Oxygen-Vacancy-Dependent Photocatalysis for the Degradation of MB Dye Using UV Light and Observation of Forster Resonance Energy Transfer (FRET) in PANI-Capped ZnO

In this work, pure ZnO, pure polyaniline (PANI), and a set of PANI-capped ZnO samples of various PANI concentrations are synthesized using the coprecipitation, oxidative polymerization, and ex situ methods, respectively. The X-ray diffraction results exhibit suppression in directional growth and shifting in ZnO-related peak positions in the capped samples with increasing PANI content. Interactions between ZnO and PANI chains are confirmed by Fourier transform infrared (FTIR) spectroscopy. The morphological characterizations (scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) reveal the presence of an agglomerated spherelike structure of PANI on top of ZnO nanosheets. The X-ray photoelectron spectra (XPS) reveal a reduction in the density of surface as well as deep oxygen vacancy defects of ZnO after capping with PANI. The absorption spectra reveal an enhancement in the bipolaron band absorption for capped samples, which reinforces the presence of PANI chains. The photoluminescence spectra show a quenching in the emission intensity of ZnO after the addition of PANI; the overall quenching is discussed through concentration-dependent Forster resonance energy transfer (FRET) theory between ZnO (donor) and PANI (acceptor) between ZnO (donor) and PANI (acceptor). The sample capped with the least PANI content shows maximum quenching in the emission profile due to the encounter among the intradonor and the inter-donor- acceptor energy transfer mechanism. For rest of the capped samples, emissions are explained using the concentration-dependent inter-donor-acceptor FRET theory. Finally, the catalytic study of all capped samples and pure ZnO is performed through the degradation of methylene blue (MB) dye by irradiating UV light. Despite having quenched photoluminescence intensity, the results show a reduction in degradation efficiency for PANI-capped ZnO compared to pure ZnO contrary to the common trend. This is associated with the change in the density of oxygen vacancy sites of ZnO. Of the two oxygen vacancy sites, the surface oxygen vacancy sites play a major role as carrier trap centers and the deep oxygen vacancy sites delay the recombination process. The mechanism behind the effect of change in the density of these two vacancy types on photocatalysis is explained. Thus, by concentration-dependent FRET between ZnO and PANI and the oxygen vacancy defect density, the emission intensity and the photocatalytic activity of PANI-capped ZnO can be tuned.