DSpace at VNU: Electrochemical grafting of TiO2-based photo-anodes and its effect in dye-sensitized solar cells

Electrochemical grafting of TiO 2 -based photo-anodes and its effect indye-sensitized solar cells a Department of Science, Systems and Models, Roskilde University, DK-4000, Denmark b Fac

Electrochemical grafting of TiO 2 -based photo-anodes and its effect in

dye-sensitized solar cells

a

Department of Science, Systems and Models, Roskilde University, DK-4000, Denmark

b Faculty of Chemistry, University of Science, Vietnam National University — Ho Chi Minh City, Vietnam

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 30 April 2015

Received in revised form 17 October 2015

Accepted 21 October 2015

Available online 23 October 2015

Keywords:

Electrochemical grafting

Mesoporous metal oxides

Reduction of diazonium salts

Dye-sensitized solar cell

Back-electron-transfer processes

We demonstrate that hydroxyl-groups which are located on the surfaces of mesoporous metal oxides (in partic-ular sintered layers of F-doped tin oxide (FTO) and TiO2on glass plates) are capable of undergoing reactions with 4-nitrobenzene radicals The highly reactive benzene radicals are generated by the electrochemical reduction of 4-nitrobenzenediazonium tetrafluoroborate in acetonitrile We found that the grafting surfaces were chemically inert to strong acids and bases The grafted surfaces were characterized and analyzed by cyclic voltammetry (CV), attenuated total reflectance Fourier transform infrared spectroscopy (ATR–FTIR), X-ray photo electron spectros-copy (XPS), scanning electron microsspectros-copy (SEM), and energy-dispersive X-ray (EDX) Implementation of electro-chemically grafted TiO2particles as photo-anodes in dye-sensitized solar cells (DSCs) showed that the grafted surface is capable of suppressing the undesired back-electron-transfer processes in dye-sensitized solar cells

© 2015 Elsevier B.V All rights reserved

1 Introduction

Dye-sensitized solar cells (DSCs) as well as its solid state version

(SSDSC) are low cost alternatives to traditional silicon solar cells[1–5]

So far however, the light-to-energy efficiencies of DSCs and SSDSC are

too low in order to rival with existing silicon solar cells One strategy

to increase the efficiency of DSCs is based on preventing the dark

current of the cell The dark current is caused by spontaneous loose of

electrons from the FTO and the TiO2surface The electron leaking causes

the undesired reduction of the dye (S) in its oxidized form (S+) as well

as of the redox mediator/hole conductor[6] One promising strategy to

prevent these undesired back-electron transfers would be to cover both

the FTO and the TiO2surfaces with an electrical isolating layer on The

concept of blocking nanometer thick isolation layers on FTO and TiO2

nanoparticles have earlier been evaluated by spray pyrolysis[7],

elec-trodepositions[8], atomic layer deposition (ALD)[9], TiCl4treatment

[10,11]as well as by the deposition of inorganic oxides such as Al2O3

[12–14]and silica[15] TiO2photo-anodes have also been modified by

adsorption of steric-demanding organic molecules that are

non-covalently bound to the surface including phosphonate esters[16],

phosphinate amphiphiles[17,18]and poly(methylsiloxane)[19], and

sensitizing dyes with build-in steric constrains such as the organic dye

D35[1,20]

Here we describe our investigation of generating blocking layers on

FTO│TiO2DSC photo-anodes by means of electrochemical reductions of

diazonium salts This methodology has been investigated and found to

be excellent for the preparation of insulating layers on pure metal and carbon electrodes[21,22] In contrast, the electrochemical grafting of metal oxides surfaces like TiO2[23]and SnO2[24]with aryl diazonium salts have yet only been sparsely reported and, to the best of our knowl-edge, they have so far neither been designed nor applied in thefield of DSCs Recently, Ceccato et al reported the electrochemical coating of gold and carbon surfaces[22]using 4-nitrophenyldiazonium salts The authors found that this type of coating is leading to oligomeric and branched nitrophenyl strings (coined as oligomer brushes) on the metal surface rather than a mono-molecular layer of 4-nitrophenyl frag-ments Therefore, we expected to observe a similar behavior on a TiO2 surface as proposed inScheme 1

