Download Simple two-phase systems involving organophosphorous

SYNTHESIS OF NANOMETRIC SILVER PARTICLES FROM
ORGANIC PHASES FROM THE LIQUID-LIQUID EXTRACTION
ROSA LINA TOVAR TOVAR, MARÍA GUADALUPE SÁNCHEZ LOREDO
Instituto de Metalurgia, Universidad Autónoma de San Luis Potosí, Sierra
Leona 550, Lomas 2ª. Sección, C.P. 78210, San Luis Potosí, México. Correo
electrónico: [email protected]
ABSTRACT
Metallic nanoparticles have acquired great importance in many fields of
science, and a variety of methods have been developed for their synthesis.
Among these methods, precipitation from aqueous or organic media is
commonly used because it is versatile, easy and economic. Ligands are
usually used to passivate the surface and control growth. In order to obtain
high-purity metallic powders, the starting materials must be purified using
separation techniques. The liquid-liquid extraction systems used in
hydrometallurgy provide “inexpensively prepared, non-aqueous media in
which the precipitation can be controlled (Doyle, 1992)”. In this work, the
synthesis of silver nanocrystals is achieved from extractant solutions of the
organophosphorous reagents Cyanex® (Cytec Co.) family. The particles
were prepared by reducing silver species using borohydride ions or ascorbic
acid. Cyanex®471x is particularly useful for nanoparticle preparation, and, as
expected, a decrease in particle size gave blue shifts in the absorption
spectra of the particles. Particle size, agglomeration degree and optical
properties are strongly dependent of the type of reagent employed.
Keywords: Nanoparticles, silver, Cyanex® 471x, reductive stripping
SÍNTESIS DE PARTÍCULAS NANOMÉTRICAS DE PLATA
OBTENIDAS A PARTIR DE FASES ORGÁNICAS DE LA
EXTRACCIÓN LÍQUIDO-LÍQUIDO
RESUMEN
Las nanopartículas metálicas han adquirido gran importancia en diversos
campos, habiéndose desarrollado una variedad de métodos para
sintetizarlas. La precipitación de medios acuosos u orgánicos se usa
comúnmente al ser económica y versátil. Normalmente se utilizan ligandos
orgánicos para pasivar la superficie y controlar el crecimiento. A fin de
obtener polvos metálicos de gran pureza, los materiales de partida deben ser
purificados usando técnicas de separación. Los sistemas de extracción
usados en hidrometalurgia proporcionan a bajo costo medios no acuosos
donde la precipitación es controlada (Doyle, 1992).
En este trabajo se reporta la síntesis de nanocristales de plata a partir de
soluciones de extractantes de la familia de reactivos organofosforados
Cyanex® (Cytec Co.). Las partículas fueron preparadas por reducción de las
especies metálicas usando iones borohidruro o ácido ascórbico.
Particularmente Cyanex® 471x es útil para la preparación de nanopartículas,
y la reducción en el tamaño de partícula dio desplazamientos hacia el azul
en el espectro de absorción. El tamaño de partícula, grado de aglomeración
y las propiedades ópticas son fuertemente dependientes del tipo de reactivo
aplicado.
