Remarkable Photocatalytic H2O2 Production Efficiency over Ultrathin g-C3N4 Nanosheet with Large Surface Area and Enhanced Crystallinity by Two-Step Calcination

Abstract

The generation of hydrogen peroxide (H₂O₂) from water and oxygen redox reaction by photocatalysis has acquired increasing attention owing to its green and clean properties. Aiming at the low intrinsic photocatalytic activity of carbon nitride (g-C₃N₄), here, an ultrathin g-C₃N₄ nanosheet photocatalyst with a large surface area and enhanced crystallinity was fabricated by a two-step thermal polymerization technique. The calcination parameters showed a significant impact on the structural properties and catalytic performance of g-C₃N₄. The remarkable H₂O₂ yield (3177.0 μmolꞏg⁻¹ꞏh⁻¹) of CN-T-1 (by two-step calcination, 1 °Cꞏmin⁻¹ optimal heating rate) was 3.7 times that (858.6 μmolꞏg⁻¹ꞏh⁻¹) of CN-O-1 (by one-step calcination, 1 °Cꞏmin⁻¹ heating rate) and higher than those of pure g-C₃N₄ in literature. Most of the H₂O₂ yield for CN-T-1 remained after five cycles, showing good stability. The enhanced catalytic performance of CN-T-1 than CN-O-1 is owing to its larger specific surface area, enhanced crystallinity, higher oxygen adsorption ability and photogenerated carrier separation efficiency, longer lifetime of carriers, and slightly larger bandgap (3.07 eV, +0.26 eV bigger than CN-O-1) with more positive valence band position owing to ultrathin layers. The $\cdot {\text{O}}_2^{-}$ radicals were verified to be the primary active species. A two-step single electron ORR pathway $({\text{O}}_2+{\text{e}}^{-}\to \cdot {\text{O}}_2^{-}\to {\text{H}}_2{\text{O}}_2)$ was confirmed for H₂O₂ production over CN-T-1.

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