Synthesis of Silver Nanoparticles Using Different Silver Phosphine

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Synthesis of Silver Nanoparticles Using Different Silver Phosphine Precursors: Formation Mechanism and Size Control Amandine Andrieux-Ledier, Benoît Tremblay, and Alexa Courty J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4040248 • Publication Date (Web): 24 Jun 2013 Downloaded from http://pubs.acs.org on June 25, 2013

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The Journal of Physical Chemistry

Synthesis of Silver Nanoparticles Using Different Silver Phosphine Precursors: Formation Mechanism and Size Control. Amandine Andrieux-Ledier†, Benoit Tremblay‡ and Alexa Courty†* †

UPMC Univ. Paris 06 et Centre National de la Recherche Scientifique, UMR 7070, Laboratoire des

Matériaux Mésoscopiques et Nanométriques (LM2N), BP 52, 4 place Jussieu, 75252 Paris Cedex 05, France. ‡

UPMC Univ. Paris 06 et Centre National de la Recherche Scientifique, UMR 7075, Laboratoire de

Dynamique, Interactions et Réactivité (LADIR), F-75005, Paris, France.

ABSTRACT. Silver nanoparticles (AgNPs) ranging from 2.5 to 7.1 nm in diameter with a narrow size distribution are prepared by reducing different silver phosphine precursors RAg(PPh3)n (R=Cl, Br or NO3 and n=1 or 3) with tert-butylamine borane (TBAB) in the presence of dodecanethiols (C12) at a temperature between 100 and 160°C. The study of the formation process of AgNPs by UV-visible and IR spectroscopy reveals that the phosphine (PPh3) derived from the metal precursor and thiols coat the NPs surface. In addition, the PPh3/C12 ligand ratio is shown to decrease during the NPs growth. PPh3 are indeed progressively replaced by thiols. The rate of the PPh3-thiols exchange is shown to depend on the nature of the silver precursor and to influence the final NPs size. By using NO3Ag(PPh3), the surface poisoning by PPh3 is shown to be the most efficient leading to the smallest NPs size (2.5 nm in diameter). Furthermore, we get evidence that the nanoparticle size is controlled by the nature of R in the precursor. Finally, it is found that the influence of the reaction temperature and the thiol chains length on the final NPs size depends on the silver precursor used.

KEYWORDS. Nanoparticle Growth, Salt Precursor, IR and UV-visible Spectroscopy, Transmission Electron Microscopy.

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INTRODUCTION Silver is an especially attractive metal due to its highest electrical and thermal conductivity among all metal. It is also known for its high optical reflectivity. As size of Ag particles decreases down to the 1, 2

nanometer scale, they exhibit many unique optical

3

4

, electronic and chemical properties that cannot

be observed in the bulk. This is largely explained by the effects of surface in nanoparticles that cannot 5

6

be neglected. The silver nanoparticles have been exploited for applications in photonics , catalysis , sensing7-10 and so forth. Because these properties are very sensitive to the size, size distribution and shape, it is crucial to develop synthesis method capable to form silver nanoparticles of controllable sizes and shapes. Furthermore, NPs with a narrow size distribution can self-organize in 2D and 3D high ordered superlattices that exhibit new collective physical properties.

11, 12

A rich variety of chemical synthesis methods are available for preparing AgNPs with a narrow size distribution as stable colloid dispersion in polar or apolar solvent including organometallic decomposition13, polyol14, the liquid-liquid phase transfer15, micro-emulsion followed by size selective precipitation

16

17,18

or digestive ripening

and metallic salt reduction methods

19-23

. Regarding this last

approach, many results have been obtained for the production of silver nanoparticles from salts 19

20

precursors such as AgCF3COO , AgNO3 , AgCH3COO

21

or (PPh3)3Ag-R (R= NO3 or Cl)

22, 23

,

reduced at different temperatures by more or less strong reducing agent such as TBAB19,22 or oleylamine

