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hexacyanoruthenate film modified electrodes
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 566 (2004) 291–303 www.elsevier.com/locate/jelechem
Preparation, characterization and electrocatalytic properties of polynuclear mixed-valent ruthenium oxide/hexacyanoruthenate film modified electrodes Shen-Ming Chen *, Sheh-Hung Hsueh Department of Chemical Engineering, National Taipei University of Technology, No. 1, section 3, Chung-Hsiao East Road, Taipei 10643, Taiwan, ROC Received 23 June 2003; received in revised form 20 October 2003; accepted 15 November 2003
Abstract Polynuclear mixed-valent ruthenium oxide/ruthenocyanide (ruthenium oxide/hexacyanoruthenate or mvRuO/RuCN) films were prepared using consecutive cyclic voltammetry directly from the mixing of Ru3þ and Ru(CN)4 6 ions from solutions of two divalent cations (Ba2þ and Ca2þ ), and seven monovalent cations (Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ ). The films exhibited three redox couples with Ba(NO3 )2 or BaCl2 aqueous solutions, and the formal potentials of the redox couples showed a cation and pH effect. An electrochemical quartz crystal microbalance (EQCM), cyclic voltammetry, UV–visible spectroscopy, and the stopped-flow method (SFM) were used to study the growth mechanism of the mvRuO/RuCN films. The results indicated that the redox process was confined to the immobilized ruthenium oxide/ruthenocyanide. The EQCM results showed a Ba2þ ion exchange reaction for the 2 two most negative redox couples. The electrocatalytic reduction properties of SO2 5 , and S2 O8 by the ruthenium oxide/ruthenocyanide films were determined. The electrocatalytic oxidation of NADH and dopamine were also determined, and revealed two 2 different types of properties. The electrocatalytic oxidations of SO2 3 , S2 O3 , and N2 H4 were also investigated. The electrocatalytic reactions of the ruthenium oxide/ruthenocyanide films were investigated using the rotating ring-disk electrode method. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Ruthenium ruthenocyanide; Film-modified electrodes; Electrocatalysis; Sulfur oxoanions; NADH; Dopamine
1. Introduction Ruthenium oxide/ruthenocyanide (mvRuO/RuCN) is the ruthenium analogue of iron hexacyanoferrate. Metal hexacyanoferrates and metal hexacyanoruthenates show interesting redox chemistry that is accompanied by changes in their electrochromic, ion exchange, and electrocatalytic properties. Chemically modified film electrodes of metal hexacyanoruthenates [1–8] and metal hexacyanoferrates [9–14] also show interesting redox properties. They are used in both chemistry and in material science, in such areas as electroanalysis, chemical sensing and electrocatalysis [15–22], in studies on interfacial charges, in ion exchange and electron *
Corresponding author. Tel.: +886-2270-17147; fax: +886-227025238. E-mail address: [email protected] (S.-M. Chen). 0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2003.11.040
transfer studies [23–25], and in research on surface chemical composition [26]. In general, an mvRuO/RuCN film can be deposited onto a working electrode by cycling the potential between 0.5 and 1.0 V in an aqueous solution containing Ru(CN)4 6 and 0.5 M KCl at pH 2 [1–8]. Two redox couples occur as a result, and these films have been used in amperometric determination and in the electrocatalysis of As(III) [2], insulin [4], methionine [27], amino acids [28], polypeptides [29], nitrosamines [30], and hydrazine [31]. We will discuss the successful formation and properties of mvRuO/RuCN films synthesized from electrolytic aqueous solutions containing the following cations: Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ , which are of scientific interest. To realize practical mvRuO/ RuCN-film-modified electrodes for ion sensors, the deposition processes of mvRuO/RuCN films must also be fully characterized.
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The results of the electrocatalytic activity measurements on the mvRuO/RuCN film are applicable to analytical applications, and to the electrochemical reaction transfer activity of low electroactive compounds. The analytical methods used are well established, and they are important for the determination of both these analytes. The electrocatalytic reduction properties of SO2 5 , and S2 O2 8 by the mvRuO/RuCN films were determined, and shown to develop directly from the film redox couple as the cathodic current was increased. The electrocatalytic oxidation of NADH, dopamine, SO2 3 , S2 O2 3 , and N2 H4 were also investigated. The electrochemical formation of mvRuO/RuCN films on a glassy carbon electrode was carried out using consecutive cyclic voltammetry in aqueous barium nitrate solutions. The mvRuO/RuCN films showed three redox couples with formal potentials between )0.4 and 1.1 V in divalent cationic Ba(NO3 )2 and BaCl2 aqueous solutions. This paper reports on the successful preparation of mvRuO/RuCN films formed by directly mixing Ru3þ and Ru(CN)4 6 , and deposit onto a working electrode by cycling the potential between )0.4 and 1.0 V in aqueous solutions at pH 7, and in acidic aqueous solutions using consecutive cyclic voltammetry for seven electrolyte monovalent cation solutions (Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ ). Films also showed three redox couples in divalent cationic Ba(NO3 )2 and BaCl2 aqueous solutions. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth processes of the mvRuO/RuCN films, and their electrochemical properties. The cyclic voltammograms of the mvRuO/RuCN films show an obvious cation effect: Ba2þ , Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ . The EQCM, cyclic voltammetry, UV–visible spectroscopy, and the stopped-flow method were used to study the growth mechanisms of the mvRuO/RuCN 2 films. The electrocatalytic reduction of SO2 5 , and S2 O8 by the mvRuO/RuCN film showed obvious activity. The electrocatalytic oxidation of NADH, dopamine, SO2 3 , S2 O2 , and N H also showed activity. The electrocat2 4 3 alytic reactions of dopamine with the mvRuO/RuCN films were also investigated using the rotating ring-disk electrode (RRDE) method.
trodes. The glassy carbon electrode was polished with 0.05 lm alumina on Buehler felt pads and then cleaned ultrasonically for 1 min. The auxiliary compartment contained a platinum wire, which was separated by a medium-sized glass frit. All cell potentials were taken using either an AgjAgCljKCl (saturated solution) reference electrode or an HgjHg2 Cl2 jKCl (saturated solution) reference electrode. The working electrode for the EQCM measurements was an 8 MHz AT-cut quartz crystal with gold electrodes. The diameter of the quartz crystal was 13.7 mm and the gold electrode diameter was 5 mm. UV–visible spectra were measured using a Hitachi Model U-3300 spectrophotometer. The RRDE experiments were performed using a Pine Instrument Co. electrode in conjunction with a CH Instruments CHI-750 potentiostat connected to a model AFMSRX analytical rotator. The rotating ring-disk electrode (RRDE), purchased from the Pine Instrument Co., consisted of a glassy carbon disk electrode and a glassy carbon (or platinum) ring electrode. Kinetic measurements were performed using a BioLogic stopped-flow module SFM-20, which consisted of a mechanical subsystem and a power supply. The fast reaction spectrometer was an optical system composed of an MOS-250 spectrophotometer and spectrofluorometer. The kinetics were recorded at a wavelength of 600 nm. All the chemicals used were of analytical grade. The K4 Ru(CN)6 and RuCl3 were purchased from the Aldrich Chemical Co., USA, and the NADH and dopamine were purchased from the Sigma Chemical Co., USA. The aqueous solutions were prepared using doubly distilled deionized water, and the solutions were deoxygenated by purging with pre-purified nitrogen gas. The electrochemical formation of the mvRuO/RuCN films was performed by continuous cycling of the potential of the working electrode in a defined potential range in a suitable aqueous solution containing Ru3þ and Ru(CN)4 6 ions. Typical electrochemical formation of mvRuO/RuCN film was performed by repetitive cyclic voltammetry on cycling the potential of the working electrode in a defined potential range in a supporting electrolyte aqueous solution containing K4 Ru(CN)6 with Ru3þ ions (with the balancing anion being chloride or nitrate).
2. Experimental The electrochemistry was performed using a Bioanalytical Systems Model CV-50W, and CH Instruments CHI-400 and CHI-750 potentiostats. Cyclic voltammetry was conducted using a three-electrode cell in which a BAS glassy carbon electrode, a platinum electrode, and an indium tin dioxide (ITO) electrode (which was composed of indium tin oxide that was sputter deposited on a glass substrate) were used as the working elec-
3. Results and discussion 3.1. Ruthenium oxide/ruthenocyanide film deposition in aqueous Ba(NO3 )2 solutions The electrochemical formation of mvRuO/RuCN films on a glassy carbon electrode was carried out using consecutive cyclic voltammetry in aqueous barium ni-
S.-M. Chen, S.-H. Hsueh / Journal of Electroanalytical Chemistry 566 (2004) 291–303
trate solutions. The mvRuO/RuCN films showed three redox couples with formal potentials between )0.4 and 1.0 V in aqueous Ba(NO3 )2 or BaCl2 divalent cation solutions. Barium nitrate (or barium chloride) was chosen as the salt for the electrolyte solution because the mvRuO/RuCN films showed three redox couples with formal potentials between )0.4 and 1.0 V in aqueous Ba(NO3 )2 or BaCl2 divalent cation solutions. Fig. 1(A) shows the consecutive cyclic voltammograms of an mvRuO/RuCN film deposited from Ru3þ and Ru(CN)4 6 in an aqueous 0.1 M Ba(NO3 )2 solution at pH 6.0. Three redox couples characterize the voltammograms, with formal potentials occurring between )0.4 and 1.0 V (vs. AgjAgCl), at about 0.87, 0.61, and – 0.02 V (vs. AgjAgCl). In acidic and neutral aqueous solutions, the results show that mvRuO/RuCN films can be formed directly from mixing Ru3þ and Ru(CN)4 in aqueous 0.1 M 6 Ba(NO3 )2 solutions by consecutive cyclic voltammetry. These mvRuO/RuCN films show three redox couples
with formal potentials between )0.4 and 1.0 V in Ba(NO3 )2 divalent cation solutions. Fig. 1(B)(b) shows the consecutive cyclic voltammograms of ruthenium oxide deposition from Ru3þ in an aqueous 0.1 M Ba(NO3 )2 solution at pH 6.0. Two redox couples characterize the voltammograms, with formal potentials occurring between )0.2 and 1.0 V (vs. AgjAgCl). The two redox couples of the ruthenium oxide film are proposed to arise from the [RuII –O]/ [RuIII –O] and [RuIII –O//[RuIV –O] redox couples, with formal potentials at about 0.8 and 0.25 V (vs. AgjAgCl), respectively. Fig. 1(B)(a) shows the cyclic voltammograms of Ru(CN)4 in an aqueous 0.1 M Ba(NO3 )2 6 solution at pH 6.0. Fig. 2(A) shows that the mvRuO/RuCN films obtained on a glassy carbon electrode from a 0.1 M aqueous KNO3 solution at pH 2.0 resulted in two chemically reversible redox couples for various scan rates. The inset of Fig. 2(A) shows a plot of cathodic peak current, Ipc , vs. scan rate, illustrating a close linear dependence of Ipc on the scan rate, and that the ratio of the anodic to the cathodic peak current (Ipa =Ipc ) was equal to unity. Fig. 2(B) shows that the mvRuO/RuCN films obtained on a glassy carbon electrode from a 0.1 M aqueous Ba(NO3 )2 solution at pH 2.0 exhibited three chemically reversible redox couples for various scan rates. The inset of Fig. 2(B) show plots of Ipc vs. scan rate at potentials of about 0.6 and 0.0 V, respectively. These illustrate a linear dependence of Ipc on the scan rate, and that the ratio of Ipa =Ipc was approximately equal to unity. The behavior in Fig. 2(A) and (B) is consistent with a diffusionless, reversible electron transfer process at low scan rates. The peak current and scan rate are related as follows [32,33] Ip ¼ n2 F 2 vACo =4RT
Fig. 1. (A) Repetitive cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film synthesized from 1 103 M Ru(CN)4 added to 1 103 M Ru3þ in an aqueous 0.1 M 6 Ba(NO)3 solution at pH 6.0. (B) (a) Cyclic voltammograms of 1 103 M Ru(CN)4 6 in an aqueous 0.1 M Ba(NO)3 solution at pH 6.0, and (b) repetitive cyclic voltammograms of a glassy carbon electrode modified with a ruthenium oxide film synthesized from 1 103 M Ru3þ in an aqueous 0.1 M Ba(NO)3 solution at pH 6.0. Scan rate ¼ 0.1 V/s.
293
ð1Þ
where Co , v, A, and Ip represent the surface coverage concentration, the scan rate, the electrode area, and the peak current, respectively. This result indicates that the redox process was confined to the surface of the mvRuO/RuCN film on the glassy carbon electrode, confirming the immobilized state of the film. In the EQCM experiments, the change in mass (or coverage) at the quartz crystal was calculated from the change in the observed frequency using the Sauerbrey equation. Using this, the film coverage (in ng/cm2 or mol/cm2 ) deposited on the electrode surface during the electrochemical process can be calculated. This result indicates that the redox process was confined to the surface of the mvRuO/RuCN film on the glassy carbon electrode, confirming the immobilized state of the mvRuO/RuCN [32,33]. The three redox couples of the mvRuO/RuCN film are proposed to be from the [RuII –O/[RuII (CN)6 ]/ [RuIII –O/[RuII (CN)6 ], [RuIII –O/[RuII (CN)6 ]/[RuIII –O/ III [Ru (CN)6 ], and [RuIII –O/[RuIII (CN)6 ]/[RuIV –O/ III [Ru (CN)6 ] redox couples.
