Di- and Tri-iodide Reactivity at Illuminated Titanium Dioxide Interfaces

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Di- and Tri-iodide Reactivity at Illuminated Titanium Dioxide Interfaces John G. Rowley and Gerald J. Meyer* Departments of Chemistry and Materials Science & Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States ABSTRACT: A photoelectrochemical cell designed to characterize interfacial electron transfer at potentiostatically controlled mesoporous nanocrystalline (anatase) TiO2 thin film electrodes was employed to characterize the reactivity of TiO2 with di- and tri-iodide in acetonitrile solution. Tri- and di-iodide, I3- and I2•-, were generated by direct excitation of iodide (266 nm), band gap excitation of TiO2 (355 nm), or both. The first iodide oxidation product observed spectroscopically after pulsed laser excitation was di-iodide, I2•-. The yield of I2•- measured 0.1 μs after pulsed 355 nm laser excitation decreased with the application of a forward bias. Under all conditions studied, there was no direct evidence for a reaction between TiO2 and I2•-, even when the concentration of trapped electrons, TiO2(e-)s, was increased with a forward bias. Di-iodide was found instead to disproportionate to yield tri-iodide, I3-, and iodide with a disproportionation rate constant that was within experimental error the same as that measured in fluid acetonitrile solution, k = 3
109 M-1 s-1. Spectroscopic evidence for a reaction between TiO2(e-) and I3- was observed. The kinetics for this reaction were complex and highly dependent on the TiO2(e-) concentration, behavior qualitatively consistent with a multiple trapping model. These findings may have relevance to unwanted charge recombination processes in dye-sensitized solar cells.

’ INTRODUCTION Optimized dye-sensitized solar cells under one sun and lower irradiance convert incident photons to electrical current with near unity efficiency at short circuit conditions.1,2 The photocurrent decreases only slightly at the point of maximum power conversion. In contrast, the photovoltage remains poorly optimized. The predominant photovoltage loss mechanism is thought to arise from recombination of TiO2 electrons with acceptors present in a nitrile solvent that predominately contains mixtures of lithium iodide and tri-iodide.1,2 This recombination is so inefficient that it has a negligible effect on the photocurrent yet may significantly lower the quasi-Fermi level of the sensitized TiO2 and, hence, the photovoltage. This putative recombination pathway remains poorly understood.1-8 Voltammetry responses under forward bias as well as photovoltage measurements of dye-sensitized cells have provided some insights into how sensitizer binding,9 molecular structure,10-13 and the presence of redox-active ligands influence the thermodynamic and kinetics of charge recombination.12 However, such electrochemical data provides limited information, and key mechanistic details, such as the identity of the acceptor(s), remain elusive. In principle, transient optical measurements of sensitized electrodes under forward bias can provide such molecular level information; this approach has proven successful for characterization of interfacial charge transfer r 2011 American Chemical Society

reactions at sensitized TiO2 in inert electrolyte.4,14-17 Forward bias allows potentiostatic control of the TiO2 quasi-Fermi level and can be used to fill trap levels, abbreviated herein TiO2(e-). The recombination reaction, in inert electrolytes, between TiO2(e-)s and oxidized molecular sensitizers bound to the TiO2 surface has been investigated in the past with electrochemical techniques coupled to transient absorption measurements.18,19 Electrolytes that contain redox mediators provide new challenges for transient optical measurements with potentiostatic control because the applied bias can result in thermal electron transfer reactions that consume the redox mediator of interest. Here, we describe a photochemical approach that circumvents this problem and enables mechanistic studies of iodide reactivity at TiO2 interfaces. Previous studies have demonstrated that the TiO2(e-) lifetime decreases with the increased concentration of oxidized iodide species.20 However, to our knowledge, experiments in which the the quasi-Fermi level of TiO2 and, hence, the TiO2(e-) concentration were controlled have not been performed.21 Here, we report studies of this type with an Received: December 2, 2010 Revised: January 22, 2011 Published: March 07, 2011 6156

