J . Phys. Chem. 1986,90, 6657-6662 more polarization is required to increase the current than in the former region. In other words, the accumulation of electrons near the surface occurs owing to a “kinetic barrier” for the HER, which plays a similar role as an insulator of MIS structure, leading to the increase of PL intensity. Thus, in the latter region, the large Tafel slope means more accumulation of electrons and stronger PL. If the above reasoning is correct, removal of the kinetic barrier for H E R should result in no accumulation of electrons near the surface and no increase of P L intensity by the increase of the forward bias. It is known that the surface treatment of n-GaAs by a noble metal which has a high activity for the HER increases the H E R rate,I0 and therefore, we studied the effect of Pt treatment on the current-PL intensity-potential relation. Figure 5 shows current-potential and P L intensity-potential relations of the electrode (sample 1) subjected to dip treatment in H2PtCls solution. The H E R current increased to a large extent, for example, about 600 times a t -1.0 V of that a t the bare electrode. At potentials more negative than ca. -1.2 V, the electrode process is so fast that the current-potential relation became linear, showing an ohmic relation with a slope of ca. 1.75 Q cm2. However, no potential dependence of the P L intensity16was observed when the
6657
electrode potential was swept in the negative direction. This result was exactly what we expected according to the present model. Thus, the Pt treatment removes the kinetic barrier for the HER, leading to the increase of the H E R rate and no accumulation of electrons at the surface, Le., no increase of PL intensity under larger cathodic bias condition.” In conclusion, we demonstrated for the first time that the PL measurements provide useful information for the electrochemical reaction mechanism at semiconductor electrodes under forward bias.
Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture. Prof. A. B. Ellis and Dr. W. S . Hobson are acknowledged for the donation of GaAs single crystals. Registry No. GaAs, 1303-00-0; HZ, 1333-74-0; Pt, 7440-06-4; H2S04, 7664-93-9. (16) The PL intensity of the Pt-treated GaAs electrode at -0.4 V was nearly equal to that observed at the bare electrode at -0.7 V. (17) In the present situation, the potential distribution within the semiconductor is not affected by the external bias.
Photoelectrochemical Transients Induced by Pulsed Laser Illumination of the SnO,/Ru(bpy);+ Solution Interface at Different pH’s A. Kirsch-De Mesmaeker,*+M. Rochus-Dewitt, and J. Nasielski Uniuersite Libre de Bruxelles. Service de Chimie Organique, CP 160, Facultd des Sciences, B- 1050 Bruxelles, Belgium (Received: April 9, 1986; In Final Form: August 12, 1986)
Pulsed laser-induced open-circuit photopotentials have been measured at a highly n-doped polycrystalline SnO2/Ru(bpy),CI2 solution interface as a function of pH. Although no quantitative analysis can be presented, because of the complexity of the system, different transient electroactive species are distinguished; their kinetic behavior is qualitatively compared with the well-known Sn02/rhdamine B solution interface. In oxygenated solution at pH 0.5, the excited complex R ~ ( b p y ) , ~ + * is oxidatively quenched by 02,and the electroreduction of the generated Ru(bpy)?+ masks all other processes. The sluggishness of the photopotential rise at pH 13 is attributed to the slow electron injection from the excited complex across an insulating hydroxide layer formed at the surface on Sn02.
Introduction
an electron into poorly or highly doped Sn02. We have shown’* that, on highly doped Sn02,the behavior of this system strongly Richardson, Perone et al.,’ and Gottesfeld et al? have developed depends on the pH of the solution. In the presence of oxygen and a technique allowing the measurement of photoelectrochemical transients generated by short light pulses at absorbing electrode/transparent electrolyte solution interfaces. Similar studies (1) Perone, S. P.; Richardson, J. H.; Deutscher, S. B.; Rosenthal, J.; of photopotentials or photocurrents triggered by a laser flash were Ziemer, J. N. J. Electrochem. SOC.1980, 127, 2580. Richardson, J. H.; presented by other authors, at semiconductors such as M O S ~ ~ , ~Perone, S. P.; Deutscher, S. B. J . Phys. Chem. 1981, 85, 341. (2) Harzion, 2.; Croitoru, N.; Gottesfeld, S. J . Electrochem. SOC.1981, Ti02,4WSe2,5 FezO3,6 PtS,,’ or CdSee8 128, 551. Gottesfeld, S.;Feldberg, S.W. J. Electroanal. Chem. 1983, 146, We have extended this method to systems where the light is 47. absorbed by a dye solution in contact with a transparent semi(3) Kawai, T.; Tributsch, H.; Sakata, T. Chem. Phys. Lett. 1980, 69, 336. conducting e l e ~ t r o d e . ~ ,The ’ ~ pulsed laser-induced open-circuit (4) Kamat, P. V.; Fox, M. A. J. Phys. Chem. 1983,87, 59. (5) Prybyla, S.; Struve, W. S.;Parkinson, B. A. J. Electrochem. SOC.1984, photopotentials at the Sn02/aqueous rhodamine B-hydroquinone 131, 1587. interface and their time evolution gave the rate constants associated (6) Itoh, K.; Nakao, M.; Honda, K. J . Appl. Phys. 1985,57, 5493. with the photosensitization and supersensitization processes in this (7) Jaegermann, W.; Sakata, T.; Janata, E.; Tributsch, H. J . Electroanul. system. Chem. 1985, 189, 65. (8) Wilson, R.H.; Sakata, T.; Kawai, T.; Hashimoto, K. J . Electrochem. The purpose of the present work is to apply this methodology SOC.1985, 132, 1082. to a photosensitizer having well-known photophysical and pho(9) Frippiat, A.; Kirsch-De Mesmaeker, A.; Nasielski, J. J. Electrochem. tochemical properties, but different from those of rhodamine. We SOC.1983, 130, 238. chose to work with tris(2,2’-bipyridine)ruthenium(II), Ru(bpy):+. (10) Frippiat, A.; Kirsch-De Mesmaeker, A. J . Phys. Chem. 1985, 89, 1285. Frippiat, A.; Kirsch-De Mesmaeker, A,, submitted for publication in The photoelectrochemistry of Ru(bpy),2+ under steady illuJ. Electrochem. SOC. mination has been thoroughly examined. Memming et aI.’1-’3 (1 1) Gleria, M.; Memming, R. 2.Phys. Chem. (Munich) 1975, 98, 303. have shown that the triplet excited state of the complex injects Memmine. R. Surf. Sci. 1980. 101. 551. (12) Gemming R.; Schroppel, F. Chem. Phys. Lett. 1979, 62, 207. Research Associate of the National Fund for Scientific Research (Belgium).
(13) Memming, R.; Schroppel, F.; Bringmann, U. J. Electroanab Chem. 1979, 100, 307.
0022-3654/86/2090-6657.$01.50/0 0 1986 American Chemical Society
6658
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986
in acidic m e d i ~ m , ' ~a steady J~ photoreduction current is observed instead of a photooxidation; MemmingI3 gave two possible origins for this: either an oxidative quenching of Ru2+*by O2 (reaction l ) , the back electron transfer (2) being inhibited by the fast protonation (3) of the peroxide radical anion,15 or a biphotonic disproportionation (4) between two excited triplet complexes, the back-reaction (5) competing now with the oxidation (6) of Ru+ by 0,. In both cases, the resulting Ru3+, either adsorbed or diffusing from the bulk, is reduced at the electrode. Process 4 was, however, excluded under continuous illumination because this second-order process should lead to a photocurrent proportional to the square of the light intensity which was not found.I3 Additionally, we never detected Ru(bpy),+ by laser flash photolysis of Ru(bpy)32+,with or without oxygen. Moreover, under the experimental conditions used to study the laser-induced photopotentials (Le., with a higher complex concentration, thus with a higher excited-state concentration than in laser photolysis), the luminescence lifetime of Ru(bpy),,+* is not affected. Thus, for this present study, biphotonic processes in solution such as reaction 4 can also be excluded.
-
+ O2 Ru(bpy),,+ + 02'R ~ ( b p y ) , ~++ 02'- Ru(bpy),,+ + 0, 02*+ H+ H02'
Ru(bpy),*+*
-
2 R ~ ( b p y ) ~ ~ + Ru(bpy)33+ * + Ru(bpy),+ Ru(bpy),+
+ O2
-
Ru(bpy),,+
(1) (2) (3)
(4)
+ 02*-
(6) Since the behavior of this system strongly depends on the amount of oxygen and on the acidity of the medium, we set out to perform the present pulsed laser study at three different pH values (0.5, 4.5,and 13) and with vacuum degassed solution or saturated with oxygen. Experimental Section Reagents. Solutions were prepared from triply distilled water and are bubbled with oxygen under atmospheric pressure or degassed by four successive freeze-pumpthaw cycles; it was found that bubbling with pure nitrogen is insufficient to deoxygenate the solutions and that the residual oxygen introduced irreproducibilities. Ru(bpy),Cl2.6H20 was made according to Burstall's procedureI6 and was characterized by its elemental analysis and absorption and 'H N M R spectra. All experiments were performed with a Ru(bpy),,+ concentration of M; the solutions were held at pH 0.5 with HCI or H2S04,at pH 13 with NaOH, and M acetate buffer containing 0.1 M at pH 4.5 with a 5 X LiN0, as supporting electrolyte. When the reference electrode is Ag/AgCl, the solutions are 0.1 M in KC1. Instrumentation. A Molectron UV24 nitrogen laser (337 nm) is used to pump a Molectron DLII tunable dye laser, giving the excitation pulse at 452 nm by using coumarine 440 as the lasing dye; the pulse has an 8-ns width. The optically isolated electronic system for triggering the laser and the oscilloscopes and the experimental conditions used to record the signals were described previously.10 The laser light is split into a transmitted and a reflected beam; the transmitted beam is defocused before irradiating the solution through the SnO, electrode, and the reflected beam is directed to a pyroelectrical joulemeter (Molectron 73-05 DW) connected to an oscilloscope (Philips PM 3234) to measure the relative light intensity of the pulse. (This value is given in millivolts in the figures.) The procedure for open-circuit measurements with a prepolarization potential (PP) of the electrode involves first an adjustment of the SnO, electrode potential to the selected value (vs. (14) Kirsch-De Mesmaeker, A,; Nasielski, J.; Willem, R. Bull. Soc. Chim. Belg. 1982, 91, 73 1. (15) Winterle, J. S.; Kliger, D. S . ; Hammond, G. S . J . Am. Chem. Soc. 1976, 98, 3719. (16) Burstall, J. M. J . Chem. Soc., Trans. 1936, 173. (17) Miller, S . ; Zahir, K.; Haim, A. Inorg. Chem. 1985, 24, 3980
