m. koelsch 2004

Thin Solid Films 451–452(2004) 86–92

0040-6090/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2003.11.150

Electrochemical comparative study of titania(anatase, brookite and rutile)nanoparticles synthesized in aqueous medium

M. Koelsch , S. Cassaignon *, C. Ta Thanh Minh , J.-F. Guillemoles , J.-P. Joliveta a, b b a

Laboratoire Chimie de la Matiere Condensee, UMR-CNRS 7574, Universite P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, Francea ` ´ ´Laboratoire d’Electrochimie et de Chimie Analytique UMR-CNRS 7575, Ecole Nationale Superieure de Chimie de Paris, 11,b ´

rue Pierre et Marie Curie, 75231 Paris Cedex 05, France

Abstract

Titanium oxide TiO has found extensive use in a great variety of applications among which electrode materials for dye-2

sensitized solar cells. The polymorphs of TiO , rutile, anatase and brookite exhibit specific physical properties, band gap,2

electronic surface states. For many applications the size of particles was an important parameter because it determines the surfaceto volume ratio, which greatly influences many properties. TiO anatase was the most used phase for photovoltaic applications2

and brookite seems potentially interesting. Nanometric particles of the three polymorphs were synthesized in aqueous medium inorder to compare the electronic properties of these materials for photovoltaic devices. No significant difference was observedbetween the phases by cyclic voltammetry. The surface and consequently the electrode preparation seemed to be the mainparameter. The measurement of double layer capacitance has shown the film activation initially insulating and strong frequencydependence. After dye sensitization, it has been observed that at low I concentration in solution(with a fixed concentration of2

I ), the open circuit voltage(V ) was independent of this concentration while above a certain threshold for I ,V decreasedyoc 2 oc

logarithmically with the iodine concentration.� 2003 Elsevier B.V. All rights reserved.

Keywords: TiO ; Anatase; Brookite; Rutile; Electrochemistry; Capacitance; Dye-sensitized solar cells; Sensitization2

1. Introduction

Much attention has been focused on the use of thetitanium oxide nanoparticles for various technical appli-cations, such as electrochromic materialw1x, organicdepollutant on windoww2x, dielectricsw3x, lithium-ionsbatteriesw4x and dye-sensitized solar cells(DSSC) w5x.In all these devices the photoelectrode is of a thin filmof nanoparticulate TiO . The properties of these films2

certainly depend on the phase, morphology and prepa-ration method that were used, and it is of interest toinvestigate what kind of surface is the most suitable fora given application and therefore which is the propertythat makes TiO so preferred for all these applications.2

TiO films can be directly synthesized by a wide2

variety of techniques such as chemical vapor depositionw6x, aerosol pyrolysisw7x, electrodepositionw8,9x andsol–gel processingsw10,11x. Most of them lead to

*Corresponding author. Tel.:q33-1-44-27-30-45; fax:q33-1-44-27-47-69.

E-mail address: [email protected](S. Cassaignon).

amorphous or to crystalline anatase. The other phases,rutile or brookite have not been investigated as much.Another possibility, very versatile, is to form nanopar-ticles in solution by soft chemistry and to deposit themon a substrate afterwards. The three polymorphs ofTiO can be synthesized by thermolysis of TiCl or2 4

TiCl in aqueous medium and the control of the precip-3

itation conditions (acidity, nature of anions, ionicstrength, titanium concentration, etc.) allows the controlof crystalline structure, size and morphology of particles.Spheroidal nanoanatasew12x, platelets of pure brookitewith nanometric sizew13x and rutile with various shapes(needle, rod or platelets) w14x were obtained. In thisway, it becomes possible to investigate systematicallythe influence of phase and morphology of the particles.For DSSC, TiO anatase is still considered to be the2

best candidate, but recently, the first DSSC using brook-ite as the semiconducting electrode was reportedw15x,with promising results. Owing to the small scale of theconstituents of the thin films, charge carriers in transitget frequently close to the surface, implying that the

87M. Koelsch et al. / Thin Solid Films 451 –452 (2004) 86–92

Fig. 1. TEM photos of TiO nanoparticles:(a) anatase(A9); (b) anatase(Alo) (c1) brookite (B17); (d1) rutile (F9); (e1) rutile (Ru4); (f1)2

rutile (Ru5).

