Hiroki Nishijima

Phase stability and high conductivity of ScSZ nanofibers: effect of the crystallite size

10 mol% Sc2O3-doped ZrO2 (10ScSZ) nanofibers were prepared through electrospinning followed by calcination. The phase structures and electrical conductivities of the nanofibers have been investigated as a function of the crystallite size. The cubic (c) phase can be stabilized in 10ScSZ nanofibers when the average crystallite size is smaller than 26 nm. The generated phase stability endows the nanofibers with an enhanced conductivity which increases with the decrease of crystallite size. As the average crystallite size decreased from 37 nm to 7 nm, the conductivity of the nanofibers increased by more than 20 times. An exceptionally high oxide ion conductivity of 0.023 S cm−1 for the nanofibers was observed at 500 °C, which is more than 900 times higher than that of bulk 10ScSZ.

Enhanced oxide-ion conductivity in highly c-axis textured La10Si6O27 ceramic

Here, we report a highly c-axis textured apatite-type La10Si6O27 ceramic with conductivity of 1.3 × 10−2 S cm−1 at 500 °C, which is an 11.2 times enhancement compared with isotropic La10Si6O27 ceramic. The highly c-axis textured La10Si6O27 was fabricated by a arc-melting process with a unilateral temperature gradient as the main driving force for the formation of the La10Si6O27 texture. The conductivity enhancement is mainly attributed to the enhanced mobility contribution along the c-axis with low activation energy and the relatively small grain boundary blocking effect along the c-axis.

Electrospun assembly: a nondestructive nanofabrication for transparent photosensors

Transparent electrodes based on a metal nanotrough network show superior electrical and optical properties. However, most metal networks fabricated by electrospinning are formed as film electrodes and are hard to pattern for the geometry shape of the device without any loss. Herein, we fabricate a highly transparent and flexible photodetector (PD) via a simple controlled electrospinning method. Owing to the trough- and belt-like geometry of Pt network electrodes, up to 83% transmittance can be obtained when the sheet resistances is 16 Ω sq-1, which may be the best performance for Pt-based transparent electrodes at present. The benefit of this advantage, is that a wearable UV PD could be obtained by a facile electrospun assembly. This all-transparent device achieves an extraordinary transparency of 90% at 550 nm and an even superior response sensitivity compared with that of a Pt film-based sensor (14 Ω sq-1 at 50% transparency). More importantly, this assembly approach has the versatility to enable us to fabricate highly transparent and flexible electronics in wearable applications, especially for the integration of oxide semiconductors and adhesive photoelectric hybrids.

Stretchable Platinum Network‐Based Transparent Electrodes for Highly Sensitive Wearable Electronics

A platinum network-based transparent electrode has been fabricated by electrospinning. The unique nanobelt structured electrode demonstrates low sheet resistance (about 16 Ω sq-1 ) and high transparency of 80% and excellent flexibility. One of the most interesting demonstrations of this Pt nanobelt electrode is its excellent reversibly resilient characteristic. The electric conductivity of the flexible Pt electrode can recover to its initial value after 160% extending and this performance is repeatable and stable. The good linear relationship between the resistance and strain of the unique structured Pt electrode makes it possible to assemble a wearable high sensitive strain sensor. Present reported Pt nanobelt electrode also reveals potential applications in electrode for flexible fuel cells and highly transparent ultraviolet (UV) sensors.

Fabrication of high performance oxygen sensors using multilayer oxides with high interfacial conductivity

The effective enhancement of the ionic conductivity of solid oxide electrolytes by manipulating the multilayered hetero-nanostructures has been recognized for more than a decade; however, such fantastic nanostructures have not been applied for practical applications yet. Here we fabricated Ce0.8Sm0.1Nd0.1O2−δ/Al2O3 (SNDC/AO) multilayered electrolyte materials with high oxygen ionic conductivity and explored their applications in oxygen sensors with low operating temperature. The multilayer oxide electrolytes show a conductivity of 2 orders of magnitude higher than that of traditional yttria-stabilized zirconia (YSZ) ceramics at 400 °C, which endows the sensors with high performances including a rapid response (∼0.1 s), high sensibility (down to 1 vol%) and high cycling stability (no performance degradation after 1000 cycles), and most importantly, low operating temperature (200 °C lower relative to YSZ-based sensors). The excellent performances of our multilayered electrolytes can be attributed to their high interfacial conductivity and low activation energy which might be related to the highly disordered microstructures. This study shows good prospects for the efficient and practical use of multilayered oxide-based electrolytes for a range of applications including sensors, oxygen separation, solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs).

