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A multifunctional organic–inorganic multilayer film based on tris(1,10-phenanthroline)ruthenium and polyoxometalate
Electrochimica Acta 51 (2006) 4965–4970
A multifunctional organic–inorganic multilayer film based on tris(1,10-phenanthroline)ruthenium and polyoxometalate Huiyuan Ma a,b,∗ , Tao Dong b , Fuping Wang a,∗ , Wei Zhang b , Baibin Zhou b a
Postdoctoral Station of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China b Chemistry Department of Harbin Normal University, Harbin 150025, PR China Received 24 November 2005; received in revised form 22 January 2006; accepted 24 January 2006 Available online 28 February 2006
Abstract The novel multifunctional thin films composed of transition metal complex tris(1,10-phenanthroline)ruthenium(II) Ru(phen)3 Cl2 (abbr Ru(phen)3 , phen = 1,10-phenanthroline), and Dawson-type polyoxometalate [P2 Mo18 O62 ]6− (abbr P2 Mo18 ) were fabricated on quartz, silicon and ITO substrates by layer-by-layer (LBL) method. The LBL films deposition were found to be linearly related to the number of bilayers as monitored by UV–vis spectroscopy. And the compositions of the films were measured by X-ray photoelectron spectra (XPS). The result of atomic force microscopy (AFM) revealed a relatively uniform surface morphology of the multilayer films. In particular, the films exhibited the photo-luminescence arising from * –t2g ligand-to-metal transition of Ru(phen)3 and catalytic activity to the reduction of NO2 − attributing to molybdenum-centered redox processes of P2 Mo18 . © 2006 Elsevier Ltd. All rights reserved. Keywords: Layer-by-layer self-assembly; Polyoxometalate; Luminescent film; Ruthenium; Electrocatalysis
1. Introduction Polyoxometalates (POMs) represent a well-defined class of inorganic compounds with remarkable structural, and electronic versatility [1–3], and possess diverse properties such as catalytic activity, molecule-based conductivity, magnetism, photochromism, electrochromism and luminescence, etc. [4–7]. However, practical applications of polyoxometalates in these areas depend on the successful preparation of thin polyoxometalate-containing films [8,9]. The layer-by-layer (LBL) method, being based on sequential adsorption of oppositely charged species from dilute solutions and relying primarily on electrostatic attraction of the oppositely charged components, provides a facile synthetic route towards uniform POM based multilayer films [10–13]. This method permits the combination of POMs with many other functional components as part of the polycationic macromolecules with good control over the layer composition and thickness, which renders them potentially
∗
Corresponding authors. Tel.: +86 451 86418409. E-mail addresses: [email protected] (H. Ma), [email protected] (F. Wang). 0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.01.040
attractive for a host of applications in photochemistry, electrochemistry, catalysis, and sensing [14,15]. Transition metal complexes have been the subject of extensive electrochemical and spectroscopic studies, due to their excellent photo-luminescence properties and excellent stability in multiple redox states. In particular, ruthenium(II) polypyridyl complexes can display strong luminescence with high quantum yield and long natural lifetime, and serve as luminescence sensors for monitoring oxygen concentration, pH, chloride, or temperature [16–22]. Meanwhile, the intercalation of chiral metal complex tris(1,10-phenanthroline)ruthenium(II) by a smectite has been studied to understand the nature of the host–guest systems as well as to construct an adsorbent, optical resolving agent, and a chiral catalysis [23–25]. It has been found that cationic ruthenium(II) polypyridyl complexes can been combine with POM anion in film materials by LBL method [26–28]. To develop multifunctional films, in the present work, we fabricated organic–inorganic composite self-assembled ultrathin films from tris(1,10-phenanthroline)ruthenium(II) Ru(phen)3 Cl2 (Ru(phen)3 ) and Dawson-type polyoxometalate [P2 Mo18 O62 ]6− (P2 Mo18 ) (shown in Fig. 1) by LBL method. The growth process of the film was monitored by UV–vis spectra and the morphology of the film was observed by atomic
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ing with deionized water. After washing, a nitrogen stream was blown over the film surface until the adhering water layer was completely removed. The PEI-coated substrates then were alternately dipped into 1 × 10−3 M P2 Mo18 , and 1 × 10−3 M Ru(phen)3 for 20 min, also rinsed with deionized water, and dried in a nitrogen stream after each dipping. Multilayer film {PEI/[P2 Mo18 /Ru(phen)3 ]n } was formed on polymer matrix. ITO glass (on one side only) substrate was cleaned by immersing into a H2 SO4 :H2 O2 = 7:3 (v/v) solution for a few minutes followed by rinsing copiously with deionized water and drying in a nitrogen stream [31]. Layer-by-layer deposition was performed according to the process as described above. Fig. 1. The structure of (a) P2 Mo18 and (b) Ru(phen)3 .
force microscopy (AFM). Photo-luminescent property and electrochemical behavior of the multilayer films were studied. The combination of the unique properties of POM and functional molecule Ru(phen)3 might provide a opportunity in the applications of luminescence sensors, electrontransfer, electrocatalysis, and so on.
