Thermal analysis on conversion of MoO3 to MoO2 and its silicothermic reduction

Synthesis of Vertical MoO2/MoS2 Core–Shell Structures on an Amorphous Substrate via Chemical Vapor Deposition

“…Further reduction at 780 °C (Stage III) induces a phase transition from MoO 2 to MoS 2 , as verified by the Raman spectra in Figure . The vertical MoO 2 crystals formed in Stage II remain in the solid state, even up to 890 °C, due to the high MoO 2 sublimation temperature of 1100 °C. , In Stage III (above 780 °C), the sulfur-induced transition to MoS 2 proceeds from the edges of the substrate to the center, as shown in Figure , demonstrating the possible dependence of this transition on the amount of sulfur supplied. Therefore, an increase in the number of MoS 2 atomic layers is expected upon increasing the sulfurization time in Stage III.…”

“…As shown in Figure 3c, stacks of perpendicularly grown plates were deposited on the substrate and the Raman spectra of these species verified the formation of various lower-valence-state Mo oxides 34,35 (see Figure S3d). Furthermore, the XRD data (Figure 3d) for the black-colored powder remaining in the boat indicated that most of the MoO 25,26 In Stage III (above 780 °C), the sulfur-induced transition to MoS 2 proceeds from the edges of the substrate to the center, as shown in Figure 2, demonstrating the possible dependence of this transition on the amount of sulfur supplied. Therefore, an increase in the number of MoS 2 atomic layers is expected upon increasing the sulfurization time in Stage III.…”

“…With progress of the CVD process, melting of the sulfur powder was initiated (at a CVD temperature over 680 °C, which corresponds to a temperature exceeding 105 °C at the site of the sulfur-containing boat), and the reaction zone quickly changed to a highly reducing atmosphere under a sufficient sulfur-vapor supply. Under these highly reducing conditions, it was expected that most of the deposited MoO 3– x would be quickly transformed to a nonvolatile compound such as MoO 2 − before resubliming, given that the ramping rate (>50 °C min –1 ) was much higher than that used for conventional CVD processes (generally, <20 °C min –1 ). ,− …”

“…This indicates that the vaporized sulfur largely suppressed sublimation of MoO 3 by quickly transforming MoO 3 to nonvolatile species when the temperature approached 800 °C. MoO 2 is thought to be the nonvolatile compound formed during the TG/DTA measurement based on its formation temperature (775 °C in the case of sulfur-induced reduction of MoO 3 ) and sublimation temperature (1100 °C). , Consequently, it was expected that the nonvolatile intermediate MoO 2 formed during the CVD process would be sulfurized during the prolonged process at high temperature.…”