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Water is detrimental to electronic devices because it easily causes short circuits. The use of sealing to prevent water permeation is considered as the current conventional solution; however, the t...

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.

Optically transparent cellulose nanopaper is one of the best candidate substrates for flexible electronics. Some types of cellulose nanopaper are made of mechanically or chemically modified cellulose nanofibers. Among these, nanopapers produced from chemically modified cellulose nanofibers are the most promising substrate because of their lower power consumption during fabrication and higher optical transparency (lower haze). However, because their thermal durability is as low as plastics, paper devices using chemically modified nanopaper often do not have sufficiently high performance. In this study, by decreasing the carboxylate content in the cellulose nanofibers, the thermal durability of chemically modified nanopaper was drastically improved while maintaining high optical transparency, low coefficient of thermal expansion, and low power consumption during fabrication. As a result, light-emitting diode lights illuminated on the chemically modified nanopaper via highly conductive lines, which were obtained by printing silver nanoparticle inks and high-temperature heating.

A novel one-pot low temperature preparation of Fe-doped anatase–rutile TiO2 (Fe-TiO2) films is demonstrated using plasma electrolytic oxidation (PEO). Pale yellow TiO2 films are obtained by PEO treatment of Ti metals in the electrolyte dispersing TiO2 and Fe2O3 particles. The oxidized layer on Ti metal have a sponge-like structure with thickness and pore size of 10 and 0.1–1 μm, respectively. Investigation by X-ray diffraction, X-ray photoelectron spectroscopy, and UV–Vis absorption spectroscopy all indicate that dissolved Fe3+ ions in the strong acidic electrolyte are doped into the TiO2 structure during PEO. The photocatalytic activity of Fe-TiO2 samples was investigated by studying the photocatalytic decomposition of acetaldehyde. Fe-TiO2 samples doped with optimum Fe content show visible light photocatalytic activity and further increased photocatalytic activity under UV illumination compared with that of pure TiO2 films.

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.