The Electronic Structure Regulation Of Lead-Free Perovskite Nanocrystals And Their Photocatalytic Reaction Mechanism

Solar-driven photocatalytic technology,which mimics the redox reactions in natural photosynthesis,is considered one of the promising approaches to address energy shortages and environmental pollution.Compared to traditional methods,photocatalysis offers advantages such as zero secondary pollution,green energy conservation,and mild reaction conditions,making it the focus of current research in environmental catalysis and energy conversion.The activation of inert sp³ C-H bonds and C=O bonds in CO₂,in particular,has become a challenging task,often referred to as the"Holy Grail"reaction in the field of photocatalysis.However,the low adsorption activation efficiency of inert molecules and sluggish carrier dynamics during catalytic reactions still severely limit the efficiency improvement of these elementary reactions.Therefore,in-depth investigation of the reaction mechanisms of heterogeneous photocatalysis,guided by theory to develop efficient and selective photocatalysts,is crucial to overcoming these challenges.Lead-free perovskite nanocrystals Cs₃B₂X₉（B=Bi,Sb;X=Br,Cl）have attracted significant attention due to their low surface defect density and long carrier diffusion distance.Compared to lead-based nanocrystals,antimony-based or bismuth-based perovskite nanocrystals possess non-toxicity and stability,making them ideal alternatives for environmental purification and energy conversion.Studies have shown that precise control over the B and X sites of perovskite nanocrystals can alter their crystal structure and electronic structure,thereby optimizing their band structure and carrier dynamics.In this thesis,we systematically investigate the electronic structure regulation of lead-free perovskite nanocrystals Cs₃B₂X₉（B=Bi,Sb;X=Br,Cl）photocatalysts and their impact on photocatalytic performance and enhancement mechanisms,as follows:（1）By employing a halogen-site regulation strategy,we achieved the controllable preparation of Cs₃Bi₂X₉ quantum dot photocatalysts.Precisely controlling the charge transfer path effectively promoted molecular adsorption activation,which is proposed to achieve both high selectivity and yield of benzyl-alcohol oxidation.The Cs₃Bi₂Br₉catalyst exhibited outstanding photocatalytic efficiency,with a conversion rate of benzyl alcohol reaching 97.9%and a corresponding aldehyde selectivity of 99.6%,representing respective enhancements of 56.9 and 1.54 times compared to the Cs₃Bi₂Cl₉ catalyst.In-situ photoluminescence（PL）and in-situ Raman spectroscopy further elucidate that the Bi-Br covalent bonds in the form of Br coordination significantly promote the separation and transfer of charge carriers.Meanwhile,as active sites,Bi-Br covalent bonds facilitate the activation of benzyl alcohol molecules,generating more carbon-centered radicals.Additionally,a combination of quasi-in-situ electron paramagnetic resonance spectroscopy（EPR）,in-situ attenuated total reflection Fourier transform infrared spectroscopy（ATR-FTIR）and density functional theory（DFT）calculations revealed that the reaction pathway from C₆H₅-CH₂OH*to C₆H₅-CH₂*governed the rate-limiting step of the entire catalytic reaction,thereby improving its selectivity and photocatalytic efficiency.（2）By taking g-C₃N₄ nanosheets as a platform,the controlled construction of Cs₃Bi₂Br₉@g-C₃N₄ heterojunction was achieved through in-situ surface modification of Cs₃Bi₂Br₉,whereby experimental and theoretical research methods were combined to unveil the interface Br-N covalent bond as its intrinsic catalytic active site.Furthermore,modulation of the charge carrier dynamics and electronic structure of Cs₃Bi₂Br₉@g-C₃N₄heterojunction catalysts led to a significant enhancement in photocatalytic activity and product selectivity.The benzaldehyde selectivity of the Cs₃Bi₂Br₉@g-C₃N₄ photocatalyst reached 98.1%,with a toluene conversion rate of 55.2%.Investigation into the catalytic reaction mechanism revealed that the interface Br-N bond and built-in electric field synergistically promoted charge separation and transfer within the Cs₃Bi₂Br₉@g-C₃N₄catalyst.Moreover,combined with quasi-in-situ electron EPR,in-situ ATR-FTIR spectra,and DFT calculations,further evidence was provided for the crucial role of the interface Br-N covalent bond in facilitating the activation of small molecule adsorption.This finding elucidates the vital role of the interface Br-N covalent bond in promoting directed charge transfer in photogenerated carriers,providing new insights for the rational design of efficient photocatalysts.（3）By taking Cs₃Sb₂Br₉ nanocrystals as a platform,ferrocene-functionalized antimony-based perovskite catalysts were controllably synthesized for the photocatalytic oxidation of benzyl alcohol coupled with CO₂ reduction.The Fc-functionalized Cs₃Sb₂Br₉ nanocrystals（CSB-Fc NCs）exhibited exceptional photocatalytic synergistic redox performance,with both selectivity for CO and benzaldehyde exceeding 97.7%and a CO yield of approximately 45.56μmol·g^-1·h^-1.Furthermore,in-situ Raman and UV-Vis diffuse reflectance spectroscopy revealed that the reversible Fe³⁺/Fe²⁺metal valence sites within Fc served as intrinsic catalytic active sites,effectively promoting the adsorption activation of molecules and the transfer of hydrogen protons.Moreover,utilizing isotopically labeled in-situ ATR-FTIR spectroscopy further demonstrated that hydrogen ions dissociated from hydroxyl groups on the benzene ring could be activated at the electron-rich Fc sites to form active hydrogen protons,achieving efficient photocatalytic CO₂ reduction through subsequent protonation reactions.Therefore,this strategy not only enhanced charge carrier separation efficiency and promoted molecular activation but also successfully realized the coupling reaction of CO₂ photoreduction with organic synthesis.（4）By precisely controlling the B site of Cs₃Sb₂Br₉ nanocrystals,a series of novel photocatalysts Cs₃Sb_xBi_2-xBr₉（0≤x≤2）were successfully synthesized and applied to organic synthesis coupling reactions.Among them,Cs₃Sb_0.5Bi_1.5Br₉ nanocrystals exhibited exceptional photocatalytic performance while maintaining high selectivity for CO and benzyl alcohol.Photoluminescence and transient photocurrent response spectroscopy confirmed that the introduction of Bi significantly enhanced the separation and transfer of photogenerated charges,effectively prolonging the carrier lifetime.Furthermore,combining in-situ ATR-FTIR spectroscopy with DFT calculations demonstrated that the introduction of Bi disrupted the crystal structure symmetry of Cs₃Sb₂Br₉ nanocrystals,leading to localized charge redistribution,enhancing the transfer and conversion of photogenerated electrons,lowering the activation energy barrier for adsorbates,and thereby promoting the adsorption activation of benzyl alcohol and CO₂molecules.Therefore,the inclusion of Bi not only optimized the electronic structure of Cs₃Sb₂Br₉ nanocrystals but also revealed the inherent relationship between the performance,structure,and mechanism of photocatalysts.In this work,effective modulation of the microelectronic structure and crystal structure of lead-free perovskite Cs₃B₂X₉ nanocrystals（B=Bi,Sb;X=Br,Cl）significantly enhanced photocatalytic performance,elucidating the mechanism underlying the improvement in photocatalytic efficiency and reaction mechanism.This study provides a new perspective for a profound understanding of the photocatalytic reaction process,further confirming the critical role of lead-free perovskite nanocrystals in environmental and energy catalysis,laying a solid foundation for the practical application of photocatalytic technology.