Gold Nanoparticles Incorporated MCM

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Zinc Phthalocyanine and Silver/Gold Nanoparticles Incorporated MCM-41 Type Materials as Electrode Modifiers Manas Pal and Vellaichamy Ganesan* Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi-221 005, UP, India Received May 20, 2009. Revised Manuscript Received August 16, 2009 Mercaptopropyl functionalized ordered mesoporous silica spheres were prepared (MPS). Ag or Au nanoparticles (NPs) were anchored onto the MPS materials (Ag-MPS or Au-MPS). Further, zinc phthalocyanine (ZnPc) was adsorbed into the channels and surface (MPS-ZnPc, Ag-MPS-ZnPc, Au-MPS-ZnPc). Diffuse reflectance studies revealed the successful incorporation of Ag or Au NPs inside the silica spheres with and without ZnPc. TEM images showed the uniform distribution of Ag or Au NPs in the silica spheres of different size ranging from 4 to 22 nm or 6 to 31 nm, respectively. XRD pattern showed average crystallite particle size of 18 or 28 nm for Ag or Au NPs respectively which were reduced to 14 or 16 nm on introduction of ZnPc which oxidizes the metal NPs partially. Chemically modified electrodes were prepared by coating the colloidal solutions of the silica materials on the glassy carbon (GC) electrodes. Electrocatalytic reductions of O2 and CO2 at the modified electrodes were studied. The presence of Ag or Au NPs was found to increase the electrocatalytic efficiency of ZnPc toward O2 reduction by 290% or 70% based on the current density measured at -0.35 V and toward CO2 reduction by 150% or 120% based on the current density measured at -0.60 V respectively. Catalytic rate constants were increased 2-fold for O2 reduction and 8-fold for CO2 reduction due to Ag or Au NPs, respectively, which act as nanoelectrode ensembles. The synergic effect of ZnPc and metal NPs on the electrocatalytic reduction of O2 is presented.

1. Introduction Chemically modified electrodes have received much attention in the last 3 decades because they display significant improvement in the electron transfer kinetics with respect to unmodified electrodes by combining the intrinsic properties of the modifier to a selected redox process. They have found applications as electrochemical sensors and biosensors.1 Among the wide range of electrode modifiers, electrodes modified with silicates have received increasing attention in recent years.1 Mesoporous materials comprising the M41S family have become the focus of profuse research due to their potential applications, both as catalysts themselves or catalyst supports.2 Mesoporous molecular sieves represent a new class of inorganic materials, first recognized by scientists from the Mobil Corporation.3 The most popular structure of this family, mesoporous molecular sieves MCM-41, possess a regular array of uniform nanostructured pores, which can be systematically varied from about 15 to 100 A˚ in diameter.3 These characteristics, together with their extraordinary high surface and distinct adsorption properties, open up many potential applications in electrocatalysis.4 On the other hand, metal nanoparticles (NPs) are attracting increasing attention in recent years *Corresponding author. Telephone: þ 91-542-2307321. Fax: þ 91-5422368127, E-mail: [email protected] and [email protected]. (1) (a) Walcarius, A. Electroanalysis 2001, 13, 701. (b) Walcarius, A. Chem. Mater. 2001, 13, 3351. (c) Walcarius, A. Electroanalysis 2008, 20, 711. (d) Walcarius, A. Ordered Porous Solids 2009, 523. (e) Walcarius, A.; Kuhn, A. Trends Anal. Chem. 2008, 27, 593. (f) Murray, R. W., Ed. Molecular Design of Electrode Surfaces; J. Wiley & Sons: New York, 1992. (g) Kaneko, M.; Wohrle, D. Adv. Polym. Sci. 1988, 84, 141. (2) (a) Corma, A. Chem. Rev. 1997, 97, 2373. (b) Boveri, M.; Pliego, J. A.; Pariente, J.; Sastre, E. Catal. Today 2005, 107, 868. (3) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (b) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (4) (a) Jena, B. K.; Raj, C. R. Anal. Chem. 2006, 78, 6332. (b) Jena, B. K.; Raj, C. R. J. Phys. Chem. C 2007, 111, 15146. (c) Chandra, D.; Jena, B. K.; Raj, C. R.; Bhaumik, A. Chem. Mater. 2007, 19, 6290. (d) Jena, B. K.; Raj, C. R. J. Phys. Chem. C 2007, 111, 6228. (e) Jena, B. K.; Raj, C. R. Langmuir 2007, 23, 4064.

13264 DOI: 10.1021/la901792b

due to their unique size dependent optical, electronic, and catalytic properties. New physical, chemical, and thermodynamic properties are expected to occur in such systems, arising from the large fraction of low coordinated atoms at the surface and the confinement of electrons to a rather small volume, respectively.5 From a technical point of view, the development of future applications will need to adhere/support the nanoscaled materials on suitable substrates in two- or three-dimensional arrangements in order to take advantage of their fascinating properties under different conditions.6 In particular, colloidal gold/silver NPs have been the subject of strong interest in the areas of material science, biotechnology, and analytical chemistry due to their functions as molecular markers, diagnostic imaging, and catalyst in recent years.4,7,8 Metal NPs incorporation into a variety of matrixes to form so-called nanocomposite films are attracting much attention. The sol-gel method offers a simple and versatile route to prepare a 3-D silicate network through the hydrolysis and condensation of silicon alkoxide precursors and to combine the favorable properties of these organic-inorganic materials with metal NPs and catalyst. The sol-gel-derived three-dimensional (5) Boyen, H.-G.; Herzog, T.; Kastle, G.; Weigl, F.; Ziemann, P.; Spatz, J. P.; Moller, M.; Wahrenberg, R.; Garnier, M. G.; Oelhafen, P. Phys. Rev. B 2002, 65, 75412. (6) Akolekar, D. B.; Bhargava, S. K. J. Mol. Catal. A: Chem. 2005, 236, 77. (7) (a) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042. (b) Xiao, Y.; Pavlov, V.; Shlyahovsky, B.; Willner, I. Chem.;Eur. J. 2005, 11, 2698. (c) Zayats, M.; Baron, R.; Popov, I.; Willner, I. Nano Lett. 2005, 5, 21. (d) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem 2000, 1, 18. (e) Xiao, Y.; Pavlov, V.; Levine, S.; Niazov, T.; Markovitch, G.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 4519. (8) (a) Bond, G. C.; Thompson, D. T. Catal. Rev. Sci. Eng. 1991, 41, 319. (b) Hutchings, G. J.; Haruta, M. Appl. Catal., A 2005, 291, 2. (c) GrzybowskaSwierkosz, B. Catal. Today 2006, 112, 3. (d) Haruta, M. Catal. Today 1997, 36, 153. (e) Zheng, S.; Gao, L. Mater. Chem. Phys. 2002, 78, 512. (f) Araki, H.; Fukuoka, A.; Sakamoto, Y.; Inagaki, S.; Sugimoto, N.; Fukushima, Y.; Ichikawa, M. J. Mol. Catal. A: Chem. 2003, 199, 95. (g) Lu, G.; Ji, D.; Qian, G.; Qi, Y.; Wang, X.; Suo, J. Appl. Catal., A 2005, 280, 175. (h) Wang, A.; Liu, J.-H.; Lin, S. D.; Lin, T.-S.; Mou, C.-Y. J. Catal. 2005, 233, 186. (i) Akolekar, D. B.; Bhargava, S. K. J. Mol. Catal. A: Chem. 2005, 236, 77. (j) Okumura, M.; Tsubota, S.; Haruta, M. J. Mol. Catal. A: Chem. 2003, 199, 73.

