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Electrochemistry of heteropolyanions in coulombically linked self-assembled monolayers
Journal of Electroanalytical Chemistry 458 (1998) 87 – 97
Electrochemistry of heteropolyanions in coulombically linked self-assembled monolayers Shaoqin Liu, Zhiyong Tang, Aili Bo, Erkang Wang, Shaojun Dong * Laboratory of Electroanalytical Chemistry, and National Analysis and Research Center of Electrochemistry and Spectroscopy, Changchun Institute of Applied Chemistry, Academic Sinica, Changchun, 130022, People’s Republic of China Received 24 March 1998; received in revised form 15 September 1998
Abstract A composite film containing heteropolyanion was fabricated on gold by attaching the Keggin-type heteropolyanion, PMo12O340− , on a 4-aminothiophenol SAM via Au–S bonding. Reflection FTIR, cyclic voltammetry and XPS were used for the characterization of the composite film. Reflection FTIR studies indicate that there is some Coulombic interaction between PMo12O340− and the surface amino group in the composite film, which greatly improves the film stability and prevents effectively the destructive intermolecular aggregation. The composite film shows three reversible redox couples within the pH range pH 57.0, attributed to three two-electron and two-proton electrochemical reduction – oxidation processes of PMo12O340− . Compared with PMo12O340− in the solution, the PMo12O340− of the composite film electrode can exist in a larger pH range, and shows smaller peak-to-peak separation, and more reversible reaction kinetics. Moreover, the composite film obtained shows a good catalytic activity for the reduction of BrO3− . © 1998 Elsevier Science S.A. All rights reserved. Keywords: Coloumbic interaction; FTIR; Reduction–oxidation; XPS
1. Introduction Heteropolyanions, [Xx Mm Oy ]p − (x 5 m) (where M represents Mo, W, V…; X: P, Si, As…) receive increasing interest in material science, catalysis, biology and medicine owing to their chemical, structural and electronic versatility [1 – 4]. One of the most important properties of polyoxometalate anions is their ability to accept various numbers of electrons giving rise to mixed-valency species (heteropolyblue and heteropolybrown). In order to take advantage of their specific properties observed in solution, there has been much exploitation of the attachment of the heteropolyanions on the substrate, such as electrodeposition [5 – 9], adsorption [10–12] and heteropolyanions as the dopant immobilized into a polymer matrix [13 – 27]. These elec* Corresponding author. [email protected].
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trode coating techniques usually result in a random spatial and orientation arrangement of the redox sites in the film. Recently, Faulkner et al. [28] and Sun et al. [29] developed the fabrication of multilayer films by alternating deposition. Moreover, Clemente-Leon et al. [30] prepared an ultrathin film containing heteropolyanion by using the Langmuir–Blodgett (LB) technique. These techniques were shown to be a rapid, and experimentally very simple, way to obtain organized molecular assemblies with precise control of layer composition and thickness. In this paper, our primary interest in self-assembled monolayers (SAMs) is that the SAM technique can create films with large-scale order, advantageous in device preparation, and can also provide new insight on electron transfer reactions at the interface. During the past several years, considerable attention has been focused on SAMs. These well-defined organic films have proven to be extremely useful for studying protein
0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 8 ) 0 0 3 6 3 - 5
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Scheme 1. represents PMo12O340− anion
adsorption [31], interfacial electron transfer [32–35], biomolecular [36] and cell immobilization [37], ion binding [38,39] and coupled chemical reactions [40]. Organic thiols chemisorbed on gold are particularly suitable for electrochemical and chemical studies due to their excellent stability and highly characterized structures [41–48], by which interfacial properties can be controlled. Through the proper selection of the terminal functional group of the SAMs, specific surface analyte interactions can be exploited to immobilize molecules at the interface. So applying the SAM technique to the preparation of assemblies of heteropolyanions should yield better control of their properties. In this paper, we demonstrate that a novel composite film in which the PMo12O340− anion and SAM film are linked with each other by a Coulombic attractive interaction can effectively improve the film stability, and the novel composite film shows reversible electrochemical behavior. The supporting Au substrate is first modified by a 4-aminothiophenol SAM via Au – S bonding, and then PMo12O340− can be linked with the 4-aminothiophenol group through electrostatic attraction during potential cycling in acidic media. The electrostatic interaction among the differently charged terminal groups is expected to stabilize the film structure and to prevent the possible molecular aggregation, leading to the formation of an ordered monolayer of heteropolyanions. Electrochemistry, reflection FTIR and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure and the stability of the films.
2. Experimental
2.1. Materials 4-Aminothiophenol (ATh) was purchased from Aldrich. 2-Aminoethanethiol (AET) was obtained from
Acros. All other chemicals were of analytical grade and used as received. Solutions were freshly prepared using a Millipore Milli-Q water purification system and deaerated by passing argon through them. Buffer solutions for pH\ 5 consisted of 0.2 M Na2HPO4 + NaH2PO4 mixtures, for 5 ] pH\ 3 consisted of 0.2 M solutions of CH3COOH+ CH3COONa mixtures and for pHB3, 0.2 M H2SO4 + Na2SO4 mixtures.
2.2. Electrochemical experiments Electrochemical experiments were carried out on CH Instruments (Model 600 Voltammetric Analyzer) in a conventional one-compartment cell with a Au disk electrode (d =0.8 mm) as the working electrode, a Ag AgCl sat. KCl electrode as the reference electrode, and a Pt electrode as the counter electrode.
2.3. Procedure The Au electrodes were polished successively with 1.0 and 0.3 mm a-Al2O3 and washed ultrasonically with ethanol and water between each experiment. Before chemical modification and subsequently they were rinsed thoroughly with pure water and ethanol. The real electrode surface area was estimated from a cyclic voltammogram by integration of the cathodic peak for the reduction of the oxide layer [49], and a roughness factor of ca. 1.2 9 0.1 was obtained. The clean gold electrodes were soaked in ethanol solution containing 10 mM ATh or AET for 24 h, then rinsed carefully with pure water to remove the non-chemisorbed materials. Subsequently, the ATh or AET modified Au (Au ATh or Au AET) electrodes were cycled between − 0.1 and 0.6 V at 100 mV s − 1 for 50 cycles in 2 M H2SO4 aqueous solution containing 5 mM PMo12O340− . Scheme 1 shows the reaction steps in the preparation of the Au 4-ATh or AET –PMo12O340− (denoted briefly as the PMo12O340− –SAMs) composite film.
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Fig. 1. Cyclic voltammograms of 10 mM Fe(CN)36 − /4 − on a naked gold electrode (…), AET SAM modified gold electrode ( – .– ), and ATh SAM modified gold electrode (—). Scan rate: 100 mV s − 1.
