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Electrochimica Acta 55 (2009) 458–464

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

hotoelectrochemical kinetics of Eosin Y-sensitized zinc oxide films investigatedy scanning electrochemical microscopy under illumination with different LED

an Shena,1, Ushula Mengesha Tefashea, Kazuteru Nonomurab,2, Thomas Loewensteinb,erck Schlettweinb, Gunther Wittstocka,∗

Department of Pure and Applied Chemistry, Faculty of Mathematics and Natural Sciences, Carl von Ossietzky University of Oldenburg, D-26111 Oldenburg, GermanyInstitute of Applied Physics, Justus Liebig University of Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany

r t i c l e i n f o

rticle history:eceived 20 March 2009eceived in revised form 14 August 2009ccepted 21 August 2009vailable online 4 September 2009

a b s t r a c t

The overall efficiency of the light-induced charge separation in dye-sensitized solar cells depends onthe kinetic competition between back electron transfer and dye regeneration processes by a redox elec-trolyte. In a previous study, the reduction of the intermittently formed photo-oxidized dye molecules byiodide ions in the electrolyte phase was investigated using the feedback mode of a scanning electrochem-ical microscope (SECM) and a quantitative model had been derived. Here we provide a more thorough

eywords:ye-sensitized solar cellslectron transferineticsanoporous zinc oxide

experimental verification of this model by variation of the excitation wavelength, light intensities andmediator concentrations. Nanoporous ZnO/Eosin Y films prepared by self-assembly were used as modelelectrodes and were used with an iodide/triiodide electrolyte. The experimentally found effective rateconstants could be related to the rate constant for the reaction of the dissolved donor with photo-oxidizedEosin Y bound to ZnO and the absorption spectrum of the dye and confirmed the assumption made in

el. Folight

riiodide electrolytecanning electrochemical microscopy

the derivation of the modlight emitting diodes and

. Introduction

The requirement to develop inexpensive renewable energyources has stimulated new approaches for the preparation offficient, low-cost photovoltaic cells. Among the approaches,ye-sensitized photoelectrochemical solar cells (DSSC) based onanocrystalline metal oxide electrodes are a promising class oflternative photovoltaic systems [1]. The function of DSSC is basedpon electron injection from a photoexcited state of the dye intohe conduction band of a wide band gap semiconductor. The result-ng dye cation is then re-reduced by redox species in the electrolyte2]. In such devices, the kinetics of electron transfer pathways at the

etal oxide sensitizer dye electrolyte interface are critical to device

unction. The pathway includes: the electron injection process kinj,nd the dye ground state regeneration by the redox electrolyte kox

Fig. 1).

∗ Corresponding author. Fax: +49 441 798 3979.E-mail address: gunther.wittstock@uni-oldenburg.de (G. Wittstock).

1 Present address: Science Research Center, The Academy of Fundamental andnterdisciplinary Science, Harbin Institute of Technology, Heilongjiang, Harbin,50001, China.

2 Present address: School of Chemical Science and Engineering, KTH Royal Insti-ute of Technology, Teknikringen 36, 10044 Stockholm, Sweden.

013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.08.062

r the regeneration process of Eosin Y, a rate constant of kox with differentintensities is determined.

© 2009 Elsevier Ltd. All rights reserved.

In dye-sensitized photoelectrochemical cells, the I−/I3− redoxcouple in organic solvents is typically employed to reduce thedye cation anchored on the surface of the semiconductor andshuttling electrons from the counter electrode (cathode) to thedye-sensitized semiconductor electrode (photoanode). Develop-ing alternatives to the iodide/triiodide couple, including solid stateelectrolytes [3], is an active area of research although device effi-ciencies remain relatively low to date. The success of this redoxcouple has generally been attributed to its slow exchange currentdensity on the semiconductor nanoparticles surfaces, minimiz-ing unwanted interfacial charge recombination losses (the darkcurrent). However, the parameters affecting the dye re-reductionkinetics by I− have received only limited attention to date, hinder-ing the modeling and optimization of devices employing this couple[4]. After electron injection from the photoexcited dye into thesemiconductor conduction band, regenerating the dye moleculesby electron transfer (ET) from dissolved electron donors is crucialto the operation of a photoelectrochemical cell, because it is a nec-essary elementary step to produce a photocurrent [5]. Fast kineticsof this regeneration step can suppress competing decompositionreactions and the back transfer of electrons from the conduction

band of the semiconductor, a major recombination route and henceloss mechanism in DSSC [6–10].

