Mild synthesis of single-nanosized plasmonic copper nanoparticles and their catalytic reduction of methylene blue

Colloid and Interface Science Communications 31 (2019) 100187

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Mild synthesis of single-nanosized plasmonic copper nanoparticles and their catalytic reduction of methylene blue

T

Kousuke Kurodaa, Philip Kellera,b, Hideya Kawasakia,

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a b

Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita-shi, Osaka 564-8680, Japan Institute of applied physics, Justus-Liebig-University, Germany

ARTICLE INFO

ABSTRACT

Keywords: Copper nanoparticles Sub-10 nm Ascorbic acid Methylene blue Clock reaction Catalysis

In this study, we report a mild synthesis of sub-10 nm Cu nanoparticles (NPs) in ethylene glycol (EG) at room temperature using L-ascorbic acid (AA) as the reducing agent in the presence of 1-amino-2-propanol (AMIP). To the best of our knowledge, this work is the first report of the room-temperature synthesis of sub-10 nm Cu NPs using a nontoxic reducing agent. The as-prepared colloidal suspension of Cu NPs exhibited surface plasmon resonance absorption at 515 nm. Transmission electron microscopy observations revealed well-dispersed spherical Cu NPs with an average particle size of 4.1 ± 0.8 nm. The use of AMIP as a ligand was found to be critical to producing sub-10 nm Cu NPs in EG under such mild conditions. Moreover, the sub-10 nm Cu NPs were an effective catalyst for the reversible methylene blue–AA reaction system.

1. Introduction

reaction solution [12,13,17]. The properties of Cu NPs are size-dependent, and Cu NPs < 10 nm in size (sub-10 nm Cu NPs) often exhibit high catalytic activity and are particularly effective for conductive inks requiring low-temperature sintering [18–24]. However, a strong and often toxic reducing agent such as NaBH4 or hydrazine or high-temperature reaction conditions are critical for the synthesis of sub-10 nm Cu NPs via conventional methods. In the present paper, we report on the mild synthesis of sub-10 nm Cu NPs with a narrow size distribution in ethylene glycol (EG) at room temperature, using a complex of Cu(II) and 1-amino-2-propanol (AMIP) as the precursor ion and L-ascorbic acid (AA) as the reducing agent. The use of AA, which is a mild and nontoxic reducing agent, and the roomtemperature conditions are advantages over previously reported chemical reduction approaches. This synthesis method is considered ecofriendly and should scale easily. To the best of our knowledge, this work is the first report of the room-temperature synthesis of sub-10 nm Cu NPs using a nontoxic reducing agent. The prepared sub-10 nm Cu NPs are shown to effectively catalyze the reduction of methylene blue (MB).

According to the International Union of Pure and Applied Chemistry definition, nanoparticles (NPs) are particles of any shape with dimensions in the range 1–100 nm [1]. Metal NPs tend to exhibit novel optical, thermal, magnetic, catalytic, antimicrobial, and electronic properties that differ from those of the corresponding bulk metals [2,3]. Among various metal NPs, Au, Ag, and Pt NPs have been extensively investigated because of their applications in electronic and optical devices, energy conversion devices, conductive inks, catalysts, sensors, and biomedicines [4–11]. More recently, Cu NPs have attracted attention because of the high natural abundance and low cost of Cu as well as its numerous practical applications in numerous industries [12,13]. Thus, Cu NPs are a promising candidate to replace expensive noblemetal NPs. Despite the advanced characteristics of Cu NPs, their application is currently limited because of the inherent instability of Cu against oxidation in the ambient atmosphere, which alters Cu NPs' physical and chemical properties [14–16]. Over the past decade, wet chemical reduction methods for the synthesis of Cu NPs (i.e., chemical reduction of Cu salts by reducing agents in solution) have been developed, where size control can be achieved through variation of experimental factors such as the molar ratio between the stabilizer and the Cu salt precursor, the fraction of reducing agent relative to the precursor salt, the concentration of the Cu salt, the type of reducing agent, and the pH and temperature of the

