Morphology evolution and quantitative analysis of β-MoO3 and α-MoO3

Abstract The morphology evolution from monoclinic molybdenum trioxide (β-MoO3) to orthorhombic molybdenum trioxide (α-MoO3) and quantitative analyses of their mixtures were examined. It was found that the morphology (from spherical to elliptical shape) and color (from green to white) displayed obvious changes when β-MoO3 converted to α-MoO3 in ambient air at 773 K. The transformation from β-MoO3 to α-MoO3 resulted from a change of the internal crystalline structure. The mass percent of β-MoO3 in MoO3 mixtures showed an excellent linear relationship with the relative intensity ratio of the strongest peaks in X-ray diffraction patterns. This approach provides a simple and time-saving method to evaluate the amount of β-MoO3, which is a promising material in catalyst and electrochemical applications, in such mixtures. This finding may provide guidance for the analysis of catalytic performance of MoO3 mixtures. In addition, it was found that β-MoO3 can be easily decomposed into suboxides such as MoO2 and Mo4O11 in pure argon gas atmosphere. The possible decomposition mechanism of β-MoO3 is discussed.


Introduction
Transition metal oxides, such as V 2 O 5 , CrO 3 , WO 3 , and MoO 3 , show several types of complex structures, formed mainly by two-or three-dimensional frameworks of octahedral or tetrahedrals [1][2][3]. Among these materials, MoO 3 has been recognized as a promising material for a rapidly increasing number of applications such as chemical synthesis, petroleum refining, gas sensor devices, photoluminescence, photochromism, electrochromism, smart windows, catalysis, and display devices [4]. MoO 3 has different structures that can be divided into four polymorphs [5][6][7][8][9]: (1) thermally stable orthorhombic phase, α-MoO 3 ; (2) metastable monoclinic phase, β-MoO 3 ; (3) metastable phase at high-pressure conditions, β′-MoO 3 ; and (4) hexagonal phase, h-MoO 3 . In all these MoO 3 structures, the MoO 6 octahedron is the primary unit, and its arrangement results in differences in the structures. The two most commonly studied polymorphs are α-MoO 3 and β-MoO 3 ; however, β-MoO 3 is believed to possess more novel and enhanced properties in catalysis and electrochemical applications when compared with α-MoO 3 [10]. It is regrettable that the synthesis of pure β-MoO 3 is usually difficult at ambient conditions [11,12], whereas mixtures of α-MoO 3 and β-MoO 3 are much easier to produce [13][14][15][16]. To make full use of the mixtures of α-MoO 3 and β-MoO 3 that are usually produced, it is necessary to quantitatively analyze the mixtures and evaluate the amount of β-MoO 3 , which may be an evaluation index, and to better understand the properties of the mixtures.
X-ray diffraction (XRD) is widely used for the quantitative analysis of geological samples [17,18]. Hillier [19] conducted the accurate quantitative analysis of clay and other minerals in sandstones by XRD using the relative intensity ratio (RIR), which gave accuracy within ±3 mass % at the 95% confidence level. Vaverka and Sakurai [20] investigated the composition of steelmaking slag, and the amount of free lime was determined by the X-ray powder diffraction and the standard addition method. Recently, Shu et al. [21] adopted the quantitative XRD analysis to calculate the ratio of mass percentages of reactant (CaWO 4 ) and product (W) from the intensities of the strongest peaks, from which fractional conversion was calculated, thus enabling the kinetics of reduction of CaWO 4 by Si to be successfully described.
Although many methods have been used for quantitative determination of different sample mixtures, XRD is nondestructive, and the samples can be used for other chemical analyses. However, there are no specific reports on the quantitative relationship between α-MoO 3 and β-MoO 3 . In the present study, the quantitative XRD analysis was used to determine the quantitative relationship between α-MoO 3 and β-MoO 3 and to evaluate the amount of β-MoO 3 in mixtures. The morphology evolution from β-MoO 3 (spherical) to α-MoO 3 and the possible decomposition mechanism of β-MoO 3 were also elucidated. Pure ultra-fine β-MoO 3 (green), prepared by the method of sublimation [22,23], was used. The X-ray diffraction pattern of the sample is shown in Figure 1. The intensity of the strongest peak for this sample was located at 2θ = 23.04°with a reflection of (011). Field-emission scanning electron microscopy (FE-SEM) images of samples at different magnifications are shown in Figure 2. All powders appeared to have a spherical shape and fine crystalline size although their size was nonuniform.

