Investigation on existing states and photoluminescence property of silver in the SiO₂ three-dimensionally ordered macroporous materials

Abstract

The photoluminescence properties of silver species, including Ag⁺, Ag⁺–Ag⁺, Ag⁰, and Ag nanoparticles in various matrices, such as gel and glass have been extensively reported. In the present study, we present the preparation of silver including SiO₂ three-dimensionally ordered macroporous (3DOM) materials and investigate the existing states and photoluminescence property of silver in the SiO₂ 3DOM materials. The results show that only Ag⁺ ions exist in the SiO₂ 3DOM materials sintered at temperature below 400 °C. With the increasing sintering temperature, the Ag⁺ ions gradually transform into Ag⁺–Ag⁺, where simultaneously, a part of Ag⁺–Ag⁺ transform into Ag nanoparticles. The Ag⁺–Ag⁺ and Ag nanoparticles are formed in the SiO₂ 3DOM materials sintered at temperature from 450 to 650 °C. Finally, only Ag nanoparticles occur in the SiO₂ 3DOM materials prepared at 750 °C. The formation mechanisms of Ag species were discussed in the SiO₂ 3DOM materials.

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Introduction

Colloidal crystals self-assembled by mono-dispersed microspheres are extensively used to prepare macroporous materials as ordered templates. After filling and removing the colloidal crystals templates by chemical etching or calcinations, the macroporous material having sub-micrometer pores with long range ordering, known as three-dimensional ordered macroporous material, is prepared. The three-dimensional ordered macroporous (3DOM) material has advanced properties attributed to its large surface areas, pore volumes and macrospores with uniform size and shape, which can be used in adsorption, separation and catalyst applications.^1–6 In addition, the most exciting application of these macroporous materials is in the form of photonic crystals, which can diffract photons from a lattice of dielectric planes.^7–10 Another novel application of 3DOM materials is to combine the feature of porous structure for the storage delivery of specific species with photoluminescence property for the simultaneous tracking or monitoring of the species, which are expected to have important applications in medicine, biosensor and even military affairs.^11–13 At present, further investigation of 3DOM materials to expand their application to other areas is underway.

Noble silver nanoparticle has a long and rich history over its preparation, characterization and applications. Based on its unique properties, noble metal silver nanoparticle has numerous potential applications in medicine, catalysis and optoelectronics.^14,15 Understanding both existing states and photoluminescence properties of silver embedded in matrices is essential for the applications of silver; therefore, extensive studies were conducted for this purpose. At present, existing states and luminescence properties for silver in solution, gel and glass matrices have been established.^16–20 Previous investigations demonstrated that existing states of silver is abundant, including Ag⁺, Ag⁺–Ag⁺, Ag⁰, Ag clusters (Ag₂–Ag₁₂) and nanoparticles.^18–20

When silver species are embedded into the wall of 3DOM materials, such composites may be used as novel photonic media in optoelectronics or non-linear optics. The silver-containing 3DOM materials may become an exciting field of nanoscience and nanotechnology because of the possibility of a better control over existing states and location of silver. The 3DOM materials containing silver nanoparticles may have potential applications in optical switching, filtering and high capacity optical recording media. In this work, silver-containing silica 3DOM materials were first prepared by sol-gel method, and the existing states of silver in the SiO₂ 3DOM materials were investigated by transmission electron microscopy, UV-vis absorption and photoluminescence spectra.

Experimental

The commercially available suspensions of polystyrene (PS) microspheres with a diameter of 380 nm were used to prepare ordered templates, and the volume fraction of PS microspheres in the suspension was about 10%. The ordered colloidal crystals templates were prepared by the vertical deposition process. Clean quartz slides were then placed vertically into the PS microsphere solution to allow the self-assembly of microspheres via solvent evaporation at a temperature of 50 °C over a period of 5 to 6 days. After the water was evaporated, highly-ordered PS templates were formed on the quartz substrates.

