J. Phys. Chem. C 2007, 111, 14911-14916
14911
Lithium Ion Diffusion into Self-Assembled Films Composed from WO3 and Polyallylamine Nelson A. Galiote and Fritz Huguenin* Departamento de Quı´mica, Faculdade de Filosofia, Cieˆ ncias e Letras de Ribeira˜ o Preto, UniVersidade de Sa˜ o Paulo, 14040-901 Ribeira˜ o Preto (SP), Brazil ReceiVed: May 9, 2007; In Final Form: July 7, 2007
Self-assembled films from WO3 and polyallylamine (PAH) were prepared by the Layer-by-Layer (LbL) method. The electroinsertion of lithium ions into these LbL films treated at room temperature and at 150 °C was examined. More specifically, the electrochemical and chromogenic properties were investigated by cyclic voltammetry, chronopotentiometry, and current pulses under visible light beams. We employed the quadratic logistic equation (QLE) to estimate the amount of electroactive sites and electrochromic efficiency (η) by fitting the absorbance changes (∆A) as a function of the injected charge (q). The η values decreased with increasing cyclic voltammetry scan rate (from 1 mV s-1 to 100 mV s-1). Our results were discussed on the basis of feedback effects for absorbance changes, which are associated with the W5+ f W6+ intervalence transfer flux. A model based on QLE and spectroelectrochemical data was developed to determine the lithium ion diffusion coefficient (DLi) within the LbL matrix. The DLi values decreased as a function of the amount of inserted lithium ions, which contributed to increasing the interaction forces and feedback effects. The method employed here revealed that the diffusion coefficients of the LbL film treated at 150 °C were higher and lower than those obtained for the LbL film that was not thermally treated, for a small and a large amount of inserted lithium ions, respectively.
Introduction The compound WO3 has been extensively investigated for a number of applications, including photoelectrochemistry,1,2 photocatalysis,3 photochromism,4 electrocatalysis,5 energy storage,6 sensors,7 and gaschromism.8 Other applications exploit its electrochromic properties,9-15 because tungsten ions from WO3 exhibit different oxidation states, and the intervalence electron transfer from the W(V) to the W(VI) state leads to a broad absorption in the red region of the visible spectrum. In order words, WO3 undergoes a color change from white to blue when tungsten ions are reduced, a process that is affected by the amount of ions inserted into the oxide matrix, which can be represented by the following process,
WO3 + xM+ + xe- f MxWO3
(1)
where M+ can be H+, Li+, K+, etc. For each electron injected (removed) into (from) the conduction band, a cationic species is inserted (removed) into (from) the structure, to compensate for the charge. The Coulombic reversibility and the coloration efficiency observed for this material are successfully used in display devices and smart windows.9-15 It is therefore important to analyze the dependence of the WO3 optical properties on the ionic insertion/deinsertion process. Models such as the site-saturation model and the quadratic logistic equation (QLE) have been used to predict the change in absorbance (∆A) and the electrochromic efficiency (η, absorbance change per unit charge) as a function of the injected charge (q).16,17 The QLE has been used to assess temporal changes in the properties of electrochemical systems,18-19 such as the feedback effect associated with charge transport. In fuel * To whom correspondence should be addressed. E-mail: fritz@ ffclrp.usp.br.
