Observing the Growth of Metal Dendrites in Specimens Prepared by

Laboratory Experiment pubs.acs.org/jchemeduc

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

Observing the Growth of Metal Dendrites in Specimens Prepared by Fabricating Galvanic Cells and Electrolytic Cells Yizhou Ling,*,†,⊥ Zhizhen Yu,‡ Pengwen Chen,§ Xiaohong Yan,*,† and Jian Yang† †

Jiangsu University, Zhenjiang, Jiangsu 212013, China College of Teacher Education, East China Normal University, Shanghai 200062, China § School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu 210023, China ⊥ Nanjing Senior High School of Jiangsu Province, Jiangyin, Jiangsu 214437, China Downloaded via NOTTINGHAM TRENT UNIV on August 19, 2019 at 23:43:35 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: This paper introduces a safe, interesting, convenient, and lowcost experiment. The products of galvanic cells and electrolytic cells are compressed into specimens that are placed on a slide to stretch the metal dendrites in one plane and ensure they are not easily destroyed by external factors. The details and growth process of the metal dendrites can be clearly observed with the naked eye and optical microscopy, which arouses interest from students and allows them to enjoy the beautiful images while obtaining a large amount of macroscopic information. By allowing the students to observe the macroscopic phenomena and the corresponding microscopic changes among the microparticles, it is possible for them to explain the morphological origins and growth rates of the metal dendrites, which deepens their understanding of the reactions that occur in galvanic and electrolytic cells. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Public Understanding/Outreach, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Electrochemistry, Electrolytic/Galvanic Cells/Potentials, Metals

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zinc particles on filter paper impregnated with a copper chloride solution to form radial copper dendrites on the filter paper, but the copper dendrites became entangled with the fibers in the filter paper.7−9 X. Xu et al. used agar as the reaction medium for the copper−aluminum displacement reaction so that the generated copper dendrites were not easily disturbed and destroyed, but a large number of copper dendrites overlapped and interfered with each other.10,11 Ikemoto et al. poured a thin layer of sodium polyacrylate gel into a Petri dish and used two copper wires as the electrodes. After electrification, the cathode formed copper dendrites, but they readily formed clusters.12,13 With these methods, it is not possible to prepare flat, stretched, and dispersed metal dendrites, and it is also not possible to observe clear structures with an optical microscope. Other researchers have observed metal dendrite microstructures using atomic force microscopy (AFM), scanning electron microscopy (SEM), and highdefinition cameras equipped with macroscopic lenses. Despite the clear images, these devices are expensive and complex to operate and thus are not suitable for routine teaching purposes.14−16 In summary, the existing methods cannot meet the requirements of microscopic observation and low

INTRODUCTION Processes that result in the loss of electrons occur during displacement and electrolysis reactions, both of which are categorized as oxidation−reduction (redox) reactions.1,2 A galvanic cell generates a current through a redox reaction that converts chemical energy into electrical energy, whereas electrolytic cells drive a redox reaction through the application of electrical energy.3,4 The metal formed during displacement reactions and electrolytic reactions has a dendritic shape and is referred to as a metal dendrite. Metal dendrites not only look novel and aesthetically pleasing but also reflect a wealth of information that can be used as an explanation for the existence of galvanic cells or electrolytic cells. In the teaching process, metal dendrites can stimulate interest and help students understand the components of a battery, the behavior of electrodes, and particle changes at the microscopic level. However, it is difficult to observe metal dendrites by conventional experimental methods, such as during a displacement reaction that occurs upon inserting a wire into a test tube containing a copper sulfate solution5 or during copper refinement in an electrolytic cell.6 The product (metallic copper) can become entangled, disturbed, or destroyed, so the detailed structure and formation process of the metal dendrites cannot be observed. To observe metal dendrites, some researchers have adopted methods to improve the reaction conditions. Y. Xu et al. placed © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: June 6, 2019 Revised: July 29, 2019

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DOI: 10.1021/acs.jchemed.9b00532 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

cost at the same time (see the lecturer notes in the Supporting Information for details).

(i) As shown in Figure 2, two copper foils in the shape of an anode and a cathode are cut and pasted on the glass slide with a single-sided adhesive.

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MATERIALS Consider the following copper dendrite experiment as an example. The materials include copper sulfate, zinc foil (0.05 mm thick), copper foil (0.025 mm thick), agar powder, an electronic scale, a test tube, a heater, a glass rod, a glass slide (25 × 75 mm), a rectangular coverslip (24 × 60 mm), scissors, single-sided tape, a dropper, an optical microscope, a 9 V battery, wires, and a copper sulfate−agar solution (prepared by adding 0.1 g of agar powder, 1.2 g of copper sulfate, and 8.7 mL of distilled water to a test tube, heating it to approximately 90 °C in a water bath, and keeping it warm). Agar is not necessary, and the copper sulfate-agar solution can be simplified to a copper sulfate solution (concentration of 1 M or more) without heating.

