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30Condensed Matter and Interphases. 2021;23(4): 543-547
ISSN 1606-867X (Print) ISSN 2687-0711 (Online)
¡нп Condensed Matter and Interphases
SC3Q Kondensirovannye Sredy i Mezhfaznye Granitsy
v'ilSL J https://journals.vsu.ru/kcmf/
Original articles
Research article
https://doi.org/10.17308/kcmf.2021.23/3674
Synthesis, structure, and photo-Fenton activity of PrFeO3-TiO2 mesoporous nanocomposites
A. S. Seroglazova1'2H, M. I. Chebanenko2, V. I. Popkov2
'Saint-Petersburg State Institute of Technology, 26 Moskovsky pr., Saint Petersburg 190013, Russian Federation 2Ioffe Institute,
26 Politekhnicheskaya str., Saint Petersburg 194021, Russian Federation Abstract
Porous nanocomposites based on PrFeO3-TiO2 were synthesized using the glycine-nitrate combustion method with different values of mass content of TiO2 (0-7.5 %) and subsequent heat treatment in air. The results of X-ray phase analysis and Raman spectroscopy confirmed the presence of ultradispersed TiO2, structurally close to that of anatase. The morphology, specific surface area, and porous structure of the obtained powders were characterized by scanning electron microscopy and adsorption-structural analysis, the results of which showed that the samples had a foam-like mesoporous structure. The specific surface area and the average pore size were in the ranges of 7.6-17.8 m2/g and 7.2-15.2 nm, respectively, and varied depending on the TiO2 content. The optical properties of the nanocomposites were studied by UV-visible diffuse reflection spectroscopy, the energy of the band gap was calculated as 2.11-2.26 eV. The photocatalytic activity of PrFeO3-TiO2 nanocomposites was investigated in the process of photo-Fenton-like degradation of methyl violet under the action of visible light. It was shown that the maximum reaction rate constant was 0.095 min-1, which is ten times higher than the value for the known orthoferrite-based analogs. The obtained photocatalysts were also characterized by their high cyclic stability. Based on the studies carried out, the obtained porous PrFeO3-TiO2 nanocomposites can be considered to be a promising basis for photocatalysts applied in advanced oxidative processes of aqueous media purification from organic pollutants.
Keywords: Solution-combustion synthesis; Praseodymium orthoferrite; Titanium Oxide; Nanocomposites; Photocatalysts; Fenton-like reactions
Acknowledgments: this work is supported by the Grant of President of the Russian Federation МК-795.2021.1.3. The DTA-TGA, XRD and SEM studies were performed on the equipment of the Engineering Center of Saint Petersburg State Institute of Technology.
For citation: Seroglazova A. S., Chebanenko M. I., Popkov V. I. Synthesis, structure, and photo-Fenton activity of PrFeO3-TiO2 mesoporous nanocomposites. Kondensirovannye sredy i mezhfaznye granitsy = Condensed Matter and Interphases. 2021;23(4): 548-560. https://doi.org/10.17308/kcmf.2021.23/3674
Для цитирования: Сероглазова А. С, Чебаненко М. И., Попков В. И. Синтез, структура и фотофентоноподобная активность мезопористых нанокомпозитов PrFeO3-TiO2 Конденсированные фазы и межфазные границы. 2021;(23)4: 548-560. https://doi.org/10.17308/kcmf.2021.23/3674 .
И Anna S. Seroglazova, e-mail: [email protected] © Seroglazova A. S., Chebanenko M. I., Popkov V. I., 2021
The content is available under Creative Commons Attribution 4.0 License.
A. S. SerogLazova et al. Synthesis, structure, and photo-Fenton activity of PrFeO-TiO .
1. Introduction
Most industries for the manufacture of plastics, fabrics, paper, and rubber use various dyes and organic substances, leading not only to environmental pollution but also to the development of various diseases in humans, since they are often carcinogenic and/or highly toxic compounds. Traditional methods of removing organic dyes such as adsorption, filtration, ozonizing filtration, electrochemistry, used in modern industries, are quite effective, but still do not achieve a complete decomposition of the organic dyes due to their complex aromatic structure and resistance to various environmental factors, including oxidation. Therefore, there is an urgent need for the development of an alternative, for example, photocatalytic methods of water purification using environmentally friendly photocatalysts, which make it possible to effectively destroy toxic organic substances under the action of visible light.
