New one-pot synthesis of Au and Ag nanoparticles using green rust reactive particle as a micro-reactor
© Ayadi et al.; licensee Springer. 2013
Received: 18 January 2013
Accepted: 9 February 2013
Published: 22 February 2013
A new, simple, and fast one-pot synthesis of supported Au or Ag nanoparticles is implemented, for which a reactive Fe(II)-bearing green rust inorganic particle is used as an individual micro-reactor acting as both the reducing agent and support for the resulting metal nanoparticles. The mechanism involves both the solid-state oxidation of the green rust support (sulfate or carbonate) and the reduction-precipitation of soluble metal precursor. The resulting nanohybrids have a platy inorganic part supporting about one to ten nanoparticles with sizes in the 20 to 120 nm range.
KeywordsGold Green rust Micro-reactor Nanoparticles Silver
Nanoparticles of noble metals exhibit unique optical, chemical, catalytic, and electronic properties which make them attractive for a wide range of applications in many domains. The most common way for preparing such nanoparticles, named as ‘wet chemistry’, consists in reducing a soluble metal precursor (AuIII or AgI) by a soluble reducing agent in the presence of a stabilizing species which keeps the formed nanoparticles from aggregation. Turkevich-Fens’s method uses AuCl4− ions and sodium citrate as both reducer and stabilizing agent and gives approximately 20-nm spherical nanoparticles [1, 2]. Numerous other stabilizing agents have been further used. In Brust’s synthesis, a two-phase aqueous-organic solution with tetraoctylammonium bromide transfer species and a strong stabilizing thiol agent are implemented and the reaction of AuCl4− and NaBH4 in these conditions allows the preparation of stable 1- to 5-nm Au clusters . Regarding silver nanoparticles, the most common synthesis is the reduction of silver cation/complex by chemical agents such as borohydride or hydrazine [4, 5]. From the so-called polyol process displaying ethylene glycol as both reductant and solvent, various nanoparticles including Au and Ag could be obtained [6, 7]. As hazardous products occur and may generate biocompatibility or environment problems, a recent development of ‘green synthesis’ was stimulated, for which environmentally friendly reducing agents are used, including saccharides or natural extracts . Suspensions of supported metal nanoparticles on inorganic solids can be formed by wetness impregnation or alkaline (co-) precipitation [9, 10]. These routes give low metal loads (wt.%) and require a final gas reduction treatment by H2 or CO, with some possible efficiency problems for the complete conversion to metal.
Fe2+ ion is a ‘green’ reducing species present in the crystalline structure of various solids including sulfides, carbonates, hydroxisalts, and clays. As the oxidation of structural FeII ions usually occurs in a very cathodic potential domain, the transfer of electrons to numerous oxidants is therefore possible. The reduction of AgI and AuIII soluble species by iron sulfide minerals has already been reported [11, 12]. However, the mineral samples available for laboratory experiments usually display very large dimensions, which preclude any potential applications.
Green rusts (GR) are layered FeII-FeIII hydroxisalts composed of positively charged Fe(OH)6 octahedra sheets alternating with interlayers filled with charge-compensating anions and water molecules . Early studies on the reduction of AgI or AuIII by green rusts were reported in 2003, from Heasmann et al. and O’Loughlin et al. [14, 15]. The presence of Au or Ag metal was evidenced by X-ray absorption spectroscopy and transmission electron microscopy. Later, these green rusts doped with very low metal loads were utilized as reducing compounds for the removal of some chlorinated hydrocarbons [16, 17]. In these studies, the reaction mechanisms between green rust and soluble metal precursor were not detailed and none of the studies gave an evidence of metallic particles by X-ray diffraction (XRD). The proposed mechanism involves the oxidation of sulfate green rust into magnetite Fe3O4, coupled to the reduction of AuIII or AgI to Au or Ag.
The oxidation mechanisms of green rusts have been extensively studied. This reaction can imply transformations via solution, i.e., dissolution, oxidation, and precipitation of the resulting ferric oxy-hydroxides, lepidocrocite, and goethite [18, 19]. Otherwise, a solid-state oxidation of green rusts involving both the conversion of FeII to FeIII inside the crystal lattice and the charge-compensating loss of protons is also possible [19–22]. The latter mechanism especially occurs when high oxidation rate is imposed, for example, by reaction with some soluble oxidizers such as H2O2. The resulting ferric products, named as ‘exGR-Fe(III)’ or as ‘ferric green rust’, keep the same apparent morphology as the initial green rusts; only local disorders at nanometric scale are induced, as indicated by the disappearance or the large decrease of (00l) lines in diffraction patterns [19, 21, 22].
In the present paper, we introduce a new one-pot synthesis of supported noble metal nanoparticles in which the green rust particle is an individual micro-reactor acting as both the reducing agent and the support for the resulting metal nanoparticles. Carbonate (GRc) or sulfate (GRs) green rust suspensions were obtained from the oxidation by air of slightly alkaline solutions containing ferrous species and carbonate or sulfate anions and the reactions with AuIII or AgI were operated shortly after, in the same solution . Our purpose is the production of Au or Ag nanoparticles by this new method and we therefore target high metal loads. This simple synthesis is carried out at near ambient temperature, in aqueous solution, and requires only common salts; it is environment friendly since no organic solvents/additives are used and the filtrates do not represent a problem for recycling.