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3.2. Ruthenium oxide/ruthenocyanide film deposition and in situ EQCM measurements We successfully grew mvRuO/RuCN films on a gold electrode from an aqueous 0.1 M Ba(NO3 )2 solution at pH 6.0. The cyclic voltammograms are shown in Fig. 3(A), with Fig. 3(B) showing the growth of the mvRuO/RuCN film on the gold electrode, and the resulting change in the EQCM frequency. The mvRuO/ RuCN film obtained from the aqueous 0.1 M Ba(NO3 )2 solution showed three redox couples occurring between potentials of –0.2 and 1.0 V, with formal potentials at approximately 0.9, 0.6, and 0.0 V (vs. AgjAgCl), respectively. Fig. 3(B) shows the change in EQCM frequency recorded during the first seven cycles of the consecutive cyclic voltammetry. The voltammetric peak current in Fig. 3(A) and the frequency decrease (or mass increase) in Fig. 3(B) are consistent with the growth of an mvRuO/RuCN film on the gold electrode. The EQCM results show that the deposition of the film occurred between potentials of 0.8 and 1.0 V (vs. AgjAgCl). This potential range is where [RuIII –O/[RuIII -
Fig. 2. (A) Cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film in an aqueous 0.1 M KNO3 solution at pH 2.0 for various scan rates: (a) 0.02 V/s, (b) 0.03 V/s, (c) 0.045 V/s, (d) 0.06 V/s, (e) 0.08 V/s, (f) 0.1 V/s, (g) 0.12 V/s, (h) 0.14 V/s, (i) 0.16 V/ s, and (j) 0.18 V/s. The inset shows a plot of the two cathodic peak currents Ipc vs. scan rate at potentials of: (a) about 0.65 V and (b) about 0.0 V. (B) Cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film in an aqueous 0.1 M Ba(NO3 )2 solution at pH 4.0 for various scan rates: (a) 0.01 V/s, (b) 0.02 V/s, (c) 0.03 V/s, (d) 0.045 V/s, (e) 0.06 V/s, (f) 0.08 V/s, (g) 0.1 V/s, (h) 0.12 V/s, (i) 0.14 V/s, (j) 0.16 V/s, and (k) 0.18 V/s. Inset shows a plot of the two cathodic peak currents Ipc vs. scan rate at: (a) about 0.6 V, and (b) about 0.0 V.
The mvRuO/RuCN film electron transfer rate, ks , was estimated using an equation derived by Laviron [34] for a diffusionless electron transfer cyclic voltammogram with nDEp > 200 mV log ks ¼ a logð1 aÞ þ ð1 aÞ log a logðRT =nFvÞ að1 aÞnF DEp =2:3RT :
ð2Þ
We used an electrochemical transfer coefficient a ¼ 0:5, and assumed that n ¼ 1 for RuO/[Ru(CN)6 ], for the electron transfer process of the [RuII O/[RuII (CN)6 ]/ [RuIII O/[RuII (CN)6 ] or [RuIII O/[RuII (CN)6 ]/[RuIII O/ [RuIII (CN)6 ] redox couples. The average ks had a value of about 1.1 s1 for scan rates between 200 and 1000 mV/s.
Fig. 3. (A) Consecutive cyclic voltammograms of a gold electrode modified with a mvRuO/RuCN film synthesized from 1 103 M 3 Ru(CN)4 M Ru3þ in an aqueous 0.1 M Ba(NO3 )2 6 added to 1 10 solution at pH 6.0. Scan rate ¼ 0.02 V/s. (B) The change in EQCM frequency recorded concurrently with the consecutive cyclic voltammograms of Figure 3(A). Inset (a) shows a plot of the change in cathodic peak current at about 0.0 V vs. scan cycle, and (b) the total frequency change vs. scan cycle.
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(CN)6 ] is oxidized to [RuIV –O/[RuIII (CN)6 ], and is deposited on the electrode surface. In the EQCM experiments, the change in mass at the quartz crystal was calculated from the change in the measurement frequency using the Sauerbrey equation [35,36] Mass change ðDmÞ ¼ ð1=2Þðfo2 ÞðDf ÞAðkqÞ
1=2
;
ð3Þ
where A is the area of the gold disk coated onto the quartz crystal, q is the density of the crystal, k is the shear modulus of the crystal, Df is the measured frequency change, and fo is the oscillation frequency of the crystal. A frequency change of 1 Hz is equivalent to a 1.4 ng change in mass. During the first cyclic voltammetry scan, approximately 721 ng/cm2 of mvRuO/RuCN was deposited on the gold electrode. Approximately 3815 ng/ cm2 of mvRuO/RuCN was deposited on the gold electrode after seven cyclic voltammetric scans. The relatively large electrodeposition that occurred during the first scan cycle was reflected in the increased frequency change from the mvRuO/RuCN film adhering to the (initially) clean gold electrode surface (see Fig. 3(B)). The rate of film growth (from the change in frequency) was slower in later scan cycles (Fig. 3(B)). The inset of Fig. 3(A)(a) shows a plot of the change in cathodic peak current at a potential of about 0.0 V vs. scan cycle, and Fig. 3(A)(b) shows the total frequency change vs. scan cycle. EQCM measurements are a very effective method for monitoring the in situ growth of mvRuO/RuCN films on gold electrodes (see Fig. 3(B)), because mvRuO/ RuCN films grow steadily with time on the gold electrode. The EQCM measurements show that deposition of the film occurred at potentials between 0.8 and 1.0 V (vs. AgjAgCl) (Fig. 3(B)), and that no deposition occurred at a potential more negative than 0.8 V. The results also show that before the film formed, Ru3þ and RuII (CN)4 6 reacted to form a new species. When the applied working electrode was between potentials of 0.8 and )0.2 V (vs. AgjAgCl), no deposition occurred, as the EQCM showed no frequency change between 0.8 V and )0.2 V. The Ru3þ and RuII (CN)4 6 species reacted to yield a soluble form of RuIII –RuII (CN)6 (or RuIII –O/ RuII (CN)6 ) after RuIII –RuII (CN)6 (or RuIII –O/RuII (CN)6 ) oxidized to RuIII –RuIII (CN)6 (or RuIII –O/RuIII (CN)6 ). Then, the RuIII –RuIII (CN)6 (or RuIII –O/ RuIII (CN)6 ) species was oxidized to RuIV –O/RuIII (CN)6 , which was subsequently deposited on the electrodeÕs surface to form the mvRuO/RuCN film. Fig. 4 shows the reaction of Ru3þ and RuII (CN)4 6 in an aqueous 0.1 M BaCl2 solution at pH 6.0. Fig. 4(a) shows the UV–visible absorption spectra of 1.3 103 3þ M Ru(CN)4 was gradually added to the 6 , where Ru solution. Fig. 4(a) shows the UV–visible absorption spectrum of an aqueous 3 102 M Ru3þ solution that was added to 10 ml of an aqueous 1.3 103 M
Fig. 4. (a) UV–visible absorption spectra of Ru3þ gradually added to an aqueous 1.3 103 M Ru(CN)4 6 solution. The inset shows a plot of the absorbance vs. the ratio of [Ru3þ ]/[Ru(CN)4 6 ] from Fig. 3(A), with the wavelength set at (b) k ¼ 360 nm and (c) k ¼ 600 nm.
Ru(CN)4 6 and 0.1 M BaCl2 solution. The UV–visible absorption spectrum changed when the 3 102 M Ru3þ solution was gradually added (1 ml on average added 25 times) to the aqueous 1.3 103 M Ru(CN)4 6 and 0.1 M BaCl2 solution. The results show that the Ru3þ and RuII (CN)4 two-colorless species react to 6 form a violet-colored solution that exhibited absorption peaks at k ¼ 360 and 600 nm, with no obvious absorption peaks of Ru3þ and RuII (CN)4 6 . Fig. 4(b) and (c) show the changes in UV–visible absorption spectra at k ¼ 360 nm and 600 nm, respectively, when Ru3þ was gradually added to 1.3 103 M Ru(CN)4 in an 6 aqueous BaCl2 solution at pH 6.0. The results show that the absorption spectral peaks of the new species formed from Ru3þ and RuII (CN)4 6 with absorption peaks at k ¼ 360 and 600 nm, increase as the concentration of Ru3þ increases up to a concentration of 1.3 103 M, as can be seen form the plots of absorbance vs. the ratio of [Ru3þ ]/[Ru(CN)4 6 ] in Fig. 4(b) and (c). The results show that the absorption increases up to a ratio of [Ru3þ ]/[Ru(CN)4 6 ] ¼ 1. The III Ru3þ and RuIII (CN)4 react to form Ru [RuII (CN)6 ] 6
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(or RuIII –O[RuII (CN)6 ]), which has a ratio of [Ru3þ ]/ [Ru(CN)4 6 ] ¼ 1. Stopped-flow kinetic measurements at k ¼ 360 nm of 2 103 M Ru(CN)4 in an aqueous 6 BaCl2 solution at pH 6.0 mixed with Ru3þ in concentrations of 5 104 M, 1 103 M, and 2 103 M were carried out. From the change in absorption of the stopped-flow kinetic measurements, as shown in Fig. 5, we can conclude that the RuIII [RuII (CN)6 ] (or RuIII –O/ RuII (CN)6 ) species formed by the Ru3þ reacts with the RuII (CN)4 to dive a voltammogram having a shape 6 that has two plateaus yielding a soluble ruthenium(III) hexacyanoruthenate(II) (RuIII [RuII (CN)6 ]) (or RuIII –O/ RuII (CN)6 ) species with a half life (t1=2 ) of approximately 0.025 s. The two plateaus of the stopped-flow kinetic measurements could be caused by more than one stage existing in the reaction of Ru3þ with RuII (CN)4 6 to yield a soluble species. The cyclic voltammetric results also show that Ru3þ reacts instantaneously with RuII (CN)4 to yield a soluble species. The cyclic voltam6 metry results show that when 1 103 M Ru3þ was added to the 1 103 M Ru(CN)4 6 aqueous solution, the Ru(CN)4 6 redox couple disappeared (see Figs. 1 and 3(A)), and the film grew by the electrochemical oxidation of the ruthenium(III) hexacyanoruthenate(II) (RuIII [RuII (CN)6 ] or RuIII –O/RuII (CN)6 ) species. This was oxidized to (RuIII [RuIII (CN)6 ] or RuIII -O/ RuIII (CN)6 ), followed by further oxidation and film deposition to form the ruthenium oxide/ruthenocyanide(RuIV –O/RuIII (CN)6 ) film. The reaction of the solution and film formation processes of 1 103 M Ru(CN)4 and 1 103 M 6 Ru3þ in an aqueous 0.1 M Ba2þ solution are proposed as follows 4
Ru3þ þ RuðCNÞ6
! RuIII ½RuII ðCNÞ6 ðor RuIII –O=RuII ðCNÞ6 Þ
ð4Þ
Fig. 5. Stopped-flow kinetic measurements at a wavelength of k ¼ 360 nm for 2 102 M Ru(CN)4 6 in an aqueous 0.1 M BaCl2 solution mixed with Ru3þ at concentrations of: (a) 5 104 M, (b) 1 103 M, and (c) 2 103 M.
RuIII ½RuII ðCNÞ6 ðor RuIII –O=RuII ðCNÞ6 Þ
RuIII ½RuIII ðCNÞ6 ðor RuIII –O=RuIII ðCNÞ6 Þ þ e ð5Þ RuIII ½RuIII ðCNÞ6 ðor RuIII –O=RuIII ðCNÞ6 Þ ! RuIV –O=RuIII ðCNÞ6 þ e ! film deposition:
ð6Þ
The film grows by the electrochemical oxidation of RuIII [RuII (CN)6 ] (or RuIII –O/RuII (CN)6 ), which is oxidized to RuIII [RuIII (CN)6 ] (or RuIII –O/RuIII (CN)6 ) (Eq. (5)), followed by the reaction described by Eq. (6), to form the film. The ‘‘polynuclear mixed-valent ruthenium oxide/ hexacyanoruthenate (mvRuO/RuCN film)’’ is denoted as Ru–O/Ru(CN)6 , and occurs after the Ru3þ and RuII (CN)4 6 species have reacted to yield a soluble form that is oxidized to RuIV –O/RuIII (CN)6 . Before RuIV –O/ RuIII (CN)6 is produced by electrochemical oxidation, the soluble Ru3þ species reacts with RuII (CN)4 6 to yield either RuIII –RuII (CN)6 or RuIII -O/RuII (CN)6 . The soluble species is then oxidized to RuIV –O/RuIII (CN)6 , which is subsequently deposited on the electrode surface to form the mvRuO/RuCN film. 3.3. The electrochemical properties of the ruthenium oxide/ruthenocyanide films Fig. 6 shows the results of cyclic voltammetry and EQCM measurements on an mvRuO/RuCN film in an aqueous 0.1 M Ba(NO3 )2 solution. The results in Fig. 6(A) show that the three redox couples exhibit voltammetric currents (and frequency changes) in the potential range of )0.2 to 0.4 V (frequency increase, mass decrease), from 0.40 to 0.87 V (frequency decrease, mass increase), and from 0.87 to 1.0 V (no obvious change in frequency), respectively. The frequency increase (or mass decrease) in Fig. 6(A) is consistent with a Ba2þ ion exchange with the redox couple at potentials of )0.2 to 0.4 V, and the frequency decrease (or mass increase) in Fig. 6(A) is consistent with a Ba2þ ion exchange with the redox couple at potentials of 0.40 to 0.87 V, with the potential range of 0.87 to 1.0 V showing no obvious frequency change. The kinetics of the potential switching responses of an mvRuO/RuCN film in Ba(NO3 )2 to assess its performance as a display device was investigated, and the results are shown for potentials between )0.2 to 0.4 V in Fig. 6(B) and from 0.40 to 0.87 V in Fig. 6(C), respectively. In these experiments, a square wave potential was applied over a 2 s period to assess the film cation exchange capability. The reversibility of the mvRuO/ RuCN film during the cycling and the frequency change was good, and the cation exchange is obvious from the redox couples.
