CONTENTS
13.2 Review of Methods of Obtaining Nanosilver
13.3 Significance of Protective Coating
13.4 Mechanism of Action of Coatings
13.4.1 Electrostatic (Charged) Stabilization
13.4.2 Steric (Polymer) Stabilization
13.5 Examples of Coating Examination (Experimental)
13.6 Characterization of the Tested Silver Nanoparticles
13.7 Examination of Protective Coatings of Nanosilver
13.7.1 Quantitative Analysis of Protective Coating
13.7.2 The Dynamics of Removal of Protective Coatings
13.7.4 Characterization of the Tested Polymer Coating P
13.8 Example of Ink for Ink-Jet Printing Technology
One of the fastest developing fields of nanotechnology is obtaining metals or chemical compounds on the nanometer scale. As it turns out, the vast majority of materials reveal their completely new properties when reduced to such small sizes. This allows us not only to better explore the properties of materials but also to use them in quite unexpected applications.
One such material of great significance in electronic applications is undoubtedly silver. One of the best conductors of electricity and heat, and used in microelectronics for a long time, silver, among other applications as a filler for electrically conductive adhesives (ECAs), is used in assembling electronic elements. The possibility of splitting silver into nanosized particles has enabled the development of a new group of materials, i.e., electrically conductive inks (ECIs). These inks are applied using mainly roll-to-roll (R2R) methods or ink-jet printers, and therefore their viscosity must be very low and they must have a stable form in both static (storage) and dynamic (printing) states, i.e., the characteristics closest to those of a homogeneous liquid.
These requirements are of particular importance in ink-jet printing since this technology is based on pulse ink dispersing. Each pulse causes a small droplet of ink (with a volume of up to several picoliters—10−12 L) to be dispensed with a very high acceleration, reaching 100,000 g. The manner of dispensing being so dynamic, the homogeneity of liquid (ink) is a necessary condition for the components of liquid not to be separated in the process of printing.
It is widely known that in order for the dispersion of silver to be stable in time, each metal particle is covered with an organic protective layer. It is the type and properties of these layers that the subsequent properties of inks and their operating parameters mainly depend on.
13.2 REVIEW OF METHODS OF OBTAINING NANOSILVER
At present, there are several methods of obtaining silver powders with nanometric dispersion. However, the common feature of all the methods is that at a definite moment the produced nanopowders are coated with an organic protective layer.
Among the most popular are the following technologies:
• DC plasma sputtering of silver particles
• Metal vapor deposition
• Chemical methods—through reduction of silver compounds
• Thermal decomposition of silver fatty acids under inert atmosphere
In each of the foregoing methods, a chemical compound is administered in an appropriate manner to produce on the surface of the forming metal particles a thin organic coating that effectively prevents coagulation of particles and their aggregation into bigger structures (Figure 13.1).
13.3 SIGNIFICANCE OF PROTECTIVE COATING
It is only possible to obtain single particles of metallic silver by coating them with protective stabilizing substances in the process of production. The stabilizer is adsorbed or bound to the surface of silver and forms a protective layer, thus preventing agglomeration with other particles. Only unprotected particles tend to aggregate and form compact microscale structures. The protective layer is popularly called a surfactant.
FIGURE 13.1 Organic layer (protective shell).
The selection of protective coating is one of the most important parameters in the synthesis of nanosilver. Besides protecting against sedimentation, the surfactant significantly influences the morphology of nanoparticles, including their sizes, distribution, or even their shape. Zhang et al. [1] have analyzed the influence of the amount of stabilizer on the grain size distribution of nanosilver. It has been shown that as the molar ratio of surfactant to silver salts grows, smaller particles with a more regular and spherical shape are formed.
There is also evidence that under certain conditions of reaction the stabilizer can play the role of reducer. Slistan-Grijalva et al. have demonstrated that the stabilizer reduces silver ions to metallic silver if the solution is heated to 100°C and the reaction takes place in darkness for 1 h [2]. This method is often accepted since stabilizers are more environmentally friendly than reducers. Furthermore, during such chemical reduction there are no undesired reaction substrates (reducer residue) that need to be washed out [3,4]. Thus it is very important for the protective coating to be reactive and ensure the solubility of metal nanoparticles in a medium. It is only thanks to solubility that there is the possibility of producing stable ink with a very good dispersion of metal nanoparticles and low resistance.
