Article Cite This: Langmuir XXXX, XXX, XXX−XXX
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Se−C Cleavage of Hexane Selenol at Steps on Au(111) Zahra Besharat,†,‡ Milad Ghadami Yazdi,† Deborah Wakeham,‡ Magnus Johnson,‡ Mark W. Rutland,‡,§ Mats Göthelid,*,† and Henrik Grönbeck∥ †
Material Physics, MNF, SCI, KTH Royal Institute of Technology, Stockholm SE-164 40, Sweden Department of Chemistry, Division of Surface and Corrosion Science, KTH Royal Institute of Technology, Stockholm SE-100 44, Sweden § Chemistry, Materials and Surfaces, SP Technical Research Institute of Sweden, Box 5607, Stockholm SE-114 86, Sweden ∥ Department of Physics and Competence Centre for Catalysis, Chalmers University of Technology, Göteborg SE-412 96, Sweden ‡
ABSTRACT: Selenols are considered as an alternative to thiols in self-assembled monolayers, but the Se−C bond is one limiting factor for their usefulness. In this study, we address the stability of the Se−C bond by a combined experimental and theoretical investigation of gasphase-deposited hexane selenol (CH3(CH2)5SeH) on Au(111) using photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory (DFT). Experimentally, we find that initial adsorption leaves atomic Se on the surface without any carbon left on the surface, whereas further adsorption generates a saturated selenolate layer. The Se 3d component from atomic Se appears at 0.85 eV lower binding energy than the selenolate-related component. DFT calculations show that the most stable structure of selenols on Au(111) is in the form of RSe−Au−SeR complexes adsorbed on the unreconstructed Au(111) surface. This is similar to thiols on Au(111). Calculated Se 3d core-level shifts between elemental Se and selenolate in this structure nicely reproduce the experimentally recorded shifts. Dissociation of RSeH and subsequent formation of RH are found to proceed with high barriers on defect-free Au(111) terraces, with the highest barrier for scissoring R−Se. However, at steps, these barriers are considerably lower, allowing for Se−C bond breaking and hexane desorption, leaving elemental Se at the surface. Hexane is formed by replacing the Se−C bond with a H−C bond by using the hydrogen liberated from the selenol to selenolate transformation. find that the Se−C bond breaks at steps on Au(111) without the presence of impurities, whereas on the Au(111) terraces, a smooth selenolate layer in the form of RSe−Au−SeR complexes is formed. At steps, the hydrogen from the Se−H bond recombines with the carbon chain to form hexane that readily desorbs from the surface at room temperature. In addition, the Se 3d core-level spectrum contains three components at room temperature: the main peak at 54.27 eV from selenolate, a small component at 55.22 eV from selenol, and a component at 53.42 eV. The low-binding-energy component is identified as atomic Se bound to gold, as the surface is free from carbon at the lowest coverage.
1. INTRODUCTION Self-assembly presents a fast and flexible way to prepare smooth molecular monolayers (MLs) on surfaces, and much effort has been put into understanding and engineering such selfassembled monolayers (SAMs).1 They find technological use in controlling biocompatibility,2 corrosion resistivity,3 and tribology,4 and also in electronic devices, sensors, quantum dots, and photocatalysis.5−7 The anchoring group, or atom, plays a crucial role in determining bond strength, electronic coupling, and charge flow between molecule and substrate.8 Thiols are commonly used for SAMs on coinage metals,9−12 but selenols and tellurols have also obtained increasing interest.13 Studies on alkane thiolates and alkane selenolates on Au(111) show that the Se−Au bond is stronger than the S− Au bond. However, the Se−C bond is weaker than the S−C bond,8,14,15 which may lead to dissociation of the Se−C bond. For example, methane selenolate deposited on Au(111) from solution has been shown to result in atomic Se.16 The S−C dissociation process has also been studied previously;13,17−19 preparation conditions and surface morphology13 as well as the presence of impurities15 were suggested as possible reasons for the chalcogenide−carbon bond dissociation. In this study, we report a combined photoelectron spectroscopy (PES), scanning tunneling microscopy (STM), and density functional theory (DFT) study of hexane selenol adsorption on Au(111). We © XXXX American Chemical Society
