Burow, A.

Freund, H.

Ganduglia-Pirovano, M.

Jerratsch, J.

Nilius, N.

Popa, C.

Sauer, J.

Shao, X.

English

MPG.PuRe

Electron Localization in Defective Ceria Films: A Study with Scanning-Tunneling Microscopyand Density-Functional TheoryJan-Frederik Jerratsch,1 Xiang Shao,1 Niklas Nilius,1,* Hans-Joachim Freund,1 Cristina Popa,2M. Veronica Ganduglia-Pirovano,2,† Asbjo¨rn M. Burow,2 and Joachim Sauer2,‡1Fritz-Haber-Institut der MPG, Faradayweg 4-6, 14195 Berlin, Germany2Humboldt-Universita¨t zu Berlin, Unter den Linden 6, 10099 Berlin, Germany(Received 5 January 2011; published 14 June 2011)Scanning-tunneling microscopy and density-functional theory have been employed to identify thespatial correlation between an oxygen vacancy and the associated Ce3þ ion pair in a defective CeO2ð111Þfilm. The two Ce3þ ions can occupy different cationic shells around the vacancy. The resulting variation inthe chemical environment leads to a splitting of the filled Ce3þ f levels, which is detected with STMspectroscopy. The position of the Ce3þ ion pair is reflected in characteristic defect patterns observed inempty-state STM images, which arise from the bright appearance of Ce4þ ions next to the defect while theCe3þ remain dark. Both findings demonstrate that at least one excess electron localizes in a Ce ion that isnot adjacent to the O vacancy.DOI: 10.1103/PhysRevLett.106.246801 PACS numbers: 73.20.Hb, 68.37.Ef, 71.15.Mb, 73.61.NgCeria is a widely used oxide support for several oxida-tion reactions, such as the water-gas shift and NOx reduc-tion [1–3]. The outstanding performance of ceria inpromoting oxidation processes relies to a large extent onits facile reducibility and the associated ability to releaselattice oxygen [4]. The vacancies are easily healed again inan O-rich ambience, which makes the oxide perfectlysuited to balance the oxygen supply during a catalyticreaction. On removing an O atom, two electrons are leftbehind in the lattice that form two reduced Ce3þ species[4]. The electrons occupy split-off states of the initiallyempty Ce 4f band, lying inside the O2p  Ce5d band gapof ceria and being highly localized in space.The reduction of ceria can be followed with photoelec-tron spectroscopy, by using the presence of filled Ce 4fstates or characteristic shifts in the Ce 3d core levels as ameasure [5,6]. This approach provides only spatially aver-aged information, and the localization of the O vacanciesand associated Ce3þ ion pairs is not revealed. In contrast,scanning-tunneling (STM) and atomic force microscopyhave been successfully employed to identify single Ovacancies in surface or subsurface ceria layers [7,8].However, no information on the electronic properties ofthe defects was acquired in this case, and the question ofelectron localization remained unsolved. Early density-functional theory (DFT) calculations assumed a stabiliza-tion of the extra electrons in Ce ions next to the defect[7,9,10], while recent studies suggested the occupancy ofmore distant cationic sites [11–13]. In ceria nanoclusters,the Ce3þ ions were found to form preferentially at low-coordinated edge sites, even if the O vacancy is far away[14]. Those differences demonstrate the sensitivity ofelectron-localization phenomena on the computationalapproach.In this work, we use a combined STM and DFTapproachto provide insight into the spatial correlation between Odefects and the associated Ce3þ ion pair in a CeO2 (111)surface. We demonstrate that electrons left behind uponvacancy formation occupy Ce sites that are not in a next-neighbor position to the defect. This possibility to exploreelectron-localization effects opens a direct route to monitoroxidation and reduction processes in the catalytically rele-vant ceria system [15].The experiments have been performed in an ultrahighvacuum STM operated at 10 K. The oxide film is preparedby depositing Ce onto a sputtered and annealed Ru(0001)surface in 5 106 mbar O2, whereby half of the materialis dosed at 100 K and half at 450 K [16]. After annealing to1000 K in O2, the film displays a sharp (p2p2) LEEDpattern that is characteristic for the CeO2 (111) surface.The mean film thickness is adjusted to five O-Ce-O tri-layers. Highly resolved STM measurements are mainlyobtained at positive sample bias (unoccupied states), in-dicating that the Ce4þ ions are responsible for the contrast(Fig. 1). At negative bias, the filled O 2p orbitals governthe contrast; however, atomic resolution is more difficult toobtain. In both cases, the observed atom