Highly sensitive NO2 sensors based on core-shell array of silicon nanowires/polypyrrole and new insight into gas sensing mechanism of organic/inorganic hetero-contact

Uniform core-shell hetero-array of polypyrrole-wrapped silicon nanowires (LNWs/PPy C-S nanowires) with enhanced gas-sensing response at room temperature was prepared via a two-step process. The aligned silicon nanowires were first formed by metal-assisted chemical etching (MACE). To achieve a loose silicon nanowires (LNWs) array facilitating the uniform deposition of PPy shell on whole wire surface, a repeated MACE treatment was further developed based on reoxidation of the MACE-produced Ag dendrites. The PPy-shell with adjustable thickness was then formed uniformly on the LNWs via vapor chemical polymerization (VCP) of pyrrole monomer (Py). The gas sensor based on the well-defined LNWs/PPy C-S heteronanowires array exhibits about 8.2 times response enhancement to 5 ppm NO2 gas compared with the pristine one at room temperature. Very low detection resolution of 50 ppb NO2 is observed when PPy shell has thickness of about 10 nm. The organic/inorganic C-S composite of LNWs/PPy nanowires shows a different characteristic of shell thickness effect on sensing response from the general semiconductor oxides-based C-S sensor; much thinner PPy shell is extremely preferred for higher sensing response. A sensing mechanism model was proposed to demonstrate the featured effect of organic shell thickness on sensing behavior of LNWs/PPy C-S heteronanowires. POLYM. COMPOS., 40:3275-3284, 2019. (c) 2019 Society of Plastics Engineers