cells, the QLE has been employed to interpret limitations in charge transport,19 which has been shown to be mainly because of the transport of protons in the ionic exchange membrane in a modeling study.20 The concepts used in that work have been extended to interpret experimental results in intercalation processes, as in the case of the intercalation of Li ions in V2O5,17 WO3,21,22 and TiO2 films.23 In this paper, the QLE is employed to describe the evolution of electron transfer in WO3 films, which consist of donor-bridge-acceptor (DBA) systems. Oxygen atoms placed between the tungsten ions function as bridges, whereas the W5+ and W6+ sites are donors and acceptors, respectively. In this work, therefore, the QLE is used to characterize the optical and electrochemical properties of novel composites from WO3 and polyallylamine (PAH) obtained by the layer-by-layer (LbL) technique. Lithium ion diffusion is discussed on the basis of spectroelectrochemical data, and it is associated with interaction forces and feedback effects described by the QLE. From this analysis, one can predict the diffusion coefficient, the number of electroactive sites, and the electrochromic efficiency, all of which may be important factors for the design of LbL structures with improved properties, particularly because of the possibility of LbL film manipulation at the molecular level. Indeed, recent works have indicated that LbL films are promising materials for microbatteries and electrochromic devices because of their enhanced charge storage capability and uniformity.21-24 PAH was chosen to build the LbL films because of its ability to interact with oxides in solutions,25 which allows uniform film growth as a function of the number of bilayers. Considering that only a small amount of this polymer is deposited in the LbL method, PAH does not affect the electrochromic properties of the transition metal oxide. Moreover, the thermal stability of PAH is high.26
10.1021/jp073546h CCC: $37.00 © 2007 American Chemical Society Published on Web 09/19/2007
14912 J. Phys. Chem. C, Vol. 111, No. 40, 2007
Galiote and Huguenin
Theoretical Section The QLE has been used to describe population growth in biological systems and to model physicochemical processes.27 The differential form of the QLE is shown in eq 2,
y dy ) sy 1 dt P
(
)
(2)
where y is the population at a time t, dy/dt is the rate of the change of the population, and P and s are positive constants. For low y values, dy/dt = sy, so s represents the maximum rate of change of the population, and P is related to the ability of the system to sustain the population growth. The mathematical expression that relates the absorbance changes with the injected charge is a direct application of the QLE in its difference form (eq 3),
(
∆A ) rq 1 -
q K
)
(3)
where the s constant is substituted by another positive constant r, which can be now interpreted as the electrochromic efficiency (η ) ∆A/q) for q ≈ 0. For these DBA electron-transfer systems, features such as the bridge chemical structure, the DBA energy gap, and the donor-acceptor distance determine the electrontransfer rate.28 Parameters related to these features are included in the r constant of eq 3. The P constant is also substituted by another positive constant K, which now corresponds to the ability of the system to sustain the increase in absorbance, representing the maximum q value. The term (1 - q/K) represents the feedback effects, which increase with q. Comparing eq 3 with eq 2, it is also implied that the injected charge is the population and that ∆A is related to dq/dt. In fact, during the charge injection (lithium ions and electrons) into the WO3/ PAH LbL films, the magnitude of the absorbance change in the visible range depends on the frequency of the charge (electron) transfer between the W(V) and W(VI) sites, in addition to the dependence on the amount of injected charge. The electrons that participate in this charge-transfer process are those injected electrochemically. As the injected charge increases, so does the number of transferred electrons from the W(V) to the W(VI) sites and the number of intercalated ions, which are transported simultaneously to maintain the electroneutrality. This increase leads to interactions between the lithium ions, delaying the ionic transport and, consequently, decreasing the frequency of the intervalence charge transfer. That is to say, the interaction effect increases with increasing numbers of charge carriers, which contributes to the feedback effect for the absorbance changes and to decreasing the electrochromic efficiency. The feedback effect is also caused by the decrease in the number of sites available for the intervalence transfer, as described in the site-saturation model.16 Considering that the absorbance changes depend on the slow lithium ion diffusion, as mentioned above, we can use spectroelectrochemical methods and the QLE to calculate the lithium ion diffusion coefficient (DLi). To determine the time dependence of the lithium ion concentration (CLi) at the film/ electrolytic solution interface (x ) 0), Fick’s second law was used (eq 4).