Studies of iodide re-reduction of the dye cation are compli-cated by the fact that the I−/I3− system is a two electron redoxcouple, requiring the reaction to proceed through one or more

Y. Shen et al. / Electrochimica A

FY

iipittwbcnti[fsicwpicsmeacmLt[tcst

2

2

vr

film was attached to the cell bottom and sealed by an O-ring. Anextra Pt wire was connected to the ZnO/Eosin Y sample with theelectrolyte in order to operate the photoelectrochemical cell in ashort-circuit setup (Fig. 2) [33]. All potentials are given with respect

ig. 1. Schematic of the processes considered in the SECM investigation of Eosin-sensitized ZnO. The figure is not to scale.

ntermediate states. Previous studies have largely addressed theodide oxidation kinetics following UV excitation of TiO2 colloidalarticles. Results obtained with Ru(dcbpy)3− sensitized colloids

n aqueous iodide have been interpreted in terms of the forma-ion of an iodide–ruthenium(III) complex intermediate, enhancinghe efficiency of this reaction [11]. In contrast, Kamat and co-orkers have indicated that the kinetics of dye cation re-reduction

y iodide are approximately first-order for a small range of iodideoncentrations with no evidence for complex formation [12]. Scan-ing electrochemical microscopy (SECM) has been demonstratedo be an effective technique of determining ET kinetics at variousnterfaces including polymer/liquid [13], liquid/liquid interfaces14–17], redox enzymes on solid supports [18,19]. Electron trans-er reactions at semiconductor/electrolyte interfaces have beentudied under various perspectives [20–29]. Various authors haventroduced the idea of using SECM to study electron transfer pro-esses at dye-sensitized semiconductor surfaces [30–32]. Recently,e have first studied the charge transfer kinetics between I− andhoto-oxidized dye molecules (Eosin Y+) adsorbed on ZnO dur-

ng illumination with a blue LED [33]. A pseudo-first-order rateonstant for the reaction of I− in a model electrolyte with the sen-itized nanoporous film was obtained from steady-state feedbackeasurements by a procedure inspired by Ref. [34], in which the

lectron transfer kinetics was studied at ferrocene-terminated self-ssembled monolayers. In the initial report, the pseudo-first-orderharacter of response was tested under the condition of differentediator concentrations under constant illumination with a blue

ED. In the present report we provide a more thorough experimen-al verification for the validity of the expression obtained in Ref.33] by testing it under different excitation wavelength and excita-ion intensities. The results of the measurements under a variety ofonditions could be correlated with the absorption spectrum of theensitizer on the electrode to yield a valid effective reaction rate ofhe sensitizer regeneration.

. Experimental

.1. Chemicals

Acetonitrile (Spectrochem, HPLC grade), used as the sol-ent for the electrolyte solution, was purified as described ineference [35]. A 0.1 M anhydrous tetrabutylammonium trifluoro-

cta 55 (2009) 458–464 459

methanesulfonate (TBAS; electrochemical grade, Fluka, Buchs,Germany) was used as supporting electrolyte without furtherpurification. I2 was obtained from Merck and was purified bysublimation before use. KI was pretreated by heating at 150 ◦Cfor 3 h and then dried in vacuum before use. Solutions of KI3 inacetonitrile-TBAS were prepared by mixing equimolar amountsof 4 mM solutions of KI and I2 in acetonitrile-TBAS and dilut-ing the resulting stock solution to the required concentration byacetonitrile-TBAS electrolyte solution.

2.2. Preparation of the sample ZnO/Eosin Y film

The preparation of the ZnO/Eosin Y hybrid thin films used assamples was carried out in a three electrode single compartmentcell with a saturated calomel electrode (SCE) as reference electrode,a Zn wire as counter electrode and F-doped SnO2 on glass (ASAHIGLASS) as working electrode [36,37]. The F-doped SnO2 glass sub-strate was mounted as a rotating electrode in a stainless steelholder providing mechanical and electrical attachment to a rotat-ing disk electrode (RDE) system and was operated at 500 rpm. TheZnO/EosinY films were deposited at −1.0 V (vs. SCE) for 20 min at70 ◦C from an oxygen-saturated aqueous solution containing 5 mMZnCl2 (Fluka), 0.1 M KCl (Roth, Karlsruhe, Germany) and 50 �MEosin Y (Aldrich, Schnelldorf, Germany). The solution was purgedwith O2 at a volume flow rate of 200 mL/min. Following deposi-tion, the ZnO/Eosin Y film was exposed to a diluted aqueous KOHsolution (pH 10.5) for 24 h to desorb the loaded Eosin Y molecules.The films were dried in air for 1 h at 150 ◦C. Eosin Y was re-adsorbedfrom 250 �M Eosin Y aqueous solutions at 80 ◦C to provide efficientsamples of Eosin Y-sensitized ZnO [36].

2.3. Instruments and procedures

SECM experiments were performed on a home-built instru-ment [38]. The Teflon cell contained a Pt wire counter electrode,a Pt wire quasi-reference electrode, and the ZnO/Eosin Y sample

Fig. 2. Schematic of SECM setup to investigate redox processes at a DSSC. (1) UME,(2) F-doped glass with coating of Eosin Y-sensitized ZnO, (3) illumination path, (4)short contact of the DSSC, (5) potentiostat with UME as working electrode and ref-erence and counter electrode. In case of intensity dependent measurement (dashedlines) an regulated LED light source (6) was powered with an potentiostat (7) andchecked by a light sensor (8).

460 Y. Shen et al. / Electrochimica A

Table 1Summary of measured wavelength, incident light power on the illuminated areaof 0.0565 cm2 and photon flux density Jh� of LEDs used as light source in theexperiment.

