollable oven, a special air flow system is added. The air flow system enables decrease of RH by removing water vapor at the water/air interface during the earlier period, thus fabricating clear transparent nanopaper in a relatively short time. Therefore, this humidity-controlled multi-stage drying method will help reduce the manufacturing time and encourage the widespread use of future nanopaper-based flexible electronics.

Y. Huang, Y. Morishita, K. Uetani, M. Nogi and H. Koga. Cellulose paper support with dual-layered nano–microstructures for enhanced plasmonic photothermal heating and solar vapor generation. Nanoscale Adv., 2020, 2, 2339. https://doi.org/10.1039/D0NA00163E., Plasmonic nanoparticles, such as gold nanoparticles (AuNPs), have been actively applied in solar vapor generation for seawater desalination and water purification, owing to their photothermal heating performances. Such nanoparticles have been frequently anchored within porous supporting materials to ensure easy handling and water absorption. However, there has been limited progress in improving the transport efficiency of light to nanoparticles within porous supports to achieve more effective photothermal heating. Here, we show an enhanced light absorption of AuNPs by supporting on a cellulose paper with tailored porous structures for efficient photothermal heating. The paper consists of AuNP-anchored cellulose nanofibers and cellulose pulp as the top and bottom layers, respectively, which provides dual-layered porous nano-microstructures in the perpendicular direction. Then, the bottom layer with pulp-derived microstructures reflects the transmitted light back to AuNPs within the top layer, which improves their light absorptivity. Thus, under 1 sun illumination, the dual-layered paper demonstrates superior performance in photothermal heating (increases from 28 °C to 46 °C) and solar vapor generation (1.72 kg m-2 h-1) compared with the single-layered AuNP-anchored cellulose nanofiber paper even at the same AuNP content. Furthermore, the water evaporation rate per AuNP content of the dual-layered paper is more than 2 times higher than those of the state-of-the-art AuNP-anchored porous materials under the same light irradiation. This strategy enables the efficient use of precious plasmonic nanoparticles for further development of solar vapor generation.

K. Uetani, S. Izakura, H. Koga and M. Nogi. Thermal diffusivity modulation driven by the interfacial elastic dynamics between cellulose nanofibers Nanoscale Adv., 2020, 2, 1024. https://doi.org/10.1039/C9NA00734B., Thermal transport modulating materials show great potential to address the heat problems in a wide range of engineering fields. However, tuning the thermal conductivity of solid-state materials is practically difficult because it requires specific or extreme stimulation, such as chemical composition change, a phase transition, or large applied fluctuations, to change the internal bulk structures. Here, we report reversible switching of the in-plane thermal diffusivity of densely packed cellulose nanofiber (CNF) films by ∼15% by simple mechanical strain as small as 0.3%. From analysis of the stress relaxation profiles and the different bulk densities of the CNF films, the interfacial elastic dynamics between the strongly hydrogen bonded CNFs were found to exhibit thermal diffusivity modulation by tuning the interfacial thermal resistance, rather than changing the bulk structure of the CNFs. Our concept of interfacial-elasticity-driven thermal diffusivity switching has the potential to enhance the on/off rate and extensibility toward practical use owing to the high designability of the interfacial conditions.

Uetani, K.; Koga, H.; Nogi, M. Checkered Films of Multiaxis Oriented Nanocelluloses by Liquid-Phase Three-Dimensional Patterning. Nanomaterials 2020, 10, 958. https://doi.org/10.3390/nano10050958., It is essential to build multiaxis oriented nanocellulose films in the plane for developing thermal or optical management films. However, using conventional orientation techniques, it is difficult to align nanocelluloses in multiple directions within the plane of single films rather than in the thickness direction like the chiral nematic structure. In this study, we developed the liquid-phase three-dimensional (3D) patterning technique by combining wet spinning and 3D printing. Using this technique, we produced a checkered film with multiaxis oriented nanocelluloses. This film showed similar retardation levels, but with orthogonal molecular axis orientations in each checkered domain as programmed. The thermal transport was enhanced in the domain with the oriented pattern parallel to the heat flow. This liquid-phase 3D patterning technique could pave the way for bottom-up design of differently aligned nanocellulose films to develop sophisticated optical and thermal materials.

This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Applied Materials and Interfaces, copyright © American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acsami.9b01287., Progress toward the concept of "a trillion sensor universe" requires sensor devices to become more abundant, ubiquitous, and be potentially disposable. Here, we report a paper-based disposable molecular sensor device constructed from a nanowire sensor based on common zinc oxide (ZnO), a wood-derived biodegradable cellulose nanofiber paper substrate, and a low-cost graphite electrode. The ZnO nanowire/cellulose nanofiber composite structure is embedded in the surface of the cellulose nanofiber paper substrate via a two-step papermaking process. This structure provides a mechanically robust and efficiently bridged network for the nanowire sensor, while ensuring efficient access to target molecules and allowing reliable electrical contact with electrodes. The as-fabricated paper sensor device with pencil-drawn graphite electrodes exhibits efficient resistance change-based molecular sensing of NO2 as a model gas. The performance of our device is comparable to that of noble metal electrodes. Furthermore, we demonstrate cut-and-paste usability and easy disposal of the sensor device with its uniform in-plane sensing properties. Our strategy offers a disposable molecular sensing platform for use in future sensor network technologies.

