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Electrochromic switching in the visible and near IR with a Ru–dioxolene complex adsorbed on a nanocrystalline SnO2 electrode
Electrochemistry Communications 5 (2003) 416–420 www.elsevier.com/locate/elecom
Electrochromic switching in the visible and near IR with a Ru–dioxolene complex adsorbed on a nanocrystalline SnO2 electrode Jorge Garcıa-Ca~ nadas a, Andrew P. Meacham b, Laurence M. Peter a
c,* ,
Michael D. Ward
b
Departamento de Quımica, Universidad, Aut onoma de Madrid, Madrid 28049, Spain School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK c Department of Chemistry, University of Bath, Bath BA2 7AY, UK
b
Received 24 March 2003; received in revised form 8 April 2003; accepted 8 April 2003
Abstract The ruthenium complex bis(2,20 -bipyridine-4,40 -dicarboxylic acid) (tetrachlorocatecholato)-ruthenium(II) has been used to modify a thin nanocrystalline transparent layer of antimony-doped SnO2 on a conducting glass electrode. The surface-bound complex shows promise as the basis for an electrochromic window operating in the near infrared region. It undergoes a reversible ligand-centered catecholate/semiquinone oxidation, and the oxidised form has a metal to ligand charge-transfer band transition in the near IR. The redox process in the adsorbed layer causes a change of colour from blue-grey (reduced) to pink (oxidised), and the increase in transmission in the visible (630 nm) is accompanied by a decrease of transmission in the near infrared region (940 nm). The electrochromic system has been studied by cyclic voltammetry, electrochemical impedance spectroscopy and frequency-resolved potential-modulated transmittance at 630 and 940 nm. The results show that the speed of the electrochromic switching process appears to be limited by the RC time constant of the system rather than by the rate constant for electron exchange. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Dioxolene; Electrochromic; Frequency response analysis; Impedance; Nanocrystalline; Ruthenium
1. Introduction Thin layers of high surface area nanocrystalline oxides such as TiO2 are attractive substrates for electrochromic redox systems [1,2]. They are also used to achieve fast switching in electrochromic devices based on lithium intercalation [3,4]. The best-known electrochromic system is based on the reduction of surface attached viologens [5–7]. The range of colours that can be achieved with viologens is fairly limited. Reduction of the unsubstituted viologen dication to the radical cation induces a dark blue colour, and Cinnsealach et al. [7] have shown that the colour change can be tuned from blue to green by substitution of the viologen. Here we report a more versatile approach involving the use of a novel surface-attached ruthenium complex that allows optical switching to extended into the near infrared. *
Corresponding author. Tel.: +44-1225-826-502; fax: +44-1225-826231. E-mail address: [email protected] (L.M. Peter).
The well-known redox activity of transition metal complexes makes them excellent candidates for use as electrochromic materials, although this activity is usually confined to the visible region of the spectrum. We have studied a series of ruthenium–dioxolene and oxo– Mo(V) phenolate complexes whose redox chemistry is characterised by the appearance and disappearance of intense charge-transfer transitions in the near infrared [8]. In the case of the Ru–dioxolene systems, oxidation of the dioxolene ligand from its catecholate form to the semiquinone as well as to the quinone forms results in the appearance of strong metal to ligand charge-transfer transitions in the near infrared region [8,9]. Our initial attempts to develop an electrochromic system based on adsorption of a ruthenium dioxolene complex on nanocrystalline TiO2 proved unsuccessful, since the redox potential of the catechol/semiquinone couple is too positive to allow reversible electron exchange with the conduction band of the oxide. SnO2 is more suitable since it has a higher electron affinity than TiO2 , and also when it is highly doped, electron
1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00092-4
J. Garcıa-Ca~nadas et al. / Electrochemistry Communications 5 (2003) 416–420
417
electrodes, respectively. The electrolyte solution was 0.1 M Bu4 NPF6 in dry acetonitrile. A small amount of ferrocene was added to the solution after the measurements to provide an internal calibration of the electrode potential (the Fcþ =Fc potential is +0.4 V vs. SCE). Optical transmission measurements were performed at 940 and at 630 nm using light emitting diodes. The transmitted light was detected by a silicon diode connected to a current amplifier. Frequency-resolved electrical and optical measurements were performed at fixed dc potential using a Solartron 1250 frequency-response analyser. Fig. 1. Absorption spectra of ruthenium dioxolene complex (broken line) and semiquinone form (solid line) in aqueous 0.1 M NaClO4 .
