Thin Films - American Chemical Society

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Growth and Surface Structure of Ti-Doped CeOx(111) Thin Films Yinghui Zhou and Jing Zhou* Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071

ABSTRACT Well-ordered (111)-oriented ceria thin films with Ti dopant were prepared by simultaneous introduction of Ce and Ti onto Ru(0001) at 700 K with oxygen pressures higher than 2 10-7 Torr. XPS data indicate that the incorporation of Ti in ceria causes the partial reduction of Ce from the þ4 to þ3 state. Ti is formally in the þ3 state. The films are of high quality, consisting of flat terraces with surface features of ceria lattices, oxygen vacancies, as well as Ti dopants. Compared to pure ceria, Ti-doped ceria displays a large number of domain boundaries, which could be the result of the strain within the film due to the different Ce-O and Ti-O bond lengths. It is demonstrated that the modified structures of ceria by Ti dopant can significantly prevent the sintering of supported Au nanoparticles upon heating to higher temperatures. SECTION Surfaces, Interfaces, Catalysis

C

eria supports, successfully used as additives in threeway automobile emission-control catalysis, have attracted great attention in recent years. Many research groups have reported that ceria-supported metal particles can exhibit promising reactivity in many catalytic processes, including steam re-forming of hydrocarbons, the water-gas shift reaction, as well as the CO oxidation reaction.1-4 It is demonstrated in the literature that ceria plays a major role in the chemistry of supported metal nanoparticles.5-8 The unique redox properties and oxygen storage capacity of ceria supports, which is manifested by the readily reversible transformation from the Ce4þ state to the Ce3þ oxidation state and the formation of oxygen vacancies on the surface, can result in the unique reactivity of supported metal catalysts.9-15 One main issue regarding the use of pure ceria as real-world catalytic supports is its poor thermal stability at high temperatures.16,17 It can undergo sintering, which causes the loss of its crucial oxygen storage capacity. Doped ceria can potentially provide a better catalytic support for metal catalysts for practical applications compared to pure ceria. The addition of different metal dopants, such as Ti and Zr, can result in structural and electronic modifications of ceria, which not only can enhance the thermal stability of ceria but also improve its redox properties and oxygen storage capacity.18-23 The enhanced redox properties of doped ceria by metal dopants could lead to superior stability, activity, and selectivity of supported metal nanocatalysts. Evidence in the literature has shown that doped ceria can promote the reactivity of metal catalysts.24-28 To achieve a rational design of doped ceria-supported metal catalysts as new catalytic materials, it is of fundamental and significant importance to address the critical role of the dopant on the properties of ceria at the atomic/nanoscale. While computational studies have provided crucial insights

r 2010 American Chemical Society

into the doped ceria systems, detailed surface science studies remain limited in the current literature.21,29-36 The fundamental question regarding the doping mechanism still remains. To our best knowledge, little is known regarding the detailed atomic structures of the doped ceria surfaces. In this paper, we report the successful growth of well-ordered Tidoped CeOx(111) (1.5 < x < 2) surfaces. Utilizing this model doped ceria surfaces, as well as a combination of X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM) techniques under ultrahigh vacuum (UHV) conditions allows for the study of the effect of Ti doping on the morphology, structure, and electronic properties of ceria in detail. In our previous study, we have demonstrated that wellordered CeOx(111) thin films can be prepared by evaporating Ce on Ru(0001) at 700 K in the presence of oxygen gas followed by annealing at 1150 K for 2 min.37 We followed the growth recipe from the previous work in the literature.38-40 The fully oxidized CeO2 was grown with an oxygen pressure of 2 10-7 Torr. The reduced ceria could be obtained by decreasing the oxygen pressure. For example, oxygen pressures of 8 10-8 and 2 10-8 Torr were used to obtain CeO1.88 and CeO1.76 thin films. Ti-doped ceria thin films were grown by simultaneously introducing Ce and Ti onto the Ru(0001) during the film grown. As the first step in the study of Ti-doped ceria, we focus on the effect of Ti-induced modification of surface structures of ceria before the addition of other complexity such as oxygen vacancies formed due to the insufficient oxygen gas during the film growth. Therefore, oxygen pressures for the growth of CeO2 and higher were Received Date: April 2, 2010 Accepted Date: May 5, 2010 Published on Web Date: May 17, 2010

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DOI: 10.1021/jz1004297 |J. Phys. Chem. Lett. 2010, 1, 1714–1720

