Nano-mineral induced nonlinear optical LiNbO3 with abundant oxygen vacancies for photocatalytic nitrogen fixation: boosting effect of polarization

Abstract

Defective crystalline semiconductor plays a significant role in photocatalytic nitrogen fixation, however the rapid recombination of photoexcited electron/hole pairs largely reduces the efficiency. Spontaneous polarization of the nonlinear optical (NLO) material has emerged as a promising strategy for addressing the problem. Herein, a defects-rich NLO crystalline LiNbO3 modulated by natural nano-mineral palygorskite (LiNbO3/Pal) has been synthesized via sol–gel method. The intrinsic polarization of LiNbO3/Pal is investigated by second harmonic generation (SHG) measurements, which indicates an intensity of about 3.1 times that of typical NLO material KH2PO4 (KDP). The strong polarization can facilitate the separation of photogenerated charge carriers in the bulk phase and on the surface of LiNbO3/Pal. The existence of oxygen vacancies in LiNbO3/Pal may originate from doping of metal ions and active groups on the surface of Pal during crystallization of LiNbO3, which shortens the bandgap of LiNbO3 and facilitates the absorption and activation of N2 molecules. As a result, the polarization effect and rich oxygen vacancies make the LiNbO3/Pal an exceptional photocatalyst for N2 fixation. The 40 wt% LiNbO3/Pal composite achieves an average rate of photocatalytic NH4+ production of 52.57 μmol gcat−1 h−1 under simulated solar light, which is about sixfold higher than that of bulk LiNbO3. This work not only presents a new perspective on mineral modification of NLO material but also offers a cost-effective photocatalyst for nitrogen fixation.