Orthoferrites are complex oxides based on rare earth elements (REE) and iron with the general chemical formula of RFeO3 (R = Sc, Y, Ln), having a distorted perovskite-like structure with the Pbnm/Pnma space group [1-4]. Recently, there has been an increased interest in the study of REE orthoferrites due to their unique physical and chemical properties, which opens a wide range of possibilities for their practical application as a basis for new functional materials [5-8]. In addition, the structural, electromagnetic, and catalytic properties of REE orthoferrites make it possible to use them in the production of ceramics, magnetic devices, gas sensors, and magnetically controlled photocatalysts [9-11].
Earlier, various methods of synthesis were used to obtain nanocrystalline orthoferrites of rare earth elements: microwave, hydrothermal, sol-gel, co-precipitation, and others [1,12-15]. Most of these methods are labor-intensive and energy-consuming and do not always allow the control of such important parameters as particle size, shape, morphology, and structure, which determine the functional properties of REE orthoferrites. However, as was shown earlier in works [16-18], the solution combustion mthod is free from these drawbacks and allows varying the porous structure of nanoparticle aggregates, and thereby controlling the functional properties of
the obtained materials based on REE orthoferrites. The two-stage synthesis of REE orthoferrites with the amorphous combustion products and their further thermal treatment to obtain pure product is particularly promising. Powders obtained by this method are characterized by their porous structure, foamy morphology, and developed surface. The latter is an important factor in the development of photocatalytic materials [19].
Among the most promising photocatalysts based on REE orthoferrites, PrFeO3 stands out, which has an unusually high activity under the action of visible light [20]. Interest in such materials is caused by the possibility of their participation in heterogeneous Fenton-like processes occurring in an aqueous-organic solution. The mechanism of Fenton-like oxidation consists of the formation of powerful oxidizing hydroxyl radicals (O№) during the reversible transition from Fe2+ to Fe3+ under the action of visible light, which makes it possible to achieve the highly efficient decomposition and oxidation of organic pollutants [21-23]. Despite the high catalytic activity, reverse recombination processes significantly impair the photocatalytic activity of such materials, including PrFeO3. As reported in [19], the reverse recombination processes can be suppressed through the creation of a heterojunction structure during the transition from PrFeO3 to a composite based on it. For several simple oxides, it has been shown that an improvement in the photocatalytic properties can be achieved by adding a second component, due to the presence of which the transfer of charge carriers becomes possible immediately after the formation of an electron-hole pair under the action of irradiation [24-28]. Thus, the transition to heterostructures based on nanocomposites of rare-earth elements and other oxides is one of the most effective strategies for suppressing electron-hole recombination. In the presented work, TiO2 was chosen to improve the photocatalytic characteristics of PrFeO3, which, according to modern concepts, should lead to a drop in recombination. In addition, TiO2 is an active, stable, and environmentally friendly photocatalyst in the near-UV region with a strong oxidizing ability [29-31]. Interest in studying its catalytic properties when paired with PrFeO3 is also associated with the relatively large band
gap of TiO2 and the high intrinsic generation of electron-hole pairs.
This paper is dedicated to obtaining PrFeO3-TiO2 nanocomposite materials by the method of solution combustion with different values of TiO2 content (0-7.5 wt%). The obtained samples were studied using a wide range of physicochemical methods of analysis, allowing a detailed study of their structure, morphology, magnetic and optical properties, etc. Photocatalytic properties were studied using the example of Fenton-like oxidation of methyl violet (MV) under the action of visible light. The data obtained were compared with the results of photocatalytic studies of pure PrFeO3 and other REE orthoferrites. PrFeO3-TiO2 nanocomposite materials demonstrated a significant increase in the efficiency of MV photocatalytic oxidation. Thus, the obtained powders can be used in modern advanced oxidative processes for the purification of contaminated aqueous media.
