Langmuir 1996,11, 849-852
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Atomic Iodine Desorption from Single Crystal Nickel Surfaces K. B. Myli and V. H. Grassian" Department of Chemistry, University of Iowa, Iowa City, Iowa 52242 Received March 21, 1994. I n Final Form: November 14, 1994@ Atomic iodine desorption from single crystal nickel surfaces, Ni(100) and Ni(lll), has been monitored by temperature-programmeddesorption (TPD). CF3I was used as a precursor molecule for iodine adsorption. At low coverages, &dine = 0.016 ML, the temperature of the desorption rate maximum ( T m a x ) is near 1035 K for desorption from Ni(100) whereas at higher coverages, &dine = 0.20 ML, the temperature decreases to 970 K. There is a similar decrease in T,,, with increasing coverage for iodine desorption from Ni(ll1). Analysis ofthe TPD data shows that iodine desorption follows first-order reaction kinetics with a coveragedependent activation energy. We have modeled the coverage-dependent activation energy of desorption, E d e & 8 ) , in terms of a repulsive dipole-dipole interaction, E d i p ( e ) , in the adsorbed layer, where = Edes(€' = 0) - Edi,(B). The activation energy of desorption at low coverages, E d e s ( 8 = O), was determined to be 249 and 214 kJ/mol from Ni(100) and Ni(lll), respectively. The activation energy decreases by approximately 7% in the coverage range studied, 0 = 0 - 0.20 ML. Simulated desorption curves agree well with the experimental data.
Introduction There have been numerous studies of halogen adsorption on metal surfaces. These studies are relevant to areas of technological importance, including corrosion, catalysis, and electrochemistry. In general, halogens adsorb dissociatively on most transition metal surfaces a t submonolayer coverages and temperatures between 100 and 300 K. For some metals the metal-halide salt forms readily, whereas for many metals the chemisorbed state is chemically distinct from the sa1t.l There are several excellent review articles on the topic of halogen ad~orption.l-~ The adsorption of halogens on single crystal Ni surfaces has been studied with several techniques, including lowenergy electron diffraction, work function measurements, and photoelectron and Auger electron spectro~copy.~-~ In all studies reported thus far, XZ dissociates on Ni a t submonolayer coverages and forms ordered overlayers. ~
~
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* Author to whom correspondence should be addressed. @
Abstract published in Advance A C S Abstracts, February 1,
1995. (1)Dowben, P. A. CRC Crit. Reu. Solid State Muter. Sci. 1987, 13, 191. (2) Jones, R. G. Prog. Surfi Sci. 1988,27, 25. (3) Grunze, M.; Dowben, P. A. Appl. Surf. Sci. 1982, 10, 209. (4) Farrell, H. H. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, King, D. A. and Woodruff, D. P., Eds.; Elsevier, 1984. (5) Fluorine-Nickel: Veremeenko,M. D.; Alenchikova, I. F.; Nefedov, V. I. Pouerkhnost 1985, 5 , 123. (6) Chlorine-Nickel: (a) Erley, W.; Wagner, H. Surf. Sci. 1977,66, 371. (b) Kisinova, M.; Goodman, D. W. Surf. Sci. 1981, 108, 64. (c) Matsudaira. T.: Onchi. M. J . Phvs. C: Solid State Phvs. 1979.12.3381. (d) Erley, W. Surf. Scz. 1982, 134, 47. (e) Takata, k.; Sato, H.: Yagi, S.; Yokoyama, T.; Ohta, T. Surf. Sci. 1992, 265, 111. (7) Bromine-Nickel: (a) Dowben, P. A,; Sakisaka,Y.; Rhodin, T. N. J . Vac. Sci. Technol. A 1985,3,1855. (b) Dowben, P. A.; Mueller, D.; Rhodin, T. N.; Sakisaka, Y. Surf. Sci. 