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Identification of Vertical and Horizontal Configurations for BPE Adsorption on Silver Surfaces Tao Chen, Amrita Pal, Jie Gao, Yun Han, Hui Chen, Svetlana A. Sukhishvili, Henry Du, and Simon G. Podkolzin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07831 • Publication Date (Web): 29 Sep 2015 Downloaded from http://pubs.acs.org on October 12, 2015

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The Journal of Physical Chemistry

Identification of Vertical and Horizontal Configurations for BPE Adsorption on Silver Surfaces Tao Chen, Amrita Pal,1 Jie Gao,2 Yun Han,3 Hui Chen, Svetlana Sukhishvili,4* Henry Du,* Simon G. Podkolzin* Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, New Jersey 07030 United States. Present Addresses: 1

National University of Singapore, Environmental Research Institute, 5A Engineering Drive 1, Singapore 117411. Integrated Analytical Laboratory, 271 Franklin Road, Randolph, NJ 07869 United States. 3 Applied Materials, 2911 U.S. 9, Ballston Spa, NY 12020 United States. 4 Departments of Materials Science and Engineering, Texas A&M University, 575 Ross Street, College Station, TX 77843 United States. 2

KEYWORDS: Trans-1,2-bis(4-pyridyl)ethylene, pyridine, silver, gold, adsorption, SERS, DFT.

ABSTRACT: Adsorption of trans-1,2-bis(4-pyridyl)ethylene (BPE), a molecule with two pyridine rings connected with a C=C double bond, was studied on Ag surfaces with surface-enhanced Raman spectroscopic (SERS) measurements and density functional theory (DFT) calculations. Spectroscopic measurements were collected using well-defined 48-nm monodispersed Ag and Au nanoparticles supported on SiO2. Effects of Ag oxidation were evaluated by varying the duration of an ozone treatment prior to adsorption. Effects of surface coverage were evaluated by exposing unoxidized and oxidized Ag samples to solutions with a variable BPE concentration. Periodic unit-cell DFT calculations were performed using Ag(111), p(4 × 4)-O/Ag(111) and Ag2O(111) surfaces. Two adsorption configurations were identified: vertical and horizontal. In the vertical configuration, BPE adsorbs nearly orthogonal to the surface by binding through one of its N atoms to a single surface Ag atom. In the horizontal configuration, BPE adsorbs nearly parallel to the surface by binding through both of its two N atoms to two separate surface Ag atoms. BPE adsorbs initially as a mixture of the vertical and horizontal configurations. As the BPE surface coverage increases, the vertical configuration becomes preferential due to geometric constraints. In contrast, the horizontal configuration becomes preferential with increasing extent of Ag oxidation due to its greater stability on oxidized surfaces. Similarities in spectroscopic results for metallic Ag and Au nanoparticles suggest that the BPE adsorption trends with increasing surface coverage are the same for both metals.

1. INTRODUCTION Trans-1,2-bis(4-pyridyl)ethylene (BPE) is a hydrocarbon molecule that is structurally similar to pyridine (two pyridine rings connected with a C=C double bond). BPE is widely used as a sensitive non-resonant spectroscopic probe for metal surfaces due to its large scattering cross section, conjugated π-bond electrons, and two nitrogen atoms on the opposite sides of the molecule.1-4 Each nitrogen atom has a lone electron pair, which allows BPE to adsorb relatively strongly on metal surfaces. Measurements of BPE adsorption on Ag nanoparticles are es-

pecially widely utilized in the development of ultra-high sensitive surface-enhanced Raman spectroscopic (SERS) techniques.5-7 Interpretation of spectroscopic results and development of improved measurement techniques require a molecular-level understanding of BPE adsorption. Infrared (IR) and Raman spectra of BPE adsorbed on Ag surfaces were analyzed initially with Hartree-Fock and later with density functional theory (DFT) calculations.8-9 Although these theoretical studies assigned all experimental vibrational frequencies, they considered only metallic Ag and assumed only one mode of BPE adsorption in a vertical orientation. In contrast, recent microscopy and

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DFT studies for pyridine adsorption on Ag showed that pyridine changes its preferential adsorption mode from horizontal to vertical with increasing adsorbate coverage.10-11 Our earlier study reached the same conclusion for p-nitrophenol (PNP) adsorption on Ag based on the evolution of Raman spectra with increasing adsorbate coverage, where in-plane PNP vibrational bands increased at the expense of out-of-plane bands.1 In addition, our recent results showed that relative intensities of Raman bands for BPE adsorbed on Ag change with the extent of surface oxidation, which also suggests that BPE has more than one adsorption mode and that these modes change with surface oxidation.5 It is, therefore, important for further development of spectroscopic and sensor applications to identify modes of BPE adsorption on Ag surfaces and evaluate their dependence on surface oxidation and adsorbate coverage. Our current study addresses these issues. In addition to spectroscopy, Ag surfaces are widely used as catalysts in selective oxidation of hydrocarbons, most notably for converting ethylene to ethylene oxide12-14 and methanol to formaldehyde.15-17 Despite the generally acknowledged dynamics of catalytic Ag surfaces,18-23 which may dynamically change their extent of oxidation due to the presence of adsorbed, subsurface and lattice oxygen species under reaction conditions in the presence of gas-phase O2, most molecular models consider hydrocarbon adsorption only on metallic Ag. As a result, current models of Ag catalysts are not able to incorporate effects of dynamic surface changes for a better description of catalytic performance because adsorption even of simplest hydrocarbons, such as ethylene and methanol, as a function of the extent of Ag oxidation is not well understood. Similarly for pyridine, a common probe in catalyst characterization, studies of its adsorption on Ag are mostly limited to metallic surfaces.10-11, 24-28 Therefore, the current study for BPE adsorption on Ag as a function of surface oxidation provides a useful benchmark for future investigations of hydrocarbons adsorbed on catalytic Ag surfaces under oxidation reaction conditions. One of the difficulties in the evaluation of Ag oxidation effects is the complexity of Ag surface transformations in reactions with oxygen. Several atomic-level structures of partially oxidized Ag surfaces have been recently identified based on extensive studies using X-ray photoelectron spectroscopy (XPS), surface X-ray diffraction (SXRD) and DFT calculations.19, 29 Our current study employs one of these recently identified surface structures for evaluating effects of Ag oxidation on BPE adsorption. The current study combines Raman spectroscopic measurements with DFT calculations for evaluating changes in BPE adsorption on Ag as a function of surface oxidation and adsorbate coverage. Our methodology of combining spectroscopic measurements and electronic-structure calculations with vibrational analyses has previously allowed us to successfully identify Mo oxides structures supported on zeolites at the molecular level.30 Spectroscopic measurements were collected using well-defined monodispersed Ag nanoparticles. The Ag nanoparticles were supported on SiO2, making them particularly suitable for SERS measurements. The employed Ag/SiO2 samples also represent a model system for hydrocarbon oxidation catalysts. Thermodynamic stability of various BPE adsorption modes on metallic, partially oxidized, and oxidized Ag surfaces were evaluated with DFT calculations. Computational results suggest that BPE adsorbs in two configurations: vertical and horizontal. Although both adsorption modes are energetically similar on metallic Ag

