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Versatile electrochromic displays based on TiO2 nanoporous films modified with triruthenium clusters
Electrochemistry Communications 8 (2006) 1628–1632 www.elsevier.com/locate/elecom
Versatile electrochromic displays based on TiO2 nanoporous films modified with triruthenium clusters Sergio H. Toma, Henrique E. Toma
*
Instituto de Quı´mica, Universidade de Sa˜o Paulo, C. Postal 26077, CEP 05513-970 Sa˜o Paulo, SP, Brazil Received 7 July 2006; received in revised form 30 July 2006; accepted 31 July 2006 Available online 30 August 2006
Abstract We report a new versatile electrochromic device based on nanocrystalline titanium dioxide films modified with triruthenium acetate clusters of the type [Ru3O(OOCCH3)6(py)2(BPEB)]PF6 (BPEB = 1,4-bis[2(4-pyridyl)etenyl]benzene). As evaluated from cyclic voltammetry and spectroelectrochemistry, the most relevant electrochromic process in this system is associated with the first reduction of the [Ru3O] cluster core. Typical electrochromic response is observed within a millisecond time scale, covering a wide range optical window (up to 1100 nm), and exhibiting excellent stability over a thousand cycles between 0.5 and 1.0 V. 2006 Elsevier B.V. All rights reserved. Keywords: Triruthenium clusters; Electrochromism; TiO2 films; Spectroelectrochemistry; Cyclic voltammetry
1. Introduction Trinuclear ruthenium acetate clusters of general formula [Ru3O(CH3COO)6(L)3]n (L = solvent or N-heterocyclic ligands) are versatile molecular species, exhibiting triangular structures in which the metal ions are strongly held together by l-oxo and carboxylate bridges, as well as by metal–metal bonds [1–3]. From the electrochemical point of view, the small [Ru3O] cluster center constitutes an attractive moiety, since it displays up to five reversible redox processes, ranging from 1.5 V to 2.5 V (vs. SHE). In fact, the electronic interaction among the ruthenium ions is so strong that the [Ru3O] central core is considered as fully delocalized, behaving as single center [1,4] which can be electronically and electrochemically modulated by the terminal and/or bridging ligands [5,6]. Moreover, each monoelectronic process is accompanied by drastic chromatic changes [7,8], providing a suitable combination of electronic stimuli and spectroscopic response for application in electrochromic devices. Relevant studies have been *
Corresponding author. Tel.: +55 11 30913887; fax: +55 11 38155579. E-mail address: [email protected] (H.E. Toma).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.07.043
carried out using these ruthenium clusters as building blocks for systems displaying electron transfer reactions and catalysis [9–11], as well as acceptors for photoinduced processes in supramolecular assemblies [12], NO release [13], and logic gates [14]. In spite of the increasing interest in electrochromic organic and polymeric materials for display applications [15], to the best of our knowledge, up to the present time, applications of such cluster species in electrochromic devices have not yet been addressed. Recently, electrochromic devices based on wide bandgap nanocrystalline semiconductors functionalized with charge-transfer compounds have been proposed in the literature [16,17]. We here report a new exciting possibility, based on a ruthenium cluster containing BPEB ancillary ligands as the electroactive species, adsorbed onto nanocrystalline TiO2 films. 2. Materials and methods All solvents and reactants were of analytical grade and employed without further purification. Degussa P25 TiO2, containing 80% anatase and 20% rutile, and average particle size of 30 nm, was used in this work. The starting
S.H. Toma, H.E. Toma / Electrochemistry Communications 8 (2006) 1628–1632
[Ru3O(CH 3COO)6(py)2(MeOH)](PF6) compound was obtained as reported in the literature [18]. Trans-1,4bis[2-(4-pyridyl)ethenyl]-benzene was prepared by the Heck method [19,20] and purified by recrystallization from ethanol–water mixture. Anal. Found: C, 83.4%; H, 5.8%; N, 9.7%. Required for C21H16N2 (MW = 284.36 g mol1): C, 84.5%; H, 5.7%; N, 9.6%. Observed [M + H]+ : m/z 285.12. 2.1. Synthesis of the [Ru3O(CH3COO)6(py)2(BPEB)] PF6 complex The [{Ru3O(CH3COO)6(py)2}2(BPEB)](PF6)2 complex was prepared by adapting a previously reported procedure for the related binuclear species [21]. Ru3O(CH3COO)6 (py)2(MeOH)](PF6) (132 mg; 0.13 mmol) and trans-1,4bis[2-(4-pyridyl)ethenyl]-benzene (112 mg; 0.39 mmol) ligand were dissolved in a minimum volume CH2Cl2. The mixture was stirred for 48 h and eventual solvent evaporation was compensated adding CH2Cl2 to the mixture. Ammonium hexafluorophosphate (260 mg; 1.6 mmol) dissolved in a minimum volume of methanol was added to the deep green solution resulting in a dark green precipitate which was collected on a filter, washed with methanol, water and diethyl ether and dried under vacuum. Anal. Found: C, 38.9%; H, 3.7%; N, 4.1%. Required for C42H44N4 O13PF6Ru3 Æ 2H2O (MW = 1297.0 g mol1): C, 38.9%; H, 3.7%; N, 4.3%. 