Constructing quantum dots@flake g-C₃N₄ isotype heterojunctions for enhanced visible-light-driven NADH regeneration and enzymatic hydrogenation

NAD(P)H is a critical cofactor (biological source of hydrogen) that participates in many enzymatic hydrogenations for energy conversion and storage with resultant oxidation to NAD(P)+. Due to its high cost, the regeneration of NAD(P)H is a critical and feasible way to ensure the sustainability of these enzymatic hydrogenations. Intrigued by the photoreaction process in thylakoid membrane, we explored a two-dimensional (2D) isotype heterojunction photocatalyst, termed as quantum dots@flake graphitic carbon nitride (QDs@Flake g-C3N4), for visible-light-driven NAD(P)H regeneration. The catalyst was synthesized by one-step calcination using cyanamide-treated cyanuric acid-melamine (CM) complex as starting material, where cyanamide plays dual roles: a) assisting the transformation of CM from bulky to stacked structure and further to flakes after calcination, and b) acting as raw material for the generation of QDs on the flakes. To replicate both the functional and structural properties of the natural photoreaction system, QDs@Flake g-C3N4 exploited the two types of g-C3N4 (i.e., QDs and flake) and a heterojunction interface to, respectively, mimic the functional components of light-harvesting systems (LHSs, i.e., PS I and PS II) and electron transport chains (ETCs), and utilized the flake structure as the analogue of 2D thylakoid membrane. Therefore, QDs@Flake g-C3N4 showed remarkably improved capability in visible light harvesting and charge separation, and exhibited elevated performance in photocatalytic NADH regeneration with a regeneration yield of up to 40%. The NADH regeneration approach was then coupled with alcohol dehydrogenase-catalyzed hydrogenation of formaldehyde, achieving continuous methanol production.