Preparation Of Two-dimensional MXene Quantum Dots And Applications In Photocatalytic/Electrocatalytic Water Splitting For Hydrogen Production

With the rapid development of economy and the improvement of human living standards,the global demand for energy is constantly increasing.Facing the limited fossil fuel storage and the increased serious environmental pollution problem,developing clean,high energy density and renewable hydrogen energy has important practical significance.Up to now,photocatalytic and electrocatalytic water splitting for hydrogen production are considered the two most promising approaches due to the advantages of non-pollution and renewable.However,photocatalytic and electrocatalytic water splitting to produce hydrogen still exist numerous problems that restrict their practical application.In photocatalytic water splitting,single-component photocatalysts suffer from low photogenerated carrier separation efficiency,poor light absorption ability and limited active sites,leading to a low solar energy conversion efficiency.In addition,the cathodic hydrogen evolution reaction in electrocatalytic water splitting involves multielectron transfer process,which produces the overpotential and causes the energy loss.Therefore,the rational design and preparation of efficient and low-cost catalyst is the urgent problem in the field of photocatalysis and electrocatalysis.In recent years,two-dimensional MXene-derived quantum dots have attracted widespread attention due to the inheritance of the electrical conductivity,easy functionalization,hydrophilicity and biocompatibility of the parent phase MXene.In addition,the MXene quantum dots also acquire new physicochemical properties,such as quantum confinement effect,higher specific surface area,tunable photoluminescence properties,and richer surface functional groups,showing promising applications in the field of photochemistry and electrochemistry.This paper aims to design and prepare a variety of high performance and stable MXene QDs-based composite photocatalysts and electrocatalysts,which will then be applied in the research of hydrogen production from photocatalytic and electrocatalytic water splitting.Details of the research are as follows:1.The nitrogen-doped Ti₃C₂ QDs（N-Ti₃C₂ QDs）have successfully prepared via the combination of reflow and hydrothermal methods using two-dimensional Ti₃C₂MXene as the precursor.Then N-Ti₃C₂ QDs/Cd S composite photocatalysts are successfully synthesized by loading N-Ti₃C₂ QDs on the surface of one-dimensional Cd S nanorods through an electrostatic self-assembly strategy.The experimental results demonstrate that the introduction of N-Ti₃C₂ QDs significantly improves the photocatalytic activity of the composites.In particular,N-Ti₃C₂ QDs/Cd S-3%exhibit excellent photocatalytic hydrogen production efficiency rate of 17.15 mmol g^-1 h^-1,which is 14.79 times higher than pure Cd S.The significant enhancement in photocatalytic performance is mainly due to the N-Ti₃C₂ QDs loaded with Cd S not only improving the visible light absorption capacity,but also increasing the specific surface area of the photocatalyst and enhancing the density of active sites.In addition,the tight interfacial contact between the highly conductive N-Ti₃C₂ QDs and Cd S nanorod greatly accelerates the separation and transfer of photogenerated carriers.N-Ti₃C₂ QDs serve as active sites for hydrogen production and effectively promote the reduction reaction,thus enhancing photocatalytic hydrogen production activity.2.Bimetallic Mo₂Ti₂C₃ QDs are successfully prepared based on the combination of reflow and hydrothermal methods,the Mo₂Ti₂C₃ QDs/g-C₃N₄ heterojunction have successfully constructed by homogeneous loading Mo₂Ti₂C₃ QDs on the surface of g-C₃N₄ nanosheet through an electrostatic self-assembly strategy.The Mo₂Ti₂C₃ QDs/g-C₃N₄ heterojunction presents an efficient and stable photocatalytic H₂ evolution activity up to 2816μmol g^-1 h^-1,which is 8.12 times higher than g-C₃N₄ nanosheet,prominently surpassing many reported photocatalysts.Kelvin probe force microscopy（KPFM）technology and density functional theory（DFT）calculation results comprehensively prove that the close contact interface between Mo₂Ti₂C₃ QDs and g-C₃N₄ induces the construction of a build-in electric field,leading to the rapid transfer of photogenerated electrons from g-C₃N₄ to Mo₂Ti₂C₃ QDs,which greatly promote the separation and transfer of photogenerated carriers,thus enhancing the charge utilization and obtaining efficient photocatalytic performance.3.The thermodynamically stabilized Ti₃C₂ QDs/1T-Mo S₂ heterostructure are prepared by uniformly dispersing Ti₃C₂ QDs on the surface of 1T-Mo S₂ microspheres through an interface engineering strategy.The strongly chemical bonds have formed between the highly conductive Ti₃C₂ QDs and 1T-Mo S₂,whereby the Ti₃C₂ QDs can transfer electrons to 1T-Mo S₂,resulting in the electron in the Mo 4d orbitals changing from distorted octahedral distribution to symmetric occupation,thus effectively stabilizing the 1T phase Mo S₂ and improving the conductivity and structural stability of the composite.The electrochemical experiments demonstrate that the Ti₃C₂ QDs/1T-Mo S₂ electrocatalysts exhibit ultra low overpotentials of 46,59 and 88 m V in acidic,basic and neutral electrolytes at a current density of 10 m A cm^-2,and the Tafel slope are 38.4,42.9 and 68.9 m V dec^-1,respectively.The excellent stability of Ti₃C₂ QDs/1T-Mo S₂ has been confirmed by structural characterization and continuous performance experiments.4.Heteroatomic N and S co-doped Ti₃C₂ MXene quantum dots have been successfully synthesized as carriers for anchoring Ru nanoparticles,and then N,S-Ti₃C₂QDs/Ru electrocatalyst are prepared by the simple high-temperature calcination methods.The electrochemical test shows that N,S-Ti₃C₂ QDs/Ru exhibit excellent hydrogen evolution activity in 0.5 M H₂SO₄,1 M KOH and 1 M PBS solutions.The overpotentials at 10 m A cm^-2 current densities are 28,37 and 56 m V,respectively,which are superior to the current commercial Pt/C and Ru-based catalysts.In addition,the N,S-Ti₃C₂QDs/Ru exhibit extremely low Tafel slope and long-term stability over wide p H range.The high specific surface area of N,S-Ti₃C₂ QDs can confine the metal between N,S-Ti₃C₂ QDs,prevent the agglomeration and growth of Ru nanoparticles,thus ensuring the high dispersion of metal Ru.DFT calculations indicate that the interaction between Ru and N,S-Ti₃C₂ QDs can modulate the electronic structure of the active center Ru and optimize the Gibbs free energy（ΔGH*）,thus effectively improving the electrocatalytic activity of N,S-Ti₃C₂ QDs/Ru.