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Adsorption of Water and Ammonia on Graphene: Evidence for Chemisorption from X-ray Absorption Spectra Stefan Böttcher, Hendrik Vita, Martin Weser, Federico Bisti, Yuriy Dedkov, and Karsten Horn J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01085 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 23, 2017

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Adsorption of Water and Ammonia on Graphene: Evidence for Chemisorption from X-ray Absorption Spectra Stefan Böttcher†,‡, Hendrik Vita†, Martin Weser†, Federico Bisti§, Yuriy S. Dedkov†,||, and Karsten Horn†,* † Fritz Haber Institut of the Max Planck Society, 14195 Berlin, Germany ‡ SPECS Surface Nano Analysis GmbH, 13355 Berlin, Germany § Dipartimento di Science Fisiche e Chemiche, Università dell’Aquila, 67100 L’Aquila, Italy || Fachbereich Physik, Universität Konstanz, Germany

Abstract

While the bonding of molecular adsorbates to graphene has so far been characterized as physisorption, our study of adsorbed ammonia and water using near-edge x-ray absorption spectroscopy provides unambiguous evidence for a chemical contribution to the adsorption bond. We use the situation, unique to graphene, to characterize the unoccupied valence band states of the partners in the bond on the basis of the complementary adsorbate and substrate x-ray absorption K edges. New adsorbate-induced features on the substrate (carbon) K edge are interpreted as hybrid states in terms of a simple model of chemical interaction.

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Graphene, with its unusual electronic structure and “massless Dirac Fermion” charge carriers, has given rise to high expectations for its use in novel electronic and related devices1. In this context, its chemical robustness is particularly useful: carbon atoms in graphene “are completely naked from above and below”, yet “they are largely immune to further bonding”2. The fact that many adsorbates only weakly interact with graphene is thought to be useful for applications in sensors.3 Such applications require a thorough understanding of adsorption processes on graphene in its pristine (“free standing”) form as well as on a variety of substrates. Issues such as adsorption energy and the influence of the adsorbate on doping type and level and carrier mobility have received particular attention, from a theoretical4-10 as well as experimental11-14 point of view; for a recent review see15. Adsorption processes are also important for controlling the properties of graphene16,17. Theoretical and experimental studies suggest that the binding energy of a water molecule to graphene is firmly within the range of physisorption (~100 meV per molecule), with the binding being dominated by van der Waals (vdW) interaction7. In order to elucidate the binding mechanism of simple molecules to graphene, here we study its electronic structure upon adsorption of NH3 and H2O, which induce an opposite type of doping in graphene/SiO23; we show that a sizeable orbital mixing between adsorbates (ammonia or water) and graphene occurs, demonstrating a chemical contribution to the adsorption bond. We use near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to examine the unoccupied region of the electronic structure18. Whereas in previous NEXAFS investigations of adsorption processes18-20 only the adsorbate electronic states were probed, we are able to study both partners in the adsorption complex, i.e. the electronic structure rearrangement upon adsorption in both the adsorbate and the substrate, through K edge absorption from the carbon atoms of graphene. In x-ray absorption studies of adsorption on bulk substrates, the signal from the surface region of the substrate is generally masked by emission from deeper layers not involved in the adsorption



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process; here, however, the single atom thickness of graphene (“all surface and no bulk”) permits to identify changes induced in the substrate electronic states. We use graphene prepared on two markedly different substrates: on lattice-matched Ni(111), often termed a “strongly interacting” interface where the Ni 3d state massively affect the graphene π band21,22, and on Ir(111), where the π band and the characteristic Dirac cone is left largely intact23, suggesting that graphene is almost entirely decoupled from the substrate24,25. The comparison of adsorbate and substrate NEXAFS features shows that considerable differences in the charge exchange occur for adsorption on graphene/Ni(111) and graphene/Ir(111). The photoemission and NEXAFS studies were performed at the UE56/2-PGM-1 and PGM-2 beamlines of BESSY II, Berlin (Germany) and at the beamline D1011 at MAX-Lab in Lund (Sweden), respectively. Graphene/Ni(111) was prepared on a layer of Ni(111), thicker than 100 Å on a W(110) crystal via thermal decomposition of propylene according to the recipe described in ref.11. Graphene/Ir(111) was prepared by thermal cracking of ethylene on the clean Ir(111) surface at a partial pressure of 1 x 10-6 mbar and T = 1250 °C14. The deposition of water and ammonia was performed from high purity sources at a partial pressure of 5 x 10-8 mbar and a sample temperature of 80 K. This temperature was kept constant for all measurements. NEXAFS spectra were collected at the carbon K, oxygen K and nitrogen K absorption edges in the surface sensitive partial electron yield mode (PEY) using a repulsive potential of 100 V and 150 V respectively, with an energy resolution of 80 meV. The base pressure during measurements was less than 7 x 10-11 mbar. The adsorbate coverage was slightly less than one monolayer, which can be clearly distinguished from the second and higher layers in valence level photoemission, as shown by the peaks indicated by the blue and green bars in Figure S1 (supporting material). The low photon flux at the D 1011 bending magnet beamline was spread out over an area of 1 x 3 mm on the sample 26. We also did not find evidence for dissociation products in core and valence level photoemission. Defects in the


