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J. Phys. Chem. 1996, 100, 4900-4908
Photosensitization of Nanocrystalline Semiconductor Films. Modulation of Electron Transfer between Excited Ruthenium Complex and SnO2 Nanocrystallites with an Externally Applied Bias Prashant V. Kamat,* Idriss Bedja,† Surat Hotchandani,† and Larry K. Patterson Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: May 5, 1995; In Final Form: August 29, 1995X
In situ spectroelectrochemical measurements have been carried out to probe the charge injection from excited Ru(bpy)2(dcbpy)2+, Ru(II), into the SnO2 nanocrystallites. The dependence of luminescence yield and lifetime at various applied potentials suggests that the heterogeneous electron transfer from excited sensitizer into the semiconductor can be controlled by the externally applied electrochemical bias. The maximum quenching is seen at positive potentials while an increase in the luminescence yield and lifetime is seen at negative potentials. Laser flash photolysis of Ru(II)-modified SnO2 nanocrystalline film has been carried out to record the transient absorption spectra at different applied potentials. The yield of electron transfer product, Ru(III), decreases as the applied bias is switched to negative potentials. At an applied bias of -0.7 V the only observable transient is the excited Ru(II) complex (Ru(II)*). The maximum apparent electron transfer rate constant, ket (∼4 × 108 s-1), observed at positive bias agrees with the previously determined electron transfer rate constants from emission lifetime and microwave conductivity experiments. The apparent rate constant for heterogeneous electron transfer is dependent on the applied bias, and it decreases as the difference between the pseudoFermi level of SnO2 and oxidation potential of Ru(II)* decreases. These results suggest that the decreased rate of charge injection is responsible for lower IPCE (incident photon-to-photocurrent efficiency) observed in photoelectrochemical cells under negative bias. No significant change in the rate of reverse electron transfer was observed at potentials greater than -0.4 V.
Introduction Recent demonstration of efficient sensitization of semiconductor nanocrystallites with inorganic1-4 and organic5-9 dyes has stimulated interest in developing nanostructured semiconductor films as photosensitive electrode materials for photoelectrochemical cells.10,11 One of the sensitizer systems that has attracted greater attention is the family of ruthenium(II) bipyridyl complexes.1-4,12-22 Net power conversion efficiencies up to 10% have been reported in diffuse daylight using a TiO2Ru complex.2 Although an ideal Schottky barrier is absent in nanocrystalline semiconductor films,4,23-27 a potential gradient arising from varying degree of electron accumulation within the semiconductor particles acts as a driving force for the transport of injected electrons across the films (Figure 1). Efforts are currently underway in several laboratories to explore the mechanism of charge transport within the nanocrystalline film and to obtain the kinetic details of the charge injection process. We have recently shown that transparent SnO2 films prepared from colloids of 30-50 Å diameter exhibit excellent photoelectrochemical activity.4 Because these films are transparent in the visible, it is convenient to employ them for various in situ spectroscopy techniques. It has been reported that the charge injection process is an extremely fast process and is completed within a few picoseconds at low submonolayer coverages on the semiconductor surface.28-30 Such an estimate should be taken with caution since nanocrystalline films employed in photoelectrochemical * Address correspondence to this author (e-mail: Kamat@ marconi.rad.nd.edu). † Permanent address: Centre de Recherche en Photobiophysique, Universite du Que´bec a` Trois Rivie`res, Trois Rivie`res, Que´bec, Canada G9A 5H7. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4900$12.00/0
Figure 1. Schematic diagram illustrating the principle of photosensitization in a nanostructured semiconductor film modified with a sensitizing dye (S). In the absence of any externally applied bias the electrons injected from excited sensitizer (S*) accumulate within the semiconductor nanocrystallites and alter their pseudo-Fermi level, thus creating a potential gradient within the film. This potential gradient facilitates electron transport toward the collecting surface of an opticaly transparent electrode (OTE).
