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J. Phys. Chem. B 2006, 110, 24486-24491
MnO Octahedral Nanocrystals and MnO@C Core-Shell Composites: Synthesis, Characterization, and Electrocatalytic Properties Sangaraju Shanmugam and Aharon Gedanken* Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan UniVersity Center for AdVanced Materials and Nanotechnology, Bar-Ilan UniVersity, Ramat-Gan, 52900, Israel ReceiVed: September 5, 2006; In Final Form: October 2, 2006
We present a simple and facile synthesis of MnO octahedral nanocrystals and MnO@C core-shell composite nanoparticles. The synthesis is accomplished by a single-step direct pyrolysis of cetyltrimethylammonium permanganate in specially made Let-lock union cells. The products are characterized by HRSEM, HRTEM, Raman spectroscopy, and cyclic voltammetry (CV). The product consists mainly of octahedral MnO nanocrystals and MnO coated with carbon (MnO@C). The core-shell particles are observed only when the core size is smaller than 150 nm. The shape of the nanocrystals can be controlled by varying parameters such as reaction temperature and duration. As the temperature increases from 600 to 800 °C, the octahedral MnO crystals observed are without any carbon shell. The effect of time and temperature on the octahedral MnO nanocrystal formation is described. The electrocatalytic activities of the products are studied for oxygen reduction reaction in aqueous basic medium and are compared with bulk MnO. The MnO nanocrystals and core-shell composites exhibit higher activity than that of bulk MnO.
Introduction Nanostructured materials have received enormous interest in recent years because of their unusual properties when compared with bulk materials. The design and preparation of nanomaterials with tunable physical and chemical properties is still a challenge for the scientific community.1 During the past decade, intensive efforts have been invested in the design and synthesis of nanocrystals with anisotropic morphologies, because the size and shape of nanocrystals determines their physical and chemical properties.2 Anisotropic inorganic nanocrystals have been synthesized by various methods.3 Recently, octahedral SnO2,4 iron nanocubes,5 Pd nanocubes,6 and Pt-functionalized nanosilica cages7 have been reported, and researchers have developed methods for producing unusual forms such as belts, stars, trees, and tetrapods of different materials in the nanoscale range.8 A large number of different oxides of manganese are possible due to the existence of Mn in various oxidation states (II, III, IV). The magnetic, structural, and transport properties of these manganese oxides are of considerable interest in understanding their unique properties from a fundamental point of view.9-11 Manganese oxides have been used as electrochromic materials, and intensive research work was carried out on these materials.12-14 Among them, MnO, Mn2O3, and Mn3O4 have a wide range of applications in catalysis and battery technologies.15 Manganese oxide and oxyhydroxide one-dimensional nanostructured materials have attracted a great deal of attention because of their low cost, high natural abundance, and environmental compatibility.16 Recently, nanohexapods of MnO were synthesized using manganese formate in an amine/carboxylic acid mixture at 340360 °C.17 Yin et al. synthesized MnO capped with organic ligands (trioctylamine and oleic acid) using manganese acetate at 320 °C resulting in 7-20-nm nanocrystals.18 These methods * Corresponding author. E-mail:
[email protected].