1985, 155, 567. (8) Iodine-Nickel: (a) Jones, R. G.; Woodruff, D. P. Vacuum 1981, 31, 411. (b) Somerton, C.; McConville, C. F.; Woodruff, D. P.; Jones, R. G. Surf. Sci. 1984,136,23. (c) Jones, R. G.; Ainsworth, S.;Crapper, M. D.; Somerton, C.; Woodruff, D. P. Surf. Sci. 1987, 179, 425. (d) Jones, R. G.; Ainsworth, S.; Crapper, M. D.; Somerton, C.; Woodruff, D. P. Surfi Sci. 1987, 179, 442. (e) Somerton, C.; McConville, C. F.; Woodruff, D. P.; Jones, R. G. Vacuum 1983,33,858. (0 Woodruff, D. P. Appl. Surf. Sci. 1985, 22/23, 459. (g) Jones, R. G.; McConville, C. F.; Woodruff, D. P. Surface Science 1983, 127, 424. (h) Jones, R. G.; Ainsworth, S.; Crapper, M. D.; Somerton, C.; Woodruff, D. P.; Brooks, R. S.; Campuzano, J . C.; King, D. A,;Lamble, G.; Prutton, M. Surf. Sci. 1985, 152/153, 443. (i) McConville, C. F.; Woodruff, D. P. Surf. Sci. 1985, 152/153, 434.
0743-7463/95/2411-0849$09.00/0
Halogen atoms can also be adsorbed on metal surfaces from the corresponding alkyl halides because C-X bond dissociation can be quite facile on metal s u r f a c e ~ . ~The J~ halogen adlayer can be cleanly prepared in this manner if the surface is heated to high enough temperatures to remove hydrocarbon fragments or unreacted parent (precursor) molecules. For example, CH31has recently been used to study iodine adsorption and desorption on Pt(lll).ll In other studies, bromine atoms thermally deposited on nickel surfaces from CH3Br are used as a spacer layer in photochemical studies of CH3Br to decrease quenching of the excited state.l2 Iodine from alkyl iodide precursors have also been used to study the effect of iodine adsorption on the chemistry of alkyl fragments adsorbed on copper.13 Despite extensive studies of IZadsorption on Ni8 and recent studies of alkyl iodide1* and fluorinated alkyl iodide15 adsorption on Ni, little is known about the desorption kinetics of iodine from Ni surfaces. Auger electron spectroscopy has shown that iodine is completely lost from Ni(100) a t temperatures near 1000 K.8a The mechanism for iodine loss from the surface, i.e., whether iodine desorbs into the gas phase atomically, molecularly, or as nickel iodide, NiIz, is unknown. In the course of our studies of CF3I adsorption on Ni surfaces,16we have determined that iodine desorbs from Ni(100) and Ni(ll1) as iodine atoms. In the low coverage regime, &dine < 0.20 ML, iodine desorption follows firstorder kinetics with a coverage-dependent activation energy. We have modeled the coverage-dependent activation energy of desorption in terms of a repulsive dipoledipole interaction in the adsorbed layer using an electrostatic model developed by Albano.17 (9) Zaera, F. Acc. Chem. Res. 1992,25, 260. (10) Zhou, X.-L.; Zhu, X.-Y.; White, J. M. Surf. Sci. Rep. 1991, 13, 79
.I.
(11)Jo, S. K.; White, J. M. Surf. Sci. 1992, 261, 111. (12) Marsh, E. P.; Tabares, F. L.; Schneider, M. R.; Cowin, J . P. J . Vuc. Sci. Technol. A 1987, 5 , 519. (13) Jenks, C. J.; Paul, A,; Smoliar, L. A,; Bent, B. E. J . Phys. Chem. 1994, 98, 572. (14) (a)Zaera, F.; Tjandra, S. J . A m . Chem. SOC.1993,115,5851. (b) Zhou, X.-L.; White, J. M. Surf. Sci. 1988, 194, 438. (15) Jones, R. G.; Singh, N. K. Vacuum 1988, 38, 213. (16) (a) Ni(100): Myli, K. B.; Grassian, V. H. J . Phys. Chem. 1995, 99, 1498. (b) Ni(ll1): Myli, K. B.; Grassian, V. H. J . Phys. Chem., in press. (17)Albano, E. V. J. Chem. Phys. 1986,85, 1044.