The grafting is initiated by a dissociative electron transfer from the TiO2 electrode to an aryl diazonium salt such as 4-nitrobenzenediazonium tetrafluoroborate In this initial step a 4-nitrobenzene radical and elemental nitrogen are formed Supported

by recent XPS results of Mahmoud et al.[25], we suggest that the

initial-ly formed 4-nitrobenzene radical abstracts an H-atom from one of the hydroxyl groups located on the TiO2surface The generated titanium-bound oxygen radicals combine with the excess of 4-nitrobenzene radicals by which 4-nitrobenzene becomes covalently bound to the TiO2surface through an oxygen–carbon bond When multiple CV scans are applied, the excess of produced free 4-nitrobenzene radicals will start to attack the TiO2-bound 4-nitrobenzene and oligomeric aryl brushes are expected to be formed Very recently Charlton et al have pro-posed a similar mechanism for the grafting of indium doped tin oxide by the reduction of di(4-nitrophenyl) iodonium tetrafluoroborate[26]

⁎ Corresponding author.

E-mail address: tlund@ruc.dk (T Lund).

http://dx.doi.org/10.1016/j.jelechem.2015.10.021

Contents lists available atScienceDirect

Journal of Electroanalytical Chemistry

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / j e a c

In this paper we report the electrochemical grafting of FTO and

FTO│TiO2electrodes as applied in DSCs as well as the analysis of

elec-trode surfaces after grafting with the help of ATR–FTIR, XPS, SEM, and

EDX methodologies In addition we report a CV method that allowed

us to measure the insulation effect of grafting using ferrocene and

1,4-dicyano-naphtalene as sensitive one-electron probes[27] Finally we

re-port primary results regarding the effect of grafting on the performance

of DSCs prepared with ferrocene/ferrocenium as the redox mediator

2 Experimental section

2.1 Reagents and materials

Acetonitrile (HPLC grade) was obtained from Lab scan and used as

received Tetrabutylammonium tetrafluoroborate (Bu4NBF4) was

synthesized using standard procedures 4-Nitrobenzenediazonium

tet-rafluoroborate was synthesized from nitroaniline according to

proce-dures published elsewhere[28] The synthesized diazonium salt was

purified by dissolution in acetonitrile followed by precipitation with

diethyl ether Afterfiltration and drying in vacuum, the product was

stored at−18 °C 1,4-Dicyanonaphthalene was synthesized according

to the protocol of Heiss et al.[29] Ferrocene, elemental iodine, lithium

iodine, tetrabutylammonium iodide, 4-tert-butylpyridine, and

3-methoxy-propionitrile (3-MPN) were obtained from

commercially-available sources and used as received Transparent (Ti-Nanooxide,

HT/SP), opaque titania paste (Ti-Nanooxide D/SP), Pt-catalyst T/SP as

well as transparent and opaque titania kit electrodes with a surface

area of 0.36 cm2were obtained from Solaronix, Aubonne, Switzerland

Transparent electrodes (Solaronic catalog number 74,111) (TP) consist

of three layers of the active dye absorption titania, and the total

thick-ness of the 3 layers is about 10μm The opaque titania electrodes

(Solaronix catalog number 74,101) (OP) consist of three layers of active

titania and one layer of reflective titania The thickness of the active

layer is about 10μm and the thickness of the reflective layer is about

three micrometer[30] Electrical isolation tape (Kapton HN Polyimide

film, Dupont, thickness 50 μm) was obtained from RS components, Denmark DSC electrolyte A was comprised of a mixture of 0.05 M I2, 0.1 M LiI, 0.6 M tetrabutylammonium iodide and 0.5 M 4-tert-butylpyridine in 3-MPN DSC electrolyte B was comprised of 0.13 M fer-rocene, 0.013 M ferrocenium tetrafluoroborate, 0.2 M Bu4NBF4,and 0.5 M 4-tert-butylpyridine in propylene carbonate

2.2 Preparation of electrodes For the grafting experiments both the commercial available trans-parent (TP) and the opaque (OP) square-shaped titania kit electrodes were applied together with proprietary electrodes prepared according

to the following protocol: FTO electrodes (2 × 3 cm) were cut from conducting glass sheets (TCO22-15) purchased from Solaronix, Aubonne, Switzerland The glass plates were cleaned with ethanol and dried at 100 °C TiO2-based electrodes with a circular surface area of 0.38 cm2(d = 6.1 mm) were prepared by doctor blading of either the transparent or the opaque titania paste from Solaronix Note: The doctor blading must be performed onto the conducting side of the FTO glass pieces The plates were sintered for 30 min at 450 °C In order to avoid unwanted electrochemistry within the“naked” FTO area all electrodes were covered carefully with Kapton polyimide tape except for the active FTO and TiO2surface electrode located in the middle of the electrode with a circular surface of 0.36 cm2

