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Topochemical synthesis of two-dimensional molybdenum carbide (Mo2C) via Na2CO3-Assited carbothermal reduction of 2H–MoS2
Journal Pre-proof Topochemical Synthesis of Two-Dimensional Molybdenum Carbide (Mo2C) via Na 2
CO3-Assited Carbothermal Reduction of 2H-MoS2
He-Qiang Chang, Guo-Hua Zhang, Kuo-Chih Chou PII:
S0254-0584(20)30092-4
DOI:
https://doi.org/10.1016/j.matchemphys.2020.122713
Reference:
MAC 122713
To appear in:
Materials Chemistry and Physics
Received Date:
29 October 2019
Accepted Date:
21 January 2020
Please cite this article as: He-Qiang Chang, Guo-Hua Zhang, Kuo-Chih Chou, Topochemical Synthesis of Two-Dimensional Molybdenum Carbide (Mo2C) via Na2CO3-Assited Carbothermal Reduction of 2H-MoS2, Materials Chemistry and Physics (2020), https://doi.org/10.1016/j. matchemphys.2020.122713
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Journal Pre-proof
Topochemical Synthesis of Two-Dimensional Molybdenum Carbide (Mo2C) via Na2CO3-Assited Carbothermal Reduction of 2H-MoS2 He-Qiang Chang, Guo-Hua Zhang*, and Kuo-Chih Chou State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, China HIGHLIIGHTS An efficient and high-yield method was proposed for the preparation of 2D Mo2C. Natural 2H-MoS2 can serve as an ideal precursor for the synthesis of 2D Mo2C. The addition of Na2CO3 can greatly promote the carburization and exfoliation rates of 2H-MoS2. ABATRACT Due to its promising applications on catalysis, electronics and electrochemical energy storage, 2D Mo2C has emerged as an important member in the 2D family. Herein, an efficient and high-yield method was proposed for the preparation of ultra-thin 2D Mo2C via Na2CO3-assited carbothermal reduction of 2H-MoS2. It was found that 2H-MoS2 can be completely carbonized to Mo2C at a MoS2:Na2CO3:C molar ratio of 2:4.2:11 at 850oC for 2 hours. Meanwhile, the prepared Mo2C were intercalated by the co-product Na2S. After leaching treatment with deionized water, the intercalated Na2S was removed and high crystalline 2D Mo2C were successfully prepared. Furthermore, the addition of Na2CO3 as a desulfurizer can greatly promote the carburization and exfoliation rates of 2H-MoS2. As can be seen from the FE-SEM images, the prepared Mo2C has a pretty regular layered structure with the layer thickness of 5-15 nanometers. KEYWORDS: 2H-MoS2; Carbothermal Reduction; Desulfurizer; Two-Dimensional Mo2C (2D Mo2C) Corresponding author. Contact e-mail addresses: [email protected]
Journal Pre-proof 1. Introduction Ultra-thin two-dimensional (2D) nanomaterials represent a new class of nanomaterials having a sheet-like structure, with the lateral size of a few hundred nanometers to a few micros, and the thickness of only a few nanometers [1]. After Novoselov et al. exfoliated graphene from graphite using the mechanical cleavage method in 2004 [2], investigations on 2D nanomaterials have grown exponentially in material science, chemistry and condensed matter physics due to their excellent physical and chemical properties, as well as their various potential applications [3-17]. Over the past fifteen years, promising research on graphene and graphene-like 2D materials has greatly enriched the exploration of 2D family members, such as hexagonal boron nitride (h-BN) [3], graphitic carbon nitride (g-C3N4) [4], transition metal dichalcogenides (TMDs) [5], layered metal oxides [6], layered double hydroxides (LDHs) [7], MXenes [8, 9], metal-organic frameworks (MOFs) [10], covalent-organic frameworks (COFs) [11, 12], polymers [13], black phosphorus (BP) [14, 15], silicene [16], arsenene and antimonene [17]. 2D transition metal carbides (TMCs) belong to MXenes. TMCs combine the properties of metals and covalent compounds due to the incorporation of C atoms into the metal lattice [9]. On the one hand, they have high electrical and thermal conductivity along with good mechanical strength. On the other hand, they show excellent catalytic activity of challenging noble metals, as well as many other attractive properties both in the bulk form and films. In addition, many TMCs, such as Mo2C, WC, W2C, TaC and Ti3C2, exhibit superconductivity [18-21]. 2D Mo2C is a member of the TMCs family, which is promising for many applications such as catalysis, electronics and electrochemical energy storage [22-26]. Previously, the strategies to produce 2D Mo2C are mainly by the methods of selective etching-assisted liquid exfoliation and chemical vapor deposition (CVD). Selective etching-assisted liquid exfoliation is exfoliation of the corresponding laminated MAX phase upon selective etching of the “A” element using an appropriate etchant
Journal Pre-proof (typically HF) [8]. 2D Mo2C had been prepared by selectively etching the MAX phase precursors (Mo2/3Sc1/3)2AlC and Mo2Ga2C [9, 27], while etching of other carbide MAX phases has not been successful so far. In addition, ultra-thin 2D Mo2C was also prepared by CVD from methane on a bilayer substrate of copper foil sitting on a molybdenum foil [18]. Besides, various morphology of ultrathin 2D Mo2C crystals can be prepared by CVD to tuning the carbon supersaturation [28]. Although the above two methods can be used for the preparation of 2D Mo2C, there is still an urgent need for an efficient and high-yield way for the synthesis of 2D Mo2C. The topochemical synthesis method may be a promising strategy, which can be broadly defined as adding, extracting or substituting elements to or from precursors for synthesis of new materials which retains the structure or the morphology of parent materials [29]. Herein, we developed an efficient and high-yield topochemical synthesis method of 2D Mo2C via Na2CO3-assisted carbothermal reduction natural 2H-MoS2. Firstly, the raw materials mixture with different MoS2:Na2CO3:C molar ratios were annealed in the argon atmosphere at 850oC. Secondly, the acquired samples were leached several times in deionized water and centrifuged for the separation of the synthesized 2D Mo2C powders. 2. Materials and methods 2.1. Materials Molybdenum disulfide (2H-MoS2, Jinduicheng Molybdenum Co., LTD, China, 98 wt% purity) was used as the molybdenum source. Na2CO3 (Sinopharm Chemical Reagent Co., LTD, China, 99.8 wt% purity) was used as the desulfurizer. Activated Carbon (Sinopharm Chemical Reagent Co., LTD, China, > 90% purity, with impurities of volatile matters) was used as the carburizing agent. Molybdenite is a frequently-used mineral for molybdenum extraction, and molybdenum in molybdenite mainly presents in the form of natural 2H-MoS2. 2H-MoS2 is similar to graphite and has a layered structure. The weak van der Waals forces between the layers are easily destroyed, dividing bulks into the layered compounds [5]. The XRD pattern of the used natural MoS2 is shown in Figure 1a, and
Journal Pre-proof the peaks are well assigned to 2H-MoS2 (PDF card no 37-1492). Meanwhile, the diffraction peak (002) is sharp and intense, which indicates its highly crystalline nature. Additionally, the 2H-MoS2 crystallizes in a hexagonal structure with space group of P63/mmc. The overhead and side view FE-SEM micrographs of 2H-MoS2 are shown in Figures 1(b) and 1(c), from which it can be clearly seen that the 2H-MoS2 has a distinct layered morphology. In addition, the unit crystal cell diagram and atoms structure images of 2H-MoS2 were drawn and shown in Figure 1(d), in which a unit cell includes two MoS2 units, and there is no covalent bonding between each layer.
