Citation: Hussain, A.; Lin, C.; Cheruiyot, N.K.; Huang, W.-Y.; Lin, K.-S.; Hussain, A. Bismuth Sulfide Doped in Graphitic Carbon Nitride Degrades Nitric Oxide under Solar Irradiation. Nanomaterials 2022, 12, 3482. https://doi.org/10.3390/ nano12193482 Academic Editors: Alessia Irrera and Antonio Alessio Leonardi Received: 16 September 2022 Accepted: 3 October 2022 Published: 5 October 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/). nanomaterials Article Bismuth Sulfide Doped in Graphitic Carbon Nitride Degrades Nitric Oxide under Solar Irradiation Adnan Hussain 1 , Chitsan Lin 2,3, * , Nicholas Kiprotich Cheruiyot 4,5 , Wen-Yen Huang 3 , Kuen-Song Lin 6, * and Abrar Hussain 6 1 Institute of Aquatic Science and Technology, National Kaohsiung University of Science and Technology, Kaohsiung 811213, Taiwan 2 Ph.D. Program in Maritime Science and Technology, College of Maritime, National Kaohsiung University of Science and Technology, Kaohsiung 81157, Taiwan 3 Department of Marine Environmental Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 81157, Taiwan 4 Super Micro Mass Research and Technology Center, Cheng Shiu University, Kaohsiung City 8333031, Taiwan 5 Center for Environmental Toxin and Emerging-Contaminant Research, Cheng Shiu University, Kaohsiung City 8333031, Taiwan 6 Department of Chemical Engineering and Materials Science, Environmental Technology Research Center, Yuan Ze University, Taoyuan City 32003, Taiwan * Correspondence: ctlin@nkust.edu.tw (C.L.); kslin@saturn.yzu.edu.tw (K.-S.L.); Tel.: +886-07-3617141 (C.L.); +886-3-4638800 (K.-S.L.) Abstract: This study developed and examined the application of bismuth sulfide doped on graphitic carbon nitride (Bi 2 S 3 @g-C 3 N 4 ) in the degradation of NO under solar irradiation. Bi 2 S 3 @g-C 3 N 4 was prepared through the calcination method. The morphological structure and chemical properties of the synthesized photocatalyst were analyzed before the degradation tests. After doping with Bi 2 S 3 @g-C 3 N 4 , the bandgap was reduced to 2.76 eV, which increased the absorption of solar light. As a result, the Bi 2 S 3 @g-C 3 N 4 achieved higher NO degradation (55%) compared to pure Bi 2 S 3 (35%) and g-C 3 N 4 (45%). The trapping test revealed that the electrons were the primary species responsible for most of the NO degradation. The photocatalyst was stable under repeated solar irradiation, maintaining degradation efficiencies of 50% after five consecutive recycling tests. The present work offers strong evidence that Bi 2 S 3 @g-C 3 N 4 is a stable and efficient catalyst for the photocatalytic oxidation of NO over solar irradiation. Keywords: air pollution; heterojunction photocatalyst; solar light degradation; thermal decomposition 1. Introduction Air pollutants that are highly reactive include nitrogen oxides (NO x ), usually caused by anthropogenic activities, particularly fuel combustion [1]. There are two approaches to controlling NO x emissions: primary measures to prevent NO x formation and secondary measures to reduce the NO x already formed [2]. Common primary measures include staged combustion [3] and exhaust gas recirculation [4], while secondary measures include the adoption of pollution control devices including selective catalytic reduction [5] and photocatalysts [6]. Photocatalytic oxidation (PCO), one of the most effective and easiest secondary control technologies for the removal of NO x emissions, has been researched in the last twenty years [7–9]. Titanium dioxide (TiO 2 ) has been frequently used as a photocatalyst in NO degradation [10] because of its long-term durability, non-toxicity, PCO activity, and long-term photostability [11,12]. However, it has several limitations including a large band gap of 3.0–3.2 eV, and it is only effective under UV light at a wavelength of <380 nm [13,14]. The graphitic carbon nitride (g-C 3 N 4 ) semiconductor is a polymeric narrow-band-gap metal-free material that is functional under solar light and has gained increasing popular- Nanomaterials 2022, 12, 3482. https://doi.org/10.3390/nano12193482 https://www.mdpi.com/journal/nanomaterials