2.3 Fabrication of DSCs The dye-sensitized solar cells were constructed by two electrodes prepared from Solaronix glass substrates (TCO22-15) coated with F-doped SnO2(FTO) A counter electrode was formed on the opposite glass plate prepared by spreading Pt-catalyst T/SP by doctor blading The counter electrode was sintered 30 min at 450 °C A one or two-layered TiO2photo-electrode was made from Ti-Nanoxide HT or/and Ti-Nanoxide D pastes in a doctor blading fashion Afterwards the elec-trode was sintered for 30 min at 450 °C In some cases, the cells were fabricated with Solaronix kits which were complete sets of electrodes including ready-to-use titania photo-anodes and platinum counter elec-trodes The kits were re-sintered at 450 °C for 15 min before used The surface modification of the FTO│TiO2photo-anode was performed by the electrochemical method before dye loading as described vide infra The dye solution (0.5 mM) of N719 or D5 was prepared in absolute eth-anol The two glass substrates were assembled into a cell using the poly-meric sealant Surlyn® 1707 (Dupont™) Finally, the cell was filled with either the electrolyte (A) or (B), as described above, and thefilling holes were sealed with Surlyn®

2.4 Cyclic voltammetry Cyclic voltammograms were recorded with a standard three-electrode electrochemical setup comprised of a VersaSTAT 3F potentiostat from Princeton Applied Research, a FTO or FTO│TiO2 work-ing electrode, a Ag/AgCl pseudo reference electrode and a platinum wire

as counter electrode

2.5 Electrochemical surface coating The surface of the photoanodes was coated in a solution of 4-nitrobenzene diazonium (10 mM) and tetrabutylammonium tetra fluo-roborate (0.10 M) in acetonitrile CV scans (1–27) were performed in the potential interval 0 to−0.8 V with a sweep rate of 20 mV/s Grafting experiments were performed both with and without careful protection

of the voltammetry cell from day-light by aluminum foil The effect of the surface coating was evaluated by comparing the performances in cyclic voltammetry experiments between non-grafted and grafted electrodes in solutions of either ferrocene (0.5 mM) or 1,4-dicyanonaphtalene (1 mM) in acetonitrile each containing additionally

Scheme 1 Proposed general mechanism for the grafting of nanometer-sized TiO 2 particles

(illustrated by the square symbol) by the electrochemical reduction of aryl diazonium salts

such as 4-nitrobenzenediazonium tetrafluoroborate Bottom: Example for a possible

olig-omeric 4-nitrophenyl fragment on the TiO 2 surface which are expected to be generated

after multiple CV scans.

Bu4NBF4(0.1 M) All solutions were carefully purged with nitrogen prior

to CV experiments

2.6 ATR–FTIR, XPS, SEM and EDX analysis

ATR–FTIR spectra were obtained with a Perkin Elmer FTIR

spectrom-eter 2000 equipped with a PIKE MLRacle ATR accessory The XPS

analy-sis was obtained using a Kratos Axis Ultra-DLD spectrometer (Kratos

Analytical Ltd., Manchester, UK) The analyzer was operated in the

con-stant analyzer energy (CAE) mode at a pass energy = 160 eV for the

sur-vey spectra and a pass energy = 20 eV for high-resolution spectra of the

elements of interest Mono chromatic Al Kα X-ray at power = 150 W

with an analysis area = 300 × 700μm2was used Charge compensation

was achieved using an electronflood gun The binding energy (BE) =

285.0 eV for C–C/C–H components of C1s peak was used as reference

for charge correction Spectral processing was carried out using the

computer software CasaXPS (v 2.3.15) provided by Casa Software Ltd

(Teignmouth, UK)

