J. Phys. Chem. C 2009, 113, 9275–9283
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Size-Dependent Stability of Supported Gold Nanostructures onto Ceria: an HRTEM Study J. Majimel,*,† M. Lamirand-Majimel,‡ I. Moog,† C. Feral-Martin,† and M. Tre´guer-Delapierre† ICMCB/CNRS, UniVersite´ de Bordeaux, 87 aVenue du Docteur Schweitzer, 33608 Pessac, France, and ENSCPB, 16 aVenue Pey Berland, 33607 Pessac, France ReceiVed: January 6, 2009; ReVised Manuscript ReceiVed: March 23, 2009
Gold nanoparticles of various sizes, supported onto ceria, were synthesized using both deposition-precipitation and coprecipitation methods. Whatever the size, the study of the Au/CeO2 interface confirms the existence of two preferential orientation relationships with a dislocation network which compensates the 25% interfacial lattice mismatch. Behaviors of supported gold nanostructures under the electron beam were examined by high-resolution transmission electron microscopy (HRTEM). The thermal stability of the gold nanostructures was found to be strongly affected by the particle size. For small nanostructures (5 nm).
Majimel et al. Typical HRTEM images of the Au/CeO2 interface of samples prepared by both methods are visible in Figures 2, b and d. They sit at the metal-oxide interface. One can recognize the well-established dodecahedral shape of the particle when formed on ceria support. Figure 2 shows HRTEM images where the incident electron beam is parallel to the [1j10] and [11j0] zone axis of the gold structure, respectively. Gold nanoparticles are exclusively deposited on the lower energy {111}CeO2 surfaces59 and two orientation relationships between the gold islands and the ceria support particles coexist and can be determined as (111)[1j 10]CeO2//(111)[1j 10]Au (A-type) and (111)[1j 10]CeO2// (111)[11j0]Au (B-Type). These orientation relationships are in good agreement with those already observed for the Au/CeO2 catalysts and more generally speaking for noble-metal/CeO2 materials.40-44 The reticular planes facing each other from both sides of the interface plane can be {111}Au and {111}CeO2 or {111}Au and {002}CeO2 as represented in Figure 3. The lattice spacings are 0.235, 0.312, and 0.271 nm for {111}Au, {111}CeO2, and {002}CeO2, respectively. The lattice misfit is thus about 25%. It is too large to be matched elastically (Figure 3). As a consequence, a dislocation network is created in the gold phase for compensation. It is represented by the additional reticular half-planes shown in the insets of Figures 2, b and d for both samples. A dislocation can be seen every four lattice planes. It is in good agreement with the evaluated lattice misfit value of 25%. Similar dislocation network was already observed for another gold-based system, Au2S/Au.60 Note that the Au/CeO2 system could have favored a lower ∆a/a misfit value obtained for example with a (002)[11j0]CeO2//(002)[200]Au orientation relationship with (220)CeO2 and (020)Au planes perpendicular to the interface plane (∆a/a ≈ 6.6%). Nevertheless, the crystallographic nature of the interface planes seems to be a key point of such a gold/ceria interaction. These observations are comparable to those obtained by recent theoretical calculations performed on the Pd/MgO system. It was shown that the lattice mismatch between the substrate and the supported metal induces tetragonal distortion in the small clusters, with a dilatation in the interface plane and a contraction in the direction perpendicular to the interface.61 However, when the supported metal nanoparticle’s size increases, the energy gain due to the interface adhesion is no longer sufficient to compensate for the strain due to the lattice misfit. Rows of metal atoms could then leave their preferential adsorption sites and an interface misfit dislocation is created. For larger metal nanoparticles, the dislocation could rearrange and form a periodic network. Stability of Gold Particles onto Ceria under Electron Irradiation. The response of the gold supported particles to a focused electron beam excitation was examined in situ by HRTEM. The series of HRTEM images and videos recorded for the same specimen region as the electron beam was focused continuously, showing the evolution process of gold particles on ceria support prepared by DP and CP, are presented in Figures 4 and 5respectively. The entire video sequences are available at http://www.promethee.cnrs.fr/spip.php?article154. Small-Sized Gold Islands (5 nm). Similar experiments were performed for the Au/CeO2-CP sample. This exhibits similar gold nanoparticles to those produced by deposition-precipitation, except for their size (>5 nm) and consequently their morphology.
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Figure 6. (a) HRTEM micrograph of as-deposited Au/CeO2-CP material. (b) HRTEM micrograph extracted from the video sequence illustrating the beginning of the encapsulation process of the bigger gold island by a CeO2-x layer (pointed by arrows), while the smallest one has already disappeared.
