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Formation and Photocatalytic Properties of Nanocomposite Films Containing Both Tetracobalt Dawson-Derived Sandwich Polyanions and Tetracationic Porphyrins Delphine Schaming,† Rana Farha,‡,§ Hualong Xu, Michel Goldmann,‡,^ and Laurent Ruhlmann*,†,#
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† Laboratoire de Chimie Physique, Groupe TEMiC, UMR 8000 au CNRS, Universit e Paris-Sud (11), B^ atiment 349, e Paris 6, 140 F - 91405 Orsay Cedex, France, ‡Institut des NanoSciences de Paris, UMR 7588 au CNRS, Universit rue de Lourmel, F - 75015 Paris, France, §Laboratoire d’Analyse et Contr^ ole des Syst emes Complexes -LACSC- ECE Paris Ecole d’Ing enieurs, 37 Quai de Grenelle, F - 75015 Paris, France, Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials and Laboratory of Advanced Materials, Fudan e Paris Descartes, 45 rue des Saint P eres, University, Shanghai 200433, People’s Republic of China, and ^Universit F - 75006 Paris, France. #Current address: Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai 200433, China
Received June 18, 2010. Revised Manuscript Received October 5, 2010 Films based on electrostatic interactions between tetracationic zinc porphyrins, ZnOEP(py)44þ or ZnTMePyP4þ, and the tetracobalt Dawson-derived sandwich polyanion RββR-[Co4(H2O)2(P2W15O56)2]16- are formed by the so-called layer-by-layer method. These films have been characterized by UV-visible absorption spectroscopy, atomic force microscopy and electrochemistry. The composition of the film was measured by X-ray photoelectron spectrum (XPS). The XPS data confirm the presence of the expected elements. The photocatalytic properties of these films have been also studied for the reduction of silver and gold ions. Indeed, in these systems, porphyrins can be excited by visible light and then play the role of photosensitizers able to give electrons to POM known to be good catalysts. Silver nanowires and gold nanosheets have been obtained.
Introduction Polyoxometalates (POMs) are metal-oxygen cluster polyanions constituted of early metal elements in their highest oxidation state with many applications in catalysis, medicine, materials science, etc.1 In particular, they appear to be effective photocatalysts both in oxidation and reduction processes. Indeed, upon photolysis, the high oxidizing ability of excited POMs can lead to the degradation of a variety of organic compounds such as pollutants, leading to a process for water purification.2 Moreover, the photolysis performed in the presence of a sacrificial electron donor conducts to the formation of reduced POMs that can in turn reduce metallic ions. It could be a good alternative for metals recovery or synthesis of metal nanoparticles.3,4 However, POMs absorb in the UV spectral domain, which is a drawback for environmental applications. Indeed, it seems preferable to use solar light, principally in the visible domain. The development of photosensitized systems could overcome this issue. Recently, we have shown the formation of electrostatic complexes between tetracationic zinc porphyrins and tetracobalt Dawson-derived sandwich polyanions and their application in photocatalysis for the reduction of silver cations.5 In these complexes, porphyrins have been used as photosensitizers because *To whom correspondence should be addressed. E-mail: laurent.ruhlmann@ u-psud.fr. Tel: þ33 (0)1 69 15 44 38. Fax: þ33 (0)1 69 15 61 88. (1) Hill, C. L. Chem. Rev. 1998, 98, 1–389. (2) Mylonas, A.; Hiskia, A.; Papaconstantinou, E. J. Mol. Catal. A: Chem. 1996, 114, 191–200. (3) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Appl. Catal., B 2003, 42, 305– 315. (4) Costa-Coquelard, C.; Schaming, D.; Lampre, I.; Ruhlmann, L. Appl. Catal., B 2008, 84, 835–842. (5) Schaming, D.; Costa-Coquelard, C.; Sorgues, S.; Ruhlmann, L.; Lampre, I. Appl. Catal., A 2010, 373, 160–167.
