Stable Photochromism and Controllable Reduction Properties of

Jun 19, 2008 - stable and reversible photochromism of POMs. In addition ... to encapsulate POMs, the obtained surfactant-encapsulated POM clusters (SE...
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J. Phys. Chem. B 2008, 112, 8257–8263

8257

Stable Photochromism and Controllable Reduction Properties of Surfactant-Encapsulated Polyoxometalate/Silica Hybrid Films Wei Qi, Haolong Li, and Lixin Wu* State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: February 8, 2008; ReVised Manuscript ReceiVed: April 14, 2008

In this paper, we present a novel strategy for fabricating polyoxometalate (POM)-based photochromic silica hybrid films. To combine metal nanoparticles (NPs) into the POMs embedded silica matrix, furthermore, we realized the controllable in situ synthesis of metal NPs in the film by utilizing the reduction property of POMs existing in the reduced state. Through electrostatic encapsulation with hydroxyl-terminated surfactants, the POMs with good redox property can be covalently grafted onto a silica matrix by means of a sol-gel approach, and stable silica sol-gel thin films containing surfactant-encapsulated POMs can be obtained. The functional hybrid film exhibits both the transparent and easily processible properties of silica matrix and the stable and reversible photochromism of POMs. In addition, well-dispersed POMs in a hydrophobic microenvironment within the hybrid film can be used as reductants for the in situ synthesis of metal NPs. More significantly, the size and location of NPs can be tuned by controlling the adsorption time of metal ions and mask blocking the surface. The hybrid film containing both POMs and metal NPs with patterned morphology can be obtained, which has potential applications in optical display, memory, catalysis, microelectronic devices and antibacterial materials. Introduction Photochromism is defined as reversible photoinduced transformation of a chemical species between different existing states that exhibit different photophysical properties such as absorption, luminescence, refractive index, and so forth. The exploitation of novel photochromic materials is of interest in fundamental research over recent years, owing to their intriguing potential applications as photoswitches and erasable optical memories.1 Polyoxometalates (POMs) are a kind of well-defined metal oxygen clusters with diverse functions such as redox, catalysis,2 antivirus activities,3 and magnetism.4 Because of the existence of the mixed valence transition metal W or Mo ions, the reduced POMs are often blue in color, which is unlike its colorless high oxidized state, and are considered to be a kind of potential photochromic material.5 However, from the point of view of devices and materials fabrication, the utilization of the chromic property of POMs is seriously restricted because of their poor processibility. Therefore, it is still a challenge to organize and assemble POMs into compatible and applicable solid matrices for utilizing their photochromic property in a much more precise procedure. Besides photochromic properties, the strong redox capability of POMs also attracts considerable interest. For example, the POMs in their reduced state can in situ reduce metal ions into nanoparticles (NPs).6 The metal NPs with POMs as stabilizers can be easily obtained, and such an interesting system provides a possibility for developing novel catalysts for special reactions.7 Furthermore, many research groups have focused on synthesizing metal NPs with defined nano- or micrometer-scale assembled morphologies using POMs, and several constructive results have already been achieved.8 However, because the acquisition of reduced POMs by chemical-, electrical- or photoirradiation * To whom correspondence should be addressed. E-mail: wulx@ jlu.edu.cn.

methods requires plenty of certain sacrificial reagents as reductants, the correlative research regarding the synthesis of metal NPs with POMs mainly concentrated on solution systems, and it can hardly perform in real applicable films or block materials. The application of this interesting multifunctional material is thus restricted to some extent. If the metal NPs can be synthesized with POMs in solid substrates, especially in preassigned locations in nano- or micrometer scale, it may have intriguing applications in microelectronics, miniaturized sensors, and microelectromechanical systems.9 Recently, we have developed a new method to organize and assemble POMs into easily processed silica sol-gel materials.10 By employing terminal hydroxyl-modified cationic surfactants to encapsulate POMs, the obtained surfactant-encapsulated POM clusters (SECs) can link to the silica backbones covalently and disperse uniformly in the silica matrix. The intrinsic property of POMs and the easily processible property of silica are cooperatively combined in the obtained hybrid materials.10 Because of the nonselectivity of this method to all POMs, we believe that the photochromic and strong reductive properties of some POMs can be well performed in this system. Aimed at that purpose, we herein introduced [EuP5W30O110]12- (POM1), which exhibits strong photochromic and redox ability,5,11 into the silica matrix with a terminal-OH-modified surfactant, 11-hydroxylundecyldimethylamine hydrobromide (HUDAH), as a bridge to construct transparent and stable POM/SiO2 sol-gel hybrid film (Scheme 1). The hybrid film exhibits stable and reversible photochromic property, which may have potential applications as high-density optical memories and optical displays and switches. Meanwhile, utilizing the reductive capability of the colored film containing reduced POM-1, the in situ controllable synthesis of metal NPs can also be achieved in the film. In addition, by controlling the UV irradiation process, the metal NPs can be obtained in the preassigned locations in

