Growth and Structure of Tb2O3(111) Films on Pt(111)

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

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Growth and Structure of TbO(111) Films on Pt(111) Christopher J Lee, Vikram Mehar, and Jason F. Weaver J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01723 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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The Journal of Physical Chemistry

Growth and Structure of Tb2O3(111) Films on Pt(111) Christopher Lee1, Vikram Mehar1, Jason F. Weaver1*

1

Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA

*To whom correspondence should be addressed, [email protected] Tel. 352-392-0869, Fax. 352-392-9513

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Abstract We investigated the formation and structure of Tb2O3(111) thin films grown on Pt(111) using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We find that the Tb2O3(111) films adopt an oxygen-deficient cubic fluorite structure wherein the Tb cations form an approximately (1.32 × 1.32) lattice in registry with the Pt(111) substrate. STM shows that terbia film growth follows the Stranski-Krastanov mechanism in which Tb2O3(111) initially forms a well-connected wetting layer up to a Tb2O3 coverage of ~2 ML (monolayer), whereas multilayer Tb2O3 islands develop as the coverage increases thereafter. The Tb2O3(111) wetting layer yields a sharp (3 × 3) LEED pattern relative to the primary Tb spots that we attribute to diffraction from a 3/4 coincidence lattice formed at the Tb2O3(111)-Pt(111) interface. STM further shows that oxygen vacancies are randomly distributed within the lattice of the Tb2O3(111) wetting layer. The (3 × 3) LEED pattern diminishes during the transition to island growth, but reemerges as the islands ripen at Tb2O3 coverages beyond about 5 ML. We attribute the re-emergent LEED pattern to a (3 × 3) superstructure of oxygen vacancies within the Tb2O3(111) islands, the formation of which is likely mediated by the (3 × 3) Tb2O3-Pt(111) coincidence lattice. The (3 × 3) structure persists to Tb2O3 coverages of at least 10 ML and remains stable after annealing to temperatures up to 1100 K. An implication of this study is that Tb2O3(111) surfaces with well-defined, ordered oxygen vacancies can be stabilized to better study the role of structure and oxygen vacancies on the chemical reactivity of TbOx surfaces.


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Introduction The rare earth oxides (REOs) have received considerable attention due to worldwide demand and potential in a wide range of applications including semiconductors, fuel cells, and heterogeneous catalysis. The two most abundant REOs, lanthanum and cerium, are primary constituents in the catalytic fluid cracking processes and automobile catalytic converters widely used today.1 One of the defining catalytic properties of the rare earth metals is their vigorous reactivity with oxygen to form very thermally stable oxides.2 The majority of REOs are most stable in the trivalent, sesquioxide state Ln2O3 with metal Ln assuming the 3+ oxidation state. However, Ce, Pr, and Tb have the characteristic of stabilizing the 4+ oxidation state and are thus able to utilize the dioxide LnO2 stoichiometry.2 In particular, the oxides of Tb and Ce, known respectively as terbia (or TbOx) and ceria (CeOx), tend to have reactive and structural similarities. Because of the flexibility to store and release oxygen, there is much current interest in REOs for their potential to promote oxidation chemistry such as the oxidative coupling of methane (OCM).3-8 Researchers have made considerable advances in understanding of the properties of ceria thin films grown on various transition metal substrates. High quality, crystalline CeO2 films, particularly the FCC (111) facet, have been grown and stabilized on a number of low index, close- packed transition metal surfaces including Cu(111)9-10, Ru(0001)11, Pt(111)12, Ni(111) 13, Pd(111) 14, and Rh(111).15-17 This has proven to be a reliable method of producing model CeO2 surfaces ideal for surface reactivity studies. In contrast, however, highly ordered surface facets of other REO thin films have not been characterized in UHV to the same extent, leaving the general understanding of trends in REO surface catalysis unknown. TbOx films provide a particularly interesting avenue in which to broaden this understanding as they share structural and oxidative similarities with CeOx.18-19 Though TbOx films are expected to follow similar oxidation



pathways as observed with CeOx due to similarity in achievable oxidation states, TbO2 is more easily reduced than CeO2 and may exhibit different chemical properties stemming from dissimilar affinity for the 4+ oxidation state. Prior research has shown that unlike CeOx, TbOx deposited on Pt(111) in UHV under slight O2 pressures (< 10-6 Torr) stabilizes as Tb2O3(111) and can then be further oxidized to TbO2 via exposure to atomic oxygen.20-21 Reactivity experiments have also shown that TbO2 is more selective in promoting the partial oxidation of methanol to formaldehyde compared with CeO2.17, 22 In the present study, we investigated the growth of Tb2O3(111) thin films on Pt(111) produced via e-beam reactive physical vapor deposition of Tb metal (RPVD). When grown in UHV under oxidizing conditions, we find that terbia thin films initially develop in the Tb2O3 stoichiometry and conform to an oxygen-deficient fluorite structure, commensurate with the Pt(111) substrate. This is in contrast to the structure of bulk Tb2O3, which adopts the bixbyite conformation. Continual development of Tb2O3 domains on Pt(111) is characteristic of StranskiKrastanov growth. Of particular significance is that at larger thicknesses, the oxygen vacancies in the Tb2O3 lattice assume long-range periodic ordering which is out of character with the oxygen-deficient fluorite structure frequently observed in the higher rare earth oxides. Our results provide insight into the stabilization of model Tb2O3(111) surfaces and can serve to advance the fundamental understanding of terbia-based catalysts.

