Nanomaterials with their unique properties allow for attractive applications in devices. Composite assemblage of nanomaterials offers great prospects of multiple functional components to be combined. Hence one of the major directions of research in the area of materials science is the fundamental understanding of the interaction between different nanomaterials and their nearby molecular species. Carbon-based materials exist in all dimensions, zero (fullerenes), one (carbon nanotubes), two (graphene) and three (graphite) dimensions and are very well-known for their versatility in various studies. Of all the carbon- based materials, graphene stands on the top of the list to provide various multifunctional materials. As a result, they are employed in various applications in nanoelectronics, polymer composites, hydrogen production and storage, intercalation materials, drug delivery, sensing, catalysis, photovoltaics etc.
Graphene possesses several interesting properties such as remarkably high surface area, electrical conductivity and mechanical strength, interesting electrochemical behavior and importantly is also naturally abundant. In particular chemical methods for the synthesis of graphene-like material called graphitic oxide or graphene oxide (GO), offer economical and easy routes. All these advantageous properties allow for the use of graphene-based mats in designing tailored composite materials. GO-semiconductor or GO-metal nanoparticle composites have the potential to function as efficient, multifunctional materials for solar energy conversion and storage, selective detection and destruction of trace environmental contaminants or achieve single-substrate, multistep heterogeneous catalysis. In this dissertation, I aim to understand the various electron transfer reactions between methyl viologen molecules, semiconductor and metal nanoparticles with GO.
In the first chapter, the electron transfer from photochemically generated methyl viologen radicals with graphene oxide (GO) is studied. This charge transfer interaction results in the reduction of GO to reduced graphene oxide (RGO) as well as storage of electrons in the carbon network. The stored electrons can be utilized to reduce Ag+ ions and thus anchor silver nanoparticles onto the RGO platform. The spectroscopic experiments allow the elucidation of quantitative electron transfer into GO and the growth mechanism of silver nanoparticle growth as well as the estimation of apparent Fermi level of GO. Transmission electron microscopy highlights the RGO-Ag structure and the potential of designing metal-RGO assemblies.
In the following chapters the electron transfer from photoirradiated semiconductors (ZnO and TiO2) nanoparticles to GO sheets suspended in ethanol is studied. Photoexcited ZnO and TiO2 particles are capable of transferring electrons to GO readily. The semiconductor-RGO composites are further decorated with Ag nanoparticles by reducing Ag+ ions quantitatively with excess electrons stored in RGO. Further the galvanic exchange process is employed to transform photocatalytically deposited Ag on RGO sheet into Au nanoparticles. Then the charge- discharge phenomenon on the GO sheet was further probed by methyl viologen. Improved charge separation and selectivity in the reduction process was achieved in these graphene based photocatalytic assemblies. We have developed a photocatalyst assembly by anchoring semiconductor and metal nanoparticles on RGO mat.
Excited state interactions of visible light active CdSe quantum dots (QDs) of different sizes with GO have been studied. Electron transfer rates from photoexcited CdSe QDs to GO were found to be dependent on QD particle size. Smallest QDs with more negative conduction band potentials show around three times faster electron transfer rates than the largest QDs used. Furthermore emission quenching of colloidal CdSe QDs was found to be dependent upon the extent of GO reduction. Electrophoretic deposition (EPD) was used to deposit CdSe and CdSe-GO composites in a controlled way. Also ?rainbow? CdSe films were assembled for optimal absorption of visible light. Photoelectrochemical results highlight the ability to alter the photoresponse via size control of CdSe QDs and enhancement in photoconversion efficiency by incorporating GO which suppresses charge recombination.