Synthesis and gas sensing property of nanostructured strontium titanate ferrite
Chow, Chee Lap
Date of Issue2013
School of Electrical and Electronic Engineering
Gas sensor technology is one of the most important key technologies for future development with a constantly increasing number of applications in environmental monitoring, pollution control, healthcare, automobiles, hydrogen economy and technical processes control. Among all available sensor technologies, the semiconducting metal oxide gas sensors have received most attention due to their advantages of simple design, small size, high sensitivity, low cost and ease of fabrication. Strontium titanate ferrite (SrTi1−xFexO3−δ or STFx in short) appeared to be a promising sensing material due to its attractive properties such as excellent gas sensitivity, mixed ionic-electronic conductivity, excellent doping flexibility and able to accommodate large amount of dopants and defects. Oxygen vacancies generated by the Fe substitution make STFx highly sensitive to oxygen partial pressure and hence a potential candidate for oxygen sensor. In this thesis, semiconducting metal oxide oxygen sensors based on nanosized STFx materials have been successfully fabricated and characterized. High-energy ball milling has been proven to be an effective method to prepare nanosized gas sensing materials. For the first part of the thesis, the STFx nanoparticles were synthesized using high-energy ball milling method for the first time. The phase formation and structural change at different stages of milling process were systematically studied. Single cubic perovskite STFx materials were successfully obtained for iron content up to 40% (x = 0.4). Miniaturized STF20 (x = 0.2) nanoparticles based gas sensors with gold interdigitated microelectrodes were designed and fabricated using standard silicon microfabrication techniques. Excellent gas sensitivity value of 2540 at 20 % oxygen gas in nitrogen was obtained for the sensor annealed at 550 °C for 1 hour. The optimum operating temperature was found to be around 250 to 300 °C. The improvement in the oxygen sensing performance was due to the smaller grain size and higher surface defects of the high-energy ball milled STF20 nanoparticles. For the second part of the thesis, STFx sensing materials have been prepared using modified sol-gel spin-coating method which has advantages such as low synthesis temperature, simple, thin film formability, capable for large area coating and good homogeneity. Factors such as molarity, chelating agent and amount of deionized water were optimized to obtain stable and precipitation-free STFx sol-gel precursors. The crystallinity and surface morphology of the sol-gel derived STFx thin films with different annealing temperatures and iron contents were systematically characterized. For the first time, a change of sensing response type was observed for the STFx sol-gel thin film sensors. This sensing response transition was observed for a huge oxygen concentration range and also for reducing gases such as carbon monoxide and ammonia. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis revealed that the anomalous n-type sensing response of STFx thin films was caused by amorphous TiO2 and Fe2O3 phases which existed for the film with low crystallinity, i.e. when the annealing temperature was insufficient or when the iron contents was too high. The electrical transport property was studied and proposed to provide in-depth understanding on the sol-gel derived STFx materials. The oxygen sensitivity factor (m) calculated from the oxygen partial pressure test was found to be smaller than the reported value, indicating that STF40 sol-gel thin film exhibited better gas sensing performance. Next, defect chemistry based on singly-charged oxygen vacancies was proposed to explain the electrical transport behavior of STF40 thin film sensors. The presence of singly-charged oxygen vacancies was further supported by the activation energy change estimated from the Arrhenius plot of STF40 thin film. Finally, ac impedance spectroscopy was successfully carried out on the STF40 sol-gel thin film sensors. An equivalent circuit model was then proposed based on the understanding of actual physical and chemical reactions that occurred at the gold electrode, grains and grain boundaries of the sensing material. Lastly, a schematic model was proposed to illustrate the electrical conduction and gas sensing behavior for n-type and p-type STF40 semiconductor metal oxide gas sensors.
DRNTU::Engineering::Electrical and electronic engineering