transport and the charge transfer processes are stronglyrelated to the electronic surface states. These processesare then expected to be sensitive to the crystal structure,size and morphology of the exposed lattice planes as itwas shownw16,17x, as well as to the band gap and tothe flat band potentialsw18,19x.Solar cell photopotential is especially sensitive to the

nature of the semiconductor surface that determineslargely the occurrence of reverse reactions(i.e. recom-bination). The best actual solar cells work with the Iy2I (or Br yBr ) couplew20x, because of a slow kineticsy y

2

for I reduction on SnO and specially on TiO surfaces2 2 2

w21x. This process is in competition with the powergeneration in the cell that involves fast electron injectionfrom dye’s LUMO into TiO ’s conduction band and dye2

cation regeneration by the redox couplew5x. Therefore,the open circuit voltage of an illuminated cell is a goodmeasure also of the surface reactivity with respect tothe redox couple measured against that of the adsorbeddye.Here we report on the electrochemical behavior in

aqueous solution of several films of TiO(anatase,2

brookite and rutile) and the influence of parameterssuch as the crystalline structure and the morphology onthe electrochemical response. Double layer capacitancewas measured by impedance spectroscopy to completethis study and to have an idea of the energetic densityof states(DOS) at the interface. Finally, to compare theinfluence of the different reverse reaction on theV ,octhe TiO has been sensitized with a dye and theV was2 oc

measured as a function of thewI xywI x ratio.y2

2. Experimental

For the synthesis of solids, a known volume ofTiCl or TiCl was added to a hydrochloric acid solution.4 3

The acid concentration was adjusted between 0.5 and 5mol l or sodium hydroxide has been added to fix they1

pH between 1 and 6. These solutions were heated andaged at 60, 100 or 1208C over 24 h. Rutile, anataseand brookite were obtained in various proportions. Theparticles were then centrifuged, washed with distilledwater and peptized with nitric acid to obtain stable sols.The sols were concentrated to get approximately 1mol l titanium concentrate solutions. Depending ony1

the synthesis conditions anatase spherical particles(A9)(�s5 nm) and(A10) (�s9 nm) w13x, needles with asize of 300=15 nm (Ru5), rod-like with a size of2

100=15 nm (Ru4) and small rods with a size of 15=52

nm rutile nanoparticles(F9) w14x or pure nanometric2

brookite platelets(B17) (�s9 nm) w12x were obtained(Fig. 1). BET measurements were made on TiO pow-2

ders, for anatase(A9) the specific surface wasSs210m g , for brookite(B17) Ss117 m g and finally2 y1 2 y1

for rutile (Ru4) Ss125 m g .2 y1

Nanostructured TiO electrodes were prepared by2

depositing the concentrate sol on a conducting fluorine-doped tin oxide(F-SnO) glass substrate. The sol was2

spread using a glass rod. The films were dried in anoven at 608C for a few minutes and sintered for 30min at 4508C. White films of TiO were obtained. It2

has been verified that the crystalline structure and theparticle size were keptw18x. The surface was homoge-neous and the films were porousw18x. The thickness

88 M. Koelsch et al. / Thin Solid Films 451 –452 (2004) 86–92

Fig. 2. Cyclic voltammetry of anatase(A9), brookite(B17) and rutile(F9) and (Ru5) in H SO pH 3yK SO 0.1 M, scan speed 502 4 2 4

mV s .y1

was nearly 10–15mm for rutile and brookite and thinnerfor anatase.The electrochemical study was realized in a conven-

tional three electrodes cell at room temperature, usingan EGGyPAR 273A potentiostatygalvanostat. Thepotential of the working titania electrode(Ss0.125cm ) was measured vs. a saturated mercurous sulfate2

electrode(SME, E s0.658 VySHE). All potentialsSME

are reported against this electrode. The auxiliary elec-trode was a platinum wire. The electrolyte was anaqueous solution of H SO pH 3yK SO 0.1 M. The2 4 2 4

cell was deoxygenated before and during measurementsby argon. Impedance spectra were recorded between10 and 10 Hz using a Perkin Elmer PAR 263A6 y3

potentiostatygalvanostat connected to a Solartron 1260frequency response analyzer. The amplitude of the super-imposed sinusoidal potential signal was 10 mV.The sensitization of the films was conducted with

ruthenium 535(Solaronix) a bipyridile ruthenium deriv-ative, in dry ethanol for 24 h in the dark. Dark-red filmswere obtained. For the measurement of theV as aoc

function of the wI xywI x ratio in solution, the initialy2

electrolyte solution was a KI 0.2 M pH 3 solution inwater, and additions of I in the cell were followed by2

measuring theV vs. the potential of the auxiliaryoc

electrode.

