A mechanism of ultraviolet activation of the α-AlH3 decomposition

• UV irradiation leads to formation of metallic clusters on the surface of hydride particles.
• UV activation involves hydrogen vacancies.
• A mechanism of formation of metallic clusters at UV activation is suggested.

We investigated the mechanism of activation of α-AlH3 powder decomposition by irradiation with ultraviolet (UV) light using barometry, photoluminescence (PL), cathode luminescence (CL) and X-ray diffraction (XRD) methods. Exposing the samples under a mercury lamp through the filters we have shown that only the line, corresponding to the energy of 4.88 eV, leads to activation, whereas the spectral lines of lower energy are ineffective. XRD shows that after continued UV activation the aluminum clusters with metallic properties are formed on the surface of the AlH3 powder particles at room temperature. The analysis of results of performed calculations of the absorption spectrum of the perfect crystal α-AlH3 using DFT method showed that hydrogen vacancies are involved in activation by UV light. Studying of dynamics of CL and PL spectra at different UV expositions permitted to suggest the following mechanism of activation. Absorption of a photon implies that vacancy captures an electron from a neighboring hydrogen atom. Hydrogen atom then can leave its regular position in the lattice and form a new vacancy next to the first. Clustering of vacancies occurs. Continued UV activation is accompanied by merging of vacancies, which deprives the aluminum atoms of hydrogen bonds connecting them. Thus the aluminum clusters with metallic properties are formed.