Nitrogen-Doped Activated Carbon-Based Ammonia Sensors: Effect of Specific Surface Functional Groups on Carbon Electronic Properties

Wood-based commercial activated carbon (BAX) and its oxidized counterpart (BAX-O) were treated with melamine and then heated at 450 degrees C in nitrogen. Further oxidation with nitric acid was also applied. The carbons were tested for ammonia sensing (45-500 ppm of NH3). Even though all samples exhibit p-type conduction, their exposure to NH3 led to different electrical outcomes. It was found that the electronic and transport properties of the carbons strongly depend on the type of nitrogen groups/surface defects, their concentration, and distribution in the carbon matrix. Pyridines and nitropyridines are the most important. A competition between the structural and chemical features of the carbons as those governing the sensing signals was observed. Exposure to ammonia altered the surface chemistry of the samples, and therefore their electrical properties. When sensitivity to H2S was tested to evaluate the selectivity of our sensors, the results showed that the chips are selective to NH3 in terms of the response time and magnitude of the signal changes.