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In Situ High Temperature Surface Enhanced Raman Spectroscopy for the Study of Interface Phenomena: Probing a Solid Acid on Alumina Eric V. Formo,† Zili Wu,*,†,‡ Shannon M. Mahurin,‡ and Sheng Dai*,†,‡ † ‡
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
bS Supporting Information ABSTRACT: Herein, we utilize surface enhanced Raman spectroscopy (SERS) for the in situ analyses of catalyst structure while operating at elevated temperatures in various atmospheres. In order to accomplish this, robust SERS substrates were generated by depositing an ultrathin protective coating of alumina on top of silver nanowires (NWs) via atomic layer deposition (ALD). In situ studies were then conducted by analyzing the effects of heating a solid acid, phosphotungstic acid (PTA), on the alumina surface in either an oxygen or hydrogen environment at temperatures up to 400 °C. Interestingly, the distance-dependent decay of the enhancement factor of the SERS signal from the underlying NWs allowed us to probe with great detail the interfacial region between the PTA and the alumina surface. The ability to analyze the area closest to the alumina surface was further confirmed by assembling vanadia onto the substrate and monitoring the intensity differences between the VOAl and outer V d O bonds.
’ INTRODUCTION Surface enhanced Raman spectroscopy (SERS) has broad appeal in numerous fields as an analytical technique due to its ability to detect minute levels of a wide variety of chemical species.1,2 However, in the field of catalysis SERS has played a relatively restricted role.36 The inability to extend the SERS technique into the study of catalytic systems stems from the use of silver nanostructures as the SERS active species, as they are prone to rapid surface oxidation causing a steep reduction in the enhancement factor.7,8 This is especially unfortunate as SERS may allow for the investigation of the interface interactions on catalytic systems, which would provide invaluable insights into both the structure of active sites and surface reaction mechanism. Moreover, when exposed to harsh conditions such as high temperatures, the destruction of the enhancement capabilities are accelerated.8 To extend the use of SERS into the examination of such catalytic systems, there has been great interest in the development of extremely stable SERS substrates. Many of these attempts have revolved around placing a protective layer on top of the SERS active moiety.911 Recently, our group has developed a bottom-up approach to generate a robust SERS substrate that utilizes atomic layer deposition (ALD) to deposit an ultrathin layer of alumina on top of silver nanowires. Further, these substrates have been shown to withstand high temperatures of up to 400 °C in air for 24 h while still maintaining SERS capabilities.12 However, there is essentially no report of in situ SERS studies of catalysts at high temperatures. The selective observation of interfacial mode between metal oxide and support is of great relevance to catalysis as the interface bond has been considered as the active site in metal oxide catalyzed r 2011 American Chemical Society
reactions.13,14 Among the most widely exploited supported catalytic agents are solid acid catalysts that have been assembled onto a metal oxide support.1517 One class of solid acids of particular interest is the Keggin polyoxometalate, phosphotungstic acid (PTA), because of its superior catalytic abilities in such reactions as the dehydration of alcohols into hydrocarbons or iso-butane alkylation.18,19 Previously, Raman spectroscopy has been used to determine structural characteristics of PTA with strong peaks corresponding to WdO and WOW components of the molecule.2022 Specifically, PTA has multiple strong peaks at 1015 cm1 and 996 cm1 which are ascribed to the symmetric (νs) and asymmetric (νas) WdO modes, respectively.23 The peaks for the bridging WOW bonds are between 930 and 850 cm1 for the νas(WOW) with the νs(WOW) at a range of 650500 cm1.21,22 Further, when a thin layer of PTA was previously analyzed by using Ag colloids to enhance the signal, the peaks associated with the νs(WdO) and νas(WdO) vibrations reversed their intensities in comparison to the PTA powder spectra.24 It was determined that this effect was due to the differing polarizability on the Ag surface in which the νs(WdO) band was parallel to the Ag surface, while the νas(WdO) vibrations were perpendicular, thereby causing the inversion of signal strength.23,24 The article herein is to our knowledge the first study of its kind that involves the use of SERS for in situ analysis of catalysts Received: December 16, 2010 Revised: February 22, 2011 Published: April 19, 2011 9068
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The Journal of Physical Chemistry C structure at high temperatures. This capability was unlocked through the utilization of a robust substrate generated by a bottom-up approach that uses Ag nanowires (NWs) as the SERS active species, which were coated with an ultrathin layer of alumina via ALD. Subsequently, these substrates were decorated with a thin layer of PTA on top. By probing the PTA, we were able to record the various alterations in the spectra that occurred by heating in either oxidative or reductive environments in situ. Most notably, these SERS substrates appear to have an enhanced capability of analyzing the area closest to the surface of the alumina with great specificity. Further, this effect of primarily analyzing the interfacial region was confirmed by observing the intensity of the peripheral VdO in comparison to the VOAl bond of vanadia that had been deposited on the alumina surface.
