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Benzoic Acid and Phthalic Acid on Atomically Well-Defined MgO(100) Thin Films: Adsorption, Interface Reaction, and Thin Film Growth Tao Xu, Susanne Mohr, Max Amende, Mathias Laurin, Tibor Doepper, Andreas Goerling, and Jörg Libuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07591 • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on October 31, 2015

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The Journal of Physical Chemistry

1

Benzoic Acid and Phthalic Acid on Atomically

2

Well-Defined MgO(100) Thin Films: Adsorption,

3

Interface Reaction, and Thin Film Growth

4

Tao Xu1, Susanne Mohr1, Max Amende1, Mathias Laurin1, Tibor Döpper2, Andreas Görling2,3,

5

Jörg Libuda1,3 *

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1

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg,

7 8

Egerlandstraße 3, D-91058 Erlangen, Germany 2

Lehrstuhl für Theoretische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg,

9 10

Egerlandstraße 3, D-91058 Erlangen, Germany 3

Erlangen Catalysis Resource Center and Interdisciplinary Center Interface Controlled

11

Processes, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany

12

Keywords: benzoic acid, phthalic acid, organic thin films, oxide thin films, magnesium oxide,

13

infrared reflection absorption spectroscopy

14 15

Abstract

16

To better understand the interaction and the growth of thin films of functionalized organic

17

molecules on oxide surfaces, we have studied the adsorption, reaction and desorption of benzoic

ACS Paragon Plus Environment

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acid (BA) and phthalic acid (PA) on a well-ordered MgO(100) thin film grown on a Ag(100)

2

single crystal surface. We have applied isothermal time-resolved infrared reflection absorption

3

spectroscopy (TR-IRAS) and temperature-programmed IRAS (TP-IRAS) under ultrahigh-

4

vacuum (UHV) conditions. BA is dosed using a supersonic molecular beam (SSMB) source

5

while PA is deposited by physical vapor deposition (PVD). For both molecules we have explored

6

the film growth as a function of temperature, both in the monolayer and in the multilayer regime.

7

We have also investigated structural transitions and desorption by temperature-programmed

8

experiments in the range from 100 K to 400 K. In addition we carried out density-functional

9

(DF) calculations. We find that both molecules BA and PA bind through the carboxyl groups to

10

the MgO(100) surface. Upon adsorption at 100 K BA binds in an asymmetric bidentate geometry

11

which exhibits a small tilting angle between the aromatic plane and the surface. Beyond the

12

monolayer, a disordered multilayer film grows, which crystallizes under formation of dimers at

13

around 180 K as indicated by a characteristic splitting of the IR bands. The BA multilayer

14

desorbs at 240 K. Upon adsorption at 300 K, only a BA monolayer forms. Again BA forms an

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asymmetric bidentate, but with a larger tilting angle compared to low temperature adsorption.

16

For PA adsorption at 100 K, the adsorption mechanism is observed to change with coverage. At

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low coverage, both carboxyl groups are deprotonated and the molecule forms an asymmetric bis-

18

bidentate carboxylate with the aromatic plane nearly perpendicular to the surface. At high

19

coverage, only one carboxyl group binds to the surface and forms an asymmetric bidentate

20

carboxylate while the molecules adopt an upright standing orientation. During PVD of PA, a

21

small fraction of phthalic anhydride (PAA) is formed which co-adsorbs at low temperature.

22

Upon annealing, the PAA desorbs around 250 K, triggering a structural transformation of the PA

23

multilayer during which the PA adopts a more flat lying orientation. The PA multilayer itself


2

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desorbs around 310 K. Therefore only monolayer of PA is stable around 300 K. Again the

2

adsorption mechanism is coverage dependent, changing from a bis-bidentate carboxylate at

3

lower coverage to a mono-bidentate carboxylate at higher coverage.

4 5 6

1. Introduction Thin layers of functional organic molecules on oxide surfaces hold great potential in 1-2

7

applications like solar cells and molecular electronics

. In spite of the importance of such

8

application, little is known about the interaction mechanisms of organic layers with oxide

9

surfaces at the atomic scale. A detailed understanding of the mechanisms, kinetics and energetics

10

of growth and structure formation at the organic-oxide hybrid interface, however, is the key to

11

control the growth and structure of organic thin films on dielectric substrates (see e.g. 3-9).

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Such an atomic level understanding can be obtained following a surface science approach

13

under ultrahigh vacuum (UHV) conditions. Many oxide surfaces can be prepared in atomically

14

clean and ordered fashion, either in form of bulk single crystals or in form of ordered films on

15

metal single crystal substrates (see e.g.

16

most surface science methods such as photoelectron spectroscopy (PES), infrared reflection

17

absorption spectroscopy (IRAS) or scanning tunnelling microscopy (STM) can be easily applied.

18

In the present work we use a thin ordered MgO(100) film grown on a Ag(100) substrate

19

The thickness of this film can be varied, so that charging can be avoided and the substrate is

20

compatible with STM and PES. For IRAS the metallic substrate ensures high sensitivity and the

21

metal surface selection rule (MSSR) allows us to extract information on the molecular

22

orientation 17.

10-13

). The latter approach provides the advantage that

14-16

.


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In the present work we explore the mechanisms of bond formation to the oxide surface within

2

the first monolayer and the subsequent growth of the molecular film. Using time-resolved (TR)

3

IRAS during deposition and growth of the organic layer both processes can be followed in-situ.

4

Subsequently, it is possible to use temperature-programmed (TP) IRAS experiments to monitor

5

the thermally induced structural rearrangements and phase transformation in the organic layer

6

19

18-

. Further information is gained by calculations using density-functional theory (DFT).

7

In the absence of interfacial reactions, organic molecules interact only weakly with oxide

8

surfaces, mostly via van-der-Waals and electrostatic interaction. To control the structure

9

formation at the interface properly, stronger and more specific interactions are required. Towards

10

this aim, functional molecules are often linked to the oxide surface using specific anchor groups

11

which reactively interact with the surface

12

carboxylic acid group is among the most common ones. Using this anchor in large organic

13

molecules, self-assembled monolayer (SAMs) have been prepared on many different oxides, in

14

most cases from solution

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cations forming a carboxylate, however, the adsorption geometry and atomic structure of

16

adsorption site remain unknown in most cases.

3, 23

5, 20-22

. Among different anchors that are used the

. After deprotonation the molecule is attached to the surface metal

17

In the present study we investigate two test molecules, containing the carboxylic acid groups,

18

benzoic acid (BA, C6H5COOH) and phthalic acid (PA, benzene-1,2-dicarboxylic acid, o-

19

C6H4(COOH)2). Depending on the structural properties of the surface and the chemical

20

interactions the carboxylic groups may bind in different adsorption geometry, i.e. in form of a

21

monodentate, a bidentate or a distorted bidentate and others (see e.g.