2.2. Preparation of mesoporous TiO2 films coated with ruthenium cluster A viscous suspension of nanocrystalline TiO2 nanoparticles (P25-Degussa) was obtained by grinding 6 g TiO2 in 2 ml of nanopure water and 0.2 ml of acetylacetone, in a mortar with a pestle for about 40 min, followed by the slowly addition of 8.0 ml of distilled water and 0.1 ml of Triton X-100. The mesoporous TiO2 films were obtained by spreading this colloidal paste by doctor blade technique onto F-doped SnO2 conducting glass (sheet resistance 15 X/cm2 – TEC15, Pilkington) delimited by adhesive tapes (1 cm2 active area) [22]. After drying the film on air, the tapes were removed and the TiO2 particles fired in a furnace at 450 C C for 30 min. About 4 lm thick mesoporous TiO2 films (measured with a Dektak 3 profilometer) were obtained by this method. Functionalized films were prepared by immersing the mesoporous TiO2 films in a 104 mol dm3 alcoholic solution of [Ru3O(CH3COO)6(py)2(BPEB)]PF6 for 12 h. Strong adsorption has been observed in spite of the single positive charge of the complex. The modified films were thoroughly washed with ethanol and acetone, and stored under vacuum. 2.3. Assembly of the electrochromic devices The assembly of the devices was performed by sandwiching the working electrode coated with the TiO2 nano-
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particles and the triruthenium cluster with a transparent counter electrode (F-doped SnO2 conducting glass). To avoid short circuit between the electrodes, and moisture contamination of the device, a 100 lm layer of a thermo-sealing plastic was applied at the edges of the working electrode, but leaving small uncovered areas in order to introduce the electrolytic and mediator solution, composed by 0.1 mol dm3 of lithium trifluoromethanesulfonate and 0.05 mol dm3 ferrocene, respectively, in acetonitrile. The thermo-sealing plastic was then melted on a heating plate, and immediately clamped to ensure a tight sealing. After cooling, the electrolyte was introduced into the cell by immersing one of the small openings into the corresponding solution, and applying vacuum suction at the other hole, with special care to remove any air bubble. Finally, the holes were closed with melted sealing plastic. 2.4. Measurements UV-visible and visible spectra were recorded on a HP8453 diode array or a Guided Wave fiber optics spectrophotometer. Cyclic voltammetry was carried out with a Princeton Applied Research model 283 or an Autolab PGSTAT 30 potentiostat. A platinum disk electrode was employed for the measurements, using a conventional Luggin capillary arrangement in 0.100 mol dm3 tetrabutylammonium hexafluorophosphate (TBAPF6) acetonitrile solution. Ag/AgNO3 (0.010 mol dm3 in acetonitrile containing 0.100 mol dm3 TBAPF6) was used as reference electrode. A platinum wire was used as the auxiliary electrode. In the spectroelectrochemical measurements, a three electrode system with a gold minigrid transparent working electrode, mounted inside a conventional quartz cell with restricted internal optical path length (0.025 cm) was used. All the E1/2 values presented here were converted to SHE by adding 0.503 V to the experimentally obtained values. 3. Results and discussion 3.1. Electrochromic behavior in solution The features of the electrochromic processes for the triruthenium acetate cluster {[Ru3O(OOCCH3)6(py)2(BPEB)]} PF6 were studied in solution by cyclic voltammetry and electronic spectroscopy. The electronic spectrum of the complex is dominated by the BPEB intra-ligand (IL) p p* transitions centered at 370 nm, the cluster-to-ligand charge transfer (CLCT) around 400 nm and the characteristic internal transitions (IC) of the [Ru3O] core between 600 and 800 nm. The assignment of the major bands of the complex was based on the spectroelectrochemical measurements and on comparisons with the dimeric cluster reported before [21]. In Fig. 1 it is shown the behavior of the electronic spectra recorded during the electrochemical processes related to a–e. The first process (a) corresponds to the first oxidation of the triruthenium core [Ru3O]+1/+2 at 1.16 V (vs. SHE)
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Fig. 1. Cyclic voltammetry and spectroelectrochemical of the {[Ru3O(OOCCH3)6(py)2(BPEB)]}PF6 complex.
