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2007, 111, 21-26 Published on Web 12/14/2006
Cation Effect on the Electrochemical Formation of Very High Aspect Ratio TiO2 Nanotube Arrays in Formamide-Water Mixtures Karthik Shankar, Gopal K. Mor, Adriana Fitzgerald, and Craig A. Grimes* Department of Electrical Engineering, Department of Materials Science and Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: September 27, 2006; In Final Form: NoVember 17, 2006
Highly ordered TiO2 nanotube arrays are of considerable interest for their use as gas sensors, materials for water photoelectrolysis, and as photoanodes in dye sensitized solar cells. For example, under UV illumination highly ordered TiO2 nanotube arrays ≈35 µm in length achieve a light-to-chemical energy photoconversion efficiency of 16.25% [M. Paulose et al., J. Phys. Chem. B 2005, 110, 16179-16184]. It is now well-established that the properties of the nanotube arrays are dependent upon their specific architecture, including nanotube array length, wall thickness, pore diameter, and tube-to-tube spacing. In this work we investigate the effect of five different cationic species on the formation of TiO2 nanotube arrays by potentiostatic anodization of titanium in formamide-water mixtures containing fluoride ions. We find the cation choice to be a key parameter influencing both the nanotube growth rate and resulting nanotube length. The length and aspect ratio of the nanotubes increases with increasing cation size. Under similar conditions, electrolytes containing the tetrabutylammonium cation resulted in the longest nanotubes (∼94 µm), while the shortest nanotubes (∼3 µm) were obtained when H+ ions were the sole cationic species in the anodization electrolyte. We attribute this difference in nanotube growth to the inhibitory effect of the quarternary ammonium ions that restrict the thickness of the interfacial (barrier) oxide layer; a thinner interfacial oxide layer facilitates ionic transport thus enhancing the nanotube growth. The aspect ratio of the resulting nanotubes is also voltage dependent, with the highest aspect ratio of ≈700 obtained at an anodizing voltage of 20 V in an electrolyte containing tetrabutylammonium ions. In a saturated solution of NaF in formamide, nanotubes with a pore diameter of 12 nm were obtained, while a formamide Bu4NF solution resulted in nanotubes of 5 nm wall thickness; both values are, to the best of our knowledge, the smallest reported values for anodically formed TiO2 nanotube arrays.
Controllable production of nanometer-sized structures is an important goal of nanotechnology, and vertically oriented TiO2 nanotubes offer exciting potential for use in sensors, actuators, data storage, displays, and fuel cells.1 Furthermore, materials with aligned porosity in the submicron regime have wide applications in organic electronics, microfluidics, molecular filtration, drug delivery, and tissue engineering.2,3 Arrays of TiO2 nanotubes fabricated by anodization constitute a vertically oriented self-organized architecture with some unique advantages. One advantage is the increase in effective internal surface area without a concomitant decrease in geometric and structural order. A second advantage is the ability to influence the absorption and propagation of light through the architecture by precisely designing and controlling the geometrical parameters of the architecture.4 Another key advantage is that the aligned porosity, crystallinity, and oriented nature of the nanotube arrays makes them attractive electron percolation pathways for vectorial charge transfer between interfaces.5-7 Fabrication of high aspect ratio semiconducting metal oxide nanotubes with lengths of several tens of micrometers is a challenge but is key to boosting the performance of a variety * Corresponding author. E-mail:
[email protected] 10.1021/jp066352v CCC: $37.00
of nanotube-based devices. The so-called first generation nanotubes anodized using an aqueous HF based electrolyte grew to a length of about 500 nm.8,9 While the nanotube length was increased to about 7 µm in the second generation by controlling the pH of the anodization electrolyte and reducing the chemical dissolution of TiO2 during anodization,10,11 smoother more uniform nanotubes of similar length were synthesized in nonaqueous viscous electrolytes in the third generation by reducing thickness variations in the sidewall profile.12,13 Recently, Yoriya et al.14 reported fourth generation ultralong nanotubes grown for the first time to a thickness of 220 µm by anodization in various polar organic electrolytes. In this report we investigate the effect of five different cationic species on the formation of TiO2 nanotube arrays with specific attention paid to the time of formation, and the length and the aspect ratio of the resulting nanotubes. We find the cation choice to be a key parameter influencing both the nanotube growth rate and resulting nanotube length, with the length and aspect ratio of the nanotubes increasing with increasing cation size. The aspect ratio of the resulting nanotubes is also voltage dependent, with the highest aspect ratio of approximately 700 obtained at an anodizing voltage of 20 V in an electrolyte containing tetrabutylammonium ions. © 2007 American Chemical Society
22 J. Phys. Chem. C, Vol. 111, No. 1, 2007
Letters TABLE 1: Physical Properties (measured at 22 °C) of Fluoride Compound Solutions Containing Different Cations, within a Formamide Electrolyte Containing 5% Water
Figure 1. Molecular structure of (a) tetrabutylammonium fluoride, and (b) benzyltrimethylammonium fluoride.
