Polarized IR MEIRAS Study of Surface Orientations of CO Molecules

Mar 31, 2014 - Detailed polarization and incidence angle experiments were performed later on selected samples using a variable angle accessory (Seagul...
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Polarized IR MEIRAS Study of Surface Orientations of CO Molecules Adsorbed on Pt Nanowires Catalysts P. Deshlahra† and E. E. Wolf* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Multilayer enhanced infrared reflection spectroscopy (MEIRAS) experiments were performed on parallel Pt nanowires nanofabricated on TiO2 and SiO2 substrates, while being exposed to 1% CO at atmospheric pressure and 75 °C. We studied the effect of nanowire widths and IR polarization and incidence angle on peak wavenumbers and intensities of absorption bands of CO molecules adsorbed on Pt surfaces. The samples consist of 4 mm × 4 mm arrays of rough polycrystalline parallel Pt nanowires placed 200 nm apart, 15−90 nm wide, and approximately 8 nm high/thick. The nanowires have somewhat straight side-faces because of the directional nature of the Pt deposition process used. We also studied 5 nm thick polycrystalline Pt film samples. Spectra from unpolarized IR at small incidence angles (20° from surface normal) showed a decrease in linearly adsorbed CO peak wavenumbers with decreasing nanowire width, from about 2090 cm −1 on Pt film to less than 2055 cm −1 on thin nanowires, indicating differences in Pt−CO interactions of the CO populations being sensed on different samples. MEIRAS results at small incidence angles allows selective sampling of vibrational modes tangential to the multilayer structure, which are more abundant on the side faces of the nanowires than on their top surfaces. This expectation is confirmed by polarization dependent experiments, which show high intensity when IR polarization is perpendicular to the sides of nanowires, thus aligned with CO adsorbed on the sides, and little intensity when polarization is parallel to the sides of the wires. Lower wavenumber on thinner wires is consistent with increased fraction (in the sampled population) of side CO molecules that are closer to the metal−support interface and thus potentially affected by metal−support charge transfer effects well-known for Pt/TiO2 interfaces; these and other possible contributing effects such as the presence of impurities and lowcoordination sites are discussed. Sensitivity to top CO relative to CO adsorbed on the edges and sides of nanowires is improved by increasing the incidence angle with p-polarization, and these top-CO vibrations appear at slightly higher wavenumbers that the side-CO on the same sample. These results demonstrate that polarized MEIRAS can be used to detect effects of adsorbate molecular orientation on metal catalysts on flat supports and selectively sense molecules with tangential dipole projections as opposed to surface normal only sensitivity in IRAS on metal surfaces or metal particles on ultrathin oxides. methods,18−20 but these concepts have not been used to probe adsorbate orientations on supported catalyst particles. Ability to selectively sense surface-normal as well as tangential molecular orientations on supported catalysts can allow distinction between adsorbates on top of a catalyst particle and those on the sides that are in closer proximity to its flat support, thus potentially helping in improving our understanding of metal−support interactions. We previously reported a new multilayer enhanced infrared reflection adsorption spectroscopy (MEIRAS) technique21 that improved IR absorption signal from adsorbates on low area supported catalysts through optical interference effects on metal-dielectric-metal structures. These structures consist of a thin, semitransparent top metal layer such as Pt catalyst particles or film, a dielectric layer such as SiO2 or TiO2 oxide of

1. INTRODUCTION Infrared (IR) spectroscopy is widely used to probe the vibrational signature of molecules, in bulk-phases or as adsorbates on surfaces. Typical IR measurements on solid catalysts during gas-phase reactions are performed, either by transmission through a wafer or by diffuse reflection from a flatbed of catalyst particles dispersed on a support.1,2 Both techniques sample a volume average of molecular orientations on randomly dispersed particles rather than selectively sampling specific orientations. Specular reflection techniques selectively sense molecules with dynamic dipole projections normal to the surface of single-crystals or particles on ultrathin oxide films, under high-vacuum3 or closer to reaction conditions,4 but they are virtually insensitive to tangential projections.5,6 Several studies have probed different orientations of uniform molecular monolayers in Langmuir−Blodgett films7−9 and on singlecrystal catalytic solids10−13 using polarized IR spectroscopy in transmission,11,12 reflection8−10,14−16 and attenuated totalinternal reflection modes,7,13,17 and other spectroscopic © 2014 American Chemical Society

Received: December 1, 2013 Revised: March 25, 2014 Published: March 31, 2014 8369

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Figure 1. Schematics showing (a) cross-sectional view of a Pt nanowire multilayer catalyst. (b) Optical setup to perform in situ polarization and incidence angle dependent MEIRAS experiments.

