J. Phys. Chem. C 2010, 114, 1885–1891
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Synthesis, Characterization, and Electrocatalytic Properties of Ultra Highly Densely Packed Carbon Sub-Micrometer Sphere Chains and Sheathed Carbon Microfiber Composites Ze´hira Hamoudi, Brahim Aı¨ssa, My Ali El Khakani, and Mohamed Mohamedi* Institut National de la Recherche Scientifique-E´nergie, Mate´riaux et Te´le´communications, 1650 BouleVard Lionel Boulet, Varennes, Que´bec, J3X 1S2, Canada ReceiVed: August 26, 2009; ReVised Manuscript ReceiVed: December 20, 2009
In this work, we introduce the on-substrate preparation and characterization of ultra highly densely packed submicrometer carbon of perfect sphericity and well-connecting to each other forming chainlike compounds. These carbon sub-microsphere (CSMS) chains are highly pure with no catalyst remaining encapsulated within their core as revealed by transmission electron microscopy. X-ray photoelectron spectroscopy survey revealed a surface concentration of carbon being of 97.68 at %, whereas micro-Raman assessment of their quality showed that they are of high graphitic structure. In addition, the static contact angle between a water droplet and the CSMS chains surface is about 151° indicating superhydrophobicity feature. Thermogravimetric analysis showed that CSMS chains are very resistant to oxidation up to 637 °C. Furthermore, the CSMS chains possess enhanced electronic transfer properties. Given these interesting physicochemical properties, the CSMS chains developed here present a distinctive opportunity for creating novel multifunctional composites for potential applications as high temperature structural composites, self-cleaning surfaces, and catalyst supports in energy conversion such as fuel cells. 1. Introduction Compared to the tremendous interest shown to carbon nanotubes and fullerenes, carbon spheres were always considered as byproducts or impurities and separated in early stage of the synthesis. However, over the past decade, carbon spheres with different structures are surprisingly gaining extensive attention likely because they exhibit interesting physicochemical properties but also are cheaper to produce with very high reproducibility contrary to carbon nanotubes. Carbon spheres are being produced by various methods: from sucrose by a hydrothermal method by replication through nanocasting of solid core/mesoporous shell silica particles,1 by a catalytic chemical vapor deposition (CCVD) method using Kaolin supported cobalt catalyst,2 from CH4/H2 mixtures by an iron catalytic process, which is derived from that used to prepare vapor-grown carbon fibers,3 by a hydrothermal method with sugar as the precursor,4 by the medial-reduction route using metallic Mg powder, and the inorganic salts Na2CO3 and CCl as reactants in benzene solvent,5 by a catalyzed solvent thermal reaction at 350 °C using both calcium carbide and chloroform as carbon sources with ferrocene as catalyst,6 by reduction of supercritical CO2 with metallic Li at 650 °C,7 from CS2 and a disproportionation reagent Hg2Cl2 in a sealed system at 750 °C for 12 h with postacid treatment,8 by CVD of benzene with templates of silica spheres at 950 °C,9 by a mixed-valent oxide-catalytic carbonization (MVOCC) process10 using silica particle and sucrose as a template and carbon precursor, respectively, under a hydrothermal condition,11 at temperatures of 400 °C by the reaction reduction of hexachlorobenzene with metallic sodium,12 through a ZnSe nanoparticle template route,13 and from hollow mesoporous aluminosilicate spheres via an incipient-wetness impregnation technique.14 As can be seen from this literature survey, the interest in carbon spheres is very recent and mainly limited to their * To whom correspondence should be addressed. E-mail: mohamedi@ emt.inrs.ca.