Published on Web 10/14/2009

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Article Scheme 1

network is particularly attractive in the development of sensing devices, because the network exhibits tunable porosity, high thermal stability, and chemical inertness.4 Direct electrochemical reduction or oxidation of substrate molecules at the bare electrodes are often not suitable for analytical applications as the electrode reactions are subjected to a high overpotential. Modifying the surface of an electrode with a redox mediator is a wellestablished strategy for achieving wider applicability of electroanalytical methodology. Despite the advantageous properties of the mesoporous MCM-41 type materials,1a-e major hurdle with these materials is the lack of conductivity when they are used as electrode modifiers and a significant fraction of redox species which are immobilized inside the silica matrix is not electrochemically active and thereby reducing the sensitivity of the electrodes. In order to overcome this problem, in this work, metal NPs (Ag or Au) are anchored on the mercaptopropyl functionalized MCM-41 type materials. Further, zinc phthalocyanine (ZnPc) was adsorbed on the surface of the above materials and used for electrocatalytic reduction studies. Synthesis and characterization of metal phthalocyanine integrated MCM-41 materials were reported by different groups for various applications.9 Metal phthalocyanines and (9) (a) Ernst, S.; Selle, M. Microporous Mesoporous Mater. 1999, 27, 355. (b) Lu, X.-B.; Wang, H.; He, R. J. Mol. Catal. A: Chem. 2002, 186, 33. (c) Wohrle, D. Macromol. Rapid Commun. 2001, 22, 68. (d) Armengol, E.; Corma, A.; Fornes, V.; Garcia, H.; Primo, J. Appl. Catal., A 1999, 181, 305. (10) (a) D’Souza, F.; Hsieh, Y.-Y.; Deviprasad, G. R. J. Electroanal. Chem. 1997, 426, 17. (b) Wiesener, K.; Ohms, D.; Neumann, V.; Franke, R. Mater. Chem. Phys. 1989, 22, 457. (c) Zagal, J. H. Coord. Chem. Rev. 1992, 119, 89. (d) Fuerte, A.; Corma, A.; Iglesias, M.; Morales, E.; Sanchez, F. J. Mol. Catal. A: Chem. 2006, 246, 109. (e) Fuerte, A.; Corma, A.; Iglesias, M.; Morales, E.; Sanchez, F. Catal. Lett. 2005, 101, 99. (f) Premkumar, J.; Ramaraj, R. J. Mol. Catal. A: Chem. 1999, 142, 153. (g) Casaletto, M. P.; Longo, A.; Venezia, A. M.; Martorana, A.; Prestianni, A. Appl. Catal., A 2006, 302, 309. (h) Veen, J.A.R.v.; Visser, C. Electrochim. Acta 1979, 24, 921. (i) Fish, J. R.; Kubaszewski, E.; Peat, A.; Malinski, T.; Kaczor, J.; Kus, P.; Leszek Czuchajowski, L. Chem. Mater. 1992, 4, 795. (11) (a) Magdesieva, T. V.; Yamamoto, T.; Tryk, T. A.; Fujishimab, A. J. Electrochem. Soc. 2002, 149, D89. (b) Premkumar, J.; Ramaraj, R. J. Mol. Catal. A: Chem. 1997, 110, 53. (c) Yoshida, T.; Kamato, K.; Tsukamoto, M.; Iida, T.; Schietiwein, D.; Woehrle, D.; Kaneko, M. J. Electroanal. Chem. 1995, 385, 209. (d) Mahmood, M. N.; Masheder, D.; Harty, C. J. J. Appl. Electrochem. 1987, 17, 1223.

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metal porphyrins are known to catalyze the reduction of O2,10 CO2,11 NO2-,12 and NO3-.12c,13 Simultaneous reduction of NO2- or NO3- with CO2 is also reported.14 Herein we describe the electrocatalytic properties of metal nanoparticles and ZnPc loaded MCM-41 type matierials. Thus, the main objective of the present investigation is to examine the influence of metal NPs on the electrocatalytic activity of the ZnPc toward the reduction of O2 and CO2 expecting that the imbedded metal NPs can act as a nanoelectrode ensemble4a and hence increase the sensitivity/ catalytic current.

2. Experimental Section Chemicals and Reagents. All reagents used were analytical grade, and solutions were prepared with triply distilled water. Tetraethoxysilane (TEOS, >98%), mercaptopropyltriethoxysilane (MPTS, >80%), (Aldrich), cetyltrimethylammonium bromide (CTAB), (Himedia), Silver nitrate (Merck), Hydrogen tetrachloroaurate (III) solution (HAuCl4, 99.99%) (Aldrich), zinc phthalocyanine (ZnPc), (Aldrich) Polystyrene (Aldrich) were used as received. K4[Fe(CN)6] and 2,20 -bipyridyl (bpy) were obtained from sd fine chemicals. Fe(bpy)3(SO4)2 was prepared according to the literature procedure.15 Preparation of Mercaptopropyl Functionalized MCM-41 Type Materials.16-18 One-pot synthetic approach was used to

synthesis mercaptopropyl functionalized silica.16-18 Typically, (12) (a) Silva, S. D.; Shan, D.; Cosnier, S. Sensors Actuators B 2004, 103, 397. (b) Li, H.-L.; Chambers, J. Q.; Hobbs, D. T. J. Appl. Electrochem. 1988, 18, 454. (c) Chebotareva, N.; Nyokong, T. J. Appl. Electrochem. 1997, 27, 975. (13) (a) Pournaghi-Azar, M. H.; Dastangoo, H. J. Electroanal. Chem. 2004, 567, 211. (b) Tenne, R.; Patel, K.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1993, 347, 409. (c) Hwang, S.; Lee, J.; Kwak, J. J. Electroanal. Chem. 2005, 579, 143. (d) Gauthard, F.; Epron, F.; Barbier, J. J. Catal. 2003, 220, 182. (14) (a) Shibata, M.; Furuya, N. Electrochim. Acta 2003, 48, 3953. (b) Shibata, M.; Furuya, N. J. Electroanal. Chem. 2001, 507, 177. (15) Burgess, J.; Prince, R. H. J. Chem. Soc. 1965, 6061. (16) Etienne, M.; Lebeau, B.; Walcarius, A. New J. Chem. 2002, 26, 384. (17) (a) Walcarius, A.; Delacote, C. Chem. Mater. 2003, 15, 4181. (b) Ganesan, V.; Walcarius, A. Langmuir 2004, 20, 3632. (c) Walcarius, A.; Ganesan, V. Langmuir 2006, 22, 469. (18) (a) Walcarius, A.; Etienne, M.; Lebeau, B. Chem. Mater. 2003, 15, 2161. (b) Walcarius, A.; Etienne, M.; Bessiere, J. Chem. Mater. 2002, 14, 2757.