which leads to the split of the IC band. This process also affects the 370 nm band, in which the contributions of the IL and CLCT transitions are simultaneously present. As the oxidation of the complex proceeds, the CLCT band decays leaving the IL band at 385 nm. The reduction process [Ru3O]+1/0 (b) at 0.09 V leads to a bathochromic shift of the IC band from 680 nm to 910 nm, reflecting an increase of the energy of the electronic levels of the metallic cluster. As expected, the CLCT transition also undergoes a red shift from 400 to 510 nm. The next step (c), at 1.19 V, can be attributed to the second reduction of the [Ru3O]0/1 unit. In analogy to the preceding process, a red shift of the IC band to 980 nm is also observed, due to the rise of the cluster energy levels. Curiously, this process is accompanied by a decrease of the IL band at 385 nm, while a new band appears at 750 nm. This behavior seems to be associated with the electronic delocalization of the complex, since the addition of a second electron in the triruthenium core is strongly influencing the electronic levels of the BPEB ligand, leading to the decrease of the IL band and the rising of a second band ascribed to the radical BPEBp ! p* at 750 nm [21]. The fourth and fifth redox processes (d) and (e) correspond to the first and second reduction of the BPEB ligand at 1.15 and 1.69 V (vs. SHE). The reduction of BPEB0/ is characterized by the partial decrease of the 351 nm IL band and the increase of another IL band at 750 nm ascribed to the radical BPEB. Finally, the second reduction of the BPEB/2 leads to the rise of a IL band at 312 nm and the decrease of the 351 and 750 nm bands. It also can be observed changes in the 400–700 nm region, where a new band appears at 510 nm, typical of fully reduced RuIIRuIIRuII species lacking the central O atom [7,23]. In fact, it is known that in the RuIIIRuIIIRuIIO species, the central oxygen atom plays an important role in stabilizing the Ru3O core, by means of a multicentric p(p)–d(p) bonding. However, as it is successively reduced, electrons are added to the dp-orbitals of the ruthenium
ions, such that they become completely filled in the RuIIRuIIRuIIO species. In this case, the interaction between the ruthenium ions and the central oxygen atom is no longer possible, prompting its release from the Ru3O core [7]. This process can be observed even for the RuIIIRuIIRuIIO species. Interestingly, the RuIIIRuIIRuII species generated in the subsequent chemical reaction exhibits a redox potential dramatically shifted to positive values. In other words, in the time scale of the spectroelectrochemical measurements, the electrochemically generated RuIIIRuIIRuIIO species loses the central oxygen atom yielding the intermediate species RuIIIRuIIRuII, which is immediately reduced to the RuIIRuIIRuII form, exhibiting an absorption band at 510 nm. This event has not been observed in the cyclic voltammograms, because of the short time scale involved in such measurements. 3.2. Electrochromic behavior on TiO2 films and device In the nanocrystalline TiO2 modified films, two electrochromic processes can be observed: (1) The electrochromism due to the reversible ions insertion/desertion, typically Li+, in the semiconductor network; [24] and (2) the color changes due to the electroactivity of the species anchored to semiconductor surface [25–27]. In the last case, if the redox potential of the electrochromophore is located above the liquid/solid interface of the semiconductor conduction band edge (CBE), electrons can flow reversibly from the oxide to the anchored molecules. In this situation, the semiconductor behaves as a conductor, allowing to the device a reversible coloration/discoloration process. Accordingly, the semiconductor should behave as a conductor, allowing to the device a reversible coloration/ discoloration process. If the redox potential of the electrochromophore is below the CBE a typical charge trap situation is originated, which blocks the re-oxidation of the reduced molecules on the surface. In addition, the mesoporous TiO2 films flat band potential (Vfb) can be modulated by the choice of solvent or adding H+, Li+ or Na+
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to the solution. When this happens, the Vfb shifts to more positive potential. For example, the Vfb for the TiO2 semiconductor in water is proton dependent, according to the relationship Vfb = (0.15 0.06 · pH) in V vs. SHE. In aprotic solvents like acetonitrile, Vfb is located below 1 V, but in the presence of Li+ it can be shifted to values around 0.1 V. In the case of the mesoporous TiO2 films modified with the triruthenium cluster, the electrochromic behavior is related to the redox process of the [Ru3O] core. In fact, Fig. 2 presents the cyclic voltammograms of the [Ru3O(CH3COO)6(py)2(BPEB)]PF6 in solution and anchored onto TiO2 surface, obtained in DMF solution with 0.1 mol dm3 Li(CF3SO3). Analogously to the behavior in solution, a reversible electrochromic process is observed. It is important to note that in the control experiments, in which the bare TiO2 was submitted to voltammetric cycles, no color changes due to Li+ intercalation was observed up to 1.6 V vs. SHE. In Fig. 3 it is shown the spectroelectrochemistry and chronoamperometry of the electrochromic device between 0.5 and 1.0 V. As one can see, the spectral changes in this potential range are related to the first reduction process of the Ru3O core in the {[Ru3O(OOCCH3)6(py)2(BPEB)]} PF6 complex, showing a decrease of the IC band at 680 nm and the rise of the CLCT and IC bands at 522 and 918 nm, respectively. The chronoamperometric behavior recorded, successively, after 5 s intervals between 1.0 and +0.5 V, is shown in Fig. 3b. At 1.0 V the sharp initial electrochemical response is associated with the cluster reduction process onto the TiO2 surface, leading to a drastic color change. This process is followed by a fast current decrease, up to a steady state condition (parasitic current). In the electrochromic device, this process is concomitant with the oxidation of the ferrocene (Fc) to ferrocenium ion (Fc+) on the counter-electrode. This observation suggest that the steady-state currents arise from the continuous re-oxidation of the reduced clusters species, presumably
Fig. 3. (a) Spectroelectrochemistry and (b) chronoamperometry of the electrochromic device between 0.5 and 1.0 V.