exchange can occur by tunnelling. In this communication, we describe characterisation of an electrochromic window fabricated by coating a conducting glass electrode with a thin layer of nanocrystalline antimonydoped SnO2 , which was then modified with the complex bis(2,20 -bipyridine-4,40 -dicarboxylic acid)(tetrachlorocatecholato) ruthenium(II) (Fig. 1) [10]. The behaviour of the electrochromic electrode was studied by cyclic voltammetry, optical transmittance, electrical impedance and frequency-resolved potential-modulated transmittance.
2. Experimental Bis(2,20 -bipyridine-4,40 -dicarboxylic acid)(tetrachlorocatecholato) ruthenium(II) was prepared by refluxing tetrachlorocatechol with ½RuðdcbpyÞ2 Cl2 (dcbpy ¼ 2, 20 -bipyridine-4,40 -dicarboxylic acid) with tetrachlorocatechol in aqueous methanol and purified by chromatography on Sephadex-G25 with MeOH. Further details are given in [10]. The conductive glass substrate (fluorine-doped SnO2 , Libby Owens Ford, 10 X/square) was cleaned with 5% Decon solution, washed with milli-Q water and then immersed for 30 min in boiling 2-propanol before being rinsed with Milli-Q water and dried. The antimony-doped SnO2 (Alfa, 15% colloidal dispersion in H2 O) was spread over the glass with a glass rod using adhesive tape spacers. The coated plate was fired at 450 °C for 30 min in air. This process was repeated four times in order to increase the thickness of the SnO2 film to around 2–3 lm. The geometric electrode area was 1.5 cm2 . The nanocrystalline SnO2 film was immersed in a 3 104 M solution of the ruthenium dioxolene complex in 50:50 methanol/ethanol for 20 h, then removed and dried prior to use. Electrochemical measurements were carried out under dinitrogen at room temperature in a three-electrode cell. Ag and Pt wires were used as reference and counter
3. Results and discussion Details of the exploratory cyclic voltammetry of the ruthenium dioxolene complex in solution are given in reference [10] The voltammograms exhibit a reversible catecholate/semiquinone couple at +0.18 V vs. SCE, followed at higher potential (+0.94 V) by an irreversible semiquinone/quinone couple. Fig. 1 shows the electronic spectra of the complex (in aqueous solution) in the catecholate and semiquinone forms (produced in an OTTLE setup). In the reduced state, the complex does not absorb in the near infrared, but the electrochemically generated semiquinone species exhibits near IR absorbance arising from a metal to ligand (i.e., RuðIIÞ ! semiquinone) charge-transfer transition. Data for the reduced catecholate state (k; e): 310 nm, 34 500; 381 nm; 9300; 556 nm, 8800. Data for the semiquinone form (k; e): 307 nm, 37 800; 505 nm, 3600; 964 nm, 6000. The absorbance spectrum of the adsorbed complex is similar, but less well resolved due to scattering. The spectroscopic properties of the complex are similar to those observed for the non-carboxylated parent complex ½RuðbpyÞ2 ðCl4 CatÞ [9]. Here we concentrate on the behaviour of the reversible catecholate/semiquinone couple in the adsorbed complex, which forms the basis of the electrochromic system. The multicycle voltammogram in Fig. 2 illustrates the oxidation and reduction peaks associated with the catecholate/semiquinone couple. The symmetry of the peaks in the voltammogram suggests that the complex behaves as an almost ideal surface-attached reversible redox system. When the voltammetry was repeated using an unmodified electrode, only the featureless background charging current typical of highly doped nanocrystalline SnO2 electrodes was observed. This background current corresponds to the charging of the electrical double layer of the high surface area conducting SnO2 film. Its magnitude indicates that the internal surface area of the porous oxide film is at least 500 times larger than the geometric area. Based on this internal surface area, the charge under the voltammetric peaks (after subtraction of the double layer component)
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Fig. 2. Multicyle voltammogram of a porous nanocrystalline SnO2 film modified with ruthenium–dioxolene complex. Electrolyte 0.1 M Bu4 NPF6 in acetonitrile. Sweep rate 10 mV s1 . Electrode area 1.5 cm2 .
