Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, USA.
Acc Chem Res. 2012 Nov 20;45(11):1973-81. doi: 10.1021/ar200317y. Epub 2012 Jun 13.
Carbon materials have mechanical, electrical, optical, and tribological properties that make them attractive for use in a wide range of applications. Two properties that make them attractive, their hardness and inertness in many chemical environments, also make them difficult to process into useful forms. The use of atomic oxygen and other forms of oxidation has become a popular option for processing of these materials (etching, erosion, chemical functionalization, etc.). This Account provides an overview of the use of theory to describe the mechanisms of oxidation of diamond and graphite using hyperthermal (few electronvolts) oxygen atoms. The theoretical studies involve the use of Born-Oppenheimer molecular dynamics calculations in which on-the-fly electronic structure calculations have been performed using either density functional theory or density-functional-tight-binding semiempirical methods to simulate collisions of atomic oxygen with diamond or graphite. Comparisons with molecular-beam scattering on surfaces provide indirect verification of the results. Graphite surfaces become oxidized when exposed to hyperthermal atomic oxygen, and the calculations have revealed the mechanisms for formation of both CO and CO(2). These species arise when epoxide groups form and diffuse to holes on the surface where carbonyls are already present. CO and CO(2) form when these carbonyl groups dissociate from the surface, resulting in larger holes. We also discuss mechanisms for forming holes in graphite surfaces that were previously hole-free. For diamond, the (111) and (100) surfaces are oxidized by the oxygen atoms, forming mostly oxy radicals and ketones on the respective surfaces. The oxy-covered (111) surface can then react with hyperthermal oxygen to give gaseous CO(2), or it can become graphitized leading to carbon removal as with graphite. The (100) surface is largely unreactive to hyperthermal atomic oxygen, undergoing large amounts of inelastic scattering and supporting reactions that create O(2) or peroxy radicals. We did not observe a mechanism for the removal of carbon for this surface. These results are consistent with experimental studies that show formation of CO and CO(2) in graphite oxidation and preferential etching on (111) CVD diamond surfaces in comparison with (100) surfaces.
碳材料具有机械、电气、光学和摩擦学性能，这使得它们在广泛的应用中具有吸引力。使它们具有吸引力的两个特性是它们在许多化学环境中的硬度和惰性，这也使得它们难以加工成有用的形式。原子氧和其他形式的氧化作用已成为加工这些材料的一种流行选择（蚀刻、侵蚀、化学官能化等）。本账户提供了使用理论来描述使用超热（几个电子伏特）氧原子氧化金刚石和石墨的机制的概述。理论研究涉及使用 Born-Oppenheimer 分子动力学计算，其中使用密度泛函理论或密度泛函紧束缚半经验方法在飞行中进行电子结构计算，以模拟原子氧与金刚石或石墨的碰撞。与表面的分子束散射的比较提供了结果的间接验证。当暴露于超热原子氧时，石墨表面会被氧化，并且计算揭示了形成 CO 和 CO(2) 的机制。当环氧化物形成并扩散到表面上已经存在羰基的孔中时，会形成这些物质。当这些羰基从表面上离解时，会形成 CO 和 CO(2)，从而形成更大的孔。我们还讨论了在以前无孔的石墨表面上形成孔的机制。对于金刚石，（111）和（100）表面被氧原子氧化，在各自的表面上形成主要的氧基自由基和酮。然后，氧覆盖的（111）表面可以与超热氧反应生成气态 CO(2)，或者它可以石墨化导致碳作为与石墨一样被去除。（100）表面对超热原子氧基本没有反应性，会经历大量的非弹性散射，并支持生成 O(2) 或过氧基自由基的反应。我们没有观察到这种表面去除碳的机制。这些结果与实验研究一致，实验研究表明在石墨氧化中形成 CO 和 CO(2)，并且与（100）表面相比，在 CVD 金刚石（111）表面上优先蚀刻。
