Synthesis of a superparamagnetic MFNs@SiO₂@Ag₄SiW₁₂O₄₀/Ag composite photocatalyst, its superior photocatalytic performance under visible light illumination, and its easy magnetic separation

Abstract

A novel superparamagnetic Ag@silver-based salt photocatalyst, MFNs@SiO₂@Ag₄SiW₁₂O₄₀/Ag, was created, which demonstrated highly efficient photocatalytic performance under visible light illumination in both the degradation of methylene blue (MB) and the disinfection of Escherichia coli (E. coli) bacteria. In this composite photocatalyst, well-dispersed, superparamagnetic magnesium ferrite nanoparticles (MFNs) were used as the core because of their easy magnetic separation capability. A passive SiO₂ mid-layer was used to separate MFNs and Ag₄SiW₁₂O₄₀ and form a strong bond with silver ions for their loading after –SH surface modification. The Ag₄SiW₁₂O₄₀ layer was subsequently formed by the reaction with silicotungstic acid to avoid the commonly adopted calcination procedure after deposition/precipitation, and silver nanoparticles were formed on the surface of Ag₄SiW₁₂O₄₀ layer after UV irradiation to further enhance their photocatalytic performance and stability under visible light illumination. The surface modification on the passive SiO₂ mid-layer and the bridging procedure for material loading developed in our approach could be readily applied to other material systems for the creation of novel composite materials with various functions.

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1. Introduction

Semiconductor-based photocatalysis could be a promising approach to solve energy and environmental problems faced by human beings now and in the near future if solar energy is efficiently utilized.¹ Among various semiconductor-based photocatalysts, TiO₂ has been extensively studied during the last several decades because of its high chemical stability, good photoactivity, relatively low cost, and nontoxicity.² However, its photocatalytic capability is limited to only ultraviolet (UV) light (wavelength λ < 400 nm), which only constitutes about 4% of the solar spectrum. Therefore, to enhance their solar efficiency, extensive research efforts have been made to explore photocatalysts that could be activated by visible light, including modified-TiO₂,^3,4 multimetal oxides,^5–7 sulfides,⁸ oxynitrides,⁹ graphite oxide,¹⁰ C₃N₄,¹¹ BiVO₄ (ref. 12) and heterojunctions.¹³

Recently, a series of silver-based compounds, including AgX (X = Cl, Br, I),^14,15 Ag₃PO₄,¹⁶ Ag₂CO₃,¹⁷ Ag₃VO₄,¹⁸ AgSbO₃,¹⁹ AgNbO₃,²⁰ AgIn(WO₄)₂,²¹ Ag₄SiW₁₂O₄₀,²² silver titanates²³ and delafossite AgMO₂ (M = Al, Ga, In),²⁴ have attracted considerable research attention due to their excellent performance as visible-light-driven photocatalysts. Due to the enhancement by the localized surface plasmon resonance (LSPR) effect of silver nanoparticles in the visible light region, their photocatalytic performance and stability were further improved under visible light illumination.^11,21,22 For example, Huang et al. reported that plasmatic Ag@AgX (X = Cl, Br, I) exhibited highly efficient and stable photocatalytic activity under visible light illumination,^14,15 and Gondal et al. found that Ag@Ag₃PO₄ showed enhanced photocatalytic activities under both UV and visible light for the degradation of rhodamine B.²⁵ The photocatalytic performance of Ag@silver-based salt photocatalysts could also be tuned by altering the negatively charged anions in this material system, and it has been reported that anions with higher charges led to a stronger photocatalytic capability.²² Thus, Ag@Ag₄SiW₁₂O₄₀ might be a promising candidate with a highly efficient visible-light-driven photocatalytic activity, and it should be further explored for various technical applications, including the degradation of organic pollutant, disinfection of pathogen microorganism, and water splitting under visible light illumination.

To remove nanomaterials from an aqueous environment efficiently and selectively after water treatment, magnetic separation was considered as a promising alternative compared with the conventional centrifugation or filtration processes.²⁶ Magnetic photocatalysts of Ag@silver-based salt (ASS), such as Fe₃O₄@SiO₂@AgCl–Ag,²⁷ AgCl-doped Fe₃O₄@SiO₂,²⁸ Ag–AgI/Fe₃O₄@SiO₂ (ref. 29) and Ag/AgBr/Fe₃O₄@SiO₂,³⁰ have been reported; however, there were several drawbacks in the synthetic procedures for ASS. First, silver based compounds were immobilized on Fe₃O₄@SiO₂ cores with deposition/precipitation procedures, resulting in nonuniform morphology.²⁷ Second, the calcination procedure after deposition/precipitation increased energy consumption/cost and usually led to their aggregation and a subsequent lower surface-to-volume ratio, deteriorating their photocatalytic activity.²⁷ Third, incorporated magnetic cores may contain remanent magnetism and magnetic attraction existed between these nano-ASSs even without the presence of external magnetic field; therefore, it was difficult to disperse them in water for a better contact efficiency with pollutants.³¹ Thus, new approaches should be developed to overcome these difficulties in the design and synthesis of novel magnetic ASSs with high photocatalytic performance.