2. EXPERIMENTAL SECTION The Au(111) single crystal was purchased from Surface Preparation Laboratory, The Netherlands. Surface cleaning involved cycles of argon-ion sputtering (1 kV, 20 min), followed by annealing at 500 °C in ultrahigh vacuum (UHV) until a sharp (22 × √3) pattern was observed in low-energy electron diffraction (LEED) and large flat terraces with the herringbone structure were observed in STM. Hexane selenol (CH3(CH2)5SeH) was purchased from AF ChemPharm with a purity of 98%. Deposition of hexane selenol was done Received: October 25, 2017 Revised: January 18, 2018 Published: February 6, 2018 A
DOI: 10.1021/acs.langmuir.7b03713 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. (a) Se 3d, (b) C 1s, and (c) Au 4f spectra recorded from the clean and hexane selenol-exposed Au(111) surfaces. Selenol doses, in Langmuir, are given in the figure. Depositions and measurements were done at 300 K. through a precision leak valve at 2 × 10−7 Torr. Doses are given in Langmuir (L), where 1 L = 1 × 10−6 Torr s. Hexane selenol was purified by pumping with mild heating (40−60 °C) until no impurities were detected in mass spectrometry. PES was carried out in the SPECIES beamline at the MAX II storage ring (Lund, Sweden). Photons were generated by an undulator and monochromatized by a plane-grating monochromator. Electron spectra were recorded with a SPECS Phoibos 150 NAP analyzer. The system was equipped with two UHV chambers with a base pressure below 5 × 10−10 Torr. The preparation chamber comprised a sputter gun, LEED, and sample heater. The details of this setup can be found in refs 20 and 21. All spectra were normalized to the background, and the energy was calibrated to the Fermi level measured on Au(111). Photoelectron spectra were collected using different photon energies: C 1s at 400 eV, Au 4f and Se 3d at 150 eV. Overview spectra were also collected to check cleanliness of the sample. Numerical curve fitting was done by Voigt functions and a linear background within the FitXPS 2 program. For Se 3d, we use the spin−orbit split functions with a 0.86 eV spin− orbit split and a (3d5/2/3d3/2) branching ratio of 1.5. The ratios and splits are kept the same for all components at all coverages. The Lorentzian width was fixed to 0.2 eV, but the Gaussian width was allowed to vary. Se 3d binding energies are given for the Se 3d5/2 component. For Au 4f7/2, we use the Lorentz width of 0.2 eV and a freely adjustable Gaussian width. To check for possible beam damage, the surface was left in the synchrotron beam and spectra were recorded repeatedly. We also scanned the surface during measurements to ensure that beam damage does not influence our measurements. STM was done in UHV at room temperature using an Omicron VT-STM, operated in constant-current mode, using electrochemically etched tungsten tips. The STM chamber is connected to a preparation chamber with LEED, sputtering, heating, and a leak valve for selenol deposition. The base pressure in both the chambers is 2 × 10−10 Torr.
variationally for each element are: Au(11), Se(6), C(4), and H(1). A plane-wave kinetic energy cutoff of 380 eV (420 eV when evaluating surface core-level shifts (SCLS’s)) is used to expand the Kohn−Sham orbitals. Adsorption on Au(111) is investigated in a (3 × 2√3) surface cell, which corresponds to the surface cell observed for thiol adsorption.25,26 Au(211) with a p(1 × 3) surface cell is considered to model a step on a Au(111) surface. Au(111) and Au(211) are represented by five and nine atomic layers, respectively. Repeated slabs are separated by 14 Å. Reciprocal space integration over the Brillouin zone for Au(111) [Au(211)] is approximated with a finite sampling of eight [10] special k-points. Structural optimization is performed without any constraints, and transition states are evaluated using a combined linear synchronous transit and quadratic synchronous transit method.27 Structures and transition states are considered to be converged when the largest force is less than 0.05 eV/Å. The considered structures for Au(111) contain four CH3Se units per unit cell. The energy difference (ΔE) between a structure α and the CH3Se radicals adsorbed in bridge sites [4RSe/ Au(111)] is calculated with bulk Au as a metal-atom reservoir: ΔE(α) = E[α] − E[4RS/Au(111)] − xE[Au(bulk)]. The surface core-level shifts (SCLS’s) are evaluated using the pseudopotentials generated with an electron hole in the Se 3d and Au 4f shells. To describe the electronic relaxation in better detail, the potential for Au includes the 5s5p semicore in the valence when evaluating the Au shifts. A Au 4f core hole in the center of the slab is used to model the bulk reference. For Se 3d, the shifts are calculated with respect to elemental Se adsorbed in an face-centered cubic (fcc) position.
