SCC-DFTB Study on Structure, Electronic and Sensing Properties of Polypyrrole

Abstract

Polypyrrole (PPy) have been considered as one of the most promising conducting polymers for gas sensor application because it can be efficiently operated at room temperature, light weight, thermal stability and ease of preparation. In this work, we have investigated the geometric and electronic structures of PPy based on B3LYP/3-21G*, B3LYP/6-31G* and SCC-DFTB methods. The effects of polymer chain length of PPy on structural and electronic properties including bond length, bond angle, torsion angle, the highest occupied orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) and energy gap have been studied from the optimized PPy oligomers (n=1-10). The results show that the bond length, bond angle and torsion angle from SCC-DFTB method are C-N = 1.38 Å, N-H = 1.04 Å, C-C = 1.45 Å, N-C-C = 122 °, C-C-C = 131 °, N-C-C-C = 179 ° and C=C-C=C = 179 °, respectively. The HOMO and LUMO are in range of -5.49 to -4.23 eV and -0.078 to-1.92 eV, respectively. The energy gap decreases with increasing polymer chain length from 5.41 to 2.31 eV. Furthermore, sensing behavior of PPy for detection of toxic gases such as ammonia (NH3), carbon dioxide (CO2) and nitrogen dioxide (NO2) has been studied and discussed in details.