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Figure 1. (a) The photoelectrochemical cell utilized that consisted of a Ag+/Ag reference electrode (RE orange), a Pt counter electrode (CE pink), and mesoporous TiO2 thin films (blue) deposited on a FTO substrate. A side view is also shown that exaggerates the 10 μm separation between the quartz coverslip (yellow) and the FTO substrate (green). (b) A top view of the cell and the optical alignment of the 266 or 355 nm excitation and white light probe beams, monochromator and detector. The Surlyn film used to contain the acetonitrile electrolyte is also shown. (c) UV-vis absorption spectra of a TiO2 thin film at the indicated applied potentials (vs Fe(Cp)2þ/0) in 0.5 M LiI/CH3CN. The inset shows the absorption measured at 800 nm as a function of the applied potential.

approach that circumvents the difficulties associated with dark electrochemistry. The key idea was to use known iodide photochemistry to generate di- and tri-iodide, I2•- and I3-, within a potentiostatically controlled mesoporous TiO2 thin film in a sandwich-type photoelectrochemical cell. Forward bias reduces the TiO2 thin film and induces a color change from transparent to bluish-black due to the production of TiO2(e-)s. Since an iodide acetonitrile electrolyte is not easily reduced as the quasi-Fermi level is raised toward the vacuum level, large forward biases can be applied without electrolyte redox chemistry. In a previous Letter, this photoelectrochemical cell was utilized to study TiO2 reactivity with oxidized iodide in 0.5 M tetrabutyl ammonium iodide acetonitrile solutions, TBAI/CH3CN.22 The oxidized iodide species were generated with 266 nm light excitation of iodide to yield iodine atoms that subsequently react to form I2•- and I3-, reactions 1-3: I- þ hv266nm f I• þ e-

ð1Þ

I• þ I- f I2 •-

ð2Þ

2I2 •- f I3 - þ I-

ð3Þ -

Reactivity between TiO2(e )s and I3 was evident in the kinetic data; however, there was no evidence for a corresponding reaction with I2•-. The result was somewhat surprising on thermodynamic grounds because the reduction of I2•- was more energetically favored than was I3-. It was postulated that the observed reactivity had a kinetic origin; disproportionation of I2•- was more rapid than interfacial electron transfer from TiO2. Herein, we report analogous studies with 355 nm light excitation of TiO2 in 0.5 M LiI. Small cations are known to intercalate into or adsorb (or both) onto anatase TiO2.23,24 It was postulated that the presence of Liþ stabilizes the TiO2(e-)s such that it is possible to electrochemically generate higher TiO2(e-) concentrations than that obtained with TBAþ. The lower-energy 355 nm photons used in this investigation were insufficient to directly excite iodide and, thus, precluded potential complications from solvated electrons and assured that all redox chemistry was initiated at the TiO2 interface.

