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Electrochemistry and electrocatalysis of polyoxometalate-ordered mesoporous carbon modified electrode
Analytica Chimica Acta 587 (2007) 124–131
Electrochemistry and electrocatalysis of polyoxometalate-ordered mesoporous carbon modified electrode Ming Zhou, Li-ping Guo ∗ , Fan-yun Lin, Hai-xia Liu Faculty of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China Received 1 August 2006; received in revised form 7 January 2007; accepted 10 January 2007 Available online 16 January 2007
Abstract In this work, we have developed a convenient and efficient method for the functionalization of ordered mesoporous carbon (OMC) using polyoxometalate H6 P2 Mo18 O62 ·xH2 O (P2 Mo18 ). By the method, glassy carbon (GC) electrode modified with P2 Mo18 which was immobilized on the channel surface of OMC was prepared and characterized for the first time. The large specific surface area and porous structure of the modified OMC particles result in high heteropolyacid loading, and the P2 Mo18 entrapped in this order matrix is stable. Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption–desorption isotherm and X-ray diffraction (XRD) were employed to give insight into the intermolecular interaction between OMC and P2 Mo18 . The electrochemical behavior of the modified electrode was studied in detail, including pH-dependence, stability and so on. The cyclic voltammetry (CV) and amperometry studies demonstrated that P2 Mo18 /OMC/GC electrode has high stability, fast response and good electrocatalytic activity for the reduction of nitrite, bromate, idonate, and hydrogen peroxide. The mechanism of catalysis on P2 Mo18 /OMC/GC electrode was discussed. Moreover, the development of our approach for OMC functionalization suggests the potential applications in catalysis, molecular electronics and sensors. © 2007 Elsevier B.V. All rights reserved. Keywords: P2 Mo18 ; Ordered mesoporous carbon; Cyclic voltammetry; Electrocatalysis
1. Introduction Polyoxometalates (POMs) are a large and rapidly growing class of compounds that have attracted much attention in catalysis, medicine, bioanalysis and materials science owing to their chemical, structural and electronic versatility [1,2]. In recent years electrochemical properties of POMs have been intensively studied [2]. An important reason is the ability of POMs anions to accept various numbers of electrons giving rise to mixed-valency species, which has made these compounds very attractive in electrode modification and electrocatalytic research [2]. Immobilization of POMs on electrodes not only simplifies their electrochemical study but also facilitates their applications [3–5]. In general, there are three main strategies for POMs immobilization: (a) Electrochemical deposition [6,7], only on metal electrode. (b) Immobilization as a dopant in conductive polymeric matrices [8]. However, the polymer environment
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Corresponding author. Tel.: +86 431 85099762; fax: +86 413 85099762. E-mail address: [email protected] (L.-p. Guo).
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affects the electrochemical behavior and the electrocatalytic properties of the immobilized POMs [8]. (c) Adsorption [9,10]. POMs are able to adsorb on the surface of carbon-based materials [14], gold and mercury [15,16]. The coated electrodes can be rinsed without loss of the POMs coating, which is actively mediating the reduction of bromate, chromate, hydrogen peroxide, and nitrite just as in the case of the dissolved POMs [2]. It is reported that POMs can been immobilized on ordered mesoporous carbon (OMC) to improve the catalytic activity [11–13], or on MCM-41 (m) to maintain the eletrocatalytic activity by physical or chemical absorption [4]. And it has already been established that POMs molecules can be firmly physically adsorbed to graphite materials [17–19]. This physical adsorption was an electrostatic attraction caused by proton transfer from POMs to graphite for modifying graphite materials with POMs [17–19]. Recently, a series of inorganic porous materials such as clay minerals [20], montmorillonite [21], porous aluminosilicates, and sol–gel matrix etc. [22–29] have been shown to be promising as immobilization matrices. In 1992, an ordered mesoporous material MCM-41 was obtained in Mobil Oil Cor-
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poration [30,31]. In 1998, a novel family of mesoporous silicas (SBA) was synthesized by using neutral organic triblock copolymers [32–34]. These new materials have larger pores, thicker pore walls and higher hydrothermal stability compared to MCM41. Mesoporous crystalline materials exhibit extremely high surface area and well defined pore size as well as high thermal stability and flexible framework composition. In addition, the unique structural and catalytic properties of molecular sieves for structuring an electrochemical/electron transfer environment and resistance to biodegradation have attracted considerable attention [28,29,35]. An important line of research has focused on the enlargement of the pore sizes into the mesopore range, allowing large molecules to enter the pore system, to be processed there and to leave the pore system again since the first ordered mesoporous carbon, CMK-1, was synthesized in 1999 [36,37]. CMK-3 carbon is particularly promising because of the fact that SBA-15 template is inexpensive [38] and easy to synthesize [36]. Ordered mesoporous carbon exhibits extremely high surface area and well defined pore size as well as high thermal stability and flexible framework composition. The material has great interest for many advanced applications [39,40]. However, there are no reports of the chemical modified electrode about OMC in the applications of electroanalytical chemistry. Contrary to the intrinsically conductive mesoporous carbon [36,37], most nanostructured silica-based materials are electronic insulators. Their use in connection to electrochemistry would, therefore, require a close contact to an electrode surface or a composite conductive matrix likely to act as an electron donor or receptor, as previously reported for zeolite- or silica-modified electrodes [28,29]. Hence, ordered mesoporous carbon could have more interests and potential advantages for many advanced applications than other mesoporous materials [22–27]. In this paper, we developed a facile method to immobilize P2 Mo18 on the channel surface of OMC for the construction of P2 Mo18 /OMC and investigated its electrochemical characteristics and electrochemical catalysis. The interaction between P2 Mo18 and OMC is characterized with Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption–desorption isotherm, XRD and CV. The CV and amperometry studies demonstrated that P2 Mo18 /OMC/GC electrode has high stability, fast response and good electrocatalytic activity for the reduction of nitrite, bromate, idonate, and hydrogen peroxide. These make P2 Mo18 /OMC/GC electrode potential candidates for efficient and durable electrochemical sensor for the detection of nitrite, bromate, idonate, and hydrogen peroxide.
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Without special comments, electrolyte in CV and amperometry studies was 1 mol L−1 H2 SO4 throughout. Electrochemical experiments were performed with a CHI 830b Electrochemical Analyzer (CH Instruments, Shanghai Chenhua Instrument Corporation, China) in a conventional three-electrode cell. The working electrode (WE) used was glassy carbon (GC) electrode (Model CHI104, 3 mm diameter). A platinum electrode was applied as the counter electrode (CE) and an Ag/AgCl (in saturated KCl solution) electrode served as reference electrode (RE). All potentials in this paper were measured and reported versus Ag/AgCl. The sample solutions were purged with purified nitrogen for at least 15 min to remove oxygen prior to the beginning of a series of experiments. Small angle X-ray diffraction (XRD) patterns were obtained on an Xray Diffractor D4 (Brucker, Germany) operating at 40 kV and 20 mA and using Cu K␣ radiation (λ = 0.15406 nm). Nitrogen adsorption and desorption isotherms were measured on ASAP 2020 (Micromeritics, USA). The pore size distributions (PSD) were calculated by the BJH method. The loading levels of H6 P2 Mo18 O62 in final composites are determined by a Leeman Prodigy inductively coupled plasma atomic emission spectrometer (ICP-AES). Infrared spectrum of the sample was recorded with Nicolet Magna 560 FT-IR Spectrometer with KBr plate. 2.2. Preparation of OMC and P2 Mo18 /OMC
2. Experimental
SBA-15, as the template, was prepared as discussed in the literature [33]. And detailed information on the preparation and properties of OMC can be found in [33,42,43]. The specific procedure of immobilizing P2 Mo18 on the channel surface of OMC was as follows: First, 40 mg OMC was heated at 250 ◦ C for 0.5 h, and added to 2 mmol L−1 cool P2 Mo18 aqueous solution (200 mL) immediately. Second, the mixture was centrifuged at 3500 rpm to get P2 Mo18 /OMC from the mixture, and then P2 Mo18 /OMC was dried at 100 ◦ C for 3 h. Third, P2 Mo18 /OMC was rinsed with double distilled water and acetone for several times so that free P2 Mo18 can be eliminated, then P2 Mo18 /OMC was obtained. For the comparison purpose, P2 Mo18 was supported on bare OMC by impregnation method (P2 Mo18 /OMC-IM) [12,23], which would cost more time than our specific annealing method. This means that our specific annealing method is more convenient than traditional impregnation method. The properties of OMC and P2 Mo18 /OMC were characterized by FTIR, nitrogen adsorption–desorption isotherm and XRD (Figs. 1 and 2), and pore characterization of OMC and P2 Mo18 /OMC by nitrogen adsorption–desorption isotherm and XRD was listed in Table 1.
