Journal of Nanomaterials
Volume 2019 (2019), Article ID 6368789, 11 pages
https://doi.org/10.1155/2019/6368789

Research Article

Ultrasonic-Assisted Synthesis, Characterization, and Photocatalytic Application of SiO₂@TiO₂ Core-Shell Nanocomposite Particles

Newaz Mohammed Bahadur,1,2 Farhana Chowdhury,² Md. Obaidullah,³ Md. Shahadat Hossain,² Rumana Rashid,⁴ Yeasmin Akter,² Takeshi Furusawa,¹ Masahide Sato,¹ and Noboru Suzuki1,3

¹Department of Material and Environmental Chemistry, Graduate School of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan
²Department of Applied Chemistry and Chemical Engineering, Noakhali Science and Technology University, Noakhali 3814, Bangladesh
³Department of Innovation Systems Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan
⁴Department of Public Health Nutrition, Primeasia University, Banani, Dhaka, Bangladesh

Correspondence should be addressed to Newaz Mohammed Bahadur; nmbahadur@yahoo.com

Received 9 February 2019; Revised 11 July 2019; Accepted 23 July 2019; Published 10 September 2019

Academic Editor: Hiromasa Nishikiori

Copyright © 2019 Newaz Mohammed Bahadur 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.

Abstract.

In this study, we report the synthesis of SiO₂@TiO₂ core-shell nanocomposite particles by ultrasound irradiation of a mixture of dispersed SiO₂ nanoparticles, titanium-tetra-n-butoxide (TBOT), and ammonia in an ethanol-water solution medium. The resulting core-shell nanocomposite particles were characterized by SEM, TEM, FT-IR, XPS, XRF, zeta potential measurements, XRD, and UV-visible spectroscopy. Results showed that TiO₂ nanoparticles coated on the surface of SiO₂ were 6–10 nm in size and retained an anatase crystalline phase. Zeta potential measurements confirmed that the surface property of the SiO₂ changed after TiO₂ coating. SiO₂@TiO₂ core-shell particles demonstrated better decolorization of methylene blue dye compared to commercial TiO₂ in aqueous solution under UV light. After treatment, the catalysts were separated with low-speed centrifugation and successfully reused four times without loss of activity. This study may provide some inspiration for the synthesis of other metal oxide-metal oxide core-shell nanocomposite materials through ultrasound irradiation.

1. Introduction

The boom in industrialization and mass production undoubtedly benefit humans with numerous latest products and services. However, different industries such as textile, paper, leather, ceramic, cosmetics, and ink and food processing release various organic and inorganic pollutants, which cause a great health hazard to humans and the aquatic environment when discharged to the environment without further treatment [1]. Among the pollutants, synthetic organic dyes used as coloring agents are considered as one of the potential sources of nonaesthetic pollutants which contaminate surface and ground water and can harm ecological resources including water quality, soils, plants, and animals and also human health [2, 3]. A number of technologies have been developed to remove the organic pollutants including chemical treatment such as chlorination [3] and ozonation [4], electrochemical treatment [5], physical treatment such as adsorption by activated carbon [6] and membranes [7], biological treatment, and a combined chemical-biological method [8]. Although the biological method is cost-effective, the dyes are only adsorbed on the sludge and are not degraded in this method [9]. Physical methods also have the disadvantages such as the pollutants simply transfer from one phase to another rather than their destruction. It has been found that photocatalysis is a promising technique for the destruction of dyes using semiconductor catalysts under light irradiation [10]. In addition, as per literature reports, enough research work has also been carried out related to the purification of water by photocatalysis [11]. Various types of photocatalysts such as TiO₂, ZnO, Al₂O₃, WO₃, Fe₃O₄, CeO₂, and their composites have been used for the degradation of organic pollutants [12]. Among these metal oxides, titanium dioxide (TiO₂) has attracted much attention due to its wide range of applications such as paint industry, biomedicine, electronics, and environmental engineering [13]. TiO₂ has also been widely used as the photocatalyst because of its chemical and biological inertness, high stability against photocorrosion, nontoxicity, low cost, and excellent degradation for organic pollutants [2, 13, 14].