spectra with unpolarized IR indicate a gradual red-shift in CO band position with decreasing width of the nanowires which are interpreted to be caused, at least in part, by charge transfer from the support to Pt−CO and is further explored here through polarization and incidence angle dependence. Different optical configurations that can be used to probe orientations of CO molecules using polarized IR are described. The results indicate high selectivity to CO on the side faces of nanowires that are oriented tangential to multilayer surface, and demonstrate ability to improve relative sensitivity to normal CO on top of nanowire by increasing the incidence angle. Such orientation dependent probes can be useful in studying electronic effects involved in the recently demonstrated changes in adsorbate binding and reactivity through external voltage biasing of metal−support junctions.25−27

thickness close to a quarter of the wavelength of the IR absorption band, and a bottom reflecting metal layer; we explained the mechanism of signal enhancement in these structures using simulations22 based on Fresnel’s equations23 (see the Supporting Information, section S.1). IR radiation incident on a reflecting (bottom) layer forms a standing wave with its surface normal component (pz, Figure 1a) at maximum amplitude (i.e., antinode) but the tangential components (py and sx) at a minimum (i.e., node). In MEIRAS, the added dielectric layer creates a gap with the reflecting layer, such that the ambient-dielectric interface is now at or near a node for pz component but at an antinode for tangential (py and sx) components. The top Pt layer provides sites for adsorbates to bind at this ambient-dielectric interface and also acts as a radiation absorber, decreasing the reflectance of sample to close to zero, without significantly decreasing the surface intensity. Increased surface amplitude (mainly py and sx) and decreased reflectance cause a net signal enhancement of 2−3 orders of magnitude compared to the signal on a single reflecting metal layer under same condition.22 Therefore, MEIRAS allows one to measure IR signals from small area catalysts that otherwise will not be measurable and to tune tangential versus normal sensitivity by changing dielectric layer thickness and/or incidence angels and polarization, limited, however, by the inherent trade-off between the two components. Our previous MEIRAS measurements using unpolarized IR showed shifts in peak position for CO adsorbed on Pt(film)/ SiO2/Au multilayer samples after repeated oxidation−reduction pretreatment due to cracks in the Pt film and exposed sides of the Pt layer to CO. Similar preliminary results showed peak shifts on Pt(nanowire)/TiO2/Au catalysts with change in width of nanowires, suggesting that these factors affect Pt−CO interactions.24 Furthermore, we also studied the effect of metal−support charge transfer controlled by an external bias voltage applied to a catalytic diode with a rough Pt film on TiO2, using unpolarized as well as polarization and incidence angle dependent MEIRAS.25 We showed that the effect of charge transfer on binding of CO is stronger at the sides of cracks in Pt film that are closer to Pt/TiO2 interfaces. Thus, theoretical and experimental analyses performed using MEIRAS suggested distinct properties of CO adsorbed near metal−support interfaces of supported multilayer catalysts, which can be selectively studied by MEIRAS. In this work we report the use of polarized IR MEIRAS on multilayer catalysts with uniform parallel Pt nanowires of varying widths to probe orientations of adsorbed CO. MEIRAS

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Pt nanowire and thin-film catalysts were prepared with a multilayer structure21 (Figure 1a) of different dielectric layer thickness and pattern of Pt layer, as shown in Table 1. A 50 nm or thicker Au film was deposited Table 1. Pt Nanowire and Thin-Film Multilayer Samples Used for Polarization and Incidence Angle Dependent MEIRAS sample 1 2 3 4 5

= 25Pt/250Ti = 60Pt/250Ti = 90Pt/250Ti = Pt/250Ti = 15Pt/4Si/ 550Ti 6 = 40Pt/4Si/ 550Ti 7 = Pt/4Si/550Ti

width of nanowires (nm) 25 60 90 Pt film 15 40 Pt film

thicknesses of layers in substrate (nm) 250 TiO2/ 60 (reflecting) 250 TiO2/ 60 (reflecting) 250 TiO2/ 60 (reflecting) 250 TiO2/ 60 (reflecting) 4 SiO2/ 550 TiO2/60 (reflecting) Au 4 SiO2/ 550 TiO2/60 (reflecting) Au 4 SiO2/ 550 TiO2/60 (reflecting) Au

Au Au Au Au

on a Si(100) wafer by electron beam evaporation to make the bottom reflecting layer. The TiO2 dielectric layer on top of Au was prepared by depositing Ti metal using electron beam evaporation, followed by thermal oxidation in dry air. For samples 1 through 4 (in Table 1), a TiO2 layer approximately 250 nm thick was prepared by depositing a 150 nm of Ti and oxidizing it at 475 °C for 8 h. Samples 5 through 7 consisted of a thicker, 550 nm TiO2 layer prepared by depositing 250 nm of Ti and oxidizing at 525 °C for 24 h. X-ray diffraction (XRD) 8370

