A Generalized Strategy for the Synthesis of Large‐Size Ultrathin Two‐Dimensional Metal Oxide Nanosheets

Two‐dimensional (2D) nanomaterials show unique electrical, mechanical, and catalytic performance owing to their ultrahigh surface‐to‐volume ratio and quantum confinement effects. However, ways to simply synthesize 2D metal oxide nanosheets through a general and facile method is still a big challenge. Herein, we report a generalized and facile strategy to synthesize large‐size ultrathin 2D metal oxide nanosheets by using graphene oxide (GO) as a template in a wet‐chemical system. Notably, the novel strategy mainly relies on accurately controlling the balance between heterogeneous growth and nucleation of metal oxides on the surface of GO, which is independent on the individual character of the metal elements. Therefore, ultrathin nanosheets of various metal oxides, including those from both main‐group and transition elements, can be synthesized with large size. The ultrathin 2D metal oxide nanosheets also show controllable thickness and unique surface chemical state.     

A Surfactant‐Free and Scalable General Strategy for Synthesizing Ultrathin Two‐Dimensional Metal–Organic Framework Nanosheets for the Oxygen Evolution Reaction

Metal–organic framework (MOFs) two‐dimensional (2D) nanosheets have many coordinatively unsaturated metal sites that act as active centres for catalysis. To date, limited numbers of 2D MOFs nanosheets can be obtained through top‐down or bottom‐up synthesis strategies. Herein, we report a 2D oxide sacrifice approach (2dOSA) to facilely synthesize ultrathin MOF‐74 and BTC MOF nanosheets with a flexible combination of metal sites, which cannot be obtained through the delamination of their bulk counterparts (top‐down) or the conventional solvothermal method (bottom‐up). The ultrathin iron–cobalt MOF‐74 nanosheets prepared are only 2.6 nm thick. The sample enriched with surface coordinatively unsaturated metal sites, exhibits a significantly higher oxygen evolution reaction reactivity than bulk FeCo MOF‐74 particles and the state‐of‐the‐art MOF catalyst. It is believed that this 2dOSA could provide a new and simple way to synthesize various ultrathin MOF nanosheets for wide applications.     

Generalized Low‐Temperature Fabrication of Scalable Multi‐Type Two‐Dimensional Nanosheets with a Green Soft Template

Two‐dimensional nanosheets with high specific surface areas and fascinating physical and chemical properties have attracted tremendous interests because of their promising potentials in both fundamental research and practical applications. However, the problem of developing a universal strategy with a facile and cost‐effective synthesis process for multi‐type ultrathin 2 D nanostructures remains unresolved. Herein, we report a generalized low‐temperature fabrication of scalable multi‐type 2 D nanosheets including metal hydroxides (such as Ni(OH)₂, Co(OH)₂, Cd(OH)₂, and Mg(OH)₂), metal oxides (such as ZnO and Mn₃O₄), and layered mixed transition‐metal hydroxides (Ni‐Co LDH, Ni‐Fe LDH, Co‐Fe LDH, and Ni‐Co‐Fe layered ternary hydroxides) through the rational employment of a green soft‐template. The synthesized crystalline inorganic nanosheets possess confined thickness, resulting in ultrahigh surface atom ratios and chemically reactive facets. Upon evaluation as electrode materials for pseudocapacitors, the Ni‐Co LDH nanosheets exhibit a high specific capacitance of 1087 F g^?1 at a current density of 1 A g^?1, and excellent stability, with 103 % retention after 500 cycles. This strategy is facile and scalable for the production of high‐quality ultrathin crystalline inorganic nanosheets, with the possibility of extension to the preparation of other complex nanosheets.     

Ultrathin Hexagonal Hybrid Nanosheets Synthesized by Graphene Oxide‐Assisted Exfoliation of β‐Co(OH)2 Mesocrystals