∂2[CLi(x, t)] ∂[CLi(x, t)] ) DLi ∂t ∂x2
(4)
The CLi(x, t) values are obtained at the initial and boundary conditions shown below (eqs 5-7),
CLi(x, 0) ) Co(0 e x e L)
( ) ( )
-DLi
∂CLi ∂x
∂CLi ∂x
)
x)0
x)L
(5)
i (t g 0) SF
(6)
) 0 (t g 0)
(7)
where L is the thickness of the film, S is the geometric area, i is the applied current, and F is Faraday’s constant. Equation 5 indicates that CLi is constant through the film before lithium ion insertion (for t ) 0), equal to Co. Equation 6 indicates that the CLi gradient at the film/electrolytic solution interface (x ) 0) follows Fick’s first law as lithium ions are inserted into film (for t g 0). Equation 7 indicates that the interface between the film and the substrate material (x ) L) is impermeable to lithium ions. For t , L2/DLi, the solution to eq 4 is29 eq 8.
CLi(0, t) ) Co +
2ixt
(8)
SFxDLiπ
Reordering and substituting ∆CLi ) CLi(0, t) - Co, we obtain eq 9.
ixt )
SFxDLiπ∆CLi 2
(9)
Substituting i for (dq/dt) and dt for 2t0.5 dt0.5 in eq 9, eq 10 is obtained.
dq ) SFxDLiπ∆CLi dxt
(10)
Expanding eq 10 by d(∆A), we obtain eq 11.
dq d(∆A) ) SFxDLiπ∆CLi d(∆A) d(xt)
(11)
So, the diffusion coefficient can be obtained according to eq 12.
DLi )
[
]
( ) d(∆A)
1
SFxπ∆CLi
d(xt) d(∆A) dq
( )
2
(12)
The d(∆A)/dt0.5 and d(∆A)/dq terms can be determined from the slope of the current pulses and the chronopotentiometric curves, respectively. The change in the lithium ion concentration can be substituted for the injected charge for each current pulse (∆q).30
∆CLi )
∆q SFL
(13)
[ ]
Substituting ∆CLi from eq 13 into eq 12, we obtain eq 14.
( ) d(∆A)
DLi )
d(xt) xπ∆q d(∆A) dq L
( )
2
(14)
Lithium Ion Diffusion into Self-Assembled Films
J. Phys. Chem. C, Vol. 111, No. 40, 2007 14913
Employing the QLE, the d(∆A)/dq derivative corresponds to eq 15,
d(∆A) 2q )r1dq K
(
[
)
(15)
which is substituted into eq 16.
DLi )
( ) d(∆A)
d(xt) xπ∆qr 1 - 2q K L
(
)
]
2
(16)
Now, the lithium ion diffusion coefficient is associated with the (1 - 2q/K) term, which associates feedback effects with the DLi values due to repulsive forces between Li+sLi+ and to interactions between Li+ and the network. Moreover, the QLE is useful in the study of the LbL films because it gives an estimate of the amount of electroactive material (K/F) in the investigated sample.17,21According to the model shown above, we can analyze the influence of the diffusion process (based on the DLi values) and of the amount of electroactive sites (based on the K values) on the charge capacity for different films separately. Another advantage of using spectroelectrochemical methods over other methods described in the literature is that we can investigate lithium ion diffusion in different components of hybrid materials. However, it is necessary that each component absorbs at different regions of the electromagnetic spectrum. Experimental The WO3 xerogel was synthesized by the sol-gel method.31,32 Briefly, an aqueous solution of sodium tungstate (Na2WO4, 0.2 M) was added to a proton exchange resin at room temperature, leading to a yellowish liquid containing tungstic acid (H2WO4). Condensation occurred via oxolation, and colloids were formed during the first condensation steps. This dispersion became turbid as time evolved, and it later became a gel. A precipitate was formed after a few hours. Then, N-butylamine (Riedel-de Ha¨en; 1 × 10-3 mol) was added to the mixture containing precipitated WO3, and the resulting mixture was stirred for two weeks. The pH of the resulting dispersion was adjusted to 2.0 with HCl. The commercial PAH was purchased from Aldrich. The LbL films were assembled onto a fluorine-doped tin-oxide (FTO) coated glass purchased from Flexitec (Curitiba, Brazil). The glass had a sheet resistance of e20 Ω and a geometrical area of 1 cm2. The layers were obtained via ionic attraction of oppositely charged materials by alternate 1 min immersions of the FTO substrate into the polycationic (PAH, 1.6 g L-1) and anionic (WO3) dispersions. After each layer deposition, the substrates were rinsed in an HCl solution (pH ) 2) for 30 s, and they were then dried under a nitrogen flow at room temperature. Some WO3/PAH LbL films were heated to 150 °C for 1 day. The X-ray diffraction of these LbL films was recorded on a Siemens D5005 diffractometer using monochromatic Cu-KR radiation. The diffractogram displayed an amorphous structure for the WO3/PAH LbL films, whether they were submitted to thermal treatment or not. For the electrochemical and spectroelectrochemical experiments, a platinum sheet with an area of 10 cm2 was used as the counter electrode, and Ag/AgNO3 saturated in propylene carbonate (PC) was used as the quasireference electrode. A LiClO4/PC electrolytic solution (0.5 M) was used in all of the electrochemical experiments, which were carried out using an
