Electrochemical Detection And Structure-activity Relationship Of Transition Metal Compounds And Their Complexes For Heavy Metal Ions

Heavy metal ion pollution affects the stability of the earth’s ecosystem and poses a serious threat to human health.The rapid and highly sensitive detection of trace heavy metal ions using a simple method is of considerable significance to protect the ecological environment.Owing to the unfilled valence electron layer,transition metal compounds possess excellent activities and are widely used in applications such as catalysis,capacitors and batteries.Nano-sized transition metal compounds exhibit a strong size effect and strong bonding ability with organic and inorganic compounds.Moreover,through doping,these compounds are combined with modification methods to improve the electrochemical performance of the material itself,laying a foundation for the highly sensitive detection of heavy metal ions.In this study,transition metal compounds are used as catalysts,combined with graphene and used as substrates or doping atoms to improve the properties of intrinsic materials and increase their binding ability with heavy metal ions.X-ray photoelectron spectroscopy(XPS)and density functional theory(DFT)calculations were used to explore the electron transfer and adsorption energy associated with the adsorption process from experimental and simulated perspectives,revealing the mechanism of different material modification methods to improve the detection of heavy metal ions.The research content of this paper is as follows:(1)25%NH3·H2O was used as the surfactant to limit the growth of crystal planes,and Co3O4/N-RGO with three different morphs was synthesised using a simple solvothermal method.The lattice spacing was measured using high-resolution transmission electron microscopy,and the exposed crystal planes of Co3O4 in different coordination environments on reduced graphene oxide(GO)were determined.The surface valence states were further determined.Results show that the proportion of surface Co3+and Co2+is different among the three Co3O4/N-RGO types and that the content of surface Co3+is the highest among the three Co3O4-NP/N-RGO types.Furthermore,the sensitivity of the glassy carbon electrodes made from the three materials to Hg2+ varies greatly.Co3O4-NP/N-RGO on the exposed(112)surface exhibited the highest square wave anodic stripping voltammetry(SWASV)response sensitivity,followed by Co3O4-NTO/N-RGO on the exposed(111)surface.However,Co3O4-NC/N-RGO exposed on the(001)surface showed poor detection performance for Hg2+.Moreover,Co3O4-NP/N-RGO also exhibited excellent signal intensity in natural water and human serum.XPS and DFT calculations were used to reveal the electron transfer and orbital coupling strength of three different Co3O4/N-RGO before and after adsorption to explore the mechanism of forming such differences.Combining Co and Hg displaced the XPS peak,indicating the presence of chemical adsorption.Simultaneously,the binding of Co3+ and Hg2+forms a new peak on the electron state density diagram,showing that the coupling strength is high,but this phenomenon is not visible in the other two materials.Moreover,the binding process of Co3+and Hg2+ in Co3O4-NP/N-RGO has relatively large adsorption energy and appropriate bond length compared with the other two materials,which is conducive to the desorption and adsorption of Hg2+ at the solid-liquid interface of the electrode.Therefore,we believe that Co3+ is the active site for efficiently detecting Hg2+.(2)CoCl2 was used as a dopant to form a co-doped Mo3O10(C6H5NH3)22H2O precursor,which was calcined in a tubular furnace in an Ar2 atmosphere to form uniform Co-Mo2C nanorods with a diameter of approximately 200 nm.The contrast material Mo2C was prepared to explore the influence of doping on the adsorption properties of the system.The SWASV response curves of these two materials to Pb2+,Cu2+,Zn2+and Hg2+ were studied using a three-electrode system.The results show that the Co-Mo2C modified glassy carbon electrode exhibited excellent electrochemical sensing efficiency for Pb2+ compared with Mo2C.Further,the sensitivity was up to 129.17 μM/μA,and the Co-Mo2C exhibited the best sensing performance for Pb2+ among different heavy metal ions.Through XPS,we confirmed that Mo obtained from Co-Mo2C lost more electrons in the adsorption reaction with Pb2+,and the peak shift was larger than that of Mo2C.Furthermore,the DFT calculations proved that Co doping promoted the activation of Mo atoms,and compared with Mo before doping,more electrons gathered around Co,providing a good binding site for the adsorption of heavy metal ions.Moreover,for different heavy metal ions,we found that Co-Pb exhibited appropriate bond length and large adsorption energy,and all the orbitals of Pb and Co exhibited a certain coupling effect,whereas for other ions,the orbitals were partially coupled or there was no obvious coupling effect.Therefore,we believe that the high sensitivity of Co-Mo2C to Pb2+ is mainly caused by the strong coupling effect of Co and Pb.(3)The reduced GO/tungsten disulphide(GWS2)nanomaterials were prepared using a simple solvothermal method,and the contrast materials tungsten disulphide(WS2)nanomaterials were prepared.The sensitive electrochemical interfaces of GWS2 and WS2 were used to explore their Pb2+ detection performance.The results show that the GWS2 film exhibited a smooth and uniform appearance and a sensitivity of 108.01 μM/μA for Pb2+detection with a detection limit of approximately 0.041 μM,which is considerably greater than that of WS2.Furthermore,XPS and DFT were used to explore the structure-activity relationship.Compared with WS2,the binding energy of S of GWS2 exhibited a greater shift after the adsorption of Pb2+,indicating that its electrons possessed more transfer.Moreover,the bandgap of GWS2 in the DFT calculation table was considerably lower than that of WS2.Compared with WS2,the interface adsorption of Pb2+using the GWS2 system exhibited higher adsorption energy and an appropriate bond length.Furthermore,introducing C provides more electrons for Pb,and the coupling effect between C and Pb was extremely obvious from the electron state density diagram.However,GWS2 exhibited poor performance for other heavy metal ions.After comparison,we found that the excellent sensing performance of GWS2 on Pb was caused by the synergistic effect of C and S on Pb.However,in other ion adsorption systems,there was no obvious coupling between the S element and heavy metal ions.Therefore,we believe that the enhanced electrochemical performance of the combined system of transition metal compounds and carbon-based materials was most likely due to the introduction of the C atom layer,leading to the generation of interface adsorption and the specific adsorption of the C atom to heavy metal ions and transition metal compounds.