SiO 2 @TiO 2 Composite Synthesis and Its Hydrophobic Applications: A Review

: Titanium dioxide is well known for its photocatalytic properties and low toxicity, meanwhile, silicone dioxide exhibits hydrophobic and hydrophilic properties and thermal stability. The union of these two materials offers a composite material with a wide range of applications that relate directly to the combined properties. The SiO 2 -TiO 2 composite has been synthesized through physical methods and chemical methods and, with adequate conditions, morphology, crystallinity, boundaries between SiO 2 -TiO 2 , among other properties, can be controlled. Thus, the applications of this composite are wide for surface applications, being primarily used as powder or coating. However, the available research information on this kind of composite material is still novel, therefore research in this ﬁeld is still needed in order to clarify all the physical and chemical properties of the material. This review aims to encompass the available methods of synthesis of SiO 2 -TiO 2 composite with modiﬁers or dopants, the application and known chemical and physical properties in surfaces such as glass, mortar and textile, including aspects for the development of this material.


Introduction
The imitation of nature gives inspiration for the development of new technologies for pro-environmental remediation. Biomimetic is the term employed in the literature and in engineering for understanding the natural process for describing a perfect functional system where, over years of evolution, it yields the best adaptation to the environment [1]. In this kind of technology, we can find silicon oxide (SiO 2 ) and titanium dioxide (TiO 2 ). Both oxides are considered an example of biomimetic materials.
To give the biomimetic term to a material, a clear example of it needs to be presented. The "lotus effect" has extraordinary hydrophobic performance and self-cleaning features. With the rolling of water droplets, dust is removed from the surface with the movement of the water; in other words, it sweeps away contaminating particles. This phenomenon is called the self-cleaning property [2]. Hydrophobic and self-cleaning surfaces show great potential in industrial applications due to their morphology structure [3].
Many materials are inspired by nature, such as the structure of butterfly wings [4], the needle-shaped seta of water stride [5], gecko feed [6], and the morphology of the lotus leaf [7]. The lotus leaf has hierarchical micro and nanostructures on its surface. It is determined by a combination of surface chemistry and surface architecture [8]. Generally, the most hierarchical topography consists of multiple levels of roughness and micro and nanostructures on the surface [9]. It was observed that the lotus leaf was covered by about 10 µm sized protrusions at random with about 20 µm intervals among them, giving low-surface energy [10]. The surface architecture of hierarchically rough surfaces reduces available contact area and the ability for water droplets to adhere to the surface. The measured contact angle is then a composite of the surface of air, as described by the Cassie-Baxter theory of wettability [11]. This is one of the classical models, and corresponding formulae about the surface wettability were established to understand the mechanism of hydrophobic phenomena from a theoretical perspective [12]. Based on these models, scientists not only theoretically explained the hydrophobic mechanism, but also calculated the contact angle (CA) of the corresponding surface [13].
The materials SiO 2 and TiO 2 have been used in separate ways, and have been widely reported in the literature with different synthesis methods and applications; however, the use of SiO 2 -TiO 2 as a composite is reported in fewer papers. The lack of information about this kind of composite material limits the synthesis methods and applications. For that reason, more research in this field is needed in order to achieve a more complete review with all physical and chemical properties.
The review will focus on the hydrophobic surfaces of SiO 2 and TiO 2 with developments in synthesis methods, physicochemical properties, reaction mechanisms and applications in the fields of energy, environment, industry and others. The content is divided into three parts: a brief summary of basic theories including the hydrophobic phenomenon, self-cleaning properties in different surfaces (Section 2); Section 3 presents the methodologies for the synthesis of hydrophobic surfaces and their nanostructures and wettability. Practical applications of hydrophobic surfaces are shown in Section 4 such as in water self-cleaning, hydrophobic and anti-reflectivity. The last section summarizes the results and the challenges in the development of hydrophobic surfaces and the trends among this kind of surface.

