Effective photocatalytic chromium (VI) detoxification with metal-free heterojunction based on g-C3N5 and ZIF-8 carbon under visible light

Abstract

A metal-free heterojunction (CNZ8C) photocatalyst prepared using g-C₃N₅ (CN) and zeolitic imidazolate framework (ZIF)-derived carbon (Z8C) is reported for sequential reduction and elimination of toxic Cr (VI) from water. Recombination of photogenerated charge carriers in CN was evicted completely through heterojunction formation of CN with Z8C. This is evidenced from the transient photocurrent responses (TPR), photoluminescence (PL) measurements and electrochemical impedance spectroscopy (EIS). The band structure analysis from the valence band X-ray spectroscopy and diffused reflectance spectroscopy revealed that the synergy in the synthesized CN-Z8C heterojunction is responsible for the reduction of CN band gap. The experimental results obtained confirm that the positions of the valence and conduction bands are aligned to more negative values facilitating the Cr (VI) reduction. Incorporation of the highly conducting porous CZ8 to CN improves the charge separation and also brings the Cr (VI) ions close to CZ8 surface via adsorption, which together favors the overall reduction and removal of reduced ions. The rate of reduction/removal significantly increased with CNZ8C 5:5 (with nearly 100 % removal efficiency) heterojunction compared to that of CN. This heterojunction can be reutilized up to 5 cycles without any significant loss in efficiency. Mechanistic studies revealed that proper bandgap engineering leads to increased reduction and removal of chromium. The post treatment X-ray photoelectrons spectroscopy of the heterojunction confirms the +3 state of chromium. These observations suggest that the successful synergy between the CN and Z8C will bring new insights into chromium remediation.