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Multifunctional Multilayer Films Containing Polyoxometalates and Bismuth Oxide Nanoparticles Chunxiang Li,† Kevin P. O’Halloran,‡ Huiyuan Ma,*,† and Shilin Shi† Department of Chemistry, Harbin Normal UniVersity, Harbin, P. R. China 150025, and Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322 ReceiVed: January 7, 2009; ReVised Manuscript ReceiVed: April 25, 2009
Multifunctional multilayer films consisting of the Keggin-type polyoxometalate [SiW9V3O40]7- (SiW9V3) and bismuth oxide nanoparticles (Bi2O3) were prepared by the layer-by-layer assembly method. For the first time, electrochromic and photochromic studies were done on a film containing both polyoxometalates and nanoparticles. The films were characterized by UV-vis absorption, emission spectra, and atomic force microscopy. Their electrochromic and photochromic properties were investigated by cyclic voltammetry and UV-vis spectroscopy. The results show that the reduction of SiW9V3 is very reversible and tunable with the addition of Bi2O3 layers into the film. The electrocatalytic activity of the films toward oxidation of L-cysteine hydrochloride hydrate (L-cysteine) and reduction of nitrite were studied with cyclic voltammetry. The results show that the incorporation of Bi2O3 nanoparticles into the films changed the films’ photoluminescence properties and electrocatalytic efficiency. 1. Introduction Some common applications of polyoxometalates (POMs) include a variety of catalysis, medicine, analysis, materials science, and biochemistry, since they cannot only be adsorbed strongly on solid surfaces but also exhibit reversible, stepwise multielectron-transfer reactions without decomposition.1-5 Substitution of the metal ions in POMs modifies the electrochemical properties and allows for a wide range of applications. For example, substitution of vanadium for tungsten into the framework of POMs increases the negative charge of the resulting polyanions and shifts the stability of the plenary species to higher pH, which is important in several catalytic and electrocatalytic processes.6-9 The electrocatalytic processes by some vanadiumsubstituted POMs proved to be in both reductive and oxidative reactions, making them potential candidates for electrocatalysis and electrochemical devices.10,11 A challenge in POM chemistry is to implement the molecules into well-defined functional materials in the device architecture to facilitate their diverse application in the field of molecular and electronic devices. Kurth and co-workers explored some fine ordered POM-containing multilayer films showing potential application for NO sensing,12 electro- and photochromic devices,13 and a remarkable pH-sensitivity meter.14 Sequentially, incorporation of nanoparticles into POM-containing multilayer films has been developed by the layer-by-layer assembly (LBL) method.15 The benefit of assembling nanoparticles into thin films is that the main functions and properties of each component are maintained rather than changed, for example, enhancing the electrocatalytic activity toward the hydrogen evolution reaction (HER),16 modifying the charge propagation mechanism,17 increasing the emission of the material,18 and so on. Kulesza et al. deposited platinum nanoparticles on electrocatalytic network films with polyoxometalates as inorganic templates,19,20 while * To whom correspondence should be addressed. E-mail: mahy017@ 163.com. Fax: +86-451-88067319. † Harbin Normal University. ‡ Emory University.
Dong et al. alternately deposited iron oxide nanoparticles and P5W30 on quartz and ITO substrates.16 Gu et al. demonstrated the POM/TiO2 film has great potential for gas sensitivity at lower temperatures.21 These examples indicate that the materials containing nanoparticles and POMs not only assemble into multilayer films, but those films can also have specific properties. Here, the multilayer films contain [SiW9V3O40]7- (SiW9V3) electrostatically assembled with the Bi2O3 nanoparticles coated by polyethylenimine (PEI) (PEI-Bi2O3). The reason for selecting Bi2O3 nanoparticles is that they are a high-band-gap semiconductor. The band character is very important for applications, e.g., photochromic and electrochromic materials.22 By comparing the films with and without Bi2O3 nanoparticles, one of the great strengths of {PEI/[SiW9V3/PEI-Bi2O3]n/ SiW9V3} films is the ability to modify the properties such as rapid response times and high optical contrasts in photochromic and electrochromic experiments. Additionally, this film allows for an interesting combination of the properties supplied by each monomer unit, such as the photoluminescence of the films arising from Bi2O3 nanoparticles and SiW9V3. Following the fabrication of multilayer films, the morphology, photoluminescent, photochromic, and electrochromic properties of the films were studied in detail. 2. Materials and Methods 2.1. Materials and Methods. SiW9V3 was prepared according to the literature method.23 PEI and L-cysteine were obtained from Aldrich, and used as received. Bi2O3 nanoparticles (e20 nm) were prepared by sol-gel methods and examined by a Hitachi s-4800 scanning electron microscope. The solution of PEI-coated Bi2O3 nanoparticles (PEI-Bi2O3) was formed by sonication of Bi2O3 nanoparticles (0.0093 g) in 2 mM PEI (the concentration was calculated on the basis of their repeating units) solution. All of the other chemicals are reagent grade. The water used in all experiments was deionized to a resistivity of 18 MΩcm. UV-vis absorption spectra were recorded on a quartz slide using a U-3010 UV-vis spectrophotometer. Atomic force
10.1021/jp9001498 CCC: $40.75 2009 American Chemical Society Published on Web 05/18/2009
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Figure 1. UV spectrum of {PEI/[SiW9V3/PEI-Bi2O3]n/SiW9V3} film with n ) 0-10 (from bottom to top) for the precursor film-modified quartz substrate (on both sides); the inset shows plots of the absorbance values at 238 and 271 nm.
