Facile synthesis of uniform yolk–shell structured magnetic mesoporous silica as an advanced photo-Fenton-like catalyst for degrading rhodamine B

Abstract

Through an ultrasound assisted etching method, uniform yolk–shell structured magnetic mesoporous silica (Fe₃O₄@void@mSiO₂) nanospheres have been fabricated and for the first time demonstrated as an efficient catalyst for degrading rhodamine B under photo-Fenton-like conditions.

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The Fenton reaction based on ferrous ions and hydrogen peroxide has been widely investigated as an effective way to degrade organic pollutants through hydroxyl radical oxidation. However, the narrow working pH range (<4), difficulties in recovering the dissolved metal ions and necessity for further treatment of ferric hydroxide sludge greatly hinder its wide application for practical water treatment.¹ To this end, synthesis of a magnetically separable Fenton-like catalyst is highly desired.² More importantly, magnetite nanoparticles (NPs) has been extensively demonstrated as the most effective heterogeneous Fenton catalyst among Fe-based materials as it contains higher content of structural Fe²⁺.³ However, Fe₃O₄ NPs still have some shortcomings such as spontaneous aggregation due to high surface energy, limited adsorption ability owing to its low surface area and small pore volume, readily contaminated catalytic sites when exposed to bulk solution and relevant low H₂O₂ activation activity because of the fully oxidized surface.⁴ Therefore, immobilizing Fenton catalysts into a porous matrix and irradiation with UV light has been regarded as an ideal way to address the above difficulties.⁵

Recently, yolk–shell structured materials with a typical core@void@shell architecture have received considerable attention. With the appealing structures of movable cores, interstitial hollow spaces and multifunctional shells, this kind of material holds an important role in modern science and technology as potential candidates for tremendous applications such as confined catalysis,⁶ lithium-ion batteries,⁷ drug delivery,⁸ and sensors.⁹ Among them, the Fe₃O₄@void@mesoporous SiO₂ (mSiO₂) nanostructures are of great interest and importance owing to their multifunctional properties including abundant catalytic sites, strong magnetic separation ability, and excellent biocompatibility and versatility in surface functionalization as well as the high surface area, ordered mesoporous structure and large porosity of mSiO₂ greatly facilitating guest molecule adsorption and diffusion.¹⁰ To date, substantial research efforts have been made to synthesis of this type of material. One common strategy involves the use of polymeric carbon interlayer as a hard template, which can be selectively removed by calcination or solvent extraction, for the formation of yolk–shell structures.^6a,11 However, this strategy is often associated with some disadvantages such as requiring high energy cost, producing materials with collapsed mesoporous structures, etc.¹² To solve these problems, an alternative method termed as “surface protected etching approach” has been developed to synthesize high-quality yolk–shell/hollow structured silica materials. Solid silica spheres or a silica interlayer was used as templates to create desired structures with the protection of some polymeric ligands or templates.^8b,13 However, the investigation of yolk-shell structured magnetic mesoporous silica as a photo-Fenton-like catalyst has rarely been reported so far.

Herein, we report an ultrasound assisted etching method to synthesize uniform yolk–shell structured magnetic mesoporous silica nanospheres consisting of a movable magnetite core, interstitial void spaces and a mesoporous silica shell. This etching strategy is facile, mild and effective. The resultant material has been demonstrated as a catalyst in the photo-Fenton-like process, showing a remarkable degradation performance for rhodamine B (RhB) in a wide range of pH values (3–7). More importantly, the catalysts can be easily recycled within a short time (<2 min) by using an external magnetic field and a constant catalytic activity is retained even after five cycles. This study paves a promising way for the design and synthesis of multifunctional catalyst for the photo-Fenton like process.

