Core-shell SiO2/Ag composite spheres with dense, complete and nanoscaled silver shell were prepared by using a novel facile chemical reduction method without surface modification of silica at room temperature. The core-shell composites were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), UV-Vis spectroscopy and energy dispersive X-ray spectroscopy (EDX). The photocatalytic properties towards the degradation of methyl orange (Mo) of the prepared SiO2/Ag composites were also tested. The studies showed that the surface of SiO2 microspheres was homogeneously and completely covered by Ag nanoparticles and the composite exhibited excellent photocatalytic activities. The possible reaction mechanisms for the formation of the silica-silver core-shell spheres were also discussed in this paper.
- SiO/Ag composite
- photocatalytic property
During the past decades, the design and con-trolled preparation of core-shell composites have attracted considerable attention and become an im-portant research field in materials science, chemistry, medicine, and engineering [1-5]. Among the numerous kinds of core-shell structured materials, silica-silver structures consisting of silica cores and silver shells have been of significant interest because of their unique properties and potential applications ranging from optical devices and surface-enhanced Raman scattering to catalysis and medicine [6-9]. To date, there has been much effort on the synthesis of SiO2/Ag core-shell composites, including chemical reduction method, template, micro-emulsion method, layer by layer (LBL), solgel method, thermal deposition, and so on [10-15]. However, SiO2/Ag composites with dense, complete, uniform nanoscaled silver layer and high purity are still difficult to be obtained. Moreover, the fabrication process of the SiO2/Ag coreshell spheres generally involves a complex, timeconsuming multi-step procedure. So, it is of great interest to develop a simple and easy-handling procedure for the fabrication of SiO2/Ag core-shell composites with complete, uniform nanoscaled silver layer.
In the present study, a facile, convenient and more efficient method for the preparation of mono-dispersed SiO2/Ag composite with uniform and complete silver shells was explored. The silica particles were prepared firstly by Stöber process . Then the mixture of polyvinylpyrrolidone stabilized Ag nuclei and the surplus [Ag(NH3)2]+ ions was adsorbed on the surface of the silica microspheres. At last, the SiO2/Ag composite particles were formed by a simple reduction process in the presence of sodium borohydride (NaBH4). All the reactions were carried out at ambient temperature. From the viewpoint of further applications, the photocatalytic activity of SiO2/Ag composites was evaluated in the degradation of methyl orange (MO) under visible light irradiation.
Polyvinylpyrrolidone (PVP, with an average molar mass of 55000 g·mol-1), silver nitrate (AgNO3), absolute ethanol (EtOH), tetraethoxysilane (TEOS), sodium borohydride (NaBH4), methyl orange (MO) were supplied by Sigma-Aldrich Company. All chemicals and reagents were used as received without further purification. Home-made Milli-Q water was used.
Silica microspheres were prepared by hydro-lysis and polycondensation of TEOS following Stöber method. In a typical experiment, a 50.0 mL ethanol solution containing 4.5 mL TEOS was added to a 16 mL ethanol solution containing 9 mL ammonia and 24 mL H2O. The mixture was stirred at room temperature for 3 h with a stirring speed of 300 rpm. The resulting silica microspheres were centrifugally separated from the suspension and then ultrasonically washed with water and ethanol for three times separately. At last, they were dried in an oven at 60 °C for 12 h.
Briefly, about 0.3 mL 0.14 M freshly prepared NaBH4 aqueous solution was dropped into 140 mL aqueous solution containing 0.12 g PVP and [Ag(NH3)2]+ (0.1 M) which was produced by the addition of ammonia to AgNO3 solution. In a separate beaker, 0.025 g of the as-prepared silica microspheres was dispersed in 10 mL deionized water by sonication and it was quickly added into the formerly prepared mixture with vigorous magnetic stirring. Subsequently, 50 mL of 0.14 M NaBH4 solution, as reducing agent, was dropped at a rate of 0.5 mL/min into the suspensions under stirring. After that the solution was allowed to react for 24 h at room temperature. The solution gradually changed to dark color, indicating the formation of Ag nanoparticles. Finally, the resulting composites were collected by centrifugation, washed with ethanol and dried at 60 °C in a vacuum oven for 12 h.
The morphology and dimensions of the prod-ucts were examined with a high-resolution scanning electron microscope (SEM, JEOL JEM-3000) operated at 20 kV. The compositional information was obtained with an energy-dispersive spectrometer (EDX) installed on SEM. The crystallographic structures of the samples were studied using X-ray diffraction (XRD, Shimadzu XRD6000) with CuK
The photocatalytic performance of SiO2/Ag core-shell composites was measured by the photocatalytic decomposition of organic dye methyl orange (MO) under visible light irradiation at ambient conditions. The experiments were conducted with 20 mg of SiO2/Ag composite suspended in 20 mL of MO aqueous solution (10 mg•L-1). The solution was pre-stirred in dark for 30 min to establish adsorption-desorption equilibrium. Then, the solutions were exposed to a visible light source (wavelength approximately: 400 nm to 700 nm, 55 W/cm2) to irradiate the suspension under stirring. The catalytic performance of the core-shell composites was analyzed quantitatively for the absorption peak at 464 nm of MO in aqueous heterogeneous solution suspensions. The visible-light photocatalytic performance of prepared bare SiO2 particles was also tested for the photocatalytic degradation of MO under the same conditions.
