Fabrication Of Novel Titanium Metal-Organic Frameworks For Photocatalytic Hydrogen Production

In the face of severe environmental and energy problems,clean and renewable energy is very needed globally,but its development is still a major challenge.The use of solar photocatalytic H₂evolution from water is one of the most promising solutions.As a new type of porous material,metal-organic frameworks?MOFs?are 1D,2D or 3D crystalline porous materials self-assembled by metal ions or metal clusters and organic ligands through coordination bonds.The advantages of controlled pore size,high specific surface area and modification show broad application prospects in the fields of optical devices,biological materials,gas storage and separation,catalytic hydrogen production and catalytic reduction of carbon dioxide,and are very valuable and potential materials in the field of energy.Since titanium dioxide was discovered in the 1970s to absorb hydrogen,to decompose water,to produce hydrogen,it has been extensively studied in the field of photocatalysis because of its stability,simple synthesis,and low toxicity.However,due to its large forbidden bandwidth,it cannot use the sun efficiently.Light,while titanium metal ions have unique Ti⁴⁺/Ti³⁺reversible redox properties under illumination and have excellent thermal and chemical stability and reusability in water and organic systems.They are natural photocatalysts and therefore it is extremely advantageous to introduce titanium ions into MOFs to construct new Ti-MOFs.However,the development of Ti-MOFs is slower than that of other metal based on MOFs because of the high decomposability of ionic titanium precursors.So in order to overcome this problem,we have done a lot of research on the synthesis of new Ti-MOFs,and have studied the photocatalytic properties of the synthesized materials.The details are as follows:?1?Using the solvothermal synthesis method,we have improved the structure design and synthesis process of the new Ti-MOFs.First,we have selected TCA as the organic ligand of the new Ti-MOFs,and have selected isopropyl titanate as the titanium source,in DMF was reacted at180°C for 24 hours to obtain a new type of Ti-MOFs?named ZSTU-1?.Two different isomorphic Ti-MOFs were synthesized by the same method of selecting BTB and BTCA,named ZSTU-2 and ZSTU-3.Through the homogeneous synthesis method,the particle size of the obtained MOFs is relatively uniform,and the structure of the material has been characterized by SEM,TEM,XRD,BET,UV,XPS,IR,etc,which proves that the material has good light absorption properties and A higher specific surface area can provide more catalytically active sites.Through photoelectric performance test,it has proved that ZSTU-N series materials have photocurrent responsiveness and exhibit good photocatalytic effect in triethanolamine/acetonitrile/water system.The materials have been analyzed by EPR and ultrafast transient absorption spectroscopy.The mechanism of photocatalytic hydrogen production is in accordance with the LMCT mechanism.The photo-excited organic ligand generates electron transfer to the titanium oxide cluster to undergo redox reaction,and accelerates the electron conduction through the Pt atom,thereby suppressing the recombination of photogenerated electrons and holes.Through the cycle stability test,the ZSTU-N material still maintains a good stability after three cycles.This work focuses on the design and synthesis of novel Ti-MOFs,which promotes the synthesis of Ti-MOFs,and validates the electron transfer paths and catalytic mechanisms of MOFs for photocatalytic hydrogen production,which designs and develops future Ti-MOFs.?2?Based on the improvement of photocatalytic properties of porphyrin-sensitized titanium dioxide,we have envisaged the introduction of the traditional photosensitive material porphyrin TCPP into the synthesis of novel Ti-MOFs.It has the same idea as the synthesis of ZSTU-N series Ti-MOFs.Through continuous experiments,it hss proved that for the synthesis of TCPP-Ti,low-concentration can synthesize better quality Ti-MOFs crystals.In the methylformamide,TCPP-Ti has been synthesized by one-step synthesis method by regulating the ratio of TCPP to TPOT.Compared with PCN-22,the crystal particles of MOFs obtained by homogeneous synthesis of TCPP-Ti are more uniform,and by UV,BET,etc,TCPP-Ti has successfully broadened the absorption of visible light and has a large specific surface area.Providing more catalytically active sites and having excellent photocatalytic properties in the triethanolamine/acetonitrile/water system proves that our experimental design ideas are correct.This work provides ideas for the development of porphyrin sensitizing materials and porphyrin titanium MOFs,which not only utilizes the photo-oxidation-reduction activity of titanium ions,but also combines the advantages of porphyrins into Ti-MOFs,which proves that the materials have great advantages in the photocatalysis field.?3?Graphene and metal-organic frameworks are the two most popular materials in the field of materials in the past two decades,and graphene,as a zero-bandgap semiconductor material,which has shown great potential advantages in the research of photocatalytic composites.Therefore,we have synthesized ZSTU-1 and graphene oxide in situ by hydrothermal synthesis.Through the characterization of the material structure,graphene oxide can be obtained by the titanium oxide cluster and N,N-dimethylformamide during the synthesis.The co-reduction has become a reduced graphene oxide,so we have successfully prepared the ZSTU-1@rGO composite,which still maintains a high specific area of the raw material and greatly improves the intervening conductivity of the reduced graphene oxide.According to the photocurrent and impedance spectrum,the photocurrent is 3.5 times higher than that of ZSTU-1,and the impedance value is about 80%smaller.It also has proved by photocatalysis that ZSTU-1@rGO has superiority to ZSTU-1.The photocatalytic decomposition of hydrogen production by water has a 1.5-fold increase in the efficiency of photocatalytic splitting of water,and the stability of the composite has been demonstrated by photocatalytic cycling experiments.Through our experiments,it has proved that graphene plays a role in improving conductivity and can transfer electrons rapidly in composite photocatalytic materials,and because of the intervention of graphene,it can have more catalytic active sites,which is also a photocatalytic material,that can provide new ideas for synthesis of new photocatalytic material.