Article pubs.acs.org/JPCC
Scanning Tunneling Microscopy and Molecular Dynamics Study of the Li2TiO3(001) Surface Kisaburo Azuma,† Coinneach Dover,‡,§ David C. Grinter,‡,§ Ricardo Grau-Crespo,‡ Neyvis Almora-Barrios,∥ Geoff Thornton,*,‡,§ Takuji Oda,† and Satoru Tanaka† †
Department of Nuclear Engineering and Management, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom § London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London, WC1H 0AH, United Kingdom ∥ Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain ‡
ABSTRACT: We have investigated the (001) surface structure of lithium titanate (Li2TiO3) using auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Li2TiO3 is a potential fusion reactor blanket material. After annealing at 1200 K, LEED demonstrated that the Li2TiO3(001) surface was well ordered and not reconstructed. STM imaging showed that terraces are separated in height by about 0.3 nm suggesting a single termination layer. Moreover, hexagonal patterns with a periodicity of ∼0.4 nm are observed. On the basis of molecular dynamics (MD) simulations, these are interpreted as a dynamic arrangement of Li atoms.
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INTRODUCTION Lithium titanate (Li2TiO3) has been investigated as a promising candidate for tritium breeding materials in fusion reactors.1 Li2TiO3 pebbles have good thermal stability and fast tritium release performance. The addition of hydrogen to helium sweep gas has been proposed for enhancing the tritium release rate of the material. However, its color changes from transparent to black due to oxygen deficiency under hydrogen purge conditions.2 This nonstoichiometric oxide has a different hydrogen release rate.3 Because the surface tritium inventory occupies the major part of the inventory in the blanket, the importance of understanding hydrogen desorption process on surfaces of Li2TiO3 is emphasized. However, not only is the atomic-scale mechanism of hydrogen isotope desorption unknown but also the surface structure itself is still not well characterized. All of the experimental studies previously reported have been devoted to sintered polycrystalline samples. Such samples can provide only limited information about the surface structure at the atomic scale. The surface structure, particularly with respect to defects and other inhomogeneities, may have a critical role in the transport of species such as lithium and tritium within Li2TiO3 and therefore represents an important avenue of research. The experimental growth conditions and chemical characterization of substantial (2 × 4 × 0.5 mm3) Li2TiO3 single crystals, which are large enough for experimental use can be found in the literature.4 Single crystals of monoclinic Li2TiO3 with a space group of C2/c were grown by a flux method. The monoclinic Li2TiO3 unit cell contains eight formula units (48 © 2013 American Chemical Society
atoms) with lattice constants of a = 0.506 nm, b = 0.878 nm, c = 0.975 nm, β = 100°.4 The crystal structure of Li2TiO3 can be described as a stacking of three different layers along the [001] direction. Figure 1 displays a ball and stick model of the geometry of the unreconstructed Li2TiO3(001) surface terminated by the Li layer, where all atoms are located at ideal bulk positions. The Li6−layers and O6−layers, where the subscript represents the number of atoms in each layer of unit cell, are occupied exclusively by lithium and oxygen atoms,
Figure 1. Ball and stick models of side views the Li2TiO3 bulk unit cell. The blue balls represent lithium, red oxygen, and the TiO6 octahedra are gray. Potential terminations have their layer compositions labeled and their separations marked. Received: December 5, 2012 Revised: February 19, 2013 Published: February 20, 2013 5126
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respectively. In Li2Ti4−layers, one-third of the sites are occupied by lithium atoms and two-thirds by titanium atoms. The structure consists of consecutive Li6, O6, Li2Ti4, and O6 layers. Hence, the Li2TiO3(001) surface has four distinct terminations: Li6−termination (Li6−O6−Li2Ti4−O6−R), O6− termination (O6−Li2Ti4−O6−Li6−R or O6−Li6−O6−Li2Ti4− R), and Li2Ti4−termination (Li2Ti4−O6−Li6−O6−R), where R denotes the continuing sequence in the bulk. There are two different interlayer distances in the bulk structure: 0.13 nm between the Li6−layer and O6−layer, and 0.11 nm between the O6−layers and Li2Ti4−layers. The nearest−neighbor spacing of atoms in each layer is 0.28−0.32 nm. All of the potential terminations of Li2TiO3(001) are of type III in the classification given by Tasker,5 that is each layer is charged and there is a dipole moment perpendicular to the surface; therefore it can only be stabilized by substantial reconstruction. A numerical analysis using the METADISE code6 indicates that the only possible nonpolar termination, excluding radical reconstruction or adsorption of other species, is the one ending in the Li only layer, but with 50% of Li vacancies (Li3-termination). However, it is not clear what the distribution of Li atoms and vacancies should be at the surface, or whether this termination remains stable when Li2TiO3(001) contains oxygen deficiencies. This study utilizes low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) in combination with molecular dynamics simulations to investigate the structure of the Li2TiO3(001) surface under ultrahigh vacuum conditions at room temperature. To our knowledge, this is the first investigation of the Li2TiO3(001) surface structure at the nanoscale.
