Activity Cite This: J. Chem. Educ. 2019, 96, 1438−1442
pubs.acs.org/jchemeduc
Experimenting with Plasmonic Copper Nanoparticles To Demonstrate Color Changes and Reactivity at the Nanoscale Alexey V. Markin* and Natalia E. Markina Saratov State University, Astrakhanskaya 83, Saratov 410012, Russia
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S Supporting Information *
ABSTRACT: The authors describe a series of simple experiments related to the synthesis, oxidation, and aggregation of plasmonic copper nanoparticles (CuNPs) stabilized by iodide ions. These experiments can help with (i) substituting noble-metal-based plasmonic nanoparticles for nanotechnology-oriented lessons, (ii) demonstrating high reactivity of nanosized objects, and (iii) attracting students’ attention by colorful experiments. The main compounds required for the experiments are CuSO4, KI, NaBH4, and diluted H2SO4. These compounds are relatively safe, available, cost-effective, and do not require special recycling. The color of the reaction mixture rapidly changes from colorless (diluted CuSO4) to wine red (fresh CuNPs) and then to yellow (oxidized CuNPs) or dark green and blue (agglomerated CuNPs), making the reactions more visible to the students. The fast oxidation of CuNPs by the oxygen in air was proposed to demonstrate the high reactivity of nanosized copper and to enrich the nanotechnology lessons with chemical reactions. The authors also highlight and discuss the importance of accounting for potential side reactions (e.g., hydrolysis) for the synthesis and oxidation of CuNPs. The important role of iodide ions in providing colloidal stability of CuNPs is discussed, and several comparative experiments with other stabilizers are proposed. In addition, the authors compare the optical properties of CuNPs with those of gold- and silver-based nanoparticles. The experiments are designed to be completed in less than 45 min and have been regularly used at chemistry club lessons for secondary school students in 2017 and 2018. The efficiency of the experiments has also been tested several times with undergraduate students at Saratov State University. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Demonstrations, Interdisciplinary/Multidisciplinary, Hands-On Learning/Manipulatives, Materials Science, Nanotechnology, Synthesis, Oxidation/Reduction, Physical Properties
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INTRODUCTION Currently, copper nanoparticles (CuNPs) are intensively studied as a promising and cost-effective alternative to gold and silver nanoparticles (AuNPs and AgNPs). For example, CuNPs have been proposed as the main components of conductive inks,1 effective catalysts in organic chemistry,2 and materials for the plasmon-assisted enhancement of infrared and Raman vibrations.3,4 However, in contrast to AuNPs and AgNPs, the plasmon resonance of CuNPs does not have a strong dependence on the morphology of particles because of the strong influence of the 3d electrons.5 Thus, CuNPs are generally colored to various shades of red. Additionally, the low chemical stability of CuNPs is another challenge that restricts the applicability of CuNPs in science and industry. Nevertheless, the listed limitations are not critical in the case of education, where long chemical stability and exceptional plasmonic properties are not the main purpose. For example, CuNPs were used as a model object for introducing students to the supercritical fluid-facilitated synthesis of nanoparticles.6 Another laboratory experiment is based on the synthesis of © 2019 American Chemical Society and Division of Chemical Education, Inc.
dendrimer-encapsulated CuNPs and the evaluation of their catalytic properties.7 Unfortunately, the results of these studies cannot be used to demonstrate the plasmonic properties of CuNPs. The low chemical stability of CuNPs can also be successfully used as an advantage, e.g., demonstrating the high reactivity of nanosized objects, thus enriching nanotechnology lessons with colorful chemical reactions. Therefore, this report describes the simple and available synthesis of plasmonic CuNPs that is suitable for routine use in teaching. The synthesis is based on the results of Kapoor et al.,8 who found that iodide-stabilized CuNPs possess an intense plasmon resonance. In addition, we propose a series of experiments with synthesized CuNPs (oxidation and aggregation) that demonstrate their optical and chemical properties. Received: December 23, 2018 Revised: May 9, 2019 Published: May 24, 2019 1438
