Continuous Surface Electric Field Contraction Accompanying Electron

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Continuous Surface Electric Field Contraction Accompanies Electron Transfer from TiO2 to Oxidized Sensitizers Renato N. Sampaio, Guocan Li, and Gerald J. Meyer ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00380 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 29, 2016

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Continuous Surface Electric Field Contraction Accompanies Electron Transfer from TiO2 to Oxidized Sensitizers Renato N. Sampaio, Guocan Li, and Gerald J. Meyer* Department of Chemistry, The University of North Carolina at Chapel Hill, Murray Hall 2202B, Chapel Hill, NC 27599-3290, USA AUTHOR INFORMATION Corresponding Author *[email protected]

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ABSTRACT

The electric field present while electrons injected into TiO2 recombine with oxidized sensitizers has been quantified for the first time. This advance was enabled by transient study of [Ru(NH3)5(ina)]2+, where ina is isonicotinic acid, anchored to the mesoporous TiO2 thin films used in dye-sensitized solar cells. Light excitation of the characteristic metal-to-ligand chargetransfer (MLCT) resulted in a significant change in the molecular dipole, ∆µ = 9.1 D, that enabled the surface electric field to be transiently quantified after pulsed light excitation. The field present 70 ns after excited state injection was E = 0.35 MV/cm and this value decreased continuously with charge recombination. The observed behavior is most consistent with these surface anchored sensitizers experiencing a continuous contraction of the electric fields due to delocalized electrons, rather than a discrete number of sensitizers experiencing a localized field, as recombination proceeds.

TOC GRAPHICS


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The electric fields present at illuminated molecular-semiconductor interfaces are relevant to energy applications that range from photovoltaics to light emitting diodes.1-5 In the case of dyesensitized solar cells it was assumed for some time that electric fields generated at the TiO2 interface by excited state injection of electrons into the nanoparticles, were completely screened from the sensitizer molecules by the large dielectric constant of this oxide as well as the commonly employed high ionic strength polar acetonitrile electrolyte.6-11 In 2010, the assumption of quantitative charge screening was proven to be incorrect.2,3,12 Electrons injected into TiO2 electrochemically or by excited states were shown to induce a uni-directional shift of the dye absorption spectrum whose magnitude reported directly on the electric field strength.2,3,12-16 This ‘electro-absorption’ feature provides herein the first quantification of the electric field as the electrons in TiO2 recombine with oxidized sensitizers. The newly discovered ‘electro-absorption’ signature has previously been utilized to quantify the field strength during electrolyte ion migration within the mesoporous TiO2 thin films commonly used in dye-sensitized solar cells.2,3,12-17 Importantly, the electric fields have been quantified by direct electrochemical reduction of the TiO2 in a standard spectro-electrochemical cell15,16 as well as by excited state injection.2,3,13,15-17 Surface adsorption of electrolyte cations creates local fields that induce unidirectional spectral shifts in a direction opposite to that observed with electrons.2,12,15,18,19 Studies with pulsed lasers have shown that excited state injection results in instrument response limited creation of the electric field, k > 108 s-1. The time-dependency of the magnitude of the electric field during charge recombination has not been reported, presumably due to the inherent difficulties associated with quantifying small spectral shifts in transient absorption measurements that coincide with more noticeable kinetic processes, such as charge recombination to the oxidized sensitizers. Indeed, it is not clear how the electric field would


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change as charge recombination occurs. For example, if the injected electron traps in a surface state that influences the absorption spectrum of a fixed number of sensitizers, the amplitude of the electro-absorption signature may simply decrease with time as recombination occurs and their number decreases, Scheme 1 (upper panel). Alternatively, if the injected electrons are more delocalized and collectively influence the field experienced by all the sensitizers, a continuous contraction of the field would be expected as recombination occurs, Scheme 1 (lower panel). The results described herein support this latter view and show for the first time that the electric field decays continuously with the transient TiO2(e-) concentration.


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Scheme 1. Pictorial description of the surface electric fields under the conditions of a localized versus delocalized injected electron in a dye-sensitized TiO2 anatase nanocrystal. The scheme denotes a simplified picture that idealizes a single sensitized TiO2 nanoparticle that would comprise one of many that are interconnected in the mesoporous TiO2 thin film. Under experimental conditions described in the text, approximately 500 sensitizers were anchored to each nanocrystallite and about 10 injected electrons were detected on a nanosecond time scale. Here a sensitizer that directly reports on the electric field strength during charge recombination is described: [Ru(NH3)5(ina)](PF6)2, where ina is isonicotinic acid, Scheme 2. This compound has a single pyridine ligand for metal-to-ligand charge transfer (MLCT) excitation with a single carboxylic acid group for surface binding. The [Ru(NH3)5(ina)]2+ compound was anchored to the mesoporous TiO2 thin films by overnight reactions in acetonitrile and are abbreviated herein as Ru(NH3)5(ina)|TiO2. Surface coverages of ~ 4 x 10-8 mol/cm2 were measured typical of these thin films. Figure 1 shows the visible absorption spectrum of Ru(NH3)5(ina)|TiO2 in neat acetonitrile. The intense band at 480 nm (ε ~ 14,000 M-1 cm-1) is assigned to a Ru → ina MLCT absorption. There was no evidence of room temperature photoluminescence from Ru(NH3)5(ina)|TiO2. Density functional theory was used to calculate a significant dipole moment change of ∆µ = 9.1 D between excited- and ground-state, consistent with Boxer’s Stark measurements of related Ru ammine compounds.20 We note also that ruthenium ammine compounds have been widely studied for their remarkable photochromic properties.21-24 Indeed these compounds were extraordinarily sensitive to the details of the TiO2 surface functionalization conditions and shifts in the absorption maximum of about 15 nm were observed when the 0.1 M LiClO4 was present


