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A novel structurally stable 2D-boron nitride material, namely di-BN, is predicted by means of the first-principles simulations. This monolayer BN system is composed of the azo (N N) and diboron (B B) groups. Its in-plane stiffness is close to the monolayer h-BN. Usually, the boron nitride materials are semiconductors with large band gaps. However, the monolayer di-BN possesses a moderate direct band gap of 1.622 eV obtained from our HSE06 calculation. Although the GW correction enlarges the band gap to 2.446 eV, this value is still in the range of the visible light. The detailed investigation of its band arrangement reveals that this material is able to product hydrogen molecules in a photocatalytic water splitting reaction. Furthermore, its charge carrier mobilities are significantly higher than the other popular 2D semiconductors, e.g., MoS 2 and phosphorene. Therefore, this 2D-BN material could have huge application potentials in the electronics and solar energy conversion fields. • A new boron nitride monolayer system, di-BN composed of B B and N N pairs, is proposed. • Unlike the other boron nitride systems with large band gaps, di-BN possesses an optimal direct band gap about 1.622 eV at HSE06 level. • The di-BN is a promising photocatalytic material for hydrogen evolution by water splitting. [ABSTRACT FROM AUTHOR]

• Heterostructure formed between Melem and g-C 3 N 4 suppress the electron-hole rebombination. • The composite material of g-C 3 N 4 /melem by molecular engineering strategy is prepared. • The photocatalytic efficiency of g-C 3 N 4 /melem was twice as high as that of pristine g-C 3 N 4. The low efficiency of photogenerated charge separation is a critical factor to restrict the efficiency of photocatalytic reaction. Extensive research efforts have been devoted to charging kinetics to improve the efficiency of photogenerated charge separation and transfer to catalytic sites. In this research, the heterostructure and interfacial electron polarization are formed between melem and graphitic carbon nitride (g-C 3 N 4) through first-principles calculation, which can greatly suppress the electron-hole recombination. The synergistic advantage of g-C 3 N 4 /melem hybrid reveals great enhancement in photocatalytic hydrogen activity. Subsequently, we prepared the composite material of g-C 3 N 4 /melem by molecular engineering strategy, and found that the photocatalytic efficiency of g-C 3 N 4 /melem was twice of pristine g-C 3 N 4. This study provides an insightful idea to improve the efficiency of photogenerated charge separation. [ABSTRACT FROM AUTHOR]