Preparation of MoO 3 powder by hydrothermal method

: Based on the raw material of ammonium tetra-molybdate in the experiment, molybdenum trioxide was prepared by hydrothermal synthesis, and the e ﬀ ects of different pH, hydrothermal reaction time, ﬁ lling degree, and calcination temperature on molybdenum trioxide powder were studied. Meanwhile, the molybdenum trioxide powder was characterized through scanning electron microscopy, transmission electron microscopy, X-ray di ﬀ raction, and thermal analysis di ﬀ erential scanning calorimetry analysis so as to study the morphology and phase changes during the experiment. As is evident from the research ﬁ ndings, MoO 3 powders with uniform and suitable size, smooth and clear surface, good dispersibility, and no adhesion can be obtained at the conditions of pH = 1, 16 h hydrothermal reaction, 90% of ﬁ lling degree, and 550°C of calcination temperature. In the calcination process, MoO 3 crystal undergone crystal transformation and was completely transformed from h-MoO 3 to α -MoO 3 at the calcination temperature of 350°C.


Introduction
As a high-performance material with high melting point, excellent strength, strong hardness, good wear resistance, outstanding electrical conductivity, small expansion coefficient, and superior thermal shock resistance as well as thermal fatigue resistance, molybdenum metal has become one of the indispensable raw materials for modern high-tech development [1][2][3][4][5][6][7][8].Now, it is widely used in fields such as metallurgy, machinery, petroleum, chemical industry, national defense, aviation, aerospace, electronics, nuclear industry, etc. [5,[9][10][11][12].By means of calcinating ammonium molybdate, molybdenum trioxide is prepared and then reduced to obtain the molybdenum powder [6,13,14].According to the research studies on the refining mechanism of molybdenum powder, the morphology of molybdenum powder "inherits" from that of molybdenum trioxide to some extent.That means both the size and shape of molybdenum trioxide will greatly affect the preparations of molybdenum powder and molybdenum alloy [9,[14][15][16][17][18]. MoO 3 prepared based on the traditional process is so rough (coarse particles, poor size uniformity) that it can no longer meet the needs of nano-molybdenum powder, high-purity molybdenum powder, and high-performance molybdenum alloy, so it is of great significance to study the preparation process for MoO 3 powder [10,[19][20][21][22][23][24][25].At present, there are many methods to prepare nano-MoO 3 materials, including vapor deposition, precipitation, hydrothermal, solvothermal, and sol-gel method.For the sake of obtaining high-quality MoO 3 , MoO 3 powder was prepared through hydrothermal synthesis in this experiment, and the effects of different hydrothermal times, pH values, calcination temperatures, and filling degrees on the morphology of MoO 3 powder were studied.

Materials and methods
Ammonium tetramolybdate and dilute nitric acid solution, after being added into the autoclave, were stirred evenly for hydrothermal reaction, and then the MoO 3 powder was obtained via washing, filtering, drying, and calcinating the hydrothermal synthesis product.After that, the effects of different pH values, hydrothermal reaction times, filling degrees, and calcination temperatures on molybdenum trioxide powder were studied.
In the experiment, the phase analysis of the powder was performed using a D8 ADVANCE X-ray diffractometer, the morphology of MoO 3 powder was analyzed using a Regulus 8220 scanning electron microscope and a JEM-2100 URP/JEM-2100 transmission electron microscope, and thermal analysis of the samples was carried out by using DSC 204F differential scanning calorimetry.During the hydrothermal process, the conversion of (NH 4 ) 2 Mo 4 O 13 •2H 2 O solution to MoO 3 is the reaction of H + electrophilic addition followed by dehydration:

