  Recently, a honeycomb borophene was reported to grow successfully on Al(111)
surface. Since the metallic $\sigma$-bonding bands of honeycomb boron sheet
play a crucial role in the 39 K superconductivity of MgB$_2$, it is physically
interesting to examine whether similar property exists in this material. We
have calculated the electronic structures and the electron-phonon coupling for
honeycomb borophene by explicitly considering the substrate effect using
first-principles density functional theory in conjunction with the Wannier
interpolation technique. We find that the $sp^2$-hybridized $\sigma$-bonding
bands of honeycomb borophene are metallized due to moderate charge transfer
from the Al substrate, similar as in MgB$_2$. However, the electron-phonon
coupling in honeycomb borophene is much weaker than in MgB$_2$ due to the
hardening of the bond-stretching boron phonon modes and the reduction of phonon
density of states. Nevertheless, the interlayer coupling between Al-associated
phonons and electrons in borophene is strong. Based on this observation, we
predict that a 6.5 K superconducting transition can be observed in a
free-standing borophene decorated by a single Al layer, namely monolayer
AlB$_2$. Accordingly, similar superconducting transition temperature could be
expected in honeycomb borophene on Al(111).
