Lithium Ion Electro-Insertion and ... - ACS Publications

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Lithium Ion Electro-Insertion and Spectroelectrochemical Properties of Films from Hexaniobate Alex R. Rezende,† Marcos A. Bizeto,‡ Vera R. L. Constantino,§ and Fritz Huguenin*,† Departamento de Quı´mica, Faculdade de Filosofia, Cieˆncias e Letras de Ribeiraõ Preto - UniVersidade de Saõ Paulo, 14040-901 Ribeiraõ Preto (SP), Brazil, Departamento de Cieˆncias Exatas e da Terra, Campus Diadema, UniVersidade Federal de Saõ Paulo, 09972-270, Diadema (SP), Brazil, and Departamento de Quı´mica Fundamental, Instituto de Quı´mica, UniVersidade de Saõ Paulo, CP 26077, 05513-970, Saõ Paulo (SP), Brazil ReceiVed: February 3, 2009; ReVised Manuscript ReceiVed: May 11, 2009

Layer-by-layer (LbL) films from K2Nb6O172- and polyallylamine (PAH) and dip-coating films of H2K2Nb6O17 were prepared on a fluorine-doped tin-oxide (FTO)-coated glass. The atomic force microscopy (AFM) images were carried out for morphological characterization of both materials. The real surface area and the roughness factor were determined on the basis of pseudocapacitive processes involved in the electroreduction/ electrooxidation of gold layers deposited on these films. Next, lithium ion insertion into these materials was examined by means of electrochemical and spectroelectrochemical measurements. More specifically, cyclic voltammetry and current pulses under visible light beams were used to investigate mass transport and chromogenic properties. The lithium ion diffusion coefficient (DLi) within the LbL matrix is significantly higher than that within the dip-coating film, ensuring high storage capacity of lithium ions in the self-assembled electrode. Contrary to the LbL film, the potentiodynamic profile of absorbance change (∆A) as a function of time is not similar to that obtained in the case of current density for the dip-coating film. Aiming at analyzing the rate of the coloration front associated with lithium ion diffusion, a spectroelectrochemical method based on the galvanostatic intermittent titration technique (GITT) was employed so as to determine the “optical” diffusion coefficient (Dop). In the dip-coating film, the method employed here revealed that the lithium ion rate is higher in diffusion pathways formed from K2Nb6O172- sites that contribute more significantly to ∆A. Meanwhile, the presence of PAH contributed to the increased ionic mobility in diffusion pathways in the LbL film, with low contribution to the electrochromic efficiency. These results aided a better understanding of the potentiodynamic profile of the temporal change of absorbance and current density during the insertion/ deinsertion of lithium ions into the electrochromic materials. Introduction Electrochromic cells based on thin oxide films have been extensively studied over the last decades.1-3 These materials are particularly relevant for electrochromic devices because of the changes in absorption (∆A) that take place in the visible region when these oxides are electrochemically reduced/ oxidized. These processes can occur simultaneously with the insertion/deinsertion of lithium ions into the oxide matrix, to compensate the electron injection/removal into (from) the conduction band. Moreover, these processes are reversible, allowing for the use of these host matrices in displays and smart windows. So the absorbance changes and the number of inserted/ deinserted lithium ions may be electrochemically controlled through the following process:

MOy+ xLi++ xe- f LixMOy

(1)

where MOy is the transition metal oxide matrix. Among the transition metal oxides, WO3 has been the most studied as electrochromic material because it displays high coloration * Corresponding author. E-mail: [email protected]. † Departamento de Quı´mica, Universidade de Saõ Paulo. ‡ Universidade Federal de Saõ Paulo. § Departamento de Quı´mica Fundamental, Universidade de Saõ Paulo.

efficiency.4-6 However, other oxides have also been investigated, such as those based on Mo, Ir, Ti, V, Ni, and Nb, aiming at enhancing coloration efficiency and, among other aspects, absorption in other regions of the visible spectrum, and reducing the time needed for the materials to change from (to) the colored state to (from) the bleached state.7-11 Normally, the limiting-rate step of the electrochemical reaction (reaction 1) is lithium ion diffusion into the oxide matrices.12-16 In this sense, efforts have been made toward increasing the lithium ion mass transport rate, decreasing the response time of absorbance changes, and, consequently, improving the performance of the electrochromic cells. Manipulating polymers and metal oxides at the nanometer scale is a means to increase the ionic diffusion rate. In fact, an intimate contact between the components can partially shield the interactions between the lithium ions and the host matrix, increasing the lithium ion mobility.17 An approach to manufacturing electrochromic nanocomposites is to employ the layerby-layer (LbL) technique, which is based on electrostatic interactions of oppositely charged layers.18-20 One advantage of this method is the high control of the thickness and nanoarchitecture of the thin films, which allows for the formation of visually homogeneous organic/inorganic nanocomposites with new electrochemical and chromogenic properties.21,22

