The Effect of Molar Ratios of Ti/Si on Core-Shell SiO2@TiO2 Nanoparticles for Photocatalytic Applications

After the core-shell SiO2@TiO2 nanoparticles (CSTNs) were synthesized by hydrothermal method, we investigated the influence of different molar ratios of Ti/Si on morphology, structure, and photocatalytic activity of the CSTNs. It was found that the CSTNs showed different size and surface morphology as the Ti/Si molar ratio changed. Besides, the TiO2 and the CSTN had the anatase phase after hydrothermal process and calcination at 450C for 2 h. The N2 adsorptiondesorption isotherms demonstrated the CSTNs with the molar ratio of Ti/Si increased from 1 : 1 to 8 : 1 can be categorized as type IV with hysteresis loop of type H2 and showed to be mesoporous materials. In addition, the CSTNs with the Ti/Si molar ratio of 5 : 1 had the highest surface area of 176.79m/g. Surface charges showed the isoelectric point (IEP) of the CSTNs ranged between silica (IEP at pH 3.10) and titania (IEP at pH 5.29). Since the molar ratio of Ti/Si increased from 1 : 1 to 8 : 1 by degradating both colorless organic pollutant of phenol and colored substances of methylene blue (MB) under UV irradiation, the photocatalytic activity of CSTNs exhibited higher photodegradation efficiency compared with TiO2. What is more, the experimental results also showed the CSTNs with Ti/Si molar ratio of 5 : 1 had the highest photocatalytic activity and showed higher photocatalytic efficiency compared with other TiO2-SiO2 composites reported for photodegradation of phenol and MB.


Introduction
Titanium dioxide (TiO 2 ), one of the most used photocatalysts for photodegradating inorganic and organic pollutants, has the advantages of low cost, chemical stability, low toxicity, high physical, easy availability, and excellent photoactivity [1][2][3]. The major drawbacks of TiO 2 arise from the wide band gap (3.2 eV for anatase) and rapid charge recombination of the electron-hole pairs, which restrict the light absorption (just ultraviolet region with wavelength < 390 nm) and suppressing the quantum efficiency [4][5][6]. In addition, the physical properties of the TiO 2 such as nanoparticles' agglomeration, phase transformation, morphology, and particle size will also limit the catalytic efficiency and practical applications [7][8][9].
In order to conquer these problems, core-shell nanoparticles with titania as shell have been studied for photocatalytic degradation due to their stability, dispersibility, and higher surface area, reducing the recombination efficiency of electron hole and improving the quantum efficiency of photocatalyst compared to either the core or the shell materials [10][11][12]. The core nanoparticles include SiO 2 , polystyrene (PS), ZrO 2 , and Fe 2 O 3 [13][14][15]. SiO 2 , due to its stable surface chemistry, low cost, mechanical stability, and facilitation of good dispersion of TiO 2 , is one of the significant core materials for synthesizing the core-shell nanoparticles [16]. Besides, SiO 2 can be easily and controllably prepared by the Stöber method [17].
Some researchers had focused their attention on photocatalytic application of the core-shell SiO 2 @TiO 2 nanoparticles (CSTNs) with SiO 2 as a core and TiO 2 as a shell, and CSTNs showed better photocatalytic performance and improved photoactivity than pure TiO 2 for improved adsorption, large surface area, and suitable porous structure [12,[18][19][20][21]. However, the research that has been reported mainly focus on the morphology of the CSTNs or how to control the uniform and spherical in shape, and just a few different weights or molar ratios of Ti/Si have been synthesized for photocatalysis with CSTNs. For example, Ullah et al. [10] synthesized core@shell SiO 2 @TiO 2 nanoparticles with four of different weight proportions of titanium (IV) isopropoxide (TTIP) (10.4%, 20.6%, 31%, and 35%) towards degradation of crystal violet (CV). Kitsou et al. [11] produced SiO 2 @TiO 2 core-shell nanospheres with only three weight ratios of titanium (IV) isopropoxide (TTIP) : SiO 2 (1 : 1, 2 : 1, and 3 : 1) for NO x degradation. Besides, few works indicated the photocatylytic applications for both colorless organic pollutants (such as phenol) and colored substances (such as methylene blue).
