Multifunctional Janus XHfPH (X = S, Se, Te) semiconductors for solar water splitting: a first-principles analysis.

Using first-principles density functional theory, we design and systematically investigate a family of highly stable Janus XHfPH (X = S, Se, Te) monolayers. Comprehensive stability and Raman spectra calculations indicate that these monolayers are theoretically stable and may warrant further investigation toward potential experimental realization. Notably, these monolayers exhibit robust in-plane and out-of-plane piezoelectricity, with SHfPH and TeHfPH showing exceptional promise for nanoscale electromechanical devices. Our calculated results indicate that XHfPH monolayers are semiconductors with moderate indirect bandgaps from 1.07 to 1.57 eV. Furthermore, their intrinsic structural asymmetry generates a built-in electric field that drives efficient spatial separation of photogenerated carriers, enabling favorable band-edge alignment for photocatalytic water splitting. Consequently, SHfPH and SeHfPH achieve impressive theoretical solar-to-hydrogen conversion efficiencies of 10.03% and 9.97%, respectively. These findings establish Janus XHfPH monolayers as highly promising, multifunctional 2D materials for next-generation optoelectronic, piezoelectric, and solar-energy conversion applications.