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these compounds absorb in the visible spectral domain. After excitation, porphyrins can transfer electrons to POMs, which are able in a second step to reduce silver cations by a similar way to that reported for the POM alone.4 Using such process, silver nanoparticles have been obtained. Nevertheless, these electrostatic complexes have been used in solution for homogeneous photocatalysis for the reduction of silver cations. So it seems preferable to develop supported photosensitized systems with POMs to perform heterogeneous catalysis. Within this context, we have lately obtained copolymers POM-porphyrin which have been used in heterogeneous catalysis for the photoreduction of silver cations.6 However other strategies can be explored to obtain supported POM-porphyrin systems. Many published works concern the entrapment of POMs in polymeric networks7,8 or the sandwiching of POMs between polymers in layer-by-layer assemblies.9,10 This layer-by-layer method has been also used to obtain POMs-chromophors films, like POMsphthalocyanines,11,12 POMs-ruthenium complexes,13,14 and more (6) Schaming, D.; Allain, C.; Farha, R.; Goldmann, M.; Lobstein, S.; Giraudeau, A.; Hasenknopf, B.; Ruhlmann, L. Langmuir 2010, 26, 5101–5109. (7) Otero, T. F.; Cheng, S. A.; Huerta, F. J. Phys.Chem. B 2000, 104, 10522– 10527. (8) Cheng, S.; Fernandez-Otero, T.; Coronado, E.; Gomez-Garcia, C. J.; Martinez-Ferrero, E.; Gimenez-Saiz, C. J. Phys. Chem. B 2002, 106, 7585–7591. (9) Liu, S.; Kurth, D. G.; Bredenk€otter, B.; Volkmer, D. J. Am. Chem. Soc. 2002, 124, 12279–12287. (10) Gao, S.; Cao, R.; Li, X. Thin Solid Films 2006, 500, 283–288. (11) Jin, Y.; Xu, L.; Zhu, L.; An, W.; Gao, G. Thin Solid Films 2007, 515, 5490– 5497. (12) Yang, Y.; Xu, L.; Xu, B.; Du, X.; Guo, W. Mater. Lett. 2009, 63, 608–610. (13) Ma, H.; Peng, J.; Chen, Y.; Feng, Y.; Wang, E. J. Solid State Chem. 2004, 177, 3333–3338. (14) Dong, T.; Ma, H.; Zhang, W.; Gong, L.; Wang, F.; Li, C. J. Colloid Interface Sci. 2007, 311, 523–529.
Published on Web 12/02/2010
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Scheme 1. Representation of (A) ZnOEP(py)44þ, (B) ZnTMePyP4þ, (C) Co4POM16-, and (D) [P2W18O62]6-
rarely, POMs-porphyrins systems.15-17 The first examples of POMs-porphyrins films have been reported by Dong et al.15,16 Their hybrid films consist in the alternately deposition of cobaltmeso-tetrakis(N-methyl-4-pyridinium)porphyrin (CoTMePyP5þ) and different POMs ([P2W18O62]6- or [SiW12O40]4-) on a modified glassy carbon electrode. These films exhibit good electrocatalytic activity for the reduction of dioxygen. More recently, Drain et al. have used the layer-by-layer method to obtain films between tetracationic porphyrins and POM deposited onto glass, quartz, ITO or mica.17 Observation of the morphology of these films by AFM measurements unexpectedly shows that the surface appears constituted of a multitude of small aggregates. Such morphology might be surprising for a system built according to a superposition of layers deposited homogeneously onto the surface. Thus the term “layer-by-layer” for the porphyrin-POM films formed by sequential dippings into solutions of porphyrin and POM is somewhat misleading and “successive dippings” seems more appropriated. It would be very useful and interesting to know if the phenomenon is always obtained even by changing the nature of the porphyrin or the POM. Moreover, as far as we know, no report has undertaken a study on the ability of porphyrins to photosensitize the near-UV absorbing polyoxometalates with the aim of exploiting their photocatalytic properties with visible excitation. Indeed, porphyrins absorb in the visible spectral domain and present a variety of redox properties according to the metal and the substituents born by the macrocycle. It will be also interesting to know the effect of the aggregatetype architecture formed (size and shape) on the efficiency of the activity of the catalysis. It would be useful and interesting to compare structures of the nanoparticles obtained from the reduction of metallic cations Mnþ in regard with the morphology of the films. In this work, we have also exploited electrostatic interactions between two different tetracationic porphyrins and an anionic Dawson-type POM to obtain deposited layers with opposite charges onto quartz. To form these films, we have chosen the same compounds that were used for the electrostatic POMporphyrins complexes in solution:5 the tetracobalt Dawson-derived (15) Shen, Y.; Liu, J.; Jiang, J.; Liu, B.; Dong, S. Electroanalysis 2002, 14, 1557– 1563. (16) Shen, Y.; Liu, J.; Jiang, J.; Liu, B.; Dong, S. J. Phys. Chem. B 2003, 107, 9744–9748. (17) Bazzan, G.; Smith, W.; Francesconi, L. C.; Drain, C. M. Langmuir 2008, 24, 3244–3249. (18) Finke, R. G.; Droege, M. W. Inorg. Chem. 1983, 22, 1006–1008.
