  Citation: Abdelmadjid, K.; Gheorghiu, F.; Abderrahmane, B. Synthesis, Characterization, and Photocatalytic Activity of Ba-Doped BiFeO 3 Thin Films. Materials 2022, 15, 961. https://doi.org/10.3390/ ma15030961 Academic Editor: John T. Kiwi Received: 4 January 2022 Accepted: 24 January 2022 Published: 26 January 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/). materials Article Synthesis, Characterization, and Photocatalytic Activity of Ba-Doped BiFeO 3 Thin Films Khiat Abdelmadjid 1 , Felicia Gheorghiu 2, * and Boughelout Abderrahmane 1 1 Research Center in Industrial Technologies (CRTI), P.O. Box 64 Cheraga, Algiers 16014, Algeria; khiatmadjid2018@gmail.com (K.A.); abderrahmanebough@gmail.com (B.A.) 2 RAMTECH, Department of Exact and Natural Sciences, Institute of Interdisciplinary Research, Alexandru Ioan Cuza University of Iasi, Blvd. Carol I, nr. 11, 700506 Iasi, Romania * Correspondence: felicia.gheorghiu@uaic.ro Abstract: In the present paper, Bi 1−x Ba x FeO 3 (BBFO) thin films (where x = 0, 0.02 and 0.05) were prepared by a combined sol-gel and spin-coating method. The influence of Ba substitutions on the structural, microstructural, optical properties, and photocatalytic activity of BiFeO 3 thin films has been studied. X-ray diffraction pattern correlated with FTIR analysis results confirms that all the films have a perovskite structure of rhombohedral symmetry with an R3m space group. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to investigate the surface morphology and reveals microstructural modifications with the increase in Ba concentration. The optical properties show that the band gap is narrowed after doping with Ba ions and decreases gradually with the increase of doping content. The photocatalytic investigations of deposited films revealed that Ba doping of BFO material leads to the enhancement of photocatalytic response. The present data demonstrates that Bi 1−x Ba x FeO 3 (BBFO) thin films can be used in photocatalytic applications. Keywords: perovskites; films; optical properties; doped BaTiO 3 ; photocatalytic activity 1. Introduction Today, evolution of modern society leads to a continuous increasing of environmental pollution. Therefore, degradation of organic pollutants shows a great interest for photo- catalytic applications [1]. To date, several semiconductors can act as photocatalysts: TiO 2 , ZnO, Fe 2 O 3 , CdS, ZnS) [2–6]. TiO 2 is the most studied photocatalysts [2,7,8], but the wide band gap (3.2 eV) [8] provides low efficiency of absorption in the visible-light region. This problem leads to the searching for new oxide-based photocatalysts with strong absorbance for visible- light applications. Semiconductor-based photocatalysts are promising materials for degradation of a large number of organic contaminants. In the last years, the multiferroic materials have become an attractive research area due its interest in photocatalytic field [9,10]. The advantage of multiferroics in comparison with usual photocatalysts is their low band gap energy. Among multiferroic materials, perovskite BiFeO 3 (BFO) is one of the few and best known room—temperature multifer- roics, that has a distorted perovskite structure that belongs to rhombohedral R3c symmetry (with a = 3.958 and α = 89.30 ◦ ), with a high ferroelectric Curie temperature (T C = 830 ◦ C) and antiferromagnetic order (T N = 370 ◦ C) [11–13]. The preparations of BiFeO 3 powder material were reported by different synthesis methods, as for example: solid-state re- action method [14], hydrothermal synthesis route [15], sol-gel process [16] combustion method [17], Pechini method [18], and microwave synthesis [19]. However, the preparation of BiFeO 3 is very difficult due to: (i) The kinetics of phase formation in the Bi 2 O 3 -Fe 2 O 3 system that leads to the formation of impurities along with the perovskite major phase [20] and thermal stability; (ii) valence fluctuation of iron (Fe 3+ /Fe 2+ transitions) which leads to the variation of oxygen stoichiometry and appearance of the oxygen vacancies as defects; (iii) Bi volatilization because bismuth oxide has a low melting point. Therefore, in spite of Materials 2022, 15, 961. https://doi.org/10.3390/ma15030961 https://www.mdpi.com/journal/materials