Elsevier

Applied Surface Science

Volume 427, Part B, 1 January 2018, Pages 687-694
Applied Surface Science

Full Length Article
On the nature of citrate-derived surface species on Ag nanoparticles: Insights from X-ray photoelectron spectroscopy

https://doi.org/10.1016/j.apsusc.2017.09.026Get rights and content

Highlights

  • Surface analysis of citrate-stabilized AgNPs immobilized from a dense sol was performed.

  • Capping species are largely products of citrate decomposition.

  • Species adsorbed on AgNPs having various sizes are different.

  • Ligands bound to surface Ag via one or two carboxylate and alcohol groups.

  • No surface ketone group was found.

Abstract

Citrate is an important stabilizing, reducing, and complexing reagent in the wet chemical synthesis of nanoparticles of silver and other metals, however, the exact nature of adsorbates, and its mechanism of action are still uncertain. Here, we applied X-ray photoelectron spectroscopy, soft X-ray absorption near-edge spectroscopy, and other techniques in order to determine the surface composition and to specify the citrate-related species at Ag nanoparticles immobilized from the dense hydrosol prepared using room-temperature reduction of aqueous Ag+ ions with ferrous ions and citrate as stabilizer (Carey Lea method). It was found that, contrary to the common view, the species adsorbed on the Ag nanoparticles are, in large part, products of citrate decomposition comprising an alcohol group and one or two carboxylate bound to the surface Ag, and minor unbound carboxylate group; these may also be mixtures of citrate with lower molecular weight anions. No ketone groups were specified, and very minor surface Ag(I) and Fe (mainly, ferric oxyhydroxides) species were detected. Moreover, the adsorbates were different at AgNPs having various size and shape. The relation between the capping and the particle growth, colloidal stability of the high-concentration sol and properties of AgNPs is briefly considered.

Introduction

The optical, chemical, bioactive, and other unique properties of Ag nanoparticles (AgNPs) and other nanomaterials can be tuned by varying the size, shape, and surface composition, particularly capping ligands [1], [2]. Citrate ions are widely utilized in the wet chemical synthesis of silver [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], gold [15], [16], [17], [18], [19], [20], [21], platinum [22], and other nanoparticles, acting as reducing (direct reduction of aqueous Ag+ ions by citrate occurs under boiling or hydrothermal conditions [9], [10], [11], [12], [13]), complexing, and stabilizing agent, although the precise mechanisms are far from being fully understood. Сitrate is a key reagent for the preparation of silver nanoplates, cubes, disks, etc., as the selective adsorption of citrate on Ag (111) facets impedes their growth and promotes the yield of anisotropic particles [12], [23], [24], [25]. Transformation of rounded AgNPs to anisotropic ones under illumination [26], [27], [28] is believed to be due to decomposition of the citrate capping. Under the synthetic conditions, and also upon the biochemical oxidation of citric acid that is vital for aerobic organisms (the Krebs cycle) [29], citrate is known to oxidize to acetonedicarboxylic and acetoacetic acids, other intermediates, and CO2 as the final product.

The solution-based synthesis and modification of AgNPs typically involve diluted sols (at most 10 mM) due to the restricted colloidal stability. High-concentration fluid dispersions such as inkjet inks can be obtained using large amounts of surfactants or/and polymer stabilizers [30], which generally should be removed to attain required characteristics of materials. The method of reduction of aqueous Ag+ with ferrous ions in the presence of sodium citrate proposed by Carey Lea [3] as long ago as 1889 is still a rare example of metal colloids stable up to 1 M concentration without high-molecular-weight reagents [4], [5], [6], [7], [8], [9], [10]. It has been reported [5], [6], [7] that the AgNPs are negatively charged due to adsorption of citrate anions, and their behavior obeys the Derjaguin-Landau-Verwey-Overbeek (DLVO) model, but the studies have been performed for diluted solutions, and the reasons behind stability of the dense sols remain elusive. Several forms of citrate bonded to the Ag surface through carboxylate groups have been proposed mainly on the base of surface-enhanced Raman scattering (SERS) [8], [9], [10], [11], [12], [23], and DFT simulation [24], [25]. A hydrogen bond involving an alcohol hydroxyl group has been suggested too, while oxidized derivatives of citrate have been observed on the AgNPs only after heating in air [9], [10]. In general, however, the results on the surface composition of nanosilver are still scarce and inconclusive, in particular as SERS is not appropriate for quantitative surface analysis.

In this research we attempted to clarify the nature of citrate-related adsorbates at various fractions of AgNPs manufactured using the Carey Lea method, i.e. avoiding direct oxidation of citrate, applying X-ray photoelectron spectroscopy (XPS) in conjunction with soft X-ray absorption near-edge spectroscopy (XANES), and other techniques, as a prerequisite for understanding the properties and behaviour of AgNPs in the synthesis, dense colloids, the environment and biological systems.