The SiO₂ : x% Ag (x = 0, 0.2, 0.4, 0.6 and 0.8) 3DOM materials were prepared using tetraethyl orthosilicate [Si(CH₃CH₂O)₄, TEOS] and AgNO₃ as raw materials. The AgNO₃ and Si(CH₃CH₂O)₄ were dissolved in ethanol, respectively, then the above solutions were mixed together. The mixture was stirred to form a homogeneous SiO₂ : Ag solution. Infiltration of the PS ordered colloidal crystal templates with the SiO₂ : Ag solution with Ag concentrations ranging from 0 to 0.8 mol% was performed at ambient temperature. Subsequently, the filled samples were sintered in an air furnace at different temperatures to remove the PS microspheres. Finally, the SiO₂ : Ag 3DOM materials were obtained.

The SEM images of SiO₂ : Ag 3DOM materials were obtained by the field-emission scanning electron microscope (QUANTA200) after sputtering the samples with a thin layer of gold. An energy dispersive X-ray spectroscopy (EDS) attached to a QUANTA200 scanning electron microscope was used to study the chemical composition of SiO₂ : Ag 3DOM materials, which gave the elemental information. Transmission electron microscopy (TEM) images were taken using a JEOL 2100 transmission electron microscope operating at an acceleration voltage of 200 kV. The elemental analysis of SiO₂ : Ag 3DOM material was characterized by transmission electron microscopy-energy dispersive X-ray spectrometry (TEM-EDS, JEOL 2100). Absorption spectra of SiO₂ : Ag 3DOM materials were recorded using a Hitachi U-4100 spectrometer in air at room temperature in the 200–500 nm regions. Photoluminescence spectra of SiO₂ : Ag 3DOM materials were recorded on a Hitachi F-7000 spectrophotometer using a Xenon-lamp as light source under the excitation of 280 or 345 nm. Photoluminescence excitation spectra were recorded with the same Hitachi F-7000 spectrophotometer at room temperature.

Results and discussion

1. Microstructure analysis of SiO₂ : Ag 3DOM materials

To obtain 3DOM materials of SiO₂ : Ag, the rising temperature rate was carefully controlled and the rate of 50 °C h⁻¹ was used. As a typical case, the SEM images of SiO₂ : 0.6% Ag 3DOM materials sintered at 450, 650 and 750 °C are shown in Fig. 1. The SEM images show that the SiO₂ : 0.6% Ag samples sintered at 450, 650 and 750 °C exhibit a long-range ordered hexagonal arrangement. The center-to-center distance of SiO₂ : 0.6% Ag 3DOM sample sintered at 450 °C is about 325 nm, which is about 15% smaller than the original size of PS microspheres because of the shrinkage of the microspheres during calcinations. The microstructure characteristics of SiO₂ : 0.6% Ag 3DOM samples sintered at other temperatures are similar to that of the sample sintered at 450 °C, as shown in Fig. 1(b) and (c). Fig. 1(d) shows the EDS spectrum of 3DOM material of SiO₂ : 0.6% Ag sintered at 650 °C. The Si, O and Ag elements were observed in this sample.

Fig. 1 SEM images of Ag–SiO₂ 3DOM materials sintered at 450 °C (a), 650 °C (b), and 750 °C (c), the EDS of Ag–SiO₂ 3DOM materials sintered at 650 °C (d).

