Dispersed uniform submicron-sized silver spheres were prepared by selective laser heating in the silver-containing precursor solution, which was produced by dissolving the irregular Ag2O in aliphatic amine. By optimizing the process conditions, silver spheres in the range of 578 ± 109 nm were obtained. The smooth surface morphology and solid structure were studied by SEM and TEM. The silver content was characterized by XRD and EDS. Finally, the mechanism of the silver spheres formation was also discussed in detail.
© 2011 OSA
CorrectionsXiangyou Li, Naoto Koshizaki, Alexander Pyatenko, Yoshiki Shimizu, Hongqiang Wang, Jianguo Liu, Xiaoye Wang, Ming Gao, Zemin Wang, and Xiaoyan Zeng, "Preparation of silver spheres by selective laser heating in silver-containing precursor solution: erratum," Opt. Express 19, 12855-12855 (2011)
As a good conductive metal, silver particles have been used widely in many fields. For example, the silver nanoparticles have been researched extensively due to their applications in surface-enhanced Raman spectroscopy, medicine, and biology . Larger silver particles of micron and submicron sizes, which might have specific and practical usage, such as conductive elements in plasma display panels (PDP), solar cells, printed circuit boards (PCB), and many other thick film components, also attract great interest during the past decades [2–6]. Furthermore, the recent increasing concern about nanosize dependent cytotoxicity in vivo also makes researcher’s attention change onto micron or submicron spheres [7,8]. Therefore, the preparation of spherical particles has become more and more important recently.
Nano-sized silver particles have been prepared extensively by various methods including reduction of silver salts , thermolysis  and laser ablating silver targets in liquid solutions [11–13] etc. However, larger silver spheres are a little difficult to prepare because the conventional heating preparation method, including nucleus formation and subsequent preferential crystal growth, which it is kinetically difficult to inhibit anisotropic crystal growth, was tended to form none-spherical particles . At present, only several approaches were reported about the silver spheres preparation. For example, Krassimir P. Velikov and associates  demonstrated a simple technique to synthesize silver spheres with a wide range of sizes (up to 1200 nm in radius) by reducing silver nitrate with ascorbic acid in aqueous solutions in the presence of a polymeric steric stabilizer and the final sphere radius can be controlled by the pH of the reaction mixture and the concentration of the reactants. Ionel Halaciuga and Dan V. Goia  also presented a rapid and convenient method for producing micrometer- and submicron-sized dispersed silver spheres by the rapid aggregation of nano-sized silver subunits. However, above methods need to use the chemical reactants such as acid, which might bring some potential hazards to circumstance and the procedures for preparation was also a litter complex. Moreover, the as-prepared spheres’ performance was influenced by the lack of close contact between nanostructures.
In order to make the procedure simpler and easier, and get the metallurgical bonding silver spheres, here we describe an innovative approach to prepare submicron-sized silver spheres. Different from the above methods, the unfocused pulsed laser beam was used to irradiate silver-containing precursor solution and silver spheres with smooth surface were prepared by laser decomposition of the precursor and growing up consequently.
2. Experimental setup and methods
In a typical procedure, a water solution of the silver-containing precursor was prepared as following steps firstly: At room temperature (25 °C) and with stirring, silver (I) oxide (Ag2O) was added into the milli-Q water solution of an aliphatic amine. In a few of minutes, most of the Ag2O was dissolved into the solution, and the saturated solution was filtered through a membrane filter (Advantech Corporation, filtering size: 0.5 μm). A colorless, transparent and clear filtrate without solid particles was obtained, which was silver-containing precursor. Then 100 μl of as-prepared silver-containing precursor was added into 4 ml ethanol and adequately mixed in a glass cell by ultrasonication for laser irradiation. An Nd:YAG pulsed laser (Spectra-Physics, repetition rate 30 Hz, pulse width 8 ns, beam diameter 8 mm) with a second harmonic wavelength of 532 nm was used without focusing to irradiate the silver-containing precursor solution. Figure 1 shows the schematic map of the laser experimental setup. During irradiation, a magnetic stirrer was used to prevent gravitational settling of the previously formed particles. After laser irradiation, the solution was centrifuged (6000 rpm, 30 min) and the supernatant was removed. A small droplet of the sediment containing prepared silver spheres was deposited onto silicon substrate for SEM (Hitachi S-4800 microscope with 30 kV acceleration and 1 nm point-to-point resolution) observation and X-Ray Diffraction (XRD, Rigaku Ultima IV/PSK) analysis after naturally dried. A small drop of above solution was also deposited on a copper grid for transmission electron microscopy (TEM, JEM-2000FX) observations. Optical absorption spectrum was measured with a UV–vis spectrophotometer (Shimadzu, UV-3700PC, Japan)
3. Results and discussion
Figure 2 shows the SEM morphology of silver spheres under different process conditions. Figure 2(a) shows the morphology of raw Ag2O particles, which are irregular sheet shape obviously and the particle size is larger than 1 μm. Figures 2(b)–2(e) show the morphology under different laser energy at the same irradiation time (30 min), respectively. The results showed that, at low laser energy, most of the particles are very small (roughly less than 100 nm), and with increasing laser energy, the particles grow up and become larger gradually. Under enough laser energy (e.g. 231 mJ/pulse), most of the small nano-particles disappeared and swallowed by the large spheres that formed previously. Moreover, the size of the large spheres also becomes larger and more uniform with increasing laser energy. Figure 2(f) is the morphology under low laser energy (16.5 mJ/pulse) and long irradiation time (120 min). Obviously, most particles are still very small and the big spheres can seldom be formed, which shows that the laser energy is a key factor of silver spheres preparation. Figure 2(g) is the magnification of Fig. 2(e), and Fig. 2(h) gives the size distribution of Fig. 2(g) (The data was gotten by measuring manually), which shows the smooth surface of as-prepared silver spheres and the sphere size ranges about 578 ± 109 nm. The EDS (Fig. 3(a) ) and XRD (Fig. 3(b)) characterization of as-prepared silver spheres proved the resulting products only containing crystal silver.
