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J. Phys. Chem. C 2009, 113, 4038–4041
Delocalization of 4f Electrons in Gadolinium Oxide on the Nanometer Scale M. Ou,† V. Mauchamp,†,‡ B. Mutelet,†,§ T. Epicier,† J. C. Le Bosse,† S. Roux,§ O. Tillement,§ and P. Perriat*,† Mate´riaux, Inge´nierie et Sciences (MATEIS), CNRS UMR 5510, UniVersite´ de Lyon, INSA-Lyon, Domaine scientifique de La Doua, 7 aVenue Jean Capelle 69621 Villeurbanne Ce´dex, France, Laboratoire de Physique des Mate´riaux (PHYMAT), CNRS UMR 6630, SP2MI - BouleVard 3, Te´le´port 2 - BP 30179, 86962 Futuroscope, Chasseneuil Cedex, France, and UniVersite´ de Lyon, UniVersite´ Lyon 1, CNRS UMR 5620, Laboratoire de Physico-Chimie des Mate´riaux Luminescents (LPCML), Domaine scientifique de La Doua, Baˆt Kastler, 10 rue Andre´ Marie Ampe`re 69622 Villeurbanne Ce´dex, France ReceiVed: October 9, 2008; ReVised Manuscript ReceiVed: January 20, 2009
Some metals such as silver are known to become strong reducers on the nanometer scale. Electron energyloss spectroscopy (EELS) shows that modifications of the electronic structure of metal ions also arise in nanoparticles of gadolinium oxide. Indeed, the oxygen K and gadolinium N4,5 edges present modifications for particles size below 1.5 nm which are the signature of an hybridization of the initially atomiclike Gd 4f states with the valence band and especially the O p states. These modifications evidence that, like in metals, electron delocalization from metal sites also occurs in oxides on the nanometer scale. 1. Introduction The physical properties of nanoparticles (smaller than 10 nm) differ strongly from those of bulk material especially when their size is small. Two different reasons explain this particular behavior. First, in the size-regime where the wavelength of the electrons is of the same order as the particle itself, quantum size effects can appear.1 Second, when the size decreases, the surface becomes predominant relatively to the bulk so that the thermodynamics properties of the whole particle (including surface and bulk ones) are modified.2 This leads for instance to grain-size driven phase transitions (for example, the silver melting point is decreased by 700 °C)3 and to a modification of the electrochemical properties of the material.4 For instance, the standard redox potential of the nanoparticle shifts negatively from that of the corresponding bulk.5 In the case of silver, the monomer Ag is a strong electron-donating reagent which can reduce many inorganic and organic compounds6 and Ag clusters undergo a loss of electrons compared to bulk. This paper aims at showing that size-induced modifications of the electronic structure can also be found in other types of materials such as metal oxides. This will be made by investigating gadolinium oxide nanoparticles by electron energy-loss spectroscopy (EELS).7 2. Experimental Methods Gadolinium oxide is a promising material in the field of multimodal (optical and magnetic resonance) imaging8 which can be prepared at various sizes.9,10 In this work, it is prepared according to the polyol method using diethylene glycol (DEG) as the solvent11,12 at four different sizes: 1.1, 1.5, 2.7, and 4 nm with a mean standard deviation of 0.3, 0.6, 0.9, and 1.4 nm, respectively. For all samples, the size distribution determined by photon correlation spectroscopy (PCS) is given in Figure 1 * Corresponding author. † Mate´riaux, Inge´nierie et Sciences (MATEIS), CNRS UMR 5510. ‡ Laboratoire de Physique des Mate´riaux (PHYMAT), CNRS UMR 6630. § Universite´ Lyon 1, CNRS UMR 5620.
Figure 1. Size distribution determined by photon correlation spectroscopy of Gd2O3 nanoparticles with a size of (a) 1.1 nm, (b) 1.5 nm, (c) 2.7 nm, and (d) 4 nm. Insets show high resolution TEM images of corresponding particles.
