Fabrication Of Titanium Metal-organic Frameworks For Photocatalytic Hydrogen Production

In recent years,the global demand for energy has reached an unprecedented level.The consumption of non-renewable energy such as coal and oil accounts for a large proportion of global energy consumption,resulting in massive emissions of greenhouse gases into the atmosphere,causing serious environmental pollution and energy crisis.The development and utilization of renewable energy is very necessary for the development of human society,but this method is still facing huge challenges.As a potential energy carrier,hydrogen energy can be used in fuel cells to meet the increasing energy demand.Therefore,the use of solar photocatalytic decomposition of water to produce hydrogen is an urgent problem to be studied.Titanium dioxide（Ti O₂）has significant advantages such as non-toxic,good chemical inertance,long photoelectric charge life,long carrier migration distance,easy availability of raw materials and high reserves,so it is widely used as photocatalytic decomposition of water to produce hydrogen.However,titanium dioxide has a wide band gap（3.1-3.4 e V）and can only utilize a fraction of the solar spectrum（λ<387 nm）.Metal-organic framework materials（MOFs）are a new type of porous materials that bind metal ions or metal clusters with organic ligands through coordination bonds.Due to their highly ordered pore distribution,adjustable structure,high specific surface area and flexible framework,MOFs show broad application prospects in many fields,such as industrial synthesis,energy development,environmental governance and catalytic conversion.The Ti⁴⁺cation in Ti O₂not only shows strong metal-ligand bonding ability,but also has good REDOX capacity.By introducing titanium ions into the framework of MOFs to construct titanium-based metal organic framework materials（Ti-MOFs）,the light response range can be expanded to the visible region.Due to their high optical responsiveness and photoredox properties,Ti-MOFs have been widely used in photocatalytic carbon dioxide reduction,photocatalytic pollutant degradation,photocatalytic hydrogen production and other practical application fields.However,due to the high reactivity and harsh synthesis conditions of titanium precursors,the number of structures and development of titanium-based metal organic framework materials are very limited compared with other MOFs.Therefore,it is of great significance for the design,synthesis and modification of the new generation of Ti-MOFs.This paper focuses on the synthesis and post-synthesis modification of Ti-MOFs to improve their photocatalytic hydrogen production performance.The specific research content is as follows:（1）On the basis of ZSTU-2（Ti-MOFs）,we successfully synthesized monoamino-functionalized and triamino-functionalized Ti-MOFs named NH₂-ZSTU-2and 3NH₂-ZSTU-2 for photocatalytic hydrogen production under visible light by modifying organic ligands with amino functional groups.They consist of two triangular organic linkers,2,4,6-tris（4-carboxyphenyl）aniline（NH₂-BTB）and 2,4,6-tris（3-amino-4-carboxyphenyl）aniline（3NH₂-BTB）,and an infinite Ti-oxo chain.PXRD,SEM,BET,TEM,FTIR and other methods were used to fully study the structure of NH₂-ZSTU-2 and 3NH₂-ZSTU-2,the results showed that the Ti-MOFs synthesized by solvothermal method had isomorphic structure with ZSTU-2,the structure was porous,the size and the distribution was uniform,They could provide more reactive active sites for the photocatalytic process,indicating that the materials were potential photocatalysts;The band structure information of NH₂-ZSTU-2 and 3NH₂-ZSTU-2 was obtained by using Solid UV-visible diffuse absorption spectrum and Mott-Schottky measurement,the results showed that the band gaps of NH₂-ZSTU-2 and 3NH₂-ZSTU-2 were smaller than that of the original ZSTU-2,which was favorable for the photocatalytic hydrogen production;the potential photocatalytic hydrogen production capability of NH₂-ZSTU-2and 3NH₂-ZSTU-2 was further demonstrated by characterizing the generation of photogenerated electron-hole pairs through transient photocurrent responses and electrochemical impedance measurements;thanks to these structural features,NH₂-ZSTU-2 and 3NH₂-ZSTU-2 showed stable rates of hydrogen production under visible light,the average photocatalytic hydrogen production rates of NH₂-ZSTU-2 and3NH₂-ZSTU-2 were 431.45μmol g^-1h^-1and 287.11μmol g^-1h^-1,respectively,under the photocatalytic system with triethanolamine as sacrifice agent and Pt nanoparticles as co-catalyst,they were almost 2.5 times and 1.7 times higher than the hydrogen production rate of isomorphic ZSTU-2.In addition,NH₂-ZSTU-2 and 3NH₂-ZSTU-2have high stability,permanent porosity and high photocatalytic activity,which are beneficial to the photocatalytic hydrogen production process.Finally,the cyclic stability and photocatalytic mechanism of NH₂-ZSTU-2 and 3NH₂-ZSTU-2 were discussed.This work further improves the photocatalytic performance of catalysts by functional groups of organic ligands,and broadens the new vision of efficient visible light photocatalysts for hydrogen evolution reaction.（2）The NH₂-ZSTU-2@r GO composite was synthesized by in situ growth of NH₂-ZSTU-2 on the surface of graphene oxide（GO）by a simple solvent thermal method,which was used to split water to produce hydrogen in visible light.PXRD characterization showed the characteristic peaks of reduced graphene oxide（r GO）and NH₂-ZSTU-2,indicating the successful preparation of NH₂-ZSTU-2@r GO composite;SEM showed that MOFs crystal particles were uniformly dispersed on the surface of r GO,providing a large number of active sites for photocatalysis;BET test proved that it had high specific surface area;because the composite material has excellent physical and chemical properties of r GO,through the transient photocurrent response and electrochemical impedance characterization under visible light,the results showed that it had high photocurrent response ability and small impedance.Moreover,the performance of photocatalytic hydrogen production was 1.55 times higher than that of NH₂-ZSTU-2,and the cyclic stability of the catalyst was proved by three photocatalytic hydrogen production cycle experiments.This method provides a new research idea for the preparation of efficient photocatalytic materials.