Composites of V2O5 xerogel with polyaniline (PANI) are prepared from a vanadyl tris(isopropoxide) precursor and aniline monomer by in situ oxidative polymerization of aniline in the sol solution, followed by gelation with the inorganic oxide. The composite is characterized using Fourier transform infrared spectroscopy, X-ray diffraction, electroacoustic impedance, electrochemical quartz crystal microbalance (EQCM), electrochemical impedance, in situ resistance, cyclic voltammetry, UV-visible (UV-vis) spectroscopy, and multiple charge/discharge measurements. Electrochemical impedance data at −0.7V give a value for the Li+ diffusion coefficient in the composite of 2×10−11 cm2 s−1, in contrast to the value of 3×10−12 cm2 s−1 obtained for V2O5. EQCM data show that charge compensation in the composite is achieved predominantly by Li+ electromigration, as is also the case for V2O5. In situ resistance measurements reveal that the composite has a very high conductivity over the potential range 0.4 to −0.5V. In contrast, V2O5 exhibits much lower conductivity over this range and also shows a deep minimum in conductivity near −0.2V. Both materials become more resistive at potentials more negative than −0.5V. UV-vis spectroscopy of the materials reveals changes in the optical bandgap that are consistent with these resistance changes. Finally, the composite shows excellent stability toward repeated charge/discharge cycling. © 2002 The Electrochemical Society. All rights reserved.

Nanoscale composites of polyaniline (PANI) and vanadium oxide (V 2 O 5 ) were assembled via the electrostatic layer-by-layer (LBL) technique, with a thickness per bilayer of 2.5 nm. Interactions between PANI and V 2 O 5 are maximized in comparison to the usual xerogel films due to the nanostructured nature of the LBL films, in which V 2 O 5 causes PANI to be oxidized. This has been demonstrated in Raman spectroscopy measurements and is consistent with electrochemical data. These strong interactions make the LBL film to display a color that is different from the colors of the individual materials. Furthermore, they promote a cooperative effect that enhances the charge storage capability of the films, with a total charge of 2.25 mC cm -2 for the PANI/ V 2 O 5 LBL films, in comparison with 1.86 mC cm -2 for the sum of the isolated contributions from PANI and V 2 O 5 .

This work describes how manipulation at the molecular level of V2O5/polyaniline (PANI) nanocomposites built with the layer-by-layer (LBL) technique promotes enhancement of charge storage capability, new electrochromic effects, and control of ionic flux. By changing the film architecture we control the amount of PANI participating in the redox processes of the LBL films. As a consequence, the films display the electrochemical profile of V2O5 and the chromogenic properties of PANI. Impedance spectroscopy data show the presence of distinct phases in the nanocomposite, which allows a conducting path for the whole potential range between 0.5 and −1.5 V. Further control of the properties of the nanoarchitectures is achieved by adsorbing V2O5/PANI LBL films onto a cast PANI film. By changing the time of immersion of the PANI−V2O5/PANI system into a solution of LiClO4/propylene carbonate (PC), we were able to monitor the mass gain/loss (Δm) with an electrochemical quartz crystal microbalance (EQCM) as a function of charge (q) and control the intercalating/deintercalating species. The Δm/q ratio shifted from 0.77 to −0.12 mg C-1 when a 10-bilayer LBL film was immersed into a solution of LiClO4/PC during 24 h, indicating a change from an anionic contribution to a cationic one for the charge compensation process. It is envisaged that the molecular-level control demonstrated here may be exploited in producing efficient lithium batteries as well as electrochromic devices and sensors.