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Figure 2. Metal dendrite specimen for an electrolytic cell (top view).

EXPERIMENTS

(ii) Two to three drops of a hot copper sulfate−agar solution are added to the glass slide. (iii) The coverslip is placed so that the solution fills all the space between the coverslip and the glass slide. (iv) Two copper foil electrodes are connected to the anode and cathode of the power supply with two wires, and the copper dendrites grow gradually at the tip of the cathode, which completes the specimen (see the Supporting Information for detailed steps). Other metal dendrites can be made in the same way.

Preparation of a Metal Dendrite Specimen from a Reaction in a Galvanic Cell

The preparation of dendritic copper specimens in a galvanic cell is introduced as an example of the displacement reaction in a zinc and copper sulfate solution. The experimental process includes the following brief steps: (i) A small piece of zinc foil is obtained and placed in the center of the slide. (ii) Two to three drops of copper sulfate−agar solution are added to the side with the zinc foil. (iii) The coverslip is placed so that the solution fills all the space between the coverslip and glass slide (Figure 1).

Visual and Microscopic Observation of Specimens

During the experiment, the whole morphology of the metal dendrites can not only be observed directly with the naked eye, but the growth rates of the metal dendrites can also be visually observed. With the help of optical microscopy, the experimental phenomena can be magnified 40 to 400 times, and the fine morphology of the metal dendrites can be clearly observed. The brief steps for microscopic observation include: (i) Selecting the appropriate eyepiece and objective magnification. (ii) Placing the specimen on the stage. (iii) Adjusting the coarse focus and fine focus knobs to produce a clear image. (iv) Saving the image.

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Figure 1. Metal dendrite specimen for a galvanic cell (top view).

HAZARDS The metal salt solution used in the experiment is toxic via oral contact and can cause eye and skin irritation. To avoid getting hurt, personal protective equipment should be worn for the entire time the experiments are being conducted, including goggles, a laboratory coat, protective gloves, and a mask. Hands should be washed after the experiments are completed. All used chemicals must be collected in waste containers and disposed in correct ways according to local regulations.

(iv) Soon afterward, copper dendrites are formed around the zinc foil, and the specimen is prepared (see the Supporting Information for detailed steps). If copper sulfate is replaced by silver nitrate, lead acetate, or stannous chloride, other specimens such as silver dendrites, lead dendrites, and tin dendrites, respectively, could be produced in the same manner. Preparation of a Metal Dendrite Specimen from a Reaction in an Electrolytic Cell

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The preparation of a copper dendritic specimen in an electrolytic cell is accomplished by using the electrochemical reaction of a copper sulfate electrolyte and copper electrode. The experimental process includes the following brief steps:

This experiment usually takes 2 h. More than 50 students from different segments participated in the experiment, and all students successfully prepared the metal dendrites. After finishing the experimental steps, the metal dendrites were gradually formed in the flat space between the glass slide and

RESULTS AND PEDAGOGY

Visual Observation of the Metal Dendrites

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Figure 3. Metal dendrites observed with the naked eye: (a) copper dendrites, (b) silver dendrites, (c) lead dendrites, (d) tin dendrites, and (e) magnified copper dendrites.

Figure 4. Different microscopic images of metal dendrites under an ordinary light microscope: (a,e) copper dendrites, (b,f) silver dendrites, (c,g) lead dendrites, and (d,h) tin dendrites at (a−d) 100 and (e−h) 400 times magnification.

beautiful structures and developed a strong curiosity about the microstructure of metal dendrites.

the coverslip. The metal dendrites from the galvanic reaction were produced from the edges of the zinc foil, and within minutes, the dendritic structure was visible to the naked eye, whereas the surface of the zinc foil was corroded (i.e., the zinc foil was gradually consumed). After the agar gel was cooled and solidified, the growth rate of the metal dendrites decreased to an extent, but the growth rate remained approximately uniform, and the radial metal dendrites shown in Figure 3 were ultimately formed. The metal dendrites in the specimens with an electrolyte were generated from the tip of the sharp angle of the cathode, grew toward the anode, and rapidly formed a dendritic structure visible to the naked eye. At the same time, the copper anode became corroded and lost its luster (i.e., the copper was gradually consumed). Generally, the growth rate of the metal dendrites was slow at the beginning of the process and increased gradually with time. After a few tens to a few hundred seconds, the reaction stopped when the metal dendrites contacted the anode. Although the students observed the experimental phenomena, the teachers guided them to understand the characteristics of the metal dendrites, such as noticing that the color of the copper dendrites comprised a beautiful gradient from dark red to bright red and that they radiated outward, like the growth of branches in nature. As a result, the students appreciated the