synthesis by different methods. Many of their physicochemical properties altogether with their studies for various applications have not been explored yet. In addition, most of the synthesis methods involve a large number of byproducts and the mass percent is very high in the products which require subsequent difficult purification handling, resulting in a great handicap for industrial production and application. Of high interest, solid carbon submicrosphere (CSMS) chains are totally new carbon materials with good physicochemical properties, which make them not only attractive for electronics, support of catalysts, electrode materials for electricity storage and biomedical sensors but also promising for a variety of other industries including, magnetic material, gas storage medium, fuel additives, drugs delivery, textile (self-cleaning surfaces), and structural reinforcement in airframe coatings, protective armor and textile applications. Still, carbon spheres are often produced in soot/or powder form so when used in functional materials either as active or as reinforcement in composites habitually dispersion processes are employed. Dispersion preparation processes have proved rather difficult and still present considerable challenges, especially in the adhesion of carbon spheres to the host matrix material. Indeed, the effective utilization of carbon spheres in composite applications depends strongly on the ability to disperse them individually and uniformly throughout the host matrix without destroying their integrity. In addition, the processing of carbon spheres to promote film formation for many applications requires the use of binders, which not only can affect the performance but also adds to the cost of forming the product. Improved dispersion of carbon spheres within the host matrix and better interfacial coating are thus essential. It is therefore further desirable to develop binderless fabrication technique, which enables to lay down films of carbon spheres onto substrates without the use of binder materials, thereby avoiding the abovementioned disadvantages associated therewith. In this work, by means of the CVD technique, we report a simple direct cost-effective growth of CSMS chains onto a
10.1021/jp908223k 2010 American Chemical Society Published on Web 01/08/2010
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carbon paper substrate to form “free-standing composites” intended for multiple applications. The composite is “freestanding” in that it does not require a binder to provide mechanical integrity. Furthremore, the carbon paper substrate used here is electrically conductive and a highly porous threedimensional network of carbon microfibers (CMFs) which can act as the current collectors for several electricity generation applications. The physicochemical properties of CSMS chains were thoroughly investigated, and prospective applications are demonstrated. 2. Experimental Methods 2.1. Material Preparation. Nickel layer of 5 nm thickness was first deposited on one side of 3D network of carbon microfibers (CMF) substrate by pulsed laser deposition (PLD) technique. The nickel layer was deposited by ablating under vacuum, a pure (99.95%) polycrystalline nickel target by means of a pulsed KrF excimer laser (wavelength ) 248 nm, pulse duration ≈ 14 ns, repetition rate of 20 Hz) with a fluence of 5 J/cm2. To obtain a uniform ablation over the target surface, the target was continuously rotated and translated. The CMF substrates were placed at 50 mm from the target, and the deposition was performed at room temperature. For the CSMS chain growth, a chemical vapor deposition (CVD) technique was employed. First, the Ni-coated CMF substrate was placed in a furnace and heated to 700 °C with a 5 °C/min rate under an 80 sccm argon gas flow. When the temperature of 700 °C is reached in the furnace, the samples were kept under the same heating conditions for 1 h in order to break the Ni film into spherical Ni nanoparticles (10-50 nm diam.) covering uniformly the carbon fibers. Subsequent to the Ni nanoparticles formation, the reactive gas (acetylene in the present study) was introduced simultaneously with argon (in a 5:4 ratio) into the furnace for the CSMS chains synthesis and synthesis times between 40 and 60 min. After synthesis, the acetylene flow was cut off and the furnace cooled down to room temperature under flowing argon. 2.2. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The surface morphology of the as-prepared samples was examined by means of a scanning electron microscope (JEOL, JSM 6300F apparatus) operated at an accelerating voltage of 5 kV and a transmission electron microscope (JEOL JEM-2100F) operating at 200 kV. 2.3. Textural Properties. Brunauer-Emmet-Teller (BET) surface area and pore size measurements were determined for the catalysts with a Quantachrome Instruments Autosorb-1. The samples were weighed and placed on the analysis port. Prior to analysis, the samples were heated to 200 °C for 20 h under vacuum. Adsorption and desorption isotherms were measured at 77 K with N2 as adsorbate. The adsorption data was analyzed with the BET theory and for the pore size distribution, with the nonlocal density functional theory (NLDFT) assuming a slitpore geometry (Autosorb software from Quantachrome Instruments). 