22

For PA, bonding via a single carboxyl group (carboxylate) will compete with bonding via both

24

and references therein).


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groups (bis-carboxylate). The mechanism will have an influence not only on the energetics but

2

also on the molecular orientation of the molecule.

3

So far most surface science studies on the adsorption of BA and PA have been focusing on 25-27

4

metal surfaces (see e.g.

5

Cu(110), which has been intensively investigated by HREELS

6

and other methods. It was proposed that after deprotonation of the carboxyl group the latter binds

7

to Cu(110) forming a bidentate with the aromatic plane oriented perpendicular to the surface at

8

room temperature. Only very few studies were reported so far on the adsorption of BA on oxide

9

surfaces

33-36

). Among the best studied systems is the adsorption of BA on 28

, LEED

29

, STM

30-31

, TPD

32

. For example, benzoate dimers were formed on TiO2(110) and a rotation of the

10

phenol ring occurred 34. Apart from the commonly reported bidentate, less common carboxylate

11

structures may be formed on oxides. For example Wöll and co-workers recently identified a

12

“quasi-bidentate” upon adsorption of formic acid on ZnO by means of polarization-dependent

13

IRAS, i.e. a species that is bound via one carboxylate oxygen and one hydrogen bridge 24.

14

For di-carboxylic acids, studies are even scarcer. The most detailed picture of the bonding

15

could be obtained for the adsorption of terephthalic acid (benzene-1,4-dicarboxylic acid, p-

16

C4H6(COOH)2) on TiO2(110)-(1×1) 8. Here, the molecule binds via one bidentate carboxylate to

17

two 5-fold coordinated Ti ions. The molecules adopt a nearly upright geometry, but tilt to form

18

dimers and the aromatic plane rotates with respect to the carboxylate unit. STM and XPS of

19

isophthalic acid (benzene-1,3-dicarboxylic acid, m-C4H6(COOH)2) on Cu modified Au(111)

20

suggest that both carboxyl groups could be utilized to link the molecule onto the surface where

21

the aromatic plane stands up-right but with a larger tilting angle 37-38.

22

To the best of our knowledge no surface science studies have been performed on the

23

adsorption of PA on oxide surfaces so far. On MgO surfaces neither BA nor PA were studied


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yet. In this work we present insight into the interaction of both molecule with MgO(100) as a

2

function of coverage and temperature.

3 4 5


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2. Experimental and computational details

2

IRAS instrumentation. . The measurements were conducted in an UHV system (base

3

pressure of 1.0×10-10 mbar) which was described in detail elsewhere

39

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consists of a preparation chamber for sample cleaning and preparation and a measurement

5

chamber for the IRAS experiments. The latter is equipped with a FTIR spectrometer (Bruker IFS

6

66v/S) connected via differentially pumped KBr windows. The spectra were taken with a

7

resolution of 2 cm-1. During the adsorption measurements and temperature programmed

8

measurements, spectra were continuously acquired with typically 256 scans per spectrum

9

corresponding to an acquisition time of one minute per spectrum. For the isothermal adsorption

10

experiments, the surface was exposed to the reactants for 60 minutes. For the temperature-

11

programmed experiments the heating rate was 2 K/minute.

. The UHV system

12

Preparation of MgO(100)/Ag(100). A Ag(100) single crystal (MaTeck, purity 99.999%) was

13

used as substrate for the growth of the MgO film. The preparation of the film has been described

14

in the literature previously 14-16. Briefly, the surface was cleaned by cycles of annealing to 900 K

15

and sputtering with Ar+ (1.8 keV) at room temperature. The quality of the Ag(100) was checked

16

by low energy electron diffraction (LEED). Sharp and low background LEED patterns of Ag(100)

17

were achieved before the growth of MgO. To prepare the MgO film, the Ag(100) substrate was

18

kept at 150 K and Mg was deposited from a resistively heated thermal evaporator source. During

19

Mg deposition, O2 was continuously dosed into the chamber at a background pressure of 1.0×10-

20

6

21

around 10 ML in the present experiments. After film growth, the sample was annealed at 650 K

22

in O2 atmosphere for 5 min and then cooled down to room temperature. The quality of the

23

MgO(100) thin film was checked by LEED.

mbar. The thickness of MgO(100) film was calibrated with a quartz microbalance and was


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Deposition of BA and PA. BA (Sigma-Aldrich, 99.9%) was deposited using a supersonic

2

molecular beam (SSMB) source. The method has been described in detail elsewhere 40. Briefly, a

3

stream of Ar (Linde, 99.9999%) runs through a BA reservoir kept at elevated temperature. From

4

the BA/Ar mixture, a molecular beam is generated by expansion through a nozzle into the

5

vacuum. The method allows well-controlled deposition of organic molecules at low rate avoiding

6

any contamination of the UHV chamber. By adjusting the temperature of BA reservoir, the

7

deposition rate was controlled. For the present experiments, the reservoir was heated up to

8

around 200°C. Decomposition products were observed neither in the BA reservoir after the

9

heating (by transmission FTIR) nor in the deposited film on the surface (see below). We find that

10

the BA monolayer at low temperature saturates after approximately 5 min yielding a deposition

11

of approximately 0.2 ML/min.

12

PA (Sigma-Aldrich, 99.9%) was dosed in from a home-built Knudsen source. The evaporator

13

was baked out over night before first usage. PA was placed into a glass reservoir with an orifice.

14

The glass reservoir with the PA was heated to 410 K for at least 30 min while being pumped

15

through a separate pumping line, before the gate valve to the chamber was opened.

16

Subsequently, the evaporator was placed in front of the sample and the sample shutter was

17

opened. The evaporation temperature was adjusted to obtain a similar deposition rate as for the

18

BA, i.e. a deposition rate of around 0.2 ML/min as determined from IRAS at low temperature.

19

DFT. We performed density-functional calculations for several adsorption geometries using 41-44

20

the VASP software

21

(PBE) 45 was employed in combination with a plane wave basis set and the projector-augmented

22

wave (PAW) method

23

damping

47-48

46

. The exchange-correlation functional of Perdew, Burke and Ernzerhof

. The vdW correction scheme from Grimme (D3) with Becke-Johnson

and adjusted parameters for the ionic magnesium was adopted

49

. The super cells


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used for the periodic calculations consisted of a six layer slab, of which the three bottom layers

2

where frozen to the calculated bulk coordinates with an oxygen-magnesium distance of 2.12 Å.

3

A 20 Å vacuum layer between the slabs was applied. The plain-wave basis cutoff was set to 450

4

eV and a 2x2x1 Monkhorst-Pack 50 k-point grid as well as Methfessel-Paxton 51 smearing of 0.1

5

eV was used. The geometry was optimized until all forces were smaller than 0.01 eV/Å.

