Salt-Templated Synthesis of 2D Metallic MoN and Other Nitrides Xu Xiao,†,§,∥ Huimin Yu,†,∥ Huanyu Jin,† Menghao Wu,‡ Yunsheng Fang,† Jiyu Sun,† Zhimi Hu,† Tianqi Li,† Jiabin Wu,† Liang Huang,† Yury Gogotsi,*,§ and Jun Zhou*,† †
Wuhan National Laboratory for Optoelectronics and ‡Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, Hubei, P. R. China § Department of Materials Science and Engineering and A. J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: 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, Ti4N3, 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−3 in sulfuric acid electrolyte with an excellent rate performance. We expect that the synthesis of metallic 2D MoN and two other nitrides (W2N and V2N) demonstrated here will provide an efficient way to expand the family of 2D materials and add many members with attractive properties. KEYWORDS: salt-templated, two-dimensional, metal nitride, supercapacitor, high-rate
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the production of 2D nitrides are mainly exfoliation of layered materials (only applicable to graphite-like dielectric hBN) and gas-phase syntheses (2D GaN, only as monolayers on graphene).19,24,25 Considering that most metal nitrides do not form layered structures, their exfoliation is not a promising synthesis method. Recently, nitride MXene-Ti4N3 has been successfully synthesized from its MAX phase precursor,17 but etching in a high-temperature molten salt was required. Etching of other nitride MAX phases has not been successful so far. Hence, it is of critical importance to develop an efficient and scalable synthesis process for 2D metal nitrides (including those that do not form layered structures), which can be used for energy storage, electrocatalysis, electromagnetic interference shielding, and other applications that require high electronic conductivity. Herein, we report the scalable salt-templated synthesis of metallic 2D MoN nanosheets. Both the theoretical and experimental analyses demonstrate that 2D MoN nanosheets are metallic with zero bandgap and excellent electronic conductivity. A schematic of the synthesis of 2D MoN
wo-dimensional (2D) atomic crystals are among the hottest candidates for energy storage, electrocatalysis, and other applications due to their large electrochemically active surface areas, resulting in a fast ion diffusion rate and high electrochemical performance.1−4 A typical example is graphene, which shows a good volumetric capacitance of as high as 300 F cm−3.5−7 Another example is 2D metal oxides/ hydroxides, which provide additional pseudocapacitance due to surface redox reactions.8−11 The problem with most oxide materials is their relatively poor conductivity that hinders the high-rate performance.12,13 Non-oxide 2D materials with high conductivity and hydrophilicity include 1T MoS2 that has shown a capacitance of 650 F cm−3 and Ti3C2 MXene that has exhibited an ultrahigh volumetric capacitance exceeding 900 F cm−3, but it should be noted that their highest sweep rate is currently limited to 1 V s−1.14,15 While about 20 2D transition-metal carbides have been reported,1,16 the number of 2D metal nitrides published to date is limited to one MXene (Ti4N3),17 MoN,18 and very recently synthesized GaN.19 Transition-metal nitrides, such as TiN, show a higher conductivity than their respective carbides and even find applications in plasmonics.20,21 While 2D transitionmetal nitrides offer the advantage of high-rate energy storage due to their higher conductivity,22,23 the current strategies for © 2017 American Chemical Society
Received: December 21, 2016 Accepted: February 3, 2017 Published: February 3, 2017 2180
DOI: 10.1021/acsnano.6b08534 ACS Nano 2017, 11, 2180−2186
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Figure 1. Synthesis process and characterization. (a) Schematic of synthesis of 2D MoN. First, 2D hexagonal MoO3-coated NaCl powders were produced by annealing a Mo precursor-coated NaCl in an Ar atmosphere. Second, by annealing the samples in a NH3 atmosphere, we obtained 2D MoN-coated NaCl samples. Finally, by washing the samples in water, NaCl was dissolved, which left 2D MoN dispersed in water. (b) XRD pattern of 2D MoN powder. The inset shows the Tyndall effect of the 2D MoN colloidal solution in water, demonstrating the good dispersity of 2D MoN in water. (c) N 1s and Mo 3p XPS spectrum of 2D MoN. (d) SEM image shows the 2D morphology of the assynthesized MoN. Scale bar, 500 nm.
