J. Phys. Chem. zyxwvut 1984,88, zyxwvu 709-71 zyxwvu 1 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK 709 - '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). 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 exci- tations. 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, zyxwv 0-1000 Berlin 39, Federal Republic zyxwvutsr 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 zyxwvut 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 Ti02 was used, were carried out to detect short-lived intermediate^.^^^ For ex- ample, 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 in- termediates 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 ac- cording to which photocatalyzed reactions in T i 0 2 sols can be achieved with sizable yield only if two scavengers (one for elec- trons, 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. zyxwvutsrqpo Am. Chem. SOC. 1977, 99, 7729. Bard, A. J. Science 1980, 207, 139. Izumi, I., Fan, zyxwvutsrq 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. adsorbed one. Despite the numerous chemical effects which have been observed with TiO2 sols 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 Ti02 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%. 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 Colloidal platinum was prepared by reducing a 3 X 0022-3654/84/2088-0709$01 .50/0 0 1984 American Chemical Society