Reactivity of Gas-Phase Radicals with Organic Surfaces - The Journal

Perspective pubs.acs.org/JPCL

Reactivity of Gas-Phase Radicals with Organic Surfaces David Y. Lee, Natalie A. Kautz, and S. Alex Kandel* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ABSTRACT: In chemical reactions at the gas−surface interface, the heterogeneity in structure of reaction sites plays a critical role in determining surface reactivity. This Perspective describes reaction mechanisms in such systems and details the use of in situ scanning probe microscopy to investigate reactions of gas-phase radicals with selfassembled alkanethiolate monolayers on gold surfaces. For both atomic hydrogen and atomic chlorine reagents, the presence of defects in the alkanethiolate surface order has a substantial influence on what reactions can occur and the speed at which they do so. Data acquired from a series of images were modeled using kinetic Monte Carlo simulations, and a surface radical reaction model was developed to explain the observed evolution of surface structure as the reactions proceed.

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directions and angles. Particularly clever experiments can begin to decompose the ensemble by comparison of different, wellcharacterized surfaces, for example, reactivity as a function of vicinal angle (and thus step density).15,26−28

or the past century, the physical interactions and chemical reactions of gas-phase molecules with solid surfaces have been the topic of experimental and theoretical investigations.1 This chemistry has significant practical implications and applications in combustion, catalysis, materials science, atmospheric chemistry, and astrochemistry.2−14 From a reductionist standpoint, the reaction of an atom or a small molecule with a single-crystal surface is the first fundamental step toward understanding the chemistry at interfaces. Studies of gas−surface interactions have typically focused either on the surface or on the gas-phase reagents and products; a true coincidence experiment, already difficult for unimolecular gas-phase interactions, is not possible for gas−surface systems. When studying adsorption or desorption of gas-phase species, ensemble measurements (which include measurement of sticking coefficients and temperature-programmed desorption) can give insight into how gas−surface interactions are involved in the overall surface chemistry.15,16 Resolution of the thermal ensemble through scattering measurements can yield additional information, through measurements of product angular and energy distributions and the dependence of these on incident angle and energy.17−21 Internal state measurements (and control) of reagents and products are possible with laserbased techniques.22−24 Measurements of the surface can focus on overall surface order (LEED), as well as the elemental and chemical composition of the surface; all of these can be measured both for the clean surface and after any adsorption or reaction has occurred. Various surface science techniques are well summarized in Table 1.1 of the book by Somorjai and Li,25 with a detailed description and primary surface information to be obtained by each technique. Common spectroscopic tools such as AES, SIMS, XPS, IR, and EELS are all ensemble measurements, though unlike ensemble measurements of gasphase species, they average over many surface sites and (depending on the technique) species, as opposed to many © XXXX American Chemical Society

Scanning probe techniques, on the other hand, allow complete resolution of reactions taking place at different sites of the same sample, with images capable of resolving each atomic site and every molecular adsorbate. This potentially confers the ability to study how chemical reactivity varies as a result of surface heterogeneity. Site-dependent surface reactivity can also be studied by spectroscopic methods via direct comparison between two samples with different concentrations of surface defects. Recently, Zhou et al.29 studied surface-site-dependent photodissociation of methanol on stoichiometric and reduced TiO2(110) surfaces with time-dependent photoemission spectroscopy. Their data indicated that the methanol photodissociation rate on the reduced (more defected) TiO2(110) surface was more than an order of magnitude higher than that on the smoother stoichiometric surface. Scanning probe techniques, on the other hand, allow complete resolution of reactions taking place at different sites of the same sample, with Received: August 8, 2013 Accepted: November 14, 2013

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dx.doi.org/10.1021/jz401691w | J. Phys. Chem. Lett. 2013, 4, 4103−4112

