| Molybdenum oxide has superior physicochemical properties and is widely used in the fields of electricity,optics,and photocatalysis.There are three main structures of molybdenum oxide:the orthorhombic phase(α-MoO3),the monoclinic phase(β-MoO3),and the hexagonal phase(h-MoO3).The orthorhombic phase is thermodynamically stable,while the monoclinic and hexagonal phases are thermodynamically metastable.h-MoO3 is susceptible to changes in crystal structure due to environmental factors.Therefore,the study of the structural phase transition of h-MoO3 can provide theoretical guidance for the application of h-MoO3.In this paper,pure phase h-MoO3 was synthesized by hydrothermal method,and the structural phase transition process of molybdenum oxide from hexagonal phase to orthorhombic phase was investigated by in situ Xray diffractometer.The details of the study are as follows:(1)The preparation of hexagonal-phase molybdenum oxide crystals and the structural features of h-MoO3 and α-MoO3.The h-MoO3 crystals were prepared by hydrothermal method,and α-MoO3 crystals were obtained by sintering.The crystal structures of h-MoO3 and α-MoO3 were resolved by X-ray diffraction(XRD),and the morphological differences between the samples were observed by scanning electron microscopy(SEM).The lattice constants of h-MoO3 were obtained as a=10.569 (?),b=10.569 (?),c=3.712 (?),and the space group was P63/m;the lattice constants of α-MoO3 were a=3.954 (?),b=13.825 (?),c=3.694 (?),and the space group was Pbnm.(2)The in-situ investigation of the structural phase transition from h-MoO3 toα-MoO3.Based on the in-situ XRD data,the lattice constant,cell volume,thermal expansion coefficient,phase content,and the variation law of channel radius in hMo03 with temperature were extracted by using the Rietveld refinement method.According to the variation law of lattice constants,it is found that the lattice of hMoO3 expands anisotropically with the increase of temperature,which shows that the lattice constants a and b increase with the increase of temperature,c decreases,and the cell volume increases.The calculated thermal expansion coefficients indicate that the phase transition process of h-MoO3 is a first-order phase transition.The radius of the tunnel structure in h-MoO3 increased continuously with the increase of temperature.The basic process of phase transition was obtained by analyzing the change of atomic spacing combined with the mass loss in thermogravimetric analysis:when the temperature increases,the radius of h-MoO3 channel increases due to the release of the water molecular or other small molecules intercalated in the tunnel.When the phase transition occurs,the channel collapses into[MoO6]octahedral chains and then reconstructs into α-MoO3.(3)The bandgap and the photocatalysis of h-MoO3 and α-MoO3 mixed crystals.The diffuse reflectance spectra showed that values of the bandgap are 2.71,2.46,2.15 eV for the hexagonal h-MoO3,the orthorhombic α-MoO3,and the mixture phase,respectively.The photocatalytic experiments indicate that the photocatalytic performance of the mixed crystals with a reaction rate constant of 0.0184 min-1,is better than the pure h-MoO3 phase and the pure α-MoO3 phase.The reasons for the higher catalytic efficiency of the mixed crystals are as follows:first,the specific surface area of the sample becomes larger due to the change of crystal morphology,which increases the number of catalytic active sites on the surface;second,the formation of heterojunctions makes it easier to generate photogenerated electron-hole pairs;third,the dislocations and defects generated during the phase transition process are beneficial to the catalytic efficiency of the samples. |
PDF Full Text Request