Journal of Nanomaterials

Journal of Nanomaterials / 2020 / Article
Special Issue

Smart Nanostructured Materials: From Molecular Self-Assembly to Advanced Applications

View this Special Issue

Research Article | Open Access

Volume 2020 |Article ID 5312376 | https://doi.org/10.1155/2020/5312376

Ning Fu, Xue-chang Ren, Jian-xin Wan, "The Effect of Molar Ratios of Ti/Si on Core-Shell SiO2@TiO2 Nanoparticles for Photocatalytic Applications", Journal of Nanomaterials, vol. 2020, Article ID 5312376, 11 pages, 2020. https://doi.org/10.1155/2020/5312376

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

Guest Editor: Pietro Calandra
Received17 Jan 2020
Revised02 Mar 2020
Accepted08 Apr 2020
Published27 Apr 2020

Abstract

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 450°C for 2 h. The N2 adsorption-desorption 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 176.79 m2/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.

1. Introduction

Titanium dioxide (TiO2), 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 [13]. The major drawbacks of TiO2 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  nm) and suppressing the quantum efficiency [46]. In addition, the physical properties of the TiO2 such as nanoparticles’ agglomeration, phase transformation, morphology, and particle size will also limit the catalytic efficiency and practical applications [79].

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 [1012]. The core nanoparticles include SiO2, polystyrene (PS), ZrO2, and Fe2O3 [1315]. SiO2, due to its stable surface chemistry, low cost, mechanical stability, and facilitation of good dispersion of TiO2, is one of the significant core materials for synthesizing the core-shell nanoparticles [16]. Besides, SiO2 can be easily and controllably prepared by the Stöber method [17].

Some researchers had focused their attention on photocatalytic application of the core-shell SiO2@TiO2 nanoparticles (CSTNs) with SiO2 as a core and TiO2 as a shell, and CSTNs showed better photocatalytic performance and improved photoactivity than pure TiO2 for improved adsorption, large surface area, and suitable porous structure[12, 1821]. 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 SiO2@TiO2 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 SiO2@TiO2 core-shell nanospheres with only three weight ratios of titanium (IV) isopropoxide (TTIP) : SiO2 (1 : 1, 2 : 1, and 3 : 1) for NO 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 , 1 : 2, 1 : 5, 1 : 8, 2 : 1, 5 : 1, and 8 : 1) were presented with SiO2 as a core and TiO2 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. Experimental

2.1. Chemicals

Titanium (IV) isopropoxide 95% (TTIP, Sigma-Aldrich), tetraethyl orthosilicate 99.9% (TEOS, Xiya Reagent Company), isopropanol, ethanol, and NH4OH (25%) were used in the synthesis process of CSTNs. KCl, KOH, and HCl (Sinopharm Chemical Reagent Company) were used in the test of surface charges. Phenol (Sinopharm Chemical Reagent Company) and methylene blue (MB, Sinopharm Chemical Reagent Company) were employed as the substrates for photocatalytic applications.

2.2. Synthesis of SiO2 Core Spheres

SiO2 core spheres were prepared using the Stöber method by hydrolysis and condensation of tetraethyl orthosilicate (TEOS) [10]. Firstly, 4 mL NH4OH and 15 mL H2O 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 (°C). Thirdly, the mixture was neutralized with 5 mol L−1 HCl and centrifuged at 4000 rpm for 10 min. Finally, the SiO2 core spheres were washed four times with ethanol and distilled water, then dried at 70°C for at least 20 h to obtain SiO2 core spheres.

2.3. Synthesis of Core-Shell SiO2@TiO2 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 SiO2 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 H2O : 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 H2O 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 TiO2 was also synthesized using 3 mL of TTIP by the same above procedures without SiO2 in the mixture.

2.4. 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 G2, 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% (). 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 TiO2-coated SiO2 nanoparticles can be calculated from the IEP data of the components by the following equation [22]: where and were the molecular weights of TiO2 and SiO2, respectively, and the subscript Ti/Si referred to the different molar ratios of Ti/Si nanoparticles, respectively.