The synthesis strategy for the yolk–shell structured magnetic mesoporous silica nanospheres is depicted in Fig. 1. First, the uniform magnetite particles were coated with a nonporous silica layer through a sol–gel approach in the presence of tetraethyl orthosilicate (TEOS) (denoted as Fe₃O₄@SiO₂).¹⁴ Then, a further sol–gel coating process was utilized to deposit a mesoporous silica shell onto the nonporous silica layer with the help of a cationic template hexadecyl trimethyl ammonium bromide (CTAB) (defined as Fe₃O₄@SiO₂@mSiO₂). Finally, an ultrasound assisted etching method was used to etch off the nonporous silica layer in a weak alkaline media, after which the CTAB was removed in an ammonia nitrate ethanol solution, leading to the resultant yolk–shell structured magnetic mesoporous silica nanospheres (designated as Fe₃O₄@void@mSiO₂).

Fig. 1 Schematic illustration for the synthesis of the yolk–shell structured Fe₃O₄@void@mSiO₂ nanospheres via an ultrasound assisted etching method.

The scan electron microscopy (SEM) images reveal that the obtained Fe₃O₄ particles possess a uniform spherical shape with an average diameter of ∼130 nm (Fig. S1A and B†). After the first sol–gel coating process, transmission electron microscopy (TEM) images clearly show that a nonporous silica layer with a thickness of ∼30 nm is uniformly coated onto the magnetic cores, resulting in a well-defined core–shell structure (Fig. S1C and D†). A subsequent sol–gel coating process leads to the deposition of a mesoporous silica shell onto the surface of Fe₃O₄@SiO₂ nanospheres. TEM images (Fig. 2A and B) clearly depict that Fe₃O₄@SiO₂@mSiO₂ nanospheres possess a typical sandwich-like core–shell–shell structure with an outer mesoporous silica shell (∼30 nm). After the ultrasound assisted etching and extracting processes, a well-defined yolk–shell nanostructure with perpendicular mesopores (∼2.8 nm) is obtained (Fig. 2C and D). The spacing between the inner magnetite cores and the outer mesoporous silica shells is measured to be ∼30 nm, well corresponding to the thickness of the nonporous silica layer, suggesting that the condensed silica layer can be selectively removed in Na₂CO₃ solution under ultrasound irradiation. The template CTAB served as a “surface protecting agent”, greatly increasing the stability of outer mesoporous silica shell against etching.¹⁵ In contrast, it is hard to get unambiguous yolk–shell structures under the conditions without ultrasound, even extending the reaction time to 12 h (Fig. S2†). However, when ultrasonic irradiation was used, uniform yolk–shell structure can be obtained in a very short aging time (6 h). This can be attributed to that the high-speed microjects (100 ms⁻¹) and enormous localized temperature (5000 K) generated from cavitation bubbles collapse greatly accelerate the diffusion and etching process.¹⁶ Moreover, hollow mesoporous silica (H-mSiO₂) can be obtained by etching off the inner magnetite cores with HCl solution (Fig. S3†).

Fig. 2 TEM images of the Fe₃O₄@SiO₂@mSiO₂ nanospheres (A and B) synthesized through two-steps sol–gel method and Fe₃O₄@void@mSiO₂ nanospheres (C and D) fabricated via an ultrasound assisted etching method.

