Using First Principles Thermodynamics to Predict the Shape Surface Structure and Reactivity of Solvated Nanoparticles at
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Using First Principles Thermodynamics to Predict the Shape Surface Structure and Reactivity of Solvated Nanoparticles at High Temperatures and Pressures. Christopher J. O’Brien and Donald W. Brenner Department of Materials Science and Engineering, North Carolina State University Engineering Building 1, Campus Box 7907 Raleigh, NC 27695-7907, U.S.A. ABSTRACT Hydrothermal nanoparticle synthesis uses high temperature and pressure water to control the chemical processes that lead to specific compositions and structures. Analyses of the chemistry associated with this process have been mainly restricted to bulk thermodynamics in the form of quantities such as solubilities and empirical models based on experimental observations. In this paper we demonstrate for NiO and NiFe2O4 particles how effective reference chemical potentials derived from first principles calculations can be used to predict cluster shapes, nucleation barriers and surface reactivity. Implications for controlling the nanoparticle size and shape by adjusting pH and temperature will be discussed, as well as implications of these results in forming nanostructured materials by cluster condensation. INTRODUCTION Nanoparticles and structures assembled from nanoparticles are an area of increasing importance to the materials science and engineering community. Applications of nanoparticles include enhancing energy generation in solar cells, information storage, drug delivery, luminescent markers for biological systems, free radical scavengers, coating additives to enhance wear resistance, paint and sunscreen additives to block harmful radiation, anti-microbial additives, and high surface area electrodes and catalysts. Nanoparticles are synthesized by different routes, including inert gas condensation and condensation from plasma and RF torches, solid-phase ball milling, and solution phase sol-gel and hydrothermal processes [1]. Hydrothermal synthesis uses high temperature and pressure water to control the chemical processes that lead to specific nanostructures [2–5]. Sue et al., for example, created metal oxide nanoparticles (including NiO) with sizes less than 50 nm by exposing aqueous solutions of metal nitrates and potassium hydroxide to supercritical conditions followed by a quench [3]. The supercritical water dissolves the species and enhances reaction, while the reduction in solubility during the quench leads to a large nucleation rate and hence small particles with a relatively uniform size. Using thermodynamic data for non-critical water, they demonstrated a monotonic relation between the degree of supersaturation and particle size as expected from nucleation theory. In related work, Takami et al. exposed an aqueous mixture of Ni nitrate and hydrogen peroxide to supercritical conditions, and then quenched the system to produce nickel oxide nanoplates with thicknesses of about 10nm and lateral dimensions of 100-500 nm [4]. They speculate that the shape of the nanoplates reflects a β-Ni(OH)2 precursor that dehydrates to form the rock salt crystal structure
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