Despite the favorable properties of TiO₂ in the decomposition of dyes, it is not thermally stable when applied independently as a powder form and tends to dissolve with losing its surface area in the solution. It is also known that when TiO₂ nanoparticle is used as free-suspending systems in photocatalysis, it shows better efficiency and photocatalytic activity compared to the encapsulated ones [15–17]. However, there are a number of disadvantages of using free-suspending systems such as loss of catalyst, instability of the catalyst particles, and difficulties for the separation of the particles after treatment. Nevertheless, one of the major drawbacks of TiO₂ photocatalyst is the low quantum efficiency of TiO₂ owing to the fast recombination of photogenerated electron-hole pairs, which significantly reduces the photocatalytic activity of TiO₂. Therefore, the low photocatalytic efficiency of the TiO₂ photocatalyst still remained a challenge and is insufficient for industrial-scaled applications. In order to minimize the above drawbacks, TiO₂ is incorporated in high surface area materials. Incorporation comprises an increase in the surface area of TiO₂, controlling the crystalline phase of TiO₂ as anatase even at high-temperature treatment to increase its crystallinity. In addition, composites made from TiO₂ and other metal oxides were often used to improve the photocatalytic activity of TiO₂ [18]. Among various types of metal oxides, SiO₂ has drawn large interest due to its chemical inertness and stability at high temperature [19, 20]. Incorporation of TiO₂ in silica has many advantages including easy separation of the nanocomposites by low-speed centrifugation when the size of the carrier spheres (SiO₂) is large but small enough for easy dispersion in the system. Besides, silica is a cost-effective support possessing good mechanical strength for long-time operations compared to polymer-based carriers. Moreover, the encapsulation of TiO₂ on high surface adsorbent silica spheres significantly facilitates reactant molecules to reach the active sites of TiO₂. Therefore, the photocatalytic activity increases [18, 19]. SiO₂ traps the photogenerated electron of TiO₂ and reduces the electron-hole recombination time of TiO₂. Although a number of methods have been developed for the incorporation of TiO₂ in silica spheres including gas phase synthesis [21], dry coating synthesis [22], sol-gel process [23, 24], and microemulsion method [25], the above methods have the disadvantages including impurities, high cost, long time to encapsulate TiO₂, multistep coating process, and complicated experimental procedure.

Ultrasound is a promising technology whose applications have been rapidly growing due to its unique effects compared to the conventional agitation, such as rapid volumetric heating, increased reaction rates and shortened reaction time, enhanced reaction selectivity, and energy saving [26]. In addition, in many cases, reactions under ultrasound irradiation represent environmentally friendly processes, using small amounts of solvents and consuming less energy [27]. Furthermore, ultrasonic irradiation provides minimal side reactions [28]. In this study, we report a fast, simple, and low temperature ultrasonic method to synthesize SiO₂@TiO₂ core-shell nanocomposite particles.

2. Experimental

2.1. Materials

The precursors used for the preparation of SiO₂ and TiO₂ were tetraethoxysilane (TEOS), ethanol (99.5%, KANTO Chemical Co., Japan), and titanium-tetra-n-butoxide (TBOT, Sigma-Aldrich) solutions. Ammonium hydroxide (NH₄OH) was purchased from WAKO Pure Chemical Industries Ltd., Japan. Degussa P-25 was used as commercial pure TiO₂ in all the experiments. All other reagents were of analytical grade and were used in experiments without further purification. Milli-Q water with a resistivity of 18.3 MΩ cm was used in all the preparations.

2.2. Synthesis of Silica Nanoparticles

The ultrasound synthesis of silica powders was performed using an ultrasound instrument (20 kHz, 700W; Q 700, QSONIC, USA). To 20 mL ethanol solution of TEOS, the desired amount of water and ammonium hydroxide were added under magnetic stirring so that the concentrations of TEOS, H₂O, and NH₄OH were 0.22 M, 6 M, and 2 M, respectively. Immediately, the mixture was then ultrasound irradiated at room temperature for 1 hr. After the synthesis, the resulting silica spheres were separated by centrifugation and washed with ethanol and water. Finally, the prepared silica powder, SiO₂(s), was dried at 60°C for 24 hrs.

