In the Laboratory
Surface Oxidation Kinetics: A Scanning Tunneling Microscopy Experiment
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Jordan C. Poler Department of Chemistry, University of North Carolina at Charlotte, Charlotte, NC 28223;
[email protected] The study of surfaces and materials is underrepresented in the undergraduate chemistry curriculum. Topics concerning materials science (synthesis, structure, properties, and performance) are too often presented as trivial examples within a general chemistry course and are never revisited. This is unfortunate because chemistry plays such a central role in the development of new materials. The performance of surfaceactive materials (e.g., sensors or catalysts) can be related to microscopic surface structure and kinetic processes. I describe a physical chemistry laboratory experiment that explores chemical kinetics at surfaces. Overview This experiment explores the kinetics of the oxidation of a carbon surface. An interesting aspect of the reaction of an oxidant at the flat surface of carbon is the morphology of the reacted material. Typically, the reaction is initiated at a surface defect and propagated parallel to the surface. A distribution of pits is formed across the surface of the sample. Most of the pits are only one atomic layer deep, which is both instructive and aesthetic. Several concepts are explored from these results: crystallography, surface diffusion, bonding, atomic defects, and reaction kinetics. A vast literature exists for the oxidation of carbon. Reactions of gases with graphite surfaces have been studied by both decorated transmission electron microscopy (TEM) (1, 2) and scanning tunneling microscopy (STM) (3–6 ). Our students use an STM to collect their data. The advantages are that TEM is too expensive and with STM, students can directly measure the depth of the reaction pit. Reaction of various gases can be studied, water vapor and O2 being the easiest to obtain and handle. We chose nitric oxide to stimulate a discussion about the environmental impact of internal combustion engines and catalytic converters (i.e., removal of NOx and SOx ). Before students do the kinetics study, they complete an experiment that introduces them to the STM and the crystal structure of graphite. Graphite is described by the hexagonal crystal lattice where covalently bonded carbon atoms form sp2 hybridized planar sheets. These sheets are the basal planes, stacked upon each other and weakly bound by van der Waals forces. Because of the differences in lattice bonding within and between basal planes, properties such as the electrical and thermal conductivities of graphite are anisotropic. Additionally, reaction rates at the surface of graphite are anisotropic. Reaction of NO(g) with C(graphite) to form CO2(g) and N2(g) is thermodynamically favorable. The kinetic mechanism is complicated and is discussed in detail elsewhere (5). After nitric oxide chemisorbs to the graphite surface, its oxygen atom diffuses across the exposed basal plane. Binding sites for the oxygen atoms are at steps from one basal plane up to the next. Eventually two oxygen atoms are interacting with 1198
one carbon atom at this monatomic step and the resulting CO2(g) desorbs from the surface. Alternatively, the oxygen can react with an sp2 hybridized basal plane carbon atom. This mechanism then requires abstraction of this carbon atom from its three bonded nearest neighbors. Since the latter mechanism has a higher activation energy (and a commensurately smaller rate constant), the removal of carbon from the graphite will be anisotropic. The reaction rate parallel with the basal plane is much faster than the rate perpendicular to the surface, resulting in the formation of circular, monolayerdeep etch pits on the surface of the graphite. The concentration of carbon atoms at the step edge is related to the circumference of the circular pit. Therefore, it is appropriate to measure the reaction rate by measuring the diameter of the etch pit. Since we are concerned only with the activation energy of this process, we do not need to measure absolute reaction rates. Initiation of the reaction occurs at a defect on the surface. When the initiation site is a point defect, monolayer-deep pits are observed. Occasionally multilayer pits, many basal planes deep, are also observed. Typically, these result from initiation at line defects, which propagate perpendicular to the graphite surface. Multilayer pit diameters increase faster than monolayer pit diameters because of increased basal plane exposure to the adsorbed oxidants. Infrequently, homogeneous initiation of the reaction can occur by abstraction of a carbon atom from a defect free basal plane. This mechanism is not easily observed at the temperatures used in this study. Experiment Highly oriented pyrolytic graphite (HOPG) (Union Carbide) was used as the reaction substrate. We used grades ZYB and ZYH. Accurate kinetic data are easier to obtain using the ZYB grade, whereas a more interesting exploration of surface defects is possible when using the ZYH grade (it’s also less expensive). HOPG is very easy to use for surface