ARTICLE pubs.acs.org/JPCC
Plasmonic Based Kinetic Analysis of Hydrogen Reactions within AuYSZ Nanocomposites Nicholas A. Joy, Charles M. Settens, Richard J. Matyi, and Michael A. Carpenter* College of Nanoscale Science and Engineering, University at Albany—State University of New York, 257 Fuller Road, Albany, New York 12203, United States ABSTRACT: The kinetics of the hydrogen reaction on a nanocomposite film comprised of gold (Au) nanoparticles embedded within yttria-stabilized zirconia (YSZ) was determined through an analysis of the localized surface plasmon resonance (LSPR) of the Au nanoparticles. The LSPR peak of the Au nanoparticles was monitored as a function of time during exposure to 0.51.00% H2 in a N2 background at temperatures of 300, 400, and 500 °C. An analysis of the reaction kinetics shows a linear relation between the observed rate of reaction and PH21/2, indicative of a H2 dissociative adsorption mechanism. Signal transduction is thought to be due mainly to charge exchange from the chemisorption of atomic hydrogen. An Arrhenius analysis of the data determined the activation energy to be about 0.2 eV for the H2 reaction.
’ INTRODUCTION Emissions sensing in harsh environments is a challenging task. Not only are sensors exposed to extreme environments but the required detection levels for emission gases are decreasing to meet future needs of “zero-emission energy sources”. In addition to these challenges, the difficulty in obtaining a selective response toward the target gases is an issue that faces almost all new sensor technologies and requires information about the reaction mechanisms. A sensing method that is under development by this group is based on an optical interrogation of the localized surface plasmon resonance (LSPR) of Au nanoparticles embedded within an yttria-stabilized zirconia (AuYSZ) nanocomposite thin film as a function of gas exposure at elevated temperatures.14 Exposures to gases such as H2, NO2, and CO have all been done, but always in an oxygen-containing environment. While this is the more realistic condition for typical sensing environments, the reaction kinetics have been too fast to resolve within the current experimental parameters, and furthermore, there are competing reactions pertaining to O2 and H2. Consequently, the only type of analysis previously done has been based on the equilibrium response. However, it was found that, when no oxygen is present during H2 exposures, the reaction kinetics can be resolved due to slower reaction rates induced by the activation energy barrier for H2 dissociation. The present work shows that, in addition to the equilibrium responses, the all-optical method of LSPR analysis can be used to observe the high-temperature kinetics of this fundamental reaction. While there have been some reports of kinetic analysis using LSPR, these have mostly been limited to room temperature biological reactions5,6 or other room temperature processes.7 A recent elevated temperature study by Larsson et al. demonstrated the measurement of catalytic r 2011 American Chemical Society
reactions through the use of Au nanoplasmonic probes.8 These studies detected the kinetic phase change between an O2 and a H2 saturated Pt surface for the hydrogen oxidation reaction; however, the reaction kinetics was not determined. While the current work is a benchmark study, the usefulness of reaction kinetics comes from the need to improve sensor selectivity via an understanding of the fundamental reaction pathway. It is expected that a broader use of the Au LSPR for kinetics analysis of high-temperature reactions relevant either to catalysis or to sensor applications can be realized. The Au nanoparticles can serve as an active catalyst or a spectator within a variety of metal oxide nanocomposites, and changes in either the free electron density of the Au nanoparticle or the dielectric function of the matrix will cause a measurable change in the LSPR. The reaction kinetics can then be monitored by acquiring the LSPR data at a rate faster than the reaction kinetics while minimizing the gas cycle times through the use of a microchamber reaction environment. For this experiment, AuYSZ films were exposed to H2 gas in a N2 background at temperatures of 300, 400, and 500 °C, which resulted in a reversible blue shift in the peak position of the LSPR absorption spectrum. This can either be due to a charge transfer mechanism or a decrease in the dielectric constant of the YSZ matrix. Since no oxygen was admitted during exposure, the signal change is not a result of charge exchange associated with O2formation and filling of oxygen vacancies (or the reverse process), which is something well-known for oxygen ion conductors such as YSZ. The results of the kinetics analysis point to Received: December 23, 2010 Revised: February 23, 2011 Published: March 15, 2011 6283
dx.doi.org/10.1021/jp112228h | J. Phys. Chem. C 2011, 115, 6283–6289
The Journal of Physical Chemistry C
ARTICLE
the dissociative adsorption of H2, where charge exchange may result from chemisorption on Au nanoparticles or from interactions with the YSZ itself, causing a signal change in the form of a LSPR peak shift.