2.4. X-ray Photoelectron Spectroscopy (XPS). XPS was conducted for elemental composition of the material’s surface and its chemical environment. The XPS study was performed with a VG Instruments Escalab 220i-XL surface microanalysis system equipped with hemispherical analyzer and Al KR X-ray source (1486.6 eV). Survey scans in the range 0-1000 eV were recorded at 100 eV pass energy with a step size of 1 eV. Core level spectra were obtained for C 1s and recorded at 20 eV pass energy with a step size of 0.1 eV. Curve fitting of the XPS data was carried out with casaXPS version 2.2.107. A semi-
Hamoudi et al. quantitative evaluation of relative atomic surface concentrations was obtained by considering their corresponding sensitivity factors: C1s (1.0) and O1s (2.93). The binding energies were corrected for surface charging by referencing them to the designated hydrocarbon C1s binding energy of 285.5 eV. 2.5. Micro-Raman Spectroscopy. The Raman measurements were performed by using the 514.5 nm (2.41 eV) laser radiation of an Ar+ laser with a circular polarization. The laser beam was focused onto the sample to a spot size of 1 µm (microRaman spectroscopy, Renishaw Imaging Microscope Wire). 2.6. Electrocatalytic Properties. Electron transfer properties of the CSMS chains as well as their electrical connectivity with the substrate were assessed by cyclic voltammetry in a benchmark solution consisting of 1.0 mM potassium ferrocyanide and 1.0 M KCl solution. Fuel cell reactions were studied in 0.5 M H2SO4 and 1 M CH3OH solutions. All measurements were conducted at room temperature using a three-electrode cell with the reference electrode and counter electrode being an Ag/ AgCl and a platinum coil, respectively. Measurements were controlled and recorded with a potentiostat/galvanostat Autolab from EcoChemie. 3. Results and Discussion 3.1. Morphology. Typical SEM micrograph data of the carbon paper substrate surface before and after the CSMS chains deposition at 700 °C are shown in Figure 1. It can be seen that an ultradense film of carbon particles that covers uniformly and completely the entire carbon paper surface is obtained (parts b and c of Figure 1). At higher magnifications observations demonstrate that the products consist of 100% carbon solid spheres, not hollow and with smooth surfaces (Figure 1d). Multiple conglomerates of the carbon spheres are abundant, and in some cases, chains of beadlike accretions of the carbon spheres are observed in SEM investigations (parts d and e of Figure 1). Fascinating coral-like buildup of carbon spheres such as shown in Figure 1f could be found in some parts of the substrate. Both SEM micrographs of parts e and f of Figure 1 show that the carbon spheres grow perpendicularly to the substrate. These carbon spheres have diameters of 300 to 1200 nm of which 80% were found to have sizes between 600 and 800 nm (over five different samples but synthesized under similar experimental conditions). The carbon spheres were further characterized by TEM. Figure 2 confirms the perfect sphericity of the closed carbon particles, the intimate connection between the spheres, and their strong adhesion to the carbon paper substrate respectively. In addition, the TEM pictures did not reveal the presence of nickel catalyst used for the growth of the CSMS chains. The crystallographic orientation was further studied by recording the selected area electron diffraction (SAED) pattern (Figure 2d). The SAED pattern reveals the presence of (002) and (100) planes. On the basis of this route, great deals of CSMS with excellent morphology are prepared. The products are with high purity that purification is unnecessary. Therefore the conventional fussy methods to prepare carbon spheres are simplified, and it provides an original way of preparation to study the large scalepreparation of high quality CSMSs and their applications. 3.2. Textural and Structural Features. The specific surface area of the CSMS chains was measured by means of the wellknown BET method. The adhesion CSMS chains to the carbon paper substrate was strong as they could not be detached from the latter, even after extensive sonication or solvent separation for BET sample preparation. Therefore, the BET was conducted
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Figure 1. SEM analyses of (a) a bare three-dimensional network of carbon microfiber paper substrate. (b-e) As-synthesized CSMS chains at different magnifications, (f) SEM micrograph of occasionally observed coral-like carbon spheres growing perpendicularly to the substrate.
