Design,Synthesis And Properties Of Photocatalysts Based On Semiconductor Heterostructures

Energy and environmental crisis have attracted global attention,and the development of renewable and clean energy sources becomes highly important.Photocatalytic water splitting provides an approach to directly convert solar energy into hydrogen energy,which is considered as a clean way to harness energy sources.In particular,the use of semiconductors in photocatalytic water splitting is currently a hot research topic.The process of photocatalytic water splitting includes three steps:?i?the semiconductor absorbs sunlight to produce electron-hole pairs;?ii?the photo-generated charges are separated and migrate to the semiconductor surface;and?iii?the charges ar transferred to water,producing hydrogen and oxygen.However,bare semiconductors typically exhibit limited capabilities in absorbing sunlight,separating electron-hole pairs and transfering charges.To address the challenges,this dissertation presents four design schemes:1.In order to broaden the absorption spectrum of g-C₃N₄ semiconductor,take advantage of the microscopic frame structure of g-C₃N₄ and consider this organic semiconductor as a polymeric ligand with N coordination sites.The photocatalytic efficiency of g-C₃N₄ is restricted in terms of light absorption and charge behavior.First,the light absorption of g-C₃N₄??<460 nm?cannot utilize most of the visible-near-infrared?vis-NIR?photons.Second,the fast recombination of electron-hole pairs and low carrier mobility largely restrict the utilization of photo-induced carriers for reactions.In efforts to facilitate the future applications of g-C₃N₄,it is thus imperative to tune light absorption and enhance charge carrier utilization by employing a different mechanism.With the experiential results,we demonstrate that the metal-to-ligand charge transfer?MLCT?between Pt²⁺ and g-C₃N₄ can play such an important role in photocatalysis.The MLCT is an excited state generally occurring in complexes,and it is known in inorganic chemistry that its photoexcitation energy is lower than that for HOMO-to-LUMO transition.The experimental results and theoretical simulation show that the HOMO-to-LUMO transition and MLCT transition can be relatively independent in the N-conjugated aromatic units,and as such,the electron-hole recombination in the long-distance transport is avoided.As a result,the photocatalytic performance can be improved.2.In order to overcome the limitation that TiO₂ and Ag₂S possess a straddling alignment of band structures,we have proposed a design that interfacial Ag,which can upshift the energy band of Ag₂S,enables constituting Z scheme between TiO₂ and Ag₂S.TiO₂ is known as an n-type semiconductor with 3.2 eV bandgap that can offer high photocatalytic activities in UV region,while the n-type semiconductor Ag₂S can absorb both visible and NIR light according to its 1.0 eV bandgap.However,narrow-bandgap semiconductors Ag₂S has straddling band structure alignments with TiO₂,constituting the obstacle to form the Z scheme for photocatalytic hydrogen production.In our work,we demonstrate by employing Ag₂S as a model system that the energy band upshift of the narrow-bandgap semiconductor by interfacing with a metal can overcome this limitation.The Ag₂S-?Ag?-TiO₂ ternary structure exhibits a significant increase in the photocatalytic hydrogen generation under full spectrum irradiation.The precise control over Ag interface layer is the key to our design.To fulfill the design,we have developed a unique approach to synthesize Ag₂S-?Ag?-TiO₂ hybrid structures.In our synthesis,the TiO₂ shells are grown on the surface of Ag nanocrystals,protecting the Ag core to complete vulcanization.As Ag and TiO₂ have a strong chemical bonding,the interfacial Ag can be free from vulcanization,forming a unique Ag₂S-?Ag?-TiO₂ hybrid structures.3.In order to solve the problem that plasmonic hot electrons cannot be efficiently injected into semiconductor and participate in reactions,we propose a strategy by using TiO₂ spatial charge distribution and Pt co-catalyst to improve the utilization of hot electrons.The contact of metal and semiconductor forms a Schottky junction,whose electron flow direction is opposite to that of plasmonic hot electron injection.To overcome this limitation,we utilize the facet-dependent charge distribution in TiO₂ as a driving force for injecting hot electrons into semiconductor.The TiO₂ nanosheets are enclosed by {001} and {101} facets.Our results indicate that Au-TiO₂{001}sample possesses a higher ability in charge separation,induced by the facet-dependent charge distribution.To further improve the performance,we integrate the co-catalyst Pt with the designed Au-TiO₂{001} sample,achieving a highly efficient catalyst for photocatalytic hydrogen production.4.In order to improve the efficiency of TiO₂/graphene heterojuncion,we propose to synthesize thin two-dimensional TiO₂ srtuctures which can reduce the electron loss during charge transfer and improve the efficiency of photocatalytic hydrogen production.In the system of TiO₂/graphene heterojuncion,the photoexcited eletrons in semicondutor TiO₂ need to pass through the bulk phase to reach the surface of graphene.The commonly used TiO₂ nanowires or naospheres usually have a small contact area with graphene,disfavoring interfacial charge separation.Moreover,the photoexcited electrons have to pass through a long distance to reach the interface.Our thin TiO₂ nanosheets not only can reduce the probability of electron loss during charge transport,but also enlarge the interface between TiO₂ nanosheets and grephene.The design significantly improves the activity of photocatalytic hydrogen production.We have also systematically investigated the dependence of photocatalytic performance on the location of Pt co-catalyst integrated with the TiO₂/graphene system.