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.
KeywordsPolyvinyl pyrrolidone Molecular weight Nanowire Nanosphere
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.
- Personick ML, Langille MR, Zhang J, Wu J, Li S, Mirkin CA: Plasmon-mediated synthesis of silver cubes with unusual twinning structures using short wavelength excitation. Small 2013, 9: 1947–1953. 10.1002/smll.201202451View Article
- Zhang XY, Hu AM, Zhang T, Lei W, Xue XJ, Zhou YH, Duley WW: Self-assembly of large-scale and ultrathin silver nanoplate films with tunable plasmon resonance properties. ACS Nano 2011, 5: 9082–9092. 10.1021/nn203336mView Article
- Zhang X-Y, Zhang T, Hu A, Song Y-J, Duley WW: Controllable plasmonic antennas with ultra narrow bandwidth based on silver nano-flags. Appl Phys Lett 2012, 101: 153118. 10.1063/1.4759122View Article
- Butun S, Sahiner N: A versatile hydrogel template for metal nano particle preparation and their use in catalysis. Polymer 2011, 52: 4834–4840. 10.1016/j.polymer.2011.08.021View Article
- Harish S, Sabarinathan R, Joseph J, Phani KLN: Role of pH in the synthesis of 3-aminopropyl trimethoxysilane stabilized colloidal gold/silver and their alloy sols and their application to catalysis. Mater Chem Phys 2011, 127: 203–207. 10.1016/j.matchemphys.2011.01.060View Article
- Hong Y, Huh Y-M, Yoon DS, Yang J: Nanobiosensors based on localized surface plasmon resonance for biomarker detection. J Nanomater 2012, 2012: 1–13.
- Stewart ME, Anderton CR, Thompson LB, Maria J, Gray SK, Rogers JA, Nuzzo RG: Nanostructured plasmonic sensors. Chem Rev 2008, 108: 494–521. 10.1021/cr068126nView Article
- Valsecchi C, Brolo AG: Periodic metallic nanostructures as plasmonic chemical sensors. Langmuir 2013, 29: 5638–5649. 10.1021/la400085rView Article
- Yang J, Wang Z, Zong S, Song C, Zhang R, Cui Y: Distinguishing breast cancer cells using surface-enhanced Raman scattering. Anal Bioanal Chem 2012, 402: 1093–1100. 10.1007/s00216-011-5577-zView Article
- Zhu SQ, Zhang T, Guo XL, Wang QL, Liu X, Zhang XY: Gold nanoparticle thin films fabricated by electrophoretic deposition method for highly sensitive SERS application. Nanoscale Res Lett 2012, 7: 613. 10.1186/1556-276X-7-613View Article
- Yang J, Wang Z, Tan X, Li J, Song C, Zhang R, Cui Y: A straightforward route to the synthesis of a surface-enhanced Raman scattering probe for targeting transferrin receptor-overexpressed cells. Nanotechnology 2010, 21: 345101. 10.1088/0957-4484/21/34/345101View Article
- Pietrobon B, Kitaev V: Photochemical synthesis of monodisperse size-controlled silver decahedral nanoparticles and their remarkable optical properties. Chem Mater 2008, 20: 5186–5190. 10.1021/cm800926uView Article
- Ray PC: Size and shape dependent second order nonlinear optical properties of nanomaterials and their application in biological and chemical sensing. Chem Rev 2010, 110: 5332–5365. 10.1021/cr900335qView Article
- Pignataro B, De Bonis A, Compagnini G, Sassi P, Cataliotti RS: The role of micro- and nanomorphology of rough silver surfaces of different nature in surface enhanced Raman scattering effect: a combined study of scanning force microscopy and low-frequency Raman modes. J Chem Phys 2000, 113: 5947. 10.1063/1.1311782View Article
- Wiley B, Sun YG, Mayers B, Xia YN: Shape-controlled synthesis of metal nanostructures: the case of silver. Chemistry 2005, 11: 454–463. 10.1002/chem.200400927View Article
- Wiley B, Sun YG, Xia YN: Synthesis of silver nanostructures with controlled shapes and properties. Acc Chem Res 2007, 40: 1067–1076. 10.1021/ar7000974View Article
- Wiley BJ, Im SH, Li ZY, McLellan J, Siekkinen A, Xia YN: Maneuvering the surface plasmon resonance of silver nanostructures through shape-controlled synthesis. J Phys Chem B 2006, 110: 15666–15675. 10.1021/jp0608628View Article