The wide-angle X-ray diffraction (XRD) patterns of the obtained SiO2 microspheres and SiO2/Ag composites are illustrated in Fig. 1. For pure silica particles, there is only a broad scattering maximum centered at 22.5°, corresponding to amorphous silica [15, 17]. Yet, for core-shell composites, except amorphous silica characteristic diffraction peak, it also exhibits four well-resolved diffraction peaks at 2θ angles of 37.9°, 44.1°, 64.3° and 77.2° in the range of approximately 10° to 80°, which can be indexed to the (1 1 1), (2 0 0), (2 2 0), and (3 11) reflections of face-centered cubic metal silver (JCPDS Card No. 87-0720), indicating that the Ag nanoparticles with high crystallinity could be obtained on the surface of SiO2. In addition, we also can find that the peaks are a little broader than that of bulk silver because the silver grain size is relatively small.
The chemical composition of resulting SiO2/Ag composite has been analyzed by EDX elemental analysis as shown in Fig. 2. In this pattern, only Si, O, and Ag peaks are clearly shown and no other peaks are detected. The atomic ratio of Si and O is about 1:2, and the total content of Ag element is about 32 wt.%. This means that the silver shell with high purity has been obtained on the silica micro-spheres in the present study.
SEM images of as-prepared silica microspheres with smooth surface and homogeneous size are shown in Fig. 3a. All of the particles are spherical in shape and the average diameter is about 390 nm. For the core-shell composites shown in Fig. 3b, the silica spheres have been completely covered by consecutive silver nanoparticles. The continuously distributed silver nanoparticles covered the silica surface completely to form a silver shell due to their quite high filling factor on the silica surface. Furthermore, the dispersed composite particles in SEM remained spherical in shape.
The UV-Vis spectroscopy is one of the most widely used techniques for structural characterization of silver nanostructures [18, 19]. The absorption spectra of the prepared bare silica and core-shell structure are shown in Fig. 4. The bare silica colloids do not show any UV-Vis absorption in the range of 300 nm to 800 nm, but the core-shell composites show an obvious absorption peak at around 418 nm due to the Mie plasmon resonance excitation from the silver nanoparticles on the surface of SiO2. Compared to pure Ag NPs, the ab-sorption peak shifts from ~400 nm to 418 nm [20, 21]. The possible reason for the red shift can be attributed to much larger size of silver nanoparticles and higher coverage on the silica surface as indicated in Fig. 3b (SEM). The results are consistent with the previous reports [18, 19, 22, 23]. The strong dipole-dipole interactions between neighboring nanoparticles and Mie scattering of silver shell would promote red shift and broadening of the plasmon bands for silver clusters attached on silica spheres.
Fig. 5 describes the mechanism of fabrication of the SiO2/Ag core-shell composites. In the first step, because the amount of NaBH4 is inadequate, both PVP stabilized Ag nuclei (zeta potential ~ +2.3 mV) and surplus [Ag(NH3)2]+ ions are ex-isting in the mixture. After the addition of silica microspheres, PVP stabilized Ag nuclei and surplus [Ag(NH3)2]+ ions are adsorbed on the surface of the silica spheres (zeta potential ~ -2.3 mV) via chemisorption and electrostatic attraction [12, 24, 25]. After that, when additional NaBH4 is dropped into, the reduction process would be initiated. In this process, [Ag(NH3)2]+ ions adsorbed on the surface of silica spheres are reduced to Ag nanoparticles. The newly reduced Ag and PVP stabilized Ag nuclei conjugated on the silica surface act as the seeds which provide nucleation sites for the new growth of silver shell [18, 26]. The remaining [Ag(NH3 )2]+ ions in the solution are further reduced by NaBH4 and Ag nanoparticles grown gradually on the surface of SiO2, eventually lead to the formation of completely covered silver-coated silica spheres.
The photocatalytic activities of the SiO2/Ag composites have been investigated by the degradation of methyl orange (MO) under visible light. Pure SiO2 microspheres have been used for comparison.
The photodegradation dynamic curves of the MO dye are displayed in Fig. 6, in which C0 represents the initial concentration of MO and C rep-resents the concentration at a given time. As can be seen in Fig. 6, more than 80 % MO dye can be decomposed within 30 min and eventually degraded completely after 60 min with the assistance of SiO2/Ag composites under the visible-light ir-radiation, indicating the excellent photocatalytic activity of the as-prepared core-shell composites, while the photodecomposition ability of pure SiO2 microspheres almost can be ignored during the same time interval.
In summary, we have demonstrated a relatively mild and facile route for the formation of SiO2/Ag core-shell composites. Their microstructure, chemical composition, optical properties, visible lightdriven photocatalytic performance were investigated. The results of XRD, SEM showed that the surface of SiO2 was completely surrounded by pure silver nanoparticles, and the silver nanoparticles had fcc structure. With MO degradation efficiency, the SiO2/Ag composites demonstrated excellent photocatalytic activity.