Single-crystalline MoO3 nanoplates: topochemical synthesis and enhanced ethanol-sensing performance

Molybdate-based inorganic–organic hybrid disks with a highly ordered layered structure were synthesized via an acid–base reaction of white molybdic acid (MoO3·H2O) with n-octylamine (C8H17NH2) in ethanol at room temperature. The thermal treatment of the as-obtained molybdate-based inorganic–organic hybrid disks at 550 °C in air led to formation of orthorhombic α-MoO3 nanoplates. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal analysis (TG–DTA), Fourier-transform infrared (FT–IR) spectra, Raman spectra, and a laser-diffraction grain-size analyzer were used to characterize the starting materials, the intermediate hybrid precursors and the final α-MoO3 nanoplates. The XRD, FT–IR and TG–DTA results suggested that the molybdate-based inorganic–organic hybrid compound, with a possible composition of (C8H17NH3)2MoO4, was of a highly ordered lamellar structure with an interlayer distance of 2.306(1) nm, and the n-alkyl chains in the interlayer places took a double-layer arrangement with a tilt angle of 51° against the inorganic MoO6 octahedra layers. The SEM images indicated that the molybdate-based inorganic–organic hybrids took on a well-dispersed disk-like morphology, which differed distinctly from the severely aggregated morphology of their starting MoO3·H2O powders. During the calcining process, the disk-like morphology of the hybrid compounds was well inherited into the orthorhombic α-MoO3 nanocrystals, showing a definite plate-like shape. The α-MoO3 nanoplates obtained were of a single-crystalline structure, with a side-length of 1–2 μm and a thickness of several nanometres, along a thickness direction of [010]. The above α-MoO3 nanoplates were of a loose aggregating texture and high dispersibility. The chemical sensors derived from the as-obtained α-MoO3 nanoplates showed an enhanced and selective gas-sensing performance towards ethanol vapors. The α-MoO3 nanoplate sensors reached a high sensitivity of 44–58 for an 800 ppm ethanol vapor operating at 260–400 °C, and their response times were less than 15 s.