Synthesis, Characterization, Electrochemical, and ... - ACS Publications

Langmuir 2001, 17, 1975-1982

1975

Synthesis, Characterization, Electrochemical, and Spectroelectrochemical Studies of an N-Cetyl-trimethylammonium Bromide/V2O5 Nanocomposite Marco A. Gimenes,† Luciene P. R. Profeti,† Taˆnia A. F. Lassali,‡ Carlos F. O. Graeff,‡ and Herenilton P. Oliveira*,† Departamento de Quı´mica, Departamento de Fı´sica e Matema´ tica, FFCLRP, USP, Ribeira˜ o Preto, SP, 14040-901, Brasil Received July 3, 2000. In Final Form: October 23, 2000 Complex structures based on “host-guest” systems make possible the development of new materials with promising technological applications. In this work, we synthesized a V2O5‚nH2O/surfactant (N-cetylN,N,N,-trimethylammonium bromide) composite that presented an improved charge capacity and stability on electrochemical response in relation with a V2O5‚1.9H2O matrix. CTA+ intercalated into the V2O5 lamellar structure favors the kinetics of VIV/VV solid-state redox transitions. Moreover, the spectroelectrochemical behavior was verified even after several successive redox cycles.

Introduction The search for materials that provide a longer life cycle and efficient charge storage in high-energy batteries for a large variety of applications (portable electronics and electric vehicles, for instance) is today a challenging task.1 Moreover, this need must follow some constraints arising from environmental legislation, availability of raw materials, and low cost of production. In particular, much effort has been devoted to lithium ion batteries in which the most common system is based on a lithium anode, lithium ion conducting electrodes, and cathode materials in which lithium ions can intercalate.1,2 Concerning the last item, insertion compounds have been intensively investigated as new materials for battery cathodes. The main reason is that they have an open structure that makes the insertion and deinsertion of high quantities of Li+ per mole of cathode (host structure) possible, from a reversible electrochemical process without breaking the overall structure or, at least, inducing little modifications on the host structure even after several redox cycles. Among the insertion compounds, vanadium pentoxide xerogel is considered as a promising material for battery cathodes, because it combines a layered structure (1.18 nm of interlamellar spacing) suitable for the insertion reaction and a mixed ionic-electronic conductivity.3 However, in V2O5 xerogel a decrease of its chargedischarge capacity is normally observed after some cycles that could be attributed to the solvent exchange,4 irreversible changes in the structure, and the steric hindrance limiting the rechargeability.5 One possibility to minimize these restrictions and enhance the Li+ diffusion through the interlamellar spacing is manipulat* To whom correspondence should be addressed. Fax: +55 16633-8151. Tel: +55 16-602-3759. E-mail: [email protected]. † Departamento de Quı´mica. ‡ Departamento de Fı´sica e Matema ´ tica. (1) Scrosati, B. In Electrochemistry of Novel Materials - Frontiers of Electrochemistry; Liokowski, J., Ross, P. N., Eds.; VCH: New York, 1994; Chapter 3, p 111. (2) Owens, B. B.; Passerini, S.; Smyrl, W. H. Electrochim. Acta 1999, 45, 215. (3) Shembel, E.; Apostolova, R.; Nagirny, V.; Aurbach, D.; Markovsky, B. J. Power Sources 1999, 80, 90. (4) West, K.; Zachau-Christiansen, B.; Jacobsen, T.; Skaarup, S. Electrochim. Acta 1993, 38, 1215. (5) Scarminio, J.; Taledo, A.; Andersson, A. A.; Passerini, S.; Decker, F. Electrochim. Acta 1993, 38, 1637.