2.1. Reagents and apparatus
2.3. Preparation of P2 Mo18 /OMC/GC electrode
H6 P2 Mo18 O62 ·xH2 O (P2 Mo18 ) was synthesized according to the literature method [41]. Polyvinyl alcohol (PVA, average degree of polymerization, 1800 ± 100) was purchased from Shanghai Laize Factory of Fine Chemicals. All other chemicals not mentioned here were of analytical reagent grade and were used as received. Double distilled water was used throughout.
GC electrodes were polished before each experiment with 1, 0.3 and 0.05 m alumina powder, respectively, rinsed thoroughly with doubly distilled water between each polishing step, then washed successively with 1:1 nitric acid, acetone and doubly distilled water in ultrasonic bath and dried in air. Dry P2 Mo18 /OMC (10 mg) was added to 10 mL water and sonicated
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Table 1 Characterization data for OMC, P2 Mo18 /OMC and P2 Mo18 /OMC-IM
OMC P2 Mo18 /OMC P2 Mo18 /OMC-IM
P2 Mo18 loading (wt.%)
BET surface area (m2 g−1 )
Mesopore volume (cm3 g−1 )
Pore size (nm)
0 18.0 10.7
1520 328 –
1.3 0.4 –
4.5 1.9 –
Fig. 1. FTIR spectra of OMC (a); P2 Mo18 (b); and P2 Mo18 /OMC (c).
Fig. 2. (A) The powder XRD patterns of OMC (a) and P2 Mo18 /OMC (b). (B) The nitrogen adsorption–desorption isotherms for OMC (c) and P2 Mo18 /OMC (d).
for 0.5 h. Then 100 L of the obtained suspension was mixed with 5 L 3% PVA solution of ethanol/water (v:v = 1:1) to produce P2 Mo18 /OMC colloid. P2 Mo18 /OMC colloid (3 L) was dropped on the pretreated GC electrode surface and allowed to dry under ambient conditions. P2 Mo18 /OMC/GC electrode was obtained. 3. Results and discussion 3.1. Interaction between OMC and P2 Mo18 Immobilization of P2 Mo18 on OMC was confirmed by FTIR, as shown in Fig. 1. In Fig. 1b, the primary Dawson structure of P2 Mo18 could be identified by four characteristic IR bands
appearing within the range 700–1200 cm−1 [41]. The OMC support showed no characteristic IR bands within the range 700–1200 cm−1 (Fig. 1a). It is noticeable that the characteristic IR bands of P2 Mo18 /OMC (Fig. 1c) appeared at almost the same positions without significant band shifts compared to those of P2 Mo18 (Fig. 1b) indicating that P2 Mo18 species were effectively and physically (not chemically) supported on OMC in P2 Mo18 /OMC [4,11,12,45]. These demonstrate that P2 Mo18 species in the P2 Mo18 /OMC still keep the Dawson structure after adsorption [11,12,41,45]. Accordingly, the active properties of P2 Mo18 should be preserved in P2 Mo18 /OMC, for example, as catalysts for redox reactions [11,12,44] indicating P2 Mo18 /OMC may have the similar electrochemical property compared with P2 Mo18 . Fig. 2A (a) shows powder X-ray diffraction (XRD) patterns of OMC. The ordered arrangement of carbon nanorods can be observed by the well-resolved XRD peaks, which can be assigned to (1 0 0), (1 1 0) diffractions of hexagonal (p6mm) structure. The long-range periodic carbon structure is primarily due to the interconnecting carbon spacers. The XRD peaks in Fig. 2A(a) demonstrate that the carbon nanorods synthesized in this study are rigidly interconnected into a highly ordered hexagonal array by the carbon spacers, which are the inverse replica of the SBA-15. And as shown in Fig. 2A(b), the XRD pattern of P2 Mo18 /OMC exhibits the characteristic peaks of OMC (2θ = 0.94◦ ) and loading of P2 Mo18 gives rise to a decrease in intensity of the peak at 0.94◦ (2θ) that shows the similar behavior with the XRD pattern of PMo12 -MCM-41 (m) [4] and PW/MCM-41 [23]. This indicates that P2 Mo18 /OMC has lost the long-range order of OMC [11]. And it should be noticed that no patterns of any bulk P2 Mo18 crystal phase are observed for the fresh composite materials, indicating that the P2 Mo18 is finely dispersed on the OMC supporter [4]. To clarify the effect of P2 Mo18 on mesoporous materials, the nitrogen adsorption–desorption isotherms before (Fig. 2B(c)) and after P2 Mo18 loading were investigated (Fig. 2B(d)). The pore volume of OMC decreased upon the immobilization treatment. In Table 1, the pore volume of P2 Mo18 /OMC was 42.2% of that of OMC, indicating that P2 Mo18 intercalated into the inner wall of OMC. The loading level of H6 P2 Mo18 O62 in P2 Mo18 /OMC (18.0%) is more than that in P2 Mo18 /OMC-IM (10.7%) in Table 1, which means that our specific annealing method is more efficient than traditional impregnation method. High loading (compared with P2 Mo18 /OMC-IM), fine dispersion and good stabilization of P2 Mo18 may result from the specific annealing procedure of immobilizing P2 Mo18 aqueous solution in the channel of OMC. When the heated OMC was added into cool POMs aqueous solution, the channel of OMC and the gas inside get cool immediately, and the pressure inside
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is lower than atmospheric pressure outside. Then POMs aqueous solution could be spontaneously and effectively kept in the channel of OMC by atmospheric pressure. The POMs aqueous solution could be kept firmly in the channel of OMC without flowing out due to the capillarity [46]. It is reported that POMs can been immobilized on the surface of certain porous supports to improve its catalytic activity, and it has already been established that POMs molecules can be firmly physically adsorbed to graphite materials [17–19]. This physical adsorption was an electrostatic attraction caused by proton transfer from POMs to graphite for modifying graphite materials with POMs. By the adsorption, P2 Mo18 was physically adsorbed on the channel surface of OMC. Therefore, our specific annealing method is more convenient and efficient than traditional impregnation method [12]. 3.2. Electrochemical behavior of P2 Mo18 /OMC/GC electrode The electroactive surface areas of OMC/GC and GC were studied according to Eq. (1) (Randles–Sevcik equation) [47,48]: Ip = 2.69 × 10−5 AD1/2 n3/2 v1/2 c
(1)
where n is the number of electrons participating in the redox reaction, A is the area of the electroactive surface area (cm2 ), D is the diffusion coefficient of the molecule in solution (cm2 s−1 ), c corresponds to the bulk concentration of the redox probe (mol cm−3 ), and v is the scan rate of the potential perturbation (V s−1 ). The [Fe(CN)6 ]3−/4− redox system used in this study is one of the most extensively studied redox couples in electrochemistry and exhibits a heterogeneous one-electron transfer (n = 1). c is equal to 5 mmol L−1 , and the diffusion coefficient (D) is (6.70 ± 0.02) × 10−6 cm2 s−1 [48]. According to the equation, the average value of the electroactive surface area for OMC/GC electrode was 0.109 cm2 , which is more than that of GC electrode (0.068 cm2 ). The increase of the estimated active surface area clearly shows that OMC/GC has relativity better electrochemical reacting ability. The observed capacities of OMC/GC and GC can be calculated from Eq. (2) [49]: C=
I Av
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Fig. 3. Cyclic voltammograms of bare GC (a); OMC/GC (b); and P2 Mo18 / OMC/GC (c) electrodes. Electrolyte: 1 mol L−1 H2 SO4 ; scan rate: 100 mV s−1 .
