Effect of the Specific Surface Sites on the Reducibility of α-Fe2O3

Sep 3, 2013 - Helmholtz-Zentrum Berlin, Abt. Solar energie forschung AG Energie katalyse, Albert-Einstein-Str. 15 12489 Berlin, Germany. ∥ Fritz-Hab...
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Effect of the Specific Surface Sites on the Reducibility of α‑Fe2O3/ Graphene Composites by Hydrogen V. Papaefthimiou,*,† I. Florea,‡ W. Baaziz,† I. Janowska,† W. H. Doh,† D. Begin,† R. Blume,§ A. Knop-Gericke,∥ O. Ersen,‡ C. Pham-Huu,† and S. Zafeiratos† †

ICPEES, UMR 7515 du CNRS-Université de Strasbourg, ECPM, 25 Rue Becquerel, F-67087 Strasbourg Cedex 8, France IPCMS, UMR 7504 du CNRS-Université de Strasbourg, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France § Helmholtz-Zentrum Berlin, Abt. Solar energie forschung AG Energie katalyse, Albert-Einstein-Str. 15 12489 Berlin, Germany ∥ Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: The reducibility of iron oxide nanoparticles (NPs) supported over few-layer thick graphite upon annealing in hydrogen is investigated by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS), and high-resolution transmission electron microscopy (HR-TEM). It is found that the stability of the iron oxide NPs toward reduction is enhanced by the interaction with the graphene nanosheets as compared to the bulk iron oxide. Postannealing TEM micrographs reveal the existence of both core/shell and homogeneous iron oxide NPs with the latter forming irregular trenches into the graphene sheets. EELS analysis and TEM images clearly demonstrate that the reducibility of iron oxide particles depends on the specific graphene site on which they are attached. Furthermore, we show that graphene etching can be mediated by iron oxide NPs at relatively mild reduction conditions. extensively studied in the past.13,14 The stability of bulk iron oxides as a function of the oxygen pressure and temperature is also well described by thermodynamic phase diagrams.15 However, because of the large fraction of surface atoms and the interaction with the support, the reduction of NPs is often significantly altered compared to their bulk counterparts.16,17 Monitoring the reduction of iron oxides under flow of hydrogen (in situ) is essential because iron and especially NPs are chemically unstable and can be readily reoxidized when exposed to air. Not only the oxidation state but also the morphology and function of iron oxide NPs can be influenced by the gas atmosphere. For example, in oxidizing atmospheres, and at relatively elevated temperature, hollow iron oxide NPs can be formed as a consequence of the Kirkendall process,18,19 while in reducing atmospheres thermally activated iron nanoparticles can create trenches on graphene sheets.12 The latter has emerged as a promising method toward tailoring of the graphene sheet shape with concomitant effects in its incorporation in the electronics industry.20 Lately, it has been implied that not only metallic but also oxidized nanoparticles can be involved in the drilling process.20,21 Even though applications of hybrid structures of irondecorated graphene have been extensively studied within the

1. INTRODUCTION Iron oxides are critical components in many diverge technological areas including magnetic storage media, corrosion, and catalysis.1,2 Reduction of carbon (coal) in the presence of iron-based catalyst is the core process for direct coal liquefaction (DCL) and in this context has been studied extensively in the past.3 Recently, graphene/iron oxide composite materials have attracted much interest as advanced electrode materials for lithium ion batteries and electrochemical capacitors, magnetic resonance imaging, and so forth.4−10 Design of such graphene-based materials with tailored properties requires improved understanding of the interfacial interaction between the graphene sheets and the iron oxide particles.11 It is of primary importance to show how the reduction−oxidation chemistry of iron nanoparticles (NPs) is affected by the graphene support. This knowledge is essential taking into account that the performance of the composite material critically depends on the specific phase of the iron oxides, which in turn is dictated by the oxidation state and the crystal structure of iron. Furthermore, possible interaction at the interface of the two components could lead to an ultimate modification of the final composite, that is, catalytic etching and formation of graphite with high edge density.12 Several iron oxide compounds are known with α-Fe2O3 (hematite), Fe3O4 (magnetite), and FeO (wustite) being the most common. Reduction of bulk iron oxide and thin films epitaxially grown on single crystalline substrates has been © XXXX American Chemical Society

