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Adhesion of Pt Nanoparticles Supported on #-AlO Single Crystal Zhongfan Zhang, Long Li, and Judith C Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp407798b • Publication Date (Web): 20 Sep 2013 Downloaded from http://pubs.acs.org on October 1, 2013

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The Journal of Physical Chemistry

Adhesion of Pt Nanoparticles Supported on γ-Al2O3 Single Crystal

Zhongfan Zhang1,* , Long Li 1,2 , Judith C. Yang 1,2 1

Department of Mechanical Engineering and Materials Science, University of Pittsburgh,

Pittsburgh, PA 15261 2

Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA

15261

Abstract: A model catalyst system of Pt nanoparticles (2~5 nm) dispersed on γ-Al2O3 (111) was prepared to study the support effects on nanoparticles shape and structure by cross-sectional high-resolution electron microscopy. A dewetting equilibrium shape of Pt nanoparticles was observed with a low-index plane orientation relation: Pt(111)[211]||γ−Al2O3(111)[211] and Pt(100)[011]||γ−Al2O3(111)[211]. Lattice-matching epitaxy of Pt(110)|| γ−Al2O3(110) was found across their interface to maintain a minimum interface strain during particle growth. To quantitatively interpret the interfacial energy and adhesion energy of the nanoparticles, the Wulff-Kaischew theorem was applied by taking into account the γ-Al2O3 support modifications to the equilibrium shape of nanoparticles. The present approach quantitatively solved the adhesion energy of the particle/support and could help to interpret nanoparticles equilibrium shape and stability.

Keywords: Pt nanoparticles, γ-Al2O3 support, structural relation, equilibrium shape, adhesion energy.

* Zhongfan Zhang, Department of Mechanical Engineering and Materials Science, Swanson School of Engineering, University of Pittsburgh, 3700 O'Hara Street, Pittsburgh, PA 15261, Office: (412) 624-9753, Email: [email protected], [email protected]

ACS Paragon Plus Environment


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Introduction

Platinum nanoparticles (NPs) dispersed on γ-Al2O3 oxide shows the highest efficiency in oxygen reduction reaction and represents the most famous heterogeneous catalyst in the chemical refining industry, fuel cell as well as gas sensor applications

1–4

. In the heterogeneous catalysis

studies, to understand the support impacts to the nanoparticle size, shape and surface structure is of the utmost importance due to their significant effects on the catalytic activities

1,4–10

. In

general, because of the severe condition of the catalyst application environment, the nanoparticles would be adapted to a more stabilized geometrical structure with respect to the support

11,12

. The study of the nanoparticle/support relationship, particularly the role of their

interface, plays a key role in interpreting the surface morphology and sintering kinetics of catalyst NPs. It is clear that the bonding strength at the interface between the metal and the oxide must dictate the metal particles’ morphology and sintering kinetics. This will guide the design direction for heterogeneous catalysts, and will also shed a light on the understanding of materials with novel growth and metal/oxide contacts in microelectronics 13,14. Experimental measurements15–21 and theoretical simulations

16,22,23

were initiated to

quantitatively study the interfacial energy and adhesion energy between NPs and their support. Among which, single crystal model catalysts have been extensively used to resolve the nanoparticle/support structure and shape relations, and can be directly compared to the theoretical simulations 24,25. In former studies, the Wulff theorem 26,27 was extensively applied to understand the nanoparticle shape and surface structure 18,24,28,29. Considering the catalyst support, especially the γ-Al2O3, has strong impacts on the structural shape and the electron density around the catalytic nanoparticle which would modify the catalysts activity, the Kaischew theorem was taken into account to analyze the support effects on NPs.


30–33

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γ-Al2O3 has a spinel structure which is face-centered cubic (FCC) with a lattice constant of 7.908Å. Because the commercialized γ-Al2O3 is polycrystalline and irregular in shape, a model Pt/γ-Al2O3 catalyst was prepared and characterized by cross-sectional transmission electron microscopy (TEM). Based on the Wulff-Kaishew theorem 32,34, an experimental method to quantitatively evaluate the adhesion energy and interfacial energy of the equilibrium shape (ES) Pt NPs on γ-Al2O3 is generalized. The interpretation of the equilibrium shape nanoparticles was analyzed to solve the discrepancies between the former experimental studies and theoretical calculations. From studying the interface of particle/support, it provides a way to fundamentally understand the adhesion aspects and equilibrium shape construction in the catalyst studies.

