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

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 O-2. Moreover, as an "electron transfer bridge", Ni(I)-N active sites promote electron transfer from the catalyst to the adsorbed O-2 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 O-2 molecules with high adsorption energy and elongate the O=O bond, leading to its activation. Charge density difference results confirm the electron transfer from the Ni+ doping sites to the O-2 molecules. The Mulliken charge is -0.42 when O-2 is adsorbed on the Ni+ doping site, indicating that the O-2 molecule is enriched by a large number of electrons. This electron-rich environment is beneficial to the H+ attack to form H2O2.