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at low coverage, the vertical configuration becomes preferable with increasing adsorbate coverage due to geometric constraints. In contrast, the horizontal configuration with bonds to two separate Ag atoms becomes energetically preferable with increasing extent of surface oxidation due to polarization of Ag adsorption sites. These changes in BPE adsorption modes provide an explanation for the observed changes in experimental Raman spectra as a function of surface oxidation and adsorbate coverage. In addition to Ag, Raman spectroscopic measurements were collected for monodispersed Au nanoparticles. Similarities in spectroscopic results for Ag and Au suggest that the BPE adsorption trends identified for Ag are also applicable to Au surfaces. 2. EXPERIMENTAL AND COMPUTATIONAL METHODS a. Materials Chemical reagents were purchased from the following suppliers and used without purification: sodium citrate dihydrate (Na3C6H5O7·2H2O, Fisher Scientific, enzyme grade), silver nitrate (AgNO3, Acros, ultra-pure grade), gold(III) chloride hydrate (HAuCl4·xH2O, Aldrich, 99.999% trace metals basis), sodium thiocyanate (NaSCN, Aldrich, 99.99%), trans-1,2-bis(4pyridyl)ethylene (Aldrich, 97%), polyallylamine hydrochloride (Aldrich, weight-average molecular weight 15,000 g/mol), and ethanol (Aldrich, anhydrous, 99.8%). Water was filtered with Barnstead ion-exchange columns and further purified by passing through Millipore (Milli-Q) columns. All glassware and substrates were cleaned in Nochromix solution (Godax Laboratories, Inc., Maryland) in concentrated sulfuric acid overnight, followed by thorough rinsing with Milli-Q water. b. Synthesis of Ag and Au Nanoparticles Ag and Au nanoparticles were synthesized using a modified Lee and Meisel procedure31, as described in our previous studies about the dependence of SERS measurements on the extent of Ag oxidation.1, 5 Briefly, a 1 wt % sodium citrate solution in water was added drop-wise into a 1 mM AgNO3 (4 mM HAuCl4 for Au nanoparticles) solution under continuous stirring for approximately 1 min. The obtained mixture was transferred into an ultraviolet (UV) chamber (UV Flood Curing System, Cure Zone 2 from CON-TROL-CURE, Chicago, IL) maintained at 313 K using a water bath and kept under UV irradiation with continuous stirring in a glove box filled with Ar for 2 h (1 h for Au). The resultant Ag and Au colloidal solutions were wrapped in an aluminum foil to avoid light exposure and stored under an Ar atmosphere. The ζ-potential of both the Ag and Au colloids, measured using a Zetasizer Nano Series (Malvern Instruments), was -40 mV, corresponding to monodispersed Ag and Au particles with a size of 48 nm. The polydispersity index for Ag was 0.12. The high level of monodispersity of Ag nanoparticles in solution was confirmed with an ultraviolet-visible (UV-vis) absorption spectrum obtained with a Synergy HT Multi-Detection Microplate Reader (BioTek Instruments). The spectrum exhibited a single absorbance peak at 410 nm. The prepared colloidal metal nanoparticles were deposited onto SiO2 glass surfaces of about 1 cm2 pre-covered by polyallylamine hydrochloride (PAH) polymer, which minimized agglomeration of the metal nanoparticles. Prior to the metal deposition, glass cells with SiO2 supports were filled with a 1 M NaOH water solution for 5 min, followed by a thorough rinse


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Figure 1. Scanning electron microscope image of monodispersed 48-nm Ag nanoparticles supported on SiO2.

with Milli-Q water. The cells were then exposed to a 0.2 mg/ml PAH solution at pH = 9 for 30 min and then thoroughly rinsed with Milli-Q water, producing a PAH coating on SiO2. The nanoparticles were deposited at pH = 5.6 by an exposure of the PAH-coated cells with SiO2 to the Ag colloidal solution for 24 h (1 h for Au), followed by rinsing with Ar-purged Milli-Q water. Images of nanoparticles supported on SiO2 were collected with a scanning electron microscope (SEM, Zeiss Auriga) at 3 kV with a working distance of 7 mm. A representative SEM image for Ag/SiO2 is shown in Figure 1. The SEM images for Au/SiO2 in Figure S1 are similar. An analysis of the SEM images with the ImageJ software confirmed that Ag and Au nanoparticles were mostly monodispersed with a size of 48 nm. The surface density of Ag nanoparticle was 207±3 per μm2 with only a small fraction of 19±1 dimers, 6±1 trimers, and 3±1 larger clusters. We note that although the fraction of the isolated Ag particles exceeded 86%, it is still possible that the collected Raman spectra for BPE adsorption were influenced by Ag clusters, which are known to serve as “hot” spectroscopic spots and enhance Raman intensities. It is, however, likely that BPE adsorption configurations on individual Ag particles and clusters were similar. c. Oxidation of Ag Nanoparticles For evaluating effects of Ag oxidation, a separate set of oxidized samples was prepared. The initial Ag/SiO2 samples ("metallic") were exposed simultaneously to ozone at a controlled concentration of 17±1 ppm at room temperature and to longwavelength UV irradiation at different time intervals: 180, 240, 300 and 360 min, generating four samples with an increasing extent of Ag oxidation. The extent of Ag oxidation was analyzed with XPS measurements. Ozone was generated with an Air-Duct 2000 instrument (Air-Zone Inc., Suffolk, VA) and measured with an OS-4 Ozone Switch sensor (Eco Sensors, Inc.) with the detection range from 10 ppb to 20 ppm. The UV exposure was performed with a 400 W metal halide lamp with a spectral range of 320-400 nm in the same Cure Zone 2 UV Flood Curing System used in the preparation of the Ag colloids. The intensity of the UV irradiation was 80 mW/cm2 at a distance of 5 cm from the protective shield of the lamp. The samples were placed at a distance of 22.5 cm from the protective shield of the UV lamp. SEM images (not shown for brevity) of the Ag nanoparticles after the oxidation treatments were similar to those of metallic nanoparticles in Figure 1, indicating that the oxidation did not measurably affect the Ag dispersion. A duplicate additional set of metallic and oxidized Ag/SiO2 samples was prepared specifically for XPS measurements. These samples were prepared identically to those used in the

spectroscopic experiments, except for an addition of a 90-nm thick Au coating layer onto the SiO2 support. The Au layer was added using an atomic layer deposition prior to the deposition of Ag nanoparticles. This was done in order to avoid an oxygen signal from the SiO2 support and, therefore, obtain a more accurate evaluation of the extent of Ag oxidation after the controlled ozone exposures. d. Surface-Enhanced Raman Spectroscopic (SERS) Measurements Prior to the Raman spectroscopic measurements, the Ag/SiO2 and Au/SiO2 samples were dried under an Ar flow and then kept in an Ar-filled glove box to prevent oxidation. The spectra were collected at room temperature after adding ~400 µl of a BPE solution to a transparent glass sample cell and allowing the system to equilibrate for 20 min, similarly to our previously reported measurements.5 This equilibration time was found to be sufficient for collecting consistent spectra. The laser beam was focused on the surface of nanoparticles. The spectra were collected using a 2.8 mW laser for Ag and a 10 mW laser for Au at 532 nm excitation with an acquisition time of 20 sec. A Nikon 20×0.4 N.A. objective was used to produce a large laser-focusing beam (~1 mm in diameter). These large, defocused excitation spots as well as the uniform nanoparticle coverage density allowed us to obtain consistent measurements with small (less than 15%) spot-to-spot variations in Raman intensities. For Ag, spectra were collected in three different sample spots for each measurement and then averaged. The standard deviations obtained based on these multiple measurements are shown as error bars for reported Raman band intensities. For Au, spectra were collected in two different sample spots, and the corresponding band intensities are reported without averaging. The band intensities were calculated and analyzed using the Grams32 version 5 and Origin version 7.5 software packages. Under all experimental conditions, the SERS signal-to-noise ratio could be clearly resolved. Adsorbate coverage effects were evaluated with SERS measurements by varying the concentration of a BPE solution in ethanol from 1×10-5 to 1.8×10-2 mg/ml. Higher concentrations were not used to restrict spectra to SERS measurements, as opposed to regular Raman spectroscopic measurements. e. X-ray Photoelectron Spectroscopy (XPS) Measurements The XPS spectra were collected using a Kratos Axis Ultra Xray photoelectron spectrometer equipped with monochromatic Al Kα X-rays and a concentric hemispherical analyzer combined with a multi-channel delay-line detector (DLD). The base pressure of the analytical chamber was 1×10-9 Torr. The hemispherical analyzer was operated in the hybrid mode using electrostatic and magnetic lenses. To minimize air exposure, samples were kept under Ar prior to installation into the load lock. High-resolution scans at a spectral resolution of 0.1 eV for the Ag3d, Au4f, C1s, and O1s regions were collected at the 20 eV pass energy at three separate spots on each sample. Peak areas were averaged for the repeated measurements and quantified using the relative sensitivity factors of 0.78 for O1s and 5.987 for Ag3d. A low-energy electron flood gun was used to minimize charging. All peaks were corrected by referencing the primary peak of the Au4f signal at 84.0 eV. f. Density Functional Theory (DFT) Calculations Raman spectra with intensities for a laser at 532 nm excitation were obtained with DFT frequency calculations for an isolated BPE molecule and for BPE adsorption configurations on