2.3. Preparation of SiO₂@TiO₂ Core-Shell Nanocomposite Particles

The coating reaction was performed in ethanol at room temperature by the hydrolysis and condensation of titanium-tetra-n-butoxide (TBOT). At first, dispersion of SiO₂ nanoparticles in ethanol was performed by sonication. Typically, 0.40 g of prepared SiO₂ nanoparticles was taken in a 100 mL Pyrex beaker, and then, 20 mL of ethanol was added into it. The mixture was then ultrasonicated for 5 min to ensure good dispersion of the SiO₂ nanoparticles. After that, an appropriate amount of TBOT ( to mol), ammonia (to maintain the concentration of 0.8 M), water (to maintain the concentration of 11 M), and ethanol was added into the dispersion. The mixture was exposed to high intensity ultrasound irradiation with a Ti probe having a diameter of 6 mm and an amplitude of 120 μm for 2 hrs. The probe was inserted to an optimum depth of the reaction mixture during the experiment. The ultrasound equipment produces acoustic waves at a frequency of 20 kHz. After ultrasonic irradiation, the resulting suspensions were centrifuged at 4000 rpm for 10 min and washed with ethanol for three times and water to remove all unreacted reagents. The SiO₂@TiO₂ core-shell nanocomposite particles were then dried at 120°C for 5 h. The prepared sample names are denoted as SiO₂-TiO₂-1, SiO₂-TiO₂-2, SiO₂-TiO₂-3, SiO₂-TiO₂-4, and SiO₂-TiO₂-5 for , , , , and of TBOT loading, respectively.

2.4. Material Characterization

The morphological structure of the samples was investigated by field emission scanning electron microscopy (FE-SEM) using S-4500 (Hitachi Ltd., Japan) and transmission electron microscopy (TEM) using JEM-2010 (JEOL Ltd., Japan). FE-SEM observation was performed at an accelerating voltage of 5 kV and current of 10 μA. Prior to the observation, the powder was dropped onto carbon tape. Then, samples were coated with a few nanometers of platinum using a Pt-Pd ion coater (Hitachi, E-1030, Japan). For TEM measurement, powder specimens were suspended in ethanol, and an aliquot of 50 μL was deposited on a copper grid coated with a holey carbon film. The copper grids were allowed to dry at room temperature in air.

X-ray photoelectron spectroscopy (XPS) measurements were performed using PHI 5000 Versa Probe-II (ULVAC-PHI Inc., Japan) by dropping the powder samples on the carbon tape using Mg Kα X-ray source operating at 200 W (15 kV). The vacuum level in the main chamber was under . The emission angle of the specimen from the surface normal was 45°. A wide scan (binding energy range: 0-900 eV, pass energy: 187.85 eV) was performed to detect all types of element on the surface, and narrow scan was performed for each element, C 1s, Si 2p, O 1s, Ti 2p2/3, and Ti 2p1/2, with the pass energy of 58.7 eV. The adventitious C 1s peak at 284.8 eV was used as reference for all binding energies.

FT-IR spectra were carried out using a system (FT/IR-4100 spectrometer, Jasco Corp., Japan) in the range 4000-400 cm⁻¹ with a resolution of 2.0 cm⁻¹. Measurements were performed with a KBr pellet containing a small amount of sample powder. The weight ratio between sample powder and KBr was 1 : 10.

The UV-vis spectroscopic measurements were performed with a UV-vis spectrophotometer (V-560, Jasco Corp., Japan) in the wavelength region of 300-700 nm and band width of 5 nm.

A Zeta PALS apparatus (Zetasizer Nano, Malvern Instrument Ltd., Malvern, UK) was used to investigate the electrophoretic behavior of the materials.

X-ray powder diffraction (XRD) spectra were recorded with a powder diffractometer (Ultima IV, Rigaku Corp., Japan) with Cu-Kα radiation.

