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Topochemical synthesis of holey 2D molybdenum nitrides nanosheets via lime-assisted nitridation of layered MoS2
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Topochemical synthesis of holey 2D molybdenum nitrides nanosheets via lime-assisted nitridation of layered MoS2 Guo-Dong Sun, Guo-Hua Zhang∗, Kuo-Chih Chou State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: 2D nanosheets Holey structure Molybdenum nitrides Topochemical synthesis
In this study, we introduced a facile and efficient method for the large-scale synthesis of porous 2D molybdenum nitrides nanosheets via the topochemical nitridation of MoS2 by NH3 with the assistance of CaO at 750 °C–820 °C. The presence of CaO can noticeably promote the reduction rate of MoS2 by generating CaS. After acidic leaching, porous 2D MoN, a mixture of 2D MoN and Mo2N, as well as Mo2N nanosheets with the thickness of about 7 nm and pore size of about 10.5 nm were successfully prepared at different temperatures. When the temperature increased from 750 °C to 820 °C, the product gradually changed from MoN to Mo2N. Due to the weak Van der Waals forces between the layers of MoS2, the topochemical substitution of S in MoS2 by N led to the exfoliating of layers. The large difference of crystal structures and volume between precursor and product, as well as the high reaction rate led to the formation of porous structure of molybdenum nitrides.
1. Introduction Recent years, porous nanomaterials with unique surface structure and morphological stability have received intensive attention for promising applications: such as energy conversion and storage, as well as catalysis [1-18]. Porous 2D nanomaterials have the fascinating synergetic chemical/physical properties of both porous structure and 2D architecture, such as abundant active surface, high electronic conductivity in-plane, enabling fast transport of ions and electrolyte through the porous flakes [2,3,7,10,12,19]. As a demonstration of the application of porous 2D transition metal nitride (TMN) nanosheets [2], in a lithium–sulfur battery, a high initial capacity of > 1000 mAh g−1 at 0.2 C under a high areal sulfur loading (> 5 mg cm−2) can be achieved with only about 13% degradation over 1000 cycles at 1 C. In Lithium-Ion Batteries, porous 2D Co3S4 nanosheets delivered a high discharge capacity of about 968 mA h g−1 with excellent cycling stability [20]. Therefore, the construction of porous 2D nanomaterials is an important trend in 2D materials [1,5,7]. 2D molybdenum nitrides nanosheets have excellent electrical, magnetic and catalytic activity properties, making them be hot candidates in many fields ranging from catalyst to energy conversion and storage [2,21-27]. For example, it was reported by Xiao et al. [25] that 2D molybdenum nitride nanosheets exhibited a very high volumetric capacitance of 928 F cm−3 in sulfuric acid electrolyte with an excellent rate performance. Until now, even though some methods have been
∗
developed for synthesis of 2D molybdenum nitrides nanosheets with smooth surface, such as: liquid exfoliating method [28], chemical solution deposition [29], and topochemical synthesis methods [22,25,26,30], few literature has been reported to prepare holey 2D molybdenum nitrides nanosheets. Nowadays, it is still a challenge to produce holey 2D molybdenum nitrides nanosheets, especially via a simple, efficient and low-cost pathway. In this study, a facile method was proposed to prepare porous 2D molybdenum nitrides nanosheests via topochemical nitridation of MoS2 with the assistance of lime in NH3 atmosphere at 750–820 °C. MoS2 is the major form of molybdenum source in nature, which has layered S–Mo–S structure and is easy to be exfoliated due to the weak Van der Waals forces between the layers [31-33]. The lime can dramatically promote the reduction and nitridation rate via combining with S to generate CaS. In the nitridation process, 2D molybdenum nitrides nanosheests with porous structure and a by-product of CaS were produced. After acidic leaching, the CaS was removed and porous 2D molybdenum nitrides nanosheests were obtained. 2. Experimental section MoS2 (purity, > 98%) with an average size of around 3 μm was used as molybdenum source. Fig. 1(a) and (b) show the Field emission scanning electron microscope (FE-SEM) micrographs of the raw MoS2, which has schistose morphology. From the crystal structures images of
Corresponding author. E-mail address: [email protected] (G.-H. Zhang).
https://doi.org/10.1016/j.ceramint.2019.10.061 Received 15 July 2019; Received in revised form 28 September 2019; Accepted 7 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Guo-Dong Sun, Guo-Hua Zhang and Kuo-Chih Chou, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.061
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Fig. 1. (a) and (b) FE-SEM and (c) and (d) corresponding crystal structure images of raw MoS2 powder. Fig. 2. Temperature dependence of the change of standard Gibbs free energy.