SEM and EDX analysis were performed with afield emission Zeiss

XB-1540 Scanning Electron Microscope (Carl Zeiss GmbH, Oberkochen,

Germany) equipped with an energy-dispersive X-ray spectroscopy

sys-tem (Oxford Instrumentation, Oxfordshire, UK) which was used for

ele-mental identification and mapping

2.7 Dye-soaking experiments

Solaronix transparent kit electrodes (TP) after treatment with

defined numbers of CV scans in a diazonium salt solution were kept

overnight in a dye bath of N719 (0.5 mM) in methanol The dye was

desorbed by treatment with 0.1 M NaOH (1 ml) The extract was

acidi-fied by 20 μl of formic acid and analyzed by HPLC-UV/Vis according to a

previously described analysis protocol[31,32]

3 Results and discussion

3.1 Electrochemical grafting

Fig 1reveals the efficiency of electrochemical surface coating of a FTO

electrode by electrochemical reduction of 4-nitrobenzenediazonium

tetrafluoroborate the 4-nitrobenzene radical in acetonitrile In the first

forward CV scan a large and broad diazonium salt reduction signal is

observed followed by an irreversible reverse scan explainable by the

fast elimination of nitrogen under the radical formation (Scheme 1)

As visible inScheme 1, already at the second scan the current is clearly

reduced indicating a generation of an insulation layer on the FTO

elec-trode This layer is presumably generated by the formation of covalent

C–O bond formations between the 4-nitrobenzene radical and the

surface-bonded hydroxyl groups of tindioxide Impressively, only after

five scans the reduction signal at −0.4 V of the diazonium salt is vanished Similar observations has been reported by Maldonado et al who found that a FTO surface may be completely covered byfive CV scans in solutions (1 mM) of para substituted aryl diazonium salts[24]

Fig 2shows cyclic voltammetry experiments with a FTO electrode in

a ferrocene solution before and after the grafting process A nice revers-ible ferrocene signal is observed at the non-coated FTO electrode whereas the ferrocene signal is almost completely eliminated after grafting (5 CV scans)

The transparent kit electrode (TP) consists of a FTO coated glass support with a sintered transparent layer of approximately 20 nm sized TiO2particles The opaque electrode (OP) is prepared similar to the TP electrode, however with an additional diffraction layer of approx-imately 300 nm sized TiO2particles on top of a transparent layer.Fig 3

shows seven selected scans of the TP electrode in a solution of 4-nitrobenzenediazonium tetrafluoroborate (10 mM) in acetonitrile The voltammetry cell was carefully protected against light by aluminum foil In thefirst CV scan the reduction peak of the diazonium salt is ob-served at−0.4 V vs Ag/Ag+ After 15–20 scans the reduction peak moves to more negative values This clearly indicates that the heteroge-neous electron transfer from the FTO and the transparent TiO2electrode surfaces to the diazonium salt is reduced First after about 24–27 scans the CV signal of the diazonium salt is completely vanished and the ear-lier colorless and transparent TiO2layer has become a red-brownish and opaque appearance Similar observations were made with the grafting

of the OP electrode However, in this case only the transparent layer is grafted whereas the top diffraction layer is visually unaffected The TP electrode currents are 20–30 times higher compared to the FTO currents (seeFig 1) and require also 5–7 times more CV scans than the FTO elec-trode in order to produce a full coverage of the elecelec-trode with an organic isolation layer One possible explanation is that the surface area of the mesoporous layer of transparent nano-crystallinic TiO2particles is about 1000 times higher than the geometrical area[33] In other words, this TiO2-layer needs a much higher reduction charge to cover compared to the FTO electrode With the exception of a slightly larger reduction current of the TP electrode, the TP as well as the OP electrodes behaves identical Apparently, the grafting of the top and transparent TiO2layer has resulted in a decreased pore size and porosity, which in return has decreased the diffusion of the diazonium salt into the opaque TiO2and the active FTO electrode surface underneath

The number of mole diazonium salt which is reduced at the elec-trode pr cm2in each of the individual CV scans may be calculated according to the Faraday Electrolysis law Q/FA, where Q = number of charge exchanged in each CV scan, F = Faradays number, and A = elec-trode surface InFig 4, Q/FA, is plotted as a function of the scan number The plot shows a linear decrease of Q/FA from 3 × 10−7to 0 mol/cm2 after about 23 CV scans with a total charge exchange ∑Q/FA of

Fig 1 Cyclic voltammograms (1–5 scans) of 4-nitrobenzenediazonium tetrafluoroborate

