Hydrothermal Synthesis and Gas Sensing of Monoclinic MoO(3) Nanosheets

Effects of different reaction parameters in the hydrothermal synthesis of molybdenum oxides (MoO(3)) were investigated and monoclinic (β-) MoO(3) 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 CrCl(3) additives were also tested. Both the reaction temperatures and durations played a significant role in the formation of the products. At 90 °C, h-MoO(3) was obtained, while at 240 °C the orthorhombic (α-) MoO(3) 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-MoO(3), but 6 h led to pure α-MoO(3). With CTAB the product was bare o-MoO(3), however, when CrCl(3) was applied, pure metastable m-MoO(3) formed with the well-crystallized nanosheet morphology. The gas sensing of the MoO(3) polymorphs was tested to H(2), which was the first such gas sensing study in the case of m-WO(3). Monoclinic MoO(3) was found to be more sensitive in H(2) sensing than o-MoO(3). This initial gas sensing study indicates that m-MoO(3) has promising gas sensing properties and this MoO(3) polymorph is promising to be studied in detail in the future.