The lead-free perovskites Cs3
B
2
X
9 (B = Sb, Bi; X = Cl, Br, I) as the popular photoelectric materials have excellent optical properties with lower toxicity. In this study, we systematically investigate the stable monolayer Cs3
B
2
X
9 and bilayer vertical heterostructure Cs3
B
2
X
9/Cs3
B
2
′
X
9 (B,
B′ = Sb, Bi; X = Cl, Br, I) via first-principles simulations. By exploring the electrical structures and band edge positions, we find the band gap reduction and the band type transition in the heterostructure Cs3
B
2
X
9/Cs3
B
2
′
X
9 due to the charge transfer between layers. Furthermore, the results of optical properties reveal light absorption from the visible light to UV region, especially monolayer Cs3Sb2I9 and heterostructure Cs3Sb2I9/Cs3Bi2I9, which have absorption peaks in the visible light region, leading to the possibility of photocatalytic water splitting. These results provide insights for more two-dimensional semiconductors applied in the optoelectronic and photocatalytic fields.
Herein, the density functional theory is used to study the structural, mechanical, electrical, optical, and hydrogen evolution reaction (HER) properties of monolayer structures and . They are claimed to be stable monolayer structures by phonon dispersion and ab initio molecular dynamics (AIMD). Compared to graphene (≈340 N m−1), the shows a higher Young's modulus (≈429, 414 N m−1). Moreover, the piezoelectric stress coefficient of α‐ (=2.65 pm V−1) is comparable with bulk piezoelectric α‐quartz (=2.3 pm V−1). Exciton effect at M point causes redshift and enhancement of light absorption of α‐ and . The overpotential of is only –0.072 V. Besides, the gives a small effective electron mass of about 0.2(m0). It is worth mentioning that the monolayer structures show the semimetal phases. Due to the unique semimetal phase, the absorption of has an excellent performance in the near‐infrared (NIR) region. It can be enhanced at the NIR region and extended to the middle infrared region when applying tensile stress. These results provide more choices for 2D materials in electrocatalysis, piezoelectric sensors, visible light devices, and infrared photodetectors.
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