Experimental Methods

The creation of single crystal γ-Al2O3 thin films is crucial for this study and the formation was systematically studied in former works

35–37

. Through oxidation of NiAl(110) at

850℃ in dry air, the γ-Al2O3(111) thin films were prepared. Pt was evaporated onto the γ-Al2O3 surface by UHV electron beam evaporation at 700 ℃ with a deposition rate of 1 Å/sec. Heating stage was applied to assist metal atoms migration and nucleation process during particle growth where the temperature was chosen corresponding to nano-sized Pt particles melting point38–41. Hence, Pt particles was precipitated from its supersaturated vapor phase under relatively high stage temperature, the Pt NPs are expected to find the thermodynamically equilibrium condition on the γ-Al2O3 support during nucleation and growth to form energetically favorable shape. Cross-sectional TEM samples were then prepared by cutting a 50 nm thin Pt/γ-Al2O3 section using a FEI DB235 dual-beam focused ion beam (DB-FIB), and further thinned with low energy Ar+ ion beam at low angles using a Fischione Model 1040 NanomillTM to remove the surface



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damage layer introduced from the DB-FIB preparation. High-resolution electron microscopy (HREM) characterization were carried out with a JEM 2100FEG, Cs-corrected Hitachi HD-2700 FE-STEM and FEI Titan 80-300 respectively. Considering that Pt NP may not be edge-on particle in cross-sectional view, and not on the same defocus plane as substrate, through focal series method was used with a 2 nm increment step to obtain a practically better resolution of Pt NP and γ-Al2O3 substrate at different defocus value.

Results and Discussion

Platinum NPs (2~5nm) were deposited onto the γ-Al2O3 surface under 700℃ heating by electron beam evaporation in ultrahigh vacuum. The experimental approach for the system to achieve equilibrium condition is using physical vapor deposition method on the heating stage. This processing condition created 2-5 nm Pt NPs, with a mean diameter of 3.5 nm in statistical measurement, Fig. 1. Cross-sectional high-angle annular dark field (HAADF) imaging and HREM imaging (Fig. 1a and b insets) indicate a large portion of Pt nanoparticles was deposited onto the γ−Al2O3 support surface. Instead of the very small contact area, one would expect for a significant fraction of particle covering on the γ−Al2O3 (111) surface with direct contact interface for HREM characterization. The ES of Pt NPs preserved a dewetting behavior on the γ−Al2O3 support through our observation. The dewetting shape of Pt NPs could be directly compared to the former findings of its wetting behavior on TiO2(110) support 42, where the support plays the dominant role in determining its energetically favorable ES. The epitaxial relation of the Pt/γ−Al2O3

observed

in

Fig.1b

(top-left

and

bottom-right

insets)

follows

Pt(111)[211]||γ−Al2O3(111)[211] with Pt(110)|| γ−Al2O3(110) as their lattice-matching across the interface.


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In Fig.2, the HREM image at the Pt/γ-Al2O3 interface and its corresponding Fast Fourier transforms (FFT) (Fig. 2a insets) indicates the ES Pt is oriented normal to the electron beam direction.

The

orientation

relation

of

Pt/γ−Al2O3

could

be

describe

as

Pt(100)[011]||γ−Al2O3(111)[211] with an interface plane of Pt(110)|| γ−Al2O3(110). In this orientation, the geometrical structure of ES Pt particle could be interpreted from its profile shape. —

The structure of platinum has a face-centered cubic (FCC) structure (space group Fm3m) with a lattice constant of 3.923Å. Because of the large unit cell of γ-Al2O3 (7.90Å), the interface lattice misfit could be the low index Pt(220) planes to match with higher index plane γ-Al2O3(440) which forms the classical lattice-matching epitaxy (LME) growth. The epitaxial mismatch m is obtained to be 1.08%, where m = (b-a)/a. The a and b are the interface plane spacing of the support and particles. By using Wulff construction, theoretical calculations have been applied to interpret the ES of fcc crystal which predicts a truncated octahedron with exposing surface of {100} and {111} low-index plane

