Electrochemical, High-Resolution Photoemission Spectroscopy and

Article pubs.acs.org/Langmuir

Electrochemical, High-Resolution Photoemission Spectroscopy and vdW-DFT Study of the Thermal Stability of Benzenethiol and Benzeneselenol Monolayers on Au(111) F. P. Cometto,† E. M. Patrito,*,† P. Paredes Olivera,‡ G. Zampieri,*,§,∥ and H. Ascolani§ †

Departamento de Fisico Química, Instituto de Fisicoquímica de Córdoba (INFIQC) and ‡Departamento de Matemática y Física, Instituto de Fisicoquímica de Córdoba (INFIQC), Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Córdoba, Argentina § Centro Atómico Bariloche, Comisión Nacional de Energía Atómica, Bariloche, Argentina ∥ Instituto Balseiro, Universidad Nacional de Cuyo, Bariloche, Argentina S Supporting Information *

ABSTRACT: The preparation and thermal stability of benzenethiol and benzeneselenol self-assembled monolayers (SAMs) grown on Au(111) have been investigated by electrochemical experiments and high-resolution photoemission spectroscopy. Both techniques confirm the formation of monolayers with high packing densities (θ = 0.27−0.29 ML) and good degrees of order in both cases. Despite many similarities between the two SAMs, the thermal desorption is distinctly different: whereas the benzenethiol SAM desorbs in a single steplike process, the desorption of the benzeneselenol SAM occurs with a much lower activation energy and involves the cleavage of some Se−C bonds and a change in molecular configuration from standing up to lying down. This behavior is explained by considering the different nature of the bonding of the headgroup with the metal surface and with the phenyl ring. Density functional theory calculations show that the breakage of the Se−C bond has a lower activation energy barrier than the breakage of the S−C bond.

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INTRODUCTION Self-assembled monolayers (SAMs) formed by the adsorption of organothiols or dialkyl disulfide molecules on metal surfaces have attracted much attention in the last few years. Most details of the self-assembly mechanism, the packing and orientation of the molecules, as well as the thermal and long-term stabilities of the SAMs have been revealed.1−4 In comparison, much less is known about SAMs made of aromatic molecules with a selenium headgroup, which have conducting and optical properties that make them very attractive for applications in the field of molecular electronics.5,6 The two simplest aromatic molecules that can be used for the preparation of SAMs on metallic surfaces are benzenethiol and benzeneselenol. The study of these SAMs, however, has been hampered by poor reproducibility and/or poor ordering, at least in comparison with what is achieved with molecules with two or more aromatic rings.7,8 Diphenyl disulfide (DPhDS) and diphenyl diselenide (DPhDSe) precursors yield essentially the same final product because both types of molecules adsorb dissociatively as the corresponding thiolate (benzenethiolate, BT) or selenolate (benzeneselenolate, BS). The use of an S− or Se− headgroup is expected to influence different aspects of the SAM, and in fact, the studies performed so far have reported differences in the type of ordering, electrical conductance and/or strength of the bond to the surface. © 2012 American Chemical Society

Scanning tunneling microscopy (STM) studies suggest in general that BS SAMs are more ordered than BT SAMs. Whereas STM images of BS SAMs reveal the existence of longrange-ordered domains,9,10 only disordered phases and small ordered regions with lateral dimensions of less than 15 nm are seen in BT SAMs.10−14 With regard to the orientation of the phenyl ring of BT SAMs, some reports found the rings nearly perpendicular to the surface14,15 whereas others reported a nearly flat configuration.10,16 In the case of BS SAM, the perpendicular configuration has been reported.10 The electrical conductance depends on the nature of the headgroup and the carbon chain. While aromatic selenols have a larger conductance than aromatic thiols,17 the opposite is trend is observed for aliphatic carbon chains.18 The thermal, chemical, and electrochemical stability of BT and BS SAMs shows different trends. From exchange19,20 and reductive desorption experiments,21 it was concluded that the Se−Au bond is stronger than the S−Au bond. Competitive adsorption experiments showed that DPhDSe displaces benzenethiolate from gold, but DPhDS does not displace benzeneselenolate. In this study, it was found that the adsorption of DPhDSe is more favorable by 0.7 kcal/mol.19 Received: June 19, 2012 Revised: August 14, 2012 Published: September 4, 2012 13624

dx.doi.org/10.1021/la3024937 | Langmuir 2012, 28, 13624−13635

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chemical cell with separate compartments for the reference (Ag/AgCl/ Cl−) and auxiliary electrode (a Pt sheet). The gold substrate was the working electrode; the part of the substrate in the electrochemical cell was confined to just the Au(111) face by means of the meniscus technique. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were performed with a Solartron 1260 electrochemical interface. The electrolyte (0.1 M NaOH) was thoroughly deaerated by bubbling with nitrogen prior to each experiment; a N2 atmosphere was maintained during the experiments. Voltammograms were measured at a scan rate of 50 mV/s. Impedance spectra were recorded in the frequency range of 0.1 Hz−10 kHz. The signal amplitude to perturb the system was 0.01 V. SAM Preparation. The self-assembled monolayers were prepared by immersing the gold substrates into 1 mM ethanolic solutions of the corresponding adsorbate. CV profiles were used to identify the best preparation conditions. After a series of experiments as a function of dipping time, we found that immersion for 15 min in the ethanolic solution produced the most reproducible monolayers as evaluated from the full width at half-maximum (fwhm) of the reductive desorption current peak and the charge under the peak. Therefore, we are confident that under these preparation conditions the monolayers could be formed in a reliable way. At longer dipping times, a lower reproducibility was observed. We also formed monolayers in hexane and chloroform solvents, but we did not observed major differences as evaluated with electrochemical measurements. In all cases, the dipping time was the most important parameter. Thermal Treatment. After SAM formation in the dipping solution, some substrates were heated for 30 min in a N2 atmosphere at a fixed temperature in the range from 298 to 573 K (±5 K). After this thermal treatment, the samples were quickly transferred under N2 to the cell where the electrochemical experiments were carried out. Because reductive desorption destroys the monolayer, a freshly prepared sample was used for each temperature. Photoelectron Spectroscopy. The photoemission experiments were carried out at the D08A-SGM beamline of the Brazilian Synchrotron Light Laboratory (Campinas, Brazil). The pressure in the analyzer chamber was in the low 10−9 Torr range. Electron energy spectra were collected with a 150 mm hemispherical analyzer with its axis placed 90° from the light beam and in the plane of the light polarization; all of the spectra were recorded with the sample oriented such that the surface normal made an angle of 45° with both the photon beam and the electron emission direction. Survey spectra were collected at a photon energy of 600 eV to check the sample cleanliness. S 2p and Se 3d core-level spectra were measured at a photon energy of 300 eV. Before and after each spectrum, we measured the Au 4f core-level spectra for count normalization and to calibrate the binding energies (BE) against that of the Au 4f7/2 core level at 84.0 eV. Typical survey spectra are shown in Figure S1 of the Supporting Information. To fit the S 2p spectra, we used up to three elemental components, each composed of a pair of Voigt functions separated by 1.18 eV and a fixed intensity ratio of 2:1. Similarly, for the fit of the Se 3d spectra we used up to three elemental components, each one made of a pair of Voigt functions separated by 0.86 eV and with an intensity ratio of 3:2. The Lorentzian widths were 0.15 and 0.2 eV for S 2p and Se 3d, respectively. The intensities, positions, and Gaussian widths of the components were varied during the fittings.