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Fig. 6. (A) Cyclic voltammograms of a gold electrode modified with a mvRuO/RuCN film synthesized from 1 103 M Ru(CN)4 6 added to 1 103 M Ru3þ in an aqueous 0.1 M Ba(NO)3 solution. The EQCM frequency change recorded for a film in an aqueous 0.1 M Ba(NO)3 solution and the EQCM data of a mvRuO/RuCN film in an aqueous 0.1 M Ba(NO)3 solution during potential switching from Eappl: ¼ 0:2 to +0.4 V vs. AgjAgCl (B), and Eappl: ¼ 0:4 to 0.87 V vs. AgjAgCl (C), respectively, using time pulses of 2 s.
The QCM provides the film mass, and the integrated current response can provide the effective molar mass of a redox couple. Approximately 3815 ng/cm2 of mvRuO/ RuCN was deposited on the gold electrode after seven cyclic voltammetric scans. The relation between the peak current and the scan rate is expressed by Ip ¼ n2 F 2 vACo =4MRT , where Co (in g/cm2 ), v, A, Ip , and M (in g/mol) represent the surface coverage concentration, the scan rate, the electrode area, the peak current, and effective molar mass. For a one electron redox couple, n ¼ 1, and the effective molar mass, M, is estimated to be about 1328 g/mol. Fig. 6(A) shows the EQCM results of a mvRuO/ RuCN film that was deposited on a gold electrode, at a coverage of about 3815 ng/cm2 (about 3.2 109 mol/ cm2 ), assuming that one effective molar mass has two mole positive charges (i.e., one mole of Ba2þ ) on exchange. The EQCM results in Fig. 6(B) show that an
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increase in frequency (or a decrease in mass) had occurred during the oxidation of the mvRuO/RuCN film. During the cyclic voltammetry, the frequency change was about 21 Hz (or 29.4 ng) for a 0.196 cm2 area gold electrode, assuming that the total Ba2þ ion exchange reaction had a frequency change of about 57 Hz. This frequency change is larger than the frequency exchange that occurred at the film. This result shows that there was perhaps water transfer in the switching of the mvRuO/RuCN film, as water could be exchanged in an mvRuO/RuCN film on cation insertion. The mvRuO/RuCN film could be synthesized from aqueous 0.1 M Ba2þ , Ca2þ , Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ nitrate and chloride aqueous solutions. Fig. 7 shows the consecutive cyclic voltammograms of an mvRuO/RuCN film synthesized from aqueous 0.1 M RbNO3 , Ba(NO3 )2 , and RbNO3 solutions at pH 6.0 (using diluted HNO3 and H3 PO4 to adjust the pH of the solution carefully) on a glassy carbon electrode. The voltammetric characteristics and the formal potential of the three redox couples were cation dependent (see Fig. 7 and Table 1). The results show that the formal potential and the voltammetric properties of films depended on the monovalent electrolyte cation used, (Hþ (pH dependent), Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ ), and on the divalent electrolyte cation used (Ba2þ or Ca2þ ). Fig. 8(A) and (B) show the results of consecutive cyclic voltammetry measurements of mvRuO/RuCN films in aqueous 0.1 M KNO3 solutions at pH 1.0 and 7.0. The cyclic voltammetry of an mvRuO/RuCN film in an aqueous KNO3 solutions at various pH values shows that the formal potentials of all the redox couples showed a negative shift over one cycle between the potentials of )0.2 and 1.05 V. However, the anodic peak potential exhibited almost the same value, with only the peak current value decreasing (see Fig. 8(A)). These results show the successful formation of mvRuO/RuCN films, and demonstrate that the voltammetric properties of the mvRuO/RuCN films depend on the seven monovalent electrolyte cations used (Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ ) and on the divalent electrolyte cation used (Ba2þ or Ca2þ ). The experimental results also show that mvRuO/RuCN films can be formed directly from the mixing of Ru3þ and Ru(CN)4 using consecutive cyclic voltammetry 6 and the nine cations above in the electrolyte solution (see Table 1).
3.4. Two types of ruthenium oxide/ruthenocyanide film deposition in aqueous Csþ and Ca2þ solutions The electrochemical formation of mvRuO/RuCN films showed two different types of cyclic voltammograms
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Fig. 7. Repetitive cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film synthesized from 1 103 M 3 Ru(CN)4 M Ru3þ in (A) 0.1 M RbNO3 , 6 added to aqueous 1 10 (B) 0.1 M Ba(NO)3 , and (C) 0.1 M KNO3 aqueous solutions at pH 6.0. Scan rate ¼ 0.1 V/s. Table 1 The formal potentials of ruthenium (III) ruthenocyanide films synthesized from different electrolytes Electrolyte
E1 °0 (V)
E2 °0 (V)
E3 °0 (V)
Electrode
pH
Ba(NO3 )2 Ba(NO3 )2 HNO3 LiNO3 NaNO3 KNO3 RbNO3 CsNO3 GaNO3
)0.02 )0.01 0.00 )0.02 )0.01 0.01 0.02 0.05 0.02
0.61 0.80 0.83 0.70 0.72 0.75 0.71 0.65 0.66
0.87 1.00 1.03 0.90 0.91 0.94 0.95 0.90 0.90
GC, Au GC GC GC GC GC GC GC GC
7.0 1.5 1.5 7.0 7.0 7.0 7.0 7.0 7.0
when the films were prepared using consecutive cyclic voltammetry in aqueous solutions with a glassy carbon electrode over different potential scan ranges.
Fig. 8. Repetitive cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film synthesized from 1 103 M 3 M Ru3þ in an aqueous 0.1 M Ru(CN)4 6 added to aqueous 1 10 KNO3 solution at (A) pH 1.0 and (B) pH 7.0. Scan rate ¼ 0.1 V/s.
Fig. 9(A)(a) and (b) show the repetitive cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film synthesized from 1 103 M 3 Ru(CN)4 M Ru3þ in an aqueous 0.1 6 added to 1 10 M CsCl solution at pH 6.0 for potential scan ranges between )0.2 and 0.95 V (9A(a)), and between )0.2 and 1.05 V (9A(b)). Three redox couples characterize the voltammograms, with formal potentials occurring between )0.2 V and +0.95 V (vs. AgjAgCl) when the scan potential ranges between )0.2 V and +0.95 V (Fig. 9(A)(a)). But the third redox couple (with an anodic peak potential Epa at about 1.0 V) of the mvRuO/ RuCN film was irreversible. The cathodic peak (with a cathodic peak potential Epc at about 0.84 V (Fig. 9(A)(a)) disappeared, leaving two anodic peaks (with Epa about 1.0 and 0.78 V) with only a single cathodic peak (Epc at about 0.7 V in Fig. 9(A)(a)) between
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potential scan range between )0.2 to +1.0 V, the third redox couple shows irreversibility, and one cathodic peak disappears to leave two anodic peaks, but only one cathodic peak in the potential scan range between 0.4 and 1.2 V. 2 3.5. Electrocatalytic reduction of SO2 5 and S2 O8 by a ruthenium oxide/ruthenocyanide film
The electrocatalytic reduction efficiency of SO2 5 and S2 O2 8 by mvRuO/RuCN films was investigated using a nitrogen-saturated and oxygen-free aqueous solution at 0 pH 5.0. The Eo value of the mvRuO/RuCN film was found to be about 0.0 V (vs. AgjAgCl) (see
Fig. 9. Repetitive cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film synthesized from 1 103 M 3 M Ru3þ at pH 6.0 in: (A) 0.1 M Ru(CN)4 6 added to aqueous 1 10 CsCl with different scan ranges between (a) –0.2 and 0.95 V and (b) between )0.2 and 1.05 V, and (B) in 0.1 M Ca(NO3 )2 with different scan ranges between: (a) –0.2 and 1.0 V and (b) between )0.2 and 1.1 V. Scan rate ¼ 0.1 V/s.
the potential scans of 0.4 and 1.15 V when the scan potential ranges between )0.2 V and +1.15 V (Fig. 9(A)(b)). Fig. 9(B) (a) and (b) show the repetitive cyclic voltammograms of a glassy carbon electrode modified with an mvRuO/RuCN film synthesized from 1 103 M 3 Ru(CN)4 M Ru3þ in an aqueous 0.1 6 added to 1 10 M Ca(NO3 )2 solution at pH 6.0 for potential scan ranges between )0.2 and 1.0 V (A), and between )0.2 and 1.1 V (B). Three redox couples also characterize these voltammograms, with formal potentials occurring between potential scan ranges between )0.2 and +1.02 V (see Fig. 9(B)(a)). However, the third redox couple (with Epa at about 0.98 V) of the mvRuO/RuCN film was irreversible. The cathodic peak (Epc at about 0.87 V in Fig. 9(B)(a)) and two anodic peaks (Epa about 0.98 and 0.78 V in Fig. 9(B)(a)) formed a single cathodic peak (Epc about 0.62 V, in Fig. 9(B)(b)) and two anodic peaks (Epc about 0.64 and 0.87 V) for potential scans in the range 0.4 and 1.1 V. These consecutive cyclic voltammograms also show that mvRuO/RuCN films are deposited from Ru3þ and Ru(CN)4 in aqueous monovalent cationic solutions 6 containing Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , Gaþ and divalent cationic Ba2þ ,and Ca2þ solutions. Three redox couples characterize the voltammograms for the potential scan range between )0.2 and 1.0 V. However, for the
Fig. 10. Cyclic voltammograms of a mvRuO/RuCN film adhered to a glassy carbon electrode in a 0.1 M KNO3 solution at pH 5.0. (A) 3 [SO2 M, (c) 1.0 102 M, (d) 1.5 102 5 ] ¼ (a) 0.0 M, (b) 5 10 0 M, and (e) 2.0 102 M. (a ) ¼ Bare glassy carbon electrode and 2 3 M. (B) [S2 O2 M, (c) [SO2 5 ] ¼ 2.0 10 8 ] ¼ 0 (a) 0.0 M, (b) 6 10 1.2 102 M, and (d) 2.4 102 M. (a ) ¼ Bare glassy carbon elec2 trode and [S2 O2 M. 8 ] ¼ 2.4 10
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Fig. 10(A)(a)). Fig. 10(A) shows the cyclic voltammograms of an mvRuO/RuCN film in an aqueous KNO3 solution at pH 5.0 in the presence of SO2 5 . The cathodic peak current of the mvRuO/RuCN film redox couple increased, while its anodic peak current decreased, as the concentration of SO2 5 increased. Fig. 10(B) also shows the cyclic voltammograms of an mvRuO/RuCN film in an aqueous KNO3 solution at pH 5.0 in the presence of S2 O2 8 . The cathodic peak current of the mvRuO/RuCN film redox couple also increased, while its anodic peak current decreased, as the concentration of S2 O2 in8 0 0 creased. Fig. 10(A)(a ) and (B)(a ) show cyclic voltammograms of SO2 and S2 O2 5 8 , respectively, between potentials of 0.8 and )0.3 V using bare glassy carbon electrode. The electrocatalytic reduction current developed directly from the redox couple of the catalyst at a potential of about 0.0 V when performing the electro2 catalytic reduction of SO2 5 and S2 O8 by the mvRuO/ RuCN films. In contrast, in the electrocatalytic reduc2 tion of SO2 5 and S2 O8 by iron hexacyanoferrate and nickel hexacyanoferrate [22], both electrocatalytic currents develop indirectly from the cathodic peak, as the electrocatalytic peak potential is more negative than the cathodic peak potential of both catalysts. The electrocatalytic reaction can be tested using cyclic voltammetry and bulk electrolysis. The current vs. time is controlled by the potential of the bulk electrolysis, and product analysis is carried out using ion chromatography. The results show obvious differences between the cases of (a) only reactants in the absence of a catalyst, (b) only the catalyst present, and (c) when the reactants and catalyst are both present. The current and product analyses both show that the reaction rate increases when both the reactant and catalyst are present. In the cyclic voltammetric experiments, the cathodic (or anodic) peak current of the mvRuO/RuCN film redox couple increased, while its anodic (or cathodic) peak current decreased through the electrocatalytic redox couple, as the concentration of reactant increased. The peak current increase is connected to the electron transfer rate (the rate of reactant transfer to product), which is faster in the presence of a catalyst than when the catalyst is absent and only the reactants are present. 2 The electrocatalytic reduction of SO2 5 and S2 O8 by mvRuO/RuCN films in an acidic aqueous solution is described by the following equations 2 þ SO2 5 þ 2H þ 2e ! SO4 þ H2 O
ð7Þ
2 S2 O2 8 þ 2e ! 2SO4 :
ð8Þ
3.6. Electrocatalytic oxidation of NADH and dopamine by a ruthenium oxide/ruthenocyanide film We were able to deposit an mvRuO/RuCN film on a glassy carbon electrode and perform the electrocatalytic
oxidation of NADH and dopamine. The electrocatalytic oxidation of NADH and dopamine shows different electrocatalytic properties. Fig. 11(A)(a) and (B)(a) show the cyclic voltammograms of an mvRuO/RuCN film in an aqueous 0.1 M Ba (NO3 )2 solution at pH 2.0. The electrocatalytic oxidation of NADH was performed at the mvRuO/RuCN film electrode, with the oxidation involving the transfer of two electrons and one proton in the reaction. The oxidation equation is NADH ! NADþ þ Hþ þ 2e :
ð9Þ
From the results shown in Fig. 11(A), the cyclic voltammogram of the mvRuO/RuCN film shows three redox couples. The electrocatalytic oxidation current increased significantly as the concentration of NADH in the aqueous solution increased, and the electrocatalytic oxidation current developed directly from the more
Fig. 11. Cyclic voltammograms of a mvRuO/RuCN film adhered to a glassy carbon electrode in an aqueous 0.2 M Ba(NO3 )2 solution at pH 2.0. (A) [NADH] ¼ (a) 0.0 M, (b) 5 104 M, (c) 1.0 103 M, and 0 (d) 1.5 103 M. (a ) ¼ Bare glassy carbon electrode with [NADH] 3 ¼ 1.5 10 M. (B) [Dopamine] ¼ (a) 0.0 M, (b) 5 104 M, (c) 0 1.0 103 M, and (d) 1.5 103 M. (a ) ¼ Bare glassy carbon elec3 trode with [Dopamine] ¼ 1.5 10 M.
