J. Phys. Chem. 1984, 88, 706-709
706
10
by syn correlation of donor-acceptor orbitals and the yield of separated radical ions is accordingly described by eq IX with = 1 under the conditions cited. (b) The exciplex state '\kIS(CR),stabilized with respect to the zeroth-order state 'ID+A-) by CI with I(DA*),is a lower-energy conformational isomer of the EDA complex state '\kla(CR) which is destabilized with respect to 'ID+A-) by CI with the ground state 'IDA). Since the stable donor-acceptor orientation (permitting ACMO) in the EDA complex ground state is preserved during electronic excitation, the absorption product '\kla(CR) may relax to the exciplex state l\kls(CR) by a viscosity-dependent donoracceptor reorientation to achieve the requisite syn correlation of donor-acceptor orbitals. As shown schematically in Figure 2, this orbital correlation switching provides the absorption-competitive emission sequence '\koa(CR)
+ hul + '\kIa(CR)
+
'\klS(CR)
+
D
Figure 3. Illustration of benzene (B)-ethylene (E) conformational isomers exhibiting SCMO (a) and ACMO (b). Filled (and empty) circles denote in-phase C2p(x) orbitals normal to the molecular plane. and anti correlation as illustrated for the benzene (D)-ethylene (A) system in Figure 3. If either isomeric configuration may be adopted by the various molecular or radical-ion states of donor and acceptor to achieve the relative orientation of lowest energy, then the following applies. (a) The geminate charge neutralization of radical-ion pairs produced by excited EDA complex dissociation may be controlled (19) Isomeric 1:l complexes have previously been proposed to account for multiple CT transitions in EDA complexes, e.g., Holder, D. D.; Thompson, C. C. Chem. Commun. 1972, 211, and references therein.
+A +h~2
manifest as the large viscosity-dependent Stokes shift (v, > u2) characteristic of EDA complex fluorescence.z0~2ZIt is moreover consistent with the current interpretation of this phenomenon based on2' "the change in geometrical and electronic structures of the complex occurring in the relaxation process from the FranckCondon state to the (equilibriumz2) fluorescent state", if these states are identified as '\kla(CR) and '\k.,'((CR), respectively. ~
~
~~~
~
(20) Nagakura, S.In "Excited States"; Lim, E. C., Ed.; Academic Press: New York, 1975; Vol. 2, p 321. Note, however, that the very large (- 12000 cm-') Stokes shifts exhibited by TCNB in pure donor solvents are attributed to emission by ternary complexes D2+A-. (21) It is concluded that ground-state orientations permitting ACMO are inaccessible to donor/acceptor pairs exibiting exciplex behavior since, by definition, CT absorption bands are not observed for these systems. (22) Thomas, M. M.; Drickamer, H. G. J . Chem. Phys. 1981, 74, 3198. 1981, 75, 5246.
Surface-Enhanced Raman Spectroscopy of Platinum.'i2 3. Enhanced Light Scattering of Electrogenerated Bromine and Tribromide Ions Coadsorbed on a Platinum Electrode B. H. Loo* and Y. G. Lee Department of Chemistry, The University of Alabama in Huntsville, Huntsville, Alabama 35899 (Received: June 20, 1983)
Surface-enhancedRaman scattering has been observed from anodically electrogenerated bromine and tribromide ions coadsorbed on a platinum electrode with the 488- and 514.5-nm Art laser lines as well as the 647.1- and 676.4-nm Kr' laser lines. The observed Raman band at 161 cm-I is attributed to surface vibrations of the Br3- ions adsorbed on Pt whereas the 305-cm-' band is attributed to surface vibrations of coadsorbed Br2 molecularly bound to the Pt electrode. This is the first report of surface vibrational spectra of Br, and Br,- on a metal surface.