In this paper, the different molar ratios of the CSTNs (seven different molar ratios of Ti/Si = 1 : 1, 1 : 2, 1 : 5, 1 : 8, 2 : 1, 5 : 1, and 8 : 1) were presented with SiO 2 as a core and TiO 2 as a shell by hydrothermal method. The surface morphology and physical properties of the CSTNs were investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) specific surface, and surface charges. It was found that high photocatalytic efficiency was shown with low photocatalyst addition by studying the photocatalytic activity of the CSTNs for both colorless organic pollutants (such as phenol) and colored substances (such as methylene blue).
2.2. Synthesis of SiO 2 Core Spheres. SiO 2 core spheres were prepared using the Stöber method by hydrolysis and condensation of tetraethyl orthosilicate (TEOS) [10]. Firstly, 4 mL NH 4 OH and 15 mL H 2 O were added to 100 mL ethanol and stirring for 30 min in a Teflon reactor. Secondly, 3.0 mL of TEOS was quickly added to the above mixture under continuous stirring for 3 h at room temperature (25 ± 2°C). Thirdly, the mixture was neutralized with 5 mol L −1 HCl and centrifuged at 4000 rpm for 10 min. Finally, the SiO 2 core spheres were washed four times with ethanol and distilled water, then dried at 70°C for at least 20 h to obtain SiO 2 core spheres.
2.3. Synthesis of Core-Shell SiO 2 @TiO 2 Nanoparticles (CSTNs). The CSTNs with different molar ratios of Ti/Si were synthesized according to the literatures with minor modifications [10]. The process is as follows: different weight of SiO 2 core spheres were dried at 110°C for 1 h and then sonicated in 80 mL isopropanol for 1 h; then, different volumes of titanium (IV) isopropoxide (TTIP) was quickly added with vigorous stirring for 24 h (different molar ratios of Ti/Si were 1 : 1, 2 : 1, 5 : 1, 8 : 1, 1 : 2, 1 : 5, and 1 : 8). Subsequently, 15 mL water-alcohol mixture (5 mL H 2 O : 10 mL isopropanol) was slowly added (2 mL min −1 ) to the above mixture and stirred for 3 h. The resulting precipitates were washed once with isopropanol and twice with deionized water at 8000 rpm. Then, the amorphous CSTNs were suspended in 50 mL H 2 O and processed to hydrothermal treatment at 105°C for 24 h, then centrifuged at 8000 rpm for 10 min. Finally the obtained CSTNs were dried at 70°C for 20 h and calcined at 450°C for 2 h. The unsupported TiO 2 was also synthesized using 3 mL of TTIP by the same above procedures without SiO 2 in the mixture.

Characterization
Methods. The size and surface morphology of the synthesized nanoparticles were examined using scanning electron microscopy (SEM) (JSM-6701F, Japan) and transmission electron microscopy (TEM) (Tecnai G 2 , American). The crystalline structure of the samples was investigated by X-ray diffraction (XRD) using Cu-Kα radiation at 40 kV and 30 mA. The specific surface area and pore size of the samples were estimated from nitrogen adsorption curves using the Brunaer-Emmett-Teller (BET) method employing ASAP 2020 (Micromeritics Instrument Corporation, USA) surface area analyzer. The pore size distribution was determined based on using the Barrett-Joyner-Halenda (BJH) model by the same instrument.
The surface charges of the samples were measured using an electrophoresis instrument (Model ZEN3690, Malvern, UK). The samples were dispersed in a 0.003 mol L −1 KCl electrolyte solution to give a final concentration of 0.01% (w/v). The pH of the suspension was varied by adding 0.01 mol L −1 HCl or 0.01 mol L −1 KOH to the suspension, and the isoelectric point (IEP) was taken at which the zeta potential was zero. The apparent surface coverage (ASC) of TiO 2 -coated SiO 2 nanoparticles can be calculated from the IEP data of the components by the following equation [22]: where M TiO2 and M SiO2 were the molecular weights of TiO 2 and SiO 2 , respectively, and the subscript Ti/Si referred to the different molar ratios of Ti/Si nanoparticles, respectively.

Photocatalytic Activity Experiments.