In the present study, we report the synthesis of a high‐quality, single‐crystal hexagonal β‐Co(OH)₂ nanosheet, exhibiting a thickness down to ten atomic layers and an aspect ratio exceeding 900, by using graphene oxide (GO) as an exfoliant of β‐Co(OH)₂ nanoflowers. Unlike conventional approaches using ionic precursors in which morphological control is realized by structure‐directing molecules, the β‐Co(OH)₂ flower‐like superstructures were first grown by a nanoparticle‐mediated crystallization process, which results in large 3D superstructure consisting of ultrathin nanosheets interspaced by polydimethoxyaniline (PDMA). Thereafter, β‐Co(OH)₂ nanoflowers were chemically exfoliated by surface‐active GO under hydrothermal conditions into unilamellar single‐crystal nanosheets. In this reaction, GO acts as a two‐dimensional (2D) amphiphile to facilitate the exfoliation process through tailored interactions between organic and inorganic molecules. Meanwhile, the on‐site conjugation of GO and Co(OH)₂ promotes the thermodynamic stability of freestanding ultrathin nanosheets and restrains further growth through Oswald ripening. The unique 2D structure combined with functionalities of the hybrid ultrathin Co(OH)₂ nanosheets on rGO resulted in a remarkably enhanced lithium‐ion storage performance as anode materials, maintaining a reversible capacity of 860 mA h g^?1 for as many as 30 cycles. Since mesocrystals are ubiquitous and rich in morphological diversity, the strategy of the GO‐assisted exfoliation of mesocrystals developed here provides an opportunity for the synthesis of new functional nanostructures that could bear importance in clean renewable energy, catalysis, photoelectronics, and photonics.     

Two-Dimensional MOF and COF Nanosheets: Synthesis and Applications in Electrochemistry

Metal–organic framework (MOF) and covalent organic framework (COF) nanosheets are a new type of two-dimensional (2D) materials with unique design principles and various synthesis methods. They are considered ideal electrochemical devices due to the ultrathin thickness, easily tunable molecular structure, large porosity and other unique properties. There are two common methods to synthesize 2D MOF/COF nanosheets: bottom-up and top-down. The top-down strategy mainly includes ultrasonic assisted exfoliation, electrochemical exfoliation and mechanical exfoliation. Another strategy mainly includes interface synthesis, modulation synthesis, surfactant-assisted synthesis. In this Review, the development of ultrathin 2D nanosheets in the field of electrochemistry (supercapacitors, batteries, oxygen reduction, and hydrogen evolution) is introduced, and their unique dimensional advantages are highlighted.     

Zeolitic Imidazolate Framework/Graphene Oxide Hybrid Nanosheets as Seeds for the Growth of Ultrathin Molecular Sieving Membranes

A defect‐free zeolitic imidazolate framework‐8 (ZIF‐8)/graphene oxide (GO) membrane with a thickness of 100 nm was prepared using two‐dimensional (2D) ZIF‐8/GO hybrid nanosheets as seeds. Hybrid nanosheets with a suitable amount of ZIF‐8 nanocrystals were essential for producing a uniform seeding layer that facilitates fast crystal intergrowth during membrane formation. Moreover, the seeding layer acts as a barrier between two different synthesis solutions, and self‐limits crystal growth and effectively eliminates defects during the contra‐diffusion process. The resulting ultrathin membranes show excellent molecular sieving gas separation properties, such as with a high CO₂/N₂ selectivity of 7.0. This 2D nano‐hybrid seeding strategy can be readily extended to the fabrication of other defect‐free and ultrathin MOF or zeolite molecular sieving membranes for a wide range of separation applications.     

A Molecular Pillar Approach To Grow Vertical Covalent Organic Framework Nanosheets on Graphene: Hybrid Materials for Energy Storage

Hybrid 2D–2D materials composed of perpendicularly oriented covalent organic frameworks (COFs) and graphene were prepared and tested for energy storage applications. Diboronic acid molecules covalently attached to graphene oxide (GO) were used as nucleation sites for directing vertical growth of COF‐1 nanosheets (v‐COF‐GO). The hybrid material has a forest of COF‐1 nanosheets with a thickness of 3 to 15 nm in edge‐on orientation relative to GO. The reaction performed without molecular pillars resulted in uncontrollable growth of thick COF‐1 platelets parallel to the surface of GO. The v‐COF‐GO was converted into a conductive carbon material preserving the nanostructure of precursor with ultrathin porous carbon nanosheets grafted to graphene in edge‐on orientation. It was demonstrated as a high‐performance electrode material for supercapacitors. The molecular pillar approach can be used for preparation of many other 2D‐2D materials with control of their relative orientation.     

Single-layered graphene oxide nanosheet/polyaniline hybrids fabricated through direct molecular exfoliation

In this study, we used direct molecular exfoliation for the rapid, facile, large-scale fabrication of single-layered graphene oxide nanosheets (GOSs). Using macromolecular polyaniline (PANI) as a layered space enlarger, we readily and rapidly synthesized individual GOSs at room temperature through the in situ polymerization of aniline on the 2D GOS platform. The chemically modified GOS platelets formed unique 2D-layered GOS/PANI hybrids, with the PANI nanorods embedded between the GO interlayers and extended over the GO surface. X-ray diffraction revealed that intergallery expansion occurred in the GO basal spacing after the PANI nanorods had anchored and grown onto the surface of the GO layer. Transparent folding GOSs were, therefore, observed in transmission electron microscopy images. GOS/PANI nanohybrids possessing high conductivities and large work functions have the potential for application as electrode materials in optoelectronic devices. Our dispersion/exfoliation methodology is a facile means of preparing individual GOS platelets with high throughput, potentially expanding the applicability of nanographene oxide materials.     

