1696
J . Phys. Chem. 1987, 91, 1696-1698
from 1506 to 1501 cm-I. However, other structure-sensitive bands above 1450 cm-’ remain unchanged upon adsorption to the silver colloid; the reason for this is at present unclear. v4, the al, symmetric stretching mode C,-N (the porphyrin “ring breathing” mode), is shifted from 1375 to 1370 cm-I. This is a marker band for the oxidation state of the central metal atom as well as for alteration in the ring a bonding.I4 The 5-cm-I decrease relative to the R R band of the solution species suggests that a partial transfer of charge from the silver metal to the heme complex is taking place. Overall, these data indicate that the vibrational mode wavenumbers are less sensitive to electronic state perturbations than are the band intensities and excitation profiles. Other evidence has pointed to an edge-on binding of the porphyrin macrocycle to the silver surface, probably through the propionate functional groups, as suggested by Stockburger and Hildebrandt’ for adsorption of cytochromes onto silver colloids and by McMahon6 and CottonIs for adsorption of porphyrin model compounds onto silver electrodes. Adsorption of oxyhemoglobin through the negatively charged proprionate groups, which extend out through the protein envelope, would not result in large perturbation of the porphyrin electronic states, since the propionate groups appear not to be involved in coupling the molecular ground state to the electronic excited state, as evidenced by their absence from the R R spectra.6 Adsorption of the porphyrin macrocycle to the silver surface via overlap of the A electrons of the pyrrole rings would result in the porphyrin plane lying parallel to the surface. This seems unlikely since it would be expected to lead to significant perturbation in the vibrational mode energies, but no such perturbations are observed. It would also require major conformational changes to the protein structure to enable the silver to come into contact with the porphyrin macrocycle (e.g., opening of the heme pocket). Moreover, we have shown that this binding geometry can be ruled out since we now have established that the adsorbed hemoglobin remains functionally active in reversible ligand binding of O2and CO. The detailed experimental evidence in support of this statement will be the subject of a separate publication. Hence, the sixth axial ligand position of the heme iron does remain available for reversible ligand binding. The (14) Spiro, T. G. In Iron Porphyrins, Part I I ; Physical Bioinorganic Chemistry Series; Leuer, A. B., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1982. (15) Cotton, T. M.; Schultz, S. G.: Van Duyne, R . P. J . Am. Chem. Soc. 1982, 104, 6528.
Fe-N,,, stretching band of adsorbed deoxyhemoglobin (obtained by 44 1.6-nm laser photolysis of adsorbed carbonylhemoglobin) is found at 215 cm-’. This indicates that the adsorbed deoxyhemoglobin is in the T-state quaternary structure and shows that the adsorption at the silver surface imposes no additional strain on the Fe-N,,, bond. The reversible ligand-binding property of the adsorbed hemoglobin is a clear indication that the hemoglobin does not denature to form (Fe111PP)20decomposition products. Assuming that oxyhemoglobin (- 6-nm molecular diameter) is bound to the silver colloid (- 35-nm average particle diameter) through the propionate groups, with the heme plane lying perpendicular to the silver surface, the iron atom will be approximately 1.5 nm from the silver surface. In this configuration the porphyrin macrocycle lies parallel to the electric field resulting from the net positive charge on the colloidal metal. Hence, the interaction of the porphyrin IT electrons with the electric field will be favored, even though somewhat distant from the silver surface. Although the absorbances are similar at 457 nm, the SERR signals of oxyhemoglobin are approximately 500 times less intense than those of hemin chloride and (Fe”’PP),O. It has been shown16 that the intensity of SER signals is a function of the distance of the adsorbate from the metal surface. Since it is believed that these heme complexes all bind to the silver surface in the same manner, it seems likely that the increase in the SERR signals of the protein-free heme complexes is due to the fact that they are able to approach closer to the silver surface than the heme groups in the oxyhemoglobin. These are encased in a protein envelope that not only restricts the movement of the porphyrin macrocycle but also shields it from the electric field surrounding the silver particle. This interpretation is consistent with our overall conclusion that oxyhemoglobin is not denatured but remains intact when adsorbed on colloidal silver.
