AD-W12 009 THE INFRARED SPECTRA OF SURFACE METAL ATOM YIIRATIONS i/1SNIFTIRS STUDIES IN..(U) UTAH UNIV SALT LAKE CITY DEPTOF CHEMISTRY S PONS ET AL. 38 JUL 86 TR-59

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The Infrared Spectra of Surface Metal Atom Vibrations.SNIFTIRS Studies in the Far Infrared Region using

Time Resolved FTIR Techniques

By

Stanley Pons, J. Li, J. Daschbach, J. Smith, M. Morse

Prepared for Publication in

Journal of Electroanalytical Chemistry D T ICUniversity of Utah APR 1 4 188

Department of Chemistry

Salt Lake City, Utah 84112 SJuly 30, 1986 .

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5. TYPE OF REPORT 6I PEntoo COVERtEDe'Id b"aIed spectra of Surface Metal Ato

Vibrations. SN4IFTIRS Studies in the Far [nt'rare, Technical Report# *

Region using Time Resolved FTIR TechniquesI416 PERFORMING OAG. REPORT mNuADUER

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Stanley Pons, J. Li, J. Daschbach. J. Smith. M 14OO0l4-83-K-O47O-POO03Morse

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Aspectroeiectrochemistry Time Resolved Spectroscopy. Metal Atom Spectros~t.i)y

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IR spectra of metal atoms on electrodes is discussed.

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Short communication

THE INFRARED SPECTRA OF SURFACE METAL ATOM VIBRATIONS

SNIFTIRS STUDIES IN THE FAR INFRARED REGION USING TIMERESOLVED FTIR TECHNIQUES

JIANGUO LI. JOHN DfSCHBACH. JERRY J. SMITH *. MICHAEL D. MORSEand STANLEY PONS "*

epameof Chenusrq. Un vesty of Uwa. Salt Lake Ci. UT 84112 (U.S.A.)

(Rad 16th April 1986: in revised form 23th May 1986)

Studies of the vibrations of pure metals have historically been pursued byinelastic neutron scattering (which is sensitive to vibrations of the bulk crystal) andmore recently by high-resolution electron energy loss spectroscopy. HREELS (whichis more sensitive to vibrations of the metal surface). Direct infrared absorbancetechniques provide advantages over both of these methods. since they may be usedin more hostile environments (such as at the interface between metal and chemicaisolutions). They have rarely been applied with much success, however, because ofthe very effective shieldihg of the electromagnetic radiation by the metal conductionclectrons. This results in an exponential damping of the radiation field as it entersthe metal phase. with typical skin depths (1/. damping distances) of only a few tensof nm. This damping improves the sensitivity of infrared absorption measurementsto the surface vibrations as opposed to bulk phonons. but limits the magnitude ofthe absorbance considerably. In this report we demonstrate that reflection infraredvibrational spectroscopy may be used to observe the vibrational structure of metallicspecies deposited on a metal surface which is under electrochemical control.

The surface FTIR spectroscopic technique SNIFTIRS [1] has been shown to beuseful for the observation of the vibrational structure of monlayer (or less) quanti-ties of materials adsorbed at the surface of metal electrodes while under electro-chemical control. We have now extended the method by modifying the cell anddetector design to permit observations in the far infrared region, even below 100cmn- 1. In addition, a signal/timer/controller/sequencer and associated software hasbeen developed to allow time resolved infrared spectral measurements to berecorded with 10 ps resolution (2].

* Permanent address: Naval Weapons Center. Physics Division, China Lake CA 93555. U.S.A." To whom conrsporndene should be addressed.

0022.0728/96/S03.50 -C 1986 Elsevier Sequota S.A.

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Fig. 1. Surface far infrared difference spectrum of the systm described on 130-170 cm - rqson as afunction of electrode potential. The curves represmt potentials of (top to botom) - 1.70 V. - 1.90 V.- 2.60 V. and - 2.90 V respectively. Refernce potential - 1.50 V.

Underpotential. solid solution, alloy formation, and bulk metal deposition stud-ies were made for lithium deposited on gold from acetonitrile solution. At potentialsbetween - 2.40 and - 2.60 V, lithium adatoms are underpotentially deposited on apolycrystalline Sold surface where they are oxidized rapidly by trace water to solidlithium hydroxide. Between -2.60 and - 2.80 V, this insulating precipitate isreduced to lithium atoms in solid solution with gold, and between - 2.80 and -3.10 VV a chemically resistant gold + lithium alloy is formed. At potentials more negativethan - 3.10 V. bulk lithium is formed and reduction of solvent proceeds sponta-neously.