periodicity of4.0 A˚ is in line with the CeO2 lattice constant of 3.88 A˚.Ceria films are prepared in an excess of O2 and exhibitonly a small defect concentration. Oxygen vacancies arethus generated by injecting 50 eVelectrons (1 mC dose) at100 K. Because of the highly nonthermal formation pro-cess, the defects are not expected to be in an equilibriumdistribution; however, most of them are located in the oxidesurface given the low excitation energies. At positive bias,they appear as paired and tripled protrusions of 0:4 Aheight, which mark the Ce4þ ions around the vacancy asshown below [Figs. 1(d) and 1(e)]. At negative polarity,PRL 106, 246801 (2011) P HY S I CA L R EV I EW LE T T E R Sweek ending17 JUNE 20110031-9007=11=106(24)=246801(4) 246801-1  2011 American Physical Societythey transform into atom-sized holes, confirming theiridentity as a surface O vacancy [Fig. 1(c), arrow] [7,8].Also, subsurface vacancies that appear as three protrudingmaxima at negative bias are occasionally observed buthave been disregarded in this study [Fig. 1(c), dashedtriangle].The electronic properties of surface defects are analyzedwith STM conductance spectroscopy performed with thelock-in technique (10 mV=1073 Hz). The stoichiometricfilm exhibits a band gap ranging from2:5 (valence band)to þ3:5 V (conduction band) [Fig. 2(a)]. The gap size of6.0 eV is close to the bulk value obtained from electron-energy loss and optical spectroscopy [5]. An extra dI=dVpeak at 2.3 eV is detected inside the gap and assigned to theunoccupied Ce 4f band for the following reasons. Itsenergy position fits well to the f band center derivedfrom electron-energy loss and inverse photoelectron spec-troscopy [5,17]. The peak is visible only at small tip-sample distances, adjusted with a bias set point close tothe conduction band onset, as expected for a highly con-tracted electronic state. And a negative-differential resist-ance behavior is revealed for higher set-point currents(I > 50 pA), as reflected in the negative dI=dV signalbeyond the peak [Fig. 2(a), spec 1*]. The negative-differential resistance effect originates from the loss ofconductivity when the f-related transport channel closesin the spectral run. It is symptomatic for electron transportthrough spatially and energetically confined states [18,19]and therefore in line with our assignment of the gap statesto the empty 4f band.On defective ceria, also filled f states should appear atthe position of the Ce3þ ions [4,20]. Their detection ischallenging due to the small tip overlap with the localizedstates in the gap. Sufficient spectral sensitivity is achievedonly if the tip is stabilized inside the empty f band and thenramped to negative bias [Fig. 2(b), left]. Whereas no filledstates are observed on defect-free oxide patches (spec 4),new dI=dV peaks appear at around 1:5 V in the vicinityof surface vacancies. The signal intensity strongly dependson a precise positioning of the tip above the defect andquickly drops to zero if the tip is displaced by more than5 A˚. Also, the spectral shape changes when going from onedefect to another (specs 1–3). Whereas defects appearingas a triple protrusion always display a single peak at around1:2 V (spec 2), a splitting with maxima at 1:5 and1:1 V is revealed for certain paired protrusions (spec1). However, the latter also appear with only one peak inthe conductance spectra. We note that similar gap states aredetected for other defect types, such as voids in the Celattice (spec 3) and step edges. Common to all defects is anempty-state spectrum that is similar to the one of thestoichiometric film [Fig. 2(b), right].The high spatial confinement and energy position of thegap states suggest a relation to the filled 4f levels localizedat the Ce3þ ions, as similar states were detected withelectron-energy loss and photoelectron spectroscopy be-fore [5,15]. However, a conclusive interpretation requiresDFT calculations and have been performed at the PBEþU level (U ¼ 4:5 eV), by using the VASP code with a400 eV cutoff for the plane-wave basis set [21]. Atomiccores are described by the projected augmented wavemethod. The O vacancy is positioned in the top layer ofthe (3 3) supercell representing theCeO2 (111), while allcationic sites of the slab are considered as potential posi-tions for the Ce3þ ions. The electronic structure is deducedfrom a (2 2 1) k-point sampling of the Brillouin zone.FIG. 2 (color online). Conductance spectra of(a) stoichiometric and (b) reduced CeO2ð111Þ taken at negative(left) and positive bias (right). The spectral positions are markedin the insets (a) 2.0 V, 15 15 nm2; (b) 1.2 V, 8 4 nm2. Allcurves have been taken with 5 pA current, apart from 1* (50 pA).While the dI=dV maximum at 2.3 V reflects the empty Ce 4fband, the peaks around 1:5 V are assigned to filled f states.FIG. 1 (color online). (a) STM image of a five trilayer thickCeO2 film grown on Ru(0001) (3.1 V, 100 100 nm2).