Ferroelectric and strain-tuned MoSe2/Bi2O2Se van der Waals heterostructure with band alignment evolution.

The vertically stacked two-dimensional van der Waals heterostructure (2D vdWH) provides a unique platform for integrating distinctive properties of various 2D materials by functionalizing the interfacial interaction and regulating its band alignment. Herein, we theoretically propose a new MoSe2/Bi2O2Se vdWH material, in which a zigzag-zipper structure of the Bi2O2Se monolayer is constructed to model its ferroelectric polarization and maintain a small interlayer mismatch with MoSe2. The results show a typical unipolar barrier structure with a large band offset in the conduction band and nearly zero offset in the valence band of MoSe2/Bi2O2Se when the ferroelectric polarization of Bi2O2Se is back to MoSe2, in which the electron migration is blocked and holes can migrate unimpeded. It is also found that the band alignment lies between the type-I and type-II heterostructures and band offsets can be flexibly modulated under the joint action of ferroelectric polarization of Bi2O2Se and in-plane biaxial tensile and compressive strains. This work would facilitate the development of multifunctional devices based on the MoSe2/Bi2O2Se heterostructure material.

Basal Plane-Activated Boron-Doped MoS2 Nanosheets for Efficient Electrochemical Ammonia Synthesis.

Under the dual pressure of energy crisis and environmental pollution, ammonia (NH3 ) is an indispensable chemical product in the global economy. The electrocatalytic synthesis of NH3 directly from nitrogen and water using renewable electricity has become one of the most attractive and important topics. Basal plane-activated boron-doped MoS2 nanosheets (B-MoS2 ) as a non-noble metal catalyst with excellent performance for N2 electroreduction are synthesized by a facile one-step hydrothermal method. In 0.1 m Na2 SO4 solution, MoS2 nanosheets doped with 300 mg boric acid (B-MoS2 -300) give rise to a good ammonia yield rate of 75.77 µg h-1 mg-1 cat. at -0.75 V vs. RHE, and an excellent Faradaic efficiency of 40.11 % at -0.60 V vs. RHE. In addition, the B-MoS2 -300 nanosheets show good selectivity and chemical stability, and no hydrazine (N2 H4 ) by-product is generated during the reaction. 15 N isotopic labeling confirms that nitrogen in produced ammonia originates from N2 in the electrolyte. On the one hand, the high conductivity of MoS2 guarantees guarantees a high electron transfer rate from nitrogen to ammonia; on the other hand, the successful incorporation of heteroatom B enlarges the interlayer spacing of MoS2 , and the B atom can act as an active site for basal plane activation, providing more active sites for the nitrogen reduction reaction (NRR). Density functional theory calculations show that the doping of B activates the base plane of 1T-MoS2 , which makes the adsorption of N2 on the base plane easier and promotes the NRR.

Porous graphitic carbon nitride with nitrogen defects and cobalt-nitrogen (CoN) bonds for efficient broad spectrum (visible and near-infrared) photocatalytic H2 production.

The porous graphitic carbon nitride (g-C3N4) with nitrogen defects and cobalt-nitrogen (CoN) bonds (g-C3N4-Co-K) is prepared by controllable copolymerization of melamine with KOH and Co(NO3)2·6H2O. The method not only provides g-C3N4 with porous structure and CoN bonds that accelerate photoexcited carrier transfer and endow numerous active sites, but also redshifts the g-C3N4 absorption edge into near-infrared (NIR) light region through the introduction of nitrogen defects and thus is suitable for H2 evolution. The g-C3N4-Co-K exhibits significantly superior photocatalytic hydrogen generation performance (808 µmol h-1 g-1) under simulated solar light irradiation, about 15.5 times higher than pure g-C3N4, about 5.2 times higher than g-C3N4 with CoN bonds (g-C3N4-Co), and about 2.1 times higher than g-C3N4 with nitrogen defects (g-C3N4-K). Interestingly, it is for the first time revealed that the synergistic effect of nitrogen defects and CoN bonds result in enhanced H2 generation activity (470 µmol h-1 g-1) under NIR light irradiation.