’ EXPERIMENTAL SECTION Materials. All chemicals were reagent grade and were used without further purification. The following reagents were used as received: acetonitrile (Burdick & Jackson, spectrophotometric grade); fluorine-doped SnO2-coated glass (FTO: Hartford Glass Co., Inc., 2.3 mm thick, 15 Ω/square); lithium iodide (LiI; Aldrich, 99.999%); microscope slides (Fisher Scientific, 1 mm thick); quartz microscope slides (Ted Pella, Inc., 1 mm thick); ntetrabutylammonium iodide (TBAI; Aldrich, >99%); titanium(IV) isopropoxide (Sigma-Aldrich, 97%); argon gas (Airgas, >99.998%); vinyl film (Warps, 8 mil Vinyl-Pane). Photoelectrochemical Measurements. Shown in Figure 1 is a simplified scheme that displays the key features of the photoelectrochemical cell. Mesoporous nanocrystalline (anatase) TiO2 doctor-bladed and sintered onto a fluorine doped tin oxide (FTO) conductive electrode in two parallel strips separated by ∼1 cm served as the working electrode. A Vycor-capped nonaqueous silver/silver nitrate reference and Pt mesh counter electrode were utilized. A quartz microscope slide secured over the FTO/TiO2 thin film fixed the path length to be ∼10 μm. The uppermost TiO2 strip serves as a spacer to ensure that the quartz microscope slide remained parallel to the FTO electrode. This sandwich cell arrangement thus contained two distinct experimental zones: one consisted only of 0.5 M lithium iodide (LiI) electrolyte, and the second consisted of the mesoporous TiO2 film and the same electrolyte. The optical path length of the cell was measured spectroscopically with a potassium permanganate solution assuming that Beer’s Law was applicable and a TiO2 porosity of 0.5. The steady state concentration of TiO2(e-)s was measured spectroscopically. The transmitted light was monitored before, during, and after pulsed laser excitation. Transient Absorption Spectroscopy. Nanosecond transient absorption measurements were obtained with an apparatus similar to that which has been previously described. Briefly, samples were excited by a pulsed Nd:YAG laser (Quantel USA (BigSky) Brilliant B; 5-6 ns full width at half-maximum, 1 Hz, ∼10 mm in diameter) tuned to 355 or 266 nm with the appropriate nonlinear optics. The excitation fluence was measured by a 6157

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Figure 2. Absorption difference spectra measured 0.1 (b) and 50 μs (O) after pulsed (a) 355 nm light excitation of a TiO2 thin film immersed in 0.5 M LiI/CH3CN and (b) 266 nm light excitation of the 0.5 M LiI/CH3CN electrolyte in which the TiO2 film was immersed. The TiO2 thin film was held at an applied potential of -930 mV (vs Fe(Cp)2þ/0). Overlaid in blue lines are simulations based upon the known extinction coefficients of I2•-, I3-, and TiO2(e-). Insets show the time-dependent concentrations of the indicated species abstracted from simulations of the observed spectroscopic data.

thermopile power meter (Molectron) and was typically 3-5 mJ/ cm2. A 150 W xenon arc lamp (OSRAM; Applied Photophysics) served as the probe beam and was aligned orthogonal to the laser excitation light. For detection at sub-100 μs time scales, the lamp was pulsed. Detection was achieved with a monochromator (Spex 1702/04 spectrometer) optically coupled to an R928 photomultiplier tube (Hamamatsu). The overall instrument response time was ∼10 ns. In some cases, the signal to noise ratio was improved with a moving average filter.

’ RESULTS AND DISCUSSION Figure 1 shows the photoelectrochemical cell that was employed and spectroelectrochemical data of a TiO2 thin film as the applied potential was raised from -700 to -1000 mV vs Fe(Cp)2þ/0 in a 0.5 M LiI/CH3CN electrolyte. The increase in absorption throughout the visible region and the bleach in the ultraviolet region have been previously reported and are consistent with the presence of trapped electrons, abbreviated TiO2(e-)s.25-27 The concentration of the trapped electrons was found to increase exponentially with the forward bias (Figure 1c inset).25 The anatase TiO2 band gap is 3.2 eV.28 Therefore, pulsed 355 nm excitation generates electron-hole pairs within the solid (reaction 4). TiO2 þ hv f TiO2 ðe- , hþ Þ

ð4Þ

It is well accepted that some fraction of these photogenerated holes are trapped at sites where they can undergo interfacial electron transfer reactions with the surrounding electrolyte.29,30 Of particular interest here was the reaction of trapped holes with iodide, a reaction that is known to generate di-iodide (reaction 5). TiO2 ðhþ Þ þ 2I- f TiO2 þ I2 •-