Before the electron beam was focused, Figure 5a shows a 12 nm diameter gold island with an hemispherical morphology and with the presence of {111} surface small steps. On the first stage of the electron beam illumination, diffusion of gold atoms occurs. Some {111} terraces are formed and the gold island is flattened: the contact angle between gold and ceria decreases. The video sequence ends while the morphology of the island has notably changed. No more terraces are visible and a contrast perturbation has occurred at the interface between gold and ceria. A thin layer, not yet organized, covering the gold particle starting from its left upper hand side appears. Figure 5c has been captured only a few minutes after the end of the video sequence while the electron beam is still focused on the Au/CeO2 interface. The layer covering the gold island is crystallized. The reticular distance is {022}CeO2. Moreover, this layer is bending to perfectly fit the gold island surface curvature. The gold lattice plane contrast has also disappeared and is now blurred by the one of the underlying oxide: the whole supported gold particle is encapsulated. At the last observation stage (Figure 5d), the encapsulating layer is still organized and exhibits a zone axis on the left upper hand side of the gold island. In contrast to Au/CeO2-DP sample, the diffusion of gold atoms is reduced for Au/CeO2-CP. Only surface reorganization of gold particles is detected. This difference between both samples is attributed to size difference and not to the chemical difference of the substrate resulting from the conditions of preparation. The melting temperature of gold particles of 12 nm diameter is comparable to the bulk gold.63 In addition, the large particles exhibit small surface tension and large adhesion energy with the substrate. These different factors limit the atomic gold diffusion. The size effect will be confirmed below with additional experiments which have been performed on samples containing both small and large gold particles on ceria surface. Encapsulation of the gold island is highlighted. Such decoration of supported metal nanoparticles has already been observed and takes part in the so-called strong metal-support interaction (SMSI).40,41,43,66-70 These SMSI are considered to significantly affect the microstructure and properties of catalysts. Liotta et al. summarized the explanations that have been given for the influence of oxidative/reductive treatments on the activity of
the Pt catalyst supported on CeO2 or CeO2-ZrO2: (i) alloy formation between Pt and Ce; (ii) decoration or encapsulation of Pt by partially reduced ceria (500 °C < Tred < 900 °C); and (iii) pure electronic interactions.71 To encapsulate supported metal particles, cerium species are thought to migrate during the reductive treatment from the bulk to the surface of the support, resulting in the formation of ceria-rich phase on the surface. The migration of a reduced support layer to the top of the metal crystallite, rather than the burial of this layer into the bulk, is the most likely decoration mechanism. This process is noteworthy for the catalytic activity since encapsulated particles are inaccessible to gas-phase molecules and cannot thus participate on catalysis. For the studied Au/CeO2 system, there is a possibility that the Au/CeO2 interface becomes Ce-rich under the present irradiation conditions. There should occur the Au-Ce alloy formation at or near the interface due to the presence of the Au-Ce alloy phase with low melting point in the phase diagram, as suggested by Akita.43 This could explain the contrast perturbation observed near the Au/CeO2 interface region in both Figure 5b and the video sequence. Mobility of the cerium atoms is then allowed by the creation of oxygen vacancies. In Figure 5b, they start to decorate the gold island, which is finally encapsulated by a 2 nm thick CeO2 layer (Figure 5c). Thanks to the measurement of the {022}CeO2 reticular distance, we give here the experimental proof that nonreduced cerium species are the constituents of the first encapsulating layer. During the decoration process, the gold particle surface is no more formed by steps or terraces. We cannot identify the structural nature of the interface plane between the gold nanoparticle and the encapsulating layer, but we can reasonably think that {022}CeO2 planes and {002}Au ones are facing each other (Figures 5, a and c). The interface mismatch is only about 6.6%. Contrary to the 25% mismatch observed earlier, the 6.6%’s one can be matched elastically. When gold, the softer material, is deposited on ceria, the system favors a compact {111} interface plane to the detriment of the lattice mismatch which is compensated by the creation of a dislocation network into the gold nanoparticle. On the contrary, when ceria is
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encapsulating a gold island and then forming a CeO2/Au interface, the system tends to minimize the lattice misfit. The last stage of this encapsulation process is the reduction of the CeO2 layer. Akita et al. performed electron energy loss spectroscopy (EELS) experiments on a gold nanoparticle decorated by a cerium-rich layer.43 They pointed out that this surface layer is not Ce4+ but Ce3+ or a mixture of both, which can correspond to several CeO2-x compounds with x varying from 1/6 to 1. The geometry of the zone axis presented in Figure 5d coupled with the reticular distances measured discards crystallographic structures other than cubic ones. Only five cubic structures belonging to Fm3jm and Ia3j space groups and with 0.305 < x < 1 remain. From measurements on HRTEM images, we are able to propose a lattice parameter for both Fm3jm and Ia3j hypothesis: 5.7 Å < a
