structure-dependent anchoring of organic molecules to atomically defined oxide surfaces

8/18/2019 Structure-Dependent Anchoring of Organic Molecules to Atomically Defined Oxide Surfaces

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& Carboxylic Acid Adsorption

Structure-Dependent Anchoring of Organic Molecules toAtomically Defined Oxide Surfaces: Phthalic Acid on Co3O4(111),

CoO(100), and CoO(111)Tao Xu,[a] Matthias Schwarz,[a] Kristin Werner,[a] Susanne Mohr,[a] Max Amende,[a] andJçrg Libuda*[a, b]

Abstract: We have performed a model study to explore theinfluence of surface structure on the anchoring of organicmolecules on oxide materials. Specifically, we have investi-gated the adsorption of phthalic acid (PA) on three different,well-ordered, and atomically defined cobalt oxide surfaces,

namely 1) Co3O4(111), 2) CoO(111), and 3) CoO(100) onIr(100). PA was deposited by physical vapor deposition(PVD). The formation of the PA films and interfacial reactionswere monitored in situ during growth by isothermal time-re-solved IR reflection absorption spectroscopy (TR-IRAS) underultrahigh vacuum (UHV) conditions. We observed a pro-nounced structure dependence on the three surfaces withthree distinctively different binding geometries and charac-teristic differences depending on the temperature and cov-erage. 1) PA initially binds to Co3O4(111) through the forma-tion of a chelating bis-carboxylate with the molecular planeoriented perpendicularly to the surface. Similar species wereobserved both at low temperature (130 K) and at room tem-

perature (300 K). With increasing exposure, chelating mono-carboxylates became more abundant and partially replacedthe bis-carboxylate. 2) PA binds to CoO(100) in the form of a bridging bis-carboxylate for low coverage. Upon pro-longed deposition of PA at low temperature, the bis-carbox-

ylates were converted into mono-carboxylate species. Incontrast, the bis-carboxylate layer was very stable at 300 K.3) For CoO(111) we observed a temperature-dependentchange in the adsorption mechanism. Although PA binds asa mono-carboxylate in a bridging bidentate fashion at lowtemperature (130 K), a strongly distorted bis-carboxylate wasformed at 300 K, possibly as a result of temperature-depen-dent restructuring of the surface. The results show that theadsorption geometry of PA depends on the atomic structureof the oxide surface. The structure dependence can be ra-

tionalized by the different arrangements of cobalt ions atthe three surfaces.

Introduction

Organic thin films on oxide surfaces have great potential inphotovoltaics[1] and molecular electronics.[2] For such applica-tions, the organic entities are often bound to the interfacethrough anchoring groups, for example, carboxylic acids, phos-phonates, or hydroxy groups. Knowledge of the associatedbinding mechanisms, kinetics, and energetics is essential forunderstanding the growth and structure formation processes

at organic oxide interfaces. They are the key to controlling the

growth and structure of the film and, therefore, their electronicand chemical properties (see, for example, ref. [3]).

Although the general behavior of these anchoring groups iswell understood, very little is known of the mechanisms of in-teraction on the atomic scale. What is the structure of thebonding site at the surface and how does it influence the sta-bility, geometry, energetics, and formation kinetics of the or-ganic film? It is the intrinsic complexity of such interfaces andthe associated experimental challenges that are the main rea-

sons for this lack of understanding.Atomically well-defined oxide surfaces can be prepared fol-

lowing a surface science approach under ultrahigh vacuum(UHV) conditions. A number of oxide materials and crystallo-graphic orientations are available either as a bulk single-crystalsurface or in the form of ordered thin films on metal singlecrystals (see, for example, ref. [4]). The thin-film approach pre-vents charging when working with photoelectron spectrosco-py or scanning tunneling microscopy (STM). In addition, itallows the stoichiometry and surface structure to be varied, atleast in some cases. In this work we used the thin-film ap-proach to study molecular anchoring as a function of the sur-

face structure.

[a] T. Xu, M. Schwarz, K. Werner, S. Mohr, M. Amende, Prof. J. LibudaLehrstuhl fr Physikalische Chemie II

Friedrich-Alexander-Universitt Erlangen-Nrnberg

Egerlandstraße 3, 91058 Erlangen (Germany)

Fax: ( +49)9131-8527308

E-mail: [email protected]

[b] Prof. J. LibudaErlangen Catalysis Resource Center

and Interdisciplinary Center Interface Controlled Processes

Friedrich-Alexander-Universitt Erlangen-Nrnberg

Egerlandstraße 3, 91058 Erlangen (Germany)

Supporting information for this article is available on the WWW under

http://dx.doi.org/10.1002/chem.201504810. Colored figures are available.

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Specifically, we scrutinized cobalt oxides, which exhibit inter-esting catalytic[5] and magnetic[6] properties. As a multivalentmetal, cobalt forms oxide phases with different stoichiometries.Of these, the most important are CoO and Co3O4. The structur-al and chemical properties of their surfaces depend on the di-rection of surface plane.[7] Most remarkably, differently orderedCoO and Co3O4 thin-film systems are available by growth onmetallic substrates, specifically on Ir(100). Many of these sys-tems have been structurally characterized in great detail byHeinz and Hammer, both by STM and low-energy electron dif-fraction (LEED) I-V analysis (see their review[8] and referencestherein). In this work, we scrutinized three different surfaces,Co3O4(111), CoO(111), and CoO(100). All the films were grownunder UHV by the reactive deposition of cobalt onto Ir(100) inan oxygen atmosphere and subsequent annealing.[8]

The atomic structures of the three surfaces are very differ-ent. The Co3O4(111) surface is very similar to that of a bulk-ter-minated spinel. The surface is terminated by coordinatively un-saturated cobalt ions occupying tetrahedral sites of the bulk

lattice.[9] Similarly to Co3O4(111), the surface of CoO(111) ispolar in nature. To compensate for the polarity, the structureswitches from the bulk rock-salt structure to a wurtzite struc-ture close to the surface.[10] The cobalt ions are embedded inthe subsurface and are less accessible to adsorbates. Finally,the structure of CoO(100) is identical to that of bulk rock salt.[8]

The surface is nonpolar and cobalt and oxygen ions are ex-posed in a similar fashion. This toolbox of structures makes it

possible to probe the role of surface structure on the interac-tions with anchoring groups.