was used to estimate the coverage of the internal surface with the ruthenium dioxolene complex. This order of magnitude estimate (based on the geometric area of the complex) indicates a surface coverage of the order of 1013 cm2 , indicating that the nanocrystalline SnO2 is covered with a monolayer of the ruthenium complex. When the complex is in the catecholate form, the colour of electrode is blue-grey. It changes to pink during oxidation to the semiquinone form and returns to blue-grey during the reverse sweep. Fig. 3 shows how the optical transmittance of the film changes during a voltammetric scan at 10 mV s1 . In the visible part of the spectrum (630 nm), the transmittance increases on oxidation, and returns with only a small amount of hysteresis to its initial value on reduction, indicating that the electrochromic process is fully reversible. By contrast, the transmittance of the electrode at 940 nm decreases on the oxidation sweep due to the appearance of the MLCT transition, and returns to its initial value on the reverse sweep. Measurements of the background optical transmittance of an unmodified SnO2 electrode at the two wavelengths showed no significant changes over the potential range studied here [11]. It was clear from these results that the ligand-based redox processes of the ruthenium complex should be suitable for the fabrication of an electrochromic system operating both in the visible and near infrared. The optical switching speed of a surface-attached electrochromic redox system is determined by the rate of the oxidation/reduction process. In principle, a fast outer-sphere redox process is suitable for rapid switching, but in practice the current that flows to the porous electrode is required not only to effect the redox switching but also to charge the electrical double layer of the high surface area nanocrystalline SnO2 . These contributions to the current are evident in the cyclic voltammogram shown in Fig. 2. The faradaic and nonfaradaic components of the charging current must flow through the resistances associated with the conducting
Fig. 3. Changes in optical transmittance at 630 and 940 nm of a SnO2 film modified with Ru–dioxolene complex, Electrolyte 0.1 M Bu4 NPF6 in acetonitrile. Sweep rate 10 mV s1 . Note that the transmission has been normalised to the value at the beginning of the voltage sweep in each case.
glass substrate, the nanocrystalline film and the electrolyte. These resistances, combined with the double layer capacitance and the faradaic pseudo-capacitance give rise to an effective RC time constant that limits the dynamic optical response of the system. This study set out to characterise this effect using electrochemical impedance spectroscopy and frequency-resolved potentialmodulated transmittance measurements. Fig. 4 shows that the impedance response of the electrochromic electrode is almost purely capacitive at low frequencies. At high frequencies, the complex plane plot tends towards the limiting behaviour expected for a porous electrode (45° phase angle). The high frequency intercept gives the series ohmic resistance as 95.5 X. The low frequency impedance response can therefore be modelled adequately by a series RC circuit element, and this is confirmed by Fig. 5(a), which shows the fit to the semicircular response expected for the electrode admittance in this case. In the case of a reversible surface-attached redox system the faradaic component of the ac modulation of the total charge can be written as
J. Garcıa-Ca~nadas et al. / Electrochemistry Communications 5 (2003) 416–420
419
low frequency limit of the capacitance derived from the fit can be compared with the capacitance derived from the cyclic voltammetric response, which is obtained by noting that dQ dQ dt i ¼ ¼ : ð2Þ dE dt dE m The double layer capacitance at +0.4 V estimated from Fig. 2 is 2 mF (since the geometric surface area is 1.5 cm2 , this corresponds to 1.3 mF cm2 ), whereas the pseudo-capacitance associated with the surface-attached redox system is 6 mF (4 mF cm2 ). The total capacitance estimated from the cyclic voltammogram is 8 mF (5.3 mF cm2 ), which is similar in magnitude to the low frequency limit of the complex capacitance plot shown in Fig. 5(b). The RC time constant, s, can be obtained from the frequency at which the maximum occurs in the admittance and complex capacitance plots:
Cx!0 ¼
Fig. 4. Impedance of the modified electrode at 0.4 V, showing the capacitive behaviour at low frequencies and the approach at high frequencies to the transmission line behaviour typical of a porous electrode.
