Hydrogen reduction of molybdenum oxide at room temperature

Andreas Borgschulte,Frank Nüesch,Roland Hany,Sandra Jenatsch,Olga Sambalova,Renaud Delmelle

doi:10.1038/srep40761

Abstract

The color changes in chemo- and photochromic MoO3 used in sensors and in organic photovoltaic (OPV) cells can be traced back to intercalated hydrogen atoms stemming either from gaseous hydrogen dissociated at catalytic surfaces or from photocatalytically split water. In applications, the reversibility of the process is of utmost importance, and deterioration of the layer functionality due to side reactions is a critical challenge. Using the membrane approach for high-pressure XPS, we are able to follow the hydrogen reduction of MoO3 thin films using atomic hydrogen in a water free environment. Hydrogen intercalates into MoO3 forming HxMoO3, which slowly decomposes into MoO2 +1/2 H2O as evidenced by the fast reduction of Mo6+ into Mo5+ states and slow but simultaneous formation of Mo4+ states. We measure the decrease in oxygen/metal ratio in the thin film explaining the limited reversibility of hydrogen sensors based on transition metal oxides. The results also enlighten the recent debate on the mechanism of the high temperature hydrogen reduction of bulk molybdenum oxide. The specific mechanism is a result of the balance between the reduction by hydrogen and water formation, desorption of water as well as nucleation and growth of new phases.

Highlights

With oxygen having a much larger electronegativity than hydrogen, most binary d-metal oxides are much more stable than the corresponding hydrides[1,2]
While energy minima can be related to thermodynamic parameters, the barriers and the reaction path requires complex calculations of the electronic structure
We give the enthalpy of formation of the various phases expected to occur to illustrate their location on the energy potential surface shown in Fig. 5

Summary

Introduction

With oxygen having a much larger electronegativity than hydrogen, most binary d-metal oxides are much more stable than the corresponding hydrides[1,2]. The phase transformation causes only small crystallographic rearrangements (topotactic reduction): hydrogen atoms occupy sites in the van der Waals gaps between double layers of MoO6 octahedra as well as intralayer sites on zigzag chains along the channels[12,13] This results in a relatively small increase of the cell volume and slight distortion of the lattice changing the overall crystal symmetry from orthorhombic orthorhombic (phase I) to monoclinic (phases II, IV)[12,14]. Lattice distortion and hydrogen ordering increase the complexity of the system[12], the electronic structure may be described using the semiconductor model with hydrogen as dopant (Fig. 1) In this model, the MoO3 structure motif (MoO6 octahedrons) remains unchanged and defines the overall electronic structure, but is perturbed by hydrogen atoms as new band gap states[10]. For the application as an electron injection/extraction layer, the formation of hydrogen bronzes by photolysis of hydrogen containing molecules is an undesired effect, because it diminishes the favorable electronic properties[24]

Methods

Results

Conclusion