Among the different anchors,[3e,11] the carboxylic acid groupis one of the most common. Self-assembled monolayers

(SAMs) of large organic molecules with carboxy anchoringgroups have been prepared on many oxide surfaces, mostlyfrom solution.[3f,12] After deprotonation the carboxylate attach-es to the surface metal cations in the form of a monodentate,bridging or chelating bidentate, or distorted bidentate species

(see Figure 1a). The stability of these species should criticallydepend on the arrangement and accessibility of the cations.However, experimental evidence of atomically defined surfacesto prove this hypothesis is pending, mainly because there arevery few ordered oxides that allow the surface structure to bechanged whilst keeping a high degree of ordering and controlat an atomic level.

In this work we studied the mechanism of binding to thecobalt oxide surfaces by using in situ time-resolved IR reflec-tion absorption spectroscopy (TR-IRAS). The method is particu-larly appropriate because it provides information on both theanchoring mechanism and the molecular orientation. Thelatter may be straightforwardly derived from the metal surfaceselection rule (MSSR),[13] which is valid for thin oxide films onmetallic substrates. We chose phthalic acid (PA, benzene-1,2-di-carboxylic acid, o-C6H4(COOH)2) as a test molecule. With twocarboxy anchoring groups, phthalic acid is particularly sensitiveto the surface structure: The two carboxy groups may eitherbind through a single carboxy group (mono-carboxylate) or

through both (bis-carboxylate) (see Figure 1b). In addition,each carboxy group may bind in different geometries, asshown in Figure 1a (see ref. [14] and references therein).

So far there have only been a few surface science studies of PA at oxide surfaces. The most detailed picture has been ob-tained for terephthalic acid (benzene-1,4-dicarboxylic acid, p-C4H6(COOH)2) on TiO2(110)-(11) (see, for example,ref. [3d,15]). Here, the molecule binds through a single biden-

tate carboxylate to two five-fold coordinated titanium ions.The molecules adopt a nearly upright geometry, but tilt toform dimers and the aromatic ring rotates with respect to thecarboxylate unit. STM and X-ray photoelectron spectroscopic

(XPS) analyses of isophthalic acid (benzene-1,3-dicarboxylicacid, m-C4H6(COOH)2) on copper-modified Au(111), on theother hand, suggest that both carboxy groups can bind to thesurface with the aromatic plane being forced to a smaller tilt-ing angle with respect to the surface. [16]

Figure 1. Possible binding geometries of a) a carboxylate group on a metaloxide surface and b) phthalic acid carboxylates on a metal oxide surface.

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In this work we have shown that PA can bind to cobaltoxides in many different geometries depending on the cover-age, temperature, and, most importantly, the surface structure.These findings underline the importance of surface structurefor the anchoring of molecular films onto oxide surfaces.

Experimental Section

All measurements were conducted in a UHV system (base pressureof 1.01010 mbar), which has been described in detail else-where.[17] The UHV system consists of a preparation chamber forsample cleaning and preparation, and a measurement chamber forthe IRAS experiments. The latter is equipped with an FTIR spec-trometer (Bruker VERTEX 80v) connected by differentially pumpedKBr windows. The spectra were recorded with a resolution of 4 cm1. During the adsorption measurements, IR spectra were con-tinuously acquired with at a rate of 30 s per spectrum.

Preparation of Co3O4(111)/Ir(100): A Co3O4(111) thin film was pre-pared by reactive deposition of cobalt atoms in an O 2 atmosphereaccording to the method described in the literature,[9] but with

small modifications of parameters such as the partial pressure of O2, annealing temperature, and the amount of cobalt. The detailedprocedure is as follows: First, the Ir(100) single crystal (MaTeck) wascleaned by cycles of Ar+ sputtering (1.8 keV, room temperature,1 h; Ar, Linde, 6.0) and annealing (11008C, 3 min) until a clear LEEDpattern (room temperature) of the Ir(100)-(51) reconstructed sur-face could be seen. Secondly, the Ir(100)-(51) surface was heatedto 1000 8C in 5108 mbar O2 (Linde, 5.0) for 3 min and cooled toroom temperature in O2 atmosphere. This led to an Ir(100)-(21)Oreconstructed surface that shows a clear (21) pattern in LEED.Starting with the Ir(100)-(21)O, cobalt was evaporated onto thesurface (cooled to temperatures below 0 8C) by using a commercialelectron beam evaporator (Focus EFM3, 2 mm cobalt rod, AlfaAesar 99,995%, Goodfellow 99.99%) in an atmosphere of 1.0

106 mbar O2 for 18 min. The evaporation rate of cobalt was deter-mined to be 2 min1 by means of a quartz crystal microbalance.After the growth, the film was annealed in O2 (1.010

6 mbar) at250 8C for 2 min and then under UHV at 430 8C for 5 min. The filmwas analyzed qualitatively by comparing the LEED I-V curves withthe literature.[9]

