H2-induced copper and silver nanoparticle precipitation inside sol-gel silica optical fiber preforms
© Chahadih et al.; licensee Springer. 2012
Received: 2 July 2012
Accepted: 5 August 2012
Published: 31 August 2012
Ionic copper- or silver-doped dense silica rods have been prepared by sintering sol-gel porous silica xerogels doped with ionic precursors. The precipitation of Cu or Ag nanoparticles was achieved by heat treatment under hydrogen followed by annealing under air atmosphere. The surface plasmon resonance bands of copper and silver nanoparticles have been clearly observed in the absorption spectra. The spectral positions of these bands were found to depend slightly on the particle size, which could be tuned by varying the annealing conditions. Hence, transmission electron microscopy showed the formation of spherical copper nanoparticles with diameters in the range of 3.3 to 5.6 nm. On the other hand, in the case of silver, both spherical nanoparticles with diameters in the range of 3 to 6 nm and nano-rods were obtained.
Keywordscopper nanoparticles silver nanoparticles surface plasmon resonance sol-gel process dense silica preforms.
Nanoparticles (NPs) of noble metals, namely gold, copper and silver, have unique optical, electrical, and magnetic properties. It is well known that these properties, and in particular the optical absorption, differ drastically from the properties of bulk metals. The physical origin of the light absorption by metallic NPs is the excitation of coherent oscillations of the conduction band electrons by light, which is known as surface plasmon resonance (SPR) . Recent investigations showed that SPR-based devices have enhanced properties useful for many optoelectronic materials and devices, such as solar cells  or emitters . Dense silica matrices, doped by NPs of noble metals, have stimulated considerable interest in basic research as well as from the applied standpoint in connection with enhancement of catalytic behavior , sensors , linear and nonlinear optical properties [6–9].
The major efforts in the synthesis of metallic nanoparticles inside a silica glass are based on two methods: melting process [8–12] and sol-gel route [7, 13, 14]. One of the most important features of doped sol-gel silica materials is their ability to preserve chemical and physical properties of the dopants at high temperature. Precipitations of copper and silver NPs in sol-gel silica glasses have been reported by Yeshchenko et al. [13, 14]. Examining the synthesis details, one can note that their porous silica matrices were produced by the conventional sol-gel technique. Then, the samples were soaked into an alcoholic solution of copper nitrate or silver nitrate before being densified at 1,200°C under hydrogen atmospheres. These densifications led to the formation of NPs. However, the synthesis of such silica glass doped with noble metal NPs has been limited to powders or thin films. To our best knowledge, the present work reports on the first precipitation of these NPs inside a bulk silica optical preform.
Recently, our research team reported the fabrication of a microstructured optical fiber drawn at about 2,000°C with a sol-gel silica cylindrical rod doped with gold NPs as a fiber core . The gold NPs were precipitated during the sintering process under air. Such optical fibers presented interesting linear and nonlinear optical properties. In this work, we report on the precipitation of copper or silver NPs inside sol-gel silica bulk rods. To this purpose, a reducing thermal processing has been applied after the glass densification. The interest of this method lies in the possible preservation of these NPs inside silica glass preforms even at a high temperature, which is promising for the achievement of silver- or copper-doped optical fiber cores.
Tetraethyl orthosilicate (TEOS) (≥99%), copper (II) hexafluoroacetylacetonate hydrate, silver (I) hexafluoroacetylacetonate (1,5-cyclooctadiene) and methanol CHROMASOLV were employed for the synthesis of ionic copper-doped and ionic silver-doped silica glasses. All these products were purchased from Sigma-Aldrich Chimie S.a.r.l. (Lyon, France).
Preparation of samples
Before any characterization, the obtained cylindrical glass rods were cut into pieces of 1-mm thick, which were polished to get high optical transparency. The samples were characterized by absorption spectroscopy and transmission electron microscopy (TEM). The absorption spectra were recorded at room temperature using a Perkin-Elmer Lambda 19 UV-vis-IR double beam spectrometer (MA, USA). The TEM characterization was performed on a microscope FEI Tecnai G2-20 twin with a 200-kV acceleration voltage (FEI Company, Eindhoven, The Netherlands). For TEM imaging, the sample was cut and polished to a 50-μm thickness and deposited on a molybdenum microscope grid. Then, an argon ion gas milling process allowed to obtain a thin area before a carbon film was evaporated on it.
Results and discussion
Cu-doped silica glass rod
Calculated diameters of Cu NPs as a function of heating temperature
Ag-doped silica glass rod
The absorption spectra of Figure 6, almost identical before and after the hydrogenation process, exhibit a band at 230 nm attributed to silver ions  and a smooth shoulder around 330 nm, corresponding to atomic Ag0. Heating the sample at 100°C under air condition did not change the spectrum, but from an annealing temperature of 300°C up to 1,100°C, the peak at 330 nm became more intense, revealing a much larger amount of reduced species. Heating at 500°C leads to the aggregation of the silver atoms into small clusters, as indicated by the presence of a silver SPR at 367 nm, which is considerably blue-shifted with regard to the normal position of the SPR band in silica (410 nm) . This conventional position (410 nm) is attained when the temperature is increased up to 700°C, as shown in Figure 6e. Moreover, the absorbance of the SPR band strongly increased and was slightly red-shifted with the heating temperature up to 1,100°C. The maximum NP concentration is attained after heat treatment at 1,100°C, as attested by the saturation of the spectrophotometer. On the contrary, after annealing at 1,200°C, the SPR band decreased probably due to oxidation of Ag NPs. Meanwhile, for this latter annealing temperature, the SPR band was divided into two sub-bands, the first one being centered at 390 nm and the second one at 450 nm. The appearance of these two peaks could be explained by the formation of elongated shapes of silver NPs, such as nanorods . Equation 1 was used to roughly estimate the mean particle size of NPs from the SPR peak width after annealing at 500°C, 700°C and 900°C, yielding diameters of 1, 1.6 and 2.7 nm, respectively.
Mechanism of NP formation
The red or yellow color observed inside the volume of the dense silica rods indicates that hydrogen molecules are allowed to penetrate the matrix at 80°C under a pressure of 140 bars in order to reduce the metal ions. Nevertheless, the diffusion of hydrogen inside the silica glass, i.e., near the copper or silver ions, is not sufficient to reduce them. To that purpose, the hydrogenated sample should be subsequently annealed at a temperature which depends on the ion. From this point of view, Cu+ ions are more difficultly reduced (annealing temperature of 700°C required) than Ag+ ions (temperature of 500°C is required).
Finally, NPs of noble metals were precipitated in silica preforms prepared by the sol-gel process. The method consists of a heat treatment of the densified Cu- or Ag-doped glasses under hydrogen atmosphere, followed by an annealing in air atmosphere. It has been shown that the Cu NP size increased as a function of this last annealing temperature, and the obtained diameters ranged between 3 and 6 nm. Similarly, it has been shown that the control of the annealing temperature allows the adjustment of both size and shape of the Ag NPs. The formation of both copper oxide (Cu2O) and silver oxide (Ag2O) NPs at high temperature has also been confirmed by TEM analysis.
The preservation of metal NPs at temperatures as high as 1,100°C is very promising. In the next future, each of these Cu or Ag cylindrical rods will be drawn into a small capillary and then stacked with pure silica tubes in order to get a holey preform.
microstructured optical fiber
transmission electron microscopy
surface plasmon resonance.
This study was supported by the French Agence Nationale de la Recherche (ANR-P2N, ‘POMESCO’ Project), the ‘Conseil Régional Nord Pas de Calais Picardie’ and the ‘Fonds Européen de Développement Economique des Régions’.
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