Kirsch-De Mesmaeker et al. S C E or Ag/AgCl) for a few seconds with a potentiostat. A solid-state electronic switch (Analog Devices AD75 12 DI) then opens the circuit. From that time on, the potential of the electrode is no longer held constant; previous experiments have shown that the spontaneous return to the equilibrium potential takes almost a second.I0 After a 20-ps delay, the laser is triggered, and the transient potential difference AV between the working and reference electrodes is monitored on a 100-MHz dual trace storage oscilloscope (Philips PM 3266, 1-Ma input) in ac mode. This means that the time dependence of the photopotentials cannot be followed on a time scale longer than the 8-ms time constant of the oscilloscope in ac mode, except when the PP value is the equilibrium potential of the S n 0 2electrode, the oscilloscope being then operated in dc mode. The sign convention for the oscilloscope traces is a positive A V value for electron injection into the electrode, which then acquires a negative charge corresponding to the oxidation of electroactive species; negative values reflect reductions. Cells and Electrodes. The cell contains the working semiconducting electrode (ND= 7 X lOI9~ m - a~ large-area ) platinum counter electrode, and a dual reference consisting of a saturated calomel electrode capacitively coupled (470 nF) to a platinum quasi-reference.ls10 Vacuum degassed cells contain only the working electrode and a large silver/silver chloride electrode playing the role of both the counter and reference electrodes. The electrical contacts on SnO, and the connections to the conducting wires are made with silver paint, and the SnO, electrodes are mounted on the cell glass with epoxy glue (Perfekta-Chemie, Holland). Results and Discussion The flat-band potential (FBP) of our polycrystalline S n 0 2 electrodes varies with the pH of the solutions according to the 60 mV/pH unit law; at pH 0.5 it is -0.02 V (SCE), at pH 4.5, -0.26 V, and at pH 13, -0.77 V. For a given prepolarization potential (PP), the thickness of the space charge depends thus on the pH, and in order to be able to discuss kinetics of electron transfers under comparable conditions, we present the data found for PP values such that the SnO, band bending is similar for all three pH values. The bending values chosen are near 0.0, 0.4, and 0.7 V vs. the FBP. Since we try to extract kinetic data from the time dependence of the photopotentials, the response time of our electrochemical setup has to be known. We had observed previouslygJOthat the rise time of the open-circuit photopotential induced by a laser pulse lasting 8 ns is near 20 ns, corresponding to the RC time constant given by the 2 0 0 4 resistance of the cell and the 100-pF capacitance of the wiring. This 20-11s response time sets thus a limit between the processes occurring in that time scale or shorter, and whose kinetics will be completely obscured, and processes which are slower and thus amenable to a kinetic analysis. For example, electron transfers involving a photogenerated electroactive species diffusing from the bulk of the solution toward the electrode may give AVevolutions lasting as long as the lifetime of that species. Before we proceed to the description and the analysis of our experiments, we must introduce the steps which lead to electron transfers to and from the electrode after the light pulse. Reactions 1-3 involve electroactive intermediates whose electrochemical fate is given by the following steps:27
-
Ru(bpy)32+*
--
R~(bpy),~+*
electrode
--
R~(bpy),~+
Ru(bpy)3,+
-
electrode
+ eCB
Ru(bpy),,+
+e
(7)
+ ecB
Ru(bpy)32+
(9) (10)
Reaction 7 is the very rapid injection of an electron from the excited complex over the lower edge of the conduction band of the semiconducting electrode, and step 8 is the back electron transfer of an electron to the oxidized complex by tunneling across the variable thickness of the space charge; these two processes pertain to intermediates adsorbed onto the electrode surface. Steps
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 6659
S n O , / R ~ ( b p y ) ~Solution ~+ Interface
TABLE I: Prepolarization Potential (PP), Capacitance ( C ) Measured by Impedance at lo4 Hz, Initial Photopotential (AV,) Normalized for the Same Laser Power ( P = 40 mV), and Calculated Initial Injected Charge (AQo) of Degassed and Oxygen-Saturated Solutions degassed saturated with O2 PP, c, A Vo, AQo = AVOC Avo, AQo = AVoC mV/SCE nF mV (&lo%), nC mV (*IO%), nC pH 0.5, FBP -0.02 V/SCE 0 -0 -0 -0 -0
-
pH 4.5, FBP
-
-0.26 V/SCE
pH 13, FBP
-
-0.77 V/SCE
+200 +500 +700 0 +200 +500 +700 0 +200 +500 +700
365 185 180 365 250 180 175 180 175 170 167
22 39 46.1" 20 27.3 42.4 43.7" 18.6b 226 25.6b 28.9b
8.0 7.2 8.3 7.3 6.8 7.6 7.6 3.3 3.8 4.4 4.8
5 25 38.4 27.1 45.6 54.4 60 17.6b 20.5b 25b 23b
1.8 4.6 6.9 9.9 11.4 9.8 10.5 3.2 3.6 4.2 3.8
'Corresponds to the total initial photopotential. Le.. after the fast and slow rise (see Figure IC). bCorrespondsto the initial photopotential reached after the slow rise (see Figure 5A,C). I
AV/mv
A
t i 1101.
