Influences of Ar Flow-Rate and Sublimation Temperature on the Sublimation Product of Analytical Reagent MoO₃

Abstract

In this work, the influences of the Ar flow-rate and sublimation temperature on the phase composition and morphological structure of the sublimation products of analytical reagent MoO₃ are investigated. The results show that the sublimation products are always composed of thermodynamically stable orthorhombic molybdenum trioxide (α-MoO₃) and metastable monoclinic molybdenum trioxide (β-MoO₃) under different reaction conditions, among which the proportion of β-MoO₃ gradually increases with the increase in Ar flow-rate and the decrease in sublimation temperature. The formation temperature of α-MoO₃ is mainly between 780 K and 847 K, with the particles exhibiting an obvious sheet-like morphology. This work also finds that β-MoO₃ is mainly generated below 500 K; however, due to the co-actions of the deposition of gaseous MoO₃ molecules, the adsorption of Ar molecules, and the collision effect between the different particles, the newly formed β-MoO₃ is more inclined to take a spherical-shaped morphology in order to maintain its lowest energy state.

1. Introduction

Molybdenum trioxide (MoO₃) possesses multiple excellent physicochemical properties; hence, it is widely used in numerous contexts, such as batteries [1,2], catalysts [3,4], sensors [5,6,7], and supercapacitors [8,9]. Additionally, as an important intermediate material, MoO₃ is usually used to prepare ultrafine Mo powder [10,11], molybdenum carbide [12], and molybdenum disulfide [13,14,15]; thus, its study has attracted significant attention in recent years [16,17]. In general, MoO₃ has four kinds of crystal types, and the different types have different crystal structures and physicochemical properties. It has been reported [18] that the metastable monoclinic molybdenum trioxide (β-MoO₃) usually has a better catalytic efficiency and electrochemical performance than that of the thermodynamically stable orthorhombic molybdenum trioxide (α-MoO₃). Therefore, the large-scale preparation of β-MoO₃ has received increasing attention in recent decades [19,20,21].

β-MoO₃ was first prepared by McCarron via a solution method [22]; since then, numerous preparation methods have been proposed [23,24]; however, the issues of high cost and route complexity still exist. The physical vapor deposition (PVD) method has the advantages of low cost and process simplicity and has thus gradually become a common method of preparing micro- and nano-sized powder materials. In view of this, the PVD method has also been used in the preparation of micro-/nano-sized β-MoO₃ in previous studies [25,26,27], in which water vapor, oxygen, and air atmospheres were adopted, respectively.

As a common gas, Ar is widely used in various industrial fields; however, there have been few studies concerning the preparation of β-MoO₃. The present study is intended to fill this gap; in this paper, the influences of the Ar flow-rate and sublimation temperature on the phase composition and morphological structure of the sublimation products are illustrated.

The chemical vapor deposition (CVD) reaction between gaseous MoO₃ and H₂ for the purpose of preparing ultrafine Mo powder had been reported in our previous work [28]. The results indicated that pure Mo powder can be obtained when the carrier gas (Ar) and reducing gas (H₂) are 300 and 500 mL/min, respectively; however, under other conditions, ultrafine MoO₂ powder can also be obtained. Herein, the formation of MoO₂ was understood to result from the chemical vapor reaction. In our preliminary experiment, carried out before this work, the sublimation–condensation process of MoO₃ under 1273 K and Ar atmosphere conditions was conducted, and the results indicated that MoO₂ can also be generated under an extremely high Ar flow-rate. Therefore, in order to further clarify the formation mechanism of different ultrafine powders during the CVD process, the authors carried out the following study.

