Phosphonic Acids on an Atomically Defined Oxide ... - ACS Publications

Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 1937−1943

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Phosphonic Acids on an Atomically Defined Oxide Surface: The Binding Motif Changes with Surface Coverage Christian Schuschke,† Matthias Schwarz,† Chantal Hohner,† Thais N. Silva,† Lukas Fromm,‡ Tibor Döpper,‡ Andreas Görling,‡,§ and Jörg Libuda*,†,§ †

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany ‡ Lehrstuhl für Theoretische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany § Erlangen Catalysis Resource Center and Interdisciplinary Center for Interface Controlled Processes, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany S Supporting Information *

ABSTRACT: We have studied the anchoring mechanism of a phosphonic acid on an atomically defined oxide surface. Using time-resolved infrared reflection absorption spectroscopy, we investigated the reaction of deuterated phenylphosphonic acid (DPPA, C6H5PO3D2) with an atomically defined Co3O4(111) surface in situ during film growth by physical vapor deposition. We show that the binding motif of the phosphonate anchor group changes as a function of coverage. At low coverage, DPPA binds in the form of a chelating tridentate phosphonate, while a transition to a chelating bidentate occurs close to monolayer saturation coverage. However, the coverage-dependent change in the binding motif is not associated with a major change of the molecular orientation, suggesting that the rigid phosphonate linker always maintains the DPPA in a strongly tilted orientation irrespective of the surface coverage. unctional organic films on oxides are at the heart of emerging technologies, with applications in molecular electronics, solar energy conversion, catalysis, sensors, and biointerface engineering.1−5 In most cases, the performance of organic/oxide hybrid materials, i.e., the efficiency and stability of solar cells or the reliability and power consumption of molecular electronic devices, depends on the properties of the interface. Usually, the formation of this interface is mediated by an anchor group, which controls ordering of the organic film, packing density, molecular orientation, electronic coupling, and stability.4,6 The most common linker for oxides is the carboxylate group,4,7 but there are many alternatives for specific applications such as phosphonates,8 catechols,9 or hydroxamates.10 Phosphonate anchors are of particular interest as they provide much higher stability than carboxylic acids in aqueous media.11,12 In particular, surface phosphonates are stable even under alkaline conditions, thus opening new fields of applications, e.g., in ion-exchange materials, catalysts, and sensors.12,13 Surprisingly, very little is known about the anchoring mechanisms and binding motifs of phosphonates on oxide surfaces.8,11 The reason is the complex surface chemistry of phosphonic acids and the fact that nearly all studies have been performed under ambient conditions with little control over the atomic structure of the oxide surface. A variety of binding motifs has been proposed ranging from monodentate via bidentates to tridentates (with different degrees of deprotonation), often with contradictory or unclear results.14−17 To understand the anchoring mechanism at the

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fundamental level, it is essential to use atomically controlled surfaces and study the anchoring reaction under ultraclean conditions, i.e., in ultrahigh vacuum (UHV). Surprisingly, very few surface science experiments have been performed on phosphonates to date, although it is possible to prepare phosphonate films in situ under UHV conditions. More than 10 years ago, Tsud and Yoshitake showed that phenylphosphonic acid (PPA) films can be deposited by physical vapor deposition (PVD).18 More recently, Wagstaffe et al. investigated the adsorption of PPA on anatase TiO2(101) by near-edge X-ray adsorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) and proposed a transition from a bidentate to a mixed monodentate−bidentate structure.19 Ostapenko et al. studied the adsorption PPA monolayers on ZnO surfaces by NEXAFS, XPS, temperature-programmed desorption (TPD), and density functional theory (DFT) and suggested adsorption in the form of bidentate and tridentate phosphonates.20 For the determination of the binding motifs, vibrational spectroscopy is the experimental method that typically provides the highest chemical sensitivity.21 Very surprisingly, however, no vibrational spectroscopy study has been performed to date on the adsorption of phosphonic acids on an atomically defined oxide surface. In this work, we present the first study of this type. We investigate anchoring of (deuterated) PPA in situ Received: March 2, 2018 Accepted: March 29, 2018 Published: March 29, 2018 1937

DOI: 10.1021/acs.jpclett.8b00668 J. Phys. Chem. Lett. 2018, 9, 1937−1943

Letter

The Journal of Physical Chemistry Letters

Figure 1. (a) Comparison of the IR spectra of PPA (recorded in KBr in transmission geometry, top spectrum) and a DPPA multilayer film prepared by PVD (recorded in reflection geometry after deposition on Co3O4/Ir(100) at 180 K, middle spectrum). The lower trace shows the IR spectrum of an isolated DPPA molecule as calculated by DFT (see text for details). (b) Selected vibrational modes from DFT, visualized by QVibeplot.24

during the film growth on a well-ordered cobalt oxide surface and show that the binding motif changes with coverage, an observation which may explain some of the contradicting results in the literature. Surprisingly, the molecular orientation of the anchored films hardly changes with the binding motif,

which suggests that the phosphonate is a rather rigid anchor that permits only slight changes of the adsorption geometry. In our study, we used a well-ordered Co3O4(111) film which was grown on an Ir(100) single-crystal surface.22,23 DPPA was deposited by PVD under UHV conditions while the anchoring 1938