Preparation of α-MoO 3
The transformation temperature from β-MoO 3 to α-MoO 3 obtained from the previous literature [9,10,15] is around at 673-723 K. Therefore, in the present study, α-MoO 3 was prepared by roasting β-MoO 3 at 773 K in the air; the higher roasting temperature was used to complete the transformation within a short time and to control the MoO 3 vapor. After confirming that the prepared products were all pure α-MoO 3 , samples were prepared for morphology observation and used to synthesize mixed MoO 3 specimens.

Preparation of mixtures of β-MoO 3 and α-MoO 3
To identify the quantitative relationship between β-MoO 3 and α-MoO 3 , standard mixtures with a mass ratio (W) of β-MoO 3 to the total mass of β-MoO 3 and α-MoO 3 that varied from 0 to 1 were prepared, i.e.: where W was in the range of 0-1. After carefully weighing and mixing β-MoO 3 and α-MoO 3 based on the specified mass ratio, the mixtures were homogenized for 30 min by milling in an agate mortar and then subjected to the quantitative XRD analysis.
The total mass of mixtures of β-MoO 3 and α-MoO 3 was fixed at 500 mg. The morphologies of β-MoO 3 and α-MoO 3 were observed by FE-SEM (ZEISS SUPRA 55, Oberkochen, Germany). Phase compositions were analyzed by the XRD (Model TTR III, Rigaku Corporation, Japan) using Cu Kα-filtered radiation with a scanning speed of 6°/min and scanning step of 0.02°. Figure 3 shows the XRD pattern of the roasted products. It can be seen that pure α-MoO 3 can be prepared by roasting β-MoO 3 at 773 K in air. In addition, the color was converted from green to white, which demonstrated that the transformation from β-MoO 3 to α-MoO 3 is photochromic. The intensity of the strongest peak of α-MoO 3 was located at 2θ = 27.36°with a reflection of (021). FE-SEM micrographs of the as-prepared α-MoO 3 at different magnifications are shown in Figure 4. The morphologies of the asprepared α-MoO 3 no longer maintained the perfect spherical shape of β-MoO 3 as shown in Figure 2. Numerous spiral fringes formed around the oval α-MoO 3 particles, which led to the formation of a layer structure.

Crystalline modification and morphology evolution
The changes of the morphology and the color on conversion from β-MoO 3 to α-MoO 3 indicated that the structures of the two phases were different. The crystal structure of β-MoO 3 shown in Figure 5 indicated that β-MoO 3 has a ReO 3 -type structure in which the MoO 6 octahedrons only share corners with each other; each oxygen atom is shared by two octahedrons. In contrast, the crystal structure of α-MoO 3 (α-MoO 11 O 22 O 33 ) has a unique twodimensional layer structure in which each layer is built up of MoO 6 octahedrons connected along ac-planes by common edges and corners to form zigzag rows and along ab-planes by common corners only, as shown in Figure 6. The interlayer interaction is weak and bounded in the aaxis direction by van der Waals forces. The transformation from β-MoO 3 to α-MoO 3 is explained by the metal offcenter displacement toward O 1 (and a little less toward O 2 ) centers, which is stabilized by an increase in covalence between the Mo and O atoms [24]. When heating β-MoO 3 at T = 773 K in air, the crystals mainly grow by coalescence with neighboring crystallites, driven by the heat treatment process, and the crystal has a tendency to form a layer structure, so the morphology of α-MoO 3 has many spiral fringes.