In a study of two fully analogous homologue series of thiols and selenol-based aromatic self-assembled monolayers on Au(l11) (with an alkyl spacer between the phenyl and the headgroup), selenium-based monolayers were also found to be more stable than their sulfur analogues in exchange experiments at room temperature.20 From electrochemical reductive desorption experiments, it was concluded that benzeneselenolate chemisorption on gold is more stable as compared to that at benzenethiolate, because the reduction peak potential of benzeneselenolate is ca. 200 mV more negative than that of benzenethiolate.21 However, the thermal and chemical stability of BS and BT monolayers shows the opposite trend to that observed by electrochemistry. Thermal desorption spectroscopy measurements of Käfer et al10 indicated that BS is less strongly bound to the gold substrate than BT. A recent analysis of this issue with density functional theory did not help to solve the controversy because it found similar adsorption energies for both thiols and selenols.22 Concerning the chemical stability, BS SAMs are less stable to air oxidation than BT monolayers because the former readily oxidize.19 The objective of this work is to elucidate the causes responsible for the different stabilities of BT and BS monolayers. We investigate the thermal stability of SAMs made of BT and BS species on Au(111) using high-resolution photoemission spectroscopy and electrochemistry. We show that the use of both techniques allows the identification and quantification of adsorbed species because surface coverages are readily obtained after the annealing procedure by cyclic voltammetry. In turn, activation energy barriers can be calculated from the variation of surface coverage with annealing temperature, as we presented in a previous work.23 Although BT and BS SAMs can be prepared with high packing densities, we found that their thermal behavior is quite different. The BS SAM desorbs quickly at temperatures slightly above room temperature and partially decomposes, producing atomic Se, whereas the BT SAM withstands prolonged annealings at more than 400 K. With the help of density functional theory calculations (including van der Waals interactions), the results are interpreted by considering the strengths of C−S and C−Se bonds as well as the strength of the surface bond between the headgroup and the Au(111) substrate in the presence of Au adatoms.

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EXPERIMENTAL DETAILS

Chemicals. All aqueous solutions were prepared with AR chemicals and deionized water (Milli-Rho, Milli-Q system). Dipping solutions were prepared using diphenyl disulfide (DPhDS), diphenyl diselenide (DPhDSe), benzeneselenol (BSe) (Sigma-Aldrich), and absolute ethanol (Baker). Gold Substrates. A gold crystal (MaTeck, Jülich, Germany), 4 mm in diameter, oriented better than 1° toward the (111)-face and polished down to 0.03 μm, was used as a working electrode for the electrochemical experiments. The cleaning of this substrate involved repeated cycles of annealing on a H2 flame during 2 min and cooling in a N2 atmosphere. In some cases, typically after experiments that left atomic species such as S or Se, the surface was exposed to a hot piranha solution (70:30 H2SO4/H2O2) for 10 s and then washed copiously with Milli-Q water. Au films (500 nm thick) evaporated on heat-resistive glasses were employed as substrates for the photoemission experiments. These substrates were annealed repeated times in a butane flame for 2 min and cooled to room temperature in a stream of nitrogen. Electrochemical Characterization. The electrochemical experiments were carried out in a conventional three-electrode electro-

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COMPUTATIONAL DETAILS Periodic density functional theory (DFT) calculations were performed using the PBE exchange and correlation functionals24 as implemented in the Quantum Espresso code.25 Norm-conserving ultrasoft pseudopotentials26 were used for the core electrons. The electron wave functions were expanded in a plane-wave basis set up to a kinetic energy cutoff of 28 Ry (180 Ry for the density). Brillouin zone integration was performed using a (4 × 4) Monkorst-Pack mesh.27 13625


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Figure 1. (a) Voltammogram (50 mV/s) and (b) S 2p photoemission spectrum of a film prepared by 15 min of immersion in a solution of DPhDS.

The periodic supercell approach was employed to describe a 3 × 3 unit cell of the Au(111) surface containing the adsorbate molecule. The surface was modeled using a slab with three gold layers. There are nine gold atoms in the unit cell for each layer; therefore, the surface coverage is 0.11 when there is one molecule in the cell. The positions of all of the adsorbate atoms as well as those of the two top-most Au layers were fully optimized. To ensure that there are not residual forces in the unit cell, we determined the equilibrium bulk structure of Au obtained for lattice constant a0 = 4.1768 Å. A vacuum thickness of 10 Å was introduced between the slabs. For the sake of clarity, in the figures we will show only the first layer of atoms. To take into account van der Waals (vdW) interactions, we employed the vdW density functional method (vdW-DFT) as implemented in the Quantum Espresso code.25 It uses the vdW density functional28,29 with an efficient evaluation of the double integral of the functional according to the procedure described by Soler and coworkers.30 We performed calculations with and without the vdW functional. We will refer to the latter as regular DFT.