DOI: 10.1021/acs.jchemed.8b01050 J. Chem. Educ. 2019, 96, 1438−1442
Journal of Chemical Education
Activity
Table 1. Reaction Equations Used During the Lesson Equation Number
Reaction Equations by Type Synthesis of CuNPs 4CuSO4 + NaBH4 + 3H 2O → 4Cu + H3BO3 + NaHSO4 + 3H 2SO4
1
2CuSO4 + NaBH4 + 3H 2O → 2Cu + 2H 2 ↑ + H3BO3 + NaHSO4 + H 2SO4 a
Cu 2 + + 2e− ↔ Cu
a
2b
H3BO3 + 7H+ + 8e− ↔ BH−4 + 3H 2O
2ca
BO−2
3
NaBH4 + O2 → NaBO2 + 2H 2↑
2a
(E° = + 0.3419 V)
−
+ 6H 2O + 8e ↔
BH−4 8OH−
(E° = − 0.481 V)
(E° = − 1.24 V) Side Reactions
NaBH4 + 2O2 → NaBO2 + 2H 2O
NaBH4 + 2H 2O → NaBO2 + 4H 2↑ 4
4NaBO2 + H 2O ↔ 2NaOH + Na 2B4 O7
5
4KI + 2CuSO4 → 2CuI ↓+ I 2 + 2K 2SO4
Na 2B4 O7 + 7H 2O ↔ 2NaOH + 4H3BO3 Oxidation of CuNPs
H2O/H3O+
6a
2Cu + O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2CuO
6ba
4Cu + O2 ⎯⎯⎯⎯→ 2Cu 2O
Cu 2O + H 2O + 2e− ↔ 2Cu + 2OH−
4Cu + O2 + 4KI + 2H 2O → 4CuI + 4KOH
CuI−2
7a a
OH−
−
−
−
−
+ e ↔ Cu + 2I
(E° = − 0.360 V)
(E° = 0.00 V)
(E° = + 0.401 V)
8
O2 + 2H 2O + 4e ↔ 4OH
9
2Cu 2O + NaBH4 → 4Cu + 2H 2 ↑ + NaBO2
a
E° is a standard electrode potential of the half-reaction; all E° values were taken from ref 9.
Figure 1. Photographs and absorbance spectra of the fresh (a), oxidized (b), and aggregated (c) copper nanoparticles.
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EXPERIMENTAL DETAILS The description of reagents and equipment used for synthesis of CuNPs and experiments with them (oxidation and aggregation) are provided in the Supporting Information. Safety notes and an example of the handouts for students are also provided.
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leads to the appearance and growth of H2 release. The reaction starts immediately after the reagents are mixed and is completed within 5 min. The high reaction speed is a result of the strong oxidizing and reducing properties of copper(II) and borohydride ions at a neutral pH (eqs 2a and 2b, respectively). It is remarkable that the reduction properties of NaBH4 additionally grow in time due to the increase in pH of the reacting medium (this will be explained further) (eq 2c). During the course of the reaction, the color of the solution changes from colorless to red due to the formation of CuNPs with sizes ranging from 10 to 20 nm. The suspension of freshly synthesized CuNPs has a deep red color due to strong light absorption in the range from violet (400 nm) to orange (610 nm) while being transparent to red light (exceeding 635 nm wavelength) (Figure 1a). The band at 570 nm (green-yellow light) corresponds to the surface plasmon resonance (SPR)
OBSERVATIONS AND EXPLANATIONS
Synthesis and Side Reactions
The synthesis of plasmonic CuNPs described here is based on the reduction of copper(II) ions by NaBH4 in the presence of iodide ions used as a stabilizer.8 The net and half-reactions corresponding to the redox process are shown in Table 1 (eqs 1−2c). Equation 1 describes the reactions with both a small and an excessive amount of NaBH4 that students can observe during the experiment; specifically, an increase in NaBH4 concentration 1439
DOI: 10.1021/acs.jchemed.8b01050 J. Chem. Educ. 2019, 96, 1438−1442
Journal of Chemical Education
Activity
the color from red to yellow if the particle size decreases below 100 nm.12 Copper can have two oxides (CuO and Cu2O); therefore, the students should be informed that the formation of copper(II) and copper(I) compounds depends on the pH value. Copper is mainly oxidized to copper(II) compounds at neutral and acidic pH values (eq 6a) and to Cu2O in alkaline media (eq 6b). However, copper is a very weak reducer in neutral and acidic media (eq 2a), while its reduction properties grow significantly in alkaline solutions (eq 6b). As the pH of the CuNP solution gradually grows due to the degradation of NaBH4 and hydrolysis of NaBO2 (eqs 3 and 4), CuNPs are oxidized to Cu2O only. It is remarkable that the CuNPs can also be oxidized to CuI (eq 7) if an excess of iodide ions is used. For undergraduate students, we noted that CuNP oxidation and oxygen reduction (eq 8) are actually solvent-assisted processes without direct redox interactions between them. Last, the oxidation process is reversible, and Cu2O can be reduced back to CuNPs by adding a new portion of NaBH4 (eq 9).
