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CHARACTERIZATION OF ELECTROCHROMIC PROPERTIES OF NANOPOROUS TIO2 FILMS MADE BY DOCTOR-BLADE METHOD



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CHARACTERIZATION OF ELECTROCHROMIC PROPERTIES OF NANOPOROUS TIO2 FILMS MADE BY DOCTOR-BLADE METHOD

Nguyen Nang Dinh*, Nguyen Minh Quyen, Tran Thi Thao, Do Ngoc Chung


University of Engineering and Technology, VNU, Hanoi 144, Xuan Thuy, Cau Giay, Hanoi,Vietnam.
Abstract

Nanostructured porous nc-TiO2/ITO films were deposited on by doctor-blade method using a colloidal TiO2 solution. Cyclic voltammeter spectra of the films were out in HCl, NaOH, KOH and LiClO4 + PC. The best electrolyte used for the ECD performance was showed as LiClO4 + PC. Reversible coloration and bleaching process was obtained. In situ transmittance spectra the nc-TiO2/ITO demonstrated the insertion/extraction of Li+ ions into anatase TiO2. The response time of the ECD coloration was found to be as small as 2 s. As large-area TiO2 films can be prepared by the doctor blade method, nc-TiO2 films constitute a good candidate for ECD applications, taking advantage of its excellent properties in terms of chemical stability.




  1. INTRODUCTION

Electrochromism is a topic that has been intensively researched from two decades of the last century for its potential applications in many fields such as electronics, optics, photonics, etc [1]. However, up to now, electrochromic devices have not been the commercial one, because of the limitation in their large price, small efficiency and slow response time, etc. It is known that electrochromic properties can be found in almost transition-metal oxides and these oxide films can be colored with anode potential or cathode potential. Among these materials, WO3 and WO3 based electrochromic devices (ECD) are the most popular for its largest ECD efficiency and the best response time for each changing in voltage in comparison with devices using other materials. There have been many technological solutions to enhance the coloration properties of electrochromic materials. For instance, doping titanium particles into WO3 film by using magnetron co-sputtering metallic titanium and tungsten in Ar/O2 atmosphere [2], with fabrication of nanostructured WO3 films Beydaghyan et al [3] have shown that porous and thick WO3 films can produce a high CE. The open structure, fast response and high normal state transmission made them good candidates for use in practical applications. Recently we have shown that nanocrystalline TiO2 anatase thin films on ITO prepared by sol-gel dipping method exhibited a good reversible coloration and bleaching process [4]. The lowest transmittance of 10% was obtained at the wavelength of 510 nm for full coloration (65% at the same wavelength in open circuitry). The coloration state was attributed to the formation of the compound Li0.5TiO2 with Li+insertion into the TiO2 electrode. However the full coloration time was found to be very large (i.e. 45 min) and the coloration efficiency was still small (i.e. 15 cm2C-1).

The nano-porous thick TiO2 films for dye-sensitized solar cells (DSSC) have been prepared by using “doctor-blade” technique [5]. However, one can suggest that the porous TiO2 films with a suitable thickness can also be used for ECD performance. The aim of this work is to investigate electrochromic properties of the nanocrystalline films prepared by the doctor blade method.