’ EXPERIMENTAL METHODS Thin-film AuYSZ nanocomposites were prepared by radio frequency co-magnetron confocal physical vapor deposition (PVD). Films were deposited on two separate sapphire substrates to allow for different annealing conditions in order to produce samples with two different Au particle sizes. The two films were deposited separately, with one of the samples having a small strip masked with aluminum foil during the deposition process in order to have a flat field reference spectrum for normalization of the transmitted intensity during gas exposures. Small pieces of silicon were also included during the sapphire substrate deposition process for materials analyses requiring conductive substrates. The PVD targets consisted of 99.99% pure Au and 99.9% pure 5 wt % YSZ. Sputtering took place in an Ar environment at 10 mTorr with the Au and YSZ targets at 25 and 200 W, respectively. After a warm up period to allow the sputtering rate to come to equilibrium, the YSZ shutter was opened for 15 s to deposit a barrier layer on the substrate. Following that, the Au shutter was opened and co-sputtering was performed for 255 s. A final surface layer of YSZ was deposited for 15 s by closing the shutter on the Au target. Following deposition, the two films on sapphire substrates were annealed separately along with the films deposited on silicon. One of the films was annealed at ∼970 °C for 3 h, while the second film was annealed at ∼920 °C for 1.5 h. Both samples were annealed in an Ar environment having a flow of 2000 sccm. A half-hour ramp up and natural cool down were also used for both samples. Previous work by this group has shown that similar deposition and annealing procedures have led to particle sizes that were around 20 and 10 nm on average for the films annealed at the higher and lower temperatures, respectively. For ease of discussion, these two films are respectively labeled as the large and small particle film to indicate the film annealed at the higher and lower temperature. For the H2 reaction characterization studies, sections of the large and small particle samples were cut with a diamond saw and both were placed in a Macor sample holder, allowing for the simultaneous measurement of the LSPR changes as a function of gas exposure and reaction temperature. The Macor sample holder was placed within a quartz flow tube that lies on the optical centerline within a tube furnace, as shown in Figure 1. For this experiment, five concentrations of H2, 0.05, 0.10, 0.20, 0.50, and 1.00 vol %, were admitted to the flow tube in a N2 background for periods of 1 h, followed by 1-h exposures to pure N2 at temperatures of 300, 400, and 500 °C. Each H2/N2 exposure was repeated three times at each temperature, and quantitative measurements were averaged over the three data sets. Gas flow was kept at a constant 2000 sccm using MKS mass flow controllers with both the H2 and N2 passing through Matheson Tri-Gas purifiers capable of removing oxygen, water vapor, and other impurities down to ppb levels. During the exposure tests, transmission spectra were collected using collimated white light that was directed into an Oriel Instruments MS257 spectrograph equipped with a Peltier cooled CCD detector. The CCD detector allowed for the collection of multiple spectra from defined regions of interest on the image of the
Figure 1. Schematic illustration of the experimental setup used for conducting gas exposures.
Figure 2. Example of the LSPR absorption spectra acquired during the experiment and the corresponding Lorentzian fits. The inset illustrates the ∼35 nm peak shift upon gas exchange.
samples. In this case, the regions of interest were the large and small particle films along with the uncoated sapphire substrate for use as a reference. The absorption intensity was calculated and recorded as a function of wavelength for each film relative to the flat field, as shown in Figure 2. A Lorentzian fit was made between 550 and 775 nm, allowing for extraction of the LSPR peak position, which was recorded as a function of time to give the sensing signal. Both of the films were characterized using XRD, SEM, and RBS. XRD with a fixed incidence angle (0.5°) and a parallel beam detection geometry was used to determine the average Au particle size throughout the film by analysis of the breadth of the Au{111} and Au{200} peaks using TOPAS software.9 The particular method used was an empirical convolution fit using a reference sample having very large Au particles within a YSZ film in order to obtain the instrument function. This was used to remove the instrument broadening contribution to the peak width. SEM was used on a sample cross-section to measure film thickness and to view the surface of the large and small particle films. Surface Au particles were analyzed with ImageJ software 6284
dx.doi.org/10.1021/jp112228h |J. Phys. Chem. C 2011, 115, 6283–6289
The Journal of Physical Chemistry C
ARTICLE
using automated particle analysis.10 Finally, RBS was performed to quantify the Au content within both films.
