14256 Phys. Chem. Chem. Phys., 2013, 15, 14256--14261 This journal is c the Owner Societies 2013

Cite this: Phys. Chem.Chem.Phys.,2013,15, 14256

Redox characterization of semiconductors based onelectrochemical measurements combined with UV-Visdiffuse reflectance spectroscopy

Elzbieta Swietek,a Kacper Pilarczyk,ab Justyna Derdzinska,a Konrad Szaciłowski*ac

and Wojciech Macyk*a

Several techniques can be applied to characterize redox properties of wide bandgap semiconductors,

however some of them are of limited use. In this paper we propose a new modification of the old

spectroelectrochemical method developed two decades ago. A procedure based on measurements of

the reflectance changes as a function of potential applied to the electrode coated with the studied

material appears to be a very convenient tool for characterizing redox properties of semiconductors,

forming either transparent or opaque films at a platinum electrode. A discussion on the measured

redox parameters of semiconductor films concludes with a definition of a new term, the absorption

onset potential of the material.

Introduction

Wide bandgap semiconductors are commonly used materialsin the processes of solar energy utilization, involving photo-catalysis and photovoltaics.1–5 Their applicability is determinedby redox properties of ground and excited states. Interfacialcharge transfer processes taking place at the semiconductorsurface are, in the first approximation, dependent on thepotentials of conduction and valence band edges. Both irreversibleredox reactions involving the photogenerated charges, takingpart in overall photocatalytic reactions (e.g. pollutant oxidation),and reversible redox reactions of electron mediators (e.g. I3

�/I�

commonly used in dye sensitized solar cells) can operate onlywhen suitable energies of electrons and holes in conduction andvalence bands are assured.2,3 Therefore determination of redoxproperties of semiconductors in the form of colloids, powdersor films is crucial while studying any photoinduced electrontransfer processes taking place at the surface of such materials.Surprisingly, this fact is often underestimated by researchersworking in the field of photocatalysis. Instead of measuring the

potentials of band edges some ‘‘standard’’ potentials are com-monly cited, despite the fact that redox properties of the materialare influenced by several factors, like crystal structure, particlesize, doping level, surface modification, environment (solvent orpH), etc.6 A difficulty in determining the potentials of band edgesis certainly a reason why such measurements are often omittedin the literature dedicated to photocatalysis. The choice of asuitable method is often problematic. In this paper we would like topropose a new, relatively simple procedure for characterization ofredox properties suitable for description of semiconductors innearly every form (powders, colloids, and films).

Due to the adsorption of various molecules and ions at thesemiconductor surface, as well as to the different distributionof defects in the subsurface region when compared to thedistribution in the bulk, bending of semiconductor bandsoccurs.7 Band bending is also due to the potential drop whenthe space charge region is formed beneath the semiconductorsurface. The width of this layer is given by:8

W ¼ 2ee0eN

E � Efb �kT

e

� �� �12

¼W0 E � Efb �kT

e

� �12

(1)

where W0 denotes the width of the depletion layer for apotential change of 1 V across it, T stands for temperature,k is the Boltzmann constant, E is the electrode potential,Efb stands for the flatband potential, N is the charge carrierdensity of the examined material, e is the elementary charge,

a Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow,

Poland. E-mail: macyk@chemia.uj.edu.pl; Fax: +48 12634 0515;

Tel: +48 12663 2222b Faculty of Physics and Applied Computer Science, AGH – University of Science

and Technology, al. A. Mickiewicza 30, 30-059 Krakow, Polandc Faculty of Non-Ferrous Metals, AGH – University of Science and Technology,

al. A. Mickiewicza 30, 30-059 Krakow, Poland. E-mail: szacilow@agh.edu.pl;

Tel: +48 12617 2931

Received 20th May 2013,Accepted 3rd July 2013

DOI: 10.1039/c3cp52129j

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e and e0 are the semiconductor dielectric constant and permit-tivity of a vacuum, respectively.

