Characterization of Micro-and Nanoscale Silver Wires Synthesized

Laboratory Experiment pubs.acs.org/jchemeduc

Characterization of Micro- and Nanoscale Silver Wires Synthesized Using a Single-Replacement Reaction between Sputtered Copper Metal and Dilute Silver Nitrate Solutions Wesley C. Sanders,* Peter D. Ainsworth, David M. Archer, Jr., Michael L. Armajo, Cariann E. Emerson, Joven V. Calara, Matthew L. Dixon, Samuel T. Lindsey, Holly J. Moore, and Jonathon D. Swenson Engineering Department, Salt Lake Community College, Salt Lake City, Utah 84123, United States S Supporting Information *

ABSTRACT: This paper describes an experiment developed to provide students enrolled in introductory nanotechnology and introductory chemistry courses with an opportunity to synthesize silver nanowires using a bottom-up nanofabrication technique. The reaction between copper metal sputter coated on silicon dioxide and a dilute silver nitrate solution is used to create the nanowires. Included in this experiment is a characterization component allowing students to analyze the nanowires using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Microscale silver wires are also produced in the reaction, making it possible for students to utilize optical microscopy, in addition to AFM and SEM, for silver wire characterization. The silver nanowire synthesis reported in this paper does not require the use of nanoporous templates or hazardous purification steps to liberate wires from template confinement. KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Upper-Division Undergraduate, Anaytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Nanotechnology, Reactions, Materials Science, Surface Science

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Cu(s) + 2AgNO3 (aq) → Cu(NO3)2 (aq) + 2Ag(s)

xposing two-year and four-year college students to nanoscience and nanotechnology is critical, because two million workers with nanotechnology related skill sets will be needed worldwide by 2015.1 Academic institutions and funding agencies, such as the National Science Foundation’s Nanotechnology in Undergraduate Education program, suggest that early incorporation of nanotechnology in college curriculum exposes students to exciting research and prepares them for career endeavors associated with nanotechnology.2 Additionally, faculty at various academic institutions have developed undergraduate experiments that demonstrate the synthesis and utility of nanomaterials.3−11 One academic organization in particular, the Interdisciplinary Education Group from the University of WisconsinMadison’s Materials Research and Science Engineering Center, developed a series of experiments to provide college students with hands-on opportunities to explore nanotechnology.12 A report suggests that exposing students to experiments related to nanotechnology and nanomaterial synthesis does not have to be costly, because some experiments can be conducted using standard laboratory equipment and procedures.13 The experiment presented in this paper is proof of this assertion. Silver nanowire synthesis is accomplished using a single replacement reaction between sputter-coated copper metal and a dilute solution of silver nitrate (eq 1). It is noteworthy to mention that this reaction is used in the classic “silver tree” demonstration.14 © XXXX American Chemical Society and Division of Chemical Education, Inc.

(1)

The prevalence of chemistry concepts associated with the nanowire synthesis experiment suggests that it could be used in an introductory chemistry course to illustrate the frequent intersection of nanotechnology with the physical sciences. This is important because there is little opportunity in most undergraduate chemistry courses for students to prepare nanoscale materials and explore how these materials will be used in the technologies of the future.4

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SILVER NANOWIRE SYNTHESES There are several procedures that can be used to synthesize silver nanowires, such as the polyol method, which involves the reduction of silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone) or PVP.15−17 The use of supercritical fluids for nanowire synthesis is also reported.18 A solutionphase approach employing the use of surface capping reagents to produce nanowires is reported by Zhang and co-workers.19 According to their report, silver nanowires are generated on the surface of a glass slide by reacting silver nitrate with ascorbic acid and polymethyl methacrylate (PMMA). Their technique, however, requires additional isolation and purification steps to remove unreacted polymeric materials.19 An additional nanowire synthesis found in the literature is template-assisted