ing the interlayer spacing by using appropriate intercalated species.6 Several works have focused their attention in the production of conductive polymer/transition metal oxide nanocomposites such as polyaniline, melanin, or polypyrrole into V2O5 or MoO3 matrixes with an increase of interlamellar distance in order to investigate their application as electrodes for lithium batteries.7-12 These authors verified an increased lithium ion diffusion and an improved electrochemical stability and that both host and guest species played a role in the redox processes. However, little is known about the electrochemical behavior of the vanadium pentoxide matrix intercalated with nonelectroactive species. Bouhaouss and Aldebert13 intercalated alkylammonium and alkylamines into vanadium pentoxide xerogel, investigating the influence of guest species on structural changes and the thermal stability. More recently, Soga and Senna14 synthesized a V2O5/CTA+ (CTAB: N-cetyl-N,N,N,-trimethyl-ammonim bromide) intercalation compound in order to study the mechanical stress of the matrix during the intercalation as well as the kinetics of the process. However, the authors did not focus on the electrochemical properties. Here, we report the synthesis and characterization of the V2O5/ CTA+ nanocomposite and the study of the influence of surfactant molecules on the conductivity, electrochemical, and spectroelectrochemical properties along with its potential application as a battery cathode. Experimental Section Materials. All chemicals used were reagent grade, and they were used without any previous treatment. Acetonitrile was chromatography grade, the vanadium pentoxide xerogel was prepared from sodium metavanadate (Fluka), and the ionexchange resin was Dowex-50X in acid form. (6) Shouji, E.; Buttry, D. A. Langmuir 1999, 15, 669. (7) Morita, M.; Miyazaki, S.; Ishikawa, M.; Matsuda, Y.; Tajima, H.; Adachi, K.; Anan, F. J. Electrochem. Soc. 1995, 142, L3. (8) Leroux, F.; Goward, G.; Power, W. P.; Nazar, L. F. J. Electrochem. Soc. 1997, 144, 3886. (9) Wong, H. P.; Dave, B. C.; Leroux, F.; Harreld, J.; Dunn B.; Nazar, L. F. J. Mater. Chem. 1998, 8, 1019. (10) Goward, G. R.; Leroux, F.; Nazar, L. F. Electrochim. Acta 1998, 43, 1307. (11) Lira-Cantu´, M.; Go´mez-Romero, P. Int. J. Inorg. Mater. 1999, 1, 111. (12) Oliveira, H. P.; Graeff, C. F. O.; Zanta, C. L. P. S.; Galina, A. C.; Gonc¸ alves, P. J. J. Mater. Chem. 2000, 10, 371. (13) Bouhaouss, A.; Aldebert, P. Mater. Res. Bull. 1983, 18, 1247. (14) Soga, N.; Senna, M. Solid State Ionics 1993, 63-65, 471.

10.1021/la0009386 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/16/2001

1976

Langmuir, Vol. 17, No. 6, 2001

Synthesis of V2O5/CTA+ Nanocomposite. The nanocomposite was prepared by directly reacting the vanadium pentoxide gel (10 mL), synthesized by ion-exchange methodology,13,15 with 2 mL of 0.2 M aqueous solution of CTA+ (CH3(CH2)15(CH3)N+Br-), at room temperature (24 °C) and under constant stirring for 15 h. A pale orange suspension was obtained, and the V2O5/CTA+ nanocomposite was prepared in film form by casting onto a substrate and dried at room temperature; then, the film was rinsed with deionized water and dried again at room temperature. The obtained material, V2O5/CTA+, has the following formula evaluated by elemental analysis and thermal analysis: (CTA+)0.47V2O5‚0.43H2O (C, 36.5%; N, 2.4%; H, 6.6%). Equipment and Procedure. The powder X-ray diffraction (PXRD) data were recorded on a Siemens D5005 diffractometer using a graphite monochromator and Cu KR emission lines. The samples, in film form, were obtained on a glass plate, and the data were collected at room temperature over the range 2° e 2θ e 50°. The transmission electronic spectra (ultraviolet/visible spectra) were recorded on a Varian Cary 50 spectrophotometer with the intercalation compound formed on a quartz plate (also used as the reference) or onto an indium tin oxide (ITO) electrode. Scanning electron microscopy (SEM) studies were carried out on a Zeiss DSM 940 microscope, operating at 20 kV, and the samples were formed into film on a plate sample holder. Electron paramagnetic resonance (EPR) spectra were obtained at room temperature using a computer interfaced Varian E-4 spectrometer operating at 9.5 GHz (X band). The g value was obtained with reference to the standard diphenyl-β-picrylhydrazyl (DPPH). The dc conductivity was measured against temperature in the 150-350 K range. The reproducibility of the experiments was tested twice for each sample. The measurements were performed in an evacuated chamber using a dc bias of 1 V between silver electrodes. The infrared spectra were recorded from 2000 to 400 cm-1 on a Nicolet 5ZDX Fourier transform infrared (FT-IR) spectrometer. The samples were dispersed in KBr and pressed into pellets. The electrochemical experiments were carried out with an Autolab (EcoChemie) model PGSTAT30 (GPES/FRA) potentiostat/galvanostat interfaced to a computer. The conventional three-electrode arrangement was used, consisting of an ITO supporting electrode, a platinum wire auxiliary electrode, and a saturated calomel electrode (SCE) as reference electrode, containing 0.1 mol dm-3 of supporting electrolyte (LiClO4). The experiments were carried out in an inert atmosphere by bubbling N2 through the solution at room temperature. Ac impedance spectra were measured as a function of potential using an ac perturbation signal of 5 mV (p/p), covering the 25 mHz-100 kHz frequency range. The potentials analyzed were chosen from voltammetric behavior in the same solution. The experimental impedance spectra were interpreted on the basis of equivalent electrical circuits using a fitting software GPES/FRA version 4.6, Autolab-EcoChimie. For the spectroelectrochemical experiments, the potentiostat was coupled with a spectrophotometer, and the three-electrode system was assembled in a quartz cell of 1.00 cm optical path length. The thermogravimetric data were registered on a Thermal Analyst equipment model 2100-TA in air atmosphere and at a heating rate of 5 °C min-1. Elemental analysis (carbon, nitrogen, and hydrogen amounts) was performed using an Elemental Analyzer CE Instruments, model EA-1110, and the data were obtained by the dynamic flash combustion method.