microscopy (AFM) images were taken on a silicon slide by a Digital Nanoscope IIIa instrument operating in the tapping mode with silicon nitride tips. Fluorescence spectra were performed with a SPEX FL-2T2 fluorescence spectrophotometer using a 450 W xenon lamp as the excitation source. Cyclic voltammetry (CV) experiments were carried out in a three-compartment cell (10 mL) with a CHI660B voltammetric analyzer at ambient temperature (20 ( 2 °C). All potentials are given with respect to commercial Ag/AgCl as the reference electrode and a twisted platinum wire as the counter electrode, a bare ITO or {PEI/ [SiW9V3/PEI-Bi2O3]n/SiW9V3} multilayer-film-coated ITO electrode as the working electrode. Photochromic experiments were carried out using a high-pressure mercury lamp which was fixed inside the UV-vis spectrophotometer during the photochromism test. The distance between the lamp and sample is about 5 cm. 2.2. Multilayer Assembly. The substrate (quartz slide or silicon wafer) was first thoroughly cleaned by treatment with Piranha solution (H2O2:H2SO4 ) 3:7 v/v) at 80 °C for 20 min, followed by rinsing with deionized water. Further purification was carried out by immersing in NH3 · H2O:H2O2:H2O (1:1:5 v/v) solution at 70 °C for 20 min and then extensively washing with water and drying with N2. Then, the cleaned substrate was immersed in 2 mM PEI solution for 20 min and a precursor layer of PEI was modified on the surface of the substrate. Next, the precursor film was alternately dipped into the 2 mM POM solution and 2 mM PEI-Bi2O3 solution for 20 min, rinsed with deionized water, and dried in a N2 stream after each dipping. The procedure results in the build-up of the multilayer films containing both SiW9V3 and Bi2O3, which can be expressed as {PEI/[SiW9V3/PEI-Bi2O3]n/SiW9V3}, where n is the number of bilayers. 3. Results and Discussion 3.1. Ultraviolet-Visible Absorption Spectra. UV-vis spectroscopy was used to characterize the growth process of the multilayer films. Figure 1 shows the UV spectra of the {PEI/ [SiW9V3/PEI-Bi2O3]n/SiW9V3} multilayer films with n ) 0-10 deposited on the precursor film of PEI. In the range 200-700 nm, the spectra showed two characteristic absorptions at 234 nm owing to Bi2O3 and 271 nm assigned to SiW9V3. However, there is a slight red shift of 2-6 nm for the absorption peak of SiW9V3 (271 nm) of the multilayer films compared to that of the SiW9V3 solution (Figure 2), which may be associated with the strong interactions between PEI cation and the polyoxometalate anion. On the other hand, the absorption band of the Bi2O3 (232 nm) exhibits a blue-shift as compared with that of purchased materials. It is well-known that the absorption of a photon, leading to excitation of an electron from the valence
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Figure 2. Comparative UV spectra of {PEI/[SiW9V3/PEI-Bi2O3]10/ SiW9V3} film, Bi2O3 nanoparticles, SiW9V3 solution, and {PEI/[SiW9V3/ PEI]10/SiW9V3} film.
