**1. Introduction**

Current environmental problems observed in big cities, such as air pollution and associated infrastructure deterioration, encourage research for the development of new technologies and products that mitigate these modern, urban threats. Among the different environmentally-friendly technologies, heterogeneous photocatalytic oxidation using TiO2 has become an interesting technology due to its durability and high photocatalytic activity [1]. Recently, the incorporation of TiO2 (e.g., coatings or additives) into construction materials used in urban infrastructure, such as concrete and mortars, has been an interesting approach to reduce NO*<sup>x</sup>* and VOCs (volatile organic compounds) at outdoor concentrations using sunlight as the only energy source; these are the so-called air purifying properties. TiO2 under UV-A light irradiation can generate oxidative (·OH) and reductive (·O2) species, which are able to degrade different organic and inorganic compounds [2–4]. Furthermore, exposure to UV-A light enhances the superhydrophilic effect on the TiO2 surface, which makes it easier to remove the fouling substances on TiO2 loaded surfaces; this is the so-called self-cleaning ability [5–7]. However, recent applications of photocatalytic building materials in urban pilot projects have demonstrated that maintaining the durability of the air-purifying and self-cleaning properties remains challenging,

especially for the application of photocatalytic building materials under outdoor conditions [6]. Among other environmental factors, dust and oil accumulation have been reported as major factors affecting the properties of photocatalytic construction materials at an urban scale [7].

On the other hand, hydrophobic surfaces have also received attention for their self-cleaning, anti-flogging, anti-adherent and anti-polluting properties. The natural model for the design of superhydrophobic synthetic films is the lotus plant, which is known for its self-cleaning properties that allow the capture of air under water droplets that contribute to the rolling water droplet, a characteristic of well-designed superhydrophobic surfaces [8]. Due to the nano-manufacturing technologies that have been established for silicon substrates, silicon has been widely used for producing superhydrophobic surfaces; moreover, this kind surfaces, for instance, promotes durability in structures by avoiding the incrustation of corrosive salts (Cl− and SO− <sup>4</sup> ) that promote cracking or surface erosion [9]. To make superhydrophobic surfaces of intrinsically hydrophilic materials, a two-step process is usually required, i.e., first, make a rough surface and second, modify it with a coating of chemicals, such as organosilane, which may offer low surface energy after binding to the rough surface [9,10]. This is the case for polydimethylsiloxane (PDMS), which can be easily processed to make a hydrophobic surface with a rough texture and reduced free surface energy [11,12]. The methods to create hydrophobic surfaces have very long reaction times and strict chemical conditions. A method that uses sonochemistry has smaller reaction times, is more likely to undergo a complete chemical reaction and more ordered crystallization. Sonochemistry is a process of cavitation that refers to the rapid growth and collapse of implosion bubbles in a liquid in an unusual reaction environment [13,14]. Therefore, this article reports the development of a SiO2@TiO2 coating applicable to cement based materials, such as mortars and glass. The SiO2 matrix, based on PDMS (polydimethylsiloxane), has the potential to increase the adherence of TiO2 particles and to improve their photocatalytic efficiency [15].

#### **2. Materials and Methods**

As a strategy to develop an efficient SiO2@TiO2 coating, pure TiO2 and pure SiO2 coatings that used the same precursors, proportions, and two different synthesis methods were evaluated.