~ F ¼ C^F V~ ; ð1Þ Q where C~F is the redox pseudo-capacitance. At low frequencies, the faradaic and non-faradaic capacitances appear in parallel to give the total capacitance, Ctot . Analysis of the response of the system can be made conveniently by using the complex capacitance, which is obtained by integration of the admittance [12]. Fig. 5(b) demonstrates the fit of the complex capacitance response to the semicircle expected for a series RC circuit. The
RC s ¼ : ð3Þ 2p 2p The value of the RC time constant obtained in this way is 0:6 0:1 ms. For a fast one-electron redox process involving reduced and oxidised species with optical cross-sections red rox opt and ropt , respectively, the normalised modulated transmittance is related to the faradaic component of the complex capacitance by ox red ~ ox red ^ ~ r r r r Q CF V ~ F opt opt opt opt DT ð4Þ ¼ ¼ q T q
Fig. 5. (a) Admittance of the modified electrode at 0.4 V, showing the semicircular behaviour characteristic of a series RC equivalent circuit. (b) Complex capacitance plot showing semicircular fit and extrapolated low frequency limit of the capacitance.
(q is the elementary charge). The potential-modulated transmission responses at 630 and 940 nm are compared in Figs. 6(a) and (b). As expected, the two responses have opposite signs (cf. Fig. 3). It can be seen that the time constants of the optical responses (cf. Eq. (3)) are remarkably similar to those of the electrical (admittance and complex capacitance) responses, as predicted by Eq. (4). It is evident from this result that the redox electron transfer process is intrinsically fast, as indicated by the shape of the voltammogram in Fig. 2. The relatively slow dynamic response of the system is determined simply by the fact that current has to flow through a series resistance in order to charge and discharge the electrochromic layer and the electrical double layer. Clearly careful design will be required to minimise series resistance in a practical electrochromic device based on the present system. A sandwich cell configuration with a narrow interelectrode gap to eliminate the solution resistance is a first step. The series resistance due to the conducting glass can be minimised in a sandwich cell by better contact geometry and small size. Work is in progress to evaluate the limiting performance of optimally designed test devices.
fmax ¼
420
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tem. The analysis shows that series resistance rather than interfacial electron transfer kinetics limits the switching speed.
Acknowledgements The authors thank Upul Wijayantha for assistance with fabrication of the tin oxide electrodes and Johnson Matthey for financial support.
References
Fig. 6. (a) Frequency-resolved potential-modulated transmission at 630 nm. (b) Frequency-resolved potential-modulated transmission at 940 nm.
4. Conclusions This study has shown that an electrochromic layer with responses in the visible and near infrared can be fabricated by adsorbing bis(2,20 -bipyridine-4,40 -dicarboxylic acid)(tetrachlorocatecholato)–ruthenium(II) on a thin porous film of highly doped nanocrystalline SnO2 . The reversible behaviour of the surface-attached ligandcentered catecholate/semiquinone redox couple has been demonstrated, and the dynamic electrochromic response has been related to the electrical properties of the sys-
[1] P. Bonhote, E. Gogniat, F. Campus, L. Walder, M. Gr€atzel, Displays 20 (1999) 137. [2] F. Campus, P. Bonhote, M. Gr€atzel, S. Heinen, L. Walder, Sol. Energ. Mat. Sol. Cells 56 (1999) 281. [3] A. Hagfeldt, N. Vlachopoulos, M. Gr€atzel, J. Electrochem. Soc. 141 (1994) L82. [4] P. Bonhote, E. Gogniat, M. Gr€atzel, P.V. Ashrit, Thin Solid Films 350 (1999) 269. [5] A. Hagfeldt, L. Walder, M. Gr€atzel, Proc. Electrochem. Soc. 95-8 (1995) 143. [6] R. Cinnsealach, G. Boschloo, S.N. Rao, D. Fitzmaurice, Sol. Energ. Mat. Sol. Cells 55 (1998) 215. [7] R. Cinnsealach, R.G. Boschloo, S.N. Rao, D. Fitzmaurice, Sol. Energ. Mat. Sol. Cells 57 (1999) 107. [8] M.D. Ward, J.A. McCleverty, J. Chem. Soc. Dalton 275 (2002). [9] M. Haga, E.S. Dodsworth, A.B.P. Lever, Inorg. Chem. 25 (1986) 447. [10] J. Garcia-Ca~ nadas., A.P.Meacham, L.M. Peter, M.D. Ward, Angewandte Chemie (submitted). [11] U. zum Felde, M. Haase, H. Weller, J. Phys. Chem. B 104 (2000) 9388. [12] M. Kalaji, L.M. Peter, J. Chem. Soc. Faraday 87 (1991) 853.