Theory behind the Physico-Chemical Characteristics in SiO 2 -TiO 2
Silicone dioxide (SiO 2 ) is a semiconductor compound with low toxicity, high thermal stability, and has both an amorphous and crystalline structure [14]. The properties have been extensively studied in different areas, such as electronics, photo-tonic, chemicals, environmental, electrochemistry, and biomedical fields [15][16][17]. The nobility of SiO 2 allows its union with different materials, such as TiO 2 , one of the most studied photocatalysts. The SiO 2 -TiO 2 composite provide the properties of both, where the SiO 2 is a support for the TiO 2 particles, with chemical, electronic and hydrophobic properties [18].
Self-cleaning is one of the most desired properties of the intelligent materials. This property is a typical phenomenon in nature; for thousands of years, natural evolution and selection has optimized functional systems, creating the ability of surfaces to repel contaminants such as solid particles and liquids. The first time this phenomenon was observed was in the leaves of the Lotus plant. Droplets of water deposited on the surface of the leaves maintained an almost perfect spherical shape, and rolled over the surface removing the particles of powder. Exhibiting not only a self-cleaning property but also extraordinary hydrophobic properties [2,19]. This phenomenon is called the "lotus effect" [20]. It is important to mention that this effect is not particular to the lotus plant, other plants and insects have hydrophobic surfaces [21]. For this reason, the lotus leaf only opened the doors for hydrophobic investigation.
In this context, the first studies in 1990 of hydrophobic surfaces discovered that some hydrophobic surfaces in nature exhibit other functionals properties such as low wettability, antifogging, anti-reflection, and resistance [22]. Barthlott and Neinhuis et al. conducted the first research reporting the microstructure of the lotus leaf through scanning electron microscopy (SEM) [23]. Subsequently, Jian et al. found that a combination of multistructural micro and nanorugosity structures are fundamental for obtaining the self-cleaning phenomenon [24].
The presence of micro and nano structures on the surface provides low surface energy and surface tension, creating a rugosity in the surface, therefore obtaining a hydrophobic surface. Wettability is usually determined through the measurement of the contact angle of the water droplet on the surface. The contact angle is defined as the liquid-steam interface with respect to the solid-liquid interface. This angle is classified in three categories as shown in Figure 1 [25], hydrophilic surfaces exhibit a contact angle of less than 90 • ; hydrophobic surfaces present a contact angle higher than 90 • and super hydrophobic surfaces present a contact angle higher than 150 • . Thomas Young was the first person to describe the contact angle in 1980, established by the mechanical balance of the drop, acting with three interfacial tensions in an ideal solid surface [26]. Considering the contact angle is stable, it corresponds to a minimum of Gibbs free energy. Where Gibbs free energy states that the system must achieve a minimum of free energy when there is a liquid vapor balance. At constant temperature and pressure the surface tension is equal to Gibbs free energy per surface area [27,28]. The reduction of the surface's energy increases the hydrophobicity of the surface. Micro/nano surface texturing is another way to decrease the surface energy. In summary, the movement of the fluid over a solid surface can be achieved by two means, namely, the liquid follows the solid state, or it moves in the air trapped within the micro / nano texture on the surface [11].
Usually, self-cleaning surfaces need to be prepared in advance, namely, making a rough surface for low surface energy materials and chemically modifying the surface roughness to create a rough surface from low-surface energy materials. In this context, self-assembly organ silanes are one of the most employed strategies due to the high efficiency of making a surface hydrophobic by reducing surface energy [29].
The structure of the Polydimethil Siloxane (PDMS) is a chain with an Si-O-Si structure and side chains of the -CH 3 groups. The structure exhibits thermal stability and elasticity, due to high bond energy and large bond angle. Furthermore, the -CH 3 groups have proportionate low surface energy and hydrophobicity as a result of the non-polar groups [30]. Moreover, PDMS with terminating methyl groups is hydrophobic silicone and can firmly adhere to the surface without using other additives. The hydroxyl groups of PDMS, which hydrolyzes during stirring, can interact with SiO 2 antiparticles by hydrogen bonding [31].
On the other hand, the Hexamethyldisilazane (HMDS) has methyl terminations, making it hydrophobic and making it an attractive material for creating a super hydrophobic surface and eliminating the need for complex chemical pathways [32]. HMDS forms the (CH 3 ) 3 -Si-O-group on Si wafer surfaces and on glass surfaces which changes these surfaces to lipophilic and indirectly facilitates the formation of a surface susceptible to the adhesion of organic impurities [33]. These modifiers of the surface enhance the self-cleaning properties. Moreover, compounds such as metallic oxides, ceramics or organic compounds offer different properties, such as photocatalytic activity. Among these compounds, TiO 2 is one of the materials with promising results. TiO 2 is known for the diverse applications in different areas such as pharmaceutical, make up, environmental remediation, biomedical applications [34], devices and others [35]. Among the qualities of the material, low toxicity, chemical stability, and high availability stand out as advantages over other semiconductors. Also, due to these characteristics, TiO 2 is one of the most employed photocatalyst [36][37][38].
TiO 2 exhibits a high effectiveness in environmental remediation in the degradation of inorganic and organic pollutants. The photocatalytic properties are exhibited under irradiation with UV light, due to the band gap value of 3.2 eV, limiting the application. However, these properties can be increased by diminution of the band gap [39,40]. The photocatalytic process begins with the photogeneration of holes and electrons, which are diffusing to the surface, where reactive species react with the pollutant; as a result, a series of oxidation and reduction phenomena take place as shown in Figure 2 [41]. On the other hand, the physical properties of the TiO 2 such as morphology, surface area, crystalline phase and particle size of TiO 2 , are well-known factors that directly impact on the photocatalysis mechanism [42,43]. One of the main drawbacks that limits the use of TiO 2 is the method of application, since it is normally used in suspension. Typically, a loss of up to 30% of catalyst material is assessed for suspension applications [42]. For this reason, there is increasing interest in novel research in which TiO 2 is supported on a different material, SiO 2 has been employed as substrate for this purpose.
The union of SiO 2 -TiO 2 has been achieved by different techniques such as sol-gel, impregnation, or precipitation [44]. In order to improve the yields of the desired reactions, the TiO 2 film must have certain characteristics such as well defined textural morphology and a proper thickness [45]. Moreover, the molar ratio of SiO 2 -TiO 2 directly affects the crystallinity of the material, crystal size and porosity of the TiO 2 . Furthermore, this material offers optical properties due to the high scattering of TiO 2 as a consequence of its large band gap and high refractive index. These properties allow the manufacture of antireflective and transparent surfaces of SiO 2 -TiO 2 [46,47].

