The Controllable Synthesis And Photocatalytic Properties Study Of Samarium Tungstates

Due to their unique 4f electron orbits and interesting crystal structures, rare-earth tungstates are widely applied in various fields, such as optical, electrical, magnetical and photocatalytic applications, receving much attention. This study focused on the crystal- and micro- structure controllable synthesis of a typical rare-earth tungstate, samarium tungstate. X-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), diffuse reflection spectrum(DRS) and other techniques were employed for characterization. Also photocatalytic properties were explored. The main research contents and results are as follows:(1) Five samarium tungstates with different crystal structures were successfully synthesized by adjusting technological factors via hydrothermal or microwave-hydrothermal method using Na2WO4·2H2O and Sm Cl3·6H2O as starting materials. The five samarium tungstates of various phases are Sm2W4O14(OH)2(H2O)2, Sm WO4(OH), Sm2W3O12, Na0.77Sm3.08W5O20 and Sm2W2O9. Chemical mechanism analysis reveals that the hydrolysis degree and different existing forms of wolframate radical are strongly responsible for the phase change. Decomposition during sintering is another cause for phase change. The influences of various technological factors on the crystal structure of samarium tungstates are in different levels: p H ≥ sintering >samarium-tungsten rario> microwave > concentration > reaction time.(2) By adjusting technological parameters and selecting synthesis routes, the morphologies of samarium tungstates were successfully modulated without any surfactant. Results show that hydrothermal product Sm2W4O14(OH)2(H2O)2 tends to have a spatial flower-like structure; Sm WO4(OH) presents a sheet-like morphology with irregular profile; Sm2W2O9 presents a mixed morphology of dodecahedrons and rhombohedrons; while hydrothermal-sintering products Sm2W3O12 and Na0.77Sm3.08W5O20 remain flower-like and sheet structures from their precursors respectively. Most of the above products can be modulated to present a cluster microstructure, which is found to be beneficial to material’s photocatalytic property.(3) Characterization of semiconductor features of 5 samarium tungstates were performed by electrochemical and optical methods. Results show that all 5 samarium tungstates are n-type semiconductors with broad band gaps: Sm2W3O12(3.33 e V), Na0.77Sm3.08W5O20(3.86 e V), Sm2W4O14(OH)2(H2O)2(4.27 e V), Sm2W2O9(4.32 e V) and Sm WO4(OH)(4.64 e V). The positions of conduction bands of these materials are alike, while the positions of valence bands are obviously different.(4) Experiment results indicate that the photocatalytic properties of the 5 samarium tungstates in squence are Sm2W3O12 > Sm2W4O14(OH)2(H2O)2 > Na0.77Sm3.08W5O20 > Sm2W2O9 > Sm WO4(OH). This order is similar to their Eg order. Except for Sm2W2O9-Zn S, other four Zn S-samarium tungstates are all effective co-photocatalytic systems with notable photocatalytic activity improvements. Reinforced photo absorption, promoted photo-generated carries separation and de-ethylation reaction are the major reasons for the photocatalytic activity enhancement.