A Simple Two-Phase System Involving the Commercial Organophosphorous Extractant Cyanex^® 471x for the Preparation of Silver Sulphide Nanoparticles

Rosa Lina Tovar-Tovar, Octavio Domínguez-Espinós, Adriana Gaona-Couto, Azdrubal Lobo-Guerrero, Salvador Palomares-Sánchez, María G. Sánchez-Loredo

Facultad de Ciencias, San Luis Potosí, México.
Instituto de Metalurgia, San Luis Potosí, México.

DOI: 10.4236/msa.2012.312123                

Abstract

The facile chemical synthesis of silver sulphide nanocrystals from metal-loaded organic media, containing a silver-selective organophosphorous ligand as extractant, is reported. The method involves the phase-transfer of silver species from aqueous nitrate media to organic solution using the commercial extractant Cyanex^? 471x (tri-isobutylphosphine sulphide, Cytec Co.) as extractant, followed by precipitation stripping using ammonium sulphide as strip reagent. The nanoparticles were structurally characterized, and some aspects of the synthetical process, are briefly discussed. Under the conditions studied, the extractant Cyanex^? 471x was able to act as stabilizer adsorbing on the particles surface, maintaining the size of the particles nanometrical.

Keywords

Silver Sulphide; Solvent Extraction; Precipitation Stripping; Cyanex^® 471x; Tri-Isobutylphosphine Sulphide

1. Introduction

Several approaches are utilised for the colloidochemical preparation of nanoparticles, but, in all cases, the metal precursors need to be highly pure in order to obtain high quality materials. High purity metal-containing solutions can be obtained using separation techniques such as the two-phase system applied in hydrometallurgy, also known as the solvent extraction process. An interesting modification of the conventional extraction is the addition of a crystallization operation (crystallization or precipitation stripping), where low-solubility salts are formed. Yttrium oxalate powders from metal-loaded D2EHPA (di-2-ethylhexylphosphoric acid) were synthesised by Combes et al. [1]. By heat-treatment in air the oxalates become the corresponding oxides [2]. Preparation of cobalt ferrite from Versatic Acid solutions was reported [3]. Fu et al. reported the preparation of organic fluids containing Ag, Ag₂S, Bi₂S₃, CdS and ZrO₂ from organophosphorous extractants solutions [4-6].

The first report of thiol-stabilized gold nanoparticles prepared by metal extraction, the two-phase approach, appeared in 1993, and a year later a simple but very successful method for preparing larger amounts of nanocrystals appeared, involving the phase transfer of an anionic Au³⁺ complex from aqueous to organic solution using a reagent-grade anion exchanger, followed by reduction with sodium borohydride using a long chain thiol as stabilizer [7]. Similar preparation of silver was also reported [8]. Well-known solvating organophosphorous extractants, such as trioctyl phosphine and trioctyl phosphine oxide, were used for preventing particle growth and aggregation in the one-pot chemical synthesis of quantum dots [9]. CdS nanowires and Bi₂S₃ nanorods were prepared by extraction of Cd²⁺ or Bi³⁺ ions using Cyanex^® 301, followed by solvothermal treatment [10]. Therefore we hoped other extractants from the Cyanex^® family (Cytec Co.), and particularly Cyanex^® 471x (tri-isobutylphosphine sulphide, TIBPS), could behave as phase transfer reagents, and as effective capping ligands during the synthesis of semiconductor nanoparticles by the facile two-phase approach. The effectiveness of different ligands in nanoparticles stabilisation is influenced by the difference in Lewis base character, where a firmly bound moiety stabilises growth. As Ag⁺ is a soft acid, the stability of the moiety would increase with increasing ligand softness, and so was expected that the sulphur containing TIBPS could act as good capping agent during the synthesis of Ag₂S nanocrystals. Silver sulphide, an intrinsic n-type semiconductor with a band gap of 1.03 eV and relatively high absorption coefficient, 104 M^–1·cm^–1 [15- 17], has attracted attention owing to its potential as optoelectronic and thermoelectric material, for uses in devices such as photovoltaic cells, photoconductors, IR detectors, superionic conductors and photosensitizer for photographic purposes [11-14]. Radiation of wavelengths under 1210 nm (e.g. ambient light) may induce photoconductivity in Ag₂S, accelerating the rate of electron transfer process. Ag₂S nanocrystals were synthesized by thermal decomposition, reverse micelles, precipitation from thiolate glutathione, and L-Cysteine complexes, or using the already mentioned two-phase system [5,14,18-21].

Depending on the leaching media, different reagents are suitable for separating silver ions from aqueous solutions. Paiva et al. revised examples of application of trialkyl-, triarylthiophosphates and triphenylphosphine to silver extraction [22-24]. TIBPS is a well-known extractant for silver and Pd/Pt separation, extracting Ag⁺ from nitrate solutions via a “solvating” mechanism by coordination through a sulphur atom, with simultaneous transfer of a nitrate to form an ion-pair [22-28]:

(1)

(2)

where (org) represents the organic phase.

We expected the reactions involved during precipitation stripping could involve the reversal of (1) or (2) promoted by the formation of silver ammonium complexes (3), which are not well extracted by TIBPS [25]:

(3)

Finally, the low solubility product appears:

(4)

In this work, the chemical synthesis and the characterisation of the Ag₂S nanocrystals obtained from organic phases containing Cyanex^® 471x and silver ions using a very simple method involving solvent extraction and precipitation stripping, are reported.