and is confirmed by the high resolution transmission electron microscopy (TEM) images shown in insets. The TEM samples were prepared by depositing a drop of a diluted colloidal solution of Gd2O3 nanoparticles on a holey carbon grid. EELS was carried out in image coupled configuration13 using a JEOL 2010F microscope operating at 200 kV and equipped with a Gatan Digipeels 766 spectrometer. The probe size was of the order of 80 nm, and the collection angle was 9 mrad. In all experiments, the energy resolution derived from the zero loss peak (ZLP) full width at half-maximum (FWHM) was around 0.9 eV. Two series of experiments were successively achieved with a dispersion of 0.1 and 0.3 eV/channel each. The lower dispersion was used to locate the absolute positions of oxygen K and gadolinium N4,5 edges comparatively to the carbon K one. All the measurements were performed at room temperature. After acquisition, the spectra were corrected from the spectrometer dark current and pixel to pixel gain variations of the
10.1021/jp808931k CCC: $40.75 2009 American Chemical Society Published on Web 02/18/2009
Delocalization of 4f Electrons in Gadolinium Oxide
J. Phys. Chem. C, Vol. 113, No. 10, 2009 4039
detector array. They were deconvolved from the zero loss spectrum using a Fourier ratio technique available in the Gatan EL/P software. 3. Results and Discussion Gadolinium oxide can crystallize within two structures, monoclinic or cubic. Cubic f monoclinic size-driven transitions were already reported below 2.8 nm for Gd2O3 particles prepared by low energy cluster beam deposition (LECBD) and maintained in a clean and controlled environment.14 However, the analysis of the luminescence spectra of Eu introduced as a doping element (5% at) in gadolinium oxide undoubtedly proved that the cubic structure is maintained even for smaller sizes when the particles are synthesized with the polyol method.11,12 This is confirmed here by the TEM images (insets of Figure 1) where the same inter-reticular distance of 0.311 nm is measured for all sizes. This distance corresponds to the inter-reticular (222) distance of the Ia3 gadolinium oxide structure (cell parameter: 1.082 nm). Consequently, it can be stated that a cubic f monoclinic transition is observed in the nanometer range when particles are prepared according to the LECBD method but not with the polyol method. This is explainable by the difference of surface energies between both particles. The surface energy of the particles prepared by LECBD is of a solid/gas type and then significantly higher than that (of a solid/liquid type) of the particles prepared by the polyol method which are strongly capped by the solvent. The cubic f monoclinic transition being surface-driven, it should then arise at a smaller size for particles prepared chemically. To give an order of magnitude, the critical size is decreased by at least a factor of 10 for barium titanate2 and iron oxide15 when the surface shifts from a solid/gas type (ceramics) toward a solid/solid type (powder). Spectra obtained at the oxygen K and gadolinium N4,5 and M4,5 edges are shown in Figure 2. In Figure 2a, the four O K spectra recorded for the four different sizes are compared to the multiple scattering simulation of Botton et al. obtained for cubic Gd2O3 (gray curve).16 A clear evolution can be observed when the size increases from 1.1 to 4.0 nm. For the bigger particles (2.7 and 4 nm), the spectra are, in agreement with the presented simulation, dominated by the broad peak B at 535 eV. For the smaller particles, two peaks (A and B) are observed. Peak B lies at the same energy (535 eV) regardless of particles size, whereas the intensity and the energy of peak A both increase when the size decreases. The O K edge being essentially described as a one-particle process (direct transitions from O 1s to O p levels),17 it is closely related to the empty O p density of states (DOS).18 In rare earth oxides, these states are mainly mixed with the rare earth 5d states and, to a lesser extent, with the 6s and 4f levels to form the valence and conduction bands.19 The changes observed in Figure 2a are thus a signature of the modification of the Gd-O chemical bond as a function of the particle size. Such changes only occur for very small particles, and bulk properties are recovered for sizes above 2.7 nm. To go deeper in the understanding of these effects, we have also recorded the Gd N4,5 and M4,5 edges. In the dipole approximation, these two edges correspond to transitions from the initially occupied 4d (for N4,5 edge) or 3d (for M4,5 edge) to the unoccupied 4f states. These edges are very different from the O K one, since they are dominated by multiplet effects arising from the strong overlap between the initial and final states involved in the transition.20,21 Focusing first on the Gd N4,5 edge (Figure 2b), the multiplet structures are composed of narrow pre-edge peaks between 135 and 145 eV (arrow in Figure 2b) and a main peak around 149 eV.22 X-ray photoelectron
Figure 2. O K (a), Gd N4,5 (b), and M4,5 (c) edges recorded for four nanoparticle sizes (1.1, 1.5, 2.7, and 4 nm). Dots are a guide for the eyes in (a) and (b).
spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and calculations performed on various rare earth (RE) oxides show that these 4d-multiplet structures (and especially the pre-edge peaks) are very sensitive to the solid-state hybridization between the RE 4f and O p states.21,23,24 Careful examination of Figure 2b reveals that the pre-edge structures are broader for the 1.1 and 1.5 nm particles. Also, the all-edge is shifted by 1.7 eV toward higher energies for the 1.1 nm particles and by 0.6 eV for the 1.5 nm particles compared to that related to the 2.7 and 4 nm particles or the bulk reference of Ahn and Krivanek (gray curve).25 In agreement with the changes observed at the O K edge, it is found that size-induced effects are again only sensitive for particles below 1.5 nm. An equivalent broadening of the pre-edge structures was already reported in the case of the Ce N4,5 edge in different Ce-based compounds and attributed to an increase of the hybridization strength between the Ce 4f states and the valence band (especially the O p states in oxides).23,24 Similarly, the modifications of the multiplet structure are here evidence of the modification of the Gd 4f wave function. They reflect the delocalization of a small part of the Gd 4f electrons via their hybridization with the Gd 5d and 6s and O 2p states. This delocalization mechanism was proposed from first principal
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Figure 3. Sketch of the evolution of the Gd2O3 nanoparticle DOS as a function of their size: Gd DOS in gray (full lines for 4d and 4f states, dots for 6s and 5d states) and O DOS in black. Structures corresponding to peak A and B of the O K edge are indicated by arrows.
Ou et al. with the O 2p states is quasi-negligible, and the intensity of peak A vanishes (Figure 3, left). When particle size decreases, there is an increase of the hybridization between the 4f states and the valence band. This change from an atomic to a more “bandlike” behavior of the Gd 4f states allows their mixing with the O 2p states. As illustrated by Figure 3 right, this mixing increases the O p density of states in the energy range corresponding to the Gd 4f empty (and occupied) states leading to a correlative increase of peak A. At the same time, the energy of the antibonding states increases as one can infer from the energy increase of both peak A and Gd N4,5 edge. That an energy increase be related to 4f electrons delocalization is in complete coherence with XPS observations which show that the binding energy of the Gd 4f states is increased upon Gd oxidation (case of extreme delocalization with loss of electron).29 In Figure 3, the gap between the valence and the conduction bands increases for nanosized particles, which was directly proved by UV spectroscopy performed on Gd2O3 nanoparticles doped with Eu.31 The Gd 4f and Gd 4d states are plotted at higher binding energy than in bulk as deduced, for Gd 4f, from analogy with electron delocalization induced by oxidation29 and, for Gd 4d, from the existence of a size-induced N4,5 edge shift. 4. Conclusion
calculations by Strange et al. in 1999 in order to understand the valence of rare earths in various compounds.26 To better characterize this size-induced electron delocalization, we focus now on the M4,5 edge. In the case of RE compounds, the multiplet effects give rise to two white lines whose branching ratio is correlated to the occupation of the 4f band.27 Figure 2c exhibits these two lines at energies of around 1185 and 1217 eV. Due to the large distance between Gd M4,5 and C K edges, these energies were not recalibrated but adjusted at the same average value. There is no evolution of the Gd M4,5 edge with size. This implies that the occupancy of the Gd 4f states remains the same whatever the particle size. Then, the size-induced effects evidenced here do not suppose any change in the valence of Gd (no charge transfer to oxygen anion) but only a modification in the Gd orbital symmetries via the hybridization of the Gd 4f states with the valence band. In order to summarize the modifications observed on the EELS spectra, the DOS of cubic Gd2O3 has been schematized in Figure 3 (Gd-DOS in gray, O-DOS in black). Only the 1s and p levels of the oxygen atoms are given, since we are focusing on the O K edge. From bottom to top of the energy scale (left panel), one finds the deep O 1s core levels, the Gd 4d levels, and the Gd 4f states which, according