A method for ammonia-free preparation of two-dimensional molybdenum nitride and crystalline Mo2N nanosheets

Abstract

The present application relates to the technical field of nano energy storage material preparation, and particularly relates to a two-dimensional molybdenum nitride and crystalline Mo2N nanosheet ammonia-free preparation method. A two-dimensional molybdenum boride compound derived from a rare earth molybdenum aluminum boride compound is used as a precursor to prepare two-dimensional molybdenum nitride and crystalline Mo2N nanosheet with a six-fold symmetric layered structure under an ammonia-free condition. A compound with a hexagonal crystal system formed by metal molybdenum, heavy rare earth metal, metal aluminum and boron is used as a precursor. After wet etching of an aluminum atom layer and a rare earth atom in the compound by using a fluorine-containing acid solution, intercalation and exfoliation are performed to form a two-dimensional derivative with a certain energy storage density. The two-dimensional derivative is annealed at different temperatures under nitrogen to control the atomic layer spacing, surface chemical state, surface functional group and chemical bond in the two-dimensional derivative, realize in-situ conversion from the two-dimensional molybdenum boride to the two-dimensional molybdenum nitride and crystalline Mo2N with a six-fold symmetric layered structure, and significantly improve the specific capacitance.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of nano energy storage material preparation, and particularly relates to a two-dimensional molybdenum nitride and a method for preparing Mo2N nanosheets without ammonia gas. BACKGROUND

[0002] New energy development and utilization has become the mainstream of future energy development in China, including energy technologies such as wind energy, water energy and nuclear energy. However, energy storage is an inevitable issue, and supercapacitors are a new type of energy storage and conversion device, which not only has high energy density, but also has high power density. It is a technology that can solve the growing energy demand in an energy shortage era. Two-dimensional materials represented by graphene have wide application in the field of new energy.

[0003] In recent years, a new type of two-dimensional transition metal carbon / nitride (MXene) has appeared. This two-dimensional material is prepared by chemical etching of three-dimensional MAX phase materials, has the characteristics of rich composition elements, diverse and adjustable surface functional groups, and shows great application potential in energy storage, electromagnetic shielding and electrocatalysis. Inspired by MXene, some researchers have predicted a series of two-dimensional transition metal borides (MBene) with excellent electrocatalytic, energy storage and conductive properties through theoretical calculation, and have tried to synthesize different precursors MAB phase and MBene in experiments. Alameda et al. used single crystal MoAlB as a precursor, and carried out chemical etching under certain conditions using NaOH, HF, LiF / HCl and other traditional etchants. Only a small amount of MBene was observed in the high dislocation area of the single crystal edge, accompanied by Mo6Al5B6, Mo4Al3B4, Mo3Al2B3 and Mo2AlB2 symbiotic structures. Continued etching will lead to the dissolution of boride layers. Among them, Mo2AlB2 is considered to be the most promising MBene precursor, but the Mo2AlB2 obtained by experiment is a thermodynamic metastable phase. "H. Zhang, F.-Z. Dai, H. Xiang, X. Wang, Z. Zhang, Y. Zhou, J. Mater. Sci. Technol. 2019, 35, (8) 1593-1600." Zhang et al. found that soaking Cr2AlB2 in HCl can etch the Al layer, but the product cannot be exfoliated into single-layer or few-layer borides. "J. Wang, T.-N. Ye, Y. Gong, J. Wu, N. Miao, T. Tada, H. Hosono, Nat. Commun. 2019, 10, (1) 2284." Other methods such as dealloying cannot obtain two-dimensional derivatives with the topological structure of three-dimensional precursors. "M. Dahlqvist, Q. Tao, J. Zhou, J. Palisaitis, M. Halvarsson, M. Edstrom, J. Mater. Chem. A 2019, 7, (32) 17927-17934." Persson, J. Rosen, J. Am. Chem. Soc. 2020, 142, (43) 18583-18591.,” J. Zhou, J. Palisaitis, J. Halim, M. Dahlqvist, Q. Tao, I. Persson, L. Hultman, Persson Per, J. Rosen, Science 2021, 373, (6556) 801-805.,” P. Helmer, J. Halim, J. Zhou, R. Mohan, B. Wickman, J. Dahlqvist, Tao, Zhou, Helmer et al. predicted a series of thermodynamically stable in-plane chemically ordered quaternary layered borides (i-MAB) by theory and synthesized Mo 4 / 3 Y 2 / 3 AlB2, Mo 4 / 3 Sc 2 / 3 AlB2. The following year, using HF etching Mo 4 / 3Y 2 / 3 AlB2, Mo 4 / 3 Sc 2 / 3 AlB2 obtained two-dimensional MBene derivatives with certain electrocatalytic application potential. However, they only reported the preparation method of two-dimensional MBene and poor electrochemical hydrogen evolution performance.

