Metal Single Atom-Hydroxyl Incorporation in Poly(heptazine imide) to Create Active Sites for Photocatalytic Water Oxidation

Poly(heptazine imide) (PHI) salts are extensively researched crystalline carbon nitride photocatalysts, but their photocatalytic water oxidation (PWO) performance is scarcely researched because of the difficulty in creating efficient active sites. Interference of metal ion (e.g., Na⁺ and K⁺) loss from the PHI salts in their PWO research has hardly been considered. Herein, metal single atom─OH (e.g., Co─OH) groups are incorporated into PHI to create efficient PWO active sites, via simple ion metathesis, hydrolysis, and deprotonation. The Co─OH modified PHI exhibits 9.3-fold higher PWO (oxygen evolution) activity than PHI, with an external quantum yield reaching 0.44% even at 600 nm. Excluding interference of the metal ion loss, the function of the Co─OH incorporation is evidenced mainly to facilitate the oxygen evolution reaction, as well as to promote photogenerated charge separation and raise visible light absorption, with the role of the OH especially revealed. Moreover, it is discovered that Na⁺ loss from sodium PHI will decrease its PWO activity, protonation of PHI has a detrimental effect on its PWO performance, and some other metal single atom─OH incorporation in PHI can also enhance its PWO activity. Overall, this work provides a general way to create PWO active sites in PHI.