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2. Experimental 2.1. Materials EG, propylene glycol (PG), triethylene glycol (TEG), glycerol (Gly), ethanol, AMIP, 2-amino-1-butanol (AB), 2-amino-2-ethyl-1,3-

Corresponding author. E-mail address: [email protected] (H. Kawasaki).

https://doi.org/10.1016/j.colcom.2019.100187 Received 7 March 2019; Received in revised form 6 June 2019; Accepted 10 June 2019 Available online 14 June 2019 2215-0382/ © 2019 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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propanediol (AEP), Cu(II) acetate, MB, polyethyleneimine (PEI; Mw ~70,000), and AA were purchased from Wako Chemical, Japan. All chemicals used were of analytical grade.

the presence of metallic Cu NPs (Fig. 1b) [25]. The position of the plasmon absorption peak of metal NPs is known to be dependent on their particle size [26]. Cu NPs with a particle size of 150 nm have been reported to exhibit a surface plasmon resonance peak at ~600 nm [17], and 5 nm Cu NPs have been reported to exhibit a surface plasmon resonance peak at 530 nm [22]. The shorter wavelength of the surface plasmon resonance peak (~515 nm) observed for the present Cu NPs suggests the formation of sub-5 nm Cu NPs. This result is consistent with the TEM images of the Cu NPs, which indicate the presence of spherical NPs with an average diameter of 4.1 ± 0.8 nm (Fig. 1c and d). The increase in the surface plasmon resonance absorbance of the Cu NPs accompanies an increase in absorbance at 385 nm. Thus, the absorbance at 385 nm is attributable to dehydroascorbic acid (i.e., the oxidation product of AA) [27]. The XRD pattern of the synthesized Cu NPs shows no clear diffraction peaks because of their sub-5 nm particle size [28] (Fig. 2a). Therefore, we sintered the sub-10 nm Cu NPs at 250 °C for 1 h under a nitrogen atmosphere to induce an increase in particle size. The sintered Cu NPs showed clear diffraction peaks attributable to metallic Cu with no evidence of a Cu oxide (Cu2O) phase (Fig. 2a). Fig. 2b shows the FT-IR spectra of the sub-10 nm Cu NPs and free AMIP in the range 800–4000 cm−1. The absorption bands are assigned as follows [29]. Absorption bands b1, b2, b3, and b4 observed in the spectrum of the Cu NPs are also observed in the spectrum of free AMIP ligands. The FT-IR spectrum includes absorption bands in the range 2850–2950 cm−1 arising from CeH stretching vibrations of eCH3 and eCH2. Absorption bands b1 (1585 cm−1) and b2 (1420 cm−1) are assigned to NeH bending and OH in-plane bending, respectively. Absorption bands b3 (1035 cm−1) and b4 (~870 cm−1) are attributed to CeN stretching and NeH wagging, respectively. These absorption-band assignments indicate that the Cu NPs are protected by AMIP ligands. AA is a weak reducing agent; thus, the synthesis of Cu NPs using AA has previously required high temperatures to facilitate the reduction of Cu(II) salts to metallic Cu [19,30]. We also previously reported the synthesis of sub-10 nm Cu NPs via the reduction of the AMIP–Cu(II) complex with hydrazine [21]. However, with this previous method, we could not suppress the surface oxidation of the as-prepared Cu NPs, resulting in the lack of a plasmon peak in the UV–Vis spectra of the Cu NPs. In addition, hydrazine is a toxic, dangerous reagent and its use is environmentally unfriendly. By contrast, with the present synthesis method, we successfully prepared plasmonic sub-10 nm Cu NPs via AA reduction of an AMIP–Cu(II) complex under mild conditions. The use of AMIP ligands is critical for obtaining sub-10 nm Cu NPs under such mild conditions. In the absence of AMIP, Cu(II) acetate was not reduced to metallic Cu by reduction with AA at room temperature after 24 h (Fig. 3). Surprisingly, the use of other alkanol-amine ligands such as AB and AEP also failed to produce plasmonic Cu NPs by AA reduction at room temperature after 24 h, despite the similar ligand structures of AMIP and AB (as shown in Fig. 3). By contrast, extending the reaction time of the room-temperature AA reduction of the AB–Cu (II) complex from 24 h to 1 week led to Cu NPs with a surface plasmon resonance at ~520 nm in the corresponding UV–Vis spectrum (as shown in a blue arrow of Fig. S1). These results indicate that the AA reduction rate of the AB–Cu(II) complex is much lower than that of the AMIP–Cu(II) complex. We speculate that the slower reduction of the AB–Cu(II) complex is attributable to the greater stability of the AB–Cu (II) complex compared with that of the AMIP–Cu(II) complex. To examine the difference in stability between the AB–Cu(II) and AMIP–Cu(II) complexes, we evaluated the stability constants (Kf) of both complexes in EG solvent spectrophotometrically using Job's continuous variation method [31]. From the Job plot shown in Fig. 4, a stoichiometric ratio of 4:1 between the AB or AMIP ligand and the Cu (II) ions was obtained. The Kf values were 2.2 × 1012 and 6.6 × 1014 for the AMIP–Cu(II) complex and the AB–Cu(II) complex, respectively. These results indicate that the AB–Cu(II) complex is more stable than the AMIP–Cu(II) complex, resulting in the slower reduction of the