3. Results and discussion 3.1. Ultraviolet–visible absorption spectra UV–vis spectroscopy was used to monitor the assembling process of the films, the absorbencies of per two-layer cycle being obtained for film assembled on two sides of a quartz slide. Fig. 2a exhibits the UV spectra of {PEI/[P2 Mo18 /Ru(phen)3 ]n }
2. Experimental 2.1. Materials and instruments Poly(ethylenimine) (PEI; MW 750,000) and tris(1,10phenanthroline)ruthenium(II) Ru(phen)3 Cl2 were commercially obtained from Aldrich and used without further purification. Polyoxometalate ␣-K6 P2 Mo18 O62 ·14H2 O was synthesized by the literature [29]. The water used in all experiments was deionized to a resistivity of 16–18 M cm−1 . All other reagents were of reagent grade. UV–vis spectra were recorded on a U-3010 UV–vis spectrophotometer made in Japan. X-ray photoelectron spectra (XPS) were performed on an Escalab MKII photoelectronic spectrometer with Mg K␣ (1253.6 eV) as X-ray source. AFM image were obtained by using 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 excitation source. A CHI 660 electrochemical workstation was used for control of the electrochemical measurements and data collection. A conventional three-electrode system was used, with a bare ITO electrode or a {PEI/[P2 Mo18 /Ru(phen)3 ]n } multilayer film coated ITO electrode as a working electrode, platinum foil as a counter electrode, and Ag/AgCl as a reference electrode. 2.2. Layer-by-layer assembly The fabrication of the multilayer film was carried out as follows. The substrate (silica or quartz glass slide) was cleaned according to the literature [30], which made its surface become hydrophilic, rinsed with deionized water, dried under a nitrogen stream. The hydrophilized substrate slide was immersed in 1 × 10−3 M PEI solution (the concentration was calculated based on their repeating units) for 20 min, followed by wash-
Fig. 2. (a) UV spectra of {PEI/[P2 Mo18 /Ru(phen)3 ]n } multilayer film (n = 1–20 and 50) deposited on quartz substrate (on both sides). Bottom to top: n = 0, 1, 2, 3, . . ., 20, and 50, respectively. Inset: plots of the absorbance values at 203, 225, 269, 293, and 317 nm. (b) Comparative UV spectra of P2 Mo18 , Ru(phen)3 solutions, and {PEI[P2 Mo18 /Ru(phen)3 ]10 } film.
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(n = 0–20 and 50) multilayer film. The spectra showed the five characteristic absorptions in 190–350 nm at 225, 269 and 293 nm owing to ligand-centered –* and metal-centered d–d transitions of Ru(phen)3 cation, and at 203 and 317 nm assigned to the overlap peaks of both Ru(phen)3 and P2 Mo18 , which confirms the incorporation of Ru(phen)3 and P2 Mo18 into the multilayer films. However, there is a slight red shift of 2–6 nm for every absorption peak of the multilayer films compared to that of the Ru(phen)3 solution (Fig. 2b), which may be associated with the strong interactions between Ru(phen)3 cation and the polyoxometalate anion. Further, one can see from the inset of Fig. 2a that the absorbance values of the multilayer films at 203, 225, 269, 293, and 317 nm increase linearly with the number of P2 Mo18 /Ru(phen)3 bilayers, suggesting that the growth of the films for each deposition cycle was uniform up to at least 20 bilayers. 3.2. Atomic force microscopy The morphology of the film was observed by AFM. Fig. 3 shows the AFM image of the thin film {PEI/[P2 Mo18 / Ru(phen)3 ]4 /P2 Mo18 }. The surface of the film assembled on silica slide is uniform and smooth, consisting of a multitude of small grains with ca. 52 nm mean grain size. The mean interface roughness is ca. 1.9 nm, calculated from an area of 1.0 m × 1.0 m.
Fig. 4. Emission and excitation spectra of (a) Ru(phen)3 solid and (b) {PEI/[P2 Mo18 /Ru(phen)3 ]}50 film.
3.3. Luminescent property
Fig. 3. AFM image of {PEI/[P2 Mo18 /Ru(phen)3 ]4 P2 Mo18 } multilayer film: (a) the planar image and (b) dimensional image.
