The preparation of a recyclable catalyst of silver nanoparticles dispersed in a mesoporous silica nanofiber matrix

Abstract

A recyclable catalyst of sliver nanoparticles well dispersed in mesoporous silica was successfully synthesized via a straight-forward strategy combining an electrospinning technique with post-calcination. The resulting mats containing silver nanoparticles 10 ± 4 nm in size well dispersed in mesoporous silica fibers of 380 ± 80 nm diameter presented surface plasma resonance of silver nanoparticles at 420 nm. The catalytic behavior of the nanofibers on the reduction of methylene blue and Congo red by NaBH₄ was tracked and showed that the silver nanoparticles immobilized on the silica nanofiber matrix possessed excellent catalytic properties, which may hold great promise in effective and eco-friendly waste water treatment.

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Introduction

Metal nanoparticles differing from the corresponding bulk metals in various fields of science have attracted great interest for their unique properties, ranging from chemistry to physics, and biotechnology.^1–3 They have been exploited for potential use as key components in biosensors,^4,5 optical switches,⁶ biological markers,⁷ nanoelectronic devices,⁸ and catalysts.^9,10 Among these metal nanoparticles, silver is particularly highlighted because of its outstanding physical and optical properties determined by the size, shape, composition, and structure.^11,12 In particular, its application in catalysis is most important, since it often combines the characteristics of high reactivity and selectivity. Especially, the 4-nitrophenol (4-NP) reduction by borohydride in the presence of Ag nanoparticles (Ag NPs) catalysts has been accepted as an alternative effective and eco-friendly route to produce 4-aminophenol (4-AP), which has important industrial roles as a antioxidant, a corrosion inhibitor, and as an important intermediate in the preparation of analgesic and antipyretic drugs.^13–16 However, the main problems encountered with metal NPs are their inconvenience for recycle and their susceptibility to coagulation and agglomeration by the London-van der Waals forces among the particles,¹⁷ resulting in a remarkable reduction in their catalytic activities.

In order to solve these issues, Ag NPs have been immobilized onto various supports such as polymeric¹⁸ and inorganic materials, including silica,¹⁹ zeolite,²⁰ alumina,²¹ ceria,²² titania,²³ activated carbon²⁴ and carbon nanotube (CNT),²⁵ etc. Among these carrier materials, mesoporous silica is regarded as a promising candidate because of its adjustable pore diameter, large surface area, and environmental tolerance.²⁶

Electrospinning is a widely employed method in recent years to produce polymer nanofiber mats with ultrahigh surface/volume ratio, which have be used as metal supporting substrates for catalytic applications.^27–33 One of the advantages of this technique is its effective preparation of nano-scale fibers from various organic and inorganic materials which do not form fibers by conventional methods. By electrospinning technique, the prepared mesoporous silica fibers possess unique physical properties, such as easy control of density of metal NPs in the fibers, resistance to high temperature and strong solvents, and recyclability/reusability. Compared with other techniques,^34,35 electrospinning is a simple, economical but efficient approach to prevent the aggregation of Ag NPs, increase catalytic activities and enable recyclability/reusability.

Therefore, in this study we combined sol–gel processing with the electrospinning technique to prepare Ag-doped mesoporous silica fibers. Tetraethylorthosilicate (TEOS), polymer polyvinyl alcohol (PVA) and silver nitrate (AgNO₃) are the main precursors used to synthesize composite fibers. Compared to other polymers to the preparation of silica composite fiber,^33,36,37 the water-soluble PVA can better disperse the silver nitrate. The electrospun fibers were then heat treated to produce Ag NPs via AgNO₃ reduction and to pyrolyze the PVA component. The morphology and chemical composition were characterized. The catalytic efficiency of the Ag NPs encapsulated inside porous silica fibers was evaluated in a Ag catalyzed reduction of a cationic dye, methylene blue dye (MB) and an ionic dye, Congo red (CR) by sodium borohydride (NaBH₄) under nitrogen. The dyes are used in textile and often found in waste water. Due to their toxicity to aquatic creatures and human beings, their removal from aqueous solution is environmentally important. Therefore, they were used to evaluate catalytic activity of the formed composite nanofibers in the experiment. The reaction was detected using UV-Vis spectroscopy.^38,39

Experimental

Materials

Tetraethylorthosilicate (TEOS, 98%), silver nitrate (AgNO₃), nitric acid (HNO₃), copolymer Pluronic P123 (EO₂₀PO₇₀EO₂₀), polyvinyl alcohol (PVA, M_w = 15 000), methylene blue (MR), Congo red (CR) and sodium borohydride (NaBH₄, 99.9%) were all purchased from Sigma-Aldrich Chemical Co. All chemicals were used without further treatment.