In 1 mol L−1 H2 SO4 aqueous solution, no redox peaks were obtained at bare GC electrode (Fig. 3a) and OMC/GC electrode (Fig. 3b) in the potential range +0.80 to 0.00 V. The background current at the OMC/GC electrode is larger than that at the bare GC electrode due to the high surface area of OMC [50]. However, at P2 Mo18 /OMC/GC electrode (Fig. 3c), three reversible redox peaks can be observed clearly in the same potential range. The formal potentials ((Ea + Ec )/2) for the peaks I–I , II–II and III–III are +0.54, +0.43, +0.26 V, respectively. The corresponding peak potential separations are 49, 43, 32 mV, respectively, because the redox processes of P2 Mo18 on electrode surface are not ideal reversible reactions. These redox waves corresponded to P2 Mo18 multi-step electron transfer processes with an earlier report of consecutive reversible two-, two-, and two-electron processes [52]. The pH has a marked effect on the electrochemical behavior of P2 Mo18 /OMC/GC electrode. With increasing pH, the three redox formal potentials all gradually shift to the negative direction and the peak currents decrease (Fig. 4). Plots of peak potentials of the three successive redox waves versus pH for P2 Mo18 /OMC/GC electrode exhibit linearity in the pH
(2)
where I is the current, A is the electroactive surface area of the electrode, and v is the scan rate. The background current is attributed to surface charging and faradaic surface reactions, and is discussed in [50]. In Fig. 3b, the current of OMC/GC was nearly uniform over the whole working potential range, and the observed capacity of OMC/GC is 834.4 pF cm−2 which is larger than that of GC electrode (285.7 pF cm−2 ). These can explain and confirm the electrochemical behavior (Fig. 3) that the background current of OMC/GC is larger than that of GC [49]. The P2 Mo18 anion is unstable in neutral and basic aqueous solution and undergoes a series hydrolysis processes [51], therefore, electrochemical studies of P2 Mo18 /OMC/GC electrode were carried out in 1 mol L−1 H2 SO4 aqueous solution.
Fig. 4. The cyclic voltammograms for P2 Mo18 /OMC/GC electrode in H2 SO4 aqueous solution at different pH: 0.00 (a); 1.00 (b); 2.00 (c); 3.00 (d). Scan rate: 100 mV s−1 . Inset shows the relationship between peak potentials and pH.
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range from 0.0 to 3.0, as shown in inset of Fig. 4. Slopes in this pH range are 53, 60 and 59 mV pH−1 for I–I , II–II and III–III couples, respectively, which are close to the theoretical value 60 mV pH−1 for the 2e− /2H+ redox process at the experimental temperature [52,53]. Reduction of P2 Mo18 /OMC/GC electrode is accompanied by the evolution of protons from solution to P2 Mo18 /OMC/GC electrode to maintain charge neutrality. Along with increasing pH, slower penetration of protons to P2 Mo18 /OMC/GC electrode should be the reason for the current decrease, and the more negative reduction potentials can be elucidated using the Nernst equation [54]. It means that the electrochemistry processes of getting or losing protons of P2 Mo18 /OMC/GC electrode consist with the electrochemical behavior of P2 Mo18 anions in H2 SO4 aqueous solution. Therefore, the three redox processes of P2 Mo18 /OMC/GC electrode can be described as follows:
better electrocatalytic activity for the reduction of the nitrite at P2 Mo18 /OMC/GC electrode (Fig. 5). Because the pKa of HNO2 is 3.3, most nitrite ions are protonated in the acidic solutions. Nitrite might be reduced to NO instantly [56] by reacting with H4 P2 Mo18 O62 6− and H6 P2 Mo18 O62 6− as soon as HNO2 arrives at the P2 Mo18 /OMC films on electrode surface. The resulting H2 P2 Mo18 O62 6− and H4 P2 Mo18 O62 6− are then reduced to H4 P2 Mo18 O62 6− and H6 P2 Mo18 O62 6− to facilitate the mediation effect for the reduction of nitrite. On the basis of the voltammetric results described above, P2 Mo18 /OMC/GC electrode can effectively catalyze reduction of nitrite, and it appears likely that amperometric detection of nitrite by P2 Mo18 /OMC/GC electrode is possible. The relationship between applied potential and nitrite electrocatalytic reduction current shows that the reduction of nitrite starts at
P2 Mo18 VI O62 6− + 2e− + 2H+ H2 P2 Mo2 V Mo16 VI O62 6− (H2 P2 Mo18 O62 6− )
(3)
H2 P2 Mo2 V Mo16 VI O62 6− + 2e− + 2H+ H4 P2 Mo4 V MoVI 14 O62 6− (H4 P2 Mo18 O62 6− )
(4)
H4 P2 Mo4 V Mo14 VI O62 6− + 2e− + 2H+ H6 P2 Mo6 V Mo12 VI O62 6− (H6 P2 Mo18 O62 6− )
(5)
The electrochemical behavior of P2 Mo18 /OMC/GC electrode was also studied at various scan rates. Plot of peak currents versus scan rates (peak III and peak III are used to study) is linear at v ≤60 mV s−1 , which predicts that the electrode has the characteristics of thin-layer electrochemistry. At higher sweep rates, over 70 mV s−1 the peak currents become proportional to the square root of the sweep rates, which indicates diffusion behavior in charge transport. In general, the modified electrode has a disadvantage of leakage of the modifier. So the stability study of P2 Mo18 /OMC/GC electrode is necessary. After 100 cycles the cathodic peak current (the peak III was used to study) decreased by 1.8% in 1 mol L−1 H2 SO4 aqueous solution at a scan rate of 100 mV s−1 . When P2 Mo18 /OMC/GC electrode was stored in the atmosphere, the cathodic peak current decreased by 3.1% for 1 month. These results prove the good stability of P2 Mo18 /OMC/GC electrode.
approximately +0.40 V and the cathodic peak current increased with potential shifting negatively. According to the potential dependence of the nitrite electrocatalytic reduction current at stirring conditions, the optimum electrode potential was selected at +0.00 V for amperometric measurements in order to obtain good repeatability and high sensitivity. Fig. 6 shows, with operating potential controlled at +0.00 V, the typical current–time recording was obtained at P2 Mo18 /OMC/GC electrode. The response time of P2 Mo18 /OMC/GC electrode to nitrite is less than 25 s at that potential. The nitrite added was transferred quickly through the surface so as to be reduced by the immobilized P2 Mo18 , owing to the property of high surface area, uniform pore size and controlled structure of the modified substrate. These indicate that P2 Mo18 /OMC/GC electrode not only has high sensitivity, but also has a fast response to nitrite with operating potential controlled at 0.00 V in 1 mol L−1 H2 SO4
3.3. Electrocatalytic reduction of nitrite at P2 Mo18 /OMC/GC electrode At bare GC electrode, the electroreduction of nitrite requires a large over potential [55,56]. In Fig. 5b, no obvious response was observed at OMC/GC electrode in 1 mol L−1 H2 SO4 aqueous solution containing 2.40 × 10−3 mol L−1 nitrite in the potential range of +0.80 to 0.00 V. Yet the catalytic reduction of nitrite at P2 Mo18 /OMC/GC electrode became easy with the same condition. With addition of nitrite, reduction currents of peak II and III gradually increase while the corresponding oxidation peak currents decrease (Fig. 5c–e), which indicates the four- and six-electron reduced species of P2 Mo18 anions present electrocatalytic activity at P2 Mo18 /OMC/GC electrode for the reduction of nitrite. So we can conclude that P2 Mo18 /OMC/GC electrode can catalyze the reduction of nitrite and the sixelectron reduced product of P2 Mo18 anions (peak III) shows
Fig. 5. Cyclic voltammograms of OMC/GC electrode containing 0.00–4.80 × 10−3 mol L−1 (a, b) nitrite and P2 Mo18 /OMC/GC electrode containing 0.00– 9.60 × 10−3 mol L−1 (c–e) nitrite in1 mol L−1 H2 SO4 . Scan rate: 100 mV s−1 .
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centration did not show interference to nitrite detection, while Fe2+ , Cu2+ , I− , BrO3 − , and IO3 − in a 20-fold concentration have been found to exhibit serious interference. So P2 Mo18 /OMC/GC electrode has good selectivity for nitrite. We also investigated the repeatability of P2 Mo18 /OMC/GC electrode for catalytic effect. The relative standard deviation of the peak currents to eight repeated injections of 8.00 × 10−5 mol L−1 nitrite by the same one P2 Mo18 /OMC/GC electrode is 2.67%. This means that P2 Mo18 /OMC/GC electrode has good stability and repeatability. Thus, P2 Mo18 /OMC/GC electrode can be used to detect nitrite as an amperometric sensor.
