A Simple Synthetic Approach To Prepare Silver Elongated

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

A Simple Synthetic Approach To Prepare Silver Elongated Nanostructures: From Nanorods to Nanowires Giovanni Ferraro and Emiliano Fratini* Department of Chemistry “Ugo Schiff” and CSGI, University of Florence, via della Lastruccia 3-Sesto Fiorentino, I-50019 Florence, Italy

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S Supporting Information *

ABSTRACT: A procedure for one-pot preparation and characterization of silver 1D nanostructures is described. The main advantages of this synthetic approach are the simplicity and reproducibility, where the size of the final product can be controlled just by changing the reaction time. This experiment is designed as a laboratory introduction to colloidal chemistry for undergraduate and graduate students; they can appreciate the role of the different reagents in a relatively complex reaction and understand the relation between the morphology of a metal at the nanoscale and some macroscopic properties, such as its visible absorption spectrum (i.e., color).

KEYWORDS: Nanotechnology, Colloids, UV−Vis Spectroscopy, Synthesis, First-Year Undergraduate/General, Second-Year Undergraduate, Hands-On Learning/Manipulatives, Laboratory Instruction, Physical Chemistry, Metals

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INTRODUCTION Silver elongated nanostructures, especially nanowires, are useful in a wide variety of electronic,1 optical,1 and antimicrobial2 applications. Nanowires are defined as 1D nanostructures with diameters on the order of tens of nanometers (always below 100 nm) and lengths above 100 nm. Ag-based metallic wires compete with expensive conductive materials, such as indium tin oxide (ITO) and carbon nanotubes (CNTs), because they can offer higher light transmission (especially with respect to CNT-based devices), greater flexibility, bendability, durability, and cost effectiveness.3 For these reasons, silver nanowires are highly desired components for touchscreen displays, photovoltaics, conductive adhesives, and light emitting devices.4−6 In addition, these nanostructures can be implemented in medical devices, food packages, and air/water purification apparatus,7 considering the well-known antimicrobial properties of silver.8 In recent years, several synthetic approaches have been proposed to produce metal nanowires of different elements: chemical synthesis,9,10 electrochemical techniques,11 hydrothermal methods,12 ultraviolet irradiation photodetection techniques,13 synthesis using porous materials as templating agents,14 and polyol processes.15 All of the mentioned methods allow researchers to obtain good control of the final morphological properties of the nanowires, but they are expensive and difficult to perform in a laboratory experience. In this paper, we report on an easy synthetic approach based on an optimized polyol process to produce silver elongated nanostructures.16 This approach involves the use of ethylene © XXXX American Chemical Society and Division of Chemical Education, Inc.

glycol (EG) as both solvent and reducing agent, AgNO3 as Ag precursor, and polyvinylpyrrolidone (PVP) as capping agent. Finally, Cu+ and Cl− ions are added to facilitate the growth of Ag nanostructures. Students will obtain Ag 1D nanoparticles of different lengths by quenching the reaction at different times in the range 1−60 min. Considering that nanosized metals possess peculiar size-dependent optical properties, as reported for example in the case of Cu2O particles,17 the different dimensions of the obtained silver nanostructures can be immediately appreciated from the direct observation of the color change as time passes. Scanning electron microscopy (SEM) allows to directly visualize the morphology and the aspect ratio of the nanowires obtained at different reaction times while UV−vis spectra can be acquired to assess the relationship between the optical properties of the nanomaterial and its size. This laboratory experiment was successfully performed by 40 students during the first year of a master’s degree program in Chemical Sciences at the University of Florence. Groups of four students each are suggested, and the whole experiment can be accomplished in a time frame of about 3−4 h (excluding SEM analysis). Received: September 7, 2018 Revised: November 22, 2018

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DOI: 10.1021/acs.jchemed.8b00628 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. Summary of reaction pathways leading to fcc metal nanocrystals with different shapes. Adapted with permission from ref 23. Copyright 2007 Wiley-VCH.

