**1. Introduction**

TiO2 is one of the most widely researched photocatalytic semiconductor materials to date. When irradiated with UV light, TiO2 can decompose the organic pollutants present on its surface, and in addition the surface turns to be highly hydrophilic [1]. Water spread out instantaneously by forming a thin layer on the superhydrophilic TiO2 thin films and steadily carries away dust particles from the surface while flowing (Figure 1). Photocatalytic superhydrophilic thin films with excellent self-cleaning properties are receiving much research attention worldwide because such smart surfaces can save time and reduce maintenance costs [2]. Today, the optically transparent, superhydrophilic and photocatalytic TiO2 thin films are finding increasing industrial applications. Many reports on application of self-cleaning TiO2 thin films on glass substrates are available in the literature [3–6]. An attractive soft polymer material, polycarbonate (PC) has great demand in the optical industry. PC can replace heavy glass in the optical and electronic industries in the coming future due to its extremely low weight, durability, low-cost, and optical transparency [7]. Many attempts have been made to improve the scratch resistance and UV degradation property of the PC by coating the surface by numerous photocatalytic materials with suitable binders.

**Figure 1.** Schematic showing the self-cleaning phenomena on superhydrophilic surface.

Hwang and coworkers [8] have prepared mechanically durable and optically transparent organic-inorganic SiO2-TiO2 nanocomposite coatings on PC by simple spray coating. The coating solution was prepared by incorporating nanosized TiO2 sol into the silica polymeric sol. This nanocomposite coating also protected PC from photo-degradation under UV illumination. Lam *et al.* [9] reported the comparative study on the influence of NaOH etching and UVC irradiation on the mechanical stability of the TiO2 thin films prepared on PC. They observed adherent TiO2 film on UVC irradiated PC than NaOH treated PC due to increase in –OH and –COOH groups on the UVC-treated PC. However, the strong hydrophilic, antifogging, photocatalytic and self-cleaning properties were observed on the TiO2 film coated on NaOH treated PC. Recently, Razan Fateh *et al.* have prepared optically transparent, highly hydrophilic, mechanically stable and photocatalytic TiO2/SiO2 [7,10], and TiO2/ZnO coating [11] on PC for self-cleaning applications. In the present research work, we prepared transparent, adherent, highly wettable and self-cleaning SiO2-TiO2 coating on PC by simple spin coating. The coating showed faster wettability switching (hydrophilic to superhydrophilic) after 30 min. of UV irradiation. The effect of different vol% of SiO2 sol in TiO2 and its influence on the surface morphology, mechanical stability, wettability, and photocatalytic properties of the coatings were studied in detail.
