Langmuir 1988,4, 1202-1206
1202
limiting transfer ratio should be about 0.2, well within the range of what is actually observed. Acknowledgment. This work was supported under the National Science Foundation's MRL program by the
Materials Research Center, Northwestern University (Grant no. DMR-85-20280). Registry No. Lead stearate, 1072-35-1;cadmium stearate,
2223-93-0.
Dissociative Chemisorption of Water on Titanium Subhodaya Aduru and J. Wayne Rabalais* Department of Chemistry, University of Houston, Houston, Texas 77004 Received February 2, 1988. I n Final Form: M a y 5, 1988 The chemisorption of water on polycrystalline titanium at room temperature has been investigated by using time-of-flight direct recoil spectrometry (TOF-DRS), X-ray photoelectron spectroscopy (XPS), and depth profiling with TOF-DRS analysis. The relative surface concentrations of H, 0, and Ti obtained from TOF-DRS and XPS along with the core-level chemical shifts of 0 and Ti show that dissociative chemisorption is the dominant interaction at room temperature. The results show that the species present on the surface are mainly hydroxyl and oxide moieties. Thermodynamic analysis shows that there is a strong driving force toward dissociative chemisorption. Evidence for displacement of surface hydrogen by oxygen or hydroxyl is observed during the early stages of water chemisorption.
Introduction Chemisorption of water on metal surfaces is important with regard to the corrosion, catalytic, and electrochemical properties of surfaces. Adsorption of water has been studied on metals such as Al,' CU,~+ Pt>6Ni>* Co? Fe,l0~l1 Ti,12-14 and Re.15 Thiel and Madey have recently reviewed16the interactions of water with solid surfaces. The mechanism and kinetics of adsorption have been followed by using a variety of surface analytical techniques such as XPS, UPS, AES, ESD, PSD, ELS, and LEED. The difficulties associated with determination of the nature of the species and stoichiometry on a water-chemisorbed surface are due to the inability of most surface analytical techniques to directly detect and determine the surface hydrogen concentration. Even though the features associated (1)Ding, M. Q.;Williams, E. M. Surf. Sci. 1985,160,189. (2)Spitzer, A.; Lueth, H. Surf. Sci. 1985,160,353. (3)Spitzer, A.; Ritz, A.; Lueth, H. Surf. Sci. 1985,152, 543. (4)Bange, K.;Grider, D. E.; Madey, T. E.; Sass, J. K. Surf. Sci. 1984, 137,38. (5) Melo, A. V.; O'Grady, W. E.; Chottiner, G. S.; Hoffman, R. W. Appl. Surf. Sci. 1985,21,160. (6)Bonzel, H.P.; Pirug, G.; Winkler, A. Chem. Phys. Lett. 1985,116, 133. (7)Benndorf, C.; Noebl, C.; Thieme, F. Surf, Sci. 1982, 121, 249. (8)Benndorf, C.; Noebl, C.; Ruesenberg, M.; Thieme, F. Surf. Sci. 1981,111, 87. (9)Heras, J. M.; Papp, H.; Spiess, W. Surf. Sci. 1982,117,590. (10)Dwyer, D. J.;Kelemen, S. R.; Kaldor, A. J. Chem. Phys. 1982,76, 1832. (11)Muessig, H.J.; Arabczyk, W. Cryst. Res. Technol. 1981,16,827. (12)Stockbauer, R.; Hanson, D. M.; Flodstroem, A. S.; Madey, T. E. Phys. Rev. B: Condens. Matter 1982,26,1885. (13)Bertel, E.;Ramaker, D. E.; Kurtz, R. L.; Stockbauer, R.; Madey, T. E. Phys. Reu. B Condens. Matter 1985,31,6840. (14)Stockbauer, R. L.;Hanson, D. M.; Flodstrom, S. A,; Madey, T . E. J. VUC.Sci. Technol. 1982,20, 562. (15)Fusy, J.; Alnot, M.; Jupille, J.; Pareja, P.; Ehrhardt, J. J. Appl. Surf. Sci. 1984,17,415. (16)Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 7 1987,211.
