TOF-SIMS characterisation of spark-generated nanoparticles made from pairs of Ir–Ir and Ir–C electrodes

This paper constitutes a comprehensive study on the use of time-of-flight secondary ion mass spectrometry (TOF-SIMS) for determining the composition of spark-generated nanoparticles as well as for deriving information on their size. This kind of nano-sized matter is often used in animal studies to explore and quantify lung inflammatory response to inhalation or instillation of insoluble particles. Two iridium samples, Ir(#1) and Ir(#2), were generated with a pair of electrodes made of pure iridium. A mixed C(Ir) sample was produced with one electrode of carbon and the other of iridium. The primary particles of Ir had a size (“diameter”) Dp ≤ 5 nm, but they were agglomerated in chain-type aggregates with different mean electrical mobility diameter, depending on the spark frequency. The atomic fraction of Ir in the C(Ir) particles was less than 1% (Dp ∼ 13 nm). The generated particles were collected on cellulose acetate-nitrate filters. An Ir electrode with a polished surface served as a reference sample. SIMS analysis of negative secondary ions was performed under impact of 34 keV Ar+. The mass spectra of the Ir samples revealed surprisingly high yields of cluster ions of the type IrkXY−, with k up to 20 and XY indicating combinations of oxygen, nitrogen and carbon. A detailed evaluation of cluster composition based on the isotopic pattern of Ir revealed a high oxygen content in the particles located at the surface of fresh samples. The true oxygen content of these “oxide” particles appears to be reduced by preferential sputtering. Depth dependent information was derived by sputter erosion in between a series of TOF mass spectra. Surprisingly, the (primary) oxide particles in Ir(#1) featured a very high cross-section for sputter ejection, reminiscent of monolayer removal. The particles uncovered by sputtering were much larger and mostly made of carbon-rich Ir particles (“carbides”), with the addition of some nitrogen. Combined with the high cluster ion yields, the results suggest that the Ir oxide was contained in flakes or chunks with sizes as small as 1 nm or even less. The same or similar oxide species were present in Ir(#2), but piled up to thicker layers. Very similar Ir oxide and carbide particles were identified in the C(Ir) sample, but at a very low yield, roughly corresponding to the atomic fraction of Ir.