N₂ sorption isotherms of the yolk–shell structured magnetic mesoporous silica nanospheres (Fig. 3A) show a characteristic IV curve with a hysteresis loops close to H₁-type and an increase in the adsorption branch at a relative pressure of P/P₀ = 0.2–0.5, further suggesting that the outer silica shells contain uniform mesopores. In addition, the loop with parallel branches at the relative pressure between 0.1 and 0.4 is attributed to the cavity between the magnetic core and the outer shell.¹⁷ The BET surface area and pore volume of the Fe₃O₄@void@mSiO₂ nanospheres are measured to be 406.1 m² g⁻¹ and 0.34 cm³ g⁻¹, respectively. Correspondingly, the pore size distribution (Fig. 3B) calculated from the adsorption branch using the Barrett–Joyner–Halenda (BJH) method reveals a uniform pore size centered at ∼2.8 nm, which is similar to the TEM results. The X-ray diffraction pattern (XRD) of the Fe₃O₄@void@mSiO₂ nanospheres (Fig. 3C) shows six well resolved characteristic diffraction peaks, which are typical for Fe₃O₄ crystalline phase. In addition, a broad amorphous silica peak can be clearly distinguished compared with XRD pattern of pure Fe₃O₄ NPs. The magnetization saturation values of pristine Fe₃O₄ and Fe₃O₄@void@mSiO₂ nanospheres are measured to be ∼58.4 and 18 emu g⁻¹, respectively (Fig. 3D). As a result of the superparamagnetic property and high magnetization, the Fe₃O₄@void@mSiO₂ nanospheres in their homogeneous dispersion show fast motion under the applied magnetic field and quick dispersibility upon a slight shake when the magnetic field is removed (Fig. 3D insert).

Fig. 3 (A) N₂ sorption isotherms and (B) pore-size distributions of the Fe₃O₄@void@mSiO₂ nanospheres; (C) XRD patterns of (a) the Fe₃O₄ particles and (b) uniform yolk–shell structured Fe₃O₄@void@mSiO₂ nanospheres and (D) The magnetic hysteresis loops at 300 K of (a) Fe₃O₄ spheres and (b) uniform yolk–shell structured Fe₃O₄@void@mSiO₂ nanospheres. Inset shows the simple magnetic separation process via a hand-held magnet.

The performance of the resultant magnetic mesoporous silica on the photo-Fenton-like catalytic degradation of RhB was examined at pH 3 (Fig. 4A). As a control, the degradation performances of RhB under H₂O₂, UV/H₂O₂, and Fe₃O₄@void@mSiO₂/H₂O₂ condition were also tested, exhibiting the removal efficiencies of 5.0, 39.5, and 25.5%, respectively. The higher degradation efficiency of UV/H₂O₂ process was due to the decomposition of H₂O₂ into hydroxyl radicals through UV light (UVA is not able to decompose RhB molecules).¹⁸ However, when Fe₃O₄@void@mSiO₂ nanospheres were used as a Fenton catalyst without UV light, the efficiency was not significant compared with the adsorption efficiency of Fe₃O₄@void@mSiO₂ (18.7%, Fig. 4B), which might result from the fully oxidized Fe₃O₄ surface inhibiting the activation of H₂O₂. Interestingly, when combine UV, H₂O₂ and Fe₃O₄@void@mSiO₂ catalyst together, the degradation performance of RhB was greatly improved and 98.3% degradation of RhB was observed in 160 min, indicating the importance of irradiation of UV light. This is because the irradiation of light can not only initiate the Fenton reaction by recovering the surface Fe³⁺ into Fe²⁺ based on light induced electron transfer according to the Haber–Weiss mechanism, but also prevent self recombination of hydroxyl radical by rapidly decomposing H₂O₂ into hydroxyl radical, thus increasing the utilization efficiency of H₂O₂ and Fe²⁺.¹⁹ To demonstrate the priority of Fe₃O₄@void@mSiO₂ nanospheres, the degradation performances of magnetite and H-mSiO₂ under photo-Fenton condition were also tested. Clearly, the degradation efficiencies of magnetite and H-mSiO₂ are 82.0 and 55%, respectively, which are much smaller than that of Fe₃O₄@void@mSiO₂ catalyst. The remarkable performance of the magnetic mesoporous silica can be attributed to the synergistic effect from its unique textual structures, as illustrated in Fig. 4C. First, the large pore size (∼2.8 nm) and ordered mesopores favored the mass transfer of both H₂O₂ and RhB (1.5 × 0.43 × 0.98 nm) molecules between aqueous and solid phases. Second, the presence of mesoporous silica shell can greatly enhance the adsorption of RhB molecules and enrich them in the void space of the nanoreactor,²⁰ which would be beneficial for surface hydroxyl radical oxidation rate. The adsorption test (Fig. 4B) clearly shows that the magnetic mesoporous silica possesses a 2 times higher adsorption efficiency than that of Fe₃O₄ (8.5%). Finally, the recovered Fe²⁺ reacts with the adsorbed H₂O₂ to produce large amount of hydroxyl radicals. These hydroxyl radicals can directly in situ oxidize the RhB molecules confined in the void space into small molecules or even CO₂ and H₂O, thus lowering the self recombination reaction rate of ˙OH that usually occurs in the oxidation process within the bulk phase. The effect of pH on the degradation performance of Fe₃O₄@void@mSiO₂ (Fig. S4A†) was investigated. We found that the removal efficiency at pH = 6.2 is still 88.0%, suggesting that such catalysts can work in a neutral pH. However, when the pH was extended to 10, the efficiency sharply decreased, which is possibly due to the self decomposition of H₂O₂ into water and oxygen.²¹ The recycle test of the yolk–shell structured Fe₃O₄@void@mSiO₂ at pH 3 was examined (Fig. S4B†). After five recycles, a constant degradation performance was retained, indicating the excellent reusability of this material. As a comparison, the recycle test of Fe₃O₄ was also tested. Notably, the removal efficiency of RhB decreased from 82.0% to 71.0% (13.4% loss) after recycling five times (Fig. S4B(b).† The poor reusability of Fe₃O₄ can be attributed to the high total Fe leaching rate and rapid weight loss during recycling (Fig. S5†).

