Preparation Of Inorganic Functional Nanomaterials By Molten Salts Method

This paper is focused on the controlled synthesis of inorganic functional nanomaterials by Molten Salts Method, including controlled synthesis of 0D, 1D and 2D nanocrystals, and the study on their novel properties. The contents are related to preparation of nanoparticles dispersed in the water, synthesis of multi-metal oxides such as perovskite and spinel, their formation mechanism and so on. We also investigate the magnetic, electrochemical, and fluorescent properties of the sample we obtained, and have received satisfied results. At the same time, these novel properties are related with the structure, shape and size of nanocrystals. So, we believe that all of these must have help to preparation of nanomaterials by molten salt method, and have help to study the effect of molten salt system on the properties of nanomaterials.1. La_1-xSr_xMnO₃ (x=0, 0.3, 0.5, 0.7) Nano-particles Nearly-Freestanding in Water: Preparation and Magnetic PropertiesA series of crystalline La_1-xSr_xMnO₃ (x = 0,0.3,0.5,0.7) nanoparticles without agglomeration in water through the molten salt method following reaction in 2-pyrrolidone have been prepared. We use NaNO₃ and KNO₃ as molten salt, La (NO₃)₃·6H₂O, Sr(NO₃)₂, Mn(NO₃)₂·4H₂O as reagents to react for 3hs. The resulting molten solid was placed with hot 2-pyrrolidone at 240℃, and the precipitate was separated by centrifugation, washed by ethanol and dried. XRD patterns show that the prepared LaMnO₃ is cubic perovskite structure, and with increasing content of doped Sr in the samples, their structures transfer from cubic to Orthorhombic gradually. All the La_1-xSr_xMnO₃ (x = 0, 0.3, 0.5, 0.7) nanoparticles can be dispersed in the water without precipitate in 1-2 weeks. The representative TEM images reveal the LSMO particles have the size of ca. 20±5 nm, which is roughly closed to the calculated results from the XRD patterns. All the samples appear almost aggregate-free spherical particles in water. The EDS analyses indicate that all samples are consistent with their element signals and stoichiometry as expected within the error. HR-TEM images show an organic layer coating on the nanoparticle surface and the high crystal under surface. The organic compounds on the as-prepared LSMO nanoparticles' surface were studied by the thermal gravity (TG), FT-IR analyses. We confirm the organic composition on the LSMO particles is the nitryl-compound (C₃H₇NO₂), indicating 2-pyrrolidone is decomposed during the refluxing process. This nirtyl-compound might be coordinated to the nanoparticle by two styles. Besides this, the reasonable process of reactions is assumed: at the first, the LSMO nanopaticles are formed in the molten salt and the anions such as NO₃^- adsorbed on the nanoprticle's surface; then, when the obtained mixed solid is placed in the 2-pyrrolidone, the molten salts are dissolved and the organic solvents gradually decompose to form the nirtyl-compound, which replaces the anions around the nanoparticle's surface. These organic compounds around nanoparticle not only reduce the surface energy of the nanoparticle but also prevent them form aggregating in polar solvents such as water.The magnetic properties of the as-prepared nanoprticles were investigated. The results show that their Curie temperatures (Tc) are lower than their bulk materials and increase with the doped Sr atom enhancement. At the same time, the hysteresis loops become more and more asymmetric with the increase in Sr doping content, meaning exchange bias occurs, which is rare in pure nanoparticles.2. Facile preparation and electrochemical properties of cubic-phase Li₄Mn₅O₁₂ nanowiresAs the molten salts, LiNO₃ and NaNO₃ with the molar ratio of 1: 2 are heated to 480℃. Quantificational MnSO₄ is put into the molten salts and reacted for 10 min. Then they are cooled to room temperature, and dried after washed by dehydronium water for several times. The XRD pattern indicates that the product is high crystalline Li₄Mn₅O₁₂ with cubic spinel structure. Representative TEM image shows that