2.4. XPS The XPS spectra were recorded with an ESCALAB MK II spectrometer. The Au samples were prepared as Au discs (diameter 2 cm). Prior to use, the gold discs were sonicated in chloroform and subsequently immersed in piranha solution (a hot solution of 30% H2O2 and 70% concentrated H2SO4, volume ratio 1:3) followed by rinsing with Milli-Q water and absolute ethanol. The gold discs were then dried quickly with a stream of hot air followed by heating to incandescence in a hydrogen-air flame. The self-assembled monolayer of ATh was then fabricated by immersing the Au substrate into an ethanol solution of ATh for 24 h. Upon removal from the deposition solution, the substrate was extensively rinsed with absolute ethanol and then water. The PMo12O340− of the composite film was deposited onto the Au ATh film according to the same procedure as Section 2.3.
2.5. FTIR measurements FITR measurements were conducted using a Nicolet 520 SX FITR spectrometer with a DTGS detector. The external reflectance system was constructed from a variable angle specular reflectance attachment (Spectra Tech). All the spectra were obtained with an average of
100 scans with 4 cm − 1 resolution. The sample chamber was purged with dry N2 to eliminate the spectral interference from water vapor in air. The Au samples were prepared according to the same procedure as XPS samples. For the IR spectra of H3PMo12O40, 1 mg H3PMo12O40 was mixed with a large excess of KBr and pelleted. The PMo12O340− dip-coated film on a Au plate was prepared by dropping a drop of 1 M H2SO4 aqueous solution containing 10 mM H3PMo12O40 on a clear Au surface and allowing it to dry in air for 45 min at room temperature.
3. Results and discussion
3.1. Effect of the monolayer on the preparation of PMo12O 340− — SAM composite film electrode To obtain a stable PMo12O340− —SAM composite film on a Au electrode, the SAM has to be formulated first. In the present work, studies were carried out with AET and ATh both containing NH2 groups for comparison. Firstly, the packing degree of the ATh and AET SAMs were assessed by investigating their blocking effect on the redox electrochemistry of water-soluble Fe(CN)3 − /4 − ion. Fig. 1 shows the well-known reversible voltammetric behavior of Fe(CN)3 − /4 − on a
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naked gold electrode (…), an AET SAM modified gold electrode (— .— ) and an ATh SAM modified gold electrode (—). Obviously, the ATh Au electrode has a lower peak current and larger peak potential separation, indicating that the ATh monolayer has a relatively close packing structure and less permeability to Fe(CN)3 − /4 − ion than that of the AET monolayer. The ATh or AET monolayer films were further examined by conducting the following voltammetric experiments. The AET or ATh Au electrode is cycled between − 0.1 and 0.6 V at 100 mV s − 1 for 50 cycles in 2 M H2SO4 containing 5 mM PMo12O340− , rinsed thoroughly with 1 M H2SO4, and its electrochemical behavior is examined in pure 1 M H2SO4 supporting electrolyte (Fig. 2C, D). For the purpose of comparison, cyclic voltammograms of the PMo12O340− — ATh or AET composite film, PMo12O340− anion in the solution (Fig. 2A) and PMo12O340− film electrodeposited on the naked Au electrode (Fig. 2B) are also presented in Fig. 2. The voltammetric responses are considerably influenced by the preparation method adopted. In the potential range from 0.60 to − 0.10 V, the PMo12O340− —ATh composite film modified Au electrode displays three sharp and well-defined redox couples with mean peak potentials of 0.362, 0.226 and − 0.007 V; the peak-to-peak separations are 34, 27 and 40 mV, respectively, corresponding to three two-electron processes(Fig. 2D). The cyclic voltammograms of PMo12O340− anion in solution and the PMo12O340− film electrodeposited on the naked Au electrode show three poor redox waves, respectively, as shown in Fig. 2A, B. Although the PMo12O340− — AET composite film gives three redox waves, the peak at −0.007 V, is strongly masked by hydrogen produced on the electrode surface and displays poor redox behavior, as shown as Fig. 2C. By comparing the curves of Fig. 2D with the curves of Fig. 2A, C, it is found that the PMo12O340− —ATh composite film modified Au electrode shows more reversible electrochemical behavior than the PMo12O340− anion in solution, and its E1/2s are not affected by the immobilization on the ATh group. This is because the ATh monolayer is relatively closely packed and effectively inhibits protons produced on the gold electrode surface, moreover, the easy electron flow in the benzene ring of ATh molecules benefits the electron transfer between the PMo12O340− ions on the ATh monolayer and the gold electrode surface. It also seems that the interaction of the oxometallates with the ATh group in the film does not interfere with the voltammetric behavior, which just meets the requirement of molecular design for fabrication of an ideal modified electrode surface. So we chose the PMo12O340− — ATh composite film modified Au electrode as the target system for study.
3.2. Electrochemical beha6ior of the PMo12O 340− — SAM composite film The PMo12O340− —ATh composite film modified Au electrode has good stability. The stability of the composite film electrode was examined by measuring the decrease in voltammetric currents of the composite film electrode during potential cycling. For example, the film electrode was subjected in 1 M H2SO4 solution to 100 potential cycles in the potential range of 0.6 and − 0.1 V at 100 mV s − 1; a decrease in the cathodic current of less than 10% was observed. After soaking the PMo12O340− —ATh composite film modified Au electrode in 1 M H2SO4 solution for 10 days, almost no change of the electrochemical response of PMo12O340− was observed. This shows that a significant activation barrier impedes the break-up of the electrostatic binding within the composite film. Fig. 3 shows cyclic voltammograms of the composite film electrode prepared from 2 M H2SO4 +5 mM PMo12O340− at different scan rates, and the inset shows plots of peak current versus scan rate. At scan rates up to 500 mV s − 1, all the peak potentials remain unchanged, and the DEp does not increase with increasing scan rate. Moreover, the cathodic peak current is almost the same as the corresponding anodic peak current. A good linearity in the plot of peak current versus scan rate up to 500 mV s − 1 reveals that the electrochemical behavior of PMo12O340− anion being electrostatically linked with the surface amino group shows prominently a fast, diffusionless electron transfer process. However, the peak potential separation (Epa −Epc) is B 40 mV instead of the value zero expected for a reversible surface redox process [50], which might arise due to non-ideal behavior of adsorbed moieties. Given the non-ideal behavior, the surface coverage of PMo12O340− could be calculated using [50]: Ip = n 2F 26AG0/RT(4−2gG0) =nFQ6/RT(4− 2gG0) (since Q= nFAG0) where Ip, 6, A, G0, g and Q represent the peak current (A), scan rate (V s − 1), electrode area (cm2) measured with the same cyclic voltammogram by integration of a cathodic peak for the reduction of an oxide layer performed electrochemically on the electrode surface [16], surface coverage of the redox species (mol cm − 2), the interaction term and the background corrected adsorbed charge (C) obtained by integration of the peak, respectively. The background corrected charge obtained by integration of the cathodic peaks I, II and III of the negative sweep of the voltammograms is invariant with the scan rate, indicating that the PMo12O340− electrostatically linked with the surface amino group exhibits the characteristic behavior of an immobilized couple with fast redox transition [51]. That is to say, mechanisms involving either a diffusion-controlled electrode reac-
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Fig. 2. Cyclic voltammograms of 5.0 mM PMo12O340− on a naked gold electrode (A), PMo12O340− film electrodeposited on the Au electrode (B), a PMo12O340− – AET composite film modified Au electrode (C) and a PMo12O340− – ATh composite film modified Au electrode (D). Supporting electrolyte: 1 M H2SO4. Scan rate: 100 mV s − 1.