Results and Discussion The diffraction pattern of the resulting material is consistent with the intercalation of CTA+ into the matrix and indicates that the framework of the host is preserved after the reaction, coherent with a topotactic reaction, as shown in Figure 1. An increase of the interlayer spacing with the insertion of the polymeric species was also observed with d-spacing equal to 1.87 nm, whereas the matrix has 1.18 nm (∆d ) 0.71 nm). Taking into account the chain length of CTA+ (almost 2.777 nm14) and the increase of basal distance, it is reasonable to assume that (15) Gharbi, N.; Sanchez, C.; Livage, J.; Lemerle, J.; Nejem, L.; Lefebvre, J. Inorg. Chem. 1982, 21, 2758.

Gimenes et al.

Figure 1. Cu KR X-ray diffraction patterns of (a) the hydrated vanadium pentoxide matrix (V2O5‚1.9H2O), (b) the (CTA+)0.47V2O5 nanocomposite, and (c) CTAB (CH3(CH2)15(CH3)NBr).

Figure 2. FT-IR spectra of (a) the hydrated vanadium pentoxide matrix (V2O5‚1.9H2O), (b) the (CTA+)0.50V2O5‚0.43H2O nanocomposite, and (c) CTAB (CH3(CH2)15(CH3)NBr).

the orientation of the guest species is parallel to the V2O5 sheets. In addition, few diffraction peaks of low intensity related to the surfactant were observed in the composite PXRD pattern (Figure 1b), evidence that the guest species was mainly between the V2O5 sheets instead of adsorbed on the matrix. In Figure 2, the FT-IR spectrum of the nanocomposite showed the typical bands of the matrix (in parentheses) [1016 cm-1 (1009 cm-1), ν(VdO); 757 cm-1 (760 cm-1), ν(VOV); 506 cm-1 (531 cm-1), δ(VOV)] and of the surfactant [2910 cm-1, ν(CH); 1425 cm-1, ν(CN); 1300 cm-1, ν(CH2)]. This spectrum clearly shows the presence of surfactant molecules and corroborates that the matrix preserved its structure after the reaction. We also noted little shifts of the main V2O5 bands, as expected because of the distortion of the framework. The ratio of organic and inorganic phase band intensities suggests that the nanocomposite contains a large amount of surfactant molecules, which is in agreement with elemental analysis (C, 36.5%; N, 2.4%; H, 6.6%). Thermogravimetric curves of the matrix, hybrid compound, and surfactant are shown in Figure 3. For the matrix, the curve (Figure 3a) shows two main weight loss waves. The first weight loss extends to around 125 °C and is attributed to the loss of weakly bounded water. The second wave of weight loss extends up to around 280 °C, attributed to the release of the intramolecular water and the water molecules bound to vanadium. For temperatures

CTAB/V2O5 Nanocomposite

Figure 3. Thermogravimetric analysis/differential thermal analysis data for (a) the hydrated vanadium pentoxide matrix (V2O5‚1.9H2O), (b) the (CTA+)0.50V2O5‚0.43H2O nanocomposite, and (inset) CTAB (CH3(CH2)15(CH3)NBr).

Figure 4. EPR spectra of (a) the hydrated vanadium pentoxide matrix (V2O5‚1.9H2O) and (b) the (CTA+)0.50V2O5‚0.43H2O nanocomposite in film form and at room temperature (24° C).

above 330 °C, there is the formation of crystalline vanadium pentoxide. From this curve, the dehydration process at 125 °C leads to a vanadium pentoxide with a low amount of water, V2O5‚0.5H2O, related to structural water that retains the lamellar structure, in agreement with previous reports.2,4 From this thermogravimetric curve, the formula of the vanadium pentoxide xerogel is V2O5‚1.9H2O. For the nanocomposite, the thermogravimetric curve (Figure 3b) shows 2.5% of mass loss up to 180 °C that could be attributed to weakly bonded water molecules, n ) 0.43. In the 180-400 °C temperature range, the mass loss (42%) in three stages (190-285 °C, 290380 °C, and 380-425 °C) could be related mainly to decomposition of the surfactant molecule, shown in Figure 3 (inset). Moreover, with the increase of temperature there was a gain in mass that could be related to the oxidation of VIV centers by oxygen concomitant with the conversion to crystalline vanadium pentoxide. By combination of elemental analysis and thermogravimetric analysis data, the obtained material has the following formula: (CTA+)0.50V2O5‚0.43H2O. EPR spectra obtained at room temperature of vanadium pentoxide xerogel and the (CTA+)0.50V2O5‚0.43H2O intercalation compound are shown in Figure 4. For these measurements, the samples have been positioned so that the magnetic field was perpendicular to the sample surface

