Template Route to Chemically Engineering Cavities at Nanoscale: A Case Study of Zn(OH)2 Template
© The Author(s) 2010
Received: 17 June 2010
Accepted: 19 July 2010
Published: 1 August 2010
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© The Author(s) 2010
Received: 17 June 2010
Accepted: 19 July 2010
Published: 1 August 2010
A size-controlled Zn(OH)2 template is used as a case study to explain the chemical strategy that can be executed to chemically engineering various nanoscale cavities. Zn(OH)2 octahedron with 8 vertices and 14 edges is fabricated via a low temperature solution route. The size can be tuned from 1 to 30 μm by changing the reaction conditions. Two methods can be selected for the hollow process without loss of the original shape of Zn(OH)2 template. Ion-replacement reaction is suitable for fabrication of hollow sulfides based on the solubility difference between Zn(OH)2 and products. Controlled chemical deposition is utilized to coat an oxide layer on the surface of Zn(OH)2 template. The abundant hydroxyl groups on Zn(OH)2 afford strong coordination ability with cations and help to the coating of a shell layer. The rudimental Zn(OH)2 core is eliminated with ammonia solution. In addition, ZnO-based heterostructures possessing better chemical or physical properties can also be prepared via this unique templating process. Room-temperature photoluminescence spectra of the heterostructures and hollow structures are also shown to study their optical properties.
Owing to the potential applications in the fields of drug delivery, catalysis, artificial cell, lightweight fillers and protection for the light- or chemical-sensitive materials, hollow structures have received increasing research interests [1–6]. So far, hollow structures have been synthesized by means of various methods, among which template directing is the most straightforward way to yield hollow structures effectively. The commonly used templates include polystyrene (PS) latex spheres, silica spheres, carbon colloid spheres, gas bubbles and emulsion droplets [7–13]. Self-template routes by Kirkendall effect and Ostwald ripening have also been applied to realize hollowing process [14–17]. However, the general synthesized hollow products are mostly spheres. Although several previous works have been devoted to synthesize nonspherical hollow structures [18–22], it still remains a challenge to fabricate well-defined nonspherical hollow structures especially with tunable size.
The merits of using Zn(OH)2 as template are as follows: (1) it provides a unique template to synthesize nonspherical hollow structures; (2) the good solubility of Zn(OH)2 facilitates the surface chemical conversion; (3) owing to its inherent amphoteric habit, both alkaline and acid solution can be used to remove the inner core according to the nature of the outside coating materials; (4) due to the abundant hydroxyl groups which exhibit strong combining ability, positive charged metal cations could be enriched on the surface of the template without surface modification; (5) the diameter of the template can be tuned from ~1 to ~30 μm by slightly altering the reaction condition. Moreover, after a low temperature calcination (135 °C), this Zn(OH)2 template can be easily converted into ZnO crystal, which can transform the intermediate Zn(OH)2/shell structures into ZnO-based heterostructures. Therefore, Zn(OH)2 octahedra can be used as a promising hard template for synthesizing nonspherical hollow structures and ZnO-based heterostructures.
Synthesis of the Zn(OH)2 template: Octahedral Zn(OH)2 template (3–4 μm in diameter) was obtained using a convenient way. 0.83 g Zn(NO3) 2·6H2O was first dissolved into 15 mL distilled water, and then mixed with 10 mL aqueous solution containing 0.96 g NaOH under stirring. Then, the clear solution was placed in 50 °C water bath for 2 h. In order to control the diameter of the template, the reaction parameter was slightly adjusted. For 25–30 μm Zn(OH)2 octahedron, 15 mL 0.19 M Zn(NO3)2 solution was kept in ice water bath (0 °C), and then mixed with 10 mL aqueous solution containing 0.96 g NaOH. The solution was aged under room temperature water bath for 2 days. For 5–7 μm Zn(OH)2 octahedron, 15 mL 0.19 M Zn(NO3)2 solution was mixed with 10 mL 0.24 M NaOH solution. The solution was placed in 30 °C water bath for 20 h. For 1–2 μm Zn(OH)2 octahedron, 0.02 g poly(vinylpyrrolidone) (PVP) was added 15 mL 0.19 M Zn(NO3)2 solution. The mixture was stirred under room temperature for 30 min. Subsequently, 10 mL 0.24 M NaOH solution was added dropwise. The solution was placed in 30 °C water bath for 12 h. All the white precipitation was collected by centrifugation and washed thoroughly with distilled water and absolute ethanol several times. The product was dried in vacuum for 6 h at 50 °C.
