Growth and Sintering of AuâPt Nanoparticles on Oxidized and

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Growth and Sintering of Au-Pt Nanoparticles on Oxidized and Reduced CeOx(111) Thin Films by Scanning Tunneling Microscopy Yinghui Zhou and Jing Zhou* Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071

ABSTRACT Bimetallic Au-Pt particles were prepared by sequential deposition of Pt first followed by Au on both oxidized CeO2(111) and reduced CeO1.88(111) thin films. The structure of Au-Pt particles was studied using scanning tunneling microscopy and compared to that of pure Au and Pt particles. Au forms larger particles on both ceria surfaces compared to Pt due to its greater mobility. Upon heating, Au significantly sinters and forms large hexagonal particles. Deposition of Au on pre-existing Pt particles on ceria with a low coverage of 0.1 ML at 300 K produces pure Au and Au-Pt particles with more Au particles on a reduced CeO1.88 surface. A bimodal size distribution is formed upon heating to high temperatures due to the different sintering behavior of Au and Au-Pt on ceria. By increasing the Pt coverage, bimetallic Au-Pt particles are more easily formed at 300 K, and a monodispersed particle size distribution is obtained. SECTION Nanoparticles and Nanostructures

I

t is of scientific interest to study ceria-supported Au-Pt bimetallic particles. Current research has demonstrated that ceria-supported Au nanoparticles can exhibit promising reactivity in many applications, including syngas reaction, hydrocarbon conversion, the water-gas shift (WGS) reaction, NO reduction, as well as the CO oxidation reaction.1-9 One of the major issues related with Au nanoparticles in catalysis is sintering at high temperatures, which results in the loss of the reactivity.10-14 The presence of Pt in bimetallic Au-Pt particles could potentially inhibit the Au sintering on ceria. As demonstrated by previous studies in the literature, the addition of Pt or Ag to Au particles on TiO2 can effectively suppress the sintering of Au.15,16 Furthermore, the Pt-ceria catalysts also show high activity in CO oxidation and WGS reactions, and bimetallic Au-Pt nanoparticles can exhibit unique reactivity different than pure Au and pure Pt due to the synergistic effect between the two metals for these catalytic applications.17-20 To elucidate the chemistry of ceria-supported Au-Pt nanoparticles as potential new catalysts, it is important to gain a fundamental understanding of their growth and sintering behavior. It is known that the catalytic reactivity of metal nanoparticles can be affected by their size and structure on the support as well as their interaction with the ceria support.21-31 Up to date, much progress has been made on the chemistry study of Au-Pt/ceria.32-34 The detailed understanding of the structure of Au-Pt nanoparticles is still lacking, and this motivates the current study. In this paper, we report the results of the growth of Au-Pt nanoparticles on ceria at 300 K and upon heating using scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. Particularly, the effect

r 2010 American Chemical Society

of Pt on the growth and sintering of bimetallic Au-Pt particles is examined. Considering the fact that the degree of ceria reduction can affect the growth of metal particles and their catalytic reactivity, the growth of Au-Pt particles was investigated on both oxidized and reduced ceria supports.24,25,35-39 Reducible CeO2(111) and CeO1.88(111) thin films grown on Ru(0001) were used as the model supports in this study to understand the Au-Pt growth and sintering at the fundamental level. The ceria thin films grown have the same thickness, which is estimated to be six layers. The degree of Ce reduction in the film was calculated by X-ray photoelectron spectroscopy (XPS) based on the method described previously.40 Low-energy electron diffraction studies show that both films exhibit a ceria p(1.4 1.4) pattern. The atomic structure of the (111) surface of the films revealed by STM agrees well with that of the bulk. Compared with CeO2, CeO1.88 has surface defects present related to oxygen vacancies.31,41-43 To understand the growth and sintering behavior of Au-Pt particles, pure Au and Pt particles on ceria supports were first studied. Figure 1 shows the STM results of ∼0.4 ML of Au and Pt deposited on fully oxidized CeO2(111) and reduced CeO1.88(111) thin films at 300 K and subsequently annealed to various temperatures as indicated. One monolayer (ML) of Au and Pt was calculated to be 1.40 1015 and 1.50 1015 atoms/cm2 with respect to the packing densities of their (111) surfaces, Received Date: December 13, 2009 Accepted Date: January 7, 2010 Published on Web Date: January 12, 2010

