Adsorption Structure of Cobalt Tetraphenylporphyrin on Ag(100) - The

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The Adsorption Structure of Cobalt Tetraphenylporphyrin on Ag(100) Daniel Wechsler, Matthias Franke, Quratulain Tariq, Liang Zhang, Tien-Lin Lee, Pardeep Kumar Thakur, Nataliya Tsud, Sofiia Bercha, Kevin Charles Prince, Hans-Peter Steinrück, and Ole Lytken J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00518 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017

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The Journal of Physical Chemistry

The Adsorption Structure of Cobalt Tetraphenylporphyrin on Ag(100) Daniel Wechsler,a Matthias Franke,a Quratulain Tariq,a Liang Zhang,a Tien-Lin Lee,b Pardeep Kumar Thakur,b Nataliya Tsud,c Sofiia Bercha,c Kevin Charles Prince,d,e Hans-Peter Steinrück,a and Ole Lytken*,a a

Chair of Physical Chemistry II, University Erlangen-Nürnberg, Egerlandstraße 3, 91058

Erlangen, Germany b

Diamond Light Source, Harwell Science and Innovation Campus, Oxfordshire OX11 0DE,

United Kingdom c

Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma

Science, V Holešovičkách 2, 18000 Prague, Czech Republic d

Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14, km 163.5, 34149 Basovizza-Trieste,

Italy e

IOM, Strada Statale 14, km 163.5, 34149 Basovizza-Trieste, Italy

ACS Paragon Plus Environment

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ABSTRACT

Using a combination of LEED, high-resolution XPS, XSW and NEXAFS, we have studied the adsorption of cobalt(II) 5,10,15,20-tetraphenylporphyrin (CoTPP) on Ag(100) at 300 K. In agreement with previous studies on Ag(111), we find a charge transfer from the silver surface to the porphyrin molecule, reducing the metal center. At high coverages we observe a squareshaped 1.41 × 1.41 nm² adsorption structure, which becomes more open at lower coverages. Because of the superior energy resolution of the Diamond i09 beamline we are able to resolve a low-binding-energy shoulder in the C 1s spectrum, originating from the lower carbon atoms in the rotated phenyl rings. This is confirmed by XSW measurements, which also gives the adsorption heights of the other atoms in the molecule. In addition, the XSW and complementary NEXAFS measurements yield information about the rotation of the phenyl rings and the deformation of the macrocycle.


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INTRODUCTION Porphyrins are a very versatile class of molecules: By incorporating different metal centers or attaching different side groups their structural and electronic properties can be changed. The functionalities of porphyrins observed in nature, like oxygen binding in mammalian blood cells by heme1 or light absorption during photosynthesis by chlorophyll2, are interesting for many scientific and industrial applications, including solar cells3-4, gas sensors5, OLEDs6 and catalysts7. All these devices involve interfaces between porphyrin molecules and solid substrates, and understanding these interfaces is therefore important. Extensive research has been carried out in this area with photoelectron spectroscopy, scanning tunneling microscopy (STM) and lowenergy electron diffraction (LEED). These studies have focused on the bonding between the surface and the molecule, especially the metal center8-14, the influence and orientation of substituents such as phenyl groups,11, 15-24 and the molecular long-range order.14-16, 19, 24-31 However, the vast majority of work has focused on close-packed (111) metal surfaces. In this work, we now investigate the adsorption of cobalt 5,10,15,20-tetraphenylporphyrin (CoTPP) (Fig. 1) on the more open surface Ag(100) surface, in order to search for a possible influence of the crystallographic orientation of the substrate. To obtain a most detailed picture of the rather complex adsorption behavior on the Ag(100) surface, we present a multi-method approach, that is, we combine four methods – LEED, high-resolution X-ray photoelectron spectroscopy (XPS), normal incidence X-ray standing wave (XSW) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) –.32-33 Our results show that a comprehensive understanding can be obtained even without using real-space imaging techniques.