i
,
ct
100
,
,
,
,
,
,
tins
,AVfmv
B
II tb I
0
I
t--1
,
,
,
,
,
,
,
tips
Figure 1. Oscilloscopic traces of the pulsed laser-induced A V recorded with the degassed complex solution at pH 0.5. The prepolarization
potential (PP), the potential vs. the flat-band potential (V/FBP), and the relative laser power (P) measured at the joulemeter are respectively (A) PP = +0.4 V/SCE, 0.42 V/FBP, P = 45 mV; (B) PP = +0.4 V/SCE, 0.42 V/FBP, P = 46 mV; (C) PP = +0.7 V/SCE, 0.72 V/FBP, P = 38 mV; (D) PP = +0.7 V/SCE, 0.72 V/FBP, P = 40 mV. 9 and 10 involve species which are photogenerated in the bulk of the solution; the characteristic time for process 9, contributing an oxidative component to AV, is close to the lifetime of the excited state, whereas step 10 may have a very long characteristic time and is a reductive contribution. Reactions 7-9 are relevant in oxygen-free systems, whereas the presence of oxygen adds step 10 through the occurrence of reaction 1 . 1. Solution a t p H 0.5. Degassed Solution. Figure 1A shows the evolution of AV on a short time scale, with the degassed solution at pH 0.5 and a band bending of 0.42 V; the photopotential reaches its maximum value in approximately 20 ns. There is no indication for a slow component in this A V rise. Two photoinjection processes have to be considered: the fast one (7) from adsorbed excited states and the slow one (9) from bulk excited states diffusing to the electrode. Total reflection spectra of adsorbed R ~ ( b p y ) ~have ~ + been recorded at an SnOz/acidic aqueous Ru(bpy),2+ solution interface1*and at an SnO, surface which has been first in contact with such a solution and then rinsed with the electrolyte solution without complex;18 in both cases a slight red shift, of about 10 nm, of Ru(bpy),Z+ was observed. This confirms Memming's previous results" and indicates indeed that the complex is adsorbed on SnOz. The 20-ns rise corresponds thus to the oxidation of adsorbed excited states; a contribution from solution excited states diffusing toward the electrode will be very small during these 20 ns since it would originate from a diffusion length I = (Dt)'I2 of only 4.5 nm. The absence of a slow (580 ns) oxidative component is probably due (18) The total reflection spectra at the SnO,/solution interface have been recorded by A. Ryan in Prof. M. Splitler's laboratory, at Mount Holyoke College, South Hadley, MA.
