The basic and important way to prepare diamond is from graphite by the phase transforming under the condition of high pressure high temperature (HPHT). However, this method needs stringent equipment and high investment costs. Recently, we proposed a method to prepare the diamond by phase transformation of graphite at atmospheric pressure with monodispersed Ta atoms. It was found that H atoms could be involved in this atmospheric pressure phase transition, but the role of O atoms was not investigated. Here, we used tantalum wires as Ta source and heating filaments to prepare vertical graphene containing Ta atoms in hot filament chemical vapor deposition (HFCVD) system. And then the vertical graphene layers were, annealed in oxygen-containing environment, and nanodiamonds are obtained by phase transformation from the vertical graphene under atmospheric pressure. The results show that the sample morphology is the same as the untreated vertical graphene when the annealed ambient air pressure is at 10 Pa and 50 Pa with oxygen atom contents of 1.96% and 2.04%, respectively; TEM tests reveal TaC and graphite in these samples but no diamonds. Nanodiamond grains with the size range of 2-4 nm are observed in the amorphous carbon region of samples annealed at 100 Pa and 500 Pa air pressure with oxygen atom contents increasing to 2.77% and 3.11%, respectively, indicating that oxidation facilitates the phase change from Ta-containing vertical graphene to diamond at atmospheric pressure. When the air pressure of the annealing environment rises to 1000 Pa with the oxygen atom contents of 3.54%, the sample is extensively oxidised and the graphite structure is severely damaged; it’s means that a large number of oxygen atoms tend to disrupt the graphite structure rather than promote the phase change to diamond. These results supply a way to prepare nanodiamond and show the effect of O atoms in the graphite phase transition at atmospheric pressure.