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sandwich POM, RββR-Na16[Co4(H2O)2(P2W15O56)2] (abbreviated Co4POM16-),9,18-20 and two different tetracationic porphyrins, the zinc-meso-tetrakis(N-methyl-4-pyridinium)porphyrin (ZnTMePyP4þ, 4Cl-) and the zinc-meso-tetrakis(N-pyridinium)β-octaethylporphyrin (ZnOEP(py)44þ, 4Cl-)21,22 (Scheme 1). Moreover, the Dawson-type POM [P2W18O62]6- has been used to see the influence of the size and the charge of the POM. Concerning the porphyrins, these two zinc tetracationic porphyrins have been chosen for their different structural and physicochemical properties. Indeed, ZnTMePyP4þ presents a planar structure while ZnOEP(py)44þ, fully substituted, adopts a nonplanar saddle-shape conformation.21-23 Distorted porphyrins are known to have different photophysical properties (and particularly possess shortened excited state lifetimes),22,24,25 and better reactivity than planar ones. Redox properties of distorted porphyrins are also changed.25 For all these reasons, one can expect different behaviors between the films prepared with these two porphyrins for photocatalytic applications. The films have been first characterized by UV-visible absorption spectroscopy, atomic force microscopy, XPS and electrochemistry. Then, we have undertaken a study of the ability of these films to photoreduce metallic ions as AgI and AuIII under visible light in the presence of propan-2-ol (sacrificial electron donor). The formation of silver nanowires and gold nanosheets is reported.
Experimental Section Materials. Most common laboratory chemicals were reagent grade, purchased from commercial sources and used without further purification. Water was obtained by passing through a Milli-RO4 unit and subsequently through a Millipore Q water purification set. The tetranuclear cobalt Dawson-derived sandwich complex Co4POM16- and the Dawson-type POM [P2W18O62]6- were prepared in acidic medium as described previously.20,26 [(py)ZnOEP(py)44þ, 4PF6-] was synthesized by published method.23 Then the PF6(19) Finke, R. G.; Droege, M. W.; Domaille, P. J. Inorg. Chem. 1987, 26, 3886– 3896. (20) Ruhlmann, L.; Nadjo, L.; Canny, J.; Contant, R.; Thouvenot, R. Eur. J. Inorg. Chem. 2002, 975–986. (21) Schaming, D.; Giraudeau, A.; Lobstein, S.; Farha, R.; Goldmann, M.; Gisselbrecht, J.-P.; Ruhlmann, L. J. Electroanal. Chem. 2009, 635, 20–28. (22) Karakostas, N.; Schaming, D.; Sorgues, S.; Lobstein, S.; Gisselbrecht, J.-P.; Giraudeau, A.; Lampre, I.; Ruhlmann, L. J. Photochem. Photobiol., A 2010, 213, 52–60. (23) Giraudeau, A.; Lobstein, S.; Ruhlmann, L.; Melamed, D.; Barkigia, K. M.; Fajer, J. J. Porphyrins Phthalocyanines 2001, 5, 793–797. (24) Gentemann, S.; Medforth, C. J.; Forsyth, T. P.; Nurco, D. J.; Smith, K. M.; Fajer, J.; Holten, D. J. Am. Chem. Soc. 1994, 116(16), 7363–7368. (25) Fajer, J. J. Porphyrins Phthalocyanines 2000, 4, 382–385. (26) Contant, R. Inorg. Synth. 1990, 27, 104–109.
