ARTICLE pubs.acs.org/JPCC
From Nondissociative to Dissociative Adsorption of Benzene-thiol on Au(111): A Density Functional Theory Study Xiaoli Fan,*,†,‡ Qiong Chi,† Chong Liu,† and Woonming Lau*,‡,§ †
School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China Beijing Computational Science Research Center, Beijing 100084, China § Chengdu Green Energy and Green Manufacturing Technology R&D Center, Chengdu, Sichuan 610207, China ‡
ABSTRACT: The adsorption of the benzene-thiol (C6H5SH) molecule on an Au(111) surface was investigated using the density functional theory method. Unlike prior studies that assume the dissociation of C6H5SH to C6H5S + H and the subsequent chemisorption of C6H5S/Au, the present computational work first clarifies the sites and energetics of both the nondissociative molecular adsorption and the dissociative adsorption and then charts the dissociative chemisorption pathways and transition states. The calculations took into account of the reaction steps in these processes, steps including SH cleavage, C6H5S/Au formation, H/Au diffusion, and H2 desorption. The thoroughness of this approach yields the discovery of a molecular nondissociative chemisorption state with the S atom sitting on top of a gold atom. This state is stable at room temperature as its adsorption energy amounts to 0.28 eV. Its direct molecular dissociation to form C6H5S/Au and H/Au is barred by an activation barrier of 0.58 eV, and the dissociation is endothermic. However, the presence of neighboring H/Au can assist the dissociative reaction to form C6H5S/Au and gaseous H2 by lowering the dissociation activation barrier from 0.58 to 0.35 eV. More importantly, the dissociation changes from endothermic to exothermic, and it can proceed with mild heating. Finally, this work also gives adequate simulation results to interpret the surface configurations of the nondissociative molecular adsorption of C6H5SH/Au and dissociative adsorption C6H5S/Au, which were previously observed by STM experiments.
1. INTRODUCTION Self-assembly monolayers (SAMs) of organosulfur on metal surface have a wide range of applications in several fields of nanotechnology, such as corrosion inhibition, biosensor, molecular electronics and molecular recognition.1 The most studied SAM system in the past decades probably is the SAMs of alkane-thiols on gold. Recently, arene-thiols have drawn much attention because their high conductivity and nonlinear optical properties are readily exploited in molecular electronics and other applications.24 However, to exploit these properties fully, it is essential to know the detailed moleculesubstrate structures as well as the formation kinetics of these SAMs. Accordingly, many experimental and theoretical investigations focusing on such adsorption kinetics and surface structures have been made.1,58 Previously, thiols on Au(111) and related substrates were commonly described as thiolates bound on the surface at hollow sites, and thus, their studies were artificially simplified with the assumption of dissociation of thiols to thiolates by discarding the hydrogen atom bound to the sulfur atom of each thiol molecule.9 Unsurprisingly, this artificial simplification has been questioned recently, and thus, nondissociative adsorption with intact SH for both short-chain10,11 and long-chain12 alkane-thiols on Au have been explored. Indeed, both recent theoretical13 and experimental investigations10,11,14 conclude that methane-thiol (CH3SH), the simplest and most studied alkane-thiol molecule, adsorbs on Au(111) nondissociatively. Although the pathways from r 2011 American Chemical Society
nondissociative adsorption to dissociative adsorption are not yet clear, the dissociation of thiols to thiolates is not uncommon. For example, it is now generally believed that the presence of surface defects and the perturbation by a scanning tunneling microscopy (STM) tip during STM measurements may induce SH bond cleavage. Adding weight to this belief, Lustemberg et al. calculated that the reaction pathways for the CH3SH bond scission on the Au(111) surface and showed that the released H atom is chemisorbed on the surface (referred as H/Au therein).15 Benzene-thiol (C6H5SH), as the simplest aromatic-thiol, was also believed to be dissociative chemisorbed on Au as phenylthiolate (C6H5S).16,17 According to recent theoretical studies, the formation energy of the nondissociative adsorption of C6H5SH on the Au(111) surface is merely 0.02 eV18 or even endothermic19 (0.07 eV). If so, the adsorption should be considered as reversible physisorption. In contrast, once C6H5SH dissociates to C6H5S, the bonding between C6H5S and the substrate in the dissociative adsorption is strong and irreversible. Probably due to the above facts, recent theoretical studies focused on the structure and energies of the adsorption of the C6H5S radical, as well as dimerization of C6H5S on surface.1820 In short, both the nature of nondissociative adsorption of C6H5SH and the Received: October 9, 2011 Revised: November 16, 2011 Published: November 29, 2011 1002