Ag (~2.7% above the experimental value of 0.41 nm) and 0.49 nm for Ag2O (~4.0% above the experimental value of 0.47 nm) were used to generate the unit cells from the respective bulk crystals. A vacuum spacing of 3 nm between the slabs in the c direction was used. The Ag(111) surface was modeled with a 5×5×3 unit cell (a 5×5 atom surface with 3 layers) with a total of 75 Ag atoms (Figure 2a). A partially oxidized surface was modeled by a single layer of a p(4 × 4)-O structure with an Ag2O stoichiometry epitaxed on a 5×6×2 Ag(111) unit cell with a total of 84 Ag and 12 O atoms (Figure 2b). Lastly, the Ag2O(111) surface was modeled with a 6×9×3 unit cell with a total of 108 Ag and 54 O atoms (Figure 2c). The two top surface layers in each Figure 2. Unit cells of DFT model surfaces: (a) Ag(111), representing metallic Ag, (b) p(4 × 4)-O/Ag(111), representing partially oxidized Ag, and (c) Ag2O(111), representing oxidized model were optimized during geometry Ag as an Ag(I) oxide. Top row: side views, bottom row: top views. Ag atom labels: Ag(number optimizations, simulating surface relaxaof neighboring Ag surface atoms, number of neighboring O surface atoms). tion on adsorption. The bottom layer in each model was fixed in order to better the Ag27 cluster shown in Figure S2. The cluster had 14 atoms account for the underlying bulk structure. Reciprocal-space inin the first layer, 9 atoms in the second layer and 4 atoms in the tegration over the Brillouin zone was approximated through kthird layer. It was fully optimized prior to BPE adsorption and point sampling using the Monkhorst-Pack grid: (4×4×1) for the then fully re-optimized with a BPE molecule in different admetallic and partially oxidized surfaces and (1×1×1) for the sorption configurations. These calculations were performed usAg2O(111) surface. The density mixing fraction of 0.1 with diing the DMol3 code in Materials Studio 7.0 software by rect inversion in the iterative subspace (DIIS) and orbital occuBIOVIA Corp. pancy with smearing of 0.005 Ha were used. The orbital cutoff 3 distance was set at 0.4 nm for all atoms. All other calculations were performed with the DMol code in Materials Studio 4.0 software, similarly to our previous periBPE adsorption modes were evaluated by generating multiodic unit-cell calculations for adsorption of C2 hydrocarbons on ple geometries with binding through one or two N atoms to varPt(111) and Pt-Sn/Pt(111) surfaces.32-33 The calculations used ious Ag surface sites. Adsorption energies were calculated with the double numerical with polarization (DNP) basis set and the one BPE molecule in a unit cell at 0 K without zero-point engeneralized gradient-corrected Perdew-Wang (GGA PW-91) ergy corrections. The sum of energies for a surface and an isofunctional. Tightly bound core electrons for Ag were reprelated BPE molecule calculated separately was used as a refersented with semi-core pseudopotentials. Effects of Ag oxidation ence. Adsorption energies are reported as positive -ΔEads numwere evaluated by comparing BPE adsorption on three model bers. Lateral adsorbate interactions were evaluated by comparsurfaces shown in Figure 2: (a) Ag(111), (b) p(4 × 4)ing adsorption of a single BPE molecule on the Ag(111) surface O/Ag(111) and (c) Ag2O(111). The (111) crystal plane is thermodynamically the most stable and, therefore, is expected to be Ag3d dominant for the 48 nm metallic Ag nanoparticles used in this 373.8 Oxidation 367.8 study. Partially oxidized surfaces are complex, and their structime, min 29 tures and stoichiometries depend on experimental conditions. 360 Our XPS results suggest the formation of a surface layer with an Ag2O stoichiometry on top of metallic Ag for our oxidized samples. The atomic-scale geometry for such an overlayer with 300 the overall Ag2O stoichiometry has been recently reported for an Ag(111) surface exposed to gas-phase O2 at 10 Torr and 500 240 K. Its structure has been identified as p(4 × 4)-O/Ag(111) with Ag2O triangles of six Ag atoms surrounded by O atoms in the top sur180 face layer, as shown in Figure 2b.29, 34 This surface with an oxide 374.2 368.2 overlayer was chosen to represent a partially oxidized Ag surmetallic 0 face. Finally, an Ag2O(111) model in Figure 2c was chosen to represent an Ag surface with a higher extent of oxidation. All 360 365 370 375 380 surfaces were modeled as infinite periodic slabs constructed Binding energy, eV with the unit cells shown in Figure 2. We note that these are not the elementary unit cells for each surface. The elementary unit Figure 3. High-resolution Ag3d X-ray photoelectron spectra of cells were combined into the shown larger supercells in order to Ag/SiO2 as a function of the duration of the oxidation treatment with simultaneous exposures to ozone (17±1 ppm) and UV irradiconstruct sufficiently large unit cells for examining BPE adation (320-400 nm). sorption. The optimized lattice constants of 0.42 nm for metallic

Intensity, cps

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1588

1618

b. Oxidized Ag

a. Metallic Ag 1588

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Raman intensity, a.u.

Raman intensity, a.u.

532 nm laser

BPE concentration in ethanol, mg/ml 5×10-3 1×10-3 1×10-4 1×10-5 background

1400 1500 1600 Wavenumber, cm-1

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1.5×10-2 5×10-3 1×10-3 5×10-4 background Ag2O

1400 1500 1600 Wavenumber, cm-1

1700

Figure 4. Surface-enhanced Raman spectra for BPE adsorbed on (a) metallic Ag and (b) partially oxidized Ag after 180-min oxidation treatment for increasing BPE concentrations in an ethanol solution.

1.6

I1618 cm-1/I 1588 cm-1

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0.4 0 0

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BPE concentration in ethanol, 10-2 mg/ml

Figure 5. Ratio of the intensity of the Raman band at 1618 cm-1 to that of the band at 1588 cm-1 as a function of the BPE concentration in an ethanol solution for (a) metallic Ag and (b) partially oxidized Ag after the 180-min oxidation treatment. Error bars represent standard deviations for repeated measurements.

increased from 180 to 360 min, the positions of the peaks remained unchanged. Previous studies with calibration measurements showed that shifts of the Ag3d peaks to lower binding energies by 0.3-0.4 eV indicate oxidation to Ag2O and by 0.8-1.0 eV to AgO.35-36 The spectra in Figure 3 with a shift by 0.4 eV, therefore, suggest that the ozone treatment for 180 min formed a surface layer of Ag2O on top of metallic Ag. Ag2O was present only as a surface layer, because extending the oxidation treatment from 180 to 360 min did not lead to the formation of an AgO phase with larger binding energy shifts. As the oxidation treatment time increased, the thickness of the Ag2O surface layer was likely growing. This conclusion is consistent with our previously reported analysis of the O1s XPS spectra for the same Ag/SiO2 samples.5 A new O1s peak at 529.2 eV due to the formation of an Ag2O surface layer35 was detected after the ozone treatment for 180 min, and the intensity of this peak increased for longer durations of the ozone treatment. The XPS results were used to select computational models of Ag surfaces with increasing extent of Ag oxidation in Figure 2: (a) Ag metal, (b) a surface overlayer with an Ag2O stoichiometry on top of Ag metal and (c) Ag2O. b. SERS Measurements The SERS measurements for BPE adsorbed on Ag surfaces as a function of the surface coverage were collected for two Ag/SiO2 samples: the initial metallic (unoxidized) Ag (Figure 4a) and partially oxidized Ag after the ozone treatment for 180 min (Figure 4b). The BPE coverage on the Ag surfaces was increased by increasing the BPE concentration in ethanol solutions. The Raman spectra in Figure 4 exhibit two dominant bands at 1588 and 1618 cm-1 for both metallic and oxidized Ag. The positions of these bands do not change with increasing BPE surface coverage. The increasing BPE surface coverage with increasing BPE concentration in ethanol solutions is evidenced by the observed increases in the intensities of these Raman bands. Although the intensities of both bands increase, their increases are not proportional. The ratio of the band intensities is different

in Figure 2a (fractional surface coverage θ=1/25 ML) to that of two molecules (θ=2/25 ML). Frequency calculations were performed with a partial Hessian matrix for an adsorbed BPE molecule. All calculated frequency values were scaled by a factor of 0.9987, which was obtained by adjusting the frequencies calculated for an isolated BPE molecule in Materials Studio 4.0 to the positions of experimental Raman bands for a 10-3 M BPE solution in ethanol. 3. RESULTS a. XPS Measurements The XPS measurements were performed for the initial metallic Ag/SiO2 sample and samples obtained after oxidation with ozone for 180-360 min. The Ag3d spectrum for the metallic Ag/SiO2 sample in Figure 3 exhibits two peaks at 368.2 (Ag3d5/2 signal) and 374.2 eV (Ag3d3/2 signal), which correspond to Ag metal.35 After the oxidation with ozone for 180 min, these peaks shifted to lower binding energies by 0.4 eV: to 367.8 and 373.8 eV, respectively. When the oxidation time was