the empty states at the surface. The use of negative sample bias voltages did not yield high-quality images. Computational. Our calculations of the potential energies of the bulk and surface models are based on the Born model of solids,7 where ions are assumed to interact via long-range electrostatic forces and additional short-range forces given by simple parametric functions which represent electron-mediated interactions (e.g., Pauli repulsions and van der Waals dispersion attractions between neighboring electron clouds). The electronic polarizability of the ions is accounted for by using the shell model of Dick and Overhauser,8 in which each polarizable ion is represented by a core and a massless shell, connected by a spring. The potential parameters for Li2TiO3 were taken from previous work by Vijayakumar et al.,9 which was based on a potential set for TiO2 polymorphs derived by Matsui and Akaogi.10 Static relaxations of the bulk and surface were performed with the programs GULP11 and METADISE,6 whereas the molecular dynamics simulation was performed using the DL_POLY2 program.12 All surface simulations were run at constant volume (cell parameters obtained from constant pressure calculations in the bulk) and temperature (300 K). The total simulation time for each job was 120 ps, including 10 ps of equilibration, using the Nosé−Hoover algorithm for the thermostat.13 The integration of the equation of motion was performed using the Verlet leapfrog scheme14 and a time step of 0.2 fs. The motion of oxygen shells was treated using a small mass (0.2 au) to simulate the fast adaptation of the ion polarizabilities to the change in ionic positions.
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RESULTS AND DISCUSSION STM and LEED Observations. After a number of sputter/ anneal cycles had been carried out, the sample was sufficiently reduced to permit the application of LEED without undesirable charging occurring. A typical (1 × 1) LEED pattern from the Li2TiO3(001) surface is displayed in part a of Figure 2. By calibration of the LEED setup using the rutile TiO2(110) surface, measurement of the spacing of the primary reflexes was possible and yielded a separation of 0.42 ± 0.07 nm. Repeated sputter/annealing cycles did not result in any changes to the observed LEED pattern beyond improving the contrast between the reflections and the background. As noted above, charging was observed before extended sputter/annealing. This indicates that this procedure introduces charge carriers although the extent of the reduction cannot be quantified. Although it was also not possible to perform a quantitative analysis of the relative elemental intensities in the Auger electron spectra collected during the course of the preparation cycles, there were no major qualitative changes observed. An example spectrum is displayed in part b of Figure 2 displaying the Ti LMM (387 and 418 eV) and O KLL (503 eV) peaks with an insert plot of the low energy region for the Li KLL (43 eV). Figure 3 presents an STM image of the Li2TiO3(001) surface after a number of surface preparation cycles with an annealing temperature of ∼1200 K. A number of bright adsorbates/ defects are observed on the (001) terraces and the step edges are poorly defined. From the line profile along the blue line, the step heights between terraces are measured to be in the range of 0.2 to 0.3 nm, which corresponds to the distance between any two equivalent layers (0.22−0.26 nm). No step heights corresponding to the interlayer bulk distance (0.11−0.13 nm) were observed. This leads to the conclusion that the terraces