In order for printed structures to conduct electricity, they must be heated at a temperature that enables effective removal (as a result of thermal decomposition, desorption, or evaporation) of the isolating protective layer. The coating separates silver nanoparticles and prevents their contact, thereby causing lack of electrical connection between them. The removal of surfactant is accompanied by a relatively small decrease in resistance of printed layer; however, it is only the process of sintering the filler nanoparticles that enables us to achieve a sufficiently high level of electrical conductivity (Figure 13.2).
Thus it is very important that the protective coating should be easily removed from the surface of silver in the sinterization process at a possibly low temperature, which offers a possibility of printing on a flexible polymer foil substrate. So, the protective coating, on the one hand, plays the role of stabilizer for filler nanoparticles, while, on the other hand, it impedes obtaining conductive structures. Magdassi et al. [5] have demonstrated in their paper that it also functions as an adhesion promoter. Silver nanoparticles were obtained with a coating that acted as an adhesive, ensuring good adhesion of inks to typical glass and flexible substrates.
FIGURE 13.2 The mechanism of removing protective coating from nanosilver surface.
13.4 MECHANISM OF ACTION OF COATINGS
When particles are of very small sizes—up to 100 nm—van der Waals forces and Brownian motion have great influence, while gravitational forces are not too important. Van der Waals forces are very weak and their range is very limited; however, Brownian motion makes nanoparticles collide, and then as a result of action of van der Waals forces, aggregates can be formed.
Thus one of the important functions of protective coatings is to prevent aggregation of metal nanoparticles, and this function is usually classified in two categories: electrostatic or steric stabilization.
13.4.1 ELECTROSTATIC (CHARGED) STABILIZATION
Electrostatic stabilization is achieved by producing a surface charge on the surface of nanoparticles.
FIGURE 13.3 Mechanism of electrostatic stabilization. (From G. Cao, Nanostructures and Nanomaterials, Imperial College Press, London, 2004.)
The mechanisms of this process may be various, e.g.:
• Preferential adsorption of ions of one sign
• Isomorphic substitution of ions
• Accumulation or transfer of electrons
• Dissociation of surface charged particles
• Physical adsorption of charged particles
A charge formed around a particle is known as the zeta potential (ζ) and strictly depends on the pH of the nanoparticle environment. According to Magdassi et al.’s study, the most stable AgNP colloids have negative potentials (–33) at a pH range of 6–8. Tests were made for both nonstabilized silver particles and those stabilized with sodium citrate. A layer of counterions, a so-called double layer, is formed around charged particles to neutralize the charge of particles.
Distant particles do not interact with each other because van der Waals forces have a short range, and the formed double layer neutralizes the electrical charge of particles (Figure 13.3a). When particles are so close that their double layers partially overlap, there emerges the resultant electrostatic repulsive force (Figure 13.3b). (From G. Cao, Nanostructures and Nanomaterials, Imperial College Press, London, 2004.)
The examples of electrostatic stabilizers are the compounds containing such functional groups as sulfo, carboxyl, and amino, including citrates, sodium dodecyl sulfate (SDS), amines, amides, saccharides, fatty acids, surface active agents, and many others (Figure 13.4).
13.4.2 STERIC (POLYMER) STABILIZATION
Steric stabilization consists in attaching polymer chains to the surface of particles; it results in the fact that because of spatial (steric) action the particles cannot come close to each other and remain dispersed (suspended) in liquid.
FIGURE 13.4 Electrostatic (charged) stabilization by the same protective coating. (From www.cabot-corp.com. With permission.)
FIGURE 13.5 Mechanism of steric (polymer) stabilization.
