Langmuir 1987, 3, 543-547 phase. In the case of water exchanging between two (or more) sites of opposite residual anisotropies, the present study shows that the ratio IAv("O)/AV(~H)Ibecomes much smaller than the value of -6. However, the direction of the deviation in this exchanging situation may not be definite and probably depends on the equilibrium constant of the exchange and the modes of binding a t each site. A mesophase system which can allow a wide range of variation of the anionic/cationic surfactant ratio while keeping the amounts of the other constituents constant such as those reported recently26should facilitate a more quan(26)Tracey, A. S.;Radley, K. Mol. Cryst. Liq. Cryst. 1985,122,77.
543
titative account of the equilibrium of the exchange and the structure of binding of water at each site.
Acknowledgment. The NT-300 spectrometer was purchased through a grant from the National Science Foundation (PCM-8115599). A gift of a sample of C14TEAB from Professor Raymond Venable is gratefully acknowledged. T.C.W. was supported by a Summer Research Fellowship from the Research Council of the University of Missouri. Registry No. C,,TEAB, 18144-35-9;CTAB, 57-09-0;KDodec, 10124-65-9;DDTMABr, 1119-94-4; DTMABr, 2082-84-0; HzO, 7732-18-5; pentanol, 71-41-0; decanol, 112-30-1.
Initial Stage of Titanium Oxidation Studied by Direct Recoil Spectrometry S. Aduru and J. W. Rabalais* Department of Chemistry, University of Houston-University
Park, Houston, Texas 77004
Received November 3, 1986. In Final Form: February 12, 1987 The initial stage of oxidation of polycrystalline titanium for low oxygen exposures (C2 langmuirs) has been studied by time-of-flight (TOF) direct recoil spectrometry (DRS). The DRS signals from surface hydrogen, oxygen, and titanium obtained by use of 4-keV Na+ and K+ and two different recoil angles are monitored as a function of exposure to OD Desorption of this oxygen induced by 3-keV Ar+ is also monitored by oxygen DRS. The results show that there are two distinct forms of oxygen below 2 langmuirs. The initial form at low dose (C0.25 langmuir) exhibits increasing O(DR) and decreasing Ti(DR) signals as a function of exposure, a rapid O2uptake rate, and a high Ar+-induced oxygen desorption rate, suggesting top-layer sites. The second form at doses of 0.25-2.0 langmuirs has slowly increasing O(DR) and constant Ti(DR) signals and lower rates of O2 uptake and Ar+-induced desorption, suggesting subsurface sites. Hydrogen migrates to the surface upon heating and is displaced by oxygen upon chemisorption.
I. Introduction These investigations have concluded that there are two distinct forms of oxygen chemisorbed on Ti. At low exThe oxidation of titanium metal has been studied exposures a tightly bound a-state corresponding to an oxide tensively by a variety of surface spectroscopic techniques, underlayer is formed and a t higher exposures a @-state including ultraviolet and X-ray photoelectron spectroscopy corresponding to an oxide overlayer is formed. The a-state (UPS and XPS),1-4 Auger electron spectroscopy,"1° lowis characterized by a well-ordered p(2 X 2) LEED pattern, energy electron diffraction (LEED),'J1J2 electron energy a work function lower than the clean Ti surface, very low loss spectroscopy (EELS),13 secondary ion mass specPSD and ESD yields, a binding energy structure at 5.9 eV trometry (SIMS),14*16 laser fluorescence spectroscopy of in UPS, and a high Ti/O sputtering ratio. The @-stateis sputtered atoms,16 photon and electron stimulated decharacterized by a (1X 1)LEED pattern, a work function sorption (PSD and ESD)," appearance potential spectroscopy (APS),' and work-function m e a s ~ r e m e n t s . ~ J ~ J ~higher than the clean surface, high PSD yield, additional UPS features, and a lower Ti/O sputtering ratio. The a-state is believed to correspond to oxygen atoms below (1)Krauss, A. R.; Gruen, D. M. Nucl. Instrum. Methods 1978, 10, the surface plane in high-coordination sites while the p1304. (2)Eastman, D.E.Solid State Commun. 1972,10,1993. state corresponds to low-coordination sites on top of or in (3)Platau, A.; Johansson, L. I.; Hagstrom, A. L.; Karlsson, S. E.; the surface plane. The a-state is observed at initial O2 Hanstrom. S. B. M. Surf. Sci. 1977.63. 153. exposures with evidence for the @-statebeginning near a (qi)Fukuda, Y.;Honda, F.; Rabdais, J. W. Surf. Sci. 1980,91,165. (5)Shih, H. D.;Legg, K. 0.;Jona, F. Surf. Sci. 1976,54,355. dose of 2 langmuirs. These two states are readily observed (6)Berel, E.;Stockbauer, R.; Madey, T. E. Phys. Rev. B 1983,27,1939. on both polycrystalline16and single-crystal surfaces.'J3J7 (7)Jonker, B. T.; Morav, J. F.; Park, R. L. Phys. Rev. E 1981,24,2951. The purpose of this paper is to apply the technique of ( 8 ) Pellerin, F.;Langeron, J. P. Surf. Sci. 1983,126,444. (9)Bignolas, J. B.; Bujor, M.; Bardolle, J. Surf. Sci. 1981,108,L453. time-of-flight (TOF) direct recoil spectrometry (DRS)19s20 (10)Brugniau, D.;Argile, C.; Barthes-Labrouase, M. G.; Rhead, G. E. to study the initial stage of Ti oxidation. The reason why Surf. Sci. 1984,141, 639. this technique was applied to this already well-studied (11)Shih, H.D.;Legg, K. 0.;Jona, F. Surf. Sci. 1976,60,355. problem is that (1)DRS is highly sensitive to the outer(12)Shih, H. D.;Jona, F. Appl. Phys. 1977,12,311. (13)Strong, R. L.; Erskine, J. L. J. Vac. Sci. Technol. 1985,A3, 1428. most atomic layers of a material, (2) it can detect extremely Biwer, B. M.; Bernasek, S. L. Surf. Sci. 1986,167,207. small adsorbate concentrations on a surface (cO.1mono(14)Dawson, P. H. Surf. Sci. 1977,65,41. layer), (3) it can directly decipher the role of hydrogen on (15)Krauss, A. R.; Gruen, D. M. Nucl. Instrum. Methods 1978,149, 547. (16)Pellin, M. J.; Young, C. E.; Gruen, D. M.; Afatons, Y.; Dewald, A. B. Surf. Sci. 1985,151,477. (17)Hanson, D. M.: Stockbauer, R.: Madev, T. E. Phvs. Rev. B 1981, 24, 5513. (18)Brearley, W.; Surplice, N. A. Surface Sci. 1977,64,372.
(19)Schultz, J. A.; Contarini, S.; Rabalais, J. W. Surf. Sci. 1985,154,
"1
r
JlD.
(20)Schultz, J. A.; Jo, Y. S.; Tachi, S.; Rabalais, J. W. Nucl. Instrun. Methods 1986,B15, 134.
0143-1463f 81f 2403-0543$01.50f 0 0 1987 American Chemical Society
A d u r u and Rabalais
544 Langmuir, Vol. 3, No. 4 , 1987 the "clean" Ti and the 02-exposedTi surfaces, and (4) it can identify the initial chemisorption sites at very low exposures, i.e.,