Electrochromic switching in the visible and near IR with a Ru–dioxolene complex adsorbed on a nanocrystalline SnO2 electrode Jorge Garcıa-Ca~ nadas a, Andrew P. Meacham b, Laurence M. Peter a
c,* ,
Michael D. Ward
b
Departamento de Quımica, Universidad, Aut onoma de Madrid, Madrid 28049, Spain School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK c Department of Chemistry, University of Bath, Bath BA2 7AY, UK
b
Received 24 March 2003; received in revised form 8 April 2003; accepted 8 April 2003
Abstract The ruthenium complex bis(2,20 -bipyridine-4,40 -dicarboxylic acid) (tetrachlorocatecholato)-ruthenium(II) has been used to modify a thin nanocrystalline transparent layer of antimony-doped SnO2 on a conducting glass electrode. The surface-bound complex shows promise as the basis for an electrochromic window operating in the near infrared region. It undergoes a reversible ligand-centered catecholate/semiquinone oxidation, and the oxidised form has a metal to ligand charge-transfer band transition in the near IR. The redox process in the adsorbed layer causes a change of colour from blue-grey (reduced) to pink (oxidised), and the increase in transmission in the visible (630 nm) is accompanied by a decrease of transmission in the near infrared region (940 nm). The electrochromic system has been studied by cyclic voltammetry, electrochemical impedance spectroscopy and frequency-resolved potential-modulated transmittance at 630 and 940 nm. The results show that the speed of the electrochromic switching process appears to be limited by the RC time constant of the system rather than by the rate constant for electron exchange. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Dioxolene; Electrochromic; Frequency response analysis; Impedance; Nanocrystalline; Ruthenium
1. Introduction Thin layers of high surface area nanocrystalline oxides such as TiO2 are attractive substrates for electrochromic redox systems [1,2]. They are also used to achieve fast switching in electrochromic devices based on lithium intercalation [3,4]. The best-known electrochromic system is based on the reduction of surface attached viologens [5–7]. The range of colours that can be achieved with viologens is fairly limited. Reduction of the unsubstituted viologen dication to the radical cation induces a dark blue colour, and Cinnsealach et al. [7] have shown that the colour change can be tuned from blue to green by substitution of the viologen. Here we report a more versatile approach involving the use of a novel surface-attached ruthenium complex that allows optical switching to extended into the near infrared. *
Corresponding author. Tel.: +44-1225-826-502; fax: +44-1225-826231. E-mail address: [email protected] (L.M. Peter).