Synthesis Methods
Nanoparticles of TiO 2 and SiO 2 are found to have different structures. The methodologies employed yield different structural and physical properties.

Physical Methods
The physical methods are optimizable processes due to the precise conditions they use, such as high temperatures, pressures and high purity of precursors, which is why they are characterized by their high cost and limited applications.

Sputtering
Magnetron sputtering is a deposition technique involving a gaseous plasma which is generated and confined to a space containing the material to be deposited. First, pressure is stabilized typically in the mTorr range employing a pressure control system. After, the sputtering gas which is the plasma is flowed into the chamber. The surface of the sample is eroded by high-energy ions within the plasma, and the liberated atoms travel through the vacuum environment and deposit onto a substrate to form a thin film [48]. These collisions cause an electrostatic repulsion which 'knock off' electrons from the sputtering gas atoms, inducing ionization. The positive sputter gas atoms are now accelerated towards the negatively charged cathode, causing high energy collisions with the surface of the target [49]. In the interest of generating as many high energy collisions as possible, a high molecular weight gas such as argon or xenon is used as sputtering gas. The oxygen or nitrogen gases are employed for a reactive sputtering process [50]. Mezur et al. present the technique of magnetron sputtering with an additional microwave source to improve plasma ionization during deposition for the synthesis of TiO 2 -SiO 2 thin films. The conditions of magnetron sputtering were of 10 kW DC power supply and 3 kW plasma source with Ti and Si targets with dimensions of 120 mm × 372 mm. The deposition rate of titania and silica was maintained at 1.1 Å/s oxygen and argon flow was equal to 56 sccm and 190 sccm, respectively. The microwave power was 2.7 kW, while power supplied to Ti and Si targets was 4 kW. The microwave plasma was activated for 5 min, providing additional cleaning of the substrates to remove organic residues. The results yield an anti-reflective, crack free coated surface, homogeneous and composed of small grains with maximum height of circa 7 nm to 12 nm [51].
Jeong et al. presents the deposition of dielectric films such as SiO 2 and TiO 2 at low substrate temperatures (<100 • C) by means of a sputtering technique. The process can be controlled with ease, which makes it a useful way to deposit high quality layers while maintaining good adhesion as well as coating uniformity [52]. The employed methodology of the sputtering technique involved the deposition of high purity (99.999% purity) ceramic targets of the materials (SiO 2 and TiO 2 ) on silicon wafers. The process was carried out in a high vacuum chamber (7X10 −6 mbar) in the presence of different O 2 /Ar+O 2 ratios. Further analysis of cross sectional transmission electron microscopy (TEM) images corroborated the variation of film structure towards a more dense and crystalline structure with the increase of temperature and also the presence of the anatase phase of TiO 2 [53].

