Graphitic carbon nitrides (CNx) have received enormous attention as a metal-free photocatalyst because of their visible-light photoactivity, high chemical stability, and eco-friendliness. Particularly, CNx is attractive for photoreductive catalysis due to the higher conduction band-edge position compared to other semiconductors. Despite the intriguing characteristic, their applications have been limited by their insufficient photoactive spectrum, inherent n-type characteristics, and hydrophobic texture. Here, we present the covalent conjugation of catechol to the surface of CNx through the 1,3-dipolar cycloaddition of azomethine ylides, generating a catechol-CNx conjugate (Cat-CNx). The catechol conjugation widens the photoactive spectrum via the larger range delocalization of π-electrons. The electron attracting property of catechol/quinone pairs induces the accumulation of photo-excited electrons at the surface of the conjugates, increasing electron transfer through the conjugated catechol while suppressing the charge recombination in the CNx. Accordingly, Cat-CNx produces a 6.3 times higher reductive photocurrent density than pristine CNx. The conjugate is investigated as a photoactive adsorbent for urban mining, the recovery of precious metals from waste resources. Gold ion reduction dramatically increased due to the enhanced electron transfer activity of Cat-CNx, catechols’ inherent hydrophilicity, and metal chelating property. Cat-CNx exhibits a 4.3 times higher maximum adsorption capacity for gold ions under simulated sun light illumination compared to pristine CNx under the same condition.

Catechol-conjugated graphitic carbon nitride with enhanced photoreductivity for environmental applications

ACS Spring Meeting 2021

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