The Journal of Physical Chemistry Letters

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STM has allowed our group to examine the effect of radical reactions on SAMs from a perspective that is not accessible via spectroscopic techniques, with a goal of realizing how surface chemical reaction mechanisms and rates are affected by the native morphological inhomogeneities of the self-assembled structures. In order to do this, exactly the same surface region is imaged before, during, and after the reaction with the gas-phase radicals. This allows for a quantitative analysis of any small changes in the surface structure, as well as an unambiguous differentiation of changes due to reaction from native defects and impurities. Series of images, acquired in a time-lapsed, “stop motion” fashion, also allow for statistical inferences about reaction rates and product yields. Reactions are studied using a differentially pumped gas-phase radical source, interfaced to a UHV STM. A schematic of the system is shown in Figure 1.80 Chambers I and II are diffusion-

images capable of resolving each atomic site and every molecular adsorbate. This potentially confers the ability to study how chemical reactivity varies as a result of surface heterogeneity. Using STM, Wang et al.30 were able to monitor the change of coverage of O2 species adjacent to either bridging O vacancies or terminal five-fold-coordinated Ti sites of the TiO2(110) surface as a function of UV light exposure. Self-assembled monolayers (SAMs), and in particular alkanethiolate monolayers on noble metals, are model organic surface systems that offer tractable experimental control of structure and functionality.31,32 Consequently, these organic surfaces have been the subject of a number of gas−surface reaction studies, including experimental investigations by the Morris,20,33−35 McKendrick,36,37 Minton,19,38,39 and Sibener17,40−44 groups and theoretical works by the Schatz,45,46 Troya,47−49 and Hase50−55 groups. These studies have focused on scattering of gas-phase reagents, typically measuring or calculating angle-dependent kinetic energy distributions. Fairbrother and co-workers, meanwhile, have measured the chemical changes that occur on alkanethiolate SAMs during such reactions through a variety of surface-specific experimental techniques.56−59

This Perspective discusses the surface physical and chemical changes that occur upon reaction with gas-phase radicals, with an emphasis on how reaction depends on the heterogeneous surface structure. The structural changes on an organic surface that accompany these gas−surface reactions have been investigated by our group using STM.60−69 This Perspective discusses the surface physical and chemical changes that occur upon reaction with gas-phase radicals, with an emphasis on how reaction depends on the heterogeneous surface structure. Our group has employed a commercial thermal gas cracker (Oxford Scientific) and a home-built high-temperature gas source70 to generate effusive gas-phase radicals. The Fairbrother group has used a similar pyrolytic source to generate H, O, and Cl radicals.56−59 In addition, other methods have been utilized to generate gasphase radicals to study gas−surface reactions; for example, Cl• and O• have been generated by the Minton group using a radio frequency discharge source,19,38,39 and H•, O•, and OH• have been produced by the Buntin and McKendrick groups using laser photolysis.36,37,71,72 Alkanethiolate SAMs provide significant heterogeneity in their structure; while the close-packed phases are crystalline, the SAMs as prepared are invariably highly defected.73 This is a direct consequence of the large number of additional degrees of freedom added by an organic adsorbate. Most notably, the alkane chain is usually angled ∼30° from the surface normal; while neighboring alkane chains tend to align to form small, ordered domains, different tilt angles between neighboring domains result in boundary defects and a polycrystalline surface. In addition, alternate phases with lower packing density can occur between two densely packed regions,74−77 and other surface defects, such as substrate vacancy islands and adsorbate molecular vacancies,78,79 are common.

Figure 1. Schematic diagrams of (a) the gas UHV STM chamber and (b) the custom-designed STM, which provides easy access to the tip− sample junction for the gas molecules.

pumped to accommodate high gas loads as well as to handle gases (especially He) that are difficult to pump with other pumping technologies. Chamber I houses a heated capillary gas cracker (Oxford Scientific OS-Crack) to produce atomic hydrogen; in past experiments, it has also been equipped with a pulsed and skimmed supersonic molecular beam. Chamber II is outfitted with a pyrolytic source (not shown in the figure) based upon a graphite furnace70 that we recently designed to generate atomic chlorine; for this source, the low 4104



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Figure 2. XPS data showing the changes in surface carbon and sulfur coverage as alkanethiolate SAMs are exposed to atomic hydrogen. Nonanethiol monolayers (top) show concomitant decreases in carbon and sulfur, indicating a dominant role for reaction 1. Hexadecanethiolate monolayers, in contrast, lose all sulfur before any significant decrease in carbon, suggesting carbon chain cross-linking after reaction 2. (From Fairbrother and coworkers, ref 59.)