to Mo−O bonding, which should be attributed to a surface termination, similar to the case of MXene.30 The morphology of 2D MoN nanosheets was verified by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The 2D nanosheets were observed in SEM, as shown in Figure 1d. The lateral size of some flakes (inset of Figure 3e) exceeded 20 μm, owing to the templating effect of the salt crystals, which were larger than 50 μm. Such a large size is beneficial for manipulation and fabrication of single 2D nanosheet devices. In addition, when filtrating the 2D MoN dispersion to fabricate the restacked film for use in electrodes, the large-sized nanosheets would improve the mechanical properties and electrical conductivity. The low-resolution TEM image of the as-synthesized MoN is shown in Figure 2a. First, the energy-dispersive spectroscopy (EDS) mapping of 2D MoN nanosheets shown in Figure S5 suggests that Mo and N are distributed uniformly. The flexibility and presence of wrinkles imply the 2D morphology of MoN nanosheets. Moreover, a high-resolution TEM image shown in Figure 2b reveals overlapping flakes. Apparently, we can distinguish the 2D morphologies of each flake, and the high transparency suggests very thin thickness of the 2D MoN nanosheets. The structure and phase composition of the 2D MoN were further verified using high-resolution TEM (HRTEM, Figure 2c) and selected area electron diffraction (SAED, inset of Figure 2c). MoN has a hexagonal structure with lattice constants a = b = 5.75 Å and c = 5.62 Å.31 Here, the hexagonal atomic structure is confirmed, in accordance with the XRD results, elucidating the formation of 2D morphology MoN with dominating (l00) facets. Furthermore, on the basis of a HRTEM image of the flake edge (Figure 2d), the thickness of that 2D MoN was around 0.71 nm. To understand the electronic structure of 2D MoN nanosheets, we performed first-principle calculations. Figure 3a,b displays the optimized geometric structure of a MoN monolayer from side and top views, respectively, where a MoN slab of 6 atoms in thickness is adopted and the thickness is around 0.7 nm, in accordance with the TEM measurement. Mo atoms are arranged in a hexagonal structure which can be
nanosheets is shown in Figure 1a. As previously demonstrated, salt-templated synthesis is an efficient and scalable way to fabricate 2D transition-metal oxides by virtue of the lattice match between salt template surfaces and target crystals.26 Accordingly, these nonlayered 2D metal oxides can serve as precursors for synthesis of 2D metal nitrides via ammoniation. In our experiments, first, we obtained the 2D hexagonal MoO3coated NaCl (2D h-MoO3@NaCl) by annealing the Mo precursor@NaCl powders in Ar atmosphere at 280 °C (Figure 1a). Second, 2D h-MoO3@NaCl powders were slowly ammoniated in a NH3 atmosphere at 650 °C. Here the salt may act as the stabilizer to avoid the morphology change during transformation from h-MoO3 to MoN (Figure S1).27 Finally, 2D MoN@NaCl powders were washed in deionized water with further filtration to remove the salts. This method can expand the family of 2D materials and produce large and high-quality 2D metal nitride flakes even for nonlayered compounds.
