Gas Sensing Studies of a n-n Hetero-Junction Array Based on WO3 and ZnO Composites

A composite metal-oxide-semiconductor sensor array based on tungsten oxide (WO₃) and zinc oxide (ZnO) was fabricated using a simple mechanical mixing method. The array was characterized using scanning electron microscopy, X-ray diffraction, Raman scattering, and X-ray photoelectron spectroscopy and tested against a variety of oxidizing and reducing gases at various operating temperatures in the range of 300°C-500°C. It was found that the composites generally had an enhanced response to the analytes. The largest observed response toward ethanol was of a 50 wt.%:50 wt.% WO₃:ZnO sensor, which displayed a response of 36 toward 100 ppm of the analyte at 350°C, a 1.5-fold enhancement compared with a pure WO₃ sensor and a 6.5-fold compared with a pure ZnO sensor. A 10 wt.%:90 wt.% WO₃:ZnO composition displayed the highest response of 148 toward 800 ppb NO₂ at 300°C, a 3.5-fold enhancement with respect to a pure WO₃ sensor and a 7-fold with respect to a pure ZnO sensor. Cross sensitivity studies toward a variety of reducing gases at 350°C, showed selectivity of the array towards ethanol. The enhanced behavior of the mixed oxide materials was influenced by the packing structure, junction effects, composition, and microstructure. The results show that it is possible to tune the sensitivity and selectivity of a composite sensor, through a simple change in the compositional contribution of each metal oxide.