band to the conduction band in the semiconductor, is referred to as band gap energy (Eg). Compared to macrocrytalline material, the nanoparticles often show a shift of the band gap energy (Eg) toward higher value due to a decrease in the size of the particle,24-26 so a blue shift of the absorption band is observed in the absorption spectra. In addition, the increase of absorptions in the range 400-700 nm owes to the increased amount of adsorbed SiW9V3 and Bi2O3, and their overlap peaks, which further confirms the incorporation of both SiW9V3 and Bi2O3 into multilayer films.27 Furthermore, the inset of Figure 1 presents the plots of the absorbance values for these multilayer films at 234 and 271 nm as a function of the number of deposition cycles, which indicates that the same amount of SiW9V3 and Bi2O3 is adsorbed after each deposition cycle, and further confirms that the deposition process is very consistent from layer to layer and highly reproducible. The absorbance increment for the first bilayer {PEI/[SiW9V3/PEI-Bi2O3]1/ SiW9V3} is larger than that for any of the other bilayers up to 10 bilayers, indicating that a greater amount of SiW9V3 penetrated the precursor {PEI/SiW9V3}.13,28-30 Comparing the UV spectra of the {PEI/[SiW9V3/PEI-Bi2O3]10/SiW9V3} and {PEI/[SiW9V3/PEI]10/SiW9V3} films, an enhancement of the characteristic absorbance is found in the Bi2O3-containing film (Figure 2). According to the method Liu et al. reported,13,28 the surface coverage (Γ) for the films can be calculated from the UV-vis spectra. The average surface coverage in {PEI/ [SiW9V3/PEI-Bi2O3]10/SiW9V3} films amounts to ca. 1.03 × 1016 SiW9V3 anions/cm2 or 1.70 × 10-9 mol cm-2 (271 ) 1.48 × 104 M-1 cm-2; λ, the extinction coefficient, is calculated according to ref 28), while that in {PEI/[SiW9V3/PEI]10/SiW9V3} films amounts to ca. 2.98 × 1014 SiW9V3 anions/cm2 or 4.95 × 10-10 mol/cm2 (268 ) 1.66 × 104 M-1 cm-2). Apparently much more SiW9V3 anions are absorbed in the Bi2O3-containing films compared to Bi2O3-free films under the same conditions, indicating that Bi2O3 nanoparticles facilitate absorption of SiW9V3 to the films. Thus, the absorbance for the Bi2O3containing films obviously increased compared to that for Bi2O3free films. Therefore, compared to {PEI/[SiW9V3/PEI]n/SiW9V3} films, {PEI/[SiW9V3/PEI-Bi2O3]n/SiW9V3} multilayer films have enhanced photochromic properties (which will be discussed in detail later). The surface morphology and the homogeneity of the deposited {PEI/[SiW9V3/PEI]3} and {PEI/[SiW9V3/PEI-Bi2O3]3} films were investigated to explore the difference between Bi2O3-free and Bi2O3-containing films. The surface of the {PEI/[SiW9V3/ PEI]3} film with PEI as the outer layer is very flat with some small holes on it, as shown in Figure 3a, and its root-meansquare (rms) roughness value calculated from an area of 1.0 ×
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Figure 3. AFM images of {PEI/[SiW9V3/PEI]3} multilayer films (a) and {PEI/[SiW9V3/PEI-Bi2O3]3} (b).
1.0 µm2 is 2.883 nm.31 The surface morphology of the {PEI/ [SiW9V3/PEI-Bi2O3]3} film with PEI-Bi2O3 as the outer layer, into which the Bi2O3 nanoparticles are introduced, is very different compared to that of the {PEI/[SiW9V3/PEI]3} film. A lot of protuberant peaks are observed in the three-dimensional AFM image of {PEI/[SiW9V3/PEI-Bi2O3]3} film, as shown in Figure 3b, suggesting that Bi2O2 nanoparticles were incorporated into film with slight aggregation. Furthermore, the packing density of the particles and the rms roughness value (3.761 nm) for the {PEI/[SiW9V3/PEI-Bi2O3]3} film are much larger than those for the {PEI/[SiW9V3/PEI]3} film, in agreement with the results of the UV-vis spectra. 3.2. Photochromic Behavior and Mechanism. In the region 400-1000 nm, the multilayer films exhibit photochromic properties in response to different UV irradiation times (Figure 4a). When the {PEI/[SiW9V3/PEI-Bi2O3]11/SiW9V3} films are irradiated with UV light, electrons are excited from the lowenergy electronic states to the high-energy states (OfM LMCT), and reduced mixed-valence species of SiW9V3 are produced. Thus, the d0 electronic configurations of partial metal ions in SiW9V3 become d1, and the excited electrons have a thermally activated delocalization within the whole polyanion. The d-d transition of M6+ to M5+ between the adjacent metal centers with different valence states will occur, resulting in the films turning slightly blue.13,29,32 In order to minimize the delay between UV irradiation and collecting absorption spectroscopy data for films, we fixed a high-pressure mercury lamp inside a UV-vis spectrophotometer above the quartz slide without hindering the optical path, and set the monitor range suitably smaller during the photochromic experiments. After irradiation for 10 s, the {PEI/[SiW9V3/ PEI-Bi2O3]11/SiW9V3} films display light blue and their absorbance increased by 0.026 units at ca. 800 nm. During a prolonged irradiation time of 15 s, the absorbance of multilayer films gradually reached saturation, and the films display deep blue. After the UV light was turned off, the blue films began to discolor gradually in air. The response speed of decoloration is fairly fast, and the bleaching process is almost complete after 20 s for {PEI/[SiW9V3/PEI-Bi2O3]11/SiW9V3} films. To make
Figure 4. Absorption spectra of {PEI/[SiW9V3/PEI-Bi2O3]10/SiW9V3} (a) and {PEI/[SiW9V3/PEI]10/SiW9V3} (b) multilayers as a function of irradiation time.
