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

Journal of Electroanalytical Chemistry 758 (2015) 85–92 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jeac Electrochemical grafting of TiO2-based photo-anodes and its effect in dye-sensitized solar cells Torben Lund a,⁎, Phuong Tuyet Nguyen a,b, Thomas Ruhland a a b Department of Science, Systems and Models, Roskilde University, DK-4000, Denmark Faculty of Chemistry, University of Science, Vietnam National University — Ho Chi Minh City, Vietnam 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 a b s t r a c t We demonstrate that hydroxyl-groups which are located on the surfaces of mesoporous metal oxides (in particular sintered layers of F-doped tin oxide (FTO) and TiO2 on 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 spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) Implementation of electrochemically grafted TiO2 particles 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 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 TiO2 surface 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 TiO2 surfaces 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], electrodepositions [8], atomic layer deposition (ALD) [9], TiCl4 treatment [10,11] as well as by the deposition of inorganic oxides such as Al2O3 [12–14] and silica [15] TiO2 photo-anodes have also been modified by adsorption of steric-demanding organic molecules that are noncovalently 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│TiO2 DSC photo-anodes by means of electrochemical reductions of ⁎ Corresponding author E-mail address: tlund@ruc.dk (T Lund) http://dx.doi.org/10.1016/j.jelechem.2015.10.021 1572-6657/© 2015 Elsevier B.V All rights reserved 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 knowledge, they have so far neither been designed nor applied in the field 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 fragments Therefore, we expected to observe a similar behavior on a TiO2 surface as proposed in Scheme The grafting is initiated by a dissociative electron transfer from the TiO2 electrode to an aryl diazonium salt such as 4nitrobenzenediazonium tetrafluoroborate In this initial step a 4nitrobenzene radical and elemental nitrogen are formed Supported by recent XPS results of Mahmoud et al [25], we suggest that the initially formed 4-nitrobenzene radical abstracts an H-atom from one of the hydroxyl groups located on the TiO2 surface The generated titaniumbound oxygen radicals combine with the excess of 4-nitrobenzene radicals by which 4-nitrobenzene becomes covalently bound to the TiO2 surface 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 proposed a similar mechanism for the grafting of indium doped tin oxide by the reduction of di(4-nitrophenyl) iodonium tetrafluoroborate [26] 86 T Lund et al / Journal of Electroanalytical Chemistry 758 (2015) 85–92 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-tertbutylpyridine in 3-MPN DSC electrolyte B was comprised of 0.13 M ferrocene, 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 transparent (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 × 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 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 TiO2 surface electrode located in the middle of the electrode with a circular surface of 0.36 cm2 Scheme Proposed general mechanism for the grafting of nanometer-sized TiO2 particles (illustrated by the square symbol) by the electrochemical reduction of aryl diazonium salts such as 4-nitrobenzenediazonium tetrafluoroborate Bottom: Example for a possible oligomeric 4-nitrophenyl fragment on the TiO2 surface which are expected to be generated after multiple CV scans In this paper we report the electrochemical grafting of FTO and FTO│TiO2 electrodes as applied in DSCs as well as the analysis of electrode 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 1,4dicyano-naphtalene as sensitive one-electron probes [27] Finally we report primary results regarding the effect of grafting on the performance of DSCs prepared with ferrocene/ferrocenium as the redox mediator 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 tetrafluoroborate was synthesized from nitroaniline according to procedures published elsewhere [28] The synthesized diazonium salt was purified by dissolution in acetonitrile followed by precipitation with diethyl ether After filtration 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 3methoxy-propionitrile (3-MPN) were obtained from commerciallyavailable 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 cm2 were 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 thickness of the layers is about 10 μm 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 10 μm and the thickness of the reflective layer is about three micrometer [30] Electrical isolation tape (Kapton HN Polyimide 2.3 Fabrication of DSCs The dye-sensitized solar cells were constructed by two electrodes prepared from Solaronix glass substrates (TCO22-15) coated with Fdoped 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 at 450 °C A one or twolayered TiO2 photo-electrode was made from Ti-Nanoxide HT or/and Ti-Nanoxide D pastes in a doctor blading fashion Afterwards the electrode was sintered for 30 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 electrodes The kits were re-sintered at 450 °C for 15 