Nanocomposite Multilayer Film of Preyssler-Type Polyoxometalates

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J. Phys. Chem. B 2004, 108, 9780-9786

Nanocomposite Multilayer Film of Preyssler-Type Polyoxometalates with Fine Tunable Electrocatalytic Activities Minghua Huang, Lihua Bi, Yan Shen, Baifeng Liu, and Shaojun Dong* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, People’s Republic of China ReceiVed: February 8, 2004; In Final Form: March 19, 2004

Through layer-by-layer (LBL) assembly technique, iron oxide (Fe3O4) nanoparticles coated by poly (diallyldimethylammonium chloride) (PDDA) and Preyssler-type polyoxometalates (NH4)14NaP5W30O110‚31H2O (P5W30) were alternately deposited on quartz and ITO substrates, and 4-aminobenzoic acid modified glassy carbon electrodes. Thus-prepared multilayer films were characterized by UV-visible spectroscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. It was proved that the multilayer films are uniform and stable. And the electrocatalytic activities of the multilayer films can be fine-tuned by adjusting the assembly conditions in the LBL assembly process, such as the pH of the assembly solution. The multilayer films fabricated from P5W30 solutions dissolved in 0.1 M H2SO4 exhibit high electrocatalytic response and sensitivity toward the reduction of two substrates of important analytical interests, HNO2 and IO3-, whereas the films assembled with P5W30 solutions dissolved in 1.0 M H2SO4 show remarkable electrocatalytic activity for the hydrogen evolution reaction (HER). Furthermore, the electrocatalytic properties of the HER of the latter film can be obtained from the former film upon exposure to 1.0 M H2SO4 for several hours. The high electrocatalytic activity and good stability for the HER make the P5W30-containing multilayer films potential candidates for efficient and durable hydrogen cathode material of fuel cells.

1. Introduction Polyoxometalates (POMs), a well-known class of nanoclusters with much diversity in size, composition, and function, are attracting increasing attention worldwide.1 One of the most attractive features of POMs is that the metal-oxygen framework can undergo reversible, stepwise, multielectron-transfer reactions, which are the basis for many catalytic processes.2,3 In particular, B. Keita and L. Nadjo had reported previously some POMs deposited on the electrode surfaces showing a strikingly high electrocatalytic activity toward the hydrogen evolution reaction (HER) in acid media.4-8 But this method of deposition showed that no useful derivatization of the electrode surface for the HER was obtained unless the potential of the working electrode was at least as negative as a proton-dependent wave; the method was also time-consuming. Usually, the realization of POM-based materials requires some methods to orient and integrate the nanoclusters in the device architecture. Traditionally, thin films of POMs are typically made by dip coating,3,9 the Langmuir-Blodgett technique,10-12 electrodeposition,7,8,13 or doping in conducting polymers.14 Recently, the electrostatic layer-by-layer (LBL) self-assembly technique provided a viable approach for the formation of various nanostructured thin films with many fascinating properties.15,16 The method allows for the fabrication of robust, homogeneous films with fine controlled film thickness and amount of the components. The general utility and vitality of these systems are further broadened when quantum dots and nanoparticles are incorporated into these multilayer films.17-19 It has been applied to the design of POM multilayer films with certain large cationic species, 20-22 polymeric materials,15,23-25 and macromolecular dendrimers.16,26 * To whom correspondence should be addressed. Fax: (+86) 4315689711. E-mail: [email protected].

Nanoparticles as components of multilayer films are of interest, in part because of the high ratio of surface area to volume that is the basis of their special sensor effects. Incorporating such functional building blocks into high-surface-area surface-confined nanostructured thin films with good stability using the LBL assembly technique might be significant for sensor applications. By preparing composites with alternate layers of polyoxometalates and nanoparticles, we envision obtaining an effective electrochemical catalyst with high permeability of the substrate to the active sites. To our knowledge, the multilayer film comprising POMs and iron oxide (Fe3O4) nanoparticles based on the LBL assembly method was first constructed. Here, the multilayer films contain Preyssler-type POMs (NH4)14NaP5W30O110‚31H2O (P5W30) electrostatically assembled with the Fe3O4 nanoparticles coated by poly(diallyldimethylammonium chloride) (PDDA) (PDDA:Fe3O4). The resulting multilayer films were characterized by UV-visible spectroscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. The multilayer films have good uniformity and stability. They also have some advantages over other positively charged polyion-assembled multilayer films, such as high electroactivity and high stability due to the high surface activity of nanoparticles. Interestingly, we note that the microenvironment of the assembly condition in the LBL process has an effect on the electrochemical behavior of the multilayer films. Consequently, simply adjusting the pH of the assembly solution in the LBL process can tune the electrocatalytic activities of the multilayer films. The multilayer films fabricated from P5W30 solutions dissolved in 0.1 M H2SO4 exhibit high electrocatalytic response and sensitivity toward the reduction of two substrates of important analytical interests, HNO2 and IO3-, whereas the films assembled with P5W30 solutions dissolved in 1.0 M H2SO4 show