Figure. 1 (a), XRD pattern of the 2H-MoS2. (b) and (c), overhead and side view FE-SEM micrograph of the used 2H-MoS2. (d), unit crystal cell diagram and atomic structure diagram of 2H-MoS2. Na2CO3 was selected as desulfurizer in the present study, for the two reasons: i) the bond strength between the Na and sulfur is stronger than between molybdenum and sulfur; ii) the bond strength between the Na and carbon is weaker than that between molybdenum and carbon. Thus, Na from Na2CO3 could combine with S in MoS2 and
Journal Pre-proof release Mo to combine with C to generate Mo2C. 2.2. Synthesis of 2D Mo2C powders In order to prepare 2D Mo2C powders, the molybdenum disulfide, Na2CO3 and activated carbon with different molar ratios were thoroughly mixed. Then, the mixed sample of about 5g was placed into the alumina crucible which was put in the constant temperature zone of a horizontal furnace. Before heating, argon gas (200 ml/min) was introduced to remove the air. After that, the furnace was heated with a ramping rate of 5 oC/min to different temperatures (800oC and 850oC) and maintained at this temperature for different times (1hour and 2 hours). After the reaction, the samples were cooled to ambient temperature, and then were leached several times in deionized water and centrifuged for the separation of the synthesized 2D Mo2C powders. 2.3. Characterizations. X-ray diffraction (XRD; TTR III, Rigaku Corporation, Japan; X-ray wavelength, 1.5418 Å [Cu-Kα]) and X-ray photoelectron spectroscopy (XPS; ESCALAB 250 Xi; Thermo Scientific, American) analysis were carried out to identify the phase compositions. The morphologies of the samples were characterized using field emission scanning electron microscopy (FE-SEM; ZEISS SUPRA 55, Oberkochen, Germany). After sonication treatment, the 2D layered Mo2C were investigated by transmission electron microscopy (TEM; Tecnai-G2-F20, FEI, American). The residual sulfur and carbon content were analyzed using infrared carbon-sulfur analyzer (EMIA-920V2; HORIBA, Japan). Additionally, The Thermal-gravimetric analysis-Mass spectrometry (TGA-MS) (MS, QMG 220, Pfeiffer, German) characterization was performed to determine the composition of produced gas. 3. Results and discussion 3.1. XRD analyses Figure 2(a) presents the XRD patterns of products acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:11. From which it can be seen that when the mixed sample was annealed at 800oC for 2 hours, there is still a distinct 2H-MoS2 diffraction peak
Journal Pre-proof (corresponding to its [002] crystal plane). When the temperature increased to 850oC, the 2H-MoS2 diffraction peak disappeared. Additionally, when the mixed sample was annealed at 850oC for 1 hour, there is also a distinct diffraction peak of 2H-MoS2. Therefore, both the increase of temperature and time contribute to the carbonization of MoS2. Accordingly, the experimental conditions for the preparation of Mo2C were selected as 850oC and 2 hours.
Figure. 2 XRD patterns and FE-SEM micrographs of the products. (a), XRD patterns of the product acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:11. (b), XRD patterns of the product obtained by adding different amounts of Na2CO3. (c), XRD
Journal Pre-proof patterns of the product obtained by adding different amounts of activated carbon. (d), FE-SEM micrograph of the product acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:1. (e), XRD patterns of the product after leaching treatment (corresponding to (c)). (f), Rietveld-refined XRD pattern of the prepared Mo2C acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:11. Figure 2(b) plots the XRD patterns of products acquired at different MoS2:Na2CO3:C molar ratios of 2:4:9, 2:4.1:9 and 2:4.2:9, respectively. It can be found that most of the 2H-MoS2 transformed to Mo2C, and the main co-product is Na2S. In additional, it can be found that there is still a distinct MoS2 diffraction peak when the MoS2:Na2CO3:C molar ratio was 2:4:9 and 2:4.1:9. When the MoS2:Na2CO3:C molar ratio is 2:4.2:9, the diffraction peak of MoS2 disappeared, indicating that all MoS2 was completely reacted. The reason for the using of 1.05 times of the theoretical amount of Na2CO3 may be due to the slow reaction kinetics of the heterogeneous phase reaction. Therefore, the increase in amount of Na2CO3 is beneficial for the carbonization of MoS2. As can be seen from Figure 2(b), there is always an obvious diffraction peak of Na2MoO4. Since the excessive activated carbon can prevent the generation of Na2MoO4 due to the reaction between them, the experiments with different addition amounts of activated carbon were conducted, and the XRD patterns are shown in Figure 2(c). In Figure 2(c), the MoS2:Na2CO3 molar ratio was set to 2:4.2 based on the experimental results shown in Figure 2(b). With the increase of activated carbon content, the intensity of the diffraction peak of Na2MoO4 decreased gradually, and totally disappeared until the MoS2:Na2CO3:C molar ratio was 2:4.2:11. Consequently, the 2H-MoS2 can be completely carbonized to Mo2C at a MoS2:Na2CO3:C molar ratio of 2:4.2:11. The main reaction of 2H-MoS2, Na2CO3 and activated carbon can be described by reaction (1). In addition, the morphological characteristics of the acquired product were investigated by FE-SEM. The FE-SEM micrograph of the product acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:11 was shown in Figure 2(d), and there were many sheets stacked and wrapped by another phase.