Section snippets

Materials and sample preparation

Silver nitrate (AgNO3), iron sulfate (FeSO4·7H2O), trisodium citrate (Na3cit·2H2O), sodium nitrate (NaNO3), and potassium nitrate (KNO3) were analytical grade and used as received. Deionized water (Millipore Milli-Q grade) was utilized to prepare all the solutions, to redisperse AgNPs and to rinse the samples deposited onto substrates. Silver nanoparticles were synthesized using the Carey Lea method [3], [4], slightly modified here. In a typical procedure, 7 mL of Na3cit·2H2O solution (400 g/L)

TEM, SEM, SAXS characterization

Fig. 1 shows a representative TEM micrograph of AgNPs from the Carey Lea sol diluted to ∼1 g/L Ag, and a SEM image of the as-prepared dense sol (about 100 g/L Ag) deposited onto highly oriented pyrolytic graphite (HOPG) and then rinsed with water. Particle size distribution calculated from the SAXS pattern is also presented. The Ag NPs are mainly spheroids of 10–12 nm with a share of ∼5 nm in the diameter, and bigger particles, many of which are nanoplates of 20–50 nm in the lateral dimension. The

Composition of surface species

The above findings can be explained as follows. Uncompromised citrate anions, which bind to surface Ag atoms via two carboxylate and alcohol oxygen, appear to exist, in a mixture with other adsorbates, on spherical AgNPs of ∼10–12 nm (Fig. 5, a). The rounded nanoparticles Ag NPs of ∼6 nm in diameter are capped with ligands composed of one carboxylate anion and one alcohol group, both bound to the surface Ag atoms, and one or two aliphatic carbon atoms. Fig. 5, b exemplifies 3-hydroxybutyrate

Conclusions

In summary, the surface analysis based mainly on the photoelectron C 1s spectra revealed that ligands covering the Ag nanoparticles prepared via Fe2+-mediated reduction of Ag+ are composed of an alcohol group and one or two carboxylate bound to the surface Ag, and minor unbound carboxylate group; alternatively, the adsorbates may represent mixtures of citrate and the products of its decomposition. The surface species differ for various fractions of AgNPs (about 10 nm spheroids, nanoplates, and ∼6

Acknowledgements

We thank the Russian-German bilateral program “Russian-German laboratory at BESSY II” and the staff of the RGLab and HZB for their kind assistance with XANES experiments.

References (50)

  • T.I.T. Okpalugo et al.

    High resolution XPS characterization of chemical functionalised MWCNTs and SWCNTs

    Carbon

    (2005)
  • K. Heymann et al.

    C 1 s K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy for characterizing functional group chemistry of black carbon

    Org. Geochem.

    (2011)
  • X. Ou et al.

    Photocatalytic reaction by Fe (III)?citrate complex and its effect on the photodegradation of atrazine in aqueous solution

    J. Photochem. Photobiol. A

    (2008)
  • J. Carrola et al.

    Insights into the impact of silver nanoparticles on human keratinocytes metabolism through NMR metabolomics

    Arch. Biochem. Biophys.

    (2016)
  • Y. Mao et al.

    Depletion force in colloidal systems

    Physica A

    (1995)
  • C.T. McKee et al.

    Interaction forces between colloidal particles in a solution of like-charged, adsorbing nanoparticles

    J. Colloid Interface Sci.

    (2012)
  • Yu.A. Krutyakov et al.

    Synthesis and properties of silver nanoparticles: advances and prospects

    Russ. Chem. Rev.

    (2008)
  • B. Calderón-Jiménez et al.

    Silver nanoparticles: technological advances, societal impacts, and metrological challenges

    Front. Chem.

    (2017)
  • M. Carey Lea

    Allotropic forms of silver

    Am. J. Sci.

    (1889)
  • G. Frens et al.

    Carey Lea's colloidal silver

    Kollod Z. Z. Polym.

    (1969)
  • O.V. Dement’eva et al.

    Comparative study of the properties of silver hydrosols prepared by citrate and citrate–sulfate procedures

    Colloid J.

    (2008)
  • O. Siiman et al.

    Surface-enhanced Raman scattering by citrate on colloidal silver

    J. Phys. Chem.

    (1983)
  • C.H. Munro et al.

    Characterization of the surface of a citrate-reduced colloid optimized for use as a substrate for surface-enhanced resonance raman scattering

    Langmuir

    (1995)
  • M. Li et al.

    Bimodal sintered silver nanoparticle paste with ultrahigh thermal conductivity and shear strength for high temperature thermal interface material applications

    ACS Appl. Mater. Interfaces

    (2015)
  • A. Henglein et al.

    Formation of colloidal silver nanoparticles: capping action of citrate

    J. Phys. Chem. B

    (1999)
  • Cited by (0)

    View full text