Fig. 6. Current–time curves obtained at P2 Mo18 /OMC/GC electrode to successive addition of 2.00 × 10−3 mol L−1 nitrite into 1 mol L−1 H2 SO4 with applied potential at 0.00 V. Inset is the calibration curve for nitrite at P2 Mo18 /OMC/GC electrode.
aqueous solution. Therefore, an applied potential of 0.00 V was used in further experiments. The calibration graph for nitrite at P2 Mo18 /OMC/GC electrode is shown in inset of Fig. 6 (background subtraction). In Table 2, P2 Mo18 /OMC/GC electrode exhibits a good linear relationship with the concentration of nitrite, which is wider than that in literatures [40,57–59]. The detection limit is about 1/3 of 5.00 × 10−6 mol L−1 [57] and even 1/390 of 7.00 × 10−4 mol L−1 [60]. Possible interference for the detection of nitrite at P2 Mo18 / OMC/GC electrode was investigated by addition of various ions to 1 mol L−1 H2 SO4 aqueous solution in the presence of 8.00 × 10−5 mol L−1 sodium nitrite. The common ions such as Na+ , K+ , Ca2+ , Mg2+ , Zn2+ , Cr3+ , Ba2+ , Cl− , NO3 − , ClO− , H2 PO4 − , HPO4 2− , CO3 2− , SO4 2− and Br− in a 500-fold con-
3.4. Electrocatalytic reduction of bromate, iodate and hydrogen peroxide at P2 Mo18 /OMC/GC electrode The catalytic reduction of BrO3 − by P2 Mo18 /OMC/GC electrode is shown in Fig. 7A. With the addition of BrO3 − to the cell produced an obvious increase in cathodic current and a concomitant decrease in anodic current, which indicates the four- and six-electron reduced species of P2 Mo18 anions in P2 Mo18 /OMC/GC electrode can catalyze the reduction of bromate. Bromate is reduced to Br− instantly by reacting with H4 P2 Mo18 O62 6− and H6 P2 Mo18 O62 6− as soon as arriving at the P2 Mo18 /OMC/GC electrode. According to the potential dependence of the bromate electrocatalytic current under steady-state conditions, the optimum operating potential was selected at 0.00 V. Typical current–time recording at P2 Mo18 /OMC/GC electrode with operating potential controlled at 0.00 V was obtained by adding bromate successively to continuously stirred 1 mol L−1 H2 SO4 aqueous solution, as shown in Fig. 7 (B) (background subtraction). A good linear relationship with the concentration
Table 2 Comparison of the parameters of POMs, and some other inorganic materials modified electrodes as sensors for different species Species
Electrode
Regression equation (I: A; c: mmol L−1 )
Linear range (mol L−1 )
NO2 −
P2 Mo18 /OMC/GC -(Bu4 N)7 SiW9 O37 (CpTi)3 droped silica/GC [59] PMo12 -l-Cys/Au [57] PMo12 -AT/Au [58] Tungsten oxide [60]
I = 2.89c –
5.34–24000 2.5–1500
– I = −0.1746lgc − 0.0048 –
2–200 0.1–100 1600–20000
5 0.02 700
– 0.9965 >0.98
BrO3−
P2 Mo18 /OMC/GC CoW11 Co/PVP/TiO2 /GC [61] Tungsten oxide [60] [Cu(bpy)2 ]Br2 /carbon ceramic [62]
I = 6.18c – – I = 19.5c + 0.0159
2.77–4000 20–4400 300–45000 0.5–200
0.922 5.0 100 0.1a
0.999 0.9997 > 0.98 0.998
IO3−
P2 Mo18 /OMC/GC CoW11 Co/PVP/TiO2 /GC [61] Tungsten oxide films/GC [63] Amorphous mixed-valent molybdenum oxide film/GC [64] P2 Mo18 /OMC/GC PMo12 /Ppy/carbon paste [65] ␣2 K7 P2 W17 VO62 /graphite/organoceramic composite [66]
I = 1.51c – I = 71c + 0.035 I = 11c + 0.007
1.13–6250 2.0–280 5–5000 1–200
I = 0.107c – –
160–44000 200–30000 100–20000
H2 O2
a
S/N, 3.
Detection limit (mol L−1 ) 1.78a 1.2a
0.377a 0.8 1.2a 0.5a 53.4a 50a 40a
Correlation coefficient 0.999 –
0.999 0.9995 0.9990 0.9997 0.999 0.997 0.998
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area (1520 m2 g−1 ), which would be very beneficial for POMs adsorption. Moreover, most nanostructured silica-based materials are electronic insulators and their use in connection to electrochemistry would therefore require a close contact to an electrode surface or a composite conductive matrix likely to act as an electron donor or receptor, as previously reported for zeolite- or silica-modified electrodes [28], in the contrary, OMC is the intrinsically conductive conductor which could play an important role in electron transfer with most of its redox partners. 4. Conclusion
Fig. 7. (A) Cyclic voltammograms of OMC/GC electrode containing 0.00–8.00 × 10−3 mol L−1 (a, b) bromate and P2 Mo18 /OMC/GC electrode containing 0.00–8.00 × 10−3 mol L−1 (c–f) bromate in 1 mol L−1 H2 SO4 . Scan rate: 100 mV s−1 . (B) Current–time response of P2 Mo18 /OMC/GC electrode with successive addition of 5.00 × 10−4 mol L−1 bromate into 1 mol L−1 H2 SO4 , applied potential: 0.00 V. Inset is the calibration curve for bromate at P2 Mo18 /OMC/GC electrode.
of bromate was exhibited by amperometry in the range of 2.77 × 10−6 –4.00 × 10−3 mol L−1 , which is wider than that in literatures [60–62]. The detection limit is 9.22 × 10−7 mol L−1 , which is 1/5 of 5.0 × 10−6 mol L−1 [61] and even 1/100 of 1.00 × 10−4 mol L−1 [60] (Table 2). We also investigated the electrocatalytic activity of P2 Mo18 / OMC/GC electrode for the reduction of the iodate and hydrogen peroxide by the same method. According to the potential dependence of the iodate electrocatalytic current under steadystate conditions, the optimum operating potential was selected at 0.00 V. In Table 2, good linear ranges from 1.13 × 10−6 to 6.25 × 10−3 mol L−1 for the detection of iodate and 1.60 × 10−4 to 4.40 × 10−2 mol L−1 for the detection of hydrogen peroxide have been shown. Table 2 displays the function parameters of the P2 Mo18 / OMC/GC electrode prepared in this paper with those of previously reported modified electrodes based on POMs, and some other inorganic materials. It has been found that our electrode presents a wider linear range, and a lower detection limit as nitrite [40,57,58,60], bromate [60,61] or iodate sensor [61,63,64], and has the similar parameter with any other modified electrodes as hydrogen peroxide sensor [65,66]. These advantages could be attributed to the unique surface property of OMC. Firstly, the OMC possesses large surface
In this work, we have developed a convenient and efficient method for the functionalization of OMC using P2 Mo18 . The immobilization of P2 Mo18 on the channel surface of OMC and the parameters of the corresponding P2 Mo18 /OMC/GC electrode have been investigated. The CV and amperometry studies demonstrated that P2 Mo18 /OMC/GC electrode has high stability, fast response and good electrocatalytic activity for the reduction of nitrite, bromate, idonate, and hydrogen peroxide. All the advantages of the P2 Mo18 /OMC/GC electrode could be contributed to the remarkable POM-adsorption ability of OMC, and proper and stable interactions between OMC and P2 Mo18 . These make P2 Mo18 /OMC/GC electrode potential candidates for durable electrochemical sensors for the detection of nitrite, bromate, iodate, and hydrogen peroxide respectively, which opened up a new approach to develop chemically modified electrode in the actual applications. The development of our approach for OMC functionalization suggests the potential applications in catalysis, molecular electronics and sensors. The method might be extended beyond nanoparticles or biomolecules on the channel of P2 Mo18 /OMC, to other systems with desired properties. Further research on functional device development (supercapacitors and biosensors) is in progress. Acknowledgement The authors gratefully acknowledge the financial support by the Bureau of Science and Technology of Jilin Province (No. 20060573). References [1] M.T. Pope, A. Muller, Angew. Chem. Int. Ed. Engl. 30 (1991) 34. [2] M. Sadakane, E. Steckhan, Chem. Rev. 98 (1998) 219. [3] P.J. Kulesza, G. Roslonek, L.R. Faulkner, J. Electroanal. Chem. 280 (1990) 233. [4] L. Li, W. Li, C. Sun, L. Li, Electroanalysis 14 (2002) 368. [5] D. Pan, J. Chen, L. Nie, W. Tao, S. Yao, J. Electroanal. Chem. 579 (2005) 77. [6] B. Keita, L. Nadjo, J. Electroanal. Chem. 227 (1987) 265. [7] B. Keita, L. Nadjo, J. Electroanal. Chem. 247 (1988) 157. [8] H. Sung, H. So, W.K. Palk, Electrochim. Acta 39 (1994) 645. [9] L. Cheng, J.A. Cox, Chem. Mater. 14 (2002) 6. [10] D. Martel, A. Kuhn, Electrochim. Acta 45 (2000) 1829. [11] A. Lapkin, B. Bozkaya, T. Mays, L. Borello, K. Edler, B. Crittenden, Catal. Today 81 (2003) 611. [12] H. Kim, P. Kim, S.H. Kwan-Young Lee, J. Yeom, I.K. Yi, Song, Catal. Today 111 (2006) 361.