negatively in hydrocarbon film deposition.81 However, it has also been shown that polymer-like hydrocarbon films can be formed by employing radical sources of atomic hydrogen and methyl radicals.82 Fairbrother and co-workers used X-ray photoelectron spectroscopy (XPS) to study the reaction between H• and alkanethiolate SAMs of various chain lengths.59 Their observation of concerted sulfur and carbon desorption for short-chain SAMs (up to 12 carbons) clearly indicated that the attack of H• at the film/substrate interface

gas load obviates the need for additional pumping in chamber I . Chamber II also includes a mechanical shutter to control radical dosing periods. Chamber III separates the gas sources from the STM chamber and includes a gate valve, entrance and exit apertures, and a small ion pump. Chamber III also incorporates an edge-welded bellows for vibrational isolation of the STM from the source chambers. The microscope (shown in Figure 1b) is suspended inside of the STM chamber, which is maintained at a base pressure below 10−10 Torr by a 500 L s−1 diode ion pump, with the sample positioned at the same height as the gas beams and ∼25 cm from the radical source. This open-design STM can accommodate gas-phase reactants in the form of either a supersonic beam or a simple effusive flux. The tunneling junction is indicated by the arrow in the middle of Figure 1b, and the microscope is depicted so that the direction of the incoming reactant flux is from the reader’s eyes into the figure. During each dose, the tip is retracted a few μm from the sample and laterally moved ∼300 nm from the imaging spot to avoid interference with the gas flux. Atomic Hydrogen Interaction with SAMs. Atomic hydrogen is a major unavoidable byproduct in generating hydrocarbon plasmas, and thus, its surface eroding effect contributes

H• + R−S−Au → R−S−H ↑ + Au

(1)

was the main process with ΔH = −197 kJ/mol. For SAMs of longer chain lengths (C16 and C18 in their study), sulfur mainly desorbed as H2S, followed by cross-linking and erosion of the hydrocarbon chains on the surface. This observation was attributed to the inability of H• to directly reach the RS−Au bond. As the consequence, then, the reaction 0

H• + HR−S−Au → •R−S−Au + H 2↑

(2)

became increasingly important. The abstraction of hydrogen from either a terminal methyl or from one of the methylene groups in the alkane chain creates a reactive surface radical, 4105



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•

R−S−Au, in the SAM, leading to new C−C bond formation and creating a cross-linked network of hydrocarbons on the surface. Figure 2 shows the XPS data that demonstrates the difference between short-chain (top) and long-chain (bottom) thiols, along with schematic diagrams describing H• reactions on SAMs of different chain lengths. Because access to the RS−Au bond is a determining factor in the reaction mechanism, we would expect the reaction mechanism would also depend on the local surface structure. In particular, surface defects that expose the sulfur or the lower part of the alkane chain should facilitate sulfur hydrogenation and chain desorption. There are complicating factors, however, as chain desorption will create additional defects that can then further modify reactivity. Initial results at low H-atom exposure are shown in Figure 3, with increasing reaction time resulting in

Figure 4. Superimposition of a series of STM images from a single Hatom bombardment on an octanethiolate SAM experiment. The extent of initial monolayer coverage of the surface is shown in blue, and this is overlaid with green, yellow, and finally white to show the decrease in coverage as well as the movement of defects and domain boundaries as the reaction progresses. See the text for details (ref 67).

Figure 3. Consecutive 308 × 288 Å2 images of low-flux H-atom bombardment on octanethiolate SAMs illustrating the annealing and reordering movements (reprinted from ref 63, Copyright 2007, with permission from Elsevier).

dark featureslikely due to alkanethiolate desorptionthat indeed aggregate along domain boundaries 1−3.63 We note that by imaging the exact same area throughout the entire experiment, changes due to reaction (dark spots near the stripe phases) can be easily isolated from the original defects (dark areas at the top of the first frame).

