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Structure and reactions of carbon and hydrogen on Ru(0001): A scanning tunneling microscopy study
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View: Figures


Image of FIG. 1.
FIG. 1.

STM images of Ru(0001) acquired at . (a) Surface containing approximately 0.03 ML of C prepared by segregation from the bulk. The C atoms appear as depressions (black spots). (b) After introducing H atoms (from water dissociation in this experiment), C is converted to CH (bright protrusion surrounded by a dark ring). A similar transformation occurs with H obtained from dissociation. Individual nonreacted H atoms appear as smaller dark spots. Tunneling condition in (a) is and , and (b) and . The total scale is adjusted to be 50 pm in both images.

Image of FIG. 2.
FIG. 2.

STM image acquired at 6 K (, , and ) after introducing H atoms from water dissociation followed by heating to 180 K. Oxygen atoms (dark circular depressions forming small aggregates), CH, H, and complexes can be observed. The area enclosed within the broken line had been previously scanned at higher bias voltages that caused the displacement of most of the H atoms away from the region.

Image of FIG. 3.
FIG. 3.

STM image at 6 K (, and ) showing C, H, CH, and complexes on Ru(0001). The surface was prepared by dosing at 150 K on the Ru(0001) surface containing 0.03 ML of C, followed by annealing to 200 K. Small dots indicate the position of the Ru atoms. Two circles mark the position of (top) and (bottom) complexes. The few O atoms visible at the bottom and top left of the image originated from water dissociation. All species reside on hcp hollow sites, except H which occupies fcc sites. Notice also the dark lines due to H atoms aligned in the compact crystallographic directions near the bottom left and center of the image, connecting complexes.

Image of FIG. 4.
FIG. 4.

STM images at 6 K (, , and ) acquired before (a) and after (b) CH dissociation induced by applying a voltage ramp from 0 to 0.5 V with an initial set point of 10 mV and 50 pA with the tip stationed over the CH species marked by the arrow in (a). The CH species is transformed to C and the dissociated H has been displaced away from the image by the high electric field from the tip during the voltage ramp.

Image of FIG. 5.
FIG. 5.

Log-log plot of the CH dissociation rate vs tunneling current at the time of event (during the pulse). The bias voltage was kept at 475 mV. From the slope of the plot, the rate was found to be proportional to . This indicates that two electrons are necessary to excite two quanta of C–H stretch oscillation, which results in the rupture of the bond.

Image of FIG. 6.
FIG. 6.

[(a)–(c)] Enlarged STM images (, , and ) showing three complexes and (d) CH. The grayscale of all four images is adjusted to be 90 pm for easier comparison of contrast. [(e)–(h)] Schematic models of the geometrical configuration employed to simulate the experimental images. All geometries were optimized by energy minimization in DFT calculations, as explained in the text. [(i)–(l)] Simulated STM images for each optimized model employing a W tip (tunneling conditions set to and ).

Image of FIG. 7.
FIG. 7.

q(a) STM image (, , and ) extracted from Fig. 3 containing a linear structure of H atoms (vertical dark band connecting two complexes). The feature in the center is due to a CH species, not H. The location of the Ru atoms is indicated by the small superimposed light gray (yellow) dots. (b) Simulated STM image corresponding to the schematic geometrical configuration in (c). Black dots in (b) indicate the site of H and CH.

Image of FIG. 8.
FIG. 8.

(a) Graph of total energy of the system calculated by DFT, as a function of the CH–H distance . The origin of energy has been set at the configuration of the most stable structure, with H at an fcc site far from the CH. The plotted energy values give the energy difference with respect to this site, with positive values corresponding to less stable arrangements. (b) Sketch of the Ru(0001) surface with the CH adsorbed at the hcp site, corresponding to . The arrows indicate two H trajectories used in the calculations as it approaches the CH from the right and left directions. (c) Solid line: CH–H bond energy in the gas phase , where gives the total energy of species and the superscript stands for the gas phase, as a function of the CH–H distance. Dashed line: CH–H bond energy with both species adsorbed on the Ru surface , where refers to the total energy of the Ru slab with species adsorbed on it and gives the total energy of the bare Ru slab. The formation of a CH–H bond, favorable in the gas phase, is suppressed by the presence of Ru.

Image of FIG. 9.
FIG. 9.

Average adsorption energy per H atom for the systems as a function of the total number of H atoms in the cell. Gray, solid, dashed, and dotted lines correspond to the most stable configurations found containing a complex with , 1, 2, and 3, respectively. Additional data points are for different, less stable configurations. The insets sketch the H arrangement for three different cases satisfying . The linear arrangement of H atoms (top inset) is the most stable of the three.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Structure and reactions of carbon and hydrogen on Ru(0001): A scanning tunneling microscopy study