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Nanosized multilayer films with concurrent photochromism and electrochromism based on Dawson-type polyoxometalate
Applied Surface Science 253 (2007) 3190–3195 www.elsevier.com/locate/apsusc
Nanosized multilayer films with concurrent photochromism and electrochromism based on Dawson-type polyoxometalate Bingbing Xu, Lin Xu *, Guanggang Gao, Yana Jin Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun 130024, PR China Received 6 April 2006; received in revised form 22 June 2006; accepted 3 July 2006 Available online 27 October 2006
Abstract An inorganic–organic composite multilayer film constructed of poly(vinyl alcohol) (PVA) with Dawson-type phosphotungstate anion [P2W18O62]6 (P2W18) and poly(allylamine hydrochloride) (PAH) were fabricated on quartz, ITO, silicon and CaF2 substrates by a layer-bylayer self-assembly method. The film was provided with concurrent photochromism and electrochromism. IR spectra showed that the structure of the PVA was fully maintained in the multilayer film. And their photochromic and electrochromic properties were investigated by UV–vis spectra, cyclic voltammetry (CV), chronoamperometry (CA) measurement and X-ray photoelectron spectra (XPS). Atomic force microscopy (AFM) was used to investigate the surface topography. This study provides a new route to explore the possibility of application to polyoxometalate-based hybrid inorganic–organic materials. # 2006 Published by Elsevier B.V. Keywords: Photochromism; Electrochromism; Polyoxometalate; Layer-by-layer
1. Introduction Polyoxometalates (POM) as a well-known class of structurally well-defined metal oxide clusters with wealthy topological, electronic versatilities [1,2], have been applied in many fields such as catalysis [3], molecular conduction [4], magnetism [5], medicine [6], luminescence as well as materials science [7]. One of the most promising properties of these metal oxide clusters is that they can accept electrons to become mixed-valence colored species namely ‘‘heteropoly blue’’. And this excellent property makes them good candidates for photochromic and electrochromic materials [8]. In recent years, studies on separate photochromic or electrochromic properties of hybrid materials were well performed since their potential application in information display devices [9]. At present, several kinds of inorganic–organic photochromic ultrathin films have been prepared by self-assembly of polyoxometalates and organic amines [10–12], and so do the electrochromic ultra-thin films based on polyoxometalates and polyelectrolyte [13–16]. But a combination of photo-
* Corresponding author. Tel.: +86 431 5099668; fax: +86 431 5099668. E-mail address: [email protected] (L. Xu). 0169-4332/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.apsusc.2006.07.034
chromism and electrochromism based on polyoxometalates has been investigated rarely [17]. Such combination should be of considerable significance for developing polyoxometalatebased functional materials, which makes the system ideally suited as the basis for optical and electronic memory system with multiple storage, anti-counterfeiting label paper, smart windows and chemical sensors, etc. [18–22]. Therefore, the design and fabrication of polyoxometalate-based ultra-thin film materials with concurrent photochromism and electrochromism have become a current challenge. In despite of enormous progress, the traditional methods to prepare films still show drawbacks in the way of high cost and complicated device fabrication such as vacuum deposition, chemical vapor deposition and sputtering method. However, Decher and Hong developed a simpler and more versatile technique to prepare thin films of defined thickness and composition, generally named as the layer-by-layer (LBL) self-assembly method [23]. It has now been extensively applied in chemistry and material science as an effective strategy for fabricating a wide variety of ordered film structures, which are impossible or difficult to prepare in conventional ways [24–26]. To date, we have placed Dawson-type phosphotungstates in polyelectrolyte multilayer by means of LBL method [16]. In this work, we managed to keep the favorite properties of each of
B. Xu et al. / Applied Surface Science 253 (2007) 3190–3195
the POM and PVA while eliminate their some limitations, and we successfully prepared PVA–POM (PVA and Dawson-type of [P2W18O62]6 anion) multilayer films with concurrent photochromism and electrochromism by LBL self-assembly technique. The layered nanocomposite films were studied by UV–vis absorption, cyclic voltammetry (CV), chronoamperometry (CA) measurement, X-ray photoelectron spectra (XPS) and atomic force microscopy (AFM). 2. Experimental details 2.1. Preparation of the aqueous solution of hybrid PVA–P2W18 After addition of 0.01 g poly(vinyl alcohol) (PVA) (MW 86,000, analytical reagent, A.R.) powder into 40 ml deionized water, the mixture solution was heated at 80 8C with stirring until absolutely dissolved. Then 0.12 g K6P2W18O6214H2O (P2W18) prepared according to the literature method [27] was added into the solution with stirring. Till the solution was evaporated to about 30 ml, the solution was cooled to room temperature. 2.2. Preparation of the [PVA–P2W18/PAH] films The [PVA–P2W18/PAH] self-assembly films were fabricated by a literature method [28,29]. 3-Aminopropyltrimethoxysilane (APS) modified substrates (quartz or ITO glass) were immersed into 0.1 M HCl solution to get an amino cation surface, and then immersed into poly(stryenesulfonate) (PSS; MW 70,000) and poly(allylamine hydrochloride) (PAH; MW 70,000) (1 103 M) solution for 20 min, respectively (Fig. 1). After that they were rinsed with deionized water and dried in nitrogen after each immersion. Then the substratesupported precursor film was alternately dipped into the PVA– P2W18 and PAH solution for 5 min, rinsed with deionized water and dried in nitrogen stream after each dipping. Repeat the above steps until the desired number of bilayers of [PVA–P2W18/PAH] was achieved. The film preparation was performed in dark to avoid light irradiation. Scheme 1 displays a schematic illustration of the self-assembly multilayer films
Fig. 1. Structure of P2W18, 3-aminopropyltrimethoxysilane (1), poly(allylamine hydrochloride) (2) and poly(stryenesulfonate) (3).
