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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.

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a Weight change and heat flow recorded for Na7(H3O)Nb6O19·14H2O nanobelts. b XRD pattern of NaNbO3 nanobelts (a) and the standard pattern of bulk NaNbO3 (b). c UV–Vis spectra of (a) Na7(H3O)Nb6O19·14H2O nanobelts, (b) NaNbO3 nanobelts, and (c) bulk NaNbO3. d Room temperature PL spectra of NaNbO3 nanobelts, the inset shows an enlarged spectrum.
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Figure 5: a Weight change and heat flow recorded for Na7(H3O)Nb6O19·14H2O nanobelts. b XRD pattern of NaNbO3 nanobelts (a) and the standard pattern of bulk NaNbO3 (b). c UV–Vis spectra of (a) Na7(H3O)Nb6O19·14H2O nanobelts, (b) NaNbO3 nanobelts, and (c) bulk NaNbO3. d Room temperature PL spectra of NaNbO3 nanobelts, the inset shows an enlarged spectrum.

Mentions: The thermal decomposition process of Na7(H3O)Nb6O19·14H2O is shown in Figure 5a. As seen in TG curve, there is only one evident step involving dehydration. The weight of the sample significantly decreases in the temperature range of 80~290°C. Furthermore, between 290 and 515°C, the mass loss becomes quite slow and ceases at higher temperature (around 515°C). The total mass loss is about 9%, slightly smaller compared with standard value (11.3%). It is possible that some impurities, such as the microparticles below the nanobelt arrays, bring the difference. DSC plot for the decomposition recorded in nitrogen gas shows two peaks: one is endothermic event corresponding to the rapid release of H2O, and the other is exothermic peak at around 490°C corresponding to the transformation into NaNbO3 phase, which can be completed at around 515°C. Thermal decomposition of the precursor nanobelts under normal atmospheric conditions gives rise to the formation of a pure monoclinic NaNbO3 phase. As indicated in the Figure 5b, all the peaks in XRD pattern can be indexed well as the pure phase (JCPDS card no. 74-2441). During the thermal conversion process, NbO6 octahedra change from edge-sharing to corner-sharing. This structural difference has an important effect on the gap between the valence band and conductive band of niobates, which can be reflected in the optical absorption spectra of the as-prepared samples. UV–Vis spectra of the precursor, final product, and bulk NaNbO3 are shown in Figure 5c. The precursor has an absorption peak at around 250 nm (a) due to the Lindquist units including six edge-sharing NbO6 octahedra. However, the peak shifts to above 300 nm in final NaNbO3 product (b) that comprises corner-sharing NbO6 octahedra. The change may originate from the difference in Nb–O bond distances in configuration of NbO6 octahedra, which also further confirms that corner-sharing NbO6 octahedra are more stable than edge-sharing ones. When compared with UV absorption peak of bulk NaNbO3 (c) at around 362 nm, the optical absorption edge of NaNbO3 nanobelts shifts towards the lower wavelength, indicating an increase in band gap. Due to the quantum size effect in nanosized semiconductors, the band gap increases when the size of belt-like nanomaterials is decreased, resulting in a blueshift of absorption bands. PL spectra of NaNbO3 nanobelts were also been measured, as shown in Figure 5d. The spectra consist of a UV emission peak and two violet emission peaks in visible region. It is found that the UV peak position is at approximately 368 nm which can be attributed to free exciton emission. Two strong visible peaks dominate the PL spectra, which locate at λ = 421 and 433 nm. The spectra suggest that niobate framework is directly involved in the photoluminescence effect. The heat treatment increases the stability of Nb–O–Nb bonds and introduces different kinds of defect centres acting as traps for charge carriers, therefore increasing the probability for electrons to reach an electron trap, such as oxygen vacancy, and leading to the luminescence effect. The optical properties of NaNbO3 thus open up opportunities for exploiting advanced NaNbO3-based optical nanodevices.


