Research On The Synthesis, Application And DFT Calculation Of Semiconductor MOF Materials Based On Tetrakis (4-carboxyphenyl) Silane

Energy shortage and environmental pollutions caused by burning fossil fuel have become worldwide concerns.Solar energy is the inexhaustible and clean energy source worldwide.UV light accounts for only 4%while visible light contributes 43%of solar energy.Thus preparing and developing visible light absorbing semiconductors which can be used in converting light into electricity or used as photocatalysts to store solar energy into useful chemicals is rather imperative.Metal-organic frameworks（MOFs）can be synthesized as visible-light absorbing semiconductors due to their designability in structures and functionalities.Studies relating to the semiconducting behavior of MOFs are still in their initial stages,and further investigations are highly desired.The GGA-PBE functional or GGA-PBE plus U method in Vienna Ab initio Simulation Package（VASP）are applied to calculate the band structures,density of states（DOS）and band gaps of three isostructural MOFs,M₂（TCS）（BPY）（M=Cu（Ⅱ）（1）,Co（Ⅱ）（2）and Ni（Ⅱ）（3）,H₄TCS=tetrakis（4-carboxyphenyl）silane,BPY=4,4’-bipyridine）.The calculated band gaps is very close to the experimental values.Thus a general,accurate and economic theoretical method of calculating band gaps,band structures and density of states of transition-metal-ion based MOFs has been found.The calculated results explained the experimental results.In addition,this study suggest that Co（Ⅱ）might be a better candidate as metal ion nodes of MOFs to raise the caltalytic efficiencies than Cu（Ⅱ）and Ni（Ⅱ）ions.The UV-vis reflectance spectra of[Cu（H₂TCS）（H₂O）]（4）,[Co（H₂TCS）（BPB）]（BPB=1,4-bis（pyridyl）benzene）（5）and[Ni（H₂TCS）（BPB）]（6）have been measured and the experimental band gaps were determined as 0.64,1.77 and 1.49 e V.The photoelectric properties of MOF 4-6 were also studied.Rapid and stable photocurrent responses of compounds 4-6 under UV and visible light illuminations are observed.This work demonstrates that using electron rich Cu²⁺,Co²⁺,or Ni²⁺as metal-ion nodes of MOFs can effectively decrease the band gaps and increasing the visible light absorbance of MOFs.Furthermore it is found that introducing electron rich and visible light absorbing ligands can increase the visible light absorbance of MOFs.Two novel visible light absorbing Ni（Ⅱ）MOFs（as suggested by their green colors）,Ni₂（TCS）（BIMB）₂（7）and[Ni₂（H₂TCS）₂（BIMB）₂]·H₂O（8·H₂O）1,4-bis（2-methyllimidazol-3-ium-1-yl）benzene were synthesized.MOF 7 has a two-fold interpenetrating 3D structure,a porosity of 44.6%,a surface area of 820 m²/g,and is stable in air for at least 7 d.The gas（N₂,H₂,CO₂,and CH₄）adsorption properties of MOF 7 were also investigated.MOF 8 has a 2D dense structure,is stable in acidic aqueous solution.Orthogonal experimental design method was applied to optimize the synthetic conditions of MOF 4,demonstrating that this is an efficient method for finding optimized reaction conditions of growing large single crystals and obtaining pure product.This systematic analysis tool is widely used in biochemistry and various fields of chemistry,^[1-5]however few reports were published in the field of coordination networks.