Article pubs.acs.org/cm
Thermal Oxidation of Size-Selected Pd Nanoparticles Supported on CuO Nanowires: The Role of the CuO−Pd Interface Stephan Steinhauer,† Junlei Zhao,‡ Vidyadhar Singh,† Theodore Pavloudis,§ Joseph Kioseoglou,§ Kai Nordlund,‡ Flyura Djurabekova,‡ Panagiotis Grammatikopoulos,† and Mukhles Sowwan*,† †
Nanoparticles by Design Unit, Okinawa Institute of Science and Technology (OIST) Graduate University, 1919-1 Tancha, Onna-Son, Okinawa 904-0495, Japan ‡ Department of Physics and Helsinki Institute of Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland § Department of Physics, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece S Supporting Information *
ABSTRACT: The structure of heterogeneous nanocatalysts supported on metal oxide materials and their morphological changes during oxidation/reduction processes play a crucial role in determining the resulting catalytic activity. Herein, we study the thermal oxidation mechanism of Pd nanoparticles supported on CuO nanowires by combining in situ environmental transmission electron microscopy (TEM), ex situ experiments, and ab initio density functional theory (DFT) calculations. High-resolution TEM imaging assisted by geometric phase analysis enabled the analysis of partially oxidized, fully oxidized, and distinct onion-like Pd nanoparticles with subsurface dislocations. Furthermore, preferential crystalline orientations between PdO nanoparticles and the CuO nanowire support have been found. Hence, the CuO−Pd interface is crucial for the thermal oxidation of Pd nanoparticles, as corroborated by electron energy loss spectroscopy and DFT calculations. The latter revealed a considerably lower energy barrier for penetration of oxygen into the Pd lattice at the CuO−Pd interface, promoting nanoparticle oxidation. The obtained results are compared with those of literature reports on different material systems, and potential implications for catalysis and chemoresistive sensing applications are discussed.
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INTRODUCTION
Herein, we study the thermal oxidation of size-selected Pd nanoparticles supported on CuO nanowires, a configuration of important practical relevance and, at the same time, a model system with one-dimensional geometry and high crystallinity. We employ magnetron sputtering inert-gas condensation, an emerging technique for the synthesis of single-component and multicomponent nanoparticles,23−25 which is ideally suited for the functionalization of nanowire-based sensor devices.22,26 CuO nanowires are decorated with well-defined Pd nanocrystals with sizes around 5 nm by means of deposition in the soft landing regime. Using a combination of in situ environmental transmission electron microscopy (TEM) and ex situ thermal oxidation, we observe different types of nanoparticle morphologies, which are analyzed in terms of their crystalline structure. Furthermore, density functional theory (DFT) calculations are performed to find energy barriers for oxygen atom diffusion processes related to Pd nanoparticle oxidation, in particular migration on Pd surfaces and inside both the Pd lattice and the PdO lattice. The impact of CuO−Pd and PdO− Pd interfaces that are close to these diffusion processes is
Among the variety of hybrid nanomaterials, nanowiresupported nanoparticles have attracted considerable attention in diverse applications such as battery systems,1 heterogeneous catalysts,2,3 dye-sensitized solar cells,4,5 and chemoresistive sensor devices.6−8 In particular, metal oxide materials decorated with noble metal clusters have been extensively studied,9,10 leading to the identification of different types of nanoparticle− support interactions relevant for catalytic activity, including charge transfer and spillover of dissociated species.6,11,12 Several factors influencing nanocatalyst properties have been identified, for instance size, shape, composition, and oxidation state.13 CuO nanowires can be grown by a simple, low-cost thermal oxidation process14 and typically show p-type semiconducting properties.15 They have been successfully employed as the anode material in lithium ion batteries16,17 and as sensing elements for toxic-gas detection.18−20 In the latter case, surface decoration with Pd nanoclusters was found to result in considerably enhanced sensor performance.21,22 However, only limited information about structural changes of CuOsupported Pd nanoparticles due to oxidative treatments resembling sensor operation is available, preventing further insights into the improved sensing activity. © 2017 American Chemical Society
Received: June 1, 2017 Revised: June 22, 2017 Published: June 26, 2017 6153
DOI: 10.1021/acs.chemmater.7b02242 Chem. Mater. 2017, 29, 6153−6160
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Chemistry of Materials
spacings were measured by a projection method based on the methodology of Bierwolf et al.32 DFT Calculations. Atomistic modeling of oxygen migration based on DFT was performed using the Vienna ab initio simulation package (VASP) implementation of the projector-augmented wave (PAW) method.33,34 Electron−ion interactions were described by Perdew− Burke−Ernzerhof (PBE) functionals35 for the exchange correlation energy. For the Hubbard-corrected local spin-density approximation (LSDA+U), we adopted the simplified rotationally invariant approach formulated by Dudarev et al.36 Parameters optimized for the electronic structure of CuO (U = 7 eV, J = 0 eV, and Ueff = 7 eV) were chosen according to ref 37. The plane wave energy cutoff was set at 520 eV to ensure convergence of the calculation. Transition barriers for oxygen atom migration via interstitial sites were calculated by the nudged elastic band (NEB) method.38,39 Surfaces and interfaces were approximated using flat, two-dimensional slabs due to system size limitations associated with DFT modeling. As the CuO nanowire diameters were considerably larger than the size of the Pd nanoparticles, no additional effects due to the one-dimensional nanowire geometry are expected. Further details about the DFT calculations can be found in the Supporting Information.