Laser Ablation
Laser ablation is a combination of both vaporization and melt expulsion [54]. A focused beam of laser radiation strikes a surface, the electrons present in the substrate are excited by laser photons, the excitation results in generation of heat by absorption of photon energy. As consequence, the heating produces vaporization or fusion of the material. This transition results in the formation of a plasma plume. The initial heat generates a melt pool at the laser-substrate interaction zone. The temperature increases more with incoming pulses and the melt pool reaches the vaporization state, creating a high pressure, called recoil pressure, which pushes molten materials from the pool where they are ejected, increasing the temperature at the laser-substrate. The liquid reaches an explosive liquid-vapor phase transition stage, yielding geometric changes in the ablated features due to the resolidification of molten material [55,56]. Barberio et al. report that nanoparticles can be grown by laser ablation irradiated with the first harmonic of a Laser Nd:YAG (fluence of 500 mJ/cm 2 , pulse of 7 ns and a frequency of 20 Hz) in solution. Obtaining TiO 2 and grains or wafer of SiO 2 . The shape and dimensions of the produced nanoparticles were controlled measuring the optical absorption of the solution during the irradiation. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) images confirm the presence of particles with an average size of 9.82 nm and 14.83 nm for SiO 2 and TiO 2 nanoparticles, respectively, and very low standard deviations (in the range of 0.02-0.04 nm for both particles species). Also, this layer is perfectly transparent, uniform, and hydrophobic [57].

Chemical Methods
Chemical methods are processes that produce a chemical compound from a chemical precursor. The SiO 2 -TiO 2 compound is mainly obtained by chemical synthesis.

Sol-Gel
Sol-gel reactions have been extensively studied as a method to prepare organic, inorganic and ceramics compounds, at low temperature and pressure [58]. The process is divided into two reactions; hydrolysis and condensation. The hydrolysis replaces alkoxide groups with hydroxyl groups, generally catalyzed by the presence of an acidic or alkaline medium. During the hydrolysis and condensation reactions, secondary products of low-molecular-weight as ethanol and water molecules are generated. These molecules must be removed from the solutions [59,60]. Much research has focused on synthesizing ceramic compounds such as TiO 2 and SiO 2 independently by the sol-gel method as well as TiO 2 -SiO 2 composites due to its low cost, operation conditions and single step doping in situ.
Son et al. fabricated SiO 2 /TiO 2 core shell nanoparticles to investigate the influence of the size and refractive index of light-scattering particles on light-scattering properties [61,62]. On the other hand, Lee et al. synthesized SiO 2 /TiO 2 core shell particles with variable shell thickness by a multistep sol-gel coating process. Titania coating on the silica particles increased the electrophoretic mobility and the specific surface area increased depending on the number of coating steps, attributed to the textured titania coating layer [63].
The feasibility of the sol-gel method allows the modification of the synthesis of the SiO 2 -TiO 2 composite by adding modifiers and dopants that enhance its properties. Ang et al. prepared a series of nitrogen-doped TiO 2 -SiO 2 were prepared by a sol-gel method through varying the TiO 2 /SiO 2 molar ratio, employed TTIP and TEOS as precursors of TiO 2 and SiO 2 respectively. The results showed photocatalytic activity was modified for structural and morphology properties such as, amount of SiO 2 on the surface, TiO 2 particle size, surface area of TiO 2 , and the formation of Brønsted acid sites on the surface [64].On the other hand, Kim et al. reports the synthesis of a nitrogen doped SiO 2 /TiO 2 core shell by the sol-gel method in a stepwise manner, obtaining a completely encapsulated SiO 2 nanoparticle by the TiO 2 shell with uniform size distribution and enhanced photocatalytic activity [65]. Finally, Den et al. report a simple and practical approach to the facile production of TiO 2 -SiO 2 @PDMS -HDMS versatile hybrid films via a sol-gel process. Obtained by hydrolyzing TEOS in the presence of acetic acid and anhydrous ethanol, and a constant temperature of 70 • C for 20 h in a closed container [66].
Kitsou et al. presented the synthesis of SiO 2 @TiO 2 core shell nanoparticles modified with hyperbranched poly(ethylene)imine (HBPEI). HBPEI was added as dispersant to form a think homogeneous shell of nanostructure. The suspension was kept under magnetic stirring for 24 h to remove the excess of precursor. Obtaining a shell constituted of pure anatase particles of 7 nm in diameter in comparison with pure TiO 2 , which yields greater particles between 15 and 30 nm and a mixture of anatase and rutile phases. As a consequence, there was an increment of 2.5 times in the surface area [67].
Kapridaki et al. present the design and synthesis of a SiO 2 -TiO 2 coating with PDMS for potential application in monument conservation due to its optical and hydrophobic properties. An analysis of X-ray diffraction (XRD) presents the existence of an anatase phase of TiO 2 , with a crystallite size of 5 nm [68]. Continuing with the investigation, Kapridaki et al. studied different concentrations of oxalic acid. The study exhibits the high importance of oxalic acid as a catalyst during the synthesis, a crack-free agent over application and a hole-scavenger over photocatalytic properties [69]. On the other hand, Jianeje et al. studied the variation of amounts of PDMS over the SiO 2 -TiO 2 coating and found that the gelation time decreased as the amount of PDMS increased [70] Furthermore, over processing of condensation of the PDMS facilitates the increase in the degree of cross and consequence?a major flexibility [24].