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pattern of the 550 nm TiO2 films showed a rutile structure consistent with similar preparations in the literature.28,29 Electrical characterization indicates a semiconducting TiO2 with a small number of bulk vacancies in TiO2. The top Pt layer deposited on TiO2 consisted of either a 5 nm thick Pt thin-film or arrays of parallel Pt nanowires as noted in Table 1. The nanowires, prepared using electron beam lithography, were 15−90 nm wide with 200 nm pitch and 8 nm thickness (Figure 2). Pt metal was deposited using electron beam evaporation, in

behavior of the Schottky junctions and slightly modified the junction barrier height.22 Sample 7, when used in the presence of applied voltage (−2 to +2 V) showed small reversible shifts over a rage of about 10 cm −1 in the peak position of adsorbed CO as shown in our previous work.25 The nanowire samples, despite being prepared as catalytic diodes, did not show a diode-like electrical behavior presumably due to nanoscale electrical discontinuities in the wires, but permitted to study the effect of polarization and incidence angle dependent during CO adsorption experiments in the absence of the bias voltage used in the catalytic diode experiments. The wires appeared continuous at micrometer scale (comparable to IR wavelengths) when observed under SEM, suggesting that the discontinuities are not large enough to adversely affect the optical measurements performed in the present work. 2.2. IR Measurements. MEIRAS experiments were performed using a Bruker Equinox 55 FTIR spectrometer equipped with deuterated-triglycine-sulfate (DTGS) and mercury−cadmium-telluride (MCT) detectors, an optical assembly to reflect the IR beam from the sample surface and vary its angle of incidence, and a ZnSe wire grid polarizer (New Era Enterprises; Figure 1b). Initial experiments on some samples were performed with and without an infrared polarizer using a reflection accessory designed for DRIFTS measurements (Praying Mantis, Harrick Scientific). In these experiments an incidence angle of 20−30° was used. Detailed polarization and incidence angle experiments were performed later on selected samples using a variable angle accessory (Seagull, Harrick Scientific). CO chemisorption on the Pt surface of the multilayer structure was studied in an in situ stainless steel diffuse reflectance infrared (DRIFTS) reaction cell (HVC-DRP4, Harrick Scientific) modified by replacing the top dome with a flat CaF2 window that minimized the IR signal from the gas phase CO as previously shown elsewhere21 and in the Supporting Information. Flow rates of gases and the cell temperature were controlled by electronic mass flow and temperature controllers, respectively. After placing the sample in the reaction cell, a background IR measurement was taken at 75 °C with helium flowing in the cell at atmospheric pressure. The samples were heated to 175 °C in helium, treated in 10% O2 balance helium for 60 min, reduced in 5% CO balanced with helium for 0.5 h, and then cooled to 75 °C. This pretreatment procedure was chosen to first oxidize the surface and burnoff any carbonaceous impurities, and then to reduce the surface, leaving a reduced Pt surface saturated with CO. Treatment in 1% CO at temperatures above 150 °C has been shown to reduce Pt surface.31 Similar oxidation−reduction (using NO2 and CO) cycles have been shown to be effective to clean and reduce the surface even under high vacuum conditions.32 IR measurements were taken at 75 °C with 1% CO balance helium flowing in the reaction chamber at atmospheric pressure, and in some cases immediately after flushing out CO with helium. These conditions are known to keep Pt surface saturated with CO. High CO coverage conditions have been shown to cause significant surface reconstruction,33 which diminishes the effect of low coordinated edge and corner sites of a Pt crystallite on the adsorbates.34 Reflectance of a multilayer sample in the absence of any adsorbed CO (R0) is defined as the ratio of the intensity measured (by the detector) when the IR beam is reflected from it to the intensity measured when reflected from a 100 nm thick reflecting Au film used as a reference:

Figure 2. SEM images of parallel Pt nanowires of different widths prepared by electron beam lithography: (a) 15 nm (sample 5), (b) 25 nm (sample 1), (c) 40 nm (sample 6), and (d) 60 nm (sample 2). The wires are separated by a center-to-center distance of 200 nm.