to the competition of the oxidative process (9) with the back electron transfer (8) from the conduction band to the already oxidized Ru3+ adsorbed at the electrode. The back electron transfer (8) is responsible for the nonexpo, in Figure 1B. The tunneling nential decay, lasting for 5-6 ~ sseen through the space charge corresponding to a band bending of 0.42 V is thus fairly slow. When this bending is close to zero, only a small AVspike has been observed because the reduction of Ru3+ is now almost instantaneous. If the band bending is increased to 0.72 V, the 20-11s fast AV rise is followed by a slower component lasting for about 600 ns (Figure 1C). This slower rise would correspond to process 9, in agreement with the 580-11s lifetime of R ~ ( b p y ) , ~ +under * these conditions, and is now observed because the thicker space charge inhibits the back electron transfer (8), as shown in Figure 1D by the absence of a AVrelaxation. The small amplitude of this slow rise precludes, however, any further quantitative analysis. Finally, AVdecays exponentially to zero with a characteristic time of 200 ms (for PP = 0.4 V) corresponding to the 1-MR resistance of the oscilloscope and the capacitance of the tin oxide electrode. Such slow processes have to be monitored in dc mode and can thus be observed only if the PP of the electrode is identical with its equilibrium potential, Le., $0.4 V/SCE in this case. Table I gives the initial photopotential AVOfor various PP values and normalized to the same laser power. Knowing the capacitances determined by impedance measurements a t 10 kHz, the amount of injected charge AQo = CAV,, can be calculated; it is found that AQo is PP independent, as was observed previously with rhodamine B.Io It has to be noticed here that irreproducibilities were observed, but only at pH 0.5. In some experiments, it was found that, instead of the 5-6-ps decay, leading to a longer lasting nonzero potential value, the signal decreases steadily, goes through zero, and then assumes a negative value of a few millivolts. Similar sign inversions will be discussed later and are found in oxygenated solutions, where Ru3+is the major electroactive species participating in electron transfers at the electrode. These anomalies suggest that oxygen is strongly adsorbed on SnO, in very acidic media. Oxygen-Saturated Solution. Here again a fast A V rise is observed during 20 ns, but contrary to what was observed before, the amount of charge injected AQo is now strongly dependent on the PP value (Table I); it is also seen that the AQo values are now less than those found in the degassed solutions. Since the presence of oxygen may lead to the photogeneration of R ~ ( b p y ) , ~ +an, explanation for these findings may be that AQo results from two contributions: the PP-independent oxidation of the excited complex and the reduction of Ru3+which would be less efficient at more positive potentials. This model is however unsatisfactory because, as was seen before, the reduction of Ru3+is a slow process occurring in a few microseconds and is thus unable to compete with processes which are completed in 20 ns. A much more consistent hypothesis would be to assume again that O2 is very
Kirsch-De Mesmaeker et al.
6660 The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 Av/mv
150r AV/mV
Figure 3. A V measured from 0 to 8 ms after the laser pulse with the oxygenated complex solution at pH 0.5 and plotted as a function of the square root of the time. PP = 0 V/SCE, 0.02 V/FBP.
I av/mv
Av/mV
difficult to imagine a degradation reaction destroying Ru3+at such a high rate, considering the well-known stability of Ru(bpy),,+ at that pH.19 Another explanation would be to assume that a triply charged cation Ru(bpy)d+ located at the surface of the electrode causes a very strong perturbation inducing a local change in the bending of the bands: the barrier at that mecific site would thus be lower;d, allowing an accelerated leakage of the electron efficiently adsorbed On Sn02 at pH O.5 and that a fast quenching back to the oxidized complex. Such a major influence of the as that described in reactions 1-3 but process, barrier shape on the rate of electron return is shown by the results adsorbed is now to compete with the fast published by Itoh, Nakao, and Honda," who found that the decay injection of step 7. corresponding to the back electron transfer to oxidized rhodamine The pp dependence Of *Qo under these conditions may be B from an Sn02electrode having a higher doping level than ours, ascribed to a potential-dependent Of '2. If One and thus a thinner space charge, lasts only 5 ps as compared with assumes, indeed, that the amount of O2 bound to the electrode our 200 ps. decreases at more positive potentials, the rate of quenching will The 5-6-ps relaxation is suppressed for a PP value of 0.7 V, decrease and be less to compete with where it is replaced by a second potential rise lasting for about the rate of the PP-independent electron injection (7). It has to 600 ns; this is identical with what was observed at pH o,5 and be noted that a biphotonic process such as reaction 4, which cannot is illustrated in Figure lc, be excluded in the adsorbed layer and where Ru(bpy),+ would An attempt was made to trap Ru(bpy),,+ with a reducing agent. be the adsorbed electroactive species instead of R ~ ( b p y ) ~ ~ + * Although the effect of triethanolamine (TEOA) cannot be per(reaction 7), would lead to the same conclusions