Effect of titanium doping on the structure and reducibility of nanoparticle molybdenum dioxide
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Molybdenum dioxide (MoO2) has potential not only in heterogeneous catalysis, but also as an electrode material in lithium-ion batteries and fuel cells. However, the reduction tendency to molybdenum metal is a limiting factor in the use of this oxide. In the present work, MoO2 was doped with Titanium (Ti4+) to increase the stability to reduction in a hydrogen-rich environment. Rather than using a conventional high-temperature solid-state reaction, a novel solution-based synthesis method followed by hydrothermal reduction was used. Nano-sized Ti-doped MoO2 particles with Ti/Mo atomic ratios of 0, 0.03, and 0.05 were prepared and characterized. X-ray and time-of-flight neutron powder diffraction (XRD and NPD) confirmed the presence of only the MoO2 phase. Transmission electron microscopy showed that the nanoparticles had an acicular morphology with particle sizes in the long dimension between 30 and 50 nm. Elemental mapping obtained by scanning electron microscopy with energy dispersive spectroscopy showed homogenous distribution of the dopant. Attenuated total reflectance-Fourier transform infrared spectroscopy revealed that the solubility limit of the synthesized Ti-doped MoO2 was reached at a Ti/Mo ratio of 0.05. This observation indicates that the bulk thermodynamic solubility limit can be exceeded at the nanoscale. X-ray photoelectron spectroscopy performed on 0.03 Ti-doped MoO2 samples showed that the Ti dopant is present as Ti4+ and the depth profile analysis indicated the presence of this species even in the bulk of the sample. This agrees with the data obtained from Rietveld refinement of the XRD and NPD results, which showed changes in the lattice parameters and Mo–Mo interatomic distance upon doping, suggesting a substitution of Mo4+ by Ti4+. The effect of Ti doping on the reducibility of MoO2 was demonstrated by in situ XRD in a hydrogen-rich environment.
KeywordsNanoparticle molybdenum dioxide Ti-doped Rietveld refinement Reducibility
This work was supported by the National Science Foundation (CBET1034308) and the Office of Naval Research (N00014-12-1-0830). Use of the Spallation Neutron Source is supported by the Division of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy.
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