Molten Salt-Directed Catalytic Synthesis of 2D Layered Transition-Metal Nitrides for Efficient Hydrogen Evolution

•Facile synthesis of 2D layered transition-metal nitrides is realized•Molten salt can act as a catalyst for the synthesis of high-energy 2D materials•2D layered transition-metal nitrides exhibit excellent activity for electrocatalysis Exploratory synthesis of 2D layered materials is important in various research fields, such as electronics, physics, and chemistry. However, the preparation of 2D layered materials with a high formation energy is hindered by sluggish growth thermodynamics. 2D layered transition-metal nitrides (TMNs) represent an important class of materials, the synthesis of which is challenging. Here, we report a molten salt method that permits catalytic synthesis of a family of single-crystal 2D layered TMNs under relatively mild conditions. With the assistance of molten salt, the atomically thin 2D layered TMN nanosheets can be synthesized directly without further exfoliation process. We also investigated the effect of different alkali metal ions (Li+, Na+, and K+) on the growth of 2D TMNs, and both binary and ternary 2D layered TMNs were obtained. The catalytic molten salt method is a generic synthesis strategy, which opens opportunities for the exploration and preparation of 2D materials. Facile synthesis of single-crystal 2D layered transition-metal nitrides (TMNs) is of crucial importance for the development of forthcoming technologies, such as superconducting, electromagnetic interference shielding, and energy-related applications. However, the fabrication of TMNs with natural 2D layered structure is thermodynamically difficult, in which stringent synthesis constraints have limited the exploration of this important class of functional materials. Here, we employed alkali molten salts as catalysts to achieve facile and large-scale (over decagram) synthesis of a family of 2D layered TMNs, such as MoN1.2, WN1.5, and Mo0.7W0.3N1.2, under atmospheric pressure. Ex-situ experiments reveal that the molten salt can lower the formation energy of 2D layered TMNs by assuring a liquid-gas synthesis and forming a TMN-salt-TMN superstructure as an intermediate. The resultant 2D layered TMNs show superior performance in hydrogen evolution reaction, demonstrating the immense potential of 2D layered TMNs for energy-related applications and beyond. Facile synthesis of single-crystal 2D layered transition-metal nitrides (TMNs) is of crucial importance for the development of forthcoming technologies, such as superconducting, electromagnetic interference shielding, and energy-related applications. However, the fabrication of TMNs with natural 2D layered structure is thermodynamically difficult, in which stringent synthesis constraints have limited the exploration of this important class of functional materials. 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Furthermore, it can intercalate into the interlayer space of 2D TMNs during the reaction to form a TMN-salt-TMN superstructure, which can lower the formation energy and stabilize the 2D layered structure. We also investigated the effect of different alkali metal ions (Li+, Na+ and K+) in the synthesis of 2D TMNs, and both 2D layered ternary and binary TMN nanosheets were obtained. In this work, we successfully synthesized binary 2D layered MoN1.2, WN1.5, and Mo0.7W0.3N1.2 nanosheets for the first time. These 2D layered TMNs demonstrated superior electrocatalytic performance in hydrogen evolution reaction. Notably, the 2D layered Mo0.7W0.3N1.2 exhibits small overpotentials of 129 and 122 mV at a current density of 10 mA cm−2 in 0.5 M H2SO4 and 1 M KOH, respectively, surpassing most of the 2D layered electrocatalysts. They also exhibit excellent oxidation resistance and film-forming property for practical applications. Most metals or metal oxides have high melting points, so their direct ammoniation under NH3 atmosphere is a solid-gas reaction. Such process is limited by the thermodynamically sluggish stuffing of N atoms into metal lattice.28Chen J.G. Carbide and nitride overlayers on early transition metal surfaces: preparation, characterization, and reactivities.Chem. Rev. 1996; 96: 1477-1498Crossref PubMed Scopus (553) Google Scholar Consequently, most of metal nitrides are nitrogen deficient (nitrogen over metal ratio < 1) with simple chemical compositions and crystal structures (e.g., Mo2N, W2N, and Ni3N). On the contrary, mixed salts can significantly reduce the melting point of the metal and metal oxide precursors, and lead to a liquid-gas synthesis.10Zhou J. Lin J. Huang X. Zhou Y. Chen Y. Xia J. Wang H. Xie Y. Yu H. Lei J. et al.A library of atomically thin metal chalcogenides.Nature. 