Preparation of CoO(111)/Ir(100): The CoO(111) film was preparedfrom a Co3O4(111) thin film prepared by the procedure describedabove. Simple heating of the Co3O4(111) film under UHV leads toloss of oxygen and the transformation to CoO(111). [10b] In our ex-periments, Co3O4(111) thin films (cobalt amount equivalent of 36 cobalt metal) were heated at 620 8C for 5 min. The quality of theCoO(111) thin film was verified by LEED.[10b]

Preparation of CoO(100)/Co/Ir(100): To obtain the CoO(100) thinfilm, a metastable Ir(100)-(1 1) reconstructed surface was first pre-pared. To this end, we started with a clean Ir(100)-(51) surfaceand prepared the Ir(100)-(21)O reconstructed surface followingthe procedure described above for the Co3O4(111) film. Then, thesample was heated at 275 8C in 1 107 mbar H2 (Linde, 5.3) for1 min and then heated under UHV at 275 8C for 1 min before cool-ing to room temperature. This procedure yielded an Ir(100)-(11)surface characterized by a sharp (1 1) pattern in LEED. Subse-quently, metallic cobalt was deposited onto the surface (50 8C) ata rate of 2 min1 for 6 min. After preparation of the cobalt bufferlayer, the sample was cooled to a temperatures below 50 8C.Then the cobalt was deposited reactively (2 min1) i n 4 107 mbar O2 for 3 min. Subsequently, the film was annealed at

1008C for 1 min to afford an ordered CoO(100) structure. For thick-

er and better-ordered CoO(100) films, a second reactive depositionof cobalt in O2 was conducted over 18 min while keeping thesample at a low temperature (


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To investigate the growth of the film in more detail, the in-tensities of the bands at 1419 and 1710 cm1 were analyzed asa function of deposition time. The result is shown in Figure 2c.Three regions of growth can be distinguished in this plot (Fig-ure 2c, I–III). In region I (0 to 10 min), the carboxylate band at

1419 cm

1

rapidly gains in intensity whereas the free acid

band at 1710 cm1 remains nearly invisible. In region II (10–30 min), the band at 1419 cm1 approaches saturation and theband at 1710 cm1 starts to grow more quickly. Finally, in re-gion III (from 30 min), the intensity of the band at1419 cm1 re-mains constant whereas the peak at 1710 cm1 continues toincrease at a constant rate.

We rationalize the origin of the three growth regions in Fig-ure 2c as follows. At low coverage, both carboxy groups aredeprotonated upon adsorption and anchor to the surface.Therefore no n(C=O) band at 1710 cm1 can be observed. In-stead, we observe the ns(OCO) band of the surface-bound car-boxylate dominating at 1419 cm1. This implies that the PA isbound to the Co3O4(111) surface through both carboxylategroups, that is, it forms a bis-carboxylate at the surface. In re-gion II, the intensity of the free carboxylic acid C=O band in-creases whereas the intensity of the bands associated with car-boxylate decreases. There are two possible scenarios that canexplain this behavior. Either there are intact PA molecules co-adsorbing as bis-carboxylate species are continuously formed

or, alternatively, PA may be adsorbed in the form of mono-car-boxylates. Later we will show that free carboxylic acid groupsare formed even at temperatures well above the desorptiontemperature of PA. This observation supports the formation of surface-anchored mono-carboxylates. We suggest that the lim-ited surface mobility of the bis-carboxylate leads to the forma-tion of a disordered monolayer in which isolated sites persist.PA can bind to the latter only as a mono-carboxylate. Finally,

a multilayer PA film grows in region III and no further surfacecarboxylates are formed.

Detailed information on the molecular orientation of the sur-face carboxylate can be derived from IRAS-active vibrations of

the hydrocarbon backbone. First, we observe that the intensityof the CH out-of-plane deformation mode at 743 cm1 is verylow for sub-monolayer coverage. If we take into account thefact that the dipole moment of this mode is perpendicular tothe molecular plane and use the MSSR, which states that onlythe component of the dynamic dipole moment perpendicularto the surface contributes to IR absorption, we can concludethat the aromatic ring is oriented nearly perpendicularly to thesurface. Secondly, we observe that the asymmetric stretchingmode nas(OCO) at around 1548 cm

1 is nearly invisible in thesub-monolayer region. This observation suggests that the car-boxylate units adsorb in a nearly symmetric bidentate geome-try with little distortion. This is in contrast to our previous

work on MgO(100) in which a substantial distortion of the bis-bidentate carboxylate was observed.[19] Note that the out-of-plane signal at 743 cm1 can be observed in the multilayer re-gion III, which indicates a smaller average tilting angle of thearomatic ring with respect to the surface in the multilayer.

A schematic model of the adsorption geometry is displayedin Figure 3a. We have used the surface structure of theCo3O4(111) films previously established by LEED I-V analysis.

[9]

The internal molecular bond lengths correspond to those inthe free molecule. As outlined above, the IRAS experiments in-dicate adsorption of a practically symmetric bis-carboxylatewith the aromatic ring oriented nearly perpendicularly to the

surface. Note that the Co3O4(111) surface is terminated by Co2+

Figure 2. a) IR spectra recorded during the deposition of PA ontoCo3O4(111)/Ir(100) for 60 min at 130 K. b) Comparison of a sub-monolayerspectrum (5 min of deposition), a multilayer spectrum (60 min of deposi-tion), and the spectrum of a thicker multilayer film (obtained at a higherdosing rate). c) Integrated peak intensities at 1419 cm1 (bound carboxylate)and 1710 cm1 (free carboxylic acid) as a function of deposition time.