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DOI: 10.1021/jz900397y |J. Phys. Chem. Lett. 2010, 1, 609–615

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Figure 1. STM images of 0.4 ML of pure Au and pure Pt deposited on CeO2 and CeO1.88 at 300 K and after annealing to 700 and 800 K, respectively. All images are 100 nm 100 nm.

heating, but to a larger extent (Figure 1e, f). At 800 K, Au particles with a particle diameter between 5.0 and 11.5 nm and a height range of 1.4-4.0 nm were formed. Deposition of 0.4 ML of Pt onto the fully oxidized CeO2 thin film at 300 K produces Pt particles that are 2.1 nm wide and 0.5 nm high with a particle density of 8.2 1012/cm2 (Figure 1g). Heating the Pt nanoparticles also causes an increase in the particle size and a decrease in the particle density (Figure 1h,i). On the reduced CeO1.88 surface (Figure 1j-l), Pt particles exhibit a smaller size and a higher particle density compared to those on the oxidized CeO2 surface. The mean height of Pt particles on CeO1.88 at 300 K is ∼0.4 nm, less than 2 atomic-layers thick compared to bulk Pt. Our STM data of Pt growth on ceria are consistent with the results from previous studies of the Pt growth on CeOx(111) thin films by CO adsorption and XPS experiments, which also indicate that Pt can better wet the reduced ceria surfaces than

respectively. The diameter, height, and density of metal particles were measured from STM images, which are listed in Table 1. At 300 K, Au shows a preferential nucleation at the step edges on CeO2(111), which is consistent with previous studies of Au on ceria as well as on titania.37,38,44-46 Heating the surface to 500 K causes a slightly increase in the Au particle size (data not shown). However, a substantial change in the Au morphology was observed after annealing the surface to 700 (Figure 1b) and 800 K (Figure 1c). Well-defined hexagonal Au particles were formed at 800 K. Compared to the Au growth on CeO2(111), deposition of Au on CeO1.88 at 300 K produces uniformly distributed Au particles with a smaller size and a larger particle density (Figure 1d). The different growth behavior of Au on reduced ceria can be attributed to the surface defects associated with missing oxygen on the surface.47,48 Similar to Au on CeO2, Au particles on CeO1.88 at 300 K aggregate to form larger particles upon


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Table 1. Mean Particle Diameter, Height, And Density of Au, Pt, and Au-Pt with a Total Metal Coverage of 0.4 ML on Oxidized CeO2 and Reduced CeO1.88 Thin Films As a Function of Annealing Temperaturesa metal Au

temperature (K)

2.0

300

2.5 ( 0.6

0.7 ( 0.3

4.1

700

3.8 ( 0.8

1.0 ( 0.3

0.9

1.88

Pt

2.0

1.88

Au-Pt

2.0

1.88

mean particle height (nm)

