Air stable colloidal copper nanoparticles: Synthesis, characterization and their surface-enhanced Raman scattering properties

https://doi.org/10.1016/j.physe.2015.11.002Get rights and content

Highlights

  • Air stable colloidal copper nanoparticles are synthesized by a simple chemical reduction method without any inert gas.

  • The resulting nanoparticles were characterized by various spectroscopic techniques such as UV-visible, FT-IR, XRD and TEM.

  • The copper nanoparticles do not suffer significant oxidation even after being stored for 6 months under ambient conditions.

  • They exhibit large surface-enhanced Raman scattering (SERS) signals.

Abstract

Air stable colloidal copper nanoparticles are synthesized by a simple chemical reduction method using octadecylsilane as a reducing agent and octadecylamine as a stabilizing agent in toluene without any inert gas. The formation of nanosized copper was confirmed by its characteristic surface plasmon absorption peaks in UV–visible spectra. The transmission electron microscopic (TEM) images show that the resulting copper nanoparticles are distributed uniformly with a narrow size distribution. The X-ray diffraction (XRD) demonstrated that the obtained copper nanoparticles are single crystalline nanoparticles. Fourier transform infra-red (FT-IR) spectroscopic data suggested that the silane Si–H is responsible for the reduction of copper ions. And also the resulting colloidal copper nanoparticles exhibit large surface-enhanced Raman scattering (SERS) signals.

Introduction

Metallic nanoparticles (MNPs) have been the subject of intense research during the recent years because of their unique properties compared to bulk metals [1], [2], [3], [4], [5], [6]. Among the different metal particles, copper nanoparticles have received considerable attention because copper is a versatile material which finds various applications in different fields of research like advanced microelectronics for conductive lines, medicine as an antibacterial agent, catalytic and biocatalytic transformations [1], [2], [3], [4], [5], [6], [7], [8] or for surface-enhanced Raman spectroscopy applications [9], [10]. It is well known that Raman spectroscopy is one of the nondestructive spectroscopic techniques widely used to characterize the dynamic behavior of molecules interacting with the electromagnetic radiation. The Raman vibrational spectrum is a molecular “fingerprint” that often provides unambiguous sample (solids, liquids, and gases) identification. Unfortunately, Raman scattering is an inherently weak process, unable to provide spectral information from very small amounts of sample. However, the sensitivity problems can be overcome by the employment of surface-enhanced Raman scattering spectroscopy (SERS) [11]. SERS enhancement effect has been observed with alkaline metals [12], various transition and noble metals [12], [13], metal oxides [14] and even semiconductor materials like silicone and graphene [14], [15]. Typically, large SERS is observed with coinage metals such as gold, silver, and copper because they exhibit localized surface plasmon resonance (LSPR) bands in the visible region due to excitation of the conduction electrons after irradiation with light. Colloidal suspensions of metal nanoparticles are the most common SERS substrates, which enable us to provide Raman intensification up to 107–108 enhancement factors with respect to the normal Raman signal of non-adsorbed molecules. The colloidal suspensions of metal nanoparticles acquire intense Raman signals due to the Brownian motion of the metal particles in the colloidal dispersions compared to other SERS active substrates such as rough metal electrodes, and metal island films. Among the group 11 metals, Cu is significantly cheaper than Ag and Au but the formation of a surface oxide layer on Cu is inevitable in an ambient atmosphere because the oxide phases are thermodynamically more stable, consequently, they are hard to synthesize chemically.