adopt various surfactants or structure-directing agents to obtain controlled shapes and size of nanocrystals and require tedious procedures. It has been shown experimentally that MnO nanoclusters have ferromagnetic properties, while bulk MnO is antiferromagnetic.19 Seo et al. reported on the size-dependent magnetic behavior of colloidal Mn3O4 and MnO nanoparticles.20 It was reported that the capacitance of carbon nanotubes can be improved by introducing various metal oxides such as MnO2 and RuO2.21 These systems were fabricated by introducing the active oxides into/onto the carbon surface by various methods. RaymundoPinero et al. studied the pseudocapacitance behavior of Mn oxide by modifying it with carbon nanotubes and observed an enhancement in the capacitance.22 Mn3O4 is known to be an active catalyst for the decomposition of waste gas NOx and the selective reduction of nitrobenzene.23 Zoltowski et al. showed the electrocatalytic properties of MnO2 for an oxygen reduction reaction.24 The catalytic activity of various other manganese oxides (Mn2O3, Mn3O4, Mn5O8, and MnOOH) for an oxygen reduction reaction in alkaline aqueous solutions was also studied.25 Manganese oxides (MnOx) were used as catalysts for NO reduction with NH3 at low temperature and have been prepared by a simple precipitation method using sodium carbonate.26 The catalysts thus obtained have exhibited excellent catalytic activity in the temperature range of 75-200 °C as compared with other MnOx-based catalysts. Matuski and Kamada examined the oxygen reduction activity on crystalline manganese oxides in alkaline solutions using a rotating ringdisk and Teflon-bonded electrode. The γ-MnOOH-based electrode exhibited much higher electrochemical activity than γ-MnO2.27 MnOx/C synthesized by the reduction of KMnO4 with active carbon black and doped with various metal ions were employed for oxygen reduction in alkaline electrolyte.28 The advantage of the present method is that a single step using a single precursor is used to prepare MnO and its carbon
10.1021/jp0657585 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/11/2006
MnO Nanocrystals and Core-Shell Composites
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TABLE 1: Reaction Parameters, Morphology of Product, and Oxygen Reduction Activities of MnO Crystals expt no. 1 2 3 4 a
reaction parameters (temp, time, atmosphere) 600 °C, 6 h, air 700 °C, 3 h, air 700 °C, 12 h, air 800 °C, 3 h, air
C (wt %)
product morphology
size of crystalsa (nm)
ORR activityb (mA/cm2)
54.7 54.2 53.4 52.5
cubes, octahedral, core-shell cubes, octahedral + core-shell octahedral + core-shell octahedral
120-200 200-450 450-600 350-600
0.54 2.50 5.18 3.12
From TEM. b Specific activity at -0.4 V vs Ag/AgCl.
composite. To the best of our knowledge, MnO and its composite were not used for the oxygen reduction reaction. Herein, we present a novel method for preparing octahedral MnO and MnO@C core-shell nanoparticles by the direct pyrolysis of a composite gel of potassium permanganate (KMnO4) and cetyltrimethylammomium bromide (C16H33N(CH3)3Br, CTAB) in a specially made Let-lock union. The shapecontrolled synthesis was carried out by varying the reaction temperature and the duration. As the temperature was increased, the MnO octahedral crystals evolved without any shell. By controlling the reaction parameters, it is possible to obtain naked MnO octahedral shapes and also core-shell nanoparticles exclusively. The electrochemical properties of the products were studied for the oxygen reduction reaction in alkaline electrolyte solution. For comparison, the oxygen reduction reaction was also studied over commercial bulk MnO without any carbon. The oxygen reduction activity is higher for MnO nanocrystals than for bulk MnO. Experimental Section The composite gel of cetyltrimethylammonium permanganate was prepared by using an aqueous solution of KMnO4 (0.01 M) and cetyltrimethylammomium bromide (CTAB, 0.01 M). The ratio between the cation and anion is 1:1. The aqueous solution of CTAB was added drop by drop to the KMnO4 solution with vigorous stirring. A purple gel was formed and was aged in air overnight, then filtered and washed with water several times. From the C, H, N analysis, the ratio of the cation to anion was found to be 1. The excess of CTAB (0.02 M) is also employed to prepare the composite. The C, H, N analysis reveals that the ratio of the cation to anion is 1. It indicates that the excess CTAB is removed while washing with water. The purple solid was used to prepare MnO nanocrystals by using a specially made Let-lock cell under autogenic pressure. The synthesis of MnO nanocrystals was carried out using 3/4-in. union parts that were plugged from both sides by standard caps.29 For a typical synthesis, 0.30 g of cetyltrimethylammonium permanganate was introduced into the cell at room temperature under atmospheric conditions. The