Direct visualization of radiation-induced transformations at alkali halide–air interfaces

Abstract

Radiation driven reactions at mineral/air interfaces are important to the chemistry of the atmosphere, but experimental constraints (e.g. simultaneous irradiation, in situ observation, and environmental control) leave process understanding incomplete. Using a custom atomic force microscope equipped with an integrated X-ray source, transformation of potassium bromide surfaces to potassium nitrate by air radiolysis species was followed directly in situ at the nanoscale. Radiolysis initiates dynamic step edge dissolution, surface composition evolution, and ultimately nucleation and heteroepitaxial growth of potassium nitrate crystallites mediated by surface diffusion at rates controlled by adsorbed water. In contrast to in situ electron microscopy and synchrotron-based imaging techniques where high radiation doses are intrinsic, our approach illustrates the value of decoupling irradiation and the basis of observation.

Direct Measurement of Mineral Dissolution Enhanced by X-ray Irradiation

“…AFM studies can enable quantitative insights into morphology change, nucleation and growth or step retreat rates, and volume increase or loss of individual or ensembles of particles under a variety of conditions. − Recently, high-resolution AFM has been used to characterize anisotropic step retreat rates for individual gibbsite particles in situ in NaOH solutions . The current study complements this prior work by exploiting a unique atomic force microscope equipped with an integrated X-ray source that enables particle dissolution to be studied with irradiation on or off, both in situ during imaging and ex situ by irradiating while not imaging and then returning to the same particles for detailed characterization. , This AFM platform uniquely enables direct measurements of irradiation effects on the dissolution of individual particles under controlled solution conditions that can be compared with dissolution under identical conditions without irradiation.…”

“…As such, care was taken to ensure consistent image quality, often requiring frequent AFM probe changes when tip convolution increased or tip artifacts became apparent. An X-ray source was chosen as the irradiation method due to its small footprint relative to other sources, allowing for integration with the AFM while allowing for freedom of experimental conditions as described in detail in previous work. , The cell was positioned so that the X-ray source was aligned with the center of the TiO 2 substrate with a separation of ∼3 mm (Figure S1). The dose rate experienced by particles was estimated by irradiating thin radiochromic films (HD-V2 Gafchromic film, International Specialty Products) at full power (12 Watt, 20 kV), as described in previous work, and determined to be reduced to 0.80 kGy/h due to attenuation through the TiO 2 substrate, resulting in a total dose of 6.4 kGy. , A custom cooling system allowed for precise control over the cooling rate of the X-ray source such that the temperature fluctuation measured in the fluid cell was limited to <1 °C during irradiation.…”

“…10 The current study complements this prior work by exploiting a unique atomic force microscope equipped with an integrated X-ray source that enables particle dissolution to be studied with irradiation on or off, both in situ during imaging and ex situ by irradiating while not imaging and then returning to the same particles for detailed characterization. 11,12 This AFM platform uniquely enables direct measurements of irradiation effects on the dissolution of individual particles under controlled solution conditions that can be compared with dissolution under identical conditions without irradiation.…”

“…An Xray source was chosen as the irradiation method due to its small footprint relative to other sources, allowing for integration with the AFM while allowing for freedom of experimental conditions as described in detail in previous work. 11,12 The cell was positioned so that the X-ray source was aligned with the center of the TiO 2 substrate with a separation of ∼3 mm (Figure S1). The dose rate experienced by particles was estimated by irradiating thin radiochromic films (HD-V2 Gafchromic film, International Specialty Products) at full power (12 Watt, 20 kV), as described in previous work, and determined to be reduced to 0.80 kGy/h due to attenuation through the TiO 2 substrate, resulting in a total dose of 6.4 kGy.…”

“…The dose rate experienced by particles was estimated by irradiating thin radiochromic films (HD-V2 Gafchromic film, International Specialty Products) at full power (12 Watt, 20 kV), as described in previous work, and determined to be reduced to 0.80 kGy/h due to attenuation through the TiO 2 substrate, resulting in a total dose of 6.4 kGy. 11,12 A custom cooling system allowed for precise control over the cooling rate of the X-ray source such that the temperature fluctuation measured in the fluid cell was limited to <1 °C during irradiation.…”