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C: Physical Processes in Nanomaterials and Nanostructures

Shape-Controlled Synthesis of Ultrafine Mo Crystals via Salt-Assisted Reduction of MoO2 with H2 Guo-Dong Sun, Guo-Hua Zhang, Shuqiang Jiao, and Kuo-Chih Chou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01236 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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The Journal of Physical Chemistry

Shape-Controlled Synthesis of Ultrafine Mo Crystals via Salt-Assisted Reduction of MoO2 with H2 Guo-Dong Sun, Guo-Hua Zhang*, Shu-Qiang Jiao, Kuo-Chih Chou State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China ABSTRACT In the current investigation, the salt-assisted hydrogen reduction of MoO2 was proposed to prepare ultrafine Mo single crystals with controllable shape and size. By adjusting the type and amount of added chlorine salts (NaCl, KCl, CaCl2 and MgCl2) and temperature, ultrafine Mo single crystals with different shapes and sizes were prepared successfully. When no salt was added, the produced Mo approximately keeps the shapes and size of the raw MoO2 in the temperature range of 805-1000 °C. However, as adding 0.1 mass% chlorine salts, ultrafine, uniformed and spherical Mo cystals were obtained. As increasing the amount of added salt, the shape of Mo crystal would change from sphere to polyhedron. With increasing the amount of added salts to 0.5 mass%, ultrafine Mo single crystals with polyhedral shape were synthesized. The effects of the added salts on the nucleation, growth and shape control of Mo crystal were investigated in detail. 1. INTRODUCTION Molybdenum (Mo) possesses high melting point, excellent mechanical, thermal, electrical and chemical properties, good corrosion and creep resistance, which makes it attractive for electronics, metallurgical, aerospace and electrical industries. In particular, ultrafine Mo powder with suitable shape, size and purity has critical application in many fields such as electronic base, electrical contacts, thick film inks, metal injection molding, thermal spraying, luminescent materials and other fields

1-7

. Therefore, Mo

powder with ultrafine size and special shape has been received wide attentions and investigations 8. Kim et al

5

and Saghafi et al

9

investigated the synthesis of

nanocrystalline Mo powder via mechanical activation of MoO3 powder following by 1

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hydrogen reduction. Shibata et al.

10

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prepared ultrafine Mo powder using the vapour

phase reaction of the MoO3-H2 system. Lamprey and Ripley

11

reported a method to

produce Mo metal powders with the particle size of 0.01–0.1 µm via reducing molybdenum chloride by the hydrogen. Liu et al

12

described a microwave plasma

chemical vapour deposition method to prepare nano-sized Mo powder. Won et al.

13

introduced the salt-assisted combustion reaction technique (SACR) for preparing nanocrystalline Mo. In our previous works, ultrafine Mo powder was successfully prepared by the hydrogen reduction of ultrafine MoO2 powder 14 and industrial grade MoO3 15. Although a great deal of methods have been proposed to prepare molybdenum powders, hydrogen reduction of molybdenum oxides is an important and well established industrial process for the manufacturing of high quality Mo powder. The industrial production of metallic molybdenum is a stepwise process including the hydrogen reduction of MoO3 to MoO2 in the temperature range of 450 °C-650 °C and the further hydrogen reduction of MoO2 to Mo at the relative higher temperatures (850 °C-950 °C) 3, 6, 14, 16. The reduction mechanisms of molybdenum oxides using H2 have been investigated by many researchers 4, 6, 14, 16-18. It is known that the morphology and size of Mo are mainly determined by the process of MoO2→Mo. Schulmeyer and Ortner

17

reported that the hydrogen reduction of MoO2 to Mo obeys pseudomorphic

transformation mechanism at low local concentration of water vapor, while obeys chemical vapor transport (CVT) mechanism at high local concentration of water vapor. For the pseudomorphic transformation, the reaction interface moves from the particle surface towards the core of the grain, forming a similar shape to the starting material. With increasing the concentration of water vapor, an intermediate gaseous transport phase (TP) increases, and reduction could be dominated by CVT mechanism, in which the reduction path could be presented as: MoO2→TP(g)→Mo 6, 14, 17, 19. The generation, transposrtation, reducion and deposition of molybdenum bearing gaseous phase always lead to a large difference of morphology between products and raw materials. Therefore, it was thought that the morphology and size of the final molybdenum powder is mainly determined by the transformation mechanismsn 6. The effects of various reaction parameters, such as the reduction temperature, concentration of water vapor, hydrogen content and the flow rate of hydrogen gas on 2


