Growth and Characterization of Barium Oxide Nanoclusters on YSZ(111)

Jul 20, 2009 - In situ reflection high-energy electron diffraction, ex situ X-ray diffraction (XRD), atomic force microscopy, and X-ray photoelectron ...
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Growth and Characterization of Barium Oxide Nanoclusters on YSZ(111) P. Nachimuthu,*,† Y. J. Kim,‡ S. V. N. T. Kuchibhatla,† Z. Q. Yu,†,§ W. Jiang,† M. H. Engelhard,† V. Shutthanandan,† Ja´nos Szanyi,† and S. Thevuthasan† EMSL, Pacific Northwest National Laboratory, Richland, Washington 99352, Department of Chemical Technology, Hanbat National UniVersity, Taejon, Korea, and Department of Chemistry, Nanjing Normal UniVersity, Nanjing, China ReceiVed: March 4, 2009; ReVised Manuscript ReceiVed: June 13, 2009

Barium oxide (BaO) was grown on a yttria-stabilized zirconia (YSZ) substrate by oxygen plasma-assisted molecular beam epitaxy. In situ reflection high-energy electron diffraction, ex situ X-ray diffraction (XRD), atomic force microscopy, and X-ray photoelectron spectroscopy (XPS) have confirmed that BaO grows as clusters on YSZ(111). During and following growth under ultrahigh vacuum conditions, we found BaO remained in single phase. When exposed to ambient conditions, the clusters transformed to BaCO3 and/or Ba(OH)2 H2O. However, in a few attempts of BaO growth, XRD results show a fairly single-phase cubic BaO with a lattice constant of 0.5418(1) nm. XPS results show that exposing BaO clusters to ambient conditions resulted in the formation of BaCO3 on the surface and partly Ba(OH)2 throughout the bulk. On the basis of the observations, it is concluded that the BaO nanoclusters grown on YSZ(111) are highly reactive in ambient conditions. Introduction NOx storage and reduction (NSR) catalysts are being developed in order to reduce the NOx emission from gasoline-based internal combustion engines that use “lean-burn” technology.1-3 The engines operate at a higher air-to-fuel ratio for a better fuel efficiency in lean-burn technology; however, the reduction of NOx generated during combustion is a challenge. NSR catalysts are designed to function between lean and rich modes in a cyclic fashion, unlike the conventional three-way catalysts that are efficient only when the engine operates at a stoichiometric airto-fuel ratio.4 Engine operation in a lean mode does not favor NOx reduction; hence, the NOx is temporarily stored in a catalytic converter. When the engine switches to a fuel-rich mode, the stored NOx is reduced to N2 and released into air. NSR catalysts consist of alkaline earth metal oxides as active NOx storage and precious metals as the main redox components. Among the many metal oxides considered for NOx storage, BaO is shown to store NOx more effectively.5 There is a general understanding that the oxidation of NO to NO2 over precious metal sites occurs prior to NOx storage on BaO.1-5 Furthermore, efficient removal of NOx from exhaust depends on effective storage of NO2 on BaO. Among all of the steps involved in the NSR catalytic processes, NOx storage on BaO has attracted a great deal of attention in terms of experimental and theoretical studies.1-3,6-19 Therefore, it is imperative to understand the stability and reactivity of BaO prior to considering it as the main NOx storage component in NSR catalysts. Recent experimental studies by Cheol-Woo Yi and Ja´nos Szanyi8-10 on the thickness-dependent reactivity of BaO on an Al2O3/NiAl(110) support show, that at submonolayer thickness, nitrite and nitrate formation occur at low and elevated exposures of NO2, respectively. In the case of thick BaO, a nitrite-nitrate ion pair and amorphous nitrate formation occur at low and * Corresponding author. E-mail: [email protected]. † Pacific Northwest National Laboratory. ‡ Hanbat National University. § Nanjing Normal University.