Experimental Details Experiments for this study were carried out in a UHV system with a typical base pressure of 2 × 10-10 Torr.23 This system is equipped with a scanning tunneling microscope, a four-grid LEED optics with RFA capabilities (SPECS), a quadrupole mass spectrometer (Hiden), a tabletop


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Fourier transform infrared spectrometer (Bruker), an ion sputter source, and an electron-beam metal evaporator (McAllister Technical Services) for the reactive vapor deposition of Tb. The equipment used for STM was produced by RHK and includes a UHV 300 “beetle-type” scan head operated with a SPM 100 controller. Follow up studies were performed in a separate UHV system described in more depth in other papers.24-25 The Pt(111) crystal used in this investigation is a circular top-hat shaped disk (10 mm × 1.65 mm) from SPL cut to isolate the (111) surface plane within a tolerance of ± 0.1°. The sample holder consists of a TiN coated top ramp for STM tip approach with the sample sandwiched between sapphire washers and secured by tungsten springs. The sample holder is cooled via thermally conductive contact between a copper braid and a liquid nitrogen reservoir. This design allows for radiative and e-beam heating of the sample up to 1100 K, cooling down to 89 K, and full electrical insulation from electrical heating contacts to maintain a stable voltage bias for STM. We monitored the sample temperature with a type-K thermocouple attached to the bottom of the sample. We cleaned the sample by sputtering with 2 keV Ar+ ions at elevated sample temperatures up to 600 K followed by post annealing the sample at 1000 K. We also employed an oxygen cycling treatment to remove any residual surface carbon by flashing the sample several times to 1000 K in an oxygen environment of 5 × 10-7 Torr. The Pt(111) surface was considered clean when Auger electron spectra exhibited no discernable signal for the C(KLL) and O(KLL) peaks. We grew several terbium oxide films of varying thickness by the reactive physical vapor deposition (RPVD) of terbium metal (Alfa Aesar, 99.9%) onto the clean Pt(111) substrate. The terbium metal was vaporized from a tantalum crucible and deposited in an O2 background of 5 × 10-7 Torr. We held the sample at 600 K during deposition followed by post annealing the film at



1000 K in the same oxygen background. This approach has proven to be effective for preparing ordered, high-quality thin films of other rare earth oxides including Sm2O3(111)26 and CeO2(111).27 In this study, one monolayer (ML) is defined in reference to bulk c-type Tb2O3 as the O-Tb-O trilayer separation in the [111] direction. This corresponds to a height of 3.01 Å and a Tb density of 8.57 × 1014 cm-2 per ML. Film thicknesses were calibrated using attenuation of the Auger electron Pt(MNN) 237 eV peak through the film. The inelastic mean free path (IMFP) for this beam energy is 7.26 Å as calculated by the average of the TPP-2M and Gries equations.28 We estimate that the average deposition rate was 0.10 ± 0.05 ML/min of Tb2O3 and categorized films with thicknesses between 0.8 – 10 ML for these experiments. STM measurements of the TbOx films were taken with the sample at room temperature (300 K) and the tunneling interaction set to constant current feedback mode. Successful images were taken with typical scan settings in the range of +0.2 to +2.5 V sample bias and a tunneling current of 0.2 to 1.0 nA. The lateral distances of the probe raster were calibrated from atomic level images taken from Pt(111) normalized to a lattice constant of a = 2.77 Å. The z-height measurements were calibrated based on the height deflection of 2.3 Å taken over the monatomic step edge of a Pt(111) terrace.

Results Growth of Tb2O3(111) Thin Films on Pt(111) We used LEED and STM to investigate the structural evolution of TbOx grown on Pt(111) as a function of the TbOx coverage. Our results reveal distinct changes in the TbOx structure with increasing terbia coverage, whereas XPS demonstrates that terbia forms in the nominally Tb2O3 stoichiometry for all of the terbia coverages investigated (see Supporting Information (SI)).


