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In situ Precursor-Template Route to Semi-Ordered NaNbO 3 Nanobelt Arrays

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ABSTRACT

We exploited a precursor-template route to chemically synthesize NaNbO3 nanobelt arrays. Na7(H3O)Nb6O19·14H2O nanobelt precursor was firstly prepared via a hydrothermal synthetic route using Nb foil. The aspect ratio of the precursor is controllable facilely depending on the concentration of NaOH aqueous solution. The precursor was calcined in air to yield single-crystalline monoclinic NaNbO3 nanobelt arrays. The proposed scheme for NaNbO3 nanobelt formation starting from Nb metal may be extended to the chemical fabrication of more niobate arrays.

No MeSH data available.


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Structural transformation from Lindquist precursor to NaNbO3.
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Figure 7: Structural transformation from Lindquist precursor to NaNbO3.

Mentions: Those Lindquist ions in Na7(H3O)Nb6O19·14H2O extend along the [001] direction and build the backbone of the 1D structure. In subsequent calcination stage, the Lindquist units with the edge-sharing Nb–O polyhedra are ruptured to form more stable corner-sharing polyhedron groups. As the transitions mainly involve the breakage of chemical bonds such as Nb–O and rotation of NbO6 octahedra, the temperature as high as 500–550°C is needed to drive rate-limiting diffusion in solid-state phase conversion. It is well known that during the thermal dehydroxylation process, the water molecules are formed and lost between the two adjacent layers of hydroxyl ions. Under the high temperature, the dehydration process occurs quickly (as evidenced by the TG curve in Figure 5a) producing a great many atomic vacancies, which results in low thermal stability of Na7(H3O)Nb6O19·14H2O in the state. Therefore, to minimize the overall system energy and stability the crystal structure, the diffusion of Nb, O, and Na atoms is accelerated. During the structural transformation process, owing to the conventional six-coordinate microstructure, niobium atoms work as central atoms and coordinate with oxygen in the 1D precursor, then many small networks comprising corner-sharing NbO67- units generate as the crystalline nuclei. With the heat treatment process proceeds, a whole rearrangement atomic network is built on pre-existing nuclei in the restricted space of the precursor. That is, a steadier framework of corner-sharing NbO6 octahedra with Na atoms occupying the cavities is generated across the whole volume (Figure 7). This dehydration process requires long-range diffusion of Nb and Na atoms, and this reaction cannot be topochemical. However, in the heat treatment condition, sudden collapse of the precursor nanobelt can be avoided. As a result, the formation of NaNbO3 nanobelt derived from the atomic rearrangement in the crystal structure of Na7(H3O)Nb6O19·14H2O is observed during the decomposition process. It is noteworthy to point out, because of the nanosized diffusion distances for atoms moving between the contact areas, that the wire-like aggregates of Na7(H3O)Nb6O19·14H2O nanobelts can be converted into single-crystalline NaNbO3 nanobelts conveniently under high temperature. Moreover, the high temperature also brings lots of thermal defects in original sublattices, some of which may expand to the surface finally and result in some pits (see the area indicating by arrowhead in Figure 6c).


In situ Precursor-Template Route to Semi-Ordered NaNbO 3 Nanobelt Arrays
Structural transformation from Lindquist precursor to NaNbO3.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211187&req=5

Figure 7: Structural transformation from Lindquist precursor to NaNbO3.
Mentions: Those Lindquist ions in Na7(H3O)Nb6O19·14H2O extend along the [001] direction and build the backbone of the 1D structure. In subsequent calcination stage, the Lindquist units with the edge-sharing Nb–O polyhedra are ruptured to form more stable corner-sharing polyhedron groups. As the transitions mainly involve the breakage of chemical bonds such as Nb–O and rotation of NbO6 octahedra, the temperature as high as 500–550°C is needed to drive rate-limiting diffusion in solid-state phase conversion. It is well known that during the thermal dehydroxylation process, the water molecules are formed and lost between the two adjacent layers of hydroxyl ions. Under the high temperature, the dehydration process occurs quickly (as evidenced by the TG curve in Figure 5a) producing a great many atomic vacancies, which results in low thermal stability of Na7(H3O)Nb6O19·14H2O in the state. Therefore, to minimize the overall system energy and stability the crystal structure, the diffusion of Nb, O, and Na atoms is accelerated. During the structural transformation process, owing to the conventional six-coordinate microstructure, niobium atoms work as central atoms and coordinate with oxygen in the 1D precursor, then many small networks comprising corner-sharing NbO67- units generate as the crystalline nuclei. With the heat treatment process proceeds, a whole rearrangement atomic network is built on pre-existing nuclei in the restricted space of the precursor. That is, a steadier framework of corner-sharing NbO6 octahedra with Na atoms occupying the cavities is generated across the whole volume (Figure 7). This dehydration process requires long-range diffusion of Nb and Na atoms, and this reaction cannot be topochemical. However, in the heat treatment condition, sudden collapse of the precursor nanobelt can be avoided. As a result, the formation of NaNbO3 nanobelt derived from the atomic rearrangement in the crystal structure of Na7(H3O)Nb6O19·14H2O is observed during the decomposition process. It is noteworthy to point out, because of the nanosized diffusion distances for atoms moving between the contact areas, that the wire-like aggregates of Na7(H3O)Nb6O19·14H2O nanobelts can be converted into single-crystalline NaNbO3 nanobelts conveniently under high temperature. Moreover, the high temperature also brings lots of thermal defects in original sublattices, some of which may expand to the surface finally and result in some pits (see the area indicating by arrowhead in Figure 6c).

View Article: PubMed Central - HTML - PubMed

ABSTRACT

We exploited a precursor-template route to chemically synthesize NaNbO3 nanobelt arrays. Na7(H3O)Nb6O19·14H2O nanobelt precursor was firstly prepared via a hydrothermal synthetic route using Nb foil. The aspect ratio of the precursor is controllable facilely depending on the concentration of NaOH aqueous solution. The precursor was calcined in air to yield single-crystalline monoclinic NaNbO3 nanobelt arrays. The proposed scheme for NaNbO3 nanobelt formation starting from Nb metal may be extended to the chemical fabrication of more niobate arrays.

No MeSH data available.


Related in: MedlinePlus