elevated exposures of NO2, respectively, and this amorphous nitrate eventually crystallizes over 500 K. Similarly, Lei Cheng and Qingfeng Ge11,12 have studied the interaction between NO2 and BaO in different morphologies with and without Al2O3 support by first-principles density functional theory calculations. This study shows that the nonstoichiometric clusters and defective surfaces with either O or Ba vacancies exhibit much stronger affinities toward NO2, up to 4.09 eV for NO2 adsorption at the edge oxygen site of Ba14O13. Furthermore, the presence of the low-coordinated sites on the small BaO clusters and at the edge of the stepped BaO(310) surface strengthens the binding of NO2 at these sites and creates new NO2 binding modes compared to a flat BaO(100) surface. These reports, in addition to a number of other experimental and theoretical studies in this area, suggest that NO2 can be effectively trapped as a result of a strong interaction between NO2 and BaO, when BaO is present as nonstoichiometric BaO clusters rather than bulk BaO.8-10,13-19 In most of these studies, Al2O3 has been used as a support. However, the fundamental scientific issues associated with BaO on other support oxides are not well understood.1-19 Furthermore, growing BaO on cerium oxide (CeO2) and yttria-stabilized zirconia (YSZ) has advantages. Functional oxides such as CeO2 and YSZ have strong abilities in oxygen storage and release. These properties may significantly influence the reactivity of BaO and provide new avenues in developing NSR catalysts. Therefore, the present study is an attempt to grow and characterize BaO nanoclusters on YSZ. Experimental Details Growth of BaO on YSZ(111) by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) and its in situ characterization were carried out in a dual chamber ultrahigh vacuum (UHV) system described in detail elsewhere.20-23 The UHV chamber consists of metal evaporation sources, a UHV compatible electron cyclotron resonance (ECR) oxygen plasma source, and reflection high-energy electron diffraction (RHEED) for real-time characterization of thin film growth. High purity

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Figure 1. (a) RHEED pattern and (b) ex situ AFM image from BaO clusters grown on YSZ(111) at 600 °C with ∼1.0 × 10-5 Torr of O2 plasma.

barium (Ba) metal was used as the source material in an effusion cell. Growth rate of the film was monitored using quartz crystal oscillators (QCO). YSZ(111) substrates were ultrasonically cleaned in acetone for ∼10 min prior to loading in the UHV system. The substrate in the chamber was then cleaned by annealing at 650 °C in oxygen plasma operating at 200 W under ∼2 × 10-5 Torr of O2. Film growth was monitored using RHEED with a 15 kV e- beam at an incidence angle of ∼3-5°. A predetermined quantity of Ba metal flux was directed at the substrate in activated oxygen plasma. The substrate temperature, Ba deposition rate, and oxygen partial pressure were systematically varied to establish optimum growth conditions for high-quality BaO clusters. Glancing incidence X-ray diffraction (GIXRD) measurements were performed using a Philips X’pert multipurpose diffractometer operating at 45 kV and 40 mA with a fixed Cu anode. In situ high-temperature XRD measurements were carried out using a HTK 1200 cell from Anton Parr. The analysis of diffraction data was carried out using JADE 8.5 from Materials Data, Inc. and a PDF4+ database from ICSD. Ex situ atomic force microscopic (AFM) studies were carried out using a Digital Instrument (DI) nanoscope IIIa multimode scanning probe microscope under tapping mode. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Phi 5000 VersaProbe. This system consists of a monochromatic focused Al KR X-ray (1486.7 eV) source and a hemispherical analyzer. The X-ray beam was incident normal to the sample, and the emitted photoelectrons were collected at an emission angle of 45° relative to the sample normal. Wide scan data were collected using pass energy of 117.4 eV. High-resolution scans were obtained using pass energy of 46.95 eV. The XPS spectra were referenced to an energy scale with binding energies for Cu 2p3/2 at 932.67 ( 0.05 eV and Au 4f at 84.0 ( 0.05 eV. Low-energy electrons at ∼1 eV and 40 µA and low-energy Ar+ ions were used to minimize surface charging. The XPS depth profile data were collected using 2 kV Ar+ sputtering over a 3 mm × 3 mm area of the specimen. The sputter rate for 2 kV Ar+ over a 3 mm × 3 mm raster area is determined to be ∼6.5 nm/min, using SiO2/Si reference material with known thickness from X-ray reflectivity and ellipsometry.