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EXPERIMENTAL OVERVIEW Different procedures based on chemical methods for the synthesis of silver nanostructures were reported in this Journal: readers are referred to the articles by Mulfinger18 and Orbaek19 for the preparation of Ag nanoparticles or the one by Frank et al. in the case of Ag nanoprisms.20 The procedure proposed here to obtain Ag nanowires is based on a previous literature report15 and consists of the controlled growth of silver seeds mediated by a PVP capping agent. Ag+ ions are reduced by ethylene glycol (EG) at high temperature, and the size of the obtained nanostructures is controlled by quenching the solution at room temperature. The addition of Cu2+ and Cl− ions ensures the preferential growth of nanowires instead of other nanostructures. The presence of the desired silver nanowires is inferred by the change of the color of the solution. Experimental details on the synthesis and the characterization techniques are reported in Supporting Information.

Student Learning Objectives

Hazards

The formation of silver nanostructures involves two main steps.21 The first step is the formation of silver seeds in solution through the reduction of Ag+ to Ag0 by ethylene glycol at high temperature (∼152 °C). The growth at 110 °C produces small Ag structures; between 130 and 150 °C the amount of rod-shaped structures increases, while in the range 150−170 °C the formation of Ag nanowires is maximized.21 The presence of chloride ions in solution helps to reduce the concentration of free Ag+ through AgCl formation (Ksp of AgCl = 1.7 × 10−10 ≪ Ksp of AgNO3 = 5.8 × 10−4).22 The decrease in free Ag+ ensures the formation of multiply twinned seeds required for the growth of nanowires,23 as shown in Figure 1. A deeper explanation of the role played by Cl− can be found in Supporting Information.

The key learning objectives of this experience can be summarized as follows: (1) perform the synthesis of metal nanostructures with controlled shapes and dimensions and understand the role of each reagent and reaction parameters in a complex process; (2) correlate the size of nanostructures with their optical properties; (3) obtain hands-on experience with different techniques such as electron/optical microscopy and UV−vis spectroscopy; (4) improve the knowledge of image analysis software to obtain information from micrographs.

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RESULTS AND DISCUSSION

Reaction Mechanism

Students must follow general safety precautions in chemistry laboratories: rubber gloves, goggles, and lab coats have to be worn for all the experiments to prevent contact with chemicals. All manipulations must be carried out in a standard operating fume hood. Always consult the materials safety data sheet (MSDS) for chemicals before the experiment. Ethylene glycol and copper dichloride may cause irritation to skin and eyes on contact. Silver nitrate is poisonous, corrosive and a strong oxidizer. It causes irritation (i.e. to skin, eyes, and respiratory tract) and it is harmful if swallowed or inhaled. Specific hazards associated with each reagent are listed in Supporting Information. B




In the second step, silver seeds grow to form nanostructures with different shapes. The different shapes can be tuned by controlling the growth on the different facets of the silver seeds. Briefly, Cu2+ ions act as atomic oxygen scavengers, thus removing the oxygen adsorbed on the silver surface and avoiding the stop of silver growth. 24 Moreover, PVP preferentially adsorbs on (100) faces of silver seeds thus preventing the growth on these faces and favoring the anisotropic growth in the other directions, as illustrated in Figure 2. Further details on the role of each reagent are given in Supporting Information.

Figure 2. Role of PVP (red stripes) in the growth of silver nanowires.

The schematic representation of the entire silver nanowires growth process is reported in Figure 3.

Figure 4. Picture showing the color of the different Ag nanowires aliquots taken at 1, 3, 4, 5, 8, and 30 min of reaction, respectively (top). UV−vis spectra of all the aliquots between 1 and 60 min (bottom).

Figure 3. Schematic representation of the main steps of silver nanowire growth: (1) silver seeds formation through the reduction of Ag+ ions by ethylene glycol, (2) multiply twinned seed formation assisted by Cl− ions, and (3) nanowire growth along the (111) face thanks to the presence of PVP as capping agent and Cu+ ions as atomic oxygen scavengers.

nanowire cross-section. The second absorption peak is not visible in the investigated energy range because of the formation of very long nanowires. Scanning Electron Microscopy of Ag Nanowires

The morphology of the nanowires can be assessed by performing scanning electron microscopy experiments. Figure 5 shows the characteristic geometry of the edge originating from a multiply twinned seed.