LED Wavelength (nm) Power (�W) Photo flux (10−9 mol s−1 cm−2)

tAdpPiUMPgposdmatZMFtwop(dpa

3

3fi

butficcItLsmpiaawsemt

I

Blue 474 58.0 4.06Green 529 33.4 2.61Yellow 593 21.1 1.85

o the quasi-reference electrode used in the particular experiment.s solution compositions vary this reference value may be slightlyifferent depending on solution composition. The positioning waserformed with an x–y–z stepper motor system from Scientificrecision Instruments (Oppenheim, Germany). For all SECM exper-ments, a monopotentiostat �-P3 (M. Schramm, Heinrich Heineniversity, Düsseldorf, Germany) was used. The in-house softwareIRA was used to process and analyze data [39]. A 25-�m-diameter

t wire (Goodfellow, Cambridge, UK) was sealed into a 5 cm Pyrexlass capillary under vacuum. The ultra microelectrode (UME) wasolished by a wheel with 180 grid Carbimet paper disks and microp-lishing cloth with 1.0, 0.3 and 0.05 �m alumina. Then the UME washarpened conically to a RG of 10, where RG is the ratio between theiameters of the glass sheath and the Pt disk. Before each experi-ent, the UME was polished with 0.3 and 0.05 �m alumina powder

nd rinsed with water. All experiments were carried out at roomemperature. The irradiation was focused onto the backside of thenO/Eosin Y film from different light emitting diode (LED, 2000CD Blue, Green and Yellow. Reichelt Elektronik, Sande, Germany).

or the variation of the light intensity, a LDA emitter (Zahner elek-rick, Kronach, Germany) was used. The emitted light intensityas kept constant by a feedback control established via a photodi-

de and an XPOT potentiostat (Zahner). The light intensity at theosition of the sample was measured using a laser power meterFieldMaster, Coherent Inc., Santa Clara, USA). The excitation lightoes not contain contributions below 420 nm that can lead to directhotoexcitation of ZnO. The emission spectra of the light sourcesre given in the electronic Supporting information SI 2.

. Results and discussion

.1. SECM feedback mode investigation of Eosin Y-sensitized ZnOlm illuminated with different LED

In order to investigate the kinetics of dye molecule regenerationy electron transfer from an electron donor in the electrolyte wesed SECM. In our previous report [33], we have experimentallyested this concept by investigating electrochemically depositedlms of Eosin Y-sensitized nanoporous ZnO by SECM approachurves in the feedback mode under illumination with a blue LED ofonstant light intensity and with different mediator concentrations.n this paper, we provide a more rigorous experimental testing ofhe proposed approximate models by using blue, green and yellowEDs as light sources and systematically varying the light inten-ity. Table 1 lists the measured wavelength of the three LEDs ataximum emission intensity and summarizes the incident light

ower and photon flux. Despite the complex electrochemistry ofodide electrolytes, I−/I3− has ideal kinetic properties as a mediatornd therefore it is often used in DSSCs. Especially, the regener-tion of the dye by I− is very fast and the recombination of I3−

ith photoinjected electrons in semiconductor oxide is extremelylow [4,7,9,40]. It is also suitable to obtain high energy conversion

fficiencies in nanoporous ZnO based substrates [41]. In this experi-ent, equimolar amounts of I2 and KI solutions were mixed in order

o form I3− according to Eq. (1)

− + I2 → I3− (1)

cta 55 (2009) 458–464

The cyclic voltammetry of a Pt UME in I3− mediator in acetoni-trile yields steady-state anodic and cathodic waves (Supportinginformation SI 1). Since the equilibrium constant related to thereaction (1) is very large (107 L mol−1) in acetonitrile solution [42],it is reasonable to assume a complete conversion from I2 and I−

to I3− and the electrolyte contains almost exclusively I3− as redoxactive compound which is the pre-requisite for quantitative SECMfeedback experiments [17]. Diffusion-limited electrolysis of the I3−

mediator at the UME in the bulk solution according to reaction (2)

I3− + 2e− → 3I− (2)

results in a steady-state limiting current iT,∞ at negative potentialswhich is given by

iT,∞ = 4nFD[I−3 ]∗rT (3)

where n = 2 is the number of electrons transferred, F is Faraday’sconstant, rT is the UME radius, D and [I3−]* are the diffusion coef-ficient and bulk concentration of the I3−, respectively. For SECMfeedback mode, the potential ET applied onto the UME was set at avalue in the diffusion-limited region (in this experiment ET = −0.7 Vvs. Pt QRE), in which a stable current response on the UME wasfound in the potential range between −0.8 and −0.6 V (Supportinginformation SI 1). Fig. 2 illustrates the principle of the SECM feed-back mode in a DSSC system under short-circuit conditions: infeedback mode, I− is generated by an electrochemical reaction (2)at the UME and diffuses to the dye-sensitized electrode. If theZnO/Eosin Y hybrid thin film is illuminated, dye molecules areexcited, and inject electrons into the conduction band. This processleads to photo-oxidized dye molecules D+ which can be re-reducedby I− produced at the UME. The redox reactions at the substrate canbe represented as:

D+ + 1.5I− → D + 0.5I3− (ZnO/EosinYfilm) (4)

Although the reaction Eq. (4) represents the stoichiometry of thedye regeneration reaction, the exact mechanism of the dye regener-ation is not exactly known and has indeed been the topic of severaldiscussions [43–45]. The absence of any significant photoelectro-chemical effect at the Pt UME was verified by SECM approach curveson a glass substrate in the dark and under illumination in the media-tor solution. The approach curves agreed within experimental errorwith the theoretical approach curves proposed by Amphlett andDenuault [46] to an insulating and inert substrate [Eq. (5)] expectedfor an UME geometry of RG = 10.2.