20,22

in presence of different ligands (usually amines or thiols). The precise control of size,

size distribution, shape and composition of nanoparticles pass through the control and adjustment of various reaction parameters (temperature, nature and concentration of reactants, and stabilizing agents). Nevertheless, to our knowledge, the role of the nature of the salt precursor on the formation mechanism and on the size control of silver NPs is poorly discussed

24

. We can just cite the works of

Tang et al.22 who have obtained AgNPs between 8 and 20 nm in diameter, by reducing ClAg(PPh3)3 and NO3Ag(PPh3)3 precursors with oleylamine that also plays the role of ligands. They show that depending on the nature of functional group in the precursor and the presence or absence of oxygen they can control the crystallinity of the nanoparticles. In this article, we have prepared AgNPs from the reduction of different silver-phosphine precursors: RAg(PPh3) (R=Cl, Br or NO3) or ClAg(PPh3)3 by tert-butylamine borane (TBAB) in presence of thiols (from C10 to C14). An intensive study of the effects of the salt precursor nature, thiol chain length and reaction temperature on the formation and final size of the AgNPs has been


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performed. By IR and UV-visible spectroscopy, we get evidence that PPh3 can coat the NPs, and are progressively replaced by thiols during the NPs growth. The PPh3-thiol exchange rate is shown to depend on the nature of the silver salt precursor and on the reaction temperature. It is further found that the effects of thiol chain length on the final NPs size depend also on the nature of the salt precursor.

EXPERIMENTAL SECTION MATERIALS. The following chemicals were used without further purification. 1-decanethiol (96%, D1602), 1-dodecanethiol (≥98%, 471364), 1-hexadecanethiol (99%, 674516) were purchased from Sigma-Aldrich. Tert-butylamine borane complex (≥97%, 05-0101) and o-dichlorobenzene (≥99%, 222050025) were purchased from STREM Chemicals and ACROS Organics, respectively. The synthesis procedure of the silver phosphine precursors are build upon the works of Ouyang and Knobler groups (see supporting information).22,25 The infrared spectra of these molecules are given in the Figure S1 of the Supporting Information. The spectra agree well with detailed spectroscopic studies on PPh3 and its complexes.26

SYNTHESIS OF AgNPs. The synthesis of silver nanoparticles stabilized by thiols of different chain 19, 23

length Cn (n=10, 12 and 14), are built upon Stucky’s procedure

. In a typical synthesis, 0.25 mmol

of RAg(PPh3)n (R= Cl, Br or NO3 and n= 1 or 3) is dissolved in 25 mL of o-dichlorobenzene (oDCB) under nitrogen atmosphere. After the solution is heated to a desired temperature (between 100 and 160°C) then 500 µL of thiols is quickly injected into the solution with vigorous stirring. The silver precursor is then reduced by adding a solution of TBAB (2.5 mmol dissolved in 15 mL of oDCB). The formation of AgNPs is evidenced by a slow color change of the solution from colorless to yellow and finally to brown red. The reaction mixture is stirring continuously for different times depending on the nature of the silver precursor and on the reaction temperature (Table 1). After the reaction is stopped, the solution is cooled to room temperature and the NPs are precipitated out by adding ethanol. It allows to eliminate by-products and to reduce the size distribution via a size selective precipitation process.

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The precipitate is then dispersed in hexane.


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CHARACTERIZATION TECHNIQUES Transmission electronic microscopy (TEM) Sample for TEM were prepared under nitrogen atmosphere by putting a drop of solution on a carboncoated copper grid. The size distributions are determined with Image J software by measuring the diameter of around 500 NPs. For the study of the NPs growth, all TEM samples are made using NPs solution extracted directly from the reaction vessel before washing by ethanol (without size selection). For the study of the final NPs size evolution with the nature of the functional group R in the precursor, the temperature and the thiol chain length, all TEM samples are made using the NPs solution after washing by ethanol.