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and the exchange between bleaching state and colored state of the films using either electro- or photoinduced stimulation. 2.3. Characterization FT-IR spectrogram was measured with Perkin-Elmer 580B infrared spectrophotometer. UV–vis absorption spectra of quartz- and ITO-supported films were recorded on a 756 CRT UV–vis spectrophotometer. The cyclic voltammetric and chronoamperometry measurements were carried out in 0.1 M HOAc–NaOAc buffer solution (pH 4.0) at ambient temperature (25 8C). X-ray photoelectron spectra (XPS) were performed on an Escalab MKII photoelectron spectrometer with ALK2 (1486.6 eV) as the excitation source. Atomic force microscopy (AFM) image was taken on ITO substrate using a SPVA 400 instrument. The light source was a 125 W high-pressure mercury lamp (HPML, output mainly at 313.2 nm). 3. Results and discussion 3.1. Photochromic behavior of the composite film The substrate is modified with a precursor APS/PSS/PAH. The layer-by-layer self-assembly of the anionic [PVA–P2W18] and the cationic PAH onto the positive surface of the film basically depends upon the electrostatic attraction between the oppositely charged species. It is essential for the film to achieve a more homogeneous, positive charge distribution on the surface of the substrate so as to keep the subsequent reproducible deposition [30]. It is also a necessary performance to rinse the films with deionized water and then dry with nitrogen after each adsorption step. With the fabrication procedure above mentioned, highly reproducible films with controlled thickness can be obtained. The growth process of the self-assembled films on quartz substrates was monitored by UV–vis spectra. Fig. 2 shows the UV–vis spectra of [PVA–P2W18/PAH]n multilayer (n = 0–20) films with organic polymer PAH as the outermost layer. The inset displays the plots of the absorbance values for quartz-supported [PVA–P2W18/PAH]n multilayer (n = 0–20) films at 200 nm as a function of the number of PVA–P2W18/PAH bilayers. As shown in Fig. 2, these films all exhibit the characteristic absorption of the hybrid PVA–P2W18 in the UV–vis region with characteristic bands at 200 and 280 nm. The former is owing to the terminal oxygen to tungsten (Od ! W) charge transfer transitions, and the latter is due to the charge transfer transitions from bridge-oxygen to tungsten (Ob/Oc ! W) [31]. And the slight red shift is due to the strong electrostatic attraction between the anionic [PVA–P2W18] and the cationic PAH. At the same time, this also confirms that the P2W18 still keep the Dawson–Wells structure in the composite films. Fig. 3 shows the IR spectra of the [PVA–P2W18/PAH]50 composite films. The bands between 3500 and 3600 cm1 are the characteristic bands of O–H group. The very strong bands below 1200 cm1 belong to the [P2W18O62]6. The bands in the films of the anion are slightly red shifted. It means that intermolecular H-bonding has been formed between POM and PVA.
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Scheme 1. [PVA–P2W18/PAH] multilayer films can be colored by either irradiation with UV light or electrochemical induce. And the film can be bleached with visible light or oxidation.
Typical absorption spectra of the [PVA–P2W18/PAH]50 composite films before and after UV irradiation are displayed in Fig. 4. A visually noticeable optical contrast at 650 nm manifests clear photochromism of the films. Before UV irradiation, no characteristic absorption bands of the
Fig. 2. UV spectrum of [PVA–P2W18/PAH]n films with n = 0–20 on the precursor film-modified quartz substrate (on both sides); the inset shows plots of the absorbance values at 200 and 280 nm.
Fig. 3. IR spectra of [PVA–P2W18/PAH]50 films.