10.1021/jp801188e CCC: $40.75  2008 American Chemical Society Published on Web 06/19/2008

8258 J. Phys. Chem. B, Vol. 112, No. 28, 2008 SCHEME 1: Overall Procedures for the Preparation of POM-1 Based Hybrid Film, the Photochromic Process and the Synthesis of Metal NPs in the Thin Film by Different Routes

micrometer scale, which would be significant for fabricating microelectronic devices or miniaturized sensors. Experimental Section Chemicals. Dimethylamine [(CH3)2NH], tetraethyl orthosilicate (TEOS), silver nitrate (AgNO3), and hydrogen tetrachloroaurate (HAuCl4) were of analytical grade and were purchased from Beijing Chemical Reagents Company. 11-bromoundecanol [Br(CH2)11OH] was purchased from Sigama-Aldrich. All the chemicals are used directly without any further purification. Preparation of Hybrid Films. POM-1 with K+ as counterions was synthesized according to a published procedure.11 The surfactant HUDAH was synthesized using the following procedure: 1.26 g (28 mmol) (CH3)2NH and 1 g (4 mmol) Br(CH2)11OH were dissolved in 50 mL of ethanol, and the initial molar ratio of (CH3)2NH to Br(CH2)11OH was controlled at 7:1. The reaction mixture was refluxed with stirring for 24 h and then cooled to room temperature. The solvent and excess dimethylamine were removed under reduced pressure to give a white solid. The crude product was purified over silica gel chromatography with 20:1 (v/v) of CHCl3 and CH3OH as eluent, and pure HUDAH was obtained as a white powder. (Yield: 50%). 1H NMR (500 MHz, DMSO-d6, δ): 1.25 (m, 14H), 1.40 (m, 2H), 1.59 (m, 2H), 2.75 (s, 6H), 3.01 (t, 2H), 3.37 (m, 2H), 4.32 (t, 1H), 9.29 (s, 1H). The surfactant HUDAH-encapsulated POM-1 cluster (SEC1) was prepared according to a previously reported method.10 Five milliliters of POM-1 aqueous solution (0.007 mmol/mL) was added to a 20 mL aqueous solution of HUDAH (0.02 mmol/ mL) with stirring. Considering the charge replacement, the initial molar ratio of HUDAH to POM-1 was controlled at 12:1. After the reaction finished, the precipitate was filtered and washed

Qi et al. with deionized water several times, then dried under vacuum until the weight remained constant, giving the complex SEC-1. The photochromic hybrid film was fabricated using the following procedure: 0.1 g of SEC-1 was dissolved in the mixed solvent of 50 mL of ethanol and 0.5 mL of water. The solution was stirred vigorously, following by the addition of 1.0 g of TEOS. When the homogenization appeared, the stirring was stopped, and the solvent was concentrated to ca. 2-3 mL. The resulting sol was cast onto the quartz slide or silicon wafer. The covered substrates were aged and dried in air for about 2 days, giving the hybrid thin film with 10% of SEC-1 content in weight. Photochromic Procedure. The obtained hybrid film was irradiated with a 300-W high-pressure mercury lamp under room temperature cooling by a small fan. The sample was set at a distance of 10 cm from the light source. The photochromism became saturated after the thin film encountered 2 min of irradiation. When the irradiation stopped, the hybrid film faded gradually, and the decoloration process was completed in 15 min in the dark. In-Situ Preparation of Metal NPs. We performed the in situ preparation of metal NPs in the film by two different routes, as shown in Scheme 1. One is to irradiate the hybrid film first to obtain the reduced POM-1, and then we adsorbed and reduced metal ions in the film to obtain metal NPs. The other route is just the opposite of the first one, in which the metal ions are adsorbed in the hybrid film in advance, and the metal NPs are in situ prepared in the film through subsequent photoreduction. These two routes are denoted as “pre-irradiation” and “preadsorption”, respectively. In both cases, Ag+ and [AuCl4]- have been employed for the preparation of Ag and Au NPs, respectively. For the pre-irradiation route, the saturated blue hybrid film obtained through the above-mentioned procedure was dipped into a deaerated aqueous solution of metal ions (1 mM of AgNO3 or HAuCl4), and the container was sealed with parafilm. After a certain time, the blue color faded, and the film turned into a brownish color for Ag and purple for Au. Then the film was taken out, washed with water, and was ready for further measurements. For the pre-adsorption route, the hybrid film was immersed into a deaerated metal ions aqueous solution (1 mM of AgNO3 or HAuCl4) for a certain time (2-14 h). Then the film that had adsorbed metal ions was taken out, washed with water, and dried. The film containing metal ions was then put into the sample chamber of a fluorescence spectrometer and irradiated with the excitation light from the fluorescence spectrometer (λ ) 300 nm, slit ) 5 nm) for 4 min. After that procedure, the film containing metal NPs was ready for further measurements. In a particular case, a copper grid was employed to cover the film as a mask for a patterned irradiation, so that the metal NPs can grow in confined areas. Measurements. 1H NMR spectra (tetramethylsilane (TMS)) were recorded on a Bruker UltraShieldTM 500 MHz spectrometer. The elemental analysis (EA) was carried out on a Flash EA1112 from ThermoQuest Italia SPA. The thermal gravimetric analysis (TGA) was carried out on a Perkin-Elmer 7 series thermal analysis system. Fourier transform infrared (FT-IR) spectra were performed on a Bruker IFS66V FT-IR spectrometer equipped with a DGTS detector (32 scans). The spectra were recorded with a resolution of 4 cm-1. The type of the fluorescence spectrophotometer used as the light source was a HITACHI F-4500. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ES-CALAB Mark (VG Company, U.K.) photoelectron spectrometer using a monochro-