Figure 1 shows the SNIFTIRS difference spectrum in a region where one wouldexpect to observe the gold-gold fundamental stretch. Diatomic gold has a vibrationfrequency of 190.9 cm - 1. with a reduced mass of 98.5 ainu [3]. If the gold atom werevibrating with the same force constant against an infinite mass the reduced masswould by 197 amu, and a vibrational frequency of 135 cm -' would be expected. As "

the potential is made more negative we observe that the intensity of a band at 145cm - ' increases. At these potentials. the surface is being increasingly covered with ! /underpotentially deposited lithium. Due to the sign convention used in these spectrathe increase in intensity is an indication of the loss of absorption by a species, herethe gold adatom on the gold surface. The close correspondence between theobserved 145 cm-I frequency and that calculated for a gold atom vibrating againstan infinite mass suggests that the adatom is bonded to a single surface atom. thatmost of the motion involved in the optically active vibration occurs on the adatoms.and that the force constant is nearly the same on the surface as the gas-phase dimer.

At 440 cm -' (Fig. 2), a simultaneous and parallel increase in absorbance occurs.This compares to a vibrational frequency of matrix-isolated "1Au 7 Li of 705 cm- .I" .

3

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-O.mN

700 400 no0 400 300

Fig. 2. Surface far infrared difference spectrum of the system described in Fig. I in the 200-700 cm-'reion as a function of electrode potential. TM curves represent potentials of (top to bottom) - 1.70 V.- 2.90 V. and - 3.0 V respectively. Reference potential - 1.50 V.

and probably corresponds to a gold-lithium surface species. The diatomicgold-lithium molecule probably derives much of its high vibrational frequency andlarge bond strength (2.92 eV) (5] from ionic interactions, Li Au-. This is expectedsince apart from the halogen atoms, gold has the highest electron affinity of anyelement (2.31 eV) [6). On a surface or in the bulk, the electron donated to gold maybe delocalized into the gold conduction ban, resulting in a smaller Coulombic forcebetween the atoms in Li-Au (surface) than in diatomic Li-Au. Other explanationsare possible, of course, but this does explain the lower than expected value for theLi-Au vibrational frequency, however.

No bands attributed to Li-Li vibrations are observed at potentials more positivethan those required for bulk deposition of lithium. As soon as three dimensionalgrowth of lithium begins, however, a band at 395 cm -I rapidly grows in. This maybe compared to the 7L 2 vibrational frequency of 351 cm - ' in the gas phase [3). Theshift of 44 cm-' to higher frequency is relatively small considering that it corre-sponds to condensation of a dimer onto a solid surface. It is therefore probable thatthe 395 cm-I peak does correspond to a surface vibration of Li-Li.

A cell was equipped with a very small electrode (0.5 mm diameter) so that thetime constant of the cell could be decreased to suitably small values, and timeresolved spectra were obtained for the fact nucleation and growth process of lithium esson Foron gold. Figure 3a shows the time and wavenumber resolved spectral response GRA&Iobtained at 10 ms intervals following application of the potential. The time profile3 TA

alone is shown in Fig. 3b. The growth of the absorption transient follows a 13 - TAB C3dependence (a plot of absorbance/t 3 vs. t is linear with a correlation coeffcient - 'ounced 00.9989). With this information, one may consider various models of nucleation and Iloation

crystal growth to determine which possibilities are consistent with this time depend-I..trbutton/Avellabllity Codem

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Fig. 3. Time resolved surface (a infrared difference spectrum of lithium nucleation and growth. Totalexperimeftal nine is 100 mns: spectral width is 45 cm. peak is at 396 cmn (a) 3-dimensional plot: (b)time profile of 395 cm - peak.

ence. In this particular example, one finds that a mechanism involving instanta-neous nucleation of lithium atoms followed by three dimensional growth is con-sistent with the observed time dependence. One also notes that the absorption peakbroadens as it grows, as a result of lateral interactions and surface defects which areincorporated into the new-grown crystallite.

In addition to the metal stretch bands reported in this note, we point out thatthere are other bands observed in other regions of the infrared spectrum. Thesecorrespond to adsorbed supporting electmlyte and solvent, as is evidenced bychanging the system components. The behavior and nature of these vibrations willbe the subject of a forthcoming report.

In conclusion, we have shown that two new powerful variations of infraredspectroelectrochemistry may be used for the study of a fast reactions at metalsurfaces involving direct bonding to the metal surface. Finally. we report the directobservation of metal atom vibrations at surfaces by reflection FTIR spectroscopy.

ACKNOWLEDGEMENT

We thank the Office of Naval Research for support of this work.

REFERENCES

I i.W. Foley. C. Korzaniewski J. Dascbbach and S. Pon in AJ. Bard (Ed.). ElectroanayticaiCbemisty. Marel Dekker. Now York. in pres.---- 1 .. ~.-,. ,

2 J. Dascech 0. Heisler and S. Po"a Appl. Spectroec.. in press.-3 X.P. Huber and G. Henber. Molecular Spectra and Molecular Structure IV. Constants of Diatomic

Molecules. Van Nostrand Reinhold. New York. 1979. "rp4 H.R. IW E.L Langenscheidt and B. Zinbora J. Chemn Pitys.. 66 (1977) 5105.3 A. Neuaen and X.F. Zmbov. J. Chain. Soc. Faraday Trans.. 70 (1974) 2219.6 HI. Hotop and W.C. Lineberger. J1. Pitys. Chain. Ref. Data. 4 (1975)5339.

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