which the Pt atoms or fragments travel in straight line from the source to the substrate because of absence of scattering under high vacuum conditions inside the evaporation chamber. This directional property leads to formation of nanowires with side faces that are straighter than conformal deposition techniques such as sputtering30 (as opposed to pseudohemispherical, tapered, or with irregular shapes). Additional details of the preparation of the TiO2 layer, its XRD and electrical characterization, and electron beam lithography are presented in the Supporting Information. Samples listed in Table 1, except sample 1, had an additional top Au electrode with a very coarse grid consisting of three thin Au lines (compared to 20 000 parallel nanowires over the entire active area) parallel to nanowires and three across, connected to Au contacting pads on the sides as they were prepared as catalytic diode devices described in previous work.25 No external voltage was applied to any of the experiments reported in this work, however. The gold electrode in the absence of an external voltage does not affect the active area of the device that is probed by IR, and hence, for the present work, it can be considered as just the metal−TiO2−Au structure as shown in Figure 1a. Samples 5 through 7 contained an additional thin 4 nm SiO2 layer sandwiched between Pt and TiO2, deposited using plasma enhanced chemical vapor deposition (PECVD). The ultrathin SiO2 film, optically too thin of a dielectric to affect the optical properties of multilayer, was introduced to smooth out the contact between the wires and the discontinuities and defects on the underlaying TiO2 layer. It also constituted a tunnel barrier that improved the rectifying 8371

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I0 I0 , Au_ref

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minimum at a wavenumber beyond the mid-infrared region of 400−4000 cm−1. Sample 6 [40Pt/Si/550Ti], on the other hand, shows a minimum in reflectance near the infrared absorption band of linear CO at 2100 cm−1. This reflectance minimum indicates optimum MEIRAS enhancement conditions achieved by a dielectric (TiO2 in this case) thickness close to the quarter-wave matching condition.21 Thus, sample 1 will exhibit some MEIRAS enhancement due to favorable optical interference, but much less than the sample 6 which has optimum interference conditions. Apart from absolute signal enhancement, the dielectric thickness also affects the relative contribution of normal and tangential dipoles to the signal. For example, sensitivity to tangential component increases relative to the surface normal with increase in dielectric thickness and it reaches a maximum at quarter-wave thickness. Pt layer thickness as well as IR incidence and polarization angles also affect the interference conditions and the resulting absolute signal and relative sensitivities. Quantitative effects of these factors are described in a previous work describing theoretical analyses of multilayer structure22 and in similar analyses in the literature.6,8,16,35 Direct comparisons between peak intensities on different samples cannot be made without taking these factors into account. Nonetheless, the peak positions can provide useful chemical information reflected in the vibrational frequency of the adsorbates being sampled. Changes in sample properties such as particle size (or nanowire width) and measurement conditions such as polarization and incidence angle can provide useful qualitative information about the effects of changing sampling populations. These effects are examined next. 3.2. Effect of Nanowire Width on CO Vibrational Frequency Using Unpolarized IR. The initial oxidation and reduction pretreatment of each sample was followed by background reflectance measurement in He, introduction of CO in the reaction cell, and measurement of spectra. Figure 4a shows the linear CO absorption bands on samples 1 [25Pt/ 250Ti], 4 [Pt(film)/250Ti], and 7 [Pt(film)/4Si/550Ti]. The thin film samples (1 and 4) show a peak near 2090 cm −1 while the thin (25 nm) nanowire is shifted to lower wavenumbers by about 40 cm −1. IR bands corresponding to CO adsorption of TiO236 were not detectable. Figure 4b shows the peak positions

(1)

The infrared absorption signal from the CO molecules adsorbed on a sample is expressed in commonly used reflectance unit ΔR/R0, defined as a normalized change in the reflectance of the sample due to infrared absorption by CO: R − R0 ΔR = R0 R0

(2)

where R0 and R are the reflectance of the bare sample and the reflectance after adsorption of CO, respectively.

3. RESULTS AND DISCUSSION 3.1. Reflectance of Multilayer Structure. Figure 3 shows the reflectance of samples 1 and 6 (eq 1) and of a reference Au

Figure 3. Wavelength dependent reflectance of multilayer nanowire samples at 20° incidence angle.

film as a function of wavenumber. Unlike the reference film, which has reflectance close to unity for all mid-infrared wavelengths, the reflectance of multilayer structures varies with wavelength. The reflectance of sample 1 [25Pt/250Ti], which has a thin dielectric (250 nm TiO2), decreases with increasing infrared wavenumber (=1/wavelength, cm−1) and reaches a

Figure 4. (a) MEIRAS spectra of linear CO adsorbed on a Pt thin film and a 25 nm Pt nanowire sample at 20° incidence angle. The weak peak apparent near 2140 cm −1 in sample 1 is the middle of the doublet signal from gas-phase CO and not a surface species. (b) Peak positions of CO adsorbed on different Pt nanowire multilayer samples. The dashed lines are included to guide the eye. 8372