43,44

. To better understand the morphologies of the observed faceted Pt

nanoparticles and the relations to the HREM image obtained, a three-dimensional schematic with its corresponding profile were constructed to compare with the present resolve nanoparticle shape, as shown in Fig. 2 b and c. It could be seen that the equilibrium shape of Pt NPs is mainly composed of the low surface energy facets with {111} and {100} planes. The schematic diagrams with corresponding profiles illustrate the observed equilibrium shape of Pt particle follows the predicted energetically favorable truncated octahedron. By clearly identifying the shape and structural relation of Pt/γ−Al2O3, Wulff-Kaishew theorem 26,27,31–33,45,46 were applied to quantitatively interpret the equilibrium shape and adhesion behavior of Pt nanoparticles. In the Wulff-Kaishew construction, the Pt/γ-Al2O3 interface energy



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was taken into account to interpret the particle shape modification where the total Gibbs free energy of Pt/γ−Al2O3 formation, ∆F, is composed of the chemical formation energy, the surface energy and interfacial energy, and the elastic energy stored by system, as given by, ∆ F = − ∆ µV +

∑γ S i

i

+ S AB (γ AB − γ B ) + ε 0 m 2VR ,

(1)

i ≠ AB

where ∆µ is the supersaturation per unit volume, V is volume, γi is the surface energy of particle and Si is the corresponding surface area, γΑΒ is the interfacial energy and SAB is the interface contacting area, γΒ is the surface energy of substrate, εo is the elastic coefficients of particle and R is relaxation energy factor. The adhension energy β can then be introduce to equation (1) by Dupre’s equation,

γ AB = γ A + γ B − β , (2) which reflects the bonding properties of the metal NPs to the support oxide. In Wulff-Kaishew theorem, the elastic energy is disregarded at the conditon of trivial lattice misfit, m→0. The equilibrium condition can thus be obtained by taking the first order derivative of euqation (1) to be zero. The nanoparticle equilibrium shape condition could be determined as:

λ=

(γ A − β ) γ i = h AB hi

(3)

where hi and hAB are the normal distances from Wulff point O to the surfaces i and the interface AB (Fig.3). If γA > β, hAB > 0, i.e. the particle is dewetting on support, the Wulff point O is above the interface AB with more surface area exposed which is observed in the Pt/γ-Al2O3 catalyst system. The Prerequisites of applying Wulff- Kaishew theorem could be summarized as below,


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1. An assumption of elastic energy is negligible along the particle-support interface. This can be obtained when the lattice mismatch between particle and support across the interface is close to 0 or the particle is fully relaxed on the support. In our observation, the main Pt/γ-Al2O3 interface misfit is LME with Pt(220)||γ-Al2O3(440) and m = 1.08% → 0. A common Wulff point of the equilibrium shape particle would also imply the absence of elastic energy 32,47. 2. Cross-sectional view of NPs is necessary to outline the transversal profile and the structural relation of particle/support. This is achieved by Pt NPs seating perpendicular to the transmitted beam and looking along a zone axis in Pt crystal to see sets of planes in the edge-on orientation, i.e. symmetrical two dimensional lattice fringes are observable in HREM images. Three-dimensional views are a plus to understand geometric construction of crystallite (Fig.2b) by indicating the project beam direction. By adhering to those requirements, adhesion energy, β, and interfacial energy, γΑΒ, can be quantitatively solved for interpreting the shape and stability of Pt NPs. Several parameters need to be measured in the profile view of the Pt particle, i.e., the hi and hAB. Caution should be taken when determining the terminal plane for each surface which may cause 1~2 Å errors in different orientations. Considering the importance of meticulous measurements and systematic error may be induced, average(λ) factor might be introduced to reduce the systematic error by measuring all hi of the profile and get the average ratio, λ

γ / /n .