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Figure 1a shows a representative LEED pattern obtained after growing a 0.8 ML TbOx film on Pt(111). The diffraction pattern exhibits sharp spots that organize in a (4 × 4) pattern with respect to the Pt(111) basis. Researchers have attributed similar LEED patterns, observed for CeO2(111) and Sm2O3(111) thin films grown on hexagonally close-packed metal surfaces, to diffraction from the coincidence lattice that forms at the oxide-metal interface.15-16, 26 On the basis of these prior studies as well as STM measurements (see below), we conclude that the TbOx thin film on Pt(111) initially adopts an oxygen-deficient, cubic fluorite structure and grows in the (111) orientation with in-plane lattice vectors that align with those of the Pt(111) substrate. We attribute the (4 × 4) LEED pattern to diffraction from a commensurate structure defined by a (3 × 3) TbOx(111) lattice superposed on a (4 × 4) Pt(111) lattice. In this case, the (¾, 0) and (1, 0) spots correspond to the primary diffraction spots from the TbOx(111) and Pt(111) lattices, respectively, while the (¼, 0) and (½, 0) spots result specifically from the (3 × 3)/(4 × 4) TbOx(111)/Pt(111) coincidence lattice. Hereafter, we refer to the superstructure LEED pattern as a (3 × 3) pattern in the TbOx(111) basis. (a)

(b)

(c)

(d)

1.7 ML film

3.1 ML film

7 ± 2 ML film

Pt (1,0) TbOx (1,0)

0.8 ML film

Figure 1: LEED images taken at 65 eV beam energy for the corresponding Tb2O3 film coverages on Pt(111). Film coverage increases from (a) to (d). Circles mark the primary diffraction spots from Pt(111) (blue) and TbOx(111) (red).

Measurement of the nominal (1, 0) spot position (TbOx basis) in the observed LEED patterns gives a lattice constant of 1.32a for the TbOx(111) lattice on Pt(111), in excellent agreement (< 1%) with the lattice constant for an ideal (3 × 3) TbOx(111) structure in the TbOx basis. Good 7 ACS Paragon Plus Environment


lattice matching between TbOx(111) and Pt(111) is expected since the TbOx(111) lattice constant (3.69 Å) predicted for cubic-fluorite TbO2 (5.22 Å)29 is equal to the value (1.33a) for the ideal 3/4 TbOx(111)/Pt(111) structure. The (3 × 3) pattern remains sharp as the film grows to nearly 2 ML thickness (Figure 1b), suggesting that the TbOx(111) film forms large and atomically-flat domains during growth of the first two TbOx(111) trilayers. We find that TbOx(111) growth beyond ~2 ML initially causes the (3 × 3) superstructure pattern to fade. For a ~3.1 ML thick TbOx(111) layer, the LEED pattern exhibits clear first-order TbOx(111) spots, but the other (3 × 3) reflections, including the primary Pt(111) spots, change from sharp spots to faint streaks (Figure 1c). These changes support the idea that the initial (3 × 3) LEED pattern arises from the 3/4 TbOx(111)/Pt(111) coincidence lattice because electron diffraction peaks from the oxide-metal interface should attenuate sharply with increasing film thickness due to the electron energies used in LEED. We have reported similar changes in LEED patterns obtained from Sm2O3(111) films grown on Pt(111) with increasing film thickness.26 The background intensity of the LEED pattern also becomes higher as the TbOx film thickens to 3.1 ML, indicating a larger degree of surface roughening and disorder in the film. The streaking of the LEED spots may also arise from the development of vacancy-superstructures with several, distinct periodicities, analogous to previous observations for partially-reduced CeOx(111) films.30 Increasing the film thickness above about 5 ML causes spots within the LEED pattern to regain clarity and brightness, suggesting a restructuring at the TbOx(111) surface (Figure 1d). The LEED pattern exhibits broad spots that approximately overlap the locations of the spots of the (3 × 3) pattern observed for the TbOx/Pt(111) coincidence structure at low coverage. The first three spots seen in the LEED pattern obtained from a 5 ML film are in the same location as the 8 ACS Paragon Plus Environment