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METHODS Experimental Section. Single crystals of monoclinic Li2TiO3 were grown by a flux method at 1373 K using a LiBO2−Li2O system flux.4 Colorless platelet crystals with a maximum size of 2.0 mm × 4.0 mm × 0.5 mm were grown. Laue diffraction patterns confirmed the symmetry and orientation of the crystal. The Laue patterns clearly showed the plate to be (001) terminated. From our powder X-ray diffraction measurements the monoclinic cell units for the platelet crystals of Li2TiO3 are: a = 0.506 nm, b = 0.879 nm, c = 0.971 nm, β = 100°, in good agreement with the reported values.4 Chemical mechanical polishing (CMP) was employed along a natural (001) growth face in order to produce high-quality surface finishes. Li2TiO3 (001) surfaces were prepared invacuum by cycles of 1 keV Ar+ ion sputtering and annealing by electron bombardment of the Ta backplate in ultrahigh vacuum (UHV) at 1200−1400 K for 10−20 min. The temperature was monitored during annealing using an optical pyrometer (Land). During this process, the single crystal changed its color from transparent to black due to reduction and the resulting formation of color centers. Following the cleaning cycles, retarding field analysis auger electron spectroscopy (RFA-AES), LEED, and STM were carried out in an adjoining UHV chamber with a base pressure below 1 × 10−10 mbar at room temperature. AES indicated the presence of only lithium, titanium, and oxygen; with any contamination under the detection limit. STM was carried out at room temperature using an Omicron VT-STM with electrochemically etched tungsten tips. Positive sample bias voltages of 1.5−3.0 V were employed for STM imaging, corresponding to tunnelling into 5127
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Figure 4. STM image, 100 × 100 nm2, (Vs = +3.0 V, It = 0.05 nA) of Li2TiO3(001) following annealing to ∼1400 K in UHV. The step edges show better alignment than before and the surface is generally more ordered.
Figure 2. (a) Typical LEED image of Li2TiO3(001)(1 × 1) (74 eV) with real-space distances highlighted. (b) Auger electron spectra of Li2TiO3(001) after 10 sputter/anneal cycles with the Li KLL (43 eV, insert; different y axis scaling), Ti LMM (387, 418 eV), and O KLL (503 eV) peaks displayed.
Figure 5. (i) Atomically resolved STM image, 20 × 20 nm2, (Vs = +3.0 V, It = 0.05 nA) of Li2TiO3(001) with features highlighted in (ii). Two distinct domains are observed across the terraces. The region with well-ordered hexagonal symmetry is highlighted with green rectangles and the some of the disordered region is marked with red circles. A number of depressions are marked with blue circles.
Figure 5 shows two clearly different regions of the surface: local hexagonal ordering (marked with green rectangles) covering ∼10% of the terraces, and a disordered area of bright features such as that which is marked with red circles. The minority, ordered region exhibits a hexagonal lattice, shown more clearly in Figure 6, with the bright protrusions having a periodicity of 0.40 ± 0.08 nm. Some dark depressions with diameters of ∼0.15 nm and depths of ∼0.1 nm are observed randomly distributed across the terraces and are likely defects or minor contamination. From Figure 5, it is observed that the step edges which generally run along the ⟨100⟩ direction for all of our samples are composed of alternating short sections aligned along the ⟨110⟩ directions, the diagonals of the surface unit cell. Further annealing at ∼1400 K led to more smoothed step edges as seen in the STM image shown in Figure 7. All of the terraces have a similar appearance and consist exclusively of the more disordered structure, with no sign of the ordered domains observed in Figures 5 and 6. In higher resolution images (not shown), it is observed that the density of the dark depressions seen in Figure 5 is significantly reduced suggesting either the healing of vacancies or the removal of some unknown adsorbate species after progressive preparation cycles.
Figure 3. STM image, 60 × 60 nm2, (Vs = +1.5 V, It = 0.05 nA) of Li2TiO3 (001) following annealing in UHV at ∼1200 K. Flat (001) terraces and step edges are clearly observed. A line profile demonstrating the step height is shown in blue.
