Cylindrical silk fiber (SF) was coated with Graphene oxide (GO) for capacitive humidity sensor applications. Negatively charged GO in the solution was attracted to the positively charged SF surface via electrostatic force without any help from adhesive intermediates. The magnitude of the positively charged SF surface was controlled through the static electricity charges created on the SF surface. The GO coating ability on the SF improved as the SF's positive charge increased. The GO-coated SFs at various conditions were characterized using an optical microscope, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and LCR meter. Unlike the intact SF, the GO-coated SF showed clear response-recovery behavior and well-behaved repeatability when it was exposed to 20% relative humidity (RH) and 90% RH alternatively in a capacitive mode. This approach allows humidity sensors to take advantage of GO's excellent sensing properties and SF's flexibility, expediting the production of flexible, low power consumption devices at relatively low costs.
Developing eco-friendly and cost-effective
processes for the synthesis
of graphene oxide (GO) is essential for its widespread industrial
applications. In this work, we propose a green synthesis technique
for GO production using recycled sulfuric acid and filter-processed
oxidized natural graphite obtained from a Couette–Taylor flow
reactor. The viscosity of reactant mixtures processed from Couette–Taylor
flow was considerably lower (∼200 cP at 25 °C) than that
of those from Hummers’ method, which enabled the simple filtration
process. The filtered sulfuric acid can be recycled and reused for
the repetitive GO synthesis with negligible differences in the as-synthesized
GO qualities. This removal of sulfuric acid has great potential in
lowering the overall GO production cost as the amount of water required
during the fabrication process, which takes a great portion of the
total production cost, can be dramatically reduced after such acid
filtration. The proposed eco-friendly GO fabrication process is expected
to promote the commercial application of graphene materials into industry
shortly.
A zero-power, low-cost ultraviolet (UV)-C colorimetric sensor is demonstrated using a gallium oxide and reduced graphene oxide (rGO) hybrid via photoelectrochemical reactions. A wide bandgap semiconductor (WBS) such as gallium oxide with an energy bandgap of 4.9 eV generates electron-hole pairs (EHPs) when exposed under a mercury lamp emitting 254 nm. While the conventional UVC sensors employing WBS convert the generated EHPs into an electrical signal via a solid-state junction device (SSD), our newly proposed UVC sensory system works by converting EHPs into an electrochemical reaction. The electrochemical reaction causes the degradation of a cationic thiazine redox dye, methylene blue (MB) and thereby spontaneously changes its color. As more rGO was hybridized with the gallium oxide, MB degradation was effectively expedited. Thus, the level of MB degradation under UVC can be evaluated as a UVC indicator. Unlike conventional SSD-based UVC sensors, our responsive colorimetric sensor can be applied where needed inexpensively and zero power.
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