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Structural changes of soda-lime silica glass induced by a two-step ion exchange

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2010 IOP Conf. Ser.: Mater. Sci. Eng. 15 012069

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Structural changes of soda-lime silica glass induced by a two-

step ion exchange

M Suszynska, L Krajczyk and B Macalik

Institute of Low Temperature and Structure Research, Polish Academy of Sciences,

50-950 Wroclaw, ul. Okolna 2, Poland

M.Suszynska@int.pan.wroc.pl

Abstract. Introduction of silver or copper ions in soda-lime silica glass has been investigated

by studying the data obtained from optical absorption measurements and the transmission

electron microscopy performances. It has been stated that the optical and structural

characteristics of the doped specimens are effectively controlled by the dopant concentrations

and parameters of both, the ion-exchange procedures and the annealing treatments applied

afterwards. By sequential ion exchange of as received specimens in molten baths of Cu2Cl2and

(AgNO3 in NaNO3), the composition and electronic structure of the elemental-nanoclusters

have been altered. Moreover, depending on the order of annealing, different microstructures

were created in the fabricated composites. Like this, the two-step exchange determines a new

engineering way new materials with desired optical properties could be produced.

1. Introduction

Metallic nanoparticles embedded in a dielectric matrix are well known for their attractive properties,

strong optical resonance and fast non-linear optical polarizability associated with the plasmon

frequency of the conduction electrons in the particles [1]. Our previous investigations [2,3] of a

multicomponent soda-lime silica (SLS) glass, exchanged either with silver or copper ions, suggest that

the structure-sensitive properties of the glass composites are strongly affected not only by the peculiar

behaviour of the quantum dots, but also by the microstructure of the host matrix.

A two-step ion exchange of the SLS glass in chemically different salts is expected to alter the

composition and the electronic structure of the dopant-nanoclusters, which would affect both the linear

and non-linear optical properties of the fabricated materials that are not possible with single elemental

colloids [4].

In this work, we report the optical response of metallic nanoclusters formed by sequential ion

exchange of copper and silver in the SLS glass. To obtain a deeper insight into the processes, which

control the related phenomena, transmission electron microscopy (TEM), extended by the high

resolution TEM (HRTEM), and the selected area electron diffraction (SAED) analysis have been

performed. The results are compared with those obtained for the same glass matrix containing the

dopant in the form of elemental silver and copper nanoparticle [2,3].

2. Experiments

The content of the main SLS glass components (SiO2/73 mol % and Na2O/13 mol %) corresponds tothe miscibility gap in the SiO2-Na2O system [5]. The applied two-step ion exchange was as follows. As

1

mailto:M.Suszynska@int.pan.wroc.plmailto:M.Suszynska@int.pan.wroc.plmailto:M.Suszynska@int.pan.wroc.pl

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received specimens were firstly dipped in a bath of molten Cu2Cl2 at 4500C for 1h.The second

exchange was in a molten mixture of AgNO3(2%) in NaNO3at 3500C for 2h. Afterwards, the doubly

doped specimens underwent two types of thermal treatment. Namely, either 1h of air annealing at600

0C was followed by 5h of annealing in gaseous hydrogen at 500

0C or 5h of the hydrogen annealing

at 5000C was followed by the air annealing at 600

0C performed versus time up to about 1500h.

According to our knowledge [2,3], these procedures would secure the transformation of both ions into

the colloidal silver and copper nanoparticle.

It should be stressed that the co-doping of glasses, described in the literature, was usually

performed by ion implantation of pure silica [6-8]. Local heating of the doped matrix works as the

reduction agent and leads to the ion/atom-transformation. However, one has to remember that ion

implantation is accompanied by the formation of some radiation damage, and the effects of these

defects are usually omitted in discussion of the obtained results. On the contrary, we have adapted the

ion exchange method for silver as well as copper ions [2,3], and these procedures have been used for

the silver-after-copper exchange applied for the soda lime silica glass.

To characterize the composite materials, optical absorption (OA) and the transmission electronmicroscopy (TEM), extended by the high resolution TEM (HRTEM) and the selected area electron

diffraction (SAED) performances, have been used. After each treatment, the optical absorption was

measured in the range from 200 to 1600 nm on a Cary-5E UV-VIS spectrophotometer at room

temperature. The TEM-micrographs were obtained by means of a Philips-CM-20 microscope while

the Philips Scanning Microscope SEM-515 with a roentgenographic analyzer (EDAX-9800) was

exploited to control the composition of the specimens. The sample surfaces were selectively etched in

diluted water solutions of HF and replicated by a single stage technique. These replicas have been

examined under TEM operating at 200 kV with a 0,24 nm point-to-point resolution.