2. Results

2.1. XRD for Phase Analysis

The phase identification of the sublimation products obtained at 1273 K under different Ar flow-rates are mainly analyzed by “Search-Match 2.1.1.0” software, with the corresponding results shown in Figure 1. The results show that the sublimation products always consist of α-MoO₃ (PDF card No. 5-508; space group: Pbnm; lattice parameter: a = 3.962 Å, b = 13.858 Å, and c = 3.697 Å) and β-MoO₃ (PDF card No. 47-1081; space group: P21/c; lattice parameter: a = 7.118 Å, b = 5.366 Å, and c = 5.568 Å), with a sharp and strong diffraction peak, indicating that both have an excellent crystallinity. It can be observed that the peak intensity of β-MoO₃ increases with the increasing Ar flow-rate, which suggests that the proportion of β-MoO₃ in the mixed MoO₃ crystals gradually increases. Herein, the proportion of β-MoO₃ is obtained by calculating the ratio of the peak area of β-MoO₃ to that of both α-MoO₃ and β-MoO₃ according to the XRD pattern, using the common “Origin” software for the calculation. To be more specific, when the Ar flow-rates are 300, 500, 700, 900, 1100, 1500, and 2000 mL/min, the proportion of β-MoO₃ in the mixed MoO₃ crystals is 4.35, 6.29, 27.69, 19.73, 35.53, 33.18, and 49.96%, respectively; that is, the proportion of β-MoO₃ gradually increases with the increase in the Ar flow-rate, even if some deviations exist, as illustrated in Figure 1h. Because a partial overlap exists between the diffraction peaks of the two MoO₃ crystals, the calculation results of their respective peak areas may not be very accurate; therefore, the relationship between the proportion of β-MoO₃ and the Ar flow-rate does not seem to indicate a strong correlation. Even so, the calculated deviation parameter R² is still up to 0.8198, and the fitting equation can be written as y = −0.2924 + 0.0255x, where y denotes the proportion of β-MoO₃ in the mixed MoO₃ crystals(%), and x presents the Ar flow-rate (mL/min).

When the Ar flow-rate is 1100 mL/min, the influence of the sublimation temperature on the phase composition of the sublimation products is also analyzed. Herein, in order to allow for the diffraction peaks to be seen more clearly, only scanning angles of 10–40° are selected. Meanwhile, the standard PDF card patterns for α-MoO₃ (No. 5-508) and β-MoO₃ (No. 47-1081) are also provided. The corresponding results are shown in Figure 2. The figure shows that the sublimation products contain both α-MoO₃ and β-MoO₃, similar to the cases observed under different Ar flow-rates. These results suggest that the change in sublimation temperature has no obvious influence on the phase composition. From Figure 2, it can also be deduced that the crystallinities of the two MoO₃ species both gradually increase with the increase in sublimation temperature due to the increasing intensity of the strongest diffraction peak. However, the proportion of β-MoO₃ within the sublimation product gradually decreases. In other words, the proportion of α-MoO₃ gradually increases with the increase in the sublimation temperature; moreover, due to the relatively strong peaks of the crystal indices of (020), (040), and (060), the newly formed α-MoO₃, with a certain anisotropy and preferred growth characteristics, can also be deduced.

2.2. FESEM for Morphology Observation

Figure 3 illustrates the morphological structure of the sublimation products obtained at 1273 K under different Ar flow-rates. It shows that the sublimation products mainly include two different morphologies—one is spherical, and the other is platelet-shaped. The results also show that the amount of the spherical particles gradually increases, while that of the platelet-shaped particles gradually decreases with the increase in Ar flow-rate. Combining the results shown in Figure 1 and those in the literature [27], it can be inferred that the spherical particle is β-MoO₃ and the platelet-shaped particle is α-MoO₃. The diameter of the spherical β-MoO₃ particle is also measured, and the results indicate that it is in the range of 1–5 μm, regardless of the Ar flow-rate. In our previous work [27], the diameter of the spherical β-MoO₃ was found to gradually decrease with the increase in the pumping speed of the vacuum pump, with the average diameter being about 0.25 μm. The deviation may result from the difference in the flow-rate of the gaseous molecules. In our previous work, the flow-rate of the gaseous molecules was as high as 40 L/min; while in the current work, the Ar flow-rate is only in the range of 300–2000 mL/min. Due to the small and narrow range of the Ar flow-rate, the particle diameter of the spherical β-MoO₃ prepared in this work seems to be unchanged. Combining the previous and current results, this work suggests that the reasonable regulation of the Ar flow-rate during the sublimation process is important for controlling the particle dimension of the spherical β-MoO₃ powder.