Journal Pre-proof 2MoS2 +4Na 2 CO3 +9C=Mo 2 C+4Na 2S+12CO
(1)
In order to eliminate the inserted Na2S and acquire layered Mo2C, the products were washed several times with deionized water and centrifuged to separate the Mo2C powders. The phase compositions after leaching treatment of the products obtained by adding different amounts of activated carbon are shown in Figure 2(e) (corresponding to Figure 2(c)). It can be seen from Figure 2(e) that Na2CO3, Na2S and Na2MoO4 were removed completely after leaching treatment, and only Mo2C remained. The Rietveld-refined XRD pattern of the prepared Mo2C is shown in Figure. 2f. The value of agreement factor Rwp is as low as 9.42%, indicating highly reliable results. The Rietveld-refined lattice parameters of the prepared Mo2C are a =3.0124 Å and c = 4.7356 Å, and the value of c is little larger than the date of bulk Mo2C (PDF card no 35-787). Additionally, when the molar ratio of MoS2:Na2CO3:C was 2:4.2:11, the amount of 2H-MoS2 in 5g of the mixed raw materials was about 1.82g. After leaching treatment, about 1.05g of Mo2C powder was obtained. Based on mass conservation of molybdenum, the yield could be calculated to about 92%. Further, the carbon and sulfur content of the synthesized Mo2C powder were measured by using an infrared carbon-sulfur analyzer, and the results were 5.98% (close to the theoretical carbon content of Mo2C, 5.89%) and 0.33%, respectively. In order to determine the composition of gases evolved, the TGA-MS analysis was performed for sample with a MoS2:Na2CO3:C molar ratio of 2:4.2:11 with a ramping rate of 10 oC/min from room temperature to 850oC. Mass scanning was carried out in the selected-ion monitoring mode. The ions with the mass-to-charge ratio (m / z) of 28 (CO), 44 (CO2), 64 (SO2), 76 (CS2) and 80 (SO3) were monitored, and the TGA-MS thermogram is shown in Figure. 3. From TGA curves it can be seen that the reaction begins at about 600oC, and when the temperature increases to 850oC and maintains at this temperature for about 10 minutes the mass loss rate reaches a maximum of 39.1%, which is close to the theoretical mass loss rate of 39.43% for reaction (1). From the MS curve it can be found that only CO (m / z=28) and CO2 (m / z=44) were produced. When the temperature increases to 600oC, CO2 was generated and
Journal Pre-proof disappeared when the temperature rises to about 800oC, while the main gaseous product is CO. It must be noted that the m/z ratio of N2 is also equal to 28, and the blue curve before 600 oC indicates the existence of N2, since the reduction reaction does not start. During the heating process, the proportion of air continues to decrease, resulting in a continuous decrease in the intensity ion current of N2 (m/z =28). In the current study, the samples were rapidly heated to 800 or 850oC, so it can be considered that the gaseous produced is mainly CO during the reaction process. Thus, the main reaction among 2H-MoS2, Na2CO3 and activated carbon can be described by reaction (1).
Figure. 3 TGA-MS thermogram of the sample acquired for sample with a MoS2:Na2CO3:C molar ratio of 2:4.2:11 with a ramping rate of 10 oC/min from room temperature to 850oC. 3.2. XPS analyses
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Figure. 4 X-ray photoelectron spectroscopy spectra of prepared Mo2C at 850oC for 2 hours. (a), Mo 3d spectrum. (b), S 2p spectrum. (c), C 1s spectrum. (d),O 1s spectrum. The chemical states and surface termination of the synthesized Mo2C were further studied by XPS measurements. High-resolution XPS spectra peak fittings for Mo 3d, S 2p, C 1s and O 1s are shown in Figures 4(a-d), respectively. Figure 4(a) plots the Mo 3d spectrum, which shows the Mo-C species (Mo 3d5/2, 228.1 eV; Mo 3d3/2, 232.2 eV) and Mo–O species [24, 30]. The Mo-oxide species should be attributed to MoO2 (Mo4+) and MoO3 (Mo6+). It was noted that no S 2s peak (MoS2, 226.8 eV) [31] was detected in this region, suggesting that MoS2 was totally carbonization. Furthermore, the S 2p spectrum was fitted by a component with a binding energy of 163.6 eV as shown in Figure 4(b), which may correspond to S species [32]. During the leaching treatment with deionized water, a part of Na2S would react with H2O to generate H2S gas (Reaction (2)), and a part of H2S is dissolved in the water which may be oxidized by the air to form S element (Reaction (3)). Furthermore, the sulfur content of the prepared Mo2C was measured by using an infrared carbon-sulfur
Journal Pre-proof analyzer, and the result was 0.33%.