3. Results and discussion

3.1. Optical absorption dataOptical absorption spectra obtained for the commercial SLS glass doubly exchanged with silver-after-

copper, thermally treated afterwards in a different atmosphere and recorded at room temperature, are

shown in figure 1. Figure 2 shows additional data for the exchanged samples annealed in air-after-

hydrogen at 6000C for times up to about 1500h.

Figure 1: Optical absorption of the

SLS glass after: 1) ion exchange

with copper and silver (black & red

lines, respectively), 2) annealing of

the exchanged specimens either in

air or hydrogen (green & blue lines,

respectively), and 3) additional

annealing either in hydrogen after

air or in air after hydrogen (cyan &

magenta lines, respectively).

250 500 750 1000

0,0

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556

401

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2Cl

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oC

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The linear-OA-measurements made in air from 200 to 1600 nm show significant changes in the

magnitude, width and location of the surface plasmon resonance (SPR) bands when compared with

those of the pure elements, cp. figures 1 and 3 in [2] and [3] for the Ag and Cu ion exchanged SLSglass, respectively.

Figure 2: Optical absorption

of the doubly exchanged

SLS sample which after the

H2- treatment(2b/5h/5000C)

was air-annealed at 6000C

for time from 1h (C48.4)

up to 1474h. Attention: to

see some of the effectsdescribed in the text, the

OA scale should be strongly

reduced (0.01-0,30).

It has been ascertained that although the exchange with copper ions may be effectively performed

in the air atmosphere, the formation of Cu-nanoclusters was possible only after additional annealing of

the exchanged specimens in the gaseous hydrogen. In contrast to copper, the silver ions can be reduced

during a high temperature treatment of the exchanged specimens in air as well as in the hydrogen

atmosphere. In the first case the Fe2+

ions, present as impurities in the SLS glass, act as the reduction

agent owing to the (Fe2+Fe

3++ e

-) redox reaction. Moreover, already during exchange some of the

silver ions transform into atoms which are the seeds for the nanosized particles formed during the

high-temperature treatment. This effect has also been detected previously [9,10].

During the two-stage exchange of the SLS specimens, copper enters the glass matrix in the form of

mono- and divalent ions. While the monovalent copper and silver ions are not detectable in the studied

spectral range, divalent copper is registered at about 780 nm.

Short annealing of the exchanged specimens in air at 6000C (sample 2a) results in appearing of the

SPR band related with nanoparticles of silver at about 411 nm. The H2-treatment of this sample

(3=2a+H2) induces a slight increase of this band, its shift to 401 nm and distinct broadening. The

hydrogen-treatment induces the formation of additional very small Ag-particles in a superficial layerof the sample. The band-broadening is a sign of the presence and/or formation of Ag nanoparticles

with different dimensions.

Hydrogen-treatment of the exchanged specimens (2b) at temperatures above 2500C may result in

reduction of the silver and copper ions. Because at 5000C this process is more effective for copper than

for silver, the SPR-band of silver is small and "covered" by the bands at 380 nm and 556 nm

corresponding to the interband transitions and the surface plasmon resonance band of metallic copper,

respectively [11]. It is suggested that the optical absorption due to the interband transitions dominates

the SPR absorption at the beginning of the reduction process when the nanosized copper particles are

small. The OA data also show that the Cu2+

ions are not completely reduced during this treatment.

Long air annealing of specimens after the first hydrogenation (2b+air) shows a lot of interesting

changes. First of all, strong increase of the SPR-silver-band occurs. After 23 h of annealing the silver

band decreases and shows a red shift to 425 nm. Moreover, a decrease and shift to 563 nm appear forthe copper-

SPR-band. After 64 h of annealing the OA band related to the SPR of copper disappears. It

250 500 750

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is possible that the already formed colloidal copper particles either dissolve inside the matrix or

diffuse toward the sample surface where they become entirely oxidized into the Cu2O particles. As will

be shown by the TEM-observations (cp. 3.2), the Cu2O components form a shell configurationaround the Cu-core-nanoparticles. The position of the related Cu-SPR peak strongly depends upon the

form and thickness of the Cu2O nanoshell.

Further annealing of samples up to 250h and 1474h results in further decrease and red-shift of the

Ag-SPR-band to 443 nm and 465 nm, respectively. At these stages the band becomes broad and the

area under the absorption curves remains nearly constant indicating that the volume fraction of the

formed nanoparticles corresponds to the total amount of silver. The detected red-shift of the band

could be related with some shape changes of the particles, as it was shown previously [12] for silver

doped SLS glass specimens subjected to deformation. At this moment it is not clear how the Ag-

spherical-particles could change their shape to induce the detected OA band.

All the above mentioned changes of the OA spectra are accompanied by a shift of the UV-

fundamental absorption edge. As far as for samples (2b) and (2b+air up to 3h) the shift is toward lower

energies, the reverse behavior of samples was detected for the remain treatments. Moreover, startingfrom the annealing for 250h the band related with matrix impurities (different from iron) becomes

detectable at 354 nm.