(b) Atomically resolved image showing the Ce sublattice(1.2 V, 12 12 nm2). The protrusion in the lower part isassigned to a single O vacancy. (d),(e) Similar defects imagedwith a tip configuration that is sensitive to the Ce lattice (1.2 V,2:4 2:4 nm2) and (c) to the O lattice ( 2:2 V). The arrow in(c) marks a surface O vacancy, while the triangle denotes asubsurface defect characterized by three protruding O ions.PRL 106, 246801 (2011) P HY S I CA L R EV I EW LE T T E R Sweek ending17 JUNE 2011246801-2We find six configurations of the Ce3þ ion pair with defect-formation energies below 2.6 eV (Table I). The Ce3þpositions are hereby labeled in an nm scheme, withm beingthe cationic layer counted from the surface and n denotingthe shell of cations surrounding the vacancy. For example,21 describes a position in the surface Ce layer that is secondneighbor to a defect. The properties of the Ce3þ ions arenow governed by the number of O ligands, i.e., the coor-dination number (CN). A Ce3þ in the surface trilayer hasseven and six O2 neighbors if it sits in a second- (21 site)and first-neighbor position (11 site) to the defect, respec-tively, but eight in a bulk environment. There are twoconfigurations in Table I with equal CN for the two Ce3þions, namely, the 21  21 and 11  11 arrangements. Inboth cases, the two ions have filled f orbitals with the sameenergy as they experience the same chemical environment.For Ce3þ ions with different CNs, we find a splitting of thef orbitals due to the different distances from the vacancy.The splitting is of the order of 0.1 eV if the CN differs byone (as in 11  21 and 11  31 pairs) but increases to 0.5 Vfor a CNmismatch of two (Fig. 3). The latter case occurs inthe 11  42 pair that consists of a sixfold and an eightfoldcoordinated Ce3þ ion in the surface and subsurface layer,respectively. From those results, it becomes clear why theexperimental spectra show either a double- or a single-peakstructure. The two Ce3þ ions in spec 1 of Fig. 2(b) areapparently located in rather different environments, whilethe pair in spec 2 has a comparable CN. The localization ofthe Ce3þ ions with respect to the defect is not deducedfrom the splitting that is sensitive only to differences in thelocal environment.This information is, however, contained in empty-stateSTM topographies, which also reflect the local electronicstructure of the ceria surface. We have therefore simulatedTABLE I. Geometric structure, Ce3þ coordination numbers, vacancy formation energy (Edef)with respect to 12 O2, and splitting of occupied f states for different O vacancy=Ce3þ configu-rations. The ion with the larger CN has the lower f state energy.Ce3þconfigurationCe3þ-Ce3þdistance [A˚]Ce3þ-defectdistances [A˚]Coordinationnumbers of Ce3þ Edefð12 O2Þ½eVf statesplitting [eV]21  21 3.91 4.52; 4.52 7; 7 2.06a 0.0011  21 3.90 2.54; 4.53 6; 7 2.16a [11] 0.0511  11 4.17 2.54; 2.54 6; 6 2.22a [11] 0.0011  31 3.74 2.51; 5.95 6; 7 2.28a 0.1011  42 5.43 2.51; 7.08 6; 8 2.45 0.4722  42 3.90 4.47; 7.08 8; 8 2.59 0.06aSimilar defect-formation energies have been calculated for a 3 4 cell in Ref. [12].FIG. 3 (color online). Total and atom-projected density ofstates for (a) the energetically favored 21  21 and (b) the 11 42Ce3þ ion pair. Dark solid line: Ce3þ; bright solid line: Ce4þnext to an O vacancy; dashed line: other surface Ce4þ; anddotted line: total density of states. All spectra are normalizedwith respect to the number of Ce ions. The respective unit cellsand defect coordination spheres are shown in the insets.FIG. 4 (color online). DFT simulations of empty-state STMimages (0–2 eV). Small blue dots denote the Ce4þ ions, red dotsthe O2, and the red circle the O vacancy. While Ce3þ ions(large blue circles) are nearly invisible, Ce4þ ions next to the Odefect appear brighter. This results in different patterns depend-ing on the number of Ce4þ ions around the defect. The origin ofthe contrast is the larger expansion of the Ce4þ 4f orbitals inpresence of the vacancy (b).PRL 106, 246801 (2011) P HY S I CA L R EV I EW LE T T E R Sweek ending17 JUNE 2011246801-3STM images for bias