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cations with a Co2+–Co2+ distance of about 5.7 . The distancebetween the two oxygen ions in the carboxylate group(COO), however, is only about 2.3 . This implies that a singlecarboxylic acid group cannot bridge two surface Co2+ ions.Therefore, the only symmetric adsorption geometry for PA that

is compatible with the structure of the Co3O4(111) surface isa chelating bis-carboxylate, as illustrated in Figure 3a.

PA adsorption on Co3O4(111)/Ir(100) at 300 K

Next, we investigated the growth of PA on Co3O4(111)/Ir(100)at 300 K by using the experimental procedure describedabove. The IR spectra recorded as a function of exposure timeare shown in Figure 4a, the spectra of samples with low andhigh PA coverage are compared in Figure 4b, and the intensi-ties of the symmetric OCO band of the bound carboxylate(1425 cm1) and the C=O stretching band of the free carboxylic

acid (1703 cm

1

) are plotted in Figure 4c.

In the IR spectra we observe one prominent peak at1425 cm1, which is the most intense feature over the com-

plete deposition time. Other peaks are observed at 1494, 1548,and 1703 cm1. Comparing the spectra in Figure 4b, we canconclude that the changes in relative intensities and positionsof the bands are moderate over the deposition time. The ex-ception is the broad band at around 1703 cm1, which is notpresent in the first spectrum, but is clearly seen in the last. InFigure 4c, we display the integrated peak areas of the bands at1425 and 1703 cm1 as a function of deposition time. We have

subdivided the deposition experiment into two regions. In thefirst, the region from the beginning to 30 min deposition time,we observe an increase in intensity and the saturation of thedominant band at 1425 cm1, whereas the peak at 1703 cm1

remains invisible. At deposition times exceeding 30 min, thepeak at 1425 cm1decreases slowly in intensity whereas thefeature at 1703 cm1 grows continuously.

On the basis of the much lower intensities of the bands andthe saturation behavior, we conclude that at 300 K onlya single monolayer of PA is adsorbed onto Co3O4(111)/Ir(100).For the peak assignments, refer to Table 1. Only slight frequen-cy shifts of the IR signals are observed in comparison with thedeposition at low temperature. The dominant bands are theOCO symmetric stretching mode at 1425 cm1, the antisym-metric OCO mode at 1548 cm1, the ring mode at 1494 cm1,and the C=O band of the free carboxylic acid group at1703 cm1.

In accord with the discussion above, we attribute the spec-tral features in region I to the formation of a symmetric bis-bi-dentate carboxylate bound to the oxide surface, similar to thatobserved for thin-film growth at low temperature. In region II,the appearance of the band at around 1700 cm1 indicates theformation of free carboxylic acid groups. As the experimentwas performed well above the multilayer desorption tempera-ture and the free carboxylic acid can be observed on the sur-face up to much higher temperatures (data not shown), we ex-clude the co-adsorption of unbound PA. Consequently, wehave assigned the free carboxylic acid band to the very slowformation of singly bound PA species, that is, a mono-carboxyl-

ate. The decrease in intensity of the band at 1425 cm

1

sug-

Table 1. Vibrational frequencies and assignments for phthalic acid on Co 3O4(111)/Ir(100), CoO(111)/Ir(100), and CoO(100)/Ir(100).[a]

CoO(100) CoO(111) Co3O4(111) Assignment[b] Comment

Monolayer [cm1] Multilayer [cm1] Monolayer [cm1] Multilayer [cm1] Monolayer [cm1] Multilayer [cm1]

743 743 743 g(CH)ring+g(CC)ring1262 1259 ns(COC) phthalic anhydride

1309 1312 1312 n(CO)+n(CC)+d(OH)

1415 1417 1416 1416 1419 1419 nsym(OCO)1497 1495 1495 1494 1494 n(CCring)+nCCring+n(CH)

1552 1550 1548 nasym(OCO)1585 1583 1583 n(CC)ring1602 1601 1603 n(CC)ring1712 1700 1712 1710 n(C=O)1776 1776 1775 nasym(C=O) phthal ic anhydride1793 1793 1790 d(CH)+d(CC)ring phthalic anhydride1853 1855 1853 nsym(C=O) phthal ic anhydride

[a] n=stretching, g=out-of-plane bending, and d= in-plane bending. [b] See ref. [18].

Figure 3. Schematic model of a PA molecule adsorbed onto a) Co 3O4(111)b) CoO(111), and c) CoO(100) surfaces at 130 K. The atoms of PA are depictedas follows: oxygen=white, carbon= light gray, and hydrogen=dark gray.

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gests that, upon continuous dosing, a portion of the bis-car-boxylates is slowly converted into the mono-carboxylate spe-cies. This interpretation is supported by the change in shapeof the antisymmetric OCO band at 1548 cm1 in region II. Wesuggest that the formation of two singly bound mono-carbox-ylates is energetically preferred at certain surface binding sites

because of the strain that is induced when the PA binds

through two anchoring groups. Finally, we note that withinthe signal-to-noise ratio of the experiment, no out-of-planepeak at 743 cm1 is observed, which indicates that the PA isadsorbed with the molecular plane oriented nearly perpendic-ularly to the surface.

PA adsorption on CoO(111)/Ir(100) at 130 K

Next, we investigated the growth of PA on CoO(111)/Ir(100) at130 K by using the experimental procedures described above.The IR spectra as a function of exposure time are shown in Fig-ure 5a, the IR spectra recorded for sub-monolayer, monolayer,and multilayer coverages are compared in Figure 5b, and theintensities of the carboxylate (1416 cm1) and free carboxylicacid (1712 cm1) bands as a function of time are plotted in Fig-ure 5c.

At low PA exposure, the most prominent bands appear at1416, 1495, 1550, and 1700 cm1. For longer deposition times,further peaks appear at 1855, 1793, 1776, and 1312 cm1. For

a detailed assignment, please refer to Table 1 and the discus-sions above.