Table 1. Position, Half-Width at Half-Maximum, and Charge Density of the Two Peaks in the Voltammograms of Figures 1−3 DPhDS E (V) fwhm (V) Q (μC/cm2)

−0.951 0.167 13

DPhDSe −0.932 0.183 46

−1.089 0.076 58

BSe −0.787 0.031 58

−1.070 0.136 18

charge density of 54 μC/cm2. The two peaks have small replicas in the positive (anodic) sweep, at slightly less negative potentials, which correspond to the oxidative readsorption of S and BT species. The S 2p spectrum shown in Figure 1b is dominated by a 2p3/2,1/2 doublet with the 2p3/2 peak located at around 162 eV. There are two minor features on both sides of the main peak that reveal the existence of the other two components. Then we fitted the spectrum with three 2p3/2,1/2 doublets, named S1, S2, and S3, in order of increasing BE. The parameters of the best fitting are listed in Table 2. Following previous work, the first two components S1 and S2 can be assigned to S atoms35 and BT molecules,36 respectively. Therefore, the photoemission spectrum totally confirms the finding in the voltammogram of these two species adsorbed on the surface. The exact position and width of the main peak is in good agreement with the results reported by other authors.13,36 The origin of the component S3 is less clear. It is often attributed to undissociated DPhDS molecules or to Sn aggregates.35 Figure 2 shows a voltammogram and a Se 3d photoemission spectrum measured in substrates immersed for 15 min in 1 mM ethanolic solutions of DPhDSe. This voltammogram is composed of two broad peaks in the cathodic sweep, at around −0.95 and −1.08 V, and only one peak in the anodic sweep, at around −1.0 V. The two peaks in the cathodic sweep, whose parameters are listed in Table 1, can be assigned provisionally to the desorption of BS and Se anions, respectively. We have found, however, that the voltammograms of these films undergo important changes when the dipping time in the solution is increased, which denotes that this case is more complex than that of the BT films described above. The Se 3d photoemission spectrum shown in Figure 2b shows clear evidence of two 3d5/2,3/2 doublets with the 3d5/2 peaks at around 53.5 and 54.5 eV. A good fitting of the spectrum, however, calls for the inclusion of two more components: a third, relatively broad, Se 3d doublet at larger

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RESULTS AND DISCUSSION DPhDS, DPhDSe, and BSe Films. Figure 1 presents the voltammogram and the S 2p photoemission spectrum of gold substrates immersed for 15 min in 1 mM ethanolic solutions of DPhDS. The voltammogram of Figure 1a is composed of a prominent peak in the cathodic sweep centered at −0.60 V and a broad hump between −0.9 and −1.0 V. The main peak is assigned to the desorption of BT molecules according to the reaction31 (1) RS−Au + e → RS− + Au The second peak (−0.944 V) has been attributed to the desorption of different thiols or sulfur atoms present in the solution as contaminants14,32 or to the cleavage of the S−C bond during the formation of the SAM.33 Following our previous results in ref 31, we assign the broad peak to the desorption of S atoms according to the reaction34 S−Au + 2e → S2 − + Au

−0.606 0.036 54

(2)

This means that after the immersion the sample is covered not only with BT molecules but also with a small amount of atomic S. Both peaks have been fitted with Gaussian functions, and the parameters are listed in Table 1. The main peak has a full width at half-maximum of only 36 mV and an integrated 13626


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Table 2. Binding Energy, Gaussian Width, and Area (Relative to That of the Au 4f7/2 Peak) of the Three Components in the S 2p and Se 3d Photoemission Spectra of Figures 1−3 DPhDS BE (eV)a GW (eV) area a

DPhDSe

BSe

S1

S2

S3

Se1

Se2

Se3

Se1

Se2

Se3

161.10 0.52 0.012 (8.3%) 0.14619

1.17 0.69 0.097 (66.5%)

2.36 0.88 0.037 (25.2%)

53.49 0.60 0.134 (48.5%) 0.277

0.81 0.76 0.087 (31.3%)

2.29 2.03 0.056 (20.2%)

53.56 0.60 0.013 (7%) 0.19

0.81 0.69 0.148 (78%)

1.89 0.95 0.028 (15%)

The positions of the S2 and S3 components are given relative to that of the S1 component.

Figure 2. (a) Voltammogram and (b) Se 3d photoemission spectrum of a film prepared by immersion for 15 min in a solution of DPhDSe.

Figure 3. (a) Voltammogram and (b) Se 3d photoemission spectrum of a film prepared by immersion for 15 min in a solution of BSe.

Therefore, from both the voltammogram and the Se 3d spectrum it is concluded that the 15 min of immersion in the 1 mM solution of DPhDSe results in films with large amounts of atomic Se on the surface. In view of the impossibility of preparing good films of BS molecules by immersion in a DPhDSe solution, we prepared films by immersing the substrates for 15 min in 1 mM ethanolic solutions of BSe. The corresponding voltammogram and Se 3d photoemission spectrum are presented in Figure 3. It shows that in this case the voltammogram is dominated by a narrow, well-defined peak at around −0.79 V; there is another smaller, broader feature at around −1.1 V and accompanying readsorption peaks in the anodic sweep. The integrated charge densities of the two peaks of reductive desorption are 58 and 18 μC/cm2.

BE and another very broad feature centered at around 57−58 eV to represent the Au 5p3/2 peak. The parameters of the three Se 3d doublets used for the fitting are listed in Table 2, where we have adopted the same convention of numbering the components in order of increasing BE. Like in the S 2p spectrum described above, the two components at the smallest BEs (Se1 and Se2) are assigned to atomic Se and to BS molecules,37 respectively. It can be seen both in the spectrum and in Table 2 that in this case the atoms are the dominant species on the surface. The third component may be assigned tentatively to undissociated DPhDSe molecules, although it must be taken into account that the intensity and width of this component can be strongly affected by the important overlap with the broad Au 5p3/2 peak. 13627