Figure 2. Current voltage behavior of different electrolytes probed using planar platinum electrodes with a cell constant of 1.25 (ratio of the distance between the two electrodes divided by their area).
compound
molar concentration (M)
density (g cm-3)
viscosity (cP)
HF NaF NH4F Bu4NF BnMe3NF
0.27 ∼0.04 0.27 0.27 0.27
1.10 1.09 1.11 1.10 1.09
2.76 2.61 2.81 3.40 3.12
in NMF and formamide is fairly similar; however, at identical anodization potentials, nanotubes formed in NMF are slightly longer.13 All studies comparing the effect of different cations were conducted using formamide as the base electrolyte. NMF was only used to assess the maximum length that the nanotubes could be grown to in this system, confined to the case where tetrabutylammonium cations were used in combination with a lower water content. The duration of the anodization varied from 6 h to 164 h; the optimal anodization time to obtain the maximum nanotube length was determined empirically. Beyond the stated anodization times, the nanotubes decreased in length. From previous studies we know as-anodized anodic films to be amorphous, crystallized by a high temperature anneal.15 To induce crystallinity the initially amorphous as-anodized films were subjected to a 3 h anneal in oxygen at 580 °C with heating and cooling rates of 1 °C/min. Non-aqueous electrolytes have been used to anodize Si,16,17 as well as valve metals such as Ti, Nb, Zr, W, and Mg.18 However, these studies have been primarily focused either on alcohols such as ethanol, ethylene glycol, and glycerol, or on polar aprotic solvents such as dimethylsulfoxide (DMSO), N,Ndimethylformamide (DMF), acetonitrile (ACN), and propylene carbonate (PC). All of these solvents have a dielectric constant lower than that of water. In comparison, the high dielectric constant amide-based amphiprotic solvents such as formamide (FA), N-methylformamide (NMF), as well as N-methylacetamide and N-methylpropionamide have received much less attention.
Experimental Section
Results and Discussion
Titanium foils (99.8% pure) approximately 250 µm thick were cleaned in ethanol and subsequently anodized at room temperature (22 °C) in formamide (FA), and N-methylformamide (NMF), solutions containing water and fluoride bearing species. For the purposes of this study, hydrogen fluoride (HF), ammonium fluoride (NH4F), sodium fluoride (NaF), tetrabutylammonium fluoride (Bu4NF), and benzyltrimethyl ammonium fluoride (BnMe3NF) were the fluoride ion bearing species chosen. The cations of these compounds shall be referred to as H+, NH4+, Na+, Bu4N+, and BnMe3N+, respectively, in the reminder of this report. The molecular structures of Bu4NF and BnMe3NF are shown in Figure 1. The conductivity of all solutions used was probed at 22 °C under DC conditions using planar platinum electrodes with a measured cell constant of 1.25 ( 0.01 cm-1 and readings were recorded when they became invariant with time; this data is shown in Figure 2. The viscosities of the solutions were measured using a falling ball viscometer (Gilmont) and are shown in Table 1. NH4F possesses limited solubility in 95% formamide + 5% water mixtures. The much higher solubility of Bu4N+ allows higher concentrations to be employed in the anodization electrolyte. NaF has the poorest solubility among the compounds studied and a saturated solution of NaF was formed at a much lower concentration (∼0.04 M) than the other compounds. The anodization process
Nanotube formation in fluoride ion containing electrolytes occurs as a result of the interplay between three simultaneously occurring processes, namely the field assisted oxidation of Ti metal to form titanium dioxide, the field assisted dissolution of Ti metal ions in the electrolyte, and the chemical dissolution of Ti and TiO2 due to etching by fluoride ions, which is substantially enhanced by the presence of H+ ions.19 The same three processes govern the anodic formation of several other self-organized nanoporous metal oxides.20-22 The growth of TiO2 nanotubes was studied in fluoride-bearing electrolytes containing different cations while maintaining similar and in some cases identical molar concentrations of the chemical species. This ensured that each electrolyte had the same concentration of fluoride ions, thus allowing us to observe the effect of the cations on nanotube growth. Figure 3 shows the real-time potentiostatic anodization behavior of Ti anodized at 20 V in 95% formamide + 5% water solutions containing 0.27 M of NH4F, Bu4NF, HF, and BnMe3NF, respectively. The ionic radius of the cationic species23 increases in the order
H+ < Na+ < NH4+ < Bu4N+ < BnMe3N+
