Electrochemical Reduction Method For Controlling Prepared Porphyrin-based MOFs Thin Films And Their Properties

Metal-organic framework materials?MOFs?have widely potential application in the fields of catalysis,sensing,gas separation and organic small molecule adsorption due to their many advantages such as large specific surface area,high porosity and tunable pores.As a kind of large?conjugated organic ligands with high symmetry and rigidity,porphyrin has outstanding photosensitivity and high molar extinction coefficient,which effectively enhances MOFs photoresponse and conductivity.Therefore,porphyrin-based MOFs not only effectively preserve the unique structure advantages of MOFs,but also could fully exert the catalytic activity and optical activity of porphyrin molecules.After the porphyrin-based MOFs materials are prepared into a film,their application can be expand in gas membrane separation,gas sensing,membrane catalysis,etc.,and has a profound impact on industrial production.Although many methods for preparing porphyrin-based MOFs films have been developed so far,such as microwave induction method,solvothermal method,layer-by-layer method,layer spray method,electrophoretic deposition method,etc.,these conventional preparation methods still can not achieve precise regulation of the orientation and film thickness of the porphyrin MOFs films,resulting in low separation efficiency and limitation the further improvement of the application performance.In response to the above problems,the following research contents were carried out in our subject.1.The porphyrin-based MOFs films with meso-tetra?4-carboxyphenyl?porphine?H₂TCPP?as the organic ligand and Zn²⁺as the node were peepared by electrochemical reduction method.The growth orientations of the porphyrin-based MOFs films could be easily controlled by reduction voltages.By adjusting reduction potentials?-1.0 V^-0.3 V?,Zn-H₂TCPP MOFs thin film materials with different orientations were obtained,which along the?004?crystal plane growth to form a 2D planar structure in the range of-0.5 V^-0.3 V,along the?110?crystal plane growth to form 3D spatial network structure in the range of-0.9 V^-0.5 V,and along different crystal planes random growth in further increase in the voltage.The thicknesses of Zn-H₂TCPP MOFs thin film can be controlled by electrochemical reduction reaction time.At the same time,the in-situ growth method can effectively reduce the contact resistance between the Zn-H₂TCPP MOFs film and the substrate,enhance the electron conduction ability and bonding ability,and achieve the stability improvement.In order to verify the superiority of the electrochemical reduction method for the preparation of Zn-H₂TCPP MOFs film,the prepared Zn-H₂TCPP MOFs film was applied to an electrocatalytic oxidation of NaNO₂ and sensitivity detection,that showed good electrocatalytic oxidation performance with sensitivity of 285.8?A·mM^-1·cm^-2,which is 3.6 times higher than the secondary coating film Zn-H₂TCPP MOFs,and the minimum detection limit for NaNO₂ is 0.24?M,which is 3.8 times lower than the secondary coating Zn-H₂TCPP MOFs material.And compared with other reported NaNO₂ detection materials,the porphyrin MOFs film materials prepared by chemical reduction have the advantages of wide linear range,high sensitivity and low detection limit.2.The choice of metal ions,organic ligands and substrate types has a direct impact on the later application of porphyrin MOFs films.To verify the versatility of electrochemical reduction in the preparation of porphyrin MOFs films.First,with same organic ligands?H₂TCPP?,three typical reduction voltages?V=-0.4 V,-0.8 V,-1.0 V?and various metal nods?M₁=Co,Ni,Mn,etc.?were selected,to prepare M₁-H₂TCPP MOFs?-0.4 V,-0.8 V,-1.0 V?films with different orientations.It was found that?110?crystal plane oriented film materials have better photoelectric response,indicating that the channels perpendicular to the FTO substrate are more favorable for photogenerated electrons transmission.The photoelectric response of M₁-H₂TCPP MOFs film materials with the same orientation is different,indicating that metal nodes are conducive to electron transport.Secondly,with same metal nodes(Zn²⁺),different metalloporphyrins?M₂TCPP=FeTCPP,NiTCPP,PdTCPP,etc.?were applied to prepare Zn-M₂TCPP MOFs with different orientations?-0.4 V,-0.8 V,-1.0 V?.In the photoelectric test of Zn-M₂TCPP MOFs film materials with different porphyrin ligands,the photoelectric response of Zn-PdTCPP MOFs is significantly better than that of other porphyrin ligand MOFs films,indicating the metal of the porphyrin macrocycle center ions are the main catalytic sites for the photoelectric response of such MOFs films.Finally,we can adjust the different substrate types?carbon cloth,molybdenum sheet,copper sheet,zinc sheet,etc.?to obtain oriented Zn-H₂TCPP MOFs?-0.8 V?film materials,which realizes the in-situ growth of various porphyrin MOFs thin film on various substrate materials.The above studies show that the electrochemical reduction method has good versatility in the controllable preparation of oriented porphyrin MOFs films,and can regulate the growth orientation and film thickness.The porphyrin MOF film prepared by the method exhibits excellent electrochemical sensing performance.At the same time,preliminary results show that the photoelectric properties of MOF films with oriented porphyrins are closely related to their orientation,which lays a foundation for the subsequent application of photovoltaics.