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Article counteranions were exchanged by metathesis by passing three times the tetracationic porphyrin through Cl- exchange resin column. During this procedure, the axially ligated pyridine was also removed from the porphyrin and accordingly [ZnOEP(py)44þ, 4Cl-] was obtained.21 [ZnTMePyP4þ, 4Cl-] was purchased from Sigma Aldrich. Electrochemistry. Voltammetric data were obtained with a standard three-electrode system using a PARSTAT 2273 potentiostat. A one side indium-tin-oxide (ITO, Aldrich, 8-12 Ω/square) electrode with a surface of about 1 cm2 was used to record the electrochemical behavior of the deposited films. A platinum wire was used as an auxiliary electrode. The reference electrode was a saturated calomel electrode (SCE). It was electrically connected to the solution by a junction bridge filled with the electrolyte. UV-visible Spectroscopic Measurements. UV-visible absorption spectra were recorded either with a single beam HewlettPackard HP 8453 diode array spectrophotometer operated at a resolution of 2 nm or with a double beam Perkin-Elmer Lambda 9 spectrophotometer operated at a resolution of 1 nm. Fluorescence Measurements. Fluorescence emission spectra were obtained with a Spex fluorolog 1681 spectrofluorimeter equipped with a Hamamatsu R928 photomultiplier cooled to -20 C. Atomic Force Microscopy (AFM). AFM was performed directly on the surface of the ITO using a Dimension 3100 (Veeco) in the tapping mode under ambient conditions. Silicon cantilevers (Veeco probes) with a spring constant of 300 N/m and a resonance frequency in the range of 120-139 kHz were used. The scanning rate was 1.0 Hz. X-ray Photoelectron Spectroscopy (XPS). XPS experiments were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) with Mg KR radiation (hν = 1253.6 eV) or Al KR radiation (hν = 1486.6 eV). In general, the X-ray anode was run at 250 W and the high voltage was kept at 14.0 kV with a detection angle at 54. The pass energy was fixed at 23.5, 46.95, or 93.90 eV to ensure sufficient resolution and sensitivity. The base pressure of the analyzer chamber was about 5 10-8 Pa. The sample was directly pressed to a self-supported disk (10 10 mm) and mounted on a sample holder then transferred into the analyzer chamber. The whole spectra (0-1100 eV) and the narrow spectra of all the elements with higher resolution were both recorded by using RBD 147 interface (RBD Enterprises, U.S.A.) through the AugerScan 3.21 software. Binding energies were calibrated by using the containment carbon (C1s = 284.6 eV). The data analysis was carried out by using the RBD AugerScan 3.21 software provided by RBD Enterprises or XPSPeak4.1 provided by Raymund W.M. Kwok (The Chinese University of Hongkong, China). Transmission Electronic Microscopy (TEM). TEM observations were performed with a JEOL 100 CXII TEM instrument operated at an accelerating voltage of 100 kV. Samples for TEM analysis were prepared by solution drops deposited and dried on carbon-coated copper TEM grids. Fabrication of Films. Films were prepared at room temperature by soaking the solid substrate (quartz or ITO) in a 0.5 mM aqueous solution of porphyrin for 1 min, followed by dipping the substrate in unbuffered Millipore Q water to remove the excess (nonbound porphyrin solution) from the substrate. After drying, polyoxometalates were added by soaking the substrate for 1 min in a 0.5 mM aqueous solution of polyoxometalates and rinsed by dipping in Millipore Q water. The procedure was repeated until the desired number of deposition cycles was carried out. Photocatalysis. The film was prepared on a slide of quartz. Then the slide was plunged in an optical cell of 1 cm path length containing 3 mL of an aqueous solution with 8 10-5 mol L-1 Ag2SO4 or 1.6 10-4 mol L-1 HAuCl4 and 0.13 mol L-1 propan2-ol. Irradiations were performed using a 300 W Xe lamp equipped with a water cell filter to absorb the near-IR radiations and a 385 nm cutoff filter to prevent POM photoexcitation. According to the supplier, the irradiance of the lamp from 320 to 790 nm 134 DOI: 10.1021/la1024923
Schaming et al. was around 50 mW m-2 nm-1. Deaerated solutions were obtained by bubbling with argon (Ar-U from Air Liquide) before illumination. All the aqueous samples were at natural pH (initially 5.5). All experiments were carried out at room temperature and the temperature of the solution did not increase by more than 2 during light illumination.