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dx.doi.org/10.1021/ed300483p | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. (A) A metal sample with increased surface roughness immersed in a silver nitrate solution. (B) Silver nanoparticles form at the tops of surface defects. (C and D) Silver ions diffuse from the bulk solution and are reduced at the metal nanoparticles resulting in the formation of silver wires. (E) A smooth metal sample with decreased surface roughness is immersed in a silver nitrate solution. (F) Silver nanoparticles form at the surface. (G) Continued reduction of silver particles cover the substrate and (H) prevent further reduction of silver ions.

nanowire formation.4,20−22 Bentley et al. demonstrate successful implementation of a nickel nanowire synthesis experiment for undergraduates utilizing an external voltage for the reduction of nickel ions in the nanopores of an alumina filter to form the nanowires.4 Dadvand and Kipouros report the use of a nanoporous alumina template for the synthesis of nanowires without the aid of an external voltage.22 However, removal of the template poses practical problems.23 Associated with the template-assisted synthesis method is the necessity of a caustic, highly concentrated sodium hydroxide solution to liberate the nanowires.4 A simpler approach to nanowire synthesis is reported by Jiang and co-workers.23 Their nanowire synthesis involves a single replacement reaction between a metal plate and a dilute silver nitrate solution, where neutral metal atoms of a more active metal are oxidized and the silver metal cations are reduced.23 This method requires a silver nitrate concentration within a specific range.23 Jiang and co-workers state that successful silver nanowire production requires a concentration between 5 × 10−4 and 1 × 10−3 M, when iron is used as the metal reacting with the silver nitrate solution.23 Their results show that higher concentrations form dendritic structures and lower concentrations form irregularly shaped silver nanopartices that absorb oxygen and other impurities, inhibiting the growth of silver nanowires.23 While developing this experiment, reactions between copper-coated silicon dioxide wafers and various silver nitrate concentrations within the range stated in the Jiang paper were observed. Concentrations higher and lower than the reported range were tested also. It was determined that an approximate silver nitrate concentration of 1.0 × 10−2 M produced silver nanowires consistently. The silver nitrate concentration used in this experiment is higher than the optimal silver nitrate concentration stated by Jiang et al.23 This discrepancy is due to the activities of copper and iron; copper is lower on the activity series than iron.24,25 The activity of a metal is often quantified using standard reduction

potential.26 Jiang states that encouraging nanowire growth is more difficult with a less active metal, such as copper, due to its smaller reduction potential.23 Therefore, it is believed that a higher concentration of silver nitrate, higher than what is stated in the Jiang paper,23 is required to produce silver nanowires when copper metal is used. Exact details of the procedure used to determine optimal silver nitrate concentration is provided in the Supporting Information. In addition to determining effective silver nitrate concentration, surface roughness of metal substrates requires consideration as well.23 When metal substrates are immersed in aqueous silver nitrate, the behavior of the ions interacting with the metal surface is consistent with the Guoy-Chapman Stern model of the electrochemical double layer. According to this model, ions located in the bulk solution act as point charges that migrate toward the metal forming a plane of closest approach.27,28 This plane consists of a compact, inner layer of directly adsorbed ions.29,30 The products formed when the plane of silver ions reach the metal surface depend strongly on the surface roughness of the metal. Metal surfaces with increased surface roughness contain large number of active sites (surface defects) that expedite reduction of Ag+ ions directly adjacent to the metal (Figure 1A).23 This results in the formation of silver nanoparticles on the metal surface (Figure 1B).23 Ions in the bulk solution then diffuse to the tops of the silver particles as they are reduced, forming of one-dimensional wires (Figures 1C and 1D).23 Smooth metal surfaces with rounded surface features, however, form silver metal aggregates that completely cover the metal surface preventing further Ag+ reduction (Figure 1E−H).23 The metal surface used to form the silver nanowires is copper deposited on a silicon dioxide substrate using sputter coating. Sputter coating provides a safe means for increasing surface roughness. Sun et al. report that hazardous chemicals, not suitable for undergraduate experiments, are often required for nanomaterial synthesis.3 For example, the surface roughing B




procedure reported by Jiang and co-workers involves the use of highly concentrated nitric acid.23 Sputter coating does not require the use of hazardous chemicals or abrasives, and it eliminates the need for harsh purification steps. These purification steps are often required for removal of unwanted materials such as alumina templates4,22 or unreacted polymer.19

