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NANOSYSTEMS: Seroglazova A.S., Popkov V.I. Nanosystems:
PHYSICS, CHEMISTRY, MATHEMATICS Phys. Chem. Math., 2022,13 (6), 649-654.
http://nanojournal.ifmo.ru
Original article DOI 10.17586/2220-8054-2022-13-6-649-654
Synthesis of highly active and visible-light-driven PrFeO3 photocatalyst using solution combustion approach and succinic acid as fuel
Anna S. Seroglazova1'2 ", Vadim I. Popkov2'6
1Saint Petersburg State Institute of Technology, St. Petersburg, Russia 2 loffe Institute, St. Petersburg, Russia
[email protected], [email protected]
Corresponding author: Anna S. Seroglazova, [email protected]
PACS 61.46.+w
Abstract In this work, nanocrystalline powder of praseodymium orthoferrite was obtained by the solution combustion synthesis using succinic acid as organic fuel. The obtained sample is characterized by techniques of powder x-ray diffraction, scanning and transmission electron microscopy, and UV-Vis diffuse reflectance spectroscopy. The sample was discovered to have a porous, foamy morphology with an average crystallite size of 36.1 nm and a band gap value of 2.1 eV. The study of Fenton-like photocatalytic activity was carried out on the example of the decomposition of the methyl violet dye in the presence of hydrogen peroxide under visible light. The maximum value of the degradation rate constant is 0.0325 min-1. The results were compared to the available data obtained for similar systems.
Keywords praseodymium orthoferrite, solution combustion method, succinic acid, nanoparticles, photo-Fenton-like reactions, photocatalysis.
Acknowledgements This work was carried out in accordance with the Grant of the President of the Russian Federation MK-795.2021.1.3. The Authors acknowledge V.N. Nevedomskyifor the help with TEM investigations.
For citation Seroglazova A.S., Popkov V.I. Synthesis of highly active and visible-light-driven PrFeO3 photocatalyst using solution combustion approach and succinic acid as fuel. Nanosystems: Phys. Chem. Math., 2022, 13 (6), 649-654.
1. Introduction
In recent years, a large number of studies have been directed toward the development and study of new functional materials with a wide range of applications. The most interesting ones among such materials are ferrites with different
structures and compositions, in particular ferrites, of rare earth elements (REE).
Rare earth orthoferrites belong to a class of complex oxides with the general chemical formula RFeO3 (R = rare earth element: Ce, La, Gd, or Pr). The enhanced interest in this class of compounds is due to their unusual distorted perovskite-like structure with a space group of Pbnm/Pnma [1-3], which provides them with unique chemical and physical properties: electrical, magnetic, and optical. The combination of the unique properties allows the use of REE orthoferrites in the production of ceramic materials, electronic devices, gas sensors, magnetic materials, MRI contrast agents, and catalytic materials [4-6]. The use of REE orthoferrites as photocatalytic materials in the visible region is identified as the most promising area due to the low value of the band gap (2-3 eV), semiconductor properties, and chemical stability [7,8].
Among the varieties of orthoferrites of rare earth elements, PrFeO3 praseodymium orthoferrite is known to have good electromagnetic properties, but it also stands out due to its distinguished photocatalytic properties under visible light [9-11]. However, one of the problems associated with the development of catalytic materials is the production of catalysts with a developed surface and a porous structure. To date, a large number of studies devoted to the preparation of PrFeO3 nanoparticles using synthesis routes such as sol-gel, co-precipitation, the template method, and the solution combustion method have been published [12-17]. The listed methods of synthesis make it possible to obtain nanostructured materials with tuned particle sizes, morphology, and structure, but most of them are power-consuming and do not always allow one to obtain a structure with a developed porous surface, with the exception of synthesis by solution combustion [1820]. Distinctive features of the method are its versatility, speed, and the possibility of varying synthesis parameters that allow tuning of the structural, morphological, and functional characteristics of the materials, as well as the possibility of transforming it into large-scale industrial production with the manufacturing of high-purity products. The most important synthesis parameter is the type of organic fuel, which provides the process of self-ignition for the reaction mixture and acts as a chelating agent for metal ions. Glycine and metal nitrates are conventionally chosen as reagents for the combustion
© Seroglazova A.S., Popkov V.I., 2022
solution synthesis. However, this type of fuel does not allow for the formation of particles with a small average size and a developed porous surface [21-24].
The present study aims to produce pure nanopowders based on PrFeO3 by solution combustion synthesis with succinic acid as an organic fuel, followed by a mild heat treatment at 500° C, and to characterize the synthesized sample by a set of methods of physicochemical analysis. The photo-Fenton-like catalytic activity was then investigated using the model reaction of photodegradation of methyl violet in the presence of PrFeO3 as a catalyst and hydrogen peroxide.
