J. Phys. Chem. 1984,88, 2284-2281
2284
of the 8CB results the combined T2-T1-ELDOR method seems promising in research on ordering media by its option to yield directly the ordering dynamics of the nearby environment.
Acknowledgment. The authors thank Dr. Rassat (Grenoble) for his generous gift of PDT when this project started.
rex
w):
Appendix In the parameter set jFj(w,),
ELDOR analysis with a fixed value of this ratio and to refine it afterward if the final outcome would make this necessary. Simcan be added afterward in view of the small experimental ilarly deviations from eq 5 . Finally, two independent parameters are left: jm(w,) andjm(oo). By variation one finds an unambiguous optimum ELDOR fit which can be refined somewhat by adding rex. As a second ste in the approach is determined from the experimental fimp and the dimensionless products which are calculated simultaneously with the ELDOR reduction ) would factors. Finally, if further analysis of j ( w ) and ~ ' together yield rotational diffusion results which are incompatible with the initial selection of the ratio (A.2), the solution can be refined by iteration. Since the more or less spherical PDT reorients nearly isotropically, a convenient starting value is 3.0. Considering the experimental accuracies further iteration is not needed.
Fbo),
-
jDGtwo)Bo,
we,
('4.1)
one has in the standard formulation,la over a large range of N , and 7 (A.2) < F(w0)/Om(wo)Bo) < 3.4 s. So it is most useful to start the for 1 < N , < 3 and 7
1.0 mM. Results on competitive adsorption between NHQ and I- indicated that, in contrast to the other halides, I- is sufficiently surface active to enforce molecular reorientations even at comparatively low aromatic concentrations.
Introduction Aromatic molecules are spontaneously and irreversibly adsorbed on smooth polycrystalline Pt electrodes.' The organometallic chemisorptive interactions are unimpeded by the presence of weakly coordinating solvents (water) or anions (C104-, PF6HS04-, SOP, H2P04-, HPO:-, The inability of these aqueous electrolyte solutions to interfere with irreversible adsorption has also been demonstrated on structurally and chemically well-defined Pt single-crystal surfaces.24 When an organic
monolayer was allowed to interact with iodide/iodine5-' solutions, displacement and/or reorientation of the initially adsorbed organic along with halogen coadsorption were observed.lbq3 Displacement reactions between halides and aromatic compounds at smooth polycrystallinePt in aqueous solutions, analogous to ligand substitution reactions in molecular coordination compounds, are described in the present paper: (1) adsorption of aromatics on halide-pretreated surfaces in relation to the ease of displacement of preadsorbed halide; (2) displacement of preadsorbed aromatic by halides including the influence of adsorbed aromatic orientation, halide concentration, and electrode potential; (3) competitive adsorption from solutions containing various proportions of aromatic and halides. Hydroquinone (HQ) and 1,4-naphthohydroquinone (NHQ) were employed as model aromatic compounds since the adsorptive properties of these substances on Pt have been studied extensively.'
(1) (a) Soriaga, M. P.; Hubbard, A. T. J . Am. Chem. SOC.1982, 104, 2735. (b) Ibid. 1982,104,2742. (c) Ibid. 1982,104,3937. (d) Soriaga, M. P.; Wilson, P.H.; Hubbard, A. T.; Benton, C. S. J . Elecfroanal Chem. 1982, 242, 317. (e) Chia, V. K.F.; Soriaga, M. P.; Hubbard, A. T.; Anderson, S. E. J . Phys. Chem. 1983,87, 232. ( f ) Soriaga, M. P.; White, J. H.; Hubbard, A. T. Ibld. 1983,87, 3048. (8) Stickney, J. L.; Soriaga, M.P.;Hubbard, A. T.; Anderson, S. E. J. Elecfroanal. Chem. 1981, 125,73. (h) Soriaga, M. P.; Stickney, J. L.; Hubbard, A. T. Ibid. 1983, 144,207. (i) J. Mol. Cafal.1983, 21,211. (j) Soriaga, M. P.;Hubbard, A. T. J . Electroanal. Chem. 1983,159, 101. (k) J . Phys. Chem., in press. (I) Ibid., 1984, 88,1089. (m) Soriaga, M. P.;Chia, V. K. F.; White, J. H.; Song, D.; Hubbard, A. T. J. Electroanal.