entire degradation process is illustrated by overlaying sequential images on top of one another, with the edge of each image color-coded to show time progression, unreacted = blue → dark green → green → yellow →white. A quantitative analysis of reactivities at the edges of defect sites and in the close-packed areas is based in Figure 5, which shows the monolayer coverage of each image in Figure 4 plotted against its total H• exposure time. The rate of monolayer degradation accelerates as the reaction proceeds, resulting in a decay curve with a distinct concave-downward shape that cannot be fit to either linear or exponential functional forms. We modeled the reaction using kinetic Monte Carlo (kMC), which treats the reaction phenomenologically in order to describe overall behavior either for large systems or for long time scales.84 The kMC procedure used was developed using MATLAB specifically for the radical gas−surface reactions on alkanethiolate SAMs, and details can be found in ref 67. Figure 5 displays kMC simulations of monolayer coverage versus time; the single adjustable parameter was the factor by which defected sites increase their reactivity compared to close-packed sites. This defect versus close-packed reactivity ratio was varied between 1 and 8100, and those 500 or higher are highlighted in red. The excellent agreement with the experimental data is consistent with the vast majority of reaction occurring at monolayer defect sites. Initially, the number of defect sites is relatively low, and the overall reaction proceeds slowly. Then, the creation of new defects by alkanethiolate desorption further promotes reaction, leading to the observed acceleration. The small size of H• suggests that it should penetrate relatively deeply into an octanethiol film, and this was the model presented in ref 59 to explain the major role played by reaction 1 for short-chain alkanethiolates. This suggests that the reaction rate should be relatively uniform across all surface sites,

By imaging the exact same area throughout the entire experiment, changes due to reaction can be easily isolated from the original defects. There are other significant changes in film morphology, however. In addition to the bright spots that appear (which are likely due to the release of Au adatoms that accompanies thiolate desorption64,65), there is reorganization of close-packed regions, with annealing occurring across the striped phases from frame (b) to (c). This is similar to previously observed surface annealing of the octanethiolate SAM during rare gas bombardments.60−62 In this case, the movement of thiolate molecules observed in Figure 3 likely results from diffusion and readsorption after reaction 1 takes place. Additionally, desorption decreases the packing density, and at lower coverages, alkanethiolates are known to be mobile.74−76,83 Figure 3 shows the majority of structural changes near extant domain boundaries, suggesting that molecules near these defects react more readily with hydrogen atoms than those at close-packed regions. Experiments that extend the reaction time confirm this, and Figure 4 shows that the initial domain boundaries (depicted by the dotted white lines) are nucleation points from which the monolayer erodes.67 In this figure, the 4106



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For H•, the reactions start slowly due to the limited number of defect sites. For Cl•, the reaction is more likely to occur at any close-packed site, and the initial reaction rate is correspondingly high.

Figure 5. Decrease in monolayer coverage as a function of H-atom exposure. The symbols (○, ●, and □) show three experimental data sets, and results of the kMC simulations are plotted as solid lines. Gray lines are for defect to close-packed reactivity ratios under 500, while red lines (which best fit the functional form of the experimental data) are for reactivity ratios in excess of 500.

which is the opposite of what we observe in STM measurements. One possibility that resolves this apparent disagreement is that reaction is relatively uniform but that desorption preferentially occurs at defect sites, with alkanethiol products formed within close-packed regions simply readsorbing. Our current experiments cannot distinguish between reaction, desorption, and readsorbtion as all are likely to be very fast processes relative to STM image acquisition (which typically takes several minutes per image). Atomic Chlorine. It is particularly interesting to compare the reaction of atomic hydrogen with that of atomic chlorine. Because the HCl and H2 bonds are nearly isoenergetic, the hydrogen abstraction reactions are potentially very similar Cl• + HR−S−AuAu → •R−S−Au + HCl↑

Figure 6. Symbols again show experimental measurements of the close-packed monolayer coverage as a function of Cl• exposure time. The kMC simulations here assume that close-packed regions are more reactive than defected ones, with reaction rate ratios of 1−5 (gray curves) and 5−10000 (red curves).69 (Reproduced by permission of the PCCP Owner Societies.)