’ EXPERTMENTAL SECTION Fabrication of the PTA Coated Substrates. Ag nanowires were prepared by placing 5 mL of ethylene glycol (EG, J.T. Baker, lot H24B27) into a vial with a Teflon-coated stir bar and heating it in air at 140 °C for 1 h. One milliliter of a 3 mM HCl solution in EG was then added to the hot EG. The solution was heated for an additional 10 min. Two solutions of 94 mM AgNO3 in EG and 147 mM PVP (Mw ≈ 5.5 104) were dissolved separately in 3 mL aliquots of EG and were added simultaneously into the vial at a rate of 45 mL per hour. The reaction mixture was continuously heated at 140 °C in air for 19 h, after which a gray opaque solution had formed. The solution was then washed with acetone to remove excess organics and centrifuged, yielding a gray precipitate. The nanowires were then stored in ultrapure 18 MΩ dionized water. The Ag nanowires were deposited onto the silicon substrate via drop-casting and allowed to dry in air at ambient conditions. Subsequently, the Ag NW substrate was placed in the center of the atomic layer deposition (ALD) reaction chamber, which consisted of a stainless steel flow tube (3.5 cm diameter; 25 cm length) that was heated to 67 °C. The alumina precursor, trimethylaluminum (TMA), and high-purity water were alternately pumped to the reaction chamber using nitrogen as a carrier gas. One complete reaction cycle was 42 s in duration and was composed of the following: (i) TMA reactant exposure time = 1 s; (ii) N2 gas purging time = 20 s; (iii) water vapor exposure time = 1 s; and (iv) N2 gas purging time = 20 s. The SERS substrates were placed in the ALD reaction chamber for a total of 6 cycles. The average growth rate per cycle was determined by exposing the Ag nanowire to 40 cycles of the ALD process, which yielded a coating thickness of roughly 12 nm from which we can estimate that the average growth rate was ∼3 Å/cycle. The Al2O3-coated substrates were pretreated by heating them to 400 °C for 12 h to remove any organic materials on the surface that could obscure Raman findings. The substrate was proved to be pinhole free as no SERS signal was observed when we attempted to absorb octadecanethiol on to the surface. These substrates were then placed in a 1 mM phosphotungstic acid (PTA) solution in water. After one hour of treatment, they were removed and allowed to dry before SERS studies were conducted. For the PTA powder tests, 20 μL of a 100 mM PTA solution was deposited onto a Si chip and allowed to dry. For the deposition of vanadia, 20 μL of a 0.01 mM solution of NH4VO3 (99þ%, Sigma-Aldrich) in water was dropped onto the alumina coated SERS substrate. The vanadia precursor coated substrate was then calcined in a 4% O2/Ar atmosphere at 400 °C for 2 h before Raman measurement. Characterization. SEM images were taken of as-prepared samples for Raman measurements using a field-emission scanning SEM (Hitachi S4700SEM) operated at an accelerating voltage of
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10 kV. STEM samples were prepared by drop-casting a dispersion of the nanowires onto carbon coated copper grids (Formvar/Carbon, 200 mesh, Ted Pella). STEM images were acquired using a Hitachi HD2000 microscope operated at 200 kV. Raman scattering was collected via an ellipsoidal mirror into fiber optics connected directly to the spectrograph stage of a triple Raman spectrometer (Princeton Instruments Acton Trivista 555). A value of 632.8 nm (20 mW at sample) is obtained from a high power HeNe laser (Melles Griot). The laser spot size is ca. 100 μm at the sample position. A RazorEdge filter (Semrock) was used in front of the UVvis fiber optic bundle (Princeton Instruments) to block the laser irradiation. A liquid N2-cooled CCD detector (Princeton Instrument) was employed for signal detection. The silicon wafer Raman signal at 520 cm1 was used for the calibration of the Raman shifts. For the in situ studies, all samples were heated in a Raman cell (Linkam THMS 600) at a rate of 10 °C per min to the target temperature. Further, the following gases were used to generate the reductive, oxidative, or inert environments: 4% hydrogen in ultra high purity (UHP) argon, 5% O2 in UHP helium, and UHP helium, respectively. Spectra were taken at several different spots on the sample after each treatment, and the spectra were essentially very similar to each other, indicating a homogeneous surface enhancement effect across the substrate. XPS was performed by mounting the samples on a stainless steel XPS sample carrier and were then placed in a vacuum load lock of the Thermo Fisher Scientific K-Alpha XPS instrument with no other preparation. After suitable evacuation, the samples were transferred into the analysis chamber (base pressure of 6 1010 mbar) and positioned at the focal point of the X-ray gun (monochromatic Al-kR X-rays), the ion gun (3 kV Ar-ions), and the electron energy analyzer (hemispherical). Surface composition was determined for the as-received samples over the binding energy range from 0 to 1350 eV using a 400 μm X-ray spot.