revealed. By correlating experimental and theoretical results, we propose a model for the oxidation of Pd nanoparticles supported on CuO nanowire surfaces. Eventually, we compare our findings with those of literature reports on different materials systems and discuss implications for catalytic activity.
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METHODS
Deposition of Size-Selected Pd Nanoparticles by Magnetron Sputtering Inert-Gas Condensation. Size-selected Pd nanoparticles were deposited using a magnetron sputtering inert-gas condensation system27 equipped with a quadrupole mass filter (Mantis Deposition Ltd.). Before nanoparticle synthesis, the main chamber base pressure was in the low 10−8 mbar range, while during deposition pressures were in the 10−1 mbar and 10−4 mbar range for the aggregation zone and the main chamber, respectively. A single 1 in. Pd target (99.95% pure) was used with a magnetron power of 15 W and a constant Ar flow of 70 sccm. Environmental TEM: In Situ CuO Nanowire Growth and Thermal Oxidation of Pd Nanoparticles. Experiments were performed using an FEI Titan Environmental TEM instrument (operation voltage of 300 kV) equipped with a spherical aberration image corrector and a postcolumn energy filter for electron energy loss spectroscopy (EELS). A thermal oxidation method was used for in situ CuO nanowire growth in the environmental TEM instrument,28 relying on the Protochips Aduro 500 TEM holder platform and membrane-based heating chips with closed loop temperature control. Cu microstructures were fabricated on these membranes using electron beam evaporation of a Cu thin film (thickness of ∼650 nm) combined with a photolithographic lift-off process. Underneath the Cu, SiN was sputter-deposited (thickness of ∼175 nm) to ensure electrical insulation to the TEM heating chip, followed by a thin Ti adhesion layer deposited by electron beam evaporation. Micromachined rectangular through-holes were realized by focused ion beam milling (FEI Helios G3 UC FIB-SEM; ion acceleration voltage of 30 kV) from the sample backside to minimize sample damage and Ga ion implantation. After CuO nanowire growth, the TEM heating chip was transferred to the magnetron sputtering inert-gas condensation system for Pd nanoparticle decoration. Next, the samples were transferred back to the environmental TEM instrument for in situ thermal oxidation experiments. To avoid any beam effects related to chemically active species resulting from electron-induced ionization of oxygen gas molecules,28 TEM imaging was performed at room temperature under vacuum after the samples had been heated at 20 mbar O2 pressure. Furthermore, the electron beam was blanked during the in situ thermal oxidation treatments. Ex Situ Thermal Oxidation of Pd Nanoparticles Supported on CuO Nanowires. CuO nanowires were synthesized by thermal oxidation of a Cu wire (0.1 mm diameter, 99.9999% pure) on a hot plate in ambient air at ∼430 °C for 3 h. A nanowire suspension was achieved by ultrasonication in isopropanol, which was drop-coated on holey SiN TEM grids, followed by Pd nanoparticle deposition by magnetron sputtering inert-gas condensation. Ex situ thermal oxidation was performed by placing the TEM grid on a hot plate in ambient air at ∼350 °C for 4 h. Geometric Phase Analysis and TEM Image Simulation. Geometric Phase Analysis (GPA)29 implemented in the Digital Micrograph (Gatan) software package was used to obtain Braggfiltered images as well as amplitude and phase images of Bragg spots in the fast Fourier transform (FFT) of experimental images. In particular, the strain field along specific reflections (spatial frequencies) of the FFT has been computed on the basis of the theory and methodology described in ref 30. High-resolution TEM image simulation and diffraction pattern analysis were performed using the EMS-JEMS software31 in combination with additional numerical methods. Taking into account imaging conditions and crystal thickness variations, we performed image simulations along the ⟨001⟩, ⟨010⟩, ⟨101⟩, ⟨110⟩, and ⟨111⟩ PdO zone axes using the multislice method. Experimental d
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RESULTS AND DISCUSSION A schematic illustration of the Pd nanoparticle synthesis process by magnetron sputtering inert-gas condensation is shown in Figure 1a. During the plasma process, atoms are