2. Influence of sintering temperature on existing states of Ag in the SiO₂ 3DOM materials

The photoluminescence spectra of SiO₂ : 0.6% Ag 3DOM materials sintered at different temperatures is shown in Fig. 2 under the excitation of 345 nm. The emission spectra in SiO₂ : 0.6% Ag 3DOM materials present various shapes along with increasing sintering temperature. No obvious emission band was observed in the 3DOM materials sintered below 400 °C, and the shape of emission spectrum is similar to that of ordered PS template filling with SiO₂ : Ag solution at room temperature without sintering. This indicates that only Ag⁺ exists in the SiO₂ : Ag 3DOM materials sintered below 400 °C. For the SiO₂ : Ag 3DOM materials sintered at 450, 550 and 650 °C, an obvious broad emission band in the 380–750 nm regions was observed, which consists of two emission bands located at 407 and 540 nm. The emission band located at 540 nm disappeared in the 3DOM materials when sintered at 750 °C. The contribution of both SiO₂ : Ag sol and PS microspheres to the 540 nm emission band can be excluded, because no emission can be detected in the PS ordered template infiltrated with SiO₂ : Ag solution without sintering excited by 345 nm light, as shown in Fig. 2. The 540 nm emission band of SiO₂ : Ag 3DOM materials sintered at 450, 550 and 650 °C may arise from the silver species. It is well known that the photoluminescence properties of silver species, including Ag⁺, Ag⁺–Ag⁺, Ag⁰, and Ag nanoparticle in the various matrices, such as gel and glass can be observed.^18,19 In addition, the Ag⁰–Ag⁺ and (Ag₃)⁺ under the excitation of UV light can exhibit 600 nm red emission.²¹ The Ag⁺–Ag⁺ can emit 550 nm light under the excitation of 350 nm.²² The emission band from 340 to 410 nm of Ag⁺ was also reported under the excitation of 275 nm.²³ The broad band at 540 nm may be assigned to Ag⁺–Ag⁺ in the present work. In addition, the 407 nm emission band is attributed to the defects of pure SiO₂, which is confirmed by the emission spectrum of pure SiO₂ 3DOM materials under the excitation of 345 nm, as shown in Fig. 7(b). Inset in Fig. 2 presents the change of 540 nm emission intensity with varying sintering temperature. It can be seen from the inset of Fig. 2 that the intensity of the 540 nm emission increases when the sintering temperature of 3DOM material increases from 450 to 650 °C. This result indicates that Ag⁺ gradually transform into Ag⁺–Ag⁺ with the increase of sintering temperature. The emission band from Ag⁺–Ag⁺ disappeared in the SiO₂ : Ag 3DOM materials formed at the 750 °C, indicating that existing state of Ag⁺–Ag⁺ was changed, which may have become silver nanoparticles.

Fig. 2 The emission spectra of SiO₂ : 0.6% Ag 3DOM sintered at different temperatures under the excitation of 345 nm. The inset pattern is the change of 540 nm emission intensity with varying sintering temperatures.

Fig. 3 shows the excitation spectra of SiO₂ : 0.6% Ag 3DOM materials sintered at different temperatures. It can be clearly seen that no obvious excitation band was observed in the 3DOM materials sintered at 350 and 400 °C, which suggested that no Ag⁺–Ag⁺ was formed. The excitation spectra monitored at 540 nm emission in the SiO₂ : 0.6% Ag 3DOM materials formed from 450, 550 and 650 °C consist of a broad excitation band ranging from 230 nm to 420 nm with peaks at 345 nm, which was associated with Ag⁺–Ag⁺.²² Previous investigations demonstrated that the excitation band of Ag⁺–Ag⁺ is located at 350 nm, which is consistent with the present result.²² The excitation band intensity of SiO₂ : Ag 3DOM materials grow with the increase of sintering temperature from 450 to 650 °C, which further indicated Ag⁺–Ag⁺ concentration increased with increasing sintering temperature. Upon increasing the heating temperature up to 750 °C, the excited band vanished again, indicating the disappearance of Ag⁺–Ag⁺ species. The broad excitation band ranging from 220 to 420 nm is advantageous to the excitation of samples. Another shoulder peak centered at 280 nm in the excitation spectra is shown in Fig. 3 at the sintering temperature of 450, 550 and 650 °C. The emission spectra of SiO₂ : 0.6% Ag 3DOM materials were measured under the excitation of 280 nm, as shown in Fig. 4(a). The emission spectra in the visible region are similar to those of SiO₂ : 0.6% Ag 3DOM materials excited at 345 nm. However, besides the visible broad band emission, a novel emission band located at 350 nm is observed in the 3DOM materials prepared above 450 °C excited by 280 nm, as shown in Fig. 4(a). In order to confirm the origin of the additional 350 nm emission band, pure SiO₂ 3DOM material without Ag was prepared. Fig. 4(b) shows the pure SiO₂ 3DOM material without Ag under the excitation of 280 nm. A 350 nm broad band with a shoulder at 455 nm is observed in the emission spectrum of pure SiO₂ 3DOM materials without Ag sintered at 550 °C, which indicates that the 350 nm emission band may be attributed to the emission of defects of pure SiO₂ host under a 280 nm excitation. The 350 nm emission band and shoulder at 455 nm may have come from the ≡Si–O–O–Si≡ and ≡Si–Si≡ defects, respectively, which were discovered in previous studies.^24–26 With increasing temperature, a relative sharp emission band located at 350 nm was obtained in the SiO₂ : Ag 3DOM materials prepared at above 650 °C, which suggested that more ≡Si–O–O–Si≡ defects were obtained in the SiO₂ : Ag 3DOM materials. The formation of ≡Si–O–O–Si≡ defects is dependent on the concentration of Ag nanoparticles and temperature.^27,28 The reactions between Ag and SiO₂ may result in the formation of ≡Si–O–O–Si≡ defects, and the formation mechanism was proposed in a previous study.²⁸ In addition, at high temperature, ≡Si–Si≡ defects may transform into ≡Si–O–O–Si≡ defects by the reaction between ≡Si–Si≡ defects and O₂.²⁴ The presence of defect centers from pure SiO₂ was further confirmed by excitation spectra, as shown in the inset of Fig. 4(b). The excitation spectrum monitored at 350 nm emission in the pure SiO₂ 3DOM material formed at 650 °C consists of two excitation bands located at 240 and 280 nm, which suggests that the emission band at 350 and excited band at 280 nm may be attributed to the defects in the SiO₂ 3DOM materials.