The potential drop associated with the space charge regionin the material vanishes as the charge on the semiconductorbecomes zero. The potential at which this condition is met iscalled the flatband potential. In other words, bands are notbent when no excess charge exists in the semiconductor andthere is neither an electric field nor a space charge region. Inthe case of heavily doped n-type semiconductors the flatbandpotential (Efb) is usually very close to the Fermi level potential(EF). It is important to note that the flatband potential dependson the concentration of potential determining ions (for metaloxides, typically OH� and H+) in the electrolyte.

That relationship can be expressed by the formula (2):

Efb ¼ E0fb þ

Xni¼1

ki log ci (2)

where E0fb is the flatband potential for the arbitrarily chosen

conditions, ki, is a set of coefficients characteristic of individualions (in the case of metal oxides it usually equals to 59 mV) andci stands for the concentration of a particular ion.

In the case of nanoparticles the band bending is negligiblewhen the radius of the particle is much smaller than theexpected width of the space charge layer. As a consequenceall nanoparticles are in a pseudo flatband state. If suchagglomerated nanopowder is contacted neither with the con-ducting support nor the electrolyte, all the particles attainsimilar band edge potentials and no space charge layer isformed. In this kind of random network wave functions ofelectrons/holes are confined to individual nanoparticles andcannot propagate through the assembly. Therefore all theelectronic states become localized, which may be interpretedas a manifestation of Anderson localization.9–13 At the sametime, numerous studies indicate that even in the absence ofband bending photovoltage and photocurrent can be observedin the multilayer structures of semiconducting nanoparticlesupon illumination.14,15

The model, in which an emphasis was put on the electron-scattering and trapping–detrapping phenomena, was proposedby D. Vanmaekelbergh. In that approximation the linear gradientin the electron Fermi level within porous film is considered to beresponsible for the electron transport.16,17

This picture changes when a film made of nanoparticles isilluminated. Irradiation leads to the splitting of the Fermi levelinto two quasi-Fermi levels, *EFn for electrons and *EFp forholes, which are displaced in the crystal towards the bottom ofthe conduction band and the top of the valence band, respec-tively (Fig. 1). The extent of the splitting depends on the radiantpower of the radiation – the larger the photon flux, the strongerthe splitting of the Fermi level.18 In the case of films composedof nanoparticles, the positions of *EFn and *EFp levels changewith respect to the depth of light penetration across the film,still they remain flat within a single nanoparticle. Therefore,the disparity in the illumination conditions on both sides ofthe film may cause an additional displacement of *EFn and*EFp levels.

Taking into account the change associated with the irradiationof a semiconductor, one may expect that determined redoxproperties will also be influenced. Recently photoelectrochemicalmethods used for determination of band edge potentials havebeen reviewed.19 Among the methods used for the character-ization of redox properties of semiconducting materials, thefollowing deserves particular attention: (i) a method describedby Roy;20 (ii) a method described by Bard;21 (iii) a method based onthe open-circuit photovoltage measurements;22–24 (iv) a methodbased on photocurrent measurements and Butler–Wilson(BW) theory;25–29 (v) a method based on capacitance-measurements,26,30–33 and (vi) a method based on spectro-electrochemical measurements.34–39 In the first four methodsthe measurements refer to the excited states of the semi-conductor, and therefore the potential of the quasi-Fermi levelfor electrons (*EFn) is measured. In the methods based oncapacitance and spectroelectrochemical measurements(described below), the studied semiconductor is not excited,thus the flatband potential is measured. While studying photo-catalytic processes, which usually require excitation of thephotocatalyst, determination of the quasi-Fermi level shouldbe considered, since photoinduced primary redox processestaking place at the surface of the photocatalyst are relatedto the quasi-Fermi levels. However, when the semiconductoris not excited but reduced, e.g. as a result of the electroninjection from the excited surface-bound molecule, informa-tion on the redox properties of the semiconductor in its groundstate becomes more desirable. In the first case the electron–hole pair is formed within the semiconductor particle, whilein the second one the semiconductor particle is reduced asa result of photosensitization or an indirect photocatalyticprocess.18

