Localized Charge Transfer in Two-Dimensional Molybdenum Trioxide

Molybdenum trioxide is an interesting inorganic system in which the empty 4d states have potential to hold extra electrons and therefore can change states from insulating opaque (MoO₃) to colored semimetallic (H_xMoO₃). Here, we characterize the local electrogeneration and charge transfer of the synthetic layered two-dimensional 2D MoO₃-II (a polymorph of MoO₃ and analogous to α-MoO₃) in response to two different redox couples, i.e., [Ru(NH₃)₆]³⁺ and [Fe(CN)₆]^3- by scanning electrochemical microscopy (SECM). We identify the reduction of [Ru(NH₃)₆]³⁺ to [Ru(NH₃)₆]²⁺ at the microelectrode that leads to the reduction of MoO₃-II to conducting blue-colored molybdenum bronze H_xMoO_3. It is recognized that the dominant conduction of the charges occurred preferentially at the edges active sites of the sheets, as edges of the sheets are found to be more conducting. This yields positive feedback current when approaching the microelectrode toward 2D MoO₃-II-coated electrode. In contrast, the [Fe(CN)₆]^4-, which is reduced from [Fe(CN)₆]^3-, is found unfavorable to reduce MoO₃-II due to its higher redox potential, thus showing a negative feedback current. The charge transfer on MoO₃-II is further studied as a function of applied potential. The results shed light on the charge transfer behavior on the surface of MoO₃-II coatings and opens the possibility of locally tuning of their oxidation states.