In situ Precursor-Template Route to Semi-Ordered NaNbO 3 Nanobelt Arrays
a Weight change and heat flow recorded for Na7(H3O)Nb6O19·14H2O nanobelts. b XRD pattern of NaNbO3 nanobelts (a) and the standard pattern of bulk NaNbO3 (b). c UV–Vis spectra of (a) Na7(H3O)Nb6O19·14H2O nanobelts, (b) NaNbO3 nanobelts, and (c) bulk NaNbO3. d Room temperature PL spectra of NaNbO3 nanobelts, the inset shows an enlarged spectrum.
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Figure 5: a Weight change and heat flow recorded for Na7(H3O)Nb6O19·14H2O nanobelts. b XRD pattern of NaNbO3 nanobelts (a) and the standard pattern of bulk NaNbO3 (b). c UV–Vis spectra of (a) Na7(H3O)Nb6O19·14H2O nanobelts, (b) NaNbO3 nanobelts, and (c) bulk NaNbO3. d Room temperature PL spectra of NaNbO3 nanobelts, the inset shows an enlarged spectrum.
Mentions: The thermal decomposition process of Na7(H3O)Nb6O19·14H2O is shown in Figure 5a. As seen in TG curve, there is only one evident step involving dehydration. The weight of the sample significantly decreases in the temperature range of 80~290°C. Furthermore, between 290 and 515°C, the mass loss becomes quite slow and ceases at higher temperature (around 515°C). The total mass loss is about 9%, slightly smaller compared with standard value (11.3%). It is possible that some impurities, such as the microparticles below the nanobelt arrays, bring the difference. DSC plot for the decomposition recorded in nitrogen gas shows two peaks: one is endothermic event corresponding to the rapid release of H2O, and the other is exothermic peak at around 490°C corresponding to the transformation into NaNbO3 phase, which can be completed at around 515°C. Thermal decomposition of the precursor nanobelts under normal atmospheric conditions gives rise to the formation of a pure monoclinic NaNbO3 phase. As indicated in the Figure 5b, all the peaks in XRD pattern can be indexed well as the pure phase (JCPDS card no. 74-2441). During the thermal conversion process, NbO6 octahedra change from edge-sharing to corner-sharing. This structural difference has an important effect on the gap between the valence band and conductive band of niobates, which can be reflected in the optical absorption spectra of the as-prepared samples. UV–Vis spectra of the precursor, final product, and bulk NaNbO3 are shown in Figure 5c. The precursor has an absorption peak at around 250 nm (a) due to the Lindquist units including six edge-sharing NbO6 octahedra. However, the peak shifts to above 300 nm in final NaNbO3 product (b) that comprises corner-sharing NbO6 octahedra. The change may originate from the difference in Nb–O bond distances in configuration of NbO6 octahedra, which also further confirms that corner-sharing NbO6 octahedra are more stable than edge-sharing ones. When compared with UV absorption peak of bulk NaNbO3 (c) at around 362 nm, the optical absorption edge of NaNbO3 nanobelts shifts towards the lower wavelength, indicating an increase in band gap. Due to the quantum size effect in nanosized semiconductors, the band gap increases when the size of belt-like nanomaterials is decreased, resulting in a blueshift of absorption bands. PL spectra of NaNbO3 nanobelts were also been measured, as shown in Figure 5d. The spectra consist of a UV emission peak and two violet emission peaks in visible region. It is found that the UV peak position is at approximately 368 nm which can be attributed to free exciton emission. Two strong visible peaks dominate the PL spectra, which locate at λ = 421 and 433 nm. The spectra suggest that niobate framework is directly involved in the photoluminescence effect. The heat treatment increases the stability of Nb–O–Nb bonds and introduces different kinds of defect centres acting as traps for charge carriers, therefore increasing the probability for electrons to reach an electron trap, such as oxygen vacancy, and leading to the luminescence effect. The optical properties of NaNbO3 thus open up opportunities for exploiting advanced NaNbO3-based optical nanodevices.

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