2. Materials and methods
2.1. Preparation of the titanyl nitrate (TiO(NO)) solution
The synthesis of nanocomposite powders based on PrFeO3-TiO2 included the preliminary preparation of titanyl nitrate TiO(NO3)2 from titanium tetrachloride TiCl4 by hydrolysis. For this, distilled water was added to 20 ml of TiCl4 with constant stirring until a white precipitate of TiO(OH)2 was formed, followed by its complete dissolution in concentrated nitric acid (HNO3).
2.2. Synthesis of PrFeO3-TiO2 nanocomposites
PrFeO3-TiO2 nanocomposites were prepared
by the solution combustion method. To prepare the reaction mixture, we used the nitrates of the corresponding metals, Pr(NO3)3-6H2O and Fe(NO3)3-9H2O. Glycine with a glycine-nitrate ratio G/N = 2.0 was used as a fuel.
During the synthesis, iron (III) and praseodymium nitrates, as well as glycine, were dissolved in a small amount of distilled water, followed by the addition of a titanyl nitrate solution TiO(NO3)2. The resulting reaction mixture was stirred until the nitrates and glycine were completely dissolved. The reaction solution prepared in this way was heated in a sand bath until the water evaporated and the reaction
mixture ignited. As a result, porous brown powders were formed. Similarly, pure PrFeO3 was synthesized, as well as a series of PrFeO3-TiO2 nanocomposite powders with different mass ratios of PrFeO3:TiO2 = 100:0, 97.5:2.5, 95:5, and 92.5:7.5%, which were then heat-treated at 700 °C for 1 hour in the air to remove unreacted impurities and form crystalline praseodymium orthoferrite from amorphous combustion products.
2.3. Physico-chemical characterization
The elemental composition and morphology of the synthesized samples were studied by energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) using a Tescan Vega 3 SBH scanning electron microscope.
To determine the phase composition of the samples, X-ray phase analysis was carried out on a Rigaku SmartLab X-ray powder diffractometer in the range of Bragg angles (20) from 20° to 60°. Qualitative X-ray phase analysis was performed using the ICSD database. Based on the broadening of the X-ray diffraction lines, the average crystallite size was calculated using the Scherrer formula:
D =
kl
p cos 0
where k - shape factor, equal to 0.94; l - X-ray wavelength (CuKa = 0.15406 nm); p — line broadening at half the maximum in radians; 0 - Bragg's angle in radians.
The structural features of the nanocomposites were studied using Raman spectroscopy (excitation wavelength l = 532 nm) on a SINTERRA Raman microscope.
The specific surface of the samples, pore volume, and pore size distribution were determined by adsorption-structural analysis (ASA) using a Micromeritics ASAP 2020. Sorption-desorption isotherms were obtained at a liquid nitrogen temperature of 77 K.
Diffuse reflectance spectra (DRS) were measured with an Avaspec-ULS2048 spectrometer equipped with an AvaSphere-30-Refl integrating sphere.
2.4. Photocatalytic measurements
The photocatalytic activity of the synthesized samples was estimated during the photocatalytic
degradation of methyl violet (MV) under the action of visible light. The dye concentration was measured using a Shimadzu UV1600 spectrophotometer. Two 100W xenon lamps with a UV filter of l > 420 nm were used as a light source. The oxidation process was carried out in an experimental setup in the form of an insulated box, which included a 50 ml beaker, a magnetic stirrer, and lamps.