3. Results and discussion

3.1. Cyclic voltammetry

Fig. 2 shows the reduction of the different films. Avery small cathodic peak is apparent approximatelyy0.5 V at pH 3, which increased significantly if asecond scan going at anodic potential(1 V) wasperformed(not shown). Because of the large specificsurface of the nanocrystalline electrodes, the surfacestates show clearly on the electrodes of TiO . The2

reduction of a nanocrystalline film of titania entails the

injection of electrons, these charge carriers can betrapped in a energy level localized under the conductionband. It has been established that this level correspondedto the energy level of Ti , that is one of the identified3q

surface states of the titaniaw18,22x. This cathodic peakwas observed to increase after prolonged oxidation ofthe films at anodic potentials above 1 V. At morenegative potential this peak is followed by a second oneat y0.8 V and by another large peak(Efy1.2 V) asthe film was becoming black. For some experiments(not shown) this large peak could be decomposed intotwo distinct peaks nearly located aty1.2 andy1.6 V,respectively. It has been shown by spectroelectrochem-istry that these peaks corresponded to the reduction ofthe surface Ti into Ti w23x. Finally, at the most4q 3q

negative potential, approximatelyy1.8 V, the reductionof the solvent takes place. This peak corresponded tothe reduction of the Ti in the bulk probably accom-4q

panied by insertion of hydrogen in the materialsw24,25x.When the potential was reversed the reoxidation of thesolvent and of the film was observed. At the mostpositive potential, 0.7 V, the electronic surface states,reduced aty0.5 V, were reoxidized just before thesolvent. No major differences were observed betweendifferent films composed of different phases with similarparticle sizes((A9), (B17) and(F9)) or of same phasewith different morphology((Ru5) and(F9)) suggestingthat the phase is not the main parameter influencing thesurface electrochemical behavior, but rather details ofthe film processing: difference between different prepa-rations of the same phase was just as significant.Possibly the electronic behavior of the outer layer ofthe particle is not completely determined by the corestructure one, as would be the case if that outer layerwas somewhat amorphous.

3.2. Double layer capacitance

The Nyquist diagram for titania particles at pH 3(notshown) exhibits an evolution of the curves with thepotentials. At the less negative potentials where no peakwas visible on the cyclic voltammetry a semicircle wasobserved, whereas when the potential corresponded tothe reduction of the electronic surface states a differentcurve appeared. These curves were well-modelized by aclassical equivalent circuit(the double layer capacitanceC is in parallel with the polarization resistanceRd p

themselves in series with the non-compensated resis-tance R ) without taking into account effects of thee

diffusion. The values of the double layer capacitanceC as a function of the potential could be calculated atd

high frequencies. Capacitances have been compared forthe three polymorphs of titania in Fig. 3 for threefrequencies(11.7, 1.08 and 204 Hz). No significantvariation between the different phases was observed.However, it was interesting to notice that whatever the


Fig. 4. Mott–Schottky diagram for thin films of anatase(A9), rutile(Ru4) and rookite(B17).

Fig. 3. Double layer capacitance of thin film of titania in H SO pH 3yK SO 0.1 M as a function of the potential.(a) Brookite (B17), (b)2 4 2 4

anatase(A9) (c) rutile (Ru4).