Preparation of spinning solution

Firstly, 0.3 g P123 was dissolved in 3.6 g ethanol and 4.12 g distilled water, and vigorously stirred for 0.5 h at room temperature. Secondly, 0.09 g AgNO₃ and 0.2 mL HNO₃ (2 mol L⁻¹) were slowly added to the solution mixture, and further stirred for 0.5 h. Then, 3.32 g TEOS was dropped slowly into the mixture and a silica gel was obtained after reacting for 2 h at room temperature. Finally, 2.3 g of 10 wt% PVA solution was dropped slowly into the silica gel, then the reaction solution was constant stirred for another 10 h. Thus, a viscous gel of PVA/silica/AgNO₃ composites was obtained.

Electrospinning

The spinning solution was collected in a 5 mL syringe with a stainless steel capillary metal hub needle. A high-voltage power supply was used to provide 15 kV voltage to the electrospinning solution. The spinning solution was pumped through the needle tip at a rate of 0.5 mL h⁻¹ by a syringe pump (New Era Pump System, USA). The tip-to-collector distance was set to 20 cm. The as-prepared fibers were subsequently calcinated at 200 °C for 2 h, and 600 °C for 3 h. The heating rate was 1 °C min⁻¹.

Characterization

Scanning electron microscope (SEM, NOVA-600) operated at an accelerating voltage of 15 kV, and equipped with an energy dispersive X-ray analyzer (EDX), was used to analyze the surface morphology and elemental analysis, respectively. A transmission electron microscope (TEM, Tecnai G2 Spirit), operated at the accelerating voltage of 120 kV, was used to study the internal morphology of the fiber, including Ag NPs distribution. The surface area of heat-treated silica fibers was calculated by the N₂ sorption characterization method (Quantachrome NOVA). X-ray photoelectron spectroscopy (XPS) measurements were performed on AXIS Ultra spectrometer using Al Kα (E = 1486.6 eV) radiation. The crystallinity of the fiber mats before and after heat treat, was characterized by an X-ray diffraction [XRD, Bruker D8 Advance, Cu Kα radiation (λ = 1.542 Å) using 40 kV, 20 mA, scanning speed 2 °C min⁻¹] and UV-Vis reflectance spectrophotometer (UV-Vis, Shimadzu-1800, 200–800 cm⁻¹ wavelength), respectively.

Catalysis

Following the previous reports,^40,41 30 mg silica composite fiber containing 5 wt% Ag NPs as catalyst was homogeneously dispersed in 2 mL 0.005 M MB water solution, and 10 mM aqueous NaBH₄ was added with vigorous stirring under nitrogen atmosphere. The original blue color of the mixture gradually vanished, indicating Ag-catalyzed reduction of the dye. The rate of color disappearance, as indicative of catalytic activity of Ag NPs, and the intensity of spectral feature absorption of MB at 665 nm were monitored with a UV-Vis spectrophotometer (UV-Vis, Shimadzu-1800). The catalytic activities of Ag NPs/silica nanofiber was also evaluated by the catalytic reduction of Congo red (CR) in the presence of NaBH₄. 30 mg of the Ag NPs/silica nanofiber was added into 100 mL of the CR solution (20 mg L⁻¹) with 10 mmol L⁻¹ of NaBH₄. The intensity of spectral feature absorption of CR at 504 nm was monitored with a UV-Vis spectrophotometer (UV-Vis, Shimadzu-1800).

Results and discussion

The morphological properties of the composite nanofibers were observed using SEM. Fig. 1 shows the SEM images and the histogram of nanofibers diameter distribution for as-prepared precursor fibers and calcinated composite fibers. Fig. 1A indicated the smooth and super-long precursor nanofiber had been prepared successfully by electrospinning. The average diameter of precursor fiber was calculated to be 470 ± 120 nm (Fig. 1C). After being calcinated at 600 °C, the surface morphology of nanofibers was fine and continuous but the average fiber diameter was decreased to 380 ± 80 nm due to the decomposition of the organic species (Fig. 1B and D).