are formed of a single termination type as is commonly observed on other oxide systems such as SrTiO3(001).15 After the Li2TiO3(001) surface was annealed at ∼1400 K in UHV, the STM images showed straighter step edges and a generally better ordered surface, as shown in Figure 4. These edges separate several tens of nanometer wide terraces. Step heights were from 0.2 to 0.3 nm. Because these heights are in the same range as that on the sample annealed at 1200 K, the stacking of atomic layers at the surface is assumed to have not changed for temperatures between 1200 and 1400 K. A higher-resolution image of a similar area of the sample is displayed in Figure 5. The highlighted image in part (ii) of 5128
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corresponds to the interlayer distance (0.1 nm). This would suggest that these dark spots are vacancies or clusters similar to oxygen vacancies on the SrTiO3 surface.16 However, the assignment of these to oxygen or lithium vacancies does not seem valid due to the expectation that their concentration should increase under reducing conditions, the opposite to what we have observed.2 Although no contamination was detected by AES, these features are at a very low concentration and as such are likely to be under the sensitivity limit. We now discuss which are more stable terminations of the Li2TiO3(001) surface, anion layers or cation layers. Take the case of anion-layer termination. There are two types of structure for this termination, Ox−Li6 and Ox−Li2Ti4. In general, these terminations are unstable because of dangling bond creation. Moving on to the nature of the cation layers, the Lix−termination should be the most stable since it has no dangling bonds. Considering the charge and the dipole moment borne by the unit cell, which starts from the outermost layer, we can consider the stoichiometric Li2TiO3(001) surface to be a Li 3 −O 6 −termination, where the surface consists of terminated by half-occupation of lithium atoms. By contrast, LixTiy−termination needs to induce surface impurity adsorption or reconstruction due to the dangling bond associated with Ti−O termination. Zainullina has reported that the electron energy spectrum of Li2TiO3 has a conduction band that is a mixture of mainly of Ti 3d states with a minority O 2p state contribution.17 It is likely that Li atoms, which do not contribute to the conduction band, are not imaged in the empty-state mode. However, surface linearized augmented plane-wave (SLAPW) calculations have shown that adsorption of Li atoms on a thin Rh(001) film affects the local density of states (LDOS) and increases the charge density of the surface.18 This effect of alkali metal adsorption on the charge density has also reported for semiconductor surfaces; for instance, potassium atoms on the Si(111) 7 × 7 surface are imaged brightly in the empty-state STM images, as discussed by Watanabe et al.19 Considering the Li2TiO3(001) surface to be Lix−terminated, we suggest that the bright protrusions of STM images are attributed to lithium atoms in the top layer. This assumption can explain the observation that the distances between bright protrusions are longer than nearest neighbor spacing of atoms because only half lithium sites are occupied on the most stable Li3−termination. Although the cause for disordering of bright protrusions after 1373 K annealing remains uncertain, this may be reflected in an increased number of oxygen vacancies in the second layer affecting the outermost lithium position. MD Calculations. In part a of Figure 8, we show all the Li positions before the introduction of vacancies. There are many different configurations in which Li vacancies can be introduced at the surface. For example, taking a 2 × 2 supercell of the surface unit cell, and assuming that the Li ions can occupy the same kind of sites they occupy in the bulk (as in part a of Figure 8), there are 12 sites to be occupied by 6 Li ions, which leads to 924 configurations. Out of these, there are 246 symmetrically different ones, which were identified using the SOD code.20 These configurations were used as starting points for energy minimizations with the GULP code11 relaxing the ion positions. The final energies form the spectrum of Li configurations at the surface (Figure 9). The most stable final configuration, which is 0.05 eV lower than the second most stable one, is shown in part b of Figure 8. The Li ions occupy two different
Figure 6. Enlarged section of the ordered region (green rectangle) in Figure 5. Eight × 8 nm2 Atomically resolved, filled-states STM image of Li2TiO3(001) (Vs = +3.0 V, It = 0.05 nA). An enlarged section of the ordered region (green rectangle) in Figure 5. The unit cell is highlighted.
Figure 7. STM image, 100 × 100 nm2, (Vs = +2.0 V, It = 0.10 nA) of the Li2TiO3(001) after further annealing at ∼1400 K showing a uniform, disordered surface structure.