POM-Based Photochromic Silica Hybrid Films

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matic Al KR X-ray source. Transmission electron microscopic (TEM) measurements were finished on a JEOL-2010 electron microscope operating at 200 KV. Scanning electron microscopic (SEM) images were acquired with a JEOL FESEM 6700F electron microscope. Microscopic photographs were obtained on an Olympus BX51 optical microscope. X-ray diffraction (XRD) data were collected on a Rigaku X-ray diffractometer (D/max rA, using Cu KR radiation at a wavelength of 1.542 Å). Results and Discussion Structure Characterization of SEC-1 and Hybrid Film. Based on our previous experiences in synthesizing SECs, the anionic POM-1 and cationic surfactant HUDAH could combine with each other through electrostatic interaction, and thus the formed SEC-1 possesses an inorganic POM-1 core and an organic shell composed of hydrophobic alkyl chains with hydroxyl groups at the terminal, as shown in Scheme 1. The following detailed characterizations support that speculation for the structure of SEC-1. The UV absorption spectrum of SEC-1 shows the characteristic OfW ligand-to-metal charge transfer (OfW LMCT) band, suggesting the presence of POM-1 in SEC-1 (Figure S1, Supporting Information). The 1H NMR spectrum of SEC-1 proves the presence of surfactant HUDAH in the complex. In contrast to the 1H NMR signal of pure HUDAH, the characteristic proton peak of the methyl group binding to the ammonium head of HUDAH in SEC-1 broadens, and the integral intensity diminishes by ca. 21%, indicating that the ammonium heads of the surfactants are immobilized on the surface of POM-1 as a result of the strong electrostatic interaction between HUDAH and POM-1 (Figures S2 and S3).12 The FT-IR spectrum exhibits the characteristic absorption of both HUDAH at V ) Vas (O-H) 3403 cm-1, Vas (CH2) 2924 cm-1, Vs (CH2) 2851 cm-1, V (NH+) 2746 cm-1, δ (CH2) 1468 cm-1, Vs (C-O) 1054 cm-1, and POM-1 at V ) V (P-Oa) 1158, 1065 cm-1, Vas (W-Od) 980 cm-1, Vas (W-Ob-W) 914 cm-1, Vas(W-Oc-W) 785 cm-1, also confirming the successful encapsulation (Figure S4). Based on the data of EA, the structural formula of SEC-1 is suggested as (HUDAH)11HEuP5W30O110 · 3H2O (calcd: C 17.15, H 3.39, N 1.53; found: C 17.18, H 3.36, N 1.45, Table S1). The TGA measurement also verifies the estimated structural formula exactly. The 0.22 wt % loss before 110 °C in TGA result just matches the weight content of crystal water, which is consistent with the value of 0.63 wt % calculated from the structural formula. Assuming that the organic component has decomposed completely and all the inorganic residuals are Eu2O3 and WO3 at 800 °C, the exact residue of 74.29 wt % in total from TGA also displays in perfect agreement with the calculated value of 74.73 wt % from the given structural formula (Figure S5). These data imply that 11 negative charges of one POM-1 cluster are neutralized with 11 HUDAH cations, thus the same amount of hydroxyl groups should exist on one SEC-1(Scheme 1). SEC-1 is soluble in the mixed solvent of ethanol and deionized water because of the coverage of the hydroxyl groups on its surface. Moreover, through condensation with the sol-gel precursor TEOS, these hydroxyl groups can connect with the silica backbones covalently and keep SEC-1 monodisperse in the silica matrix.10,13 The hydrolysis of TEOS and condensation between TEOS and SEC-1 occur when the two components are blended together in the mixed solvent.10,13 The formed homogeneous sol affords a transparent and stable POM-1-based hybrid film by simply casting or spin-coating it onto the solid substrate (quartz matrix or silicon wafer). Because of the similarity of