Contrary interfacial effects for textured and non-textured multilayer solid oxide electrolytes

The relationship between the microstructure and the conductivity for nanocrystallized oxygen ionic conducting thin films has been receiving great interest since it provides guidelines for designing electrolytes with high performances which might find applications in fuel cells and oxygen and fuel separation membranes. Here, we present a strategy for using the multilayered structure to tune the microstructures and ionic transport properties of solid electrolyte. Textured and non-textured Ce0.8Sm0.2O2−δ/Al2O3 (SDC/AO) solid electrolyte multilayers were prepared, and the dependence of conductivity on layer number was studied. We found that non-textured and textured multilayers show a positive and a negative interfacial conduction contribution to the total ionic conductivity, respectively. The decrease of conductivity with the increase of layer number for textured SDC/AO was attributed to that the multilayered structure introduces random grain orientations to the interfacial region which results in more pronounced grain boundary blocking effects. In contrast, non-textured SDC/AO show rich structural defects in the interfacial regions which facilitate the oxygen ionic transport and lead to a higher ionic conductivity. These insights into the effect of the interfacial interaction on the structure and the conductivity allow a better control of the electrical properties of multilayered electrolytes, which might foster their applications in electrochemical devices operable at lower temperatures.

High photodetectivity of low-voltage flexible photodetectors assembled with hybrid aligned nanowire arrays

One-dimensional (1-D) hybrid nanostructures with high opto-electronic performance, flexible features, and a rationally designed device assembly have an exponentially growing demand for their use in transparent and flexible photodetectors. To meet these requirements, a cost effective and facile processing route is mandatory for their fabrication and integration into the device assembly. Herein, we have successfully assembled photodetectors from high quality well-oriented uniaxially aligned hybrid ZnO–ZnGa2O4 nanofibers. Our prepared flexible devices demonstrate outstanding performance in terms of large responsivity (1.73 × 102 A W−1), high photodetectivity (∼1.02 × 1012 Jones), and external quantum efficiency (7.10 × 104%) along with fast rise and recovery times of less than 400 and 500 ms, respectively. In addition, the prepared photodetectors have shown outstanding flexibility and stability with more than 70% retention in photosensitivity at a very small bending radius of 2 mm. The synthesized hybrid nanofibers have also shown more than 80% transparency in the visible range (400–700 nm). The assembled hybrid photodetectors indicate that ZnO–ZnGa2O4 nanofibers are highly valuable for optoelectronic devices. It is also worth emphasizing that our assembled flexible photodetectors based on hybrid nanofibers with high surface-to-volume ratio have promising applications in soft electronics and invisible optoelectronic devices.

Stabilizing Nanocrystalline Oxide Nanofibers at Elevated Temperatures by Coating Nanoscale Surface Amorphous Films

Nanocrystalline materials often exhibit extraordinary mechanical and physical properties but their applications at elevated temperatures are impaired by the rapid grain growth. Moreover, the grain growth in nanocrystalline oxide nanofibers at high temperatures can occur at hundreds of degrees lower than that would occur in corresponding bulk nanocrystalline materials, which would eventually break the fibers. Herein, by characterizing a model system of scandia-stabilized zirconia using hot-stage in situ scanning transmission electron microscopy, we discover that the enhanced grain growth in nanofibers is initiated at the surface. Subsequently, we demonstrate that coating the fibers with nanometer-thick amorphous alumina layer can enhance their temperature stability by nearly 400 °C via suppressing the surface-initiated grain growth. Such a strategy can be effectively applied to other oxide nanofibers, such as samarium-doped ceria, yttrium-stabilized zirconia, and lanthanum molybdate. The nanocoatings also increase the flexibility of the oxide nanofibers and stabilize the high-temperature phases that have 10 times higher ionic conductivity. This study provides new insights into the surface-initiated grain growth in nanocrystalline oxide nanofibers and develops a facile yet innovative strategy to improve the high-temperature stability of nanofibers for a broad range of applications.