raw MoS2 (Fig. 1(c) and (d)), it can be seen that MoS2 owns a layered S–Mo–S structure. CaO powder was obtained by roasting CaCO3 at 1000 °C for 6 h. The MoS2 and CaO powders with a CaO and MoS2 molar ratio of 3.0 were mixed in a mortar for 20 min. In each experiment, about 5 g sample with a thickness of about 1.5 cm was reduced and nitridized in flowing NH3 (200 ml/min) at desired temperatures for a certain time (heating/cooling rate 20 °C/min). Then the obtained product was leached by dilute hydrochloric acid (3 mol/L) and deionized water to remove the by-product and unreacted CaO. Finally, molybdenum nitrides powder was obtained after filtration. The phase composition and crystalline structure of sample were analyzed by X‐ray diffraction (XRD; TTR III, Rigaku Corporation, Japan). The chemical states and termination species of product were analyzed by X‐ray photoelectron spectroscopy (XPS; ESCALAB 250 Xi; Thermo Scientific, American). The morphology, thickness and crystalline structure of particles were characterized by FE‐SEM (ZEISS SUPRA 55, Oberkochen, Germany) with energy dispersive X‐ray spectroscopy (EDS) and transmission electron microscopy (TEM). The thickness of sample was analyzed by Atomic force microscopy (AFM, Dimension FastScan, Bio-Logic Science Instruments). The contents of N and O were determined using the O–N–H analyzer (EMIA-830, HORIBA, Japan). The N2 adsorption-desorption isotherms and pore size were analyzed by a Micromeritics ASAP2460 analyzer. The UV–vis absorption spectra were carried by using a spectrophotometer (Hitachi, U-3900). The thermodynamic calculations were conducted by FactSage 7.0 with pure substances database.
seen that the presence of CaO can dramatically decrease the critical temperature of the reduction-nitridation reaction of MoS2 to 360 °C. Additionally, since CaO can fix the generated H2S, it may be also possible to noticeably improve the reaction rate.
4 5 MoS2 + NH3 = 0.5Mo2 N + 2H2 S + N2 3 12
Δr G (reaction(1 )) = Δr Gθ(reaction(1 )) + RT ln(
(pH2S /pθ )2⋅(p N2 /pθ )5/12 ) (pNH3 /pθ )4/3
CaO + H2 S = CaS + H2 O
(1)
(2) (3)
4 5 MoS2 (s) + NH3 + 2CaO = 0.5Mo2 N + 2CaS + 2H2 O + N2 3 12 (4) Fig. 3(a) shows the XRD patterns of products obtained after reaction at different temperatures for 3 h. It can be seen that after reaction at 750 °C, MoS2 were not completely reduced and nitridized, and there were still lots of MoS2. Additionally, the solid products were only molybdenum nitrides and CaS without other phase, indicating that molybdenum nitrides could be directly generated from MoS2. When the temperature was increased to 800 °C and 820 °C, all the MoS2 were completely reduced and nitrided to molybdenum nitrides within 3 h, which was tremendously less than the dozens of hours for the case without the addition of CaO [30]. Fig. 3(b) shows the XRD patterns of products obtained at different temperatures and time after leaching. It can be seen that the excessive CaO and by-product CaS were completely removed. For the product obtained after reacting at 750 °C for 7 h, MoN product (PDF# 89–4318) was generated. With the further increase of temperature, the relatively intensity of the diffraction peaks of MoN gradually decreased. As the temperature was increased to 775 °C and 800 °C, the mixture of MoN and Mo2N (PDF# 15–1366) was obtained. At the temperature of 820 °C, the diffraction peaks of MoN almost disappeared, indicating that MoN was unstable at higher temperatures. However, at 860 °C, a few diffraction peaks of Mo appeared, indicating that Mo2N decomposed to Mo. To further investigate the reaction process, the reactions at 820 °C for different time were conducted, and the XRD patterns of products were shown in Fig. 3(c). It can be seen that there were only Mo2N and unreacted MoS2 after the reaction for 1 h. Even the reaction time was prolonged to 5 h, Mo2N can still stably exist. Therefore, MoN, the mixture of MoN and Mo2N, and Mo2N were successfully prepared by adjusting the reaction temperatures. To further quantitatively analyze the N and O amounts, the prepared molybdenum nitrides nanosheets were analyzed by O–N–H analyzer, and the contents of O and N of the molybdenum nitrides prepared at
3. Results and discussion The reduction and nitridation of MoS2 to Mo2N by NH3 can be described by reaction (1). The change of standard Gibbs free energy of reaction (1) is shown in Fig. 2, from which it can be found that the critical temperature (ΔrGθ = 0) for reaction (1) is as high as 945 °C. Therefore, from the thermodynamic perspective, the reduction of MoS2 by NH3 at below 900 °C is impossible under the standard condition. Eq (2) shows the change of actual Gibbs free energy of reaction (1). It can be seen that the decrease of partial pressure of H2S can decrease the critical temperature of reaction (1) (ΔrG = 0). For example, according to Eq. (2), when the partial pressure of H2S is reduced to 0.01 atm, the critical temperature can be reduced to about 650 °C. It is well known that CaO is a cheap and common-used sulfur fixing agent [34-36]. The reaction between H2S with CaO can be described by reaction (3), and its change of standard Gibbs free energy is depicted in Fig. 2, which is always thermodynamically feasible. Thus, in the presence of CaO, the reaction can be described by reaction (4), and the corresponding change of standard Gibbs free energy is also shown in Fig. 2. It can be 2
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Fig. 3. XRD patterns of products, (a) after reaction in NH3 atmosphere, (b) after leaching and (c) after reacting at 820 °C for different time; XPS spectra of Mo2N prepared at 820 °C for 3 h, (d) Mo 3p and N 1s, (e) Mo 3d and (f) O 1s.
