Green synthesis and structural control of metal and mineral nanostructures

Christian Engelbrekt

Research output: Book/ReportPh.D. thesis

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Nanomaterials are widely used in modern technology, especially as catalysts in energy technology. Nanocatalysts are often prepared from expensive elements that constitute a large part of the total device cost. A critical component in the successful implementation of sustainable energy technology is efficiency which in large part comes down to the efficiency of the catalyst. Global issues arising from the use and reliance on fossil fuels urges the transition to sustainable alternatives and the development of new highly efficient nanomaterials. The efficiency depends on structure so controlling size, shape and composition at the atomic level is essential. To this adds low cost if massive use is anticipated. In this work, nanostructures of gold, platinum/gold, platinum/palladium, clinoatacamite (Cu₂(OH)₃Cl), cupric oxide and gold/titanium dioxide were synthesized and broadly characterized, their formation mechanisms studied, and their application as catalysts in relevant systems tested. Solution synthesis of nanomaterials offers large-scale, mild production but control of size, shape and composition is often challenging. This may be improved by expanding the understanding of the complex processes of nanoparticle formation which, however, entails the development of new methods. Two approaches to the advancement of solution synthesis of gold nanomaterials for energy technology were exploited, namely the development of techniques to study nanoparticle formation and the synthesis of active, composite nanomaterials. In the first approach, time-resolved chronopotentiometry, pH, conductivity and turbidity, and ultraviolet-visible light spectroscopy were employed to follow the green synthesis of gold nanoparticles. Several distinct phases were observed with all techniques providing a broad picture of the complex processes. Strong indications of sequential reduction were found and details about ligands and surface immobilized molecules disclosed. This platform is a widely available alternative to traditionally used synchrotron techniques. In the second approach, systematic efforts toward size and shape control of gold nanostructures provided controlled synthesis of not only spherical gold nanoparticles of 8-80 nm but also graphene oxide templated preparation of flat, ring-shaped gold nanostructures up to 1 μm in diameter mainly exposing (111) facets. Increased efficiency of new platinum nanocatalysts was attempted by two parallel strategies. One entailed avoiding inactive bulk platinum using a gold core and leaving only vi an atomically thin platinum shell. The specific catalytic activity of this nanocomposite will be studied in the future. The other strategy was to alloy platinum with a cheaper element, i.e. palladium, while retaining high activity. Supported PdPt alloy nanoparticles were successfully prepared and showed promising performance as catalysts in direct methanol and formic acid fuel cells. Catalysts made from earth-abundant elements may provide other low-cost anternatives to traditional, scarce and expensive catalysts. This may be achieved through niche applications and nanoscale engineering. Copper mineral nanoparticles were prepared and studied for this purpose. A buffered synthesis offering the preparation of phase-pure clinoatacamite Cu₂(OH)₃Cl and tenorite CuO by controlling pH was developed. Detailed crystallographic characterization was obtained by combining X-ray diffraction and infrared spectroscopy. The synthesis of CuO was further optimized and the flat, rod-shaped nanostructures applied as heterogeneous catalysts for oxidative dehydrogenation reactions. High activity and good reusability was found and the potential of this noble metal-free system will be explored further. Solar energy is appealing as a plentiful and free energy source. A plasmonic photoelectrocatalytic 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.
Original languageEnglish
PublisherDepartment of Chemistry, Technical University of Denmark
Number of pages230
Publication statusPublished - 2014

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