Elucidating the mechanism for the chemical vapor deposition growth of vertical MoO2/MoS2 flakes toward photoelectrochemical applications

To control the growth of layered two-dimensional transition metal dichalcogenide (TMD) materials or complex structures such as heterostructures, it is important to understand the growth mechanism. In this study, we demonstrated the chemical vapor deposition (CVD) growth of vertically aligned MoO2/MoS2 flakes by using sulfur and molybdenum trioxide (MoO3) powders as precursors. Moreover, based on various experimental measurements, a detailed growth mechanism is proposed for elucidating the conversion of MoO2 to MoS2 in the CVD process to achieve vertically aligned MoO2/MoS2 flakes. Our results reveal that two competing pathways for sulfurization reactions occur during the CVD growth process: the dominance of the reactions in the vapor-phase pathway facilitates the formation of parallel-aligned MoS2 flakes, while the dominance of the reactions in the solid-phase pathway determines the formation of vertically aligned MoO2/MoS2 flakes. In addition, the photoelectrochemical (PEC) activity of the vertically aligned MoO2/MoS2 flakes was evaluated for hydrogen production applications based on PEC water splitting. Our results open the door to promote the process for designing the growth of other TMD materials or complex structures, which can be beneficial for use in various applications.