10.1021/jp0494335 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/11/2004

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remarkable electrocatalytic activity for the hydrogen evolution reaction (HER) in acid media. The dynamic constants of the HER were estimated by hydrogen evolution voltammograms. In addition, when the multilayer films fabricated from P5W30 solutions dissolved in 0.1 M H2SO4 were immersed in 1.0 M H2SO4 for several hours, electrocatalytic activity for the HER in acid media can also be observed. The high electrocatalytic activity and good stability for the HER make the P5W30containing multilayer films potential candidates for efficient and durable hydrogen cathode material of fuel cells. 2. Experimental Section 2.1. Reagents. 4-Aminobenzoic acid (4-ABA) was purchased from Aldrich. The absolute ethanol was dried over 3 Å molecular sieves before use and lithium perchlorate was dried at about 90 °C in a vacuum oven before use. Poly (diallyldimethylammonium chloride) (PDDA, MW 20 000) was purchased from Aldrich and was used without further treatment. The PDDA-coated Fe3O4 nanoparticle (PDDA:Fe3O4) solution was prepared by a published method.27 Preyssler-type polyoxometalates ((NH4)14NaP5W30O110‚31H2O (P5W30)) were synthesized as previously reported.28 Other chemicals were of analytical reagent grade and used as received. Water used for preparation of aqueous solutions was purified using a Millipore-Q water purification system. Buffer solutions were prepared from 0.1 M NaAc + HAc (pH ) 3-6) and 0.1 M Na2SO4 + H2SO4 (pH < 3). 2.2. UV-Visible (UV-vis) Absorption Spectroscopy. UVvis absorption spectra were recorded using a Cary 500 Scan UV-vis-NIR spectrometer (Varian Co.) on a quartz slide, which was cleaned with “piranha solution”, a 3:7 solution mixture of 30% hydrogen peroxide (H2O2) and concentrated sulfuric acid (H2SO4) at 80 °C for 1 h (Caution: since piranha solution reacts Violently with many organic compounds, extreme care must be taken when handing it.), followed by rinsing with water, and drying with a high-purity nitrogen stream. The freshly prepared quartz slide was soaked for 30 min in an aqueous solution containing 8.0 wt % PDDA, forming a precursor PDDA layer with positive charge. The treated quartz slide was alternately immersed for 30 min in 1 mM P5W30 + 0.1 M H2SO4 and the as-prepared PDDA:Fe3O4 solution. The resulting films were washed with water, dried under nitrogen, and used to record UV-vis spectra to follow the deposition processes. The spectra were background subtracted from a reference sample of a PDDA-functionalized quartz slide. 2.3. X-ray Photoelectron Spectroscopy (XPS). XPS measurement was performed on an ESCALAB-MKII spectrometer (VG Co., United Kingdom) with Al Ka X-ray radiation as the X-ray source for excitation and analyzer pass energy of 50 eV. Typically, the operating pressure in the analysis chamber was below 10-9 Torr. The resolution of the spectrometer was 0.2 eV. XPS measurement was performed on an ITO-coated glass slide assembled with P5W30 multilayer films in a manner similar to the above procedure. 2.4. Electrochemical Measurements. Electrochemical experiments were performed with a CHI 660 electrochemical workstation (United States) in a conventional three-electrode electrochemical cell using a glassy carbon electrode (GCE, 3 mm diameter) as the working electrode, a twisted platinum wire as the auxiliary electrode, and a Ag/AgCl reference electrode in aqueous media or Ag/Ag+ (0.01 M AgNO3) in anhydrous ethanol solutions. The GCEs were polished with 1.0 and 0.3 µm R-Al2O3 powders, successively, and sonicated in water for about 3 min after each polishing step. Finally, the electrodes

