Mesoporous TiO₂ encapsulating a visible-light responsive upconversion agent for enhanced sonocatalytic degradation of bisphenol-A

Abstract

Herein, we report the integration of a visible-light active upconversion agent (Er:Y₂O₃) with mesoporous TiO₂ via a modified two-step sol–gel coating method. The resultant material possesses a well-defined core–shell structure with a good upconversion property and exhibits uniform worm-like mesopores (∼3.8 nm), a high BET surface area (∼151.5 m² g⁻¹) and a large pore volume (∼0.23 cm³ g⁻¹). The composite was demonstrated as an advanced sonocatalyst, showing a superior degradation performance for bisphenol-A (BPA). The effect of the erbium dopant content on the upconversion property and sonocatalytic performance was evaluated for the production of the best sonocatalyst. We found that the maximum pseudo first-order reaction rate constant in the presence of the composite with an Er³⁺ content of 3 wt% in Y₂O₃ nanocrystals is calculated to be 0.155 min⁻¹, which is 2.9 and 2 times larger than that of US alone (0.054 min⁻¹) and hollow structured mesoporous TiO₂ (0.077 min⁻¹), respectively. More importantly, the degradation rate is much higher than that of the sonocatalysts reported previously for treatment of BPA (0.09–0.14 min⁻¹). The superior catalytic activity can be attributed to an intensified cavitation reaction zone and an enhanced amount of photo-generated charges. Moreover, the recycling test shows that a constant catalytic activity is retained even after 4 cycles. This study paves a promising way for the development of a multi-functional catalyst for sonochemical processes.

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Introduction

Recently, advanced oxidation processes (AOPs) which involve the generation of highly reactive oxidation species such as hydroxyl radicals, have emerged as a promising technology for the decomposition and mineralization of emerging organic pollutants. Among various AOPs, sonolysis is of great importance because it is environmentally friendly and simple to operate.^1–3 When ultrasound (US) is irradiated into an aqueous phase, acoustic cavitation including the nucleation, growth, and violent collapse of microbubbles occurs, leading to the formation of sonoluminescence (SL, λ = 300–600 nm) and “hot spots” with extremely high temperature (5000 K) and pressure (1000 atm).^4,5 As a result, water is dissociated to produce ·OH, ·H, and ·OOH, which can mineralize organic pollutants into CO₂ and H₂O. However, a significant amount of electric energy is typically required to obtain a fast ultrasonic degradation rate using one-fold US owing to the massive loss of input energy in thermal dissipation (>50%), which hinders the wide application of sonolysis for practical water treatment.⁶ To this end, development of a heterogeneous sonocatalyst is greatly desired. Till now, TiO₂-based materials are extensively studied because of their good photocatalytic properties, nontoxicity, and cost-effectiveness. More importantly, it has been demonstrated that TiO₂-based materials exhibited high sonocatalytic activities due to the formation of electron–hole pairs excited by SL, which is similar to the photocatalytic mechanism.^7,8 However, the efficiency of TiO₂ is still limited because it can only be activated by ultraviolet (UV) light. In this perspective, numerous strategies have been reported to extend the absorption spectra of TiO₂ to visible-light region such as surface hydrogenation, metal or nonmetal ions doping, narrow band gap semiconductors coupling, etc.^9–13 Nonetheless, the overall catalytic performance was sometimes found to decrease due to an increased recombination rate of photo-generated electron–hole pairs. Therefore, it is still a challenge to develop an appropriate way to address this issue. Recently, the incorporation of an upconversion (UC) agent with TiO₂ has been extensively investigated as a novel strategy for enhancing the degradation performance of TiO₂ under near-infrared light.^14–16 However, the application of visible-light active UC agent is still restricted.^17,18 In addition, one unique mechanism that governs the sonocatalytic efficiency is the heterogeneous nucleation process, which can be significantly affected by the catalyst surface properties such as roughness, pore size, wettability, etc. To date, substantial research efforts have been made for evaluating the effects of wettability and roughness on heterogeneous cavitation nucleation,^19–21 whereas the patterning of a sonocatalyst surface with a nanoporous structure was rarely considered. Thus, to develop a catalyst with a UC capacity for utilizing visible light and a nanoporous surface for facilitating nucleation is highly desirable.