We report here a novel superparamagnetic ASS, i.e., magnesium ferrite nanoparticles (MFNs)@SiO₂@Ag₄SiW₁₂O₄₀/Ag. In this photocatalyst material system, well-dispersed, superparamagnetic magnesium ferrite nanoparticles were used as the core.^32–34 A passive SiO₂ mid-layer was introduced to coat the MFNs, which separated MFNs and Ag₄SiW₁₂O₄₀, to prevent the charge carrier recombination in MFNs for the enhancement of their photoactivity and provide an easy surface modification potential. After –SH modification on the SiO₂ mid-layer surface, silver ions could be uniformly bridged onto the surface of MFNs@SiO₂, and Ag₄SiW₁₂O₄₀ layer was subsequently formed by the reaction with silicotungstic acid to avoid the commonly adopted calcination procedure after deposition/precipitation. Finally, silver nanoparticles were formed on the surface of Ag₄SiW₁₂O₄₀ layer after UV irradiation to further enhance their photocatalytic performance and stability under visible light illumination, which was demonstrated in both the degradation of methylene blue (MB) and the disinfection of Escherichia coli (E. coli) bacteria.

2. Experimental section

2.1. Chemicals and material

Well-dispersed, superparamagnetic magnesium ferrite nanoparticles were synthesized as detailed in our previous report.³³ Tetraethyl orthosilicate (TEOS, C₈H₂₀O₄Si, 99.9%, Tianjin Kermel Chemical Reagents Development Center, Tianjin, P. R. China), aqueous ammonia (NH₃·H₂O, 25–28%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, P. R. China), mercaptopropyltriethoxysilane (MPTES, C₉H₂₂O₃SSi, 98%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, P. R. China) and methylbenzene (C₇H₈, 99.9%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, P. R. China) were used to coat the SiO₂ layer on MFN to obtain MFN@SiO₂ and modify the SiO₂ layer surface with –SH. Cetyltrimethylammoniumbromide (CTAB, C₁₉H₄₂BrN, 99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, P. R. China), silver nitrate, sodium chloride, sodium carbonate, sodium phosphate and silicotungstic acid (99.9%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, P. R. China) were used to synthesize MFNs@SiO₂/Ag₄SiW₁₂O₄₀ photocatalyst. Methylene blue (C₁₆H₁₈ClN₃S₂, 99%, Shanghai Huyu biotechnology Co., Ltd, Shanghai, P. R. China) and E. coli bacteria (ATCC 15597, American type culture collection) were used for the photocatalytic degradation and disinfection experiments, respectively. Commercially available Degussa P25 TiO₂ nanoparticles (Evonik Industries, Germany) were used for comparison.

2.2. Synthesis of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst

Fig. 1 schematically shows the synthetic procedure of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst. Superparamagnetic MFNs (0.05 g) were firstly dispersed into a mixed solution of 80 mL ethanol and 20 mL deionized water, and ultrasonicated for 30 min. Then, 100 μL TEOS was added dropwise into the suspension with mechanical stirring at 200 rpm, and 2 mL NH₃·H₂O was quickly added into it after 5 min. The suspension was stirred for 6 h before MFN@SiO₂ nanoparticles were magnetically separated and re-dispersed in 5 mL ethanol. These steps were repeated to get sufficient amount of MFN@SiO₂ nanoparticles dispersed in ethanol. 50 mL of such suspension was transferred into a 250 mL three-necked flask and kept at 80 °C in a water bath. 200 μL MPTES was then added dropwise into the flask, and the suspension was refluxed for 12 h. After magnetic separation and washing with deionized water and ethanol for three times, MFNs@SiO₂ with –SH surface modification were obtained.

Fig. 1 The schematic illustration of the synthetic procedure of the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst (A: MFNs, B: MFNs@SiO₂, C: MFNs@SiO₂ with –SH surface modification, D: MFNs@SiO₂/Ag⁺, E: MFNs@SiO₂/Ag₄SiW₁₂O₄₀, and F: MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag).

These MFNs@SiO₂ with –SH surface modification were then dispersed into 50 mL silver nitrate solution (0.01 M) by ultrasonication, and the Ag⁺ adsorption lasted for 12 h. Nanoparticle samples were magnetically separated and washed with deionized water until no Ag⁺ was found in water. Then, they were re-dispersed in 50 mL deionized water with a proper amount of CTAB, and 50 mL silicotungstic acid solution (0.0025 M) was slowly added into the suspension to react with Ag⁺ bridged on the surface of MFNs@SiO₂ to form Ag₄SiW₁₂O₄₀. After being magnetically separated and dried at 80 °C in a vacuum oven for 8 h, the MFNs@SiO₂/Ag₄SiW₁₂O₄₀ samples were obtained. Finally, MFNs@SiO₂/Ag₄SiW₁₂O₄₀ samples were irradiated for 0.5 h, 1 h, 2 h, 4 h and 8 h, respectively, by UV light (365 ± 15 nm, 5 mW cm⁻²) to form Ag nanoparticles on the surface of the Ag₄SiW₁₂O₄₀ layer.

