Preparation of nanodiamonds based on phase transformation of vertical sheet under atmospheric pressure

A basic and important way to prepare diamond is to make graphite experience the phase transformationunder the high-pressure high-temperature (HPHT) condition. However, this method needs stringent equipmentand high investment cost. Recently, we proposed a method to prepare the diamond by phase transformation ofgraphite at atmospheric pressure with monodispersed Ta atoms. It is found that a phase transformationhappens to H atoms under atmospheric pressure, but the role of O atoms has not been investigated. Here, weuse tantalum wires as Ta source and heat the filaments to prepare vertical graphene containing Ta atoms in hotfilament chemical vapor deposition (HFCVD) system. And then the vertical graphene layers are annealed inoxygen-containing environment, and nanodiamonds are obtained by phase transformation from the verticalgraphene under atmospheric pressure. The results show that the sample morphologies are the same as theuntreated vertical graphene's, when the annealed ambient air pressure is at 10 Pa and 50 Pa with oxygen atomcontent of 1.96% and 2.04%, respectively; TEM tests reveal TaC and graphite but no diamond in these samples .Nanodiamond grains with the size range of 2-4 nm are observed in the amorphous carbon region of samplesannealed at 100 Pa and 500 Pa air pressure with oxygen atom content increasing to 2.77% and 3.11%,respectively, indicating that oxidation facilitates the phase change from Ta-containing vertical graphene todiamond at atmospheric pressure. When the air pressure of the annealing environment rises to 1000 Pa with theoxygen atom content of 3.54%, the sample is extensively oxidized and the graphite structure is severelydamaged,which means that a large number of oxygen atoms tend to disrupt the graphite structure rather thanpromote the phase change into diamond. These results supply a way to prepare nanodiamond and show theeffect of O atoms in the graphite phase transition at atmospheric pressure