they are uniform nanowires with the diameter of 20-30 nm, the length of several micrometers and the aspect ratio of about 100. These characters are accorded with what observed under FE-SEM. EDX shows the sample is consist with Mn and O elements without any other impurity such as Na, K, and S, which confirms its purity too. The photo of HRTEM and SAED illustrate its high crystalline and the growth direction of [110]. Based on the XPS, the molar ratio of 0 to Mn is calculated to be ca.2.38, which is closed to the theoretical value (2.40) of the spinel Li₄Mn₅O₁₂. It is estimated from the result that the molar ratio of Mn(IV) to all the Mn elements in the product is ca.0.97. The formation process of the spinel Li₄Mn₅O₁₂ is tracked by the time-dependent experiments. The experiments indicate that the product is mainly cubic Mn₂O₃ at the first, and following a few of spinel Li₄Mn₅O₁₂ with the reaction performed. So, we propose the reasonable mechanism of the progress that the insertion of Li⁺ into Mn₂O₃ and the oxidation of Mn³⁺ to Mn⁴⁺ are simultaneously carried out. The reaction takes place in two discrete steps: oxidation of Mn²⁺ to Mn³⁺ and Li⁺ insertion. The Mn²⁺ cations were firstly oxidized to Mn₂O₃ with elimination of NO₂ and O₂, and then the Li⁺ cations inserted into Mn₂O₃ to form Li₄Mn₅O₁₂, accompanying with the oxidation of Mn³⁺ to Mn⁴⁺. To study the morphology evolution of the nanowores, the morphology evolution of the product during the reaction process is tracked by TEM. TEM images show the flake-like product at first and SAED illustrates it is Mn₂O₃ which is accorded with the XRD result. With the reaction proceeding, the Li⁺ cations insert into the crystal lattice of Mn₂O₃ to form the Li₄Mn₅O₁₂ on the lamellas' edges, and then the lamellae have curled to form the tubular and wire-like morphologies, the SAED patterns demonstrate the coexistance of Mn₂O₃ and Li₄Mn₅O₁₂ phases at this stage. When the reaction time lasts for 10 min, all the lamellas transform to the wire-like morphology and the SAED pattern illustrates that the product is the pure Li₄Mn₅O₁₂. So, the results confirm that a lamella-rolling mechanism is reasonable for the formation of Li₄Mn₅O₁₂ nanowires.To study the electrochemical properties, we assemble a lithium ion second cell in which Li₄Mn₅O₁₂ nanowires as working electrode and lithium metal as opposite electrode. The as-prepared Li₄Mn₅O₁₂ nanowires show preferable electrochemical properties, high charge/ discharge capacity and well CE. So, this work should be beneficial to the study safe, steady and preferable lithium ion second cells.3. Size-controlled synthesis LnF₃: Eu nanoplates by molten salt and luminescent property NaNO₃ and KNO₃ (molar ratio 2:1) as molten salts, La(NO₃)₃·6H₂O, Eu₂O₃ and NH₄F are mixed in the crucible at 350℃C to react for 1 hour. After the system cooled to room time, the as-prepared solids are washed by distilled water and dried. The size of nanoplate can be controlled by changing the mass of NH₄F. The purity and structure of products are studied by XRD, and it shows LaF₃: Eu phase without impurity. The shape and size of as-prepared samples are observed by FE-SEM and TEM, which illustrate they are nanoplate of hexagon and the sizes are changed under different reaction conditions.The luminescent properties of LaF₃: Eu samples with different doped Eu are studied in our work. The results illustrate that the intensity of emission increases when the content of Eu changes from 10% to 20%, which is caused by the increasing of luminescent center. However, the intensity of emission falls down as the content of Eu changes from 20% to 30%, which is caused by luminescent quenching for excessive Eu. Comparing to the I_(595nm)/I_(617nm) in the spectrums of different samples, the symmetry in local environments around Eu³⁺ ions gradually grow with the increasing Eu content. The luminescence of different size nanoplate shows that some emitting peaks happen broad-phenomena and the symmetry in local environments around Eu³⁺ ions gradually fall down with the increasing Eu content with the reducing size of nanoplates.