DOI: 10.1021/la901792b

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Article CTAB (2.4 g) was dissolved in 50 mL of deionized water and 45 mL of ethanol to which 14.6 mL of 25% aqueous ammonia was added. The precursor mixture was prepared by dissolving MPTS and TEOS in 20:80 molar ratios in 5 mL of ethanol, which was then added to the surfactant-plus-catalyst solution under vigorous stirring condition. Condensation occurred within 5 min, however stirring was continued further for 2 h at room temperature. Product was then isolated by vacuum filtration and washed alternatively with excess water and ethanol. The resulting powder was dried in vacuum for 24 h. The surfactant was removed from the hybrid material by acid/solvent extraction by suspending the 1 g of solid particles in 1 M HCl (8.8 mL) in ethanol (92 mL), which was then allowed to reflux during 18 h. Solution was filtered and washed with excess ethanol, and dried in vacuum for 24 h. The functionalized (mercaptopropyl-silica or thiol-functionalized silica), materials have been named hereafter as MPS.

Preparation of Ag-MPS and Au-MPS Materials.19,20 Synthesis of Ag-MPS and Au-MPS are as follows: Calculated amount of MPS suspended with AgNO3 or HAuCl4 in 50 mL of distilled water (the ratio of -SH: Ag or Au was kept as 1: 1). After equilibrating for 12 h under stirring, the mixture was poured to a beaker containing 10 mL of NaBH4 solution (3-5 °C) and stirred for another 2 h. The metal particle loaded MPS was filtered and washed with water and ethanol and dried in vacuum desiccator in dark. Hereafter they are represented as Ag-MPS or Au-MPS.

Preparation of MPS-ZnPc, Ag-MPS-ZnPc, and Au-MPS-ZnPc Materials. MPS or Ag-MPS or Au-MPS material was poured into 25 mL of DMF solution containing ZnPc and allowed to equilibrate for 12 h under stirring condition, filtered, washed with excess DMF and dried in vacuum desiccators in dark. The ratios of -SH:Zn and -SH:Ag or Au:Zn were kept as 1:1 and 1:1:1 respectively. Here after they are represented as MPS-ZnPc, Ag-MPS-ZnPc, or Au-MPS-ZnPc, respectively. Schematic representation of preparation of the above materials is shown in Scheme 1.

Preparation of Colloidal Ag-MPS and Au-MPS Materials.21 A 1 mL aliquot of of silica solution with MPTS (20%)

and TEOS (80%) in water-ethanol-ammonia mixture (500 μL355 μL-145 μL) was prepared. To this was added excess NaBH4 dissolved in 98 mL of water under vigorous stirring. Then Ag or Au precursors (the ratio of -SH:Ag or Au is 1:1) dissolved in 1 mL water was added (3-5 °C), and the reaction was allowed to proceed for a further 2 h. Metal NPs incorporated colloidal organo silica spheres were used within 24 h. Preparation of Modified Electrodes. Typically, 0.5% or 1.0% colloidal solution of the respective material (MPS, Ag-MPS, Au-MPS, MPS-ZnPC, Ag-MPS-ZnPc, and Au-MPS-ZnPc) with 0.01% poly(vinyl alcohol) (PVA) as a binder was prepared. Glassy carbon (GC) electrodes were polished with alumina on a Buehler-felt pad, washed with water followed by methanol and sonicated for 5 min in water. A 5 μL aliquot of the colloidal solution was coated onto 0.07 cm2 of the GC electrode, which was air-dried under dark conditions and used within 24 h for electrochemical studies. Another type of electrodes were also prepared using Calzaferri’s monograin approach22 for the zeolite modified electrodes.22,23 In this method, in the first step, GC electrodes were coated with 5 μL of 0.5% or 1.0% colloid without PVA and air-dried. In the second step, 5 μL of 0.01% of polystyrene in tetrahydrofuran was applied and air-dried. Both types of electrodes showed similar behavior and most of the results reported in this study are based on the electrodes prepared in the first method. (19) Mukherjee, P.; Patra, C. R.; Ghosh, A.; Sastry, M.; Kumar, R. Chem. Mater. 2002, 14, 1678. (20) Mukherjee, P.; Patra, C. R.; Kumar, R.; Sastry, M. Phys. Chem. Commun. 2001, 4, 24. (21) (a) Maduraiveeran, G.; Ramaraj, R. J. Electroanal. Chem. 2007, 608, 52. (b) Jiang, Z.-J.; Liu, C.-Y.; Sun, L.-W. J. Phys. Chem. B 2005, 109, 1730. (22) Li, J.-w.; Calzaferri, G. Chem. Commun. 1993, 1430. (23) Ganesan, V.; Ramaraj, R. Langmuir 1998, 14, 2497.

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Instrumentation. Diffuse reflectance spectra were recorded from UV/vis (UV 1700 Pharma Spec, Shimadzu) spectrophotometer, (powdered samples were mixed well with one or two drops of Nujol to make a mull and this mull was coated on a filter paper strip). The absorption spectral studies were carried out with UV/vis spectrophotometer (2802 PC UNICO, USA). X-ray diffraction (XRD) studies were carried out using a classical powder diffractometer (1D 3000 SEIFERT, Germany) with transmission geometry, using Cu KR radiation and a fixed powder source. Samples were scanned over a range of 2θ = 10-80°. Cyclic voltammograms (CV) were recorded using CH Instruments (CHI-660C) potentiostat/galvanostat. A three electrode one compartment cell with silica materials modified GC electrode as working electrode, Pt wire as counter electrode and an Ag/AgCl as reference electrode, were used for the electrochemical studies. All electrochemical studies were carried out at room temperature under nitrogen atmosphere unless otherwise mentioned. Transmission electron microscope (TEM) pictures were collected from TECNAI 20G2 FEI microscope, operating at 200 kV. Calculation of Rate Constants. Choronoamperometry was used for the evaluation of the catalytic rate constants by several groups.24 According to Pariente et al.,25 I cat =I L ¼ γ1=2 ½π1=2 erfðγ1=2 Þ þ erfð -γÞ=γ1=2 

ð1Þ

where Icat and IL are the currents of the modified electrodes in the presence and absence of substrate (i.e O2 or CO2) and γ=kCt is the argument of the error function. k is the catalytic rate constant, C is the bulk concentration of substrate, and t is elapsed time (s). In the cases where γ > 1.5, erf (γ1/2) is almost equal to unity, and the above equation can be reduced to I cat =I L ¼ γ1=2 π1=2 ¼ π1=2 ðkCtÞ1=2

ð2Þ

From the slope of the Icat/IL vs t1/2 plot, the k for the catalytic reduction was calculated. Calculation of Active Surface Area. Capacitive currents (iC) were measured from the CV response of the modified electrodes in 50 mM NaNO3 (Figure S1). iC can be related to active surface area (Aact) of the modified electrodes by eq 326-28 iC ¼ -νAact C d

ð3Þ

where ν is the scan rate (V s-l), Aact is the active surface area (cm2) of the electrode and Cd is the double-layer capacitance (μF cm-2). From the slope of the plot of iC vs ν and using 21 μF cm-2 for Cd,26c Aact of the modified electrodes were estimated (average of three experiments). iC was measured at 0.2 at 0.1 V s-1 scan rate for GC/MPS, GC/MPS-ZnPc, GC/Au-MPS, and GC/ Au-MPS-ZnPc electrodes and at -0.05 at 0.1 V s-1 scan rate for GC/Ag-MPS and GC/Ag-MPS-ZnPc electrodes.