Figure 4 presents the morphological structure of the sublimation products obtained under different sublimation temperatures. It shows that the sublimation products also exhibit two distinct morphologies, namely, platelet-shaped α-MoO₃ and spherical β-MoO₃. Similar to the results observed under different Ar flow-rates, the diameter of the spherical particles is also in the range of 1–5 μm; however, due to the fast growth rate of particles at a higher temperature, the number of spherical particles with large dimensions gradually increases with the increase in the sublimation temperature.

3. Discussion

MoO₃ has a strong sublimation property, and its sublimation rate can be very large, even at 973 K [29,30,31]. When solid MoO₃ raw material is in the high-temperature-zone of the Si-C electronic furnace, it will sublimate into gaseous forms, in which the gaseous (MoO₃)_n (n = 2, 3, 4, and 5) species dominate [32,33]. Due to the continuous introduction of carrier gas (Ar), the newly formed gaseous MoO₃ species will be rapidly carried through the quartz tube toward the collection bottle. During this process, the temperature of the gaseous MoO₃ gradually decreases, which can be seen in the temperature distribution diagram in the quartz tube, as shown in Figure 5a. It has been reported [22,34] that when the temperature is below 723 K, gaseous MoO₃ is preferentially condensed into β-MoO₃, while when the temperature exceeds that value, α-MoO₃ is preferentially formed. Because the temperature of the collection bottle is lower than 373 K, β-MoO₃ will form in the collection bottle. The larger the Ar flow-rate, the more gaseous MoO₃ there will be, and the higher the relative content of deposited β-MoO₃ in the collection bottle will be. Meanwhile, a certain amount of gaseous MoO₃ will condense on the wall surface of the quartz tube, which is beneficial for the formation of α-MoO₃ due to its relatively high temperature. On the one hand, α-MoO₃ has an orthorhombic crystal structure with a double-layered network of [MoO₆] octahedral. These layers are stacked along the b-axis (010) through van der Waals forces. Moreover, the structure is beneficial for the formation of sheet-like images [35]. On the other hand, the newly formed α-MoO₃ in the quartz tube wall is still easily sublimated, even if its sublimation extent is weakened; in this case, the newly formed gaseous MoO₃ will transform to the relatively low-temperature zone and condensate on the edge of the original α-MoO₃ crystals; that is, parts of the volatilization–condensation process are still undertaken in this zone. As a result, α-MoO₃ is more inclined to form a sheet-like structure at a high temperature. One thing that must be noted is that when the α-MoO₃ forms in the high-temperature zone, the Ar airflow will carry parts of the α-MoO₃ into the collection bottle. Therefore, the sublimation products in the collection bottle always contain both α-MoO₃ and β-MoO₃. Even though an increment in the Ar flow-rate would enhance its carrying capacity, this capacity is not sufficient to take away all the sheet-like α-MoO₃; that is, most of the α-MoO₃ are still adhered to the wall of the quartz tube, as seen in the inset of Figure 5a. For this reason, even if all the analytical reagent MoO₃ is completely sublimated, and no residue exists in the alumina crucible, the mass of the sublimation product in the collection bottle will still only be in the range of 1–2 g; that is, the yield is about 20–40%. Herein, the temperature of the α-MoO₃ deposited on the quartz tube is determined to be in the range of 780 K to 847 K, and that of the β-MoO₃ is lower than 500 K; in other words, the above temperature ranges denote the stable existence intervals of α-MoO₃ and β-MoO₃, respectively. Due to the use of an Ar atmosphere and the lack of O₂ in the reaction system, as well as the unstable characteristics of β-MoO₃, a certain number of oxygen vacancies will form in the β-MoO₃ crystal during the condensation process [36], and this is the reason why the colors of the β-MoO₃ deposited on different sites differ, as shown in Figure 5b.