Two decades ago Fitzmaurice and coworkers describeda spectroelectrochemical method of flatband potential deter-mination.34–39 The concept of this method is based on electro-chemical reduction of a transparent electrode with depositedtransparent, thin film of studied semiconducting material.The electrode, immersed in the quartz cuvette filled with anelectrolyte solution, is placed inside a UV-Vis spectrophotometer inthe way enabling the light beam to pass through the electrode(Fig. 2A). Together with the swept potential, changes in absorbanceof the electrode are monitored. At potentials lower than the

Fig. 1 The Fermi level for an n-semiconductor nanoparticle in the dark (A) andits splitting into *EFn and *EFp levels upon irradiation with lower (B) and higher (C)light intensity.

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flatband potential, reduction of the conduction band starts.This process is associated with the increase of the measuredcurrents as well as with spectral changes combined withchanges in the oxidation state of the corresponding metalion. In the case of titanium dioxide reduction of its conductionband leads to formation of titanium(III) centers characterized bya broad peak in the absorption spectrum, with maximumlocalized at 780 nm.38 An increase in absorbance at 780 nmcorrelated with the electrode potential enables a precise deter-mination of the flatband potential. A similar procedure wasalso successfully applied to determine redox properties oftransparent Fe2O3 and Fe2O3–TiO2 films.39 Unfortunately, thespectroelectrochemical method developed by Fitzmaurice has alimited applicability, since it may be used only for transparentfilms synthesized at the surface of transparent electrodes. Thisrestriction implies application of ITO (indium tin oxide) orF:SnO2 (fluorine doped SnO2) materials, which may appear tobe redox active at more negative potentials.

Recently, the spectroelectrochemical method has beenapplied by Ondersma and Hamann.40 The authors used themethod to study the influence of temperature on the potentialof the conduction band edge for nanostructured, transparentTiO2 electrodes. A method suitable for the determination ofextinction coefficients of free conduction band electrons wasalso presented.

Here we propose a modification of the described spectro-electrochemical method. Instead of absorption spectroscopy weapplied diffuse reflectance spectroscopy (Fig. 2B). The advan-tages of such modification are obvious: instead of a transparentworking electrode a platinum plate can be used. Also transparencyof the semiconducting material is not required. In contrast to theabsorbance, the reflectance can be recorded for films made ofnano- and microcrystalline coatings, irrespectively of their lightscattering properties. The changes in reflectance are associatedwith the density of electrons in the conduction band. Measure-ments of reflectance as a function of electrode potential (sweptfrom high to low values) enable determination of the potential atwhich the electrochemical reduction of the semiconductor starts.

The measurements carried out at various pH values allow us todetermine the k constant from eqn (2).

In this paper we demonstrate applicability of the modifiedspectroelectrochemical method to study the redox properties oftitanium dioxide material (Evonik-Degussa P25).

Experimental sectionMaterials

Evonik-Degussa P25 was used as a TiO2 material. Other chemicals(KNO3, HNO3, and NaOH) were supplied by Sigma-Aldrich.

Spectroelectrochemical measurements

All measurements were carried out in an aqueous solution of0.1 mol dm�3 KNO3 as an electrolyte. Oxygen was thoroughlyremoved from the electrolyte by purging with argon prior toexperiments. pH values were adjusted with NaOH and HNO3

solutions. Working electrodes were prepared by casting of TiO2

material, previously ground in a mortar and suspended in distilledwater, at the surface of a platinum plate (ca. 2 � 2 cm2). Theelectrodes were dried at approx. 100 1C, resulting in formation of anopaque film.

Relative reflectance changes, recorded versus the reflectancemeasured for the same system directly prior to the measure-ment, were recorded using a Perkin Elmer UV-Vis Lambda12 spectrometer equipped with a 5 cm diameter integratingsphere. The measurements were carried out in a three-electrodecell (quartz cuvette), with platinum wire and Ag/AgCl as counterand reference electrodes, respectively. The quartz cuvette wasplaced in front of the sphere, facing the working electrode(platinum plate with deposited TiO2) towards the light beam(Fig. 2B). Potential control was provided using the electro-chemical analyser Autolab PGSTAT302N. The scan rate wasvaried in the range of 0.2 to 10 mV s�1. Reproducible resultswere obtained when the electrolyte solution was intensivelypurged with argon during the measurements.

