Construction And Performance Of Epitaxially Grown Layered Double Hydroxides Nanofiber Membranes

Layered Double Hydroxides（LDHs）are a class of two-dimensional inorganic layered materials composed of positively charged host layers and negatively charged interlayer anions.With the adjustable laminated metal elements and interlayer anions,controllable morphology,low cost of raw materials,simple preparation and easy large-scale production,LDHs have attracted much attention.To expand the application of LDHs-based materials,our team have developed"Embedded Seed-Epitaxial Growth"strategy to construct LDHs-based nanofiber membrane materials,achieving uniform dispersion and stable immobilization of LDHs.On this basis,aiming at the emerging problems in thermal insulation,solar-driven interfacial evaporation and heavy metal adsorption,the two strategies of"Embedded Seed-Epitaxial Growth-Template Etching"and"Embedded Seed-Epitaxial Growth-Ion Exchange"were further proposed,the novel LDHs-based nanofiber membrane materials were constructed,which realized the precise regulation of the LDHs structure and the optimization of LDHs performance.The main contents of this thesis are shown as follows:1.Thermal insultation materials can diminish heat loss effectively during generation,transportation,storage and utilization of thermal energy,which plays a vital role in civil and military fields.To development the high-performance thermal insulation materials,the"Embedded Seed-Epitaxial Growth"strategy was employed to achieve the interpenetration growth of LDHs on nanofibers,which constructed abundant interfaces with different length-scales containing inorganic-organic interfaces,lattice interfaces and atomic interleaved interfaces.PAN@Mg Al-LDH（PAN=polyacrylonitrile;Mg Al-LDH=magnesium aluminum layered double hydroxides）composite membranes were prepared,and the regulation of LDHs loading of composite membranes was realized by adjusting the doping amount of the Al OOH.Experimental results showed that the increase of LDHs loading enhanced the thermal insulation performance of the composite membranes.The PAN@Mg Al-LDH-70%composite membrane（LDHs loading was 70%）with a thin layer of 0.6 mm showed an impressive temperature difference of 28.1°C in the hot stage of 80°C.When covered with 3-layers PAN@Mg Al-LDH-70%composite membranes,the human hand realized infrared invisible.The efficient thermal insulation effect of PAN@Mg Al-LDH composite membrane can be attributed to that 1)the in-situ growth of LDHs constructed micrometer porous structure,which restricted thermal convection;2)the abundant interface structure with different length-scales enhanced phonon scattering,resulting in the decreased thermal conduction;3)the selective infrared absorption of LDHs and the interpenetration growth structure reduced the transmission of infrared radiation,which decreased thermal radiation.Based on the abundant modification sites on the surface of LDHs,the hydrophobic modification of LDHs by DTMS（Dodecyltrimethoxysilane）was realized,which expanded the application of LDHs-based composite membranes in high humidity conditions.2.Solar-driven interfacial evaporation can realize the effective utilization of seawater resources,which alleviate the pressure of water shortage.Photothermal conversion materials are essential for solar-driven interfacial evaporation.The solar absorption range and photothermal conversion performance of traditional Co-based materials were limited by the wide bandgaps.To address these issues,the"Embedded Seed-Epitaxial Growth-Template Etching"strategy was proposed to achieve the in-situ growth of different Co-based LDHs,which constructed the hierarchical structure.The bandgap width of the composite membranes can be precisely controlled by adjusting the laminate metal elements of LDHs.The results exhibited that the PAN@Co Mn-LDH（Co Mn-LDH=cobalt manganese layered double hydroxide）composite membrane possessed the best photothermal conversion performance.Under 1.0 k W m^-2 solar irradiance,the surface temperature of the composite membrane rapidly rose to 85.7°C.The PAN@Co Mn-LDH composite membrane demonstrated excellent water evaporation performance with an evaporation rate as high as 3.12 kg m^-2 h^-1 and a solar-to-vapor conversion efficiency of 94.5%.When applied for seawater,the evaporation rate can reach to 3.06 kg m^-2 h^-1 by PAN@Co Mn-LDH and remained stable for the whole 8 h in the aid of the salt resistant evaporator.The structure-function relationship of the PAN@Co Mn-LDH composite membrane was revealed in depth:1)the in-situ growth of Co Mn-LDHs constructed a hierarchical structure,enhancing the multiple absorption and reflection for sunlight,which improved solar energy utilization;2)the abundant-OH groups of Co Mn-LDHs broke the hydrogen bonding between water molecules,reducing the evaporation enthalpy to 1307k J kg^-1,which facilitated the efficient water evaporation;3)the narrow bandgap of the composite membrane（1.79 e V）greatly broadened the absorption range of sunlight and enhanced the conversion of solar energy into thermal energy.3.The high selective and trace removal of Cu²⁺from industrial wastewater is of great significance.The traditional LDHs adsorbents suffered from easy agglomeration,difficulty in recovery and the lack of selectivity for Cu²⁺.To address these issues,the"Embedded Seed-Epitaxial Growth-Ion Exchange"strategy was proposed to achieve the in-situ growth of 3,5-pyrazoledicarboxylic acid-intercalated LDHs,which constructed the hierarchical structure.The PAN@Mg Al-LDH-Hpdc（Hpdc=3,5-pyrazole dicarboxylate）composite membrane showed a removal efficiency of 99.6%for Cu²⁺in 25 min and the concentration of Cu²⁺can be decreased to 3 ppb in 60 min.When the concentration of Cu²⁺,Ni²⁺,Zn²⁺and Co²⁺was the same in the solution,the PAN@Mg Al-LDH-Hpdc composite membrane exhibited a distribution coefficient for Cu²⁺of 4.4×10⁶,which was 3 to 4 orders of magnitude higher than that for the other three metal ions,demonstrating an extremely high selectivity for Cu²⁺.The excellent Cu²⁺adsorption performance of PAN@Mg Al-LDH-Hpdc composite membrane can be attributed to that 1)the immobilization of LDHs addressed the issues of easy agglomeration and loss,which achieved the in-situ removal of heavy metals and avoided secondary pollution;2)the hierarchical structure increased the specific surface area and the exposed active sites of LDHs,which improved the adsorption efficiency;3)the 3,5-pyrazoledicarboxylate anions of LDH interlayer exhibited the strongest affinity for Cu²⁺,greatly enhancing the selective adsorption performance of the composite membrane towards Cu²⁺;4)the coordination between the-OH groups in LDHs laminate and Cu²⁺further promoted the adsorption efficiency.