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graphene film are also not thought to have an influence on our data, since we followed graphene preparation procedures that lead to films of low defect concentration as determined by STM and AFM 27,28 . Consider the carbon K edge NEXAFS spectra of clean graphene on Ni(111) and Ir(111) shown in Figure 1 a) and b). They exhibit clear differences in the shape of the π* (at around 285 eV) and σ* (at around 291 eV) resonances. On Ir(111), the shapes of the π* and σ* absorption lines closely matches those of graphite29 except for a shoulder on the low photon energy side (inset to Figure 1b) which has been assigned to a small hybridization with the Ir states30,31 in the fcc and hcp position of the graphene supercell. In graphene on Ni(111), however, the π* absorption line is split into a doublet peak at 285.5 and 287.1 eV. This clearly shows that valence level orbital mixing leads to the formation of hybrid states between graphene and the nickel substrate at this interface32,33. The strong influence of the Ni 3d states on the band structure of graphene22,34, and the fact that the distance between nickel and graphene is only ~ 2.1 Å, much smaller compared to other, less strongly interacting metal substrates such as Cu (~ 3.2 Å)7 gives evidence for a massive rearrangement of electronic levels. Turning now to the carbon K edge NEXAFS spectra upon adsorption of ammonia and water on graphene/Ni(111) (Figure 1c) and graphene/Ir(111) (Figure 1d) recorded at normal light incidence, we note that new features (a1, a2 for ammonia, and w1 for water) appear in the energy region of the π* resonance. These lines have considerable strength for adsorption of both molecules on graphene/Ni(111), but are much weaker for graphene on Ir(111) where, however, only one line for ammonia occurs. The lines are also considerably sharper [0.3 … 0.5 eV full width at half maximum (FWHM)] than those of the π* and σ* resonances (~1 eV). The adsorbate-induced lines appear in a region of the spectrum in which orbital mixing between states from the metal substrate with graphene has been identified30,31.



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Figure 1: a,b) NEXAFS spectra of graphene/Ni(111) and graphene/Ir(111) recorded at two different angles of incidence α; inset in b) shows leading peak on an enlarged scale. Changes induced by NH3 and H2O adsorption are shown for α = 0° in c) and d), and on an enlarged energy scale, in e) and f). Inset in a) shows geometry of the experiment. Several messages are immediately evident from the data in Figure 1. First, there is hybridization between the graphene and adsorbate states as evidenced by new contributions to the carbon density of states as reflected in the carbon K edge NEXAFS. Second, ammonia and water induce different ACS Paragon Plus Environment

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lines (“fingerprints”) in the spectrum, underlining the relation of these to differently hybridized states. Third, the intensity of the adsorbate-induced lines in K edge NEXAFS depends on the type of metal substrate on which graphene is prepared; the intensity of the a1 line induced by ammonia on graphene/Ir(111) has only about 20% of that on graphene/Ni(111). We can speculate that the interaction between adsorbate and graphene thus appears to depend on that between the metal and graphene; indeed, Ambrosetti and Silvestrelli have recently found evidence for enhanced chemical reactivity in graphene on a Ni(111) substrate35. Note that the detection of features due to transfer of charge from the adsorbate to the carbon atoms is a specific advantage of NEXAFS; similar conclusions cannot be readily be derived from ARPES where signals from the occupied adsorbate and substrate levels are superimposed 11. We should then expect a complementary influence in the unoccupied states of the adsorbate. The nitrogen and oxygen K edge NEXAFS of ammonia and water adsorbed on graphene on the two metal substrates are shown in Figure 2, together with a spectrum of a thick condensed ammonia layer. Based on this comparison, and taking into account the NEXAFS spectrum of free NH336,37, we assign the peak at 400.6 eV in nitrogen K absorption to the 4a1 state, the lowest unoccupied molecular orbital (LUMO) in the free molecule and the condensed phase. The increase in signal around 401.7 eV is assigned to the next unoccupied state, the 2e orbital, and the onset of Rydberg and continuum states to features around 404 eV. The separation between the 4a1 and 2e lines is 2.2 eV as in gas phase and condensed NH3. The dominant peak at 398.7 eV is absent for adsorption of NH3 on ZnO38 and only very small on Cu(110)37. We assign this peak to N 1s transitions into a (partially empty) antibonding level made up of the interaction between the 3a1 level and a graphene/Ni(111)interface state32. Data for ammonia adsorption on graphene/Ir(111) (Figure 2b) show similar features, although their relative intensities differ compared to graphene/Ni(111), with