800

5.2 ( 0.6

2.0 ( 0.5

0.1

300

2.0 ( 0.4

0.6 ( 0.2

6.5

700

7.7 ( 1.4

1.3 ( 0.3

0.1

800

7.0 ( 2.0

2.1 ( 0.8

0.1

300

2.1 ( 0.4

0.5 ( 0.2

8.2

700

2.8 ( 0.5

0.8 ( 0.2

2.2

800

3.5 ( 0.7

0.8 ( 0.2

1.0

300 700

1.9 ( 0.3 2.4 ( 0.4

0.4 ( 0.1 0.5 ( 0.1

14.1 4.2

800

3.0 ( 0.5

0.6 ( 0.2

2.1

300 700

2.2 ( 0.6 (S) 3.3 ( 0.4

0.5 ( 0.2 0.6 ( 0.2

6.4 1.1

(L) 6.0 ( 1.0

1.6 ( 0.4

0.2

800

(S) 2.6 ( 0.6

0.6 ( 0.2

0.5

(L) 7.2 ( 0.7

2.0 ( 0.5

0.1

1.8 ( 0.4

0.5 ( 0.2

8.4

300 700 800

a

mean particle diameter (nm)

particle density (1012 cm-2)

x in CeOx

(S) 2.1 ( 0.4

0.4 ( 0.2

1.3

(L) 5.5 ( 1.0

1.7 ( 0.5

0.1

(S) 2.3 ( 0.5 (L) 7.1 ( 1.0

0.5 ( 0.2 1.8 ( 0.7

0.6 0.04

Au-Pt particles exhibit two particle sizes labeled as S (small) and L (large) after annealing.

formed on CeO1.88(111) under the same conditions. Therefore, they are assigned to pure Au. The smaller particles were formed due to the sintering of the bimetallic Au-Pt at 300 K. The different sintering behavior of pure Au and bimetallic Au-Pt can be explained by the different metal-metal bond strengths in the particles. As demonstrated in the literature, detachment of atoms from the existing particles is the ratinglimiting step for the metal particle sintering on oxides, which can scale with the metal-metal bond strength.15,49 The Au-Au bond strength (2.04 eV) is much smaller than the Pt-Pt bond strength (3.29 eV).50 Therefore, pure Au particles can sinter much more easily than Pt particles upon heating. The presence of Pt in the Au particles can cause the formation of the Au-Pt bond with a strength of 2.30 eV and the Pt-Pt bond and thus can inhibit the Au particle sintering.50 Further annealing of the surface to 800 K slightly increases the size of the particles while maintaining the bimodal size distribution (Figure 2d). Deposition of 0.3 ML of Au on 0.1 ML of Pt/ CeO2(111) at 300 K results in an increase of the particle diameter from 1.6 to 2.2 nm, the particle height from 0.4 to 0.5 nm, as well as the particle density from 5.1 1012 to 6.4 1012/cm2 (Figure 2e,f). The 25% increase in the particle density also suggests the formation of pure Au particles on CeO2, although it is estimated that ∼70% of Au favors nucleation on the existing Pt particles and form Au-Pt bimetallic particles. It is not surprising that less Au formed pure Au particles on CeO2 than that on CeO1.88, considering

on the oxidized CeO2.39 Compared to Au, Pt exhibits a much smaller particle size upon deposition on both fully oxidized and partially reduced ceria supports at 300 K as well as after annealing to higher temperatures. The different Pt growth behavior compared to Au can be explained by the strong interaction between the Pt and the ceria.32,39 The Au-Pt particles with the same total metal coverage of 0.4 ML were prepared on CeO2 and CeO1.88(111) supports by the deposition of 0.1 ML of Pt first followed by 0.3 ML of Au at 300 K and annealing to high temperatures (Figure 2a-h). The STM image of 0.1 ML of Pt deposited on the reduced CeO1.88 surface (Figure 2a) shows the formation of small Pt particles with a mean diameter and height of 1.5 and 0.3 nm and a particle density of 5.5 1012/cm2. Subsequent deposition of 0.3 ML of Au onto the surface increases the particle density by 53% and the particle size to 1.8 nm in diameter and 0.5 nm in height (Figure 2b). The increase in the particle density indicates the formation of pure Au particles on the surface. Besides the pure Au particles, it is estimated that 60% of Au was deposited on existing Pt particles to form Au-Pt bimetallic particles based on the increased particle size. A bimodal particle size distribution of the particles was observed after annealing to 700 K (Figure 2c). Large particles with an average diameter of 5.5 nm and a height of 1.7 nm as well as small particles that are 2.1 nm wide and 0.4 nm high were formed on the surface. These large particles exhibit a hexagonal shape and have a particle size comparable to pure Au particles