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Figure 1. Cobalt 5,10,15,20-tetraphenyl porphyrin (CoTPP). The tilt of the pyrrole rings out of the molecular plane is described by δ and the rotation of the phenyl rings by θ. Both angles at 0° would result in a completely flat molecule.

EXPERIMENTAL XPS, XSW and LEED experiments were performed at the beamline i09 at Diamond Light Source in Oxfordshire, UK. The end station of the beamline has a base pressure below 3 × 10-10 mbar and contains a VG Scienta EW4000 HAXPES hemispherical electron analyzer and a micro-channel plate LEED optic. The Ag(100) crystal (Surface Preparation Laboratory, 6 N) was cleaned by repeated cycles of Ar+ sputtering and annealing to 700 K. The surface cleanliness was checked with XPS and LEED. CoTPP was purchased from Porphyrin System and evaporated using a homebuilt Knudsen cell. The binding energy positions of the features in the XP spectra were aligned with those measured with a monochromatized Al Kα X-ray source in our home


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laboratory at the University of Erlangen-Nürnberg, which was calibrated to the Fermi edge of Ag(100). NEXAFS experiments were performed at the Materials Science beamline at Elettra in Trieste, Italy. The end-station of the Materials Science beamline has a base pressure of 2 × 10-10 mbar and is equipped with a SPECS Phoibos 150 hemispherical energy analyzer. The photon flux in the N K-edge region was recorded with a gold mesh and found to be almost constant. However, due to carbon impurities on the gold mesh, the photon flux in the C K-edge region had to be measured by following the photoemission intensity of the background between 190 – 200 eV binding energy of the clean Ag(100) surface as a function of photon energy. With this method, the lowest photon flux in the C K-edge region was found to be just 40 % of the highest flux in the same region, an effect caused by carbon impurities on the X-ray optics. The C and N K-edges were measured by following the C and N KLL Auger intensity (250 – 270 eV and 360 – 385 eV, respectively) as a function of photon energy. This method is sensitive to photoemission features travelling through the regions of the Auger lines, specifically in our case Ag 4p, and we corrected for this by subtracting identical images measured for the clean Ag(100) surface and scaled to the intensities of the travelling photoemission features. Throughout the paper, we define 1 ML as the coverage of CoTPP that remains on the surface when multilayers are heated to 650 K for 2 minutes. This is often close to, but not necessarily the same as, the coverage of the completed first layer.

RESULTS AND DISCUSSION Low energy electron diffraction


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When investigating the long-range order of CoTPP on Ag(100) at 300 K, we found two characteristic LEED patterns. Figure 2a shows the LEED pattern of a coverage of 1.7 ML CoTPP, as determined from XPS. All diffraction spots in the pattern are well described by two square-shaped mirror domains rotated 26.4 ± 0.3° clockwise and anticlockwise relative to the Ag(100) unit cell, with lattice vectors of 1.41 ± 0.12 nm, and an area of 1.96 nm². This shape and size of the unit cell is typical for metalated (and often also for free base) tetraphenylporphyrins, which interact weakly with many metal surfaces, and therefore tend to form square-shaped domains with unit cell lengths of 1.29 – 1.42 nm,32, 34-35 irrespective of the substrate symmetry. Since the unit cell is too small to accommodate more than one molecule (Figure 3a), we estimate that the fully-closed first layer of this structure contains 0.50 molecules/nm². Therefore, for the denoted coverage of 1.7 ML, molecules in a second layer must also exist. Since no additional diffraction spots are visible, these second layer molecules must either be in registry with the first layer or in a disordered structure. Ruggieri et al. reported similar results for ZnTPP on Ag(100).35 Their unit cell of 1.29 × 1.29 nm² lies within the error bars of our measurements and would result in a slightly higher coverage of 0.60 molecules/nm². Figure 2 (bottom) shows the LEED pattern for 0.7 ML CoTPP at two electron energies. This structure turns out to be very complex and cannot be described by two simple square or rectangular shaped mirror domains. Nevertheless, some information can be deduced: similar to the 1.7 ML pattern, the 0.7 ML pattern displays eight first-order spots with identical distance to the central spot, indicative of two mirror domains, rotated by 7.9° ± 0.7° clockwise and anticlockwise with respect to the substrate unit cell. Considering only these inner spots, we can estimate the longest lattice vector of the unit cell to be 1.90 nm ± 0.11 nm. Assuming a squareshaped unit cell, we arrive at a unit cell area of 3.61 nm² and a coverage of 0.28 molecules/nm²


