Electrochemical Preparation and Characterization of Silicotungstic

ated functional group on carbon surface, whose nature and number are difficult ..... (64) Song, I. K.; Kaba, M. S.; Coulston, G.; Kourtakis, K.; Barte...
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Electrochemical Preparation and Characterization of Silicotungstic Heteropolyanion Monolayer Electrostatically Linked Aminophenyl on Carbon Electrode Surface Shaoqin Liu, Zhiyong Tang, Zhong Shi, Li Niu, Erkang Wang,* and Shaojun Dong*,† Laboratory of Electroanalytical Chemistry, Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, People’s Republic of China Received November 24, 1998. In Final Form: April 28, 1999 In this paper, an organic-inorganic composite film of heteropolyanion was formed by attaching a Keggintype heteropolyanion, SiW12O404-, on carbon electrode surface derivatized by 4-aminophenyl monolayer. The composite film thus grafted on carbon electrode surface has good stability because of the ionic bonding character between SiW12O404- and surface aminophenyl groups. X-ray photoelectron spectroscopy, scanning tunneling microscopy, and cyclic voltammetry were used to characterize the composite film. Compared with SiW12O404- electrodeposited on a bare glassy carbon electrode (GCE), the composite film gives three more sharp and well-defined redox couples attributed to two one- and two-electron processes, and the analyses of the voltammograms of SiW12O404- anion in the composite film modified on GCE shows that its surface coverage is close to a closest packing monolayer. STM characterization shows that a twodimensional order heteropolyanion monolayer was formed on HOPG substrate. The composite film provides a favorable environment for electron and proton transfer between SiW12O404- ion and electrode surface, which may make it suitable for various applications in sensors and microelectronics devices.

1. Introduction Interest in heteropolyanions, [XxMmOy]p- (x e m) (where M represents Mo, W, V, etc.; X: P, Si, As, etc.) is growing in the fields of 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), which makes them suitable as electrocatalysis and electrochromic materials.5 To take advantage of their specific properties observed in solution, the attachment of such species onto electrode surfaces is of great interest. The attachment of the Keggin [XM12O40]p- or Dawson [X2M18O62]q- parent structure has been largely explored: (i) heteropolyanions are adsorbed on carbon electrode,6-8 which often involves vigorous oxidation processes resulting in the formation of oxygenated functional group on carbon surface, whose nature and number are difficult to identify and control; (ii) heteropolyanions as a dopant are immobilized into polymer films9-26 (e.g., polypyrrole, polyaniline, polythiophene, and their derivatives). However, in some cases, the polymer * To whom correspondence should be addressed. † FAX: +86-431-5689711. Phone: +86-431-5682801-5562. Email: [email protected]. (1) Pope, M. T.; Muller A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. (2) Pope, M. T. Heteropoly and Isopoly Oxometalates; SpringerVerlag: Berlin, 1983. (3) Pope, M. T.; Muller A. Polyoxometalates: from Platonic Solids to Antiretrovival Activity; Kluwer: Dordrecht, 1994. (4) Misono, M. Catal. Rev. Sci. Eng. 1987, 29, 269. (5) Shimidzu, T.; Ohtami, A.; Aiba, M.; Honda, K. J. Chem. Soc., Faraday Trans. 1988, 84, 3941. (6) Dong, S.; Wang, B. Electrochim. Acta 1992, 37, 11. (7) Wang, B.; Dong, S. Electrochim. Acta 1992, 37, 1859. (8) Wang, B.; Dong, S. J. Electroanal. Chem. 1992, 328, 245. (9) Bidan, G.; Genies, E. M.; Lapkowski, M. J. Electroanal. Chem. 1988, 251, 297. (10) Bidan, G.; Genies, E. M.; Lapkowski, M. Synth. Metals 1989, 31, 327.

