Hollow Sodium Tungsten Bronze (Na0.15WO3) Nanospheres: Preparation, Characterization, and Their Adsorption Properties
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We report herein a facile method for the preparation of sodium tungsten bronzes hollow nanospheres using hydrogen gas bubbles as reactant for chemical reduction of tungstate to tungsten and as template for the formation of hollow nanospheres at the same time. The chemical composition and the crystalline state of the as-prepared hollow Na0.15WO3nanospheres were characterized complementarily, and the hollow structure formation mechanism was proposed. The hollow Na0.15WO3nanospheres showed large Brunauer–Emment–Teller specific area (33.8 m2 g−1), strong resistance to acids, and excellent ability to remove organic molecules such as dye and proteins from aqueous solutions. These illustrate that the hollow nanospheres of Na0.15WO3should be a useful adsorbent.
KeywordsSodium tungsten bronze Hollow nanosphere Adsorption property
Hollow structure materials exhibit usually extraordinary adsorbing capacities to a wide range of species (i.e., metal ions, organic molecules, and biomolecules) and have found practical applications in catalysis [1, 2], water treatment , and drug delivery . The hollow nanospheres, because of their unique physical and chemical properties, have attracted more significant interest during the last few years [5, 6, 7, 8, 9]. Up to now, several synthetic strategies have been developed, and a range of hollow nanospheres, especially metal oxides and sulfides, have been fabricated [3, 6, 8, 10, 11, 12], but it is still challenging to develop simple and reliable synthetic methods for hollow nanospheres with diverse chemical compositions, desired chemical/physical stabilities, and controlled size and shell structures (shell thickness and porosity), which are critical for their practical applications.
Sodium tungsten bronzes (NaxWO3, 0 < x ≤ 1), besides their unique electronic/electric properties that vary greatly with their compositions [13, 14, 15, 16, 17], have inert chemical properties, such as insolubility in water and resistance to most acids except hydrofluoric , which make NaxWO3 promising for use in many extreme chemical cases. Nanosized NaxWO3, predictably, should have more enriched properties differing from that of the corresponding bulk materials and might find more novel applications, but have barely been explored . We report herein a facile strategy for the fabrication of hollow nanospheres of sodium tungsten bronzes, NaxWO3, and their potential applications in water treatment. The fabrication, including the control on sizes of the spheres and hollow feature of the hollow NaxWO3 nanospheres, was achieved through reduction of aqueous sodium tungstate (Na2WO4) solution by sodium borohydride (NaBH4) powder under well-controlled pH and temperature. The chemical composition, crystalline state, size, and morphology of the as-prepared hollow NaxWO3 nanospheres were characterized complementarily using scanning electron microscopy (SEM), transmission electron microscopy (TEM, including HRTEM), energy dispersive spectrum (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD). Their application in the removal of organic molecules from water was illustrated using different molecules, such as Coomassie brilliant blue, Albumin Bovine, and Lysozyme.
Sodium tungstate, sodium borohydride, hydrochloric acid (37%), and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received. Coomassie Brilliant blue, Lysozyme, and Albumin Bovine were from Sino-American Biotechnology Co. (Shanghai, China). Pure water (electric resistance of 18.2 MΩ cm−1) was produced through an HF Super NW water purification system (Heal Force Co. Shanghai, China). A typical procedure for the preparation of hollow Na0.15WO3nanospheres is as follows: 40 mL of 0.25 M Na2WO4aqueous solution was put in a 250 mL flask and the pH of the solution was adjusted to 6.8 using concentrated HCl (37%). Then, 0.025 mol of NaBH4powder was added gradually into the Na2WO4solution, and the mixture was stirred at room temperature (~25 °C) for 2 h. After the reaction, the brown precipitate was separated from the reaction system by centrifugation, washed three times with pure water and two times with ethanol, and finally dried at 80 °C under a vacuum. Solid Na0.15WO3nanospheres were prepared under almost the same conditions used above except that the reaction temperature was 100 °C and that the NaBH4powders must be added step-by-step because the reaction at 100 °C takes place vigorously.