Synthesis of hollow structures: for ZnS hollow structure, 200 mg Zn(OH)2 template (3–4 μm in diameter) was dispersed in 40 mL 0.20 M Na2S solution and stirred for 12 h in a 70 °C oil bath. Then, dark gray precipitation was centrifuged and subsequently placed in 25% (wt) ammonia solution for 50 min under mild stirring. Finally, the as-prepared products were centrifuged, washed with distilled water and absolute ethanol several times. The as-prepared sample was dried in vacuum for 6 h at 50 °C. For Ag2S hollow structure, after the surface of the template was sulfured by the Na2S solution, the participation was washed thoroughly with water to eliminate the S2− in the product. Then, the core/shell product was placed in 30 mL 0.05 M AgNO3 solution. The suspension was stirred for 30 min under room temperature to obtain black products, and then the pH value of the solutions was adjusted to 2 by several drops of diluted HNO3. Another 20 min was allowed to obtain Ag2S hollow octahedron. Finally, the product was washed by water and ethanol several times and dried in vacuum for 8 h at 60 °C. For Synthesis of SiO2 hollow structure, 200 mg Zn(OH)2 template (1–2 μm in diameter) was dispersed into 20 mL ethanol, then 9 mL water and 0.5 mL, 25% (wt) ammonia was added. The as-formed suspension was placed into ultrasonic irradiation and 0.5 mL TEOS was added. The mixture was subsequently stirred at room temperature for 3 h, the final white product was collected by centrifugation and washed with water and absolute ethanol for several times. The product was dried in vacuum at 80 °C for 3 h and treated with 0.10 M HCl to remove the inner template. The SiO2 hollow octahedron was collected by centrifugation, washed with water and absolute ethanol and dried in vacuum at 80 °C for 3 h. For Synthesis of the CeO2 hollow structure, 200 mg as-prepared Zn(OH)2 octahedron template (1–2 μm in diameter) was dispersed into 16 mL alcohol by ultrasonic irradiation. Then, 2 mL double-distilled water containing 0.50 mmol Ce(NO3)3 was added. The solution was stirred for 30 min to form a homogenous suspension. Subsequently, 5 mmol urea was added into the suspension and stirred for another 30 min to dissolve the urea completely. Then, the suspension was placed into an oil bath at 60 °C for 12 h under vigorous stirring. The final white product was collected by centrifugation, washed thoroughly with distilled water and absolute ethanol several times and dried in vacuum for 5 h at 50 °C. Then, the as-prepared product was annealed at 150 °C for 2 h to generate light yellow ZnO/CeO2 heterostructure. Finally, the heterostructure was washed with 0.10 M HCl to remove the inner ZnO core and obtain CeO2 hollow octahedron.
The phase purity of the products were characterized by X-ray diffraction (XRD) patterns, using a Bruker advance-D8 XRD with Cu Kα radiation (λ = 0.154178 nm). The accelerating voltage was set at 40 KV with a 100 mA flux. Scanning electronic microscopy (SEM) images were taken on JEOL JSM-6390LV. Low-magnification transmission electronic microscopy (TEM) images were obtained from JEOL JSM-100 while the high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) images were taken on FEI Tecnai G220. Thermogravimetry and differential scanning calorimetry (TG–DSC) of the samples were carried out with a Netzsch STA 409 PC analyzer at a heating rate of 10 °C/min. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a Bio-Rad FTS-40 Fourier transform infrared spectrometer. The photoluminescence (PL) was performed on JASCO FP-6500 fluorophotometer at room temperature.
A general template route has been designed to chemically engineering nanoscale cavities, which provides a simple scheme for the fabrication of highly crystalline hollow nanostructures with tailorable size. Zn(OH)2 octahedra were facilely synthesized at low temperature. Then, they were used as sacrificial template to construct octahedral hollow structures with controlled sizes. Two strategies can be adopted to fabricate different types (sulfides and oxides) of octahedral hollow structures. In addition, Zn(OH)2 can be transformed into ZnO via a low temperature calcination. Therefore, ZnO-based heterostructures possessing better chemical or physical properties can also be prepared via this facile templating process. These nanostructures can be of special interest for a variety of applications, including catalysis, gas sensing, and nanoelectronics.
This work was supported by the National Natural Science Foundation of China (20571025) and Henan Innovation Project for University Prominent Research Talents (2005KYCX005).
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