Synthesis and exfoliation of Co2+-Fe3+ layered double hydroxides: an innovative topochemical approach

This paper describes a topochemical synthetic approach to Co2+-Fe3+ layered double hydroxides (LDHs). Micrometer-sized hexagonal platelets of brucite-like Co2/3Fe1/3(OH)2 were first prepared by a homogeneous precipitation of an aqueous solution of divalent cobalt and ferrous ions through hexamethylenetetramine (HMT) hydrolysis under a nitrogen gas atmosphere. A subsequent oxidative intercalation process, by the action of iodine (I2) in chloroform (CHCl3), transformed the precursory brucite-like Co2+-Fe2+ hydroxides into hydrotalcite-like Co2+-Fe3+ LDHs, in which the oxidization of Fe2+ into Fe3+ induced positive charges to the octahedral hydroxyl layers while anions (I-) were intercalated into the interlayer space. Co2+-Fe3+ LDHs inherited the high crystallinity and hexagonal platelet morphology from their brucite-like precursor due to the topotactic nature of the transformation, which was verified by abundant microscopic and spectroscopic characterizations. After a normal ion-exchange process, Co2+-Fe3+ LDHs accommodating perchlorate anions were exfoliated into unilamellar nanosheets in formamide by an ultrasonic treatment.     

二元类水滑石层板组成、结构与性能的理论研究

采用晶体学理论建立二元类水滑石(LDHs)微观结构模型与静电势能模型，将层板金属离子间距、层板电荷密度、层间阴离子间距等微观结构参数定量化，并将层间阴离子的静电势能表示成层板金属离子半径和物质的量之比、插层阴离子尺寸和电荷的函数。研究结果表明：LDHs层板金属离子间距应用离子紧密堆积来估算和孔径按阴离子平面六方点阵分布来计算是可行的；调变层板金属离子种类与物质的量之比影响层间阴离子的稳定性，势能计算值与文献报道的LDHs热稳定性次序一致。所以该模型可用于预测LDHs的微观结构参数以及热稳定性，为新型层状双羟基材料的定向合成提供思路。     

层状复合金属氢氧化物：结构、性质及其应用

本文评述层状复合金属氢氧化物(layered double hydroxides，LDHs)的化学及其应用。层状复合金属氢氧化物具有结合紧密的氢氧化物层和处于层间的阴离子，其比较突出的化学性质是可逆的阴离子交换和热分解性质。不过，因为受到氢氧化物层的局限，处于层间的阴离子表现出的特殊化学性质近年来也越来越受到重视。利用上述三个方面的性质，人们发掘了层状复合金属氢氧化物在材料(包括有机无机复合材料)制备、离子交换与有害阴离子脱除、异构体化学分离、化学反应控制、阻燃材料、活性分子储存与缓释、局部化学反应合成、催化剂及催化剂载体等方面的应用。     

层状双金属氢氧化物的剥离方法及其应用

层状双金属氢氧化物(LDHs)是由带结构正电荷的片层和层间阴离子有序组装而成的层状无机化合物, 近期其剥离研究受到关注. 剥离后的LDHs纳米片可被看做“无机高分子”, 具有纳米尺度的开放结构, 既可作为理论研究模型, 又可作为新型基元组装功能复合纳米结构或材料, 具有显著的应用潜力. 本文对LDHs的剥离方法、剥离产物的表征方法及其应用研究现状进行了综述, 并对今后的研究方向进行了展望.     

层状材料及催化

LDHs（1ayered double hydroxides）是一类结构可调的阴离子层状及插层结构功能材料,近些年来在催化领域得到了广泛的关注．本文综述了有关LDHs材料构筑原则的理论研究、组装方法及其在多相催化领域应用的最新进展．     

Recent progress on preparation and applications of layered double hydroxides

The properties of layered double hydroxides (LDHs), including the adjustability of cations in host layers, exchangeability of anions between layers, and tunability of the crystal structure, render them unique characteristics in preparation and applications. Relating to the structural characteristics of LDHs, this work analyzes the research status, advantages and disadvantages of the synthetic methods for LDHs, including hydrothermal, electrodeposition, co-precipitation and anion exchange methods. In addition, the application status and prospects are reviewed, such as photo/electrocatalysis, electrochemical energy storage, magnetic materials, pollutant adsorption, and other fields. Lastly, the critical issues and solutions in the developing process of LDHs are analyzed and proposed.     