3. Results and Discussion Characterization of Materials. Mercaptopropyl-functionalized silica spheres containing Ag or Au NPs with and without ZnPc were characterized by diffuse reflectance spectral measurements (24) (a) Elahi, M. Y.; Heli, H.; Bathaie, S. Z.; Mousavi, M. F. J Solid State Electrochem. 2007, 11, 273. (b) Sabzi, R. E.; Pournaghi-Azar, M. H. Anal. Sci. 2005, 21, 689. (c) Razmi-Nerbin, H.; Pournaghi-Azar, M. H. J Solid State Electrochem. 2002, 6, 126. (d) Galus, Z. Fundamentals of Electrochemical Analysis; Horwood: New York, 1976; p 313. (e) Zhang, L.; Dong, S. J. Electroanal. Chem. 2004, 568, 189. (25) Pariente, F.; Lorenzo, E.; Tobalina, F.; Abruna, H. D. Anal. Chem. 1995, 67, 3936. (26) (a) Cheng, I. F.; Whiteley, L. D.; Martin, C. R. Anal. Chem. 1989, 61, 762. (b) Penner, R. M.; Martin, C. R. Anal. Chem. 1987, 59, 2625. (c) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920. (27) (a) Pereira, F. C.; Moretto, L. M.; Leo, M. D.; Zanoni, M. V. B.; Ugo, P. Anal. Chim. Acta 2006, 575, 16. (b) Leo, M. D.; Kuhn, A.; Ugo, P. Electroanalysis 2007, 19, 227. (28) (a) Baker, W. S.; Crooks, R. M. J. Phys. Chem. B 1998, 102, 10041.

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Figure 1. UV-vis diffuse reflectance spectra of (a) MPS, (b) Ag-MPS, (c) Ag-MPS-ZnPc, and (d) MPS-ZnPc materials. Curve e shows the spectrum of ZnPc in dichloromethane.

(Figure 1 and 2). MPS material does not show any characteristic absorption peak in the wavelength range of 300-900 nm (Figure 1a and Figure 2a).29 On the other hand, Ag and Au nanoparticle loaded silica material display characteristic surface plasmon band at 465 and 550 nm, respectively, supporting the existence of nanosized particles on the silica material (Figure 1b and Figure 2b). It is known that the organosulfur compounds can chemisorb on the coinage metal surface.4,20,30-32 In our case, it is believed that the Ag and Au metal particles are anchored on the mercaptopropyl moiety of the materials.21,33,34 Ag-MPS-ZnPc and Au-MPS-ZnPc materials showed a broad peak around 680 nm (Figures 1c and 2c), which is attributed to the Q-band of the ZnPc. Broad bands of ZnPc extending from 500 to 800 nm suggests the presence of dimers or higher aggregates of ZnPc at the organo silica spheres. In Ag-MPS-ZnPc or Au-MPS-ZnPc material, the peak due to the metal NPs is not observed clearly. This could be due to (1) the metal nanoparticles are completely covered by ZnPc and blocked the absorbance (2) formation of aggregates of metal NPs which has the absorption band at higher wavelength that merges with the absorption band of ZnPc, as observed by Jia et al.35 (3) introduction of ZnPc partially oxidizes the metal NPs and induce a reorientation/ deaggregation process (vide infra) thereby decreasing the size of the gold NPs and shifting the surface plasmon band to lower wavelengths which merges with the ZnPc absorption band in the blue region. It is known that the wavelength of maximum absorption and the bandwidth of the plasmon resonance depend on the size and shape of the metal particles or aggregates on the substrates.33-36 (29) Rodrigues, S.; Uma, S.; Martyanov, I. N.; Klabunde, K. J. J. Mol. Catal. A: Chem. 2005, 233, 405. (30) (a) Kudelski, A. Langmuir 2003, 19, 3805. (b) Sharma, J.; Mahima, S.; Kakade, B. A.; Pasricha, R.; Mandale, A. B.; Vijayamohanan, K. J. Phys. Chem. B 2004, 108, 13280. (c) Aslam, M.; Mulla, I. S.; Vijayamohanan, K. Langmuir 2001, 17, 7487. (d) Venkataramanan, M.; Murty, K. V. G. K.; Pradeep, T.; Deepali, W.; Vijayamohanan, K. Langmuir 2000, 16, 7673. (e) Bandyopadhyay, K.; Patil, V.; Vijayamohanan, K.; Sastry, M. Langmuir 1997, 13, 5244. (31) (a) Liu, T.-X.; Gli, G.-H.; Li, F.-B. Environ. Sci. Technol. 2008, 42, 4540. (b) Luo, N.; Mao, L.; Jiang, L.; Zhan, J.; Wu, Z.; Wu, D. Mater. Lett. 2009, 63, 154. (32) (a) Ghosh, A.; Patra, C. R.; Mukherjee, P.; Sastry, M.; Kumar, R. Microporous Mesoporous Mater. 2003, 58, 201. (b) Haruta, M. Catal. Today 1997, 36, 153. (c) El-Deap, M. S.; Ohsaka, T. Electrochem. Commum. 2002, 4, 288. (33) (a) Zheng, J.; Wu, X.; Wang, M.; Ran, D.; Xu, W.; Yang, J. Talanta 2008, 74, 526. (b) Kundu, S.; Mandal, M.; Ghosh, S. K.; Pal, T. J. Collid Inter. Sci. 2004, 272, 134. (c) Shao, K.; Yao, J. Mater. Lett. 2006, 60, 3826. (34) (a) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (b) Nakamura, T.; Yamada, Y.; Yano, K. J. Mater. Chem. 2007, 17, 3726. (c) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396. (35) Jia, H.; Zeng, J.; Song, W.; An, J.; Zhao, B. Thin Solid Films 2006, 496, 281. (36) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148.

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Figure 2. UV-vis diffuse reflectance spectra of (a) MPS, (b) Au-MPS (c) Au-MPS-ZnPc and (d) MPS-ZnPc materials. Curve e shows the spectrum of ZnPc in dichloromethane.

