J. Phys. Chem. C 2010, 114, 4051–4056
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Structures of Water Molecules Adsorbed on a Gold Electrode under Negative Potentials Sai Duan,†,‡ De-Yin Wu,† Xin Xu,*,† Yi Luo,*,‡,§ and Zhong-Qun Tian† State Key Laboratory of Physical Chemistry of Solid Surface and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, P. R. China, Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, S-106 91 Stockholm, Sweden, and Hefei National Laboratory for Physical Science at the Microscale, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P. R. China ReceiVed: NoVember 21, 2009; ReVised Manuscript ReceiVed: January 26, 2010
Two stable conformations of water hexamer clusters on gold electrode under negative potentials have been identified by density functional theory calculations. Both form a ring structure but with different orientations of free OH bonds. In one of the structures, labeled as F-Type, four free OH bonds of the water molecules point toward the gold surface and remain stable over a wide range of the negative potential. The other structure, labeled as S-Type, starts with five such free OH bonds pointing toward the gold surface at the low negative potential and ends up with six of them at higher negative potential. From the energetic point of view, the S-Type structure is more stable than the F-Type under the same potential. By comparing the calculated Raman spectra with the experiment, it is found that S-Type structures are the most possible surface adsorption state of water molecules at the electrochemical interface under very negative electrode potentials. It is believed that such a novel water structure could also exist on other negative charged surfaces. Introduction Water, as the most widely used solvent, and its structure on solid interface has attracted continuous interest in surface science.1-4 It is particularly important for understanding electrochemistry since the water molecules on metal-electrolyte interfaces play a vital role in the electrochemical double-layer.5 However, the in situ structures of water adsorbed on metal electrode surfaces are difficult to be determined straightforward and uncontroversial from conventional electrochemical methods.6,7 The use of multiple experimental technologies in combination with sophisticated theoretical studies is highly desirable. With the ex-situ technology, it is possible to obtain direct molecular pictures of water molecules adsorbed on metal surfaces in ultrahigh vacuum (UHV) conditions. For instance, from scanning tunneling microscopy (STM) images, water hexamer clusters are found to be formed as six-membered ring on M(1 1 1) (M ) Cu, Ag, Pt, Pd) surfaces.8-11 However, high resolution STM images for the water adsorbed on gold surfaces have not been reported yet,12 which might be attributed to the weak interaction between the gold surface and the water.13 By means of in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), Osawa and co-workers found that the water conformation on the gold surface undergoes a “flip-flop” transformation when the electrochemical potential passes through the potential of zero charge (PZC),14 which it does on other metal surfaces.15-18 In principle, surface-enhanced Raman spectroscopy (SERS) could be used to study this interface. However, earlier results showed that SERS signals of the interfacial water could only be presented in high concentration halide solutions and mainly on silver electrodes.19,20 A recent * To whom correspondence should be addressed: E-mail: xinxu@ xmu.edu.cn;
[email protected]. Phone +86 (0)592 2182219; +46 (0)8 55378414. Fax+86 (0)592 2183047; +46 (0)8 55378590. † Xiamen University. ‡ Royal Institute of Technology. § University of Science and Technology of China.
study has revealed that SERS signals of the interfacial water could be resumed under negative potentials.21 Changes of high resolution Raman spectra under different potentials have also been observed recently for water molecules adsorbed on gold surfaces.22 How the spectral change is related to the structural change of the adsorbed water molecules is a question that can not be directly answered by the experiments without the help of theoretical modeling. For neutral species, structures of water molecules adsorbed on metal surfaces have been extensively investigated theoretically. It was found that the water monomer is often tilted on the face-centered cubic (FCC) metal surfaces via the oxygen atom,23,24 while for water monolayers, six-membered ring structures are formed.25 All these theoretical findings are in good agreement with experiments. The conformation change controlled by surface charges or electric fields was also calculated for water molecules on Ag, Pd, and Pd on Au(111) surfaces.26-29 For the water monolayer on gold surfaces, classical molecular dynamics (MD) simulations,30-32 and first principles studies33 were also performed. To the best of our knowledge, the structure and its relation to Raman spectra of water molecules on charged gold surfaces have not yet been investigated. It is well-known that gold electrodes are widely used in electrochemistry due to their stability and biological activity.34-36 The focus of the present work is to study structures and corresponding Raman spectra of water clusters on gold electrodes under negative potentials. The calculated Raman spectra are compared with high resolution experimental SERS spectra of ref 22 to identify the actual structure of water molecules in realistic electrochemical environments. Computational Details Our calculations were carried out using density functional theory (DFT) with Beckes’s three-parameters exchange and Lee-Yang-Parr correlation hybrid functionals (B3LYP).37,38 Sadlej pVTZ basis sets were chosen for main elements while
10.1021/jp911072g 2010 American Chemical Society Published on Web 02/17/2010
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Figure 1. Optimized geometries for neutral (a) and charged Au9 cluster with one (b) and two (c) excess electrons at B3LYP/Lanl2dz level.
