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As-deposited amorphous MoO3 films were found to crystallize into a stable orthorhombic phase on annealing in air at 350°C. The feasibility of using Mg ions as intercalants, in polycrystalline MoO3 thin films has been investigated. These ions were intercalated galvanostatically at a constant current density of 166 µA/cm2. Structural studies carried out on intercalated films, show lattice expansion along b axis by 2.35% for x = 0.1 and 3.17% for x = 0.3. Partial restoration of b-axis is observed on deintecalation. The chemical status of Mo and O ions, before and after intercalation has been investigated using Photoemission Spectroscopy (core level spectra). Core level (CL) of Mo 3d levels indicate the presence of Mo+6 oxidation state only in unintercalated films, with a stoichiometry of MoO3. The intercalated ions are found to reduce Mo ions as revealed by the presence of Mo+5, Mo+4 and Mo+3 states in addition to Mo+6 state. Formation of Mg–O bonds is seen from the CL spectrum of O1s levels. From cyclic voltammetry experiments the estimated diffusion coefficient of Mg ions is found to be of the order of 10-19 cm2/s and the films are stable even after 300 cycles.

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Mg ions were intercalated in MoO 3 films with different microstructure/stoichiometry and the observed changes in absorption are explained in terms of the estimated diffusivity of Mg ions in these films. Optical studies on unintercalated and intercalated films revealed two absorption peaks at 850 and 1550 nm; first associated with Mo 5+ states and the second with Mo 4+ states. The changes in absorption with increasing intercalated fraction of ions ͑x͒ showed saturation behavior and the x value at which it occurred was found to be strongly dependent on microstructure/stoichiometry. These changes are satisfactorily explained based on Mg ion diffusivity.Reversible absorption in inorganic oxides has long been obtained using monovalent intercalant ions, but the reported values of coloration efficiencies ͑͒ still need further improvements for their effective use in electrochromic ͑EC͒ devices. Several materials such as V 2 O 5 , WO 3 , MoO 3 , and Co 3 O 4 , have been investigated during the last three decades for their use in EC devices. 1 Few investigations on WO 3 established that optical, 2 chemical, 3 and photoluminescence 4 properties depend on microstructure/ stoichiometry. However the role of these parameters on EC properties of MoO 3 have not been investigated to date. Further, the advantage of MoO 3 is that a thin film of this material can be deposited in many ways, including thermal evaporation, sputtering, chemical vapor deposition, sol-gel, and electrochemical methods. 1 In all of these methods, MoO 3 films with desired stoichiometry and microstructure can be obtained by suitably adjusting the deposition conditions. 1 Hence, MoO 3 is well suited for investigating the role of microstructure/stoichiometry on EC properties. For the first time, a direct relationship between film microstructure/stoichiometry, ion diffusivity, and spectral dependence of absorption has been established and the results obtained are presented in this paper.The decrease in transmittance ͑T͒ of an EC device following ion insertion has been explained using color center model, 5 intervalence charge transfer transitions ͑ICTT͒ 6 of electrons and polaron absorption. 7 Recently, we reported another absorption mechanism which explains the absorption process in Mg intercalated MoO 3 films. 8 In this model, reduced oxidation states of Mo, located within the bandgap region and behaving as defect levels, are referred to as lower valence states ͑LVS͒. The incident photons are basically utilized to liberate electrons from these levels. The released electron may attach to neighboring Mo of higher oxidation state thus reducing it. Mg has been chosen as the intercalant for reasons that were discussed in an earlier paper. 9 We have reported that MoO 3 films with varying degree of crystallinity and stoichiometry can be prepared by adjusting postdeposition annealing conditions. 10 The crystal structure of the films can be varied from amorphous to partly crystalline to polycrystalline and stoichiometry varied from highly nonstoichiometric to nearly stoi...

The effect of stoichiometry on the coloration efficiency (η) and cycling life of MoO3 films has been investigated and underlying reasons discussed in the paper. Optical studies reveal that as-deposited and 150 °C annealed films are nonstoichiometric while films deposited by activated reactive evaporation are stoichiometric. Nonstoichiometry in unintercalated MoO3 is associated with lower valence states of Mo is responsible for absorption in the 400–1000-nm spectral region. It is found that this nonstoichiometry affects η and the cycling life of the films adversely.