Geometric Selectivity Diamond Oxidation

Oxidation and graphitization of synthetic diamond is an important issue for practical applications. Oxidation limits the performance of diamond used in cutting tools, optical windows, and electronic devices when the diamond exposed to air or other gaseous ambient at high temperatures. The infrared transmittance of diamond windows drops by 6-12 % after being heated in air at 1,070 K for 255 s [85]. 

Diamond oxidation occurs at 1,070 K preferentially at grain boundaries, local defects, and in the diamond-like carbon phase [86, 87], Molecular oxygen adsorption happens to the clean (111) and (110) surfaces of diamond at room temperature [88]. Thermal desorption produces CO from both surfaces. Apart firom a low-temperature desorption peak, TDS shows two CO desorption peaks at 1,060 and 1,300 K for the C(lll)—(2 x 1) surface, whereas only one desorption peak presents in the 1,030-1,160 K range for the C(llO) surface. 

According to the periodic bond chain theory [89-92] and the defect density mechanism, diamond (111) surface is most stable in resisting oxidation than other faces [92]. However, dry oxygen roughens the (111) surface very fast, while the (100) surface is largely inert to oxygen below 1,220 K [93]. 

One needs to understand why the densely packed diamond (111) planes are oxidized easier than the (001) or the (220) planes and how the oxygen penetrates into the (111) face during the course of reaction. The geometrical selectivity of oxide tetrahedron formation and the mechanism of oxide bond switching could resolve the discrepancy. 

It is important to note that the lone pairs of oxygen possess the special ability of inducing dipoles. Interaction between the dipole and the oxygen ion, and hence the dipole to the bulk, is rather weak ( 50 meV, Chap. 6) as there is no electron sharing between the dipole and the oxygen. Due to the strong repulsion of the non-bonding lone pairs, the dipoles tend to locate at the open end of a surface. The loosely bounded dipoles therefore tend to be eroded away from the surface in the process of corrosion. 

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