The exploration of novel and stable two-dimensional (2D) materials holds considerable significance in the development of optoelectronic devices and photocatalytic water splitting. Herein, $\mathit{MNXY}(M\text{/}N=\mathrm{Al},\mathrm{Ga},X\text{/}Y=\mathrm{N},\mathrm{P},\text{As})$ monolayers are constructed based on 2D double-layer honeycomb structures. Six stable semiconductor types were screened through first-principles calculations, and their mechanical, electrical, optical, and transport properties as well as their applications in photocatalytic water splitting were investigated. We found that the six stabilized MNXY monolayers had band gaps of 0.846–3.806 eV. The mechanical properties indicate that ${\mathrm{AlGaN}}_2$ has a Young's modulus exceeding 100 N/m, whereas the remaining five monolayers exhibit values below this threshold. The hole carrier mobility of the ${\mathrm{AlGaN}}_2$ monolayer along the armchair direction reaches an ultrahigh value of 3649.21 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$; thus it has the potential for application in optoelectronic devices. Furthermore, we observed that ${\mathrm{AlGaP}}_2$ monolayers exhibit band-edge potentials spanning the redox potential of water, a considerable difference in electron-hole carrier mobility, strong visible light absorption, and a high solar to hydrogen efficiency (17.51%) in the absence of strain, making them suitable for photocatalytic water splitting. We expect that our results will pave the way forward for material selection for next-generation optoelectronic devices and photocatalysts.

The search for contact materials with low contact resistance and tunable Schottky barrier (SB) height of two-dimensional (2D) materials is important for improving the electronic performance. Inspired by the recently synthesized metallic Janus MoSH, this study employs first-principles calculations to investigate the electronic structure, mechanical properties, and interface characteristics of Janus MoSH/GaN and MoHS/GaN van der Waals (vdW) heterostructures. We find that both heterostructures exhibit isotropic mechanical properties and form p-type Schottky barrier contacts (p-ShC) and the SB height of MoHS/GaN is smaller than that of the MoSH/GaN heterostructure. The variation in SB height and contact type under biaxial strain and electric field is also studied for both vdW heterostructures, respectively. Compared to the MoSH/GaN heterostructure, the MoHS/GaN heterostructure can transition to Ohmic contact (OhC) under biaxial strain and electric field, making the S-face contact of MoSH with GaN a more effective contact approach. These findings could provide a new pathway for the design of controllable Schottky nanodevices and high-performance electronic devices on GaN-based vdW heterostructures.