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Scheme 2. The model of the photoreduced site and the process of photoreduced reaction. Fig. 4. Spectral change at 400–800 nm upon UV irradiation of 30 min of the [PVA–P2W18/PAH]50 multilayer films at room temperature.
[PVA–P2W18/PAH]50 composite films appear in visible region, because separate PVA, P2W18, PAH and PSS do not show absorption bands in visible region. After UV light irradiation, a broad absorption band is observed and the color of the films change from transparent to deep blue with increasing irradiation time. When the UV light is taken off, the films began to bleach gradually under ambient conditions. The photochromism of the films is similar to that reported in previous publication and the coloring/bleaching is reversible at room temperature [31–33]. We infer thus the occurrence of intermolecular oxidation– reduction reaction. Photoexcitation of the O W in WO6 leads to the transfer of one hydrogen from PVA to the oxygen atom at the photoreduced site in the edge-shared WO6 octahedral lattice, so that the POM is reduced and the PVA is oxidized through radical process. The change of color from deep blue to light blue suggests an unstable oxidation state for the organic matrix. Fig. 5 shows the change curve of the bleaching time. Scheme 2 denotes the environment of the photoreduced site and the process of photoreduced reaction. 3.2. Electrochromic behavior of the composite film Cyclic voltammograms (CV) of 1 mM P2W18 aqueous solution (in 0.1 M HOAc–NaOAc buffer at pH 4), using a bare ITO-coated glass electrode (immersion area 1.0 1.0 cm2), shows four anodic peaks of A1 = 0.1060 V, A2 = 0.0713 V,
Fig. 5. The decaying curve of the absorbance (At/A0) at 650 nm of the irradiated [PVA–P2W18/PAH]50 multilayer films at room temperature.
A3 = 0.4622 V and A4 = 0.7256 V, and four cathodic peaks of C1 = 0.0214 V, C2 = 0.2128 V, C3 = 0.6786 V and C4 = 1.000 V (see Fig. 6 dotted line). The C1/A1, C2/A2 and C3/A3 pairs of peaks correspond to 1e/1H+, 1e/1H+ and 2e/2H+, respectively. The last pair of peaks C4/A4 corresponds to irreversible process in HOAc–NaOAc buffer [34]. Cyclic voltammograms of the [PVA–P2W18/PAH]50-modified ITO electrode, with immersion area of 1.0 1.0 cm2 in HOAc– NaOAc buffer solution (pH 4.0), using a platinum coil as the counter electrode and Ag/AgCl/KCl (3 M) as reference electrode, displays three pairs of anodic/cathodic peaks (see Fig. 6, solid line). The peak potential values of a1 = 0.2013 V, a2 = 0.3737 V, a3 = 0.7451 V and their counterparts of c1 = 0.1242 V, c2 = 0.6896 V and c3 = 0.9920 V have some differences compared with the peaks of 1 103 M P2W18 solution. However, in comparison with the first and second pair of peaks of the P2W18 solution, we can only observe one pair of broad peaks for [PVA–P2W18/PAH] multilayer films. This may result from the possible interaction between P2W18 and PAH. Additionally, the protonation plays an important role in the charge compensation of the electrochromic self-assembly films [35]. When protons are difficult to incorporate into the reduction process in a selfassembly film, the neighboring two cathodic peaks will be combined into one [14]. Similarly, deprotonation of reduced
Fig. 6. Cyclic voltammograms of 1 mM P2W18 in solution (dotted line, ITOcoated glass as working electrode) and a [PVA–P2W18/PAH]50-modified ITO electrode in HOAc–NaOAc buffer solution (solid line) [platinum coil as the counter electrode and Ag/AgCl/KCl (3 M) as reference electrode with a scan rate = 50 mV s1].
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Fig. 8. Tungsten atom peaks (W 4f7/2 and W 4f5/2) in the X-ray photoelectron spectrum for the [PVA–P2W18/PAH]50 films: (a) before colored and (b) after light colored.
Fig. 7. AFM images for the [PVA–P2W18/PAH]1 film on silicon matrix substrate.
order to identify the elemental composition and the binding energy of the atoms. Although the XPS measurement gives only semi-quantitative elemental composition, the presence of P, W, C, O and N elements in the films can be confirmed by the observed signal peaks, and the expected molar ratio of 2:18 for P to W, also approximately established. The XPS peaks of W element before coloration and after light irradiation are shown in Fig. 4 for the films. The alternation of the spectrum in (a) compared to the spectrum in (b) is due to the overlap in binding energy of the W 4f states, namely, the tungsten atom is reduced (Figs. 7–9).