before used The surface modification of the FTO│TiO2 photo-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 ethanol The two glass substrates were assembled into a cell using the polymeric sealant Surlyn® 1707 (Dupont™) Finally, the cell was filled with either the electrolyte (A) or (B), as described above, and the filling holes were sealed with Surlyn® 2.4 Cyclic voltammetry Cyclic voltammograms were recorded with a standard threeelectrode electrochemical setup comprised of a VersaSTAT 3F potentiostat from Princeton Applied Research, a FTO or FTO│TiO2 working 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 4nitrobenzene diazonium (10 mM) and tetrabutylammonium tetrafluoroborate (0.10 M) in acetonitrile CV scans (1–27) were performed in the potential interval 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 1,4dicyanonaphtalene (1 mM) in acetonitrile each containing additionally T Lund et al / Journal of Electroanalytical Chemistry 758 (2015) 85–92 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 spectrometer 2000 equipped with a PIKE MLRacle ATR accessory The XPS analysis was obtained using a Kratos Axis Ultra-DLD spectrometer (Kratos Analytical Ltd., Manchester, UK) The analyzer was operated in the constant analyzer energy (CAE) mode at a pass energy = 160 eV for the survey 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 × 700 μm2 was used Charge compensation was achieved using an electron flood 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 a field emission Zeiss XB-1540 Scanning Electron Microscope (Carl Zeiss GmbH, Oberkochen, Germany) equipped with an energy-dispersive X-ray spectroscopy system (Oxford Instrumentation, Oxfordshire, UK) which was used for elemental 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 acidified by 20 μl of formic acid and analyzed by HPLC-UV/Vis according to a previously described analysis protocol [31,32] Results and discussion 3.1 Electrochemical grafting Fig reveals 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 in Scheme 1, already at the second scan the current is clearly reduced indicating a generation of an insulation layer on the FTO electrode 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 Fig Cyclic voltammograms (1–5 scans) of 4-nitrobenzenediazonium tetrafluoroborate (10 mM, acetonitrile) at a FTO electrode with surface area of 0.36 cm2 87 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 by five CV scans in solutions (1 mM) of para substituted aryl diazonium salts [24] Fig shows cyclic voltammetry experiments with a FTO electrode in a ferrocene solution before and after the grafting process A nice reversible 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 TiO2 particles The opaque electrode (OP) is prepared similar to the TP electrode, however with an additional diffraction layer of approximately 300 nm sized TiO2 particles on top of a transparent layer Fig shows seven selected scans of the TP electrode in a solution of 4nitrobenzenediazonium tetrafluoroborate (10 mM) in acetonitrile The voltammetry cell was carefully protected against light by aluminum foil In the first CV scan the reduction peak of the diazonium salt is observed at − 0.4 V vs Ag/Ag+ After 15–20 scans the reduction peak moves to more negative values This clearly indicates that the heterogeneous electron transfer from the FTO and the transparent TiO2 electrode 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 earlier colorless and transparent TiO2 layer 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 (see Fig 1) and require also 5–7 times more CV scans than the FTO electrode in order to produce a full coverage of the electrode with an organic isolation layer One possible explanation is that the surface area of the mesoporous layer of transparent nano-crystallinic TiO2 particles 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 TiO2 layer has resulted in a decreased pore size and porosity, which in return has decreased the diffusion of the diazonium salt into the opaque TiO2 and the active FTO electrode surface underneath The number of mole diazonium salt which is reduced at the electrode pr cm2 in 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 = electrode surface In Fig 4, Q/FA, is plotted as a function of the scan number The plot shows a linear decrease of Q/FA from × 10−7 to mol/cm2 after about 23 CV scans with a total charge exchange ∑ Q/FA of Fig Voltammograms of ferrocene at a FTO electrode (0.36 cm2) before and after grafting by CV scans (20 mV/s) in nitrophenyldiazonium tetrafluoroborate (10 mM) in acetonitrile 88 T Lund et al / Journal of Electroanalytical Chemistry 758 (2015) 85–92 Fig Cyclic voltammograms in the absence of light of 4-nitrobenzenediazonium tetrafluoroborate (10 mM) in acetonitrile at a TP kit electrode (0.36 cm2) Scan number 1, 5, 9, 13, 17, 19 and 25 is shown Scan rate = 20 mV/s Fig 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 ≈3.5 × 10−6 mol/cm2 Given that the grafting mechanism in Scheme 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 TiO2 powder had previously been measured to be Γmax = · 10−6 mol/m2 corresponding to 1.54 · 10−4 mol/g TiO2 (BET = 51.4 m2/g) [34] Given that the titan dioxide of the transparent electrode has the same Γmax as the Degussa powder, then the total number of OH groups on a photoanode with a 10 μm thick active TiO2 