2.3. Material characterization

The crystal structures of samples were analyzed by D/MAX-2004-X-ray powder diffractometer (Rigaku Corporation, Tokyo, Japan) with Ni-filtered Cu (0.15418 nm) radiation at 56 kV and 182 mA. Their morphologies were examined by transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FESEM). TEM observations were carried out on a JEM 2100 transmission electron microscope (JEOL Corporation, Tokyo, Japan) operated at 200 kV. TEM samples were made by dispersing a thin film of samples on Cu grid pre-coated with thin and flat carbon film. SEM images were obtained with a SUPRA35 field-emission scanning electron microscope (ZEISS, Germany). SEM samples were prepared by applying a drop of the sample on a conductive carbon tape, followed by drying in air. Prior to imaging, the sample was sputtered with gold for 20 s (Emitech K575 sputter coater, Emitech Ltd., Ashford Kent, UK). BET surface area was measured by N₂ adsorption–desorption isotherm with an Autosorb-1 series surface area and pore size analyzers (Quantachrome Instruments, Boynton Beach, FL, U.S.A.). The surface chemical states of the samples were examined by X-ray photoelectron spectroscopy (XPS) using an ESCALAB250 X-ray photoelectron spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, U.S.A.) with an Al K anode (1486.6 eV photon energy, 0.05 eV photon energy resolution, 300 W). FTIR spectrometer (Bruker TENSOR 27, MCT detector) was used to investigate the surface organic functional groups. Samples for FTIR observation were ground with spectral grade KBr in an agate mortar. Then, a fixed amount of sample (1% w/w) in KBr was used to prepare all the pellets. The UV-vis spectra of samples were measured on a UV-vis 2550 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). To explore the influence of SiO₂ coating and Ag salt loading on the magnetic properties of samples, MPMS-XL superconducting quantum interference device magnetometer (Quantum Design, U.S.A.) was used to measure the magnetization curve in external magnetic field of 0–1 T.

2.4. Photocatalytic degradation of methylene blue under visible light illumination

Methylene blue (MB) was used as a model organic pollutant to evaluate the photocatalytic activity of samples under visible light illumination. The initial MB concentration was 1 × 10⁻⁵ M, and a fixed concentration of 0.5 mg photocatalyst per mL solution was used in the photocatalytic degradation experiments. Photocatalysts were firstly dispersed in MB solutions by mechanical stirring for 30 min in the dark to reach adsorption equilibrium before visible light illumination was applied. A 300 W xenon lamp (PLS-SXE300, Beijing Perfect Light Technology Co., Ltd., Beijing, P. R. China) was used as the light source, which has a glass filter to provide zero light intensity below 400 nm. The light intensity striking the MB solution was at ca. 23 mW cm⁻², as measured by an FZ-A optical radiometer (Photoelectric Instrument Factory of Beijing Norman University, Beijing, P. R. China). At each time interval, photocatalysts were magnetically separated, and the light absorption of the clear solution at ∼663.5 nm was measured by the UV-2550 spectrophotometer to determine the remaining MB concentration. P25 TiO₂ nanoparticles were also used in the photocatalytic degradation of MB experiments for comparison purpose under the same experimental conditions. All analyses were performed in triplicates.

2.5. Photocatalytic disinfection of Escherichia coli (E. coli) bacteria under visible light illumination

Wild-type E. coli AN 387 (ATCC 15597, American Type Culture Collection, Manassas, VA, USA), a commonly used non-pathogenic bacterium with a short reproductive cycle, was used for the photocatalytic disinfection experiment under visible light illumination. After an overnight culture, E. coli cells were diluted to a cell suspension (ca. 10⁷ cfu mL⁻¹) in a buffer solution (0.05 M KH₂PO₄ and 0.05 M K₂HPO₄, pH 7.0) prior to the use in photocatalytic disinfection experiments. The fixed concentration of 1 mg photocatalyst per mL cell suspension was used for Degussa P25 TiO₂ nanoparticles, while 0.05 mg photocatalyst per mL cell suspension was used for MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalysts. All solid or liquid materials were autoclaved for 30 min at 121 °C before use. The same visible light source was used in the photocatalytic degradation of MB. In the photocatalytic disinfection experiment, an aliquot of 10 mL E. coli cell suspension was pipetted into a sterile 50 × 10 mm Petri dish with photocatalytic powder samples placed at the bottom. At regular time intervals, 100 μL of aliquots of the powder-treated cell suspensions were withdrawn in sequence. After appropriate dilutions in a buffer solution, an aliquot of 100 μL was spread onto an agar medium plate and incubated at 37 °C for 48 h, and the number of viable cells in terms of colony-forming units was counted. The survival ratio of E. coli was determined by the ratio of N_t/N₀, where N₀ and N_t are the numbers of colony-forming units at the initial and each following time interval, respectively. Tests were also performed in the dark in the presence of the photocatalyst for comparison. Analyses were done in triplicates, and control runs were carried out each time under the same experiment conditions, but without any photocatalytic materials.

