Thermally Evaporated MoO3 Nanowires as Oxygen Evolution Reaction Catalysts for Water Splitting Applications

In this work, we report the growth of MoO3 nanowires (NWs) by thermal evaporation with the aim of evaluating their use as catalysts for oxygen evolution reaction (OER) in water splitting application. Growth was performed on two different kinds of substrates, Si(100) and Si(111), and at different growth temperatures between 200 and 400 °C. The effect of such experimental parameters on the morphological, structural, and compositional properties was investigated. The deposited materials were characterized by in situ reflection high-energy electron diffraction, scanning electron microscopy, transmission electron microscopy, Rutherford backscattering spectroscopy, and X-ray diffraction. For a growth temperature of 200 °C, a flat and amorphous oxide layer is obtained, while as the temperature is increased, one-dimensional (1D) growth prevails on 2D layer formation. In particular, at 400 °C, the deposited material consists of a high-density array of MoO3 crystalline NWs a few nm in diameter and up to about 0.5 μm in length. Only in the case of Si(111) substrates have small three-dimensional structures formed and coexisted with NWs since the beginning of the growth process. Finally, the OER activity of MoO3 NWs was evaluated. The nanostructures were deposited on graphene paper, and their electrochemical activity was investigated and compared to other nonprecious metal-based catalysts for alkaline OER. Our MoO3-based electrodes showed overpotentials of 330 and 340 mV at 10 mA/cm2 (η10) depending on the average length of NWs and a quite high mass activity associated with a low catalyst loading, thus fitting the best performing electrocatalysts. Our results demonstrate that MoO3 NWs are suitable candidates for the development of anode electrodes for water splitting application.