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per molecule in the unit cell. Using the results of the high-coverage structure, the coverage 0.7 ML corresponds to 0.35 molecules/nm². Therefore, we conclude that the unit cell must contain two molecules, and that the surface is not completely covered with this ordered phase. This would be even more pronounced with the smaller unit cell reported by Ruggieri et al.35 (see above). There is also the possibility of coexisting domains, but since we have based our analysis on the innermost diffraction spots we only consider the domain with the largest unit cell. In Figure 3, we have depicted a possible adsorption structure for the low-coverage LEED pattern, together with the high coverage structure. The two structures are very similar; the only differences being a slight rotation of the central molecule and a lower density of molecules for the 0.7 ML structure.


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Figure 2. LEED patterns of 1.7 ML (top) and 0.7 ML (bottom) CoTPP on Ag(100). All images were recorded at 300 K. Unit cells based on the eight first-order spots are indicated for both structures, showing a good agreement for the high-coverage structure, but a less good agreement for the low-coverage structure.


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Figure 3. Structural models of CoTPP on Ag(100) based on the measured LEED patterns at 1.7 ML (top) and a very tentative model based on the 0.7 ML pattern (bottom) shown in Figure 2. The two structures are almost identical, the only differences being the density and the rotation of the central molecule. The green boxes indicate the unit cells with areas of 1.41 × 1.41 nm² (top) and 1.90 × 1.90 nm² (bottom).


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X-ray photoelectron spectroscopy Figure 4 shows N 1s and Co 2p XP spectra of CoTPP on Ag(100) at 300 K. The N 1s spectra exhibit the single peak expected for a metalated porphyrin, which, as the coverage is increased beyond one monolayer, shifts by 0.5 eV to higher binding energies. This is typical for metal surfaces where molecules in the first layer, in close proximity to the surface, experience an increased final-state screening of the created core-holes, shifting the photoemission peaks to lower binding energies.36 Notably, the occurrence of the shifted N 1s peak (and also C 1s peak – see below) just above the completion of 1 ML is a strong confirmation of our coverage calibration. A shift is also observed in the Co 2p region, but this shift of 2.1 eV is much larger than the shift in the N 1s region. For the multilayer, the Co 2p peak shows a pronounced multiplet, as expected for a Co2+ complex.12, 34, 37 For the monolayer, this multiplet structure is much less pronounced. This substantial shift and change in shape for submonolayer coverages has been seen for many porphyrins on many metal surfaces.12, 38-39 It has been attributed to a covalent bond formation between the Ag 5s and Co dz2 orbitals, resulting in a charge transfer from the metal surface and an apparent reduction of the Co2+ metal center. This effect is also visible in UPS, and it has been shown that the interaction with the surface can be switched off by NO ligands attached to the metal center.12, 32, 38-39


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Figure 4.: N 1s and Co 2p3/2 XP spectra of CoTPP on Ag(100) recorded at 300 K. At coverages below 1 ML (green traces) only one N 1s peak at 398.3 eV and one Co 2p3/2 peak at 778.1 eV are present. For multilayer coverages (blue traces) new peaks at 398.8 eV and 780.2 eV appear.

