| To solve current energy problems and environmental crises,designing and developing efficient catalyst materials is an effective means.This thesis was devoted to the study of low-dimensional nanomaterials to improve the catalytic properties of materials,which can be control constructing microstructures in nanomaterials and modulating their physical and chemical properties.The purpose of this dissertation is to explore the structure-activity relationship between the microstructure and catalytic properties of nanomaterials,to further improve the catalytic activity of materials in different reactions and to explore the mechanism of catalytic reactions.The strategy of constructing ultra-thin structure,defect engineering and surface interface structure to regulate and optimize the catalytic materials was investigated.the fine structure of nanomaterials,important factors in the catalytic reaction process and in-depth understanding on reaction mechanisms was revealed by combining synchrotron radiation X-ray absorption near-edge spectra,quasi in-situ synchrotron radiation XPS and in-situ DRIFTS measurements.This dissertation provides a new direction and understanding for the design and development of efficient low-dimensional catalytic materials with superior structure in the future.The details of this dissertation are summarized briefly as follows:1.The electrical conductivity and adsorption ability with water are two key points that influence the activity of OER catalysts,both of which are strongly correlated with the electronic configuration,especially for the spin states of catalysts.Herein,by taking the nickle-chalcogenides(Ni?-S/Ni?-Se)as examples,the ultrathin NiSe2/NiS2 nanosheet with non-layered cubic structure and clean surface were successfully synthesized by an inorganic-organic hybrid intermediate-assisted liquid exfoliation strategy.By introducing structural distortion to their confined two-dimensional atomic layers,we show that the spin states of Ni? atoms in their ultrathin nanosheets were successfully regulated.Theoretical calculations indicate that after reducing the dimension of bulk NiSe2 to 2D-confined atomic layer,spin states in the atomic layers would be more delocalized,which can realize the delocalization of electrons and enhance the electron transfer between the active sites and the reaction intermediates.In additon,the spin state delocalization can reduce the adsorption energy of water molecules on the catalyst and increases the catalytic activity of the active sites.Further electrocatalytic test results show that the NiSe2/NiS-nanosheet exhibits an improved OER activity with high stability.This work indicates that the introduction of ultra-thin structure and spin delocalization regulation has great potential application prospects in the field of advanced OER electrocatalysts.2.Identification of active sites in electrocatalyst is essential for understanding on the mechanism of electrocatalytic water splitting.Herein,taken the ultrathin Fe7S8 nanosheets as an example,we studied the role of Fe played during the oxygen evolution reactions.As directly revealed by the synchrotron-based XANES spectra,the d orbitals of Fe?and Fe? in the ultrathin Fe7S8 nanosheets are intent to overlap with each other for the enhanced electron transfer process,resulting in the highly catalytic activity of Fe sites.Based on the above ultra-thin structure regulation,the Fe7S8 nanosheets display improved OER catalytic activity and manifest a small overpotential of 0.27 V and a large current density of 300 mA cm-2 at 0.5 V,which is 5 times larger than Fe7S8 bulk.This work gives deep understanding on the role of Fe played in the OER process,and also paves a practical way for the design of efficient electrocatalysts.3.In response to the problem of direct photocatalytic fixation of CO2 under mild conditions,the authors have developed a method of single-electron transfer to effectively immobilize CO2 by regulating oxygen vacancies in ultra-thin nanosheets.The Bi2O3 atomic layers were prepared via in-situ oxidation of the fresh exfoliated Bi nanosheets for their high surface energy,during which the oxygen deficiencies would be readily introduced.Theoretical simulations showed that the oxygen vacancy can not only provide abundant localized electrons,but also lower the adsorption energies of CO2 on Bi2O3 atomic layer.The Bi2O3 nanosheets with single-layer thickness were obtained by X-ray diffraction,high-resolution transmission electron microscopy,and atomic force microscopy.The presence of oxygen vacancies in Bi2O3 ultrathin nanosheets was demonstrated by X-ray photoelectron spectroscopy,electron spin resonance spectrometry and photoluminescence spectroscopy.Both synchrotron-based in-situ DRIFT and quasi in-situ XPS reveal that oxygen vacancies in Bi2O3 nanosheets could enhance the generation of·CO2-species,which is the rate determining step for CO2 photofixation.Moreover,the introduction of oxygen vacancies greatly increases the yield of the final product dimethyl carbonate.4.Through the regulation of the interface structure,we realized the efficient synthesis of dimethyl carbonate directly by thermal catalytic CO2 and methanol.In this study,Cu2O/Cu nanosheets were successfully prepared through a facile and straightforward hydrothermal process.The interface structure in the prepared Cu2O/Cu nanosheets was confirmed by HAADF-STEM.The synchrotron-based quasi in-situ XPS shows that CO2 can be activated at the interface to form carbon dioxide ions(·CO2-),which is an important intermediate in the direct synthesis of dimethyl carbonate.The synchrotron-based in-situ DRIFT confirmed that the Cu2O/Cu nanosheets can efficiently catalyze the preparation of dimethyl carbonate during the reaction,which is much higher than the pure phase of Cu2O and Cu nanosheets.This work introduces the surface interface in the two-dimensional material,which greatly increases the reactive site of the catalyst material and the yield of the catalytic preparation of dimethyl carbonate,which provides a further investigation for the thermal catalytic CO2 reaction. |
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