2.2. Synthesis of sub-10 nm Cu NPs Sub-10 nm Cu NPs were synthesized by chemical reduction of an AMIP–Cu(II) complex precursor salt with AA. Typically, Cu(II) acetate (0.908 g) was completely dissolved in 3.85 mL of AMIP (Cu:AMIP molar ratio of 1:10) and 44 mL of EG under ultrasonic agitation, resulting in a blue solution of the AMIP–Cu(II) complex. Solid AA (3.52 g) was then added to this AMIP–Cu(II) complex solution and the resultant solution was stirred continuously at 1000 rpm for 24 h at room temperature (~23 °C). 2.3. Characterization Transmission electron microscopy (TEM) images of the Cu NPs were obtained using a JEOL JEM1400 microscope operated at 120 kV. The UV–Vis absorption spectra of colloidal Cu NP solutions were recorded using a UV–Vis–near-infrared (NIR) spectrophotometer (Jasco V670, Tokyo, Japan). Fourier transform infrared (FT-IR) spectra of the dried samples were recorded on an FT-IR 4200 spectrometer (Jasco, Japan) using the attenuated total reflection technique with a ZnSe prism. Powder X-ray diffraction (XRD) patterns were obtained on a Bruker AXS D2 Phaser X-ray diffractometer equipped with a Cu Kα radiation source. Solid samples for FT-IR and XRD analyses were prepared by transferring the as-prepared Cu NP suspensions to a 3 kDa cutoff dialysis bag and dialyzing them with 3 L of ethanol while stirring constantly at room temperature for 1 day. After the dialysis, dried samples of Cu NPs were obtained by evaporating the ethanol under reduced pressure at room temperature. 2.4. Determination of binding strengths using spectroscopy methods Job's method of continuous variations was carried out using spectroscopic methods to determine the binding strengths (formation constant Kf) between the AB ligands or the AMIP ligands and the Cu(II) ions. For this purpose, different volumes (2, 5, 7, 10, and 12 mL) of EG solution containing 0.02 M Cu(II) acetate were mixed with a sufficient volume of 0.02 M ligand solution to fill a 25 mL standard volumetric flask. Similarly, different volumes (2, 5, 7, 10, and 12 mL) of EG solution with 0.02 M AB or 0.02 M AMIP were mixed with a sufficient volume of 0.02 M Cu(II) acetate to fill a 25 mL standard volumetric flask. In total, we prepared 10 samples for the Job's method. The absorbance was recorded at 500 nm. This wavelength was selected as the absorbance of AMIP–Cu(II) complex-specific. The absorbance values were plotted against the mole fractions of the ligands, thereby providing the Kf values associated with formation of the complexes. 2.5. Catalytic tests In a typical catalytic reaction test, 1 × 10−4 M Cu NPs (Cu atom concentration), 1.5 × 10−5 M MB, and 0.1 wt% PEI were mixed in 3 mL of water and an aqueous AA solution of a given concentration was added to the reaction mixture while stirring. The time-dependent absorption spectrum of MB was recorded using a UV–Vis spectrophotometer at room temperature (~23 °C). 3. Results and discussion The formation of Cu NPs was evident from a gradual color change of the solution due to the reduction of the AMIP–Cu(II) complex by AA: the solution changed from blue to wine-red after 24 h because of the plasmonic resonance of the Cu NPs (Fig. 1a). The UV–Vis spectrum exhibits a surface plasmon resonance peak at ~515 nm, which confirms 2

