Ni2P-Modified Ta3N5 and TaON for Photocatalytic Nitrate Reduction

Abstract

Self-sustaining photocatalytic NO3− reduction systems could become ideal NO3− removal methods. Developing an efficient, highly active photocatalyst is the key to the photocatalytic reduction of NO3−. In this work, we present the synthesis of Ni2P-modified Ta3N5 (Ni2P/Ta3N5), TaON (Ni2P/TaON), and TiO2 (Ni2P/TiO2). Starting with a 2 mM (28 g/mL NO3−−N) aqueous solution of NO3−, as made Ni2P/Ta3N5 and Ni2P/TaON display as high as 79% and 61% NO3− conversion under 419 nm light within 12 h, which correspond to reaction rates per gram of 196 μmol g−1 h−1 and 153 μmol g−1 h−1, respectively, and apparent quantum yields of 3–4%. Compared to 24% NO3− conversion in Ni2P/TiO2, Ni2P/Ta3N5 and Ni2P/TaON exhibit higher activities due to the visible light active semiconductor (SC) substrates Ta3N5 and TaON. We also discuss two possible electron migration pathways in Ni2P/semiconductor heterostructures. Our experimental results suggest one dominant electron migration pathway in these materials, namely: Photo-generated electrons migrate from the semiconductor to co-catalyst Ni2P, and upshift its Fermi level. The higher Fermi level provides greater driving force and allows NO3− reduction to occur on the Ni2P surface.