层状双金属氢氧化物(LDH)基光催化剂在太阳能燃料生产领域的研究进展

半导体光催化剂吸收太阳光分解水制氢或还原CO2,实现了太阳能燃料生产,不仅可获取清洁、可再生、高热值的太阳能燃料,还能有效减少CO2的排放.层状双金属氢氧化物(LDHs)是一类基于水镁石结构的二维阴离子黏土矿物材料,具有独特的层状结构、主体层金属阳离子可调性、客体阴离子可交换、多维结构和可分层等优势,在CO2还原、光电催化水产氧及光解水制氢等领域研究广泛,有望成为新型半导体光催化材料.但单纯LDHs载流子迁移率低和电子空穴复合率高,在太阳辐射下的量子效率非常低.因此,研究人员采用缺陷控制、设计多维结构或偶联不同类型半导体构建异质结等方法,获得高能量转换效率的LDH基光催化剂.本文首先总结了传统光催化剂的优缺点及其能带分布,阐述了LDHs的六个主要方面特性,包括主体层板金属阳离子可调性、客体阴离子插层、热分解、记忆效应、多维结构特征及分层,进而提出LDH基光催化材料在增强反应物吸附活化、扩宽吸光范围、抑制光生载流子与空穴复合三个方面的改性策略.然后,分析了LDH光催化剂的光催化水解产氢反应机理,并从材料结构与性能的关联,概述LDH基复合光催化剂(金属硫化物LDH复合材料、金属氧化物LDH复合材料、石墨相氮化碳LDH复合材料)、三元LDH基光催化剂及混合金属氧化物光催化剂在水分解制氢领域的研究进展.最后,分析了LDH光催化还原CO2反应机理,归纳石墨相氮化碳复合LDH材料、MgAl-LDH基复合光催化剂、CuZn-LDH光催化剂及其它半导体系列LDH的结构特点和在还原CO2领域的研究进展.尽管LDH基光催化剂研究取得了一定的进展,但是对LDH的结构调控及其光催化机理仍需进一步探索,光催化活性位点、不同组分之间的协同作用和界面反应机理还有待进一步研究.未来LDH在光催化领域的应用可以微观尺度调控和宏观性能为导向设计,进一步研究不同组分的协同效应、界面反应及材料组成对物理化学性质的影响,不断完善LDH基光催化剂的理论系统和开发其应用潜能.     

Synthesis of Carbon Nanotubes:A New Solid-state Approach

In recent years, investigation on layered double hydroxide materials (LDHs, anionic clays) becomes an active field in layered materials research owing to their many important applications^[1-7]. Among the LDHs, hydrotalcite-like compounds (HTlcs) have attracted great attention due to their synthetic flexibility in preparing catalyst and ceramic precursors, and in tailor-making adsorbents, medicine stabilizers, and ion exchangers^[1,2]. In the structure of HTlcs, divalent and trivalent cations are located in the center of oxygen octahedron formed by six hydroxyl groups of the two-dimensional brucite-like sheets^[1,2]. To balance the extra charges carried by trivalent cations, anions have to be intercalated into the inter-brucite-like-sheet space (interlayer space) during the synthesis, which leads to the formation of a sandwich-like structure alternatively stacked in vertical direction of the sheets (c-axis), forming a 3D structure.     

