Green Synthesis of Nanostructures

Nanoparticles already play an essential role as catalysts in energy conversion, e.g. solar energy to electricity, fuel to electricity, and solar to chemical energy, and will become even more important in a future society less dependent on fossil fuels. Precise structural control of nanomaterials is crucial for catalytic efficiency.

Further, it provides the technology to limit the amounts of needed precious catalysts needed, which are known to be efficient, but also expensive and requiring rare raw materials. The thesis presents new fundamental understanding of the formation of metallic nanoparticles and several applications for these in energy technology. Gold (Au) is a very stable metal with well-defined chemistry. Nanoparticles of gold are well suited for several catalytic purposes, and at the same time an ideal model system for studying nanoparticle synthesis. Gold is thus central to all topics in the thesis. Time-resolved chrono-potentiometry, pH, conductivity and turbidity, and ultravioletvisible light spectroscopy were employed to follow the synthesis of gold nanoparticles under mild reaction conditions, also denoted as “green” synthesis of gold nanoparticles.

Several distinct phases were observed. Strong indications of sequential reduction in several identified steps were found and details about ligands and surface immobilized molecules disclosed.

This platform is a widely available and facile alternative to traditionally used synchrotron techniques. Structural control lead to the synthesis of both spherical 8-80 nm gold nanoparticles and graphene oxide templated flat, ring-shaped gold nanostructures up to 1 μm in diameter mainly exposing Au(111) facets. A synthesis protocol to produce 8 nm nanoparticles with a gold core and an atomically thin platinum (Pt) shell in one pot was also developed. As platinum is an expensive and scarce raw material, it would be highly desirable if the use of all-platinum catalysts could be replaced by Au core/ Pt shell nanostructures. The catalytic activity of this nanocomposite will be studied in a future project.

In a parallel approach, platinum was alloyed with a cheaper element, namely palladium. Supported palladium-platinum alloy nanoparticles showed promising performance as catalysts in direct methanol and formic acid fuel cells. Further, syntheses of copper mineral nanoparticles were developed. Copper is an abundant element available at a lower cost than most highly efficient catalyst materials. It is important to explore copper-based nanoparticles as catalysts, even if only in niche applications. A buffered synthesis of phase-pure clinoatacamite Cu2(OH)3Cl and tenorite CuO was developed. The synthesis of CuO was further optimized. The flat and rod-shaped nanostructures were obtained and used as heterogeneous catalysts for oxidative dehydrogenation reactions. High activity and good reusability was found, and the potential of this system is being explored further in the future.

Finally, a plasmonic photo-electro-catalytic system was prepared to utilize visible light by incorporating gold nanoparticles in titanium dioxide. The composite material showed improved optical properties compared to pure titanium dioxide, and preliminary catalytic tests were promising.

Caption: (A) Monitoring AuNP formation. Nucleation is highlighted with grey. (B) High-resolution TEM image and schematic of nanoparticle with a gold core and an atomically thin platinum shell.