Results and Discussion Solution Phase Interactions. Solution-phase experiments have been done to garner information on the interactions between the two components, but the hierarchical organization of the molecular components in the films is likely quite different from the structures in the solution. Titration of an aqueous solution of ZnTMePyP4þ or ZnOEP(py)44þ by an aqueous solution of Co4POM16- in water, illustrated Figures S1A and B in Supporting Information, shows well-resolved isosbestic points until 0.25 equiv of the POM has been added to the aqueous solution. Job’s diagram at λ = 436 nm (maximum of the porphyrin B band) shows a unique slope break at XPOM = 0.2 indicating the formation of only one complex with a stoichiometry of 1/4, that is, one POM for four porphyrins. Moreover, the β-association constant is estimated to be 7 1030 M-4 for [Co4POM16-][ZnTMePyP4þ]4 (1 1031 M-4 for [Co4POM16-][ZnOEP(py)44þ]4) indicating a total reaction. It seems that the electrostatic interaction between the two subunits is very strong. The fluorescence intensity of the porphyrin (Figures S1A and S1B in Supporting Information) decreases linearly and is totally quenched when 0.25 equiv of the POM is added. The evolution of the luminescence at the maxima of the band clearly shows two parts that can be fitted by a straight line intersecting at XPOM = 0.2. This result is also in agreement with the formation of one complex formed by four porphyrins and one POM. Absorption and luminescence behavior during the titration of a solution of ZnTMePyP4þ by a solution of [P2W18O62]6- shows a unique slope break at XPOM = 0.33 indicating the formation of only one complex with a stoichiometry of 1/2, i.e. one POM for two porphyrins (see Figure S1C in Supporting Information). Formation of Alternate Films of Co4POM16-/Porphyrin. The films are prepared by alternate deposition of positively charged porphyrin, ZnOEP(py)44þ or ZnTMePyP4þ, and negatively charged Co4POM16- on a slide of quartz by layer-by-layer method. Since the porphyrins have characteristic spectral absorption in the UV-visible region (Soret and Q bands), UV-visible absorption spectroscopy has been employed to follow the deposition process of the films. Figure 1A shows the UV-visible absorption spectra of [Co4POM16-/ZnOEP(py)44þ]n films after each dipping. We can see that the absorption intensity increases progressively for each deposition cycle. As shown in Figure 1B, the plot of the absorbance at 468 nm (maximum of the Soret band (B band) of the metalloporphyrin) versus the number n of deposition cycles of [Co4POM16-/ZnOEP(py)44þ] results in a nearly straight line, showing a linear growth of the film up to at least an n value of 25. It indicates that an approximately equal amount of porphyrin or POM is deposited upon each dipping cycle and the [Co4POM16-/ZnOEP(py)44þ] film grows uniformly and homogeneously. The Soret absorption band of the metalloporphyrin in the films shows a slight red shift by ca. 15 nm accompanied by a broadening of the absorption bands compared to that in solution.5 The red shift and the broadening of the UV-visible spectra are likely ascribed to interactions of porphyrins either with other porphyrins or with POMs. It must be noted that the O to W transfer band of the polyoxometalate is observed at ca. 265 nm. Indeed, deposition of these Langmuir 2011, 27(1), 132–143
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Figure 1. (A) UV-visible absorption spectra of [Co4POM16-/ZnOEP(py)44þ]n films (onto quartz) with different numbers of deposition cycles (after porphyrin and POM depositions). (The measured absorption corresponds to the deposition of material on both sides of the quartz.) (B) Plots of the absorbance at 265 and 468 nm as a function of the number n of deposition cycles of [Co4POM16-/ZnOEP(py)44þ] in pure aqueous solution. (C) Plots of the absorbance at 444 nm as a function of the number n of deposition cycles of [Co4POM16-/ ZnTMePyP4þ] in pure aqueous solution and in 0.1 M NaCl aqueous solution.
films onto quartz allows us also to monitor the increasing absorbance due to the addition of the POM. But there are no significant shift in the UV-visible spectra for the POM in the films compared to solution. Similar observations can be done for [Co4POM16-/ZnTMePyP4þ]n film (Figure S2 in Supporting Information). It can be noticed that the amount of porphyrins deposited upon each cycle is quasi-equal for the two different systems. Indeed, the increase of the intensity of the Soret absorption band is more important in the case of the [Co4POM16-/ZnTMePyP4þ]n film, but ZnTMePyP4þ has an extinction coefficient for the Soret band higher than ZnOEP(py)44þ (εSoret = 180 000 and 76 000 L mol-1 cm-1 for ZnTMePyP4þ and ZnOEP(py)44þ, respectively). Similar behavior has been also observed by changing the nature of POM, for instance with [P2W18O62]6-. Influence of Salt Concentration. To investigate the effect of ionic strength on the growth, films were prepared from 0.5 mM aqueous solutions of porphyrin and POM containing 0.1 M NaCl by sequential dipping cycles. Figure 1C shows the dependence of