environment affects the electrochemical behavior and the electrocatalytic properties of the immobilized heteropolyanions.18,19,24,26 (iii) Heteropolyanions are electrodeposited on metal electrode.27-32 Despite the amount of literature the many methods used for the attachment of heteropolyanions on substrate surface, little study has been reported on how to design a well-organized and structurecontrolled molecular assemblies of heteropolyanion, while maintaining and/or enhancing their above beneficial properties. Recently, several composite film containing heteropolyanion modified electrodes have been prepared by several groups.33-35 These techniques were shown to (11) Lapkowski, M.; Bidan, G.; Fournier, M. Synth. Metals 1991, 41-43, 407. (12) Bidan, G.; Lapkowski, M.; Travers, J. P. Synth. Metals 1989, 28, C113. (13) Fabre, B.; Bidan, G.; Lapkowski, M. J. Chem. Soc., Chem. Commun. 1994, 1509. (14) Dong, S.; Song, F.; Wang, B.; Liu, B. Electroanalysis 1992, 4, 643. (15) Dong, S.; Wang, B.; Song, F. Chem. Lett. 1992, 215, 414. (16) Dong, S.; Jin, Z. Chin. Chim. Acta 1989, 47, 922. (17) Dong, S.; Jin, W. J. Electroanal. Chem. 1993, 354, 87. (18) Dong, S.; Liu, M. Electrochim. Acta 1994, 39, 947. (19) Xi, X.; Dong, S. J. Mol. Catal. A. 1996, 114, 257. (20) Keita, B.; Belhouari, A.; Nadjo, L.; Contant, R. J. Electroanal. Chem. 1995, 381, 243. (21) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1988, 255, 303. (22) Keita, B.; Essaadi, K.; Nadjo, L. J. Electroanal. Chem. 1989, 259, 127. (23) Keita, B.; Mahmoud, A.; Nadjo, L. J. Electroanal. Chem. 1995, 386, 245. (24) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1988, 240, 325. (25) Sung, H.; So, H.; Palk, W.-K. Electrochim. Acta 1994, 39, 645. (26) Kasem, K. K.; Schultz, F. A. Can. J. Chem. 1995, 73, 858. (27) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1987, 227, 1019. (28) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1987, 230, 85. (29) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1988, 243, 87. (30) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1988, 247, 157. (31) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1993, 354, 295. (32) Savadogo, O.; Allard, C. J. Appl. Electrochem. 1991, 21, 73. (33) Ingersoll, D.; Kulesza, P. J.; Faulkner, L. R. J. Electrochem. Soc. 1994, 141, 140.

10.1021/la981641z CCC: $18.00 © 1999 American Chemical Society Published on Web 09/24/1999

Silicotungstic Heteropolyanion Monolayers

be rapid and simple to obtain organized molecular assemblies experimentally with precise control of layer composition and thickness. Sun et al.36 have extended the technique of selfassembled monolayers of alkanethiols on gold to development of heteropolyanion multilayer system, but some undesirable characteristics of a thiol monolayer, such as its unstability in the typical hydrothermal conditions (high pH and high temperature), preclude the use of this kind of film. These lead us to seek routes which permit chemical functionalization yet provide an interface that maintains its integrity under exposure to harsh environments. We approached our goal of developing a well organized monolayer of heteropolyanions on a carbon electrode surface by using electrochemical method as follows: the carbon substrate was first modified by 4-nitrophenyl group via C-C bond through electrochemical reduction of diazonium salts, and NO2 group was reduced into NH2 group,37-42 and then the heteropolyanion can be linked with 4-aminophenyl 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 heteropolyanion. As is well-known, glassy carbon, with good conductivity and resistance to chemical attack,43 is a substrate potentially attractive for various applications of heteropolyanion film in the field of sensors. Covalent bonding of 4-nitrophenyl group on carbon surface may provide a much robust substrate attractive for developing heteropolyanion monolayer and multilayer systems44 by a layer-by-layer (LBL) method. Such a system can then lead to a large number of materials with various properties associated with the heteropolyanions. In the present work, we described our results obtained from electrochemistry, scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS) characterization of heteropolyanion monolayer prepared by the above method. 2. Experimental Section 2.1. Materials. 4-Nitrophenyl diazonium fluoborate (Aldrich) and tetrabutylammonium tetrafluoroborate (n-Bu4NBF4) (Fluka) was used as received. Acetonitrile (ACN) was distilled prior to use. All other chemicals were of reagent grade and used as received. Solutions were prepared using a Millipore Milli-Q water purification system and deaerated by passing argon through them before experiments. 2.2. Electrochemical Experiments. Electrochemical experiments were carried out on CH Instruments (model 600 voltammetric analyzer) in a conventional one-compartment cell with a glassy carbon electrode (GCE, Crystal Research Institute, Beijing) as working electrode, a Ag|AgCl|sat. KCl or Ag|Ag+ (0.01 M AgNO3 in ACN) electrode as reference electrode, and a Pt electrode as counter electrode. (34) Kulesza, P. J.; Roslonek, G.; Faulkner, L. R. J. Electroanal. Chem. 1990, 280, 233. (35) Kuhn, A.; Anson, F. C. Langmuir 1996, 12, 5481. (36) Sun, C.; Zhao, J.; Xu, H.; Sun, Y.; Zhang, X.; Shen, J. J. Electroanal. Chem. 1997, 435, 63. (37) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883. (38) Hitmi, R.; Pinson, J.; Saveant, J. M. Fr. Pat., 91 011172. (39) Bourdillon, C.; Demaille, C.; Hitmi, R.; Moiroux, J.; Pinson, J. J. Electroanal. Chem. 1992, 336, 113. (40) Bhugun, I.; Saveant, J. M. J. Electroanal. Chem. 1995, 395, 127. (41) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1997, 119, 201. (42) Liu, Y.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254. (43) McCreery, R. L. J. Electroanal. Chem. 1991, 17, 221. (44) Liu, S.; Tang, Z.; Wang, E.; Dong, S. Manuscript in preparation.

