Energetics at the nanoscale: Impacts for geochemistry, the environment, and materials
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Introduction Nanomaterials and, in particular, nanoparticles are everywhere. They exist in air, water, and soil. They have existed through geologic time, and they probably account for much of the chemical reactivity in the Earth and on other planets, as well as in interplanetary space. We have discovered them, named them, made them fashionable, and of course, made them technologically useful. The main question about nanomaterials is, “how are they different from bulk materials?” Certainly, they are more reactive, but they are also different in their fundamental thermodynamic properties. Many of the behaviors that we have previously ascribed to kinetics are, in fact, thermodynamically driven. Kinetic constraints obviously do exist, but there are often other underlying drivers, including thermodynamics. Current issues such as energy, environment, waste, and climate are interrelated in that they all essentially reflect the physics and chemistry of materials. These encompass materials in the solid state ranging all the way from the Earth’s core to its crust, in soils, and in particulates in the atmosphere, as well as materials in the liquid and dissolved state, as found
in the Earth’s molten metallic outer core, in magmas and lavas, in oceans, lakes, and rivers, and in the gaseous atmosphere. Materials issues are important in the balance of energy in the planet and in the balance of materials in geochemical cycles. Materials are also important in how we interact with the planet. With the human population at its current level, using resources and emitting CO2 into the atmosphere, materials, and especially nanomaterials, have global impacts. To understand these impacts (e.g., the persistence of CO2 in the atmosphere) one has to understand geochemical cycles and nanomaterials. For example, the precipitation of carbonates in the ocean and elsewhere, which is crucial to the removal of atmospheric carbon dioxide, may occur through nanophase and amorphous intermediates.1 We know that the rates at which processes occur are controlled by kinetics, and their end products are eventually controlled by thermodynamics. With nanomaterials, as well as with all processes technological or natural, lack of knowledge of the sign and magnitude of a free energy change makes it difficult to judge what reactions (e.g., synthetic pathways, corrosion processes) are possible.
Alexandra Navrotsky, University of California, Davis, USA; [email protected] DOI: 10.1557/mrs.2015.336
© 2016 Materials Research Society
MRS BULLETIN • VOLUME 41 • FEBRUARY 2016 • www.mrs.org/bulletin
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ENERGETICS AT THE NANOSCALE: IMPACTS FOR GEOCHEMISTRY, THE ENVIRONMENT, AND MATERIALS
In many cases, such as in precipitation, sol-gel processing and hydrothermal synthesis in the natural or anthropogenic environment, particles that initially form are small. These nanoscale particles often persist because coarsening requires a higher temperature to strip off their hydration shells to enable them to come together. One is left with nanomaterials that are metastabl
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