UV-visible and IR absorption spectroscopy The UV-visible absorption spectra of AgNPs solutions were obtained on a Cary 5000 spectrophotometer, at room temperature, between 300 and 650 nm and using a quartz cell. Infrared spectra of the samples were recorded using a Bruker Equinox 55 spectrometer equipped with a Ge/KBr beamsplitter and a liquid nitrogen-cooled MCT detector. Using a single reflection accessory, we have obtained the IR spectra of the deposited particles on a horizontal mirror (fused silica -1

substrate and aluminium coating) at a spectral resolution of 4 cm .

RESULTS AND DISCUSSION Growth process of AgNPs obtained from the reduction of ClAg(PPh3)n (n=1 or 3) precursors: role of phosphine. We have prepared AgNPs using ClAg(PPh3) or ClAg(PPh3)3 as silver precursors following the procedure described in the experimental section, the reaction temperature being fixed at 100°C in presence of C12. To follow the NPs growth process, small amounts of NPs solutions are extracted from the reaction vessel at various reaction times and are analyzed directly without washing by ethanol, by TEM and UV-visible absorption spectroscopy (Figures 1 and 2). We observe for both samples (Figure 1a-c and 1d-h) that the large particles grow at the expense of smaller ones that dissolve. Thus, it is likely that 28

the size growth occurs via the Ostwald ripening process . At the end of the growth process, the


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nanoparticles have similar sizes (4.7 nm in diameter) (Figure 1c and 1h) and are characterized by a broad size distribution σ~20%, requiring a subsequent selection in size obtained by adding ethanol (see experimental section). Nevertheless they are obtained for reaction times of 420 and 900 min for ClAg(PPh3) and ClAg(PPh3)3, respectively. These results show that a decrease of the PPh3 concentration in the reaction solution accelerates the formation of silver NPs. The NPs size increase with the reaction time for both samples is also reflected in the UV-visible spectra in Figure 2, in which the intensity of the surface plasmon resonances (SPR) increases with the reaction time, excepted for the spectrum obtained at 900 min from the reduction of ClAg(PPh3)3. In this case, we have observed indeed small aggregation responsible of this decrease in intensity. The SPR bandwidths of both samples are broad in agreement with the broad size distributions observed in Figure 1. Blue shifts of the SPR maximum are observed with the reaction time from 464 to 441 nm for ClAg(PPh3) and from 473 to 444 nm for ClAg(PPh3)3 (Table 2). The SPR maximum is known to be sensitive to the shape, particle size and refractive index n of the medium. According to Figure 1 for both samples, the NPs shape remains spherical whatever the reaction times are. Otherwise, the size increase with the reaction time should induce a slight red shift of the SPR maximum.29 As a blue shift of the SPR maximum is expected when the medium dielectric constant decreases

30-32

, the changing

SPR maximum could be explained by ligand exchange of PPh3 (n=1.59) by thiols (n=1.45) during the formation process. For both precursor, the average variation in wavelength (∆λ = 26 nm) of the plasmon band maximum is coherent with a variation of the medium refractive index (∆n=0.14)

31

and

thus with silver nanoparticles coated initially by PPh3 and at the end of the growth process by thiols. This implies thus that PPh3 can act as ligands for silver NPs. This is supported by previous studies showing that PPh3 can coat gold or cobalt NPs either alone or mix with other ligands33,

34

.

Furthermore, for both samples the final positions of the SPR maxima obtained at 420 and at 900 min are very similar (Table 2) while the initial position at 30 min for ClAg(PPh3)3 is red shifted compared to ClAg(PPh3). This cannot be attributed to a size effect as the average NPs sizes at 30 min are around 3.4 nm and 2.1 nm for ClAg(PPh3) and ClAg(PPh3)3, respectively. In this case, we will thus expect a blue shift. This suggest an increase of the medium dielectric constant due to a larger amount of PPh3 that coat initially the NPs synthesized from ClAg(PPh3)3, although the final PPh3/C12 ratio are similar for both samples. In order to explain the higher formation time of silver NPs from ClAg(PPh3)3 compared to ClAgPPh3, we can make two hypotheses: i) an increase of PPh3/C12 ligand ratio could