tion in solution [52,53] or immobilized films with a smaller diffusional charge transport constant [54] are not present. Thus Q at any 6 obtained in the case of the
PMo12O340− —ATh composite film corresponds to the total charge of the reducible species in the corresponding potential range. Therefore, the PMo12O340− —ATh
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Fig. 3. Cyclic voltammograms of a PMo12O340− –ATh composite film modified Au electrode prepared in 2 M H2SO4 at different scan rates. The inset shows variation of the cathodic and anodic peak current with scan rate.
composite film prepared in 2 M H2SO4 gives a surface coverage of 2.2× 10 − 10 mol cm − 2, a value slightly more than the closest packing concentration, 2.15× 10 − 10 mol cm − 2. In view of these values, we may conclude that the PMo12O340− electrostatically linked with the surface amino group is a closely-packed monolayer.
3.3. pH-Dependent electrochemical beha6ior of the PMo12O 340− — SAM composite film In general, the reduction of heteropolyanion is accompanied by protonation, therefore, the pH of the solution has a great effect on the electrochemical behavior of heteropolyanions. The PMo12O340− anion is unstable in aqueous solution and undergoes a series of hydrolysis processes [55], but it is fairly stable in acidic solutions of pH less than 2. Fig. 4 shows the dependence of E1/2 of the PMo12O340− —ATh composite film modified Au elec-
trode on solution pH. In the range of pH5 7.0, as pH increases, the shapes of redox peaks remain unchanged, the Eps of all three redox couples shift negatively. Plots of E1/2 versus pH for the composite film have a linear region from pH 1 up to 7. Average slopes of the three redox couples in this pH range are − 61, − 63 and − 65 mV pH − 1, which are close to the theoretical value − 60 mV pH − 1 for 2e − /2H + . It is confirmed that, in the composite film, the two-electron process is accompanied by a two-protonation reaction. The slope of the plots of E1/2 versus pH may suggest that the reduction of the PMo12O340− from Mo(VI) to Mo(V) induces the uptake of protons from solution to the film to maintain charge neutrality of the film. From the above comparison, we can understand that the electrostatic interaction of the composite film can improve the stability of the PMo12O340− within this pH range. As a matter of fact, analogous behavior has been described and discussed previously [56–59]. At higher pH, the CV behavior of the film electrode becomes unstable, which is believed
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Fig. 4. Relationship between peak potentials and pH values for PMo12O340− – ATh composite film modified Au electrode.
to originate from the cleavage of the ionic bonding due to deprotonation of – NH3+ to – NH2 in the phenyl group. The above results allow us to describe the three overall redox processes of the PMo12O340− — ATh composite film modified Au electrode in acidic solution of pH 5 7.0 as follows: PMo12O340− +2e − + 2H + X H2PMo12O340− H2PMo12O340− +2e − +2H + X H4PMo12O340− H4PMo12O340− +2H+ +2e − X H6PMo12O340− The same redox processes of the PMo12O340− —ATh composite film as that of PMo12O340− anion in solution may suggest that protonation – deprotonation is almost unhindered by the electrostatic interaction. The composite film can provide a very favourite environment for electron and proton transferring.
3.4. Structural characterization of the PMo12O 340− — ATh composite film 3.4.1. Infrared reflectance FTIR To improve the stability and high ordering of the heteropolyanion film modified electrode, we first modify the gold substrate by the self-assembled monolayer ATh. The pKb value of the surface amino group of ATh Au was evaluated according to the method described in the literature [38], suggesting that the ATh Au surface is positively charged more easily within a wide pH range. On the other hand, PMo12O340−
is negatively charged, therefore, a Coulombic attractive interaction is expected when depositing PMo12O340− anion onto the ATh Au surface. This additional adhesion force may then effectively prevent the destructive molecular aggregation. In Fig. 5 IR spectra of the PMo12O340− —ATh composite film (Fig. 5B) are presented and compared with those of the ATh SAM (Fig. 5A), the PMo12O340− dip-coated Au plate (Fig. 5C) and the crystalline heteropolyacid (Fig. 5D). Note that the IR spectrum of the PMo12O340− —ATh composite film reveals the three clear peaks: 1124, 1025, 875 cm − 1 (Fig. 5B). The band at 875 cm − 1 is diagnostic of Keggin structural units, corresponding to asymmetric stretching vibrations of the Mo–Oc –Mo bridges [60]. Compared to the spectrum of the H3PMo12O40, clearly, the bands in the composite film associated with the anions are shifted. The shift observed for different peaks is probably related to the organization and in particular to the presence of positively charged ATh in the composite film. In order to obtain some insight into the mechanism by which the shift of the PMo12O340− —ATh composite film IR spectrum was influenced, we also prepared PMo12O340− dip-coated on a Au plate by dipping a droplet of 10 mM PMo12O340 on a clean Au plate and conducted the IR experiments. By comparing Fig. 5B and C, D, it is obvious that the shift of IR bands was induced by the Coulombic interactions between heteropolyanions and the protonated ATh groups. Indeed, Rocchiccioli-Deltcheff et al. [61] observed a shift of the Keggin infrared bands depending on the
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Fig. 5. Reflectance FTIR spectra of ATh SAM (A), the PMo12O340− – ATh composite film (B), the PMo12O340− dip-coated film (C) and H3 PMo12O40(D).
size of the tetralkylammonium cations used as a counterion in the solid state. The values reported for the larger cation used by these authors are in close agreement with those measured for the corresponding Keggin polyanion in the composite film. Hasik et al. [62] also observed that a shift in the Mo – Oc – Mo vibration of PMo12O340− incorporated into the polyaniline was induced by the Coulombic interactions between heteropolyanions and the protonated polymeric support. On the basis of results obtained and studies reported in the literature, we propose that the shift of
PMo12O340− infrared bands is attributed to the formation of ionic bonds –NH3+ —PMo12O340− at the PMo12O340− ATh interface as expected.