2.5. Photocatalytic Activity Measurement

The photocatalytic activities of uncoated SiO₂ nanoparticles and SiO₂@TiO₂ core-shell nanocomposite particles were determined by methylene blue (MB) degradation under UV radiation. Photocatalytic experiments were performed at 293 K using a 100 mL Pyrex beaker in a reaction chamber () containing five bulbs (22 inches, 15 W) with the strongest band at the wavelength of 352 nm. The distance between the UV lamp and the surface of the solution was about 15 cm. The average UVA and UVB intensities of exposure for all measurements were around 80 and 0.25 W/m² measured by a UV monitor (MS-211-1, Eiko Instruments Co. Ltd.) with the peak intensity of 352 nm. The details of the experimental procedure were described somewhere else [29]. The initial concentration of the dye was , and different amounts of SiO₂@TiO₂ composite particles were added in the dye solution so that the TiO₂ content in all the composites were 0.1 g. MB solution and photocatalysts were mixed in a beaker and kept at dark for 30 min with magnetic stirring to ensure the adsorption/desorption equilibrium between photocatalyst and dye. After switching on the UV lamp, at each time step, an aliquot of 2 mL of the aqueous suspension was taken from the beaker and centrifuged to separate the photocatalytic particles. The total volume of this mixture was kept constant by adding the same amount of water. The remaining MB dye concentration after irradiation was measured by UV-visible spectroscopy.

3. Results and Discussion

For the preparation of nanomaterials by using ultrasonic irradiation, phenomena responsible for sonochemistry can be characterized into primary sonochemistry, secondary sonochemistry, and physical modifications. It is important to note that the chemical and physical effects of ultrasound do not arise from a direct interaction between chemical species and sound waves but rather from the physical phenomenon of acoustic cavitation, namely, the formation, growth, and implosion of bubbles [30]. The bubbles collapse implosively in less than a microsecond under ultrasonic irradiation and produce intense local heating (~5000 K), high pressures (~1000 atm), and large heating and cooling rates (>109 K/s), in a microscopic region of the sonicated liquid [30, 31]. Furthermore, the implosive collapse of bubbles facilitates the production of intense shock waves and microjetstream in the solutions, which promote the transfer of reactants and prevent the nanoparticles from aggregation [32]. Besides, it could enhance the dissolution processes and promote the chemical reactions and kinetics [24]. In the present study, the cavitation bubbles are filled with ethanol, water, ammonia, and TBOT vapors. Since the vapor pressure of ethanol (bp 78°C) at 32°C is much higher than that of TBOT (bp 312°C), the concentration of TBOT molecules inside the bubble is negligibly low and the ultrasound-driven hydrolysis of TBOT occurs in the liquid reaction zone rather than in the gas phase of the cavitation. The temperature of the hot spot in the vicinity of the collapsing bubble has been estimated to be about several thousand Kelvin. Consequently, transient temperature jumps in the liquid shell surrounding the cavitation bubble would lead to a strong local acceleration of titania nucleation [33].

3.1. SEM Images

The surface morphology of the uncoated SiO₂, pure TiO₂, and SiO₂@TiO₂ core-shell nanocomposite particles was investigated using SEM, and the results are shown in Figures 1(a), 1(b), and 1(c), respectively. The particle size of uncoated SiO₂ was around 250 nm. Pure TiO₂ had a particle size of 30–50 nm, whereas the particle size of TiO₂ in SiO₂@TiO₂ core-shell particles was around 6-10 nm (Figure 1(c), inset). The elemental compositions of the prepared materials measured by XRF and XPS techniques are shown in Table 1. It was observed that the amount of titania content increased with an increase in TBOT loading which indicates the thicker titania layer formation with higher TBOT loading.

(a)

(b)

(c)

Figure 1: FE-SEM images of (a) uncoated SiO₂ nanoparticles, (b) pure TiO₂ nanoparticles, and (c) SiO₂@TiO₂ core-shell nanocomposite particles (SiO₂-TiO₂-4).

Table 1: Elemental analysis of SiO₂@TiO₂ samples using EDX and XPS analysis.

