WO2006038045A1

WO2006038045A1 - Size-selective synthesis of metal nanoparticles

Info

Publication number: WO2006038045A1
Application number: PCT/GR2004/000049
Authority: WO
Inventors: Anastasia Hiskia; Elias Papaconstantinou; Aristides Troupis - Koukoutsis
Original assignee: National Center Of Scientific Research 'demokritos'
Priority date: 2004-10-05
Filing date: 2004-10-05
Publication date: 2006-04-13

Abstract

The present invention relates to the size-selective preparation of metal nanoparticles in polyoxometalates (POM) solutions. In the process, reduced POM can act as catalytic electron donors and possibly also as stabilizers that restrict the dimensions of the particles to nanoscale. By choosing the appropriate kind of reduced POM, and/or the appropriate concentrations of the reduced POM and/or and/or the appropriate concentrations of metal ions, the size of the formed particles is controlled.

Description

SIZE-SELECTIVE SYNTHESIS OF METAL NANOPARTICLES

Technical Field

The present invention deals with the size-selective synthesis (preparation) of uniform metal particles and more specifically with the control of the reduction of metal ions in the presence of reduced polyoxometalates (POM), which act as electron donors and possibly also as stabilizers, in order to adjust the size of the obtained particles.

Background of the Invention Solutions of metal nanoparticles are of intriguing interest due to their photocatalytic, photovoltaic and optoelectronic properties, their catalytic role in various chemical reactions and even their antiseptic ability. Since these properties are a function of the size of the dispersed metal particles, the establishment of methods with which one could control and select the size of the produced particles is of paramount interest in order to maximize the efficiency of the particles.

Traditionally, metal nanoparticles have been obtained by thermal, photochemical, radiolytic, electrochemical or sonochemical reductive methods, which methods have certain disadvantages. More particularly, almost all these methods are reagents-consuming since two kind of substances are needed: the electron donor in stoichiometric (large) amounts and the stabilizer that stops the growth of the particles to nanometer scale.

Recently, POM have been proposed as both catalytic electron donors and stabilizers for the synthesis of metal nanoparticles in aqueous solutions using either UV/near-Vis light [A. Troupis, A. Hiskia, E. Papaconstantinou, Angew. Chem. Int. Ed. 2001, 41, 1911] or γ-rays [A V. Gordeev, N.I. Kartashev, B. G. Ershov, High Energy Chem. 2002, 36, 102]. The use of POM provides reagents-economy since the addition of a second chemical (stabilizer) is avoided, and besides, only traces of POM are needed (catalytic amounts). However, despite these practical benefits of th-is use, up to now POM have never been utilized for controlling the metal particle size.

The size of the metal nanoparticles, as is known in nanochemistry, is crucial since it dictates their physical and chemical behavior, such as the ability to accept or release electrons, light absorption, photoredox, photocatalytic, photovoltaic or even bactericidical properties among others. By presenting methods that enable to control the size of these nanoparticles, one could also control the corresponding properties and applications of these particles. It is the object of this invention to provide a process that is able to control the size of the obtained metal nanoparticles at will, using POM as electron donors and as a result define certain properties of the nanoparticles. More particularly, the present invention aims at providing a process with particular advantages, namely a rapid and simple process (simple mixing at room temperature, reagents-economy) by which metal nanoparticles of well- controllable size can be made

Summary of the Invention

The invention provides a process for the manufacture of metal nanoparticles in a solution in which a POM (polyoxometalate) is used as catalytic reducing reagent, to induce reduction of a metal cation to form the metal nanoparticles, which process is characterized in that the size of the metal nanoparticles formed is controlled by the selection of POM of appropriate redox potentials, and /or the selection of appropriate concentration of reduced POM, and/or the selection of appropriate concentration of metal ion. Advantageously, in the process described above, POM can also serve as stabilizer.

Conveniently, the solution within which the metal nanoparticles are manufactured may be liquid, or ionic liquid.

Advantageously, the liquid solution within which the metal nanoparticles are manufactured is aqueous. Conveniently, the metal nanoparticles manufactured are silver nanoparticles.

Advantageously, the silver is provided by silver nitrate, silver perchlorate or other soluble silver salt.

Preferably, the metal nanoparticles manufactured are gold, palladium, copper or platinum nanoparticles. Conveniently, for the manufacture of the metal nanoparticles the two reactants,

POM and metal ions, are premixed in a POM/organic/metal ion solution (one-pot process).