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Figure 2. (a) IR spectra of DPPA recorded during deposition onto Co3O4(111)/Ir(100) at 180 K. (b) Selected spectra in the limit of low coverage (top), at monolayer coverage (middle), and of the multilayer (bottom). (c) Integrated peak areas of selected peaks marked in panel b. (d) Schematic representation of the in situ TR-IRAS experiment.

spectrum calculated by density functional theory (DFT) in the gas phase (see the Supporting Information). We find that several calculated bands are in good agreement with the experimental data, whereas some bands between 950 and 1200 cm−1 show deviations in position and width. These disagreements are due to the formation of hydrogen-bonded networks, which are not accounted for in the calculation. Taking this effect into account and using the analysis from previous work,25,26 we can assign the bands observed in the experimental spectrum (see the Supporting Information). Here, we focus on a few selected bands which are of relevance for the analysis of the binding motifs. The intense feature in IRAS around 1175 cm−1 is attributed to a superposition of the C−P stretching mode ν(CP) at 1159 cm−1 and the P=O stretching mode ν(P=O) at 1190 cm−1. The signals at 1029 and 946 cm−1 correspond to the symmetrically and antisymmetrically coupled P−O stretching modes νas(P(OD)2) and νs(P(OD)2), while a broad feature at 2140 cm−1 originates from OD stretching modes (ν(OD)) of the phosphonic acid group. The band at 716 cm−1 is assigned to

reaction was monitored in situ during the growth process by time-resolved infrared reflection absorption spectroscopy (TRIRAS). PVD of Organic Phosphonates in UHV. For an in situ growth study in UHV, it is essential that the organic compound can be deposited by PVD without decomposition. To verify whether this is the case, we prepared a multilayer film (10 monolayers, ML) of DPPA on Co3O4(111) at 180 K by PVD and compared the corresponding IR spectrum recorded in reflection geometry to a transmission spectrum recorded in KBr (Figure 1a). Both spectra are in good agreement concerning band intensities and peak positions. Only in the spectral region between 1050 and 1250 cm−1 we observe deviations, which are attributed to differences in Hbonding between the phosphonic acid groups in the multilayer film and in the KBr pellet. We conclude that the DPPA can indeed be deposited by PVD in UHV, without any detectable contamination. Assignment of the Vibrational Bands. In Figure 1a, we also show a comparison between the IR spectra of DPPA and the 1939


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Figure 3. (a) IR spectra of DPPA recorded during deposition onto Co3O4(111)/Ir(100) at 380 K. (b) Selected IR spectra in the limit of low coverage (top) and at full monolayer coverage (bottom). (c) Integrated peak area of selected peaks marked in panel b. (d) Schematic representation of the binding motif for DPPA changing as a function of coverage.

the CH out-of-plane deformation mode, γ(CH), whereas the two smaller signals at 1441 and 1596 cm−1 are characteristic of CC stretching modes of the phenyl ring, νas(CC) and νs(CC). To facilitate the analysis, we visualized selected modes from the DFT calculations using QVibeplot24 (Figure 1b). Briefly, bond stretching is represented by lines, bending contributions by arcs, and torsions by curves, with the phase indicated by different colors (see ref 24 for details). The ν(P=O), νas(P(OD)2), and νs(P(OD)2) bands are indicators for chemical interactions with the surface as they are sensitive to deprotonation and adsorption. The ν(CP), νas(CC), and γ(CH) modes, on the other hand, are polarized nearly along the principal axes of the phenyl unit. To a first approximation, their dynamic dipole moments form an orthogonal basis set which allows us to obtain information on the molecular orientation.27 This analysis is based on the metal surface selection rule (MSSR), stating that only the component of the dynamic dipole moment perpendicular to the surface can be observed in IRAS.28 The MSSR is also valid for thin oxide films on metal substrates. Therefore, changes in relative band

intensities indicate changes in the molecular orientation on the surface. In the analysis, the transmission spectra with randomly oriented molecules can be used as a reference to determine the relative magnitude of the dynamic dipole moments. The comparison of IRAS in the multilayer and transmission IR (Figure 1a) shows that the intensity ratios are similar, indicating a nearly random orientation of DPPA in the frozen multilayer. Anchoring and Film Growth on Co3O4(111). In the next step, we studied the anchoring and film growth of DPPA on Co3O4(111) by in situ TR-IRAS. The Co3O4(111) surface was prepared in UHV in the form of an ordered Co3O4(111) film on Ir(100).22,23 Its surface structure has been characterized in great detail by Heinz, Hammer, and co-workers using STM and LEED I−V analysis.22,23 Briefly, the surface is terminated by a layer of undercoordinated Co2+ ions in the tetrahedral positions of the spinel lattice. The Co2+ ions form a hexagonal unit cell with a Co2+− Co2+ distance of 5.7 Å (see Figure 2 d). The DPPA was deposited in UHV from a Knudsen cell (at a rate of 1940