species also depends on a more positive potential, which may make the adjacent two anodic peaks one. The CV curves of P2W18 in solution and the multilayer films demonstrate that the electrochromic properties of P2W18 are fully maintained in the multilayer films. The response time of the films was recorded by doublepotential experiment as well as the absorbance measurement. The coloration (tc) and bleaching (tb) time was 8 and 11 s, respectively, and the maximum absorbance at about 650 nm increased by 0.36 U. At the same time, the electrochromic reversibility of [PVA–P2W18/PAH]50 films was evaluated by performing repetitive double potential steps from 800 to 800 mV. 3.3. XPS spectra The X-ray photoelectron spectra (XPS) measurements on the [PVA–P2W18/PAH]50 multilayer films were carried out in
Fig. 9. Potential current and absorbance at 650 nm of the [PVA–P2W18/ PAH]50-modified ITO during subsequent double-potential steps (800 to 800 mV).
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3.4. Morphology of the film AFM image of a [PVA–P2W18/PAH] film shows a uniform distribution of the aggregate nanoclusters on uniform surface. This should be explained not only as a result of PAH deposition but also as an indirect reflection of PVA aggregation. According to the AFM image, the film thickness could be estimated from the height of valley-to-peak to be about 3 nm, a sum of both the surface thickness (2.5 nm) and the nanocluster height (3 nm). 4. Conclusion Both photo- and electrochromic multilayer films consisting of poly(vinyl alcohol), Dawson-type polyoxometalate and poly(allylamine hydrochloride) have been prepared successfully by self-assembly method. The composite films can be photochromic and electrochromic from transparent to deep blue by ether irradiation with UV light or electrochemical induce, and the photochromism and electrochromism is reversible. Furthermore, this work shows a promising way to develop functional ultrathin film materials with concurrent photochromism and electrochromism based on polyoxometalates. The fabricated composite films can keep their fine properties from each component as well as reduce some particular limitations. Moreover, the dual-mode film material has the possibility of practical application in a wide field such as anti-counterfeiting technology, sensors, data storage, smart window, etc. Acknowledgements The authors are thankful for the financial supports from the National Natural Science Foundation of China (grant no. 20371010) and the Natural Science Foundation of Jilin Technology Office of China (no. 20030512-1). References [1] M.T. Pope, A. Mu¨ller, Polyoxometalates: From Platonic Solid to Antiretroviral Activity, Kluwer Academic Publishers, Dordrecht, 1994. [2] C.L. Hill, Chem. Rev. 98 (1998) 1. [3] O.V. Branytska, R. Neumann, J. Org. Chem. 68 (2003) 9510 (Technical note).
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[4] E. Coronado, J.R. Gala´n-Mascaro´s, C. Gime´nez-Saiz, C.J. Go´mez-Garcı´a, E. Martı´nez-Ferrero, M. Almeida, E.B. Lopes, Adv. Mater. 16 (2004) 324. [5] E. Coronado, M. Clemente-Leo´n, J.R. Gala´n-Mascaro´s, C. Gime´nez-Saiz, E. Martı´nez-Ferrero, J. Chem. Soc., Dalton Trans. 21 (2000) 3955. [6] F. Busserean, M. Piacard, C. Malik, A. Teze, J. Blancen, Ann. Inst. Pasteur/Virol. 32 (1988) 33. [7] L. Cheng, J.A. Cox, Chem. Mater. 14 (2002) 6. [8] T. Yamase, Chem. Rev. 98 (1998) 307. [9] C.J. Schoot, J.J. Ponjee, H.T. van Dam, R.A. van Doorn, P.T. Bolwijn, Appl. Phys. Lett. 23 (1973) 64. [10] F. Arnaud-Neu, M. Schwing-Weill, J. Bull. Soc. Chim. Fr. (1973) 3225. [11] T.R. Zhang, W. Feng, Y.Q. Fu, R. Lu, C.Y. Bao, X.T. Zhang, B. Zhao, C.Q. Sun, T.J. Li, Y.Y. Zhao, J.N. Yao, J. Mater. Chem. 12 (2002) 1453. [12] M. Jiang, E.B. Wang, G. Wei, L. Xu, Z.H. Kang, Z. Li, New J. Chem. 27 (9) (2003) 1291. [13] S.Q. Liu, D.G. Kurth, H. Mo¨hwald, D. Volkmer, Adv. Mater. 