Directed wet-chemical synthesis of metallic nanoparticles of different sizes and shapes, control mechanisms = Síntesis química de nanopartículas metálicas de diferentes tamaños y formas, mecanismos de contro

Resumo

The successful applications of nanoparticles require the ability to tune their properties by controlling size and shape at the nanoscale. Therefore, it is very important to prepare nanoparticles of well-defined sizes and shapes in order to control their physicochemical characteristics. In this work we described an improved seed mediated synthesis to prepare gold nanoparticles of different shapes (Spheres, Rods, Prisms,¿..). New absorption features different from that of the classical surface plasmon absorption bands of rods and spheres are found to be associated with the appearance of nanoprisms, and snapped prism shaped gold nanoparticles. The effect of [Ag-ions], [seed], [ascorbic acid], [Cl-] and the aging of the seed solution were studied in order to get a deeper understanding of the growth mechanism. Our results indicate that the ratio of [Ag+] to [seed] is one of the key parameters for controlling the shape of the particles, e.g. rods, prisms or snapped prisms. We proposed a mechanism for the formation of the different gold shapes that takes in account the reduction of Ag ions by ascorbic acid forming intermediate small Ag clusters, which can then act as catalysts for the formation of gold nanoparticles. The mechanism has been proved using externally prepared Ag clusters instead of adding Ag ions. The results also indicate that, to get such different shapes, the used CTAB concentration should be above the CMC and is independent of the CTAB supplier. It seems therefore that, the catalytic activity of clusters needs the presence of micelles, whose micellar surface can be viewed as a kind of catalyst nanosupport. These results open a new way of thinking in the interpretation of the mechanisms involved in the anisotropic growth of nanoparticles. In order to confirm that mechanism, we developed a simple method to synthesize gold nanorods with high aspect ratios using gold clusters (Au2- xxvi Au6) stabilized by CTAB. These clusters were added to gold ions in the growth solution without adding any surfactant or polymer. The mechanism proposed for the formation of gold nanorods with high aspect ratios in the absence / presence of the seeds is based on the deposition of these catalytic clusters on active sites of the gold surface. The deposition and the corresponding reduction reaction mainly occur at the end (tips) of the nanorods. The concentration of these clusters was calculated from the UVvis absorption of the solutions at different reaction times. The effects of the dilution of CTAB in the clusters solution and the dilution of the seed solution on the growth of gold nanorods were also studied. The results showed that gold nanorods can be formed without addition of the seeds indicating that the seed particles do not directly affect the growth of gold nanorods. The effect of the gold nanoparticle shape on the stability (thermal heating and photo stability by UV-light and laser) was also studied. It was found that the mechanism of the particle dissociation in the case of nanoprisms is different from than that of nanorods under thermal heating. Great enhancement of the thermal stability has been achieved by adding specific amounts of polyvinyl pyrrolidone (PVP) to the gold nanoparticles of different shapes capped with cetyltrimethylammonium bromide (CTAB). It is worth to mention that gold nanorods stabilized by PVP are totally stable up to 220 0C. The effect of irradiation with UV-light on gold nanoparticles also showed different mechanisms depending on the shape of the NPs. Under long irradiation times (up to approx. 30 h), gold nanorods totally decomposed and another band started to appear at 398nm , corresponding to the formation of the Au(III)-CTAB complex, indicating that the gold(0) NRs are dissolved and transformed to gold-ions. The photocorrosion of the Au nanoparticles was explained assuming that semiconducting Ag-clusters are attached to the tips of the NRs as we assumed in the growth xxvii mechanism. This photocatalytic activity of Ag clusters was confirmed adding ethanol to the irradiated rods, which acts as hole scavenger and avoids in this way the dissolution of the rods. The inhibition of the Au photocorrosion can also be done by pre-heating the irradiated rods to 1300C which corresponds to the fusion temperature. Further experiments were used to analyze the Au ions produced during the photocorrosion and to confirm the high stability of the photocatalytic Ag clusters, which can be reused to direct again the formation of Au NRs after its photodissolution. Finally, the effect of different types of nanosecond lasers (CW and pulsed) on different aspect ratio gold nanorods was studied and the corresponding mechanisms (fragmentation and melting) were proposed to explain the different observed nanosecond pulsed laser effects.