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change in binding motif as a function of coverage. Specifically, the higher intensity of the antisymmetric P−O mode indicates that the symmetry of the tridentate phosphonate is broken. In addition, the appearance of the shoulder at 1190 cm−1 is attributed to ν(P=O), suggesting the formation of a P=O double bond. On the basis of these observations, we propose the formation of a chelating bidentate phosphonate as majority species. Accordingly, we assign the bands at 1052 cm −1 to νas(PO22−) and at 971 cm−1 to νs(PO22−). The high intensity of the νas(PO22−) band suggests an asymmetric adsorption geometry. Such binding modes have been previously suggested for PPA derivatives, both in experimental and theoretical work.16,19,20,34 It is noteworthy that all P−O bands broaden substantially, suggesting that they are involved in a hydrogenbonded network. Further deposition of DPPA then leads to the linear increase in intensity of the multilayers bands (see Figure 2c, III) superimposed on the monolayer spectrum. This indicates the growth of the disordered multilayer on top of the anchored monolayer. In the next step, we investigated the anchoring behavior at 380 K, i.e., at a temperature at which no multilayer is formed. The corresponding data from TR-IRAS are shown in Figure 3. At low exposure, we observe two dominating bands in the P−O region at 971 and 1144 cm−1, along with a series of additional bands that are listed and assigned in the Supporting Information. With increasing coverage, the intensity ratio of the antisymmetric νas(PO22−) (1064 cm−1) and the symmetric νs(PO22−) (980 cm−1) bands changes in favor of νas(PO22−) and a broad shoulder develops in the region of the ν(P=O) band at 1190 cm−1. This behavior is very similar to that observed at 180 K (Figure 2) at low DPPA exposure. Following the above discussion, we attribute the bands at low coverage to the formation of a chelating tridentate phosphonate and the coverage-dependent changes to the transition to a chelating bidentate. The width of the P−O and O−D bands suggests that the surface phosphonates are part of a hydrogen-bonded network. The proposed adsorption geometries are illustrated in Figure 3d. Note that the saturation coverage cannot be directly extracted from the IRAS data. An upper limit for the saturation coverage is given by the density of anchoring Co2+ surface ions, which is 3.6 nm−2. Coverage-Dependent Adsorption Geometry and Reorientation. The integrated areas of the νas(PO32−), νs(PO32−), and νs(CC) bands are shown in Figure 3c as a function of the DPPA exposure. At large deposition times, all three bands show saturation, indicating that adsorption occurs in the monolayer only and no multilayer is formed. The intensities of the νas(PO32−) and νs(PO32−) bands show a completely different coverage dependence, with the νs(PO32−) band going through a maximum while the νas(PO32−) band does not. This clearly proves the presence of two different binding motifs as a function of coverage, as illustrated in Figure 3d. The coverage-dependent change in the binding motif might be associated with a change in the molecular orientation on the surface. Such coverage-dependent reorientations are a common phenomenon in self-assembled monolayers and have, for example, been observed for organic films anchored by carboxylic acid groups to Co 3O4(111).31,32 To obtain information on the reorientation, we analyzed the evolution of the orthogonal set of modes νas(CC), ν(CP), and γ(CH) at 1441, 1140, and 715 cm−1, respectively. Qualitatively, the slight decrease of the relative intensity of the γ(CH) mode with increasing coverage indicates a slight increase of the tilting