Figure 4 shows the normalized optical absorption spectrum of as-prepared silver particles under different laser input energy. The redshift of maximum peaks (From left to right: 426 nm, 450 nm, 492 nm, 518 nm) were observed clearly with increasing the laser energy. According to the Mie theory of absorption and scattering of light by small particles, the wavelength of the maximum optical absorption and the shape of the spectra depend on the dielectric function of the medium, size, shape and material type of the nanoparticles . In our case, the environment and conditions were same except the size increasing of the particles. Therefore, this result agrees well with the SEM results (Fig. 2).
The mechanism of above silver spheres formation can be described as follows. Figure 5 shows the schematic procedures. At the beginning, the irregular Ag2O big particles dissolve into aliphatic amine and form the silver-containing precursor by the chemical reaction, just as stated in literature 3. The transparent, uniform silver-containing solution was formed and ready for laser treatment. After laser irradiation, the silver-containing precursor was decomposed by the pulsed laser selective heating as the equation (Precursor → Ag + aliphatic amine), and the nano-sized silver particles precipitated to form the nano silver containing colloidal suspension. Due to the aggregation of nano materials, the nano silver particles are easy to unite and melt by the laser irradiation. When a pulsed laser is applied onto the colloidal solution, only the particles not the solvent are heated. This selective heating relies on the laser energy absorption of solid particles as well as the lack of thermal energy transfer to the solvent. If sufficient laser energy is absorbed, the particles will be melted to form large silver particles. At the same time, the absorption of laser irradiation between small and large particles is quite different , so the large particles will swallow the small particles by the subsequent laser pulses and be melted together to form larger spheres. On the other hand, because the instantaneous heating can be completed within 8 ns (defined by laser pulse width), and the followed quenching process usually takes 10−6 to 10−4 s [17,18], as well as the pulse interval is typically 10−2 s (laser repetition rates are 30 Hz), so the heating process is a temporally discontinuous heating process. Figure 6 shows the time difference of heating, quenching and pulse interval. As we known, the crystal growth strongly depends on the cooling speed. If the cooling speed is very fast, the anisotropic growth will be blocked because rapid cooling makes the periodic lattice arrangement very difficult. In our case, the laser heating and quenching is very fast compared with conventional heating and quenching, which will make the growing of silver spheres isotropic and finally results that the silver spheres with smooth surface and metallurgical bonding structure are formed.
In conclusion, an innovative and convenient approach to prepare submicron-sized silver spheres by pulsed laser selective heating in solution was proposed in this express. Different from other methods, the raw irregular Ag2O was dissolved into aliphatic amine to form silver-containing precursor firstly, and the silver nanoparticles were generated by laser irradiation consequently, finally the silver spheres formed by the discontinuously pulse laser selective heating and rapid quenching, which does not require polymeric dispersing agents or potential hazard chemicals as other methods. The simplicity of the process makes it an advantageous route to manufacture cost effectively submicron-sized silver spheres for the possible applications in plasma display panels (PDP), low temperature co-fired ceramics (LTCC), multilayer ceramic capacitors (MLCC), and solar cells.
This work was supported by the National Natural Science Foundation of China (Grant No. 60806035). We thank the Nanosystem Research Institute of Advanced Industrial Science and Technology (AIST) for the partial work, characterization and test.
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