The typical STM images presented earlier in this article for the Li2TiO3(001) surface allow a number of observations to be made regarding the surface structure. First, step edges on the surface changed from ill-defined shapes to straight when annealing temperatures increased from 1200 to 1400 K. This transition of step structure reflects the movement of atoms on the surface toward step edges in order to decrease energetically unfavorable kink sites. As noted above, only double-layer height steps are observed. Based on the bulk structure, the surface terminations of these layers are Ox−layers (anion layers) or Lix−and LixTiy−layers (cation layers), where x and y denotes the number of atoms below six in the surface unit cell. Dark spots with diameters of about 0.15 nm in typical images of the sample annealed at 1400 K disappeared after the sample was annealed for longer durations. The depth of dark spots 5129
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stable configuration has either one or three Li ions in each hexagonal hole, there are configurations with all possible combinations of one, two, and three Li ions per hexagonal hole. We observed that in all the optimized configurations, whenever there is only one Li ion in a hexagonal hole, it occupies a Λ site. In all cases, when there are two or three Li ions in the same hexagonal hole, they occupy Ω sites. The least stable configuration (∼2.75 eV above the configurational groundstate) is one in which all Li ions aggregate and order by occupying all the Ω sites in a row of hexagons leaving the adjacent rows unoccupied. The electrostatic repulsion between the Li ions clearly makes Li aggregation at the surface very unfavorable. A fully ordered configuration, as in part b of Figure 8, is not compatible with the periodic hexagonal pattern observed experimentally (Figure 6). This comparison, together with the existence of a dense spectrum of configurations, suggests that the hexagons in the STM images could be generated dynamically, that is, by the hopping of Li ions from one hexagon to another. To test this hypothesis, we perform molecular dynamics (MD) simulations at 300 K, starting from the lowestenergy configuration of part b of Figure 8, and using a 4 × 4 supercell of the surface unit cell. Indeed, the simulation shows that Li ions hop from one hexagon to another, at an estimated rate of ∼13 hops per square nanometer per nanosecond. The distribution of the Li ions after 0.12 ns of simulation is shown in part c of Figure 8. In their motion, Li ions remain most of the time in the hexagonal holes, in either Λ sites (if there is only one Li in the hexagon) or in Ω sites (if there are two or three Li ions in the hexagon); the time duration of the hops is very short in comparison. Therefore, the expected average over time corresponds to the periodic hexagonal pattern observed in the experiment.
Figure 8. Top view of the Li3-termination of Li2TiO3(001) (a) without Li vacancies, with ions in bulk positions, (b) the most stable configuration with 50% of surface Li vacancies (also the starting point for the molecular dynamics simulation), and (c) snapshot after 0.12 ns of molecular dynamics simulation at 300 K showing that some surface Li cations have hopped to neighboring positions. Smaller spheres are at a lower level.
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CONCLUSIONS Room temperature LEED and STM have been employed to investigate the structure of Li2TiO3(001). The surfaces prepared by annealing at 1200−1400 K in UHV appear well−ordered on the basis of LEED patterns. Atomically resolved STM images reveal the presence of a minority, ordered hexagonal phase with features spaced at 0.40 ± 0.08 nm, consistent with an interatomic spacing measured from LEED of 0.42 ± 0.07 nm. The majority of the surface was composed of a disordered phase, which was observed to cover the entire surface after subsequent annealing. STM images also showed that terraces are separated by steps of about 0.3 nm height. These results suggest that the surface has a single stable termination. This will be a Li3−layer on the basis of the dipole moment perpendicular to the surface as well as the number of dangling bonds. Bright protrusions in the empty-state STM images are assigned to lithium atoms on the basis of molecular dynamics simulations. These suggest that the hexagonal pattern observed in the STM images are formed dynamically by Li ions hopping between the hexagons defined by the subsurface TiO6 octahedra. These results have important implications for understanding the surface nature of Li2TiO3.
Figure 9. Energy spectrum of Li configurations in a 2 × 2 surface cell. All energies are in eV per cell relative to the most stable configuration.
types of sites, as defined by the geometry of the subjacent Li− Ti−O layer. When there is only one Li ion on the hexagonal hole defined by the network of TiO6 octahedra, that Li ion occupies a position in the center of the hexagonal hole, directly on top of the subsurface Li ion; we label this position as Λ in part b of Figure 8. However, when there are three Li ions sharing the same hexagonal hole, they occupy the same kind of sites as in the bulk, that is, on top of a subsurface O anion − this site is labeled as Ω in part b of Figure 8. Although the most
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AUTHOR INFORMATION
Corresponding Author
*Tel: +44 (0)20 7679 7979, fax: +44 (0)20 7679 0595, e-mail:
[email protected]. 5130
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Notes
(20) Grau-Crespo, R.; Hamad, S.; Catlow, C. R. A.; de Leeuw, N. H. Symmetry-Adapted Configurational Modelling of Fractional Site Occupancy in Solids. J. Phys. Condens. Matter 2007, 19, 256201.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for JSPS Fellows, the EPSRC (U.K.), COST1104, and the European Research Council Advanced Grant ENERGYSURF. We would like to thank Dr. Chi-Ming Yim for several helpful discussions.
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REFERENCES
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