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EXPERIMENT

Instructor Preparations

Several preparations were made by the instructor before students participated in the experiment. Copper-coated silicon dioxide plates were prepared using a Denton Vacuum Desk V Sputter Coater equipped with a copper source. Scanning tunneling microscopy (STM) was used to compare the surfaces of sputter-coated copper surfaces with solid copper surfaces. Exact details of the STM imaging procedure are listed in the Supporting Information. It is important to note that students did not perform STM imaging, and no information regarding STM operation was provided during the pre-lab lecture. After STM imaging, the sputter-coated plates were placed in a clean, dry Petri dish until use. Dilute silver nitrate solutions were prepared ahead of time by the instructor. Exact details regarding silver nitrate solution preparations are included in the Supporting Information. The instructor added 150 μL of 1.02 × 10−2 M silver nitrate solution to the surface of several 5 cm2 squares of solid copper metal. This was done to show students that silver dendrites are formed when solid copper metal is used in the reaction with dilute silver nitrate, instead of sputtered copper.

Figure 2. Illustration of a sputter coater utilizing physical vapor deposition (PVD) for thin film deposition.

Pre-Lab Instruction

Before facilitating the experiment, faculty presented a pre-lab lecture describing the processes involved with nanowire formation. During the lecture, students were informed that the silver nanowire synthesis employed is a form of bottom-up nanofabrication because the reduction of cationic particles (Ag+ ions) forms silver nanowires. An explanation of top-down nanofabrication, a process that relies on a gradual reduction from the macroscale to the nanoscale to form structures,31 was included for comparative purposes. Students learned that the substrate used to facilitate Ag+ reduction was prepared using sputter coating (Figure 2), a form of physical vapor deposition (PVD), and one of the most frequently used methods for thin film deposition.32,33 It was also mentioned that PVD uses plasma for removal of atoms from a metal target under reduced pressure which deposit on the surface of a substrate.34,35 Faculty used STM images for the purpose of comparing sputter-coated copper surfaces (Figure 3A) to surfaces of solid copper metal (Figure 3B). Data from the STM images was used to prepare cross-sectional profiles of both surfaces (Figure 4). The cross-sectional profiles were shown to students during the pre-lab lecture, which allowed them to observe that sputter coating produced “jagged copper mountains” with a maximum height of 7 nm and that features on the surface of the pure copper metal were smoother with a maximum height of 2 nm. Atomic force microscopy (AFM) was used to characterize the wires. Incorporation of a characterization component using specialized microscopes is consistent with existing literature precedents involving the use of scanning probe microscopes in undergraduate experiments.31−49 During the pre-lab lecture, students were provided with a general description of AFM

Figure 3. STM images of copper sputtered on (A) a silicon dioxide plate for 15 min and (B) solid copper metal.

Figure 4. Cross-sectional profile of copper sputtered on a silicon dioxide plate and solid copper metal.

operation (Figure 5). Several concepts were included in the lecture such as how changes in the z movements of a tip scanning a sample surface is monitored using a laser beambounce detection system, how a reflected laser beam strikes a photodetector to generate signals representative of z movements of the tip, and how these signals are collected by a computer and used by image processing software to produce a topographical image of the sample surface.50,51 C




soluble copper nitrate product. This prevented the accumulation of salt crystals that would interfere with characterization of the wires. Students characterized the silver wires with an optical microscope, AMF, and SEM.

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HAZARDS Isopropyl alcohol is flammable; do not use alcohol near an open flame or a heat source. Silver nitrate is a strong oxidizer, causes irritation to skin, eyes, and respiratory tract and is harmful if swallowed or inhaled. Students should wear rubber gloves and goggles for all the experiments to prevent contact with chemicals used in this lab.