Figure 1. (a) Schematic illustration of Pd nanoparticle synthesis by magnetron sputtering inert-gas condensation. (b) High-resolution TEM micrograph of a representative Pd nanoparticle close to the [101] zone axis with a size of ∼5 nm and single-crystalline structure (scale bar of 1 nm). (c) CuO nanowire decorated with Pd nanoparticles (scale bar of 5 nm).
sputtered off the Pd target and cooled by collisions with Ar gas atoms, enabling bond formation. Cluster nucleation, growth, and coalescence lead to the formation of Pd nanoparticles, which pass through a quadrupole mass filter (not shown) for selection according to their sizes.40 The resultant Pd nanoparticles with a narrow size distribution22,41 are transported to the main chamber through a pressure differential and are subsequently deposited on the sample in the soft landing regime.42 A high-resolution TEM micrograph of a representative Pd nanoparticle can be seen in Figure 1b, which exhibits 6154
DOI: 10.1021/acs.chemmater.7b02242 Chem. Mater. 2017, 29, 6153−6160
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Chemistry of Materials single-crystalline structure with lattice parameters corresponding to the face-centered cubic Pd lattice. To enable in situ TEM studies of the thermal oxidation of supported Pd nanoparticles, CuO nanowires were grown inside the environmental TEM instrument at well-defined positions on membrane-based TEM heating chips (Figure S1). Thermal oxidation of a microstructured Cu thin film most commonly led to bicrystalline CuO nanowires with diameters of ∼20 nm (Figure S2) that are quite comparable to those achieved by ex situ synthesis at atmospheric pressure.20 After in situ CuO nanowire growth in the environmental TEM instrument, samples were transferred to the magnetron sputtering inertgas condensation system and decorated with size-selected Pd nanoparticles, exhibiting a narrow size distribution around 5 nm. A typical TEM micrograph at intermediate magnification is shown in Figure 1c with a CuO nanowire and three supported nanoparticles, allowing for heating to elevated temperatures upon operation of the membrane-based device. A major advantage of this unique experimental platform is that CuO nanowires are suspended in vacuum for optimal TEM imaging conditions. The thermal oxidation process of the CuO nanowiresupported Pd nanoparticles was studied by in situ heating to 300 °C at 20 mbar O2 pressure for 1 h. High-resolution TEM imaging and analysis of the lattice spacings showed that the majority of the nanoclusters was oxidized to single-crystalline PdO particles (see a representative example in Figure S3). However, several nanoparticles with metallic Pd as well as PdO regions were found, which is attributed to an intermediary stage of the oxidation process. Residual Pd was observed at positions away from the nanoparticle center, suggesting that nanoparticle oxidation is not proceeding through homogeneous oxide shell formation. The example in Figure 2a shows PdO formation close to the nanowire interface and a misfit dislocation at the Pd−PdO boundary, indicated by a red circle in the GPA phase and Bragg-filtered images. Further details of the analysis and interpretation of the experimental results are shown in Figure S4. Electron microscopy combined with GPA is a well-known tool for the analysis of crystal lattice displacements,29 in particular around dislocations.43 The strain fields close to the defect at the Pd−PdO interface obtained here (see the enlarged GPA strain image in Figure S4f) are in good qualitative agreement with misfit dislocations at other types of interfaces reported in the literature.44−46 Moreover, a GPA strain value of ∼17% for the PdO 101 spatial frequency with respect to Pd 111 was found, in agreement with the theoretical value. After an additional thermal oxidation step at 350 °C and 20 mbar O2 pressure for 1 h, the same nanoparticle was found to be fully oxidized with no signs of dislocations, which can be seen from the high-resolution TEM micrograph and the homogeneous phase image obtained by GPA strain analysis. These observations provide a first indication that the CuO−Pd interface plays an important role during the thermal oxidation of Pd nanoparticles. Apart from the cases of partial oxidation described above, other types of nanoparticle morphologies were also observed after the first thermal oxidation step at 300 °C, i.e., fully oxidized PdO nanoparticles and distinct onion-type structures. A high-resolution TEM micrograph and the corresponding Bragg-filtered image of a fully oxidized PdO nanoparticle exhibiting an onion-like structure are shown in Figure 3a. Subsurface dislocations were observed as discontinuities of the PdO (101) lattice fringes between adjusted PdO domains. After
