Layered Double Hydroxides/Carbon Nanotubes Hybrid Composites: Preparation, Structure And Their Properties

Since discovered in 1991, carbon nanotube (CNTs), by virtue of its unique molecular geometry structure and physical and chemical properties, have been paid much attention in the field of CNTs-based hybrid materials. Of its property, high special surface area, high ratio of length to diameter and high thermal stability have made great contribution in exploration of CNTs as support, especially catalyst support, which have become a rapidly expanding research field. CNTs have been widely used in electrochemistry including electrode and sensor because of good conductivity and biocompatibility. It was reported that CNTs own excellent mechanical properties, and its high Young's modulus and strong strength is 1 TPa and 20 GPa, respectively, which is nearly 5 times and 100 times stronger than these of steel. Therefore, CNTs have been used as enhanced materials. However, CNTs are inert and are not susceptible to dissolve common reagents or disperse in matrix. This disadvantage becomes the troublesome bottleneck in the preparesion of CNTs-based materials with stable and uniform structure. Therefore, the modification of CNTs surface by non-covalent bond or covalent bond is carried out in the international carbon nanotube science research, which is not only improve the distribution in organic and inorganic reagent, but also is favorable to reaction with other materials.Layered double hydroxides (LDHs), known as a family of synthetic anionic clays, are a class of brucite-like layered inorganic materials. Within the layers, the cations are uniformly distributed on an atomic level without segregation of "lakes" of separate cations. Due to the characteristics of tunable compositions and exchangeable anions, LDH have been widely used in various fields like catalysts or catalyst support, adsorbent, electrochemistry materials, drug delivery materials, and retardant. Calcination of LDHs may lead to the collapse of LDHs layer and conversion of LDHs to metal oxides and/or spinel, which have been exploited in catalyst and adsorption. However, LDHs nanoparticles aggregate to each other to some extent, and the calcinated product can occur second reunion during the process of calcination. These phenomenon lead to the poor distribution of nanoparticles, low special surface area and the decrease of active site amount, which inhibits the application of LDHs as function materials. In order to overcome this shortage and improve distribution, we utilized different methods to assemble LDHs nanoparticles on the surface of CNTs to prepare a serial of LDH/CNTs composites with excellent hybrid structure. The high distribution of LDHs facilitates the increase of specific surface area and the exposure of more active site. Furthermore, the remarkable structure and property of CNTs are advantageous for the property of composite.Nanostructured Ni-Al layered double hydroxide/carbon nanotubes (NiAl-LDH/CNTs) composites, where NiAl-LDH nanocrystallites could highly disperse on the surface of CNTs matrix through the interfacial electrostatic interaction between the positively-charged layers of NiAl-LDH and the negatively-charged functional groups of modified CNTs, have been successfully synthesized by a simple one-pot coprecipitation method. The results reveal that the surface coverage of NiAl-LDH nanoparticles onto CNTs could be tuned easily by changing the mass ratios of CNTs to NiAl-LDH. The strong electrostatic interaction between NiAl-LDH and CNTs gives rise to both the weakened affinity between the layers and the interlayer species of NiAl-LDH and the high positive shift of binding energy for metal and oxygen elements (around 1.8 eV) in NiAl-LDH/CNT composite. Further the electrode modified by NiAl-LDH/CNTs nanocomposite exhibits eight times higher electrocatalytic activity for glucose electrooxidation than those modified by either pristine NiAl-LDH or CNTs, which is attributable to the fact that CNTs can efficiently promote the charge transport between the active Ni centers and the electrode and the high affinity of NiAl-LDH to CNTs matrix in composite favors the stabilization of electro-active NiAl-LDH nanoparticles on the surface of CNTs at the operating potentials of electrode. In addition, porous network-like microstructure of composite on the electrode favorites the diffusion of reactant molecules. On the one hand, we chose poly(sodium styrenesulfonate) (PSS) to modify the surface of CNTs and prepare the ZnAl-LDH/CNTs nanocomposites. The negatively charged polyelectrolyte PSS can graft the surface of CNTs throughπ-πinteraction, thus providing a homogeneous distribution of negative charges on CNTs, which is beneficial for the homogeneous adsorption of metal ions through electrostatic interactions and the nucleation and growth of ZnAl-LDH nanoparticles. The results reveal that the composite with well distribution and stable structure can be obtained by this method. On the other hand, a facile and effective strategy has been utilized for assembling hybrid ZnAl-layered double hydroxide/carbon nanotubes (ZnAl-LDH-cy-CNTs) nanocomposites in the presence of L-cysteine molecules. L-cysteine can exist in the form of -NH3+ and -COO- in the solution. The electrostatic interaction between -NH3+ of L-cysteine and-COO- on the modified CNTs contributes greatly to the immobilization of L-cysteine onto the surface of CNTs. Furthermore,-COO- groups of immobilized L-cysteine can selectively coordinate and/or electrostatically interact with Zn2+ cations in the solution. Subsequently, with the successive titration of alkali, ZnAl-LDH nucleates in situ onto CNTs through coprecipitation process. The results indicate that L-cysteine as bridging linker plays a key role for enhancing both adhesion and dispersion of ZnAl-LDH nanocrystallites onto the surface of CNTs matrix through the interfacial interaction, and effectively inhibits the in situ growth of ZnAl-LDH crystallites, thus resulting in remarkably reduced ZnAl-LDH crystallite sizes. The Eu(Ⅲ) fluorescence quenching in intercalated-Eu(Ⅲ) complex ZnAl-LDH(EY)-cy-CNTs nanocomposite can occur because of the interaction between ZnAl-LDH crystallites and CNTs matrix. Furthermore, it is found that as-assembled hybrid ZnAl-LDH/CNTs nanocomposites exhibit excellent performance for photodegradation of methyl orange molecules under UV irradiation, which is closely related to the unique hybrid nanostructure and composition of composites.We researched the thermal catalytic property of CoAl-MMO-cy-CNTs compsite towards ammonium perchlorate decomposition, which was prepared by the calcination of CoAl-LDH-cy-CNTs composite precursor at 500℃under N2 atmosphere. After calcinated at 500℃, CoAl-LDH is transformed to CoO and CoAl2O4 mixture. The distribution of CoAl-LDH on the surface of CNTs, which influences the composition of calcainated product, can be controlled by the dosage of L-cysteine in the synthesis process of CoAl-LDH. CoAl-MMO-cy-CNTs composite exhibited excellent cooperative performance of CoAl-MMO and CNTs for the decomposition of ammonium perchlorate, the decrease of decomposition temperature to 271.3℃, and the increase of decomposition rate to 13.0 mg/min.