Pure and Mo-doped WO3 films were formed on ITO-coated glass substrate held at 473？K by RF magnetron sputtering technique. The structural, morphological, and optical properties of pure and Mo-doped WO3 thin films have been systematically studied. The structural properties revealed that the pure WO3 films exhibited a (020) reflection related to the orthorhombic phase of WO3, whereas Mo-doped films showed (200) reflection. The surface morphology revealed that pure WO3 films showed the dense surface and Mo-doped films contained agglomerated grains which were uniformly distributed on the surface of the substrate. The optical transmittance decreased from 85% to 75% for pure and Mo-doped WO3 films, respectively. The electrochromic properties of the films were measured by cyclic voltametry in 1？M Li2SO4 electrolyte solution. The optical modulation of pure WO3 films at near IR was 50%, and the calculated color efficiency was 33.8？cm2/C, while in Mo-doped WO3 the efficiency improved to 42.5？cm2/C. 1. Introduction Tungsten oxide (WO3) is the most widely used electrochromic material because of easiness in synthesis and favorable electrical and optical properties. The application of electrochromic materials for smart windows, displays, and antiglare mirrors and several applications have been developed such as control of incoming daylight into buildings, smart windows, rearview mirrors, and aphotochromic and electrochromic devices [1–3]. Tungsten oxide is the extensively studied electrochromic material [4, 5]. Doping of vanadium, niobium, nitrogen, titanium, or nickel to WO3 enhances in the electrochromic properties. Muthu Karuppasamy and Subramanyam [6] reported that the color efficiency decreased from 121 to 13？cm2/C with increase of vanadium doping of 9 at. % in tungsten oxide films deposited by DC magnetron sputtering. Bathe and Patil [7] studied the electrochromic properties of niobium-doped WO3 films, and the coloration efficiency decreased with the increase of niobium doping. Sun et al. [8] studied the nitrogen-doped WO3 films formed by reactive DC pulsed sputtering and the color efficiency achieved to 45？cm2/C at 5 at. % nitrogen doped films. Karuppasamy and Subrahmanyam [9] studied the electrochromic properties of titanium doped tungsten oxide films and realized the improvement in the electrochromic properties with the increase of titanium doping. Gesheva et al. [10] studied MoO3-WO3 films formed by chemical vapour deposition method and showed the color efficiency of 141？cm2/C when compared to 84？cm2/C for WO3 and 39？cm2/C for MoO3 films. Valyukh et al. [11]
References
[1]
W. L. Kwong, N. Sarvides, and C. C. Sorrell, “Electrodeposited nanostructured WO3 thin films for photoelectrochemical application,” Electrochimica Acta, vol. 75, pp. 371–380, 2012.
[2]
S. Balaji, Y. Djaoued, A. S. Albert, R. Brüning, N. Beaudoin, and J. Robichaud, “Porous orthorhombic tungsten oxide thin films: synthesis, characterization, and application in electrochromic and photochromic devices,” Journal of Materials Chemistry, vol. 21, no. 11, pp. 3940–3948, 2011.
[3]
G. A. Niklasson and C. G. Granqvist, “Electrochromics for smart windows: thin films of tungsten oxide and nickel oxide, and devices based on these,” Journal of Materials Chemistry, vol. 17, no. 2, pp. 127–156, 2007.
[4]
J. Zhang, X. L. Wang, X. H. Xia, C. D. Gu, and J. P. Tu, “Electrochromic behavior of WO3 nanotree films prepared by hydrothermal oxidation,” Solar Energy Materials and Solar Cells, vol. 95, no. 8, pp. 2107–2112, 2011.
[5]
B. Baloukas, J. M. Lamarre, and L. Martinu, “Electrochromic interference filters fabricated from dense and porous tungsten oxide films,” Solar Energy Materials and Solar Cells, vol. 95, no. 3, pp. 807–815, 2011.
[6]
K. Muthu Karuppasamy and A. Subrahmanyam, “Results on the electrochromic and photocatalytic properties of vanadium doped tungsten oxide thin films prepared by reactive dc magnetron sputtering technique,” Journal of Physics D, vol. 41, no. 3, Article ID 035302, 2008.