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of different samples from Table 1. The x-axis in this figure represents the length of Pt/TiO2 interface per unit Pt area based on geometrical considerations. For example, a continuous wire of length L and width W has a Pt/TiO2 interface length of 2 × L (one corresponding to each side of the wire) and a total area of L × W (neglecting the nanowire wire height/thickness which is approximately the same in all cases). Thus, the length of interface per unit area is given as 2/W. Although the Pt film in samples 4 and 7 is unlikely to be completely continuous with no cracks, it will have much fewer interfaces than the widest of the nanowires. Therefore, as an approximation for the x-axis position for these samples in Figure 4b, the value of length of interface per unit area is taken to be zero for a Pt film and it increases with the inverse of the wire width. The error-bars represent two times the standard deviation in peak positions in several measurements performed on these samples. Sample 1 has a particularly large error bar because data was collected on this sample for more than ten times over a period of two years and it experienced some type of aging which shifted the peak position to higher wavenumbers, but it still remained significantly lower than wavenumber of a continuous film. Some samples do not show error bars because only one set of measurements was performed on them. The thin-film samples or wide nanowires showed either a single peak (sample 7) near 2090 cm−1 or a double-peak shape (samples 3 and 4) with one peak near 2090 cm−1 and another one shifted to slightly lower wavenumbers. Other samples showed a single positive, negative or derivative-like band and the peak position shifted to lower wavenumbers with decreasing width of the nanowires. The shape of the MEIRAS peaks can be positive, negative and positive−negative (derivative-like) due to superposition of the effect of refractive index of CO monolayer, which has a derivative-like shape, and its absorption coefficient (see the Supporting Information).16 The peaks position’s appear to decrease with the fraction of total Pt sites that are closer to the Pt-TiO2 interface (Figure 4b). These trends may be caused by several contributing factors related to metal−support charge transfer effects and geometric effects, discussed next. The dielectric layer in MEIRAS measurement places the tangential (to the film surface) component of the incident radiation at an antinode (i.e., maximum amplitude)22 increasing tangential sensitivity that leads to selective sampling of CO linearly adsorbed on the side faces of the thin nanowires, which are closer to the Pt/TiO2 interface. These close-to-interface sites are most strongly affected by the Pt-TiO2 charge transfer effects,25 which would polarize the CO bond and cause a frequency shift. The nature and strength of electric fields at and around the interface depend on the position of electronic bands in Pt and TiO2 and on the local geometry. These effects can be elucidated using a 2 or 3-dimentional electrostatic simulation of metal−semiconductor Schottky junctions.37,38 Schematic description of charge accumulation at such junctions along with estimations of field strength and width of the charge depletion region are presented in the Supporting Information. It is often stated39 that only the single layer of boundary atoms are affected by charge transfer because the bulk of metallic Pt particle is screened by surface-charges on the metal. The atoms on the sides of particles close to the interface, however, are exposed surface atoms that are not screened and thus affected by charge transfer. Fringe electric fields near these exposed interfaces can even be stronger than the unexposed regions where Pt is directly in contact with TiO2.37,38 On wider wires, because of nanoscale roughness of the substrate, a fraction of

CO adsorbed on the top surface of the wires (away from interface) would also have tilted or tangential orientation that contributes to the MEIRAS peak, and thus shift its position to higher wavenumbers. Moreover, the effective optical thickness of the Pt layer increases with wires’ width which increases normal sensitivity,22 thus decreasing the contribution of low wavenumber tangential CO to the spectra. The shift was higher for nanowires directly deposited on TiO2, as compared to the sample with a thin SiO2 tunneling layer and reduced the effective charge transfer from the TiO2, which caused a lower shift. These factors suggest that shifts in peak positions observed in Figure 4 may be due to metal−support charge transfer effects. Apart from charge-transfer, shifting of peaks to lower wavenumber may result from low-coordinated Pt sites or presence of impurities that affect Pt-CO binding. Concentration of any impurities, however, should be similar for samples prepared by the same methods and does not lead to the observed trends. Similarly, sample roughness and concentration of low coordinated sites should be similar for the wires’ top and sides, and for all samples, unless the metal particles are very small (1−5 nm), in which case, the concentration of low coordinated sites would change with particle size. Twenty-five nm wires on Pt/TiO2 substrate (Figure 4) show IR bands at lower wavenumber than thinner (15 nm) wires on Pt/SiO2/TiO2, consistent with the increasingly importance of the support and with charge transfer at Pt/TiO2 interfaces being a significant contributor to observed peak shifts. “Decoration effects”, resulting from TiO2‑x moieties migrating over the Pt surface have also been proposed to affect metal−support interfaces. Such decoration effects,40 however, occur at high temperature treatments (above 500 °C) and are more localized (like ligand effects) than charge-transfer effects dominated by longer-rage electrostatic forces. Our samples were not treated above 175 °C during or after Pt deposition or during experiments, therefore, any significant decoration effects are unlikely. MEIRAS bands are prone to distortions caused by changes in optical interference effects and, even in the absence of distortions, the background reflectance, and hence the signal enhancement changes with thickness of dielectric and top metal layers. Therefore, shapes and intensities of bands on nanowires of different widths could not be compared directly. In general, however, the peak intensity decreased for thinner nanowires, due to the combined effect of a decrease in the amount of adsorbed CO on the lower Pt surface area and an increase in the background reflectance. The optical distortions of the band can in principle be corrected through methods based on Kramers−Kronig transforms41 to decouple the refraction signal (which causes distortion) from the absorption signals. Similarly, the differing extents of signal enhancements in different samples can be quantified via optical simulations based on Fresnel’s equations.23 Nonetheless, the results in Figure 4 suggest that the use of polarized IR, which has been used in studies of IR dichroism of adsorbate monolayers,14 could allow us to selectively sense different populations of CO molecules adsorbed on either on top or on the sides of the wires and show the effect of the metal−support interface on CO adsorbed at the sides and edges of wires. 3.3. Effect of Infrared Polarization Relative to Nanowires. Infrared absorption by C−O bond vibrations is strongest when the electric field vector of the infrared radiation is aligned with the molecular dipole of CO (maximum overlap) 8373