6

Adsorption energies of the systems were calculated by subtracting the total energies of the intact

7

molecules in the gas phase plus the total energy of the pristine MgO surface from the total

8

energy of the geometry optimized combined system. In all calculations required for the

9

determination of the adsorption energy the same unit cell was used.

10 11

3. Results and discussion

12

3.1

Benzoic Acid/MgO(100)/Ag(100): Adsorption and time-resolved IRAS at 100K

13

To study the growth of BA on MgO(100)/Ag(100) at low temperature (100 K), BA was

14

deposited from a SSMB source onto the freshly prepared surface for 60 min. During deposition,

15

IR spectra were acquired at a rate of 1 spectrum/minute. A reference spectrum was measured

16

immediately before the experiment. The results are displayed in Fig. 1a. For easier comparison

17

of the monolayer and the multilayer region, spectra recorded after 1 min of deposition (sub-

18

monolayer) and after 60 min of deposition (multilayer) are shown in Figure 1b. The two most

19

prominent bands in the submonolayer region are those at 719 cm-1 and at 1422 cm-1. In addition,

20

a weak and broad feature appears around 1540 cm-1. In the multilayer spectrum a large number

21

of bands grow linearly with deposition time. The most intense bands are those at 719, 1260,

22

1320, 1385, 1450, 1692, 1708, 2853, and 2927 cm-1.

23

A more complete list of the bands observed in the monolayer and in the multilayer region is

24

provided in Table 1. Considering the multilayer region first, all listed peaks can be


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straightforwardly assigned to molecular BA 52-54. Briefly, the band at 719 cm-1 corresponds to the

2

characteristic out-of-plane CH deformation in the aromatic ring. The band in the fingerprint

3

region from 1000 to 1400 cm-1 can be assigned to modes with contributions from CC stretching,

4

CH deformation, CO deformation and CO stretching. The two most intense peaks at 1692 cm-1

5

and 1708 cm-1 correspond to the C=O stretching mode, which is split due to dimer formation

6

and coupling

7

assignments of the bands in Table 1. In addition, we have performed density-functional

8

calculations of the isolates BA (monomer and dimer) and have analysed the vibrational modes

9

using the visualization software QVibePlot

10

52, 54-56

. CH stretching modes appear at around 3000 cm-1. We summarize the

57

. The corresponding graphs are shown in the

Supporting Information.

11

In the monolayer region the three observed bands can be attributed to the out-of-plane CH

12

deformation of the aromatic ring (719 cm-1), the symmetric OCO stretching mode of the

13

carboxylate (νs(OCO) = 1420 cm-1) and the antisymmetric OCO stretching mode of the

14

carboxylate (νas(OCO) = 1540 cm-1). The assignment has been discussed in the literature,

15

previously

16

undergoes quantitative deprotonation even at 100 K.

58-60

. The band at 1700 cm-1 is absent in the monolayer region, indicating that BA

17

Of special interest is the polarization of these three modes with respect to the molecule and

18

with respect to the surface. This is because of the metal surface selection rule (MSSR) which

19

holds strictly for thin oxide films on metallic substrates

20

component of the dynamic dipole moment that is perpendicular to the surface contributes to IR

21

absorption. First we consider the out-of-plane CH deformation mode at 719 cm-1 which is

22

polarized perpendicularly to the aromatic ring. The fact that the corresponding band appears with

23

high intensity shows that the BA cannot be adsorbed in upright standing geometry but must have

61

. The MSSR states that only the


10

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a small tilting angle between aromatic plane and surface. The intensities of the symmetric and

2

asymmetric OCO stretching modes provide information on the bonding geometry. For a

3

symmetrically adsorbed bidentate carboxylate the antisymmetric OCO mode is polarized parallel

4

to the surface whereas the symmetric mode is polarized perpendicular to the surface. The fact

5

that the asymmetric OCO mode is weak but visible, therefore, indicates that the carboxyl group

6

is bound in a slightly distorted bidentate geometry.

7

Fig 1d shows three adsorption geometries of BA that result from DFT optimizations for a

8

coverage of one BA per 18 MgO(100) unit cells The geometries were obtained from different

9

starting geometries and all represent local minima on the potential energy surface. Structures of

10

further local minima can be found in the SI. Geometries A and B have the same energy within

11

the accuracy of the employed method and represent the minima in energy. Structure A exhibits

12

an upright standing molecule, structure B a tilted geometry. This shows that the tilting angle has

13

a minor effect on the energy only. The tilting angle therefore can most likely be influenced by

14

packing effects, which are not present at the low coverage considered in the calculations. In

15

geometry C the proton originating from the carboxyl group is located further away from the

16

adsorbed molecule. This leads to an increase of the energy by 0.28 eV. Other structures with

17

protons located further apart also exhibit a higher energy. This means that the position of the

18

proton is strongly correlated to the position of the carboxyl group. In the energetically lowest

19

structures A and B the proton is located symmetrically between the oxygen atoms of the

20

carboxyl group and no distortion of the bidentate geometry is found. Geometry C shows that

21

external effects can lead to a distortion of the bidentate geometry . Here the asymmetric position

22

of the proton leads to a distortion. Because geometry C is energetically unfavourable and,

23

therefore, not likely to be present in experiment, other effects, most likely packing effects, have


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Page 12 of 36

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to be responsible for the experimentally observed distortion of the bidentate geometry. In

2

summary the DFT calculation confirms that BA adsorbs on the MgO(100) surface in a bidentate

3

geometry. The tilting of the aromatic plane and the distortion of the bidentate geometry are

4

attributed to packing effects.

5

Interestingly, the feature at 1540 cm-1 increases in intensity with increasing BA exposure in the

6

multilayer region. This observation indicates that the adsorption geometry of the carboxylate at

7

the interface changes to a more strongly distorted orientation with increasing BA coverage.

8

In Figure 1c, we show the intensities of the peaks at 719, 1422 and 1708 cm -1 plotted against

9

deposition time. The band at 1422 cm-1 is attributed to the surface bound carboxylate which

10

saturates after a deposition time of approximately 5 min. The other two bands at 1708 cm-1 and

11

719 cm-1 linearly increase in intensity with deposition time. Within the accuracy limits of the

12

experiment the intensity ratio of these two bands remains constant, indicating that the average

13

orientation of the molecularly adsorbed BA does not change as a function of coverage.

14 15

3.2

Benzoic Acid/MgO(100)/Ag(100): Temperature-programmed IRAS

16

In the next step we investigated the thermal behaviour of the BA layer. To this end we

17

performed a temperature-programmed IRAS experiment by heating the BA multilayer from 113

18

K to 309 K with a constant rate of 2 K/min. IRAS spectra were taken during the heating ramp

19

with an acquisition time of 1 min/spectrum. The development in selected spectral regions and the

20

corresponding band intensities are plotted in Figure 2 (for better visibility spectra are plotted

21

with a temperature interval of 4 K).