Figure 2. (a-b) TEM micrographs of CSMS chains, (c) HR-TEM micrograph of a typical carbon sphere, and (d) electron diffraction patterns of such carbon spheres.
on the composite, i.e., carbon paper-CSMS chains composite (results not shown here). A specific surface area of 7 m2/g was obtained. Next, the pore size distributions were calculated using the nonlocal density functional theory (NFDFT). The deduced size distributions revealed maximum pore volume values appearing at around 0.016 cm3/g. Because the absence of micropores (e20 Å), there is no adsorption activity when the pressure is low, and the adsorption isotherms are horizontal at first. With the increase of pressure, because of a small quantity of pores existing on external surfaces, the adsorbed amount has
a little increase. The results clearly indicate that the CSMS can be considered as nonporous surface. The carbon spheres are so closely packed that the low BET surface area should arise just from the external surfaces of the spheres. Accordingly, it can be concluded that there are nearly no void and deficiency in the carbon microspheres developed here, and they have integrate and perfect structures, in agreement with HR-TEM analysis. Figure 3a shows an XPS spectrum from the as-prepared CSMS chains. The sample shows a strong C1s peak and a weak O1s; the latter almost certainly due to adsorbed water. Atomic
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Figure 3. (a) X-ray photoelectron spectroscopy spectrum for carbon spheres chains (inset is high-resolution spectra (C1s region)) (b) microRaman spectrum (514.5 nm laser wavelength) of the as-grown CSMS chains, (c) thermogravimetric analyses (TGA) where the black line shows the total weight of the sample and the blue line is the corresponding weight derivative curve, and (d) optical picture of water drop on the superhydrophobic CSMS chains solid sheet surface.
concentrations survey revealed that the surface concentration of carbon being 97.68 at % and that of oxygen equal to 2.32 at %. The high-resolution C1s core level peak could be fairly deconvoluted into four major components centered around 284.3, 285.6, 287.4, and 290.1 eV (insert in Figure 3a), which are attributed to CsC, CsOH, CdO functional groups and π-π transitions, respectively, in accordance with literature data.15,16 The micro-Raman spectrum of the CSMS chains samples is shown in Figure 3b. The sample exhibits mainly two Raman bands, at around 1327 cm-1 (D band) and 1590 cm-1 (G band). The D and G bands reflect the defective (nongraphitic) structure and the graphitic structure (sp2), respectively. Generally, the ratio of the intensities of G to D band (IG/ID) can be used as an indicator of the quality of the carbon. The intensity ratios of the D band to the G band (IG/ID) were determined and were found to be 1.41, indicating a high level of graphitic structure. TGA was employed to characterize the thermal stability of the as-synthesized CSMS chains toward high-temperature air oxidation. The TGA (TA Instrument HR 2950 TGA) was conducted by heating the carbon paper substrate-CSMS chains composite in purged air from 25 to 900 °C at a rate of 10 °C/ min. The TGA results are given in Figure 3c, where the black line shows the total weight of the sample and the blue line is the corresponding weight derivative curve. The results showed that the blocks are stable below 500 °C and a first decreasing stage begins at ca. 547 °C up to 637 °C at which 28% weight
loss was reached. Maximum weight loss occurred at 830 °C, with all of the carbon being oxidized with no residual. The absence of any residual confirms that no nickel catalyst remained which further corroborates the purity of the CSMS chains. The TGA derivative curve shows that the CSMS chains-carbon paper composite burning process consisted of two oxidation events at 602, and 764 °C ascribed to CSMS chains and the carbon fibers, respectively. Wettability properties are crucial for applications involving liquids. By use of the sessile drop method, the static contact angle between a water droplet and the CSMS chains surface was found to be about 151 ( 2° demonstrating that CSMSs surfaces are superhydrohobic (Figure 3d). Such properties are extremely interesting for self-cleaning surfaces applications. 