Oxidation of Diamond Surfaces

Concentrated oxidizing acids (sulfuric, perchloric or nitric acid, or mixtures thereof) are also suitable to oxidize noncovered as well as prefunctionahzed diamond materials. Both hydroxylated and carboxylated surfaces can be generated this way. However, characterizing exactly the kind of carbonyl function with either XPS or IR-spectroscopy is rather complicated. An actual example is the reaction with so-called piranha water, that is, a mixture of concentrated sulfuric acid and 

---

The photocurrent decay, under pulsed illumination, is strongly accelerated upon anodic or O-plasma oxidation of diamond surface. This effect is believed to be due to removal of the above-mentioned subsurface hydrogen [169]. 

There are several methods for the oxidation of diamond surfaces. 

Oxygen surface functionalities can be introduced by chemical and or electrochemical oxidation of diamond electrodes. The XPS atomic O/C ratio increases from approximately 0.01 to 0.15 after extensive electrochemical oxidation in HCIO4 or H2SO4 [40, 89, 90]. The types of oxygen... 

This type of treatment can convert the hydrogen termination to oxygen termination. An additional benefit is that possible nondiamond carbon impurities, as well as metallic impurities, can be removed from the surface in this way. The chemical oxidation of diamond is closely related to electrochemical oxidation, discussed later, and which is also discussed in Chapters 8 and 10. 

Diamond behaves somewhat differently in that n is low in air, about 0.1. It is dependent, however, on which crystal face is involved, and rises severalfold in vacuum (after heating) [1,2,25]. The behavior of sapphire is similar [24]. Diamond surfaces, incidentally, can have an oxide layer. Naturally occurring ones may be hydrophilic or hydrophobic, depending on whether they are found in formations exposed to air and water. The relation between surface wettability and friction seems not to have been studied. 

The adsorption of oxygen on diamond was studied by Barrer (156). Essentially no chemisorption was observed at —78°. From 0 to 144° oxygen was chemisorbed, but no carbon oxides were liberated. Some carbon dioxide was formed as well from 244 to 370° by interaction of oxygen and diamond surface not covered with surface oxides. Surfaee oxide formation was observed at low pressures. The coefficient of friction of diamond increases considerably after heating in a high vacuum. The measurements by Bowden and Hanwell (157) showed a decrease in the friction on access of oxygen, even at very low pressures. 

A chemical investigation of the surface oxides on diamond was undertaken by Boehm et al. (35). Using a fine particle size diamond powder with a specifie surface area of 17 m /gm, the oxidation was studied by use of a vacuum microbalance. Formation of surface oxides started at a measurable rate with pure oxygen at 260°. A weight loss due to formation of carbon oxides became apparent above 360°. 

Diamond surfaces after anodic oxidation treatment involve oxygen-containing surface functional groups. The electron-transfer kinetics for ions and polar molecules are expected to be quite different. Fe(CN)l /4 was highly sensitive to the surface termination of diamond. For an anionic reactant, there was an inhibition of the electron transfer for the oxygen-terminated diamond electrodes compared with the hydrogen-terminated diamond electrodes, and there was also an acceleration of the electron transfer for oxygen-terminated diamond for some cationic reactants such as Ru(NH3) +/3+ and Fe2+/3+. These results can be explained by electrostatic effects, which interact between the ionic... 

Figure 10.4 shows SEM photographs of the surface of SiC-coated diamond particles coated at 1350°C. Tiny granules of SiC were deposited and aggregated with an increase in coating time. Even for samples treated for 1 min, the entire surface is considered to be covered with a thin SiC layer formed by the direct reaction of diamond and SiO(g) because the samples show good oxidation resistance, to be discussed later. EDX analysis shows a uniform distribution of Si atoms on the entire surface of the SiC-coated diamond particle. 

Chromium(H)-bearing minerals are expected to be highly susceptible to pressure-released oxidation of Cr2+ to Cr3+ ions under the relatively lower pressures and more oxidizing conditions existing in the Earth s Crust. However, observations that high confining pressures are maintained in crystals included in diamonds may account for the retention of the Cr(II) oxidation state in the olivine inclusions brought to the Earth s surface. 

---