IinsT (L) = iT

iT,∞

= 10.40472 + (1.60185/L) + 0.58819 exp(−2.37294/L)

(5)

where iT the measured UME current at a certain distance d, IinsT is

the normalized UME current, over an inert and insulating sample, Lis the normalize distance L = d/rT. From this fitting, rT and the pointof closest approach d0 have been determined, because the inde-pendent knowledge of these parameters increases the accuracy ofcurve fits for finite kinetics described below [47]. Subsequently,the corresponding experiments were carried out at the ZnO/EosinY films. In the dark, iT is equal to that of glass (curve 1 in Figs. 3–5)for any [I3−]*. This indicates that in the absence of illumination, theflux of I− generated at the UME is not oxidized at ZnO/Eosin Y filmsubstrate and as a result the Eosin Y-sensitized ZnO film behaves as

an inert and insulating substrate in the dark. This is expected fromthe semiconducting characteristics of ZnO but is remarkable sinceit shows efficient blocking of the conductive FTO back electrode bythe porous ZnO. This fact is important in DSSC to hinder the backtransfer of electrons (shunt in the cell).

Y. Shen et al. / Electrochimica A

Fig. 3. Normalized SECM feedback approach curves for the approach of a Pt diskelectrode towards a ZnO/Eosin Y film under illumination by a blue LED [I3

−]* inmM: (2) 2.00, (3) 1.238, (4) 0.512, (5) 0.248, (6) 0.13, (7) 0.063; scan rate = 1 �m s−1,ET = −0.7 V, rT = 16.5 �m. Solid lines are calculated curves for an approach of an UMEwith RG = 10 towards an inert insulating surface (curve 1), and to samples with first-order kinetics of mediator recycling using normalized rate constants �: (2) 0.03, (3)0.085, (4) 0.167, (5) 0.196, (6) 0.265, (7) 0.45.

Fig. 4. Normalized SECM feedback approach curves for the approach of a Pt UMEtowards a ZnO/Eosin Y film under illumination by a green LED [I3

−]* in mM: (2) 2.00,(3) 1.238, (4) 0.626, (5) 0.248, (6) 0.13, (7) 0.063; scan rate = 1 �m s−1, ET = −0.7 V,rto(

tTot

Femlir

T = 16.5 �m. Solid lines are calculated curves for an approach of an UME with RG = 10owards an inert insulating surface (curve 1), and to samples with first-order kineticsf mediator recycling using normalized rate constants �: (2) 0.04, (3) 0.055, (4) 0.081,5) 0.14, (6) 0.25, (7) 0.43.

When ZnO/Eosin Y film was illuminated from the back (Fig. 2),he tip current became significantly larger than that in the dark.he increase of current on the UME is attributed to a higher fluxf I3− [Eq. (4)]. Figs. 3–5 present a set of normalized experimen-al approach curves to an Eosin Y-sensitized ZnO film for various

ig. 5. Normalized SECM feedback approach curves for the approach of a Pt disklectrode towards a ZnO/Eosin Y film under illumination by a yellow LED [I3

−]* inM: (2) 1.05, (3) 0.248, (4) 0.063; scan rate = 1 �m s−1, ET = −0.7 V, rT = 16.5 �m. Solid

ines are calculated curves for an approach of an UME with RG = 10 towards an inertnsulating surface (curve 1), and to samples with first-order kinetics of mediatorecycling using normalized rate constants �: (2) 0.017, (3) 0.036, (4) 0.174.

cta 55 (2009) 458–464 461

[I3−]* under illumination of the film with blue, green and yellowLEDs. In each Figure, the effect of [I3−] on the approach curves wasstudied. Note that similar to the experiments with blue and greenLEDs, approach curves with yellow LED at various other [I3−]* werecarried out but only part of the data are presented in Fig. 5 becauseall curves overlap with those actually plotted in Fig. 5. The UMEcurrent under illumination with all three LEDs at various [I3−]* istypical for SECM feedback experiments with finite electron transferkinetics at the sample. Under this particular experimental condi-tion, it is reasonable to assume that UME current response in thisregion was governed by a photo-induced charge transfer reaction atthe illuminated film-electrolyte interface. Experimental approachcurves iT(z) were normalized to IT(L) and the normalized heteroge-neous rate constants � have been extracted by fitting them to ananalytical approximation of Cornut and Lefrou [48] for a first-orderreaction at the sample and infinitely fast reaction at UME [Eq. (6)].

IT(L, �, RG) = IcondT

(L+ 1

�, RG

)+ Iins

T (L, RG)−1

(1+2.47RG0.31L�)(1+L0.006RG+0.113�−0.0236RG+0.91)(6)

where IinsT and Icond

T , the normalized UME current, when the reactionat the sample is diffusion-controlled, are calculated according to[Eq. (7)] and [Eq. (8)], respectively.