the Lanl2dz for metal elements, which is a good combination recommended for calculations of Raman intensities.39-41 The convergence of the energy and the dipole moment were checked by adding more diffuse basis sets, whose exponents were generated by scaling outermost diffuse functions, s function for hydrogen atoms as well as s and p functions for gold and oxygen atoms, by a factor of 0.125. The calculated expectations for the square of the spin angular moment (〈S2〉) were around 0.00 in the singlet and 0.75 in the doublet states, respectively, which indicated that the spin contamination could be neglected in the calculations. As mentioned above, water molecules are formed in a sixmembered ring on many metal surfaces. The bilayer like sixmembered ring water cluster, (H2O)6, was also chosen as the initial geometry here. It should be mentioned that the sixmembered ring water cluster is one of the isomers with the lowest energy in water hexamers.42 Moreover, this six-membered ring water cluster could be easily transferred to the basic bilayer structure of the liquid water. A planar Au9 cluster was used to represent the gold electrode, which should be big enough to accommodate the water hexamer cluster. Our test calculations with one-layer Au14 cluster43 have also shown that the geometry of the water cluster is almost identical to the one on the Au9 cluster. Excess electrons were added to the Au9 cluster to mimic
Duan et al. the negative potential used in experiments. The neutral and negative charged Au9 clusters are depicted in Figure 1. The average Au-Au bond length of Au9, Au9-, and Au92- clusters is found to be 2.81, 2.83, and 2.85 Å, respectively. This result shows that the planar Au9 cluster is stable even under very negative potential, i.e., Au92-, which is in good agreement with previous results.43 Full geometry optimizations have been done for all species with analytical frequency analysis to confirm that each geometry is a true local minimum. In the current investigation, we focus on the vibrations from water clusters. It is known that DFT calculations tend to overestimate vibrational frequencies of polyatomic molecules due to the lack of proper description of anharmonicity. Certain scaling factors are often applied to theoretical results to obtain a better agreement with corresponding experiments. We have used dual scaling factors 0.96 and 0.90, as proposed by Halls et al.,44 for bending and stretching vibrational frequencies of water clusters, respectively. The calculated Raman spectra are broadened by a Lorentzian function with a full width at the half-maximum (fwhm) of 100 cm-1 for the bending modes and of 600 cm-1 for the stretching modes, respectively, in order to direct compare with experimental observations. It is noted that the use of larger broadening factor for the stretching modes is to take into account the considerable temperature effects on Raman spectra in this region. All calculations were performed with Gaussian 03 suite of programs.45 Results and Discussions We have identified two stable ring conformations for (H2O)6 cluster adsorbed on Au9δ surfaces (δ ) -1) from the same initial structure. As shown in Figure 2b, one of them, named as F-Type, has four “free” OH bonds pointing toward the gold surface and
Figure 2. Initial structure (a) and optimized structures of [(H2O)6 Au9]- (b) and [(H2O)6 Au9]2- (c) at B3LYP/Sadlej pVTZ/Lanl2dz level.
Structures of Water Molecules under Negative Potentials TABLE 1: Calculated Absolute Energies in Hartree and Total Dipole Moments in Debye for [(H2O)6 Au9]δ Species at B3LYP/Sadlej pVTZ/Lanl2dz Level energy
dipole
δ
F-Type
S-Type
F-Type
S-Type
-1 -2 -2a
-1678.4335 -1678.4376 -1678.4393
-1678.4365 -1678.4451 -1678.4465
3.95 6.62 6.62
7.83 9.36 9.38
a Results at B3LYP/Sadlej pVTZ+diff/Lanl2dz level+diff level with the same geometries.