with H, OH, and H20 can be identified in UPS, this assignment is not devoid of ambiguities. In XPS, the chemical shift between the 0,OH and H20 Is lines is small and the peaks usually overlap; however, quantification can be obtained by deconvolution. The desorption studies of H+, 0+,and OH+ induced by photons or electrons do not result in unambiguous determinations of the surface concentrations. To the best of our knowledge only a limited number of papers have been published on chemisorption of H 2 0 on titanium metal'2-14 and oxide"-21 surfaces. The adsorption of water on titanium is of particular interest due to its good corrosion resistance and the photocatalytic properties of its oxide. The purpose of this work is to apply the time-of-flight direct recoil (TOF-DRS) technique to the problem of water chemisorption on titanium for determination of the nature and concentration of surface hydrogen. Atoms can be directly recoiled22from a surface by means of a single-collision encounter from a primary ion beam incident on the surface at a grazing angle. The energy E R of the target atom of mass M 2 recoiling from a primary ion of energy Eoand mass M 1 is given by 4A cos2 CP E R = Eo(1
+ A)2
where A = M 2 / M l and CP is the recoil angle. In our experiments a pulsed ion beam (1-5 keV) is used, and the times-of-flight (TOF) of the directly recoiled (DR) and scattered (S) particles are measured at a forward scattering angle. The flight times of light elements such as H, C, and 0 are resolvable by judicious choice of the primary ion (17)De Pauw, E.;Marien, J. J.Phys. Chem. 1981,85, 3550. (18)Raupp, C. B.; Dumesic, J. A. J. Phys. Chem. 1985,89, 5240. (19)Sham, T. K.;Lazarus, M. S. Chem. Phys. Lett. 1979,68, 426. (20)Smith, P.B.; Bernasek, S. L. Surf. Sci. 1984,188, 241. (21)Henrich, V. E.; Dresselhaus, Cr.; Zeiger, H. J. Solid State Commun. 1977,24, 623. (22)Schultz, J. A,: Contarini, S.; Jo, Y. S.; Rabalais, J. W. Surf. Sci. 1985,154, 315
0743-746318812404-1202$01.50/0 0 1988 American Chemical Society
Langmuir, Vol. 4,No. 5, 1988
Dissociative Chemisorption of H20on T i mass and energy. The sampling depth is of the order of a single monolayer, since most of the direct recoils originate from the topmost atomic layer. The most important feature of TOF-DRS is its high sensitivity for hydrogen. The capability of TOF-DRS to detect and monitor the surface H, 0, and Ti simultaneously as a function of H 2 0 exposure makes the technique ideally suited to studies of the nature of the adspecies in H 2 0 chemisorption. XPS analysis combined with TOF-DRS intensity ratios is used herein to determine the nature of H 2 0 on the titanium surface.
Experimental Section The instrumental requirements for low-energy ion scattering and recoiling with TOF analysis of ions and neutrals have been described e l s e ~ h e r e . ~The ~ - ~primary ~ ions are generated by heating an alkali zeolite ion source. The ion beam is velocity selected and pulsed, and the detector is a Channeltron electron multiplier. The experiment is performed by triggering a timeto-amplitude converter (TAC) t~ start when the primary ion pulse reaches the sample surface with a delayed synchronizedpulse from the pulse generator; the TAC is stopped by a pulse from the channeltron when it detects a particle. The TAC produces voltage pulses proportional in height to the flight times of scattered and recoiled particles. Pulse height analysis is accomplished by using a multichannel analyzer, and hence the channel number is proportional to the particle flight time. Repetition of this procedure results in a TOF spectrum. The operating conditions are as follows: (1)primary ion beam 4 keV Na+,30-40-na pulse width, 40-kI-I~pulse rate; (2)73.5' angle of incidence from the surface normal, 33' scattering and recoiling angle; (3)base chamber pressure 14 X Torr. The total ion dose required to obtain each DR spectrum is =lo" ions/cm2,thus minimizing surface damage due to the probing primary ion beam. Typical spectral acquisition times were kept down to 1-2 min in order to minimize surface contamination from residual gases during the course of an experiment. The sample is a 99.999% nominal purity Ti foil, spot welded onto tantalum posts with a filament positioned behind the sample facilitating heating by electron impact. Sample cleaning for TOF studies is accomplished by 3-keV Ar+ bombardment from a Varian Associates ion gun and by annealing at 1100 "C.All gas exposures were made after the annealed sample had cooled to room temperature. The photoelectron spectra were obtained with a Perkin-Elmer PHI Model 550 ESCA/SAM system. Since the sample could only be annealed to low temperatures in the ESCA/SAM system, all XPS measurements were made on sputter-cleaned surfaces. Wide-scan surveys were carried out to check for impurities; adsorbed carbon and oxygen were the only contaminants observed with a total concentration of