cell was closed tightly with another plug and placed inside an iron pipe at the center of the furnace. The closed cell was heated at 700 °C for 3 h at its autogenic pressure and cooled to room temperature. The yield of product is 0.16 g which corresponds to 55% relative to the starting material. Similar experiments were carried out at different temperatures (600, 700, and 800 °C) and different duration periods. The carbon content in the products was determined by using C, H, and N elemental analysis. The theoretical carbon content in the starting material is 56.6 wt %, and the observed carbon content is 54.2 wt %. A comparison of product morphology, reaction parameters, and carbon content is presented in Table 1. The bulk MnO (99%, +170 mesh) was purchased from Aldrich and used for comparison for the oxygen reduction reaction. Structural Characterization. The X-ray diffraction measurements were carried out with a Bruker AXSD Advance
Powder X-ray diffractometer with a Cu KR (λ ) 1.5418 Å) radiation source. The diffraction measurements were collected from 30 to 80° at a speed of 1.2°/min. High-resolution scanning electron microscopy (HRSEM) of the obtained product was carried out on a JEOL-JSM 840 scanning electron microscope operating at 10 kV. The particle morphology was studied with transmission electron microscopy on a JEOL-JEM 100 SX microscope working at an 80-kV accelerating voltage and a JEOL-2010 HRTEM instrument with an accelerating voltage of 200 kV. Samples for TEM and HRTEM were prepared by ultrasonically dispersing the products into absolute ethanol, then placing a drop of this suspension onto a copper grid coated with an amorphous carbon film and then drying under air. The elemental analysis of the sample was carried out by an Eager C, H, N, S analyzer. An Olympus BX41 (Jobin Yvon Horiba) Raman spectrometer was employed, using the 514.5-nm line of an Ar laser as the excitation source to analyze the nature of the carbon present in the products. Electrochemical Characterization. The electrochemical studies were carried out with a Potentiostat/Galvanostat model 273 A. For electrochemical measurements a conventional threeelectrode single glass compartment cell was employed. Pt wire and Ag/AgCl saturated with KCl were used as counter and reference electrodes, respectively. A 0.076-cm2-area glass carbon (GC) served as the working electrode. An amount of 10 mg of MnO octahedra was dispersed in 0.3 mL of ethanol for 20 min in an ultrasonicator. The dispersed composite (10 µL) was placed on GC and dried in an oven at 90 °C for 1 min, and then 5% Nafion (5 µL) was dropped on GC and dried at room temperature. The solvent was evaporated, and the Nafion acted as a binder to hold the MnO crystals to the electrode. For the electrochemical oxygen reduction, the electrolyte was saturated by purging with oxygen gas. The electrolyte was degassed with nitrogen gas for 30 min before the electrochemical measurements were carried out. Results and Discussion Figure 1 shows the XRD patterns of the product synthesized at 700 °C. The diffraction patterns were observed at 2θ ) 34.9, 40.6, 58.8, 70.3, and 73.9° and are assigned as (111), (200), (220), (311), and (222) reflections. These reflections can be readily indexed to cubic rock salt MnO (Manganosite), with an Fm3m (225) space group and a lattice constant of 4.446 Å. This value matches well with the literature values (JCPDS 01-0751090). It can be seen that no other phases of Mn oxides are present. The SEM images of product show the presence of various nanocrystals in the shapes of cubes, hexagons, cubooctohedra-like nanocrystals, most of which are octahedra. The product also illustrates the presence of carbon replica-like structures indicating that the MnO nanocrystals were originally formed in those templates and left them upon their further growth. It also demonstrates that the MnO nanocrystals are partially encaged on the carbon matrixes. The EDX measurements of individual nanocrystals showed the presence of Mn, O, and C.
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Shanmugam and Gedanken
Figure 1. XRD pattern of the MnO product synthesized at 700 °C for 3 h.
Figure 3. (a) TEM image of an individual octahedral MnO crystal, (b) tilted nanocrystal, edge outlines are depicted in the insets, (c) MnO@C core-shell nanoparticles, arrow shows the thickness of the carbon shell, and (d) HRTEM image of an edge of an MnO nanocrystal shows resolved lattice fringes of the (200) plane of MnO.
Figure 4. Raman spectrum of product obtained at 700 °C, showing the presence of disorder graphitic carbon. Figure 2. (a) TEM image shows anisotropic MnO nanocrystals, arrows indicate the carbon shell sorrounding the MnO core, (b) selected area diffraction pattern of sample, indexed to cubic rock salt structure, and (c) an individual MnO octahedron and carbon replica alike are shown with arrows.