22

We focus on the bands at around 1700 cm-1 (C=O stretching region), 1450 cm-1 (in-plane CH

23

deformation and CO stretching region) and 719 cm-1 (out of pane CH deformation). All spectra

24

remain largely unchanged up to a temperature of 177 K. In the temperature region between 177


12

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K and 200 K both the intensity and the shape of the peaks change dramatically. Specifically, we

2

observe that the band at 1696 cm-1 splits into two sharp features at 1692 and 1708 cm-1. The band

3

at 1452 cm-1 and 1426 cm-1 shifts and two intense bands develop at 1454 and 1431 cm-1 and the

4

band at 716 cm-1 decreases in width and shifts to 711 cm-1.

5

The changes in peak width and intensity indicate a restructuring process in the BA multilayer.

6

The decrease in the peak width suggests a transition to a more homogeneous molecular

7

environment and, thereby, an increase in ordering. The altered intensities are due to coupling in

8

the crystalline layer and due to changes of the average molecular orientation. Consequently, we

9

attribute the transformation at around 180 K to a crystallization process in the BA multilayer. A 62-64

10

similar behaviour has been observed for other organic thin films

. Note that in the bulk BA

11

crystallizes in the monoclinic system with four BA molecules in the unit cell (space group P2 1/c)

12

55

13

dimers in a tilted stacking.

. In this structure, the BA molecules form planar, centrosymmetric and hydrogen-bonded

14

The enhanced splitting that is observed in the C=O stretching region is consistent with the

15

formation of dimers upon crystallization. Interestingly, the intensity of the CH out-of-plane mode,

16

which is polarized perpendicular to the aromatic plane decreases upon crystallization, whereas

17

the two other modes which are polarized parallel to the aromatic plane increase in intensity.

18

Qualitatively, these changes indicate that the crystallites adopt a preferential orientation with

19

respect to the surface, in which the BA has a larger tilting angle than in the disordered film at

20

100 K. Above 200 K, no further reorientation or restructuring processes are observed. The BA

21

multilayer is found to desorb at 237 K.

22


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3.3

Page 14 of 36

Benzoic Acid/MgO(100)/Ag(100): Adsorption and time-resolved IRAS at 300K

2

To probe the interaction of BA with MgO(100)/Ag(100) at room temperature (300 K)

3

isothermal dosing experiments were performed with the same deposition rate as used at low

4

temperature. IR spectra were recorded for 60 min during deposition. In Figure 3a the full set of

5

IRAS spectra is shown. We observe a peak at 1431 cm-1 which remains dominant over the

6

complete exposure range. In addition a number of weaker features are observed, the most intense

7

of which is the one at 1605 cm-1. The intensity of these bands as a function of deposition time is

8

shown as inset in Figure 3a. We observe that the band intensity almost reaches saturation after 2

9

min. Thereafter the bands grow very slowly and the relative absorption never exceeds values

10

larger than about 1%.

11

In Figure 3b we compare the spectra after 1min and 60 min deposition time, in Figure 3c we

12

provide a comparison between the spectra in the monolayer region at 100 K and at 300 K,

13

respectively. From the dependence of the peak intensity as a function of exposure we conclude

14

that at 300 K only a monolayer of chemisorbed BA is stabilized on the surface. The spectra

15

suggest that one dominant species is formed over the full coverage range (see Figure 3b), very

16

similar to the one formed at 100 K. Following the arguments in Section 3.1 we assign the bands

17

to a distorted bidentate carboxylate. The similar dependence on exposure of the monolayer bands

18

at 100 K and at 300 K indicates that the sticking coefficient is also similar in both temperature

19

regions.

20

However, upon closer inspection two minor differences are identified, between the BA

21

monolayer at 100 K and at 300 K. First, the symmetric OCO stretching mode at 1431 cm-1 shows

22

a broadening and/or splitting at 300 K with a second weaker band at observed at 1450 cm -1. The

23

latter band increases in intensity with increasing exposure. We may tentatively assign this band

24

to a second carboxylate species, for example at defect sites. The most prominent difference


14

Page 15 of 36

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1

between the spectra at 100 K and 300 K is related to the out-of-plane CH deformation mode at

2

719 cm-1, which is not visible at 300 K. This observation points to a larger tilting angle between

3

the aromatic plane and the surface with a nearly perpendicular orientation.

4

The fact that the molecule adsorbs in a nearly perpendicular orientation at 300 K is in

5

agreement with the DFT calculations which show that there are only small energy differences

6

between tilted and upright geometries in the low coverage limit. Therefore small changes in the

7

experimental conditions could easily lead to a change from tilted to upright geometries.

8

3.4

Phthalic Acid on MgO(100)/Ag(100): Adsorption and time resolved IRAS at 100K

9

In the second part of this work we compare the behaviour of BA to that of PA. In contrast to

10

BA, PA may bind to the surface either via one or via two carboxylic acid groups, thereby

11

forming a mono- or a bis-carboxylate.

12

In the first experiment the MgO(100)/Ag(100) surface was exposed to PA for 60 min at a

13

temperature of 100 K. During deposition IRAS spectra were continuously recorded with an

14

acquisition speed of 1 min/spectrum. The corresponding set of spectra is displayed in Figure 4a.

15

In Figure 4b, we compare a spectrum after a deposition time of 1 min (monolayer region) and

16

after a deposition time of 60 min (multilayer region).

17

In the monolayer region we observe a prominent band at 1420 cm-1 and a number of much

18

weaker and broader features, for example at around 1600 and around 1700 cm-1. In the

19

multilayer region a large number of bands appear, with the most intense ones at 1260, 1310,

20

1420, 1520, and 1710 cm-1. A complete list of bands observed in the monolayer and the

21

multilayer region together with the assignments based on the literature are given in table 2. In

22

addition, we have performed density functional (DF) theory calculations of the isolated PA and

23

have analysed the vibrational modes using the visualization software QVibePlot

24

corresponding graphs are shown in the Supporting Information.

57

. The


15


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Page 16 of 36

1

Considering the monolayer region first, we note that the spectra are dominated by the band at

2

1420 cm-1. As discussed for the BA the band is associated with the symmetric OCO stretching

3

mode of a bidentate carboxylate. The fact that weaker features are observed in the region

4

between 1500 and 1600 cm-1 suggests that, similar as for BA, the carboxylate is adsorbed as a

5

form of a slightly asymmetric bidentate. Interestingly, a weak feature at around 1700 cm-1 is

6

observed even in the monolayer region. This observation suggests that, in addition to the

7

majority of carboxylic acid groups deprotonating and forming carboxylates, a smaller fraction of

8

intact carboxylic acid units is preserved. This implies that in addition to the majority of bis-

9

carboxylates a smaller fraction of mono-carboxylates coexists on the surface.