Langmuir, Vol. 17, No. 6, 2001 1977

orientation. The spectrum of the matrix is typical of the VIV ion in an axially distorted crystal field, presenting several lines due to the hyperfine coupling of one unpaired electron (S ) 1/2) with the nuclear spin (I ) 7/2).15 EPR spectra are anisotropic with respect to the external magnetic field and can be simulated by combining an amorphous part with a c-axis preferentially oriented polycrystalline part. From the simulations for the amorphous part, the EPR parameters found are g| ) 1.940, g⊥ ) 1.976, A| ) 179 G, and A⊥ ) 67 G, where A| and A⊥ are the hyperfine tensor components. For the polycrystalline part, g| ) 1.996, g⊥ ) 1.977, A| ) 168 G, and A⊥ ) 71 G. Both results are in reasonable agreement with the EPR parameters reported in the literature.15 After the reaction with the surfactant molecules, we did not observe any apparent change in the oxidation state of the vanadium ions. However, the anisotropy is practically absent and it can be clearly seen that the line width decreases from approximately 26 to 20 G. On one hand, EPR results indicate an increase in disorder, as will be discussed next, in the film structure after the intercalation: lack of anisotropy. On the other hand, the results confirm that the interlayer spacing is affected. A higher interlayer spacing induces a decrease in the VIV ion interaction between layers and consequently a decrease in line width. Figure 5 shows scanning electron micrographs of the matrix and the (CTA+)0.50V2O5‚0.43H2O intercalation compound. From the SEM image of the hydrated vanadium pentoxide matrix (Figure 5a), we can note the presence of a network of interconnected chains associated with a nucleation center,16,17 consistent with a fibrous texture. On the other hand, the SEM images of (CTA+)0.50V2O5‚0.43H2O (Figure 5b,c) show how the morphology changed dramatically from a network of chains to a platelike structure, forming aggregates with a 5- 20 µm range of diameter. Despite the morphological differences, the microcrystallites of the compound formed kept their bidimensional structure. In addition, the nanocomposite surface is inhomogeneous and rough and presents holes between the aggregates. The effect of intercalation of CTA+ on the conductivity of V2O5‚1.9H2O is shown in Figure 6. The room-temperature conductivity changed from 2.1 × 10-6 to 2 × 10-7 (Ω cm)-1 with the introduction of CTA+ into the V2O5 structure. The activation energy also changed from 0.31 eV in V2O5‚1.9H2O to 0.28 eV in (CTA+)0.50V2O5‚0.43H2O. This change in the transport properties may arise from different factors. From the small polaron model,18 the activation energy (W) of the dark conductivity at higher temperatures reflects the polaron binding energy and the structural disorder as well as a transfer integral (the coupling potential between two hopping sites). In the case of V2O5 xerogel, it is especially difficult to separate these contributions to the activation energy, which can vary from 0.17 to 0.65 eV.19 Higher activation energy is normally attributed to the disorder term. In our system, however, the disorder has indeed increased with the incorporation of CTA+. Thus, assuming that the small polaron hopping model is valid (which may not be the case because our samples are clearly inhomogeneous) a possible explanation comes from a change in the potential barrier sensed by the electron because of the presence of a charged CTA+ (16) Oliveira, H. P.; Graeff, C. F. O.; Rosolen, J. M. Mater. Res. Bull. 1999, 12-13, 1891. (17) Bailey, J. K.; Pozarnsky, G. A.; Mecartney, M. L. J. Mater. Res. 1992, 7, 2530. (18) Austin, I. G.; Mott, N. F. Adv. Phys. 1969, 18, 41. (19) Sayer, M.; Mansingh, A. J. Non-Cryst. Solids 1983, 58, 91.

1978

Langmuir, Vol. 17, No. 6, 2001

Gimenes et al.

Figure 6. Conductivity results for (a) the hydrated vanadium pentoxide matrix (V2O5‚1.9H2O) and (b) the (CTA+)0.50V2O5‚ 0.43H2O nanocomposite.

Figure 7. Cyclic voltammograms and stability essay of the hydrated vanadium pentoxide in acetonitrile containing 0.1 mol L-1 LiClO4, in the potential range from +1.00 to -0.60 V (SCE) and v ) 100 mV/s.