Figure 1. UV-vis absorption spectra of the PDDA:Fe3O4/P5W30 multilayer films on the PDDA-coated ITO substrate (P5W30 as the outmost layer) with different number of layers: 3, 4, 5, 6, 7, 8, 9, 10, and 11 (from bottom to top), respectively. The inset displays the absorbance at 208 and 282 nm vs the number of layers.

were sonicated in ethanol, washed with ethanol, and dried with a high-purity nitrogen stream immediately before use. Solutions were deaerated for at least 20 min with a high-purity nitrogen stream and kept under a pressure of this gas during the experiments. An EG & PARC model 636 rotating ring-disk electrode system and an EG & PARC model 366 bipotentiostat were used for rotating disk voltammetric experiments. A rotating GC disk electrode (4 mm diameter) was used as a working electrode. 2.5. Modification Procedure. A GCE was first treated with 4-ABA according to the published procedures.29,30 The 4-ABA/ GCE was first placed in an aqueous solution containing 8.0 wt % PDDA, to provide a stable, positively-charged surface to which the polyanionic P5W30 can be bound electrostatically. Then, the PDDA/ 4-ABA/GCE was immersed in 1 mM P5W30 + 0.1 M H2SO4 with cyclic potential scanning between 0 and -0.5 V at a scan rate of 100 mV s-1 for 360 cycles. After thoroughly rinsing with water and drying with a nitrogen stream, the modified electrode with P5W30 layer was transferred to the as-prepared PDDA:Fe3O4 solution, resulting in one layer of PDDA:Fe3O4 by scanning between 0 and 0.5 V, as mentioned above. When the resulting modified electrode was alternately placed in P5W30 and PDDA:Fe3O4 solutions in the cyclic fashion, the P5W30 multilayer films could be formed. The thickness of the multilayer films was readily adjusted by choosing different numbers of cycles by the LBL modification method. 3. Results and Discussion 3.1. UV-visible (UV-vis) Absorption Spectroscopy Characterization. Assuming that the absorption intensity is proportional to the concentration of the deposition material with characteristic absorption peaks in the UV-vis region, UV-vis spectroscopy was used to characterize the growth process of the multilayer films. Figure 1 shows the UV-vis absorption spectra of the P5W30/PDDA:Fe3O4 films with different layer numbers, and P5W30 existed in the outmost layer. It can be seen that the absorptions of the P5W30 multilayer films occur at the same wavelengths with sequential deposition. The absorbances at 208 and 282 nm, corresponding to the oxygen-tungsten charge transfer (CT) of P5W30, are observed to increase linearly with the number of layers as shown in the inset of Figure 1. The linear increase of the intensity of the absorption peaks as a function of the number of layers indicates that regular and uniform multilayer films have been constructed.

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Figure 2. (A) Cyclic voltammograms of 4PDDA:Fe3O4/5P5W30/PDDA/ 4-ABA/ GCE. The inset shows a cyclic voltammogram of l mM P5W30 + 0.1 M H2SO4 solution at a bare GCE. (B) Cyclic voltammograms of PDDA:Fe3O4/P5W30 multilayer films (P5W30 as the outmost layer, and fabricated from 0.1 M H2SO4) with a different number of layers: n ) 1, 3, 4, 5, 6, 7, 8, 9, and 10 (from inside to outside), respectively. The inset shows the relationship of the number of layers vs the first reduction peak current. Supporting electrolyte: N2-saturated 0.1 M H2SO4. Scan rate: 20 mV s-1.

3.2. X-ray Photoelectron Spectroscopy (XPS) Measurement. Direct measurements of the elements presented by XPS in the ITO-supported multilayer films were also conducted (the figure is not shown). Two characteristic W 4f peaks appear at about 35.0 eV (7/2) and 37.2 eV (5/2), respectively. In addition, the P 2p peak is present at ca. 131.2 eV. Also, XPS signals from the Fe 2p3/2 level appear at ca. 710.2 eV. Hence, the presence of Fe3O4 nanoparticles and polyanion P5W30 in the multilayer film was confirmed by XPS data. 3.3. Fabrication of Alternate P5W30 and PDDA:Fe3O4 Multilayer Films on 4-ABA Modified GCE. Through the attachment of the 4-ABA containing a COOH group to the GCE, the 4-ABA modified GCE was first placed in 8.0 wt % PDDA aqueous solution, and then it was alternately immersed in P5W30 and PDDA:Fe3O4 solutions with cyclic potential scanning in a suitable potential range. First, we assembled the multilayer films from P5W30 solutions dissolved in 0.1 M H2SO4. Figure 2A shows a cyclic voltammogram (CV) of the film with five P5W30 layers in 0.1 M H2SO4 solution. The 4PDDA: Fe3O4/5P5W30/PDDA/4-ABA/GCE exhibits four couples of redox waves with the formal potentials (Ef) of -0.26, -0.33, -0.39, and -0.59 V, respectively. This pattern is different from that of P5W30 in 0.1 M H2SO4 solution at a bare GCE. The latter shows two four-electron redox processes each at -0.23