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The Journal of Physical Chemistry C transition from nondissociative to dissociative adsorption remain uncertain. The first logical question about dissociative adsorption of C6H5SH is the fate of the H atom of the SH group. Different scenarios have been proposed:1,13,15,18,19,21,22 (i) the H atom is retained by the chemisorbed thiol with the presence of a weakened SH bond; (ii) the SH bond cleaves, and the H atom chemisorbs on the surface; and (iii) the SH bond cleaves, and the H atom desorbs as H2. For example, Yates et al. observed the nondissociative adsorption of C6H5SH molecules on the Au(111) surface using STM at low temperature.21 According to their experiments, C6H5S can be formed by thermal dissociation of a C6H5SH molecule on the surface at 300 K. However, this conclusion differs from the predication of previous theoretical studies,18,19 which point to the weak and reversible physisorption state of C6H5SH since the physisorbed C6H5SH molecules with formation energy of 0.02 eV will desorb during thermal activation at 300 K. Furthermore, in contrast to the previous theoretical prediction of the release of the H atom from the SH group,15 some experimental evidence has been obtained at 500 K that the released H atom desorbs from the surface instead of chemisorbs on the surface.22 In short, intensive works are being carried out with the discovery of new experimental data and theoretical models on bonding, structure, and energetics of thiols on the Au surface.2328 However, the structure of thiolAu bonding, the nature and mechanism of SH bond scission, and the fate of the released H atom are not yet clear, a knowledge gap that continues to drive the research of the basic aspects of SAMs. In our present work, we studied the adsorption of a C6H5SH molecule on the Au(111) surface at low coverage using the firstprinciples method. We examined the nondissociative adsorption as an intact molecule, as well as two dissociative adsorption cases where (i) the released H atom chemisorbs on surface and (ii) the released H atom desorbs into vacuum. We also investigated the possible dissociation pathways for scenarios (i) and (ii) to study the mechanism of SH bond dissociation and the fate of the H atom. By comprehensively studying the sites and energetics of the nondissociative adsorption and all the dissociative reaction pathways leading to C6H5S/Au adsorption, H/Au adsorption and diffusion, and H2 desorption, we attempt to bridge the knowledge gap that exists so far in the study of the benzene-thiol adsorption.
2. CALCULATION METHODS The first-principles calculations were performed by using the Vienna ab initio simulation package (VASP)2932 based on the density functional theory (DFT).33,34 In these calculations, the electron-ion interactions were described by the projector augmented wave (PAW) method.35,36 The generalized gradient approximation (GGA)37 of the PerdewBurkeErnzerhof (PBE) formula was used for the electronic exchange-correlation potential. The wave functions were expanded in a plane wave basis with an energy cutoff of 400 eV. The minimum energy paths (MEPs) for the dissociation reactions were mapped out using the nudged elastic band (NEB) method developed by Jonsson and co-workers.38,39 The Au(111) surface was modeled as a repeated slab with four layers of Au atoms and a vacuum region of seven atomic layers. For the adsorption of a single C6H5SH molecule on the Au(111) surface, we used the 4 4 super cell. In the total energy calculation, the bottom layer of Au atoms were fixed at the bulk
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Figure 1. Angles describing the orientation of benzene-thiol on Au(111). θ is the tilt angle between the surface normal and molecular axis; ϕ is the angle between the projection of the molecular axis and the x axis of the surface unit cell.
positions, while the adsorbed molecule and the other three layers of Au atoms were fully relaxed. The Monkhorst Pack of a 3 3 1 k mesh was used to sample the first Brillouin zone. The convergence criteria of 0.02 eV/Å in total energy calculation and 0.03 eV/Å in MEP calculation for force are adopted.
3. RESULTS AND DISCUSSIONS 3.1. General Description of the Adsorption. On the basis of the discussions in the introduction part, we write the nondissociative adsorption and the dissociative adsorption processes of the C6H5SH molecule on the Au(111) surface as
C6 H5 SH þ Auð111Þ f C6 H5 SH=Auð111Þ
ð1Þ
C6 H5 SH þ Auð111Þ f C6 H5 S=Auð111Þ þ H=Auð111Þ
ð2aÞ
C6 H5 SH þ Auð111Þ f C6 H5 S=Auð111Þ þ 1=2H2
ð2bÞ
where reaction 1 corresponds to the nondissociative molecular adsorption, and reactions 2a and 2b represent the dissociative adsorption where the released H atom chemisorbs on the Au surface and desorbs in the molecular form into the vacuum, respectively. We refer the dissociative adsorptions represented by reactions 2a and 2b as DS_I and DS_II, respectively. The adsorption geometry of a thiol molecule on the Au(111) surface can be described by four structure parameters: (a) the adsorption site of the S atom on the surface; (b) the distance between the S atom and the Au substrate; (c) the tilt angle θ, which is defined as the angle between the surface normal and the molecular axis; and (d) the azimuthal angle ϕ, which is defined as the angel between the projection of the molecular axis and the x axis of surface unit cell on the surface. Figure 1 shows the angles describing the orientations of the C6H5SH molecule on the Au(111) surface. The bonding interaction between the headgroup and Au substrate as well as the van der Waals interactions between the hydrocarbon groups are the two factors determining the structure and the physical and chemical properties of the SAMs. In this work, the calculations were carried out for a single C6H5SH molecule within a 4 4 supercell, the corresponding coverage is Θ = 1/16, which is sufficiently low and the molecules in the adjacent supercell are well separated to ignore the van der Waals interactions. 1003