2.5. 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 adsorption-desorption 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 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.

3. Results and Discussion

3.1. Morphological Characterization

The morphology of the core-shell SiO2@TiO2 nanoparticles (CSTNs) was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in Figures 1 and 2. As shown in Figures 1(a) and 2(a), the uncoated SiO2 particles showed smooth and spherical shape. The morphology of (Figure 1(b)) and (Figures 1(d) and 2(c)) had rough and textured surfaces compared with the SiO2 core particles, which confirmed that the titania was coated on SiO2 core particle [23]. Besides, the uniform coating of silica in and 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 (Figure 1(c)) and (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 , the free or coreless TiO2 nanoparticles agglomerated in addition to the deposition on the SiO2 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 SiO2 core particles had a narrow size distribution ( nm) (Figure 3(a)) and TiO2 nanoparticles had a size distribution of average 16.4 nm (Figures 2(b) and 3(b))). Furthermore, the average size of the was 333 nm for the uniform TiO2 shell coated on SiO2 particle. The size of CSTNs increased as the content of TiO2 increased in the Ti/Si molar ratio shown from Figure 3(b)3(f).

3.2. X-Ray Diffractometer (XRD) Analysis

The phase compositions and crystallinity of the SiO2, TiO2, and core-shell SiO2@TiO2 nanoparticles (CSTNs) were investigated by XRD in Figure 4. As shown in Figure 4(a), the TiO2 with 2θ peaks of 25.2°, 37.8°, 48.0°, 53.9°, 62.6°, and 75.1° corresponded to the anatase phase of TiO2 [28]; the average TiO2 crystallite sizes calculated by the Scherrer equation were around 18.0 nm which basically agreed with the transmission electron microscopy (TEM) results. Figures 3(b)3(e) indicate that the CSTNs with different Ti/Si molar ratios (1 : 1, 2 : 1, 5 : 1, and 8 : 1) showed the same 2θ peaks of anatase TiO2 phase but without sharp peaks due to the doped silica, which also confirmed that titania was coated on the SiO2 particles. The wide diffraction peak at was ascribed to the amorphous SiO2 in Figure 4(i), which would explain the spherical morphology and lower surface area values of SiO2 without preferential directions for crystal growth [2931]. Similarly, no detectable characteristic anatase TiO2 diffraction peaks occurred in (Figure 4(g)) and (Figure 4(h)) because of more SiO2 amorphous structure coated on it. It was reported that the crystallinity of SiO2-TiO2 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. N2 Adsorption-Desorption Isotherm Analysis

Figure 5 illustrates N2 adsorption-desorption isotherms of the synthesized samples. As shown in Figure 5(a), the adsorption isotherm of TiO2 nanoparticles can be categorized as type IV with hysteresis loop of type H2, which indicated the characteristic type of TiO2 nanoparticles with mesoporous materials [33]. The isotherms of the core-shell SiO2@TiO2 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 TiO2 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 SiO2 particles and CSTNs with the Ti/Si of 2 : 1, 5 : 1, and 8 : 1. The isotherms of exhibited the similar shape of type IV with H2 hysteresis loops and indicated the presence of mesopores. However, the isotherms of and showed the same isotherm shape of SiO2 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 TiO2 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 ( and ) did not show the pore size distributions like SiO2 (Figure 6(b)). In general, the pore size distributions presented the same trend of adsorption-desorption isotherms of the CSTNs as shown in Figure 5.


