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In Situ X‑ray Absorption Fine Structure Probing-Phase Evolution of CuFe2O4 in Nanospace Confinement Pongtanawat Khemthong,*,†,§ Chanapa Kongmark,*,‡,§ Nopparuj Kochaputi,‡ Sompin Mahakot,∥ Somboonsup Rodporn,∥ and Kajornsak Faungnawakij*,†,§,⊥

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 05/01/19. For personal use only.

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National Science and Technology Development Agency (NSTDA), National Nanotechnology Center (NANOTEC), Pathumthani 12120, Thailand ‡ Department of Materials Science, Faculty of Science, and §Research Network of NANOTECKU on NanoCatalysts and NanoMaterials for Sustainable Energy and Environment, Kasetsart University, Bangkok 10900, Thailand ∥ Synchrotron Light Research Institute (SLRI), 111 University Avenue, Muang, Nakhon Ratchasima 30000, Thailand ⊥ Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand S Supporting Information *

the structure, stoichiometry, particle size, and shape, which holds the key to optimizing their functionality and properties. However, a detailed study on the formation mechanism of the CuFe2O4 spinel has rarely been reported.12,13,15−18 A few ex situ X-ray diffraction (XRD) experiments have been carried out to study the structural evolution and formation of CuFe2O4 from a gel precursor (CuFe2),12,18 metal salt precursor in a mesoporous template,13 and mechanical milled oxide precursors (CuO and α-Fe2O3 hematite)15−17 at various calcination temperatures (ca. 300−1100 °C). Because these XRD data were collected after the samples were cooled to room temperature, some chemical reactions that occurred at high temperatures could not be identified. The atomic rearrangement from octahedrally coordinated cations of metal oxide or salt precursors into the spinel structure, a network of edge-sharing octahedra and corner-sharing tetrahedra, has not been completely clarified. It is therefore challenging to elucidate how the CuFe2O4 spinel was formed from metal salt precursors during heat treatment. In the present work, we carried out an in situ X-ray absorption spectroscopy (XAS) study at Cu and Fe K-edges during the formation of CuFe2O4 spinel nanoparticles in the confined space of mesoporous silica SBA-15. An in situ XAS cell was specially designed and developed to suit the beamline system (Schemes S1 and S2). This allowed monitoring of the structural evolution at atomic scale around copper and iron atoms and determination of the nature of intermediate phases formed during the reaction from room temperature to 750 °C. The synthesis of CuFe2O4 nanoparticles in confined spaces has been investigated together with a conventional synthesis method, i.e., sol−gel combustion method, in order to study the effect of the preparation method and the role of the mesoporous template. We aimed at understanding the crystallization process of the CuFe2O4 spinel nanoparticles to optimize the production of this material and to inspire the creation of newly designed spinel structures. Prior to calcination, the structures of the precursors prepared by sol−gel combustion (as-synthesized CuFe2O4_SG) and the confined space method (as-synthesized CuFe2O4_SBA-15)

ABSTRACT: The thermal transformation of Cu(NO3)2 and (Fe(NO3)3 into a CuFe2O4 spinel structure in the confined space of SBA-15 has been investigated. Interestingly, we observed the new formation mechanism of CuFe2O4 in SBA-15 via isolated metal ions (Cu2+ and Fe3+) surrounded by oxygen atoms, which gradually transformed to CuO and ferrihydrite. The latter evolved to maghemite spinel ferrite and reacted with CuO to form CuFe2O4 as the final species. In contrast, in the nonconfined space where the spinel was produced via a sol−gel combustion method, the nanostructure of CuFe 2O 4 immediately formed during the sol−gel combustion process and its crystallinity was improved after calcination. This is the first report on probing-phase formation using high-temperature in situ X-ray absorption fine structure.

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pinel nanoparticles have gained considerable attention for a wide range of applications such as gas sensors,1,2 magnetic hyperthermia,3 supercapacitors,4 and catalysts.5−11,13 Knowledge on structural transformation of the spinel is key to the development of this class of material because it dictates the properties of these materials such as magnetic and electronic characteristics in these applications. As an example of this class of materials, CuFe 2 O 4 nanoparticles can exist in two polymorphic forms, including cubic CuFe2O4, where all Fe3+ cations are located at the octahedral sites and Cu2+ cations at the tetrahedral sites, and tetragonal CuFe2O4, with all Cu2+ cations occupying the octahedral sites and the Fe3+ cations distributed evenly over both sites.14,15 These two phases show a large difference in the magnetic, optical, and electrical properties due to cation distribution between the tetrahedral and octahedral sites. Over the past few decades, several synthetic routes have been designed for the preparation of CuFe2O4 spinel nanoparticles such as the solid-state reaction,16,17 the soft-templating route,6 impregnation,7 coprecipitation,4,8 sol−gel combustion,9−12 and confined space synthesis.13 A fundamental understanding of the chemical process involved in the formation of materials is essential for controlling © XXXX American Chemical Society

Received: February 24, 2019

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DOI: 10.1021/acs.inorgchem.9b00540 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 1. In situ Cu K-edge XAS during formation of the CuFe2O4 spinel on SBA-15 at 25−750 °C.