cells require high surface coverages for maximizing the lightharvesting efficiency. Moreover, luminescence decay alone cannot yield complete information regarding the charge injection process. Nonradiative processes such as excited state annihilation, ground state quenching, energy transfer to surface, or surface impurities can compete with the heterogeneous electron transfer. Recent lifetime measurements of Ru(II)-modified SnO24,31 and TiO2 nanocrystallites32,33 have shown that the charge injection process occurs with a rate constant in the range 108-109 s-1. The emission decay in these studies was found to be multiexponential, indicating the existence of multiple injection sites on the semiconductor surface. The trapping and detrapping processes could also play a major role in influencing the kinetics of charge injection process.34,35 In a recent study we have made an effort to correlate the luminescence decay with the increase in microwave conductivity.36 The energy difference between the conduction band of the semiconductor and oxidation potential of the excited sensitizer © 1996 American Chemical Society
Photosensitization of Semiconductor Films is the major driving force for the excited state charge transfer.37,38 Different approaches have been considered to study the energy gap dependence of the photosensitization efficiency. Hashimoto et al.39,40 have shown that the excited state lifetime of Ru(bpy)32+ adsorbed on a metal oxide semiconductor is dependent on the conduction band energy of the semiconductor. Tani41,42 has made an effort to establish the energy gap dependence of the electron transfer between silver halides and J-aggregates of the dye. Spitler and co-workers43,44 have varied the pH to examine the energetic threshold for dye-sensitized photocurrent generation at SrTiO3 and TiO2 electrodes. The energy gap is dependent on pH since the conduction band of metal oxide semiconductor shifts 0.059 V/pH. Spectroelectrochemical measurements of metal oxide films have shown that externally applied electrochemical bias causes electron accumulation in semiconductor nanocrystallites.45-51 The onset potential at which the electron accumulation is seen corresponds to the flat-band potential of the semiconductor. In the case of InP semiconductor the applied potential was shown to influence the hot electron injection process. Several researchers have observed an increase in the quenching of the excited state of the sensitizer adsorbed onto an n-type semiconductor electrode or increased production of oxidized sensitizer by biasing the electrode at positive potentials.17-43,52 Recently, resonance Raman spectroscopy53 and transient absorption spectroscopy54 have been employed to monitor the changes that occur on the nanocrystalline semiconductor surface at positive and negative bias potentials. O’Regan et al.55 have investigated the influence of externally applied bias on the charge injection efficiency and reverse electron transfer process in a TiO2/Ru(II) system. It is known that the photosensitization efficiency of a dyemodified semiconductor electrode is strongly dependent on the applied bias.38 It significantly decreases when the applied potential is more negative than the flat-band potential of the semiconductor. Often such a decrease is attributed to the increase in reverse electron transfer arising from the lack of potential gradient within the semiconductor to drive away the injected charge toward the collecting surface. A series of spectroelectrochemical measurements have now been performed to investigate the role of applied bias in modulating the charge injection process in nanocrystalline SnO2 films. To the best of our knowledge, this is the first such attempt to present direct evidence for the suppression of the charge injection process at negative bias using transient absorption spectroscopy. Experimental Section Materials. Optically transparent electrodes (OTE) were cut from an indium tin oxide-coated glass plate (1.3 mm thick, 20 ohms/square) obtained from Donnelly Corp., Holland, MI. SnO2 colloidal suspension (18%) was obtained from Alfa Chemicals and used without further purification. Absorption spectra were recorded with a Perkin-Elmer 3840 diode array spectrophotometer. Emission spectra were recorded with SLM S-8000C spectrofluorometer. Preparation of SnO2 Particulate Films. The synthetic procedure for casting a transparent thin film of SnO2 on an optically transparent electrode has been reported earlier.4 A small aliquot (usually 0.1 mL) of the diluted SnO2 colloidal suspension (2%) was applied to a conducting surface of 0.8 × 3 cm2 of OTE and was dried in air on a warm plate. The SnO2 colloid-coated glass plates were then annealed at 673 K for 1 h. The thin film semiconductor electrode is referred to as OTE/ SnO2. The thickness of the film was e1 µm. Modification with Ru(2,2′-bipyridine)2(2,2′-bipyridine-4,4′dicarboxylic acid)2+. We modified the OTE/SnO2 electrodes
J. Phys. Chem., Vol. 100, No. 12, 1996 4901
Figure 2. Schematic diagram of the experimental setup for spectroelectrochemical measurements. The cell design is magnified for illustrating details.