Preliminary calculations of the extinction spectra, based on the MIE theory [13], support the

suggestion that the produced heterosystem of metal nanoparticles corresponds to solid solutions rather

than some core shell structures.

3.2. TEM data

Figures 3-5 show the most interesting data obtained by using the TEM-related procedures. The main

results are as follows.

Annealing of the exchanged specimens in selected atmospheres results in the formation of new

microstructures within the glass matrix, cp. figure 3. The Na 2O-rich droplets, present already in as

exchanged samples, grow in size, alter their shape and distribution during the thermal treatmentapplied. The small, well separated, droplets characteristic of as exchanged specimens, grow in size

during the air-treatment, and are changing into interconnected structures after hydrogenation. These

effects resemble those observed previously [2,3] either for Ag- or the Cu-exchanged specimens,

respectively.

(a) (b) (c)

Figure 3: TEM-micrographs of

the glass surface showing the

morphology of samples: (a) as

exchanged, (b) after short air-

annealing, (c) after hydroge-nation.

With respect to the dopant, the TEM-micrographs reveal that nearly spherical colloids are formed

for the exchanged specimens annealed afterwards. Figure 4 exemplifies the particles of copper

surrounded by a layer of Cu2O and a particle of silver, the size of which is distinctly smaller than that

related with a copper cluster. The two fast-Fourier-transforms (FFT) correspond to the particles of

Cu2O and silver. The SAED-patterns show the presence of both Ag and Cu FCC spotty rings

confirming that separation instead of alloying of the two crystalline species took place. The

interplanar spacing of the nanoparticles are comparable with those of bulk Ag and Cu being in accord

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with the equilibrium Cu-Ag phase diagram. This means that "our Cu-Ag" particles may be a solid

solution in which Cu-atoms replace the Ag-atoms in the FCC-structure.

Figure 4: Copper and silver

nanosized particles in soda lime

silica glass doubly exchanged and

hydrogenated afterwards.

Two other results deserved our attention. Namely, the formation of crystalline Na2O particles in the

direct vicinity of the nanosized silver particles and the presence of Cu2O particles round the separated

copper nanoparticles. Examples of both have been shown in figure 5.

Figure 5: Two HRTEM micrographs of the

sequentially exchanged SLS glass samples in

which the Cu-nanoparticle is surrounded by

Cu2O layers, and the Ag-nanoparticles are

accompanied by the crystalline Na2O

nanoparticle.

The electron diffraction patterns confirm that the quantity of crystalline Na 2O is largest for the

highest exchange time. This amorphous-to-crystalline transformation of the Na2O-rich phase is

probably induced by the presence of the copper ions/clusters and can result in the detected

improvement of the mechanical characteristics of the near-surface layer of these specimens [14].

For the Cu-related particles the presence of core/shell structures has been detected. The {111}

lattice fringes of Cu indicate that the cores of nanoclusters are Cu while the shells are Cu2O either in

the form of separated particles or as a continuous layer. The geometrical form of the Cu 2O phase

depends on the time during which the sample is in contact with air. It is possible that the core-

Cu/shell-Cu2O structures appear during ion exchange performed in air when the first Cuoparticles are

working as seeds for the Cu2O aggregates. During this process, the Cu2O shells are changing their

geometrical form from separated small spherical particles distributed around the large Cu-cores to a

continuous layer surrounding small cores. Sometimes the Cu-nanoparticle become entirely changedinto the Cu2O particles. Controlling the formation of Cu2O, one obtains additional species in the newly

created multi-structure, which give additional possibilities for manipulating the properties of the

exchanged SLS glass.

4. Summary

In this paper, we demonstrated how sequential ion exchange can modify the wavelength of the SPR

bands of metal colloids in a dielectric matrix.

Combined measurements of the optical and structural characteristics of the (Cu+Ag)-doped SLS

glass showed that:

clear changes of the matrix morphology occur after the high temperature treatment, and the

changes are probably related with the formation of strong chemical bonds between the dopant and

non-bridging oxygen;

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the amorphous-to-crystalline transformation of the Na2O-rich phase is probably induced by the

presence of the copper ions/clusters and improves the detected mechanical characteristics of the

near-surface layer of the doped SLS glass [14];heterostructured nanoparticle, comprising a metal/Cu core and oxide/Cu2O shell are formed; they

are interesting for further studies because of their interesting electrical properties;

the copper-plus-silver nanoparticles also as solid solution in the SLS glass specimens seem be

more attractive over monometallic nanoclusters because they induce improved optical

performances of the doped dielectric matrix.

In a general case, the composition of the ion exchanged SLS glass could be an additional degree of

freedom, which provides new possibilities in tailoring the properties of bimetallic nanoparticle besides

the usual size and shape manipulation, and the nucleation of various multicomponent nanoclusters

would be a novel method by which many of the glass properties could be controlled.

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