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.     

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.     

Liquid‐Phase Epitaxial Growth of Two‐Dimensional Semiconductor Hetero‐nanostructures

Although many two‐dimensional (2D) hybrid nanostructures are being prepared, the engineering of epitaxial 2D semiconductor hetero‐nanostructures in the liquid phase still remains a challenge. The preparation of 2D semiconductor hetero‐nanostructures by epitaxial growth of metal sulfide nanocrystals, including CuS, ZnS and Ni₃S₂, is achieved on ultrathin TiS₂ nanosheets by a simple electrochemical approach by using the TiS₂ crystal and metal foils. Ultrathin CuS nanoplates that are 50–120 nm in size and have a triangular/hexagonal shape are epitaxially grown on TiS₂ nanosheets with perfect epitaxial alignment. ZnS and Ni₃S₂ nanoplates can be also epitaxially grown on TiS₂ nanosheets. As a proof‐of‐concept application, the obtained 2D CuS–TiS₂ composite is used as the anode in a lithium ion battery, which exhibits a high capacity and excellent cycling stability.     

Two‐Dimensional Metal–Organic Framework Nanosheets for Membrane‐Based Gas Separation

Metal–organic framework (MOF) nanosheets could serve as ideal building blocks of molecular sieve membranes owing to their structural diversity and minimized mass‐transfer barrier. To date, discovery of appropriate MOF nanosheets and facile fabrication of high performance MOF nanosheet‐based membranes remain as great challenges. A modified soft‐physical exfoliation method was used to disintegrate a lamellar amphiprotic MOF into nanosheets with a high aspect ratio. Consequently sub‐10 nm‐thick ultrathin membranes were successfully prepared, and these demonstrated a remarkable H₂/CO₂ separation performance, with a separation factor of up to 166 and H₂ permeance of up to 8×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at elevated testing temperatures owing to a well‐defined size‐exclusion effect. This nanosheet‐based membrane holds great promise as the next generation of ultrapermeable gas separation membrane.     

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.     

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.     

Two‐Dimensional Co@N‐Carbon Nanocomposites Facilely Derived from Metal–Organic Framework Nanosheets for Efficient Bifunctional Electrocatalysis

Metal–organic frameworks (MOFs) and MOF‐derived nanomaterials have recently attracted great interest as highly efficient, non‐noble‐metal catalysts. In particular, two‐dimensional MOF nanosheet materials possess the advantages of both 2D layered nanomaterials and MOFs and are considered to be promising nanomaterials. Herein, we report a facile and scalable in situ hydrothermal synthesis of Co–hypoxanthine (HPA) MOF nanosheets, which were then directly carbonized to prepare uniform Co@N‐Carbon nanosheets for efficient bifunctional electrocatalytic hydrogen‐evolution reactions (HERs) and oxygen‐evolution reactions (OERs). The Co embedded in N‐doped carbon shows excellent and stable catalytic performance for bifunctional electrocatalytic OERs and HERs. For OERs, the overpotential of Co@N‐Carbon at 10 mA cm^?2 was 400 mV (vs. reversible hydrogen electrode, RHE). The current density of Co@N‐Carbon reached 100 mA cm^?2 at an overpotential of 560 mV, which showed much better performance than RuO₂; the largest current density of RuO₂ that could be reached was only 44 mA cm^?2. The Tafel slope of Co@N‐Carbon was 61 mV dec^?1, which is comparable to that of commercial RuO₂ (58 mV dec^?1). The excellent electrocatalytic properties can be attributed to the nanosheet structure and well‐dispersed carbon‐encapsulated Co, CoN nanoparticles, and N‐dopant sites, which provided high conductivity and a large number of accessible active sites. The results highlight the great potential of utilizing MOF nanosheet materials as promising templates for the preparation of 2D Co@N‐Carbon materials for electrocatalysis and will pave the way to the development of more efficient 2D nanomaterials for various catalytic applications.     

