Polaron and bipolaron mediated photocatalytic activity of polypyrrole nanoparticles under visible light

The present research investigates the photocatalytic activity of pyrrole and polypyrrole (PPy) under visible light for the treatment of contaminated water with the organic dye methylene blue (MB). Pyrrole is inert for photo-catalytic activities, because of its large HOMO-LUMO gap. However, when PPy was synthesized, the electronic structure changes, making PPy an active photo-catalyst. PPy can be utilized to produce photo-excited charges on any type of light source (UV or visible) since it has broad absorbance spectra. The UV-Vis spectroscopy shows absorbance bands in the UV and visible region, which correspond to absorption maxima at 380 nm, 470 nm and 525 nm. Additionally, a broad tail near the IR region was observed which corresponds to polaronic and bipolaronic transitions. These charges actively contribute to the breakdown of dye during the degradation phenomenon. The presence of polaron was confirmed by electron spin resonance (ESR) having g value of 1.99, and its intensity decreases on light exposure. The interaction between PPy and MB through polaron-bipolaron was explained by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). N 1s and C 1s XPS results show the shift in the B.E. of –N–H+·, =N–H+, –CN+ and =C–N+·when PPy interacts with MB which gives the signature of charge transfer. Furthermore, the interactions within the system were analyzed by ultraviolet photoelectron spectroscopy (UPS) data which provided descriptive information on the molecular orbital level and also gave an indication of charge transfer state formation between PPy and MB at − 7.83 eV.