2. Procedures

The extractant Cyanex^® 471x was kindly donated by Cytec Co. and used as received. The silver nitrate solutions used were: 0.005 and 0.01 M Ag⁺ in 0.1 M HNO₃. The silver phase-transfer was carried out at room temperature (about 20˚C) in separatory funnels shaken mechanically at a fixed frequency of 90 min^–1. Unless otherwise stated, the metal loading was performed by shaking 0.01 L of Ag⁺ aqueous solution with 0.01 L of the organic phase (0.005, 0.01 and 0.05 mol/L of Cyanex^® 471x in p-xylene). In order to obtain enough material for XRD (powder X-ray diffraction) characterisation, some experiments were performed by mixing 0.5 L of silver aqueous solution with 0.5 L of the selected organic phase. After 30 minutes shaking, the two phases were allowed to separate. The metal concentration of the aqueous phase before and after extraction was determined using an Atomic Absorption Spectrometer Varian model Spectra AA 220 (AAS) with graphite furnace GTA-110 and the organic phase metal content was calculated by mass balance.

At the beginning of this work, the silver-loaded organic phases were transferred into separatory funnels, and the precipitates were obtained by adding a 0.01 L (or 0.5 L in case of materials prepared in order to perform the XDR characterisation) of a freshly prepared stripping solution (0.005, 0.01, 0.05, 0.1 mol/L ammonium sulphide). After 30 minutes shaking, both phases were allowed to separate and filtered. The obtained powders were washed with methanol and acetone and dried. Depending on the experimental conditions, some of the particles prepared do not precipitated and remained dispersed, in the organic or aqueous solutions. They were separated by centrifugation, washed and dried. Residual metal concentration in the organic phases was determined after digestion using AAS. After improving of the preparation method, the precipitation stripping was carried out in a stirred reaction vessel, both phases were stirred for 30 minutes, and after that the two phases were transferred to a separatory funnel for phase separation and particles recovery.

The X-ray diffraction pattern was obtained in a Rigaku 2200 diffractometer equipped with a nickel monochromator using Cu Kα (λ = 1.54 Å) radiation. The Rietveld analysis was performed using the program MAUD (Materials Analysis Using Diffraction), software developed to analyze diffraction spectra and obtain crystal structures, quantity and microstructure of phases along with the texture and residual stresses. It applies the RITA/ RISTA method as developed by Wenk et al. and Ferrari and Lutterotti [29,30]. As initial model for the refinement, the structure of silver sulfide (monoclinic, space group: P21/n) was used [31]. The SEM images were obtained using a XL-30 scanning electron microscope (Philips, Netherlands). The size and morphology of selected nanocrystals were examined using a JEOL JEM 2000 FXII operating at 100 kV, and a high resolution transmission electron microscopy (HRTEM Tecnai, 300 kV), the crystallinity was confirmed by FFT simulation. The samples were prepared by dispersing of the powders in acetone using an ultrasound bath, and putting a drop of the dispersion over the carbon-supported copper grid and letting it dry at room temperature. Dynamic Light Scattering analysis (DLS) was performed in a Malvern Zetasizer Nano S. Organic adsorption on the precipitates was checked by thermal analysis (Perkin Elmer TGA), using nitrogen or argon as a purging gas, at a scanning rate of 10˚C/min. FTIR measurements were carried out using a Perkin-Elmer FT-IR coupled to a diffuse reflectance device. Since the sample was diluted, to minimize interfereences from impurities in the KBr matrix, the spectra were rationed to the background spectrum taken for a blank KBr pellet. In order to find out if the extractant was adsorbed on the surface of the materials prepared, the spectrum of one of the samples was obtained using pure Ag₂S synthesized with this purpose.

3. Results

During the experiments the extraction conditions were chosen such as they ensure the extent of silver extraction by the commercial organophosphorus reagent was in all cases quantitative. Under the stripping conditions chosen, the quantitative and efficient precipitation of Ag₂S from organic solutions could be observed. MEB images (not shown) showed the products to be nanometrical in size, but the agglomeration degree was in general high, as confirmed in experiments varying the sulphide concentration (0.1, 0.05, 0.01 to 0.005 M [], metal and extractant concentrations were kept constant at 0.005 mol/ L), where the particles average size obtained from dynamic light scattering measurements varied from 160 nm to 301 nm (Figure 1).

In order to obtain information about the crystal structure of the powders, for the following experiment the extraction conditions were 0.5 L aqueous phase (0.01 M Ag⁺ in HNO₃ 0.1 mol/L), and 0.5 L organic phase (0.05 mol/L of Cyanex^® 471x). Ammonium sulphide concentration during stripping was 0.1 mol/L and the volume

Figure 1. Average size of the Ag₂S powders as a function of sulphide ions concentration obtained from dynamic light scattering measurements.

was also 0.5 L. The reflections on the XRD patterns of the product could be indexed to the monoclinic acanthite (a-Ag₂S) crystal structure (Figure 2, JCPDS Card File 14-0072), which is the stable form of silver sulphide at room temperature in the bulk phase, also called the bAg₂S phase [14].

Figure 2(a) represents the experimental (original data) and calculated diffractograms (computed data from Rietveld analysis), with good agreement between them indicating that the used initial structural model was correct. Figures 2(b) and (c) show that the materials are ellipsoidal in shape with some other irregular particles. The FFT simulated pattern (Figure 2(c)) contains spots, confirming the high crystalline quality. Table 1 shows the refined atomic positions from the computed data and Table 2 the unit cell parameters along with the X-ray

Conflicts of Interest

The authors declare no conflicts of interest.

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