Adbwantageously, the preparation of the catalytic reducing agent (reduced POM) is produced by treatment of a solution containing oxidized POM (or less reduced POM) and oxidizable species by means of exposure to electric current, γ-rays, ultrasound, temperature, UV/near- Visible light, or sunlight.

Preferably, the preparation of the catalytic reducing agent (reduced POM) is achieved with the use of oxidizable reagents.

Conveniently, the oxidizable reagent is an alcohol, phenol, hydride, chorophenol, cresol, benzene derivative, organohalogenated compounds aliphatic, or a pesticide Advantageously, the process for the formation of silver nanoparticles in a two-step process is as follows:

(step 1) An aqueous solution of the electron donor-stabilizer is prepared (reduced POM) by illuminating with UV/near-Vis light (according to an embodiment of the invention) a deaerated aqueous solution of oxidized POM / propan-2-ol. Upon illumination, electron transfer from propan-2-ol to POM resulted in the formation of reduced POM, (POM(e-)),

■ according to the reaction: POM + S → POM (e-) + S_0x (1)

(step 2) An aqueous silver salt solution such as silver nitrate, silver perchlorate or other water soluble silver salt solutions is prepared, is deaerated and is added to the solution from step 1 or vice versa to produce silver nanoparticles, according to the reaction:

POM (e-) + Ag⁺ → POM + Ag°_co,,_oida, (2) The whole process is schematically illustrated in Figure 3.

The mechanism for the formation of silver nanoparticles, reaction (step) (2), comprises firstly a slow nuclealion step and, then, a fast reductive growth on the already formed nuclei.

Conveniently, these two stages and, subsequently, the size of the formed metal particles can be affected via two ways: (i) By changing the rate of metal reduction one can adjust the nucleation process. In general, faster reduction leads to greater number of seeds and, for a fixed initial concentration of silver, the same quantity of silver has to be distributed on greater number of seeds and smaller particles are expected (the "rate rule").

(ii) Alternatively, the initial amount of silver can affect the extent of the growth process. By increasing silver concentration, for roughly the same number of nuclei, more amount of silver has to be deposited on the same number of seeds and larger particles are expected.

Preferably, the size-selectivity of the process is achieved by varying operational parameters such as the initial concentration of reduced POM, the kind of POM or the extent of reduction of the same POM (which affects the rate of the reaction) or the initial concentration of silver ions (which affects the amount of silver to be deposited).

Possible POM are all reducible POM, i.e the Keggin, Dawson, Pryessler, Lindqvist or Anderson structured polyoxometalate anions, or the decatungstate POM. Preferably, the Keggin and Dawson POM are used.

In particular, for different POMs, those ones with more negative redox potentials, such as H₂Wi₂O₄₀ ^7" compared to SiWi₂O₄₀ ^5", react faster with silver ions. For the same POM, the one that is more extensively reduced exhibits more negative redox potential and reacts faster with silver ions (P₂Wi₈O₆₂ ^8" compared to P₂Wi₈O₆₂ ^7'; P₂MOi₈O₆₂ ^10" compared to P₂MOi₈O₆₂ ^8"). Moreover, increase of the concentration of reduced POM leads to increase of the-reaction with silver ions. In all the above embodiments, faster reactions result in smaller nanoparticles. On the contrary, increase of silver ion concentration did not alter drastically the reaction rate, that is the number of nuclei and, since more amount of silver is to be deposited on the same number of nuclei, led to an increase of the nanoparticles size.

Advantageously, for the purpose of making silver nanoparticles of well-controlable size, a variety of POMs selected from the two classical types of Keggin and Dawson structrure (Fig. 1) are used. In particular, the following photochemically produced reduced POM were used to reduce Ag⁺ to Ag⁰ nanoparticles: the 1-e-reduced Keggin POM SiWi₂O₄₀ ^4" and H₂W_nO₄₀ ^6" (SiWj₂O₄₀ ^5" and H₂Wj₂O₄₀ ^7"), the 1-e- and 2-e-reduced Dawson phosphotungstate P₂Wi₈O₆₂ ^6" (P₂Wi₈O₆₂ ^7" and P₂Wi₈O₆₂ ^8") and the 2-e- and 4-e-reduced Dawson phosphomolybdate P₂MOi₈O₆₂ ^6" (P₂MOI₈OO₂ ^8" and P₂MOi₈O₆₂ ^10"). These reagents exhibit widely-ranged redox potentials dependent on either the nature of POM or the extent of reduction in the same POM (Fig. 2).