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in the acetonitrile, behavior that is interesting in its own right but did not influence the key conclusions reported herein.

Scheme 2. The sensitizer molecule [Ru(NH3)5(ina)](PF6)2 used in this study serves as a sensitive probe of electric fields at the dye-sensitized TiO2 interface. A) Orientation of the change in dipole moment, Δ , of Ru(NH3)5(ina)|TiO2 and an electric field from a injected electron. B) The effect of electronic transitions under a small perturbation due to an external electric field.


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Figure 1. A) Ground-state UVvis absorption spectra of Ru(NH3)5(ina)|TiO2. B) Ground-state reference spectra of Ru(NH3)5(ina)|TiO2 reporting on the bleach of the oxidized molecules (solid lines) and the 1st derivative of the ground-state absorption spectra (dotted lines). The color coding indicates the composition of the electrolyte solution. Pulsed laser excitation of Ru(NH3)5(ina)|TiO2 in neat acetonitrile led to the absorption changes shown in Figure 2A. A prompt bleach of the MLCT absorption centered near 500 nm was observed consistent with kinj > 108 s-1 that returned to pre-excitation levels on a microsecond time scale. An MLCT bleach was not observed under the same conditions in the absence of TiO2, consistent with the sensitizer excited state lifetime of τ < 10 ns. Even a quick inspection of the temporal data in Figure 2 reveals a blue shift in the maximum absorption bleach with time. Transient spectra were quantitatively modelled by a sum of two components: 1) a first-derivative of the ground-state absorption due to the electric field; and 2) the absorption difference spectrum between oxidized and ground-state due to the [RuIII(NH3)5(ina)|TiO2 (e-)] charge separated state that was generated independently by spectro-electrochemical methods. The first derivative feature arises from a uni-directional shift of the ground state MLCT absorption induced by the injected electrons.2,15 Significantly, the spectral shift associated with the surface electric field are readily apparent without the need to regenerate the oxidized sensitizer. In previous studies, the electric field was most accurately quantified only after regeneration by an external donor like iodide,2,15,18 and attempts to do so before regeneration were less quantitative due to the much larger spectral changes that resulted from photo-oxidation of the sensitizers.19 Regeneration solved this issue, however at the expense of time resolution as one must wait for complete regeneration of the oxidized sensitizers. In addition, there is fundamental interest in quantifying interfacial electric fields in the absence of ions like iodide.


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Figure 2. A) Transient absorption spectra of Ru(NH3)5(ina)|TiO2 after pulsed 532 nm laser excitation at the indicated time delays in neat acetonitrile. Overlaid to the experimental data are the simulated spectra to a least square fitting function. B) The transient spectra deconvoluted into components reporting on the change in electric field and the surface coverage decay of oxidized molecules as a function of time due to charge recombination. The transient spectra measured enables one to quantify the electric field strength, as well as the concentration of injected electrons and oxidized sensitizers as the charge recombination reaction. The magnitude of the spectral shift reports directly on the electric field strength and the [RuIII(NH3)5(ina)|TiO2 (e-)] concentration was calculated through Beer’s law. Equation 1 relates

∆

∆∙

Equation 1

the measured spectral shift ∆ν (m-1) to the electric field strength, E (V/m), and the dipole moment change, ∆µ (C/m). The spectral shift was calculated from a Taylor expansion as derived previously.3 With a ∆µ = 9.1 D and assuming collinearity with E, the electric field is calculated as a function of time after excitation and compared with the kinetics for back electron transfer, Figure 2B. The field clearly contracts continuously as charge recombination occurs.