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the morphology and size of molybdenum particles were investigted in several studies 6, 17, 19

. However, it is difficult to control the shape and size of Mo crystals succefully by

simplified adjusting these parameters. In the literatures, the method of assisted molten salt syntheses (MSS) were widely used to prepare nano and microcrystals with controlled shapes and sizes 20-22. In this paper, the salt assisted hydrogen reduction of MoO2 was proposed to prepare the ultrafine Mo crystals. A small amount of chlorine salts (NaCl, KCl, CaCl2 and MgCl2) were employed as assisted additives, respectively, to control the mophology and size of Mo crystal. 2. EXPERIMENTAL SECTION 2.1 Materials The raw materials used in this study are commercially MoO2 powders, which is manufactured by hydrogen reduction of ultrafine MoO3 at 600 °C from Jinduicheng Molybdenum Co., Ltd, Xi’an, China. The X-ray diffraction pattern (XRD) and Field emission scanning electron microscope (FE-SEM) image of raw MoO2 material are presented in Figure 1 and Figure 2, respectively. It can be seen from Figure 1 that all the peaks are well defined with MoO2 (PDF card No 32-671). Platelet-shaped morphology can be seen in Figure 2. Reagent-grade chlorine salts, NaCl, KCl, CaCl2 and MgCl2, were employed as assisted additives in this study, which have a high solubility in water and are easy to be removed by water washing. The additives were dissolved in deionized water and sprayed into the MoO2 evenly by an atomizing sprayer. Mixed MoO2 powders with NaCl (0.1, 0.5 and 2 mass%), KCl (0.1and 0.5 mass%), CaCl2 (0.5 mass%) and MgCl2 (0.5 mass%) were prepared, respectively. Then the mixtures were dried at 100 °C for 6 hours before experiments. 2.2 Experimental procedure The schematic diagram of the experimental apparatus used in this study were described in detail in our previous paper 23. Alumina crucibles of 45 mm in length, 15 mm in width, and 20 mm in height was used. In each experimental run, a sample of about 2 g was used and filled into the alumina crucible. The thickness of sample in crucible was about 4 mm. After the crucible with the sample was placed into the quartz tube, argon gas was passed into the quartz to purge air. Then the quartz tube 3



was put into the electrical furnace with the heating element of Si-C rods, the temperature of which was already raised to the desired value of 805 °C, 900 °C or 1000 °C. When the temperature was stabilized, the argon gas was switched to H2 to start the reduction experiment. After reacting for 40 minutes, which is enough to reduce MoO2 to Mo completely at above 800 °C

3, 14, 19

, the gas inlet was switched to

argon gas again, and then the quartz tube was taken out quickly from the furnace to cool the sample to the room temperature. The phase composition of reduction product is determined by X-ray diffraction technology (XRD) (TTR III, Rigaku Corporation, Japan). The morphology of reduction product is observed by field emission scanning electron microscope (FE-SEM) (ZEISS SUPRA 55, Oberkochen, Germany) with energy dispersive X-ray spectroscopy (EDS) and transmission electron microscope (TEM).

Figure 1. XRD patterns of the raw MoO2.

Figure 2. FE-SEM images of the raw MoO2, (a1) ×5000; (a2) ×20000. 4


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3. RESULTS 3.1 X-ray diffraction analyses According to the weight loss rates (defined as the ratio of mass loss to the total mass of the initial sample) and XRD patterns, for the most of the experiments, molybdenum oxide powders were completely reduced within 40 min. However, when the sample with 0.5 mass% MgCl2 or 0.5 mass% CaCl2 was reduced at 805 °C for 40 min, the weight loss rate of the samples was 23.48% (reaction extent, 0.939) and 18.12% (reaction extent, 0.725), respectively. As shown in Figures 3 (c) and (d), the characteristic peaks of MoO2 appear in XRD patterns of the two incomplete samples mentioned above, indicating that compared to adding NaCl or KCl, the addition of MgCl2 or CaCl2 has an obvious inhibition effect on the reduction rate. Besides, no diffraction peaks of chlorine salts (NaCl, KCl, CaCl2 or MgCl2) is detected due to their very small amounts.