’ RESULTS AND DISCUSSION Table 1 summarizes the large and small particle film characterization results. SEM analysis of film thickness from the cross-section was done on films supported on silicon. From SEM images of the annealed films, surface Au particles are clearly visible on the large particle film but not the small particle film, as shown in Figure 3. This is not to say there are no surface Au particles in Figure 3A, but undoubtedly, the higher temperature and longer anneal time for the large particle film, as well as the thickness difference, allowed more Au to diffuse through to the surface. ImageJ particle analysis was performed on SEM images with a total of 287 surface particles to get the average particle size and standard deviation. Particles were assumed to be spherical, since ImageJ calculates the projected area, which must then be converted to particle diameter. Also from Figure 3, areas of separation between YSZ grains are seen on both films. While no measurement of porosity has been made, this will allow gas to access more than just the top surface of the films. In order to get an average particle size of Au throughout the entire film thickness, XRD was used with TOPAS software to analyze the Au{111} and Au{200} peaks relative to a standard sample having very large Au particles within a YSZ film. It is expected that, with larger surface mobilities, the surface Au particles are significantly larger than Au particles incorporated within the large particle YSZ film. These differences in the size
distributions likely result in the calculated size for the large particle film having a large error. XRD results for this film can therefore only be used to show qualitatively that incorporated particles are smaller than surface particles. The average size of the embedded particles in the small particle film was checked using TEM with Z-contrast as shown in Figure 4, resulting in a measured diameter of 12 ( 4.0 nm, within reasonable agreement with XRD. To date, these films have shown repeatable sensing results after approximately 214 h of a variety of H2 in N2 exposures, the majority of which were performed at 500 °C. This does not include 105 h of preliminary experiments at 500 °C prior to the
Table 1. Characterization Results for the Large and Small Particle Films technique large particle small particle film thickness (nm)
SEM
62 ( 6
gold content (atom %)
RBS
3.7
120 ( 12 3.9
surface Au particle diameter (nm) average particle diameter (nm)
SEM TEM
42 ( 15 N/A
N/A 12 ( 4.0
average Au particle diameter (nm)
XRD
25 ( 26
14 ( 6
Figure 4. High-resolution TEM image of the small particle film. Diameter measurements of the darker particles were done on 80 separate particles to get an average size. The lines in the figure are measured diameters.
Figure 3. SEM images of the AuYSZ nanocomposites. (A) Small particle film annealed at ∼920 °C for 1.5 h. No surface Au particles could be positively identified given the resolution of the instrument. (B) Large particle film annealed at ∼970 °C for 3 h showing the formation of surface Au particles with an average diameter of 42 nm determined by ImageJ analysis. 6285
dx.doi.org/10.1021/jp112228h |J. Phys. Chem. C 2011, 115, 6283–6289
The Journal of Physical Chemistry C
ARTICLE
Figure 5. Sensor response as a function of time during H2 exposures. (A) The initial change in peak position squared of the small particle film at 500 °C relative to the gas-on point with the five H2 concentrations overlaid for comparison. Linear fits using the method of initial rates are also shown. (BD) Peak position squared vs time and hydrogen exposure at 500, 400, and 300 °C for both films.
