Investigation on the role of the molecular weight of polyvinyl pyrrolidone in the shape control of high-yield silver nanospheres and nanowires
- Yuan-Jun Song†1, 2, 3,
- Mingliang Wang1Email author,
- Xiao-Yang Zhang†2, 3,
- Jing-Yuan Wu†2, 3 and
- Tong Zhang2, 3Email author
© Song et al.; licensee Springer. 2014
Received: 21 November 2013
Accepted: 26 December 2013
Published: 13 January 2014
Serving as shape control agent, polyvinyl pyrrolidone (PVP) has been widely used in chemical synthesis of metal nanoparticles. However, the role of molecular weight (MW) of PVP has been rarely concerned. In this study, we show a facile method to control the shapes of silver nanocrystals using PVP with different MWs. PVPMW=8,000, PVPMW=29,000, PVPMW=40,000, and PVPMW=1,300,000 are compared in the present study. Surprisingly, high-yield silver rodlike nanostructures, nanospheres, and nanowires can be obtained under the same growth environment and reactant concentrations by simply changing the MW of PVP. The mechanism studies of the role of PVP with different MWs in the growth process were carried out systemically using the morphology and spectroscopic measurement, FT-IR spectrum analysis, and seed crystallization monitoring. The results indicate that the MW of PVP plays a determinant role in the morphology and optical property control of the silver nanocrystals. Meantime, the concentration of PVP was found to be an assistant factor to further improve the shape and the yield of the synthesized nanocrystals.
The synthesis of metal nanoparticles with high uniformity attracts considerable attentions due to their fantastic optical properties arising from localized surface plasmon resonance (LSPR) [1–3]. Such plasmonic nanoparticles, especially silver, are widely used in catalysis [4, 5], biological and chemical sensors [6–8], and surface-enhanced Raman spectroscopy [9–11]. It has been recognized that the optical spectral signatures of plasmonic nanoparticles are primarily dependent on their shapes [12–14]. Leading works in the synthesis of silver nanoparticles have focused on the shape control of silver nanocrystals via various routes. Wiley et al.  controlled the shapes of silver nanocrystals by varying reaction conditions such as the precursor concentration, molar ratio of the surfactant, and silver ions. As well known, the final structure of the nanocrystals are mainly determined by the crystallinity of seeds produced in the early stage of the reaction. Xia's group prepared silver pentagonal nanowires, nanocubes, and bipyramids from multiply twinned decahedral seeds, single-crystalline seeds, and single-twinned seeds, respectively . As for the crystals' control of seeds, Xia et al. introduced Cl- or Br- as etchants combined with oxygen to avoid the formation of undesired seeds . Another factor that influences the shape uniformity of the nanocrystals is self-nucleation in the reaction process. Self-nucleation of reductive silver atoms usually blocks the seed growth process resulting in the formation of spherical by-productions. The solution to the problem is to decrease the reduction rate of silver ions. Zhang et al.  applied a weak reductant to control the reduction rate. Meantime, citrate ligands used can also decrease the reduction rate because of complexation between silver ions and citrate ligands. Using polyol reduction method in the presence of polyvinyl pyrrolidone (PVP), Sun and co-workers successfully prepared silver nanowires [19–22]. Alternatively, the addition of as-prepared seeds  in the initial growth step has been suggested to induce the formation of nanowires preferentially. However, these reaction processes are usually complex or difficult to control. Without fine control of reactant concentrations and growth process, the obtained silver nanowires are always in low yield accompanied by large amounts of by-products such as nanocubes or nanospheres growing from isotropic seeds. In these cases, the post processing, such as low rotation-rate centrifugation  or special separation technique  to purify nanowires, is usually indispensable. Therefore, it is highly desirable to develop a reliable and facile method for the synthesis of silver nanocrystals in high yield with uniform size.
In the polyol process, acting as stabilizer, PVP plays an important role in controlling the shape. Chou et al.  compared the ability of PVP to stabilize silver colloids in the presence of NaOH or Na2CO3. Liu et al.  also proposed that the crystal structure shape was related to the capping modes between PVP with different molecular weights (MWs) and silver nanocrystals. Although the changes arising from the addition of PVP with different MWs have been observed in previous works, the exact function of the MW of PVP on the formation of silver nanocrystals has not been clarified until now. In this work, we deeply studied the role of MW of PVP in the shape control of silver nanocrystals. According to optical spectroscopic analysis and statistic of the yield and average size of each product prepared by varying the MW and concentration of PVP, we obtained the relationship between the MW of PVP and preferential products. By analyzing the interaction between PVP with different MW and silver crystals by Fourier transform infrared (FT-IR)spectroscopy, we deduce the role of PVP in the nucleation and growth processes. The results suggest that we provide a facile and robust strategy for the synthesis of well-shaped silver nanocrystals in high yield.
Silver nitrate (AgNO3 99 + %), sodium chloride (NaCl), and ethylene glycol (EG) were all purchased from Nanjing Chemical Reagent Co. Ltd (Nanjing, People's Republic of China). Polyvinyl pyrrolidone (PVPMW=8,000, PVPMW=1,300,000) were purchased from Aladdin (Shanghai, People's Republic of China). PVPMW=29,000 and PVPMW=40,000 were purchased from Sigma-Aldrich (St. Louis, MO, USA).
