Two-dimensional atomic crystals, such as two-dimensional oxides, have attracted much attention in energy storage because nearly all of the atoms can be exposed to the electrolyte and involved in redox reactions. However, current strategies are largely limited to intrinsically layered compounds. Here we report a general strategy that uses the surfaces of water-soluble salt crystals as growth templates and is applicable to not only layered compounds but also various transition metal oxides, such as hexagonal-MoO3, MoO2, MnO and hexagonal-WO3. The planar growth is hypothesized to occur via a match between the crystal lattices of the salt and the growing oxide. Restacked two-dimensional hexagonal-MoO3 exhibits high pseudocapacitive performances (for example, 300 F cm−3 in an Al2(SO4)3 electrolyte). The synthesis of various two-dimensional transition metal oxides and the demonstration of high capacitance are expected to enable fundamental studies of dimensionality effects on their properties and facilitate their use in energy storage and other applications.

Two-dimensional (2D) transition-metal nitrides just recently entered the research arena, but already offer a potential for high-rate energy storage, which is needed for portable/wearable electronics and many other applications. However, a lack of efficient and high-yield synthesis methods for 2D metal nitrides has been a major bottleneck for the manufacturing of those potentially very important materials, and only MoN, TiN, and GaN have been reported so far. Here we report a scalable method that uses reduction of 2D hexagonal oxides in ammonia to produce 2D nitrides, such as MoN. MoN nanosheets with subnanometer thickness have been studied in depth. Both theoretical calculation and experiments demonstrate the metallic nature of 2D MoN. The hydrophilic restacked 2D MoN film exhibits a very high volumetric capacitance of 928 F cm in sulfuric acid electrolyte with an excellent rate performance. We expect that the synthesis of metallic 2D MoN and two other nitrides (WN and VN) demonstrated here will provide an efficient way to expand the family of 2D materials and add many members with attractive properties.

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Because of their exotic electronic properties and abundant active sites, two-dimensional (2D) materials have potential in various fields. Pursuing a general synthesis methodology of 2D materials and advancing it from the laboratory to industry is of great importance. This type of method should be low cost, rapid and highly efficient. Here, we report the high-yield synthesis of 2D metal oxides and hydroxides via a molten salts method. We obtained a high-yield of 2D ion-intercalated metal oxides and hydroxides, such as cation-intercalated manganese oxides (Na0.55Mn2O4·1.5H2O and K0.27MnO2·0.54H2O), cation-intercalated tungsten oxides (Li2WO4 and Na2W4O13), and anion-intercalated metal hydroxides (Zn5(OH)8(NO3)2·2H2O and Cu2(OH)3NO3), with a large lateral size and nanometre thickness in a short time. Using 2D Na2W4O13 as an electrode, a high performance electrochemical supercapacitor is achieved. We anticipate that our method will enable new path to the high-yield synthesis of 2D materials for applications in energy-related fields and beyond.

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2D materials have demonstrated good chemical, optical, electrical, and magnetic characteristics, and offer great potential in numerous applications. Corresponding synthesis technologies of 2D materials that are highquality, high-yield, low-cost, and time-saving are highly desired. Salt-assisted methods are emerging technologies that can meet these requirements for the fabrication of 2D materials. Herein, the recent process for the salt-assisted synthesis of 2D materials and their typical applications are summarized. First, the properties of salt crystals and molten salts are briefly introduced, and then some examples of 2D materials synthesis with the assistance of salt as well as their representative applications are presented. The underlying mechanisms of salts with different states on the formation of 2D morphology are discussed to aid in the rational design of synthetic route of 2D materials. At last, the challenges and future perspectives for salt-assisted methods are briefly described. This review provides guidance for the controllable synthesis of 2D materials based on the salt-assisted approaches.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201908486.MoO 3 , and LaNb 2 O 7 ), [14][15][16] layered double hydroxides (LDHs, e.g., Mg 6 Al 2 (OH) 16 CO 3 •4H 2 O), [17] hexagonal-boron nitride (h-BN), [2,18] transition metal halides (such as MoCl 2 and CrCl 3 ), [19] black phosphorus, [20] graphitic carbon nitride (g-C 3 N 4 ), [5] MXene (such as Ti 2 C and Ti 3 CN), [21][22][23][24][25] and clays (for instance, [(Mg 3 )(Si 2 O 5 ) 2 (OH) 2 ] and [(Al 2 )(Si 3 Al) O 10 (OH) 2 ]K). [19] Notably, many nonlayered structure species can also form 2D morphology with specific synthetic methods, largely expanding the 2D family (such as hexagonal-MoO 3 (h-MoO 3 ), hexagonal-WO 3 (h-WO 3 ), [15] transition metal nitrides (TMNs), [26,27] and transition metal phosphides (TMPs)). [28] To exploit the applications of 2D materials, the development of a synthetic method should be prioritized. Currently, synthesis processes of 2D materials could be divided into two categories, naming the top-down and bottom-up approaches. [29][30][31] Generally, exfoliation is the most common top-down method to produce monolayer or few-layer 2D materials. [19,32] Graphene was first prepared by mechanical exfoliation of highly oriented pyrolytic graphite with sellotape in 2004. [33] The exfoliation can weaken the interlayer Van der Waals force for bulk materials with layered crystal structures while maintain the covalent bonding in plane to produce monolayer or few-layer nanosheets. Although the productivity based on this method is limited due to the low efficiency, the exfoliation approach provides a new synthesis methodology for 2D materials. A series of studies on liquid exfoliation have been conducted by Coleman and co-workers. [19,34,35] By sonicating the bulk material in an appropriate solvent with similar interface energy, 2D material ink can be prepared. After the liq...

