Characterization and Oxidation of Fe Nanoparticles Deposited onto

6418

J. Phys. Chem. C 2009, 113, 6418–6425

Characterization and Oxidation of Fe Nanoparticles Deposited onto Highly Oriented Pyrolytic Graphite, Using X-ray Photoelectron Spectroscopy D.-Q. Yang† and E. Sacher* Regroupement Que´becois de Mate´riaux de Pointe, and De´partement de Ge´nie Physique, E´cole Polytechnique, C.P. 6079, succursale Centre-Ville Montre´al, Que´bec H3C 3A7 Canada ReceiVed: NoVember 19, 2008; ReVised Manuscript ReceiVed: February 24, 2009

The characterization of Fe0 nanoparticles (NPs), both before and during oxidation, has been of concern for the last two decades. We have studied the 2p and 3p XPS core levels of Fe NPs evaporated onto highly oriented pyrolytic graphite (HOPG) under ultrahigh vacuum. Both components of the 2p spectrum of Fe0 are found to be highly asymmetric to higher binding energy; each is composed of a major photoemission peak, plus several smaller peaks attributable to a vacancy cascade, a process known to occur in Fe. In contrast, the two Fe 3p spectral components are too close to be separated with precision, and were treated as one single component; as with the 2p components, it, too, is asymmetric, due to the vacancy cascade. The onset of oxidation affects both spectra somewhat differently, causing the introduction, and subsequent increase, of components on the high binding energy side of the 2p3/2 spectrum, superimposed on the vacancy cascade; this is not as obvious for the 3p spectrum because of a larger probe depth, to which the surface contributes less. These new components represent the FeO, γ-Fe2O3, and Fe3O4 formed on oxidation; their oxidation kinetics indicate that the initially formed FeO is rate controlling. Introduction Fe-based nanoparticles (NPs), including oxides and alloys, have received substantial attention for several decades because (1) such NPs constitute the key materials behind the recent development of rewritable electronic media, (2) improvements in their production have led to increased efficiency and reduced component size in many electronic products, (3) they can be used in the diagnosis and treatment of medical diseases and as electronic sensors,1,2 (4) they represent an important catalyst used in the formation and cleavage of C-C bonds and, (5) under the name nanoscale zeroValent iron, and often abbreviated as nZVI, they are an effective reagent for the treatment of toxic and hazardous chemicals.3,4 X-ray photoelectron spectroscopy (XPS) has been widely used to characterize the surface compositions of these materials.5-10 The complexities of the Fe 2p core level peaks in the various Fe oxides (and halides) have been extensively noted in these references, although efforts have been made to use them in estimating chemical compositions and surface electronic states. Whereas the complexities have been discussed using many-body effects,11,12 there is, nonetheless, substantial disagreement among the various models13-18 that have been proposed. Early XPS studies focused on bulk13-15 and thin film samples of Fe19-22 and some of its compounds,5-10 but comparatively little work has been carried out on Fe NPs. Here, we have used XPS to characterize the chemical state of Fe NPs, evaporated onto highly oriented pyrolytic graphite (HOPG), with which it reacts minimally. Such minimal interaction avoids surface wetting, permitting the surface retraction of the deposited material, to form NPs. Our motivations have been to acquire more detailed information on the core level peak shape as a function of deposited NP size, to furnish a better † Present address: Surface Science Western, Room G-1, Western Science Centre, The University of Western Ontario, London, Ontario N6A 5B7, Canada.

