In situ Precursor-Template Route to Semi-Ordered NaNbO3 Nanobelt Arrays
© Wu and Xue. 2010
Received: 7 July 2010
Accepted: 12 August 2010
Published: 26 August 2010
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.
One-dimensional (1D) nanostructures are receiving an ever-increasing amount of attention from researchers in various disciplines because of their unusual quantum properties to their bulk counterparts and potential use as building blocks for the next generation of nanoscale optical, electronic, photonic, and biological devices [1, 2]. Ordered functional arrays or chemically defined surfaces with fascinating quantum behaviour are more attractive nanostructures owing to their applications in high-density memories, sensors, lasers, and photonic crystals. Although numerous efforts have been invested in developing simple and low-cost fabrication techniques for the growth of high-quality 1D materials in a relatively large scale [3–10], the ability to fabricate ordered 1D micro- and nanostructures in a desired pattern with controllable size and shape uniformity is a key challenge in enabling their improved technological applications and has opened up the minds of new generation of materials scientists about the potential of nanoscience and technology [11, 12].
Alkaline niobates is one class of widely investigated ternary materials because of their optical, ferroelectric, and piezoelectric properties [13–19]. To date, many niobium-containing perovskite materials have been synthesized through various kinds of methods and exhibit wide applications in nonlinear optics, pyroelectric detectors, and optical memories, etc. NaNbO3 belongs to a technologically important group of perovskite materials, which comprises a three-dimensional framework of corner-sharing NbO6 octahedra with Na cations occupying their cavities [20–22]. Tilting of NbO6 octahedra at different temperature brings different phases (orthorhombic, monoclinic, and cubic phase). Generally, the shape of crystalline particles depends on their internal structures . This means that materials with a cubic or pseudocubic structure will normally form isotropic particles in a thermodynamic decided process. Actually, till now there are few reports about high aspect ratio 1D NaNbO3 micro/nano structures, which provide a good system to study the size and dimensionality dependences of the physical properties. Herein, we exploit a facile precursor-template route to chemically fabricate NaNbO3 nanobelts. After a solid-phase transformation of Na7(H3O)Nb6O19·14H2O precursors in air, semi-ordered NaNbO3 nanobelt arrays were yielded without morphology deformation. In this proposed scheme, the aspect ratio and uniformity of the precursor and NaNbO3 nanobelts are controllable. Low concentration of NaOH in this process also avoids strong corrosive effect.
A typical synthesis was performed as follows. A piece of Nb foil (6 × 6 × 0.5 mm) was pretreated by sonication in ethanol for 10 min and laid flat in a Teflon-lined stainless steel autoclave (capacity, 30 mL). Twenty millilitres 1.0 M NaOH solution mixed with 3 mL H2O2 (PH = 13.2) was then filled into the autoclave that was sealed and put into an electric oven. The concentration of NaOH was changed from 0.5 to 1.5 M to tune the aspect ratio of precursor nanobelts. The temperature of the electric oven was set at 150–220°C and kept the reaction lasted for 10–24 h under autogenous pressure. After the autoclave was air-cooled to room temperature, Nb foil was dissolved completely, and the white sheet-like powders were collected, rinsed with deionized water, and dried at 40–60°C for 2–5 h in air. NaNbO3 nanobelt arrays could be obtained by calcining the precursor at 500–550°C for 1–4 h.
The as-prepared samples were characterized by an X-ray diffractometer (XRD) on a Rigaku-DMax 2400 diffractometer equipped with the graphite monochromatized Cu Kα radiation flux at a scanning rate of 0.02°/s in the 2θ range 5–80°. Scanning electron microscopy (SEM) images were taken with a JEOL-5600LV scanning electron microscopy, using an accelerating voltage of 20 kV. Energy-dispersive X-ray (EDX) microanalysis of the samples was performed during SEM measurements. The structures were investigated by transmission electron microscopy (TEM, Philips, Tecnai G220, operated at 200 kV). Thermogravimetric analysis and differential scanning calorimetry (TG/DSC, SDT Q600, TA) were employed to analyse the thermal behaviours of the synthesized precursor in N2 atmosphere at a heating rate of 10°C/min. UV–visible (UV–Vis) spectra of the samples were measured on a UV–Vis-NIR spectrophotometer (JASCO-V570). The photoluminescence (PL) spectra were measured at room temperature in the range of 310–700 nm using a Xe lamp with a wavelength of 290 nm as the excitation source. The infrared (IR) spectrum was measured by KBr pellet method (using a Nicolet NEXUS infrared spectroscopy) in the range of 400–4,000 cm-1.
Results and Discussion
It has been reported that as the concentration of AOH (A = Na, K) increases in solution, the Lindquist compound is salted out as a major product . In each Lindquist ion of Na7(H3O)Nb6O19·14H2O, six NbO6 octahedra compose a bigger octahedron by sharing edges, Na atoms are scattered within the vacancies between the Lindquist octahedral sites. Further examination of the crystal structure of Na7(H3O)Nb6O19·14H2O reveals that the preferential orientation of the Na7(H3O)Nb6O19·14H2O is along c-axis. Considering that no templates and surfactants are used, it is the internal crystal structure that induces 1D growth of the Lindquist composite. Because of the coexistence of H2O2 and NaOH, alkaline oxidant solution is formed in the autoclave. The metal substrate is oxidated continuously by H2O2 into Nb2O5 which is subsequently dissolved by NaOH. Metastable phases often crystallize first at low temperature because their nucleolus may require lower free energy and lower supersaturation to form in the nucleation-controlled regime. Therefore, Na7(H3O)Nb6O19·14H2O is crystallized and precipitated out of the solution.
We have successfully developed a facile precursor-template route to chemically fabricate dense semi-ordered NaNbO3 nanobelt arrays with tunable aspect ratio, which may be thermodynamically inaccessible structural and morphological features. During the thermal conversion process, atoms gradually rearrange in the restricted space of 1D Na7(H3O)Nb6O19 14H2O precursor until single-crystalline NaNbO3 nanobelt forms without loss of the original shape. The study facilitates to advance the understanding of the crystal phase control and transformation during solid-state reactions. We also established the controlled organization of the film surface with NaNbO3 nanocubes, which may be also useful for optical and piezoelectric devices. The proposed chemical strategy for NaNbO3 film formation may be extended to the fabrication of more niobate arrays.
Financial support from the Natural Science Foundation of China (grant Nos. 50872016, 20973033) is acknowledged.
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