Flexible thermoelectric materials have been investigated for developing wearable power generators and self-powered wireless physiological sensors. One of ways to enhance the thermoelectric properties is the introduction of nanostructures. Therefore, we have investigated to form the oxide semiconductor nanomaterials on fabric substrates, as a flexible thermoelectric material, by solvothermal synthesis [1-7]. In the present study, with the aim of realizing oxide-nanostructure/fabric materials appropriate for wearable devices, we investigated the vertical growth of taller ZnO nanorods on conductive NiCu fabrics by microwave-assisted solvothermal synthesis.
ZnO nanostructures were formed on a NiCu fabric substrate using two-step microwave-assisted solvothermal synthesis. Based on solvothermal synthesis with microwave, some seeds of ZnO were fixed on the fabric in the first step, and ZnO nanorods grew up in the second step. In this study, we repeated the growth process (second step) for making the ZnO nanorod longer.
Figure shows scanning electron microscope (SEM) images of ZnO morphological structures grown on a NiCu fabric after (a) one-time and (b) four-times growth process, for microwave power of 100 W with an irradiation time of 10 min in the crystal-growth step. It is found that the fabric surface is fully covered with dense nanorods. In the case of conventional solvothermal synthesis with a furnace, it spent several hours for ZnO growth, and ZnO nanosheets and nanopillars were frequently observed [5,6]. Therefore, the microwave has effects of shortening the synthesis time and of forming ZnO nanorods. In addition, the nanorods seem decently vertical and become larger by repeating the growth process. Actually, it was confirmed that the diameter and height of nanorods evaluated from the SEM images increase linearly with the repeat count of the growth process, which indicates that the nanorod length can be controlled to obtain the desired length.
This work was financially supported in part by a Grant-in Aid for Challenging Exploratory Research (No.20K21886) from the Japan Society for the Promotion of Science and by the Cooperative Research Project on Research Centre for Biomedical Engineering.
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