Autolab PGSTAT30 potentiostat/galvanostat. Chromogenic analysis was carried out concomitantly with the electrochemical experiments using a microprocessor-controlled solid-state light source (WPI, Inc.). Plastic fiber-optic cables up to 1 mm in diameter were used to deliver red light (660 nm) 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. The experimental data were fitted using the Origin software, with the values of r and K in QLE as the fitting parameters. The thickness of the samples was measured with a Talystep profilometer. The thickness values measured for the 10- and 20-bilayer WO3/PAH LbL films were ca. 200 and 350 nm, respectively. The geometrical area of all of the LbL films was 1 cm2. 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 the galvanostatic intermittent titration technique (GITT).29 The experiments were performed in the following way. A chronopotentiometric curve at a current density of 1 µA cm-2 was obtained for each of the LbL films under light beams at 660 nm. The curve of the absorbance change as a function of the injected charge is fitted with the QLE, whose r and K values can be determined. Afterward, starting from the open circuit potential, a 10 µA cm-2 current density is applied for 2 s. On the basis of the d(∆A)/dt0.5 slope of this pulse and on the values of r, K, L, and ∆q (obtained during the current pulse), the DLi value can be determined from eq 16. This procedure was repeated at five and at three different degrees of charging for the 10-bilayer WO3/PAH LbL films prepared at room temperature and 150 °C, respectively. A current of 1 µA cm-2 was applied for some time between current pulses to obtain the several charging degrees. Before each current pulse, the circuit was open for 2 h for the system to reach a novel equilibrium potential (Eo). In the case of the WO3/PAH LbL films prepared at room temperature, the current pulses were applied for the previously injected charge of 0.075 mC cm-2 (Eo ) -1.64 V), 0.15 mC cm-2 (Eo ) -1.67 V), 0.20 mC cm-2 (Eo ) -1.69 V), 0.25 mC cm-2 (Eo ) -1.71 V), 0.325 mC cm-2 (Eo ) -1.72 V), and 0.45 mC cm-2 (Eo ) -1.73 V). In the case of the WO3/PAH LbL films prepared at 150 °C, the current pulses were applied for the previously injected charge of 0.11 mC cm-2 (Eo ) -1.63 V), 0.22 mC cm-2 (Eo ) -1.65 V), 0.33 mC cm-2 (Eo ) -1.67 V), and 0.48 mC cm-2 (Eo ) -1.68 V). Results and Discussion Figure 1 displays the cyclic voltammograms for (a) the 20and (b) the 10-bilayer WO3/PAH LbL films, either submitted to thermal treatment or not, obtained at a sweeping rate of 50 mV s-1. Lithium insertion is depicted in the negative potential scan of these cyclic voltammograms, whereas the deinsertion process takes place during the positive potential scan. The current density values indicate that a larger amount of lithium was inserted into the LbL film treated at 150 °C as compared with the as-prepared LbL film. The inserted charges were 1.54 mC cm-2 and 0.91 mC cm-2 for the thermally treated 20-bilayer WO3/PAH LbL film and the corresponding nontreated film, respectively. This enhancement in the charge inserted into the self-assembled materials obtained at 150 °C is also observed in the case of the 10-bilayer WO3/PAH LbL film. Charges of 0.65 mC cm-2 and 0.41 mC cm-2 were inserted into the thermally treated 10-bilayer WO3/PAH LbL film and the nontreated one,
14914 J. Phys. Chem. C, Vol. 111, No. 40, 2007
Figure 1. The cyclic voltammograms of (a) 20- and (b) 10-bilayer WO3/PAH LbL films treated at (‚‚‚) room temperature and at (s) 150 °C for 1 day. V ) 50 mV s-1.