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1/3, 2/3, 1 spots observed with the TbOx/Pt(111) coincidence structure, but the fourth spot is centered at the (1.27, 0) position in the TbOx(111) basis. The smaller spacing of the fourth spot in the LEED pattern is consistent with a periodicity that is nearly 3.2 times greater than the primary TbOx(111) lattice constant. We conclude that the reemergence of superstructure LEED spots arises from a restructuring wherein oxygen vacancies near the surface arrange into a nominal 3 × 3 structure relative to the cubic Tb sub-lattice as the film thickens beyond ~5 ML. Further, the considerable breadth of the LEED spots is indicative of ordered domains that are relatively small, suggesting that the surface roughens at higher TbOx coverages. Our LEED results reveal an interesting structural evolution as a function of the TbOx(111) film thickness on Pt(111) in that a (3 × 3) superstructure develops during growth of the first ~ 2 ML of the oxide but diminishes as the TbOx layer thickens to ~3 ML, and then reemerges beyond about 5 ML thickness. For the thin film (< 2 ML), we assert that the (3 × 3) superstructure arises from a 3/4 coincidence structure at the TbOx(111)/Pt(111) interface that results from ordering of the cubic Tb sub-lattice relative to the Pt substrate whereas oxygen vacancies are randomly distributed. Below we show STM results that support this interpretation. For the thicker films (> 5 ML), we contend that the (3 × 3) superstructure observed in LEED no longer arises from the oxide-metal coincidence lattice, but instead results from a surface reconstruction produced by ordering of oxygen vacancies. We reach this conclusion by recognizing that electron diffraction from the metal-oxide interface is unlikely to be detected for the 5 ML TbOx(111) layer (~15 Å) because the electron energies used the LEED experiments (< 70 eV) correspond to short IMFPs so the scattered electrons have very low probabilities of propagating through the oxide layer without losing energy. The initial diminution of the (3 × 3) LEED pattern that we observe (Figure 1c) is consistent with the idea that diffracted electrons from the metal-oxide interface 9 ACS Paragon Plus Environment


attenuate below the detection limit as the film thickness increases above ~3 ML. Prior studies also report that oxygen vacancies arrange into a (3 × 3) structure on partially-reduced CeOx(111) films,10, 30-31 thus revealing a tendency for such vacancy-ordering in REO surface structures. We note that the (3 × 3) LEED pattern is evident for TbOx(111) film thicknesses up to ~10 ML, and that the (3 × 3) pattern persists even after extended annealing of thick TbOx(111) layers up to 1000 K in both UHV and background O2 at a pressure of 5 × 10-7 Torr. These annealing treatments did not induce a measurable transformation to the (4 × 4) structure (in the TbOx basis) for bixbyite Tb2O3(111). Below we suggest how the (3 × 3) structure could develop for a stoichiometry close to the sesquioxide (LnO1.5). STM Measurements of Film Morphology We identified three distinct growth regimes as a function of the TbOx coverage on Pt(111) using STM. Below coverages of 1 ML, TbOx(111) forms small hexagonal domains on the surface with diameters generally between 10-20 nm. The TbOx(111) domains self-order along the exposed Pt(111) crystallographic directions and the domain edges form sharply defined 120° angles with one another. The STM images reveal a tendency for the terbia to form aggregate structures along the Pt step edges that span distances of hundreds of nm (Figure 2a,b). However, we also observe more fine dispersion of the terbia even in the presence of Pt(111) step edges. For example, Figures 2b and 2c each show Pt(111) substrate domains with smaller terraces on the order of 20 nm as well as sub-monolayer coverages of TbOx. Figure 2c shows a preference for the nucleation of small, compact TbOx domains on the order of 10 nm rather than formation of the longer, aggregate step-edge structures seen in Figures 2a and 2b. This comparison suggests that the tendency for TbOx to nucleate on step-edges rather than terraces depends on factors other than the local step-edge density and terrace width. For example, the Tb vapor flux to the surface 10 ACS Paragon Plus Environment

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during growth could influence the formation of TbOx islands on step-edges vs. terraces, with nucleation on terraces being more probable at higher Tb incident flux. Additional studies are needed to determine how the growth conditions and Pt(111) step density influence the nucleation and growth of TbOx at sub-monolayer coverages. Low coverage: 0.3 to 1.0 ML (a)

(b)

0.5 V, 1.0 nA

(c)

0.5 V, 1.0 nA

0.5 V, 1.0 nA

Medium coverage: 1.0 to 2.0 ML (e)

(d)

(f)

0.5 V, 1.0 nA

0.5 V, 1.0 nA

High coverage: 3.1 ML

High coverage: ~5.0 ML

(g)

(h)

2.0 V, 0.3 nA

2.5 V, 0.8 nA


2.0 V, 0.5 nA


Figure 2: STM images show (a) nucleation on terraces and agglomeration along step edges (b) predominantly agglomeration along step edges, and (c) predominantly island nucleation (0.3 – 1.0 ML). (d), (e) and (f) show continuous network growth along terraces, discontinuous along step edges, and (g) and (h) illustrate island growth at coverages above ~2.5 ML.