3. Results and discussion

3.1. Creation of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst

Fig. 2a shows the TEM image of MFNs, in which well-dispersed nanocrystallites were observed and their size was around several nanometers. After coating with SiO₂ by the Stöber reaction,³⁵ well-dispersed MFNs@SiO₂ core/shell structured nanoparticles were created, in which SiO₂ shell wrapped around a bunch of MFNs functioned as the superparamagnetic core to provide a strong magnetic attraction for their effective magnetic separation. This is demonstrated clearly in the inset to the TEM image in Fig. 2b. The particle size distribution of these MFNs@SiO₂ nanoparticles was relatively small at ∼100–200 nm, and their surface was relatively smooth. The MFNs/TEOS ratio was carefully modulated to obtain samples with good dispersity, small size, and enough amount of MFNs to act as the superparamagnetic core for proper magnetic separation.

Fig. 2 (a) The TEM image of MFNs. (b) The TEM image of MFNs@SiO₂ core/shell structured nanoparticles (inset image shows the TEM image of a MFNs@SiO₂ nanoparticle with higher magnification). (c) The SEM image of MFNs@SiO₂/Ag₄SiW₁₂O₄₀ nanoparticles (inset image shows the SEM image of a MFNs@SiO₂/Ag₄SiW₁₂O₄₀ nanoparticle with higher magnification). (d) The FTIR spectra of MFNs, MFNs@SiO₂, MFNs@SiO₂–SH, and MFNs@SiO₂/Ag₄SiW₁₂O₄₀ samples. (e) The XRD patterns of MFNs@SiO₂/Ag₄SiW₁₂O₄₀ nanoparticles after UV illumination for different durations. (f) The high-resolution XPS scans over Ag 3d peaks of MFNs@SiO₂/Ag₄SiW₁₂O₄₀ nanoparticles after UV illumination for different durations.

To enhance the dispersion and subsequent contact efficiency with pollutants in water, a non-traditional, three-step process was adopted for the deposition of Ag₄SiW₁₂O₄₀ layer onto the surface of MFNs@SiO₂ nanoparticles. In the first step, the surface of MFNs@SiO₂ nanoparticles was modified with the –SH functionality by reacting with MPTES. According to Pearson's HSAB principle,³⁶ –SH is a typical soft alkaline coordination group, which has a strong attraction towards soft acidic coordination groups like Ag⁺. Thus, Ag⁺ could be strongly adsorbed onto the surface of MFNs@SiO₂ nanoparticles in the second step because of its interaction with the –SH group. In the third step, the active Ag₄SiW₁₂O₄₀ layer was formed on the surface of MFNs@SiO₂ nanoparticles by the reaction with silicotungstic acid. This process eliminated the calcination procedure commonly adopted in the deposition of silver salts onto substrates, and could form silver salt layers strongly attached to the substrates. The Ag₄SiW₁₂O₄₀ loading amount could be estimated by the change in sample weight before Ag⁺ adsorption and after the reaction with silicotungstic acid, and a relatively large loading amount of ∼24 wt% was achieved in this process. Fig. 2c shows the SEM image of MFNs@SiO₂/Ag₄SiW₁₂O₄₀ nanoparticles, which demonstrated that a rough layer was formed on the smooth surface of MFNs@SiO₂ nanoparticles. The inset to Fig. 2c shows the high magnification SEM image of an MFNs@SiO₂/Ag₄SiW₁₂O₄₀ nanoparticle, which clearly demonstrates that this rough layer was composed of fine nanoparticles with a size of ∼10–20 nm. A rough layer of Ag₄SiW₁₂O₄₀ could increase their contact area with pollutants in water, which is beneficial to their photocatalytic performance.

Fig. 2d compares the FTIR spectra of samples synthesized during this process, which confirms the formation of MFNs@SiO₂/Ag₄SiW₁₂O₄₀ nanoparticles. In the FTIR spectrum of MFNs (curve a), the two characteristic absorption bands at ∼593 cm⁻¹ and 442 cm⁻¹ correspond to the vibration of tetrahedral and octahedral sites, respectively.³⁴ After SiO₂ coating, a characteristic Si–O bond peak (980–1220 cm⁻¹)³⁷ occurred in the FTIR spectrum of MFNs@SiO₂ nanoparticles (curve b in Fig. 2d), while the intensity of the two characteristic peaks of MFNs (∼593 and 442 cm⁻¹) was largely depressed. These changes clearly suggest that SiO₂ coating was formed on MFNs, which is in accordance with the TEM observation (Fig. 2b). After the reaction with MPTES, the characteristic –SH peak (∼2547 cm⁻¹) could be observed in curve c, indicating the successful surface –SH modification. Moreover, curve d shows the FTIR spectrum of MFNs@SiO₂/Ag₄SiW₁₂O₄₀ nanoparticles. After Ag⁺ adsorption and the reaction with silicotungstic acid to form the Ag₄SiW₁₂O₄₀ layer, the –SH peak disappeared, while the vibration absorption peak of W–O bond and W–O–W bond appeared at 978.2 cm⁻¹ and 794.4 cm⁻¹, respectively, demonstrating that –SH on the surface of MFN@SiO₂ nanoparticles played the key role in the bridging of Ag₄SiW₁₂O₄₀ onto the surface of MFN@SiO₂ nanoparticles.