respectively. The reason for this enhancement can be associated with the lithium ion diffusion rate and/or the amount of exposed electroactive sites in the self-assembled matrices. Spectroelectrochemical measurements were carried out to better understand the processes involved in lithium ion insertion. Figure 2a shows the absorbance change (∆A) as a function of inserted charge at different wavelengths: 480, 525, 590, 623, and 660 nm. These spectroelectrochemical data were obtained in situ during the cyclic voltammetry performed for the thermally treated 10-bilayer WO3/PAH LbL films. Figure 2b shows ∆A versus q for the nontreated 10-bilayer WO3/PAH LbL film at 660 nm. Note that the electrochromic efficiency decreases as more lithium ions are injected and that the profile of these absorbance changes as a function of the injected charge is parabolic. In this case, the QLE (eq 3) can be used to predict the profile of these curves. Figure 2 shows the fitting from the absorbance change as a function of the injected charge using the QLE. The K value obtained from the fitting is 0.95 mC cm-2, which indicates that the number of electroactive sites is ca. 9.8 × 10-9 mol (K/F) for the 10-bilayer WO3/PAH LbL film treated at 150 °C. This value is larger than that obtained for the nonthermally treated LbL film (7.2 × 10-9 mol of electroactive sites, K ) 0.7 mC cm-2), indicating that the number of electroactive sites is one of the factors responsible for the difference in the charge capacity of the LbL films. Although the total amount of WO3 in the LbL films is the same for the 10-bilayer LbL films, local structural changes due to the thermal treatment may have facilitated the access of lithium ions to the WO3 sites. The r values obtained for the LbL film treated at 150 °C are 40, 32, 29, and 11 cm2 C-1 at 660, 623, 525, and 480 nm, respectively. These values correspond to the electrochromic efficiency for q ≈ 0. Because the largest electrochromic efficiency is observed at 660 nm, the electrochromic and electrochemical properties of the LbL films were then investigated under light beams at
Galiote and Huguenin
Figure 2. (a) Absorbance changes (∆A) as a function of the injected charge for the thermally treated 20-bilayer WO3/PAH LbL film at (9) 480, (4) 525, (2) 623, and (O) 660 nm. (b) The absorbance changes as a function of the injected charge for the nonthermally treated WO3/ PAH LbL film without thermal treatment at (O) 660 nm. The solid line (s) corresponds to the theoretical data obtained with the QLE; V ) 50 mV s-1.