Figure 2. In situ Fe K-edge XAS during formation of the CuFe2O4 spinel on SBA-15 at 25−750 °C.

and 2. Significant changes in the spectra were clearly observed upon heat treatment. In the Cu K-edge XANES spectra (Figure 1A), the white line’s intensity decreased upon heating while the shoulder at 9015 eV became more pronounced. Additionally, a magnified Cu K-edge XAS spectrum at the preedge region (Figure S3A) suggests formation of the CuO phase at a temperature range below 350 °C. Moreover, the development of a second Fourier transform peak at 2.4 Å (Figure 1B) would suggest that the local structure of copper in copper nitrate with the nearest-neighbor oxygen atoms transformed to a more complex structure with several shells of neighboring atoms. However, at temperatures above 410 °C, the white line’s intensity increased, the preedge feature slightly decreased, and the development of a peak at the shoulder feature was clearly observed. These different trends indicate an indirect phase transformation and the presence of an intermediate phase. The principal-component analysis identified three different species, which are attributed to copper nitrate, CuO, and the CuFe2O4 spinel ferrite structure. Consequently, linear combination fit (LCF) analysis of the XANES data was performed in order to examine the composition change of copper compounds during calcination, as shown in Figure 1C. The transformation from copper nitrate to CuO occurred above 80 °C. The maximum amount of the CuO intermediate phase (56.0%) is observed at 383 °C. Upon heating, CuFe2O4 formed at the expense of CuO and copper nitrate, which diminished at 750 °C. Fe K-edge XAS, presented in Figure 2A,B, provided new insight into the structural change of the as-synthesized

were carefully examined by Cu and Fe K-edge XAS at ambient conditions and compared with reference compounds (Figure S1). The spectral features of as-synthesized CuFe2O4_SG were identical with those of the CuFe2O4 standard in the X-ray absorption near-edge structure (XANES) region and in the extended X-ray absorption fine structure (EXAFS) region. In contrast, the as-synthesized CuFe2O4_SBA-15 obtained by the confined space method exhibited spectral features similar to those of the ferrihydrite structure [Fe10O14(OH)2], which belongs to the hexagonal lattice system (space group P63mc) and contains 20% tetrahedrally (corner-sharing) and 80% octahedrally (edge-sharing) coordinated iron atoms.19 In line with previous studies, Tüysüz et al. found that ferrihydrite can be formed via a nanocasting route using iron nitrate and SBA-15 templates.20 In situ XAS spectra of the as-synthesized CuFe2O4_SG collected during calcination from room temperature to 750 °C are shown in Figure S2A,B. No significant modifications were observed. All spectra exhibited spectral features similar to those of the CuFe2O4 standard, which confirmed that the formation of a spinel structure occurred during sol−gel autocombustion before the calcination process. It is obvious that the energy released during the combustion process could be high enough to initiate the formation of a copper ferrite structure, and calcination was subsequently performed to ensure the crystallinity and phase purity.12 Regarding to the confined space synthesis, the evolution of the Cu and Fe K-edge XAS spectra of the as-synthesized CuFe2O4_SBA-15 during calcination is illustrated in Figures 1 B

DOI: 10.1021/acs.inorgchem.9b00540 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 3. Formation mechanism of CuFe2O4 spinel on SBA-15.

