Nitric oxide sensors using combination of p- and n-type semiconducting oxides and its application for detecting NO in human breath

Mixtures of WO3 and Cr2O3 in varying weight ratios as well as adjacent alignment of these two powders were both examined as possible designs for NO-selective sensor. Studies focused on resistance measurements toward NO and CO at 300 °C were carried out. Using the sensor design with the adjacent alignment of n-type WO3 and p-type Cr2O3 resulted in optimal performance. This sensor design exploits the different majority carriers in these two semiconducting oxides for selective NO gas sensing. The advantage of such a sensor system is that the device can be sensitive to NO at low ppb level (18 ppb detection limit) and discriminate against CO at concentrations a thousand-fold higher (20 ppm). The sensing mechanism was investigated by in situ diffuse reflectance infrared studies. Characterization of the adjacent p–n interface was done by scanning electron microscopy (SEM) and Raman imaging study. Practical application of this device is demonstrated by measuring NO in human breath samples.