Solvothermal synthesis of uniform bismuth nanospheres using poly(N-vinyl-2-pyrrolidone) as a reducing agent
© Wu et al; licensee Springer. 2011
Received: 26 July 2010
Accepted: 12 January 2011
Published: 12 January 2011
Uniform bismuth nanospheres were successfully prepared from bismuth nitrate in the presence of poly(N-vinyl-2-pyrrolidone) (PVP) by solvothermal process. The product was characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and energy-dispersive X-ray. PVP plays a critical role both as a reducing agent and a capping agent in the formation of bismuth nanospheres. Shape and size of bismuth nanospheres could be tuned by changing the employed PVP/bismuth salt ratio. It was also found the solvent had an effect on the morphologies of bismuth nanomaterials. The possible formation and growth mechanism of bismuth nanospheres were also discussed and proposed to explain the reduction step.
Recently, metal nanostructures with functional properties have been extensively studied due to their wide applications. Among the various metal nanostructures, monodisperse metallic nanospheres have attracted considerable attention because of their unique optical, catalytic, as well as novel chemical and biological properties [1–8]. For example, the controlled silver nanospheres were applied for optical studies and antibacterial applications [9, 10]. Thus, much effort has been devoted to the preparation of new metal nanospheres and their functional transformation [11–14]. As a semimetal, bismuth has unusual properties due to its highly anisotropic Fermi surface, low carrier densities, small carrier effective masses, and long carrier mean free path . Bismuth nanostructures have received steadily growing interests, since they play important roles in many diverse applications [16, 17]. Different bismuth nanomaterials, such as single-crystal bismuth thin film and nanowire, have been investigated for their large magnetoresistance, finite-size effects, and enhanced thermal conductivity [18–20]. It was also reported that spherical bismuth nanomaterials were a good catalyst for the growth of SnS2 nanotubes and germanium nanowires [21, 22]. Up to now, a wide variety of bismuth nanostructures such as nanoparticles , nanowires , nanorods , nanotubes [26–28], triangular nanoplates , bunch-like nanostructures , nanocubes , as well as nanospheres (>100 nm) [31–34] have been prepared by using different methods. Among these methods, solvothermal synthesis has become an important and promising approach to prepare controlled inorganic nanocrystals [35–37].
In the reported literatures, the alcohol [normally ethylene glycol (EG)] was used as both solvent and reducing agent to generate bismuth nanospheres with a wide size distribution [31–34]. Wang and Xia  have developed the synthesis of uniform bismuth nanospheres by thermal decomposition bismuth acetate in EG under the protection of nitrogen. The use of protection of inactive gas and organic metal precursor, however, made it complicated and costly. Hence, a simple, low-cost approach for the preparation of uniform bismuth nanospheres is highly desired.
In this communication, we describe a solvothermal process for the synthesis of uniform bismuth nanospheres in the presence of poly(N-vinyl-2-pyrrolidone) (PVP). In most cases, PVP was used as a protecting agent to cap and stabilize the particles to avoid aggregation in the solution . It was also reported that PVP was used as reducing agent in the preparation of noble metallic nanostructures [40, 41]. In this synthesis, PVP acts as both reducing and capping agent. To the best of our knowledge, there are few publications relevant to the synthesis of bismuth nanomaterials using PVP as a reducing agent.
Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) was purchased from Fuchen Chemicals Reagent (China). N,N-dimethylformamide (DMF), ammonia, mannitol, and EG were purchased from Sinopharm Chemical Reagent (China); PVP (with average M w of 10,000) was purchased from Sigma-Aldrich Inc. All the reagents were analytical grade and used directly without further purification.
Synthesis of uniform bismuth nanospheres
Experimental conditions for the synthesis of bimuth nanomaterials
The amount of PVP (g)
The bismuth nanostructures were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive X-ray (EDX). The powder XRD pattern was recorded on Bruker AXS D8 Discover (Cu Kα radiation, λ = 1.5406 Å). The scanning rate is 1°/min in the 2θ range from 20 to 70°. The SEM images were taken on a Hitachi S4800 field emission scanning electron microscope operating at 3 or 5 kV (FESEM, Japan). TEM images and SAED pictures were recorded on a Philips Technai G2 20 electron microscope (Manufacturer: FEI Company), using an accelerating voltage of 200 kV. The EDX analysis was performed on an Oxford Instruments Inca with a scanning range from 0 to 20 keV. The samples for TEM observations were prepared by dispersing some of the solid products into absolute alcohol and then sonicated for several minutes. A few drops of the suspension were deposited on the copper grid, which was then put into the desiccator. The thermogravimetric and differential analysis was done by DIAMOND TG/DTA instrument (PerkinElmer Instruments, Waltham, CT, USA) with a heating rate of 10°C/min.