layer (ρanatase = 2.89 g/cm3, porosity ≈50%) and a geometrical area of cm2 may be calculated to approximate 2.9 · 10−7 mol As seen from Fig approximately 5–6 times more nitrobenzene groups were attached to the TiO2 surface 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 observed that the number of CV scans needed to fully coat the surface of the TiO2 electrode 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 cation and nitrogen [35] followed by coupling of the of the arene cation with the OH groups on the TiO2 surface Some of the grafting of the TiO2 surface 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 ferrocene test and the results are shown in Fig A nicely reversible ferrocene CV was obtained with the non-grafted OP electrode, whereas the ferrocene signal completely vanished after surface coating within five CV scans The n-type semiconductor TiO2 electrode is not able to accept electrons from a redox mediator like ferrocene with a redox potential between the conduction band and the valence band [27,36,37] The voltammetry currents observed in Fig are therefore exclusively due to the reduction/oxidation reactions in the environment of the FTO layer The TiO2 of the OP electrode is electrochemically “silent” in the scanning interval of Fig The ferrocene test shows that pretreatment of the FTO surface with five CV scans in a 10 mM diazonium salt solution is able to produce an electrical isolation layer which prevents electron exchange between the FTO electrode and the ferrocene mediator According to the theory of semiconductor electrochemistry the ntype semiconductor should be able to function as a normal metal electrode with the potential more negative than the conduction band edge [36,37] The aromatic redox mediator 1,4-dicyanonaphtalene (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 TiO2 electrode Fig shows reasonable reversible CVs (without IR compensation) of DCN obtained with both a FTO and an untreated OP electrode of similar size (0.36 cm2) The charge exchanged 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 Mole electrons exchanges pr TP electrode (geometric) area as a function of CV scan number in 10 mM diazonium salt solution Fig CV of 1,4-dicyanonaphtalene (1 mM) at a FTO electrode (blue), a Solaronix opaque two layer FTO│TiO2 electrode (black) and after grafting with six CV scans in a solution of 4-nitrobenzenediazonium tetrafluoroborate (10 mM) in acetonitrile (red curve) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) T Lund et al / Journal of Electroanalytical Chemistry 758 (2015) 85–92 Fig ATR–FTIR spectrum of the TP electrode treated with 27 CV scans in a 10 mM 4nitrobenzene diazonium tetrafluoroborate solution before heating (blue) and after heating (red) of the electrode for 15 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.) 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 3.2 Surface analysis with the help of ATR–FTIR, XPS, SEM, and EDX methodologies Fig shows 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−1 and 89 1350 cm−1 at 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 with five CV scans High-resolution scans of Ti, C and N regions are shown in Fig 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 of Fig 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 accordance with similar findings of Mahmoud et al [25], the absence of a C–Ti peak at ca 281.5 eV (C s) in Fig 8B and the absence of a peak at ca 454.6 eV (Ti2p3/2) in Fig 8C, confirm that the benzene ring is not directly 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 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 nitrobenzene group as proposed in Scheme 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 functionalities in the grafting layers might explain why electrodes become brownish-yellow colored after more than ten CV scans Fig (A) Survey XPS spectra of a the surface of a transparent Solaronix kit FTO│TiO2 electrode after grafting with five 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(2p1/2) at 464.4 eV and Ti(2p3/2) 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 to color in this figure legend, the reader is referred to the web version of this article.) 90 T Lund et al / Journal of Electroanalytical Chemistry 758 (2015) 85–92 Fig (a) SEM micrographs of a none-grafted and (b) of diazonium salt grafted TP electrode (seven CV scans) In order to further sustain our findings we performed scanning electron microscopy (SEM) from plain transparent kit electrodes and from CV-grafted transparent kit electrodes As shown in Fig 9, the surface coated electrode appears more dark indicating that the TiO2 surface 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 bands with lower intensity at 1379 and 1644 cm−1 appear Obviously, thermal treatment reduces the amount of material grafted to surface and clearly changes its chemical composition Despite that, the ferrocene test, however, reveals that thermal treated electrodes are still efficiently 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 indicating the existence of a strong covalent attachment of 4-nitrobenzene units to the TiO2-surface 3.3 Stability of the grafting layer 3.4 Dye absorption on surface grafted electrodes When a grafted TP electrode (after 27 CV) is heated to 450 °C for 15 the color of the electrode changes from red-brownish to carbon