SampleBET surface area (m2g−1)Pore size (nm)Pore volume (cm3g−1)

SiO28.5141.570.004
TiO233.7813.390.136
122.537.670.258
167.066.620.311
176.796.190.312
174.705.920.297
102.028.670.242
24.9327.440.0946
14.1945.780.101

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 SiO2 nanoparticles had a specific surface area of 8.51 m2g−1 with an average pore size of 41.57 nm; the TiO2 nanoparticles had a specific surface area of 33.78 m2g−1 with an average pore size of 13.39 nm. The CSTNs of had the biggest specific surface area of 176.79 m2g−1, which was more than 5 times higher than the specific surface area of TiO2. 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 had the highest pore volume of 0.313 cm2g−1. It can be concluded that the CSTNs with the appropriate molar ratio of Ti/Si had significant effects on the samples’ specific surface area, pore size, and pore geometry [35].

3.4. 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 SiO2@TiO2 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 TiO2 while the IEP trends of the molar ratios of Ti/Si = 1 : 2, 1 : 5, and 1 : 8 were gradually close to SiO2. 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 TiO2 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.


SampleIEP (pH)ASC (%)

TiO25.29/
4.6074.3
4.7279.1
4.7881.4
4.8885.3
4.4568.2
3.8641.4
3.7636.5
SiO23.10/

3.5. Photocatalytic Activity

The photocatalytic activity of core-shell SiO2@TiO2 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 SiO2 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. TiO2 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 TiO2. 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 m2g−1 as shown in Table 1. Due to the higher content of TiO2 could lead to the formation of free coreless TiO2 nanoparticles and the subsequent aggregation in aqueous dispersions, the photoactivity of Ti/Si molar ratio of 8 : 1 was slightly lower than 5 : 1 [1012].

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 the photodegradation of phenol, similar trends can be observed as shown in 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 TiO2, 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 SiO2 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].

Compared with reported paper about the TiO2-SiO2 composites for photodegradation of phenol and MB as shown in Table 3 [3647], the CSTNs with Ti/Si molar ratio of 5 : 1 in this paper showed higher photocatalytic efficiency with less concentration of photocatalysis (0.25 g/L).


PhotocatalystSynthesis methodLight sourceInitial concentration of substrate (mg/L)Concentration of photocatalysis (g/L)Reaction time (min)Efficiency (%)Ref.

TiO2-SiO2 photocatalystSol-gelUV light (150 W)50 (phenol)112048[36]
Titania-silica compositesNonaqueous approachUV light (8 W)100 (phenol)318067[37]
TiO2-SiO2 catalystsHomogeneous precipitation methodUV light (125 W)500 (phenol)2.5640100[38]
TiO2-SiO2 aerogelsSol-gel methodUV light (15 W)50 (phenol)118042[39]
CSTNsHydrothermal methodUV light (500 W)20 (phenol)0.2512099.4This paper
TiO2/SiO2 nanoparticlesHydrothermal methodUV light (30 W)10 (MB)13596.4[40]
Titania-modified mesoporous silicatesImpregnation methodUV light (400 W)10 (MB)124096.0[41]
Silica–titania mixed oxidesSol-gel methodXenon lamp (0.68 W/m2) mol/L (MB)1218095[42]
TiO2-SiO2 mesoporous materialsHydrothermal methodUV light (125 W)40 (MB)0.311065[43]
Silica–titania photocatalystsHydrothermal methodUV light (125 W)20 (MB)536090[44]
SiO2/TiO2 nanoparticlesSol-gel methodXenon lamp (300 W)10 (MB)18097.7[45]
TiO2-SiO2 mixed oxidesSol-gel methodUV light (39 W) mol/L (MB)0.312076.7[46]
TiO2/SiO2 compositesSol-gel methodXenon lamp (300 W) mol/L (MB)0.36098[47]
CSTNsHydrothermal methodUV light (500 W)20 (MB)0.2512099.2This paper

There were several reasons for better photocatalytic activity with the proper molar ratio of 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].

4. Conclusions

In summary, we synthesized the core-shell SiO2@TiO2 nanoparticles (CSTNs) with different molar ratios of Ti/Si by hydrothermal method. The study showed that the TiO2 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 g/L photocatalyts compared with the other reported TiO2-SiO2 composites [3647].

Data Availability

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

Conflicts of Interest

The authors declare that there is no conflict of interests regarding the publication of this paper.