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3.2. Nondissociative Molecular Adsorption States. The C6H5SH molecule was initially placed near the surface on the four sites: fcc and hcp hollow sites and top and bridge sites. On each site, the molecule was tilted from the normal direction (upright configuration) to the parallel direction (lying down configuration) at a tilt angle of 15, 30, 45, 60, 75, 90°. It has been reported that the adsorption energies are insensitive to ϕ.19 Hence, we eliminated ϕ by simply choosing ϕ = 0°. In total, we constructed 24 initial structures to study the configurations and
Table 1. Computational Results on the Nondissociative Adsorption of C6H5SH on Au(111) θi (deg)a initial siteb θo (deg)a optimized siteb dSAu (Å)c Efnon_diss (eV) 15
30
45
60
75
90
a
bri fcc
36.0 35.6
top-hcp top-fcc
2.85 2.95
0.16 0.16
hcp
38.1
top-bri
3.19
0.10
top
39.5
top-fcc
2.95
0.15
bri
38.7
top-fcc
2.95
0.16
fcc
32.4
top-bri
3.12
0.13
hcp
43.1
top-hcp
2.87
0.16
top
49.8
top
2.77
0.22
bri fcc
48.3 69.4
top top
2.76 2.79
0.22 0.22
hcp
52.3
top
2.79
0.23
top
56.8
top
2.72
0.23
bri
57.4
top
2.71
0.25
fcc
68.9
top
2.78
0.24
hcp
59.4
top
2.77
0.25
top
67.9
top
2.72
0.26
bri fcc
79.2 68.0
top-bri top
3.45 2.74
0.13 0.25
hcp
63.1
top
2.74
0.25
top
72.2
top
2.72
0.25
bri
77.2
top-bri
3.35
0.13
fcc
73.3
top-bri
3.33
0.13
hcp
64.0
top
2.76
0.23
top
70.7
top
2.70
0.28
The title angle between the surface normal and molecular axis in the initial and optimized structure, respectively. b The adsorption site in the initial and optimized structure, respectively. c The dSAu distance between the S atom and the nearest neighboring Au atom on the surface.
energies of nondissociative adsorption. Then, we performed the geometry optimization and calculated the adsorption energies for the nondissociative adsorptions by Efnon_diss ¼ EðAuÞ þ EðC6 H5 SHÞ EðC6 H5 SH=AuÞ
ð3Þ
A summary of our calculation results for the nondissociative adsorption of C6H5SH is presented in Table 1. There are basically three species of adsorption according to the formation energies: the adsorptions on top sites, on sites away from top toward hollow sites labeled as top-fcc and top-hcp, and on sites away from top toward bridge sites labeled as top-bri. The stability of these adsorptions follows the trend top-bri < top-fcc/hcp < top. The adsorption energy of the least stable adsorption geometry with the S atom located on the top-bri site is 0.10 eV, and the distance between the S atom and the nearest neighbor Au atoms is 3.19 Å. The adsorptions with the S atom on the top-fcc/ hcp sites are a little more stable than those with the S atom on the position of top-bri. The lowest energy is obtained for the adsorption with the S atom located on the top position, and the tilt angle between the surface normal and molecular axis is 70.7°. The geometry of this most stable molecular adsorption is labeled as MCS-a. The adsorption energy is 0.28 eV, and the distance between the S atom and the nearest neighbor Au atom is 2.70 Å. As we have mentioned in the Introduction section, the nondissociative adsorption configurations calculated by Bilic18 and Nara19 belong to physisorption. In the present study, the adsorption energy and the distance between the S atom and Au substrate indicate that the nondissociative molecular adsorption geometry MCS-a should belong to the chemisorption. This was further checked by our electronic structure calculations. The decomposed S 3P density of states (DOS) are shown in Figure 2a,b for the C6H5SH molecule before and after the adsorption, respectively. One can see the localized 3P atomic orbitals becoming delocalized in MCS-a, and Pz being the orbital that interacts significantly with the substrate Au atom. More specifically, the interaction changes the position of the highest occupied Pz orbital from 0.4 eV to 1.2 eV and decreases its peak height to 20%. These changes imply the formation of a chemical bond between the S and Au substrate atom. In short, we have discovered a stable nondissociative molecular chemisorption of C6H5SH, a discovery that has never been reported before. In addition to the chemisorption state of MCS-a with
Figure 2. Decomposed local density of states of the S atom in (a) a single C6H5SH molecule and (b) a molecular chemisorption state of MCS-a. 1004
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Figure 3. Top view of molecular chemisorption states of C6H5SH on Au(111) with the S atom located on the top site. (a) MCS-a; ϕ is 24.5° and the angle between the SH bond and SC bond is 97.0°; (b) MCS-b; ϕ is 81.7° and the angle between the SH bond SC bond is 97.0°; (c) MCS-c; ϕ is 34.5° and the angle between the SH bond and SC bond is 97.0°.