The first part of the experiment consisted in determining the most efficient catalyst among the samples PrFeO3, PrFeO3-TiO2 2.5%, PrFeO3-TiO2 5%, PrFeO3-TiO2 7.5%. For this, colloidal solutions of the corresponding nanopowders with a concentration of 0.5 g/L were preliminarily prepared. Then, 12.5 ml of a colloidal solution containing the catalyst, 0.6 ml of methyl violet (C = 1 g/L) and 6 ml of hydrogen peroxide (C = 1 mol/L), and 5.9 ml of distilled water were added to each of the four cuvettes. The final concentrations of these components in the resulting 25 ml volume were 0.25 g/L, 0.0232 g/L, and 0.24 mol/L, respectively. Before the start of the photocatalytic experiment, the solutions were stirred in the dark for 30 minutes to establish adsorption equilibrium. After this time, the solutions were irradiated with visible light with continuous stirring for 15 minutes. Then, 5 ml samples were taken from each solution to determine the concentration of the dye. Methyl violet removal efficiency was evaluated using the formula:
Rem. Eff. = . 100 %,
C
0
where C0 - initial dye concentration, C - final dye concentration after Fenton-like oxidation.
After the determination of the most active photocatalyst (PrFeO3-TiO2 5%), the corresponding kinetic parameters of the photo-Fenton-like oxidation of methyl violet were studied. For this, 25 ml of a solution containing 12.5 ml of catalyst (C = 0.25 g/L), 1.2 ml of methyl violet (C = 0.0464 g/L), 6 ml of H2O2 (C = 0.24 mol/L), and 4.1 ml of H2O were prepared. With constant stirring, the solution was irradiated for 15 minutes with a sample taken every 3 minutes to obtain the dependence of the efficiency of dye removal on the duration of irradiation.
In the final part of the experiment, the cyclic activity of the nanocomposite photocatalyst
PrFeO3-TiO2 5% was studied. As in the second experiment, a solution was prepared with the same concentrations of catalyst, dye, and hydrogen peroxide, which was irradiated for 15 minutes with constant stirring with sampling every 3 minutes. After 15 minutes, the final concentration of the decolorized dye was measured. Based on the obtained values of the final concentration MV, add the required volume of the dye to the initial solution so that its concentration becomes the same (C = 0.0464 g/L). Then this procedure was repeated.
3. Results and discussion
3.1. Energy-dispersive X-ray spectroscopy and element mapping
The results of X-ray energy dispersive spectroscopy showed that the powder synthesized without the addition of titanyl nitrate TiO(NO3)2 contains 3 main elements Pr, Fe, and O with the ratio Pr:Fe = 49.9:50.1 at%. Thus, the obtained sample corresponds in chemical composition to pure praseodymium orthoferrite (Fig. 1a). Powders synthesized with the addition of TiO(NO3)2, in terms of the ratio of key elements (Pr and Fe), also correspond to praseodymium orthoferrite within deviations of 0.1-0.6 at%, and additionally, contain titanium in the amount specified during the synthesis.
For the sample with the highest TiO2 content (7.5%), energy dispersive elemental mapping was additionally carried out (Fig. 1 b-f). According to the multi-element image (Fig. 1b), the obtained sample is characterized by a uniform distribution of the key elements Ti, Fe, O, and Pr. Singleelement images (Fig. 1 c-f) confirm the absence of regions enriched in individual chemical elements in the sample.
3.2. Powder X-ray diffraction
To determine the phase composition of the synthesized samples, powder X-ray diffraction was carried out, the results of which are shown in Fig. 2a. According to the obtained X-ray diffraction patterns, the reference sample (TiO2-0%) is a phase-pure crystalline praseodymium orthoferrite with an orthorhombic structure. The position of the diffraction lines and their intensity are fully consistent with the known data on the structure of PrFeO3 (JCPDS No: 18-9725).
