Quasihorizontal projectile motion of carriers in two-dimensional piezoelectric photocatalyst Janus <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mi>GeN</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:m

At present, the research on two-dimensional (2D) piezoelectric photocatalysts for water splitting lacks relevant physical quantities to describe the time for the progress of photogenerated carriers to transport from the interior of the material to the surface and participate in the photocatalytic reaction under the built-in electric field. Herein, we proposed the quasihorizontal projectile motion model of carriers and then quantified the time (\ensuremath{\tau}) for the photogenerated carriers to participate in the above process in a series of 2D piezoelectric photocatalyst Janus ${\mathrm{GeN}}_{2}$-XY monolayers through first-principles calculations. We found that the magnitude of \ensuremath{\tau} in ${\mathrm{GeN}}_{2}$-XY is in the femtosecond range, which is much smaller than the recombination time of photogenerated carriers in most 2D semiconductors (which is in the picosecond or longer range). This means that the photogenerated carriers in ${\mathrm{GeN}}_{2}$-XY are likely to migrate to the material surfaces before recombination and participate in the photocatalytic reaction. This highlights the advantages of 2D piezoelectric photocatalysts. In addition, we propose the elasticity--bond order (BO) mechanism and the two-step strategy. The BO mechanism explains that the in-plane elastic stiffness coefficient of 2D materials increases with the bond-order sum of all the atomic bonds that contribute along the in-plane direction in the structure. This provides important inspiration for the design of 2D flexible wearable devices. And the two-step strategy provides a summary of the method for controlling the photocatalytic water splitting of 2D materials through piezoelectric effects clearly.