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Electrochemistry and electrocatalysis of polyoxometalate-ordered mesoporous carbon modified electrode Ming Zhou, Li-ping Guo ∗ , Fan-yun Lin, Hai-xia Liu Faculty of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China Received 1 August 2006; received in revised form 7 January 2007; accepted 10 January 2007 Available online 16 January 2007
Abstract In this work, we have developed a convenient and efficient method for the functionalization of ordered mesoporous carbon (OMC) using polyoxometalate H6 P2 Mo18 O62 ·xH2 O (P2 Mo18 ). By the method, glassy carbon (GC) electrode modified with P2 Mo18 which was immobilized on the channel surface of OMC was prepared and characterized for the first time. The large specific surface area and porous structure of the modified OMC particles result in high heteropolyacid loading, and the P2 Mo18 entrapped in this order matrix is stable. Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption–desorption isotherm and X-ray diffraction (XRD) were employed to give insight into the intermolecular interaction between OMC and P2 Mo18 . The electrochemical behavior of the modified electrode was studied in detail, including pH-dependence, stability and so on. The cyclic voltammetry (CV) and amperometry studies demonstrated that P2 Mo18 /OMC/GC electrode has high stability, fast response and good electrocatalytic activity for the reduction of nitrite, bromate, idonate, and hydrogen peroxide. The mechanism of catalysis on P2 Mo18 /OMC/GC electrode was discussed. Moreover, the development of our approach for OMC functionalization suggests the potential applications in catalysis, molecular electronics and sensors. © 2007 Elsevier B.V. All rights reserved. Keywords: P2 Mo18 ; Ordered mesoporous carbon; Cyclic voltammetry; Electrocatalysis
1. Introduction Polyoxometalates (POMs) are a large and rapidly growing class of compounds that have attracted much attention in catalysis, medicine, bioanalysis and materials science owing to their chemical, structural and electronic versatility [1,2]. In recent years electrochemical properties of POMs have been intensively studied [2]. An important reason is the ability of POMs anions to accept various numbers of electrons giving rise to mixed-valency species, which has made these compounds very attractive in electrode modification and electrocatalytic research [2]. Immobilization of POMs on electrodes not only simplifies their electrochemical study but also facilitates their applications [3–5]. In general, there are three main strategies for POMs immobilization: (a) Electrochemical deposition [6,7], only on metal electrode. (b) Immobilization as a dopant in conductive polymeric matrices [8]. However, the polymer environment
∗
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affects the electrochemical behavior and the electrocatalytic properties of the immobilized POMs [8]. (c) Adsorption [9,10]. POMs are able to adsorb on the surface of carbon-based materials [14], gold and mercury [15,16]. The coated electrodes can be rinsed without loss of the POMs coating, which is actively mediating the reduction of bromate, chromate, hydrogen peroxide, and nitrite just as in the case of the dissolved POMs [2]. It is reported that POMs can been immobilized on ordered mesoporous carbon (OMC) to improve the catalytic activity [11–13], or on MCM-41 (m) to maintain the eletrocatalytic activity by physical or chemical absorption [4]. And it has already been established that POMs molecules can be firmly physically adsorbed to graphite materials [17–19]. This physical adsorption was an electrostatic attraction caused by proton transfer from POMs to graphite for modifying graphite materials with POMs [17–19]. Recently, a series of inorganic porous materials such as clay minerals [20], montmorillonite [21], porous aluminosilicates, and sol–gel matrix etc. [22–29] have been shown to be promising as immobilization matrices. In 1992, an ordered mesoporous material MCM-41 was obtained in Mobil Oil Cor-
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poration [30,31]. In 1998, a novel family of mesoporous silicas (SBA) was synthesized by using neutral organic triblock copolymers [32–34]. These new materials have larger pores, thicker pore walls and higher hydrothermal stability compared to MCM41. Mesoporous crystalline materials exhibit extremely high surface area and well defined pore size as well as high thermal stability and flexible framework composition. In addition, the unique structural and catalytic properties of molecular sieves for structuring an electrochemical/electron transfer environment and resistance to biodegradation have attracted considerable attention [28,29,35]. An important line of research has focused on the enlargement of the pore sizes into the mesopore range, allowing large molecules to enter the pore system, to be processed there and to leave the pore system again since the first ordered mesoporous carbon, CMK-1, was synthesized in 1999 [36,37]. CMK-3 carbon is particularly promising because of the fact that SBA-15 template is inexpensive [38] and easy to synthesize [36]. Ordered mesoporous carbon exhibits extremely high surface area and well defined pore size as well as high thermal stability and flexible framework composition. The material has great interest for many advanced applications [39,40]. However, there are no reports of the chemical modified electrode about OMC in the applications of electroanalytical chemistry. Contrary to the intrinsically conductive mesoporous carbon [36,37], most nanostructured silica-based materials are electronic insulators. Their use in connection to electrochemistry would, therefore, require a close contact to an electrode surface or a composite conductive matrix likely to act as an electron donor or receptor, as previously reported for zeolite- or silica-modified electrodes [28,29]. Hence, ordered mesoporous carbon could have more interests and potential advantages for many advanced applications than other mesoporous materials [22–27]. In this paper, we developed a facile method to immobilize P2 Mo18 on the channel surface of OMC for the construction of P2 Mo18 /OMC and investigated its electrochemical characteristics and electrochemical catalysis. The interaction between P2 Mo18 and OMC is characterized with Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption–desorption isotherm, XRD and CV. The CV and amperometry studies demonstrated that P2 Mo18 /OMC/GC electrode has high stability, fast response and good electrocatalytic activity for the reduction of nitrite, bromate, idonate, and hydrogen peroxide. These make P2 Mo18 /OMC/GC electrode potential candidates for efficient and durable electrochemical sensor for the detection of nitrite, bromate, idonate, and hydrogen peroxide.
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Without special comments, electrolyte in CV and amperometry studies was 1 mol L−1 H2 SO4 throughout. Electrochemical experiments were performed with a CHI 830b Electrochemical Analyzer (CH Instruments, Shanghai Chenhua Instrument Corporation, China) in a conventional three-electrode cell. The working electrode (WE) used was glassy carbon (GC) electrode (Model CHI104, 3 mm diameter). A platinum electrode was applied as the counter electrode (CE) and an Ag/AgCl (in saturated KCl solution) electrode served as reference electrode (RE). All potentials in this paper were measured and reported versus Ag/AgCl. The sample solutions were purged with purified nitrogen for at least 15 min to remove oxygen prior to the beginning of a series of experiments. Small angle X-ray diffraction (XRD) patterns were obtained on an Xray Diffractor D4 (Brucker, Germany) operating at 40 kV and 20 mA and using Cu K␣ radiation (λ = 0.15406 nm). Nitrogen adsorption and desorption isotherms were measured on ASAP 2020 (Micromeritics, USA). The pore size distributions (PSD) were calculated by the BJH method. The loading levels of H6 P2 Mo18 O62 in final composites are determined by a Leeman Prodigy inductively coupled plasma atomic emission spectrometer (ICP-AES). Infrared spectrum of the sample was recorded with Nicolet Magna 560 FT-IR Spectrometer with KBr plate. 2.2. Preparation of OMC and P2 Mo18 /OMC
2. Experimental
SBA-15, as the template, was prepared as discussed in the literature [33]. And detailed information on the preparation and properties of OMC can be found in [33,42,43]. The specific procedure of immobilizing P2 Mo18 on the channel surface of OMC was as follows: First, 40 mg OMC was heated at 250 ◦ C for 0.5 h, and added to 2 mmol L−1 cool P2 Mo18 aqueous solution (200 mL) immediately. Second, the mixture was centrifuged at 3500 rpm to get P2 Mo18 /OMC from the mixture, and then P2 Mo18 /OMC was dried at 100 ◦ C for 3 h. Third, P2 Mo18 /OMC was rinsed with double distilled water and acetone for several times so that free P2 Mo18 can be eliminated, then P2 Mo18 /OMC was obtained. For the comparison purpose, P2 Mo18 was supported on bare OMC by impregnation method (P2 Mo18 /OMC-IM) [12,23], which would cost more time than our specific annealing method. This means that our specific annealing method is more convenient than traditional impregnation method. The properties of OMC and P2 Mo18 /OMC were characterized by FTIR, nitrogen adsorption–desorption isotherm and XRD (Figs. 1 and 2), and pore characterization of OMC and P2 Mo18 /OMC by nitrogen adsorption–desorption isotherm and XRD was listed in Table 1.