Layer double hydroxides (LDHs)- based electrochemical and optical sensing assessments for quantification and identification of heavy metals in water and environment samples: A review of status and prospects

One of the most severe environmental problems is heavy metal contamination, putting the world's sustainability at risk. Much effort has been put into developing sensors that can be taken anywhere to detect the environmental effects of heavy metals. Sensitivity, selectivity, multiplexed detection ability, and mobility enhance significantly when nanoparticles and nanostructures are incorporated into sensors. LDHs (layered double hydroxides) have gotten much attention in analytical chemistry in recent years because of their benefits, including their large specific surface area, ease of synthesis, low cost, and high catalytic efficiency and biocompatibility. LDHs are often manufactured as nanomaterial composites or created with specialized three-dimensional structures depending on the application. However, in these settings, LDHs (as color indicators, extracting sorbents, and electrochemical sensing) are usually restricted. Upcoming signs of progress and development possibilities of LDHs in analytical chemistry are reviewed in this paper to assist overcome these problems. Furthermore, the approaches used in the design of LDHs, including structural aspects, are defined and assessed in preparation for future analytical applications. The latest advances in optical and electrochemical sensors to detect heavy metals are described in this review. The sorts and characteristics of LDHs will be explored first. We will then go into microelectrode (or nanoelectrode) arrays, nanoparticle-modified electrodes, and microfluidic optical and electrochemical sensing assays in detail. This paper also discusses design strategies for LDH-based nanostructured sensors and the advantages of using nanomaterials and nanostructures.     

二氟尼柳/水滑石插层组装结构、氢键及水合特性的分子动力学模拟

采用分子动力学方法模拟二氟尼柳插层水滑石(DIF/LDHs)的超分子结构, 研究复合材料主客体间形成的氢键以及水合膨胀特性.结果表明, 当水分子总数与DIF分子总数之比Nw≤3时, 层间距dc保持基本恒定, 约1.80 nm; 当Nw≥4时, 层间距逐渐增大, 且符合dc=1.2611Nw+13.63线性方程. 随着水分子个数增加, 水合能驻UH逐渐增大. 当Nw≤16时, 由于⊿UH<-41.84 kJ·mol-1, LDHs-DIF可以持续吸收水, 从而使材料层间距不断膨胀. 但当Nw≥24时, ⊿UH>-41.84 kJ·mol-1, 此时LDHs-DIF层间不能再进一步水合, 因此LDHs-DIF在水环境中膨胀具有一定的限度. 水滑石层间存在复杂的氢键网络. DIF/LDHs水合过程中, 水分子首先同步与层板和阴离子构成氢键; 当阴离子趋于饱和后, 水分子继续与层板形成氢键, 并逐步发生L-W型氢键取代L-A型氢键, 驱使阴离子向层间中央移动, 与层板发生隔离; 最后水分子在水滑石羟基表面形成有序结构化水层.     

Layered-double-hydroxide-modified electrodes: electroanalytical applications

Two-dimensional inorganic solids, such as layered double hydroxides (LDHs), also defined as anionic clays, have open structures and unique anion-exchange properties which make them very appropriate materials for the immobilization of anions and biomolecules that often bear an overall negative charge. This review aims to describe the important aspects and new developments of electrochemical sensors and biosensors based on LDHs, evidencing the research from our own laboratory and other groups. It is intended to provide an overview of the various types of chemically modified electrodes that have been developed with these 2D layered materials, along with the significant advances made over the last several years. In particular, we report the main methods used for the deposition of LDH films on different substrates, the conductive properties of these materials, the possibility to use them in the development of membranes for potentiometric anion analysis, the early analytical applications of chemically modified electrodes based on the ability of LDHs to preconcentrate redox-active anions and finally the most recent applications exploiting their electrocatalytic properties. Another promising application field of LDHs, when they are employed as host structures for enzymes, is biosensing, which is described considering glucose as an example.

Variation of benzyl anions in MgAl-layered double hydroxides: Fire and thermal properties in PMMA

Magnesium aluminum layered double hydroxides (MgAl-LDHs) intercalated with a range of benzyl anions were prepared using the coprecipitation method. The benzyl anions differ in functionality (i.e. carboxylate, sulfonate, and phosphonate) and presence or absence of an amino substituent. Various methods for preparing LDHs (i.e. ion exchange, coprecipitation and rehydration of the calcined LDH methods) have been compared with the MgAl-benzene phosphonate and their effect on fire and thermal properties was studied. After characterization, the MgAl-LDHs were melt-blended with poly(methyl methacrylate) (PMMA) at loadings of 3 and 10% by weight to prepare composites. Characterization of the LDHs and the PMMA composites was performed using FTIR, XRD, TGA, transmission electron microscopy (TEM) and cone calorimetry. FTIR and XRD analyses confirmed the presence of the charge balancing benzyl anions in the galleries of the MgAl-LDHs. Improvements in fire and thermal properties of the PMMA composites were observed. The cone calorimeter revealed that the addition of 10% MgAl-LDHs reduces the peak heat release rate by more than 30%.     