IinsT (L, RG)

= (2.08/RG0.358)(L − (0.145/RG)) + 1.585

(2.08/RG0.358)(L + 0.0023RG)+1.57 + (ln RG/L) + (2/�RG) ln(

1 + (�RG/2L))

(7)

IcondT (L + �−1, RG) = ˛(RG) + �

4ˇ (RG) arctan(L + �−1)

+(

1 − ˛(RG) − 12ˇ(RG)

)2�

arctan(L + �−1)

(8a)

˛(RG) = ln 2 + ln 2(

1 − 2�

arccos(

1RG

))

− ln 2

(1 −

(2�

arccos(

1RG

))2)

(8b)

ˇ(RG) = 1 + 0.639(

1 − 2�

arccos(

1RG

))

− 0.186

(1 −

(2�

arccos(

1RG

))2)

(8c)

In [Eq. (6)] � is a normalized, dimensionless, first-order rate con-stant which is obtained by fitting the experimental approach curveto the analytical approximations. The apparent heterogeneous rateconstant keff for the regeneration of the photo-oxidized dye by I−

is obtained from Eq. (9) with the diffusion coefficient of I3− in ace-tonitrile of 1.37 × 10−5 cm2 s−1 [42] and rT of the particular UMEused.

keff = �D

rT(9)

The results are summarized in Table 2a–c for blue, green andyellow LED illumination, respectively, and variation of [I3−]*. Ateach LED illumination � obtained from the best fit of experimentalapproach curves to the theory of finite kinetics [Eq. (6)], decreaseswith increased [I3−]* because the diffusion of I3− from the solution

bulk dominates over the flux resulting from dye regeneration at thesample. A similar behaviour is known from SECM feedback mea-surements at enzyme-modified insulating surfaces [18,49]. Thissimilarity points at the decisive role of light absorption in the sen-sitizer as trigger of the photoelectrochemical redox cycle.

462 Y. Shen et al. / Electrochimica A

Table 2Normalized apparent heterogeneous first-order rate constants � and apparent het-erogeneous first-order rate constants keff = �D/rT obtained for the reduction ofphotoexcited Eosin Y+ by I−; rT = 16.5 �m, RG = 10, D = 1.37 × 10−5 cm2 s−1.

keff [I3−]* [10−6 mol cm−3]a �b keff (10−3 cm s−1)

(a) Blue LED0.063 0.45 3.74770.13 0.265 2.20700.248 0.196 1.63230.512 0.167 1.37421.238 0.085 0.69132.00 0.03 0.250

(b) Green LED0.063 0.43 3.99760.13 0.25 2.08210.248 0.14 1.1660.626 0.081 0.67461.238 0.055 0.45812.00 0.04 0.3331

(c) Yellow LED0.063 0.174 1.44910.248 0.036 0.29981.05 0.017 0.1416

e

oriotrstet

3i

eadniua

F(

ues were 1.47 × 10 , 1.39 × 10 and 3.67 × 10 s for blue,green and yellow LEDs, respectively. The result shows that khv,eff

a Total concentration of I3− .

b Dimensionless normalized pseudo-first-order rate constant obtained by fittingxperimental approach curves to Eq. (6).

Because of the strong spectral dependence of the absorptionf Eosin Y, the normalized approach curves in Figs. 3–5 thereforeesult in different values of keff for Eosin Y regeneration by I− forllumination by the three different wavelengths of the blue, greenr yellow LED. A larger value of keff for the green LED compared tohe blue LED fits to the maximum absorption coefficient of Eosin Yound 520 nm as it is evident from the absorption spectrum of theensitized film (Fig. 6). At longer wavelengths (such as yellow LED),he photon absorption by Eosin Y is very weak (Fig. 6). Hence, asxpected the keff values for the yellow LED are significantly lowerhan those for the blue or green LED at a given [I3−]*.

.2. Comparison of dye regeneration kinetics with different LEDllumination

The three consecutive microscopic processes must be consid-red at the illuminated interface: the diffusion-controlled reactiont UME [Eq. (2)], the heterogeneous reaction of I− with the oxidized

ye molecule [Eq. (4)] and diffusion of photo injected electrons inanoporous ZnO film into the back contact. From the mathemat-

cal treatment of the limiting current for each of these processesnder simplifying assumption regarding the structure of the DSSC,n expression for normalized substrate current was determined

ig. 6. Comparison of absorbance (solid line, left axis) and excitation cross-sections�, right axis) for ZnO/Eosin Y film plotted as a function of excitation wavelength.

cta 55 (2009) 458–464

and related to a simple first-order rate law at the sample [33]. Inthe following discussion we consider Eq. (18) of Ref. [33] to analyzethe kinetics of Eosin Y regeneration with different LED illumination:

1keff

= 2

3√

3�Do kox

√[I−3

]∗+

2[I−3

]∗

�Do ϕhvJhv+

2[I−3

]∗

�Do kinj(10)

where �Do is the surface concentration of the dye in the groundstate, kox is the heterogeneous rate constant for the reduction ofphoto-oxidized Eosin Y by I−, ϕhv is the excitation cross-section ofthe adsorbed dye, kinj is the first-order rate constant for injectionof electrons from excited dye into the conduction band of ZnO, andJhv is the incident photon flux density. Eosin Y-sensitized ZnO filmprepared by electrochemical self-assembly is porous with a filmthickness of 3 �m and a dye loading �Do of 6 × 10−8 mol cm−2, andthe dye is homogenously distributed within a film [36].