Palabras clave: nanopartículas, plata, Cyanex®, despojo reductivo
INTRODUCTION
Several approaches are utilised to prepare silver ultrafine particles, but, in all
cases, the precursors need to be highly pure in order to obtain high quality
materials. High purity can be achieved using the separation techniques such
as the solvent extraction process applied in hydrometallurgy. Paiva (2000)
revised several examples of application of phosphorus compounds to silver
extraction from different aqueous media. A few works involve the use of
phosphoric acids or trialkylphosphates, but phosphorus compounds with
sulphur atoms are more utilised. The use of Cyanex® 301 (bis-(2,4,4trimethylpentyl)dithiophosphinic
acid),
Cyanex®
302
(bis-(2,4,4trimethylpentyl)monothiophosphinic acid) and Cyanex® 272 (bis-(2,4,4trimethylpentyl)phosphinic acid) as extractants for silver ion from nitrate
aqueous media was reported (Sole, 1994). Extraction shifted to lower pH with
increasing sulphur substitution in the phosphinic acid (Cyanex 272 > Cyanex
302 > Cyanex 301). Cyanex® 471x is a relatively new extractant for silver
and Pd/Pt separations, available also from Cytec Inc. Its composition is
based on tri-isobutylphosphine sulphide (TIBPS). The extractant has been
investigated either for silver ion extraction from nitric or concentrated chloride
media (Zuo, 1995; Capela, 2002; Paiva 1993; Paiva 1993a; Paiva 2000;
Hubicki, 1995). Asano y col. (1999) extracted silver from chloride media and
recovered the metal as a powder using as reducing agent sulphite ions under
alkaline conditions.
The use of reducing agents has been proposed as an alternative to
conventional stripping in solvent extraction (Demopoulos, 1986). According to
Demopoulos y col. (1988), gold could be recovered directly from the organic
solvent by hydrolytic stripping. The first report of thiol-stabilized gold
nanoparticles of good quality synthesized by the two-phase approach
appeared in 1993, and a year later a simple but very successful method for
preparing larger amounts of gold nanocrystals appeared, involving the phase
transfer of an anionic Au3+ complex from aqueous to organic solution in a
two-phase system, followed by reduction with sodium borohydride in the
presence of a long chain thiol-stabilising ligand (Brust, 1994). Similar
procedures for the preparation of silver particles in two-phase systems were
also reported, but the mechanism of the phase transfer step was not clear,
since silver was present in the aqueous phase as a cation (Brust, 2002).
Stabilizers are capable of preventing particle growth and aggregation during
the chemical synthesis of nanoparticles. The most important factor
discriminating the effectiveness of different ligands is the difference in Lewis
base character, where a firmly bound moiety stabilises growth. Because Ag +
is a soft acid, the stability of the moiety would increase with increasing ligand
softness, so we expected that particularly the sulphur-containing extractants
could behave as good capping agents. In this work, we report the chemical
synthesis and characterisation of silver nanocrystals obtained from organic
phases containing the Cyanex reagents using reductive stripping.
PROCEDURES
The aqueous media used were: nitrates (0.001-0.01 M Ag+ in water, 0.5 M
NaNO3 and 0.1-0.5 M HNO3). The extractants Cyanex® 471x, Cyanex® 272,
Cyanex® 302 and Cyanex® 301 were kindly donated by Cytec Co. 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 (0.005-0.05 mol/L of
Cyanex reagent in p-xylene or kerosene). In order to obtain enough material
for DRX characterisation, some experiments were performed by shaking
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 with graphite furnace GTA-110 and the organic phase metal content
was calculated by mass balance.
The silver-loaded organic phase was transferred into a stirred reaction
vessel, and the precipitates were obtained by adding a 0.01 L of a freshly
prepared stripping solution (0.1 mol/L sodium borohydride). After 30 minutes
stirring, the two phases were transferred to a separatory funnel, allowed to
separate and filtered. The obtained powders were washed with methanol and
acetone and dried. Dispersions were characterised as obtained. X-ray
diffraction patterns (P-XRD) were recorded with a GBC-Difftech MMA
diffractometer. The nickel filtered Cu Ka (l=1.54Å) radiation was used at 34.2
mA y 35 kV. Organic adsorption on the precipitates was checked by thermal
analysis (TA Instruments DSC system, model Q600), using nitrogen or argon
as a purging gas, at a scanning rate of 10°C/min. The morphology and size
were observed by scanning (SEM) and transmission electron microscopy
(TEM). The SEM images were obtained using a XL-30 scanning electron
microscope (Philips, Netherlands). The chemical composition of the obtained
powers was determined by energy dispersive X-ray spectrometry (EDS),
carried out using an X-ray microanalyzer (DX-4I, EDAX) built on the scanning
electron microscope. The TEM images were obtained using a Jeol electron
microscope model 1230 (120 kV). 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. The optical properties were measured in an OceanOptics
S2000
UV/Vis
spectrophotometer
and
an
OceanOptics
NIR
spectrophotometer model 512 using a tungsten lamp as the light source.
