Fabrication And Properties Of Oxyasalt/Metal-organic Framework Heterogenous Materials

It is urgent to develop and utilize renewable energy due to the shortage of fossil energy.With the help of photocatalyst,hydrogen evolution with high energy density from water reduction under sunlight has been a green and efficient way to solve the problem of energy shortage.While the design and preparation of those photocatalysts with high activity are closely related to the photoreduction of water to hydrogen.In this thesis,niobates and titanates with high chemical stability,good hydrophilicity and low toxicity were used as the main basis,a series of novel and efficient oxyasalt-based composite photocatalysts were constructed by coupling porphyrin-based metal organic frameworks（MOFs）,and the assembly mechanism,photocatalytic properties and photogenerated carrier transfer mechanism of the nanocomposites were further studied.The main research contents are as follows:1.By constructing Co（II）meso-tetra（4-carboxyphenyl）porphyrin-metal organic frameworks（CPMOF）on K₄Nb₆O₁₇ microflowers,a special K₄Nb₆O₁₇-based nanocomposite(K₄Nb₆O₁₇/CPMOF)was achieved.The CPMOF constructed on the K₄Nb₆O₁₇ microflowers was employed as strong light-harvesting sensitizer and electron transfer carrier,and thus the light absorbance range of K₄Nb₆O₁₇ microflowers could be efficiently expanded as well as relatively high separation of photo-generated charges was obtained.Only introducing 9 wt%CPMOF,the K₄Nb₆O₁₇/CPMOF nanocomposite showed about 17.5 times higher photocatalytic activity than that of K₄Nb₆O₁₇ microflowers.Impressively,the stability of the K₄Nb₆O₁₇/CPMOF can be improved due to the protection of the K₄Nb₆O₁₇ microflowers and the constructed heterostructure.The reason for the enhanced photocatalytic activity was explored in detail.This research will provide a new perspective for the rational design of oxysalt-based photocatalysts used in energy conversion.2.A promising direct Z-scheme KNO MF/Co-PMOF nanohybrid was facilely fabricated by means of cobalt meso-tetrakis（4-carboxyphenyl）porphyrin metal organic frameworks（Co-PMOF）in-situ grown on potassium niobate microflowers（KNO MF）.The tight combination of the two semiconductors can be confirmed by HRTEM.It was found that the spectral response range can be effectively expanded,and also the carrier separation and migration efficiency in the nanohybrid can be effectively improved by the establishment of the built-in electric field at the interface.Under irradiation without adding any cocatalyst,the optimized KNO MF/Co-PMOF exhibited 14.5 times higher activity than that of pristine KNO MF.Moreover,the KNO MF/Co-PMOF showed a good durability in five continuous recycles（30 hours）.This study will provide a promising strategy for designing Z-type photocatalysts for enhanced H₂ release.3.The ultrathin two-dimensional copper carboxyl porphyrin MOF（Cu-PMOF）nanosheets were in situ assembled on the Ti O₂ nanotubes（TNTs）riveted with copper ions by hydrothermal method with the aid of non-covalent interaction and coordination.Subsequently,a novel TNTs/Cu-PMOF nanocomposite was constructed,which displayed both increased visible light absorption and improved charge transfer.The results showed that the nanocomposites possessed higher photocatalytic activity for hydrogen production from water reduction and better cycling stability.It was mainly attributed to the strong interface interaction between TNTs and Cu-PMOF in the in-situ constructed TNTs/Cu-PMOF nanocomposites.It was also found that,after in-situ growth of the Cu-PMOF,the specific surface area of the nanocomposite was increased,and thus more active sites for the photocatalytic hydrogen evolution reaction were produced.It was beneficial to the improvement of the photocatalytic hydrogen evolution activity.Furthermore,a type-II mechanism for photocatalytic hydrogen production in the presence of TNTs/Cu-PMOF nanocomposites was clarified by XPS.In this thesis,by coupling niobate and titanate with porphyrin-based MOFs,respectively,the construction of a series of oxyasalt-based nanocomposites with novel structure and high photocatalytic hydrogen production and the exploration of the mechanism about photogenerated electron transfer provide new ideas and experimental basis for the design and synthesis of other highly efficient oxysalt-based photocatalytic systems.