an adsorption energy of 0.28 eV, our results in Table 1 show some of the less stable adsorption states with much lower adsorption energy. For those states with low adsorption energies, the adsorption becomes physisorption in nature; we believe that previous studies of C6H5SH/Au concluded the nature of adsorption being physisorption merely because these studies might not have conducted a search of all adsorption potential wells and thus missed the chemisorption configurations found in the present work. For the molecular chemisorption state, we have checked that the azimuthal angle ϕ does not affect the adsorption energy. For instance, the energy of MCS-a with a ϕ of 24.5°, shown in Figure 3a, is almost the same as the energy of MCS-b with a ϕ of 81.7° shown in Figure 3b. The features of the MCS-a and MCS-b configurations of C6H5SH/Au are consistent with the theoretical prediction and experimental observation on the MCS of CH3SH/Au that the adsorbed CH3SH can rotate almost freely around the top site.40 Additionally, the direction of the SH bond does not affect the adsorption energy significantly; for example, the energy of MCS-c with the SH bond being 97° to the SC bond, as shown in Figure 3c, is merely 0.01 eV different from the energy of MCS-a with the SH bond being 97° to the SC bond. In previous calculations reported in the literature,18,19 only the top layer of Au atoms and the C6H5SH molecule are relaxed, while other substrate layers are fixed at the bulk positions. In the present work, we examined if relaxing the substrate layers is important. More specifically, we examined the following two scenarios: (a) relaxing the top three layers of Au atoms and the C6H5SH molecule and fixing only the bottom layer of Au atoms at their bulk positions; and (b) same as part a, except fixing the bottom three layers of Au atoms. In both scenarios, the MCS-a configuration was found. From scenario a to b, the adsorption energy changes from 0.28 to 0.25 eV, and the distance between the S atom and nearest Au atom changes from 2.70 to 2.76 Å. Hence, the exact condition of relaxing the Au(111) substrate does not affect the adsorption results significantly. In previous studies of the molecular chemisorption state of CH3SH/Au(111), the predicted adsorption energies are shown to be around 0.5 eV with the discrepancy less than 0.2 eV depending on the function and surface coverage.13,15,41 For aromatic-thiol molecules, our discovery of stable nondissociative molecular chemisorption on Au(111) is new. 3.3. Dissociative Adsorption States. 3.3.1. Phenyl-Thiolate Adsorption. In the dissociative adsorption state, the C6H5SH bond is cleaved, and the C6H5S group chemisorbs in a stable configuration. The released H atom may either chemisorb on the surface or desorb into vacuum as represented by eqs 2a
Table 2. Computational Results on the Adsorption of C6H5S on Au(111) θi (deg)a initial siteb θo (deg)a optimized siteb dSAu(Å)c EfPhS (eV) 15
30
45
60
75
90
bri
21.8
bri-fcc
2.46
1.38
fcc
13.9
bri-fcc
2.45
1.39
hcp
17.7
bri-hcp
2.47
1.26
top bri
26.7 32.1
fcc bri-fcc
2.47 2.46
1.34 1.34
fcc
29.2
fcc
2.47
1.32
hcp
37.7
bri-hcp
2.48
1.34
top
26.4
bri-fcc
2.46
1.34
bri
33.6
bri-fcc
2.47
1.36
fcc
35.6
fcc
2.49
1.29
hcp
47.1
bri
2.48
1.39
top bri
57.8 43.1
top bri-fcc
2.39 2.47
1.12 1.35
fcc
62.7
bri
2.50
1.40
hcp
56.5
bri
2.48
1.43
top
68.4
top
2.40
1.14
bri
64.8
top
2.40
1.15
fcc
65.2
bri
2.49
1.42
hcp
61.6
bri
2.49
1.43
top bri
75.2 43.7
top bri
2.40 2.46
1.14 1.38
fcc
64.2
bri
2.49
1.42
hcp
63.1
bri
2.49
1.43
top
75.6
top
2.39
1.12
a
Thr title angle between the surface normal and molecular axis in the initial and optimized structure, respectively. b The adsorption site in the initial and optimized structure, respectively. c The dSAu distance between the S atom and the nearest neighboring Au atom on the surface.