by Fc+ formed at the counter electrode. In addition, the Fc+ can also react with TiO2 or SnO2 surface, increase the leakage current [28]. At +0.5 V, the reduced clusters are re-oxidized injecting electrons onto conduction band CB of the TiO2 electrode. This reaction is followed by the reduction of the Fc+ generated on the counter-electrode. At this point, a zero current flows should be expected, however a small leakage current was also observed. The leakage current in this step is due to the permeation of some Fc to the semiconductor electrode resulting in its oxidation to Fc+. In Fig. 4 it is shown the current response of the [Ru3O(CH3COO)6(py)2(BPEB)]PF6 electrochromic device at 692 and 920 nm. The coloration time, which is the time to produce 75% change in the optical density, was typically less than 0.5 s. The electrochemical and optical changes were shown to be stable up to a hundred cycles studied, showing that the electrochromic conversion is reversible. The coloration efficiency CE at 692 and 920 nm can be defined in terms of absorbance changes (DA) and charge involved (DQ) by CEðkÞ ¼
Fig. 2. Cyclic voltammograms of the [Ru3O(CH3COO)6(py)2(BPEB)] PF6 complex in DMF solution with 0.1 mol dm3 Li(CF3SO3), and adsorbed onto TiO2.
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DAðkÞ DQ
In our case, CE were 50 and 190 cm2 C1 at 692 and 920 nm respectively, which is quite high, if compared with TiO2 itself (8 cm2 C1 at 633 nm) and to other electrochromic transition metal oxides cited in the literature [29–33],
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Fig. 4. Spectral variations at 692 and 920 nm and chronoamperometric behavior of the [Ru3O(CH3COO)6(py)2(BPEB)]PF6 device.
like NiOx (40 cm2 C1), CoOx (50 cm2 C1), WO3 (70– 100 cm2 C1) and to the bis-(2-phosphonoethyl)-4,4 0 -bipyridinium dichloride based device (170 cm2 C1) [28]. More accurate comparisons could be made, as recently proposed in the literature [34] using coloration efficiencies (CCE) based tandem chronoabsorptometry/chronocoulometry measurements, considering as point of reference, the charge passed at 95% of the full optical switch. 4. Conclusions A new class of electrochromic devices can be generated using triruthenium acetate clusters of the type [Ru3O(OOCCH3)6(py)2(L)] PF6, directly applied onto nanocrystalline TiO2. In the case of the BPEB ruthenium cluster, a good color efficiency and reversibility is found in 0.5 to 1.0 V working potential ranges. These versatile, multicolor electrochromophores provide interesting options to be exploited in electrochromic devices, since their performance can be conveniently modulated by the donor/acceptor nature of the ancillary ligands [1]. Acknowledgements We are thank to the financial Brazilian agencies CNPq (Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico), FAPESP (Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo), RENAMI (Rede de Nanotecnologia Molecular e Interfaces) and IMMC (Instituto do Mileˆnio de Materiais Complexos), and to Degussa, for the generous supply of TiO2 P-25 nanoparticles. References [1] H.E. Toma, K. Araki, A.D.P. Alexiou, S. Nikolaou, S. Dovidauskas, Coord. Chem. Rev 187 (2001) 219–221.
[2] A.D.P. Alexiou, S. Dovidauskas, H.E. Toma, Quı´mica Nova 23 (2000) 785. [3] T.M. Busleva, S.N. Red’kina, O.V. Rudnitskaya, Russ. J. Coord. Chem. 25 (1999) 3. [4] R.C. Rocha, H.E. Toma, Quim. Nova 25 (2002) 624. [5] H.E. Toma, C.J. Cunha, C. Cipriano, Inorg. Chim. Acta 154 (1988) 63. [6] A.D.P. Alexiou, H.E. Toma, J. Chem. Research (S) (1993) 464. [7] H.E. Toma, C. Cipriano, J. Electroanal. Chem. 263 (1989) 313. [8] H.E. Toma, F. Matsumoto, C. Cipriano, J. Electroanal. Chem. 346 (1993) 261. [9] H.E. Toma, K. Araki, A.D.P. Alexiou, S. Nikolaou, S. Dovidauskas, Coord. Chem. Rev. 219 (2001) 187–234. [10] H. Winnischofer, V.Y. Otake, S. Dovidauskas, M. Nakamura, H.E. Toma, K. Araki, Electrochim. Acta 49 (2004) 3711– 3718. [11] G.S. Nunes, A.D.P. Alexiou, K. Araki, A.L.B. Formiga, R.C. Rocha, H.E. Toma, Eur. J. Inorg. Chem. (2006) 1487–1495. [12] M. Itou, M. Otake, Y. Araki, O. Ito, H. Kido, Inorg. Chem. 44 (2005) 1580–1587. [13] H.E. Toma, A.D.P. Alexiou, A.L.B. Formiga, M. Nakamura, S. Dovidauskas, M.N. Eberlin, D.M. Tomazela, Inorg. Chim. Acta 358 (2005) 2891–2899. [14] L.F.O. Furtado, A.D.P. Alexiou, L. Gonc¸alves, H.E. Toma, K. Araki, Angewandte Chemie Int. Ed. 45 (2006) 3143–3146. [15] R.J. Mortimer, A.L. Dyer, J.R. Reynolds, Displays 27 (2006) 2–18. [16] M. Biancardo, R. Argazzi, C.A. Bignozzi, Displays 27 (2006) 19–23. [17] A.F. Nogueira, S.H. Toma, M. Vidotti, A.L.B. Formiga, S.I.C. Torresi, H.E. Toma, New J. Chem. 29 (2005) 320–324. [18] J.A. Baumann, D.J. Salmon, S.T. Wilson, T.J. Meyer, W.E. Hatfield, Inorg. Chem. 17 (1978) 3342. [19] R.F. Heck, Org. React. 