5. Selective Crystal Structure Synthesis and Sensing Dependencies

Chemo-resistive sensors utilizing meal oxides form a very important type of sensors for gas detection. They are based on the interaction between gas molecules and surface ionosorbed oxygen species accompanied by electron transfer, which eventually leads to the change of material resistance. This process is controlled by a few external parameters (working temperature) and internal parameters (microstructure, chemical composition and crystal structure). While most parameters have been paid sufficient attention to, the influence of crystal structures is still largely unexplored. On the other hand, metal oxides exist in more than one crystalline form. The structural and property difference between different structures is expected to affect the sensing behavior of the material. Taking TiO₂ and WO₃ as examples, this chapter reviews how to selectively synthesize desired crystal structures and how they are related to the performance as agas sensor. TiO₂ exists in two major polymorphs, with rutile being the thermodynamically stable phase and anatase being the metastable one. Compared to rutile, anatase is more open-structured and more chemically active and has lower surface energy. The hydrothermal method has been proved to be very effective in anatase synthesis as long as particle size is well controlled (normally under 20 nm) and dopants could stabilize this phase. Studies have found that anatase shows higher sensitivity as a gas sensor which is believed to be attributed to its higher chemical activity.WO₃ undergoes a series of phase transition when it is cooled down and γ-WO₃ is usually the room-temperature (RT) stable phase. The low-temperature stable phase, ε-WO₃, is the least symmetric among all the phases and is the only one with a ferroelectric feature. By a rapid solidification method called flame spray pyrolysis, ε-WO₃ is able to be synthesized in high purity at RT. Doping with silicon and chromium could effectively stabilize this phase up to 500 °C by forming boundary domains or surface layers. The dopant-stabilized ε-WO₃ shows high sensitivity and unique selectivity to polar gas molecules, esp. acetone, which may be due to the strong interaction between the ε-WO₃ surface dipole and polar molecules.