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positive of the two redox couples of the mvRuO/RuCN film at potentials of about 0.7 and 0.9 V when performing the electrocatalytic oxidation of NADH and dopamine, respectively. The results show two different types of electrocatalytic oxidation for NADH and dopamine by mvRuO/ RuCN films. The cyclic voltammograms of dopamine in 0 an aqueous buffered solution at pH 2.0 (Fig. 11(B)(a )) shows one redox couple with an anodic oxidation peak at a potential of about 0.55 V (vs. AgjAgCl), and a cathodic peak at a potential of 0.28 V (vs. AgjAgCl). The electrocatalytic oxidation current increased significantly as the concentration of dopamine in the aqueous solution increased. The electrocatalytic oxidation current peak potentials at 0.75 and 1.0 V are both more positive than the electrochemical oxidation peak potential of dopamine in the absence of catalyst. The cyclic voltammograms of NADH in an aqueous buffered solution 0 at pH 2.0 (Fig. 11(A)(a )) shows an anodic oxidation peak at potentials more positive than 1.1 V (vs. AgjAgCl). The electrocatalytic oxidation current increased significantly as the concentration of NADH in the aqueous solution increased. The electrocatalytic oxidation current peak potentials at 0.75 and 0.97 V are both more negative than the electrochemical oxidation peak potential of NADH in the absence of a catalyst. The electrocatalytic oxidation and reversible reduction of dopamine by a mvRuO/RuCN film in an acidic aqueous solution is described by the following chemical reaction [37,38]
ð10Þ 3.7. The discussion on the electrocatalytic oxidation of 2 SO2 3 , S2 O3 , and N2 H4 by a ruthenium oxide/ruthenocyanide film 2 The electrocatalytic oxidation of SO2 3 , S2 O3 , and N2 H4 by an mvRuO/RuCN film in a 0.1 M KNO3 solution at pH 5.0 was studied. Fig. 12(A) shows the cyclic voltammograms of an mvRuO/RuCN film in a 0.1 M aqueous KNO3 solution at pH 5.0 with various concentrations of SO2 3 . The anodic current peaks of the two redox couples of the mvRuO/RuCN film have formal potentials between 0.4 and 1.0 V (vs. AgjAgCl), with the two anodic peak currents occurring at about 0.76 and 0.95 V (vs. AgjAgCl). These increased with increasing concentration of SO2 3 . Similar results were observed with various concentrations of S2 O2 3 (Fig. 12(B)), and N2 H4 (Fig. 12(C)). The Ipcat values were found to increase with increasing
Fig. 12. Cyclic voltammograms of a mvRuO/RuCN film adhered to a glassy carbon electrode in an aqueous 0.2 M Ba(NO3 ) solution at pH 3 2 3 5.0, with: (A) [SO2 3 ] ¼ 0–1.5 10 0 M, (B) [S2 O3 ] ¼ 0–1.5 10 M, (C) [N2 H4 ] ¼ 0–1.5 103 M. (a ) ¼ Bare glassy carbon electrode, and the substrate is at maximum concentration.
concentration of substrate. When the various concenand N2 H4 were added, the anodic trations of S2 O2 3 peak current increased noticeably, with a catalytic peak current occurring at about 0.76 V (vs. AgjAgCl), which was more marked than in the electrocatalytic oxidation of SO2 3 . These results may point to a practical application of mvRuO/RuCN-film-modified electrodes as electrochemical sensors to determine substrates. The anodic peak current increases noticeably, and develops directly from the redox couple in suitable aqueous cation solutions. This is interesting, as the Ipcat value increases when the concentration of the substrate increases. The electrocatalytic oxidation of various substrates by an mvRuO/RuCN film is summarized in Table 2. Fig. 13 shows the electrocatalytic oxidation of dopamine by an mvRuO/RuCN film using the rotating ring-disk electrode (RRDE) method. Fig. 13(A) shows the dopamine present in a pH 1.5 aqueous solution
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Table 2 The electrocatalytic properties of mvRuO/RuCN films with various substrates Substrate
Peak potential (V)a
Electrocatalytic type
pH
SO2 5 S2 O2 8 SO2 3 S2 O2 3
)0.07 )0.09 0.61, 0.98 0.80, 0.97 0.74, 0.98 0.76, 1.00 0.76, 0.96
Reduction Reduction Oxidation Oxidation Oxidation Oxidation Oxidation
5.0 5.0 5.0 5.0 2.0 2.0 5.0
NADH Dopamine N2 H4 a
Epcat ¼ the anodic peak potential when substrate is added.
when the mvRuO/RuCN-film-modified disk glassy carbon electrode and ring glassy carbon electrode had an applied potential of 0.1 V (vs. AgjAgCl) for 2500 rpm. In Fig. 13(A), the value of ID is the oxidation current, and IR is the reduction current from the chemical reaction described by Eq. (10). These results are consistent with the IR current being the reduction current shown in Eq. (10). The results show ID and IR increasing as the concentration of dopamine increases. Fig. 13(B) shows the dopamine present for various rotation rates in a pH 1.5 aqueous solution when the mvRuO/RuCN-film-modified glassy carbon disk electrode had an applied potential of 0.1 V (vs. AgjAgCl) at a constant concentration of dopamine. These results are consistent with ID being the oxidation current and IR being the reduction current described in Eq. (10) [37,38]. In Fig. 13(A) and (B), the zero current was found between the potentials of )0.2 and 0.3 V. The values of ID and IR are the oxidation and reduction currents at a potential of 0.78 V (where IR reached a plateau) from the baseline zero current (at a potential of about +0.25 V) of ID and IR . The best straight line through these points was selected (if selection of the best straight line were difficult, then perhaps the errors in ID and IR would be higher). The RRDE data were analyzed using the 1 Koutecky–Levich equation [39,40], and plotted as Ilim;c 1=2 vs. x , as shown in the inset of Fig. 13(B). 1=I ¼ 1=Ik þ 1=Ilim;c ;
ð11Þ
where 2=3
Ilim;c ¼ 0:62 nFAD0 x1=2 u1=6 c0 :
Fig. 13. RRDE voltammogram of a mvRuO/RuCN film adsorbed on a glassy carbon disk electrode in an aqueous 0.2 M Ba(NO3 )2 solution at pH 4.0. (A)with [dopamine] ¼ (a) 0.0 M, (b) 1 104 M, (c) 2 104 M, (d) 3 104 M, and (e) 4 104 M at a rotation rate of 2500 rpm. ER ¼ 0:1 V. (B) [dopamine] ¼ 3 104 M and rotation rate ¼ (a) 200, (b) 400, (c) 900, (d) 1200, (e) 1600, and (f) 2500 rpm. Electrode ¼ glassy carbon disk and ring (ER ¼ 0:1 V vs. AgjAgCl). Scan rate ¼ 1 0.015 V/s. Inset of (B) shows plots of Ilim vs. x1=2 .
ð12Þ
The parameter I is the measured limiting current of the disk, x is the rotation rate, D0 and c0 are the diffusion coefficient and the bulk concentration of dopamine respectively, and u is the kinematic viscosity of water in the experimental rotating rates. The inset of Fig. 13(B) 1 shows a plot of Ilim vs. x1=2 , Ik ¼ nFAkCc0 , where k is the rate constant of the chemical reaction between the film and dopamine, and C is the coverage of the catalyst on the electrode surface. Plots of 1==ID;lim vs. x1=2 are shown in the inset of Fig. 13(B). The surface coverage, C, was estimated to be C ¼ 1 108 mol/cm2 of the mvRuO/RuCN in the film from the chronocoulometry charge and the EQCM frequency change. If we assume that the film undergoes a layer-type reaction mechanism, then the rate constant of the chemical reaction, j, can be estimated to be an average value of j ¼ 3:5 103 M1 s1 from three epinephrine concentrations: 1 104 , 2 104 and 3 104 M.