3. COMPUTATIONAL DETAILS DFT is employed in the implementation with plane waves and pseudopotentials.22 The spin-polarized Perdew−Burke−Ernzerhof approximation is used for the exchange and correlation functional,23 and ultrasoft scalar-relativistic pseudopotentials are used to describe the interaction between the valence electrons and the core.24 The number of electrons treated
4. RESULTS AND DISCUSSION Core-level spectra recorded from the Au(111) surface after different selenol exposures are presented in Figure 1 (a) Se 3d, (b) C 1s, and (c) Au 4f. At the lowest dose, 10 L, there is only one Se 3d component (Se1) at 53.42 eV and zero C 1s intensity; at higher doses (50 and 130 L), two new Se 3d B
DOI: 10.1021/acs.langmuir.7b03713 Langmuir XXXX, XXX, XXX−XXX
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Langmuir components appear: Se2 at 54.27 eV and Se3 at 55.22 eV in addition to Se1 at 53.42 eV. These components represent three different Se’s on the surface: atomic Se (Se1), selenolate chemisorbed to the Au(111) surface (Se2), and a small contribution (Se3) from second-layer selenol, as suggested by Tong et al.13 This assignment is also in agreement with our recent report on hexane selenol adsorption on Cu.28 For thiols, other interpretations of the S 2p components have been put forward, combining both experimental and theoretical works, where the low-binding-energy component is instead interpreted as an alternative binding site of thiolate.29−37 Our results cannot rule out other adsorption sites, but the absence of carbon on the surface at low coverage clearly favors the Se−C scission scenario. In C 1s, a strong peak appears at 283.82 eV at 50 L, related to the hexane selenolate adsorbed on the surface. At 130 L, C 1s shifts 0.2 eV to higher binding energy and the peak broadens. An explanation of the observed C 1s development is that core-level electron energies are sensitive measures of the ability of the local environment to screen the core hole. Geometric changes, such as reorientation of the alkane chain from lying to standing, can result in core-level energy shifts because the ionized carbon is further from the Au surface. A similar shift and broadening can also be due to the creation of a second layer. The two scenarios are not simply disentangled, but given the rather small contribution from the second layer in the Se 3d spectrum, it is tempting to suggest that lifting the alkane chain from lying to standing is the largest contribution to the C 1s change. This is a well-known mechanism during thiol and selenol adsorption; when the adsorbate density increases on the surface, the van der Waals interaction between the alkane chains forces them into a more upright orientation.38−40 Au 4f spectra are presented in Figure 1c. The spectrum from the clean surface was fitted by two components representing bulk (B) and surface (S) atoms, in agreement with previous studies.41−44 B is located at 84.00 eV, whereas S is at 83.67 eV. S is broader than B, also in agreement with previous reports and explained as due to surface inhomogeneities, surface phonon broadening, and crystal field splitting.45,46 Upon selenol adsorption, S is reduced and shifts slightly closer to B, from 0.33 to 0.26 eV. We also add a component (A) on the high-binding-energy side, as previously observed for methyl thiolate and butyl thiolate on Au(111).35,47 S narrows, the Gaussian full width at half-maximum goes from 0.42 eV on the clean surface to 0.36 eV at 130 L, whereas simultaneously B broadens from 0.28 to 0.36 eV. The relative intensity (S + A) at 130 L is smaller than S on the clean surface, and broadening indicates that there is an unresolved surface contribution within the B peak. The energetically most stable surface structure, for both thiolates and selenolates,25,26,45,48,49 involves a Au-adatomdichalcogenate arrangement; a gold adatom resides in a bridge site, with respect to the first Au layer below, and binds to two S (or Se) with their respective alkane chains pointing away from the gold adatom. We have chosen the most stable configurations for the present study. Models and relative energies of different methylselenolate structures on Au(111) are presented in Figure 2. In the case of alkyl thiolates, the short methyl thiolate has shown to represent the RS/Au interface with longer R-chains.49,50 Model (a) corresponds to RSe adsorbed on an unreconstructed surface and is used as energy reference. Figure 2b shows the relaxed structure with point
Figure 2. Structural models of the considered structures for CH3Se (RSe) on Au(111). Atomic color code: Au (orange), Se (green), C (gray), and H (white). The surface cell is indicated by white lines.