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and it decreases with a cos2 θ dependence leading to no IR absorption when the electric field vector is at 90° to the dipole (no overlap). In transmission IR experiments on powder catalysts, the metal particles and adsorbed molecules are randomly oriented, therefore the same absorption signal would be observed for all polarizations, and unpolarized radiation is commonly used. Polarized IR radiation, however, can be utilized for probing orientation of molecules adsorbed on welldefined surfaces such a parallel uniform nanowires. Figure 5

the wires are placed with along orientation, py will have no overlap (AL-pP), whereas sx (AL-sP) radiation will have maximum overlap. Thus, at normal incidence or small incidence angles (ϕ < 30°) AC-pP will be similar to AL-sP and AC-sP will be similar to AL-pP. On changing the polarizer angle between 0 and 90 deg we can gradually change between these two limiting cases. Figure 6a shows the MEIRAS peak on sample 1 [25Pt/ 250Ti] (placed in the reaction cell with AL orientation) for

Figure 6. (a) Polarization angle dependent MEIRAS spectra of CO adsorbed on sample 1[25Pt/250Ti] at ϕ = 20°. (b) Peak areas as a function of polarization angle obtained by integrating the peaks in (a). The peak intensity changes from maximum value near P = 0° to close to zero near P = 90° as the measurement conditions change between limiting cases of AL-sP and AL-pP shown in Figure 5. Solid curve is included to guide the eye.

Figure 5. Optical setup with different nanowire orientations and polarizer angles: (I) AC-pP: nanowires oriented across the plane of incidence (i.e., across nanowire orientation) and p-polarized infrared (P = 90°); (II) AC-sP: across nanowire orientation and s-polarized infrared (P = 0°); (III) AL-pP: nanowires oriented along the plane of incidence (i.e., along nanowire orientation) and p-polarized infrared (P = 90°); and (IV) AL-sP: along nanowire orientation and s-polarized infrared (P = 0). Plane of the paper is the plane of incidence of the infrared in all side views.

different polarizer angles of the infrared beam at incidence angle of (ϕ) 20° from the surface normal. The band area changes significantly with changing the polarization angle (Figure 6b); it reaches a maximum near the polarizer angle of P = 0° (AL-sP case in Figure 5) with a maximum overlap between side-CO and infrared electric field vector. Change in polarizer angle decreases the overlap of side-CO dipoles with infrared polarization vector leading to decreased band intensity that reaches a minimum at P = 90°, corresponding to AL-pP. These results are consistent with negligible pz intensity, and with dipoles of CO adsorbed on side faces aligned in-plane (i.e., tangential to surface) but perpendicular to the direction of wire. Similar results were obtained on the other thin nanowire samples. 3.4. Effect of Incidence Angle (ϕ) at Different Polarization Conditions. From Figure 5, we can see that the optical configurations AC-pP and AL-sP are identical at ϕ = 0° and very similar at small incidence angles (ϕ < 30°) because the infrared polarization vector is pointing perpendicular to the side faces of nanowires, allowing maximum overlap with sideCO. Similarly, AC-sP and AL-pP are identical at normal incidence both having no overlap with side-CO. At larger incidence angles, however, these configurations start to differ significantly as the p-polarized components start to gain surface normal sensitivity corresponding to CO on top of the wires and loose tangential sensitivity. s-polarization, on the other hand is only sensitive to tangential modes at all incidence angles. These effects of changing between different conditions at low incidence angle and that of changing the incidence angle are shown in Figures 7 and 8, respectively.