Langmuir, Vol. 15, No. 21, 1999 7269 Prior to its modification, the GCE was successively polished with 1.0 and 0.3 µm R-Al2O3 and ultrasonically washed with acetone and water between each experiment. The preparation of silicotungstic heteropolyanion monolayer was as follows: the carbon electrode was first immersed into a ACN + 0.1 M n-Bu4NBF4 solution containing 5 mM 4-nitrophenyl diazonium fluoborate and modified by 4-nitrophenyl radicals via C-C bond through cyclic scanning between 0.0 and -1.0 V or electrolysis at -1.0 V vs Ag|Ag+ (0.01 M AgNO3 in ACN). After rinsed and ultrasonically washed with water, the electrode derivatized by 4-nitrophenyl radical group was transferred to the protic solution (90:10 H2O/EtOH + 0.1 M KCl), and the reduction of NO2 group into NH2 group on electrode surface was carried out by applying a potential of -1.2 V during 600 s.41 The electrode was then immersed in the aqueous solutions containing 5 mM SiW12O404-, and cyclic scanning between -0.7 and 0.1 V at 100 mV s-1 was performed. The surface concentration of SiW12O404- was controlled by the number of potential scans. 2.3. XPS. XPS spectra were obtained with a vacuum ESCALAB MK II spectrometer operated in the constant analyzer energy mode (20 eV). The pressure in the analyzer chamber was in the 10 to 8 mbar range. After they were carefully polished and ultrasonically washed and rinsed, the clean GC plates (φ1 cm × 0.5 mm) were first examined by XPS to control their surface composition. Then SiW12O404-/4-aminophenyl group composite film was modified on the same plate by the same procedure described above for the GCE. Before XPS experiments were performed, the GC samples were rinsed with water and dried with Ar stream. 2.4. STM. The STM images were obtained in air with a Topometrix (Santa Clara, CA) TMX 2000 instrument. Electrochemically etched Pt/Ir tips were used.45 Scanning was in the constant current mode at a positive sample bias of 100 mV and tunneling current of 1.5 nA. The images were only equalized to enhance the contrast. Fresh surfaces of HOPG were obtained by pealing off the top layers before use. Modification was carried out by the same procedure described above for GCE but with no ultrasonic agitation. The fresh surfaces of HOPG obtained were carefully rinsed with 0.5 M H2SO4 solution before imaging.

3. Results and Discussion 3.1. Preparation of a SiW12O404-/4-Aminophenyl Composite Film Modified GCE. To obtain a stable, closest SiW12O404- coating on GCE, the grafting of 4-nitrophenyl group on GCE has to be formulated in the first place. In the present work, we have used two methods grafting 4-nitrophenyl group on GCE:37-42 cyclic scanning between -1.0 and 0.0 V at 100 mV s-1 or preparativescale electrolysis at -1.0 V vs Ag|0.01 M Ag+ in a CH3CN + 0.1 M n-Bu4NBF4 solution containing 5 mM 4-nitrophenyl diazonium fluoborate. In view of the modified surface properties, such as the thickness and permeation of an organic film, measurements of the double-layer capacitance can provide valuable complementary information. In present work, the doublelayer capacitance was evaluated by differentiating the cyclic voltammetry background current curve in cases where Faradic contributions are of minor importance, which is expressed as follows:46 C ) ∆i/(2vA) where, C is the double-layer capacitance (µF/cm2); ∆i, the difference between cathodic and anodic currents (A) at 0.05 V; v, the scan rate (V/s); A, the electrode area (cm2) measured with a Fe(CN) 63-/4- redox couple. Figure 1 shows the variation of double-layer capacitance with (A) electrolysis time and with the (B) number of the scan cycles. It can be seen that the double-layer capacitance decreases rapidly to a plateau within a short electrolysis time or at initial values of potential cycles, indicating that the reaction kinetics appears to be quite fast. According (45) Zhang, B.; Wang, E. Electrochim. Acta 1994, 39, 103. (46) McEvoy, A. J.; Gratzel, M. J. Electroanal. Chem. 1986, 200, 391.

