Nanocasting synthesis of BiFeO3 nanoparticles with enhanced visible-light photocatalytic activity Thomas Cadenbach*1, Maria J. Benitez*2, A. Lucia Morales1, Cesar Costa Vera2, Luis Lascano2, Francisco Quiroz3, Alexis Debut4 and Karla Vizuete4 Full Research Paper Open Access Address: Beilstein J. Nanotechnol. 2020, 11, 1822–1833. 1Universidad San Francisco de Quito, Colegio de Ciencias e https://doi.org/10.3762/bjnano.11.164 Ingenierias, El Politécnico, Diego de Robles y Vía Interoceánica, 170901, Quito, Ecuador, 2Departamento de Física, Facultad de Received: 24 July 2020 Ciencias, Escuela Politécnica Nacional, Ladrón de Guevara E11-253, Accepted: 11 November 2020 Quito 170517, Ecuador, 3Departamento de Ciencia de los Alimentos y Published: 07 December 2020 Biotecnología DECAB, Escuela Politécnica Nacional, Ladrón de Guevara E11-253, Quito 170517, Ecuador and 4Centro de Associate Editor: W.-J. Ong Nanociencia y Nanotecnología, Universidad de las Fuerzas Armadas ESPE, Av. Gral. Rumiñahui s/n, Sangolquí, PO Box 171-5-231B, © 2020 Cadenbach et al.; licensee Beilstein-Institut. Ecuador License and terms: see end of document. Email: Thomas Cadenbach* - [email protected]; Maria J. Benitez* - [email protected] * Corresponding author Keywords: bismuth ferrite (BiFeO3); dye; nanocasting; nanoparticles; photocatalysis; rhodamine B; SBA-15 Abstract In this work, monodisperse BiFeO3 nanoparticles with a particle diameter of 5.5 nm were synthesized by a nanocasting technique using mesoporous silica SBA-15 as a hard template and pre-fabricated metal carboxylates as metal precursors. To the best of our knowledge, the synthesized particles are the smallest BiFeO3 particles ever prepared by any method. The samples were character- ized by X-ray powder diffraction, transmission electron microscopy and UV–vis diffuse reflectance spectroscopy. The phase purity of the product depends on the type of carboxylic acid used in the synthesis of the metal precursors, the type of solvent in the wet impregnation process, and the calcination procedure. By using tartaric acid in the synthesis of the metal precursors, acidified 2-methoxyethanol in the wet impregnation process and a calcination procedure with intermediate plateaus, monodisperse 5.5 nm BiFeO3 nanoparticles were successfully obtained. Furthermore, the nanoparticles were applied in photodegradation reactions of rhodamine B in aqueous solution under visible-light irradiation. Notably, the cast BiFeO3 nanoparticles demonstrated very high efficiencies and stability under visible-light irradiation, much higher than those of BiFeO3 nanoparticles synthesized by other syn- thetic methods. The possible mechanism in the photodegradation process has been deeply discussed on the basis of radical trapping experiments. 1822 Beilstein J. Nanotechnol. 2020, 11, 1822–1833. Introduction In the face of a continuously growing demand, the production Bismuth ferrite (BiFeO3) is one of those photocatalysts and has of safe and readily available water is one of the biggest chal- been intensively researched in the past few years due to its lenges humanity is facing currently. Studies suggest that by narrow bandgap in the visible-light region (2.1–2.8 eV), high 2025, half of the world’s population will be living in water- chemical stability, and ferroelectric and ferromagnetic stressed areas [1,2]. Consequently, water-related crises are properties at room temperature [19-27]. Furthermore, slow rising around the globe due to population growth, climate electron–hole recombination as a result of efficient separation change, and environmental damage, making the scenario worse of the charge carriers has yielded an enhancement of its photo- than ever, especially in low- and middle-income countries. catalytic activity [20]. Since the photocatalytic degradation of In particular, water pollution is one of the most challenging organic molecules using a metal oxide photocatalyst is a hetero- problems to face [2-4]. The quantity of wastewater produced geneous process, it is obvious that efficiency and overall cata- as well as the overall pollution level are continuously increas- lytic performance are strongly correlated to the number of ing around the globe [5,6]. For instance, more than 80% of the active sites on the catalyst surface area and, thus, to the particle globally generated wastewater flows back into the environment size. The smaller the particle, the greater the specific surface untreated. One of the most polluting industries is the textile area is and, thus, the more active sites are available for photo- industry, in which vast quantities of toxic and harmful organic degradation [15,18]. In addition, it was shown that size and and inorganic chemicals are utilized [7]. The effluents resulting shape of