14 (2002) 225. [14] G.G. Gao, L. Xu, W.J. Wang, W.J. An, Y.F. Qiu, J. Mater. Chem. 14 (2004) 2024. [15] G.G. Gao, L. Xu, W.J. Wang, W.J. An, Y.F. Qiu, Z.Q. Wang, E.B. Wang, J. Phys. Chem. B. 109 (18) (2005) 8948. [16] G.G. Gao, L. Xu, W.J. Wang, Z.Q. Wang, Y.F. Qiu, E.B. Wang, Electrochim. Acta 50 (2005) 1101. [17] S.Q. Liu, H. Mo¨hwald, D. Volkmer, D.G. Kurth, Langmuir 22 (2006) 1949. [18] S. Kawata, Y. Kawata, Chem. Rev. 100 (2000) 1777. [19] D.R. Rosseinsky, R.J. Mortimer, Adv. Mater. 13 (2001) 783. [20] O. Kyeongsik, J. Namju, K. Jaeho, Synth. Met. 117 (2001) 131. [21] C.M. Lampert, Mater. Today 3 (2004) 28. [22] R. Stewart, L. Li, D. Thomas, J. Mater. Process. Technol. 114 (2001) 161. [23] G. Decher, J.D. Hong, Ber. Bunsenges. Phys. Chem. 95 (1991) 1430. [24] I. Ichinose, T. Kawakami, T. Kunitake, Adv. Mater. 10 (1998) 535. [25] S.Y. Oh, Y.J. Yun, D.Y. Kim, S.H. Han, Langmuir 15 (1999) 4690. [26] M. Dreja, I.T. Kim, Y.D. Yin, Y.N. Xia, J. Mater. Chem. 10 (2000) 603. [27] N. Haraguchi, Y. Okaue, T. Isobe, Y. Matsuda, Inorg. Chem. 33 (1994) 1015. [28] Z.H. Chen, Y.A. Yang, J.B. Qiu, J.N. Yao, Langmuir 16 (2000) 722. [29] Z.H. Chen, Y. Ma, T. He, R.M. Xie, K. Shao, W.S. Yang, J.N. Yao, New J. Chem. 26 (2002) 621. [30] I. Ichinose, H. Tagawa, S. Mizuki, Y. Lvov, T. Kunitake, Langmuir 14 (1998) 187. [31] J. Gong, Y.G. Chen, L.Y. Qu, Polyhedron 15 (1996) 2273. [32] J. Gong, X.D. Li, C.L. Shao, B. Ding, D.R. Lee, H.Y. Kim, Mater. Chem. Phys. 79 (2003) 87. [33] G.C. Yang, Y. Pan, F.M. Gao, J.S. Park, J. Gong, Mater. Lett. 59 (2004) 450. [34] M. Sadakane, E. Steckhan, Chem. Rev. 98 (1998) 219. [35] J.H. Isamu Moriguchi, Fendler, Chem. Mater. 10 (1998) 2205.
Nanosized multilayer films with concurrent photochromism and electrochromism based on Dawson-type polyoxometalate Bingbing Xu, Lin Xu *, Guanggang Gao, Yana Jin Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun 130024, PR China Received 6 April 2006; received in revised form 22 June 2006; accepted 3 July 2006 Available online 27 October 2006
Abstract An inorganic–organic composite multilayer film constructed of poly(vinyl alcohol) (PVA) with Dawson-type phosphotungstate anion [P2W18O62]6 (P2W18) and poly(allylamine hydrochloride) (PAH) were fabricated on quartz, ITO, silicon and CaF2 substrates by a layer-bylayer self-assembly method. The film was provided with concurrent photochromism and electrochromism. IR spectra showed that the structure of the PVA was fully maintained in the multilayer film. And their photochromic and electrochromic properties were investigated by UV–vis spectra, cyclic voltammetry (CV), chronoamperometry (CA) measurement and X-ray photoelectron spectra (XPS). Atomic force microscopy (AFM) was used to investigate the surface topography. This study provides a new route to explore the possibility of application to polyoxometalate-based hybrid inorganic–organic materials. # 2006 Published by Elsevier B.V. Keywords: Photochromism; Electrochromism; Polyoxometalate; Layer-by-layer
1. Introduction Polyoxometalates (POM) as a well-known class of structurally well-defined metal oxide clusters with wealthy topological, electronic versatilities [1,2], have been applied in many fields such as catalysis [3], molecular conduction [4], magnetism [5], medicine [6], luminescence as well as materials science [7]. One of the most promising properties of these metal oxide clusters is that they can accept electrons to become mixed-valence colored species namely ‘‘heteropoly blue’’. And this excellent property makes them good candidates for photochromic and electrochromic materials [8]. In recent years, studies on separate photochromic or electrochromic properties of hybrid materials were well performed since their potential application in information display devices [9]. At present, several kinds of inorganic–organic photochromic ultrathin films have been prepared by self-assembly of polyoxometalates and organic amines [10–12], and so do the electrochromic ultra-thin films based on polyoxometalates and polyelectrolyte [13–16]. But a combination of photo-
