Highly efficient Cr(VI) photoreduction by C/N vacancies and hydroxyl co-modified g-C3N4: The new insight into the key role of citric acid

Photocatalysis is an energy-efficient and environment-friendly method to reduce Cr(VI) in water. The rapid spatial separation of photogenerated electrons and holes is still full of challenge. Microenvironment regulation of photocatalyst is a promising strategy. In this work, defined C- and N-vacancies were simultaneously successfully introduced to g-C3N4, accompanied by surface hydroxyl modification. Unlike traditional bulk g-C3N4, constructing C-/N- defects and surface hydroxyl can significantly improve the surface-interface microenvironment, thus leads to the enhanced photocurrent density and efficient charge separation. In a citric acid involved solution, the apparent rate constant of CN-160 for Cr(VI) photoreduction under visible light irradiation (λ ≥420 nm) reaches up to 2.02×10−2 min−1, which is 3.89-fold higher compared to bulk g-C3N4. While there is almost no photoactivity in the absence of citric acid. The experimental and DFT calculations results show that C- and N-vacancies and surface OH can significantly promote the charge transfer from g-C3N4 to citric acid, and subsequently to Cr(VI). Most importantly, citric acid not only plays a vital role of charge transfer bridge between Cr(VI) and g-C3N4, but also provides H proton to activate Cr(VI) as well as adjusts the solution pH values. All of these finally facilitate the photoreduction activity of Cr(VI). This study firstly demonstrates the definite role of citric acid during photocatalysis and provides a deep insight into the microenvironment regulation on the effect of charge separation.