Takaaki Kasuga, Hitomi Yagyu, Kojiro Uetani, Hirotaka Koga, and Masaya Nogi. “Return to the Soil” Nanopaper Sensor Device for Hyperdense Sensor Networks. ACS Applied Materials & Interfaces 2019 11 (46), 43488-43493. https://doi.org/10.1021/acsami.9b13886., A nanopaper sensor device that combines humidity sensing, wireless information transmission, and degradability has been fabricated using wood-derived nanopaper as the substrate and dielectric layers. The nanopaper shows excellent suitability for capacitor dielectric layers because of its high dielectric constant, insulating properties suitable for thin-film formation, and lamination properties. A wireless transmission circuit containing the nanopaper capacitor can transmit radio signals in the megahertz band, and the relative humidity change can be output as a change in the radio signal owing to the humidity sensitivity of the nanopaper capacitor. More than 95% of the total volume of the nanopaper sensor device decomposes in soil after 40 days. Because the nanopaper sensor device does not need to be recovered, it is expected to greatly contribute to a sustainable society through realization of hyperdense observation networks by mass installation of sensor devices.

Uetani, K.; Izakura, S.; Kasuga, T.; Koga, H.; Nogi, M. Self-Alignment Sequence of Colloidal Cellulose Nanofibers Induced by Evaporation from Aqueous Suspensions. Colloids Interfaces 2018, 2, 71. https://doi.org/10.3390/colloids2040071., Cellulose nanopapers fabricated by drying aqueous colloidal suspensions of cellulose nanofibers (CNFs) have characteristic hierarchic structures, which cause the problem that their optical properties, including their transparency or haze, vary due to the drying processes affecting CNF alignment. It is unclear when and how the colloidal CNFs align in the evaporation–condensation process from the randomly dispersed suspension to form the nanopaper. In this study, we found that the CNFs undergo a self-alignment sequence during the evaporation–condensation process to form chiral nematic nanopaper by observing the birefringence of the drying suspensions from both the top and side for two suspensions with different initial CNF concentrations. The layer structures of the CNFs first form on the surface by condensation of the suspension, owing to water evaporation from the surface. The thickness of the layered structure then increases and the CNFs begin to align within each layer plane, finally forming chiral nematic structures. A birefringence difference also occurs for dried nanopapers with similar transparency or haze because of the initial CNF concentration.

Kasuga, T.; Isobe, N.; Yagyu, H.; Koga, H.; Nogi, M. Clearly Transparent Nanopaper from Highly Concentrated Cellulose Nanofiber Dispersion Using Dilution and Sonication. Nanomaterials 2018, 8, 104. https://doi.org/10.3390/nano8020104., Nanopaper prepared from holocellulose pulp is one of the best substrates for flexible electronics because of its high thermal resistance and high clear transparency. However, the clearness of nanopaper decreases with increasing concentration of the starting cellulose nanofiber dispersion—with the use of a 2.2 wt % dispersion, for example—resulting in translucent nanopaper with a high haze of 44%. To overcome this problem, we show that the dilution of this high-concentration dispersion with water followed by sonication for 10 s reduces the haze to less than 10% while maintaining the high thermal resistance of the nanopaper. Furthermore, the combination of water dilution and a short sonication treatment improves the clearness of the nanopaper, which would translate into cost savings for the transportation and storage of this highly concentrated cellulose nanofiber dispersion. Finally, we demonstrate the improvement of the electrical conductivity of clear transparent nanopaper prepared from an initially high-concentration dispersion by dropping and heating silver nanowire ink on the nanopaper. These achievements will pave the way toward the realization of the mass production of nanofiber-based flexible devices.

Hsieh, MC., Koga, H., Suganuma, K. et al. Hazy Transparent Cellulose Nanopaper. Sci Rep 7, 41590 (2017). https://doi.org/10.1038/srep41590.

This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Applied Materials and Interfaces, copyright © American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acsami.7b14827., Functional molecules play a significant role in the development of high-performance composite materials. Functional molecules should be well dispersed (ideally dissolved) and supported within an easy-to-handle substrate to take full advantage of their functionality and ensure easy handling. However, simultaneously achieving the dissolution and support of functional molecules remains a challenge. Herein, we propose the combination of a nonvolatile ionic liquid and an easy-to-handle cellulose paper substrate for achieving this goal. First, the photochromic molecule, i.e., diarylethene, was dissolved in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([bmim]NTf2). Then, diarylethene/[bmim]NTf2 was supported on cellulose fibers within the paper, through hydrogen bonding between [bmim] cations of the ionic liquid and the abundant hydroxyl groups of cellulose. The as-prepared paper composites exhibited reversible, rapid, uniform, and vivid coloration and bleaching upon ultraviolet and visible light irradiation. The photochromic performance was superior to that of the paper prepared in the absence of [bmim]NTf2. This concept could be applied to other functional molecules. For example, lithium perchlorate/[bmim] tetrafluoroborate supported within cellulose paper acted as a flexible electrolyte to provide a paper-based electrochromic device. These findings are expected to further the development of composite materials with high functionality and practicality.

