Elsevier

Journal of Non-Crystalline Solids

Volume 475, 1 November 2017, Pages 71-75
Journal of Non-Crystalline Solids

Distinguishing the nature of silver incorporated in sol-gel silica

https://doi.org/10.1016/j.jnoncrysol.2017.08.033Get rights and content

Highlights

  • Silica glass doped with silver was synthesized using the sol-gel method.

  • FT-IR confirms the formation of the silica network and the purity of all samples.

  • UV–Vis and XRD spectroscopy confirm the formation of Ag NPs in annealed sample.

  • HRTEM and XPS confirm the formation of Ag NPs in the as-prepared sample.

  • The ratio between Ag NPs/Ag ions increases by increasing annealing temperature.

Abstract

Silica glass doped with silver was synthesized using the sol-gel method. The formation of the silica network and the purity of samples were confirmed by Fourier transform infrared spectroscopy. The nature of silver as a function of annealing temperature was investigated using X-ray diffraction (XRD), UV–Vis spectrophotometry, high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) techniques. The obtained results from XRD and UV–Vis absorption showed similar information which both confirmed the formation of Ag nanoparticles (NPs) in the samples annealed at high temperature, but revealed nothing about the nature of Ag in the as-prepared samples and those annealed at low temperature. The images and diffraction patterns from HRTEM showed the interesting result that even for the as-prepared sample some of the Ag ions were converted to Ag NPs. This result was further confirmed by XPS data. Moreover, the HRTEM images showed that an increase in the annealing temperature increases the size of the Ag NPs, although the shape of the NPs remains almost unchanged. The quantitative information extracted from XPS results revealed that the ratio of Ag NPs to Ag ions increases with increasing annealing temperature.

Introduction

Preparation and characterization of glasses doped with noble metal nanoparticles (NPs) have attracted much attention due to their promising optical properties resulting from the localized surface plasmon resonance (LSPR) [1], [2], [3], [4]. Among the noble metals, NPs of silver exhibit a sharp and distinct absorption band related to LSPR in the visible region of the electromagnetic spectrum, which is extremely important for optoelectronic applications [2], [5]. This absorption band is strongly dependant on the metal, distribution of particle sizes and shapes, as well as the dielectric constant of the surrounding material [6]. The interaction between metal NPs and light (at the resonance frequency) produces a strong electric field around the NPs which make it desirable for use to enhance the 4f-4f absorption cross section of rare earth (RE) ions [2], [6]. This effect is the so-called plasmonic enhancement and is associated only with Ag NPs and not Ag ions.

Silver can be incorporated in silica either as Ag ions (e.g. Ag2O) or metallic Ag NPs, depending on the experimental conditions. Hence two possibilities of luminescence enhancement have been considered: energy transfer associated with Ag ions and the plasmonic effect associated with Ag NPs [1], [6]. Moreover, there are a variety of interaction mechanisms (associated with energy transfer or the plasmonic effect) between luminescent material and silver which have been reported [1], [7]. However, these have proved challenging to distinguish [8]. Knowledge about the nature of Ag doped in sol-gel silica is vital to identify the origin of the luminescence enhancement and the mechanism of interaction. Additionally, in the case of the plasmonic effect, the distribution of size and shape of the NPs also has a significant effect on the luminescence enhancement [6]. Therefore, to obtain information about the nature of Ag as well as the distribution of size and shape of Ag NPs requires a variety of spectroscopic techniques. In previous studies, researchers have used limited spectroscopic techniques to motivate the formation of Ag NPs in glass [2], [9], [10], ignoring the possible effect of residual Ag ions which may suppress the plasmonic effect or quench the luminescence of RE ions instead of leading to enhancement.

In this work we synthesized pure silica and silica doped with Ag using the sol-gel method. All samples were characterized using a variety of complementary techniques, giving information on the nature of Ag and the shape and size of Ag NPs in silica in order to compare them and to investigate the nature of the Ag as a function of the annealing temperature. Further novelty of this work is providing further information on the challenging questions such as the formation of Ag NPs in the as-prepared sample and the optimum temperature to convert all Ag ions to Ag NPs in sol-gel silica. Moreover, the XPS results confirmed that the formation of Ag NPs after thermal treatment is accompanied with the decomposition of Ag2O.

Section snippets

Materials and methods

Silica (SiO2) pure host and material doped with Ag were synthesized by the sol-gel method. The starting solutions used to prepare these samples consisted of tetraethylorthosilicate (TEOS, 99.7%), ethanol (C2H5OH, 99.0%), and distilled water. The molar ratio was taken as 1:5:10 for TEOS:ethanol:water, respectively [11]. For doped samples, silver nitrate (AgNO3, 99.9%) was used as the source of Ag. For all samples TEOS was dissolved in ethanol by magnetic stirring for 30 min. To catalyze the

Results and discussion

Fig. 1 shows FT-IR spectra in the region 400–2000 cm 1 of (a) undoped silica samples, as-prepared and annealed at 1000 °C (b) silica samples doped with different amounts of Ag annealed at 1000 °C. As can be seen from Fig. 1a, there are seven peaks for the as-prepared sample. A very strong and broad band at 1082 cm 1 with a shoulder at 1180 cm 1 is attributed to the transverse optical (TO) mode and longitudinal optical (LO) mode of Sisingle bondOsingle bondSi asymmetric stretching vibrations, respectively [12], [13]. The

Conclusion

We have successfully synthesized silica glass doped with Ag using the sol-gel method. The samples were characterized using a variety of complementary techniques to establish the nature of the Ag dopant. Results showed that the XRD and UV–Vis techniques were not effective to investigate the nature of silver, particularly for the as-prepared samples and those annealed at lower temperature. Therefore, other complementary techniques, namely HRTEM and XPS were used. The HRTEM results revealed that

Acknowledgements

The authors thank Professor M. E. Lee at the Centre for HRTEM, Nelson Mandela Metropolitan University for facilitating HRTEM measurements. This work is based on the research supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation of South Africa. The University of the Free State Cluster programme is acknowledged for financial support.

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