Surface/Interface Engineering On Two-dimensional Bismuth-Based Materials For CO₂ Photoreduction

Solar-driven photocatalytic CO₂ reduction reaction（CO₂RR）is highly promising for the conversion of value-added chemicals and solar fuels,able to alleviate environmental and energy scarcity issues for achieving a carbon-neutral society.Owing to the chemical inertness of the CO₂ molecule and the intrinsically low activity of photocatalysts,the obtained photoconversion efficiency of CO₂RR is still very low.In particular,the multielectron elementary reactions involved in CO₂RR further reduce the selectivity of the target product,bringing an extra cost for the product separation.Surface/interface engineering of catalysts can precisely modulate the sluggish dynamics of CO₂RR,in terms of adsorption and activation of CO₂ molecules,transport and separation of photogenerated charges,as well as adsorption/desorption behaviors of catalytic intermediates.Based on this,two-dimensional（2D）bismuth-based materials were selected as the platform for further surface/interface engineering,which concerns heterojunction construction,interfacial interaction modulation,interfacial electric field modulation,surface strain engineering,and transition metal heteroatom doping.The induced effects were systematically investigated during the catalytic reduction process from the perspectives of charge transfer dynamics and adsorption/desorption behaviors of catalytic intermediates.Meanwhile,a series of advanced characterizations such as photo-irradiated Kelvin probe force microscopy（KPFM）,in-situ Raman spectroscopy,in-situ diffuse reflectance infrared Fourier transform spectroscopy（DRIFTS）in combination with theoretical simulations,were carried out to further elucidate the intrinsic catalytic nature.In this dissertation,the main concents and results are as follows:1.Design and interfacial modulation of 2D/2D BiVO₄/Cs Pb Br₃ heterojunction for CO₂RR.This work achieved the construction of as-designed Bi VO₄/Cs Pb Br₃ with a2D/2D face-to-face structure.The unique S-scheme charge transfer was verified by photo-irradiated KPFM characterization and density functional theory（DFT）calculations.An intimate heterointerface between Bi VO₄ and Cs Pb Br₃ nanosheets with the maximally intensified interfacial interaction was obtained by precise heterostructure regulation,which largely boosts the charge transfer across the interfacial conjunction.Under visible light irradiation,the CO yield reached 68μmol g^-1 with a selectivity of more than 95%.The performance was increased by 201.7%compared with that of pure Bi VO₄ nanosheets.By establishing the parameter effective contact ratio(R_eff),the interfacial interaction intensity endowed by their heterostructures was accurately evaluated in this work,which coincides well with X-ray photoelectron spectroscopy（XPS）results.Based on the optimal structure of Bi VO₄/Cs Pb Br₃ heterojunction,the intrinsic oxygen vacancy of Bi VO₄ nanosheets was further tailored with the resultant gradient Fermi level shift towards its valence band,yielding an enlarged Fermi level gap and an enhanced internal electric field（IEF）over Bi VO₄/Cs Pb Br₃ heterojunctions certificated from electron paramagnetic resonance（EPR）and Mott-Schottky test.Such intensified IEF provides a more powerful driving force for charge migration.Therefore,the high CO yield reaching103.5μmol g^-1 with over 97%CO selectivity could be obtained under visible light irradiation without any co-catalyst or sacrificial agent.2.Highly strained Bi-MOF on bismuth oxyhalide support with tailored intermediate adsorption/desorption capability for CO₂RR.The surface-mounted Bismuth-based metal-organic framework（Bi-MOF）material on two-dimensional BiOBr support is specifically designed,subsequently as an operable platform to accurately tailor the intermediate adsorption/desorption capability in multi-electron CO₂ reduction reaction,through site-specific strain engineering.Revealed by high resolution transmission electron microscope（HRTEM）images and geometric phase analysis（GPA）as well as in situ Raman characterization,site-specific strain on Bi-MOF with a value as high as 7.85%in the as-designed Bi-MOF/catalyst hybrid is successfully engineered.XPS characterization and DFT calculations further revealed that the introduced compressive strain dramatically alters the electronic structure of Bi-MOF and the unsaturated state of the bismuth nodes.The consequences are the downshift of the p band center from 0.35 e V to-5.4 e V.Further,in-depth investigations on the Gibbs free energy changes,p-p（Bi 6p and CO₂/CO 2p）orbital hybridization and crystal orbital Hamilton population（COHP）validated the successful modulation of the adsorption/desorption capacities of the corresponding intermediates.Specifically,for the CO desorption,strain engineering levels up 1πand 5σfrontier orbitals of CO 2p and decreases the overlapping between Bi 6p orbital and 5σorbital of CO 2p,signifying favorable dissociation of*CO intermediate.As for CO₂RR,a remarkable CO yield of 87.83μmol g^-1 was obtained within 4 h without the assistance of any sacrificial agent or co-catalyst,and the CO₂-to-CO selectivity could reach 93%.3.Transition metal doped two-dimensional Bi₃O₄Br with functional orbital evolution for CO₂RR.2D Bi₃O₄Br nanosheets as an operable platform is adopted for heteroatom doping of various transition metals with distinguished outmost electron configurations to identify the active orbital evolution in terms of CO₂ photoreduction.High-resolution XPS and TEM characterization confirmed the successful introduction of heterogeneous transition metal atoms.Based on the results of DFT theoretical calculations,the built distance between the d/p-band center of dopants/bismuth sites and the p-band center of the free CO₂ molecule（Δd/p-p）are used as primary indicators to evaluate their interaction with CO₂ molecule.Among them,the Zn doped Bi₃O₄Br showed a smallestΔp-p（1.32）,indicative of the enhancement of p-p interaction.In addition,high-resolution XPS mapping further revealed that the spin-orbital splitting energy of bismuth was linearly correlated with the reactivity as increased atom number of transition metal dopants,indicating the intrinsic contribution of p orbitals of the Bi site.Furthermore,by determining the electron transfer,it was found that the electron loss of the transition metal from the Fe site to the Ni site and then to the Zn site gradually decreases from 0.169 to 0.088 and then to 0.033,while that of the neighboring Bi site gradually increases from 0.001 to final 0.121.The charge transfer behavior revealed that the gradual augmentation of p orbital function of Bi sites with the faded d orbital function of transition metal sites.Meanwhile,in situ DRIFTS analysis and Gibbs free energy change suggest the rapid emergence of the critical*COOH intermediate in a thermodynamically preferred pathway.Under visible light irradiation,without the assistance of any sacrificial agent or co-catalyst,the CO yield of Zn-Bi₃O₄Br within 3 h reaches 82.6μmol g^-1,increased by a factor of four.This dissertation focuses on surface/interface engineering towards 2D bismuth-based materials in modulating the interfacial interaction,interfacial internal electric field,surfacial strain,surfacial heterodoping to achieve favorable charge transfer dynamics and adsorption and desorption capabilities of specific intermediates in the coordinate of CO₂RR.Heterojunction engineering enables the CO₂ reduction application of oxidation bismuth-based semiconductors.Strain engineering reveals the intrinsic p-p orbital interactions of bismuth-based materials in CO₂ catalytic reduction.Transition-metal doping strategy further explores the effect of d orbital on the p-p orbital hybridization of reduced bismuth-based semiconductors in CO₂ reduction.