Sonochemistry Coupled Sol-Gel
Sonochemistry is a new technique in the process of polymerization, catalytic chemistry reaction, synthesis of materials and synthesis of inorganic nanostructure with applications in science and technology [71,72]. This technique is called "green chemistry", for the short times of reactions [73]. Sonochemistry is a process of acoustic cavitation involving the fast growing and collapse of the microbubbles in liquid media in a very short period, generating hot spots with temperatures and pressure of around 5000 K and 1000 bar, respectively, with a collapse velocity of around −1360 m/s [74,75]. The principal reaction occurs inside the bubble. Due to these extreme conditions organic molecules could be destroyed directly or decomposed with radicals ( − O 3 H-, − O 2 , OH 2 , etc.) formed during the collapse of the microbubble. In new research, the use of sonochemistry makes a difference in reaction times and exhibits different structural properties [62,71]. Rosales et al. coupled the sol-gel method with sonochemistry for the synthesis of a SiO 2 @TiO 2 coating. The aim was to reduce the synthesis time of conventional sol-gel compared to a sonochemistry coupled method. It was proven that the sonochemistry energy was enough for the crystallization of the anatase phase of TiO 2 given the photocatalytic properties and the preparation of nano and micro roughness for hydrophobic properties, with the amorphous phase of SiO 2 [76].

Hydrothermal
Hydrothermal methods are an important branch of inorganic synthesis. We et al. describes the hydrothermal synthesis of new materials including single crystals, microporous, complex oxides, and inorganic-organic hybrid materials [77]. Grover et al. report a simple method of synthesizing titanium dioxide nano rods via a hydrothermal route using P25 TiO 2 as a precursor, obtained TiO 2 nanorods were calcined at 500 • C. SiO 2 -TiO 2 composite was prepared by mixing the previously obtained TiO 2 , SiO 2 precursor with ethanol, and concentrated H 2 SO 4 . The system was refluxed at 80-90 • C for 1 h. The results show a large difference in the surface area, showing major catalytic activity in the degradation of contaminants such as naphthalene and anthracene with respect to other catalysts [78]. On the other hand, Zhang et al. present the synthesis of SiO 2 @TiO 2 hybrid nanoparticles by a solvothermal method employing tetrabutyl titanate (Ti(OBu) 4 ) as a precursor of TiO 2 . As a result, the SiO 2 @TiO 2 hybrid nanoparticles, presented a core diameter of 437 nm and shell thickness of 50 nm. The XRD analysis confirm the presence of anatase phase of TiO 2 in the shell structure [79]