concerning the formed at pH 4,5, we added 10-2 M of TEOA and adjusted the O2 effect on AQo. pH to 7 with 0.1 N H2S04. As expected, we observe the disapAfter the initial AV rise, and for a PP of 0.4 V, the potential pearance of the 5-bmps decay and the appearance of the 6 0 0 - n ~ d a x e s in a few microseconds (Figure 2A) and, contrary to the growth after the initial 2 0 - n ~rise. Considering the well-known degassed solutions, continues to drop at longer times, goes through reduction of Ru(bpy),3+ by TEOA, these findings lend strong zero (Figure 2B), and increases negatively for some 100 ms until support to our hypothesis for the 600-ns rise. a steady value of -300 mV is reached (Figure 2C). These negative Oxygen-Saturated Solution. The major difference with the AVvalues are due, as discussed before, to the reduction of Rubehavior at pH 0.5 is that the AQo values are no pp de(bpy)33+generated during the light pulse by reactions 1-3 in the pendent and are close to those obtained without oxygen. This bulk of the solution. When the PP is 0.7 V, the fast AV rise is suggests that the adsorption of o2onto tin oxide is pH dependent again followed by a slow decay, and also with a sign inversion, and is much weaker at pH 4.5 than at pH o.5, despite the thicker space charge. The long-term polarity reversal found at pH 0.5 is observed For a PP value of 0 V, a small A v s ~ i k eis d ~ e ~ -just ~ e after d here also, but with a much smaller amplitude (-20 m~ instead the light pulse, and the potential immediately a ~ ~ mnegative - e ~ of -150 m ~ 16, ms after the pulse for a 0.4-v band bending), values (Figure 2D). This experiment, involving almost only so3. Solution at p H 13. At pH 13, the FBP of S n 0 2 shifts to lution species, can be analyzed in more detail. The concentration -0.77 V/SCE; since the potential of Ru(bpy)?+* is -0.81 profile Of RU2+* from the electrode to the bulk follows the V/SCEZI( ~ 4), ,.losei to the ~ sno2 ~ conduction ~ band ~ edge, Beer-Lambert law, but it is quite safe to assume that, in a it is expected that the photoinjection ofan electron from the excited 0.Ol-mm optical path from the electrode surface, this concentration complex to the semiconductor will be less efficient than in + a lifetime which is very should be constant. Since R u ( b p ~ ) , ~has acidic solutions. It was observed, however, that under continuous long compared with the time scale of our experiments, the reillumination and potentiostatic contro114 the oxidation duction corresponds to the diffusion-controlled process (10); this to pH 3. This of Ru(bpy)32+ at SnOz increases from pH is in very g o d agreement with the excellent fit of the AVvariation current enhancement was attributed to the generation and the as a function of the square root of time (Figure 3). Moreover, accumulation of a long-lived electroactive species which would Since the reduction Of Ru3+ involves the bulk Of the Solution, it be produced photochemically but with a very low quantum yield the 'low takes a long time and ends as a process "pensating since it cannot be detected optically, neither by the growth of a discharge of the system through the external 1-Mi-I resistance of new absorption nor by the bleaching ofthe starting complex, after the oscilloscope, explaining thus the long-lasting nonzero value one light flash. This new species must thus be unable to yield of the potential (Figure 2C). a measurable photopotential after a laser pulse. 2. Solution at p H 4.5. Degassed Solution. The results are Degassed Solution. As shown in Figure 5A and in Table I, to those Obtained at pH 0.5 regarding the quite the photopotential and corresponding AQo values are, as expected, reached after 2o ns and the 5-6*ps decay dependent *QO smaller than at pH 0.5 or 4.5. What deserves closer attention for a 0.4-V band bending. is the fact that this photopotential is reached after 200-300 ns however be noticed that, at the Same pH and the Same It instead of 20 ns; this sluggishness will be discussed in section 4. value of the band bending, the relaxation observed in the case of rhodamine B is much slower and lasts some 200 pslo instead of ( 1 9 ) Creutz, C.; Sutin, N . Proc. Natl. Acad. Sci. U.S.A. 1975, 72, 2858. 5-6 ps. In the case of rhodamine, the 200-ps decay was ascribed ( 2 0 ) Itoh, K.; Nakao, M.; Honda, K. Chem. Phys. Left. 1984, 1 1 1 , 4 9 2 . to the sum of two processes: the back electron transfer to, and (21) Bock, C . R.; Meyer, T. J.; Whitten, D. G. J . Am. Chem. SOC.1975, 97, 2909. the chemical degradation of, the oxidized dye. It is, however, ~
Figure 2. Oscilloscopic traces of the pulsed laser-induced A V recorded with the oxygenated complex solution at pH 0.5: (A)PP = +0.4 V/SCE, 0.42 V/FBP, P = 38 mV; (B) PP = +0.4 V/SCE, 0.42 V/FBP, P = 36 mV; (C)PP = +0.4 V/SCE, 0.42 V/FBP, P = 44 mV; (D)PP = 0 V/SCE, 0.02 V/FBP, P = 40 mV.
The Journal of Physical Chemistry. Vol. 90, No. 25, 1986 6661
SnO,/R~(bpy),~+ Solution Interface
Sn02
SOL
I
TABLE II: Measurements with the Complex Solution at pH 13 solution 7(Av),’’ ~(lum),” ns (DH131 ns degassed 280 580 saturated with N2 280 530 saturated with O2 150 160
“Rise time of the initial photopotential corresponding to 90% of the photopotential amplitude and resulting from the average of three measurements. Luminescence lifetime of the complex excited state in the three solutions.
1
+1 V / SCE
l-
i
Figure 4. Energetic diagram of the Sn02/Ru(bpy)?+ solution interface at pH 13.