10

In order to investigate this phenomenon in more detail, we plot the intensities of the bands at

11

1420 cm-1 and 1710 cm-1 as a function of coverage (see Figure 4c). We find that the intensity of

12

the two bands behaves very differently. Initially, the band at 1710 cm-1 grows only slowly but

13

shows an accelerated increase in intensity after about 2 min of deposition. In contrast, the band at

14

1420 cm-1 dominates at low deposition times, but then stops to grow and even decreases slightly

15

after 5 min deposition. At larger deposition times its intensity increases linearly again.

16

This behaviour can be explained as follows. At low coverage the dominance of the symmetric

17

OCO stretching band (1420 cm-1) indicates that most of the PA molecules are linked via both

18

carboxylate groups forming a surface bis-carboxylate. The low intensity of the out-of-plane

19

vibration at 743 cm-1 points to a large tilting angle between the aromatic plane and the surface.

20

Indeed DFT calculations with a coverage of one PA molecule per 18 MgO(100) unit cells

21

confirm a bis-bidentate adsorption geometry of the bis-carboxylate with the protons located in

22

close vicinity to the carboxylate group. Fig 4d shows two adsorption geometries A and B

23

obtained by optimization from different starting points for this coverage. Both geometries


16

Page 17 of 36

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1

represent local minima on the potential energy surface. Further local minima with the protons

2

located further away from the carboxylate groups, as in the case of BA, exhibit higher energies,

3

see SI. Geometry A represents the structure with the lowest energy and features an upright

4

standing PA. Structure B with a somewhat higher energy exhibits a tilting angle between the

5

aromatic plane and the surface. Structure C results from an optimization at higher coverage (two

6

PA molecules per 8 MgO(100) unit cells) and also shows tilting of the molecule. This suggests

7

that the tilting found in experiment is again due to packing effects and adsorbate-adsorbate

8

interactions. Fig. 4d shows that the adsorption of PA leads to a corrugation of the MgO(100)

9

surface which results from optimizing the interaction of the carboxylate groups with the

10

magnesium ions of the surface.

11

The fact that the free carboxylate band at around 1700 cm-1 remains visible indicates the

12

coexistence of a minor fraction of singly-bound mono-carboxylate. With increasing surface

13

coverage the ratio of species changes in favour of the mono-carboxylate. We propose that the

14

slight decrease in intensity observed for the νs(OCO) band in the upper monolayer region

15

(around 5 min deposition time) may be associated with a change in adsorption geometry from a

16

more symmetrically to a more asymmetrically adsorbed carboxylate. This process may be

17

associated with the transition from bis-carboxylate to mono-carboxylate bonding. In the

18

multilayer region (after 10 min deposition time) the linear increase of the band at 1700 cm -1

19

indicates growth of the PA multilayer. Noteworthy, the band at 1420 cm-1 also grows linearly in

20

this region which is due to a weak PA bulk band (OH, CH deformation, see Table 2 and SI) at

21

similar frequency.

22

Noteworthy are features in the multilayer spectrum at 1774, 1790 and 1853 cm-1 (and a

23

shoulder at 1263 cm-1) which cannot be attributed to molecular PA. In fact these bands can be


17


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65

Page 18 of 36

1

assigned to molecular phthalic anhydride (PAA)

. In our previous work we have investigated

2

the vibrational spectrum of PAA on MgO as a function of coverage and temperature

3

conclude that during PVD of PA a small fraction of PAA is formed which is then co-deposited

4

with the PA multilayer. We have also analysed the content of the PA evaporator after prolonged

5

operation and found no indication for dehydration. Therefore, we conclude that the PAA is

6

formed during evaporation in UHV.

66-67

. We

7 8

3.5

Phthalic Acid on MgO(100)/Ag(100): Temperature programmed IRAS

9

The temperature-dependent behaviour of PA multilayer films was investigated by TP-IRAS

10

experiments similar to the one described in Section 3.2 for BA. The temperature was ramped

11

from 107 K to 401 K, with a heating rate of 2 K/min while IR spectra were recorded at a rate of 1

12

spectrum/min. Selected spectra are plotted in Figure 5a together with the intensity of

13

characteristic bands in Figure 5b. Specifically, we analyse the intensity of three bands which

14

originate from PA (743 cm-1, 1420 cm-1, 1711 cm-1) and three bands which are due to the co-

15

deposited PAA (1774 cm-1, 1790 cm-1, 1853 cm-1).

16

In the temperature range between 110 K and 250 K all bands undergo minor changes in

17

intensity only. This allows us to exclude thermally induced changes of the average molecular

18

orientation. The absence of restructuring phenomena at low temperature is in strong contrast to

19

both BA (see Section 3.2) and pure PAA (see

20

180 K. For pure PAA we previously found a reorientation to a flat-lying structure at 150 K and a

21

phase transition to a geometry with more upright standing molecules at 180 K. We attribute the

22

absence of reorientation transitions to the more rigid hydrogen bonded network in PA, which

23

requires higher activation energies to induce restructuring.

66

). For BA we found crystallization to occur at


18

Page 19 of 36

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1

Between 250 and 260 K the spectra change drastically. First, we observe a strong decrease of

2

the three bands associated to the PAA. In parallel, changes occur in the bands associated with the

3

PA. We observe a very strong increase in the intensity of the band at 743 cm -1 and a moderate

4

increase of the band at 1711 cm-1. The band at 1420 cm-1 slightly decreases in intensity instead.

5

The sudden decrease in the PAA signals suggests that the observed structural transformation of

6

PA is triggered by desorption of the minor fraction of PAA co-deposited. In our previous work

7

on pure PAA films, we have shown that the PAA multilayer desorbs at 240 K

8

higher desorption temperature observed in the present work is attributed to the fact that the PAA

9

is embedded in the solid PA matrix. It is noteworthy that the anhydride bands do not disappear

10

completely at 260 K but weak anhydride features remain up to about 320 K. The latter may be

11

due to a small fraction of PAA remaining buried in the PA layer.

66

. The slightly

12

After desorption of the major part of PAA at 260 K the remaining bands become narrower and

13

their intensities change drastically. These changes indicate a phase transition to a crystalline

14

structure, triggered by the loss of PAA. The pronounced increase in the intensity of the out-of-

15

plane CH deformation mode at 743 cm-1 implies that the average orientation of the PA in the

16

crystalline phase is more parallel to the surface. This is in sharp contrast to the PAA multilayer,

17

for which a transition to a phase with more upright standing molecules was observed above 180

18

K

19

data alone due to the complex spectrum and the low symmetry of the molecule. Above 260 K no

20

restructuring phenomena are observed until PA finally desorbs around 310 K.