- Zhang Q, Hu Y, Guo S, Goebl J, Yin Y: Seeded growth of uniform Ag nanoplates with high aspect ratio and widely tunable surface plasmon bands. Nano Lett 2010, 10: 5037–5042. 10.1021/nl1032233View Article
- Sun YG, Yin YD, Mayers BT, Herricks T, Xia YN: Uniform silver nanowires synthesis by reducing AgNO3 with ethylene glycol in the presence of seeds and poly(vinyl pyrrolidone). Chem Mater 2002, 14: 4736–4745. 10.1021/cm020587bView Article
- Sun YG, Xia YN: Large-scale synthesis of uniform silver nanowires through a soft, self-seeding, polyol process. Adv Mater 2002, 14: 833–837. 10.1002/1521-4095(20020605)14:11<833::AID-ADMA833>3.0.CO;2-KView Article
- Sun YG, Gates B, Mayers B, Xia YN: Crystalline silver nanowires by soft solution processing. Nano Lett 2002, 2: 165–168. 10.1021/nl010093yView Article
- Korte KE, Skrabalak SE, Xia YN: Rapid synthesis of silver nanowires through a CuCl- or CuCl2-mediated polyol process. J Mater Chem 2008, 18: 437–441. 10.1039/b714072jView Article
- Pradel KC, Sohn K, Huang J: Cross-flow purification of nanowires. Angew Chem Int Ed Engl 2011, 50: 3412–3416. 10.1002/anie.201100087View Article
- Chou KS, Lai YS: Effect of polyvinyl pyrrolidone molecular weights on the formation of nanosized silver colloids. Mater Chem Phys 2004, 83: 82–88. 10.1016/j.matchemphys.2003.09.026View Article
- Liu XH, Zhang F, Huang R, Pan CF, Zhu J: Capping modes in PVP-directed silver nanocrystal growth: multi-twinned nanorods versus single-crystalline nano-hexapods. Cryst Growth Des 2008, 8: 1916–1923. 10.1021/cg701128bView Article
- Tang X, Tsuji M, Jiang P, Nishio M, Jang S-M, Yoon S-H: Rapid and high-yield synthesis of silver nanowires using air-assisted polyol method with chloride ions. Colloids Surf A Physicochem Eng Asp 2009, 338: 33–39. 10.1016/j.colsurfa.2008.12.029View Article
- Chen D, Gao L: Large-scale growth and end-to-end assembly of silver nanorods by PVP-directed polyol process. J Cryst Growth 2004, 264: 216–222. 10.1016/j.jcrysgro.2003.12.019View Article
- Li H, Xia H, Wang D, Tao X: Simple synthesis of monodisperse, quasi-spherical, citrate-stabilized silver nanocrystals in water. Langmuir 2013, 29: 5074–5079. 10.1021/la400214xView Article
- Sun YG, Mayers B, Herricks T, Xia YN: Polyol synthesis of uniform silver nanowires: a plausible growth mechanism and the supporting evidence. Nano Lett 2003, 3: 955–960. 10.1021/nl034312mView Article
- Zhang XY, Zhang T, Zhu SQ, Wang LD, Liu X, Wang QL, Song YJ: Fabrication and spectroscopic investigation of branched silver nanowires and nanomeshworks. Nanoscale Res Lett 2012, 7: 596. 10.1186/1556-276X-7-596View Article
- Li L, Sun J, Li X, Zhang Y, Wang Z, Wang C, Dai J, Wang Q: Controllable synthesis of monodispersed silver nanoparticles as standards for quantitative assessment of their cytotoxicity. Biomaterials 2012, 33: 1714–1721. 10.1016/j.biomaterials.2011.11.030View Article
- Tsuji M, Tang X, Matsunaga M, Maeda Y, Watanabe M: Shape evolution of flag types of silver nanostructures from nanorod seeds in PVP-assisted DMF solution. Cryst Growth Des 2010, 10: 5238–5243. 10.1021/cg101041mView Article
- Kim JH, Min BR, Kim CK, Won J, Kang YS: Spectroscopic interpretation of silver ion complexation with propylene in silver polymer electrolytes. J Phys Chem B 2002, 106: 2786–2790. 10.1021/jp010481oView Article
- Gao Y, Jiang P, Liu DF, Yuan HJ, Yan XQ, Zhou ZP, Wang JX, Song L, Liu LF, Zhou WY, Wang G, Wang CY, Xie SS, Zhang JM, Shen AY: Evidence for the monolayer assembly of poly(vinylpyrrolidone) on the surfaces of silver nanowires. J Phys Chem B 2004, 108: 12877–12881. 10.1021/jp037116cView Article
- Peng P, Liu L, Gerlich AP, Hu A, Zhou YN: Self-oriented nanojoining of silver nanowires via surface selective activation. Particle & Particle Systems Characterization 2013, 30: 420–426. 10.1002/ppsc.201200099View Article
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