Two-step high-pressure high-temperature synthesis of nanodiamonds from naphthaleneProject supported by the National Key R&D Program of China (Grant No. 2018YFB0406500), the National Natural Science Foundation of China (Grant Nos. U1804155, U1604263, and 11804307), and the China Postdoctoral Science Foundation (Grant Nos. 2018M630830 and 2019T120631)

Nanodiamonds have outstanding mechanical properties, chemical inertness, and biocompatibility, which give them potential in various applications. Current methods for preparing nanodiamonds often lead to products with impurities and uneven morphologies. We report a two-step high-pressure high-temperature (HPHT) method to synthesize nanodiamonds using naphthalene as the precursor without metal catalysts. The grain size of the diamonds decreases with increasing carbonization time (at constant pressure and temperature of 11.5 GPa and 700 °C, respectively). This is discussed in terms of the different crystallinities of the carbon intermediates. The probability of secondary anvil cracking during the HPHT process is also reduced. These results indicate that the two-step method is efficient for synthesizing nanodiamonds, and that it is applicable to other organic precursors.