which both structures contain tetrahedrally and octahedrally coordinated iron. In the present study, the maghemite produced in the first step was later consumed in the second step of the reaction at temperatures above 383 °C. The combination of Fe and Cu K-edge XAS results revealed that maghemite spinel ferrite (γ-Fe2O3) reacted with CuO and transformed to CuFe2O4 spinel ferrite as a final product. It seems that the energy barrier of γ-Fe2O3 formation is lower than that of CuFe2O4 and the γ-Fe2O3 spinel could act as a host structure for the incorporation of copper cations, leading to the formation of a CuFe2O4 spinel structure with a mixture of iron and copper cations at the octahedral sites. On the basis of these results, the formation mechanism of CuFe2O4 spinel on SBA-15 support could be proposed (Figure 3). It is worth noting that previous studies on the solid-state reaction15−17 and sol−gel combustion synthesis12,18 reported the formation of CuFe2O4 spinel through CuO and α-Fe2O3 intermediates. This difference is probably related to the effect of the preparation method and the role of the SBA-15 mesoporous template that favor the formation of ferrihydrite precursor and γ -Fe2O3 maghemite intermediate. For the ex situ XRD studies of CuFe2O4 spinel synthesized in confined spaces and calcined at different temperatures (450−750 °C),13 the observation of αFe2O3 intermediate could possibly be explained by the phase transition of γ -Fe2O3 to α-Fe2O3, which can take place at around 400 °C.26 In addition, the structure and morphology of the CuFe2O4 spinel nanoparticles obtained after in situ studies have been confirmed by XRD, EXAFS analysis, and transmission electron spectroscopy (TEM) for CuFe2O4_SBA-15, as shown in Figures S5−S7, respectively. The structural parameters obtained from EXAFS analysis are shown in Table S1. TEM images of CuFe2O4_SBA-15 (Figure S7) revealed the confinement of CuFe2O4 inside the regular hexagonal pore array of SBA-15. Here we successfully demonstrated the formation mechanism of the CuFe2O4 spinel nanoparticles using the confined space method. This is ideally suited to the design of desired species of spinel oxides during the crystal growth. We expected that this approach also offers a great advantage with respect to the preparation of nanocrystalline spinel ferrites, which are suitable for catalysis, sensing, and hyperthermia applications.

CuFe2O4_SBA-15 during calcination. A slight increase of the absorption peak intensity at the preedge region (the electric quadrupole-allowed transitions) was observed, indicating partial conversion from the octahedral to tetrahedral environment of iron ions (Figure S3B). A stacked plot of the first-order derivative of XAS spectra is shown in Figure S4. The threshold energy can be defined as the energy of the maximum derivative of the XAS spectrum. Interestingly, the absorption threshold energy of the CuFe2O4_SBA-15 catalyst remained the same along the reaction, and it was close to that of Fe2O3 and CuFe2O4, suggesting that the iron cation in this system was mainly Fe3+. Principal-component analysis identified three iron(III) compounds, which were attributed to iron nitrate, ferrihydrite, and the spinel ferrite structure. LCF analysis of the XANES data was subsequently applied to determine the composition change of the iron compounds during calcination, as shown in Figure 2C. CuFe2O4 (pure phase) was used as a standard spectrum for the spinel ferrite structure. The possibility of forming spinel ferrite (with trivalent Fe3+) with a copper/iron composition ratio different from that of CuFe2O4 could not be neglected. At the beginning of the reaction, the as-synthesized CuFe2O4_SBA-15 was composed of iron nitrate (40.8%) and ferrihydrite (59.2%). As the temperature increased, iron nitrate progressively transformed to ferrihydrite, giving the maximum amount of ferrihydrite (84.2%) at 135 °C. Further heating caused formation of the spinel ferrite structure, while iron nitrate and ferrihydrite gradually diminished at 245 and 750 °C, respectively. Interestingly, formation of spinel ferrite was noticed to occur via two successive elementary steps. The first step occurred within the temperature range of 135−383 °C. It was suggested that ferrihydrite transformed to maghemite spinel ferrite (γFe2O3), a ferromagnetic cubic form of iron(III) oxide, because Cu K-edge XAS gave no indication of CuFe2O4 but Fe K-edge XAS affirmed formation of the spinel ferrite structure. The structure of γ-Fe2O3 is closely related to magnetite (Fe3O4), in which iron atoms are in tetrahedral and octahedral coordination with oxygen atoms in a ratio of 1:2, with the presence of iron vacancies.21 A random arrangement of iron vacancies gives rise to a cubic structure with the space group Fd3̅m. When the vacancies are ordered, the symmetry of the spinel structure can reduce to P41212 or P4332.22 Several studies have reported the transformation of ferrihydrite into maghemite (γ -Fe2O3) as an intermediate phase upon heating to around 150−300 °C before conversion into hematite at higher temperatures.23−25 It was also suggested that this transformation occurs because of the structural similarity between ferrihydrite and maghemite, in