Finally, UV light illumination was used to form Ag nanoparticles on the surface of Ag₄SiW₁₂O₄₀ layer to further enhance its photocatalytic performance and stability. Fig. 2e shows the XRD patterns of MFNs@SiO₂/Ag₄SiW₁₂O₄₀ nanoparticles after UV illumination for different durations. Without UV illumination, only the diffraction peaks of Ag₄SiW₁₂O₄₀ could be observed, which further illustrates that the SiO₂ layer could effectively wrap around MFNs to prevent their contact with the Ag₄SiW₁₂O₄₀ layer. After UV illumination, Ag nanoparticles were formed due to the photo-decomposition of silver salts. Diffraction peaks of Ag (111) (2θ ∼ 38.08°), Ag (220) (2θ ∼ 64.48°) and Ag (311) (2θ ∼ 77.36°) could be observed on samples after UV illumination for 2 h and longer, which clearly demonstrates the formation of Ag nanoparticles. With the increase of UV illumination time, their peak intensities increased gradually, indicating the increase in the Ag nanoparticles formed. Fig. 2f shows the high-resolution XPS scans over Ag 3d peaks of MFNs@SiO₂/Ag₄SiW₁₂O₄₀ nanoparticles after UV illumination for different durations. Without UV illumination, the Ag 3d_5/2 peak was at ∼368.5 eV and the Ag 3d_3/2 peak was at ∼374.5 eV, indicating that silver was at its oxidized state as Ag⁺. With the increase in UV illumination time, the Ag 3d peaks became broader and the peak positions gradually moved to the lower end, indicating the formation of metallic silver (3d_5/2 peak ∼ 368.0 eV) from light reduction on these samples.³⁸

3.2. The magnetic properties of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst

The magnetic properties of the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst are critical to its separation performance after water treatment. Fig. 3a compares the magnetic field-dependent behaviors of MFNs, MFNs@SiO₂ nanoparticles, and MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag nanoparticles at room temperature. All three samples demonstrated the typical superparamagnetic behavior with zero remanence and zero coercivity, which was not affected by SiO₂ coating or the further coating of the Ag₄SiW₁₂O₄₀/Ag layer. The saturation magnetization, M_s, could be obtained by extrapolating a graph of M versus 1/H to 1/H → 0 (for H > 10 kOe). At room temperature, MFNs possessed a high M_s of ∼27.4 emu g⁻¹. The M_s of MFNs@SiO₂ nanoparticles dropped to ∼16.3 emu g⁻¹ due to the inclusion of the nonmagnetic SiO₂ coating, while the M_s of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag nanoparticles did not decrease considerably after further loading of photocatalytic Ag₄SiW₁₂O₄₀/Ag layer. Fig. 3b shows that all three samples could disperse well in DI water when there was no external magnetic field, which could be attributed to their supermagnetic behavior. Thus, no magnetic attraction existed when there was no external magnetic field being applied during the water treatment, which is beneficial to their better dispersion and the subsequent better contact efficiency with pollutants in water. Note that when an external magnetic field was applied for only 5 min, all the three samples were efficiently separated from water as shown in Fig. 3b. This observation demonstrates that although the M_s of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag nanoparticles was lower than that of MFNs, it was strong enough for the efficient separation of these photocatalysts from water, which is very desirable for its easy recovery after water treatment and enhances its application potential in real samples.

Fig. 3 (a) The magnetic field-dependent behaviors of MFNs, MFNs@SiO₂, and MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag at room temperature. (b) Magnetic separation of MFNs, MFNs@SiO₂, and MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag from water under an external magnetic field for 5 min.

3.3. Optical properties of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst

The optical properties of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalysts with different UV treatment durations were investigated by measuring their diffuse reflectance spectrum. From the reflectance data, optical absorbance can be approximated by the Kubelka–Munk function, as given by eqn (1):where R is the diffuse reflectance.³⁹ Fig. 4 shows their light absorbance spectra (in term of the equivalent Kubelka–Munk absorbance units). Without UV treatment, Ag₄SiW₁₂O₄₀ demonstrated an evident light absorption into the visible light region, and its absorbance stopping edge was found at ∼460 nm. After UV treatment, all Ag₄SiW₁₂O₄₀/Ag samples demonstrated enhanced visible light absorption than Ag₄SiW₁₂O₄₀. With the increase of the UV treatment time, their visible light absorption gradually increased until the UV treatment time reached 4 h. Further UV treatment did not show more enhancement effect. A broad absorption peak could be observed for all Ag₄SiW₁₂O₄₀/Ag samples from ∼500 nm to 600 nm, which should correspond to the absorption by Ag nanoparticles from LSPR effect.

Fig. 4 The light absorbance of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalysts under different UV treatment durations.