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hinder the NPs from growing if PPh3 was stronger bind to the NPs surface than C12. Nevertheless, several studies have reported the opposite

33,34

. ii) PPh3 could be exchanged by Ag(I)thiolate

generated by the digestive etching of small particles that could favor even more growth of nanoparticles when the amount of phosphine decreases and would allow a more rapid NPs growth synthesized by using ClAgPPh3 as precursor. This last hypothesis appears the most probable and is 33

supported by the results of Song et al. on the growth of gold NPs capped by thiols and PPh3.

To further probe the nature of the ligands adsorbed on the AgNPs during the synthesis, we have analyzed the NPs samples obtained from the reduction of ClAg(PPh3) for different reaction time by IR spectroscopy. Figure 3 shows the IR spectra of (a) free PPh3, (b) free thiol, and AgNPs obtained using ClAg(PPh3) by extracting successively small amount of the solution from the reaction vessel for (c) 30, (d) 120, and (e) 420 min reaction times. When we compare the spectra, we can observe several characteristic bands from the phenyl ring and for the thiol. However, for the AgNPs, several bands have changed in their relative intensity and position compared to those of the free PPh3 and thiol. Among the PPh3 bands, it was possible to easily measure the shifts, in comparison with free -1

PPh3, on four bands: +1, +15, -4, and +2 cm for the bands at 1434(νC-C), 1090(δC-C-H), 744(γC-H), and 696 cm-1 (γC-C), respectively35 (these bands are marked with asterisks on the Figure 3c). Also, for the thiol, we observed similar modifications on the methylene and the terminal methyl stretching modes 36

groups

-1

observed around 2900 cm , especially on νa(CH2) and νs(CH2) with shifts of about -11 and -7

-1

cm , respectively. Similar band shifts have been reported for aromatics upon absorption on a metallic surface35, 37, for BINAP adsorption on palladium38 and for thiol layer adsorbed on an AgNPs surface15. All of these observed band shifts are consistent with the coordination of thiol and PPh3 to the Ag nanoparticles. Also, a noticeable intensity evolution occurs on the main bands in the IR spectra of the AgNPs for the reaction times, what means that quantity of thiol and PPh3 is not constant. For each reaction time, we can estimate the integrated intensity of the thiol bands (using the methylene and -1

methyl bands around 3000 cm ), named Ithiol, and for each of the four mentioned characteristic bands from the phenyl ring, named IPPh3. To know the variation of the PPh3 quantity, in comparison with the thiol quantity, for a given reaction time, we have calculated the ratio IPPh3/Ithiol for each selected band and the values are reported in Table 3. We can observe that the ratio decreased with the reaction time, thus a decrease of the PPh3 and an increase of the thiols, which implies a ligand exchange of PPh3 by thiols during the NPs formation process. This observation is totally in agreement with the UV-


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visible results. Also, these results show that PPh3 and thiols remain adsorbed on the NPs surface at the end of the formation process. But since we don’t know the absolute infrared intensity of the PPh3 and thiol bands, it was not possible to know the real quantity of molecules.