The well-known redox activity of transition metal complexes makes them excellent candidates for use as electrochromic materials, although this activity is usually confined to the visible region of the spectrum. We have studied a series of ruthenium–dioxolene and oxo– Mo(V) phenolate complexes whose redox chemistry is characterised by the appearance and disappearance of intense charge-transfer transitions in the near infrared [8]. In the case of the Ru–dioxolene systems, oxidation of the dioxolene ligand from its catecholate form to the semiquinone as well as to the quinone forms results in the appearance of strong metal to ligand charge-transfer transitions in the near infrared region [8,9]. Our initial attempts to develop an electrochromic system based on adsorption of a ruthenium dioxolene complex on nanocrystalline TiO2 proved unsuccessful, since the redox potential of the catechol/semiquinone couple is too positive to allow reversible electron exchange with the conduction band of the oxide. SnO2 is more suitable since it has a higher electron affinity than TiO2 , and also when it is highly doped, electron
1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00092-4
J. Garcıa-Ca~nadas et al. / Electrochemistry Communications 5 (2003) 416–420
417
electrodes, respectively. The electrolyte solution was 0.1 M Bu4 NPF6 in dry acetonitrile. A small amount of ferrocene was added to the solution after the measurements to provide an internal calibration of the electrode potential (the Fcþ =Fc potential is +0.4 V vs. SCE). Optical transmission measurements were performed at 940 and at 630 nm using light emitting diodes. The transmitted light was detected by a silicon diode connected to a current amplifier. Frequency-resolved electrical and optical measurements were performed at fixed dc potential using a Solartron 1250 frequency-response analyser. Fig. 1. Absorption spectra of ruthenium dioxolene complex (broken line) and semiquinone form (solid line) in aqueous 0.1 M NaClO4 .
exchange can occur by tunnelling. In this communication, we describe characterisation of an electrochromic window fabricated by coating a conducting glass electrode with a thin layer of nanocrystalline antimonydoped SnO2 , which was then modified with the complex bis(2,20 -bipyridine-4,40 -dicarboxylic acid)(tetrachlorocatecholato) ruthenium(II) (Fig. 1) [10]. The behaviour of the electrochromic electrode was studied by cyclic voltammetry, optical transmittance, electrical impedance and frequency-resolved potential-modulated transmittance.
2. Experimental Bis(2,20 -bipyridine-4,40 -dicarboxylic acid)(tetrachlorocatecholato) ruthenium(II) was prepared by refluxing tetrachlorocatechol with ½RuðdcbpyÞ2 Cl2 (dcbpy ¼ 2, 20 -bipyridine-4,40 -dicarboxylic acid) with tetrachlorocatechol in aqueous methanol and purified by chromatography on Sephadex-G25 with MeOH. Further details are given in [10]. The conductive glass substrate (fluorine-doped SnO2 , Libby Owens Ford, 10 X/square) was cleaned with 5% Decon solution, washed with milli-Q water and then immersed for 30 min in boiling 2-propanol before being rinsed with Milli-Q water and dried. The antimony-doped SnO2 (Alfa, 15% colloidal dispersion in H2 O) was spread over the glass with a glass rod using adhesive tape spacers. The coated plate was fired at 450 °C for 30 min in air. This process was repeated four times in order to increase the thickness of the SnO2 film to around 2–3 lm. The geometric electrode area was 1.5 cm2 . The nanocrystalline SnO2 film was immersed in a 3 104 M solution of the ruthenium dioxolene complex in 50:50 methanol/ethanol for 20 h, then removed and dried prior to use. Electrochemical measurements were carried out under dinitrogen at room temperature in a three-electrode cell. Ag and Pt wires were used as reference and counter
3. Results and discussion Details of the exploratory cyclic voltammetry of the ruthenium dioxolene complex in solution are given in reference [10] The voltammograms exhibit a reversible catecholate/semiquinone couple at +0.18 V vs. SCE, followed at higher potential (+0.94 V) by an irreversible semiquinone/quinone couple. Fig. 1 shows the electronic spectra of the complex (in aqueous solution) in the catecholate and semiquinone forms (produced in an OTTLE setup). In the reduced state, the complex does not absorb in the near infrared, but the electrochemically generated semiquinone species exhibits near IR absorbance arising from a metal to ligand (i.e., RuðIIÞ ! semiquinone) charge-transfer transition. Data for the reduced catecholate state (k; e): 310 nm, 34 500; 381 nm; 9300; 556 nm, 8800. Data for the semiquinone form (k; e): 307 nm, 37 800; 505 nm, 3600; 964 nm, 6000. The absorbance spectrum of the adsorbed complex is similar, but less well resolved due to scattering. The spectroscopic properties of the complex are similar to those observed for the non-carboxylated parent complex ½RuðbpyÞ2 ðCl4 CatÞ [9]. Here we concentrate on the behaviour of the reversible catecholate/semiquinone couple in the adsorbed complex, which forms the basis of the electrochromic system. The multicycle voltammogram in Fig. 2 illustrates the oxidation and reduction peaks associated with the catecholate/semiquinone couple. The symmetry of the peaks in the voltammogram suggests that the complex behaves as an almost ideal surface-attached reversible redox system. When the voltammetry was repeated using an unmodified electrode, only the featureless background charging current typical of highly doped nanocrystalline SnO2 electrodes was observed. This background current corresponds to the charging of the electrical double layer of the high surface area conducting SnO2 film. Its magnitude indicates that the internal surface area of the porous oxide film is at least 500 times larger than the geometric area. Based on this internal surface area, the charge under the voltammetric peaks (after subtraction of the double layer component)
418
J. Garcıa-Ca~nadas et al. / Electrochemistry Communications 5 (2003) 416–420
Fig. 2. Multicyle voltammogram of a porous nanocrystalline SnO2 film modified with ruthenium–dioxolene complex. Electrolyte 0.1 M Bu4 NPF6 in acetonitrile. Sweep rate 10 mV s1 . Electrode area 1.5 cm2 .