The excitation and emission spectra of the Ru(phen)3 solid and {PEI/[P2 Mo18 /Ru(phen)3 ]50 } LBL film self-assembled on smooth quartz substrates at room temperature were showed in Fig. 4. Compared with the excitation spectrum of the Ru(phen)3 solid, certain variations occurred in the band number, band position, band shape, and relative intensity of some individual bands in the spectrum of the film. Particularly, in contrast to the spectrum of Ru(phen)3 solid, the most intense band arising from the singlet metal-to-ligand (MLCT) d–* transitions [32], has a blue-shift of 11 nm, while the intense of broad band centered at 421 nm decrease greatly, and the much less intense bands at 472, 506, and 515 nm nearly disappear in the film. The emission spectra for Ru(phen)3 solid and the thin film are quite similar and all show a characteristic broad band, arising from * –t2g ligand-to-metal transition of Ru(phen)3 [32]. The luminescence band is, however, blue-shifted from 595 to 582 nm in thin film. All these changes as mentioned above may be associated with the strong interactions between tris(1,10phenanthroline)ruthenium(II) and the polyanion, which have influence on the molecular orbitals of ligands and metal in Ru(phen)3 [33–35]. The electroluminescence behavior of the multilayer film is underway.
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3.4. X-ray photoelectron spectrum XPS measurements were performed to identify elemental composition of {PEI/[P2 Mo18 /Ru(phen)3 ]6 } LBL film deposited on the single-crystal silicon substrate. The film exhibited peaks corresponding to C 1s (BE = 284.6 eV), N 1s (BE = 399.5 eV), O 1s (BE = 532.2 eV), P 2p (BE = 132.6 eV), Mo 3d 5/2 (BE = 231.9 eV), and Mo 3d 3/2 (BE = 234.9 eV) shown in SFig. 1. Whereas that for Ru was not observed due to its very low percentages in the film (detection limit required by the instrument is ≥2%). The N 1s is attributed to the nitrogen atom of the N–Ru coordination bond in Ru(phen)3 . The C 1s signal can be assigned to the carbon in Ru(phen)3 ; the O 1s is ascribed to the oxygen atoms in P2 Mo18 . These XPS results confirm the existence of the P2 Mo18 , and Ru in the multilayer film in conjunction with the results of UV–vis and fluorescent spectra. 3.5. Voltammetric behavior and electrocatalytic activity of the film assembled on ITO glass substrate The electrochemical studies of {PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 }-modified ITO electrode were carried out in 0.5 M Na2 SO4 + H2 SO4 buffer solution (pH 0.5). The comparative cyclic voltammograms of a bare ITO electrode just after being immersed into 0.5 M Na2 SO4 + H2 SO4 buffer solution (pH 0.5) containing 0.1 mM P2 Mo18 and a multilayer film {PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 }-modified ITO electrode in the same buffer solution were showed in Fig. 5. It can been see that in the range −100 and 700 mV, the thin film undergo three redox waves assigned to molybdenum-centered two-electron redox processes of P2 Mo18 [36], while certain variation occurred in peak position, peak relative intensity, and peak-to-peak separation in contrast to that of P2 Mo18 in aqueous solution perhaps due to strong interactions between Ru(phen)3 cation and the polyoxometalate anion.
Fig. 6. The cyclic voltammograms for the ITO/{PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 } multilayer film in Na2 SO4 + H2 SO4 buffer solution with different pH: (a) 0.5, (b) 1.0, (c) 1.5, (d) 2.0, (e) 2.5, and (f) 3.0. Scan rate: 20 mV s−1 . Inset: the relationship between peak potentials of the three redox waves and pH.
In general, the reduction of polyoxometalate anions is accompanied by protonation, Fig. 6 exhibits the cyclic voltammograms for the multilayer film {PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 }modified ITO electrode in 0.5 M Na2 SO4 + H2 SO4 aqueous solutions with different pH. In the range of pH 0.5–3, the peak potentials for all three redox couples shift negatively with the increase in pH, and plots of peak potentials of the three successive redox waves versus pH show good linearity (see inset). The slopes of the E/pH lines are about 61 mV for wave I, 59 mV for wave II, and 61 mV for wave III, respectively, which indicating the addition of one proton to the reduced forms of P2 Mo18 when one electron is transferred [37]. So the electrochemical behavior of the multilayer film {PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 }modified ITO electrode can be summarized by the following equations: P2 Mo18 O62 6− + 2e + 2H+ H2 P2 Mo2 V Mo16 VI O62 6− H2 P2 Mo2 V Mo16 VI O62 6− + 2e + 2H+ H4 P2 Mo4 V Mo14 VI O62 6− H4 P2 Mo4 V Mo14 VI O62 6− + 2e + 2H+ H6 P2 Mo6 V Mo12 VI O62 6−
Fig. 5. Comparative cyclic voltammograms of 0.1 M P2 Mo18 (dot line, ITO electrode), and a {PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 } multilayer film-modified ITO electrode (solid line) in 0.5 M Na2 SO4 + H2 SO4 buffer solution (pH 0.5). Scan rate: 20 mV s−1 .
