Article pubs.acs.org/JPCC
Size-Dependent Room Temperature Oxidation of In Nanoparticles E. Sutter* and P. Sutter Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States ABSTRACT: Using transmission electron microscopy, we study the size-dependent room-temperature oxidation of indium nanoparticles. From the investigation of arrays of In nanoparticles with broad size distribution, we evaluate, under identical conditions, the In2O3 thickness as a function of size at a given time. The results of our measurements are in excellent agreement with predictions for a Mott−Cabrera model corrected for a spherically symmetric electric field and directly demonstrate the accelerated oxidation kinetics of nanoscale particles with high curvature. We show that the size-dependent oxidation kinetics of nanoparticles can be probed by considering polydisperse arrays of nanoparticles as snapshots of the oxidation kinetics of differently sized particles.
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oxidation via the Kirkendall effect,8 in which vacancies form and coalesce to generate a void in the metal core due to a large asymmetry between the diffusion rates of metal ions (fast) and oxygen ions (slow). Ostensibly the experimental determination of the size dependence of the accelerated oxidation rate of metal nanoparticles requires time-dependent in situ observations during the oxidation of individual particles, which would be very challenging. Here we demonstrate that the size-dependent oxidation kinetics of nanoparticles can be probed instead by considering polydisperse particle arrays. Following their exposure to oxidizing conditions, such arrays represent a snapshot of the oxidation kinetics of differently sized particles. Specifically, using transmission electron microscopy (TEM) we investigate the room-temperature oxidation of indium nanoparticles. While representing a convenient model system for studying nanoparticle oxidation, semiconducting indium oxide has widespread applications as a transparent conductor due to its high optical transparency paired with good electrical conductivity9 and represents a promising photocatalyst for visible-light water splitting.10 The use of arrays of In nanoparticles with broad size distribution allows us to evaluate, under identical conditions the In2O3 thickness as a function of size at a given time. The results of our measurements are in excellent agreement with predictions for a Mott−Cabrera model corrected for a spherically symmetric electric field and directly demonstrate the accelerated oxidation of nanoscale particles with high curvature. In nanoparticles for control experiments were formed by room temperature evaporation of 2 and 20 nm In onto SiO2 and carbon membranes supported on standard TEM grids. The
he oxidation of nanoparticles has been an area of active research both because of basic scientific interest and possible applications. Nanoparticles of most metals react readily with oxygen at room temperature, forming thin oxides on the surface or oxidizing completely. Understanding and controlling oxidation is crucial for devising strategies for either avoiding it when detrimental to nanoparticles functionality or taking advantage of it to create novel nanostructures. The formation of a thin oxide layer can negatively affect performance in applications that require pure metal surfaces. Oxidation can be advantageous because it opens a way for engineering of more complex nanostructures. Examples are metal-oxide core−shell nanoparticles or hollow metal (Co,1 Al,2 Fe,3 Ni,4 among others) oxide nanocrystals that can be used as cages or porous material or can be filled with different materials1 for storage or protection, for example, of a catalytically active component. The low-temperature growth kinetics of thin oxides are generally described by the Cabrera−Mott model.5 The oxide growth depends critically on the electric-field-enhanced transport of ions. The driving force for this ion transport is the Mott potential, VM, established as electrons pass from the metal to the oxide surface, leaving behind metal ions and ionizing oxygen atoms or molecules adsorbed at the oxide−gas interface. In a planar