Polymer chains may be chemically attached to the surface of the particle (Figure 13.5 left) or physically adsorbed on its surface (Figure 13.5 right). The mechanism of steric stabilization (also called polymer stabilization) depends on the degree of coating of the surface of particles with macroparticles, as well as on the type of solvent. When the surface of particles is densely covered by polymer chains, the formed layers prevent the particles from approaching each other. When the degree of coating is relatively low, stabilization relies on the solvent. In a so-called good solvent, whose particles interact with the polymer, the chains of macroparticles are straight, while in a weak solvent the chains are curled up (Figure 13.6).
Typical polymers used for protection against agglomeration include poly(vinyl pyrrolidone) (PVP), poly(ethylene glycol) (PEG), poly(methacrylic acid) (PMAA), poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA), and others.
Steric stabilization can be combined with electrostatic stabilization:
• Polymer chains are attached to the surface of an electrically charged particle.
• Polyelectrolyte chains are attached to the surface of an electrically neutral particle.
FIGURE 13.6 Steric (polymer) stabilization by the same protective coating. (From www.cabot-corp.com. With permission.)
13.5 EXAMPLES OF COATING EXAMINATION (EXPERIMENTAL)
The basic problem while producing nanocomposites (for instance, silver nanocomposites) where nanoparticles should be uniformly dispersed and stable in the medium is to ensure their proper stabilization and to split the aggregated metal clusters into individual nanoparticles. The experiments were performed for colloids with nanosilver with various types of coatings:
• Carboxyl-coated nanosilver (Ag1)
• Amino-coated nanosilver (Ag2)
• Polymer-coated nanosilver (Ag3)
The first two coatings provide the electrostatic stabilization of nanoparticles in the medium, while the polymer layer stabilizes nanosilver sterically (Figure 13.7).
The first objective of the experiment was to characterize nanoparticles. The obtained nanoparticles with different protective coatings required a full description in order to assess their sizes and shapes (SEM), size distribution (Malvern), and optical properties (UV-Vis). In the second stage of testing, attention was mainly focused on quantitative analysis of the protective coatings used, their chemical composition (energy-dispersive x-ray spectroscopy, or EDX), and the removal dynamics. The tests aimed at assessing the behavior of coating at the sintering (synthesizing) temperature of printed structures.
FIGURE 13.7 The preparation of silver nanoparticles with different protective coatings via reduction.
Three types of nanosilver were used for the tests: carboxyl-coated Ag1, amino-coated Ag2, and polymer-coated Ag3, which were the final products of three different types of synthesis reaction.
13.6 CHARACTERIZATION OF THE TESTED SILVER NANOPARTICLES
The morphology of the tested silver samples was determined using scanning electron microscopy (SEM). The electron microscope uses a beam of electrons for imaging and enables us to examine the structure of matter at the atomic level. The greater the energy of electrons, the shorter their wave and the higher the microscope resolution.
The exemplary photographs from numerous measurements made in various enlargements are shown in Figure 13.8.
The methodology of tests consisted of applying a sample in the dry form to a substrate placed in vacuum. The sample of Ag1 was in the form of powder, while imaging the morphology with a microscope and Ag2 and Ag3 silver required the application of a colloid droplet, and next the evaporation of solvent. The obtained results clearly indicate that all nanoparticles are spherical and have a uniform structure of an average size of 50–70 nm for amino-coated and polymer-coated Ag, and ca. 80–100 nm for carboxyl-coated nanoparticles.
FIGURE 13.8 SEM pictures of nanosilver with different protective coatings: (a) Ag1. (b) Ag2, and (c) Ag3.
In order to investigate grain size distribution of samples, tests were performed on a zetasizer, which made it possible to measure the size of particles within the range between 0.6 nm and 6 microns. Examples of the obtained results are shown in Figure 13.9.
The presented diagrams of grain size distribution in the function of their quantity show that the diameters of tested particles were ca. 100, 50, and 70 nm, respectively, for Ag1, Ag2, and Ag3. The measurements were characterized by a very narrow distribution range and very high accuracy and repeatability. Despite using different coating sizes, the samples show similar grain size distributions.
The UV-Vis spectroscopic investigations were performed using a spectrometer with a wavelength range of 190–1100 nm for Ag1, Ag2, and Ag3 samples. For all the samples, absorption spectra were obtained with the absorption maximum in a wavelength range of 415–430 nm (Figure 13.10).