In order to understand the interaction of ZnPc with gold and silver NPs, colloidal solutions of the respective NPs were prepared. (It should be noted that, for this study the colloidal metal NPs in MPS matrix was prepared by mixing the necessary amounts of TEOS and MPTS in situ (vide supra). Surfactant template (CTAB) is not used and no attempt was made to collect the solid samples.) To a colloidal solution of Ag-MPS material, ZnPc in DMF was added incrementally. Control experiment was carried out by mixing the same amount of ZnPc in DMF to water to understand the shifts in absorbance due to solvent. Careful analysis reveals that addition of ZnPc induces a blue shift (∼14 nm) to Ag NPs absorption band (figure not shown) with a significant increase in the peak intensity. However, there is no change in the absorption band due to ZnPc in the longer wavelength region. These results show that there is a charge transfer interaction between ZnPc and Ag NPs37 and possibly this interaction can induce a partial oxidation to the Ag NPs. However it should be noted that the oxidized NPs can be reduced back by the silanol groups and/or by mercaptopropyl groups in the MPS materials19,20 resulting in the deaggregation and reorientation of Ag NPs. Since there is no change in the absorption band due to ZnPc (either in intensity or in λmax) in the longer wavelength region, it is believed that complete oxidation of Ag NPs back to its ions is not taking place nevertheless, possibility of partial oxidation can not be neglected. Similar studies and careful analysis with Au-MPS material reveals that, addition of ZnPc induces a blue shift to Au NPs absorption band (∼10 nm) (Figure not shown) with a small decrease in absorption band of ZnPc in the longer wavelength region. The small decrease in absorbance intensity could be due to the partial oxidation of Au NPs back to its ions. However, it is possible that the oxidized Au NPs can be autoreduced back to Au NPs by the silanol groups or by mercaptopropyl groups of the MPS materials.19,20 Thus, the blue shift could be attributed to the change in size of the Au NPs due to deaggregation/reorientation process in the presence of ZnPc. In addition to the partial oxidation of metal NPs, ZnPc may cause a strain to the metal NPs resulting in the decreased size of the metal NPs.38 The prepared materials were further characterized by TEM. Figures 3 and 4 show the TEM pictures of the materials studied. MPS samples (Figure 3a) are in a shape of typical silica microspheres (250-600 nm in diameter) usually obtained in (37) (a) Naito, T.; Sugawara, H.; Inabe, T.; Kitajima, Y.; Miyamoto, T.; Niimi, H.; Asakur, K. Adv. Functional Mater. 2007, 17, 1663. (b) Naumov, S.; Kapoor, S.; Thomas, S.; Venkateswaran, S.; Mukherjee, T. J. Mol. Struct. (THEOCHEM) 2004, 685, 127. (38) (a) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (b) Yadav, T. P.; Mukhopadhyay, N. K.; Tiwari, R. S.; Srivastava, O. N. Mater. Sci. Eng. A. 2007, 449, 1052. (c) Sharma, R. K.; Sharma, P.; Maitra, A. J. Colloid Interface Sci. 2003, 265, 134.

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Figure 4. TEM images of (a) Au-MPS and (b) Au-MPS-ZnPc materials.

Figure 3. TEM images of (a) MPS, (b) Ag-MPS and (c) AgMPS-ZnPc materials.

water-alcohol-ammonia solution, which is consistent with the first report on the preparation of mesoporous spherical silica particles containing an organic moiety covalently attached to an ordered silica network.16 The presence of Ag or Au in the Ag/ Au-MPS materials can be clearly seen from Figures 3b and 4a, respectively. The shape of the silica microspheres and the porous wormhole like mesostructures are not affected by the introduction of Ag or Au NPs. Ag NPs were of different sizes ranging from 4 to 22 nm and Au NPs ranging from 6 to 31 nm (Figure 3b, 4a and Table 1). As the MCM 41 materials are known to contain pores 13268 DOI: 10.1021/la901792b

around 38 A˚ in diameter,18a the mercaptopropyl groups are expected to be distributed uniformly throughout the channels,16,17,18a and considering the chemisorptions property of mercaptopropyl groups with metal,20,30-32 it is expected that Ag or Au NPs are distributed uniformly in the MPS materials. However, presence of large sized metal NPs indicates that they are formed on the surface of the silica spheres also. Introduction of ZnPc to the metal NPs incorporated MPS materials decreases the size of the metal NPs which is reflected by Figures 3c and 4b also supported by UV-vis spectroscopic studies (vide supra). In Ag-MPS-ZnPc and Au-MPS-ZnPc materials, after the introduction of ZnPc, (compare parts b and c of Figure 3 and parts a and b of Figure 4, respectively) the number of Ag or Au NPs decreased. This could be due to the fact of partial oxidation of Ag or Au NPs back to its ions by ZnPc, and thus formed metal ions were leached from the silica backbone into solution during equilibration with ZnPc in DMF. Figure 5 shows the XRD patterns of MPS, Ag-MPS, and Ag-MPS-ZnPc materials within the range of 2θ = 10-80°. A typical halo pattern of SiO2 around 22.3° is observed in MPS which could be attributed to the characteristic of wormhole like mesostructures,39 indicating a significant degree of orderness in (39) (a) Walcarius, A.; Sayen, S.; Gerardin, K.; Hamdoune, S.; Rodeh€user, L. Colloids Surf. A: Physicochem. Eng. Asp. 2004, 234, 145. (b) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (c) Prouzet, E.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 516. (d) Mercier, L.; Pinnavaia, T. J. Chem. Mater. 2000, 12, 188.

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Article Table 1. XRD and TEM Data for Metal NPs in MPS Materials d spacing (nm) and (hkl)

particle size (nm)

material Ag-MPS Ag-MPS-ZnPc Au-MPS Au-MPS-ZnPc

2.36 (111) 2.36 (111) 2.35 (111) 2.35 (111)

2.04 (200) 2.04 (200) 2.04 (200) 2.07 (200)

1.44 (220) 1.45 (220) 1.44 (220) 1.44 (220)

1.23 (311) 1.23 (311) 1.23 (311) 1.22 (311)

XRD

TEM

18 14 28 16

4 to 21 not measurable 6 to 31 3 to 7

Figure 5. XRD patterns of (a) MPS, (b) Ag-MPS, (c) AgMPS-ZnPc, and (d) MPS-ZnPc materials.