proceeds. The kMC simulations fit the data only when molecules at close-packed regions are more reactive than molecules at defects. The curves shown are for simulations with close-packed versus defect reaction rate ratios between 1 and 10000, with good fits (red curves) when close-packed regions are more than 5 times more reactive than defects. This explains the observed rate data, with defects created by reaction acting to slow down further reaction of surrounding molecules. Qualitative analysis of STM images also shows that initial reaction with Cl• results in more new defects within closepacked areas than at domain boundaries; this is shown in Figure 1 of ref 68. The reaction of atomic chlorine on surface hydrocarbon chains has been investigated also by the Fairbrother group using in situ XPS.57 Their observation suggests that after eq 3 takes place, further attack of Cl• would produce surface species like −Ċ Cl− and −CCl2−, evaporate more gas-phase HCl, and be possibly followed by secondary processes such as crosslinking between the surface radicals. We made ex situ XPS measurements on reacted monolayers to get an approximate comparison of our chlorine-atom exposure compared to that of ref 57, and we found that under our experimental conditions, significant disorder occurs in STM images before any appreciable increase in chlorine or decrease in sulfur in the monolayer. However, an increase in intensity of the Cl(2p)

(3)

Chlorine, however, has a much larger (55 pm) van der Waals radius (rw) than hydrogen,85 and an atom incident upon the surface in a region where the alkanethiol chains are closepacked will have little access to the surface-bound sulfur. A similar effect has been observed experimentally as oxygen atoms (with rw 23 pm smaller than Cl) cannot penetrate deeper beyond the seventh methylene group in a C12 alkanethiol SAM.37 Steric factors, then, should shut down the reaction pathway of reaction 2, a process that is further disfavored because the S−Cl bond is significantly weaker than the S−H bond. A plot similar to Figure 5 was generated for longer attack of atomic chlorine following the same procedure described above and is shown in Figure 6.69 In this figure, the abscissae of three data sets are scaled to fit into one kMC simulation (kMC units of time are arbitrary and need to be fit to the actual reaction time scale in seconds; the fitting can be done equivalently in reverse). Qualitatively, the coverage versus time behavior is nearly opposite to that observed for H-atom reaction; the data are subexponential, with a rate that decreases as the reaction 4107



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Figure 7. Direct comparison between H-atom and Cl-atom interactions with an octanethiol monolayer. An area of the SAM after reaction with H• is shown in (a), and the overall coverage-overexposure behavior is illustrated in (b), with experimental data drawn as black dots and kMC results plotted as a red line. Similarly, results from Cl• reactions are presented in (c) and (d). (a) 950 Å × 950 Å. (c) 870 Å × 820 Å.

reaction proceeds, however, the close-packed areas degrade, increasing the ratio of defected to undefected sites and resulting in a subexponential decay for coverage versus exposure. The cartoon in Figure 8 compares and contrasts reaction mechanisms for H-atom and Cl-atom reactions with alkanethiolate monolayers. For octanethiolate, the relatively short hydrocarbon chain means that the hydrogenation/ desorption reaction pathway dominates. While incident Hatoms have access to every sulfur, a SH reaction product is significantly more likely to desorb when it is formed near a defect site, while readsorption is the typical outcome for thiols generated within close-packed domains. This is depicted in panels (f) and (g). Hydrogen abstraction (panel (a)) is undoubtedly occurring as well, but the surface-bound alkyl radicals (panel (b)) will encounter another H-atom before they can participate in further surface reactions. The ability of Hatoms to penetrate the monolayer, then, is also responsible for quenching any further reactivity as alkyl radicals react to form alkane chains. Chlorine atoms, meanwhile, do not have the ability to penetrate into the molecule, and hydrogen abstraction will be confined to defect sites and to hydrogens on terminal CH3 groups in close-packed areas. This, by itself, is not sufficient to explain the differences between chlorine atom and hydrogen atom reactions as the radicals formed in this process are equally likely to be quenched (forming chlorinated species) by incident Cl and, potentially, Cl2. Panel (d) shows the critical difference, however; primary radicals formed at terminal CH3 groups can undergo further intra- and intermolecular reactions that transfer the radicals to secondary sites, effectively burying them within