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AB–Cu(II) complex. We calculated the electronegativity of the amino groups of AB or AMIP by estimating the nitrogen charge (Mulliken charge) at the semiempirical PM3 level. The results (Fig. S2) indicate that the charge (−0.0435) of the nitrogen of AB is more negative than that (−0.0396) of AMIP. Consequently, the amino group of AB behaves as a stronger Lewis base, resulting in the observed high stability of the AB–Cu(II) complex. We also examined the critical parameters for producing plasmonic sub-10 nm Cu NPs via AA reduction. Cu NPs were also synthesized using PG, TEG, or Gly as an alternative polyol solvent to EG or using reaction temperatures (40, 50, 60, and 80 °C) other than room temperature (~23 °C) while using otherwise identical synthesis procedures. However, the plasmonic peak of the Cu NPs prepared under these

conditions is weak (Fig. S3). The high reaction temperature (> 60 °C) promotes the reduction of Cu salts but also leads to oxidation of the Cu NPs, suppressing the plasmonic peak. Thus, plasmonic sub-10 nm Cu NPs were only obtained via the AA reduction of the Cu–AMIP complex in EG solvent at room temperature. Cu2O NPs can catalyze the hydrazine reduction of MB to leuco-MB (LMB, colorless), causing a color change from blue to colorless within a short time [32–36]. In the MB–hydrazine system, an oscillation reaction between blue MB and colorless LMB has been observed upon periodic shaking [32,33]. This phenomenon is known as the Cu2O NP-catalyzed clock reaction involving conversion between MB and LMB [34]. Herein, we examined the sub-10 nm Cu NP-catalyzed reduction of MB in the presence of AA. However, the surfaces of the sub-10 nm Cu NPs were 3

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Wavelength / nm Fig. 3. UV–Vis spectra of Cu NPs in EG solution, as prepared from AMIP–Cu(II), AB–Cu(II), AEP–Cu(II) complexes; the spectrum for Cu(II) acetate (no ligand) is included. The chemical structures of 1-amino-2-propanol (AMIP), 2-amino-1-butanol (AB), 2-amino-2-ethyl-1,3-propanediol (AEP) are shown for reference.

rapidly oxidized in aqueous media. When the Cu NPs were diluted with water, the reddish-brown suspension became yellowish within 1 min and the plasmonic absorption peak dramatically decreased in intensity after 10 min because of the oxidation of the Cu NPs (Fig. S4a). By contrast, the surface oxidation of the Cu NP colloidal aqueous suspension was suppressed in the presence of trace amounts of 0.1 wt% PEI (Fig. S4b). This behavior is attributed to the PEI adsorbing onto the Cu NPs through CueN coordination to form a layer that suppresses the oxidation reaction [37]. Thus, we examined the Cu NP-catalyzed reaction in the presence of a trace amount of PEI. We found that the oxidation of the Cu NPs proceeded after the NPs were exposed to air for 24 h even when 0.1 wt% PEI was present; the UV–Vis spectrum of the oxidized Cu NPs correspondingly showed no plasmonic absorption peak. However, the plasmonic peak of the Cu NPs in the PEI aqueous solution was maintained when the catalytic reaction proceeded for < 30 min. MB has a basic dye skeleton comprising a thiazine group, and it has an absorption maximum at 665 nm due to an n–π* transition in aqueous media. The progress of the Cu NP-catalyzed reduction of MB by AA was monitored by measuring the changes in the absorbance maxima of MB using a UV–Vis spectrometer. Fig. 5a and b show the absorption change of MB at 665 nm as a function of time for different AA concentrations. In the absence of Cu NPs, MB reduction by AA proceeds slowly and is incomplete after 15 min when 10 mM AA is present (Fig. 5b). In aqueous MB/AA solutions containing Cu NPs, a rapid decrease in absorbance of MB resulting in a colorless solution was observed, suggesting that the Cu NPs catalyzed the clock reaction involving MB and LMB. We