10.1021/jp901006u CCC: $40.75  2009 American Chemical Society Published on Web 05/29/2009

Lithium Ion Electro-Insertion

J. Phys. Chem. C, Vol. 113, No. 25, 2009 10869 of WO3,10 these materials display a high electrochromic efficiency and can color in other regions of the visible spectrum along with the lithium ion electro-insertion. Moreover, the lithium ion diffusion process into these self-assembled materials is analyzed by means of spectroelectrochemical data, aiming at evaluating the rate of the coloration front correlated with the ionic transport. Theoretical Section

Figure 1. Schematic structure of K4Nb6O17.

In the search of new materials to produce nanostructured films, some layered metal oxides have been attracting attention due to the possibility of delamination into nanosheets. The nanosheets are considered a new class of nanoparticles due to their strong morphological anisotropy characterized by a thickness of a few nanometers and lateral dimensions in the micrometer scale. Added to this fact, nanosheets are easily produced as a colloidal dispersion of charged particles, which facilitates their use in LbL techniques. Hence, in this work, we considered the use of the layered hexaniobate of K4Nb6O17 composition as a precursor for the production of heterostructured thin films. K4Nb6O17 is a transparent semiconductor consisting of [NbO6] octahedral units joined by vertices and/or edges, and potassium ions positioned in the interlayer space.23 The stacked niobate layers produce two distinct interlayer regions, usually designated by I and II, as seen in Figure 1. The regions are crystallographically distinct and also show different intercalation properties, which can be related to the fact that only interlayer region I is capable of hydration. The acidic form of hexaniobate (H2K2Nb6O17) can be exfoliated in aqueous solution using a delaminating agent such as n-butylamine, which results in a colloidal dispersion of hexaniobate single sheets.24 The plate-like niobate nanoparticles can be reassembled by layer-by-layer technique,25,26 electrostatic selfassembly deposition (ESD),27,28 and Langmuir-Blodgett techniques forming multilayer films.29 Thin films of niobate can be easily prepared and investigated, with possible use as lithium ion insertion electrode for electrochromic devices. So visually homogeneous H2K2Nb6O17 films can be obtained by the dipcoating method, and LbL films of this material can also be prepared with polications due to its negatively charged layers. In fact, LbL films based on K2Nb6O172- and poly(allylamine hydrochloride) (PAH) were grown due to the electrostatic interactions between them, allowing for the formation of heterostructures with high control of thickness.30 In this work, electrochemical and chromogenic properties of dip-coating and LbL films from K2Nb6O172- are investigated, because this material can be an alternative to electrochromic devices. Although the materials formed from this oxide have an electrochromic efficiency lower than that obtained in the case

Several techniques have been used to determine the diffusion coefficient of lithium ions inserted into electrochromic films, for instance, the galvanostatic intermittent titration technique (GITT),31 potentiostatic intermittent titration technique (PITT),32 and electrochemical impedance spectroscopy.33 In some cases, the relation between mass transport and chromogenic properties has also been discussed, once the coloration wave accompanies the ionic diffusion into the insertion matrices.34-38 So the lithium ion diffusion coefficient can also be determined by absorbance changes, allowing for investigation of the rate of the coloration front and also providing a practical method for testing the ionic diffusion rate. Considering that the coloration wave accompanies the lithium ion diffusion during the ionic insertion/deinsertion process into the host matrices,34 spectroelectrochemical methods can be used to calculate the “optical diffusion coefficient” (Dop). To determine the time (t) dependence of the lithium ion concentration (CLi) at the film/electrolytic solution interface (x ) 0), Fick’s second law is used:

∂[CLi(x, t)] ∂2[CLi(x, t)] ) Dop ∂t ∂x2

(2)

Using the same initial and boundary conditions shown in ref 30, and for t , L2/Dop (where L is the thickness of the film), the solution to eq 2 is

CLi(0, t) ) Co +

2i√t SF√Dopπ

(3)

where S is the geometric area, i is the applied current, and F is the Faraday constant. By reordering eq 3 above, we have that:

dCLi(0, t) d√t

)