and 2b, respectively. To study the dissociative adsorption, we first calculated the adsorption of the C6H5S group on the Au(111) surface. Adopting the same way as we studied the nondissociative adsorption, we constructed a total of 24 initial configurations and placed them on the four possible adsorption sites. On each adsorption site, C6H5S was tilted from the normal direction at angles of 15, 30, 45, 60, 75, and 90°. The calculated results for the adsorption of C6H5S are summarized in Table 2, where the adsorption energies are calculated via EfPhS ¼ EðAuÞ þ EðC6 H5 SÞ EðC6 H5 S=AuÞ 1005
ð4Þ
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The results indicate that the C6H5S group prefers to chemisorb on the bridge site, and the distance between the S atom and the two nearest Au atoms are about 2.5 Å. In the most stable adsorption geometry, the adsorption energy is 1.43 eV. The second and third most stable adsorption configurations are the geometries with the S atom located on the bri-fcc/hcp and fcc sites, respectively. The geometry with the S atom located on the top position is the least stable one. Our calculation results on the adsorption of C6H5S summarized above are in good agreement with the previous theoretical calculations.18,19 3.3.2. Dissociative Adsorption with the Released H Atom Chemisorbed on the Au(111) Surface. In the dissociative adsorption case of DS_I, both the C6H5S and H atom chemisorb in their stable configurations and are far apart from each other. The formation energy is computed as EfDS_I ¼ EðAuÞ þ EðC6 H5 SHÞ EðC6 H5 S=Au þ H=AuÞ
ð5Þ
According to our calculation results summarized in Table 3, the H atom chemisorbs on the fcc hollow site on the Au(111) surface, while the adsorption on the hcp hollow site is less stable by merely 0.01 eV. This agrees well with the previous theoretical studies.15 To study DS_I, we first chose the most stable adsorption geometry of C6H5S, put one H atom in the neighboring fcc or hcp hollow site around the original SH bond direction, and then performed a full geometry optimization of the H atom and C6H5S simultaneously. Figure 4 shows the three geometries of the DS_I state where the H atom is away from the S atom at 4.86, 3.86, and 4.43 Å. Table 4 shows the calculation results for the three geometries. The DS_I state with the dissociated C6H5S group and H atom chemisorbed on the surface is slightly endothermic, with the formation energy of (0.02)∼(0.08) eV. Our results indicate that direct dissociative adsorption with the C6H5S group and H atom being adsorbed on the gold surface is almost energy neutral, and the released H atom can wander around quite freely. In this sense, dissociation is favored by an increase in entropy.
3.3.3. Dissociative Adsorption with the Released H Atom Desorbing in the Molecular Form of H2. In the dissociative adsorption case of DS_II, the C6H5S group is chemisorbed on the surface and the released H atom is desorbed into the vacuum by forming molecular H2. The formation energy of the DS_II state can be computed by the following formula using the energy of chemisorption of C6H5S and the energy of H2 in gas phase: EfDS_II ¼ EðAuÞ þ EðC6 H5 SHÞ EðC6 H5 S=AuÞ 1=2EðH2 Þ
ð6Þ The formation energy of the H2 molecule in gas phase is calculated to be 4.53 eV, which agrees well with the previous experimental and theoretical results.42 From the calculation results for the adsorption of C6H5S summarized in Table 2, we picked up the most stable one among those four configurations with the same initial tilt angle to compute the formation energy for DS_II. The calculation results are summarized in Table 5. The results indicate that the dissociative adsorption with the released H atom desorbing in the molecular form of H2 is exothermic, with a formation energy of 0.080.17 eV. A comparison of DS_I and DS_II clearly shows that while the most stable DS_I is endothermic by 0.02 eV, the most stable DS_II is exothermic by 0.17 eV. In other words, the dissociative adsorption with the released H atom desorbing in the molecular form of H2 is energetically more favorable. However, both the two dissociation adsorption cases, DS_I and DS_II, are less stable than the nondissociative molecular chemisorption. The total energy increases by 0.30 eV from MCS to DS_I and increases by 0.11 eV from MCS to DS_II, but in both DS_I and DS_II, increase in entropy is in their favor. 3.4. Dissociation Reaction Pathways. To further examine the kinetics of the dissociation of the C6H5SH molecule on the Au(111) surface and investigate the fate of the H atom, we studied the reaction pathways leading to the dissociation of C6H5SH/Au Table 4. Computational Results on the Dissociative Adsorption of C6H5SH with the Released H Atom Chemisorbed on Au(111)
Table 3. Computational Results for Atomic H Chemisorbed on Au(111)
θo
adsorption
adsorption
dSH
EfDS_I
ref
(deg)a
site of S
site of H
(Å)b
(eV)
adsorption site
Ef (eV)
DS_I-a
62.9
bri
fcc
4.86
0.02
fcc
2.03
DS_I-b
62.9
bri
hcp
3.86
0.06
hcp
2.02
DS_I-c
62.8
bri
fcc
4.43
0.08
bri
2.01
top
1.87
a
The tilt angle between the normal of the surface and the molecular axis of the C6H5S group. b The distance between the S atom and H atom.
Figure 4. Dissociative adsorption geometries with the released H atom chemisorbed on Au(111). (a) Configuration DS_I-a with the H atom chemisorbed on the fcc site and away from the S atom at 4.86 Å. (b) Configuration DS_I-b with the H atom chemisorbed on the hcp site and away from the S atom at 3.86 Å. (c) Configuration DS_I-c with the H atom chemisorbed on the fcc site and away from the S atom at 4.43 Å. 1006
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via DS_I and DS_II. The additional information on activation energies of transition states in these pathways further enhances our understanding of the intriguing case of C6H5SH/Au(111). 3.4.1. Dissociation Reaction with the Released H Atom Chemisorbing on the Au(111) Surface. Two reaction pathways, Path_I and Path_II were calculated to study the dissociation reaction from the MCS to the DS_I. Along Path_I, the H atom dissociates from the SH group and moves to the fcc site to form the dissociative adsorption of DS_I-a shown in Figure 4a. Table 5. Computational Results on the Dissociative Adsorption of C6H5SH with the Released H Atom Desorbing in the Molecular Form of H2 refa EfDS_II a
(eV)
15-fcc
30-bri
45-hcp
60-hcp
75-hcp
90-hcp
0.13
0.08
0.13
0.17
0.16
0.16
The dissociative adsorption with the released H atom desorbing in the molecular form of H2 and C6H5S chemisorbed in different geometries. For instance, 15-fcc refers to the adsorption with chemisorption of C6H5S calculated from the initial structure with a tilt angle of 15° and the S atom located on the Fcc site.