Fig. 1 SEM images of non-heat-treated precursor composite nanofibers (A), and heat-treated silica fibers at 600 °C (B). The insets show lower magnify image. Diameter distribution of the fibers before heat-treatment (C) and after heat-treatment (D).

The internal morphology of the fiber was further characterized by TEM. Fig. 2 shows the typical TEM image of the silica composite nanofiber before and after heat treatment at 600 °C. From Fig. 2A, the fibers were smooth and contained no NPs. The TEM micrograph images (Fig. 2B) of the heat-treated silica fibers clearly show a distribution of Ag NPs inside the porous silica fibers. The average size of NPs is about 10 ± 4 nm. The diameter of silica fiber is about 350 nm, which is consistent with that observed from the SEM image. The TEM image clearly showed the formation of stable, spherical, and dispersed NPs in the polymeric matrices. The corresponding EDX elemental mapping further confirms the existence of the Ag NPs.

Fig. 2 TEM micrographs of the nanofibers before heat treatment (A) and after heat treatment (B), and EDX elemental mapping of composite fiber after heat treatment.

The N₂ adsorption/desorption isotherms of mesoporous silica fibers was shown in Fig. S1.† It can be seen that the silica fiber samples show similar VI isotherms and typical H1-hysteresis loops, demonstrating the properties of a typical mesoporous material. The surface area is about 443.62 m² g⁻¹, and the average pore size is about 5.3 nm.

The optical properties of heat-treated silica fibers further proved the existence of Ag NPs in the fibers. Before heat treatment, the AgNO₃/silica fiber membrane is white (Fig. 3A). After heating, these fiber membranes change to brown in color (Fig. 3B). The phenomenon is resulted from the surface plasma resonance (SPR) of the Ag NPs produced in the silica fibers during heat treatment. SPR is a characteristic feature of metal NPs with the sizes of 2 and 50 nm.^42,43 As show in Fig. 3C, the appearance of a broad plasmon peak at 420 nm after heat treatment indicates formation of Ag NPs after calcination at 600 °C.

Fig. 3 White electrospun PVA modified silica fiber mats containing silver nitrate (AgNO₃) before heat treatment (A). Golden yellow porous silica fiber mats containing Ag NPs after heat treatment at 600 °C (B). UV-Vis spectra of porous silica nanofibers containing Ag NPs before heat treatment and after heat treatment (C).

Fig. 4A shows XRD spectra of the fibers before and after heat treatment. The diffused peak at 2θ value of 20° observed in all samples can be attributed to the amorphous nature of silica materials.⁴⁴ The diffraction peaks with 2θ values of 38.7°, 65.1°, 77.9° are corresponded to the (111), (200), (311) crystal planes of Ag NPs, respectively.^45,46 The calculated d-spacing of the Ag NPs was found to be consistent with the International Center for Diffraction Data obtained from JCPDS files (no. 41-1402), indicating that the Ag NPs have cubic symmetry (Fig. 4B). XRD patterns confirmed that the polymeric blend was semicrystalline in nature and Ag was presented as NPs forms in the composite nanofibers.

Fig. 4 (A) X-ray diffraction patterns for silica fibers containing Ag NPs: before heat treated fiber and after heat treated at 600 °C. (B) Comparison of calculated d-spacing (d_hkl) values of Ag NPs with data from the International Centre for Diffraction Data obtained for Ag from JCPDS files (No. 41-1402).

To further investigate the chemical states of the Ag species in the composite nanofiber, XPS was performed on the composite nanofiber and pure silica fiber. Compared to the XPS spectrum of pure silica fiber, the new peaks, ascribed to the Ag3d, were observed from composite nanofiber (Fig. 5A). The typical fully scanned spectra demonstrated that Ag, Si, O and C elements existed in the Ag NPs/silica composite nanofiber. The binding energy for the C1s peak at 284.6 eV is used as the reference for calibration. The high-resolution XPS spectra (Fig. 5B) shows the two peaks at binding energy of 368.5 and 374.5 eV, assigned to Ag⁰3d_5/2 and Ag⁰3d_3/2, respectively, demonstrating the metallic nature of Ag NPs.⁴⁷

Fig. 5 XPS spectra of composite nanofiber: (A) full XPS spectrum of Ag NPs/silica nanofiber and pure silica nanofiber, (B) Ag3d of Ag NPs/silica nanofiber.