H. Koga, N. Namba, T. Takahashi, M. Nogi, Y. Nishina. Renewable Wood Pulp Paper Reactor with Hierarchical Micro/Nanopores for Continuous‐Flow Nanocatalysis. ChemSusChem 2017, 10, 2560. https://doi.org/10.1002/cssc.201700576., Continuous-flow nanocatalysis based on metal nanoparticle catalyst-anchored flow reactors has recently provided an excellent platform for effective chemical manufacturing. However, there has been limited progress in porous structure design and recycling systems for metal nanoparticle-anchored flow reactors to create more efficient and sustainable catalytic processes. In this study, traditional paper is used for a highly efficient, recyclable, and even renewable flow reactor by tailoring the ultrastructures of wood pulp. The “paper reactor” offers hierarchically interconnected micro- and nanoscale pores, which can act as convective-flow and rapid-diffusion channels, respectively, for efficient access of reactants to metal nanoparticle catalysts. In continuous-flow, aqueous, room-temperature catalytic reduction of 4-nitrophenol to 4-aminophenol, a gold nanoparticle (AuNP)-anchored paper reactor with hierarchical micro/nanopores provided higher reaction efficiency than state-of-the-art AuNP-anchored flow reactors. Inspired by traditional paper materials, successful recycling and renewal of AuNP-anchored paper reactors were also demonstrated while high reaction efficiency was maintained.

Reproduced from Ref. Green Chem., 2016,18, pp. 1117-1124. with permission from the Royal Society of Chemistry., Recent progress in portable and wearable electronics has promoted a growing demand for high-performance and flexible energy-storage devices that are abundant and affordable. Because reduced graphene oxide (rGO), originating from inexpensive graphite, serves as a higher-performance energy-storage electrode than conventional activated carbons and carbon nanotubes, research and development of rGO/polymer composite electrodes for flexible supercapacitors have become a center of attraction. However, the fabrication of rGO-based flexible electrodes frequently requires a long time with high-temperature treatment or toxic chemical treatment, resulting in the lack of scalability and eco-friendliness. Here we show a fast, scalable, and environment-compatible route to fabricate a high-performance rGO/cellulose paper supercapacitor electrode. Single-layer graphene oxide (GO) sheets and recycled waste pulp fibers were successfully fabricated into a paper composite by a well-established scalable papermaking process, followed by a room-temperature, additive-free, and millisecond-timescale flash reduction process. The as-prepared rGO/paper electrode had a high specific capacitance, up to 212 F g-1, for an all paper-based flexible supercapacitor, comparable to those of state-of-the-art rGO-based electrodes, while dramatically decreasing the reduction time of GO from the conventional hour timescale to milliseconds. This work will pave the way for green, flexible, and mass-producible energy-storage paper in future wearable electronics.

Celano, U., Nagashima, K., Koga, H. et al. All-nanocellulose nonvolatile resistive memory. NPG Asia Mater 8, e310 (2016). https://doi.org/10.1038/am.2016.144., Single-use disposable nonvolatile memory devices hold promise for novel applications in internet of everything (IoE) technology by storing the health status of individual humans in daily life. However, conventional memory devices are not disposable because they are mostly composed of non-renewable, non-biodegradable and sometimes toxic materials, causing serious damage to ecological systems when they are released to the environment. Here, we demonstrate an environment-friendly, disposable nonvolatile memory device composed of 99.3 vol.% nanocellulose. Our memory device consists of a nanocellulose-based resistive-switching layer and a nanopaper substrate. The device exhibited nonvolatile resistive switching with the capability of multilevel storage and potential scalability down to the single nanofiber level (ca. 15 nm). The biodegradability of our memory device was confirmed by burying it in natural soil for 26 days.

This is the peer reviewed version of the following article: Inui, T., Koga, H., Nogi, M., Komoda, N. and Suganuma, K. (2015), A Miniaturized Flexible Antenna Printed on a High Dielectric Constant Nanopaper Composite. Adv. Mater., 27: 1112-1116., which has been published in final form at https://doi.org/10.1002/adma.201404555. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving., A high-dielectric-constant and flexible cellulose nanopaper composite is prepared by mixing a small amount of silver nanowires with cellulose nanofibers. The nanopaper antenna is downsized by about a half when using the nanopaper substrate. The nanopaper antenna has potential in wearable wireless communication devices.