Huang et al. and -0.35 V, and one additional multielectron process at -0.56 V28,31 (the inset of Figure 2A). It can be seen that the two redox waves of five P5W30 layers at -0.33 and -0.39 V in the multilayer films in 0.1 M H2SO4 solution are from the second four-electron redox wave of P5W30 at -0.35 V in 0.1 M H2SO4 solution at a bare electrode according to the similar potential of peaks. The currents of the two split redox waves in the mutliayer films are almost equal, and it is reasonable to consider that the second four-electron process of P5W30 in 0.1 M H2SO4 solution has split into two two-electron processes of P5W30 in the multilayer films. It is possible that the structure of the multilayer films, with strong electrostatic interactions of P5W30 and PDDA:Fe3O4, influences the electron transport process of the redox processes, therefore causing this split to occur into a pair of two-electron processes.26 With P5W30 as the electroactive anion cluster, the multilayer deposition process on the above-mentioned modified GCE was monitored by cyclic voltammetry. Because overlap with the hydrogen evolution reaction complicated the measurement, we took the first three redox waves to monitor the multilayer deposition process in subsequent voltammetric experiments. Figure 2B reveals CVs of the as-prepared multilayer films (P5W30 dissolved in 0.1 M H2SO4 as the outmost layer) with different numbers of P5W30 layers (curves from inside to outside: n ) 1, 3, 4, 5, 6, 7, 8, 9, and 10, respectively) in 0.1 M H2SO4 solution. With the number of P5W30 layers increasing, the redox peak currents increase gradually. Taking the first cathodic peak as an example, a good linear relationship between the layer numbers and the first cathodic peak currents of P5W30, as shown in the inset of Figure 2B, demonstrates that equal amounts of P5W30 are adsorbed in each deposition cycle, confirming the film grows uniformly. In addition, we find that with the increase in the number of layers changes in the shift of formal potentials and the increase of peak potential separations are almost negligible. There is no peak in the above-studied potential region with only one PDDA:Fe3O4 layer, indicating that the PDDA:Fe3O4 layer has no effect on the electrochemistry of the PDDA:Fe3O4/P5W30 composite layers. There are almost negligible changes in the shape and height of the redox waves of one P5W30 layer in the presence and absence of a PDDA: Fe3O4 layer (see Supporting Information), clearly indicating that the P5W30 layer mainly contributes to the electrochemistry of the PDDA:Fe3O4/P5W30 composite layers. These results also demonstrated that the PDDA:Fe3O4 layers acted as nanoparticle conductive layers in the multilayer films and did not block the electron transfer of P5W30 sandwiched between them. In addition, CVs of the 4PDDA:Fe3O4/5P5W30-modified GCE show the characteristic of chemically reversible surface redox electrochemistry. A plot of the cathodic and anodic peak currents, taking the first redox peak as representative, and as a function of scan rate, is linear up to 289 mV s-1 (the figure is not shown). In addition, the formal potentials are independent of scan rates and the cathodic and anodic peak currents ratio is unity at all scan rates. All of this demonstrates that the redox processes of surface-confined P5W30 anions are fast and not diffusion limited, a result that is consistent with the electrochemical behavior of surface-confined POMs.32,33 Therefore, the surface coverage Γ of P5W30 anions in the films can be calculated by the equation15

Γ) ipRT[4 - 2γΓ]/n2F2νA = 4ipRT/n2F2νA

(1)

where ip is the peak current, γ is the interaction term, n is the number of electrons transferred per electroactive species, and ν is the scan rate. A is the geometric area of the electrode and

