Nanophotothermolysis of Poly-(vinyl) Alcohol Capped Silver Particles
© to the authors 2008
Received: 15 January 2008
Accepted: 4 April 2008
Published: 15 April 2008
Laser-induced thermal fusion and fragmentation of poly-(vinyl) alcohol (PVA)-capped silver nanoparticles in aqueous medium have been reported. PVA-capped silver nanoparticles with an average size of 15 nm were prepared by chemical reduction technique. The laser-induced photo-fragmentation of these particles has been monitored by UV-visible spectroscopy and transmission electron microscopy. The morphological changes induced by thermal and photochemical effects were found to influence the optical properties of these nanoparticles.
KeywordsNanoparticles PVA Silver Surface plasmon Thermolysis
Ultrafine metal particles in the nanometer regime have various interesting properties compared with bulk metals because of their quantum size effects, so they hold promise as advanced materials with new electronic, magnetic, optical, and thermal properties, as well as new catalytic properties . Metal nanoparticles (NPs) are certain to be the building blocks of the next generation of electronic, optoelectronic and chemical sensing devices . One of the most important applications of metal NPs is as catalysts . The activity of a catalyst largely depends on the particle size. Several methods have been employed to control the particle size in solution. A common feature of those methods is that the size control is achieved by changing the reaction conditions, for example, adding surfactants as protective agents, changing pH, concentration of reactants, etc. However, once metal NPs are synthesized, it is very difficult to break them effectively, particularly, when their diameter is less than 50 nm. Thus, the size-manipulation of metal NPs remained a major challenge for materials scientists and physical chemists. The first successful attempt was reported by S. Koda et al. [4, 5]. They have employed short-laser pulses to induce photofragmentation of pure metal NPs (gold and silver). However, in recent time polymer-protected metal NPs have gained much attention than that of pure metal NPs, owing to their extensive applications in bio-medical engineering (as sensors and drug-delivery agents), site-selective catalysts, and optoelectronic components, etc. Hence, it is necessary to apply the Koda-technique to achieve the smaller polymer-capped metal NPs. The only appreciable effort was made by P. V. Kamat and co-workers to investigate the photofusion and fragmentation of thionicotinamide capped gold NPs . However extension of this technique to other metal NPs capped with other polymers has not been explored. In the present communication, we have made an approach to investigate the fusion and fragmentation of PVA capped silver NPs induced by continuous laser irradiation. The laser induced photo-fragmentation of these particles has been monitored by UV-visible spectroscopy and transmission electron microscopy. The morphological changes induced by thermal and photochemical effects were found to influence the optical properties of these NPs.
PVA capped silver NPs were synthesized by a chemical reduction technique using NaBH4(Extra pure, Junsei Chemicals Co., Ltd.) as reductant, poly-(vinyl) alcohol (1,500) (98.0%, Showa Chemical Co., Ltd.) as a stabilizer and AgClO4 · H2O (99.999%, Aldrich Chem. Co.) as the source for the Ag4+ion. Exact experimental procedures are as follows: 97 mL of distilled water was placed in a 250 mL glass beaker in an ice bath. A calculated quantity of 1 mM silver perchlorate followed by 100 mM sodium borohydride and 3 mM of trisodium citrate was added to the above beaker under vigorous stirring. This solution was used as the reference colloid. Then PVA capped samples were prepared by inserting 1 wt% of poly-(vinyl) alcohol to the reaction mixture instead of trisodium citrate. This was used as the experimental colloid. A transparent bright yellow color was observed immediately in both the cases due to the formation of the silver colloid. UV-vis spectra were taken by a UV-visible spectrophotometer (UV-2550, Shimadzu). TEM images were collected to investigate the morphology of NPs (JEM-2010, JEOL). Laser irradiation experiments was carried out in a quartz cuvette (10 mm × 2 mm) by using 325 nm continuous laser radiation with a power of 9 mW/cm2.
Results and Discussion
Two possible physical mechanisms were suggested that could lead to the laser-induced explosion of NPs; thermal explosion through electron-phonon excitation-relaxation, and Coulomb explosion through multiphoton ionization. We have tried to explain our results by considering the thermal explosion via electron-phonon excitation-relaxation concept. This phenomenon was expected to be the melting (fusion) of aggregates to form larger spherical particles during initial stages of laser irradiation. Since surface-modified silver NPs exists as aggregates it was expected that the energy gained from the absorbed photons to be dispersed as excess heat to the neighboring particles and thus to induce their fusion. Similar laser-induced fusion is not observed in bare silver NPs [7, 8]. When laser was irradiated for longer time, particle promptly approaches the melting point. If there are some cracks in the parent particle, then it may explode to fragments. Smaller particles of about 10 nm may be thus produced [9, 10].
In conclusion, at long-term continuous laser irradiation we have observed the photothermal fragmentation of PVA capped silver NPs. Similar results have been observed by pulsed laser irradiations by other researchers. It was expected that the photoejection of electrons followed by the charging-up of the metal surface is a possibility that could lead to the particle fragmentation. The surface-complexed PVA may also play a role by capturing the photoejected electrons at the silver surface.
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