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00540. C

DOI: 10.1021/acs.inorgchem.9b00540 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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(9) Luadthong, C.; Khemthong, P.; Nualpaeng, W.; Faungnawakij, K. Copper ferrite spinel oxide catalysts for palm oil methanolysis. Appl. Catal., A 2016, 525, 68−75. (10) Faungnawakij, K.; Viriya-Empikul, N. Catalytic behavior toward oxidative steam reforming of dimethyl ether over CuFe2O4-Al2O3 composite catalysts. Appl. Catal., A 2010, 382 (1), 21−27. (11) Oar-Arteta, L.; Remiro, A.; Aguayo, A. T.; Olazar, M.; Bilbao, J.; Gayubo, A. G. Development of a bifunctional catalyst for dimethyl ether steam reforming with CuFe2O4 spinel as the metallic function. J. Ind. Eng. Chem. 2016, 36, 169−179. (12) Yadav, R. S.; Kuřitka, I.; Vilcakova, J.; Havlica, J.; Masilko, J.; Kalina, L.; Tkacz, J.; Hajdúchová, M.; Enev, V. Structural, dielectric, electrical and magnetic properties of CuFe2O4 nanoparticles synthesized by honey mediated sol−gel combustion method and annealing effect. J. Mater. Sci.: Mater. Electron. 2017, 28, 6245−6261. (13) Yang, S.-C.; Su, W.-N.; Lin, S. D.; Rick, J.; Cheng, J.-H.; Liu, J.-Y.; Pan, C.-J.; Liu, D.-G.; Lee, J.-F.; Chan, T.-S.; Sheu, H.-S.; Hwang, B.-J. Preparation of nano-sized Cu from a rod-like CuFe2O4: Suitable for high performance catalytic applications. Appl. Catal., B 2011, 106 (3), 650−656. (14) Krishnan, V.; Selvan, R. K.; Augustin, C. O.; Gedanken, A.; Bertagnolli, H. EXAFS and XANES investigations of CuFe2O4 nanoparticles and CuFe2O4− MO2 (M= Sn, Ce) nanocomposites. J. Phys. Chem. C 2007, 111 (45), 16724−16733. (15) Zhang, R.; Yuan, Q.; Ma, R.; Liu, X.; Gao, C.; Liu, M.; Jia, C.-L.; Wang, H. Tuning conductivity and magnetism of CuFe2O4 via cation redistribution. RSC Adv. 2017, 7 (35), 21926−21932. (16) Lalanne, M.; Barnabé, A.; Mathieu, F.; Tailhades, P. Synthesis and Thermostructural Studies of a CuFe1−xCrxO2 Delafossite Solid Solution with 0 ≤ x ≤ 1. Inorg. Chem. 2009, 48 (13), 6065−6071. (17) Marinca, T. F.; Chicinaş, I.; Isnard, O. Influence of the heat treatment conditions on the formation of CuFe2O4 from mechanical milled precursors oxides. J. Therm. Anal. Calorim. 2012, 110, 301−307. (18) Köferstein, R.; Walther, T.; Hesse, D.; Ebbinghaus, S. G. Crystallite-growth, phase transition, magnetic properties, and sintering behavior of nano-CuFe2O4 powders prepared by a combustion-like process. J. Solid State Chem. 2014, 213, 57−64. (19) Michel, F. M.; Ehm, L.; Antao, S. M.; Lee, P. L.; Chupas, P. J.; Liu, G.; Strongin, D. R.; Schoonen, M. A. A.; Phillips, B. L.; Parise, J. B. The Structure of Ferrihydrite, a Nanocrystalline Material. Science 2007, 316, 1726−1729. (20) Tüysüz, H.; Salabaş, E. L.; Weidenthaler, C.; Schüth, F. Synthesis and Magnetic Investigation of Ordered Mesoporous Two-Line Ferrihydrite. J. Am. Chem. Soc. 2008, 130, 280−287. (21) Petkov, V.; Cozzoli, P. D.; Buonsanti, R.; Cingolani, R.; Ren, Y. Size, Shape, and Internal Atomic Ordering of Nanocrystals by Atomic Pair Distribution Functions: A Comparative Study of γ-Fe2O3 Nanosized Spheres and Tetrapods. J. Am. Chem. Soc. 2009, 131 (40), 14264−14266. (22) Jørgensen, J.-E.; Mosegaard, L.; Thomsen, L. E.; Jensen, T. R.; Hanson, J. C. Formation of γ-Fe2O3 nanoparticles and vacancy ordering: An in situ X-ray powder diffraction study. J. Solid State Chem. 2007, 180 (1), 180−185. (23) Eggleton, R. A. New Data and a Revised Structural Model for Ferrihydrite. Clays Clay Miner. 1988, 36, 111−124. (24) Barrn, V.; Torrent, J. Evidence for a simple pathway to maghemite in Earth and Mars soils. Geochim. Cosmochim. Acta 2002, 66 (15), 2801−2806. (25) Manceau, A. Critical evaluation of the revised akdalaite model for ferrihydrite. Am. Mineral. 2011, 96, 521−533. (26) El Mendili, Y.; Bardeau, J.-F.; Randrianantoandro, N.; Greneche, J.-M.; Grasset, F. Structural behavior of laser-irradiated γ-Fe2O3 nanocrystals dispersed in porous silica matrix: γ-Fe2O3 to α-Fe2O3 phase transition and formation of ε-Fe2O3. Sci. Technol. Adv. Mater. 2016, 17, 597−609.