understanding of the XPS spectra, as well as to obtain standard spectra to be subsequently used in the quantitative analysis Fe/ oxide core-shell NP structures.23,24 Experimental Section XPS were carried out on a VG ESCALab 3 Mark II, in which the sample preparation chamber is separated from the instrument analysis chamber by a gate valve, avoiding air exposure on sample transfer. Grade ZYA HOPG was obtained from Advanced Ceramics, Inc.; it was cleaved with adhesive tape just prior to each experiment and immediately inserted into the spectrometer. This technique assures that an almost undetectable trace of oxygen is found on the HOPG, at the step edges where free radicals are created by the cleavage process. Measurements on samples so prepared indicate the relative concentration of oxygen to be ∼0.1%. High-purity Fe (>99.9995%) was evaporated in the preparation chamber of our spectrometer, using an e-beam evaporator, at a deposition rate of 0.2 nm/min and a pressure of 2 × 10-8 Torr (the base pressure was 20 at the higher deposition thicknesses, to produce a contaminant peak the size of the asymmetry, as previously noted; • multiple ionizations, due to multiphoton absorption,43,44 are expected to contribute little, if anything, due to a lack of sufficient photon intensity. It appears, then, that a distinctly possible explanation for the surprisingly similar 2p asymmetries found for all Fe spectra is offered by the vacancy cascade cited above, with a possible minor contribution from multiphoton absorption. As noted earlier, theoretical calculations36,37 have so far been confined to inner-shell vacancy creation in isolated atomic systems. It is not clear just how the probabilities of the various contributions to the overall process will vary with NP size and shape; however, with the omission of 1s photoemission, ref 36 indicates that the processes still contributing to the cascade (i.e., 2p f 3s, 2p f 3d, 3p f 3d) are actually enhanced in intensity. Further, whereas processes involving the deep inner-shell vacancy states would be expected to be much less altered in comparison with the processes involving the outer electron states, small changes in the energies of the many-electron states are known to have an important effect on the energetic permissibilities of certain Auger and Coster-Kronig transitions, so that the elementary transition probabilities would also be modified in comparison with those for an isolated atom. Thus, a presently undetermined number of vacancy cascade peaks is expected for the 2p and 3p processes (we found 4 and 2 respectively using parameters similar to those for the main photoemission peak) with presently undetermined intensity ratios. Atypical Fe 2p and Fe 3p Peaks at Low Coverages. Both the Fe 2p and Fe 3p spectra contain peaks, at low coverages, which are not typical of those formed at higher coverages: the Fe 2p spectrum, in Figure 1, manifests an additional peak at a higher binding energy of ∼711 eV and, the Fe 3p spectrum, in Figure 10, one at a lower binding energy of ∼ 51 eV. The data evolve with deposition, making it clear that, as deposition continues, these spectra are subsumed into the more typical spectra, at ∼707 and ∼53 eV, respectively. That is, a buildup of material over that originally deposited conceals the original spectrum, as we previously found for Ni.42 This means that we are dealing with an initial layer of Fe that has deposited onto, and interacted with, HOPG. In the present case, this interaction is stronger than for Ni because the reaction there was too weak to form a Ni-C bond, whereas, here, the C 1s spectrum, in part a of Figure 4, shows that a Fe-C bond has been formed. In support of this, Figure 11 shows the O 1s: C 1s and Fe 2p3/2: C 1s ratios of a 0.5 nm Fe deposit, as a function of time. The lack of change shows the absence of surface diffusion, as expected for interfacial bond formation. The shift of the Fe 2p spectrum to higher binding energy indicates that the interaction slightly depletes the electron density in that orbital, whereas the shift of the Fe 3p spectrum to lower binding energy indicates that the opposite happens there. This is visualized to happen in the following way: the interaction of the electrons in the Fe valence orbital, with the π orbital system of the HOPG substrate, by backbonding42 (in which a filled π orbital on the HOPG overlaps an empty p orbital of the Fe, whereas a filled d orbital of the Fe overlaps an empty π* orbital

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6423

Figure 11. Evolutions of the O 1s: C 1s and Fe 2p3/2: C 1s relative intensity ratios, for a deposit of 0.5 nm, as a function of oxygen exposure.

of the HOPG, thus attempting to maintain electroneutrality) causes changes in the electron densities of the 2p and 3p core orbitals, in which that in the 2p orbital is somewhat depleted, and that in the 3p orbital, somewhat enhanced. While we are presently planning density functional simulations, in an effort to verify this, we note that similar behavior has been found for the Fe 2p spectrum of nanoparticles deposited onto Al2O3, and is similarly attributed.45 Fe NP Oxidation. When considering the various possible Fe oxidation products, one must also be careful to consider their crystalline structures, which differ, one from another. The importance of doing this may be illustrated by considering Co3O4.46 This oxide exists in a spinel structure, where CoII is tetrahedrally surrounded by O, and CoIII, octahedrally surrounded. While the octahedral O atoms lie at the apexes of the CoIII px, py, pz, dx2-y2 and dz2 orbitals, there is far less Co-O overlap in the case of tetrahedrally coordinated CoII, where extensive overlap occurs only in the cases of one lobe each of the pz and dz2 orbitals. Thus, whereas the Co 2p spectrum manifests two peaks, that for CoIII is at a lower binding energy than that for CoII. While not discussed by us,46 this is due to the fact that extensive orbital overlap reduces the positive charge of the formal CoIII to below that of the formal CoII. Only one O 1s peak is found, meaning that both the tetrahedral and octahedral O atoms have the same electron density. In a similar fashion, the present results show four new peaks in the Fe 2p spectrum but only one Fe-O peak in the O 1s spectrum. Thus, for all the actual oxides formed, the electron density at the O atoms remains virtually constant. It is instructive to consider the various possible oxides (hydroxides are excluded because pure, dry oxygen was bled into the apparatus). These include • FeO, whose surface is unstable to oxidation and which is generally written as Fe0.95O; in the bulk, it exists in a rock salt structure; at the surface, it disproportionates to Fe0 and Fe3O4 [4FeO f Fe0 + Fe3O4]; • Fe2O3, which exists in two common forms, R, in the corundum structure, and γ, in the cubic close-packed structure; • Fe3O4, which exists in an inVerse spinel structure, where the FeII is in an octahedral oxygen environment along with half the FeIII, whereas the other half of the FeIII exists in a tetrahedral oxygen environment; the oxidation of the FeII content to FeIII, without a change in crystal structure, leads to γ-Fe2O3. A serious obstacle to the precise analysis of the surface oxidation spectra is the lack of consistency, in the literature, as