2

Fig 2 Voltammograms of ferrocene at a FTO electrode (0.36 cm 2

) before and after grafting

by 5 CV scans (20 mV/s) in nitrophenyldiazonium tetrafluoroborate (10 mM) in

≈3.5 × 10−6mol/cm2 Given that the grafting mechanism inScheme 1

is correct this result corresponds to a maximum grafting 4-nitrophenyl

layer concentration = 1.7 × 10−6 mol/cm2 on the transparent

electrode

The maximum number of available OH groups (Γmax) on the surface

of commercial Degussa TiO2powder had previously been measured to

beΓmax= 3 · 10−6mol/m2corresponding to 1.54 · 10−4mol/g TiO2

(BET = 51.4 m2/g)[34] Given that the titan dioxide of the transparent

electrode has the sameΓmaxas the Degussa powder, then the total

num-ber of OH groups on a photoanode with a 10μm thick active TiO2layer

(ρanatase= 2.89 g/cm3, porosity≈50%) and a geometrical area of 1 cm2

may be calculated to approximate 2.9 · 10−7mol As seen fromFig 4

approximately 5–6 times more nitrobenzene groups were attached to

the TiO2surface compared to the number of OH groups, which clearly

indicates that the initial layer of 4-nitrobenzene units resulted in a

layer of branched oligomeric 4-nitrophenyl brushes

The grafting process was affected by ambient day-light We

ob-served that the number of CV scans needed to fully coat the surface of

the TiO2electrode was less when the CV cell was carefully protected

against light An explanation for this observation is that diazonium

salts may be cleaved by UV–visible light under formation of an aryl

cat-ion and nitrogen[35]followed by coupling of the of the arene cation

with the OH groups on the TiO2surface Some of the grafting of the

TiO2surface in the CV experiments without light protection is therefore

likely to occur by photolytic initiated grafting

The effect of grafting of the OP electrode was checked by the

ferro-cene test and the results are shown inFig 5 A nicely reversible

ferro-cene CV was obtained with the non-grafted OP electrode, whereas the

ferrocene signal completely vanished after surface coating withinfive

CV scans

The n-type semiconductor TiO2electrode is not able to accept elec-trons from a redox mediator like ferrocene with a redox potential be-tween the conduction band and the valence band[27,36,37] The voltammetry currents observed inFig 5are therefore exclusively due

to the reduction/oxidation reactions in the environment of the FTO layer The TiO2of the OP electrode is electrochemically“silent” in the scanning interval of Fig 5 The ferrocene test shows that pre-treatment of the FTO surface withfive CV scans in a 10 mM diazonium salt solution is able to produce an electrical isolation layer which pre-vents electron exchange between the FTO electrode and the ferrocene mediator

According to the theory of semiconductor electrochemistry the n-type semiconductor should be able to function as a normal metal elec-trode with the potential more negative than the conduction band

(DCN) has a standard potential (E°DCN/DCN− = −1.2 V vs NHE) which is more negative than the conduction band edge of TiO2 (Ecb=−0.9 V vs NHE)[2].Therefore it should be possible to obtain a reversible CV of DCN at a TiO2electrode.Fig 6shows reasonable revers-ible CVs (without IR compensation) of DCN obtained with both a FTO and an untreated OP electrode of similar size (0.36 cm2) The charge ex-changed at the OP electrode is much larger than at the FTO electrode which demonstrate that a substantial part of the DCN redox reactions (DCN + e−= DCN−.) of the OP electrode are taking place at the TiO2

Fig 3 Cyclic voltammograms in the absence of light of 4-nitrobenzenediazonium

tetraflu-oroborate (10 mM) in acetonitrile at a TP kit electrode (0.36 cm 2

) Scan number 1, 5, 9, 13,

17, 19 and 25 is shown Scan rate = 20 mV/s.

Fig 4 Mole electrons exchanges pr TP electrode (geometric) area as a function of CV scan

Fig 5 Cyclic voltammograms of ferrocene (0.5 mM) at an opaque Solaronix kit electrode (OP see text) shown before and after diazonium salt grafting with three and five CV scans Scan rate = 20 mV/s.