Na 2S+2H 2 O=2NaOH+H 2S
(2)
2H 2S(l)+O 2 =2S +2H 2 O
(3)
Figure 4(c) plots the C 1s specture that was fitted by components corresponding to Mo-C(283.5 eV), C-C(284.6 eV), CHx(285.6 eV) and C-O(286.6 eV) species [33-35]. The O 1s specture was shown in Figure 3(d), which was fitted by components corresponding to: MoO2 (530.2 eV) [36], MoO3 (531.1 eV) [37], Mo2COx (-O terminated,
531.1
eV),
Mo2C(OH)2
(-OH
terminated,
532.1
eV)
and
Mo2C(OH)x-H2Oads (-OH terminated, 533.4 eV) species [24]. 3.3. FE-SEM and TEM analyses
Figure. 5 FE-SEM images of the Mo2C prepared at different MoS2:Na2CO3:C molar ratios at 850oC for 2 hours. (a) and (b), 2:4.2:11. (c) and (d), 2:4.2:10. (e) and (f), 2:4.2:9. (g) and (h), 2:4.1:9. (i) and (j), 2:4:9. (k), low magnification FE-SEM images of the prepared Mo2C. (l), EDS pattern of the Mo2C obtained at a MoS2:Na2CO3:C molar ratio of 2:4:11 at 850oC for 2 hours. The morphology and structure of the synthesized 2D Mo2C were further
Journal Pre-proof investigated by FE-SEM and TEM. The FE-SEM images of the 2D Mo2C acquired after leaching treatment are shown in Figure 5. Figures 5(a) and (b) show the FE-SEM images of the Mo2C acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:11, where the layers are clearly separated from each other, similar to the exfoliated graphite and MXenes [38, 8]. The pretty regular layered structure proves the perfect 2D morphology of the synthesized Mo2C. Furthermore, it can be seen from Figures 5(a) and (b) that the thickness of the Mo2C is varied between 5 to 15 nm and its lateral size was several micrometers. Figures 5(c-j) show the FE-SEM images of the Mo2C prepared at different MoS2:Na2CO3:C molar ratios, from which it can be found that there is no apparent difference in the morphology of the prepared Mo2C, and the thickness of each layer is varied between 5 to 15 nm. The low magnification SEM images of the synthesized Mo2C is shown in Figure 5(k), from which it can be seen that the product has the 2D layered structure. Furthermore, the EDS spectrum of the obtained Mo2C is shown in Figure 5(l), and it can be seen that the atomic percentages of Mo atoms and C atoms are 63.68% and 36.32%, respectively, which is close to the stoichiometric ratio of Mo to C in Mo2C. Prior to the TEM test, the prepared Mo2C powders were sonicated in an ethanol solution for 300 seconds to exfoliate the layered Mo2C. Due to the 2D nature of ultra-thin Mo2C crystals, it is easy to obtain an atomic-resolution image along its [100] zone axis [39]. An atomic model of the Mo2C crystal project in the [100] direction was drawn and shown in Figure 6(a). The TEM image of the 2D Mo2C obtained at a MoS2:Na2CO3:C molar ratio of 2:4.2:11 is shown in Figure 6(b), which also revealed that each layer of the synthesized 2D Mo2C is very thin because of its high transparency, in which the underlying layers are clearly visible beneath the top Mo2C layer. Figures 6(c) and (d) show the HRTEM images of Mo2C corresponding to Figure 6(b), while Figure 6(d) has a higher magnification. In Figure 6(d), blue sphere represents Mo atoms, and red sphere represent C atoms. This is consistent with the atomic model shown in Figure 6(a), where the Mo atoms are arranged in a hexagonal closepacked (HCP) configuration, showing a graphene-like honeycomb lattice in the [100] projected direction. Figure 6(e) shows the TEM images of the 2D Mo2C
Journal Pre-proof prepared at a MoS2:Na2CO3:C molar ratio of 2:4.2:10, from which it is concluded that the prepared Mo2C has the multi-layer sheets structure. Figures 6(f) and (g) show the HRTEM images corresponding to different position of Figure 6(e). From the HRTEM images of the layer edge, the layered Mo2C had a pretty small thickness.
Figure. 6 (a), Atomic model of an Mo2C crystal project in the [100] direction. (b) and (c), Transmission electron mircroscopy (TEM) image with corresponding SAED pattern and high-resolution TEM (HRTEM) image of the product after leaching treatment obtained at a MoS2:Na2CO3:C molar ratio of 2:4.2:11. (d), HRTEM image of Mo2C (same as (c) with higher magnification). (e), (f) and (g), TEM image and HRTEM image of the product after leaching treatment obtained at a MoS2:Na2CO3:C molar ratio of 2:4.2:10. From the above results, the prepared Mo2C retains the layered structure similar to 2H-MoS2, which indicates that the Na2CO3-assisted carbothermal reduction of 2H-MoS2 is a topochemical reaction. Accordingly, during Na2CO3-assisted carbothermal reduction of 2H-MoS2, the S atoms in 2H-MoS2 were replaced by C atoms, forming Mo2C with a layered hexagonal structure. Meanwhile, the S atoms combined with Na atoms to form Na2S. Therefore, 2H-MoS2 with inherent 2D crystal structure can be considered as an ideal precursor for synthesizing 2D Mo2C. Moreover, it can be seen from Figure 2d that the co-product Na2S was inserted into the generated Mo2C, which could play a crucial role on the exfoliating of the 2D
Journal Pre-proof Mo2C. Firstly, the inserted Na2S can separate the produced layered Mo2C, hindering the combination of each layer of Mo2C at high temperature. In addition, even if Na2S has a high solubility in water, a part of Na2S would react with H2O to generate H2S gas (reaction (2)), and the presence of H2S can further promote the separation of each layer of Mo2C. Therefore, after removing the intercalated Na2S, ultrathin 2D Mo2C with a thickness of several nanometers was successfully obtained. In our previous research [40], the temperature of the direct carbothermal reduction of MoS2 to prepare Mo2C without the addition of Na2CO3 was 1450oC. By comparison, it can be found that the addition of Na2CO3 as a desulfurizer can promote the carburization of 2H-MoS2 and greatly reduce the temperature for the preparation of Mo2C. 4. Conclusions In this work, an environmentally friendly, efficient and high yield approach is proposed to prepare 2D Mo2C via Na2CO3-assited carbothermal reduction of 2H-MoS2. Natural 2H-MoS2 have a layered structure was used as an ideal precursor for preparing 2D Mo2C. It can be found that the addition of Na2CO3 as a desulfurizer can promote the carburization rate of 2H-MoS2 and greatly reduce the preparation temperature of Mo2C. At the same time, the sulfur contained in MoS2 could be fixed in Na2S. After reacting at 850oC for 2h, a mixture of 2D Mo2C and co-product Na2S were successfully prepared. After leaching treatment with deionized water, ultra-thin 2D Mo2C were successfully obtained. FE-SEM and TEM images indicate that each layer of Mo2C has a thickness of 5-15 nanometers. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (51734002). Conflicts of interest There are no conflicts to declare.
