Final Report Summary - ADAPTNANO (Adaptive nanostructures prepared by hierarchical self-assembly)
Project context and objectives
With constantly increasing demand for electrical energy and chemical fuels for the world's population and with diminishing reserves of fossil fuels, new concepts for energy production, conversion and storage, as well as alternative carbon sources for chemical syntheses are required. As sunlight is the most abundant energy source, tremendous efforts are being devoted to exploring novel systems for harvesting solar energy and its efficient storage by conversion into fuel like hydrogen. Within a couple of years, a few percent of the available electrical power is being fed by photovoltaic installations in European countries. The large- and small-scale facilities are mainly based on inorganic semiconductor technology. Further progress is envisioned by employing flexible and lightweight organic-based devices. In particular, the hybrid dye-sensitised solar cells draw significant attention due to low-cost manufacturing and the promising energy conversion efficiency. Alternative routes in energy conversion explore artificial carbon-based combustibles (e.g. methanol) converted from CO2 exhaust gas from heat engines. Following this path, efficient catalysts for CO2 reduction and conversion are required enabling the use of the abundant yet inert CO2 as a carbon source for chemical synthesis.
The design of novel catalytic materials for the industrial-scale conversion and production of chemical fuel requires the knowledge of the fundamental processes at the atomic and molecular level. Advanced methods and strategies need to be developed to fabricate controllably and reproducibly functional nanostructures with structural control down to sizes of small atomic clusters or even single atoms. Knowledge of the exact binding geometry of molecules on the surface of catalysts is the key to understand the functionality of the devices.
Supramolecular self-assembly is a promising route to build complex structures from molecular building blocks with atomic precision. In particular, metal-organic frameworks are a promising class of materials as they combine unique functionalities of organic molecules and metal containing secondary building blocks. For catalytic applications, low-dimensional analogues are of high interest, having been demonstrated to readily self-assemble on noble metal surfaces by the evaporation of the constituents followed by an annealing step. The realisation of extended surface-confined metal-organic networks was hindered by their low tolerance to intrinsic surface defects, restricting their application to single crystalline metal surfaces. Moreover, the rather low stability and robustness compared to conventional catalysts hindered the further functionalisation of the networks by catalytic metal atoms and clusters.
Project work
Our research is based on self-assembled molecular systems at surfaces with focus on adaptive metal-organic coordination networks and their functionalisation with catalytically active atoms. The synthesis and investigation of the two-dimensional metal-organic coordination networks were performed with scanning tunnelling microscopy under ultra-high vacuum conditions that allow the structural characterisation at the molecular level. The supramolecular networks were prepared using a specially synthesised ligand molecule. The central part of molecular ligand (i.e. butadiyne) shows a high degree of internal flexibility when adsorbed on a metal surface. As a result, the molecular ligand is capable of adapting its shape and mediating the formation of extended supramolecular networks that can cross multiple surface step edges and adapt to structural defects of the substrate. Despite the ligand's flexibility, the supramolecular networks exhibit considerable robustness against high temperatures. The central part does not only account for the ligand's flexibility but can be further utilised for the incorporation of a second transition metal (here nickel) in the metal-organic matrix without impairing the structural integrity of the network. Interestingly, we found that the underlying substrate strongly influences the interaction of the Ni atoms with the organic ligand. On a silver substrate a unique interaction was revealed, in which the metal-organic template imposes the nucleation of Ni atoms within the top-most substrate layer underneath the network. Thereby, few-atomic Ni clusters with high spatial density were prepared that are stable at high temperatures (~ 450 K), which are required for catalytic reactions. These findings show that mesoscale self-assembled functional architectures with a high degree of substrate error tolerance can be realised with metal coordination networks. The decorated networks will be further explored toward their catalytic activity with a prime focus on CO2 conversion.
As regards the efficient utilisation of solar energy we studied model dye-functionalised surfaces at the molecular level. Even though significant progress in the overall efficiency has been made, the fundamental questions, e.g. the exact adsorption geometry and the type of dye-substrate interaction, remain to be solved. To study the bonding geometry of individual dye molecules on TiO2 anatase (101) substrates at the atomic and molecular level we employed a unique way to deposit intact dye molecules from a solution on to atomically clean substrates under vacuum conditions, i.e. by electrospray ionisation, and studied them under well-defined conditions using a low-temperature scanning tunnelling microscope. The atomically sharp tip of the microscope does not only provide information on the precise adsorption geometry but can also probe local electronic properties revealing the energy level alignment with the substrate. Further, the adsorption-induced changes as well as the impact of lateral interactions on the electronic structure and response to illumination are addressed. Atomistic understanding of the fundamental interactions is expected to have a high impact on the further development and understanding of dye-sensitised solar cells and may lead to the design of novel dye-sensitisers or interface materials.
Project results
The results obtained for both research lines revealed several new ideas and possible new research directions, which open future cooperation in areas relevant to energy-related research. The project brought together physical and chemical approaches for the preparation of functional surface nanostructures. This strong interdisciplinary character bridges the fields of surface science with organic chemistry giving rise to new functional devices with a wide range of potential application, i.e. in solar energy conversion and catalysis. The ability to develop new nanodevices and to be able to transfer this knowledge to the industry is of fundamental relevance for the EU, and plays a vital role in the transition to a sustainable economy.