3.4.2. XPS The presence of the PMo12O340− –ATh composite film on a gold electrode surface can be confirmed by XPS data. The relative molar ratio of C1s, N1s, S2p, P2p and Mo3d levels in the XPS spectrum of the PMo12O340− —ATh composite film are presented in Table 1.
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Table 1 XPS spectra of PMo12O3− 40 –ATh composite film on gold disc (assuming the molar quantity of S2p as 1)
PMo12O3− 40 –ATh
C1s% (285 eV)
N1s% (401.7 eV)
S2p% (172.4 eV)
P2p% (140.8eV)
Mo3d% (Mo3d5/2 =235.8 eV Mo3d2/3 = 232.6 eV)
5.8
1.0
1.0
0.6
6.9
The deconvolution was carried out assuming the same weighting of Gaussian (75%) and Lorentzian (25%) functions in each case. Under these conditions the C(1s) signal consisted of two peaks at 285 –286.2 eV. The peak at 285 eV is attributed to the aromatic C – C bond, the peak at 286.2 eV to the carbon bond to the nitrogen (C–N) and to the sulfur (C – S). The two deconvoluted peaks at 399.4 – 401.9 eV of N(1s) can be ascribed to the amine state( – NH – ) and a protonated nitrogen species (N + ) [62].
the reduction of ClO3− in homogeneous aqueous solution [67] and the catalytic reduction of ClO3− on PMo12O340− —modified carbon fiber electrodes [21,68]. The inset of Fig. 6 shows the dependence of the catalytic current on the concentration of BrO3− . A good linearity in the catalytic current versus the concentration of BrO3− reveals that the catalytic process corresponds to an EC catalytic mechanism. For the PMo12O340− –ATh composite film modified Au electrode, the catalytic process can be expressed as:
H2PMo12O340− + 2e − + 2H + X H4PMo12O340− H4PMo12O340− + BrO3− + 4H + X H2PMo12 O340− + Br − +3H2O The adsorption of PMo12O340− on a SAM Au disc was disclosed by the presence of two doublets at 235.6– 232.5 eV arising from Mo(3d5/2). These values are consistent with spin orbit splitting of the Mo 3d level in the oxidation state 6 (MoVI (Mo(3d5/2) =235.8 eV and Mo(3d5/2)=232.6eV)) [62].
3.5. Electrocatalytic effect of the PMo12O 340− – ATh composite film on the reduction of BrO − 3 anion The reduction of BrO3− is totally irreversible at a glassy carbon electrode in acidic aqueous solution and does not take place prior to the evolution of hydrogen. However, the reduction of BrO3− can readily be catalyzed by the mixed-valence molybdenum(VI, V) [65,66] or tungsten(VI, V) [63,64] species, such as WO3 film [65] and MoO3 film [66]. In this paper, we also found that the PMo12O340− –ATh composite film modified Au electrode shows catalytic activity toward the reduction of BrO3− . The electrocatalytic reduction of BrO3− with the PMo12O340− –ATh composite film modified Au electrode is recorded in Fig. 6. The electrocatalysis occurs at the third wave (most negative one) of the PMo12O340− –ATh composite film, as shown in Fig. 6; with the addition of BrO3− , the cathodic current of the third wave is enhanced and the corresponding oxidation peak decreases, while peak I and II are almost unaffected by the addition of BrO3− . The result indicates that the cathodic wave corresponding to the reduction of the phosphomolybdic anion from four-electron to six-electron reduction products has catalytic properties toward the reduction of BrO3− . A similar phenomenon has been observed for the catalytic effect of PMo12O340− on
4. Conclusion We have prepared PMo12O340− –ATh or AET SAM composite films on gold substrates by electrochemical methods. The PMo12O340− is strongly adsorbed onto the Au-grafted surface, which is demonstrated by electrochemistry, XPS and reflectance FTIR. Reflectance FTIR studies show that there is some Coulombic interaction between –NH3+ – PMo12O340− in the PMo12O340− ATh interface. The PMo12O340− –ATh composite film modified gold electrode exhibits three reversible redox waves, corresponding to two two-electron, two-proton processes. Compared with the PMo12O340− in aqueous solution, the composite film shows more reversible kinetics with smaller peak-to-peak separation and narrower peak shape. Otherwise, the PMo12O340− anion within the composite film is stable at a larger pH range. This suggests that the Coulombic attractive interaction between the PMo12O340− anions and the surface amino group improves the stability of PMo12O340− anion. We have also found that this film possesses a good catalytic activity for the reduction of BrO3− in sulfuric acid solution. The composite film can provide a sensitive assembly for the electrochemical detection of BrO3− . The preliminary results obtained for PMo12O340− clearly indicate that the method described in this paper can be extended to all kinds of polyanions, and selecting the exact nature of the polyoxometalate should enable the elaboration of new catalytic electrodes having various properties. The above results gives us significant implications: one can effectively improve the structural stability of a
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Fig. 6. Cyclic voltammograms of a PMo12O340− –ATh composite film modified Au electrode in 2 M H2SO4 containing BrO3− concentrations of 0, 0.5, 1, 1.5, 2, 3, 4, 5 mmol l − 1 (reading from top to bottom). Scan rate: 100 mV s − 1. The inset shows the relation between catalytic current (Icat.) versus BrO3− concentration (cBrO − ). 3
heteropolyanion modified electrode through molecular level designing of the substrate surface.
Acknowledgements This work has been supported by the National Natural Science Foundation of China. Useful discussions with Dr J. Yan were appreciated.