Figure 6. (a) Experimental and (b) calculated Raman spectra for BPE in solution with band assignments. The experimental band at 1592 cm-1 is assigned to the symmetric pyridyl ring breathing vibrational mode with coupled C-C stretching and C-H in-plane bending. The experimental band at 1631 cm-1 is assigned to the stretching of the C=C double bond between the two pyridyl rings.


The Journal of Physical Chemistry Metallic Au Raman intensity, a.u.

for the metallic and oxidized Ag surfaces, and it also changes with increasing BPE concentrations. The ratios of the intensity of the band at 1618 cm-1 to that of the band at 1588 cm1 (I -1 /I -1 ) are

1616 1583

532 nm laser

BPE concentration in ethanol, mg/ml 1.8×10-2 1.8×10-3

1618 cm

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1.8×10-5 background

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1700

1.6

1588 cm

presented in Figure 5 as a function of the BPE solution concentration. At low BPE concentrations, the band at 1588 cm-1 is more prominent, and the I -1 /I -1 band 1618 cm

Figure 7. Surface-enhanced Raman spectra for BPE adsorbed on metallic Au for increasing BPE concentrations in an ethanol solution.

I1616 cm-1/I 1583 cm-1

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intensity ratio is low: 0.7 for the metallic and 0.2 for the oxidized Ag. As the BPE concentration increases, the band at 1618

532 nm laser

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Figure 8. Ratio of the intensity of the Raman band at 1616 cm-1 to that of the band at 1583 cm-1 as a function of the BPE concentration in an ethanol solution for metallic Au.

racm-1 becomes more prominent, and the I /I 1618 cm -1 1588 cm -1 tio increases and then stabilizes at values above unity: ~1.4 for the metallic and ~1.1 for the oxidized Ag. The I -1 /I -1 ratio remains higher for the metallic Ag 1618 cm

1588 cm

than for the oxidized Ag for all BPE concentrations. The Raman spectrum for BPE in solution (without Ag) in Figure 6a exhibits the same two dominant bands at 1592 and 1631 cm-1, with a band intensity ratio of ~1.8. Since the positions of these bands change only slightly after adsorption on Ag surfaces and no new bands are observed, BPE must be adsorbing molecularly, without decomposition. Furthermore, its adsorbed geometry under all evaluated conditions must be similar to that of a BPE molecule in solution because any significant geometry changes would change these bands or shift their positions significantly. The observed changes in the I -1 /I -1 band intensity ratio in Figure 5 as a function 1618 cm

1588 cm

of the adsorbate coverage and Ag oxidation suggest that BPE molecules change their adsorption modes and orientation on the surface. Opposite effects are observed: the I -1 /I -1 1618 cm

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values increase with increasing adsorbate coverage and, in contrast, decrease with Ag oxidation. The Raman spectra for Au/SiO2 as a function of the BPE concentration in Figure 7 are similar to those for the metallic (unoxidized) Ag in Figure 4a. The two dominant bands are slightly shifted to 1583 (1588 for Ag) and 1616 (1618 for Ag) cm-1. The positions of these bands practically do not change with increasing BPE surface coverage. Similarly to Ag, the intensities of these bands increase, but not proportionally. The ratios of the intensity of the band at 1616 cm-1 to that of the band at 1583 cm-1 ( I -1 /I -1 ) for Au in Figure 8 are initially low 1616 cm

1583 cm

at 0.7-0.9 and then increase and stabilize at 1.2-1.3. The similarity of this trend to the changes in the band intensity ratio for the metallic Ag in Figure 5a suggests that changes in BPE adsorption modes caused by increasing coverage are similar on Ag and Au surfaces. c) DFT Calculations The DFT calculations were employed to examine BPE adsorption sites, geometries and energies on three surfaces: (a) Ag(111) (Figure 2a), representing metallic Ag, (b) a single layer of a p(4 × 4)-O with the stoichiometry of Ag2O epitaxed on Ag(111) (Figure 2b), representing partially oxidized Ag, and (c) Ag2O(111) (Figure 2c), representing oxidized Ag as an Ag(I) oxide. The computational results are summarized in Table 1. Multiple experimental and computational studies for pyridine adsorption on Ag examined the adsorption modes in vertical, tilted and horizontal configurations.10-11, 24-28 Therefore, BPE adsorption configurations with the same binding modes were evaluated. On the Ag(111) surface, adsorption sites and configurations were initially examined with a single BPE molecule in the 5×5 unit cell, corresponding to a fractional surface coverage value of 1/25 ML. These calculations show that when BPE adsorbs by binding through one of its N atoms to a surface Ag atom, its adsorption configuration is vertical, with the molecular plane being nearly orthogonal to the surface (Figure 9a). The calculated N-Ag bond distance is 0.25 nm with an adsorption energy of 105 kJ/mol (Table 1). These values are consistent with the numbers of 0.24 nm and 95 kJ/mol reported for pyridine adsorption in the vertical configuration on Ag(110) based on DFT calculations.10-11 After BPE adsorption, the bonding N atom becomes less electronegative: its Hirshfeld partial charge increases from -0.17 to -0.12. The bonding Ag atom, correspondingly, becomes electropositive: its Hirshfeld partial charge increases from 0 to 0.05 (Table 1). When BPE adsorbs through both N atoms, its adsorption configuration is horizontal, with the molecular plane being almost parallel to the surface (Figure 9b). In this horizontal configuration, each N atom binds to a separate Ag atom with a bond distance of 0.33 nm. The N atom partial charges practically do not change after adsorption, remaining at -0.17. The partial charges on the bonding Ag atom increase slightly to 0.02-0.03. The calculated Ag-N bond distance of 0.33 nm is larger than the values of 0.23-0.25 nm reported for pyridine adsorption in the horizontal configuration on Ag(110) based on DFT calculations.10-11, 28 It is, therefore, possible that our calculations underestimate van der Walls interactions. The effect of van der Walls interactions on the adsorption energy for pyridine in a horizontal configuration on Ag(110) was estimated at 28 kJ/mol.28 Our calculated adsorption energy for BPE in the horizontal configuration with two

