  Superconductivity in two-dimensional compounds is widely concerned, not only
due to its application in constructing nano-superconducting devices, but also
for the general scientific interests. Very recently, borophene (two-dimensional
boron sheet) has been successfully grown on the Ag(111) surface, through direct
evaporation of a pure boron source. The experiment unveiled two types of
borophene structures, namely $\beta_{12}$ and $\chi_3$. Herein, we employed
density-functional first-principles calculations to investigate the
electron-phonon coupling and superconductivity in both structures of borophene.
The band structures of $\beta_{12}$ and $\chi_3$ borophenes exhibit inherent
metallicity. We found electron-phonon coupling constants in the two compounds
are larger than that in MgB$_2$. The superconducting transition temperatures
were determined to be 18.7 K and 24.7 K through McMillian-Allen-Dynes formula.
These temperatures are much higher than theoretically predicted 8.1 K and
experimentally observed 7.4 K superconductivity in graphene. Our findings will
enrich the nano-superconducting device applications and boron-related material
science.