Figure 1. Photochromic process of a POM-1-based hybrid film. (a) Photographs of the hybrid film (1) before and (2) after irradiation for 2 min. (b) UV absorption spectra of the hybrid film before (black solid line) and after (blue dashed line) irradiation for 2 min.

the synthesis method, the obtained hybrid film possesses a microstructure identical to that having been confirmed in our previous results.10 The characteristic vibration absorptions belonging to HUDAH at Vas (O-H) 3403 cm-1, Vas (CH2) 2925 cm-1, Vs (CH2) 2851 cm-1, V (NH+) 2748 cm-1, δ (CH2) 1468 cm-1, and POM-1 at Vas (W-Ob-W) 920 cm-1, Vas(W-Oc-W) 785 cm-1, clearly appear in the IR spectrum of the hybrid, confirming the existence of SEC-1 in the silica matrix. Besides those vibrations, a band centered at 1068 cm-1 (V Si-O-Si) indicates the formation of silica networks. The absorption at 1143 cm-1 (V Si-O-C) confirms the successful condensation between TEOS and hydroxyl groups covered on SEC-1 (Figure S6).10,13 Besides IR measurement, XPS is also used to identify that the coupling reaction between SEC-1 and that the silica matrix is in effect (Figure S7). In the XPS spectra of the C1s level of the hybrid film, four types of C atoms (peak binding energy at 284.65, 286.10, 287.83, and 288.86 eV, respectively) are detected. They just correspond to C atoms in four different environments (C-C, C-N, C-O-H, and C-O-Si) in the hybrid film. The relative peak area ratio of C-C, C-N, and C-O is 9:3.24:1.44, as shown in the spectra, which is close to the corresponding C atoms ratio at 9:3:1 calculated from the molecular structure and suggesting that the assignment is proper and acceptable. In addition, by comparing the relative peak areas of C-O-H and C-O-Si, we find that ca. 37.5% of -OH groups on SEC-1 are covalently linked to silica matrix. That is to say, more than four -OH groups of one SEC-1 are covalently linked to silica backbones on average. In our previous studies, SEC was known to exhibit characteristic packing reflections in the small-angle region of its XRD pattern, and we could detect the existence of the aggregation of SEC with these specific diffractions.10,12 However, the XRD pattern of the as-prepared hybrid film shows no packing reflections of SEC-1 in the smallangle region, and the replacements are two broad reflections centered around 2θ ) 10° and 25°, suggesting that SEC-1 disperses in the hybrid film uniformly and that the silica matrix exists in a amorphous state (Figure S8).10 Comparing to pure silica films produced by sol-gel procedure, the brittleness of hybrid films decreased largely because of the introduction of the flexible alkyl chains, and therefore it is easier to get cracked decreased silica thin films with this method. Photochromism of Hybrid Film. The obtained hybrid film exhibits interesting photochromic behavior. After 2 min of irradiation with UV light, the transparent and colorless hybrid film turns blue (Figure 1a), and the typical absorption spectra of the hybrid film before and after photoirradiation are shown

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Figure 2. XPS spectra of W 4f level of the hybrid film (a) before and (b) after UV irradiation for 2 min.

Figure 3. (a) Bleaching process of the colored film monitored by absorption at 700 nm versus time, and (b) photochromic cycles by the irradiation of UV light and following relaxation in the dark, monitored by absorption at 700 nm.

in Figure 1b. The characteristic OfW LMCT band of POM-1 at ca. 280 nm can be clearly seen in both of the spectra (Figure 1b inset), suggesting that the structure of POM-1 is well retained during the photochromic process. The new broad absorption band appearing in the visible region at ca. 700 nm after the irradiation can be clearly assigned to the W5+fW6+ intervalence charge transfer (IVCT), the characteristic absorption band of heteroblue, indicative of the formation of reduced POM-1.5,14 The XPS analysis also confirms the existence of W5+ after photoirradiation. As shown in Figure 2, the valence of W atoms is detected to be +6 in the freshly prepared hybrid film, and there is no evidence of W5+ signal. After the photoirradiation, W5+ ion appears and its concentration reaches ca. 39.8%, determined from the peak area ratio of W6+ and W5+, indicating that some of W6+ ions have been reduced to W5+ accompanied by the coloration process. The blue thin film faded gradually when the irradiation stopped, and the bleaching could be completed in 15 min at room temperature in the dark (Figure 3a). The absorption and

