Catalysts 2023, 13, 409. https://doi.org/10.3390/catal13020409 www.mdpi.com/journal/catalysts Article Biomass Mediated Synthesis of ZnO and ZnO/GO for the Decolorization of Methylene Blue under Visible Light Source Salma Ahmed Al-Zahrani 1, *, Khalid Umar 2, *, Saleh Ali Tweib 3 , Jebrel Abdeljawad M. Rashd 4,5 , Saima Khan Afridi 2 , Showkat Ahmad Bhawani 6 , Ahmed Al Otaibi 1 , Najat Masood 1 , Dorsaf Mansour 1 , Anish Khan 7 and Manikandan Ayyar 8 1 Chemistry Department, Faculty of Science, University of Ha’il, P.O. Box 2440, Ha’il 81451, Saudi Arabia 2 School of Chemical Sciences, Universiti Sains Malaysia, Gelugor 11800, Pulau Pinang, Malaysia 3 Chemical and Process Engineering, Zawia Higher Institute of Science and Technology-Central Zawia, Msehel 201300, Libya 4 School of Industrial Technology, Universiti Sains Malaysia, Gelugor 11800, Pulau Pinang, Malaysia 5 Chemical Engineering Department, Higher Institute of Science & Technology. Souq Khamis Emsihel, Tripoli 199002, Libya 6 Faculty of Resource Science and Technology, Universiti Malaysia Sarawak, Kota Samarahan 94300, Sarawak, Malaysia 7 Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia 8 Department of Chemistry, Bharath Institute of Higher Education and Research (BIHER), Bharath University, Chennai 600073, Tamil Nadu, India * Correspondence: s.alzahrane@uoh.edu.sa (S.A.A.-Z.); khalidumar@usm.my (K.U.); Tel.: +60-46533567 (K.U.) Abstract: In this study, zinc oxide (ZnO) as well as ZnO/GO (zinc oxide/graphene oxide) were successfully synthesized. The Carica papaya leaf extract was used to synthesize ZnO and oil palm empty fruit bunch biomass to obtain graphene, which was further used to obtain graphene oxide. The samples were characterized through a variety of analytical methods such as scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, Fourier transform infra- red spectroscopy and UV–Visible spectroscopy in order to understand their morphology, size, structural phase purity, functional groups and optical properties. Various peaks such as O-H, Zn-OH and Zn-O were found in the case of ZnO. Some additional peaks, such as C-C and C=C, were also been detected while analyzing the sample by Fourier-transform infrared spectroscopy. The results of the XRD and SEM studies demonstrated that the synthesized material shows the crystalline nature of the substance in the case of ZnO, and the crystallinity decreases for ZnO/GO. The average crystallite size was found to 80.0 nm for ZnO and 74.0 nm for ZnO/GO. Further, a red shift was shown in the case of ZnO/GO, which was indicated by the UV–Vis absorption spectrum. In the TEM analysis, the particles were shown to be nanosized. For instance, the highest number of particles was found in the range of 100 to 120 nm in the case of ZnO, while 80–100 nm sized parti- cles were found for ZnO/GO. Using synthesized ZnO and ZnO/GO, the decolorization of meth- ylene blue was found to be 64% and 91%, respectively. Keywords: ZnO; ZnO/GO; photocatalyst; photodecolorization; green synthesis; methylene blue 1. Introduction Water pollution has threatened the survival of human beings, as many industries have been expelling harmful substances into the environment in recent decades [1–3]. Even though chemical compound-based companies have played an important role in modern life (especially those that make dyes, insecticides, pharmaceuticals, etc.), a lot of waste is thrown into the environment by them, which potentially disturbs our ecosystem Citation: Al-Zahrani, S.A.; Umar, K.; Tweib, S.A.; Rashd, J.A.M.; Afridi, S.K.; Bhawani, S.A.; Otaibi, A.A.; Masood, N.; Mansour, D.; Khan, A.; et al. Biomass Mediated Synthesis of ZnO and ZnO/GO for the Decolorization of Methylene Blue under Visible Light Source. Catalysts 2023, 13, 409. https://doi.org/10.3390/catal13020409 Academic Editors: Fengxia Deng and Xiaoxiao Zhang Received: 11 December 2022 Revised: 30 January 2023 Accepted: 09 February 2023 Published: 14 February 2023 Copyright: © 2023 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/license s/by/4.0/).