Graphene‐Assisted Room‐Temperature Synthesis of 2D Nanostructured Hybrid Electrode Materials: Dramatic Acceleration of the Formation Rate of 2D Metal Oxide Nanoplates Induced by Reduced Graphene Oxide Nanosheets

A new prompt room temperature synthetic route to 2D nanostructured metal oxide–graphene‐hybrid electrode materials can be developed by the application of colloidal reduced graphene oxide (RGO) nanosheets as an efficient reaction accelerator for the synthesis of δ‐MnO₂ 2D nanoplates. Whereas the synthesis of the 2D nanostructured δ‐MnO₂ at room temperature requires treating divalent manganese compounds with persulfate ions for at least 24 h, the addition of RGO nanosheet causes a dramatic shortening of synthesis time to 1 h, underscoring its effectiveness for the promotion of the formation of 2D nanostructured metal oxide. To the best of our knowledge, this is the first example of the accelerated synthesis of 2D nanostructured hybrid material induced by the RGO nanosheets. The observed acceleration of nanoplate formation upon the addition of RGO nanosheets is attributable to the enhancement of the oxidizing power of persulfate ions, the increase of the solubility of precursor MnCO₃, and the promoted crystal growth of δ‐MnO₂ 2D nanoplates. The resulting hybridization between RGO nanosheets and δ‐MnO₂ nanoplates is quite powerful not only in increasing the surface area of manganese oxide nanoplate but also in enhancing its electrochemical activity. Of prime importance is that the present δ‐MnO₂–RGO nanocomposites show much superior electrode performance over most of 2D nanostructured manganate systems including a similar porous assembly of RGO and layered MnO₂ nanosheets. This result underscores that the present RGO‐assisted solution‐based synthesis can provide a prompt and scalable method to produce nanostructured hybrid electrode materials.     

A Two‐Dimensional Lamellar Membrane: MXene Nanosheet Stacks

Two‐dimensional (2D) materials are promising candidates for advanced water purification membranes. A new kind of lamellar membrane is based on a stack of 2D MXene nanosheets. Starting from compact Ti₃AlC₂, delaminated nanosheets of the composition Ti₃C₂T_x with the functional groups T (O, OH, and/or F) can be produced by etching and ultrasonication and stapled on a porous support by vacuum filtration. The MXene membrane supported on anodic aluminum oxide (AAO) substrate shows excellent water permeance (more than 1000 L m⁻² h⁻¹ bar⁻¹) and favorable rejection rate (over 90 %) for molecules with sizes larger than 2.5 nm. The water permeance through the MXene membrane is much higher than that of the most membranes with similar rejections. Long‐time operation also reveals the outstanding stability of the MXene membrane for water purification.     

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 %).     

Solution‐Processed Two‐Dimensional MoS2 Nanosheets: Preparation,Hybridization, and Applications

As one member of the emerging class of ultrathin two‐dimensional (2D) transition‐metal dichalcogenide (TMD) nanomaterials, the ultra‐thin MoS₂ nanosheet has attracted increasing research interest as a result of its unique structure and fascinating properties. Solution‐phase methods are promising for the scalable production, functionalization, hybridization of MoS₂ nanosheets, thus enabling the widespread exploration of MoS₂‐based nanomaterials for various promising applications. In this Review, an overview of the recent progress of solution‐processed MoS₂ nanosheets is presented, with the emphasis on their synthetic strategies, functionalization, hybridization, properties, and applications. Finally, the challenges and opportunities in this research area will be proposed.     

The Rapid Exfoliation and Subsequent Restacking of Layered Titanates Driven by an Acid–Base Reaction

Two‐dimensional (2D) (hydro)oxide materials, that is, nanosheets, enable the preparation of advanced 2D materials and devices. The general synthesis route of nanosheets involves exfoliating layered metal (hydro)oxide crystals. This exfoliation process is considered to be time‐consuming, hindering their industrial‐scale production. Based on in situ exfoliation studies on the protonated layered titanate H_1.07Ti_1.73O₄?H₂O (HTO), it is now shown that ion intercalation‐assisted exfoliation driven by chemical reaction provides a viable and fast route to isolated nanosheets. Contrary to the general expectation, data indicate that direct exfoliation of HTO occurs within seconds after mixing of the reactants, instead of proceeding via a swollen state as previously thought. These findings reveal that ion intercalation‐assisted exfoliation driven by chemical reaction is a promising exfoliation route for large‐scale synthesis.     