Molybdenum carbide (Mo 2 C) has been widely applied in energy conversion, electrocatalysis, and other electronic devices, but its nanostructure with certain morphology and porous structure is tough to control. In this work, 1D or 2D porous Mo 2 C nanostructures can be synthesized by carburizing cobalt-based zeolitic imidazolate framework (ZIF-67) cladding MoO 3 nanowires or nanosheets hybrid structure under high temperature. The obtained 2D porous Mo 2 C shows a low onset overpotential (η = 25 and 36 mV) and a small Tafel slope (40 and 47 mV dec −1 ) in 0.1 m HClO 4 and 0.1 m KOH as well as great stability. This work highlights a new strategy for the design and synthesis of porous nanostructure Mo 2 C electrocatalysts. and 1D nanowires. The approach is through carburizing cobalt or zinc-based zeolite-type metal-organic framework (MOF) (ZIF-67 or ZIF-8) cladding MoO 3 nanosheets or nanowires under high temperature. The resulting nanostructures are periodically porous, largely preserving the precursor morphology. The porous Mo 2 C materials may provide an exceptionally large number of active sites. For instance, 2D Mo 2 C nanosheets may show excellent catalytic performance for hydrogen evolution over a broad range of pH value (0.1 m HClO 4 and 0.1 m KOH), including a low onset overpotential (η = 25 and 36 mV vs RHE), a small Tafel slope (40 and 47 mV dec −1 ), and remarkable stability.

layered MnO 2 materials, composed of exotic electronic properties and accessible active sites with alkali metal ions, provide a comprehensive platform for developing catalysts with chemical modification. Significantly, K + -contained layered MnO 2 catalysts have been verified as strong candidates toward catalytic oxidation of formaldehyde (HCHO). Unveiling the effects of alkali metal ions on active sites is critical to understand the interaction between reactants and active centers. Through a combination of analytical tools with periodic computational density functional theory modeling, the surface structures and the exposing specific defects of alkali metal ions affiliated to oxygen vacancies (Vo) are figured out by comparing three typical alkali metal ionintercalated (Na + , K + , and Cs + ) layered MnO 2 materials. These materials have been synthesized via a molten salt method, with high yield, large lateral size, and nanometer thickness in a few moments. We demonstrate that the alkali metal ions could remarkably alter the formation energy of Vo by the sequence of CsMnO (1.94 eV) < KMnO (1.97 eV) < NaMnO (2.07 eV) < ideal MnO 2 surface without the intercalated ion (2.23 eV). As a result, CsMnO with the most surface Vo sites could achieve efficient HCHO oxidation to CO 2 , with a HCHO consumption rate of about 0.149 mmol/(g•h) at 40 °C in 200 ppm HCHO/humid air [gas hourly space velocity = 80,000 mL/(g•h)]. Different from the Mars−van-Krevelen process, quantum chemical calculations and in situ diffuse reflectance infrared Fourier transform spectroscopy revealed that the main reaction pathway might be HCHO(ad) + [O](ad) → DOM → [HCOO − ] s → CO 2 via a Langmuir−Hinshelwood (L−H) mechanism. Alkali metals remarkably promoted the HCHO conversion by trapping oxygen through Vo sites and accelerating the facile reaction among adsorbed oxygen with adsorbed HCHO to deep degradation products (CO 2 and H 2 O).