crystalline phase of titania, an abrupt and strong increaseof the capacitance(for 100–1000 times) is visible at apotential ofy0.8 V corresponding to the reduction ofthe surface titanium as it has been seen on cyclicvoltammetry. The capacitance of the doped tin oxide(F-SnO) alone has also been reported on the graph,2

and it is readily apparent that before the reduction ofthe electronic surface states the behavior of titania filmsand SnO were similar. It can be assumed that the2

observed limiting phenomenon corresponds to SnO .2

This result confirms that only a capacitive current wasmeasured on cyclic voltammetry. The interpretation fol-lows that proposed earlier by Zaban et al.w26x that asthe film is reduced, it becomes conducting and electro-chemically active. Initially, only SnO(and TiO close2 2

to it) can participate in electrochemical reactions, asattested(i) by the value of the capacitance, close fromthe Helmoltz capacitance of a flat SnO film and(ii)2

by the Mott–Schottky type behavior of the capacitancegiving a free carrier concentration in the film corre-sponding to that of tin oxide(f10 ) and a flat band21

potential of tin oxide close fromy0.8 V (Fig. 4) was

deduced. As can be seen in Fig. 3, all the films displaythe same characteristics in the anodic potential rangegiving a strong support to the hypothesis that the filmare not electrically active in this potential range. By


comparison of the SnO and titania films it is possible2

to have an idea of the active surface of the TiO films.2

Indeed, if one supposes that SnO and TiO after2 2

reduction are sufficiently conductive, the capacitancescorrespond essentially to the Helmoltz capacitances,therefore the ratio of the capacitances of TiO and2

SnO gives the ratio of the surfaces. For the brookite,2

this ratio being approximately 1500, the active surfacecan be estimated toS s190 cm . This result is in2

a

agreement with the generally accepted value of theporosity (50%) of a titania thin film of thicknessapproximately 15mm and particles size of 5 nmw27x.Moreover, the charge passed for the film reduction atthe y0.8 V wave would correspond roughly to thereduction of all surface Ti atoms.The influence of the frequency has also been studied.

The increase of the capacitance with the decrease of thefrequency indicates that the capacitance is related to theelectronic surface states of the titania since these specieshave a relatively slow reaction rate: when the frequencyis too high, the electrochemical reaction cannot takeplace within a cycle and the capacitance response islowered.

3.3. Energetics

From the experiments above, the interface energeticscan be tentatively inferred. Both capacitance and voltam-metric measurements suggest that the respective posi-tions of the SnO and TiO conduction bands are not2 2

the same before and after surface reduction as thecharging of the TiO induces a shift in its Fermi level.2

Fig. 5 presents the energetics diagram. In the ‘oxidized’state, as the SnO is highly doped(to the point of2

degeneracy), its conduction band(given by the flat bandpotential) must be close to the equilibrium Fermi levelE , i.e. aty0.8 V. This is also the position for whichf

the TiO film starts getting reduced, and this level2

corresponding to electronic surface states is thereforepinning E in the oxidized film. Upon reduction elec-f

tronic surface states are reduced, which causes anEfincrease with respect to the TiO conduction band in2

the particles. Interestingly, during reduction the energet-ics of the film is ‘scanned’ as all the levels of TiO are2

being reduced in turn. If the voltammetry is conductedslowly enough, the current is proportional to the numberof states reduced, i.e. to the DOS at the given potential.Since the standard hydrogen electrode(SHE) can beplaced on the absolute electron energy scale(y4.5 eVwith electron in vacuum as reference), the DOS can beplaced on the electron energy scale. If moreover thisscanning is done irrespective of the distance betweenthe particle and the electrode, the TiO DOS can be2

inferred directly from either voltammetry or capaci-tance–voltage measurements.

3.4. Photosensitization

In the following step, interface energetics was probedin dye-sensitized cells for different TiO phases. More-2

over, various statements can be found in the literatureregarding factors affecting the photovoltage(V ), andoc

for instance low values ofwI xywI x are sometimesy y3

regarded as preferable.Two models have been proposed:(i) that a low value

of the ratio would tend to increase the interfacial electricfield and therefore increase the photovoltagew28x and(ii) that a high value of the ratio increases the energydifference betweenE and the HOMO of the dyeredox

which would unavoidably hamper the photovoltage bythe same amountw29x unless under illumination theHOMO of the dye shifts to higher energies with respectto the redox level. Alternatively, one could suggest thatthe photopotential is essentially limited by the recom-bination of the electrons injected in TiO with either the2

dye cation(D ) or the iodine, and that the dye cationq

regeneration depends on the I concentration. Moreover,y

one could argue that a fast D regeneration, being theq

expression of a quasi equilibrium between the redoxcouple with the holes trapped on the D specie, thisq

imposes a shift of the dye and TiO levels with respect2

to E . This model regards highwI xywI x as prefer-y yredox 3

able for theV . We note first that the expression foroc

the redox level with respect to the iodideyiodine con-centrations is(Eq. (1))

yw xI3kTEsE8q Ln . (1)y 3w x2q I

Therefore, previous experiments such as in Ref.w30x atconstantwI xywI x have not been carried out at constanty y