With constantly increasing demand for electrical energy and chemical fuels for the world's population and with diminishing reserves of fossil fuels, new concepts for energy production, conversion and storage, as well as alternative carbon sources for chemical syntheses are required. As sunlight is the most abundant energy source, tremendous efforts are being devoted to exploring novel systems for harvesting solar energy and its efficient storage by conversion into fuel like hydrogen. Within a couple of years, a few percent of the available electrical power is being fed by photovoltaic installations in European countries. The large- and small-scale facilities are mainly based on inorganic semiconductor technology. Further progress is envisioned by employing flexible and lightweight organic-based devices. In particular, the hybrid dye-sensitised solar cells draw significant attention due to low-cost manufacturing and the promising energy conversion efficiency. Alternative routes in energy conversion explore artificial carbon-based combustibles (e.g. methanol) converted from CO2 exhaust gas from heat engines. Following this path, efficient catalysts for CO2 reduction and conversion are required enabling the use of the abundant yet inert CO2 as a carbon source for chemical synthesis.
The design of novel catalytic materials for the industrial-scale conversion and production of chemical fuel requires the knowledge of the fundamental processes at the atomic and molecular level. Advanced methods and strategies need to be developed to fabricate controllably and reproducibly functional nanostructures with structural control down to sizes of small atomic clusters or even single atoms. Knowledge of the exact binding geometry of molecules on the surface of catalysts is the key to understand the functionality of the devices.
Supramolecular self-assembly is a promising route to build complex structures from molecular building blocks with atomic precision. In particular, metal-organic frameworks are a promising class of materials as they combine unique functionalities of organic molecules and metal containing secondary building blocks. For catalytic applications, low-dimensional analogues are of high interest, having been demonstrated to readily self-assemble on noble metal surfaces by the evaporation of the constituents followed by an annealing step. The realisation of extended surface-confined metal-organic networks was hindered by their low tolerance to intrinsic surface defects, restricting their application to single crystalline metal surfaces. Moreover, the rather low stability and robustness compared to conventional catalysts hindered the further functionalisation of the networks by catalytic metal atoms and clusters.
Project work
Our research is based on self-assembled molecular systems at surfaces with focus on adaptive metal-organic coordination networks and their functionalisation with catalytically active atoms. The synthesis and investigation of the two-dimensional metal-organic coordination networks were performed with scanning tunnelling microscopy under ultra-high vacuum conditions that allow the structural characterisation at the molecular level. The supramolecular networks were prepared using a specially synthesised ligand molecule. The central part of molecular ligand (i.e. butadiyne) shows a high degree of internal flexibility when adsorbed on a metal surface. As a result, the molecular ligand is capable of adapting its shape and mediating the formation of extended supramolecular networks that can cross multiple surface step edges and adapt to structural defects of the substrate. Despite the ligand's flexibility, the supramolecular networks exhibit considerable robustness against high temperatures. The central part does not only account for the ligand's flexibility but can be further utilised for the incorporation of a second transition metal (here nickel) in the metal-organic matrix without impairing the structural integrity of the network. Interestingly, we found that the underlying substrate strongly influences the interaction of the Ni atoms with the organic ligand. On a silver substrate a unique interaction was revealed, in which the metal-organic template imposes the nucleation of Ni atoms within the top-most substrate layer underneath the network. Thereby, few-atomic Ni clusters with high spatial density were prepared that are stable at high temperatures (~ 450 K), which are required for catalytic reactions. These findings show that mesoscale self-assembled functional architectures with a high degree of substrate error tolerance can be realised with metal coordination networks. The decorated networks will be further explored toward their catalytic activity with a prime focus on CO2 conversion.
As regards the efficient utilisation of solar energy we studied model dye-functionalised surfaces at the molecular level. Even though significant progress in the overall efficiency has been made, the fundamental questions, e.g. the exact adsorption geometry and the type of dye-substrate interaction, remain to be solved. To study the bonding geometry of individual dye molecules on TiO2 anatase (101) substrates at the atomic and molecular level we employed a unique way to deposit intact dye molecules from a solution on to atomically clean substrates under vacuum conditions, i.e. by electrospray ionisation, and studied them under well-defined conditions using a low-temperature scanning tunnelling microscope. The atomically sharp tip of the microscope does not only provide information on the precise adsorption geometry but can also probe local electronic properties revealing the energy level alignment with the substrate. Further, the adsorption-induced changes as well as the impact of lateral interactions on the electronic structure and response to illumination are addressed. Atomistic understanding of the fundamental interactions is expected to have a high impact on the further development and understanding of dye-sensitised solar cells and may lead to the design of novel dye-sensitisers or interface materials.
Project results
The results obtained for both research lines revealed several new ideas and possible new research directions, which open future cooperation in areas relevant to energy-related research. The project brought together physical and chemical approaches for the preparation of functional surface nanostructures. This strong interdisciplinary character bridges the fields of surface science with organic chemistry giving rise to new functional devices with a wide range of potential application, i.e. in solar energy conversion and catalysis. The ability to develop new nanodevices and to be able to transfer this knowledge to the industry is of fundamental relevance for the EU, and plays a vital role in the transition to a sustainable economy.