Figure 6. XRD patterns of (a) MPS, (b) Au-MPS, (c) AuMPS-ZnPc, and (d) MPS-ZnPc materials.

this material which is confirmed by the TEM picture of silica spheres (Figure 3a), where the framework porosity can be seen. The structure of MPS is similarly ordered to those characteristic of aminopropyl- or mercaptopropyl or quaternary ammoniumfunctionalized organo silicas produced according to the similar synthetic procedure, which display typical MCM-41 long-range packing.16,17,39a XRD reflections of Ag-MPS, show four peaks around 38.1°, 44.3°, 64.5°, and 77.5°. These peaks were respectively assigned to (111), (200), (220), and (311) reflection lines of the cubic structure of Ag (Table 1).40 Average crystallite particle size of Ag NPs were calculated from XRD reflections (Table 1), using the Debye-Scherrer equation,41 adopting the reported method32a,40a and found to be 18 nm which are in agreement with the TEM measurements. In Ag-MPS-ZnPc material, reflections due to ZnPc are observed in addition to the Ag NPs. However, a significant decrease in the average crystallite particle size (14 nm) with a decrease in peak intensity of Ag NPs reflections are observed which may be due to charge transfer interaction between Ag NPs and ZnPc (vide supra) and the surrounding of ZnPc molecules around Ag NPs which hinders the diffraction reflections. It has been observed that organic components and clusters of Ag cause a decrease in peak intensity of the low-angle reflections of MCM-41 materials.40b,42 Similarly XRD patterns of Au-MPS and Au-MPS-ZnPc materials are shown in Figure 6. The observed results (Table 1) were similar to Ag materials. Characterization of Modified Electrodes. CV can be used to evaluate the response characteristics of the nanoelectrode ensembles. The characterization and evaluation of nanoelectrode ensembles (or ultramicro electrode ensembles) were elegantly demonstrated by Martin and co-workers26 and latter by others

by different methods.27,28,43 CV responses of the modified electrodes were examined in solutions containing different redox probes. Two different solutions were tested namely (1) 0.5 mM K4[Fe(CN)6] with 50 mM NaNO3, and (2) 2.5 mM Fe(bpy)32þ with 0.1 M Na2SO4. The plot of log of anodic peak current (log (ipa)) and log of scan rate in V s-1 (log υ) were examined for the systems studied (Figures S2 and S3, respectively). The peak current at lower scan rate is very similar to those obtained on a macroelectrode of same geometrical surface area. The plot is linear in the case of macroelectrode. However, on the modified electrode the plot deviates from linearity at high scan rate (g20 mV/s). This voltammetric feature is in close agreement with the characteristics of the nanoelectrode ensemble based electrodes.26a It has been described that a plot of log (ipa) and log υ for the nanoelectrode ensembles deviate from the current observed at the macroelectrodes.26a The deviation from linearity on the modified electrode points out the nanoelectrode ensemble behavior of our electrodes. In the case of GC/Ag-MPS and GC/ Ag-MPS-ZnPc electrodes, the high current observed (curves are not shown) for the redox probe is attributed to the catalytic effect of Ag NPs and the overlap of the peak current of Ag NPs and K4[Fe(CN)6]. As observed from the TEM images (Figure 3b and 3c) the nanoparticles are present on silica spheres and the distance between the particles are high and hence the modified electrode can be considered as nanoelectrode ensemble.43 The reproducible and stable CV responses recorded for MPS-ZnPc, Ag-MPS, and Ag-MPS-ZnPc coated electrodes in 0.05 M HClO4 are shown in Figure 7A. GC/MPS-ZnPc electrode shows a redox wave with an E1/2 value of -0.228 V due to the ZnPc redox couple (i.e. [ZnIIPc(-2)]/[ZnIIPc(-3)]-)44 (Figure 7A-a). GC/Ag-MPS electrode shows a redox wave with an E1/2 value of 0.242 V due to Ag NPs oxidation-reduction

(40) (a) Mukherjee, P.; Patra, C. R.; Kumar, R.; Sastry, M. Phys. Chem. Commun. 2001, 5, 1. (b) Zhao, H.; Zhou, J.; Luoa, H.; Zeng, C.; Lia, D.; Liu, Y. Catal. Lett. 2006, 108, 49. (41) Jeffrey, J. W. Methods in Crystallography; Academic Press: New York, 1971. (42) Zhao, X.-G.; Shi, J.-L.; Hu, B.; Zhang, L.-X.; Hua, Z.-L. Mater. Lett. 2004, 58, 2152. (43) (a) Zoski, C. G.; Yang, N.; He, P.; Berdondini, L.; Koudelka-Hep, M. Anal. Chem. 2007, 79, 1474. (b) Welch, C. M.; Banks, C. E.; Simm, A. O.; Compton, R. G. Anal. Bioanal. Chem. 2005, 382, 12. (c) Baron, R.; Sljukic, B.; Salter, C.; Crossley, A.; Compton, R. G. Russ. J. Phys. Chem. A 2007, 81, 1443.

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(44) (a) Gantchev, T. G.; Lier, J. E. V.; Hunting, D. J. Radiat. Phys. Chem. 2005, 72, 367. (b) Kandaz, M.; Yarasir, M. N.; Koca, A. Polyhedron 2009, 28, 257. (c) Masilela, N.; Idowu, M.; Nyokong, T. J. Photochem. Photobiol. A: Chem. 2009, 201, 91. (d) Esenpnar, A. A.; Ozkaya, A. R.; Bulut, M. Polyhedron 2009, 28, 33. (e) Yarasir, M. N.; Kandaz, M.; Senkal, B. F.; Koca, A.; Salih, B. Dyes Pigments 2008, 77, 7. (f) Giraudeau, A.; Louati, A.; Gross, M.; Andre, J. J.; Simon, J.; Su, C. H.; Kadish, K. M. J. Am. Chem. Soc. 1983, 105, 2917.

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Figure 8. Current density-potential curves for (a) GC/MPS-ZnPc, (b) GC/Ag-MPS-ZnPc, and (c) GC/Ag-MPS electrodes in 0.05 M HClO4 in oxygen saturated solution. Curve d is for GC/ Ag-MPS electrode under N2 saturated condition. The inset shows the corresponding CV responses at a scan rate of 50 mV s-1.

Figure 7. (A) CVs of (a) GC/MPS-ZnPc, (b) GC/Ag-MPS, and (c) GC/Ag-MPS-ZnPc electrodes in 0.05 M HClO4. Scan rate= 50 mV s-1. (B) CVs of (a) GC/MPS-ZnPc, (b) GC/Au-MPS, and (c) GC/Au-MPS-ZnPc electrodes in 0.05 M HClO4. Scan rate= 50 mV s-1. Inset shows the CV response of bulk Au electrode under similar conditions.