1 AV/mV
C
t
Figure 5. Oscilloscopic traces of the pulsed laser-induced A V recorded with the complex solution at pH 13, degassed (A, B) and saturated with O2 (C, D): (A) PP = 0 V/SCE, 0.77 V/FBP, P = 36 mV; (B) PP = -0.4 V/SCE, 0.37 V/FBP, P = 43 mV; (C) PP = 0 V/SCE, 0.77 V/ FBP, P = 32 mV; (D) PP = -0.4 V/SCE, 0.37 V/FBP, P = 32 mV.
In connection with this observation, a rather low electron injection rate constant from the excited complex into colloidal T i 0 2 at pH la has been determined recently.22 The 5-6-ps relaxation is still observed at a band bending of 0.37 (Figure 5B) and is suppressed a t a more anodic PP with a band bending of 0.77 V. This decay was not expected to occur, however, since it was s ~ g g e s t e d ’ ~that , ’ ~ Ru(bpy)?+ is very rapidly reduced by OH-, thus regenerating the starting material. If one uses the kinetic data for this proce~s,’~ it is found that the lifetime of Ru3+ in 0.1 M N a O H should be near 60 ms; it is thus clear that this reaction is too slow to compete with the usual back electron transfer. Oxygen-Saturated Solution. The AV rise is now slightly faster than in the degassed solution and takes 150-200 ns (Figure 5C); the final AQ values are, nevertheless, the same as in the degassed solutions and are PP independent (Table I). This will also be discussed in section 4. The photopotentials decay nonexponentially to zero (Figure 5D) in 4-8 ms for a band bending of 0.37 V; this is much faster than the time constant of the external circuit and should thus correspond to the reduction of Ru(bpy)k+ generated by reaction 1 or of species originating from Ru3+ during its d e c o m p o ~ i t i o n . ~As ~ . ~before, ~ ~~
~
(22) Dcsilvestro, J.; Grltzel, M.; Kavan, L.; Moser, J. J . Am. Chem. SOC. 1985, 107, 2988.
this relaxation is suppressed for a band bending of 0.77 V. 4. Sluggishness of the AVRise at p H 13. As mentioned before, the photoinjection of an electron from the excited complex takes about 200 ns at pH 13, which is 10 times longer than the 20 ns observed in more acidic media, but definitely shorter than the lifetime of the excited state as measured in solution. Moreover, preliminary experiments using a focused and unfocused laser indicate that this slow rise time is not affected by the exciting light intensity. Since this AVrise is not exponential, we chose to use the time required to reach 90% of the maximum value as a measure of the characteristic time for the charge injection; the values are collected in Table I1 under the heading T(AV). The corresponding lifetimes in solution, as determined by the luminescence decay, are given under the heading T(1um). It is seen that under vacuum or under Nz T ( A V is definitely shorter than T(lum), whereas under Oz, both 7 ’ s become shorter, but are now much less different. The charge injection might be slow because the donor level of Ru2+* is very close to the conduction band edge and would thus require tunneling through the space charge. That this is probably not the main reason is shown by the fact that the amount of charge injected is only slightly increased (less than 50%) and the rate ’is not affected at more anodic potentials, whereas one would expect very large changes for a 700-mV variation of the electrode potentia1.l0 A more attractive explanation would be to consider that the electron has to cross an insulating layer formed, as shown by G e r i s ~ h e r at , ~ the ~ SnO, surface at high pH values and corresponding thus to a hydroxide film. In agreement with this hypothesis are the facts that the injected charge is only slightly smaller than in more acidic solutions and that it is almost independent of the electrode bias since the rate-determining barrier is now the insulating layer. Actually, we have some additional experiments suggesting the presence of an insulating layer on the surface of SnOz at pH 13, based on a comparison with the behavior of rhodamine B on SnO, and on ZnO. No evidence for a hydroxide film on zinc oxide has been found by G e r i s ~ h e rin ; ~agreement ~ with this, when a ZnO single crystal is in contact with a rhodamine solution at pH 13, a fast laser-induced AV rise is obsewed, showing that the rate of electron injection is fast enough to overrun the 1-ns lifetime of the excited dye. The donor level of excited rhodamine is thus well above the conduction band edge of ZnO. Since the E, value for SnO, at the same pH is at a more positive potential than that of ZnO, the injection from excited rhodamine should be direct on SnO, also, and very fast. We found, however, that no light-induced AV is detected with the Sn02/rhodamine system at pH 13. The insulating layer present at the surface of S n 0 2 is thus a barrier sufficiently high to completely inhibit the injection from species having a lifetime of only 1 ns, but which may be overcome by an excited state having a lifetime between 160 and 600 ns. We found also that when an SnOzelectrode, which has been in contact with a solution at pH 13, is placed in a rhodamine solution at pH 4.5, a laser-induced AVrising in 20 ns is again observed but with an attenuated amplitude; the formation of the insulating film is thus at least partially reversible. In connection with this, it is interesting (23) Gosh, P.; Brunschig, B.; Chou, M.; Creutz, C.; Sutin, N. J . Am. Chem. SOC.1984, 106, 4112. (24) Lay, P.; Sasse, W. Inorg. Chem. 1985, 24, 4701. (25) Bressel, B.;Gerischer, H. Ber. Bunsenges. Phys. Chem. 1983, 87, 298.