Herein, we report a successive sol–gel coating process to integrate a UC agent with mesoporous TiO₂. The resultant material consists of a inner UC core – erbium doped yttrium oxide (Er:Y₂O₃) that can absorb visible light and then emit UV light, a middle silica layer for preventing dissociation of the core and a mesoporous TiO₂ shell with a high surface area (∼151.5 m² g⁻¹), a large pore volume (0.23 cm³ g⁻¹) and a uniform pore size distribution (∼3.8 nm). The composite is used as an advanced sonocatalyst for degrading bisphenol-A (BPA), exhibiting a superior performance especially when the dopant content of erbium in the Y₂O₃ nanocrystals is 3 wt%. The high catalytic activity is associated with the intensified cavitation reactive zone resulted from the mesoporous structure and the generation of additional photo-excited charge carriers achieved through UC mechanism. Moreover, the catalyst can be easily recycled and exhibit a constant activity even after 4 cycles. This study paves a promising way for the design and development of multi-functional catalyst for sonochemical process.

Experimental section

Materials

ErCl₃·6H₂O, YCl₃·6H₂O, urea, tetraethyl orthosilicate (TEOS), ethanol, concentrated ammonia solution (28 wt%), titanium(IV) isopropoxide (TIPO), and bisphenol-A (BPA) were of analytical grade and purchased from Sigma-Aldrich (USA). All chemicals were used as received without further purification. Millipore water was used for all experiments.

Preparation of Er³⁺ doped Y₂O₃ NPs

The erbium doped yttrium oxidation (Er:Y₂O₃) NPs were prepared via a urea assisted co-precipitation method reported previously.²² Briefly, YCl₃·6H₂O (0.28 g), ErCl₃·6H₂O (0.5–1.34 mL, 0.05 mmol mL⁻¹), and urea (4.0 g) were dissolved in water (50 mL) with agitation. The mixture was stirred vigorously for 1 h at room temperature and then heated at 90 °C for 3 h. The white products were washed with deionized water and ethanol for 3 times, respectively.

Fabrication of core–shell Er:Y₂O₃@SiO₂ nanospheres

The core–shell Er:Y₂O₃@SiO₂ nanospheres were prepared through a versatile Stöber sol–gel method. For a typical synthesis, an ethanol dispersion of the Er:Y₂O₃ particles obtained above was added into a three-neck round-bottom flask with a mixture of ethanol (280 mL), deionized water (70 mL) and concentrated ammonia solution (4.0 mL, 28 wt%). The mixed solution was sonicated for 15 min. Then, 2.0 mL of TEOS was added dropwise in 10 min, and the reaction was allowed to proceed for 12 h at room temperature under continuous mechanical stirring (400 rpm). The resultant products were separated and washed with deionized water and ethanol for 3 times, respectively.

Synthesis of core–shell Er:Y₂O₃@SiO₂@mTiO₂ nanospheres

The coating of mesoporous TiO₂ shell was prepared via a kinetically controlled Stöber method, followed by an ultrasound assisted post-hydrolysis for the formation of mesopores and a calcination process for removing organic species and improving crystallinity.^23,24 Typically, core–shell structured Er:Y₂O₃@SiO₂ nanospheres obtained above were dispersed in ethanol (100 mL), and mixed with concentrated ammonia solution (0.40 mL, 28 wt%) under ultrasound for 15 min. Subsequently, 0.75 mL of TIPO was added dropwise in 5 min, and the reaction was allowed to proceed for 24 h at 45 °C under continuous mechanical stirring. The resultant products were separated and collected, followed by an additional ultrasonic post-hydrolysis process in water according to that reported previously. Finally, the resultant samples were collected by centrifuge and calcined at 600 °C for 2 h to remove the organic species and improve crystallinity.

Synthesis of nonporous Er:Y₂O₃@SiO₂@TiO₂ nanospheres and hollow structured mesoporous TiO₂

As a comparison, nonporous Er:Y₂O₃@SiO₂@TiO₂ nanosphere was synthesized without ultrasound assisted post-hydrolysis process. Moreover, the hollow structured mesoporous TiO₂ sphere was obtained by etching off the inner Er:Y₂O₃ core. Typically, the core–shell structured Er:Y₂O₃@SiO₂@mTiO₂ nanospheres was dissolved into 10 mL of HCl (0.1 mol L⁻¹) and subjected to ultrasonic treatment for 2 h. The products were washed with H₂O and ethanol for several times, and dried at 60 °C for 6 h.

