Electrodeposition of Silver Micro- and Nanoscale Wires in the

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

pubs.acs.org/jchemeduc

Electrodeposition of Silver Micro- and Nanoscale Wires in the Capillaries of PDMS Stamps Modified with Hydrophilic Polymers Wesley C. Sanders,*,† Glen Johnson,† Ron Valcarce,‡ Peter Iles,‡ Hunter Fourt,† Kyne Drystan,† Dakota Edwards,† Jarom Vernon,† Spencer Ashworth,† Amar Barucija,† and Zachary Curtis† †

Engineering Department, Salt Lake Community College, Salt Lake City, Utah 84123, United States Chemistry Department, Salt Lake Community College, Salt Lake City, Utah 84123, United States

‡

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S Supporting Information *

ABSTRACT: A new experiment for use in introductory nanotechnology courses is described. This experiment allows students to fabricate metallic wires with microscale lateral dimensions and nanoscale vertical dimensions. Fabrication occurs in the capillaries of polydimethylsiloxane (PDMS) stamps modified with hydrophilic polymers. This experiment provides students with an opportunity to conduct templateassisted electrodeposition of micro- and nanomaterials, utilizing a reusable template instead of the commonly used porous, anodic alumina oxide (AAO) membranes that require dissolution to examine the wires. Fabrication of the metal wires is accomplished via the reduction of metal cations in the channels of modified PDMS stamps. In addition, this experiment introduces students to characterization using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and atomic-force microscopy (AFM). The microscale dimensions of the silver wires accommodate imaging with optical microscopy for institutions possessing limited characterization capabilities. KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Polymer Chemistry, Hands-On Learning/Manipulatives, Nanotechnology, Noncovalent Interactions, Undergraduate Research, Oxidation/Reduction

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anodic alumina oxide (AAO) membranes.1 The report describes modification of the AAO membranes with conductive materials to facilitate reduction of metal ions in the pores. In addition, the report describes the dissolution of the template to release the metal nanowires.1 This experiment involves the reduction of metal cations in the capillaries of PDMS stamps modified with hydrophilic polymers. Dissolution of the template is not required in this experiment. Students can characterize the nanowires by peeling the stamp away from the substrate. PDMS stamps and substrates can be reused in successive experiments with proper cleaning and deposition of a hydrophilic polymer. All of the steps involved in the electrodeposition of silver wires in the capillaries of PDMS stamps are shown in Figure 1.

INTRODUCTION The emerging emphasis on nanotechnology is evident in media publications, academic research programs, and industry.1 The importance of nanotechnology is further emphasized by the National Science Foundation (NSF). The NSF estimates nanotechnology has a $3 trillion impact on the global economy and employs six million workers.2 The emergence of this field has resulted in the creation of nanotechnology coursework, programs of study, and certificates at 2 and 4 year colleges.2 As nanotechnology-based curricula find their way into undergraduate education, a need emerges for laboratory experiments that address basic nanotechnology concepts in fabrication and characterization. In addition, nanotechnology-based laboratory experiments need to be rooted in the physical sciences, particularly chemistry. Chemistry plays a central role in the development of nanoscale materials because it gives precise control over the composition of the nanostructures.1 This Journal has published several articles describing nanotechnology-based experiments, including experiments involving nanoparticle synthesis,3 self-assembled monolayers,4 fabrication of polymeric nanostructures,5,6 graphene synthesis,7 nanofabrication,8−11 quantum-dot synthesis,12 and nanowire synthesis.1,13 An earlier report describes student investigations of templateassisted electrodeposition of metal nanowires in the pores of © XXXX American Chemical Society and Division of Chemical Education, Inc.

Template-Assisted Electrodeposition Using AAO Membranes

The template synthesis technique was first popularized more than 10 years ago; this method of nanowire synthesis has become widely used.1,14,15 Nanoporous AAO or polycarbonate membranes are used as templates to direct the growth of Received: December 27, 2018 Revised: March 21, 2019

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DOI: 10.1021/acs.jchemed.8b01053 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. Main steps associated with the electrodeposition of silver wires in the capillaries of PMDS stamps.

nanowires. The pores are filled with one or more metals via a technique such as electrodeposition. The membrane can be chemically removed, leaving metallic nanowires behind (Figure 2).