0 1995 American Chemical Society
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850 Langmuir, Vol. 11,No. 3, 1995
Experimental Section An ultrahigh vacuum (UHV) chamber with a base pressure of 2 x 10-lo Torr is used in these experiments. The chamber is equipped with a cylindrical mirror analyzer for Auger analysis, quadrupole mass spectrometer (UTI-100C; 0-300 AMU mass range) for TPD, three gas dosers, and an ion-sputteringgun. The quadrupole mass spectrometer (QMS) is mounted on a linear translator and can be positioned to within a few millimeters of the crystal face. Aglass cone shield with a 3 mm diameter pinhole is placed over the ionizer region of the quadrupole so that molecules desorbing from the front crystal face will be preferentially detected in TPD.18 Both the Auger electron and mass spectrometers are interfaced to a 286-PC computer for data acquisition and analysis. Five mass signals can be monitored simultaneously in TPD. The Ni crystal is attached t o a copper block, held by a precision translation stage and a differentially pumped rotary drive. The sample is heated resistively and can be cooled to 100 K with liquid nitrogen. The power supply used to heat the sample provides a linear temperature ramp from 100 to 1200 K. The heating rate, ,B, was 2 Ws and 3 Ws for iodine desorption from Ni(100) and Ni(lll), respectively. The Ni crystals were purchased from Monocrystals Inc. and final polished with 1 pm diamond paste followed by 0.05 pm alumina. Two different crystals of each surface face were used during the course of these experiments. Samples were cleaned by Ar ion bombardment to remove sulfur and oxygen impurities from the surface. Carbon was removed by heating the sample to 800 Kin the presence of 1 x lo-? Torr of oxygen. The sample was checked by Auger electron spectroscopy prior to each experiment. Gases are introduced by backfilling the chamber. CF3I was purchased from PCR Inc. CF3I exposures are given in units of Torr s). langmuir (1langmuir = 1 x
~/'\JI(lOO)
700
BOO
900
1000
1100
'200
Temperature (K)
Figure 1. TPD curves of iodine (AMU = 127) desorption from Ni(100) as a function of CF31exposure: 0.75, 1.0, 1.8,2.5, 3.7, and 5.0 L. The heating rate was 2 Ws. liN,(111) I
Results and Discussion Temperature-ProgrammedDesorption of Atomic Iodine from Ni(100) and Ni(ll1). We have recently investigated the chemistry of CF31 adsorbed on single crystal metal surfaces under ultrahigh vacuum conditions. CF31is very reactive on Ni(100) and N i ( l l l ) , both C-I and C-F bond activation occur below room temperature. Several reaction products form from CF31dissociation on Ni(loo), including1, N, Fz, CF31,and CF3. Similar reaction products form from reaction of CF31with Ni(ll1). A full discussion of the surface chemistry of CF31on nickel is given elsewhere.16 Relevant results described in this study are the following. First, it is evident that the C-I bond dissociates on Ni(100)(andNi(ll1)as well), leaving iodine atoms on the surface. Second, reaction products desorb from the surface below the onset temperature for iodine desorption. Jones and Singh have shown with Auger electron spectroscopy that only iodine remains on the surface after heating the sample to 820 K,15 below the onset of iodine desorption, in agreement with experiments done in our laboratory. Therefore, we conclude that CF3I can be used as a precursor for iodine adsorption on single crystal surfaces. For the iodine desorption experiments, CF31was adsorbed on Ni(100) a t low temperatures, T = 100 K, a t various exposures. Atomic iodine (mle = 127) desorbs a t temperatures near 1000 K (Figure 1). In Figure 1, iodine desorption curves are shown for several CF31exposures (0.75-5.0 langmuir). As the CF31exposure is increased, the temperature of the desorption rate maximum decreases and the area under the desorption trace increases. At a CF3I exposure of 0.75 langmuir, the temperature of the desorption rate maximum is near 1035 K. The I+ (18) Feulner, P.; Menzel, D. J. Vuc. Sci. Technol. 1980, 17,662.