Generally, colloidal copper nanoparticles are synthesized by the chemical reduction of copper salts with sodium borohydride or hydrazine in water, followed by stabilization of the metal clusters inside protective shells (reverse micelles, polymers or long alkane chains) and eventually transfer into an organic phase [16]. They can also be synthesized by other methods such as laser ablation [17], gamma irradiation [18], microwave [19], electron beam [20], thermal decomposition [21] or vapor deposition [22]. Among the above mentioned methods, the chemical reduction is an easy and rapid procedure to prepare quite stable metal nanoparticles. To better control the NP size and size distribution, which are fundamental prerequisites for their employment in SERS studies, their synthesis should be performed in a single organic solvent [23]. Monodisperse Au, Ag, and Au3Pd nanoparticles with narrow size distribution were prepared by direct reaction of the related metal salt with oleylamine in toluene and studied their SERS properties using Rhodamine B and 2-naphthalenethiol as model substrates [24]. Zhang et. al. reported the large scale synthesis of monodisperse CuNPs with high purity using Cu(OH)2 as the precursor, L-ascorbic acid as the reductant, and PEG-2000 as the protectant for the conductive ink applications [25]. However, the investigation of Cu nanoparticles synthesized in single organic phase as SERS substrate has remained largely unexplored. This has inspired us to develop synthetic methods for stable colloidal Cu nanoparticles in organic solvent and explore their SERS properties.

Polysilanes are known to have a reductive nature for reducing noble metal salts to form the corresponding noble metals by the high HOMO-electrons of the Si–Si σ-bonds [26]. However, metal salts, which can be reduced by polysilanes, are limited in metals which have high standard reduction potentials, i.e., noble metal salts, and reduction of metal salts having a low reduction potential, such as copper or nickel salts, into metals is considered to be very difficult using polysilanes [27]. Kamada group reported the preparation of a Cu nanoparticles-polysilane composite film [28]. Polymeric hydrosiloxane stabilized palladium and iron oxide NPs have been used for catalysis [29] and biomedical applications [30]. Polymerized long chain alkylsilanes were used for the synthesis of gold [31] and silver metal core-siloxane shell nanoparticles and siloxane nanowires [32]. In recent years, we have successfully demonstrated that monomeric hydrosilanes can be used both for reducing as well as stabilizing of active Pd and Ag NPs [33], [34].

Herein, we report a one-step synthetic method to produce colloidal Cu nanoparticles in toluene by reducing copper ions with octadecylsilane using octadecylamine as a stabilizing agent. The advantages of the method include the synthesis of copper nanoparticles with reasonably uniform size and relatively higher yield. The resulting nanoparticles are stable in time, due to the presence of suitable concentrations of stabilizing agents. Investigation of the SERS property of the colloidal copper nanoparticles showed that the nanoparticles gave a 103 times SERS enhancement for Rhodamine B adsorbed on the nanoparticles compared to bulk Rhodamine B.

Section snippets

Materials and reagents

Copper (II) acetate (Cu(OAc)2), octadecylsilane (ODS) and octadecylamine (ODA) were purchased from Sigma-Aldrich and toluene was obtained from Rajadhani scientifics. All chemicals were used as received without further treatment. Freshly distilled toluene over Na/benzophenone was used in the reactions.

Characterization

UV-visible absorption spectra were recorded on a Perkin Elmer Lamda 750 UV–vis spectrophotometer. The FT-IR spectra were recorded on a Bruker, Alpha T FT-IR spectrometer. The particle size and

Synthesis of copper nanoparticles

Copper nanoparticles were prepared by reducing the copper (II) ions from copper (II) acetate dissolved in dry toluene using octadecylsilane.

High molar concentrations; 10 equivalents of ODS is required to reduce the Cu+2 ions due to its low standard reduction potential. When 10 equivalents of ODS is added to the Cu(OAc)2 solution in dry toluene at room temperature, the solution turned black color after 12 h (Fig. 1A) indicates the formation of Cu nanoparticles which was further conformed by

Conclusions

We have successfully synthesized air stable colloidal copper nanoparticles by a simple chemical reduction method without any inert gas. The resulting nanoparticles were characterized by various spectroscopic techniques such as UV–visible, FT-IR, XRD and TEM. The formation of nanosized copper was confirmed by its characteristic surface plasmon absorption peaks in UV-Visible spectra. The transmission electron microscopic (TEM) images show that the resulting copper nanoparticles are distributed

Acknowledgments

T. Ramani thanks the Department of Science and Technology (DST) for a WOS-A fellowship [DST No: SR/WOS-A/CS-115/2011(G)] and L.P thanks the DST fast track young scientist fellowship (CS 342/2011). Authors sincerely acknowledge the financial support from the CSIR-Network Project INTELCOAT (CSC-0114).