Received: July 9, 2013 Revised: August 27, 2013

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Figure 1. (a) XAS Fe L3,2-edge and (b) XPS C 1s spectra of Fe/FLGs upon annealing under O2 at 250 and 400 °C. The corresponding difference curve of C 1s spectra is also shown. The C 1s spectra were acquired with photon energy of 465 eV. The original spectra are normalized to the maximum peak height.

performed at the ISISS beamline at BESSY synchrotron facility at the Helmholtz Zentrum Berlin in a setup described elsewhere.24 The XAS spectra were measured in the total electron yield (TEY) detection mode. The samples were placed on a sample holder, which could be heated from the rear by an IR laser (cw, 808 nm). Fe/FLGs were primarily annealed at 0.2 mbar O2 up to 400 °C and were subsequently stepwise annealed at 0.2 mbar H2 by 50 °C up to 500 °C. Theoretical simulation of the experimental Fe L3,2 spectra of Fe foil has been analyzed with the charge-transfer multiplet (CTM) program.25,26 The calculations have been carried out using the CTM4XAS version 5.2 program27 and using literature values for the difference between the core hole potential and the 3d−3d repulsion energy Udd as well as for the hopping parameters.28 The morphology of the Fe/FLGs and the relative presence of O inside the Fe-based particles, before (as prepared) and after (post annealing), was analyzed by transmission electron microscopy in high resolution mode (HR-TEM) combined with EELS spectroscopy in the scanning TEM mode (STEM). The analyses were performed on a JEOL 2100 F electron microscope working at 200 kV with a Cs probe corrector and a postcolumn GATAN energy filter. For the EELS-STEM analysis, the EELS spectra were recorded for various positions of the electron beam focused probe (0.25 nm in diameter) using a convergent angle of about 25 mrad and a collection angle of 30 mrad.

past few years, to our knowledge, there are only a few detailed studies focusing on the reducibility of iron oxide nanoparticles.17 Here, we report on the effect of the step edges and planar graphene sites to the reducibility and the morphology of iron oxide nanoparticles. Near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and X-ray absorption spectroscopy (XAS) have been employed to study in situ and with high surface sensitivity the reduction of the iron oxide/few-layer thick graphite (Fe/FLG) hybrid materials annealed under the flow of hydrogen. In addition, postannealing high resolution transmission electron microscopy (HRTEM) imaging combined with electron energy loss spectroscopy (EELS) probes with high spatial resolution the effect of the graphene (planar or edge sites) to the iron particles’ morphology and reducibility.

2. EXPERIMENTAL SECTION FLGs were synthesized by mechanical thinning of graphitic precursor as described elsewhere.22 The synthesis of the Fe3O4 NPs on the FLG surface consists of the thermal decomposition of iron stearate complex in high boiling point solvent (octyl ether, 287 °C) in the presence of oleic acid as surfactant and FLG.23 In more detail, 150 mg of FLG was dispersed in 20 mL of octyl ether (97%, Fluka) and was sonicated for 10 min, and then 1.382 g (2.2 mmol) of iron stearate (9% Fe, Strem Chemicals) and 1.4 mL (4.4 mmol) of oleic acid (99%, AlfaAesar) were added. The reaction medium was subsequently heated to 110 °C under stirring for 30 min in order to dissolve the reactants and to eliminate traces amount of water or organic solvent. The system was heated up to reflux (287 °C) under air with a heating rate of 5 °C·min−1 and was kept at this temperature for 2 h. The resulting black solution was then cooled to room temperature, and the NPs on FLG were washed three times by the addition of a mixture of hexane/acetone (1/ 3) followed by centrifugation (14 000 rpm, 10 min); they were filtrated and easily suspended in chloroform. For the spectroscopic characterization, the Fe/FLG solution was drop cast onto the surface of a clean Au polycrystalline foil. The procedure was repeated several times until the Au 4f photoelectron signal from the Au support was completely suppressed (estimated thickness of the Fe/FLG layer was higher than 5 nm). NAP-XPS and XAS spectroscopies were