Results and Discussion The RHEED pattern from YSZ(111) following the surface cleaning showed a streak pattern, indicating the high-quality, well-ordered single-crystal substrate, with terrace widths smaller than the electron coherence length. When the BaO growth started, the streak patterns from the substrate slowly disappeared, and the diffraction spots corresponding to BaO started appearing and remained until the end of the growth. The RHEED pattern from BaO grown on YSZ(111) at 600 °C with ∼1.0 × 10-5 Torr of O2 plasma is shown in Figure 1a. These diffraction spots indicate a three-dimensional (3D) island or cluster growth of BaO on YSZ(111).24,25 Many attempts have been made to grow single phase BaO clusters on YSZ(111). During and following the growth under UHV, we found the BaO clusters remained in single phase as shown by the in situ RHEED pattern. Previous studies showed that BaO is in a highly unstable phase under ambient conditions.26-28 This is indeed found to be the case for most attempts of the BaO growth in our experiments as well. When BaO clusters are exposed to ambient conditions, the clusters transformed to BaCO3 and/or Ba(OH)2 H2O as determined by GIXRD measurements. However, in a few attempts of BaO growth, the XRD measurements carried out on these clusters under ambient conditions within 30 min following the removal of the films from the UHV chamber showed fairly single-phase BaO. One of these GIXRD patterns from BaO clusters grown on YSZ(111) at 600 °C with ∼1.0 × 10-5 Torr of O2 plasma is shown in Figure 2. The major diffraction patterns are from cubic BaO. However, weak diffraction features can be noticed, which are from barium hydroxide hydrate, Ba(OH)2H2O (PDF 0260154). The lattice constant determined from the XRD pattern of BaO clusters is 0.5418(1) nm. This value is ∼2% lower than the lattice constant of 0.55 nm reported previously for epitaxial cubic BaO(001) on SrTiO3(001) with a SrTiO3 capped layer.26-28 A decrease in the lattice constant is probably from the formation of BaO clusters, which are expected to be fully relaxed, unlike the previous study where BaO forms an epitaxial layer over a SrTiO3 single-crystal substrate in which there could be strain because of substrate influence.26-28 The average crystallite size determined from full-width at half-maximum (fwhm) of diffraction patterns using the Scherrer equation is found to be ∼25 nm. Exposing the single-phase BaO clusters in air resulted in the formation of Ba(OH)2 H2O. Temperature-dependent X-ray

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Figure 2. Glancing incidence X-ray diffraction pattern from BaO clusters grown on YSZ(111) at 600 °C with ∼1.0 × 10-5 Torr of O2 plasma. The entire diffraction pattern from cubic BaO is marked with the corresponding (hkl). Patterns marked with * are from Ba(OH)2 H2O. Background is removed from the diffraction pattern.

diffraction measurements under rough vacuum (3.0 × 10-3 Torr) showed that Ba(OH)2 H2O formed from BaO clusters at room temperature converted to BaCO3 at ∼200 °C followed by decomposition of BaCO3 at ∼500 °C. It is worth mentioning that following decomposition of BaCO3, BaF2 formation occurred with residual fluorine from the high-temperature stage. Further heating to 700 °C resulted in sublimation of BaF2 from the YSZ(111) substrate. These observations also prove the high affinity of BaO to react with various constituents in the surroundings and form respective compounds and the ease with which their thermal decomposition occurs. No other crystalline phases were found by the XRD measurements on a number of films made during the course of this work within the experimental detection limits. It is possible that some amorphous intermediate phases may have formed during the annealing process that could not be identified by XRD. The hightemperature stage used for these experiments has been used for a variety of powder and thin film samples; however, no fluoride formation was observed except for BaO clusters. These results suggest that a highly reactive amorphous phase is forming following BaCO3 decomposition. This highly reactive amorphous phase is most likely to be BaO, and it reacts with residual fluorine from the high-temperature stage and forms BaF2. However, no direct evidence exists to support the formation of amorphous BaO. Following room-temperature XRD measurements, where we found fairly single-phase BaO, the AFM image was taken on these BaO clusters as shown in Figure 1b. The average roughness and height of these clusters are found to be ∼1.7 and ∼7 nm, respectively. About 50 individual clusters were examined for their fwhm, and the average fwhm is found to be ∼30 nm. This value is comparable to the crystallite size of ∼25 nm obtained from XRD patterns. However, it should be noted that the BaO cluster surface is no longer pristine because this film was left in air for more than 30 min. AFM measurements were not carried out on BaO clusters that transformed to BaCO3 and/or Ba(OH)2 H2O following exposure to ambient conditions as determined by XRD. The XPS depth profile from BaO clusters grown on YSZ(111) at 600 °C with ∼1.0 × 10-5 Torr of O2 plasma and the corresponding Ba 3d5/2, O 1s, and C 1s XPS spectra collected during the depth profile from the top 20 nm of BaO clusters are shown in Figure 3. Following deposition, BaO nanoclusters on the YSZ(111) substrate was transported quickly (∼10 min)

