Critical observations in the research of carbon nitride

A critical view on the research of carbon nitride (CNx) with an emphasis on the predicted α- and β-C3N4 phases derived from their Si3N4 analogues is presented. This paper highlights the reasons for the less occurence of planar nitrogen in both phases which otherwise presumed to be existed as in their silicon nitride counterparts which to date, has not been searched and reported approriately. Also, other hypothetical C3N4 phases including the very recently predicted spinel phase, c-C3N4 are discussed. Further, the recent synthesis of carbon nitrides and their characterisations are briefly reviewed. Finally, an outlook is presented which includes most promising synthesis routes for crystalline carbon nitride.