* Corresponding author. Tel.: +86 431 5099668; fax: +86 431 5099668. E-mail address: [email protected] (L. Xu). 0169-4332/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.apsusc.2006.07.034
chromism and electrochromism based on polyoxometalates has been investigated rarely [17]. Such combination should be of considerable significance for developing polyoxometalatebased functional materials, which makes the system ideally suited as the basis for optical and electronic memory system with multiple storage, anti-counterfeiting label paper, smart windows and chemical sensors, etc. [18–22]. Therefore, the design and fabrication of polyoxometalate-based ultra-thin film materials with concurrent photochromism and electrochromism have become a current challenge. In despite of enormous progress, the traditional methods to prepare films still show drawbacks in the way of high cost and complicated device fabrication such as vacuum deposition, chemical vapor deposition and sputtering method. However, Decher and Hong developed a simpler and more versatile technique to prepare thin films of defined thickness and composition, generally named as the layer-by-layer (LBL) self-assembly method [23]. It has now been extensively applied in chemistry and material science as an effective strategy for fabricating a wide variety of ordered film structures, which are impossible or difficult to prepare in conventional ways [24–26]. To date, we have placed Dawson-type phosphotungstates in polyelectrolyte multilayer by means of LBL method [16]. In this work, we managed to keep the favorite properties of each of
B. Xu et al. / Applied Surface Science 253 (2007) 3190–3195
the POM and PVA while eliminate their some limitations, and we successfully prepared PVA–POM (PVA and Dawson-type of [P2W18O62]6 anion) multilayer films with concurrent photochromism and electrochromism by LBL self-assembly technique. The layered nanocomposite films were studied by UV–vis absorption, cyclic voltammetry (CV), chronoamperometry (CA) measurement, X-ray photoelectron spectra (XPS) and atomic force microscopy (AFM). 2. Experimental details 2.1. Preparation of the aqueous solution of hybrid PVA–P2W18 After addition of 0.01 g poly(vinyl alcohol) (PVA) (MW 86,000, analytical reagent, A.R.) powder into 40 ml deionized water, the mixture solution was heated at 80 8C with stirring until absolutely dissolved. Then 0.12 g K6P2W18O6214H2O (P2W18) prepared according to the literature method [27] was added into the solution with stirring. Till the solution was evaporated to about 30 ml, the solution was cooled to room temperature. 2.2. Preparation of the [PVA–P2W18/PAH] films The [PVA–P2W18/PAH] self-assembly films were fabricated by a literature method [28,29]. 3-Aminopropyltrimethoxysilane (APS) modified substrates (quartz or ITO glass) were immersed into 0.1 M HCl solution to get an amino cation surface, and then immersed into poly(stryenesulfonate) (PSS; MW 70,000) and poly(allylamine hydrochloride) (PAH; MW 70,000) (1 103 M) solution for 20 min, respectively (Fig. 1). After that they were rinsed with deionized water and dried in nitrogen after each immersion. Then the substratesupported precursor film was alternately dipped into the PVA– P2W18 and PAH solution for 5 min, rinsed with deionized water and dried in nitrogen stream after each dipping. Repeat the above steps until the desired number of bilayers of [PVA–P2W18/PAH] was achieved. The film preparation was performed in dark to avoid light irradiation. Scheme 1 displays a schematic illustration of the self-assembly multilayer films
Fig. 1. Structure of P2W18, 3-aminopropyltrimethoxysilane (1), poly(allylamine hydrochloride) (2) and poly(stryenesulfonate) (3).
