Atomic-Hydrogen Hydrogenation of SWCNTs via the C KLL Auger D-Parameter

Motivation

Quantifying sp³ conversion in hydrogenated carbon is harder than it sounds. The standard XPS approach, decomposing the C 1s peak to find a small sp³ shoulder shifted 0.7 to 0.8 eV, is sensitive to fitting choices and easy to over-read. The C KLL Auger D-parameter, the energy separation between the extrema of the first-derivative Auger spectrum, is an established alternative probe of sp²/sp³ hybridization with clean endpoints near 23.1 eV for graphite and 13.2 eV for diamond. A literature review I conducted before designing the experiment found that no published study reports the D-parameter for hydrogenated SWCNTs: every XPS-based hydrogenation study of nanotubes relies on C 1s decomposition or NEXAFS. The closest precedent (Alanazi et al., 2018) applies the D-parameter only to irradiated SWCNT buckypaper. A three-way comparison of pristine, atomic-hydrogen-exposed, and electron-irradiated SWCNTs is therefore a genuine gap, and the group had a direct in-house precedent: a graduate student had found a drastic D-parameter change in CVD graphene under analogous treatments.

Methods

Thin films were prepared from purified HiPco-grown SWCNT nanopowder dispersed in dimethylformamide, spin-coated onto SiO₂/Si, and annealed under ultra-high vacuum. Hydrogenation used an atomic-hydrogen beam produced by dissociating H₂ on a hot tungsten filament, with a 30-minute exposure. The D-parameter was measured by laboratory XPS from the first-derivative C KLL spectrum, and TPD from 50 to 400 °C tracked m/z = 1, 2, 17, 18, and 44. A parallel electron-irradiation leg on unpurified HiPco material (1.5 keV, ultra-high vacuum) was characterized by Raman spectroscopy and TPD, continuing the lab’s irradiation program.

Results

After annealing, the films show a D-parameter of approximately 20 eV, characteristic of sp²-dominated nanotube carbon. Atomic-hydrogen exposure reduces it to approximately 15 eV, a large shift toward diamond-like values consistent with substantial conversion toward sp³-like C−H bonding on the nanotube lattice. The TPD result is the surprise: from 50 to 400 °C there is no clear H or H₂ desorption, even though recombinative desorption from flat graphitic carbon is reported to peak near 170 to 290 °C. Within detection limits, the hydrogen introduced by atomic-H exposure is either retained beyond 400 °C, as expected for curvature-stabilized C−H bonds on small-diameter tubes, or leaves through recombination-limited or alternative pathways. The electron-irradiation leg carries its own signal: I_D/I_G doubles from 0.047 to 0.097, and a secondary TPD hydrogen desorption feature shifts to higher temperature after irradiation.

Interpretation discipline

The claims are deliberately scoped. The measured D shift mixes true sp²-to-sp³ rehybridization with the chemical effect of surface hydrogen on the Auger lineshape, and no universal conversion from D-parameter to hydrogen weight percent exists in the literature; the manuscript states both. The D-parameter is presented as a hybridization probe and a practical tracking framework rather than a direct hydrogen quantification.

Current status

An abstract on this work was accepted for the 2026 MRS Fall Meeting (Symposium EN06, Materials for Effective Hydrogen Storage, Transmission and Applications). A first-author manuscript is in preparation, targeting Nano Express (IOP), chosen for its validity-based editorial policy and fast first decisions. The work is also the centerpiece of my Goldwater Scholarship research essay.

Future work

Quantified uptake, D₂ isotope labeling, and desorption above 400 °C will discriminate among the retention scenarios. An empirical calibration of the D-parameter against independently measured hydrogen coverage would convert the probe from qualitative to quantitative. The purified-versus-unpurified comparison and a dose-dependent sp³ saturation study across SWCNTs, GO, and rGO extend naturally from the same sample set.