was used to estimate the coverage of the internal surface with the ruthenium dioxolene complex. This order of magnitude estimate (based on the geometric area of the complex) indicates a surface coverage of the order of 1013 cm2 , indicating that the nanocrystalline SnO2 is covered with a monolayer of the ruthenium complex. When the complex is in the catecholate form, the colour of electrode is blue-grey. It changes to pink during oxidation to the semiquinone form and returns to blue-grey during the reverse sweep. Fig. 3 shows how the optical transmittance of the film changes during a voltammetric scan at 10 mV s1 . In the visible part of the spectrum (630 nm), the transmittance increases on oxidation, and returns with only a small amount of hysteresis to its initial value on reduction, indicating that the electrochromic process is fully reversible. By contrast, the transmittance of the electrode at 940 nm decreases on the oxidation sweep due to the appearance of the MLCT transition, and returns to its initial value on the reverse sweep. Measurements of the background optical transmittance of an unmodified SnO2 electrode at the two wavelengths showed no significant changes over the potential range studied here [11]. It was clear from these results that the ligand-based redox processes of the ruthenium complex should be suitable for the fabrication of an electrochromic system operating both in the visible and near infrared. The optical switching speed of a surface-attached electrochromic redox system is determined by the rate of the oxidation/reduction process. In principle, a fast outer-sphere redox process is suitable for rapid switching, but in practice the current that flows to the porous electrode is required not only to effect the redox switching but also to charge the electrical double layer of the high surface area nanocrystalline SnO2 . These contributions to the current are evident in the cyclic voltammogram shown in Fig. 2. The faradaic and nonfaradaic components of the charging current must flow through the resistances associated with the conducting
Fig. 3. Changes in optical transmittance at 630 and 940 nm of a SnO2 film modified with Ru–dioxolene complex, Electrolyte 0.1 M Bu4 NPF6 in acetonitrile. Sweep rate 10 mV s1 . Note that the transmission has been normalised to the value at the beginning of the voltage sweep in each case.
glass substrate, the nanocrystalline film and the electrolyte. These resistances, combined with the double layer capacitance and the faradaic pseudo-capacitance give rise to an effective RC time constant that limits the dynamic optical response of the system. This study set out to characterise this effect using electrochemical impedance spectroscopy and frequency-resolved potentialmodulated transmittance measurements. Fig. 4 shows that the impedance response of the electrochromic electrode is almost purely capacitive at low frequencies. At high frequencies, the complex plane plot tends towards the limiting behaviour expected for a porous electrode (45° phase angle). The high frequency intercept gives the series ohmic resistance as 95.5 X. The low frequency impedance response can therefore be modelled adequately by a series RC circuit element, and this is confirmed by Fig. 5(a), which shows the fit to the semicircular response expected for the electrode admittance in this case. In the case of a reversible surface-attached redox system the faradaic component of the ac modulation of the total charge can be written as
J. Garcıa-Ca~nadas et al. / Electrochemistry Communications 5 (2003) 416–420
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low frequency limit of the capacitance derived from the fit can be compared with the capacitance derived from the cyclic voltammetric response, which is obtained by noting that dQ dQ dt i ¼ ¼ : ð2Þ dE dt dE m The double layer capacitance at +0.4 V estimated from Fig. 2 is 2 mF (since the geometric surface area is 1.5 cm2 , this corresponds to 1.3 mF cm2 ), whereas the pseudo-capacitance associated with the surface-attached redox system is 6 mF (4 mF cm2 ). The total capacitance estimated from the cyclic voltammogram is 8 mF (5.3 mF cm2 ), which is similar in magnitude to the low frequency limit of the complex capacitance plot shown in Fig. 5(b). The RC time constant, s, can be obtained from the frequency at which the maximum occurs in the admittance and complex capacitance plots:
Cx!0 ¼
Fig. 4. Impedance of the modified electrode at 0.4 V, showing the capacitive behaviour at low frequencies and the approach at high frequencies to the transmission line behaviour typical of a porous electrode.