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whereas the coverages determined with voltammetry involve all the molecules adsorbed on the surface, those determined with STM are extrapolations to all the surfaces of observations made in small regions. In the case of the BS films, our coverage is in agreement with that reported by Käfer et al,10 and these values in turn are larger than that reported by Sato et al.21 If one deducts the adsorption sites occupied by the atomic species and computes the molecular coverages as the number of molecules per available sites,45 then the BT and BS coverages increase to θBTeff = 0.26 and θBSeff = 0.29. We note that immersion times longer than 15 min, although probably useful in improving the ordering of the film, do not substantially increase the coverage14,21 and, on the contrary, in some cases may result in larger numbers of atomic species. We think that these coverages must be close to the saturation limit because although they are slightly smaller than the maximum coverages obtained with alkanethiols, θalkane ≈ 1/3, they correspond to a molecular packing similar to that in a basal plane of crystalline benzene; in fact, the density of molecules in such planes divided by the atomic density in a Au(111) plane yields an equivalent coverage of θ = 0.275,46 which is quite close to the coverages θBTeff and θBSeff obtained above. The high packing densities obtained for both SAMs allow us to assume that the molecules must be oriented on the main in a standing-up configuration. This assumption is fully supported by all the studies in which the coverage and tilting angle of the molecules have been measured. Wan et al.14 have found that the phenyl rings in their BT film (θ = 0.23) were tilted about 30° from the surface normal, and Käfer et al.10 determined for their BS film (θ = 0.25) a tilt angle of 19°. Likewise, in a NEXAFS experiment with a BT film prepared by immersion for 24 h in DPhDS we found a tilt angle of 22° (Figure S2 in the Supporting Information). Therefore, both the voltammetry and the photoemission spectra show that an immersion time of 15 min in DPhDS and BSe solutions produces compact BT and BS monolayers. The narrow peaks observed for both spectroscopic methods indicate a good level of ordering, whereas the high molecular densities obtained from reductive desorption peaks indirectly imply a dominant vertical orientation. We note that the fwhm's of the S2 and Se2 components in the photoemission spectra of the BS and BT SAMs are similar to those reported by Zharnikov et al. for high-quality SAMs made of thioaromatic molecules with one, two, and three rings8 and for SAMs of hybrid 4,4′biphenyl-substituted alkaneselenolates.47 As outlined in the Introduction, from electrochemical and exchange experiments performed at room temperature, it was concluded that benzeneselenoate is more stable than benzenethiolate.19−21 We found that the reduction peak potential of benzeneselenolate is 181 mV more negative than that of benzenethiolate (Table 1), in good agreement with the value of ca. 200 mV found in ref 21. However, the elucidation of the strength of the surface bond in these experiments is not as straightforward as in thermal desorption studies. In the linear sweep voltammetry experiment, the system is not in equilibrium and the kinetics of the processes involved may result in the development of overpotentials. For example, no reductive desorption current peak is observed for alkanethiols on Pt. However, thermodynamically, it is easier to desorb alkanethiols from Pt than from Au in electrochemical experiments performed under constant-potential conditions.48 This indicates that kinetically it requires more time to desorb an alkanethiol SAM from Pt than from Au.48 We therefore

Figure 3b shows that the Se 3d photoemission spectrum is also dominated by one component, namely, the Se2 component at 54.37 eV, assigned to BS molecules. Again, a good fitting of the spectrum requires the inclusion of two smaller components, at lower and higher BE (Se1 and Se3, respectively), and the broad Au 5p3/2 feature. Because the Se1 component corresponds to atomic Se, the two peaks in the voltammogram can be safely assigned to the desorption of BS molecules (−0.787 V) and the desorption of Se atoms (−1.070 V). Possible assignments for the Se3 component in the photoemission spectrum are physisorbed benzeneselenol (RSeH) molecules or DPhDSe physisorbed molecules produced after the oxidation of BS molecules in the solution, although we recall the comment made above that this component is not well defined because of its overlap with the Au 5p3/2 peak. The integration of reductive desorption current peaks allows a quantitative determination of adsorbed electroactive species and the calculation of the surface coverage.38 The BT or BS species desorb in a reaction that involves the exchange of only one electron (eq 1). Therefore, by dividing the integrated charge density by the electronic charge, one obtains the surface density, which yields γBT = 3.35 × 1014 mol/cm2 for the film prepared with DPhDS and γBS = 3.63 × 1014 mol/cm2 for the film prepared with BSe. From these densities, one readily gets surface coverages of θBT = 0.24 and θBS = 0.26 (number of molecules per surface atom). The reductive desorption of S and Se atoms involves the exchange of two electrons (eq 2). From the reductive desorption current peaks of the atomic species in Figures 1 and 3, we obtain coverages of θS = 0.03 and θSe = 0.04. The ratios in the photoemission spectra between the intensities of the S 2p or Se 3d components and those of the Au 4f peaks can also be used to determine surface coverages.39,40 This method, however, is less reliable because one has to correct the measured intensities by the photoionization cross sections,41 attenuations of the photoelectrons,42 and analyzer transmissions, some of which may not be accurately known. Therefore, we will mention here only that all of the determinations made with the intensity ratios listed in Table 2 are in good agreement with the coverages derived from the voltammograms. In Table 3, we compare our surface coverages with those reported by other authors. In the case of BT films, it is seen that all the coverages obtained with cyclic voltammetry are coincident in θBT = 0.23−0.24. On the contrary, the coverages determined in STM experiments are rather dissimilar, varying from 0.18 to 0.31. At this point, it is important to note that Table 3. Comparison of the Surface Coverages Obtained by Different Authors θBT

θBS

0.24

0.26

CV

0.23 0.31 0.18

0.25

CV STM STM

0.23

0.17

CV

0.23 0.20

method

CV STM

preparation 15 min in 1 mM ethanol solutions of diphenyl disulfide and benzeneselenol 1 to 2 min in 0.1 mM aqueous solutions of benzenethiol

this work ref 14

12 h in 1 mM ethanol solutions of benzenethiol and benzeneselenol several minutes in 0.1 mM aqueous solutions of benzenethiol and benzeneselenol

ref 10

1 day in a 1 mM ethanol solution of benzenethiol

ref 21 ref 43 ref 44

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Figure 4. (a) Voltammograms of 10 films prepared by immersion for 15 min in an ethanolic solution of DPhDS and then annealing at the temperatures indicated on the right. (b) S 2p photoemission spectra of a single film prepared under the same conditions as in part a and after annealing at 373, 413, and 453 K (where the intensities are normalized to that of the Au 4f7/2 peak).

