APPLICATION OF NANO-STRUCTURED CONDUCTING POLYMERS TO HUMIDITY SENSING

Open Access
Author:
Park, Pilyeon
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 29, 2008
Committee Members:
  • Dr Fonash, Dissertation Advisor
  • Stephen Joseph Fonash, Committee Chair
  • Mirna Urquidi Macdonald, Committee Member
  • Barbara Shaw, Committee Member
  • Sanjay B Joshi, Committee Member
Keywords:
  • conducting polymers
  • humidity sensors
  • Nanotechnology
Abstract:
Nanostructures, such as nanowires, nanocolumns, and nanotubes, have attracted a lot of attention because of their huge potential impact on a variety of applications. For sensor applications, nanostructures provide high surface area to volume ratios. The high surface area to volume ratio allows more reaction areas between target species and detection materials and also improves the detection sensitivity and response time. The main goal of this research was to exploit the advantages and develop innovative methods to accomplish the synthesis of nanowires and nano-coulmn conducting polymers used in humidity detection. To accomplish this, two fabrication methods are used. The first one utilizes the geometric confinement effect of a temporary nanochannel template to orient, precisely position, and assemble Polyaniline (PANI) nanowires as they are synthesized. The other approach is to simply spin-coat a polymer onto a substrate, and then oxygen plasma etch to generate a nano-columned Polyethylenedioxythiophene (PEDOT) thin film. 200 nm silicon oxide coated wafers with embedded platinum electrodes are used as a substrate for both fabrication methods. The biggest advantage of this first method is that it is simple, requires a single-step, i.e., synthesizing and positioning procedures are carried out simultaneously. The second method is potentially manufacturable and economic yet environmentally safe. These two methods do not produce extra nano-building materials to discard or create a health hazard. Both PANI nanowires and nano-columned PEDOT films have been tested for humidity detection using a system designed and built for this research to monitor response (current changes) to moisture, To explain the surface to volume ratio effect, 200 nm PANI nanowires and 10 μm PANI wires were directly compared for detecting moisture, and it was shown that the PANI nanowire had a better sensitivity. It was found difficult to monitor the behaviors of the PEDOT reaction to varying moisture levels because even low humidity levels saturate the sample surface within a few minutes. Because of this, it was not perfect to distinguish the effects of etching the PEDOT film for humidity detection and difficult to apply nano-columned PEDOT films as a humidity sensors under continuously changing humidity conditions. However, nano-columned PEDOT films showed excellent performance in simulated breath tests, i.e., an area where the medical needs sensors for pulmonary monitoring. Since the polymers are sensitive to heat, it was important to characterize the influence of temperature on the sensor performance. PANI nanowires and nano-columned PEDOT sensors were tested in the environmental chamber developed in this work as a function of temperature with the humidity fixed, and only the temperature was varied. The PANI nanowires showed very fast degradation at temperatures above room temperature, while the nano-columned PEDOT film performed up to 50 °C. The influence of other gases was also tested for the potential of gas sensing, selectivity, and chemical stability. In order to exclude the moisture effect during the measurement, the samples were characterized under the lowest humidity condition, RH 14 % preserved in the system. Under these conditions the PANI nanowires responded to the gases (hydrogen and carbon monoxide were used), but the moisture inside the PANI nanowire was forced to influence the gas detection. Therefore, samples were dried overnight under a nitrogen environment and tested again. With this careful control of the moisture present, it was found that PANI nanowires respond to both hydrogen and carbon monoxide gases, however, there is no selectivity between gases. Nano-columned PEDOT films were also tested under the same experimental moisture-controlling conditions. It was shown that there was little response to other gases. Any response that may have been presented was buried in the electrical noise. Finally, both samples were tested for long-term stability. PANI nanowires showed almost linear degradation with continuous use, while nano-columned PEDOT was stable over extended periods of time. The PANI nanowires were more reactive to the environment compared to the nano-columned PEDOT films. The only disadvantage of the nano-columned PEDOT sensor structure is that it loses its durability when it is directly exposed to liquid water or highly saturated humidity for long periods of time. A surface treatment of PEDOT was developed that enhanced its durability against water without degrading its sensor performance. Nano-columned PEDOT films look promising for breath sensors for medical usage because they have sub-second response times and are relatively chemically and physically stable.