Acknowledgment. We are grateful to Mr. R. B. Girling for technical assistance, to Dr. T. Kitagawa for helpful discussions and for placing the facilities of his laboratory at the I.M.S., Okazaki, at our disposal, and to the SERC for financial support to J.deG. (16) Murray, C. A.; Allara, D. L. J . Chem. Phys. 1982, 76, 1290. (17) Choi, S.; Spiro, T. G.; Langry, K. C.; Smith, K. M.; Budd, D. L.; La Mar. G. N.J . A m . Chem. Soc. 1982, 104, 4345. (18) Spiro, T. G.; Strekas, T. C. J . Am. Chem. SOC.1974, 96, 338.
Transient Grating Method Applied to Electron-Transfer Dynamics at a Semiconductor/Liquid Interface Seiichiro Nakabayashi, Shuji Komuro, Yoshinobu Aoyagi, and Akira Kira* The Institute of Physical and Chemical Research, Wako-Shi, Saitama 351 -01, Japan (Received: September 18, 1986; In Final Form: December 2, 1986) The transient grating method utilizing a fringe pattern of excited states was first applied to a semiconductor/electrolyte interface. The dynamics of a photoelectrochemical oxidation of 2-propanol aqueous solution on a Ti02 electrode was observed by this method. The results were explained in terms of participation of additional electron injection to the conduction band from an oxidized intermediate species of 2-propanol, which has been proposed as a mechanism of the current-doubling effect. Introduction This letter reports the first application of a transient grating method to the measurement of electron-transfer dynamics at a semiconductor/electrolyte interface. The transient grating method has recently been employed for the study of the recombination and the diffusion of charge carriers in semiconductors,l-3and (1) Aoyagi, Y.; Segawa, Y.; Namba, S. Phys. Reo. B: Condenr. Matrer 1982, 25, 1453.
0022-3654/87/2091-1696$01.50/0
studies refer to a surface or a solid/solid interface.* In this method, a holographic diffraction grating consisting of a fringe of photoinduced charge carriers is utilized for the monitoring of carrier (2) Komuro, S.; Aoyagi, Y.; Namba, S.; Masuyama, A,; Okamoto, H . ; Hamakawa, y. Phys. Lett. 1983, 43,968. Newell, v. J.; Rose, T.s.; Fayer, M. D. Phys. Rev. B Condens. Marrer 1985, 32, 8035. (3) Hoffman, C. A.; Jarasiunas, K.; Gerritsen, H. J.: Kurmikko, A. V. Appl. Phys. Lert. 1978, 33, 536. Salcedo, J. R.: Siegman, A. E.; Dlott, D. D.;Fayer, M . D. Phyr. Rei!. Lert. 1978, 4 1 , 131.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 7, 1987 1697
Letters
I
'
.-I -i-.
-%
?I. 1st /--
!
!
4
-.-+ TD
i
@
HV
I
Figure 1. Diagram of the transient grating apparatus. Solid and dotdash arrows represent the excitation and the monitor beams, respectively. Principal elements: Q:YAG and Nd:YAG, pulse laser; THG, thirdharmonics generator; Kr', Krypton ion cw laser; ST, mechanical shutter; PM, photomultiplier; PD, photodiode for timing; PS, potentiostat.