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Figure 4. Cyclic voltammograms of the PDDA:Fe3O4/P5W30 multilayer films (P5W30 as the outmost layer, and fabricated from 1.0 M H2SO4) with different number of layers: n ) 1, 2, 3, 4, 5, 6, and 7 (from top to bottom), respectively. Supporting electrolyte: N2-saturated 0.1 M H2SO4. Scan rate: 20 mV s-1. Figure 3. Cyclic voltammograms of 9PDDA:Fe3O4/10P5W30/PDDA/ 4-ABA/GCE in absence (dotted line) and presence (solid line) of the substrate at a scan rate: 20 mV s-1. 0.1 M Na2SO4 + H2SO4 (pH ) 2.22) solution containing IO3- at various concentrations: (a) 0.33, (b) 0.67, (c) 1.00, (d) 1.33, (e) 1.67, (f) 2.00, (g) 2.33, (h) 2.67, (i) 3.33, (j) 4.00, (k) 4.67, (l) 6.33, and (m) 8.00 mM. The inset shows the relationship between catalytic current and concentration of IO3-.

all other terms have their usual meaning. The analysis of the plot of ip as a function of ν indicates that 4 . 2γΓ.34 Based on the result that an equal amount of P5W30 was deposited in each deposition step as obtained by cyclic voltammetry characterization, the average surface coverage of P5W30 anions per layer in the films amounts to 2.92 × 10-11 mol cm-2 or 5.68 nm2 per P5W30. From the single X-ray structure analysis, the lateral dimensions of the cluster are 2.4 × 1.8 × 1.8 nm3 or an average 2.0 nm mean spherical diameter.28 Assuming an isotropic distribution, the surface area of the P5W30 is estimated to be 4.00 nm2; that is, the electrochemically determined surface coverage corresponds to a submonolayer. 3.4. Electrocatalytic Activities on the Reduction of HNO2 and IO3- of the Multilayer Film Fabricated from P5W30 Dissolved in 0.1 M H2SO4. Our interest in the multilayer films is primarily related to electrocatalytic behaviors. Here, the 9PDDA:Fe3O4/10P5W30/PDDA/4-ABA/GCE is the working electrode, and iodate and nitrite are the test species. Figure 3 presents CVs of the multilayer films electrode in solutions containing IO3- at various concentrations. In pH ) 2.22 buffer solution, the first four-electron reduction peak at about -0.38 V becomes substantially increased on addition of IO3-, while the corresponding oxidation peak is suppressed. The other redox waves remain almost constant. Apparently, these results signify that the first four-electron reduction species of P5W30 anions can catalyze electrochemically the reduction of IO3-. From the inset of Figure 3, we can see that with the increase of the concentration of IO3-, the corresponding catalytic currents increase linearly. In addition, when one P5W30 layer is in the presence and absence of a PDDA:Fe3O4 layer, there are no apparent noticeable changes in the first four-electron redox waves in which the electrocatalysis occurred, indicating that the presence of the PDDA:Fe3O4 layer does not impede the electron-transfer reaction, and also does not make any additional contribution to the electrocatalytic activity on the reduction of IO3- (see Supporting Information). The same multilayer films electrode also exhibits high electrocatalytic response toward the HNO2 reduction (the figure is not shown). In addition, we studied the typical steady-state current-time responses at the as-prepared modified electrode with 10 layers of P5W30 on successive addition of iodate at applied potential -0.5 V (see Supporting Information). As IO3- was added into

the stirred buffer solution (pH ) 2.22), the multilayer film electrode responded rapidly to the analyte. The time required to reach 95% of the steady-state current is ca. 3 s after addition of IO3-. The nearly equal current steps for each addition of IO3- demonstrate a stable and efficient catalytic property of the electrocatalyst immobilized in the multilayer film. 3.5. Effect of pH on the Electrochemical Behavior of the P5W30 Multilayer Films. We also investigated the effect of the solution pH on the electrochemical response of the P5W30 multilayer film (the figure is not shown). With increasing pH of the solution, the potentials of the first three redox couples of the P5W30 film gradually shift to more negative values. It is confirmed that some protons participate in the reduction process in order to balance the charge of the P5W30 film, which is commonly found for POMs redox reactions.35,36 3.6. Electrocatalytic Activities for the Hydrogen Evolution Reaction of the Multilayer Film Assembled with P5W30 Dissolved in 1.0 M H2SO4. We note that the electrochemical behavior of the multilayer films can be fine-tuned by changing the solution pH of P5W30 in the LBL assembly process. In the above experiments, P5W30 was dissolved in 0.1 M H2SO4 solution to accomplish the LBL assembly process. However, when P5W30 was dissolved in 1.0 M H2SO4 to assemble the multilayer film, the electrochemical behavior of the multilayer films is different from that of the films with P5W30 dissolved in 0.1 M H2SO4 solution. In such an LBL assembly process, the modified electrode was soaked in 0.1 M H2SO4 solution for 2 h after the deposition of P5W30 in each cycle. The aims of applying this procedure are to wash 1.0 M H2SO4 physically adsorbed on the modified electrode in the LBL assembly process, and then to reach the equilibrium of the protons between the multilayer films modified electrode and the supporting electrolyte (0.1 M H2SO4). According to eq 1, the surface coverage of P5W30 per layer amounts to 1.06 × 10-10 mol cm-2, which corresponds to an average area of 1.57 nm2 per P5W30. Compared to the packing of the crystalline solid,28 this value corresponds to multilayer coverage under this condition, which is different from the result obtained from the films (P5W30 dissolved in 0.1 M H2SO4 solution). A possible explanation for the low surface coverage of P5W30 in the films from P5W30 dissolved in 0.1 M H2SO4 solution could be residual electrostatic or dipolar repulsions of adjacent cluster anions at the interface.24 When P5W30 was dissolved in 1.0 M H2SO4 solution to assemble the films, the ionic strength is increased. It might be possible to screen the electrostatic repulsion, thus allowing the cluster anions to move together more closely.24 Figure 4 shows CVs of the thus-prepared multilayer-filmsmodified GCE (P5W30 dissolved in 1.0 M H2SO4 as the outmost

