Insertion and removal of protons in single-crystal orthorhombic molybdenum trioxide under H2S/H2 and O2/N2

Abstract

Using AFM/XRD/XPS methods, in this paper, we investigate protonation and deprotonation processes in single-crystal samples of orthorhombic molybdenum trioxide (α-MoO3). At low temperatures, a small part of α-MoO3 is changed to needlelike HxMoO3 (x ≈ 0.33) along 〈203〉 directions in a H2S/H2 gas stream. When these elongated crystallites assemble into a maze structure, the growth of HxMoO3 is gradually ceased due to closing entrance for hydrogen. At higher temperatures, the needlelike HxMoO3 crystallites turn to a growth perpendicular to 〈203〉, which leads to the formation of HxMoO3 blocks. It is observed that the basal plane of α-MoO3 is severely buckled upon the protonation. Surface sulfidation is also observed. The formed HxMoO3 or surface MoS2, however, can be readily converted back to their original-phase α-MoO3 in air at 350-400 °C. This oxidation process gives rise to a flattened (010) topography (i.e., debuckling) on which shallowly divided α-MoO3 surface blocks bounded with {101} planes are formed. When an α-MoO3 (010) plane embedded with nanocrystallites is used to create surface stress or nucleation sites, the insertion mode of hydrogen along 〈001〉 is further reconfirmed in this work. A correlation of surface/bulk phases upon various chemical reactions is addressed, and a model to summarize these changes is also proposed.