Results and discussion

The general concept of spectroelectrochemical measurementsinvolving diffuse reflectance spectroscopy has been describedin the Introduction. The use of UV-Vis spectroscopy togetherwith the electrochemical reduction enables a proper assignmentof the potential at which the electron injection to availableenergy states starts to proceed.34–39 Moreover, application of adiffuse reflectance spectroscopy instead of transmission modeextends applicability of the method to materials in almostany form, independently of light scattering properties of thesemiconductor. In the case of TiO2 the increase in absorbance(or decrease in reflectance) within nearly the whole visible and apart of NIR range of radiation is attributed to the increasingdensity of electrons in the conduction band of titania.38

Electrons trapped as TiIII centers are characterized by a wideabsorption band with the maximum at 780 nm.38 The change inabsorbance is a function of the density of electrons in theconduction band, as demonstrated by Fitzmaurice et al.38,41

Fig. 2 The set-up used for the spectroelectrochemical characterization of theredox properties: (A) absorbance measurements; (B) reflectance measurements.

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On the other hand, the absorbance changes are equivalent to thechanges in the Kubelka–Munk function, defined as:

KM ¼ ð1� RÞ2

2R(3)

where R is the reflectance.The results of measurements made for P25 are shown in

Fig. 3A and B for pH = 6.0 and 10.5. Figures present bothcurrent and reflectance changes upon swept potential. Trans-formations of reflectance into the Kubelka–Munk function areshown in corresponding Fig. 3C and D. The potential, at whichthe electrons start to be trapped, Eon, may be determined fromKM(E) plots, or directly from R(E) plots, since the onsetof Kubelka–Munk function corresponds to the onset of thereflectance changes. The value of Eon should be identifiedeither as the flatband potential or as the Fermi level. This valuesimply describes the ability of material to accept electrons, i.e.its absorption onset potential.

The results obtained using the above described procedureare scan rate dependent (Fig. 4). When the scan rate is loweredthe onset potential shifts to higher values. It may be explainedby the fact that the system cannot reach an equilibrium if thepotential change is too fast.

A decrease in the scan rate below 1 mV s�1 does not resultin the further shift of the onset potential (Fig. 4 and 5).

Therefore determination of the onset potential for the testedTiO2 sample can be done based on the measurements madewith scan rates of 1 mV s�1. The kinetics of the reductionprocess may be different for other materials, so other scan ratesmay appear applicable for different semiconductors. Thedependence of the scan rate on spectral changes at givenpotential was not discussed in Fitzmaurice’s works.

Fig. 3 Current and relative reflectance measured at 700 nm as a function of applied potential for TiO2 (P25) deposited at the surface of a platinum plate at pH = 6.0(A) and 10.5 (B). Reflectance converted to Kubelka–Munk function at pH = 6.0 (C) and 10.5 (D); scan rate = 1 mV s�1, deaerated solutions.

Fig. 4 Kubelka–Munk function at 700 nm as a function of the appliedpotential and the scan rate for TiO2 (P25) deposited at the surface of a platinumplate at pH = 6.0.

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Finally, the influence of oxygen on results of the measure-ments should be considered. Reduction of titanium dioxidefilm leads to formation of TiIII which, in turn, is able to reduceoxygen according to eqn (4):

TiIII + O2 - TiIV + O2�� (4)

The consumption of electrogenerated titanium(III) is anunwanted process since recorded changes in reflectance areassociated with formation of TiIII. Therefore the presenceof oxygen should influence the results. The dependence ofdetermined potentials on time of electrolyte purging with argonprior to the measurements is presented in Fig. 6. After 15 minof purging the constant level of dissolved oxygen was achieved,which is reflected in reproducible Eon values.