27 (1981) 345. [20] A.J. Amoroso, A.M.W.C. Thompson, J.P. Maher, J.A. McCleverty, M.D. Ward, Inorg. Chem. 34 (1995) 4828. [21] S.H. Toma, S. Nikolaou, D.M. Tomazela, M.N. Eberlin, H.E. Toma, Inorg Chim. Acta 357 (2004) 2253–2260. [22] F. Campus, P. Bonhoˆte, M. Gra¨tzel, S. Heinen, L. Walder, Solar Energy Material and Solar Cells 56 (1999) 281. [23] H.E. Toma, C.J. Cunha, Can. J. Chem. 67 (1989) 1632. [24] M.P. Canta˜o, J.I. Cisnero, R.M. Torresi, Thin Solid Films 259 (1995) 70–74. [25] R. Cinnsealach, G. Boschloo, S.N. Rao, D. Fitzmaurice, Solar Energy Materials and Solar Cells 57 (1999) 107–125. [26] D. Cummins, G. Boschloo, M. Ryan, D. Corr, S.N. Rao, D. Fitzmaurice, J. Phys. Chem. B. 104 (2000) 11449–11459. [27] M. Gra¨tzel, Nature 409 (2001) 575–576. [28] R. Cinnsealach, G. Boschloo, S.N. Rao, D. Fitzmaurice, Solar Energy Materials and Solar Cells 55 (1998) 215–223. [29] S.C. Oliveira, R.M. Torresi, S.I. Cordoba de Torresi, Quim. Nova 23 (2000) 79. [30] A. Surca, B. Orel, B. Pihlar, P. Bukovec, J. Electroanal. Chem. 408 (1996) 83. [31] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Oxford, 1995. [32] S.I. Cordoba de Torresi, A. Gorenstein, R. Torresi, M.V. Vasquez, J. Electroanal. Chem. 318 (1991) 131. [33] C.N.P. Fonseca, M.A. De Paoli, A. Gorenstein, Sol. Energy Mater. Sol. Cells 33 (1994) 73. [34] C.L. Gaupp, D.M. Welsh, R.D. Rauh, J.R. Reynolds, Chem. Mater. 14 (2002) 3964–3970.
Versatile electrochromic displays based on TiO2 nanoporous films modified with triruthenium clusters Sergio H. Toma, Henrique E. Toma
*
Instituto de Quı´mica, Universidade de Sa˜o Paulo, C. Postal 26077, CEP 05513-970 Sa˜o Paulo, SP, Brazil Received 7 July 2006; received in revised form 30 July 2006; accepted 31 July 2006 Available online 30 August 2006
Abstract We report a new versatile electrochromic device based on nanocrystalline titanium dioxide films modified with triruthenium acetate clusters of the type [Ru3O(OOCCH3)6(py)2(BPEB)]PF6 (BPEB = 1,4-bis[2(4-pyridyl)etenyl]benzene). As evaluated from cyclic voltammetry and spectroelectrochemistry, the most relevant electrochromic process in this system is associated with the first reduction of the [Ru3O] cluster core. Typical electrochromic response is observed within a millisecond time scale, covering a wide range optical window (up to 1100 nm), and exhibiting excellent stability over a thousand cycles between 0.5 and 1.0 V. 2006 Elsevier B.V. All rights reserved. Keywords: Triruthenium clusters; Electrochromism; TiO2 films; Spectroelectrochemistry; Cyclic voltammetry
1. Introduction Trinuclear ruthenium acetate clusters of general formula [Ru3O(CH3COO)6(L)3]n (L = solvent or N-heterocyclic ligands) are versatile molecular species, exhibiting triangular structures in which the metal ions are strongly held together by l-oxo and carboxylate bridges, as well as by metal–metal bonds [1–3]. From the electrochemical point of view, the small [Ru3O] cluster center constitutes an attractive moiety, since it displays up to five reversible redox processes, ranging from 1.5 V to 2.5 V (vs. SHE). In fact, the electronic interaction among the ruthenium ions is so strong that the [Ru3O] central core is considered as fully delocalized, behaving as single center [1,4] which can be electronically and electrochemically modulated by the terminal and/or bridging ligands [5,6]. Moreover, each monoelectronic process is accompanied by drastic chromatic changes [7,8], providing a suitable combination of electronic stimuli and spectroscopic response for application in electrochromic devices. Relevant studies have been *
Corresponding author. Tel.: +55 11 30913887; fax: +55 11 38155579. E-mail address: [email protected] (H.E. Toma).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.07.043
carried out using these ruthenium clusters as building blocks for systems displaying electron transfer reactions and catalysis [9–11], as well as acceptors for photoinduced processes in supramolecular assemblies [12], NO release [13], and logic gates [14]. In spite of the increasing interest in electrochromic organic and polymeric materials for display applications [15], to the best of our knowledge, up to the present time, applications of such cluster species in electrochromic devices have not yet been addressed. Recently, electrochromic devices based on wide bandgap nanocrystalline semiconductors functionalized with charge-transfer compounds have been proposed in the literature [16,17]. We here report a new exciting possibility, based on a ruthenium cluster containing BPEB ancillary ligands as the electroactive species, adsorbed onto nanocrystalline TiO2 films. 2. Materials and methods All solvents and reactants were of analytical grade and employed without further purification. Degussa P25 TiO2, containing 80% anatase and 20% rutile, and average particle size of 30 nm, was used in this work. The starting
S.H. Toma, H.E. Toma / Electrochemistry Communications 8 (2006) 1628–1632