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the peak at 398.5 eV now roughly the same magnitude as the one at 400.1 eV assigned to the 4a1 LUMO peak. Adsorption of water on graphene on Ni(111) and Ir(111) leads to roughly similar oxygen K edge spectra: a shoulder at 533.5 eV that we ascribe to the unoccupied 4a1 (LUMO) and a wider peak extending from 536.3 to 539.3 eV that we assign to the 2b2 orbital, in good agreement with spectra from condensed layers of water39, and water adsorbed on Pt(111)40. By combining the complementary x-ray absorption data from the graphene substrate and adsorbate, we obtain a coherent picture of the adsorption bond. First of all, changes in the substrate (carbon K edge absorption) upon NH3 and H2O adsorption give a clear indication that hybridization between substrate and adsorbate orbitals occurs, i.e. a chemical interaction. The spectral features induced in the carbon K edge NEXAFS by ammonia and water can be explained on the basis of a straightforward molecular orbital (MO) model shown in Figure 3. We assign the peak at 398.5 eV in the nitrogen K edge NEXAFS to the former highest occupied molecular orbital (HOMO), which is partially emptied by charge transfer from the ammonia molecule to the graphene layer upon formation of the chemisorption bond. The carbon π states interact with the ammonia 3a1 and 4a1 states to form bonding and antibonding orbitals (center of Figure 3), of which the latter give rise to the new peaks a1, a2 in the carbon K and the strong 3a1 peak in the nitrogen K NEXAFS. The fact that the energy separation of 1.6 eV of the new lines induced by ammonia in the carbon K spectrum agrees with that of the peaks assigned to the 3a1 and 4a1 molecular orbitals in the nitrogen K edge spectrum strongly supports our molecular orbital model scheme. The binding energy of the carbon 1s photoemission line does not exhibit a shift upon adsorption of ammonia (see supplementary Fig. S2), suggesting an absence of net charge transfer. The interaction between ammonia and graphene is thus a donation-backdonation process, whereby charge transferred to graphene is given back to the adsorbate through interaction of the π states with


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the 4a1 MO. This analysis is supported by a comparison of ammonia on graphene on either Ni(111) or Ir(111). In the latter case, the adsorbate-induced features are much smaller, suggesting a much weaker chemisorptive component to the bond, a conclusion that is supported by the observation that the relative intensity of the feature assigned to the now partly unoccupied 3a1 is much smaller in graphene/Ir(111).

Figure 2: NEXAFS spectra for adsorption of ammonia (a,b) and water (c,d) on graphene/Ni(111) and graphene/Ir(111). The spectrum of a thick condensed layer of ammonia is also shown in a) and b). The situation for water adsorption is different. First, the carbon 1s core level photoemission data (Fig. S1) show that water induces a sizeable binding energy shift, indicative of net charge transfer; there is thus a much reduced or entirely absent backdonation onto the adsorbate. Moreover, the



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HOMO of water is much lower in energy than that of ammonia41, such that the considerable change upon adsorption found in the latter (appearance of a signal due to the emptied 3a1 HOMO in the unoccupied region) does not occur in the oxygen K edge NEXAFS of adsorbed water. It is likely that this causes the bonding strength of water on graphene to be much weaker than that of ammonia. The doping induced by water adsorption can be explained by the electric field due to its dipole which may shift the graphene bands41.

Figure 3: Schematic diagram of the electronic level interaction between water (left), ammonia (right) and graphene (center) that leads to a bonding-antibonding molecular orbital (MO) combination (center).