Experimental details, diagram, and experimental setup of an in situ XAS cell, XAS spectra of the as-synthesized and reference samples at ambient conditions, prepeak regions of Cu and Fe K-edge XAS, first-order derivative of Fe Kedge XAS spectra of CuFe2O4_SBA-15 during calcination, XRD patterns, EXAFS analysis, and TEM images of CuFe2O4 nanoparticles prepared by the confined space method (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.K.). *E-mail: [email protected] (C.K.). *E-mail: [email protected] (K.F.). ORCID

Pongtanawat Khemthong: 0000-0002-9502-3584 Chanapa Kongmark: 0000-0001-8533-3223 Kajornsak Faungnawakij: 0000-0002-4724-0613 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was co-supported by the Thailand Research Fund, SLRI, and NANOTEC through Projects TRG5780192, TRG5580014, and BRG6080015 . This work was also supported by the Research Network NANOTEC program of NSTDA, Ministry of Science and Technology, Pathumthani, Thailand.

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REFERENCES

(1) Rathore, D.; Mitra, S.; Kurchania, R.; Pandey, R. Physicochemical properties of CuFe2O4 nanoparticles as a gas sensor. J. Mater. Sci.: Mater. Electron. 2018, 29, 1925−1932. (2) Haija, M. A.; Abu-Hani, A. F. S.; Hamdan, N.; Stephen, S.; Ayesh, A. I. Characterization of H 2 S gas sensor based on CuFe 2 O 4 nanoparticles. J. Alloys Compd. 2017, 690, 461−468. (3) Kombaiah, K.; Vijaya, J. J.; Kennedy, L. J.; Bououdina, M.; AlNajar, B. Conventional and microwave combustion synthesis of optomagnetic CuFe2O4 nanoparticles for hyperthermia studies. J. Phys. Chem. Solids 2018, 115, 162−171. (4) Chandel, M.; Moitra, D.; Makkar, P.; Sinha, H.; Hora, H. S.; Ghosh, N. N. Synthesis of multifunctional CuFe2O4−reduced graphene oxide nanocomposite: an efficient magnetically separable catalyst as well as high performance supercapacitor and first-principles calculations of its electronic structures. RSC Adv. 2018, 8, 27725−27739. (5) Estrella, M.; Barrio, L.; Zhou, G.; Wang, X.; Wang, Q.; Wen, W.; Hanson, J. C.; Frenkel, A. I.; Rodriguez, J. A. In Situ Characterization of CuFe2O4 and Cu/Fe3O4 Water− Gas Shift Catalysts. J. Phys. Chem. C 2009, 113 (32), 14411−14417. (6) Haetge, J.; Suchomski, C.; Brezesinski, T. Ordered Mesoporous MFe2O4 (M = Co, Cu, Mg, Ni, Zn) Thin Films with Nanocrystalline Walls, Uniform 16 nm Diameter Pores and High Thermal Stability: Template-Directed Synthesis and Characterization of Redox Active Trevorite. Inorg. Chem. 2010, 49 (24), 11619−11626. (7) Kameoka, S.; Tanabe, T.; Tsai, A. P. Spinel CuFe2O4: a precursor for copper catalyst with high thermal stability and activity. Catal. Lett. 2005, 100 (1), 89−93. (8) Rajput, J. K.; Arora, P.; Kaur, G.; Kaur, M. CuFe2O4 magnetic heterogeneous nanocatalyst: Low power sonochemical-coprecipitation preparation and applications in synthesis of 4H-chromene-3-carbonitrile scaffolds. Ultrason. Sonochem. 2015, 26, 229−240. D

DOI: 10.1021/acs.inorgchem.9b00540 Inorg. Chem. XXXX, XXX, XXX−XXX