3. RESULTS 3.1. Oxidation of Fe/FLGs. Characterization of pristine Fe/FLG hybrids indicated that Fe3O4 (magnetite) nanoparticles with sizes ranging from 5 to 8 nm were homogenously distributed over the edge and planar areas of the graphene support (see Supporting Information S1). To remove surfactants and residual carbon species, the sample was initially stepwise annealed in 0.2 mbar O2 up to 400 °C. The Fe L-edge spectra recorded under O2 flow are presented in Figure 1a. At 250 °C, the Fe L-edge spectrum resembles that of γ-Fe2O3 for which the shoulder at ca. 708.2 eV is attenuated.29 The attenuation of this shoulder is also consistent with the smaller crystal field splitting (10 Dq) value that is an indication of larger Fe−O distance and less Fe−O interaction at this B

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temperature.30 Upon further annealing at 400 °C, the characteristic peak shape of α-Fe2O3 is observed, as confirmed by the simulations presented below, in the literature data31 and in the Fe 2p and O 1s photoelectron peaks (see Supporting Information S2) that appear at 710.9 and 529.8 eV, respectively.32−35 The O/Fe stoichiometry as calculated by the Fe 2p and O 1s peaks is 1.5 ± 0.2 in both temperatures as expected for stoichiometric Fe2O3 oxide (see Table S1 of the Supporting Information). This indicates that under 0.2 mbar O2 the Fe3O4 oxide is stepwise oxidized to γ-Fe2O3 and αFe2O3 at least to a depth equal to or higher than the probing depth of the XAS technique (ca. 5 nm). In Figure 1b, the XPS C 1s peaks measured upon oxidation are shown. A sharp asymmetric C 1s peak at 284.4 eV typical of the C−C sp2 bonds of FLG dominates the spectrum.36 Comparison of the C 1s peaks recorded at 250 and 400 °C indicates differences in the curve shape, which are clearly visible in the difference peak (bottom curve). In particular, the extra C 1s peak components at 283.9 and 284.8 eV observed at 250 °C decrease after annealing at 400 °C. The high binding energy (BE) component can be related to defects on the graphite structure and oxygen functional groups attached on the surface.37 This component can be also related to the existence of Fe−O−C bonds taking also into account the XPS results for iron and oxygen (Figure S2 of the Supporting Information) and the literature references for magnetite/graphene hybrid materials.38 The low BE broadening of the carbon main line might be related to defects formed upon the initial preparation of the composite and removed upon annealing. Consecutive annealing in H2 (spectra not shown) does not induce any modification of the C 1s spectra indicating no pronounced differences on the graphene layers after oxidation. 3.2. Reduction of Fe/FLGs. The reducibility of the αFe2O3−FLG hybrid material under 0.2 mbar H2 was studied upon stepwise annealing. Selected Fe L-edge spectra at various temperatures are shown in Figure 2. Spectra recorded on a preoxidized α-Fe2O3 iron foil under identical conditions are included for comparison. Heating up to 250 °C has no effect on the Fe L-edge spectra for both samples. Further annealing at 350 °C induces partial reduction of the α-Fe2O3 grown on the foil, while only minor changes are observed for the α-Fe2O3− FLG. Finally, at the maximum annealing temperature used here (500 °C), the iron foil is reduced to FeOx31,39,40 (0.75 < x < 1) as is also confirmed from the chemical shifts in the Fe 2p3/2 XPS peak and the O/Fe atomic ratio (Supporting Information S2). Octahedral (Oh) Fe3+ ion coordination state was found to be the best match to our experimental curve for Fe2O3 while C4v symmetry with pure d6 character best matches the FeO.41 However, the Fe L-edge spectrum of Fe/FLG sample at 500 °C is complex and cannot be simulated by any known iron oxide structure. In contrast, the spectrum can be fitted by a linear superposition of FeO and Fe3O4 spectra of ca. 1:1 ratio indicating a mixture of the two iron oxides. Considering that Fe2O3 contains ferric ions (Fe3+), FeO ferrous ions (Fe2+), and Fe3O4 (both Fe2+and Fe3+), the valence state of iron as a function of annealing temperature in H2 is presented in Figure 2c for the foil and the Fe/FLG samples. It is evident that ferric ions (due to Fe2O3 and Fe3O4) that exist in the iron oxide NPs are reduced to ferrous ions at significantly higher temperatures compared to the bulk iron oxide foil. Stabilization of metal oxide nanoparticles on graphene under highly reducing conditions has been observed by Schäffel et al.42 using ex situ TEM. Cobalt particles in two oxidation states