Fig. 3 The excitation spectra monitored at 540 nm emission of SiO₂ : 0.6% Ag 3DOM.

Fig. 4 The emission spectra of SiO₂ : 0.6% Ag 3DOM formed at different temperatures under the excitation of 280 nm (a), the emission spectrum of pure SiO₂ 3DOM materials under the excitation of 280 nm (b); inset is the excitation spectrum of pure SiO₂ 3DOM materials monitored at 350 nm emission.

The existing states of silver in the SiO₂ 3DOM materials were also investigated by the absorption spectra, as shown in Fig. 5. Whatever the sintered temperature of the SiO₂ : 0.6% Ag 3DOM materials, all the SiO₂ : 0.6% Ag 3DOM materials exhibited an absorption band located at about 510 nm, which is attributed to the Bragg diffraction of visible light from the ordered porous structure. The absorption band below 350 nm was observed in all the SiO₂ 3DOM materials, which may be from the absorption of SiO₂ hosts. The SiO₂ : 0.6% Ag 3DOM material formed at 400 °C has no other absorption in the visible region of the spectrum except for an absorption band at 510 nm. However, another absorption band located at about 410 nm was observed in the SiO₂ : 0.6% Ag 3DOM materials sintered above 450 °C, which is attributed to the plasmon resonance absorption of silver nanoparticles (usually around 400 ± 20 nm),^29,30 indicating that silver nanoparticles was formed when sintering temperature was above 450 °C.

Fig. 5 The absorption spectra of SiO₂ : 0.6% Ag 3DOM prepared at different temperatures.

In order to verify the formation of Ag nanoparticles in the SiO₂ 3DOM materials, the TEM images of the SiO₂ : 0.6% Ag 3DOM materials were measured, as shown in Fig. 6. Few small silver nanoparticles was observed in SiO₂ : Ag 3DOM material sintered at 450 °C, although no obvious plasmon resonance absorption band from silver nanoparticles was observed in this sample, as shown in Fig. 5. It can be clearly seen that the concentration and size of Ag nanoparticles increases with increasing sintering temperature, which is consistent with the results shown in the absorption spectra. The results in the TEM images further demonstrate that other Ag species such as Ag⁺–Ag⁺ transform into silver nanoparticles with increasing sintering temperature. Fig. 6(d) shows the TEM-EDS spectrum of the SiO₂ :  0.6% Ag 3DOM sintered at 750 °C. It shows that Si, O and Ag elements were observed in the sample, and this is the direct experimental evidence to prove the presence of Ag element in 3DOM SiO₂ materials.

Fig. 6 TEM images of SiO₂ : 0.6% Ag 3DOM formed at different temperatures (a) 450, (b) 650, (c) 750 °C; (d) the EDS spectrum of SiO₂ : 0.6% Ag 3DOM formed at 650 °C.

3. Formation mechanisms of Ag species in the 3DOM materials

The existing states of Ag changes from one to another in the SiO₂ : Ag 3DOM materials under varying sintering temperatures. Based on the TEM images, as well as absorption, excitation and photoluminescence spectra, it can be concluded that: (1) the Ag⁺ existed in the SiO₂ : Ag 3DOM materials formed below 400 °C, (2) part of Ag⁺ convert into Ag⁺–Ag⁺ and nanoparticles in SiO₂ : Ag 3DOM materials sintered at 450, 550 and 650 °C, and Ag⁺–Ag⁺ and silver nanoparticles co-occurred in the SiO₂ 3DOM materials sintering at 450, 550 and 650 °C, (3) at 750 °C, all Ag species completely convert into silver nanoparticles in the 3DOM materials. In the SiO₂ materials containing Ag⁺, non-bridging oxygen species (≡Si–O⁻) occurred, which was revealed by previous investigation.³¹ The following reactions may occur in the formation of Ag species in the SiO₂ 3DOM materials.