Figure 5. AFM operation.

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Like AFM, there are reports that describe successful implementation of SEM characterization in undergraduate, nanotechnology-related experiments as well.3,5 To follow this trend, a SEM component was incorporated. A general overview of SEM operation was provided in the pre-lab lecture. Information provided to the students detailed the use of a beam of electrons to scan sample surfaces (Figure 6).

RESULTS

Optical Microscope

At the end of the wire synthesis, students used a Fisher Micromaster Optical Microscope to observe that microscale silver wires were produced in the reaction (Figure 7).

Figure 7. A 400× optical image of silver wires produced by the reaction between sputtered copper and dilute silver nitrate.

Atomic Force Microscopy

Figure 6. SEM operation.

After commencement of the experiment, students signed up for time outside of class for supervised use of AFM and SEM instrumentation to characterize the silver wires. Under faculty supervision, students used an Agilent 5400 AFM operating in tapping mode to obtain images of the silver nanowires (Figure 8). Additionally, students were guided in the use of WSXM

Additional information included the following: how an electron beam is produced by a heated filament, how electromagnetic coils are used to control the diameter of the electron beam, and how raster coils are used to scan the electron beam across a sample surface.52 An explanation regarding the production of secondary electrons, backscattered electrons, and X-rays was also included along with statements explaining that these signals are used to produce topographic images, to examine sample composition, and for elemental identification, respectively.52 Execution

This experiment was used in an introductory nanotechnology course. Participating students, mostly engineering and science majors, worked individually taking approximately 20−25 min to complete the wire synthesis. Students began by rinsing the copper-coated silicon dioxide plates with 70% isopropyl alcohol and drying them with nitrogen. Next students added 150 μL of 1.0 × 10−2 M silver nitrate, prepared by the instructor, to the copper-coated silicon dioxide plate. The silver nitrate solution was allowed to react with the coated plate for 5 min, producing silver wires with varying lengths and diameters. After 5 min, students dried the plates with nitrogen. Drying with a gentle stream of nitrogen removed any unreacted silver nitrate and the

Figure 8. Student-generated AFM image of silver wires.

image processing software53 to generate cross-sectional profiles of the silver nanowires; a sample student cross-sectional profile is shown in Figure 9. These profiles allowed students to obtain silver nanowire diameters. D



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ASSOCIATED CONTENT

S Supporting Information *

Instructor notes describing more detailed procedures involved with the preparation of silver nitrate solutions and imaging with the scanning tunneling microscope (STM) and a sample student handout. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Figure 9. Cross-sectional profiles of silver nanowires.

Notes

Scanning Electron Microscopy

The authors declare no competing financial interest.

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Student-generated SEM images are shown in Figure 10. Figure 10A is an image of silver dendrites formed when 1.0 × 10−2 M

ACKNOWLEDGMENTS The authors thank the Utah Engineering Initiative for generous support. The authors also gratefully acknowledge Adrianne Gill-Sanders for useful discussions and suggestions.