Figure 5. Scanning electron micrographs of (a) the hydrated vanadium pentoxide matrix (V2O5‚1.9H2O) and (b and c) the (CTA+)0.50V2O5‚0.43H2O nanocomposite.

near the vanadium sites, similar to what we have investigated earlier in melanin-like/V2O5 hybrid compounds.12 However, contrary to what was found in the previous study CTA+ is responsible for a decrease in conductivity, which is clearly an indication that although CTA+ may have a positive effect in the conductivity on

one hand, from a topological point of view, as discussed previously, the increase of the roughness and the formation of aggregates decreases the connectivity of the sample, decreasing the conductivity. Another point that must be considered is the difference in water content that can affect the conductivity. The lower amount of water molecules in the hybrid compound can contribute to the increase in electronic resistivity, as observed in Figure 6, because the conductivity has the contribution of hopping of H+ though hydrogen bonds.20 And, according to thermogravimetric studies there is a decrease of water content in the nanocomposite material ((CTA+)0.50V2O5‚0.43H2O) in comparison with vanadium pentoxide xerogel (V2O5‚1.9H2O) that could change the conductivity, despite the presence of surfactant. This interesting fact must be studied further in relation to the water molecule content and morphological/crystalline differences, and such studies are now underway in our laboratory. Cyclic voltammograms of the V2O5‚1.9H2O and (CTA+)0.50V2O5‚0.43H2O were very similar (Figures 7 and 8, respectively), exhibiting reversible peaks in the potential range from -0.60 to +1.00 V (SCE), in acetonitrile solutions containing 0.1 M LiClO4. These peaks can be ascribed to the insertion of lithium ions (xe- + xLi+ + V2O5‚nH2O S LixV2O5‚nH2O) in two steps or can be due to two nonequivalent sites in the vanadium oxide matrix, (20) Kittaka, S.; Hamaguchi, H.; Shinno, T.; Takenaka, T. Langmuir 1996, 12, 1078.

CTAB/V2O5 Nanocomposite

Figure 8. Cyclic voltammograms and stability essay of the (CTA+)0.50V2O5‚0.43H2O nanocomposite in acetonitrile containing 0.1 mol L-1 LiClO4, in the potential range from +1.00 to -0.60 V (SCE) and v ) 100 mV/s.

as a function of different vanadium coordination spheres.21 The width and asymmetric pattern of the waves can be related to the heterogeneity of the film surface. Voltammetric data of (CTA+)0.50V2O5‚0.43H2O suggest that during the anodic process the deinsertion of Li+ occurs mainly in one step instead of two steps as for the matrix. The low potential difference between Epa2 and Epc2 (∆ ) Epa2 Epc2 ) -0.40 V) in relation to vanadium pentoxide xerogel indicates a decrease in film resistance, perhaps from the higher lithium diffusion constant through the film. Another interesting fact is that the insertion of surfactant into the matrix seems to stabilize the electrochemical response during several cycles (100 cycles) in contrast with the matrix (30 cycles) (Figure 8). For the matrix xerogel (Figure 7), after 30 successive voltammetric cycles we noted a decrease of the total charge associated with the loss of peak definition attributed to insertion/deinsertion of Li+, probably because of the production of LixV2O5 stable crystalline phases, making the release of lithium ions to the supporting electrolyte solution more difficult. Another possibility for the decay of electrochemical response for vanadium pentoxide xerogel is the loss of water from the intercalation compound into the solvent during its reduction and subsequent oxidation in acetonitrile solution. To test this hypothesis, an electrochemical essay was performed in which the V2O5 xerogel was submitted to a thermal annealing at 120° C for 1 h in order to remove the loose water molecules without collapsing the lamellar structure.2 The first and 100th cyclic voltamogramms of V2O5‚0.5H2O (composition after thermal treatment) are shown in Figure 9. The overall appearance of the voltammetry behavior is quite similar to that of vanadium pentoxide xerogel, V2O5‚1.9H2O; however, it is clear that the lower water content induced a stabilization of the electrochemical response with a decrease of only 27% of the total charge after successive reduction/oxidation cycles, in contrast to 40% from the oxide xerogel without heat treatment. These results reveal changes in solvent transport behavior, which with low water content facilitated the electromigration of solvated Li+ into and out of the network with structural changes of the matrix.22 High water molecule content probably is partially exchanged with acetonitrile, limiting the charge/ discharge process, and with the release of loosely bound (21) Nabavi, M.; Doeuff, S.; Sanches, C.; Livage, J. Mater. Sci. Eng., B 1989, 3, 203. (22) Shouji, E.; Buttry, D. A. Electrochim. Acta 2000, 45, 3757.

Langmuir, Vol. 17, No. 6, 2001 1979

Figure 9. Cyclic voltammograms and stability essay of the dehydrated vanadium pentoxide (V2O5‚0.5H2O) in acetonitrile containing 0.1 mol L-1 LiClO4, in the potential range from +1.00 to -0.60 V (SCE) and v ) 100 mV/s.