’ RESULTS AND DISCUSSION Figure 1A shows a scheme that details the steps in the generation of the coated alumina SERS substrates with a solid acid layer on top. First, silver NWs were generated via a protocol that involved the use of an oxidative etchant in the polyol reduction process (Supporting Information, Figure S1).25 The NWs were then drop-cast onto a silicon chip and dried in air (Figure 1A, i). Subsequently, these substrates were placed into an atomic layer deposition (ALD) chamber and coated with alumina for a thickness of roughly 1.6 nm (Figure 1A, ii). Afterward, the coated substrates were pretreated by heating them in air to 400 °C for 12 h to remove any organic materials on the alumina surface that could obscure the in situ Raman spectra upon heating to higher temperatures (Supporting Information, Figure S2). The preheated substrates were then placed into a 1 mM phosphotungstic acid (PTA) in water solution for 1 h for the PTA to assemble onto the surface of the alumina (Figure 1A, iii). After the substrate was removed and allowed to dry, SEM was utilized to analyze the substrates’ surface, and we determined that the coating was well dispersed on the surface when compared to the same process being conducted on a bare silicon chip (Figure 1C and Supporting Information, Figure S3). To begin our in situ Raman studies, we first analyzed PTA powder on a silicon chip (Figure 2A). We could see the predominate peaks at 1015 cm1 and 992 cm1 which we assigned to the νs (WdO) and νas(WdO), respectively, with minor peaks at 935 cm1 and 890 cm1 which correspond to bridging νas(WOW) modes (Figure 2A, black). These results are roughly similar to the values 9069
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Figure 2. In situ Raman/SERS study of the effect of heating PTA. (A) PTA powder on Si substrate; (B) thin layer of PTA on alumina-coated Ag NWs SERS substrate at room temperature (black) and heated to 400 °C in hydrogen (red) or oxygen (blue). Figure 1. (A) Scheme depicting steps toward generating the phosphotungstic acid (PTA) decorated SERS substrate: (i) bare Ag nanowire (NW) on Si; (ii) ultrathin coating of Al2O3 on the Ag nanowire via atomic layer deposition; and (iii) PTA assembly onto the preheated alumina surface. (B) SEM image of ii after the SERS substrate after pretreatment in air for 12 h. (C) SEM image of iii, the pretreated sample, after its surface had been decorated with PTA by placing it in a 1 mM PTA/H2O solution for 1 h and allowing to dry in air. The scale bars are 10 μm.
previously reported .2022,26 When the PTA was heated to 400 °C in a hydrogen atmosphere, there was a significant loss of spectral intensity, apparently due to the reduction of PTA. A weak band was observed at 1022 cm1 due to νs(WdO) along with a weak and broad feature below 1000 cm1 (Figure 2A, red). Conversely, when heated to the same temperature in oxygen, there was only a slightly broader peak at 1022 cm1 that corresponded to the νs(WdO) peak (Figure 2A, blue); the blue shift is due to the loss of crystal water.27 The observation is in good agreement with the previous Raman study of bulk PTA.27 However, when the PTA was loaded on the Al2O3coated SERS substrate, a number of differences occurred. The first difference was an inversion of the intensities between the νs(WdO) and νas(WdO) peaks as seen in the previous use of PTA adsorbed onto bare Ag colloids (Figure 2B, black).24 This was expected as the PTA was still within the electromagnetic features of the silver surface even with the alumina overlayer. Moreover, it indicates that the
orientation of PTA on the alumina surface is likely similar to that on the Ag surface so that the νas(WdO) mode gives more of the perpendicular component of polarizability change along the electromagnetic field. When the enhancement factor (EF) was calculated for the SERS substrates under the various conditions, the equation below was used: EF ¼ I SERS =I bulk Nbulk =N ads where Nbulk is the number of analyte molecules in the focal volume, and Ibulk is the intensity of the Raman signal obtained from the bulk PTA on silicon. In the case of Nads, the amount of PTA assembled on the surface was determined via the atomic ratio between Al and W when the samples were analyzed with XPS, and Isers is the intensity of the SERS spectrum (Supporting Information, Figure S4). In all cases, the intensity of the spectrum was determined for the integrated peak area between 600 and 1200 cm1 as most of the spectral information takes place within this region. For the room temperature spectrum, we noted that the enhancement factor was 2.4 103. When heated under a hydrogen environment at 400 °C, there was again an increase in the relative intensity of the WOW signal, along with the νs(WdO) peak (Figure 2B, red). There was also a small carbon signal with peaks at 1594 and 1360 cm1 corresponding to the G and D bands, respectively.28 In comparison to the bulk PTA sample, the PTA/Al2O3 substrate showed a signal enhancement of 9.1 102, but both spectra had a fairly similar shape, indicating heating in H2 would 9070