2. Materials and methods
The starting reagents used for the synthesis of PrFeO3 praseodymium orthoferrite were praseodymium nitrate hexahy-drate (Pr(NO3)3-6H2O), iron nitrate nonahydrate (Fe(NO3)3 9H2O), succinic acid (C4H6O4), as well as distilled water. All the reagents were chemically pure and used as purchased from Neva-Reactiv (Saint Petersburg, Russia). The reaction solution was prepared by dissolving metal nitrate and succinic acid, taken according to the stoichiometric ratio, in 40 ml of distilled water under constant mixing until all components were completely dissolved. The resulting solution was placed in a glassy carbon dish and heated on an electric stove until complete water removal was achieved, followed by self-ignition of the reaction mixture and the formation of the final solid product, which was ground in a mortar until a homogeneous bright brown powder was formed. The powder was then subjected to heat treatment at 500° C for 1 hour in an air atmosphere to remove unreacted residues of nitrates and organics.
The elemental composition and morphology of the synthesized PrFeO3 particles were studied by scanning electron microscopy using a Tescan Vega 3 SBH microscope equipped with an Oxford INCA 200 electron probe microanalyzer. Analysis of structure and phase composition was performed via powder x-ray diffraction on a Rigaku Smart Lab 3 diffrac-tometer using CuKa1 irradiation (A=0.154056 nm). For a more detailed assessment of the morphology and microstructure, transmission electron microscopy was performed on a JEOL TEM-100CX microscope. Diffuse reflectance spectra were measured with an Avaspec-ULS2048 spectrometer equipped with an AvaSphere-30-Refl integrating sphere.
Methyl violet (MV) was chosen as a model dye for studying photocatalytic activity. The photodegradation process was carried out in an insulated box equipped with a light source with a wavelength of A >420 nm, a magnetic stirrer for constant stirring of the reaction solution, and a 50-ml graduated cylinder.
The experiment included the preparation of a reaction solution containing a dye, a catalyst (PrFeO3), and hydrogen peroxide (H2O2) with a concentration of 0.0232 g/L, 0.25 g/L, and 0.24 mol/L, respectively. The volume of the solution was 30 mL. Before the start of the experiment, the solution was stirred in the dark for 30 minutes to establish adsorption equilibrium. After that, it was irradiated with a visible light source for 60 minutes with sampling of 5 ml every 10 minutes to determine the MF concentration. Changes in the dye concentration were recorded using an Avaspec-ULS208 spectrometer.
3. Results and discussion
Elemental analysis of the combustion products and X-ray diffraction data collected on the initial and heat-treated samples are shown in Fig. 1.
Fig. 1. The energy dispersive X-ray spectroscopy (a) and powder X-ray diffraction patterns (b) of the as-prepared and heat-treated PrFeO3 samples
According to the presented results (Fig. 1a), the elemental composition of the synthesis product corresponds to PrFeO3, as evidenced by the presence of three main spectral lines belonging to the key elements: praseodymium (Pr), iron (Fe), and oxygen (O). Within the method's error, the quantitative ratio Pr:Fe in the obtained sample was 49.16 and 50.92 at.%, which is very close to praseodymium orthoferrite in terms of stoichiometry. Detailed results of EDX measurements in terms of Fe and Pr are presented in the table in Fig. 1a.
The X-ray powder diffraction patterns shown in Fig. 1b indicate the formation of only one crystalline phase of praseodymium orthoferrite before and after the heat treatment of the combustion products of the mixture. The crystal structure of the heat-treated product was refined by the Rietveld method. According to the refinement results, the unit cell parameters are in good agreement with the data of JCPDS card No. 18-9725 and are as follows: a=5.4858(4), b=5.5756(2), c=7.7898(7) Awhich corresponds to the space group Pbnm. The data obtained are also consistent with the results of other studies on the production of PrFeO3 by the methods of glycine-nitrate synthesis, microwave, and hydrothermal synthesis [9,13,25]. The average crystallite size calculated from the broadening of X-ray diffraction lines for the initial sample was 27.9 nm. After the heat treatment, a slight increase in size up to 36.1 nm is observed, which indicates the process of recrystallization into larger crystals upon moderate heating. It should be noted that PrFeO3 synthesized by solution combustion using succinic acid at a stoichiometric ratio has smaller crystallite sizes than PrFeO3 synthesized by a similar method, but using glycine as a fuel (57.9 nm) [13].
According to SEM and TEM analysis (Fig. 2b), the morphology of the combustion product after finishing heat treatment is spongy with a developed system of micron and submicron pores, which is typical for many substances, particularly simple and complex oxides obtained by a similar synthesis method [26-28]. The formation of agglomerates consisting of individual PrFeO3 particles is also observed, which is associated both with high temperatures in the reaction zone during combustion and with the thermal treatment of the product, which leads to an increase in mass transfer processes. A more detailed study of these processes was previously studied using the examples of the YFeO3 [24] and NiO [29] systems.