Fig. 7 shows the cyclic voltammograms of the {PEI/[P2 Mo18 /Ru(phen)3 ]10 }-modified ITO electrode at different scan rate in 0.1 M Na2 SO4 + H2 SO4 solution (pH 1.5), and plots of peak current (III) versus the square root of the scan rates. As can be seen from the inset that the anodic currents were proportional to the square root of the scan rates, which indicates that the redox process is diffusion-controlled. Reduced POMs can be exploited extensively in electrocatalytic reductions due to their ability of delivering the electrons to other species. The multilayer film {PEI/[P2 Mo18 /Ru(phen)3 ]10 } also presents electrocatalytic activity toward the reduction of nitrite. Fig. 8 exhibits cyclic voltammograms for the electrocatalytic reduction of nitrite by {PEI/[P2 Mo18 /Ru(phen)3 ]10 } in
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of luminescence sensors, electrontransfer, electrocatalysis, and so on. Acknowledgments This work was supported by the China Postdoctoral Science Foundation (No. 2005037644) and the Nature Science Foundation of Heilongjiang Province in China (No. B0308). Appendix A. Supplementary data
Fig. 7. Cyclic voltammograms of {PEI/[P2 Mo18 /Ru(phen)3 ]10 } multilayer film in 0.1 M Na2 SO4 + H2 SO4 buffer solution (pH 1.5) at different scan rates (from inner to outer: 25, 50, 75, 100, 125, 150, 175, 200, 250, 300, 350, 400, 450, and 500 mV s−1 ). The inset shows a linear dependence of the third anodic current with square root of the scan rates.
Fig. 8. Cyclic voltammograms of {PEI/[P2 Mo18 /Ru(phen)3 ]10 } multilayer film in pH 1.2 buffer solutions containing NO2 − in various concentrations: 0.0, 5.0 10.0, 20.0, 40.0, and 80.0 mM, respectively. Scan rate: 25 mV s−1 . The inset shows relationship between catalytic current and concentration of NO2 − .
0.1 M Na2 SO4 + H2 SO4 buffer solution (pH 1.2). With addition of NO2 − , the Mo reduction peak current (III) substantially increases, while the corresponding oxidation peak current decreases, suggesting that the two-electron reduction of P2 Mo18 can catalyze the electrochemical reduction of the NO2 − . 4. Conclusion The well-defined organic–inorganic mutilayer films composed of transition metal complex tris(1,10-phenanthroline) ruthenium Ru(phen)3 and Dawson-type polyoxometalate P2 Mo18 were fabricated by layer-by-layer assembly method. The film exhibited photo-luminescence arising from * –t2g ligand-to-metal transition of Ru(phen)3 , and electrocatalytic activity on the reduction of nitrite attributing to molybdenumcentered redox processes of [P2 Mo18 O62 ]6− . These preliminary results in this work represent potential applications in the field
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2006.01.040. References [1] T. Yams, Chem. Rev. 98 (1998) 307. [2] L.C.W. Baker, D.C. Glick, Chem. Rev. 98 (1998) 3. [3] Y. Guo, Y. Wang, C. Hu, Y. Wang, E. Wang, Chem. Mater. 12 (2000) 3501. [4] C.L. Hill, Chem. Rev. 98 (1998) 1. [5] E. Coronado, J.R. Galan-mascaros, C. Gimenez-Saiz, C.J. GomezGarcia, Adv. Mater. 4 (1993) 283. [6] N. Casan-Pastor, L.C.W. Baker, J. Am. Chem. Soc. 114 (1992) 10384. [7] T. Yamase, Mol. Eng. 3 (1993) 241. [8] (a) D.G. Kurth, P. Lehmann, D. Volkmer, A. M¨uller, D. Schwahn, J. Chem. Soc. Dalton Trans. 21 (2000) 3989; (b) I. Ichinose, H. Tagawa, H. Mizuki, Y. Lvov, T. Kunitake, Langmuir 14 (1998) 3462. [9] L. Xu, E.B. Wang, Z. Li, D.G. Kurth, X.G. Du, H.Y. Zhang, C. Qin, New J. Chem. (2002) 782. [10] G. Decher, Science 277 (1997) 1232. [11] I. Ichinose, H. Tagawa, S. Mizuki, Y. Lvov, T. Kunitake, Langmuir 14 (1998) 187. [12] D.G. Kurth, D. Volkmer, Chem. Mater. 12 (2000) 2829. ¨ [13] S.Q. Liu, D.G. Kurth, B. BredenkOtter, D. Volkmer, J. Am. Chem. Soc. 124 (2002) 12280. [14] X. Zhang, J.C. Shen, Adv. Mater. 11 (1999) 1139. [15] M.T. Pope, A. Muller, Polyoxometalate Chemistry from Topology via Self-assembly to Applications, Kluwer Academic Publishers, 2001. [16] J.R. Bacon, J.N. Demas, Anal. Chem. 59 (1987) 2780. [17] O.S. Wolfbeis, M.J.P. Leiner, H.E. Posch, Mikrochim. Acta 3 (1986) 359. [18] J.M. Price, W. Xu, J.N. Demas, B.A. DeGraff, Anal. Chem. 70 (1998) 265. [19] C. Huber, I. Klimant, C. Krause, O.S. Wolfbeis, Anal. Chem. 73 (2001) 2097. [20] J.N. Demas, B.A. DeGraff, Proc. SPIE-Int. Soc. Opt. Eng. 1796 (1992) 71. [21] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press, New York, 1984. [22] R.G. Nuzzo, F.A. Fusco, D.L. Allara, J. Am. Chem. Soc. 109 (1987) 2358. [23] (a) N. Kakegawa, A. Yamagishi, Chem. Mater. 