H2N2 titration experiments. The data reported here and summarized with a subset of the acquired data in Figure 5 come from a continuous experiment at the three temperatures, 300, 400, and 500 °C, with simultaneous exposure and data acquisition for both films. The signal being monitored is the SPR peak position squared (Ω2), which can be mathematically described by the Drude model as shown in eq 111 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N0 e2 ð1Þ Ω¼ ð1 þ 2εm Þme ε0 where Ω is the surface plasmon resonant frequency, N0 is the density of conduction electrons, e is the elementary charge, εm is the dielectric function of the matrix, me is the electron mass, and ε0 is the permittivity of free space. From the equation, a change in the plasmon resonant frequency can be attributed to either a change in the dielectric constant or the quasi-free-electron density of the Au nanoparticles. Since YSZ is a well-known O2ion conductor above about 300 °C, any high-temperature reactions that either provide or extract O2- ions will likely change the electron density of the Au nanoparticles in AuYSZ composite
films. This charge exchange process was used as the basic model for previous experimental work2 and is why the data is typically expressed in terms of the LSPR peak position squared. The results reported here, though, are quite different from previous work, because no oxygen is introduced during gas exposure. In fact, both N2 and H2 were purified to reduce oxygen and other impurities down to ppb levels or lower. This means that the signal change is not due to O2- ions moving into or out of the YSZ. As further evidence of this, hydrogen exposures were performed at 100 °C (data not shown here), well below the turn-on point for oxygen ion conductivity in YSZ, and significant signal changes were still seen, albeit with recovery times on the order of several hours. As noted from Figure 5, besides simply observing a signal change, the rate of change also showed a measurable and repeatable response for each set of the three sequential exposures. Therefore, an analysis of these observed reaction kinetics is used for an understanding of the reaction pathway and also as an indication of the possible signal transduction mechanism. In Figure 5A, the change in the peak position squared (eV2) can be seen to follow a three-stage response when plotted as a 6286
dx.doi.org/10.1021/jp112228h |J. Phys. Chem. C 2011, 115, 6283–6289
The Journal of Physical Chemistry C
ARTICLE
Figure 6. Linear relation between the measured rate of signal change and the partial pressure of H2 raised to the one-half power for (A) the small particle film, and (B) the large particle film. The highest concentrations of H2 could not be measured at 300 and 400 °C for the small particle film, due to an insufficient linear region for measurement.
function of time. As soon as hydrogen is turned on, there is an immediate response that is too fast for the system to resolve (data points are approximately 35 s apart), but the magnitude of this initial response is only weakly dependent on H2 concentration at a given temperature. The next stage shows a linear response whose rate shows a clear dependence on H2 concentration. The linear response continues until reaching a point of saturation whose magnitude is nearly independent of H2 concentration above about 0.10%. Separation of the three stages of the response is most apparent for the 500 °C exposures. For all temperatures, the peak-squared rate of change within the linear response region was used within an initial rate kinetics analysis to provide information of the reaction rate. These were measured for each H2 concentration and at each reaction temperature for both the large and small particle films, as the example shows in Figure 5A. At temperatures below 500 °C it becomes harder to distinguish between the different stages of the response, so fitting was done to the most linear portion following the initial immediate response. Fitting was done for three replications at each temperature and the average value was taken as the final result. When the measured rate is plotted against the partial pressure of hydrogen raised to the one-half power, a direct relation is seen for both the small and large particle films at all three temperatures, as shown in parts A and B of Figure 6, respectively. This suggests that hydrogen dissociation is the rate-limiting step, as expressed in the following 1 ðgÞ H2 þ S T HðadÞ 2 1=2 r µ PH2
ð2Þ
where S is an available surface site for adsorption. Since it is the initial part of the reaction that is under analysis and not the equilibrium case, the reverse reaction is presumed to be negligible and the change in the number of adsorption sites is small, so the rate is proportional to the partial pressure of H2 raised to the one-half power. This result is similar to a study done on titaniasupported Au, where hydrogen dissociation was also found to be
