Identification of a Fe(OH)2-like phase in the core–shell structure of nano-zero-valent Fe and its evolution when interacting with Pd2+aq ions by Mössbauer spectroscopy, XPS, and TEM
Graphical abstract
Introduction
Zero-valent iron nanoparticles (Fe-NPs) have been synthesized using various methods including (i) ball milling of bulk Fe particles, (ii) reduction of Fe oxides to Fe0 with hydrogen gas and (iii) aqueous reduction of ferric (FeIIIaq) or ferrous (FeIIaq) species [1,2]. In aqueous solution, the most commonly used method is the reduction of ferric species by sodium borohydride (NaBH4) as initially proposed by Wang et al. [3]. In comparison with the classical micrometric-sized Fe0 granular materials, Fe-NPs present generally higher chemical reactivity due to their higher surface to volume ratio. Fe-NPs have been used to eliminate organic and inorganic pollutants when directly injected in groundwater [4,5]. Furthermore, when the surface of these NPs is modified by organic stabilizers, their agglomeration decreases and their dispersion into the reactive media increases [6]. Additionally, Fe-NPs were used as a substrate to stabilize metallic clusters such as Cu0 or Pd0 on their surface [3,7,8]. The preparation of NPs with such modified surfaces was performed relatively easily by reducing CuII or PdII salt at the NP–water interface. In addition, metal-coated Fe-NPs where shown to be effective catalyzers to oxidize formate and reduce carbon dioxide to formate [8] or to catalyze the reaction between iodobenzene with benzene boronic acid to form a biphenyl product (Suzuki−Miyaura cross-coupling reactions) [7]. Many core–shell structures containing metals and oxides have been studied for their original magnetic properties (e.g. Fe(1-x)O/Fe3O4 [9] and Ni/Au [10]) or their electrocatalytic properties (e.g. Pd/Pt [11]). Zero-valent NPs have the advantage of being relatively cheap and ecofriendly NPs with Fe0 or FeII redox active species present in their structure. Another important potential application of Fe-NPs free of metallic Fe0 and containing Fe3O4 or maghemite γ-Fe2O3, is as materials for the treatment of cancers by magnetic hyperthermia [[12], [13], [14], [15]]. The numerous implications of Fe-based materials in various technological applications make the full characterization of these materials very important.
The nanostructure and physicochemical properties of the Fe-NPs synthesized in the presence of NaBH4 have been studied using several techniques, including X-ray diffraction (XRD) [3,7,8,[16], [17], [18], [19]], scanning electron microscopy (SEM) [16,19,20], high-resolution transmission electron microscopy (HR-TEM) coupled with energy dispersive X-ray spectroscopy (EDX) or electron energy loss spectroscopy (EELS) [7,8,[16], [17], [18]], atomic force microscopy (AFM) [13], X-ray photoelectron spectroscopy (XPS) [8,17,20], X-ray absorption near edge structure (XANES) [7,17], Raman [18] and Mössbauer spectroscopies [[19], [20], [21]] and finally electrochemically using cycling voltammetry or chronoamperometry [8]. In all these studies the NPs were constituted by a metallic Fe0 core and a Fe oxide shell. The chemical composition and the phase partition in the core–shell were shown to depend on the synthesis conditions such as the nature of the Fe salt containing either FeII or FeIII species and the contact time between the Fe salt and the reducing agent NaBH4 [18]. The XRD analyses provide essential information about the structure of the core constituted by metallic bcc Fe [3,7,[16], [17], [18]]. In some studies, very low intensity or broad diffraction peaks attributed to Fe oxides were observed [17]. Nevertheless, XRD is certainly not the most suitable technique to characterize more or less crystalized thin Fe oxide coatings present at the surface of Fe-NPs. The presence of a mixture of FeII and FeIII species in the shell was clearly shown in XANES and XPS studies [7,8,17]. Using a series of reference samples, Yao et al. [7] estimated the relative proportion of Fe0, FeII, and FeIII species in Fe-NPs. The determination of such a speciation was also possible using XPS [8,17], which probes the extreme surface of the analyzed materials (maximal analysis depth <10 nm). Since the average sizes of the synthesized Fe-NPs were 10–50 nm [7,8,17,18,20], XPS provided a good picture of the chemical composition of the NPs. In addition, such results should be carefully analyzed by considering the various fitting procedures used [22,23]. The unique technique 57Fe Mössbauer spectroscopy is used to determine the mineralogical nature of the Fe-rich phases as well as their relative proportions. Relatively complex Mössbauer spectra of Fe-NPs were recorded at room temperature (RT, 298 K) [19,21]. The complexity of the spectra is directly related to the reduced size of the grains and then their superparamagnetic behavior [[24], [25], [26], [27]]. In all the Mössbauer spectra previously shown, Fe0 was clearly identified but the exact chemical nature of the Fe oxides present in the shell was not. Moreover, some of the Mössbauer spectra at 298 K exhibited a paramagnetic ferrous doublet that was not fully interpreted [[19], [20], [21]]. Indeed, recording the Mössbauer spectra in a full range of temperatures of 4–298 K is generally required to obtain full characterization of a sample. This characterization has never been done for Fe/FeOx NPs. In our study, a combination of microscopic and spectroscopic techniques – TEM/scanning TEM (STEM) coupled with EELS/X-ray chemical imaging and electron diffraction, XPS and Mössbauer spectroscopy – were used to highlight the nanostructure of Fe-NPs and their evolution when interacting with PdIIaq ions.
Section snippets
Preparation methods
Iron(II) sulfate heptahydrate (FeSO4·7H2O, 99%), palladium(II) chloride (PdCl2, 99%), polyvinylpyrrolidone (PVP, average molecular weight = 55 000), NaBH4, and glacial acetic acid were purchased from Sigma Aldrich. Hydrochloric acid (HCl, 36.5%) was purchased from J.T. Baker.
The Fe@FeOx NPs were prepared by mixing 8.0 mL of 0.625 M FeSO4·7H2O aqueous solution and 10 mL of 2 M PVP methanol solution. The solution was then manually stirred with a glass rod for 1 min. The starting reagents were
Characterization of Fe@FeOx NPs
High-angle annular dark field (HAADF) (Z contrast; Z, atomic number) micrographs showed the Fe@FeOx NP structure (Fig. 1a). The NPs were mostly spherical in shape, with a core (very bright) and shell (less bright) as previously observed [7]. They also appeared in a chain-like aggregate conformation (Fig. 2). The diameter range of these NPs was ∼13–20 nm with a shell thickness of ∼3–5 nm (Fig. 1a and b). The bright field (BF)-TEM micrographs and the corresponding selected area electron
Conclusion
Mössbauer spectroscopy and XPS were used to determine the nature of the phases present in the core–shell nanostructure of Fe@Fe0x NPs. Metallic Fe0, a Fe(OH)2-like phase, magnetite, and a top layer of ferric oxides were identified. Interestingly, the Fe(OH)2-like phase was fully transformed when the Fe@Fe0x NPs reacted with PdIIaq and concomitantly the proportion of Fe0 did not vary significantly. The relative proportion of Fe species present in the core and the shell was revealed by TEM images
CRediT authorship contribution statement
M. Abdelmoula: Investigation, Conceptualization, Methodology, Software. C. Ruby: Writing – original draft, Writing – review & editing, Supervision. M. Mallet: Investigation, Writing – review & editing. J. Ghanbaja: Investigation. R. Coustel: Conceptualization, Writing – review & editing. Louis Scudiero: Writing – review & editing, Supervision. Wei-Jyun Wang: Chemical synthesis.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to thank Jason Levy DeMers from Washington State University for his contributions in measuring the pH values at each step of the synthesis of the NPs. Aurélien Renard (Université de Lorraine) is acknowledged for his help with the XPS measurements. The Mössbauer spectroscopy and XPS experiments were supported by means of the Spectroscopy and Microscopy Core Facility of SMI LCPME (Université de Lorraine, CNRS, LCPME- http://www.lcpme.cnrs-nancy.fr), which is gratefully acknowledged.
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