Figure 10. Cyclic voltammograms and stability essay of the dehydrated (CTA+)0.50V2O5 nanocomposite in acetonitrile containing 0.1 mol L-1 LiClO4, in the potential range from +1.00 to -0.60 V (SCE) and v ) 100 mV/s.

water, the electrochemical response of the dehydrated material becomes more reversible.2,4 Concerning the intercalation compound (Figure 8), although the current peak intensities have decreased we verified that from the 15th to the 100th successive voltammetric cycles the electrochemical response has stabilized in relation to that of vanadium pentoxide xerogel. Probably, this phenomenon can be related to an enhanced Li+ diffusion through the film because of a decrease in steric and electrostatic effects promoted by the higher interlamellar distance or because of a low quantity of water present in the nanocomposite. So, the material was submitted to a thermal treatment at 125 °C for 1 h to remove weakly bound water molecules, n ) 0.43 (evaluated by thermal analysis, as shown in Figure 3b). It should be noted that there is some uncertainty in this value because it was not possible to separate the fraction corresponding to water mass loss and to the organic phase from the total mass loss verified in the thermogravimetric curve for temperatures above 290 °C (Figure 3b and inset). Although there is an error arising from the determination of structural water molecules, we can assume that this error does not affect the qualitative discussions. Assuming that the material retains some strongly bound water molecules, the impact of this error in the real composition of the nanocomposite could be neglected and does not affect the qualitative discussions and conclusions. On the basis of these assumptions, the approximate formula of dehydrated material is (CTA+)0.50V2O5. Figure 10 shows the 1st and 100th cyclic voltammograms for the dehydrated

1980

Langmuir, Vol. 17, No. 6, 2001

Figure 11. Complex-plane plots of V2O5‚1.9H2O in acetonitrile containing 0.1 mol L-1 LiClO4, in the potential range from +1.00 to -0.60 V (SCE).

material. As can be seen, the electrochemical behavior is very similar to that of the material before thermal treatment (Figure 8); however, the dehydrated material is much more stable against successive reduction/oxidation cycles in comparison with its hydrated form (decrease of 10% of total charge after successive reduction/oxidation cycles, compared to 35% from hydrated material) as well as with the dehydrated vanadium pentoxide (decrease of 27%). Perhaps, besides the fact that Li+ diffusion through the film is enhanced because of a decrease in steric and electrostatic effects promoted by the higher interlamellar distance, as mentioned above, we can speculate that the solvent can reversibly enter and exit the film during Li+ intercalation and deintercalation in such way that the volume and mechanical stress changes in the film are minimized after the surfactant intercalation. Overall, we can suppose that the contribution of the guest species is, from what has been discussed, only structural, that is, acting as pillars between the V2O5 sheets, with a higher interlamellar space, and decreasing the structural changes in the films provoked by the solvated Li+ insertion/ deinsertion. However, at this point more work is under way to discern the magnitude of the contribution of the surfactant and the amount of water molecules on electrochemical stabilization. Complex-plane plots, representative of the systems, are shown in Figures 11 and 12 as a function of potential for V2O5‚1.9H2O and (CTA+)0.50V2O5‚0.43H2O films, respectively. The potentials presented in these figures were selected to be representative of the behavior of the impedance spectra before and after the VV/VIV redox transition. In both systems, impedance spectra always show the following characteristics at positive potential values: a depressed arc in the high-frequency range, followed by a line inclined at a constant angle to the real axis in the low-frequency range, and a second straight line with an angular coefficient close to 90° in the frequency range below 100 mHz. These features are in good agreement with those reported in the literature under similar experimental conditions.23,24 A complete analysis of the impedance spectrum of an electrochemical system involving lithium diffusion was performed in the pioneering work of Ho et al. on tungsten trioxide thin films.25 According

Gimenes et al.

Figure 12. Complex-plane plots of the (CTA+)0.50V2O5‚0.43H2O nanocomposite in acetonitrile containing 0.1 mol L-1 LiClO4, in the potential range from +1.00 to -0.60 V (SCE).

