Layered Double Hydroxide Nanosheets with Multiple Vacancies Obtained by Dry Exfoliation as Highly Efficient Oxygen Evolution Electrocatalysts

Layered double hydroxides (LDHs) with two-dimensional lamellar structures show excellent electrocatalytic properties. However, the catalytic activity of LDHs needs to be further improved as the large lateral size and thickness of the bulk material limit the number of exposed active sites. However, the development of efficient strategies to exfoliate bulk LDHs into stable monolayer LDH nanosheets with more exposed active sites is very challenging. On the other hand, the intrinsic activity of monolayer LDH nanosheets can be tuned by surface engineering. Herein, we have exfoliated bulk CoFe LDHs into ultrathin LDH nanosheets through Ar plasma etching, which also resulted in the formation of multiple vacancies (including O, Co, and Fe vacancies) in the ultrathin 2D nanosheets. Owing to their ultrathin 2D structure, the LDH nanosheets expose a greater number of active sites, and the multiple vacancies significantly improve the intrinsic activity in the oxygen evolution reaction (OER).     

Water-swellable MgAl-LDH (layered double hydroxide) hybrids: synthesis, characterization, and film preparation

LDH hybrids were synthesized from Cl (-)MgAl-LDHs by anion exchange with short-chain alkyl carboxylate intercalants: C n H 2 n+1 COO (-) ( n = 0-3). Among them, LDH3 (LDH with Mg/Al = 3) hybrids containing acetate ( n = 1) and propionate ( n = 2) exhibited swelling behavior in water. The action of water on acetate-LDH3 (AcO-LDH3) and propionate-LDH3 (PrO-LDH3) led to semitransparent suspensions via a viscous gel state. From the X-ray diffraction profiles of the gels, only a broad feature was observed by the loss of the sharp reflections. The reflections reappeared for the films obtained by drying the gel, indicating the restacking of the LDH nanosheets into the original stacked structure. Observation using atomic force microscopy revealed delaminated nanosheets with a thickness of 1.1-1.5 nm with the same morphological features as the starting LDHs. XRD measurement and AFM observation supported the formation of unilamellar LDH sheets. Semitransparent self-standing LDH films were obtained by peeling off the films formed on a PE (polyethylene) substrate by drying the colloidal suspension thereon. The thickness of the obtained flexible films ranged from 10 to 25 microm, and they could be anion exchanged with inorganic and organic anions in the film state.     

Mg-Ni-Al-EDTA柱撑LDHs层状材料的水热合成、结构及性能

以Mg(NO3)2.6H2O、Ni(NO3)2.6H2O、Al(NO3)3.9H2O和[CH2N(CH2COOH)2]2为原料,采用水热合成法,合成了Mg-Ni-Al三元EDTA柱撑LDHs层状材料。采用ICP、元素分析仪、XRD、FTIR、TG-DSC、SEM等手段对样品进行了表征。探讨了pH值、反应温度、反应时间和原料配比对EDTA柱撑LDHs材料合成的影响。结果表明,在pH=8、反应温度控制在140℃、反应时间为24 h时,可以合成出结构规整、晶形良好、各层间排列紧密有序的含不同比例金属阳离子的EDTA柱撑LDHs材料。Mg-Ni-Al三元EDTA柱撑LDHs层状材料通过层间EDTA对Co2+的螯合作用,可以在较短时间内吸附溶液中的Co2+,去除率在97%以上。pH值、吸附时间、吸附温度、固体投加量及初始Co2+浓度对去除率均有不同程度的影响。     

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%.     

Layer-by-layer assembly and spontaneous flocculation of oppositely charged oxide and hydroxide nanosheets into inorganic sandwich layered materials

Exfoliated oxide nanosheets such as Ti0.91O2 and Ca2Nb3O10 and layered double hydroxide (LDH) nanosheets of Mg2/3Al1/3(OH)2 were restacked into inorganic sandwich layered materials. Sequential adsorption of these oppositely charged nanosheets from their colloidal suspensions yielded multilayer ultrathin films while their simple mixing produced lamellar flocculates. Eliminating carbonate ions from the reaction system was found to be essential for successfully achieving the sandwich structures. The flocculated materials as well as the films were characterized by atomic force microscopy (AFM), UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and chemical analysis, which all supported the formation of the ordered sandwich structures. AFM observations revealed alternate dense tiling of LDH nanosheets and oxide nanosheets onto a substrate surface. UV-visible absorption spectra exhibited progressive enhancement of optical density due to oxide nanosheets as a function of deposition cycles, providing strong evidence for regular growth of multilayer films. The combinations of Mg2/3Al1/3(OH)2/Ti0.91O2 and Mg2/3Al1/3(OH)2/Ca2Nb3O10 produced XRD Bragg peaks having multilayer spacings of 1.2 and 2.0 nm, respectively. These basal spacing values are compatible with the sum of thickness of LDH nanosheets and corresponding oxide nanosheets. TEM images of flocculated samples displayed lamellar features with two different constituent layers appearing alternately.     