2018; 556: 355-359Crossref PubMed Scopus (597) Google Scholar,47Wang H. Sandoz-Rosado E.J. Tsang S.H. Lin J. Zhu M. Mallick G. Liu Z. Teo E.H.T. Elastic properties of 2D ultrathin tungsten nitride crystals grown by chemical vapor deposition.Adv. Funct. Mater. 2019; 29: 1902663Crossref Scopus (8) Google Scholar Under molten state, the metal precursors melt into monomers, which have higher reaction activity and faster reaction rate.44Hu Z. Xiao X. Jin H. Li T. Chen M. Liang Z. Guo Z. Li J. Wan J. Huang L. et al.Rapid mass production of two-dimensional metal oxides and hydroxides via the molten salts method.Nat. Commun. 2017; 8: 15630Crossref PubMed Scopus (164) Google Scholar Using this method, we have synthesized a family of 2D layered materials, such as MoN1.2, WN1.5, and Mo0.7W0.3N1.2. The composition of Mo0.7W0.3N1.2 was confirmed using inductively coupled plasma mass spectrometry (ICP-MS). Figure 1 shows the flow chart for the synthesis of 2D layered TMNs using the molten salt method. Typically, metal oxide powders (e.g., MoO3) were mixed with their alkali metal salts (e.g., Na2MoO4) via ball milling. Then the precursor was annealed under 5% NH3 in Ar. Before ammoniation, the mixed precursor melted first, and then a bottom-up 2D vertical growth on the molten mixture surface took place. After water washing to remove the salts, the freestanding 2D layered TMN nanosheets were obtained. Since the final products were atomically thin nanosheets, no further exfoliation process was required. It should be noted that, without adding the salts, the final products are conventional 3D non-van der Waals MoNx and W2N nanoparticles (Figure S1). The resulting 2D layered TMNs nanosheets exhibited a good dispersity and stability in water, which is shown in Figure S2. After 60 days, no oxidation was observed, demonstrating good oxidation resistance of the produced 2D layered TMNs in water. We further investigated the stability of the 2D layered TMNs using X-ray photoelectron spectroscopy (XPS). After exposing MoN1.2 to air for 8 months, this sample was shown to be stable without any obvious compositional change (Figures S2C and S2D). The observed good stability originates from the surface O terminations (Figure S2C), which can prevent further oxidation of 2D layered TMN. It should be noted that over a decagram of 2D layered TMNs powder can be obtained (Figure S3A) by one-time synthesis, which reveals the immense potential of this approach for large-scale production. In the meantime, the lyophilized 2D TMN powder can be directly rolled into a freestanding film with good hydrophilicity (Figures S3B and S4). The film is flat and thin, which is suitable for energy conversion and storage application.48Tao Q. Dahlqvist M. Lu J. Kota S. Meshkian R. Halim J. Palisaitis J. Hultman L. Barsoum M.W. Persson P.O.Å. Rosen J. Two-dimensional Mo1.33C MXene with divacancy ordering prepared from parent 3D laminate with in-plane chemical ordering.Nat. Commun. 2017; 8: 14949Crossref PubMed Scopus (241) Google Scholar The morphology and atomic structure of the 2D crystals are revealed by scanning electron microscopy (SEM) and spherical-aberration-corrected scanning transmission electron microscopy (STEM). Figures 2A, 2E, and 2I show the SEM images of MoN1.2, WN1.5, and Mo0.7W0.3N1.2 after washing with water. Compared with the samples before washing (Figures S5 and S6), the 2D morphology was successfully preserved, demonstrating the structure stability of these 2D TMNs. Notably, the samples before washing were atomically thin 2D nanosheets instead of bulk particles. The atomic structure of the as-prepared 2D crystals exhibited a hexagonal molecular phase with similar in-plane crystal lattice of 0.25 nm (Figures 2B, 2F, and 2J). The selected area electron diffraction (SAED) in Figure S7 shows only one hexagonal pattern, indicating the single-crystal phase of 2D TMNs. The layered structure of the 2D crystals can be clearly seen in high-angle annular dark-field (HAADF) STEM images (Figures 2C, 2G, and 2K). The MoN1.2 and Mo0.7W0.3N1.2 materials show similar layered structure with single-layer thickness of about 0.6 nm (consisting 3 layers of Mo atoms per unit) and an interlayer distance of about 0.5 nm. Whereas a single layered WN1.5 consists of 4 layers of W atoms with single-layer thickness of 0.9 nm and interlayer space of 0.8 nm, which is different from MoN1.2 and the known W2N3.35Jin H. Li L. Liu X. Tang C. Xu W. Chen S. Song L. Zheng Y. Qiao S.