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 12345678. For example, the controlled silver nanospheres were applied for optical studies and antibacterial applications 910. Thus, much effort has been devoted to the preparation of new metal nanospheres and their functional transformation 11121314. 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 15. Bismuth nanostructures have received steadily growing interests, since they play important roles in many diverse applications 1617. 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 181920. It was also reported that spherical bismuth nanomaterials were a good catalyst for the growth of SnS₂ nanotubes and germanium nanowires 2122. Up to now, a wide variety of bismuth nanostructures such as nanoparticles 23, nanowires 24, nanorods 25, nanotubes 262728, triangular nanoplates 29, bunch-like nanostructures 17, nanocubes 30, as well as nanospheres (>100 nm) 31323334 have been prepared by using different methods. Among these methods, solvothermal synthesis has become an important and promising approach to prepare controlled inorganic nanocrystals 353637.

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 31323334. Wang and Xia 38 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 39. It was also reported that PVP was used as reducing agent in the preparation of noble metallic nanostructures 4041. 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.

Figure 1a shows the typical powder XRD pattern of the as-synthesized product (sample 1). All the diffraction peaks could be readily indexed to be rhombohedral phase of bismuth, which were consistent with the literature values (JCPDS No. 05-0519). No impurity was detected in this pattern, indicating the formation of pure bismuth products under current synthetic condition. The sharp and strong diffraction peaks also confirm the well crystallization of the products.

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.

The low- (Figure 2a) and high-magnification (Figure 2b) TEM images further demonstrated the morphologies of these bismuth nanospheres clearly. It also reveals a well-defined spheric structure and smooth surface. The statistical analysis showed that bismuth nanospheres had a narrow size distribution and the average diameter was about 180 nm with a standard deviation of 10 nm (Figure 2c). SAED pattern taken from an individual bismuth particle showed clear diffraction spots, indicating a good single-crystal structure (Figure 2d). The elemental composition of the nanospheres was also analyzed by EDX, and characteristic bismuth absorption peaks were observed (Figure S1 in Additional File 1). The copper and carbon peaks are due to the carbon-coated copper grid.

To investigate the composition of the white precipitate obtained in the synthetic procedure, thermogravimetric and differential thermal analysis (TG/DTA) and EDX measurements were carried out. Figure 3a shows the results of the thermal analysis of the white precipitate at a heating rate of 10°C/min in the nitrogen atmosphere. It indicates a two-step weight loss in the temperature range from 25 to 200 and 200 to 300°C. The first weight loss in the TG trace could be attributed to the desorption of physically adsorbed water, and the second one could be recognized in according with the exothermal reaction in DTA trace (Figure 3a). The exothermal reaction thermal was found to arise from the decomposition of the white precipitate. The white precipitate was further investigated by EDX spectrum. As shown in Figure 3b, the white precipitate was composed of Bi, O, and C and no other additional elements were detected. The Si signal is attributed to the silicon wafers. When the white precipitate was calcined at 350°C for 90 min, the XRD pattern of the calcined product was very close to the standard data (Bi₂O₃, JCPDS No. 71-2274) (Figure S2 in Additional File 2). Hence, it was proposed that the possible component of the white precipitate was bismuth complexes (Bi^III-mannitol), which were formed in ammonia solution.

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 4042. 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 434445. 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 3346. 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.

When the amount of PVP was increased to 0.310 g (the molar ratio of repeating units of PVP to Bi³⁺ is about 6), bismuth nanospheres and nanowires were formed, as shown in Figure 4a. The diameters of obtained bismuth nanospheres were in the range from 250 to 500 nm, and the length of the nanowires varies from 0.5 to more than 6 μm. When the molar ratio of repeating units of PVP to Bi³⁺ was further increased to about 9 (the amount of PVP is 0.465 g), a mass of bismuth nanowires were obtained and few bismuth nanospheres remained, as shown in Figure 4b. The length of bismuth nanowires is up to 10 μm and the diameters of nanospheres are in range from 200 to 500 nm. The elemental composition of these nanowires and few nanospheres was also analyzed by EDX, and characteristic bismuth absorption peaks were observed (Figure 4e). Thus, PVP is considered to act both as reducing agent and capping agent in this synthesis. It was believed that the formation of bismuth nanowires was based on the Ostwald ripening process 4748. At first, bismuth nanoparticles were formed in the solution according to eqs 1 or 2 with PVP as reducing or nucleating agent. Then, the excess PVP molecules could cap onto the surface of bismuth particles 33, and bismuth nanowires were produced through this polyol process. In this process, bismuth particles can continuously grow into nanospheres, nanorods, or nanowires. The co-effect of PVP and EG resulted in a wide size distribution of the diameter of bismuth nanospheres in this experiment and literatures 313233.

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 49. 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.

DMF: N,N-dimethylformamide; EDX: energy-dispersive X-ray; EG: ethylene glycol; PVP: poly(N-vinyl-2-pyrrolidone); SAED: selected area electron diffraction; SEM: scanning electron microscopy; TEM: transmission electron microscopy; XRD: X-ray diffraction.

The authors declare that they have no competing interests.

JW participated in the acquisition of data and drafted the manuscript. FQ carried out the SEM characterization and participated the analysis and interpretation of data. ZL and HJY helped to draft and revise the manuscript. RC made contributions to conception and design of the study, revised the manuscript critically for important intellectual content, and has given final approval of the version to be published.