We used a colloidal synthesis method improved from the literature . The method is one of the main methods for silver nanowire preparation. However, we found that when PVPMW=40,000 was used in this method, there are always plenty of by-products such as nanospheres and nanocubes unless the reaction condition was strictly controlled. It provides us an opportunity to exhibit the role of MW and the concentration of PVP in the synthesis process using this method. In each synthesis, l-mL EG solution of AgNO3 (0.9 M) and 0.6-mL EG solution of NaCl (0.01 M) were added into 18.4-mL EG solution of PVP (0.286 M). Then, the mixture was refluxed at 185°C for 20 min. After these processes, the excess PVP and EG were removed by adding deionized water centrifuged at 14,000 rpm for 10 min, three times. The centrifugation ensures that all the products can be collected for the sake of statistics of shapes and size.
The morphologies of the prepared silver samples were observed by transmission electron microscopy (TEM; JEM-2100, JEOL Ltd., Akishima, Tokyo, Japan) and scanning electron microscopy (SEM; SIRION, Durham, NH, USA). FT-IR analysis was conducted on the FT-IR spectrum (NICOLET 5700, Thermo Fisher Scientific, Waltham, MA, USA). UV-visible near-infrared (NOR) spectra were recorded by a fiber-optic spectrometer (PG2000, Ideaoptics Technology Ltd., Shanghai, People's Republic of China).
Results and discussion
Statistic of the yield and average size of each product prepared by varying concentrations of PVP
Concentration of PVP (M)
Diameter (nm)/length (μm)
100 ± 10/1 ± 0.5
100 ± 20
100 ± 10/0.6 ± 0.1
60 ± 10
100 ± 10/0.4 ± 0.1
50 ± 10
100 ± 10/1.5 ± 0.2
100 ± 50
100 ± 10/0.6 ± 0.1
100 ± 50
100 ± 10/0.6 ± 0.1
60 ± 10
200 ± 100/2 ± 0.5
200 ± 50
100 ± 20/4 ± 2
200 ± 50
100 ± 10/6 ± 1
200 ± 50
Optical property characterization
Figure 4b,c,d shows the extinction spectra of the silver nanostructure solution obtained at different concentrations of PVPMW=29,000, PVPMW=40,000, and PVPMW=1,300,000, respectively. We find that when the concentration of PVP increases, the resonance peaks blue-shifted and extinction bands became narrow. The reason is that with the decrease of the nanoparticle size, the resonance peak will shift towards the shorter wavelength and uniform size will cause narrow extinction bands , which correspond to our experimental results.
Supporting evidence for the function of MW of PVP
Positions of free and coordinated C = O bands in Ag/PVP with four kinds of MWs
Another factor influencing the morphology of silver nanocrystals with different PVPs is the steric effect. Shorter chains of PVP cause a smaller steric effect which can combine PVP with silver nanoparticles in the colloid better but also results in incomplete coating of silver nanocrystals. In this case, silver nanocrystals may aggregate together. On the contrary, PVP with longer chains can protect silver nanocrystals from aggregation. However, a thicker coating on the surface of silver nanocrystals may decrease the strength of the coordination interaction between Ag+ ions and PVP.
Compared with PVPMW=8,000, PVPMW=29,000 with longer chains is able to offer more protection against aggregation, but weakest selective adsorption of PVP on the (100) facets of silver nanocrystals leads to the formation of isotropic seeds. Hence, in Figure 6b, one can see seeds prepared at 100°C mainly involving quasi-spherical seeds. Finally, these seeds evolved into nanospheres. The moderate selective adsorption of PVPMW=40,000 on the (100) facets results in exits of anisotropic seeds such as nanoplate and twinned pentahedron as shown in Figure 6c. Because each facet has different growth resistances, in different conditions, silver seeds evolve into different shapes . According to our observations, this is the main reason why PVPMW=40,000 is the commonest capping agent used for the preparation of silver nanoparticles. However, any undesired disturbance can greatly influence the morphologies of silver nanocrystals. For example, Tsuji et al.  demonstrated that there was a significant difference in the yield and average size of silver nanowires when they varied the reaction temperature or reaction atmosphere with PVPMW=40,000. As a result, although numerous nanocrystals have been obtained, PVPMW=40,000 is not the best choice for high-yield synthesis of silver nanocrystals due to limitations in production efficiency, yield, and reproducibility. PVPMW=1,300,000 has both the strongest interaction of PVP on the surface of silver nanocrystals and the ability of anti-agglomeration arising from longest chains, inducing the formation of twinned pentahedron seeds which can be observed in Figure 6d. According to the growth mechanism of silver nanowires reported by Xia et al. , twinned pentahedron seeds will evolve into nanowires finally.
In this study, we exhibit that the MW of PVP plays a critical role in the shape control of silver nanocrystals. The function of PVP on the shape control of silver nanocrystals can be discussed from two aspects: adsorption effect and steric effect. Results suggest that adsorption effect holds the dominated position in the selective adsorption of PVP on (100) facets of silver nanocrystals when the MW of PVP is very small, while with the increase of MW, the chemical adsorption gradually takes the place of the former. Therefore, different silver nanocrystals can be obtained by varying MWs of PVP. In addition, compared with the products obtained by varying the concentrations of PVP, we find that the MW of PVP plays a more efficient role in shape control. Our study on the effect of PVP with different MWs paves the way for the synthesis of silver monodisperse nanospheres and nanowires in high yield.
This work is supported by NSFC under grant number 61307066, Doctoral Fund of Ministry of Education of China under grant numbers 20110092110016 and 20130092120024, Natural Science Foundation of Jiangsu Province under grant number BK20130630, the National Basic Research Program of China (973 Program) under grant number 2011CB302004, and the Foundation of Key Laboratory of Micro-Inertial Instrument and Advanced Navigation Technology, Ministry of Education, China under grant number 201204.
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