[0004] The existing technology for preparing two-dimensional layered transition metal nitrides is very limited, often by etching ternary nitride precursors in fluorine-containing eutectic molten salt or fluorine-containing acidic solution or using ammonia gas, sodium amide to nitride transition metal carbide. However, due to the instability of the ternary nitride precursor, the nitride layer is easily dissolved in the fluorine-containing acidic solution, and the use of ammonia gas and sodium amide also has potential safety risks. In view of the excellent energy storage performance of transition metal nitrides, a safe and green preparation method of two-dimensional transition metal nitrides is urgently needed. SUMMARY

[0005] The present application provides a preparation method for synthesizing a two-dimensional derivative molybdenum boride of a rare earth molybdenum aluminum boride compound as a precursor, in-situ generating a two-dimensional molybdenum nitride with a six-fold symmetric layered structure and a crystalline Mo2N nanosheet; and the two-dimensional derivative has certain capacitance performance.

[0006] The technical scheme of the present application is as follows: a two-dimensional molybdenum nitride and crystalline Mo2N nanosheet preparation method without ammonia gas, taking a two-dimensional derivative molybdenum boride of a rare earth molybdenum aluminum boride compound as a precursor, and performing nitrogenization treatment at different temperatures under a specific atmosphere without ammonia gas, so as to control the interlayer spacing, surface functional groups, internal chemical bonds and structure of the two-dimensional derivative molybdenum boride, and prepare a two-dimensional molybdenum nitride and crystalline Mo2N nanosheet with a six-fold symmetric layered structure, that is, in-situ obtain a two-dimensional layered molybdenum nitride with the structure of the two-dimensional derivative molybdenum boride of the rare earth molybdenum aluminum boride compound, and a crystalline Mo2N nanosheet.

[0007] The preparation steps of the two-dimensional derivative molybdenum boride are as follows:

[0008] Step (1), (Mo 2 / 3 R 1 / 3 )2AlB2 precursor is etched by a fluorine ion-containing solution, R is a rare earth element, and a two-dimensional derivative with a molybdenum and boron atom ratio no longer being (Mo 2 / 3 R 1 / 3 )2AlB2 precursor is formed, wherein the ordered vacancies formed by etching of the rare earth atoms are contained; a negative ion functional group is bonded to the surface of the two-dimensional derivative during the etching process; after the etching is completed, a molybdenum-based MBene is formed, and after intercalation treatment and hand shaking treatment, a molybdenum-based MBene suspension of one layer or more is obtained.

[0009] Step (2), the molybdenum-based MBene of one layer or more is stably suspended in deionized water; the molybdenum-based MBene suspension is freeze-dried to obtain an MBene aerogel, which is a two-dimensional derivative molybdenum boride of a rare earth molybdenum aluminum boride compound.