( )
During the hydrothermal process, H + is involved in the reaction and its pH value has a great influence on the morphology of MoO 3 powder.As shown in Figure 1, which is an observation of the morphology of MoO 3 powders with pH values of 0, 0.5, 1, 1.5, and 2, the pH value seriously affects the morphology of MoO 3 powder.Specifically, at the pH value of 0 (Figure 1a), the grains take on the shape of a complete hexagonal prism with basically equal side lengths and height at around 15 µm.When increasing the pH value to 0.5 (Figure 1b), fine crystals begin to appear, which are distributed in the vicinity of the hexagonal prisms.The hexagonal prism disappears, and MoO 3 in flaky structure comes out once the pH value reaches 1. MoO 3 in Figure 1c has a uniform size without adhesion, which is not only the best state among all products with a pH value of 0-2, but also the best pH value of MoO 3 powder prepared by the hydrothermal method.With the increase of the pH value (Figure 1d), large bulk MoO 3 and small unreacted (NH 4 ) 2 Mo 4 O 13 •2H 2 O appear (Figure 1e).With the further increase of the pH value, a large amount of unreacted (NH 4 ) 2 Mo 4 O 13 •2H 2 O begins to appear, as shown in Figure 1e.A study of the reaction process and the hydrothermal method's preparation of MoO 3 revealed that the pH of the solution has a significant impact on the hydrothermal reaction progress due to the involvement of H + in the reaction.When the pH is lower than 1, rod-shaped MoO 3 is formed, and the ideal MoO 3 form appears as the pH reaches 1.With the further increase of the pH value, unreacted (NH 4 ) 2 Mo 4 O 13 •2H 2 O begins to appear because H + is insufficient to support the hydrothermal reaction, and the pH of the supernatant after the reaction at pH 1 is measured to be greater than 4.

Effects of different hydrothermal reaction time on MoO 3 powder
In view of needing to both ensure the complete reaction process and prevent the powder particles from being too large due to the long hydrothermal time, proper control of the hydrothermal time is critical to the hydrothermal process.Because the time is too short to observe the difference between the powders visually, 8 h is selected as the interval (as shown in Figure 2) to observe the morphology of MoO 3 at the reaction time of 8, 16, 24, 32, 40, and 48 h.In Figure 2a

Effects of different filling degrees on MoO 3 powder
Filling degree refers to the percentage of the liquid volume in the lining of the hydrothermal kettle to the total volume.Different filling degrees have a great influence on the MoO 3 powder prepared in the experiment, and pressure generated by the lining with different filling degrees during the experiment is varied, so that different growth conditions of the MoO 3 powder are produced.The effects of 45, 60, 75, and 90% of filling degrees on the morphology of prepared MoO 3 powders were studied, as shown in Figure 3.In Figure 3a, due to the small filling degree, the size of MoO 3 powder is large with poor uniformityn of MoO 3 powder decreases while the size uniformity improves (Figure 3b and c).The size and size uniformity of MoO 3 powder (Figure 3d) are optimal when the final filling degree reaches 90%.

Effects of different calcination temperatures on MoO 3 powder
The hydrothermally generated MoO 3 is composed of [MoO 6 ] primitives, which are in the shape of an octahedron and connected in different ways.On this basis, MoO 3 is in two different states.When the vertices are connected, the Mo-O-Mo bond is developed, so that the formed MoO 3 is in a hexagonal metastable state.When the common edge is connected, two Mo-O-Mo bonds are developed, which, compared with the two octahedrons connected by the common point, is a stable orthorhombic structure with lower energy.Therefore, calcination is necessary to make MoO 3 in a stable orthorhombic structure.
Calcination is an essential link in the preparation of MoO 3 powder.After the drying and calcination of powder with a filling degree of 90% at 180°C × 16 h and pH = 1.0, there is still a large amount of crystal water in the dried powder, and MoO 3 generated after hydrothermal is unstable that it needs to remove part of the crystal water by calcination.
Figure 4 presents the SEM images of the hydrothermally synthesized MoO 3 powders at different calcination temperatures.When the temperature is lower than 500°C, the MoO 3 powder appears as nearly circular flakes (Figure 4a-d) and tends to agglomerate, and the grain size is as small as 2-5 µm.When the temperature increases to 550°C, the MoO 3 powder particles become lath-like (Figure 4e).With the further temperature increase, the lath-like MoO 3 continues to grow and shows a block-like structure (Figure 4f).When the calcination temperature reaches 550°C, MoO 3 powder is completely transformed with a small and uniform size, which is a suitable calcination temperature that can ensure complete transformation without making the calcined MoO 3 powder grow into a larger lath shape.

