Effects of Mo vapor concentration on the morphology of vertically standing MoS₂ nanoflakes

Vertically standing MoS₂ nanoflakes are favourable in applications such as energy storage devices, hydrogen evolution reactions, and gas sensors due to their large surface area and high density of exposed edges. In this work, we report the effect of Mo vapor concentration on the morphology of vertical MoS₂ nanoflakes prepared by chemical vapor deposition at atmospheric pressure. A series of MoS₂ samples were grown under different Mo vapor concentrations by varying the separation distance (x) between the MoO₃ source and the substrate. Field emission scanning electron microscopy showed the sample grown at x = 1 cm had a high density of vertical flakes (7 vertical flakes µm^-2) with an average flake length of ~770 nm and thickness of ~10 nm. As x increased to 4 cm, the average flake length was reduced to ~150 nm while the flake orientation changed from vertical to lateral. That is, high Mo vapor concentration favours the formation of large and vertical MoS₂ nanoflakes. However, oversupply of Mo vapor results in significantly thicker flakes. Raman spectra of all samples showed two main peaks at 380 and 407 cm^-1 that correspond to the E¹ _{2 g} and A_{1 g} vibrational peaks of MoS₂. As x decreased from 4 to 1, the peak intensity ratio (E¹ _2g/A_{1 g}) reduced from 0.58 to 0.42, suggesting greater dominance of vertical flakes at low x. X-ray diffraction data showed a prominent peak at 14.4°, which corresponded to the (002) diffraction peak of 2H MoS₂. Transmission electron microscopy verified the flakes consist of eight layers with an interlayer spacing of 0.62 nm. Based on hydrogen evolution reaction measurements, samples with thin flakes have high catalytic activity. This work highlights the importance of optimizing Mo vapor concentration to obtain a high density of thin, large, and vertically standing MoS₂ nanoflakes.