1588 cm


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When a BPE molecule changes its orientation on the surface from horizontal to vertical, six Ag atoms previously covered by this molecule become available as adsorption sites. The configuration in Figure 10b was generated by changing the orientation of a half of the BPE molecules at 2/25 ML from horizontal to vertical. The average adsorption energy remained practically unchanged at 106 kJ/mol, indicating that the horizontal and vertical configurations remain energetically similar with increasing coverage. When all BPE molecules are in the vertical configuration (Figure 10c), the adsorption energy is slightly lower at 102 kJ/mol. Therefore, the calculations suggest that BPE molecules adsorb initially Figure 9. Most stable BPE adsorption geometries identified with DFT calculations for the vertical configuration (top row) and the horizontal configuration (bottom row) on surfaces shown in both the horizontal and vertical conin Figure 2: (a-b) Ag(111), representing metallic Ag, (c-d) p(4 × 4)-O/Ag(111), representing figurations. Increases in the adsorbate coverage require some initially adsorbed partially oxidized Ag, and (e-f) Ag2O(111), representing oxidized Ag as an Ag(I) oxide. BPE molecules to change their orientation from horizontal to vertical due to gebonds to the surface is 109 kJ/mol. For comparison, the calcuometric constraints. Therefore, the fraction of BPE molecules lated adsorption energies for pyridine in the horizontal configin the vertical configuration must increase with increasing sururation on Ag(110) with a single bond to the surface were reface coverage. 10-11 ported in the range of 68-98 kJ/mol. Results of frequency calculations with Raman intensities for The calculated adsorption energy of 109 kJ/mol for the horian isolated BPE molecule are presented in Figure 6b as a calcuzontal BPE configuration in Figure 9a is similar to 105 kJ/mol lated spectrum. A comparison of this calculated spectrum with for the vertical configuration in Figure 9b. This similarity sugthe experimental Raman spectrum for BPE in an ethanol solugests that the horizontal and vertical configurations are roughly tion in Figure 6a allows to assign the two dominant Raman equally likely to be present on Ag metal surfaces at low adsorbbands. The first experimental Raman band at 1592 cm-1 is due ate coverage under equilibrated adsorption conditions. to the symmetric pyridyl ring breathing vibrational mode with In addition to the vertical and horizontal adsorption configucoupled C-C stretching and C-H in-plane bending. The second rations (Figures 9a and b) with Ag-N-N angles of 173 and 97°, band at 1631 cm-1 is due to the stretching of the C=C double respectively (Table 1), tilted configurations were examined by bond between the two pyridyl rings. These two vibrational optimizing initial trial geometries obtained by placing the BPE modes are illustrated schematically in Figure 6 and with animamolecular plane at intermediate positions. The bonding Ag tion in Movies S1 and S2. atom was used for the tilting angle measurements. Tilted conWe note that there are two separate vibrational modes for figurations with Ag-N-N bond angles of 107, 122, 133 and 153° pyridyl ring breathing: symmetric and asymmetric. Our calcuwere examined. On geometry optimization, the initial configulations show that the symmetric mode has a high Raman intenrations with 122, 133 and 153° angles were approaching the sity (Figure 6b) and a low IR intensity. The asymmetric mode vertical position with a 173° angle. The configuration with a is predicted to be at higher wavenumbers and, in contrast, to 107° angle was approaching the horizontal position with a 97° have a low Raman intensity and a high IR intensity. The domiangle. These results suggest that only the vertical and horizontal nant IR peak at 1595 cm-1 reported for a BPE solution9 can, configurations are stable at low adsorbate coverage. therefore, be assigned not to the symmetric, but to the asymmetEffects of surface coverage were examined by increasing the ric pyridyl ring breathing vibrational mode. fractional coverage from 1/25 to 2/25 ML with two BPE moleResults of frequency calculations for a BPE molecule adcules in the 5×5 Ag(111) unit cell. The adsorption energy in the sorbed on Ag(111) in the vertical and horizontal configurations arrangement with all BPE molecules in the horizontal configuat 1/25 ML (Figures 9a and b) confirm the Raman band assignration in Figure 10a is 107 kJ/mol. This value is only slightly ments. Since the geometry of the BPE molecule does not change lower than the adsorption energy of 109 kJ/mol for the same significantly after adsorption, the calculated frequencies remain configuration at θ=1/25 ML (Table 1). This result indicates that similar to those for an isolated molecule. The symmetric pyridyl the lateral repulsive interactions among BPE molecules at the ring breathing vibrational mode was calculated to be at 1588 increased coverage of 2/25 ML are still minimal. However, the cm-1 for the vertical configuration and at 1573 cm-1 for the horgeometric arrangement in Figure 10a demonstrates that the surizontal configuration. Both of these values are close to the poface cannot accommodate any additional BPE molecules in the sition of the experimental Raman band at 1588 cm-1 in Figure horizontal configuration because of insufficient space between 4. The stretching of the C=C double bond between the two adsorbed molecules. The coverage, therefore, cannot increase pyridyl rings is calculated to be at 1618 and 1622 cm-1, for the from 2/25 to 3/25 ML with all BPE molecules being in the horvertical and horizontal configurations, respectively. Both of izontal configuration due to geometric constraints. Any addithese values are close to the position of the experimental Raman tional BPE molecules can adsorb only in the vertical configuration.



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Figure 10. DFT results for BPE adsorption on Ag(111) at 2/25 ML fractional coverage in different configurations: (a) all horizontal, (b) half horizontal and half vertical, and (c) all vertical. For clarity, the figure shows top views of the surface layer with four neighboring unit cells for each configuration. The calculations demonstrate that due to geometric constraints, BPE molecules must transform and change their orientations from horizontal into vertical in order to accommodate additional adsorbate molecules by making more surface sites available for adsorption.

band at 1618 cm-1 in Figure 4. The similarity between the calculated vibrational frequencies for the vertical and horizontal configurations indicates that the positions of the Raman bands will remain practically the same when BPE changes its orientation on the surface. However, according to the surface selection rules, the relative intensities of the two bands should be dependent on the BPE orientation because of differences in motion components orthogonal to the surface. This dependence was evaluated by calculating Raman band intensity ratios for the vertical and horizontal adsorption configurations using the Ag27 cluster model in Figure S2. The asymmetric pyridyl ring breathing becomes Raman active for the adsorbed BPE molecule, and the calculated ratios include both symmetric and asymmetric modes. For the horizontal configuration, the I -1 /I -1 band inten1618 cm

1588 cm

sity ratio is 0.05 (Figure S3a). For vertical configurations, the ratio increases to 0.14-0.57, depending on the type of the Ag adsorption site. The ratio progressively increases from 0.14 for the vertical configuration on an edge site with 3 surface and 4 total neighboring Ag atoms (Figure S3b) to 0.24 on a partially coordinated surface site with 6 surface and 8 total neighboring Ag atoms (Figure S3c) and then to 0.57 on a surface site with 6 surface and 9 total neighboring Ag atoms (Figure S3d). The calculations, therefore, suggest two trends. First, the I -1 /I -1 band intensity ratio increases when BPE 1618 cm

1588 cm

changes its orientation on an Ag surface from horizontal to vertical. And second, the ratio progressively increases for the vertical configuration with increasing coordination of the bonding Ag atom. These two trends are consistent with reported calculations for BPE adsorption on Au surfaces. DFT calculations for BPE adsorption in the vertical configuration on a single Au adatom/Au(111), Au trimer/Au(111), Au pentamer/Au(111) and on Au(111) surface showed that the ratio of Raman intensities for the similar bands at 1650 and 1593 cm-1 increased with increasing coordination of the bonding Au atom and became the highest for the flat Au(111) surface.37 The same trend was reported for the DFT calculated bands at 1638 and 1581 cm-1 for adsorption sites with different coordination on an Au20 model cluster.38 In addition, the same study reported that the ratio of Raman intensities increased from 0.22-0.47 for adsorption configurations between two Au20 clusters, which are structurally similar to the horizontal configuration in Figures 9b, to 0.59-0.78 for vertical

configurations.38 The computationally predicted increase in the I -1 /I -1 Raman band intensity ratio with the change 1618 cm

1588 cm

in the BPE orientation from horizontal to vertical allows to interpret the experimental I -1 /I -1 ratios for Ag in 1618 cm

1588 cm

Figure 5. The initial increases in the I ratio /I 1618 cm -1 1588 cm -1 with increasing BPE solution concentration indicate increases in the fraction of BPE molecules adsorbed in the vertical configuration with increasing surface coverage. The consistently lower I -1 /I -1 values for the oxidized sample indi1618 cm

1588 cm

cate that the horizontal configuration becomes more preferable after Ag oxidation. The dependence of adsorption configurations on the extent of oxidation was examined with additional calculations using the oxidized Ag surfaces. Unlike the Ag(111) metal surface where all Ag atoms are identical, the p(4 × 4) oxygen overlayer has two types of Ag atoms. The surface is composed of triangular ensembles with six Ag atoms in each ensemble: three atoms are in the vertices and additional three atoms are in the middle of each side of the triangle (Figure 2b). The atoms in the vertices are designated as Ag(2,2), where the first index indicates two neighboring Ag atoms and the second index indicates two neighboring O atoms. The atoms in the middle of each triangle side with four neighboring Ag atoms and two neighboring O atoms are, correspondingly, designated as Ag(4,2). For comparison, atoms on the Ag(111) surface can be described as Ag(6,0) using this nomenclature (Figure 2a). The triangular Ag ensembles on the p(4 × 4)-O surface form larger hexagonal structures, which are illustrated in Figure 11 by simultaneously showing two unit cells of Figure 2b. Each larger hexagonal structure is composed of six triangular Ag ensembles. The evaluated horizontal BPE configurations are indicated in Figure 11 with arrows. The first horizontal configuration on the Ag(2,2)-Ag(4,2) sites is the most stable with an adsorption energy of 123 kJ/mol. This geometry is shown in Figure 9d. The N-Ag bond distances are 0.27 and 0.30 nm (Table 1). The N atoms become slightly less electronegative: their partial charges increase from -0.17 to -0.15. The bonding Ag atoms become slightly more electropositive: the charge on the Ag(2,2) atom increases from 0.17 to 0.19 and the charge on the Ag(4,2) atom increases from 0.12 to 0.16. The BPE molecular plane with N-


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Figure 11. DFT results for BPE adsorption in the horizontal configuration on the p(4 × 4)-O/Ag(111) surface (top view). The black arrows (1 and 2) represent stable geometries described in Table 1. The green arrows (3-6) represent unstable trial geometries.