[0010] The two-dimensional derivative molybdenum boride of the rare earth molybdenum aluminum boride compound mainly consists of molybdenum Mo, boron B, F, Cl and O; the F, Cl and O serve as surface functional groups; the two-dimensional derivative molybdenum boride of the rare earth molybdenum aluminum boride compound contains ordered vacancies introduced by rare earth atom vacancies, and has the same six-fold symmetric structure as the (Mo 2 / 3 R 1 / 3 )2AlB2 precursor.

[0011] The specific atmosphere is nitrogen.

[0012] The nitrogenization treatment at different temperatures specifically refers to that the nitrogenization temperature is 300-700 DEG C.

[0013] The fluorine ion-containing solution is an HF aqueous solution or a mixed solution of a fluorine-containing salt and hydrochloric acid; the intercalation agent for intercalation treatment is tetramethylammonium hydroxide TMAOH or tetrabutylammonium hydroxide TBAOH.

[0014] The fluorine-containing salt is LiF, NaF, KF, NH4HF2 or NH4F.

[0015] The present application has the following beneficial effects: the two-dimensional molybdenum nitride and the crystalline Mo2N nanosheet prepared by the ammonia-free preparation method of the present application, by taking the two-dimensional derivative molybdenum boride of the rare earth molybdenum aluminum boride compound as the precursor, the two-dimensional molybdenum nitride and the crystalline Mo2N nanosheet with a six-fold symmetric layered structure are prepared under the condition of no ammonia gas. By performing chemical selective etching on the synthesized layered rare earth molybdenum aluminum boride compound with an element ratio of Mo:R:Al:B = 1.33:0.67:1:2 (wherein R is Tb, Dy, Ho, Er, Tm, Lu), the Al layer and the rare earth atom are etched away, and the six-fold symmetry of the precursor is retained. By nitriding at a specific temperature under a high-purity nitrogen atmosphere, the MBene interlayer ion, the surface functional group and the Mo-N bond introduced therein are controlled, so that the two-dimensional layered molybdenum nitride with the morphology and the six-fold symmetric structure of the two-dimensional MBene can be in-situ converted. The corresponding capacitance can be greatly increased to 219 F / g (at a scanning rate of 2 mV / s), the rate performance is increased to 40%, and the specific capacity can be maintained at 87% after 5000 cycles at a current density of 10 A / g. The crystalline Mo2N nanosheet originally converted has a specific capacitance of 228 F / g (at a scanning rate of 2 mV / s), the rate performance is increased to 69.8%, and the specific capacity can be maintained at 89% after 5000 cycles at a current density of 10 A / g. BRIEF DESCRIPTION OF DRAWINGS

[0016] Fig. 1(a) is an X-ray energy dispersive spectrum of the rare earth molybdenum aluminum boride precursor before etching and after etching;

[0017] Fig. 1(b) is a layered MBene suspension;

[0018] Fig. 1(c) is a digital photo of the MBene suspension filter sheet;

[0019] Fig. 1(d) is a scanning electron microscope photo of the cross section of the MBene suspension filter sheet;

[0020] Fig. 2(a) is a STEM-HAADF picture of the MBene;

[0021] Fig. 2(b) is a local enlarged view based on Fig. 2(a);

[0022] Fig. 3(a) is an XRD spectrum of the MBene annealed at different temperatures;

[0023] Fig. 3(b) is an XPS spectrum of the MBene annealed at different temperatures;

[0024] Fig. 4(a) is a transmission electron microscope photo and a selected area electron diffraction of the two-dimensional layered molybdenum nitride with a six-fold symmetric structure generated in-situ from the MBene;

[0025] Figure 4(b) shows the transmission electron microscope image and selected area electron diffraction of the Mo2N nanosheets generated in situ by MBene;

[0026] Figure 5(a) shows the CV curve of the original MBene;

[0027] Figure 5(b) shows the GCD curve of the original MBene;

[0028] Figure 5(c) shows the CV curves of Mo2N nanosheets generated in situ by MBene;