Nachimuthu et al. in a closed container with minimum air exposure to the VersaProbe chamber for XPS measurements. The Ba 3d5/2, O 1s, and C 1s XPS spectra shown in Figure 3 correspond to the surface of the BaO clusters to the bulk for the first 20 nm. The surface XPS spectra were charge corrected using the C 1s from adventitious carbon at 284.8 eV, whereas the bulk XPS spectra were shifted assuming that the O 1s from BaO is at 528.3 eV.29-32 The Ba 3d5/2, O 1s, and C 1s XPS from the surface of the clusters occur at the binding energies of 779.6 eV, 530.9, and 289.1 eV, respectively. These binding energies are in good agreement with the values reported for BaCO3.29-32 Elemental quantification from Ba 3d5/2, O 1s, and C 1s XPS of a cluster surface is found to be 20 atom % Ba, 60 atom % O and 20 atom % C, respectively (the atom % values were rounded up to nearest integer). However, one-fourth of the carbon detected on the BaO cluster surface is from adventitious carbon, suggesting that the cluster surface is not necessarily stoichiometric BaCO3. Sputtering with Ar+ for 1 min (∼6.5 nm) removed all of the adventitious carbon and most carbonates from the cluster surface. However, an additional peak started appearing at ∼528.2 eV in the O 1s XPS. Subsequent sputtering cycles (depth of ∼20 nm) resulted in removal of all carbonates. The Ba to O ratio remained constant throughout the bulk. Ba 3d5/2 XPS shows a peak at 779.3 eV, and a shoulder at 780.5 eV. O 1s XPS shows two prominent peaks at 528.3 and 531.3 eV with an energy difference of 3 eV. Elemental quantification from Ba 3d5/2 and O 1s XPS of the bulk film is found to be 42.5 atom % Ba and 57.5 atom % O, respectively. The relative area of the O 1s XPS peaks at 528.3 and 531.3 eV are found to be 58.3% and 41.7%, respectively. Comparing these results with previous reports, we assigned the O 1s XPS peaks at 528.3 and 531.3 eV to oxygen from BaO and Ba(OH)2, respectively.8,9,29-32 Assuming that the film contains a stoichiometric mixture of 58.3% BaO and 41.7% Ba(OH)2 based on the relative area in the O 1s XPS peaks at 528.3 and 531.3 eV, we find the film should have 44.2 atom % Ba and 55.8 atom % O. However, elemental quantification from Ba 3d5/2 and O 1s XPS show the film contains 42.5 atom % Ba and 57.5 atom % O, suggesting that the bulk of the film contains excess oxygen. This excess oxygen and the shoulder at 780.5 eV in Ba 3d5/2 XPS are probably associated with nonstoichiometric Ba(OH)2 H2O, which is in agreement with XRD measurements. In order to further confirm the feature at 780.5 eV in Ba 3d5/2 XPS and excess oxygen in the bulk of the film, a XPS depth profile was repeated on a clean spot on the surface of the film left in the UHV chamber with a base pressure of 3 × 10-10 Torr for 12 h. The XPS results show that all of the adventitious carbon was desorbed from the surface, and carbon from BaCO3 is significantly reduced. Elemental quantification of Ba 3d5/2, O 1s, and C 1s XPS from the cluster surface shows 28.3 atom % Ba, 65.1 atom % O, and 6.1 atom % C, respectively. Following a few sputter cycles, the Ba to O ratio and relative area of O 1s XPS peaks at 528.3 and 531.3 eV remained constant throughout the bulk of the film similar to the previous depth profile. The Ba 3d5/2 and O 1s XPS from the bulk of the film (depth of ∼30 nm), where the BaO to Ba(OH)2 ratio is constant, for the first depth profile and the corresponding XPS from the depth profile collected following 12 h in vacuum are compared in Figure 4. The shoulder at 780.5 eV in Ba 3d5/2 XPS was not detected. The elemental quantification from Ba 3d5/2 and O 1s XPS of the bulk film from the depth profile following 12 h in vacuum are found to be 44.5 atom % Ba and 55.5 atom % O, respectively. The relative area of the O 1s XPS

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Figure 3. (a) XPS depth profile from BaO clusters grown on YSZ(111) at 600 °C with ∼1.0 × 10-5 Torr of O2 plasma. Corresponding (b) Ba 3d5/2, (c) O 1s, and (d) C 1s XPS spectra were collected during the depth profile from the top 20 nm of BaO clusters.