(1)
Figure 2 shows that the conductivities of the various electrolytes
Letters
J. Phys. Chem. C, Vol. 111, No. 1, 2007 23
TABLE 2: Effect of Cation Type, Cation Concentration, and Anodization Duration on the Morphological Features of TiO2 Nanotube Arrays for a Given Potentiala molar (M) cation concentration
cation
anodization duration (hr)
outer diameter (nm)
wall thickness (nm)
nanotube length (µm)
H+ H+ NH4+ NH4+ Na+ Bu4N+
0.14 0.27 0.14 0.27 saturated solution (∼0.04) 0.27
(a) Anodization Potential ) 35 V 101 256 48 214 88 208 30 159 48 48 48 190
21 20 17 15 18 22
5.6 7.3 29.2 37.4 9.6 68.9
H+ NH4+ NH4+ Bu4N+
0.27 0.14 0.27 0.27
(b) Anodization Potential ) 20 V 48 99 55 90 24 90 34 90
22 19 17 16
2.9 14.4 19.6 35.2
NH4+ Bu4N+ BnMe3N+
0.14 0.27 0.27
(c) Anodization Potential ) 15 V 110 81 46 80 42 70
29 15 18
8.2 20.0 7.2
a In every case, the electrolytes contain 5% water in formamide and the anodization was performed at room temperature, 22 °C. the anodization duration is the approximate time required to obtain maximum nanotube length. Anodization potential is (a) 35 V; (b) 20 V; (c) 15 V.
respectively. Figure 2 also shows the benzyltrimethylammonium fluoride solution to be the most conductive among all the electrolytes investigated. The solution viscosities increase with the size of the cationic species, with solutions containing Bu4N+ and BnMe3N+ exhibiting much higher viscosities than solutions of the remaining cations. The electrolyte containing NH4F has a lower viscosity than the one containing Bu4NF but is more conductive, resulting in a similar value for the viscosityconductivity product. The NaF electrolyte has a much lower concentration than the other electrolytes and is hence excluded from this comparison. The product of the viscosity and conductivity increases in the following order for the strong electrolytes:
H+ < NH4+, Bu4N+ < BnMe3N+ Figure 3. Current-time behavior during 20 V potentiostatic anodization of a Ti foil (99.8% pure) in a formamide solution containing 5% H2O and identical molar concentrations (0.27 M) of fluoride ion bearing compounds with four different cationic species: hydrogen (H+), ammonium (NH4+), tetrabutylammonium ([C4H9]4N+), and benzyltrimethylammonium ([C6H5CH2][CH3]3N+).
increased in the order
H+ < Na+ < Bu4N+ < NH4+ < BnMe3N+
(2)
The Stokes radius of an ion in solution provides information about the solvation of the respective ion and is given by24
r)
|z|F2 6πNηλi0
(3)
where z is the charge on the ion, F is the Faraday constant, and N is the Avogadro number. Equation 3 shows the Stokes-law radii rs of ions to be inversely related to the product of the viscosity η and the limiting equivalent conductance λi0 of the ion. Replacing 6 by 4 in the denominator of eq 3 yields the hydrodynamic radius of the ion. While we have not calculated the limiting equivalent conductance of the electrolytes, a qualitative trend may be established by reviewing the viscosity and conductivity information gathered in Table 2 and Figure 2
Thus the resulting stokes radii and hydrodynamic radii may be expected to decrease in the following order:
H+ > NH4+, Bu4N+ > BnMe3N+ Thus we observe that the species with smallest ionic radii (H+ and NH4+) have the largest Stokes radii as well as hydrodynamic radii. This suggests that the H+ and NH4+ cations have larger solvation shells than Bu4N+ and BnMe3N+ ions. This is understandable considering that formamide is a polar protophilic solvent with good hydrogen bond donating ability,24 wherein small cations, such as H+ and Na+, are strongly solvated; due to the hydrogen bond donating ability of formamide, the Fions are strongly solvated as well. From the solvent-berg model, the solvent molecules immediately adjacent to the ion are rigidly bound to it and thus the ion moves as a kinetic unit.24 Consequently, the conductivity of HF in the formamide-water mixture is limited by the solvent molecule mobility and the anodization currents seen in Figure 3 are relatively small. On the other hand, larger ions such as Bu4N+ and BnMe3N+ are weakly solvated and their higher ionic mobility translates into a higher anodization current. However, at high cation concentrations the viscosity of the solution increases, with a corresponding decrease in solution conductivity. For large cations such as Bu4N+ and BnMe3N+, the Stokes radii are independent of solvent and Walden’s rule25 is obeyed whereby
24 J. Phys. Chem. C, Vol. 111, No. 1, 2007
Letters
Figure 4. Anodization current-time behavior of a Ti foil (99.8% pure) anodized at 20 V in an electrolyte containing 0.27 M NH4F in formamide of variable water content.