3.4. Photocatalytic degradation of methylene blue by MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst under visible light illumination

The photocatalytic activities of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalysts were first demonstrated by their degradation effect on a model organic contaminant, i.e., methylene blue (MB), under visible light illumination. Prior to the visible light illumination, photocatalysts were dispersed in MB solutions by mechanical stirring for 30 min in the dark to reach the adsorption equilibrium. Fig. 5a shows the representative light absorption spectra of MB solutions at different treatment durations by the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst irradiated by UV light for 4 h. During adsorption in the dark, a large portion of MB was removed as demonstrated by the decrease in light absorption for MB solution from the “original” curve to the “0 min” curve. After the visible light illumination began, the light absorption of MB solution decreased steadily, indicating the continuous decrease of MB concentration. After only 10 min, the light absorption of MB solutions dropped to near zero, indicating a near complete degradation of MB. A clear change was observed on the shape of MB light absorption curve as new peaks occurred, in addition to the characteristic MB peaks at 664 nm and 615 nm after the visible light illumination began. The inset to Fig. 5a shows MB solutions at different treatment durations. After visible light illumination began, the MB solution color changed from pure blue to light purple, which was in accordance with their light adsorption spectra. This observation suggested that intermediate products were produced during the photocatalytic degradation process.⁴⁰

Fig. 5 (a) The representative light absorption spectra of MB solutions at different treatment times by the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst with 4 h prior UV irradiation (inset image shows MB solutions at different treatment durations). (b) The relative residual MB concentration after being treated by MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalysts with different prior UV irradiation durations under visible light illumination compared with that treated by P25 TiO₂ nanoparticles under the same visible light illumination.

Fig. 5b shows the relative residue MB concentration after being treated by MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalysts with different prior UV irradiation durations under visible light illumination compared with that treated by P25 TiO₂ nanoparticles under the same visible light illumination. MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalysts demonstrated a better adsorption and a much faster degradation on MB than P25 TiO₂ nanoparticles. With only 10 minute visible light illumination, the relative MB concentration dropped to around zero by MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalysts, while the relative MB concentration treated by P25 TiO₂ nanoparticles was still round 90% and most of the drop was due to its adsorption onto the P25 TiO₂ nanoparticles. The prior UV irradiation treatment affected their photocatalytic degradation effect on MB. With the increase of the UV irradiation time from zero up to 4 h, the photocatalytic degradation efficiency of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalysts increased, while a further increase of UV irradiation time to 8 h demonstrated a deteriorated effect on their photocatalytic degradation efficiency. The slope of the MB degradation curve in Fig. 5b represents the MB degradation rate at certain treatment time. The initial MB degradation rate was ∼0.0038 mg (g⁻¹ min⁻¹) when Degussa P25 TiO₂ nanoparticles were used. The initial MB degradation rate by MFNs@SiO₂/Ag₄SiW₁₂O₄₀ photocatalyst without prior UV irradiation increased to ∼0.115 mg (g⁻¹ min⁻¹), ∼30 times as that of Degussa P25 TiO₂ nanoparticles. The initial MB degradation rate by the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst irradiated by UV light for 4 h further increased to ∼0.144 mg (g⁻¹ min⁻¹), ∼38 times as that of Degussa P25 TiO₂ nanoparticles and ∼1.27 times as that of the MFNs@SiO₂/Ag₄SiW₁₂O₄₀ photocatalyst without prior UV irradiation. Thus, these MFNs@SiO₂/Ag₄SiW₁₂O₄₀ photocatalysts demonstrated superior photocatalytic performance under visible light illumination.

3.5. Photocatalytic disinfection of the Escherichia coli bacteria by MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst under visible light illumination

The superior photocatalytic performance of these MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalysts under visible light illumination was further demonstrated by their photocatalytic disinfection effect on the viability of E. coli cells. Fig. 6a shows the survival ratio of E. coli under treatments by the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst irradiated by UV light for 4 h under visible light illumination and in the dark compared with that by Degussa P25 TiO₂ nanoparticles under visible light illumination. When there was no photocatalyst presence, no obvious E. coli disinfection was observed under visible light illumination (not shown in Fig. 6a). Note that Degussa P25 TiO₂ nanoparticles demonstrated a weak disinfection capability on E. coli bacteria. After a 10 min treatment, the survival ratio of E. coli bacteria was still ∼40%, which could be attributed to weak photocatalytic activity of the Degussa P25 TiO₂ nanoparticles under visible light illumination, caused by the mixture of both anatase and rutile phases. When the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst was present, it demonstrated a moderate disinfection effect on E. coli without light illumination. After the 10 min treatment, the survival ratio of E. coli dropped to ∼3%, which should due to the well-known bactericidal effect of silver. Under visible light illumination, however, the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst demonstrated a much better disinfection effect on E. coli. The survival ratio of E. coli continuously decreased with the increase of the treatment time. After only 10 min of treatment, the survival ratio of E. coli dropped to ∼3 × 10⁻⁶, which was more than 5 orders of magnitude lower than that treated by Degussa P25 TiO₂ nanoparticles, although its amount was only 5% higher that of Degussa P25 TiO₂ nanoparticles. The E. coli survival ratio treated by the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst under visible light illumination was about 4 orders of magnitude lower than that without visible light illumination. From this comparison, it is clear that the superior bactericidal effect on E. coli under visible light illumination could be mainly attributed to the superior photocatalytic performance of these MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalysts, and not to the modest bactericidal effect from silver itself.

