Preparation, Structure And Optical Properties Of Sm₂O₃Crystallites Synthesized By Hydrothermal And Solvothermal Process

As a typical kind of metal oxides with4f electronic structure, three crystalphases are found in Sm₂O₃materials, of which the band gaps range from4to6eV. Sm₂O₃is a multiphase oxide which is found transiting from the monoclinicphase to cubic phase at850℃and then to hexagonal phase at2000℃. Theyhave been deeply studied for their high resistivity, dielectric constant chemicalstability and thermal stability with excellent electrical, optical and magneticproperties, which are considered to be widely used in fields including ceramiccapacitor, samarium metal preparation, automobile tail gas treatment, catalystsand medical treatments; Besides, Sm₂O₃materials are also found to have nuclearproperties to be used as structure, shielding and controlling materials of theatomic furnaces. Herein, hydrothermal and solvothermal methods with varioustemplates were employed to prepare Sm₂O₃microcrystals, of which the Smsources were SmCl₃·6H₂O and Sm(NO₃)₃·6H₂O. The synthesis process weresystematically investigated by using XRD, SEM and UV-Vis to characterize theproducts crystal structure and corresponding properties; The influences ofhydrothermal temperature, reaction time, precursor concentration, pH value ofthe precursor solutions, different Sm sources, templates, reaction solvents andratio between solvent and templates were completely investigated to control thecrystal structure and morphologies of the calcined products. The following arethe main research contents and results: Pure Sm₂O₃particles were successfully prepared by using SmCl₃·6H₂O andSm(NO₃)₃·6H₂O as the start materials. By changing the reaction temperature,precursor concentration, precursor solution pH value, reaction time and Smsources, the synthesis process were systematically controlled to study the productphase, morphologies and reaction mechanisms. XRD and SEM were used for thecharacterization. Results show that under the optimum reaction conditions (Smsources: SmCl₃·6H₂O; Reaction temperature:200oC; Precursor concentration:0.4M; pH value:9; Reaction time:48h.), monoclinic Sm₂O₃particles with goodcrystallinity and dispersing properties are obtained. When the precursor ischanged concentration to1.0M, the as-obtained products present a crystal phaseof cubic Sm₂O₃with nanorod-like microstructure. By studying the influences ofdifferent reaction conditions, it is found that the calcined product phase andmorphology are greatly influenced by the hydrothermal products. Differentreaction temperature, precursor concentrations, pH values and Sm sources arefound to influence the product crystal structure; while the precursorconcentration, Sm sources and reaction times are found to have great effects onthe products morphologies.By studying influences of different organic solvent ratio on the productphase and morphology, the Sm₂O₃nanoparticles were systematically preparedwith controllable phases and morphology to investigate the Sm₂O₃reactionmechanism. Results show that monoclinic Sm₂O₃particles could be obtained indifferent solvent conditions with obvious morphology changes. It is found thatthe crystal orientation of the as-obtained product decrease clearly in isopropylalcohol system. Same results were found in both ethylene glycol and alcoholsystem. Besides, cubic Sm₂O₃particles could be obtained in pure alcoholcondition with high productivity. This phase could be also found in mixture ofisopropyl alcohol and water system, rather than in ethylene glycol, which suggestthat the cubic Sm₂O₃possess a metastable crystal structure with high sensitivityto solvent content. Comparing the different properties of these solvents suggeststhat the polarity and the molecular hindering effect of different solvents may bethe main influence factors when changing product phases and crystallizations.The influences of different hydro/solvothermal templates on the productphase and morphology were investigated. Aiming at the differences of the reaction mechanism between hydrothermal and solvothermal system, differenttemplates were employed to control the size and shape of the products. Theresults show that, under hydrothermal conditions, trisodium citrate, PVP,urotropin and oxalic acid may give rise to the formation of cubic Sm₂O₃phasewith low growth orientation, spheres-like and irregular particle morphologies.However, CTAB, EDTA and NaNO₃lead to the formation of monoclinic Sm₂O₃rods, which diameter and length that varies with different templates. Theproducts from solvothermal conditions are found more sensitive to the change oftemplates, which exhibit various morphologies: Mixed short rod and hexagonalsheet-like structure with CTAB as the template; Uniform spheres-likenanoparticles with carboxymethylcellulose sodium; Mixed rods andirregularly-shaped particles with PEG-4000and oleic acid as the templates.According to these changes of product phase and morphology, the influences ofthe templates were analyzed to propose possible growth mechanisms in differentconditions.The as-obtained Sm₂O₃particles with different morphologies werecharacterized by UV-vis absorption spectra to calculate their corresponding bandgaps. The results show that the obtained products under hydrothermal conditionscould have high absorption in ultraviolet range of200—250nm, where thehighest peak were found at230nm. Besides, a weak absorption peak could alsobe found at410nm to suggest possible optical properties in visible-light range.The products prepared under solvothermal conditions exhibit high ultravioletabsorption peaks at the range from200to280nm with the highest peaks at230—250nm. Moreover, another weak visible light absorption peak could be foundat410nm. The corresponding band gaps of these hydro/solvothermal-treatedparticles present various value from4.820eV to4.928eV, which is suggested tobe related with their different sizes and microstructures. These differences arebelieved to results in the change of quantum size effect and microscopic stress ofthe products to show different optical properties.