studies because a new surface is obtained by simply delaminating the top layers of the graphite with adhesive tape. Freshly cleaved surfaces were reacted with a flowing gas of 5–10% NO in N2 (Air Products) in a quartz tube furnace at 500 to 700 °C. The temperature was measured with a quartzencased thermocouple near the sample position within the tube. Students estimated the error in temperature by moving the thermocouple around and by observing the small fluctuation in the furnace temperature as a function of time. Reactions were carried out in our class-100 clean-room. Because nitric oxide and nitrogen dioxide gas can cause deleterious respiratory complications, exposure to inhalation of the gas must be minimized. Since our lab is a clean-room, there is a very large exchange of air. When the reactive gases were vented into the room, their resultant concentration was five orders of magnitude below OSHA’s TLV and 7500 times below the 30-min exposure limit STEL for both NO(g)
Journal of Chemical Education • Vol. 77 No. 9 September 2000 • JChemEd.chem.wisc.edu
In the Laboratory
and NO2(g). In general, these gases should be vented into a fume hood. Each sample was heated at reaction temperature in N2(g) for 20 min. The tube was then flushed with the NO(g)–N2(g) mixture at a high flow rate (ca. 2 L/min) for ca. 30 s. Some of the NO reacts in the tube to form NO2, which absorbs strongly in the visible and appears brown (seen in smog on a hot day). We estimated the NO2 concentration in the tube by a colorimetric method to be ca. 5 torr. The flow was reduced to 0.2 L/min during the reaction. At the end of the reaction, the NO was turned off and pure N2 flushed out the tube (ca. 2 L/min) as the sample was removed quickly from the furnace. Students estimated the error in reaction time due to these concentration transients. After the sample cooled in air, its back side was affixed to a steel disk with graphite paint (SPI supplies) and heated on a hot-plate (80 °C) to drive off the paint’s solvent. After cooling to room temperature the sample was analyzed using STM. All STM measurements were made with a Burleigh Instruments Metris 1000 in air. Tunneling conditions were not critical and were typically successful with a tip bias of ca. 2 V and a tunneling current of ca. 7 nA. Images were recorded in constant-current (topographic) mode in order to measure etch pit depths. STM tips were formed by shearing Pt–Ir (20%) wire (Alfa Aesar). This was the second STM experiment completed by these students. The first was an introduction to STM, where they learned the theory of STM and its variants, studied metrology issues and feedback control, and made their first atomic-scale measurements (i.e., the interplane distance on
Figure 1. Left: STM micrographs of ZYH HOPG surfaces oxidized for 20 min in 10% NO(g). Right: ZYB HOPG surfaces oxidized for 45 min with 5% NO(g). Reaction temperatures were (top left) 550 °C; (bottom left) 650 °C; (top right) 568 °C; and (bottom right) 614 °C. Data were plane-filtered to remove sample tilt. Scale bar represents 2500 Å. Monolayer and multilayer etch pits and basalplane etching at step edges are observed on the ZYH HOPG. Multilayer etch pits were not observed on the ZYB HOPG.
the Au (111) surface and the atomically resolved distance in the HOPG basal plane). During the first STM experiment, students were required to make their own tips. To save time during this experiment the instructor cut the tips for the students. Although lateral atomic resolution is not required to observe these etch pits, a poorly cut tip will result in excessive noise and poorly defined images. There can be several goals for this experiment. Ours was to determine the activation energy of the defect-nucleated basal plane reaction. Alternatively, one could study relative rates of various reactants, or different sets of groups could study the reaction mechanism by varying the oxidant concentration. For our experiment, each student studied a separate reaction temperature. Individual students prepared and oxidized their own sample and performed their own STM measurements and analysis. To save time, we assumed that the reaction rate was time invariant. Therefore, we could calculate a rate from just one oxidation. We did not observe any etch pits on the graphite for the control experiments in pure N2(g). All samples were reacted for 20–45 min. Each student needed about two hours of lab time (out of a sixhour allotment), to acquire images of the surface. This was enough time to find about 5–15 acceptable etch pits for analysis. Students then submitted (via email) a preliminary analysis of their data (Excel workbook) to the instructor. These data were then distributed to the entire class for an Arrhenius analysis. Results and Discussion Typical STM images of the reacted HOPG surface are shown in Figure 1. Etch pits are never seen on freshly cleaved HOPG. Some of the pits appear to be slightly elliptical, not circular. This is most likely a thermal drift artifact and is typically resolved by rotating the scan direction. When obtaining an STM image the tip is scanned rapidly in the x-coordinate, but slowly