Qi et al. XPS spectra of the faded transparent film are identical with its original state, and no W5+ signal is detected, indicating that all W5+ are oxidized to W6+ again during the bleaching process. In addition, it should be noted that the blue color could last for a quite long time if the hybrid film is stored in a vacuum, implying that O2 plays an important role in the decoloration process. This phenomenon is consistent with previous results in the literature14 and O2 acts as a catalyst for the oxidation of the reduced POMs in the hybrid film during the bleaching process. The color change of the hybrid film is reversible, and the coloration-decoloration cycle can repeat over 20 times (Figure 3b), implying highly photochromic stability and potential application in the areas of photoswitches and memory devices. The photochromic reversibility and stability of the hybrid film should be attributed to the special structure of the surfactant HUDAH that was applied to modify POM-1 and the designed artificial microenvironment in SEC-1 grafted onto silica matrix. The active H atom at the ammonium head can provide a proton to POM-1, which stabilizes the reduced state of POM-1 and makes the transition between original and reduced state of POM-1 reversible. The mechanism of this process can be well explained based on the reported process, as shown in Scheme 2.14 The photoexcitation to the OfW LMCT band of POM-1 would generate a transfer of the active proton at the ammonium head of HUDAH to a bridging oxygen atom at the photoreducible site of WO6 octahedral lattice (Scheme 2b). Then the d1 electron of the bridging oxygen binds to the proton provided by HUDAH, and a hole left at the other oxygen atom due to the OfW LMCT transition interacts with the nonbonding electrons of the N atom of the amino group, forming a chargetransfer complex (Scheme 2c). Such a process implies the separation of the electron and hole charges, and the close binding between HUDAH and POM-1 in SEC-1 keeps the colored charge-transfer complex stable and the photochromism reversible. The color bleaching of the blue film in the presence of O2 in the dark can be explained to be just the opposite process of the reduction. The electron moves back to the bridging oxygen atom from W5+, the charge transfer complex disassembles, and SEC-1 gets back to its original state. To testify that the surfactant HUDAH is not oxidized during photoirradiation and the photochromic process occurs according to the route mentioned above, we investigated the structure of the hybrid film under the condition before and after the irradiation. The IR spectrum of the hybrid film after irradiation shows typical vibration absorption bands of HUDAH, POM-1, and silica matrix (Figure S9). The band positions are almost

SCHEME 2: Schematic Drawing of the Photochromic Mechanism

POM-Based Photochromic Silica Hybrid Films

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Figure 4. (a) Photographs and (b) UV absorption spectra of the POM-1-based hybrid films containing (1) Ag and (2) Au NPs, respectively, prepared by pre-irradiation procedure. (c) The cross section SEM image and inserted amplification of a local place of the hybrid film containing Ag NPs.

identical, comparing to the hybrid film without irradiation, suggesting that the structure of the hybrid film is well retained during irradiation. In addition, we also compared the structure of pure SEC-1 before and after UV irradiation with 1H NMR spectra for a control experiment. As a result, we found that the chemical shifts of HUDAH in these two states were almost the same except that the peak of the active H atom became broadened, and the peak integral intensity decreased to some extent (reduced by 19%) after the irradiation (Figure S3). These results confirm that the reduction of POM-1 is not caused by direct oxidization of the organic part, but resulted from the transfer of an active H atom from HUDAH, and the photochromic process operates toward the route we mentioned, which leads to the photochromism performing reversibly. In-Situ Synthesis and Patterning of Metal NPs. The colored POM-1 in its reduced state can be treated as reductants for the preparation of metal NPs6 and the in situ synthesis of NPs in the hybrid film can be carried out in two different routes, as shown in Scheme 1. In a typical pre-irradiation process, as described in the experimental part, a blue-colored hybrid film that has undergone 2 min of irradiation is immersed into a deaerated AgNO3 or HAuCl4 aqueous solution (1 mM) for adsorbing and in situ reducing metal ions. The blue color of the film fades in a few seconds and turns brown or pink at a certain time, indicating the formation of Ag and Au NPs, respectively (Figure 4a). It should be noted that, the synthesis of Au NPs needs much longer time than the preparation of Ag NPs. Generally, the formation of Ag NPs could complete in 30 min, but it takes 10 h or more for the synthesis of Au NPs. The reason can be considered that [AuCl4]-ions are much more difficult than Ag+ to approach POM-1 and get reduced in the hybrid film, as a result of the cooperative interaction of the electrostatic repulsion and the size effect. UV absorption, XPS, TEM, and SEM measurements were employed for the characterization of the obtained metal NPs. From the absorption spectra of the hybrid film containing metal NPs, we can find that the absorption band of POM-1 existed in its reduced state at 700 nm disappears, and in the meantime, the typical plasmon-resonance absorption at 452 nm for Ag NPs and 597 nm for Au NPs appears, respectively (Figure 4b). These spectral features just correspond to the color change of the hybrid film from blue to brown or pink for Ag and Au NPs, suggesting that the reduced POM-1 in the hybrid film turned back to its original high oxidized state and the metal NPs in situ formed in the film. The XPS measurements also confirm the reduction of Ag+ and [AuCl4]- by reduced POM-1 (Figure