Oxyhydroxide Nanosheets with Highly Efficient Electron–Hole Pair Separation for Hydrogen Evolution

The facile electron–hole pair recombination in earth‐abundant transition‐metal oxides is a major limitation for the development of highly efficient hydrogen evolution photocatalysts. In this work, the thickness of a layered β‐CoOOH semiconductor that contains metal/hydroxy groups was reduced to obtain an atomically thin, two‐dimensional nanostructure. Analysis by ultrafast transient absorption spectroscopy revealed that electron–hole recombination is almost suppressed in the as‐prepared 1.3 nm thick β‐CoOOH nanosheet, which leads to prominent electron–hole separation efficiencies of 60–90 % upon irradiation at 350–450 nm, which are ten times higher than those of the bulk counterpart. X‐ray absorption spectroscopy and first‐principles calculations demonstrate that [HO?CoO_6?x] species on the nanosheet surface promote H⁺ adsorption and H₂ desorption. An aqueous suspension of the β‐CoOOH nanosheets exhibited a high hydrogen production rate of 160 μmol g^?1 h^?1 even when the system was operated for hundreds of hours.     

Ag@TiO2 Nanoprisms with Highly Efficient Near‐Infrared Photothermal Conversion for Melanoma Therapy

Melanoma is a primary reason of death from skin cancer and associated with high lethality. Photothermal therapy (PTT) has been developed into a powerful cancer treatment technique in recent years. Here, we created a low‐cost and high‐performance PTT agent, Ag@TiO₂ NPs, which possesses a high photothermal conversion efficiency of ≈65 % and strong near‐infrared (NIR) absorption about 808 nm. Ag NPs were synthesized using a two‐step method and coated with TiO₂ to obtain Ag@TiO₂ NPs by a facile sol‐gel method. Because of the oxide, Ag@TiO₂ NPs exhibit remarkable high photothermal conversion efficiencies and biocompatibility in vivo and in vitro. Cytotoxicity and therapeutic efficiency of photothermal cytotoxicity of Ag@TiO₂ NPs were tested in B16‐F10 cells and C57BL/6J mice. Under light irradiation, the elevated temperature causes cell death in Ag NPs‐treated (100 μg mL^?1) cells in vitro (both p<0.01). In the case of subcutaneous melanoma tumor model, Ag@TiO₂ NPs (100 μg mL^?1) were injected into the tumor and irradiated with a 808 nm laser of 2 W cm^?2 for 1 minute. As a consequence, the tumor volume gradually decreased by NIR laser irradiation with only a single treatment. The results demonstrate that Ag@TiO₂ NPs are biocompatible and an attractive photothermal agent for cutaneous melanoma by local delivery.     

Electric‐Field Assisted In Situ Hydrolysis of Bulk Metal–Organic Frameworks (MOFs) into Ultrathin Metal Oxyhydroxide Nanosheets for Efficient Oxygen Evolution

Facile preparation of low‐cost electrocatalysts for efficient oxygen evolution reaction (OER) remains a big challenge. Herein, a novel strategy for ultrafast (20 s) transformation of bulk metal–organic frameworks (MOFs) into ultrathin metal oxyhydroxide nanosheets for efficient OER has been developed. For two isomeric MOFs ( FJI‐H25Fe and FJI‐H25FeCo ), only the metastable FJI‐H25FeCo bulk can immediately transform into FeCo‐oxyhydroxides nanosheets through electric‐field assisted hydrolysis. The potential evolution process from MOF bulk to FeCo‐oxyhydroxides nanosheets has been investigated in detail. The as‐made nanosheets exhibit excellent OER performances, showing an extremely low overpotential of 231 mV at the current density of 10 mA cm^?2, a relatively small Tafel slope of 42 mV dec^?1, and long‐term durability of at least 30 h. This work not only provides a novel strategy for facile preparation of low‐cost and efficient OER electrocatalysts, but also represents a new way for preparation of metal oxyhydroxides nanosheets with good crystallinity and morphology, and a fresh method for mild synthesis of nanosized derivatives from MOF materials.     

Cobalt–Manganese Mixed‐Sulfide Nanocages Encapsulated by Reduced Graphene Oxide: In Situ Sacrificial Template Synthesis and Superior Lithium Storage Properties

This work demonstrates a facile in situ synthesis of cobalt–manganese mixed sulfide (CoMn‐S) nanocages on reduced graphene oxide (RGO) sheets by using a crystalline Co–Mn precursor as the sacrificial template. The CoMn‐S/RGO hybrid was applied as the anode for Li‐ion storage and exhibited superior specific capacity, excellent cycling performance, and great rate capability. In particular, lithium storage testing revealed that the hybrid delivered high discharge–charge capacities of 670 mA h g⁻¹ at 1.0 A g⁻¹ after 400 cycles and 925 mA h g⁻¹ at 0.1 A g⁻¹ after 300 cycles. The outstanding electrochemical performance of CoMn‐S/RGO is attributed to the close entanglement of nanocages with RGO nanosheets achieved by the synthetic method, which greatly improves ion/electron transport along the interfaces and efficiently mitigates volume dilation during lithium reactions. This rational design of both the composition and architecture of mixed metal sulfides can be expanded to other composite systems for high‐capacity Li‐ion batteries and provides a unique insight into the development of advanced hybrid electrode materials.     

