Langmuir 1985,1, 131-135 We believe that these results are truly representative of the two-dimensional diffusion in the planar layers between the bilayers.
Acknowledgment. We acknowledge the financial support of the donors of the Petroleum Research Fund,
131
administered by the American Chemical Society, Research Corporation, National Science Foundation, the Drexel University Graduate School, and Computer Center. We also thank Prof. C. Rorris for helpful discussions. Registry No. SHBS,67267-95-2;water, 7732-18-5.
Kinetics of Displacement and Charge-Transfer Reactions Probed by SERS: Evidence for Distinct Donor and Acceptor Sites on Colloidal Gold Surfaces C. J. SandrofPt and D. R. Herschbach*t Exxon Research and Engineering Co., Annandale, New Jersey 08801 Received August 17, 1984. In Final Form: October 19, 1984 Both the electron donor W F and the electron acceptor TCNQ give large SERS signals upon adsorption onto colloidal gold particles, but only TTF displaces previously adsorbed pyridine, indicating that molecular donors and acceptors ockupy different surface sites. The acceptor (Lewis acid) site probably involves a reducible metal complex of Au+ while the donor (Lewis base) site may be associated with AuO. The kinetics for the charge-transfer reaction involving two distinct TTF oxidation states, TTF0.3+ TTF'.O+, can be explained by migration of the adsorbate from the donor to the acceptor site.
-
Introduction The degree of charge transfer between certain molecular adsorbates and metal surfaces can be determined from vibrational frequency shifts observed by surface-enhanced Raman scattering.l We have found that electron donors and acceptors based on tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) form with the noble metals particularly harmonious adsorbate/metal systems. These molecules absorb very strongly on silver and gold surfaces, transfer considerable amounts of charge, and give rise to large surface-enhanced Raman (SERS)signals. In experiments with these charge-transfer systems, we observed chemical transformations on gold colloidal surfaces involving two distinct oxidation states of adsorbed TTF a partially oxidized TTF species, TTF0.3+, disappeared in favor of the radical cation, TTF'.O+. Here we report a kinetic study of this surface oxidation and competitive adsorption experiments with pyridine, TTF and TCNQ. Our results can be rationalized by assuming that molecular donors and acceptors occupy different sites on the gold colloidal surface. The surface sites which act as acceptors or Lewis acids seem to involve a gold complex with the metal in a high oxidation state. These sites are readily reduced by proficient electron donors like 'ITF which have exceptionally low ionization potentials. The surface sites that serve as donors or Lewis bases behave like the neutral metal and readily give up charge to acceptors like TCNQ with sizable electron affmities. The donor sites, in contrast to acceptor sites, seem to possess substantial amphoteric character; they can behave as Lewis acids or bases depending on the difference between the work function of gold and the electron affinity or ionization potential of the adsorbate molecule. Thus TTF, when adsorbed as the partially oxidized TTF0.3+,binds to the Auo site, while TTF'.O+ attaches strongly to the Au+ site. The relative
populations of and T"F1.'H do not remain constant in time after the initial adsorption onto the gold colloids. Rather, TTF0.3+converts to the radical cation with a fmliorder rate constant of 1 X lo4 s-l. The two-site model suggests that this interfacial charge transfer involves migration from a donor to an acceptor site.
Experimental Section Aqueous colloidal suspensions consisting of 150-&diametergold particles at a concentration of 2 X lo1' cm-3 were prepared by the sodium citrate method of Turkevich et al.2 Adsorbates were introduced to 20 cm3 of the sol by adding 2 drops of a 5 X M solution. (Solventswere water for pyridine and acetone for the charge-transfer compounds.) The colloid turned from red to blue 30 min after the addition of pyridine, indicating partial aggregation of the sol particle^."^ Addition of TTF turned the sol to blue instantly, showing that TTF is much more effective at reducing the net (negative)charge on the colloidal surface than is pyridine. The very fast aggregation caused by "Fis consistent with the flocculation time predicted by Smoluchowski's6theory of diffusion-limitedaggregation. For our dilute colloidal suspension, aggregation should occur in about 1s once the repulsive interactions between the charged particles are significantly reduced.' As in previous work? only after aggregation had occurred could strong SERS signals be seen. In contrast to electron donon like pyridine and TIT, the electron acceptor TCNQ did not cause any significant aggregation, and SERS spectra of TCNQ were obtained only after the colloid was aggregated by pyridine or TTF. (1) Sandroff, C. J.; Weitz, D. A.; Chung, J. C.; Herschbach, D. R. J. Phvs. Chem. 1983 87. 2127. 12) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday SOC. 1947. 11. 58. ( 3 ) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. SOC., Faraday Trans. 2, 1979, 75, 790. (4) See: Creighton, J. A. In 'Surface Enhanced Raman Scattering"; Chang, R. K., Furtak, T. E., Eds.; Plenum Press: New York, 1982; "Metal
Colloids".
(5) Turkevich, J.; Garton, G.; Stevenson, P. C. J. Colloid Sci., Suppl. 1954, 1, 26. (6) Von Smoluchowski, M. Physic 2. 1916, 17, 557, 858; Z. Phys. Chem. 1917,92, 129. ~~
'Present address: Bell Communications Research, Murray Hill, NJ 07974. Exxon Faculty Fellow from Harvard University.
