Atomic-scale Surface Modification Of Two-dimensional Nanomaterials And Their Catalytic Application

The increasing consumption of fossil fuels and environmental problems worldwide have made the storage and conversion of clean energy a consensus and focus of research.Supported heterogeneous metal catalysts,which are the most widely used catalytic forms in the catalytic industry,are important media for efficient energy storage and conversion.The size of the supported metal plays a crucial role in the catalytic performance of the catalyst.The reduced size of metal particles can bring more unsaturated coordination environment,thus exposing more active sites and promoting the catalytic performance.When the size of the metal particles is reduced to a single atom which could bind with the supports and form well-ordered catalytic units,we call the systems single-atom catalysts.Single-atom catalysts,containing single metal atoms anchored on the supports,achieve 100%dispersion of metal atoms on the support,maximizing the usage efficiency of metal active sites.In addition,the uniform active sites and coordination structures endow single-atom catalysts with the advantages of both homogeneous and heterogeneous catalysts,showing high activity,high selectivity and high stability,thus bridging the gap of homogeneous and heterogeneous catalysis.Two-dimensional nanomaterials have larger specific surface area and rich physical and chemical properties,making them ideal support for single-atom metal sites.With atomic-level thickness,the surface and bulk phases of two-dimensional nanomaterials become equally important.More importantly,the physical and chemical properties of the two-dimensional material system can be modulated by surface modification.The construction of single-atom metal sites on the surface of two-dimensional nanomaterials which achieves surface atomic modification not only modulates the electronic structure of two-dimensional material supports,but also brings rich physical/chemical properties for the metal sites,ultimately modulating the catalytic properties of the catalysts.Based on this,the surface atomic modification for two-dimensional material are expected to inject new vitality into the further development of catalysis.In this dissertation,the single-atom metal sites were constructed by selecting the appropriate two dimensional supports system.Through ultrafast transient absorption spectroscopy,synchrotron X-ray absorption spectroscopy and other characterization methods and also the first-principle calculations,the influence of the local structure of single-atom active sites on the catalytic performance were explored.And the clear relationship between structure and activity was established,providing theoretical and experimental support for the design and development of high-efficiency single-atom catalytic systems.The research content of this dissertation includes the following aspects:1.We have developed isolated single Pt atoms as a new form of co-catalyst,by embedding them in sub-nanoporosity in 2D g-C3N4.The single-atom platinum acts as a co-catalyst to maximize the utilization of atom efficiency while greatly improving the photocatalytic hydrogen production performance of g-C3N4,achieving a photocatalytic H2 generation rate of 318μmol·h-1,50 times of that for blank g-C3N4.High angle annular dark field scanning transmission electron microscopy demonstrates the single-atom form of Pt on g-C3N4.The extended X-ray absorption fine structure spectroscopy clearly confirms that Pt atoms have been dispersed on the five-membered N/C ring of the C3N4 network.Ultrafast transient absorption spectroscopy reveals that the single-atom Pt induces the intrinsic change of the surface trap states of g-C3N4,leading to 1.8-time longer lifetime of photogenerated electrons and thus offering more opportunities for photogenerated electrons to participate in the H+ reduction,eventually leading to tremendously enhanced photocatalytic H2 generation performance.Besides,due to the unique N/C coordination environment that traps the high-activity single atoms,the single-atom cocatalyst shows excellent stability.We believe that the single-atom co-catalyst strategy will provide a promising way to improve the atom efficiency and reduce the high cost of noble metals,and pave a new avenue for the development of highly efficient co-catalysts performance.2.We demonstrate a topo-chemical transformation strategy,which could construct exclusive Ni-N4 sites on the surface of carbon materials,which shows excellent activity for CO2 reduction to CO.g-C3N4 as a precursor could trap single-atom Ni ions and construct Ni-Nx structure.Then the outer carbon layer coated on the precursor ensures the preservation of the Ni-N4 structure to a maximized extent and avoids agglomeration of Ni atoms to particles during the high-tempeture anneal process,providing abundant active sites for catalytic reaction.The Ni-N4 structure exhibits excellent activity for electrochemical reduction of CO2 with particularly high selectivity,achieving high faradaic efficiency over 90%for CO in the potential range from-0.5 to-0.9 V and gives a maximum faradaic efficiency of 99%at-0.81 V with a current density of 28.6 mA.cm-2.Besides,the Ni-N4 active sites shows excellent stability in the catalytic process.Topo-chemical transformation strategy provides a feasible way to construct abundant and exclusive active sites,avoiding aggregation of metal atoms and the resulting loss of active sites.We anticipate this work will lead to promising path for design of high-efficiency catalysts and should inject new vitality not only to CO2 reduction research but also to the extensive electrocatalytic field.3.We have developed the surface ion adsorption strategy for two-dimensional nanosheets to optimize the photocatalytic CO2 reduction activity and selectivity.Isolated bismuth(Bi)ions confined on the surface of TiO2 nanosheets through a simple ionic adsorption method could promote the photogenerated charge carrier separation,offering more opportunities for photogenerated electrons to participate in the CO2 reduction reaction.Density functional theory calculation and surface photovoltage measurement demonstrate Bi ions adsorbed on surface as an electron donor induce charge redistribution on the surface of TiO2,forming a built-in electric field which benefits the separation of charge carriers.The accelerated charge transfer facilitates the further reduction of CO,eventually leading to preferred CO2 reduction to CH4.We believe surface ion adsorption strategy will provide an effective way for the design of highly efficient photocatalytic systems,thus shedding new light on development of CO2 reduction.