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Figure 1. (a) Ti 2P and (b) Ce 3d XPS spectra from three types of Ti-doped ceria thin films (I: Ce0.91Ti0.09O1.89; II: Ce0.92Ti0.08O1.90; and III: Ce0.80Ti0.20O1.76) grown on Ru(0001). The Ti 2p region overlaps with the Ru 3p3/2 peak, which can be removed from the spectrum by the subtraction of a Ru 3p3/2 peak collected from a fully oxidized CeO2 film grown on Ru with a similar film thickness. The red dotted line indicates the fitted spin-orbit doublets of Ti 2p3/2 and 2p1/2 peaks. Table 1. Peak Position, Full Width at Half Maixumum (fwhm), and Area Ratio of Fitted Ti 2p3/2 and 2p1/2 Peaks from Three Types of TiDoped Ceria Thin Films as Well as a Pure TiO2(110) Surface along with the Percentages of Ce4þ Cations and the Ti Metal Composition in the Doped Ceria Ti 2p Ti 2p3/2

fwhm

Ti 2p1/2

fwhm

2p3/2/2p1/2 ratio

Ce4þ/Ce

Ti%

type I (Ce0.91Ti0.09O1.89)

458.0

1.9

463.7

2.6

2.2

85.6%

8.7%

type II (Ce0.92Ti0.08O1.90)

458.0

1.8

463.6

2.5

2.3

87.3%

7.7%

type III (Ce0.80Ti0.20O1.76)

458.1

1.9

463.8

2.5

2.3

65.4%

20.0%

pure TiO2(110)

459.0

1.9

464.7

2.4

2.2

Ti-doped ceria

2p3/2 and 2p1/2 positions (459.0 and 464.7 eV) from a bulk stoichiometric TiO2(110) sample (CrysTec GmbH, 10 mm 10 mm 1 mm, one side polished, tolerance < 0.1 degree), the binding energy of Ti 2p peak from the Ti-doped ceria shifts to the lower energy by 0.9-1.0 eV. This core-level binding energy shift of the Ti 2p peak can be attributed to the reduction of Ti cations from the þ4 state to the þ3 state. The assignment of Ti ions in the ceria films to Ti3þ cations based on the 2p XPS binding energy is consistent with studies of pure titanium oxides.41 Furthermore, literature studies of titania-ceria mixed oxides prepared by solution-based methods reported the 2p3/2 XPS binding energy for Ti4þ cations in the range of 458.7-459.1 eVand for Ti3þ cations with a value of 457.9 eV.42,43 The core-level binding energy shift of the Ti 2p peak could also be associated with tetravalent Ti ions in the ceria films due to the fact that they exhibit a different structure unit than those in a bulk TiO2 sample. Such a peak shift was observed for Ti4þ cations in ultrathin TiOx films grown on Mo(112) due to the formation of a Ti4þ-O-Mo4þ linkage.44 Doping Ti into ceria forms Ti-O-Ce structures that could result in the observed shift in the Ti 2p3/2 binding energy position. However, our study of Ti deposition on the grown pure CeOx(111) surfaces, which can also form Ti-O-Ce structures, shows a higher Ti 2p3/2 XPS binding energy of 458.5 eV under a similar Ti composition as that in the doped ceria films. Therefore, the Ti ions in ceria are more consistent

used. Three types of Ti-doped ceria films were prepared and are shown in the paper. The type I film was grown with Ce and Ti fluxes of 0.2 and 0.02 ML/min, respectively, under an oxygen pressure of 3 10-7 Torr for 25 min. The oxygen pressure was increased to 5 10-7 Torr for the growth of the type II film while keeping other conditions the same. Compared to the type II, the type III film was obtained with a reduced Ce flux of 0.1 ML/min and a longer growth period of 50 min to increase the Ti composition in the film. The prepared ceria thin films have the same film thickness, which is ∼6 equivalent layers measured by scanning tunneling microscopy. One equivalent layer is defined as O-Ce-O trilayers normal to the (111) plane. Ti 2p and Ce 3d XPS spectra were taken from these three ceria films (type I, type II, and type III) to examine the oxidation states of Ti as well as Ce (Figure 1). As shown, the Ti 2p XPS region overlaps with the Ru 3p3/2 peak. The contribution of Ru was removed from the Ti 2p spectrum by the subtraction of a Ru 3p3/2 peak collected from a fully oxidized CeO2 film grown on Ru(0001) with a similar film thickness. The remaining signal is from the Ti 2p and can be fitted with two GaussianLorentzian peaks. The analyzed data of these two fitted peaks for all three ceria films are tabulated in Table 1. On the basis of the measured area ratio of these two peaks (2.2-2.3) and their split (∼5.7 eV), they can be attributed to spin-orbit split doublets of Ti 2p (2p3/2 and 2p1/2). Compared to the Ti