seen that porous MoN nanosheets were obtained from the bulk MoS2. Fig. 4(b) shows the FE-SEM micrograph of the side of MoN particles. It can be seen that the basic planes were spread apart with a thickness of about 10 nm, resulting from the reduction and nitridation treatment. Fig. 4(c) and (d) show the TEM images of nanosheets of MoN, which indicates that they were quite thin due to the high transparency. From the high‐resolution TEM (HRTEM) and fast Fourier transform (FFT) images (Fig. 3(d)), it can be seen that MoN nanosheets had a high crystallinity with hexagonal atomic arrangement and an interplanar spacing of 2.46 Å, coinciding with the (200) crystal plane of MoN [42]. Fig. 4(e-h) show the FE-SEM and TEM images of the mixture of MoN and Mo2N prepared at 800 °C, in which the morphology and thickness (about 12 nm, Fig. 4(f)) of product were similar to that of 750 °C. Meanwhile, the interplanar spacing of (200) (MoN) was 2.49 Å (Fig. 4(h)). Fig. 4(i) and (j) show the FE-SEM micrographs of Mo2N prepared at 820 °C, in which Mo2N nanosheets had a good dispersity with a thickness of about 9 nm. The HRTEM and FFT show the Mo2N also had a hexagonal atomic arrangement with a interplanar spacing of 2.11 Å, corresponding to the crystal plane of (200) (Mo2N) [24]. Therefore, porous MoN, mixture of MoN and Mo2N, as well as Mo2N nanosheets with a thickness of about 10 nm were successfully prepared. These porous structures are noticeably different from the 2D MoN or Mo2N nanosheets with smooth surfaces reported in the previous literatures [22,25,26,28,30,42]. Additionally, the thickness of the prepared molybdenum nitride was also further analyzed by AFM, as shown in Fig. 4(m)-(o). It can be seen that the thickness of the prepared porous 2D molybdenum nitrides nanosheets was about 6.2~7.2 nm, which is slightly smaller than the corresponding values by FE-SEM analyses (around 10 nm). The Nitrogen adsorption/desorption isotherms and Barret-JoynerHalenda (BJH) pore size distribution of the prepared porous 2D molybdenum nitride (820 °C-3h, without any treatment of ultrasonic dispersion or chemical activation) are shown in Fig. 5(a) and (b), respectively. The Brunauer-Emmett Teller (BET) specific surface area of sample was about 42.28 m2/g, which shows a visible mesoporous structure with an average pore size of 10.5 nm, which is smaller than
Table 1 The contents of N and O of the prepared molybdenum nitrides nanosheets. Reaction temperature and time
O content/(wt%)
N content/(wt%)
750 °C-7h 775 °C-5h 800 °C-3h 820 °C-3h
0.9325 1.1604 1.0783 1.2160
13.4783 10.6841 9.2309 7.4934
different temperature are shown in Table 1. It can be seen that the N contents of sample produced at 750 °C for 7 h was 13.4783%, which is higher than the stoichiometric nitrogen content of MoN (12.73%), indicating that the molybdenum nitrides contain nitrogen-rich phase [37]. With the increase of temperature from 750 °C to 820 °C, the N content gradually decreased to 7.4934%, which is still slightly higher than the stoichiometric nitrogen content of Mo2N (6.73%). Additionally, the oxygen contents of products were around 1 wt%. XPS analysis was also conducted to investigate the surface composition and chemical states of the Mo2N product produced at 820 °C (3h). Fig. 3(d)-(f) show the high high‐resolution XPS spectra peaks of Mo 3p and N 1s, Mo 3d and O 1s. The spectrums of Mo 3p and N 1s region were fitted by components of: Mo–N (N 1s, 397.5 eV), Mo–N (Mo 3p3/2, 394.7 eV) and MoO3 (Mo 3p3/2, 399.5 eV), confirming the bonding of N to Mo [26]. In the Mo 3d region, the fitting peaks (binding energies) at 228.5 eV and 231.6 eV were corresponded to the Mo 3d5/2 and Mo 3d3/ 2 of Mo in Mo2N [38-40]. The binding energies at 235.5 eV and 232.8 eV could be in accordance to Mo6+, while 232.3 eV and 229.3 eV corresponded to Mo4+, resulting from the surface oxidation [26,38,41]. This surface oxidation was similar to the 2D molybdenum nitrides prepared by other methods [25,26]. The high-resolution spectrum of O 1s region is shown in Fig. 3(f), in which the binding energies at 530.2 eV, 531.1 eV and 532.0 eV were coincided with oxygen in MoO3, Mo–N-Ox, Mo–N-(OH)x [26]. Fig. 4 shows the FE-SEM and TEM micrographs of the produced molybdenum nitrides at different temperatures. Fig. 4(a) and (a) are FESEM images of product obtained at 750 °C for 7 h, from which it can be 3
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Fig. 4. FE-SEM and TEM images of products, (a)-(d) 750 °C-7 h, (b)-(h) 800 °C-3 h and (i)-(l) 820 °C-3 h. AFM images of the 2D molybdenum nitrides nanosheets prepared at (m) 800 °C-3 h and (n)-(o) 820 °C-3 h.