Figure 2. (a) Pd nanoparticle on a CuO nanowire surface after in situ thermal oxidation at 20 mbar O2 pressure and 300 °C for 1 h (scale bar of 2 nm). The left inset shows GPA strain analysis of the Pd 111 spatial frequency in relation with PdO 101, whereas the Bragg-filtered image is shown in the right inset (the red circle indicates dislocation at the Pd−PdO interface). (b) The same nanoparticle after an additional thermal oxidation treatment at 20 mbar O2 pressure and 350 °C for 1 h is fully oxidized to PdO, which can be seen from the homogeneous phase in the GPA analysis (scale bar of 2 nm).
the second thermal oxidation step (350 °C, 20 mbar O2, 1 h), the nanoparticle morphology was preserved to a large extent (see Figure 3b). These specific nanoparticle structures were furthermore analyzed by GPA amplitude images, which are equivalent to dark-field images and can be helpful for revealing boundaries between crystalline domains. The same PdO nanoparticle supported on a CuO nanowire can be seen in the GPA amplitude images of the PdO 101 spatial frequency after the first thermal oxidation step [300 °C (Figure 3c)] and after the second thermal oxidation step [350 °C (Figure 3d)]. The onion-like morphology was visualized with bright adjusted crystalline domains and the dark subsurface region exhibiting dislocations between them. Furthermore, ex situ thermal oxidation experiments were performed using CuO nanowires grown by thermal oxidation of a Cu foil, which were transferred to TEM grids and subsequently decorated with Pd nanoparticles by magnetron sputtering inert-gas condensation. The nanoparticles were found to be fully oxidized to PdO after thermal oxidation in ambient air at ∼350 °C for 4 h, which is shown for a representative example in Figure 4. Both FFT analysis and TEM image simulation are in very good agreement with the [001] PdO projection. The [100] PdO crystalline direction was aligned in parallel with respect to the [110] direction of the CuO nanowire support. In other cases, the onion-like PdO structure exhibiting subsurface dislocations was occasionally observed. On several different CuO nanowires, specific PdO lattice planes were found to be preferentially aligned in parallel to CuO nanowire crystalline directions (sometimes with 6155
DOI: 10.1021/acs.chemmater.7b02242 Chem. Mater. 2017, 29, 6153−6160
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Chemistry of Materials
mismatches of a few degrees). The utilized gas phase deposition method is linked with random nanoparticle orientations upon landing, and hence, one can expect that the observed preferential CuO−PdO relative orientations arise during the thermal oxidation process, emphasizing again the importance of the nanoparticle−nanowire interface. In additional EELS measurements, no significant diffusion of Cu atoms to the surface or bulk of the oxidized Pd nanoparticles was detected (Figure S5); however, the possibility that strong metal support interactions such as nanoparticle decoration with the support material47,48 act as secondary influencing factors on Pd nanoparticle oxidation cannot be excluded, which will be the subject of future investigations. To study the role of the CuO nanowire interface further, DFT calculations using the NEB method were performed, breaking down the process of Pd nanoparticle oxidation into diffusion of oxygen atoms. It is known from the literature that Pd oxidation at elevated temperatures and O2 pressures proceeds via several steps, involving oxygen chemisorption followed by formation of two-dimensional surface oxides [for the cases of Pd(100) and Pd(111)], oxygen at subsurface sites, and eventually bulk PdO.49−51 The two-dimensional surface oxides formed on Pd(100) and Pd(111) present kinetic barriers for bulk oxide formation, which is not the case for Pd(110) showing facile bulk oxide