Facile synthesis of novel nest-shaped Sb₂O₃ micro/nanostructures and their optical properties

Abstract

Novel nest-shaped Sb₂O₃ micro/nanostructure consisting of numerous nanorods grown along the [111] direction has been synthesized via a wet chemical method. A possible formation mechanism was proposed. Furthermore, optical properties suggest that the nest-shaped Sb₂O₃ material has great potential applications in electronics and optoelectronics.

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Recently, micro/nanomaterials have been developed with self-assembled three-dimensional structures; these materials have peculiar physical and chemical properties due to their unique size and morphology.¹ For instance, the flower-like,² hierarchical,³ and leaf-like⁴ ZnO micro/nanomaterials possess different gas sensing properties. Microsphere,⁵ branch-like,⁶ and flower-like⁷ TiO₂ micro/nanomaterials have demonstrated increased photocatalytic and photoelectrochemical performance. Nanoscale WO₃ based composites have exhibited enhanced electrochromic properties.⁸ Flower-like⁹ and octahedron-like¹⁰ SnO₂ micro/nanomaterials have showed improved optical and electrochemical performances. Sphere-like, cubic and octahedron-like Cu₂O exhibited enhanced electromagnetic,¹¹ biological,¹² catalytic and sensing properties.¹³ Sb₂O₃, as an important semiconducting V–VI main group compound, has been widely used in various industrial fields, e.g., flame retardants of rubber, plastics and textile products,¹⁴ catalysts,¹⁵ gas sensors,¹⁶ functional fillers and clarifying agent of glasses.¹⁷ Sb₂O₃ is also used in optical, optoelectronic, and magnetic materials.¹⁸ In addition, the various Sb₂O₃ nanostructures (grass-like,¹⁹ flower-like,²⁰ bow-tie-like,²¹ branched²² and octahedron-like²³ structures) that are already known have confirmed that the properties of Sb₂O₃ semiconductor micro/nanomaterials depend not only on their chemical compositions but also on their shapes and sizes.²⁴

In this study, cubic phase Sb₂O₃ with a nest shape was synthesized by a wet chemical method. The formation of a unique nest-shaped micro/nanostructure was studied by adjusting the reaction time, temperature, and concentrations of didodecyldimethylammonium bromide (DDAB) and urea. In addition, optical properties, i.e., UV-vis diffuse reflectance, Fourier transform infrared, Raman and photoluminescence of the as-prepared nest-shaped Sb₂O₃, were also evaluated. This work presents a novel Sb₂O₃ nanostructure with potential applications in electronics and optoelectronics.

A simple wet chemical method was developed for the synthesis of Sb₂O₃ (see ESI†). Typical SEM images of the as-prepared Sb₂O₃ samples at different magnifications are shown in Fig. 1. From the overall view image (Fig. 1a), it is observed that the sample is mainly nest-shaped Sb₂O₃ micro/nanostructures with uniform morphology and size. The high-magnification image (Fig. 1b) reveals a detailed morphology of a single Sb₂O₃ crystal with a diameter of ∼5 μm, which is composed of many nanorods. The diameter of these nanorods is only about 200–500 nm, according to the high-magnification FESEM image (Fig. 1c). The nanorods have two orientations: parallel to each other in one direction and crisscross in different direction. The EDS spectrum (Fig. 1d) indicates that the sample is composed of Sb and O, with the atom ratio of Sb to O being 56.72 : 43.28 or close to 1 : 1.5, which is in agreement with the stoichiometry of Sb₂O₃. No other element was identified, confirming that the product is of high purity.

Fig. 1 SEM and EDS images of the as-prepared Sb₂O₃ samples at different magnifications: (a) a panorama; (b) one typical single microparticle; (c) local image; and (d) EDS image.

The room temperature powder XRD pattern of the sample was recorded for phase confirmation and chemical state determination. As can be observed from Fig. 2a, the XRD pattern exhibits standard peaks of the cube phase Sb₂O₃ (JCPDS Card no. 05-0534) with lattice constants a, b, c = 11.152 Å. No peak from any other phase of Sb₂O₃ or impurities can be observed, indicating that it is a single-phase sample. The diffraction peaks are narrow and sharp, proving that the product is of high crystallinity.

Fig. 2 (a) XRD pattern; (b and c) TEM images; (d) HRTEM image; (e) SAED pattern of nest-shaped Sb₂O₃ micro/nanostructures.

Transmission electron microscopy (TEM) was used to verify the microstructure of Sb₂O₃. Fig. 2b and c show low-magnification TEM images of Sb₂O₃, exhibiting a self-assembled nest-shaped microstructure, which is consistent with the SEM results (Fig. 1b). The high-resolution transmission electron microscopy (HRTEM) image shown in Fig. 2d was taken from the square-closed area in Fig. 2b. The lattice spacing was calculated to be around 6.42 Å in different spots, which corresponded to the (111) lattice planes spacing of Sb₂O₃ in the cube phase. The electron diffraction (SAED) pattern of the selected area (in Fig. 2e) provides additional evidence of the single crystalline nature of the sample. This testifies that nanorods prefer to grow along the [111] crystallographic direction and have good crystallinity.