Introduction In previous studies, we have demonstrated surface-enhanced Raman scattering (SERS) from electrogenerated I, and 13coadsorbed on a Pt electrode' and from electrogenerated CI2 molecularly bound to a Pt electrode., In the present work, which represents a part of our study on platinum as a substrate for surface-enhanced Raman spectroscopy, we investigate SERS from electrogenerated Br, and Br,- coadsorbed on a Pt electrode. The study of Pt as a substrate for surface-enhanced Raman spectroscopy is important because Pt does not have dielectric functions like those of the group 1B metals (Cu, Ag, and Au) to support
B. H. Loo,Soiid Stare Commun., 43, 349 (1982). (2) B. H. Loo,J . Phys. Chem., 87, 3003 (1983). (1)
0022-3654/84/2088-0706$01SO10
electromagnetic resonances on surfaces. Electromagnetic resonances on surfaces are crucial for some theoretical models (known as electrodynamic model^)^ proposed to account for the observed enormous enhancement in the Raman intensity from molecules or ions adsorbed on metal surfaces. Experimental Section The experimental setup for in situ Raman experiments was described previously.'t2 Reagent grade potassium bromide (J. T. (3) (a) S.S. Jha, J. R. Kirtley, and J . C. Tsang, Phys. Reu. B: Condens. Matter, 22, 3973 (1980); (b) S.L. McCall, P. M. Platzman, and P. A. Wolff, Phys. Lett. A , 77A, 381 (1980); (c) D. S. Wang, M. Kerker, and H. Chew, Appl. Opt., 19,4159 (1980); (d) J. L. Gersten and A. Nitzan, J . Chem. Phys., 73, 3023 (1980); (e) F. J. Adrian, Chem. Phys. Letr., 78, 45 (1981).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 4, 1984 707
SERS of Br2 and Br3- on Pt
0 653 659 653 659 653 659 VOLTAGE, V
vs SCE
Figure 1. Cyclic voltammogram for polycrystalline Pt in 0.1 M KBr solution. Sweep rate is 2 V/min.
Figure 3. Surface-enhanced Raman spectra for Pt in 0.1 M KBr solution for a sequence of applied potentials: (a) 0.8, (b) 1.0, (c) 1.2, (d) 1.3 V (SCE) (A, = 647.1-nrn Kr+ line; laser power 175 mW). 1
\ i\
- 6i
m
-> 0,
Z
!
I
\ A
t tn
EI-
z -
z a
2 653
659
665
WAVELENGTH,
671 nm
Figure 2. Surface-enhanced Raman spectrum for Pt in 0.1 M KBr solution at 1.2 V (SCE). The Stokes line at 161 cm-I is due to surface vibrations of the adsorbed Br< whereas the 305-crn-l band is due to surface vibrations of the coadsorbed Br2 molecularly bound to the Pt electrode (A, = 647.1-nm Kr+ line; laser power 175 mW).
Baker) was used in preparing 0.1 M KBr solutions. A polycrystalline Pt working electrode was prepared as described before.l12 The Pt electrode was introduced at a cathodic potential, and surface-enhanced Raman (SER) spectra were recorded during the anodic scan. The oxidation-reduction cycling treatment of the electrode surface was not necessary for observation of the SERS signals. All potentials are reported with respect to saturated calomel electrode (SCE).
Results and Discussion Figure 1 shows the cyclic voltammogram of Pt in 0.1 M KBr solution between -0.4 and +1.4 V (SCE). The bromide ion strongly adsorbs on the Pt electrode and greatly inhibits the cathodic hydrogen- and anodic oxygen-evolution reactions. At high anodic potentials where there is a flow of anodic currents, the bromide ion could possibly be oxidized to Br2 and Br3- according to the following reactions, re~pectively.~
+ 2e- 2BrBr3- + 2e- * 3BrBr,
0
653 659
WAVELENGTH (nm)
E o = 0.845 V (SCE) E o = 0.808 V (SCE)
Other species such as BrO-, BrOy, Br03-, and PtBr," could also possibly be formed at high anodic potential^.^ Figure 2 shows SERS for Pt in 0.1 M KBr solution at 1.2 V (SCE). The excitation wavelength was the Kr+ 647.1-nm line. Two bands are visible, one with a very broad shoulder at 161 cm-', the other at 305 cm-l. Similar SER spectra were obtained when the Kr+ 676.4-nm and the Ar' 488- and 514.5-nm lines were used. Figure 3 shows SER spectra for Pt in 0.1 M KBr solution at four different applied electrode potentials with 175-mW Kr+ 647.1-nm excitation. Figure 4 shows the voltage dependence of the SERS signals with 145-mW Ar+ 514.5-nm excitation for a sequence of applied potentials (0.8, 1.O, and 1.2 V (SCE)). The Pt electrode was first scanned from a cathodic potential to an (4) W. M. Latimer, 'Oxidation Potentials", 2nd ed., Prentice-Hall, Englewood Cliffs, NJ, 1952.