Description of Figures

The invention in its different embodiments is illustrated from the following figures:

Figure 1 shows the structures of the characteristic POM used herein. (A) XM₁₂O₄₀" . Keggin structure. They are composed of MO₆ octahedra sharing corners and edges. The heteroatom X=P, Si or H2 is within the central (shaded) tetrahedron XO₄. (B) P₂M₁₈O₆₂ -

Wells-Dawson structure, comes from the Keggin ion by removing three MO₆ octahedra and joining the two 9-metalo half units (M = Mo or W).

Figure 2 shows the various redox potentials exhibited by polyoxometalate anions and silver ions (Volts vs. NHE).

Figure 3 shows a schematic diagram of synthesis and stabilization of Ag nanoparticles in the presence of polyoxometalates.

Figure 4 shows the diagram of the variation of the initial rate of SiWnO₄₀ ^5" reoxidation with the initial concentration Of SiWi₂O₄₀ ^5'. Figure 5 shows the particle size distribution of the silver particles prepared in solutions of various initial concentrations Of SiWi₂O₄₀ ^5".

Figure 6 shows the UV absorbance of the silver particles prepared in solutions of various initial concentrations of SiW]₂O₄₀ ^5". Figure 7 shows the diagram of the variation of the UV absorbance peak of the silver particles prepared in solutions of various initial concentrations Of SiWi₂O₄O^5".

Figure 8 shows the diagrams of the decrease of reduced POM concentration with reaction time upon addition of silver ions, for various kinds of reduced POM. (a) H₂WnO₄₀ ^7", (b) SiWi₂O₄₀ ^5", (c) P₂WIgO₆₂ ^7". The inset shows the variation with time of the UV absorbance spectra OfP₂Wi₈Oo₂ ^7" upon addition of silver ions. - Figure 9 shows the influence of the kind of reduced POM on the size distribution and the

TEM images of the obtained silver particles.

Figure 10 shows the UV absorbance of the silver particles obtained in the presence of the same POM but with different extent of reduction.

Figure 11 shows the diagram of the variation of the initial rate of SiWi₂O₄O^5" reoxidation with the initial concentration of Ag⁺.

Figure 12 shows the particle size distribution and the TEM pictures of the silver particles prepared in solutions of various initial concentrations of Ag¹. Figure 13 shows the UV absorbance of the silver particles prepared in solutions of various initial concentrations of Ag' .

Figure 14 shows the diagram of the variation of the UV absorbance peak of the silver particles prepared in solutions of various initial concentrations of Ag⁺.

Description of the Invention

The preparation of the electron donor solution (reduced POM) can be achieved with various ways for different embodiments of the invention; such ways are treatment of a solution containing oxidized POM (or reduced POM) and oxidizable species by means of exposure to electric current, γ-rays, ultrasound, temperature or UV/near- Visible (Vis) light. In one of the examples shown in the present, the photolytic method was used. In all cases electrons were transferred to POM and the reduced POM of characteristic blue color was formed. Alternatively, instead of a mild organic reductant combined with the energy input, other reductants, usually strong, such as hydrides can be used (chemical energy).

The reaction media can be a solvent, water or organics, where POM are dissolved (i.e, polar organics such as acetonitrile, acetone, protic solvents such as alcohols e t c.), films where POM are immobilized through i.e. a sol-gel or layer-by-layer technique or ionic liquids, where reduction of POM can also take place. In the examples illustrated in the present water was the reaction media. In one embodiment of the present invention, the metal that was used as target metal was silver. However, many other metals can also be efficient as target metals, such as gold, palladium, platinum and copper, that are known to be converted to nanoparticles by reduced POM [A. Troupis, A. Hiskia, E. Papaconstantinou, Angew. Chem. Int. Ed. 2001, -//, 191 1] In different embodiments of the present invention, POM from two representative series were used, of the type XMi₂O₄O^11" or X₂MI₈OG₂"^", as an example. However, many other - reducible types from the enormous class of POM compounds, such as the lacunar/ types or other structured POM, can be used.

The use of propan-2-ol for the production of reduced POM in the photolytic method is the example presented as an embodiment of this invention. However, since almost any oxidizable reagent, i.e., organic pollutants such as phenols, chorophenols, cresols, benzene derivatives, organohalogenated compounds, aliphatics, triazine pesticides e t c [E Papaconstantinou, A. Hiskia, in Polyoxometalate Molecular Science, Borras-Almenar, JJ. ; Coronado, E.; Mϋller A.; Pope M., Eds.; Kluwer Academic Publishers: The Netherlands, 2003, p 381] can deliver electrons to POM in the presence of UV/near-Visible light, any of these could also be used.

