Preparation and photoelectric properties of semiconductor MoO2 micro/nanospheres with wide bandgap

Semiconductor molybdenum dioxide (MoO2) micro/nanospheres with a band gap of 3.85 eV were synthesized by the hydrothermal method. Their structures, morphologies and photoelectric properties were carefully studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis, photoluminescence (PL) photometer and variable temperature resistance tester. The results show that MoO2 micro/nanostructures are spheral with the diameters ranging from tens to hundreds nanometers and their dispersivity is good. The UV–vis spectrum at room temperature demonstrates that the samples are direct-gap semiconductor and the absorption peak is at about 300 nm. Especially, the band-gap is estimated to be approximately 3.85 eV, which makes it different from all previously reported metallic MoO2 materials. The PL observation exhibits fluorescent emission peaks at 340 nm (3.65 eV), 355 nm (3.49 eV), 370 nm (3.35 eV), 395 nm (3.13 eV) and 407 nm (3.04 eV) when the 300 nm light excitation is applied at room temperature and fluorescent emissions originate from the defects of the samples and the electron transition between valence band and conduction band, which confirmed the samples wide band gap semiconductor. The electrical resistivity of the as-prepared MoO2 materials was measured in the temperature range from 77 to 300 K, which shows that the resistivity decreases with the increase of testing temperature, which indicates that the samples have semiconductor characteristics again and a low resistivity (0.2 mΩ cm at 300 K). On the results obtained, theoretical explanations are proposed.