another, named as S-Type, has five. Increasing the excess electron (δ) from -1 to -2, the F-Type conformation remains stable, whereas the S-Type structure changes to have six “free” OH bonds pointing to the gold surface instead. Such a structural change is well demonstrated in Figure 2c. We have also started geometry optimization from a flat water hexagon, which has led to the same final products. Unfortunately in the neutral case, the water cluster always interacts strongly via oxygen atom with edge gold atoms and a bigger Au cluster is thus needed. It is interesting to note from Figure 2 that the optimized water cluster has rotated nearly 30° from its initial structure, which is attributed to the increase of electrostatic repulsion between negatively charged gold atoms and electronegative oxygen atoms. It can also be observed that the charged gold surfaces has a minor distortion with an angle less than 6°. The calculated energy and total dipole moment for different structures are listed in Table 1. It can be seen that the SType structures are 1.9 and 4.7 kcal/mol more stable than the F-Type structures with δ ) -1 and -2, respectively. It indicates that the more negative the potential is, the more favorable the S-Type structure will be. We have also examined basis sets dependence by using more diffuse basis sets to calculate species of δ ) -2. It is found that both the total energy and dipole moment are well converged with respect to the increase of the diffusion basis set. It strongly indicates that the computational method employed is indeed adequate. In electrochemical experiments when the electrode potential is negative, the electrode surface should be charged with excess electrons. We have used a negatively charged Au9 cluster to represent the gold electrode under the negative electrochemical
J. Phys. Chem. C, Vol. 114, No. 9, 2010 4053 potential, therefore, the location of excess electrons becomes critical. We have plotted all orbitals with excess electrons in Figure 3. It can be seen that the excess electron is always located at the Au9 surface even in the most negative system, i.e. [(H2O)6 Au9]2-. It thus shows that the model adopted in the present study is quite reasonable. For both F-Type and S-Type structures, vibrational frequencies and Raman intensities have been calculated. It is shown that the normal modes with large Raman intensity are distributed mainly in two regions: the bending modes around 1660-1690 cm-1 and the stretching modes at 3690-3890 cm-1. All other normal modes are too weak to be observed in Raman spectra. We will focus our discussion on those modes with strong intensity. The calculated Raman frequencies and intensities of three bending modes and six stretching modes for [(H2O)6 Au9]δ species are listed in Table 2. It can be seen that when more excess electrons are added, vibration frequencies are red-shifted for both two regions of the F-Type, but only the stretching region of the S-Type. Such behaviors can be well explained by simple electrostatic interaction. However, the observed blue shift in the bending region of the S-Type is quite unusual. An analysis has shown that it could be strongly associated with the structure change of the water cluster. It has already been pointed out that when one more excess electron is added, one of the OH bonds undergoes a “flip-flop” transformation, i.e. its direction changes from in-plane to normal to the gold surface. After such a transformation, all free OH bonds in the S-Type point toward the gold surface. Hence the electrostatic interaction between the negatively charged gold surface and electropositive hydrogen atoms in the water molecules is drastically increased, which hinders the bending modes and results in the blue shift.46 The calculated Raman spectra are compared with experimental spectra of ref 22 in Figure 4. The electrochemical potential dependent experimental spectra do show some noticeable features. In the bending region, there is only one peak centered around 1610 cm-1 after scaling, showing no shift in energy but large enhancement in intensity when the potential becomes more negative. In general, the calculated spectra of S-Types at δ ) -1 and -2 reproduce the experimental ones very well. It is interesting to note that for the S-Type the vibration frequencies of the calculated three visible modes should be blue-shifted when
Figure 3. Molecular orbitals with the excess electron in the F-Type (up) and the S-Type (bottom) obtained at B3LYP/Sadlej pVTZ/Lanl2dz level. (a) Highest occupied molecular orbital (HOMO) of δ ) -1, (b) HOMO-1 of δ ) -2, and (c) HOMO of δ )- 2. The value of the isosurface is 0.015 au.
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TABLE 2: Calculated Three Lowest Frequencies in Bending Region and Six Largest Frequencies in Stretching Region in cm-1 and Corresponding Raman Intensities in Å4/amu for [(H2O)6 Au9]δ Species at B3LYP/Sadlej pVTZ/Lanl2dz Level species F-Type δ ) -1
S-Type δ ) -1
a
freq.
intensity
Max Max IBend /IStret
dipolea
species
freq.
intensity
Max Max IBend /IStret
dipolea
1674.4 1682.0 1683.7 3746.8 3750.1 3754.5 3788.7 3875.0 3882.7
6.6 4.5 4.0 118.8 65.5 454.5 155.3 90.6 140.3
0.015
2.87
δ ) -2
1647.9 1666.1 1675.3 3683.3 3695.2 3709.7 3721.4 3730.2 3883.2
27.5 40.3 4.9 154.2 129.6 167.2 16.2 423.1 121.3
0.095
5.78
1661.0 1678.5 1687.4 3753.0 3755.3 3760.7 3795.6 3802.1 3881.6
3.3 5.1 5.3 73.5 172.9 261.1 109.7 162.9 83.1
0.020
6.24
δ ) -2
1684.7 1687.8 1693.8 3692.6 3698.5 3719.0 3736.5 3740.4 3759.8
46.3 19.3 13.7 132.7 162.4 95.4 102.7 399.9 196.9
0.12
7.38
Calculated total dipole moment of neutral (H2O)6 fragment in Debye at B3LYP/Sadlej pVTZ level.