Although the light region presented in the embodiment of the invention as presented in the examples was in the range of greater than 320 nm, the process was also effective at even larger wavelengths, i.e. using sunlight. In one embodiment of the invention, to produce silver nanoparticles the two reactants,

(POM and Ag⁺), can be premixed in a POM/propan-2-ol/Ag^f solution. In this case, the reduced POM is produced in situ in the photolyzed solution and the already present Ag¹ is directly converted to Ag nanoparticles in a one-pot process. In another embodiment of the invention, the two reactants above (reduced POM and Ag⁺) are separated in a two- step (two-pot) process, which may be realized as follows: Typical procedure

Step 1. In order to produce reduced POM 4 ml of an aqueous solution of POM H₄SiWi₂O₄O (m.w. 3096, Aldrich) or K₄SiWi₂O₄O (m.w. 3137, specially synthesized) Na_OH₂Wi₂O₄₀ (m.w. 3370, specially synthesized) KeP₂Wi_SO₆₂ (m.w. 4705, home-made) or (NH₄)₆P2Mo I₈O₆₂ (m.w. 3175, specially synthesized) usually at ca 10^"4-2xl 0^"4 M and propan-2-ol (ca IM), at pH ca 5 without adjustment, were put into a spectrophotometer cell (1 cm path length), deaerated with Ar and then covered with a serum cap. Photolysis was performed with a 1000 W Xe arc lamp at which the light intensity was reduced mechanically by ca. 40%, while cut-off filters 320 nm were used to avoid direct photolysis of organic substrates. Upon illumination the solution turned blue due to the formation of reduced POM, POM(e-), reaction (1) as above. The illumination time was adjusted (varied in minutes timescale, depending on the POM and the concentrations) in order to obtain the desired concentration of reduced POM. Higher illumination times resulted in higher concentrations of reduced POM while prolonged illumination led to the formation of extensively reduced POM (2-e-reduced for P₂W₁₈O₆₂ ^6" and 4-e-reduced for P₂MOI₈OO₂ ^6") The concentration of reduced POM was measured with a UV/Visible Absorption

Spectrometer, a Perkin Elmer Lambda 19 Spectrometer, by monitoring the characteristic absorbance of the blue POM(e-) species [SiWi₂O₄₀ ^5" ε₇₃ M^"1 cm^"1; H₂Wi₂O₄₀ ^7" 8690nm =2100 M^"' cm^"1; P₂W]₈O₆₂ ^7" ε_714nm =3600 M^"1 cm^"1; P₂W₁₈O₆₂ ^8" ε₆94nm =10600 M^"1 cm^"1; P₂Mo₁₈O₆₂ ^8" S_75611111 =1 1000 M^"1 cm^"1; P₂MOi₈O₆₂ ^10" ε_675llm =19400 M^"1 cm^"1]. The concentration of Ag⁰ nanoparticles was measured from the increase of the absorbance at ca 420 nm, attributed to the plasmon resonance peak of colloidal silver, taking as absorption coefficient the one calculated at the saturation point of the reaction, when all silver had been reduced.

Step 2. Then, in the absence of light, silver nanoparticles were obtained by injecting a deaerated aqueous solution of AgNO₃ from Panreac (μl of a 3.83xlO^"3 M or 0.01 15 M solution were added in order to obtain concentration of silver ions of ca 10^"4 M) to the already prepared POM(e-) solution (4ml). The solutions were mixed, agitated for ca 3 seconds and allowed to stand. The initially blue solution turned green and finally yellow within time that spanned from seconds to hours depending on the POM used. Silver nanoparticles were formed according to reaction (2). The reduced POM donated electrons to silver ions, then the produced silver atoms started aggregating and the POM surrounded them on their surface to arrest their aggregation in nanometer scale. The obtained silver nanoparticles were characterized using Transmission Electron Microscopy. The corresponding images were obtained using a Philips 20OkV microscope, while the samples were prepared by placing microdrops of colloid solution on a Fprmvar/Carbon coated copper grid. The subsequent analysis for the size-distribution of the particles was based on the counting of ca 150 particles. The initial rate of POM(e-) reoxidation was calculated by the slope of the curve of the concentration of POM(e-) vs. time of the reaction with silver ions, for conversion less than 30 %

As will be shown below, in the examples illustrating embodiments of this invention, control of the size of the nanoparticles was achieved by changing either the reaction rate [by variation of POM(e-) initial concentration, kind of POM or extent of reduction of the same POM] or the amount of silver ions. , More particularly:

1. Size-control by POM(e-) concentration By changing the illumination time in step 1, solutions of various [POM(e-)]o were prepared.