rate-limiting.12 By using the reaction shown in 2 as a simple model for the data, there is an assumption that the measured peak-squared is linearly proportional to the amount of adsorbed hydrogen. From eq 1, Ω2 is proportional to 1/εm, and it is not known how εm would change with increasing hydrogen adsorption. However, there is a linear relation with the quasi-free electron density, N0, and as described below, a charge exchange mechanism that could take place from the chemisorption of hydrogen on Au is a potential reaction that would induce this response. Interactions between hydrogen and the YSZ matrix itself are also considered in the following discussion. Hydrogen Reaction with Gold. Typically Au is not considered to be an active catalyst for hydrogen dissociation. While the 1s state of hydrogen can form a strong bond with the 6s state of Au, interaction with the filled d-band of Au results in a filled antibonding state, leading to a repulsive force that tends to dominate. In addition, there is a large overlap between the s and d states that requires wave functions to orthogonalize and thus a rise in potential energy.13 For bulk Au, DFT calculations show that dissociative chemisorption requires an increase in energy to reach a local energy minimum that is not very stable.13 This means the process is activated and will not readily proceed on bulk Au. However, a significant increase in catalytic activity has been well-established for Au in nanoparticle form.1416 This is typically attributed to an increased number of edge and corner adsorption sites and has been found experimentally as well as with theoretical DFT calculations.1720 The dissociation process is usually still an activated one, but with greatly reduced activation energy, as experiments and theoretical calculations have shown.16,21,22 The average Au particle sizes in this experiment are very large compared to the small clusters typically investigated with DFT, but the results should still be relevant to corner and edge sites. Two attributes that would indicate activated adsorption are (1) an increase in coverage with increasing temperature and (2) a slow reaction rate due to the activation barrier. Both of these are seen from the data presented here. The first is apparent from the increasing magnitude of the maximum peak shift upon exposure 6287
dx.doi.org/10.1021/jp112228h |J. Phys. Chem. C 2011, 115, 6283–6289
The Journal of Physical Chemistry C
ARTICLE
Figure 7. Arrhenius plots for (A) the small particle film and (B) the large particle film.
to the same hydrogen concentrations at increasing temperatures, as seen by comparing parts BD of Figure 5. For a metal with no activation barrier, such as platinum, a smaller signal change would be expected for increasing temperatures, assuming the signal correlates with the amount of adsorbed hydrogen. The second attribute is also seen from the data in Figure 5BD because the signal change takes place over an extended period, even over the full 1-h exposure in some cases, indicative of a barrier-induced slow reaction rate. From the literature, experimentally reported activation energies for H2 dissociation on Au nanoparticles typically range between 0.3 and 0.6 eV. Theoretical calculations have a much wider variation due to the structure type chosen for analysis, but the calculated values are typically less than 1 eV. After dissociation of H2, atomic hydrogen is chemisorbed on the surface of Au. Since chemisorption involves a chemical bond, there will be a certain amount of charge transfer during the process, the direction of which depends on the nature of the metaladsorbate interaction.23 This has been studied theoretically for hydrogen adsorption on Au(111), leading to the conclusion that electrons are transferred to the Au upon chemisorption.24 With regard to the work here, if electrons are also transferred to the Au nanoparticles, this will cause a blueshift in the LSPR peak position, as described in eq 1. From inspection of Figure 5, it is apparent that upon H2 exposure a blue shift is observed, lending support to this possible mechanism. This is not the first time this mechanism has been proposed to cause a LSPR peak shift in metal nanoparticles. Sodium and potassium nanoparticles have been studied optically under highvacuum exposures, and it was found that charge transfer to chemisorbed oxygen was the cause of a red shift seen in the LSPR peak upon exposure.25 For these sodium and potassium experiments, changes in the dielectric constant were also taken into account, and it was determined that the change in the free electron density was the cause for the shift in the LSPR peak. Hydrogen Reactions with the Matrix. While it seems likely that H2 would chemisorb on Au, there is also the possibility that it is adsorbing on the YSZ matrix. There have been many examinations of hydrogen interactions with YSZ and undoped ZrO2, both theoretically and experimentally. The general finding is that atomic hydrogen acts as an electron donor in these materials.2629 For ZrO2, the adsorption process may either be homolytic,
