High-performance photocatalytic and piezoelectric properties of two-dimensional transition metal oxyhalide $\mathrm{ZrO}{X}_2 (X=\mathrm{Br},I)$ and their Janus structures

As an emerging type of two-dimensional (2D) family, transition metal oxyhalides with the chemical formula ${\mathrm{MOX}}_2$ have been studied in recent years. Inspired by the successful synthesis of $\mathrm{ZrO}{I}_2$ monolayer with excellent photocatalytic and piezoelectric properties, we conducted a systematic and comprehensive investigation of $\mathrm{ZrO}{X}_2 (X=\mathrm{Br},I)$ and its Janus ZrOBrI monolayers using first-principles calculations. The results show that the mechanically, dynamically, and thermally stable $\mathrm{ZrO}{X}_2$ and Janus ZrOBrI are indirect-gap semiconductors with band gaps ranging from 1.89 to 3.48 eV by the hybrid density functional HSE06 method. Besides, their valence-band minimum and conduction-band minimum can straddle the redox potential of water at $pH=0$, respectively. Interestingly, due to the optimal band alignment mechanism, the band-edge position of Janus ZrOBrI with an intrinsic electric field does not expand. Moreover, biaxial compressive strain (within −6%) effects on the band alignments and band gaps are discussed. What is more, strong anisotropy and high optical absorption in the visible-ultraviolet region ($\sim {10}^{5}c{m}^{-1}$) render these monolayers fantastic polarizers and photoelectronic devices. Besides, the orientation-dependent carrier mobility of these monolayers is much higher than that of many other 2D semiconductors, making them potential electronic and photocatalytic devices. For piezoelectric performance, all of these monolayers exhibit a considerable in-plane transverse piezoelectric coefficient $d_{21}$, reaching about 20 pm/V. Furthermore, the Janus ZrOBrI possesses additional out-of-plane piezoelectric responses due to structural mirror asymmetry. Under AC stacking mode, multilayer Janus ZrOBrI has appreciable vertical piezoelectric coefficients $d_{31}$ and $d_{32}$, reaching 1.17 and 4.61 pm/V, respectively. Our findings highlight that all three monolayers are multifunctional devices, especially in the fields of photocatalysis and piezoelectricity.