Effect of Fe/Sn doping on the photocatalytic performance of multi-shelled ZnO microspheres: experimental and theoretical investigations

A series of multi-shelled Fe3+/Sn4+-doped ZnO microspheres were synthesized by calcining carbon microspheres as sacrificial templates. The crystal structure, morphology and optical properties of prepared photocatalysts were investigated by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Scanning Transmission Electron Microscopy (STEM), X-ray Photoelectron Spectroscopy (XPS), UV-Vis absorption spectroscopy (UV-Vis DRS), Photoluminescence (PL) spectroscopy and transient photocurrent techniques. FESEM and STEM images demonstrated that the samples had a multi-shelled structure and the doping elements were uniformly dispersed in the ZnO matrix. DRS and PL spectra and photocurrent measurements show that the Fe/Sn doped-ZnO samples exhibited higher visible absorption and lower recombination rates of photogenerated electron-hole (e(-)/h(+)) pairs. These advantages resulted in higher photodegradation efficiency of these catalysts towards methylene blue (MB) and ciprofloxacin (CIP). Moreover, the band structure and density of states of the un-doped ZnO and Fe/Sn-doped ZnO models with three different doping ratios were calculated based on the CASTEP code. The roles of the Fe3+/Sn4+ dopant in the ZnO matrix were found to be band gap control and electron density adjustment due to the Fe(3d) and Sn(5s, 5p) orbitals. This work has a certain reference value for constructing multi-shelled photocatalysts with doping modification and investigating the mechanism of enhanced photoactivity at the microscopic level.