producing two OH groups, or heterolytic, producing an OH group and a ZrH hydride, depending on whether the ZrO2 is monoclinic or tetragonal.30 In the case of tetragonal ZrO2, OH groups are stable at least up to 600 °C, whereas the reversible formation of OH groups is expected for the monoclinic form.30 Such a reversible process could be a factor in the observed sensing response if there is a resulting change in the dielectric constant. On the basis of the reports that H acts as donor, electron doping of YSZ by chemical functionalization, whether electrons go to the conduction band or somewhere else, should increase the polarizability and correspondingly the dielectric constant, implying a blue-shift, as was seen upon H2 exposure. However, a reversible change in the peak position would only be expected for monoclinic ZrO2. In addition, only surface hydrogen complexes would be expected at these temperatures with an activation energy that has been calculated to be in the range of 0.280.65 eV, similar to values for dissociation on Au.31 However, the present study uses YSZ having a 5 wt % yttria doping level that serves to stabilize the crystal structure in the tetragonal form. From previous AuYSZ samples prepared in a similar fashion to the ones tested here, it has been found that the YSZ is in the tetragonal form. Thus, if the formation of the H functional groups is not reversible on tetragonal ZrO2 at these operating temperatures, this is an unlikely mechanism for the observed response. Hydrogen Spillover. One more process that may be occurring is the phenomenon known as spillover, where atomic hydrogen will diffuse off the surface of the metal and form complexes with the metal oxide. This process is related to the above case of H2 reacting with the YSZ matrix; however, it first requires dissociation on the Au nanoparticle. Spillover was demonstrated with TiO2 supported Au12 and ZrO2 supported platinum32 both of which used FTIR for analysis. With LSPR analysis, it is difficult to tell if the spillover process is occurring, since a reference film with no Au nanoparticles, as used in the FTIR experiments, would give no LSPR absorption spectrum for signal analysis. Generally speaking, though, ZrO2 is less reducible than TiO2, and spillover becomes more likely as the support becomes more reducible.32 If the spillover step was a rate-limiting first-order process, the measured rate would still be expected to be proportional with PH21/2, since H2 dissociation is a necessary 6288
dx.doi.org/10.1021/jp112228h |J. Phys. Chem. C 2011, 115, 6283–6289
The Journal of Physical Chemistry C first step. This means spillover can not be ruled out, although the mechanism of signal transduction would still be charge exchange and/or changes in the dielectric constant. Arrhenius Analysis. As a final analysis, the apparent activation energy was determined from repeated experiments at three different temperatures. Figure 7 shows the Arrhenius plots for both the large and small particle film at each H2 concentration. The apparent activation energy was found for each concentration of H2 tested, although it is typically not considered as a function of concentration, and the average of these values was used in determining Ea. The results show fairly consistent slopes (and, hence, activation energies) for the different H2 concentrations with Ea = 0.26 ( 0.05 eV for the small particle film and Ea = 0.17 ( 0.02 eV for the large particle film, which are close in magnitude to each other as well as to values found in the literature. However, due to the wide distribution of Au particle sizes within these two samples and variations between the two films, such as film thickness and the amount of surface Au, a direct comparison cannot be made on the basis of particle size.
’ CONCLUSIONS The LSPR of Au nanoparticles embedded in YSZ has been used as a method of monitoring reaction kinetics at temperatures of 300, 400, and 500 °C. Upon H2 exposure in a N2 background, a repeatable reaction rate was found to be proportional to PH21/2, indicating a process dominated by the dissociative adsorption of hydrogen. An explanation for the correlation between hydrogen adsorption and signal change may be based on charge exchange due to chemisorption; however, no measurements were made of the dielectric constant, which is the other possible variable that could affect the sensing response. Spillover of adsorbed atomic hydrogen from Au nanoparticles to the YSZ cannot be ruled out either, but the process would not be expected to affect the signal transduction mechanism. From repeated experiments at 300, 400, and 500 °C, the activation energy was found to be 0.17 ( 0.02 eV for the large particle film and 0.26 ( 0.05 eV for the small particle film; however, due to particle size distributions and differences between the films, a conclusion regarding the effect of particle size cannot be determined from these samples. It is expected that the broader use of the Au LSPR for kinetics analysis of high-temperature reactions relevant either to catalysis or to sensor applications, within metal oxide nanocomposites, can be realized by acquiring the LSPR data at a rate faster than the kinetics and minimization of the gas cycle times through the use of a microchamber reaction environment. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the United States Department of Energy National Energy Technology Laboratory under contract number DE-NT0007918. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the United States Department of Energy National Energy Technology Laboratory. TEM image acquisition by Dr. Tom Murray is gratefully acknowledged.
