Photocatalytic Activity and Humidity Sensor Studies of Magnetically Reusable FeWO <sub/>4</sub> –WO <sub/>3</sub> Composite Nanoparticles

摘要： Different mole ratios of (8:2, 6:4, 4:6 and 2:8) iron tungstate–tungsten trioxide (FeWO4–WO3) composite nanoparticles were synthesized by solid state method. The synthesized composite nanoparticles were characterized by powder X-ray diffraction analysis (XRD), field-emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) studies. The crystalline nature and particle size of the samples were characterized by powder X-ray diffraction analysis (XRD). The morphology was confirmed by field-emission scanning electron microscopy (FE-SEM) analysis and transmission electron microscope (TEM). The energy dispersive X-ray spectroscopy (EDX) proved the purity of nanocomposites. Vibrating sample magnetometer reveals that the sample shows paramagnetic property based on the metal present in the prepared nanocomposites at room temperature. The magnetic property is due to the structural defects rather than the impurity phase. Magnetization saturation value (Ms = 398.7 emu/g) of FWWO-46 composite nanoparticles is high enough to be magnetically removed by applying a magnetic field. The composites were subjected to DC conductance measurement as a function of relative humidity in the range of 5–98%, achieved by different water vapour buffers thermostated at room temperature. The sensitivity factor, Sf = R5%/R98%, where R5% and R98% the values of the resistances measured at different RH respectively, are evaluated in Table I. If the composite has the greater value of Sf, then those materials possess the greater sensitivity towards moisture. The sensitivity factor (Sf) of the prepared composite nanoparticles was FWWO-10 (473), FWWO-82 (209), FWWO-64 (323), FWWO-46 (3956), FWWO-28 (361) and FWWO-01 (373). From this experimental value FWWO-46 exhibit the maximum Sf value of 3956 among the composites. This was due to the presence of more pores and cavities in the morphology of FWWO-46 then the other composite nanoparticles. Meanwhile the composite FWWO-46 can interact with water molecules easily then the others. The absorption and desorption of water molecules vary from the other composites. In the presence of water molecules on the morphology of FWWO-46 shows higher conductivity and higher sensitivity factor (Sf). At low relative humidity, water adsorption on the surface of the sample was likely the dominant factor for electronic conduction. The adsorbed water increases the surface electrical conductivity of the ceramic due to the increased charge carrier, protons in the ceramic/water system. The conductivity was further increased by the presence of pores on the sample surface. In the initial stage of water adsorption, a few water vapour molecules chemisorbed on the grain surface by a dissociative mechanism to form two surface hydroxyls per water molecule. In this chemisorbed layer charge transport occurs by the hopping mechanism. Conduction probably occurs by the Grotthus transport mechanism.