3.3. Electronic Transfer Properties. Electronic properties are central for various electrochemical applications. To assess both the electronic transfer properties and the quality of the electrical contact between the CSMS chains and the CMF substrate, we employed classical cyclic voltammetry with 5 mV/s potential scan rate in 1.0 mM K4Fe(CN)6 and 1.0 M KCl solution (Figure 4), which usually serves as a benchmark for investigating electrocatalytic properties of different carbon structures.17-19 The results clearly show that the Fe(CN)4-6/ Fe(CN)3-6 redox pairs is highly resolved at the CSMS chains at which the redox current is 1.5-fold higher than that at the bare CMF substrate. The electroactive surface area (ESA) of
CSMS Chains
Figure 4. Electron transfer properties with cyclic voltammetry in a 1.0 mM K4Fe(CN)6 + 1.0 M KCl solution at bare carbon paper (black line) and CSMS chains (blue line). The potential scan rate was 5 mV s-1.
the electrodes was estimated from voltammetry recorded at various potential scan rates by means of the Randles-Sevcik equation.20 The ratio of ESA of CSMS chains-CMF composite over that of bare CMF paper is around 1.30, which means 30% more surface area achieved after the CSMS chains decoration. It is further apparent that the electron-transfer kinetics at the CSMS chains surface has been fostered compared to a bare CMF surface as demonstrated by the decreased overpotential required to oxidize the ferrocyanide. All these results demonstrate that not only CSMS chains possess very good electrocatalytic properties but are also electrically well-connected to the CMF substrate (the current collector) when used as electrodes in electrochemical reactions.
J. Phys. Chem. C, Vol. 114, No. 4, 2010 1889 3.4. Fuel Cell Electrode Reactions. Because of the fact that CSMS chains exhibit enhanced electron transfer rates, they could offer higher ability to stabilize the high dispersion of catalyst particles. To explore this avenue, Pt nanoparticles onto CSMS chains have been deposited by the pulsed laser deposition (PLD) technique following the conditions reported elsewhere.21 The amount of platinum deposit in our case here is 104 µg/cm2 as determined by TGA. TEM image of Pt deposited onto CSMS chains is shown in Figure 5a. As can be seen an excellent particle dispersion of Pt particles was achieved with particle size ranging between 1.5-2.3 nm (mean diameter of 2 nm ( 0.5 nm). High-resolution XPS Pt 4f core level spectrum of Pt nanoparticles (NPs) deposited onto CSMS chains is reported in Figure 5b. The Pt 4f core level spectrum displays two peaks whose maximum intensities are located at ∼71 and ∼74.3 eV. The binding energy difference (∆E bind) 3.3 eV between these two maxima is that expected from 4f7/2 (71 eV) and Pt 4f5/2 (74.3 eV) core level peaks. The position of these two peaks is consistent with the fact that Pt is in a metallic state. Next, the electrocatalytic properties the PtNps-CSMS chains composite were investigated always in 1.0 mM K4Fe(CN)6 and 1.0 M KCl solution electrode. Figure 6 shows a cyclic voltammogram with 5 mV/s potential scan rate. By examination of the voltammogram of Figure 6, three important observations have to be pointed out. First, it is clear that the redox waves of the K3Fe(CN)6/K4Fe(CN)6 couple are much well-defined at the PtNps-CSMS chains compared to that observed at the bare CSMS chains shown in Figure 4. The redox current at the PtNpsCSMS chains is 1.26-fold higher than that at the bare CSMS chains. The peak-to-peak separation ∆Ep of ∼89 mV (closer to 60 mV, the ideal value for reversible one-electron-transfer reaction) is smaller than that obtained with bare CSMS chains (171 mV). Finally, ferrocyanide is oxidized at a less positive potential than bare CSMS chains shown. All these observations indicate that for our purposes the platinum nanoparticles can be considered as the principal electroactive sites on this heterogeneous electrode surface.