[7]
S. R. Bathe and P. S. Patil, “Influence of Nb doping on the electrochromic properties of WO3 films,” Journal of Physics D, vol. 40, no. 23, pp. 7423–7427, 2007.
[8]
X. Sun, Z. Liu, and H. Cao, “Electrochromic properties of N-doped tungsten oxide thin films prepared by reactive DC-pulsed sputtering,” Thin Solid Films, vol. 519, no. 10, pp. 3032–3036, 2011.
[9]
A. Karuppasamy and A. Subrahmanyam, “Studies on electrochromic smart windows based on titanium doped WO3 thin films,” Thin Solid Films, vol. 516, no. 2-4, pp. 175–178, 2007.
[10]
K. A. Gesheva, T. Ivanova, M. Kozlov, and S. Boyadzhiev, “Atmospheric pressure chemical vapour deposition of electrochromic Mo-W thin oxide films: structural, optoelectronic and vibration properties,” Journal of Crystal Growth, vol. 312, no. 8, pp. 1188–1192, 2010.
[11]
I. Valyukh, S. V. Green, C. G. Granqvist, G. A. Niklasson, S. Valyukh, and H. Arwin, “Optical properties of thin films of mixed Ni-W oxide made by reactive DC magnetron sputtering,” Thin Solid Films, vol. 519, no. 9, pp. 2914–2918, 2011.
[12]
J. M. O. R. De León, D. R. Acosta, U. Pal, and L. Casta？eda, “Improving electrochromic behavior of spray pyrolised WO3 thin solid films by Mo doping,” Electrochimica Acta, vol. 56, no. 5, pp. 2599–2605, 2011.
[13]
C. E. Patil, P. R. Jadhav, N. L. Tarwal, H. P. Deshmukh, M. M. Karanjkar, and P. S. Patil, “Electrochromic performance of mixed V2O5-MoO3 thin films synthesized by pulsed spray pyrolysis technique,” Materials Chemistry and Physics, vol. 126, no. 3, pp. 711–716, 2011.
[14]
N. Ozer and C. M. Lampert, “Electrochromic performance of sol-gel deposited WO3-V2O5 films,” Thin Solid Films, vol. 349, no. 1, pp. 205–211, 1999.
[15]
W. Wang, Y. Pang, and S. N. B. Hodgson, “Design and fabrication of bimodal meso-mesoporous WO3 thin films and their electrochromic properties,” Journal of Materials Chemistry, vol. 20, no. 39, pp. 8591–8599, 2010.
[16]
S. Y. Lin, C. M. Wang, K. S. Kao, Y. C. Chen, and C. C. Liu, “Electrochromic properties of MoO3 thin films derived by a sol-gel process,” Journal of Sol-Gel Science and Technology, vol. 53, no. 1, pp. 51–58, 2010.
[17]
M. Deepa, A. K. Srivastava, S. N. Sharma, Govind, and S. M. Shivaprasad, “Microstructural and electrochromic properties of tungsten oxide thin films produced by surfactant mediated electrodeposition,” Applied Surface Science, vol. 254, no. 8, pp. 2342–2352, 2008.
[18]
K. Hari Krishna, O. M. Hussain, and C. M. Julien, “Electrochromic properties of nanocrystalline WO3 thin films grown on flexible substrates by plasma-assisted evaporation technique,” Applied Physics A, vol. 99, no. 4, pp. 921–929, 2010.
[19]
K. Sauvet, L. Sauques, and A. Rougier, “IR electrochromic WO3 thin films: from optimization to devices,” Solar Energy Materials and Solar Cells, vol. 93, no. 12, pp. 2045–2049, 2009.
[20]
K. W. Weng, S. Han, Y. C. Chen, and D. Y. Wang, “Charactrization of tungsten-titanium electrode for electrochromic application,” Applied Surface Science, vol. 255, pp. 3848–3853, 2008.
[21]
P. R. Patil and P. S. Patil, “Preparation of mixed oxide MoO3-WO3 thin films by spray pyrolysis technique and their characterization,” Thin Solid Films, vol. 382, no. 1-2, pp. 13–22, 2001.
[22]
C. G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, The Netherlands, 1995.