Size control of the AgNPs: Influence of the nature of the functional group R in the silver precursor. Subsequently, we have studied the influence of the nature of the functional group R in the silver precursor on the AgNPs formation. Its influence was revealed by a comparative study of the final size of the AgNPs, by performing the synthesis at 100°C for the three different silver precursor RAg(PPh3) (R= Cl, Br or NO3) for 420 min stirring in presence of C12. Note that the TEM analyses have been performed on AgNPs obtained after washing by ethanol. The TEM images on Figure 4a-c show the AgNPs obtained with ClAg(PPh3), BrAg(PPh3) and NO3Ag(PPh3), respectively. For the first two samples the size distribution is narrow (around 8%) although the average size decreases between 4.8 and 3.4 nm. For the third sample, the average size is decreased to 2.5 nm, nevertheless the size distribution is broad (σ~15%). This can be explained by the inefficiency of the size selective precipitation process. All the nanoparticles are in this case precipitated after the addition of ethanol. The decrease of the NPs size from Cl to NO3 can be related to the evolution of the Ag+/Ag potential upon complexation.39 We have thus measured the potential differences between an Ag electrode immersed into a diluted solution of ClAg(PPh3) (10-4 M), used as reference and another Ag electrode immersed into a solution of RAg(PPh3) (R= Br or NO3) at the same concentration: E(BrAg(PPh3))E(ClAg(PPh3))= +10 mV and E(NO3Ag(PPh3))-E(ClAg(PPh3))=+12 mV. The redox potential variations that we have measured are small. A redox potential variation of 10 mV corresponds indeed to an enthalpy variation ∆rG of only 0.965 kJ/mol. This small enthalpy change may however be important for inducing the nucleation since the redox potential of the couple Ag+/Ag1 atom is known to be very small +

40

compared to the one of Ag /Agbulk . Furthermore, we can exclude a nanoparticle size control by surface poisoning effects via selective adsorption of the Cl-, Br- or NO3- anions provided by the precursors. We would obtain in this case formation of nanoparticles of various shapes (rods, cubes…) 41

, that is not our case as we observe only spherical nanoparticles. Finally, Stuky et al.19 reported that

the use of weak reducing agents as amine borane complexes is essential for the formation of monodisperse gold nanoparticles from the reduction of ClAuPPh3 precursor. However, they found that


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the polydispersity of the nanoparticles increases significantly with ammonia borane compared to trimethylamine borane or terbutyamine borane. Thus a slight variation of the reducing ability is shown to induce strong difference in the final size polydispersity. We can thus assume that a small increase of the redox potential (E(NO3Ag(PPh3))>E(BrAg(PPh3))> E(ClAg(PPh3)) can induce a significant increase of the nucleation rate and therefore a decrease in the 40, 42

average NPs size from Cl to NO3 and Cl to Br.

Nevertheless, the insignificant change of the redox

potential between NO3Ag(PPh3) and BrAg(PPh3) (∆E=2 mV) is probably not the cause of the change in size of 1 nm of the nanoparticles obtained by the reduction of the both precursors. The growth process of the NPs obtained from the reduction of NO3Ag(PPh3) and BrAg(PPh3) precursor has been investigated as previously by UV-visible measurements by extracting successively small amount of the solution from the reaction vessel. The solutions are thus not washed by ethanol. On the UV-visible spectra for Br or NO3, we observe a blue shift of the SPR band with the reaction time as observed previously with Cl (Table 2 and Figure 4d and e). Nevertheless, the final position of the SPR maximum is red shifted for NO3 compared to the other two samples. It suggests that the final PPh3/C12 ligand ratio is higher and that the PPh3-C12 exchange is note as efficient as for the other two salt precursors that induces the formation of nanoparticles of smaller sizes. From these results, we get evidence the role of nature of the silver precursor (via its functional group and its number of PPh3) on the size of the AgNPs .

Size control of the AgNPs: thiol chain length, and reaction temperature effects The synthetic strategy was applied for other thiols Cn (n=10, 12 and 14) by using ClAg(PPh3)3, ClAg(PPh3) and NO3Ag(PPh3) (see experimental section and Table 1). Whatever the thiol chain length, the NPs are stable and have a narrow size distribution (see Supporting Information Figures S2 and S3). All the TEM images and thus size distributions have been performed on AgNPs obtained after washing by ethanol. The dependence of the NPs diameter with the thiol chain length for the three different silver precursors and for a reaction temperature of 100°C is shown on Figure 5a. Similarly NPs sized (around 5 nm in diameter) AgNPs are obtained whatever the thiol chain length for ClAg(PPh3)3 and ClAg(PPh3). For NO3Ag(PPh3), the NPs diameter reaches a maximum at around 3.7 nm for C10 and remains constant around 2.5 nm for C12 and C14. By increasing the temperature up to 140°C, the size evolution in function of the thiol chain length is modified (Figure 5b). We observe


