  Two-dimensional materials provide remarkable platforms to uncover intriguing
quantum phenomena and develop nanoscale devices of versatile applications.
Recently, AlSb in the double-layer honeycomb (DLHC) structure was successfully
synthesized exhibiting a semiconducting nature [ACS Nano 15, 8184 (2021)],
which corroborates the preceding theoretical predictions and stimulates the
exploration of new robust DLHC materials. In this work, we propose a Janus DLHC
monolayer Al$_2$SbBi, the dynamical, thermal, and mechanical stabilities of
which are confirmed by first-principles calculations. Monolayer Al$_2$SbBi is
found to be a nontrivial topological insulator with a gap of about 0.2 eV,
which presents large spin splitting and peculiar spin texture in the valence
bands. Furthermore, due to the absence of inversion symmetry, monolayer
Al$_2$SbBi exhibits piezoelectricity and the piezoelectric strain coefficients
d$_{11}$ and d$_{31}$ are calculated to be 7.97 pm/V and 0.33 pm/V,
respectively, which are comparable to and even larger than those of many
piezoelectric materials. Our study suggests that monolayer Al$_2$SbBi has
potential applications in spintronic and piezoelectric devices.
