Construction Of Two-dimensional Metal Complexes/Nitrogen-doped TiO₂ Z-scheme Nanocomposites For Visible-light Catalytic CO₂ Reduction

Dramatic climate change caused by the high concentration of CO₂ in the atmosphere and the imminent depletion of fossil energy have become important challenges affecting the ecological environment of the earth and the basic living environment of human beings.Photocatalytic technology,with the help of clean solar energy as the driving force,can realize the resource reuse of excess CO₂ and convert it into valuable chemical fuels and chemicals,which is one of the sustainable ways to solve the current global warming and energy shortage problems.At present,a large number of photocatalytic materials have been developed,where Z-scheme photocatalysts based on natural photosynthesis can effectively promote the transfer and separation of photogenerated charges and maintain the higher thermodynamic ability of photogenerated electrons and holes,which has become a hot spot of current research.Titanium dioxide（TiO₂）,as a widely studied photocatalytic material,is a better-oxidized semiconductor in the Z-scheme photocatalytic system because of its stable photochemistry and deep valence band position.However,TiO₂ has a wide band gap（3.2 e V）and no visible light response,which makes it less utilizable for sunlight.Non-metallic nitrogen doping is considered to be a common strategy to extend the photoresponsive range of TiO₂,however,lower nitrogen doping still limits the utilization of visible light,while achieving higher nitrogen doping remains a challenge.Therefore,this thesis develops a high nitrogen-doped method to address the scientific problems of low doping and limited photoresponsive range of nitrogen-doped TiO₂;and selects metal complexes as reduced semiconductors to construct novel Z-scheme composites with broad spectral response and adjustable catalytic active sites to further improve the photocatalytic CO₂ reduction performance of nitrogen-doped TiO₂,in response to the scientific problems of the ease of compounding of nitrogen-doped TiO₂photogenerated charges and the lack of catalytic sites.And the charge transfer mechanism as well as the mechanism of the photocatalytic conversion process were further explored therein.The main research contents and results of the thesis are as follows:（1）High nitrogen-doped TiO₂ nanosheets（NPT）were prepared by a phosphoric acid-assisted high-temperature ammoniation synthesis strategy,and the broad spectral-responsive Cu Pc/NPT two-dimensional Z-scheme complexes were further constructed by the hydroxyl-induced assembly method.The experimental results show that the phosphoric acid molecule modification can effectively facilitate the doping of elemental N.High nitrogen doping was achieved under high-temperature ammoniation at 600℃,and the visible-light performance of the prepared NPT nanosheets was enhanced by three times compared with the reactivity of ordinary nitrogen-doped TiO₂.On this basis,a novel Z-scheme system was constructed by selecting Cu Pc,which does not overlap with the light absorption range of NPT,as a reduced semiconductor.Cu Pc significantly expands visible light absorption range and effectively promotes its photogenerated charge transfer and separation of NPT,and while the Cu Pc center metal provides catalytically active sites for CO₂ conversion.Under the condition of no sacrificial agent,the visible-light driven catalytic CO₂ conversion yield of Cu Pc/NPT nanocomposite with optimal loading could reach 5.4μmol·g^-1·h^-1,which is 3.6 times higher than that of NPT alone.The catalytic conversion pathway of CO₂ to CO through the formation of*COOH key intermediates at the reducing-end catalytic site was further clarified.（2）Aiming at the problem of limited CO₂ adsorption and activation capacity of metal phthalocyanine macrocyclic molecules as reduced semiconductors,choosing porphyrin macrocyclic molecules with similarπ-electron conjugation structure as phthalocyanine as ligands,covalent organic framework（COF）materials with unique two-dimensional lamellae,wide visible-light absorption and pore structure,were synthesized by spatial site-barrier method to increase CO₂ adsorption.And then,the novel Z-scheme Cu-COF/GO/NPT composite with tight interfacial connection was also constructed by interfacial modulation with functionalized graphene（GO）and NPT.The experimental results show that GO interfacial modulation effectively improves the interfacial charge transfer between the two components of the composite and enhances the dispersion of Cu-COF with the optimal loading from 5 wt.%to 8 wt.%.In addition,the highly loaded Cu-COF has a better CO₂ enrichment ability and its abundant ligand-centered metal can effectively catalyze the CO₂ conversion,which in turn substantially enhances its photocatalytic CO₂ reduction reaction activity.In the absence of sacrificial reagents,the CO production rate of visible-light catalytic CO₂ reduction of the optimally loaded Cu-COF/GO/NPT composite was 14.3μmol·g^-1·h^-1,which was 8times higher than that of NPT alone.（3）Based on porphyrin Cu-COF,the ligand center metal Ni,which can synergistically catalyze the conversion of CO₂,was further introduced,and then the bimetallic Cu_xNi_y-COF/GO/NPT Z-scheme composite was constructed with NPT under the modulation of graphene.Under the condition of no sacrificial reagent,the visible-light catalytic CO₂ conversion yield of the Cu₁Ni₂-COF/GO/NPT composite with the optimal metal ratio can reach 35.7μmol·g^-1·h^-1,which is 20 times higher than that of NPT alone.The experimental results showed that the bimetallic Cu（II）and Ni（II）served as reaction catalytic centers for both promoting Z-scheme charge transfer and catalyzing different reaction core steps.It was observed that the central metal Cu was superior to activating CO₂ adsorption,whereas the introduced metal Ni played a crucial role in activating H₂O molecules to generate the·H radical,which could synergize with the metal Cu in triggering the·H radical-mediated CO₂ conversion process,ultimately enhancing the overall conversion of CO₂.In this thesis,a series of novel Z-scheme composite photocatalysts were constructed with high nitrogen-doped TiO₂ as an oxidized semiconductor,and metal complexes such as phthalocyanine for CO₂ conversion,and in-depth exploratory studies were carried out to reveal the mechanism of photogenerated charge transfer and the process of the photocatalytic CO₂ conversion,which provided a new idea for the design of highly efficient Z-scheme photocatalytic materials for the production of solar fuels.