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and the exchange between bleaching state and colored state of the films using either electro- or photoinduced stimulation. 2.3. Characterization FT-IR spectrogram was measured with Perkin-Elmer 580B infrared spectrophotometer. UV–vis absorption spectra of quartz- and ITO-supported films were recorded on a 756 CRT UV–vis spectrophotometer. The cyclic voltammetric and chronoamperometry measurements were carried out in 0.1 M HOAc–NaOAc buffer solution (pH 4.0) at ambient temperature (25 8C). X-ray photoelectron spectra (XPS) were performed on an Escalab MKII photoelectron spectrometer with ALK2 (1486.6 eV) as the excitation source. Atomic force microscopy (AFM) image was taken on ITO substrate using a SPVA 400 instrument. The light source was a 125 W high-pressure mercury lamp (HPML, output mainly at 313.2 nm). 3. Results and discussion 3.1. Photochromic behavior of the composite film The substrate is modified with a precursor APS/PSS/PAH. The layer-by-layer self-assembly of the anionic [PVA–P2W18] and the cationic PAH onto the positive surface of the film basically depends upon the electrostatic attraction between the oppositely charged species. It is essential for the film to achieve a more homogeneous, positive charge distribution on the surface of the substrate so as to keep the subsequent reproducible deposition [30]. It is also a necessary performance to rinse the films with deionized water and then dry with nitrogen after each adsorption step. With the fabrication procedure above mentioned, highly reproducible films with controlled thickness can be obtained. The growth process of the self-assembled films on quartz substrates was monitored by UV–vis spectra. Fig. 2 shows the UV–vis spectra of [PVA–P2W18/PAH]n multilayer (n = 0–20) films with organic polymer PAH as the outermost layer. The inset displays the plots of the absorbance values for quartz-supported [PVA–P2W18/PAH]n multilayer (n = 0–20) films at 200 nm as a function of the number of PVA–P2W18/PAH bilayers. As shown in Fig. 2, these films all exhibit the characteristic absorption of the hybrid PVA–P2W18 in the UV–vis region with characteristic bands at 200 and 280 nm. The former is owing to the terminal oxygen to tungsten (Od ! W) charge transfer transitions, and the latter is due to the charge transfer transitions from bridge-oxygen to tungsten (Ob/Oc ! W) [31]. And the slight red shift is due to the strong electrostatic attraction between the anionic [PVA–P2W18] and the cationic PAH. At the same time, this also confirms that the P2W18 still keep the Dawson–Wells structure in the composite films. Fig. 3 shows the IR spectra of the [PVA–P2W18/PAH]50 composite films. The bands between 3500 and 3600 cm1 are the characteristic bands of O–H group. The very strong bands below 1200 cm1 belong to the [P2W18O62]6. The bands in the films of the anion are slightly red shifted. It means that intermolecular H-bonding has been formed between POM and PVA.
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Scheme 1. [PVA–P2W18/PAH] multilayer films can be colored by either irradiation with UV light or electrochemical induce. And the film can be bleached with visible light or oxidation.
Typical absorption spectra of the [PVA–P2W18/PAH]50 composite films before and after UV irradiation are displayed in Fig. 4. A visually noticeable optical contrast at 650 nm manifests clear photochromism of the films. Before UV irradiation, no characteristic absorption bands of the
Fig. 2. UV spectrum of [PVA–P2W18/PAH]n films with n = 0–20 on the precursor film-modified quartz substrate (on both sides); the inset shows plots of the absorbance values at 200 and 280 nm.
Fig. 3. IR spectra of [PVA–P2W18/PAH]50 films.
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Scheme 2. The model of the photoreduced site and the process of photoreduced reaction. Fig. 4. Spectral change at 400–800 nm upon UV irradiation of 30 min of the [PVA–P2W18/PAH]50 multilayer films at room temperature.
[PVA–P2W18/PAH]50 composite films appear in visible region, because separate PVA, P2W18, PAH and PSS do not show absorption bands in visible region. After UV light irradiation, a broad absorption band is observed and the color of the films change from transparent to deep blue with increasing irradiation time. When the UV light is taken off, the films began to bleach gradually under ambient conditions. The photochromism of the films is similar to that reported in previous publication and the coloring/bleaching is reversible at room temperature [31–33]. We infer thus the occurrence of intermolecular oxidation– reduction reaction. Photoexcitation of the O W in WO6 leads to the transfer of one hydrogen from PVA to the oxygen atom at the photoreduced site in the edge-shared WO6 octahedral lattice, so that the POM is reduced and the PVA is oxidized through radical process. The change of color from deep blue to light blue suggests an unstable oxidation state for the organic matrix. Fig. 5 shows the change curve of the bleaching time. Scheme 2 denotes the environment of the photoreduced site and the process of photoreduced reaction. 3.2. Electrochromic behavior of the composite film Cyclic voltammograms (CV) of 1 mM P2W18 aqueous solution (in 0.1 M HOAc–NaOAc buffer at pH 4), using a bare ITO-coated glass electrode (immersion area 1.0 1.0 cm2), shows four anodic peaks of A1 = 0.1060 V, A2 = 0.0713 V,
Fig. 5. The decaying curve of the absorbance (At/A0) at 650 nm of the irradiated [PVA–P2W18/PAH]50 multilayer films at room temperature.