By adding the same [Ag ' ]₀ in each solution, the influence of [POM(e-)]₀ on the size of the

- particles was studied. Experiments with the SiW₁₂O₄₀ ^5", as an example, were executed.

Using solutions of increased initial concentration of the 1 -electron-reduced POM

SiWi₂O₄₀ ^5", [SiWi₂O₄₀ ^5-]O, and adding a fixed Ag⁺ concentration, an increase of the initial rate of SiWj₂O₄₀ ⁵ reoxidation was observed (Fig. 4). Thus, smaller silver nanoparticles were formed, Fig. 5. In all cases, POM(e-) was in excess in order to ensure that the same, all, quantity of silver was always reduced. These results based on TEM measurements were also in consistence to the visible absorption spectra of the corresponding nanoparticles (see Fig. 6), which exhibited a blue shift, denoting decrease of their size [S. M. Heard, F. Grieser, CG. Barraclough J. Colloid Interface Sci. 1983, 93, 2, 545], with increasing [SiW₁₂O₄₀ ^5"]₀ (see Fig. 7). In the case of the lowest concentration of SiWi₂O₄₀ ^5", 1.0x10⁴ M, even larger particles were obtained, however not so uniform. It seems that the slowest reduction in this case, let another pathway, this of the agglomerative growth, to prevail, producing less uniform particles.

2. Size-control by nature of POM

By using various POM and for the same [POM(e-)]₀ and [Ag⁺]₀, the effect of the nature of POM(e-) to the size of the obtained particles was examined. More drastic changes to the rates and thus to particle size were seen by using different POM of various redox potentials. For instance, examining the influence of the kind of POM of the same series, the 1-e-reduced Keggin species H₂W₁₂O₄₀ ^7" and SiWi₂O₄₀ ^5" were compared. The former, which exhibits a more negative redox potential [E⁰ (H₂W₁₂O₄₀ ^6" / H₂Wj₂O₄₀ ⁷;) = -0.337 V] reacted ca 10 times faster than the latter [E⁰ (SiWj₂O₄₀ ^4" / SiWi₂O₄₀ ^5") = 0.054 V] with Ag¹ ions (Fig. 8), leading to smaller silver particles of a mean diameter 6.6 nm, compared to these obtained by SiWi₂O₄₀ ^5", 29.2 nm (all the experimental conditions remain the same in both cases, Fig. 9). This great difference in the size of the silver particles is corroborated by the fact that H₂Wi₂O₄₀ ^7" is more negative than SiWi₂O₄₀ ^5", acting as a stronger stabilizer that restricts more efficiently the size of the particles. When coming to collate the results obtained by a completely different POM, P₂Wi₈O₆₂ ^7" of the Dawson series, caution is needed concerning the "rate-rule". In this case, although larger particles were expected according to the "rate rule", smaller were obtained than these with SiWi₂O₄₀ ^5". This could be attributed to a better stabilizing action of the Dawson POM, since both its negative charge and volume is ca twice this of Keggin POM (note that POM owe their stabilizing ability both to electrostatic and stereochemical repulsions).

3. Size-control by extent of POM(e-) reduction

By changing the illumination time in step 1 (prolonged illumination), solutions of POM(e-) with higher extent of reduction were prepared. By adding the same [Ag⁺Jo in each solution, the influence of the extent of reduction of POM on the size of the particles was studied.

The extent of reduction was decisive to the redox potential (Fig. 2) and thus to the rate of reaction (2). Accumulation of electrons on POM drives the redox potential to more negative values, rendering the oxide stronger electron donor. Thus, a faster reduction of

Ag⁺ took place.

For instance, no reaction was marked between the 2-e-reduced phosphomolybdate,

P₂MOi₈O₆₂ ^8' [E⁰ (P₂MOi₈O₆₂ ^6" / P₂MOi₈O₆₂ ^8") = 0.664 V] and Ag⁺. On the contrary, when using the 4-e-reduced POM, P₂MOi₈O₆₂ ^10" [E⁰ (P₂MOi₈O₆₂ ^8" / P₂MOi₈O₆₂ ¹⁰ ) = 0.514 V], silver nanoparticles were formed, following also a stoichiometry [P₂Mo _I8O₆₂'^0"]: [Ag⁰] = 1 :2 throughout the reaction, suggesting the reoxidation of P₂MOi₈O₆₂ ^10" to P₂MOi₈O₆₂ ^8" and its action as a 2-electron donor.