Microscopic Observation of Metal Dendrites

The growth of the metal dendrites was confined to a thin plane between the slide and the coverslip as a result of clever experimental design. The sample design not only reduced the thickness of the metal dendrites to the depth of field of the microscope but also allowed a clear image to be produced as well as an extremely large surface-to-volume ratio, which enhanced the visual effects. The results of the experimental observations are shown in Figure 4, where it can be seen that the metal dendrites had stable and regular microstructures. The microstructures of the same metal that formed during the galvanic reaction and during the electrolytic reaction were always similar, but the microstructures from the different kinds of metals were quite different. Observing specimens with a microscope satisfied the curiosity of the students, who were amazed by the fine structures and regularity of the metal dendrites. However, students were then puzzled by the morphogenesis and growth rates of the metal dendrites, which made them ask some deep questions. These questions cannot be answered by relying only on macroscopic evidence (including the results of the above visual observations and microscopic observations). Therefore, C




these problems should be discussed in combination with the changes of particles at the microscopic level.

about particle movements) to form a one-to-one correspondence.

Discussion about the Morphogenesis of Metal Dendrites

Discussion about the Growth Rate of Metal Dendrites

Before the experiment, students guessed that the copper produced in the reaction would wrap tightly around the surface of the reactant. In contrast, the copper grew like a tree branch, and the origin of the growth was always exactly at the tips of the metal dendrites. This indicated that at the microscopic level, copper ions near the tips of copper dendrites could be reduced to copper atoms and deposited on the tips of copper dendrites without direct contact with the zinc foil (or the sharp corner of the cathode). This experimental phenomenon caused confusion among the students. To resolve their confusion, the teachers guided them to analyze the changes in these phenomena at both the macroscopic and microscopic levels. In the experiment that involved preparing a copper dendrite specimen in a galvanic cell, the macroscopic level change on the surface of the zinc foil involved its corrosion (observed evidence), which corresponded to the zinc atoms being oxidized to zinc ions and entering into the solution at the microscopic level. This is also the reaction in eq 1. At a position away from the zinc foil, the macroscopic change was the continuous formation of copper at the tips of the copper dendrites, similar to the growth of branches in nature (observed evidence), which corresponded to the reduction of copper ions to copper atoms and deposition at the copper dendrite tip at the microscopic level. This reaction is described by eq 2.

The growth rates of the metal dendrites could be calculated by measuring them with a ruler on the microscope and could also be determined by visual observation. Different metal dendrites had different growth rates,18,19 which aroused the curiosity of the students. On the one hand, even if it was the same metal dendrite, its growth rates in the galvanic cell and electrolytic cell could be significantly different. In the galvanic cell reaction, the growth rates of metal dendrites remained roughly the same. However, in the electrolytic cell reaction, the growth rates of metal dendrites accelerated with time. This is because in the galvanic cell reaction, the metal dendrites were continuously growing outward, and the concentration of the copper ions near the tips of the metal dendrites was always constant. The electromotive force and current of the galvanic cell did not change, so the reaction rate remained unchanged. Regarding the electrolytic cell, the metal dendrites grew toward the anode, which decreased the distance between the cathode and the anode and thus reduced the resistance. As the voltage was constant, the current increased, so the reaction rate continuously increased. However, under the same experimental conditions, the growth rates of different metal dendrites were also different. The tin dendrites grew rapidly, whereas the copper dendrites grew the slowest among the samples considered in this experiment.20 This phenomenon occurred because of the different migration rates of different ions at the microscopic level. Compared with stannous ions, copper ions have a smaller radius and a smaller mobility, so the deposition amount per unit time is less.21,22 Considering that students below the undergraduate level lack sufficient background knowledge, teachers only need to explain the difference in the ion radii in a general way.

Zn(s) − 2e− → Zn 2 +(aq)

(1)

Cu 2 +(aq) + 2e− → Cu(s)

(2)

In eq 1, the zinc foil loses electrons; in eq 2, the tips of the copper dendrites gains electrons. Here, the teacher reminded the students that the copper dendrite itself could be a good conductor. Then, the students quickly realized that an electric charge could be directed to move through the copper dendrite. They understood that the zinc foil was a positive electrode and that the copper dendrite was a negative electrode in the displacement reaction. Similarly, during the experiment that involved copper dendrite specimens in the electrolysis cell reaction, the anode was corroded at the macroscopic level, which corresponds to eq 3 at the microscopic level. The copper was continuously generated at the tips of the copper dendrites, which corresponds to eq 2. Because the copper dendrite itself was electrically conductive, it formed a new cathode together with the original cathode and formed a new electrolytic cell under the condition of electrification. Cu(s) − 2e− → Cu 2 +(aq)

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EXTENDING THE EXPERIMENT Most students were generally very interested in the experiment and asked for extra time to continue it. Figure 5 is a schematic

Figure 5. Expanding the experiment with metal dendrites in an electrolytic cell.