A. S. SerogLazova et al.
Synthesis, structure, and photo-Fenton activity of PrFeO-TiO .
content, % Ti Pr Fe
0 0 50.1 49.1
2.5 48.8
5 47.3
7.5 KZKH 45.9
К ' s Urn ■--
11 К a 1
■ЭДИ1
^ m m ЦГП
e) Fe К al
5 цт
Fig. 1. EDS results (a) and EDS multi-element (a) and single element (c - O, d - Ti, e - Fe, f - Pr) mapping of the PrFeO3-TiO2-7.5% sample
The observed X-ray reflections of the samples with 2.5%, 5%, and 7.5% TiO2 content also relate to orthorhombic praseodymium orthoferrite. The most intense PrFeO3 X-ray diffraction lines do not have a noticeable shift, which indicates the absence of titanium being incorporated into the structure of praseodymium orthoferrite and the separate existence of the corresponding TiO2 and PrFeO3 phases. However, it should be noted, that there are no additional reflections in all diffractograms that could be attributed to other structural forms of TiO2, which indirectly indicates its presence in an amorphous or ultradispersed (with crystallite sizes less than 5 nm) state. An increase in the TiO2 content in this nanocomposite from 2.5% to 7.5% leads to a noticeable broadening of the PrFeO3 diffraction lines (Fig. 2b), which indicates a decrease in the crystallite size in the series Ti02-0 > TiO2-2.5 > TiO2-5 > TiO2-7.5. The calculation of the average crystallite size of praseodymium orthoferrite confirms its decrease in this series from 47.2 nm (pure PrFeO3) to 28.5 nm (TiO2 content of 7.5%) (Fig. 2c). As previously shown in [25,26], such
a decrease in the crystallite size is associated with the effect of the second TiO2 phase on the mass transfer processes, which complicates the recrystallization of praseodymium orthoferrite during the thermal treatment of solution combustion products.
3.3. Raman spectroscopy
To study the structural features of the TiO2 and PrFeO3 phases in the obtained nanocomposites, Raman spectroscopy was performed (Fig. 3). The results of the study showed the presence of combination modes belonging to titanium oxide with an anatase structure with the space group D4h19 (14/amd) [32]. The most intense peak of the Raman scattering of TiO2 at 145 cm-1 corresponds
to the E mode. The less intense is at 197 cm-1
g
and is attributed also to the E mode. The other
g
modes A - 285.8 cm-1, B - 330.6 cm-1, B, -
g ' 3g ' 2g
428 cm-1, A - 447.8 cm-1, B - 647.9 cm-1 confirm
g 3g
the formation of the PrFeO3 phase with the space group Pbnm [33].
The data obtained indicate the co-existence of two separate phases: orthorhombic PrFeO3
A. S. SerogLazova et al.
Synthesis, structure, and photo-Fenton activity of PrFeO-TiO .
Fig. 2. PXRD patterns of the PrFeO3-TiO2 (0-7.5%) samples (a); FWHM of main (112) X-ray reflex of PrFeO3 as a function of the TiO2 content (b); average crystallite size of PrFeO3 (c)
Fig. 3. Raman spectrum of the PrFeO3-TiO2-7.5% nanocomposite
with a perovskite structure and tetragonal TiO2 with an anatase structure, which indicates the successful synthesis of the nanocomposite. Comparing these data with the results of powder X-ray diffraction, it should be concluded that TiO2 in this nanocomposite occurs in the form of ultrafine nanoparticles characterized by weak crystallinity with structural motives of anatase.
3.4. Scanning electron microscopy
The morphology of the synthesized PrFeO3 and PrFeO3-TiO2 samples was studied by the SEM method, the results of which are presented in Fig. 4. For all the samples presented, a highly porous microstructure with a developed surface and foamy morphology is observed, which is typical for oxides of powders obtained by solution combustion [3, 18, 34, 35]. The formation of a developed porous structure is associated with the abundant release of gaseous products (CO, CO2, N2, NO2, etc.) during the redox combustion of a glycine-nitrate mixture.
It should be noted that with an increase in the proportion of TiO2 in the composition of the PrFeO3-TiO2 composite, a visual increase in the average pore size is observed, and their size distribution becomes less uniform. Thus, in a sample with 7.5% TiO2 content, pores with a size of several microns are observed, while pure PrFeO3 is characterized mainly by submicron pores. In all the cases, no individual nanocrystals are observed; they are strongly aggregated into large formations (more than ten microns in size) and occupy the interpore space of the composites.
A. S. Seroglazova et al.
Synthesis, structure, and photo-Fenton activity of PrFeO-TiO .