700
800
900 Temperature
iaao
1100
1200
(Y)
Figure 2. TPD curves of iodine (AMU = 127) desorption from Ni(ll1) as a function of CF3I exposure: 0.1, 0.4, 0.8, 1.5, 2.4, and 3.6 L. The heating rate was 3 Ws.
peak saturates a t CF31 exposures '5 langmuir with a T,,, near 970 K. The high coverage desorption curves show a n increasing background signal due to the desorption of CF31 from the crystal support wires. (I' is formed from the fragmentation of CF31in the QMS ionizer.) Molecular iodine, 1 2 (mle = 254), was not observed in TPD. Although Ni12+could not be detected with our QMS (mle = 312), the expected mass fragments, Nif (mle = 58) and Nil+ (mle = 185), were not detected by TPD. Similar desorption studies of atomic iodine from Ni(ll1) were done. In Figure 2, atomic iodine desorption traces from Ni(ll1) are shown as a function of CF3I exposure. At low CF31exposures (0.1 langmuir), the desorption rate maximum is near 1000 K. The desorption rate maximum shifis to 950 K a t a n exposure of 3.6 L. The If signal saturates near a CF31exposure of 3.5 langmuir. Kinetic Modeling of Iodine Desorption. As the data show, the temperature of the desorption rate maximum is a function of iodine coverage. At the lowest coverages, atomic iodine desorbs from Ni(100) with a temperature maximum that is nearly 65 K higher than at the highest
Langmuir, Vol. 11, No. 3, 1995 851
Atomic Iodine Desorption from Ni(lO0) and Ni(l11)
The coverage dependent activation energy of desorption, can be written as
a) Ni(100)
-
Edes(o),
5 v
-10 I 00090
00095
00100
n = P
00105
I
where &e,(@ = 0 ) is the activation energy in the limit of zero coverage and &ip( 0) is the coverage-dependent repulsive interaction term. The repulsive term, E&,(@, is expressed as
00110
Temperature-’ (K-’) b) Ni(’l1)
O
-Job95
7
00100
00105
00110
where y is the static dipole moment of the I-surface bond in the limit of zero coverage, a is the polarizability of the adsorbed atom, and N , as defined earlier, is equal to the surface coverage in atoms/cm2. The repulsive term, given in eq 2, is the destabilization energy from aligned dipoles which are uniformly spaced. Simulated desorption curves are obtained by solving the first-order desorption rate equation
Od115
Temperature-’ (K.l)
Figure3. ln(Rate)-n InNversus 1/Tisplotted for three values of n: 0, 1,and 2. A linear relationship is obtained when n = 1.
