Theoretical Study of Size Effects on Surface Chemical Properties for Nanoscale Diamond Particles

Nanodiamond has displayed some unique physical and chemical properties compared to bulk diamond, which broadens its applications in various areas. However, a more detailed picture of nanodiamond quantum confinements is still missing from a theoretical point of view. This investigation presents a study where the effects of one-dimension (i.e., diamond thin films) and three-dimension (i.e., nanodiamond particles) confinement on surface reactivity, and properties, have been calculated using density functional theory (DFT) and tight binding density functional theory (DFTB) methods. Surface specific parameters like (i) surface CH bond length, (ii) atomic charges, (iii) H adsorption energy, (iv) highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), (v) band gap, and (vi) Fukui functions were thereby carefully calculated and compared. For both the one-dimensional diamond thin films of different surface planes, quantum confinements have strong influences on these factors from thickness of 0.2 to similar to 1 nm, while for thin films thicker than 1 nm, the values stabilize around a plateau value. For three-dimensional situations, these factors were found to change within a range of nanodiamond diameter of 0.4 to similar to 2 nm, followed by oscillations around specific values as well. These results reveal that nanoscale diamond quantum confinements exist for a nanodiamond particle of a diameter smaller than 2 nm, but not for larger particle sizes. It must here be stressed that all surface specific parameters did independently show the existence of the here presented size ranges for quantum confinement.