XPS spectrum was observed in the end of our typical experiment.69 We have also monitored the octanehiolate SAM after dosing with high flux of Cl• and found that initial domain boundaries were no longer observable and only a small portion of close-packed area remained,68 as in agreement with the XPS measurements by the Fairbrother group.57 Combined, the data imply that the observed loss of monolayer order is primarily due to alkane chain cross-linking instead of desorption. A comparison of the results presented here for Cl-atom reaction to our earlier results for H-atom reaction with octanethiolate monolayers is shown in Figure 7; panels (a) and (b) show an image and the coverage versus time behavior for H-atom reaction, while panels (c) and (d) show the results for the Cl-atom reaction. The most obvious difference between the two STM images is the loss of monolayer coverage; in panel (a), more than 50% of the monolayer has desorbed after hydrogen atom exposure, while in panel (c) the chlorineexposed monolayer remains largely on the surface. In addition, the remaining ordered monolayer areas in (a) are relatively large and compact, while the image in (c) shows defects and vacancies that wind and branch in a convoluted pattern between small ordered areas. Panels (b) and (d) show the vastly different reaction kinetics observed for H• and Cl•. For H•, the reactions start slowly due to the limited number of defect sites. Progression of the reaction, however, creates more defect sites, which accelerates the rate of the reaction; this leads to a concave-down shape for the coverage versus exposure plot. For Cl•, the reaction is more likely to occur at any close-packed site, and the initial reaction rate is correspondingly high. As the 4108



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Figure 8. Cartoon illustrating the differences in the reaction mechanism for H-atom and Cl-atom reactions with octanethiolate monolayers. For the H-atom reaction, hydrogenation/desorption is shown in (a) and (b), while the hydrogen abstraction reaction is depicted in (f) and (g). Cl-atom hydrogen abstraction that leads to monolayer cross-linking is depicted in (c−e). See the text for details.

the monolayer where they can no longer react with chlorine. The consequential buildup of radical species within the monolayer then leads to C−C cross-linking and the observed morphological changes in the STM images. As defect sites provide access at secondary radical sites to incident Cl, radicals are quenched at these locations, leaving chlorinated molecules on the defected edges. Overall, reactivity is still enhanced at defect sites, but the reactions involved (hydrogen abstraction plus chlorination) have a relatively minor impact on monolayer order compared to the cross-linking reactions that happen in the close-packed regions.69 Our studies have focused on octanethiolate, which belongs to the short-chain category depicted in the top part of Figure 2, where both our STM measurements and Fairbrother’s XPS studies show that the major reaction pathway is sulfur

Primary radicals formed at terminal CH3 groups can undergo further intra- and intermolecular reactions that transfer the radicals to secondary sites, and the consequential buildup of radical species within the monolayer then leads to C−C cross-linking and the observed morphological changes in the STM images. 4109



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reaction systems described here or others. Vibrational spectroscopy, whether by infrared- or electron-based methods or by using inelastic tunneling measurements is a particularly useful tool and would provide information currently missing from our understanding of these systems. We are also excited about the prospect of thermal desorption mass spectrometry, which could also allow detailed chemical identification of surface reaction products; we have explored such measurements in a preliminary way in our recent work.69 Finally, and perhaps most importantly, detailed chemical dynamics calculations are necessary. Such calculations should work toward determining the effect of surface heterogeneity on reaction rates and outcomes and in tandem should explore how reaction creates additional types of surface heterogeneity. The comparison of theory with multiple experimental results provides the best hope for significantly increasing our understanding of gas− surface chemistry.