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Figure 2. Cyclic voltammograms of a SiW12O404-/4aminophenyl group modified GCE during continuous growth by cycling in (A) 2 M H2SO4 and (B) pH 2 + 0.5 M Na2SO4 containing 5 mM SiW12O404-. Scan rate: 100 mV s-1.

Figure 1. Variation of capacitance of 4-nitrophenyl on a (A) GCE with electrolysis time and (B) the number of cycles.

to the above results, 10 min was chosen as the optimal electrolysis time for the derivatization of GC surfaces. The packing degree of a 4-nitrophenyl film on the GCE can also be assessed by investigating its blocking effect on the redox reaction of Fe(CN)63-/4- ions. When the derivatization by electrochemical reduction of diazonium leads to total disappearance of the redox peak currents of Fe(CN)63-/4-; that is, the redox reaction of Fe(CN) 63-/4was totally hindered by the grafting, indicating that a closest packing of 4-nitrophenyl radicals was achieved on the GC electrode. However, on HOPG, the grafting of 4-nitrophenyl radical group is slower. Since GC surfaces contain both edge and basal planes, and edge carbons are more reactive than basal carbons, it thus appears that the GC surface is more reactive toward 4-nitrophenyl radicals than the basal plane of HOPG. The GCE derivatized by 4-nitrophenyl group, after rinsed and ultrasonically washed with acetone and water, was transferred to the protic solution (90:10 H2O/EtOH + 0.1 M KCl) and electrolyzed at -1.2 V vs Ag|AgCl to reduce NO2 group into NH2 group on electrode.41 The electrode was then immersed in aqueous solutions containing 5 mM SiW12O404- and cyclic scanning between -0.7 and 0.1 V at 100 mV s-1. Figure 2 shows cyclic voltammograms obtained during the film growth on GCE derivatized by 4-aminophenyl radicals in (A) 2 M H2SO4

and (B) dilute H2SO4 (pH 2) + 0.5 M Na2SO4 in the presence of SiW12O404-. Three redox couples corresponding to SiW12O404- appear in the potential range 0.1 to -0.63 V in 2 M H2SO4, and the peak currents increase continuously with increasing cycling time until it keeps almost unchanged. As described by Saveant et al.,41 the conversion of NO2- into NH2 is not complete in above protic solution, while only protonated 4-aminophenyl group can interact with SiW12O404- via electrostatic interaction to form complex on electrodes. Therefore, the peak currents increase continuously with increasing cycling time, that is, with increasing the extent of the reduction of NO2- to NH2. When NO2- is completely converted into NH2, the peak currents of SiW12O404- keep almost unchanged and is close to that of SiW12O404- at bare GCE. The above results demonstrated that the presence of SiW12O404- is benefit for the conversion of NO2- into NH2, which is also confirmed by XPS results (see the corresponding detailed discussion in section 3.3). Compared with the voltammetric behavior of SiW12O404- on bare GCE (Figure 3A), it is obvious that the peaks at about -0.5 V are smaller than those of the former two redox couples. The difference in electrochemical behavior of SiW12O404- on bare GCE, or on GCE derivatized by 4-aminophenyl group, may be due to the formation of complex between reduced SiW12O404and protonated 4-aminophenyl group. The adsorbed layer prevents the third reduction of the unabsorbed anions. However, when the potential step is extended to the third oxidation potential, the oxidation of the dissolved reactant occurs by means of rapid electron transfer between the oxidized adsorbed layer and the anions in solution. A

Silicotungstic Heteropolyanion Monolayers

Figure 3. Cyclic voltammograms of (A) 5.0 mM SiW12O404- on GCE, (B) a SiW12O404-/4-aminophenyl group modified GCE, (C) SiW12O404- film on the GCE preanodized at 2.0 V for 60 s, and (D) SiW12O404- film on the GCE precathodized at -2.0 V for 60 s. Supporting electrolyte: 0.5 M H2SO4. Scan rate: 100 mV s-1.

similar example of redox mediation of the electrode reaction of an electrostatically blocked anion was reported.47 In contrast with the case of the 2 M H2SO4 solution, only ill-defined peak currents are observed in the pH 2 solution during the potential scans as shown in Figure 2B, and they increase with an increasing of the number of scans. With the pH of the solution for modification purposes, the cyclic voltammograms become more ill-defined. The electrodes are taken out after the peak currents of SiW12O404- are almost unchanging, and the electrodes rinsed thoroughly with a 0.5 M H2SO4 electrolyte solution, and their electrochemistry is examined in a supporting electrolyte. 3.2. Electrochemistry of the SiW12O404-/Aminophenyl Composite Film Modified GCE. Figure 3A shows cyclic voltammograms of SiW12O404- at a bare GCE in a 0.5 M H2SO4 solution. The homogeneous solution response of SiW12O404- agrees with the previously reported (47) Lee, C.; Anson, F. C. J. Electroanal. Chem. 1992, 323, 381.