Path_II describes the reaction that the H atom dissociates from the SH group and moves to the hcp site to form DS_I-b shown in Figure 4b. We chose MCS-a shown in Figure 3a as the starting geometry for Path_I and Path_II. The calculated MEPs and transition states along Path_I and Path_II are shown in Figure 5ad, respectively. The activation energy barrier along Path_I and Path_II are both 0.58 eV. In addition, the geometrical configurations of the transition states along the two reaction pathways are also similar. In both cases, the C6H5S group remains close to its position in MCS with the S atom located on the top site, while the H atom is dissociated from the S atom and stays around the nearest top site. Since the H atom and the C6H5S group are separated during the dissociation, we checked the reaction by the spin-unrestricted calculations. It turns out that the spin-unrestricted calculations and spin-restricted calculations give the same results. This is because the cleavage of the SH bond and the chemisorption of the H atom to the top site happen simultaneously. Figure 6a,b shows the motion trace of the released H atom along Path_I and Path_II, where the C6H5S group moves from the original location on the top site in MCS to the bridge site in
Figure 5. Minimum energy paths of the dissociation reactions with the released H chemisorbed on the surface and the geometries of transition states in top and side views. (a) Minimum energy path along Path_I; (b) transition state along Path_I; (c) minimum energy path along Path_II; (d) transition state along Path_II. 1007
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Figure 6. Motion trace of the H atom along dissociative relevant reaction pathways: (a) Path_I; (b) Path_II. Numbers from 1 to 5 show the order of the positions reached by the H atom.
Figure 7. (a) Minimum energy path for the H atom transferred from the fcc to the hcp site through the top site and from the hcp to the fcc site through the bridge site; (b) Trace of the H atom on the surface, the number in the circle represents the image number along the MEP.
DS. It indicates that the H atom first dissociates from the adsorbed C6H5SH molecule to stay at the top site, then moves to the neighboring hollow site, and finally stays at the most stable chemisorption state. As shown by the reaction path in Figure 7, the barrier for the H atom moving between the hollow sites through the top site is 0.18 eV. However, the energy changes little when the H atom moves through the bridge sites, with the barrier being less than 0.05 eV. Since it is always possible for the H atom to find a way to avoid the top site, the released H atom is free to move on the surface between hollow sites. Experimentally, the perturbation with a voltage pulse of 2.5 V from the tip of a scanning tunneling microscope (STM) is enough to cleave the SH bond of a CH3SH molecule on Au(111),11 but for a C6H5SH molecule on Au(111), a voltage pulse of >2.7 V is needed.21 The experimental results thus suggest that the activation energy barrier for CH3SH dissociation is less than that of C6H5SH. In the literature, the calculated activation barrier for the dissociation of a CH3SH molecule on Au(111) is known to be 0.41 eV,15 which is 0.17 eV less than the barrier calculated in the present study for the C6H5SH molecule. The difference between the calculated activation barriers to dissociate a CH3SH and C6H5SH molecule on the Au(111) surface is therefore consistent with the experimental results. 3.4.2. Dissociation Reaction with the Released H Atom Desorbing in the Molecular Form of H2. The reaction pathways, Path_III and Path_IV, were calculated to study the dissociation reaction from the MCS to the configuration with the C6H5S
group chemisorbed on the surface and the H atom desorbing in the forms of a single atom and a H2 molecule. We first computed the reaction pathway Path_III along which the H atom dissociates from the SH group and desorbs into vacuum in the single atom form. Since the H atom is dissociated from the C6H5S group and far away from the surface, the MEP has been mapped out by the spin unrestricted calculations. The results are shown in Figure 8. Path_III starts at MCS-c shown in Figure 3c. Along Path_III, the SH bond cleaves and then the released H atom desorbs into vacuum. Meanwhile, the C6H5S group moves from the top position in the MCS state to the bridge position in the DS state. In the transition state shown in Figure 8b, the distance between the H atom and S atom is 3.48 Å, and the C6H5S group is very close to the position in the DS state. The activation barrier along Path_III is 1.82 eV. In the case of Path IV and eq 2b in which H2 is released, the formation energy of H2 amounting to 4.52 eV favorably drives this dissociation reaction. As shown in Figure 9, Path IV starts with the initial configuration having C6H5SH in the MCS state and one additional H atom nearby in its stable chemisorption state. The presence of such an adsorbed H atom is not unreasonable because, as shown earlier, any hydrogen atoms released from the dissociation of C6H5SH/Au can move freely on the gold surface. Along Path_IV, the adsorbed H atom moves from the hcp site toward the H atom of the adsorbed C6H5SH molecule, which results in the formation of the transition state. The geometry of the initial configuration and transition state along Path_IV are given 1008
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Figure 8. (a) Minimum energy path along Path_III of the dissociation reaction with the H atom released in the single atom form; (b) top and side views of the transition state along Path_III.