Between TbOx coverages of 0.8 and 1.5 ML, we observe a transition where the hexagonal islands and step-edge aggregate structures begin to agglomerate into a well-connected oxide (Figure 2d-f). The resulting structure may be described as a wetting layer of TbOx regularly interspersed with pits that expose the underlying Pt(111) substrate. At the lower range of these coverages, the pits are larger and more irregularly shaped, but retain a 60° edge termination (Figure 2d). The pits begin to close off and assume a hexagonal geometry as the coverage of TbOx increases. At a coverage of 1.7 ML, the characteristic size of the oxide domains is 20-40 nm in length with hexagonal pits characterized by widths on the order of 10-20 nm (Figure 2e,f). Finally, the first Tb2O3 trilayer reaches a critical coverage where the second trilayer begins to form hexagonal domains on top. This represents the Stranski-Krastanov growth mechanism in that growth of the initial wetting layer is favorable as opposed to island growth.32 However, growth of the second and third trilayers does commence before the first trilayer is completely closed. We find that TbOx begins to form irregularly-shaped islands as the oxide coverage increases beyond ~3 ML and that the morphology generally becomes less organized. STM shows a high density of TbOx islands of ~2-5 nm in diameter dispersed across the surface after depositing ~3.1 ML of TbOx (Figure 2g). Increasing the coverage to greater than 5 ML causes the islands to ripen and reach sizes of ~15-25 nm, though the islands continue to exhibit irregular shapes and are randomly dispersed (Figure 2h). The increase in island size with increasing TbOx coverage from ~3 to 5 ML coincides with the observed change in the LEED pattern from a streaky to a better defined (3 × 3) pattern (Fig. 1c,d). An implication is that the (3 × 3) structure that emerges at 12 ACS Paragon Plus Environment

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high TbOx coverages originates from the oxide islands. As the TbOx coverage increases beyond 3 ML, the surface appears to become rougher and more disorganized in that the surface is covered by oxide islands that become sparsely connected before islands begin to form in the next layer. It is worth noting that tunneling on the thicker TbOx layers (> 3 ML) became unstable at bias voltages less than ~1 V, and that biases above about 1.5 V were needed to resolve the oxide islands on these surfaces.

Height and Length Measurements from STM

Figure 3 compares STM images obtained from a 1.7 ML TbOx(111) film at tunneling biases of 0.7 and 2.0 V as well as line profiles measured across the same area of the surface. The image obtained at 0.7 V bias shows flat, contiguous domains with pits interspersed throughout the layer. The flat domains exhibit a more granular structure in the image obtained at 2.0 V bias, where the height variations measured within these domains is typically less than 0.5 Å. This small corrugation varies over characteristic in-plane distances of 1 to 3 nm, without exhibiting an obvious periodicity. Rather than representing a true geometric roughness, the observed granularity likely arises from variations in the local electronic structure resulting from randomlydistributed vacancy structures (see below) as well as modulations in the bonding at the oxidemetal interface. We also find that height differences between contiguous domains are larger in the images obtained at higher tunneling bias. Luches et al. have reported a similar bias-dependent behavior in STM images obtained from CeO2(111) films on Pt(111).12 The line profiles measured at 2.0 V bias reveal that the domains lie about 3 and 6 Å above the bottom of the dark pits (e.g., Figure 3c), where these values are consistent with the stacking of one and two trilayers



of cubic-fluorite TbOx(111), respectively. The line profile obtained across the same surface features exhibits smaller height variations when measured at 0.7 V bias (Figure 3d).

(a)

(b)

(d)

(c)

Figure 3: STM images from the same area of a 1.7 ML TbOx film taken at (a) 0.7 V, 0.37 nA and (b) 2.0 V, 0.38 nA. Height profiles of a characteristic TbOx terrace step and depression region shown for the black and red line scans respectively (c and d).

Figure 4 shows an atomically-resolved image obtained from a flat domain on a 1.7 ML TbOx layer. The image reveals a hexagonal arrangement of bright, atomic features that exhibits the periodicity of the Tb sub-lattice of cubic-fluorite TbOx(111). We attribute the bright features to the Tb cations in the lattice because the applied sample bias was positive and tunneling thus probed the empty Tb 4f states of the Tb2O3 film. The average spacing between the bright features


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measured along the three high-symmetry directions is 4.04 Å, in good agreement (~11%) with the TbOx(111) lattice constant of 3.67 Å as estimated from LEED. The few atomic features that appear particularly bright in the image may arise from H2O or OH species produced by background adsorption during prolonged scanning at room temperature. (a)

(b)

Figure 4: Topographic survey and FFT filtered atomic resolution STM images obtained after growing a 1.7 ML Tb2O3 film on Pt(111) followed by annealing at 1000 K for 10 minutes in 5 × 10-7 torr O2: (a) shows a 500 Å × 500 Å terrace (0.6 V, 0.4 nA), (b) shows an 100 Å × 100 Å FFT filtered atomic resolution scan of the region outlined by the blue box in Figure 4a (0.7 V, 0.7 nA).