High-yield synthesis of ultrathin silica-based nanosheets and their superior catalytic activity in H2O2 decomposition

Ultrathin silica-based nanosheets with a thickness of ～1 nm are fabricated in a low-cost and high-yield system excluding alkali metal ions. Unlike the Na-zeolites with a long capillary and microporous structure, the ultrathin nanosheets exhibit highly effective surface and active sites. As a result, the 0.4 wt% Pt-loaded nanosheets exhibit superior catalytic activity in H(2)O(2) decomposition with an O(2) evolution rate of 342.5 mL g(-1) min(-1) in a clean aqueous system.     

规整Mn-Zn-Mg-Al-CO3四元LDHs层状材料的水热合成、结构及性能

以Mn(NO3)2.6H2O、Zn(NO3)2.6H2O、Mg(NO3)2.6H2O和Al(NO3)3.9H2O为原料,采用水热合成法,一步合成了Mn-Zn-Mg-Al-CO3四元LDHs层状材料。采用ICP、元素分析仪、XRD、FTIR、TG-DSC、SEM、低温氮吸附-脱附等对样品进行了表征。探讨了pH值、反应温度、反应时间和原料配比对Mn-Zn-Mg-Al-CO3四元LDHs层状材料合成的影响。结果表明,在pH=10、反应温度控制在140℃、反应时间为24 h的条件下,可以合成出结构规整、晶形良好、各层间排列紧密有序的含不同比例金属阳离子的Mn-Zn-Mg-Al-CO3四元LDHs层状材料。其吸附等温线符合V型吸附,H3滞后环,晶体内层间存在2 nm以上的孔,晶体内部结构的有序性高,层间碳酸根离子的排列整齐,通道内孔密度大、孔径小、比表面积大。     

单层类水滑石纳米片的可控合成及规模生产展望

水滑石(LDHs)是一种阴离子黏土材料,由于其主体层板厚度的可调性,使其在光/电催化、电池、超级电容器、传感器以及生物医药等领域都具有广泛应用。降低层厚至单层可使材料的物理化学性质发生根本改变,从而优化催化性能。近期研究表明,利用自上而下,自下而上的方法,可以实现单层LDHs类材料的合成,但是受限于产量(g级)以及成本设备等问题,目前规模化制备高质量单层LDHs类材料还没有工业案例。成核晶化隔离法是目前唯一规模化合成纳米LDHs的工业化方法,具有成本低,产量可吨级放大等优点。本综述从合成方法、表征手段、应用三个角度讨论了单层及超薄LDHs的精准调控,详细论述了近期关于单层及超薄LDHs合成突破以及LDHs的规模化生产进展,并对其性能进行了总结,为后续设计高性能单层LDHs提供思路。     

Ordered blue luminescent ultrathin films by the effective coassembly of tris(8-hydroxyquinolate-5-sulfonate)aluminum and polyanions with layered double hydroxides

This article reports a novel method to assemble a small anion with exfoliated Mg-Al-layered double hydroxide (LDH) nanosheets into ordered ultrathin films (UTFs) by employing the layer-by-layer assembly technique. The premixing solution of tris(8-hydroxyquinolate-5-sulfonate)aluminum(III) (AQS(3-)) with three kinds of polyanions-poly(acrylic acid), ((C(3)H(4)O(2))(n), PAA), poly(styrene 4-sulfonate) ([CH(2)CH(C(6)H(4))SO(3)](m), PSS), and poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylene vinylene] (C(12)H(13)O(5)S)(n), PPV)-has been used as building blocks to assemble alternatively with LDH nanosheets. The UV-vis absorption and fluorescence spectroscopy of (AQS-polyanion/LDH)(n) UTFs presents stepwise growth upon increasing deposited cycles in comparison with the (AQS/LDH)(n) film under the same experimental process. (AQS-PPV/LDH)(n) UTF displays complex fluorescence originating from AQS and PPV. The (AQS/LDH)(n) and (AQS-polyanion/LDH)(n) UTFs exhibit higher blue-polarized photoemission character with a luminescence anisotropy (r) of ca. 0.12-0.20 and a longer fluorescence lifetime than that of the Na(3)AQS film with r = 0.04. X-ray diffraction, scanning electron microscopy, and atomic force microscopy demonstrated that the UTFs were orderly periodically layered structures with a thickness of ca. 3.0 nm per bilayer. Therefore, this work gives a feasible method for immobilizing small anions into the gallery of LDHs.     