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Figure 2. (a) Large-scale (100 nm 100 nm) and (b) high-resolution (30 nm 30 nm) STM images of Ce0.92Ti0.08O1.90(111). (c) STM image of a pure reduced CeO1.88(111) surface. The image size is 100 nm 100 nm. The inset in (a) shows the LEED pattern of the Ti-doped ceria surface. The insets in (b) and (c) are the atomically resolved STM images of the surfaces. The size for the insets is 3 nm 3 nm. Line profiles taken from the surfaces as indicated are shown below the images.

with the assignment to Ti3þ cations. Ti metal compositions in the type I, type II, and type III films are determined to be 9, 8, and 20% using the equation Ti(%) = (ITi/STi)/[(ITi/STi) þ (ICe/SCe)]. Here, I refers to the XPS peak intensity obtained from the integration of the XPS peak after the subtraction of a Shirley background. S is the atomic sensitivity factor, which is 1.8 for Ti 2p and 10 for Ce 3d.45 Cerium has two oxidation states (Ce4þ and Ce3þ). The satellite peaks (u000 , u00 , u, v000, v00, v) are associated with the tetravalent Ce cations. The presence of Ce3þ cations in the films results in four additional peaks indicated as u0 , u0, v0, and v0. The labels u and v refer to the 3d3/2 and 3d5/2 spin-orbit components, respectively. The spin-orbit splitting is 18.3 eV, which is consistent with the reported reference data.46 By fitting the Ce 3d XPS spectra based on the reference spectra from stoichiometric ceria films (CeO2 and CeO1.5) taken with our XPS system, percentages of Ce4þ cations are calculated to be 87 in type I and II films and 65 in type III films. According to the global charge equilibrium, the stoichiometry of the Ti-doped ceria can be expressed with the formula of Ce1-yTiyO2-x. The x and y correspond to the loss of the oxygen from the fully oxidized surface as well as the Ti metal composition, respectively. On the basis of our XPS data analysis, the films (type I-III) can be expressed as Ce0.91Ti0.09O1.89, Ce00.92Ti0.08O1.90, and Ce0.80Ti0.20O1.76, respectively. Our results demonstrate that reduction of Ce4þ is facilitated with the doping of Ti in the film. Increasing the oxygen pressure from 2 10-7 Torr for the growth of pure CeO2 thin films to 5 10-7 Torr does not cause any significant change in the concentration of Ce4þ ions in the doped ceria films. However, with the increase of Ti composition in the film, the degree of Ce reduction increases.


Doping ceria with Ti maintains the ceria lattice as the p(1.4 1.4) spots of the CeO2(111) structure are still observed in the LEED pattern (inset, Figure 2a). A 12 spot LEED pattern (not shown) is also observed for some of the doped ceria surfaces, which is the result of ceria structures grown on the Ru substrate with a 30° difference in the domain orientation. Such a LEED pattern was also observed on pure ceria surfaces from our studies and the literature.39 STM images of the type II film are shown in Figure 2. The film consists of flat terraces that are 5-60 nm wide. As indicated in the line profile (Figure 2a), the height between adjacent terraces is about 0.3 nm, consistent with the height of a O-Ce-O triple layer in the CeO2(111) fluorite bulk structure.47 Compared to pure ceria surfaces, the terraces of Ce0.92Ti0.08O1.90(111) consist of domains which are 5-20 nm wide, as shown in the highmagnification STM image (Figure 2b). The depth of the domain boundaries measured from the STM image ranges from 0.05 to 0.25 nm. The structure of the type III film is similar to that of the type II, except that the number of domain boundaries in the type III film increases with the increase of Ti composition. The formation of the domain boundaries can be attributed to the lattice mismatch between the film and the Ru substrate as well as the strain within the film. Domain boundary structures were also observed on pure ceria films grown on Ru(0001) in the literature as result of the VolmerWeber growth of ceria.39 Our study of the growth of pure ceria thin films on Ru also indicates the presence of domain boundaries. Figure 2c shows the STM image of a CeO1.88 surface with domain boundaries indicated by arrows. However, the density of the domain boundaries in the Ti-doped ceria thin film is much higher than that on the pure ceria,

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Figure 3. STM images of 0.3-0.5 ML Au deposited on (a) oxidized CeO2, (b) reduced CeO1.88, and (c) Ti-doped ceria thin films at 300 K followed by heating to 800 K. Image sizes are 100 nm 100 nm. (d) Plots of mean height of Au particles deposited on these three ceria surfaces as a function of annealing temperatures. (e) Line profile measurements of representative Au particles formed on surfaces (a)-(c).