Fig 6 CV of 1,4-dicyanonaphtalene (1 mM) at a FTO electrode (blue), a Solaronix opaque two layer FTO│TiO 2 electrode (black) and after grafting with six CV scans in a solution of 4-nitrobenzenediazonium tetrafluoroborate (10 mM) in acetonitrile (red curve) (For in-terpretation of the references to color in this figure legend, the reader is referred to the

surface After the OP electrode has been surface coated with six CV scans

the CV signal of DCN is clearly reduced as seen by the red CV curve of

Fig 6

3.2 Surface analysis with the help of ATR–FTIR, XPS, SEM, and EDX

methodologies

Fig 7shows the spectrum of the transparent (TP) electrode which

was surface grafted by 27 CV scans in the diazonium salt acetonitrile

solution The grafting of 4-nitrobenzene groups is demonstrated by

the observation of the strong N_O IR absorptions at 1536 cm−1and

1350 cm−1at frequencies with are close to the characteristic IR signals

of nitrobenzene The IR absorptions of a grafted FTO layer are naturally weaker (typically 10 times) due to the much thinner light absorption pathway in comparison with the microporous structure of the TP electrode

Fig 8A shows an XPS survey spectrum of a transparent TP electrode surface grafted withfive CV scans High-resolution scans of Ti, C and N regions are shown inFig 8B, C, and D, respectively The experimental atomic C/N ratio = 9/1 is slightly higher than the calculated 6/1 ratio for a 4-nitrobenzene layer The analysis of the Ti region shown in

Fig 8B shows that the Ti is present mainly as TiO2, as indicated by the peaks for Ti(2p1/2) at 464.4 eV and Ti(2p3/2) at 458.4 eV A peak for Ti bounded directly to carbon at 454.6 eV[38]was not observed The C(1 s) peak ofFig 8C was analyzed by deconvolution into two distinct peaks centered at 284.8 eV and the other at 286.1 eV The 284.8 eV peak corresponds to C(1 s) of sp2-hybridized carbon in aromatic ring

of 4-nitrophenyl group and the other at 286.1 eV represents a phenyl carbon atom attached to either nitrogen or oxygen atoms[39] In accor-dance with similarfindings of Mahmoud et al.[25], the absence of a C–Ti peak at ca 281.5 eV (C 1 s) inFig 8B and the absence of a peak at ca 454.6 eV (Ti2p3/2) inFig 8C, confirm that the benzene ring is not

direct-ly attached to the titanium atom The coupling of the 4-nitrobenzene radical is therefore most likely through a Ti–O–C bond as indicated in

Scheme 1 The N1s XPS spectrum (Fig 8D) shows two main peaks at

400 and 406 eV corresponding to the N1s binding energies of a azo (N_N) and a nitro (NO2) group, respectively[40] The formation of the azo group is most likely due to an electrophilic aromatic substitution between the 4-nitrobenzene diazonium salt and the TiO2-bound nitro-benzene group as proposed inScheme 1 This type of azo formations has been observed earlier in the electro grafting of carbon electrodes

by nitrobenzene diazonium salts[22] The formation of azo functionali-ties in the grafting layers might explain why electrodes become brownish-yellow colored after more than ten CV scans

Fig 7 ATR–FTIR spectrum of the TP electrode treated with 27 CV scans in a 10 mM

4-nitrobenzene diazonium tetrafluoroborate solution before heating (blue) and after

heating (red) of the electrode for 15 min at 450 °C (For interpretation of the references

to color in this figure legend, the reader is referred to the web version of this article.)

Fig 8 (A) Survey XPS spectra of a the surface of a transparent Solaronix kit FTO│TiO 2 electrode after grafting withfive CV scans in a solution of 4-nitrobenzenediazonium tetrafluoroborate (10 mM) in acetonitrile (B) High-resolution spectrum in the Ti region showing peaks correspond to Ti(2p 1/2 ) at 464.4 eV and Ti(2p 3/2 ) at 458.4 eV (C) High-resolution C1s envelope (blue) containing a C–C/C–H hydrocarbon peak at 284.8 eV (black), a peak at 286.1 eV attributed to C–N/C–O (red) and a peak at 291.4 eV (green) assigned as a shake-up satellite π → π* transition

in the phenyl ring.(D) High-resolution spectrum of the nitrogen region showing an azo N(1 s) peak at 399.9 eV and a nitro N(1 s) peak at 406 eV [38] (For interpretation of the references

figure legend, the reader is referred to the web version of this article.)