s
2
4-
(a) 0.8V
(b) 1.OV
(c) 1.2
v
2-
5:9'5:2'5:5
5;9'5:2'5:5 5:8'512'5:5 WAVELENGTH fnm)
Figure 4. Surface-enhanced Raman spectra for Pt in 0.1 M KBr solution at different applied potentials: (a) 0.8, (b) 1.0, (c) 1.2 V (SCE) ( h =~ 514.5-nm Ar+ line; laser power 145 mW).
anodic potential of 0.8 V (SCE), before the onset of the anodic current (see Figure 1). At this potential no SERS signals were detected (Figures 3a, 4a) since there was no anodic current flow. When the voltage was scanned to 1.0 V, two Raman peaks were apparent (Figures 3b, 4b). These Raman peaks became more visible at higher anodic potentials (Figures 3c,d, 4c). The Raman intensities exhibited reversible voltage dependence, a characteristic associated with SERS. The Raman signals vanished when the potential of the Pt electrode was scanned from 1.3 to 0.8 V and reappeared when it was scanned back to 1.0 V. We now examine our results with specific reference to the published Raman results on possible reaction products: Br,, Br3-, BrO-, Br02-, BrOC, and PtBr4-. Johnson and Br~ckenstein~ studied the adsorption and desorption of Br- at a platinum electrode in 1.O M H2S04solution using a rotating ring-disk electrode. They found bromide is oxidized to BrO- at E > 1.25 V (SCE). However, under our present experimental conditions, we have detected no BrO- species. The Raman frequency for diatomic BrO- is 609 cm-1.6 Additionally, we have not detected the Raman signals due to the Br0,- and ,) Br03- ions; the bromite ion, Br02-, is bent (with symmetry C and has three vibrational modes: 775 (q),400 (uZ), and 800 cm-l (4.7 All three modes are Raman active. The bromate ion, BrO;, which is pyramidal and has symmetry C3",has four vibrational and 358 cm-l (v4).' All four modes: 850 ( q ) ,418 (Y,), 805 (4, modes are Raman active. Recently, we have studied the electrochemistry of gold in aqueous solutions of CI-, Br-, and I- using surface-enhanced Raman spectroscopy.8 We found that square-planar complexes ( 5 ) D. (1970).
C. Johnson and S. Bruckenstein, J . Electrochem. SOC.,117, 460
(6) J. C. Evans and G. Y.-S.Lo, Inorg. Chem., 6, 1483 (1967). (7) K. Nakamoto, 'Infrared and Raman Spectra of Inorganic and Coordination Compounds", 3rd ed., Wiley, New York, 1978.
708 The Journal of Physical Chemistry, Vol. 88, No. 4, 1984
Loo and Lee
TABLE I: Reported Raman Frequencies (in em-') for Br, and Br; and the Observed SERS Frequencies and Their Assignments gasa
liquidb
162 (v,)
Br, aq solnd
Are
Kr, Xef
silicag
zeolitesh
obsd SERS freq
302.5
315.6
305
3 14
307
305 (Br,)
295 Ag
318.5
solid'
solidC
in C,H,NO,J 162 (u,)
in (CH,Cl),h
in CH,ClI
aqueousm
zeolites"
obsd SERS freq
162 (v,) 196 (v,)
163 (v,) 192 (v,)
170 (v,)
170 (u,)
161 (u,, Br,-)
a Reference 14. Reference 9. Reference 13. Reference 9, saturated (0.2 M BrJ e Reference 16, matris. Reference 15, matrix Reference 19, adsorption. Reference 20, adsorption. Reference 10, Me,NBr, and Bu,NBr,. Reference 10, Bu,NBr,. Reference 10, Me,NBr,. Reference 10, Bu,NBr,. Referenee 6. Reference 21, adsorption.
