Boosted photocatalytic nitrogen fixation by bismuth and oxygen vacancies in Bi2MoO6/BiOBr composite structures

Abstract

Photocatalytic nitrogen fixation performance is mainly hampered by slow carrier transport and inefficient surface reaction, where surface oxygen vacancies have been proved to alleviate these limitations. However, there are a few reports on the introduction of metal vacancies into photocatalysts and the effect of metal vacancies on the N₂ photofixing properties. Herein, we injected bismuth vacancies (V_Bi) into the surface of BiOBr nanospheres with oxygen vacancies (V_O) via an ion exchange strategy to form hierarchical Bi₂MoO₆/BiOBr composite structures. The intentionally introduced V_Bi adjust the band structures of V_O–BiOBr and act as charge separation centers in coordination with V_O, which improves the separation efficiency of electron–hole pairs. The presence of V_Bi and the Bi₂MoO₆ phase enhances the light absorption of the composite materials. Additionally, the hierarchical nanosheet assembly structure facilitates the surface adsorption and activation of N₂ on the catalyst. In particular, the optimal defect-rich Bi₂MoO₆/V_Bi+O–BiOBr exhibits the best photocatalytic ammonia production activity. After two hours, the NH₃ yield was 412.18 mol L⁻¹ without any noble metal cocatalyst and sacrificial agent, and was nearly 4 times higher than that of the original V_O–BiOBr (96.08 mol L⁻¹). This work provides a new inspiration for the design of efficient N₂ immobilizing photocatalysts through synergistic metal and oxygen vacancy engineering.