Sample
TBOT loading (mol)
Elemental analysis from XRF analysis
Elemental analysis from XPS analysis

Si
Ti
Si
C
O
Ti

Uncoated SiO₂
—
100

25.4
21.0
53.6
0.00

Uncoated TiO₂

100
0.00
23.8
51.6
24.6

SiO₂-TiO₂-1

95.7
4.3
23.5
16.7
55.7
4.1

SiO₂-TiO₂-2

90.3
9.7
20.2
14.8
56.2
8.8

SiO₂-TiO₂-3

85.7
14.3
15.6
11.6
55.4
15.4

SiO₂-TiO₂-4

80.3
19.7
8.7
12.1
59.4
19.8

SiO₂-TiO₂-5

71.2
28.8
3.2
12.2
59.8
24.8

3.2. TEM Images

Figures 2(a), 2(b), and 2(c) show the TEM images of uncoated SiO₂, pure TiO₂, and SiO₂@TiO₂ core-shell particles, respectively. It can be found from Figure 2(a) that the diameter of the uncoated SiO₂ was about 250 nm. The surface of SiO₂ core-shell nanoparticles was coated with uniform TiO₂ layer, and the particle size of TiO₂ was 6-10 nm, as shown in Figure 2(b). The phase of TiO₂ on the surface of SiO₂ was confirmed by XRD analysis.

(a)

(b)

(c)

Figure 2: TEM images of (a) uncoated SiO₂ nanoparticles, (b) TiO₂ nanoparticles, and (c) SiO₂@TiO₂ core-shell nanocomposite particles (SiO₂-TiO₂-4).

3.3. XRD Patterns

The XRD patterns of the uncoated SiO₂, pure TiO₂, and SiO₂@TiO₂ core-shell nanocomposite particles are shown in Figure 3. Before taking the XRD spectra, pure SiO₂, pure TiO₂, and prepared SiO₂@TiO₂ core-shell particles were heated at 500°C for 1 hour in order to improve the crystallinity of the particles. The XRD pattern of SiO₂ nanoparticles indicates that the prepared SiO₂ was amorphous in nature. In addition to this, the pure TiO₂ showed peaks that correspond to both the anatase and rutile phases. The prepared SiO₂@TiO₂ core-shell particles did not show any peak that corresponds to TiO₂, which confirms that the crystalline phase of TiO₂ coated on SiO₂ was amorphous. However, after heating the samples at 500°C, the anatase phase of TiO₂ was observed. The absence of a rutile peak in the SiO₂@TiO₂ samples indicates that the prepared nanocomposite particles retained their anatase phase to anatase crystalline phase. It has been established from the literature that the anatase form of titania is very important for the photocatalysis [34, 35].

Figure 3: X-ray diffraction patterns of (a) SiO₂ nanoparticles, (b) SiO₂@TiO₂ core-shell nanocomposite particles, SiO₂-TiO₂-4 (as prepared), (c) SiO₂@TiO₂ core-shell nanocomposite particles, SiO₂-TiO₂-4 (annealed at 500°C), and (d) pure TiO₂ nanoparticles.

3.4. XPS Spectra

The presence of TiO₂ coating on SiO₂ nanoparticles was further confirmed from the XPS wide scan spectra of uncoated SiO₂, SiO₂@TiO₂, and pure TiO₂ samples. The XPS wide spectra of the samples as shown in Figure 4 indicate that commercial TiO₂ and SiO₂@TiO₂ samples had Ti 2p peaks whereas no such peak was observed in the case of uncoated SiO₂. In addition, the peak intensities of Ti 2p increased and Si 2p decreased with increasing TBOT loading while the intensity of O 1s remained almost unchanged. The above results indicate successful TiO₂ coating on the surface of SiO₂ particles. The XPS narrow scan spectra of Ti 2p and O 1s are illustrated in Figures 5 and 6, respectively. The spin-coupled Ti (2p3/2 and 2p1/2 peaks) doublet was located at 458.4 and 464.3 eV in pure TiO₂. The shifting of binding energy to higher values in the SiO₂@TiO₂ sample compared to pure TiO₂ is attributed to the formation of Ti-O-Si bonds at the interface of titania coating layer and silica particle surface [36, 37]. Since the electronegativity of Si is higher and polarizability is lower than that of Ti atom, the effective positive charge on Ti atom is increased, electron density around Ti atom is decreased, and the shielding effect is weakened, which results in the increased binding energy. The doublet for O 1s peaks was observed at 530.9 eV and 533.4 eV, which corresponds to Ti-O-Ti and Si-O-Si bonds [36, 37], respectively. Compared to the O 1s peak of pure TiO₂ (530.4 eV) (Supplementary Information-1 (SI-1)), the binding energies were shifted towards higher values in SiO₂@TiO₂ core-shell nanocomposites due to the greater electronegativity of Si than that of Ti [36, 37]. From the chemical shift of O 1s and Ti 2p peaks, it can be concluded that SiO₂ nanoparticles were successfully coated with TiO₂ and that Si-O-Ti bond was formed between the interface of SiO₂ and TiO₂.