Again, we have subdivided the deposition of PA into two re-gions of growth (see Figure 5c). In region I, from the beginningof the deposition to approximately 30 min deposition time,the symmetric OCO stretching band at 1416 cm1 increases inintensity and finally approaches saturation. At the same time,the C=O band of the free carboxylic acid groups at 1712 cm1

increases almost linearly. This behavior is very different fromthat observed for growth on Co3O4(111); it implies that rightfrom the beginning of the deposition, the surface concentra-tions of both the free and bound carboxylic acid groups in-

crease linearly, which suggests the formation of mono-carboxy-lates even at low coverages. With increasing coverage, the in-tensity of the carboxylate band grows more slowly, which indi-cates an increasing fraction of intact PA molecules. Finally, inregion II (>30 min), no further surface-bound carboxylates areformed and a multilayer film of intact PA starts to grow.

The atomic structure of the CoO(111) surface is well knownfrom previous LEED I-V analyses, in which it was shown thatthe polar surface undergoes reconstruction to form a wurtzite-type top layer (see Figure 3b[10b]). The Co2+ ions at the surfaceare arranged in a hexagonal lattice and are separated by a dis-tance of 3.0 , which is similar to that of bulk rock salt. TheseCo2+ ions are located, however, slightly below the terminating

layer of O2

ions. From a steric perspective this may restrictaccess to the Co2+ ions and therefore limit the range of possi-ble binding geometries of the carboxylate groups. We proposethat this steric restriction prevents the PA from bindingthrough both carboxylate units. For the adsorbed carboxylate,both adsorption geometries, bridging and chelating, would bepossible from a geometric point of view and both geometrieswould also be compatible with the IR data. Previous experi-mental and theoretical studies of different carboxylic acids onmetal oxide surfaces showed, however, that the bridging car-boxylate binding mode is typically preferred from an energeticpoint of view (see, for example, formic acid on ZnO(101̄0),

rutile TiO2(110),[21]

anatase TiO2(110),[22]

and CeO2(111),[23]

and

Figure 4. a) IR spectra recorded during the adsorption and growth of PA onCo3O4(111)/Ir(100) at 300 K. b) Comparison of the IR spectra recorded after1 and 60 min of deposition. c) Integrated peak intensities at 1425 cm 1

(bound carboxylate) and 1703 cm1 (free carboxylic acid) as a function of deposition time.



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benzoic acid on rutile TiO2(110)[15] or MgO(100)[19]). Therefore

we propose that at 130 K, PA adsorbs on CoO(111) as a bridgingmono-carboxylate. Some possible adsorption geometries areschematically depicted in Figure 3b. It is worth noting that at300 K a more complex and roughened (

p 3

p 3)R 308 struc-

ture is formed.[10b] Nevertheless, Figure 3b should convey the

image of the bonding of a mono-carboxylate on the surface.


Next, we explored the growth of PA on CoO(111)/Ir(100) at300 K by using the experimental procedure described above.The IR spectra as a function of exposure time are shown in Fig-ure 6a, the IRAS spectra recorded at low and high PA coverageare compared in Figure 6 b, and the intensities of the symmet-

Figure 5. a) IR spectra recorded during the deposition of PA onto CoO(111)/Ir(100) for 60 min at 130 K. b) Comparison of the sub-monolayer spectrum(2 min of deposition), a multilayer spectrum (60 min of deposition), and thespectrum of a thicker multilayer film (obtained at higher dosing rate). c) Inte-grated peak intensities at 1416 cm1 (bound carboxylate) and 1712 cm1

(free carboxylic acid) as a function of deposition time.

Figure 6. a) IR spectra recorded during the adsorption and growth of PA onCoO(111)/Ir(100) at 300 K. b) Comparison of the IR spectra recorded after 2and 60 min of deposition. c) Integrated peak intensities at 1428 cm1 (boundcarboxylate) and 1700 cm1 (free carboxylic acid) as a function of deposition

time.



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ric OCO band of the bound carboxylate (1428 cm1) and theC=O stretching band of the free carboxylic acid (1700 cm1) asa function of time are plotted in Figure 6c.

As discussed above, no PA multilayer was formed at 300 K and adsorption occurs in the monolayer region only. We ob-serve the symmetric OCO band of the bound carboxylate peak at 1428 cm1 and this band is the most prominent throughoutthe deposition experiment. Smaller bands are observed at1493, 1550, and 1700 cm1. As discussed above (see Table 1),these features have been assigned to the ring mode of PA(1493 cm1), the antisymmetric OCO mode (1550 cm1) of thebound carboxylate, and the C=O band (1700 cm1) of the freecarboxylic acid group.

As indicated in Figure 6c, we have subdivided the experi-ment into two regions of growth. In region I, up to a depositiontime of approximately 45 min, the symmetric OCO mode(1428 cm1) increases in intensity and finally saturates. Forlonger PA exposures, no further changes are observed. Inter-estingly, the free carboxylic acid band at 1700 cm1 remains

very weak throughout the whole experiment, which clearly in-dicates that the PA is adsorbed as a bis-carboxylate. This be-havior is in sharp contrast to that observed at 130 K; at thistemperature singly bound mono-carboxylates are formed atlow coverage. At 300 K, the band of the free carboxylic acidgroup at 1700 cm1 is observed at long PA exposure only, butremains very weak. We associate this weak band with the for-mation of a small fraction of mono-carboxylates at defects and

vacancies in the adsorbate layer. The out-of-plane CH deforma-tion mode at 743 cm1 is practically absent, which indicatesthat the aromatic plane is oriented nearly perpendicularly tothe surface. Of special interest is the asymmetric OCO peak at

1550 cm

1

. Note that this band is absent in perfectly symmetriccarboxylates and that it increases in intensity with increasingasymmetry of the OCO bridge. The relatively high intensity of this feature indicates the formation of a strongly distorted car-boxylate species.

