Heterostructure Based on Titanium-Based Semiconductor for Hydrogen Generation

Author:Yu Yan

Supervisor:song shuang he zhi qiao


Degree Year:2019





The global demand for energy and the consequences of prolonged dependence on fossil fuels are major issues facing humanity in the 21st century.Semiconductor photocatalytic technology that uses sunlight and water to generate hydrogen provides a safe,clean and sustainable technology.The ideal photocatalysts should possess the characteristics of efficient light absorption,rapid charge separation and transport,and superior stability in the harsh environment.The titanium-based semiconductors has been deeply researched due to its low cost,high activity and stability.However,the fast recombination of photogenerated electrons and holes in the photocatalytic reaction of titanium-based semiconductor resulted in low quantum efficiency.In this dissertation,the semiconductor heterojunction based on TiO2 and titanium-based metal organic framework materials(Ti-MOF)is constructed to promote the directional separation of photo-generated charges and to inhibit the recombination of photo-generated carriers during migration,thereby improving the photocatalytic hydrogen production performance.Graphene(Gr)modified highly exposed{001}crystal facet TiO2 nanosheet heterostructure photocatalyst(Gr/TiO2)was synthesized by two-step hydrothermal method using tetrabutyl titanate and graphene oxide(rGO)as precursors.Three-dimensional TiO2 microspheres(3DTiO2MSs/Ti)grown in situ on a titanium substrate were prepared by hydrothermal method,and poly-2,6-diaminopyridine(PDAP)modified3DTiO2MSs/Ti were successfully prepared by electrochemical polymerization to form a new heterostructure photoelectrode(PDAP-3DTiO2MSs/Ti).In addition,using titanium tetraisopropoxide,2-aminoterephthalic acid and 2,5-dihydroxyterephthalic acid as precursors,two-step hydrothermal synthesis of p-type Ti-MOF(NTU-9)and n-type Ti-MOF(NH2-MIL-125)was carried out to form a heterogeneous heterostructure photocatalyst(NH2-MIL-125@NTU-9).Analyze the morphology,composition,and properties of materials using physicochemical characterization and photoelectric performance testing:X-ray diffraction(XRD),field-emission scanning electron microscopy(FESEM),and transmission electron microscopy(TEM),X-Ray photoelectron spectroscopy(XPS),Electron paramagnetic resonance(EPR)spectroscopy,UV-vis diffuse reflectance spectroscopy(UV/Vis DRS),Raman spectroscopy(Raman),Photoluminescence spectroscopy(PL),Thermogravimetric analysis(TGA),Cyclic voltammetry(CV),Mott–Schottky measurements(M-S),Electrochemical impedance spectroscopy(EIS).In the PEC water oxidation reaction,the 1-Gr/TiO2 exhibits a maximum photocurrent density of 60μA cm-2 at 1.23 V vs.RHE.The enhanced PEC performance of Gr/TiO2 can be attributed to the heterostructure formed between TiO2 nanosheets and graphene,{001}active crystal facet,Ti3+active site and C doping level in TiO2nanosheets.The PDAP-3DTiO2MSs/Ti could achieve higher applied bias photon-to-current efficiency(ABPE)at lower bias.The maximum ABPE efficiency for PDAP-3DTiO2MSs/Ti was 0.87%at 0.41 V vs.RHE and a photocurrent density of 1.56 mA cm-2at 1.23 V vs.RHE.The PDAP-3DTiO2MSs/Ti could maintain impressive photoelectrochemical stability,with 92.86%of the initial photocurrent retained after 4 h reaction.The superior PEC water splitting performance of PDAP-3DTiO2MSs/Ti should benefit from the coexistence of Ti3+and Ti4+in 3DTiO2MSs,the light harvest capability of PDAP and the type II heterojunction between 3DTiO2MSs and PDAP,resulting in enhanced generation and separation of photocarriers.Thereby,in-situ grown titanium oxide-based heterojunction photoelectrode realized the maximum photon utilization efficiency in water splitting.The homologous heterojunction photocatalyst rapidly separates and transfers photogenerated charges between the organic linker and the titanium-oxygen cluster through a p-n heterostructure formed between NH2-MIL-125 and NTU-9,thereby making the Ti3+active site preserved and thus exhibits high photocatalytic activity.The hydrogen production of the NH2-MIL-125@NTU-9 was 847.20μmol g-1.And the NH2-MIL-125@NTU-9 exhibits higher current density,charge storage capacity and lower charge transfer resistance than the NH2-MIL-125 and NTU-9 photocatalysts in the photoelectric performance test.In this dissertation,the titanium-based semiconductor-based heterojunction powder photocatalyst and photoelectrode designed have enhanced light absorption,improved charge separation and transfer,and increased reaction specific surface area,and further photocatalytic decomposition of water to produce hydrogen.Should provide effective support.