The effect of Ni(i)–N active sites on the photocatalytic H2O2 production ability over nickel doped graphitic carbon nitride nanofibers

Abstract

Hydrogen peroxide (H₂O₂) is a highly efficient and green oxidant because it has the highest content of active oxygen (47% w/w) and produces only H₂O as a by-product. In this work, nickel doped graphitic carbon nitride (g-C₃N₄) nanofibers with outstanding photocatalytic H₂O₂ production ability are prepared. The characterization results demonstrate that Ni is not only present as the oxide but inserts at the interstitial position through coordinative Ni(I)–N bonds. These Ni(I)–N bonds can act as chemical adsorption sites to activate molecular O₂. Moreover, as an “electron transfer bridge”, Ni(I)–N active sites promote electron transfer from the catalyst to the adsorbed O₂ molecules. The as-prepared nickel-doped g-C₃N₄ displays a much higher H₂O₂ equilibrium concentration and formation rate than neat g-C₃N₄ prepared by calcination without post-treatment does, as well as excellent structural stability. Density functional theory simulations show that Ni(I)–N active sites can adsorb O₂ molecules with high adsorption energy and elongate the OO bond, leading to its activation. Charge density difference results confirm the electron transfer from the Ni⁺ doping sites to the O₂ molecules. The Mulliken charge is −0.42 when O₂ is adsorbed on the Ni⁺ doping site, indicating that the O₂ molecule is enriched by a large number of electrons. This electron-rich environment is beneficial to the H⁺ attack to form H₂O₂.