4. Conclusions Polynuclear mixed-valent mvRuO/RuCN films were successfully synthesized using consecutive cyclic vol-
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tammetry on various electrodes directly from Ru3þ and Ru(CN)4 in various electrolyte solutions containing 6 Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ cations. The cyclic voltammograms showed three redox couples in aqueous solutions of Ba(NO3 )2 or BaCl2 , which contain a divalent cation, and the formal potentials of the redox couples of the cyclic voltammograms depended on the electrolyte cation and the pH. EQCM, cyclic voltammetry, UV–visible spectroscopy, and the stopped-flow method were used to study the growth mechanism of the mvRuO/RuCN films, with the EQCM and cyclic voltammetry used to study the in situ growth of the mvRuO/RuCN films. The results indicate that the redox process was confined to the surface, confirming the immobilized state of the mvRuO/ RuCN. A growth mechanism of the mvRuO/RuCN films was proposed whereby the film grew by electrochemically oxidizing the ruthenium(III) hexacyanoruthenate(II) (RuIII [RuII (CN)6 ] (or RuIII –O/RuII (CN)6 ) species. This species was then oxidized to (RuIII [RuIII (CN)6 ] (or RuIII –O/RuIII (CN)6 ), followed by film deposition to form a ruthenium oxide/ruthenocyanide(RuIV –O/RuIII (CN)6 ) film. The mvRuO/RuCN films can reduce SO2 5 , and S2 O2 electrocatalytically and are electrocatalytically 8 active. The experimental results also show that mvRuO/ RuCN films are electrocatalytically active in the oxida2 tion of NADH, dopamine, SO2 3 , S2 O3 , and N2 H4 . Acknowledgements This work was supported by the National Science Council of the Republic of China. References [1] [2] [3] [4] [5] [6]
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[8] J.A. Cox, K. Lewinski, Electroanalysis 6 (1994) 976. [9] K. Itaya, I. Uchida, V.D. Neff, Acc. Chem. Res. 19 (1986) 162. [10] D.N. Upadhyay, D.M. Kolb, J. Electroanal. Chem. 358 (1993) 317. [11] V.D. Neff, J. Electro. Chem. Soc. 132 (1985) 1382. [12] K. Honda, H. Hayashi, J. Electrochem. Soc. 134 (1987) 1330. [13] T.R.I. Cataldi, G.E. De Benedetto, J. Electroanal. Chem. 458 (1998) 149. [14] L.M. Siperko, T. Kuwana, J. Electrochem. Soc. 130 (1983) 396. [15] K. Ogura, I. Yoshida, Electrochim. Acta 32 (1987) 1191. [16] C. Lin, A.B. Bocarsly, J. Electroanal. Chem. 300 (1991) 325. [17] B.D. Humphrey, S. Sinha, A.B. Bocarsly, J. Phys. Chem. 91 (1987) 586. [18] P.J. Kulesza, K. Brajter, E. Dabek-Zlotorzynska, Anal. Chem. 59 (1987) 2776. [19] S.-M. Chen, J. Electroanal. Chem. 521 (2002) 29. [20] S.-M. Chen, J. Electroanal. Chem. 401 (1996) 147. [21] S.-M. Chen, Electrochim. Acta 43 (1998) 3359. [22] S.-M. Chen, C.-M. Chan, J. Electroanal. Chem. 543 (2003) 161. [23] R.J. Mortimer, D.R. Rosseinsky, J. Chem. Soc. Dalton Trans. (1984) 2059. [24] K. Tanaka, R. Tamamushi, J. Electroanal. Chem. 380 (1995) 279. [25] H. Kahlert, U. Retter, H. Lohse, K. Siegler, F. Scholz, J. Phys. Chem. 102 (1998) 8757. [26] L.T. Kubota, Y. Gushikem, J. Electroanal. Chem. 362 (1993) 219. [27] J.A. Cox, E. Dabek-Zlotorzynska, Electroanalysis 3 (1991) 239. [28] J.A. Cox, E. Dabek-Zlotorzynska, J. Chromatogr. 543 (1991) 226. [29] A. Przyjazny, J.A. Cox, Electroanalysis 5 (1993) 657. [30] J.A. Cox, K. Lewinski, Electroanalysis 6 (1994) 976. [31] J. Wang, Z. Lu, Electroanalysis 1 (1989) 517. [32] A.J. Bard, L.R. Faulkner, Electrochemical Method Fundamentals and Applications, John Wiley and Sons, New York, NY, 1980. [33] A.P. Brown, F.C. Anson, Anal. Chem. 49 (1977) 1589. [34] E. Laviron, J. Electroanal. Chem. 101 (1979) 19. [35] D.A. Buttry, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 17, Marcel Dekker, New York, NY, 1992. [36] S.J. Reddy, A.D. Fritz, J. Scholz, J. Electroanal. Chem. 403 (1996) 209. [37] P. Hernandez, I. Sanchez, F. Paton, L. Hernandez, Talanta 46 (1998) 985. [38] S.-M. Chen, K.-C. Lin, J. Electroanal. Chem. 523 (2002) 93. [39] A.J. Bard, L.R. Faulkner, Electrochemical Method Fundamentals and Applications, Wiley, New York, 2001. [40] R.R. Durand Jr., C.S. Bencosme, J.P. Collman, F.C. Anson, J. Am. Chem. Soc. 105 (1983) 2710.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 566 (2004) 291–303 www.elsevier.com/locate/jelechem
Preparation, characterization and electrocatalytic properties of polynuclear mixed-valent ruthenium oxide/hexacyanoruthenate film modified electrodes Shen-Ming Chen *, Sheh-Hung Hsueh Department of Chemical Engineering, National Taipei University of Technology, No. 1, section 3, Chung-Hsiao East Road, Taipei 10643, Taiwan, ROC Received 23 June 2003; received in revised form 20 October 2003; accepted 15 November 2003
Abstract Polynuclear mixed-valent ruthenium oxide/ruthenocyanide (ruthenium oxide/hexacyanoruthenate or mvRuO/RuCN) films were prepared using consecutive cyclic voltammetry directly from the mixing of Ru3þ and Ru(CN)4 6 ions from solutions of two divalent cations (Ba2þ and Ca2þ ), and seven monovalent cations (Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ ). The films exhibited three redox couples with Ba(NO3 )2 or BaCl2 aqueous solutions, and the formal potentials of the redox couples showed a cation and pH effect. An electrochemical quartz crystal microbalance (EQCM), cyclic voltammetry, UV–visible spectroscopy, and the stopped-flow method (SFM) were used to study the growth mechanism of the mvRuO/RuCN films. The results indicated that the redox process was confined to the immobilized ruthenium oxide/ruthenocyanide. The EQCM results showed a Ba2þ ion exchange reaction for the 2 two most negative redox couples. The electrocatalytic reduction properties of SO2 5 , and S2 O8 by the ruthenium oxide/ruthenocyanide films were determined. The electrocatalytic oxidation of NADH and dopamine were also determined, and revealed two 2 different types of properties. The electrocatalytic oxidations of SO2 3 , S2 O3 , and N2 H4 were also investigated. The electrocatalytic reactions of the ruthenium oxide/ruthenocyanide films were investigated using the rotating ring-disk electrode method. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Ruthenium ruthenocyanide; Film-modified electrodes; Electrocatalysis; Sulfur oxoanions; NADH; Dopamine
1. Introduction Ruthenium oxide/ruthenocyanide (mvRuO/RuCN) is the ruthenium analogue of iron hexacyanoferrate. Metal hexacyanoferrates and metal hexacyanoruthenates show interesting redox chemistry that is accompanied by changes in their electrochromic, ion exchange, and electrocatalytic properties. Chemically modified film electrodes of metal hexacyanoruthenates [1–8] and metal hexacyanoferrates [9–14] also show interesting redox properties. They are used in both chemistry and in material science, in such areas as electroanalysis, chemical sensing and electrocatalysis [15–22], in studies on interfacial charges, in ion exchange and electron *
Corresponding author. Tel.: +886-2270-17147; fax: +886-227025238. E-mail address: [email protected] (S.-M. Chen). 0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2003.11.040
transfer studies [23–25], and in research on surface chemical composition [26]. In general, an mvRuO/RuCN film can be deposited onto a working electrode by cycling the potential between 0.5 and 1.0 V in an aqueous solution containing Ru(CN)4 6 and 0.5 M KCl at pH 2 [1–8]. Two redox couples occur as a result, and these films have been used in amperometric determination and in the electrocatalysis of As(III) [2], insulin [4], methionine [27], amino acids [28], polypeptides [29], nitrosamines [30], and hydrazine [31]. We will discuss the successful formation and properties of mvRuO/RuCN films synthesized from electrolytic aqueous solutions containing the following cations: Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ , which are of scientific interest. To realize practical mvRuO/ RuCN-film-modified electrodes for ion sensors, the deposition processes of mvRuO/RuCN films must also be fully characterized.
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The results of the electrocatalytic activity measurements on the mvRuO/RuCN film are applicable to analytical applications, and to the electrochemical reaction transfer activity of low electroactive compounds. The analytical methods used are well established, and they are important for the determination of both these analytes. The electrocatalytic reduction properties of SO2 5 , and S2 O2 8 by the mvRuO/RuCN films were determined, and shown to develop directly from the film redox couple as the cathodic current was increased. The electrocatalytic oxidation of NADH, dopamine, SO2 3 , S2 O2 3 , and N2 H4 were also investigated. The electrochemical formation of mvRuO/RuCN films on a glassy carbon electrode was carried out using consecutive cyclic voltammetry in aqueous barium nitrate solutions. The mvRuO/RuCN films showed three redox couples with formal potentials between )0.4 and 1.1 V in divalent cationic Ba(NO3 )2 and BaCl2 aqueous solutions. This paper reports on the successful preparation of mvRuO/RuCN films formed by directly mixing Ru3þ and Ru(CN)4 6 , and deposit onto a working electrode by cycling the potential between )0.4 and 1.0 V in aqueous solutions at pH 7, and in acidic aqueous solutions using consecutive cyclic voltammetry for seven electrolyte monovalent cation solutions (Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ ). Films also showed three redox couples in divalent cationic Ba(NO3 )2 and BaCl2 aqueous solutions. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth processes of the mvRuO/RuCN films, and their electrochemical properties. The cyclic voltammograms of the mvRuO/RuCN films show an obvious cation effect: Ba2þ , Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ . The EQCM, cyclic voltammetry, UV–visible spectroscopy, and the stopped-flow method were used to study the growth mechanisms of the mvRuO/RuCN 2 films. The electrocatalytic reduction of SO2 5 , and S2 O8 by the mvRuO/RuCN film showed obvious activity. The electrocatalytic oxidation of NADH, dopamine, SO2 3 , S2 O2 , and N H also showed activity. The electrocat2 4 3 alytic reactions of dopamine with the mvRuO/RuCN films were also investigated using the rotating ring-disk electrode (RRDE) method.
trodes. The glassy carbon electrode was polished with 0.05 lm alumina on Buehler felt pads and then cleaned ultrasonically for 1 min. The auxiliary compartment contained a platinum wire, which was separated by a medium-sized glass frit. All cell potentials were taken using either an AgjAgCljKCl (saturated solution) reference electrode or an HgjHg2 Cl2 jKCl (saturated solution) reference electrode. The working electrode for the EQCM measurements was an 8 MHz AT-cut quartz crystal with gold electrodes. The diameter of the quartz crystal was 13.7 mm and the gold electrode diameter was 5 mm. UV–visible spectra were measured using a Hitachi Model U-3300 spectrophotometer. The RRDE experiments were performed using a Pine Instrument Co. electrode in conjunction with a CH Instruments CHI-750 potentiostat connected to a model AFMSRX analytical rotator. The rotating ring-disk electrode (RRDE), purchased from the Pine Instrument Co., consisted of a glassy carbon disk electrode and a glassy carbon (or platinum) ring electrode. Kinetic measurements were performed using a BioLogic stopped-flow module SFM-20, which consisted of a mechanical subsystem and a power supply. The fast reaction spectrometer was an optical system composed of an MOS-250 spectrophotometer and spectrofluorometer. The kinetics were recorded at a wavelength of 600 nm. All the chemicals used were of analytical grade. The K4 Ru(CN)6 and RuCl3 were purchased from the Aldrich Chemical Co., USA, and the NADH and dopamine were purchased from the Sigma Chemical Co., USA. The aqueous solutions were prepared using doubly distilled deionized water, and the solutions were deoxygenated by purging with pre-purified nitrogen gas. The electrochemical formation of the mvRuO/RuCN films was performed by continuous cycling of the potential of the working electrode in a defined potential range in a suitable aqueous solution containing Ru3þ and Ru(CN)4 6 ions. Typical electrochemical formation of mvRuO/RuCN film was performed by repetitive cyclic voltammetry on cycling the potential of the working electrode in a defined potential range in a supporting electrolyte aqueous solution containing K4 Ru(CN)6 with Ru3þ ions (with the balancing anion being chloride or nitrate).
2. Experimental The electrochemistry was performed using a Bioanalytical Systems Model CV-50W, and CH Instruments CHI-400 and CHI-750 potentiostats. Cyclic voltammetry was conducted using a three-electrode cell in which a BAS glassy carbon electrode, a platinum electrode, and an indium tin dioxide (ITO) electrode (which was composed of indium tin oxide that was sputter deposited on a glass substrate) were used as the working elec-
3. Results and discussion 3.1. Ruthenium oxide/ruthenocyanide film deposition in aqueous Ba(NO3 )2 solutions The electrochemical formation of mvRuO/RuCN films on a glassy carbon electrode was carried out using consecutive cyclic voltammetry in aqueous barium ni-
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trate solutions. The mvRuO/RuCN films showed three redox couples with formal potentials between )0.4 and 1.0 V in aqueous Ba(NO3 )2 or BaCl2 divalent cation solutions. Barium nitrate (or barium chloride) was chosen as the salt for the electrolyte solution because the mvRuO/RuCN films showed three redox couples with formal potentials between )0.4 and 1.0 V in aqueous Ba(NO3 )2 or BaCl2 divalent cation solutions. Fig. 1(A) shows the consecutive cyclic voltammograms of an mvRuO/RuCN film deposited from Ru3þ and Ru(CN)4 6 in an aqueous 0.1 M Ba(NO3 )2 solution at pH 6.0. Three redox couples characterize the voltammograms, with formal potentials occurring between )0.4 and 1.0 V (vs. AgjAgCl), at about 0.87, 0.61, and – 0.02 V (vs. AgjAgCl). In acidic and neutral aqueous solutions, the results show that mvRuO/RuCN films can be formed directly from mixing Ru3þ and Ru(CN)4 in aqueous 0.1 M 6 Ba(NO3 )2 solutions by consecutive cyclic voltammetry. These mvRuO/RuCN films show three redox couples
with formal potentials between )0.4 and 1.0 V in Ba(NO3 )2 divalent cation solutions. Fig. 1(B)(b) shows the consecutive cyclic voltammograms of ruthenium oxide deposition from Ru3þ in an aqueous 0.1 M Ba(NO3 )2 solution at pH 6.0. Two redox couples characterize the voltammograms, with formal potentials occurring between )0.2 and 1.0 V (vs. AgjAgCl). The two redox couples of the ruthenium oxide film are proposed to arise from the [RuII –O]/ [RuIII –O] and [RuIII –O//[RuIV –O] redox couples, with formal potentials at about 0.8 and 0.25 V (vs. AgjAgCl), respectively. Fig. 1(B)(a) shows the cyclic voltammograms of Ru(CN)4 in an aqueous 0.1 M Ba(NO3 )2 6 solution at pH 6.0. Fig. 2(A) shows that the mvRuO/RuCN films obtained on a glassy carbon electrode from a 0.1 M aqueous KNO3 solution at pH 2.0 resulted in two chemically reversible redox couples for various scan rates. The inset of Fig. 2(A) shows a plot of cathodic peak current, Ipc , vs. scan rate, illustrating a close linear dependence of Ipc on the scan rate, and that the ratio of the anodic to the cathodic peak current (Ipa =Ipc ) was equal to unity. Fig. 2(B) shows that the mvRuO/RuCN films obtained on a glassy carbon electrode from a 0.1 M aqueous Ba(NO3 )2 solution at pH 2.0 exhibited three chemically reversible redox couples for various scan rates. The inset of Fig. 2(B) show plots of Ipc vs. scan rate at potentials of about 0.6 and 0.0 V, respectively. These illustrate a linear dependence of Ipc on the scan rate, and that the ratio of Ipa =Ipc was approximately equal to unity. The behavior in Fig. 2(A) and (B) is consistent with a diffusionless, reversible electron transfer process at low scan rates. The peak current and scan rate are related as follows [32,33] Ip ¼ n2 F 2 vACo =4RT
Fig. 1. (A) Repetitive cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film synthesized from 1 103 M Ru(CN)4 added to 1 103 M Ru3þ in an aqueous 0.1 M 6 Ba(NO)3 solution at pH 6.0. (B) (a) Cyclic voltammograms of 1 103 M Ru(CN)4 6 in an aqueous 0.1 M Ba(NO)3 solution at pH 6.0, and (b) repetitive cyclic voltammograms of a glassy carbon electrode modified with a ruthenium oxide film synthesized from 1 103 M Ru3þ in an aqueous 0.1 M Ba(NO)3 solution at pH 6.0. Scan rate ¼ 0.1 V/s.
