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Objective:
To obtain a comprehensive understanding of the fundamental properties and mechanisms controlling titanium dioxide (TiO2) photoreactivity and the effect that photoreactivity has on the service life of polymeric materials. This research effort employs an integrated metrological and scientific approach using facilities and expertise unique to BFRL and NIST. The general goals associated with this objective are as follows:
• Develop novel metrologies for the measurement of photoreactivity, including non-contact methods and methods for use with nanostructured materials.
• Develop analytical techniques for characterizing bulk and surface properties of nanostructured TiO2 materials.
• Establish correlation(s) between semiconductor photoreactivity, material properties, and heterogeneous photochemistry.
Problem:
High volumes of semiconductor metal oxides such as titanium dioxide (TiO2) are utilized each year as fillers and UV absorbers for building materials such as paints, sealants and bulk plastics. A common issue shared by each of the abovementioned applications is that their service life and durability is affected by the photocatalytic properties of these semiconductor metal oxides. When a semiconductor is irradiated with light of a given wavelength, electrons in the material’s valence band are excited into the conduction band. The electron’s departure leaves behind a positively charged species in the valence band, known as a hole. Both electrons and holes are capable of causing the rapid destruction of organic materials that they come into contact with. This property of TiO2 can also be exploited to deliberately destroy harmful organic materials, such as viruses, bacteria and toxic chemicals, and potentially could be a key component in applications related to homeland security.
There is a broad range of photoreactivity for metal oxide nanomaterials, particularly titanium dioxide (TiO2). The mechanism for this large range in photoreactivity for TiO2 is not well understood. Additionally, TiO2 can exist in three basic crystal phases and in many different forms, such as powders, vapor deposited films, and single crystals. With the increased interest in nanosized TiO2, there are even more variations of crystal phases and forms. Currently, each industry utilizing TiO2 has developed its own test methods for assessing photoreactivity. At both ends of the photoreactivity spectrum, there is a need to develop methods of measuring TiO2 photoreactivity and identify TiO2 properties that contribute to photoreactivity. The major impact of this research would be to establish quantitative, scientifically-based techniques for the measurement of photoreactivity in TiO2 and other semiconductor materials.
Approach:
Techniques for the measurement of photoreactivity that will continue to be developed and investigated in this program include the following:
• Electron Paramagnetic Resonance (EPR).
• Chemical assays and probes, including methyl viologen, horseradish peroxidase and isopropyl alcohol.
• Photoconductivity.
Each of these techniques yields specific information on each of the three primary processes in a photocatalytic reaction, which are (1) photo-induced charge carrier generation in the semiconductor, (2) interfacial charge transfer to surface species on the surface of the semiconductor, and (3) reaction of charge carriers and activated species with adsorbed materials on the semiconductor surface. Information obtained using each of these analytical techniques will contribute toward a more comprehensive understanding of the entire photoreactive process, which will aid in the elucidation of physical and chemical mechanisms in photoreactivity.
Another thrust in this program involves the characterization of the oxide materials themselves, particularly in regards to the surface properties, which affect adsorption of and subsequent reactivity of the particles with reagents. Characterization of the surface physical and chemical properties of the TiO2 particles will be carried out in BFRL and also in conjunction with researchers in CSTL and MSEL. This year, a focus on methods to separate fractions of specific particle-sizes from our current series of TiO2 samples will be investigated. This will allow for a systematic investigation of the role of particle size on the resulting photoreactivity.
EPR methods, which measure unpaired electron species of free radicals, will continue to be employed to measure different aspects (charge carrier generation, carrier trapping/interfacial charge transfer, and oxidation/reduction reactions) of photoreactivity in the various TiO2 specimens. Research has been focused on EPR methods using nitroxide spin traps in TiO2 aqueous suspensions and solid state analysis of TiO2 specimens to quantitatively measure photoreactivity. The EPR results will be correlated with TiO2 properties, such as crystal phase, surface area, and particle size, and the end-application performance tests.
End-application performance tests will be a major focus this fiscal year. These tests include the characterization of pigmented and nanocomposite polymeric coatings. The pigmented coatings will be made from the addition of a select series (based on particle size and photoreactivity) of TiO2 samples in different polymeric coatings. The films will be analyzed using EPR before, during, and after UV exposure in the SPHERE and the EPR results will be correlated with the photoreactivity ranking of the TiO2 pigment determined from the EPR spin trap and solid state studies.
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