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.