Figure 2a shows a typical low-magnification TEM image of the product. The various morphologies of MnO nanocrystals that are observed include cubes, truncated cubes, hexagons, spheres, squares, and tetrahedra. MnO crystals varying from 100 to 400 nm in size were found without any carbon shell and were truncated cube-, octahedral-shaped. Most of the crystals exhibited octahedral shapes. When the particle size was smaller, a carbon shell was observed whose thickness was 30-40 nm (marked with arrows in Figure 2a). The HRTEM measurements and EDX of MnO@C revealed that the core is MnO and the shell is carbon. It is important to point out that the smaller particles are completely surrounded by a carbon shell. These particles are highlighted with dotted circles in Figure 2a. The
interesting fact is that the imprinting carbon matrixes are also observed. The SAED measurements reveal that it consists entirely of carbon. The diffraction pattern of the product is presented in Figure 2b indicating the crystalline nature from which the lattice parameter is calculated to be 4.436 Å, which is well matched with the XRD analysis. Figure 2c shows a typical TEM image of the product consisting of an individual MnO nanocrystal together with carbon imprints. The carbon imprints were highlighted with arrows in Figure 2c. These carbon imprints may be formed when the MnO nanocrystal is grown out of the carbon shell leaving the imprinting carbons. Figure 3a shows an individual 220-nm-size nanocrystal of MnO. To understand the nature of facets, we tilt the sample and observed a three-dimensional structure. The resulting image is shown in Figure 3b, where it can be seen that the edge size is increased to 67 nm, indicating the existence of the crystal as a truncated octahedron. A typical MnO@C core-shell particle is shown in Figure 3c. The thickness of the carbon shell is 23.8
MnO Nanocrystals and Core-Shell Composites
J. Phys. Chem. B, Vol. 110, No. 48, 2006 24489 SCHEME 1: Schematic Representation of the Formation of MnO Nanocrystals and MnO@C CoreShell Particles
Figure 5. TEM images of product synthesized at different temperatures (a) 600 °C for 6 h, (b) 700 °C for 12 h, and (c) 800 °C for 3 h. Arrows show the carbon shell on the MnO core in (a) and (b)
nm and the size of the core is approximately 120 nm, when the core size increases; it grows out of the shell, leaving its replica as a carbon shell. The lattice fringes of MnO are well resolved (Figure 3d). The lattice fringes with a d spacing of 0.221 nm are in close match with the (200) plane of MnO (JCPDS 01-
075-1090). The formation of various morphologies of MnO is not clear at this stage. Early experiments suggest that the CTAB is necessary in order to obtain faceted nanocrystals. A control experiment (without CTAB) was carried out under identical conditions. The process yielded spherical nanoparticles and agglomerated particles of Mn3O4 and Mn2O3, suggesting that the cetyltrimethylammonium cation facilitates the formation of the shapes and phase of Mn oxide. The source of the carbon is the CTAB. The nature and type of carbon in the product synthesized at 700 °C is analyzed by Raman spectroscopy. The carbon exhibits both graphitic and nongraphitic forms. The product shows peaks at 1339 and 1603 cm-1 which originate from disordered and ordered graphitic carbon, respectively (Figure 4). The bands at 1339 and 1603 cm-1 are D and G bands, respectively. The ratio between the D and G band is found to correlate to the nature of carbon. The measured ID/IG ratio is found to be 0.79, suggesting that the carbon exists in the more graphitic form. When the reaction was conducted at 700 °C for 3 h, we observed different shapes of MnO. We also detected carboncoated MnO core-shell particles. To synthesize only octahedral MnO crystals or to obtain entirely core-shell nanoparticles, several experiments have been performed at different reaction temperatures and for different durations. The observed results prove that by adopting proper reaction parameters, it is possible to get the desired product, either a core-shell or the naked octahedral MnO. The TEM image of the reaction product carried out at 600 °C for 6 h is presented in Figure 5, and it shows that the product consists mainly of core-shell-structured nanoparticles. The size of the MnO is around 120 nm, and the thickness of the shell is about 35 nm. When the reaction was carried out at 700 °C for 12 h at its autogenic pressure, it resulted in an octahedral MnO crystal partially encaged with a carbon shell, while most of the crystals are without any shell. This observation indicates that the shell is initially formed around the MnO core. When the MnO core continues to grow, over a longer reaction time, the carbon shell is removed (Figure 5b). It is seen from Figure 5b that the carbon shell is broken and also partially attached to the core (marked with arrows). When the reaction was carried out at a higher temperature (800 °C) for 3 h, only octahedral MnO crystals were obtained. The TEM image of the product (800 °C) is shown in Figure 5c, and it reveals that the octahedral MnO crystals are without any carbon shell. On the basis of the TEM results, we speculate the formation of coreshell nanoparticles, that the process take place initially then as the time of reaction increased, and that the MnO core grows larger in size leaving the carbon shell as replica. As the temperature increases further, the MnO crystals were observed without any shell. The formation of core-shell or octahedral crystals is depicted in Scheme 1.