Optical Properties

The color of metal nanostructure dispersions is strongly dependent on the shape and the size of the objects in solution according to Mie-Gans theories detailed in the Supporting Information. It is highly recommended that the reader refer also to the work by Xia and Campbell25 published in this Journal for an overview of the interaction between light and nanostructured metals. Students can directly observe the change of the silver nanowire aspect ratio by simply monitoring the color of the different aliquots at different reaction times, as shown in Figure 4. The dispersion after 1 min of reaction presents just one absorption peak centered at approximately 410 nm, while the presence of a second absorption band is visible for the aliquots taken between 3 and 8 min of reaction. This observation is consistent with the presence of spherical silver nanostructures in the first aliquot, while upon increasing the reaction time, the formation of elongated structures is responsible for the appearance of the second peak ascribable to the longitudinal motion of electrons in the nanowires. Since the diameter of the nanowire is about the same as the diameter of the seed, the position of the first peak is initially unchanged. Furthermore, the second absorption peak shifts from 500 to 665 nm increasing the reaction time, thus indicating an increase of the aspect ratio of the objects in solution. Finally, the absorption spectra of the samples obtained at 30 and 60 min of reaction show a broad first absorption peak shifted to a higher wavelength, which is compatible with the increase in the

Figure 5. SEM micrographs of Ag nanowires taken after 60 min of reaction (right) and at 10× magnification (left).

The growth of silver nanowires can be confirmed by acquiring SEM micrographs at different magnifications on the aliquots taken after 10, 25, and 60 min of reaction reported in Figure 6 as an example. During the first 10 min of reaction, the formation of spherical nanoparticles together with some low aspect ratio nanorods occurs. Increasing the reaction time up to 25 min, we can observe an increase of the mean dimension of these objects, while long nanorods appear after 60 min of reaction. This growth mechanism is already reported in the C



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00628.

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Student handout and notes for the instructor (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: emiliano.fratini@unifi.it. ORCID

Emiliano Fratini: 0000-0001-7104-6530 Notes

The authors declare no competing financial interest.

Figure 6. SEM micrographs at different magnifications of Ag nanowires taken after 10, 25, and 60 min of reaction.

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ACKNOWLEDGMENTS The authors acknowledge financial support from Consorzio per lo sviluppo dei Sistemi a Grande Interfase (CSGI) and Ministero dell’Istruzione, dell’Università e della Ricerca (MiUR).

literature26,27 and described as a growing process in which larger nanostructures form through the Ostwald ripening mechanism in which smaller particles solubilize and increase the size of larger structures that are thermodynamically more stable. In particular, in the micrographs acquired after 60 min, we can observe the presence of wires with different length and thickness together with other silver morphologies, like rods or platelets. In addition, it is possible to evaluate both the length and the diameter of the synthesized nanostructures by analyzing SEM micrographs with the appropriate image processing software, such as ImageJ.28 An example of the obtained results is reported in Supporting Information together with the instructions for the use of ImageJ software.