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Figure 2. STM images of 0.1 ML of Pt followed by 0.3 ML of Au deposition at 300 K and subsequent annealing to 700 and 800 K on (a-d) CeO1.88 and (e-h) CeO2; (i-l) STM images of 0.3 ML of Pt followed by 0.3 ML of Au deposition on CeO2 at 300 K and after annealing to 700 and 800 K. All images are 100 nm 100 nm.

the fact that the oxidized CeO2 surface has fewer surface defects as nucleation sites for Au compared to CeO1.88. After annealing to 700 and 800 K (Figure 2g,h), the particles aggregated, and a bimodal particle size distribution was also observed as a result of the different sintering behavior of pure Au and Au-Pt. In the study, the coverage of Pt prior to Au deposition was varied to investigate its effect on the growth of bimetallic Au-Pt particles (Figure 2i-l). Figure 2i shows the STM image of 0.3 ML of Pt deposited on the fully oxidized CeO2 thin film. The average size of Pt particles is 1.8 nm in diameter and 0.5 nm in height. The particle density is 6.6 1012/cm2. When 0.3 ML of Au was deposited (Figure 2j), the particle density increased only by 8%. This suggests that most of Au is deposited on Pt instead of the ceria substrate, which is consistent with the observed increase in the particle size.


Upon heating, larger particles were formed with a relatively uniform size distribution. At 700 K, the average size of Au-Pt particles is 3.4 nm in diameter and 0.9 nm in height (Figure 2k). After annealing to 800 K, the particle diameter and height increased to 3.5 and 1.1 nm, respectively, as shown in Figure 2l. Histograms of the particle height and diameter based on the measurement of 100 particles formed at 800 K are shown in Figure 3a, which are compared to those from particles formed upon deposition of 0.3 ML of Au on 0.1 ML of Pt/CeO2 and annealed to 800 K (Figure 3b). For the lower Pt coverage of 0.1 ML, a clear bimodal particle size distribution was observed. Both small particles (0.6 nm in height and 2.6 in diameter) and large particles (2.0 nm in height and 7.2 nm in diameter) were formed, which is the result of the presence of nonuniformity of particles (Au and Au-Pt) on CeO2, as discussed previously. With the increase of

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A Ru(0001) single crystal (Princeton Scientific Corp., one side polishing, roughness < 0.03 μm, orientation accuracy < 0.1°) was used as a substrate for the growth of ceria thin films. Ru(0001) with a diameter of 10 mm and a thickness of 2 mm was held in thermal contact to an Omicron-style tantalum single plate using Ta straps. The Ru crystal was cleaned by Ar ion bombardment with a sample current of ∼3 μA followed by annealing at 1300 K for 45 s via electron bombardment from a tungsten filament. The reported temperatures were based on our own temperature calibrations of the sample heater at various power settings with a thermal couple (type C) spot-welded at the center of the solid Ta plate without the Ru crystal. Fully oxidized CeO2(111) thin films were grown on Ru(0001) by deposition of Ce with a flux of ∼0.25 ML/min for 25 min from a homemade water-cooled electron beam evaporator in 2 10-7 Torr of O2 at 700 K and subsequently annealing of the surface to 1150 K for 2 min. Reduced CeO1.88(111) thin films could be grown by decreasing the oxygen pressure to 8 10-8 Torr during Ce deposition. Au and Pt were evaporated on both ceria thin films at 300 K with a flux of 0.25 ML/min and 0.07 ML/min using homemade evaporation sources, respectively. The metal sources were composed of pure Pt/Au metal wire (Alfa Aesar, Au wire, diameter 0.25 mm, 99.999%; Pt wire, diameter 0.25 mm, 99.997%) wrapped around a tungsten wire (diameter 0.25 mm, 99.95%). After deposition at 300 K, the sample was postannealed by e-beam heating to the desired high temperatures (500, 700, and 800 K) and held for 4 min. The growth and sintering behavior of Au, Pt, and Au-Pt were studied by STM. All of the STM images were recorded using an etched tungsten tip at room temperature in a constant current mode (0.05-0.1 nA, 2-3 V). The reported average particle height and diameter for each experiment in the paper were obtained by measuring the sizes of 100 particles using the line profile mode in the scanning probe imaging software. It is known that the STM tip can influence the size and structure of the metal particles. Therefore, during the STM experiments, special attention was paid to ensure the good quality of the tip. Furthermore, the images shown in this paper were selected only after the repeated scans of the same surface region as well as the evaluation of numerous similar images obtained to minimize any artifact from the tip and STM electronics. The reported sizes of the metal particles in the paper can be overestimated due to the tip convolution effects as well as the different electronics of the ceria thin films and the metal particles, but they are self-consistent.