λ∞‚η ) constant λ∞
(4)
where is the limiting molar conductivity of the cation in the solution and η is the viscosity. Figure 4 contrasts the real time potentiostatic anodization behavior of Ti anodized at 20 V in electrolytes containing an identical concentration of NH4F (0.27 M) but with different amounts of water ranging from 100% water (no formamide) to 2.5% water + 97.5% formamide. In the 100% aqueous
electrolyte, the anodization current drops sharply from a high initial value (>100 mA/cm2) in the first few seconds of anodization. This is attributed to the formation of an initial insulating oxide layer that reduces the current. Thereafter, pitting of the oxide layer by fluoride ions commences and the anodization current increases to reach a local maximum after about 5000 s. On the other hand, the anodization current in the electrolytes containing formamide remains relatively constant in the first 100-1000 s of anodization monotonically decreasing thereafter. As a result, while the total charge passed after 500 min of anodization was similar for both kinds of electrolytes, a much larger amount of charge is passed in the formamide based electrolytes during the first 500 s of anodization. The region of stable current extends for a longer period of time in electrolytes containing a smaller concentration of water. The current plots are broadly similar in that they all exhibit a sigmoidal curve with an initial region of stable or near stable anodization current followed by a region of falling current of larger slope and a then a region where the current reaches a lower level and continues to decrease at a smaller slope. During the initial period of high current gas evolution at the anode is observable. Since gas evolution requires electronic charge transfer, this is indicative that in the early stages of the anodization electronic conduction dominates. With organic electrolytes the donation of oxygen is more difficult in comparison to water and results in a reduced tendency to form oxide.16 The reduction in water content allows for thinner and/or lower quality barrier layers
Figure 5. Illustrative cross-sectional FESEM images of TiO2 nanotubes formed in formamide electrolyte containing 5% water at 35 V with different cations. (a) 0.27 M HF. (b) 0.27 M Bu4NF. (c) 0.27 M NH4F. (d) NaF-saturated solution (∼0.04 M). Insets show top surface and enlarged crosssectional views.
Letters
Figure 6. FESEM images of nanotubes formed in 0.5 M solution of Bu4NF in formamide electrolyte containing 5% water.