8
AuXL (X = C1, Br, I) are the predominant species formed at high anodic potentials, resulting in anodic dissolution of the gold electrode in halide solutions. However, we have not detected the presence of the square-planar complexes PtX4,- (X = C1, Br, I) for Pt electrodes in halide solutions at anodic potentials.'*2 For PtBr4,-, the vibrational frequencies are 208 ( u 1 ) , 106 ( Q ) , 105 ( u 3 ) , 194 ( u 4 ) , 227 (v6), and 112 cm-' (u7); only vI, vz, and u4 are Raman active.' Vibrational spectra of Br, and B r r in different phases and environments have been well studied and The fundamental Raman vibrational frequency for gaseous Br, is at 318.5 cm-', which is shifted to 306.4 cm-' for liquid Br2. In the solid state, this fundamental band splits into two components: one corresponds to the two molecules stretching in phase (the A, mode at 295 cm-I), the other to the out-of-phase motion (the B3*mode at 300 cm-I). The Raman spectra of Br2 were also studied in aqueous s o l ~ t i o nin, ~Ar/Kr/Xe matrices,I5J6and on silicalg and zeolitez0 substrates. The results are tabulated in Table I. The frequency of the observed Raman band at 305 cm-l is very close to the fundamental frequencies (295-318.5 cm-I) reported for Br, in different phases and environments. Therefore, we confidently assign the 305-cm-I band to surface vibrations of adsorbed Br,, electrogenerated at the Pt electrode surface. Molecular bromine exists in the three different isotopic species 79Br79Br,79Br81Br,and 81Br81Brwith natural abundance of 1:2:1. Raman signals due to these three isotopic species in the gas phase have been resolved," and the spacings between the Raman signals are small (2-3 an-'). Because of the interaction between the adsorbed Brz and the Pt substrate, the observed Raman band is considerably broadened (half-width 15 cm-l) and cannot be resolved into its isotopic components. Tribromide ions occur with both symmetric and asymmetric configurations. For the symmetric configuration (symmetry Om,,), there is only one Raman-active mode of vibration ( u l , symmetric stretching mode). However, for the asymmetric configuration (symmetry C,,), the three modes u1, u2 (doubly degenerate bending mode), and u3 (asymmetric stretching mode) are all Raman active. The tribromide ion maintains its symmetric configuration in the solid state,l0 in aqueous solution,6in C6H5N02,'0and on zeolites.21 In these cases, only the u1 mode was observed. However, in
-
(8) B. H. Loo, J . Phys. Chem., 86, 433 (1982). (9) H. Stammreich, R. Forneris, and Y. Tavares, Spectrochim. Acta, 17, 1171 .. - (1961). I----,-
(IO) W. B. Person, G. R.Anderson, J. N. Fordemwalt, H. Stammreich, and R. Forneris, J . Chem. Phys., 35, 908 (1961). (11) M. Suzuki, T. Yokayama, and M. Ito, J . Chem. Phys., 51, 1929 (1969). (12) J. E. Cahill and G. E.Leroi, J . Chem. Phys., 51, 4514 (1969). (13) A. Anderson and T. S . Sun, Chem. Phys. Lett., 6, 611 (1970). (14) W. Holzer, W. F. Murphy, and H. J. Bernstein, J . Chem. Phys., 52, 399 (1970). (15) D. H. Boa1 and G. A. Ozin, J . Chem. Phys., 55, 3598 (1971). (16) B. S.Ault, W. F. Howard, and L. Andrews, J . Mol. Spectrosc., 55, 217 (1975). (17) P. Baierl and W. Kicfcr, J . Chem. Phys., 62, 306 (1975). (18) E. Halac and H . Bonadeo, J . Chem. Phys., 78, 643 (1983). (19) P. J. Hendra and E. J. Loader, Trans. Faraday Soc., 67,828 (1971). (20) J. C. Rubim and 0. Sala, J. Raman Spectrosc., 9, 155 (1980). (21) J. C. Rubim and 0. Sala, J. Raman Spectrosc., 11, 320 (1981). (22) W. Gabes and H. Gerding, J . Mol. Struct., 14, 267 (1972).