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REFERENCES

(1) Murphy, C. J.; Gole, A. M.; Hunyadi, S. E.; Orendorff, C. J. OneDimensional Colloidal Gold and Silver Nanostructures. Inorg. Chem. 2006, 45 (19), 7544−7554. (2) Valodkar, M.; Sharma, P.; Kanchan, D. K.; Thakore, S. Conducting and Antimicrobial Properties of Silver Nanowire−Waxy Starch Nanocomposites. Int. J. Green Nanotechnol. Phys. Chem. 2010, 2 (1), P10−P19. (3) Madaria, A. R.; Kumar, A.; Zhou, C. Large Scale, Highly Conductive and Patterned Transparent Films of Silver Nanowires on Arbitrary Substrates and Their Application in Touch Screens. Nanotechnology 2011, 22 (24), 245201. (4) Lee, P.; Lee, J.; Lee, H.; Yeo, J.; Hong, S.; Nam, K. H.; Lee, D.; Lee, S. S.; Ko, S. H. Highly Stretchable and Highly Conductive Metal Electrode by Very Long Metal Nanowire Percolation Network. Adv. Mater. 2012, 24 (25), 3326−3332. (5) Lee, M.-S.; Lee, K.; Kim, S.-Y.; Lee, H.; Park, J.; Choi, K.-H.; Kim, H.-K.; Kim, D.-G.; Lee, D.-Y.; Nam, S.; et al. High-Performance, Transparent, and Stretchable Electrodes Using Graphene-Metal Nanowire Hybrid Structures. Nano Lett. 2013, 13 (6), 2814−2821. (6) Lee, J.; Lee, P.; Lee, H.; Lee, D.; Lee, S. S.; Ko, S. H. Very Long Ag Nanowire Synthesis and Its Application in a Highly Transparent, Conductive and Flexible Metal Electrode Touch Panel. Nanoscale 2012, 4 (20), 6408−6414. (7) Ojha, A. K.; Forster, S.; Kumar, S.; Vats, S.; Negi, S.; Fischer, I. Synthesis of Well-Dispersed Silver Nanorods of Different Aspect Ratios and Their Antimicrobial Properties against Gram Positive and Negative Bacterial Strains. J. Nanobiotechnol. 2013, 11, 42. (8) Sondi, I.; Salopek-Sondi, B. Silver Nanoparticles as Antimicrobial Agent: A Case Study on E. Coli as a Model for Gram-Negative Bacteria. J. Colloid Interface Sci. 2004, 275 (1), 177−182. (9) Kim, S. H.; Choi, B. S.; Kang, K.; Choi, Y.-S.; Yang, S. I. Low Temperature Synthesis and Growth Mechanism of Ag Nanowires. J. Alloys Compd. 2007, 433 (1−2), 261−264. (10) da Silva, A. G. M.; Rodrigues, T. S.; Parussulo, A. L. A.; Candido, E. G.; Geonmonond, R. S.; Brito, H. F.; Toma, H. E.; Camargo, P. H. C. Controlled Synthesis of Nanomaterials at the Undergraduate Laboratory: Cu(OH)2 and CuO Nanowires. J. Chem. Educ. 2017, 94 (6), 743−750. (11) Cui, S.; Liu, Y.; Yang, Z.; Wei, X. Construction of Silver Nanowires on DNA Template by an Electrochemical Technique. Mater. Eng. 2007, 28 (2), 722−725.

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CONCLUSIONS This paper describes a very simple procedure to synthesize silver nanowires with different dimensions. The main advantage of the proposed synthetic approach is that it is a one-pot synthesis with good reproducibility, in which the size of the final product can be controlled just by changing the reaction time. This experience was successfully performed by 40 students during the first year of a master’s degree program in Chemical Sciences. The students really appreciated this experiment since it allows understanding the role of the different reagents in a relatively complex reaction and the relation between the morphology of a material at the nanoscale and some macroscopic properties, such as the perceived color. Indeed, the growth of silver structures at the nanoscale can be followed in situ by monitoring changes in the light absorption of the solution. In this particular instance, the formation of silver nanostructures starts immediately after the reagents are mixed, and their growth can be followed for up to 60 min looking at the noticeable change of the color of the solution, making the event quite memorable. This experiment is optimized for student laboratory courses, but the possibility of using scanning electron microscopy to investigate the size and the morphology of the obtained nanowires makes the experience interesting also for students attending upper-level courses. In addition to the SEM experiment, the micrograph analysis to estimate the size of the obtained nanostructures represents a good opportunity to approach the use of image processing software, such as ImageJ. D