through which ionic transport may be enhanced. The greater ionic conduction allows faster movement of the Ti/TiO2 interface into the Ti substrate allowing for a larger final nanotube length. Higher anodization voltages provide a greater driving force for both electronic and ionic conduction and are accompanied by higher anodization currents. The surface morphology is a function of anodization potential with the pore diameter remaining roughly constant in electrolytes containing different cations. Field emission scanning electron microscopy (FESEM) images of the anodic TiO2 films formed in the FA/NMF based electrolytes containing 5% water are shown in Figures 5 and 6. At an anodization potential of 35 V, nanotubes with outer diameters in the range 175-215 nm were obtained in all electrolytes investigated. However, the length and aspect ratio increased when the cations in the electrolytes were larger. The shortest nanotubes were obtained when the cation was H+ (Figure 5a). The longest nanotubes were obtained when the cation was Bu4N+ (Figure 5b) with nanotubes of intermediate length formed when the cation was NH4+ (Figure 5c). Anodization in BnMe3N+ containing electrolytes were unstable at anodization potentials larger than 20 V. When Na+ was the cation, the solubility in the electrolyte was poor and the anodization was performed for a saturated solution of concentration much smaller than 0.27 M. Nanotubes with an outer diameter of 48 nm were obtained in this electrolyte (Figure 5d). Table 2 summarizes the effect of different cations and cation concentrations on the nanotube morphology for formamide based electrolytes containing 5% water at different anodization potentials. A trend of decreasing wall thickness at higher fluoride ion concentrations is attributable to the enhanced chemical etching afforded by higher F- concentrations. At larger anodization voltages, the driving force for ionic transport through the barrier layer at the bottom of the pore is greater and results in faster movement of the Ti/TiO2 interface into the Ti metal. We attribute the higher nanotube length obtained at larger anodization voltages to this enhanced pore deepening effect. As previously indicated, nanotubes of greater length were obtained in the Bu4N+ containing electrolytes. The maximal length was obtained with a shorter anodization duration implying a faster growth rate. We attribute this behavior to (a) the presence of a thinner interfacial oxide layer that promotes faster migration of the oxide-metal interface deeper into the oxide, (b) a decrease in the rate of chemical etching, and (c) the higher conductivity and smaller solvation shell of the Bu4N+ ion enabling a greater amount of charged to be passed during anodization. Quaternary ammonium salts effectively inhibit the
J. Phys. Chem. C, Vol. 111, No. 1, 2007 25
Figure 7. Illustrative cross-sectional FESEM image of 94 µm long nanotubes obtained by anodization at 35 V in an NMF solution containing 1% water. Inset shows a portion of the nanotube array at enhanced magnification.
Figure 8. Histogram showing effect of anodization potential, cation species (0.27 M concentration), and water concentration on aspect ratio of the resulting nanotubes.
acid corrosion of metals in the presence of halide ions.23 The inhibitory effect of quaternary ammonium salts is explained by structuring of the solvent in the diffuse part of the electrical double layer and by a decrease in the mobility of hydronium ions in the near-electrode layer.26 The inhibiting action of organic compounds containing polar groups such as nitrogen and sulfur is also based on the adsorption ability of the molecules where the resulting adsorbed film protects the metal from the corrodent.27 The chemical etching of TiO2 is assisted by H+ ions and has a marked pH dependency.10 Therefore, chemical etching of the TiO2 is highest in the HF containing electrolyte. The conductivity of the ammonium ion bearing solution is higher than the tetrabutylammonium containing solution and their viscosity-conductivity products are similar. The anodic current transient is larger in the ammonium bearing electrolyte. Yet the growth rate and final formation length of TiO2 nanotubes are much larger in the electrolyte containing tetrabutylammonium cations. Chemical etching is also higher when the cation is NH4+ compared to Bu4N+, since the ammonium ion is a stronger acid than tetrabutylammonium and can donate a proton more easily. During the course of our study, nanotubes with the smallest wall-thickness of approximately 5 nm, shown in Figure 6, were obtained in an electrolyte containing 0.5 M Bu4NF in formamide with 5% water. When the wall thickness is this small the nanotubes are rendered almost transparent to the electron beam used to perform the scanning electron microscopy. This effect is seen clearly in Figure 6 where when, looking at any particular nanotube, features belonging to the nanotube immediately behind it are visible within its contours. A lower water content
26 J. Phys. Chem. C, Vol. 111, No. 1, 2007 was found to increase the nanotube length while previous studies13 indicated that NMF, which has a higher dielectric constant than formamide, result in slightly longer nanotubes. Therefore, a combination of NMF and a lower water content was used to examine the maximum nanotube length obtainable in this system. Figure 7 shows nanotubes formed at 35 V in an NMF based electrolyte containing 1% water and 0.27 M Bu4NF with a length of 94 µm and an outer diameter of 180 nm resulting in an aspect ratio of 522, formed after 48 h of anodization. Nanotubes formed at 20 V in the same electrolyte had a length of 63 µm and an outer diameter of 90 nm resulting in an aspect ratio of 700. For a given electrolyte we also observe an increase in the nanotube diameter as the anodization voltage is increased. This observation agrees with the reported behavior of nanotubular TiO2 as well as other anodically formed metal oxides.9,28 At anodization potentials from 10 V to about 20 V, the nanotube length increases faster than nanotube diameter with increasing voltage resulting in an increasing aspect ratio. At potentials larger than 20 V, the increase in length is less dramatic and generally matched by the increasing pore diameter. Consequently, as Figure 8 demonstrates, for the electrolytes we have investigated the highest aspect ratio nanotubes are obtained at an anodization potential of 20 V. Acknowledgment. Support of this work by the Department of Energy under grant DE-FG02-06ER15772 is gratefully acknowledged. A.F. was supported by National Science Foundation, Research Experience for Undergraduates, Grant No. EEC-0244030. The authors wish to thank the Reviewers for their helpful comments and suggestions. References and Notes (1) Hueso, L.; Mathur, N. Nature 2004, 427, 301-304. (2) Zhang, H. F.; Hussain, I.; Brust, M.; Butler, M. F.; Rannard, S. P.; Cooper, A. I. Nat. Mater. 2005, 4, 787-793. (3) Popat, K. C.; Leary-Swan, E. E.; Desai, T. A. Biomaterials 2005, 26, 1969-1976. (4) Ong, K. G.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Nanosci. Nanotechnol. 2005, 5, 1801-1808.