In order to explore the formation process of nest-shaped Sb₂O₃ micro/nanostructures, we investigated the influence of the concentration of DDAB and urea, reaction temperature and time on the shape of the final product (Fig. S1–S4, see ESI†). On the basis of those observations, we speculated that the growth process of the nest-shaped Sb₂O₃ might include three stages: crystal nuclei formation, self-assembly and oriented attachment. At the initial stage, numerous nuclei of Sb₂O₃ were formed. The chemical reactions were as follows:

(NH₂)₂CO + 3H₂O → 2NH₄OH + CO₂↑ (1)

2K(SbO)C₄H₄O₆ + 2OH⁻ → Sb₂O₃↓ + K₂C₄H₄O₆ + H₂O + C₄H₄O₆²⁻ (2)

We found that DDAB is the key factor in determining the final morphology of Sb₂O₃ microcrystals (Fig. S1, see ESI†). It is well known that DDAB in an aqueous solution with ammonium compounds has two long-chain alkyl groups. Kunitake et al. found that DDAB could form stable bilayer structures, which further form vesicles and lamellae in water spontaneously.²⁵ The formation of these structures may be related to interaction of its non-polar long chain alkyl groups with the polar aqueous media. After the initial formation of Sb₂O₃ crystal nuclei, the particles dissolved gradually and absorbed to the surface of micelles to grow again due to Ostwald ripening mechanisms. The subsequent stages, self-assembly and oriented attachment, are thought to be strongly dependent on the interaction with DDAB molecules. As the aging time increased, Sb₂O₃ nanorods appeared owing to selective growth in the favorable crystallographic direction [111]. Finally, surfactant micelles were removed by washing with distilled water and absolute alcohol, and the final Sb₂O₃ materials were obtained. This process was used previously for the formation of spindle-like Sb₂O₃ products.²⁶ The formation mechanism consisting of dissolution–recrystallization, self-assembly and oriented attachment is illustrated in the Scheme 1 and Fig. S4 (see ESI†).

Scheme 1 Schematic of the formation process of nest-shaped Sb₂O₃ micro/nanostructures.

Diffuse reflectance spectra of the Sb₂O₃ sample were obtained in order to determine the energy gap (E_g). As shown in Fig. 3a, the as-prepared nest-shaped Sb₂O₃ micro/nanostructures can only absorb ultraviolet light with wavelength smaller than 320 nm. The photonic energy band gap (E_g) value can be obtained from the Tauc plot,²⁷ which describes the αhν value versus photon energy (hν),

αhv = (hv − E_g)ⁿ (3)

where α is absorbance coefficient, h is Planck's constant, and ν is the photo frequency. The exponent n is 2 for indirect-allowed Sb₂O₃ semiconductor, and the E_g value is obtained by extrapolation of the linear portion of the curve to the energy axis. The calculated E_g of as-prepared Sb₂O₃ sample is 3.48 eV, showing its great potential applications in optoelectronics. The Fourier transform infrared (FT-IR) absorption spectrum of the as-prepared Sb₂O₃ micro/nanostructure displays two peaks at 725.1 and 954.6 cm⁻¹, which can be ascribed to the stretching vibration of Sb–O bond (Fig. 3b). The bands observed here are slightly shifted (740 and 940 cm⁻¹) as compared to the absorption peaks in other reports due to surface defects and special self-assembly morphologies.²⁸

Fig. 3 (a) Diffuse reflectance absorption spectrum of the as-prepared Sb₂O₃ sample. The inset shows the corresponding plots of (αhν)^1/2 versus photon energy. (b) Fourier transform infrared spectrum; (c) room-temperature Raman spectrum; (d) photoluminescence spectrum of nest-shaped Sb₂O₃ micro/nanostructure.

A Raman spectrum for the nest-shaped Sb₂O₃ material in the range from 100 to 800 cm⁻¹ is shown in Fig. 3c. The strong peaks correspond to Raman shifts of 123.5, 194.1, 259.1, 377.8, 455.1, and 711.7 cm⁻¹. Comparison with previous experimental and theoretical Raman investigations of Sb₂O₃ (121, 193, 256, 376, 452, 717 cm⁻¹)²⁹ enabled assignment of the main peaks in this spectrum to the cubic Sb₂O₃ phase. According to previous reports for Sb₂O₃ materials, the peaks observed for the nest-shaped material can be ascribed to the vibration modes of Sb–O–Sb groups: all signals below 400 cm⁻¹ belong to the external lattice mode regime, whereas those above 400 cm⁻¹ can be ascribed to internal vibration.³⁰

Fig. 3d shows the photoluminescence (PL) spectrum of nest-shaped Sb₂O₃ using an excitation wavelength of 325 nm. It shows an intense blue emission at 525 nm (2.36 eV), which is associated with oxygen vacancy related defect centers in the Sb₂O₃ lattice. Deng et al. found that defect centers appeared during the processes of oriented attachment and assisted self-assembly.³¹ Due to the recombination of photo-generated electrons and holes, a UV emission band at about 385 nm (3.22 eV) was observed. Additional broad peaks at approximately 415, 432, 564 and 611 nm were observed as well and can be attributed to either of the oxygen vacancies, dangling bonds or defect centers.³²

In summary, cubic phase Sb₂O₃ nest-shaped micro/nanostructure has been successfully prepared via a simple wet chemical method in the presence of DDAB. The nest-shaped microparticle has a typical diameter of 5 μm and is composed of nanorods. The formation mechanism of unique nest-shaped micro/nanostructure was attributed to dissolution–recrystallization, self-assembly and oriented attachment process. The UV-vis diffuse reflectance spectrum demonstrates that the band gap energy of as-prepared nest-shaped Sb₂O₃ is about 3.48 eV, and the PL spectrum reveals an intense blue photoluminescence at 525 nm. These excellent characteristics indicate potential for applications in functional semiconductor materials and photoelectric devices.

Acknowledgements

This project was financially supported by the National Natural Science Foundation of China (No. 51442002).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic process and mechanism analysis. See DOI: 10.1039/c6ra20558e

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