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indeed that, for ClAg(PPh3), the NPs size increases slightly with the temperature for C12 and C14 and more significantly for C10, where the size increases by 10%. For ClAg(PPh3)3, whatever the chain length, the NPs size increases by 20%. The most significant NPs size variation with the temperature is observed for NO3Ag(PPh3) (Figure 5). The average size is indeed multiplied around by 2. To explain this large increase of size, we have compared for C12, the size and final position of the SPR band at the final reaction time for AgNPs obtained at 100°C and 140°C (Figure 4c, 4d and 6). We can note that the SPR band is shifted from 457 nm (at 100°C) to 443 nm (at 140°C) (Figure 4d and 6c). This suggest that the increase in the reaction temperature favors the desorption of PPh3 ligands that are less strongly bound to the AgNPs33, 34 than thiols and activate the NPs growth. The successful size control of the NPs obtained via the reduction of NO3Ag(PPh3) by the temperature prompted us to extend it to higher temperature such as 160°C, in presence of C12. After an optimal reaction time of 60 min, AgNPs of 7.1 ± 0.5 nm in diameter are obtained (Figure 7a-b). Surprisingly, the final position of the SPR band at the final reaction time is at 453 nm suggested that a higher amount of PPh3 remains adsorbed on the NPs than at 140°C (Figure 7c). Nevertheless, as the reducing rate increases with the reaction temperature, a higher reaction temperature gives rise to NPs 19

of larger size in a shorter time . At 160°C, the reaction time is probably too short to allow a good exchange of PPh3 by thiol as at 140°C (Table 1). That could thus explain the mean position of the SPR maximum between those obtained at 100°C and 140°C. Theses results show thus that the average size of nanoparticles is mainly controlled by the balance of nucleation and growth rate, in which PPh3 and temperature play a crucial role.

CONCLUSION Silver nanoparticles have been synthesized by the reduction of different silver phosphine precursors by TBAB in presence of thiols. With UV-visible and IR spectroscopy, the formation process of silver NPs via a ligands exchange of PPh3 by C12 has been established. It is also demonstrated that PPh3 and thiols remain adsorbed on the NPs surface at the end of the formation process. The final NPs size distributions are broad (σ~20%) but can be reduced by a size selective precipitation method. NPs diameter ranging from 2.5 to 7.1 nm have been thus obtained with a narrow size distribution (σ~8% between 3.4 and 7.1 nm and σ ~15% for 2.5 nm). It is found that the final NPs size depends on the


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nature of the functional group R in the precursor. Furthermore, the NP size increase with the reaction temperature. This effect is attributed to a higher PPh3/C12 ligand exchange rate. Finally it is demonstrated that the influence of the thiol chain length on the final NPs size depends on the silver precursor used.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors want to thanks Pr Emmanuel Maisonhaute from the Laboratory “Interfaces et Systèmes Electrochimiques” (LISE) at the University Pierre and Marie Curie, for fruitful discussions.

SUPPORTING INFORMATION AVAILABLE. Synthesis procedure and characterization of the silver precursors by elementary analyses, and infrared spectroscopy (Figure S1). TEM images of the AgCn NPs (n=10, 12 and 14) synthesized from the reduction of ClAg(PPh3)3, ClAg(PPh3), NO3Ag(PPh3) at 100 (Figure S2) and 140 °C (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.


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Table 1. AgNPs synthesis procedures depending on the nature of the silver precursors, reaction temperature, reaction time, thiol chain length (Cn). In all procedures, TBAB and oDCB are used as reducing agent and solvent, respectively.