Chemical Vapor Deposition
Chemical Vapor Demposition (CVD) is currently used for depositing a wide variety of films on the float glass process in solid thin films. Obtaining a large density, good stoichiometry and uniformity over a large area. One of the main drawbacks of this method is that it requires a high temperature to produce high quality materials, and the substrate cannot tolerate being heated [80]. Nevertheless, it does not require as high a vacuum as some other processing methods, and films can be favorably deposited even at atmospheric pressure. The description of the deposition of SiO 2 -TiO 2 using CVD has rarely been reported. Lee et al. describe the deposition behavior of SiO 2 -TiO 2 thin films, using CVD processing and volatile alkoxide precursors. It was found that the process of deposition of SiO 2 and TiO 2 are not the same, due to disposition of the alkoxide radicals. Furthermore, the refractive indices of SiO 2 -TiO 2 thin films were determined with values of 1.5 and 2.3, respectively [81].
A summary of different synthesis methods of SiO 2 -TiO 2 composites are shown in Table 1. It is possible to observe that physical and chemical methods differ in equipment and synthesis conditions, note that physical methods requires sophisticated equipment with rigorous control on conditions. Furthermore, the resulting morphology and structure of the SiO 2 -TiO 2 composite can be tuned easily by applying chemical methods rather than the physical methods which have been used mostly for layer deposition [82]. Methods such as sputtering and laser ablation allow the control of the crystallite size, crystalline phase, uniform and thickness of layer of the SiO 2 -TiO 2 composite, such as Barberio et al.'s work on growing nanoparticles of TiO 2 with SiO 2 , controlling size, with a low deviation. On the other hand, the chemical methods are more frequently employed due to less sophisticated equipment and variable conditions of synthesis, which yields different properties of the material such as photocatalytic, hydrophobic, high porosity, and antireflectant. These methods allow the control of the size of the crystal, morphology, structure and dopants or modifiers such as PDMS [83], HDMS [84], nitrogen [85], among others. Work such as that by as Kapridaki  The different tunable morphology obtained through the different synthesis methods yields materials with specific physicochemical properties, which can be useful for a particular application. Figure 3 shows some of the different morphology that can be obtained through the physical and chemical synthesis methods, particularly, the core-shell morphology is better controlled through the sol-gel method. However, the versatility of the method allows the obtaining of other morphologies such as raspberry or nanoparticles. Nevertheless, the sol-gel method is not the only option as these morphologies can be achieved through hydrothermal synthesis or sonochemical synthesis. On the other hand, physical methods such as laser ablation are one of the better options when synthesizing thin films, given the sensibility of the technique that allows better thickness control and particle size.

Hydrophobicity
Kapridaki et al. reported the application of a SiO 2 -TiO 2 hybrid coating on marble surfaces. The marble hydrophobicity enhancement was presented with a 34% decrease of the water vapor permeability and a reduction of the water capillary coefficient. As a consequence, a notable increment of the contact angle in the treated surfaces was observed. The application of the coating on the marbles surfaces was carried out by brushing. The contact angle showed a diminution of 1-2 • after 20 min of UV-Vis irradiation [68]. On the other hand, Wu et al. reported an antireflection coating of modified SiO 2 @TiO 2 on glass substrates fabricated by using a dip-coating machine. The glass substrates was coated with urb-hSiO 2 @TiO 2 , after the coating was dried, a calcination process was carried out at 550 • C for 2 h. The initial CA was 107.9 • . Afterward, the coated glass substrates were irradiated with UV light. Results show that with a short time of irradiation the CA was rapidly decreased. After 8.5 h of irradiation the final CA was 0 • . This effect is attributed to the photocatalytics properties of TiO 2 . Furthermore, the decrease of the CA is attributed to the degradation of the surface hydrophobic alkyl chain [87]. Moreover, Mezur et al. determined the wettability of the antireflection SiO 2 -TiO 2 coating, based on the contact angle measurement. The CA was equal to 91.1 • and a roughness lower of 1.4 nm [51].