*
0743-7463/85/2401-0131$01.50/0
(7) 'Colloid Science"; Kruyt, H. R., p 278.
Ed.; Elsevier:
0 1985 American Chemical Society
Amsterdam, 1952;
132 Langmuir, Vol. 1, No. I, 1985
Sandroff and Herschbach Table I. Characteristic Raman Frequencies (cm-') adsorbed on gold homogeneous colloids mode and phase" nomina 1 symmetry neutral ion obsd freq chargeb TTFO TTF+
PYRIDINE DISPLACED BY T T F AU COLLOID
PYRIDINE
-i
v,(A,)' 1)g(Ag)
1512 468
1416 506
TCNQO V~(AJ v4(Ag)'
+,(Ag)
1410, 1482 507, 476
+1.1, +0.3 +1.0, +0.3
1600 1386 1210
-1.1
TCNQ-
1602 1454 1207
1614 1392 1209
From ref 8 and 9 for TTF, from ref 10 for TCNQ. Estimated as described in ref 1; estimated error in nominal oxidation state is about 0.1 electron charge. cThesemodes are most sensitive to the degree of charge transfer. 500
600
700
EGO
RAMAN SHIFT
900
1000
1100
12K
(cm-')
Figure 1. SERS spectra showing displacement of pyridine by TTF. Trace a was recorded before the addition of TTF to a sol previously aggregated with pyridine. Trace b was taken about 1min after addition of TTF to that sol. Most of the TTF exists on the surface as TTF0.3+. GOADSORPTION OF PYRIDINE AND TCRO AU COLLOID
I 1
1
1
1350
1450
1550
i~
1150
' 250
M 1650
RAMON SHIFT (cm-';
Figure 3. Results of adding TCNQ to colloidal surface with adsorbed TTF. SERS spectra pertain (a) to system 3 min after initial addition of TTF and (b) to same system after addition of TCNQ. When TCNQ'.@signals appear a decrease in TTF0.3+ population and increase in TTF'.O+ is seen.
RAMAhr S H I F T (Cm-')
Figure 2. Same as Figure 1,except that TCNQ was added rather than TTF. The TCNQ species on the surface, TCNQ',@,does not displace pyridine. The SERS spectra were recorded with an optical multichannel analyzer (PAR OMA 11) using 80 mW of 647.1-nm radiation from a Kr+ laser.
Results Competitive Adsorption. Figures 1 and 2 show the effect of adding TTF and TCNQ to colloidal gold particles previously aggregated with pyridine. ?TF is quite efficient at displacing weakly chemisorbed pyridine as shown by the nearly complete disappearance of the intense ring breathing mode of pyridine (at 1014 cm-') shortly after the introduction of TTF. The SERS spectra in Figure 1were obtained with 30 s of integration, but we could detect decreases in pyridine signal intensity approximately 1 s after adding TTF. The behavior of TCNQ is markedly different from TTF. Although signals from absorbed TCNQ appear as rapidly and are as intense as those from TTF, TCNQ does not displace previously adsorbed pyridine. While changes in peak intensities permit displacement kinetics to be measured, it is peak frequencies that identify the displacing species. Thorough spectral assignments exist for both TTF and TCNQ in environments where different oxidation states of the molecules obtain.8-10
Table I compares vibrational frequencies of several donor and acceptor species in the solid state to those of the most intense SERS bands. As reported earlier,l TTF exists on the surface in two forms, which from vibrational frequency shifts can be assigned as TTF0.3+and TTF1.O+. On the other hand, TCNQ adsorbs in predominantly one form as the radical anion, TCNQ'.". Figure 3 shows the effect of adding TCNQ to the gold colloid after TTF had been permitted to adsorb. The SERS spectrum characteristic of TCNQ'." appeared almost immediately, accompanied by a sharp decrease in the intensity of the u3 band of TTF0.3+ and an increase in the us band of TTF'.O+. Temporal Behavior of TTF Species. The spectrum in Figure 1 represents only the initial stages of TTF adsorption, in which TTF exists primarily as TTF0.3+.In Figure 4 we display the temporal evolution of the SERS bands corresponding to TTF0.3+and TTF1.O+,showing that the surface coverage of the latter increases in time while that of the former decreases. The rates of disappearance and appearance of TTF'.O+ are roughly equal of TTF0.3+ and characterized by a first-order rate constant of 1 X lo4 s-l. SERS intensities of the two oxidation states are plotted in Figure 5 as a function of time, for the case when pyridine is previously adsorbed and when TTF is allowed to adsorbed onto a fresh colloidal surface. Whether pyr(8) Bozio, R.; Zanon, I.; Girlando, A.; Pecile, C. J . Chem. Phys. 1979, 71, 2282. (9) Seidle, A. R.; Candela, G. A.; Finnegan, T. F.; VanDuyne, R. P.; Cape, T.; Kokoszka, G. F.; Woyciejes, P. M.; Hashmall,J. A. Inorg. Chem. 1981,20, 2635. (10) Bozio, R.; Girlando, A,; Pecile, C. J . Chem. SOC.,Faraday Trans. 2 1975, 171, 1237.
Langmuir, Vol. 1, No. 1, 1985 133
Kinetics of Displacement and Charge- Transfer Reactions T T F o 3 * -TTF'Ot A U COLLOID