2.1. Reagents and apparatus
2.3. Preparation of P2 Mo18 /OMC/GC electrode
H6 P2 Mo18 O62 ·xH2 O (P2 Mo18 ) was synthesized according to the literature method [41]. Polyvinyl alcohol (PVA, average degree of polymerization, 1800 ± 100) was purchased from Shanghai Laize Factory of Fine Chemicals. All other chemicals not mentioned here were of analytical reagent grade and were used as received. Double distilled water was used throughout.
GC electrodes were polished before each experiment with 1, 0.3 and 0.05 m alumina powder, respectively, rinsed thoroughly with doubly distilled water between each polishing step, then washed successively with 1:1 nitric acid, acetone and doubly distilled water in ultrasonic bath and dried in air. Dry P2 Mo18 /OMC (10 mg) was added to 10 mL water and sonicated
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Table 1 Characterization data for OMC, P2 Mo18 /OMC and P2 Mo18 /OMC-IM
OMC P2 Mo18 /OMC P2 Mo18 /OMC-IM
P2 Mo18 loading (wt.%)
BET surface area (m2 g−1 )
Mesopore volume (cm3 g−1 )
Pore size (nm)
0 18.0 10.7
1520 328 –
1.3 0.4 –
4.5 1.9 –
Fig. 1. FTIR spectra of OMC (a); P2 Mo18 (b); and P2 Mo18 /OMC (c).
Fig. 2. (A) The powder XRD patterns of OMC (a) and P2 Mo18 /OMC (b). (B) The nitrogen adsorption–desorption isotherms for OMC (c) and P2 Mo18 /OMC (d).
for 0.5 h. Then 100 L of the obtained suspension was mixed with 5 L 3% PVA solution of ethanol/water (v:v = 1:1) to produce P2 Mo18 /OMC colloid. P2 Mo18 /OMC colloid (3 L) was dropped on the pretreated GC electrode surface and allowed to dry under ambient conditions. P2 Mo18 /OMC/GC electrode was obtained. 3. Results and discussion 3.1. Interaction between OMC and P2 Mo18 Immobilization of P2 Mo18 on OMC was confirmed by FTIR, as shown in Fig. 1. In Fig. 1b, the primary Dawson structure of P2 Mo18 could be identified by four characteristic IR bands
appearing within the range 700–1200 cm−1 [41]. The OMC support showed no characteristic IR bands within the range 700–1200 cm−1 (Fig. 1a). It is noticeable that the characteristic IR bands of P2 Mo18 /OMC (Fig. 1c) appeared at almost the same positions without significant band shifts compared to those of P2 Mo18 (Fig. 1b) indicating that P2 Mo18 species were effectively and physically (not chemically) supported on OMC in P2 Mo18 /OMC [4,11,12,45]. These demonstrate that P2 Mo18 species in the P2 Mo18 /OMC still keep the Dawson structure after adsorption [11,12,41,45]. Accordingly, the active properties of P2 Mo18 should be preserved in P2 Mo18 /OMC, for example, as catalysts for redox reactions [11,12,44] indicating P2 Mo18 /OMC may have the similar electrochemical property compared with P2 Mo18 . Fig. 2A (a) shows powder X-ray diffraction (XRD) patterns of OMC. The ordered arrangement of carbon nanorods can be observed by the well-resolved XRD peaks, which can be assigned to (1 0 0), (1 1 0) diffractions of hexagonal (p6mm) structure. The long-range periodic carbon structure is primarily due to the interconnecting carbon spacers. The XRD peaks in Fig. 2A(a) demonstrate that the carbon nanorods synthesized in this study are rigidly interconnected into a highly ordered hexagonal array by the carbon spacers, which are the inverse replica of the SBA-15. And as shown in Fig. 2A(b), the XRD pattern of P2 Mo18 /OMC exhibits the characteristic peaks of OMC (2θ = 0.94◦ ) and loading of P2 Mo18 gives rise to a decrease in intensity of the peak at 0.94◦ (2θ) that shows the similar behavior with the XRD pattern of PMo12 -MCM-41 (m) [4] and PW/MCM-41 [23]. This indicates that P2 Mo18 /OMC has lost the long-range order of OMC [11]. And it should be noticed that no patterns of any bulk P2 Mo18 crystal phase are observed for the fresh composite materials, indicating that the P2 Mo18 is finely dispersed on the OMC supporter [4]. To clarify the effect of P2 Mo18 on mesoporous materials, the nitrogen adsorption–desorption isotherms before (Fig. 2B(c)) and after P2 Mo18 loading were investigated (Fig. 2B(d)). The pore volume of OMC decreased upon the immobilization treatment. In Table 1, the pore volume of P2 Mo18 /OMC was 42.2% of that of OMC, indicating that P2 Mo18 intercalated into the inner wall of OMC. The loading level of H6 P2 Mo18 O62 in P2 Mo18 /OMC (18.0%) is more than that in P2 Mo18 /OMC-IM (10.7%) in Table 1, which means that our specific annealing method is more efficient than traditional impregnation method. High loading (compared with P2 Mo18 /OMC-IM), fine dispersion and good stabilization of P2 Mo18 may result from the specific annealing procedure of immobilizing P2 Mo18 aqueous solution in the channel of OMC. When the heated OMC was added into cool POMs aqueous solution, the channel of OMC and the gas inside get cool immediately, and the pressure inside
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is lower than atmospheric pressure outside. Then POMs aqueous solution could be spontaneously and effectively kept in the channel of OMC by atmospheric pressure. The POMs aqueous solution could be kept firmly in the channel of OMC without flowing out due to the capillarity [46]. It is reported that POMs can been immobilized on the surface of certain porous supports to improve its catalytic activity, and it has already been established that POMs molecules can be firmly physically adsorbed to graphite materials [17–19]. This physical adsorption was an electrostatic attraction caused by proton transfer from POMs to graphite for modifying graphite materials with POMs. By the adsorption, P2 Mo18 was physically adsorbed on the channel surface of OMC. Therefore, our specific annealing method is more convenient and efficient than traditional impregnation method [12]. 3.2. Electrochemical behavior of P2 Mo18 /OMC/GC electrode The electroactive surface areas of OMC/GC and GC were studied according to Eq. (1) (Randles–Sevcik equation) [47,48]: Ip = 2.69 × 10−5 AD1/2 n3/2 v1/2 c
(1)
where n is the number of electrons participating in the redox reaction, A is the area of the electroactive surface area (cm2 ), D is the diffusion coefficient of the molecule in solution (cm2 s−1 ), c corresponds to the bulk concentration of the redox probe (mol cm−3 ), and v is the scan rate of the potential perturbation (V s−1 ). The [Fe(CN)6 ]3−/4− redox system used in this study is one of the most extensively studied redox couples in electrochemistry and exhibits a heterogeneous one-electron transfer (n = 1). c is equal to 5 mmol L−1 , and the diffusion coefficient (D) is (6.70 ± 0.02) × 10−6 cm2 s−1 [48]. According to the equation, the average value of the electroactive surface area for OMC/GC electrode was 0.109 cm2 , which is more than that of GC electrode (0.068 cm2 ). The increase of the estimated active surface area clearly shows that OMC/GC has relativity better electrochemical reacting ability. The observed capacities of OMC/GC and GC can be calculated from Eq. (2) [49]: C=
I Av
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Fig. 3. Cyclic voltammograms of bare GC (a); OMC/GC (b); and P2 Mo18 / OMC/GC (c) electrodes. Electrolyte: 1 mol L−1 H2 SO4 ; scan rate: 100 mV s−1 .
In 1 mol L−1 H2 SO4 aqueous solution, no redox peaks were obtained at bare GC electrode (Fig. 3a) and OMC/GC electrode (Fig. 3b) in the potential range +0.80 to 0.00 V. The background current at the OMC/GC electrode is larger than that at the bare GC electrode due to the high surface area of OMC [50]. However, at P2 Mo18 /OMC/GC electrode (Fig. 3c), three reversible redox peaks can be observed clearly in the same potential range. The formal potentials ((Ea + Ec )/2) for the peaks I–I , II–II and III–III are +0.54, +0.43, +0.26 V, respectively. The corresponding peak potential separations are 49, 43, 32 mV, respectively, because the redox processes of P2 Mo18 on electrode surface are not ideal reversible reactions. These redox waves corresponded to P2 Mo18 multi-step electron transfer processes with an earlier report of consecutive reversible two-, two-, and two-electron processes [52]. The pH has a marked effect on the electrochemical behavior of P2 Mo18 /OMC/GC electrode. With increasing pH, the three redox formal potentials all gradually shift to the negative direction and the peak currents decrease (Fig. 4). Plots of peak potentials of the three successive redox waves versus pH for P2 Mo18 /OMC/GC electrode exhibit linearity in the pH
(2)
where I is the current, A is the electroactive surface area of the electrode, and v is the scan rate. The background current is attributed to surface charging and faradaic surface reactions, and is discussed in [50]. In Fig. 3b, the current of OMC/GC was nearly uniform over the whole working potential range, and the observed capacity of OMC/GC is 834.4 pF cm−2 which is larger than that of GC electrode (285.7 pF cm−2 ). These can explain and confirm the electrochemical behavior (Fig. 3) that the background current of OMC/GC is larger than that of GC [49]. The P2 Mo18 anion is unstable in neutral and basic aqueous solution and undergoes a series hydrolysis processes [51], therefore, electrochemical studies of P2 Mo18 /OMC/GC electrode were carried out in 1 mol L−1 H2 SO4 aqueous solution.