Another case was the one with P₂Wi₈O₆₂ ^6'. The 1-e-reduced phosphotungstate,

P₂Wi₈O₆₂ ^7" [E⁰ (P₂W]₈O₆₂ ^6' / P₂Wi₈O₆₂ ^7") = 0.344 V], was measured to be ca 3 times slower in reducing silver than the 2-e-reduced phosphotungstate, P₂Wi₈O₆₂ ^8" [E⁰ (P₂Wi₈O₆₂ ^7" /

P₂Wi₈O₆₂ ^8") = 0.144 V]. The corresponding visible absorption spectra in Fig. 10 suggest that, once again, the faster POM, that is P₂Wi₈O₆₂ ^8", resulted in smaller silver particles as indicated by the blue shift and the higher absorbance of the plasmon resonance peak, compared to the one obtained by P₂Wi₈O₆₂ ^7*.

4. Size control by Ag amount

Control of the size of the particles was also achieved through variation of silver ions concentration. For the same illumination time the same [POM(e-)]₀ was produced in each case and by adding different volumes of a Ag⁺ solution, the effect of [Ag⁺J₀ was examined. By changing the initial concentration of Ag⁺, [Ag⁺J₀, the rate of POM(e-) reoxidation and, thus this of silver formation, remained almost the same for [Ag⁺]₀>1.0x l0^~4 M (Fig.

1 1), Thus, roughly the same number of seeds was formed during the first stages of the process. In turn, by increasing [Ag⁺J₀ more amount of silver had to be deposited on the same number of seeds and larger particles were expected, in accordance to TEM measurements, Fig. 12. These results were in consistence to the visible absorption spectra

• of the corresponding nanoparticles (Fig. 13), which exhibited a red shift, denoting increase of their size, with increasing [Ag⁺J₀ (Fig. 14).

In addition, by using even lower [Ag⁺Jo, 0.2x10^"4 M, even smaller silver particles were produced (not shown in Fig. 12 since they were reoxidized upon saturation with air and TEM measurements could not be obtained). Their smaller size was verified by: (i) their plasmon resonance peak (Fig. 13) which was at even lower wavelengths and (ii) their instability to air oxidation, providing that, in general, smaller metal nanoparticles are more amenable to oxidations.

From the size-distribution analysis in Fig. 5, 9 and 12 it can be noticed that uniform silver particles were obtained. In all cases, a relative standard deviation value (r.s.d.) less than 22 % was observed.

The following examples are presented by way of illustration of particular embodiments of the present invention.

EXAMPLES

1. Size-control by POM(e-) concentration 4 ml of a deaerated solution containing K₄SiWi₂O₄₀ (IxIO^'3 M) and propan-2-ol ( 1.0 M) were photolyzed until the 1 -equivalent reduced POM, SiWi₂O₄₀ ⁵ , was formed at concentration 1.OxIO ⁴M. This procedure was repeated for 3 more sets of 4ml solutions, increasing the illumination time so that to also obtain solutions of 2.3xlO^"4, 5.3xlO^"4 and 8.3xlO^"4 M in SiWi₂O₄₀ ^5'. In turn, 90μl of a deaerated aqueous solution Ag⁺ 3.83xl O^"3 M were injected in each solution, to obtain 0.8x10^'4 M of Ag⁺ (initial concentration in the mixed solution). Decrease of the [SiWi₂O₄₀ ^5"]₀ from 8.3 to 5.3, 2.3 and 1.0x10^'4 M resulted in the linear decrease of the initial rate of formation of SiWi₂O₄₀ ⁵ reoxidation (Fig. 4) and the gradual increase of the size of silver particles (Fig. 5-7). Larger nanoparticles of mean diameters 28.2, 38.0 and 44.0 nm for initial concentration of SiWi₂O₄₀ ^5" 8.3, 5.3 and 2.3 x 10^'4 M respectively were formed (Fig. 5).