Letters (5) Tenne, R.; Rao, C. N. R. Philos. Trans. R. Soc. London Ser. A 2004, 362, 2099-2125. (6) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215-218. (7) Frank, A. J., Kopidakis, N.; van de Lagemaat, J. Coord. Chem. ReV. 2004, 248, 1165-1179. (8) Mor, G. K.; Varghese, O. K.; Paulose, M.; Grimes, C. A. AdV. Funct. Mater. 2005, 15, 1291-1296. (9) Gong, D.; Grimes, C. A.; Varghese, O. K.; Hu, W. C.; Singh, R. S.; Chen, Z.; Dickey, E. C. J. Mater. Res. 2001, 16, 3331-3334. (10) Cai, Q. Y.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Mater. Res. 2005, 20, 230-236. (11) Macak, J.; Tsuchiya, H.; Schmuki, P. Angew. Chem., Int. Ed. 2005, 44, 2100-2102. (12) Ruan, C. M.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Phys. Chem. B 2005, 109, 15754-15759. (13) Paulose, M.; Shankar, K.; Yoriya, S.; Prakasam, H. E.; Varghese, O. K.; Mor, G. K.; Latempa, T. A.; Fitzgerald, A.; Grimes, C. A. J. Phys. Chem. B 2006, 110, 16179-16184. (14) Yoriya, S.; Prakasam, H. E.; Varghese, O. K.; Shankar, K.; Paulose, M.; Mor, G. K.; Latempa, T. J.; Grimes, C. A. Sens. Lett. 2006, 4, 378386. (15) Varghese, O. K.; Gong, D. W.; Paulose, M.; Grimes, C. A.; Dickey, E. C. J. Mater. Res. 2003, 18, 156-165. (16) Foll, H.; Langa, S.; Carstensen, J.; Christophersen, M.; Tiginyanu, I. M. AdV. Mater. 2003, 15, 183-198. (17) Harraz, F. A.; Kamada, K.; Kobayashi, K.; Sakka, T.; Ogata, Y. H. J. Electrochem. Soc. 2005, 152, C213-C220. (18) Melody, B.; Kinard, T.; Lessner, P. Electrochem. Solid State Lett. 1998, 1, 126-129. (19) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191-195. (20) Parkhutik, V. P.; Shershulsky, V. I. J. Physics D 1992, 25, 12581263. (21) Shobha, T.; Sarma, C. S. N.; Sastry, K. S.; Anjaneyulu, C. Bull. Electrochem. 2001, 17, 519-522. (22) Lu, Q.; Hashimoto, T.; Skeldon, P.; Thompson, G. E.; Habazaki, H.; Shimizu, K. Electrochem. Solid State Lett. 2005, 8, B17-B20. (23) Varela, H.; Torresi, R. M.; Buttry, D. A. J. Electrochemical Soc. 2000, 147, 4217-4223. (24) Izutsu, K. Electrochemistry in Nonaqueous Solutions; WileyVCH: New York, 2002. (25) Rice, M. J.; Kraus, C. A. Proc. Natl. Acad. Sci. U.S.A. 1953, 39, 1118-1124. (26) Pletnev, M. A.; Shirobokov, I. B.; Ovechkina, O. E.; Reshetnikov, S. M. Prot. Met. 1995, 31, 317-320. (27) Oguzie, E. E.; Okolue, B. N.; Ebenso, E. E.; Onuoha, G. N.; Onuchukwu, A. I. Mater. Chem. Phys. 2004, 87, 394-401. (28) Furneaux, R. C.; Rigby, W. R.; Davidson, A. P. Nature 1989, 337, 147-149.