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REFERENCES

(1) Roco, M. C. Nanoscale Science and Engineering Education Activities in the United States (2001−2002). J. Nanopart. Res. 2002, 4, 271−274.1. (2) Winkelmann, K.; Noviello, T.; Brooks, S. J. Preparation of CdS Nanoparticles by First-Year Undergraduates. J. Chem. Educ. 2007, 84, 709−710. (3) Sun, L.; O’Reilly, J. Y.; Tien, C. W.; Sue, H. J. J. Preparation of Electrically Conductive Polystyrene/Carbon Nanofiber Nanocomposite Films. J. Chem. Educ. 2008, 85, 1105−1107. (4) Bentley, A. K.; Farhoud, A. B.; Ellis, A. B.; Lisensky, G. C.; Crone, W. C. J. Template Synthesis and Magnetic Manipulation of Nickel Nanowires. J. Chem. Educ. 2005, 82, 765−767. (5) de Dios, M.; Salgueirino, V.; Perez-Lorenzo, M.; Correa-Duarte, M. A. Synthesis of Carbon Nanotube-Inorganic Hybrid Nanocomposites: An Instructional Experiment in Nanomaterials Chemistry. J. Chem. Educ. 2012, 89, 280−283. (6) Soukupova, J.; Kvitek, L.; Kratochvilova, M.; Panacek, A.; Prucek, R. Silver Voyage from Macro- to Nanoworld. J. Chem. Educ. 2010, 87, 1094−1097. (7) Meenakshi, V.; Babayan, Y.; Odom, T. W. Benchtop Nanoscale Patterning Using Soft Lithography. J. Chem. Educ. 2007, 84, 1795− 1798. (8) 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, 322−325. (9) Vohs, J. K.; Bentz, A.; Eleamos, K.; Poole, J. Chemical Vapor Deposition of Aluminum Oxide Thin Films. J. Chem. Educ. 2010, 87, 1102−1104. (10) Sahar-Halbany, A.; Vance, J. M.; Drain, C. M. Lithography of Polymer Nanostructures on Glass for Teaching Polymer Chemistry and Physics. J. Chem. Educ. 2011, 88, 615−616. (11) Christenson, A. M.; Corder, G. W.; DeVore, T. C.; Augustine, B. H. A Photolithography Laboratory Experiment for General Chemistry Students. J. Chem. Educ. 2003, 80, 183−186. (12) University of WisconsinMadison Materials Research Science and Engineering Center. Exploring the Nanoworld Video Lab Manual. http://education.mrsec.wisc.edu/nanolab/index.html (accessed Mar 2014). (13) Gottfried, D. S. Review of Nanotechnology in Undergraduate Education. J. Chem. Educ. 2011, 88, 544−545. (14) Carmody, W. R.; Wiersma, J. A Study of the Silver Tree Experiment. J. Chem. Educ. 1967, 44, 417−420. (15) Yin, Y.; Lu, Y.; Sun, Y.; Xia, Y. Silver Nanowires Can Be Directly Coated with Amorphous Silica to Generate Well-Controlled Coaxial Nanocables of Silver/Silica. Nano Lett. 2002, 2, 427−430.

Figure 10. Student generated SEM images of (A) silver dendrites at 4000× on solid copper metal and (B) silver wires at 1000× on a SiO2 surface sputter coated with copper. A total of 1.0 × 10−2 M silver nitrate was used for both samples.

silver nitrate was added to the surface of pure copper metal. Figure 10B is an image of silver wires produced when the same silver nitrate solution was added to a silicon dioxide plate sputter coated with copper. Both images were obtained using a Hitachi TM 3000 Benchtop SEM. With these images, students were able to see the drastic differences between silver structures formed on pure copper and silver structures formed on silicon dixode surfaces sputter coated with copper. The SEM results are consistent with claims made by Jiang et al. which state that the type of product formed in the reaction between metals and dilute silver nitrate solutions depends strongly on the surface roughness of the metal substrate.23

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SUMMARY Since 2011, this experiment has been successfully implemented in introductory nanotechnology courses, allowing students to fabricate silver nanowires using a bottom-up technique. It can also be used in an introductory chemistry course, to reinforce the fact that nanotechnology encompasses a wide range of disciplines, including materials science, physics, chemistry, biology, mathematics, and engineering.54 Chemistry concepts associated with the experiment, including single-replacement reactions, oxidation, reduction, activity series of metals, concentration, atoms, ions, and plasma, have to be mentioned in the pre-lab lecture to provide students with a basic understanding all steps involved in the bottom-up nanofabrication process used to make the silver nanowires. This experiment also provides students with exposure to characterization techniques (AFM and SEM) that are frequently used in laboratories pursuing nanotechnology-related endeavors. E




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Characterization of Micro-and Nanoscale Silver Wires Synthesized

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