[0029] Figure 5(d) shows the GCD curves of Mo2N nanosheets generated in situ by MBene. Detailed Implementation

[0030] This invention designs an ammonia-free preparation method for two-dimensional molybdenum nitride and crystalline Mo2N nanosheets. Its key feature lies in the synthesis of a layered molybdenum rare-earth aluminum boron compound (Mo2N) with an elemental ratio of Mo:R:Al:B = 1.33:0.67:1:2 (where R represents Tb, Dy, Ho, Er, Tm, and Lu). 2 / 3 R 1 / 3 The Al layer and rare earth atoms were selectively etched away using a chemical method, while retaining the Mo layer. 2 / 3 R 1 / 3 The six-fold symmetry of the AlB2 precursor yields a two-dimensional layered molybdenum boride with a large interlayer spacing. Further in-situ nitridation in a nitrogen atmosphere generates a two-dimensional layered molybdenum nitride with a six-fold symmetry structure and crystalline Mo2N nanosheets.

[0031] A mixed solution of fluoride salts (LiF, NaF, KF, NH4HF2, NH4F) and hydrochloric acid was used. 0.5 g of (Mo) 2 / 3 R 1 / 3 After adding AlB2, the mixture is continuously stirred in a water bath at 35–55°C for 2–3 days to selectively etch away the Al atomic layer and R atoms. Figure 1a After intercalation, exfoliation, and centrifugation, the supernatant is collected to obtain a colloidal aqueous solution exhibiting the Tyndall effect. Figure 1b This colloidal solution is composed of (Mo). 2 / 3 R 1 / 3 Two-dimensional Mo derived from the 2AlB2 type i-MAB phase 1.33 B 2-x It is composed of MBene nanosheets. After vacuum filtration, only MBene fragments were obtained due to their poor mechanical properties. Figure 1c The cross-section of the fragment still shows a mesoscale layered stacking morphology. Figure 1d The aerogel powder obtained by freeze-drying the colloidal solution retains (Mo). 2 / 3 R1 / 3 )2AlB2 precursor (Figure 2). The specific capacitance is 4 F / g at a scan rate of 2 mV / s.

[0032] Nitridation at a specific temperature under a high-purity nitrogen atmosphere can regulate intercalated ions between MBene layers, surface functional groups, and Mo-N bonds introduced therein, and in-situ transform into two-dimensional layered molybdenum nitride with a morphology and six-fold symmetry structure of the two-dimensional MBene and a crystalline Mo2N nanosheet. As shown in Figure 3(a), after nitridation at 300°C, the interlayer spacing of the MBene is reduced from to The XPS peak of carbon (C) is also reduced, which can prove that the intercalated ions are deintercalated from the interlayer after decomposition, and the MBene still retains the six-fold symmetry structure. When the nitridation temperature is increased to 500°C, the layered structure can still be observed from the XRD spectrum, and the interlayer spacing remains unchanged. As shown in Figure 4(a), the electron diffraction result shows that the MBene still has a six-fold symmetry structure at this time, and the XPS analysis shows that Mo-N bonds have been introduced therein, which indicates that a six-fold symmetry molybdenum nitride with a two-dimensional layered structure is generated in-situ at this temperature. When the nitridation temperature is further increased to 700°C, the small-angle diffraction peak caused by the large interlayer spacing cannot be observed in the XRD spectrum, which indicates that the layered structure has been destroyed at this time. However, two diffraction peaks are observed at 37.4° and 43.4°, which are labeled as Mo2N diffraction peaks. Through transmission electron microscopy observation, the sample after nitridation at 700°C contains cluster-like substances, and electron diffraction analysis shows that the phase at this time is Mo2N, which is consistent with the XRD analysis result, indicating that a crystalline Mo2N nanosheet is generated in-situ after nitridation at 700°C. The capacitance of the two-dimensional molybdenum nitride can be greatly increased to 219 F / g (at a scan rate of 2 mV / s) compared with the original MBene, and the rate performance is increased to 40%, and the specific capacity can still be maintained at 87% after 5000 cycles at a current density of 10 A / g; the crystalline Mo2N nanosheet has a specific capacitance of 228 F / g (at a scan rate of 2 mV / s), and the rate performance is increased to 69.8%, and the specific capacity can still be maintained at 89% after 5000 cycles at a current density of 10 A / g. We believe that the reason why the MBene can be nitridized into a two-dimensional molybdenum nitride and a crystalline Mo2N nanosheet under an ammonia-free condition is that the two-dimensional MBene obtained by chemical etching and peeling loses Al atoms and Ho atoms, and the charge balance in the crystal is broken. To balance the charge, the MBene surface will adsorb negative ion functional groups such as F, Cl, and OH through electrostatic attraction, and in the high-temperature nitridation process, the molybdenum-boron bond with a weak bond is broken, and the functional groups are lost, and the nitrogen atom with a strong electronegativity is combined with the molybdenum atom to form a two-dimensional molybdenum nitride and a crystalline Mo2N nanosheet. Therefore, the present application develops a safe and green preparation method of a two-dimensional transition metal nitride and a crystalline Mo2N nanosheet.