dynamics. Since such a grating is produced by the refractive index difference caused by the excited-state formation, this method is useful for the study of transient charge carriers in a semiconductor that lacks the transient absorption bands. The transient grating method can be applied to observation of electron-transfer dynamics a t semiconductor electrode/electrolyte interfaces if the grating is produced on the surface of a semiconductor electrode, since electron transfer must affect the time evolution of charge carriers. This new optical method to measure the dynamics at the interface will be advantageous to conventional transient photocurrent or photovoltage measurements, since it is free from the effect of the photoinduced charging of the double layer which seriously affects the current and voltage measurement, especially in the short time regi~n.~
Experimental Section Our apparatus is schematically illustrated in Figure 1. A pulse beam (with a 6-ns width) of the third harmonics (A = 355 nm) of a Quanta Ray DCR-2A Nd:YAG laser is split into two passes that cross each other with an angle of 0 on the semiconductor electrode surface to form an interference pattern, which causes the transient grating with a pitch: A = A/(2 sin (0/2)). The grating must be formed near the surface, since the absorption coefficient indicates that half of the incident light should be absorbed within ca. 1 pm on the surface. A beam from a Spectra-Physics 166 krypton ion cw laser (647 nm) is focused on the spot of the transient grating to monitor its diffraction intensity. The first-order diffraction of the monitor beam was picked up through a pin hole and detected by an EM1 9268QB photomultiplier. The photoelectric signal is fed into an Iwatsu TS-8123 digital oscilloscope and analyzed with a Hewlett-Packard 98 16 computer. Single crystals of TiO, purchased from Nakazumi Crystal, cut perpendicular to the c axis, were reduced in hydrogen gas at 600 OC and optically polished. The electrochemical condition was controlled by a Tohou Giken 2020 potentiostat in a conventional three-electrode system. The electrode potential was measured against an Ag quasi-reference electrode. Results First, the transient-grating signals of a TiO, crystal were measured in air without solution. The decay was exponential with respect to time. The lifetime, 7, is given by3 1/ T = 1 /To + (47r2/A2)D (1) where T~ and D denote the recombination lifetime and the diffusion coefficient of the carriers, respectively. The data for different pitch values fit eq 1, giving values of T~ = 75 p s and D = 1.34 X loW2 cm2 s-I. This value of D agrees with the one reported (4) Kamat, P.V.; Fox, M. A. J . Phys. Chem. 1983, 87,
59.
1
I
I
I
0
100
200
300
I
400
t / ns Figure 2. Signal profiles of the diffraction of transient gratings on a TiO, electrode at +1.5 V vs. Ag in aqueous solutions (with 1 M sodium perchlorate) of 2-propanol at concentrations as indicated. Insertion: sche, the first (photoinduced) and matic illustration of the half-width, w , , ~and
the second (chemically induced) components.
P
7 20
40
[ 2 PrOHI/vol% Figure 3. Half-widths plotted as a function of the 2-propanol concentration at potentials of +2.0 V ( O ) , +1.5 V (0),and -1.5 V (0) vs. Ag.
previou~ly.~This result ensures that the grating observed is due to the fringe pattern of the carrier population. The decay was identical with the above when the crystal was used as an electrode in a 1 M perchlorate aqueous solution at an flat-band electrode potential. When 2-propanol was added to this solution and an extremely anodic potential of +1.5 V vs. Ag was applied to the crystal, the signal changed with increasing 2-propanol concentration, as shown in Figure 2. This photoelectrochemical system, on illumination with a 150-W xenon lamp, showed a saturated photocurrent for electrode potentials above + 1 .O V vs. Ag; thus, the change in the transient grating signal corresponds to photoinduced interfacial electron transfer. A slight decrease in the lifetime caused by the addition of 2-propanol at 20% is probably due to hole transfer to 2-propanol. At higher 2-propanol concentrations, the decay is not enhanced markedly, but the total profile changes, viz., the crest becomes flat. The half-width of the profile, w,,,,as defined in Figure 2,6 changes with 2-propanol concentration as shown in ( 5 ) Goodenough, J. B.; Hammett, A. In Semiconductors: Physics of Non-Tetrahedrally Bonded Binary Compounds 111, Landolt-Boernstein 111/17gMadelung, O.,Schultz, M., Weiss, H., Eds.; Springer-Verlag: Berlin, 1984; p 133. ( 6 ) Note that the half-width thus defined differs from a usual decay half-life.
1698 The Journal of Physical Chemistry, Vol. 91, No. 7, 1987
Letters The second component appearing at high 2-propanol concentrations can be ascribed to electrons injected into the conduction band from the oxidized species X, as schematically shown in Figure 4
-I ----___
'*
2-PrOH
$--
Ti02
Soln.
