Highly Sensitive Ammonia Sensor Based on Modified Nanostructured Polypyrrole Decorated With MAF-6 to Reduce the Effect of Humidity

An ammonia gas sensor based on nanostructured polypyrrole functionalized by carboxylic groups (PPyCOOH) was prepared and characterized. The active layer was deposited on a ceramic substrate with gold interdigital electrodes by a low-cost airbrush-spray coating technique, and the sensor exhibited a strong response to ammonia (Z/Z(0) = 5.48 at 100 ppm) in comparison with pure PPy (Z/Z(0) = 1.5 at 100 ppm). Cross-sensitivity measurement showed a more significant response to relative humidity (RH) and so two approaches were used to reduce this undesirable humidity dependence: 1) the creation of a hydrophobic metal-organic framework (so-called MAF-6) on the PPy-COOH layer and 2) the fabrication of a heterostructure of PPyCOOH with multiwall carbon nanotubes (MWCNTs). Both approaches resulted in a significant reduction in the humidity dependence of the sensor, from the value Z/Z(0) = 0.02 (PPyCOOH) to 0.404 and 0.427 for PPy-COOH-MAF-6 and PPyCOOH/MWCNT, respectively. However, the samples coated with MAF-6 showed a smaller decrease in response to ammonia. In addition, the proposed approach with MAF-6 functionalization allowed ammonia detection within a broader RH range (up to 60% RH) compared to the PPy-COOH sensing layer. The presence of MAF-6 on the surface of PPy-COOH was further verified by infrared spectroscopy (FTIR), X-ray diffraction (XRD) spectroscopy, and scanning electron microscopy together with energy-dispersive X-ray spectroscopy (SEM-EDX). The sensing mechanisms of ammonia and RH of the PPy-COOH material are also discussed in this article.