Preparation and Characterisation of Li-Al-glycine Layered Double Hydroxides (LDHs)-Polymer Nanocomposites

LiAl-Layered Double Hydroxides, containing glycinate anions, have been prepared using LiAlO₂. The glycine containing LDHs were then exfoliated in chloroform. Dispersions of approximately 0.03 g of the LDH in 15 ml of the solvent were possible. Nanocomposites using the exfoliated LiAl-glycine LDH and polyethyleneglycol were prepared by adding appropriate amounts of polymer to the LDH-chloroform dispersion. The clay-polymer nanocomposites were then characterised using powder X-ray diffraction. The thermal and mechanical properties of the composites are reported. The results suggest that composites containing individual (exfoliated) LDH layers were obtained with the mechanical and thermal properties of the composites noticeably superior to those of the parent polymer.     

Structure-property relationships of new polystyrene nanocomposites prepared from initiator-containing layered double hydroxides of zinc aluminum and magnesium aluminum

Polystyrene/layered double hydroxides (PS/LDHs) nanocomposites were prepared by free radical polymerization of styrene monomer in the presence of LDHs intercalated with 4,4′-azobis(4-cyanopentanoate) anions (LDH-ACPA). XRD and ATR-IR are used to confirm that the materials produced are layered and the presence of the azo-initiator anions in these LDHs. These LDHs were used successfully to polymerize styrene and both XRD and TEM images of the composites support the formation of a mixed exfoliated-intercalated nanocomposite for ZnAl-ACPA but a microcomposite for MgAl-ACPA. The magnesium-containing LDHs decreased the glass transition temperature (T_g) of the composites while ZnAl-ACPA did not affect T_g significantly. The T_g depression is related to enhanced polymer dynamics due to the extra free volume at the LDH additive-polymer interface. A reduction in the onset of thermal decomposition temperature was observed in PS/LDH compared to neat PS, likely due to the early decomposition of the LDH. The fire performance, as evaluated by the cone calorimeter, reveal that PS-ZnAl-ACPA shows enhanced fire properties compared to PS-MgAl-ACPA.     

The Effect of Interlayer Anion Grafting on Water Oxidation Electrocatalysis: A Comparative Study of Ni- and Co-Based Brucite-Type Layered Hydroxides,Layered Double Hydroxides and Hydroxynitrate Salts

The urge for carbon-neutral green energy conversion and storage technologies has invoked the resurgence of interest in applying brucite-type materials as low-cost oxygen evolution reaction (OER) electrocatalysts in basic media. Transition metal layered hydroxides belonging to the brucite-type structure family have been shown to display remarkable electrochemical activity. Recent studies on the earth-abundant Fe³⁺ containing mössbauerite and Fe³⁺ rich Co−Fe layered oxyhydroxide carbonates have suggested that grafted interlayer anions might play a key role in OER catalysis. To probe the effect of such interlayer anion grafting in brucite-like layered hydroxides, we report here a systematic study on the electrocatalytic performance of three distinct Ni and Co brucite-type layered structures, namely, (i) brucite-type M(OH)₂ without any interlayer anions, (ii) LDHs with free interlayer anions, and (iii) hydroxynitrate salts with grafted interlayer anions. The electrochemical results indeed show that grafting has an evident impact on the electronic structure and the observed OER activity. Ni- and Co-hydroxynitrate salts with grafted anions display notably earlier formations of the electrocatalytically active species. Particularly Co-hydroxynitrate salts exhibit lower overpotentials at 10 mA cm⁻² (η=0.34 V) and medium current densities of 100 mA cm⁻² (η=0.40 V) compared to the corresponding brucite-type hydroxides and LDH materials.     