The most surprising finding for this system is the exclusiveformation of the bis-carboxylate at room temperature, whereasat low temperature the mono-carboxylate is preferred. The de-pendence on temperature implies that the bis-carboxylate isformed in an activated process. Although the nature of thisprocess is not understood yet, we may speculate on its originbased on the structural properties of the CoO(111) film. In theprevious section, we argued that the formation of the mono-

carboxylate is driven by steric hindrance due to the presenceof oxygen ions in the top layer. Following this argument, wepropose that the activated formation of the bis-carboxylate at300 K is associated with a restructuring of the surface that fos-ters the accessibility of the second carboxylate group. This hy-pothesis is supported by the fact that the CoO(111) film isknown to undergo a structural phase transition close to roomtemperature.[10b] The phase transition leads to a roughenedsurface on which we would expect the formation of a morestrongly distorted bridging carboxylates species, as is indeedobserved in the experiment.


Next, we investigated the growth of PA on CoO(100)/Ir(100) at130 K by using the experimental procedures described above.It needs to be stated again that the evaporation rate usedhere was about 1.5 times higher than that used in the previousexperiments. The IR spectra are shown as a function of expo-sure time in Figure 7a, the IR spectra recorded of samples withsub-monolayer and multilayer coverage are compared in Fig-ure 7b, and the intensities of the carboxylate (1417 cm1) andfree carboxylic acid (1712 cm1) bands as a function of timeare plotted in Figure 7c.

For the peak assignments, again refer to Table 1. Similarly, asdiscussed for PA deposition on Co3O4/Ir(100) and CoO(111)/Ir(100) at low temperature, we observe the characteristic fea-tures of free PA, surface-bound carboxylates, and traces of phthalic acid anhydride. We have subdivided the growth intothree regions (Figure 7). In region I (up to 15 min), a strongsymmetric OCO peak at 1417 cm1 appears and finally satu-

rates, whereas the carboxylic acid band at 1712 cm

1 is practi-cally absent. This observation indicates a strong preference forthe formation of bis-carboxylates. The corresponding spectrumin Figure 7b (upper trace) is particularly simple. Neither theasymmetric OCO stretching mode of the carboxylate nor theout-of-plane CH deformation mode of the organic ring is ob-served. This shows that the PA forms a nearly perfectly sym-metric bridging bis-carboxylate adsorbed with the aromatic

ring oriented perpendicularly to the surface.In region II (deposition times of 15–30 min), the symmetric

OCO peak decreases whereas the C=O band of the free car-boxylic acid increases with a similar slope. This suggests the

formation of a mono-carboxylate. The substantial loss of inten-sity of the OCO band in this region can be explained either bythe conversion of bis-carboxylates into mono-carboxylates orby the dynamic coupling of the surface-bound PA to PA in themultilayer. As we do not observe similar effects for the othersurfaces, however, we conclude that a substantial fraction of the bis-carboxylates is converted into singly bound species. Inregion III (beyond 30 min), we observe the linear growth of a PA multilayer film. Some loss of the perpendicular alignmentin the growing multilayer film is indicated by the small out-of-plane peak at 743 cm1 at higher coverage (see Figure 7b).

The adsorption geometry of PA on CoO(100) at low cover-age and low temperature is schematically shown in Figure 3c.

CoO(100) has a nonpolar surface terminated by both Co2+ andO2 ions.[8] In contrast to Co3O4(111), the distance between thecobalt ions at the surface is relatively small (3.0 ), which per-mits the formation of a bridging carboxylate. In contrast toCoO(111), the Co2+ ions are easily accessible from a stericpoint of view. Thus, there is little structural restriction of theadsorption of anchoring molecules and the energetically mostfavorable adsorption geometry, the bridging bis-bidentatephthalate, can be realized. At high PA coverage, the formationof a densely packed layer of mono-carboxylates becomes morefavorable and, as a result, one carboxylate group again de-taches from the surface.



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Finally, we investigated the growth of PA on CoO(100)/Ir(100)at 300 K by using the experimental procedures describedabove. The IR spectra as a function of exposure time areshown in Figure 8a, the IR spectra of sub-monolayer and multi-layer coverage are compared in Figure 8b, and the intensities

of the carboxylate (1429 cm

1

) and free carboxylic acid

(1700 cm1) bands are plotted as a function of time in Fig-ure 8c.

The overall band intensities observed in Figure 8 a are com-patible with the formation of a monolayer. In the first regionof growth (0–25 min), the intensity of the symmetric OCOband (1429 cm1) increases and finally saturates. In this region

the C=O band of the free carboxylic acid (1700 cm

1

) peak is

Figure 7. a) IR spectra recorded during the deposition of PA on CoO(100)/Ir(100) for 60 min at 130 K. b) Comparison of a sub-monolayer spectrum(3 min of deposition) and a multilayer spectrum (60 min of deposition). c) In-tegrated peak intensities at 1417 cm1 (bound carboxylate) and1712 cm1band (free carboxylic acid) as a function of deposition time.

Figure 8. a) IR spectra recorded during the adsorption and growth of PA onCoO(100)/Ir(100) at 300 K. b) Comparison of the IR spectra recorded after1 and 60 min of deposition. c) Integrated peak intensities at 1429 cm 1

(bound carboxylate) and 1700 cm1 (free carboxylic acid) as a function of deposition time.