Figure 5. (a) TEM micrograph of platinum nanoparticles deposited by pulsed laser deposition onto CSMS chains, and its corresponding (b) XPS spectrum.
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Hamoudi et al. of methanol to CO2. In our experiments, the ratio was estimated to be 2.2 for the Pt-CSMS chains electrode. Such a high value indicates that most of the intermediate carbonaceous species were oxidized to CO2 in the forward scan. For comparison, the ratio 0.87 was reported with a nanosized Pt on XC-72 electrode22 and 1.4 for carbon nanotube-supported Pt nanoparticles catalysts.23 The high activity for methanol oxidation observed at carbon nanotube-supported Pt nanoparticles catalysts might have been the results of the high surface area of carbon nanotube and the nanostructure of PtNPs.23 However, the enhanced activity obtained with our PtNps on CSMS chains cannot be due the surface area of the latter that is low but to the fact that to the difference of carbon nanotubes CSMS chains possess multiple points of electrical conductivity and excellent dispersions characteristics for the PtNPs that allow the latter to be largely accessible for methanol electro-oxidation compared to other carbon support structures including nanotubes. Such unique features CSMS chains are promising in designing novel fuel cell electrodes.
Figure 6. Electron transfer properties of platinum nanoparticles supported on the CSMS chains with cyclic voltammetry in a 1.0 mM K4Fe(CN)6 + 1.0 M KCl solution at. The potential scan rate was 5 mV s-1.
Figure 7. Cyclic voltammogram showing methanol electro-oxidation of the Pt-NP-decorated CSMS chains, in 0.5 M H2SO4 and 1 M Methanol solution. The potential scan rate was 50 mV s-1.
We have a strong interest in the electrocatalysis of methanol oxidation, one reaction that is of prime importance to direct methanol fuel-cell technology. Figure 7 shows the typical cyclic voltammogram for methanol electrochemical oxidation of the Pt catalyst supported on the CSMS chains obtained at a scan rate of 50 mV/s in 0.5 M H2SO4 and 1 M CH3OH solution. In the forward scan, methanol oxidation produced a prominent symmetric anodic peak around 0.69 V vs Ag/AgCl. In the backward scan, an anodic peak appeared at around 0.53 V. This anodic peak in the backward scan could be attributed to the removal of the incompletely oxidized carbonaceous species formed in the forward scan.22 The ratio of the forward anodic peak current (If) to the backward anodic peak current (Ib) can be employed to depict the catalyst tolerance to carbonaceous species accumulation.22 A high If/Ib value implies good oxidation
4. Conclusions In a simple and cost-effective way, we have developed with the chemical vapor deposition route 100% solid CSMS chains of perfect sphericity with smooth surface, of very high purity, without any catalyst encapsulated or any other byproduct in the products (as demonstrated by SEM and TEM observations). The CSMS chains are further ultra highly densely packed and found to grow perpendicularly to the substrate. It is further important to emphasize that such material was prepared without using any template or binding agents which opens a diverse functionality that is beneficial for numerous applications. Their high thermal stability to