A3 = 0.4622 V and A4 = 0.7256 V, and four cathodic peaks of C1 = 0.0214 V, C2 = 0.2128 V, C3 = 0.6786 V and C4 = 1.000 V (see Fig. 6 dotted line). The C1/A1, C2/A2 and C3/A3 pairs of peaks correspond to 1e/1H+, 1e/1H+ and 2e/2H+, respectively. The last pair of peaks C4/A4 corresponds to irreversible process in HOAc–NaOAc buffer [34]. Cyclic voltammograms of the [PVA–P2W18/PAH]50-modified ITO electrode, with immersion area of 1.0 1.0 cm2 in HOAc– NaOAc buffer solution (pH 4.0), using a platinum coil as the counter electrode and Ag/AgCl/KCl (3 M) as reference electrode, displays three pairs of anodic/cathodic peaks (see Fig. 6, solid line). The peak potential values of a1 = 0.2013 V, a2 = 0.3737 V, a3 = 0.7451 V and their counterparts of c1 = 0.1242 V, c2 = 0.6896 V and c3 = 0.9920 V have some differences compared with the peaks of 1 103 M P2W18 solution. However, in comparison with the first and second pair of peaks of the P2W18 solution, we can only observe one pair of broad peaks for [PVA–P2W18/PAH] multilayer films. This may result from the possible interaction between P2W18 and PAH. Additionally, the protonation plays an important role in the charge compensation of the electrochromic self-assembly films [35]. When protons are difficult to incorporate into the reduction process in a selfassembly film, the neighboring two cathodic peaks will be combined into one [14]. Similarly, deprotonation of reduced
Fig. 6. Cyclic voltammograms of 1 mM P2W18 in solution (dotted line, ITOcoated glass as working electrode) and a [PVA–P2W18/PAH]50-modified ITO electrode in HOAc–NaOAc buffer solution (solid line) [platinum coil as the counter electrode and Ag/AgCl/KCl (3 M) as reference electrode with a scan rate = 50 mV s1].
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Fig. 8. Tungsten atom peaks (W 4f7/2 and W 4f5/2) in the X-ray photoelectron spectrum for the [PVA–P2W18/PAH]50 films: (a) before colored and (b) after light colored.
Fig. 7. AFM images for the [PVA–P2W18/PAH]1 film on silicon matrix substrate.
order to identify the elemental composition and the binding energy of the atoms. Although the XPS measurement gives only semi-quantitative elemental composition, the presence of P, W, C, O and N elements in the films can be confirmed by the observed signal peaks, and the expected molar ratio of 2:18 for P to W, also approximately established. The XPS peaks of W element before coloration and after light irradiation are shown in Fig. 4 for the films. The alternation of the spectrum in (a) compared to the spectrum in (b) is due to the overlap in binding energy of the W 4f states, namely, the tungsten atom is reduced (Figs. 7–9).
species also depends on a more positive potential, which may make the adjacent two anodic peaks one. The CV curves of P2W18 in solution and the multilayer films demonstrate that the electrochromic properties of P2W18 are fully maintained in the multilayer films. The response time of the films was recorded by doublepotential experiment as well as the absorbance measurement. The coloration (tc) and bleaching (tb) time was 8 and 11 s, respectively, and the maximum absorbance at about 650 nm increased by 0.36 U. At the same time, the electrochromic reversibility of [PVA–P2W18/PAH]50 films was evaluated by performing repetitive double potential steps from 800 to 800 mV. 3.3. XPS spectra The X-ray photoelectron spectra (XPS) measurements on the [PVA–P2W18/PAH]50 multilayer films were carried out in
Fig. 9. Potential current and absorbance at 650 nm of the [PVA–P2W18/ PAH]50-modified ITO during subsequent double-potential steps (800 to 800 mV).
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3.4. Morphology of the film AFM image of a [PVA–P2W18/PAH] film shows a uniform distribution of the aggregate nanoclusters on uniform surface. This should be explained not only as a result of PAH deposition but also as an indirect reflection of PVA aggregation. According to the AFM image, the film thickness could be estimated from the height of valley-to-peak to be about 3 nm, a sum of both the surface thickness (2.5 nm) and the nanocluster height (3 nm). 4. Conclusion Both photo- and electrochromic multilayer films consisting of poly(vinyl alcohol), Dawson-type polyoxometalate and poly(allylamine hydrochloride) have been prepared successfully by self-assembly method. The composite films can be photochromic and electrochromic from transparent to deep blue by ether irradiation with UV light or electrochemical induce, and the photochromism and electrochromism is reversible. Furthermore, this work shows a promising way to develop functional ultrathin film materials with concurrent photochromism and electrochromism based on polyoxometalates. The fabricated composite films can keep their fine properties from each component as well as reduce some particular limitations. Moreover, the dual-mode film material has the possibility of practical application in a wide field such as anti-counterfeiting technology, sensors, data storage, smart window, etc. Acknowledgements The authors are thankful for the financial supports from the National Natural Science Foundation of China (grant no. 20371010) and the Natural Science Foundation of Jilin Technology Office of China (no. 20030512-1). References [1] M.T. Pope, A. Mu¨ller, Polyoxometalates: From Platonic Solid to Antiretroviral Activity, Kluwer Academic Publishers, Dordrecht, 1994. [2] C.L. Hill, Chem. Rev. 98 (1998) 1. [3] O.V. Branytska, R. Neumann, J. Org. Chem. 68 (2003) 9510 (Technical note).
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