Fig. 4. The cyclic voltammograms for P2 Mo18 /OMC/GC electrode in H2 SO4 aqueous solution at different pH: 0.00 (a); 1.00 (b); 2.00 (c); 3.00 (d). Scan rate: 100 mV s−1 . Inset shows the relationship between peak potentials and pH.
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range from 0.0 to 3.0, as shown in inset of Fig. 4. Slopes in this pH range are 53, 60 and 59 mV pH−1 for I–I , II–II and III–III couples, respectively, which are close to the theoretical value 60 mV pH−1 for the 2e− /2H+ redox process at the experimental temperature [52,53]. Reduction of P2 Mo18 /OMC/GC electrode is accompanied by the evolution of protons from solution to P2 Mo18 /OMC/GC electrode to maintain charge neutrality. Along with increasing pH, slower penetration of protons to P2 Mo18 /OMC/GC electrode should be the reason for the current decrease, and the more negative reduction potentials can be elucidated using the Nernst equation [54]. It means that the electrochemistry processes of getting or losing protons of P2 Mo18 /OMC/GC electrode consist with the electrochemical behavior of P2 Mo18 anions in H2 SO4 aqueous solution. Therefore, the three redox processes of P2 Mo18 /OMC/GC electrode can be described as follows:
better electrocatalytic activity for the reduction of the nitrite at P2 Mo18 /OMC/GC electrode (Fig. 5). Because the pKa of HNO2 is 3.3, most nitrite ions are protonated in the acidic solutions. Nitrite might be reduced to NO instantly [56] by reacting with H4 P2 Mo18 O62 6− and H6 P2 Mo18 O62 6− as soon as HNO2 arrives at the P2 Mo18 /OMC films on electrode surface. The resulting H2 P2 Mo18 O62 6− and H4 P2 Mo18 O62 6− are then reduced to H4 P2 Mo18 O62 6− and H6 P2 Mo18 O62 6− to facilitate the mediation effect for the reduction of nitrite. On the basis of the voltammetric results described above, P2 Mo18 /OMC/GC electrode can effectively catalyze reduction of nitrite, and it appears likely that amperometric detection of nitrite by P2 Mo18 /OMC/GC electrode is possible. The relationship between applied potential and nitrite electrocatalytic reduction current shows that the reduction of nitrite starts at
P2 Mo18 VI O62 6− + 2e− + 2H+ H2 P2 Mo2 V Mo16 VI O62 6− (H2 P2 Mo18 O62 6− )
(3)
H2 P2 Mo2 V Mo16 VI O62 6− + 2e− + 2H+ H4 P2 Mo4 V MoVI 14 O62 6− (H4 P2 Mo18 O62 6− )
(4)
H4 P2 Mo4 V Mo14 VI O62 6− + 2e− + 2H+ H6 P2 Mo6 V Mo12 VI O62 6− (H6 P2 Mo18 O62 6− )
(5)
The electrochemical behavior of P2 Mo18 /OMC/GC electrode was also studied at various scan rates. Plot of peak currents versus scan rates (peak III and peak III are used to study) is linear at v ≤60 mV s−1 , which predicts that the electrode has the characteristics of thin-layer electrochemistry. At higher sweep rates, over 70 mV s−1 the peak currents become proportional to the square root of the sweep rates, which indicates diffusion behavior in charge transport. In general, the modified electrode has a disadvantage of leakage of the modifier. So the stability study of P2 Mo18 /OMC/GC electrode is necessary. After 100 cycles the cathodic peak current (the peak III was used to study) decreased by 1.8% in 1 mol L−1 H2 SO4 aqueous solution at a scan rate of 100 mV s−1 . When P2 Mo18 /OMC/GC electrode was stored in the atmosphere, the cathodic peak current decreased by 3.1% for 1 month. These results prove the good stability of P2 Mo18 /OMC/GC electrode.
approximately +0.40 V and the cathodic peak current increased with potential shifting negatively. According to the potential dependence of the nitrite electrocatalytic reduction current at stirring conditions, the optimum electrode potential was selected at +0.00 V for amperometric measurements in order to obtain good repeatability and high sensitivity. Fig. 6 shows, with operating potential controlled at +0.00 V, the typical current–time recording was obtained at P2 Mo18 /OMC/GC electrode. The response time of P2 Mo18 /OMC/GC electrode to nitrite is less than 25 s at that potential. The nitrite added was transferred quickly through the surface so as to be reduced by the immobilized P2 Mo18 , owing to the property of high surface area, uniform pore size and controlled structure of the modified substrate. These indicate that P2 Mo18 /OMC/GC electrode not only has high sensitivity, but also has a fast response to nitrite with operating potential controlled at 0.00 V in 1 mol L−1 H2 SO4
3.3. Electrocatalytic reduction of nitrite at P2 Mo18 /OMC/GC electrode At bare GC electrode, the electroreduction of nitrite requires a large over potential [55,56]. In Fig. 5b, no obvious response was observed at OMC/GC electrode in 1 mol L−1 H2 SO4 aqueous solution containing 2.40 × 10−3 mol L−1 nitrite in the potential range of +0.80 to 0.00 V. Yet the catalytic reduction of nitrite at P2 Mo18 /OMC/GC electrode became easy with the same condition. With addition of nitrite, reduction currents of peak II and III gradually increase while the corresponding oxidation peak currents decrease (Fig. 5c–e), which indicates the four- and six-electron reduced species of P2 Mo18 anions present electrocatalytic activity at P2 Mo18 /OMC/GC electrode for the reduction of nitrite. So we can conclude that P2 Mo18 /OMC/GC electrode can catalyze the reduction of nitrite and the sixelectron reduced product of P2 Mo18 anions (peak III) shows
Fig. 5. Cyclic voltammograms of OMC/GC electrode containing 0.00–4.80 × 10−3 mol L−1 (a, b) nitrite and P2 Mo18 /OMC/GC electrode containing 0.00– 9.60 × 10−3 mol L−1 (c–e) nitrite in1 mol L−1 H2 SO4 . Scan rate: 100 mV s−1 .
M. Zhou et al. / Analytica Chimica Acta 587 (2007) 124–131
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centration did not show interference to nitrite detection, while Fe2+ , Cu2+ , I− , BrO3 − , and IO3 − in a 20-fold concentration have been found to exhibit serious interference. So P2 Mo18 /OMC/GC electrode has good selectivity for nitrite. We also investigated the repeatability of P2 Mo18 /OMC/GC electrode for catalytic effect. The relative standard deviation of the peak currents to eight repeated injections of 8.00 × 10−5 mol L−1 nitrite by the same one P2 Mo18 /OMC/GC electrode is 2.67%. This means that P2 Mo18 /OMC/GC electrode has good stability and repeatability. Thus, P2 Mo18 /OMC/GC electrode can be used to detect nitrite as an amperometric sensor.