which could be due to the strain within the doped ceria film as a result of the different bond lengths between Ce-O and Ti-O.36,48 The formation of the domain boundaries can reduce the strain within the film. Despite the presence of a large number of domains, the grown doped ceria film is very smooth, with a surface roughness between 0.1 and 0.2 nm measured by STM. Besides domain boundaries, the Ti-doped ceria film (Figure 2b) displays surface features associated with lattice Ce, oxygen vacancies, as well as Ti dopants. Defect-free wellordered atomic structures of ceria were observed in the Tidoped ceria film. The measured distance between two adjacent bright atoms in the image is about 0.4 nm, which is consistent with the distance between the Ce atoms on the surface.39,49-52 However, there are surface defects present on the doped ceria surface. Depressions with different depths as indicated by the line profile i and ii are related to oxygen vacancies on the surface.39,51 Such features were also observed on the reduced ceria in our study (inset, Figure 2c). Compared to the pure ceria surfaces, there are new features on the doped ceria surface shown as a triangular depression with a bright protrusion in the center. The triangular depression is about 0.04 nm deep with a side length of 0.8 nm. The bright protrusion is 0.4 nm wide. It has the same apparent height but not the same registry compared to the lattice Ce cations as indicated by the profile curve iii in Figure 2b. These features are uniformly distributed on the surface, the number of which correlates with the amount of Ti dopants in the film. Furthermore, they were not observed on the pure ceria


surfaces and therefore can be attributed to Ti doping. The addition of Ti metal dopant into ceria ideally replaces the Ce cation lattice sites and forms a solid solution of a mixed oxide (Ce1-yTiyO2; 0 < y < 1). However, due to the size difference between the Ce and the metal dopant as well as nonequivalent metal-dopant bond distances,34,36,42,48,53 doping ceria can result in a change in its unit cell size and a perturbation of its atomic structures around the dopant. Furthermore, recent experimental and theoretical studies have also shown that the doping of ceria with metals, such as Au, Ti, and Zr, would facilitate the formation of oxygen vacancies around the dopant and thus significantly modify its atomic structure around the dopant,33,35,36 which could explain the observed complicated structures of ceria upon doping Ti in our study. The protrusions within the triangular depressions could be the doped Ti cations, of which the dark features surrounding can be due to the formation of oxygen vacancies. However, it should be pointed out that further experimental and computational investigation is needed to elucidate the exact nature of these structures. It is well-known in the literature that nanostructures of ceria can affect the dispersion and morphological and electronic properties of supported metal nanoparticles and thus influence their catalytic reactivity.40,54-56 In the study, we investigated the effect of modified structures of ceria by Ti dopant on the growth and sintering behavior of Au particles (Figure 3). As a comparison, the results of the Au growth on pure ceria surfaces were also shown. Both oxidized CeO2(111) and reduced CeO1.88(111) thin films were employed as