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behavior of this compound.48,49 The voltammograms were obtained by two one-electron process as followed by a one two-electron process which occurs at the mean peak potentials (E1/2 ) (Epa + Epc)/2) of -0.195, -0.420, and -0.591 V. As can be observed, the voltammogram of the SiW12O404-/aminophenyl composite-film-modified GCE presents the same features described for the solution of SiW12O404- on bare GCE and displays three redox couples with the surface formal potentials E′surs(E′sur ) (Epc + Epa)/2 of -0.205, -0.440, and -0.617 V, and the peakto-peak separations ∆Ep are 60, 40, and 30 mV, respectively (as shown in Figure 3B). These voltammograms do not change upon cycling. It is noteworthy that the E′s for the three redox couples were near the formal potentials for the solution of SiW12O404- on bare GCE. This good correspondence is desired for predicative design of electrocatalytic system.50 Realization that the electrochemistry of SiW12O404- is sensitive to its environment prompted an examination of its reactivity after attached on electrode surface pretreated with different methods. Figure 3C and D show the cyclic voltammograms of the SiW12O404- film on the preanodized GCE at 2.0 V for 60 s and those of SiW12O404- film on the precathodized GCE at -2.0 V for 60 s, respectively. However, the SiW12O404- film fabricated on precathodized or preanodized GCE in 0.5 M H2SO4 gives only two poor redox waves, as shown in Figure 3C and D. By comparing the curves in B-D with the curve in A, it is found that only SiW12O404-/aminophenylmodified GCE gives sharp and well-defined redox peaks, which is analogous to that of dissolved SiW12O404-. It seems that the interaction of the oxometalates with 4-aminophenyl group in the film does not interfere with the voltammetric behavior. Figure 4 shows cyclic voltammograms of the composite film electrode being prepared from (A) 2 M H2SO4 + 5 mM SiW12O404- and from (B) 0.5 M Na2SO4 (pH 2) in 0.5 M H2SO4 at different scan rates, and the inset showing plots of peak current versus scan rate. The two composite film electrode prepared in different medium shows similar cyclic voltammetric response in same medium. At a scan rate below 200 mV s-1, all the peak potentials do not change with increasing scan rate, and the cathodic peak current is almost as 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 SiW12O404- electrostatically linked with the surface amino group shows a fast, diffusionless electrontransfer process. However, the ∆Ep is 60, 40, and 30 mV, respectively, instead of the value zero expected for a reversible surface redox process.51 The deviations might arise due to nonideal behavior of absorbed moieties.51,52 Given a nonideal behavior, the surface coverage of SiW12O404- could be calculated using51,52

Ip ) n2F2AΓ0v/RT(4 - 2γΓ0) ) nFQv/RT(4 - 2γΓ0) (since Q ) nFAΓ0) where Ip, v, A, Γ0, γ, and Q represent the peak current (A), scan rate (V s-1), electrode area (cm2) measured with Fe(CN)63-/4-, surface coverage of the redox species(mol cm-2), interaction term (cm2 mol-1), and the absorbed charge (C), respectively, and all the other terms have their usual significance. Quantitative determination of the absorbed (48) Keita, B.; Nadjo, L. J. Electroanal. Chem 1987, 217, 287. (49) Dong, S.; Xi, X.; Tian, M. J. Electroanal. Chem. 1995, 385, 227. (50) Moses, P. R.; Murray, R. W. J. Electroanal. Chem. 1977, 77, 393. (51) Brown, A. P.; Anson, F. C. Anal. Chem. 1977, 49, 1589. (52) Smith, D. F.; Willman, K.; Kuo, K.; Murray, R. W. J. Electroanal. Chem. 1979, 95, 217.

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Figure 5. Cyclic voltammogram of a SiW12O404-/4-aminophenyl composite film modified HOPG plate in 0.5 M H2SO4 at different scan rates: 100, 150, 200, 250, 300, 400, 500 mV s-1 (from inside to outside).