black The IR spectrum of electrode after heating are missing the characteristic nitro IR absorption bands at 1341 and 1523 cm−1 while two new TP electrodes treated with between and 16 CV scans were exposed 16 h to a solution of the ruthenium dye N719 (0.5 mM) in methanol Afterwards the excess of N719 was then extracted from the TiO2 surface T Lund et al / Journal of Electroanalytical Chemistry 758 (2015) 85–92 91 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 this figure legend, the reader is referred to the web version of this article.) by under mild basic conditions, specifically with a solution of aqueous NaOH (0.1 M) followed by analysis of the extract by HPLC Table 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 TiO2 surface is reduced by ca 40% Our explanation for this observation is that the progressive attachment of 4-nitrobenzene to the surface decreases 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 experiment Apparently all the OH groups on the TiO2 surface have been completely covered by nitrophenyl groups with no space for the N719 attachment on this iodide mediator is very low [41] Therefore further reduction of the dark current by surface grafting does not improve the DSC performance However, when ferrocene/ferrocenium is used as a redox mediator in a DSC prepared with a none-coated FTO│TiO2 photo-anode both parameters Voc and Jsc are almost zero This indicates that the rate of the back-electron transfer to the ferrocenium equals the rate electron injection resulting in a zero output of the DSC This finding is in agreement with previously reported observations made in particular by Feldt et al [19] Surface grafting of the FTO│TiO2 electrode with 1–3 scans of an electrical isolation layer of nitrobenzene groups increases the Voc from V to 400–600 mV depending on the nature of the applied dye Unfortunately the Jsc is a factor of 100 lower than a typical DSC functioning with I−/I− as mediator Surface coating with 5–10 CV scans, however, reduced the Voc from the maximum value The grafting was performed in daylight and after 10 CV scans the electrode was fully surface grafted On the other hand TiO2 surface coating by silylation [19] increased the Voc from to 500 mV with Jsh ≈ mA/cm2 at standard AM1.5 irradiation 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 Jsc values of our ferrocene/ferrocenium based DSCs are even lower relative to the 3.5 DSC measurements Table shows the open-circuit voltage (Voc) and the short current (Jsc) of DSCs with electrochemical grafted FTO│TiO2 photo-anodes and irradiated with a green 550 nm LED lamp light source When I−/I− 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 Table 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 ΓN719 x 108mol/cm2 % of scan 1 16 – 10 10 10 10 10 10 4.1 4.1 4.0 3.6 3.9 3.1 2.5 1.6 100 101 98 89 94 75 78 58 Table Comparison of Voc and Jsc values obtained from DSCs with integrated with electrochemical grafted TiO2 layers.a Photoanode CV scans b FTO│TiO2 (HT) FTO│TiO2 (HT) 3d FTO│TiO2 (HT) FTO│TiO2 (HT) 2d FTO│TiO2 (HT) 5d FTO│TiO2 (HT) 10d FTO│TiO2 (D)c FTO│TiO2 (D) (FTO)e FTO│TiO2 (D) 2d FTO│TiO2 (D) (FTO)e + 2d a Dye N719 N719 N719 N719 N719 N719 D5 D5 D5 D5 Mediator − I /I− I−/I3 +f Fc/Fc Fc/Fc+ Fc/Fc+ Fc/Fc+ Fc/Fc+ Fc/Fc+ Fc/Fc+ Fc/Fc+ Voc/mV Jsc/μA/cm2 720 672 568 179 94 200 155 236 5840 5370 40 23 50 44 78 Green LED light source; λ = 550 nm Light intensity ≈ mJ/cm2 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 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 f Fc/Fc+ = Ferrocene (0.13 M) / ferrocenium tetrafluoroborate (0.013 M) b 92 T Lund et al / Journal of Electroanalytical Chemistry 758 (2015) 85–92 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 grafted first, followed by addition of a TiO2 layer, sintering, and finally grafting the assembled electrode by two CV scans The CV grafting results shown in Figs and suggest that the application of relative few CV scans (b3) primarily covers the FTO layer of the photo-anode The main increase in Voc is therefore likely due to the formation of a blocking layer on the FTO part of the photo electrode Conclusion In conclusion we report a straightforward electrochemical methodology to reduce the dark currents of DSCs by grafting an insulating organic layer onto the surfaces of FTO and FTO│TiO2 electrodes The efficiency 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│TiO2 electrodes by means of XPS, ATR–FTIR, SEM and EDX support the grafting mechanism as proposed in Scheme DSCs prepared with none-coated FTO│TiO2 electrodes show zero performance with Voc = V when the ferrocene/ferrocenium is applied as mediator Surface grafting of the electrodes with 2–3 CV scans in a solution of 4nitrobenzene diazonium tetrafluoroborate (10 mM, acetonitrile) increases the Voc from to 600 mV depending on the dye The increase in Voc demonstrates clearly that the dark currents of the DSCs have been reduced One the other side, the observed short currents Jsh of the investigated DSCs are approximately 100 times smaller compared to traditional DSCs At this stage we not have a plausible explanation for this undesired effect Therefore work is in progress in order to identify the underlying problems This includes in particular the systematic investigation of the DSC performance as 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