Inductively coupled plasma spectroscopy for heteroatom-doped carbonaceous materials: Limitations and acid choice for digestion

Heteroatom-doped carbonaceous materials have garnered significant attention in the fields of catalysis, energy conversion and storage, and pollutant recovery. However, accurately determining the doping extent remains a delicate task in a number of cases (e.g. phosphorus, boron, selenium), often leading to conflicting data from different characterization techniques. Inductively Coupled Plasma (ICP) spectroscopy stands out as a routine technique; nevertheless, reliable results necessitate appropriate digestion protocols. In this study, we demonstrate on a series of P-doped samples, with P contents ranging from 2 to 14 wt.%, how the choice of acids (HCl, HNO3, H2SO4) and oxidizing compounds (H2O2) for digestion drastically affects the results, with variations of up to 620 %. Though commonly used, aqua regia proved highly unreliable, particularly for P-doped carbon nitrides, while piranha solution appeared as a promising alternative, with a precision, i.e. a coefficient of variation, ca. 5.0 %. The contents deriving from piranha solution digestion were subsequently comforted through X-ray Photoelectron Spectroscopy. We finally put in perspective the use of ICP in terms of sensitivity and accuracy with the main analytical techniques employed in literature to determine the element composition of carbon materials.