Fig. 4. SEM images of the PrFeO3-TiO2 nanocomposites synthesized at 0 (a), 2.5 (b), 5 (c) and 7.5 (d) % of TiO2 content
3.5. Low-temperature nitrogen adsorption-desorption
A more detailed study of the fine porous structure of powders based on PrFeO3 and TiO2, as well as the determination of their specific surface area, was carried out using adsorption-structural analysis during the low-temperature (77 K) adsorption-desorption of nitrogen, the results of which are presented in Fig. 5.
According to the results obtained, all isotherms are of type II, and the hysteresis loops are of type H3, which indicates a developed meso- and macroporous structure of the obtained nanopowders with a wide pore distribution. For a pure PrFeO3 sample, the smallest area of the hysteresis loop is observed, suggesting relatively low micro- and mesoporosity [29]. For samples containing the anatase TiO2 phase, the area of the hysteresis loops increases with an increase in the TiO2 content, which confirms an increase in porosity in this series. Based on this, it can be concluded that the addition of TiO2 to PrFeO3 facilitates pore formation in the composite at the stage of solution combustion of the glycine-nitrate reaction mixture.
The average pore size and specific surface area of the obtained nanocomposite powders were determined by the BJH and BET methods, respectively. According to the data presented in the inset in Fig. 5, the pure PrFeO3 sample has the smallest pore size (7.2 nm) and a specific surface area of 7.6 m2/g. For samples with variable TiO2 content, the pore size and specific surface area increase in the following sequence: 5% > 7.5% > 2.5%. For TiO2-2.5, these values were 10.3 nm and 13 m2/g, for TiO2-5 they were 15.2 nm and 17.8 m2/g, and for TiO2-7.5 they were 11.2 nm and 17.7 m2/g. Earlier, it was shown [25] that, in similar cases, an increase in the average pore size and specific surface area while transforming into the composite could be associated with both suppressions of the growth of crystals of the main phase (PrFeO3) and a higher specific surface area of the secondary phase (TiO2), the portion of which is growing. However, with an increase in the second fraction content to 7.5%, the growth of the specific surface area and the average pore size stops, which is explained by the intensification of aggregation processes and the blocking of some of the pores by TiO2 particles [36].
A. S. SerogLazova et al.
Synthesis, structure, and photo-Fenton activity of PrFeO-TiO .
Fig. 5. N2 adsorption-desorption isotherms of PrFeO3-TiO2 nanocomposites depending on the TiO2 content. The inset shows the specific surface area and average pore size of the samples
3.6. UV-visible diffusion reflectance spectroscopy
To determine the edge of the optical absorption band and the band gap of the obtained PrFeO3-TiO2 nanocomposite powders, as well as pure PrFeO3, diffusion reflection spectroscopy (DRS) in the UV-visible region was carried out (Fig. 6).
According to the DRS presented in Fig. 6a, the prepared samples are characterized by a broad absorption band from 500 to 800 nm, corresponding to the visible region of radiation. The edge of the absorption band for pure PrFeO3 is in the region of slightly longer wavelengths than for PrFeO3-TiO2 nanocomposites and slowly decreases in the following sequence: TiO2-5% > TiO2-7.5% > TiO2-2.5%.
To determine the band gap, Tauc plots were plotted using the transformation to the Kubelka-Munk function for the optical absorption spectra of the samples (Fig. 6b). The values of the band gap were determined from the tangent lines of the dependences concerning the photon energy [27, 37]. For pure praseodymium orthoferrite, the band gap was 2.11 eV, which is in good agreement with the results of previous studies [10, 38]. For the samples containing the anatase phase, an insignificant increase in the band gap values from 2.11 eV to 2.26 eV was observed; however, this delta is within the analysis error and the method for determining this value. Since in the case of the incorporation of titanium into the structure of
praseodymium orthoferrite, a significant change
in the value of E would be observed, and in our
g
case, it can be neglected, the results obtained are in good agreement with the data of PXRD and Raman spectroscopy, once again confirming the co-existence of the PrFeO3 and TiO2 phases in the nanocomposite in the form of separate phases.
