J. Phys. Chem. 1985,89, 211-213 I
c
/
'"1
I
n
1
I
I
I
I
1/ I1
0.01 0
/
I
I
I
I
I
I
I
HNTIETHANOL
c
I
,',V, I
600
I
1200
I
,
I
I
1600
RELATIVE T I M E
I
I
1
2400
I
! 3000
/ ps
Figure 4. Time-resolved normal fluorescence spectrum of HNT in ethanol. The solid curve was generated from a 650-ps exponential decay convoluted with the instrument response function. The sharp peak is explained in the Figure 3 caption.
state equilibrium between the molecules which are intramolecularly hydrogen bonded (intra-H-bonded) and those which are hydrogen bonded only to the solvent molecule(s) (inter-H-bonded). It follows from the kinetic data that only those molecules which are intra-H-bonded at the time of excitation undergo ESIPT and that proton transfer proceeds very rapidly (
AU SURFACE
t
1418
z X
A I Pt COLLOID
4
1
1300
1400
1500
RAMAN S H I F T
~
250
350
,
450
,
550
Figure 2. A partial 416-nm Raman spectrum of a Pt colloid with lo4 650
750
WAVELENGTH ( N M I
Figure 1. Absorbance spectra for the Pt colloid with added TTF. Trace
A without TTF shows the true absorbance scale. Traces B and C are vertically offset for clarity. The major band of TTF’ near 430 nm is labeled in C.
(surface enhanced Raman scattering) effect exhibited by molecules adsorbed on partially aggregated Cu, Ag, and Au colloid^.^-'^ Sandroff and Herschbach have reported that colloidal Au is able to oxidize adsorbed TTF (tetrathiafulvalene) to TTP and reduce adsorbed TCNQ (tetracyanoquinodimethane) to TCNQ-, at different surface sites.s This intriguing observation appears to imply that these two surface sites have different redox potentials; Le., the metal surface is not characterized by one Fermi energy. In the Au colloid, adsorbed TCNQ- and TTF+ were detected by the SERS phenomenon. The use of SERS gives high, surface specific detection sensitivity, yet limits the experiment to a few metals, at specific Raman scattering wavelengths, and in partial states of aggregation.8-’0 We attempt now to generalize these experiments by using the molecular resonance Raman effect to observe the surface redox products of TCNQ and TTF in colloidal systems where SERS effects are absent.
Experimental Section Our pulsed laser, multichannel detection Raman apparatus has been previously described.” Absorption spectra were taken on a Perkin-Elmer 330 spectrophotometer. TTF and TCNQ were purified by recrystallization and used as stock solutions in tetrahydrofuran. 0.2 g/L Pt colloids were made by a procedure similar to that described by Turkevich and co-workers.’ A 632-cm3 solution, containing lO-’M PtC1,- and 0.2 wt % sodium citrate, is refluxed in a 2-L flask for 4 h. The initially colorless solution becomes blackish (but not cloudy) and has a relatively flat (free electron metal-like) absorbance spectrum (Figure 1A) after refluxing. Turkevich reports the Pt particles have diameters near 30 A. High-resolution TEM images directly show an internal lattice structure characteristic of crystalline Pt.2 The small crystallites are prevented from aggregating and sedimenting by (2) R. S. Miher, S. Namba, and J. Turkevich, Stud. Surf. Sci. Curd., 7, 160 (1981). (3) P. A. Brugger, P. Cuendet, and M. Gratzel, J. Am. Chem. Sor., 103, 2923 (1981). (4) A Henglein, J . Am. Chem. SOC.,102, 3461 (1980). (5) A. Henglein and J. Lille, J . Am. Chem. Soc., 103, 1059 (1981). (6) D. S. Miller, A. J. Bard, G. M. McLendon, and J. Ferguson, J . Am. Chem. SOC.,103, 5336 (1981). (7) C. J. Sandroff, D. A. Weitz, J. C. Chung, and D. R. Herschbach, J . Phys. Chem., 87, 2128 (1983). (8) C . J. Sandroff and D. R. Herschbach, Longmuir, submitted for pub-
lication. (9) M. Kerker, 0. Silman, and D. S . Wang, J . Phys. Chem., 88, 3168 (1984).
(IO) J. A. Creighton, M. S. Alvarez, D. A. Cuertz, S. Garoff, and M.W. Kim, J . Phys. Chem., 87, 4793 (1983). (1 1) R. Rossetti, S. M. Beck, and L. E. Brus, J . Phys. Chem., 87, 3058 (1983).
M injected TTF. The strong TTFC resonance Raman line at 1436 cm-I
-
is labeled. This line is present immediately after injection; it decays on a I-h time scale. An unidentified product line labeled X at 1477 cm-‘ grows in as TTF+ disappears.
electrostatic double layers formed by adsorption of citrate ions on the Pt surface.