Figure 5. (a) Voltammograms of seven films prepared by immersion for 15 min in an ethanolic solution of BSe and then annealing at the temperatures indicated on the right. (b) Se 3d photoemission spectra of a single film prepared under the same conditions as in part a and after annealings at 373, 413, and 453 K (where the intensities are normalized to that of the Au 4f7/2 peak).

effects are expected to be more important for BS than for BT, thus shifting the potential to more negative values. Therefore, we think that the thermal treatment is the most efficient way to evaluate the strength of the surface bonding and the overall stability of the monolayer. Thermal Treatments. In the previous section, we have shown that the immersion of a gold substrate for 15 min in

think that the more negative reductive desorption potential of BS as compared to that of BT is influenced by kinetic considerations. Among other contributions, the reductive desorption potential depends on the diffusion of desorbed species from the surface into the solution. As diffusion coefficients decrease with the increase in molecular size, kinetic 13629


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C bonds may be the destabilizing factor in the BS−surface bonding. Indeed, in the DFT section, we will show that the coadsorption of BS and Se atoms decreases the binding energy of BS. Figure 6 shows the evolution with the annealing temperature of the BT and BS coverages (determined from the areas of the

DPhDS and BSe solutions produces compact SAMs of BT and BS molecules, respectively. In this section, we investigate their thermal stability. Figure 4 shows voltammograms and photoemission spectra of samples prepared by immersion for 15 min in a DPhDS solution and then annealing for 30 min at different temperatures. Both the voltammogram and the photoemission spectrum of the samples measured before the annealing (RT) are in good agreement with the voltammogram and photoemission spectrum shown in Figure 1; this illustrates the good level of reproducibility achieved in the preparation of the films.49 The voltammograms in Figure 4a show that the main peak, assigned to desorption of the BT species, decreases steadily with the annealing temperature until its complete disappearance at around 453 K. The S 2p photoemission spectra of Figure 4b corresponds to a single sample annealed in vacuum at the three temperatures indicated on the right. It is seen that between room temperature and 373 K there is only a small decrease in the BT component. Further annealing at 373 K produces a marked decrease of the BT component without much change in the other components. Finally, the annealing at 453 K produces the disappearance of all of the components. Figure 5 shows a similar series of voltammograms and photoemission spectra corresponding to samples prepared by immersion for 15 min in a BSe solution and then annealing for 30 min at the temperatures indicated in the panels. The voltammogram and photoemission spectrum acquired at room temperature are again in very good agreement with those shown in Figure 2. Figure 5a shows that mild annealing at only 318 K is enough to cause an important loss of intensity of the main peak; as the temperature is increased, the peak shifts to less-negative potentials, develops a marked asymmetry, and eventually splits into two peaks. The most important finding, however, is that no traces of the peak are found after the annealing at 398 K, a temperature substantially lower than that found for the films prepared with DPhDS. This is confirmed by the photoemission spectra shown in Figure 5b. These spectra were measured in a single sample annealed in vacuum at three temperatures. Two important differences with the spectra of the BT SAM are to be noted. In the first place, it is observed that the first annealing at 373 K produces a significant decrease of component Se2, assigned to BS molecules, whose intensity decreases almost 3-fold. In the second place, it is also seen that the disappearance of BS molecules is accompanied by an increase in the amount of atomic Se on the surface. This is evidenced by the increase in component Se1, which increases almost 4-fold. This latter observation is a direct indication that annealing produces the cleavage of the C−Se bond. We note, however, that the increase in the Se1 component does not equal the decrease in the Se2 component, so the cleavage must occur in only a fraction of the molecules. Further annealing at 373 K causes the BS molecules to fall to the limit of detection, leaving the Se atoms as the dominant species on the surface, and after the annealing at 453 K, all of the species have disappeared. Figure 4 shows that the reductive desorption current peak of BT broadens only with increasing temperature whereas Figure 5 shows that the corresponding peak for BS broadens and shifts to less-negative potentials. The broadening is usually interpreted as a disordering of the monolayer, whereas the changes in the reductive desorption potential may be indicative of changes in the surface bonding.23,31 We think that the appearance of selenium atoms upon the breakage of some Se−

Figure 6. Plots of the capacitance and of the coverage as a function of the annealing temperature of SAMs prepared by immersion for 15 min in ethanolic solutions of (a) DPhDS and (b) BSe. The meaning of the arrows in the lower panels is explained in the text; Käfer and Noh in the plot of the BT SAM correspond to refs 10 and 13, respectively; α and β in the plot of the BS SAM correspond to the two phases found in ref 10. Typical error bars are 5% of the values shown in the plots. The initial coverages for the magenta and blue lines were calculated from the total coverage of BT (0.24, Table 3) multiplied by 0.8 and 0.2, respectively.

main desorption peaks in the voltammograms) and of the capacitances of the films. The latter were obtained by electrochemical impedance measurements performed after annealing and just before voltammetry. It is clearly seen in both cases that the disappearance of the SAM is accompanied by a change in the capacitance from that of the substrate covered with the molecular layer to that of the bare substrate. It is interesting that the capacitance of the BS film at room temperature, 5.8 μF/cm2, is slightly lower than that of the BT film, 7.2 μF/cm2. As discussed above, both the electrochemical and the XPS experiments show that BT and BS form compact monolayers with a molecular density that is very close to that of a basal plane of crystalline benzene. We therefore conclude that the differences in the capacitances reveal the fact that the BS monolayer is slightly thicker than the BT monolayer. This is in agreement with the molecular size of BS being larger than that of BT and the tilt angle of BS (19°) being lower than that of BT (30°).12,16 The two lower panels of Figure 6 reveal that besides the different desorption temperatures, the evolutions of the coverages with the annealings are also different. Whereas the BT SAM desorbs in two steplike changes that occur at around 340 and 430 K, the BS SAM desorbs in a more continuous way. To analyze these desorption curves, we use the Polanyi− Wigner (PW) equation, which gives the instantaneous rate of desorption at temperature T dq ⎛ ΔE ⎞ n ⎟θ = −νn exp⎜ − ⎝ kT ⎠ dt 13630