Figure 5. TEM micrographs of the hybrid thin films containing (a) Ag and (b) Au NPs prepared by pre-irradiation strategy, and the corresponding size distribution of (c) Ag and (d) Au NPs.

S13). For Ag NPs, the peaks observed at 367.5 and 373.5 eV can be attributed to electron binding energy of Ag 3d5/2 and 3d 3/2 respectively, confirming the formation of metallic Ag. Similarly, the Au 4f7/2 and 4f5/2 signals appear at 83.5 and 86.7 eV, proving the formation of pure Au. Besides that, we also find that the peak belonging to W 4f appears at 39.7 eV, which means that the valence of element W is +6 and no W5+ is detected. That is to say, all of the W5+ took part in reducing metal ions to NPs and was oxidized back to W6+. This result is consistent with the phenomena observed in the absorption spectrum and proves that the reduced POM-1 in the hybrid film can in situ reduce metal ions into a corresponding simple substance, just as we expected. As shown in Figure 5a,b, from TEM measurements we could find that Ag and Au NPs produced by pre-irradiation strategy exhibit almost regular spherical morphology and could exist stably for at least two weeks, although no stabilizers were added to protect them. It is believed that the structure of the hybrid film plays an important role in protecting the metal NPs. The modification of POMs with surfactants bearing amine groups can help in forming spherical metal NPs readily, and the hydrophobic alkyl chains of the surfactant HUDAH can stabilize the metal NPs to some extent just as those that have been reported in the literature.15 The size distribution of metal NPs

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Qi et al.

Figure 6. UV absorption spectra of the (a) Ag and (b) Au NPs growth in hybrid films with different UV irradiation times.

is also estimated based on the statistical results from TEM measurements (Figure 5c,d). The Ag NPs display a diameter of 9.0 ( 4.5 nm (sample population 198), and the Au NPs are 8.5 ( 3.0 nm in diameter (sample population 168). In fact, the dimension statistical result suggests that the size of metal NPs is still not very consistent, and the size distribution is broad, which is probably attributed to the fusion of NPs during the sample preparation. Considering the porous structure of the silica matrix and the uniform distribution of POM-1 in the hybrid film, we believe that the adsorption and reduction of metal ions perform both on the surface and inside of the film. The SEM image of the cross section of the hybrid film containing Ag NPs proves this hypothesis and exhibits nearly even distribution of Ag NPs from top to bottom in the hybrid film (Figure 4c). Besides the pre-irradiation process, Ag and Au NPs also can be synthesized by the pre-adsorption process. Because metal NPs are synthesized in a “dry” state within this route, it provides a much more convenient way for controlling the size of NPs and the patterned growth of NPs in certain locations effectively. In a typical procedure, the hybrid film that had been presoaked in metal ions aqueous solution for a definite time was irradiated by UV light for 1-4 min, and then the metal NPs formed in situ in the hybrid film. It is known that the silver or gold salt is photosensitive and can be reduced directly by UV light. However, this photoreduction process normally needs a strong UV light source and long irradiation time.16 To avoid the direct reduction of metal ions to the utmost extent and to ensure that the metal NPs are synthesized through the reduction of POMs, in the present case, we utilized a gentle light from the fluorescent spectrophotometer as the light source for photoreduction. To prove that the obtained metal NPs are produced by POMs rather than direct UV reduction, we employed a pure silica sol-gel film as a reference for adsorbing and photoreducing metal ions under the same condition. As a result, we found that no typical plasmon-resonance absorption of metal NPs was observed in the absorption spectra of this silica film. That is to say, the metal ions reduced by direct UV irradiation have little influence on our results in the given experimental conditions, and the metal NPs are mostly synthesized from the reduction of POMs. By examining UV absorption spectra of synthesized metal NPs in the hybrid film with regard to the irradiation time (Figure 6a and 6b), we can find that the intensity of plasmon-resonance absorption becomes stronger gradually with the irradiation going along, indicative of the increase of the formed metal NPs. According to our results, 4 min of UV irradiation can make the growth of Ag NPs completed if the adsorption time for Ag+ is controlled at 2 h. The maximal plasmon-resonance absorption

Figure 7. TEM micrographs of the hybrid thin films containing (a) Ag and (b) Au NPs prepared by the pre-adsorption strategy, and the corresponding size distribution of (c) Ag and (d) Au NPs.