shows four possible combinations of nanowire placement in the IR cell and optical conditions that can be used to probe different molecular orientations. The terms used to specify directions and orientations of infrared, CO molecules, and nanowires (parallel/tangential/along versus perpendicular/ normal/across) are shown in Figure 5 and further summarized in the Supporting Information (Table S.1). The sample can be placed with wires oriented along (AL) or across (AC) the plane of incidence and the polarization of the incident beam with respect to the plane of incidence can be gradually changed by changing the polarizer angle. In the two limiting cases, the beam is polarized perpendicular (s; polarizer angle P = 0°) and parallel (p; P = 90°) to the plane of incidence. At small incidence angles (ϕ), there is little surface normal intensity (pz) for any configuration because: (i) surface normal projection proportional to sin2(ϕ) is small and (ii) z-component is at a node (i.e., minimum amplitude) when the TiO2 thickness is close to 500 nm. Therefore, only the components tangential to the surface of the multilayer structure (py and sx) can lead to IR absorption by CO. In AC nanowire placement, the py component of radiation (AC-pP configuration) will have maximum overlap with CO dipoles on the sides of the wire leading to maximum signal from side-CO molecules, whereas sx component (AC-sP) will have no overlap because there are no CO molecules oriented in that direction. On the other hand, if 8374

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Figure 7. (a) Projections of infrared polarization vectors onto the sample surface for different optical conditions shown in Figure 5. The double arrows reflect magnitude and direction of surface electric field, relative to the nanowire orientation. MEIRAS spectra of CO adsorbed on (b) sample 5 [15Pt/4Si/550Ti] and (c) sample 6 [40Pt/4Si/550Ti] at these optical conditions.

Figure 8. Effect of incidence angle on MEIRAS spectra on sample 6 [40Pt/4Si/550Ti] obtained with (a) parallel polarization (AC-pP) and (b) perpendicular polarization (AL-sP). The double arrows reflect magnitude and direction of surface electric field, relative to the nanowire orientation.

samples have been of opposite sign (positive at higher and negative at lower wavenumbers) than the 2093 cm−1 peak in Figure 8, which is a negative feature at higher wavenumbers. This peak is not very intense partly because the pz component is at a node on the multilayer structure22 and partly because the background reflectance of the sample also increases at larger angles leading to a larger denominator in eq 2. The peak intensities in AL-sP (Figure 8b) initially do not decrease with ϕ because the sx component of s-polarization is independent of ϕ, but the intensities eventually decrease due to increased background reflectance from the sample as well as from the reaction cell window (see the Supporting Information). The band areas for AC-pP and AL-sP are almost identical for ϕ = 5° (Figure 9). At ϕ = 55°, however, AC-pP retains only 34% of the area at ϕ = 5°, while AL-sP retains 88% of it. This change occurs despite the fact that the effect of increased background reflectance from the CaF2 window of the reaction cell on decreasing the peak intensity is more significant for spolarization (otherwise it would have dropped even less) and less significant for p-polarization which is almost fully transmitted by the window due to Brewster’s angle42 of CaF2 near 55°.

Figure 7 shows MEIRAS spectra of CO adsorbed on sample 5 [15Pt/4Si/550Ti] and sample 6 [40Pt/4Si/550Ti] at the four limiting conditions described in Figure 5 at a small incidence angle (ϕ = 20°). The spectra for both samples are consistent with expectations from Figure 5 as there is large signal and similar peak shapes for AC-pP and AL-sP, whereas a much smaller signal is observed for AC-sP and AL-pP. The shape of the peaks is derivative-like for sample 5 due to optical distortions (see the Supporting Information) and positive for sample 6. Figure 8 shows the effect of changing incidence angle on MEIRAS peaks of sample 6 [40Pt/4Si/550Ti] for conditions AC-pP and AL-sP. At near-normal incidence, the two conditions show similar peaks, but increases in incidence angle for AC-pP causes the peak intensity at 2073 cm −1 to decrease significantly because the tangential component of radiation (py, Figure 5) decreases with some increase in the normal component (pz). In addition to decreased intensity, a new peak appears near 2093 cm−1, which may correspond to CO on top of the wires with a surface normal orientation (overlapping with pz) or to optical distortions leading to derivative-like peaks similar to Figure 7a. We attribute this peak to the former scenario because distortions observed on other 8375