coverages. A smaller shift of 50 K i n Tmaxis observed for to lower Ni(111). Below we will show that the shift in Tmax temperatures as a function of iodine coverage is due to a first-order desorption process with a coverage-dependent activation energy. From the TPD data the reaction order, n , of the desorption kinetics was determined from a plot of ln(Rate) - n In N versus l/T,19 where N is equal to the surface coverage in atoms/cm2. For Ni(100), the coverage is determined by using a value of 0.20 ML for saturation coverage, i.e., N,, is equal to 3.2 x l O I 4 atoms/cm2. The saturation coverage of iodine from the low-temperature adsorption of CF31 on Ni(100) has been determined by Jones and Singh.15 The coverage is then obtained by integrating the area under each desorption curve and calibrating to a saturation coverage of 0.20 ML. Although the saturation coverage for iodine from the adsorption of CFJ on Ni(ll1) has not been independently determined, we have used a value of 0.17 ML as an estimate of the coverage. A coverage of 0.17 ML for Ni(ll1) would correspond to the same surface density of iodine atoms, i.e., 3.2 x 1014atoms/cm2. After baselining the raw TPD data, ln(Rate)-n 1 n N was plotted as a function of T-l for three values ofn; n = 0, 1,and 2. A straight line is obtained for n = 1 (see Figure 3). Therefore, atomic iodine desorption from Ni(100) and Ni(ll1) follows first-order reaction kinetics and the shift in T, is attributed to a coverage-dependent activation energy and not reaction order. We have modeled the coverage-dependent desorption kinetics using a simple electrostatic method developed by Albano.17 The coverage-dependent activation energy is assumed to be entirely due to a repulsive dipole-dipole interaction in the adlayer. The Albano model has been successfully applied to several systems including desorption of potassium from metal surfaces,17 desorption of potassium in the presence of coadsorbed oxygen,17and the molecular desorption of CH3Cl from Pd( (19) Parker, D. H.; Jones, M. E.; Koel, B. E. Surf. Sci. 1990,233,65.
using E d e s ( 0 ) given in eq 1, with p = the linear heating rate, v = a pre-exponential factor, N =the surface coverage, R = the gas constant, and T = the temperature. Berko et al. have discussed the effect of the various parameters on the simulated desorption curves by varying each factor independently.z1 It can be seen from the simulated desorption curves given in ref 20 that the shape of the desorption curves is most affected by the polarizability ( a )and the dipole @). The pre-exponential factor and the activation energy only slightly change the shape of the desorption curves. All of the above parameters, a, y, v, p and &ea, have a n effect on the temperature of the desorption rate maximum.z1 As for most models, the parameters are, of course, interdependent, and a n incorrect value for one of the parameters could be compensated by some of the other parameters. As with most models, i t is difficult to determine if a unique fit has been achieved and, therefore, as much information as possible must be derived from the experimental data. As discussed below, we have determined from the experimental data the activation energy a t low coverage and the pre-exponential factor. The pre-exponential factor is assumed to be independent of coverage; problems associated with this assumption will be discussed. The low coverage desorption energy, Edes(@) = 0, is determined from the slope of a plot of ln(rate) - n In N versus 1/T using the low-coverage data. From this analysis, E d e s ( 8 = 0 ) is equal to 249 and 214 kJ/mol for Ni(100) and Ni(ill), respectively. The pre-exponential factor, Y , is calculated a t low coveragesz0to be 1.4 x l o l l and 1.0 x 1O1O s-l for Ni(100) and N i ( l l l ) , respectively. Finally, two parameters,y and a, need to be determined. The overall quality of the fit was evaluated by two experimental observables, the peak maximum and the overall shape of the desorption curve a t each coverage. The rising background due to CFJ desorption in the high coverage data was subtracted out when determining the optimum fit.22 For Ni(100), a value of 5.0 A3 for the polarizability (a)and a value of 3.0 D for the surface dipole (p) gave simulated curves that agreed well with the experimental data. For Ni(l l l ) , a value of a = 5.0 A3and y = 2.2 D gave the best results. The value of a is near (20) Habenschaden, E.; Kuppers, J. Surf. Sci. 1984, 138, L147. (21)Berko, A.; Erley, W.; Sander, D. J . Chem. Phys. 1990,93,8500. (22)The rising background at high converages was subtracted from the raw data using the CF31+desorption trace multiplied by the ratio of the ion intensities for I+ and CF31+from CF3I fragmentation.