References

  1. N. S. Waldmann and Y. Paz, “Away from TiO2: a critical minireview on the developing of new photocatalysts for degradation of contaminants in water,” Materials Science in Semiconductor Processing, vol. 42, no. 6, pp. 72–80, 2016. View at: Publisher Site | Google Scholar
  2. J. Schneider, M. Matsuoka, M. Takeuchi et al., “Understanding TiO2 photocatalysis: mechanisms and materials,” Chemical Reviews, vol. 114, no. 19, pp. 9919–9986, 2014. View at: Publisher Site | Google Scholar
  3. S. Liu, N. Zhang, and Y. J. Xu, “Core-shell structured nanocomposites for photocatalytic selective organic transformations,” Particle & Particle Systems Characterization, vol. 31, no. 5, pp. 540–556, 2013. View at: Publisher Site | Google Scholar
  4. E. B. Moushoul, Y. Mansourpanah, K. Farhadi, and M. Tabatabaei, “TiO2 nanocomposite based polymeric membranes: a review on performance improvement for various applications in chemical engineering processes,” Chemical Engineering Journal, vol. 283, no. 124, pp. 29–46, 2016. View at: Publisher Site | Google Scholar
  5. B. Szczepanik, “Photocatalytic degradation of organic contaminants over clay-TiO2 nanocomposites: a review,” Applied Clay Science, vol. 141, no. 29, pp. 227–239, 2017. View at: Publisher Site | Google Scholar
  6. H. Zangeneh, A. A. L. Zinatizadeh, M. Habibi, M. Akia, and M. Hasnain Isa, “Photocatalytic oxidation of organic dyes and pollutants in wastewater using different modified titanium dioxides: a comparative review,” Journal of Industrial and Engineering Chemistry, vol. 26, pp. 1–36, 2015. View at: Publisher Site | Google Scholar
  7. N. Mandzy, E. Grulke, and T. Druffel, “Breakage of TiO2 agglomerates in electrostatically stabilized aqueous dispersions,” Powder Technology, vol. 160, no. 2, pp. 121–126, 2005. View at: Publisher Site | Google Scholar
  8. D. A. H. Hanaor, M. H. N. Assadi, S. Li, A. Yu, and C. C. Sorrell, “Ab initio study of phase stability in doped TiO2,” Computational Mechanics, vol. 50, no. 2, pp. 185–194, 2012. View at: Publisher Site | Google Scholar
  9. K. Raj and B. Viswanathan, “Effect of surface area, pore volume and particle size of P 25 titania on the phase transformation of anatase to rutile,” Indian Journal of Chemistry, vol. 48A, pp. 1378–1382, 2009. View at: Google Scholar
  10. S. Ullah, E. P. Ferreira-Neto, A. A. Pasa et al., “Enhanced photocatalytic properties of core@shell SiO2@TiO2 nanoparticles,” Applied Catalysis B: Environmental, vol. 179, no. 5, pp. 333–343, 2015. View at: Publisher Site | Google Scholar
  11. I. Kitsou, P. Panagopoulos, T. Maggos, M. Arkas, and A. Tsetsekou, “Development of SiO2@TiO2 core-shell nanospheres for catalytic applications,” Applied Surface Science, vol. 441, no. 5, pp. 223–231, 2018. View at: Publisher Site | Google Scholar
  12. E. Pakdel, W. A. Daoud, S. Seyedin et al., “Tunable photocatalytic selectivity of TiO2/SiO2 nanocomposites: Effect of silica and isolation approach,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 552, pp. 130–141, 2018. View at: Publisher Site | Google Scholar
  13. S. Son, S. H. Hwang, C. Kim, J. Y. Yun, and J. Jang, “Designed synthesis of SiO2/TiO2 core/shell structure as light scattering material for highly efficient dye-sensitized solar cells,” ACS Applied Materials & Interfaces, vol. 5, no. 11, pp. 4815–4820, 2013. View at: Publisher Site | Google Scholar