Tuning the Reactivity of Ultrathin Oxides: NO Adsorption on Monolayer FeO(111)

Ultrathin metal oxides exhibit unique chemical properties and show promise for applications in heterogeneous catalysis. Monolayer FeO films supported on metal surfaces show large differences in reactivity depending on the metal substrate, potentially enabling tuning of the catalytic properties of these materials. Nitric oxide (NO) adsorption is facile on silver‐supported FeO, whereas a similar film grown on platinum is inert to NO under similar conditions. Ab initio calculations link this substrate‐dependent behavior to steric hindrance caused by substrate‐induced rumpling of the FeO surface, which is stronger for the platinum‐supported film. Calculations show that the size of the activation barrier to adsorption caused by the rumpling is dictated by the strength of the metal–oxide interaction, offering a straightforward method for tailoring the adsorption properties of ultrathin films.     

Preparation of Ultrathin Two‐Dimensional TixTa1−xSyOz Nanosheets as Highly Efficient Photothermal Agents

Although two‐dimensional (2D) metal oxide/sulfide hybrid nanostructures have been synthesized, the facile preparation of ultrathin 2D nanosheets in high yield still remains a challenge. Herein, we report the first high‐yield preparation of solution‐processed ultrathin 2D metal oxide/sulfide hybrid nanosheets, that is, Ti_x Ta_1−x S_y O_z (x =0.71, 0.49, and 0.30), from Ti_x Ta_1−x S₂ precursors. The nanosheet exhibits strong absorbance in the near‐infrared region, giving a large extinction coefficient of 54.1 L g⁻¹ cm⁻¹ at 808 nm, and a high photothermal conversion efficiency of 39.2 %. After modification with lipoic acid‐conjugated polyethylene glycol, the nanosheet is a suitable photothermal agent for treatment of cancer cells under 808 nm laser irradiation. This work provides a facile and general method for the preparation of 2D metal oxide/sulfide hybrid nanosheets.     

Preparation of Ultrathin Two-Dimensional TixTa1−xSyOz Nanosheets as Highly Efficient Photothermal Agents

Although two-dimensional (2D) metal oxide/sulfide hybrid nanostructures have been synthesized, the facile preparation of ultrathin 2D nanosheets in high yield still remains a challenge. Herein, we report the first high-yield preparation of solution-processed ultrathin 2D metal oxide/sulfide hybrid nanosheets, that is, Ti_xTa_1−xS_yO_z (x=0.71, 0.49, and 0.30), from Ti_xTa_1−xS₂ precursors. The nanosheet exhibits strong absorbance in the near-infrared region, giving a large extinction coefficient of 54.1 L g⁻¹ cm⁻¹ at 808 nm, and a high photothermal conversion efficiency of 39.2 %. After modification with lipoic acid-conjugated polyethylene glycol, the nanosheet is a suitable photothermal agent for treatment of cancer cells under 808 nm laser irradiation. This work provides a facile and general method for the preparation of 2D metal oxide/sulfide hybrid nanosheets.     

Two-dimensional Nitrogen-doped Mesoporous Carbon/Graphene Nanocomposites from the Self-assembly of Block Copolymer Micelles in Solution

The self-assembly of block copolymer in solution has proven to be an effective strategy for building up a wide range of nanomaterials with diverse structures and applications. This paper reports a facile self-assembly approach towards two-dimensional(2D) sandwich-like mesoporous nitrogen-doped carbon/reduced graphene oxide nanocomposites(denoted as m NC/r GO) with well-defined large mesopores. The strategy involves the synergistic self-assembly of polystyrene-block-poly(ethylene oxide)(PS-b-PEO) spherical micelles, m-phenylenediamine(m PD) monomers and GO in solution and the subsequent carbonization at 900 ℃. The resultant m NC/r GO nanosheets have an average pore size of 19 nm, a high specific surface of 812 m~2·g~(-1) and a nitrogen content of 2.2 wt%. As an oxygen reduction reaction(ORR) catalyst, the unique structural features render the metal-free nanosheets excellent electrocatalytic performance. In a 0.1 mol·L~(-1) KOH alkaline medium, m NC/r GO exhibits a four-electron transfer pathway with a high half-wave-potential(E_(1/2)) of +0.77 V versus reversible hydrogen electrode(RHE) and a limiting current density(JL) of 5.2 mA·cm~(-2), which are well comparable with those of the commercial Pt/C catalysts.     

