SENSORS FOR DETECTING SUB-PPM NO2 USING PHOTOCHEMICALLY PRODUCED AMORPHOUS TUNGSTEN-OXIDE

This paper describes a new approach to making the active elements of g as sensors using a photochemical deposition method. The method involve s making amorphous films of photochemically active precursors. These p recursors are then exposed to light and, in air, convert to metal oxid es. Amorphous films constructed of W(CO)(4)(Et-2-en) are deposited on interdigitated microelectrodes. Photolysis of these films, in air at r oom temperature, results in the deposition of amorphous films of tungs ten oxide. This forms the sensing element of an NOx sensor. Films thus prepared were also annealed to yield sensors whose active elements we re crystalline tungsten oxide. An investigation of the conduction mech anism in the sensor materials was performed. The conduction in the pol ycrystalline materials is controlled by grain boundaries through therm ionic emission. Absorption of NO2 leads to a modification of the grain boundary resulting in the change in current. In contrast, the amorpho us materials conduct via variable range hopping. In this case, absorpt ion of NO2 leads to a reduction in the number of carriers and a change in conductivity of the material. The response of the amorphous materi als has been fit to a model based on a Langmuir isotherm.