MoS2 and MoN/Mo2N had huge differences in crystal structures and volumes. For example, the molar volume of MoS2 is about 32.52 cm3/ mol (density, 4.92 g/cm3) [43], while the values of MoN and MoN0.5 are about 11.99 cm3/mol (density, 9.17 g/cm3) [44] and 10.96 cm3/ mol (density, 9.40 g/cm3) [45]. Therefore, in the presence of CaO, a relatively high reduction and nitridation rate of MoS2 would lead to a large change of crystal structures and volume, which further leaded to the formation of porous structure via the etching and recrystallizing of flakes [2]. As a comparison, similar experiment without CaO was also conducted at 800 °C for 30 h, and the TEM image of product is shown in Fig. 6(b). It can be seen that the prepared nanosheets did not have porous morphology, but single crystalline 2D flakes (MoN, interplanar spacing of (200), 2.47 Å) with smooth surfaces. Additionally, it was reported that the Mo2N prepared via nitridation of 2D molybdenum carbides (Mo2C) also had single crystalline 2D flakes without holey structure, due to the similar crystal structures and volume between the Mo2N and Mo2C [26]. Therefore, the large difference of crystal structures and volume (between precursor and product) and the high reaction rate were the crucial factors for producing the porous structure of molybdenum nitrides nanosheets.
the FE-SEM and TEM results. Ojha et al [21] prepared the porous Mo2N nanosheets and nanowires (using ammonium molybdate and g-C3N4 as molybdenum and N sources) with specific surface areas of 16 m2/g (thickness, about 30 nm (SEM result); pore radius, 2.9 nm) and 3 m2/g (thickness, about 100 nm (SEM result); pore radius, 3.2 nm), respectively. Therefore, the porous 2D molybdenum nitrides nanosheets prepared in the present study have relatively higher specific surface area due to the thicker sheets. Furthermore, to investigate the dispersibility of the porous 2D molybdenum nitrides nanosheets, the MoS2 or 2D molybdenum nitride nanosheets were dissolved into water or alcohol (3 mg/ml), and sonicated for 20 min. After standing for 72 h, the supernatants were removed and their photos are shown in Fig. 5(c) (insert pictures). It can be seen that the colors of water and alcohol with dissolved 2D molybdenum nitrides were dark brown and light brown, respectively. However, for the water or alcohol with MoS2, the color of solution was colorless, indicating that there was almost no MoS2 dissolving in water or alcohol. Furthermore, UV–vis analyses of the supernatants have been conducted and the results are also shown in Fig. 5(c). It can be seen that the absorbance intensity of the 2D molybdenum nitride in water solvent was higher than in alcohol, which could indicate the higher dispersibility of the 2D molybdenum nitride in water than alcohol. However, for the raw bulk MoS2, it can be seen that the corresponding absorbance intensities of both in water and alcohol are much weaker than those of the 2D molybdenum nitride. As discussed above, it can be found that compared to the raw bulk MoS2, the prepared molybdenum nitride nanosheets had much better dispersibility in different solvents. As discussed above, it can be found that the synthesis of 2D porous molybdenum nitrides is a typical topochemical synthesis process [2,22]. The raw MoS2 has a sandwich-like (S–Mo–S) layered structure with weak Van der Waals forces among the layers. Therefore, during the reduction and nitridation process, the S was topochemically substituted by N in ammonia, and H2S gas was also generated, which would lead to the exfoliating of layers, as shown in Fig. 4. Additionally,
4. Conclusions In this study, a facile method was reported for the efficient and large-scale synthesis of 2D porous molybdenum nitrides nanosheets via CaO-assisted topochemical transformation of MoS2 by NH3 at 750 °C–820 °C. The presence of CaO made the reaction between MoS2 and NH3 become thermodynamically feasible at below 750 °C, and also noticeably promoted the reduction rate of MoS2. Porous 2D MoN, a mixture of 2D MoN and Mo2N, as well as Mo2N nanosheets with the thickness of about 7 nm and pore size of about 10.5 nm were successfully prepared by adjusting the reaction temperature. The weak Van der Waals forces between the layers of MoS2 and the topochemical substitution of S in MoS2 by N via ammonia led to the exfoliating of layers. 4
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Fig. 5. (a) Nitrogen adsorption/desorption isotherms and (b) Barret-Joyner-Halenda (BJH) pore size distribution of the porous 2D molybdenum nitride prepared at 820 °C-3h. (c) UV–vis spectra of MoS2 and porous 2D molybdenum nitrides nanosheets (820 °C-3h) dissolved in different solvents (water and alcohol).
MoN was unstable at higher temperatures, and when the temperature was increased from 750 °C to 820 °C, the product gradually changed from MoN to Mo2N. The large difference of crystal structures and volume between precursor and product and the high reaction rate were the crucial factors for producing porous structure of molybdenum nitrides.
interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (51734002).