2i SF√Dopπ

(4)

By expanding eq 4 by dA, which is equal to d(∆A), one obtains:

dCLi dA 2i ) dA d√t SF√Dopπ

(5)

where A is the absorbance and ∆A is the absorbance change, which are a function of the amount of lithium ion in the host matrix. In the equilibrium state, the change in the lithium ion concentration can be substituted for the injected charge (q):39

dCLi )

dq SFL

(6)

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Substituting dCLi from eq 6 into eq 5, and dt0.5 by dt, eq 7 is obtained:

Dop

[ ]

dA iL dq ) √tπ dA dt

( ) ( )

2

(7)

where the dA/dq term can be determined from the slope of the steady-state (equilibrium state) curves. Meanwhile, the dA/dt term can be determined from the slope of current pulses. To determine Dop by this method, it is necessary that the molar absorptivity is a function of the lithium ion concentration in the host matrix (ensuring different absorbance changes between the current pulses and the steady state), allowing for investigation of the lithium ion transport (due to the concentration gradient) by means of spectroelectrochemical data. According to the model shown above, the optical properties can be directly associated with the lithium ion diffusion. Expanding eq 4 by potential change and using eq 6, an equation similar to that developed by Weppner and Huggins is obtained:31

DLi

[ ( )]

dE ( dq ) 4 ) iL π

2

dE d√t

(8)

where the dE/dq term is the slope of the potential as a function of the charge at the steady state, and the dE/dt0.5 term is the slope of the potential as a function of the time square root during current pulses. Hence, the chemical diffusion coefficient of lithium ions (DLi) can be determined, independent of the optical properties. The lithium ion self-diffusion coefficient (Dki) can be determined according to eq 9, once the electronic transport number tends to unit value (tef1):

Dk )

| |

DLi RT d ln q te F dE

(9)

Experimental Section a. Hexaniobate Synthesis and Exfoliation. K4Nb6O17 was prepared by heating a stoichiometric mixture of Nb2O5 (CBMM - Companhia Brasileira de Metallurgia e Mineraçaõ) and K2CO3 (Merck) in a platinum crucible at 1100 °C in two 5 h steps, with one grinding process between them.40 Conversion to its acid phase H2K2Nb6O17 was carried out by suspending K4Nb6O17 in a 6 mol L-1 HNO3 solution for 3 days, at 60 °C. After this period, the acid solution was substituted for a new one and allowed to react for another 3 days. The solid was centrifuged, washed with deionized water, and dried in a dessicator with silica gel under vacuum. Later, H2K2Nb6O17 was exfoliated with n-butylamine by addition of 0.5 g of niobate to 250 mL of n-butylamine aqueous solution containing an amine/ H+-niobate molar ratio of 0.5.24 The pH of the resulting dispersion was 7.5 and should be maintained above 7.0 to avoid nanosheet curling.41 b. Preparation of 10-Bilayer LbL Films from K2Nb6O172-/ PAH and Dip-Coating from H2K2Nb6O17. The commercial PAH was purchased from Aldrich. LbL films were assembled onto a fluorine-doped tin-oxide (FTO)-coated glass purchased

Figure 2. AFM height images of 10-bilayer LbL films from K2Nb6O172-/ PAH (a) without and (c) with gold deposition. Inset in (a) shows AFM height image of the dip-coating film from H2K2Nb6O17. The z-scale is shown in the figures.

from Flexitec (Curitiba, Brazil). The glass had a sheet resistance e20 Ω and a geometrical area of 1 cm2. The layers were obtained via ionic attraction of oppositely charged materials, by alternated immersion of the FTO substrate into the PAH (1.6 g L-1) and niobate dispersions for 3 min. KOH solution was added to the PAH dispersion, so that the pH ) 7.5 would be maintained. After each layer deposition, the substrates were rinsed in KOH solution (pH ) 7.5) for 30 s, and dried under nitrogen flow at room temperature. The dip-coating method was used for preparation of films from H2K2Nb6O17, with an immersion and submersion rate of 20 mm/min. The FTO substrate was kept immersed in the H2K2Nb6O17 dispersion for 1 min, and the drying time was 2 min. This process was repeated 10 times. c. Characterization Measurements. X-ray diffraction patterns of LbL and dip-coating films were recorded on a Siemens D5005 diffractometer using monochromatic Cu KR radiation. The diffractogram displayed an amorphous structure for these films. Film thickness was analyzed by specular reflectance using the Nanocalc 2000 program coupled with a single channel 2048 pixel CCD spectrophotometer with halogen lamp as light source. The thickness values measured for the 10-bilayer H2K2Nb6O17/ PAH LbL and dip-coating films were 100 ( 4 and 45 ( 4 nm, respectively. The geometrical area of both films was 1 cm2. Atomic force microscopy (AFM) height images were obtained on a digital Shimadzu microscope. An electrochemical method was employed for the determination of the real surface area of the LbL and dip-coating films. First, the real area of a gold electrode was determined in a solution of 1 mol L-1 H2SO4, according to the method adopted by Rand and Woods.42 A potential of 1.6 V vs reversible hydrogen electrode (RHE) was applied for 10 s. Next, N2 was bubbled for 90 s at 1.46 V, to eliminate O2. A linear scan from 1.46 to 0.68 V was