3.7. Fenton-like catalytic activity analysis
Based on the results of SEM, adsorption-structural analysis, and DRS, it was concluded that the obtained PrFeO3 and PrFeO3-TiO2 powders could be promising photocatalysts. Therefore, their functional properties were investigated in the photocatalytic process of Fenton-like oxidation of methyl violet under the action of visible light. The results of these studies are shown in Fig. 7.
Fig. 7a contains the typical absorption spectra of a methyl violet solution after a survey photocatalytic test with PrFeO3 and PrFeO3-TiO2 nanopowders. It follows from these results that the PrFeO3-TiO2-5% sample has the highest catalytic activity, for which, according to the results of low-temperature adsorption analysis, the highest values of the specific surface area and average pore size (17.8 m2/g and 15.2 nm) have been established. A higher activity, in this case, is achieved due to higher accessibility of the surface, the increased total number of active centers, easier access of reagents, and removal
A. S. Seroglazova et al.
Synthesis, structure, and photo-Fenton activity of PrFeO-TiO .
Fig. 6. UV-visible diffusion reflectance spectra of PrFeO3 and PrFeO3-TiO2 samples (a) and corresponding Tauc plots (b)
of reaction products from the pore structure of the nanocomposite [23-25]. However, the nanocomposite photocatalyst PrFeO3-TiO2-7.5%, which has a higher specific surface area and average pore size (17.7 m2/g and 11.2 nm)
compared to the PrFeO3 and PrFeO3-TiO2-2.5% samples, exhibits the lowest photocatalytic activity among them. In this case, a decrease in the efficiency of dye removal may be associated
with a decrease in the efficiency of radiation absorption by photoactive PrFeO3 due to its shielding by TiO2 particles located on the surface of large PrFeO3 particles (Fig. 8). Nevertheless, the process of generation of oxidative radicals from water molecules (H2O ^ •OH) during the photo-Fenton-like reaction (Fig. 8) can be suppressed, which was previously reported [21, 39]. In this case, an increase in the photocatalytic
Fig. 7. UV-visible absorption spectra of MV dye during photo-Fenton-like oxidation (a); removal efficiency of PrFeO3 and PrFeO3-TiO2 nanopowders depending on the TiO2 content (b); MV concentration and kinetic curves of the PrFeO3-TiO2-5% sample (c)
A. S. SerogLazova et al.
Synthesis, structure, and photo-Fenton activity of PrFeO-TiO .
activity of nanopowders based on praseodymium orthoferrite and the addition of titanium dioxide is provided by the formation of a heterojunction, which allows charge carriers to transfer to the second phase and thereby reduce the negative effect of reverse recombination of electron-hole pairs [19]. Thus, the catalytic activity of the samples decreases in the sequence TiO2-5% > TiO2-2.5% > Ti02-0% > TiO2-7.5% and the optimum positive effect of the introduction of TiO2 on the photocatalytic activity of the nanocomposite is observed in the TiO2-5% sample.
For the PrFeO3-TiO2-5% sample, providing 100% MV photodegradation under previously indicated experimental conditions, kinetic studies were carried out and the results obtained are presented in Fig. 7c. As the irradiation time of the solution increases, the relative concentration of the dye naturally decreases, and the kinetic dependence corresponds to the pseudo-first-order of the reaction. The rate constant of this process was calculated based on the linearization of the kinetic dependence in logarithmic coordinates and amounted to 0.095 min-1. When comparing the results obtained with the results of photocatalytic tests of other nanocrystalline orthoferrites (Table 1), it was shown that the PrFeO3-TiO2-5% sample is superior to the best analogs known.