9784 J. Phys. Chem. B, Vol. 108, No. 28, 2004 layer) with different numbers of P5W30 layers (curves from top to bottom: n ) 1, 2, 3, 4, 5, 6, and 7, respectively) in 0.1 M H2SO4 solution. It can be seen that the multilayer films exhibit remarkable electrocatalytic activity for the hydrogen evolution reaction (HER). With the increase of the layer numbers, the HER starts at more and more positive potentials and the current for the HER increases drastically. This suggests that the capability of electrocatalytic activities toward the HER is enhanced gradually with the increase in the number of layers. From this phenomenon, we demonstrate that the microenvironment of assembly condition in the LBL assembly process has an effect on the electrochemical behavior of the multilayer films. We can tailor the favorable electrochemical properties by adjusting the pH of the assembly solution. When the pH range of the assembly solution was broadened, we prepared the multilayer films assembled with P5W30 dissolved in pH ) 5.45 and 7.00 buffer solutions and the Fe3O4 nanoparticles. We found the electrochemistry and electrocatalytic activities of the asprepared multilayer films are almost similar to that of the multilayer films assembled with P5W30 dissolved in 0.1 M H2SO4 solutions (the figure is not shown). As seen from the previous section, the surface coverage of P5W30 clusters in the multilayer films can be controlled from submonolayer to multilayer coverage by adjusting the ionic strength of the assembly solution. It is known that the surface coverage can affect the average cluster spacing and, in turn, the microenvironment of the POM clusters in the multilayer films, thus obtaining different properties of surface-confined POMs.23,24 In particular, subtle differences in the microenvironment have been demonstrated by electrochemistry.23 Under the different assembly conditions as mentioned above, the surface-confined P5W30 clusters in the different multilayer films may also experience the different microenvironments, which may give rise to different electrocatalytic activities. The multilayer films from P5W30 dissolved in 1.0 M H2SO4 can be used as the HER material, whereas the multilayer films from P5W30 dissolved in 0.1 M H2SO4 cannot be so used. In addition, an indication of the catalytic effects on HER of the multilayer films containing P5W30 dissolved in 1.0 M H2SO4 solution is obtained under successive potential cycling of the surfaceimmobilized P5W30. After potential scanning 1000 cycles between 0.6 V and - 1.0 V at 100 mV s-1, there was no observable changes in the electrocatalytic current and the electrocatalytic potential of the HER. Up to 2000 cycles (ca. 9 h), there is slight decrease (ca. 5.0%) in the electrocatalyitc current, but with the electrocatalytic potential unchangeable. Therefore, the multilayer films exhibit electrocatalytic effect on the HER with good stability and not just electrolysis of film protons. The kinetic constants for HER of the multilayer film in 0.1 M H2SO4 were comparatively studied at different layers by hydrogen evolution voltammetry. Figure 5A shows polarization curves for HER on the multilayer-films-modified GCE (P5W30 as the outmost layer) with different numbers of layers. With the increase in the layer numbers, HER starts at progressively more positive potentials. The influences of PDDA:Fe3O4 on the electrocatalytic activities of the multilayer film are supported by experiments in which the outmost layer is PDDA:Fe3O4. Figure 5B provides a comparison of polarization curves obtained on the multilayer film with different P5W30 layers in the presence and absence of a PDDA:Fe3O4 overlayer. It can be seen that the presence of the PDDA:Fe3O4 overlayer makes the overpotential for the HER shifts more positive than that of the films in the absence of the PDDA:Fe3O4 overlayer, which indicates

Huang et al.