293
ð1Þ
where Co , v, A, and Ip represent the surface coverage concentration, the scan rate, the electrode area, and the peak current, respectively. This result indicates that the redox process was confined to the surface of the mvRuO/RuCN film on the glassy carbon electrode, confirming the immobilized state of the film. In the EQCM experiments, the change in mass (or coverage) at the quartz crystal was calculated from the change in the observed frequency using the Sauerbrey equation. Using this, the film coverage (in ng/cm2 or mol/cm2 ) deposited on the electrode surface during the electrochemical process can be calculated. This result indicates that the redox process was confined to the surface of the mvRuO/RuCN film on the glassy carbon electrode, confirming the immobilized state of the mvRuO/RuCN [32,33]. The three redox couples of the mvRuO/RuCN film are proposed to be from the [RuII –O/[RuII (CN)6 ]/ [RuIII –O/[RuII (CN)6 ], [RuIII –O/[RuII (CN)6 ]/[RuIII –O/ III [Ru (CN)6 ], and [RuIII –O/[RuIII (CN)6 ]/[RuIV –O/ III [Ru (CN)6 ] redox couples.
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3.2. Ruthenium oxide/ruthenocyanide film deposition and in situ EQCM measurements We successfully grew mvRuO/RuCN films on a gold electrode from an aqueous 0.1 M Ba(NO3 )2 solution at pH 6.0. The cyclic voltammograms are shown in Fig. 3(A), with Fig. 3(B) showing the growth of the mvRuO/RuCN film on the gold electrode, and the resulting change in the EQCM frequency. The mvRuO/ RuCN film obtained from the aqueous 0.1 M Ba(NO3 )2 solution showed three redox couples occurring between potentials of –0.2 and 1.0 V, with formal potentials at approximately 0.9, 0.6, and 0.0 V (vs. AgjAgCl), respectively. Fig. 3(B) shows the change in EQCM frequency recorded during the first seven cycles of the consecutive cyclic voltammetry. The voltammetric peak current in Fig. 3(A) and the frequency decrease (or mass increase) in Fig. 3(B) are consistent with the growth of an mvRuO/RuCN film on the gold electrode. The EQCM results show that the deposition of the film occurred between potentials of 0.8 and 1.0 V (vs. AgjAgCl). This potential range is where [RuIII –O/[RuIII -
Fig. 2. (A) Cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film in an aqueous 0.1 M KNO3 solution at pH 2.0 for various scan rates: (a) 0.02 V/s, (b) 0.03 V/s, (c) 0.045 V/s, (d) 0.06 V/s, (e) 0.08 V/s, (f) 0.1 V/s, (g) 0.12 V/s, (h) 0.14 V/s, (i) 0.16 V/ s, and (j) 0.18 V/s. The inset shows a plot of the two cathodic peak currents Ipc vs. scan rate at potentials of: (a) about 0.65 V and (b) about 0.0 V. (B) Cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film in an aqueous 0.1 M Ba(NO3 )2 solution at pH 4.0 for various scan rates: (a) 0.01 V/s, (b) 0.02 V/s, (c) 0.03 V/s, (d) 0.045 V/s, (e) 0.06 V/s, (f) 0.08 V/s, (g) 0.1 V/s, (h) 0.12 V/s, (i) 0.14 V/s, (j) 0.16 V/s, and (k) 0.18 V/s. Inset shows a plot of the two cathodic peak currents Ipc vs. scan rate at: (a) about 0.6 V, and (b) about 0.0 V.
The mvRuO/RuCN film electron transfer rate, ks , was estimated using an equation derived by Laviron [34] for a diffusionless electron transfer cyclic voltammogram with nDEp > 200 mV log ks ¼ a logð1 aÞ þ ð1 aÞ log a logðRT =nFvÞ að1 aÞnF DEp =2:3RT :
ð2Þ
We used an electrochemical transfer coefficient a ¼ 0:5, and assumed that n ¼ 1 for RuO/[Ru(CN)6 ], for the electron transfer process of the [RuII O/[RuII (CN)6 ]/ [RuIII O/[RuII (CN)6 ] or [RuIII O/[RuII (CN)6 ]/[RuIII O/ [RuIII (CN)6 ] redox couples. The average ks had a value of about 1.1 s1 for scan rates between 200 and 1000 mV/s.
Fig. 3. (A) Consecutive cyclic voltammograms of a gold electrode modified with a mvRuO/RuCN film synthesized from 1 103 M 3 Ru(CN)4 M Ru3þ in an aqueous 0.1 M Ba(NO3 )2 6 added to 1 10 solution at pH 6.0. Scan rate ¼ 0.02 V/s. (B) The change in EQCM frequency recorded concurrently with the consecutive cyclic voltammograms of Figure 3(A). Inset (a) shows a plot of the change in cathodic peak current at about 0.0 V vs. scan cycle, and (b) the total frequency change vs. scan cycle.
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(CN)6 ] is oxidized to [RuIV –O/[RuIII (CN)6 ], and is deposited on the electrode surface. In the EQCM experiments, the change in mass at the quartz crystal was calculated from the change in the measurement frequency using the Sauerbrey equation [35,36] Mass change ðDmÞ ¼ ð1=2Þðfo2 ÞðDf ÞAðkqÞ
1=2
;
ð3Þ
where A is the area of the gold disk coated onto the quartz crystal, q is the density of the crystal, k is the shear modulus of the crystal, Df is the measured frequency change, and fo is the oscillation frequency of the crystal. A frequency change of 1 Hz is equivalent to a 1.4 ng change in mass. During the first cyclic voltammetry scan, approximately 721 ng/cm2 of mvRuO/RuCN was deposited on the gold electrode. Approximately 3815 ng/ cm2 of mvRuO/RuCN was deposited on the gold electrode after seven cyclic voltammetric scans. The relatively large electrodeposition that occurred during the first scan cycle was reflected in the increased frequency change from the mvRuO/RuCN film adhering to the (initially) clean gold electrode surface (see Fig. 3(B)). The rate of film growth (from the change in frequency) was slower in later scan cycles (Fig. 3(B)). The inset of Fig. 3(A)(a) shows a plot of the change in cathodic peak current at a potential of about 0.0 V vs. scan cycle, and Fig. 3(A)(b) shows the total frequency change vs. scan cycle. EQCM measurements are a very effective method for monitoring the in situ growth of mvRuO/RuCN films on gold electrodes (see Fig. 3(B)), because mvRuO/ RuCN films grow steadily with time on the gold electrode. The EQCM measurements show that deposition of the film occurred at potentials between 0.8 and 1.0 V (vs. AgjAgCl) (Fig. 3(B)), and that no deposition occurred at a potential more negative than 0.8 V. The results also show that before the film formed, Ru3þ and RuII (CN)4 6 reacted to form a new species. When the applied working electrode was between potentials of 0.8 and )0.2 V (vs. AgjAgCl), no deposition occurred, as the EQCM showed no frequency change between 0.8 V and )0.2 V. The Ru3þ and RuII (CN)4 6 species reacted to yield a soluble form of RuIII –RuII (CN)6 (or RuIII –O/ RuII (CN)6 ) after RuIII –RuII (CN)6 (or RuIII –O/RuII (CN)6 ) oxidized to RuIII –RuIII (CN)6 (or RuIII –O/RuIII (CN)6 ). Then, the RuIII –RuIII (CN)6 (or RuIII –O/ RuIII (CN)6 ) species was oxidized to RuIV –O/RuIII (CN)6 , which was subsequently deposited on the electrodeÕs surface to form the mvRuO/RuCN film. Fig. 4 shows the reaction of Ru3þ and RuII (CN)4 6 in an aqueous 0.1 M BaCl2 solution at pH 6.0. Fig. 4(a) shows the UV–visible absorption spectra of 1.3 103 3þ M Ru(CN)4 was gradually added to the 6 , where Ru solution. Fig. 4(a) shows the UV–visible absorption spectrum of an aqueous 3 102 M Ru3þ solution that was added to 10 ml of an aqueous 1.3 103 M
Fig. 4. (a) UV–visible absorption spectra of Ru3þ gradually added to an aqueous 1.3 103 M Ru(CN)4 6 solution. The inset shows a plot of the absorbance vs. the ratio of [Ru3þ ]/[Ru(CN)4 6 ] from Fig. 3(A), with the wavelength set at (b) k ¼ 360 nm and (c) k ¼ 600 nm.
Ru(CN)4 6 and 0.1 M BaCl2 solution. The UV–visible absorption spectrum changed when the 3 102 M Ru3þ solution was gradually added (1 ml on average added 25 times) to the aqueous 1.3 103 M Ru(CN)4 6 and 0.1 M BaCl2 solution. The results show that the Ru3þ and RuII (CN)4 two-colorless species react to 6 form a violet-colored solution that exhibited absorption peaks at k ¼ 360 and 600 nm, with no obvious absorption peaks of Ru3þ and RuII (CN)4 6 . Fig. 4(b) and (c) show the changes in UV–visible absorption spectra at k ¼ 360 nm and 600 nm, respectively, when Ru3þ was gradually added to 1.3 103 M Ru(CN)4 in an 6 aqueous BaCl2 solution at pH 6.0. The results show that the absorption spectral peaks of the new species formed from Ru3þ and RuII (CN)4 6 with absorption peaks at k ¼ 360 and 600 nm, increase as the concentration of Ru3þ increases up to a concentration of 1.3 103 M, as can be seen form the plots of absorbance vs. the ratio of [Ru3þ ]/[Ru(CN)4 6 ] in Fig. 4(b) and (c). The results show that the absorption increases up to a ratio of [Ru3þ ]/[Ru(CN)4 6 ] ¼ 1. The III Ru3þ and RuIII (CN)4 react to form Ru [RuII (CN)6 ] 6
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(or RuIII –O[RuII (CN)6 ]), which has a ratio of [Ru3þ ]/ [Ru(CN)4 6 ] ¼ 1. Stopped-flow kinetic measurements at k ¼ 360 nm of 2 103 M Ru(CN)4 in an aqueous 6 BaCl2 solution at pH 6.0 mixed with Ru3þ in concentrations of 5 104 M, 1 103 M, and 2 103 M were carried out. From the change in absorption of the stopped-flow kinetic measurements, as shown in Fig. 5, we can conclude that the RuIII [RuII (CN)6 ] (or RuIII –O/ RuII (CN)6 ) species formed by the Ru3þ reacts with the RuII (CN)4 to dive a voltammogram having a shape 6 that has two plateaus yielding a soluble ruthenium(III) hexacyanoruthenate(II) (RuIII [RuII (CN)6 ]) (or RuIII –O/ RuII (CN)6 ) species with a half life (t1=2 ) of approximately 0.025 s. The two plateaus of the stopped-flow kinetic measurements could be caused by more than one stage existing in the reaction of Ru3þ with RuII (CN)4 6 to yield a soluble species. The cyclic voltammetric results also show that Ru3þ reacts instantaneously with RuII (CN)4 to yield a soluble species. The cyclic voltam6 metry results show that when 1 103 M Ru3þ was added to the 1 103 M Ru(CN)4 6 aqueous solution, the Ru(CN)4 6 redox couple disappeared (see Figs. 1 and 3(A)), and the film grew by the electrochemical oxidation of the ruthenium(III) hexacyanoruthenate(II) (RuIII [RuII (CN)6 ] or RuIII –O/RuII (CN)6 ) species. This was oxidized to (RuIII [RuIII (CN)6 ] or RuIII -O/ RuIII (CN)6 ), followed by further oxidation and film deposition to form the ruthenium oxide/ruthenocyanide(RuIV –O/RuIII (CN)6 ) film. The reaction of the solution and film formation processes of 1 103 M Ru(CN)4 and 1 103 M 6 Ru3þ in an aqueous 0.1 M Ba2þ solution are proposed as follows 4
Ru3þ þ RuðCNÞ6
! RuIII ½RuII ðCNÞ6 ðor RuIII –O=RuII ðCNÞ6 Þ
ð4Þ
Fig. 5. Stopped-flow kinetic measurements at a wavelength of k ¼ 360 nm for 2 102 M Ru(CN)4 6 in an aqueous 0.1 M BaCl2 solution mixed with Ru3þ at concentrations of: (a) 5 104 M, (b) 1 103 M, and (c) 2 103 M.