approximately 0.15 ML/min) while IR spectra were taken continuously (1 spectrum/min). The IR reference was recorded before starting the deposition. In a first set of experiments, we investigated the film formation at 180 K (see Figure 2a). This temperature was chosen to prevent adsorption of water from the background during the experiment, while still enabling the formation of DPPA multilayers. Note that the multilayer desorption temperature is 340 K as derived by temperature-programmed IRAS (data not shown). The IR spectra taken during deposition over a period of 60 min are shown in Figure 2a. For deposition times greater than 20 min, the spectra are very similar to the multilayer spectrum in Figure 1. In Figure 2c, we show the integrated intensity in the spectral region of the νas(P(OD)2), νs(P(OD)2), and νas(CC) bands. All bands grow in parallel with nearly constant intensity ratio (small changes in the slope are due to changes in the deposition rate). This behavior is consistent with the growth of a disordered multilayer. During the initial phase of deposition, pronounced deviations are observed from a linear growth (see Figure 2c). We attribute these deviations to anchoring of DPPA to the Co3O4(111) surface. It is worth noting that the spectra at low coverage differ clearly from the multilayer spectra. In Figure 2b, we show a comparison of the spectra obtained at deposition times of 2 min (I, submonolayer), 6 min (II, monolayer), and 59 min (III, multilayer) for comparison. At low coverage (I, submonolayer), the bands at 1140 and 960 cm−1 dominate the spectrum, while weaker features are observed at 695, 716, 753, 1050, and 2400 cm−1. We attribute the band at 1140 cm−1 to the ν(CP) mode, while the ν(P=O) band is not observed. The second band in the spectral region of the anchor group is observed at 960 cm−1, while the feature at 1050 cm−1 is very weak. This spectral pattern is indicative of a fully deprotonated tridentate phosphonate. The formation of a partially deprotonated phosphonate can be ruled out as no band appears in the range from 920 to 950 cm−1, which would be indicative of a ν(P−OD) vibration.25,26,29,30 Accordingly, we assign the bands at 1050 and 960 cm−1 to the antisymmetric stretching mode, νas(PO32−), and the symmetric stretching mode, νs(PO32−), of a surface tridentate phosphonate. The frequencies observed are similar to those of fully deprotonated phosphonates in solution.26 The low intensity of the νas(PO32−) mode indicates a rather symmetric adsorption geometry in which νas(PO32−) is polarized mostly parallel to the surface. It should be noted that the Co−Co distance of 5.7 Å on the Co3O4(111) surface is too large to accommodate a bridging phosphonate between different Co2+ centers. Therefore, we propose that the DPPA is bound in the form of a chelating surface tridentate in the limit of low coverage. Note that chelating species have been reported previously for carboxylic acid anchoring on the same Co3O4(111) film.31,32 Tridentate phosphonates were previously proposed for films on metal oxides prepared by wet chemical approaches (without structural control over the surface)17,25,29,30,33 and by surface science methods (without spectroscopic proof for the binding motif).20 With increasing DPPA exposure, the spectra change drastically. At monolayer coverage (see Figure 2c, II), we observe a strong increase in intensity of the phosphonate band at 1052 cm−1, a decrease in intensity and blue-shift of the phosphonate band at 970 cm−1, and an additional shoulder at 1190 cm−1, along with an increase in intensity of the remaining bands at 2400, 753, 716, and 695 cm−1. The dramatic change in the intensity ratio between the phosphonate bands indicates a 1941


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(Bridge Funding) in the framework of the Excellence Initiative is gratefully acknowledged.

angle; however, the effect is rather weak. For a quantitative analysis, we use the peak areas normalized to the relative dynamical dipole moments from the transmission IR and calculate the tilt angle using a method adapted from Mohr et al.27 (see the Supporting Information for details). In particular, we determined the angle spanned by the phenyl group and the surface normal. This angle decreases from 42° ± 5° at low coverage (spectrum I) to 37° ± 4° at full monolayer coverage (spectrum II). This shows that the reorientation effect is rather weak and the tilting angle is nearly independent of the binding motif. In contrast, much stronger reorientation effects were observed previously for carboxylic acid anchors.31,32 These differences suggest that the phosphonate anchor provides much less flexibility in terms of the molecular orientation in comparison to the carboxylate group. In summary, we present the first vibrational spectroscopy study on the anchoring of phosphonic acids to an atomically defined oxide surface. By in situ TR-IRAS, we have investigated the reaction of DPPA with a well-ordered Co3O4(111) surface under UHV conditions. We show that the bonding motif of the phosphonate anchor group changes as a function of coverage. At low coverage, DPPA binds in the form of a chelating tridentate phosphonate, while a transition to a chelating bidentate is observed when approaching full monolayer coverage. The adsorption structure is stabilized by a hydrogen-bonded network. It is noteworthy that the coveragedependent change in the binding motif is not associated with strong changes of the molecular orientation. This finding suggests that organic films with phosphonate linkers are rigid; that is, they show a pronounced preference for adsorption in tilted orientation irrespective of the binding motif.

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EXPERIMENTAL AND THEORETICAL METHODS Details of the experimental and theoretical methods are given in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b00668. Experimental and theoretical methods, data analysis, determination of the molecular tilting angle, assignment of molecular vibrations (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49 9131 8527308. ORCID

Christian Schuschke: 0000-0002-5635-1112 Matthias Schwarz: 0000-0002-7308-3172 Jörg Libuda: 0000-0003-4713-5941 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Deutsche Forschungsgemeinschaft (DFG) within the Reasearch Unit FOR 1878 “funCOS-Functional Molecular Structures on Complex Oxide Surfaces”. Further financial support of the DFG within the Excellence Cluster “Engineering of Advanced Materials” 1942


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