in the y-coordinate. Therefore, thermal drift of the sample relative to the tip results in expansion or compression of the y-coordinate dimension of an object. Accurate results are obtained by using the xcoordinate cross section of the pit. The rate of removing carbon from a monolayer-deep pit is proportional to the number of exposed basal plane carbon atoms around the circumference of the etch pit. Therefore, this reaction rate is proportional to the diameter of the etch pit. Since the concentration of the carbon substrate and the gaseous oxidant are constant throughout the reaction, this rate can be related to the kinetic rate constant for this process. Consequently, the rate of change of the pit diameter is directly proportional to the reaction rate constant. Diameters of the pits were measured and their distribution analyzed. The average pit diameter was divided by the reaction time to give a relative rate constant at a specific temperature. Standard errors in the average pit diameter and the estimated error in reaction time were propagated to give the error (95% confidence) in the rate constant. When using the lower grade of ZYH graphite, at higher reaction temperatures, the distribution in etch pit diameters appeared to be bimodal. Multilayer pits do etch at a faster rate than monolayer pits (5). Moreover, lower grade HOPG has a higher defect density and is more prone to multilayer
JChemEd.chem.wisc.edu • Vol. 77 No. 9 September 2000 • Journal of Chemical Education
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In the Laboratory
pit formation (6 ). A second group of students used the higher grade ZYB HOPG and did not observe multilayer etching. The distribution in etch pit diameter using the ZYB graphite was Gaussian for all temperatures and the calculated activation energy was more accurate. Figure 2 shows the Arrhenius plot of these data for the ZYB HOPG. A linear regression analysis yielded the slope and estimated standard error of the slope. From this the activation energy for this process and the propagated uncertainty was determined to be 95 ± 7 kJ/mol (95% confidence). The literature value for monolayer etching of 91 ± 3 kJ/mol (5) is within our experimental uncertainty. A separate group of students did this study on the ZYH HOPG and obtained an activation energy of 81 ± 25 kJ/mol. Conclusion This lab was well received by the students. Results from a course evaluation survey showed that many of them thought that this was the lab from which they learned the most and that provided the greatest challenge to their intellectual and problem-solving skills. This experiment was successfully performed for two consecutive years. In the second year we used the higher grade ZYB HOPG and allowed more time for data acquisition. This lowered our uncertainties and yielded a more accurate analysis of the activation energy. In general, students like to reproduce the literature value of a result. However, I believe it is also instructive when the experiment does not produce the expected result. The first group used the lower grade ZYH HOPG and had to deal with the inclusion of various pit depths and wide-ranging pit diameters in their analysis. In trying to rationalize these data they hypothesized a more varied model. They proposed other experiments to test their hypotheses. I believe the “poorer” data from the first group’s experiment taught them more about the scientific process, whereas the “better” data from the second set only taught them about the experiment and the chemistry. I prefer the former for students who are scientifically mature enough. In general, I find it difficult to simulate a research experience in the teaching lab under our self-imposed time constraints. However, I believe that the open-ended nature of both the experiment and the data analysis will result in a better-educated and more productive scientist.
1200
ln[rate / (nm/min)]
3
2
Ea = 95 ± 7 kJ/mol 1
0 1.1 0
1.1 5
1.2 0
1.2 5
1.3 0
1.3 5
(1/T ) / K-1 Figure 2. Arrhenius plot for the oxidation of ZYB HOPG by 5% NO(g).
Acknowledgments I would like to acknowledge the financial support of the college of Arts and Science at the University of North Carolina at Charlotte and an ILI grant from the National Science Foundation (DUE-9751183). I also thank the undergraduate students who worked through this lab and acquired most of the data: Veronica R Bossert, Natalie R. Carrol, Leo Darnell Ferguson Jr., David Andrew Nicewicz, Peter Ivo O’Daniel, Nathaniel J. Read, Mark A. Reyes, Brent Harper Sellers, Michael F. Streeter, and Yancie Weathers. W
Supplemental Material
Supplemental material for this article is available in this issue of JCE Online. Literature Cited 1. Yang, R. T.; Yang, K. L. Carbon 1985, 23, 537–547. 2. Duan, R.-Z.; Yang, R. T. Chem. Eng. Sci. 1984, 39, 795–798. 3. Chang, H.; Bard, A. J. J. Am. Chem. Soc. 1990, 112, 4598– 4599. 4. Chu, X.; Schmidt, L. D. Carbon 1991, 29, 1251–1255. 5. Chu, X.; Schmidt, L. D. Surf. Sci. 1992, 268, 325–332. 6. Stevens, F.; Kolodny, L. A.; Beebe, T. P. J. Phys. Chem. 1998, 102, 10799–10804.
Journal of Chemical Education • Vol. 77 No. 9 September 2000 • JChemEd.chem.wisc.edu