Numerous theoretical simulations of Pt NPs and γ-Al2O3 have been done to calculate their respective surface energy44,48–51, yet, it is still a challenge to determine their interfacial information and adhesion energy, and very few results can be found. By utilizing the deliberately calculated surface energy of Pt and γ-Al2O3, the adhesion energy (see Table 1) and interfacial



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energy can be obtained from Eq. (2) and (3), where γ Pt (100) / γ − Al O (111) is equal to 215.2 ± 5 µJ/cm2, 2

3

see Supplemental Information. In this manner, the obtained quantitative information for the Pt/γ-Al2O3 interface could be directly compared with theoretical prediction. In the former study, by applying the Wulff theorem, K. Hansen et al. 18 quantitatively solved the adhesion energy of Pd/Al2O3 system where a large discrepancy was found compared to the density-functional theory obtained results by A. Bogicevic et al. [32], and Vitos et al. 48,52, as shown Table 1. The substantial difference of former findings has been later addressed by Varga where it is due to the deposition condition on a different Al2O3 system53. From a theoretical perspective, it might also be partially due to overlook the γ-Al2O3 support effects on the equilibrium shape of Pd NPs. By discerning the profile and truncation of the Pt particle in cross-sectional HREM imaging, the Wulff-Kaischew theorem included the shape modifications by support to precisely interpret the adhesion behavior of the Pt nanoparticle on γ-Al2O3 support and demonstrated a consistent result with the theoretical calculations while the Wulff theorem calculations tends to achieve a higher adhesion energy, Table 1. The dewetting behavior of Pt particles could also be analyzed by the Wulff-Kaischew construction where the Wulff point wanders outside of the substrate. According to equation (3), when γA > β or γAB > γB , one will find hAB > 0, i.e., the particle is dewetting on the support. In our observation, due to the fact that β Pt (100 ) / γ − Al O (111 ) is smaller than the surface energy of the Pt 2

3

(100) top facet, a dewetting shaped Pt nanoparticle will be preferred to form on γ-Al2O3 surface with an island growth behavior which would lead to a more exposed surface area. This dewetting behavior could be directly compared to the Pt/TiO2 system where a truncated wetting Pt nanoparticle was observed which suggests a higher Pt adhesion energy on the TiO2 support 3.


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Therefore, the dewetting shape of Pt on γ-Al2O3 support would be more beneficial to its catalytic activities since a higher specific surface area would be present as the catalytic reaction sites for a certain volume. However, the lower adhesion energy suggests a weak bonding of the particle/support which results in a low thermal stability during its application. Thus, a compromise needs to be justified between the Pt catalytic activity and thermal stability. In fact, the resovled wetting criteria of particle growth could also be extended to interpret the thin film wetting behavior comparing with the more generally accepted Young’s equation, as given below 54

,

γ A cos θ + γ AB = γ B

(4)

In this case, γ is the surface energy of the liquid, θ is the equilibrium contacting angle between the support and the liquid drop. Combining with Eq. (3), a non-wetting growth mode of liquid (or dewetting behavior for crystallites) could be established when the sessile drop is greater than half-way wetting on the support with θ > 90°, which leads to the equllibrium condition

(for liquid from Young’s equation) or γ

(for crystallite from

Wulff-Kaischew theorem).

Conclusion

In summary, the Pt/γ-Al2O3 model catalyst was prepared and characterized by cross-sectional TEM. The in-plane orientation relation of Pt/γ-Al2O3 follows a low-index plane epitaxy: Pt(111)[211]||γ−Al2O3(111)[211] and Pt(100)[011]||γ−Al2O3(111)[211], where the interfacial

plane

matching

all

followed

with

Pt(110)|| γ−Al2O3(110).

The

classical

lattice-matching epitaxy was found to be the main growth epitaxy across the Pt/γ-Al2O3(111)



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interface to maintain a minimum interface strain during particle growth. The equilibrium shape of Pt NPs on γ-Al2O3 preserves a dewetting behavior. Truncated octahedron shape of Pt NPs were observed with surface composed by {100} and {111} facets. Based on the Wulff-Kaischew theorem, the quantitative measurement of particle/support interfacial information was obtained with a high degree of consistency to theoretical calculation. This generalized method is expected to provide a better understanding of the interfacial studies in heterogeneous catalysts and metal/oxide contacts in semiconductor industry.

Support Information

Wulff-Kaischew theorem construction with the presence of support was quantitatively deduced to describe the equilibrium shape of nanoparticles.