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Declaration of interests ■The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
CO3-Assited Carbothermal Reduction of 2H-MoS2
He-Qiang Chang, Guo-Hua Zhang, Kuo-Chih Chou PII:
S0254-0584(20)30092-4
DOI:
https://doi.org/10.1016/j.matchemphys.2020.122713
Reference:
MAC 122713
To appear in:
Materials Chemistry and Physics
Received Date:
29 October 2019
Accepted Date:
21 January 2020
Please cite this article as: He-Qiang Chang, Guo-Hua Zhang, Kuo-Chih Chou, Topochemical Synthesis of Two-Dimensional Molybdenum Carbide (Mo2C) via Na2CO3-Assited Carbothermal Reduction of 2H-MoS2, Materials Chemistry and Physics (2020), https://doi.org/10.1016/j. matchemphys.2020.122713
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Topochemical Synthesis of Two-Dimensional Molybdenum Carbide (Mo2C) via Na2CO3-Assited Carbothermal Reduction of 2H-MoS2 He-Qiang Chang, Guo-Hua Zhang*, and Kuo-Chih Chou State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, China HIGHLIIGHTS An efficient and high-yield method was proposed for the preparation of 2D Mo2C. Natural 2H-MoS2 can serve as an ideal precursor for the synthesis of 2D Mo2C. The addition of Na2CO3 can greatly promote the carburization and exfoliation rates of 2H-MoS2. ABATRACT Due to its promising applications on catalysis, electronics and electrochemical energy storage, 2D Mo2C has emerged as an important member in the 2D family. Herein, an efficient and high-yield method was proposed for the preparation of ultra-thin 2D Mo2C via Na2CO3-assited carbothermal reduction of 2H-MoS2. It was found that 2H-MoS2 can be completely carbonized to Mo2C at a MoS2:Na2CO3:C molar ratio of 2:4.2:11 at 850oC for 2 hours. Meanwhile, the prepared Mo2C were intercalated by the co-product Na2S. After leaching treatment with deionized water, the intercalated Na2S was removed and high crystalline 2D Mo2C were successfully prepared. Furthermore, the addition of Na2CO3 as a desulfurizer can greatly promote the carburization and exfoliation rates of 2H-MoS2. As can be seen from the FE-SEM images, the prepared Mo2C has a pretty regular layered structure with the layer thickness of 5-15 nanometers. KEYWORDS: 2H-MoS2; Carbothermal Reduction; Desulfurizer; Two-Dimensional Mo2C (2D Mo2C) Corresponding author. Contact e-mail addresses: [email protected]
Journal Pre-proof 1. Introduction Ultra-thin two-dimensional (2D) nanomaterials represent a new class of nanomaterials having a sheet-like structure, with the lateral size of a few hundred nanometers to a few micros, and the thickness of only a few nanometers [1]. After Novoselov et al. exfoliated graphene from graphite using the mechanical cleavage method in 2004 [2], investigations on 2D nanomaterials have grown exponentially in material science, chemistry and condensed matter physics due to their excellent physical and chemical properties, as well as their various potential applications [3-17]. Over the past fifteen years, promising research on graphene and graphene-like 2D materials has greatly enriched the exploration of 2D family members, such as hexagonal boron nitride (h-BN) [3], graphitic carbon nitride (g-C3N4) [4], transition metal dichalcogenides (TMDs) [5], layered metal oxides [6], layered double hydroxides (LDHs) [7], MXenes [8, 9], metal-organic frameworks (MOFs) [10], covalent-organic frameworks (COFs) [11, 12], polymers [13], black phosphorus (BP) [14, 15], silicene [16], arsenene and antimonene [17]. 2D transition metal carbides (TMCs) belong to MXenes. TMCs combine the properties of metals and covalent compounds due to the incorporation of C atoms into the metal lattice [9]. On the one hand, they have high electrical and thermal conductivity along with good mechanical strength. On the other hand, they show excellent catalytic activity of challenging noble metals, as well as many other attractive properties both in the bulk form and films. In addition, many TMCs, such as Mo2C, WC, W2C, TaC and Ti3C2, exhibit superconductivity [18-21]. 2D Mo2C is a member of the TMCs family, which is promising for many applications such as catalysis, electronics and electrochemical energy storage [22-26]. Previously, the strategies to produce 2D Mo2C are mainly by the methods of selective etching-assisted liquid exfoliation and chemical vapor deposition (CVD). Selective etching-assisted liquid exfoliation is exfoliation of the corresponding laminated MAX phase upon selective etching of the “A” element using an appropriate etchant
Journal Pre-proof (typically HF) [8]. 2D Mo2C had been prepared by selectively etching the MAX phase precursors (Mo2/3Sc1/3)2AlC and Mo2Ga2C [9, 27], while etching of other carbide MAX phases has not been successful so far. In addition, ultra-thin 2D Mo2C was also prepared by CVD from methane on a bilayer substrate of copper foil sitting on a molybdenum foil [18]. Besides, various morphology of ultrathin 2D Mo2C crystals can be prepared by CVD to tuning the carbon supersaturation [28]. Although the above two methods can be used for the preparation of 2D Mo2C, there is still an urgent need for an efficient and high-yield way for the synthesis of 2D Mo2C. The topochemical synthesis method may be a promising strategy, which can be broadly defined as adding, extracting or substituting elements to or from precursors for synthesis of new materials which retains the structure or the morphology of parent materials [29]. Herein, we developed an efficient and high-yield topochemical synthesis method of 2D Mo2C via Na2CO3-assisted carbothermal reduction natural 2H-MoS2. Firstly, the raw materials mixture with different MoS2:Na2CO3:C molar ratios were annealed in the argon atmosphere at 850oC. Secondly, the acquired samples were leached several times in deionized water and centrifuged for the separation of the synthesized 2D Mo2C powders. 2. Materials and methods 2.1. Materials Molybdenum disulfide (2H-MoS2, Jinduicheng Molybdenum Co., LTD, China, 98 wt% purity) was used as the molybdenum source. Na2CO3 (Sinopharm Chemical Reagent Co., LTD, China, 99.8 wt% purity) was used as the desulfurizer. Activated Carbon (Sinopharm Chemical Reagent Co., LTD, China, > 90% purity, with impurities of volatile matters) was used as the carburizing agent. Molybdenite is a frequently-used mineral for molybdenum extraction, and molybdenum in molybdenite mainly presents in the form of natural 2H-MoS2. 2H-MoS2 is similar to graphite and has a layered structure. The weak van der Waals forces between the layers are easily destroyed, dividing bulks into the layered compounds [5]. The XRD pattern of the used natural MoS2 is shown in Figure 1a, and
Journal Pre-proof the peaks are well assigned to 2H-MoS2 (PDF card no 37-1492). Meanwhile, the diffraction peak (002) is sharp and intense, which indicates its highly crystalline nature. Additionally, the 2H-MoS2 crystallizes in a hexagonal structure with space group of P63/mmc. The overhead and side view FE-SEM micrographs of 2H-MoS2 are shown in Figures 1(b) and 1(c), from which it can be clearly seen that the 2H-MoS2 has a distinct layered morphology. In addition, the unit crystal cell diagram and atoms structure images of 2H-MoS2 were drawn and shown in Figure 1(d), in which a unit cell includes two MoS2 units, and there is no covalent bonding between each layer.
