High Capacity Hydrogen Absorption in Transition Metal-Ethylene Complexes Observed via Nanogravimetry

Using a surface acoustic wave based high resolution gravimetric technique where samples close to a monolayer are measured we observe high weight percentage hydrogen (${\mathrm{H}}_2$) uptake with rapid kinetics at room temperature in transition metal (TM) ethylene (${\mathrm{C}}_2{\mathrm{H}}_4$) complexes formed by laser ablation. By ablating titanium (Ti) in ${\mathrm{C}}_2{\mathrm{H}}_4$ we obtain a complex that exhibits 12 wt % uptake of ${\mathrm{H}}_2$ with substitution by deuterium providing a doubling. Mass spectroscopic studies during ablation of Ti show presence of a species, with a $\mathrm{\text{mass}}=78\mathrm{amu}$, a likely candidate for the high ${\mathrm{H}}_2$ uptake.

Figure 1

Hydrogen uptake weight percentage in TM-ethylene complexes. A surface acoustic wave device operating at 315 MHz is used as a gravimetric sensor to measure the mass increase due to ${\mathrm{H}}_2$ uptake by the TM-ethylene complexes formed in a PLD chamber. The ethylene pressure during ablation was below 13 Pa in all cases.

Figure 2

Comparison of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ absorption in the Ti-Ethylene complex. The expected near doubling of the uptake with ${\mathrm{D}}_2$ is readily seen for this complex which was synthesized in 6.5 Pa of ethylene. Also shown is the response of an empty SAWR when exposed to ${\mathrm{D}}_2$ (with exposure to ${\mathrm{H}}_2$ being similar).

Figure 3

TEM picture of titanium ablated in 6.5 Pa ethylene for 5000 pulses (left) and its corresponding (right) electron energy loss spectrum (EELS). The TEM picture reveals no significant structure on the surface down to a scale of 1 nm, but the EELS scan shows two peaks that correspond to titanium thus suggesting that the titanium-ethylene complex is uniformly deposited through the PLD method.

Figure 4

Shows the increase in the partial pressure (in arbitrary units) of the 78 AMU species that is unique to experimental runs when Ti is ablated in ethylene. The results for ablation of Ti in vacuum are similar to the case of no ablation. The two vertical lines mark the beginning and end of the ablation process. In our setup the RGA is positioned at an angle of 15 deg. behind the plane of the target with the position of the RGA filament being 50 cm away from the ablation region.

Figure 5

Hydrogen uptake, weight percentage, as a function of the total quantity of the Ti-ethylene complex accumulated on the transducer face. The data are presented in two different ways. The top vertical axis measures the thickness of the film calculated using the lattice spacing of titanium carbide and the bottom axis is the total number of laser pulses used to accumulate the material. The left vertical axis (circles) presents the uptake that occurs in 1 h and the right vertical axis (squares) the total time it takes to reach “full” saturation of 12%.