17 (2005) 2997; (b) H. Sato, A. Yamagishi, S.J. Kato, J. Phys. Chem. 96 (1992) 9377; (c) H. Sato, A. Yamagishi, S.J. Kato, J. Phys. Chem. 96 (1992) 9382; (d) H. Sato, A. Yamagishi, S.J. Kato, J. Am. Chem. Soc. 114 (1992) 10933. [24] T. Okada, T. Morita, M. Ogawa, Appl. Clay Sci. 29 (2005) 45. [25] (a) J.X. He, H. Sato, P.J. Yang, A. Yamagishi, J. Electroanal. Chem. 560 (2003) 169; (b) T. Hikita, K. Tamaru, A. Yamagishi, T. Iwamoto, Inorg. Chem. 28 (1989) 2221.
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A multifunctional organic–inorganic multilayer film based on tris(1,10-phenanthroline)ruthenium and polyoxometalate Huiyuan Ma a,b,∗ , Tao Dong b , Fuping Wang a,∗ , Wei Zhang b , Baibin Zhou b a
Postdoctoral Station of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China b Chemistry Department of Harbin Normal University, Harbin 150025, PR China Received 24 November 2005; received in revised form 22 January 2006; accepted 24 January 2006 Available online 28 February 2006
Abstract The novel multifunctional thin films composed of transition metal complex tris(1,10-phenanthroline)ruthenium(II) Ru(phen)3 Cl2 (abbr Ru(phen)3 , phen = 1,10-phenanthroline), and Dawson-type polyoxometalate [P2 Mo18 O62 ]6− (abbr P2 Mo18 ) were fabricated on quartz, silicon and ITO substrates by layer-by-layer (LBL) method. The LBL films deposition were found to be linearly related to the number of bilayers as monitored by UV–vis spectroscopy. And the compositions of the films were measured by X-ray photoelectron spectra (XPS). The result of atomic force microscopy (AFM) revealed a relatively uniform surface morphology of the multilayer films. In particular, the films exhibited the photo-luminescence arising from * –t2g ligand-to-metal transition of Ru(phen)3 and catalytic activity to the reduction of NO2 − attributing to molybdenum-centered redox processes of P2 Mo18 . © 2006 Elsevier Ltd. All rights reserved. Keywords: Layer-by-layer self-assembly; Polyoxometalate; Luminescent film; Ruthenium; Electrocatalysis
1. Introduction Polyoxometalates (POMs) represent a well-defined class of inorganic compounds with remarkable structural, and electronic versatility [1–3], and possess diverse properties such as catalytic activity, molecule-based conductivity, magnetism, photochromism, electrochromism and luminescence, etc. [4–7]. However, practical applications of polyoxometalates in these areas depend on the successful preparation of thin polyoxometalate-containing films [8,9]. The layer-by-layer (LBL) method, being based on sequential adsorption of oppositely charged species from dilute solutions and relying primarily on electrostatic attraction of the oppositely charged components, provides a facile synthetic route towards uniform POM based multilayer films [10–13]. This method permits the combination of POMs with many other functional components as part of the polycationic macromolecules with good control over the layer composition and thickness, which renders them potentially
∗
Corresponding authors. Tel.: +86 451 86418409. E-mail addresses: [email protected] (H. Ma), [email protected] (F. Wang). 0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.01.040
attractive for a host of applications in photochemistry, electrochemistry, catalysis, and sensing [14,15]. Transition metal complexes have been the subject of extensive electrochemical and spectroscopic studies, due to their excellent photo-luminescence properties and excellent stability in multiple redox states. In particular, ruthenium(II) polypyridyl complexes can display strong luminescence with high quantum yield and long natural lifetime, and serve as luminescence sensors for monitoring oxygen concentration, pH, chloride, or temperature [16–22]. Meanwhile, the intercalation of chiral metal complex