Figure 2. XAS Fe L3,2 spectra for Fe/FLGs (left) and Fe foil (right) taken upon annealing at various temperatures under H2. The Fe/FLG spectrum at 500 °C is analyzed by deconvoluting FeO and Fe3O4 spectra. The amount of Fe3+ is given at the bottom for the Fe/FLGs and the Fe foil as a function of the annealing temperature. Blue lines: theoretically simulated Fe L3-edge absorption spectrum for Fe3+ (Octahedral (Oh) coordination, crystal field splitting 10 Dq = 1.1 eV, charge transfer energy Δ = 3, hopping eg electrons T(eg) = 2, hopping t2g electrons T(t2g) = 1) and Fe2+ (C4v symmetry, 10 Dq = 0.6 eV, Dt = 0.05).

attached on different graphene sites were found after H2 treatment to 775 °C. In particular, cobalt oxide NPs were exclusively attached to flat basal graphite planes while metallic Co was found to be attached to FLG edges. However, as in most ex situ studies, it was not possible to clarify if oxidized particles could partly withstand the reducing procedure or if they were formed during transfer from the reactor to the microscope because of ambient air exposure. Here, we confirm by in situ methods that Fe3O4 nanoparticles attached on FLG can resist in high temperature hydrogen treatment in contrast to Fe3O4 grown onto a bulk iron foil. This observation implies a strong interaction between the iron oxide and specific graphene sites. To clarify if the differences in the oxidation state of iron NPs are related to the FLG support anchoring sites, we performed TEM/EELS analysis of the post-treated samples. A typical large area TEM micrograph (Figure 3a) shows that iron NPs with sizes of 10−20 nm and rarely bigger aggregates decorate the surface. Compared to their initial size (ca. 10 nm, see Supporting Information S1), particle agglomeration after annealing is rather limited indicating relatively strong iron− FLG interaction. Similar results have been previously reported for the Pt/FLG system and were attributed to a strong interaction between the metal NPs and the graphene surface C

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Figure 3. Postannealing TEM image of Fe/FLGs (a) wide area image, (b) core−shell iron NPs, (c) initial stage of trench formation/drilling, and (d) nanotrenches formation in FLGs.

defects.43,44 A closer look at the TEM images reveals the existence of three shapes of iron NPs (Figure 3b−d): (a) Spherical core−shell iron oxide NPs (S-CS) with different gap width (void) between the core and the shell, which contribute to about 45% of the overall iron particles (Figure 3b). Analysis of TEM images showed that these particles are located on flat basal graphene planes and do not etch tracks on FLG layers. (b) Semispherical core−shell iron oxide NPs (Sm-CS), which consist of about 40% of the overall iron particles (Figure 4c). The Sm-CS particles are preferentially located at the edge of the FLG layers but do not induce any linear trench on graphene. (c) Homogeneous solid iron oxide NPs (HS) (Figure3d), which are about 15% of the overall iron particles. These particles are located at edges and create short irregular trenches with sizes of a few nanometers on the FLGs. In contrast to the other two morphologies, these NPs consist of a single type of iron oxide and not of a core− shell structure. HAADF-STEM (high angle annular dark field-scanning TEM) images and EELS spectra of the three iron oxide NP morphologies are presented in Figure 4. The Fe L-edge and O K-edge EELS spectra of selected areas (see right part of Figure 4) or within line profile mode (see Supporting Information S4) are indicative of the iron oxidation state. In particular, the presence of a prepeak at the O K-edge (below the dominant contribution around 540 eV) and the energy position of the principal peak of the Fe K-edge at around 714 eV observed at the shell areas of S-CS and Sm-CS NPs are characteristic of αFe2O3 or Fe3O4 oxides.45,46 The absence of O K-edge signal of the EELS spectra recorded at the center of Sm-CS particles reveals that they consist of metallic iron at the core and αFe2O3 or Fe3O4 at the shell (see Figure 4). This is also confirmed by the line profile analysis of the nanoparticles presented in the Supporting Information S4. Reduction of iron oxide to metallic iron can be facilitated by a strong interaction between the Fe particles and the FLG support as has been previously proposed for Fe2O3 particles inside the channels of carbon nanotubes47 (also see Supporting Information S5). Furthermore, there is an intense C K-edge signal on the EELS