Figure 4. Raman spectra of water molecules adsorbed on the gold surface in bending (up) and stretching (bottom) vibration regions. (a) Experimental in-site electrochemical SERS spectra from ref 22. Calculated Raman spectra of (b) F-Types and (c) S-Types at B3LYP/Sadlej pVTZ level. All theoretical spectra have been broadened with a fwhm of 100 cm-1 in the bending and of 600 cm-1 in the stretching regions with scaling factors of 0.96 and 0.90, respectively, for the frequency.
the potential goes from -1 to -2. However, the final spectra do not change due to the redistribution of the Raman intensity. This result clearly demonstrates that it is not adequate for interpreting spectra with only the information of vibrational frequency. In the stretching region, experimental spectra show red shift with the increase of the potential and the narrowing of
spectral profile. Both F-Type and S-Type structures produce the red-shift of the spectral peaks although less than what has been observed in the experiment. By inspecting the spectral profile, one might argue that the spectral peak of the S-Type becomes slightly narrower from -1 to -2 and it seems to fit the experimental result slightly better. Moreover, for the absolute
Structures of Water Molecules under Negative Potentials
J. Phys. Chem. C, Vol. 114, No. 9, 2010 4055 Ministry of Science and Technology (973 program Nos. 2007CB815303 and 2009CB930703), and Swedish Research Council (VR). The high performance computational center of Xiamen University and the Swedish National Infrastructure for Computing (SNIC) are acknowledged for computer time. D.Y.W. thanks NCETFJ for support.
Figure 5. Structures of water clusters when the gold surface is replaced by an excess electron at B3LYP/Sadlej pVTZ level. (a) Initial geometry and (b) optimized geometry.
intensity of the spectral maximum, the F-Type shows no enhancement whereas the S-Type does. In this sense, the latter agrees with the experiment.22 From both energetics and Raman spectra points of view, the S-Type structure should be favored over the F-Type. Both F-Type and S-Type structures are dipole bounded electron system alike since excess electrons are localized on the Au9 surface.47 The calculated total dipole moments of neutral (H2O)6 fragments in all species at B3LYP/Sadlej pVTZ level are listed in Table 2. The dipole of water fragments in the S-Type is larger than that in the F-Type. This might be the reason why the S-Type structure is more stable than the F-Type structure. Kim and co-workers argued that there often exist a set of “free water” motif structures in traditional dipole bounded electron systems of water clusters.47 Here the largest dipole moment (7.38 D) of the neutral water fragment in our system is comparable with that (7.08D) of a “free water” structure cluster.47 Our results might imply that the “free water” motif is not necessary for bounding an excess electron on the watermetal interface. We have found that when the gold surface is replaced by an excess electron, the planar structure of the water hexamer is distorted as shown in Figure 5 and the dipole moment of the neutral water cluster reduces to the value of 5.89 D. It means that the localized electron can induce a novel dipole bounded water cluster at the interface. Furthermore the planar structure could be suitable for creating a water network which was predicted in traditional dipole bounded electron systems of water clusters.48 Since the interaction between the dipole of the interfacial water cluster and excess electrons on metal surfaces is the determinant factor for the formation of water structures, we could anticipate that such conformations could also exist on other negative metal surfaces like Pt or Pd, since experimental Raman spectra of water molecules on Au, Pt, and Pd electrodes show similar changes when the potential goes more negative.22 Conclusion With density functional theory, we have studied conformations of water hexamer clusters adsorbed on negative charged gold electrodes. Two possible stable conformations at different negative potentials have been identified. By comparing the calculated and experimental Raman spectra, the structure with the lowest energy, the S-Type, has been confirmed to be the most possible structure in the experiments. Calculations have shown that the S-Type structure can undergo a flip-flop transformation for one of its free OH bonds during the change of the potential. This study clearly demonstrates that a combination of theoretical modeling and experimental measurements could provide rich structural information for molecules adsorbed on metal surface under electrochemical conditions. Acknowledgment. This work is supported by the Natural Science Foundation of China (Grants 20433040 and 20973143),
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