tris(1,10-phenanthroline)ruthenium(II) by a smectite has been studied to understand the nature of the host–guest systems as well as to construct an adsorbent, optical resolving agent, and a chiral catalysis [23–25]. It has been found that cationic ruthenium(II) polypyridyl complexes can been combine with POM anion in film materials by LBL method [26–28]. To develop multifunctional films, in the present work, we fabricated organic–inorganic composite self-assembled ultrathin films from tris(1,10-phenanthroline)ruthenium(II) Ru(phen)3 Cl2 (Ru(phen)3 ) and Dawson-type polyoxometalate [P2 Mo18 O62 ]6− (P2 Mo18 ) (shown in Fig. 1) by LBL method. The growth process of the film was monitored by UV–vis spectra and the morphology of the film was observed by atomic
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ing with deionized water. After washing, a nitrogen stream was blown over the film surface until the adhering water layer was completely removed. The PEI-coated substrates then were alternately dipped into 1 × 10−3 M P2 Mo18 , and 1 × 10−3 M Ru(phen)3 for 20 min, also rinsed with deionized water, and dried in a nitrogen stream after each dipping. Multilayer film {PEI/[P2 Mo18 /Ru(phen)3 ]n } was formed on polymer matrix. ITO glass (on one side only) substrate was cleaned by immersing into a H2 SO4 :H2 O2 = 7:3 (v/v) solution for a few minutes followed by rinsing copiously with deionized water and drying in a nitrogen stream [31]. Layer-by-layer deposition was performed according to the process as described above. Fig. 1. The structure of (a) P2 Mo18 and (b) Ru(phen)3 .
force microscopy (AFM). Photo-luminescent property and electrochemical behavior of the multilayer films were studied. The combination of the unique properties of POM and functional molecule Ru(phen)3 might provide a opportunity in the applications of luminescence sensors, electrontransfer, electrocatalysis, and so on.
3. Results and discussion 3.1. Ultraviolet–visible absorption spectra UV–vis spectroscopy was used to monitor the assembling process of the films, the absorbencies of per two-layer cycle being obtained for film assembled on two sides of a quartz slide. Fig. 2a exhibits the UV spectra of {PEI/[P2 Mo18 /Ru(phen)3 ]n }
2. Experimental 2.1. Materials and instruments Poly(ethylenimine) (PEI; MW 750,000) and tris(1,10phenanthroline)ruthenium(II) Ru(phen)3 Cl2 were commercially obtained from Aldrich and used without further purification. Polyoxometalate ␣-K6 P2 Mo18 O62 ·14H2 O was synthesized by the literature [29]. The water used in all experiments was deionized to a resistivity of 16–18 M cm−1 . All other reagents were of reagent grade. UV–vis spectra were recorded on a U-3010 UV–vis spectrophotometer made in Japan. X-ray photoelectron spectra (XPS) were performed on an Escalab MKII photoelectronic spectrometer with Mg K␣ (1253.6 eV) as X-ray source. AFM image were obtained by using 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 excitation source. A CHI 660 electrochemical workstation was used for control of the electrochemical measurements and data collection. A conventional three-electrode system was used, with a bare ITO electrode or a {PEI/[P2 Mo18 /Ru(phen)3 ]n } multilayer film coated ITO electrode as a working electrode, platinum foil as a counter electrode, and Ag/AgCl as a reference electrode. 2.2. Layer-by-layer assembly The fabrication of the multilayer film was carried out as follows. The substrate (silica or quartz glass slide) was cleaned according to the literature [30], which made its surface become hydrophilic, rinsed with deionized water, dried under a nitrogen stream. The hydrophilized substrate slide was immersed in 1 × 10−3 M PEI solution (the concentration was calculated based on their repeating units) for 20 min, followed by wash-
Fig. 2. (a) UV spectra of {PEI/[P2 Mo18 /Ru(phen)3 ]n } multilayer film (n = 1–20 and 50) deposited on quartz substrate (on both sides). Bottom to top: n = 0, 1, 2, 3, . . ., 20, and 50, respectively. Inset: plots of the absorbance values at 203, 225, 269, 293, and 317 nm. (b) Comparative UV spectra of P2 Mo18 , Ru(phen)3 solutions, and {PEI[P2 Mo18 /Ru(phen)3 ]10 } film.