Treatment of heavy metal ions in wastewater using layered double hydroxides: A review

Heavy metal ions are toxic, and their toxicities change with different valence states, charges, and radii. Among the methods used for heavy metal ion removal, adsorption is widely employed due to its low cost and simple operation. As natural anionic clays, layered double hydroxides (LDHs) have drawn considerable attention for their use in the removal of anionic pollutants (such as heavy metal anions) due to their high removal efficiency and environmental friendliness. This article reviews the effects of the charge, type, and radius of the cations in the laminates of LDHs and the anions in the LDH interlayers, as well as the charge and radius of the heavy metals and the conditions (such as pH, coexisting ions, and temperature) on removing heavy metal ions with LDHs. The removal mechanisms have also been discussed. LDHs are hugely promising as an application for removing heavy metal ions that exist in different ionic forms by controlling the type and condition of LDHs.     

Preparation and characterization of composites that contain small carbon nano-onions and conducting polyaniline

Small multilayer fullerenes, also known as carbon nano-onions (CNOs; 5-6 nm in diameter, 6-8 shells), show higher reactivity than other larger carbon nanostructures. Here we report the first example of an in situ polymerization of aniline on phenyleneamine-terminated CNO surfaces. The green, protonated, conducting emeraldine polyaniline (PANI) was directly synthesized on the surface of the CNO. The functionalized and soluble CNO/PANI composites were characterized by TEM, SEM, DSC, Raman, and infrared spectroscopy. The electrochemical properties of the conducting CNO/PANI films were also investigated. In comparison with pristine CNOs, functionalized carbon nanostructures show dramatically improved solubility in protic solvents, thus enabling their easy processing for coatings, nanocomposites, and biomedical applications.     

Synthesis and UV absorption properties of 2,3-dihydroxynaphthalene-6-sulfonate anion-intercalated Zn-Al layered double hydroxides

An organic UV absorbent has been intercalated into a layered double hydroxide (LDH) host by ion exchange of a Zn-Al-LDH-nitrate precursor with a solution of 2,3-dihydroxynaphthalene-6-sulfonic acid (DNSA) sodium salt in water. After intercalation of the UV absorbent, the powder X-ray diffraction (XRD) pattern shows that the interlayer distance in the LDHs increases from 0.90 to 1.59 nm. The possible structure is that the interlayer DNSA anions arrange in a monolayer and in a perpendicular orientation toward the hydroxide layers. Infrared spectra and TG-DTA curves reveal the presence of a complex system of supramolecular host-guest interactions between layers. The thermal stability of the intercalated UV absorbent was investigated by TG-DTA and it was found that this material is more stable than the original organic UV absorbent at high temperature, showing that the thermostability is markedly enhanced after intercalation into the LDH host. The UV absorbent-intercalated LDHs exhibit excellent UV photostability in polypropylene composites.     

乙二胺四乙酸柱撑镁锌铝三元水滑石的制备与表征

采用共沉淀法合成了Mg2ZnAl-CO3水滑石;以其为前驱体,利用离子交换法进行插层组装得到Mg2ZnAl-EDTA三元柱撑水滑石;采用X射线衍射仪和傅立叶变换红外光谱仪对产物进行了表征.结果表明,合成的Mg2ZnAl-CO3水滑石纯度高、晶型良好,其层间CO32-可被乙二胺四乙酸(EDTA)阴离子取代形成Mg2ZnAl-EDTA三元柱撑水滑石;柱撑水滑石的层间距离明显增加,EDTA阴离子在层间倾斜排列.     

Conducting polymer nanostructures and their application in biosensors

Nowadays, functionalized conducting polymer nanomaterials have been received great attention in nanoscience and nanotechnology because of their large surface area. This article reviews various methods for synthesis of conducting polymer nanostructures and their applications in sensing materials, focusing on hard-template, soft-template and other methods and the formation mechanism of conducting polymer nanostructures by these methods. Conducting polymer nanostructures, such as nanotubes, nanowires, and nanoparticles, as sensing platforms for various applications are also summarized.