Article pubs.acs.org/JPCC
Kinetic Resolution of Charge Recombination and Electric Fields at the Sensitized TiO2 Interface Cassandra L. Ward, Ryan M. O’Donnell, Brian N. DiMarco, and Gerald John Meyer* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States ABSTRACT: The sensitizer [Ru(dtb)2(dcb)]2+, where dtb is 4,4′-di-tert-butyl-2,2′-bipyridine and dcb is 4,4′-dicarboxylic acid-2,2′-bipyridine, was anchored to mesoporous TiO2 thin films and characterized by visible spectroscopy in 0.1 M Na+, Li+, Mg2+, and Ca2+ perchlorate acetonitrile solutions on nanosecond and longer time scales. Relative to neat acetonitrile, the presence of these electrolyte cations induced a red shift in the metal-to-ligand charge transfer (MLCT) absorption of Ru(dtb)2(dcb)/TiO2. The magnitude of the shift increased with increasing valence of the metal cation. Pulsed 532 nm light excitation of Ru(dtb)2(dcb)/TiO2 resulted in the appearance of a long-lived bleach that returned to preexcitation values on an approximately millisecond time scale under all conditions studied. Global analysis, spectral modeling, and single wavelength kinetic analysis revealed that two dynamic processes were operative: (1) charge recombination, RuIII(dtb)2(dcb)/TiO2(e−) → RuII(dtb)2(dcb)/TiO2, and (2) an electric field created by the injected electron. These two distinct nonexponential processes were observed in the same spectral region and on similar time scales. The ability of global analysis, specifically the decay-associated spectra, to kinetically and spectrally resolve these two processes was assessed. Single wavelength kinetic measurements and spectral modeling provided quantitatively different rate constants, but both led to the surprising conclusion that there was no evidence for charge screening of the electric field by cations present in the electrolyte. The decay of the electric field was cation independent, behavior very different from that previously reported in the presence of redox mediators. The charge recombination kinetics revealed a small yet measurable dependence on the nature of the cation present in the electrolyte with the divalent cations inducing the fastest recombination.
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INTRODUCTION There is considerable interest in light-driven electron transfer reactions occurring at dye-sensitized TiO2 interfaces as they may be exploited for the conversion of sunlight into electrical power1 or into chemical fuels.2−4 It has been known for some time that a favorable mismatch of interfacial electron transfer rates underlies the usefulness of these materials for solar energy conversion, as outlined in Scheme 1.5,6 For gold-standard Ru(polypyridyl) sensitizers, excited-state electron injection occurs on ultrafast femto- to pico-second time scales, while recombination of the injected electron to the oxidized sensitizer is orders of magnitude slower, requiring microseconds for completion.5,7 This fortuitous mismatch of the interfacial electron transfer rates can be rationalized in at least three different ways. First, the sensitizer’s molecular orbitals differ between the two interfacial processes. Excited state injection occurs from the π* orbitals of a bipyridyl ligand, which is directly anchored to TiO2, while charge recombination occurs to the d orbitals of the oxidized Ru sensitizer that is more distant and more weakly coupled to the TiO2. Second, the thermodynamic driving force for excited state injection is activationless and occurs into a continuum of states while recombination occurs with a large free energy change in the Marcus kinetic inverted region. Finally, local electric f ields at the interface rapidly separate the injected electron from the © XXXX American Chemical Society
oxidized sensitizer. The relative importance of each of these three contributors to this remarkable interfacial rectification remains unknown. Fundamental studies designed to better understand the factors that control these interfacial electron transfer processes are important and represent ongoing research efforts in laboratories around the world.8−13 This study aims to understand how the electric fields generated by the injected electron influence charge recombination when alkali and alkaline earth cations, which can “screen” the field,14−16 are present in the electrolyte. There is compelling evidence that, besides incomplete solar light harvesting in the red and infrared regions, the main loss mechanism in dye-sensitized solar cells (DSSCs) is charge recombination to the oxidized sensitizer, which lowers efficiency near the power point and open circuit conditions when the number of injected electrons in each TiO 2 nanocrystallite is large.17 Recombination of the injected electron with the oxidized sensitizer has been quantified by transient absorption spectroscopy, which can in principle provide direct observation of the injected electron and oxidized sensitizer.18 Because there is no geminate recombination Received: September 3, 2015 Revised: October 14, 2015
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DOI: 10.1021/acs.jpcc.5b08617 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Scheme 1. After Visible Light Absorption, Excited State Electron Injection Occurs from the π* Orbitals of the dcb Ligand to the Acceptor States in TiO2 with kinj Typically >109 s−1a
a
The acceptor states are shown pictorially in purple, which is based on a density of states model. Charge recombination to the oxidized metal center is typically orders of magnitude slower, kcr < 106 s−1.