Experimental Section The measurement of absolute surface coverages based on thin-layer coulometry* has been described.lc*eJ' The amount ad-
Chem., in press. (n) Soriaga, M. P.; Hubbard, A. T. Ibid.in press. ( 0 ) Chia, V. K.F.; Soriaga, M. P.;Hubbard, A. T. Ibid.,in press. (2) (a) Hubbard, A. T.; Stickney, J. L.; Rosasco, S. D.; Soriaga, M. P., Song, D. J. Electroanal Chem. 1983,150, 165. (b) Stickney, J. L.; Rosasco, S. D.; Song, D.; Soriaga, M.P., Hubbard, A. T.Surf.Sci. 1983, 130, 326. (3) Katekaru, J. Y.; Garwood, G. A.; Hershberger, J. F.; Hubbard, A. T. SurJ Sci. 1982, 121, 396.
(4) (a) Hubbard, A. T. Acc. Chem. Res. 1980,13, 177. (b) J . Vac. Sci. Technol. 1980, 17, 49. (5) Felter, T. E.; Hubbard, A. T.J. Elecrroanal. Chem. 1979, 100, 473. (6) Garwood, G. A.; Hubbard, A. T. Surf.Sci. 1980, 92, 617. (7) Lane, R. F.; Hubbard, A. T. J. Phys. Chem. 1975, 79, 808.
0022-3654/84/2088-2284$01.50/00 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2285
Ligand Substitutions a t Metal Surfaces sorbed is determined from the disappearance of aromatic compound from the solution phase upon contact with a Pt electrode. For the subject compounds, detection of unadsorbed material takes advantage of their reversible quinone diphenol reaction, since attachment through the electroactive center alters the electrochemical properties of the adsorbed intermediate relative to the unbound species.la,d,g-hr is given by i[ ( e - Qb) - (Q2 - &)I r = [ ( e - Qb) - (QI - Q i d lnFA
where (Q - &)des is the charge for desorbed diphenol. H Q was studied with F, CP, and Br-; overlap of the 12/1- and benzoquinone/hydroquinone redox couples (in l M H+) made accurate quantitation of (Q for HQ difficult. The effects of I- and Br- were explored with NHQ. Molecular orientation of aromatics coadsorbed with iodine was determined from the molecular crosssection, u (A2/molecule), given bylb 10I6/NA- uIFI
r
40
35
30
25
(1)
where Q1, Q2, and Q, respectively, are the electrolytic charges for one, two, and multiple (three or more) fillings of the thin-layer cavity, Qb the appropriate background charge, A the electrode surface area: and n (equals 2 ) the number of Faradays per mole of diphenol. Equation 1 takes into account the extreme sensitivity of r to concentration in the orientational transition region;'" at concentrations where r is constant, (Q - Qb) is generally equal to (Q2 - QZb). For very dilute solutions, multiple rinses may be necessary to satisfy the adsorptive demands of the surface, and the procedure follows that described elsewhere.lctd Equation 1 is applicable whether the electrode is clean or previously treated with halide, and whether the solutions contain halide or not. Halide pretreatment was done by exposure of the clean Pt thin-layer electrode to aqueous 1 mM halide solution followed by removal of unadsorbed species by rinsing with pure solvent. Pretreatment with aromatic was carried out similarly, except that the exposure was made at selected concentrations when a specific adsorbed aromatic orientation was desired." Electrodes pretreated in this way were then given a single rinse with halide solution. Desorbed starting material remained inside the thin-layer cavity; detection was by thin-layer voltammetry, the amount displaced (ArdeS)quantitated by coulometry:
u=
45
(3)
where the subscript I refers to coadsorbed iodine and NA is Avogadro's number. rIwas measured by oxidation of adsorbed iodine followed by coulometry of the five-electron reduction of IO; to 12,1b*'uI is constant:lb uI(expt) = 14.1 A2, uI(calcd) = 14.5 A2; molecular area calculations have been To minimize ohmic drops during electrolysis the halide solutions contained 1 M HC104in pyrolytically distilled water: except when F was studied for which bufferedIa 1 M NaClO,, was used. Smooth polycrystalline Pt electrodes4 were employed.la.d In between experimental trials, the electrodes were cleaned by electrochemical oxidation [ 1.2 V (AgCl reference) 1 M H+; 0.90 V at pH 71 and reduction (-0.20 V in 1 M H+; -0.65 V at pH 7). The time allowed for adsorption was 3 min. Adsorption was carried out a t a fixed potential (0.20 V in 1 M H+; -0.20 V at pH 7), although adsorption without potential control gave identical results.la'o The time allowed for the displacement reactions was varied from 3 to 10 min; in some cases, desorption potential was also varied. The experiments were made at room temperature.