[Ru3O(CH 3COO)6(py)2(MeOH)](PF6) compound was obtained as reported in the literature [18]. Trans-1,4bis[2-(4-pyridyl)ethenyl]-benzene was prepared by the Heck method [19,20] and purified by recrystallization from ethanol–water mixture. Anal. Found: C, 83.4%; H, 5.8%; N, 9.7%. Required for C21H16N2 (MW = 284.36 g mol1): C, 84.5%; H, 5.7%; N, 9.6%. Observed [M + H]+ : m/z 285.12. 2.1. Synthesis of the [Ru3O(CH3COO)6(py)2(BPEB)] PF6 complex The [{Ru3O(CH3COO)6(py)2}2(BPEB)](PF6)2 complex was prepared by adapting a previously reported procedure for the related binuclear species [21]. Ru3O(CH3COO)6 (py)2(MeOH)](PF6) (132 mg; 0.13 mmol) and trans-1,4bis[2-(4-pyridyl)ethenyl]-benzene (112 mg; 0.39 mmol) ligand were dissolved in a minimum volume CH2Cl2. The mixture was stirred for 48 h and eventual solvent evaporation was compensated adding CH2Cl2 to the mixture. Ammonium hexafluorophosphate (260 mg; 1.6 mmol) dissolved in a minimum volume of methanol was added to the deep green solution resulting in a dark green precipitate which was collected on a filter, washed with methanol, water and diethyl ether and dried under vacuum. Anal. Found: C, 38.9%; H, 3.7%; N, 4.1%. Required for C42H44N4 O13PF6Ru3 Æ 2H2O (MW = 1297.0 g mol1): C, 38.9%; H, 3.7%; N, 4.3%. 2.2. Preparation of mesoporous TiO2 films coated with ruthenium cluster A viscous suspension of nanocrystalline TiO2 nanoparticles (P25-Degussa) was obtained by grinding 6 g TiO2 in 2 ml of nanopure water and 0.2 ml of acetylacetone, in a mortar with a pestle for about 40 min, followed by the slowly addition of 8.0 ml of distilled water and 0.1 ml of Triton X-100. The mesoporous TiO2 films were obtained by spreading this colloidal paste by doctor blade technique onto F-doped SnO2 conducting glass (sheet resistance 15 X/cm2 – TEC15, Pilkington) delimited by adhesive tapes (1 cm2 active area) [22]. After drying the film on air, the tapes were removed and the TiO2 particles fired in a furnace at 450 C C for 30 min. About 4 lm thick mesoporous TiO2 films (measured with a Dektak 3 profilometer) were obtained by this method. Functionalized films were prepared by immersing the mesoporous TiO2 films in a 104 mol dm3 alcoholic solution of [Ru3O(CH3COO)6(py)2(BPEB)]PF6 for 12 h. Strong adsorption has been observed in spite of the single positive charge of the complex. The modified films were thoroughly washed with ethanol and acetone, and stored under vacuum. 2.3. Assembly of the electrochromic devices The assembly of the devices was performed by sandwiching the working electrode coated with the TiO2 nano-
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particles and the triruthenium cluster with a transparent counter electrode (F-doped SnO2 conducting glass). To avoid short circuit between the electrodes, and moisture contamination of the device, a 100 lm layer of a thermo-sealing plastic was applied at the edges of the working electrode, but leaving small uncovered areas in order to introduce the electrolytic and mediator solution, composed by 0.1 mol dm3 of lithium trifluoromethanesulfonate and 0.05 mol dm3 ferrocene, respectively, in acetonitrile. The thermo-sealing plastic was then melted on a heating plate, and immediately clamped to ensure a tight sealing. After cooling, the electrolyte was introduced into the cell by immersing one of the small openings into the corresponding solution, and applying vacuum suction at the other hole, with special care to remove any air bubble. Finally, the holes were closed with melted sealing plastic. 2.4. Measurements UV-visible and visible spectra were recorded on a HP8453 diode array or a Guided Wave fiber optics spectrophotometer. Cyclic voltammetry was carried out with a Princeton Applied Research model 283 or an Autolab PGSTAT 30 potentiostat. A platinum disk electrode was employed for the measurements, using a conventional Luggin capillary arrangement in 0.100 mol dm3 tetrabutylammonium hexafluorophosphate (TBAPF6) acetonitrile solution. Ag/AgNO3 (0.010 mol dm3 in acetonitrile containing 0.100 mol dm3 TBAPF6) was used as reference electrode. A platinum wire was used as the auxiliary electrode. In the spectroelectrochemical measurements, a three electrode system with a gold minigrid transparent working electrode, mounted inside a conventional quartz cell with restricted internal optical path length (0.025 cm) was used. All the E1/2 values presented here were converted to SHE by adding 0.503 V to the experimentally obtained values. 3. Results and discussion 3.1. Electrochromic behavior in solution The features of the electrochromic processes for the triruthenium acetate cluster {[Ru3O(OOCCH3)6(py)2(BPEB)]} PF6 were studied in solution by cyclic voltammetry and electronic spectroscopy. The electronic spectrum of the complex is dominated by the BPEB intra-ligand (IL) p p* transitions centered at 370 nm, the cluster-to-ligand charge transfer (CLCT) around 400 nm and the characteristic internal transitions (IC) of the [Ru3O] core between 600 and 800 nm. The assignment of the major bands of the complex was based on the spectroelectrochemical measurements and on comparisons with the dimeric cluster reported before [21]. In Fig. 1 it is shown the behavior of the electronic spectra recorded during the electrochemical processes related to a–e. The first process (a) corresponds to the first oxidation of the triruthenium core [Ru3O]+1/+2 at 1.16 V (vs. SHE)
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Fig. 1. Cyclic voltammetry and spectroelectrochemical of the {[Ru3O(OOCCH3)6(py)2(BPEB)]}PF6 complex.