Figure 4. Cyclic voltammograms of a SiW12O404-/4-aminophenyl group modified GCE prepared in (A) 2 M H2SO4 and (B) pH 2 + 0.5 M Na2SO4 in 0.5 M H2SO4 at different scan rates. The inset shows variation of the cathodic and anodic peak current with scan rates.

charge Q is obtained by the integration of the peaks of the negative sweep of voltammograms. The background corrected Q of the reduction peaks is invariant with the inverse square root of the scan rate, indicating that the SiW12O404- electrostatically linked with the surface aminophenyl group on GCE exhibits the characteristic behavior of an immobilized couple with fast redox transition.53 That is to say, mechanisms involving either a diffusion-controlled electrode reaction in solution54,55 or immobilized film with a smaller diffusional charge transport constant56 are not present. Thus the background corrected Q of the reduction peaks obtained at any v corresponds to the total charge of the reducible species in the corresponding potential range. According to the above equation, the SiW12O404-/aminophenyl modified GCE prepared in 2 M H2SO4 gives a surfaces coverage of 2.20 × 10-10 mol cm-2, a value slightly larger than the closest packing concentration, 2.15 × 10-10 mol cm-2, while the composite film electrode prepared in 0.5 M H2SO4 gives a surfaces coverage of 1.95 × 10-10 mol cm-2. In view of these values, we may conclude that the SiW12O404electrostatically linked with the surface aminophenyl group is a monolayer. (53) Oldham, K. B. J. Electroanal. Chem. 1981, 121, 341. (54) Pickup, P. G.; Osteryoung, K. A. J. Electroanal. Chem. 1985, 186, 99. (55) Myland, J. C.; Oldham, K. B.; Zoski, C. G. J. Electroanal. Chem. 1985, 182, 221. (56) Daum, P.; Lenhard, J. R.; Rolisan, D.; Murray, R. W. J. Am. Chem. Soc. 1980, 102, 4649.

The SiW12O404-/aminophenyl modified GCE has good stability. The SiW12O404-/aminophenyl-modified GCE was transferred into 0.5 M H2SO4 after rinsing with 0.5 M H2SO4. Initially, the peak currents decrease slightly with potential scanning, and then reach a steady state after only a few cycles. After potential cycling between 0.1 and -0.70 V at 100 mV s-1 in 0.5 M H2SO4 for 1 h, a decrease in the cathodic current of about 10% was observed. After the SiW12O404-/aminophenyl modified GCE was soaked in 0.5 M H2SO4 solution for several days, almost no change of the electrochemical response of SiW12O404- was observed. It could be considered that a significant activation barrier impedes the break-up of the electrostatic binding within the composite film. Similar phenomena were observed with heteropolyanion incorporated in PVP or QPVP.57,58 Moreover, these voltammograms do not changed in the range of pH e 6.0; at higher pH, the CV behavior of the film electrode becomes unstable, which is believed to originate from the cleavage of the ionic bonding due to deprotonation of -NH3+ to -NH2 in phenyl group, indicating the electrostatic interaction of the composite film can improve the stability of SiW12O404-. The detailed electrochemical reduction mechanism of the composite film involves the uptake of proton from the solution to film. The details are described in other works.59 Similar results were obtained at HOPG and carbon fiber.59 Figure 5 shows the cyclic voltammograms obtained under similar conditions on HOPG electrodes modified with SiW12O404-/4-aminophenyl composite film. As can be observed, at a scan rate below 200 mV s-1, all the peak potentials do not change with increasing scan rate, and the cathodic peak current is almost as 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 SiW12O404electrostatically linked with the surface amino group shows a fast, diffusionless electron-transfer process. This indicates that the physicochemical properties of basal plane of HOPG do not retard the electron-transfer rate of the composite film. (57) Keita, B.; Nadjo, L. Mater. Chem. Phys. 1989, 22, 77. (58) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1988, 240, 325. (59) Shi, Z.; Liu, S.; Tang, Z.; Dong, S. Manuscript in preparation.

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Figure 6. 6 × 6 nm STM images of HOPG surfaces. (A) clean surface, (B) surface derivatized by electrochemical reduction of 4-nitrobenezenediazonium salt (at a potential of -1.0 V during 10 min in a 5 mM solution), then reduced in the protic solution (at a potential of -1.2 V during 10 min), (C) grafted surface modified with SiW12O404- (cyclic scanning in the potential range of 0.1 to -0.7 V in 2 M H2SO4 containing 5 mM SiW12O404-), (D) 500 nm × 500 nm STM images of C. Scan rate: 200 nm/s for A, B, and C; 1500 nm/s for D.

