Hydrothermal synthesis of well crystallized C8 and diamond nanocrystals and pH-controlled C8 ↔ diamond phase transition

Abstract

Diamond is a major carbon allotrope, whereas C₈ is a rare and mysterious carbon allotrope with a super-cubic crystal structure that is even denser than diamond (by 15%). Little is known about the properties of C₈ nanocrystals (NCs) because carbon NCs synthesized under usual conditions have either a graphitic or diamond crystal structure. We develop a hydrothermal method to synthesize well-crystallized C₈ (∼6.1 nm) and diamond (∼4.7 nm) NCs. The crystal phase of the synthesized carbon NCs is determined by the pH of the initial precursor solution: acid solution results in diamond NCs while basic solution results in C₈ NCs. These two phases of carbon NCs can be reversibly converted to each other by changing the pH from acidic to basic or vice versa under thermal conditions.