L
-
aoi 000 L
-
. 1 mM1c,d)was identical with that for the clean electrode. Displacement of adsorbed F by HQ occurred spontaneously. (ii) No adsorption occurred on I-pretreated Pt; at least up to 3 mM, HQ did not displace adsorbed I. The inertness of iodine superlattices on Pt has been demonstrated in ultrahigh v a c ~ u mand ~*~ s o l ~ t i o studies. n ~ ~ ~ (iii) ~ ~ The ~ ease of halide displacement, F >> C1- > Br- >> I, reflects the relative strengths of halide coordination in Pt(I1) and Pt(1V) mM." (iv) The inhibition of HQ adsorption by C1- and Br- pretreatment was more pronounced at low than at high concentrations; above 3mM, the preadsorbed C1and Br- were eventually displaced, yielding an organic layer of I' indicative of v2 structures.lc~d The stronger suppression of H Q adsorption at @ < 0.1 m M by preadsorbed C1- or Br- may be related partly to the fact that v6 adsorption requires a greater number of contiguous sites. On the other hand, since adsorption of one large aromatic molecule would liberate several small halide particles, entropy effects might be expected to favor halide displacement by v6-adsorbed species.
( 8 ) Hubbard, A. T. Crit. Rev. Anal. Chem. 1973, 3, 201. (9) Conway, B. E.; Angerstein-Kozlowska,H.; Sharp, W. B. A,; Criddle, E.E.Anal. Chem. 1973, 45, 1331.
(10) Lane, R. F.; Hubbard, A. T. J . Phys. Chem. 1975, 81, 734. (1 1) Hartley, F.R."The Chemistry of Platinum and Palladium"; Applied Science Publishers: London, 1973.
e
2286 The Journal of Physical Chemistry, Vol. 88, No. 1 1 , 1984 TABLE I: Displacement of Adsorbed 1,4-Naphthohydroquinone by Br- and Ipotential Ardcs,bPc concn: v vs. nmol Aracs/ halide M AgCl cm-2 rc Flat ( q l 0 ) NHQ Orientation Br0.001 0.20 0.0 14 0.06 0.30 -0.10 0.075
0.010
-0.20 0.40 0.20 0.00
-0.10 0.10 1.o
0.20 -0.10 0.20
-0.10 -0.20
1-
0.001 0.010
0.00 -0.10 0.00 -0.10
Br-
0.10 0.16 0.11 0.21 0.31 0.21 0.38 0.25 0.43 0.25
0.176 0.188 0.212 0.228
0.71 0.76 0.86 0.92
Vertical (2,3-q2) NHQ Orientation 0.001 0.30 0.059
0.010
0.10
1.o 1-
0.024 0.040 0.026 0.052 0.078 0.052 0.094 0.061 0.106 0.061
0.20 0.00 -0.10 -0.20 0.40 0.10 0.00 -0.10 -0.20 0.20 0.00
-0.10 0.20 0.00 -0.10
0.001
0.00 -0.10
0.010
0.00 -0.10
0.10
Soriaga et al. TABLE 11: Displacement of Adsorbed Hydroquinone by F,CI-, and Br-
halide
-
Ardes/ r c
10
0.0
0.0
0.10
10
0.0
0.0
c1-
0.001 0.010
10
0.0
10
0.10
3 10 3 10
0.0 0.007 0.007 0.009 0.009
0.0 0.0 0.02 0.02 0.03 0.03
0.014 0,019 0.024 0.031 0.035 0.045 0.059 0.075
0.042 0.056 0.073 0.095 0.1 1 0.14 0.18 0.24
1.o
Br-
0.001
3
10 0.010
0.10 1.o
0.10
-
0.18 0.22 0.16 0.16 0.16 0.21 0.25 0.19 0.20 0.26 0.31 0.24 0.32 0.37
0.355 0.374 0.470 0.497
0.63 0.66 0.83 0.88
Chemical potential considerations cannot be ignored. For example, the concentration at which the I? vs. log C curves starts to increase rapidly is identical for both C1- and Br- pretreated surfaces, -0.5 mM. On clean surfaces, -0.5 mM is the minimum concentration at which q2 adsorption is completed. Also, on surfaces purposely precoated with flat-adsorbed intermediates, formation of vertically oriented species (severely retarded relative to untreated surfaces) does not begin until -0.5 mM." Displacement of Adsorbed Aromatic by Halide. Figure 2 provides evidence for displacement of starting material when an electrode precoated with N H Q was exposed (single rinse) to 0.5 mM I- at open circuit. It can be seen that (i) adsorbed N H Q was not displaced by water or 1 M HC104, (ii) N H Q was chemisorbed on Pt largely without decomposition, and (iii) Arda for N H Q was higher for q2 than for qIo orientations. Quantitative results on N H Q displacement by I- and Br- are summarized in Table I: (i) virtually all of the initially adsorbed N H Q was displaced by 10 mM I- at -0.10 V, regardless of orientation. This is evidence that N H Q is not decomposed when adsorbed either in the flat or edge orientation. (ii) while N H Q was displaced quantitatively by 10 m M I-, the amount desorbed by Br- was considerably lower. Coupled with the data in Figure 1, this indicates that the adsorption strength of aromatics is much greater than that of Br- but lower than that of I-. (iii) ATdesfor
-
Ardes,b'c time, nmol min cm-* (a6) HQ Orientation
F
0.059 0.103 0.127 0.089 0.092 0.089 0.1 18 0.141 0.106 0.113 0.148 0.173 0.136 0.179 0.207
"The solutions contained 1 M HC104. bDesorption time was 3 min. Initial r's for flat and vertical orientations were 0.247 and 0.564 nmole cm-2, respectively. The average standard deviations in r and Ardesare &3% below 1 mM and &6% above 1 mM.
