Two-Dimensional Semiconducting ZrNX (X= Cl, Br, I) with a Janus Structure for Solar Energy Utilization

Recently, two-dimensional (2D) Zr-based halides have brought an intensive interest for applications in multifunctional energy conversion. Herein, the single-layer ZrNX (ZrNCl, ZrNBr, and ZrNI) and Janus ones (Zr2N2ClBr, Zr2N2ClI, and Zr2N2BrI) are proposed using first-principles calculations. The single-layer ZrNX can be obtained from its bulk counterpart via mechanical exfoliation. The single-layer ZrNX and Janus ones show dynamic and thermodynamic stability, revealing moderate band characteristics, and band values are ∼2.25–2.90 eV. Janus structure can tune the electronic structure, band alignment, and light absorption within a certain range. In particular, the Janus structure induces a built-in electric field and a great potential drop, improving the separation efficiency of photogenerated electron holes. All the single-layer ZrNX and Janus ones, except for the Zr2N2ClI and Zr2N2BrI monolayers, are interested in being a candidate for photocatalytic water splitting. The Janus structure (Zr2N2ClBr) shows a better water oxidation ability than the others. The excellent optical absorption coefficient (∼104 cm–1, in the visible-light region) implies the significant potential of (photo)­electric devices. The proposed exciton solar cell is expected to have a large power conversion efficiency (PCE) limit of ∼12%. These present results indicate that the single-layer ZrNX and Janus ones are potential materials for photocatalytic water splitting and other (photo)­electric devices.