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where νn is the frequency factor (normally taken equal to be 1013 s−1), ΔE is the activation energy, k is the Boltzmann constant, and n = 1, 2... is the desorption order parameter. The PW equation allows the inclusion in Figure 6 of the results obtained by other authors with thermal desorption spectroscopy (TDS). This is done by assuming that desorption occurs as a first-order process in those experiments and with the reported activation energies;50 because this assumption yields steplike desorption curves, each result is represented in Figure 6 by an arrow placed at the temperature at which the coverage falls to 50% of the initial value. The red line superimposed on the data points in the lowerleft panel shows that the desorption of the BT SAM is well reproduced by assuming two successive first-order processes with activation energies of ΔE = 25.8 and 32.3 kcal/mol.51 It is seen that the first desorption process (blue line) occurs at a temperature very similar to that calculated with the activation energy reported by Käfer et al. for this film (27.0 kcal/mol).10 It can be observed, however, that this desorption involves only about 20% of the SAM; the remaining 80% desorbs at a much larger temperature. The second desorption process (magenta line), which leads to the complete disappearance of the SAM, occurs at a temperature and with an activation energy that coincides exactly with that reported by Noh et al.13 The evolution of the BS coverage with the temperature is different and cannot be fitted satisfactorily with either a firstorder or second-order process. The two arrows in this panel correspond to the results of Käfer et al, who observed in this case two well-separated desorption peaks with corresponding activation energies of 24.2 and 30.2 kcal/mol;10 the first peak was ascribed to the desorption of the initial saturated phase (α), with the molecules in a standing-up configuration, and the second peak was ascribed to the desorption of a diluted phase (β) with the molecules mostly in a lying-down configuration. This report of the coexistence of two phases during the desorption of the BS film suggests an explanation of the evolution of the coverage of this SAM in the following terms. Initially, desorption occurs only from the α phase. Then, as the coverage diminishes and the steric hindrance to adopt a lyingdown configuration gradually disappears, desorption occurs also from the phase β. Finally, when most molecules have adopted the lying-down configuration, desorption occurs only from the β phase. This type of behavior has been observed and described in great detail for hexanethiol SAMs by Kondoh et al.,52 who found that the molecules changed reversibly from one type of configuration to the other at a coverage of θ ≈ 0.13. The simplest way to account for the change in configuration during annealing is to add a term rαβθα(1 − θα − fθβ) to the PW equations for the α and β phases, with negative and positive signs, respectively.53 With this added term, the number of molecules in the standing-up configuration will decrease because of both the desorption and the change to the lyingdown configuration, with the latter occurring at a rate governed by rαβ and the availability of sites. Instead, the number of molecules in the lying-down configuration will tend to increase because of the molecules that change configuration and tend to decrease because of the thermal desorption (with a larger activation energy). One ends with a set of two coupled differential equations that can be integrated numerically (up to the time t = 30 min using initial values of θα(0) = 1 and θβ(0) = 0) to obtain the two partial coverages and thereby the total coverage after each annealing. The best fit to the experimental points is the red line shown in the lower-right panel

superimposed on the experimental curve; the green and gray lines correspond to the α- and β-partial coverages, respectively. It is seen that after the annealings at temperatures of less than 330 K the surface contains only molecules in the standing-up configuration; between 330 and 350 K both types of molecules coexist, and at higher temperatures, only molecules in the lyingdown configuration remain on the surface. We note that in this last temperature region the gradual disappearance of the molecules in the lying-down configuration is not due to desorption from this phase; it rather occurs because the rate of desorption from the standing-up configuration becomes so much faster than the rate of configuration change that progressively fewer molecules can make the switch. Because of this, the fitting is rather insensitive to the activation energy for the desorption of the β phase. The desorption energy obtained for the α phase is 25.1 kcal/mol, which is in excellent agreement with the energy determined by Käfer et al.10 for this phase through TDS. Our results mostly agree with those of Käfer et al,10 with the important exception that in ref 10 the BT SAM desorbed completely with an activation energy of 27.0 kcal/mol, whereas in our case this produces the desorption of only a minor portion of it. The complete desorption of the SAM occurs with an activation energy of 32.3 kcal/mol, in agreement with what was found in ref 13. This means that although we agree with Käfer et al. in that the BT SAM has a better thermal stability than the BS SAM, we find a much larger difference in the activation energies (32.3 vs 27.0 kcal/mol). In ref 10, this difference was attributed to the interactions of the anchoring groups with the surface. In that case, however, there were other differences between the two films, besides the headgroups, that could also play a role. Noticeably, the smaller coverage and larger tilt angle of the BT molecules could result in an additional (attractive) interaction with the surface. In our case, the structures of both SAMs are more similar; the effective densities differ by less than 10%, and all of the molecules are expected to be in similar upright configurations. Therefore, we think that to first order the interactions between phenyl rings and between the rings and the surface should be similar in both films and that any eventual difference in these interactions could not explain the large difference observed in the thermal stabilities. Therefore, we are led to conclude that the different thermal stabilities are determined mainly by the nature of the bonding of the headgroup with the metal surface. DFT Calculations. The experiments described in the previous section reveal at least three important differences in the behavior of the BT and BS SAMs when they are annealed: (a) a much better thermal stability of the BT SAM, (b) the cleavage of some Se−C bonds in the BS SAM, and (c) a change in orientation of some molecules in the BS SAM. To shed some light on these issues, we investigated both the strength of the headgroup−surface interaction and the strength of the bond between the headgroup and the phenyl ring. The adsorption of BT and BS was investigated on the unreconstructed surface as well as on Au(111) with a gold adatom. Figure 7 shows the energy profile along the reaction coordinate for the breakage of Se−C and S−C bonds of BS and BT initially adsorbed on Au(111). The structures of reactants, transition states, and products are shown in panels I− III. The reaction first proceeds by an enlargement of Se−C and S−C bonds whereas the headgroup remains bicoordinated to the surface up to the reaction coordinate of the transition state (panel II). In the transition state, the phenyl group is already 13631


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molecule in the 3 × 3 unit cell. This may be another factor contributing to the lower thermal stability of BS SAMs. The formation of thiol SAMs on gold involves a considerable rearrangement of the gold substrate, and it is well established that gold adatoms are involved in the thiolate surface bonding.54−57 The self-assembly of purely aromatic thiol SAMs is usually accompanied by the formation of gold islands,11,58,59 and a recent study of pentafluorobenzenethiols has shown that the size and concentration of gold islands depend on the dipping time in the forming solution. We therefore calculated the binding energy of the BT and BS species on the unreconstructed Au(111) surface as well as in the presence of a gold adatom. Figure 8 shows the main equilibrium configurations, and Table IV contains the corresponding binding energies. For the