formation and the absence of a thin surface oxide.52 According to surface phase diagrams, bulk PdO is favorable in the temperature and O2 pressure ranges relevant for this study.49,50,52 In the initial stages of Pd oxidation, oxygen molecules are adsorbed on the surface and dissociate easily into separate atoms, because the minimal energy barriers for the latter process were predicted to lie in the range of ∼0.12−0.2 eV.53−55 For the case of thermal oxidation of Pd nanoparticles supported on CuO nanowires presented here, the surface mobility of atomic oxygen species is considered an important factor. To predict energy barrier values for oxygen diffusion on the close-packed Pd surfaces, DFT−NEB calculations were performed (see Figure 5). Energy barriers of 0.11, 0.57, and 0.27 eV were found for Pd(100), Pd(111), and Pd(110), respectively. Recently, the authors of ref 56 have reported similar calculations using a different DFT method and taking charge transfer into account, which led to energy barriers for oxygen atom migration of 0.154 eV on Pd(100), 0.327 eV on Pd(111) (hcp site to fcc site), and 0.211 eV on Pd(110) surfaces. These values are quite comparable with our results, indicating that the obtained migration barriers are rather insensitive to the choice of DFT method or system size. The energy barriers for surface migration are relatively low compared to those for oxygen subsurface penetration that have been found by atomistic modeling (>1 eV)57 and by experimental methods (0.6−0.9 eV).50,51 Hence, we deduce that oxygen atoms on Pd nanoparticle surfaces can readily migrate toward the CuO interface via surface diffusion. As a next step, the penetration of oxygen into the Pd matrix was studied, in particular in the presence of a CuO interface. For the CuO(111)−Pd(100) case, DFT−NEB calculations predict an energy barrier of 0.50 eV for the migration of oxygen from a first-second-layer tetrahedral site to a second-third-layer tetrahedral site via an octahedral site (see Figure 6). On the other hand, in the absence of a CuO interface, an energy barrier of 0.77 eV was found for a similar path in the Pd lattice. These results clearly substantiate the idea that penetration of oxygen into the Pd matrix is facilitated at the CuO−Pd interface, and consequently, it can be assumed that the latter has a marked
Figure 3. (a) Pd nanoparticle on a CuO nanowire surface after in situ thermal oxidation at 20 mbar O2 pressure and 300 °C for 1 h (scale bar of 2 nm). On the right, the corresponding Bragg-filtered images of the PdO 101 spatial frequency reveal the onion-like structure with subsurface dislocations between adjusted PdO domains. (b) The nanoparticle structure is preserved to a large extent after an additional thermal oxidation treatment at 20 mbar O2 pressure and 350 °C for 1 h (scale bar of 2 nm). (c and d) GPA amplitude images (scale bars of 2 nm) of the PdO 101 spatial frequency after the first thermal oxidation step and the second thermal oxidation step, respectively, show adjusted crystalline domains (bright) and subsurface regions with dislocations (dark).
Figure 4. Pd nanoparticle on the CuO nanowire surface after ex situ thermal oxidation in ambient air at ∼350 °C for 4 h, resulting in a phase transformation to PdO. High-resolution TEM image simulation (overlaid in blue) of PdO along the [001] projection is found to be in agreement with the experimentally observed structure. FFT analysis of the PdO nanoparticle (FFT - NP) and the support reveals that the PdO [100] direction is parallel to the [110] direction of the CuO nanowire support (scale bar of 2 nm).
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DOI: 10.1021/acs.chemmater.7b02242 Chem. Mater. 2017, 29, 6153−6160
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Figure 5. Energy barriers for oxygen atom migration on close-packed Pd surfaces. DFT−NEB calculations of diffusion on the (a) Pd(100), (b) Pd(111), and (c) Pd(110) surfaces predict barriers of 0.11, 0.57, and 0.27 eV, respectively.
barrier was found to be significantly reduced to 0.97 eV (Figure 7a, bottom right). For the case of oxygen diffusion in the tetragonal PdO lattice, DFT−NEB calculations predicted an energy barrier 2.5 eV for an oxygen atom moving to the next tetrahedral vacancy (Figure 7b). Conversely, a small energy barrier of
