Gold nanostars: new syntheses and applications in SERS and photocatalysis

Gold nanostars have received significant attention in different fields due to their unique optical and electrical properties. However, controlling the morphology (size and shape) and achieving high synthetic yields are still important challenges, even though several studies have already been reported on this topic. Moreover, it still remains challenging to achieve monodispersity and reproducibility. Finally, it is important to fabricate the gold nanostars in a way that allows us to fully exploit their tunable localized surface plasmon resonance (LSPR) bands and the high electron density localized at the tips. In this dissertation, we design and synthesize a variety of gold nanostars with unique shape and surface properties to target specific applications in surface enhanced Raman spectroscopy (SERS) and photocatalysis, and by taking into consideration future biological applications, which is a major interest in our group. For instance, we developed a synthetic methodology to achieve gold nanostars with a unique 6-branched morphology, which possess narrow absorbance bands tunable from the visible to the short wave infrared. We investigated the role of various synthetic parameters (Triton X, ascorbic acid, AgNO3, and seeds) on the resulting shape of the gold nanostars, and revealed that the intermediate seed morphology determines the number and morphology of the branches obtained. For example, multiply-twinned and penta-twinned intermediate seeds form multibranched stars and 6-branched nanostars respectively. The evolution of gold nanostars reveals that a common intermediate exists at a 5-minute time point, which is stabilized by increasing amounts of AgNO3. Therefore, the plasmon tunability (correlated to the morphological tunability) arises from the concentration of AgNO3. For example, at 100 µM AgNO3 concentration, the LSPR band reaches its most redshifted position at 1071 nm, whereas a maximum at 734 nm is observed for 30 µM AgNO3. We developed a route for silica coating etching on gold nanostars that does not affect the morphology of the nanostar; this synthesis allowed us to investigate how the SERS enhancement depends both on the morphology of gold nanostars and the thickness of the surrounding silica shell. We used the chemoselective reagent NaBH4 to etch the silica layer so that only sharp protruding tips of the nanostars were exposed. We correlated the Raman signal enhancements obtained experimentally by employing gold nanostars with different degrees of silica shell thickness with the field enhancements calculated employing 3D finite element models, obtaining excellent agreement and highlighting the role of both the amount of silica and the degree of gold tip exposure as the two main parameters influencing the intensity of the scattered fields. Similarly, we studied the role of a thin TiO2 shell on the generation of hot electrons and their use in photocatalytic reduction reactions. We observed that when the gold nanostars are coated by a conformal crystalline TiO2 layer, hot electrons are generated at the tips of the gold nanostars and can be injected into the semiconductor through the Schottky barrier between the Fermi level of the metal (Au) and the conduction band of the semiconductor (TiO2), where they can be exploited to increase the performance of the photocatalyst (TiO2). In model hydrogen evolution reactions (HER) we have observed that these nanostructures substantially outperform previously reported systems of hybrid gold nanoparticle-TiO2 photocatalysts, most importantly because of their ability to perform under broad illumination conditions, which is a promising alternative approach to efficiently obtain clean fuels from sunlight.Ph.D.Includes bibliographical referencesby Supriya Att