Cyanex 471x stability under reducing conditions was investigated by gas
chromatography/mass spectrometry (GC-MS) at the Instituto de
Investigaciones Científicas, Universidad de Guanajuato, using a Varian CP3800 gas chromatograph coupled to a Varian Saturno 2000 mass
spectrometric detector, with autosampler CP-8200., Under the current
experimental conditions the extractant solutions did not show any
degradation.
RESULTS
The silver extraction experiments showed that, as expected, the extent of
extraction was in all cases quantitative (extraction using Cyanex 272 was
carried out at a pH of 7 and from sodium nitrate solutions to ensure high
rates). On the contrary, reduction from the different silver-loaded organic
solutions was not always feasible. As expected the silver complexes with
Cyanex 301 and Cyanex 302 were very stable and reduction with borohydride
ions did not lead to particle formation. But in general precipitation was
possible in good yields from solutions containing the extractants Cyanex 471x
and Cyanex 272. Precipitation stripping resulted in almost all cases in
uncontrolled particle growth, with material precipitating as agglomerates
within seconds after injection of the stripping solution. As a result of attractive
van der Waals forces, and the tendency to minimise the total surface,
nanostructured particles tend to form agglomerates.
Figure 1 presents images of the products obtained by stripping from Cyanex
272 solutions. The diameter and morphology vary with the diluent used.
Under high silver loading conditions (Cyanex 471x 0.005 M in p-xylene, 0.01
M Ag+ in HNO3 0.1 M, extraction rate 35 %), a stable dispersion in the
aqueous phase, and a small amount of precipitate, were obtained (Figure 2).
Analysis of the organic phase after the stripping operation demonstrated the
efficiency of the metal recovery (5 ppm silver content in the organic solution
after stripping). The silver colloids do not precipitated for at least 3 months.
The obtained particles (dispersions and powder) presented spherical
morphology, their size varied between 7.5 and 40 nm for the powders and
2.6-25 nm for the colloids. The Figure 2a show that most of the particles were
twinned, a phenomenon common for fcc structures (Sarkar, 2005). The
particles in the dispersion on the contrary showed less imperfection.
Cyanex 272 in kerosene
Cyanex 272 in p-xylene
Figure 1. Micrographs of silver particles from Cyanex 272 0.01 M in a)
kerosene, and b) p-xylene. Feed phase: Ag+ 0.001 M in 0.5 M NaNO3.
Dispersion
Powder
20 nm
Figure 2. Transmission electron microscopy images of silver a) powders and
b) dispersion. Cyanex 471x 0.005 M in p-xylene, 0.01 M Ag+ in HNO3 0.1 M.
The powder diffractogram (Figure 3) showed that a pure phase was obtained.
The reflections on the XRD pattern could be indexed to a face-centered cubic
sublattice, assigned to metallic silver according to the literature pattern (Joint
Committee on Powder Diffraction Studies, file no. 04-0783).
Intensity
20
30
40
50
60
70
80
90
100 110
2( q )
Figure 3. X-ray diffraction pattern of silver synthesized from Cyanex 471x
solution in p-xylene (silver fcc, JCPDS, file no. 04-0783).
Average grain dimensions of the nanocrystals calculated from diffraction data
using the Scherrer equation (applied to particles under 200 nm) was found to
be 30.5 nm.
