In situ synthesis of N-doped TiO2 on Ti3C2 MXene with enhanced photocatalytic activity in the selective reduction of nitrate to N2

Abstract

Photocatalysis exhibits promise in the reduction of nitrates into harmless dinitrogen. Herein, the synthesis of N-doped TiO₂/Ti₃C₂ at a low calcination temperature using NH₄Cl and Ti₃C₂ was reported for the first time. Cationic NH₄⁺ was intercalated into the interlayers of Ti₃C₂ and NH₄Cl was self-assembled on the surface of Ti₃C₂. N-doped TiO₂/Ti₃C₂ was in situ synthesized by one-step calcining the Ti₃C₂ MXene and NH₄Cl mixture. The as-prepared composite exhibited excellent activity in the photocatalytic reduction of nitrate. As characterized, the N-dopant added an impurity energy level within the band gap of TiO₂, which was favourable for improving the light absorption ability. Meanwhile, the metallic nature of Ti₃C₂ with a layered structure accelerated the charge transfer between N-TiO₂ and Ti₃C₂, and further facilitated the separation of photogenerated charge carriers. Moreover, the addition of formic acid in the solution not only acted as a hole scavenger to produce the reducing agent ·CO₂⁻, but also provided extra reactive electrons by generating the charge-transfer-complex between N-TiO₂ and formic acid, which greatly promoted the photocatalytic reduction of nitrate. Notably, NO₃⁻ with Lewis basic property was easily adsorbed onto the Lewis acid sites on the surface of N-doped TiO₂/Ti₃C₂ by the electrostatic force. All the above-mentioned superiorities contributed to the greatly enhanced photocatalytic activity of the N-doped TiO₂/Ti₃C₂ composite in the reduction of nitrate. The mechanism has also been proposed based on the testing results.