2. Size-control by nature of POM

4 ml of deaerated solutions containing Na₆H₂Wi₂O₄₀, K₄SiWi₂O₄₀ or KGP₂W_{I S}O₆₂ (1.OxIO^"4 M) and propan-2-ol (1.0 M) were photolyzed to form the 1-equivalent reduced

POM at concentration l .OxlO^"4 M. In turn, 52μl of a deaerated aqueous solution Ag

■ 3.83x10³ M were injected in each solution, to obtain 0.5xl0^"4 M of Ag¹ (initial concentration in the mixed solution). The corresponding UV absorption spectra were taken with reaction time (see the inset of Fig. 8 for P₂WiSO₆₂ ^7" case) in order to measure the decrease of concentration of reduced POM with reaction time and the increase of the concentration of silver nanoparticles (Fig. 8). H₂Wi₂O₆₂ ^7" was the fastest, reacting within less than ca 0.5 min, followed by SiWi₂O₄O^5" which reacted within ca 4 min. P₂WiKOr,.^7" was the slowest, taking ca 40 minutes to complete the formation of silver nanoparticles.

In order to obtain the corresponding TEM images, a similar procedure was followed, involving slight changes in the concentration values and a pre-treatment, filtration step. 4 ml of deaerated solutions containing Na₆H₂Wi₂O₄₀, H₄SiW₁₂O₄O or K₆P₂WiSO₆₂ (2.OxIO^"4 M) and propan-2-ol (1.0 M) were firstly filtrated and then photolyzed until the 1-equivalent reduced POM was formed at concentration 1.3xlO^"4 M. In turn, 104μl of a deaerated aqueous solution Ag^H 3.83xlO^"3 M were injected in each solution, to obtain l .OxlO^"4 M of Ag^"1 (initial concentration in the mixed solution). The TEM pictures obtained and the subsequent analysis revealed that particles of a mean diameter of 6.6 nm, 29.2 nm and 24.3 nm for H₂Wi₂O₆₂ ^7", SiWi₂O₄₀ ^5" and P₂Wi₈O₆₂ ^7" respectively were formed (Fig. 9).

3. Size-control by extent of POM(e-) reduction

4 ml of a deaerated solution containing (NH₄)₆P₂Mo I₈O₆₂ (l .OxlO ⁴ M) and propan-2-ol (1.0 M) were photolyzed to form the 2-e- reduced POM P₂MoIgO₆₂ ^8" at l.OxlO^"4 M. In turn, 52μl of a deaerated aqueous solution Ag¹ 3.83xlO^'3 M were injected in each solution, to obtain 0.5xl0^"4 M of Ag^"1 (initial concentration in the mixed solution). No reaction was marked between the 2-e-reduced phosphomolybdate, P₂MoisO₆₂ ^8" and Ag⁺ ions even after 26 hours waiting. Another 4 ml of the (NH₄)₆P₂Mθigθ6₂ / propan-2-ol solution were deaerated and illuminated for further time to form the 4-e-reduced POM P₂MOi₈O₆₂ ^10" at l .OxlO^"4 M. In turn, 52μl of a deaerated aqueous solution Ag⁺ 3.83xlO^"3 M were injected in each solution, to obtain O.5xlO^'4 M Of Ag⁺ (initial concentration in the mixed solution). In contrast to the 2-e-reduced POM, in the case of the 4-e-reduced POM, P₂MOi₈O₆₂ ^10", the formation of silver nanoparticles had effectively proceeded within 200 minutes upon then addition of

Ag⁺, following also a stoichiometry [P₂Mθι₈O₆₂ ^10"]:[Ag°] = 1 :2 throughout the reaction.

4 ml of a deaerated solution containing

K₆P₂Wi₈O₆₂ (1 2x10⁴ M) and propan-2-ol (2.0 M) were photolyzed to form the 1-e- reduced POM P₂Wi₈O₆₂ ^7" at 1.OxIO^"4 M. Another 4 ml of the K₆P₂W₁₈O₆₂ (1.2xlO^"4 M) and propan-2-ol (2.0 M) solution were deaerated and illuminated for further time to form the 2-

- e-reduced POM P₂Wi₈O₆₂ ^8" at l .OxlO^"4 M. In turn, 90μl of a deaerated aqueous solution

Ag⁺ 3.83xlO^"3 M were injected in each solution, to obtain O.δxlO^"4 M of Ag⁺ (initial concentration in the mixed solution). The reaction Of Ag⁺ with P₂Wi₈O₆₂ ^8" was ca 3 times faster than the one with P₂W₁₈O₆₂ ^7". The UV absorbance of the silver particles finally obtained were taken, Fig. 10. 4. Size control by Ag amount