Figure 3. XRD patterns of the products, (a) 0.5 mass% NaCl; (b) 0.5 mass% KCl; (c) 0.5 mass% MgCl2; (d) 0.5 mass% CaCl2. 5



3.2 FE-SEM analyses 3.2.1 Reduction of MoO2 with the addition of NaCl Pure MoO2 without any salt assistance was employed as blank reference. The FE-SEM micrographs of product Mo reduced at different temperatures are shown in Figures 4 (a), (b) and (c). From these FE-SEM images, it can be seen clearly that at 805 °C, on the whole, the Mo product obtained by reducing MoO2 with H2 (without additive) keeps the plate-like morphology as that of MoO2. With increasing the temperature from 805 °C to 1000 °C, the morphology of Mo product changes gradually to smoother and rounder shape. Figures 4 (d), (e) and (f) shows the FE-SEM images of product Mo obtained by reducing MoO2 with 0.1 mass% NaCl at different temperatures. It is worth noting that all the experiment conditions were kept the same as to the reduction experiments without salt addition. It can be seen from Figures 4 (d) that uniformed, near-spherical primary grains with an average size of about 150 nm are obtained at 805 °C. With further increasing the reaction temperature to 900 °C, the grain size become larger and the number of spherical grains decreases. When the temperature is increased to 1000 °C, as shown in Figures 4 (f), the grains have fine spherical and uniform shape with a size of about 500 nm. Therefore, the addition of 0.1 mass% NaCl has a great effect on the morphologies and sizes. In order to further understand the effect of NaCl addition on the morphology of Mo powders, the experiments of reducing MoO2 with 0.5 and 2 mass% of NaCl were also conducted, and the FE-SEM micrographs of produced Mo are presented in Figure 5. It can be seen from Figures 5 (a), (b) and (c) that as increasing the amount of NaCl to 0.5 mass%, the grains of products have pretty obvious polyhedral shapes with edges and corners. Especially at 1000 °C, one typical shape of products is dodecahedron, which is significantly different with the spherical shape obtained in the case of the 0.1 mass% NaCl addition. These polyhedral shapes are much more evident when the added NaCl is increased to 2 mass%. As shown in Figures 5 (d), (e) and (f), the particle size is much bigger but the size distribution is inhomogeneous compared to the case of the 0.1 mass% and 0.5 mass% NaCl. At above 900 °C, fine Mo single crystals with polyhedral shape are produced. With increasing the temperature from 900 °C to 1000 °C, the particle size rises to about 1500 nm. It can be found that the 6


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amount of added salt can not only influence the shape but also the size of Mo grain. When the amount of added salt increases, the produced Mo crystals changes from spherical shape to polyhedral shape and the size of Mo grain increases.

Figure 4. FE-SEM images of Mo products obtained by reducing MoO2, (a) without additive, 805 °C; (b) without additive, 900 °C; (c) without additive,1000°C; (d) 0.1 mass% NaCl, 805 °C; (e) 0.1 mass% NaCl, 900°C; (f) 0.1 mass% NaCl,1000 °C.

Figure 5. FE-SEM images of Mo products obtained by reducing MoO2 with different amount of NaCl and temperatures, (a) 0.5 mass% NaCl, 805 °C; (b) 0.5 mass% NaCl, 900 °C; (c) 0.5 mass% NaCl, 1000 °C; (d) 2 mass% NaCl, 805 °C; (e) 2 mass% NaCl, 900 °C; (f) 2 mass% NaCl, 1000 °C. 3.2.2 Reduction of MoO2 with the addition of KCl 7



Figure 6 shows the FE-SEM images of Mo reduced with the assistance of 0.1 and 0.5 mass% KCl. As shown in Figures 6 (a), (b) and (c), when the amount of added KCl is 0.1 mass%, the morphologies of products are similar to the case of adding 0.1 mass% NaCl, in which Mo particles have spherical and uniform shapes. When the added KCl rises to 0.5 mass%, it is obvious in Figures 6 (d), (e) and (f) that Mo particles with polyhedral shapes are also produced, which have a larger size than adding 0.1 mass% KCl. Accordingly, KCl have a very similar effect on the shape and size of produced Mo crystals as NaCl.