Fig. 6. Current–time curves obtained at P2 Mo18 /OMC/GC electrode to successive addition of 2.00 × 10−3 mol L−1 nitrite into 1 mol L−1 H2 SO4 with applied potential at 0.00 V. Inset is the calibration curve for nitrite at P2 Mo18 /OMC/GC electrode.
aqueous solution. Therefore, an applied potential of 0.00 V was used in further experiments. The calibration graph for nitrite at P2 Mo18 /OMC/GC electrode is shown in inset of Fig. 6 (background subtraction). In Table 2, P2 Mo18 /OMC/GC electrode exhibits a good linear relationship with the concentration of nitrite, which is wider than that in literatures [40,57–59]. The detection limit is about 1/3 of 5.00 × 10−6 mol L−1 [57] and even 1/390 of 7.00 × 10−4 mol L−1 [60]. Possible interference for the detection of nitrite at P2 Mo18 / OMC/GC electrode was investigated by addition of various ions to 1 mol L−1 H2 SO4 aqueous solution in the presence of 8.00 × 10−5 mol L−1 sodium nitrite. The common ions such as Na+ , K+ , Ca2+ , Mg2+ , Zn2+ , Cr3+ , Ba2+ , Cl− , NO3 − , ClO− , H2 PO4 − , HPO4 2− , CO3 2− , SO4 2− and Br− in a 500-fold con-
3.4. Electrocatalytic reduction of bromate, iodate and hydrogen peroxide at P2 Mo18 /OMC/GC electrode The catalytic reduction of BrO3 − by P2 Mo18 /OMC/GC electrode is shown in Fig. 7A. With the addition of BrO3 − to the cell produced an obvious increase in cathodic current and a concomitant decrease in anodic current, which indicates the four- and six-electron reduced species of P2 Mo18 anions in P2 Mo18 /OMC/GC electrode can catalyze the reduction of bromate. Bromate is reduced to Br− instantly by reacting with H4 P2 Mo18 O62 6− and H6 P2 Mo18 O62 6− as soon as arriving at the P2 Mo18 /OMC/GC electrode. According to the potential dependence of the bromate electrocatalytic current under steady-state conditions, the optimum operating potential was selected at 0.00 V. Typical current–time recording at P2 Mo18 /OMC/GC electrode with operating potential controlled at 0.00 V was obtained by adding bromate successively to continuously stirred 1 mol L−1 H2 SO4 aqueous solution, as shown in Fig. 7 (B) (background subtraction). A good linear relationship with the concentration
Table 2 Comparison of the parameters of POMs, and some other inorganic materials modified electrodes as sensors for different species Species
Electrode
Regression equation (I: A; c: mmol L−1 )
Linear range (mol L−1 )
NO2 −
P2 Mo18 /OMC/GC -(Bu4 N)7 SiW9 O37 (CpTi)3 droped silica/GC [59] PMo12 -l-Cys/Au [57] PMo12 -AT/Au [58] Tungsten oxide [60]
I = 2.89c –
5.34–24000 2.5–1500
– I = −0.1746lgc − 0.0048 –
2–200 0.1–100 1600–20000
5 0.02 700
– 0.9965 >0.98
BrO3−
P2 Mo18 /OMC/GC CoW11 Co/PVP/TiO2 /GC [61] Tungsten oxide [60] [Cu(bpy)2 ]Br2 /carbon ceramic [62]
I = 6.18c – – I = 19.5c + 0.0159
2.77–4000 20–4400 300–45000 0.5–200
0.922 5.0 100 0.1a
0.999 0.9997 > 0.98 0.998
IO3−
P2 Mo18 /OMC/GC CoW11 Co/PVP/TiO2 /GC [61] Tungsten oxide films/GC [63] Amorphous mixed-valent molybdenum oxide film/GC [64] P2 Mo18 /OMC/GC PMo12 /Ppy/carbon paste [65] ␣2 K7 P2 W17 VO62 /graphite/organoceramic composite [66]
I = 1.51c – I = 71c + 0.035 I = 11c + 0.007
1.13–6250 2.0–280 5–5000 1–200
I = 0.107c – –
160–44000 200–30000 100–20000
H2 O2
a
S/N, 3.
Detection limit (mol L−1 ) 1.78a 1.2a
0.377a 0.8 1.2a 0.5a 53.4a 50a 40a
Correlation coefficient 0.999 –
0.999 0.9995 0.9990 0.9997 0.999 0.997 0.998
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area (1520 m2 g−1 ), which would be very beneficial for POMs adsorption. Moreover, most nanostructured silica-based materials are electronic insulators and their use in connection to electrochemistry would therefore require a close contact to an electrode surface or a composite conductive matrix likely to act as an electron donor or receptor, as previously reported for zeolite- or silica-modified electrodes [28], in the contrary, OMC is the intrinsically conductive conductor which could play an important role in electron transfer with most of its redox partners. 4. Conclusion
Fig. 7. (A) Cyclic voltammograms of OMC/GC electrode containing 0.00–8.00 × 10−3 mol L−1 (a, b) bromate and P2 Mo18 /OMC/GC electrode containing 0.00–8.00 × 10−3 mol L−1 (c–f) bromate in 1 mol L−1 H2 SO4 . Scan rate: 100 mV s−1 . (B) Current–time response of P2 Mo18 /OMC/GC electrode with successive addition of 5.00 × 10−4 mol L−1 bromate into 1 mol L−1 H2 SO4 , applied potential: 0.00 V. Inset is the calibration curve for bromate at P2 Mo18 /OMC/GC electrode.
of bromate was exhibited by amperometry in the range of 2.77 × 10−6 –4.00 × 10−3 mol L−1 , which is wider than that in literatures [60–62]. The detection limit is 9.22 × 10−7 mol L−1 , which is 1/5 of 5.0 × 10−6 mol L−1 [61] and even 1/100 of 1.00 × 10−4 mol L−1 [60] (Table 2). We also investigated the electrocatalytic activity of P2 Mo18 / OMC/GC electrode for the reduction of the iodate and hydrogen peroxide by the same method. According to the potential dependence of the iodate electrocatalytic current under steadystate conditions, the optimum operating potential was selected at 0.00 V. In Table 2, good linear ranges from 1.13 × 10−6 to 6.25 × 10−3 mol L−1 for the detection of iodate and 1.60 × 10−4 to 4.40 × 10−2 mol L−1 for the detection of hydrogen peroxide have been shown. Table 2 displays the function parameters of the P2 Mo18 / OMC/GC electrode prepared in this paper with those of previously reported modified electrodes based on POMs, and some other inorganic materials. It has been found that our electrode presents a wider linear range, and a lower detection limit as nitrite [40,57,58,60], bromate [60,61] or iodate sensor [61,63,64], and has the similar parameter with any other modified electrodes as hydrogen peroxide sensor [65,66]. These advantages could be attributed to the unique surface property of OMC. Firstly, the OMC possesses large surface
In this work, we have developed a convenient and efficient method for the functionalization of OMC using P2 Mo18 . The immobilization of P2 Mo18 on the channel surface of OMC and the parameters of the corresponding P2 Mo18 /OMC/GC electrode have been investigated. The CV and amperometry studies demonstrated that P2 Mo18 /OMC/GC electrode has high stability, fast response and good electrocatalytic activity for the reduction of nitrite, bromate, idonate, and hydrogen peroxide. All the advantages of the P2 Mo18 /OMC/GC electrode could be contributed to the remarkable POM-adsorption ability of OMC, and proper and stable interactions between OMC and P2 Mo18 . These make P2 Mo18 /OMC/GC electrode potential candidates for durable electrochemical sensors for the detection of nitrite, bromate, iodate, and hydrogen peroxide respectively, which opened up a new approach to develop chemically modified electrode in the actual applications. The development of our approach for OMC functionalization suggests the potential applications in catalysis, molecular electronics and sensors. The method might be extended beyond nanoparticles or biomolecules on the channel of P2 Mo18 /OMC, to other systems with desired properties. Further research on functional device development (supercapacitors and biosensors) is in progress. Acknowledgement The authors gratefully acknowledge the financial support by the Bureau of Science and Technology of Jilin Province (No. 20060573). References [1] M.T. Pope, A. Muller, Angew. Chem. Int. Ed. Engl. 30 (1991) 34. [2] M. Sadakane, E. Steckhan, Chem. Rev. 98 (1998) 219. [3] P.J. Kulesza, G. Roslonek, L.R. Faulkner, J. Electroanal. Chem. 280 (1990) 233. [4] L. Li, W. Li, C. Sun, L. Li, Electroanalysis 14 (2002) 368. [5] D. Pan, J. Chen, L. Nie, W. Tao, S. Yao, J. Electroanal. Chem. 579 (2005) 77. [6] B. Keita, L. Nadjo, J. Electroanal. Chem. 227 (1987) 265. [7] B. Keita, L. Nadjo, J. Electroanal. Chem. 247 (1988) 157. [8] H. Sung, H. So, W.K. Palk, Electrochim. Acta 39 (1994) 645. [9] L. Cheng, J.A. Cox, Chem. Mater. 14 (2002) 6. [10] D. Martel, A. Kuhn, Electrochim. Acta 45 (2000) 1829. [11] A. Lapkin, B. Bozkaya, T. Mays, L. Borello, K. Edler, B. Crittenden, Catal. Today 81 (2003) 611. [12] H. Kim, P. Kim, S.H. Kwan-Young Lee, J. Yeom, I.K. Yi, Song, Catal. Today 111 (2006) 361.
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