Figure 3. Linear scan for (0) gold plate, (O) LbL, and (b) dip-coating with gold deposition in aqueous solution at pH ) 7. V ) 20 mV s-1.

performed at 20 mV s-1, to determine the charge (1.56 mC) associated with the electroreduction of oxygen-like species adsorbed onto the gold surface. Considering that the charge density of this gold electrode is 386 µC cm-2 in this electrolytic solution, we determined its real area (5.94 cm2 mC). Next, the electrode was washed and dipped into a solution of 0.5 mol L-1 KOH, with the pH adjusted to 7.5 by addition of H2SO4. We used the same method to determine the charge density (277 µC cm-2) in this basic medium. This allowed for determination of the real surface area of the LbL and dip-coating films, which were bathed with gold on a BALTEC Mod. SCD 050 equipment. Note that the pH change from 0 to 7.5 was carried out so as to equal the pH value to that used in the preparation of the LbL film. This avoids changes to the morphology of the LbL film, once niobate is dependent on the medium pH.43 For the spectroelectrochemical experiments, a platinum sheet with an area of 10 cm2 was used as the counter electrode, and Ag/Ag+ in propylene carbonate (PC) in LiClO4/PC 0.5 mol L-1 was used as the quasi-reference electrode. A LiClO4/PC electrolytic solution (0.5 mol L-1) was used in all of the electrochemical experiments, which were carried out using an Autolab PGSTAT30 potentiostat/galvanostat. Chromogenic analysis was carried out concomitant with the electrochemical experiments by means of a microprocessor-controlled solid-state light source (WPI, Inc.). Plastic fiber optical cables up to 1 mm in diameter were used to deliver light from the instrument to a PDA1 photodiode amplifier (WPI, Inc.). For the transmission experiments, the films were placed in a cell made of optical glass, where light beams at fixed wavelengths were transmitted across the film during the electrochemical experiments. Transport number of electrons (te) and ions (ti) was determined for LbL and dip-coating films. A potential step was applied in a symmetrical cell (film deposited between two gold electrodes with previously inserted lithium ion) for 5 s. On the basis of the current change as a function of time, the determined te and ti values were close to 1 and 0, respectively, with several amounts of lithium ion previously inserted into both films.44 Spectroelectrochemical measurements were used to calculate the lithium ion diffusion coefficient into the LbL films, according to the method shown above, which is based on GITT. The experiments were performed in the following way: the curve of the absorbance change as a function of the injected charge under light beams at 470 nm was determined, which is similar to steady state; afterward, starting from the open circuit potential, a 15 µA cm-2 current density was applied for 10 s. On the basis of the d(∆A)/dt slope of this pulse and on the values of dA/dq

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Figure 4. In situ UV-vis spectra for casting H2K2Nb6O17 at 1.0, -1.4, -1.5, -1.55, and -1.8 V vs Ag/Ag+. The arrow points to red shifting of the absorbance band maximum.

