Non-Oxidized Bare Metal Nanoparticles in Air: A Rational Approach for Large-Scale Synthesis via Wet Chemical Process

Abstract

Metal nanoparticles (MeNPs) have been used in various industrial applications, owing to their unique physical and chemical properties different from the bulk counterparts. However, the natural oxidation of MeNPs is an imminent hindrance to their widespread applications despite much research efforts to prevent it. Here, a rational approach for non-oxidized bare MeNPs in air, which requires no additional surface passivation treatment is reported. The direct synthetic route uses the [Gd₂ C]²⁺ · 2e^- electride as an exceptional electron-donating agent to reduce diverse metal precursors in alcoholic solvents. All synthesized bare Cu, Ag, and Sn nanoparticles are ultra-stable in ambient air, exhibiting no trace of metal oxides even on their outermost atomic layer. This unique resistance to oxidation is ascribed to the accumulation of excess electrons on the surface of bare MeNPs, which originates from the spontaneous transfer of anionic electrons from the electride during the nanoparticle growth process. This approach provides not only a revolutionary scheme to obtain MeNPs with non-passivated and non-oxidized surfaces, but also fundamental knowledge about metal oxidation.

Keywords: electrides; negatively charged surfaces; non-oxidized metal nanoparticles; wet chemical syntheses.

Figure 1

Synthesis of non‐oxidized bare MeNPs by the wet chemical solution process. a) Schematic illustration for the growth of MeNPs in alcoholic solutions containing the dissolved precursor compounds. Green and pink spheres represent metal cations and counter anions, respectively. Expanded schematic (middle panel) shows the reduction of metal cations and the growth of MeNPs. Orange arrows represent the transfer of anionic electrons from the [Gd₂C]²⁺ · 2e⁻ electride. b) Photographs showing the color transition before and after the synthesis of CuNPs. After reaction, the [Gd₂C]²⁺ · 2e⁻ electride flakes were removed by an external magnet.

Figure 2

Surface analysis of the non‐oxidized bare CuNPs. a) Photograph of CuNP powder synthesized by wet chemical solution process. b) SEM image of as‐prepared CuNPs with an average size of ≈30 nm. c) TEM image of as‐prepared CuNPs. d) ADF STEM image of an as‐prepared CuNP. e) EELS elemental mapping of the ADF STEM image in (d). The mapping image shows the non‐oxidized state both in the interior and on the surface of CuNP. f) EELS elemental mapping for the Cu L edge (red), C K edge (yellow), Gd M edge (green), and O K edge (blue). Scale bars: 2 nm. g) EEL spectra obtained from points marked 1−3 in (d) at the CuNP surface, showing the metallic L_3,2 edges. White lines of Cu oxides are denoted by asterisk (*). The white lines were not observed for the wet‐chemically synthesized CuNP. h) Cu L₃M₄₅M₄₅ AES data of as‐prepared CuNPs fitted to five peaks. i) Work function histogram of the as‐prepared CuNPs measured using KPFM.

Figure 3

Long‐term stability of bare CuNP powder in air. a) ADF STEM image of a CuNP after 10 days of air exposure. b) Magnified HR‐STEM image of the boxed region in (a). c) Interplanar distance profile from regions marked 1−3 in (b). The interplanar distance of 0.21 nm corresponds to fcc Cu(111). d) ADF STEM image of a CuNP after 20 days of air exposure. e) Enlarged HR‐STEM image of the boxed region in (d). f) Interplanar distance profile from regions marked 1−3 in (e). The interplanar distance of 0.18 nm corresponds to fcc Cu(200). g) ADF STEM image of a CuNP after 30 days of air exposure. h) HR‐STEM image of the boxed region in (g). i) Interplanar distance profile from regions marked 1−3 in (h). The interplanar distance of 0.21 nm corresponds to fcc Cu(200). j) XRD patterns of commercial CuNPs and the bare CuNPs in the as‐prepared state and after 10, 20, and 30 days of air exposure.

Figure 4

Rational strategy for preparing non‐oxidized bare MeNPs in air. a) Optical image of AgNP powder synthesized by wet chemical solution process. b) ADF STEM and EELS mapping image of as‐prepared AgNPs: the Ag M edge (pink), C K edge (yellow), Gd M edge (green), and O K edge (blue). c) Histogram of work function for as‐prepared AgNPs measured by KPFM (average value: ≈3.9 eV). d) Optical image of SnNP powder synthesized by wet chemical solution process. e) ADF STEM and EELS mapping image of as‐prepared SnNPs: the Sn M edge (orange), C K edge (yellow), Gd M edge (green), and O K edge (blue). f) Histogram of work function for as‐prepared SnNPs measured by KPFM (average value: ≈4.1 eV). g) Ag 3d XPS spectra of as‐prepared AgNPs. A negative shift in the binding energy of the Ag 3d_5/2 peak (367.9 eV) directly implies a high electron density on the surface. h) Sn 3d XPS spectra of as‐prepared SnNPs. A negative shift in the binding energy (484.7 eV) was observed for Sn 3d_5/2 compared to Sn metal. i) XRD patterns of as‐prepared AgNP and SnNP powders without peaks of metal oxides.

Figure 5

Air stability of non‐oxidized bare AgNPs and SnNPs. a) ADF STEM image of an air‐exposed AgNP for 30 days. b) EELS mapping image of (a): the Ag M edge (pink), C K edge (yellow), Gd M edge (green), and O K edge (blue). c) EEL spectra obtained from surface points marked 1−4 in (a). d) ADF STEM image of an air‐exposed SnNP for 1 day. e) EELS mapping image of (d): the Sn M edge (orange), C K edge (yellow), Gd M edge (green), and O K edge (blue). f) EEL spectra obtained from surface points marked 1−4 in (d). The EEL mapping image and EEL spectra show metallic M edges of air‐exposed Ag and SnNPs without any detectable O K signal.