The photocatalytic activity of the prepared samples was evaluated by measuring the photodegradation of phenol and methylene blue (MB) in a reactor under UV light irradiation (high-pressure mercury lamp with 500 W) for 120 minutes. In a typical experiment, 75 mg of prepared photocatalysts were dispersed in 300 mL phenol or MB aqueous solution with initial concentration of 20 mg/L. The mixture was first stirred in the dark for 30 minutes to achieve adsorptiondesorption equilibrium and then was carried out under UV light. 4 mL of the solution was collected from the reactor at different irradiation time intervals. The collected samples were centrifuged to separate the suspended 2 Journal of Nanomaterials particles. The concentration changes of phenol were analyzed by colorimetric method of 4-amino antipyrine at 510 nm, and the MB were analyzed by recording the maximum absorbance of MB at 664 nm.

Results and Discussion
3.1. Morphological Characterization. The morphology of the core-shell SiO 2 @TiO 2 nanoparticles (CSTNs) was    Figures 1(d) and 2(c)) had rough and textured surfaces compared with the SiO 2 core particles, which confirmed that the titania was coated on SiO 2 core particle [23]. Besides, the uniform coating of silica in Ti/Si = 1 : 2 and Ti/Si = 1 : 5 also indicated that less titania molar content was conducive to the formation of core-shell structure by the hydrolysis-polycondensation process [24]. In contrast, the morphology of Ti/Si = 2 : 1 (Figure 1(c)) and Ti/Si = 5 : 1 (Figures 1(e) and 1(f)) had a rougher and more textured surface which can also be shown from TEM in Figures 2(c) and 2(d). Especially from the SEM and TEM morphology of Ti/Si = 5 : 1, the free or coreless TiO 2 nanoparticles agglomerated in addition to the deposition on the SiO 2 surface and formed an irregular core-shell structure [25].
The particle size distributions were also determined from SEM and TEM images in Figure 3 according to the literatures [26,27]. The SiO 2 core particles had a narrow   ). Furthermore, the average size of the Ti/Si = 1 : 5 was 333 nm for the uniform TiO 2 shell coated on SiO 2 particle. The size of CSTNs increased as the content of TiO 2 increased in the Ti/Si molar ratio shown from Figure 3 3.2. X-Ray Diffractometer (XRD) Analysis. The phase compositions and crystallinity of the SiO 2 , TiO 2 , and core-shell SiO 2 @TiO 2 nanoparticles (CSTNs) were investigated by XRD in Figure 4. As shown in Figure 4 showed the same 2θ peaks of anatase TiO 2 phase but without sharp peaks due to the doped silica, which also confirmed that titania was coated on the SiO 2 particles. The wide diffraction peak at 2θ = 23°was ascribed to the amorphous SiO 2 in Figure 4(i), which would explain the spherical morphology and lower surface area values of SiO 2 without preferential directions for crystal growth [29][30][31]. Similarly, no detectable characteristic anatase TiO 2 diffraction peaks occurred in Ti/Si = 1 : 5 (Figure 4(g)) and Ti/Si = 1 : 8 (Figure 4(h)) because of more SiO 2 amorphous structure coated on it. It was reported that the crystallinity of SiO 2 -TiO 2 composites decreased while the molar ratio of silica increased, and this behaviour was explained by the fact that the silica suppressed the growth of titania [32].
3.3. N 2 Adsorption-Desorption Isotherm Analysis. Figure 5 illustrates N 2 adsorption-desorption isotherms of the 5 Journal of Nanomaterials synthesized samples. As shown in Figure 5(a), the adsorption isotherm of TiO 2 nanoparticles can be categorized as type IV with hysteresis loop of type H2, which indicated the characteristic type of TiO 2 nanoparticles with mesoporous materials [33]. The isotherms of the core-shell SiO 2 @TiO 2 nanoparticles (CSTNs) with the molar ratios of Ti/Si (1 : 1, 2 : 1, 5 : 1, and 8 : 1) exhibited the similar shape of type IV with H2 hysteresis loops ( Figure 5(a)) and showed the characteristic type of mesoporous materials as the TiO 2 nanoparticles. Besides, the adsorbed volume enhanced as the molar ratios of Ti/Si increased from 1 : 1 to 8 : 1, due to the coated amorphous silica on the CSTNs as shown in X-ray diffractometer (XRD) analysis. Figure 5(b) shows the isotherms of the SiO 2 particles and CSTNs with the Ti/Si of 2 : 1, 5 : 1, and 8 : 1. The isotherms of Ti/Si = 1 : 2 exhibited the similar shape of type IV with H2 hysteresis loops and indicated the presence of mesopores. However, the isotherms of Ti/Si = 1 : 5 and Ti/Si = 1 : 8 showed the same isotherm shape of SiO 2 particles with low adsorbed volume and type I shape without any obvious hysteresis loop, which was attributed to the less mesoporous titania coated on the CSTNs [34].