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EXPERIMENTAL METHODS

supports to differentiate the role of oxygen vacancies in ceria in the structure of the deposited Au nanoparticles. The 0.3-0.5 ML Au was deposited onto these two pure ceria surfaces as well as Ti-doped ceria at 300 K from a homemade evaporation source. One monolayer (ML) of Au is calculated to be 1.4 1015 atoms/cm2 with respect to the atomic density of the Au(111) surface. On the oxidized CeO2, Au particles that were 2.5 nm wide and 0.7 nm high were formed at 300 K. The particle density was 4.1 1012 cm-2. Smaller Au particles (2.0 nm in diameter and 0.6 nm in height) with a higher particle density (6.5 1012 cm-2) were produced on the reduced CeO1.88 thin film. When Au was deposited on the Ti-doped ceria surface at 300 K, it was found that the Au particles had a similar size compared to those on the reduced CeO1.88. Significant difference in the size and structure of the Au particles deposited on the pure and doped ceria was observed after annealing to high temperatures, in particular above 700 K. Au sinters significantly to form larger particles on either oxidized CeO2(111) or reduced CeO1.88(111). However, the aggregation of Au particles is dramatically suppressed on the doped ceria. At 800 K, the mean height of Au particles on the Ti-doped ceria is less than half of that on the pure ceria. Further increasing of the Au coverage to 1 ML on the doped ceria surface does not significantly change the Au particle size. Overcoming the sintering problem is a major issue related with Au catalysis. Our data suggest that modified surface structures of ceria as well as the increased number of surface defects by the incorporation of Ti dopant can act as the nucleation sites for the gold particles and hinder their diffusion on the surface to form larger particles upon heating to higher temperatures, which would be beneficial to promote their catalytic activity. The effect of structures of ceria on the growth of Au is consistent with previous studies of Au on pure ceria as well as titania.13,15,37,55,57,58 In summary, we have demonstrated that well-ordered CeOx(111) thin films with Ti dopant can be obtained by incorporating Ti during the ceria film growth. The grown film is stable under the UHV conditions as well as upon heating. The fraction of Ti substitution for Ce cations in the film is below 20%. XPS results indicate that doping the ceria film with Ti under the growth conditions for the CeO2(111) thin film can cause a significant reduction of Ce cations from the þ4 state to the þ3 state. Cerium displays both þ4 and þ3 states. The incorporated Ti atoms in the ceria film exhibit a þ3 oxidation state. The LEED pattern shows that the Ti-doped ceria surface preserves the lattice structure of CeO2(111). However, compared to the structure of pure ceria surfaces, a relatively large amount of domain boundaries is formed. Different atomic features associated with pure ceria lattices, oxygen vacancies, as well as Ti dopants are present on the surface. It is demonstrated that the modified structures of ceria by Ti dopant can significantly prevent the sintering of Au nanoparticles. The ability to prepare and characterize the doped ceria films at the nanoscale provides an exciting approach to correlate the redox properties and oxygen storage capacity of ceria associated with metal dopants with the stability and reactivity of ceria-supported metal particles. Such knowledge will be crucial for their rational design as future promising catalytic systems.


All 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). A Ru(0001) single crystal (Princeton Scientific Corp., diameter 10 mm, one side polishing, roughness < 0.03 μm, orientation accuracy < 0.1 degree) was used as a substrate for the growth of ceria thin films as well as Ti-doped ceria films. The Ru surface was cleaned by Ar ion sputtering (1 keV, ∼3 μA sample current) followed by annealing to 1300 K for 45 s. The reported temperatures were based on our own temperature calibrations of the sample heater at various power settings with a thermocouple (type C) spot-welded at the center of the solid Ta plate without the Ru crystal. The sputtering and annealing cycle was repeated until a clean and well-ordered Ru(0001) surface was obtained, which was confirmed by XPS, LEED, and STM. Pure ceria thin films were prepared by deposition of Ce onto Ru at 700 K from a homemade water-cooling e-beam evaporator in the presence of oxygen and subsequent annealing of the surface to 1150 K for 2 min. The degree of ceria reduction was controlled by varying the oxygen pressure during the film growth. Ti-doped CeOx(111) was prepared by codeposition of Ce and Ti onto Ru at 700 K with an oxygen pressure range between 2 10-7 and 5 10-7 Torr followed by annealing at 1150 K. The Ti source was composed of a pure Ti metal wire (Alfa Aesar, diameter 0.25 mm, 99.99%) wrapped around a tantalum wire (Alfa Aesar, diameter 0.25 mm, 99.9þ%). Au was deposited on all ceria surfaces at 300 K from a homemade evaporation source. The Au source was constructed by wrapping a Au wire (Alfa Aesar, diameter 0.25 mm, 99.999%) around a 0.25 mm tungsten wire. After Au deposition at 300 K, the sample was annealed by e-beam heating to desired high temperatures of 500, 700, and 800 K for 4 min. XPS experiments were carried out using an Mg KR anode (1253.6 eV, 15 kV, 20 mA) with a fixed electron passing energy of 50 eV and an entrance slit size of 6 12 mm2. The spectra were collected with a 0.020 eV step and averaged over two scans. The XPS spectrometer was calibrated by setting the 4f7/2 binding energy of a bulk gold foil (Alfa Aesar, 0.1 mm thick, 99.9975þ%) at 84.0 eV. LEED patterns were taken with a beam energy of 60 eV. 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, 1-3 V). The Au coverage was calculated from the particle size and density observed in the STM images. The diameter and height of Au particles were measured from STM line profiles, and the particle density was obtained by counting the number of particles in three STM images with a size of 120 120 nm2. During the STM experiments, special attention was paid to ensure the good quality of the tip. The reported Au coverage in

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the paper might be overestimated due to the tip convolution effects, but they are self-consistent.

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AUTHOR INFORMATION

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

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ACKNOWLEDGMENT The research is sponsored by University of Wyoming start-up funds, a Wyoming NASA EPSCoR grant (NNX07AM19A), as well as a Basic Research grant.

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