Flexible NO<sub>2</sub> Gas Sensor Based on Functionalized MWCNTs Modified with a Layer of Metallic Nanoparticles and Polypyrrole Conductive Polymer Works at Room Temperature

A gas sensor based on a multi-walled carbon nanotube (MWCNTs-OH) network was fabricated by filtration from the suspension method (FFS), and its properties were improved by coating the network with a polypyrrole conductive polymer. The polymer was prepared using the chemical oxidation method. Metallic nanoparticles of silver and copper were added separately to the polymer by the in situ chemical oxidation method. The fabricated networks were characterized using an X-ray diffractometer (XRD) and a Photoluminescent spectrometer. For the networks of (MWCNTs) with polypyrrole (PPy), (PPy: Cu), the (002) peak's widening diminishes, and the broadening increases when silver (PPy:Ag) is added to the MWCNTs networks. The crystalline size decreases for the networks coated by (PPy) as well as silver NPs (PPy:Ag), while it increases with the network coated by (PPy:Cu). Photoluminescence spectra of the networks were measured at different excitation wavelengths (340-380) nm and the networks gave the emission spectra in the range of (765-855) nm. The analysis revealed that the energy gap becomes larger for the networks coated with (PPy: Ag), (PPy: Cu) networks than for pure MWCNTs. A homemade sensing device was used to evaluate the sensitivity of the fabricated networks for gas concentration of 20 ppm at room temperature. The sensitivity of the fabricated sensor was (56.17%). After modifying the surface of the fabricated network by coating with the polypyrrole conductive polymer and polypyrrole composite with silver and copper metallic nanoparticles, the sensitivity became (59.29, 64.5, and 65.3) % respectively.