Unusual crystallization behaviors of anatase nanocrystallites from a molecularly thin titania nanosheet and its stacked forms: increase in nucleation temperature and oriented growth

Crystallization behaviors of anatase nanocrystallites from an ultrathin two-dimensional reactant composed of exfoliated titania nanosheets have been studied by monitoring the heating process of their well-organized films, with which the film thickness can be controlled from a molecularly thin monolayer to a stacked multilayer structure with a stepwise increment of approximately 1 nm. The heated products were identified by means of total reflection fluorescence X-ray absorption near-edge structure analysis and in-plane X-ray diffraction measurements using a synchrotron radiation source. The films composed of five or more layers of stacked nanosheets were transformed into anatase at 400-500 degrees C, which is a normal crystallization temperature of anatase from bulk reactants. As the film became thinner by decreasing the number of nanosheet layers to five or less, the crystallization temperature was found to increase and finally reached 800 degrees C for the monolayer film. Interestingly, preferential growth of anatase along the c-axis was strongly promoted for these ultrathin films. These unusual behaviors may be understood in terms of crystallization from the two-dimensional system of scarcely distributed reactants. The titania nanosheet crystallite is much thinner than the unit cell dimensions of anatase, and therefore, extensive atomic diffusion is required for the transformation particularly for the ultrathin films with a critical number (2-3) of stacked nanosheet layers. There is some structural similarity between anatase and titania nanosheet, which may account for the oriented growth of anatase nanocrystallites.     

Heterogeneous ultrathin films fabricated by alternate assembly of exfoliated layered double hydroxides and polyanion

Transparent heterogeneous ultrathin films of exfoliated layered double hydroxide (LDHs) nanosheets, fabricated alternately with polyanion, have been obtained via a layer-by-layer electrostatic self-assembly which yields a series of novel LDH films with potential multifunctionality.     

基于插层化学的单晶氧化钨纳米片的制备、表征与形成机制

结合插层化学与湿化学方法的优点, 建立了一种高比表面积、大径厚比、易分散的二维氧化钨(WO3)纳米片单晶的制备新方法. 微米级WO3与Bi2O3在800 ℃通过固相反应生成层状化合物Bi2W2O9; 所得到的Bi2W2O9经盐酸选择性溶出[Bi2O2]层后得到质子化形式的H2W2O7·xH2O相. 以H2W2O7·xH2O为钨源, 以辛胺插层所得无机-有机混杂纳米带为前驱物, 经硝酸氧化除去前驱物中的有机组分后得到正交相WO3·H2O纳米片; 将所得到的WO3·H2O纳米片在250~ 450 ℃和空气气氛中热处理2~5 h(升温速率为2 ℃/min), 得到单斜相WO3单晶纳米片. TEM与SEM分析结果表明, 单晶WO3·H2O与WO3纳米片的形貌相似, 其大小为(200~500) nm×(200~500) nm, 厚度为10~30 nm; 所得WO3·H2O与WO3纳米片单晶的厚度方向分别为[010]和[001]. N2吸附结果表明, WO3·H2O与WO3纳米片的比表面积分别可达到250与180 m2/g.     

Facile Synthesis of Ultrathin Lepidocrocite Nanosheets from Layered Precursors

Ultrathin two‐dimensional nanosheets have been widely studied because of their peculiar properties and promising applications. As a typical layered material, successful exfoliation of freestanding ultrathin lepidocrocite (γ‐FeOOH) nanosheets from the bulk material has not been reported to date. Herein, we report a facile synthetic route to prepare ultrathin lepidocrocite nanosheets with a thickness of approximately 2–3 nm from FeO_x–propanediol layered precursors through weakening of the hydrogen bonds during the crystallization process. The ultrathin morphology and single‐crystal structure of the nanosheets were confirmed by transmission electron microscopy, X‐ray diffraction, and atomic force microscopy. The formation process of these nanosheets demonstrated simultaneous exfoliation and crystallization of lepidocrocite in basic aqueous solution. The obtained ultrathin nanosheets exhibited a much lower Néel temperature (18.3 K) than bulk lepidocrocite and weak ferromagnetic behavior below this temperature.     