be used in high temperature structural composites applications. Furthermore, given their superhydrophobic properties, these carbon spheres present a distinctive opportunity for creating novel composite materials for self-cleaning surfaces applications. Finally, the CSMS chains have the ability to promote electron-transfer reactions when used as electrode materials and might constitute excellent catalyst support in the electrocatalysis of fuel cell reactions (either hydrogen fuel cells or liquid fuel cells) and may potentially offer advantages over current supports. Still the mechanism by which the CSMS chains form is not understood at this time. Work is under way by varying the experimental conditions synthesis (temperature, substrate, catalyst, synthesis time, gases flow ratio, etc.) combined with microscopic and spectroscopic techniques which would help to elucidate the growth mechanism of the CSMS chains. Acknowledgment. This work was supported by the Natural Sciences Engineering Research Council of Canada, the Fonds Que´be´cois pour la Recherche en Nature et Technologie, NanoQuebec, and the Centre Que´be´cois sur les Mate´riaux Fonctionnels. We are grateful to Mr. G. Abel for wettability measurements. References and Notes (1) Xu, C.; Cheng, L.; Shen, P.; Liu, Y. Electrochem. Commun. 2007, 9, 997. (2) Miao, J.-Y.; Hwang, D. W.; Narasimhulu, K. V.; Lin, P.-I.; Chen, Y.-T.; Lin, S.-H.; Hwang, L.-P. Carbon 2004, 42, 813. (3) Serp, P.; Feurer, R.; Kihn, Y.; Kalck, P.; Faria, J. L.; Figueiredo, J. L. J. Mater. Chem. 2001, 11, 1980. (4) Wang, Q.; Li, H.; Chen, L. Q.; Huang, X. J. Carbon 2001, 39, 2211. (5) Liu, J.; Shao, M.; Tang, Q.; Chen, X.; Liu, Z.; Qian, Y. Carbon 2002, 411, 1645.
CSMS Chains (6) Yin, C.; Huang, Q.; Xie, Y.; Wang, X.; Xie, Z; He, L.; Su, Z.; Liu, B. Carbon 2007, 45, 583. (7) Lou, Z.; Chen, Q.; Gao, J.; Zhang, Y. Carbon 2004, 42, 219. (8) Shen, J.; Li, J.; Chen, Q.; Luo, T.; Yu, W.; Qian, Y. Carbon 2006, 44, 158. (9) Li, F.; Zou, Q.-Q.; Xia, Y.-Y. J. Power Sources 2008, 177, 546. (10) Wang, Z. L.; Yin, J. S. Chem. Phys. Lett. 1998, 289, 189. (11) Joo, J. B.; Kim, P.; Kim, W.; Kim, J.; Kim, N. D.; Yi, J. Curr. Appl. Phys. 2007, 8, 814. (12) Cai, P.-Jun; Feng, L. Mater. Chem. Phys. 2008, 108, 1. (13) Geng, B. Y.; Ma, J. Z.; Du, Q. B.; Liu, X. W.; Zhang, L. D. Mater. Sci. Eng. 2007, 466, 96. (14) Li, Y.; Yang, Y.; Shi, J.; Ruan, M. Microporous Mesoporous Mater. 2008, 112, 597. (15) Kovtyukhova, N. I.; Mallouk, T. E.; Pan, L.; Dickey, E. C. J. Am. Chem. Soc. 2003, 125, 9761.
J. Phys. Chem. C, Vol. 114, No. 4, 2010 1891 (16) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992. (17) Justin Gooding, J. Electrochim. Acta 2005, 50, 3049. (18) Banks, Craig E.; Compton, Richard G. Analyst 2006, 131, 15. (19) Antiochia, R.; Lavagnini, I.; Magno, F.; Valentini, F.; Palleschi, G. Electroanalysis 2004, 16, 1451. (20) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods Fundamentals and Applications; Wiley: New York, 1982. (21) Aı¨ssa, B.; Hamoudi, Z.; Takahashi, H.; Tohji, K.; Mohamedi, M.; El Khakani, M. A. Electrochem. Commun. 2009, 11, 862. (22) Liu, Z.; Ling, X. Y.; Su, X.; Lee, J. Y. J. Phys. Chem. B 2004, 108, 8234. (23) Lin, Y.; Cui, X. J. Phys. Chem. B 2005, 109, 14410.
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