Strong Vertical Piezoelectricity and Broad-pH-Value Photocatalyst in Ferroelastic Y2Se2BrF Monolayer.

With the development of miniaturized devices, there is an increasing demand for 2D multifunctional materials. Six ferroelastic semiconductors, Y2Se2XX' (X, X' = I, Br, Cl, or F; X ≠ X') monolayers, are theoretically predicted here. Their in-plane anisotropic band structure, elastic and piezoelectric properties can be switched by ferroelastic strain. Moderate energy barriers can prevent the undesired ferroelastic switching that minor interferences produce. These monolayers exhibit high carrier mobilities (up to 104 cm2 V-1 s-1) with strong in-plane anisotropy. Furthermore, their wide bandgaps and high potential differences make them broad-pH-value and high-performance photocatalysts at pH value of 0-14. Strikingly, Y2Se2BrF possesses outstanding d33 (d33 = -405.97 pm/V), greatly outperforming CuInP2S6 by 4.26 times. Overall, the nano Y2Se2BrF is a hopeful candidate for multifunctional devices to generate a direct current and achieve solar-free photocatalysis. This work provides a new paradigm for the design of multifunctional energy materials.

Magnetic and Ferroelectric Manipulation of Valley Physics in Janus Piezoelectric Materials.

The realization of multiferroic materials offers the possibility of multifunctional electronic device design. However, the coupling between the multiferroicity and piezoelectricity in Janus materials is rarely reported. In this study, we propose a mechanism for manipulating valley physics by magnetization reversing and ferroelectric switching in multiferroic and piezoelectric material. The ferromagnetic VSiGeP4 monolayer exhibits a large valley polarization up to 100 meV, which can be effectively operated by reversing magnetization. Interestingly, the antiferromagnetic VSiGeP4 bilayers with AB and BA stacking configurations allow the coexistence of valley polarization and ferroelectricity, supporting the proposed strategy for manipulating valley physics via ferroelectric switching and interlayer sliding. In addition, the VSiGeP4 monolayer contains remarkable tunable piezoelectricity regulated by electron correlation U. This study proposes a feasible idea for regulating valley polarization and a general design idea for multifunctional devices with multiferroic and piezoelectric properties, facilitating the miniaturization and integration of nanodevices.

Efficient direct conversion of methane into methanol on CuZn hetero-diatomic catalysts with certain coordination spheres: a DFT study.

The oxidation of methane to a high-value-added chemical, methanol, is a major challenge in catalysis, requiring high energy input to overcome the CH3-H bond activation energy barrier. Based on density functional theory (DFT) calculations, methane oxidation to methanol is catalyzed by hetero-diatomic catalysts (CuZn-NG) with different coordination spheres (CSs). Valence band maximum (VBM), atomic charge and d-band center are selected as analysis methods for the pathway selection and activity of catalysis. The VBM plays a vital role in the catalytic pathway selection, CuZn-NG catalyzes the direct conversion of methane into methanol without side reactions. Alarmingly, the most important reaction step, CH3-H bond activation, is a spontaneously exothermic reaction (releasing 0.06 eV) with CuZn-NPAG as the catalyst, in contrast to most other endothermic reactions in the same activation. By analyzing the atomic charge of the Cu center and O atom, the special electronic phenomenon for this important step is summarized as the "bow-release effect". The CS affects the electronic properties of the active center and further affects the methane oxidation activity. This work provides a useful guide to understand the catalytic selectivity and activity of hetero-diatomic catalysts.

Switch effect on controlled water splitting by biaxial strain regulating the promising two-dimensional Janus X2PAs (X = Si, Ge and Sn) photocatalyst.

Two-dimensional photocatalytic materials with unique properties have been well-reported in recent decades. However, strategies for controlling the photocatalytic process are still ongoing. Herein, Janus X2PAs (X = Si, Ge and Sn) monolayers have been explored by first-principles calculations to meet this challenge. All strain-free X2PAs monolayers exhibit excellent photocatalytic properties with high carrier mobility (2.39 × 102-1.34 × 104 cm2 V-1 s-1), suitable band edge positions straddling the standard redox potential of water and large visible light absorption coefficients (up to 105 cm-1). Most importantly, a reaction switch effect is proposed for the first time towards controlling the microscopic photocatalytic process of water splitting on X2PAs monolayers through macroscopic mechanical strain. This effect renders the Janus X2PAs photocatalytic switches among the states of only oxygen evolution reaction, only hydrogen evolution reaction and the full redox reaction for controlled water splitting. This work not only provides a new avenue for designing highly tunable photocatalysts but also offers new physical insights into controlling the photocatalytic water-splitting reaction.

Active Asymmetric Electron-Transfer Effect on the Enhanced Piezoelectricity in MoTO (T = S, Se, or Te) Monolayers and Bilayers.

Development of piezoelectric materials is limited partly due to the incompleteness of internal mechanism and the lack of vertical piezoelectricity. Herein, we theoretically identify the stable MoTO (T = S, Se, or Te) monolayers and bilayers. When two elements are given but another element can be changed, the larger the electronegativity difference ratio Rratio is, the stronger the piezoelectricity will be. Vertical piezoelectric coefficient d33 of the MoTeO bilayer reaches 38.907 pm/V, which is 12 times larger than that of the bulk GaN. The "active asymmetric electron-transfer" strategy mainly contributes to the spontaneous remarkable piezoelectricity of MoTO. Importantly, we proposed the new method for calculating the piezoelectric coefficients of two-dimensional (2D) materials, which corresponds to the fact that 2D materials have a certain thickness. This study not only provides novel extraordinary candidates for energy conversion and touch-sensor nanodevices but also promotes a deeper understanding of piezoelectricity of 2D materials.