Figure. 1 (a), XRD pattern of the 2H-MoS2. (b) and (c), overhead and side view FE-SEM micrograph of the used 2H-MoS2. (d), unit crystal cell diagram and atomic structure diagram of 2H-MoS2. Na2CO3 was selected as desulfurizer in the present study, for the two reasons: i) the bond strength between the Na and sulfur is stronger than between molybdenum and sulfur; ii) the bond strength between the Na and carbon is weaker than that between molybdenum and carbon. Thus, Na from Na2CO3 could combine with S in MoS2 and
Journal Pre-proof release Mo to combine with C to generate Mo2C. 2.2. Synthesis of 2D Mo2C powders In order to prepare 2D Mo2C powders, the molybdenum disulfide, Na2CO3 and activated carbon with different molar ratios were thoroughly mixed. Then, the mixed sample of about 5g was placed into the alumina crucible which was put in the constant temperature zone of a horizontal furnace. Before heating, argon gas (200 ml/min) was introduced to remove the air. After that, the furnace was heated with a ramping rate of 5 oC/min to different temperatures (800oC and 850oC) and maintained at this temperature for different times (1hour and 2 hours). After the reaction, the samples were cooled to ambient temperature, and then were leached several times in deionized water and centrifuged for the separation of the synthesized 2D Mo2C powders. 2.3. Characterizations. X-ray diffraction (XRD; TTR III, Rigaku Corporation, Japan; X-ray wavelength, 1.5418 Å [Cu-Kα]) and X-ray photoelectron spectroscopy (XPS; ESCALAB 250 Xi; Thermo Scientific, American) analysis were carried out to identify the phase compositions. The morphologies of the samples were characterized using field emission scanning electron microscopy (FE-SEM; ZEISS SUPRA 55, Oberkochen, Germany). After sonication treatment, the 2D layered Mo2C were investigated by transmission electron microscopy (TEM; Tecnai-G2-F20, FEI, American). The residual sulfur and carbon content were analyzed using infrared carbon-sulfur analyzer (EMIA-920V2; HORIBA, Japan). Additionally, The Thermal-gravimetric analysis-Mass spectrometry (TGA-MS) (MS, QMG 220, Pfeiffer, German) characterization was performed to determine the composition of produced gas. 3. Results and discussion 3.1. XRD analyses Figure 2(a) presents the XRD patterns of products acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:11. From which it can be seen that when the mixed sample was annealed at 800oC for 2 hours, there is still a distinct 2H-MoS2 diffraction peak
Journal Pre-proof (corresponding to its [002] crystal plane). When the temperature increased to 850oC, the 2H-MoS2 diffraction peak disappeared. Additionally, when the mixed sample was annealed at 850oC for 1 hour, there is also a distinct diffraction peak of 2H-MoS2. Therefore, both the increase of temperature and time contribute to the carbonization of MoS2. Accordingly, the experimental conditions for the preparation of Mo2C were selected as 850oC and 2 hours.
Figure. 2 XRD patterns and FE-SEM micrographs of the products. (a), XRD patterns of the product acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:11. (b), XRD patterns of the product obtained by adding different amounts of Na2CO3. (c), XRD
Journal Pre-proof patterns of the product obtained by adding different amounts of activated carbon. (d), FE-SEM micrograph of the product acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:1. (e), XRD patterns of the product after leaching treatment (corresponding to (c)). (f), Rietveld-refined XRD pattern of the prepared Mo2C acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:11. Figure 2(b) plots the XRD patterns of products acquired at different MoS2:Na2CO3:C molar ratios of 2:4:9, 2:4.1:9 and 2:4.2:9, respectively. It can be found that most of the 2H-MoS2 transformed to Mo2C, and the main co-product is Na2S. In additional, it can be found that there is still a distinct MoS2 diffraction peak when the MoS2:Na2CO3:C molar ratio was 2:4:9 and 2:4.1:9. When the MoS2:Na2CO3:C molar ratio is 2:4.2:9, the diffraction peak of MoS2 disappeared, indicating that all MoS2 was completely reacted. The reason for the using of 1.05 times of the theoretical amount of Na2CO3 may be due to the slow reaction kinetics of the heterogeneous phase reaction. Therefore, the increase in amount of Na2CO3 is beneficial for the carbonization of MoS2. As can be seen from Figure 2(b), there is always an obvious diffraction peak of Na2MoO4. Since the excessive activated carbon can prevent the generation of Na2MoO4 due to the reaction between them, the experiments with different addition amounts of activated carbon were conducted, and the XRD patterns are shown in Figure 2(c). In Figure 2(c), the MoS2:Na2CO3 molar ratio was set to 2:4.2 based on the experimental results shown in Figure 2(b). With the increase of activated carbon content, the intensity of the diffraction peak of Na2MoO4 decreased gradually, and totally disappeared until the MoS2:Na2CO3:C molar ratio was 2:4.2:11. Consequently, the 2H-MoS2 can be completely carbonized to Mo2C at a MoS2:Na2CO3:C molar ratio of 2:4.2:11. The main reaction of 2H-MoS2, Na2CO3 and activated carbon can be described by reaction (1). In addition, the morphological characteristics of the acquired product were investigated by FE-SEM. The FE-SEM micrograph of the product acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:11 was shown in Figure 2(d), and there were many sheets stacked and wrapped by another phase.