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Figure 3. In situ effect of heating PTA on Al2O3-coated Ag NWs substrate in an oxidative environment. (A) Heating in an oxygen atmosphere with the SERS spectra taken at room conditions (black), 100 °C (red), 200 °C (green), 300 °C (blue), and 400 °C (gray). (B) Closeup of SERS spectra taken at 400 °C (gray) and 300 °C (blue). (C) SERS spectra taken at 400 °C (gray) and after it had been cooled to RT (purple). (D) Spectra of PTA on Al2O3coated Ag NWs substrate after reheating to 400 °C in hydrogen (orange), once the substrate had been previously heated to 400 °C in oxygen (gray), then cooled, and both spectra taken at 400 °C.
increase the WOW population at the expense of the WdO bonds of PTA. When the PTA/Al2O3 substrate was heated under an oxygen atmosphere, a strong peak at 893 cm1 formed, which indicates predominate WOW bonding and displays little signal for the WdO component of the PTA structure, both of which are a substantial shift from the initial spectrum, along with an EF of 1.4 103 (Figure 2B, blue). When we compare the different EFs, we can see the SERS substrates excelled at maintaining their enhancement capabilities across a wide range of temperatures and environments. Figure 3 details the in-depth analysis of the PTA on the alumina substrate upon heating in the oxidative environment. When the sample was heated to 100 °C, a slight blue shift in the spectrum of roughly 20 cm1 took place, most likely caused by the dehydration of the PTA, which moved the WdO stretching modes closer to the reported values of bulk PTA (Figure 3A). Upon further heating to 200 and 300 °C, the spectra red-shifted slightly upon each temperature increase, possibly caused by the PTA losing electron density (Figure 3A). However, when the PTA/Al2O3 was heated to 400 °C, a significant change occurred in which the spectrum went from two major peaks at 906 and 986 cm1 at 300 °C to one at 893 cm1 at 400 °C along with a slight shoulder around 986 cm1 (Figure 3B). These spectral changes were most likely due to distortions in the PTA’s surface interaction with alumina where the SERS enhancement was most intense. Specifically, the PTA will undergo reorientation via rotation with the WOW bridging bond component of the PTA structure more closely associating with the alumina surface, thus leading to the large change in the spectrum with the strong
νas(WOW) peak at 893 cm1.24 The ability for the PTA to rotate in such a manner has been previously reported by Teague et al. who ascribed an increase in the νas(WOW) intensity to the movement of the S4 axis of the PTA molecule moving up to 40° from the perpendicular, bringing more of the WOW component closer to the surface under cathodic conditions.24 To observe if spectral change is a temperature induced effect on Raman spectra, we cooled the sample and noted only an increase in the intensity of the spectrum. The lack of change in the spectrum upon cooling led us to conclude that the PTA strongly interacted with the Al2O3 surface sites at the elevated temperatures, leading to the PTA being at a fixed position on the substrate’s surface (Figure 3C). Further, the intensity difference between these spectra collected was due to a well-known temperature effect that causes a decreased population of the ground state phonons as given by the Boltzmann distribution when spectra are obtained at high temperatures.29 As cooling did not affect the PTA arrangement on the surface of alumina, the substrate was then reheated to 400 °C in hydrogen (Figure 3D). The reductive environment led to a large change in the spectrum, as we noted the νs(WdO) band at 1015 cm1. We determined that the PTA rotated again during reduction so that some of the WdO species would again be interacting with the alumina surface. When the substrate was exposed to the same heating cycle for a second and third time, we noted that the PTA continued to undergo reorientation on the surface of alumina with little change in the spectra in O2 or H2 environments (Supporting Information, Figure S5). 9071
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Scheme 1. Schematic Diagram Illustrating the Interplay between the Raman Signal and SERS Signal from the PTA Powder (A) and the PTA Layer on Top of the Alumina-Coated Ag Nanowire (B)
Figure 4. (A) Scheme depicting the phosphotungstic acid (PTA) or vanadia closest to the surface of the Al2O3-coated SERS substrate. (B) Raman spectra of the calcined vanadia-coated sample after it had been heated in oxygen to 400 °C (black), then cooled to room temperature (red), and once it had been rehydrated (blue).