ð5Þ

The mechanism for I-I bond formation remains unknown, but we note that evidence for iodine atom and ion-pair intermediates have been garnered from studies in fluid solution.31,32 Consistent with literature reports, band gap excitation of TiO2 did, indeed, yield I2•-.33 Representative absorption difference spectra

measured after pulsed 355 nm light excitation of a mesoporous TiO2 thin film at an applied potential of -930 mV (vs Fe(Cp)2þ/ 0 ) in 0.5 M LiI/CH3CN are shown in Figure 2. Ten nanoseconds after 355 nm excitation, the spectral data were well modeled by standard addition of the known absorption spectra of I2•- and TiO2(e-) in a stoichiometry near 1:1. The I2•- concentration decreased over time with the concomitant appearance of the characteristic absorption of I3-. Fifty microseconds after the laser pulse, the principal iodide species observed spectroscopically was I3-. The transient concentration of the photochemical products abstracted from the spectral data are shown as insets to Figure 2 and reveals that as the I2•- concentration decays to zero, the I3concentration increases to half the initial concentration of I2•within experimental uncertainty, data that is consistent with the disproportionation reaction 3. We note that pulsed 355 nm light excitation of zone 1 of the photoelectrochemical cell that did not contain TiO2, resulted in no significant absorption transients. Pulsed 266 nm excitation of either zone in the photoelectrochemical cell (i.e. with or without TiO2) resulted in the prompt appearance of an absorption transient attributed exclusively to I2•-. Under these experimental conditions, >70% of the light was absorbed by the iodide-to-CH3CN charge transfer (eq 1). The solvated electron product was not observed spectroscopically, presumably because of its weak absorption in this spectral region.22 The transmitted light was expected to directly excite TiO2, but our inability to observe the characteristic absorption of the TiO2(e-) indicates that the yield of products on the 10 ns time scale was low. In any case, I2•- was clearly identified, and its concentration decreased by disproportionation to zero as the I3- increased to half the initial I2•- value. Figure 3 shows representative absorption changes monitored at 405 nm, where I2•- and I3- absorb, and at 710 nm, where I2•and TiO2(e-) absorb light. Two distinct kinetic processes were observed at each wavelength. With observation at 405 nm, the absorption contribution from two equivalents of I2•- was much larger than that from one equivalent of I3-, and the data was reasonably assigned to I2•- disproportionation that was well described by a second-order equal concentration kinetic model 6158

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Figure 3. Transient absorption changes monitored at 405 (black) and 710 nm (blue) after pulsed (a) 355 or (b) 266 nm light excitation of mesoporous TiO2 in 0.5 M LiI/CH3CN at an applied potential of -900 mV (vs Fe(Cp)þ/0). Overlaid on the data for the first 100 μs are fits to a second-order equal concentration kinetic model with a rate constant of 3
109 M-1 s-1. On time scales longer than 100 μs, the data was overlaid by eq 6 with β = 0.7 for (a) τo = 0.54 s and for (b) τo = 0.49 s.

to a nonzero baseline. Disproportionation was also evident at 710 nm with rate constants that agreed well with those measured at 405 nm. With 266 nm light excitation, the absorption change cleanly returned to pre-excitation levels on a submilliseconds time scale, whereas a nonzero final absorption was included for 355 nm excitation due to TiO2(e-) absorption. Although some uncertainty in the spectroscopic path length exists, a rate constant of 3
109 M-1 s-1 was used to model the data, which is in reasonable agreement with the accepted value, 3.3
109 M-1 s-1.31,32,34 On >10 μs time scales, the absorption changes were attributed to the reduction of I3- by TiO2(e-). The transient data was nonexponential, yet for comparison purposes, adequately described by the Kohlrausch-William-Watts (KWW) kinetic model, eq 6. h i IðtÞ ¼ I0 exp - ðt=τ0 Þβ