Declaration of competing interest The authors declare that they have no known competing financial
Fig. 6. Comparison of TEM images of products obtained from (a) reduction of MoS2 by NH3 with the assistance of CaO at 800 °C for 3 h and (b) reduction of MoS2 by NH3 at 800 °C for 30 h. 5
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Short communication
Topochemical synthesis of holey 2D molybdenum nitrides nanosheets via lime-assisted nitridation of layered MoS2 Guo-Dong Sun, Guo-Hua Zhang∗, Kuo-Chih Chou State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: 2D nanosheets Holey structure Molybdenum nitrides Topochemical synthesis
In this study, we introduced a facile and efficient method for the large-scale synthesis of porous 2D molybdenum nitrides nanosheets via the topochemical nitridation of MoS2 by NH3 with the assistance of CaO at 750 °C–820 °C. The presence of CaO can noticeably promote the reduction rate of MoS2 by generating CaS. After acidic leaching, porous 2D MoN, a mixture of 2D MoN and Mo2N, as well as Mo2N nanosheets with the thickness of about 7 nm and pore size of about 10.5 nm were successfully prepared at different temperatures. When the temperature increased from 750 °C to 820 °C, the product gradually changed from MoN to Mo2N. Due to the weak Van der Waals forces between the layers of MoS2, the topochemical substitution of S in MoS2 by N led to the exfoliating of layers. The large difference of crystal structures and volume between precursor and product, as well as the high reaction rate led to the formation of porous structure of molybdenum nitrides.
1. Introduction Recent years, porous nanomaterials with unique surface structure and morphological stability have received intensive attention for promising applications: such as energy conversion and storage, as well as catalysis [1-18]. Porous 2D nanomaterials have the fascinating synergetic chemical/physical properties of both porous structure and 2D architecture, such as abundant active surface, high electronic conductivity in-plane, enabling fast transport of ions and electrolyte through the porous flakes [2,3,7,10,12,19]. As a demonstration of the application of porous 2D transition metal nitride (TMN) nanosheets [2], in a lithium–sulfur battery, a high initial capacity of > 1000 mAh g−1 at 0.2 C under a high areal sulfur loading (> 5 mg cm−2) can be achieved with only about 13% degradation over 1000 cycles at 1 C. In Lithium-Ion Batteries, porous 2D Co3S4 nanosheets delivered a high discharge capacity of about 968 mA h g−1 with excellent cycling stability [20]. Therefore, the construction of porous 2D nanomaterials is an important trend in 2D materials [1,5,7]. 2D molybdenum nitrides nanosheets have excellent electrical, magnetic and catalytic activity properties, making them be hot candidates in many fields ranging from catalyst to energy conversion and storage [2,21-27]. For example, it was reported by Xiao et al. [25] that 2D molybdenum nitride nanosheets exhibited a very high volumetric capacitance of 928 F cm−3 in sulfuric acid electrolyte with an excellent rate performance. Until now, even though some methods have been
∗
developed for synthesis of 2D molybdenum nitrides nanosheets with smooth surface, such as: liquid exfoliating method [28], chemical solution deposition [29], and topochemical synthesis methods [22,25,26,30], few literature has been reported to prepare holey 2D molybdenum nitrides nanosheets. Nowadays, it is still a challenge to produce holey 2D molybdenum nitrides nanosheets, especially via a simple, efficient and low-cost pathway. In this study, a facile method was proposed to prepare porous 2D molybdenum nitrides nanosheests via topochemical nitridation of MoS2 with the assistance of lime in NH3 atmosphere at 750–820 °C. MoS2 is the major form of molybdenum source in nature, which has layered S–Mo–S structure and is easy to be exfoliated due to the weak Van der Waals forces between the layers [31-33]. The lime can dramatically promote the reduction and nitridation rate via combining with S to generate CaS. In the nitridation process, 2D molybdenum nitrides nanosheests with porous structure and a by-product of CaS were produced. After acidic leaching, the CaS was removed and porous 2D molybdenum nitrides nanosheests were obtained. 2. Experimental section MoS2 (purity, > 98%) with an average size of around 3 μm was used as molybdenum source. Fig. 1(a) and (b) show the Field emission scanning electron microscope (FE-SEM) micrographs of the raw MoS2, which has schistose morphology. From the crystal structures images of
Corresponding author. E-mail address: [email protected] (G.-H. Zhang).
https://doi.org/10.1016/j.ceramint.2019.10.061 Received 15 July 2019; Received in revised form 28 September 2019; Accepted 7 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Guo-Dong Sun, Guo-Hua Zhang and Kuo-Chih Chou, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.061
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Fig. 1. (a) and (b) FE-SEM and (c) and (d) corresponding crystal structure images of raw MoS2 powder. Fig. 2. Temperature dependence of the change of standard Gibbs free energy.