Scheme 1 displays the differences in the Raman/SERS signal obtained when analyzing either the bulk PTA or the PTA assembled onto the alumina, which led to the variation in the recorded spectra as seen in Figure 2. Scheme 1A details the PTA on Si substrate with the Raman signal arising from the PTA molecules interacting with themselves, yielding a very intense νs(WdO) mode as displayed in the spectrum. Conversely, when analyzing the thin layer of PTA on alumina, the SERS effect of the underlying Ag NWs greatly impacts the spectra (Scheme 1B). Specifically, we should see spectral information for the region at the interface between the two metal oxides as the distance from the NW surface increases the decay of the plasmonic field and will lead to a direct reduction in the enhancement factor. As the signal enhancement will be the most intense at the surface, most likely we are observing the changing orientations of the PTA on the alumina surface upon heating in our in situ Raman spectra. To ensure that we were in fact probing the interface region, we elected to deposit vanadia onto the surface of the alumina in place of the PTA. A diagram of the structural differences between PTA and vanadia on alumina can be seen in Figure 4A. Specifically, in the case of PTA both the WdO and WOW bonds can associate with the surface. However, vanadia is directly bound onto the substrate via VOAl bonds.13,14,30 The interfacial VOAl modes will experience a more intense signal enhancement in comparison to the VdO bond on the periphery. Specifically, upon heating a vanadia coated Al2O3 substrate in oxygen to 400 °C, we observed a strong peak at 818 cm1 due to the out-of-phase VOAl mode31 along with a much weaker
peak at 990 cm1 which corresponds to the VdO mode (Figure 4B, black). By cooling back to room temperature, the VOAl mode remained at around 818 cm1 with the VdO stretching peak at around 1000 cm1 (Figure 4B, red). The outof-phase VOAl stretching mode, which likely undergoes a larger change in polarizability by the Ag plasmonic field than the in-phase VOAl mode (generally observed above 900 cm1), was affected by the SERS substrate and thus is selectively observed in the SERS spectra.32 Finally, when the vanadia coated substrate was exposed to ambient air, the rehydration of the VdO bond caused its disappearance from the SERS spectrum (Figure 4B, blue), while the VOAl mode blue shifts to 840 cm1. This is in general agreement with the Raman study of hydrated V/Al2O3 powder samples where only the interface VOAl mode can be observed since the VdO bond is hydrolyzed.13,32 These experiments again illustrate the distance dependent nature of the SERS effect which resulted in the relative intensity of the peak nearest to the surface receiving the largest enhancement.
’ CONCLUSIONS In summary, we have unlocked the use of the SERS technique to obtain the structural information of catalyts in situ at high temperatures in various environments. This achievement was accomplished through the utilization of robust SERS substrates that were generated via the deposition of an ultrathin alumina layer onto Ag NWs. Through these studies, we ascertained that our substrates were extremely adept at analyzing the interface region between the PTA and alumina. The ability to probe the 9072
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The Journal of Physical Chemistry C interface region was further proven through the analysis of vanadia anchored onto the surface where the VOAl modes have a much more intense signal compared to the outer bond VdO. This ability to selectively observe the interface interaction is of general significance to catalysis because the interface bonds often play a critical role in catalytic reactions.
’ ASSOCIATED CONTENT
bS
Supporting Information. SEM image of nanowires produced from the HCl mediated polyol process; Raman spectrum of the as-prepared SERS Al2O3 substrate; SEM image of PTA on a silicon surface; XPS spectra; and Raman spectra for cycling of an oxidized PTA surface on Al2O3-coated Ag NW substrate. This material is available free of charge via the Internet at http://pubs. acs.org
’ AUTHOR INFORMATION Corresponding Author
*(Z.W.) Fax: 865-574-1753. E-mail:
[email protected]. (S.D.) Fax: 865-576-5235. E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at the Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. Further, a portion of this research, performed by Shannon M. Mahurin, was conducted through The Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy. XPS measurements were conducted using the SHaRE facilities, sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. The research was supported in part by the appointment of E. V. Formo to the ORNL Postdoctoral Research Associates Program, administered jointly by ORNL and the Oak Ridge Associated Universities. The Oak Ridge National Laboratory is managed by UT-Battelle, LLC for the U.S. Department of Energy under contract DE-AC0500OR22725.
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