to these authors, the behavior observed in the complexplane plots shown in Figures 11 and 12 is related to a system where the kinetics passes from diffusion controlled at low frequencies to charge transfer controlled at higher frequencies. Following this argument, the high frequency behavior observed in the complex-plane plots is probably related to the charge-transfer reaction of solid-state VIV/ VV transitions at the electrolyte/electrode interface. This arc is severely depressed as a result of the porous and inhomogeneous character of the composite electrode.26 The tilted line, with an angular coefficient close to 45°, in the high-frequency range is attributable to Warburg impedance associated with lithium diffusion through the vanadium pentoxide matrix. The second line at low frequencies can be attributed to finite length effects and related to a pseudocapacitance observed in the insertion compounds when the film is sufficiently thin to ensure that there are no concentration gradients as the voltage perturbation inserts/releases guest species.25 In addition, Pyun et al. also suggested that this line is characteristic of a capacitive behavior and is related to the accumulation of lithium at the center of the oxide particle.23 Below -0.325 V, the film is completely reduced. The observed curvature, with no evidence of the linear Warburg region at low frequencies, is consistent with a kinetic controlled charge/electron transport inside the film. This behavior is similar to that observed by Anaissi et al.27 in vanadium pentoxide xerogel film, characterized by ac impedance in acetonitrile containing 0.1 mol dm-3 LiClO4. The equivalent circuit, EC, which best fits the experimental data in all potential values is a R(Q1(Rct(RdQ2))) combination. Figure 13 shows a schematic representation of the EC. R represents the sum of ohmic resistance of the solution and electrode, Rct is the charge-transfer resistance of Faradaic processes occurring at the oxide/solution interface, and Rd is the ionic resistance arising from the diffusion of lithium ions. The R behavior as a function of potential is shown in (23) Pyun, S.-I.; Bae, J.-S. Electrochim. Acta 1996, 41, 919. (24) Farcy, J.; Messina, R.; Perichon, J. J. Electrochem. Soc. 1990, 137, 1337. (25) Ho, C.; Raistrick, I. D.; Huggins, R. A. J. Electrochem. Soc. 1980, 127, 343. (26) de Levie, R. Electrochim. Acta 1964, 9, 1231. (27) Anaissi, F. J.; Demets, G. J. F.; Toma, H. E. Electrochem. Commun. 1999, 1, 332.

CTAB/V2O5 Nanocomposite

Langmuir, Vol. 17, No. 6, 2001 1981

Figure 13. Equivalent circuit used in the simulation (R(Qdl(Rct(RdQ))) which best fit the experimental data to all potential values.

Figure 15. Rct behavior as a function of potential applied for (a) V2O5‚1.9H2O and (b) the (CTA+)0.50V2O5‚0.43H2O nanocomposite.

Figure 14. Dependence of R-values on the applied potentials for (a) V2O5‚1.9H2O and (b) the (CTA+)0.50V2O5‚0.43H2O nanocomposite.

Figure 14 for both films. Although R-values do not show a systematic change with potential in V2O5‚1.9H2O film, the same is not true for (CTA+)0.50V2O5‚0.43H2O nanocomposites. In the intercalation compound, R shows a higher value at -0.5 V when the film is reduced, probably because of structural changes in the V2O5 lattice related to lithium saturation of the matrix. The film resistance decreases for anodic potential values reaching a minimum value at 0.25 V (this value is coincident with Epa2 in the cyclic voltammogram, Figure 8). Note that the R-values are always lower in (CTA+)0.50V2O5‚0.43H2O film independent of the potential applied when compared to V2O5‚ 1.9H2O R-values. As the experimental conditions were the same for both systems and the only difference was the nature of the film, we can suppose that the impedance associated with charge and mass transport phenomena for the hybrid material is less significant than for V2O5 xerogel alone. The Rct-value is related to V2O5 electroreduction that occurs during Li+ intercalation into the V2O5 lamellar structure.27 During lithiation, Li+ ions are incorporated into the oxide and at the same time VV sites are reduced to VIV by donating an electron. The Rct behavior as a function of applied potential is very similar in both systems (Figure 15). Rct is approximately constant until 0.25 V, rising for higher potentials. The increase is probably related to the absence of a process kinetically favorable in these conditions. Note that Rct-values are much lower in (CTA+)0.50V2O5‚0.43H2O film compared to the matrix. Moreover, these results are in good agreement with a decrease of ∆Ep observed in the oxi-reduction process of the cyclic voltammetry under the same experimental

Figure 16. Dependence of Rd on the applied potentials for (a) V2O5‚1.9H2O and (b) the (CTA+)0.50V2O5‚0.43H2O nanocomposite.

conditions. However, at this point we are uncertain of the origin of the effect, but we can associate it to an improvement in the kinetics of the charge-transfer reaction into the film, as a consequence of the release of loosely bound water molecules and a higher interlamellar distance. Rd values are similar in both systems (Figure 16). This result suggests that CTA+ intercalated into the V2O5 matrix does not induce an improvement in the diffusion of Li+ species into the matrix. This behavior can result from two opposite effects promoted by the CTA+ insertion; whereas the increase of the interlayer spacing of the matrix should facilitate the diffusion of Li+ species, the steric hindrance of guest species acts in the opposite sense. Q1 and Q2 are so-called constant phase elements (CPEs). The impedance of the CPE shows a frequency dependence that can be written as ZCPE ) 1/T(iω)n, where T is a frequency-independent term, ω is the angular velocity, and i ) (-1)1/2. The value of the exponent n ranges between zero and unity. An n-value of zero corresponds to a pure resistor, n ) 1 corresponds to a pure capacitor, and n ) 0.5 has been attributed to Warburg type impedance. Other n-values have been attributed to various causes such as porosity and fractal structure of the interface or to various distributions in the activation energy for charge migration