Ordered Ultrathin Films of Perylene Anions and Layered Double Hydroxide Using Exfoliation‐coassembly Method

Recently, the fabrication of ordered luminescent ultrathin film (UTF) materials based on layered double hydroxides (LDHs) has received much attention. However, how to obtain these UTFs assembled by small anions and LDHs nanosheets is still a challenge. Herein, perylene 3,4,9,10‐tetracarboxylate (PTCB) was firstly chosen as the anion with the aim to obtain PTCB/LDH UTFs, which cannot be formed based on typical layer‐by‐layer (LBL) method. Then, the polymer anions (such as poly 4‐styrene sulfonate (PSS) and poly vinyl sulfonate (PVS)) were further chosen as the co‐assembled units with PTCB, which can act as carriers to assemble with LDH nanosheets. The as‐obtained PTCB@PSS/LDH and PTCB@PVS/LDH UTFs present long range ordered structures confirmed by powder X‐ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Moreover, these two UTF systems show polarized luminescence with the emissive anisotropy of ca. 0.62 and 0.73. Therefore, this work presents an exfoliation‐coassembly way to develop LDH‐based ordered luminescent films, which may benefit their future optical display applications.     

Ultrathin hollow nanoshells of manganese oxide

Novel hollow nanoshells of Mn(2)O(3) with controllable ultrathin shell thickness have been fabricated through layer-by-layer assembly of exfoliated MnO(2) nanosheets and polyelectrolytes on polymer bead templates, followed by removal of the polymer cores via calcination.     

Synthesis and characterization of a strong-fluorescent Eu-containing hydrotalcite-like compound

A novel strong-fluorescent Eu-containing hydrotalcite-like compound (Eu-HTlc) was synthesized using the coprecipitation method, in which aluminum(III) was partially substituted by Europium (III) in the hydrotalcite-like layers, and thenoyltrifluoroacetone, 1,10-phenanthroline were dispersed into the anions in the interlayer region. The sample was characterized by XRD, XPS, FT-IR, ICP, TG-DSC, TEM and fluorescence spectra, and its composition and structure were determined. The results indicated that the sample exhibited a characteristic red light (614 nm). The fluorescent lifetime and fluorescence quantum yield of Eu-HTlc were measured to be respectively 893 μs and 66.44%, higher than those of Eu(III)-thenoyltrifluoroacetone-1,10-phenanthroline complex [Eu(TTA)₃phen]. The result of TG-DSC measurement showed the enhanced thermal stability of Eu-HTlc compared with that of MgAl-LDHs and Eu(TTA)₃phen. With excellent photoluminescent property and thermal stability, low contents of rare earth ions and ligands, the Eu-HTlc may become one of the novel fluorescent materials with potential applications.     

Exfoliation of Metal–Organic Frameworks to Give 2D MOF Nanosheets for the Electrocatalytic Oxygen Evolution Reaction

The structure and properties of materials are determined by a diverse range of chemical bond formation and breaking mechanisms, which greatly motivates the development of selectively controlling the chemical bonds in order to achieve materials with specific characteristics. Here, an orientational intervening bond-breaking strategy is demonstrated for synthesizing ultrathin metal–organic framework (MOF) nanosheets through balancing the process of thermal decomposition and liquid nitrogen exfoliation. In such approach, proper thermal treatment can weaken the interlayer bond while maintaining the stability of the intralayer bond in the layered MOFs. And the following liquid nitrogen treatment results in significant deformation and stress in the layered MOFs’ structure due to the instant temperature drop and drastic expansion of liquid N₂, leading to the curling, detachment, and separation of the MOF layers. The produced MOF nanosheets with five cycles of treatment are primarily composed of nanosheets that are less than 10 nm in thickness. The MOF nanosheets exhibit enhanced catalytic performance in oxygen evolution reactions owing to the ultrathin thickness without capping agents which provide improved charge transfer efficiency and dense exposed active sites. This strategy underscores the significance of orientational intervention in chemical bonds to engineer innovative materials.     

Immobilization and catalytic properties of candida lipolytic lipase on surface of organic intercalated and modified MgAl-LDHs

In this study, MgAl-LDHs (layered double hydroxides) intercalated with sodium dodecyl sulfate and outside surface modified with (3-aminopropyl)triethoxysilane (KH550) were prepared. The existence of organic part in LDHs improved immobilization efficiency and activity recovery of candida lipolytic lipase loaded. Also the positive charge in framework of LDHs was found to be beneficial to the enzyme immobilization. An immobilization efficiency of 56.4% and an activity recovery over 69.2% of the enzyme were obtained after it was loaded on the intercalated and modified LDHs, and catalytic activity of the immobilization can be kept at least five times. Moreover, the immobilized enzyme was found to have higher temperature resistance, wider pH value and better thermostability in reactive activity.     