defects in the surface layer,51 which reduces the total energy by 0.19 eV. Figure 2c shows the structure with RSe−Au−SeR complexes having the alkyl chains in a trans-configuration,25 whereas in Figure 2d, the alkyl chains are in a cisconfiguration26,52 and the complexes are ordered differently on the surface as compared to model (c). The RSe−Au−SeR complexes with the chains in a cis-configuration are clearly favored. A recent work37 suggested that the low-binding-energy S 2p component is due to thiol in an atop site. However, that work disregarded previous work. 25,26,48 Although their theoretical results agreed with experiments, no comments on the relative stability of the structures were given.37 Previously, methyl thiolate-induced Au 4f core-level shifts (CLS’s) were calculated from structure as shown in Figure 2d;53 surface atoms binding to adatoms and other surface atoms not binding to sulfur have negative CLS (i.e., to lower binding energy), whereas adatoms in the complexes have positive CLS with respect to the bulk peak. In our case, we calculate CLS from the methylselenolate structures as shown in Figure 2a,d. The results are presented in Figure 3. Only the structure with the RSe−Au−SeR complexes exhibits positive CLS, corresponding to peak A in the experimental spectra. In addition, this structure yields atoms with zero CLS as well, in agreement with the increased and broadened Au 4f bulk peak. Thus, both
Figure 3. Au 4f surface core-level shifts evaluated for the pristine Au(111) surface and RSe adsorbed in the bridge configuration (Figure 2a) and in the form of RSe−Au−SeR complexes (Figure 2d). The shifts are given with respect to a bulk Au atom. C
DOI: 10.1021/acs.langmuir.7b03713 Langmuir XXXX, XXX, XXX−XXX
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hexane selenol in the second layer. One must keep in mind that intensities can be affected by photoelectron diffraction effects. Within the Au−adatom−diselenolate structure, 1/3 ML selenolate is expected at saturation,38,42 in fair agreement with our finding. This is by no means a decisive proof, but in agreement. There is atomic Se on the surface at the lowest coverage, due to Se−C bond scission. It was previously suggested in a DFT study by Cometto et al. that the activation barriers for the Se− C bond cleavage are 32.3 and 23.5 kcal/mol for CH3Se and benzene selenol, respectively.16,19 These values are too high for facile Se−C dissociation at room temperature, and it was suggested that an oxidized surface species is needed to catalyze Se−C bond scission.19 The calculations were done on an unreconstructed (111) surface, and steps were not taken into account, although atoms at steps have lower coordination and are often more reactive than atoms on the terrace.56 STM was used to image the surface at low coverage. In Figure 5, we show one image with two terraces separated by a
energetic considerations and Au 4f CLS favor a surface structure built from RSe−Au−SeR complexes. We also calculated the Se 3d core-level shifts for different structures and compared with elemental Se in an fcc site. The results are presented in Figure 4, in which (a) corresponds to
Figure 4. Se 3d core-level shifts evaluated with respect to elemental Se adsorbed in an fcc position, denoted (a), and the corresponding adsorbate structures are shown at the top of the figure; (b) denotes RSe adsorbed in the bridge configuration; (c) denotes HSe adsorbed in a bridge configuration; (d) denotes RSe in the RSe−Au−SeR complexes; (e) is RSeH adsorbed on the surface; and (f) is RSeH physisorbed on the complex structure. Atomic color codes are as in Figure 2.
elemental Se, (b) to RSe adsorbed in the bridge configuration, and (c) to HSe adsorbed in a bridge configuration. The similar result for HSe and RSe shows that the Se shifts are not sensitive to the length of the R-group. Peak (d) corresponds to RSe adsorbed in the form of RSe−Au−SeR complexes. The clear difference in the CLS signature of the two adsorption modes (bridge and complex) indicates that the Se 3d shift may be used to discriminate between the two modes. Peak (e) is calculated from RSeH adsorbed on the surface and (f) from RSeH physisorbed on the complex structure. It is thus clear that the Se 3d CLS is insensitive to whether RSeH binds directly to the surface or to the complex. Our Se 3d results are in agreement with elemental Se and RSe−Au−SeR selenolate complexes. The intensity ratio between Se 3d and Au 4f can be utilized to calculate the selenium coverage. The intensity in a certain core-level peak is determined by the photon flux, the photoionization cross section, the electron mean free path, and the surface concentration of the species related to that core-level peak. The mean free path does not play a role when atoms at the very surface are used; here, we use the Au 4f surface peak (S) and the Se 3d peak (Se1) at the lowest coverage. We use the same photon energy and the same flux for both peaks, and it remains simply I σ θSe = Se × Au × θAu IAu σSe (1)
Figure 5. Two STM images from the Au(111) surface: the clean (80 × 80 nm2) (left) after 10 L hexane selenol dose and (90 × 90 nm2) (right) at −1.3 V sample bias and 71 pA tunnel current.