Our high-resolution images also reveal a high-density of shallow, atomic-scale depressions that arise from lattice O-atoms, as well as smaller concentrations of deeper depressions that we attribute to O-vacancies. The depths of the deeper depressions measure at ~0.2 to 0.3 Å in line scans obtained from the images (see SI). The vacancies arrange into diverse types of local configurations, without organizing into a structure with long-range periodicity. For example, the image shows several isolated structures containing three to four vacancies that are each separated by one lattice constant and arranged into either linear or triangular configurations. Prior studies report that surface O-vacancies on mildly-reduced CeO2-x(111) films also form triangular as well as linear structures33-35 that are similar to those resolved for the Tb2O3(111) film in the present study. In contrast, researchers have shown that sub-surface O-vacancies on CeO2-x(111) tend to 15 ACS Paragon Plus Environment


produce structures with local 2 × 2 periodicity.36-38 The image also shows two areas that each contain about 20 closely-spaced vacancies (Fig. 4b, near center and bottom left). The appearance of these aggregates demonstrates that a large fraction of the vacancies agglomerate at the surface of the ultrathin Tb2O3(111) film rather than remaining separated. Overall, our STM images show that oxygen vacancies, while adopting characteristic local arrangements, do not organize into configurations with long-range periodicity near the surface of the ultrathin (< 2 ML) Tb2O3(111) films grown on Pt(111). This finding lends support to our conclusion that Tb2O3(111) initially grows on Pt(111) in a cubic fluorite structure with randomly-distributed vacancies, and thus that the 3 × 3 LEED pattern observed at low Tb2O3 coverage (~ 3 ML) with STM. We found that the rough surface morphology (Figure 2g-h) limited the resolution that could be achieved on the thicker films and that the STM tip was difficult to stabilize when scanning over small areas. Duchoň et al. have also reported tip instability during imaging of CeO2-x(111) surfaces that exhibit the 3 × 3 vacancy superstructure, and suggested that water molecules accumulating on 16 ACS Paragon Plus Environment

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the surface may have induced tip instability during prolonged scanning of the CeO2-x(111)-(3 × 3) structure.10 Adsorbed water may have influenced the tip stability in our attempts to atomically-resolve the thick Tb2O3(111) films, and vacancies might also become more mobile in the thicker Tb2O3(111) films, thus further complicating STM imaging at room temperature. Future work to generate larger domains of the Tb2O3(111)-(3 × 3) surface structure to aid in structural characterization would be worthwhile to pursue. Despite these limitations, our results support the view that the (3 × 3) periodicity has distinct origins at low vs. high terbia coverage on Pt(111).

Discussion We find that the growth of Tb2O3 films on Pt(111) follows a Stranski-Krastanov growth mechanism, analogous to other REO films grown on single crystal metal surfaces. Our results show that Tb2O3(111) initially forms well-ordered trilayer islands that agglomerate with increasing Tb2O3 coverage to generate an interconnected film consisting of large domains interspersed with hexagonal pits. This contiguous film morphology persists as the Tb2O3(111) coverage increases to about 2 ML. As the coverage increases toward ~5 ML, the Tb2O3 forms islands that ripen yet tend to remain discrete rather than aggregating into a well-connected structure. STM images also show that islands begin to grow in new layers before the underlying layers are completed (e.g., Figure. 2f). The observed morphological evolution is consistent with the Stranski-Krastanov growth mechanism in that an ultrathin wetting layer of Tb2O3 initially forms up to ~2 ML thickness, while island growth ensues as Tb2O3 forms on-top of the wetting layer. Stranski-Krastanov growth signals that the Tb2O3-Pt(111) interaction is sufficiently strong to promote wetting of the Pt(111) substrate during growth of the first ~2 ML of Tb2O3(111). 17 ACS Paragon Plus Environment


Thereafter, the driving force for wetting diminishes and island growth ensues as new Tb2O3 layers develop on top of existing Tb2O3 domains that are ~2 ML or more in thickness. Our results also provide evidence of a structural change in the Tb2O3 film that occurs with increasing film thickness whereby oxygen vacancies are randomly distributed within the lattice of ultrathin Tb2O3(111) films (< 2 ML) but organize into a (3 × 3) superstructure as the film thickness increases to above ~5 ML. Similar to CeO2 films,12, 39 we find that Tb2O3(111) initially grows in the cubic fluorite structure on Pt(111) with oxygen vacancies distributed randomly across the anion lattice sites and the Tb sub-lattice forming a well-organized 3/4 commensurate lattice with the Pt(111) substrate. Our STM observations of randomly-arranged vacancies (Fig. 4b) provide strong evidence that the 3/4 commensurate lattice is the origin of the sharp (3 × 3) LEED patterns observed for the ultrathin Tb2O3(111) films. The diminution of the (3 × 3) pattern as the film thickens to ~3 ML further supports our interpretation. We speculate that favorable lattice matching with the Pt(111) substrate as well as the Tb2O3Pt interaction are important factors in causing the Tb2O3 layer to initially adopt the oxygendeficient cubic-fluorite structure rather than the bixbyite structure (i.e., c-Tb2O3), which is the favored polymorph for bulk Tb2O3 over a wide range of conditions.2 In support of this interpretation, we note that the surface lattice constant of bulk-terminated, cubic-fluorite TbO2(111) (aTbO2 = 3.69 Å)29 is nearly identical to 4/3 of the value of the Pt(111) lattice constant (aPt = 2.77 Å); the relation 3aTbO2 = 4aPt is accurate to within better than 0.1 %. The Tb sublattice of cubic fluorite TbO2(111) can thus form an essentially unstrained 3/4 commensurate lattice on Pt(111). In contrast, the in-plane lattice constant of bulk-terminated c-Tb2O3(111)2 is acTb2O3 = 15.17 Å and the vacancies form an approximately 4 × 4 superstructure relative to the cubic fluorite Tb sub-lattice. The c-Tb2O3(111) lattice can form a 1/5 commensurate structure on


