Synthesis of Vertically Standing MoS2 Triangles on SiC

Layered material MoS2 has been attracting much attention due to its excellent electronical properties and catalytic property. Here we report the synthesis of vertically standing MoS2 triangles on silicon carbon(SiC), through a rapid sulfidation process. Such edge-terminated films are metastable structures of MoS2, which may find applications in FinFETs and catalytic reactions. We have confirmed the catalytic property in a hydrogen evolution reaction(HER). The Tafel slope is about 54mV/decade.


Methods
In this work, we develop a rapid sulfidation process through a large carrier gas flow rate on SiC using a CVD method. The MoS 2 triangles are aligned vertically to the surface of the substrate. Various characterizations techniques were used to have a good understand to the mechanism of the vertically standing triangles. Furthermore, HER performance of vertically standing MoS 2 triangles was researched.
The synthesis process of vertically standing MoS 2 triangles is schematically illustrated in Fig. 1. At the beginning, MoO 3 powder was placed in the centre of the furnace, 6H SiC was placed next to the MoO 3 powder. Sulphur powder was placed inside of a steel cylinder out side of the furnace, with a heating tape around it. High pure argon was chosen as carrier gas to convey sulphur and MoO 3 vapor to downstream. The temperature of MoO 3 powder was 1000 °C, sulphur powder was heated to 260 °C, the growth pressure was atmospheric.
A typical optical image of the MoS 2 triangles grown on SiC substrate is shown in Fig. 2a. It clearly shows that most of the triangles on the surface were vertically standing, there are a few flat triangles on the surface. The SEM image demonstrates that the as grown triangles are nearly perpendicular to the substrate. There is a small angles of inclination of some triangles. The edges of different MoS 2 triangles could be clearly observed in SEM images. The lateral dimensions of the triangles are tens micrometer, the heights are from 30 nm to 2 μ m. Figure 2c is the Raman spectrum of the vertically standing triangles on SiC and monolayer MoS 2 on sapphire. Two Raman characteristic bands of vertically standing triangles at 410 cm −1 and 383 cm −1 corresponding to A 1 g and E 1 2g respectively [25][26][27] . Comparing to the flat monolayer MoS 2 on sapphire, the intensity ratio between A 1 g and E 1 2g of the vertically standing triangles is higher, revealing a higher density of exposed edges in those vertically standing triangles. Figure 2d Due to the anisotropic bonding and the general tendency to minimize the surface energy, nanoparticles of layer materials usually exhibit platelet-like morphology 30,31 . Alternatively, vertically standing triangles can also be obtained by a fast growth process 32 , the synthesis rate is mainly affected by the diffusion of product gas on the surface of the substrate. By controlling the reactant concentration, we can obtain the MoS 2 films with different morphologies. Through regulating the carrier gas flow rate, the sulfidation rate of MoO 3 can be controlled well. In addition, by changing the carrier gas flow rate, we can have a good understand of the growth process of MoS 2 film.
As shown in Fig. 3a, when the carrier gas flow rate is 100sccm, the concentration of sulfidation vapor is too low to meet the needs. Meanwhile, with a slow carrier gas flow rate, the forming of MoO 3−x is limited, and the transport rate of MoO 3−x to the surface of substrate is also affected. In this condition, there are only some nanoparticles and some small rectangles on the surface of SiC, because of the lacking of S. So we increasing the carrier gas flow rate to 180sccm, the size of the rectangles increased, but we still do not obtained the vertically standing triangles. When the carrier gas flow increased to 260sccm, the synthesis rate is faster with the increasing of sulfidation concentration. Under this condition, we obtained the vertically standing triangles on the surface of SiC, the result is shown in Fig. 3c. Some of flat triangles were observed on the surface of SiC, only a few vertically standing triangles were obtained. So we increased the carrier gas flow to 340sccm, almost all of the triangles are perpendicular to the substrate, as shown in Fig. 3d. Figure 3e,f were the SEM images of flat rectangles and triangles. Figure 3f-h show the SEM images of vertically standing triangles. From the results, we can see that, only under a fast growth rate, can we get the vertically standing triangles of MoS 2 .
To have a better understand, the growth model is shown in Fig. 4. At the beginning of the growth, there was neither buffer layer nor seed on the surface of the substrate, so nucleation process was a 3D. The triangles are all coming from the islands of Fig. 4a. When the carrier gas flow rate is small, the atoms and molecules have enough time to mobility and diffusion on the surface of the substrate, so the synthesis process of MoS 2 will be a 2D growth. With the supply of the sulfur vapor, the small islands grew into larger domain size, at last, the flat triangles of MoS 2 were obtained, as shown in Fig. 4b. When the carrier gas flow rate is high enough, the chemical  conversion occurs much faster than the diffusion of sulfur gas into the film. Under this condition, the sulfidation process will be rate-limiting. Meanwhile, with the anisotropic structure, the diffusion along the layers through van der Waals gaps is much faster than diffusion across the layers. Accordingly, the layers naturally orient perpendicular to the film, exposing van der Waals gaps for fast reaction. In this condition, the vertically structure formed, as shown in Fig. 4c.
As we know, the wettability has a great effect to the nucleation process at the begining of the growth. If the wettability of the subbstrate is good, the film will be a two dimmendional growth, along the surface of the subbstrate, then flat film will be obtained. If the wettability of the subbstrate is poor, the film will maintain a 3D growth along the layers. In order to prove our conclusion, and have a contrast, we also synthesis the MoS 2 film on sapphire with the same condition: the temperature of MoO 3 powder is 1000 °C, carrier gas flow rate were 100sccm, 180sccm, 260sccm, 340sccm respectively. The results were shown in Fig. 5. we can see that, the film on the sapphire are all flat triangles. There is no vertically standing triangles obtained on the sapphire. This is because the perfect wettability of MoS 2 on the sapphire, the growth process will be 2D. Under this condition, there will not be vertically standing triangles formed. In contrast, with a poor wettability between MoS 2 and SiC, the nucleation of the film on the surface of SiC is more difficult, and the growth process will be 3D, which is useful to synthesis the vertically standing triangles. Through the results, we can see that the wettability is also a important factor during the growth of the vertically standing MoS 2 .
HER catalytic activity of vertically standing MoS 2 triangles on SiC was tested. Typical cathodic polarization curves and corresponding Tafel plots are shown in Fig. 6a,b. The Tafel slope in our vertically standing triangles was about 54 mV/decade. The related reports about vertically standing structures of MoS 2 is about 94 mV/dec 10 and 105-120 mV/dec 23 . Tafel plots are commonly used to evaluate the efficiency of the catalytic reaction. which indicated that the surface coverage of absorbed hydrogen was relatively low. The small Tafel plots means the high efficiency of the reaction. This indicating a good catalytic property of vertically standing triangles on SiC.

Conclusions
We have developed a rapid sulfidation process for the synthesis of vertically standing MoS 2 triangles. SEM images reveal the synthesis mechanism of the triangles. It is suggested that under a high concentration of sulfur, the growth process will by a 3D growth, all these vertically standing triangles come from the small islands on the surface of SiC. In addition, by a comparision between the films grown on sapphire and SiC, we find the wettability  is another factor for the forming of vertically standing triangles. At last, the HER properties of the triangles was tested. The Tafel slope is about 54 mV/decade, which is much smaller than the related reports about vertically standing MoS 2 nanosheets.