References (41)

  • C. Dong et al.

    Physica E

    (2014)
  • C. Dong et al.

    J. Alloy. Compd.

    (2014)
  • C. Dong et al.

    J. Mol. Liq.

    (2014)
  • C. Dong et al.

    Mater. Lett.

    (2014)
  • K. Zhou et al.

    Ceram. Int.

    (2015)
  • H. Khanehzaei1 et al.

    Int. J. Electrochem. Sci.

    (2014)
  • J. Cejkova et al.

    Appl. Surf. Sci.

    (2009)
  • L.Q. Pham et al.

    Radiat. Phys. Chem.

    (2011)
  • S.F. Adil et al.

    Dalton Trans.

    (2015)
  • Y. Tao et al.

    Chem. Soc. Rev.

    (2015)
  • Y. Wang et al.

    Langmuir

    (2009)
  • E.C. Le Ru et al.

    Principles of Surface-Enhanced Raman Spectroscopy and Related Plasmonic Effects

    (2009)
  • R. Aroca

    Surface-enhanced Vibrational Spectroscopy

    (2006)
  • P. Van Duyne et al.

    J. Chem. Phys.

    (1993)
  • X. Wang et al.

    Phy. Chem. Chem. Phys.

    (2012)
  • Y.B. Wu et al.

    J. Appl. Phys.

    (2009)
  • S. Magdassi et al.

    Materials

    (2010)
  • M.-M. Maurizio et al.

    J. Phys. Chem. C.

    (2011)
  • S.I. Bin Ahmad et al.

    AIP Conf. Proc.

    (2009)
  • M. Blosi et al.

    J. Nanoparticle Res.

    (2011)
  • Cited by (21)

    • Rationally designed SERS AgNPs/GO/g-CN nanohybrids to detect methylene blue and Hg<sup>2+</sup> ions in aqueous solution

      2021, Applied Surface Science
      Citation Excerpt :

      However, metallic nanoparticles tend to get easily aggregated/oxidized in air atmospheric condition. The aggregation state is naturally unstable and tends to precipitate from solution, to end with in loss of SERS signals [12–20]. Graphitic Carbon Nitride (g-CN) has been proven to be a fascinating semiconductor for photo-catalysis.

    • 2D layer stacked metallic Cu-serine triangular pyramids and their surface plasmon resonance properties

      2021, Physica E: Low-Dimensional Systems and Nanostructures
      Citation Excerpt :

      Size and shape of the nanoparticles are well investigated factors in the case of Au nanoparticles [2,3]. Copper (Cu) is a wise economic alternative for silver and gold, with better conductivity, and luminescent properties [4]. Copper nanoparticles (Cu NPs) offer great potential in the field of clinical research including drug delivery and therapeutics due to its biocompatibility [5].

    • Cu@Au self-assembled nanoparticles as SERS-active substrates for (bio)molecular sensing

      2019, Journal of Alloys and Compounds
      Citation Excerpt :

      Most of SERS substrates for the detection of potential organic pollutants [9–12], clinical samples [13,14], drugs [2,15,16], etc. reported in the literature are Au or Ag based. Despite the lower cost, Cu has been under used because of its inherent tendency to form a stable oxide layer [17]. Another factor is that the synthesis of Cu NPs is usually performed in organic media, which allows for better control over NP shape and size distribution [18], but may be disadvantageous for some applications.

    • Well-Defined Nanoparticles for Model Studies in Sustainable Industrial Chemistry

      2019, Studies in Surface Science and Catalysis
      Citation Excerpt :

      Shorter chain silanes have also been tested and were found to be less efficient stabilizers for Ag NPs. Copper NPs were also synthesized from Cu(OAc)2 using n-octadecylsilane as a reductant in different conditions [23]. Here, Cu NPs with a broad size distribution were embedded in a polymerized octadecylsilane micelles network using 10 M equivalents of silane at room temperature.

    • Surface-Enhanced Spectroscopy for Surface Characterization

      2017, Nanolayer Research: Methodology and Technology for Green Chemistry
    View all citing articles on Scopus
    View full text