Abstract
Effects of different reaction parameters in the hydrothermal synthesis of molybdenum oxides (MoO3) were investigated and monoclinic (β-) MoO3 was prepared hydrothermally for the first time. Various temperatures (90/210 °C, and as a novelty 240 °C) and durations (3/6 h) were used. At 240 °C, cetyltrimethylammonium bromide (CTAB) and CrCl3 additives were also tested. Both the reaction temperatures and durations played a significant role in the formation of the products. At 90 °C, h-MoO3 was obtained, while at 240 °C the orthorhombic (α-) MoO3 formed with hexagonal rod-like and nanofibrous morphology, respectively. The phase transformation between these two phases was observed at 210 °C. At this temperature, the 3 h reaction time resulted in the mixture of h- and α-MoO3, but 6 h led to pure α-MoO3. With CTAB the product was bare o-MoO3, however, when CrCl3 was applied, pure metastable m-MoO3 formed with the well-crystallized nanosheet morphology. The gas sensing of the MoO3 polymorphs was tested to H2, which was the first such gas sensing study in the case of m-WO3. Monoclinic MoO3 was found to be more sensitive in H2 sensing than o-MoO3. This initial gas sensing study indicates that m-MoO3 has promising gas sensing properties and this MoO3 polymorph is promising to be studied in detail in the future.
Highlights
Molybdenum oxides (MoO3) are considerable materials among molybdenum (Mo) compounds due to their excellent physical and chemical properties
The as-prepared different MoO3 allotropes were further studied by energy dispersive X-ray spectroscopy (EDX) and specific surface area measurements, by Fourier transformed infrared (FT-IR), Raman, and ultraviolet-visible (UV–Vis) spectroscopies
The sensitivity (S) of the sensors can be calculated using the following formula [41]: S = |∆ |, where ∆R is the change in electrical resistance due to the presence of the gas, Based on scanning electron microscopy (SEM) and X-ray diffraction (XRD) results, we concluded that the heat treatment did not change the phase and morphology of o- and m-MoO3
Summary
Molybdenum oxides (MoO3) are considerable materials among molybdenum (Mo) compounds due to their excellent physical and chemical properties. Neither the different reaction times nor the role of CTAB (cetyltrimethylammonium bromide) and CrCl3 additives have been studied yet at temperatures above 200–210 ◦C in the hydrothermal synthesis of MoO3. We studied different reaction parameters in the hydrothermal synthesis of MoO3, focusing on the changes in the obtained crystalline phases and morphologies. The as-prepared different MoO3 allotropes were further studied by energy dispersive X-ray spectroscopy (EDX) and specific surface area measurements, by Fourier transformed infrared (FT-IR), Raman, and ultraviolet-visible (UV–Vis) spectroscopies. Their band gap energies were determined and their gas sensing properties were analyzed towards H2
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