with Ru(2,2′-bipyridine)2(2,2′-bipyridine-4,4′-dicarboxylic acid)2+, Ru(II), by dipping the warm OTE/SnO2 electrodes directly in an acetonitrile solution containing the Ru(II) complex for a period of 8-10 h. The electrode was then thoroughly washed with acetonitrile and stored in the dark. The yellow-orange coloration of the porous SnO2 film (A470 nm ∼ 0.5) confirmed adsorption of Ru(II) in large amounts. (These electrodes will be referred to as OTE/SnO2/Ru(II) in the following discussion.)Spectroelectrochemical and Photoelectrochemical Measurements. The measurements were carried out in a thin-layer cell consisting of a 2 or 5 mm path length quartz cuvette with two side arms attached for inserting reference, RE (Ag/AgCl), and counter, CE (Pt gauze), electrodes. The spectroelectrochemical cell employed in the present set of experiments is shown in Figure 2. The design of the cell is such that we can insert it into the sample compartment of the absorption or emission spectrophotometer and carry out the measurements under the influence of an applied bias. A Princeton Applied Research (PAR) Model 173 potentiostat and Model 175 universal programmer were used in spectroelectrochemical and photoelectrochemical measurements. Photocurrent measurements were carried out with a Keithley Model 617 programmable electrometer. A collimated light beam from a 250 W xenon lamp was used as the light source. A Bausch and Lomb high-intensity grating monochromator was introduced into the path of the excitation beam for selecting the excitation wavelength. Lifetime Measurements. Emission lifetime measurements were carried out using the electrochemical cell as explained above. The experiments were performed by a time-correlated single-photon counting technique using an apparatus that has been described elsewhere.56 The excitation source was a modelocked, Q-switched Quantronix 416 Nd:YAG laser which provided 80 ps pulses of 355 nm light with a frequency of 5 kHz and an integrated power of 10 mW. Laser Flash Photolysis Experiments. The cell described in Figure 2 was placed in the sample compartment of the nanosecond laser flash photolysis setup. The OTE/SnO2/Ru-
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(II) electrode was inserted into the cell so that the transient absorption spectrum could be recorded at the desired electrochemical bias. The excitation was carried out in a front face geometry with 532 nm laser pulses from a Quanta-Ray CDR-1 Nd:YAG laser system (∼6 ns pulse width, 5 mJ). White light from 1000 W xenon lamp was used as a probe. The photomultiplier output was digitized with a Tektronix 7912 AD programmable digitizer. A typical experiment consisted of a series of 3-6 replicate shots per single measurement. The average signal was processed with an LSI-11 microprocessor interfaced with a VAX computer.57 Results and Discussion Photoelectrochemical Behavior of OTE/SnO2/Ru(II). In our earlier study we described in detail the preparation and characterization of SnO2 films prepared from colloidal suspensions. The high porosity of this film facilitates adsorption of Ru(II) in very high concentrations. The Ru(II) complex absorbs strongly in the visible (460 ) 15 500 M-1 cm-1) and is able to sensitize large-bandgap semiconductors such as SnO2. The SnO2 films modified with Ru(II) exhibit photoresponse in the visible with maximum incident photon-to-photocurrent efficiency (IPCE) of ∼50%. The close match between the photocurrent action spectrum and the absorption spectrum shows that the photosensitization mechanism is operative in extending the photocurrent response of OTE/SnO2/Ru(II) electrode into the visible (reaction 2). kr
Ru(II) + hν f Ru(II)* 98 Ru(II) + hν′ knr
Ru(II)* 98 Ru(II) ket
Ru(II)* + SnO2 98 SnO2(e) + Ru(III)
(1a) (1b) (2)
The radiative (kr) and nonradiative (knr) decay and charge injection (ket) processes are major pathways for the deactivation of Ru(II)* (reactions 1a, 1b, and 2). Upon excitation with visible light the excited sensitizer molecules are capable of injecting electrons into the SnO2 particles. These electrons are then collected at the OTE surface to generate anodic photocurrent. The oxidized sensitizer can either react back with the injected electron (reaction 3) or be regenerated by the redox couple I3-/ I- present in the electrolyte (reaction 4). kret
Figure 3. Energy level diagram illustrating the conduction (CB) and valence band (VB) energies of SnO2 and the oxidation potential (E0) of ground and excited state of Ru(II). kr, knr, ket, and kret represent rate constants for radiative, nonradiative, charge injection, and reverse electron transfer processes, respectively.
