Structure-Property Relations of Reduced MoO3

Abstract

Molybdenum oxide is an important component in many technological applications. Its unusually high work function coupled with its transparency make it valuable in optoelectronic devices such as photovoltaic cells and light emitting diodes. Its structural sensitivity to redox reactions lends it to selective catalysis and gas sensing. It is utilized in compositions ranging from MoO3 to MoO2 with several intermediary phases, and some operational functions rely on a narrow window of oxygen non-stoichiometry. Across this range the electronic structure varies from a wide band gap semiconductor to metallic as the Mo 4d orbitals are filled with decreasing O/Mo ratio. As such, there are changes in the optical and electronic properties during reduction of MoO3, and these properties are intimately linked to the broad structural variations hosted by the layered, van der Waals structure. In this work, theoretical and experimental studies have been applied to investigate the structureproperty relationships of MoO3 in reduction. Density functional theory (DFT) has been used to treat the range of stoichiometries from the dilute limit of oxygen vacancies, to extended defect shear structures and reduced phases up to MoO2. The enthalpies of formation are calculated for oxygen vacancies at the three symmetry inequivalent oxygen sites in MoO3. Calculation of enthalpies of reduction found that the layered structure is destabilized by loss of oxygen from the structure. Modelling the electronic structures revealed changes in the density of states, with oxygen vacancies giving rise to Mo 4d gap states with energy levels dependent on the vacancy position. Further reduction and lattice collapse leads to a reduced band gap and a metallic structure dependent on the extent of Mo–Mo bonding. Experiments were focused on thin films of molybdenum oxide, as this form is most relevant to applications and reduction in this setting remains largely unknown. The DFT predictions were compared to experimental studies of molybdenum oxide films produced by pulsed laser deposition. Films had various compositions of MoO3, Mo4O11 and MoO2, determined by X-ray diffraction (XRD), and the valence band region was explored by X-ray photoelectron spectroscopy (XPS) and compared to the calculated electronic structures. MoO2 was easily distinguished from MoO3, while intermediate phases had similar metallic properties which overlapped with vacancy induced gap states. The effect of the differing band structures was seen in the electrical conductivity, optical transmittance and photoluminescence response of the films. In addition, a simple, aqueous route to MoO3 thin films is presented by which nanostructured morphologies were attained by control of solution parameters. Smooth and homogeneous thin films were achieved by control of the species in solution. Film dimensions were controlled by changing the solution concentration, and the sensitivity of film quality to pH was demonstrated. Upon reduction in dilute hydrogen gas, the nanocrystallite grain morphology was observed hosting a sequence of shear defects, nucleation and growth of reduced molybdenum oxide phases before decomposition of the film. The importance of film morphology was revealed in the compositional changes of the films during hydrogen reduction, established by XPS and XRD. The compositional and structural changes had accompanying effects on optical propert