The effective rate constant khv,eff for the excitation and electroninjection process at incident light of given intensity, wavelengthand dye molecule can be defined as in equation:

1khv,eff

= 1kinj

+ 1ϕhvJhv

(11)

Substituting Eq. (11) in Eq. (9) results in the following expres-sions:

1keff

= 2

3√

3�Do kox

√[I−3

]∗+

2[I−3

]∗

�Do khv,eff(12)

keff = 3√

3koxkhv,eff�D◦√

[I−3 ]∗

2khv,eff + 6√

3kox[I−3 ]∗3/2(13)

With each of the three LEDs keff was determined for various[I3−]*. It is expected that the kox value is similar for all the threeLED illumination for a given dye-sensitized electrode and elec-trolyte mediator; however, khv,eff should vary with the wavelengthof LEDs. Therefore, the experimental data were fitted to Eq. (13)using one kox and three khv,eff (for blue, green and yellow LED).For �Do the experimental value was used. From the best fit ofthe experimental data (Fig. 7, solid lines) the following valueswere obtained kox = 1.82 × 108 cm9/2 mol−3/2 s−1 and the khv,eff val-

−2 −21 −3 −1

values are approximately ten-fold higher with blue and green LEDthan with yellow LED illumination. A lower khv,eff value at yellowLED illumination (� > 580 nm) is consistent with low IPCE of Eosin

Fig. 7. Plot of experimental values of keff vs. [I3−]*. The lines represent fits of kox and

kh�,eff in Eq. (13) to the data: (1, �) blue LED, (2, ©) green LED and (3, �) yellow LED.

mica Acta 55 (2009) 458–464 463

YerItftt

tpmi(

kEEddisLrnRo

3

sftrctlpsLbrslpa�UfkE3tscwiftiei

Fig. 8. Normalized SECM feedback approach curves for the approach of a Pt diskUME towards a ZnO/Eosin Y film under illumination by a blue LED with intensi-ties expressed in terms of photon flux density in mol cm−2 s−1: (2) 0.978 × 10−9,(3) 2.95 × 10−9, (4) 6.95 × 10−9, (5) 11.4 × 10−9, (6) 22.4 × 10−9, (7) 28.4 × 10−9;�T = 1 �m s−1 [I3

−]* = 0.104 mM, rT = 12.5 �m, ET = −0.7 V; Solid lines are calculatedcurves for an approach of an UME towards inert insulating surface (curve 1), andto samples with first-order kinetics of mediator recycling using normalized rateconstants �: (2) 0.05, (3) 0.1, (4) 0.14, (5) 0.16, (6) 0.19, (7) 0.205.

Y. Shen et al. / Electrochi

-sensitized nanoporous ZnO based DSSC [36]. One of the inter-sting findings in our model is that a uniform kox value provides aeasonable approximation for the kinetics of dye regeneration by− in Eosin Y-sensitized ZnO cells. The constant kox is itself an effec-ive rate constant summarising the kinetics of the electron transferrom I− to the oxidized dye and all follow-up processes that leado the formation of I3−. Currently, it is impossible to further detailhose individual steps by SECM investigations.

Since the fluorescence of Eosin Y is completely quenched whenhe dye is adsorbed onto ZnO and the injection of electrons fromhotoexcited dye molecules into ZnO conduction band has beeneasured to be ultrafast [50], it is reasonable to assume that kinj is

n the same order as for a fluorescence process and therefore, Eq.11) can be reduced to:

1khv,eff

≈ 1ϕhvJhv

(14)

From the measured Jhv values in Table 1 and the values ofhv,eff from the fits in Fig. 7, the photoexcitation cross-section ofosin Y ϕhv at each LED wavelength can be estimated by usingq. (14) as 5.99 × 10−2, 8.81 × 10−2, 3.30 × 10−2 Å2 per individualye molecule for blue, green and yellow LED, respectively. Theependence of ϕhv of Eosin Y molecules on the wavelength of the

ncident LED follows closely the absorption spectrum of Eosin Y-ensitized ZnO film with a maximum at the wavelength of the greenED (Fig. 6). As expected the spectral dependence of ϕhv basicallyeflects the absorption properties of the dye molecule adsorbed inanoporous ZnO, a confirmation of the kinetic model proposed inef. [33]. It also yields a constant value of kox that is independentf the triiodide concentration.