similar to that at the steady state, the DLi value could be determined from eq 9. This procedure was repeated at several degrees of charging for the LbL and dip-coating films. Results and Discussion The access of lithium ions to the insertion sites can be facilitated in the case of host matrices with high surface area. So morphology was evaluated, and surface roughness of the insertion electrodes was estimated by means of atomic force micrographs. Figure 2 shows the AFM height images for (a) LbL and (b) dip-coating films. These images reveal a large amount of aggregates and roughly spherical colloidal particles. The aggregate size in the LbL film is larger than that in the case of the dip-coating film, probably due to the electrostatic interactions between K2Nb6O172- and PAH, which contributes to destabilizing the K2Nb6O172- colloidal particles. The smoothing surface roughness (Ra) is close to 22 and 19 nm for the LbL and dip-coating films, respectively. As explained in the Experimental Section, gold particles were deposited on these materials, and electrochemical methods were employed for estimation of the real surface areas. So it is important to verify whether the gold monolayers reproduce the LbL film morphology, thus allowing for a reliable estimation of these areas. Figure 2c shows the LbL film with gold bath (Au/LbL), whose morphology and roughness (Ra ) 23 nm) are similar to those obtained in the case of the LbL film without gold deposition. Figure 3 shows linear scan measurements for the Au/LbL and Au/dip-coating films in aqueous solution, at pH ) 7.5 and 20 mV s-1, after the pretreatment mentioned above in the Experimental Section. The electroreduction charge obtained in these linear scans was 866 and 771 µC for the Au/LbL and Au/dip-coating films, respectively. On the basis of these values and the electroreduction charge density for Au electrodes in the same conditions (277 µC cm-2), the real surface areas of the LbL and dip-coating films were determined as 3.1 and 2.8 cm2, respectively. These values are identical to the respective roughness factor, once the geometrical area of both electrodes is 1 cm2. The ratio between these roughness factors is close to that obtained in the case of Ra, suggesting that the morphology observed in the atomic force micrographs is very representative for both films. These data are important because they show that the real area of the films is closed. Thus, when the LbL and dip-coating films were used as electrodes for lithium ion insertion/deinsertion, as will be shown below, the difference in the electrochemical and spectroelectrochemical properties (in-

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Figure 5. Potentiodynamic profile of (a) j (O) and dA/dt (-), and (b) ∆A at 470 nm for the LbL film from K2Nb6O172-/PAH; potentiodynamic profile of (c) j (O) and dA/dt (-), and (d) ∆A at 470 nm for the dip-coating film from H2K2Nb6O17.

vestigated in this work) between these films will not be influenced by the surface area. On the other hand, this difference will depend on the properties associated with the interior of the matrices, such as the diffusional rate of lithium ions. Figure 4 shows the absorption spectra of casting film from H2K2Nb6O17 in 1.0 mol L-1 LiClO4/PC at 1.0, -1.4, -1.5, -1.55, and -1.8 V vs Ag/Ag+. Hexaniobate-derived materials display absorption in the UV region (absorption band edge around 300 nm), assigned to a charge-transfer transition from the 2p orbitals of oxygen to the 4d orbitals of niobium.45,46 Hence, no absorption is observed between 350 and 600 nm at 1.0 V. The electronic absorption noticed in the visible region at negative potentials (-1.5 to -1.8 V) is due to the reduction of Nb(V) to Nb (IV). Moreover, a red shift from 375 nm (at -1.4 V) to 405 nm (at -1.8 V) for the absorbance band maximum is observed. This result indicates that lithium ion insertion into the niobate matrix promotes a change in the chemical environment, which can promote changes in absorptivity, as will be discussed below. Figure 5a shows the potentiodynamic profile of the current density (j) for 10-bilayer LBL films in 1.0 mol L-1 LiClO4/PC at 50 mV s-1, which depicts the lithium ion insertion process during the negative potential scan and the lithium ion deinsertion process during the positive potential scan. These voltammograms were taken simultaneously with in situ measurements of absorbance changes (∆A) at 470 nm, shown in Figure 5b. Figure 5a also shows the dA/dt derivative as a function of the potential. The potentiodynamic profile of dA/dt is similar to that of the current density, once the absorbance change is associated with charge injection/removal in the host matrix. Figure 5c and d shows the potentiodynamic profile of the current density and the absorbance change at 470 nm for the dip-coating film, in 1.0 mol L-1 LiClO4/PC at 50 mV s-1, respectively. Note that the j and ∆A values are smaller than those in the case of the LbL film. This is associated with the difference in the thickness of the films and, consequently, the