Figure 8. Microscopic photos of patterned growth of (a) Ag and (b) Au NPs at preassigned locations using copper grids with round and square holes as masks, respectively.

of the Ag and Au NPs produced by the pre-adsorption strategy appears at 437 and 541 nm, exhibiting 15 and 56 nm of blue shift, respectively, comparing with that produced by preirradiation strategy. That is to say, the size of the metal NPs prepared by pre-adsorption route is smaller than that prepared by pre-irradiation route. As shown in Figure 7, the TEM images of the hybrid films containing Ag and Au NPs also confirm that regularity intuitively, and demonstrate that they are also in a spherical morphology with a diameter of 5.0 ( 1.5 nm (sample population 144) and 6.5 ( 2.0 nm (sample population 186), respectively, which is smaller than that obtained from the preirradiation route. Furthermore, the size distribution of the metal NPs prepared by pre-adsorption method is much narrower than that prepared by pre-irradiation strategy. Since the synthesis of NPs is governed by UV irradiation in the pre-adsorption route, and there no NPs are observed without UV irradiation, as shown by Figure 6a,b; we can simply realize the patterned growth of metal NPs by simple covering the hybrid thin film with a mask bearing a certain pattern in the photoreduction process. For example, during the irradiation of the preadsorbed Ag+ hybrid film, a copper grid is allowed to cover the film surface as a mask. After removing the copper grid, we can see that the areas that have been exposed to the UV light exhibit the typical brown color of the Ag NPs (Figure 8a), suggesting the formation of NPs at the preassigned locations. In contrast to the irradiated places, the covered places are

POM-Based Photochromic Silica Hybrid Films

J. Phys. Chem. B, Vol. 112, No. 28, 2008 8263 as in redox catalyst, and so forth. Our future work will focus on the controllable growth of the metal NPs and the catalytic property of POM-based hybrid films containing metal NPs. Acknowledgment. The authors acknowledge the financial supportfromtheNationalBasicResearchProgram(2007CB808003), the National Natural Science Foundation of China (20574030, 20731160002), PCSIRT of the Ministry of Education of China (IRT0422), and the Open Project of the State Key Laboratory of Polymer Physics and Chemistry of CAS. We also acknowledge the support from the 111 Project (B06009) for the collaboration with Prof. Loodsdrecht at the University of Groningen.

Figure 9. UV absorption spectra of the (a) Ag and (b) Au NPs growth in hybrid films with different metal ion adsorption times.

transparent, and only display the color produced by the light source of the microscope, indicating that no Ag NPs formed at the mask-covered places. That is, Ag NPs only emerge at the areas that experienced irradiation. A similar result is found for Au NPs, and the location of Au NPs can be easily controlled by simple mask blocking (Figure 8b). Moreover, we believe that if the mask is small and sharp enough, more delicate patterns of metal NPs can be achieved, which is significant for the pattern formation based on metal NPs. We also found that the size of metal NPs can be adjusted to a certain extent by tuning the adsorbed amount of metal ions, which is decided by the adsorption time. It can be seen in Figure 9a,b that the band position of plasmon-resonance absorption of Ag and Au NPs shifts 10 nm (from 437 to 447 nm) and 16 nm (from 541 to 557 nm) to the long wavelength, respectively, with the adsorption time increasing, indicative of a size increase of the metal NPs. This result is similar to that found previously in the solution system: the particle size is determined by the molar ratio of metal ions and POMs, and with this molar ratio increasing, the size of the metal NPs increases, accordingly.15 Conclusions In conclusion, we have presented a new method for fabricating reversible, stable and quick-response POM-based photochromic hybrid films through the supramolecular encapsulation of POMs with terminally modified surfactants and consecutively gelation with TEOS. Meanwhile, we have successfully realized the reduction of both cationic and anionic metal ions in the hybrid film with photoreduced POM-1 as reductants, and obtained hybrid films containing both POMs and metal NPs through two different routes. Because of the encapsulation of POMs and their uniform dispersion in the hybrid film, the obtained NPs present a spherical morphology and a considerably uniform distribution. More significantly, we can control the formation of NPs at a preassigned location and tune the size of NPs in a certain extent by a simple mask blocking method and a change of adsorption time, which may be used in the field of construction of patterned NPs. The hybrid films containing POMs and metal NPs may have potential applications in the areas of optical memories, optical displays and switches, as well