Ru(III) + SnO2(e) 98 Ru(II) + SnO2
(3)
2Ru(III) + 3I- f 2Ru(II) + I3-
(4)
In the present study we have excluded a regenerative redox couple (I3-/I-) so that only reaction 3 is responsible for the regeneration of Ru(II). The value of kret has been shown to be several orders of magnitude smaller than ket.2,55 The energy level diagram describing the band energies of SnO2 and oxidation potential of the sensitizer is illustrated in Figure 3. Under unbiased conditions, the difference between the oxidation potential of excited sensitizer, Ru(bpy)2(dcbpy)2+* (E0 ) -0.72 V vs Ag/AgCl or -0.52 V vs NHE)58,59 and the conduction band of SnO2 (ECB ) -0.2 V vs Ag/AgCl or 0 V vs NHE) provides necessary driving force for the charge injection process (reaction 2). Under unbiased conditions the pseudo-Fermi level or electrochemical potential of the nanocrystalline SnO2 film lies close to the conduction band. Ap-
Figure 4. Dependence of incident photon to photocurrent conversion efficiency (IPCE) of OTE/SnO2/Ru(II) on the applied potential (CE ) Pt; RE ) Ag/AgCl; electrolyte ) 0.04 M I2 and 0.5 M LiI in acetonitrile.) IPCE (%) was determined from the expression {100 × (1240isc)/(λIinc)}, where isc is the short-circuit current photocurrent (A/ cm2), Iinc is the incident light intensity (W/cm2), and λ is the excitation wavelength (nm).
plication of an external bias shifts the pseudo-Fermi level in such a way that it directly alters the energetics that control ket and kret in Ru(II)-modified SnO2 film. The net effect of the change in pseudo-Fermi level could be observed from the dependence of IPCE of a photoelectrochemical cell. The dependence of IPCE of Ru(II)-modified SnO2 electrode on the applied potential is shown in Figure 4. The IPCE is maximum at positive bias (V > 0 V vs Ag/AgCl) and remains almost constant at potentials greater than 0.0 V. A decrease in the IPCE is seen with increasing negative bias. At potential ∼-0.6 V the IPCE is zero, suggesting no net electron flow within the nanocrystalline semiconductor film. Such a dependence of IPCE on the applied potential has been attributed to the dependence of charge injection efficiency on the applied potential.55 There are two possible reasons for observing no net current flow under negative bias: (i) the charge recombination (reaction 3) competes with charge injection (reaction 2) and hence no net electron flow and/or (ii) the charge injection (reaction 2) becomes slower because of lack of driving force. Spectroelectrochemical experiments have been performed in the present study to answer this intriguing question. It should be noted that no direct electrochemical reduction or oxidation of Ru(II) occurs in the potential range -0.7 to +0.6 chosen in this study.
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J. Phys. Chem., Vol. 100, No. 12, 1996 4903
Figure 5. Emission spectra of OTE/SnO2/Ru(II) recorded at different applied potential. Excitation was at 460 nm. The spectra were recorded in a front face configuration and are corrected for instrument response (CE ) Pt; Re ) Ag/AgCl; electrolyte ) 0.1 M tetrabutylammonium perchlorate in acetonitrile).
