Citation: Safeen, K.; Safeen, A.; Arif, D.; Shah, W.H.; Ali, A.; Ali, G.; Hussain, F.; Imran, N.; Ullah Shah, A.; Alataway, A.; et al. Tuning the Optical Properties of ZnO by Co and Gd Doping for Water Pollutant Elimination. Water 2023, 15, 1470. https://doi.org/10.3390/w15081470 Academic Editor: Constantinos V. Chrysikopoulos Received: 2 March 2023 Revised: 3 April 2023 Accepted: 6 April 2023 Published: 9 April 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/licenses/by/ 4.0/). water Article Tuning the Optical Properties of ZnO by Co and Gd Doping for Water Pollutant Elimination Kashif Safeen 1 , Akif Safeen 2, * , Danish Arif 2 , Wiqar H. Shah 3 , Akhtar Ali 3 , Ghafar Ali 4 , Fayaz Hussain 5 , Naveed Imran 6 , Atta Ullah Shah 7 , Abed Alataway 8 , Ahmed Z. Dewidar 8,9 , Hosam O. Elansary 8,10, * , Mohamed Al-Yafrsi 10 and Khawaja Shafique Ahmad 11 1 Department of Physics, Abdul Wali Khan University Mardan, Mardan 23200, Pakistan 2 Department of Physics, University of Poonch Rawalakot, Rawalakot 12350, Pakistan 3 Department of Physics, International Islamic University H-10, Islamabad 44000, Pakistan 4 Nanomaterials Research Group, Physics Division, PINSTECH, Islamabad 45650, Pakistan 5 Department of Materials Engineering, NED University of Engineering and Technology, Karachi 75270, Pakistan 6 HITEC Schools and Colleges, HIT, Taxila 47070, Pakistan 7 National Institute of Lasers and Optronics College Pakistan Institute of Engineering and Applied Sciences, Nilore, Islamabad 45650, Pakistan 8 Prince Sultan bin Abdulaziz International Prize for Water Chair, Prince Sultan Institute for Environmental, Water and Desert Research, King Saud University, Riyadh 11451, Saudi Arabia 9 Department of Agricultural Engineering, College of Food and Agriculture Sciences, King Saud University, Riyadh 11451, Saudi Arabia 10 Plant Production Department, College of Food & Agriculture Sciences, King Saud University, Riyadh 11451, Saudi Arabia 11 Department of Botany, University of Poonch Rawalakot, Rawalakot 12350, Pakistan * Correspondence: akifsafeen@upr.edu.pk (A.S.); helansary@ksu.edu.sa (H.O.E.); Tel.: +92-5824-960217 (A.S.) Abstract: In the present study, pure, Co, Gd, and Co/Gd di-doped ZnO nanoparticles were synthe- sized via the co-precipitation synthesis route. The prepared samples were characterized through different techniques such as the X-ray diffraction method (XRD), scanning electron microscopy (SEM), UV-Vis spectroscopy, photoluminescence (PL)spectroscopy, and an impedance analyzer and vibrating sample magnetometer (VSM). The XRD pattern shows ZnO’s wurtzite hexagonal crystal structure; moreover, the shifting of characteristic peaks toward the lower angle indicates the inclusion of Co and Co/Gd in the ZnO host lattice. SEM micrographs show various morphologies such as rods, the agglomeration of particles, and spherical nanoparticles. The UV-Vis spectroscopy reveals that the absorption increased in the visible region and there was a substantial redshift for the doped samples. The bandgap decreased from 3.34 to 3.18 eV for the doped samples. The PL spectra show near-edge and inter-band transitions; the origin of inter-band transitions is attributed to the defect states present within the bands. The dielectric constant is strongly frequency dependent and decreases with Co and Co/Gd doping, while the electrical conductivity increases. A VSM study indicates that pure ZnO is diamagnetic, while the Co and Co/Gd doped ZnO nanoparticles showed ferromagnetic behavior. Under UV-visible light irradiation, the Co/Gd-ZnO nanoparticles showed higher photocatalytic activity than the ZnO nanoparticles. The enhanced photocatalytic activity may be attributed to a decreased bandgap with doping. Keywords: Co/Gd-doped ZnO nanoparticles; optical properties; photodegradation 1. Introduction The chemical industry’s significant growth has led to the release of increasingly large quantities of resistant dye effluent into water resources, causing ecological problems. Dye- contaminated wastewater adversely affects marine life and also poses a threat to human health and well-being. Therefore, researchers worldwide are interested in finding efficient Water 2023, 15, 1470. https://doi.org/10.3390/w15081470 https://www.mdpi.com/journal/water