Oxidation of diamond

Check the electrochemical properties before and after anodic oxidation of diamond electrodes by using cyclic voltammetry of K4Fe(CN)6, Ru(NH3)6C13, Fe(C104)3, and ascorbic acid. The results are shown in Fig. 14.4 and Table 14.2. 

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]. 

The oxidation of diamond is clearly an active corrosion process. At least up to 700°C diamond has a fast reacting lll -plane, an intermediate 110 and a slow 100 -plane, which indicates reaction control. At higher temperatures and/or lower oxygen pressures gas diffusion becomes rate determining in analogy with graphite [46], and this is indicated by a more even attack [47]. Hence corrosion rates are faster or start at lower temperatures for fine powders compared to films and the corrosion in air is faster than in low-oxygen environments [48]. 

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... 

C exotherm, oxidation of diamond starts at 710 °C and generation of CO 2 is observed as a mass loss. 

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. 

There are several methods for the oxidation of diamond surfaces. 

Heal content, 110. 116 change (luring a reaction, 110 of a substance, 109 Heat of combustion of diamond, 122 graphite, 122 hydrazine, 47 hydrogen, 40 methane, 123 Heat of formation, 113 Heat of reaction, 135 between elements, table, 112 oxidation of HC1, 160 oxidation of sulfur dioxide, 161 predicting, 112 Heat of reaction to form ammonia, 112 Br atoms, 290 carbon dioxide, 112 carbon monoxide, 112 Cl atoms, 290 CO + Hi, 110 ethane, 112 F atoms, 290 H atoms, 274 hydrogen chloride, 160 hydrogen iodide, 112 iron(Ill) oxide, 162 Li atoms, 290 Li + Br, 290 Li + F, 290 Na + Cl, 290 NHs products, 114 Na atoms, 290 NO, 112 NOj, 112... 

SoUd ice forms a crystal of diamond structure, in which one water molecule is hydrogen-bonded with four adjacent water molecules. Most (85%) of the hydrogen bonds remain even after solid ice melts into liquid water. The structure of electron energy bands of liquid water (hydrogen oxide) is basically similar to that of metal oxides, 6dthough the band edges are indefinite due to its amorphous structure. 

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. 

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