Figure 6. FE-SEM images of Mo products obtained by reducing MoO2 with different amount of KCl and temperatures, (a) 0.1 mass% KCl, 805°C; (b) 0.1 mass% KCl, 900°C; (c) 0.1 mass% KCl, 1000°C; (d) 0.5 mass% KCl, 805°C; (e) 0.5 mass% KCl, 900°C; (f) 0.5 mass% KCl, 1000°C. 3.2.3 Reduction of MoO2 with the additions of MgCl2 and CaCl2 Figure 7 shows the FE-SEM images of products obtained with the addition of 0.5 mass% MgCl2 or 0.5 mass% CaCl2 at different temperatures. It can be seen from Figure 7 (a) (0.5 mass% MgCl2, 805 °C) that the products maintain the same morphology of initial MoO2, with the particle composed of a lot of small primary grains with an average size of dozens of nanometers. When the temperature increases to 900 °C, the product still partially keep the overall morphology while the primary grains grow to about 200 nm. At 1000 °C, the size of grains increases dramatically, as shown in Figure 9(c). For the addition of 0.5 mass% CaCl2, the morphology evolution 8


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is almost the same as the case of adding 0.5 mass% MgCl2, except that the primary grain size of products is a little larger. As discussed above, it can be found that in the reduction process of MoO2, the additives can remarkably change the morphology and grain size of the produced Mo crystals. By adjusting the type of additive and its amount, ultrafine Mo crystals with different morphologies and grain sizes can be synthesized.

Figure 7. FE-SEM images of Mo products obtained by reducing MoO2 with different additives and temperatures, (a) 0.5mass% MgCl2, 805 °C; (b) 0.5mass% MgCl2, 900 °C; (c) 0.5mass% MgCl2,1000°C; (d) 0.5mass% CaCl2, 805 °C; (e) 0.5mass% CaCl2, 900°C; (f) 0.5mass% CaCl2,1000 °C. 3.3 TEM analyses Further characterization of the final Mo products was performed by the TEM technology. Figure 8 shows the TEM images of Mo obtained under different conditions with the additive of NaCl. From Figure 8 (a), it can be seen that the Mo crystals obtained at 805 °C with 0.1 mass% NaCl has a nearly spherical shape and a uniform size of about 150 nm, which is in good agreement with the results of FE-SEM. Besides, the selected area electron diffraction (SAED) patterns (shown in Figures 8 (a2), (b2) and (d2), insets) demonstrate that the Mo grains are single-crystalline structure. With increasing the temperature to 900 °C and 1000 °C, as shown in Figures 8 (b) and (c), spherical Mo single crystal grains with size of about 250 nm (900 °C) and 500 nm (1000 °C), respectively, can be obtained. Figure 8 (d) 9



shows the TEM pictures of Mo crystals produced with the addition of 0.5 mass% NaCl, from which it can be seen that the Mo single crystal has an obvious polyhedral shape with edges and corners.

Figure 8. TEM images of Mo products obtained by reducing MoO2 by hydrogen with the additive of NaCl, (a) 805 °C-0.1mass% NaCl; (b) 900 °C-0.1mass% NaCl; (c) 1000 °C-0.1mass% NaCl; (d) 805 °C-0.5mass% NaCl. 4. DISCUSSIONS 4.1 The mechanisms of salt-assisted reduction of MoO2 with H2 Schulmeyer and Ortner

17

investigated the reduction mechanism of MoO2 at

1100 °C under extremely low and high partial pressure of water vapor, respectively, and proposed two different reaction mechanisms: pseudomorphic transformation and chemical vapor transport (CVT). Specifically, under extremely low partial pressure of 10


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water vapor, the pseudomorphic transformation mechanism was obeyed, while the CVT transformation mechanism was obeyed when the partial pressure of water vapor is extremely high. These two mechanisms have been used extensively to analyze the reduction mechanisms of reduction of MoO2.5, 6, 14, 19 For the pure pseudomorphic transformation mechanism, the main reaction is shown as reaction (1), and Mo is generated directly and the reaction interface moves from the particle surface towards the core of the grain, forming Mo particle with a similar shape to the starting material. However, for the CVT mechanism, a gaseous intermediate transport phase is formed.14, 17 Many researchers have investigated the chemical transport (volatilization) of molybdenum and its oxides in the presence of water vapor and concluded that the volatilization occurs via the generation of gaseous MoO2(OH)2 by reaction between molybdenum or its oxides with water vapor.24-27 And the volatilization of MoO2 with water vapor can be described as reaction (2),25, 26 in which the letters “s” and “g” in brackets represent “solid” and“gas” respectively. It was also reported that the pressure of MoO2(OH)2 is proportional to the pressure of water25,

26

. The generated

MoO2(OH)2 can be reduced to Mo by H2 when the concentration of water vapor is low, described as reaction (3). Therefore, the CVT mechanism is based on the generation of the MoO2(OH)2, which is significantly influenced by local concentration of water vapor.