  14. W. Li, J. Yang, Z. Wu et al., “A versatile kinetics-controlled coating method to construct uniform porous TiO2 shells for multifunctional core-shell structures,” Journal of the American Chemical Society, vol. 134, no. 29, pp. 11864–11867, 2012. View at: Publisher Site | Google Scholar
  15. J. W. Lee, M. R. Othman, Y. Eom, T. G. Lee, W. S. Kim, and J. Kim, “The effects of sonification and TiO2 deposition on the micro-characteristics of the thermally treated SiO2/TiO2 spherical core–shell particles for photo- catalysis of methyl orange,” Microporous and Mesoporous Materials, vol. 116, no. 1-3, pp. 561–568, 2008. View at: Publisher Site | Google Scholar
  16. C. Anderson and A. J. Bard, “An improved photocatalyst of TiO2/SiO2 prepared by a sol-gel synthesis,” Journal of Chemical Physics, vol. 99, no. 24, pp. 9882–9885, 1995. View at: Publisher Site | Google Scholar
  17. T. Okuno, G. Kawamura, H. Muto, and A. Matsuda, “Photocatalytic properties of Au-deposited mesoporous SiO2–TiO2 photocatalyst under simultaneous irradiation of UV and visible light,” Journal of Solid State Chemistry, vol. 235, no. 12, pp. 132–138, 2016. View at: Publisher Site | Google Scholar
  18. X. C. Guo and D. Peng, “Multistep coating of thick titania layers on monodisperse silica nanospheres,” Langmuir, vol. 15, no. 17, pp. 5535–5540, 1999. View at: Publisher Site | Google Scholar
  19. S. A. Khan and K. F. Jensen, “Microfluidic synthesis of titania shells on colloidal silica,” Advanced Materials, vol. 19, no. 18, pp. 2556–2560, 2007. View at: Publisher Site | Google Scholar
  20. S. H. Lim, N. Phonthammachai, S. S. Pramana, and T. J. White, “Simple route to monodispersed silica-titania core-shell photocatalysts,” Langmuir, vol. 24, no. 12, pp. 6226–6231, 2008. View at: Publisher Site | Google Scholar
  21. C. Kang, L. Jing, T. Guo, H. Cui, J. Zhou, and H. Fu, “Mesoporous SiO2-modified nanocrystalline TiO2with high anatase thermal stability and large surface area as efficient photocatalyst,” The Journal of Physical Chemitry C, vol. 113, no. 3, pp. 1006–1013, 2009. View at: Publisher Site | Google Scholar
  22. J. W. Lee, S. Kong, W. S. Kim, and J. Kim, “Preparation and characterization of SiO2/TiO2 core-shell particles with controlled shell thickness,” Materials Chemistry and Physics, vol. 106, no. 1, pp. 39–44, 2007. View at: Publisher Site | Google Scholar
  23. H. Eskandarloo, A. Badiei, M. A. Behnajady, and M. Afshar, “Enhanced photocatalytic removal of phenazopyridine by using silver-impregnated SiO2-TiO2 nanoparticles: optimization of synthesis variables,” Research on Chemical Intermediates, vol. 41, no. 12, pp. 9929–9949, 2015. View at: Publisher Site | Google Scholar
  24. S. Kamaruddin and D. Stephan, “The preparation of silica–titania core–shell particles and their impact as an alternative material to pure nano-titania photocatalysts,” Catalysis Today, vol. 161, no. 1, pp. 53–58, 2011. View at: Publisher Site | Google Scholar
  25. E. P. Ferreira-Neto, S. Ullah, M. B. Simões et al., “Solvent-controlled deposition of titania on silica spheres for the preparation of SiO2@TiO2 core@shell nanoparticles with enhanced photocatalytic activity,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 570, pp. 293–305, 2019. View at: Publisher Site | Google Scholar