ð6Þ

Here, β is inversely related to the width of the underlying Levy distribution of rate constants, 0 < β < 1, and τ0 is a characteristic lifetime. The value of β was fixed at 0.7 to model the data in Figure 3a and b using eq 6. For 355 nm excitation, Figure 3a, the best fits of the data to this model were achieved using τ0 = 0.54 s, and for 266 nm excitation, Figure 3b, the best fits were achieved using τ0 = 0.49 s. For 355 nm excitation, absorption changes monitored at 405 and 710 nm occurring on the millisecond and longer time scale were within experimental error the same, although the uncertainty was larger at 710 nm than that measured at 405 nm due to the lower signal-to-noise ratio. No spectral features were observed that might be attributed to the I3reduction products. The yield of oxidized iodide photoproducts measured after 355 nm excitation was highly sensitive to the presence of TiO2(e-) (Figure 4a). The yield decreased by about a factor of 20 as the applied potential was raised from -710 to -960 mV. No corresponding decrease in yield was measured with 266 nm excitation. The ground state absorption at the excitation wavelength was sensitive to the applied potential and may contribute to the change in yield of I2•- (Figure 1c).

In addition to ground state absorption changes, the yield of oxidized iodide photo-products may have been altered by fast reactions that could not be time-resolved on our system, that is, occurring faster than 10 ns. Either the intrinsic quantum yield for I2•- formation decreases with forward bias, perhaps due to less efficient hole trapping, or rapid subnanosecond reduction of I2•- occurs. Less efficient hole trapping would be expected if the increased TiO2(e-) concentration led to short hole lifetimes.35 It is interesting to note that the yield of excited state injection from molecular dyes is also known to decrease with forward bias or removal of small cations from the surrounding electrolyte.15,18,36,37 The absence of this effect with 266 nm light excitation further indicates that iodide oxidation with these higher energy photons resulted predominately from light absorption by iodide. Forward bias was found to have a significant effect not only on the I2•- yield but also on the kinetics of I3- consumption. Representative data are shown for normalized and non-normalized data in Figure 4. As the quasi-Fermi energy of the TiO2 was raised, the effective lifetime of I3- decreased significantly. The transient data could not be fit to a first- or second-order kinetic model or to the KWW model. The data qualitatively appears biphasic, particularly at the more negative applied potentials. The data were fit to a sum of a first-order rate constant and the KWW function, eq 7, although no significance was placed on the highly correlated rate constants that were abstracted. The fits are overlaid on the data in Figure 4a. h i IðtÞ ¼ I1st exp½ - ðt=τ1st Þ þ IKWW exp - ðt=τKWW Þβ ð7Þ Qualitatively, the time-resolved absorption data recorded as a function of the TiO2(e-) concentration or quasi-Fermi level appear to follow Tachiya’s multiple trapping model in which two kinetic processes are also expected: one strongly dependent on the concentration of TiO2(e-)s and one that is not.38 The data in Figure 4a shows that the kinetics are nearly first-order when the TiO2(e-) concentration is low, yet become highly nonexponential as the steady state TiO2(e-) concentration increases. 6159

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Figure 4. (a) The absorption change monitored at 375 nm after pulsed 355 nm excitation of a TiO2 thin film in 0.5 M LiI/CH3CN with an applied bias of -960 (magenta), -910 (light blue), -860 (dark blue), -810 (green), -760 (red), and -710 mV (black) vs Fe(Cp)2þ/0. Fits shown in gray are the linear sum of an exponential and stretched exponential function. (b) Transient absorbance data from panel a normalized at 10 μs. The incident irradiance was 3 mJ/pulse.

Figure 5. The absorption change monitored after pulsed 355 nm excitation of a TiO2 thin film in 0.5 M LiI/CH3CN. (a) Absorption changes at 710 nm with an applied bias of -800 (black) and -700 mV (red) vs Fe(Cp)2þ/0. Inset shows an expansion of the initial 2.5 μs of absorption change. Overlaid on this data is a fit to a second-order equal concentration kinetic model with a rate constant of 3
109 M-1 s-1. (b) Absorption changes at 375 nm with an applied bias of -800 (black) and -700 mV (red) vs Fe(Cp)2þ/0. Inset shows an expansion of the initial 2.5 μs of absorption change.