which leads to the split of the IC band. This process also affects the 370 nm band, in which the contributions of the IL and CLCT transitions are simultaneously present. As the oxidation of the complex proceeds, the CLCT band decays leaving the IL band at 385 nm. The reduction process [Ru3O]+1/0 (b) at 0.09 V leads to a bathochromic shift of the IC band from 680 nm to 910 nm, reflecting an increase of the energy of the electronic levels of the metallic cluster. As expected, the CLCT transition also undergoes a red shift from 400 to 510 nm. The next step (c), at 1.19 V, can be attributed to the second reduction of the [Ru3O]0/1 unit. In analogy to the preceding process, a red shift of the IC band to 980 nm is also observed, due to the rise of the cluster energy levels. Curiously, this process is accompanied by a decrease of the IL band at 385 nm, while a new band appears at 750 nm. This behavior seems to be associated with the electronic delocalization of the complex, since the addition of a second electron in the triruthenium core is strongly influencing the electronic levels of the BPEB ligand, leading to the decrease of the IL band and the rising of a second band ascribed to the radical BPEBp ! p* at 750 nm [21]. The fourth and fifth redox processes (d) and (e) correspond to the first and second reduction of the BPEB ligand at 1.15 and 1.69 V (vs. SHE). The reduction of BPEB0/ is characterized by the partial decrease of the 351 nm IL band and the increase of another IL band at 750 nm ascribed to the radical BPEB. Finally, the second reduction of the BPEB/2 leads to the rise of a IL band at 312 nm and the decrease of the 351 and 750 nm bands. It also can be observed changes in the 400–700 nm region, where a new band appears at 510 nm, typical of fully reduced RuIIRuIIRuII species lacking the central O atom [7,23]. In fact, it is known that in the RuIIIRuIIIRuIIO species, the central oxygen atom plays an important role in stabilizing the Ru3O core, by means of a multicentric p(p)–d(p) bonding. However, as it is successively reduced, electrons are added to the dp-orbitals of the ruthenium
ions, such that they become completely filled in the RuIIRuIIRuIIO species. In this case, the interaction between the ruthenium ions and the central oxygen atom is no longer possible, prompting its release from the Ru3O core [7]. This process can be observed even for the RuIIIRuIIRuIIO species. Interestingly, the RuIIIRuIIRuII species generated in the subsequent chemical reaction exhibits a redox potential dramatically shifted to positive values. In other words, in the time scale of the spectroelectrochemical measurements, the electrochemically generated RuIIIRuIIRuIIO species loses the central oxygen atom yielding the intermediate species RuIIIRuIIRuII, which is immediately reduced to the RuIIRuIIRuII form, exhibiting an absorption band at 510 nm. This event has not been observed in the cyclic voltammograms, because of the short time scale involved in such measurements. 3.2. Electrochromic behavior on TiO2 films and device In the nanocrystalline TiO2 modified films, two electrochromic processes can be observed: (1) The electrochromism due to the reversible ions insertion/desertion, typically Li+, in the semiconductor network; [24] and (2) the color changes due to the electroactivity of the species anchored to semiconductor surface [25–27]. In the last case, if the redox potential of the electrochromophore is located above the liquid/solid interface of the semiconductor conduction band edge (CBE), electrons can flow reversibly from the oxide to the anchored molecules. In this situation, the semiconductor behaves as a conductor, allowing to the device a reversible coloration/discoloration process. Accordingly, the semiconductor should behave as a conductor, allowing to the device a reversible coloration/ discoloration process. If the redox potential of the electrochromophore is below the CBE a typical charge trap situation is originated, which blocks the re-oxidation of the reduced molecules on the surface. In addition, the mesoporous TiO2 films flat band potential (Vfb) can be modulated by the choice of solvent or adding H+, Li+ or Na+