a comparison between Bi2O3-containing and Bi2O3-free films, we prepared the {PEI/[SiW9V3/PEI]11/SiW9V3} film, and investigated its photochromic properties using the same experimental methods and conditions. In the same region, the bleaching process for {PEI/[SiW9V3/PEI]11/SiW9V3} is nearly complete after 180 s (Figure 4b), and the change in the absorbance of {PEI/[SiW9V3/PEI]11/SiW9V3} film at 800 nm (only 0.01 units) after UV irradiation for 120 s is smaller than that of {PEI/[SiW9V3/PEI-Bi2O3]11/SiW9V3} film (0.11 units). Therefore, the higher optical density change and shorter illumination time indicates that the Bi2O3 nanoparticles incorporated into the films have shortened the response time and enhanced the photochromic efficiency. The reason for this activity may be as follows: The SiW9V3 and Bi2O3 nanoparticles in the {PEI/[SiW9V3/ PEI-Bi2O3]n/SiW9V3} composite film are all photoactive due to their large band gaps; i.e., the HOMO-LUMO gaps for SiW9V3 are ca. 3.7-4.6 eV, and the band gaps of Bi2O3 are 3.2 eV (estimated by UV spectra, the method was reported in ref 33).30 With UV illumination of a semiconductor with energies equal to or higher than its band gap, electrons can be excited from the valence band to the conduction band, producing excited electrons in the conduction band and holes in the valence band. These excited electrons can return back nonradiatively or radiatively, or get trapped and react with acceptors adsorbed.33,34 The redox potential of SiWVI9VV3O407- is more positive than that of Bi2O3 nanoparticles under the same conditions (Figure 5). Thus, the electron transfer between Bi2O3 and SiWVI9VV3O407- is thermodynamically favorable to yield the blue SiWVI9VV2VIVO408- species, which can improve the photochromism of the film. Namely, a photoinduced electrontransfer event exists between SiWVI9VV3O407- and Bi2O3. The electron transfer between two inorganic components in composite materials has been previously documented: He et al. reported that in composite systems, such as WO3(MoO3)/CdS
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Figure 5. Cyclic voltammograms (CVs) of 2 mM SiW9V3 (solid line) and Bi2O3 (dashed line) nanoparticles in a pH 3.82 buffer solution. The scan rate was 50 mV s-1.
Figure 6. Reversibility absorption of {PEI/[SiW9V3/PEI-Bi2O3]10/ SiW9V3} multilayers in the irradiation process.