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nearly invisible, which indicates that the PA adsorbs nearly ex-clusively in the form of a bis-carboxylate. The symmetric OCOstretching band of the carboxylate at 1429 cm1 is the mostprominent feature throughout the deposition (see Table 1 fordetails). As for the deposition at 130 K, the intensities of thecarboxylate mode at 1547 cm1 and the out-of-plane CH defor-mation mode remain weak, which indicates the formation of a rather symmetric bridging bis-carboxylate perpendicular tothe surface.

In region II (from 25 min), no further change is observed inthe intensity of the symmetric OCO mode. However, a smallC=O peak originating from free carboxylic acid groups growsin a stepwise fashion for deposition times between 25 and35 min. The appearance of the C=O peak indicates the forma-tion of a small fraction of mono-carboxylate. We assign thiseffect to adsorption at defect sites on the surface or in the ad-sorbate layer; the two carboxylic acid groups of PA are unableto bind under these circumstances. Note, we do not observethe conversion of the bis-carboxylate into the mono-carboxyl-

ate, as was found, for example, for PA on Co3O4(111) orCoO(100) at low temperature. We suggest that the strongbinding of PA to the oxide surface in the form of a bridgingbis-carboxylate is also associated with a higher activation barri-er for its conversion into the mono-carboxylate. As a result, thedesorption of molecularly adsorbed PA is preferred and the for-mation of the mono-carboxylate is kinetically hindered.

Discussion of the structure dependency of PA adsorption on

cobalt oxide surfaces

Finally, we compare the interaction and growth of PA on the

three different cobalt oxide surfaces and discuss the possiblerole of the surface structure of Co3O4(111), CoO(111), andCoO(100). A direct comparison of selected spectra at low andhigh coverages and of the evolution of intensity of the carbox-ylate and acid bands are shown for the three surfaces inFigure 9. The integrated peak areas of the carboxylate ( I

ns(OCO))and acid (I

n(C=O)) bands as well as the intensity ratio I ns(OCO)/(I n(C=

O)+ I ns(OCO)) are shown in Figure 9b as a function of exposuretime. Because of the different amplitudes and orientations of the dynamic dipole moments, the intensity ratio does notquantitatively reflect the relative abundance of the species,but allows us to follow the coverage-dependent trends ina qualitative fashion.

At low temperature we can clearly differentiate betweenmono- and multilayer growth (Figure 9a). For both Co3O4(111)and CoO(100) we observe the formation of a bis-carboxylate atlow coverage, that is, a phthalate species that binds to the sur-face through both carboxylic acid groups. For both surfaces,the low intensity of the asymmetric OCO stretching mode indi-cates that the carboxylate groups are practically undistorted,and the low intensity of the CH out-of-plane deformationmode indicates that the aromatic ring is oriented nearly per-pendicular to the surface. Also, the band positions are verysimilar. Inspection of the surface structure suggests, however,that the binding geometries must be quite different: For

Co3O4(111) with a large Co–Co distance of 5.7 , a chelating

bis-carboxylate must be formed, whereas the formation of a bridging bis-carboxylate is possible for the CoO(100) surface,which has a Co–Co distance of only 3.0 .

For longer PA exposure times we observe the competingformation of mono-carboxylates and the conversion of bis-car-boxylates into mono-carboxylates (Figure 9b). The latter pro-cess is most pronounced on CoO(100). We speculate that thehigh density of surface Co2+ ions and their good accessibilityprovide the flexibility that is required to facilitate this restruc-

turing process.A completely different adsorption behavior is observed for

CoO(111). In this case the formation of mono-carboxylates isobserved from the lowest coverages. This is reflected by theI ns(OCO)/(I ns(OCO)+ I ns(OCO)) ratio at low coverage, which is much

smaller than for the other two surfaces (Figure 9b). Althoughthe density of the Co2+ ions is very similar in CoO(100) andCoO(111), the main difference between the two surfaces is thefact that in the latter the Co2+ ions are located slightly belowthe terminating layer of O2 ions. We assume that the oxygenions restrict the accessibility of the surface cobalt ions, therebyenforcing a molecular orientation that does not permit the

second carboxylic acid group to interact with the surface. Also,

Figure 9. a) Comparison of the IR spectra of a sub-monolayer and multilayerof PA on Co3O4(111), CoO(111), and CoO(100) at 130 K. b) Comparison of theintegrated peak intensities of the carboxylate band (I

ns(OCO)), the free acidband (I

n(C=O)), the sum of the carboxylate and free acid, and the intensity

ratio I ns(OCO)/(I n(C=O)+ I ns(OCO)).



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the asymmetric stretching mode of the carboxylate is found tobe more pronounced for CoO(111), which indicates that a sub-stantially distorted bridging carboxylate is formed.

Clear structure dependencies are also observed for the ad-sorption of PA at room temperature (Figure 10). At 300 K, onlychemisorbed monolayer films are stable. Interestingly, thestructure dependencies and interaction mechanisms are differ-ent to those at low temperature. On Co3O4(111) and CoO(100),bis-carboxylates are formed exclusively during the initial stagesof deposition. Based on the surface structures of the films, weagain assume the formation of chelating species on Co3O4(111)and of bridging species on CoO(100). Upon prolonged dosing,the bis-carboxylates on Co3O4(111) are slowly converted intomono-carboxylates. The mechanism of this process is not yetclear. A possible explanation involves the slow desorption of more weakly bound chelating species, the re-adsorption of PA,and, finally, the formation of mono-carboxylates at vacancies inthe PA film. On the CoO(100) surface, on the other hand,a very stable bridging bis-carboxylate is formed. Even upon

prolonged PA exposure, only a very small fraction of mono-car-boxylate species is formed on this film, which we attribute toadsorption at defects in the film, which do not accommodatedoubly linked bis-carboxylates.

