NiO/Ag heterostructure: enhanced UV emission intensity, exchange interaction and photocatalytic activity

Abstract

The effect of direct Ag coating on the optical and catalytic properties of NiO flake-like structures has been investigated in detail in this work. Ag nanoparticle decorated NiO flakes were synthesized using a facile two-step process. The phases and crystal structure were examined using an X-ray diffractometer, whereas the morphologies of the synthesized nanostructures were investigated using FESEM and HRTEM. Interfacial interaction of the synthesized heterostructures was examined using Fourier transformation infrared and Raman spectroscopy. Importantly the LO phonon and magnon scattering mode of NiO are significantly affected by the Ag coating. The band gap of NiO is found to be lowered (direct band gap: 3.52 to 2.78 eV and indirect band gap: 2.72 to 0.50 eV) due to the change in valence band energy originating from the charge transfer process between NiO and Ag. A UV emission that depends on the crystal field splitting of the Ni d orbitals is observed to possess a blue shift along with their enhancement of intensity. Variation of the magnon scattering mode also reveals that the enhancement of exchange interaction with NiO nanoflakes causes the blue shift of the UV emission. The nonradiative part corresponding to this UV emission which is parameterized using the Huang-Rhys factor gets enhanced due to the lowering of the electron–phonon interaction. The photocatalytic activity of the synthesized heterostructures was evaluated using the catalytic decomposition of the dye methylene blue in aqueous solution. The results show that the NiO/Ag heterostructures exhibit a much higher degradation rate than virgin NiO flakes that might be from improved separation of the photogenerated electrons and holes. In particular, the pseudo-first-order degradation rate constant of NiO/Ag is found to be 10 times greater than bare NiO.