Surface Modification of Polymer-Based Templates with Poly(vinyl alcohol) (PVA)

A frequently used polymer in micro- and nanofabrication, particularly in soft lithography, is the silicon-based polymer poly(dimethylsiloxane) (PDMS), shown in Figure 3. This

Figure 2. (a) Electrodeposition of metal nanowires in an AAO membrane. (b) Exposing metal nanowires by removing the AAO template. Figure 3. PDMS structure.

The polymer-based, template-assisted nanowire synthesis that is described here possesses several key differences from the widely reported nanowire synthesis using AAO templates. The differences include the following: • PDMS stamps do not require the addition of silver paint or gallium indium eutectic alloy to serve as the cathode; this is because the wires are deposited on interdigitated electrodes with metal traces serving as the anode and cathode. • PDMS stamps are directly mounted on top of interdigitated electrodes instead of copper plates. • A smaller volume of electroplating solution is used in this experiment. • No corrosive chemicals are needed to dissolve the template to liberate the wires; the wires are exposed by peeling the PDMS stamp from the interdigitated electrode. • PDMS stamps and interdigitated electrodes can be cleaned and reused.

polymer possesses several attractive properties, such as optical transparency, chemical inertness, and elasticity. The PDMS structure consists of consists of repeating (−OSi(CH3)2−) units resulting in a hydrophobic, methyl-terminated surface.16 For PDMS applications involving aqueous solvents, it is necessary to render the surface of the PDMS hydrophilic.16 It has been proven that surface-modification techniques such as plasma, corona, and ultraviolet−ozone (UVO) treatment can be used for well-controlled and rapid conversion of the methyl groups on PDMS to various hydrophilic moieties, including hydroxyls, carboxyls, aldehydes, and peroxides.17 In addition, PDMS surface-modification techniques can involve the use of poly(vinyl alcohol) (PVA). PVA (Figure 4) is a nontoxic, hydrophilic polymer with several applications, including uses in emulsifiers, coatings, food-packaging materials, biomaterials, drug-delivery systems, adhesives, and surfactants.18 PVA is water-soluble and one of the most hydrophilic polymers, with stable chemical properties, good dissolution, and strong B




three Introduction to Nanotechnology courses (a total of 50 students). The supplies used by the students in the experiment are shown in Figure 6a. After cleaning the stamps and electrodes, silver paint was deposited to the gold traces of the electrode to facilitate easier connection with alligator, clips as shown in Figure 6b. Students used laptop-fan-based spin coaters to deposit the solutions on the pattern side of the PDMS. Additional instructions for spincoater assembly can be found in the Supporting Information. Students used double-sided tape to adhere the stamp to the spin coater, with the pattern side facing up (Figure 6c). Afterward students used a clean plastic pipet to add 250 μL (0.250 mL) of the PVA solution to the pattern side of the stamp and allowed it to sit for 1 min to allow for attraction between the carbon chains of the PDMS and the PVA via London dispersion forces. Next, students operated the spin coater for 30 s to spread PVA on the surface of the stamp; a 250 mL beaker was placed over the spin coater to prevent splatter of the solution. This step converted the character of the surface of the stamp from hydrophobic to hydrophilic because of the nature of the pendant OH groups along the carbon chain of PVA. After PVA deposition, students added 250 μL (0.250 mL) of the 1.5 M silver nitrate solution to the pattern side of the stamp, and allowed it to sit for 1 min to allow the silver ions to form complexes with the OH groups found in PVA. After 1 min, the spin coater was operated for 30 s. Low spin times resulted in the formation of PVA films that retained enough moisture to facilitate the entrapment of aqueous silver ions that were subsequently reduced in an additional step. After addition of the silver nitrate, the stamp was removed from the spin coater and placed pattern side down on the larger gold traces of the interdigitated electrode (Figure 6d). It is important to note that the stamp was positioned in a manner facilitating a perpendicular arrangement between the lines of the stamp and the larger gold traces of the interdigitated electrode. Students applied voltage to the interdigitated electrode for approximately 25 min. During electrodeposition, a short lecture was conducted to provide students with the following background information:

Figure 4. Poly(vinyl alcohol) (PVA) structure.