Figure 5 shows the C 1s XP spectra of CoTPP on Ag(100). Just as for the N 1s spectra, when the coverage is increased beyond one monolayer the main peak shifts by 0.5 eV to higher binding energies, due to increased final-state screening of the molecules in the first layer. However, for submonolayer coverages an additional low-binding-energy shoulder is present in the C 1s spectra. The porphyrin molecule has eight carbon atoms bound to nitrogen atoms, but those eight carbon atoms are located on the higher binding energy side of the main peak40 and can therefore not explain the low-binding-energy shoulder. The binding energy of 283.9 eV would be in agreement with carbide, which could be an impurity in the Ag(100) sample, but the surface was clean before porphyrin deposition. We also rule out beam damage, a very real


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possibility when using synchrotron radiation, since we always kept the beam energy per area as small as possible, and even repeating the same measurement ten times in the same spot caused no change in the C 1s spectra. Since the LEED pattern for 0.7 ML suggests a unit cell with two porphyrin molecules, the difference in adsorption sites could lead to different electronic environments and, therefore, a peak shift. However, this should also give rise to a second peak in Co 2p and N 1s spectra with the same shift. While such an additional peak would be hard to detect in the Co 2p spectra, the width of the N 1s peak is narrow enough to spot an additional contribution had one been there.


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Figure 5. C 1s XP spectra of CoTPP on Ag(100) recorded at 300 K. Included are also fits for the highest and lowest coverages. The spectra can be described with a multilayer feature at 285.2 eV


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and two monolayer features at 284.7 eV and 283.9 eV. The shape of the monolayer spectrum is independent of coverage and can be described well by assuming a 36 : 8 ratio between the two monolayer components (0.3 ML graph). We explain the low-binding-energy shoulder as a surface-induced final-state screening of the lower carbon atoms of the phenyl rings (red in the upper illustration), which are closer to the silver surface.

But there is another possibility: due to steric repulsion, the phenyl rings are rotated out of the porphyrin plane, which moves two carbon atoms per phenyl ring away from the surface and two carbon atoms per phenyl ring closer to the surface. The eight carbon atoms which move closer to the surface should experience an increased final-state screening and might therefore appear at lower binding energies, and, indeed, a ratio of 36 : 8 between the two C 1s components does fit the 1.0 ML spectrum very well, see Figure 5. This shoulder has to our knowledge not been observed before, and we attribute the fact that we are able to see it to the superior energy resolution of beamline i09 at Diamond. Principally, one would expect a similar high-binding energy shoulder for the upper carbon atoms of the phenyl rings which move away from the surface. However, the screening by the surface decreases dramatically with distance and the effect on the upper carbon atoms of the phenyl rings therefore is expected to be much smaller.

X-ray standing wave measurements Normal-incidence XSW measurements provide information about adsorption heights of atoms in a molecule. By superimposing an incident and a Bragg-reflected X-ray wave from a crystal surface, a standing wave field is generated. The vertical position of nodes and antinodes of this


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wave field is a function of the X-ray beam energy. Therefore, by varying the photon energy the photoelectron yield of an atomic species changes with the vertical position of the atoms relative to the lattice planes, with a periodicity of the vertical lattice spacing of the substrate. The result of the XSW measurement is determined by the coherent position PH and the coherent fraction fH. The distance to the surface dH is obtained with dH = (PH + n)d0. Here, d0 is the lattice-spacing of the (200) reflection and n is an integer, accounting for the periodicity of the substrate lattice perpendicular to the surface. The coherent fraction fH reflects the degree of vertical order and lies between 0 (evenly distributed) and 1 (all atoms in the same plane).41 Non-dipolar corrections were applied according to Lee et al.42


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Figure 6. XSW yield and reflectivity curves for 0.7 ML CoTPP on Ag(100) at 300 K. The values for the coherent fraction (fH) and coherent position (PH) are given for each fit.

Table 1 lists the results obtained by the XSW fits (Fig. 6) for the C 1s, N 1s and Co 2p regions for 0.7 ML CoTPP on Ag(100); three C 1s and N 1s measurements and nine Co 2p measurements were averaged to produce the reported values. Table 1. Coherent fractions, positions and distances for 0.7 ML CoTPP adsorbed on Ag(100). The distances are given for n = 1 and n = 2.