3.3. Structural Characterization of SiW12O404-/4Aminophenyl Composite Film. STM. Scanning tunneling microscopy (STM) offers an opportunity to characterize heteropolyanion on various materials.60-65 HOPG is atomically flat over regions up to at least 104 Å2 and is generally devoid of gross microstructural defects, so it serves well as the nearly perfect surface for STM images. Thus, in the present study, the HOPG was chosen as the carbon substrate in the STM experiment. On the other hand, experimental results happened to show that both (60) Keita, B.; Nadjo, L.; Kjoller, K. Surf. Sci. Lett. 1991, 256, L613. (61) Watson, B. A.; Barteau, M. A.; Haggerty, L.; Lenhoff, A. M.; Weber, R. S. Langmuir 1992, 8, 1145. (62) Keita, B.; Chauveau, F.; Theobald, F.; Belanger, D.; Nadjo, L. Surf. Sci. 1992, 264, 271. (63) Zhang, B.; Wang, E. J. Electroanal. Chem. 1995, 388, 207. (64) Song, I. K.; Kaba, M. S.; Coulston, G.; Kourtakis, K.; Barteau, M. A. Chem. Mater. 1996, 8, 2352. (65) Song, I. K.; Kaba, M. S.; Barteau, M. A. J. Phys. Chem. 1996, 100, 17528.

cyclic voltammograms of the SiW12O404-/4-aminophenyl group composite film modified GC and of the modified HOPG electrodes exhibited the same voltammetric response (see Figures 4 and 5). These results demonstrated that the physicochemical properties of the basal plane of HOPG does not retard the electron-transfer rate of the composite film. After the atomic resolution of a clean HOPG shown in Figure 6A was achieved, HOPG is grafted by 4-aminophenyl groups in the conditions described in the caption of Figure 6B. Figure 6B shows a typical example of a high-resolution image, revealing an ordered 4-aminophenyl groups monolayer. The nearest-neighbor distances between the white spots are 3.4 ( 0.1 and 3.8 ( 0.1 Å, respectively. According to the model calculation, the shortest length of an aminophenyl group is 3.3-3.4 Å. So the STM images shows that the aminophenyl group perpendicularly oriented on HOPG surface through C-C bonds. We also observe that the STM image of 4-nitrophenyl group is the same as that of 4-aminophenyl group.

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The image gives a surface coverage of (1.2-1.3) × 10-9 mol cm-2, which is in excellent agreement with the value calculated from the closest packing of 4-nitrophenyl groups on the surface.41 Figure 6C shows STM image of the heteropolyanions, which are adsorbed onto the HOPGgrafted surface in the conditions described in the caption of Figure 6C. The image clearly shows the formation of two-dimensional ordered arrays on the HOPG-grafted substrate, which is obviously different from Figure 6A and B. The molecular rows cross each other at an angle of either 60° or 120° within an experimental error ((2°). The intermolecular distance along these rows is found to be 10.4 ( 0.6 Å, which is consistent with the dimensions of the heteropolyanion molecule as determined by X-ray crystallography.66-69 It is shown that the SiW12O404molecules form the hexagonal close-packed ordered array on carbon surface derivatized by 4-aminophenyl radicals. The STM image produces a surface coverage of (2.1-2.2) × 10-10 mol cm-2 of SiW12O404-, which is similar to the value obtained by electrochemical method. Considering the results obtained in section 3.2, nevertheless, the images clearly demonstrates that an ordered SiW12O404monolayer is formed on HOPG grafted by 4-aminophenyl radicals. This ordering phenomenon is quite reproducible and observable over hundreds of nanometers. In macroscale (as shown in Figure 6D), the SiW12O404-/4aminophenyl composite film seems quite smooth and homogeneous and does not show any evidence for roughness of the surface or the existence of domain structure. Less aggregation has occurred even after 30-day time aging. Such results suggest that the heteropolyanion monolayer can be constructed by the electrostatic interaction at the SiW12O404-/4-aminophenyl interface and that the interaction has effectively stabilized the film structure and prevented the heteropolyanion molecules from destructive interaggregations. Although some studies64,65 demonstrated that evaporative solution deposition of polyanions onto HOPG yields ordered two-dimensional arrays of polyanions on the HOPG surface, no measurement of the polyoxoanion film thickness had been reported and claims of monolayer formation have been supported only by tunneling spectroscopy. Combined with the electrochemical results, we can conclude that this article is the first report of the formation and the imaging of a ordered heteropolyanion monolayer deposited on HOPG surface. XPS. The presence of the SiW12O404-/4-aminophenyl composite film on the carbon electrode surface can be confirmed by XPS data. Information about the protonation of 4-aminophenyl group can be extracted from N 1s XP spectra. Figure 7A shows deconvoluted N 1s XP spectra of the SiW12O404-/4-aminophenyl composite film on GC disk. The deconvolution was carried out assuming the same weighting of Gaussian (75%) and Lorentzian (25%) functions in each case. Under these conditions the binding energy of N 1s in the amine state (-NH2) has been obtained within a very narrow range of 400.2 eV. The peak at 401.9 eV must be ascribed to a protonated nitrogen species (N+).70 No peak attributable to the nitro groups (NO2) is observed at 406 eV, indicating that the conversion of NO2 into NH2 is almost complete in our work. This result is different from that found by Saveant et al.41 The existence of SiW12O404- may be beneficial for the reduction of NO2. Considering the fact that a certain number of amine group (66) Keggin, J. F. Nature 1934, 144, 75. (67) Evans, H. T. Inorg. Chem. 1966, 5, 967. (68) Izumi, Y.; Hasebe, R.; Urabe, K. J. Catal. 1983, 94, 402. (69) Highfield, J. G.; Moffat, J. B. J. Catal. 1984, 88, 177. (70) Kang, E. T.; Neoh, K. G.; Tan, K. L. Polymer J. 1989, 21, 873.