Results and discussion
The XRD patterns of the hydrothermally synthesized MoO 3 powders at different calcination temperatures are shown in Figure 5.At the calcination temperature of 250°C (Figure 5a), MoO 3 powder basically presents the form of a metastable hexagonal phase, in which the lattice constants are a = b = 1.0531 nm and c = 1.4876 nm, and crystal planes with strong diffraction peaks are (110), ( 200), ( 210), (300), ( 220), (310),   When the calcination temperature is higher than 350°C (as shown in Figure 5c-f), the MoO 3 powders all exist in the form of an orthorhombic phase, and the intensity of diffraction peaks is strengthened with the increase of calcination temperature, but their positions are not changed, and the intensity of individual diffraction peaks is strong, indicating that MoO 3 crystals grow preferentially during the growth.According to the analysis of this experiment, only metastable h-MoO 3 powder can be obtained in the stage of hydrothermal synthesis.Under subcritical or supercritical hydrothermal conditions, the reaction in hydrothermal synthesis is at the molecular level, so the activity of reactants is enhanced, the dissolution-recrystallization process is promoted, some new chemical reactions are realized, and metastable molybdenum materials are obtained.The growth of MoO 3 crystal is divided into two stages: first, ammonium tetramolybdate reacts with nitric acid to generate a molybdenum trioxide crystal nucleus (as in formula (1)); second, once the crystal nucleus is developed, a solid-liquid interface is formed, and the crystal nucleus grows on it The growth process of h-MoO 3 crystals is quite complicated because it is controlled by the combined effect of the singlenuclear layer mechanism, the multi-nuclear layer mechanism,  and the diffusion-controlled growth mechanism rather than only one mechanism.In the hydrothermal synthesis stage of this experiment, only the thermodynamically metastable phase h-MoO 3 was obtained instead of transforming into the thermodynamically stable phase α-MoO 3 .This is because under the hydrothermal conditions of this experiment, the external energy was lower than the activation energy needed for converting h-MoO 3 into α-MoO 3 , leading to the existence of MoO 3 at the metastable hexagonal phase.As the external energy is obtained during the calcination, h-MoO 3 begins to transform to the α-MoO 3 in a stable state (as shown in Figure 5a).When the temperature continued to rise to 350°C, h-MoO 3 was all transformed into stable α-MoO 3 after obtaining enough energy (Figure 5b) [21,22,26].
Figure 6 shows the DSC analysis curve of MoO 3 powder.There is an obvious endothermic peak between 300 and 400°C, in which the crystal transformation of h-MoO 3 into α-MoO 3 occurred; for the exothermic peak between 400 and 500°C, it is caused by the decomposition of ammonium tetramolybdate with incomplete reaction; the DSC curve is relatively flat after 500°C, indicating that the decomposition of organic matter is complete.Therefore, only the temperature for the calcination of MoO 3 powder at 550°C can meet the complete decomposition of ammonium tetramolybdate as well as a complete transformation of MoO 3 .
In order to further determine the crystal type of calcined molybdenum trioxide powder, TEM analysis was conducted on the powder calcined at different temperatures.Figure 7 shows the TEM image of MoO 3 powder calcined at 250°C and its diffraction pattern, and Figure 8 shows the TEM image of MoO 3 powder calcined at 250°C and its diffraction pattern.According to Figure 7a, MoO 3 calcined at 250°C is in the form of flakes, and the diffraction pattern is marked as a hexagonal phase (Figure 7b), while as presented in Figure 8a, the MoO 3 calcined at 550°C shows a lath-shaped diffraction pattern and is marked as an orthorhombic phase (Figure 8b).

Conclusion
MoO 3 powder in smaller and uniform size and good dispersion can be obtained when pH = 1; after 16 h of hydrothermal reaction, the MoO 3 crystal grows completely, and MoO 3 crystal changes from cotton wool to flake, but cohesion will occur in the condition of long-time reaction (such as 48 h).
With the increase of filling degree, MoO 3 crystal size becomes small and uniform, and MoO 3 powder with better quality can be obtained at the filling degree of 90%.
Through the investigation of the hydrothermal process for MoO 3 powder, it has been determined that the optimal method for preparing MoO 3 powder involves hydrothermal treatment at 180°C under pH conditions of 1 and 16, a filling degree of 90%, and calcination at 550°C.This process yields uniformly fine particles with a smooth, unblemished surface, excellent dispersion, and no adhesiveness.The final product exhibits an optimal particle morphology and a stable crystal structure for MoO 3 powder.

1
Effects of different pH values on MoO 3 powder
, at the beginning of the reaction, there are still a lot of flaky (NH 4 ) 2 Mo 4 O 13 •2H 2 O, but the flaky (NH 4 ) 2 Mo 4 O 13 •2H 2 O basically disappeared in Figure 2b.With the increase of hydrothermal time, the size of MoO 3 grows further in Figure 2c-e, and finally, larger flaky MoO 3 appears at 48 h in Figure 2f.The comparison discovers that 16 h is a more suitable reaction time.

Figure 4 :
Figure 4: SEM images of MoO 3 powder synthesized by hydrothermal reaction at different calcination temperatures.