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H

f

P

dH (nm)

H

n=1

n=2

C 1s

0.2

0.52

0.310

0.514

C 1s shoulder

0.4

0.20

0.245

0.448

N 1s

0.5

0.52

0.310

0.514

Co 2p

0.5

0.47

0.300

0.504

For the C 1s data, we find a low value for the coherent fraction. This observation is attributed to the rotation of the phenyl rings and the likely saddle-shape deformation8, 16, 25, 43-45 of the macrocycle. This distortion is caused by the attraction between the molecule and the surface forcing the molecule close to the surface, which is only possible by rotating the phenyl rings making them less perpendicular to the surface. This in turn, because of the steric repulsion between the phenyl and pyrrole rings, forces two of the pyrrole rings to bend up and the other two to bend down. As a consequence, the carbon atoms are distributed over several vertical positions, yielding the observed low coherent fraction. Higher values than those in Table 1 might be expected for cobalt with only one atom per molecule. However, as already indicated by LEED the adsorption structure at 0.7 ML has two, possibly inequivalent, molecules per unit cell, which might explain the lower than expected coherent fraction. Because of the small amount of cobalt metal atoms the Co 2p signal is also weak. It is furthermore quite broad and on top of a large background signal, which scales with the silver intensity. If the fit of the Co 2p peak is not perfect the intensity change of the background might therefore slightly influence the fit and skew the coherent fraction, which could be a reason for the low coherent fraction.


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For large planar organic molecules, adsorbed mostly through van der Waals interactions, the distance between the molecular plane and the surface is expected to be between 0.24 and 0.36 nm.46-50 For a non-planar molecule such as tetraphenylporphyrin with rotated phenyl rings, the carbon atoms closest to the surface should be slightly below this equilibrium distance resulting in a slight repulsion, while the carbon atom in the macrocycle should be slightly above the equilibrium distance resulting in a slight, balancing attraction. Table 1 lists adsorption distances both for n = 1 and n = 2, but only n = 1 satisfies the above consideration, and those values are therefore considered to be the valid ones. Based on the XPS results, we speculated that the low-binding energy C 1s shoulder resulted from the lower carbon atoms of the phenyl rings which are closer to the surface and thus might experience an increased final-state screening. This is in agreement with the XSW results in Table 1, where the C 1s shoulder is indeed found to originate from carbon atoms closer to the surface. From the difference in adsorption distance, ΔdH , between the lower carbon atoms of the phenyl rings and the rest of the molecule, we can estimate the rotation angle θ of the phenyl rings, assuming an undistorted macrocycle: Δd' θ= sin-1 sin 60° DC-C 36

Thereby we use Δd'=ΔdH 44 instead of ΔdH , to account for the fact that the value in Table 1 is the average of the 8 upper carbon atoms in the phenyl rings and the 28 carbon atoms in the plane of the macrocycle. Using a carbon-carbon bond length DC-C of 0.14 nm51 for the phenyl rings, we calculate a rotation angle of 27° with respect to the macrocycle plane, which is slightly smaller than the values reported for metalated tetraphenylporphyrins on different surfaces (35° - 50°).44-


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45, 52

However, the calculated value should be seen as a lower limit, since the likely saddle-shape

deformation of the macrocycle will have moved some of the central carbon atoms closer to the surface, which we ignore in our simple calculation. At the same time, an upward tilt of the phenyl groups would also lead to an apparently lower rotation angle in the XSW measurements.

Near edge X-ray absorption fine structure spectroscopy Additional information about the adsorption geometry was acquired through N K-edge and C K-edge NEXAFS measurements of 0.8 ML CoTPP on Ag(100) at 300 K; the N K-edge provides information about the deformation of the macrocycle, while the C K-edge also provides information about the rotation of the phenyl rings. The intensities of the π* resonances at different incidence angles reflect the orientation of the electric field vector, E, of the incoming light with respect to the molecular planes. The signal has zero intensity if E is coplanar with the molecular plane and maximum intensity if E is perpendicular to it. From the relative intensities I at different incidence angles φ, the orientation of the molecular plane θ can be calculated as described by J. Stöhr.53 In the following formula A is the absolute angle-integrated intensity and P is the polarization of the X-ray beam. 3 1 I = A φ 1 − #$ θ + #$ θ & 2 2 The quantitative analysis of the first π* resonance of the N K-edge in normal (0°) and grazing (80°) incidence depicted in Figure 7 yields an angle of the macrocycle of 36° with respect to the surface plane. This angle is not attributed to a tilted molecule, but to the saddle-shaped distortion


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of the (overall flat-lying) molecule, with two opposing pyrrole rings pointing upwards and the other two down towards the surface.