(CH2C1), and CH3C1solutions Br3- ions are asymmetric,1° and both the ul and u j modes were detected (see Table I). The observed SERS signal at 161 cm-' is very close to the reported values (162-170 cm-') for the u1 mode of the Br3- ion in different environments. Since no u3 mode is observed in the SER spectra, we assign the 161-cm-' band to the uI mode of symmetric Br,adsorbed on the Pt electrode surface. Most metal electrodes will be covered or partially covered with a surface oxide film at high anodic potentials. However, halide ion adsorption generally inhibits the development of the surface oxide film. The coverage by electrodeposited oxide species was found to decrease with increasing halide ion concentration and with increasing adsorbability of the halide ions (in the order of C1- < Br- < I-). In the case of Br2 evolution, the potential at which reaction commences (0.845 V (SCE)) is less positive than that for 0,evolution (0.988 V (SCE));therefore, thermodynamically bromine should be evolved first before oxygen and the fraction of oxide surface should be small.23 Thus, it is reasonable to assume that the Pt electrode surface is nearly oxide free under the present experimental conditions. Since both Br2and Br3- are colored molecules, we must examine the possibility of resonance Raman scattering (RRS) from these molecules. RRS, in which excitation occurs close to or within strong absorption bands, would account for the large Raman intensities observed from monolayer or submonolayer quantities of Br2 and Br3- coadsorbed on the Pt electrode surface.24 The absorption spectrum of Br2 in the region of interest25 consists of a very weak banded region, 645-8 18 nm, corresponding X'Z:,'. The second banded region, to the transition A311(l,) 511-640 nm, represents the transition B311(Ouf) X'Z,' and converges at 510.8 nm. This is followed by a continuum in the region 300-5 11 nm with maximum absorption coefficient emax 165 L mol-I cm-l at 420 nm. Holzer et al.14 show that RRS of halogen gases (Clz, Br2, Iz) is very much associated with continuous absorption and the convergence limit of the B3n(O,+) X'Zgf transition. In all cases the RRS is observed only when the energy of the laser excitation is above the convergence limit. Thus, for Br, RRS is observed with Arf 488-nm excitation but not with Ar+ 514.5-nm e~citati0n.l~ We believe the SERS signal at 305 cm-' due to the adsorbed Br2 is surface enhanced with the Ar+ 514.5-nm and the Kr+ 647.1- and 676.4-nm excitation. The absorption spectrum of Br3- ions in aqueous solution26 exhibits two bands, one centered at 400 nm with cmax 180 L mol-' cm-I, and the other at 260 nm with tmax-480 L mol-' cm-I. These two bands correspond to the transitions 'Z, +- 'Z, and 'nu
-
+ -
-
-
-
(23) D. M. Novak, B. V. Tilak, and B. E. Conway in "Modern Aspects of Electrochemistry", Vol. 14, J. O M . Bockris, B. E. Conway, and R. E. White, Ed., Plenum Press, New York, 1982, Chapter 4. (24) If we assume the saturation coverage of Br- (eBr-)on Pt to be 0.6, according to V. S. Bagotzsky, Y. B. Vassilyev, J. Weber, and J. N. Pirtskhalara, J . EIecrroanaL Chem., 27, 31 (1970). then the individual coverage of Br2 and Br3- on the Pt electrode surface at any instance would be less than a monolayer. It is very unlikely that Br2 and Br3- would form multilayers on the Pt electrode surface. (25) J. G . Calvert and J. N. Pitts, Jr., "Photochemistry", Wiley, New York, 1967
(26) F. L. Gilbert, R. R. Goldstein, and T. M. Lowery, J . Chem. Soc., 1092 (1931).
J . Phys. Chem. 1984,88, 709-71 1
-
'Z,, re~pectively.~'The wavelengths of the laser excitation used in the SERS experiments, 488, 514.5, 647.1, and 676.4 nm, are either at the tail end of or far from the lowest transition '2" 'Z,, and therefore no RRS is expected with these wavelengths of excitation. In conclusion, the anodic reactions of Pt in 0.1 M KBr solution resulted in the formation of Br2 and Br3-. SERS was observed
-
(27) P. W. Tasker, Mol. Phys., 33, 511 (1977).
709
from the adsorbed Br, molecularly bound to the Pt electrode surface with the excitation wavelengths 514.5, 647.1, and 676.4 nm. SERS was also observed from Br3- coadsorbed on the Pt electrode surface with 488-, 514.5-, 647.1-, and 676.4-nm excitations. We have demonstrated that Pt, although it does not have the proper dielectric functions in the visible region to substain electromagnetic resonances on surfaces, can still give rise to the SERS effect. Registry No. Br2, 7726-95-6; Br3-, 14522-80-6; Pt, 7440-06-4.
Flash Photolysis Observation of the Absorption Spectra of Trapped Positive Holes and Electrons in Colloidal TiO, D. Bahnemann, A. Henglein,* J. Lilie, and L. Spanhel Hahn-Meitner-Institut fur Kernforschung Berlin, Bereich Strahlenchemie, 0-1000 Berlin 39, Federal Republic of Germany (Received: August 12, 1982)
When a Ti02 sol containing an adsorbed electron scavenger such as platinum or methyl viologen is flashed with a 347-nm laser, an immediate broad absorption with k- = 475 nm is observed. In acid solution the absorption decays within milliseconds. In alkaline solution it decays within microseconds, depending on the OH- concentration, and OH- ions are consumed in the process. In the presence of scavengers for positive holes the decay is faster, while oxygen does not have any effect. This absorption spectrum is attributed to excess positive holes trapped at the surface of the colloidal particles. When a TiOzsol containing an adsorbed scavenger for positive holes, such as polyvinyl alcohol or thiocyanate, is flashed, a broad absorption with A, = 650 nm is observed. It decays in the presence of electron scavengers. This spectrum is attributed to excess electrons trapped close to the surface of the colloidal particles.