concn," M Flat
3 10 3 10 3 10
Vertical (2,3-q2) HQ Orientation -
10
0.028
0.047
F
0.10
10
0.024
0.040
c1-
0.001 0.010 0.10
10 10 3 10 3
0.072 0.10 0.12 0.12 0.14
10
0.045 0.064 0.070 0.070 0.085 0.089
3 10 3 10 3 10 3 10
0.054 0.063 0.070 0.080 0.106 0.1 13 0.1 13 0.134
0.088 0.10 0.12 0.13 0.17 0.18 0.18 0.22
1.o
Br-
0.001 0.01 0.10
1.o
0.15
"The C1- and Br- solutions contained 1 M HC10,; the F solution contained 1 M NaC104, pH 7. bDisplacementreactions were at open circuit (no potential control). CInitialr for flat and vertical orientations were 0.320 and 0.602 nmol cm-2, respectively. The average standard deviations in F and ATdcsare 1 3 % below 1 mM and &6% above 1 mM. N H Q is higher for q2 structures than from q l 0 oriented layers, as expected since the surface-binding strength of q'O-NHQ is greater than that of q2-NHQ.ld However, the extent of desorption (AFdes/I?) appears to be independent of orientation. (iv) The amount of desorbed N H Q was slightly influenced by electrode potential: the lower the potential, the greater the amount displaced. Maximum desorption occurred at -0.100 V; ATdeswas lowered at -0.200 V because of hydrogenation side reactions which destroyed the quinone/diphenol functionality.'J Under the present conditions, Ardes/r (at constant halide activity) appears to be determined primarily by the extent of halide coadsorption; at a fixed halide concentration, the amount of coadsorbed halide approached a constant (optimum) value, and the final (undesorbed) N H Q orientation was the same as the initial orientation. It has been shownlb that N H Q which remained on the surface after a layer of qlo-NHQ was exposed to dilute acidic I- retained its original flat orientation. In this study, when q2-NHQ was exposed to 1.0 mM I- at 0.00 V (Table I), the amount of coadsorbed I was measured to be 0.71 nmol cm-2; from eq 3, thefinal u of coadsorbed N H Q was found to be 30.0 A2, indicating retention of the original 2,3-q2 structure ( C T ~ =I 29.0 ~ A2) ,lc,d
The potential dependence of Ardg is not consistent with specific adsorption of halide.Im Surface organoplatinum bonding con-
J . Phys. Chem. 1984,88, 2281-2293 4.0
4.5 100 mM F -
N I
5
o I mM
cl.