is 2,2′-bipyridine-4,4′-dicarboxylic acid. Decay-associated spectra (DAS), abstracted from global analysis of the transient absorption data, were compared with that obtained by standard addition of known absorption spectra, as well as by kinetic analysis of data measured at specific observation wavelengths.21,27,28 Such an analysis was reasonably self-consistent and indicated that global analysis enabled isolation of the two principle kinetic processes. Surprisingly, without the presence of the redox mediator there was very little experimental evidence for screening of the surface electric field by the cation electrolytes, but there was a measurable trend in charge recombination that was opposite to previous recombination trends observed when iodide was present.14−16,19
occurring at these interfaces, nanosecond and longer time scales enable one to quantify the kinetics for the entire recombination process. Prior to 2010, it was generally assumed that when excited state injection was quantitative, charge recombination was the only kinetic process that contributed to the measured absorption change on nanosecond and longer time scales.19,20 Kinetic measurements performed at an observation wavelength where the extinction coefficient change between the ground state and oxidized sensitizers was the largest afforded the most optimal signal-to-noise ratio and hence were the most trusted. However, the assumption of a single kinetic process was recently shown to be incorrect due to another dynamic process that occurs on the same time scale and in the same wavelength region as charge recombination. This dynamic process has been attributed to screening of the surface electric field by electrolyte cations.14−16,19−25 An important aspect of this study was to develop new modeling tools so that these two kinetic processes, charge recombination and charge screening, can be kinetically resolved and uniquely studied. It is now understood that excited state injection into TiO2 generates an electric field that induces a unidirectional shift of the ground-state absorption of the surface-anchored sensitizers, similar to that observed in traditional Stark spectroscopy.26 A single injected electron can influence the ground-state spectrum of many sensitizers, so the absorption change associated with the electric field can be quite large. After pulsed laser excitation, the magnitude of the field was found to decrease under conditions where the number of injected electrons (and hence the field strength) were constant.14,19 In particular, Lewis acidic alkali and alkaline earth cations were shown to decrease the magnitude of the field experienced by the sensitizer.14,15 This was attributed to “screening” of the electric field by reorganization of cations at the interface. Hence, transient absorption spectroscopy enables non-Faradaic processes at the sensitized TiO2 interface to be quantified. However, these previous studies were performed in the presence of the redox mediator, iodide, to remove absorption features of the oxidized sensitizer such that screening could be easily analyzed.14−16,19 In this article, the interfacial electric field and charge recombination were characterized as a function of the identity of the cations present in the external acetonitrile electrolyte by probing the spectral response of [Ru(dtb)2(dcb)]2+ sensitized to TiO2, where dtb is 4,4′-di-tert-butyl-2,2′-bipyridine, and dcb
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EXPERIMENTAL SECTION
Materials. The following reagents and substrates were used as received from the indicated commercial suppliers: aluminum perchlorate nonahydrate (Al(ClO4)3·9H2O, Sigma-Aldrich, 98%), magnesium perchlorate (Mg(ClO4)2, Sigma-Aldrich, 99%), calcium perchlorate tetrahydrate (Ca(ClO4)2·4H2O, Sigma-Aldrich, 99%), tetra-n-butylammonium perchlorate (TBAClO4, Aldrich, ≥ 99.0%), sodium perchlorate (NaClO4, Sigma-Aldrich 99%), and lithium perchlorate (LiClO4, SigmaAldrich 99.99%) were used to make the 0.1 M perchlorate acetonitrile (Burdick & Jackson, spectrophotometric grade) solutions (abbreviated Mg2+, Ca2+, TBA+, Na+, and Li+, respectively). The transparent TiO2 anatase nanocrystallites were prepared by acid hydrolysis of titanium(IV) isopropoxide (Sigma-Aldrich, 97%) using the sol−gel method previously reported.29 Using a conductive transparent fluorine-doped SnO2-coated glass (FTO; Hartford Glass Co., Inc., 2.3 mm thick), the sols were cast by the doctor blade technique using transparent cellophane tape as a mask and spacer. The 4 μm thick films were sintered at 450 °C for 30 min under O2 atmosphere and stored immediately afterward in an oven for later use. [Ru(dtb)2(dcb)](PF6)2 was available from previous studies.19 Preparations. Sensitization was achieved by immersing the thin films overnight in a 1:1 acetonitrile/t-butanol solution containing ∼1 mM of the Ru sensitizers. The films were sensitized to roughly maximum surface coverage, Γ ∼ 3 × 10−8 mol/cm2, which was determined by using a modified Beer− Lambert law.30 The sensitized films were then immersed in neat acetonitrile 30 min before experimentation. For the transient B
DOI: 10.1021/acs.jpcc.5b08617 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C absorption (TA) measurements, the sensitized thin films were purged with argon gas (Airgas, > 99.998%) for 30 min. Spectroscopy. The sensitized TiO2 thin films were positioned at a 45° angle in the 1 cm quartz cuvette for all measurements. Steady-state UV−visible absorption spectra were obtained on a Varian Cary 50 at room temperature. The steady-state photoluminescence (PL) spectra were obtained with a Spex Fluorolog spectrophotometer at the right angle configuration using a 532 nm solid-state laser (Coherent, Genesis MX-series) at room temperature. Nanosecond TA measurements were obtained with a 532 nm Q-switched, pulsed Nd:YAG laser (Quantel U.S.A. (BigSky) Brilliant B; ∼6 ns full width at half-maximum, 1 Hz) with a diameter of ∼1 cm and the power at 10 mW.31 A 150 W xenon arc lamp pulsed with 100 V served as the probe beam, which was aligned 90° to the laser excitation light. Single wavelength detection was achieved with a monochromator (Spex 1702/04) optically coupled to an R928 photomultiplier tube (Hamamatsu). Two glass filters