Supporting Information Available: The detailed fundamental characterizations for all samples such as EA, TGA, 1H NMR, XPS and FT-IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Irie, M. Chem. ReV. 2000, 100, 1683. (2) (a) Haimov, A.; Cohen, H.; Neuman, R. J. Am. Chem. Soc. 2004, 126, 11762. (b) Yoshida, A.; Yoshimura, M.; Uehara, K.; Hikichi, S.; Mizuno, N. Angew. Chem., Int. Ed. 2006, 45, 1956. (3) Judd, D. A.; Nettles, J. H.; Nevins, N.; Snyder, J. P.; Liotta, D. C.; Tang, J.; Ermolieff, J.; Schinazi, R. F.; Hill, C. L. J. Am. Chem. Soc. 2001, 123, 886. (4) Todea, A. M.; Merca, A.; Bogge, H.; Slageren, J. V.; Dressel, M.; Engelhardt, L.; Luban, M.; Glaser, T.; Henry, M.; Muller, A. Angew. Chem., Int. Ed. 2007, 46, 1. (5) (a) Liu, S.; Kurth, D. G.; Mohwald, H.; Volkmer, D. AdV. Mater. 2002, 14, 225. (b) Liu, S.; Mohwald, H.; Volkmer, D.; Kurth, D. G. Langmuir. 2006, 1949. (c) Zhang, G.; Chen, Z.; He, T.; Ke, H.; Ma, Y.; Shao, K.; Yang, W.; Yao, J. J. Phys. Chem. B. 2004, 108, 6944. (6) (a) Troupis, A.; Hiskia, A.; Papaconstantinou, E. New J. Chem. 2001, 25, 361. (b) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Angew. Chem., Int. Ed. 2002, 41, 1911. (c) Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Sastry, M. J. Am. Chem. Soc. 2003, 125, 8440. (7) (a) Maayan, G.; Neumann, R. Chem. Commun. 2005, 4595. (b) Hetterley, R. D.; Kozhevnikova, E. F.; Kozhevnikov, I. V. Chem. Commun. 2006, 782. (8) (a) Mandal, S.; Rautaray, D.; Sastry, M. J. Mater. Chem. 2003, 13, 3002. (b) Rautaray, D.; Sainkar, S. R.; Sastry, M. Langmuir. 2003, 19, 10095. (9) (a) Jackman, R. J.; Brittain, S. T.; Adams, A.; Prentiss, M. G.; Whitesides, G. M. Science 1998, 280, 2089. (b) Zhang, M.; Lenhert, S.; Wang, M.; Chi, L.; Lu, N.; Fuchs, H.; Ming, N. AdV. Mater. 2004, 16, 409. (10) Qi, W.; Li, H.; Wu, L. AdV. Mater. 2007, 19, 1983. (11) (a) Antonio, M. R.; Soderholm, L. J. Alloys Compd. 1997, 250, 541. (b) Creaser, I.; Heckel, M. C.; Neitz, R. J.; Pope, M. T. Inorg. Chem. 1993, 32, 1573. (12) (a) Volkmer, D.; Du Chesne, A.; Kurth, D. G.; Schnablegger, H.; Lehmann, P.; Koop, M. J.; Muller, A. J. Am. Chem. Soc. 2000, 122, 1995. (b) Bu, W.; Wu, L.; Zhang, X.; Tang, A. C. J. Phys. Chem. B 2003, 107, 13425. (13) (a) Judeinstein, P.; Titman, J.; Stamm, M.; Schmidt, H. Chem. Mater. 1994, 6, 127. (b) Nishio, K.; Okubo, K.; Watanabe, Y.; Tsuchiya, T. J. Sol-Gel Sci. Technol. 2000, 19, 187. (14) (a) Yamase, T. Chem. ReV. 1998, 98, 307. (b) Chen, Z.; Loo, B. H.; Ma, Y.; Gao, Y.; Ibraham, A.; Yao, J. ChemPhysChem 2004, 5, 1020. (15) Yang, L.; Shen, Y. Xie, A.; Zhang, B. J. Phys. Chem. C 2007, 111, 5300. (16) Tan, S.; Erol, M.; Attygalle, A.; Du, H.; Sukhishvili, S. Langmuir. 2007, 23, 9836. (b) Sakka, S.; Kozuka, H. J. Sol-Gel Sci. Technol. 1998, 13, 701.

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