Antireflective Properties
Zhang et al. created a double-layered TiO 2 -SiO 2 nanostructure film with self-cleaning and antireflection properties. The films were prepared layer-by-layer employing different routes of synthesis. The results demonstrated that the combined properties of self cleaning and antireflection could be created in double-layered TiO 2 -SiO 2 nanostructured films [91]. Previously, Mioa et al. prepared a SiO 2 -TiO 2 coatings by depositing double-layers onto glass substrates using a combined sol-gel dip-coating process in which the two oxide layers were deposited in succession [92]. On the other hand, Ye et al. designed a material of triple-layer broadband abrasion-resistant and antireflective (AR), employing SiO 2 , TiO 2 and SiO 2 -TiO 2 hybrid. Synthesized through the sol-gel method using TBOT and TEOS as precursors, and employing HDMS to modify the surface of the AR coatings. The results showed an average transmittance of 98.4% in the visible region and CA variation in the function of HMDS concentration [93].
Mazur et al. prepared and studied an antireflection multilayer coating, employing a combination of five TiO 2 and SiO 2 thin films on glass substrates. The results showed amorphous structures with low surface roughness. The transmittance in the visible wavelength range was increased after the deposition of the AR coating as-compared to glass substrate, obtaining transmittances above 97% in the visible spectrum range with as low a number of layers as possible. Furthermore, the TiO 2 layer on the top is narrower with a thickness of 10 nm, with the purpose of maintaining the high transparency [51]. The recent research of Wu et al. reported that the modification of the SiO 2 @TiO 2 coating with an organic chemical compound and under the optimal conditions. The results showed an increase of the average transmittance and refractive index of 99.04% and 1.218, respectively, in the range of 400-800 nm, presenting an excellent antireflection performance [87].

Anti-Fogging
Tricoli et al. synthesized nanofibers of SiO 2 and nanoparticles of TiO 2 were deposited onto glass substrates by flame spray pyrolysis of organometallic solutions and stabilized by in situ flame annealing, finding that the minimal thickness of 200-300 nm of as deposited coatings or films was necessary to obtain full anti-fogging performance. The in situ annealed coatings preserved their super-hydrophilicity and anti-fogging functionality after being flushed with water [94]. Miyauchi et al. fabricated thin films of TiO 2 /SiO 2 /WO 3 , which exhibited excellent sustainability of the anti-fogging property even in indoor conditions, attributing this property the effect of photocatalytic oxidation activity and photoinduced hydrophilicity of TiO 2 and the hydrophilic property of SiO 2 [95]. Eshaghi et al. researched the thin film of SiO 2 /TiO 2 /SiO 2 deposited on glass substrates using an electron beam physical vapor deposition technique. The SiO 2 /TiO 2 /SiO 2 film showed higher transmittance and a lower refractive index in comparison to SiO 2 /TiO 2 thin film. They demonstrated that by covering TiO 2 thin film with a SiO 2 over layer the hydrophilicity was significantly improved and an anti-fogging effect was obtained. Therefore, they suggest that SiO 2 /TiO 2 /SiO 2 thin film would be very useful where super-hydrophilicity and an anti-fogging effect are desired for automobile and optical lens applications [96]. The presence of SiO 2 inhibited the growth of TiO 2 particles and the transformation from anatase phase into rutile phase. As a consequence of the thin and uniform amorphous SiO 2 layers, that was covering the surface of TiO 2 particles, inhibiting the growth of the particles and phase change of them. Furthermore, the high presence of amorphous SiO 2 layers obstruct the redox reaction between electrons and holes on the surface of the TiO 2 particles, diminishing the photocatalytic activity of the particle, and therefore the time of degradation of the colorant. Nonetheless, samples with content 5% mol of SiO 2 present anatase phase structure TiO 2 particles with an optimal photocatalytic activity, with an average particle size of 15.3 nm and a specific surface area of 77 m 2 /g [99].
Resende et al. prepared a composite of SiO 2 -TiO 2 particles with low molar ratio between them, synthesized through a hydrothermal method with a calcination of under 800 • C, the TiO 2 particles showed an anatase-phase structure and high specific surface [100]. On the other hand, Dong et al. prepared SiO 2 -TiO 2 nanocomposite with anatase phase of TiO 2 nanocrystals and amorphous SiO 2 nanoparticles, characteristics for mesoporous structure and high specific surface area and high photocatalytic activity with a stable repeatability [101]. Kitsou et al. present an optimum core-shell nanoparticles material that showed a better performance than that of pure titania, that is, 89.71% vs. 88.54% degradation percentage and a higher degradation rate (0.62 vs 0.55 lmicrog/m 2 s) [67,102].