Claims

1. A method for the ammonia-free preparation of two-dimensional molybdenum nitride and crystalline Mo2N nanoplates, characterized in that, The two-dimensional derivative molybdenum boride of a rare earth molybdenum aluminum boride compound is used as a precursor, and is subjected to nitriding treatment at different temperatures under a specific atmosphere without ammonia gas, so as to control the interlayer spacing, surface functional group, internal chemical bond and structure of the two-dimensional derivative molybdenum boride, and to prepare two-dimensional molybdenum nitride with six-fold symmetry layered structure and crystalline Mo2N nanosheet; The preparation steps of the two-dimensional derivative molybdenum boride are as follows: Step (1), (Mo 2 / 3 R 1 / 3 )2AlB2 precursor is etched by a solution containing fluorine ions, forming a two-dimensional derivative of the original stoichiometric ratio in the (Mo 2 / 3 R 1 / 3 )2AlB2 precursor, which contains ordered vacancies formed by etching; during etching, the surface of the two-dimensional derivative bonds with negative ion functional groups; After etching, a molybdenum-based MBene is formed, and after intercalation treatment and hand shaking treatment, a molybdenum-based MBene suspension of more than one layer is obtained; Step (2), the molybdenum-based MBene of more than one layer is stably suspended in deionized water; and the molybdenum-based MBene suspension is freeze-dried to obtain an MBene aerogel, which is the two-dimensional derivative molybdenum boride of the rare earth molybdenum aluminum boride compound. The two-dimensional derivative molybdenum boride of the rare earth molybdenum aluminum boride compound mainly consists of molybdenum Mo, boron B, F, Cl and O; the F, Cl and O serve as surface functional groups; the two-dimensional derivative molybdenum boride of the rare earth molybdenum aluminum boride compound contains ordered vacancies introduced by rare earth atom vacancies, and has the same six-fold symmetry structure as the (Mo 2 / 3 R 1 / 3 )2AlB2 precursor.

2. The production method according to claim 1, characterized by, The specific atmosphere is nitrogen.

3. The production method according to any one of claims 1 or 2, characterized by, The nitriding treatment at different temperatures is specifically as follows: the nitriding temperature is 300-700 DEG C.

4. The preparation method according to claim 3, characterized in that, The fluoride ion-containing solution is an aqueous HF solution or a mixed solution of a fluoride salt and hydrochloric acid; and the intercalation agent for the intercalation treatment is tetramethylammonium hydroxide TMAOH or tetrabutylammonium hydroxide TBAOH.

5. The preparation method according to claim 4, characterized in that, The fluoride salt is LiF, NaF, KF, NH4HF2 or NH4F.

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