MoO2 (s) + 2H 2 (g) = Mo(s) + 2H 2O(g)

(1)

MoO2 (s) + 2 H 2O(g) = MoO2 (O H)2 (g) + H 2 (g)

(2)

MoO2 (O H)2 (g)+ 3H 2 (g) = Mo(s) + 4 H 2O(g)

(3)

In the previous published paper19, the influence of concentration of water vapor in H2 on the morphology of produced Mo by reducing MoO2 was investigated at 950 °C. It was found that when dry H2 was used, the produced Mo grains still maintained plate-like shape of MoO2. With increasing concentration of water vapor, the shape of Mo particles changed gradually to smoother and rounder shape. When the concentration of water vapor of H2 was about 19.67 vol%, spherical agglomerated Mo was obtained. In the current study, at 805 °C, the Mo product obtained by reducing 11



MoO2 with dry H2 (without additive) also keeps the morphology of plate-like shape of MoO2. At this low temperature, due to the slow reduction rate, the rate of water generation is relatively slow, resulting in a relatively low concentration of water vapor, and thus, there will be a low concentration of gaseous molybdenum compounds. Therefore, in this case, pseudomorphic transformation mechanism may be dominated. However, with increasing the temperature from 805 °C to 1000 °C, the morphology of Mo product changes gradually to smoother and rounder shape. The main reason could be that the rate of water generation increases with the increasing temperature and more gaseous molybdenum compounds will be generated. Therefore, the proportion of CVT mechanism will increase when the temperature increases. However, even though the CVT mechanism exists, it is hard to obtain Mo with fine morphology and size. Whereas, after adding a small amount of salt, the Mo morphology changed significantly and ultrafine Mo grains with fine morphology can be obtained. Due to that a very small amount of salt was added, it couldn’t greatly change the local concentration of water vapor. Thus, the local concentration of water vapor and the amount of generated volatile molybdenum compound should be approximately same to the case of reduction reaction without salt addition. Therefore, there must be other reasons resulting in the different morphologies and sizes of produced Mo grains with or without additive. In the previous publications, there is little attention focused on the effect of MoO2(OH)2 concentration on nucleation of Mo. However, the initial nucleus generated from gaseous molybdenum compounds may play an important role in the morphology of Mo grain. At the beginning of the reaction between MoO2 and H2, water is generated and H2 is consumed near the surface of MoO2 (reaction (1)), leading to a high partial pressure of water and low partial pressure of H2. Then the generated H2O could react with MoO2 to form MoO2(OH)2 (reaction (2)), which transfers away to the area with a relative low partial pressure of H2O (high partial pressure of H2) and is reduced to Mo (reaction (3)). According to the nucleation theory of vapor phase28-30, if the concentration of generated volatile molybdenum is pretty high, a high degree of supersaturation will be created and it will be possible to form a lot of stable Mo nuclei around the outer surface of the MoO2. Once the nuclei are formed, it would be easy 12


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for Mo atoms to deposit on formed Mo nuclei rather than continue to nucleate29, 30 and thus, the formed Mo nuclei will grow by depositing Mo atoms generated by reducing MoO2(OH)2. These could be the nucleation and growth mechanisms in the CVT process. However, if the concentration of gaseous transport phase is low, it would be difficult to form stable nuclei near the out surfaces of MoO2. Therefore, in this case, Mo layer surrounded unreacted MoO2 could be formed, and Mo atoms, generated by reducing MoO2(OH)2, would deposit on the formed Mo layer. Finally, Mo product could approximately inherit the morphology of raw MoO2 partially. Based on the above analyses, the effect of salt on the nucleation and growth could be described as follows. After adding a small amount of salt, via spraying method, it would be deposited dispersedly on the surface of MoO2 and be molten at high temperature, which could supply an ideal place for nucleation. At the initial stage of the reaction, the Mo atoms, both generated from reaction (1) and (3), would be adsorbed, concentrated, and nucleated with the help of molten salt. Then, the formed Mo nuclei would grow via the deposition of Mo atoms generated by reducing the gaseous molybdenum compounds. Figure 9 shows the FE-SEM micrographs of MoO2 (with 0.5 mass% NaCl) reduced by H2 with a reaction extent of 0.35 at 805 °C and 0.23 at 900 °C, respectively. It can be seen obviously from Figure 9 that the Mo particles form surrounding the outer surface of MoO2, which can further prove the proposed salt-assisted mechanism. Besides, the schematics, shown in Figure 10, are drawn to describe the two reaction mechanisms of with or without additive respectively.

