Integration of promising piezoelectric and photocatalytic properties in Janus <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>In</mml:mi> <mml:mi>X</mml:mi> <mml:mi>Y</mml:mi> </mml:mrow> </mml:math> ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>X</mml:mi> <mml:mo>=</mml:mo> <mml:mi mathvariant="normal">S</mml:mi> </mml:mrow> </mml:math> , Se, Te; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Y</mml:mi> <mml:mo>=</mml:mo> <mml:mi>Cl</mml:mi> </mml:mrow> </mml:math> , Br, I) monolayers and their heterojunctions

Two-dimensional (2D) Janus materials show great promise as piezoelectric materials and photocatalysts for water splitting. In this work, we systematically investigated the piezoelectric and photocatalytic properties of the hexagonal Janus $\mathrm{In}XY$ ($X=\mathrm{S}$, Se, Te; $Y=\mathrm{Cl}$, Br, I) monolayers (MLs) using first-principles calculations. Except for InSeCl ML, the remaining eight $\mathrm{In}XY$ MLs are stable and exhibit exceptionally high in-plane piezoelectric coefficients ($|{d}_{22}|=6.07--155.27\phantom{\rule{0.28em}{0ex}}\mathrm{pm}/\mathrm{V}$), which exceed those of most known 2D materials. $\mathrm{In}XY$ MLs possess band edges straddling the water redox potentials at pH = 0. Their intrinsic vertical polarization induces an intralayer polarization field ${E}_{\mathrm{intra}}$, leading to low exciton binding energies (0.44--0.78 eV). Moreover, their strong vertical piezoelectric responses ($|{d}_{32}|=0.34--0.65\phantom{\rule{0.28em}{0ex}}\mathrm{pm}/\mathrm{V}$) suggest that in-plane stress can further enhance ${E}_{\mathrm{intra}}$ and facilitate the separation of photogenerated carriers. Additionally, these $\mathrm{In}XY$ MLs exhibit high electron mobility ($101--899\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}/\mathrm{V}/\mathrm{s}$) and a pronounced anisotropy ratio in carrier mobility, which effectively suppresses charge recombination. Among them, several stand out: InSI and InSeBr MLs show high electron mobility and a large carrier mobility anisotropy ratio; InSeBr ML exhibits excellent in-plane and out-of-plane piezoelectricity; and InSeBr, InSeI, and $\mathrm{InTe}Y$ ($Y=\mathrm{Cl}$, Br, I) MLs show strong visible light absorption. To optimize performance, we constructed a van der Waals heterojunction (InSI/InSeBr), which demonstrates remarkable photocatalytic properties, including enhanced redox ability, a direct Z-scheme charge transfer pathway, strong visible light absorption, high carrier mobility, and excellent photocorrosion resistance. Our results indicate that hexagonal Janus $\mathrm{In}XY$ MLs and their heterojunctions are multifunctional materials integrating piezoelectricity and photocatalysis, paving the way for energy conversion applications.