amount of electroactive sites. However, the current density and inserted charge values normalized by volume are also higher for the LbL film. The oxidation current peak and charge normalized by the volume were 12.5 A cm-3 and 106.0 C cm-3 for the LbL film, respectively, and 5.14 A cm-3 and 56.8 C cm-3 for the dip-coating film, respectively. As will be shown below, these differences are associated with the lithium ion diffusion rate into the LbL and dip-coating films. Figure 5c shows the dA/dt derivative as a function of the potential for the dip-coating film. Contrary to the LbL film, the potentiodynamic profile of dA/dt is not similar to that obtained in the case of current density (j ) dq/dt). More significant dA/ dt changes are observed at less (more) negative potentials in comparison with the current density during the negative (positive) potential scan. Moreover, the dA/dt curve reaches a maximum at ca. -1.65 V during the negative potential scan and a minimum at ca. -1.39 V during the positive potential scan. A reduction current peak is not observed, and the oxidation current peak is at ca. -1.28 V. We suggest that there are different pathways in the host matrix formed from K2Nb6O172sites, where the lithium ion diffusion rate and optical properties are distinct. So, for the case of the dip-coating film, the diffusion rate must be the highest for those lithium ions (and electrons) that travel on pathways formed from K2Nb6O172- sites with high contribution to absorbance changes due to the local structure. Figure 6a displays the cyclic voltammograms, and Figure 6b shows the absorbance changes at 470 nm for 10-bilayer LbL films at several scan rates (V). The voltammetric profiles depend on the scan rate, with slight shifts in the reduction current peak potential and in the potential corresponding to maximum ∆A. Despite the small thickness of the LbL film, these shifts are associated with the slow lithium ion diffusion into the material. The increase in absorbance change obtained at lower scan rates (longer time) is associated with the amount of Nb(IV) and inserted lithium ion, compensating the charge injected into the self-assembled matrix. In addition, the ∆A data show that the


Figure 6. Potentiodynamic profile of (a) j and (b) ∆A for the 10bilayer LbL films from K2Nb6O172-/PAH at (yellow) 10 mV s-1, (wine) 20 mV s-1, (olive) 30 mV s-1, (black) 40 mV s-1, (red) 50 mV s-1, (cyan) 60 mV s-1, (blue) 70 mV s-1, (orange) 80 mV s-1, (purple) 90 mV s-1, and (green) 100 mV s-1. (c) Absorbance changes as a function of the injected charge at (wine) 20 mV s-1, (olive) 30 mV s-1, (black) 40 mV s-1, (red) 50 mV s-1, and (blue) 70 mV s-1. The arrow points to increasing scan rates from 10 to 100 mV s-1. (d) Plot of the values of (b) q/qV)20 mV/s and (O) ∆A/∆AV)20 mV/s ratios, and (-) fitting data.

inserted lithium ions are deinserted in the reverse process. The negligible difference in ∆A before and after the cycle indicates high chemical reversibility. Figure 6c displays the absorbance changes as a function of the charge at 470 nm for the LbL film at several scan rates. It

J. Phys. Chem. C, Vol. 113, No. 25, 2009 10873 is noteworthy that the slope of these curves slightly increases with the scan rate. This is attributed to the changes in the electronic transitions of the Nb(IV) sites, probably due to chemical environment changes associated with the increase in the lithium ion concentration close to the film/electrolytic solution interface. This accumulation is attributed to low ionic mobility as compared to the electronic mobility. Under the formed concentration gradient, lithium ions and electrons are simultaneously transported into the insertion matrix, to maintain electroneutrality. On the basis of this, the absorbance change as a function of the time (according to eq 7) can be employed to investigate the rate of the coloration wave, which accompanies the lithium ion diffusion in electrochromic pathways (sites) into the insertion matrices. Figure 6d shows how q/qV)20 mV/s (q for different scan rates normalized with respect to q at 20 mV s-1) and ∆A/∆AV)20 mV/s (∆A for different scan rates normalized with respect to ∆A at 20 mV s-1) vary with the scan rate (V). Note that the profiles of both curves are similar, once absorbance change is proportional to the amount of inserted/deinserted charge into the film (which is associated with the amount of lithium ion insertion/deinsertion). Moreover, these data indicate that the lithium ions reached inner sites of the LbL film, and not only the outer sites. This graphic also shows the slight enhancement ∆A/∆AV)20 mV/s as compared to q/qV)20 mV/s for high scan rates, which is attributed to the small increase in absorptivity. The absorbance change and the injected charge decreased from 75.8 × 10-3 and 1.17 mC cm-2 at 20 mV s-1 to 62.1 × 10-3 and 0.82 mC cm-2 at 90 mV s-1, respectively. The q/qV)20 mV/s ratio as a function of the scan rate was fitted with an empirical exponential function 0.69273 + 0.4072 exp(-V/75.59002). Extrapolating the curve for an infinite scan rate, the value of q/qV)20 mV/s is 0.69. The charge associated with the outer sites, which was ca. 0.81 mC, can be estimated as the value of q/qV)20 mV/s is multiplied by qV)20 mV/s. In fact, as the electronic mobility is much higher than in the case of lithium ions, these ions access the outer sites at high scan rate. Fitting the ∆A/∆AV)20 mV/s ratio versus scan rate with an empirical exponential function 0.74328 + 0.36408 exp(-V/57.93364), the value of ∆A () 56.3 × 10-3) can also be determined as the scan rate tends to the infinite value. So the electrochromic efficiency (η ) ∆A/q) and the molar absorptivity (ε ) Fη) are increased from 64.79 cm2 C-1 and 6.54 × 106 cm2 mol-1 at 20 mV s-1 to 69.5 cm2 C-1 and 6.70 × 106 cm2 mol-1 at scan rates tending to the infinite. Because the molar absorptivity changes as a function of the scan rate, indicating its increase due to the accumulation of lithium ions close to the film/electrolytic solution interface, we used the model shown in the theoretical section to determine the Dop values, in addition to the determination of DLi values by GITT. Several current pulses of 15 µA cm-2 were applied, and the potential and absorbance changes at 470 nm were registered. Figure 7a shows the potential change as a function of the time square root during each current pulse at several amounts of lithium ions previously inserted into the LbL film. A linear variation in these curves for times higher than 4 s occurs, which is associated with the semi-infinite lithium ion diffusion into the host matrix only. This information is important because eqs 7 and 8 involve the ionic diffusion process only, and they do not consider the effects from the double-layer charging current and from the electrolyte, bulk material, and charge transfer resistances.29,36 The dE/dt0.5 slope at several amounts of lithium ions previously inserted into the LbL film is determined and used to calculate the DLi values, according