Fig. 6 (a) The survival ratio of E. coli with the treatments by the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst irradiated by UV light for 4 h under visible light illumination and in the dark compared with that by Degussa P25 TiO₂ nanoparticles under visible light illumination. The SEM images of E. coli cells (b) before and (c) after photocatalytic treatment by the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst under visible light illumination. (d) The survival ratio of E. coli for four consecutive runs.

Fig. 6b and c show the SEM images of E. coli cells before and after photocatalytic treatment by the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst under visible light illumination. Before the photocatalytic treatment, E. coli cells had a damage-free surface; however, during the photocatalytic disinfection treatment, E. coli cells could not sustain their structure. Severe damage to the surface was clearly observed on these cells. Large holes and pits occurred on their cell membranes, and they lost their flagella completely. This observation indicates that the disinfection of E. coli cells by the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst was irreversible due to its destructive nature. The MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst also demonstrated a good stability during the photocatalytic disinfection of E. coli cells for four consecutive runs. After each run, the MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst was separated from the E. coli cell suspension by an external magnetic field, washed with DI water, and then reused for the next run. As demonstrated in Fig. 6d, the survival ratio of E. coli all dropped to ∼10⁻⁶ after only 10 min of treatment for all four runs. The good stability demonstrated is beneficial for their potential applications.

3.6. Photocatalytic mechanism of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst

Fig. 7 schematically illustrates the mechanism for the superior photocatalytic performance of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst under visible light illumination. Only Ag₄SiW₁₂O₄₀ shell and Ag nanoparticles were demonstrated in this illustration for better understanding. After being treated with UV irradiation, Ag nanoparticles were formed due to the photo-decomposition of Ag₄SiW₁₂O₄₀.²² Thus, the localized surface plasmon resonance (LSPR) effect from Ag nanoparticles on the surface of Ag₄SiW₁₂O₄₀ could enhance its light absorption in the visible light region.²² Ag nanoparticles could promote the electron/hole separation in Ag₄SiW₁₂O₄₀ when it was under visible light illumination by acting as the electron trapping center, which further enhanced its photocatalytic performance. The electron trapping on Ag nanoparticles could reduce and finally inhibit further photo-decomposition of Ag₄SiW₁₂O₄₀. Thus, prior UV treatment of the MFNs@SiO₂/Ag₄SiW₁₂O₄₀ photocatalyst could also enhance its stability.

Fig. 7 The schematic illustration of the mechanism for the superior photocatalytic performance of MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst under visible light illumination.

4. Conclusions

In summary, MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst was synthesized by our approach, which overcame the difficulties in the current synthetic practices of magnetic ASSs to create a novel magnetic ASS photocatalyst with highly efficient photocatalytic performance under visible light illumination and easy magnetic separation capability. The initial MB degradation rate by MFNs@SiO₂/Ag₄SiW₁₂O₄₀/Ag photocatalyst irradiated by UV light for 4 h was ∼38 times as that of Degussa P25 TiO₂ nanoparticles, and the survival ratio of E. coli treated by it was more than 5 orders of magnitude lower than that treated by Degussa P25 TiO₂ nanoparticles, although its amount was only 5% as that of Degussa P25 TiO₂ nanoparticles. Furthermore, the surface modification on the passive SiO₂ mid-layer and the bridging procedure for material loading developed in our approach could be readily applied to other material systems for the creation of novel composite materials with various functions.

Acknowledgements

The experimental assistance on E. coli culture by Ms Mian Song and Ms Shuang Jiao was greatly appreciated. This study was supported by the National Natural Science Foundation of China (Grant no. 51102246), the Knowledge Innovation Program of Institute of Metal Research, Chinese Academy of Sciences (Grant no. Y0N5A111A1), the Youth Innovation Promotion Association, Chinese Academy of Sciences (Grant no. Y2N5711171), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, P. R. China.