2(≡Si–O⁻) → ≡Si–O–Si≡ + O²⁻ (1)

Ag⁺ + Ag⁺ → (Ag⁺–Ag⁺) (2)

2(Ag⁺–Ag⁺) + 2O²⁻ → 4Ag⁰ + O₂ (3)

Ag⁰ + Ag⁰ →Ag nanoparticle (4)

4. Influence of Ag content on photoluminescence property of Ag existing states in the SiO₂ 3DOM materials

As shown in Fig. 2, the photoluminescence intensity of SiO₂ : 0.6% Ag 3DOM materials sintered at 650 °C is more intense than those of other samples. The dependence of luminescence on Ag concentration was investigated in the SiO₂ 3DOM materials sintered at 650 °C under the same preparation and measurement condition. Fig. 7 shows the photoluminescence spectra of SiO₂ : Ag 3DOM materials as a function of Ag concentration under 280 (a) and 345 nm excitation (b). Under the excitation of 280 nm, the emission spectra of 3DOM materials consist of two broad bands with maxima at 350 and 540 nm, which were attributed to the defects and Ag⁺–Ag⁺ in the SiO₂ 3DOM materials, respectively. Under the excitation of 345 nm, the two broad emission bands at 407 and 540 nm were attributed to the defect and Ag⁺–Ag⁺ in the SiO₂ 3DOM materials, respectively. It is clearly seen from Fig. 7 that the shape of emission spectra is similar with the SiO₂ 3DOM materials with various Ag concentrations, indicating that the Ag concentration has no influence on Ag states in the SiO₂ 3DOM materials. It can be noted that the emission intensities depend on Ag concentration. Under the 280 or 345 nm excitation, the intensity of the emission spectra was increased firstly and then decreased with increasing Ag concentration. The emission intensity is the strongest in SiO₂ : 0.6% Ag 3DOM materials. The concentration quenching was observed when the concentration of Ag is about 0.8 mol%.

Fig. 7 The emission spectra of SiO₂ : Ag 3DOM with different Ag concentrations under the excitation of 280 (a) and 345 nm (b).

Fig. 8 shows the absorption spectra of SiO₂ 3DOM materials with various Ag concentrations. It can be clearly seen that the plasmon resonance absorption peak located at about 410 nm from silver nanoparticles was observed, indicating that the silver nanoparticles were formed in the SiO₂ 3DOM materials apart from Ag⁺–Ag⁺. The plasmon resonance absorption peak from silver nanoparticles increased with increasing Ag concentration, suggesting that more silver nanoparticles were obtained in the SiO₂ 3DOM materials with higher Ag concentration.

Fig. 8 The absorption spectra of Ag–SiO₂ 3DOM materials with different Ag concentrations.

Conclusion

The Ag including SiO₂ three-dimensionally ordered macroporous materials have been synthesized. The formation mechanism and luminescence of silver species in the SiO₂ three-dimensionally ordered macroporous materials have been studied using transmission electron microscopy, ultraviolet absorption and photoluminescence spectra. The results show that only Ag⁺ exists in the SiO₂ 3DOM materials at the sintering temperature below 400 °C. At sintering temperature above 450 °C, the Ag ions gradually converted into Ag⁺–Ag⁺, where a part of Ag⁺–Ag⁺ simultaneously transformed into Ag nanoparticles. The Ag⁺–Ag⁺ and Ag nanoparticles were obtained in the SiO₂ 3DOM materials. When the sintering temperature was 750 °C, all kinds of Ag species completely converted into silver nanoparticles in the SiO₂ 3DOM materials. This is the first report of three-dimensionally macroporous materials doped with metallic nanoparticles, which may provide the possibility of extending application potentials of both three-dimensionally ordered macroporous materials and metallic species.

Acknowledgements

This work was supported by the Reserve Talents Project of Yunnan Province (2013HB068).

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