Figure 6 depicts some special morphologies of the sublimation products obtained under different conditions. Because spherical particles have the lowest surface free energy, and because the rapid reduction in the temperature of gaseous MoO₃ would increase its saturated vapor pressure, in order to maintain the lowest energy state, numerous spherical particles will form during the condensation process. However, as shown in Figure 6, some newly formed particles are not perfectly spherical-shaped, and several transition states, such as cubic or elliptic particles, with many high-energy sites on the surface, also exist. In this case, the subsequent gaseous MoO₃ molecules can be deposited on these sites to fill the voids and to make the particle spherical-shaped. In addition to gaseous MoO₃ molecules, the small volume ratio of Ar molecules may also play important roles. When Ar molecules are adsorbed on the surface of MoO₃, they can average the surface energy anisotropy, which may cause any oriented plane to hold the same surface energy and, finally, produce a spherical crystal. This is to say, that the gas molecules’ adsorption is an easy way to manipulate the crystal surface energy [37], which defines the notable Wulff shape of a single crystal. From this figure, it can also be observed that some small or platelet-shaped particles are adsorbed on the surface of the larger ones; in these cases, as time goes on, these small or platelet-shaped particles may gradually disappear and become a part of the larger one, as supported by the clear crystal boundary on the crystal surface, and this process is consistent with the classical Ostwald ripening mechanism [38]. Therefore, due to the co-action of the continuous deposition of gaseous MoO₃ molecules, the adsorption of Ar molecules, and the particle collision effect, large spherical particles will form.

Table 1. Different preparation methods for ultrafine β-MoO₃ powder.

References	Preparation Processes	Images	Microstructures
Sun et al. [25]	$\begin{array}{l}\mathrm{High}-\mathrm{grade} \mathrm{molybdenite}{\displaystyle \frac{873 \mathrm{K}}{2\mathrm{h}}}{\mathrm{MoO}}_3{\displaystyle \frac{\mathrm{water} \mathrm{vapor} \mathrm{atmosphere}, 300 \mathrm{L}/\mathrm{h}}{1173-1373 \mathrm{K}}}\\ {\mathrm{\alpha }-\mathrm{MoO}}_3{(973 \mathrm{K})+\mathrm{\beta }-\mathrm{MoO}}_3(473 \mathrm{K})\end{array}$		The sample deposited at 973 K, presenting as thin strips, was pure α-MoO3, and that deposited at 473 K took the form of spherical β-MoO3 and thin-strips α-MoO3
Ngo et al. [26]	${\mathrm{\alpha }-\mathrm{MoO}}_3 \mathrm{powder} \left(99.9\%\right){\displaystyle \frac{8{ \mathrm{L}/\min \mathrm{O}}_2}{1023 \mathrm{K},30 \min }}{\mathrm{\alpha }-\mathrm{MoO}}_3{ + \mathrm{\beta }-\mathrm{MoO}}_3$		Composed of large spherical particles with sizes of about 200 nm; some needle-shaped particles also exist
Wang et al. [27]	${\mathrm{Industrial} \mathrm{MoO}}_3{\displaystyle \frac{\mathrm{air}, 1373 \mathrm{K}, \mathrm{water} \mathrm{cooling}}{\mathrm{pumping} \mathrm{speed}: 40 \mathrm{cm}/\mathrm{s}}}{\mathrm{\alpha }-\mathrm{MoO}}_3{ + \mathrm{\beta }-\mathrm{MoO}}_3$		The obtained product is composed of spherical β-MoO3 and platelet-like α-MoO3 particles
Mariotti et al. [39]	$\mathrm{Mo} \mathrm{wire}{\displaystyle \frac{\mathrm{patch} \mathrm{antenna}-\mathrm{based} \mathrm{atmospheric} \mathrm{microplasma} \mathrm{processing}}{\mathrm{Si}(100) \mathrm{substrate} \mathrm{with} \mathrm{a} \mathrm{conducting} \mathrm{Pt} \mathrm{layer} (\mathrm{Si}-\mathrm{Pt} \mathrm{substrate})}}{\mathrm{\beta }-\mathrm{MoO}}_3$		The obtained β-MoO3 is a nanosheet with a thickness of 50–100 nm
This work	${\mathrm{MoO}}_3\left(\mathrm{AR},99.5\%, \mathrm{Aladdin}\right){\displaystyle \frac{1100 \mathrm{mL}/\min }{1273 \mathrm{K}}}{\mathrm{\alpha }-\mathrm{MoO}}_3{ + \mathrm{\beta }-\mathrm{MoO}}_3$		The sublimation products include two different species: spherical β-MoO3 and platelet-shaped α-MoO3

