Transition Metal Nitrides As Non-noble Plasmonic Photocatalysts For Water Splitting Studies

Local surface plasmon resonance（LSPR）are the collective electronic oscillations generated by the coupling of free electrons and electromagnetic fields in metal nanostructures,which have important applications value in many fields such as physics,chemistry,biomedical science,information and energy due to its unique optical property.Especially,LSPR can generate an extremely electromagnetic field enhancement and efficient light trapping,and can yield high-energy hot electrons through LSPR relaxation,therefore,it has great significance for both fundamental studies and applications,on the research of light-to-chemical energy conversion,such as photocatalytic water splitting,carbon dioxide reduction,and nitrogen fixation,etc.The traditional plasmon materials are mainly gold（Au）and silver（Ag）based noble metals.However,recent research reveals that the transition metal nitrides represented by titanium nitride（Ti N）and zirconium nitride（Zr N）have the analogous plasmon characteristics compare with Au and Ag in the visible light and near infrared regions.And it has the advantages of stable chemical properties,wide range of work functions,rich resources and low cost,therefore it can be expected to have better application prospects in the field of photoelectric catalysis.This dissertation has a depth study about Ti N and Zr N non-noble plasmon materials systems,including the controlled preparation of high crystallization quality materials,design and construction of its compound system and the exploration of photocatalytic water splitting properties,with the following results being achieved:Firstly,the shape-controlled Ti N and Zr N nanoparticles with highly crystalline quality were successfully synthesized through high temperature solid-phase reaction.Zr N nanoparticles exhibit excellent overall water splitting property.Both the as-synthesized Ti N and Zr N with significant LSPR properties exhibit strong broadband light absorption in the UV-Vis-NIR region,and Zr N has higher plasmon resonance energy than Ti N.By comparing the photocatalytic water splitting properties,Ti N nanoparticles only have superior photocatalytic efficiency for hydrogen evolution half-reaction with methanol as hole sacrificial reagent,while Zr N nanoparticles not only have excellent hydrogen evolution activity,but also displayed significant photocatalytic activity for oxygen evolution half-reaction with sodium persulfate as electron sacrificial agent and Co O_x as cocatalyst.More importantly,Zr N nanoparticles achieved the overall water splitting under the loading of Co O_x without any electron or hole sacrificial agents.The ratio of hydrogen evolution and oxygen evolution is close to 2:1.Furthermore,the dynamics of oxidation reaction could be regulated by changing cocatalyst.When the Co O_x cocatalyst was changed to Co（OH）₂,the kinetic reaction pathway of water oxidation will change from four-electron pathway of O₂ generation to the two-electron pathway of H₂O₂ generation.Secondly,full nitride metal-semiconductor heterostructure based on Ti N/Ga N was designed and constructed.The Ti N/Ga N composite system with one-dimensional mesoporous nanorod structure was prepared by multiple steps.Due to the LAPR characteristics of Ti N,the composite system exhibits strong broadband light absorption in the UV-Vis-IR region.The formation of ohmic contact between Ti N and Ga N effectively reduced the charge-transfer barrier between metal and semiconductors,thereby promoted the rapid separation of photogenerated carriers.The hydrogen evolution efficiency of Ti N/Ga N composite system is significant increased than that of pure Ti N or Ga N under the same reaction conditions,and improved with the increase of Ti N content.Furthermore,the hot carrier transfer process between Ti N and Ga N were analyzed and discussed by studying the variation of catalytic activities under light illumination with different wavelength.Thirdly,owing to the special linear dispersive energy band structure and ultra-high electron mobility at room temperature,graphene is often used as an electron acceptor in composite photocatalytic material systems.The Fermi-level adjustable N-doped graphene and Ti N nanocomposite material was obtained by loading Ti O₂ on graphene oxide in liquid phase and ammoniating at high temperature.The uniform anchoring load function of the two-dimensional structure of graphene oxide enabled the particle size of Ti N nanoparticles smaller and evenly distributed.Driven by the difference of Fermi energy level between Ti N and graphene,the hot electrons induced by Ti N LSPR would transfer to the graphene,which effectively prolong the life of the hot electrons,thereby significantly improving the efficiency of photocatalytic water splitting.The hydrogen evolution rate of compound system is up to 25 times than pure Ti N,and the photocatalytic activity of the composite system displayed the regulation with the adjustment of the Fermi energy position by the degree of N doping.Fourthly,TiN/Zr N bimetallic heterostructure was designed and constructed.In the heterogeneous structure,Ti N and Zr N formed phases respectively,instead of forming alloy structure.The efficient separation of photogenerated electron-holes was achieved by the built-in electric field generated by the contact of two metals.As-synthesed Ti N/Zr N heterostructure exhibits excellent photocatalytic activities in both hydrogen evolution and oxygen evolution half reactions without any cocatalysts.Furthermore,by changing the ratio between Ti N and Zr N,the balance between hydrogen evolution and oxygen evolution can be effectively regulated.More importantly,at the optimized ratio,Ti N/Zr N achieved photocatalytic overall water splitting and released hydrogen and oxygen simultaneously.