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Figure 7. (a) Potential changes as a function of the time square root during a constant current pulse of 15 µA cm-2 for 10-bilayer LbL films from K2Nb6O172-/PAH. (b) Potential as a function of the inserted charge for the LbL film. (c) Plots of absorbance changes as a function of time during a constant current pulse of 15 µA cm-2 for the LbL film. (d) Potential changes as a function of the time square root during a constant current pulse of 15 µA cm-2 for the dip-coating film from H2K2Nb6O17. (e) Potential as a function of the inserted charge for the dip-coating film. (f) Plots of absorbance changes as a function of time during a constant current pulse of 15 µA cm-2 for dip-coating film. The sequence of current pulses, from the first to the seventh, is represented by the following order of symbols: (∆), (6), (b), (O), (0), (9), and (1).

to eq 8. In this case, it is still necessary to determine the dE/dq slope, which is obtained from the steady-state (equilibrium) conditions for potential as a function of the charge (Figure 7b). Figure 7c shows the absorbance changes as a function of the time during the current pulses. Note that the d∆A/dt slope increases as a function of the amount of lithium ions previously inserted into the LbL film, indicating the increase in absorptivity due to changes in the chemical environment. The time range used for determining Dop in eq 7 corresponds to the same range used in Figure 7a. This guarantees that the semi-infinite lithium ion diffusion is involved in the absorbance changes, in agreement with the model shown above. The d∆A/dq slope, also included in eq 7 to calculate the Dop values, was obtained from the absorbance changes as a function of charge during the cyclic voltammetry at 30 mV s-1, which is similar to that in the steady

state. Figure 7d-f shows the potential change as a function of the time square root during the several current pulses, the potential change as a function of the charge, and the absorbance changes as a function of the time during the current pulses for the dip-coating film, respectively. Figure 8 shows the DLi values as a function of the injected charge for the LbL and dip-coating films. Note that the DLi values for both films decrease with increasing injected charge because of the interactions between the lithium ions. This behavior has already been observed for lithium ion insertion electrodes.36 In the case of the LbL film, these values are close to or higher than those of WO3, which depends on the preparation method and electrochemical technique used for determination of DLi.10 Note also that the DLi values for the LbL film are higher than those obtained in the case of the dip-


J. Phys. Chem. C, Vol. 113, No. 25, 2009 10875 Based on these data, the model developed in this work allowed for a better understanding of the coloration wave rate associated with the lithium ion mass transport in the films based on hexaniobate. However, concerning future applications, other experiments are necessary for a better evaluation of the use of these materials in electrochromic devices. For example, thicker films containing a higher amount of K2Nb6O172- sites and inserted/deinserted lithium ions must be investigated, aiming at analyzing the electrochemical and spectroelectrochemical properties under the contrast ratio desired for these applications.10 As for the electrochromic efficiency (at a wavelength of maximum absorbance change) and charge capacity, the values observed here are lower than those obtained in the case of WO3, although they are high as compared to other Nb-based oxides.8-11 Thus, novel synthetic routes, film preparation, thermal treatment, and other polymeric components can also be an alternative to improving the properties of electrochromic materials based on hexaniobate. Conclusions