Figure 4. EELS analysis of the three iron morphologies: spherical core−shell iron oxide (S-CS), semispherical core−shell iron oxide (Sm-CS), homogeneous solid iron oxide (HS). For each morphology, we present HAADF-STEM images taken on representative NPs as well as the EELS spectra recorded at the positions indicated on the images.

spectrum of Sm-CS particles (recorded on both core and shell particle areas). The observation of both metallic iron and intense C K-edge signal in the EELS spectra taken on these particles, as well as TEM image analysis (see Figure S5 of the Supporting Information), suggests that these particles are effectively protected against oxidation by being buried underneath the FLG layers.42 In the case of HS particles, the absence of the prepeak at the O K-edge is characteristic of FeO.45 Furthermore, the Fe/O ratio measured from the EELS spectra (see Figure S6 of the Supporting Information) is persistently higher than the one measured for the two other kinds of NPs (S-CS and Sm-CS), also consistent with the change in their oxidation state (since Fe/O in FeO is 1 instead of 0.66 in Fe2O3). D

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4. DISCUSSION To combine the information obtained from in situ spectroscopy (NAP-XPS and XAS) with that of ex situ TEM/EELS, one should keep in mind the differences in the surface sensitivity and the measuring environment between the two methods. In particular, because of the short photoelectron escape depth (up to 5 nm), the core of the surface-supported nanoparticles (SCS) and the particles hidden between graphene layers (Sm-CS) do not substantially contribute to the NAP-XPS and XAS signal, in contrast to the microscopy images which probe the whole volume of the iron NPs. EELS line profile analysis (see Supporting Information S4) of the HR-TEM images shows metallic Fe in the core of Sm-CS particles, while metallic iron was never observed by the XAS and NAP-XPS spectroscopic methods, confirming the differences in the depth analysis of the two methods used here. Metallic Fe should be easily detected in the Fe 2p XPS peak, since it appears at ca. 707 eV, significantly shifted with respect to the iron oxide peaks.48 Therefore, we assume that the NAP-XPS and XAS signals mainly derive from the HS and the shell of the S-CS iron oxide particles because the signal of Sm-CS is attenuated by the carbon layer. Second, the oxidation state of iron deduced from the TEM experiments might be influenced by the air exposure during the transfer of the sample from the NAP-XPS to the microscope chamber. However, this effect does not account for the formation of thick oxide layers or void/core shell particles (that have been observed only under controlled high temperature oxidation).49 In situ XPS and XAS results give solid evidence that the surface of the iron nanoparticles under 0.2 mbar H2 atmosphere and up to 500 °C remains oxidized. This observation gives a particular value on the ex situ microscopy images because it confirms that iron oxide was not formed during the sample transfer to the microscope chamber. Our results indicate that the interaction with the FLG surface sites influences the reducibility of iron oxide and, in particular, that step/edge FLG sites facilitate the reduction of α-Fe2O3 to FeO, compared to particles attached to flat basal FLG sites. Closer inspection of the TEM images (Figure 3b and c) at the interphase between NPs and FLG indicates that the support does not have the graphitic structure, but it resembles more like amorphous carbon. This observation can provide a possible explanation for the enhanced stability of iron oxide NPs on flat support areas. In particular, during the annealing treatment, iron oxide NPs may interact with the support that can form an ultrathin protective amorphous carbon shell (hard to distinguish in TEM images and EELS spectra), stabilizing the S-CS iron oxide NPs formed during the O2 treatment against hydrogen reduction. The existence of Fe−O−C bonding can be also supported by the persistence of the high BE component in the O 1s peak (see Supporting Information S2). The protection of Sm-CS particles by thick carbon layers is clearly indicated at the C K-edge EELS spectra (Figure 4). On the other hand, HS particles at the edge sites are easily reduced and form trenches by gasifying the carbon support. The proposed iron oxide stabilization mechanism can also explain the discrepancy on the relative Fe3O4 to FeOx amount obtained by the XAS and TEM results. In particular, according to XAS (Figure 2), about 50% of iron oxide reduces to FeOx after the highest temperature treatment, while statistical TEM image analysis shows that HS particles (FeOx) are only 15%. If the Fe3O4 NPs are covered by a thin carbon layer, because of the surface sensitivity of XAS

