Thermal annealing and laser-induced mechanisms in controlling the size and size-distribution of silver nanoparticles in Ag+-Na+ ion-exchanged silicate glasses
Introduction
In the last decades, a great deal of work has been devoted to the study of the metal-alkali thermal ion exchange process in silicate glasses. The process is usually realized by immersing silicate glass slides in a molten salt bath containing the dopant ions, which are driven into the glass by the chemical potential gradient, and replace alkali ions of the glass matrix that are released into the melt. In this way, metal concentration values far exceeding the solubility limit can be achieved in the glass without aggregation. Ion exchange found its first application for surface strengthening [1], then being studied as a suitable method in the fabrication of passive and active optical waveguides [2], [3], [4], [5], [6], [7], of stress modulated systems [8], and for basic questions concerned with the understanding of dopant-glass interactions [9]. The ion-exchange process was effectively used for doping silicate glasses with different metal species and then promoting the controlled cluster formation by means of proper subsequent treatments, e.g., particle or laser irradiation as well as heat-treatments [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. The possibility of controlling the clusterization in glasses has opened quite attractive perspectives, since nanocomposite materials made by metal clusters embedded in silicate glass exhibit striking optical properties interesting for photonic application, for example third order nonlinearity [20], finding application as well in catalysis and sensor technology and superparamagnetic materials [21], [22], [23]. Moreover, by introducing metal nanoclusters into the glass matrix, the fluorescence properties of rare-earth ions containing glasses can be greatly improved [24], [25], [26], because metal clusters can be used as sensitizers in rare earth glasses, which is very important in telecommunications technology. In addition to being interested in their technical applications, these glasses have also been studied from a more basic perspective, such as the kinetics of cluster nucleation and growth, their stability, and their structure in terms of composition, crystalline phase, size, and size distribution.
The nucleation and growth of silver clusters upon diffusion and thermal annealing of exchanged ions is a complex process, due to several factors: ion exchange is intrinsically a non-equilibrium process, since the diffusion and the site occupation occur below the glass softening point, so preventing the structure to accommodate the host dopant following structural and thermodynamical stability constraints. Moreover, three classes of phenomena take place in giving the resulting system, namely, diffusion, nucleation and aggregate growth. These processes exhibit different regimes, and are somewhat in conflict, giving rise to dynamic feedbacks that may actually stop or enhance one of them upon small changes in any of the involved variables, in particular, the local silver concentration, the temperature, the glass composition, or even the local stoichiometry. On the basis of Raman spectroscopy, the influence of alkali ions on the structure of silicate glass was studied in Ref. [27]. In our previous article, the effect of silver concentration on the significant structural changes of the ion sites of the glass network after the ion-exchange in silicate glass has been reported [19]. In another work, we studied the role of post-exchange thermal treatment in the clustering process of the incorporated silver in silicate glasses [28]. Moreover, in the aim to understand the mechanism of metal nanoparticles formation during thermal annealing of silver containing glasses we reported the use of spectroscopic techniques in a configuration that allows a cross-sectional analysis [29]. In general, precipitation of silver and subsequent cluster nucleation and growth occur (even during the silver diffusion) depending on the energy provided to the system, either by thermal treatments or by energetic beams of particles or electromagnetic radiation. The chemistry of the atmosphere in which a treatment is realized can also play a significant role, as shown by different experiments [30] in which annealing treatments in H-rich atmosphere were quite effective in promoting silver clusterization in glasses. Within this frame, a phenomenological model describing the transport phenomena including the hydrogen contribution was actually proposed for metal cluster formation [31,32], although limited in its effectiveness to specific range of the involved variables.
The control of metal nanoparticle size, as well as of size distribution is important for optimizing the operation of nonlinear optical [33] and sensor [34] devices fabricated using nanocomposite materials. In our recent article, the modification of the silica network structure and the Ag clustering process in silver-containing silicate glass caused by thermal annealing and 355 nm laser irradiation were studied by Raman spectroscopy [35]. However, the effect of laser radiation on the metal doped glasses depends on laser parameters such as wavelength, power and laser fluence, and the following mechanisms are not yet fully understood. In this framework, with the aim to control Ag nanoparticles size and size distribution in silicate glass and ultimately to tune the optical properties of this nanocomposite system, the impact of irradiation with different laser parameters, such as different wavelength and power density, on the silver containing glass is expected. In this contribution, micro-Raman spectroscopy have been used to investigate the direct impact of laser irradiation of different wavelength and laser fluence values on Ag+-Na+ ion-exchanged glasses to deepen our understanding in controlling the Ag nanoparticles size and size distribution in silicate glasses, and ultimately to provide more information on the formation of metal nanoparticles, including diffusion, aggregation, and possible cluster fragmentation.
Section snippets
Experimental
Commercially available soda-lime silicate glass (SLG) slides with glass transition temperature (Tg) of 570 ˚C and a thickness of 1 mm were used as starting materials. The atomic % composition of the SLG glass used in this work was: 59.6 O, 23.9 Si, 10.1 Na, 2.6 Mg, 2.4 Ca, 0.7 Mg, 0.5 K, 0.2 S and other elements in traces. The nominal composition, given by the manufacturer, was confirmed by Rutherford backscattering spectroscopy (RBS) measurements (within the experimental errors). Ag+-Na+
Effect of post-exchange thermal annealing on the vibrational dynamics and optical properties of incorporated silver species
After the ion-exchange process at 320 °C for 20 minutes an exchange layer of about 7-8 microns containing silver species is formed at the glass surface. After the thermal annealing, almost uniformly distributed silver species are found in the ion-exchanged layer, and as the annealing temperature increased, the silver species further diffuses into the glass matrix. The effect of thermal annealing of the ion-exchanged samples is clearly visible and is reflected by the change of color of the
Conclusions
Ag+-Na+ ion-exchange have been performed on commercial soda-lime silicate glasses. The ion exchange was performed at 320 °C for periods from 20 minutes, in a molten mixture of AgNO3 and NaNO3 with a molar ratio of 1:99. After the ion exchange process, different treatments, such as thermal annealing, laser irradiation and combinations thereof, were performed to synthesize and tune the size of Ag NPs and the size distribution on the surface of the glass substrate. Inspection of low- frequency
Author contributions
G.M. supervised the project. E.C. motivated and prepared the glass samples, performed the thermal annealing and A.R. wrote the main manuscript and performed spectra measurements and evaluations. E.T. did the modification of the glass samples by laser irradiation. F.G. and A.Q. provided suggestions for article writing. All authors iteratively discussed and revised the manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the Fondo Sociale Europeo (FSE) (Project no. 1695/1/12/1017/2008).
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