lists some preparation methods for β-MoO₃ powder. It shows that many parameters, such as Mo source, sublimation temperature, and reaction atmosphere, may have close relationships with the microstructure of β-MoO₃. However, it is also the case that β-MoO₃ is entirely formed at a relatively low temperature [25,26,27]. Since the Ar gas is used in this work, many oxygen vacancies form inside the MoO₃ species. When the Ar flow-rate is not very high, metastable β-MoO₃ can be obtained, while when the Ar flow-rate is sufficiently high, the oxygen content in the reaction system will be extremely low; meanwhile, the condensation rate of the gaseous MoO₃ will rapidly increase, which will lead to the formation of more oxygen vacancies. In this situation, the newly formed metastable β-MoO₃ will be decomposed into numerous sub-oxides, such as MoO₂, Mo₄O₁₁, and other oxides; this is the reason why many other sub-oxides appeared in our preliminary finding, as shown in Figure 7. Therefore, based on the current results, and those reported in our previous study, we propose that increasing the sublimation temperature appropriately may be a good option for improving the preparation efficiency of ultrafine β-MoO₃ powder; moreover, the reasonable control of the reaction atmosphere and its gas flow-rate are also very important.

4. Materials and Experimental Procedures

4.1. Materials

Analytical reagent MoO₃ (AR, 99.5%), purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., is used as a raw material. The XRD result shows that the raw material belongs to α-MoO₃, and FESEM imaging shows that the raw material exhibits a sheet-like morphology, as observed in Figure 8. The carrier gas used in the work is high-purity Ar (the amount of impurities < 5 ppm).

4.2. Experimental Procedures

Figure 9 shows the schematic diagram of the experimental device. In each of the experiments, 5 g of MoO₃ raw material was weighed and placed into an alumina crucible (50 mm × 20 mm × 20 mm). Then, the MoO₃-containing crucible was placed on the right end of the inner quartz tube (inner diameter: 30 mm; outer diameter: 38 mm). After the Si-C electronic furnace reaching the specified temperature, the inner quart tube was inserted into the external quartz tube (inner diameter: 50 mm; outer diameter: 60 mm), with the MoO₃ raw material placed into the constant-temperature zone of the furnace. Meanwhile, Ar gas with a certain flow-rate was introduced from the left end of the inner quartz tube to carry the gaseous MoO₃ towards the collection bottle and accelerate its condensation rate. After 1 h of reaction time, we turned off the Ar gas and then collected the sublimation product in the collection bottle for further testing. In order to ensure parity with our previous work [28] and make a further contribution under the same study conditions, Ar flow-rates of 300, 500, 700, 900, 1100, 1500, and 2000 mL/min and sublimation temperatures of 1223, 1273, 1323, and 1373 K were selected for the current experiments.

4.3. Product Characterization

The phase composition of the sublimation product was analyzed by an X-ray diffraction analyzer (XRD; D8 Advance, AXS Corporation, Bruker, Germany; operation voltage: 30 kV; operation current: 30 mA; scanning speed: 10°/min; scanning angle: 7–90°; Cu-Kα1 radiation with the wavelength of 1.54056 Å), and its morphological structure was observed via field emission scanning electron microscopy (FESEM; Nova 400 Nano SEM, FEI Corporation, Hillsboro, OR, USA; Operation voltage: 15 kV).

5. Conclusions

In this work, the phase composition and morphological structure of the sublimation products of analytical reagent MoO₃ are investigated. The following conclusions are drawn.

(1)

The sublimation products obtained under different Ar flow-rates and sublimation temperatures are always composed of both α-MoO₃ and β-MoO₃, among which the proportion of β-MoO₃ gradually increases with the increase in Ar flow-rate and the decrease in sublimation temperature.

(2)

Platelet-shaped α-MoO₃ is usually generated in the range of 780 K to 847 K, while spherical-shaped β-MoO₃ forms below 500 K. The diameter of β-MoO₃ is determined to be in the range of 1–5 μm and has no obvious relationship with the Ar flow-rate due to its low value and narrow range.

(3)

Due to the co-actions of the deposition of gaseous MoO₃ molecules, the adsorption of Ar molecules, and the particle collision between different particles, the newly formed β-MoO₃ particles usually exhibit a spherical-shaped morphology in order to decrease their surface free energy.