Letter pubs.acs.org/NanoLett
Anisotropic Nanocrystal Dissolution Observation by in Situ Transmission Electron Microscopy Marco A. L. Cordeiro,*,† Peter A. Crozier,‡ and Edson R. Leite*,§ †
Department of Materials Science and Engineering, Federal University of São Carlos, 13565-905, São Carlos, SP, Brazil School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287-6106, United States § Department of Chemistry, Federal University of São Carlos, 13565-905, São Carlos, SP, Brazil ‡
S Supporting Information *
ABSTRACT: In this study, some keys in the knowledge of nanocrystals dissolving by the direct phenomenon observations are provided through in situ transmission electron microscopy experiments. The new characteristic of anisotropic nanoparticles dissolving is discussed and correlated with the evolution of the crystal to reach a minimum surface free energy (Gibbs−Wulff theorem), which has an impact on the nanocrystal ripening models. The process whereby the ripening occurs was identified and correlated to the adparticle motion. KEYWORDS: Nanocrystal dissolution, in situ TEM, CeO2, nanocrystal ripening
T
he crescent ability in controlling nanocrystal shape, size, and terminal facets has widened their use, due to their outstanding physical and chemical properties. However, the task of tailoring nanoparticles can be hindered by nanocrystal instabilities, which has been a challenge in several fields such as nanocatalyst design. A major problem with nanocatalyst stability during heterogeneous catalysis is the inevitable loss of activity by chemical and/or physical changes with the timeon-stream (deactivation).1,2 Owing to the high surface area and conditions with high temperatures and reactive gases, nanocrystals lose their initial shape, and the terminal facets can change, followed by the sintering or ripening process.3−5 This phenomenon occurs not only with the nanocatalyst but also with the nanosize support. Several approaches have been made to surmount the nanocatalysis deactivation processes (poisoning, coking, etc.), usually with regeneration methods.1,2,6 Nevertheless, the sintering and/or ripening phenomena are often irreversible, and the most feasible approach is prevention. Traditional studies have used indirect phenomenon analysis; that is, the nanocatalyst is studied before and after its deactivation, and the whole ripening process is deduced by comparison. Despite the importance of these studies, real nanocatalyst deactivation processes can be overshadowed, because the real process is not analyzed directly. Consequently, recent in situ characterization techniques are becoming very important to understand catalysts during their use, inasmuch as it is possible to follow several aspects of the motion in the catalyst dynamics. These techniques open the way to reveal the dynamics of more complex physical and chemical events and identify the atomicscale structure of unknown phenomena. Among these techniques, in situ transmission electron microscopy (TEM) has a relevant role. In spite of the topic’s importance and the © 2012 American Chemical Society
important in situ TEM studies already performed, which have made a better understanding in thermal nanocrystal stability possible,7−16 additional studies are necessary to provide new breakthroughs in the design of nanostructured catalytic materials. Accordingly, in this study we provide some insights into nanocrystal stability. For this purpose, CeO2 nanocube-like (NC), with {200} exposed facets were synthesized by the hydrothermal method under two-phase conditions17 (synthesis details are described in the Supporting Information). CeO2 nanocrystals are very promising as catalysts or catalytic supports in many oxidation reactions or as components in three-way automotive catalysts. This is because of their high oxygen mobility, which allows easy cycling between reduced and oxidized states, and the strong interaction with metal clusters. In situ TEM was performed in a FEI Tecnai F20 fieldemission TEM operating at 200 kV. Self-organizations of CeO2 nanocubes on an amorphous carbon film supported on molybdenum or nickel grids were prepared by simple colloid dropping. The isothermal in situ heating experiments were performed in a Gatan 628 single-tilt heating holder (with an Inconel-based furnace). A series of experiments was performed to determine the temperature at which phenomena of interest took place within a convenient observation time. For these particles, the dissolution process seemed to activate at a temperature of about 890 °C with significant dissolution taking place within a period of about 10 min. We also conducted experiments to investigate the role of the electron beam in the Received: August 7, 2012 Revised: September 21, 2012 Published: October 5, 2012 5708
dx.doi.org/10.1021/nl3029398 | Nano Lett. 2012, 12, 5708−5713
Nano Letters
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Figure 1. CeO2 nanocrystal dissolution during the experiment: (a) 0 s; (b) 64 s; (c) 184 s; (d) 220 s.