Nanostructured Three‐Dimensional (3D) Assembly of 2D MoS2 and Graphene Directly Build From Acidic Graphite Oxide

A 3D highly interconnected macroporous network of reduced GO having finely dispersed few‐layered 2D MoS₂ nanosheets was constructed through direct use of acidic graphite oxide (GO) for the first time. This facile and technologically scalable process can afford efficient hydrodesulfurization electrocatalysts as potential anode materials at lower cost, and can circumvent the poor thermal stability and recyclability of the material. The strategy provided here can be the basis to design and develop practical processes to address the ultimate goal of large‐scale manufacturing of hybrids composed of 2D materials for various energy and catalysis applications.     

Controlled Synthesis of Ultrathin Lanthanide Oxide Nanosheets and Their Promising pH‐Controlled Anticancer Drug Delivery

Various lanthanide oxides (Sm₂O₃ and Gd₂O₃) nanostructures were synthesized by a facile hydrothermal method. The loss of surfactants on the nanocrystals surface, followed by the resultant assembly is responsible for the formation of ultrathin nanosheets. Owing to strong surface effects, the different morphologies of the Sm₂O₃:5 % Eu and Gd₂O₃:5 % Eu nanocrystals present unique photoluminescence properties. As a proof‐of‐concept application, the as‐obtained Sm₂O₃ and Gd₂O₃ ultrathin nanosheets exhibit promising pH‐controlled anticancer drug‐delivery behavior.     

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 %).     

Controllable Synthesis of Ultrathin Transition‐Metal Hydroxide Nanosheets and their Extended Composite Nanostructures for Enhanced Catalytic Activity in the Heck Reaction

We report an effective and universal approach for the preparation of ultrathin single‐ or multiple‐component transition‐metal hydroxide (TMH) nanosheets with thickness below 5 nm. The unique synthesis benefits from the gradual decomposition of the preformed metal–boron (M‐B, M=Fe, Co, Ni, NiCo) composite nanospheres which facilitates the formation of ultrathin nanosheets by the oxidation of the metal and the simultaneous release of boron species. The high specific surface area of the sheets associated with their ultrathin nature promises a wide range of applications. For example, we demonstrate the remarkable adsorption ability of Pb^II and As^V in waste water by the ultrathin FeOOH nanosheets. More interestingly, the process can be extended simply to the synthesis of composite structures of metal alloy hollow shells encapsulated by TMH nanosheets, which show excellent catalytic activity in the Heck reaction.     

Wrapping Nanocellulose Nets around Graphene Oxide Sheets

Constructing advanced functional nanomaterials with pre‐designed organized morphologies from low‐dimension synthetic and biological components is challenging. Herein, we report an efficient and universal amphiphilicity‐driven assembly strategy to construct “hairy” flexible hybrid nanosheets with a 1D cellulose nanofibers (CNFs) net conformally wrapped around 2D graphene oxide (GO) monolayers. This interface‐driven bio‐synthetic assembly is facilitated by tailoring the surface chemistry of flexible GO sheets, resulting in individual sheets tightly surrounded by dense conformal nanocellulose network. The mechanical stability of the products far exceeds the compressive instability limits of both individual components. Additionally, the CNF network significantly enhances the wetting ability of initial hydrophobic reduced GO nanosheets, allowing fast water transport combined with high filtration efficiency.     