It was speculated that the extractant molecules could be adsorbed onto the
particles surface. In case of the aqueous Ag-Cyanex 471x dispersions, the
presence of the extractant means the latter transfers also to the aqueous
phase. This is noticeable because the aqueous solubility of Cyanex 471x is
quite low (Baba, 1988). A deeper investigation was required on this aspect. In
order to clarify this subject, thermal analysis was conducted in order to
examine the organic adsorption on the particles, which is also important
because the presence of even small amounts of organic impurities on the
nanoparticles surface might alter the material properties. Thermal analysis of
the sample Ag-Cyanex 471x (not shown) did not show peaks on the DTA
curves which would result from combustion of the organic material or thermal
transitions. A featureless DTA curve was an expected result because for pure
silver only the face centered cubic crystal structure is possible. The material
presented a small mass loss of approximately 2 % between 50 and 150 °C,
due to desorption of water molecules from the particles surface. Probably,
Cyanex 471x provides some protection against growth, but the stabilizing
molecules are easily removed from the precipitates by washing.
To study the presence of quantum-confined effects, the UV/vis spectrum of
the obtained material was recorded (Figure 4). The main peak in the
absorption spectra for the silver dispersion exhibits a noticeable blue shift
compared to the bulk value. Excitations of conduction electrons in noble
metal particles are known as the particles plasmons, Mie plasmons or
surface plasmons (Sönnichsen, 2002). Silver particles have a strong and
sharp plasmon band in the visible region whose shape and position are
susceptible to surface adsorption, and therefore the particles formation and
adsorption effects can be followed spectrophotochemically (Jana, 1999,
Sönnichsen, 2002). An absorption band at 440 nm corresponds normally to
the plasmon peak associated with relatively large spherical silver particles.
Absorbance, a.u.
0.65
0.60
0.55
391.7 nm
0.50
0.45
0.40
0.35
0.30
300
350
400
450
500
550
Wavelength, nm
Figure 4. UV/vis spectrum of silver colloid. Cyanex 471x 0.005 M in p-xylene,
0.01 M Ag+ in HNO3 0.1 M.
In a previous work, we compared the optical properties of silver particles
obtained in absence of any stabilizer, related to organically-capped Ag
nanoparticles prepared for us from aqueous media (Martínez-Castañón,
2003). The absorption spectra of the samples showed a band centered at
around 440 nm (Ag without stabilizer), 400 nm (Ag-thioglycerol) and 410 nm
(Ag-mercaptoacetic acid), all of which can be identified as the surface
plasmon resonance band of metallic Ag particles. The maximum at around
392 nm was assigned to the surface plasmon resonance of our colloids. The
obvious blue shift of the absorption peak can be attributed to the small
dimension of the Ag relative to the bulk.
CONCLUSIONS
Silver nanoparticles can be easily prepared by a simple method involving
solvent extraction using organophosphorous extractants and precipitation.
Investigation into the extractants revealed that Cyanex 471x and Cyanex 272
are effective capping agents. Under the same reaction conditions, Cyanex
471x is the most effective reagent in stabilizing Ag particles. As expected,
reduction in particle size gives a noticeable blue shift in the absorption
spectrum.
Although further studies are required for a better understanding of the
correlation between the mechanism of nanoparticles stabilization and the
stripping process, solvent extraction combined with reductive or precipitation
stripping is an attractive alternative for materials synthesis, because this
purification technique is widely used in hydrometallurgical processes.
ACKNOWLEDGEMENTS
Cytec Canada is thanked for providing the extractants used in this work. This
work was supported by a grant from the Fondo de Apoyo a la Investigación
(UASLP) and a grant from the Fondos Mixtos CONACyT-Gobierno de San
Luis Potosí (Project FMSLP 2002-5630). We also thank M.Sc. Imelda
Esparza, Dr. Facundo Ruiz and Dr. Gabriel Martínez Castañón (UASLP), for
TEM characterization, and Dr. R. Navarro Mendoza from the University of
Guanajuato for the GC-MS studies.
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