Figure 9. (a) Minimum energy path along Path_IV of the dissociation reaction with the H atom released in the H2 form; (b) initial configuration with molecular chemisorbed C6H5SH and one additional H atom nearby in its chemisorption state; (c) transition state along Path_IV.
in Figure 9b,c, respectively. In the transition state, the H atom is located on the top position, and the SH distance is elongated to 1.39 Å from the normal bond-length of 1.35 Å. The activation barrier of Path IV is 0.35 eV. After passing through the transition state, the two H atoms combine and form the H2 molecule; during this process, C6H5S is trapped in a metastable state (MT). In the final steps, the H2 molecule desorbs, and the C6H5S group relaxes by overcoming a small barrier of 0.07 eV to the most stable chemisorption state. 3.5. Simulated Scanning Tunneling Microscopy Images. As we have mentioned, Yates et al. observed the nondissociative adsorption of C6H5SH on Au(111) using STM at low temperature and found that the images have two lobes as a dumbbell with different lobe-sizes. In addition, the dissociative adsorption of C6H5S/Au produced by the thermal activation of C6H5SH/Au was imaged as a large lobe and a small tail. We simulated the STM images for the nondissociative molecular adsorption of C6H5SH/ Au and dissociative adsorption of C6H5S/Au using the Tersoff Hamann theory.43 Figure 10a,b shows our simulated images of filled states at a bias voltage of 2.0 V for the most stable MCS and C6H5S/Au configurations. The image of MCS in Figure 10a shows two connected bright protrusions with different protrusion sizes, reflecting the pattern of the C6H5 group and SH group. As shown in Figure 10b, the image of C6H5S/Au shows a large
Figure 10. Simulated filled-state STM images at a bias of 2.0 V for (a) nondissociative molecular chemisorption of C6H5SH/Au(111) and (b) dissociative adsorption of C6H5S/Au(111).
bright protrusion connected to a dim and small lobe, which corresponds to the C6H5 group and S atom. Our simulated images for both the nondissociative and dissociative adsorption agree well with the STM experimental results. 3.6. Fate of the H Atom of the Adsorbed C6H5SH. Before its nondissociative adsorption on Au(111), our calculation results show that C6H5SH has a stable SH bond with a bond energy of 3.5 eV. The respective experimental data44 are about 3.3 eV. The discrepancy of 0.2 eV between our calculation and the experimental data is the typical overestimation of the PBE method 1009
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The Journal of Physical Chemistry C on the RS-H (R represents CH3 and C6H5) bond energy.41 In comparison, the nondissociative adsorption of C6H5SH on the top site of Au(111) weakens the SH bond slightly by forming a weak chemical bond with the Au atom. Hence, thermal activation or an STM-tip may dissociate the SH bond of the C6H5SH molecule as reported in some recent experiments.21 Although the DS state is slightly less favorable than the MCS state energetically, the bonding between the S atom and Au surface is stronger in the DS state than in the MCS state. As shown earlier, the activation barrier to directly dissociate the H atom from an adsorbed C6H5SH molecule into vacuum is 1.82 eV. Such a high barrier prohibits practical thermal dissociation. Our calculations show that by keeping the H atom released from the dissociation of C6H5SH on the gold surface, the chemisorption of the H atom drastically lowers the activation energy for dissociating C6H5SH to 0.58 eV. The calculations also show that the reaction is almost energetically neutral and favors by an increase in entropy; hence, the reaction can proceed by thermal activation or other means of activation. Furthermore, the released H atom can diffuse readily on the gold surface and assist the dissociation of another adsorbed C6H5SH by forming and desorbing H2, as the activation barrier of this dissociation pathway has a low energy barrier of 0.35 eV. We believe that this is the most probable pathway for the dissociative adsorption of C6H5SH on Au(111). The evidence for the H2 desorption channel was indeed reported previously in the case of self-assembly of nitroaromatic thiols formed at 500 K on Au.22 X-ray photoelectron spectroscopy shows that the nitro groups of thiols are partially reduced to amino groups. The reduction of nitro groups is attributed to the interaction with the H atom released from the dissociation of the SH bond. This supports our proposal that the mechanism of C6H5SH dissociation is assisted by the availability of chemisorbed H.