-Z. Nitrogen vacancies on 2D layered W2N3: a stable and efficient active site for nitrogen reduction reaction.Adv. Mater. 2019; 31: 1902709Crossref Scopus (163) Google Scholar Due to the large contrast of Mo or W atoms to N atoms under STEM mode, the N atom cannot be observed. The schematic illustration of the expected atomic arrangements is shown in Figures 2D, 2H, and 2L. Furthermore, Figure S8 shows the in-plane structure of Mo0.7W0.3N1.2, in which W atoms are uniformly distributed on the MoN1.2 substrate, demonstrating a doped structure similar to MoWSx chalcogenides.49Lei Y. Pakhira S. Fujisawa K. Wang X. Iyiola O.O. Perea López N. Laura Elías A. Pulickal Rajukumar L. Zhou C. Kabius B. et al.Low-temperature synthesis of heterostructures of transition metal dichalcogenide alloys (WxMo1–xS2) and graphene with superior catalytic performance for hydrogen evolution.ACS Nano. 2017; 11: 5103-5112Crossref PubMed Scopus (90) Google Scholar We also compared the sample before and after washing with water (Figures 2C, 2G, 2K, and S9). The layered atomic structure remain unchanged, which was in accordance with the SEM images. The crystal structure of the prepared 2D TMNs was further analyzed by X-ray diffraction (XRD). MoN1.2 and Mo0.7W0.3N1.2 have the similar XRD patterns as that of Mo5N6.50Jin H. Liu X. Vasileff A. Jiao Y. Zhao Y. Zheng Y. Qiao S.Z. Single-crystal Nitrogen-rich two-dimensional Mo5N6 nanosheets for efficient and stable seawater splitting.ACS Nano. 2018; 12: 12761-12769Crossref PubMed Scopus (109) Google Scholar WN1.5 has the similar crystal structure as W2N3 (Figures 3A , 3B, and S10).35Jin H. Li L. Liu X. Tang C. Xu W. Chen S. Song L. Zheng Y. Qiao S.-Z. Nitrogen vacancies on 2D layered W2N3: a stable and efficient active site for nitrogen reduction reaction.Adv. Mater. 2019; 31: 1902709Crossref Scopus (163) Google Scholar However, according to the TEM images perpendicular to the layers, the atom arrangement of MoN1.2 and WN1.5 at [001] facet clearly shows the unique layered structure, which is totally different from that of Mo5N6 (non-layered material) and W2N3 (2 layers of W atoms per unit) (Figure S11), indicating that our TMNs are newfound 2D materials. The broad peak before 10 degree for MoN1.2 and WN1.5 reflects the layered structure of 2D TMNs. To elucidate the chemical composition of 2D TMNs, we conducted XRD and XPS analysis for the samples before and after washing with water. As shown in Figure S12, the products before washing are only 2D TMNs and alkali metal salts (e.g., Na2MoO4). Apparently, in the reaction, the composition and content of the salt remains the same, demonstrating the catalytic synthesis (Table S1). As shown in Figures 3C and 3D, the sodium content is significantly reduced to an undetectable level (Figure S13), indicating that the alkali metal salts are washed-out completely. In the meantime, the energy-dispersive X-ray spectroscopy (EDS) mapping under STEM mode also reveals that the sodium salts have been washed-out (Figures S14–S16). The high-resolution XPS spectra of Mo, W, and N indicate that the washing process only removes the salt, the nitrides remain stable (Figures S17–S19). By comparing the composition of the samples before and after reaction, the chemical content of 2D TMNs is determined (N over Mo ratio ∼1.2, N over W ratio ∼1.5). Since the only invariable element during the washing process is nitrogen, we further conducted synchrotron-based X-ray absorption near edge structure (XANES) to observe the N K edge of 2D TMNs before and after washing. As expected, the N K edge spectra remain unchanged (Figures 3E and 3F), demonstrating the stable structure of 2D TMNs even after removal of salts. We further investigated the growth mechanism by a series of experiments and theoretical calculations. In this method, the key variables are the temperature and alkali metal salts. Consequently, we identified the growth mechanism by controlling these two factors. First, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed on different specimens to corroborate the temperature zone. As shown in Figure 4A, the melting point of MoO3-Na2MoO4 mixture (608°C) is lower than that of MoO3 (787°C) and Na2MoO4 (682°C), individually. Thus, we can conclude that before re