1982

Langmuir, Vol. 17, No. 6, 2001

Figure 17. Dependence of Q1 on the applied potentials for (a) V2O5‚1.9H2O and (b) the (CTA+)0.50V2O5‚0.43H2O nanocomposite.

and transfer.26-29 Figure 17 shows the Q1 and n behavior as function of potential applied to V2O5‚1.9H2O and (CTA+)0.50V2O5‚0.43H2O films. For both systems, an increase of Q1-values is observed when the potential is dislocated to the anodic direction, reaching a maximum value at 0.25 V and decreasing for subsequent values of potentials. Values of n close to 1 suggest that the CPE behaves as a capacitor to V2O5‚1.9H2O film independent of the potential applied, so Q1 ≡ Cdl (double-layer capacitance). In contrast, both parameters (Q1 and n) change with applied potential for (CTA+)0.50V2O5.0.43H2O films. A n-value close to unity is observed when the measures were performed in extreme values of potentials (for anodic and cathodic potential values). In these conditions, the Q1 values are similar for both systems, suggesting that Q1 also behaves as a capacitor for the intercalation compound. However, for potential values close to Epa2 in the cyclic voltammogram (Figure 8) n-values tend to 0.5, suggesting a change in the physical attribution of Q1, from capacitor to Warburg impedance (W), related to diffusion phenomena. One possible explanation of this behavior is an improvement in the kinetics of the charge-transfer reaction for the intercalated film. This assumption is coherent with Rct values. For potential values close to the solid-state redox transition VIV/VV, the impedance of the system could be governed by solvated Li+ movement into the structure of the (CTA+)0.50V2O5‚ 0.43H2O nanocomposite. The n-value of the exponent, associated with Q2, close to 0.5 in all potential intervals analyzed suggests that the RQ combination is a Warburg type impedance for both systems. This parameter is probably related to the Li+ diffusion process into the V2O5 matrix. Presumably, in agreement with cyclic voltammetric results surfactant molecules acted as pillars inside the matrix, giving more rigidity to the matrix and consequently making the material less susceptible to changes in volume and mechanical stress resulting from Li+ insertion/deinsertion. Another possibility is the creation of channels between the layers that facilitated the diffusion of lithium ions through the material. Moreover, we cannot discard the fact that the Li+ insertion/deinsertion into these materials is shown to be more reversible when weakly bonded water is removed. For the (CTA+)0.50V2O5‚0.43H2O system, more work is under way to clarify the stabilization of the electrochemical response and identify the contribution of each factor. The electrochromic effect was also observed in the intercalation material during the potential scans (Figure (28) Pajkossy, T.; Nyikos, L. Electrochim. Acta 1988, 33, 713. (29) Macdonald, J. R. J. Appl. Phys. 1985, 58, 1955.

Gimenes et al.

Figure 18. Dependence of Q2 on the applied potentials for (a) V2O5‚1.9H2O and (b) the (CTA+)0.50V2O5‚0.43H2O nanocomposite.

Figure 19. Spectroelectrochemical study of the (CTA+)0.50V2O5‚ 0.43H2O nanocomposite in acetonitrile containing 0.1 mol dm-3 LiClO4, in the potential range from -0.50 to +1.50 V (SCE), using ITO as a working electrode.

19). The band at 385 nm attributed to the vanadium(V) oxide CT transition decreases in intensity, and a broad band around 800 nm appears, showing up the weak transitions which should correspond to ligand field transitions within the vanadium(IV) oxide chromophore.14 We did not observe any other electronic transitions during the essays, indicating that the surfactant molecules act only in the electrochemical process and not in the spectroscopic properties. Conclusions The manipulation of interlayer spacing with nonelectroactive guest species in order to achieve a stable reversible electrochemical response appears to give promising results. Indeed, we verified a stabilization of the electrochemical process for the V2O5/CTA+ system in comparison with the matrix alone where we noted a decrease in the total charge after approximately five cycles. Electrochemical studies showed that CTA+ intercalation into V2O5 improves the kinetics of the lithium electroinsertion into the oxide matrix. In addition, independent of the potential value used, R and Rct are lower than those observed in V2O5‚1.9H2O film, suggesting that CTA+ intercalation into the V2O5 lamellar structure favors the kinetics of VIV/VV solid-state redox transitions. Besides the maintenance of total charge during successive redox cycles, the spectroelectrochemical behavior was verified even after several cycles. Acknowledgment. This research was supported by FAPESP, CNPq, CAPES, and PADCT (no. 62.0238/97-6). The authors acknowledge J. M. Rosolen for FT-IR measurements. LA0009386