and WO3(MoO3)/TiO2 composites, the photogenerated electrons and holes can transfer between the two inorganic components due to the different energy levels of these materials, which can lead to improved photochromism.32 Also, the following luminescence experiments exhibited that the characteristic emission peak arising from V4+ appeared in the emission spectrum of Bi2O3-containing film but did not in that of the Bi2O3-free film, further providing evidence for electron transfer from Bi2O3 to SiW9V3 (see the luminescence investigation below). Additionally, the increased amount of adsorbed SiW9V3 anions in Bi2O3containing film due to incorporation of Bi2O3 nanoparticles also enhanced the photochromic efficiency. In order to study the reversibility of the coloration-decoloration process for the multilayer films incorporated with Bi2O3 nanoparticles, the wavelength at 800 nm was selected to observe the changes of absorbance of multilayer films. First, the absorbance of the original films was measured. Then, the films were irradiated for 15 s and the absorbance was measured again immediately. In the following step, the blue films were stored in air and decolored over 20 s, and then, the absorbance was measured once again. By repeating the above process, Figure 6 was obtained. The absorption spectra of the completely bleached {PEI/[SiW9V3/PEI-Bi2O3]11/SiW9V3} films were almost consistent with those of the films without UV irradiation. The photochromic contrast is a measure of the degree of color change from bleached to colored state, and the highest value for the {PEI/[SiW9V3/PEI-Bi2O3]15/SiW9V3} film was at 800 nm and determined to be ∆(%T) ) 20.6% from Figure 6. This value is
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Figure 7. UV spectra of {PEI/[SiW9V3/PEI-Bi2O3]15/SiW9V3} filmmodified ITO glass electrode during the different potentials, from bottom to top: 0, -0.3, -0.6, -0.9, and -1.0 V, respectively.
much higher than that for the film not containing Bi2O3 nanoparticles, which was determined to be only ∆(%T) ) 3.4% at 800 nm from Figure 4. Therefore, the nanoparticles incorporated into the film have greatly enhanced the photochromic contrast of the film. 3.3. Electrochromism Investigation. The UV spectrum of {PEI/[SiW9V3/PEI-Bi2O3]15/SiW9V3} films modified on ITO electrode (immersion area 0.8 × 2.7 cm2) was recorded under different potentials from 0 to -1.0 V (Figure 7). When different negative potentials were applied, the {PEI/[SiW9V3/PEI-Bi2O3]15/ SiW9V3} films were reduced to a different extent of blue color, which results from the optical absorption of charge transfer (W5+-O-W6+ or W6+-O-W5+), indicating that the films were electrochromic. The response times of the films were investigated by doublepotential experiments with absorbance measurements at 678 nm. The coloration and bleaching times are 7.0 and 6.3 s, respectively, for 90% ∆A (difference between maxima). The ∆A value is 0.03 units for {PEI/[SiW9V3/PEI-Bi2O3]15/SiW9V3} films. At the same time, the electrochromic reversibility of the films was evaluated by performing repetitive double-potential steps from -1.0 to 0 V (Figure 8). The response time for coloration and bleaching as well as the absorbance of the electrochromic films do not change noticeably even after 200 cycles, except for a slight decrease of maximum absorbance, which demonstrates a stable electrochromic behavior of the self-assembled films during double-potential cycles. The electrochromic efficiency (also called the coloration efficiency) is a measure of optical density change for the {PEI/[SiW9V3/PEI-Bi2O3]15/ SiW9V3} film as a function of injected/ejected electronic charge and was determined from data in Figure 8 to be an average value of η ) 7.8 cm2/C.35 3.4. Electrocatalytic Investigation. The electrochemical studies of the {PEI(SiW9V3/PEI-Bi2O3)15SiW9V3}-modified ITO electrode were carried out in 0.2 M HOAc-NaOAc (pH 3.82) buffer solutions. L-Cysteine and nitrite were chosen as test reactants for the electrocatalytic activities of {PEI(SiW9V3/ PEI-Bi2O3)15SiW9V3} multilayer films. In the potential domain where this catalysis is observed, the direct oxidation of L-cysteine is negligible.36 With addition of L-cysteine in the medium, a current increase is observed readily at the oxidation potential of the first V-wave (Figure 9), suggesting that the oxidation of L-cysteine is effectively electrocatalyzed by the V-center of Keggin-type POM, which is seldom reported in the literature.37 Figure 10 shows the cyclic voltammograms for the electrocatalytic reduction of nitrite by {PEI(SiW9V3/PEI-Bi2O3)15SiW9V3} in 0.2 M HOAc-NaOAc (pH 3.82) buffer solutions. The
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Figure 10. CVs of {PEI/[SiW9V3/PEI-Bi2O3]15/SiW9V3} film restricted to the W-centered redox process in the absence and presence of NO2- (pH 3.82 medium). The scan rate was 50 mV s-1. The concentration of NO2- from top to bottom: 0, 1.25, 2.5, and 5 mM.
Figure 8. Potential, current and absorbance at 678 nm of the {PEI/ [SiW9V3/PEI-Bi2O3]15/SiW9V3} film-modified ITO during subsequent double-potential steps (-1.0 to 0 mV).