超薄Ni(OH)2纳米片修饰Zn0.5Cd0.5S纳米颗粒作为二维/零维异质结构光催化剂可见光下光催化制氢

设计与制备高效的光解水催化剂是解决能源问题和环境问题的策略之一.硫化镉因其可在可见光引发下分解水制氢而受到广泛关注,然而光腐蚀严重,过电势高,载流子复合快速以及表面反应动力学缓慢等缺点极大地限制了其在光解水反应中的实际应用.本文采用简单液相法将均匀的Zn_0.5Cd_0.5S纳米颗粒锚定在超薄Ni(OH)₂纳米薄片上,构建紧密的二维/零维异质结构.通过调控Ni(OH)₂纳米片的含量,制备出不同Ni(OH)₂质量比(3%,5%,7%,9%,11%)的二维/零维Ni(OH)₂/Zn_0.5Cd_0.5S复合材料,并考察其可见光激发的光催化分解水制氢性能.在可见光照射下,Ni(OH)₂/Zn_0.5Cd_0.5S复合材料的光催化性能要大幅度地优于未修饰的Zn_0.5Cd_0.5S纳米颗粒,甚至远高于贵金属Pt修饰的Zn_0.5Cd_0.5S.在不同Ni(OH)₂含量的纳米复合材料中,7%Ni(OH)₂/Zn_0.5Cd_0.5S具有最高效的产氢性能,产氢速率可达6.87 mmol·h^–1·g^–1,且在波长为420 nm的表观量子产率为16.8%.在同等条件下,二维/零维7%Ni(OH)₂/Zn_0.5Cd_0.5S复合光催化剂的光催化分解水产氢速率分别约为纯Zn_0.5Cd_0.5S纳米颗粒和Pt/Zn_0.5Cd_0.5S光催化剂的43倍和8倍,甚至要高于零维/零维7%Ni(OH)₂/Zn_0.5Cd_0.5S纳米复合材料.7%Ni(OH)₂/Zn_0.5Cd_0.5S复合光催化剂具有优异的光催化产氢循环性能,通过循环反应后样品的X射线衍射,X射线光电子能谱和透射电子显微镜等表征,结果表明Ni(OH)₂/Zn_0.5Cd_0.5S在经过20 h的使用后,其晶体结构、表面化学成分和形貌结构未发生明显改变.通过研究样品的时间分辨荧光光谱,线性扫描伏安响应,光电流性能及电化学交流阻抗等,发现二维Ni(OH)₂纳米片的修饰能一定程度降低Zn_0.5Cd_0.5S的过电势,还能有效促进Zn_0.5Cd_0.5S的光生电子-空穴的分离和光生电子的转移.本文认为二维/零维Ni(OH)₂/Zn_0.5Cd_0.5S光催化活性的大幅提升主要由于Zn_0.5Cd_0.5S与Ni(OH)₂之间独特且牢固的纳米结构,在该过程中超薄Ni(OH)₂纳米片不仅能为Zn_0.5Cd_0.5S纳米颗粒的负载提供平台,而且作为一种高效的助催化剂,促进光生电子的转移以及为制氢反应提供更多的活性位点.本文可为多功能,高效及低成本的二维-零维异质结构光催化剂的制备及在太阳能转化方面的应用提供一定借鉴.     

Universal Access to Two-Dimensional Mesoporous Heterostructures by Micelle-Directed Interfacial Assembly

Two-dimensional (2D) mesoporous heterostructures combining ultrathin nanosheet morphology, periodic porous surface structures, and diverse hybrid compositions have become increasingly important for renewable energy storage and electronics. However, it remains a great challenge to develop a universal method to prepare 2D mesoporous heterostructures. Herein, we report a composite-micelle-directed interfacial assembly method to synthesize heterostructures of an ultrathin 2D material covered with mesoporous monolayers assembled on both sides. To demonstrate the concept, we first fabricated a new sandwichlike carbon@MXene@carbon mesoporous heterostructure through the self-assembly of exfoliated MXene nanosheets and block copolymer F127/melamine-formaldehyde resin composite micelles and subsequent thermal treatment. Finally, we demonstrate that the carbon@MXene@carbon mesoporous heterostructured nanosheets manifest remarkably enhanced electrochemical performance as a cathode material for lithium–sulfur batteries.     

Ultra-Permeable Dual-Mechanism-Driven Graphene Oxide Framework Membranes for Precision Ion Separations

Two-dimensional graphene oxide (GO) membranes are gaining popularity as a promising means to address global water scarcity. However, current GO membranes fail to sufficiently exclude angstrom-sized ions from solution. Herein, a de novo “posterior” interfacial polymerization (p-IP) strategy is reported to construct a tailor-made polyamide (PA) network in situ in an ultrathin GO membrane to strengthen size exclusion while imparting a positively charged membrane surface to repel metal ions. The electrostatic repulsion toward metal ions, coupled with the reinforced size exclusion, synergistically drives the high-efficiency metal ion separation through the synthesized positively charged GO framework (PC-GOF) membrane. This dual-mechanism-driven PC-GOF membrane exhibits superior metal ion rejection, anti-fouling ability, good operational stability, and ultra-high permeance (five times that of pristine GO membranes), enabling a sound step towards a sustainable water-energy-food nexus.