Textile Application
Yuranova et al. investigated the photocatalytic activity of TiO 2 -SiO 2 coating over cotton textiles, colored with red wine. The cotton fibers were dip-coated with the mixed colloids of TiO 2 and SiO 2 . Afterward, a thermal treatment was carried out with the purpose of obtaining high dispersed TiO 2 particles surrounded by amorphous SiO 2 . Results showed a better efficiency of TiO 2 -SiO 2 -coated cotton samples than TiO 2 -coated cotton samples [103]. Moreover, Fakin et al. synthesized and characterized nanoparticles of TiO 2 -SiO 2 , with the purpose of being utilized in functional dyeing as combined reactive dyeing of cotton fabrics [104]. On the other hand, Veronoski et al. synthesized nanoparticles of TiO 2 and applied them to man-made cellulose fibers, which facilitates the photocatalysis activity [105]. Pakdel et al. functionalized wool fabrics using TiO 2 /SiO 2 nanocomposites through a low-temperature sol-gel method. The self-cleaning and hydrophilicity properties were evaluated in wool fabrics through the removal of coffee stains under UV irradiation [106]. Carrying on with the research led to the functionalization of cotton fabrics with TiO 2 /SiO 2 , yielding a synergistic function with the SiO 2 with the properties of TiO 2 [107].
Other research modified the composite with organic chemical compounds or inorganic compounds. Deng et al. obtained a TiO 2 -SiO 2 suspension with PDMS, which was applied to a polyester-cotton fabric with the purpose of obtaining flexibility, low-toxicity and a high contact angle in the coating. Results showed the initial CA was 158 • and the SA was 4 • . Afterward, the coated fabrics were tested to determine the washing durability and abrasion resistance; the diminution of the CA was of 10 • . Finally, after being attacked by strong basic the CA diminution was 32 • less. However, the analysis of TEM did not show morphology change [66]. Xu et al. prepared a coating of SiO 2 -TiO 2 modified with Methyltrimethoxysilane (MTMS). The cotton fabrics coated with the TiO 2 -SiO 2 composite particles exhibit simultaneous superhydrophobicity and a photocatalytic self-cleaning property [108]. Landi et al. studied the photocatalytic ability of SiO 2 -TiO 2 and SiO 2 -TiO 2 -HY (zeolite) coated textile substrates. Photocatalytic activity was evaluated by monitoring, over time, the degradation of Rhodamine B dye (RhB), a typical textile industry pollutant [109].
The properties and applications of the TiO 2 -SiO 2 composite are shown in Table 2. It is possible to observe that one of the most exploited properties of the TiO 2 -SiO 2 composite is the photocatalytic property, regardless of the substrate, namely, mortar, marbel, glass, or suspension due to the flexibility to adapt with the SiO 2 [110]. The TiO 2 -SiO 2 composite with photocatalytic properties present other properties such as antireflective and hydrophobic, the latter is normally complicated to achieve due to the nature of TiO 2 [111].

Conclusions
The diversity of physical and chemical synthesis methodologies allow us to obtain different morphologies, crystalline phases, and properties. Chemical methods offer a greater flexibility of synthesis with less stringent conditions. However, physical methods offer a control over thickness and uniformity layer by layer. TiO 2 and SiO 2 are typical optical thin films that exhibit high transparency and low absorption in the visible and near infrared wavelength range, but with a different reflection index.
Although SiO 2 does not exhibit photocatalytic properties, its high porosity and large specific surface area makes it suitable as an adsorbent. As a consequence, an increase of reactant concentration on the TiO 2 surface can be assessed, improving the photocatalytic activity of TiO 2 . Moreover, the presence of SiO 2 has a positive effect on the enlargement of TiO 2 specific surface area. It also inhibits the phase change and grain growth of TiO 2 and, overall, it contributes to the enhancement of the photocatalytic activity of TiO 2 , confirming the synergistic role of silica in this type of composite and in different substrates.
The hydrophobic properties of the composite SiO 2 -TiO 2 are of great interest for researchers in the field, however, the contribution of TiO 2 decreases these properties, yielding a low contact angle. Nevertheless, the addition of different compounds, such as PDMS or HDMS have proven to increase the contact angle and therefore the hydrophobic properties.
The applications of the SiO 2 -TiO 2 composite over different surfaces with notable roughness between substrates, such as textiles, mortar, glass or marble, is made possible due to the methods of synthesis, which offer different conditions of synthesis and allow us to add additives that chemically modify the surface and improve the properties of interest thus making it a tunable composite with a wide variety of applications.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
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