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Synthesis of 2D metal oxides and hydroxides - high-yield, efficient, fast and low-cost

(Nanowerk Spotlight) As two-dimensional (2D) materials gain more and more importance – thanks to their exotic electronic properties and abundant active sites – the development of high-yield, efficient, fast and low-cost synthesis methods to advance these materials from the laboratory to industry has become an urgent issue.
"Previous research has shown that ions always play a key role in the synthesis of 2D materials," Jun Zhou, a professor at Huazhong University of Science and Technology, Wuhan National Laboratory for Optoelectronics, tells Nanowerk. "However, it should be noted that when the synthesis process occurs in solution, desolvation is a necessary step because ions are in the solvated state in solution. Unfortunately, though, the energy consumption for desolvation increases the overall activation energy, thus limiting the reaction rate."
In new work, a team led by Zhou has developed a general and rapid molten salts method (MSM) – widely studied for the synthesis of nanomaterials, such as graphene and perovskite – that can synthesize various ion-intercalated 2D metal oxides and hydroxides, such as cation-intercalated manganese oxides, cation-intercalated tungsten oxides, and anion-intercalated metal hydroxides.
Proposed mechanism of the molten salts synthesis of 2D ion-intercalated metal oxides and hydroxides
Proposed mechanism of the molten salts synthesis of 2D ion-intercalated metal oxides and hydroxides. MOx or M(OH)x (M represents metal) molecules were formed when the metal ions from the precursor reacted with nitrate or H2O. During the self-assembly of these MOx or M(OH)x molecules, ions from the molten salts rapidly arranged in a 2D plane and guided the growth of the 2D structure. (© Nature Publishing Group) (click on image to enlarge)
The most significant result of this work, reported in Nature Communications ("Rapid mass production of two-dimensional metal oxides and hydroxides via the molten salts method"), is the ability to obtain high quality 2D materials within just 1 minute by using very cheap and common source materials.
This is a big step towards the commercialization of 2D materials.
"The key feature of our method is the direct use of naked ionized ions without hydration in the molten state salt to quickly induce the growth of 2D metal oxides and hydroxides," explains Zhou. "In our technique, by adding precursors into the low-cost molten salts for only 1 minute, we could obtain high-yield 2D materials simply by washing the salts. Even without centrifugation or sedimentation, we do not observe particles or nanowires in the final products."
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SEM image of Na0.55Mn2O4•1.5H2O. (Image: Wuhan National Laboratory for Optoelectronics) (click on image to enlarge)
To demonstrate the potential applications of their 2D ion-intercalated metal oxides and hydroxides in energy storage, the team printed a flexible solid-state supercapacitor based on A4 paper@CNT@Na2W4O13 electrodes.
According to the researchers, these supercapacitors show good electrochemical performance with an excellent rate capability, demonstrating the potential applications of these 2D ion-intercalated metal oxides and hydroxides in energy storage and beyond.
This video demonstrates the process for synthesis of two-dimensional metal oxides and hydroxides via the molten salts method. (Video: Wuhan National Laboratory for Optoelectronics)
"Although we only reported eight 2D ion-intercalated metal oxides and hydroxides, the versatility of this molten salt synthesis process gives us confidence that reasonably tuning the precursors and molten salts will allow various 2D oxides and hydroxides to be synthesized and the scope of the accessible oxides and hydroxides to expand to other 2D materials with attractive properties," concludes Zhou.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk
 

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