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S. Liu et al. / Journal of Electroanalytical Chemistry 458 (1998) 87–97 [25] B. Keita, K. Essaadi, L. Nadjo, J. Electroanal. Chem. 259 (1989) 127. [26] B. Keita, A. Mahmoud, L. Nadjo, J. Electroanal. Chem. 386 (1995) 245. [27] H. Sung, H. So, W. Paik, Electrochim. Acta 39 (1994) 645. [28] D. Ingersoll, P.J. Kulesza, L.R. Faulkner, J. Electrochem. Soc. 140 (1994) 840. [29] C. Sun, J. Zhao, H. Xu, Y. Sun, X. Zhang, J. Shen, J. Electroanal. Chem. 435 (1997) 63. [30] M. Clemente-Leon, B. Agricole, C. Mingotand, C.J. Gomez-Garcia, E. Coronado, P. Delhaes, Langmuir 13 (1997) 2340. [31] K.L. Prime, G.M. Whiteside, Science 252 (1991) 1164. [32] C.E.D. Chidsey, C.R. Bertozzi, T.M. Putvinski, A.M. Mujsce, J. Am. Chem. Soc. 112 (1990) 4301. [33] C.E.D. Chidsey, Science 251 (1991) 919. [34] C. Miller, P. Cuendet, M. Gra¨tzel, J. Phys. Chem. 95 (1991) 877. [35] H.O. Finklea, M. Ynch, Langmuir 3 (1987) 409. [36] L. Haussling, H. Ringsdort, F.J. Schmitt, W. Knoll, Langmuir 7 (1991) 1837. [37] S. Margel, O. Sivan, Y. Dolitzky, Langmuir 7 (1991) 2317. [38] L. Sun, B. Johnson, T. Wade, R.M. Crooks, J. Phys. Chem. 94 (1990) 8869. [39] S. Steinberg, Y. Tor, E. Sabatani, I. Rubinstein, J. Am. Chem. Soc. 113 (1991) 5173. [40] L. Sun, R.C. Thomas, R.M. Crooks, A.J. Ricco, J. Am. Chem. Soc. 113 (1991) 8550. [41] C.A. Widrig, C.A. Alves, M.D. Porter, J. Am. Chem. Soc. 113 (1991) 2805. [42] C.A. Widrig, C. Chung, M.D. Porter, J. Electroanal. Chem. 310 (1991) 335. [43] M.G. Samant, C.A. Brown, J.G. Gordon, Langmuir 7 (1991) 437. [44] L. Sun, R.M. Crooks, J. Electrochem. Soc. 138 (1991) 23. [45] S.M. Stole, M.D. Porter, Langmuir 6 (1990) 1199. [46] M.D. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, J. Am. Chem. Soc. 109 (1987) 3559.
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Electrochemistry of heteropolyanions in coulombically linked self-assembled monolayers Shaoqin Liu, Zhiyong Tang, Aili Bo, Erkang Wang, Shaojun Dong * Laboratory of Electroanalytical Chemistry, and National Analysis and Research Center of Electrochemistry and Spectroscopy, Changchun Institute of Applied Chemistry, Academic Sinica, Changchun, 130022, People’s Republic of China Received 24 March 1998; received in revised form 15 September 1998
Abstract A composite film containing heteropolyanion was fabricated on gold by attaching the Keggin-type heteropolyanion, PMo12O340− , on a 4-aminothiophenol SAM via Au–S bonding. Reflection FTIR, cyclic voltammetry and XPS were used for the characterization of the composite film. Reflection FTIR studies indicate that there is some Coulombic interaction between PMo12O340− and the surface amino group in the composite film, which greatly improves the film stability and prevents effectively the destructive intermolecular aggregation. The composite film shows three reversible redox couples within the pH range pH 57.0, attributed to three two-electron and two-proton electrochemical reduction – oxidation processes of PMo12O340− . Compared with PMo12O340− in the solution, the PMo12O340− of the composite film electrode can exist in a larger pH range, and shows smaller peak-to-peak separation, and more reversible reaction kinetics. Moreover, the composite film obtained shows a good catalytic activity for the reduction of BrO3− . © 1998 Elsevier Science S.A. All rights reserved. Keywords: Coloumbic interaction; FTIR; Reduction–oxidation; XPS
1. Introduction Heteropolyanions, [Xx Mm Oy ]p − (x 5 m) (where M represents Mo, W, V…; X: P, Si, As…) receive increasing interest in material science, catalysis, biology and medicine owing to their chemical, structural and electronic versatility [1 – 4]. One of the most important properties of polyoxometalate anions is their ability to accept various numbers of electrons giving rise to mixed-valency species (heteropolyblue and heteropolybrown). In order to take advantage of their specific properties observed in solution, there has been much exploitation of the attachment of the heteropolyanions on the substrate, such as electrodeposition [5 – 9], adsorption [10–12] and heteropolyanions as the dopant immobilized into a polymer matrix [13 – 27]. These elec* Corresponding author. [email protected].
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trode coating techniques usually result in a random spatial and orientation arrangement of the redox sites in the film. Recently, Faulkner et al. [28] and Sun et al. [29] developed the fabrication of multilayer films by alternating deposition. Moreover, Clemente-Leon et al. [30] prepared an ultrathin film containing heteropolyanion by using the Langmuir–Blodgett (LB) technique. These techniques were shown to be a rapid, and experimentally very simple, way to obtain organized molecular assemblies with precise control of layer composition and thickness. In this paper, our primary interest in self-assembled monolayers (SAMs) is that the SAM technique can create films with large-scale order, advantageous in device preparation, and can also provide new insight on electron transfer reactions at the interface. During the past several years, considerable attention has been focused on SAMs. These well-defined organic films have proven to be extremely useful for studying protein
0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 8 ) 0 0 3 6 3 - 5
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Scheme 1. represents PMo12O340− anion
adsorption [31], interfacial electron transfer [32–35], biomolecular [36] and cell immobilization [37], ion binding [38,39] and coupled chemical reactions [40]. Organic thiols chemisorbed on gold are particularly suitable for electrochemical and chemical studies due to their excellent stability and highly characterized structures [41–48], by which interfacial properties can be controlled. Through the proper selection of the terminal functional group of the SAMs, specific surface analyte interactions can be exploited to immobilize molecules at the interface. So applying the SAM technique to the preparation of assemblies of heteropolyanions should yield better control of their properties. In this paper, we demonstrate that a novel composite film in which the PMo12O340− anion and SAM film are linked with each other by a Coulombic attractive interaction can effectively improve the film stability, and the novel composite film shows reversible electrochemical behavior. The supporting Au substrate is first modified by a 4-aminothiophenol SAM via Au – S bonding, and then PMo12O340− can be linked with the 4-aminothiophenol group through electrostatic attraction during potential cycling in acidic media. The electrostatic interaction among the differently charged terminal groups is expected to stabilize the film structure and to prevent the possible molecular aggregation, leading to the formation of an ordered monolayer of heteropolyanions. Electrochemistry, reflection FTIR and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure and the stability of the films.
2. Experimental
2.1. Materials 4-Aminothiophenol (ATh) was purchased from Aldrich. 2-Aminoethanethiol (AET) was obtained from
Acros. All other chemicals were of analytical grade and used as received. Solutions were freshly prepared using a Millipore Milli-Q water purification system and deaerated by passing argon through them. Buffer solutions for pH\ 5 consisted of 0.2 M Na2HPO4 + NaH2PO4 mixtures, for 5 ] pH\ 3 consisted of 0.2 M solutions of CH3COOH+ CH3COONa mixtures and for pHB3, 0.2 M H2SO4 + Na2SO4 mixtures.
2.2. Electrochemical experiments Electrochemical experiments were carried out on CH Instruments (Model 600 Voltammetric Analyzer) in a conventional one-compartment cell with a Au disk electrode (d =0.8 mm) as the working electrode, a Ag AgCl sat. KCl electrode as the reference electrode, and a Pt electrode as the counter electrode.