Figure 4: XPS wide scan spectra of (a) SiO₂ nanoparticles, (b) SiO₂-TiO₂-1 (SiO₂@TiO₂ core-shell nanocomposite particles prepared with TBOT loading), (c) SiO₂-TiO₂-3 ( TBOT loading), (d) SiO₂-TiO₂-4 ( TBOT loading), and (e) pure TiO₂ nanoparticles.

Figure 5: The XPS narrow scan spectra of Ti 2p for (a) pure TiO₂ nanoparticles and (b) SiO₂@TiO₂ core-shell nanocomposite particles, SiO₂-TiO₂-4.

Figure 6: XPS narrow scan spectrum of O 1s of SiO₂@TiO₂ core-shell nanocomposite particles (SiO₂-TiO₂-4).

3.5. UV-vis Absorption Spectra

Figure 7 represents the UV-vis absorption spectra of uncoated SiO₂, SiO₂@TiO₂, and pure TiO₂ samples. Pure TiO₂ showed a peak at 325 nm, SiO₂ nanoparticles did not show any absorption peak, and SiO₂@TiO₂ core-shell nanocomposite particles showed a broad peak at 314 nm. It was observed that the absorption maxima of SiO₂@TiO₂ nanoparticles shifted toward the blue region compared to pure TiO₂. This result confirms not only the successful titania coating of SiO₂ particles but also the smaller particle size of TiO₂ in SiO₂@TiO₂ compared to commercial TiO₂ [38]. Therefore, the smaller particle size of TiO₂ in SiO₂@TiO₂ core-shell particles would be beneficial for the photocatalytic reaction as smaller particles provide a more effective surface for organic reactants and light absorption [39].

Figure 7: UV-visible absorption spectra of (a) SiO₂ nanoparticles, (b) SiO₂@TiO₂ core-shell nanocomposite particles (SiO₂-TiO₂-4), and (c) pure TiO₂ nanoparticles.

3.6. FT-IR Spectra

The FT-IR spectra of uncoated SiO₂, SiO₂@TiO₂, and pure TiO₂ are presented in Figure 8. The band at 1105 cm⁻¹ was attributed to Si-O-Si asymmetric stretching vibration [40]. The peaks at 800 cm⁻¹ and 475 cm⁻¹ corresponded to the symmetric stretching and deformation modes of Si-O-Si [40], respectively. A band at around 1450 cm⁻¹ was attributed to silanol bonds [41]. The broad band at around 3300 cm⁻¹ and 1625 cm⁻¹ present in all the samples corresponded to OH stretching and bending of water [42, 43], respectively. Notably, by comparing the FT-IR spectra of uncoated SiO₂ and TiO₂, the characteristic peak, observed at 935 cm⁻¹, in Figure 8(b), was attributed to Ti-O-Si bond [41, 44]. These FT-IR spectra provided an important information that TiO₂ was successfully coated on SiO₂.

Figure 8: FT-IR spectra of (a) pure SiO₂ nanoparticles, (b) SiO₂@TiO₂ core-shell nanocomposite particles (SiO₂-TiO₂-4), and (c) pure TiO₂ nanoparticles.