The most surprising observation is the temperature-depen-dent change in the anchoring mechanism observed for theCoO(111) surface. Although we find mono-carboxylates at lowtemperature, only bis-carboxylates are formed at room temper-ature. This observation implies that there is a substantial acti-vation barrier for the formation of the bis-carboxylate species.As we would not expect such a high barrier for a simple de-protonation reaction, it appears likely that bis-carboxylate for-mation requires a restructuring of the surface, thereby makingthe Co2+ ions more easily accessible for the carboxylategroups. Indeed, a structural phase transition has been ob-served in CoO(111) and the high intensity of the asymmetricOCO mode supports the proposal of a strongly distorted bridg-ing carboxylate on a more corrugated surface.

Conclusion

We have studied the adsorption and growth of phthalic acid

deposited by physical vapor deposition (PVD) onto well-or-dered Co3O4(111), CoO(111), and CoO(100) thin films on Ir(100).

We have monitored the interfacial reactions in situ during filmgrowth by using time-resolved IRAS under UHV conditions.The observations are schematically illustrated in Figure 11 andare summarized as follows.

PA on Co3O4(111)

At low temperature (130 K) and low coverage, PA binds to theCo3O4(111) surface forming a chelating bis-carboxylate with the

molecular plane oriented perpendicularly to the surface. Withincreasing coverage, the formation of a chelating mono-car-boxylate is preferred, before multilayers of physisorbed PAstart to grow on top of the chemisorbed monolayer. UponPVD of PA, a small fraction of phthalic acid anhydride isformed that is co-deposited on all three oxide surfaces.

At 300 K, only a chemisorbed monolayer of PA is stable onCo3O4(111). At low exposure the PA binds to the oxide surfaceas a chelating bis-carboxylate. On continuing exposure, thechelating bis-carboxylate is slowly replaced by an increasingfraction of chelating mono-carboxylate.

PA on CoO(100)

At low temperature (130 K) and low coverage, PA binds to theCoO(100) surface in a bridging bis-carboxylate mode with themolecular plane oriented perpendicularly to the surface. Withincreasing coverage, the bridging bis-carboxylate species isconverted into a bridging mono-carboxylate before finally thePA multilayer starts to grow. Upon deposition at 300 K, a verystable monolayer of bridging bis-carboxylates is formed. Evenupon prolonged exposure to PA, only a small fraction of mono-carboxylate is co-adsorbed, possibly onto defect sites in

the film and on the surface.

Figure 10. a) Comparison of the IR spectra of a sub-monolayer and saturatedmonolayer of PA on Co3O4(111), CoO(111), and CoO(100) at 300 K. b) Com-parison of the integrated peak intensities of the carboxylate band (I

ns(OCO)),the free acid band (I

n(C=O)), the sum of the carboxylate and free acid, and the

intensity ratio I ns(OCO)/(I n(C=O)+ I ns(OCO)).



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PA on CoO(111)

For PA on CoO(111), a temperature-dependent change in theadsorption mechanism was observed. At low temperature(130 K) and low coverage, PA binds to CoO(111) in the form of a mono-carboxylate with a distorted bridging geometry and itsmolecular plane oriented nearly perpendicular to the surface.At 300 K, the interaction mechanism changes and PA is ad-sorbed in the form of a strongly distorted bis-carboxylate. Weattribute this effect to a temperature-dependent restructuring

of the surface. The bis-carboxylate film is stable and only smallfractions of mono-carboxylates are co-adsorbed upon pro-longed exposure to PA.

These results show that the adsorption geometry and ad-sorption kinetics of PA on cobalt oxide are strongly dependentupon the surface structure. This structure dependence can berationalized on the basis of the density and accessibility of thecations at the oxide surface. The results show that the struc-ture of the surface is important for the anchoring of molecularfilms. Both a detailed understanding of the interaction mecha-nisms and a high level of control of the structure of the surfaceis essential to tailor hybrid interfaces between organic films

and oxide surfaces.

Acknowledgements

This project was financially supported by the Deutsche For-schungsgemeinschaft (DFG) within the Research Unit FOR1878 “funCOS – Functional Molecular Structures on ComplexOxide Surfaces”. Additional support is acknowledged from theExcellence Cluster “Engineering of Advanced Materials” withinthe framework of the excellence initiative. The authors also ac-knowledge additional support by the Deutsche Forschungsge-

meinschaft within the DACH Project “COMCAT”, the EuropeanCommission (“chipCAT”, Grant Agreement No. 310191), andCOST Action CM1104 “Reducible oxide chemistry, structure andfunctions”. T.X. gratefully acknowledges support by way of a Ph. D. grant from the China Scholarship Council (CSC).

Keywords: adsorption · carboxylic acids · cobalt · IRspectroscopy · thin films

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Figure 11. Schematic summary of binding geometries of PA on the three cobalt oxide surfaces at different temperatures and coverages.



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14/14

FULL PAPER

& Carboxylic Acid Adsorption

T. Xu, M. Schwarz, K. Werner, S. Mohr,

M. Amende, J. Libuda*

&&–&&

Structure-Dependent Anchoring of

Organic Molecules to Atomically

Defined Oxide Surfaces: Phthalic Acid

on Co3O4(111), CoO(100), and

CoO(111)

Surface structure matters : The adsorp-tion geometry of phthalic acid is depen-dent on the atomic structure of thecobalt oxide surface. The structure de-pendence can be rationalized by the

different arrangements of the cobaltions at the three surfaces (see figure;IRAS= IR reflection absorption spectros-copy).



Full Paper

structure-dependent anchoring of organic molecules to atomically defined oxide surfaces

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