Material characterization

X-ray diffraction (XRD) patterns were recorded on a Bruker D8 X-ray diffractometer with Ni-filtered Cu Kα radiation (40 kV, 40 mA). Nitrogen sorption isotherms were measured at 77 K with a Micromeritics Tristar 3020 analyzer (USA). Prior to measurements, the samples were degassed in a vacuum at 160 °C for 9 h. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas using adsorption data in the relative pressure range P/P₀ = 0.05–0.3. Using the Barrett–Joyner–Halenda (BJH) model, the pore size distributions were derived from the adsorption branches of the isotherms, and the total pore volumes were estimated from the adsorbed amount at the relative pressure P/P₀ = 0.995. Transmission electron microscopy (TEM) was carried out on a JEOL 2011 microscope (Japan) operated at 200 kV. For TEM measurements, the sample was suspended in ethanol and supported on a holey carbon film on a Cu grid. Time-resolved fluorescence measurements were performed by using a Fluorometer (wavelength range: excitation 200–1100 nm, emission 200–1100 nm; wavelength accuracy: <1.5 nm; wavelength setting repeatability: <0.2 nm; Cary Eclipse, Agilent Technologies). The excitation of samples at 522 nm was achieved by using a Xe pulsed lamp. Powder UV-Vis absorbance spectra were measured using a UV-2600 spectrophotometer (Shimadzu, Japan) equipped with an integrating sphere. The emission spectra were recorded on LS-50B, Perkin Elmer instrument with the excitation wavelength of 350 nm. Scanning electron microscopy (SEM) images were taken on S-4800 II field-emission scanning electron microanalyzer with an accelerating voltage (15 kV).

Sonocatalytic degradation experiment

The sonoreactor consisted of a double-layered cylindrical container with a capacity of 1.25 L (Φ 10.0 × 16.0 cm), and was equipped with a cup-horn-type ultrasonic transducer (Mirae Ultrasonic MEGA-100). The frequency of 300 kHz and electric power of 100 W were applied for carrying on the sonocatalytic reaction. A 800 mL reactor (Φ 7.0 × 20.0 cm) was emerged into the container loaded with 400 mL of water for the reaction. The solution temperature was measured using a thermometer (Tecpel DTM-318) and maintained with a water jacket. A retort stand was used to fix the emerged reactor. The distance between the bottom of the reactor and container was about 6.0 cm. Aqueous suspensions (100 mL) of BPA (5 mg L⁻¹) and catalyst (0.5 g L⁻¹) were used for the sonocatalytic degradation. At given time intervals, 0.5 mL of the suspension was removed using a 2 mL syringe and filtered by a membrane with a pore size of ∼0.45 μm. The BPA concentration in the resultant filtrate was analyzed on a High-Performance Liquid Chromatography (HPLC, Agilent 1260) with a Eclipse XDB C18 column (4.6 × 250 mm, 5 μm) and a diode array UV detector (G4212B 1260 DAD, λ = 210 nm). Mobile phase consisted of 60 vol% aqueous acetonitrile (ACN) solution (A) and 100 vol% ACN solution (B). In the recycling test, the used catalyst was separated by centrifuging (4000 rpm) and washed with water for three times. The obtained catalyst was dried at 60 °C for 12 h and collected for further recycling test.

Sonochemiluminescence observation

Sonochemiluminescence (SCL) was used to visualize the sonochemical reaction zone in the presence of various catalysts. The luminol solution was prepared by mixing 0.1 g L⁻¹ luminol (3-aminophthalhydrazide) with 1 g L⁻¹ sodium hydroxide. Luminol reacts with hydroxyl radicals generated during the collapse of cavitation bubbles to produce aminophthalate anions that exhibit blue fluorescence. SCL images were recorded for an exposure time of 30 second using a digital camera (Canon EOD 400D equipped with a Tamron AF 17–50 mm lens) in a dark room.

Results and discussion

The synthesis strategy for the core–shell structured Er:Y₂O₃@SiO₂@mesoporous TiO₂ (mTiO₂) nanospheres is depicted in Fig. 1. First, the uniform Er³⁺ doped Y₂O₃ nanoparticles (NPs) with various weight ratio (1, 3, and 5 wt%) were synthesized via a urea assisted co-precipitation method and coated with a silica layer through a sol–gel approach in the presence of tetraethyl orthosilicate (TEOS). Then, a kinetically-controlled sol–gel coating process was utilized to deposit a titania shell onto the silica layer, leading to a core–shell–shell structured amorphous Er:Y₂O₃@SiO₂@TiO₂ nanospheres. Finally, an ultrasound assisted post-hydrolysis process was used to assist the formation of mesopores on the TiO₂ shell, after which the resultant material was calcined at 550 °C to remove the organic species and improve the crystallinity. The resultant composites with different Er³⁺ loadings in Y₂O₃ nanocrystals were denoted as Er:Y₂O₃-1@SiO₂@mTiO₂, Er:Y₂O₃-2@SiO₂@mTiO₂, and Er:Y₂O₃-3@SiO₂@mTiO₂, respectively.