A prerequisite for the further practical application of photocatalysts is their high stability and cyclic reproducibility of results. The study of the cyclic photodegradation of the MV dye by visible light in the presence of PrFeO3-TiO2-5% was carried out under the same conditions as the kinetic study. The results of the cyclic stability experiment (Fig. 7d) indicate that after three cycles of the methyl violet
Fig. 8. Schematic representation of PrFeO3 shielding effect by TiO2 particles upon absorption of visible light by the nanocomposite with low (a) and high (b) TiO2 content
photodegradation process, there is only a slight loss of photocatalytic activity, which confirms its high stability of functioning under the selected conditions. This insignificant decrease in photocatalytic activity may be due to the physical loss of the catalyst during its separation from the initial solution. The amount of catalytic testing results makes it possible to consider PrFeO3-TiO2 nanocomposite powders as a promising basis for the processes of photo-Fenton-like oxidation of organic pollutants in aqueous media.
4. Conclusion
In the current study, the method of glycine-nitrate combustion followed by heat treatment in the air has been successfully applied to obtain nanocomposite porous materials based on PrFeO3-TiO2 with a variable mass content of TiO2 (0-7.5%). It is shown that the obtained powders contain nanocrystals of orthorhombic PrFeO3 and an ultradispersed TiO2 phase with an anatase structure. It is revealed that the TiO2 content has a significant effect on the average crystallite size, the specific surface area, and the average pore volume of the samples, which at the considered
Table 1. Photocatalytic characteristics comparison of the PrFeO3-TiO2-5% nanocomposite photocatalyst with other orthoferrite-based photocatalysts described in the literature
№ Photocatalyst Synthesis method Band gap, eV Lamp Pollutant K, min-1 Ref.
1 EuFeO3 nanoparticles Sol-gel 2.22 Xenon Rhodamine B 0.002 [23]
2 YbFeO3 nanopowder Solution-combustion 2.05 LED Methyl violet 0.004 [19]
3 BiFeO3 nanopowder Sol-gel 2.21 Sunlight Mordant blue 0.009 [40]
4 PrFeO3-TiO2-5% nanocomposite Solution-combustion 2.22 Xenon Methyl violet 0.095 This work
A. S. Seroglazova et al. Synthesis, structure, and photo-Fenton activity of PrFeO-TiO .
concentrations suppresses the growth of PrFeO3 nanocrystals and increases the total porosity of the nanocomposite. The amount of added TiO2 has practically no effect on the band gap values, which confirms the formation of PrFeO3-TiO2 composite consisting of two independent phases. The photocatalytic activity of the synthesized nanocomposite samples has been studied in Fenton-like oxidation of methyl violet. The optimal TiO2 content in the nanocomposite is 5 wt%, which ensures the 100% removal of methyl violet from an aqueous solution. Compared to pure catalysts based on other orthoferrites, the PrFeO3-TiO2 nanocomposites obtained in this work exhibit higher photocatalytic activity. Thus, the resulting nanopowders are promising photocatalysts for modern wastewater treatment processes.
Author contribution
All authors made an equivalent contribution to the preparation of the publication.
Conflict of interests
The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.
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Information about the authors
Anna S. Seroglazova, student at the Departament of Physical Chemistry, Saint Petersburg State Technological Institute (University), Russian Federation. Laboratory assistent, Ioffe Physical-Technical Institute of the Russian Academy of Sciences, Russian Federation; e-mail: [email protected]. ORCID: https://orcid. org/0000-0002-3304-9068.
Maria I. Chebanenko, Junior Researcher at the Laboratory of Materials and Processes of Hydrogen Energy, Ioffe Physical-Technical Institute of the Russian Academy of Sciences, Russian Federation; e-mail: [email protected]. ORCID: https://orcid. org/0000-0002-1461-579X.
Vadim I. Popkov, PhD in Chemistry, Senior Research Fellow, Head of the Laboratory of Materials and Processes of Hydrogen Energy, Ioffe Physical-Technical Institute of the Russian Academy of Sciences, Russian Federation; e-mail: [email protected]. ORCID: https://orcid.org/0000-0002-8450-4278.
Received August 11, 2021; approved after reviewing September 23,2021; accepted for publication November 15, 2021; published online December 12, 2021.
Translated by Vadim Popkov
Edited and proofread by Simon Cox
