RESEARCH PAPER Effect of particle size on the photochromic response of PWA/SiO 2 nanocomposite Feng-Hsi Huang • Ching-Chung Chen • Dar-Jong Lin • Trong-Ming Don • Liao-Ping Cheng Received: 10 November 2009 / Accepted: 13 February 2010 / Published online: 3 March 2010 Ó Springer Science+Business Media B.V. 2010 Abstract A series of photochromic phosphotung- stic acid (PWA)/SiO 2 composites were synthesized using the sol-gel method. Depending on the feeding schedule of PWA during synthesis, the size of the formed PWA/SiO 2 particles varied considerably from as small as 1.2 nm to ca. 10 nm. With decreasing silica particle size, the total contact area/interaction between SiO 2 and PWA increases, as revealed by FT- IR and solid-state 29 Si-NMR analyses. Particularly, when the size of PWA/SiO 2 is *1 nm, crystallization of PWA is inhibited, and PWA presents as amor- phous molecular entities distributing uniformly in the SiO 2 host, which is in evidence in the XRD spectroscopy and HR-TEM imaging. In contrast, substantial crystallization of PWA takes place when PWA/SiO 2 particles are as large as 10 nm, in which case less amount of surface free Si-OH is available for PWA to make bonds with. Photochromism occurs activated by ultraviolet light irradiation. The rate of coloration/bleaching is found to depend strongly on the particle size of PWA/SiO 2 ; specifically, the rate increases twice when the particle size is reduced from 10 nm to 1.2 nm. Keywords Photochromism Á Nanocomposite Á Phosphotungstic acid Á Silica Á Crystallization Á Optical display Á Smart windows Introduction Organic-inorganic hybrid photochromic materials attract a great deal attention recently because of their potential applications in optical display, catalytic, smart windows, proton-conducting membranes, and other high technology products (Kukovecz et al. 2002; Bechinger et al. 1996; Gong et al. 2003; Poulos et al. 2008). To date, significant effort has been put on developing inorganic oxides, such as WO 3 , MoO 3 , V 2 O 5 , etc. for photochromic applications (Yao et al. 1992; He et al. 2003; Katayama et al. 2006). However, for these materials, there exist inherent drawbacks that are very difficult to eliminate, for which low response speed, poor solubility in common solvents, and high cost for manufacture are most often criticized. Lately, a new class of polyoxometalates (isopoly- and hetero- polyanions) including H 4 SiW 12 O 40 , H 3 PW 12 O 40 , H 3 PMo 12 O 40 , and H 3 SiMo 12 O 40 has emerged as substitutes for the aforementioned inorganic oxides (Katsoulis 1998). These polyanions exhibit superior redox capability, high electronic density, and good solubility in water and solvents, which approve them ideal for photochromic utilization. In order to realize practical applications, polyoxo- metalates photochromic materials have to be shaped into coatings, monoliths, or objects of a desired form F.-H. Huang Á D.-J. Lin Á T.-M. Don Á L.-P. Cheng Department of Chemical and Materials Engineering, Tamkang University, Taipei 25137, Taiwan, ROC C.-C. Chen Á D.-J. Lin Á T.-M. Don Á L.-P. Cheng (&) Energy and Opto-Electronic Materials Research Center, Tamkang University, Taipei 25137, Taiwan, ROC e-mail: lpcheng@mail.tku.edu.tw 123 J Nanopart Res (2010) 12:2941–2950 DOI 10.1007/s11051-010-9884-7