Observations Injection of TTF stock solution into the Pt colloid in the 104-104 M range produces the optical changes shown in Figure 1, B and C. Near M structured absorption becomes deM (Figure tectable in addition to the Pt absorption. At 5 X 1C) this spectrum closely resembles the well-known TTF+ spectrum.12-15 T T F and the dimer (TTF+)2 are not apparently observed. The colloid aggregates, and ultimately sediments, upon injection of TTF. In the low 10” M range, the Pt spectrum M, sedimentation of flattens as aggregation occurs. Above visible aggregates occurs in about -0.1 h. TTF has very low solubility ( 3) parallel polarization. We conclude that we observe aqueous solvated T T P in the Pt colloid. M injected TTF, the intensity of the TTF+ 434-nm At 5 X absorption band (Figure 1C) indicates that -3 X M TTF+ has been made. As previously mentioned, neutral TTF has very low solubility in water. We surmise that when TTF is injected it initially adsorbs on the Pt particles, as also occurs in the Au colloids. It is oxidized to TTF+ and desorbs. We investigated the possible involvement of dissolved 02. Colloids refluxed under pure 02,pure N2, and air all have the same ability to oxidize TTF. In addition, removal of dissolved oxygen before TTF injection, in a colloid refluxed under air, had no effect on the observations. Dissolved oxygen and/or a citrate reaction product with O2 do not appear to be involved. TCNQ is slightly soluble (1 X 10-5-2 X M) in water at neutral pH. A Pt colloid with injected TCNQ, in this concentration range, shows the absorbance of neutral TCNQ (397 nm) without evidence for TCNQ-. A 395-nm resonance Raman spectra of this solution shows the expected strong lines of neutral TCNQ, without those of TCNQ-.I7 These data indicate that l(r5 M TCNQ is not substantially affected by the presence of the colloidal Pt crystallites under our experimental circumstances.
Discussion The liquid-phase redox reactions of TTF and TCNQ are governed by the standard potentials:18
-
+ eTCNQ + eTTF’
TTF E,, = +0.54 V vs. N H E
TCNQ- Eo = +0.37 V vs. N H E
The fact that the colloidal Pt crystallites spontaneously oxidize TTF, and do not spontaneously reduce TCNQ, implies that the Pt crystallites are oxidizing agents with an effective potential above N +OS4 V vs. NHE. Bulk Pt metal has a work function of 5.6 eV; this value on the electrochemical scale corresponds to the potential +0.95 V. In (17) D. L. Jeanmaire and R. P. Van Duyne, J. Am. Chem. Soc., 98,1029 (1976). (18) A. J. Bard and L. R. Faulkner, “Electrochemical Methods”, Wiley, New York, 1980, Table (2.2. These values refer to acetonitrile solutions; the values in water should be quite similar.
solution, however, the bulk work function does not determine the crystallite redox potential. Colloidal Ag, Au, and Pt crystallites are known to readily exchange electrons with solution molecular species, so that the crystalline potential tends to equilibrate with the solution redox potentiaLM Pt and Ag crystallites can acquire and store substantial exnegative charge in solutions containing electron-rich (reducing) free radicals. Redox potential shifts as large at -1 V occur upon charging with electron^.^ Conversely, in our present case, the oxidizing behavior of the Pt crystallites indicate that they have not acquired substantial excess negative charge in the reducing environment of colloid formation. They must initially sit somewhere near +0.95 V. In the Figure 1C experiment, one can calculate that, with [TTF+] 3 X M, about 25 TTF molecules are oxidized by each Pt crystallite. If we assume a typical metabelectrolyte surface capacitance of -45 FF/cm2, then there is a drop in potential of -0.3 V across the Helmholtz double layer as the oxidation proceeds. Aggregation sets in as the double layer potential changes. Raman scattering experiments as a probe of electrochemical kinetics were originally initiated by Fleischmann and Hendra,I9 and Van D ~ y n e . ’ ~ ~Sandroff ” et al. pointed out the value of Raman spectroscopy as an in situ, nondestructive probe of the adsorbed TTF oxidation state. We see now the additional ability of Raman spectroscopy to reveal something of the local enuironmenf of molecular species. The intense TTF’ vj line occurs a t 1436 cm-’ in water, 1427 cm-I in acetonitrile, and 1418 cm-’ on an Au surface. The Raman information we obtain using the molecular resonance Raman effect is complementary to SERS information. Working near 430 nm, we detect aqueous TTF+ in the Pt colloid; in the partially aggregated Au colloid, Sandroff et al. detect adsorbed TTF’ using SERS at 647 nm. This red Raman wavelength is not well suited for detection of aqueous TTF’, and there could well be aqueous TTF’ in the Au colloids. There could also be adsorbed TTF+ in our Pt colloids that we do not detect in the absence of the SERS enhancement.
-
Acknowledgment. We had extensive discussions with, and useful suggestions from, B. Miller, J. D. E. McIntyre, and C. F. Sandroff. (19) M. Fleischmann, P. Hendra, and A. J. McQuillan, J . Chem. Soc., Chem. Commun., (1973).