RESULTS AND DISCUSSION One of the features of this salt-templated method is a large yield because the synthesis can be easily scaled up by adjusting the amount of salt, and the salt could be recycled for further usage. In addition, 2D MoN nanosheets showed a good dispersity in deionized water with a zeta potential of −32 mV (Figure S2), resulting in a colloidal solution that was confirmed by the Tyndall scattering effect (inset of Figure 1b). In addition, the negatively charged 2D MoN nanosheets would be beneficial for cation intercalation.28 We can also obtain a highconcentration dispersion of 2D MoN in DI water (Figure S3), for potential applications in printable and flexible electronics.29 The X-ray diffraction (XRD) pattern shown in Figure 1b reveals the crystal structure of hexagonal MoN, which is consistent with the PDF card of MoN (01-089-5024). X-ray photoelectron spectroscopy (XPS) confirms the presence of Mo and N (Figure S4). The detailed analysis was conducted around 400 eV where the N 1s and Mo 3p peaks were found. As shown in Figure 1c, the appearance of an N 1s peak (around 397.5 eV) and Mo−N bonding (centered at around 393.7 eV) confirms the formation of MoN. Here another peak is assigned 2181
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temperature dependence of conductivity (see details in Supporting Information note 1). The SEM image of the device is shown in the inset of Figure 3e. As we all know, the conductivity of metals decreases with a rise in temperature, but semiconductors are on the contrary. The current−voltage (I− V) curves and resistivity of a single 2D MoN nanosheet were measured at different temperatures (Figure 3e,f). The 2D MoN nanosheet showed a very high conductivity, and the temperature dependence of conductivity is consistent with metals, which also elucidates the successful transformation from semiconducting 2D h-MoO3 to metallic 2D MoN in our synthesis process. The as-synthesized metallic 2D MoN nanosheets are expected to be suitable for high-rate energy storage because of the fast transport of electrons and ions in metallic 2D structures.32 Therefore, we fabricated restacked 2D MoN electrodes through vacuum filtration of the 2D MoN dispersion in deionized water onto a membrane and explored their electrochemical performance. The ideal electrode materials should be simultaneously hydrophilic and highly conductive, if used in an aqueous electrolyte. We verified that the restacked films of the 2D MoN nanosheets are very hydrophilic with a contact angle of about 20°, which is shown in Figure S7. The electrochemical energy storage was first explored in 1 M H2SO4 electrolyte by using a three-electrode configuration with Ag/ AgCl and active carbon as the reference and counter electrodes, respectively. The resulting cyclic voltammetry (CV) curves for the potential range from −0.1 to 0.6 V versus NHE at different sweep rates are shown in Figure 4a. The rectangular shape of CV curves indicates capacitive or pseudocapacitive behavior in the solution due to the highly accessible surface of 2D MoN.33 Ions could be rapidly intercalated into the restacked 2D MoN electrode and react with the exposed active surface. Importantly, the sweep rate of restacked 2D MoN electrode can reach up to 20 V s−1 due to superb electrical conductivity, as previously observed for transition-metal nitrides,23 and the CV curves do not show an obvious narrowing to the resistor shape, which indicates the extremely fast ion diffusion and electron transport in the restacked electrode attributed to the 2D nanosheet morphology and the remarkable electrical conductivity of the metallic MoN. The volumetric capacitance
Figure 2. TEM characterization of 2D MoN nanosheets. (a) Lowresolution TEM image of 2D MoN nanosheets. The wrinkles demonstrate their flexibility. Scale bar, 500 nm. (b) TEM image of overlapping single-layer 2D MoN nanosheets. Three pieces of nanosheets were labeled as 1L, 2L, and 3L. Scale bar, 20 nm. (c) HRTEM showed that 2D MoN was single-crystalline with hexagonal structure. The inset is a SAED that demonstrates the single-crystal structure of 2D MoN. Scale bar, 2 nm. (d) HRTEM image of the edge of a 2D MoN nanosheet, showing a thickness of about 0.71 nm. Scale bar, 5 nm.
clearly seen from the top-view image (Figure 3b), and N atoms occupy triangular prismatic sites between the Mo layers (Figure 3a). In addition, no bandgap was found in the MoN nanosheets, and the high electron density of states (DOS) at the Fermi level reveals its metallic properties, as shown in Figure 3c,d. It is revealed that the large partial DOS at the Fermi level is mainly distributed by the d electrons of Mo atoms, which leads to the high conductivity of 2D MoN. Accordingly, to verify the metallic conductivity of 2D MoN, we fabricated a single 2D MoN nanosheet device and measured the
Figure 3. Electronic properties of 2D MoN. (a, b) Side and top view images of the atoms structure of 2D MoN. (c, d) DOS of 2D MoN. No bandgap was found in 2D MoN, implying its metallic character. (e) I−V curves of the single-sheet microdevice at different temperatures. The inset is a SEM image of the microdevice. Scale bar, 10 μm. (f) The temperature dependency of conductivity at different temperatures. 2182
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Figure 4. Electrochemical performance of 2D MoN as a SC electrode. (a) CV curves of 2D MoN in H2SO4 at different sweep rates. The 2D MoN electrode maintains a rectangular CV shape at an ultrahigh sweep rate of 20 V/s. (b) CV curves of 2D MoN in different electrolytes. The sweep rate was 100 mV/s. (c) Volumetric capacitance versus sweep rates for different electrolytes. (d) EIS measurement in different electrolytes. (e) Long-term stability of 2D MoN in H2SO4 electrolyte. (f) Thickness effect of the electrodes on electrochemical performance at sweep rate of 100 mV s−1 in KOH electrolyte.