IO IO 01 IO
CI-
mM mM mM mM
3.5
3.0
2.5
3.5
3.0
2.5
HYDROQUINONE
Cl0,01-
W 0 40
dH
Y
m
0 x
0301
0 20
L 0. I O F 0.00
4.5
4.0
-LOG C(M) Figure 3. r vs. log C curves for hydroquinone adsorbed onto clean Pt from solutions containing halide. The solid lines connect experimental points and do not assume any theoretical fit. TABLE 111: Competitive Adsorption between I- and 1.4-Naohthohvdroauinone concn: mM NHQ I0.100 0.5 0.100 1.0 0.124 0.5 0.124 1.0 0.240 0.5 0.244 1.0 0.576 0.5 0.578 1.0
r:
nmol cm-2
"Q 0.108 0.086 0.158 0.125 0.223 0.183 0.400 0.348
1-
0.698 0.910 0.569 0.834 0.569 0.761 0.268 0.477
upy
predom
molecule 61.5 61.1 54.4 38.9 38.4 31.8 32.1 30.3
orientnd
"Q
flat (q") qIo $0
tilted tilted vertical (2,3-q2) 2,3-q2 2,3-a2
"The solutions contained 1 M HC104. bThe average relative standard deviations in r were k3%. uNHQ was obtained from eq 3 with uI = 14.1 A2. dThe calculated uNHQ values for $lo and 2,3-q2 orientations are 69.9 and 29.0 A2, respectively.'a,d siderations are indicated. If surface attachment is viewed as a balance between aromatic-to-metal electron donation (u-bonding) and metal-to-electron back-donation (a-back bonding)," an increased negative charge on the electrode would weaken the aromatic-Pt interaction. It may be mentioned that Pt(0) complexes
2287
are usually stabilized by ligands which alleviate charge buildup on the zerovalent metal." Table I1 summarizes results on H Q displacement by halides. F did not displace H Q from the surface. Comparison of data for C1- and Br- media demonstrates that $-HQ is more strongly adsorbed than q2-HQ. At a given Br- concentration, only a slight increase in Ardes was noted as the reaction time was increased, but no such variation was observed for C1-. Competitive Adsorption between Aromatic and Halide. r vs. log C curves for HQ on clean Pt in solutions which contain halide are shown in Figure 3: (i) The adsorption and reorientation of H Q was unhindered by F.(ii) Dilute (CO.l mM) C1- did not affect formation of v6 or q2 adsorbed species, but completion of the q2 oriented layer was slightly retarded. (iii) The effect of 1 mM C1- was the same as that of 0.1 mM Br-; although l7 was significantly lower at all concentrations, the overall shape of the adsorption isotherms remained unchanged. (iv) Except in the transition region, the effect of 10 mM C1- was not too different from that of 1 mM C1-. The adsorbability of C1- is such that it can retard reorientation but not null out q2 adsorption. (v) In the presence of 1 mM Br-, r was lowered dramatically, but the decrease in r was not as severe at high than at low concentrations of aromatic. The data in Figures 1 and 3 show a number of similarities: The general shapes of the isotherms suggest that, at the C1- and Bractivities studied, adsorption of H Q at C0.5 mM leads to flat orientations, adsorption of > 1.O mM to vertical structures. Data on competitive adsorption between NHQ and dilute acidic I- are summarized in Table 111. The rate of N H Q adsorption is at least equal to that of I-, since N H Q adsorption was substantial.lb The influence of NHQ/I- concentrations on adsorbed \amounts was as expected. It is of interest to note that, at 0.12 mM, NHQ adsorption in the presence of 1.0 mM I- yielded partially reoriented intermediates; at higher N H Q concentrations, adsorption was in the v2 orientation even for low I- activities. In contrast to the other halides, I- is strongly coordinating toward the Pt surface to bring about molecular reorientations even at comparatively low aromatic concentrations.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Air Force Office of Scientific Research for support of this research. Registry No. Pt, 7440-06-4; F, 16984-48-8;C1-, 16887-00-6;Br-, 24959-67-9; I-, 20461-54-5; HQ, 123-31-9; NHQ, 130-15-4.
Generalized Methods for Determining Thermal Activation Energies: Applied to Bromosodalite Witold P. Maszara* and Lee T. Todd, Jr. Department of Electrical Engineering, University of Kentucky, Lexington, Kentucky 40506 (Received: July 11, 1983)
The evaluation of thermal activation energies of electron traps located in the bandgap of crystalline materials with absorption bands in the visible spectrum is discussed. Relations between the activation energy and various parameters of the thermal decay curves are derived. Both isothermal and constant rate thermal bleaching experimentsare used to determine the activation energies of samples with established order of bleaching kinetics as well as samples with undetermined order of kinetics. The methods were applied to bromosodalite to determine an activation energy of 1.45 eV. Sodalite samples were found to obey bleaching kinetics of the intermediate order between first and second.
Introduction Basic principles of kinetics regarding the decay of luminescence in crystals were described in several early monographical works.'-* *On leave from Technical University of Wroclaw, Poland. 0022-365418412088-2287$01SO10
Two differeht mechanisms-monomolecular (kinetics of the first order) and bimolecular (kinetics of the second order)-were an(1) H. W. Leverenz, "An Introduction to Luminescence of Solids", Wiley, New York, 1950.
0 1984 American Chemical Society
I
