Synthesis, Performance And Mechanism Of 2-Dimensional Transition Metal Carbide (MXene) Drived Photocatalysts

Environmental and energy-related concerns are the problems to be solved in the development of human society.Solar photocatalysis is a promising candidate to solve those problems.Metal oxide?TiO₂,Nb₂O₅ et.al?have excellent photocatalytic activity,outstanding chemical stability,and environmental benignity,has becoming a research hotspots in the field of photocatalysis.However,metal oxide photocatalysts remains two major drawbacks till now,on the one hand,lots of metal oxides absorb light only in the UV region but not in visible light,on the other hand,the rapid recombination of photogenerated electrons and holes in bulk phase is serious,so that the proportion of the effective carriers that can reach the surface of the photocatalyst and participate in the reaction is low,leading to low-efficiency of photocatalysis.It is urgent to find suitable co-catalysts to modify the metal oxide semiconductors to improve the separation efficiency of the photogenerated carriers,or to introduce impurity energy level to extend their light absorption range to the visible light.Two-dimensional transition metal carbides?MXene?is a new family of graphene-like twodimensional materials,which have unique layered structures and high carrier mobility.They have been widely used in electrochemical energy storage,environmental pollutant adsorption and so on.However,there are few studies on their photocatalytic applications.In this work,a series of MXene-based composite photocatalysts were designed and synthesized,to reduce the photogenerated recombination efficiency and expand its light absorption to visible region.The main contents of the thesis are as follows:?1?Hybrids of Two-Dimensional Ti₃C₂ and TiO₂ Exposing?001?Facets toward Enhanced Photocatalytic ActivityEffectively harvesting light to generate long-lived charge carriers to suppress the recombination of electrons and holes is crucial for photocatalytic reactions.Exposing the highly active facets has been regarded as a powerful approach to high-performance photocatalysts.Herein,a hybrid comprised of?001?facets of TiO₂ nanosheets and layered Ti₃C₂,an emerging 2D material,was synthesized by a facile hydrothermal partial oxidation of Ti₃C₂.The in-situ growth of TiO₂ nanosheets on Ti₃C₂ allows for the interface with minimized defects,which was demonstrated by high resolution transmission electron microscopy and density functional theory calculations.The highly active?001?facets of TiO₂ afford high-efficiency photogeneration of electron-hole pairs,meanwhile the carrier separation is substantially promoted by hole trapping effect by the interfacial Schottky junction with 2D Ti₃C₂ acting as a reservoir of holes.The improved charge separation and exposed active facets dramatically boost the photocatalytic degradation of methyl orange dye,showing the promise of 2D transition metal carbide for fabricating functional catalytic materials.The optimum activity?001?TiO₂/Ti₃C₂ was prepared by regulate the ratio of TiO₂ and Ti₃C₂,the maximum degradation rate value to Methyl orange?MO?is 18.8 min^-1g^-1,exceeds that of Degussa P25 TiO₂(15.7 min^-1g^-1).?2?Ti³⁺-doped rutile?111?TiO₂-x/Ti₃C₂ hybrids and its visible light photocatalytic activity.A nanocomposite comprised of Ti³⁺ doped rutile TiO₂ octahedrons exposing active?111?facets and two-dimensional Ti₃C₂ sheets was synthesized by a hydrothermal oxidation route and subsequent hydrazine hydrate reduction.The crystal facet proportions of TiO₂ can be controlled by NH₄ F as a crystal facet controlling agent.The photocatalytic activity of the TiO₂/Ti₃C₂ composite was maximized by completely exposing the active?111?surfaces.The interfacial microstructure between Ti₃C₂ and TiO₂ can be tuned by the hydrothermal reaction duration.The charge kinetics was substantially steered by the hole trapping effect of 2D Ti₃C₂ terminated by –OH groups.Through the reduction by hydrazine hydrate,Ti³⁺ was doped into the bulk of TiO₂,thereby enabling the photocatalytic activity under visible light.?3?Nano-Cu-loaded?001?TiO₂/Ti₃C₂ composites and its photocatalytic hydrogen evolution activity.3-5 nm Cu₂O nanoparticles were selectively photodeposited on the?001?TiO₂ nanosheets embedded in?001?TiO₂/Ti₃C₂.The water splitting hydrogen evolution results showed that the composite material was first subjected to a relatively low hydrogen production period,and then high efficiently produced hydrogen with a hydrogen production rate of 1892 ?mol/h/g?CuO_x·TiO₂?.The XPS,H₂-TPR,photoluminescence and surface photovoltage pectroscopy were employed to reveal the evolution of Cu species during the initial induction period with low hydrogen production rates.It was demonstrated that the photogenerated holes were captured by Ti₃C₂-OH,and the photo-electrons were enriched on the conduction band of TiO₂ and hopped to the conduction band of Cu₂O through tunneling effect.As a result,the Cu₂O was reduced to elemental Cu.Thereafter,the metallic copper nanoparticles acted as electron trapping centers and hydrogen production sites,while Ti₃C₂-OH separated and trapped the holes.In this case,the electron-hole pairs move in the opposite direction,therefore the separation efficiency is enhanced,allowing a high activity of hydrogen evolution.?4?Preparation and water splitting activity of Ag nanoparticle supported?001?Nb₂O₅/Nb₂C photocatalysts.The layered Nb₂C MXene was prepared by etching Nb₂ AlC as the precursor.Nb₂O₅ nanorods exposing?001?facets sandwiched by layered Nb₂C sheets were fabricated by hydrothermal oxidation of Nb₂C.And then Ag nanoparticles were phogoreduced on the?001?Nb₂O₅/Nb₂C composite.The relationship between the structure and water splitting hydrogen evolution property of the ternary photocatalyst was further discussed.Similar to nanoCu-loaded?001?TiO₂/Ti₃C₂,Ag nanoparticles capture photogenerated electrons,while Nb₂C-OH captures photogenerated holes.The efficient separation of electrons and holes improves the photocatalytic activity,displaying a hydrogen evolution rate of 682.21 ?mol/g·h.