Figure 8. Plot of (a) DLi, (b) Dki, and Dop as a function of the injected charge for (b) the 10-bilayer LbL films from K2Nb6O172-/PAH and (O) the dip-coating film from H2K2Nb6O17.

coating film. Probably, the presence of PAH is responsible for the increase in lithium ion mobility. The intimate contact between the PAH and niobate can partially shield the interactions between the lithium ions and the oxygen atoms, thus increasing the ionic mobility.17 The Dki values were calculated according to eq 9 for several amounts of previously inserted lithium ions (Figure 8b), and they are ca. 10 times lower than the DLi ones. These results confirm the enhancement in the transport rate of lithium ion in the self-assembled structure of K2Nb6O172-/PAH. Contrary to the DLi values, the Dop values are slightly higher for the dip-coating film than for the LbL film (Figure 8c). This is in agreement with the discussion above on the potentiodynamic profile of dA/dt and dq/dt in Figure 5. For the dip-coating film, we suggest that the lithium ions travel faster in pathways with high contribution to electrochromic efficiency than in other diffusion pathways, in contrast to what was observed in the case of the LbL film. Although PAH does not delay the lithium ion and electron transport in pathways with high contribution for the electrochromic efficiency significantly, the presence of this polymer in the self-assembled structure guarantees a considerable enhancement in the lithium ion mobility, involving diffusion pathways where the electrons jump between K2Nb6O172- sites with small contribution to absorbance changes.

Dip-coating films of H2K2Nb6O17 and novel self-assembled films from K2Nb6O172-/PAH were prepared, aiming at evaluating their electrochemical and spectroelectrochemical properties for application as lithium ion insertion electrode in electrochromic devices. On the basis of the electroreduction of oxygen-like species adsorbed onto gold surface deposited on the films in acid medium, we determined the roughness factors for the dipcoating and LbL films. These values revealed that the real surface area is not responsible for the difference in lithium ion storage capacity between the thin films in nonaqueous medium. DLi and Dop values as a function of the amount of lithium ion previously inserted into the electrodes were obtained by GITT and the model developed in this work. Based on these values, there was an increase in the ionic diffusion rate due to the presence of PAH in the self-assembled structure, with the consequent increase in the charge storage capacity of the LbL film. However, in contrast to DLi values, the Dop values are slightly higher for the dip-coating film as compared to those obtained in the case of the LbL film. This corroborates with the spectroelectrochemical studies for both films. The DLi and Dop values and the potentiodynamic profile of dA/dt and dq/dt suggest that the lithium ion rate in diffusion pathways, which contributes more significantly to electrochromic efficiency, is faster for the dip-coating film. The model developed here for analysis of the coloration wave rate allowed for a better understanding of the absorbance changes during lithium ion insertion, which is an important issue in electrochromic studies, and can aid the choice of materials for electrochromic devices. Acknowledgment. We are grateful to FAPESP (05/00106-7 and 2005/60596-8), CNPq (555436/2006-3, 550581/2005-7 and 301149/2006-2), IMMP/MCT, and IMMC/MCT for financial support. We are also grateful to Associate Professor Jose´ Maurıćio Rosolen (DQ/FFCLRP/USP) and Assistant Professor Roge´ria Rocha Gonçalves for the availability of their laboratories. References and Notes (1) Niklasson, G. A.; Granqvist, C. G. J. Mater. Chem. 2007, 17, 127. (2) Argazzi, R.; Iha, N. Y. M.; Zabri, H.; Odobel, F.; Bignozzi, C. A. Coord. Chem. ReV. 2004, 248, 1299. (3) Bueno, P. R.; Gabrielli, C.; Perrot, H. Electrochim. Acta 2008, 53, 5533. (4) White, C. M.; Gillaspie, D. T.; Whitney, E.; Lee, S.-H.; Dillon, A. C. Thin Solid Films 2009, 517, 3596. (5) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2007, 91, 1529. (6) Deb, S. K. Appl. Opt. Suppl. 1969, 3, 192.

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