growth step on ionic strength for [Co4POM16-/ZnTMePyP4þ]n film. Obviously, more porphyrins and POMs are deposited at higher ionic strength after every dipping step. Regular film growth is also found in the presence of NaCl, but the magnitude of the growth step increases with increasing ionic strength. Then we propose that adding salt to the tetracationic porphyrin solution as well as the polyanionic POM solution reduces the mutual electrostatic repulsion of tetracationic porphyrins or polyanionic POMs.9,27 Stability. The stability of the [Co4POM16-/porphyrin] films in water, in 0.1 M NaCl water, and in 0.13 M of propan-2-ol has been investigated. For instance, the films formed from 5 deposition cycles have been immersed in the different solutions during 24 h, followed by 1 min of sonication. UV-visible spectra have been recorded for each sample. The UV-visible spectra of the films do not show significant absorption decreasing after treatment ( 1, the initial rates of the reaction are higher (between 2 and 5 10-6 mol L-1 min-1), but the silver nanowires are not very homogeneous in shape (Figure 6D). To explain the formation of the observed silver nanoparticles, a mechanism similar to the one for the electrostatic [Co4POM16-][ZnTMePyP4þ]4 complex in solution can be proposed.5 Indeed, visible illumination allows the formation of excited porphyrins, which can transfer electron to POM (Scheme 2). To justify this electron transfer, we can calculate the ΔG of this process using the Rehm-Weller equation ΔG ¼ Eðporph•þ =porphÞ - EðPOM=POMred Þ - ES0 -S 1 where E(porph•þ/porph) corresponds to the first oxidation potential of the porphyrin and E(POM/POMred) corresponds to the first reduction potential of the POM, obtained from the cyclic voltammograms of the compounds in aqueous solution (E(porph•þ/porph) = 1.15 V vs NHE22 for ZnTMePyP4þ and E(POM/POMred) = -0.20 V vs NHE20). ES0-S1 corresponds to the energy of the lowest electronic transition of the porphyrin determinated from its absorbance spectrum in solution (at 602 nm for ZnTMePyP4þ).22 This value leads to negative ΔG (-68.5 kJ mol-1), showing that the electron transfers between the excited porphyrins and the POMs are thermodynamically favorable. These electronic transfers lead to oxided porphyrins (porph•þ) and reduced POMs (POMred). Then, oxidized porphyrins are regenerated by reaction with propan-2-ol which leads to the formation of alcohol radicals (CH3)2C•OH. Indeed, this reaction is also thermodynamically favorable, since the redox potential of the couple (CH3)2C•OH/(CH3)2CHOH is estimated to 0.8 V vs NHE,34 that is lower than the oxidation potential of the porphyrin. Nevertheless, the potential of the couple POM/POMred is too high to permit the reduction of silver ions (E(Agþ/Ag0)=-1.75 V vs NHE, corresponding to the redox potential of a single silver atom, different from the one corresponding to metallic silver).35 We can also rule out the (32) Wang, Z.; Liu, J.; Chen, X.; Wan, J.; Qian, Y. Chem.;Eur. J. 2005, 11, 160–163. (33) Xu, J.; Hu, J.; Peng, C.; Liu, H.; Hu, Y. J. Colloid Interface Sci. 2006, 298, 689–693. (34) Gachard, E.; Remita, H.; Khatouri, J.; Keita, B.; Nadjo, L.; Belloni, J. New J. Chem. 1998, 1257–1265. (35) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1977, 81, 556–561.
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Figure 6. (A) Change in the UV-vis absorption spectrum of a deaerated aqueous solution of 8 10-5 mol L-1 Ag2SO4 and 0.13 mol L-1
propan-2-ol containing a slide of quartz modified with a [Co4POM16-/ZnTMePyP4þ]5 film under illumination. (B) Spectra of the silver solution before and after photocatalysis. (C,D) TEM images of the silver nanowires with the [Co4POM16-/ZnTMePyP4þ]5 film in deaerated solutions. (E) TEM image of the silver nanowires obtained with the [Co4POM16-/ZnTMePyP4þ]1 film in deaerated solutions. (F) TEM image of the silver nanowires obtained with the [Co4POM16-/ZnTMePyP4þ]25 film in deaerated solutions.
possibility of a photoinduced electron transfer from the excited porphyrins to the silver ions, because the ΔG values of such transfer is estimated equal to þ81.1 kJ mol-1. Similarly to our precedent works, we can suppose a complexation step between alcohol radical and silver ions which initiates the formation of silver clusters4,5
Then, association and coalescence reactions of the Agþ ions and initial silver clusters lead to the formation of silver particles
ðCH3 Þ2 C• OH þ Agþ f ½AgðCH3 Þ2 C• OHþ
Later, knowing that the redox potential of the silver cluster, E(Agnþ/Agn), increases with the nuclearity n,36 for sufficient large
½AgðCH3 Þ2 C• OHþ þ Agþ f Ag2 þ þ ðCH3 Þ2 CO þ Hþ
(36) Belloni, J.; Amblard, J.; Marignier, J. L.; Mostafavi, M. Clusters of atoms and molecules II; Springer-Verlag: Berlin, 1994; pp 290-311.