  26. P. Calandra, A. Ruggirello, A. Pistone, and V. Turco Liveri, “Structural and optical properties of novel surfactant coated TiO2–Ag based nanoparticles,” Journal of Cluster Science, vol. 21, no. 4, pp. 767–778, 2010. View at: Publisher Site | Google Scholar
  27. P. Calandra, D. Lombardo, A. Pistone, V. Turco Liveri, and S. Trusso, “Structural and optical properties of novel surfactant-coated Yb@TiO2 nanoparticles,” Journal of Nanoparticle Research, vol. 13, no. 11, pp. 5833–5839, 2011. View at: Publisher Site | Google Scholar
  28. S. Kalele, R. Dey, N. Hebalkar, J. Urban, S. W. Gosavi, and S. K. Kulkarni, “Synthesis and characterization of silica-titania core-shell particles,” Pramana-Journal of Physics, vol. 65, no. 5, pp. 787–791, 2005. View at: Publisher Site | Google Scholar
  29. E. Y. Kim, C. M. Whang, W. I. Lee, and Y. H. Kim, “Photocatalytic property of SiO2/TiO2 nanoparticles prepared by solhydrothermal process,” Journal of Electroceramics, vol. 17, no. 2-4, pp. 899–902, 2006. View at: Publisher Site | Google Scholar
  30. J. R. Martínez, S. Palomares-Sánchez, G. Ortega-Zarzosa, F. Ruiz, and Y. Chumakov, “Rietveld refinement of amorphous SiO2 prepared via sol–gel method,” Materials Letters, vol. 60, no. 29-30, pp. 3526–3529, 2006. View at: Publisher Site | Google Scholar
  31. G. Zhang, Y. Xu, D. Xu, D. Wang, Y. Xue, and W. Su, “Pressure-induced crystallization of amorphous SiO2 with silicon-hydroxy group and the quick synthesis of coesite under lower temperature,” High Pressure Research, vol. 28, no. 4, pp. 641–650, 2008. View at: Publisher Site | Google Scholar
  32. Z. Li, B. Hou, Y. Xu et al., “Comparative study of sol–gel-hydrothermal and sol–gel synthesis of titania–silica composite nanoparticles,” Journal of Solid State Chemistry, vol. 178, no. 5, pp. 1395–1405, 2005. View at: Publisher Site | Google Scholar
  33. H. H. Choi, J. Park, and R. K. Singh, “Nanosized titania encapsulated silica particles using an aqueous TiCl4 solution,” Applied Surface Science, vol. 241, no. 1-4, pp. 7–12, 2005. View at: Publisher Site | Google Scholar
  34. S. Falahatdoost, M. H. M. Ara, Z. Shaban, and N. Ghazyani, “Optical investigation of shell thickness in light scattering SiO2 particle with TiO2 nanoshells and its application in dye sensitized solar cells,” Optical Materials, vol. 47, pp. 51–55, 2015. View at: Publisher Site | Google Scholar
  35. N. Fu, X. C. Ren, and J. X. Wan, “Preparation of Ag-coated SiO2@TiO2 core-shell nanocomposites and their photocatalytic applications towards phenol and methylene blue degradation,” Journal of Nanomaterials, vol. 2019, Article ID 8175803, 8 pages, 2019. View at: Publisher Site | Google Scholar
  36. K. M. Fuentes, P. Betancourt, S. Marrero, and S. García, “Photocatalytic degradation of phenol using doped titania supported on photonic SiO2 spheres,” Mechanisms and Catalysis, vol. 120, no. 1, pp. 403–415, 2017. View at: Publisher Site | Google Scholar
  37. J. Jammaer, C. Aprile, S. W. Verbruggen, S. Lenaerts, P. P. Pescarmona, and J. A. Martens, “A non-aqueous synthesis of TiO2/SiO2 composites in supercritical CO2 for the photodegradation of pollutants,” ChemSusChem, vol. 4, no. 10, pp. 1457–1463, 2011. View at: Publisher Site | Google Scholar