  1. EXPERIMENTAL

The TiO2 films were made with use of the doctor blade technique followed by the sequence which was reported in [5]. It is conformable to make such a thin film with its thickness less than 1 µm by this method. With two thin adhesive tapes with thickness about 30 µm, we put them parallel and 1cm apart from each other to create a slot on the ITO substrate slide which contains the colloidal solution of commercial TiO2 (Nyacol Products). The sheet resistance of the ITO-coated glass with 200 nm thickness was about 10 Ω and the transmittance of the substrate was of 90%. The useful area taken by this method was 1cm2. The colloidal solution of titanium oxide in water was used for creating thin films. This solution had 15 wt % nano-particles in 15 nm size. For producing thinner films we added more distilled water to get ca. 5 wt % TiO2 and a few drops of the liquid surfactance were added. This diluted solution was spreaded on the ITO substrate and along the parallel tapes. Drying the samples in air about 15 minutes and then annealing them at the temperature 450oC for 1 hour. Using FE-SEM system to analyze the surface morphology of the thin films. Also using the atom force microscope, the roughness and the thickness of the nc-TiO2/ITO thin films were determined. The X-ray diffraction analysis was prepared by the Brucker “Advanced-8D” X-ray diffractometer. An Auto-Lab-Potentiostat-PGS30 electrochemical unit with a standard three-electrode cell was used for electrochemical processes. The standard three-electrode cell has a working electrode (WE) as which TiO2/ITO served, a saturated calomel electrode worked as a reference electrode (RE) and a platinum grid worked as a counter electrode (CE). The different solutions used for electrolyte in the experiments, such as 0.1M HCl, 1M LiClO4 + propylene carbonate (LiClO4+PC), 1M NaOH and 1M KOH. All the measurements were prepared and carried out at room temperature. In order to characterize the transmitting property of TiO2 in solutions, the in-situ transmittance spectra of TiO2/ITO colored in the solutions was recorded by using a JASCO “V-570” photospectrometer.

  1. RESULTS AND DISCUSSION

    1. Morphology and crystalline structure of nc-TiO2 fims

AFM micrograph of a nc-TiO2 is shown in Fig. 1. It is clear that the nc-TiO2 film has nanocrystalline structure with the thickness of ~ 700 nm. The average size of the nanoparticles is about 25 nm. The film is porous in the nanoscale.





Fig. 1. AFM micrographs of a nc-TiO2 film on the surface (a) and on the cross section (b) for the thickness determination

For such a thick TiO2 film, all XRD patterns of the ITO substrate do not appear. Thus the XRD diagram shows all the diffraction peaks corresponding to the titanium oxide. Indeed, in Fig. 2 there are three diffraction peaks which are consistent with the peaks for a single crystal of TiO2 anatase. Those are the most intense peak of the (021) direction corresponding to d = 0.240 nm and two smaller peaks (022) and (220) corresponding to 0.183 nm and 0.174 nm, respectively. The fact that the peak width is rather small shows that the TiO2 anatase film was crystallized into large grains. To obtain the grain size we used the Scherrer formula [6]:



 = (1)

w




Fig. 2. XRD patterns of a nanocrystalline porous TiO2 films made by the doctor blade technique after being annealed at 450°C in air for 1 hour.

here  is wavelength of the X-ray used ( = 0.154 nm),  the peak width of half height in radians and  the Bragg angle of the considered diffraction peak. From the XRD patterns the full width at half maximum of the (021) direction with 2 = 37.4150 was found to be  = 0.0053, consequently the size of (021) grain was determined as  25 nm. Similarly, the sizes for the (022) and (220) grains were found to be ca. 30 and 20 nm, respectively. This is in good agreement with data obtained by FE-SEM for the average size of particles when the crystalline grains were not identified (see AFMFig. 1a).

    1. E


      Fig. 3. CV of nc-TiO2/ITO cycled in 1M HCl (1), 1M NaOH (2) and 1M KOH (3)
      lectrochemical property


To investigate the electrolyte effect in electrochromic properties, we used the cyclic voltammetry (CV) characteriza-tion TiO2 thin films on ITO in solutions which contained three solutions: HCl, NaOH and KOH. The CV curves are plotted in Fig.3.

The CV curve of the nc-TiO2/ITO cycled in NaOH and KOH is quite similar, except a small shift oxidation peak to the positive scanning direction (PSD) for the KOH solution. In case of HCl solution, all the CV curves are shifted in ~ 0.7 V/SCE to the PSD. Moreover, the current density in this case is much smaller than that obtained in case of NaOH and KOH solutions. Since HCl is a strong acid, the working electrode (Nc-TiO2/ITO) was removed after the third cycling time. Thus HCl is not suitable solution for the ECD. Both the NaOH and KOH electrolytes are alkaline, so more or less the working electrode is also etched during cycling. LiClO4+PC solution is a neutral electrolyte, thus this solution further has been used for characterization of ion insertion/extraction into/ out of the nc-TiO2 electrode (Fig. 4).



From this figure one can see that in the PSD a peak of the anodic current density corresponding to a value of ca. 8.5 mA/cm2 was obtained at a potential of 1.0 V/SCE. A smaller value (0.3 mA) of the peak in the negative sweep direction (NSD) was obtained at a potential of  0.92 V/SCE.



Fig. 4. CV of nc-TiO2/ITO cycled in 1M LiClO4+PC

This CV curve proves the reversibility of Li+ ion insertion (extraction) processes from the electrolyte into (out of) the working electrode. The corresponding anodic and cathodic reactions are expressed as follows [7]:

TiO2 + x (Li+ + e-)  LixTiO2 (2)



With the in-situ Raman spectra it was confirmed that 0 < x  0.5 [6], that is distinguished from the case of WO3/FTO coloration, where 0 < x  1.

    1. Electrochromic performance

T


Fig. 5. In situ transmission spectra of the TiO2/ITO colored in 1M LiClO4 + PC at 1.20 V/SCE versus time. Curve 1 is the transmittance spectra in open circuit; 2 and 3 - the spectra corresponding to respective coloration times of 1 and 2 sec; 4 – the saturate coloration and 5 – bleaching
he
in situ transmission spectra, obtained during coloration at a polarized potential of 1.2 V/SCE are given in Fig. 5. The first spectrum (curve 1) is the transmittance in open circuit. The plots denoted by numbers from 2 and correspond respectively to coloration times of 1 and 2sec. The curve 4 is of the saturated coloration, the completely bleached state occurred also fast, after approximately 2 sec (curve 5). At  = 550 nm (for the best human-eye sensitivity) the transmittance of the open circuit state is as high as 78%, whereas the transmittance of the saturated coloration state is as low as 10% (see curves 1 and 4 in Fig. 5). This efficient coloration can be attributed to the high porosity of the nanocrystalline TiO2 film.

  1. CONCLUSION

Nanostructured porous TiO2 anatase films with a grain size of 25 nm were deposited on transparent conducting ITO electrodes by a doctor-blade method using a colloidal TiO2 solution (Nyacol Products). Electrochromic performance of TiO2/ITO was carried out in HCl, NaOH, KOH and LiClO4 + PC. The best electrolyte used for the ECD performance was found as LiClO4 + PC. Reversible coloration and bleaching process was obtained. The response time of the ECD coloration was found to be as small as 2 s. In situ transmittance spectra and XRD analysis of the TiO2/ITO working electrode demonstrated the insertion/extraction of Li+ ions into antaes TiO2. Since a large-area TiO2 electrolyte can be prepared by the doctor blade method, nc-TiO2 electrode constitutes a good candidate for ECD applications, taking advantage of its excellent properties in terms of chemical stability.

  1. ACKNOWLEDGMENTS

This work was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) in the period 2010 – 2011 (Project Code: 103.02.88.09).

REFERENCES

  1. C. G. Granqvist, Handbook of inorganic electrochromic materials, Elsevier, Amsterdam, 1995.

  2. Karuppasamy and A. Subrahmanyam, Thin Solid Films, 516, 175 (2007).

  3. G. Beydaghyan, G. Bader, P.V. Ashrit, Thin Solid Films, 516, 1646 (2008).

  4. N. N. Dinh, N. Th. T. Oanh, P. D. Long, M. C. Bernard, A. Hugot-Le Goff, Thin Solid Films 423, 70 (2003).

  5. N. N. Dinh, N. M. Quyen, L. H. Chi, T. T. C. Thuy, T. Q. Trung, AIP Conf. Proc. 1169, 25 (2009).

  6. B. D. Cullity, Elements of X-Ray diffraction, Addison-Wesley Publishing Company, Inc., Reading, MA, 1978.

  7. T. Ohtsuka, M. Masud, N. Sato, J. Electrochem. Soc. 134, 2406 (1995).




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