TiO₂@rGO-based Core-shell Structure With Vacancies And Its Photostability

The economic development of human beings is along with the increasing serious energy crisis and environmental pollution problems.Thus,exploiting and utilizing new and clean energy is becoming more and more urgent.Hydrogen is considered the alternative ideal green energy dealing with environmental pollution and energy crisis.Photocatalytic semiconductors based on water splitting are considered the key transform solar energy to hydrogen.TiO₂ has long been applied in photocatalysis for its nontoxicity,low cost,superior photocatalytic activity and long-term chemical stability.However,TiO₂ only absorbs light in the ultraviolet region due to its large band gap,visible light occupied about 50%of solar energy and cannot be utilized.Moreover,the recombination of the photo-generated electron-hole pairs dissipates the input energy in the form of heat or emitted light,which is one of the main reasons for the low efficiency of photocatalysis.Binary heterojunctions aid separating and transporting surface/interface carriers before they recombine.Recent attempts to utilize the photogenerated charges are using ternary heterojunctions such as binary composites onto larger-scale reduced graphene oxide?rGO?,they can promote the collection effect of photogenerated charges for their highly efficient utilization.To solve the above problems,started with the essential factors affecting photocatalytic efficiency,the thesis focused on the design a series of TiO₂@rGO based hierarchical composites materials to reduce the rapid combination of photogenerated electrons and holes.?1?Construction of TiO2@rGO heterojunctions and interface vacanciesUltrathin reduced graphene oxide?rGO?sheets coated mesoporous TiO₂ beads?denoted as TiO₂@rGO?with surface O-vacancy enhanced charge/ion transport channels are synthesized via a two-step calcination process.Transmission electron microscope?TEM?images show that rGO is successfully coated on the surface of TiO₂microspheres and forms nanoscale crystal fusion.The Ti-C chemical bond between the TiO₂ and the rGO is detected by X-ray photoelectron spectroscopy?XPS?,indicating the good coating structure.Electron paramagnetic resonance?EPR?spectroscopy revealed that the material calcined at a high temperature in argon forms oxygen vacancies,and then the titanium vacancies are formed by the next calcination in air at350 ^oC.The formation of O-rich interface confined by rGO at relatively low temperature is a possible explanation to our junction of oxygen and titanium vacancies.The photocatalytic performance test showed that the hydrogen production rate reached30.5 mmol h^-1g^-1 in the presence of platinum,which is 4-fold of TiO₂.?2?Construction of TiO₂@CQDs@rGO heterojunctions and bulk vacanciesHierarchical TiO₂@CQDs@rGO ternary photocatalyst was synthesized by an in situ self-assembly approach.Metal vacancies at the surface/interface are acceptor type centers for separation of inner charges,Core-shell nanostructures can spatially be integrated into the composites to form one system and result in high-performance nanocatalysts and bioinspired catalysts.In this section,CQDs act as bulky molecules doping the TiO₂ nanoparticles and forming interfacial Ti-vacancies.The ternary heterojunction shows a cascade effect of charge separation from the inner part to the Ti-vacancies and the CQDs,transport from the core of CQDs and TiO₂ to the rGO shell and utilization of the collected charge to the reactants;This approach thus provides an ideal mode of producing spatially inside-out heterojunctions,resulting in a cascade effect for photogenerated charges separation/transport/collection/utilization.Such a mechanism has been investigated and preliminarily expanded to a quaternary composite,which exhibits significant performances as to photocatalytic H₂ production?8-fold of TiO₂?and a remarkable apparent quantum yield as we expected.?3?Construction of visible light TiO₂@CdS@rGO heterojunctions and high photostabilityHierarchical TiO₂@CdS@rGOternary photocatalyst was synthesized by an electric-triple-layer hierarchical assembly approach,which exhibits high photoactivity and excellent photostability?more than twice the activity of pure CdS while 82%of initial photoactivity remained after 16 recycles during 80 hrs irradiation?.The ternary nanojunction effect of the photocatalyst has been investigated from orbitals hybrid,bonding energy to atom-stress distortion and nano-interface fusion.And a coherent separation mechanism of charge carriers in the ternary system has been proposed at an atomic/nano scale.