RuIII ½RuII ðCNÞ6 ðor RuIII –O=RuII ðCNÞ6 Þ
RuIII ½RuIII ðCNÞ6 ðor RuIII –O=RuIII ðCNÞ6 Þ þ e ð5Þ RuIII ½RuIII ðCNÞ6 ðor RuIII –O=RuIII ðCNÞ6 Þ ! RuIV –O=RuIII ðCNÞ6 þ e ! film deposition:
ð6Þ
The film grows by the electrochemical oxidation of RuIII [RuII (CN)6 ] (or RuIII –O/RuII (CN)6 ), which is oxidized to RuIII [RuIII (CN)6 ] (or RuIII –O/RuIII (CN)6 ) (Eq. (5)), followed by the reaction described by Eq. (6), to form the film. The ‘‘polynuclear mixed-valent ruthenium oxide/ hexacyanoruthenate (mvRuO/RuCN film)’’ is denoted as Ru–O/Ru(CN)6 , and occurs after the Ru3þ and RuII (CN)4 6 species have reacted to yield a soluble form that is oxidized to RuIV –O/RuIII (CN)6 . Before RuIV –O/ RuIII (CN)6 is produced by electrochemical oxidation, the soluble Ru3þ species reacts with RuII (CN)4 6 to yield either RuIII –RuII (CN)6 or RuIII -O/RuII (CN)6 . The soluble species is then oxidized to RuIV –O/RuIII (CN)6 , which is subsequently deposited on the electrode surface to form the mvRuO/RuCN film. 3.3. The electrochemical properties of the ruthenium oxide/ruthenocyanide films Fig. 6 shows the results of cyclic voltammetry and EQCM measurements on an mvRuO/RuCN film in an aqueous 0.1 M Ba(NO3 )2 solution. The results in Fig. 6(A) show that the three redox couples exhibit voltammetric currents (and frequency changes) in the potential range of )0.2 to 0.4 V (frequency increase, mass decrease), from 0.40 to 0.87 V (frequency decrease, mass increase), and from 0.87 to 1.0 V (no obvious change in frequency), respectively. The frequency increase (or mass decrease) in Fig. 6(A) is consistent with a Ba2þ ion exchange with the redox couple at potentials of )0.2 to 0.4 V, and the frequency decrease (or mass increase) in Fig. 6(A) is consistent with a Ba2þ ion exchange with the redox couple at potentials of 0.40 to 0.87 V, with the potential range of 0.87 to 1.0 V showing no obvious frequency change. The kinetics of the potential switching responses of an mvRuO/RuCN film in Ba(NO3 )2 to assess its performance as a display device was investigated, and the results are shown for potentials between )0.2 to 0.4 V in Fig. 6(B) and from 0.40 to 0.87 V in Fig. 6(C), respectively. In these experiments, a square wave potential was applied over a 2 s period to assess the film cation exchange capability. The reversibility of the mvRuO/ RuCN film during the cycling and the frequency change was good, and the cation exchange is obvious from the redox couples.
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Fig. 6. (A) Cyclic voltammograms of a gold electrode modified with a mvRuO/RuCN film synthesized from 1 103 M Ru(CN)4 6 added to 1 103 M Ru3þ in an aqueous 0.1 M Ba(NO)3 solution. The EQCM frequency change recorded for a film in an aqueous 0.1 M Ba(NO)3 solution and the EQCM data of a mvRuO/RuCN film in an aqueous 0.1 M Ba(NO)3 solution during potential switching from Eappl: ¼ 0:2 to +0.4 V vs. AgjAgCl (B), and Eappl: ¼ 0:4 to 0.87 V vs. AgjAgCl (C), respectively, using time pulses of 2 s.
The QCM provides the film mass, and the integrated current response can provide the effective molar mass of a redox couple. Approximately 3815 ng/cm2 of mvRuO/ RuCN was deposited on the gold electrode after seven cyclic voltammetric scans. The relation between the peak current and the scan rate is expressed by Ip ¼ n2 F 2 vACo =4MRT , where Co (in g/cm2 ), v, A, Ip , and M (in g/mol) represent the surface coverage concentration, the scan rate, the electrode area, the peak current, and effective molar mass. For a one electron redox couple, n ¼ 1, and the effective molar mass, M, is estimated to be about 1328 g/mol. Fig. 6(A) shows the EQCM results of a mvRuO/ RuCN film that was deposited on a gold electrode, at a coverage of about 3815 ng/cm2 (about 3.2 109 mol/ cm2 ), assuming that one effective molar mass has two mole positive charges (i.e., one mole of Ba2þ ) on exchange. The EQCM results in Fig. 6(B) show that an
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increase in frequency (or a decrease in mass) had occurred during the oxidation of the mvRuO/RuCN film. During the cyclic voltammetry, the frequency change was about 21 Hz (or 29.4 ng) for a 0.196 cm2 area gold electrode, assuming that the total Ba2þ ion exchange reaction had a frequency change of about 57 Hz. This frequency change is larger than the frequency exchange that occurred at the film. This result shows that there was perhaps water transfer in the switching of the mvRuO/RuCN film, as water could be exchanged in an mvRuO/RuCN film on cation insertion. The mvRuO/RuCN film could be synthesized from aqueous 0.1 M Ba2þ , Ca2þ , Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ nitrate and chloride aqueous solutions. Fig. 7 shows the consecutive cyclic voltammograms of an mvRuO/RuCN film synthesized from aqueous 0.1 M RbNO3 , Ba(NO3 )2 , and RbNO3 solutions at pH 6.0 (using diluted HNO3 and H3 PO4 to adjust the pH of the solution carefully) on a glassy carbon electrode. The voltammetric characteristics and the formal potential of the three redox couples were cation dependent (see Fig. 7 and Table 1). The results show that the formal potential and the voltammetric properties of films depended on the monovalent electrolyte cation used, (Hþ (pH dependent), Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ ), and on the divalent electrolyte cation used (Ba2þ or Ca2þ ). Fig. 8(A) and (B) show the results of consecutive cyclic voltammetry measurements of mvRuO/RuCN films in aqueous 0.1 M KNO3 solutions at pH 1.0 and 7.0. The cyclic voltammetry of an mvRuO/RuCN film in an aqueous KNO3 solutions at various pH values shows that the formal potentials of all the redox couples showed a negative shift over one cycle between the potentials of )0.2 and 1.05 V. However, the anodic peak potential exhibited almost the same value, with only the peak current value decreasing (see Fig. 8(A)). These results show the successful formation of mvRuO/RuCN films, and demonstrate that the voltammetric properties of the mvRuO/RuCN films depend on the seven monovalent electrolyte cations used (Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ ) and on the divalent electrolyte cation used (Ba2þ or Ca2þ ). The experimental results also show that mvRuO/RuCN films can be formed directly from the mixing of Ru3þ and Ru(CN)4 using consecutive cyclic voltammetry 6 and the nine cations above in the electrolyte solution (see Table 1).
3.4. Two types of ruthenium oxide/ruthenocyanide film deposition in aqueous Csþ and Ca2þ solutions The electrochemical formation of mvRuO/RuCN films showed two different types of cyclic voltammograms
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Fig. 7. Repetitive cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film synthesized from 1 103 M 3 Ru(CN)4 M Ru3þ in (A) 0.1 M RbNO3 , 6 added to aqueous 1 10 (B) 0.1 M Ba(NO)3 , and (C) 0.1 M KNO3 aqueous solutions at pH 6.0. Scan rate ¼ 0.1 V/s. Table 1 The formal potentials of ruthenium (III) ruthenocyanide films synthesized from different electrolytes Electrolyte
E1 °0 (V)
E2 °0 (V)
E3 °0 (V)
Electrode
pH
Ba(NO3 )2 Ba(NO3 )2 HNO3 LiNO3 NaNO3 KNO3 RbNO3 CsNO3 GaNO3
)0.02 )0.01 0.00 )0.02 )0.01 0.01 0.02 0.05 0.02
0.61 0.80 0.83 0.70 0.72 0.75 0.71 0.65 0.66
0.87 1.00 1.03 0.90 0.91 0.94 0.95 0.90 0.90
GC, Au GC GC GC GC GC GC GC GC
7.0 1.5 1.5 7.0 7.0 7.0 7.0 7.0 7.0
when the films were prepared using consecutive cyclic voltammetry in aqueous solutions with a glassy carbon electrode over different potential scan ranges.
Fig. 8. Repetitive cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film synthesized from 1 103 M 3 M Ru3þ in an aqueous 0.1 M Ru(CN)4 6 added to aqueous 1 10 KNO3 solution at (A) pH 1.0 and (B) pH 7.0. Scan rate ¼ 0.1 V/s.
Fig. 9(A)(a) and (b) show the repetitive cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film synthesized from 1 103 M 3 Ru(CN)4 M Ru3þ in an aqueous 0.1 6 added to 1 10 M CsCl solution at pH 6.0 for potential scan ranges between )0.2 and 0.95 V (9A(a)), and between )0.2 and 1.05 V (9A(b)). Three redox couples characterize the voltammograms, with formal potentials occurring between )0.2 V and +0.95 V (vs. AgjAgCl) when the scan potential ranges between )0.2 V and +0.95 V (Fig. 9(A)(a)). But the third redox couple (with an anodic peak potential Epa at about 1.0 V) of the mvRuO/ RuCN film was irreversible. The cathodic peak (with a cathodic peak potential Epc at about 0.84 V (Fig. 9(A)(a)) disappeared, leaving two anodic peaks (with Epa about 1.0 and 0.78 V) with only a single cathodic peak (Epc at about 0.7 V in Fig. 9(A)(a)) between
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potential scan range between )0.2 to +1.0 V, the third redox couple shows irreversibility, and one cathodic peak disappears to leave two anodic peaks, but only one cathodic peak in the potential scan range between 0.4 and 1.2 V. 2 3.5. Electrocatalytic reduction of SO2 5 and S2 O8 by a ruthenium oxide/ruthenocyanide film
The electrocatalytic reduction efficiency of SO2 5 and S2 O2 8 by mvRuO/RuCN films was investigated using a nitrogen-saturated and oxygen-free aqueous solution at 0 pH 5.0. The Eo value of the mvRuO/RuCN film was found to be about 0.0 V (vs. AgjAgCl) (see
Fig. 9. Repetitive cyclic voltammograms of a glassy carbon electrode modified with a mvRuO/RuCN film synthesized from 1 103 M 3 M Ru3þ at pH 6.0 in: (A) 0.1 M Ru(CN)4 6 added to aqueous 1 10 CsCl with different scan ranges between (a) –0.2 and 0.95 V and (b) between )0.2 and 1.05 V, and (B) in 0.1 M Ca(NO3 )2 with different scan ranges between: (a) –0.2 and 1.0 V and (b) between )0.2 and 1.1 V. Scan rate ¼ 0.1 V/s.