Figure 4. Energy diagram of the system. T h e bands of T i 0 2 are shown by bold lines for an anodic potential (e.g., +1.5-2.0 V) and by dotted lines for a cathodic potential (e.g., -1.5 V).
Figure 3. It should be remarked that eq 1 does not apply to the electrode/electrolyte interface, since not only the surface-reaction term should be added but also the strong electric field of the space-charge layer could affect the carrier dynamics in a complicated manner. Formulation has not been made yet for such an interface; therefore, analysis and discussion will be qualitative in the following. The results exhibited in Figures 2 and 3 are explained if the signal contains the second delayed component, as illustrated in an insertion of Figure 2. The increase in the half-width in the 2-propanol concentration region above 30 vol % is ascribed to the predominance of the second component. Measurements were also made for an electrode potential of -1.5 V vs. Ag at which no photocurrent occurs in a usual photovoltammogram. At 50 vol % 2-propanol, the signal profile was such that the second component contributes much less, and accordingly the half-width for -1.5 V was smaller than that for +2.0 V, as shown in Figure 3.
Discussion When a high anodic potential is applied to an n-TiO, electrode, the bands bend, as shown schematically in Figure 4, so that photoinduced holes come to the surface and oxidize 2-propanol
PrOH
+ h+
-
X
(2)
where X denotes an oxidized intermediate species,which is perhaps a radical produced from 2-propanol by a-hydrogen abstraction.' However, the time evolution of reaction 2 could not be measured in this experiment owing to the overlap of the second-component formation as shown in Figure 2. (7) Yamagata, S.; Nakabayashi, S.; Fujishima, A,; Honda, K.; Sancier, K. M., unpublished results.
X-Y+e(3) where Y is a higher oxidized species. Although neither identification of X nor determination of its redox potential has been made yet, it has been believed that reaction 3 is responsible for the current-doubling effect that has been reported for many organic compounds, including 2-propanol.* It is also known that the quantum yield for the current-doubling effect in most oxygen-free systems is close to 2,9 and accordingly the signal change caused by 2-propanol can be mainly due to interfacial processes 2 and 3. In a preliminary experiment, the electrode was illuminated by ca. 100 repeated laser pulses in order to produce concentrated oxygen,1° and immediately after this procedure the second component in the signal reduced markedly because of removal of X by the oxygen. Thus, the present result can be regarded as the first direct time-resolved observation of reaction 3. A delay of ca.100 ns in the appearance of the second component probably relates to an interval between the hole ejection from valence band and the electron injection to the conduction band from X. The intermediate X must keep the same fringe pattern as the photoinduced grating on the electrode surface, since the distance over which X can move during 100 ns is ca. lo-, pm, which is much shorter than a grating pitch of 3 pm in the experiments. Accordingly, the electrons injected from X reproduce again the same pattern on the electrode surface to form a chemically induced transient grating which gives the second component. The substantial decrease of the second component in the signal measured at -1.5 V vs. Ag is consistent with the above explanation. For such a cathodic potential, electron injection from X to the conduction band should not occur because of an unfavorable band bending as shown in Figure 4. In reality, small residual signals were observed: the half-width did not decrease to the value for 0% 2-propanol at -1.5 V, as shown in Figure 3. A process or mechanism responsible for this residue cannot be specified at the present. One important possibility is that the density of charge carriers induced by a strong laser pulse is high enough to disturb the space-charge layer. Another possibility is that 2-propanol produces a surface state which enhances surface recombination of the carriers. (8) Morrison, S. R.; Freund, T. J. Chem. Phys. 1967,47, 1543. Gommes, W. P.; Freund, T.; Morrison, S. R. J. Electrochem. Sac. 1968, 115, 818. (9) Maeda, Y.; Fujishima, A,; Honda, K. J . Electrochem. SOC.1981, 128, 1731. (10) Oxygen is produced by competitive photoelectrochemical oxidation of water.