Journal Pre-proof 2MoS2 +4Na 2 CO3 +9C=Mo 2 C+4Na 2S+12CO
(1)
In order to eliminate the inserted Na2S and acquire layered Mo2C, the products were washed several times with deionized water and centrifuged to separate the Mo2C powders. The phase compositions after leaching treatment of the products obtained by adding different amounts of activated carbon are shown in Figure 2(e) (corresponding to Figure 2(c)). It can be seen from Figure 2(e) that Na2CO3, Na2S and Na2MoO4 were removed completely after leaching treatment, and only Mo2C remained. The Rietveld-refined XRD pattern of the prepared Mo2C is shown in Figure. 2f. The value of agreement factor Rwp is as low as 9.42%, indicating highly reliable results. The Rietveld-refined lattice parameters of the prepared Mo2C are a =3.0124 Å and c = 4.7356 Å, and the value of c is little larger than the date of bulk Mo2C (PDF card no 35-787). Additionally, when the molar ratio of MoS2:Na2CO3:C was 2:4.2:11, the amount of 2H-MoS2 in 5g of the mixed raw materials was about 1.82g. After leaching treatment, about 1.05g of Mo2C powder was obtained. Based on mass conservation of molybdenum, the yield could be calculated to about 92%. Further, the carbon and sulfur content of the synthesized Mo2C powder were measured by using an infrared carbon-sulfur analyzer, and the results were 5.98% (close to the theoretical carbon content of Mo2C, 5.89%) and 0.33%, respectively. In order to determine the composition of gases evolved, the TGA-MS analysis was performed for sample with a MoS2:Na2CO3:C molar ratio of 2:4.2:11 with a ramping rate of 10 oC/min from room temperature to 850oC. Mass scanning was carried out in the selected-ion monitoring mode. The ions with the mass-to-charge ratio (m / z) of 28 (CO), 44 (CO2), 64 (SO2), 76 (CS2) and 80 (SO3) were monitored, and the TGA-MS thermogram is shown in Figure. 3. From TGA curves it can be seen that the reaction begins at about 600oC, and when the temperature increases to 850oC and maintains at this temperature for about 10 minutes the mass loss rate reaches a maximum of 39.1%, which is close to the theoretical mass loss rate of 39.43% for reaction (1). From the MS curve it can be found that only CO (m / z=28) and CO2 (m / z=44) were produced. When the temperature increases to 600oC, CO2 was generated and
Journal Pre-proof disappeared when the temperature rises to about 800oC, while the main gaseous product is CO. It must be noted that the m/z ratio of N2 is also equal to 28, and the blue curve before 600 oC indicates the existence of N2, since the reduction reaction does not start. During the heating process, the proportion of air continues to decrease, resulting in a continuous decrease in the intensity ion current of N2 (m/z =28). In the current study, the samples were rapidly heated to 800 or 850oC, so it can be considered that the gaseous produced is mainly CO during the reaction process. Thus, the main reaction among 2H-MoS2, Na2CO3 and activated carbon can be described by reaction (1).
Figure. 3 TGA-MS thermogram of the sample acquired for sample with a MoS2:Na2CO3:C molar ratio of 2:4.2:11 with a ramping rate of 10 oC/min from room temperature to 850oC. 3.2. XPS analyses
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Figure. 4 X-ray photoelectron spectroscopy spectra of prepared Mo2C at 850oC for 2 hours. (a), Mo 3d spectrum. (b), S 2p spectrum. (c), C 1s spectrum. (d),O 1s spectrum. The chemical states and surface termination of the synthesized Mo2C were further studied by XPS measurements. High-resolution XPS spectra peak fittings for Mo 3d, S 2p, C 1s and O 1s are shown in Figures 4(a-d), respectively. Figure 4(a) plots the Mo 3d spectrum, which shows the Mo-C species (Mo 3d5/2, 228.1 eV; Mo 3d3/2, 232.2 eV) and Mo–O species [24, 30]. The Mo-oxide species should be attributed to MoO2 (Mo4+) and MoO3 (Mo6+). It was noted that no S 2s peak (MoS2, 226.8 eV) [31] was detected in this region, suggesting that MoS2 was totally carbonization. Furthermore, the S 2p spectrum was fitted by a component with a binding energy of 163.6 eV as shown in Figure 4(b), which may correspond to S species [32]. During the leaching treatment with deionized water, a part of Na2S would react with H2O to generate H2S gas (Reaction (2)), and a part of H2S is dissolved in the water which may be oxidized by the air to form S element (Reaction (3)). Furthermore, the sulfur content of the prepared Mo2C was measured by using an infrared carbon-sulfur
Journal Pre-proof analyzer, and the result was 0.33%.