In order to further sustain ourfindings we performed scanning

elec-tron microscopy (SEM) from plain transparent kit electrodes and from

CV-grafted transparent kit electrodes As shown inFig 9, the surface

coated electrode appears more dark indicating that the TiO2surface

has been covered by an isolating layer of organic material This is also

in harmony with energy-dispersive X-ray (EDX) analysis (Fig 10) that

clearly shows that the degree of carbon increases with the number of

CV scans, while the degree of titanium decreases

3.3 Stability of the grafting layer

When a grafted TP electrode (after 27 CV) is heated to 450 °C for

15 min the color of the electrode changes from red-brownish to carbon

black The IR spectrum of electrode after heating are missing the

charac-teristic nitro IR absorption bands at 1341 and 1523 cm−1while two new

bands with lower intensity at 1379 and 1644 cm−1appear Obviously, thermal treatment reduces the amount of material grafted to surface and clearly changes its chemical composition Despite that, the ferro-cene test, however, reveals that thermal treated electrodes are still ef fi-ciently blocking the ferrocene CV signal at the FTO surface Exposing electrochemically grafted TP electrodes to strong bases and acids in water, methanol or DMSO, did not remove the yellow grafted layer indi-cating the existence of a strong covalent attachment of 4-nitrobenzene units to the TiO2-surface

3.4 Dye absorption on surface grafted electrodes

TP electrodes treated with between 0 and 16 CV scans were exposed

16 h to a solution of the ruthenium dye N719 (0.5 mM) in methanol Af-terwards the excess of N719 was then extracted from the TiO surface

Fig 9 (a) SEM micrographs of a none-grafted and (b) of diazonium salt grafted TP electrode (seven CV scans).

by under mild basic conditions, specifically with a solution of aqueous

NaOH (0.1 M) followed by analysis of the extract by HPLC.Table 1

shows the correlation between N719 adsorption on the TP electrode

as a function of the number of CV scans The amount of N719 which

can be adsorbed at the electrodes gradually decreases as a function of

the number of CV scans After 16 scans the dye binding capacity of the

TiO2surface is reduced by ca 40% Our explanation for this observation

is that the progressive attachment of 4-nitrobenzene to the surface

de-creases the number of available hydroxyl groups for binding the dye

(Scheme 1) The grafting–dye-soaking procedure has also been tested

in a reversed manner However, this reversed procedure resulted in

changing the red color of the electrode surface into a slightly yellow

one, indicating that the N719 dye has chemically altered under the

grafting conditions In terms of chemistry this is hardly surprising,

since the grafting process is accomplished with the generation of highly

reactive 4-nitrobenzene radicals that are likely attacking the aromatic

region of the adsorbed ruthenium complex

After more than 27 CV scans the TP electrodes became dark yellow/

red and the electrode is unable to absorb N719 in a dye-soaking

exper-iment Apparently all the OH groups on the TiO2surface have been

completely covered by nitrophenyl groups with no space for the N719

attachment

3.5 DSC measurements

Table 2shows the open-circuit voltage (Voc) and the short current

(Jsc) of DSCs with electrochemical grafted FTO│TiO2photo-anodes and

irradiated with a green 550 nm LED lamp light source

When I−/I3 −is applied as a redox mediator the performance of the

DSC is not significantly improved by the surface grafting This result

was expected, because it is known that the dark current of DSCs based

on this iodide mediator is very low[41] Therefore further reduction of the dark current by surface grafting does not improve the DSC perfor-mance However, when ferrocene/ferrocenium is used as a redox medi-ator in a DSC prepared with a none-coated FTO│TiO2photo-anode both parameters Vocand Jscare almost zero This indicates that the rate of the back-electron transfer to the ferrocenium equals the rate electron injec-tion resulting in a zero output of the DSC Thisfinding is in agreement with previously reported observations made in particular by Feldt et al

[19] Surface grafting of the FTO│TiO2electrode with 1–3 scans of an electrical isolation layer of nitrobenzene groups increases the Vocfrom

0 V to 400–600 mV depending on the nature of the applied dye Unfortu-nately the Jscis a factor of 100 lower than a typical DSC functioning with

I−/I3 −as mediator Surface coating with 5–10 CV scans, however, reduced the Vocfrom the maximum value The grafting was performed in day-light and after 10 CV scans the electrode was fully surface grafted

On the other hand TiO2surface coating by silylation[19]increased the Vocfrom 0 to 500 mV with Jsh≈ 1 mA/cm2

at standard AM1.5 irra-diation conditions However, this still a 10 times lower current density compared to a typical iodide electrolyte based DSCs The low current density was explained by encapsulation of N719 into an organic silylation layer, which in return decreased the rate of regeneration of the dye[19] To this end we have no explanation why the Jscvalues of our ferrocene/ferrocenium based DSCs are even lower relative to the

Fig 10 EDX analysis of a none-grafted TP electrode (green curve) and of a grafted TP electrode (red curve) after seven CV scans (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

Table 1

Amount of N719 which can be attached on a high transparency TP electrode as a function

of the number of CV scans in a 4-nitrobenzene tetrafluoroborate solution (10 mM,

acetonitrile).