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Pt(111) but the oxide would experience 8.7% of compressive strain to adopt this structure. This comparison suggests that energy penalties incurred by the vacancies randomly distributing are overcome by the favorable lattice matching achieved between the cubic-fluorite Tb sub-lattice and Pt(111). We find that the initial diminution of the (3 × 3) LEED pattern with increasing Tb2O3 coverage to ~3 ML coincides with a transition from Tb2O3 wetting to island growth, and that reemergence of the (3 × 3) pattern above ~5 ML occurs as the Tb2O3 islands ripen into larger domains of ~15 – 25 nm in width. These observations provide evidence that oxygen vacancies organize into a (3 × 3) superstructure as the Tb2O3 layer transitions from wetting to island growth and thickens, given that electron diffraction from the oxide-metal interface would be undetectable for the thick films under the LEED conditions employed. Several studies report that an analogous (3 × 3) vacancy superstructure develops during partial reduction of relatively thick CeOx(111) films (>~3 nm).10,

30-31

However, XPS measurements demonstrate that the (3 × 3)

structure forms when ceria reaches the CeO1.67 stoichiometry, whereas we observe this structure for the more reduced Tb2O3 stoichiometry. Olbrich et al. have recently reported DFT predictions of the (3 × 3) CeO1.67(111) surface structure.40 The proposed structure is described in terms of a (3 × 3) surface unit cell containing one oxygen-vacancy (Ov) site in the top layer and two Ov sites in the bottom layer of a CeO1.67(111) trilayer. For this structure, each (3 × 3) unit cell contains 9 Ce cations and 18 anion sites so the removal of three O-atoms generates a stoichiometry of Ce9O15 or CeO1.67. A possible way to achieve a (3 × 3) periodicity for a more reduced stoichiometry would be to remove four rather than three O-atoms from the (3 × 3) unit cell, where this structure yields a stoichiometry of Tb9O14 or TbO1.56. While we have reported XPS results showing that TbOx 19 ACS Paragon Plus Environment


films grown on Pt(111) in up to 10-6 Torr O2 are consistent with Tb2O3,21-22 a stoichiometry of TbO1.56 represents less than a 3% oxygen enrichment over TbO1.5 and would be difficult to identify with XPS. It thus seems plausible that the thicker TbOx films (>~ 3 ML) grown in this study preferentially form a superstructure characterized by four oxygen vacancies per (3 × 3) surface unit cell in each O-Tb-O trilayer, resulting in a stoichiometry of TbO1.56. Considering prior studies of ceria,10, 30-31 our findings reveal a tendency for oxygen vacancies to organize into (3 × 3) superstructures on both partially-reduced CeOx and TbOx surfaces. Further work is needed to atomically resolve these (3 × 3) structures, and possibly identify common features in the local arrangements of oxygen vacancies within the different REOs. Our results show that the Tb2O3(111) film avoids transformation to the bixbyite structure for film thicknesses up to ~10 ML and the growth conditions investigated. Oxygen vacancies in bulk-terminated c-Tb2O3(111) form a (4.1 × 4.1) superstructure relative to the cubic fluorite TbOx(111) lattice, and the c-Tb2O3(111) lattice would need to undergo compressive strain of only 2.8% to adopt a perfect (4 × 4) structure. However, even though the lattice strain required is modest, we find that oxygen vacancies arrange into a (3 × 3) superstructure rather than the (4 × 4) structure as the Tb2O3(111) film thickens. Considering the thermodynamic preference for bulk Tb2O3 to exist as bixbyite, we speculate that the (3 × 3) periodicity of the oxide-metal interfacial structure causes vacancies to arrange into a (3 × 3) rather than (4 × 4) superstructure under the growth conditions that we investigated. We specifically find that the (3 × 3) structure persists after repeated annealing of the Tb2O3(111) films to 1000 K in O2, demonstrating a high stability of the (3 × 3) structure. Bixbyite c-Tb2O3(111) might form during prolonged annealing at temperatures above 1000 K, but we did not examine this possibility in the present study.