66

. A more precise determination of the molecular orientation is not feasible from the IRAS

21 22

3.6

Phthalic Acid on MgO(100)/Ag(100): Adsorption and time-resolved IRAS at 300 K

23

Finally we investigate the interaction of PA with MgO(100)/Ag(100) at 300 K. As for BA, the

24

surface was exposed to PA for 60 min and IR spectra were recorded continuously. The


19


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Page 20 of 36

1

corresponding spectra are displayed in Figure 6a. Four features are observed. The most

2

prominent peak appears at 1441 cm-1, a weaker and sharp feature is observed at 1497 cm-1, and

3

two broader features are found at 1576 and 1708 cm-1.

4

All bands saturate after exposure times of less than 15 min and none of the bands exceeds a

5

relative absorption of 1%. This observation suggests that PA adsorbs in form of a single

6

monolayer only and multilayers are not stable under the present conditions. In Figure 6b we

7

compare two spectra, one taken during the initial stage of adsorption (1 min) and one recorded

8

after extended exposure (60 min). In addition we show the intensities of the four mentioned

9

peaks as a function of exposure (see Figure 6c). We find that the bands at 1441 cm -1 and 1497

10

cm-1 grow fastest at low exposure, whereas the bands at 1576 cm-1 and at 1708 cm-1 follow with

11

some delay.

12

We can rationalize the observed behaviour as follows. At low exposure the symmetric OCO

13

stretching band of the carboxylate group at 1441 cm-1 dominates and the intensity of the

14

asymmetric band at 1576 cm-1 remains comparably small. As discussed before this intensity

15

pattern indicates formation of an asymmetric carboxylate in bidentate geometry. As the signature

16

of the free carboxylic acid group at 1708 cm-1 is rather weak in the first spectra, we infer that the

17

majority of molecules adsorbs in form of a bis-bidentate carboxylate. The origin of the weaker

18

band at 1497 cm-1 is not clear. Tentatively, we may associate this band with OCO stretching

19

mode of a carboxylate in different adsorption geometry (for example originating from a mono-

20

carboxylate or from adsorption at defects). The intensity development of the C=O stretching

21

band of the free COOH (1708 cm-1) indicates that the concentration of intact carboxylic acid

22

groups is small at low exposure, but rapidly increases upon prolonged exposure. We suggest that

23

with increasing exposure, the adsorption mechanism changes from preferential formation of bis-


20

Page 21 of 36

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1

carboxylates to preferential formation of singly-bound mono-carboxylates as the surface

2

becomes more crowded. In parallel the increasing intensity of the asymmetric stretching mode

3

(1576 cm-1) suggests more asymmetric adsorption geometries at higher coverage. Finally, we

4

note that the out-of-plane CH deformation mode at 743 cm-1 remains very weak. This

5

observation indicates a nearly upright standing molecular orientation. In summary, we find that

6

the adsorption geometry and mechanism are very similar in the monolayer region both at 100 K

7

and at 300 K. The PA monolayer binds to the surface via a bis-bidentate carboxylates (low

8

coverage) and bidentate mono-carboxylates (high coverage), with a large tilting angle between

9

the aromatic plane and the surface. The molecular orientation of the multilayer, however, does

10

not follow the molecular orientation in the monolayer. This behaviour is in contrast to the case of

11

PAA on MgO(100)

12

molecular orientation at the molecule-oxide interface.

66

where the orientation of the multilayer film was shown to adopt the

13 14

4. Conclusions

15

In conclusions, we have studied the adsorption, interface reaction and growth of benzoic acid

16

(BA) and phthalic acid (PA) on a well-ordered MgO(100) film on Ag(100) using time-resolved

17

and temperature-programmed IRAS under UHV conditions.

18

At 100 K in the monolayer region BA deprotonates quantitatively and binds to the MgO(100)

19

surface forming a distorted bidentate carboxylate. The aromatic plane adopts a small tilting angle

20

with respect to the MgO(100) surface.

21

At 100 K in the multilayers region BA form poorly ordered layers which crystallize around

22

180 K. In the crystalline phase, the BA forms dimers with a smaller tilting angle with respect to


21


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Page 22 of 36

1

surface. The crystalline film is stable up to a temperature of 237 K at which the multilayer film

2

desorbs.

3

At 300 K, a monolayer of BA adsorbs on MgO(100) through deprotonation and formation of

4

an asymmetric bidentate carboxylate similar as at 100 K. The molecules adopt a nearly perfect

5

upright standing geometry.

6

PA shows a coverage dependent binding mechanism on MgO(100). At low coverage both

7

carboxyl groups undergo deprotonation and the molecule binds as an asymmetric bidentate bis-

8

carboxylate. With increasing coverage we find a transition to bonding via a single carboxylate

9

group and formation of a distorted bidentate mono-carboxylate. In both cases the molecule

10

stands nearly upright on the surface.

11

Physical vapour deposition of PA in UHV leads to co-deposition of a small fraction of phthalic

12

anhydride (PAA) formed during evaporation. The co-deposited PAA desorbs between 250 and

13

260 K, leaving behind a nearly pure PA film. Desorption of the PAA is associated with

14

crystallization of the PA multilayer. Upon crystallization the PA in the multilayer adopts a more

15

flat-lying orientation, in contrast to the strongly upright standing molecular orientation at the

16

molecule-oxide interface.

17 18

ASSOCIATED CONTENT

19

Supporting Information. Difference TP-IRAS of BA on MgO(100) (Figure S1), DF calculated

20

gas phase BA monomer IR spectrum (Figure S2), Visualization of DF calculated BA monomer

21

vibrational modes (Table S1); DF calculated BA dimer IR spectrum (Figure S3), Visualization of

22

DF calculated BA dimer vibrational modes (Table S2), IRAS of BA adsorption on

23

MgO(100)/Ag(100) at 200 K (Figure S4),. Difference TP-IRAS of PA on MgO(100) (Figure S5),


22

Page 23 of 36

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1

DF calculated gas phase PA IR spectrum (Figure S6), Visualization of DF calculated PA

2

vibrational modes (Table S3). DFT optimizations of BA, PA on MgO(100) with protons located

3

further away from carboxylate group (Figure S7). The Supporting Information is available free

4

of charge on the ACS Publications website.

5

AUTHOR INFORMATION

6

Corresponding author: [email protected], FAX: +49 9131 8528867

7

Notes. The authors declare no competing financial interest.