3

redox level position but rather for decreasingE .redox

Experimental results for TiO films are reported in2

Fig. 5, and show a very characteristic behavior: at lowI concentrations,V is insensitive to iodine, but2 oc

decreases strongly after a certain threshold in concentra-tion. Clearly, low wI xywI x are detrimental toV , andy y

3 oc

the first model is inadequate. A good fit to the experi-mental values can be obtained using the second model,with some simple assumptions, as explained below. Thephotopotential of the cell is that for which the generationcurrent equals the recombination current, i.e. whenJ sJ . In such cells, provided the illumination inten-gen rec

sity is not too large, generation of photocarriers isproportional to the absorbable photon flux:J sKF,gen

whereF is the photon flux. The recombination currentcan proceed via either of the two paths(Eqs. (2) and(3)) (Fig. 6):

y y ye q1y2I ´3y2I (2)3

y qe qD ´D8 (3)


Fig. 5. Energetics diagram for a titania film on F-SnO substrate2

before(a) and after(b) surface reduction with CB: conduction band,VB: valence band andE : Fermi level.f

Fig. 6. Open circuit voltage of a thin film of anatase(A10), rutile(Ru4) and brookite(B17) sensitized with ruthenium 535 in solutionI 0.2 M pH 3y0-I -0.01 M: reproducibility and comparison withy

2

the EPFL anatase.

With K andK being, respectively, the kinetic constant1 2

of Eqs.(2) and(3). Therefore, the recombination currentcan be expressed as(Eq. (4))

y m qw x w xJ sK n I qK n D (4)rec 1 3 2

wheren is the electron concentration in TiO andm is2

the reaction order. From the Fermi distribution,n canbe obtained as(Eq. (5))

B EE yEŽ .fn cC FnsN expcD GkT

B EE yE qE yE 8qE 8yEŽ .fn redox redox c c cC FsN expcD GkT

(5)

whereN is the conduction band DOS andE its positionc c

(E 8 is the conduction band position in the dark, thec

position of the redox level being taken as a reference)E is the quasi Fermi level of electrons in TiO underfn 2

illumination. By definition,qV sE yE , moreover,oc fn redox

we make the assumption that the conduction band shiftunder illumination is proportional toV (Eq. (6)):oc

B E1C FE 8yE s y1 qV (6)c c ocD GA

Thus,n can be expressed as(Eq. (7))

B EqVocC FnsN exp (7)cD GAkT

and thereforeV can be expressed as(Eq. (8))oc

B EAkT KFC FV s Ln (8)oc my q) ) ) )D Gq K N 8 I qK N 8D1 c 3 2 c

This expression tells us that at low I concentration, the2

V is independent of the I concentration while aboveoc 2

a certain threshold for I ,V will decrease logarithmi-2 oc

cally with the iodine concentration.

4. Conclusion

Nanometric particles of anatase, brookite and rutilewere synthesized in aqueous medium in order to com-pare the electronic properties of these materials forphotovoltaic devices. No significant difference wasobserved between these phases by cyclic voltammetry.The surface and consequently the electrode preparationseemed to be the main parameter. Although the exposedfaces were not the same, and then the coordination ofthe titanium atoms was different(more or less chargedby hydroxylation), the conditions of preparation andespecially the deposition of the sol on the substrate werethe determinant factors. Measurement of the doublelayer capacitance has allowed to show the film activationinitially insulating and a strong frequency dependenceproving one more time that it was the electronic surfacestates, which were implied. Finally, after dye sensitiza-tion the V has been measured as a function of theoc

concentration of the couple IyI and it has beeny2

observed that at low I concentration, theV was2 oc

independent of this concentration while above a certainthreshold for I ,V decreased logarithmically with the2 oc

iodine concentration showing a change in the dominantrecombination mechanism at a critical concentration ofI in the millimolar range.2


Acknowledgments

The authors express their most sincere gratitude to B.O’Regan for helping us with some useful experimentaltips.

