Design And Construction Of Two-dimensional Inorganic Bulk Heterojunction Study On The Performance Of Photocatalytic CO₂ Reduction

The environmental issues of excessive CO₂ emissions and energy scarcity can be addressed with the support of clean energy-driven conversion of CO₂to other hydrocarbon compounds.Nevertheless,the semiconductor photogenerated exciton separation efficiency,the hard to activate CO₂,and the sluggish surface reaction rate all have an impact on the efficiency of the photocatalytic CO₂reduction reaction.Although conventional planar heterojunctions（PH）based on epitaxial growth methods have been extensively explored for photocatalytic applications,their photovoltaic efficiency is negatively impacted by the many defects and dislocations at the interfaces.In order to create dense interfaces and reduce the diffusion lengths from photogenerated exciton separation to transfer,the bulk heterojunctions（BH）create three-dimensional charge regions through the mutual contact of several semiconductor types.Nevertheless,the majority of recent BH researches have concentrated on the field of organic semiconductors,and there is not a broadly-applicable method for creating inorganic BH.This thesis successfully developed a method to construct inorganic intergrowth bulk heterojunctions（IIBH）based on the structure memory effect of solid-solution layered double hydroxides（LDHs）.It also systematically investigated the principle of heterojunction construction and applied it to the photocatalytic reduction of CO₂reaction.In-situ illumination X-ray photoelectron spectra and transient absorption spectra of the IIBH demonstrate the photogenerated electron transfer routes,while in-situ infrared spectra clarified the CO₂reduction reaction pathways.The primary research findings and conclusions of this thesis are as follows:（1）Constructing IIBH:Based on the structure memory effect of NiAl-LDHs,a two-stage topological pyrolysis（TTP）method is proposed to construct pn-type IIBH NiO/CuO（Ni,Al）for photocatalytic CO₂to methane production reaction using Cu NiAl-LDHs as precursor.The intergrowth interfaces of BH can be clearly identified in high-resolution transmission electron microscopy,while synchrotron radiation extended X-ray absorption fine structure spectra provide the mechanism of the topological pyrolysis process and the validity of the synthesis of IIBHs based on the structure memory effect of LDHs by the TTP method.Transient absorption spectra are used to study the mechanism of photogenerated electron transport in NiO/CuO（Ni,Al）in accordance with pn-type heterojunctions.Lastly,density functional theory simulations and in-situ infrared spectra demonstrate that CO₂adsorption is more effectively activated by Ni²⁺and Al³⁺doping in pn-IIBH and that CO₂methanation takes place via the hydrogenation route.The feasibility of the TTP method to achieve the formation of oxide/oxide-type IIBHs on the scale of atoms is initially depicted,along with a comparison of its advantages over other heterojunctions.（2）Introduced active defect site into the IIBH:Based on the structure memory effect of Ni Ga-LDHs,Ni Ti Ga-LDHs are used as precursors to construct Z-type IIBH NiO（Ti）/Ti₃O₅（Ni,Ga）with photo-induced oxygen vacancy regeneration by the above proposed TTP method.The process of photo-induced oxygen vacancy（V_O）regeneration is investigated using in-situ illumination X-ray photoelectron spectra and electron paramagnetic resonance techniques.In the presence of light,the Ni²⁺in NiO（Ti）loses electrons to form Ni³⁺,while the Ti⁴⁺in Ti₃O₅（Ni,Ga）gains electrons to generate Ti³⁺along with the conversion of some lattice hydroxyls to V_O.In the photocatalytically reduced CO₂,V_Ois able to continue functioning owing to the photogenerated electrons tranfer between the Ti⁴⁺/Ti³⁺and Ni³⁺/Ni²⁺redox pairs.The electron concentration is proportional to the V_Oconcentration,and the electrons bound by the more active surface V_Ocan provide the electron-deficient center of CO₂with electrons to generate·CO₂^–,increasing the rate of the CO₂reduction tachyphylaxis step.The cyclic photocatalytic CO₂reduction studies show that the stability held at 96.7%and that the CO production reached a maximum of2560.1μmol/g·h over a duration of 60 h.Further,the introduction of renewable oxygen vacancies in the oxide/oxide type IIBH for CO₂activation provides a new idea for reusable defective catalysts.（3）IIBH with defect to mimic enzyme:Based on the structure memory effect of Mg Fe-LDHs,the pyrolytic topological vulcanisation（PTV）method is proposed to construct IIBH Ni S/Fe S@Mg Fe-LDHs to mimic the[Ni-4Fe-4S]active site in carbon monoxide dehydrogenase（CODH）for the photocatalytic reduction of CO₂by using Ni Mg Fe-LDHs as the precursor.The enzyme-mimicking catalysts are evaluated in terms of both microstructure and catalytic reaction sites.The structural similarity between Ni S/Fe S@Mg Fe-LDHs and CODH active site is confirmed by X-ray diffraction and X-ray absorption near edge structure characterisation.Then,in-situ illumination X-ray photoelectron spectra and transient absorption spectra show the photogenerated electron transfer of IIBH.In other words,for the photocatalytic reduction of CO₂,the electrons ultimately converge in the conduction band of Ni S,resembling the C-cluster[Ni-4Fe-4S]with Ni²⁺ions serving as the reactive site.The PTV approach provides more opportunities for the synthesis of IIBH to imitate the enzyme catalyst,and the synthesis of sulfide/LDHs type IIBH is further extended to mimic the enzyme catalyst activity center at a intergrowth interface.（4）Polaron activation of CO₂:Based on host-guest interaction of LDHs,organic/inorganic composites CuPcS/Ni Mg Fe-LDHs are constructed by electrostatic assembly method,and their unique photogenerated electron transfer mechanism is proposed.The strong coupling between charge transfer state and molecular vibration leads to the formation of molecular polaron,and the shielding effect of polaron further accelerates the separation of photogenerated excitons.Upon photoexcitation,when the photogenerated electrons are transferred from NMF-LDHs to CuPcS,the electrons in CuPcS and the holes in NMF-LDHs form a charge-transfer singlet state.The heavy-atom effect induces spin-orbit strong coupling to promote the electron spin-state flip,which generates the CuPcS excited triplet state and forms the charge-transfer triplet state.In addition,the photogenerated electrons in NMF-LDHs continuously inject electrons into CuPcS,inducing its polarisation,and when photoelectrons transfer from the ligand to the central metal Cu²⁺ion,vibrational coupling with the CuPcS molecule generates[Cu（I）Pc S]^–polaron,localising the photogenerated electrons.A series of long-range F（?）rster energy transfers extends the lifetime of the photogenerated electrons.Meanwhile,the efficient photogenerated exciton separation efficiency and active[Cu（Ⅰ）Pc S]^–polaron can effectively convert CO₂to CO.In order to match to the time scale of the surface reaction on the surface of CO₂,the composite structure extends the lifetime of the electron with continuous energy transfer.