geometry, the equilibration of the electrochemical potential in the metal and on the surface generates a uniform electric field E = −(VM/L) in the planar oxide film with thickness L. In nanoparticles, however, the metal can have very high curvature. Recent theoretical work has recognized the implications of curvature of the oxidizing surface on the electric field in the oxide, concluding that small spherical metal nanoparticles could show significantly increased oxidation rates.6,7 Several experimental studies have investigated nanoparticle oxidation and have attempted to establish the kinetics of oxide growth in nanoparticles.1−4,6,7 The primary focus has been on understanding the formation of hollow particles during © 2012 American Chemical Society
Received: June 13, 2012 Revised: August 14, 2012 Published: September 4, 2012 20574
dx.doi.org/10.1021/jp305806v | J. Phys. Chem. C 2012, 116, 20574−20578
The Journal of Physical Chemistry C
Article
contrast is, however, not uniform. The particles appear to have a core−shell structure. The outer shell is assigned to the oxidation of the particles,11 which were exposed to air at room temperature after deposition. Electron diffraction in the TEM (Figure 1b) demonstrates that the particles consist of single crystalline In12 and c-In2O3, which can be readily indexed to the cubic In2O3 bixbyite phase.11,13 The calculated DPs of the cIn2O3 phase along the [111] zone axis and the tetragonal In structure along the [201] zone axis, superimposed in Figure 1c, respectively, provide an excellent match to the experimental DP. In situ annealing at moderate temperatures in the TEM can be used to melt the metallic In core, thus demonstrating clearly the extent of the In2O3 shell and In core. Upon melting, the contrast of the In core becomes homogeneous and uniform, characteristic of the liquid state (Figure 1d).14,15 At the same time, the diffraction spots from In disappear and are replaced by one wide diffuse ring at 3.8 nm−1, as expected for liquid In (inset).11 The melting of the In core facilitates an accurate measurement of the oxide shell thickness in TEM. Our measurements on a number of particles show that particles >20 nm have uniform thickness of the In2O3 shell of 3.5 to 3.7 nm, independent of particle size, consistent with previous experiments on In films that found the thickness of indium oxide to self-terminate at about 3 to 4 nm at room temperature.16 Particles with smaller diameters show a pronounced dependence of the oxide thickness on particle size, which we studied in detail in polydisperse arrays of smaller nanoparticles. The TEM image in Figure 2a gives an overview of the In particles deposited on carbon with 10 times lower In coverage, resulting in an ensemble of particles with diameters in the range of 2−20 nm. The nanoparticle ensemble gives rise to a
as-deposited particle ensembles were exposed to ambient conditions and transferred to the TEM. The TEM investigations were performed typically within 48 h after deposition. The morphology and composition of the In nanoparticles were investigated by TEM in a JEOL 2100F field-emission microscope equipped for energy-dispersive X-ray (EDS) in scanning TEM mode (beam size: 0.2 nm). Variable-temperature (room temperature to 800 °C) TEM experiments were carried out using a Gatan-652 sample holder. The TEM image in Figure 1a gives an overview of an ensemble of In particles deposited on carbon. Identical particle morphologies were found on SiO2. The deposition of an equivalent thickness of 20 nm of In results in a wide distribution of particle sizes, in the range of 2−200 nm. The TEM image and the corresponding diffraction patterns (DPs) (Figure 1b) demonstrate that the particles are crystalline. The
Figure 1. Indium nanoparticles on carbon support at room temperature and at 180 °C. (a) Overview TEM image of In−In2O3 nanoparticles at room temperature. (b) Diffraction pattern of a large In−In2O3 particle. (c) Inverted superimposed patterns simulated for tetragonal In along the [201] zone axis and cubic In2O3 along the [111] zone axis, respectively. (d) TEM image of In−In 2 O 3 nanoparticles at 180 °C. Inset: Diffraction pattern of liquid In−solid In2O3.