Results and discussion
The morphologies and structures of the as-prepared bismuth nanospheres were investigated by SEM and TEM images. Figure 1b, c, d shows the representative SEM images of the as-prepared bismuth nanospheres with different magnifications. The low-magnification images (Figure 1b, c) indicate the large production of uniform bismuth nanospheres. No other bismuth morphologies could be found in the whole sample. The diameters of these nanospheres varied from 160 to 210 nm and most of them were about 180 nm. The high-magnification image shown in Figure 1d clearly reveals the detailed morphologies of the products, indicating the formation of uniform, regular nanospheres with smooth surfaces.
In this study, it was found that there was no black product precipitated from the solution in the absence of PVP under identical experiment condition. The XRD pattern of the obtained white product could not be assigned in the XRD standard data base (Figure S3 in Additional File 3). It is obvious that PVP plays a key role in the formation of bismuth nanospheres. It was proposed that PVP reduced Bi3+ ions to give metallic bismuth. The reduction process could be probably understood considering two main reactions: (1) direct abstraction of hydrogen atoms from the polymer by the Bi3+ ions; (2) reduction of the Bi3+ ions by the organic radicals formed in (1) or during the bismuth-accelerated degradation of PVP, which is similar to the reported literatures [40, 42]. It was reported that the capping organic molecules in the reaction system could determine the subsequent morphology of the product because of their special chemical properties and self-assembling functions [43–45]. Recently, it was also found that EG molecules could adsorb onto the surface of bismuth crystals through O-Bi bonding which could significantly decrease their growth rates and lead to highly anisotropic growth [33, 46]. Thus, when the molar ratio of repeating units of PVP to Bi3+ is about 3, that is, there is no excess PVP in the solution, these uniform bismuth nanospheres can be obtained under the only effect of EG.
Interestingly, when DMF was instead of EG, bismuth nanoparticles and nanotubes arrays were formed under identical experimental conditions (Figure 4c, d). The average diameter of bismuth nanoparticles is about 50 nm, and the diameters of the bismuth nanotubes are in a narrow range from about 3.2 to 5.2 nm, with an average diameter of 4.3 nm. EDX was used to further analyze the elemental compositions of these nanotubes lying on the hole of the copper grid. It reveals the elemental Bi signals and confirmed the purity of this bismuth nanotube (Figure 4f). The copper and carbon peaks are due to the carbon-coated copper grid. Here, DMF plays an important role in the fabrication of bismuth nanotubes. In this synthesis, the starting materials can dissolve in DMF and DMF can partially decompose into dimethylamine, which is a good coordination ligand with N-chelation. The diffusion of Bi3+ ions in DMF-dimethylamine will be faster due to its lower viscosity, which is beneficial for the formation of nanotube products . The formation mechanism is still limited and further study in progress.
In summary, uniform bismuth nanospheres were successfully synthesized via a facile solvothermal process by reduction of EG solution of bismuth salt, using PVP both as a reducing agent and a capping agent. The shapes and sizes of the bismuth nanostructures varied with the changing of the PVP/bismuth salt molar ratio. Uniform bismuth nanospheres with an average diameter of 180 nm could be routinely synthesized through this solvothermal approach by controlling the PVP/bismuth salt ratio. The solvent EG also played an important role in the formation of uniform bismuth nanospheres. Bismuth nanoparticles and nanotubes were formed when DMF was used as a solvent instead of EG. The possible formation mechanism of the bismuth nanotubes was also discussed and further study was in progress.
selected area electron diffraction
scanning electron microscopy
transmission electron microscopy
This study was supported by National Natural Science Foundation of China (Grant 20801043), the Program for New Century Excellent Talents in University (NCET-09-0136), and the Outstanding Youth Scientific Foundation of Hubei Province (2009CDA075).
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