The presented diagrams clearly demonstrate the formation of nanocrystalline silver in each sample, regardless of a protective coating used. All the peaks are smooth and have a similar profile. The absorption maximum is shifted depending on the type of coating used. In the case of carboxyl-coated nanosilver, it appears at a wavelength of 430 nm, while amino-coated nanosilver shows the maximum at 415 nm, and a shift toward longer waves (420 nm) can again be observed with a polymer coating.
13.7 EXAMINATION OF PROTECTIVE COATINGS OF NANOSILVER
Testing the properties of protective coating formed on the surface of nanosilver in the process of its production is key for further applications of nAg as a filler for electrically conductive inks. In order to determine the influence of the size of coatings as well as the dynamics of their removal, further tests were made.
13.7.1 QUANTITATIVE ANALYSIS OF PROTECTIVE COATING
To determine the interdependence of removing the protective coating surrounding the silver as the function of sintering temperature, thermal tests were made. To this end, it is necessary to begin with determining the total amount of coatings. Therefore a quantitative analysis of sample weight loss in a thermal process was performed. The simple control of weight loss at a temperature of 500°C for 1 h constituted a significant criterion allowing us to assess the content of each type of coating in the tested samples (Figure 13.11). Analyzing the thermal process of nanosilver, one can assume that under the test conditions any organic substances are removed, and after the heating process pure metallic silver is left. Percentage weight loss corresponds to the quantity by weight of protective coating.
It is possible to determine the total amount of protective coating by performing the test as described above. As can be seen, the tested silvers contain only a small amount of protective coating—less than 1.5%. The maximum amount of protective coating was determined for Ag1. In the remaining cases, 0.8% of polymer coating was for Ag3, and the smallest amount of 0.2% was assessed for amino coating (Ag2).
FIGURE 13.9 Distribution of nanosilver particles by Malvern Zetasizer: (a) Ag1, (b) Ag2, and (c) Ag3.
FIGURE 13.10 UV-Vis spectra of (a) Ag1, (b) Ag2, and (c) Ag3.
FIGURE 13.11 The result of quantitative analysis for (a) Ag1, (b) Ag2, and (c) Ag3.
13.7.2 THE DYNAMICS OF REMOVAL OF PROTECTIVE COATINGS
Having knowledge on the total amount of coatings, it is possible to perform tests of weight loss of silver samples having various coatings in the function of time at specified temperatures planned for their removal. Figure 13.12 presents the dependence of protective coating weight loss in the function of time (15, 30, 45 min, and 1 h) at assumed temperatures of silver sintering.
FIGURE 13.12 The dynamics of removal of protective coatings at temperatures of 150 and 230°C in the function of time (a) Ag1, (b) Ag2, and (c) Ag3.
An analysis of the sintering process of tested samples enables the observation that the decomposition of coatings takes place already at 150°C. The obtained results indicate that the weight loss at any temperature is time dependent. As can be seen in the figures, the total content of protective material for respective samples amounted to 1.1% Ag1, 0.2% Ag2, and 0.8% Ag3, and was determined in the diagram as an initial value (time 0 min). The diagram of changes of the coating amount at both 150°C and a higher temperature has the same profile. However, a greater weight loss was observed at a temperature of 230°C and for a longer sintering time.
To sum up the results of studies concerning the dynamics of removing the coating from the surface of silver during the thermal sintering process in the presented cases, it can be stated that this is an individual process for each of the protecting materials used. In the case of Ag1, there was 0.5 and 0.3% of the carboxyl coating left, respectively, at temperatures of 150 and 230°C. Moreover, Figure 13.12c shows that not the entire polymer material was decomposed. It is only in the case of amino-coated silver that there is a possibility of removing nearly the whole amount of protective material, even at a temperature of 150°C.
To additionally verify the methodology of the tests applied, EDX spectral analysis was performed for the Ag3 sample. The measurement consisted of using x-radiation generated by an electron beam for examining the chemical composition of a sample on a microanalyzer. The collaboration of EDX with a scanning electron microscope ensures, moreover, a qualitative analysis of chemical composition of the tested surface in a small area and a very short time, while the registration and analysis of radiation enable qualitative and quantitative measurements of chemical composition of the tested sample in these micro areas.