2.3. Procedure The Au electrodes were polished successively with 1.0 and 0.3 mm a-Al2O3 and washed ultrasonically with ethanol and water between each experiment. Before chemical modification and subsequently they were rinsed thoroughly with pure water and ethanol. The real electrode surface area was estimated from a cyclic voltammogram by integration of the cathodic peak for the reduction of the oxide layer [49], and a roughness factor of ca. 1.2 9 0.1 was obtained. The clean gold electrodes were soaked in ethanol solution containing 10 mM ATh or AET for 24 h, then rinsed carefully with pure water to remove the non-chemisorbed materials. Subsequently, the ATh or AET modified Au (Au ATh or Au AET) electrodes were cycled between − 0.1 and 0.6 V at 100 mV s − 1 for 50 cycles in 2 M H2SO4 aqueous solution containing 5 mM PMo12O340− . Scheme 1 shows the reaction steps in the preparation of the Au 4-ATh or AET –PMo12O340− (denoted briefly as the PMo12O340− –SAMs) composite film.
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Fig. 1. Cyclic voltammograms of 10 mM Fe(CN)36 − /4 − on a naked gold electrode (…), AET SAM modified gold electrode ( – .– ), and ATh SAM modified gold electrode (—). Scan rate: 100 mV s − 1.
2.4. XPS The XPS spectra were recorded with an ESCALAB MK II spectrometer. The Au samples were prepared as Au discs (diameter 2 cm). Prior to use, the gold discs were sonicated in chloroform and subsequently immersed in piranha solution (a hot solution of 30% H2O2 and 70% concentrated H2SO4, volume ratio 1:3) followed by rinsing with Milli-Q water and absolute ethanol. The gold discs were then dried quickly with a stream of hot air followed by heating to incandescence in a hydrogen-air flame. The self-assembled monolayer of ATh was then fabricated by immersing the Au substrate into an ethanol solution of ATh for 24 h. Upon removal from the deposition solution, the substrate was extensively rinsed with absolute ethanol and then water. The PMo12O340− of the composite film was deposited onto the Au ATh film according to the same procedure as Section 2.3.
2.5. FTIR measurements FITR measurements were conducted using a Nicolet 520 SX FITR spectrometer with a DTGS detector. The external reflectance system was constructed from a variable angle specular reflectance attachment (Spectra Tech). All the spectra were obtained with an average of
100 scans with 4 cm − 1 resolution. The sample chamber was purged with dry N2 to eliminate the spectral interference from water vapor in air. The Au samples were prepared according to the same procedure as XPS samples. For the IR spectra of H3PMo12O40, 1 mg H3PMo12O40 was mixed with a large excess of KBr and pelleted. The PMo12O340− dip-coated film on a Au plate was prepared by dropping a drop of 1 M H2SO4 aqueous solution containing 10 mM H3PMo12O40 on a clear Au surface and allowing it to dry in air for 45 min at room temperature.
3. Results and discussion
3.1. Effect of the monolayer on the preparation of PMo12O 340− — SAM composite film electrode To obtain a stable PMo12O340− —SAM composite film on a Au electrode, the SAM has to be formulated first. In the present work, studies were carried out with AET and ATh both containing NH2 groups for comparison. Firstly, the packing degree of the ATh and AET SAMs were assessed by investigating their blocking effect on the redox electrochemistry of water-soluble Fe(CN)3 − /4 − ion. Fig. 1 shows the well-known reversible voltammetric behavior of Fe(CN)3 − /4 − on a
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naked gold electrode (…), an AET SAM modified gold electrode (— .— ) and an ATh SAM modified gold electrode (—). Obviously, the ATh Au electrode has a lower peak current and larger peak potential separation, indicating that the ATh monolayer has a relatively close packing structure and less permeability to Fe(CN)3 − /4 − ion than that of the AET monolayer. The ATh or AET monolayer films were further examined by conducting the following voltammetric experiments. The AET or ATh Au electrode is cycled between − 0.1 and 0.6 V at 100 mV s − 1 for 50 cycles in 2 M H2SO4 containing 5 mM PMo12O340− , rinsed thoroughly with 1 M H2SO4, and its electrochemical behavior is examined in pure 1 M H2SO4 supporting electrolyte (Fig. 2C, D). For the purpose of comparison, cyclic voltammograms of the PMo12O340− — ATh or AET composite film, PMo12O340− anion in the solution (Fig. 2A) and PMo12O340− film electrodeposited on the naked Au electrode (Fig. 2B) are also presented in Fig. 2. The voltammetric responses are considerably influenced by the preparation method adopted. In the potential range from 0.60 to − 0.10 V, the PMo12O340− —ATh composite film modified Au electrode displays three sharp and well-defined redox couples with mean peak potentials of 0.362, 0.226 and − 0.007 V; the peak-to-peak separations are 34, 27 and 40 mV, respectively, corresponding to three two-electron processes(Fig. 2D). The cyclic voltammograms of PMo12O340− anion in solution and the PMo12O340− film electrodeposited on the naked Au electrode show three poor redox waves, respectively, as shown in Fig. 2A, B. Although the PMo12O340− — AET composite film gives three redox waves, the peak at −0.007 V, is strongly masked by hydrogen produced on the electrode surface and displays poor redox behavior, as shown as Fig. 2C. By comparing the curves of Fig. 2D with the curves of Fig. 2A, C, it is found that the PMo12O340− —ATh composite film modified Au electrode shows more reversible electrochemical behavior than the PMo12O340− anion in solution, and its E1/2s are not affected by the immobilization on the ATh group. This is because the ATh monolayer is relatively closely packed and effectively inhibits protons produced on the gold electrode surface, moreover, the easy electron flow in the benzene ring of ATh molecules benefits the electron transfer between the PMo12O340− ions on the ATh monolayer and the gold electrode surface. It also seems that the interaction of the oxometallates with the ATh group in the film does not interfere with the voltammetric behavior, which just meets the requirement of molecular design for fabrication of an ideal modified electrode surface. So we chose the PMo12O340− — ATh composite film modified Au electrode as the target system for study.