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2Ag2 þ f Ag4 2þ Agm xþ þ Agp yþ f Agðm þ pÞ ðxþyÞþ
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Scheme 2. (A) Schematic Proposed Mechanism for the Photoreduction of Silver Cations Using the Porphyrin-Polyoxometalate Films; (B) Redox Potentials of Involved Compounds
clusters, their direct reduction by the reduced POM become thermodynamically favorable. However, while large silver aggregates have been obtained by using the electrostatic complex in solution,5 in this present case of heterogeneous photocatalysis, the shape of the formed nanoparticles is considerably different. Indeed, silver nanowires have been obtained. Although it seems difficult to explain the shape of these silver nanoparticles, two hypotheses can be proposed. The first one considers that the surface of the films plays the role of matrix. In other words, the reduction of the Agþ ions takes place on the surface between the small micelles observed in AFM, leading to the formation of nanowires. However, this hypothesis does not explain the formation of straight nanowires. The second hypothesis is based on a growth of the nanowires perpendicularly to the surface at the interstices of the micelles, as proposed by Tung et al. in the case of the synthesis of silver nanowires on TiO2 films.37 They suggested that the nanowires grow in the [110] direction to minimize their surface energy with the substrate. In our study, the diameters of the Agn nanowires obtained fit well with this hypothesis. Experiments have been also carried out with [Co4POM16-/ ZnOEP(py)44þ]n films (Figure 7). Concerning the shape and the size of the formed silver nanoparticles, results are similar: long nanowires are obtained with more or less homogeneity in the shape distribution. Nevertheless, the initial rates of reduction are smaller than with [Co4POM16-/ZnTMePyP4þ]n systems (for example, equal to 7.6 10-7 mol L-1 min-1 with the [Co4POM16-/ ZnOEP(py)44þ]5 film). Moreover, the Soret band disappears revealing the instability of the films composed of ZnOEP(py)44þ upon illumination. However, in our precedent work, we have shown that the electrostatic [Co4POM16-][ZnOEP(py)44þ]4 complex was stable for applications in homogeneous catalysis in deaerated solution.5 Thus, to explain the difference of stability between [Co4POM16-/ ZnTMePyP4þ]n and [Co4POM16-/ZnOEP(py)44þ]n films, we cannot invoke the difference between the redox and the photophysical properties of the two porphyrins.22,30 Indeed, in this case, a similar (37) Tung, H.-T.; Chen, I.-G.; Song, J.-M.; Yen, C.-W. J. Mater. Chem. 2009, 19, 2386–2391.
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degradation would have been observed for the [Co4POM16-][ZnOEP(py)44þ]4 complex in solution. Nevertheless, the geometry of the macrocycle could be the factor modifying the stability of the films. Indeed, because of these 12 peripheral substituents, ZnOEP(py)44þ is a porphyrin having a nonplanar skeleton,21-23 and its conformation can differ within electrostatic complexes in solution and within supported films. Consequently, we can suppose that according to its conformation, the ZnOEP(py)44þ is more or less stable upon illumination. On the contrary, ZnTMePyP4þ has a planar conformation, and its geometry is not modified according to the system used (complexes in solution or supported films). Another hypothesis consists in assuming a different organization of the films according to the used porphyrin. Indeed, the distorted porphyrin ZnOEP(py)44þ could organize with POMs differently than the planar porphyrin ZnTMePyP4þ. However, if it is possible to consider a degradation of the macrocycle for the nonplanar ZnOEP(py)44þ upon illumination, we can also evoke a removing of the film during photocatalysis. In the case of the [[P2W18O62]6-/ZnTMePyP4þ]5 film, as expected, the film is also stable during irradiation (Figure S7 in Supporting Information). For application purposes, air conditions are preferred. So, the photocatalytic activity has been also tested in the presence of dioxygen for both systems. Figures S8A and S9A in Supporting Information show the absorption spectra recorded during the experiments performed respectively with the [Co4POM16-/ ZnTMePyP4þ]5 and the [Co4POM16-/ZnOEP(py)44þ]5 films in aerated aqueous solutions of Agþ ions. As in anaerobic solution, spectra present a broad absorption band in the whole visible domain, showing as before the formation of long silver nanowires. TEM micrographs (Figures S8B and S9B in Supporting Information) confirm the formation of such long silver nanowires. However, whatever the porphyrin used, a degradation of the system is observed when the experiment is carried out with dioxygen. But, in accordance with the experiments performed without dioxygen, the degradation is slower for the [Co4POM16-/ZnTMePyP4þ]n film compared to the films constituted with the other porphyrin. DOI: 10.1021/la1024923
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Figure 7. (A) Change in the UV-vis absorption spectrum of a deaerated aqueous solution of 8 10-5 mol L-1 Ag2SO4 and 0.13 mol L-1
propan-2-ol containing a slide of quartz modified with a [Co4POM16-/ZnOEP(py)44þ]5 film under illumination. (B) Spectra of the silver solution before and after photocatalysis. (C,D) TEM images of the silver nanowires.
Figure 8. (A) Spectra of an aqueous solution of 1.6 10-4 mol L-1 HAuCl4 and 0.13 mol L-1 propan-2-ol before and after photocatalysis performed without dioxygen with a [Co4POM16-/ZnTMePyP4þ]5 film. (B) TEM images of the gold nanosheets.