24490 J. Phys. Chem. B, Vol. 110, No. 48, 2006
Figure 6. XRD patterns of products obtained at (a) 600 °C, 6 h, (b) 700 °C, 12 h, and (c) 800 °C, 3 h.
Figure 7. Linear voltammetric response of products synthesized at different conditions (a) in argon and (b)-(e) in 0.1 M KOH saturated with oxygen solution. (b) 600 °C and (c) 700 °C for 3 h, (d) 700 °C for 12 h, and (e) at 800 °C for 3 h. Scan rate: 10 mV/s
The phase purity of the products obtained at different temperature conditions is shown in Figure 6. It is seen from Figure 6 that the intensity of all peaks increased as the temperature is increased, indicating the enhancement of crystallinity of MnO. In all reactions the product was MnO, without the presence of any other oxides. The carbon present in the product may be responsible for the formation of single-phase MnO. The carbon protects the MnO from further oxidation that might yield higher oxides such as Mn2O3, Mn3O4, or MnO2. As pointed our earlier, the absence of CTAB yielded a mixture of oxides (Mn3O4, Mn2O3). This observation indicates the importance of the carbon shell in obtaining single-phase MnO with faceted morphology. On the other hand, when the same composite precursor was used in a flow system pyrolysis at 500, 600, and 700 °C for 3-6 h, spherical chains of Mn3O4 resulted. These submicron spherical chains exhibit a high magnetization and coercivity compared to the bulk Mn3O4. Figure 7 shows the oxygen reduction on various products obtained at different temperatures. The oxygen reduction reaction was performed in a solution saturated with oxygen. All the voltammograms were measured at 10 mV/s. In the case of MnO/GC in argon-saturated KOH solution, the voltammetric response was featureless, indicating that the MnO composite is electro-inactive in the potential window employed (Figure 7a). From the prominent difference between the reduction currents in the two systems, the significant reduction current in the oxygen-saturated solution must be due to a catalyzed oxygen
Shanmugam and Gedanken reduction reaction. Considering the different scanning rates used by Mao et al. and in the present study, the difference of the peak positions is even smaller. In oxygen-saturated solution, the composite MnO/GC electrode showed an oxygen reduction activity. Figure 7b shows the linear voltammogram of the product obtained at 600 °C for 6 h, with significant oxygen reduction current at -0.50 V in oxygen-saturated electrolyte. It is therefore likely that the composite-MnO catalyzes a twoelectron reduction of O2 to hydrogen peroxide. The product synthesized at 700 °C for 3 h showed a peak at -0.34 V with high current response. The position of the reduction peak, -0.34 V vs Ag/AgCl, is close to the 2-e- oxygen reduction process. A similar peak was observed at around -0.30 V vs Ag/AgCl by Mao et al. for a MnOOH/Nafion-based electrode.25 In the case of MnO obtained at 700 °C for 12 h, a very high oxygen reduction current over all other products was observed (Figure 7d). The specific activity was evaluated by taking the current at -0.4 V. Among the electrodes studied, the MnO synthesized at 700 °C for 12 h showed the highest activity (Table 1). For comparison, the oxygen reduction was carried out on commercial bulk MnO, which showed a specific activity of 0.4 mA/ cm2, which is several-fold lower than the prepared composites. The products synthesized at 700 °C (3, 12 h) and 800 °C (3 h) showed higher activity and shift in reduction potential than product obtained at 600 °C for 6 h. The MnO-composite electrode showed a substanially