the potential scans of 0.4 and 1.15 V when the scan potential ranges between )0.2 V and +1.15 V (Fig. 9(A)(b)). Fig. 9(B) (a) and (b) show the repetitive cyclic voltammograms of a glassy carbon electrode modified with an mvRuO/RuCN film synthesized from 1 103 M 3 Ru(CN)4 M Ru3þ in an aqueous 0.1 6 added to 1 10 M Ca(NO3 )2 solution at pH 6.0 for potential scan ranges between )0.2 and 1.0 V (A), and between )0.2 and 1.1 V (B). Three redox couples also characterize these voltammograms, with formal potentials occurring between potential scan ranges between )0.2 and +1.02 V (see Fig. 9(B)(a)). However, the third redox couple (with Epa at about 0.98 V) of the mvRuO/RuCN film was irreversible. The cathodic peak (Epc at about 0.87 V in Fig. 9(B)(a)) and two anodic peaks (Epa about 0.98 and 0.78 V in Fig. 9(B)(a)) formed a single cathodic peak (Epc about 0.62 V, in Fig. 9(B)(b)) and two anodic peaks (Epc about 0.64 and 0.87 V) for potential scans in the range 0.4 and 1.1 V. These consecutive cyclic voltammograms also show that mvRuO/RuCN films are deposited from Ru3þ and Ru(CN)4 in aqueous monovalent cationic solutions 6 containing Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , Gaþ and divalent cationic Ba2þ ,and Ca2þ solutions. Three redox couples characterize the voltammograms for the potential scan range between )0.2 and 1.0 V. However, for the
Fig. 10. Cyclic voltammograms of a mvRuO/RuCN film adhered to a glassy carbon electrode in a 0.1 M KNO3 solution at pH 5.0. (A) 3 [SO2 M, (c) 1.0 102 M, (d) 1.5 102 5 ] ¼ (a) 0.0 M, (b) 5 10 0 M, and (e) 2.0 102 M. (a ) ¼ Bare glassy carbon electrode and 2 3 M. (B) [S2 O2 M, (c) [SO2 5 ] ¼ 2.0 10 8 ] ¼ 0 (a) 0.0 M, (b) 6 10 1.2 102 M, and (d) 2.4 102 M. (a ) ¼ Bare glassy carbon elec2 trode and [S2 O2 M. 8 ] ¼ 2.4 10
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Fig. 10(A)(a)). Fig. 10(A) shows the cyclic voltammograms of an mvRuO/RuCN film in an aqueous KNO3 solution at pH 5.0 in the presence of SO2 5 . The cathodic peak current of the mvRuO/RuCN film redox couple increased, while its anodic peak current decreased, as the concentration of SO2 5 increased. Fig. 10(B) also shows the cyclic voltammograms of an mvRuO/RuCN film in an aqueous KNO3 solution at pH 5.0 in the presence of S2 O2 8 . The cathodic peak current of the mvRuO/RuCN film redox couple also increased, while its anodic peak current decreased, as the concentration of S2 O2 in8 0 0 creased. Fig. 10(A)(a ) and (B)(a ) show cyclic voltammograms of SO2 and S2 O2 5 8 , respectively, between potentials of 0.8 and )0.3 V using bare glassy carbon electrode. The electrocatalytic reduction current developed directly from the redox couple of the catalyst at a potential of about 0.0 V when performing the electro2 catalytic reduction of SO2 5 and S2 O8 by the mvRuO/ RuCN films. In contrast, in the electrocatalytic reduc2 tion of SO2 5 and S2 O8 by iron hexacyanoferrate and nickel hexacyanoferrate [22], both electrocatalytic currents develop indirectly from the cathodic peak, as the electrocatalytic peak potential is more negative than the cathodic peak potential of both catalysts. The electrocatalytic reaction can be tested using cyclic voltammetry and bulk electrolysis. The current vs. time is controlled by the potential of the bulk electrolysis, and product analysis is carried out using ion chromatography. The results show obvious differences between the cases of (a) only reactants in the absence of a catalyst, (b) only the catalyst present, and (c) when the reactants and catalyst are both present. The current and product analyses both show that the reaction rate increases when both the reactant and catalyst are present. In the cyclic voltammetric experiments, the cathodic (or anodic) peak current of the mvRuO/RuCN film redox couple increased, while its anodic (or cathodic) peak current decreased through the electrocatalytic redox couple, as the concentration of reactant increased. The peak current increase is connected to the electron transfer rate (the rate of reactant transfer to product), which is faster in the presence of a catalyst than when the catalyst is absent and only the reactants are present. 2 The electrocatalytic reduction of SO2 5 and S2 O8 by mvRuO/RuCN films in an acidic aqueous solution is described by the following equations 2 þ SO2 5 þ 2H þ 2e ! SO4 þ H2 O
ð7Þ
2 S2 O2 8 þ 2e ! 2SO4 :
ð8Þ
3.6. Electrocatalytic oxidation of NADH and dopamine by a ruthenium oxide/ruthenocyanide film We were able to deposit an mvRuO/RuCN film on a glassy carbon electrode and perform the electrocatalytic
oxidation of NADH and dopamine. The electrocatalytic oxidation of NADH and dopamine shows different electrocatalytic properties. Fig. 11(A)(a) and (B)(a) show the cyclic voltammograms of an mvRuO/RuCN film in an aqueous 0.1 M Ba (NO3 )2 solution at pH 2.0. The electrocatalytic oxidation of NADH was performed at the mvRuO/RuCN film electrode, with the oxidation involving the transfer of two electrons and one proton in the reaction. The oxidation equation is NADH ! NADþ þ Hþ þ 2e :
ð9Þ
From the results shown in Fig. 11(A), the cyclic voltammogram of the mvRuO/RuCN film shows three redox couples. The electrocatalytic oxidation current increased significantly as the concentration of NADH in the aqueous solution increased, and the electrocatalytic oxidation current developed directly from the more
Fig. 11. Cyclic voltammograms of a mvRuO/RuCN film adhered to a glassy carbon electrode in an aqueous 0.2 M Ba(NO3 )2 solution at pH 2.0. (A) [NADH] ¼ (a) 0.0 M, (b) 5 104 M, (c) 1.0 103 M, and 0 (d) 1.5 103 M. (a ) ¼ Bare glassy carbon electrode with [NADH] 3 ¼ 1.5 10 M. (B) [Dopamine] ¼ (a) 0.0 M, (b) 5 104 M, (c) 0 1.0 103 M, and (d) 1.5 103 M. (a ) ¼ Bare glassy carbon elec3 trode with [Dopamine] ¼ 1.5 10 M.
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positive of the two redox couples of the mvRuO/RuCN film at potentials of about 0.7 and 0.9 V when performing the electrocatalytic oxidation of NADH and dopamine, respectively. The results show two different types of electrocatalytic oxidation for NADH and dopamine by mvRuO/ RuCN films. The cyclic voltammograms of dopamine in 0 an aqueous buffered solution at pH 2.0 (Fig. 11(B)(a )) shows one redox couple with an anodic oxidation peak at a potential of about 0.55 V (vs. AgjAgCl), and a cathodic peak at a potential of 0.28 V (vs. AgjAgCl). The electrocatalytic oxidation current increased significantly as the concentration of dopamine in the aqueous solution increased. The electrocatalytic oxidation current peak potentials at 0.75 and 1.0 V are both more positive than the electrochemical oxidation peak potential of dopamine in the absence of catalyst. The cyclic voltammograms of NADH in an aqueous buffered solution 0 at pH 2.0 (Fig. 11(A)(a )) shows an anodic oxidation peak at potentials more positive than 1.1 V (vs. AgjAgCl). The electrocatalytic oxidation current increased significantly as the concentration of NADH in the aqueous solution increased. The electrocatalytic oxidation current peak potentials at 0.75 and 0.97 V are both more negative than the electrochemical oxidation peak potential of NADH in the absence of a catalyst. The electrocatalytic oxidation and reversible reduction of dopamine by a mvRuO/RuCN film in an acidic aqueous solution is described by the following chemical reaction [37,38]
ð10Þ 3.7. The discussion on the electrocatalytic oxidation of 2 SO2 3 , S2 O3 , and N2 H4 by a ruthenium oxide/ruthenocyanide film 2 The electrocatalytic oxidation of SO2 3 , S2 O3 , and N2 H4 by an mvRuO/RuCN film in a 0.1 M KNO3 solution at pH 5.0 was studied. Fig. 12(A) shows the cyclic voltammograms of an mvRuO/RuCN film in a 0.1 M aqueous KNO3 solution at pH 5.0 with various concentrations of SO2 3 . The anodic current peaks of the two redox couples of the mvRuO/RuCN film have formal potentials between 0.4 and 1.0 V (vs. AgjAgCl), with the two anodic peak currents occurring at about 0.76 and 0.95 V (vs. AgjAgCl). These increased with increasing concentration of SO2 3 . Similar results were observed with various concentrations of S2 O2 3 (Fig. 12(B)), and N2 H4 (Fig. 12(C)). The Ipcat values were found to increase with increasing
Fig. 12. Cyclic voltammograms of a mvRuO/RuCN film adhered to a glassy carbon electrode in an aqueous 0.2 M Ba(NO3 ) solution at pH 3 2 3 5.0, with: (A) [SO2 3 ] ¼ 0–1.5 10 0 M, (B) [S2 O3 ] ¼ 0–1.5 10 M, (C) [N2 H4 ] ¼ 0–1.5 103 M. (a ) ¼ Bare glassy carbon electrode, and the substrate is at maximum concentration.
concentration of substrate. When the various concenand N2 H4 were added, the anodic trations of S2 O2 3 peak current increased noticeably, with a catalytic peak current occurring at about 0.76 V (vs. AgjAgCl), which was more marked than in the electrocatalytic oxidation of SO2 3 . These results may point to a practical application of mvRuO/RuCN-film-modified electrodes as electrochemical sensors to determine substrates. The anodic peak current increases noticeably, and develops directly from the redox couple in suitable aqueous cation solutions. This is interesting, as the Ipcat value increases when the concentration of the substrate increases. The electrocatalytic oxidation of various substrates by an mvRuO/RuCN film is summarized in Table 2. Fig. 13 shows the electrocatalytic oxidation of dopamine by an mvRuO/RuCN film using the rotating ring-disk electrode (RRDE) method. Fig. 13(A) shows the dopamine present in a pH 1.5 aqueous solution
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Table 2 The electrocatalytic properties of mvRuO/RuCN films with various substrates Substrate
Peak potential (V)a
Electrocatalytic type
pH
SO2 5 S2 O2 8 SO2 3 S2 O2 3
)0.07 )0.09 0.61, 0.98 0.80, 0.97 0.74, 0.98 0.76, 1.00 0.76, 0.96
Reduction Reduction Oxidation Oxidation Oxidation Oxidation Oxidation
5.0 5.0 5.0 5.0 2.0 2.0 5.0
NADH Dopamine N2 H4 a
Epcat ¼ the anodic peak potential when substrate is added.
when the mvRuO/RuCN-film-modified disk glassy carbon electrode and ring glassy carbon electrode had an applied potential of 0.1 V (vs. AgjAgCl) for 2500 rpm. In Fig. 13(A), the value of ID is the oxidation current, and IR is the reduction current from the chemical reaction described by Eq. (10). These results are consistent with the IR current being the reduction current shown in Eq. (10). The results show ID and IR increasing as the concentration of dopamine increases. Fig. 13(B) shows the dopamine present for various rotation rates in a pH 1.5 aqueous solution when the mvRuO/RuCN-film-modified glassy carbon disk electrode had an applied potential of 0.1 V (vs. AgjAgCl) at a constant concentration of dopamine. These results are consistent with ID being the oxidation current and IR being the reduction current described in Eq. (10) [37,38]. In Fig. 13(A) and (B), the zero current was found between the potentials of )0.2 and 0.3 V. The values of ID and IR are the oxidation and reduction currents at a potential of 0.78 V (where IR reached a plateau) from the baseline zero current (at a potential of about +0.25 V) of ID and IR . The best straight line through these points was selected (if selection of the best straight line were difficult, then perhaps the errors in ID and IR would be higher). The RRDE data were analyzed using the 1 Koutecky–Levich equation [39,40], and plotted as Ilim;c 1=2 vs. x , as shown in the inset of Fig. 13(B). 1=I ¼ 1=Ik þ 1=Ilim;c ;
ð11Þ
where 2=3
Ilim;c ¼ 0:62 nFAD0 x1=2 u1=6 c0 :
Fig. 13. RRDE voltammogram of a mvRuO/RuCN film adsorbed on a glassy carbon disk electrode in an aqueous 0.2 M Ba(NO3 )2 solution at pH 4.0. (A)with [dopamine] ¼ (a) 0.0 M, (b) 1 104 M, (c) 2 104 M, (d) 3 104 M, and (e) 4 104 M at a rotation rate of 2500 rpm. ER ¼ 0:1 V. (B) [dopamine] ¼ 3 104 M and rotation rate ¼ (a) 200, (b) 400, (c) 900, (d) 1200, (e) 1600, and (f) 2500 rpm. Electrode ¼ glassy carbon disk and ring (ER ¼ 0:1 V vs. AgjAgCl). Scan rate ¼ 1 0.015 V/s. Inset of (B) shows plots of Ilim vs. x1=2 .
ð12Þ
The parameter I is the measured limiting current of the disk, x is the rotation rate, D0 and c0 are the diffusion coefficient and the bulk concentration of dopamine respectively, and u is the kinematic viscosity of water in the experimental rotating rates. The inset of Fig. 13(B) 1 shows a plot of Ilim vs. x1=2 , Ik ¼ nFAkCc0 , where k is the rate constant of the chemical reaction between the film and dopamine, and C is the coverage of the catalyst on the electrode surface. Plots of 1==ID;lim vs. x1=2 are shown in the inset of Fig. 13(B). The surface coverage, C, was estimated to be C ¼ 1 108 mol/cm2 of the mvRuO/RuCN in the film from the chronocoulometry charge and the EQCM frequency change. If we assume that the film undergoes a layer-type reaction mechanism, then the rate constant of the chemical reaction, j, can be estimated to be an average value of j ¼ 3:5 103 M1 s1 from three epinephrine concentrations: 1 104 , 2 104 and 3 104 M.
4. Conclusions Polynuclear mixed-valent mvRuO/RuCN films were successfully synthesized using consecutive cyclic vol-
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tammetry on various electrodes directly from Ru3þ and Ru(CN)4 in various electrolyte solutions containing 6 Hþ , Liþ , Naþ , Kþ , Rbþ , Csþ , and Gaþ cations. The cyclic voltammograms showed three redox couples in aqueous solutions of Ba(NO3 )2 or BaCl2 , which contain a divalent cation, and the formal potentials of the redox couples of the cyclic voltammograms depended on the electrolyte cation and the pH. EQCM, cyclic voltammetry, UV–visible spectroscopy, and the stopped-flow method were used to study the growth mechanism of the mvRuO/RuCN films, with the EQCM and cyclic voltammetry used to study the in situ growth of the mvRuO/RuCN films. The results indicate that the redox process was confined to the surface, confirming the immobilized state of the mvRuO/ RuCN. A growth mechanism of the mvRuO/RuCN films was proposed whereby the film grew by electrochemically oxidizing the ruthenium(III) hexacyanoruthenate(II) (RuIII [RuII (CN)6 ] (or RuIII –O/RuII (CN)6 ) species. This species was then oxidized to (RuIII [RuIII (CN)6 ] (or RuIII –O/RuIII (CN)6 ), followed by film deposition to form a ruthenium oxide/ruthenocyanide(RuIV –O/RuIII (CN)6 ) film. The mvRuO/RuCN films can reduce SO2 5 , and S2 O2 electrocatalytically and are electrocatalytically 8 active. The experimental results also show that mvRuO/ RuCN films are electrocatalytically active in the oxida2 tion of NADH, dopamine, SO2 3 , S2 O3 , and N2 H4 . Acknowledgements This work was supported by the National Science Council of the Republic of China. References [1] [2] [3] [4] [5] [6]
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