Figure 9. FE-SEM images of MoO2 with 0.5 mass% NaCl additive reduced by H2, (a) 13



805 °C, α=0.35; (b) 900 °C, α=0.23. (α is reaction extent).

Figure 10. The schematics the reaction mechanisms of without additive and adding additives. 4.2 The mechanism of controlling crystal shape and size It is known that in chemical reaction, the thermodynamically stable product can be obtained when the system is in equilibrium. Similarly, the stable shape of ultrafine crystals is also thermodynamically favored. The system seeks paths for minimizing the total surface energy. For example, the most stable shape of liquid droplets and amorphous particles is spherical shape because it has the lowest surface energy. For the ultrafine crystals, the lowest surface energy is complex to evaluate due to their different type of facets, which could have different stabilities 29, 31, 32. The shape of the ultrafine crystals would also be spherical when the facts are equally preferred, in which case each facts has a nearly same surface energy. If there is only one type of favored facet, which has the lowest surface energy, the most stable shape in thermodynamics would be composed of the favored type of facet, leading to that the polyhedral shape with sharp edges and corners will be preferred. With reducing the difference in the stability of the facets, more type of facets will appear, leading to the change of shape from polyhedron to rounder gradually. It was reported that the 14


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relative stability of the facets can be evaluated on the basis of the bonding interactions within the facets, determined by the surface packing efficiency (surface atomic density) 29. For example, the most stable facets of fcc metal nanocrystals are normally the (111) facets, which has the highest density of atom 32, 33. Mo metal owns a bcc crystal structure, in which the (110) facts may be the most stable ones in thermodynamic, due to the atoms thereon have the largest number of neighbors. Wang et al

32

reported that the surface energies of (100), (110), and (111)

facts of Mo are 3.661, 3.174 and 3.447 J/m2 respectively, in which the (110) facts should be the most stable type of facets due to its lowest surface energy. Therefore, with the consideration that the system seeks to minimize the total surface energy, the thermodynamically stable shape of Mo nanocrystal should be polyhedron with sharp edges and corners. However, in addition to the thermodynamic driving force, the kinetic barrier is also needed to overcome. In the growth process of ultrafine crystals, the atoms or molecules are deposited onto a nucleus, but it is also possible that they will re-dissolve in the solution or migrate on the surface, which is similar to reverse chemical reactions. This reversibility is critical for crystals to form facets in equilibrium state and obtain the most stable shape in thermodynamics. However, if the deposition is a one-way process, uniform over-coating will only lead to isotropic structures and spherical shape, under which condition the shape couldn’t reach the most stable state

29, 34, 35

. The Mo crystals have nearly spherical shape when 0.1

mass% NaCl or KCl is added in MoO2. It is because that the deposition of Mo atoms on the Mo nuclei could be a one-way process, forming uniform over-coating, due to the difficult re-dissolution of Mo atoms from particles to gas. However, when the amount of added salts (NaCl, KCl, MgCl2 and CaCl2) is 0.5 mass%, as described above, the shape of the Mo crystals is polyhedral with sharp edges and corners. The mechanism can be explained as follows. The more molten salt surrounding the particles could dissolute Mo atoms, making it possible that the Mo atoms can both deposit on the particles (crystallization) and re-dissolve in the salt solution or migrate on the surface, which helps to establish the stable facets and thus form the polyhedral shapes. That is to say that sufficient molten salts (0.5 mass%) can help ultrafine crystals to reach the minimum energy state via the continuously dissolving and redepositing of the growth material and accelerating migration of the surface atoms. 15