Fig. 1 Schematic illustration for combining the UC agent with mesoporous TiO₂.

The transmission electron microscopy (TEM) images reveal that the obtained Er:Y₂O₃ NPs possess a uniform spherical shape with an average diameter of ∼260 nm (Fig. 2a and b). After the first sol–gel coating process, the TEM images clearly show that a smooth silica layer with a thickness of ∼30 nm is uniformly coated onto the UC core, resulting in a well-defined core–shell structure (Fig. 2c). A subsequent sol–gel coating process leads to the deposition of a titania shell onto the surface of Er:Y₂O₃@SiO₂ nanospheres. Followed by an ultrasound assisted post-hydrolysis and a calcination process, typical sandwich-like core–shell–shell structured Er:Y₂O₃@SiO₂@mTiO₂ nanospheres with an average diameter of 380 nm are obtained (Fig. 2d and e), indicating the presence of ∼30 nm thick TiO₂ layer. In addition, both the TEM and SEM image (Fig. 2f and g) show that the TiO₂ shells exhibit highly mesoporous structures, which are resulted from the voids between the aggregated TiO₂ oligomers. The high resolution TEM (HRTEM) image (Fig. 2h) clearly displays that the TiO₂ shells are well crystallized with a size of ∼5.5 nm and a d-spacing of 0.35 nm, well-matching to the d₁₀₁ of anatase TiO₂.²⁵ Energy-dispersive X-ray (EDX) analysis of the sample Er:Y₂O₃-2@SiO₂@mTiO₂ (Fig. 2i) taken within the red spot exhibits the characteristic peaks of Ti, Y, Er, Si and O, suggesting the possible coexistence of Er:Y₂O₃, silica and titania. In addition, the quantitative result indicates that the weight percentage of Er and Y in the resultant composites is 0.34 and 9.7 wt%, which is similar with the adding ratio. Moreover, hollow structured mesoporous TiO₂ can be obtained by etching off the inner UC core with HCl solution (Fig. 3, insert). EDX analysis only displays the characteristic peaks of O, Si, and Ti, revealing that the inner UC core was well removed (Fig. 3).

Fig. 2 TEM images of the uniform erbium doped Y₂O₃ nanospheres with dopant content of 3 wt% prepared via a urea assisted co-precipitation method (a and b), core–shell structured Er:Y₂O₃@SiO₂ nanospheres obtained by a Stöber sol–gel coating method (c), mesoporous Er:Y₂O₃@SiO₂@mTiO₂ nanospheres through a kinetically controlled sol–gel method (d, e, and g), SEM image of Er:Y₂O₃@SiO₂@mTiO₂ nanospheres (f), HRTEM image of mesoporous Er:Y₂O₃-2@SiO₂@mTiO₂ nanospheres (h), and EDX analysis taken within the red circle (i).

Fig. 3 EDX analysis for hollow structured mesoporous TiO₂. Insert is the TEM image.

N₂ sorption isotherm of the Er:Y₂O₃-2@SiO₂@mTiO₂ composites (Fig. 4A) shows a characteristic IV curve with a hysteresis loops close to H₁-type, further suggesting that the outer TiO₂ shells contain uniform mesopores. The BET surface area and pore volume of the composites are calculated to be 151.5 m² g⁻¹ and 0.23 cm³ g⁻¹, respectively. Correspondingly, the pore size distribution (Fig. 4B) derived from the adsorption branch using the Barrett–Joyner–Halenda (BJH) method reveals a uniform pore size centered at ∼3.8 nm. The X-ray diffraction patterns (XRD) of the Er:Y₂O₃@SiO₂@mTiO₂ nanospheres (Fig. 5) exhibit a sharp and strong diffraction peak at 2θ = 25.3°, which can be well indexed as the 101 reflection of anatase TiO₂ crystalline phase (JCPDS card no. 01-073-1764). Moreover, the size of TiO₂ nanocrystals (Table 1) in the Er:Y₂O₃@SiO₂@mTiO₂ composites was calculated to be ∼5.5 nm using Scherrer formula, which is similar to the HRTEM result. Additional diffraction peaks were observed in the patterns of all materials, which are typical for Y₂O₃ (JCPDS card no. 00-043-1036). No diffraction peak of erbium oxide was detected, indicating that the Er³⁺ ion was well doped into the nanocrystals of Y₂O₃.