8 9

ACKNOWLEDGMENT

10

This project was financially supported by the Deutsche Forschungsgemeinschaft (DFG) within

11

the Research Unit FOR 1878 “funCOS – Functional Molecular Structures on Complex Oxide

12

Surfaces”. Additional support is acknowledges from the Excellence Cluster “Engineering of

13

Advanced Materials” in the framework of the excellence initiative. T.X. gratefully acknowledges

14

support by PhD grant of the China Scholarship Council (CSC).

15 16


23


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Page 24 of 36

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34. Guo, Q.; Williams, E. M., The effect of adsorbate-adsorbate interaction on the structure of chemisorbed overlayers on TiO2(110). Surface Science 1999, 433, 322-326. 35. Meier, D. M.; Urakawa, A.; Baiker, A., Adsorption behavior of salicylic, benzoic, and 2methyl-2-hexenoic acid on alumina: an in situ modulation excitation PM-IRRAS study. Physical Chemistry Chemical Physics 2009, 11, 10132-10139. 36. Kittelmann, M.; Rahe, P.; Gourdon, A.; Kuhnle, A., Direct Visualization of Molecule Deprotonation on an Insulating Surface. Acs Nano 2012, 6, 7406-7411. 37. Cebula, I.; Shen, C.; Buck, M., Isophthalic Acid: A Basis for Highly Ordered Monolayers. Angewandte Chemie International Edition 2010, 49, 6220-6223. 38. Shen, C.; Cebula, I.; Brown, C.; Zhao, J. L.; Zharnikov, M.; Buck, M., Structure of isophthalic acid based monolayers and its relation to the initial stages of growth of metal-organic coordination layers. Chem Sci 2012, 3, 1858-1865. 39. Desikusumastuti, A.; Staudt, T.; Happel, M.; Laurin, M.; Libuda, J., Adsorption and reaction of NO 2 on ordered alumina films and mixed baria–alumina nanoparticles: Cooperative versus non-cooperative reaction mechanisms. J Catal 2008, 260, 315-328. 40. Amende, M., et al., Dehydrogenation Mechanism of Liquid Organic Hydrogen Carriers: Dodecahydro-N-ethylcarbazole on Pd(111). Chemistry – A European Journal 2013, 19, 1085410865. 41. Kresse, G.; Hafner, J., Abinitio Molecular-Dynamics for Liquid-Metals. Physical Review B 1993, 47, 558-561. 42. Kresse, G.; Hafner, J., Ab-Initio Molecular-Dynamics Simulation of the Liquid-Metal Amorphous-Semiconductor Transition in Germanium. Physical Review B 1994, 49, 1425114269. 43. Kresse, G.; Furthmüller, J., Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comp Mater Sci 1996, 6, 15-50. 44. Kresse, G.; Furthmüller, J., Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Physical Review B 1996, 54, 11169-11186. 45. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple. Phys Rev Lett 1996, 77, 3865-3868. 46. Blöchl, P. E., Projector Augmented-Wave Method. Physical Review B 1994, 50, 1795317979. 47. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J Chem Phys 2010, 132, 154104. 48. Grimme, S.; Ehrlich, S.; Goerigk, L., Effect of the damping function in dispersion corrected density functional theory. Journal of Computational Chemistry 2011, 32, 1456-1465. 49. Ehrlich, S.; Moellmann, J.; Reckien, W.; Bredow, T.; Grimme, S., System-Dependent Dispersion Coefficients for the DFT-D3 Treatment of Adsorption Processes on Ionic Surfaces. ChemPhysChem 2011, 12, 3414-3420. 50. Monkhorst, H. J.; Pack, J. D., Special Points for Brillouin-Zone Integrations. Physical Review B 1976, 13, 5188-5192. 51. Methfessel, M.; Paxton, A. T., High-Precision Sampling for Brillouin-Zone Integration in Metals. Physical Review B 1989, 40, 3616-3621. 52. Hayashi, S.; Kimura, N., Infrared Spectra and Molecular Configuration of Benzoic Acid (Special Issue on Physical Chemistry). Bulletin of the Institute for Chemical Research, Kyoto University 1966, 44, 335-340.


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53. Klausberger, G.; Furić, K.; Colombo, L., Vibrational spectra and normal mode calculations of benzoic acid single crystals. Journal of Raman Spectroscopy 1977, 6, 277-281. 54. Boczar, M.; Szczeponek, K.; Wójcik, M. J.; Paluszkiewicz, C., Theoretical modeling of infrared spectra of benzoic acid and its deuterated derivative. Journal of Molecular Structure 2004, 700, 39-48. 55. Sim, G.; Robertson, J. M.; Goodwin, T., The crystal and molecular structure of benzoic acid. Acta Crystallographica 1955, 8, 157-164. 56. Reva, I.; Stepanian, S., An infrared study on matrix-isolated benzoic acid. Journal of molecular structure 1995, 349, 337-340. 57. Laurin, M., QVibeplot: A Program To Visualize Molecular Vibrations in Two Dimensions. Journal of Chemical Education 2013, 90, 944-946. 58. Arenas, J. F.; Marcos, J. I., Infrared and Raman-Spectra of Phtalic, Isophtalic and Terephtalic Acids. Spectrochim Acta A 1980, 36, 1075-1081. 59. van den Brand, J.; Blajiev, O.; Beentjes, P. C. J.; Terryn, H.; de Wit, J. H. W., Interaction of Anhydride and Carboxylic Acid Compounds with Aluminum Oxide Surfaces Studied Using Infrared Reflection Absorption Spectroscopy. Langmuir 2004, 20, 6308-6317. 60. Rotzinger, F. P.; Kesselman-Truttmann, J. M.; Hug, S. J.; Shklover, V.; Grätzel, M., Structure and Vibrational Spectrum of Formate and Acetate Adsorbed from Aqueous Solution onto the TiO2 Rutile (110) Surface. The Journal of Physical Chemistry B 2004, 108, 5004-5017. 61. Sobota, M., et al., Ionic liquid based model catalysis: interaction of [BMIM][Tf2N] with Pd nanoparticles supported on an ordered alumina film. Physical Chemistry Chemical Physics 2010, 12, 10610-10621. 62. Naselli, C.; Rabe, J. P.; Rabolt, J. F.; Swalen, J. D., Thermally Induced Order-Disorder Transitions in Langmuir-Blodgett-Films. Thin Solid Films 1985, 134, 173-178. 63. Cohen, S. R.; Naaman, R.; Sagiv, J., Thermally induced disorder in organized organic monolayers on solid substrates. The Journal of Physical Chemistry 1986, 90, 3054-3056. 64. Steinruck, H. P.; Libuda, J.; Wasserscheid, P.; Cremer, T.; Kolbeck, C.; Laurin, M.; Maier, F.; Sobota, M.; Schulz, P. S.; Stark, M., Surface Science and Model Catalysis with Ionic Liquid-Modified Materials. Advanced Materials 2011, 23, 2571-2587. 65. Haq, S.; Bainbridge, R. C.; Frederick, B. G.; Richardson, N. V., Anhydride ring chemistry at a metal surface. J Phys Chem B 1998, 102, 8807-8815. 66. Mohr, S.; Xu, T.; Dopper, T.; Laurin, M.; Gorling, A.; Libuda, J., Molecular Orientation and Structural Transformations in Phthalic Anhydride Thin Films on MgO(100)/Ag(100). Langmuir 2015, 31, 7806-14. 67. Mohr, S.; Doepper, T.; Xu, T.; Tariq, Q.; Lytken, O.; Laurin, M.; Steinrueck, H.-P.; Goerling, A.; Libuda, J., Organic linkers on oxide surfaces: Adsorption and chemical bonding of phthalic anhydride on MgO(100). In Surface Science. 68. Dobson, K. D.; McQuillan, A. J., In situ infrared spectroscopic analysis of the adsorption of aromatic carboxylic acids to TiO2, ZrO2, Al2O3, and Ta2O5 from aqueous solutions. Spectrochim Acta A 2000, 56, 557-565. 69. Colombo, L.; Volovšek, V.; Lepostollec, M., Vibrational analysis and normal coordinate calculations of the o-phthalic acid molecule. Journal of Raman Spectroscopy 1984, 15, 252-256.