3.7. Zeta Potential Measurement

Zeta potential measurements of the particles were carried out to investigate the presence of TiO₂ coating on the silica surface. The zeta potential values of uncoated SiO₂, pure TiO₂, and SiO₂@TiO₂ core-shell particles are presented in Figure 9. The values of zeta potentials of uncoated SiO₂ changed after TiO₂ coating, which confirms successful TiO₂ coating of SiO₂ particles. Compared to uncoated TiO₂, SiO₂@TiO₂ core-shell nanocomposite particles showed a lowered zeta potential value at neutral pH, which means better dispersion of SiO₂@TiO₂ particles compared to uncoated TiO₂ particles.

Figure 9: Variation of zeta potential with pH for (a) SiO₂ nanoparticles, (b) SiO₂@TiO₂ core-shell nanocomposite particles (SiO₂-TiO₂-4), and (c) pure TiO₂ nanoparticles.

4. Photocatalytic Activity Measurement

To evaluate the photocatalytic activity of the SiO₂@TiO₂ core-shell particles and uncoated SiO₂ under UV light irradiation, methylene blue (MB) was chosen as a model pollutant for photocatalytic degradation. The changes in the absorption intensity of MB at 664 nm were monitored by UV-vis spectroscopy. Figure 10 shows the degradation percentages of MB with the photocatalysts. It was found that the uncoated SiO₂ nanoparticles did not show any photocatalytic activity whereas SiO₂@TiO₂ core-shell nanocomposite particles showed better photocatalytic activity compared to uncoated SiO₂ nanoparticles as well as commercial TiO₂. In addition, the photocatalytic activity increases with increasing titania loading. However, the SiO₂-TiO₂-5 (Table 1) sample showed lower photocatalytic activity compared to SiO₂-TiO₂-4 even though the former sample contained higher titania than the latter. The two curves seem, indeed, overlapped, i.e., the behavior seems comparable, which sounds reasonable if taking into account the fact that the silica nanoparticles are coated at their maximum potential and the titania in excess generates free coreless nanoparticles, which is also confirmed by SEM and TEM images (Supplementary Information-2 (SI-2)).

Figure 10: Photocatalytic activities of SiO₂ nanoparticles and SiO₂@TiO₂ core-shell nanocomposite particles prepared with different TBOT loading.

In general, photocatalytic degradation of dyes is an oxidative process, which involves several active radical formations such as hole (), superoxide anion (), and hydroxyl radicals (OH⋅). Therefore, when catalysts are illuminated by UV light with photon energy higher than the band gap of TiO₂, electrons (e⁻) in the valence band (VB) of TiO₂ excite to its conduction band (CB) with the formation of the same number of holes () left behind in the VB. The various reactions involve in the photocatalytic process can be summarized as follows [45–47]:

The mechanism of the enhancement of the photocatalytic activity of SiO₂@TiO₂ over pure TiO₂ under UV light irradiation could be illustrated in Scheme 1. In the SiO₂@TiO₂ system, the excited electron transfer from TiO₂ surface to SiO₂ surface and stored temporarily in the silica as it acts as electron sink. Accordingly, the lifetime of the photogenerated pairs is increased. During the photodegradation process, the holes in the valance band react with water and generate OH⋅ radicals and electron stored in the SiO₂ surface produces superoxide anion radicals (). Moreover, the insulating character of silica provides the stability of titania particles even at extreme conditions. Furthermore, the formation of the new Ti-O-Si bond in SiO₂@TiO₂ core-shell nanocomposite particles increases the band gap energy of the active center of TiO₂ [20]. As a result, the lifetime of photogenerated electrons (), holes (), and electrons increases which leads to enhanced photocatalytic activity of TiO₂ in SiO₂@TiO₂ samples. Finally, the uniform distribution of titania nanoparticles on the silica sphere surface, as shown in SEM and TEM images (Figures 1(c) and 2(c)), also a good indication of the significant photocatalytic performance of the synthesized nanocomposite particles.

Scheme 1: Scheme of photocatalytic degradation of SiO₂@TiO₂ core-shell nanocomposite particles.