Theoretical treatments of adsorption on graphene have so far concentrated on water, and there is general agreement that the bond is entirely van-der-Waals type, i.e. water is physisorbed. Inclusion of vdW interaction in DFT calculations of water adsorption through various schemes results in a binding energy range of roughly 50…120 meV per adsorbed molecule, depending on the substrate7. This value is in agreement with the most reliable calculations based on the coupled cluster (CCSD(T))10 or diffusion Monte Carlo4 approaches that yield values in the range of 70…110 meV, i.e. firmly in the region of physisorption. In their detailed study of water adsorption on strongly and weakly bound graphene on metal surfaces [Ni(111) vs. Cu(111)], Li et al.7 also arrive at the conclusion that water is physisorbed. They also find a response of the adsorbate in the orbitals of ACS Paragon Plus Environment

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graphene. While such processes may explain the action of the adsorbates as charge donors or acceptors3, the calculations do not explain the new peak that water induces in the carbon-projected density of states as reflected in the carbon K edge absorption spectrum, Figure 1 e) and f). Regarding ammonia adsorption, similar conclusions about a lack of electronic state mixing were reached by Zhang and coworkers13; however, Leenarts et al.8 found a small charge transfer in DFT calculations of ammonia on graphene, and concluded that donation from the 3a1 orbital and backdonation into the 4a1 may occur, in line with our above observations. Note that our temperature programmed desorption experiments (ref.26; an example is given in Figure S3) suggest a much higher binding energy (~450 meV), in line with results from Standop et al.42. The clustering observed by these authors for water on graphene/Ir(111) is not thought to affect our interpretation of the NEXAFS data, since the clusters are planar. In summary, we assign new signals in the unoccupied electronic states to hybrid states formed between the states of the adsorbates and the substrate carbon atoms. These data provide evidence for a chemical contribution to the adsorption bond. The fact that we can access both the adsorbate and substrate K edges, a situation unique to graphene, permits us to obtain complementary information on the modification of the adsorbate and substrate unoccupied states through the adsorption process. The new features on the substrate (carbon) K edge are interpreted in terms of a simple model of chemical interaction, which also takes into account differences in water and ammonia bonding. Supporting information: Valence level spectra of water on graphene/Ir(111), demonstrating how the emergence of features associated with the growth of a second layer of water as a function of exposure can be used to calibrate monolayer coverage, carbon 1s core level line shifts for water and ammonia on



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graphene/Ir(111) and Ni(111), and thermal desorption data for water on graphene/Ni(111) used to determine the binding energy of water. Author information Corresponding author: K. Horn, e-mail: [email protected] ORCID: Karsten Horn: 0000-0003-0844-5437 Yuriy Dedkov: 0000-0001-7904-2892 Stefan Böttcher: 0000-0002-1058-8480 Acknowledgements: This work has been supported by the European Science Foundation (ESF) under the EUROCORES Programme EuroGRAPHENE (project “SpinGRAPH”), and by the German Research Foundation (DFG) through Priority Programme (SPP) 1459 “Graphene”. Support by the staff from the Helmholtz-Zentrum für Materialien und Energie Berlin is gratefully acknowledged. We thank Alexej Preobrajenski for his support during the measurements at MAX-Lab, Lund, Sweden. References 1.

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32. Bertoni, G.; Calmels, L.; Altibelli, A.; Serin, V. First-principles Calculation of the Electronic Structure and EELS Spectra at the Graphene/Ni (111) Interface. Phys. Rev. B 2005, 71, 075402-1 - 8. 33. Voloshina, E.; Ovcharenko, R.; Shulakov, A.; Dedkov, Y. Theoretical Description of X-ray Absorption Spectroscopy of the Graphene-metal Interfaces. J. Chem. Phys. 2013, 138, 154706-1-8. 34. Dedkov, Y. S.; Fonin, M. Electronic and Magnetic Properties of the Graphene–ferromagnet Interface. New J. Phys. 2010, 12, 125004-1 - 20. 35. Ambrosetti, A.; Silvestrelli, P. L. Communication: Enhanced Chemical Reactivity of Graphene on a Ni(111) Substrate. J. Chem. Phys. 2016, 144, 111101-1 - 5. 36. Schirmer, J.; Trofimov, A. B.; Randall, K. J.; Feldhaus, J.; Bradshaw, A. M.; Ma, Y.; Chen, C. T.; Sette, F. K-shell Excitation of the Water, Ammonia, and Methane Molecules Using High-resolution Photoabsorption Spectroscopy. Phys. Rev. A 1993, 47, 1136-1147. 37. Hasselström, J.; Föhlisch, A.; Karis, O.; Wassdahl, N.; Weinelt, M.; Nilsson, A.; Nyberg, M.; Pettersson, L. G. M.; Stöhr, J. Ammonia Adsorbed on Cu (110): An Angle Resolved X-ray Spectroscopic and Ab Initio Study. J. Chem. Phys. 1999, 110, 4880-4890. 38. Ozawa, K.; Hasegawa, T.; Edamoto, K.; Takahashi, K.; Kamada, M. Adsorption State and Molecular Orientation of Ammonia on ZnO (1010) Studied by Photoelectron Spectroscopy and Near-edge X-ray Absorption Fine Structure Spectroscopy. J. Phys. Chem. B 2002, 106, 9380-9386. 39. Bluhm, H.; Ogletree, D. F.; Fadley, C. S.; Hussain, Z.; Salmeron, M. The Premelting of Ice Studied with Photoelectron Spectroscopy. J. Phys.: Condens. Matter 2002, 14, L227-L233.