couple (Figure 7A-b).45 Ag-MPS-ZnPc modified GC electrodes show a redox peak (Figure 7A-c) due to Ag NPs oxidation-reduction couple which is shifted to an E1/2 value of 0.252 V in addition to ZnPc redox couple ([ZnIIPc(-2)] to [ZnIIPc(-3)]-) which is shifted to an E1/2 value of -0.292 V. The shift in potentials may be due to the interaction between the Ag NPs and ZnPc. In the positive range, GC/MPS-ZnPc electrode shows a pair of redox peak (ill defined) with an E1/2 value of 0.620 V which could be attributed to the oxidation of [ZnIIPc(-2)] to [ZnIIPc(-1)]þ (Figure 7B-a).44 At more positive range, (i.e., at 1.120 V) they showed an oxidation peak which could be due to the irreversible Pc ligand oxidation. GC/ Au-MPS electrode shows an oxidation peak due to the formation of surface oxide layer at 1.272 V which is reduced at 0.833 V (Figure 7B-b).43a,c,46 GC/Au-MPS-ZnPc electrode shows (Figure 7B-c) a redox wave with an E1/2 value around 0.590 V which can be attributed to [ZnIIPc(-2)] to [ZnIIPc(-1)]þ redox couple, in addition to Au-oxide formation and reduction at 1.175 and 0.876 V, respectively. It should be noted that in the presence of Au NPs, the redox reaction of [ZnIIPc(-2)] to [ZnIIPc(-1)]þ is improved and well-defined peaks were observed. The irreversible Pc ligand oxidation is observed at 1.132 V together with the Au-oxide formation peak. CV response of bulk Au electrode is shown as an inset in Figure 7B for comparison purpose.46 (45) Li, J.-w.; Pfanner, K.; Calzaferri, G. J. Phys. Chem. 1995, 99, 2119. (46) (a) El-Deab, M. S.; Okajima, T.; Ohsaka, T. J. Electrochem. Soc. 2003, 150, A851. (b) El-Deab, M. S.; Ohsaka, T. Electrochim. Acta 2002, 47, 4255. (47) (a) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (b) Luo, J.; Maye, M. M.; Lou, Y.; Han, L.; Hepel, M.; Zhong, C. J. Catal. Today 2002, 77, 127. (c) El-Deab, M. S.; Sotomura, T.; Ohsaka, T. Electrochem. Commun. 2005, 7, 29. (d) Kumar, S.; Zou, S. J. Phys. Chem. B. 2005, 109, 15707. (e) Maye, M. M.; Lou, Y.; Zhong, C.-J. Langmuir 2000, 16, 7520. (f) Cuenya, B. R.; Baeck, S. -H.; Jaramillo, T. F.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 12928.

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Electrocatalytic Reduction of O2. Metal phthalocyanines and metal porphyrins are known to catalyze the 2 electron reduction of O2.10 The electrocatalytic property of Au nanoparticles has been investigated by different groups.4,7,46,47 Figure 8 demonstrate the electrocatalytic activity of modified electrodes toward the reduction of oxygen. Since these electrodes are having different surface area, current densities were used to evaluate the electrocatalytic efficiency rather than the catalytic current. Current densities were obtained by multiplying the current observed at the modified electrodes with the corresponding Aact. The GC/MPS electrode does not show any characteristic peak for the reduction of oxygen (figure not shown) which contains neither metal NPs nor ZnPc. Electrocatalytic reduction of oxygen to H2O2 was observed on the other electrodes studied (Figure 8a-c). At GC/MPS-ZnPc electrode, the catalytic reduction starts at -0.25 V (Figure 8a) whereas at GC/Ag-MPS-ZnPc electrode catalytic reduction starts at -0.15 V (Figure 8b). Thus, presence of Ag NPs decrease the oxygen reduction overpotential by 100 mV. At GC/Ag-MPS electrode, catalytic reduction starts at -0.10 V (Figure 8c). Unlike other electrodes, it shows a defined peak which is due to the reduction of oxygen to H2O2.4b,46 GC/ Au-MPS electrode shows catalytic reduction current starting at -0.15 V (figure not shown). Recently, Raj and co-worker4b studied the electrocatalytic reduction of O2 at the gold nanoprisms and nanoperiwinkles modified electrodes and they observed a well-defined peak at -0.165 V for the reduction of oxygen to H2O2. However, it is well documented that the catalytic properties of metal NPs greatly depend on the size and shape.48 Remarkable changes in the electrocatalytic responses have been observed when the size and shape of the NPs are changed.49 Thus, absence of well-defined peak for the reduction of O2 may be attributed to the shape and size of the metal NPs which are different from the reported. GC/Au-MPS-ZnPc electrode also shows electrocatalytic reduction of O2 current after -0.15 V (figure not shown). In comparison with GC/MPS-ZnPc electrodes, GC/Au-MPS-ZnPc electrode shows high catalytic current density which could be attributed to the presence of Au NPs and also due to the synergy between ZnPc and metal nanoparticle that led to the enhanced electrocatalytic reduction of oxygen. The low catalytic current density observed at GC/Ag-MPS-ZnPc and GC/Au-MPS-ZnPc in comparison with GC/Ag-MPS and GC/Au-MPS electrodes (48) (a) Park, S.; Xie, Y.; Weaver, M. J. Langmuir 2002, 18, 5792. (b) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (c) Wilson, O. M.; Knecht, M. R.; Gracia-Martinez, J. C.; Crooks, R. M. J. Am. Chem. Soc. 2006, 128, 4510. (49) Li, Y.; Shi, G. J. Phys. Chem. B. 2005, 109, 23787.

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Table 2. Rate Constants for the Catalytic Reduction of O2

Table 3. Rate Constants for the Catalytic Reduction of CO2

electrode

rate constant (M-1 s-1)

electrode

rate constant (M-1 s-1)

GC/MPS GC/MPS-ZnPc GC/Ag-MPS-ZnPc GC/Au-MPS-ZnPc GC/Ag-MPS GC/Au-MPS

no catalysis 8.6  103 14.1  103 11.7  103 59.8  103 34.2  103

GC/MPS GC/MPS-ZnPc GC/Ag-MPS-ZnPc GC/Au-MPS-ZnPc GC/Ag-MPS GC/Au-MPS

no catalysis 0.8  103 0.9  103 6.8  103 no catalysis no catalysis

Figure 9. Current density-potential curves for (a) GC/MPS-ZnPc, (b) GC/Ag-MPS-ZnPc, and (c) GC/Au-MPS-ZnPc electrodes in 0.05 M HClO4 in CO2 saturated solution. Curve d is for GC/Au-MPS-ZnPc electrode under N2 saturated condition. Inset shows the corresponding CV responses at a scan rate of 50 mV s-1.