¹ 4 ml of a deaerated solution containing K₄SiWi₂O₄₀ (1.OxIO^"3 M) and propan-2-ol (1.0 M) were photolyzed to form the 1-e-reduced POM, SiWi₂O₄₀ ^5", at concentration 7.OxIO ⁴M. This procedure was repeated 3 times. In turn, 7, 35, 70 and 210μl from a deaerated aqueous solution Ag^1" 0.01 15 M were injected in each solution, to obtain 0.2, 1.0, 2.0 and 5.7x10^"4 M of Ag⁺ (initial concentration in the mixed solution) respectively This increase in the [Ag⁺J₀ did not affect significantly the initial rate Of SiWnO₄₀ ^5" for [Ag⁺J₀ > l.OxlO^"4 M (Fig. 11). However an increase in the [Ag⁺J₀ from 1.0 to 2.0 and 5.7xlO^"4 M resulted in an increase of the mean diameter of the silver particles (Fig. 11-13) from 33.3 to 42.9 and 62.3nm respectively (Fig. 12). The silver particles formed upon addition of the lowest silver concentration, 0.2x10-4 M, were decolorized within some minutes upon opening to air.

Claims

CLAIMS:

1. Process for the manufacture of metal nanoparticles in a solution in which a POM (polyoxometalate) is used as catalytic reducing reagent, to induce reduction of a metal cation to form the metal nanoparticles characterized in that the size of the metal nanoparticles formed is controlled by a) the selection of POM of appropriate redox potentials, and /or b) the selection of appropriate concentration of reduced POM, and/or c) the selection of appropriate concentration of metal ion.

2. Process according to claim 1, wherein the POM also serves as stabilizer.

3. Process according to claim 1 or 2, wherein the metal nanoparticles are manufactured in solution in a liquid, including in ionic liquid.

4. Process according to claim 3 wherein the liquid is aqueous.

5. Process according to any one of claims 1 to 4, wherein the POM is immobilized.

6. Process according to any one of claims 1 to 5, wherein the metal is silver, gold, palladium, copper or platinum nanoparticles.

7. Process according to any of the preceding claims, wherein for the manufacture of the metal nanoparticles the two reactants, POM and metal ions, are premixed in a POM/organic/metal ion solution (one-pot process).

8. Process according to any of the claims 1 to 7, wherein the preparation of the catalytic reducing agent (reduced POM) is produced by treatment of a solution containing oxidized POM (or less reduced POM) and oxidizable species by means of exposure to electric current, γ-rays, ultrasound, temperature, UV/near- Visible light, or sunlight.

9. Process according to any of the claims 1 to 8, wherein the preparation of the catalytic reducing agent (reduced POM) is achieved with the use of oxidizable reagents.

10. Process according to Claim 9, wherein as the oxidizable reagent is an alcohol, phenol, hydride, chorophenol, cresol, benzene derivative, organohalogenated compounds aliphatic, or a pesticide.

11. Process according to any of the claims 1 to 10, wherein the reduction of the metal cation comprises the stages of a) a slow nucleation step and then b) a fast reductive growth on the already formed nuclei, wherein the size of the formed metal particles are affected by changing the rate of metal reduction, or by using a certain initial concentration of metal.

12. Process according to any of the claims 1 to 1 1, wherein the metal nanoparticles manufactured are formed in two steps (two-pot process), using the following reactions: a) POM + S → POM (e-) + S_0x (1) b) POM (e-) + M^{n +} → POM + M⁰ _coiι_oidai (2) where S is an oxidizeable species

13. Process according to any of claims 1 to 12 wherein the metal is silver.

14. Process according to claim 13 wherein the silver is provided by silver nitrate, silver perchlorate or other soluble silver salt.

15. Process according to any of the claims 13 to 14, wherein the silver nanoparticles manufactured are formed in two steps (two-pot process), using the following reactions: a) POM + S hv ^ POM (e-) + S_0x (1 ) b) POM (e-) + Ag⁺ -→ POM + Ag°_col,_Oid_ai (2) where S is an oxidizeable species

16. Process according to any of claim 13 to 14, wherein the two reactants, POM and Ag⁺, are mixed with propan-2-ol to form a POM/propan-2-ol/ Ag⁺ solution which is then irradiated.

17. Process according to any of the preceding claims 1 to 16, wherein the catalytic reducing agent POM is any reducible POM, i.e the Keggin, Dawson, Pryessler, Lindqvist or Anderson structured polyoxometalate anions, or the decatungstate POM.

18. Process according to claim 17 wherein the catalytic reducing agent is selected from SiW₁₂O₄₀ ^5", H₂W₁₂ O₄₀ ⁷-, P₂W₁₈O₆₂ ^7", P₂W₁₈O₆₂ ^8", P₂Mo₁₈O₆₂ ^8" and P₂MOi₈O₆₂ ¹⁰"