2D MXene-derived Nb2O5/C/Nb2C/g-C3N4 heterojunctions for efficient nitrogen photofixation

Abstract

Due to the rather high 940.95 kJ mol⁻¹ thermodynamic cleavage energy of the N–N triple bond, developing a robust catalytic process for N₂ reduction under mild conditions is a continuing scientific challenge. Here, 2D MXene-derived niobium pentoxide/carbon/niobium carbide/graphite-like carbon nitride (Nb₂O₅/C/Nb₂C/g-C₃N₄) heterojunctions were explored as photocatalysts for N₂ reduction in water. Nb₂O₅/C/Nb₂C/g-C₃N₄ heterojunctions were prepared by uniformly growing Nb₂O₅ on Nb₂C and then forming g-C₃N₄ nanosheets in situ on Nb₂O₅/C/Nb₂C. With an optimized Nb₂O₅/C/Nb₂C : g-C₃N₄ ratio of 1 : 1, Nb₂O₅/C/Nb₂C/g-C₃N₄ showed a high nitrogen reduction rate (0.365 mmol h⁻¹ g_cat⁻¹), which was 9.1 times as high as that of the MXene derived Nb₂O₅/g-C₃N₄ composite. It is worth mentioning that the enhanced performance of Nb₂O₅/C/Nb₂C/g-C₃N₄ should be attributed to the enhancement in photogenerated electron and hole separation efficiency caused by the short-range directional charge transmission over the close contact between Nb₂O₅ and conductive Nb₂C as well as the Schottky junction formed at the Nb₂O₅/Nb₂C interface. By further changing the pH of the catalytic system, the concentration of reactive electrons was adjusted and the energy barrier of proton reduction was improved. With the optimized pH of 9 adjusted with NaOH solution, the nitrogen reduction efficiency could be further promoted 2.5 times (0.927 mmol h⁻¹ g_cat⁻¹).