Figure 11. Emission and excitation spectra of (a) Bi2O3 nanoparticles and (b) {PEI/[SiW9V3/PEI-Bi2O3]10/SiW9V3} films. Solid line: excitation at 390 nm. Dot-dashed line: excitation at 471 nm.
Figure 9. CVs of {PEI/[SiW9V3/PEI-Bi2O3]15/SiW9V3} film restricted to the V-centered redox process in the absence and presence of L-cysteine (pH 3.82 medium). The scan rate was 50 mV s-1. The concentration of L-cysteine from bottom to top: 0, 2, 5, and 10 mM. Inset: the relationship between the anodic current of the wave at ca. 0.42 V versus L-cysteine concentration.
multilayer films present good electrocatalytic activity for the reduction of nitrite. It can be seen that, with the addition of nitrite, the cathodic peak current of W-waves located at ca. -0.91 V increases remarkably, while the corresponding anodic
peak current decreases, suggesting that the films containing SiW9V3 could have potential applications in the detection of nitrite.12 3.5. Luminescence Investigation. Figure 11 shows the photoluminescence spectra of Bi2O3 nanoparticles (a) and {PEI(SiW9V3/PEI-Bi2O3)11SiW9V3} film (b) at room temperature. In the excitation spectra of Bi2O3 nanoparticles and Bi2O3contianing films, the most intense characteristic peaks are 397 and 390 nm, respectively, which are very close to the 388 nm (3.2 eV), attributed to the band gap excitation of the Bi2O3 nanoparticles (3.2 eV).33 Compared to the excitation spectrum of Bi2O3 nanoparticles, certain small variations occurred in the band number, band position, and relative intensity in the spectrum of the Bi2O3-contianing film. This difference between
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Bi2O3 nanoparticles and Bi2O3-contianing films may be caused by the close packing of the Bi2O3 nanoparticles and the change in environment of the Bi2O3-contianing film.38 The emission spectra of Bi2O3 nanoparticles and the Bi2O3-contianing film all exhibit a strong green-light emission with a very weak shoulder peak. These emission peaks can be attributed to the recombination from conduction band to the energy levels of deep-trap or surface state.33 Also in the spectrum of the Bi2O3containing film, the shapes of peaks became narrow, the intensity was decreased, and the weak peak at 561 nm nearly disappeared. In addition, no luminescence was detected for Bi2O3 powder (Analytical Reagent grade). It is well-known that the deep-trap and surface state originates from the crystal defects inside or at the surface of the crystallite. Compared to Bi2O3 powder, Bi2O3 nanoparticles have a smaller size and larger surface-to-volume ratio, which increases surface defects and the probability for overlap in energy between the conduction band states and surface states, resulting in the visible green-light emission.39-42 When the excitation is set to λ ) 471 nm, the main emission occurred at 608 nm with a shoulder peak at ca. 640 nm; this fluorescence is assigned to V4+, which should be rooted in the electron transfer from Bi2O3 to SiW9V3O407-, yielding SiW9V3O408- species. This luminescence property for vanadium oxide clusters has rarely been reported.39 4. Conclusion Photochromic and electrochromic inorganic multilayer films containing SiW9V3 and Bi2O3 nanoparticles were constructed by the LBL method. When exposed to UV light, the multilayer films showed a blue color due to the formation of heteropoly blue species. Moreover, the Bi2O3 nanoparticles incorporated in the films have the ability to shorten the response time and enhance the efficiency of photochromism. The films show electrochromism with response times, bleaching times, and reversibility comparable to similar POM films. Additionally, they operate at low potential. These two results suggest that these materials may become promising candidates for the application of photochromic and electrochromic materials. The photoluminescence studies indicate the film shows emission properties due to either Bi2O3 nanoparticles or V4+ centers in the POM, depending on the excitation wavelength. The electrocatalytic studies indicate that the film can be used as a bifunctional electrocatalyst to catalyze both the oxidation of L-cysteine and the reduction of nitrite. Acknowledgment. This work was supported by the National Nature Science Foundation of China (No. 20771031) and the Foundation of Education Committee in Heilongjiang (No. 11531228). References and Notes (1) Kuhn, A.; Anson, F. C. Langmuir 1996, 12, 5481–5488. (2) Lira-Cantu, M.; Gomez-Romero, P. Chem. Mater. 1998, 10, 698– 704. (3) Yamase, T. Chem. ReV. 1998, 98, 307–326. (4) Liu, S. Q.; Kurth, D. G.; Mohwald, H.; Volkmer, D. AdV. Mater. 2002, 14, 225–228.
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