.3. Variation of the illumination intensity

In order to verify the intensity dependence of the SECM feedbackignal predicted by Eq. (13), feedback measurements were per-ormed for a fixed mediator concentration [I3−]* = 0.104 mM withhe three LED. For this purpose a LED illuminator with a feedbackegulation of intensity was employed instead of a simple LED (Fig. 2,omponents in dashed line). The spectral properties of the LEDs inhis light source are very similar to that of the blue, green and yel-ow LED applied before (Supporting information S2). The incidenthoton flux at the location of the sample was measured for eachetting of the illuminator with the same power meter as with theED light source employed before. Fig. 8 shows the results for thelue LED. With increasing illumination intensity higher currents areecorded. At an intensity of 22.4 × 10−9 mol cm−2 s−1 the responseaturates and further increase in illumination intensity does notead to increased currents (Fig. 8, curve 6). This corresponds to therediction of Eq. (13). Similar results were obtained for the greennd yellow LED (Supporting information S3). From the extractedof each curve keff can be calculated (Supporting information S3).sing approximation (14) and Eq. (13), keff can be expressed as

unction of Jhv. The corresponding data are shown in Fig. 9. Usingox and ϕh�(�) as adjustable parameters, the data can be fitted toqs. (13) and (14) yielding kox = 0.472 × 108 cm9/2 mol−3/2 s−1 and.69 × 10−2, 4.67 × 10−2, 2.24 × 10−2 Å2 molecule−1 as ϕh�(�) forhe blue, green and yellow LEDs, respectively. The kox is slightlymaller than the kox determined from the variation of the con-entration. The determination method introduces an uncertaintyhich has been estimated in Ref. [33] to about 60%. Uncertainties

n the preparation of the DSSC add to this value since the samples

or Figs. 7 and 9 originate from different batches. Since the parame-er �Do and kox are highly coupled in Eq. (13), a variation of the �Do

n a particular electrode would be reflected in a changed kox. Thexperimentally determined dye loading is an average value, wherentegration occurs across a macroscopic sample and is averaged

Fig. 9. Plot of experimental values of keff vs. Jh�(�). The lines represent fits of keff

according to Eq. (13) with kox and ϕh�(�) as adjustable parameters.

between a limited number of samples. Uncertainties in the prepa-ration will also cause slight changes in the ϕh�(�) values. However,for one and the same sample, Eq. (13) provides a consistent quanti-tative description of the observed behavior. This is remarkable sincethe applied assumption of uniform accessibility of all dye moleculesby light and by the I− ions is not necessarily obvious when look-ing at electron microscopic images of the dye-modified ZnO withstrongly differing positions of the pores within the porous network[51].

4. Conclusion

SECM has been shown to be a successful approach for theinvestigation of the kinetics of dye regeneration by iodide ionsin a DSSC. The feedback mode approach curves have been usedto quantitatively investigate the redox process at an illuminatedEosin Y-sensitized ZnO thin film interface with three differ-ent LEDs (blue, green and yellow). Experimentally determinedkox value by using one model for the three different LED was

8 9/2 −3/2 −1

1.82 × 10 cm mol s . The SECM study shows that the dif-ference in the apparent heterogeneous first-order rate constant forLEDs irradiation with different wavelength could be due to the exci-tation cross-section of the adsorbed dye, which is related to the dyemolecular properties. At present, additional experiments to deter-

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[[49] D.T. Pierce, A.J. Bard, Anal. Chem. 65 (1993) 3598.[50] K. Keis, C. Bauer, G. Boschloo, A. Hagfeldt, K. Westermark, H. Rensmo, H. Sieg-

bahn, J. Photochem. Photobiol., A: Chem. 148 (2002) 57.

64 Y. Shen et al. / Electrochi

ine structural parameters of DSSC which influence the kinetics ofegeneration of dye adsorbed in ZnO are underway.

cknowledgement

Y.S. thanks the Alexander von Humboldt Foundation and theanse Institute of Advanced Studies in Delmenhorst for fellow-

hips. U.M.T. gratefully acknowledges a grant of the Germancademic Exchange Office (DAAD). Financial support of parts of theork by the German Federal Ministry of Education and Research

BMBF 16 SV 3454) is gratefully acknowledged.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.electacta.2009.08.062.

eferences

[1] B. O’Regan, M. Graetzel, Nature 353 (1991) 737.[2] S. Lindquist, A. Hagfeldt, S. Södergren, H. Lindström, in: G. Hodes (Ed.), Electro-

chemistry of Nanomaterials, Wiley-VCH, 2001.[3] F.-T. Kong, S.-Y. Dai, K.-J. Wang, Adv. OptoElectron. (2007) 75384.[4] B.A. Gregg, F. Pichot, S. Ferrere, C.L. Fields, J. Phys. Chem. B 105 (2001) 1422.[5] S.N. Lewis, J. Phys. Chem. B 102 (1998) 4843.[6] A. Hagfeldt, M. Graetzel, Acc. Chem. Res. 33 (2000) 269.[7] S.A. Haque, E. Palomares, B.M. Cho, A.N.M. Green, N. Hirata, D.R. Klug, J.R. Dur-

rant, J. Am. Chem. Soc. 127 (2005) 3456.[8] Q. Wang, S. Ito, M. Graetzel, F. Fabregat-Santiago, I. Mora-Sero, J. Bisquert, T.

Bessho, H. Imai, J. Phys. Chem. B 110 (2006) 25210.[9] L.M. Peter, J. Phys. Chem. C 111 (2007) 6601.10] J.R. Jennings, A. Ghicov, L.M. Peter, P. Schmuki, A.B. Walker, J. Am. Chem. Soc.

130 (2008) 13364.11] J.N. Clifford, E. Palomares, M.K. Nazeeruddin, M. Graetzel, J.R. Durrant, J. Phys.