References

w1x T. Ohzuku, T. Hirai, Electrochim. Acta 27(1982) 1263.w2x I. Sopyan, S. Murasawa, K. Hashimoto, A. Fujishima, Chem.

Lett. 8 (1994) 723.w3x T. Fuyuki, T. Kobayashi, H. Matsumami, J. Electrochem. Soc.

135 (1988) 248.w4x L. Kavan, K. Kratochvilova, M. Gratzel, J. Electroanal. Chem.¨

394 (1995) 93.w5x B. O’Regan, M. Gratzel, Nature 353(1991) 737.¨w6x G.K. Boschloo, A. Goossens, J. Schoonman, J. Electrochem.

Soc. 144(4) (1997) 1311.w7x L. Kavan, M. Gratzel, Electrochem. Acta 40(1995) 643.¨w8x L. Kavan, T. Stoto, M. Gratzel, D. Fitzmaurice, V. Shklover,¨

J. Phys. Chem. 97(1993) 9493.w9x Y. Lei, L.D. Zhang, J.C. Fan, Chem. Phys. Lett. 338(2001)

231.w10x G.K. Boschloo, D. Fitzmaurice, J. Phys. Chem. B 103(1999)

7860.w11x L. Kavan, M. Gratzel, J. Rathousky, A. Zukal, J. Electrochem.¨

Soc. 143(2) (1996) 394.w12x A. Pottier, S. Cassaignon, C. Chaneac, F. Vilain, E. Tronc, J.P.´

Jolivet, J. Mater. Chem. 13(2003) 877.w13x A. Pottier, C. Chaneac, E. Tronc, L. Mazerolles, J.P. Jolivet, J.´

Mater. Chem. 11(2001) 1116.

w14x M. Koelsch, S. Cassaignon, J.P. Jolivet, in progress.w15x K.J. Jiang, T. Kitamura, H. Yin, S. Ito, S. Yanagida, Chem.

Lett. (2002) 872.w16x A. Zaban, S.T. Aruna, S. Tirosh, B.A. Gregg, Y. Mastai, J.

Phys. Chem. B 104(2000) 1104.w17x C.J. Barbe, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann,´

V. Shklover, M. Gratzel, J. Am. Ceram. Soc. 80(12) (1997)3157.

w18x S. Doherty, D. Fitzmaurice, J. Phys. Chem. 100(1996) 10732.w19x M. Koelsch, S. Cassaignon, J.F. Guillemoles, J.P. Jolivet, Thin

Solid Films 403–404(2002) 312.w20x K. Hara, T. Horigushi, T. Kinoshita, K. Sayama, H. Arakawa,

Solar Energy Mater. Solar Cells 70(2) (2001) 151.w21x J.S. Salafsky, W.H. Lubberhuizen, E. van Fassen, R.E.I.

Schropp, J. Phys. Chem. B 102(1998) 766.w22x A. Hagfeld, M. Gratzel, Chem. Rev. 95(1995) 49.¨w23x M. Koelsch, S. Cassaignon, J.F. Guillemoles, S.A. Bilmes, J.P.

Jolivet, in progress.w24x P. Clechet, C. Martele, R. Olier, J.P. Thomas, M. Falavier, J.

Electrochem. Soc. 130(8) (1983) 1795.w25x M.F. Weber, L.C. Chumacher, M.J. Dignam, J. Electrochem.

Soc. 129(9) (1982) 2022.w26x A. Zaban, A. Meier, B. Gregg, J. Phys. Chem. B 101(1997)

7985.w27x D. Cahen, G. Hodes, M. Gratzel, J.F. Guillemoles, I. Ries, J.¨

Phys. Chem. B 104(2000) 2053.w28x K. Schwarzburg, F. Willig, J. Phys. Chem. B 103(1999) 5743.w29x G. Smestad, Ph.D. Thesis, EPFL, Lausanne, 1998.w30x Y. Liu, Hagfeldt, X.R. Xiao, S.E. Lindquist, Solar Energy

Mater. Solar Cells 55(1998) 267.

m. koelsch 2004

Documents