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Pt/SiO2 interfaces. The MEIRAS band of CO adsorbed on these broken Pt film after reduction pretreatment showed a double-peak shape with the low frequency shoulder corresponding to the CO adsorbed on the edges of Pt particles (Figure 10a). Upon exposure to oxygen, the band disappears due to desorption of CO and displacement by adsorbed oxygen. When CO is readsorbed on the oxygen-exposed surface, the low frequency shoulder shifts and merges with the main peak which itself showed a much smaller shift. This shift may be caused by electronic changes due to oxidation of TiO2 vacancies by O2 molecules43 or by partial coverage of Pt surface by O atoms. Similar peak shift was observed on the nanowires of sample 1 [25Pt/250Ti] (Figure 10b), but in this case, the whole peak is initially at a lower wavenumber and shifts to higher wavenumber with CO readsorption after O2 exposure, rather than just a low frequency shoulder. These observations further support the interpretations that the CO signal obtained on thin Pt nanowires at low incidence angles predominantly comes from CO adsorbed on the sides and edges of the wires, and that these side-CO molecules are more strongly affected by changing electronic properties of the catalyst surface (through pretreatment, in this case).

Figure 9. Peak areas as a function of incidence angle on sample 6 obtained using parallel polarization (AC-pP) and perpendicular polarization (AL-sP). The lines are included to guide the eye.

Similar effects of changing incidence angle were observed on sample 5 with 15 nm wires as well as on sample 6 with similar AC-sP and AL-pP conditions. The samples showed a weak but consistent appearance of a higher wavenumber peak corresponding to normal CO on top of the nanowires, with an increase in incidence angle of p-polarization, but not with spolarization. These results are consistent with the attribution of low-wavenumber peaks on the nanowire samples, to the CO adsorbed on the edges of the wire and that we can selectively detect adsorbates on these edges. The surface roughness of the substrate and the Pt nanowires in the current samples, however, probably makes the trends in tangential versus normal sensitivity less distinct. The spectral features are also prone to optical distortion, which must be corrected before possible implementation of these concepts to other systems such as supported heterogeneous catalysis. 3.5. Comparison of Nanowire and Thin-Film MEIRAS Results. Next, we compare our previously reported24 effects of sample pretreatment on CO adsorption on a continuous film with present studies on Pt nanowires. We previously found that after a few cycles of oxidation/reduction pretreatments on Pt/ SiO2/Au multilayer structures, the top Pt film broke-up to form loosely connected islands with significant amount of exposed

4. CONCLUSION Polarized MEIRAS studies on multilayer catalysts with parallel uniform Pt nanowires of varying widths are used to probe orientations of adsorbed CO molecules. MEIRAS peaks shift to lower wavenumber with decreasing nanowires widths. The low wavenumber peaks on thin nanowires can be attributed to CO adsorbed on the side faces of nanowires, as demonstrated using the effect of polarization and comparisons with the effect of O2 exposure with previous broken-thin-film results. Sensitivity for normal CO relative to the side-CO can be improved by increasing the incidence angle with p-polarization, and these top-CO vibrations appear at slightly higher wavenumbers than the side-CO on the same sample. The results describe how optical properties of multilayers and polarized IR can be used in a novel way to probe adsorbate orientations on supported metal nanowires and thin films. Most IR measurements reported for supported catalysts reflect signals that integrate all directions and crystallographic faces, which renders them incapable of

Figure 10. Effect of exposure of surface to oxygen on the peak position of: (a) A broken Pt film measured using unpolarized IR at ϕ = 30° (adapted from ref 24) and (b) sample 1[25Pt/250Ti] using P = 160° and ϕ = 20° (this work). 8376

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discern the effect of molecular orientation near the metal− support interfaces. The polarized MEIRAS technique demonstrated here is partly limited by nonidealities such as roughness of films and wires and optical distortions of peak shapes. Future improvements in sample preparation methods and in theoretical methods to analyze the spectra, however, can make it a convenient way to probe the catalytic sites on supported metal close to a support, as distinct from the ones away from the support.

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ASSOCIATED CONTENT

S Supporting Information *

Summary of MEIRAS effect. Distortion of bands shapes in reflection spectroscopy. Summary of terms reflecting orientations of sample, electric field and CO molecules. Detailed sample preparation and experimental setup. XRD and electrical characterization of TiO2 films. Estimates of bulk O-vacancy in TiO2. Structural considerations and electric fields near metal− support interface. Effect of CaF2 window on MEIRAS signal. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 574-631-5897. Present Address †

Department of Chemical and Biomolecular Engineering, University of California, Berkeley.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation (Grant No. 0854324) and nanofabrication resources from the Notre Dame Nanofabrication Facility and the Center for Nanoscale Materials (CNM), Argonne National Laboratory are gratefully acknowledged. Use of the CNM was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. The authors thank Dr. Leonidas E. Ocola for his help with the use of nanofabrication facilities at CNM.

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