adhesion. Because of the presence of many desirable properties, PVA is often used for various pharmaceutical and biomedical applications.18 It is possible for PVA to be adsorbed onto hydrophobic surfaces.19 Adsorption of PVA on PDMS is a stable process.18 When PVA is deposited on PDMS, a hydrophobic interaction occurs between the inherently hydrophobic PDMS and the carbon backbone of PVA.18 The hydrophobic interactions between the two polymers induce the release of water from the solid−liquid interface.18 Another important phenomena associated with this experiment is the association of PVA and silver ions, which has been reported previously.20 A report states that the introduction of Ag+ ions into PVA facilitates complexation between Ag+ ions and the oxygen atoms of hydroxyl (O−H) groups21 (Figure 5).

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EXPERIMENTAL METHODS

Instructor Preparation

PDMS-stamp creation and solution preparation required approximately 1 h. Detailed instructions for PDMS-stamp creation can be found in the Supporting Information. Commercially available compact discs with the labels removed served as the hard master for PDMS-stamp preparation. To prepare the polymer solution, 0.05 g of PVA was added to 30 mL of hot deionized water. The solution was stirred with a magnetic stir bar until all the PVA had dissolved. A solution of 1.5 M silver nitrate was prepared by dissolving 7.6 g of silver nitrate in 30 mL of deionized water. Interdigitated gold electrodes on glass (Metrohm) were first cleaned with a chemwipe moistened with DI water; this was followed by cleaning with a chemwipe moistened with 100% isopropyl alcohol.

• template-assisted electrodeposition • oxidation−reduction reactions • PDMS properties and structure • PVA properties and structure

Execution

• hydrophobicity and hydrophilicity

Faculty developed this experiment with seven students enrolled in a materials science internship−co-op course during the Fall 2018 semester. After approximately five tests with the co-op students, the lab was conducted by students enrolled in

After electrodeposition and the lab lecture, students removed the voltage and peeled the PDMS from the interdigitated electrode to reveal a diffraction pattern.

Figure 5. Complexation of silver ions with poly(vinyl alcohol). C




Figure 6. (a) Supplies used to perform electrodeposition in the capillaries of PDMS stamps. (b) Silver paint added to the interdigitated electrode for better alligator-clip connection. (c) PDMS stamp mounted on a spin coater for PVA and silver nitrate deposition. (d) Coated PDMS stamp placed pattern-side down on the interdigitated electrode and connected to two AA batteries (not shown).

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HAZARDS Because of the flammability of isopropyl alcohol, do not use it near an open flame or heat source. Silver nitrate is classified as a strong oxidizer. Other hazards include irritation to the skin, eyes, and respiratory tract. Silver nitrate is also harmful if swallowed or inhaled. Silver nitrate can also stain skin and clothing. Students should wear gloves and protective eyewear during the experiment.

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RESULTS Students were provided with an opportunity to characterize the nanowires outside of normal meeting times using SEM−EDS (20 min) and AFM (20 min). Optical microscopy was also used to view the nanowires under different magnifications (20 min). Figure 8. EDS analysis of silver wires electrodeposited on an interdigitated electrode. Inset: silver-wire EDS analysis generated using Quantax software (Bruker).

SEM−EDS Data

Figure 7a,b shows SEM images of the silver wires collected at 5000× and 15,000× magnification, respectively. These images

AFM Data

AFM images were obtained using a Nanosurf Easyscan 2 AFM (Nanoscience, Inc.) operating in tapping mode. AFM images (Figure 9) show that the electrodeposited nanowires are evenly

Figure 7. Scanning-electron-microscopy images of silver wires at (a) 5000× and (b) 15,000× magnification.

were collected using a Hitachi 3000 Benchtop SEM. The brightness of the wires suggests the presence silver metal. Silver ions (Ag+) are reduced in the channels of the PDMS stamp. Two AA batteries provide a potential of 1.5 V, which is well above the reported reduction potential of silver, +0.80 V.22 By applying a DC voltage at least comparable to the reduction potential of silver ions, it is possible to reduce silver ions to elemental, metallic silver, as shown in eq 1. Ag +(aq) + 1e− → Ag(s)

Figure 9. AFM scans of silver wires electrodeposited on interdigitated electrodes: (a) 30 × 30 μm and (b) 10 × 10 μm.

spaced. Figure 10 shows cross-sectional analyses of the AFM data obtained using WXSM image-processing software.25 Cross-sectional analysis indicates that the silver wires are approximately 140 nm high and 1.5 μm wide.

