An efficient metal-free photocatalytic system with enhanced activity for NADH regeneration

Photocatalysis is a green and sustainable technique for converting solar energy into energy-storage molecules. A novel type of metal-free photocatalyst consisting of polydopamine (PDA) surface-functionalized graphitic carbon nitride (g-C3N4) was fabricated to enable the highly efficient visible-light-driven photocatalytic production of reduced nicotinamide adenine dinucleotide (NADH) through the photomediated reduction of NAD+. The photocatalytic performance and kinetic processes were investigated to evaluate the efficiency of NADH regeneration by the PDA/g-C3N4 photocatalysts. It was shown that the PDA layer thus formed could promote the charge carrier transfer of g-C3N4 to increase the solar-to-chemical conversion efficiency under visible light irradiation. A photocurrent density three times higher than that of pristine g-C3N4 was obtained. The photocatalytic activity of the PDA/g-C3N4 photocatalysts appeared to be dependent on the pH, with an increase in the pH from 3.0 to 9.5 substantially enhanced the NADH yield. The 20 wt % PDA/g-C3N4 exhibited remarkably improved photocatalytic NADH regeneration efficiency, achieving an NADH yield of 94.7% with an initial reaction rate of 2.4 mmol.h(-1).g(-1), which is approximately six times higher than that of pristine g-C3N4. Our study thus identifies a promising photocatalyst for enhancing the photocatalytic efficiency for nicotinamide cofactor regeneration.