Fig. 4 N₂ sorption isotherms (A) and pore size distribution (B) of Er:Y₂O₃-2@SiO₂@mTiO₂ nanosphere.

Fig. 5 XRD patterns of (a) Er:Y₂O₃-2, (b) Er:Y₂O₃-2@SiO₂, (c) Er:Y₂O₃-1@SiO₂@mTiO₂, (d) Er:Y₂O₃-2@SiO₂@mTiO₂, and (e) Er:Y₂O₃-3@SiO₂@mTiO₂.

Table 1 Physicochemical properties of the resultant core–shell nanospheres^a

Sample name	DA (nm)	D (nm)	SBET (m^2 g^−1)	V (cm^3 g^−1)
a DA, the particle size of anatase nanocrystals calculated by Scherrer formula; SBET, BET surface area; D, pore-size diameter; V, total pore volume.
Er:Y2O3-1@SiO2@mTiO2	5.4	3.8	150.8	0.23
Er:Y2O3-2@SiO2@mTiO2	5.5	3.8	151.5	0.23
Er:Y2O3-3@SiO2@mTiO2	5.7	3.7	153.2	0.24

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UV-Vis absorption spectra (Fig. 6A) of the Er:Y₂O₃@SiO₂@mTiO₂ nanospheres with varied Er³⁺ contents reveal that all the samples possess an absorption edge of ∼420 nm, corresponding to the bandgap energy of anatase TiO₂ (3.2 eV). Moreover, a peak at 522 nm was clearly observed and the intensity increases with rising Er³⁺ concentration, which is due to the energy transition from ⁴I_15/2 to ²I_11/2 in Er³⁺ dopant.¹⁶ To prove the UC properties of the resultant composites, the fluorescence analysis was carried out (Fig. 6B). When the samples were excited with a 522 nm laser, the UC signals were clearly shown at the wavelengths of 339, 363, 394, 403, and 423 nm, which can be assigned to ²P_3/2 → ⁴I_15/2, ²K_15/2 → ⁴I_15/2, ⁴G_11/2 → ⁴I_15/2, ²H_9/2 → ⁴I_15/2, and ⁴H_3/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively.^26,27 The UC could be triggered by the excited state absorption (ESA) mechanism which refers to that Er³⁺ in the excitation state can still absorb photon(s) to transit to higher energy levels. Afterwards, the UC luminescence would be generated when the high-energy leveled Er³⁺ ion relaxes to the ground state (Fig. S1†).²⁸ In addition, it was found that the maximum intensity of luminescence was obtained with an Er³⁺ content of 3 wt% in the Y₂O₃ nanocrystals. This is due to the fact that erbium ions in a lower concentration (1 wt%) are randomly distributed in the host lattice and the average distance between two Er³⁺ ions are too far apart to interact efficiently. However, at a higher concentration (5 wt%) the average Er³⁺–Er³⁺ distance are shortening, thus leading to the formation of Er³⁺ cluster and concentration quenching of Er³⁺ ions. As a result, the non-radiative decay will become prominent in the whole UC process, causing the loss of energy and the degradation of emission intensities.^29,30 To further evaluate the UC property of the resultant composites, a metal halide lamp (150 W) with a 420 cutoff filter (λ > 420 nm) was employed as the light source to examine the photocatalytic degradation activity for BPA over commercially available TiO₂ NPs (P25) and Er:Y₂O₃-2@SiO₂@mTiO₂ composites (Fig. 7). Before initiation of the reaction, the mixture was mechanically stirred in dark for 60 min to reach the adsorption/desorption equilibrium between the catalyst and pollutants. It was found that the P25 catalyst did not show any visible photocatalytic activity and the concentration change of BPA in the system fluctuated possibly owing to the adsorption/desorption equilibrium. Among the UC catalysts tested, Er:Y₂O₃-2@SiO₂@mTiO₂ exhibits the best degradation performance (16% decomposition of BPA in 2 h), which is higher than that of previous UC based photocatalyst (10% degradation of methyl blue in 2 h).³¹

Fig. 6 UV-Vis absorption spectra (A) and emission spectra (B) of Er:Y₂O₃@SiO₂@mTiO₂ nanospheres with various erbium concentrations excited using a Xe flash source. Black: Er:Y₂O₃-1@SiO₂@mTiO₂, red: Er:Y₂O₃-2@SiO₂@mTiO₂, and green: Er:Y₂O₃-3@SiO₂@mTiO₂.