Electrochemical impedance spectroscopy (EIS) was also performed to research electron/ion transport as shown in Figure 4d. The results in various electrolytes revealed a typical capacitive behavior. Additionally, no apparent charge-transfer resistance was observed in various electrolytes (the inset in Figure 4d), which indicates a fast electron transport in the restacked 2D MoN electrodes. These results further demonstrate the promising electrochemical performance of the 2D MoN film. The cycling stability of the restacked 2D MoN film was also tested (Figure 4e). An excess of 95% of the initial volumetric capacitance was maintained after 25,000 cycles at a sweep rate of 100 mV s−1. The high conductivity of 2D MoN definitely plays a key role in assuring the long-term stability by fast electron transport and reversible charge storage. Since electrodes with large mass loading per unit of area are preferable for many practical applications, we fabricated a restacked 2D MoN electrode with a thickness of 30 μm and tested it at a sweep rate of 100 mV s−1 (Figure 4f). Compared with the 1 μm thick electrode, the CV curve of the 30 μm-thick electrode still showed a rectangular shape and the specific capacitance did not significantly decrease, which indicated that metallic 2D MoN is promising for practical applications in large devices. Flexible solid-state supercapacitors (SCs) attracted much interest recently, owing to their small size, better performance compared to microscale Li-ion batteries, and safety demanded in portable/wearable electronics.40 Since restacked 2D MoN has excellent electrochemical performance and is both flexible and electrochemically active, we used it to fabricate a flexible solid-state SC. Figure 5a illustrates the structure of the flexible solid-state SC. It is composed of two symmetric restacked 2D MoN electrodes, a separator, two flexible substrates (polyethylene glycol terephthalate, PET), and a LiCl/PVA solid-state electrolyte. The solid-state SCs had a good flexibility and mechanical stability, as displayed in Figure 5b. The CV curves of the solid-state SC at different sweep rates are shown in Figure 5d, through which the typical capacitive behavior is revealed. The energy and power density of the solid-state SC are summarized in the Ragone plot (Figure 5e). The largest energy density is about 2 mWh cm−3, and largest power density
in 1 M H2SO4 electrolyte was calculated from CV curves at different sweep rates, as shown in Figure 4c. Encouragingly, the volumetric capacitance value reached 928 F cm−3 at a sweep rate of 2 mV s−1. This value is superior to the metal−organic frameworks (MOFs) derived carbon (10 F cm−3 in organic electrolyte),34−37 chemically converted graphene (256 F cm−3),5 restacked 1T-MoS2 (650 F cm−3),14 Ti3C2 MXene (340 F cm−3),1 and at the same level with MXene “clay” (900 F cm−3).15 Furthermore, the volumetric capacitance maintained around 200 F cm−3 at an ultrahigh sweep rate of 20 V s−1, owing to the remarkably high conductivity of the restacked 2D MoN films, which is extremely beneficial to high-rate energy storage. Galvanostatic charge/discharge measurements at high current density in 1 M H2SO4 (Figure S8) show a nearly ideal triangular shape indicative of capacitive behavior. Besides H2SO4, various electrolytes with different cationic radii including 1 M Li2SO4, Na2SO4, MgSO4, K2SO4 as well as KOH were applied to the measurements of electrochemical performance of restacked 2D MoN film (Figure 4b). For closeto-neutral electrolyte, as the cationic radius gradually increases from Li+ to K+, the CV curves keep the rectangular shape at sweep rates up to 100 mV s−1. In addition, similar results were achieved in 1 M KOH electrolyte. However, the working potential window for various electrolytes obviously differs. The largest potential window for close-to-neutral electrolyte is achieved in Li2SO4 (0.9 V), which is larger than the value in MgSO4 (0.8 V), Na2SO4 (0.7 V), and K2SO4 (0.65 V). Such a phenomenon is similar to carbon-based materials in different electrolytes in which the larger hydrated ionic radius could provide a more negative potential window.38,39 The volumetric capacitances in various electrolytes were calculated as shown in Figure 4c. Similar to the result in H2SO4, high-rate performance was also found in these aqueous electrolytes. The remarkable electrochemical performance can be summarized with the two following reasons: First, nearly all active atoms of the MoN flakes were exposed to the electrolyte due to their 2D morphology, which indicated that the usual confinement in bulk MoN due to slow ion diffusion would be largely relaxed. In addition, the intrinsic metallic conductivity facilitates the electron transport in electrodes. 2183
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devices using 2D MoN electrodes could power a commercial LED, suggesting potential applications in wearable/potable electronics. Synthesis of other nitrides, such as W2N and V2N, using the same method suggests that other 2D metal nitrides, and possibly carbides, can be synthesized by changing the precursor and synthesis parameters.
