Citation: Allali, M.; Dahamni, M.A.; Ghamnia, M.; Boukhachem, A.; Boukrédimi, D.; Tonneau, D.; Fauquet, C. Synthesis and Investigation of Pure and Cu-Doped NiO Nanofilms for Future Applications in Wastewater Treatment Rejected by Textile Industry. Catalysts 2022, 12, 931. https://doi.org/10.3390/ catal12090931 Academic Editor: John Vakros Received: 31 July 2022 Accepted: 15 August 2022 Published: 23 August 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 Synthesis and Investigation of Pure and Cu-Doped NiO Nanofilms for Future Applications in Wastewater Treatment Rejected by Textile Industry Malika Allali 1 , Mohamed Amine Dahamni 1 , Mostefa Ghamnia 1, * , Abdelwahab Boukhachem 2 , Djamel Boukrédimi 3 , Didier Tonneau 4 and Carole Fauquet 4 1 Laboratoire des Sciences de la Matière Condensée (LSMC), Université Oran 1 Ahmed Ben Bella, Oran 31100, Algeria 2 Laboratoire de Nanomatériaux et Systèmes pour Energies Renouvelables, Centre de Recherches et des Technologies de l’Energie, Technopôle de Borj-Cédria, BP 95 Hammam-Lif, Tunis 2050, Tunisia 3 Institut des Sciences et Techniques Appliquées (ISTA), Université Oran 1 Ahmed ben Bella, Oran 31100, Algeria 4 Centre Interdisciplinaire de Nanosciences de Marseille (CINaM), CNRS, Aix Marseille Université, 13288 Marseille, France * Correspondence: mghamnia@yahoo.fr Abstract: Pure and Cu-doped NiO films were synthesized via a soft chemical process. They were deposited on glass substrates heated to 400 ◦ C. Different atomic percentage ratios (2, 4, 6, 8, and 10%) of Cu-doping were used. The prepared samples were characterized by several techniques such as X-ray diffraction for crystallographic study, SEM and AFM for microstructural and morphological properties, and UV-Visible spectroscopy for optical and photocatalytical analysis. XRD results of pure and Cu-doped NiO films indicated the formation of NiO polycrystalline phases under a cubic structure with a favored orientation along the (200) plane noticed in all sprayed films. SEM images revealed the formation of NiO nanoparticles of spherical forms whose sizes increase and agglomerate with increasing Cu-doping. At 10% Cu-doping, NiO agglomeration was extended to the whole surface. AFM images showed a textured and rough surface composed of NiO nanoparticles of average size varying from 16 to 10 nm depending on Cu-doping concentration. UV-visible spectroscopy confirmed the transparency of NiO films and their semiconducting character with a band gap ranging from 3.4450 eV to 2.8648 eV. The photocatalytical properties of pure and Cu-NiO films were enhanced by Cu-doping particles as revealed by the degradation of methylene blue (MB) solution subjected to irradiation. Keywords: NiO synthesis; spray pyrolysis; Cu-doping; nanograins; photocatalytic properties 1. Introduction Transparent Conducting Oxides (TCO) such as ZnO, TiO 2 , SnO 2 , CuO, etc., continue to receive considerable attention due to their outstanding electrical, magnetic, and catalytic properties [1–4]. They are essential compounds for the development of ultra-high frequency components, gas sensors, optoelectronics, and batteries [5–7]. As the textile industries are disposing of unused direct dyes into the aquatic environment, which is posing a serious, alarming threat to aquatic life, recent studies are increasingly interested in wastewater treatment because the nanofilms of these materials allow the photodegradation of dyes rejected by the textile industry [8–11]. As a reminder, dyes are aromatic organic compounds which have the property of absorbing light and giving color in the visible region. This important property is exploited in many industries such as textiles, food, cosmetics, medicine, etc., which generate a huge amount of wastewater containing toxic dyes that pollute the environment. Among the various dyestuffs, methylene blue (MB) is the most commonly used in the textile industry Catalysts 2022, 12, 931. https://doi.org/10.3390/catal12090931 https://www.mdpi.com/journal/catalysts