3.2. Electrochemical beha6ior of the PMo12O 340− — SAM composite film The PMo12O340− —ATh composite film modified Au electrode has good stability. The stability of the composite film electrode was examined by measuring the decrease in voltammetric currents of the composite film electrode during potential cycling. For example, the film electrode was subjected in 1 M H2SO4 solution to 100 potential cycles in the potential range of 0.6 and − 0.1 V at 100 mV s − 1; a decrease in the cathodic current of less than 10% was observed. After soaking the PMo12O340− —ATh composite film modified Au electrode in 1 M H2SO4 solution for 10 days, almost no change of the electrochemical response of PMo12O340− was observed. This shows that a significant activation barrier impedes the break-up of the electrostatic binding within the composite film. Fig. 3 shows cyclic voltammograms of the composite film electrode prepared from 2 M H2SO4 +5 mM PMo12O340− at different scan rates, and the inset shows plots of peak current versus scan rate. At scan rates up to 500 mV s − 1, all the peak potentials remain unchanged, and the DEp does not increase with increasing scan rate. Moreover, the cathodic peak current is almost the same as the corresponding anodic peak current. A good linearity in the plot of peak current versus scan rate up to 500 mV s − 1 reveals that the electrochemical behavior of PMo12O340− anion being electrostatically linked with the surface amino group shows prominently a fast, diffusionless electron transfer process. However, the peak potential separation (Epa −Epc) is B 40 mV instead of the value zero expected for a reversible surface redox process [50], which might arise due to non-ideal behavior of adsorbed moieties. Given the non-ideal behavior, the surface coverage of PMo12O340− could be calculated using [50]: Ip = n 2F 26AG0/RT(4−2gG0) =nFQ6/RT(4− 2gG0) (since Q= nFAG0) where Ip, 6, A, G0, g and Q represent the peak current (A), scan rate (V s − 1), electrode area (cm2) measured with the same cyclic voltammogram by integration of a cathodic peak for the reduction of an oxide layer performed electrochemically on the electrode surface [16], surface coverage of the redox species (mol cm − 2), the interaction term and the background corrected adsorbed charge (C) obtained by integration of the peak, respectively. The background corrected charge obtained by integration of the cathodic peaks I, II and III of the negative sweep of the voltammograms is invariant with the scan rate, indicating that the PMo12O340− electrostatically linked with the surface amino group exhibits the characteristic behavior of an immobilized couple with fast redox transition [51]. That is to say, mechanisms involving either a diffusion-controlled electrode reac-
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Fig. 2. Cyclic voltammograms of 5.0 mM PMo12O340− on a naked gold electrode (A), PMo12O340− film electrodeposited on the Au electrode (B), a PMo12O340− – AET composite film modified Au electrode (C) and a PMo12O340− – ATh composite film modified Au electrode (D). Supporting electrolyte: 1 M H2SO4. Scan rate: 100 mV s − 1.
tion in solution [52,53] or immobilized films with a smaller diffusional charge transport constant [54] are not present. Thus Q at any 6 obtained in the case of the
PMo12O340− —ATh composite film corresponds to the total charge of the reducible species in the corresponding potential range. Therefore, the PMo12O340− —ATh
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Fig. 3. Cyclic voltammograms of a PMo12O340− –ATh composite film modified Au electrode prepared in 2 M H2SO4 at different scan rates. The inset shows variation of the cathodic and anodic peak current with scan rate.
composite film prepared in 2 M H2SO4 gives a surface coverage of 2.2× 10 − 10 mol cm − 2, a value slightly more than the closest packing concentration, 2.15× 10 − 10 mol cm − 2. In view of these values, we may conclude that the PMo12O340− electrostatically linked with the surface amino group is a closely-packed monolayer.
3.3. pH-Dependent electrochemical beha6ior of the PMo12O 340− — SAM composite film In general, the reduction of heteropolyanion is accompanied by protonation, therefore, the pH of the solution has a great effect on the electrochemical behavior of heteropolyanions. The PMo12O340− anion is unstable in aqueous solution and undergoes a series of hydrolysis processes [55], but it is fairly stable in acidic solutions of pH less than 2. Fig. 4 shows the dependence of E1/2 of the PMo12O340− —ATh composite film modified Au elec-
trode on solution pH. In the range of pH5 7.0, as pH increases, the shapes of redox peaks remain unchanged, the Eps of all three redox couples shift negatively. Plots of E1/2 versus pH for the composite film have a linear region from pH 1 up to 7. Average slopes of the three redox couples in this pH range are − 61, − 63 and − 65 mV pH − 1, which are close to the theoretical value − 60 mV pH − 1 for 2e − /2H + . It is confirmed that, in the composite film, the two-electron process is accompanied by a two-protonation reaction. The slope of the plots of E1/2 versus pH may suggest that the reduction of the PMo12O340− from Mo(VI) to Mo(V) induces the uptake of protons from solution to the film to maintain charge neutrality of the film. From the above comparison, we can understand that the electrostatic interaction of the composite film can improve the stability of the PMo12O340− within this pH range. As a matter of fact, analogous behavior has been described and discussed previously [56–59]. At higher pH, the CV behavior of the film electrode becomes unstable, which is believed
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93
Fig. 4. Relationship between peak potentials and pH values for PMo12O340− – ATh composite film modified Au electrode.
to originate from the cleavage of the ionic bonding due to deprotonation of – NH3+ to – NH2 in the phenyl group. The above results allow us to describe the three overall redox processes of the PMo12O340− — ATh composite film modified Au electrode in acidic solution of pH 5 7.0 as follows: PMo12O340− +2e − + 2H + X H2PMo12O340− H2PMo12O340− +2e − +2H + X H4PMo12O340− H4PMo12O340− +2H+ +2e − X H6PMo12O340− The same redox processes of the PMo12O340− —ATh composite film as that of PMo12O340− anion in solution may suggest that protonation – deprotonation is almost unhindered by the electrostatic interaction. The composite film can provide a very favourite environment for electron and proton transferring.
3.4. Structural characterization of the PMo12O 340− — ATh composite film 3.4.1. Infrared reflectance FTIR To improve the stability and high ordering of the heteropolyanion film modified electrode, we first modify the gold substrate by the self-assembled monolayer ATh. The pKb value of the surface amino group of ATh Au was evaluated according to the method described in the literature [38], suggesting that the ATh Au surface is positively charged more easily within a wide pH range. On the other hand, PMo12O340−
is negatively charged, therefore, a Coulombic attractive interaction is expected when depositing PMo12O340− anion onto the ATh Au surface. This additional adhesion force may then effectively prevent the destructive molecular aggregation. In Fig. 5 IR spectra of the PMo12O340− —ATh composite film (Fig. 5B) are presented and compared with those of the ATh SAM (Fig. 5A), the PMo12O340− dip-coated Au plate (Fig. 5C) and the crystalline heteropolyacid (Fig. 5D). Note that the IR spectrum of the PMo12O340− —ATh composite film reveals the three clear peaks: 1124, 1025, 875 cm − 1 (Fig. 5B). The band at 875 cm − 1 is diagnostic of Keggin structural units, corresponding to asymmetric stretching vibrations of the Mo–Oc –Mo bridges [60]. Compared to the spectrum of the H3PMo12O40, clearly, the bands in the composite film associated with the anions are shifted. The shift observed for different peaks is probably related to the organization and in particular to the presence of positively charged ATh in the composite film. In order to obtain some insight into the mechanism by which the shift of the PMo12O340− —ATh composite film IR spectrum was influenced, we also prepared PMo12O340− dip-coated on a Au plate by dipping a droplet of 10 mM PMo12O340 on a clean Au plate and conducted the IR experiments. By comparing Fig. 5B and C, D, it is obvious that the shift of IR bands was induced by the Coulombic interactions between heteropolyanions and the protonated ATh groups. Indeed, Rocchiccioli-Deltcheff et al. [61] observed a shift of the Keggin infrared bands depending on the
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Fig. 5. Reflectance FTIR spectra of ATh SAM (A), the PMo12O340− – ATh composite film (B), the PMo12O340− dip-coated film (C) and H3 PMo12O40(D).