ARTICLE
’ REFERENCES (1) Rogers, P. H.; Carpenter, M. A. J. Phys. Chem. C 2010, 114, 11033–11039. (2) Rogers, P. H.; Sirinakis, G.; Carpenter, M. A. J. Phys. Chem. C 2008, 112, 6749–6757. (3) Rogers, P. H.; Sirinakis, G.; Carpenter, M. A. J. Phys. Chem. C 2008, 112, 8784–8790. (4) Sirinakis, G.; Siddique, R.; Manning, I.; Rogers, P. H.; Carpenter, M. A. J. Phys. Chem. B 2006, 110, 13508–13511. (5) Myszka, D. G. Curr. Opin. Biotechnol. 1997, 8, 50–57. (6) Morton, T. A.; Myszka, D. G.; Chaiken, I. M. Anal. Biochem. 1995, 227, 176–185. (7) Novo, C.; Funston, A. M.; Mulvaney, P. Nat. Nanotechnol. 2008, 3, 598–602. (8) Larsson, E. M.; Langhammer, C.; Zoric, I.; Kasemo, B. Science 2009, 326, 1091–1094. (9) TOPAS Version 4.2, available from Bruker-AXS, Madison, WI. (10) Rasband, W. S. ImageJ; U.S. National Institutes of Health, Bethesda, MD, http://rsb.info.nih.gov/ij/, 19972009. (11) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (12) Panayotov, D. A.; Yates, J. T. J. Phys. Chem. C 2007, 111, 2959–2964. (13) Hammer, B.; Nørskov, J. K. Nature 1995, 376, 238–240. (14) Claus, P. Appl. Catal., A 2005, 291, 222–229. (15) Haruta, M. Catal. Today 1997, 36, 153–166. (16) Zanella, R.; Louis, C.; Giorgio, S.; Touroude, R. J. Catal. 2004, 223, 328–339. (17) Bus, E.; van Bokhoven, J. A. Phys. Chem. Chem. Phys. 2007, 9, 2894–2902. (18) Lopez, N.; Janssens, T. V. W.; Clausen, B. S.; Xu, Y.; Mavrikakis, M.; Bligaard, T.; Nørskov, J. K. J. Catal. 2004, 223, 232–235. (19) Iida, I. Bull. Chem. Soc. Jpn. 1979, 52, 2858–2862. (20) Corma, A.; Boronat, M.; Gonzalez, S.; Illas, F. Chem. Commun. (Cambridge, U.K.) 2007, 3371–3373. (21) Kang, G.; Chen, Z.; Li, Z.; He, X. J. Chem. Phys. 2009, 130, 034701. (22) Varganov, S. A.; Olson, R. M.; Gordon, M. S.; Mills, G.; Metiu, H. J. Chem. Phys. 2004, 120, 5169–5175. (23) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley-Interscience: New York, 1994. (24) Takagi, S.; Hoshino, J.; Tomono, H.; Tsumuraya, K. J. Phys. Soc. Jpn. 2008, 77, 054705. (25) Iline, A.; Simon, M.; Stietz, F.; Tr€ager, F. Surf. Sci. 1999, 436, 51–62. (26) Kondo, J.; Sakata, Y.; Domen, K.; Maruya, K.; Onishi, T. J. Chem. Soc., Faraday Trans. 1990, 86, 397–401. (27) Mantz, Y. A.; Gemmen, R. S. J. Phys. Chem. C 2010, 114, 8014– 8025. (28) Xiong, K.; Robertson, J.; Clark, S. J. J. Appl. Phys. 2007, 102, 083710. (29) Robertson, J.; Peacock, P. W. Thin Solid Films 2003, 445, 155– 160. (30) Trunschke, A.; Hoang, D. L.; Lieske, H. J. Chem. Soc., Faraday Trans. 1995, 91, 4441–4444. (31) Syzgantseva, O.; Calatayud, M.; Minot, C. J. Phys. Chem. C 2010, 114, 11918–11923. (32) Benvenutti, E. V.; Franken, L.; Moro, C. C.; Davanzo, C. U. Langmuir 1999, 15, 8140–8146.
6289
dx.doi.org/10.1021/jp112228h |J. Phys. Chem. C 2011, 115, 6283–6289
