One-step preparation and characterization of core-shell SiO₂/Ag composite spheres by pulse plating

2.1 Materials

The silica particles used in this experiment were provided directly by a factory rather than the widely known modified Stöber et al. [15] synthesis and used for the preparation of homogeneous silica spheres. The electroplating baths used were silver nitrate (42~50 g/l), ammonium acetate (77 g/l), ammonia (32 ml/l), KOH (45~55 g/l), and niacin (90~110 g/l), K₂CO₃ (70~82 g/l) – all of which were analytical reagents (AR) purchased from Sinopharm Chemical Reagent Co. Ltd, Shanghai, China. Ultrapure water (about 0.21 MΩ·cm) from an “up water purification system” produced by ULUPURE (Sichuan, China) was used throughout the whole experiment. In addition, titanium plates were used as the cathodes and Ag plates as the anodes in the pulse plating.

2.2 Synthesis of SiO₂/Ag core-shell spheres

The typical scheme is presented in Figure 1. The mechanism of composite spheres produced in cathodes is as follows.

Figure 1:

Schematic illustration of deposition procedure for synthesizing the SiO₂/Ag composite spheres.

In general, it is believed that particles will be in accelerated motion to the cathode under the force of the electric field because of the particles’ surface adsorption positive charge. In a uniform electric field between two electrodes, the deposition rate (V) of particles in the cathode is expressed as follows:

U is the potential difference between the electrodes, q is the quantity of the particles’ electric charge, and m is the quality of the particles.

The deposition rate is associated with both the adsorbed amount of the charge and the quantity of the particles. Therefore, in the initial plating, SiO₂ particles move quickly to the cathode under the action of an electric field while SiO₂ particles are coated with silver. In the one-step preparation of SiO₂/Ag composite spheres, the silica particles should be mixed with ultrapure water via ultrasonic processing in order to make sure the blending suspension is even, and then the power supply should be turned on at a current density of 0.2–0.4 A/dm² [16].

2.3 Thermal annealing

To obtain dense coating, the as-prepared samples should be roasted in a chemical vapor deposition (CVD) furnace (SK2-4-13A, China) under an Argon atmosphere.

3.1 X-ray diffraction (XRD)

Crystal structure identification was accomplished using a powder X-ray diffractometer (Rigaku Ultima IV with D/tex Ultra, Japan) operating at 40 KV and 30 mA with Cu Kα radiation (with the corresponding X-ray wavelength of λ=1.54056 nm) at a scanning rate of 0.02 degrees per second in the 2θ ranging from 10 to 80 degrees.

The XRD patterns of bare silica powders shown in Figure 2a indicate that the bare silica is in an amorphous state. The strongest diffraction peak is still only the crystalless state of aggregation. The XRD patterns of SiO₂/Ag composite spheres, as demonstrated in Figure 2b, exhibited diffraction peaks at 2θ angles of 38.1°, 44.3°, 64.4°, and 77.4°, corresponding to the reflections of (111), (200), (220), and (311) crystalline planes of the face centered cubic (fcc) structure of Ag (PCPDF # 893722) [17].

Figure 2:

(a) XRD patterns of bare silica powder; (b) SiO₂/Ag composite spheres prepared by pulse plating.

3.2 Scanning electron microscope (SEM) observation

The morphologies of as-prepared samples were analyzed with a field-emission SEM (JEOL JSM-7500F, Japan) operating at an accelerating voltage of 5 KV.

In Figure 3a and b, some small spheres have attached to big ones because of aggregation. But the surface of all spheres is obviously smooth. The as-prepared core-shell SiO₂/Ag composite spheres are shown in Figure 3c and d, and we can see the materials coated on the surface of the spheres are loose. However, through thermal annealing, the coating layer becomes relatively smooth and dense, like the images shown in Figure 3e and f. In summary, the SEM measurement carried out to further confirm the effect of coating shows that SiO₂ nanoparticles are homogeneously impregnated with Ag nanoparticles, as in Figure 3, from which a better understanding about whether the silver is coated onto silica spheres can be acknowledged.