’ CONCLUSIONS The key accomplishment of this work is the clarification of the nature of nondissociative adsorption of C6H5SH on Au(111), particularly the discovery of some relatively stable chemisorption configurations in addition to other physisorption configurations. The most stable configuration has the S atom locating on the top site of Au(111), with an adsorption energy of 0.28 eV. The discovery is reached by a comprehensive search of all possible adsorption configurations. In addition, this work also clarifies the detailed transition mechanisms from nondissociative to dissociative adsorption. In the first case, the SH bond cleaves, and the released H is stabilized by its simultaneous chemisorption on Au(111). The activation barrier is 0.58 eV. An alternative mechanism involves one additional H atom nearby on the surface to assist the dissociation of C6H5SH by forming and desorbing H2. The activation barrier is reduced to 0.35 eV. These results lead to the prediction that at a low coverage and low temperature, C6H5SH/Au(111) will stably assume the top site nondissociative adsorption configuration. Mildly heating the system above room-temperature will lead to dissociative adsorption, with the formation of C6H5S/Au and H/Au, and the emission of H2. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (X.F);
[email protected] (W.L.).
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’ ACKNOWLEDGMENT We thank Dr. Xiangqian Hu for valuable discussion. This work was funded by the National Natural Science Foundation of China (NSFC) (20903075) for X.F. This work was also supported by the 111 Project (B08040) in China. We also acknowledge the generous supports from the Beijing Computational Science Research Center, the Chengdu Green Energy and Green Manufacturing Technology R&D Center, the Institute of Chemical Materials, and the China Academy of Engineering Physics. ’ REFERENCES (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (2) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705–1707. (3) Romaner, L.; Heimel, G.; Gruber, M.; Bredas, J. L.; Zojer, E. Small 2006, 2, 1468–1475. (4) Heimel, G.; Romaner, L.; Bredas, J. L.; Zojer, E. Surf. Sci. 2006, 600, 4548–4562. (5) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (6) Schwartz, D. K. Annu. Rev. Phys. Chem. 2001, 52, 107–137. (7) Vericat, C.; Vela, M. E.; Benitez, G. A.; Gago, J. A. M.; Torrelles, X.; Salvarezza, R. C. J. Phys.: Condens. Matter 2006, 18, R867–R900. (8) Vericat, C.; Vela, M. E.; Salvarezza, R. C. Phys. Chem. Chem. Phys. 2005, 7, 3258–3268. (9) Liu, G.; Rodriguez, J. A.; Dvorak, J.; Hrbek, J.; Jirsak, T. Surf. Sci. 2002, 505, 295–307. (10) Rzeznicka, I. I.; Lee, J. S.; Maksymovych, P.; Yates, J. T. J. Phys. Chem. B 2005, 109, 15992–15996. (11) Maksymovych, P.; Sorescu, D. C.; Dougherty, D.; Yates, J. T. J. Phys. Chem. B 2005, 109, 22463–22468. (12) Hasan, M.; Bethell, D.; Brust, M. J. Am. Chem. Soc. 2002, 124, 1132–1133. (13) Zhou, J. G.; Hagelberg, F. Phys. Rev. Lett. 2006, 97, 045504– 045507. (14) Nenchev, G.; Diaconescu, B.; Hagelberg, F.; Pohl, K. Phys. Rev. B 2009, 80, 081401–081404. (15) Lustemberg, P. G.; Martiarena, M. L.; Martinez, A. E.; Busnengo, H. F. Langmuir 2008, 24, 3274–3279. (16) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir 1998, 14, 3570–3579. (17) Jung, H. H.; Do Won, Y.; Shin, S.; Kim, K. Langmuir 1999, 15, 1147–1154. (18) Bilic, A.; Reimers, J. R.; Hush, N. S. J. Chem. Phys. 2005, 122, 094708–094702. (19) Nara, J.; Higai, S.; Morikawa, Y.; Ohno, T. J. Chem. Phys. 2004, 120, 6705–6711. (20) Ford, M. J.; Hoft, R. C.; Gale, J. D. Mol. Simul. 2006, 32, 1219–1225. (21) Maksymovych, P.; Yates, J. T. J. Am. Chem. Soc. 2008, 130, 7518–7519. (22) Kankate, L.; Turchanin, A.; Golzhauser, A. Langmuir 2009, 25, 10435–10438. (23) Canchaya, J. G. S.; Wang, Y.; Alcami, M.; Martin, F.; Busnengo, H. F. Phys. Chem. Chem. Phys. 2010, 12, 7555–7565. (24) Stettner, J.; Winkler, A. Langmuir 2010, 26, 9659–9665. (25) Rajaraman, G.; Caneschi, A.; Gatteschi, D.; Totti, F. Phys. Chem. Chem. Phys. 2011, 13, 3886–3895. (26) Abufager, P. N.; Canchaya, J. G. S.; Wang, Y.; Alcami, M.; Martin, F.; Soria, L. A.; Martiarena, M. L.; Reuter, K.; Busnengo, H. F. Phys. Chem. Chem. Phys. 2011, 13, 9353–9362. (27) Pasquali, L.; Terzi, F.; Seeber, R.; Nannarone, S.; Datta, D.; Dablemont, C.; Hamoudi, H.; Canepa, M.; Esaulov, V. A. Langmuir 2011, 27, 4713–4720. 1010
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