Ultrathin Spinel‐Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation

Electrochemical water splitting is a clean technology for H₂ fuels, but greatly hindered by the slow kinetics of the oxygen evolution reaction (OER). Herein, a series of spinel‐structured nanosheets with oxygen deficiencies and ultrathin thicknesses were designed to increase the reactivity and the number of active sites of the catalysts, which were then taken as an excellent platform for promoting the water oxidation process. Theoretical investigations showed that the oxygen vacancies confined in the ultrathin nanosheet could lower the adsorption energy of H₂O, leading to increased OER efficiency. As expected, the NiCo₂O₄ ultrathin nanosheets rich in oxygen vacancies exhibited a large current density of 285 mA cm^?2 at 0.8 V and a small overpotential of 0.32 V, both of which are superior to the corresponding values of bulk samples or samples with few oxygen deficiencies and even higher than those of most reported non‐precious‐metal catalysts. This work should provide a new pathway for the design of advanced OER catalysts.     

Ultrathin Metal–Organic Framework Nanosheets with Ultrahigh Loading of Single Pt Atoms for Efficient Visible‐Light‐Driven Photocatalytic H2 Evolution

A surfactant‐stabilized coordination strategy is used to make two‐dimensional (2D) single‐atom catalysts (SACs) with an ultrahigh Pt loading of 12.0 wt %, by assembly of pre‐formed single Pt atom coordinated porphyrin precursors into free‐standing metal–organic framework (MOF) nanosheets with an ultrathin thickness of 2.4±0.9 nm. This is the first example of 2D MOF‐based SACs. Remarkably, the 2D SACs exhibit a record‐high photocatalytic H₂ evolution rate of 11 320 μmol g^?1 h^?1 via water splitting under visible light irradiation (λ>420 nm) compared with those of reported MOF‐based photocatalysts. Moreover, the MOF nanosheets can be readily drop‐casted onto solid substrates, forming thin films while still retaining their photocatalytic activity, which is highly desirable for practical solar H₂ production.     

Bimetallic Metal‐Organic Framework‐Derived Nanosheet‐Assembled Nanoflower Electrocatalysts for Efficient Oxygen Evolution Reaction

Non‐noble metal‐based metal–organic framework (MOF)‐derived electrocatalysts have recently attracted great interest in the oxygen evolution reaction (OER). Here we report a facile synthesis of nickel‐based bimetallic electrocatalysts derived from 2D nanosheet‐assembled nanoflower‐like MOFs. The optimized morphologies and large Brunauer–Emmett–Teller (BET) surface area endow FeNi@CNF with efficient OER performance, where the aligned nanosheets can expose abundant active sites and benefit electron transfer. The complex nanoflower morphologies together with the synergistic effects between two metals attributed to the OER activity of the Ni‐based bimetallic catalysts. The optimized FeNi@CNF afforded an overpotential of 356 mV at a current density of 10 mA cm^?2 with a Tafel slope of 62.6 mV dec^?1, and also exhibited superior durability with only slightly degradation after 24 hours of continuous operation. The results may inspire the use of complex nanosheet‐assembled nanostructures to explore highly active catalysts for various applications.     

Synthesis and Properties of Oval‐Shaped Iron Oxide/Ethylene Glycol Mesostructured Nanosheets

We have demonstrated a new and facile bottom‐up protocol for the effective synthesis of oval‐shaped iron oxide/ethylene glycol (FeO_x/EG) mesostructured nanosheets. Deprotonated ethylene glycol molecules are intercalated into iron oxide layers to form an interlayer distance of 10.6 Å. These materials display some peculiar magnetic properties, such as the low Morin temperature T_M and ferromagnetism below this T_M value. CdSe/ZnS nanoparticles can be loaded onto these mesostructured nanosheets to produce nanocomposites that combine both magnetic and fluorescence functions. In addition, iron oxide/propanediol (or butanediol) mesostructured materials with increased interlayer distances can also be synthesized. The developed synthetic strategy may be extended toward the creation of other ultrathin mesostructured nanosheets.