Figure 3. Histograms of the particle height and diameter distribution based on the measurement of 100 particles at 800 K from the surfaces of (a) 0.3 ML of Pt þ 0.3 ML of Au shown in Figure 2l and (b) 0.1 ML of Pt þ 0.3 ML of Au shown in Figure 2h.

the Pt coverage to 0.3 ML, the deposition of Au mainly causes the formation of bimetallic Au-Pt particles, and thus, a monodispersed particle size distribution would be expected and was observed in the study with the maximum distribution in diameter and height of ∼3.5 and 1.1 nm. The effect of Pt coverage on the Au-Pt growth was also studied on TiO2, the result of which is consistent with our study.15 In conclusion, the growth and sintering behavior of Au-Pt nanoparticles on both oxidized and reduced CeOx(111) thin films was studied and compared to that of pure Au and Pt. For both Pt and Au, the degree of ceria reduction can affect their growth. Smaller particles with a higher particle density are formed on the reduced ceria. Compared to Pt, Au forms larger particles on ceria surfaces at 300 K due to a greater mobility of Au on ceria. Furthermore, it exhibits a preferential nucleation at the step edges of CeO2(111). Upon heating, Au sinters to form large hexagonal particles. The growth of Au-Pt particles prepared by depositing Au on pre-existing Pt particles is dependent on the ceria oxidation state and Pt coverage. With a low coverage of Pt, the formation of pure Au particles on ceria surfaces was observed in addition to the Au-Pt particles. On the reduced ceria, more pure Au particles were formed due to the presence of missing oxygen surface defects as nucleation sites for Au. With the increase of Pt coverage, the deposition of Au produces primarily bimetallic Au-Pt particles which exhibit significantly smaller size than that of pure Au upon heating.

AUTHOR INFORMATION Corresponding Author:

EXPERIMENTAL METHODS

*To whom correspondence should be addressed. Phone: (307) 7664335. E-mail: [email protected].

The experiments were performed in a multitechnique surface analysis system manufactured by Omicron Nanotechnology with a base pressure below 5 10-11 Torr. The system consists of a variable-temperature scanning tunneling microscope (VT STM XA650), an EA 125 U1 hemispherical electron spectrometer, a DAR 400 twin-anode X-ray source, 4-grid SPECTALEED optics, an ISE 5 cold cathode sputtering ion source, as well as a quadrupole mass spectrometer (Hiden HAL/3F PIC).


ACKNOWLEDGMENT We would like to acknowledge the help received from Dr. Arthur P. Baddorf and Dr. John Wendelken at Oak Ridge National Laboratory regarding the design of the watercooled e-beam metal evaporation source. The research is sponsored by University of Wyoming start-up funds as well as the Wyoming NASA EPSCoR grant (NNX07AM19A).

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