Fig. 7 (A) Visible-light photocatalytic degradation profile of BPA in the presence of (a) commercial TiO₂ nanoparticles (P25), (b) Er:Y₂O₃-1@SiO₂@mTiO₂, (c) Er:Y₂O₃-2@SiO₂@mTiO₂ and (d) Er:Y₂O₃-3@SiO₂@mTiO₂ and (B) the intensity distribution of the light source utilized. Before initiation of the reaction, the mixture was mechanically stirred in dark for 60 min to reach the adsorption/desorption equilibrium between the catalyst and pollutants.

The performance of the resultant core–shell materials on the sonocatalytic degradation of BPA was examined using an US with a frequency of 300 kHz and an applied power of 100 W (Fig. 8 and S2†). As a control, the degradation performances of US alone, Er:Y₂O₃-2 and Er:Y₂O₃-2@SiO₂ were also tested, exhibiting the degradation rate constants of 0.054, 0.086 and 0.066 min⁻¹, respectively, when a pseudo first-order reaction was assumed. The higher degradation rate of Er:Y₂O₃-2 nanoparticles than Er:Y₂O₃-2@SiO₂ core–shell nanospheres could be due to the fact that the former one possesses a smaller particle size and a larger surface area, thus providing more nucleation sites for the cavitation bubble formation. Notably, the degradation performance was greatly improved when Er:Y₂O₃@SiO₂@mTiO₂ composites were utilized as the sonocatalysts. The sonocatalytic degradation rate of BPA (Fig. 8A and B) first increased with the rise of Er³⁺ content in the Y₂O₃ nanocrystals. A maximum sonocatalytic degradation rate was observed in the presence of the sample Er:Y₂O₃-2@SiO₂@mTiO₂ (0.155 min⁻¹), which was 2.9 and 2 times higher than that of US alone and hollow structured mesoporous TiO₂ (0.077 min⁻¹), respectively. More importantly, the degradation rate is much faster than of the sonocatalyst reported previously for treatment of BPA (0.09–0.14 min⁻¹).^32,33 In addition, BPA can be completely removed in 30 min. Further increasing Er³⁺ content to 5% did not lead to any further increase in the sonocatalytic reaction rate, and even caused a decrease, which is due to the reduced UC emission intensity (Fig. 6B). The trend of sonocatalytic activity with respect to different doping content is same as that of the intensity of luminescence, revealing the close relationship between the sonocatalytic performance and luminescence property of the catalyst. Note that the morphologies and surface areas for all the mesoporous Er:Y₂O₃@SiO₂@mTiO₂ samples (Fig. S3,† Table 1) are similar, indicating that the effects of morphology and surface area on the behavior of photo-generated charges for these samples are negligible. Interestingly, the nonporous Er:Y₂O₃-2@SiO₂@TiO₂ composites synthesized without a post-hydrolysis process exhibited an apparently slower degradation rate (0.096 min⁻¹) than mesoporous one (0.155 min⁻¹), suggesting the priority of mesoporous structure in the resultant composites. The remarkable sonocatalytic performance of the resultant catalyst can be attributed to two possible reasons, as illustrated in Fig. 9. On one hand, the presence of mesopores accelerates the nucleation rate for the formation of cavitation bubbles, producing superior cavitation effects to the nonporous material because pore corners in porous material can provide energetically preferred binding sites at which the new phase can be easier hold.^34–36 The SCL images (Fig. S4†) recorded with an exposure time of 30 s using a digital camera were taken to visualize the change of cavitation reactive zone in the presence of nonporous and porous catalyst.^33,37 It clearly shows that the sonochemical reaction zone becomes wider and scattered after adding nonporous Er:Y₂O₃-2@SiO₂@TiO₂ catalyst because the presence of solid impurities could increase the nucleation rate to some extent by altering the nucleation from the aqueous phase to the solid–liquid boundary.³⁸ However, when the mesoporous Er:Y₂O₃-2@SiO₂@mTiO₂ catalyst was used, the blue color in SCL image becomes more intense and bright, further illustrating the priority of mesoporous structure. On the other hand, the UC core could absorb the visible light photons from SL (Fig. S5†) and then emit UV light photons through an energy transfer, which can excite TiO₂ nanocrystals to form additional electron–hole pairs on the surface, increasing the concentration of reactive oxidation species in the reaction system.^39,40 Note that although the TiO₂ fabricated in this study possessed mesoporous structures and high surface areas, the adsorption effect is not significant possibly owing to the low affinity of TiO₂ towards BPA (Fig. S5†). Thus, the physical adsorption effect can be neglected compared with the high degradation performance. Therefore, the high catalytic activity of Er:Y₂O₃@SiO₂@mTiO₂ catalyst is related to the intensified cavitation reactive zone resulted from the mesoporous structure and the generation of additional photo-excited charge carriers achieved through UC mechanism. The recycle test of the core–shell structured Er:Y₂O₃-2@SiO₂@mTiO₂ was also examined (Fig. 10). After four recycles, a constant degradation performance was retained, indicating the excellent reusability of this material. In addition, TEM images (Fig. 11) clearly showed that the defined core–shell structure is well retained, suggesting the excellent mechanical stability of the resultant composites. However, SEM images revealed that the surface of the recycled sample became a little bit rougher relative to fresh one and a few nanospheres were broken, which was possibly resulted from the continuous asymmetrical pitting by the shock waves (Fig. S6†). The reusability tests further confirm that Er:Y₂O₃@SiO₂@mTiO₂ composites are promising candidates as sonocatalysts.