Number of CV scans [Diazonium]/mM Γ N719 x 10 8 mol/cm 2 % of 0 scan

Table 2 Comparison of V oc and J sc values obtained from DSCs with integrated with electrochemical grafted TiO 2 layers a

Photoanode CV scans Dye Mediator V oc /mV J sc /μA/cm 2

FTO│TiO 2 (HT) b

FTO│TiO 2 (HT) 3 d

FTO│TiO 2 (HT) 10 d

N719 Fc/Fc +

FTO│TiO 2 (D) c

FTO│TiO 2 (D) 2 (FTO) e

FTO│TiO 2 (D) 2 d

FTO│TiO 2 (D) 2 (FTO) e

+ 2 d

a

Green LED light source; λ = 550 nm Light intensity ≈ 8 mJ/cm 2

.

b

The FTO electrode was covered with one layer of highly transparent Ti-Nanooxide T/

SP paste.

c

FTO electrode covered with one layer of opaque Ti-Nanooxide D paste.

d The electrode was surface grafted by 2 CV scans in 10 mM diazonium salt solution The

CV cell was not protected against light.

e

FTO electrode was grafted first with two scans, followed by coating the electrode with the opaque Ti-Nanooxide D paste, and sintering at 450 °C for 30 min.

f + = Ferrocene (0.13 M) / ferrocenium tetrafluoroborate (0.013 M).

DSC prepared with the iodide mediator The best result was obtained

with the organic dye D5 and ferrocene as mediator Here the surface

the FTO electrode was graftedfirst, followed by addition of a TiO2

layer, sintering, andfinally grafting the assembled electrode by two CV

scans The CV grafting results shown inFigs 1 and 3suggest that the

ap-plication of relative few CV scans (b3) primarily covers the FTO layer of

the photo-anode The main increase in Vocis therefore likely due to the

formation of a blocking layer on the FTO part of the photo electrode

4 Conclusion

In conclusion we report a straightforward electrochemical

method-ology to reduce the dark currents of DSCs by grafting an insulating

organic layer onto the surfaces of FTO and FTO│TiO2electrodes The

ef-ficiency of the blocking layer was demonstrated with the aid of a reliable

ferrocene CV test in which the reversible ferrocene signal disappears

after the electrochemical reduction process Surface analysis of FTO

and FTO│TiO2electrodes by means of XPS, ATR–FTIR, SEM and EDX

sup-port the grafting mechanism as proposed inScheme 1 DSCs prepared

with none-coated FTO│TiO2electrodes show zero performance with

Voc= 0 V when the ferrocene/ferrocenium is applied as mediator

Surface grafting of the electrodes with 2–3 CV scans in a solution of

4-nitrobenzene diazonium tetrafluoroborate (10 mM, acetonitrile)

in-creases the Vocfrom 0 to 600 mV depending on the dye The increase

in Vocdemonstrates clearly that the dark currents of the DSCs have

been reduced One the other side, the observed short currents Jshof

the investigated DSCs are approximately 100 times smaller compared

to traditional DSCs At this stage we do not have a plausible explanation

for this undesired effect Therefore work is in progress in order to

iden-tify the underlying problems This includes in particular the systematic

investigation of the DSC performance as a function of the thickness of

the isolation layer, and its chemical nature

Acknowledgments

We thank the Vietnam National University in Ho Chi Minh for a

re-search grant given to P.T Nguyen (HS2014-18-01) We thank PhD

Kyoko Shimizu from the Aarhus University in Denmark (Chemistry

De-partment) for the recording and analysis of XPS spectra, We like to

thank PhD Erik Wisaeus from the Technological Institute in Taastrup,

Denmark (Nano- and Microtechnology Production), for recording SEM

micrographs and performing EDX analysis as well as his kind efforts in

discussing the results

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