Liu et al.

Figure 7. XPS spectra for GC disk: (A) N 1s, (B) W 4f.

Figure 8. Structural illustration of the SiW12O404-/4aminophenyl composite film on carbon surface. Circled “4-” represents the SiW12O404- anion.

are not protonated at all, we can calculate the protonation extent of nitrogen atom in the composite film:

z ) [N+]/([N+] + [-NH2]) The [N+] and [-NH2] is evaluated from the corresponding XPS intensities, window widths, and photoionization cross section for Mg KR. The z value is about 62, which indicates that about 62% of nitrogen atom is protonated; that is, each 4-aminophenyl group possesses about 0.62 positive charge. The adsorption of SiW12O404- on the GC disk was disclosed by the presence of two doublets at 37.0 and 38.1 eV arising from W(4f) (Figure 7B). These values are consistent with spin-orbit splitting of W 4f level in the oxidation state +6(WVI(W(4f7/2) ) 37.1 eV and W(4f5/2) ) 38.2 eV)).71 In XPS study, the charge of aminophenyl group is +0.62, while SiW12O404- is -4. Obviously, to attain the electroneutrality of the composite film, the molar ratio of 4-aminophenyl group to SiW12O404- is 6.45:1. On the other hand, according to the occupied area of SiW12O404- (ca. 85 Å2) and 4-aminophenyl group (ca. 13 Å2) determined by (71) Kazansky, L. P.; Launy, L. P. Chem. Phys. Lett. 1977, 51, 242.

Silicotungstic Heteropolyanion Monolayers

STM image, the molar ratio of 4-aminophenyl group to SiW12O404- is 6.54:1. The two ratios are in consistent with the expected composition of the composite film in parentheses (as shown as Figure 8), which demonstrates that the adsorbed polyanion are in a close packing state with a SiW12O404- molecule per 6.5 4-aminophenyl group. Derivatization of HOPG by the same technique also resulted in the appearance of the characteristic N1s and W4f XPS signal.The signals obtained on HOPG are identical to that obtained on GC surface. 4. Conclusion We have prepared the SiW12O404-/4-aminophenyl modified carbon substrate by electrochemical methods. The composite film of SiW12O404- is very stable upon cycling and at a larger pH range. This suggests that the electrostatic interaction between the SiW12O404- anions and the surface aminophenyl group improves the stability of SiW12O404- anion. Cyclic voltammetry, XPS, and STM have allowed the characterization the overlayer and an estimate of the surface coverage. Analysis of the cyclic voltammetric data indicates that a saturation value near a compactly packed monolayer is formed on carbon electrode surface. STM confirms that a two-dimensional ordered monolayer of SiW12O404- is attached on carbon surface and no destructive molecular aggregation is observed. Thus a two-dimensional ordered monolayer of

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heteropolyanion is through molecular level designing of the substrate surface. The SiW12O404-/4-aminophenyl-modified GCE exhibits three reversible redox waves which is similar to that of SiW12O404- anion in solution, corresponding to two oneelectron followed by a one two-electron process. Compared with the SiW12O404- film fabricated on an electrochemically pretreated GCE, the composite film shows reversible kinetics, smaller peak-to-peak separation, and narrower peak shape. The composite film offers a favorable environment for electron and proton transfer between SiW12O404- ion and electrode surface, which is desired for predicative design of electrocatalytic system. We have also found that this film possesses a good catalytic activity for the reduction of nitrite in sulfuric acid solution.59 Acknowledgment. This work has been supported by the National Natural Science Foundation of China. The authors also thank Dr. J. Luo for his help in calculating the dimension of molecules. SupportingInformationAvailable: ImagesofSiW12O404-/ 4-aminophenyl produced by scanning tunneling microscopy and polarization modulation IR reflection absorption spectroscopy. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. LA981641Z