6662 The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 to note that Liang and Ponte Gonvalves26 found that the luminescence lifetime of rhodamine B adsorbed on SnO, does not depend on the pH treatment of the surface, whereas we find that electron injection is completely inhibited at pH 13. These two apparently conflicting observations are in agreement with the fact that the quantum yield for photoinjection on our Sn0, electrodes meaning that the dominant process is always very low ( occurring at the interface is a pH-independent quenching of the excited state by other mechanisms. Since the injection is now rather slow, it may be perturbed by the competing spontaneous decay of the photoexcited complex; T(AVis thus shorter than T(1um) because there are two decay channels contributing to T(AV) whereas there is only one for T(1um). When oxygen is present, it introduces an additional decay channel and both characteristic times decrease. T(AV) is now given by the sum of three decay rates and s(1um) by the sum of only two; T(AV) is thus proportionately less sensitive to the presence of oxygen than T(lum), and the two lifetimes converge. The 200-ns AV rise is however not exponential, showing that adsorbed molecules, which are definitely identified at that pH,18 are not the only ones participating in the charge injection. The full equation including both adsorbed and diffusing excited states is, however, too complicated to be tested experimentally. N
Conclusion The behavior of Ru(bpy),2+ under pulsed laser-induced excitation at Sn02/electrolyte interfaces, as measured by the time evolution sf photopotentials, is much more complicated than that of rhodamine B. This difference arises essentially from the very large difference between the lifetimes of the corresponding excited states: 1 ns for the dye and 580 ns for the organometallic cation. The ruthenium complex is thus able to participate in electrode processes through bulk excited states diffusing to the electrode during their longer lifetime, and oxygen is now in state to oxidize ~~~~~
~
(26) Liang, Y.; Ponte Gonvalves, A. J. Phys. Chem. 1985, 89, 329. (27) Other conceivable processes are absent from this list because they cannot occur or are very slow at the electrode potentials considered: Ru2+* + e Ru' (&, = +0.74 V/SCE); 02.- 02 + e (pH 13; Eo = -0.32 H* O2 e (pH 0; E,, = +0.46 V/SCE). A major V/SCE); H02' uncertainty remains about the electrochemistry at S n 0 2 of singlet O2which has been shown to be formed very efficiently in step 2.''
-
-
+ +
-
Kirsch-De Mesmaeker et al. excited states, producing thus a new electroactive species. In degassed solution, the injection from Ru2+*is very fast at pH 0.5 and 4.5, and PP independent, but is slowed down at pH 13 because of the insulating hydroxide layer which forms in strongly alkaline solutions. A fairly strong adsorption of O2onto S n 0 2 at pH 0.5, and which seems to decrease at more positive electrode potentials, can explain the behavior of this system in very acidic solutions. The relaxation of the photopotentials is assigned to the back electron transfer from the electrode to the oxidized Ru3+. This is evidenced first by the suppression of this relaxation at anodic potentials; it is then possible to detect a relatively slow (600 ns) oxidative A V rise originating from the diffusion of solution excited states toward the electrode. A second argument in favor of the involvement of Ru3+in the 5-6-ps relaxation is the inhibition of this photopotential decay by added triethanolamine. In the presence of oxygen, the main process is now the photogeneration of Ru3+; its reduction then masks the oxidative transfers, and for a PP value of 0 V at pH 0.5, this reduction strictly obeys the law of diffusion from the bulk toward the electrode. The analysis of the time dependence of the laser-induced photopotentials has thus provided valuable information, complementary to those derived from studies under continuous illumination, allowing the identification of electroactive species and of specific mechanisms. It also allows to correlate the short time scale kinetic behavior of the sensitizer at the semiconductor/solution interface with its short time scale photophysics and photochemistry in homogeneous solution. The complexity of the system unfortunately precludes any quantitative kinetic analysis, but a consideration of relative rates leads to a consistent model for the photoelectrochemistry of Ru(bpy),Z+at a highly n-doped S n 0 2 semiconducting electrode.
Acknowledgment. We are grateful to Professor M. Spitler and Mrs. A. Ryan for their efficient contribution to this work, especially to A.R., who performed the measurements of the total reflection spectra at the Mount Holyoke College, South Hadley, MA. Registry No. SnOz, 18282-10-5; Ru(bpy),Cl,, 14323-06-9; 02, 7782-44-7; tris(2,2'-bipyridine)ruthenium(3+), 18955-01-6.