Figure 7. Energy profile as a function of the reaction coordinate for the breakage of S−C and Se−C bonds for BT (panel I) and BS (panel I′) adsorbed on Au(111). Panels I−III show the structures of reactants, transition state, and products for BT whereas panels I′−III′ show the corresponding structures for BS.

bonded to a gold atom that is shifted upward from the plane of the surface as shown in panel II. Figure 7 shows that the activation barrier for the breakage of the Se−C bond (23.5 kcal/mol) is lower than the barrier for the S−C bond breakage (31.0 kcal/mol). The decomposition of BS is also less endothermic than the decomposition of BT. The fact that the surface concentration of selenium atoms increases during the initial stages of the annealing of BS, whereas this does not occur for sulfur during the annealing of BT, correlates with the lower energy barrier for the breakage of the Se−C bond as compared to that of the S−C bond (Figure 6). In turn, we note that the presence of Se atoms on the surface has a destabilizing effect on the BS−surface bonding. For example, the binding energy of BS decreases to around 7 kcal/mol when two Se atoms are coadsorbed with a BS

Figure 8. Equilibrium structures of BT and BS adsorbed (a) on the unreconstructed Au(111) surface and (b, c) around an Au adatom. (b) Phenyl ring parallel to the surface and headgroup monocoordinated to the Au adatom. (c) Upright configuration with the headgroup monocoordinated to the Au adatom.

adsorption on the unreconstructed Au(111) surface, Table IV contains the binding energies at the vdW and regular DFT levels (numbers in parentheses). The binding energy of BS is around 3 kcal/mol higher than that of BT. By comparing the binding energies calculated at the vdW and regular DFT levels, it can be observed that the van der Waals interaction contributes around 5 kcal/mol to the binding energy. 13632


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The thermal studies of BT monolayers reported by Noh,13 Käfer,10 and us in this work show a dispersion in the values of activation energy barriers as discussed above. In light of the results of this section, we think that such a dispersion cannot be solely attributed to changes in the monolayer structure. The structure and the dynamics of the gold substrate must also be taken into account. A recent STM study of the structure of pentafluorobenzenethiols SAMs59 with the immersion time indeed showed a sequence of phase transitions that correlate with the changes in the substrate structure. This work showed that gold islands are formed upon adsorption of the aromatic SAM and that their size, density, and shape evolve as a function of the dipping time. We therefore think that the changes that occur at around 340 and 430 K in Figure 6 for BT could be attributed to the desorption from domains with different substrate structures.

Table IV. Binding Energies (BE) in kcal/mol of BT and BS and Binding Energy Changes (ΔBE) on the Unreconstructed Au(111) Surface as Well as in the Presence of an Au Adatoma BT BE

BSe ΔBE

BE

ΔBE

Au(111) 37.1 (32.4) Au(111) + Au adatom flat perpendicular

48.3 41.8

40.3 (35.7) 11.2 4.7

47.4 39.9

7.1 −0.4

a Calculations were performed at the vdW-DFT level in a 3 × 3 unit cell (coverage 0.11). The numbers in parentheses correspond to regular DFT calculations. The corresponding adsorption geometries are shown in Figure 7a−c.

On the unreconstructed Au(111) surface, the tilt of the phenyl ring with respect to the surface normal is 58.0° for BS and 63.8° for BT (Figure 8a). The calculated tilts are larger than those reported experimentally10,14 because our calculations were performed under low-coverage conditions that favor the interaction of the phenyl ring with the metal surface. The binding energy changes with the tilt angle show a similar pattern for both BS and BT (Figure S3 in Supporting Information), with BT being slightly more sensitive to changes in the tilt angle. The binding energy decreases linearly with decreasing tilt angles, with slopes of 0.38 kcal/mol/degree for BT and 0.34 kcal/mol/degree fo BS. There are many equilibrium structures when the molecules adsorb around an Au adatom. Figure 8b shows a configuration with the phenyl ring parallel to the surface and the headgroup monocoordinated to the Au adatom. Figure 8c shows a configuration with the headgroup monocoordinated to the adatom and the phenyl ring in an upright configuration. Taking as a reference the binding energy on the unreconstructed surface, Table IV shows that the binding energy of BT increases up to 11.2 kcal/mol for the lying-down configuration. For the upright configuration, the increase in binding energy is 4.7 kcal/mol because of a lower vdW interaction of the phenyl ring with the surface. In the case of BS, the increase in binding energy is less pronounced in the presence of an adatom. It increases 7.1 kcal/mol for the flatlying configurations (Figure 8b), and it shows a small decrease of 0.4 kcal/mol in the upright configuration (Figure 8c). In the latter configuration, the binding energy of BT (41.8 kcal/mol) is nearly 2 kcal/mol higher than that of BS (39.9 kcal/mol). The results indicate that on average the presence of adatoms on the Au(111) surface produces an increase in the binding energy that is higher for BT than for BS. Although these calculations have been performed under low-coverage conditions, we think that this trend should extrapolate to compact monolayers. Moreover, when the phenyl ring is perpendicular to the surface, which is the case under high-coverage conditions, the presence of an adatom does not produce any further stabilization in the case of BS. This result is in agreement with the fact that thiols reconstruct the gold surface54−56 (and also benzenethiol57) whereas no reconstruction has been observed for selenols. Therefore, we attribute the higher thermal stability of the BT SAM to the presence of Au adatoms whereas the desorption of BS would occur from the unreconstructed Au(111) surface.