References

1. M. R. Hoffmann, S. T. Martin, W. Y. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69–96.
2. K. Hashimoto, H. Irie and A. Fujishima, Jpn. J. Appl. Phys., Part 1, 2005, 44, 8269–8285.
3. Q. Li, Y. W. Li, P. G. Wu, R. C. Xie and J. K. Shang, Adv. Mater., 2008, 20, 3717–3723.
4. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269–271.
5. D. F. Wang, T. Kako and J. H. Ye, J. Am. Chem. Soc., 2008, 130, 2724–2725.
6. Z. G. Zou, J. H. Ye, K. Sayama and H. Arakawa, Nature, 2001, 414, 625–627.
7. J. W. Tang, Z. G. Zou and J. H. Ye, Angew. Chem., Int. Ed., 2004, 43, 4463–4466.
8. N. Z. Bao, L. M. Shen, T. Takata and K. Domen, Chem. Mater., 2008, 20, 110–117.
9. K. Maeda, K. Teramura, D. L. Lu, T. Takata, N. Saito, Y. Inoue and K. Domen, Nature, 2006, 440, 295.
10. T. F. Yeh, J. M. Syu, C. Cheng, T. H. Chang and H. S. Teng, Adv. Funct. Mater., 2010, 20, 2255–2262.
11. X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76–80.
12. A. Kudo, K. Ueda, H. Kato and I. Mikami, Catal. Lett., 1998, 53, 229–230.
13. X. Zong, H. J. Yan, G. P. Wu, G. J. Ma, F. Y. Wen, L. Wang and C. Li, J. Am. Chem. Soc., 2008, 130, 7176–7177.
14. P. Wang, B. B. Huang, X. Y. Qin, X. Y. Zhang, Y. Dai, J. Y. Wei and M. H. Whangbo, Angew. Chem., Int. Ed., 2008, 47, 7931–7933.
15. P. Wang, B. B. Huang, X. Y. Zhang, X. Y. Qin, H. Jin, Y. Dai, Z. Y. Wang, J. Y. Wei, J. Zhan, S. Y. Wang, J. P. Wang and M. H. Whangbo, Chem.–Eur. J., 2009, 15, 1821–1824.
16. Z. G. Yi, J. H. Ye, N. Kikugawa, T. Kako, S. X. Ouyang, H. Stuart-Williams, H. Yang, J. Y. Cao, W. J. Luo, Z. S. Li, Y. Liu and R. L. Withers, Nat. Mater., 2010, 9, 559–564.
17. H. J. Dong, G. Chen, J. X. Sun, C. M. Li, Y. G. Yu and D. H. Chen, Appl. Catal., B, 2013, 134, 46–54.
18. X. X. Hu and C. Hu, J. Solid State Chem., 2007, 180, 725–732.
19. T. Kako, N. Kikugawa and J. Ye, Catal. Today, 2008, 131, 197–202.
20. G. Q. Li, S. C. Yan, Z. Q. Wang, X. Y. Wang, Z. S. Li, J. H. Ye and Z. G. Zou, Dalton Trans., 2009, 8519–8524,  10.1039/b906799j.
21. B. Hu, L. H. Wu, S. J. Liu, H. B. Yao, H. Y. Shi, G. P. Li and S. H. Yu, Chem. Commun., 2010, 46, 2277–2279.
22. H. Huang, X. R. Li, Z. H. Kang, Y. Liu, H. T. Li, X. D. He, S. Y. Lian, J. L. Liu and S. T. Lee, Dalton Trans., 2010, 39, 10593–10597.
23. Q. Y. Li, T. Kako and J. H. Ye, Appl. Catal., A, 2010, 375, 85–91.
24. H. Dong, Z. H. Li, X. M. Xu, Z. X. Ding, L. Wu, X. X. Wang and X. Z. Fu, Appl. Catal., B, 2009, 89, 551–556.
25. M. A. Gondal, X. F. Chang, W. E. I. Sha, Z. H. Yamani and Q. Zhou, J. Colloid Interface Sci., 2013, 392, 325–330.
26. W. Z. Sun, Q. Li, S. Gao and J. K. Shang, J. Mater. Chem. A, 2013, 1, 9215–9224.
27. C. H. An, X. J. Ming, J. Z. Wang and S. T. Wang, J. Mater. Chem., 2012, 22, 5171–5176.
28. N. Mahmed, O. Heczko and S. P. Hannula, Adaptive, Active and Multifunctional Smart Materials Systems, 2013, vol. 77, pp. 184–189.
29. J. F. Guo, B. W. Ma, A. Y. Yin, K. N. Fan and W. L. Dai, Appl. Catal., B, 2011, 101, 580–586.
30. G. T. Li, K. H. Wong, X. W. Zhang, C. Hu, J. C. Yu, R. C. Y. Chan and P. K. Wong, Chemosphere, 2009, 76, 1185–1191.
31. X. X. Yu, S. W. Liu and J. G. Yu, Appl. Catal., B, 2011, 104, 12–20.
32. W. S. Tang, Y. Su, Q. Li, S. Gao and J. K. Shang, J. Mater. Chem. A, 2013, 1, 830–836.
33. W. S. Tang, Y. Su, Q. Li, S. A. Gao and J. K. Shang, RSC Adv., 2013, 3, 13961–13967.
34. W. S. Tang, Y. Su, Q. Li, S. A. Gao and J. K. Shang, Water Res., 2013, 47, 3624–3634.
35. W. Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968, 26, 62–69.
36. R. G. Pearson, Inorg. Chim. Acta, 1995, 240, 93–98.
37. K. El-Boubbou, C. Gruden and X. Huang, J. Am. Chem. Soc., 2007, 129, 13392–13393.
38. K. T. V. Rao, P. S. S. Prasad and N. Lingaiah, Green Chem., 2012, 14, 1507–1514.
39. J. Tauc, R. Grigorov and A. Vancu, Phys. Status Solidi, 1966, 15, 627–637.
40. G. Y. Zhang, Y. Q. Sun, D. Z. Gao and Y. Y. Xu, Mater. Res. Bull., 2010, 45, 755–760.

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