Synthesis, Characterization and Applications of Selected Transition Metal Sulfides Nanoparticles

Abstract

Currently, mankind is facing the most challenging and solution demanding energy and environment related issues. Transition metal sulfide nanoparticles (TMS NPs) have attracted significant attention for the mitigation of these problems since the previous decade, due to their diverse structural types and tremendous physicochemical properties governed by size and morphology. In this work, four different types of TMS NPs i.e, MoSx, ZnS, CdS and HgS have been synthesized by adopting the electrochemical deposition and single source precursor strategy. Among these four, MoSx was electrochemically deposited on polyethylenimine (PEI) modified reduced graphene oxide (rGO-PEI-MoSx) substrate. After characterization by FT-IR, SEM, TEM, EDS, XRD and Raman spectroscopy it was tested as a heterogeneous electrocatalyst for the reduction of CO2 to CO in CO2 saturated aqueous NaHCO3 solution with high efficiency and selectivity. The catalyst is capable of producing CO at overpotential as low as 140 mV and reaches a maximum faradaic efficiency of 85.1% at 540 mV. However, at 290 mV syngas (CO + 2H2) was formed instead of CO formation. Detailed investigations reveal that PEI works as a co-catalyst by synergetic effect. A single source precursor strategy was adopted for the synthesis of HgS, CdS and ZnS NPs by using their respective metal (II) dithiocarbamates as precursors. Prior to their use as precursors all the complexes were characterized by CHNS, FT-IR, 1HNMR and 13CNMR spectroscopy. Four of the complexes i.e. HgL1, HgL2, ZnL6 and ZnL7 were also characterized by single crystal XRD analysis. The conversion of parent complexes to their off-spring HgS, CdS and ZnS NPs was achieved by thermolysis of the corresponding precursors in ethylenediamine (en) at ambient pressure and temperature, devoid of any externally added toxic surfactant. The morphology, structure, phase and elemental composition of as-obtained MS (M = Hg, Cd and Zn) products were characterized by SEM, TEM, XRD and EDS analysis. The SEM and TEM results revealed precursor based significant morphological variation for HgS and CdS NPs assignable to precursor’s stability/solubility in en and presence of capping agent on the particle surface (as revealed by DFT studies and FT-IR analysis). The XRD pattern showed that HgS NPs has grown in both typical cystalline forms i.e. black colour cubic iv and red colour hexagonal, however, CdS NPs into hexagonal and ZnS formed in mixed crystalline phases. The optical properties assessment of MS (M = Hg, Cd, Zn) by UV-Visible spectroscopy have confirmed their good absorption ability in visible (HgS and CdS NPs) and UV (ZnS NPs) regions. Based upon the band gap suitability, CdS NPs in different morphological forms were used as solar light driven photocatalysts for the degradation of Congo red dye and photoconversion of an environmentally detrimental p-nitrophenol to pharmaceutically valuable p-aminophenol. Among them, the anisotropically grown CdS NPs showed better photocatalytic performance probably due to their good optical absorbance and longer electron hole recombination time.