to several XPS measurements,28,29 are found at the bottom of the valence band. The valence band is formed by the hybridization of the O 2p with the Gd 5d and 6s states. Above the Fermi level is the conduction band formed by the antibonding states which are probed by EELS. Following the self-interaction-corrected localspin-density calculations of Petit et al. performed on Gd2O330 and in agreement with assumptions made by Strange et al.,26 the unoccupied Gd 4f states are at the bottom of this conduction band while the unoccupied Gd d and s states are higher in energy. The proportion of the O p DOS in these bands is directly correlated to the mixing of the O 2p with the Gd 4f, 5d, and 6s states in the valence one. Following this picture, peak A in the O K edge, at the bottom of the unoccupied O p DOS, can be attributed to the mixing of the O p states with the Gd 4f ones and the broad peak B to their mixing with the more delocalized Gd 5d and 6s states. For the bigger particles (2.7 or 4 nm), the Gd 4f electrons possess a strong atomic character. Their mixing
In this paper, we have characterized the size-induced modifications of the electronic structure of Gd2O3 nanoparticles by analyzing via EELS the O K, Gd N4,5, and M4,5 edges. For particles smaller than 1.5 nm, clear modifications in the edges evidence Gd 4f electron hybridization with the valence band (and in particular the O p states). This electron delocalization is not accompanied by a change in the Gd valence but is only associated to a modification in the Gd orbital symmetries. References and Notes (1) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (2) Perriat, P.; Niepce, J. C.; Caboche, G. J. Therm. Anal. 1994, 41, 635. (3) Buffat, P.; Borel, J. P. Phys. ReV. A 1976, 13, 2287. (4) Chaki, N. K.; Sharma, J.; Mandle, A. B.; Mulla, I. S.; Pasricha, R.; Vijayamohanan, K. Phys. Chem. Chem. Phys. 2004, 6, 1304. (5) Plieth, W. J. J. Phys. Chem. 1982, 86, 3166. (6) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (7) Egerton, R. F. Electron Energy-Loss Spectroscopy in the Electron Microscope; Plenum Press: New York, 1996. (8) Bridot, J. L.; Faure, A. C.; Laurent, S.; Riviere, C.; Billotey, C.; Hiba, B.; Janier, M.; Josserand, V.; Coll, J. L.; Vander Elst, L.; Muller, R.; Roux, S.; Perriat, P.; Tillement, O. J. Am. Chem. Soc. 2007, 129, 5076. (9) Louis, C.; Roux, S.; Ledoux, G.; Dujardin, C.; Tillement, O.; Cheng, B. L.; Perriat, P. Chem. Phys. Lett. 2006, 429, 157. (10) Louis, C.; Bazzi, R.; Marquette, C. A.; Bridot, J. L.; Roux, S.; Ledoux, G.; Mercier, B.; Blum, L.; Perriat, P.; Tillement, O. Chem. Mater. 2005, 17, 1673. (11) Bazzi, R.; Flores, M. A.; Louis, C.; Lebbou, K.; Zhang, W.; Dujardin, C.; Roux, S.; Mercier, B.; Ledoux, G.; Bernstein, E.; Perriat, P.; Tillement, O. J. Colloid Interface Sci. 2004, 273, 191. (12) Bazzi, R.; Brenier, A.; Perriat, P.; Tillement, O. J. Lumin. 2005, 113, 161. (13) Keast, V. J.; Scott, A. J.; Brydson, R.; Williams, D. B.; Bruley, J. J. Microsc. 2001, 203, 135. (14) Nicolas, D.; Masenelli, B.; Me´linon, P.; Bernstein, E.; Dujardin, C.; Ledoux, G.; Esnouf, C. J. Chem. Phys. 2006, 125, 171104. (15) Millot, N.; Champion, Y.; Hytch, M. Y.; Bernard, F.; Begin-Colin, S.; Perriat, P. J. Solid State Chem. 1998, 139, 66. (16) Botton, G. A.; Gupta, J. A.; Landheer, D.; McCaffrey, J. P.; Sproule, J. P.; Graham, M. J. J. Appl. Phys. 2002, 91, 2921. (17) Paxton, A. J. Electron Spectrosc. Relat. Phenom. 2005, 143, 53. (18) Nufer, S.; Marinopoulos, A. G.; Gemming, T.; Elsa¨sser, C.; Kurtz, W.; Ko¨stlmeier, S.; Ru¨hle, M. Phys. ReV. Lett. 2001, 86, 5066. (19) Harvey, A.; Guo, B.; Kennedy, I.; Risbud, S.; Leppert, V. J. Phys.: Condens. Matter 2006, 18, 2181.
Delocalization of 4f Electrons in Gadolinium Oxide (20) Thole, B. T.; Van der Laan, G.; Fuggle, J. C.; Sawatzky, G. A.; Karnatak, R. C.; Esteva, J. M. Phys. ReV. B 1985, 32, 5107. (21) Kotani, A.; Ogasawara, H.; Okada, K.; Thole, B. T.; Sawatzky, G. A. Phys. ReV. B 1989, 40, 65. (22) Pagliara, S.; Sangaletti, L.; Cepek, C.; Bondino, F.; Larciprete, R.; Goldoni, A. Phys. ReV. B 2004, 70, 035420. (23) Wieliczka, D.; Weaver, J. H.; Lynch, D. W.; Olson, C. G. Phys. ReV. B 1982, 26, 7056. (24) Gordon, R. A.; Seidler, G. T.; Fister, T. T.; Haverkort, M. W.; Sawatzky, G. A.; Tanaka, A.; Sham, T. K. Eur. Phys. Lett. 2008, 81, 26004. (25) Ahn, C. C.; Krivanek, O. EELS Atlas; Gatan Inc.: Warrendale, PA, 1983.
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JP808931K
