Chemical Vapor Deposition of Oriented Vertical MoO₂ Nanofins in a Confined Space for Conductive Electrodes

The horizontal and vertical growth of two-dimensional (2D) materials dominates their properties and applications. Vertical 2D materials have great potential in applications like high-density transistors, sensors, catalytic and energy storage. However, the controllable growth of vertical 2D materials is still challenging and the mechanism for the vertical growth is far from well understood. In this study, MoO2 nanosheets were grown on c-Al2O3 substrate using atmospheric pressure chemical vapor deposition. A transition from horizontal MoO2 nanosheets to vertical MoO2 nanofins was observed on different regions of the substrate. On the confined contact region between the substrate and the quartz boat sides, the MoO2 nanofins grew vertically, while MoO2 nanosheets grew horizontally on the other regions of the substrate. According to our specially designed confined space experiments and the available nucleation theory, the vertical growth of MoO2 nanofins was enabled by the synergistic effect of a low precursor concentration and a low carrier gas flow rate. In particular, the vertical MoO2 nanofins were mainly oriented in three directions with the interval angle of 120°, which was explained by the local minimum angle-dependent binding energies between MoO2 and Al2O3 calculated by density functional theory. Finally, the electrical conductivity of an individual vertical MoO2 nanofin was measured to be 1.9 × 105 S/m, which is among the highest in metal oxides, making it promising for non-metal electrode in batteries and supercapacitors. Our work reveals the growth mechanism of oriented vertical MoO2 nanofins, which will benefit the preparation and practical application of vertical 2D materials.