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Optical Microscopy

Elemental-analysis data were collected using energydispersive X-ray spectroscopy (EDS). Figure 8 shows a silver peak at 3 keV, which is consistent with the silver EDS data reported in the literature.23,24

Institutions with limited characterization capabilities can use optical microscopy to view the electrodeposited silver wires. Students were able to use bright-field microscopy (Figure 11a) and dark-field microscopy (Figure 11b) to view the wires. D




The experiment presented in this manuscript reinforces the fact that nanotechnology is inclusive of a wide range of disciplines. This is evident from the chemistry concepts emphasized in this experiment, including polymers, functional groups, intermolecular interactions, ions, and redox reactions.

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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.8b01053. Instructor notes, which include the parts list and instructions for PDMS-stamp fabrication, spin-coater assembly, solution preparation, and electrodeposition, and postlab questions for students to complete at the end of the experiment (PDF, DOCX)

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Figure 10. Cross-sectional profile and silver wires. Inset: 5 × 5 μm AFM scan of silver wires.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wesley C. Sanders: 0000-0003-4472-3086 Notes

The authors declare no competing financial interest.

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(1) Bentley, A. K.; Farhoud, A. B.; 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, 765− 767. (2) Sanders, W. C. Basic Principles of Nanotechnology; CRC Press/ Taylor and Francis: Boca Raton, 2018. (3) 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. (4) Cea, P.; Martín, S.; Gonzalez-Orive, A.; Osorio, H. M.; Quintín, P.; Herrer, L. Nanofabrication and Electrochemical Characterization ofSelf-Assembled Monolayers Sandwiched between Metal Nanoparticlesand Electrode Surfaces. J. Chem. Educ. 2016, 93, 1441−1445. (5) Sanders, W. C. Fabrication of Polyvinylpyrrolidone Micro-/ Nanostructures Utilizing Microcontact Printing. J. Chem. Educ. 2015, 92, 1908−1912. (6) 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, 615−618. (7) Jacobberger, R. M.; Machhi, R.; Wroblewski, J.; Taylor, B.; Gillian-Daniel, A. L.; Arnold, M. S. Simple Graphene Synthesis via Chemical Vapor Deposition. J. Chem. Educ. 2015, 92, 1903−1907. (8) Vohs, J. K.; Bentz, A.; Eleamos, K.; Poole, J.; Fahlman, B. D. Chemical VaporDeposition of Aluminum Oxide Thin Films. J. Chem. Educ. 2010, 87, 1102−1104. (9) Meenakshi, V.; Babayan, Y.; Odom, T. W. Benchtop NanoscalePaterrning Using Soft Lithography. J. Chem. Educ. 2007, 84, 1795−1798. (10) Campbell, D. J.; Beckman, K. J.; Calderon, C. E.; Doolan, P. W.; Ottosen, R. M.; Ellis, A. B.; Lisensky, G. C. Replication and Compression of Bulk and Surface Structures with PolydimethylsiloxaneElastomer. J. Chem. Educ. 1999, 76, 537−541. (11) Sanders, W. C.; Valcarce, R.; Iles, P.; Smith, J. S.; Glass, G.; Gomez, J.; Johnson, G.; Johnston, D.; Morham, M.; Befus, E.; Oz, A.; Tomaraei, M. Printing Silver Nanogrids on Glass. J. Chem. Educ. 2017, 94, 758−763. (12) Winkelmann, K.; Noviello, T.; Brooks, S. J. Preparation of CdSNanoparticles by First-Year Undergraduates. J. Chem. Educ. 2007, 84, 709−710.

Figure 11. (a) Bright-field image of silver wires collected using an 100× objective lens and (b) dark field image of silver wires collected using a 40× objective lens.