Dependence of Emission Yield on the Applied Potential. Ru(II) complex has a strong emission in the red region with a maximum around 630 nm. Comparison between the emission behavior of Ru(II)* on nonreactive (SiO2, Al2O3) and semiconductor (TiO2) surfaces has been made in previous studies.32,33,54 At similar coverages, the emission of Ru(II)* was significantly lower on the TiO2 surface than on Al2O3. Nearly 95% quenching observed on TiO2 surface is the result of an efficient charge injection process. Similarly, Ru(II)-modified SnO2 films exhibited weak emission with a maximum around 630 nm. The emission spectra recorded at various bias potentials are shown in Figure 5. There was no significant difference between the emission spectra recorded at 0.0 and +0.6 V. However, a dramatic increase in the emission yield is seen with increasing negative bias without any changes in the emission maximum. At a bias potential more negative than -0.6 V most of the emission of excited Ru(II) can be restored. The emission quenching phenomenon could be seen again by reversing the applied potential to positive bias. (Making the bias potential more negative than -0.8 V led to some irreversibility.) Since Ru(II)* deactivates via radiative (kr), nonradiative (knr), and electron transfer (ket) processes, one can express the quantum yields for emission and net electron transfer processes by the expression (5).
Φr ) kr/(kr + knr + ket) and Φet ) ket/(kr + knr + ket)
(5)
Since nonradiative decay is not likely to be influenced by the applied bias, one can correlate
Φr + Φet ) constant
(6)
Thus, any increase in Φr should reflect a decrease in Φet and hence ket. In order to check the reproducibility of the dependence of emission yield on the applied potential, the emission at 640 nm was continuously monitored during an electrochemical scan. The scan rate was kept constant at 1 mV/s so that there was enough time for the semiconductor nanocrystallites to attain equilibrium with the applied bias. Figure 6 shows one such cycle recorded under forward and reverse bias. It is evident from this experiment that the deactivation of the excited state can be controlled by the applied potential. The sharp increase observed in the relative emission yield at negative applied potentials
Figure 6. Dependence of relative emission yield of OTE/SnO2/Ru(II) on the applied potential. The emission was monitored at 640 nm during the electrochemical cycle (scan rate 1 mV/s; CE ) Pt; RE ) Ag/AgCl; electrolyte ) 0.1 M tetrabutylammonium perchlorate in acetonitrile; excitation at 460 nm).
Figure 7. Emission lifetimes of OTE/SnO2/Ru(II) recorded at applied potentials, +0.6, 0.0, and -0.6 V versus Ag/AgCl. The solid line shows the kinetic fit as obtained from expression 7 (CE ) Pt; RE ) Ag/ AgCl; electrolyte ) 0.1 M tetrabutylammonium perchlorate in acetonitrile).
shows the suppression of the electron transfer quenching and dominance of radiative decay for deactivating the excited sensitizer. This effect parallels the observation of decreased IPCE at negative applied potentials (Figure 4). Effect of Applied Bias on Emission Decay of Ru(II)*. It has been shown earlier that the emission lifetimes are useful for obtaining the kinetic details of heterogeneous electron transfer between the semiconductor and sensitizer (reaction 2). Figure 7 shows the emission decay of Ru(II)* adsorbed on SnO2 surface at different applied potentials. When adsorbed on a neutral surface such as silica or alumina or in aqueous solutions, it shows a single-exponential decay with a lifetime of ∼0.23 µs. However, when adsorbed on the SnO2 surface, it significantly deviates from the exponential behavior. This nonexponential behavior can originate from several sources. These include the presence of different kinds of injection (active and inactive) sites and/or adsorption sites on SnO2 surface and the potential distribution across the nanocrystalline film. The multiexponential emission decay suggests that the charge injection in a nanocrystalline film is controlled by a distribution of charge transfer rate constants. The emission decay of Ru(II)* in Figure 7 was fitted to a biexponential kinetics. The kinetic analysis was carried out
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TABLE 1: Emission Decay Kinetics of OTE/SnO2/Ru(II) at Different Applied Bias Potentialsa,b V vs Ag/AgCl -0.6 0.0 +0.6
a1
τ1 (ns)
a2
τ2 (ns)
0.011 7.18 ( 0.22 0.004 127.0 ( 4.6 0.038 2.22 ( 0.09 0.003 52.5 ( 3.5 0.015 2.33 ( 0.02 0.006 45.9 ( 1.0
CHISQR 1.55 1.57 1.27
a The emission decay was analyzed with a biexponential kinetic fit as described in eq 7. b CE ) Pt; RE ) Ag/AgCl; electrolyte ) 0.1 M tetrabutylammonium perchlorate in acetonitrile. Excitation was at 355 nm.
