Bismuth Semimetal-Boosted Electron Transfer, Nitrogen Adsorption, and Nitrogen Photoreduction Activity of Bi/g-C3N4 Nanosheets

Nitrogen photoreduction is a promising ammonia production technology since it can be carried out at the atmospheric pressure by merely utilizing the abundant solar energy. However, the low conversion efficiency is still a big challenge for practical applications. In this work, Bi-decorated g-C3N4 nanosheets are successfully prepared by an in situ solvothermal method. The ultraviolet–visible diffuse reflection spectrum shows that Bi decoration conspicuously enhances the visible light absorption because of the surface plasmon resonance effect of the Bi semimetal. Moreover, the photoluminescence spectrum demonstrates that Bi decoration greatly improves the charge separation efficiency because the high work function of Bi (4.5 eV) favors electron transfer from the conduction band of g-C3N4 to the Bi surface. Meanwhile, nitrogen sorption isotherms show that Bi decoration obviously increases the Brunauer–Emmett–Teller surface area (4.3 m2 g–1 for g-C3N4 vs 7.7–8.4 m2 g–1 for Bi/g-C3N4), which is beneficial for N2 adsorption. Additionally, the density functional theory calculation shows that the adsorption energy (Ead) of N2 on Bi/g-C3N4 (−2.3 eV) is 6.2 times higher than that on g-C3N4 (−0.3 eV), which indicates that N2 is easier to adsorb on Bi/g-C3N4 thermodynamically. It is assumed that the decorated Bi can also provide new active sites that significantly improve the adsorption and activation of N2. Therefore, Bi/g-C3N4 exhibits a high nitrogen photoreduction activity under visible light irradiation (λ > 420 nm): the ammonia yield over 30-Bi/g-C3N4 (3075 μmol L–1 g–1) is 1.8 times higher than that over g-C3N4 (1076 μmol L–1 g–1) and the ammonia production rate over 30-Bi/g-C3N4 (1025 μmol L–1 g–1 h–1) is 1.8 times higher than that over g-C3N4 (359 μmol L–1 g–1 h–1). The improved activity is mainly attributed to the Bi decoration on the g-C3N4 surface, which improves the adsorption and activation of N2, the electron–hole separation efficiency, and the light-harvesting ability. This work demonstrates that similar to precious metals, Bi semimetal can also greatly improve the nitrogen photoreduction activity. Most importantly, Bi semimetal is more abundant and cheaper than precious metals, and it could be more promising for practical applications.