raw MoS2 (Fig. 1(c) and (d)), it can be seen that MoS2 owns a layered S–Mo–S structure. CaO powder was obtained by roasting CaCO3 at 1000 °C for 6 h. The MoS2 and CaO powders with a CaO and MoS2 molar ratio of 3.0 were mixed in a mortar for 20 min. In each experiment, about 5 g sample with a thickness of about 1.5 cm was reduced and nitridized in flowing NH3 (200 ml/min) at desired temperatures for a certain time (heating/cooling rate 20 °C/min). Then the obtained product was leached by dilute hydrochloric acid (3 mol/L) and deionized water to remove the by-product and unreacted CaO. Finally, molybdenum nitrides powder was obtained after filtration. The phase composition and crystalline structure of sample were analyzed by X‐ray diffraction (XRD; TTR III, Rigaku Corporation, Japan). The chemical states and termination species of product were analyzed by X‐ray photoelectron spectroscopy (XPS; ESCALAB 250 Xi; Thermo Scientific, American). The morphology, thickness and crystalline structure of particles were characterized by FE‐SEM (ZEISS SUPRA 55, Oberkochen, Germany) with energy dispersive X‐ray spectroscopy (EDS) and transmission electron microscopy (TEM). The thickness of sample was analyzed by Atomic force microscopy (AFM, Dimension FastScan, Bio-Logic Science Instruments). The contents of N and O were determined using the O–N–H analyzer (EMIA-830, HORIBA, Japan). The N2 adsorption-desorption isotherms and pore size were analyzed by a Micromeritics ASAP2460 analyzer. The UV–vis absorption spectra were carried by using a spectrophotometer (Hitachi, U-3900). The thermodynamic calculations were conducted by FactSage 7.0 with pure substances database.
seen that the presence of CaO can dramatically decrease the critical temperature of the reduction-nitridation reaction of MoS2 to 360 °C. Additionally, since CaO can fix the generated H2S, it may be also possible to noticeably improve the reaction rate.
4 5 MoS2 + NH3 = 0.5Mo2 N + 2H2 S + N2 3 12
Δr G (reaction(1 )) = Δr Gθ(reaction(1 )) + RT ln(
(pH2S /pθ )2⋅(p N2 /pθ )5/12 ) (pNH3 /pθ )4/3
CaO + H2 S = CaS + H2 O
(1)
(2) (3)
4 5 MoS2 (s) + NH3 + 2CaO = 0.5Mo2 N + 2CaS + 2H2 O + N2 3 12 (4) Fig. 3(a) shows the XRD patterns of products obtained after reaction at different temperatures for 3 h. It can be seen that after reaction at 750 °C, MoS2 were not completely reduced and nitridized, and there were still lots of MoS2. Additionally, the solid products were only molybdenum nitrides and CaS without other phase, indicating that molybdenum nitrides could be directly generated from MoS2. When the temperature was increased to 800 °C and 820 °C, all the MoS2 were completely reduced and nitrided to molybdenum nitrides within 3 h, which was tremendously less than the dozens of hours for the case without the addition of CaO [30]. Fig. 3(b) shows the XRD patterns of products obtained at different temperatures and time after leaching. It can be seen that the excessive CaO and by-product CaS were completely removed. For the product obtained after reacting at 750 °C for 7 h, MoN product (PDF# 89–4318) was generated. With the further increase of temperature, the relatively intensity of the diffraction peaks of MoN gradually decreased. As the temperature was increased to 775 °C and 800 °C, the mixture of MoN and Mo2N (PDF# 15–1366) was obtained. At the temperature of 820 °C, the diffraction peaks of MoN almost disappeared, indicating that MoN was unstable at higher temperatures. However, at 860 °C, a few diffraction peaks of Mo appeared, indicating that Mo2N decomposed to Mo. To further investigate the reaction process, the reactions at 820 °C for different time were conducted, and the XRD patterns of products were shown in Fig. 3(c). It can be seen that there were only Mo2N and unreacted MoS2 after the reaction for 1 h. Even the reaction time was prolonged to 5 h, Mo2N can still stably exist. Therefore, MoN, the mixture of MoN and Mo2N, and Mo2N were successfully prepared by adjusting the reaction temperatures. To further quantitatively analyze the N and O amounts, the prepared molybdenum nitrides nanosheets were analyzed by O–N–H analyzer, and the contents of O and N of the molybdenum nitrides prepared at
3. Results and discussion The reduction and nitridation of MoS2 to Mo2N by NH3 can be described by reaction (1). The change of standard Gibbs free energy of reaction (1) is shown in Fig. 2, from which it can be found that the critical temperature (ΔrGθ = 0) for reaction (1) is as high as 945 °C. Therefore, from the thermodynamic perspective, the reduction of MoS2 by NH3 at below 900 °C is impossible under the standard condition. Eq (2) shows the change of actual Gibbs free energy of reaction (1). It can be seen that the decrease of partial pressure of H2S can decrease the critical temperature of reaction (1) (ΔrG = 0). For example, according to Eq. (2), when the partial pressure of H2S is reduced to 0.01 atm, the critical temperature can be reduced to about 650 °C. It is well known that CaO is a cheap and common-used sulfur fixing agent [34-36]. The reaction between H2S with CaO can be described by reaction (3), and its change of standard Gibbs free energy is depicted in Fig. 2, which is always thermodynamically feasible. Thus, in the presence of CaO, the reaction can be described by reaction (4), and the corresponding change of standard Gibbs free energy is also shown in Fig. 2. It can be 2
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Fig. 3. XRD patterns of products, (a) after reaction in NH3 atmosphere, (b) after leaching and (c) after reacting at 820 °C for different time; XPS spectra of Mo2N prepared at 820 °C for 3 h, (d) Mo 3p and N 1s, (e) Mo 3d and (f) O 1s.
