Citation: Venkatesvaran, H.; Balu, S.; Chowdhury, A.; Chen, S.-W.; Yang, T.C.-K. Photo–Redox Properties of –SO 3 H Functionalized Metal-Free g-C 3 N 4 and Its Application in the Photooxidation of Sunset Yellow FCF and Photoreduction of Cr (VI). Catalysts 2022, 12, 751. https:// doi.org/10.3390/catal12070751 Academic Editor: Caue Ribeiro Received: 14 June 2022 Accepted: 5 July 2022 Published: 7 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). catalysts Article Photo–Redox Properties of –SO 3 H Functionalized Metal-Free g-C 3 N 4 and Its Application in the Photooxidation of Sunset Yellow FCF and Photoreduction of Cr (VI) Harikrishnan Venkatesvaran 1 , Sridharan Balu 1,2 , Anuradha Chowdhury 1 , Shih-Wen Chen 1,2 and Thomas C.-K. Yang 1,2, * 1 Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 106, Taiwan; harikrishnan.osv@gmail.com (H.V.); bsridharanbsc.12@gmail.com (S.B.); anuradhachowdhary24@gmail.com (A.C.); shihwen@mail.ntut.edu.tw (S.-W.C.) 2 Precision Analysis and Materials Research Centre, National Taipei University of Technology, Taipei 106, Taiwan * Correspondence: ckyang@mail.ntut.edu.tw Abstract: In this work, we synthesized a metal-free sulfonic functionalized graphitic carbon nitride using sulfuric acid through the wet impregnation technique. The functionalization of sulfonic groups (–SO 3 H) on g-C 3 N 4 will promote a high surface charge density and charge separation owing to its high electronegativity. The g-C 3 N 4 –SO 3 H shows excellent optical/electronic and surface properties towards enhanced photo–redox reactions. The sulfonic groups also facilitate the availability of more separated charge carriers for photocatalytic oxidation and reduction reactions. The as- synthesized material has been characterized by different spectroscopic tools to confirm the presence of functionalized –SO 3 H groups and optoelectronic possessions. The photocatalytic responses of g-C 3 N 4 – SO 3 H result in 99.56% photoreduction of Cr (VI) and 99.61% photooxidation of Sunset Yellow FCF within 16 min and 20 min, respectively, of visible light irradiation. The g-C 3 N 4 –SO 3 H catalyst exhibits a high apparent rate constant (K app ) towards the degradation of Cr (VI), and SSY, i.e., 0.783 min −1 and 0.706 min −1 , respectively. The intense optical–electrochemical properties and potentially involved active species have been analyzed through transient photocurrent, electrochemical impedance, and scavenging studies. Consequently, the photocatalytic performances are studied under different reaction parameters, and the plausible photocatalytic mechanism is discussed based on the results. Keywords: metal-free graphitic carbon nitride; sulfonic groups; wet impregnation; photo–redox; Cr (VI); SSY FCF; photocatalysis 1. Introduction As we human beings tend to have the urge towards innovation and exploration in various fields, we are leaving behind a lot of detrimental effects on future generations via numerous hazardous outputs. We are responsible for making the world a better place to live. The major menaces to health are present in water and food products that entered through several insignificant ways and resulted in huge hazards. Nowadays, heavy metal contaminants and food additive poisoning are dominant in these environmental and health death traps. Heavy metals are widely utilized in chemical industries, mining, paint manufacturing, textile dyeing, and so on [1]. Among these, the usage of chromium in chrome electroplating [2] and leather tanning [3] leads to an extensive release of this particular heavy metal into the environment and causes severe ecological pollution. In general, chromium exists in two different forms as trivalent [Cr(III)] and hexavalent [Cr (VI)] species; the former does not have adverse effects [4] and is immobile and less soluble and not widely absorbed by human cells. However, the hexavalent species of chromium (Cr 6+ ) is mobile, soluble, and bioavailable under oxidizing conditions and can Catalysts 2022, 12, 751. https://doi.org/10.3390/catal12070751 https://www.mdpi.com/journal/catalysts