Figure 5. (A) Polarization curves for the HER on the bare (dashed line) and the PDDA:Fe3O4/P5W30-multilayer-films-modified GCE (solid line) with different number of layers: n ) 3, 5, 7, 8, 9, 10 and 11, respectively. (P5W30 as the outmost layer, and fabricated from 1.0 M H2SO4). (B) Polarization curves for the HER on the PDDA:Fe3O4/P5W30 multilayer film with a differing number of layers 5, 9, and 10 in the absence (solid line) and presence (dashed line) of the PDDA:Fe3O4 overlayer. The inset represents polarization curves for the HER on the PDDA/P5W30 multilayer film with a differing number of layers 3, 5, 9 and 10 in the absence (solid line) and presence (dashed line) of the PDDA overlayer. Tested solution: N2-saturated 0.1 M H2SO4. Rotation rate: 3500 rpm. Scan rate: 5 m V s-1.

that the capability of the electrocatalytic activities toward the HER is enhanced. On the other hand, we also investigated the PDDA/P5W30 multilayer film in order to further support the role of PDDA:Fe3O4 in the electrocatalytic activities. As shown in the inset of Figure 5B, there is a slight reduction of the electrocatalytic current for the HER in the presence of a PDDA overlayer, indicating that the capability of the electrocatalytic activities toward the HER is slightly weakened in the presence of the PDDA overlayer. In addition, in comparison to the polarization curves of the PDDA/P5W30 multilayer film to that of the PDDA:Fe3O4/P5W30 multilayer film, obviously the current is lower and the overpotential is more negative than that of the PDDA:Fe3O4/P5W30 multilayer film. These above-mentioned experiments are controlled under a high rotating rate, eliminating the effect of mass transfer on the current, which means the process of the HER catalyzed by the P5W30 multilayer films is kinetically controlled.37 According to the following equation

η)

RT RT ln i ln i RnF 0 RnF

(2)

which is valid in the high overpotential38 approximation regime; classical Tafel analysis was applied to the HER. In Table 1, the main several kinetic parameters from the data of Figure 5A were summarized. The Tafel slopes refer to the slope of the plots of overpotential η vs. log i for the HER in 0.1 M H2SO4,

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Figure 6. Polarization curves for the HER on the 4PDDA:Fe3O4/ 5P5W30 multilayer films (P5W30 as the outmost layer, and fabricated from 0.1 M H2SO4) upon exposure to 1.0 M H2SO4 solution for 0 h, 1 h, and 20 h, respectively. The dotted line corresponds to the polarization curve of the 4PDDA:Fe3O4/5P5W30 multilayer films (P5W30 as the outmost layer, and fabricated from 1.0 M H2SO4). Tested solution: N2-saturated 0.1 M H2SO4. Rotation rate: 3500 rpm. Scan rate: 5 m V s-1.

TABLE 1: Tafel Parameters for the HER on Glassy Carbon Electrodes Modified with Different Numbers of Layers of PDDA:Fe3O4/P5W30 (P5W30 as the outmost Layer)a P5W30 layer number

-log(i0/A cm-2)

Tafel slope/V

a

3 5 7 8 9 10 11

5.13 4.03 3.76 3.61 3.26 2.98 2.74

0.302 0.364 0.383 0.398 0.463 0.510 0.585

0.195 0.162 0.154 0.148 0.127 0.116 0.101

a

Test solution contains 0.1 M H2SO4.