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Finally, it is instructive to review the conditions under which the bixbyite structure forms in ceria thin films for comparison with our results with terbia. Höcker et al. have shown that the (4 × 4) c-Ce2O3(111) structure forms during reduction of CeO2(111) films on Ru(0001) by exposure to H2 at 700 K and also by prolonged annealing at 973 K in UHV.30-31 These studies demonstrate that large H2 exposures (4800 L) and UHV annealing times of more than one hour are required to generate c-Ce2O3(111) at the temperatures considered, and further show that the (3 × 3) CeO1.67(111) structure forms prior to the (4 × 4) c-Ce2O3(111) structure. Duchoň et al. have shown that c-Ce2O3(111) can also be generated by depositing a metallic Ce layer on a CeO2(111) film on Cu(111) and subsequently annealing the Ce/CeO2(111) heterostructure to 873 K for 30 minutes in UHV.10 Those authors show that the (3 × 3) CeO1.67(111) structure forms at the surface when the metallic Ce layer is thinner than that needed to produce (4 × 4) c-Ce2O3(111). These prior studies demonstrate that formation of bixbyite c-Ce2O3(111) occurs over a range of conditions in ceria films on Ru(0001) and Cu(111), including temperatures as low as ~700 K, and that the (3 × 3) structure forms only at the CeO1.67 stoichiometry. A possible implication is that reduction of ceria to the Ce2O3 stoichiometry provides a sufficient driving force to induce bixbyite formation in these systems, at least down to 700 K, and that a (3 × 3) Ce2O3(111) structure is less stable than the (4 × 4) c-Ce2O3(111). Our results reveal a distinct behavior for terbia in that the nominally Tb2O3 stoichiometry forms a (3 × 3) TbOx(111) structure that avoids conversion to bixbyite for temperatures up to at least 1000 K. We speculate that the (3 × 3) periodicity of the terbia-Pt(111) coincidence lattice as well as the minimal strain required to form this epitaxial structure act to stabilize the (3 × 3) vacancy superstructure in the Tb2O3(111) layer. While additional work is needed to further explore the stability of the (3 × 3) TbOx(111) structure, the present results reveal an interesting contrast between ceria and terbia, and 21 ACS Paragon Plus Environment


demonstrate diversity in the structures of REO thin films. Computational studies could further illuminate the factors that cause preferential formation of the (3 × 3) TbOx(111) structure on Pt(111) under the conditions studies, particularly in determining if this structure represents a thermodynamically stable phase or a meta-stable phase.

Summary We characterized the growth and structural organization of Tb2O3(111) deposited on a Pt(111) substrate for films up to ~10 ML in coverage. We find that the Tb2O3(111) domains grown are high quality with a strong hexagonal (1.32 × 1.32) lattice match in registry with the underlying substrate. At coverages up to 2.5 ML, this is characterized by a 3/4 coincidence lattice between the Tb2O3 film and the Pt(111) substrate. STM measurements show that the Tb cations adopt a hexagonal arrangement that is consistent with cubic fluorite TbOx(111) and confirm that the oxygen vacancies at this coverage are randomly distributed. Growth is characterized by the Stranski-Krastanov mechanism in which an initial Tb2O3 wetting layer initially forms before multilayer island formation becomes dominant at coverages greater than 2.5 ML. Of particular interest is that oxygen vacancies within the fluorite lattice begin to order at these higher coverages as evidenced by the reemergence of a (3 × 3) LEED pattern with respect to the Tb2O3(111) fluorite lattice. The (3 × 3) structure persists with increasing Tb2O3 coverage to at least ~10 ML and also when the film is annealed at 1000 K, even though the bixbyite structure is the thermodynamically-favored polymorph of bulk Tb2O3. We speculate that the (3 × 3) periodicity of the Tb2O3-Pt(111) interface structure acts to stabilize the (3 × 3) vacancy superstructure of the fluorite Tb2O3(111) films and hinders conversion to the bixbyite structure during thermal annealing. Further investigation is needed to understand the stabilization 22 ACS Paragon Plus Environment

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mechanism, especially in terms of oxygen mobility. Our results show that high quality Tb2O3 thin films can be grown on Pt(111) for structural and chemical reactivity characterization. The further possibility of stabilizing well-ordered oxygen vacancies on the surface of the fluorite lattice may provide opportunities for clarifying how long-range ordering of vacancies influences the chemical reactivity and selectivity of TbOx and other rare earth oxides.

Supporting Information XPS Tb 3d5/2 spectra obtained from TbOx(111) films of various thicknesses, STM line scans over oxygen vacancy regions of an atomically-resolved 1.7 ML TbOx(111) film.

Acknowledgements We thank Simona Keil and Ameen Sayal for assistance with experiments. We gratefully acknowledge financial support for this work provided by the National Science Foundation, Division of Chemistry through grant number 1464765.

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Growth and Structure of Tb2O3(111) Films on Pt(111)

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