Study on the effect of N–H–O co-doping on diamond growth and its mechanism under HPHT by FeNi solvent

Abstract

The synthesis of diamonds with rich nitrogen (N), hydrogen (H) and oxygen (O) elements is a hot topic that needs to be solved in current research. In this work, diamonds were synthesized with C₆H₅NO₂ dopants using iron and nickel (FeNi) catalysts under conditions of 5.8 GPa and 1370 °C. We characterized N, H, O co-doped diamonds in detail and profoundly researched the effect of N, H, O on FeNi catalysts. The results indicate that with the increase in the content of C₆H₅NO₂, the crystal growth texture is increasingly obvious, and the surface morphology is increasingly uneven and is eventually cracked. Besides, we noticed that the growth rate of the crystal slowed from 2.07 mg h⁻¹ to 0.15 mg h⁻¹. Using Fourier transform infrared (FTIR) spectroscopy, we calculated the N concentration inside the crystal; it increased from 154 ppm to 1218 ppm. The Raman spectra indicate that all the synthesized crystals had a high-quality sp³ structure. X-ray photoelectron spectroscopy (XPS) proved that a small amount of H, O, N entered the lattice of the diamonds. XPS measurement of the metal solvent illustrated that in the doped carbon sources, more oxygen and nitrogen-related components were formed and the proportion of diamond was decreased. SEM-EDS mapping confirms that when the carbon sources are doped with C₆H₅NO₂, the mutual diffusion between carbon sources and the metal solvent is inhibited. XRD results confirmed that when doped, FeN, FeO and FeC₈ emerge, while indispensable intermediates for diamond growth vanish. These results ascertain that with the induction of C₆H₅NO₂ in the synthesis cavity, the metal solvent tends to react with the decomposition products of C₆H₅NO₂, thus affecting the graphite conversion to diamond from the carbon source as well as the mutual diffusion between carbon sources and the metal solvent, which eventually leads to the reduction of crystal growth rate. This work further explains the mechanism of decreasing the growth rate and improving the growth conditions of diamonds synthesized using the FeNi catalyst under the condition of N–H–O doping, which provides valuable data for synthesizing diamonds with rich N, H and O content.