In these tests, the polymer protective coating is manifested as carbon, which constitutes ca. 1.1% of the whole volume of product (Figure 13.13a), which is confirmed by previous quantitative tests. The carbon is removed in the thermal process (Figure 13.13b), in accordance with the dynamics of coating removal (Figure 13.11c).
The tests performed and their results, shown in Figures 13.10 and 13.11, allow drawing an initial conclusion that for silver nanoparticles with different coatings, the effect of steric (polymer) type stabilization prevails over other types since:
• It is a more efficient stabilization, even in the case of very concentrated suspensions, e.g., inks with a concentration of conductive nanoparticles of 20–40%.
• It can be used for multicomponent systems.
• It is a thermodynamic method.
• The agglomerates formed can be split again into single nanoparticles.
On the other hand, electrostatic stabilization is of limited practical significance because:
• It is effective only for diluted suspensions.
• It may not be applied in multicomponent systems since different particles will produce different surface charges and different double layers.
• It is a kinetic method.
• If agglomerates are formed, it is practically impossible to split them.
FIGURE 13.13 Test result before the sintering process (a) and after the process (b).
Therefore the polymer coating P has been chosen for further studies concerning production of a stable electrically conductive ink with a low thermal processing temperature and nAg content of ca. 20%.
13.7.4 CHARACTERIZATION OF THE TESTED POLYMER COATING P
The accepted amorphous polymer coating P is characterized by a small value of vitrification temperature Tg (ca. 60°C) [8], and its action as stabilizer depends to a great extent on its molecular mass. It is very well soluble in water and alcohol solvents, and therefore it is easy to wash out its unnecessary quantity immediately after the synthesis of nanoparticles. The rest of the coating is removed in the thermal process only, in order to obtain pure silver with perfect electrical properties. However, Lee et al. have demonstrated that even 5% of protective coating after the sintering of structure allows us to obtain very good electrical conductivity parameters of printed pathways [4].
13.8 EXAMPLE OF INK FOR INK-JET PRINTING TECHNOLOGY
The sample of conductive ink was based on obtained Ag nanoparticles with a polymer coating, using solvents mainly composed of ethanol and ethylene glycol, which are currently used in commercial ink. This composition is expected to be environmentally friendly. The base properties of produced ink formulation are presented in Table 13.1. Figure 13.14 shows several printed structures made on flex polymer polyethylene (PE) type foil.
Number of components |
One |
Consistency |
Very low viscous ink |
Color |
Dark green to gray |
Percentage of silver filler |
20–30% |
Viscosity |
5–6.5 m Pas |
Thixotropy index (1/10) |
~1.0 |
Surface tension value |
~35 dynes/cm |
Recommended curing and sintering conditions in convection oven |
150°C, 60 min |
Storage |
2 months at room temperature (do not keep in temperatures below 5°C) |
FIGURE 13.14 Example printout on the PE foil using tested inks at a sintering temperature of 150°C.
During the production process of nanosilver, all particles are safely dispersed without an agglomeration possibility. This is because each particle is protected by a special kind of layer, which is electrically insulating. Therefore to obtain good electrical conductivity of printed structures, the additional energy—mostly thermal by heating—has to be delivered. In case of flexible electronics, where the ink-jet printing technique is commonly used, the temperature of the sintering process plays a very important role, because of the low thermal resistance of polymeric substrates. As it was presented above, it is possible to produce silver nanoparticles with coating layers of different materials, which are characterized by different parameters. The deep study about the different coatings of silver nanoparticles presented above gives the opportunity to modify the properties of inks for ink-jet printing technology that is based on nanosilver. The sintering temperature of the printed microstructure depends on the dynamics of removal of protective coatings. So the proper selection of the protective layer during the manufacturing process of silver nanoparticles may have a significant impact on the properties of microstructures performed by the ink-jet printing technique. Moreover, other properties, like viscosity, surface tension, particle size distribution, etc., which are also important from the ink-jet printing technique point of view, depend on the properties of the organic protective layer on silver nanoparticles.
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