Figure 2. Indium particles on amorphous carbon films at room temperature. (a) Overview TEM image and diffraction pattern (inset) of the In particles. (b−d) High-resolution images of individual nanoparticles. 20575
dx.doi.org/10.1021/jp305806v | J. Phys. Chem. C 2012, 116, 20574−20578
The Journal of Physical Chemistry C
Article
Consistent with the melting of the In core and desorption of In, part of the rings in the DP disappear (Figure 3a, inset). Therefore, above the In melting temperature and under the electron beam the particles transform from an In−In2O3 core− shell structure into hollow c-In2O3 shells. Once transformed into oxide shells (Figure 3d,e), the morphology remains unchanged to high temperatures, similar to the completely oxidized small nanoparticles. The removal of the In core enables accurate measurements of the oxide thickness. The dependence on the nanoparticle diameter of (i) the In core diameter and (ii) the In2O3 thickness resulting from the room-temperature oxidation, determined by the above procedure, is given in Figure 4a. As
polycrystalline-type DP (shown in the inset) due to the contribution from several particles. The DP consists of discrete spots on rings that, similar to the larger particles, are readily indexed to a mixture of c-In2O3 and In structures. Importantly, HRTEM images (Figure 2b−d) show that the individual nanoparticles exhibit two distinct types of morphologies depending on their size. Nanoparticles smaller than ∼12 nm (Figure 2b,c) are single-crystalline and consist of only one phase with homogeneous contrast. These particles typically show lattice fringes with 0.297 nm separation, consistent with the spacing of (222) planes in c-In2O3, indicating that they have become completely oxidized during exposure to air. Nanoparticles larger than ∼12 nm (Figure 2d) are also singlecrystalline but show homogeneous contrast only at the periphery. At the center they exhibit nonuniform contrast similar to larger In particles;11 that is, air exposure has led only to partial oxidation and produced an In−In2O3 core−shell structure. Oxidation of the polydisperse nanoparticle array under identical conditions therefore has led to the complete oxidation of small particles, whereas larger particles preserve an In core. Heating of the nanoparticle array to 160−180 °C, slightly higher than the melting temperature of bulk In, results in two distinctly different behaviors (Figure 3a). Small,
Figure 4. Dependence of the In2O3 thickness on nanoparticle size. (a) Experimental In2O3 thickness and In core diameter determined from measurements on TEM images. The arrow marks the critical size, below which the nanoparticles are completely oxidized. For nanoparticles with diameters below 18 nm, each symbol corresponds to an individual particle. For larger diameters, the single data point given represents the average of measurements on several particles as well as at different facets. (b) Dependence of the In2O3 thickness on In nanoparticle size for room temperature oxidation, calculated using a modified Mott−Cabrera model for spherical nanoparticles.6,7 Whereas the model would predict a continuous increase in the critical size, the underlying assumptions are eventually no longer valid for large particles. Curves correspond to different oxidation times: 1000 (green), 10 000 (red), 13 000 (gray in panel a), and 50 000 min (blue). Arrows: largest completely oxidized particles.
Figure 3. Indium particles on amorphous carbon films at 169 °C. (a) Overview TEM image and diffraction pattern (inset) of the In particles. (b−e) High-resolution images of individual nanoparticles.
completely oxidized particles do not undergo any changes (Figure 3b,c). Even annealing to high temperatures (800 °C) leaves these In2O3 particles intact. In contrast, the cores of the partially oxidized nanoparticles melt and shortly thereafter desorb under the electron beam, leaving a hollow c-In2O3 nanoparticle behind (Figure 3d,e). The HRTEM images of such particles clearly show an empty oxide shell devoid of the In core (Figure 3d,e), similar to a process observed for oxidized In whiskers.17 In ref 17, electron beam heating was suggested to be a possible explanation for the In desorption. We find that the electron beam interaction with molten In is a more likely explanation because no In desorption was observed for crystalline samples maintained just below the melting point.
previously mentioned, In nanoparticles with diameters 12 nm exhibit a core−shell structure with an In core encapsulated by a c-In2O3 shell with varying thickness. The In core diameter increases with particle size, whereas the oxide thickness scales inversely with the particle diameter; that is, the oxide is thinner for larger particles. For particles with diameters >20 nm, the oxide thickness 20576
dx.doi.org/10.1021/jp305806v | J. Phys. Chem. C 2012, 116, 20574−20578
The Journal of Physical Chemistry C
Article
ent oxide thickness after a constant oxidation time, predicted by the Mott−Cabrera eq 1 modified for spherical geometry. Figure 4b shows the oxide thickness, L(R, t), calculated by numerically solving eq 1 as a function of particle size (R) for three different oxidation times, 1000, 10 000, and 50 000 min. The calculation uses planar room-temperature oxide growth parameters, u and qaVM, determined experimentally in ref 16. The curves show that the oxidation of small particles is a continuous process that leads to increasing oxide thickness even after prolonged time periods. The size-dependent solutions are very characteristic: for large particles the oxide thickness changes very little, whereas small particle with sizes
