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Molten-Salt Synthesis of Complex Metal Oxide Nanoparticles

Published: October 27th, 2018



1Department of Chemistry, University of Texas Rio Grande Valley, 2Radiochemistry Division, Bhabha Atomic Research Centre, 3School of Earth, Environmental, and Marine Sciences, University of Texas Rio Grande Valley

Here, we demonstrate a unique, relatively low-temperature, molten-salt synthesis method for preparing uniform complex metal oxide lanthanum hafnate nanoparticles.

The development of feasible synthesis methods is critical for the successful exploration of novel properties and potential applications of nanomaterials. Here, we introduce the molten-salt synthesis (MSS) method for making metal oxide nanomaterials. Advantages over other methods include its simplicity, greenness, reliability, scalability, and generalizability. Using pyrochlore lanthanum hafnium oxide (La2Hf2O7) as a representative, we describe the MSS protocol for the successful synthesis of complex metal oxide nanoparticles (NPs). Furthermore, this method has the unique ability to produce NPs with different material features by changing various synthesis parameters such as pH, temperature, duration, and post-annealing. By fine-tuning these parameters, we are able to synthesize highly uniform, non-agglomerated, and highly crystalline NPs. As a specific example, we vary the particle size of the La2Hf2O7 NPs by changing the concentration of the ammonium hydroxide solution used in the MSS process, which allows us to further explore the effect of particle size on various properties. It is expected that the MSS method will become a more popular synthesis method for nanomaterials and more widely employed in the nanoscience and nanotechnology community in the upcoming years.

Molten-salt synthesis (MSS) involves the use of a molten salt as the reaction medium for preparing nanomaterials from their constituent precursors. The molten salt acts as the solvent and facilitates the enhanced reaction rate by increasing the contact area between reactants and their mobility. The choice of molten salts is of paramount importance for the success of the MSS method. The salt must meet some important quality requirements such as low melting point, compatibility with reacting species, and optimum aqueous solubility. Molten salt has been used previously to enhance the rate of solid-state reactions; however, in a flux system, only a small amount of molten ....

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1. Preparation of Single-Source Complex Precursor via a Coprecipitation Route

  1. Preparation of lanthanum and hafnium precursor solution
    1. Measure 200 mL of distilled water in a 500 mL beaker and start stirring at 300 rpm.
    2. Dissolve lanthanum and hafnium precursors in the stirring water [i.e., 2.165 g of lanthanum nitrate hexahydrate (La(NO3)3•6H2O) and 2.0476 g of hafnium dichloride oxide octahydrate (HfOCl2•8H2.......

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The as-synthesized La2Hf2O7 NPs may exist in the ordered pyrochlore phase. However, chemical doping, pressure, and temperature could modify the phase to defect fluorite. It is possible for our material to have multiple phases; however, here we focus only on the pyrochlore phase for simplicity. X-ray diffraction (XRD) and Raman spectroscopy have been used to systematically characterize their phase purity, structure, and phase. The crystalline size can .......

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The chart in Figure 4 provides several reliable controlling factors of the MSS method and accounts for alternative pathways to fine-tune the features of synthesized nanomaterials. In addition, it helps identify critical steps in the MSS process.

Figure 4
Figure 4: Flowchart of the critical steps of MSS indi.......

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The authors thank the financial support provided by the National Science Foundation under CHE (award #1710160) and the USDA National Institute of Food and Agriculture (award #2015-38422-24059). The Department of Chemistry at the University of Texas Rio Grande Valley is grateful for the generous support provided by a Departmental Grant from the Robert A. Welch Foundation (Grant No. BX-0048). S.K.G. would like to thank the United States-India Education Foundation (USIEF) and the Institute of International Education (IIE) for his Fulbright Nehru Postdoctoral Fellowship (award #2268/FNPDR/2017).


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Name Company Catalog Number Comments
Acetone, ACS, 99.5+% Alfa Aesar 67-64-1 Dried over 4A sieves
Hafnium dichloride oxide octahydrate, 98+% (metals basis excluding Zr), Zr <1.5% Alfa Aesar 14456-34-9 Hygroscopic
Lanthanum(III) nitrate hexahydrate Aldrich 10277-43-7 Hygroscopic
Potassium nitrate, ReagentPlus R, ≥99.0% Sigma-Aldrich 7757-79-1 Hygroscopic
Sodium nitrate, ReagentPlus R, ≥99.0% Sigma-Aldrich 7631-99-4
Ammonium hydroxide, 28% NH3, NH4OH Alfa Aesar 1336-21-6
Filter paper, P8 grade Fisherbrand

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