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Figure 3. A) Normalized absorption changes recorded at specific time delays after pulsed 532 nm excitation of Ru(NH3)5(ina)|TiO2 in neat acetonitrile (solid symbols; the data was offset by 0.5 for better visualization) and 0.1 M LiClO4/acetonitrile solution (open symbols). The black symbols represent the kinetics for charge recombination while the red symbols report on the decay of the surface electric fields. B) Calculated relative permittivity based on the data in Figure 3A using the equation 2. Solid symbols are values calculated for experiments in neat acetonitrile solution while open symbols are those in 0.1 M LiClO4/acetonitrile solution. The inset shows a graphical scheme of screening the surface electric field by Li+ cations. Transient absorption studies were also performed in 0.1 M LiClO4/acetonitrile solution. Similar to the results observed for neat acetonitrile, pulsed laser excitation led to absorbance changes consistent with a bleach centered around 515 nm that blue-shifted with time. The data was modeled analogously to that described for the data acquired in neat acetonitrile and Figure 3A reveals that the electric field strength does not directly correlate with the charge recombination dynamics. Rather the field reported by the surface anchored sensitizers decays on about an order of magnitude faster time scale and a much longer lived fraction of [RuIII(NH3)5(ina)|TiO2 (e-)]


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resulted. This behavior is attributed to the ability of Li+ cations to screen the electric field in a process often referred to as ‘charge screening’.14-16 Processes such as ion intercalation,25-30 adsorption29,30 and increased concentration at the double layer, may all significantly contribute to such screening of the surface electric field. Knowledge of the electric field strength and the number of oxidized molecules, that is assumed equivalent to the number of injected electrons, allows estimation of the relative permittivity, εr, at a distance 0.5 nm from the nanoparticle surface, Equation 2. The 0.5 nm distance was used / 1/(4 )

Equation 2

as an approximate distance between the sensitizer and the TiO2 surface. The continuous contraction of the electric field with charge recombination observed in neat acetonitrile provides clear evidence that εr is unchanged during recombination. However, with a Li+ containing electrolyte the relative permittivity at 0.5 nm far from the TiO2 surface increases exponentially with time. This analysis indicates that Li+ cations influence the local dielectric constants of the TiO2 interface and external electric field on a charge compensation process. In summary, the electric field was shown to contract continuously with charge recombination in neat acetonitrile. When Li+ cations were present in the electrolyte, the field decayed more rapidly than did charge recombination and a longer-lived [RuIII(NH3)5(ina)|TiO2 (e-)] charge separated state was observed, behavior attributed to cation screening of the electric fields. Taken together these observations suggest that the surface electric field controls charge recombination and may underlie the non-exponential kinetics commonly observed for this process. Furthermore, as mentioned in the opening paragraphs, the data provides insights into the nature of the injected electrons. Had the electric field reported by the sensitizers been independent of the TiO2(e-)


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concentration, a localized picture would have emerged wherein each injected electron shifts the absorption of a few sensitizers whose numbers decline with recombination. Instead, the field was found to decreased continuously with the TiO2(e-) concentration. This is most consistent with delocalized electrons that generate an average field that is experienced by all of the sensitizers on the time scales of charge recombination events. As charge recombination required hundreds of microseconds for completion, electron delocalization may occur by trapping/detrapping,7,31 random flight,32 or ambipolar diffusion33 models that have previously been invoked. The data appear to rule out a static mechanism where the injected electrons remain trapped at a single site throughout recombination. It has previously been shown that excited state injection by one sensitizer molecule can influence the absorption spectrum of a different sensitizer molecule,2 but the number of sensitizers influenced by injected electrons remains unknown. The data reported herein are completely consistent with the presence of more delocalized electrons on the microsecond time scale that collectively influence the field experienced by many sensitizers. The data also indicates that [Ru(NH3)5(ina)]2+ is a remarkably sensitive in situ probe of electric fields at dye-sensitized interfaces. Whether this behavior is specific to this particular sensitizer, that is remarkably sensitive to outer-sphere interactions,21-24 remains unknown and will be the focus of future studies. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]

Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully 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 (DESC0013461).

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(28) Olson, C. L.; Nelson, J.; Islam, M. S. Defect Chemistry, Surface Structures, and Lithium Insertion in Anatase TiO2. J. Phys. Chem. B 2006, 110, 9995-10001. (29) Lyon, L. A.; Hupp, J. T. Energetics of Semiconductor Electrode/Solution Interfaces: EQCM Evidence for Charge-Compensating Cation Adsorption and Intercalation During Accumulation Layer Formation in the Titanium Dioxide/Acetonitrile System. J. Phys. Chem. 1995, 99, 15718-15720. (30) Lemon, B. I.; Hupp, J. T. Electrochemical Quartz Crystal Microbalance Studies of Electron Addition at Nanocrystalline Tin Oxide/Water and Zinc Oxide/Water Interfaces: Evidence for Band-Edge-Determining Proton Uptake. J. Phys. Chem. B 1997, 101, 2426-2429. (31) Cao, F.; Oskam, G.; Meyer, G. J.; Searson, P. C. Electron Transport in Porous Nanocrystalline TiO2 Photoelectrochemical Cells. J. Phys. Chem. 1996, 100, 17021-17027. (32) Barzykin, A. V.; Tachiya, M. Mechanism of Charge Recombination in Dye-Sensitized Nanocrystalline Semiconductors: Random Flight Model. J. Phys. Chem. B 2002, 106, 43564363. (33) Kopidakis, N.; Schiff, E. A.; Park, N.-G.; Lagemaat, J. v. d.; Frank, A. J. Ambipolar Diffusion of Photocarriers in Electrolyte-Filled, Nanoporous TiO2. J. Phys. Chem. B 2000, 104, 3930-3936.


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