Langmuir, Vol. 11, No. 3, 1995
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IiNt(100) . Simulation
216
,
1 250
212
-5 n Y
208
204
0
800
700
900
1000
1200
1100
Temperature (K)
Figure 4. Simulated TPD curves for iodine desorption from Ni(100) as a function of coverage calculated using eq 3. The following values were used in the simulation: Edes(6 = 0) = 249 kJ/mol, Y = 1.4 x lo1' s-l, /3 = 2 Ws, Nsat= 3.2 x 1014 atom/cm2,a = 5.0 A3,and p = 3.0 D. iiNi(111) - Simulation
I
700
I
800
900
1000
1100
~200
Temperature (K)
Figure 5. Simulated TPD curves for iodine desorption from Ni(ll1) as a function of coverage calculated using eq 3. The following values were used in the simulation: Edes(@= 0) = 214 kJ/mol, Y = 1.0 x 1Olo s-l, /3 = 3 Ws, Nsat= 3.2 x 1014 atom/cm2,a = 5.0 A3,and p = 2.2 D.
the experimental value for the polarizability of atomic iodine.23 It should be noted that in the model the two parameters, a and p , do compensate one another and if a increases so will p. For example, on Ni(100), if a was too small and was increased from 5 to 10 A3 then p increases by 0.6 D. A further increase in a to 20 A3would result in a n increase of 1.9 D in p. So care must be taken in not overinterpreting the values of a andp derived from the model without some other independent measurement. Simulated desorption curves for iodine desorption from Ni(100) and Ni(ll1) are shown in Figures 4 and 5, respectively. In general, the calculated curves predict the temperature of the desorption rate maxima very well. The shape of the calculated desorption curves is in good agreement with the experimental curves, especially so a t the onset of desorption. However, the desorption tails of the calculated curves do not descend as fast as the experimental data, particularly a t high coverages. (23) CRC Handbook of Chemistry and Physics, 72nd ed., CRC: Boca Raton, FL, 1991.
05
10
15
'
20
230
Q (ML) Figure 6. The calculated activation of desorption for iodine from Ni(100) and Ni(ll1) as a function of coverage. The desorptionenergydecreases by7 and5%forNi(100)andNi(lll), respectively.
The calculated activation energy of desorption as a function of coverage is shown in Figure 6. At the highest coverages formed, there is a 7%decrease in the activation energy of desorption for iodine from Ni(100) and a 5% decrease from Ni( 111). Although the simulated curves agree well with the experimental data, we should now consider some of the shortcomings of the model and how it affects the results. First, the model assumes that the decrease in activation energy is entirely due to repulsive through-space dipolar interactions; through-metal interactions are totally neglected. In the low-coverage regime this is probably a good assumption; however, a t higher coverages throughmetal interactions will contribute more. It has been shown that a t coverages above 0.3 ML through-metal interactions become increasingly important.7f If important, throughmetal interactions would contribute to the decrease in the activation energy. Neglect of this component to the decrease in Edes(8 = 0) would mean that the calculated repulsive interaction term would be too large. We have also neglected any coverage dependence of the preexponential factor. The well-known compensation effect would indicate an increase in the pre-exponential factor with coverage. Neglect of this dependence on the preexponential factor would result in a calculated repulsive interaction term that was too small. In summary, we have shown that a t low coverages (8 0.20 ML) iodine desorbs as atomic iodine from Ni(100) and Ni(ll1) at temperatures near 1000 K. Of the four congeners, F, C1, Br and I, iodine appears to be unique in that it does not desorb as the metal salt, Nix2, a t low coverages. This result suggests that the activation energy of formation for nickel iodide is larger than for the other nickel halides. We have modeled the desorption kinetics using an electrostatic model that calculates the repulsive interaction of the aligned dipoles. I t appears that, in the low coverage regime, a n electrostatic repulsive dipole interaction can account for most of the repulsive interactions in the adlayer.
Acknowledgment. Support of this work was provided by the National Science Foundation and the Carver Scientific Research Initiative Grant Program. LA940246F
