This grant provides funding to develop and study the characteristics of novel nanocomposite materials for chemical sensing. The materials involve integrating metal or semi-conductor in polymers. Such materials have exhibited unique electrical conductivity properties important for development of highly selective and sensitive gas sensors. The sensor material will be synthesized using specially-designed a chemical vapor deposition technique that permits optimization of the metal/semiconductor-to-polymer composition ratio. The synthesized material structure will be characterized including chemical composition and morphology, and a model of nanocomposite growth is developed. The sensor performance will be studied by measuring induced electrical conductivity upon exposure to specific gaseous ambience. The experimental results will be used to develop and validate a comprehensive theory of sensor behavior and sensor selectivity, sensitivity and time response to a variety of gaseous surroundings including carbon dioxide and hydrogen.
If successful, the result of this research will lead to a novel method for intelligent nanomanufacturing and optimization of materials sensing properties. A primary objective of this research is to establish the structure-property relationship ? the link between the key materials parameters (chemical composition, and microstructure) and the resulting sensing responses. The integration of synthesis, characterization, and modeling is the critical component of the proposal which will allow optimization of synthesis conditions in order to obtain materials with the desired sensing properties. The proposed work will also provide a methodology for producing chemical biosensors with selectivity towards ambient conditions that are unique to certain diseases. An example is a sensor for detection of superoxide anion radicals (oxgygen ions) that have been implicated in a number of diseases including cancer and heart disease. Such a sensor could potentially provide a means for early detection and treatment of such diseases.
Sensor technology including chemical sensing plays ever increasing role in monitoring manufacturing processes, the environment, and health/biological systems. Development of novel sensor materials tailored to specific applications is driven by the requirement to produce high volume, low cost sensors with the highest selectivity and sensitivity, and with the fastest response. Nanostructured semiconductor thin films are one of the best sensor materials employed to detect analyte gases. The sensing films are composed of sintered nanoparticles of an average diameter up to several hundred nanometers, which possess high surface to volume ratio that allows effective surface adsorption of gas phase analyte molecules. The excellent stability, high sensitivity, relatively low operating temperatures, and low cost of sensors based on semiconductor nanocomposites are responsible for their wide-spread application. In spite of extensive efforts in the research and development of nanostructured semiconductor sensors, the fundamental mechanisms of coupled chemical and electronic processes were not fully understood. Therefore, the primary goal of this project was to develop fundamental science of sensor response of novel nanoparticle semiconductor sensor materials for chemical sensing. The project established the connection between the material’s microstructure, the chemical composition, and the desired sensing properties, thus allowing an intelligent design of sensor materials to achieve the maximal sensing response. The project was focused on the fundamental physic-chemical mechanisms of sensor response, which is an increase in conductivity upon the addition of reducing gases such as H2 and CO to ambient atmosphere. It was found that pre-existing oxygen vacancies semiconductor nanoparticles act as electron donors to the conduction band. Oxygen atoms appearing upon dissociation of atmospheric oxygen at the surface of nanoparticles serve as electron traps, thus decreasing the concentration of conduction electrons. Sensor response is caused by an increase in the film conductivity upon the addition of the reducing analyte gas, which reacts with atomic oxygen at the surface of nanoparticles to form water molecules in the gas phase, and is then followed by the transfer of electrons back into the conduction band. The project developed the theoretical description of sensor response by taking into account the kinetics of surface chemical reactions that both control the concentration of electrons within the conduction band, and the physics of electron transport in nanostructured nanoparticles. By solving the stationary system of equations for chemical kinetics, the dependence of sensor response on average nanoparticle size, analyte concentration, and temperature has been predicted. The important outcome of this project is an extensive validation of the developed theory of sensor response. For this purpose, new experiments were performed by international collaborators to investigate the sensitivity of semiconductor tin dioxide nanostructured thin films as a function of temperature and concentration of analyte hydrogen gas. Concurrently, the sensor properties were calculated at experimental conditions by taking into account the increase of surface chemical reactions with temperature as well as subsequent dominance of desorption over adsorption processes at high temperatures. Both theory and experiment display a sharp maximum in the temperature dependence of sensor sensitivity, which is due to the temperature-dependent increase of adsorption and desorption processes, as well as their competition at different temperatures. In addition, detailed comparison with experimental data from other groups was performed, which demonstrates good agreement between experiment and the theory across the samples obtained at different synthesis and processing conditions. This project achieved an important educational goal of educating the next generation of scientists and engineers by providing them with an opportunity to work on challenging fundamental problems of practical significance at the boundaries of different scientific and engineering disciplines. International collaboration also increased the broader impact by providing our students with the opportunity to gain professional experience at the international level.