size of the tetralkylammonium cations used as a counterion in the solid state. The values reported for the larger cation used by these authors are in close agreement with those measured for the corresponding Keggin polyanion in the composite film. Hasik et al. [62] also observed that a shift in the Mo – Oc – Mo vibration of PMo12O340− incorporated into the polyaniline was induced by the Coulombic interactions between heteropolyanions and the protonated polymeric support. On the basis of results obtained and studies reported in the literature, we propose that the shift of
PMo12O340− infrared bands is attributed to the formation of ionic bonds –NH3+ —PMo12O340− at the PMo12O340− ATh interface as expected.
3.4.2. XPS The presence of the PMo12O340− –ATh composite film on a gold electrode surface can be confirmed by XPS data. The relative molar ratio of C1s, N1s, S2p, P2p and Mo3d levels in the XPS spectrum of the PMo12O340− —ATh composite film are presented in Table 1.
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Table 1 XPS spectra of PMo12O3− 40 –ATh composite film on gold disc (assuming the molar quantity of S2p as 1)
PMo12O3− 40 –ATh
C1s% (285 eV)
N1s% (401.7 eV)
S2p% (172.4 eV)
P2p% (140.8eV)
Mo3d% (Mo3d5/2 =235.8 eV Mo3d2/3 = 232.6 eV)
5.8
1.0
1.0
0.6
6.9
The deconvolution was carried out assuming the same weighting of Gaussian (75%) and Lorentzian (25%) functions in each case. Under these conditions the C(1s) signal consisted of two peaks at 285 –286.2 eV. The peak at 285 eV is attributed to the aromatic C – C bond, the peak at 286.2 eV to the carbon bond to the nitrogen (C–N) and to the sulfur (C – S). The two deconvoluted peaks at 399.4 – 401.9 eV of N(1s) can be ascribed to the amine state( – NH – ) and a protonated nitrogen species (N + ) [62].
the reduction of ClO3− in homogeneous aqueous solution [67] and the catalytic reduction of ClO3− on PMo12O340− —modified carbon fiber electrodes [21,68]. The inset of Fig. 6 shows the dependence of the catalytic current on the concentration of BrO3− . A good linearity in the catalytic current versus the concentration of BrO3− reveals that the catalytic process corresponds to an EC catalytic mechanism. For the PMo12O340− –ATh composite film modified Au electrode, the catalytic process can be expressed as:
H2PMo12O340− + 2e − + 2H + X H4PMo12O340− H4PMo12O340− + BrO3− + 4H + X H2PMo12 O340− + Br − +3H2O The adsorption of PMo12O340− on a SAM Au disc was disclosed by the presence of two doublets at 235.6– 232.5 eV arising from Mo(3d5/2). These values are consistent with spin orbit splitting of the Mo 3d level in the oxidation state 6 (MoVI (Mo(3d5/2) =235.8 eV and Mo(3d5/2)=232.6eV)) [62].
3.5. Electrocatalytic effect of the PMo12O 340− – ATh composite film on the reduction of BrO − 3 anion The reduction of BrO3− is totally irreversible at a glassy carbon electrode in acidic aqueous solution and does not take place prior to the evolution of hydrogen. However, the reduction of BrO3− can readily be catalyzed by the mixed-valence molybdenum(VI, V) [65,66] or tungsten(VI, V) [63,64] species, such as WO3 film [65] and MoO3 film [66]. In this paper, we also found that the PMo12O340− –ATh composite film modified Au electrode shows catalytic activity toward the reduction of BrO3− . The electrocatalytic reduction of BrO3− with the PMo12O340− –ATh composite film modified Au electrode is recorded in Fig. 6. The electrocatalysis occurs at the third wave (most negative one) of the PMo12O340− –ATh composite film, as shown in Fig. 6; with the addition of BrO3− , the cathodic current of the third wave is enhanced and the corresponding oxidation peak decreases, while peak I and II are almost unaffected by the addition of BrO3− . The result indicates that the cathodic wave corresponding to the reduction of the phosphomolybdic anion from four-electron to six-electron reduction products has catalytic properties toward the reduction of BrO3− . A similar phenomenon has been observed for the catalytic effect of PMo12O340− on
4. Conclusion We have prepared PMo12O340− –ATh or AET SAM composite films on gold substrates by electrochemical methods. The PMo12O340− is strongly adsorbed onto the Au-grafted surface, which is demonstrated by electrochemistry, XPS and reflectance FTIR. Reflectance FTIR studies show that there is some Coulombic interaction between –NH3+ – PMo12O340− in the PMo12O340− ATh interface. The PMo12O340− –ATh composite film modified gold electrode exhibits three reversible redox waves, corresponding to two two-electron, two-proton processes. Compared with the PMo12O340− in aqueous solution, the composite film shows more reversible kinetics with smaller peak-to-peak separation and narrower peak shape. Otherwise, the PMo12O340− anion within the composite film is stable at a larger pH range. This suggests that the Coulombic attractive interaction between the PMo12O340− anions and the surface amino group improves the stability of PMo12O340− anion. We have also found that this film possesses a good catalytic activity for the reduction of BrO3− in sulfuric acid solution. The composite film can provide a sensitive assembly for the electrochemical detection of BrO3− . The preliminary results obtained for PMo12O340− clearly indicate that the method described in this paper can be extended to all kinds of polyanions, and selecting the exact nature of the polyoxometalate should enable the elaboration of new catalytic electrodes having various properties. The above results gives us significant implications: one can effectively improve the structural stability of a
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Fig. 6. Cyclic voltammograms of a PMo12O340− –ATh composite film modified Au electrode in 2 M H2SO4 containing BrO3− concentrations of 0, 0.5, 1, 1.5, 2, 3, 4, 5 mmol l − 1 (reading from top to bottom). Scan rate: 100 mV s − 1. The inset shows the relation between catalytic current (Icat.) versus BrO3− concentration (cBrO − ). 3
heteropolyanion modified electrode through molecular level designing of the substrate surface.
Acknowledgements This work has been supported by the National Natural Science Foundation of China. Useful discussions with Dr J. Yan were appreciated.
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