Figure 7. N and C K-edge NEXAFS spectra of 0.8 ML CoTPP on Ag(100) at 300 K. All spectra were normalized to an edge jump of one. Since only the macrocycle contains nitrogen the first resonance in the N K-edge spectrum (orange) can be used to calculate the deformation of the macrocycle. The first resonance in the C K-edge spectrum contains two components: the macrocycle (orange) and the phenyl rings (green), and can therefore be used to calculate both angles.


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Figure 8: Fits for the N K-edge and C K-edge π* regions of the NEXAFS spectra depicted in Figure 7. The resulting peak areas are plotted against the X-ray beam angle in the bottom graph. The lines show calculated curves for the denoted molecule fragment orientation angles obtained for a linear polarization of 80%.

The π* resonances of the C K-edge consists of several overlapping features. The 80° spectra were fitted according to Diller et al.52 and Schmidt et al.54 (Figure 8). For the 0° spectra the


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positions and shapes of the peaks were held constant and only the areas were allowed to vary. The peak at 284.0 eV (marked orange) is assigned to the macrocycle C atoms and the peak at 285.0 eV (green) to the phenyl rings. The other peaks are caused by a mixture of different orbital transitions. There are uncertainties when deriving angles from overlapping NEXAFS peaks, especially when using only two incidence angles. Nevertheless, analyzing these two components leads to tilt angles of 31° for the macrocycle and 42° for the phenyl rings, with respect to the surface plane. The macrocycle angle originates from the saddle-shaped distortion of the molecule and fits very well with the angle derived from the N K-edge spectra (31° vs. 36°). Because of the saddle-shaped deformation of the macrocycle, the rotational angle of the phenyl rings with respect to the macrocycle is expected to be smaller than the angle of the phenyl rings with respect to the surface plane. The angle of the phenyl rings derived from NEXAFS of 42o is therefore an upper limit on the rotational angle of the phenyl rings with respect to the macrocycle. The true rotational angle of the phenyl rings should therefore lie in between the upper limit of 42o (determined by NEXAFS) and the lower limit of 27o (determined by XSW). CONCLUSION At high coverages, CoTPP adsorbs on Ag(100) in a square 1.41 × 1.41 nm² structure, which changes to a more open structure at lower coverages. Charge transfer from the silver surface to SOMO of the porphyrin molecule reduces the metal center. Both findings are similar to what has been observed for tetraphenylporphyrins on other substrates. The presence of a low binding energy shoulder in the high-resolution C 1s spectra is explained as originating from an increased final-state screening of the lower carbon atoms of phenyl rings; this interpretation is strengthened by XSW measurements, which find the low-binding-energy shoulder to originate from carbon


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atoms closer to the surface. XSW and NEXAFS measurements were also used to estimate the rotational angle of the phenyl rings and the deformation of the macrocycle.

AUTHOR INFORMATION Corresponding Author *Phone: +49 9131 85-27320. Fax: +49 9131 85-28867. E-mail: [email protected] (O.L.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was carried out with support of the Diamond Light Source and Elettra-Sincrotrone Trieste. The authors thank the German Research Council (DFG) for financial funding through FOR 1878 (funCOS). L.Z. thanks the Alexander von Humboldt Foundation for a research fellowship.

ABBREVIATIONS LEED, Low-energy electron diffraction; XPS, X-ray photoelectron spectroscopy; XSW, X-ray standing wave; NEXAFS, Near-edge X-ray absorption fine structure; CoTPP, cobalt 5,10,15,20tetraphenylporphyrin; SOMO, Singly occupied molecular orbital


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Adsorption Structure of Cobalt Tetraphenylporphyrin on Ag(100) - The

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