二维介孔超薄Cd0.5Zn0.5S纳米片:形成机制及光催化分解水制氢性能

太阳光驱动的光催化分解水产氢是一种绿色制氢技术,并以氢为载体可实现太阳能向化学能的转化.目前开发高效、稳定的可见光催化剂仍是本领域的研究热点.在各类光催化材料中,Cd0.5Zn0.5S固溶体比TiO2及g-C3N4具有更优异的光催化产氢活性,但它一般为团聚了的纳米颗粒或纳米微球,表面积小,比表面反应迟缓,从而限制了其实际应用.通常,超薄多孔二维结构光催化剂具有高比表面积,能够为反应物分子与催化剂之间提供大量接触界面并促进传质,此外,特定晶面暴露赋予了其大量不饱和配位表面原子,使反应物分子更容易在催化剂表面吸附活化,提升表面催化反应动力学.本文首先采用乙二胺与水的混合溶液制备了无机有机杂化的硫化锌-乙二胺(记为:ZnS(en)0.5).随后,分别以ZnS(en)0.5为硬模板、以乙二醇为反应介质、氯化镉为镉源,通过溶剂热阳离子交换得到了无机有机杂化的Cd0.5Zn0.5S(en)x中间产物.最后,将Cd0.5Zn0.5S(en)x在纯水中进行水热反应脱除晶格内乙二胺分子得到了2D介孔超薄Cd0.5Zn0.5S纳米片.TEM测试发现,纳米片表面存在大量孔洞,其主要源于Cd0.5Zn0.5S(en)x的相变过程及其晶格内乙二胺分子的逃逸导致的晶格畸变.AFM观察结果表明,最终产物Cd0.5Zn0.5S纳米片厚度约为1.5 nm;其比表面积可达63.5 m2/g,几乎是相应纳米颗粒的两倍.以三乙醇胺(TEOA)为牺牲剂时,Cd0.5Zn0.5S纳米片的产氢速率达到19.1 mmol·h^?1·g^?1,是相应纳米颗粒的两倍多.即使在纯水中,Cd0.5Zn0.5S纳米片产氢速率仍可达到1395μmol·h?^1·g^?1,超过了目前所报道的未加修饰的光催化剂的活性.其优异的活性源于其独特的结构优势,包括载流子迁移距离的缩短、表面不饱合原子及比表面积的增大.但在纯水中其严重的光腐蚀仍然亟待克服.此外,为进一步增强其活性,通过机械复合的方法得到了NiCo2S4/Cd0.5Zn0.5S二元复合光催化剂,其在TEOA为牺牲剂时制氢速率可达62.2 mmol·h^?1·g^?1,在纯水制氢速率达到2436μmol·h^?1·g^?1.电化学、UPS及EPR分析表明,NiCo2S4与Cd0.5Zn0.5S纳米片间形成了肖特基接触,进一步促进了载流子分离能力,提高了复合物的产氢活性.以本工作为基础,还可制备其他高活性的CdZnS-基功能光催化材料用于太阳能转化或其他领域.     

Ultrathin FeSe2 Nanosheets: Controlled Synthesis and Application as a Heterogeneous Catalyst in Dye‐Sensitized Solar Cells

Two‐dimensional (2D) semiconducting nanosheets have emerged as an important field of materials, owing to their unique properties and potential applications in areas ranging from electronics to catalysis. However, the controlled synthesis of ultrathin 2D nanosheets remains a great challenge, due to the lack of an intrinsic driving force for anisotropic growth. High‐quality ultrathin 2D FeSe₂ nanosheets with average thickness below 7 nm have been synthesized on large scale by a facile solution method, and a formation mechanism has been proposed. Due to their favorable structural features, the as‐synthesized ultrathin FeSe₂ nanosheets exhibit excellent electrocatalytic activity for the reduction of triiodide to iodide and low charge‐transfer resistance at the electrolyte–electrode interface in dye‐sensitized solar cells (DSSCs). The DSSCs with FeSe₂ nanosheets as counter electrode material achieve a high power conversion efficiency of 7.53 % under a simulated solar illumination of 100 mW cm^?2 (AM 1.5), which is comparable with that of Pt‐based devices (7.47 %).