Reaction temperature (°C)

Reaction time (min)

100 140 100 140 100

420 20 900 30 420

NO3Ag(PPh3)

140

120

BrAg(PPh3)

160 100

60 420

Silver precursor ClAg(PPh3) ClAg(PPh3)3

Cn C10 / C12 / C14 C10 / C12 / C14 C10 / C12 / C14 C10 C12 / C14 C12 C12

Table 2. Evolution of the SPR band position (nm) with the reaction time for the synthesis at 100 °C of AgNPs from silver phosphine precursors in presence of dodecanethiols.

Silver precursor

ClAg(PPh3)

ClAg(PPh3)3

NO3Ag(PPh3)

BrAg(PPh3)

465

454

Reaction time (min) 15 30

464

473

462

447

120

455

470

457

444

420

441

452

457

444

720

443

900

444


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Table 3. Intensity ratio of IPPh3/Ithiol measured for selected PPh3 bands at different reaction times. The -1

band positions are in cm .

Intensity ratio IPPh3/Ithiol Reaction time

1434

1096

744

695

30

0.16

0.11

0.20

0.20

120

0.14

0.09

0.11

0.13

420

0.06

0.03

0.03

0.03

(min)


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FIGURE CAPTIONS Figure 1. TEM images of AgNPs synthesized via the reduction at 100°C of ClAg(PPh3) for (a) 30, (b) 120 and (c) 420 min, and ClAg(PPh3)3 for (d) 30, (e) 120, (f) 420, (g) 720 and (h) 900 min, in presence of dodecanethiols. All the solutions have been not washed by ethanol.

Figure 2. UV-visible absorption spectra of not washed AgNPs obtained by the reduction at 100°C of (a) ClAg(PPh3) and (b) ClAg(PPh3) vs reaction time in presence of dodecanethiols.

Figure 3. IR spectra of (a) free PPh3, (b) free dodecanethiol, AgNPs obtained by the reduction at 100°C of ClAg(PPh3) in presence of dodecanethiols for (c) 30, (d) 120, and (e) 420 min reaction times. The bands used in Table 3 to calculate the intensity ratio IPPh3/Ithiol are marked with asterisks.

Figure 4. TEM images and size histograms of AgNPs synthesized via the reduction at 100°C of the silver precursors RAg(PPh3), R= (a) Cl, (b) Br and (c) NO3, in presence of dodecanethiol and washed by ethanol. UV-visible absorption spectra of not washed AgNPs solution obtained by the reduction at 100°C of (d) NO3Ag(PPh3) and (e) BrAg(PPh3) vs reaction time.

Figure 5. Variation of the average AgNPs diameter (as determined by TEM) obtained by the reduction of RAg(PPh3) (R=Cl, Br and NO3) with the chain length of the coating agent Cn (n= 10, 12, 14). The reaction temperature is (a) 100 and (b) 140°C. The error bars reflect the particle size distribution deduced from TEM. For a better visual presentation, the points are slightly shifted for a given n value. The average sizes have been determined after washing all the solutions by ethanol.

Figure 6. TEM images (a) and size histograms (b) of AgNPs synthesized via the reduction at 140°C of NO3Ag(PPh3) in presence of dodecanethiols and washed by ethanol. (c) UV-visible absorption spectra of not washed AgNPs solution vs time reaction

Figure 7. TEM images (a) and size histograms (b) of AgNPs synthesized via the reduction at 160°C of NO3Ag(PPh3) in presence of dodecanethiols and washed by ethanol. (c) UV-visible absorption spectra of not washed AgNPs solution vs time reaction


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Figure 1


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Figure 2


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0.12 e d

Absorbance

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0.08 *

*

* *

0.04

c

b a

0.00 3000

1500

1000 -1

Wavenumbers (cm ) Figure 3


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Figure 4


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Figure 6


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Table of Contents (TOC) Image


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Synthesis of Silver Nanoparticles Using Different Silver Phosphine

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