Na 2S+2H 2 O=2NaOH+H 2S
(2)
2H 2S(l)+O 2 =2S +2H 2 O
(3)
Figure 4(c) plots the C 1s specture that was fitted by components corresponding to Mo-C(283.5 eV), C-C(284.6 eV), CHx(285.6 eV) and C-O(286.6 eV) species [33-35]. The O 1s specture was shown in Figure 3(d), which was fitted by components corresponding to: MoO2 (530.2 eV) [36], MoO3 (531.1 eV) [37], Mo2COx (-O terminated,
531.1
eV),
Mo2C(OH)2
(-OH
terminated,
532.1
eV)
and
Mo2C(OH)x-H2Oads (-OH terminated, 533.4 eV) species [24]. 3.3. FE-SEM and TEM analyses
Figure. 5 FE-SEM images of the Mo2C prepared at different MoS2:Na2CO3:C molar ratios at 850oC for 2 hours. (a) and (b), 2:4.2:11. (c) and (d), 2:4.2:10. (e) and (f), 2:4.2:9. (g) and (h), 2:4.1:9. (i) and (j), 2:4:9. (k), low magnification FE-SEM images of the prepared Mo2C. (l), EDS pattern of the Mo2C obtained at a MoS2:Na2CO3:C molar ratio of 2:4:11 at 850oC for 2 hours. The morphology and structure of the synthesized 2D Mo2C were further
Journal Pre-proof investigated by FE-SEM and TEM. The FE-SEM images of the 2D Mo2C acquired after leaching treatment are shown in Figure 5. Figures 5(a) and (b) show the FE-SEM images of the Mo2C acquired at a MoS2:Na2CO3:C molar ratio of 2:4.2:11, where the layers are clearly separated from each other, similar to the exfoliated graphite and MXenes [38, 8]. The pretty regular layered structure proves the perfect 2D morphology of the synthesized Mo2C. Furthermore, it can be seen from Figures 5(a) and (b) that the thickness of the Mo2C is varied between 5 to 15 nm and its lateral size was several micrometers. Figures 5(c-j) show the FE-SEM images of the Mo2C prepared at different MoS2:Na2CO3:C molar ratios, from which it can be found that there is no apparent difference in the morphology of the prepared Mo2C, and the thickness of each layer is varied between 5 to 15 nm. The low magnification SEM images of the synthesized Mo2C is shown in Figure 5(k), from which it can be seen that the product has the 2D layered structure. Furthermore, the EDS spectrum of the obtained Mo2C is shown in Figure 5(l), and it can be seen that the atomic percentages of Mo atoms and C atoms are 63.68% and 36.32%, respectively, which is close to the stoichiometric ratio of Mo to C in Mo2C. Prior to the TEM test, the prepared Mo2C powders were sonicated in an ethanol solution for 300 seconds to exfoliate the layered Mo2C. Due to the 2D nature of ultra-thin Mo2C crystals, it is easy to obtain an atomic-resolution image along its [100] zone axis [39]. An atomic model of the Mo2C crystal project in the [100] direction was drawn and shown in Figure 6(a). The TEM image of the 2D Mo2C obtained at a MoS2:Na2CO3:C molar ratio of 2:4.2:11 is shown in Figure 6(b), which also revealed that each layer of the synthesized 2D Mo2C is very thin because of its high transparency, in which the underlying layers are clearly visible beneath the top Mo2C layer. Figures 6(c) and (d) show the HRTEM images of Mo2C corresponding to Figure 6(b), while Figure 6(d) has a higher magnification. In Figure 6(d), blue sphere represents Mo atoms, and red sphere represent C atoms. This is consistent with the atomic model shown in Figure 6(a), where the Mo atoms are arranged in a hexagonal closepacked (HCP) configuration, showing a graphene-like honeycomb lattice in the [100] projected direction. Figure 6(e) shows the TEM images of the 2D Mo2C
Journal Pre-proof prepared at a MoS2:Na2CO3:C molar ratio of 2:4.2:10, from which it is concluded that the prepared Mo2C has the multi-layer sheets structure. Figures 6(f) and (g) show the HRTEM images corresponding to different position of Figure 6(e). From the HRTEM images of the layer edge, the layered Mo2C had a pretty small thickness.
Figure. 6 (a), Atomic model of an Mo2C crystal project in the [100] direction. (b) and (c), Transmission electron mircroscopy (TEM) image with corresponding SAED pattern and high-resolution TEM (HRTEM) image of the product after leaching treatment obtained at a MoS2:Na2CO3:C molar ratio of 2:4.2:11. (d), HRTEM image of Mo2C (same as (c) with higher magnification). (e), (f) and (g), TEM image and HRTEM image of the product after leaching treatment obtained at a MoS2:Na2CO3:C molar ratio of 2:4.2:10. From the above results, the prepared Mo2C retains the layered structure similar to 2H-MoS2, which indicates that the Na2CO3-assisted carbothermal reduction of 2H-MoS2 is a topochemical reaction. Accordingly, during Na2CO3-assisted carbothermal reduction of 2H-MoS2, the S atoms in 2H-MoS2 were replaced by C atoms, forming Mo2C with a layered hexagonal structure. Meanwhile, the S atoms combined with Na atoms to form Na2S. Therefore, 2H-MoS2 with inherent 2D crystal structure can be considered as an ideal precursor for synthesizing 2D Mo2C. Moreover, it can be seen from Figure 2d that the co-product Na2S was inserted into the generated Mo2C, which could play a crucial role on the exfoliating of the 2D
Journal Pre-proof Mo2C. Firstly, the inserted Na2S can separate the produced layered Mo2C, hindering the combination of each layer of Mo2C at high temperature. In addition, even if Na2S has a high solubility in water, a part of Na2S would react with H2O to generate H2S gas (reaction (2)), and the presence of H2S can further promote the separation of each layer of Mo2C. Therefore, after removing the intercalated Na2S, ultrathin 2D Mo2C with a thickness of several nanometers was successfully obtained. In our previous research [40], the temperature of the direct carbothermal reduction of MoS2 to prepare Mo2C without the addition of Na2CO3 was 1450oC. By comparison, it can be found that the addition of Na2CO3 as a desulfurizer can promote the carburization of 2H-MoS2 and greatly reduce the temperature for the preparation of Mo2C. 4. Conclusions In this work, an environmentally friendly, efficient and high yield approach is proposed to prepare 2D Mo2C via Na2CO3-assited carbothermal reduction of 2H-MoS2. Natural 2H-MoS2 have a layered structure was used as an ideal precursor for preparing 2D Mo2C. It can be found that the addition of Na2CO3 as a desulfurizer can promote the carburization rate of 2H-MoS2 and greatly reduce the preparation temperature of Mo2C. At the same time, the sulfur contained in MoS2 could be fixed in Na2S. After reacting at 850oC for 2h, a mixture of 2D Mo2C and co-product Na2S were successfully prepared. After leaching treatment with deionized water, ultra-thin 2D Mo2C were successfully obtained. FE-SEM and TEM images indicate that each layer of Mo2C has a thickness of 5-15 nanometers. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (51734002). Conflicts of interest There are no conflicts to declare.
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Declaration of interests ■The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: