Characterization of Sol-gel Derived Ti-doped

Available online at www.sciencedirect.com

Energy Procedia 9 (2011) 446 – 451

9th Eco-Energy and Materials Science and Engineering Symposium

Characterization of Sol-gel Derived Ti-doped Tungsten Oxide Electrochromic Thin Films K. Paipitaka,b*, C. Kahatthaa,b, W. Techitdheera c, S. Porntheeraphatd, and W. Pecharapaa,b a

College of KMITL Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Rd., Ladkrabang Bangkok 10520, Thailand. b ThEP Center, CHE, 328 SiAyutthaya Rd., Bangkok 10400, Thailand. c School of Applied Physics, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang Bangkok 10520, Thailand. d Thai Microelectronics Center, TMEC 51/4 Moo 1 Suwiontawong Rd., Wangtakien District Amphur Muang Chachoengsao 24000, Thailand.

Abstract Ti-doped WO3 electrochromic thin films were deposited onto F-doped tin oxide (FTO) substrates using spin coating technique. The as-deposited films prepared with various concentration of titanium were annealed in air at 500oC. The effect of Ti-doping concentration on structural, surface morphology and optical properties of the films were characterized by X-ray diffractometer, scanning electron microscope and UV-VIS spectrophotometer. The results show that the crystalline of WO3 can be identified at 2θ values of 24.14° corresponding to 200 orientation. In addition, It was found that the electrochromic performance of WO3 can be enhanced by small doping concentration of titanium due to structural modification of the films. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of CEO of Sustainable Energy System, © 2011 Published Elsevier Ltd. All rights reserved. Rajamangala Universityby of Technology Thanyaburi (RMUTT).

Keywords: Titanium, Tungsten oxide, Spin coating technique.

1. Introduction Electrochromic is a unique optical phenomenon in which materials of reversibly changing color based on electrons insertion-extraction induced by external applied electric field. Various kind of materials including organic (viologens) or inorganic (based on an oxide film such as tungsten oxide, nickel oxide, vanadium oxide et.al.) [1] have been recognized as technologically potential electrochromic materials. Up to now, several techniques are available for metal-oxide thin film coatings including sputtering [2], chemical vapor deposition [3], electron beam deposition [4], pulsed spray pyrolysis [5], and spin coating [6]. Among

1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of CEO of Sustainable Energy System, Rajamangala University of Technology Thanyaburi (RMUTT). doi:10.1016/j.egypro.2011.09.050

447

K. Paipitak et al. / Energy Procedia 9 (2011) 446 – 451

metal oxides, tungsten oxide is profoundly well-known because of its excellent electrochromic and gasochromic performance [7]. Recently, tungsten oxide have been widely applied for smart windows, electrochromic windows, switchable devices and gas sensors [8]. Typically, the coloring and bleaching of tungsten oxide in electrochromism is attributable to the injection and extraction of electrons and cations (M+ such as H+, Li+), which is described by following equation. WO3 (transparent) + xM+ + xe-

MxWO3 (blue)

(1)

More recently, many efforts have been made to ameliorate electrochromic performance of tungsten oxide employing Ti or its oxide. Esra Ozkan Zayim [9] reported on the analysis of the optical and electrochromic properties of antireflective WO3–TiO2 thin films and results indicated that the significant structural changes were found in tungsten oxide films with titanium doping.Zhongchun Wang et.al.[10] successfully prepared TiO2-doped WO3 thin films via the peroxo sol–gel and reported on the effect of annealing temperature on electrochromic properties of films. Suvarna R. Bathe et.al. [11] synthesized Ti doped WO3 thin films by pulsed spray pyrolysis technique. It is seen that effect of Ti doping can lead to significant surface morphology changes. In this study, we report on the deposition of Ti-doped WO3 (TWO) thin films based on sol-gel spin coating technique and the effect of Ti doping on structural and electrochromic efficiency of the films. The morphology and structural of TWO thin films were characterized by scanning electron microscopy (SEM) and X-ray diffractometer (XRD). The relevant optical and electrochromic properties were interpreted by UVVIS spectrophotometer. 2. Experimental Setup Ti-doped WO3 thin films were deposited by sol-gel spin coating. The starting precursor was prepared from tungsten powder (Aldrich 12 micron; 99.9%) dissolved in 15% hydrogen peroxide. The dissolution was performed at 10-15°C which finally results to a pale yellow colored solution. After completely dissolved, ethanol solution and Ti(OBun)4 (Aldrich) were subsequently added into the stock solution. The mixture was then stirred continuously for a few hours until the homogeneous starting precursor was obtained. Ti-doped WO3 electrochromic thin films were deposited onto F-doped tin oxide (thickness~400 nm) conducting substrates with sheet resistance of 15 Ω/□. The sol-gel solution was coated by a spinner at speed of 2000 rpm. After each coating, the sample was post-baked at 100°C for 5 min and finally annealed at 500°C in ambient air for 2 hr. The effect of Ti doping on structural properties of the films were characterized by X-ray diffractometer (XRD). The surface morphology and film thickness were examined by SEM (Hitachi S-4700). The Optical properties of the films were investigated using UV-Vis spectrophotometer (Thermo Electron Corporation HeliosD). 3. Results and Discussion Fig 1 shows the XRD patterns of undoped WO 3 and TWO thin films with different Ti doping concentration of 5 wt%, 15 wt%, and 20 wt%. The XRD results exhibit that the crystalline of WO3 can be identified at 2θ values of 24.14° assigned to (200) orientation plane [12]. Explain more about XRD peaks correspond to FTO substrate [13].

448


Fig. 1. XRD patterns of Ti-doped WO3 thin films prepared with different Ti doping contents.

It is obviously noticed that the crystallinity of the WO3 film drastically decreases with increasing of titanium content.When doping, Titanium may diffuse into the crystal lattice of WO3 and create disorder in the WO3 structure leading to not only decrease in crystallization but also its grain size. As shown in Fig. 2, further interesting result was found on the significant shift of (200) peak to the lower diffraction angle as the film was doped with Ti indicating the increase in d-spacing of WO3 lattice. This feature may attribute to the good Ti substitution or intercalation on W site in WO3 crystal lattice.

Fig. 2. Plot of shifting of (200) peak position of Ti-doped WO3 thin films.

Fig. 3(a) shows the cross-section image of TWO thin film deposited on FTO substrate. The results indicate the good and uniform formation of WO3 thin film with average thickness of 150 nm. The top view SEM images giving an insight into the morphological feature is illustrated in Fig.3(b). This image indicated that morphology of as-prepared film comprised uniform defect-free grains covered on the FTO layer. The formation of TWO film by this deposition technique was also affirmed by EDX results, which indicated the presence of tungsten and titanium element in these pattern.


Fig. 3. (a) cross-section image and (b) surface morphology image of Ti-doped WO3 thin films.

Fig. 4. EDX spectrum of Ti-doped WO3 thin film.

The electrochromic measurements were carried out in 0.001 M of H2SO4 electrolyte. The corresponding transmittance spectra at the colored (Tc) and bleached (Tb) states of Ti-doped WO3 thin films with various Ti doping concentration are shown in Fig. 5 The optical transmittance of all asprepared films are greater than 60% in the visible region of the spectrum. The transmittance modulation of undoped film at colored state (Tc) is less than that of doped WO3 thin films. For TWO film with 5% Ti content, the percentage of transmittance in a bleached state was apparently higher than undoped film implying the significant improvement in bleaching of the film. For TWO film with 5% Ti content, the percentage of transmittance in a bleached state was apparently higher than undoped film implying the significant improvement in bleaching of the film. The transmission change (∆T) values of all films measured at 550 nm is concluded in table 1. It is clearly indicated that the TWO film with 5% doping content has superiority in transmission change among the others advising that the enhancement of electrochromic performance and reversibility of WO3 can be attained by certain amount of Ti additive.

449

450


Fig. 5. Transmission spectra at colored and bleached state of (a) undoped and Ti-doped WO3 thin films with (b) 5% doping content, (c) 10% doping content, (d) 20% doping content. Table 1. The transmittance measurement of undoped and Ti-doped WO3 thin films. Transmittance at bleached state Tb,(%)

Transmittance at colored state Tb,(%)

transmission change (∆T) = (Tb-Tc)

0

61.507

20.903

40.604

5

78.572

31.072

47.500

15

69.961

37.223

32.738

20

65.138

37.114

27.994

Concentration of Ti (wt%)

4. Conclusion In summary, Ti-doped WO3 electrochromic thin films were successfully deposited onto F-doped tin oxide FTO substrates using spin coating technique. XRD results validate the crystallization of WO3. Ti additive doped into WO3 thin film leads to not only the increase in d-spacing of WO3 but also the decrease in crystallinity. The EDX measurement was conducted to confirm the existence of Ti dopant in the doped films. The electrochromic measurements were conducted and the corresponding results indicated that the electrochromic performance of the films can be enhanced by the specific Ti doping content incorporated into the films.


Acknowledgements This work has partially been supported by the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology. Thailand, through its program of Center of Excellence Network. Authors would like to thank Rajamangala University of Technology Thanyaburi (RMUTT) for XRD measurement and Thai Microelectronic (TMEC) for FE-SEM measurement. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Anneke, G. and Andreas, G. 2009. Electrochromic device with a redox electrolyte. In Solar Energy Materials & Solar Cells. 93 : 1329–1337. Karuppasamy, A. and Subrahmanyam, A. 2007. Studies on electrochromic smart windows based on titanium doped WO3 thin films. In Thin Solid Films. 516 : 175–178. Garga, D., Henderson, P.B., Hollingsworthb, R.E. and Jensen, D.G. 2005. An economic analysis of the deposition of In Materials Science and electrochromic WO3 via sputtering or plasma enhanced chemical vapor deposition. Engineering. 119 : 224–231. Joraid, A. 2009. Comparison of electrochromic amorphous and crystalline electron beam deposited WO3 thin films. In Current Applied Physics. 9 : 73–79. Patil, P.S., Mujawar, S.H., Inamdar, A.I. and Sadale, S.B. 2005. Electrochromic properties of spray deposited TiO2-doped WO3 thin films. In Applied Surface Science. 250 : 117–123. Dilek, I., Metin, A. and Caner, D. 2009. Structural, electrochemical and optical comparisons of tungsten oxide coatings derived from tungsten powder-based sols. In Thin Solid Films. 518 : 104–111. Gogova, D., Thomas, L. and Camin, B. 2009. Comparative study of gasochromic and electrochromic effect in thermally evaporated tungsten oxide thin films. In Thin Solid Films. 517 : 3326–3331. Andreas, G., Anneke, G., Wolfgang, G. and Volker, W. 2008. Switchable windows with tungsten oxide. In Vacuum. 82 : 730– 735. Esra Ozkan, Z. 2005. Optical and electrochromic properties of sol–gel made anti-reflective WO3–TiO2 films. In Solar Energy Materials & Solar Cells. 87 : 695–703. Zhongchun, W. and Xingfang, H. 2001. Electrochromic properties of TiO2-doped WO3 films spin-coated from Ti-stabilized peroxotungstic acid. In Electrochimica Acta. 46 : 1951–1956. Suvarna R., B. and Patil, P.S. 2008. Titanium doping effects in electrochromic pulsed spray pyrolysed WO 3 thin films. In Solid State Ionics. 179 : 314–323. Vinay Shankar, V., Martin, H., Alan, S., Kirill, S., Dennis, K., Radim, B., Wolfgang, S. and Alfred, L. 2011. Enhanced photoelectrochemical properties of WO3 thin films fabricated by reactive magnetron sputtering. In International Journal of Hydrogen Energy. 36 : 4724-4731. Chi-Hwan, H., Sang-Do, H., Jihye, G. and Khatkar, S.P. 2007. Synthesis of indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) nano - powder by sol–gel combustion hybrid method. In Materials Letters. 61 : 1701–1703.

451