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(n = 0–20 and 50) multilayer film. The spectra showed the five characteristic absorptions in 190–350 nm at 225, 269 and 293 nm owing to ligand-centered –* and metal-centered d–d transitions of Ru(phen)3 cation, and at 203 and 317 nm assigned to the overlap peaks of both Ru(phen)3 and P2 Mo18 , which confirms the incorporation of Ru(phen)3 and P2 Mo18 into the multilayer films. However, there is a slight red shift of 2–6 nm for every absorption peak of the multilayer films compared to that of the Ru(phen)3 solution (Fig. 2b), which may be associated with the strong interactions between Ru(phen)3 cation and the polyoxometalate anion. Further, one can see from the inset of Fig. 2a that the absorbance values of the multilayer films at 203, 225, 269, 293, and 317 nm increase linearly with the number of P2 Mo18 /Ru(phen)3 bilayers, suggesting that the growth of the films for each deposition cycle was uniform up to at least 20 bilayers. 3.2. Atomic force microscopy The morphology of the film was observed by AFM. Fig. 3 shows the AFM image of the thin film {PEI/[P2 Mo18 / Ru(phen)3 ]4 /P2 Mo18 }. The surface of the film assembled on silica slide is uniform and smooth, consisting of a multitude of small grains with ca. 52 nm mean grain size. The mean interface roughness is ca. 1.9 nm, calculated from an area of 1.0 m × 1.0 m.
Fig. 4. Emission and excitation spectra of (a) Ru(phen)3 solid and (b) {PEI/[P2 Mo18 /Ru(phen)3 ]}50 film.
3.3. Luminescent property
Fig. 3. AFM image of {PEI/[P2 Mo18 /Ru(phen)3 ]4 P2 Mo18 } multilayer film: (a) the planar image and (b) dimensional image.
The excitation and emission spectra of the Ru(phen)3 solid and {PEI/[P2 Mo18 /Ru(phen)3 ]50 } LBL film self-assembled on smooth quartz substrates at room temperature were showed in Fig. 4. Compared with the excitation spectrum of the Ru(phen)3 solid, certain variations occurred in the band number, band position, band shape, and relative intensity of some individual bands in the spectrum of the film. Particularly, in contrast to the spectrum of Ru(phen)3 solid, the most intense band arising from the singlet metal-to-ligand (MLCT) d–* transitions [32], has a blue-shift of 11 nm, while the intense of broad band centered at 421 nm decrease greatly, and the much less intense bands at 472, 506, and 515 nm nearly disappear in the film. The emission spectra for Ru(phen)3 solid and the thin film are quite similar and all show a characteristic broad band, arising from * –t2g ligand-to-metal transition of Ru(phen)3 [32]. The luminescence band is, however, blue-shifted from 595 to 582 nm in thin film. All these changes as mentioned above may be associated with the strong interactions between tris(1,10phenanthroline)ruthenium(II) and the polyanion, which have influence on the molecular orbitals of ligands and metal in Ru(phen)3 [33–35]. The electroluminescence behavior of the multilayer film is underway.
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3.4. X-ray photoelectron spectrum XPS measurements were performed to identify elemental composition of {PEI/[P2 Mo18 /Ru(phen)3 ]6 } LBL film deposited on the single-crystal silicon substrate. The film exhibited peaks corresponding to C 1s (BE = 284.6 eV), N 1s (BE = 399.5 eV), O 1s (BE = 532.2 eV), P 2p (BE = 132.6 eV), Mo 3d 5/2 (BE = 231.9 eV), and Mo 3d 3/2 (BE = 234.9 eV) shown in SFig. 1. Whereas that for Ru was not observed due to its very low percentages in the film (detection limit required by the instrument is ≥2%). The N 1s is attributed to the nitrogen atom of the N–Ru coordination bond in Ru(phen)3 . The C 1s signal can be assigned to the carbon in Ru(phen)3 ; the O 1s is ascribed to the oxygen atoms in P2 Mo18 . These XPS results confirm the existence of the P2 Mo18 , and Ru in the multilayer film in conjunction with the results of UV–vis and fluorescent spectra. 3.5. Voltammetric behavior and electrocatalytic activity of the film assembled on ITO glass substrate The electrochemical studies of {PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 }-modified ITO electrode were carried out in 0.5 M Na2 SO4 + H2 SO4 buffer solution (pH 0.5). The comparative cyclic voltammograms of a bare ITO electrode just after being immersed into 0.5 M Na2 SO4 + H2 SO4 buffer solution (pH 0.5) containing 0.1 mM P2 Mo18 and a multilayer film {PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 }-modified ITO electrode in the same buffer solution were showed in Fig. 5. It can been see that in the range −100 and 700 mV, the thin film undergo three redox waves assigned to molybdenum-centered two-electron redox processes of P2 Mo18 [36], while certain variation occurred in peak position, peak relative intensity, and peak-to-peak separation in contrast to that of P2 Mo18 in aqueous solution perhaps due to strong interactions between Ru(phen)3 cation and the polyoxometalate anion.