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The design of improved materials for electrochromic applications now involves extensive use of novel composites, thus requiring an investigation of the mechanisms responsible for electrochromism in these structures. Using films of WO(3) and chitosan produced with the layer-by-layer (LBL) technique, we demonstrate that characteristics such as the number of electrochemical active sites (K), the molar absorption coefficient (epsilon), and the electrochromic efficiency (eta) can be obtained using the quadratic logistic equation (QLE). The complexation ability between chitosan and WO(3) allowed the growth of visually uniform multilayers of the composite, with the same amount of material adsorbed in each deposition cycle. By fitting the absorbance changes (DeltaA) resulting from the electronic intervalence transfer from W(V) to W(VI) sites in four-bilayer LBL films of WO(3)/chitosan and WO(3)/chitosan with ethanol in the precursor dispersion, K was estimated to be ca. 5.5 x 10(-8) mol cm(-2) and 3.6 x 10(-8) mol cm(-2), respectively. The molar absorption coefficient and electrochromic efficiency vary with the charge injected because of the saturation of W(V) sites and the dissipation and feedback effects implicit in the QLE associated with ion-network interactions, such as the proton trapping effect. The LBL film of WO(3)/chitosan showed a smaller molar absorption coefficient and electrochromic efficiency than that containing ethanol because of a greater proton trapping effect for the LBL film with no ethanol. This enhanced trapping effect was seen as a decrease in the electronic flux involved in intervalence transfer in electrochemical impedance spectroscopy experiments.

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Layer-by-layer (LbL) films from WO3, TiO2, and chitosan have been produced, in which the ability of complexation of chitosan with transition metal oxides was exploited. The electrochemical and chromogenic properties of LbL films of (chitosan/WO3)4/TiO2 and (chitosan/WO3/chitosan/TiO2)4 have been examined and compared to those of (chitosan/WO3)4. More specifically, we employed the quadratic logistic equation (QLE) to estimate the number of electrochemically active sites (K) and the molar absorption coefficient (ε) of WO3 in LbL films, by fitting the absorbance changes (ΔA) resulting from the electron transfer from W(V) to W(VI) sites. The number of electrochemical active sites for 4-bilayer LbL films of chitosan/WO3 before and after deposition of TiO2 was ca. 15.5 × 10-9 mol cm-2 and 10.7 × 10-9 mol cm-2, respectively. The incorporation of TiO2 in the LbL films led to a larger absorption coefficient and electrochromic efficiency. The differences in the molar absorption coefficient were attributed to an electron-transfer rate associated with ionic-trapping effects, which were observed by electrochemical impedance spectroscopy. These results are discussed on the basis of dissipation and feedback effects.

The combination of semiconducting oxides and polyaniline in the nanoscale range may result in hybrid materials having enhanced properties, such as electrochromism and charge capacity. This paper reports the spectroscopic, morphological and electrochromic characterization of hybrid films made up of hexaniobate one-dimensional (1D) nanoscrolls and polyaniline prepared by the layer-by-layer assembly technique (LbL). Secondary electron imaging and backscattered electron imaging techniques performed using a scanning electron microscope showed that polyaniline is adsorbed on the hexaniobate nanoscrolls, which confirms the combination of the components in the nanoscale domain. UV-VIS-NIR electronic spectra of the LbL hybrid films showed the absorption tail in the NIR region, assigned to delocalized polarons of the polyaniline. Resonance Raman spectra in the 1000-1700 cm À1 range indicated that hybrid films present a higher relative intensity of polaron bands at 1337 and 1508 cm À1 than pristine polyaniline in the emeraldine salt form. These results suggest that hexaniobate nanoscrolls induce a secondary doping of polyaniline. The cyclic voltammetry (CV) data for the hybrid film showed a specific capacity of 870 C cm À3 . According to CV results, the synergistic effect on charge storage properties of the hybrid material is attributed to the enhanced electroactivity of the hexaniobate component in the LbL film. Spectroelectrochemical experiments showed that the electrochromic efficiencies at 420 nm are ca. À41 and 24 cm 2 C À1 as the potential changes from 0.8 to À0.9 V and from À0.9 to À1.8 V, respectively, whereas at 800 nm the efficiencies are ca. À55 and 8 cm 2 C À1 for the same potential ranges. The electrochromic efficiencies and multi-colour character of the LbL film of hexaniobate nanoscrolls and polyaniline indicate that this novel hybrid material is an interesting modified electrode for electrochromic devices.