this wavelength to guarantee high sensibility for the ∆A data. The r value obtained for the nonthermally treated LbL film is 31.5 cm2 C-1, which is lower than that achieved with the thermally treated film. This difference in the electrochromic efficiency of the films has been associated with the lithium ion mass transport rate into the LbL WO3 films.21,22 As the electron accompanies the lithium ions in the mass transport, the slow ionic diffusion can retard the W5+ f W6+ electron transfer, thus decreasing the electrochromic efficiency. Figure 3a displays the potentiodynamic profile of the current density for the thermally treated WO3/PAH LbL films, at sweeping rates of 1, 10, 20, 50, and 100 mV s-1. The injected charge changed from 0.91 mC cm-2 at 1 mV s-1 to 0.39 mC cm-2 at 100 mV s-1, indicating that the lithium ions reached inner sites of the LbL film. However, these values of injected charge per area unit and per thickness unit are smaller than those for pure WO3 films found in the literature.33,34 This fact is associated with the lower concentration of WO3 sites for the LbL films compared with the pure WO3 films. Considering that the ionic diffusion is the rate-limiting step, the rate of electron into the LbL film is delayed by the slow lithium ion diffusion. So, there is great accumulation of lithium ions and electrons in the LbL film near the film/electrolytic solution interface with increasing scan rate. This fact tends to diminish the amount of neighbor W6+ sites from a given W5+ site, and it also leads to a larger amount of interactions between the neighbor lithium ions, which delay the lithium ion diffusion.35 These factors contribute to decreasing the W5+ f W6+ intervalence charge-transfer flux, and the electrochromic efficiency consequently decreases as a function of the scan rate. This behavior can be observed in Figure 3b, which displays the absorbance change as a function of the charge at sweeping
Lithium Ion Diffusion into Self-Assembled Films
J. Phys. Chem. C, Vol. 111, No. 40, 2007 14915
Figure 3. (a) Cyclic voltammograms for the thermally treated 10bilayer WO3/PAH LbL film at (ss) 1, (s0s) 10, (s2s) 20, (s Os) 50, and (sbs) 100 mV s-1. (b) Absorbance changes (∆A) as a function of the injected charge for the thermally treated 10-bilayer WO3/ PAH LbL film at (4) 1, (1) 2, (0) 10, (2) 20, (O) 50, and (b) 100 mV s-1. The arrow points to increasing scan rates from 1 to 100 mV s-1.
rates of 1, 2, 10, 20, 50, and 100 mV s-1. Note that the slope of ∆A versus q is higher at 1 mV s-1 than at 100 mV s-1 (this slope does not change at scan rates lower than 1 mV s-1), so the d(∆A)/dt derivative can be employed to investigate the lithium ion diffusion into the WO3/PAH LbL film, according to eq 14. Figure 4a shows the chronopotentiometric curve for the thermally treated 10-bilayer WO3/PAH LbL film at a current density of 1 µA cm-2. A constant flux of lithium ions is inserted into the LbL film throughout this chronopotentiometric curve. Figure 4b shows the absorbance change at 660 nm as a function of the charge obtained in situ during the insertion of lithium ions. This curve is fitted with the QLE, whose r and K values can be determined and used to calculate the lithium ion diffusion coefficient at several charging degrees, according to eq 16. In this case, it is still necessary to apply current pulses to determine the d(∆A)/dt0.5 slope at several amounts of lithium ions previously inserted into the LbL film. Figure 4c shows an example of the absorbance changes as a function of the time square root during the application of a 10 µA cm-2 current pulse for 2 s, when the injected charge is 0.11 mC cm-2. Figure 4c also shows the linear variation in the potential as a function of the time square root (in this case for 0.6 s e t0.5 e 1.4 s) during the current pulse, which is associated only with the semi-infinite lithium ion diffusion into the LbL film. This information is important because the model described above involves the ionic diffusion process only and does not consider the effects from the double-layer charging current and from the electrolyte, bulk material, and charge-transfer resistances.29,36 Figure 5 shows the lithium ion diffusion coefficient as a function of the injected charge for the WO3/PAH LbL films
Figure 4. (a) Potential as a function of the inserted charge for the thermally treated 10-bilayer WO3/PAH LbL film; j ) 1 µA cm-2. (b) Plot of the (s) theoretical and (O) experimental absorbance changes (∆A) as a function of injected charge. (c) Plots of (O) absorbance changes and (b) potential as a function of the time square root during a constant current pulse of 10 µA cm-2.