Coomassie Brilliant Blue and the proteins adsorption experiments were carried out at room temperature. The Na0.15WO3was first dispersed into water or buffer; the stock solutions of Coomassie Brilliant blue or proteins were then added to the Na0.15WO3suspension and incubated on the shaker. UV–vis absorption spectra of Coomassie Brilliant blue and proteins in the supernatant were recorded at different time intervals to follow the adsorption process. The gel electrophoresis was run on a DYY-6C electrophoresis system (Liuyi Electrophoresis Co., Beijing, China). The standard 15% SDS polyacrylamide gel was used and was run under constant voltage of 50 mV.
Scanning electron microscopy images were acquired on a SIRION 200 field emission scanning electron microscope (FEI Company, USA). TEM images and energy dispersive spectra (EDS) were taken on a JSM-2010 transmission electron microscope (JEOL Ltd., Japan) operated at 200 kV. The powders of Na0.15WO3nanospheres were first suspended in water and then transferred on to silicon substrates or copper TEM grids for the SEM and TEM measurements, respectively. XRD patterns were recorded on a D/MAX 2200/PC diffractometer (Rigaku Corporation, Japan) using Cu Kα radiation, λ = 1.54 Å. XPS measurement was performed on an Axis Ultra DLD instrument (Kratos Analytical, UK) using a monochromatized Al (Kα) source. UV–vis absorption spectra were recorded on a UV-2550 spectrometer (Shimadzu Corporation, Japan). The Brunauer–Emment–Teller (BET) specific area was measured on ASAP 2010 M/C surface area and porosimetry analyzer (Micromeritics Instrument Corporation, USA) based on N2adsorption.
Results and Discussion
In the reaction, the hydrogen generated from the hydrolysis of NaBH4 under acidic reaction condition was partially consumed to reduce tungstate to tungsten, and the rest was released from the reaction system to the air . Therefore, in practice, to prevent a rapid loss of hydrogen and to enhance the reduction ability of NaBH4, the aqueous solutions of Na2WO4 and NaBH4 were mixed first, and the initial pH of mixture solution was maintained at 11 or above. The Na2WO4 reduction was initiated subsequently by adjusting the pH of the mixture down below 7 by adding acid, such as HCl. Thus, there were not too many hydrogen gas bubbles accumulated in the reaction system, the loss of the hydrogen gas could be suppressed, and powder of bulk sodium tungsten bronzes was obtained finally. In this work, instead of mixing two pre-prepared solutions, the reaction was conducted by adding the NaBH4 powder directly into the Na2WO4 aqueous solutions. However, we found that when the pH of the Na2WO4 aqueous solution is above 10, the reaction took place very slow; under the acidic condition, pH < 6, the NaBH4 was hydrolyzed rapidly and the as-generated hydrogen bubbles escaped from the reaction system severely. Hence, in a typical procedure of preparing NaxWO4 nanospheres in the work, Na2WO4 aqueous solutions with pH near to neutral (typically, 6.9–7.2) were prepared first, and NaBH4 powder was then added gradually into the Na2WO4 solutions under moderate stirring at room temperature (~25 °C). The total amount of NaBH4 added was usually three times of Na2WO4 (molar ratio) to ensure the reduction of tungstate to tungsten. After completion of the reaction, the solid product was collected by centrifugation and was washed thoroughly using pure water and ethanol, and finally dried at 80 °C under a vacuum (0.01 Torr).
The hollow sodium tungsten bronze, Na0.15WO3, nanospheres have been successfully fabricated using the hydrogen gas bubbles as reactant to reduce the tungstate to tungsten and as template to direct the hollow structure formation as well. This, to our best knowledge, is the first example of using hydrogen gas bubbles as reactant and template at the same time to prepare nanosized hollow materials, and should provide a general means for preparing other inorganic nanosized hollow materials. The resistance to most acids and the pronounced removal capacity of the as-synthesized hollow Na0.15WO3nanospheres to small organic molecules and proteins from acidic waste water should find widespread applications in water treatment. Further studies on tailoring the surface chemistry and the shell porosity of the hollow Na0.15WO3nanospheres would be essential to their practical applications and are under current investigation.
This work was supported by the National Basic Research Program (973 program) of China (No. 2007CB936000), the National High Technology Research and Development Program (863 program) of China (No. 2006AA04Z309), and the Shanghai Pujiang Scholarship Program (Nos. 06PJ14025, 06PJ14030).