Chem. C 111 (2007) 6561.12] C. Nasr, S. Hotchandani, P.V. Kamat, J. Phys. Chem. B 102 (1998) 4944.13] C. Combellas, J. Ghilane, F. Kanoufi, D. Mazouzi, J. Phys. Chem. B 108 (2004)

6391.14] C. Wei, A.J. Bard, M.V. Mirkin, J. Phys. Chem. 99 (1995) 16033.15] Y. Selzer, D. Mandler, J. Electroanal. Chem. 409 (1996) 15.16] C.J. Slevin, J.V. Macpherson, P.R. Unwin, J. Phys. Chem. B 101 (1997) 10851.17] G. Wittstock, M. Burchardt, S.E. Pust, Y. Shen, C. Zhao, Angew. Chem., Int. Ed.

46 (2007) 1584.

[

cta 55 (2009) 458–464

18] D.T. Pierce, P.R. Unwin, A.J. Bard, Anal. Chem. 64 (1992) 1795.19] C. Zhao, G. Wittstock, Anal. Chem. 76 (2004) 3145.20] S.B. Basame, H.S. White, J. Phys. Chem. 99 (1995) 16430.21] P. James, N. Casillas, W.H. Smyrl, J. Electrochem. Soc. 143 (1996) 3853.22] S.B. Basame, H.S. White, Langmuir 15 (1999) 819.23] S.B. Basame, H.S. White, Anal. Chem. 71 (1999) 3166.24] I. Serebrennikova, S. Lee, H.S. White, Faraday Discuss. 121 (2002) 199.25] D. Mandler, A.J. Bard, Langmuir 6 (1990) 1489.26] B.R. Horrocks, M.V. Mirkin, A.J. Bard, J. Phys. Chem. 98 (1994) 9106.27] H. Maeda, K. Ikeda, K. Hashimoto, K. Ajito, M. Morita, A. Fujishima, J. Phys. Chem.

B 103 (1999) 3213.28] S.M. Fonseca, A.L. Barker, S. Ahmed, T.J. Kemp, P.R. Unwin, Chem. Commun.

(2003) 1002.29] T.J. Kemp, P.R. Unwin, L. Vincze, J. Chem. Soc. Faraday Trans. 91 (1995) 3893.30] S.M. Fonseca, A.L. Barker, S. Ahmed, T.J. Kemp, P.R. Unwin, Phys. Chem. Chem.

Phys. 6 (2004) 5218.31] B. Bozic, E. Figgemeier, Chem. Commun. (2006) 2268.32] E. Figgemeier, W.H. Kylberg, B. Bozic, Proc. SPIE-Int. Soc. Opt. Eng. 6197 (2006)

619711/1.33] Y. Shen, K. Nonomura, D. Schlettwein, C. Zhao, G. Wittstock, Chem. A Eur. J. 12

(2006) 5832.34] B. Liu, A.J. Bard, M.V. Mirkin, S.E. Creager, J. Am. Chem. Soc. 126 (2004) 1485.35] G. Rothenberger, D. Fitzmaurice, M. Graetzel, J. Phys. Chem. 96 (1992) 5983.36] T. Yoshida, M. Iwaya, H. Ando, T. Oekermann, K. Nonomura, D. Schlettwein, D.

Woehrle, H. Minoura, Chem. Commun. (2004) 400.37] T. Yoshida, D. Komatsu, N. Shimokawa, H. Minoura, Thin Solid Films 451–452

(2004) 166.38] T. Wilhelm, G. Wittstock, Langmuir 18 (2002) 9485.39] G. Wittstock, T. Asmus, T. Wilhelm, J. Fresenius, Anal. Chem. 367 (2000) 346.40] G.B. Saupe, T.E. Mallouk, W. Kim, R.H. Schmehl, J. Phys. Chem. B 101 (1997)

2508.41] H. Tsubomura, M. Matsumura, Y. Nomura, T. Amamiya, Nature 261 (1976) 402.42] V.A. Macagno, M.C. Giordano, Electrochim. Acta 14 (1969) 335.43] I. Montanari, J. Nelson, J.R. Durrant, J. Phys. Chem. B 106 (2002) 12203.44] J. Bisquert, V.S. Vikhrenko, J. Phys. Chem. B 108 (2004) 2313.45] H. Greijer, J. Lindgren, A. Hagfeldt, J. Phys. Chem. B 105 (2001) 6314.46] J.L. Amphlett, G. Denuault, J. Phys. Chem. B 102 (1998) 9946.47] C. Nunes Kirchner, G. Wittstock, in: S. Alegret, A. Merkoci (Eds.), Electrochemical

Sensor Analysis, vol. 49, Elsevier, Amsterdam, 2007.48] R. Cornut, C. Lefrou, J. Electroanal. Chem. 621 (2008) 178.

51] T. Yoshida, J. Zhang, D. Komatsu, S. Sawatani, H. Minoura, T. Pauporte, D. Lincot,T. Oekermann, D. Schlettwein, H. Tada, D. Wohrle, K. Funabiki, M. Matsui, H.Miura, H. Yanagi, Adv. Funct. Mater. 19 (2009) 17.