signal, their Fe L-edge will be preferentially attenuated and, therefore, will have lower signal intensity compared to FeOx. The above presented results reveal also some new insights on the mechanism of iron-mediated etching of graphene. Typically, graphene etching by transition-metal particles (Fe, Ni, Co) involves metallic particles at etching conditions with significantly higher temperature (>800 °C) and hydrogen pressures.12,50 We show here that FeOx NPs may also be accountable for etching FLGs layers at temperature and hydrogen pressure much lower than those previously applied for iron-mediated etching of graphene. The catalytic action of FeO has been established, and the role of unsaturated ferrous sites confined between nanostructured FeO and noble-metal (Pt, Au) substrates has been demonstrated.51,52 In our case, HR-TEM was indispensable to demonstrate that solely FeO is related to the trenches observed in the FLGs and thus to establish the catalytic action of FeO. The tracks on graphene formed by the FeO NPs are irregular and relatively broad compared to those previously observed at higher temperature and hydrogen pressure.12,51 Assuming that the etching mechanism involves dissociation of molecular H 2 and subsequent reaction of atomic hydrogen with carbon atoms at the iron/graphene interface,42 the temperature and hydrogen pressure should affect the reaction kinetics, being accountable for the short path and irregular shape of the trenches.

5. CONCLUSIONS Summarizing, our spectroscopic results reveal the oxidation state of iron upon thermal redox treatments, while HR-TEM and EELS reveal the formation of different kinds of iron nanoparticles on the FLGs after the in situ studies. α-Fe2O3 formation is observed upon annealing of the Fe/FLGs under O2. By annealing under H2, Fe2O3 is partially reduced to FeO. Some of the iron NPs drill the FLGs and are immobilized on the graphene surface. Others have enough energy to create a trench on the FLGs, even though they are oxidized. Altogether, in this study, experimental proof for the role of FeOx during the creation of trenches in the FLGs at relatively low temperatures (around 500 °C) is established. We propose that in the absence of metallic Fe on the particle surface, it is FeOx that catalyzes the hydrogenation reaction of graphene. In that sense, the present work is a step forward toward the fundamental comprehension of the interference of metal oxides during catalytic etching of graphene and the design of advanced iron oxide nanocatalysts.

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ASSOCIATED CONTENT

S Supporting Information *

TEM, XAS, and XRD of pristine Fe-FLGS. Fe 2p and O 1s XPS spectra of Fe foil and Fe/FLGs. TEM images of the nanoparticles. EELS line profile analysis of iron and oxygen concentration for the three iron morphologies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. E

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ACKNOWLEDGMENTS This work was supported by the European Community’s Seventh Framework Program (FP7/2007-2013, grant agreements n°.226716 and 245202). We acknowledge the Helmholtz-Zentrum Berlin (electron storage ring BESSY II) for provision of synchrotron radiation at ISISS beamline and financial support from REALISE network. The authors are grateful to B. Pichon and S. Begin-Colin for their assistance in the synthesis of the Fe3O4 NPs.

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