Acknowledgements

We appreciate N. T. Nuhfer, JH Liu, R. R. Cerchiara, JG Wen, J. A. Barnard for their help during this work. Gratefully acknowledges the financial support from DOE-BES Catalysis Science Initiative Program (DE-FG02-3ER15476), the facilities, scientific and technical assistance of the Materials Micro-Characterization Laboratory at the Department of Mechanical Engineering and Materials Science, Nanoscale Fabrication and Characterization Facility (NFCF), at University of Pittsburgh, Carnegie Mellon University, Frederick Seitz Materials Research Laboratory at University of Illinois at Urbana-Champaign, Fischione Instruments, Inc.


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Table 1. A comparison between experimental adhesion energy data and density-functional calculated results. LDA-DFT [17]

Wulff-Kaishew theorem

Wulff theorem [15]

(µJ/cm2)

(µJ/cm2)

(µJ/cm2)

Pd(100)/γ-Al2O3

170

－

280± 20

Pt(100)/γ-Al2O3

83.2

69.1 ± 2

110.3 ± 5



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Fig.1 (a) Cross-sectional HAADF image shows a large portion of Pt NPs directly seating on the γ-Al2O3(111) support with a dewetting shape. (b) Cross-sectional HREM image of Pt/γ-Al2O3 shows Pt(111)||γ-Al2O3(111) epitaxial relation with interface Pt(110) ||γ-Al2O3(110) lattice-matching epitaxy, Fast Fourier Transformation (FFT) patterns with indices for the Pt particle (top-left corner) and γ-Al2O3 support (bottom-right corner) are also shown. Fig.2 (a). Cross-sectional HREM image of an equilibrium shape Pt particle with coherent epitaxial relation: Pt(100) || γ-Al2O3(111) and Pt(220) || γ-Al2O3(440) along the interface, Fast Fourier Transformation (FFT) patterns with indices for the Pt particle (top-right corner) and γ-Al2O3 support (bottom-right corner) are presented to show its orientation relation, (b). Three-dimensional truncated octahedron of the Pt particle, the transmitting beam direction is indicated as the solid arrow, (c). Matching profile of the observed Pt particle. Fig.3 Schematic diagram of the observed Pt particle with equilibrium shape seating on γ-Al2O3(111) support. hi and hAB are the normal distances from Wulff point O to the surfaces i and the interface AB. hAB > 0 means Wulff point O is above the support.

Table of Contents Image


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Pt NP

Fig.1 (a) Cross-sectional HAADF image shows a large portion of Pt NPs directly seating on the -Al2O3(111) support with a dewetting shape. (b) Cross-sectional HREM image of Pt/-Al2O3 shows Pt(111)||-Al2O3(111) epitaxial relation with interface Pt(110) || Al2O3(110) lattice-matching epitaxy, Fast Fourier Transformation (FFT) patterns with indices for the Pt particle (top-left corner) and -Al2O3 support ACS Paragon Plus Environment (bottom-right corner) are also shown.


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Page 18 of 21

Fig.2 (a). Cross-sectional HREM image of an equilibrium shape Pt particle with coherent epitaxial relation: Pt(100) || -Al2O3(111) and Pt(220) || -Al2O3(440) along the interface, Fast Fourier Transformation (FFT) patterns with indices for the Pt particle (top-right corner) and Al2O3 support (bottom-right corner) are presented to show its orientation relation, (b). ThreeACS Pt Paragon Plus Environment dimensional truncated octahedron of the particle, the transmitting beam direction is indicated as the solid arrow, (c). Matching profile of the observed Pt particle.

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Pt NP - A

A hA H

hi hAB

O

Pt/-Al2O3 interface- AB

(100)

-Al2O3 (111) support - B

Fig.3 Schematic diagram of the observed Pt particle with equilibrium shape seating on -Al2O3(111) support. hi and hAB are the normal distances from Wulff point O to the surfaces i and the interface AB. hAB > 0 means Wulff point O is above the support.


3


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Table 1. A comparison between experimental adhesion energy data and density-functional calculated results.

LDA-DFT [17]

Wulff-Kaishew theorem

Wulff theorem [15]

(µJ/cm2)

(µJ/cm2)

(µJ/cm2)

Pd(100)/-Al2O3

170

－

280± 20

Pt(100)/-Al2O3

83.2

69.1 ± 2

110.3 ± 5


Page 21 of 21

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Table of Contents Graphic


5

Single Crystal - American Chemical Society

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