44


27


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 57 58 59 60

1

Table 1. Assignments of vibrational frequencies of BA on MgO(100)/Ag(100). Peak position Peak position (cm(cm-1) 1) Assignment53-54, 58-59, 68 multilayer monolayer 719

1550

Comment (see also SI)

719

(CH)ring+ (CC)ring

951

(CH)ring+ (OH)

1026

(CC)ring+(CH)

1071

(CC)ring+(CH)

1117

(Ph-COOH)+ (CC)ring+(CH)

1173

(CC)ring+(CH)

1260

(CC)ring+(OH) +(CH)

(CC)ring+ ,(OH) +(CH)

1320

(CC)ring+(OH) +(CH)

ν,(OH)+ν(CC)ring ,Kekule) +(CH)

(CC)ring+(CH)+ (C-O)

νsym(O-C-O), monolayer

1422

2

Page 28 of 36

1450

(CH)

1495

(CH) νasym(O-C-O), monolayer

1540 1585

ν(CC)ring+(CH)

1605

ν(CC)ring

1692

ν(C=O)

1708

ν(C=O)

2853

ν(C-H)

2927

ν(C-H)

ν=stretching, γ=out of plane bending, =in plane bending, see SI for details


28

Page 29 of 36

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 57 58 59 60

1


Table 2 Assignments of vibrational frequencies of PA on MgO(100)/Ag(100). Peak (cm-1)

position Peak position Assignment (cm-1) 28-29, 58-59, 65, 68-69 multilayer monolayer (743)

743

γ(CH)ring+ γ(CC)ring

1140

(CH)

1263

νsym(C-O)

1310

ν(C-O)+ν(CC)+(OH)

1420

2

(see also SI)

Phthalic anhydride

νsym(O-C-O) 1420

(OH)

1520

(CH)

1580

ν(CC)ring

(1600)

1700

Comment

νasym(O-C-O) 1600

ν(CC)ring

1710

ν(C=O)

1774

νasym(C=O)

Phthalic anhydride

1790

(CH)+(CC)ring

Phthalic anhydride

1853

νsym(C=O)

Phthalic anhydride

2850

ν(CH)

2930

ν(CH)

ν=stretching, γ=out of plane bending, =in plane bending, see SI for details

3 4 5


29


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Page 30 of 36

1 2

Figure 1. a) IR spectra recorded during the adsorption and growth of BA on MgO(100)/Ag(100)

3

at 100K as a function time. b) Comparison of the first and last IRAS spectrum during the

4

deposition. Inserted is a scheme of proposed binding mechanism. c) Developments of peak

5

intensities at 719, 1422, 1708 cm-1 against deposition time. d) Three possible adsorption

6

geometries of BA on MgO(100) obtained by DFT optimization which all represent local minima

7

on the potential energy surface. The adsorption energy of geometry A, B and C are 1.46, 1.46


30

Page 31 of 36

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 57 58 59 60


1

and 1.18 eV. (Atoms of BA molecule are depicted as follow: oxygen red; hydrogen grey; carbon

2

black.).

3

4 5

Figure 2. IRAS recorded during a programmed heating (TP-IRAS) of multilayer BA on

6

MgO(100)/Ag(100). a) Selected regions of spectra showing the evolvement of peaks centered at

7

716, 1426, 1452 and 1696 cm-1. b) Integrated area of selected peaks as a function of substrate

8

temperature.


31


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Page 32 of 36

1 2

Figure 3. a) IR spectra recorded during the deposition of BA on MgO(100)/Ag(100) at 300 K.

3

Inserted is integrated peak area at 1431 and 1605 cm-1 against deposition time. b) Comparison of

4

IRAS spectrum after 1 and 60 min deposition. c) Comparison of sub-monolayer IRAS spectra at

5

100 K and 300 K surface temperature.


32

Page 33 of 36

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 57 58 59 60


1 2

Figure 4. a) IR spectra recorded during the adsorption and growth of PA on MgO(100)/Ag(100)

3

at 100K. b) Comparison of the 1st and 60th IR spectrum during the deposition. c) Developments

4

of peak intensities at 1420, 1722 cm-1 as a function of deposition time. Inserted is a scheme of

5

proposed changing binding mode as coverage increases. d) Three possible adsorption geometry

6

of PA on MgO(100) which were obtained by DFT optimizations and represent a local minima on

7

the potential energy surface. The adsorption energy for geometry A and B are 2.49 and 2.08 eV.


33


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Page 34 of 36

1

Geometry C corresponds to an adsorption energy of 1.82 eV for each molecule. (Atoms of PA

2

molecule are depicted as follow: oxygen red; hydrogen grey; carbon black.)

3

4 5

Figure 5. a) IR spectra recorded during a temperature programmed heating of multilayer PA pre-

6

deposited on MgO(100)/Ag(100). b) Developments of integrated peak areas with substrate

7

temperature. Peak centers are at 743, 1420, 1711, 1744, 1790 and 1853 cm-1.


34

Page 35 of 36

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 57 58 59 60


1 2

Figure 6. a) IR spectra recorded during the adsorption and growth of PA on MgO(100)/Ag(100)

3

at 300K. b) Comparison of the 1st and 60th IR spectrum during the deposition. c) Developments

4

of peak intensities at 1441, 1497, 1576 and 1708 cm-1 as a function of time.

5 6 7 8 9 10 11


35


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1 2

TOC graphic


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Benzoic Acid and Phthalic Acid on Atomically Well ... - ACS Publications

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