Structural phase controlled transition metal (Fe, Co, Ni, Mn) doping in CdS nanocrystals and their optical, magnetic and photocatalytic properties

Abstract

Cadmium sulphide (CdS) generally exhibits cubic zinc blende (ZB) and hexagonal wurtzite (WZ) structural phases at room temperature. The present work reports successful doping of transition metals (TMs) (2% Fe, 2% Co, 2% Ni, 2% Mn) in the ZB and WZ phases of CdS nanocrystals, investigation of their various physical properties and evaluation of their photocatalytic performances by degradation of aqueous methylene blue under simulated sunlight irradiation. X-ray diffractometer, transmission electron microscope, field emission scanning electron microscope, energy dispersive spectrometer, Fourier transform infrared spectrometer, vibrating sample magnetometer, UV–Visible spectrophotometer and spectrofluorometer were used to investigate the physical properties of the nanocrystals. Doping did not change the structural phase of the un-doped CdS nanocrystals. The un-doped CdS nanocrystals with the ZB and WZ phases had different band gaps, photoluminescence properties and photocatalytic activities. The un-doped CdS nanocrystals were diamagnetic while the TM doped CdS nanocrystals were ferromagnetic showing the TMs were essential for inducing the room temperature ferromagnetism. The band gaps of the nanocrystals with the WZ phase were larger than those with the ZB phase. The photoluminescence spectra showed the presence of four visible emissions for the WZ CdS nanocrystals while the presence of only two visible emissions was observed for the ZB CdS nanocrystals indicating the presence of more defect types in the nanocrystals with the WZ phase. The un-doped nanocrystals with the WZ phase exhibited superior photocatalytic activities compared to those with the ZB phase. The TMs caused significant modification in their photocatalytic activities. Ni and Mn doping showed increase in photocatalytic activities while Fe and Co doping showed decrease in photocatalytic activities of both ZB and WZ CdS nanocrystals. Ni doping in the WZ CdS nanocrystals was found to result in the best photocatalytic properties among the materials studied.