The pore size distributions were shown in Figure 6 the Brunauer-Emmett-Teller (BET) surface areas and the pore size parameters were tested (Table 1) by using the Barrett-Joyner-Halenda (BJH) method from the adsorption branch of the isotherm. As shown in Figure 6(a), the TiO 2 nanoparticles had monomodal pore size distribution with a maximum pore diameter of about 13 nm. The pore size distributions of the CSTNs were more obvious, because the Ti/Si increased from 1 : 1 to 8 : 1 (Figure 6(a)); thus, more titanium dioxide was coated. However, the CSTNs with high silica molar ratios (Ti/Si = 1 : 5 and Ti/Si = 1 : 8) did not show the pore size distributions like SiO 2 (Figure 6(b)). In general, the pore size dis-tributions presented the same trend of adsorption-desorption isotherms of the CSTNs as shown in Figure 5.
Following this, we measured the specific surface area, pore size, and pore volume of the CSTNs, which are listed in Table 1. The results showed that SiO 2 nanoparticles had a specific surface area of 8.51 m 2 g −1 with an average pore size of 41.57 nm; the TiO 2 nanoparticles had a specific surface area of 33.78 m 2 g −1 with an average pore size of 13.39 nm. The CSTNs of Ti/Si = 5 : 1 had the biggest specific surface area of 176.79 m 2 g −1 , which was more than 5 times higher than the specific surface area of TiO 2 . However, the specific surface area of the CSTNs decayed as the molar ratios of Ti/Si decreased from 1 : 1 to 1 : 8. The pore volume of the CSTNs showed the same regularity of the specific surface area, and the CSTNs of Ti/Si = 5 : 1 had the highest pore volume of 0.313 cm 2 g −1 . It can be concluded that the CSTNs with the appropriate molar ratio of Ti/Si had   Journal of Nanomaterials significant effects on the samples' specific surface area, pore size, and pore geometry [35].

Isoelectric Point (IEP)
Analysis. Surface charges were measured by the isoelectric point (IEP) and apparent surface coverage (ASC) of the synthesized nanoparticles in Figure 7 and Table 2. It was reported that surface charge affected the adsorption process of organic molecules and the driving force of hole migration on the surface of photocatalyst [10,22]. As shown in Figure 7, generally, the IEP of the core-shell SiO 2 @TiO 2 nanoparticles (CSTNs) ranged between silica (IEP at pH 3.10) and titania (IEP at pH 5.29); the IEP trends of the CSTNs with the molar ratios of Ti/Si = 1 : 1, 2 : 1, 5 : 1, and 8: 1 were gradually close to TiO 2 while the IEP trends of the molar ratios of Ti/Si = 1 : 2, 1 : 5, and 1 : 8 were gradually close to SiO 2 . It can be concluded that more molar ratio of titania coating on CSTNs increased the electrophoretic mobility and shifted the IEP to the similar value of TiO 2 particles, which showed the similar trend in the literature [36]. The apparent surface coverage (ASC) of the CSTNs showed the ASC increased from 74.3% to 85.3% as the molar ratios of Ti/Si increased from 1 : 1 to 8 : 1 (Table 2); the results were in accord with IEP of the CSTNs. Besides, the ASC results also indicated that silica particles possessed a discontinuous titania nanoparticle coating layer.
