Distinguishing the nature of silver incorporated in sol-gel silica
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 SiOSi 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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