the facets reach the same size (∼130 s), both facets then decrease at almost the same rate. In other words, the larger facet size decreases rapidly until the ratio between the facets on the nanoparticle reaches the value around the unit. It is important to highlight that all of the exposed facets (S1, S2, S3, and S4) belong to the {200} family of facets; consequently they have the same surface energy per area. Then, it implies that not only the kind of facet and its surface energy per area are keys to model the dynamics of nanocrystals dissolving but also the total energy per facet, even if these facets belong to the same facet family. Further, the dissolving dynamics can be dissimilar depending on the initial facet proportion. Interestingly, the anisotropic phenomena cannot be observed when a symmetrical-like CeO2 nanocrystal is analyzed during the dissolution process. In addition to the symmetrical nanocube in the first experiment (Figure 1), Figure S2 of the Supporting Information shows a typical time-resolved image series of a spheroid CeO2 nanocrystal in dissolving process. In this last experiment, it is possible to see that the CeO2 nanocrystal shrink happens about homogeneously. The anisotropic phenomenon can be understood energetically. For any crystal, the Gibbs−Wulff theorem18,19 that expresses, in terms of a crystal shape in equilibrium, the total surface energy (Ei) for a given facet (i) is proportional to the
dissolution phenomena (Figures S2 and S3 of the Supporting Information). All images were recorded after the sample drift associated with temperature change had stabilized (typically 2 min after reached the desired temperature). Figure 1 shows a typical time-resolved image series of two CeO2 NCs during the dissolution process (the same experiment is shown in Movie 01). During the experiment, both NC’s continuously shrink and disappear, whereas the smaller CeO2 nanocrystal (SN) vanished in 30 s and the larger CeO2 nanocrystal (LN) disappeared in about 240 s (Movie 01). This feature is predicted by ripening models.3 In general, ripening models are based on Gibbs−Thompson equation and show that smaller particles tend to dissolve, while clusters larger than a critical radius are more stable and grow at the expense of smaller ones. Moreover, an interesting behavior was observed during the NC dissolving. Figure 2 shows the variation in facet size with time for several crystal facets during the dissolving process, from the same experiment. For the SN (Figure 2a), with facets sizes (S1,S2) of about same length, the facets shrink homogeneously; that is, the facet size ratio (S1/S2) remains nearly constant with a value around unit. On the contrary, for the LN, Figure 2b shows that the smaller LN facet (S3) decreases in size more slowly than the larger facet (S4). When 5709
dx.doi.org/10.1021/nl3029398 | Nano Lett. 2012, 12, 5708−5713
Nano Letters
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Anisotropic behaviors of nanocrystals due to specific exposed facets have been studied for a set of properties such wetting,20 chemical and photochemical anisotropic etching,13 adsorption,21 and so forth. However, the classical models of ripening, which have been tailored for average phenomenon prediction, do not predict this anisotropic behavior of the exposed facets. In fact, this phenomenon could provide deviations in the ripening theory models on the micro/nanoscale. Simonsen et al.9,10 have demonstrated interesting features during the Pt nanocrystals ripening by in situ TEM. In their study, the meanfield model for the Ostwald ripening generally fit their observations of the average particle behavior. However, deviations from the expected model were seen for some of the individual nanocrystal ripening behavior. These discrepancies were correlated to the local nanocrystal environment, which could influence the atom exchange process among nanocrystals. Here, additional insight is provided into nanocrystal ripening that could promote similar deviation on nanoscale analysis when classical ripening models are used to explain the process. The mass transport processes associated with the ripening in isolated oxide nanocrystals (evaporation/condensation and/or surface diffusion) are difficult to observe because these processes may take place via fast motion of atoms or small clusters. Interestingly, however, in the in situ CeO2 nanocrystals ripening experiments, small dots (adparticlesadatoms or adclustersas described as follows) leaving the nanocrystals are seen clearly throughout the sample (Figures 1 and 3 and movie 01 of the Supporting Information). Notwithstanding that evaporation may have an effect on CeO2 nanocrystal dissolution, the motion of these dots leaving the nanocrystals during their ripening infers that surface diffusion plays a significant role in the dissolution process. Aiming to determine the influence of the temperature and the electron beam effect, several experiments were performed at lower temperatures (Figures S3 and S4 of the Supporting Information). In these experiments, no dissolving processes were evidenced in any temperature below 850 °C (up to 40 min of analysis), which implies the major influence of the temperature over the electron beam. Then, the activation temperature around 890 °C can be related to the necessary energy for adparticles detachment from the NCs. Hence, the first question regards the adparticle composition (from just a single atom to a small cluster). Classical22,23 and recent24 studies have shown that it is possible to resolve individual atoms, but in some cases it is not simple to determine the particle composition directly by TEM. Several bright-field images simulations were performed (Figures S5 and S6 of the Supporting Information), starting from inputs of a single CeO2 and Ce2O3 unit cells on carbon film input to other structures through cuttings in the unit cells. It was evidenced that a single Ce adatom bright-field image simulation could match the simulations with a bigger structure simulation (CeyOx adcluster). Even the analysis of the intensity of several adparticles (Figure S7), which suggests a size of about 0.2 nm for the adparticles, could also match the different structures simulations, making the unambiguous determination of the adparticle composition difficult. Additionally, the organic shell during its elimination from the nanocrystals may generate defects on the nanocrystals surface, for example, the reduction of Ce ions or Ce vacancies, which processes can influence the adparticles composition. In spite of these uncertainties, the variation on adparticle contrast (Figure S6) can provide some
Figure 2. Graphics showing the CeO2 NCs side size and the side ratio evolution; (a) smaller nanocrystal (SN); (b) larger nanocrystal (LN).
length of a vector drawn normal to this facet to the crystal center (li), and then total energy (ET) is the sum of all surface energy, as follows:
ET =
∑ Ei = ∑ αili i
i
(1)
where αi is a constant which is dependent on the kind of facet. This theory was established for crystals in their equilibrium shape, but the same concept can be applied to each crystal during the dissolution to indicate how far each facet is from the minimum surface free energy configuration by considering the ratio of facets energy. Applying this approach to the LN, it is possible to write: α{200}lsmaller facet Esmaller facet S3 = = E bigger facet α{200}llarger facet S4
(2)
In the CeO2 nanocrystals with a cubic-like shape and only {200} exposed faces, the expected equilibrium is reached when all faces have the same size. Indeed, the facet ratios S1/S2 (∼1) and S3/S4 (
