Low-Dimensional Solid Mesoporous Nanomaterials:Synthesis And Application

Low-dimensional nanomaterials with the reduction of at least one-dimensional scale to the nanoscale,exhibit novel physicochemical properties because their size is comparable to the coherence length of electrons.Meanwhile,the low-dimensional mesoporous nanomaterials exhibit the structural characteristics of controllable pore size,adjustable skeleton composition and diverse morphology,and derive the advanges of large specific surface area,nano confinement effect,and the ability to regulate the convection and transport of gas molecules.Therefore,this kind of material show broad application prospects in thermal insulation,catalysis,and other fields.Customizing the mesoporous structure for low-dimensional nanomaterials with specific chemical compositions,not only benefits improving the application potential,but also shows great significance to explore the special physical and chemical properties of nanomaterials.This dissertation aims to adjust the physical and chemical properties of low-dimensional nanomaterials through the mesoporous structure design,so as to realize its application in multiple fields.This dissertation focuses on the low-dimensional solid mesoporous nanomaterials and analyzes the physical and chemical properties corresponding to the movement of gas molecules in the mesoporous structure.Since the property of thermal insulating material is closely related to the convection of gas molecules,and the gas transport process has a significant impact on the kinetics of electrocatalytic oxygen reduction reaction,the thermal conductivity or electrocatalytic performance of the low-dimensional nanomaterial is selected as indicator to evaluate the influence of mesoporous structures on the movement of gas molecules.By mesoporous structure design of specific low-dimensional nanomaterials,the improvement of thermal insulation performance and electrocatalysis oxygen reduction performance are achieved,and then provide new pathways to the reasonable design and controllable synthesis of mesoporous structure in low-dimensional nanomaterial.The main research contents of this dissertation are as follows:1.Mesoporous multi-shelled hollow resin nanospheres are developed via an in-situ polymerization strategy,and the ultra-low thermal conductivity was achieved in the melamine/phenolic resin system due to the inhibition of the thermal convection of gas molecules.The new configuration of hollow nanostructures possesses more than three layers of shells and simultaneously integrated mesopores on every shell,and therefore expresses characteristics of periodic interfaces.Benefiting from the suppression of gas molecules convection by boundary scattering,the thermal conductivity of mesoporous multi-shelled hollow resin nanospheres reaches 0.013 W·m-1·K-1 at 298 K and shows unusual stability from 298 K to 473 K.In this work,an elaborate interior mesostructure of hollow nanosphere is designed to elucidate the influence of mesoporous structure design on gas convection,which provides a new choice for the nano-sized structural block of thermal insulation materials.2.Two-dimensional mesoporous nanosheets are constructed by the small size nanobands,and a macroscale inorganic thermal insulation foam based on the mesoporous nanosheets is obtained by a freeze-casting method.Due to the strong interaction and large area of the two-dimensional mesoporous nanosheets,the growth of ice crystals would make them tightly stacked to form the skeleton,and therefore construct a self-supported pure inorganic foam without additional binder.The cell-like macropores derive from ice crystal template,and forms a multi-scale pore network with the mesoporous skeleton.Thanks to the high porosity of 93%,the thermal conductivity of the self-supported foam is only 1/7 of that of the skeleton material at room temperature and as low as 0.2 W·m-1·K-1 at 600℃ indicating the effective suppression of the gas convection.In this work,the mesoporous nanomaterial is constructed as the building block of thermal insulating foam to suppress gas convection,which provides a new idea for the assembly of low-dimensional mesoporous nanomaterials into macroscopic structures.3.Two-dimensional hierarchical mesoporous Fe-N-C catalysts are constructed in local nitrogen-rich environment.Mesoporous folded carbon structure is integrated into a two-dimensional support,showing high electroconductivity and specific surface area.Benefiting from the double enhancement of charge transport and mass transport at the three phase boundary to facilitate the mass transport,the as-obtained catalysts shows excellent oxygen reduction performance under alkaline conditions.The half-wave potential of the as-obtained catalysts reached 0.91 V vs RHE and lost only 12 mV after 6000 cycles.In this work,a catalyst support with two-dimensional hierarchical mesoporous structure is designed,and gas transport at the three phase boundary are optimized,which provides a new scheme for the design of efficient electrocatalysts.In summary,mesoporous multi-shelled hollow nanospheres,two-dimensional mesoporous nano sheets,two-dimensional hierarchical mesoporous nanostructure were developed in this dissertation.By regulating the thermal convection and mass transport process of gas molecules,the thermal insulation performance and electrocatalytic oxygen reduction performance are improved.Furthermore,the mechanism of action of mesoporous structures in the application fields related to gas molecular motion is summarized,which provides a new idea for the application of low-dimensional mesoporous nanomaterials.In the future,the development of green and controllable synthesis methods and microscale characterization to clarify the structure-activity relationship between mesoporous structures and physical and chemical properties will contribute to the development of low-dimensional mesoporous nanomaterials with more application potential.