respectively could be attributed to the amount of metal NPs present in the modified electrodes. As discussed previously, the amount of metal NPs present in the Ag-MPS-ZnPc and Au-MPS-ZnPc materials are less due to the leaching of metal NPs during the ZnPc adsorption process as evidenced by TEM, XRD and electrochemical results. However, increase in catalytic efficiency of ZnPc in the presence of metal NPs which act as nanoelectrode ensembles is clearly demonstrated. In order to understand the improvement of catalytic efficiency more quantitatively, catalytic rate constants for the reduction of O2 to H2O2 are calculated for all the electrodes and presented in Table 2. The rate constant reflected the observations made based on the CV responses. Even though the observed rate constants are comparable with the rate constants reported,50,51 it should be noted that the rate constants reported here are based on the steady state catalytic current. Thus, quantitative comparisons of rate constants are more appropriate to restrictions with modified electrodes, which are presented in this study only. Electrocatalytic Reduction of CO2. Various metallophthalocyanines, porphyrins, and their derivatives have been studied for the electrocatalytic reduction of CO2, in both homogeneous and heterogeneous systems.11,52 Direct CO2 electroreduction on several metal electrode which enable the reduction of CO2 in  (50) (a) Sljuki c, B.; Banks, C. E.; Compton, R. G. J. Iranian Chem. Soc. 2005, 2, 1. (b) Ganesan, V.; Ramaraj, R. J. Appl. Electrochem. 2000, 30, 757. (c) Durand, R. R., Jr.; Anson, F. C. J. Electroanal. Chem. 1980, 134, 273. (51) (a) Anson, F. C.; Ni, C.-L.; Saveant, J.-M. J. Am. Chem. Soc. 1985, 107, 3442. (b) Shi, C.; Anson, F. C. J. Electroanal. Chem. 1990, 293, 165. (52) (a) Eggins, B. R.; Irvine, J. T. S.; Grimshaw, J. J. Electroanal. Chem. 1989, 266, 125. (b) Lieber, C. M.; Lewis, N. S. J. Am. Chem. Soc. 1984, 106, 5033. (53) (a) Hori, Y.; Kikuchi, K.; Murata, A.; Suzuki, S. Chem. Lett. 1986, 897. (b) Noda, H.; Ikeda, S.; Oda, Y.; Imai, K.; Maeda, M.; Ito, K. Bull. Chem. Soc. Jpn. 1990, 63, 2459. (c) Sullivan, B. P. Platinum Met. Rev. 1989, 33, 2. (d) Sullivan, B. P., Krist, K., Guard, H. E., Eds. Electrochemical and Electrocatalytic Reactions of Carbon Dioxide; Elsevier: Amsterdam, 1993. (54) Begum, A.; Pickup, P. G. Electrochem. Commun. 2007, 9, 2525.

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water were also reported.53 Recently, Pickup and co-workers54 reported the electrocatalytic reduction of CO2 by Ru(2,20 bipyridine)2(2-(2-pyridyl)benzothiazole)2þ, Ru(2,20 -bipyridine)2(2,20 -bithiazole)2þ, and Ru(2,20 - bithiazole)32þ. However in all the systems the CO2 reduction was observed at large negative over potential (i.e., more negative than -1.0 V) and in most of the study, HCO2H is observed as reduced product due to 2Hþ and 2e- involved reduction of CO2. Figure 9 shows the electrocatalytic reduction of CO2 at the modified electrodes. GC/ MPS-ZnPc (Figure 9a), GC/Ag-MPS-ZnPc (Figure 9b) and GC/Au-MPS-ZnPc (Figure 9c) electrodes show electrocatalytic reduction of CO2 at -0.50 V. Thus, more than 500 mV positive shift in CO2 reduction potential is observed at these electrodes. Increased current densities are observed at GC/Au-MPS-ZnPc and GC/Ag-MPS-ZnPc electrodes, which could be attributed to the presence of Ag or Au NPs that act as nanoelectrode ensembles. Thus, the presence of metal NPs which increases the electrocatalytic activity of the immobilized catalyst is clearly demonstrated. There is no electroreduction of CO2 observed at GC/Ag-MPS and GC/Au-MPS electrodes under the potential window studied. Even if it is there, unfortunately, we were not able to observe it because of the dominant water reduction current at the high negative potentials (curves not shown). So that possible synergic effect of ZnPc and metal NPs which can lead to the enhanced electrocatalytic reduction current at GC/ Ag-MPS-ZnPc and GC/Au-MPS-ZnPc electrodes are not considered here. Catalytic rate constants were calculated and are shown in Table 3. Increased rate constants were observed in the presence of metal NPs which reflect the conclusions made based on the CV responses. The observed rate constants in this work are comparable with that of the literature;55 however, as already mentioned, quantitative comparisons of rate constants are more appropriate to restrict with modified electrodes which are presented in this study only. Modified electrodes used in this study are mechanically stable. Detachment of film from the electrode surface or detachment of coated materials was rarely observed. The modified electrodes after used for the catalytic reduction studies rinsed gently with excess water and stored in deoxygenated water under dark conditions. In order to check the reproducibility of the stored electrodes, electrocatalytic studies were carried out using the same electrodes (approximately 15 days and 30 times) and it has been found that there is no significant changes in the catalytic current and reproducible rate constant values were observed. Therefore, these electrodes can be used repeatedly for O2 or CO2 reductions.

4. Conclusions Micelle-templated organosilica spheres have been prepared by co-condensation of MPTS and TEOS. Ag or Au NPs were anchored onto the mercaptopropyl functionalized inorganic-organic (55) (a) Beley, M.; Collin, J.-P.; Ruppert, R.; Sauvage, J.-P. J. Chem. Soc. Chem. Commun. 1984, 1315. (b) Pearce, D. J.; Pletcher, D. J. Electroanal. Chem. 1986, 197, 317. (c) Daniele, S.; Ugo, P.; Bontempelli, G.; Fiorani, M. J. Electroanal. Chem. 1987, 219, 259. (d) Bhugun, I.; Lexa, D.; Saveant, J.-M. J. Phys. Chem. 1996, 100, 19981.

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hybrid materials with or without ZnPc and used as electrode modifiers. The prepared materials were characterized by TEM, XRD and UV-vis diffuse reflectance studies from which uniform incorporation of Ag or Au NPs with or without ZnPc was revealed. TEM studies suggested that ZnPc deaggregates and induce a reorientation of metal NPs on the organo silica spheres. Electrodes modified with ZnPc adsorbed silica spheres electrocatalyse the reduction of O2 and CO2. Electrocatalytic CO2 reduction was not observed with GC/Ag-MPS and GC/ Au-MPS electrodes. Increased electrocatalytic reduction current and rate constants were observed in the presence of metal NPs for both catalytic reductions studied which clearly demonstrates that the metal NPs act as nanoelectrode ensembles. The synergic effect of ZnPc and metal NPs which can lead to the enhanced electrocatalytic reduction is considered for O2 reduction. Acknowledgment. Generous funding from DST (SR/FTP/ CS-49/2006) and CSIR (01(2098)/07/EMR-II), New Delhi,

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India, is gratefully acknowledged. We acknowledge Prof. O. N. Srivastava, Department of Physics, Faculty of Science, for the TEM facilities, Dr. Devendra Kumar, Department of Ceramic Engineering, Institute of Technology, BHU, Varanasi, India, for the XRD. We are grateful to Dr. S. A. John, Gandhigram Rural University, Gandhigram, India, and Dr. C. R. Raj, Indian Institute of Technology, Kharagpur, India, for fruitful discussions and suggestions. We thank one of the reviewers for the suggestion of experiments for the evaluation of nanoelectrode ensemble characteristics of the modified electrodes. Supporting Information Available: Figures showing CVs of modified electrodes in sodium nitrate and plots of log (ipa) vs log υ for CVs of modified electrodes in 2.5 mM Fe(bpy)32þ with 0.1 M Na2SO4 and in 0.5 mM K4[Fe(CN)6] with 50 mM NaNO3. This material is available free of charge via the Internet at http://pubs.acs.org.

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