Sodium bicarbonate NaHCO3, sodium sulfate Na2SO4,10H2O, iron chloride FeCl2,4H20 (>99%), iron sulfate FeSO4,10H2O (>99%), potassium tetrachloroaurate KAuCl4 (98%), silver nitrate AgNO3 (>99%), ammoniac NH3(aq) (33%), sodium hydroxide NaOH (10 M solution) purchased from Sigma Aldrich (Steinheim, Germany). The solutions were prepared with 18 MΩ cm nanopure water.
For the synthesis of carbonate (or sulfate) green rusts, 50 ml of 0.4 M NaHCO3 (or 0.4 M Na2SO4) solution is put into a cylindrical glass cell thermostated at 25°C and stirred at 300 rpm under argon for 15 min. Then, 0.5 ml of 1 M FeCl2 solution or 1 M FeSO4 solution is introduced and 10 M NaOH solution is added dropwise to fix the initial pH at a value of 9.5. Finally, argon bubbling is stopped and the cell is opened to air. After about 25 min, a green rust suspension containing 333 μmol FeII is obtained. The AuIII solution contains 0.05 M KAuCl4; the AgI solution contains 0.1 M Ag(NH3)2+ and 0.3 M NH3. The reactions with green rust suspensions are conducted by adding an appropriate quantity of AuIII or AgI, expressed as a stoichiometric ratio R; R = 100% corresponds to 111 μmol AuIII or to 333 μmol AgI. The solution reactions are monitored by recording redox potential with a WTW multimeter, using a platinum working electrode (Radiometer Tacussel, La Fontaine du Vaisseau, Neuilly Plaisance, France) and a homemade AgCl/Ag-0.1 M NaCl reference electrode (0.23 V with respect to standard hydrogen electrode).
The resulting metal-inorganic nanohybrids were characterized by Fourier transform infrared spectroscopy (FTIR) spectrometry, X-ray diffraction, and scanning electron microscopy. After interactions of about 20 to 30 min, solid samples were separated by filtration, carefully rinsed with deionised water, and dried at ambient temperature for at least 24 h. They were weighted and then characterized. FTIR data were recorded on a Bruker IFS 28 spectrometer (Bruker optics, Wissembourg, France). Powder samples were pressed to pellets with KBr and analyzed by direct transmission mode. XRD measurements were carried out using a Bruker D8 diffractometer with CuKα radiation (1.5406 Å). Scanning electron microscope (SEM) examinations were performed by a LEO 1530 (Carl Zeiss AG, Oberkochen, Germany) microscope using in-lens and backscattered electron modes.
Results and discussion
In Figure 4b, Ag nanoparticles appear as polyhedrons with an apparent preferential location at the edge of exGRc-Fe(III) particles. A similar analysis as before was performed. The surface density of particles, NAg is 26 μm−2. From the size distribution in the insert and assuming a spherical shape of Ag nanoparticles, we obtained VAg = 4.2 × 10−15 cm3, a value approximately three times higher than for Au, consistent with the molar volume values, 10.3 and 10.2 cm3 mol−1 for Ag and Au, respectively. The corresponding δ value (44 nm) is very close to the one found above.
The whole previous results show that a green rust particle can be used as a micro-reactor for the synthesis of metal particles. The electrons consumed for the reduction of the soluble precursor to metal come from the oxidation of structural Fe2+ to structural Fe3+, which causes the progressive transformation of green rust to exGR-Fe(III) with no morphology change. The quantity of deposited metal is governed by the size of the GR particle. Actually, about one to ten metal nanoparticles on each inorganic particle are commonly observed.
With only one final separation step and the use of non-hazardous reagents, the synthesis of our metal/exGR-Fe(III) nanohybrids is very attractive. Due to their flat shape, the nanohybrids can be easily separated from a solution by filtration, either after their synthesis or after their operation as colloidal reagents. Moreover, their manipulation is very easy and relatively safe since mineral types such as iron compounds are generally fully biocompatible and metal nanoparticles are well attached to the inorganic matrices. The surface of inorganic and/or metal parts can be functionalized to target specific properties. The nanohybrids can be compacted to build permeable reactive membranes for remediation or disinfection treatments and heterogeneous catalysis. The formation of thin films by cast deposition, for example, may also be considered for the fabrication of modified (bio-) electrodes dedicated to analytical applications. If necessary, the inorganic part could even be partially or entirely removed by acidic or reducing treatments. This facile removal is attractive when the device requires metal nanoparticles only.
The paper reports a new, simple, and fast (40 min) one-pot synthesis of supported Au and Ag nanoparticles in which a reactive Fe(II)-bearing green rust inorganic particle is used as an individual micro-reactor acting as both the reducing agent and the support for the resulting metal nanoparticles. The reaction of carbonate or sulfate green rusts with AuCl4− or Ag(NH3)2+ involves the solid-state oxidation of green rust, and the reduction/precipitation onto the inorganic surface of Au or Ag metal. The resulting nanohybrids display a platy shape inorganic part, similar to the green rust precursor, supporting about one to ten metal nanoparticles which appear as flattened hemispheres (Au) or as polyhedrons (Ag). The size ranges are 10 to 60 nm for sulfate green rust and 20 to 120 nm for carbonate green rust. The field of potential applications of these nanohybrids is very large and deserves a careful exploration in the future.
We thank the University of Evry Val d’Essonne and the French Ministry of Research and Higher Education for the financial support of SA.
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