(A) study on the development of self-powered gas sensor using chemical color change or mechanical structure change by gas reaction

Abstract

A self-powered gas sensor based on chemical color changes or mechanical structure changes due to gas reactions was proposed in this dissertation. The sensor does not work on electrical power and instead uses ambient visible light sources that exist around it. The sensor comprises a sensing film, which changes color because of a gas reaction, or changes its structure, leading to a change in its light transmittance, and a photovoltaic cell. In the first research section, $NO_2$ gas sensor was developed by using chemical color change. N, N, N ', N'-tetramethyl-p-phenylenediamine (TMPD) was used as the colorimetric material for $NO_2$ detection and was coated on micro/nano structures by the spray coating method. TMPD reacts with nitrogen dioxide ($NO_2$) in an oxidation-reduction reaction forming a blue color. When a color change occurs on the film surface, the transmittance changes to the visible region, changing the output of the photovoltaic cell. Thus, $NO_2$ can be detected at concentrations of 1, 5, 10, and 20 ppm using the change in current output of the photovoltaic cell. The response ($deltaI/I_0$) of the sensor was improved by forming micro/nano structures on the surface of the transparent polydimethylsiloxane (PDMS) film. The higher the aspect ratio of the micropost array formed, the better the performance of the sensor. Also, nanowire formation on the micro structures increased gas sensing performance. The response for 20 ppm $NO_2$ detection was increased from 0.09 to 0.17 using the micro/nano structures. These micro/nano structures increase the surface area for the gas reaction, thus changing the transmittance of the film and improving the performance of the sensor. This sensor has highly selectivity for $H_2S$, CO, and humidity. Furthermore, the performance of the sensor was verified under real life applications using office LEDs, fluorescence light, and various light intensities. Finally, to verify its practical application, the circuit that turns the LED on when it is exposed to NO2 gas was made simply with an operational amplifier (op-amp) circuit.

In second research section, $H_2$ gas sensor was developed by using mechanical structure change of nano-grating. In the case of hydrogen ($H_2$) detection, palladium (Pd) was deposited on the polyurethane acrylate (PUA) nano-grating structures. Pd reacts with $H_2$ forming $PdH_x$, which causes volume expansion. Such a sensing system uses a method in which the amount of transmitted light varies when the polymer nanostructure is deformed because of volume expansion. To optimize the performance of the sensor, the deposition angle of Pd was simulated and compared with actual experiments. The deposition thickness was also optimized, and the deposition angle and thickness were set to $45^\circ$ and 80 nm, respectively. The sensor could detect 2% hydrogen with a response of 0.22. This sensor also showed no reaction for CO, $H_2S$, and $NO_2$ gases, and it was confirmed that the sensor performance was not changed by the presence of humidity. In addition, it operated stably against 2% $H_2$ in 125 cycles and showed high reliability. Also, it was also possible to produce a $H_2$ sensor alarm using a mobile device using a Bluetooth module. These sensor systems do not require heaters or photodetectors, and they can inform people about target gas exposures using only ambient visible light sources in the surrounding without external power consumption.