BiFeO3 particles have a direct influence on the from these processes contain residues of very stable and toxic bandgap of the material. Smaller BiFeO3 particles tend to have dyes such as rhodamine B (RhB), methyl orange, or methylene lower bandgaps than larger particles [28-30]. The synthesis of blue [8]. Due to the increasing demand for safe water, waste- pure single-phase BiFeO3 is very challenging, due to the vola- water itself, coming from all types of industries including the tile character of bismuth at temperatures higher than 400 °C, textile industry, is considered to be a reliable alternative source which leads to the formation of undesired phases such as of water. Thus, it is becoming part of the solution to the water α-Fe2O3, α- and β-Bi2O3 or Bi2Fe4O9 [24,31-33]. In some problem that we face today [5]. Regarding this, the removal of cases, secondary phases can be removed by further treatment organic dyes from industrial wastewater is absolutely essential such as leaching with nitric acid. However, acid leaching as and, consequently, it has become a focus of attention of the well as other purification steps most often lead to lower yields scientific community over the past two decades. A number of and to the formation of larger particles. A promising synthetic techniques have been reported for the removal of dyes from technique for the synthesis of pure-phase BiFeO3 is the in situ wastewater such as precipitation (chemical coagulation, floccu- formation of Fe/Bi mixed metal complexes by using different lation), membrane and electrochemical processes, as well as complexing agents such as carboxylic acids. During the calcina- biological treatment methods [9]. The main disadvantages tion process the carboxylate ligands in the Fe/Bi complexes of these treatment methods are very often incomplete dye decompose to CO2 and BiFeO3 is formed [29]. This synthetic removal, high energy consumption and capital cost, and the technique requires calcination temperatures of at least 500 °C production of secondary waste products that require further for a minimum of 1–2 h, which can lead to particle growth, treatment. Advanced oxidation processes, in general, and agglomeration and to the formation of secondary phases. It heterogeneous semiconductor photocatalysis, in particular, are should be noted that, in general, low calcination temperatures promising candidates to efficiently treat wastewater as they are favor the generation of smaller particles with narrow bandgaps cost-effective and green treatment methods in which the and promote the formation of pure-phase BiFeO3 materials complete mineralization of dyes is achieved by the generation [19,22-24,34,35]. of hydroxyl and superoxide anion radicals [10,11]. Traditional photocatalysts, such as TiO2 or ZnO, provide chemical A proposed solution to control particle growth and to guarantee stability and facile preparation methods [12,13]. However, the formation of pure phases is the use of a nanocasting tech- their environmental benefit in large-scale industrial applica- nique [36-38]. Here, previously synthesized mesoporous tions has been limited due to their relatively large bandgaps matrices act as sacrificial rigid molds, also known as hard tem- along with their susceptibility to the fast recombination of plates. The synthesis of the desired material takes place in the photogenerated electron–hole pairs, leading to inefficient photo- pores of the template, which serves as a nanoreactor for the catalytic activity under visible-light or solar irradiation [14-18]. reaction. Thus, particle growth is restricted to the pore size of Thus, the development of photocatalysts with narrow bandgaps the porous matrix. After removal of the mold, the precursors are in the visible-light region in combination with a slow characterized by a maximum particle diameter corresponding to electron–hole recombination has attracted a great deal of the pore size of the porous matrix and, consequently, by a high interest [18]. specific surface area. Silica matrices, such as Santa Barbara 1823 Beilstein J. Nanotechnol. 2020, 11, 1822–1833. Amorphous silica (SBA-15) or Korean Advanced Institute of water at pH 3. After stirring the solution for 4 h at room temper- Science and Technology silica (KIT-6), have been used suc- ature the solvent was removed by evaporation using a mem- cessfully as hard templates to synthesize metal oxide particles brane pump vacuum on a rotary evaporator at 75 °C. Then, the and, in particular, nanometer-sized ferrite particles due to their sample was dried for 16 h in a ventilated oven at 75 °C until a availability, low cost and relative inertness [36-41]. dry powder was obtained. Finally, the sample was calcined at 500 °C for 1 h with two intermediate plateaus at 200