Fig. 4 (A) The photo-Fenton catalytic degradation performance of RhB under varied conditions: (a) H₂O₂ alone, (b) Fe₃O₄@void@mSiO₂/H₂O₂ process, (c) UV/H₂O₂ process, (d) UV/mSiO₂/H₂O₂ process, (e) UV/Fe₃O₄/H₂O₂, and (f) UV/Fe₃O₄@void@mSiO₂/H₂O₂. Before initiation of the reaction, the mixture was mechanically stirred in dark for 30 min to reach the adsorption/desorption equilibrium between the catalyst and pollutants, (B) the adsorption test of (a) Fe₃O₄ NPs and (b) Fe₃O₄@void@mSiO₂ nanospheres. Other factors were controlled as constants: pH = 3, T = 25 °C, and stirring speed = 300 rpm, and (C) schematic illustration for the photo-Fenton catalytic degradation of RhB in the presence of yolk–shell structured magnetic mesoporous silica spheres: (1) enhanced mass transfer by large ordered mesopores; (2) enrichment of H₂O₂ and RhB molecules within the void space; (3) surface recovery and in situ oxidation of organic pollutant on Fe₃O₄ particles.

In summary, we report the synthesis of uniform yolk–shell structured magnetic mesoporous silica nanospheres via a straightforward route. An ultrasound assisted etching method was first demonstrated to selectively remove the nonporous silica layer, which is facile, mild and effective. The resultant Fe₃O₄@void@mSiO₂ nanospheres possess uniform mesopores (∼2.8 nm), a high BET surface area (∼406.1 m² g⁻¹) and a large pore volume (∼0.34 cm³ g⁻¹) as well as a high magnetic susceptibility (∼18.0 emu g⁻¹). Then, it was demonstrated as an advanced photo-Fenton-like catalyst, showing a remarkable performance for degrading RhB. Moreover, the catalyst can work in a wide pH range (3–7) and exhibit an excellent reusability. This study paves a great way to synthesize and design a multifunctional material for the photo-Fenton like process.

Acknowledgements

We thank the Korea Institute of Energy Technology Evaluation and Planning (KETEP, 20152510101820), National Natural Science Foundation of China (21471034) and National Research Foundation of Korea (NRF, 2013R1A1A2006586) for financial support.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15693a

‡ The contribution is equal.

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