Because of their high surface/volume ratio and unusual electronic properties,⁴⁸ Ag NPs exhibit excellent photochemical activity and have been used as catalysts in reduction reactions of nitrophenols, nitroanilines, and various dyes.^49–51 Here, reduction of MB and CR by NaBH₄ was used for determining the catalytic activity of the Ag NPs encapsulated in the porous silica fibers. The progression of the catalytic reduction of MB was followed by the change of optical absorbance at 665 nm (Fig. 6). As seen, the absorbance gradually decreases with increased reaction time and the catalytic reduction of MB proceeds successfully in the fibers contained Ag NPs (Fig. 6A). Fig. 6B shows the MB is also gradually reduced when equivalent amount of Ag NPs and NaBH₄ were added directly to the solution, but the reduction efficiency is lower. The non-immobilized Ag NPs are prone to aggregate to minimize their surface area due to their higher surface energy, resulting in a remarkable reduction in their catalytic activities. Therefore, highly distributed Ag NPs in the porous silica fiber are ideal for high electrocatalyst activity owing to their maintained large surface-to-volume ratio. As a control, the pure silica fibers containing no Ag NPs did not show any reduction to MB (in the presence of NaBH₄) for more than 240 min (Fig. 6C), confirming that the catalytic effect arises from Ag NPs. Similar results was observed for the reduction of CR (Fig. S2†).

Fig. 6 Successive UV-Vis spectra of methylene blue (MB) dye reduction, using Ag NPs/silica nanofiber as the catalyst and NaBH₄ as the reducing agent (A) (inset is digital photos of methylene blue (MB) dye reduction at different stages), bare Ag NPs as the catalyst and NaBH₄ as the reducing agent (B) and bare silica fiber as the catalyst and NaBH₄ as the reducing agent (C). (D) Catalytic kinetics for ten successive cycles with the same batch of Ag NPs/silica nanofiber.

In the catalytic reaction, the Ag NPs/silica fiber can act as an electron relay system. The Ag NPs start the catalytic reduction by acting as an electron transfer intermediate station, where the Ag NPs/silica nanofiber accepts electrons from BH₄⁻ and conveys them to the dyes. From thermodynamics considerations, this implies that the redox potential of the Ag cluster should be intermediate between the donor system D (NaBH₄), E⁰(D⁺/D), as the lower threshold and that of the acceptor system A (dye), E⁰(A/A⁻) as the upper threshold (Scheme S1†).^52,53 The reactions become accelerated due to the double electron transfer through the metal cluster relay. At the early stage, dye molecules could easily transport to the Ag surface because of the strong adsorption and reduction ability of the Ag NPs/silica nanofiber. With the process of reaction, the H₂ generated by NaBH₄ can cause the convection of the water, and remove the alteration products, thus keeping the surface fresh and maintaining the high reactivity of the NPs. The degradation of methylene blue and Congo red by NaBH₄ in the presence of Ag NPs is depicted in Scheme S2.†

The reusability of the catalysts is the main advantage of heterogeneous catalysts, which makes them useful for industrial applications. Therefore, the recyclability of the Ag NPs/silica nanofiber was investigated. The Ag NPs/silica nanofiber can be easily separated from the reaction mixture by mild centrifugation and reused for the 10 successive reactions. As shown in Fig. 6D, no significant changes were observed. This indicates that the nanofiber can be used as a promising reusable catalyst.

Conclusions

In the study, novel catalytic silica nanofibers containing Ag NPs have been successfully prepared simply combining electrospinning and calcination. The Ag NPs were uniformly dispersed in the mesoporous silica fibers. This method is simple but effective for preparing nanocomposites without aggregation. The silica composite nanofiber exhibit excellent catalytic properties with clear recyclability and reusability.

Acknowledgements

The authors gratefully acknowledge the Danish Council for Strategic Research for the funding to the ElectroMed Project at the iNANO Center, Aarhus University Research Foundation, Chinese Scholarship Council and the Carlsberg Foundation for their financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10867a

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