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REFERENCES

ASSESSMENT

Learning Objectives

The following learning objectives addressed in this experiment are associated with nanotechnology, fabrication, and basic chemistry. These objectives include: • performing nanowire synthesis using template-assisted electrodeposition • recognizing the importance of PVA polymer structure in silver-wire synthesis • identifying the redox reactions involved in silver-wire synthesis • evaluating PDMS surface-modification schemes (hydrophobic to hydrophilic) • interpreting AFM, SEM, and EDS data Postlab questions pertaining to procedures and fundamental science concepts were incorporated in quizzes and exams to test student understanding of associated concepts. A list of postlab questions is included in the Supporting Information.

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SUMMARY This experiment was developed for students enrolled in introductory nanotechnology courses. This exercise allows students to use electrodeposition to create silver nanowires in the channels of a modified PDMS stamp, a process that has never before been reported to the authors’ knowledge. In addition, students were able to observe the morphology of the wires and confirm the presence of silver using SEM and EDS, respectively. The lateral and vertical dimensions of the electrodeposited wires were assessed using AFM. E




(13) Sanders, W. C.; Ainsworth, P. D.; Archer, D. M., Jr.; Armajo, M. L.; Emerson, C. E.; Calara, J. V.; Dixon, M. L.; Lindsey, S. T.; Moore, H. J.; Swenson, J. D. Characterization of Micro- and Nanoscale Silver Wires Synthesized Using a Single-Replacement Reaction between Sputtered Copper Metal and Dilute Silver Nitrate Solutions. J. Chem. Educ. 2014, 91, 705−710. (14) Penner, R. M.; Martin, C. R. Preparation and Electrochemical Characterization of Ultramicroelectrode Ensembles. Anal. Chem. 1987, 59, 2625−2630. (15) Hulteen, J. C.; Martin, C. R. A General Template-Based Method for the Preparation of Nanomaterials. J. Mater. Chem. 1997, 7, 1075−1087. (16) Ye, H.; Gu, Y.; Gracias, D. H. Kinetics of Ultraviolet and Plasma Surface Modification ofPoly(dimethylsiloxane) Probed by Sum Frequency VibrationalSpectroscopy. Langmuir 2006, 22, 1863− 1868. (17) Ozcam, A. E.; Efimenko, K.; Genzer, J. Effect of Ultraviolet/ Ozone Treatment on the Surface and Bulk Properties of Poly(dimethyl siloxane) and Poly(vinylmethyl siloxane) Networks. Polymer 2014, 55, 3107−3119. (18) Karki, A. Adsorption of Poly(vinyl alcohol) onto Polydimethylsiloxane Substrates: Formation of Continuous Films, Honeycomb Structures, and Fractal Morphologies. Bachelors of Arts Thesis, Mount Holyoke College, South Hadley, MA, 2015. (19) Trantidou, T.; Elani, Y.; Parsons, E.; Ces, O. Hydrophilic Surface Modification of PDMS for Droplet Microfluidics Using a Simple, Quick, and Robust Method via PVA Deposition. Microsyst. Nanoeng. 2017, 3, 16091. (20) Phan, D. N.; Hasegawa, Y.; Song, K. H.; Lee, H.; Kim, I. S. Adsorption of Silver Ions from Aqueous Solution onto Thiol Modified Polyvinyl Alcohol Nanofibers. Trends Text. Eng. Fashion Technol. 2018, 1, 1−4. (21) Gan, Y.; Bai, S.; Hu, S.; Zhao, X.; Li, Y. Reaction Mechanism of Thermally-Induced Electric Conduction of Poly(vinyl alcohol)-Silver Nitrate Hybrid Films. RSC Adv. 2016, 6, 56728−56737. (22) Brown, T. L.; LeMay, H. E., Jr.; Bursten, B. E.; Murphy, C. J.; Woodward, P. M. Chemistry - The Central Science; Prentice Hall: Boston, 2012. (23) Cao, Y.; Liu, W.; Sun, J.; Han, Y.; Zhang, J.; Liu, S.; Sun, H.; Guo, J. A Technique for Controlling the Alignment of Silver Nanowires with an Electric Field. Nanotechnology 2006, 17, 2378− 2380. (24) Liu, C. H.; Yu, X. Silver Nanowire Based Transparent, Flexible, and Conductive Thin Film. Nanoscale Res. Lett. 2011, 6, 75. (25) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. WSXM: A Softwware for Scanning Probe Microscopy and Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705.

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Electrodeposition of Silver Micro- and Nanoscale Wires in the

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