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to the solution. When this happens, the Vfb shifts to more positive potential. For example, the Vfb for the TiO2 semiconductor in water is proton dependent, according to the relationship Vfb = (0.15 0.06 · pH) in V vs. SHE. In aprotic solvents like acetonitrile, Vfb is located below 1 V, but in the presence of Li+ it can be shifted to values around 0.1 V. In the case of the mesoporous TiO2 films modified with the triruthenium cluster, the electrochromic behavior is related to the redox process of the [Ru3O] core. In fact, Fig. 2 presents the cyclic voltammograms of the [Ru3O(CH3COO)6(py)2(BPEB)]PF6 in solution and anchored onto TiO2 surface, obtained in DMF solution with 0.1 mol dm3 Li(CF3SO3). Analogously to the behavior in solution, a reversible electrochromic process is observed. It is important to note that in the control experiments, in which the bare TiO2 was submitted to voltammetric cycles, no color changes due to Li+ intercalation was observed up to 1.6 V vs. SHE. In Fig. 3 it is shown the spectroelectrochemistry and chronoamperometry of the electrochromic device between 0.5 and 1.0 V. As one can see, the spectral changes in this potential range are related to the first reduction process of the Ru3O core in the {[Ru3O(OOCCH3)6(py)2(BPEB)]} PF6 complex, showing a decrease of the IC band at 680 nm and the rise of the CLCT and IC bands at 522 and 918 nm, respectively. The chronoamperometric behavior recorded, successively, after 5 s intervals between 1.0 and +0.5 V, is shown in Fig. 3b. At 1.0 V the sharp initial electrochemical response is associated with the cluster reduction process onto the TiO2 surface, leading to a drastic color change. This process is followed by a fast current decrease, up to a steady state condition (parasitic current). In the electrochromic device, this process is concomitant with the oxidation of the ferrocene (Fc) to ferrocenium ion (Fc+) on the counter-electrode. This observation suggest that the steady-state currents arise from the continuous re-oxidation of the reduced clusters species, presumably
Fig. 3. (a) Spectroelectrochemistry and (b) chronoamperometry of the electrochromic device between 0.5 and 1.0 V.
by Fc+ formed at the counter electrode. In addition, the Fc+ can also react with TiO2 or SnO2 surface, increase the leakage current [28]. At +0.5 V, the reduced clusters are re-oxidized injecting electrons onto conduction band CB of the TiO2 electrode. This reaction is followed by the reduction of the Fc+ generated on the counter-electrode. At this point, a zero current flows should be expected, however a small leakage current was also observed. The leakage current in this step is due to the permeation of some Fc to the semiconductor electrode resulting in its oxidation to Fc+. In Fig. 4 it is shown the current response of the [Ru3O(CH3COO)6(py)2(BPEB)]PF6 electrochromic device at 692 and 920 nm. The coloration time, which is the time to produce 75% change in the optical density, was typically less than 0.5 s. The electrochemical and optical changes were shown to be stable up to a hundred cycles studied, showing that the electrochromic conversion is reversible. The coloration efficiency CE at 692 and 920 nm can be defined in terms of absorbance changes (DA) and charge involved (DQ) by CEðkÞ ¼
Fig. 2. Cyclic voltammograms of the [Ru3O(CH3COO)6(py)2(BPEB)] PF6 complex in DMF solution with 0.1 mol dm3 Li(CF3SO3), and adsorbed onto TiO2.
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DAðkÞ DQ
In our case, CE were 50 and 190 cm2 C1 at 692 and 920 nm respectively, which is quite high, if compared with TiO2 itself (8 cm2 C1 at 633 nm) and to other electrochromic transition metal oxides cited in the literature [29–33],
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Fig. 4. Spectral variations at 692 and 920 nm and chronoamperometric behavior of the [Ru3O(CH3COO)6(py)2(BPEB)]PF6 device.
like NiOx (40 cm2 C1), CoOx (50 cm2 C1), WO3 (70– 100 cm2 C1) and to the bis-(2-phosphonoethyl)-4,4 0 -bipyridinium dichloride based device (170 cm2 C1) [28]. More accurate comparisons could be made, as recently proposed in the literature [34] using coloration efficiencies (CCE) based tandem chronoabsorptometry/chronocoulometry measurements, considering as point of reference, the charge passed at 95% of the full optical switch. 4. Conclusions A new class of electrochromic devices can be generated using triruthenium acetate clusters of the type [Ru3O(OOCCH3)6(py)2(L)] PF6, directly applied onto nanocrystalline TiO2. In the case of the BPEB ruthenium cluster, a good color efficiency and reversibility is found in 0.5 to 1.0 V working potential ranges. These versatile, multicolor electrochromophores provide interesting options to be exploited in electrochromic devices, since their performance can be conveniently modulated by the donor/acceptor nature of the ancillary ligands [1]. Acknowledgements We are thank to the financial Brazilian agencies CNPq (Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico), FAPESP (Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo), RENAMI (Rede de Nanotecnologia Molecular e Interfaces) and IMMC (Instituto do Mileˆnio de Materiais Complexos), and to Degussa, for the generous supply of TiO2 P-25 nanoparticles. References [1] H.E. Toma, K. Araki, A.D.P. Alexiou, S. Nikolaou, S. Dovidauskas, Coord. Chem. Rev 187 (2001) 219–221.
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