(12) Xu, J.; Hu, J.; Peng, C.; Liu, H.; Hu, Y. A Simple Approach to the Synthesis of Silver Nanowires by Hydrothermal Process in the Presence of Gemini Surfactant. J. Colloid Interface Sci. 2006, 298 (2), 689−693. (13) Liu, L.; He, C.; Li, J.; Guo, J.; Yang, D.; Wei, J. Green Synthesis of Silver Nanowires via Ultraviolet Irradiation Catalyzed by Phosphomolybdic Acid and Their Antibacterial Properties. New J. Chem. 2013, 37 (7), 2179. (14) Bentley, A. K.; Farhoud, M.; Ellis, A. B.; Nickel, A.-M. L.; Lisensky, G. C.; Crone, W. C. Template Synthesis and Magnetic Manipulation of Nickel Nanowires. J. Chem. Educ. 2005, 82 (5), 765. (15) Lee, J. H.; Lee, P.; Lee, D.; Lee, S. S.; Ko, S. H. Large-Scale Synthesis and Characterization of Very Long Silver Nanowires via Successive Multistep Growth. Cryst. Growth Des. 2012, 12 (11), 5598−5605. (16) Korte, K. E.; Skrabalak, S. E.; Xia, Y. Rapid Synthesis of Silver Nanowires through a CuCl- or CuCl2-Mediated Polyol Process. J. Mater. Chem. 2008, 18 (4), 437−441. (17) Markina, N. E.; Pozharov, M. V.; Markin, A. V. Synthesis of Copper(I) Oxide Particles with Variable Color: Demonstrating SizeDependent Optical Properties for High School Students. J. Chem. Educ. 2016, 93 (4), 704−707. (18) Mulfinger, L.; Solomon, S. D.; Bahadory, M.; Jeyarajasingam, A. V.; Rutkowsky, S. A.; Boritz, C. Synthesis and Study of Silver Nanoparticles. J. Chem. Educ. 2007, 84 (2), 322. (19) Orbaek, A. W.; McHale, M. M.; Barron, A. R. Synthesis and Characterization of Silver Nanoparticles for an Undergraduate Laboratory. J. Chem. Educ. 2015, 92 (2), 339−344. (20) Frank, A. J.; Cathcart, N.; Maly, K. E.; Kitaev, V. Synthesis of Silver Nanoprisms with Variable Size and Investigation of Their Optical Properties: A First-Year Undergraduate Experiment Exploring Plasmonic Nanoparticles. J. Chem. Educ. 2010, 87 (10), 1098−1101. (21) Coskun, S.; Aksoy, B.; Unalan, H. E. Polyol Synthesis of Silver Nanowires: An Extensive Parametric Study. Cryst. Growth Des. 2011, 11 (11), 4963−4969. (22) Speight, J. Lange’s Handbook of Chemistry; McGraw-Hill Professional: New York, 2005. (23) Xiong, Y.; Xia, Y. Shape-Controlled Synthesis of Metal Nanostructures: The Case of Palladium. Adv. Mater. 2007, 19 (20), 3385−3391. (24) Amirjani, A.; Marashi, P.; Fatmehsari, D. H. Effect of AgNO3 Addition Rate on Aspect Ratio of CuCl 2−Mediated Synthesized Silver Nanowires Using Response Surface Methodology. Colloids Surf., A 2014, 444, 33−39. (25) Xia, Y.; Campbell, D. J. Plasmons: Why Should We Care? J. Chem. Educ. 2007, 84 (1), 91. (26) Wiley, B.; Sun, Y.; Xia, Y. Synthesis of Silver Nanostructures with Controlled Shapes and Properties. Acc. Chem. Res. 2007, 40 (10), 1067−1076. (27) Murph, S. E. H.; Murphy, C. J.; Leach, A.; Gall, K. A Possible Oriented Attachment Growth Mechanism for Silver Nanowire Formation. Cryst. Growth Des. 2015, 15 (4), 1968−1974. (28) Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W. NIH Image to ImageJ: 25 Years of Image Analysis. Nat. Methods 2012, 9 (7), 671− 675.

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A Simple Synthetic Approach To Prepare Silver Elongated

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