Cédric Jaoul

Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H− yield

Highly oriented polycrystalline graphite (HOPG), boron-doped diamond (BDD), nanocrystalline diamond, ultra-nanocrystalline diamond and diamond-like carbon surfaces are exposed to low-pressure hydrogen plasma in a 13.56MHz plasma reactor. Relative yields of surface-produced H− ions due to bombardment of positive ions from the plasma are measured by an energy analyser cum quadrupole mass spectrometer. Irrespective of plasma conditions (0.2 and 2 Pa), HOPG surfaces show the highest yield at room temperature (RT), while at high temperature (HT), the highest yield (∼3-5 times compared to HOPG surface at RT) is observed on BDD surfaces. The shapes of ion distribution functions are compared at RT and HT to demonstrate the mechanism of ion generation at the surface. Raman spectroscopy analyses of the plasma-exposed samples reveal surface modifications influencing H− production yields, while further analyses strongly suggest that the hydrogen content of the material and the sp3/sp2 ratio are the key parameters in driving the surface ionization efficiency of carbon materials under the chosen plasma conditions.

Electrochemical Hydrogen Insertion in Substoichiometric Titanium Carbide TiC0.6: Influence of Carbon Vacancy Ordering

Substoichiometric titanium carbide of formula TiC0.6 with different degrees of carbon vacancy ordering has been synthesized first by reactive sintering at high temperature (2100 °C) and then by annealing at low temperature (730 °C) for different durations. The effect of annealing on the structure of the carbide and its capacity to electrochemically insert hydrogen has been investigated. The XRD study reveals two phase transitions in the carbide during annealing. First, annealing of disordered TiC0.6 (space group Fm3¯m) for 40 h leads to a trigonal superstructure of space group R3¯m thanks to ordering of carbon vacancies. After the longest annealing time of 120 h, the structure of the carbide becomes cubic anew, but with a space group of Fd3¯m. The electrochemical hydrogen insertion in these different types of TiC0.6, as studied by cyclic voltammetry, strongly depends on the crystalline structure of the carbide. The maximum storage capacity is obtained for 40 h, corresponding to the R3¯m ordered phase. Because the R3¯m structure consists of alternately empty and full (111) carbon atomic planes, we show that the hydrogen insertion and diffusion in the material is rendered possible by the presence of vacant (111) planes. This behavior is very similar to that of a cation-ordered manganese-nickel spinel vis-à-vis lithium ion insertion.