step from the clean surface and another image from the surface after 10 L hexane selenol dose. The filled-state image after selenol exposure demonstrates that terrace edges are decorated by an adsorbate, whereas the herringbone structure is left intact on the terraces. The bright spots at the bends/elbows of the herringbone structure are electronic edge states,44 and not adsorbed molecules or atoms. This observation in combination with the Se 3d results at low coverage suggests that Se−C bond scissoring occurs primarily at step edges and that atomic Se is located primarily at the steps. We calculate energies and barriers for RSeH adsorption and dissociation on the unreconstructed (111) surface and at steps. Steps are modeled by an Au(211) surface. The potential energy surfaces are presented in Figure 6 for the two cases: the red curve from Au(111) and the blue curve from Au(211). RSeH is weakly adsorbed (−0.47 eV) atop a gold atom with a long Se− Au bond of 2.71 Å. Dissociation proceeds by scissoring of the Se−H bond and H transfer to an fcc hollow position with a barrier of 0.89 eV. RSe occupies a bridge position in the final state. RSe dissociates with a high barrier of 1.47 eV over the bridge site by Se transfer to a hollow position, whereas the methyl group takes an atop position. Association of H and CH3 to methane occurs over a top site with a barrier of 0.57 eV. The formation of CH4 is energetically preferred. The potential energy landscape is markedly different on Au(211). RSeH is adsorbed atop a gold atom at the step, and the slightly shorter Se−Au distance (2.61 Å) is consistent with a stronger
where θ is the surface coverage represented by a certain peak, I is the intensity of that peak, and σ is the photoionization cross section.54 S represents 1.045 ML of surface atoms in the herringbone reconstruction.55 The total intensity of Au 4f did not change after the 10 L dose. From the cross-sectioncorrected intensities of Se 3d and Au 4f, we calculate the Se coverage at 10 L to be around 0.05 ML. Further, by comparing the Se 3d intensities at 130 L and 10 L, the total Se coverage at 130 L is around 0.42 ML. The three Se 3d components represent 0.05 ML atomic Se, 0.33 ML selenolate, and 0.04 ML D
DOI: 10.1021/acs.langmuir.7b03713 Langmuir XXXX, XXX, XXX−XXX
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Langmuir Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support from the Swedish Research Council (VR) and the Stiftelsen för Strategisk Forskning (SSF) is gratefully acknowledged. The authors thank Dr. Margit Andersson, MAXIV, for her precious support and Dr. Markus Soldemo, KTH Material Physics, for valuable discussions. The calculations were performed at C3SE (Gothenburg) via an SNIC grant.
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Figure 6. Potential energy landscape of RSeH adsorption and dissociation on Au(111) and Au(211). Au(211) is used to model a stepped Au surface. The energy is given in eV with respect to RSeH in the gas phase. The structural models for adsorption at the Au(211) step: RSeH (left), RSe (mid), and R + Se (right). Atomic color codes are as in Figure 2.
adsorption energy (−0.65 eV). RSeH dissociation proceeds by hydrogen transfer to a bridge site, which is the stable H site on the stepped surface. The barrier is 0.53 eV, and in contrast to Au(111), this step is exothermic. RSe dissociates at the edge over a bridge site at the step, where Se in the final state occupies a 4-fold hollow position. The barrier for Se−C scissoring is 1.02 eV. The barrier for CH4 formation is only 0.1 eV, and the process is also exothermic in this case. The difference in potential energy landscape for the two surfaces indicates that steps are important for R−Se bond cleavage and deposition of elemental Se on the surface.
5. CONCLUSIONS We have studied adsorption of hexane selenol on Au(111) using core-level photoelectron spectroscopy, STM, and DFT. We found that at the initial stages of adsorption the Se−C bond broke at steps on Au(111), leaving elemental Se. The hydrogen atom released from the Se−H bond recombines with the hydrocarbon chain to produce hexane that leaves the surface. On the Au(111) terraces, selenol adsorption removes the herringbone reconstruction and forms a smooth selenolate layer in the form of RSe−Au−SeR complexes. Our study clearly shows that a stepped bare gold surface is able to break the Se− C bond without impurities or oxidized surface species. Furthermore, the low-binding-energy Se 3d core-level component is unambiguously assigned to elemental Se on the surface.
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REFERENCES
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Mats Göthelid: 0000-0002-6785-8293 E
DOI: 10.1021/acs.langmuir.7b03713 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.7b03713 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.7b03713 Langmuir XXXX, XXX, XXX−XXX