prepared at 30 °C and 150 °C. Note that the diffusion coefficient values of the thermally treated LbL WO3/PAH film are higher than those obtained for the non-treated LbL film, for a small amount of inserted lithium ions. However, the DLi values obtained when a higher charge is injected into the LbL film treated at 150 °C decrease significantly as compared to those of the nonthermally treated film. This does not help probe the difference in the charge capacity between these LbL films. However, the K values determined for the LbL films also showed that the thermal treatment promoted an increase in the amount of electroactive sites (9 × 10-9 mol cm-2 and 7 × 10-9 mol cm-2 for the treated and non-treated LbL films, respectively). These facts must be associated with the local structure in the LbL films. Note that the DLi values of all of the films decrease as a function of q because of the interactions between the lithium ions and the network and the interactions between
14916 J. Phys. Chem. C, Vol. 111, No. 40, 2007
Galiote and Huguenin (2) Tatsuma, T.; Saitoh, S.; Ohko, Y.; Fujishima, A. Chem. Mater. 2001, 13, 2838. (3) Tada, H.; Kokubu, A.; Iwasaki, M.; Ito, S. Langmuir 2004, 20, 4665. (4) He, T.; Ma, Y.; Cao, Y.; Hu, X.; Liu, H.; Zhang, G.; Yang, W.; Yao, J. J. Phys. Chem. B 2002, 106, 12670. (5) Park, K.-W.; Choi, J.-H.; Ahn, K.-S.; Sung, Y.-E. J. Phys. Chem. B 2004, 108, 5989. (6) Tatsuma, T.; Saitoh, S.; Ngaotrakanwiwat, P.; Ohko, Y.; Fujishima, A. Langmuir 2002, 18, 7777. (7) Li, X.-L.; Lou, T.-J.; Sun, X.-M.; Li, Y.-D. Inorg. Chem. 2004, 43, 5442. (8) Shanak, H.; Schmitt, H.; Nowoczin, J.; Ziebert, C. Solid State Ionics 2004, 171, 99. (9) Yang, H.; Shang, F.; Gao, L.; Han, H. Appl. Surf. Sci. 2007, 253, 5553.
Figure 5. Lithium diffusion coefficient as a function of the injected charge for the WO3/PAH LbL films prepared at (b) room temperature and (O) 150 °C.
the ions. This behavior has already been observed for lithium ion insertion electrodes.35 Moreover, the magnitude of the determined values is close to that observed in the literature,37,38 suggesting that the method employed in this work is valid for the determination of diffusion coefficients. Conclusions According to the model developed here, which employs the QLE, the influence of the diffusion process and of the amount of electroactive sites in the films on the charge capacity can be analyzed separately. The proposed model associates the diffusion coefficient with the absorbance changes, which in turn is associated with the W5+ f W6+ intervalence charge-transfer flux. Interaction forces between Li+-Li+ and lithium ions and the network contribute to delaying the ionic diffusion, which, consequently, increases the feedback effects of absorbance changes as a function of the injected charge. The amount of electroactive sites (K/F) can be determined by extrapolating the absorbance change as a function of the injected charge for null ∆A using the QLE. This eliminates the difficulty in determining the amount of electroactive sites by electrochemical techniques. In fact, the potential window is widened because of the decrease in the lithium ion diffusion rate as a function of the injected charge, and parallel reactions involving the electrolytic solution can take place. Contrary to the diffusion coefficient values, the number of electroactive sites helped explain the reason why the LbL WO3/PAH film heated at 150 °C has a higher charge capacity than the nonthermally treated one. This model will be employed in the investigation of the lithium ion diffusion process within the distinct electrochromic components in hybrid composites. Acknowledgment. N.A.G. thanks Fundac¸ a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) for the scholarship (No. 2005/01877-7). We are grateful to FAPESP, IMMP/MCT, and Conselho Nacional de Desenvolviment Cientı´fico e Tecnolo´gico (CNPq) for the financial support. We are also grateful to Full Professor Ernesto R. Gonzalez (Instituto de Quimica De Sa˜o Carlos/University of Sa˜o Paulo, IQSC/USP) and Associate Professor Jose´ M. Rosolen (Faculdade de Filosofia, Cieˆncias e Letras de Ribeira˜o Preto (FFCLRP)/USP) for the spectroelectrochemical experiments.