It has also been studied that well-separated particles show better catalytic performance than aggregated particles. Two aggregated catalyst particles compete with each other for the reactants, thus lowering their usage as catalyst particles, while dispersed particles could overcome this drawback. Wu and Lu [48] found that better separation of the immobilized catalyst particles leads to complete utilization of the catalyst particles and thus resulted in the higher rate of photodegradation. The better dispersion of SiO₂@TiO₂ core-shell nanocomposite particles over pure TiO₂ nanoparticles enhances the photocatalytic activity of the catalyst. The catalytic performance of SiO₂@TiO₂ particles was better than uncoated TiO₂ that could also be due to the following reasons: (1) smaller size of TiO₂ in SiO₂@TiO₂ than commercial TiO₂, (2) complete recovery of the catalyst after treatment with low-speed centrifugation, (3) excellent reusing capability, etc. The degradation of MB can be explained by the following elementary mechanism shown in equations (2) and (3). It can involve the direct reaction of the dye with photogenerated holes in the process similar to the photo-Kolbe reaction or oxidation through successive attacks by hydroxyl radicals

or superoxide species [49]. The hydroxyl radical in particular is an extremely strong nonselective oxidant that has shown to lead to the partial or complete oxidation of many organic chemicals [50].

5. Reusability of SiO₂@TiO₂ Core-Shell Nanocomposite Particles

Photocatalyst stability is one of the most important factors for its practical application. As a consequence of that, the stability and reuse ability of SiO₂@TiO₂ core-shell nanocomposite particles were examined by conducting the photocatalytic experiments with model dye MB under UV irradiation cycled for five times as shown in Figure 11. After each run, SiO₂@TiO₂ particles were recovered by centrifugation at 4000 rpm, washed with deionized water thrice, dried, and reused for the next run. On the other hand, 11000 rpm speed was required to recover the commercial TiO₂ with a particle size of 20-30 nm. The results of the reusing test indicated that there was no considerable difference in the activity of the prepared SiO₂@TiO₂ catalyst at least for five cycles. Thus, the SiO₂@TiO₂ catalyst can be separated after dye treatment with low power centrifugation and reused successfully without loss of activity.

Figure 11: Reusability of SiO₂@TiO₂ core-shell photocatalyst (SiO₂-TiO₂-4); pH 7.0; 25°C.

6. Conclusions

We have shown a simple, fast, and environment-friendly ultrasonic irradiation method for the synthesis of SiO₂@TiO₂ core-shell nanocomposite particles using TBOT in SiO₂-ethanol suspension. Compared to the conventional methods, which need long reaction time, the ultrasonic-assisted method allows the formation of TiO₂ nanoparticles on the SiO₂ surface within short reaction time (2 hours). SEM and TEM images revealed that 6–10 nm sized TiO₂ nanoparticles were uniformly coated on the surface of around 250 nm sized SiO₂ nanoparticles. XRD analysis showed that TiO₂ deposited on the surface of the SiO₂ was amorphous, and the anatase crystalline form of TiO₂ was formed after heating the composite materials at 500°C. A new Ti-O-Si bond at the interface between SiO₂ and TiO₂ particles identified by FT-IR and XPS has confirmed the successful TiO₂ coating of SiO₂ nanoparticles. Encapsulation of anatase TiO₂ on high surface area SiO₂ resulted in better photocatalytic ability compared to commercial TiO₂, easy separation, and complete recovery of the catalyst after the dye treatment with low-speed centrifugation (4000 rpm). The prepared nanocomposite particles were reused successfully for five cycles without loss of catalytic activity. The ultrasonic-assisted approach can also be extended for the fabrication of a variety of core-shell nanocomposites such as Au-TiO₂, Ag-TiO₂, ZrO₂-TiO₂, and SnO₂-TiO₂. Research on these issues is currently underway.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was financially supported by the Ministry of Science and Technology, Bangladesh, and the University Grant Commission, Bangladesh.

Supplementary Materials

Supplementary Information-1 (SI-1): XPS narrow scan spectra of O 1s of pure TiO₂ revealed that the binding energy of O 1s of pure TiO₂ was 530.1 eV. Supplementary Information-2 (SI-2): SEM and TEM images of TiO₂-coated SiO₂ nanocomposite particles with high titanium-tetra-n-butoxide (TBOT) loading indicated the significant free TiO₂ nanoparticle formation. (Supplementary Materials)

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