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CONCLUSIONS

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ASSOCIATED CONTENT

We have investigated SAMs grown on Au(111) substrates by immersion for 15 min in ethanolic solutions of DPhDS, DPhDSe, and BSe. We have found that immersion in DPhDS and BSe solutions produces good SAMs of benzenethiolate and benzeneselenolate with effective coverages that are as high as θBT = 0.26 and θBS = 0.29. The comparison with the molecular packing in crystalline benzene suggests that these are maximum coverages and that the packing is governed by molecule−molecule interactions. The peaks in the voltammograms and the photoemission spectra are very narrow, indicating good ordering of the films. The immersion in the DPhDSe solution does not lead to the formation of good SAMs; in this case, both the voltammetry and the photoemission spectra indicate that, besides benzeneselenolate, there are large amounts of atomic Se. We have also studied the thermal stability of the BT and BS SAMs and have found that whereas the BS SAM desorbs quickly at temperatures slightly above room temperature the BT SAM withstands prolonged annealing at temperatures greater than 400 K. Being the packing of the phenyl rings, similar in both SAMs, we ascribe the different thermal stabilities to differences in the C−Se and C−S bonds as well as to the different surface bonding of the headgroups that may induce the formation of gold adatoms. The energy barrier for the breakage of the C−Se bond of BS is lower than that of the C−S bond of BT, and this correlates with the appearance of Se atoms upon heating, as observed from both the reductive desorption experiments and the photoemission spectra. The coadsorption of Se atoms with BS molecules weakens the BS−surface bonding. The vdW-DFT calculations show that in the presence of Au adatoms the binding energy of BT shows a more pronounced increase than in the case of BS. We therefore ascribe the high thermal stability of BT to the strengthening of the surface bonding produced by the substrate reconstruction.

S Supporting Information *

Additional figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org. 13633


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AUTHOR INFORMATION

Corresponding Author

*(E.M.P.) Tel: 54-351-4344972. E-mail: [email protected]. ar. (G.Z.) Fax: ++54-2944-445 299. Tel: ++54-2944-445 229. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Brazilian Synchrotron Light Laboratory (LNLS), FONCyT (grants PICT 2005-33432 and 2005-32893), CONICET (grants PIP 5903 and 112 200801 00958), and SECYT-UNC is gratefully acknowledged. This research was performed under the framework of the Argentine network for Nanociencia y Nanotecnologiá Molecular, Supramolecular e Interfaces (PAE04-22711). We are also members of CONICET, Argentina.

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REFERENCES

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(35) Rodríguez, J.; Dvorak, J.; Jirsak, T.; Liu, G.; Hrbek, J.; Aray, Y.; González, C. Coverage Effects and the Nature of the Metal−Sulfur Bond in S/Au(111): High-Resolution Photoemission and DensityFunctional Studies. J. Am. Chem. Soc. 2003, 125, 276. (36) Zharnikov, M.; Grunze, M. Spectroscopic Characterization of Thiol-Derived Self-Assembling Monolayers. J. Phys.: Condens. Matter 2001, 13, 11333. (37) Shaporenko, A.; Cyganik, P.; Buck, M.; Terfort, A.; Zharnikov, M. Self-Assembled Monolayers of Aromatic Selenolates on Noble Metal Substrates. J. Phys. Chem. B 2005, 109, 13630. (38) Note, however, that this applies only to the electroactive species; adsorbates that are not electroactive do not show up in the voltammograms. (39) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Mullenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer Corporation, Eden Prairie, MN, 1978. (40) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science; Cambridge University Press: Cambridge, U.K., 1994. (41) Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization Cross Sections and Asymmetry Parameters. At. Data Nucl. Data Tables 1985, 32, 1. (42) Powell, C. J.; Jablonski, A. Electron Effective Attenuation Lengths for Applications in Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy. J. Phys. Chem. Ref. Data 1999, 28, 19. (43) Zhong, C.-J.; Porter, M. Evidence for Carbon-Sulfur Bond Cleavage in Spontaneously Adsorbed Organosulfide-Based Monolayers at Gold. J. Am. Chem. Soc. 1994, 116, 11616. (44) Kang, H.; Park, T.; Choi, I.; Lee, Y.; Ito, E.; Hara, M.; Noh, J. Formation of Large Ordered Domains in Benzenethiol Self-Assembled Monolayers on Au(111) Observed by Scanning Tunneling Microscopy. Ultramicroscopy 2009, 109, 1011. (45) If one assumes that the atoms adsorb in a 31/2 × 31/2 arrangement, then the area per atom equals that of three Au atoms, and the ″effective″ molecular coverage becomes θmoleff = θmol /(1 − 3θat). (46) Crystalline benzene can be described as the stacking of loosely bound planes with the molecules oriented such that the normals to the rings make an angle of 77° with the c axis (i.e., almost perpendicular to the planes). The molecular arrangement in the planes is rectangular with two molecules per unit cell (at a corner and the center), with their rings oriented at right angles. At 270 K, the separation between the planes is 4.83 Å and the rectangular unit cell is 7.46 × 7.03 Å2 ( Cox, E. G. Crystal Structure of Benzene. Rev. Mod. Phys. 1958, 30, 159 ). (47) Weidner, T.; Shaporenko, A.; Müller, J.; Schmid, M.; Cyganik, P.; Terfort, A.; Zharnikov, M. Effect of the Bending Potential on Molecular Arrangement in Alkaneselenolate Self-Assembled Monolayers. J. Phys. Chem. C 2008, 112, 12495−12506. (48) Williams, J. A.; Gorman, C. B. Alkanethiol Reductive Desorption from Self-Assembled Monolayers on Gold, Platinum, and Palladium Substrates. J. Phys. Chem. C 2007, 111, 12804−12810. (49) Note that every voltammogram involves the preparation of a new sample; thus, the smooth variation of the voltammograms with the annealing temperature is also a measure of the good reproducibility achieved in the preparation of the SAMs. (50) Note that in the TDS experiments, which use a continuous heating ramp, desorption peaks occur at temperatures given by the Redhead formula; our case is different in that the samples are held 30 min at a given temperature. The point of contact between the two types of experiments is the activation energy. (51) We note that by assuming a second-order process one gets a similar result; therefore, our experiment cannot distinguish between the two cases. (52) Kondoh, H.; Kodama, C.; Sumida, H.; Nozoye, H. Molecular Processes of Adsorption and Desorption of Alkanethiol Monolayers on Au(111). J. Chem. Phys. 1999, 111, 1175. (53) In this part, the coverages are normalized in such a way that θαmax = 1, and f = θαmax/θβmax is the ratio between the areas per molecule in the lying-down and standing-up orientations. 13635


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