This mechanism can also be thought as reversible dissolution-crystallization process, in which the salt acts as a flux, and Mo could have certain solubility in it which provides condition for particles shaping. Besides, temperature could also have an influence on the crystallization process. When the temperature increases, the reversible dissolution-crystallization process could be deeper. However, compared to the effect of the amount of salt, it was found that the change of temperature (805-1000°C) has a relatively smaller effect on the shape of Mo crystals (spherical or polyhedral), but a larger effect on the size of Mo crystals. Therefore, as increasing the amount of added salt, the shape of Mo would change from sphere to polyhedron. It can be found that the sizes of crystals increase with increasing temperature because of the increase of the reaction rate, local partial pressure of H2O and concentration of ternary gaseous molybdenum compounds, which can promote the growth of grains. Besides, the amount of added salt can not only influence the shape but also the size of Mo grain. It can be found that the size of Mo grain increases when the amount of added salt increases due to the promotion of the kinetic condition for crystal growth. It is known that these four salts (NaCl, KCl, MgCl2 and CaCl2) have different properties such as molting point, dissolubility and diffusion of molybdenum and so on. Therefore, the type of salt can lead to certain differences of nucleation, growth, and shape of Mo crystals. Accordingly, the shape and size of ultrafine Mo crystals can be controlled by changing the type, amount of additives and temperature. 5. CONCLUTION In the present study, a new mechanism was proposed for preparing ultrafine Mo crystals with control size and shape via salt-assisted reduction of MoO2 with H2 at 805-1000 °C. Without additive, the produced Mo crystals approximately kept the same shapes and sizes of the raw MoO2. Ultrafine, uniformed and spherical Mo nanocystals were obtained by adding 0.1 mass % chlorine salts. As increasing the amount of added salt, the shape of Mo crystals changed from sphere to polyhedron. It was found that as increasing temperature or the amount of salt, the grains size become larger. The added salts can create an ideal place for the nucleation of Mo particles and sufficient molten salts can help Mo crystals reach the stable shape. AUTHOR INFORMATION 16


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Corresponding Author *Guo-Hua Zhang. Tel.: +86 1062333703; fax: +86 1062332570. E-mail address: [email protected]; Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (51734002). REFERENCES (1) Manukyan, K.; Davtyan, D.; Bossert, J.; Kharatyan, S. Direct reduction of ammonium molybdate to elemental molybdenum by combustion reaction. Chem. Eng. J. 2011, 168, 925-930. (2) Novoselova, L. Y., Mo and MoO3 powders: Structure and resistance to CO. J. Alloys Compd. 2014, 615, 784-791. (3) Dang, J.; Zhang, G. H.; Chou, K. C., Study on kinetics of hydrogen reduction of MoO2. Int. J. Refract. Met. Hard Mater. 2013, 41, 356-362. (4) Majumdar, S.; Sharma, I. G.; Samajdar, I.; Bhargava, P., Kinetic studies on hydrogen reduction of MoO3 and morphological analysis of reduced Mo powder. Metal. Mater. Trans. B. 2008, 39, 431-438. (5) Kim, G. S.; Lee, Y. J.; Kim, D. G.; Kim, Y. D. Consolidation behavior of Mo powder fabricated from milled Mo oxide by hydrogen-reduction. J. Alloys Compd. 2008, 454, 327-330. (6) Bolitschek, J.; Luidold, S.; O'Sullivan, M. A study of the impact of reduction conditions on molybdenum morphology. Int. J. Refract. Met. Hard Mater. 2018, 71, 325-329. (7) Huang, L.; Li, M.; Pan, Y.; Shi, Y.; Quan, X.; Puma, G. L., Efficient W and Mo deposition and separation with simultaneous hydrogen production in stacked bioelectrochemical systems. Chem. Eng. J. 2017, 327, 584-596. (8) Wang, L.; Zhang, G. H.; Jiao, S. Q.; Chou, K. C., Pyrophoric behaviour of ultrafine Mo powder. Corros. Sci. 2017, 128, 85-93. (9) Saghafi, M.; Heshmati, M, S.; Ataie, A.; Khodadadi, A. A., Synthesis of nanocrystalline molybdenum by hydrogen reduction of mechanically activated MoO3. J. Refract. Met. Hard Mater. 2012, 30, 128-132. (10) Shibata, K.; Tsuchida, K.; Kato, A. Preparation of ultrafine molybdenum 17



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Shape-Controlled Synthesis of Ultrafine Mo ... - ACS Publications

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