Measurements performed at various pH values of the electrolyteenabled determination of the k parameter (eqn (2)). A plot showingthe pH dependence of the absorbance onset is shown in Fig. 7. Theobtained value of the k parameter amounts to 0.056� 0.002 V. Thedetermined potential of the electrons, trapped as TiIII centers, is�0.19 � 0.03 V vs. SHE for P25 at pH = 7.0.

Conclusions

Various methods of flatband potential determination can beapplied to characterize semiconducting material. The methodsbased on open-circuit photovoltage measurements and photo-current measurements involve excitation of the semiconductor.The measured signals depend on the quasi-Fermi level, which,in turn, depends on light intensity. During spectroelectro-chemical and impedance measurements the semiconductor isnot excited. While considering the use of any of the methods,one has to be aware what kind of parameter can be derivedfrom the measurements.

While discussing the determination of *EFn, Efb, EF and Eon

potentials, the work function measurements, realized by theuse of a Kelvin probe, should be mentioned. This method,applied to TiO2 (P25), affords a value of work function equal to4.3 eV, which corresponds to a potential of approximately�0.2 V vs. SHE.42 Since the measurements were done for adry sample, a direct comparison to the data collected withinthese studies should be avoided. Nevertheless, the consistencyof the results obtained in this way and in the spectroelectro-chemical measurements may be interpreted as an exhibition ofa strong Fermi level pinning – associated with the high concen-tration of surface states – responsible for equalisation of EF andEon values. Also, the presence of defects should influence valuesof the obtained absorption onset potential, Eon.43

The potential at which the reduction of the material starts toproceed (Eon) may be determined using the described spectro-electrochemical procedure. The use of transparent electrodeswas essential in the case of the original version of this methoddeveloped by Fitzmaurice et al., but application of the diffusereflectance instead of transmission spectroscopy considerablyextends the use of this technique. The obtained values of Eon

are higher than the potentials reported in the literature, usuallybeing within �0.58 to �0.60 V vs. SHE.19 This discrepancy maybe explained by the fact that the electrons are injected intoavailable energy states lying beneath the conduction band edge,within the bandgap. Such energy states may be associated withthe defects of the crystal structure. The presence of surface

Fig. 5 Determined Eon values as a function of the scan rate for TiO2 (P25)deposited at the surface of a platinum plate at pH = 6.0.

Fig. 6 Eon values for TiO2 as a function of time of purging with argon. Potentialswere determined at pH = 5.58, the reflectance changes were monitored at700 nm, scan rate = 1 mV s�1.

Fig. 7 The determined Eon as a function of pH (0.1 M LiClO4). The slope of theline translates to the value of the k parameter.

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states localized at defect sites of the crystal lattice influencesthe absorption onset potential values.

A convenient experimental procedure should be easy to use andprovide reproducible results. Spectroelectrochemical measure-ments are characterized by a high reproducibility when oxygen iscarefully removed from the system. We have successfully appliedthe spectroelectrochemical procedure using DRS spectroscopy tocharacterize redox properties of other semiconducting powders likeCdS, CdO, ZnS, ZnO, WO3, Fe2O3, CuI. For each material theappropriate k parameters were easily determined.

The developed modification of the spectroelectrochemicalprocedure seems to be very useful. It allows us to determine theredox properties of electrons in the conduction band under variousconditions (different electrolytes, different pH values) which trans-lates, in practice, to the useful information concerning the redoxproperties of a semiconducting material in almost every form.

It is obvious, that in addition to already known procedures, thenew one, based on electrochemical measurements combined withdiffuse reflectance spectroscopy, can be a valuable alternative.

Acknowledgements

Authors thank Dr Zbigniew Karkuszewski for stimulating dis-cussions. This work was supported by National Science Centrewithin projects No. 2011/01/N/ST5/05542 and 2011/03/B/ST5/01495. A part of measurements was carried out with theequipment purchased thanks to the financial support of theEuropean Regional Development Fund in the framework ofthe Polish Innovation Economy Operational Program (contractNo. POIG.02.01.00-12-023/08).

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