Fig. 8 The sonocatalytic degradation performance (A) and pseudo first-order kinetic constant (B) of BPA with varied sonocatalysts. (a) US alone, (b) Er:Y₂O₃-1@SiO₂@mTiO₂, (c) Er:Y₂O₃-2@SiO₂@mTiO₂, (d) Er:Y₂O₃-3@SiO₂@mTiO₂, (e) hollow structured mesoporous TiO₂, and (f) nonporous Er:Y₂O₃-2@SiO₂@TiO₂ composites. In (B), the value above the vertical bar is the coefficient of determination (R²), which is defined as the ratio of the explained variation to the total variation.

Fig. 9 Schematic illustration for the sonocatalytic mechanism in the presence of Er:Y₂O₃@SiO₂@mTiO₂ catalyst. On one hand, the presence of mesopores serves as nucleation sites for accelerating the formation of cavitation bubbles. On the other hand, the UC agent absorbs the visible-light photons from SL and then emits the UV light photons through ESA mechanism, which can further excite anatase TiO₂ to form electron–holes pairs. G: ground state, E1 and E2: excitation state 1 and 2, CB: conduction band, VB: valence band, and SL: sonoluminescence.

Fig. 10 Recycle test for the Er:Y₂O₃-2@SiO₂@mTiO₂ sonocatalyst.

Fig. 11 TEM images of recycled Er:Y₂O₃-2@SiO₂@mTiO₂ catalyst (a and b).

Conclusion

In summary, we report the integration of a visible-light active UC agent with mesoporous TiO₂ via a versatile two-step sol–gel coating strategy. The resultant composites possess a high BET surface area (∼151.5 m² g⁻¹), a large pore volume (∼0.23 cm³ g⁻¹) and uniform mesopores (∼3.8 nm) as well as an excellent UC property. When evaluated as an advanced sonocatalyst for degradation of BPA, the catalyst with an erbium dopant content of 3 wt% in the Y₂O₃ nanocrystals is found to exhibit the best degradation performance. The pseudo first-order degradation rate constant is calculated to be 0.155 min⁻¹, which is 2.9 and 2 times larger than that of US alone (0.054 min⁻¹) and hollow structured mesoporous TiO₂ NPs (0.077 min⁻¹), respectively. The superior catalytic activity is ascribed to an intensified cavitation reaction zone and an enhanced amount of photo-generated charges. In addition, the catalyst can be easily recycled and exhibit an excellent reusability. This study gives an important insight into the design and synthesis of multi-functional catalyst for sonochemical process.

Acknowledgements

We thank the Korea Institute of Energy Technology Evaluation and Planning (KETEP, 20152510101820) and Korea Mine Reclamation Corporation (MIRECO, Q1512631) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01689h

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