6424 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Yang and Sacher

to the order of binding energies and spectral shapes of the various oxides.6-10,47,48 Nonetheless, an experimental study very similar to ours49 was recently carried out on millimeter-sized discs of Fe, with similar results. These authors used the QUASES spectral superposition software, with angle resolution, to analyze their experimental data on the oxidation process. Their results indicate that the surface oxide that is formed gradually increases, in its extent of oxidation, from Fe3O4 at the NP (core) surface, to γ-Fe2O3 at the outer oxide (shell) surface. As noted earlier, this is accomplished by the oxidation of the FeII ions to FeIII, without a change in crystal structure. Thus, the chemical distribution varies from 1:1:1 FeIIocta:FeIIIocta:FeIIItetra for the Fe3O4 at the interface to 2:1 FeIIIocta:FeIIItetra for the γ-Fe2O3 at the surface. In a recent paper,50 grazing ion scattering, Auger electron spectroscopy, and low-energy electron diffraction were used to study the oxidation of the Fe(110) surface by molecular oxygen at 420 K, over a range of oxygen exposures similar to ours. The authors found that 3-5 layers of FeO(111) were formed on the Fe(110) surface, having extremely poor long-range structural order because of crystal size incompatibility. They further showed that the low oxygen sticking coefficient, and its slow dissociation at the surface, were rate-determining. In addition, Fe(111), prepared epitaxially on Pt(111) was found51,52 to reconstruct into a p(2 × 2) LEED pattern, whose outer surface forms a layer of Fe3O4,52 whose lattice parameters are such as to induce such a reconstruction. On the basis of this, we propose that our Fe0 oxidation process must pass through the initial formation of FeO, which subsequently disproportionates to form Fe3O4; this latter material then oxidizes to form γ-Fe2O3 as oxygen permeates from the outer (shell) surface. We visualize the overall NP oxidation process as

Fe0 + [O] f FeO

(1)

4FeO f Fe + Fe3O4

(2)

Fe3O4 + [O] f γ-Fe2O3.

(3)

0

Basing ourselves on • the Co example given earlier,46 as well as the fact that, • for a given crystal structure, FeIII is expected to undergo photoemission at a higher binding energy that FeII, and that • an octahedral environment will, through more efficient overlap, lower the positive charge of a central transition metal cation to a greater extent than will a tetrahedral environment, we are led to the tentatiVe conclusion that C1 represents disordered FeIIrock salt, C2 represents FeIIocta, C3 represents FeIIIocta, and C4 represents FeIIItetra (no attempt was made, at these low concentrations, to adjust peak widths to account for the Russell-Saunders coupling that FeIII is expected to undergo). Thus, in the pure materials, the Fe 2p3/2 envelope of FeO will have a single component (C1), that of Fe3O4, three components (C2, C3, and C4, in the ratio 1:1:1) and that of γ-Fe2O3, two components (C3 and C4, in the ratio 2:1). It should be noted that the attributions are referred to as tentative because the first and third points are based on our work on Co and our knowledge of ligand field chemistry, and not on any direct comparisons of FeIII in tetrahedral and octahedral environments. We are presently carrying out such a comparison because, in the case of spinel structures,6 the XPS results do not lend themselves to