~ F ¼ C^F V~ ; ð1Þ Q where C~F is the redox pseudo-capacitance. At low frequencies, the faradaic and non-faradaic capacitances appear in parallel to give the total capacitance, Ctot . Analysis of the response of the system can be made conveniently by using the complex capacitance, which is obtained by integration of the admittance [12]. Fig. 5(b) demonstrates the fit of the complex capacitance response to the semicircle expected for a series RC circuit. The
RC s ¼ : ð3Þ 2p 2p The value of the RC time constant obtained in this way is 0:6 0:1 ms. For a fast one-electron redox process involving reduced and oxidised species with optical cross-sections red rox opt and ropt , respectively, the normalised modulated transmittance is related to the faradaic component of the complex capacitance by ox red ~ ox red ^ ~ r r r r Q CF V ~ F opt opt opt opt DT ð4Þ ¼ ¼ q T q
Fig. 5. (a) Admittance of the modified electrode at 0.4 V, showing the semicircular behaviour characteristic of a series RC equivalent circuit. (b) Complex capacitance plot showing semicircular fit and extrapolated low frequency limit of the capacitance.
(q is the elementary charge). The potential-modulated transmission responses at 630 and 940 nm are compared in Figs. 6(a) and (b). As expected, the two responses have opposite signs (cf. Fig. 3). It can be seen that the time constants of the optical responses (cf. Eq. (3)) are remarkably similar to those of the electrical (admittance and complex capacitance) responses, as predicted by Eq. (4). It is evident from this result that the redox electron transfer process is intrinsically fast, as indicated by the shape of the voltammogram in Fig. 2. The relatively slow dynamic response of the system is determined simply by the fact that current has to flow through a series resistance in order to charge and discharge the electrochromic layer and the electrical double layer. Clearly careful design will be required to minimise series resistance in a practical electrochromic device based on the present system. A sandwich cell configuration with a narrow interelectrode gap to eliminate the solution resistance is a first step. The series resistance due to the conducting glass can be minimised in a sandwich cell by better contact geometry and small size. Work is in progress to evaluate the limiting performance of optimally designed test devices.
fmax ¼
420
J. Garcıa-Ca~nadas et al. / Electrochemistry Communications 5 (2003) 416–420
tem. The analysis shows that series resistance rather than interfacial electron transfer kinetics limits the switching speed.
Acknowledgements The authors thank Upul Wijayantha for assistance with fabrication of the tin oxide electrodes and Johnson Matthey for financial support.
References
Fig. 6. (a) Frequency-resolved potential-modulated transmission at 630 nm. (b) Frequency-resolved potential-modulated transmission at 940 nm.
4. Conclusions This study has shown that an electrochromic layer with responses in the visible and near infrared can be fabricated by adsorbing bis(2,20 -bipyridine-4,40 -dicarboxylic acid)(tetrachlorocatecholato)–ruthenium(II) on a thin porous film of highly doped nanocrystalline SnO2 . The reversible behaviour of the surface-attached ligandcentered catecholate/semiquinone redox couple has been demonstrated, and the dynamic electrochromic response has been related to the electrical properties of the sys-
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