(10) Deepa, M.; Singh, D. P.; Shivaprasad, S. M.; Agnihotry, S. A. Curr. Appl. Phys. 2007, 7, 220. (11) Deb, S. K. Philos. Mag. 1973, 27, 801. (12) Wolcott, A.; Kuykendall, T. R.; Chen, W.; Chen, S.; Zhang, J. Z. J. Phys. Chem. B 2006, 110, 25288. (13) van Driel, F.; Decker, F.; Simone, F.; Pennisi, A. J. Electroanal. Chem. 2002, 537, 125. (14) Granqvist, C. G. Solar Energy Mater. Solar Cells 2000, 60, 201. (15) Bohnke, O.; Bohnke, C.; Roberst, G.; Carquille, B. Solid State Ionics 1982, 6, 121. (16) Denesuk, M.; Uhlmann, D. R. J. Electrochem. Soc. 1996, 143, L186. (17) Huguenin, F.; Nart, F. C.; Gonzalez, E. R.; Oliveira, O. N. J. Phys. Chem. B 2004, 108, 18919. (18) May, R. M. Nature 1976, 261, 459. (19) Gonzalez, E. R. J. Electrochem. Soc. 1996, 143, L113. (20) de Sena, D. R.; Ticianelli, E. A.; Gonzalez, E. R. Electrochim. Acta 1998, 43, 3755. (21) Huguenin, F.; Gonzalez, E. R.; Oliveira, O. N. J. Phys. Chem. B 2005, 109, 12837. (22) Huguenin, F.; Zucolotto, V.; Carvalho, A. J. F.; Gonzalez, E. R.; Oliveira, O. N. Chem. Mater. 2005, 17, 6739. (23) Galiote, N. A.; Carvalho, A. J. F.; Huguenin, F. J. Phys. Chem. B 2006, 110, 24612. (24) Huguenin, F.; dos Santos, D. S.; Bassi, A.; Nart, F. C.; Oliveira, O. N. AdV. Funct. Mater. 2004, 14, 985. (25) Ferreira, M.; Huguenin, F.; Zucolotto, V.; da Silva, J. E. P.; de Torresi, S. I. C.; Temperini, M. L. A.; Torresi, R. M.; Oliveira, O. N., Jr. J. Phys. Chem. B 2003, 107, 8351. (26) Kim, S. J.; Park, S. J.; Shin, M. S.; Lee, Y. H.; Kim, N. G.; Kim, S. I. J. Appl. Polym. Sci. 2002, 85, 1956. (27) Varela, H.; Torresi, R. M.; Gonzalez, E. R. Quim. NoVa 2002, 25, 99. (28) Segal, D.; Nitzan, A.; Davis, W. B.; Wasielewski, M. R.; Ratner, M. A. J. Phys. Chem. B 2000, 104, 3817. (29) Weppner, W.; Huggins, R. A. J. Electrochem. Soc. 1977, 124, 1569. (30) Maranha˜o, S. L. A.; Torresi, R. M. Electrochim. Acta 1998, 43, 257. (31) Chemseddine, A.; Henry, M.; Livage, J. ReV. Chim. Mine´ r. 1984, 21, 487. (32) Livage, J.; Guzman, G. Solid State Ionics 1996, 84, 205. (33) Burdis, M.; Siddle, J.; Batchelor, R.; Gallego, J. Proc. Soc. PhotoOpt. Instr. Eng. 1995, 2531, 11. (34) Granqvist, C. G. Solar Energy Mater. Solar Cells 2000, 60, 201. (35) Baddour, R.; Pereira-Ramos, J. P.; Messina, R.; Perichon, J. J. Electroanal. Chem. 1991, 314, 81. (36) Montella, C. J. Electroanal. Chem. 2002, 518, 61.
References and Notes
(37) Garcı´a-Can˜adas, J.; Fabregat-Santiago, F.; Porqueras, I.; Person, C.; Bisquert, J.; Garcia-Belmonte, G. Solid State Ionics 2004, 175, 521.
(1) Santato, C.; Ulmann, M.; Augustynski, J. J. Phys. Chem. B 2001, 105, 936.
(38) Ho, C.; Raistrick, I. D.; Huggins, R. A. J. Electrochem. Soc. 1980, 127, 343.