  38. L. Alemany, M. Banares, E. Pardo, F. Martin, M. Galanfereres, and J. Blasco, “Photodegradation of phenol in water using silica-supported titania catalysts,” Applied Catalysis B: Environmental, vol. 13, no. 3-4, pp. 289–297, 1997. View at: Publisher Site | Google Scholar
  39. Z. Deng, J. Wang, Y. Zhang et al., “Preparation and photocatalytic activity of TiO2-SiO2 binary aerogels,” Nanostructured Materials, vol. 11, no. 8, pp. 1313–1318, 1999. View at: Publisher Site | Google Scholar
  40. Z. Bo, R. Dong, C. Jin, and Z. Chen, “High photocatalytically active cocoons-like TiO2/SiO2 synthesized by hydrothermal process and subsequent calcination at 900°C,” Materials Science in Semiconductor Processing, vol. 72, pp. 9–14, 2017. View at: Publisher Site | Google Scholar
  41. A. A. Belhekar, S. V. Awate, and R. Anand, “Photocatalytic activity of titania modified mesoporous silica for pollution control,” Catalysis Communications, vol. 3, no. 10, pp. 453–458, 2002. View at: Publisher Site | Google Scholar
  42. P. Periyat, K. V. Baiju, P. Mukundan, P. K. Pillai, and K. G. K. Warrier, “High temperature stable mesoporous anatase TiO2 photocatalyst achieved by silica addition,” Applied Catalysis A: General, vol. 349, no. 1-2, pp. 13–19, 2008. View at: Google Scholar
  43. B. Mazinani, A. Beitollahi, S. Radiman et al., “The effects of hydrothermal temperature on structural and photocatalytic properties of ordered large pore size TiO2-SiO2 mesostructured composite,” Journal of Alloys and Compounds, vol. 519, pp. 72–76, 2012. View at: Publisher Site | Google Scholar
  44. I. Krivtsov, M. Ilkaeva, V. Avdin et al., “A hydrothermal peroxo method for preparation of highly crystalline silica-titania photocatalysts,” Journal of Colloid and Interface Science, vol. 444, pp. 87–96, 2015. View at: Publisher Site | Google Scholar
  45. L. Wu, Y. Zhou, W. Nie, L. Song, and P. Chen, “Synthesis of highly monodispersed teardrop–shaped core-shell SiO2/TiO2 nanoparticles and their photocatalytic activities,” Applied Surface Science, vol. 351, pp. 320–326, 2015. View at: Publisher Site | Google Scholar
  46. V. D. Chinh, A. Broggi, L. Di Palma et al., “XPS spectra analysis of Ti2+, Ti3+ ions and dye photodegradation evaluation of titania-silica mixed oxide nanoparticles,” Journal of Electronic Materials, vol. 47, no. 4, pp. 2215–2224, 2017. View at: Publisher Site | Google Scholar
  47. W. Chang, L. Yan, B. Liu, and R. Sun, “Photocatalyic activity of double pore structure TiO2/SiO2 monoliths,” Ceramics International, vol. 43, no. 8, pp. 5881–5886, 2017. View at: Publisher Site | Google Scholar
  48. Y. Arai, K. Tanaka, and A. L. Khlaifat, “Photocatalysis of SiO2-loaded TiO2,” Journal of Molecular Catalysis A: Chemical, vol. 243, no. 1, pp. 85–88, 2006. View at: Publisher Site | Google Scholar
  49. S. Rasalingam, R. Peng, and R. T. Koodali, “Removal of hazardous pollutants from wastewaters: applications of TiO2-SiO2 mixed oxide materials,” Journal of Nanomaterials, vol. 2014, Article ID 617405, 42 pages, 2014. View at: Publisher Site | Google Scholar

Copyright © 2020 Ning Fu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


More related articles

 PDF Download Citation Citation
 Download other formatsMore
 Order printed copiesOrder
Views433
Downloads453
Citations

Related articles

Article of the Year Award: Outstanding research contributions of 2020, as selected by our Chief Editors. Read the winning articles.