seen that porous MoN nanosheets were obtained from the bulk MoS2. Fig. 4(b) shows the FE-SEM micrograph of the side of MoN particles. It can be seen that the basic planes were spread apart with a thickness of about 10 nm, resulting from the reduction and nitridation treatment. Fig. 4(c) and (d) show the TEM images of nanosheets of MoN, which indicates that they were quite thin due to the high transparency. From the high‐resolution TEM (HRTEM) and fast Fourier transform (FFT) images (Fig. 3(d)), it can be seen that MoN nanosheets had a high crystallinity with hexagonal atomic arrangement and an interplanar spacing of 2.46 Å, coinciding with the (200) crystal plane of MoN [42]. Fig. 4(e-h) show the FE-SEM and TEM images of the mixture of MoN and Mo2N prepared at 800 °C, in which the morphology and thickness (about 12 nm, Fig. 4(f)) of product were similar to that of 750 °C. Meanwhile, the interplanar spacing of (200) (MoN) was 2.49 Å (Fig. 4(h)). Fig. 4(i) and (j) show the FE-SEM micrographs of Mo2N prepared at 820 °C, in which Mo2N nanosheets had a good dispersity with a thickness of about 9 nm. The HRTEM and FFT show the Mo2N also had a hexagonal atomic arrangement with a interplanar spacing of 2.11 Å, corresponding to the crystal plane of (200) (Mo2N) [24]. Therefore, porous MoN, mixture of MoN and Mo2N, as well as Mo2N nanosheets with a thickness of about 10 nm were successfully prepared. These porous structures are noticeably different from the 2D MoN or Mo2N nanosheets with smooth surfaces reported in the previous literatures [22,25,26,28,30,42]. Additionally, the thickness of the prepared molybdenum nitride was also further analyzed by AFM, as shown in Fig. 4(m)-(o). It can be seen that the thickness of the prepared porous 2D molybdenum nitrides nanosheets was about 6.2~7.2 nm, which is slightly smaller than the corresponding values by FE-SEM analyses (around 10 nm). The Nitrogen adsorption/desorption isotherms and Barret-JoynerHalenda (BJH) pore size distribution of the prepared porous 2D molybdenum nitride (820 °C-3h, without any treatment of ultrasonic dispersion or chemical activation) are shown in Fig. 5(a) and (b), respectively. The Brunauer-Emmett Teller (BET) specific surface area of sample was about 42.28 m2/g, which shows a visible mesoporous structure with an average pore size of 10.5 nm, which is smaller than
Table 1 The contents of N and O of the prepared molybdenum nitrides nanosheets. Reaction temperature and time
O content/(wt%)
N content/(wt%)
750 °C-7h 775 °C-5h 800 °C-3h 820 °C-3h
0.9325 1.1604 1.0783 1.2160
13.4783 10.6841 9.2309 7.4934
different temperature are shown in Table 1. It can be seen that the N contents of sample produced at 750 °C for 7 h was 13.4783%, which is higher than the stoichiometric nitrogen content of MoN (12.73%), indicating that the molybdenum nitrides contain nitrogen-rich phase [37]. With the increase of temperature from 750 °C to 820 °C, the N content gradually decreased to 7.4934%, which is still slightly higher than the stoichiometric nitrogen content of Mo2N (6.73%). Additionally, the oxygen contents of products were around 1 wt%. XPS analysis was also conducted to investigate the surface composition and chemical states of the Mo2N product produced at 820 °C (3h). Fig. 3(d)-(f) show the high high‐resolution XPS spectra peaks of Mo 3p and N 1s, Mo 3d and O 1s. The spectrums of Mo 3p and N 1s region were fitted by components of: Mo–N (N 1s, 397.5 eV), Mo–N (Mo 3p3/2, 394.7 eV) and MoO3 (Mo 3p3/2, 399.5 eV), confirming the bonding of N to Mo [26]. In the Mo 3d region, the fitting peaks (binding energies) at 228.5 eV and 231.6 eV were corresponded to the Mo 3d5/2 and Mo 3d3/ 2 of Mo in Mo2N [38-40]. The binding energies at 235.5 eV and 232.8 eV could be in accordance to Mo6+, while 232.3 eV and 229.3 eV corresponded to Mo4+, resulting from the surface oxidation [26,38,41]. This surface oxidation was similar to the 2D molybdenum nitrides prepared by other methods [25,26]. The high-resolution spectrum of O 1s region is shown in Fig. 3(f), in which the binding energies at 530.2 eV, 531.1 eV and 532.0 eV were coincided with oxygen in MoO3, Mo–N-Ox, Mo–N-(OH)x [26]. Fig. 4 shows the FE-SEM and TEM micrographs of the produced molybdenum nitrides at different temperatures. Fig. 4(a) and (a) are FESEM images of product obtained at 750 °C for 7 h, from which it can be 3
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Fig. 4. FE-SEM and TEM images of products, (a)-(d) 750 °C-7 h, (b)-(h) 800 °C-3 h and (i)-(l) 820 °C-3 h. AFM images of the 2D molybdenum nitrides nanosheets prepared at (m) 800 °C-3 h and (n)-(o) 820 °C-3 h.