Fig. 2. SEM (a) and TEM (b) images of calcined PrFeO3 nanopowder
To describe the optical characteristics of the sample, diffuse reflectance spectroscopy in the UV-Vis region was carried out, the results of which are shown in Fig. 3.
The spectrum shown in Fig. 3a shows a wide absorption band in the wavelength range from 500 nm to 700 nm, corresponding to the visible region of light. The value of the band gap, recalculated in accordance with the Kubelka-Munk transformation and presented in the Tauc coordinates, is shown in Fig. 3b and is 2.1 eV, which is consistent with the literature data [12,13,17].
The combination of the research results from powder X-ray diffraction, SEM analysis, and diffuse reflection spectroscopy makes it possible to assume that the synthesized sample can act as a promising photocatalyst in the visible region. It is possible due to its porous structure and small crystallite sizes, which provide greater access for reagents to the catalyst surface area, as well as due to the small value of the band gap, which, when irradiated with visible light, allows for an electron to pass from the valence band to the conduction band with the subsequent formation of a powerful oxidizing hydroxyl radical [13,30,31].
The study of the functional properties of PrFeO3 in a photocatalytic Fenton-like oxidation process was carried out on the example of the decomposition of the model methyl violet (MV) dye under the action of visible light. The results of the study are shown in Fig. 4.
Fig. 4a shows the typical absorption spectra of methyl violet during photo-Fenton-like degradation. According to the obtained data, in all spectra, there is a single absorption peak corresponding to 550 nm, which naturally decreases
500 550 600 650 700 2.00 2.05 2.10 2.15 2.20 2.25 2.30 2.35 Wavelength, (nm) Energy, eV
Fig. 3. UV-Vis spectrum of PrFeO3 (a) and the corresponding Tauc plot (b)
Fig. 4. UV-Vis absorption spectra of methyl violet during photo-Fenton-like degradation (a) and corresponding kinetic curves (b)
with time during prolonged irradiation with a light source, which confirms the photocatalytic activity of the synthesized sample. The most intense decrease in the dye concentration is observed in the first 10 minutes of irradiation, then the intensity of discoloration decreases, which is associated with a gradual decrease in the generation of hydroxyl radicals caused by the processes of recombination of electron-hole pairs [13, 32] and the peculiarity of the filling of catalytically active centers on the catalyst surface.
Based on the experimental data, kinetic studies were also carried out, the results of which are shown in Fig. 4b. As noted earlier, the relative concentration of the dye decreases regularly with the course of irradiation, and in accordance with the shape of the kinetic dependence, it refers to the pseudo-first order of the reaction. The rate constant was calculated by linearizing the kinetic dependence in logarithmic coordinates. The obtained value was 0.0325 min-1, which is higher compared to other rare-earth ferrites and orthoferrites (Table 1).
4. Conclusion
Thus, within the framework of this work, the possibility of obtaining pure nanocrystalline praseodymium orthoferrite by combustion in solution using succinic acid as a fuel was shown. According to the results of comparison with the literature data, the obtained particles have a smaller crystallite size (36.1 nm) than in similar synthesis using a standard fuel, glycine (57.9 nm), which makes it possible to vary the particle size, morphology, and specific surface area using different types of fuel. Analysis of photocatalytic activity showed high efficiency in the photo-Fenton-like degradation of methyl violet with a rate constant of 0.0325 min-1.
TABLE 1. Comparison of rare earth orthoferrites as visible-light-driven photocatalysts depending on rare earth element, synthesis method and crystallite size
No Photocatalyst Synthesis method Crystallite size, nm Dye k, min-1 Reference
1 YbFeOs Solution combustion synthesis 54.6 Methyl Violet 0.0040 [33]
2 EuFeÜ3 Sol-gel synthesis 25.2 Rhodamine B 0.0020 [31]
3 NiFe2O4 Solution combustion synthesis 27.0 Methylene blue 0.0080 [34]
4 PrFeO3 Solution combustion synthesis 36.1 Methyl Violet 0.0325 This work
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Submitted 28 August 2022; accepted 1 December 2022
Information about the authors:
Anna S. Seroglazova - Saint Petersburg State Institute of Technology, 26 Moskovsky prospect, St. Petersburg, 190013 Russia; Ioffe Institute, St. Petersburg, 194021 Russia; ORCID 0000-0002-3304-9068; [email protected]
Vadim I. Popkov - Ioffe Institute, St. Petersburg, 194021 Russia; ORCID 0000-0002-8450-4278; [email protected]
Conflict of interest: the authors declare no conflict of interest.