N-C bond angles of 8 and 11° is slightly more bent than on the Ag(111) surface. The second stable horizontal configuration is also bonded to the Ag(2,2)-Ag(4,2) sites, but it is now across a hexagonal ring opening (Figure 11). This configuration with an adsorption energy of 110 kJ/mol (Table 1, Ring Ag(2,2)-Ag(4,2)) is less stable than the first. The BPE molecular plane is also more bent with N-N-C bond angles of 9 and 14°. All other possible horizontal configurations, which are shown as trial configurations 3-6 in Figure 11, were found to be unstable. The trial configuration number 3 on the Ag(4,2)-Ag(4,2) sites converted into a vertical configuration after geometry optimization. The trial configuration number 4 on the Ag(2,2)-Ag(2,2) sites optimized into the horizontal configuration number 1 on the Ag(2,2)- Ag(4,2) sites. The trial configuration number 5 on the Ag(2,2)-Ag(2,2) sites across the hexagonal ring optimized into a vertical configuration. The final trial configuration number 6 on the Ag(4,2)-Ag(4,2) sites across a hexagonal ring also optimized into a vertical configuration. There are only two vertical configurations. The adsorption on the less electropositive Ag(4,2) site with an initial partial charge of 0.12 is more stable with an adsorption energy of 144 kJ/mol (Table 1). This geometry is shown in Figure 9c. The adsorption on the more electropositive Ag(4,2) site with an initial partial charge of 0.17 is less stable with an adsorption energy of 103 kJ/mol (Table 1). The N-Ag bond distances for the two vertical geometries of 0.24-0.25 nm are similar to 0.25 nm on Ag(111). The N-N-C and Ag-N-N bond angles are also similar to those on Ag(111), indicating that the vertical adsorption geometries remain practically unchanged after partial Ag oxidation. The Ag2O(111) surface has two types of Ag atoms. Atoms of the first type, Ag(0,2), without neighboring Ag atoms and with two neighboring O atoms form hexagonal rings (Figure 2c). Each hexagonal ring structure is composed of six Ag-O atom pairs. Atoms of the second type, Ag(0,1), are located inside these hexagonal rings with a single neighboring O atom. The initial partial charges of both types of Ag atoms are the same at 0.19 (Table 1). However, Ag(0,1) atoms are more unsaturated and, therefore, serve as preferable adsorption sites. There are three

possible horizontal configurations: on the Ag(0,1)-Ag(0,1), Ag(0,1)Ag(0,2), and Ag(0,2)-Ag(0,2) sites. The configuration on the Ag(0,1)Ag(0,1) sites is the most stable with an adsorption energy of 182 kJ/mol. This geometry is shown in Figure 9f. The shorter N-Ag bond distances of 0.22 nm compared to 0.33 nm on Ag(111) are consistent with the higher adsorption energy (182 vs. 109 kJ/mol on Ag(111)). The changes in the N atom partial charges from the initial -0.17 to -0.11 and -0.12 after adsorption also reflect a more stable adsorption mode (Table 1). In order to accommodate the distance of 0.69 nm between two Ag(0,1) sites, which is shorter than the distance of 0.95 nm between the N atoms in BPE, the BPE molecular plane becomes bent in the middle of the C=C double bond (Figure 9f). The calculations indicate that by bending in the horizontal configuration, BPE can accommodate Ag adsorption sites with separation distances between 0.90 and 1.15 nm. The N-N-C bond angles are 33 and 36°, higher than 7 and 7° for the almost flat geometry on Ag(111) in Figure 9b and 8 and 11° for the slightly bent geometry on p(4 × 4)-O/Ag(111) in Figure 9d. The distance between two Ag(0,2) sites on Ag2O(111) is larger, and the extent of bending is lower for the horizontal configuration on these sites with N-N-C bond angles of 12 and 17° (Table 1). This second horizontal configuration is less stable with an adsorption energy of 168 kJ/mol. The third possible horizontal configuration on the Ag(0,2)-Ag(0,2) sites is unstable because it requires bonding to Ag atoms that are below the surface with interfering O atoms at the surface level between them, and a trial horizontal configuration optimized into a vertical configuration. Similarly to the horizontal configurations, the more unsaturated Ag(0,1) atoms are preferable adsorption sites for the vertical configurations. The vertical configuration on the Ag(0,1) site with an adsorption energy of 172 kJ/mol (Table 1) is shown in Figure 9e. The N-Ag bond distance of 0.22 is the same as those for the horizontal configuration on the Ag(0,1)-Ag(0,1) sites and shorter than 0.25 nm for the vertical configuration on Ag(111). The vertical configuration on the Ag(0,2) sites is significantly less stable with an adsorption energy of 82 kJ/mol and a N-Ag bond distance of 0.27 nm. Despite the variations in the adsorption energies and N-Ag bond distances, all vertical configurations on all three examined surfaces, Ag(111), p(4 × 4)-O/Ag(111), and Ag2O(111), maintain essentially the same orientation, with the BPE molecular plane being nearly orthogonal to the surface and N-N-C (degree of bending) and Ag-N-N (degree of tilting) bond angles of 5-6° and 171-178°, respectively (Table 1). 4. DISCUSSION The structure of the BPE molecule and its adsorption on Ag surfaces were previously studied with IR and Raman spectroscopic measurements and theoretical calculations.7-9, 39 These studies analyzed the BPE vibrational modes for an isolated molecule and also after its adsorption on Ag. In addition, the dependence of Raman spectra on the laser excitation wavelength was examined as well as the mechanism of the surface enhancement in Raman measurements. These previous studies and multiple other studies that utilized BPE adsorption assumed a single adsorption configuration. Such an assumption is inconsistent with our results that demonstrate the changes in the Raman spectra with increasing BPE concentration in Figure 4. If BPE had only one adsorption configuration, all Raman bands would increase proportionately with increasing BPE concentration in solution and corresponding increases in adsorbate surface coverage. In contrast, the relative intensities of the characteristic



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Raman bands at 1588 and 1618 cm-1 change (Figure 5). The DFT calculations show that BPE can adsorb on Ag surfaces in two configurations: vertical and horizontal (Figure 9). Tilted intermediate configurations were found to be unstable at low coverage. However, tilted configurations at high coverage with predominantly vertical configurations and significant repulsive lateral interactions were not examined computationally and, therefore, cannot be ruled out completely. The frequency calculations for the vertical and horizontal adsorption configurations demonstrate that the relative intensities of the characteristic vibrational modes with the stretching of the C=C double bond (band at 1618 cm-1, Movie S2) and with the pyridyl ring breathing (band at 1592 cm-1, Movie S1) depend on the BPE orientation. The relative intensity of the band at 1618 cm-1 is enhanced and, correspondingly, the I -1 /I -1 1618 cm

1588 cm

Raman band intensity ratio becomes larger when BPE adsorbs vertically (Figure S3). Thus, the increases in the I -1 /I -1 values with increasing BPE concentration 1618 cm

