Metal-free photocatalyst for nitrogen fixation under visible light based on COF/g-C3N4/ CNT nanocomposite

Rational design of efficient semiconductors for photocatalytic nitrogen fixation is an urgent challenge for artificial ammonia (NH3) production under mild condition. Recently, graphitic carbon nitride (g-C3N4, CN) shows fascinating potential in photocatalytic nitrogen fixation under visible light. However, compared with other catalysts, the low NH3 production with pure CN leads to the restriction of practical application. Herein, we covalently coupled covalent organic frameworks (Tptta-COF) and carboxylated carbon nanotubes (CNT) onto CN with amido-linkage and imine-linkage, respectively, through the hydrothermal reaction. Compared with the pristine CN, the obtained COF/CN/CNT achieved larger specific surface area, more visible light absorption and more efficient separation of the photo-generated carriers. The optimal catalyst (COF/CN/CNT) exhibited enhanced photocatalytic activity with NH3 conversion of 864 μmol·g−1 within 4 h under visible light illumination. Moreover, the introduction of COF and CNT results in a 6.20 times improved photocatalytic NH3 conversion rate (211 μmol·g−1·h−1) under visible light in contrast to that of the pure CN. The boosting NH3 production is due to the dual covalent decoration effect of COF and CNT. On the one hand, the COF introduction is favorable for the regulation of band gap, promoting specific surface area as well as N2 adsorption ability. On the other hand, the CNT introduction not only enhances optical light harvesting, but also accelerates the transfer of photo-excited electron, maintaining high efficient separation of photo-generated carriers. This dual covalent decoration, therefore, shows a potential as a feasible strategy of combining light-harvesting and charge transfer for efficient NH3 production.