Introduction Titanium dioxide has, during the past 6 years, been studied as a catalyst of photoreactions. In their pioneering studies Bard and co-workers used TiOz suspensions and showed that the chemical reactions were brought about by the electrons and positive holes generated upon illumination with near-UV lightsi More recently laser flash photolysis studies, in which colloidal T i 0 2 was used, were carried out to detect short-lived intermediate^.^^^ For example, the respective absorptions of Br2-- and MV+. were seen in flashed TiO, sols containing Br- anions or MV2+cations (MV*+ = methylviologen; 1, I'-dimethyl-4,4'-bipyridinium ion). The present study was undertaken to detect the primary intermediates of the chemical reactions, i.e., the oxidizing and reducing species which are formed in TiO, itself. It had already been supposed by Bard that the electrons and positive holes are trapped in surface states from which they react with dissolved compounds. It seemed possible that these trapped species had optical absorptions which were detectable by flash photolysis. The optical observation of occupied and unoccupied electronic surface states would enable one to follow the kinetics of heterogeneous reactions at the Ti02/solute interface in greater detail. Our present paper serves also to emphasize the principle according to which photocatalyzed reactions in T i 0 2 sols can be achieved with sizable yield only if two scavengers (one for electrons, e-, and one for positive holes, h+) are present and at least one of them is adsorbed at the colloidal particles. It is the adsorbed scavenger which determines the yield of the reaction of the non( 1 ) Kraeutler, B.; Bard, A. J. J . Am. Chem. SOC.1977, 99, 7729. Bard, A. J. Science 1980, 207, 139. Izumi, I., Fan, F.-R.F.; Bard, A. J. J . Phys. Chem. 1981, 85, 218. Ward, M. D.; Bard, A. J. Ibid. 1982, 86, 3599. (2) Henglein, A. Ber. Bumenges. Phys. Chem. 1982, 86, 241. (3) Duonghong, D.; Ramsden, J.; Gratzel, M. J . Am. Chem. SOC.1982, 104. 2977.
0022-3654/84/2088-0709$01 .50/0
adsorbed one. Despite the numerous chemical effects which have been observed with TiO2sols or suspensions, this principle does not seem to have been stated precisely.
Experimental Section Preparation of the Colloids. Colloidal TiO, was prepared by the dropwise addition of titanium tetraisopropoxide dissolved in propanol-2 to hydrochloric acid solution of pH 1.5. The final concentrations of T i 0 2 and propanol-2 were 1.5 X and 1.2 M, respectively. This mixture was stirred overnight until it was virtually clear. After vacuum evaporation of the solvent a soluble white powder of TiO, remained. The final solution was made by dissolving 500 mg of this powder in 1 L of water. This solution had a pH of 3. Alkaline TiO, solutions were made by fast addition of the estimated amount of 0.1 M NaOH to the vigorously stirred acid solution. All these solutions were optically transparent and showed the steep increase in absorption below 380 nm which is typical for colloidal TiO,.' The absorbance at the laser wavelength of 347 nm was 50%. Colloidal platinum was prepared by reducing a 3 X M H,PtCl, solution with 1.7 X lo+ M sodium citrate (1 h and 100 "C). Excess ions were removed with Amberlite MBl ion-exchange resin until a specific conductivity of 3-5 wS.cm-' was reached. This colloidal platinum solution was added to the above acid TiO, solution in the ratio of 1:15. The solvent was then evaporated under vacuum and a brown powder obtained. This powder was redissolved to obtain the final solution of Pt-covered TiO,, the pH of which was adjusted as described above. Apparatus. The flash photolysis experiments were carried out with a frequency-doubled JK ruby laser (A = 347.1 nm; 15-11s pulse width). The changes in optical absorption and conductivity were measured. The solution was passed continuously through a quartz cell, which was equipped with two sets of glassy carbon electrodes. The technical details of the optical detection system 0 1984 American Chemical Society