resulting from the collective oscillation of the outward valence electrons (4s for copper) caused by a resonance interaction with the oscillating electric field of the incident light. Generally, side reactions are often skipped during the education process; however, from our point of view, this is not appropriate for complex systems such as nanoparticle suspensions and leads to obtaining only superficial knowledge of such objects. The primary side reactions for CuNP synthesis are (i) the decomposition of NaBH4 and (ii) the redox interaction between copper(II) and iodide ions (eqs 3−5). The high reactivity of NaBH4 leads to its fast decomposition (within 1 h) because of oxidation by air oxygen and the protons of water (hydrolysis) (eq 3). Thus, a 10-fold excess of NaBH4 was used to minimize the influence of its decomposition on the speed of CuNP synthesis and to slow the oxidation of CuNPs by the oxygen in air after their preparation. However, the application of excess NaBH4 will lead to the formation of excess decomposition products, which can also influence CuNP properties. For example, NaBO2 is affected by hydrolysis because of the low acidic properties of H3BO3 (eq 4). Therefore, as a result of these two processes, the pH value of the CuNP solution slowly increases to 9−10 within 30 min. After discussing side reactions, we usually ask the students how to minimize their impact. The correct answers are increasing pH and decreasing temperature to suppress hydrolysis. We additionally emphasize that an alkaline pH value also improves the reduction capability of NaBH4 (eq 2c). However, then we explain that we cannot increase the pH because its high value will lead to faster oxidation of the CuNPs (eq 6b, will be discussed further); thus, using cold water for the synthesis is the only reliable way to suppress hydrolysis. The reaction between CuSO4 and KI proceeds with the release of molecular iodine and the sedimentation of CuI (eq 5). This reaction is used to demonstrate the oxidizing properties of Cu(II) ions and to produce CuI.10,11 However, the rate of this reaction significantly depends on the concentration of both CuSO4 and KI and is drastically slowed down for diluted solutions. For example, the mixing of the stock solutions of the reagents (0.2 and 0.1 M for CuSO4 and KI, respectively) leads to the immediate formation of CuI sediment, and the solution turns brown due to the presence of iodine. On the other hand, the reaction between 20 times diluted reagents (5 drops per 5 mL of water) can be observed only after several hours and only in the presence of a starch solution. Thus, the speed of this reaction should be negligible in the case of the 200-fold dilution used for CuNP synthesis. However, the order of reagent addition is also important, e.g., mixing concentrated KI and diluted CuSO4 solutions leads to the partial interaction between them. This is why it is important to add KI solution first as this leads to producing CuNP with better plasmonic properties.
Colloidal Stability
As previously mentioned, iodide ions are responsible for the colloidal stability of CuNPs. The stabilization ability of iodide ions is based on their strong interaction with the copper surface, leading to the formation of a negative surface charge. As a consequence, the charged surfaces of the CuNPs experience electrostatic repulsion if they are placed in close proximity to each other. The small size of the CuNPs (10−20 nm) also contributes to better colloidal stability due to Brownian motion. Last, dropwise addition of NaBH4 is also important for producing stable CuNPs because the iodide ions will not properly stabilize the CuNPs, leading to their agglomeration if NaBH4 is added too fast. To comparatively demonstrate the stabilization properties of iodide ions, the synthesis of CuNPs can be performed without the addition of KI (control experiment) or in the presence of other compounds, e.g., chloride, bromide, or citrate ions. Such a comparison will show the students that only citrate ions can provide colloidal stability to CuNPs. However, the size of citrate-stabilized CuNPs is usually larger (20−40 nm), and their optical properties are more inferior than iodide-stabilized 2− CuNPs. In all other cases (Cl−, Br−, SO2− 4 , and B4O7 ), the CuNPs are unstable, and the sediment is a black precipitate. The comparison of iodide, bromide, and chloride ions is particularly useful, as they are all halides. If we assume that outermost surface atoms of CuNPs are the most reactive, then we can assign the stabilization process to the formation of a very thin layer of copper halide on the surfaces of the CuNPs. Therefore, the solubility product constants for CuCl (1.72 × 10−7), CuBr (6.27 × 10−9), and CuI (1.27 × 10−12)13 can be roughly (but quantitatively) used for estimating halide ion interactions with the surface of the CuNPs. A decrease in this constant means that copper(I) ions and halide ions are more attracted to each other and form stronger bonds. This observation supports the experimental results regarding the better stabilization capability of iodide ions compared to other ions. The last experiment shows an artificial decrease in the colloidal stability of the CuNPs (aggregation). Because this stability is caused by a repulsion between the charged surfaces of the CuNPs, we added the acid (H2SO4) to neutralize the negative surface charge by protonation and to decrease the interparticle repulsion. The students observed a decrease in
Oxidation
Generally, copper is described as a relatively inert metal, but this is true only for bulk copper. The large specific surface area of CuNPs leads to a significant increase in copper reactivity compared to bulk metal. Indeed, CuNPs experience quite a fast oxidation (
