Wet process-prepared thick films of WO3 for NO2 sensing

Preparation of an aqueous sol of WO3·2H2O starting from Na2WO4 through an ion-exchange method, coupled with post-treatments (ultrasonic and/or centrifugal), was found to give WO3·2H2O gels which differed in morphology and preferred orientation depending on the kinds and conditions of the post-treatments. The morphology of the gels was well inherited topotaxically by the dehydrated phases of WO3·H2O and tungsten trioxide (WO3), allowing to control the morphology of WO3 by the wet processing. The WO3 thick film devices, fabricated from the gels and calcined at 300°C, were found to show very different sensor responses to NO2 in air depending on the morphology of WO3. That is, poorly developed, small lamellas of WO3 obtained by the ultrasonic treatment for a prolonged time (5h) gave the most sensitive device at 200°C, which could respond to as low as 10ppb NO2, while well developed, thin lamellas of WO3 obtained by the centrifugal treatment for a prolonged time (10h) gave the most sensitive one at 300°C. In order to understand the dependence of sensor response on the morphology of WO3, several other methods were also adopted for preparing WO3. It was found that all of the sensor response data collected at 300°C could be related fairly well with the thickness of WO3 lamellas or the diameter of WO3 grains, whereas such a relationship became less certain at 200°C.