Development and Application of In Situ Nanocharacterization to Photocatalytic Materials for Solar Fuel Generation

Project: Research project

Description

CO2 emission associated with the combustion of fossil fuels is now recognized as a serious contributor to climate change. One potential strategy to mitigating this problem is to capture CO2 and convert it into fuels such as methane or methanol. Sunlight can provide the energy to drive these reactions forward via photocatalytic pathways creating so-called solar fuels. Photocatalysts based on inorganic materials have been developed which can convert CO2 and H2O to fuels. Such catalysts must be able to efficiently harvest incoming solar radiation, transfer charge to the surface and then transfer charge to adsorbed molecules on the particle surface to catalytic create the fuels. Each of these steps must be optimized in order to increase the efficiency of the photocatalytic process. However, at the present time, the conversion efficiency of photocatalysts is rather low especially in the visible region of the solar spectrum. There are many fundamental materials questions on the nature of the processes taking place during photocatalysis and developing new in situ methods to characterize the photocatalytic process at the nanometer and atomic level is essential in order to provide new scientific understandings upon which to develop improved materials. Here we propose to modify an in situ environmental transmission electron microscope to allow light irradiation in the presence of reactive gases. This approach will then be applied to the study of gassolid photo-reactions related to conversion of CO2 on supported metal catalysts based on titania. Titania is an excellent model for exploring the fundamentally new nanoscience that may emerge from our novel in situ microscopy approach. Titania systems are known to function as photocatalysts under ultraviolet illumination and nanostructured doped titania appears to show significant activity in the visible. A major goal is to develop a novel tool which will allow us to study photocatalytic nanomaterials under near reaction conditions. Specifically we will: Develop in situ instrumentation to permit atomic level observation of catalytically active nanomaterials in the presence of reactive gases and heat under photon irradiation of variable wavelength and intensity. Determine the phase transformations that take place in titania-based nanomaterials during in situ photon irradiation at different wavelengths and intensities under gas conditions relevant to photocatalytic conversion of CO2. Use monochromated electron energy-loss spectroscopy to investigate the changes in electronic structure of titania-based photocatalysts during in situ photon irradiation at different wavelengths and intensities. To achieve the first objective, an FEI Tecnai F20 in situ environmental transmission electron microscope (ETEM) will be modified to permit variable wavelength photon irradiation of the sample during observation. This modification will allow us to study materials under close to reaction conditions i.e. in the presence of reactive gases and photon irradiation. A similar modification will be carried out on a NION UltraSTEM (scanning transmission electron microscope) equipped with a monochromator. The combination of the monochromator and in situ photon irradiation will allow unprecedented exploration of the the nanoscale changes in the electronic structure of the materials under intense light illumination. We will investigate the nanoscale structure and chemistry in the materials under a variety of reaction and illumination conditions. By correlating these in situ nanoscale observations with activity and selectivity data from our photoreactors we can develop a deeper understanding of the relationship between catalyst structure and properties. The fundamental information derived from the proposed research should point the way forward for developing novel materials with properties that are tuned to optimize the atomic level processes necessary to generate more effici

Description

DEVELOPMENT AND APPLICATION OF IN SITU NANOCHARACTERIZATION TO PHOTOCATALYTIC MATERIALS FOR SOLAR FUEL GENERATION Applicant/Institution: Arizona State University Principal Investigator: Peter A. Crozier Photocatalytic generation of solar fuels such as hydrogen is a potential path for solar energy utilization. This approach is attractive because it not only captures the energy of the sun but allows it to be stored for future use in the form of fuel molecules. Hydrogen can be generated by water splitting and can be directly employed or use to produce other fuels such hydrocarbons or alcohols. Photocatalysts based on inorganic materials are potential pathways for converting water into hydrogen. Such catalysts must be able to efficiently harvest incoming solar radiation, transfer charge to the surface and then transfer charge to adsorbed molecules on the particle surface to catalytically create the fuels. Each of these steps must be optimized in order to increase the efficiency of the photocatalytic process. However, the performance of the current generation of photocatalysts is well below the minimum efficiency of 10% required to develop a viable techonology and the materials lack the long-term stability required to develop a viable technology. To resolve these problems many fundamental materials questions on the nature of the processes taking place during photocatalysis must be addressed. This requires the development of new in situ methods to characterize active photocatalytic materials at the nanometer and atomic level to provide new scientific understandings upon which to develop improved materials. Here we propose to develop and apply novel in situ transmission electron microscope techniques to determine changes taking place in titania-based photocatalyst under conditions related to water splitting. Titania is an excellent model system and is known to function as a photocatalyst under ultraviolet illumination and nanostructured functionalized titania shows activity in the visible. Specific objectives for the project will include: Determination of structural and/or compositional changes taking place in photocatalytic heterostructures under conditions relevant to gas phase water splitting Using graphene-based liquid cells, determine changes taking place in photocatalytic heterostructures under conditions relevant to liquid phase water splitting Identify nanoscale location of water reduction and oxidation sites on photocatalyst Use monochromated electron energy-loss spectroscopy to investigate the changes in electronic structure of titania-based photocatalytic heterostructures during in situ photon irradiation. To achieve the first 3 objectives, an FEI Tecnai F20 in situ environmental transmission electron microscope which has been modified to permit variable wavelength photon irradiation of the sample during observation will be employed. To accomplish the last objective, a similar modification will be carried out on a NION scanning transmission electron microscope equipped with a monochromator. The combination of the monochromator and in situ photon irradiation will allow unprecedented exploration of the nanoscale changes in the electronic structure of the materials under light illumination. We will investigate the nanoscale structure and chemistry in the materials under a variety of reaction and illumination conditions. By correlating these in situ nanoscale observations with activity and selectivity data from our photoreactors we can develop a deeper understanding of the relationship between catalyst structure and properties. The fundamental information derived from the proposed research should point the way forward for developing novel materials with properties that are tuned to optimize the atomic level processes necessary to generate more efficient inorganic photocatalysts for solar fuel synthesis. This research topic addresses several of the areas identified in recent DOE reports. The proposed project provides a novel approach for characterizing active photocatalysts for solar fuel production.
StatusFinished
Effective start/end date8/15/107/14/19

Funding

  • US Department of Energy (DOE): $1,525,000.00

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Photocatalysts
Irradiation
Photons
Electron microscopes
Monochromators
Lighting
Gases
Electronic structure
Water
Charge transfer
Wavelength
Catalysts
Nanostructured materials
Heterojunctions
Hydrogen
Photocatalysis
Electron energy loss spectroscopy
Solar radiation
Molecules
titanium dioxide