Figure 3:

(a, b) SEM images of bare silica powder; (c, d) SiO₂/Ag composite spheres depending on different angles of view; (e, f) SiO₂/Ag composite spheres after post-processing depending on different magnifying powers; (g) TEM images of SiO₂/Ag composite spheres; (h) the corresponding EDS pattern.

In Figure 4, the coating layer under the annealing condition of 900°C is smoother and denser than below 800°C. As is known, the melting point of Ag is about 960°C, so 800°C and 900°C were tried to explore a better annealing condition. From Figure 4a, it is clear that the Ag particles cannot be completely melted under the annealing condition of 800°C. However, a relatively ideal result can be achieved below 900°C.

Figure 4:

SEM images of SiO₂/Ag composite spheres after thermal annealing under different temperature conditions. (a) annealing below 800°C; (b) annealing below 900°C.

3.3 Transmission electron microscopy (TEM) observation

In order to have a deeper understanding of the microstructure of the obtained composite spheres, a TEM (JEOL JEM 2010, Japan) was used to observe and analyze these composite spheres. The sphere was covered by a thick and dense material. This is a typical core-shell composite. The diameter of the core is about 400 nm, and the thickness of the cover layer is about 20 nm (see Figure 3g).

Compared with the energy dispersive spectrometer (EDS) pattern, it is clear that the strong peak comes from the SiO₂ sphere and the peaks of Ag comes from the cover layer. Some of weak peaks of Cu come from the Cu grid.

3.4 Photoluminescence (PL) property of SiO₂/Ag composite spheres

The results show the PL properties of the composite particles have been enhanced significantly from the original silica [18]. Upon coating, the enhancements of the ultraviolet and visible emission are changed, as reflected by the PL intensity as function of wavelength (Figure 5).

Figure 5:

Photoluminescence spectra of pure silica powder and SiO₂/Ag composite spheres.

As is known, it is impossible for electrons in silica to jump from the valence band to the conduction band at an excitation wavelength of 325 nm. That is to say, there exists no excitation enhancement for the PL emission of the pure silica powder [19]. However, the waveform shown in Figure 5 does not match this theory. It occurs in this work due to the defect emissions caused by the impurities in silica. In addition, after coating, the defect emissions are remarkably increased in the composite spheres, which indicates that an Ag shell can enhance the defect luminescence. The reason why an Ag shell can enhance the defect luminescence may come from the combination of silver and the oxygen in silica. The specific reasons leading to this increase in defects are still in research. From the property, these composite spheres may also be applied to enhance the signals of other optical species [20].

3.5 Surface enhanced Raman scattering (SERS) property of SiO₂/Ag composite spheres

The signal of SERS was enhanced to analyze adsorbed Au or Ag materials [21, 22]. In this study, we investigated the SERS effect of SiO₂/Ag composite spheres by performing comparative experiments. Here, DTNB [5,5′-Dithiobis-(2-nitrobenzoic acid)] was chosen as Raman reporters, and the SERS measurements were performed by using the 325 nm exciting radiation. It was quite clearly found that no Raman peak was identified when the pure silica powder acted as the SERS substrate, but the Raman signal can be magnified significantly when the SERS substrate is SiO₂/Ag composite spheres, as seen in Figure 6. This result indicates that the SiO₂/Ag composite spheres have excellent SERS performance.

Figure 6:

SERS spectra of DTNB. (a) with pure SiO₂ powder; (b) with SiO₂/Ag composite spheres.

3.6 Electrical conductivity of SiO₂/Ag composite spheres

In our work, a four-point probe technique was used to investigate the electrical conductivity of SiO₂/Ag composite spheres. Silica powder is used today in a large variety of applications because it is easy to form and has good chemical resistance. However, silica powder is electrically insulating. Composite spheres overcome this shortcoming by coating a layer of silver on the surface of silica [23]. As shown in Figure 7, the current-voltage characteristic (I-V characteristic) of SiO₂/Ag composite spheres improves, the reason for which is that the SiO₂/Ag composite spheres can increase conductive contact points, forming more conductive channels [24].

Figure 7:

I-V characteristics of the pure SiO₂ powder and SiO₂/Ag composite spheres.