EXPERIMENTAL SECTION Synthesis of Metallic 2D Molybdenum Nitride Nanosheets. The 2D molybdenum nitride nanosheets were synthesized in three steps. First, 2D h-MoO3@NaCl powder was prepared using our previously reported salt-templating method.26 Typically, 0.4 g Mo powder was dispersed in 30 mL of ethanol with stirring for 30 min on a magnetic stirrer. Then, 1.4 mL of H2O2 (30%) solution was added to the Mo powder suspension dropwise. After stirring for 48 h, the solution turned blue. Then, the precursor solution was mixed with 640 g of NaCl powder and dried at 50 °C with stirring. The 2D h-MoO3@ NaCl was produced by annealing as-prepared precursor at 280 °C for 2 h at a ramp rate of 2 °C/min under Ar atmosphere. The produced 2D h-MoO3@NaCl was further annealed at 650 °C for 5 h at a ramp rate of 1 °C/min under a mixture of NH3 (5%) and Ar. Finally, the product was washed by deionized water to remove NaCl and dispersed in deionized water. After centrifugation at 2500 rpm for 20 min, we collected the supernatant for further studies. Synthesis of 2D Tungsten Nitride Nanosheets. The 2D tungsten nitride nanosheets were synthesized in three steps. First, 2D h-WO3@KCl powder was prepared using our previously reported salttemplating method.26 Typically, 3 mL of H2O2 (30%) solution were added into the 0.1 g of W powder dropwise. Thirty mL of ethanol were added to the solution with stirring for 1 h on a magnetic stirrer. Then the precursor solution was mixed with 500 g of KCl powder and dried at 50 °C with stirring. The precursor was annealed at 500 °C for 2 h at a ramp rate of 2 °C/min under Ar atmosphere. The produced 2D h-WO3@KCl was further annealed at 700 °C for 5 h at the ramp rate of 1 °C/min under the mixture of NH3 (5%) and Ar. Finally, the product was washed by deionized water to remove KCl and dispersed in deionized water. After centrifugation at 2500 rpm for 20 min, we collected the supernatant for further studies. Synthesis of 2D Vanadium Nitride Nanosheets. The 2D vanadium nitride nanosheets were synthesized by three steps. First, 2D h-VOx@NaCl powders were prepared using our previously reported salt-templating method.26 Typically, 0.2 g of V powder was dispersed in 30 mL of ethanol with stirring for 30 min on a magnetic stirrer. Then 1 mL of H2O2 (30%) solution was added to the V powder suspension dropwise. After stirring for 48 h, the solution turned tawny. Then the precursor solution was mixed with 500 g of NaCl powder, drying at 50 °C with stirring. The 2D h-VOx@NaCl was produced by annealing the as-prepared precursor at 500 °C for 2 h at a ramp rate of 2 °C/min under Ar atmosphere. The produced 2D h-VOx@NaCl was further annealed at 800 °C for 5 h at a ramp rate of 1 °C/min under a mixture of NH3 (5%) and Ar. Finally, the product was washed by deionized water to remove NaCl and dispersed in deionized water. After centrifugation at 2500 rpm for 20 min, we collected the supernatant for further studies. Theoretical Calculations of Metallic 2D MoN. Density functional theory (DFT) calculations were performed by using the Vienna ab initio simulation package (VASP).44,45 We have employed projector augmented-wave (PAW) method,46 Perdew−Burke−Ernzerhof (PBE) exchange−correlation functional, an energy cutoff of 400 eV for the plane-wave basis, and Monkhorst−Pack k-point sampling of 8 × 8 × 1. Geometry optimizations were performed with a criterion of the maximum residual force