Fig. 6. The cyclic voltammograms for the ITO/{PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 } multilayer film in Na2 SO4 + H2 SO4 buffer solution with different pH: (a) 0.5, (b) 1.0, (c) 1.5, (d) 2.0, (e) 2.5, and (f) 3.0. Scan rate: 20 mV s−1 . Inset: the relationship between peak potentials of the three redox waves and pH.
In general, the reduction of polyoxometalate anions is accompanied by protonation, Fig. 6 exhibits the cyclic voltammograms for the multilayer film {PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 }modified ITO electrode in 0.5 M Na2 SO4 + H2 SO4 aqueous solutions with different pH. In the range of pH 0.5–3, the peak potentials for all three redox couples shift negatively with the increase in pH, and plots of peak potentials of the three successive redox waves versus pH show good linearity (see inset). The slopes of the E/pH lines are about 61 mV for wave I, 59 mV for wave II, and 61 mV for wave III, respectively, which indicating the addition of one proton to the reduced forms of P2 Mo18 when one electron is transferred [37]. So the electrochemical behavior of the multilayer film {PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 }modified ITO electrode can be summarized by the following equations: P2 Mo18 O62 6− + 2e + 2H+ H2 P2 Mo2 V Mo16 VI O62 6− H2 P2 Mo2 V Mo16 VI O62 6− + 2e + 2H+ H4 P2 Mo4 V Mo14 VI O62 6− H4 P2 Mo4 V Mo14 VI O62 6− + 2e + 2H+ H6 P2 Mo6 V Mo12 VI O62 6−
Fig. 5. Comparative cyclic voltammograms of 0.1 M P2 Mo18 (dot line, ITO electrode), and a {PEI/[P2 Mo18 /Ru(phen)3 ]10 P2 Mo18 } multilayer film-modified ITO electrode (solid line) in 0.5 M Na2 SO4 + H2 SO4 buffer solution (pH 0.5). Scan rate: 20 mV s−1 .
Fig. 7 shows the cyclic voltammograms of the {PEI/[P2 Mo18 /Ru(phen)3 ]10 }-modified ITO electrode at different scan rate in 0.1 M Na2 SO4 + H2 SO4 solution (pH 1.5), and plots of peak current (III) versus the square root of the scan rates. As can be seen from the inset that the anodic currents were proportional to the square root of the scan rates, which indicates that the redox process is diffusion-controlled. Reduced POMs can be exploited extensively in electrocatalytic reductions due to their ability of delivering the electrons to other species. The multilayer film {PEI/[P2 Mo18 /Ru(phen)3 ]10 } also presents electrocatalytic activity toward the reduction of nitrite. Fig. 8 exhibits cyclic voltammograms for the electrocatalytic reduction of nitrite by {PEI/[P2 Mo18 /Ru(phen)3 ]10 } in
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of luminescence sensors, electrontransfer, electrocatalysis, and so on. Acknowledgments This work was supported by the China Postdoctoral Science Foundation (No. 2005037644) and the Nature Science Foundation of Heilongjiang Province in China (No. B0308). Appendix A. Supplementary data
Fig. 7. Cyclic voltammograms of {PEI/[P2 Mo18 /Ru(phen)3 ]10 } multilayer film in 0.1 M Na2 SO4 + H2 SO4 buffer solution (pH 1.5) at different scan rates (from inner to outer: 25, 50, 75, 100, 125, 150, 175, 200, 250, 300, 350, 400, 450, and 500 mV s−1 ). The inset shows a linear dependence of the third anodic current with square root of the scan rates.
Fig. 8. Cyclic voltammograms of {PEI/[P2 Mo18 /Ru(phen)3 ]10 } multilayer film in pH 1.2 buffer solutions containing NO2 − in various concentrations: 0.0, 5.0 10.0, 20.0, 40.0, and 80.0 mM, respectively. Scan rate: 25 mV s−1 . The inset shows relationship between catalytic current and concentration of NO2 − .
0.1 M Na2 SO4 + H2 SO4 buffer solution (pH 1.2). With addition of NO2 − , the Mo reduction peak current (III) substantially increases, while the corresponding oxidation peak current decreases, suggesting that the two-electron reduction of P2 Mo18 can catalyze the electrochemical reduction of the NO2 − . 4. Conclusion The well-defined organic–inorganic mutilayer films composed of transition metal complex tris(1,10-phenanthroline) ruthenium Ru(phen)3 and Dawson-type polyoxometalate P2 Mo18 were fabricated by layer-by-layer assembly method. The film exhibited photo-luminescence arising from * –t2g ligand-to-metal transition of Ru(phen)3 , and electrocatalytic activity on the reduction of nitrite attributing to molybdenumcentered redox processes of [P2 Mo18 O62 ]6− . These preliminary results in this work represent potential applications in the field
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