1588 cm

in Figure 5 can be assigned to progressively greater fractions of adsorbed BPE molecules being in the vertical configuration. Since the calculated adsorption energies for the vertical and horizontal configurations on Ag(111) are similar at 105 and 109 kJ/mol, respectively (Table 1), both configurations are expected to be present at low surface coverage. As the coverage increases, some BPE molecules in the horizontal configuration must convert to the vertical configuration in order to accommodate additional adsorbate molecules by making more surface sites available for adsorption (Figure 10). This orientational phase transition from a mixture of vertical and horizontal configurations to the predominantly vertical configuration, which allows to increase the surface coverage, is reflected in the sharp increase in the I -1 /I -1 Raman band intensity ratio 1618 cm

1588 cm

with increasing BPE concentration for the metallic Ag in Figure 5a. After the transition, the I -1 /I -1 values stabilize 1618 cm

1588 cm

and, therefore, the fractions of BPE molecules in the vertical and horizontal configurations remain approximately constant. Since the same trend is observed for the oxidized Ag in Figure 5b, the initial horizontal configurations on the oxidized Ag must be also converting into vertical configurations with increasing surface coverage with subsequent stabilization of the fraction of BPE molecules in the vertical configuration. The I -1 /I -1 values for the oxidized sample, however, 1618 cm

1588 cm

are consistently lower, indicating a greater stability of horizontal configurations on the oxidized Ag. This effect can be interpreted by comparing the stability of the BPE adsorption configurations on the model surfaces with the increasing extent of Ag oxidation. A comparison of adsorption energies as a function of the extent of Ag oxidation in Table 1 indicates two trends. The first trend is an increase in the adsorption energy with increasing extent of Ag oxidation for the most stable vertical and horizontal configurations. The adsorption energy for the most stable vertical configuration increases from 105 kJ/mol on Ag(111) to 144 kJ/mol on p(4 × 4)-O/Ag(111) and then increases further to 172 kJ/mol on Ag2O(111). The adsorption energy for the most stable horizontal configuration similarly increases from 109 kJ/mol on Ag(111) to 123 kJ/mol on p(4 × 4)-O/Ag(111) and then increases further to 182 kJ/mol on Ag2O(111). These in-

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creases can be attributed to the presence of O atoms that generate surface structures with more coordinatively unsaturated Ag atoms compared to those on Ag(111). In addition, partial charges on surface Ag atoms increase with the extent of Ag oxidation: from zero on Ag(111) to 0.12-0.17 on p(4 × 4)O/Ag(111), and further to 0.19 on Ag2O(111) (Table 1). More electropositive Ag atoms generally form stronger bonds with BPE. Changes in charge distributions with the extent of Ag oxidation are illustrated in Figure S4 by mapping the electrostatic potential for the adsorbed configurations in Figure 9. The electrostatic potential for a BPE molecule before adsorption is provided in Figure S5 for comparison. The second trend is the relative stability of the horizontal versus the vertical configurations. As Ag surface sites adsorb BPE more strongly with the increasing extent of Ag oxidation, the horizontal configurations, which have two surface bonds compared to one for the vertical configurations, become progressively energetically preferential. The adsorption energies for the vertical and horizontal configurations are similar on Ag(111). On p(4 × 4)-O/Ag(111), the vertical configuration on the Ag(4,2) sites is the most stable with an adsorption energy of 144 kJ/mol. However, the two horizontal configurations on the Ag(2,2)Ag(4,2) sites with adsorption energies of 123 and 110 kJ/mol are more stable than the vertical configuration on the Ag(2,2) sites with an adsorption energy of 103 kJ/mol. Finally on Ag2O(111), the horizontal configuration on the Ag(0,1)- Ag(0,1) sites becomes the most stable with an adsorption energy of 182 kJ/mol. The second horizontal configuration on the Ag(0,1)- Ag(0,2) sites with an adsorption energy of 168 kJ/mol is comparable in stability to the most stable vertical configuration on the Ag(0,1) sites, and both of these configurations are significantly more stable than the second vertical configuration on the Ag(0,2) sites with an adsorption energy of 82 kJ/mol. In summary, the calculations demonstrate that generally with increasing extent of Ag oxidation, BPE adsorbs more strongly and, importantly, the horizontal configurations become energetically preferable. Our identification of two distinct configurations for BPE adsorption on Ag surfaces is consistent with results for pyridine adsorption, which has been extensively studied because it is a common method for characterization of surfaces, especially catalytic surfaces. For example, a study with high-resolution electron energy loss (EELS), UV-photoemission (UPS) and thermal desorption spectroscopic (TDS) measurements for Ag(111) at 140 K concluded that pyridine initially adsorbs in the horizontal configuration.24 Then at the coverage of ~3×1014 molecules/cm2, the horizontal pyridine configuration transforms into a tilted configuration with a ~55° bond angle to the surface. After that, the surface coverage with the tilted configuration increases to the saturation value of ~5×1014 molecules/cm2. This analysis was performed based on differences in spectroscopic results for regular and deuterated pyridine molecules and coverage-dependent changes in relative intensities of out-of-plane and in-plane C-H deformation vibrational modes.24 Another study for pyridine on Ag(111) at 100 K reached a similar conclusion for an orientational phase transition with a transformation of an initial configuration with a 45 ± 5° tilt angle to a final configuration with a 70 ± 5° tilt angle (nearly vertical), based on near-edge X-ray-absorption fine-structure (NEXAFS) measurements.25 However, a more recent study with scanning tunneling microscopic (STM) measurements for Ag(110) at 13 K in combination with DFT calculations concluded that only the vertical and horizontal pyridine configurations are present


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at low coverage values below 0.01 ML, with the horizontal configuration being preferable.10 Another recent DFT study examined adsorption geometries and confirmed details of the vertical and horizontal pyridine configurations on Ag(110).28 Earlier reports of a tilted pyridine configuration were interpreted as an averaging effect when both vertical and horizontal configurations are present.10 Therefore, the earlier spectroscopic results mentioned above24-25 can also be interpreted as a transformation of a mixture of vertical and horizontal configurations into a predominantly vertical configuration with increasing surface coverage. The latest STM study observed this transformation directly by counting and analyzing images of more than 2000 pyridine molecules in a 25×25 nm2 area on Ag(110) at 13 K.11 Initially at the coverage of 0.04 pyridine molecules per nm2, pyridine adsorbs predominantly in the horizontal configuration, with less than 10% of all molecules being in the vertical configuration. As the coverage increases to ~0.08 pyridine molecules per nm2, the fraction of the vertical configuration gradually increases to ~15%. At the coverage of ~0.1 pyridine molecules per nm2, an orientational phase transition is observed with a sharp decrease in the number of molecules in the horizontal configuration and a corresponding sharp increase in the number of molecules in the vertical configuration. After the phase transition, the fraction of the vertical orientation increases only slightly, stabilizing at ~60%.11 Therefore, our results, which indicate that BPE adsorbs initially as a mixture of the vertical and horizontal configurations with a subsequent rapid transformation and final stabilization with the dominance of the vertical configuration, are in agreement with the pyridine adsorption studies. An orientational transformation with increasing surface coverage is not limited to N-containing hydrocarbons. It has been reported for other functional groups. For example, a study of acrolein adsorption on Ag(111) showed that this molecule changes its orientation from the horizontal at low coverage to a configuration with a strong tilt of the C=C double bond part of the molecule at high coverage, based on high-resolution synchrotron XPS, near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and temperature-programmed reaction (TPR) measurements.40 Such a coverage-dependent orientational transformation allows to control the selectivity of catalytic reactions. At low acrolein surface coverage, when the molecule adsorbs in the horizontal configuration, the C=C double is easily hydrogenated by added H2, which adsorbs dissociatively and forms H atoms on the Ag surface. In contrast, at high acrolein coverage, when the molecule transforms into the tilted configuration, the C=C double bond becomes immune to an attack by surface H atoms. In this case, only the aldehyde fragment is hydrogenated, and the corresponding unsaturated alcohol desorbs.40 The similarity in the Raman band intensity ratios at different BPE concentrations for metallic Ag and Au in Figures 5a and 8 suggests that the BPE adsorption trends on both metals are the same. It is, therefore, likely that on Au, similarly to Ag, BPE adsorbs initially as a mixture of horizontal and vertical configurations. And then, at higher coverage values, the vertical configuration becomes dominant due to geometric constraints. However, adsorption on Au, in contrast to Ag, is usually dominated by step, edge and other highly coordinatively unsaturated sites rather than by sites on flat surfaces. For example, our computational study for various adsorbates involved in H2O formation from H2 and O2 on Au surfaces showed that most adsorption geometries and energies are highly dependent on the

coordination of Au sites.41 Some species adsorb very weakly or do not adsorb at all on a flat Au(111) surface, while they readily adsorb on more coordinatively unsaturated edge and step sites.41 Therefore, further studies for BPE adsorption on Au are needed in order to examine preferential adsorption configurations and sites. Furthermore, in addition to relative thermodynamic stability of different adsorption configurations, the adsorption kinetics are likely to be important at low BPE concentrations. For example, a reverse trend for the I -1 /I -1 Raman 1616 cm