be used in this fashion and, in the case of mixed alkali metal salts (i.e., tetrahedral NaFeCl4 and octahedral Na3FeCl6), no XPS data exist. We plan to publish our findings in the near future. To the best of our ability to separate the C components, and believing that our attributions are correct, Figure 8 indicates that d(FeIIrock salt)dt ≈ d(FeIIocta)/dt ≈ d(FeIIIocta)/dt ≈ d(FeIIItetra)/ dt, implying that eq 1 is the rate-determining step; thus, the FeO formed immediately disproportionates, to give Fe3O4, which oxidizes to γ-Fe2O3; that is, the rates of eqs 2 and 3 are much greater than that of eq 1. For this reason, the rates of formation of Fe3O4 and γ-Fe2O3 cannot exceed that of FeO. This mechanism is in agreement with the formation of FeO(111), cited above,49 in which rate determination was governed by the molecular oxygen sticking coefficient and its subsequent dissociation. Conclusions We have used XPS to characterize Fe0 NPs and the oxidation product on their surfaces, when exposed to pure O2 in vacuum. The Fe 2p and 3p photoemission spectra have been found to be asymmetric to higher binding energies, which has been shown to be consistent with a vacancy cascade process, known to occur for Fe. An analysis of the oxidation process, taking crystal structure into account, indicates that the bare Fe surface is attacked by the sticking and dissociation of O2 to form FeO, which disproportionates to give Fe3O4; on exposure to further O2, the FeII present in Fe3O4 oxidizes to FeIII, resulting in the formation of γ-Fe2O3, without a change in crystal structure. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for funding, and Dr. V. L. Jacobs, Naval Research Laboratory, for his aid on vacancy cascades. References and Notes (1) Neri, G.; Bonavita, A.; Galvagno, S.; Siciliano, P.; Capone, S. Sens. Actuators, B 2002, 82, 40. (2) Schulz, H. Appl. Catal., A 1999, 186, 3. (3) Li, X. Q.; Elliott, D. W.; Zhang, W. Crit. ReV. Solid State Mater. Sci. 2006, 31, 111. (4) Ponder, S. M.; Darab, J. G.; Mallouk, T. E. EnViron. Sci. Technol. 2000, 34, 2564. (5) Graat, P.; Somers, A. J. Surf. Interface Anal. 1998, 26, 733. (6) McIntyre, N. S.; Zetaruk, D. G. Anal. Chem. 1977, 49, 1521. (7) Grosvenor, A. P.; Kobe, B. A.; Biesinger, M. C.; McIntyre, N. S. Surf. Interface Anal. 2004, 36, 1564. (8) Aronniemi, M.; Lahtinen, J.; Hautojarvi, P. Surf. Interface Anal. 2004, 36, 1004. (9) Prince, K. C.; Matteucci, M.; Kuepper, K.; Chiuzbaian, S. G.; Bartkowski, S.; Neumann, M. Phys. ReV. B 2005, 71, 085102. (10) Yamashita, T.; Hayes, P. J. Electron Spectrosc. Relat. Phenom 2006, 152, 6. (a) Paparazzo, E. J. Electron Spectrosc. Relat. Phenom. 2006, 154, 38. (b) Yamashita, T.; Hayes, P. J. Electron Spectrosc. Relat. Phenom 2006, 154, 41.. (11) Doniach, S.; Sˇunjic´, M. J. Phys. C 1970, 3, 285. (12) Kotani, A.; Toyozawa, Y. J. Phys. Soc. Jpn. 1974, 37, 912. (13) Pessa, V. M. Phys. Scr. 1977, 15, 352. (14) Berndt, K.; Brummer, O.; Mark, U. Phys. Status Solidi B 1978, 86, K93. (15) Leiro, J. A.; Heinonen, M. H. Phys. ReV. B 1999, 59, 3265. (16) (a) Yang, D.-Q.; Sacher, E. Langmuir 2006, 22, 860. (b) Yang, D.-Q.; Sacher, E. Surf. Sci. 2002, 504, 125. (17) Yang, D.-Q.; Zhang, G.-X.; Sacher, E.; Jose-Yacaman, M.; Elizondo, N. J. Phys. Chem. B 2006, 110, 8348. (18) Zhang, G.-X.; Yang, D.-Q.; Sacher, E. J. Phys. Chem. C 2007, 111, 565. (19) (a) Brundle, C. R.; Chuang, T. J.; Wandelt, K. Surf. Sci. 1977, 68, 459. (b) Corneille, J. S.; He, J.-W.; Goodman, D. W. Surf. Sci. 1995, 338, 211. (20) Castro, V. D.; Caimpi, S. Surf. Sci. 1995, 331, 294. (21) Gao, X. Y.; Qi, D. C.; Tan, S. C.; Wee, A. T. S.; Yu, X. J.; Moser, H. O. J. Electron Spectrosc. Relat. Phenom. 2006, 151, 199.