(3)

Therefore, the formation of metal dendrites could be explained by the reaction principles of galvanic cells and electrolytic cells. At the same time, the formation of metal dendrites could be used as strong evidence for the existence of galvanic cells and electrolytic cells.17 Through experimental observation and analysis, the students strengthened their understanding of the concepts of oxidation, reduction, galvanic cells, and electrolytic cells. They came to realize that in the analysis of chemical problems, it is necessary to combine changes in phenomena at the macroscopic level (observed evidence) with changes at the microscopic level (reasoning

diagram of the experimental device for an extended experiment made by students. A rectangular piece of copper foil was added to the sample of the electrolytic cell and then attached to the power. The students expected that the copper dendrites would first form at the cathode tip, touch the bottom of the rectangular copper foil, and then continue to form above the copper foil. However, the actual phenomenon is shown in Figure 6. Copper dendrites were generated simultaneously from the cathode tip and above the rectangular copper foil (typical D




Figure 6. (a−d) Experimental phenomenon of the extended experiment: (a) before energizing, (b) when copper dendrites were generated at the tip of the cathode and at the edge of the rectangular copper foil after 2.5 min of electrification, (c) when the power was on for 3 min and metal dendrites grew at the tip of the cathode and touched the lower edge of the copper foil, and (d) when metal dendrites continued to grow after 5 min of electrification. (e) Particle motion model at the microscopic level (the changes in the upper and lower electrodes are not shown).

phenomena with the microchanges to form a one-to-one correspondence and, eventually, deepen their understanding of galvanic cells and electrolytic cells.

dendrite growth is shown in Figure 6b). The anode and the underside of the copper foil began to lose their metallic luster at the same time; that is, they began to corrode. To resolve any confusion among the students, the particle motion model shown in Figure 6e was built. Under the action of an external electric field, the rectangular copper foil was polarized, and the internal charge distribution changed (the electrons moved up), thus resulting in a potential difference between the upper and lower electrodes.23,24 In this way, the top end of the rectangular copper foil gathered more electrons and reacted with the copper ions in the solution through the reduction reaction in eq 2 to generate copper dendrites. The low end of the copper foil lost electrons, and the oxidation reaction of eq 3 occurred, resulting in the formation of copper ions into the solution. In this case, the rectangular copper foil was equivalent to splitting an electrolytic cell into two electrolytic cells that were distributed up and down. The students generally felt that the extended experiment was an effective method for analyzing and solving chemical problems by applying the observed phenomena to deduce the corresponding particle changes (namely, combining macroscopic and microscopic knowledge).

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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.9b00532. Lecturer Notes (PDF, DOCX) Student Handout (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.L.). *E-mail: [email protected] (X.Y.). ORCID

Yizhou Ling: 0000-0001-9056-1976 Pengwen Chen: 0000-0002-5663-2192 Notes

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The authors declare no competing financial interest.

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CONCLUSION In this experiment, the growth of metal dendrites was compressed into the gap between the glass slide and the coverslip.25 This approach has the following advantages: • It stretches the metal dendrites in one plane to prevent them from overlapping and interfering with each other. • The general growth direction of the metal dendrites can be predicted for easy tracking and observation. • A large surface-to-volume ratio is obtained and makes the reaction phenomenon more visually appealing.26 • Reagent consumption is reduced so that the experiment can be conducted with only a few drops of the reagents. • After the experiment, the metal dendrites formed in the reaction can be stored for more than 1 day, and the dendrite structure can be preserved despite the gradual evaporation of water. In the teaching process, the details of the metal dendrites can be clearly observed, allowing students to feel the beauty and wonder of chemistry, as well as obtain a large amount of evidence in the macroscopic level. This macroscopic evidence can then promote the development of students’ thinking and guide them to analyze the changes at the microscopic level, leading the students to learn how to combine the macro-

ACKNOWLEDGMENTS This paper is supported by the Basic Education Project of the Chinese Chemical Society (No. HJ2018-0015). We would like to express our appreciation to the students of Jiangyin Experimental Primary School and Nanjing Senior High School of Jiangsu Province for conducting these experiments.

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Observing the Growth of Metal Dendrites in Specimens Prepared by

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