Figure 4. Ba 3d5/2 and O 1s XPS from the bulk of the film (depth of ∼30 nm) for the first depth profile (a) and the depth profile collected following the film left in the UHV chamber with a base pressure of 3 × 10-10 Torr for 12 h (b) for BaO clusters grown on YSZ(111) at 600 °C with ∼1.0 × 10-5 Torr of O2 plasma. Arrow indicates the shoulder in Ba 3d5/2 XPS, which is absent following 12 h.

peaks at 528.3 and 531.3 eV are found to be 63.3% and 36.7%, respectively. On the basis of the relative area in the O 1s XPS peaks at 528.3 and 531.3 eV arising from BaO and Ba(OH)2, the film should contain 44.9 atom % Ba and 55.1 atom % O, which is in good agreement with the elemental quantification using Ba 3d5/2 and O 1s XPS. These results suggest that nonstoichiometric Ba(OH)2 H2O is completely dehydrated when left in the UHV chamber with a base pressure of 3 × 10-10 Torr for 12 h at room temperature. Although in a few attempts of BaO growth, the ex situ XRD measurements confirmed single-phase BaO clusters on YSZ(111), in most attempts of growth, XRD results indicated the formation of BaCO3 and/or Ba(OH)2 H2O phases. It should be noted that the experimental conditions applied to grow BaO on YSZ(111) are fairly the same for all attempts of growth within experimental error. The substrate temperature was measured using a thermocouple. Depending on the contact of the thermocouple to the YSZ substrate surface, we found measuring the substrate temperature can have an error of ( 20 °C. Recovering the base pressure of the MBE chamber following the growth, transferring the samples, and carrying out ex situ measurements all required a certain amount of time, which can slightly vary from one growth to another. However, the ex situ XRD results between

different attempts of BaO growth are significantly different and arise from variation in the reactivity of the BaO clusters with atmosphere. High-temperature XRD data indicate that a highly reactive amorphous phase forms following BaCO3 decomposition, which further reacts with residual fluorine from the hightemperature stage and forms BaF2. Furthermore, XPS results show that exposing BaO clusters to ambient conditions results in formation of BaCO3 on the surface and partly Ba(OH)2 throughout the bulk. All of these data suggest that BaO is highly reactive in ambient conditions. However the reason for the variation in the reactivity of BaO between different attempts of growth is not clear at this point. The crystallite size and fwhm of BaO clusters as determined by ex situ XRD and AFM are ∼25 nm and ∼30 nm, respectively, in which case the clusters are stable enough to show single-phase BaO, although there was surface contamination as shown by XPS. Therefore, one possible reason for the variation in the reactivity of BaO could be the cluster size as the smaller clusters have a larger surface area and tend to absorb moisture with ease. Future studies are aimed at understanding the influence of growth parameters on the BaO cluster size on YSZ and CeO2 and their reactivity with moisture, CO2, and NO2.

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Conclusions Barium oxide nanoclusters on YSZ(111) were grown by oxygen plasma-assisted molecular beam epitaxy. The spots in the in situ RHEED pattern indicate 3D island or cluster growth of BaO on YSZ(111), and the BaO clusters remained in single phase under UHV. When BaO clusters are exposed to ambient conditions, they transform to BaCO3 and/or Ba(OH)2 H2O as determined by glancing incidence X-ray diffraction measurements. However, in few attempts of BaO growth, XRD results show a fairly single-phase cubic BaO with a lattice constant of 0.5418(1) nm. XPS results show that exposing BaO clusters to ambient conditions results in the formation of BaCO3 on the surface and partly Ba(OH)2 throughout the bulk. XRD and XPS results show that BaO nanoclusters in ambient conditions are highly reactive. Acknowledgment. The research was performed using the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy (DOE), Office of Biological and Environmental Research, located at the Pacific Northwest National Laboratory (PPNL). PNNL is operated for the U.S. DOE by Battelle Memorial Institute under Contract DE-AC05-76RL01830. References and Notes (1) Burch, R. Knowledge and know-how in emission control for mobile applications. Catal. ReV. 2004, 46, 271. (2) Epling, W. S.; Parks, J. E.; Campbell, G. C.; Yezerets, A.; Currier, N. W.; Campbell, L. E. Catal. Today 2004, 96, 21. (3) Liu, Z. M.; Woo, S. I. Catal. ReV. Sci. Eng. 2006, 48, 43. (4) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.; Tanizawa, T.; Tanaka, T.; Tateishi, S.; Kasahara, K. The new concept 3-way catalyst for automotive lean-burn engine: NOx storage and reduction catalyst. Catal. Today 1996, 27, 6369. (5) Fridell, E.; Skoglundh, M.; Westerberg, B.; Johansson, S.; Smedler, G. J. Catal. 1999, 183, 196. (6) Bowker, M. Chem. Soc. ReV. 2008, 37, 2204. (7) Epling, W. S.; Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks, J. E. Catal. ReV. Sci. Eng. 2004, 46, 163. (8) Yi, C. W.; Szanyi, J. J. Phys. Chem. C 2009, 113, 716. (9) Yi, C. W.; Szanyi, J. J. Phys. Chem. C 2009, 113, 2134.

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