This photocatalytic process can be extended to the production of other metal nanoparticles. We have also investigated the reduction of gold ions. The experiments have been performed with a similar process, using HAuCl4 (1.6 10-4 mol L-1) instead of Ag2SO4. Results are similar whatever the film used (whatever the porphyrin used) and are analogous with the ones obtained with the reduction of silver ions. The main difference consists in the shape of the formed gold nanoparticles. The absorption spectrum shows a very broad absorption band in the whole visible domain with a maximum around 550 nm (Figures 8A and S10A in Supporting Information), and the TEM micrographs show the 142 DOI: 10.1021/la1024923
formation of gold “river-shape” nanosheets (Figures 8B and S10B in Supporting Information). To explain the difference of shapes between silver and gold nanoparticles, we can suggest that the interactions with the films are not the same for Agþ and AuCl4-. Indeed, Agþ cation dimension is smaller than AuCl4- anion one. First, we can suggest that cations are more repulsed than anions by the surface of the film, favoring a 1D direction growth. But the size of the ions can also influence the direction growth. Indeed, Agþ ions can be sufficiently little to penetrate into the interstices between the micelles constituting the film, favoring a 1D direction growth Langmuir 2011, 27(1), 132–143
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(see before). The Agþ ions get reduced at the interstice of the micelles leading to the formation of the Agn nanowires. The good conductivity of Ag makes the Ag nanowire “electrode” extend into solution where the next Agþ is found and reduced on the surface of Agn. Another explanation can be founded by considering the non homogeneity of the film. The Agn nanowires grow directly from the interstice of the micelle-type architecture that is allowed by diffusion of Agþ cations. On the contrary, AuCl4should be too large to penetrate into the interstices. Thus reduction of gold ions can take place onto the micelles, leading to the formation of small spherical nanosheets. Then these nanosheets can grow and join together, leading to the formation of larger nanosheets with various shapes.
Conclusion The formation of films by alternatively dippings of a support (slide of quartz or ITO electrode) into an aqueous solution of tetracationic porphyrin (ZnOEP(py)44þ or ZnTMePyP4þ) and an aqueous solution of the tetracobalt Dawson-derived sandwich polyanion Co4POM16- has been evidenced by UV-visible absorption spectroscopy, atomic force microscopy, XPS and electrochemistry. These films have been used for the photoreduction of silver and gold ions performed with visible light. In these systems, porphyrins act as photosensitizers able to give electrons to POM known to be good catalysts. Silver nanowires and gold nanosheets have been obtained. Experiments aiming to improve the control of the size and the shape of the silver nanowires are the next steps for potential applications in nanomaterials or nanoelectronic. Indeed, it is known that silver exhibits the highest electrical conductivity (38) Zhou, G.; L€u, M.; Yang, Z.; Zhang, H.; Zhou, Y.; Wang, S.; Wang, S.; Zhang, A. J. Cryst. Growth 2006, 289, 255–259.
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among all metals.32,33,37,38 Then synthesis of silver nanowires with well-defined dimensions is particularly interesting. For instance, the use of surfactants in the metallic ions solutions could be conceivable to control the shape of the nanowires.39,40 Acknowledgment. This work was supported by the CNRS, the Universite Paris-Sud (Paris 11, Orsay), the Universite Paris Descartes (Paris 5), the Universite Pierre et Marie Curie (Paris 6), ECE Paris Ecole d’Ingenieurs, and the Fudan University. This work was also supported by the French ANR agency, Project No. JC05-52437, NCPPOM, and Fund of senior visiting professor of Fudan University (Shanghai, China). Patricia Beaunier (UPMC, Paris 6) is gratefully acknowledged for TEM images. Supporting Information Available: Results concerning formation of the electrostatic complexes in aqueous solution, UV-visible absorption spectra and AFM images obtained for [Co4POM16-/ZnTMePyP4þ]n films, proposed arrangement between Co4POM16- and tetracationic porphyrins leading to selfassembled micelles, supplementary results concerning the [[P2W18O62]6-/ZnTMePyP4þ]n films and supplementary results of photocatalysis of the reduction of AgI and AuIII. This material is available free of charge via the Internet at http://pubs.acs.org. Note Added after ASAP Publication. This article was published ASAP on December 2, 2010. A current address for author Laurent Ruhlmann has been added. The correct version was published on December 10, 2010. (39) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736–4745. (40) Hu, J.-Q.; Chen, Q.; Xie, Z.-X.; Han, G.-B.; Wang, R.-H.; Ren, B.; Zhang, Y.; Yang, Z.-L.; Tian, Z.-Q. Adv. Funct. Mater. 2004, 14, 183–189.
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