confirmed this clock reaction by observing the color change from colorless to blue under exposure to an air atmosphere (Fig. 5c). The color change from LMB to MB induced by air bubbling was reversible (Fig. 6a). A possible mechanism of the reversible reaction facilitated by the Cu NP catalyst is proposed as follows [31–33]: Cu NPs facilitate electron transfer from AA to MB, resulting in the formation of LMB. Excess AA, in turn, decreases the concentration of dissolved oxygen, which also facilitates the formation of LMB. The bubbling of air through the solution increases the dissolved oxygen concentration, resulting in oxidation of the LMB to MB. A plot of log A/A0 vs. t is linear for times < 5 min (Fig. 6b). From the slopes of the log A/A0 vs. t plots, the following rate constants, kobs (min−1), were obtained: 7.0 × 10−3 min−1 for 1 mM AA, 21 × 10−3 min−1 for 3 mM AA, 47 × 10−3 min−1 for 10 mM AA, and 268 × 10−3 min−1 for 10 mM AA +1 × 10−4 M Cu NPs. The kobs value in the presence of 1 × 10−4 M Cu NPs is 5.7 times larger than that in the absence of Cu NPs. The kobs value can be described as kobs = k0[AA] [Cu NPs]. Therefore, the k0 value is 4.5 × 103 M−2 s−1 for 10 mM AA +1 × 10−4 M Cu NPs and its value is compared with the value reported for small Cu nanoclusters (Cu NCs) on the Cu NC-catalyzed reduction of MB with hydrazine (k0 = 0.9 × 103 M−2 s−1) [38]. This result demonstrates that the sub-10 nm Cu NPs exhibit substantial catalytic activity toward MB reduction by AA in aqueous media. 4. Conclusion We have described the first synthesis of sub-10 nm Cu NPs

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Fig. 5. (a) UV–Vis spectra showing the successive decolorization of MB due to reduction by AA in the presence of Cu NPs. Conditions: [MB] = 1.5 × 10−5 M, [AA] = 10 mM, and [Cu] = 1 × 10−4 M. (b) Absorbance change of MB at 665 nm as a function of time for various AA concentrations: (i) 1 mM, (ii) 3 mM, (iii) 10 mM, and (iv) 10 mM AA +1 × 10−4 M Cu. A is the absorbance at a given time after the addition of Cu NPs, and A0 is the absorbance immediately after the addition of Cu NPs. (c) Photographs of the Cu NPcatalyzed MB−AA system.

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exhibiting surface plasmon resonance (peak wavelength ~515 nm) by AA reduction in EG at room temperature using a Cu(II)–AMIP complex as the precursor ion. The sub-10 nm Cu NPs are protected by AMIP ligands, which are critical for obtaining sub-10 nm NPs under such mild conditions. We also showed that the sub-10 nm Cu NPs exhibit high catalytic activity toward the reduction of MB by AA in aqueous media in the clock reaction involving MB and LMB.

[2] [3] [4] [5]

Acknowledgment

[6]

This work was supported by JSPS KAKENHI Grant Number JP 19H02564 and 15H03520.

[7] [8]

Appendix A. Supplementary data

[9]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.colcom.2019.100187.

[10] [11]

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