MoS2 and MoN/Mo2N had huge differences in crystal structures and volumes. For example, the molar volume of MoS2 is about 32.52 cm3/ mol (density, 4.92 g/cm3) [43], while the values of MoN and MoN0.5 are about 11.99 cm3/mol (density, 9.17 g/cm3) [44] and 10.96 cm3/ mol (density, 9.40 g/cm3) [45]. Therefore, in the presence of CaO, a relatively high reduction and nitridation rate of MoS2 would lead to a large change of crystal structures and volume, which further leaded to the formation of porous structure via the etching and recrystallizing of flakes [2]. As a comparison, similar experiment without CaO was also conducted at 800 °C for 30 h, and the TEM image of product is shown in Fig. 6(b). It can be seen that the prepared nanosheets did not have porous morphology, but single crystalline 2D flakes (MoN, interplanar spacing of (200), 2.47 Å) with smooth surfaces. Additionally, it was reported that the Mo2N prepared via nitridation of 2D molybdenum carbides (Mo2C) also had single crystalline 2D flakes without holey structure, due to the similar crystal structures and volume between the Mo2N and Mo2C [26]. Therefore, the large difference of crystal structures and volume (between precursor and product) and the high reaction rate were the crucial factors for producing the porous structure of molybdenum nitrides nanosheets.
the FE-SEM and TEM results. Ojha et al [21] prepared the porous Mo2N nanosheets and nanowires (using ammonium molybdate and g-C3N4 as molybdenum and N sources) with specific surface areas of 16 m2/g (thickness, about 30 nm (SEM result); pore radius, 2.9 nm) and 3 m2/g (thickness, about 100 nm (SEM result); pore radius, 3.2 nm), respectively. Therefore, the porous 2D molybdenum nitrides nanosheets prepared in the present study have relatively higher specific surface area due to the thicker sheets. Furthermore, to investigate the dispersibility of the porous 2D molybdenum nitrides nanosheets, the MoS2 or 2D molybdenum nitride nanosheets were dissolved into water or alcohol (3 mg/ml), and sonicated for 20 min. After standing for 72 h, the supernatants were removed and their photos are shown in Fig. 5(c) (insert pictures). It can be seen that the colors of water and alcohol with dissolved 2D molybdenum nitrides were dark brown and light brown, respectively. However, for the water or alcohol with MoS2, the color of solution was colorless, indicating that there was almost no MoS2 dissolving in water or alcohol. Furthermore, UV–vis analyses of the supernatants have been conducted and the results are also shown in Fig. 5(c). It can be seen that the absorbance intensity of the 2D molybdenum nitride in water solvent was higher than in alcohol, which could indicate the higher dispersibility of the 2D molybdenum nitride in water than alcohol. However, for the raw bulk MoS2, it can be seen that the corresponding absorbance intensities of both in water and alcohol are much weaker than those of the 2D molybdenum nitride. As discussed above, it can be found that compared to the raw bulk MoS2, the prepared molybdenum nitride nanosheets had much better dispersibility in different solvents. As discussed above, it can be found that the synthesis of 2D porous molybdenum nitrides is a typical topochemical synthesis process [2,22]. The raw MoS2 has a sandwich-like (S–Mo–S) layered structure with weak Van der Waals forces among the layers. Therefore, during the reduction and nitridation process, the S was topochemically substituted by N in ammonia, and H2S gas was also generated, which would lead to the exfoliating of layers, as shown in Fig. 4. Additionally,
4. Conclusions In this study, a facile method was reported for the efficient and large-scale synthesis of 2D porous molybdenum nitrides nanosheets via CaO-assisted topochemical transformation of MoS2 by NH3 at 750 °C–820 °C. The presence of CaO made the reaction between MoS2 and NH3 become thermodynamically feasible at below 750 °C, and also noticeably promoted the reduction rate of MoS2. Porous 2D MoN, a mixture of 2D MoN and Mo2N, as well as Mo2N nanosheets with the thickness of about 7 nm and pore size of about 10.5 nm were successfully prepared by adjusting the reaction temperature. The weak Van der Waals forces between the layers of MoS2 and the topochemical substitution of S in MoS2 by N via ammonia led to the exfoliating of layers. 4
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Fig. 5. (a) Nitrogen adsorption/desorption isotherms and (b) Barret-Joyner-Halenda (BJH) pore size distribution of the porous 2D molybdenum nitride prepared at 820 °C-3h. (c) UV–vis spectra of MoS2 and porous 2D molybdenum nitrides nanosheets (820 °C-3h) dissolved in different solvents (water and alcohol).
MoN was unstable at higher temperatures, and when the temperature was increased from 750 °C to 820 °C, the product gradually changed from MoN to Mo2N. The large difference of crystal structures and volume between precursor and product and the high reaction rate were the crucial factors for producing porous structure of molybdenum nitrides.
interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (51734002).
Declaration of competing interest The authors declare that they have no known competing financial
Fig. 6. Comparison of TEM images of products obtained from (a) reduction of MoS2 by NH3 with the assistance of CaO at 800 °C for 3 h and (b) reduction of MoS2 by NH3 at 800 °C for 30 h. 5
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