higher specific activity of 5.18 mA/cm2 at -0.4 V, compared to 0.4 mA/cm2 for a commerical bulk MnO electrode. The MnO-composite electrode shows several-fold higher activity than the commercial bulk MnO electrode catalyst. This result indicates the better performance of the composite electrode over bulk MnO. However, the MnOcomposite electrode shows a large enhancement in the reduction current, as well as a shift in the potential. The shift in the potential and higher current is expected for any economically viable electrode material. This enhancement suggests that the MnO and MnO@C could be a potential electrode catalyst for oxygen reduction reaction in alkaline medium. Conclusions In summary, octahedral MnO nanocrystals and core-shell nanoparticles were synthesized by a simple and facile single step. The formation of octahedral MnO nanocrystals is assisted by the presence of cetyltrimethylammonium cation. The product mainly consists of truncated cubes, cubes, hexagons, spheres, and tetrahedra. The formation of octahedral MnO is accompanied by imprinted carbon replicas. When the MnO crystal size is small, a shell of carbon is present, giving rise to coreshell nanocrystals. As the crystal size of MnO increases, it separated from the surrounding shell, giving rise to the imprinting carbon hollow cubelike structures. By using proper reaction parameters, it is possible to obtain core-shell nanoparticles of MnO and carbon or octahedral MnO nanoparticles. The preliminary oxygen reduction activity of the MnO composite electrode showed higher specific current density than bulk MnO in alkaline aqueous solutions. Acknowledgment. The authors thank Dr. Yudith Grinblat and Dr. Tova Tamari for the HRTEM and TEM analysis. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933; Special issue: Nanostructured Materials, in Chem. Mater. 1996, 8 (5); Jun, W. Y.; Lee, J.-H.; Choi, J.-H.; Cheon, J. J. Phys. Chem. B 2005, 109, 14795. (2) Hu, J.; Odom, T. W.; Liebes, C. M. Acc. Chem. Res. 1999, 32, 435.
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J. Phys. Chem. B, Vol. 110, No. 48, 2006 24491 H. Chem. Mater. 2006, 18, 1829. (18) Yin, M.; O’Brien, S. J. Am. Chem. Soc. 2003, 125, 10180. (19) Lee, G. H.; Huh, S. H.; Jeong, J. W.; Choi, B. J.; Kin, S. K.; Ri, H. C. J. Am. Chem. Soc. 2002, 124, 12094. (20) Seo, W. S.; Jo, H. H.; Lee, K.; Kim, B.; Oh, S. J.; Park, J. T. Angew. Chem., Int. Ed. 2004, 43, 1115. (21) Brousse, T.; Toupin, M.; Belanger, D. J. Electrochem. Soc. 2004, 151, A614; Lee, C. Y.; Tsai, H. M.; Chuang, H. J.; Li, S. Y.; Lin, P.; Tseng, T. Y. J. Electrochem. Soc. 2005, 152, A 716; Ye, S. J.; Cui, H. F.; Liu, X.; Lim, T. M.; Zhang, W. D.; Sheu, F. S. Small 2005, 5, 560. (22) Pinero, E. R.; Khomenko, V.; Frackowiak, E.; Beguin, F. J. Electrochem. Soc. 2005, 152, A229. (23) Yamashita, T.; Vannice, A. J. Catal. 1996, 163, 158; Wang, W. M.; Yang, Y. N.; Zhang, J. Y. Appl. Catal. A 1995, 133, 81. (24) Zoltowski, P.; Drazic, D. M.; Vorkapic, L. J. Appl. Electrochem. 1973, 3, 271. (25) Mao, L.; Zhang, D.; Sotomura, T.; Nakatsu, K.; Koshiba, N.; Ohsaka, T. Electrochim. Acta 2003, 48, 1015; Mao, L.; Sotomura, T.; Nakatsu, K.; Koshiba, N.; Zhang, D.; Ohsaka, T. J. Electrochem. Soc. 2002, 149, A504. (26) Kang, M.; Yeon, T. H.; Park, E. D.; Yie, J. E.; Kim, J. M. Catal. Lett. 2006, 106, 77. (27) Matsuki, K.; Kamada, H. Electrochim. Acta 1986, 31, 13. (28) Klapste, B.; Vondark, J.; Velicka, J. Electrochim. Acta 2002, 47, 2365. (29) Shanmugam, S.; Alexandra, G.; David, J. S.; Yu, J. C.; Gedanken, A. Chem. Mater. 2006, 18, 2275.