Characterization and Oxidation of Fe Nanoparticles (22) Khan, N. A.; Matranga, C. Surf. Sci. 2008, 602, 932. (23) Sun, Y.-P.; Li, X.-Q.; Cao, J.-S.; Zhang, W.-X.; Wang, H.-P. AdV. Colloid Interface Sci. 2006, 120, 47. (24) Martin, J. E.; Herzing, A. A.; Yan, W.; Li, X.-Q.; Koel, B. E.; Kiely, C. J.; Zhang, W. X. Langmuir 2008, 24, 4329. (25) Yang, D.-Q.; Sacher, E. J. Phys. Chem. B 2005, 109, 9703. (26) http://www.phy.cuhk.edu.hk/∼surface/XPSPEAK/ (27) Ho¨chst, H.; Goldmann, A.; Hu¨fner, S. Z. Physik B 1976, 24, 245. (28) (a) Yang, D.-Q.; Sacher, E. Surf. Sci. 2002, 516, 43. (b) Yang, D.-Q.; Meunier, M.; Sacher, E. Appl. Surf. Sci. 2001, 173, 134. (29) (a) Chapon, C.; Henry, C. R. Surf. Sci. 1981, 106, 152. (b) Bardotti, L.; Jensen, P.; Hoareau, A.; Treilleux, M.; Cabaud, B. Phys. ReV. Lett. 1995, 74, 4694. (c) Luedtke, W. D.; Landman, U. Phys. ReV. Lett. 1999, 82, 3835. (30) Binns, C.; Baker, S. H.; Demangeat, C.; Parlebas, J. C. Surf. Sci. Repts. 1999, 34, 105. (31) Khomanov, I. N.; Gavioli, L.; Fanetti, M.; Casella, M.; Cepek, C.; Mattevi, C.; Sancrotti, M. Surf. Sci. 2007, 601, 188. (32) Somorjai, G. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, N.Y., 1981. (33) Weisheit, J. C. Astrophys. J. 1974, 190, 735. (34) Hatchett, J.; Buff, J.; McCray, R. Astrophys. J. 1976, 206, 847. (35) Cooper, E. P. Phys. ReV. 1942, 61, 1. (36) Jacobs, V. L.; Davis, J.; Roszynai, B. F.; Cooper, J. W. Phys. ReV. A 1980, 21, 1917. (37) Jacobs, V. L.; Roszynai, B. F. Phys. ReV. A 1986, 34, 216. (38) Gupta, R. P.; Sen, S. K. Phys. ReV. B 1974, 10, 71. (39) See e.g., http://wwwchem.uwimona.edu.jm/courses/RScoupling.html.

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6425 (40) Gupta, R. P.; Sen, S. K. Phys. ReV. B 1975, 12, 15. (41) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem.sEur. J. 2002, 8, 28. (42) Yang, D.-Q.; Sacher, E. J. Phys. Chem. B 2005, 109, 19329. (43) Luk, T. S.; Pummer, H.; Boyer, K.; Shahidi, M.; Egger, H.; Rhodes, C. K. Phys. ReV. Lett. 1985, 51, 110. (44) Kotochigova, S. A. J. Opt. Soc. Am. B 1992, 9, 1215. (45) Mattevi, C.; Wirth, C. T.; Hofmann, S.; Blume, R.; Cantoro, M.; Ducati, C.; Cepek, C.; Knop-Gericke, A.; Milne, S.; Castellarin-Cudia, C.; Dolafi, S.; Goldoni, A.; Schloegl, R.; Robertson, J. J. Phys. Chem. C 2008, 112, 12207. (46) Zhang, G.-X.; Yang, D.-Q.; Sacher, E. J. Phys Chem C 2007, 111, 17200, and references therein. (47) Fujii, T.; de Groot, F. M. F.; Sawatzky, G. A.; Voogt, F. C.; Hibma, T.; Okada, K. Phys. ReV. B 1999, 59, 3195. (48) See e.g., http://srdata.nist.gov/xps/ElmComposition.aspx (NIST X-ray Photoelectron Spectroscopy Database), where the elements Fe and O should be selected. (49) Grosvenor, A. P.; Kobe, B. A.; McIntyre, N. S. Surf. Sci. 2004, 565, 151. (50) Busch, M.; Gruyters, M.; Winter, H. Surf. Sci. 2006, 600, 2778. (51) Cappus, D.; Haβel, M.; Neuhaus, E.; Heber, M.; Rohr, F.; Freund, H.-J. Surf. Sci. 1995, 337, 268. (52) Weiss, W.; Barbieri, A.; Van Hove, M. A.; Somorjai, G. A. Phys. ReV. Lett. 1993, 71, 1848.

JP810171E