hydrogenation followed by desorption. For longer-chain (C18) alkanethiolates, the XPS data of ref 59 indicates that the increased film thickness and the generally better ordering of monolayers act to greatly decrease the rate at which hydrogen atoms can reach the RS−Au bond. This minimizes the importance of the direct desorption reaction pathway, resulting in cross-linked alkanethiolate films with far longer desorption times. Our study of Cl-atom reactivity shows that when the Cl• cannot penetrate into the monolayer, inter- and intramolecular reactions can cause radical sites to diffuse into areas of the monolayer not accessible to the gas-phase reagent. We suggest that H-atom reactions with long-chain alkanethiolates could be affected similarly; limited penetration of H-atoms into the monolayer will also reduce quenching of alkyl radicals, allowing radicals to accumulate and thus promoting radical−radical reactions (especially cross-linking) at deeply buried carbons. Seen in this light, increased monolayer thickness leads to crosslinking not only because it inhibits attack at the S−Au bond but also because it inhibits quenching reactions as well. We would expect that both of these behaviors would be reversed near extant monolayer defects as the geometry at these defects will allow both hydrogenation of the sulfur and quenching of radicals all along the alkyl chain. The Sibener group has demonstrated that lowering the temperature from RT to 150 K improves ordering in long-chain C16 SAMs and reduces the penetration depth of gas-phase oxygen but has little effect on a C12 SAM.44 Surface temperature, then, provides another means to vary the depth at which gas−surface interactions can occur, potentially in a more controlled, finely tunable fashion. We are currently constructing a variable-temperature STM, and the exploration of such effects for Cl-atom and H-atom reactions could significantly improve our understanding of these systems. Prognosis for Future Research. The role of surface heterogeneity in gas−surface radical reactions has been the focus of this Perspective, and there are broad avenues of future research open. In most cases, the best insight into the chemistry of these systems results from combining experimental techniques. Scattering experiments, for example, allow the gas-phase reagents and products to be investigated, while surface spectroscopic methods allow for the chemical identity of surface species to be resolved, and scanning-probe measurements are necessary to understand the ways in which surface structure influences reactivity. Separate investigations by different groups using different techniques, both experimental and computational, will drive progress on this research topic. Given the complexity of chemistry at the gas−surface interface, there are a large number of variables that can be controlled and a correspondingly large number of future research directions to be explored. Relatively straightforward extensions of the research described here include varying the gas-phase reactant, possibly to compare hydrogen and chlorine atoms to oxygen and bromine atoms or even hydroxyl and methyl radicals. There is also a rich chemistry of alkanethiolate monolayers to take advantage of, varying the surface chemical identity and morphology through the use of functionalized alkanethiolates, as well as alternate preparation methods for single- and multiple-component monolayers. The identity of the monolayer−surface linker is also a promising target for comparative studies. The depth of any particular study could be enhanced in a number of ways, and we believe that more chemical probes of surface composition will provide necessary insight into the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies David Y. Lee is a Postdoctoral Research Fellow in the Department of Chemistry and Biochemistry at the University of Notre Dame. He received his Ph.D. from Cornell University in August 2011 studying velocity map imaging of gas-phase reaction dynamics. His current research interests involve development of novel STM instrumentation and STM investigation of gas−surface interactions. Natalie A. Kautz received her Ph.D. from the University of Notre Dame in August 2010 studying hydrogen reactions with alkanethiol SAMs using STM. She is currently a Postdoctoral Scholar in the Department of Chemistry and the James Franck Institute at The University of Chicago. Her current work probes fundamental chemical reactions on niobium substrates used in particle accelerators. S. Alex Kandel is an Associate Professor in the Department of Chemistry and Biochemistry at the University of Notre Dame. Before joining Notre Dame’s faculty in 2001, he did postdoctoral work with Paul Weiss at the Pennsylvania State University, and he received his Ph.D. in 1999 from Stanford with Richard N. Zare. His research group’s website, which contains a searchable database of over 50 000 STM images, is at http://graphite.chemistry.nd.edu/.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF Grant CHE-0848415).

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REFERENCES

(1) Richardson, L. B.; Woodhouse, J. C. The Adsorption of Mixed Gases by Charcoal. I. Carbon Dioxide and Nitrous Oxide. J. Am. Chem. Soc. 1923, 45, 2638−2653. (2) Vlachos, D. G. Homogeneous−Heterogeneous Oxidation Reactions over Platinum and Inert Surfaces. Chem. Eng. Sci. 1996, 51, 2429−2438. (3) Veser, G.; Schmidt, L. D. Ignition and Extinction in the Catalytic Oxidation of Hydrocarbons over Platinum. AIChE J. 1996, 42, 1077− 1087. (4) Veser, G.; Ziauddin, M.; Schmidt, L. D. Ignition in Alkane Oxidation on Noble-Metal Catalysts. Catal. Today 1999, 47, 219−228.

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