Regardless of the parameters used to model this data, the absorption changes were highly dependent on the quasi-Fermi level of the TiO2. We note also that the concentration of TiO2(e-)s at the most negative applied potential is over 100 times that expected for an operational solar cell.22 Taken together, the data shows that band gap excitation of TiO2 generates I2•- that quantitatively disproportionates to iodide and I3- with no evidence for a reaction between TiO2(e-)s and I2•- . To demonstrate this more clearly, a comparative study is shown in Figure 5, where absorption changes were monitored at 710 and 375 nm at two different applied potentials. The laser fluence was tuned to give the same yield of TiO2(e-)s and I2•- at the two applied potentials. The 710 nm data in Figure 5a shows the expected disproportionation of I2•- on short

time scales to a nonzero baseline due to TiO2(e-)s. With a 375 nm observation wavelength in Figure 5b, no change in absorption accompanies I2•- disproportionation; that is, it is an isosbestic point for I2•- disproportionation. Therefore, the recombination of TiO2 with I3- can be cleanly observed and showed the expected increase in rate with TiO2(e-) concentration. Furthermore, the fact that the I3- concentration measured 10 μs after excitation was the same at the two applied biases indicates that within experimental error, there was no reaction of the TiO2(e-)s with I2•-. In summary, the data in Figure 5a and b show clearly that two distinct kinetic processes occur consecutively over this time range: (1) I2•- disproportionation to yield I3- and (2) I3- reduction by TiO2(e-). The latter process is clearly dependent on the applied potential and, hence, TiO2(e-) 6160

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The Journal of Physical Chemistry C concentration. Although only two representative potentials and absorption wavelengths are shown, a large number were investigated, and the results described here were found to be general.

’ CONCLUSIONS Band gap excitation of mesoporous TiO2 thin films immersed in 0.5 M LiI/CH3CN electrolyte resulted in the appearance of diiodide, I2•-; however, the mechanism for I-I bond formation remains speculative. The I2•- yield measured 10 ns after laser excitation decreased dramatically with increased forward bias and, hence, the concentration of TiO2(e-)s. The origin of the decreased yield was not due to trivial light absorption changes but instead to an unknown rapid recombination process(es). There was no kinetic evidence for a reaction between TiO2(e-) and I2•on nanosecond or longer time scales. Instead, the I2•- concentration decreased with the second-order equal concentration kinetics expected for a disproportionation reaction. The measured rate constant was independent of whether 355 or 266 nm light was employed and was in reasonable agreement with previous studies in fluid acetonitrile solution. This indicates that the TiO2 surface and mesoporous structure had no significant influence on the diffusion and reactivity of di-iodide. In contrast, on time scales >10 μs, the concentration of tri-iodide and TiO2(e-) decreased. The similarity of the transient absorbance changes measured at wavelengths where I3- and TiO2(e-) absorb light implies a reaction between the two. No reaction products for I3- reduction were observed, but recent flash-quench experiments demonstrated that the one-electron reduction in acetonitrile yields I2•-.39 Our inability to observe this here presumably stems from the rapid disproportionation reaction, 3.3
109 M-1 s-1, that does not allow a significant concentration to accumulate. The absence of reactivity with I2•- was unexpected because I2•- reduction is energetically more favored by ∼600 mV than is I3- reduction.34 The absence of reactivity suggests that interfacial electron transfer does not complete kinetically with rapid I2•- disproportionation. It should be emphasized, however, that these measurements were performed with band gap or charge-transfer-to-solvent-band excitation at high TiO2(e-) concentrations in the absence of molecular sensitizers, and the extrapolation of these results to dye-sensitized solar cells should therefore be done with caution. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We acknowledge support by a grant from the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy (DE-FG0296ER14662). ’ REFERENCES (1) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269. (2) Ardo, S.; Meyer, G. J. Chem. Soc. Rev. 2009, 38, 115. (3) Green, A. N. M.; Chandler, R. E.; Haque, S. A.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B 2004, 109, 142. (4) Montanari, I.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B 2002, 106, 12203. (5) Fitzmaurice, D. J.; Eschle, M.; Frei, H.; Moser, J. J. Phys. Chem. 1993, 97, 3806.

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