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40. Ogasawara, H.; Brena, B.; Nordlund, D.; Nyberg, M.; Pelmenschikov, A.; Pettersson, L. G.; Nilsson, A. Structure and Bonding of Water on Pt(111). Phys. Rev. Lett. 2002, 89, 276102-1 – 276102-4. 41. Wehling, O.; Katsnelson, I.; Lichtenstein, I. Adsorbates on Graphene: Impurity States and Electron Scattering. Chem. Phys. Lett. 2009, 476, 125-134. 42. Standop, S.; Michely, T.; Busse, C. H2O on Graphene/Ir(111): A Periodic Array of Frozen Droplets. J. Phys. Chem. C 2015, 119, 1418-1423.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

hybrid states detected in NEXAFS

x-ray photons

H2O

graphene

H H

metal x-ray absorption detects charge transfer and orbital mixing between adsorbate and graphene

Page 18 of 24

graphene

NH3

σ*

O

H N H

H

1b1

π*

4a1

3a1

π

3a1

EF

H

O


H

σ

H

N H

H

Page 19 of 24

a)

E

NEXAFS ntensity (arb. units)

1s → π*

b)

α

1s → σ* 1s → σ*

α = 60°

282 284

286

288

α = 60° α = 0°

290

300

c) NEXAFS Intensity (arb. units)

graphene/Ir(111)

1s → π*

280

α = 0°

310 α = 0°

280

290

300

d)

310 α = 0°

a1 a2

NH3

NH3 w1

a1

H20

H 20 clean 280

w1

clean 290

300

e) NEXAFS Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


graphene/Ni(111)

310

α = 0°

280

290

300

f)

310

α = 0°

a1 a2

NH3

NH3

w1

w1

H2O

H 2O

clean

clean 280

a1

285

290

280

photon energy (eV)


285

290

graphene/Ni(111) graphene/Ir(111) The Journal of Physical Chemistry Page Letters 20 of 24 a)

NEXAFS Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

3a1

2e

b)

nitrogen K Edge

2e

α = 60°

α = 60°

3a1 4a1

4a1 α = 0°

α = 0°

α=50°

α=50°

thick NH3 layer

398 400 402 404 406

2b2

c)

thick NH3 layer

398 400 402 404 406

2b2

oxygen K edge

d)

α = 60° α = 60°

4a1

4a1

1

α = 0°

532 536 540 536 540 544 ACS Paragon Plus Environment

532

photon energy (eV)

α = 0°

544

Page 21 of 24


H2O 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

H H

EF

O

graphene

σ* π*

1b1 3a1

NH3 H N H

4a1 3a1

π

O H

H

σ


H

H

N H H


π

Intensity x 105 (cts)

5

II Ni 3d

I

4

t = 570 sec

3

t = 350 sec

2

t = 150 sec

1

t = 100 sec

0

t=

0 sec intensity profile

500

Depostion time (sec)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Ni 3d water 1st layer water 2nd layer graphene π

400 300 200 100 0

-100 14

12

10

8

6

4

2

EF

ACS Paragon Binding Energy (eV) Plus Environment

Page 22 of 24

Graphene/Ir(111) Page 23 of 24 Graphene/Ni(111) The Journal of Physical Chemistry Letters

Photoemission Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

NH3

(a)

NH3

(d)

H 2O

(b)

H2O

(e)

clean

(c)

clean

(f)

288

286 ACS 284Paragon 282 Plus Environment 288 286 284 282 Binding Energy (eV) Binding Energy (eV)

The Journal of Physical Chemistry Page 24 Letters of 24 H2O/graphene/Ni(111) 2

0.3 L

-27

ln(pmax)

QMS Signal (10-9 mbar)

1 0.5 L 1 L -28 2 2 L 3 L 3 -29 5 L 4 5 6.2 6.4 6.6 6.8 1 T (10 K ) 6 7 8 9 10 110 ACS Paragon Plus Environment 12 120 140 160 180 200 220 240 260 temperature (K) 13 -1

-3

-1

Adsorption of Water and Ammonia on Graphene ... - ACS Publications

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