voltages, where electron tunnelinginto the empty Ce 4f band is enabled (Fig. 4). The Ce3þ(large blue dots) are not visible in this case, because theirempty 4f states shift to higher energies due to the betterscreening of the Ce core potential in the electron-rich ions(Fig. 3). The Ce4þ ions, on the other hand, appear brightespecially if they are adjacent to an O defect (large greendots). The reason for the enhanced contrast of the latter hasbeen clarified with cluster calculations for the 21  21 ionpair, performed with PBE0 hybrid functionals and anelectrostatic embedding method [22]. Because of the miss-ing repulsive potential at the O vacancy site, the 4f orbitalslargely extend towards the empty space and partiallydelocalize over the Ce4þ ions surrounding the defect[Fig. 4(b)]. Tunneling into those expanded states givesrise to the bright appearance of the Ce4þ ions next to thevacancy. This interpretation allows us to draw conclusionson the position of the excess electrons with respect to the Odefect. If the vacancy is surrounded only by Ce4þ ions andthe Ce3þ occupy more distant coordination shells, a sym-metric triple protrusion becomes visible in empty-stateimages [Fig. 4(a)]. This triple signature is indeed observedin the STM, although its abundance is relatively small[Fig. 1(e)]. The most frequently seen defect type is thepaired protrusion, which is readily assigned to two Ce4þlocalized next to an O vacancy [Figs. 1(d) and 4(c)]. Thethird faint species marks a Ce3þ in a next-neighbor posi-tion to the defect, while the second Ce3þ cannot be iden-tified in the image. The last possible configuration reveals asingle maximum and reflects an O vacancy surrounded byone protruding Ce4þ and two faint Ce3þ ions [Fig. 4(d)].However, this configuration has not been detected in theexperiment.In conclusion, STM imaging and spectroscopy in com-bination with DFT calculations provide an unique insightinto the localization of the two excess electrons that remainin the CeO2 (111) surface upon O vacancy formation.Although the precise position of the two Ce3þ ions is notrevealed, we have demonstrated that they always sit indifferent coordination spheres around the defect, in agree-ment with earlier theoretical results [11]. The driving forcefor adopting such open configurations is the better abilityof the system to relax lattice strain induced by the morespacious Ce3þ ion compared to its Ce4þ counterpart. Ourresults bring the long-standing puzzle on electron localiza-tion in defective CeO2 surfaces closer to a final answer.We acknowledge financial support from the DFG withinthe Cluster of Excellence ‘‘UniCat’’ and the HLRN andthank J. Paier for various discussions.*Corresponding author.nilius@fhi-berlin.mpg.de†Present address: Institute of Catalysis and Petrochemistry-CSIC, Marie Curie 2, 28049 Madrid, Spain.‡Corresponding author.js@chemie.hu-berlin.de[1] A. Trovarelli, Catalysis by Ceria and Related Materials(World Scientific, Singapore, 2002).[2] Q. Fu, H. Saltsburg, and M. Flytzani-Stephanopoulos,Science 301, 935 (2003).[3] J. B. Park et al., J. Am. Chem. Soc. 132, 356 (2010).[4] N. V. Skorodumova, S. I. Simak, B. I. Lundqvist, I. A.Abrikosov, and B. Johansson, Phys. Rev. Lett. 89,166601 (2002).[5] A. 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Chem. Soc. 133, 3444(2011).[16] J.-L. Lu, H.-J. Gao, S. Shaikhutdinov, and H.-J. Freund,Surf. Sci. 600, 5004 (2006); Catal. Lett. 114, 8 (2007).[17] E. Wuilloud, B. Delley, W.D. Schneider, and Y. Baer,Phys. Rev. Lett. 53, 202 (1984).[18] N. Nilius, T.M. Wallis, and W. Ho, Phys. Rev. Lett. 90,046808 (2003).[19] M. Grobis, A. Wachowiak, R. Yamachika, and M. F.Crommie, Appl. Phys. Lett. 86, 204102 (2005).[20] We note that no spontaneous ceria reduction via the Rusupport takes place as shown in Ref. [15].[21] J. P. Perdew, J. A. Chevary, S. H. Vosko, K.A. Jackson,M. R. Pederson, D. J. Singh, and C. Fiolhais, Phys. Rev. B46, 6671 (1992); G. Kresse and J. Furthmu¨ller, Comput.Mater. Sci. 6, 15 (1996).[22] A.M. Burow, M. Sierka, J. Do¨bler, and J. Sauer, J. Chem.Phys. 130, 174710 (2009).PRL 106, 246801 (2011) P HY S I CA L R EV I EW LE T T E R Sweek ending17 JUNE 2011246801-4

Electron localization in defective ceria films: A study with scanning-tunneling microscopy and density-functional theory

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Electron localization in defective ceria films: A study with scanning-tunneling microscopy and density-functional theory

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