3.5. Photocatalytic Activity. The photocatalytic activity of core-shell SiO 2 @TiO 2 nanoparticles (CSTNs) was showed in Figure 8 by photodegradating the substrates of phenol and methylene blue (MB) under UV light irradiation. As shown in Figure 8(a), all the nanocomposites did not show photocatalytic activity during the dark reaction, which demonstrated the nanocomposites had no adsorption efficiency towards colorless phenol under dark condition. Under the UV light irradiation, both the natural degradation and SiO 2 showed very low photocatalytic efficiency, but for the photodegradation efficiency of the CSTNs gradually increased as the molar ratio of Ti/Si increased from 1: 8 to 8: 1. TiO 2 showed higher photocatalytic activity than CSTNs with silica increased in the molar ratios of Ti/Si form 1: 1 to 1: 8, while the photocatalytic activities of the CSTNs with the molar ratios of Ti/Si = 2: 1, 5: 1 and 8: 1 were higher than the TiO 2 . The Ti/Si molar ratio of 5: 1 possessed the highest photodegradation efficiency of 99.4% under UV light photodegradation for 120 minutes because of the highest specific surface area of 176.79 m 2 g −1 as shown in Table 1. Due to the higher content of TiO 2 could lead to the formation of free coreless TiO 2 nanoparticles and the subsequent aggregation in aqueous dispersions, the photoactivity of Ti/Si molar ratio of 8 : 1 was slightly lower than 5 : 1 [10][11][12].
We evaluated the photocatalytic activity of CSTNs for the photodegradation of MB which was showed in Figure 8(b). During the dark reaction, the nanoparticles had different adsorption efficiency towards MB and the Ti/Si molar ratio of 5 : 1 showed the highest adsorption efficiency, which was ascribed to the mesoporous materials with colored organic dyes adsorption [31]. Compared with   Figure 8(b); the CSTNs with the molar ratio of Ti/Si from 1 : 1 to 8 : 1 showed high photodegradation efficiency of more than 95% after UV irradiation for 80 minutes. The Ti/Si molar ratio of 5 : 1 also had the highest photocatalytic activity of 99.2% after 120 minutes of UV light photodegradation.
For CSTNs with different molar ratios of Ti/Si, the higher molar ratios of titania in Ti/Si induced agglomeration of excess TiO 2 , and poor dispersibility in aqueous solution. In addition, too much titania can reduce the specific surface area, occlude the pores, and limit the diffusion of the reactant molecules. In contrast, with higher molar ratios of SiO 2 in Ti/Si, although the CSTNs showed spherical and uniform core-shell structure, the reaction sides of titania reduced and the crystallinity was low too, so the photocatalytic efficiency would be reduced [25].
There were several reasons for better photocatalytic activity with the proper molar ratio of Ti/Si = 5 : 1 in the CSTNs. Firstly, a proper molar ratio of Ti/Si both enhanced the thermal stability and increased the surface area and surface acidity. At the meantime, higher surface area and mesoporous structure provided more adsorption sites and photocatalytic reaction centers, which can enhance the photocatalytic reaction of phenol and MB. It is noted that a suitable molar ratio of Ti/Si may lead to an increase in the surface defects, which was reported to have the ability to capture the photocatalytic carriers and increase the reactive activity of hydroxyl [48,49].

Conclusions
In summary, we synthesized the core-shell SiO 2 @TiO 2 nanoparticles (CSTNs) with different molar ratios of Ti/Si by hydrothermal method. The study showed that the TiO 2 and CSTNs had the anatase phase after hydrothermal method and calcination at 450°C for 2 h. Furthermore, we evaluated the morphology, structure, and other properties of the CSTNs by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area, and surface charges. The BET surface area results indicated that the different molar ratios of Ti/Si affected the specific surface area on the CSTN; for the meantime, the CSTNs with the molar ratios of Ti/Si increased from 1 : 1 to 8 : 1 had the mesoporous structures which were categorized as type IV and hysteresis loop of type H2. Importantly, the surface charge results indicated that isoelectric point (IEP) of CSTNs ranged between 3.10 (silica) and 5.29 (titania), and the apparent surface coverage (ASC) ranged from 36.5% (Ti/Si molar ratio of 1 : 8) to 85.3% (Ti/Si molar ratio of 8 : 1). The photocatalytic activity of the CSTNs showed different photodegradation efficiencies with different molar ratios of Ti/Si towards colorless phenol and colored methylene blue (MB) under UV irradiation caused by different consistency of surface area and suitable porous structure. Finally, the experimental results showed the CSTNs with the Ti/Si molar ratios of 5 : 1 had the highest photocatalytic activity of 99.4% towards phenol and 99.2% towards MB, which showed higher photocatalytic efficiency with the addition of 0. 25

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.