1583 cm

band intensity ratios compared to that in Figure 8 was observed for 30 nm Au particles supported on SiGe when spectra were collected by increasing the exposure time from 1 to 48 h at a constant 50 μM BPE concentration in methanol.37 Such a decrease in the Raman band intensity ratios with increasing exposure time is possibly due to a kinetic effect. If BPE adsorbs initially in a precursor vertical configuration prior to gradually transforming into a potentially more thermodynamically stable horizontal configuration, the surface can be expected to be dominated by BPE molecules in the vertical configuration at small exposure times prior to equilibration. With longer exposures, the fraction of BPE molecules in the horizontal configuration would increase and, finally, stabilize at an equilibrium value. Such a kinetic transformation can account for an initial decrease and subsequent stabilization of the I -1 /I -1 Raman 1616 cm

1583 cm

band intensity ratios with increasing exposure times. The identification of the vertical and horizontal configurations for BPE adsorption on Ag will allow to better interpret previously published spectroscopic studies for Ag, Au and other metals and will be helpful for future studies in the development of ultra-high sensitive spectroscopic techniques. In addition, the identified dependences for BPE adsorption on the surface coverage and the extent of Ag oxidation will be helpful in catalytic studies for adsorption of hydrocarbons on metal catalysts in oxidation and other reactions. 5. CONCLUSIONS 1. The Raman spectra for trans-1,2-bis(4-pyridyl)ethylene (BPE) adsorbed on monodispersed 48-nm Ag nanoparticles supported on SiO2 in Figure 4 exhibit two characteristic bands at 1588 and 1618 cm-1. The relative intensities of these two bands summarized in Figure 5 are not constant. Instead, they depend on the concentration of the BPE solution and on the extent of Ag oxidation. 2. The DFT calculations show that BPE can adsorb on Ag surfaces in two configurations: vertical and horizontal (Figure 9). In the vertical configuration, BPE adsorbs nearly orthogonal to the surface by binding through one of its N atoms to a single surface Ag atom. In the horizontal configuration, BPE adsorbs nearly parallel to the surface by binding through both N atoms to two separate surface Ag atoms. Tilted intermediate configurations were found to be unstable at low surface coverage. 3. The results of frequency calculations clarify the assignments for the two characteristic Raman bands (Figures 4 and 6). The first experimental Raman band at 1588 cm-1 (1592 cm-1 in solution) is due to the symmetric pyridyl ring breathing vibrational mode with coupled C-C stretching and C-H in-plane bending (Movie S1). The second band at 1618 cm-1 (1631 cm-1 in solution) is due to the stretching of the C=C double bond between the two pyridyl rings (Movie S2). The relative intensity of the band at 1618 cm-1 is enhanced when BPE adsorbs in the



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vertical configuration (Figure S3). 4. The computational results provide an explanation for the observed changes in the Raman band intensity ratios in Figure 5. BPE adsorbs initially as a mixture of vertical and horizontal configurations. With increasing surface coverage, a transition occurs when some BPE molecules in the horizontal configuration transform into the vertical configuration in order to accommodate additional adsorbate molecules by making more surface sites available for adsorption (Figure 10). After the orientational phase transition, the fractions of BPE molecules in the vertical and horizontal configurations do not change significantly with further increases in the surface coverage. The similarities in spectroscopic results for Ag in Figure 5a and Au in Figure 8 suggest that the BPE adsorption trends are the same for both metals. 5. The computational results in Table 1 indicate that there are two general trends for the oxidized Ag surfaces. The first trend is an increase in the adsorption energy with increasing extent of Ag oxidation due to the presence of more coordinatively unsaturated Ag atoms compared to those on metallic Ag and to progressively higher partial charges on surface Ag atoms (Figure S4). The second trend is an increase in the relative stability of the horizontal configurations compared to the vertical configurations, which explains the consistently lower Raman band intensity ratios for the oxidized Ag compared to those for the metallic Ag in Figure 5. Therefore, the horizontal orientation is preferable at low coverage and at higher extents of Ag oxidation and, conversely, the vertical orientation is preferable at high coverage and on metallic Ag.

ASSOCIATED CONTENT Supporting Information. Scanning electron microscope images of monodispersed 48-nm Au nanoparticles supported on SiO2 are provided in Figure S1. The geometry of the Ag27 cluster used in the Raman band intensity calculations is provided in Figure S2. Details of BPE adsorption configurations on the Ag27 cluster and their calculated

I1618 cm-1 /I1588 cm-1 Raman band intensity ratios are pro-

vided in Figure S3. Mapping of the electrostatic potential for the BPE adsorption geometries in Figure 9 is provided in Figure S4. Mapping of the electrostatic potential for an isolated BPE molecule is provided in Figure S5. Visualization of the BPE symmetric pyridyl ring breathing vibrational mode with coupled C-C stretching and C-H in-plane bending for the vertical configuration on Ag(111) (experimental Raman band at 1588 cm-1) is provided in Movie S1. Visualization of the BPE vibrational mode with the stretching of the C=C double bond between the two pyridyl rings for the vertical configuration on Ag(111) (experimental Raman band at 1618 cm-1) is provided in Movie S2. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (S.S.); [email protected] (H.D.); [email protected] (S.G.P.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The computational work in Prof. Simon G. Podkolzin’s group was partially supported by the National Science Foundation under grant

Page 12 of 14

CBET-1264453. Calculations were performed with Materials Studio software under a collaborative research license with BIOVIA Corp. in San Diego, California, USA. The experimental work in Prof. Svetlana Sukhishvili's and Prof. Henry Du’s groups was supported by the Defense Advanced Research Projects Agency under grant N66001-09-1-2076.

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Table 1. DFT calculated configurations for BPE adsorption on Ag surfaces.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Surface BPE Orientation

Ag(111) Horizontal

p(4 × 4)-O/Ag(111)

Vertical

Horizontal

Ag2O(111)

Vertical

Horizontal

Vertical

Ring Adsorption site Adsorption energy, kJ/mol

Ag(6,0)-Ag(6,0)

Ag(6,0)

Ag(2,2)-Ag(4,2)

109

105

123

110

103

144

182

168

172

82

N-Ag bond distance, nm Degree of bending, N-N-Ca bond angle, deg Degree of tilting, Ag-N-N bond angle, deg

0.33, 0.33

0.25

0.30, 0.27

0.26, 0.32

0.25

0.24

0.22, 0.22

0.22, 0.32

0.22

0.27

7, 7

6

8, 11

14, 9

5

6

36, 33

17, 12

6

6

97, 97

173

109, 111

101, 91

178

174

90, 88

112, 106

172

171

after adsorption Bonding Ag before adsorption

-0.17, -0.17

-0.12

-0.15, -0.15

-0.15, -0.15

-0.11

-0.12

-0.11, -0.12

-0.12, -0.12

-0.09

-0.13

0, 0

0

0.17, 0.12

0.17, 0.12

0.17

0.12

0.19, 0.19

0.19, 0.19

0.19

0.19

after adsorption

0.03, 0.02

0.05

0.19, 0.16

0.19, 0.16

0.19

0.14

0.23, 0.23

0.23, 0.23

0.20

0.21

Ag(2,2)-Ag(4,2)

Ag(2,2)

Ag(4,2)

Ag(0,1)-Ag(0,1)

Ag(0,1)-Ag(0,2)

Ag(0,1)

Ag(0,2)

Hirshfeld partial charges

Bonding N before adsorption

-0.17

a. Carbon atom opposite of the nitrogen atom in the same pyridyl ring.


Identification of Vertical and Horizontal ... - ACS Publications

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