Determination of quantitative relationship curves
Quantitative curves were determined by using the quantitative X-ray analysis based on RIR values [18]. The ratios of the mass of β-MoO 3 to the total mass of β-MoO 3 and α-MoO 3 were calculated by the intensities of the strongest peaks for β-MoO 3 (peak (011)) and α-MoO 3 (peak (021)).
The XRD patterns of mixtures of β-MoO 3 and α-MoO 3 at different mass ratios are displayed in Figure 7. The intensity of the strongest peak of β-MoO 3 gradually increased and that of α-MoO 3 gradually decreased with the increase of mass ratio (W). The intensity changes are listed in Table 1. The values of I β /(I β + I α ) had a strong linear relationship with W, as shown by the results presented in Figure 8. According to these results, it is easy to obtain the mass percent of β-MoO 3 in mixtures of β-MoO 3 and α-MoO 3 . It is worth noting that another RIR method that measures the integrated peak intensity may have advantages and higher accuracy, but it is hard to accurately measure     the integrated intensity and deal with interferences from other phases. In the present study, it was obvious that the strongest peak of β-MoO 3 had some overlap with the small peak of α-MoO 3 , as shown in Figure 7(a), so it would be difficult to accurately measure the integrated intensity. If we adopted the multiple-peak separation method, it would waste a lot of time and have no practical application. Moreover, although the application of the Riveted quantitative analysis has demonstrated its excellent potential for complex samples, the time required for data processing is very long and does little to popularize its application. In contrast, the RIR method proposed in this study (measurement of the intensity of the strongest peaks of different phases) exhibits its own advantages: it is easy, quick, time-saving, and has high accuracy. Therefore, this method can be widely applied for the rough quantitative analysis of mixtures of β-MoO 3 and α-MoO 3 .
In the present study, both β-MoO 3 and α-MoO 3 had a small particle size (spherical or oval shaped, respectively), and so preferred orientation was eliminated. If the preferred orientation existed, for example, if peaks with a reflection of (0k0) for α-MoO 3 were stronger, this method could not be applied.

Decomposition of β-MoO 3
Before successfully preparing α-MoO 3 by roasting β-MoO 3 at 773 K in air, another method was attempted, that is, roasting β-MoO 3 at 773 K in a highly pure Ar atmosphere (<5 ppm O 2 ); however, pure α-MoO 3 could not be obtained. The corresponding XRD pattern is shown in Figure 9. The roasted products were very complicated and included not only α-MoO 3 but also MoO  . In addition, the color of the roasted products was dark or gray, rather than white, which is shown in Figure 10.
It has been reported that raw green β-MoO 3 may contain a number of oxygen vacancies [25][26][27]. When roasting β-MoO 3 in air, the samples have enough opportunity to interact with O 2 . Maximum interaction between air and β-MoO 3 is important to ensure that the following chemical equilibrium is shifted to the left [28]: Oxygen exchange between the lattice and air has been investigated using isotope ( 18 O 2 ) labeling and Raman spectroscopy, which showed that gaseous O 2 is able to incorporate into the oxygen-deficient β-MoO 3 [29]. Therefore, perfect α-MoO 3 can be prepared under air; however, when roasting β-MoO 3 in the argon atmosphere, no O 2 was provided to counter the oxygen-deficient β-MoO 3 , which led to shifting of the chemical equilibrium (2) to the right and the production of low-valent molybdenum oxides, such as MoO 2 and Mo 4 O 11 . Once the oxygen defects were exhausted,   the remaining components formed perfect α-MoO 3 . Therefore, different molybdenum-oxygen compounds can coexist in the roasted products created under argon atmosphere conditions. The residue of α-MoO 3 also indicated that α-MoO 3 cannot be decomposed under the current experiment conditions, which was further supported by the data given in Figure 11. Figure 11 shows that whether using air or argon atmosphere, pure α-MoO 3 was the only phase present in the final roasted products, that is, pure α-MoO 3 cannot be decomposed, but β-MoO 3 can be easily decomposed into MoO 2 and Mo 4 O 11 . The corresponding decomposition correlations are summarized in Figure 12.

Conclusions
In the present study, the morphology evolution and the quantitative analysis of β-MoO 3 and α-MoO 3 were clarified. It was found that the morphology and color displayed obvious changes when β-MoO 3 was transformed into α-MoO 3 . Spherical-shaped β-MoO 3 had the tendency to form ovalshaped α-MoO 3 when the heating temperature was around 773 K. XRD was used to quantitatively analyze the amount of β-MoO 3 in mixtures of β-MoO 3 and α-MoO 3 . It was found that the mass of β-MoO 3 in the mixtures had a strong linear relationship with the intensities of the strongest peaks of β-MoO 3 and α-MoO 3 . This provides an easy and convenient way to determine the amount of β-MoO 3 in MoO 3 mixtures. This approach may provide guidance for evaluation of the catalytic efficiency of MoO 3 mixtures. In addition, the decomposition of β-MoO 3 under argon gas atmosphere may result from the existing oxygen defects, which may contribute to the formation of MoO 2 and Mo 4 O 11 .