and R is the cathodic transfer coefficient as defined for multistep n-electron-transfer processes with possible chemical steps included. As shown in Table 1, with the number of P5W30 deposited films increasing, the exchange current density increases, whereas the R value reduces. This means that the catalytic activity of the multilayer film is enhanced. It is also consistent with the result that at the same η, the larger the exchange current density is, the smaller the R value is.39 The exchange current density values are high and compare favorably with those quoted results in the literature, where -log(i0/A cm-2) ranges from 3.0 to 3.3 for Pt in 0.1 M H2SO4 solutions.39,40 In addition, compared to the multilayer films assembled with POMs and metalloporphyrins,40 the P5W30containing multilayer films show high exchange current density and small R value at the same layer numbers of the two multilayer films, indicating the P5W30-containing multilayer films exhibit higher electrocatalytic activities for the HER than that in the literature.40 These properties make such catalysts for the HER very good candidates for use in hydrogen cathodes of fuel cells. 3.7. Conversion of the Electrochemical Properties between the Two Above-Mentioned Multilayer Films. When the multilayer films fabricated from P5W30 solutions dissolved in 0.1 M H2SO4 were immersed in 1.0 M H2SO4 for several hours, electrocatalytic activity for the HER of such a multilayer film in acid media can also be observed. As depicted by Figure 6, the dashed line represents the polarization curve of the multilayer film with five P5W30 layers (P5W30 as the outmost layer, fabricated from 0.1 M H2SO4), and the overpotenial is very negative. In comparison to the polarization curve of the five P5W30 multilayer films upon exposure to 1.0 M H2SO4 solution for several hours to the dashed line, the overpotential shifts more positive, the current grows larger, and there is abundant

hydrogen evolution. Moreover, it can be seen that as the time of exposure to 1.0 M H2SO4 is prolonged, the capability of such multilayer films is enhanced gradually. However, at the same overpotential, the current density of such films is smaller than that of the multilayer films assembled with P5W30 solutions dissolved in 1.0 M H2SO4 (dotted line). 3.8. The Stability of the Multilayer Films. The stability of the thus-prepared multilayer films containing ten P5W30 layers (P5W30 fabricated from 0.1 M H2SO4) and nine PDDA:Fe3O4 layers was evaluated by comparing the changes in voltammetric peak currents before and after potential scanning 1000 cycles between 0.6 V and -1.0 V at 100 mV s-1 in pH 5.45 buffers. There was no observable decrease in the voltammetric currents. Furthermore, no observable change in the shape and height of the redox waves was found, after the as-prepared modified electrode was exposed in air or soaked in the supporting electrolyte for two months. In addition, when fabricated in the multilayer film, P5W30 is stable at pH 1-10. Although P5W30 dissolved in buffer solutions is also stable at pH 0-12, it is confirmed that the P5W30 films have an excellent stability, which is very useful in the preparation of a modified electrode and the catalytic reaction. We also investigated the stability of the multilayer film in which P5W30 is dissolved in 1.0 M H2SO4. Under the above test conditions, such multilayer films show the same activity toward the HER before and after the stability tests. In addition, we found that the catalyst is insensitive to impurities that are usually deleterious to platinum and palladium catalysts, by using some purposely nonpurified HCl, H2SO4 and HClO4 as the tested solutions.5 It is known that, under relatively “high-purity” conditions, activated Pt electrodes lose most of their electrocatalytic activity during a period of ca. 1 h41 (up to 90% in acid solutions). In contrast, the multilayer films-modified electrodes are very robust, durable, and efficient in longer time periods. These make such catalysts very good candidates for use in extremely diverse conditions. And it may be hoped to replace the metal Pt as the hydrogen cathode material of fuel cells. 4. Conclusions In summary, the first example of nanocomposite multilayer films of Preyssler-type polyanion cluster P5W30 and Fe3O4 nanoparticles was successfully fabricated on various substrates by layer-by-layer self-assembly technique. The linear and regular growth of the multilayer films characterized by UV-vis spectroscopy and cyclic voltammetry were observed. It is interesting that the multilayer films exhibit fine-tunable electrochemical behaviors by simply adjusting the pH of assembly solution in the LBL process. The multilayer films fabricated from P5W30 solutions dissolved in 0.1 M H2SO4 exhibit high electrocatalytic response and sensitivity toward the reduction of two substrates of important analytical interests, HNO2 and IO3-, whereas the films assembled with P5W30 solutions dissolved in 1.0 M H2SO4 show remarkable electrocatalytic activity for the HER in acid media. The dynamic constants of the HER were estimated by hydrogen evolution voltammograms. In addition, when the multilayer films fabricated from P5W30 solutions dissolved in 0.1 M H2SO4 were immersed in 1.0 M H2SO4 for several hours, electrocatalytic activity for the HER in acid media can also be observed. Although the PDDA:Fe3O4 nanoparticles layers did not obviously enhance the electrocatalytic activities on the reduction of HNO2 and IO3-, it is demonstrated that the PDDA:Fe3O4 layers acting as nanoparticle-conductive layers in the multilayer films do not block the electron transfer of P5W30 sandwiched between them.

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