using the expression
F(t) ) a1 exp(-t/τ1) + a2 exp(-t/τ2)
(7)
The values of a1, τ1, a2, τ2, and CHISQR are summarized in Table 1. The lifetimes at 0.0 V (2.22 and 52.5 ns) and +0.6 V (2.3 and 45.9 ns) are essentially the same. However, the lifetimes at -0.6 V are significantly longer (7.2 and 127 ns). This increase in lifetime at negative bias is in good agreement with the increase observed in emission yield measurements. In our earlier study, we have evaluated rate constants for the charge injection process from the luminescence decay of the sensitizer.36 For SnO2 film, the faster component had a rate constant of 3 × 108 s-1 and agreed well with the rate constant obtained from the pseudo-first-order growth of microwave conductivity. By employing a similar analysis (i.e., by assuming observed decrease in lifetime is entirely due to charge injection process), one could correlate,
ket ) 1/τs - 1/τs0
(8)
where τs and τs0 are the lifetimes of Ru(II)* on SnO2 and silica, respectively. For example, by substituting the values of τs for fast and slow components, we obtain the values for ket as 4.25 × 108 and 1.7 × 107 s-1 at +0.6 V, respectively. The two values of ket at a constant bias represent a range of rate constants with which net electron transfer from the excited Ru(II) into SnO2 particles is experimentally observed. However, ket is significantly slower under negative bias conditions. Note that this comparison of decay kinetics essentially shows a qualitative trend of increase in Ru(II)* lifetime under negative bias. The complex nature of the decay kinetics limited our efforts to obtain any further quantitative information. Efforts are underway to investigate how the potential distribution across the nanocrystalline film may influence the emission decay of Ru(II)* on SnO2 nanocrystallites. Laser Flash Photolysis of Ru(II)-Modified Nanocrystalline SnO2 Films. The emission studies presented in previous sections clearly indicate that the competition between kr and ket can be modulated with an electrochemical bias. If indeed the applied potential is responsible for controlling the charge injection process, we should be able to monitor its direct influence on the decay and formation of reactant (Ru(II)*) and product (Ru(III)) of heterogeneous electron transfer, respectively. Spectroelectrochemical experiments were carried out in a conventional laser flash photolysis setup by modifying the sample holder to accommodate the cell containing OTE/SnO2/ Ru(II) (WE), Pt (CE), and Ag/AgCl (RE) electrodes. The excitation of the Ru(II)-modified SnO2 film was carried out in a front face geometry with a 532 nm laser pulse. The cell configuration is illustrated in Figure 2. The transparency of the OTE/SnO2/Ru(II) electrode facilitated direct monitoring of the transient absorbance following laser pulse excitation. The electrode potential was maintained at a desired value during the laser flash photolysis experiment.
Figure 8. Transient spectra recorded 250 ns after laser pulse (532 nm) excitation of (a) OTE/SiO2/Ru(II) (b) and (b) OTE/SnO2/Ru(II) (O) in acetonitrile containing 0.1 M TBAP. No electrochemical bias was applied during the transient absorption measurements.
(i) Identification of Transients Produced with Visible Laser Excitation. Figure 8 shows the transient spectra recorded 250 ns after laser pulse excitation of OTE/SiO2/Ru(II) and OTE/ SnO2/Ru(II) electrode samples at no applied potentials. The energy of the laser pulse was kept sufficiently low, 5 mJ/pulse. High absorption of Ru(II) at 532 nm (∼0.3) assured nearly 50% absorption of the incident laser pulse. Excitation of Ru(II) adsorbed on silica film results in the formation of a transient with absorbance maximum at 380 nm, a bleaching corresponding to ground state depletion at 460 nm, and an isosbestic point at 397 nm. These spectral features match the reported spectral characteristics of Ru(II)*.33 The major component of this transient has a lifetime of 0.23 µs. Any contribution from the excited state annihilation or self-quenching process which is usually observed on surfaces such as silica was very small (
