Print version ISSN 1516-1439
Mat. Res. vol.5 no.1 São Carlos Jan./Mar. 2002
Electrochromic Properties of Sol-gel Coating of Nb2O5 and Nb2O5:Li+
L. Meloa, C.O. Avellanedab, R. Caramc, E. Sichieric, A. Pawlickaa*
aDFQ, IQSC - USP, C.P. 780, 13560-970 São Carlos - SP, Brazil
bLIEC, DQ - UFSCar, C.P. 676, 13565-905 São Carlos - SP, Brazil
cDAU, EESC - USP, 13560-970 São Carlos - SP, Brazil
Trabalho apresentado no 14° CBECIMAT, Águas de São Pedro, Dezembro de 2000.
Received: November 13, 2000; Revised: December 29, 2001
Thin solid films of Nb2O5 and Nb2O5:Li+ were prepared by the sol-gel process using the sonocatalytic method. The films reported here were prepared from a NbCl5 as precursor and butanol as solvent. Through the addition of lithium salt LiCF3SO3 to the precursor solution, films with different electrochemical performance were obtained. The electrochemical and optical responses of the films were determined from the cyclic voltammetry and chronoamperometry measurements using a 0.1 M LiClO4/PC as electrolyte. The electrochromic properties of the films were investigated using in situ spectroelectrochemical method. They exhibit a gray coloration under Li+ insertion with a reversible variation of the transmittance in the visible and near infrared range between 20% and 80%.
Keywords: electrochromism, Nb2O5 , sol-gel
In recent years, there has been considerable interest in the good use of solar energy. The development of new technologies in this area has increased over the past years. Such energy is very much applied and one of its utilizations is in electrochromic devices (windows, displays, sunglasses, mirrors). A large class of optically active materials has drawn scientific interest, especially the ones with transmission, absorption and reflectance control. These materials are known as chromogenous as their color change when an external field is applied. Some studies have reported the electrochromic properties of many materials such as TiO2, WO3, V2O5, Nb2O5 etc. The electrochromic properties of these materials are related with the insertion of small ions like Li+ or H+ in their structures1-3.
Pure and doped niobium(V) oxides (Nb2O5) are promising cathodic electrochromic materials. Their electro-optical performance depends strongly upon their structural morphology. They can be obtained as films by sol-gel process. This well-know process is a good method to achieve such morphology and combined with dip-coating technique is possible to deposit a large area coating at a low cost as well as to control the microstructure of the films.
Since Richman and Bard's4 observation on coloring effect in Nb2O5 under H+ or Li+ insertion, several studies of electrochromic behavior of these materials have been reported. Gomes et al.5 found an opaque blue coloration in Nb2O5 grew thermally at ~500 °C on a niobium metallic disk. Alves et al.6 has confirmed the possibility of inserting Li+ ions into an Nb2O5 ceramic prepared from a commercial powder sintered at 800 °C. The first attempt to produce sol-gel films of Nb2O5 for electrochromic purpose has been reported by Lee and Crayston7 using a sol made from a mixture of NbCl5 dissolved in ethanol. Using a sol prepared with niobium pentachloride via Na process, homogenous films without cracks have been obtained, showing good electrochromic properties8-10. Faria et al.11 prepared Nb-oxide films from an NH4H2[NbO4C2O4)3]3H2O precursor dissolved in a solution of citric acid and ethylene glicol. Also Nb2O5 films were prepared by the sonocatalytic method12. In order to improve the bleaching process, Macek et al.15 have prepared films of Nb2O5 doped with lithium salt (LiCF3SO3) according to Barros Filho et al.16 procedure. These sols were obtained by the solubilization of NbCl5 in propanol for 24 h, and the addition of glacial acetic acid and LiCF3SO3. These sols resulted in Li/Nb molar ratio of 0.1 and 0.2.
This paper deals with a study of electrochemical and optical properties of 3 layer coatings of Nb2O5 doped with LiCF3SO3 films utilizing the sonocatalytic sol-gel route and dip coating deposition technique. The sonocatalytic sol-gel route is fast and easy to prepare niobium sol. The films demonstrated excellent bleaching kinetics and gray coloration during the insertion of Li+. These very good optical quality films can be used probably in architecture applications where the minimum coloration is necessary.
2.1. Preparation of the sol
The starting solution to produce Nb2O5 films was prepared by dissolving NbCl5 powder (1.3 g, 0.005 mol) in butanol (15 mL, 0.16 mol), acetic acid (3 mL, 0.05 mol) and LiCF3SO3 salt, (10 %mol). The solution mixture was submitted for ~5 min to the action of a 95 W, 20 kHz ultrasonic irradiation from a sonicator resulting in a transparent and viscous solution.
2.2. Preparation of the films
The coatings were deposited by dip-coating method on ITO coated glass substrates (Asahi Glass 14 W/) previously cleaned and rinsed with bidestilled water, ethanol and then dried at room temperature. The ITO glasses were immersed into the solution in ambient atmosphere (RH 60%) and withdrew at rate of 10 cm/min, dried at room temperature for 5 min and then sintered at 450 °C during 5 min. The process was repeated to obtain 3 layer films. The final heat treatment was performed at 560 °C in air atmosphere for 15 min. The resulting coatings were transparent and homogeneous without any visual cracking.
2.3. Measurement technique
Cyclic voltammetry measurements were performed with AUTOLAB 30 equipment and General Purpose Electrochemical System (GPES) for Windows - version 4.7 and the chronoamperometry experiments were performed with an EG&G PAR 273 computer-controlled potentiostat/galvanostat controlled by 270 Electrochemical Analysis software. A conventional three-electrode cell was used, a platinum foil of 1 cm2 was the counter-electrode and a silver wire was the quasi-reference electrode. The electrolyte was 0.1 M solution of LiClO4 dissolved in propylene carbonate (PC) and the cell was previously purged with dry N2 gas. The UV-Vis spectra of the films were recorded in situ with a 5G model Varian spectrophotometer and the coated substrate was placed in a special electrochemical cell with two flat quartz windows.
3. Results and Discussion
The cyclic voltammograms for Nb2O5 doped and undoped films are depicted in Fig. 1, by sweeping the potential the range of -1.8V to +2.0V. It can be observed that, close to -1.0 V, there is an increase in the cathodic current associated with Nb2O5 reduction with simultaneous Li+ cation insertions. During this process it is observed a change in the optical properties of the films, from transparent to gray color. After changing the potential sweep direction, the lithium deintercalation close to -1.5 V started, followed by a peak at E =-1.2 V. Both films showed the same voltammetric shape, but the Li doped film reached higher current values when compared with non-doped films. The same tendency was observed for the charge densities as a function of time measurements during a potential step at intervals of 30 s between -1.8 V (colored state) and +2.0 V (bleaching state) (Fig. 2). The film doped with lithium showed a better electrochromic performance (42 mC/cm2) than the undoped one, where the charge density was 29 mC/cm2.
The transmittance variation as a function of time at l = 550 nm during a potential step at intervals of 30 s between -1.8 V (colored state) and +2.0 V (bleaching state) is showed in Fig. 3. Doped films exhibited intensive coloring/bleaching changes (Tb - Tc = 59%) where Tb and Tc are transmittances (in %) of bleached and colored films, respectively while undoped film showed Tb - Tc = 33%. In addition doped films exhibited higher reversibility in the coloring/bleaching process when compared with undoped films. They showed an excellent performance where their original transmitting value was restored (approximately 77%). The two curves show that the charge insertions were 30 s, but the lithiated film showed only 20 s for extraction. This fact can explain the influence of Li+ present in the structure of the film. This was assumed to be due to the increased ionic conductivity of films originated from the presence of lithium14. It probably facilitates the exit of the charges inserted when an anodic potential is applied.
The color changes for doped and undoped Nb2O5 films were performed by in situ visible measurements from 350 nm to 800 nm (Fig. 4). The results show that the films exhibit a different spectral transmission, where the lithiathed film exhibits much higher transmission difference values between colored and bleached states. For the doped films it was observed a transmission of 18% for colored and 72% for the bleaching state and for undoped film, 40% for colored and 75% for bleaching state.
The importance of using doped films with Li+ is the improvement of the bleaching kinetics, which helps their electrochromic properties15.
The analysis of the optical density variation and transmittance versus density charge for Nb2O5 films unlithiathed and lithiathed is shown in Figs. 5 and 6. It can be observed that, for all cases, the films were transparent with an initial transmission about of about 80%. As the charge was inserted, there was an initial linear increase in DOD, followed by saturation. The initial linear increase is to be expected from the Beer-Lambert law, and the slope of this gives the electrochromic efficiency (h) of 9.5 cm2/C and 19 cm2/C for undoped and doped films respectively.
A sonocatalytic method was used for the preparation of stable, cheap and easy to prepare niobium oxide sol. Sol-gel doped Nb2O5 films have shown better electrochromic properties evidenced by a larger transmittance difference between colored and bleached states than undoped films. The difference between both states was about 59% for doped films and 33% for undoped one. It probably occurs because Li+ in its structure improves the bleaching kinetics. Both films showed high charge density inserted, but the doped ones showed best coloration and reversibility behavior reaching the bleaching state faster than undoped films. The maximum charge density inserted into a tree-layer coating was 42 mC/cm2. The sonocatalytic route is easy and cheap and the preparation of the sols is quick, giving promising results.
The authors would like to acknowledge the financial support given by FAPESP.
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FAPESP helped in meeting the publication costs of this article