Photon Factory Activity Report 2005 #23 Part B (2006)
Materials Science

10B/2004G101

EXAFS study for the formation of noble metal nanoparticles prepared fromreverse micelles of metallosurfactants
Masayasu IIDA,1 Chihiro BABA,1 and Masafumi HARADA*2
Department of Chemistry, Nara Women’s University, Nara 630-8506, Japan2
Department of Clothing Environmental Science, Faculty of Human Life and Environment,
Nara Women’s University, Nara 630-8506, Japan
1Introduction
We have been studying the preparations and theformation of aggregates for the alkylethylenediamine
metal complexes in aqueous organic solvent systems.
These systems are utilized to create metal nanoparticles inhigh yields [1, 2]. In this study, we have prepared silver(Ag) nanoparticles by the chemical reduction of theiralkylethylenediamine complexes in the microemulsion
systems using a reducing agent such as NaBH4, and haveestimated the average particle sizes of the Ag
nanoparticles by means of EXAFS measurements. We
have focused on the time-course in the interactions
between the silver(0) and nitrogen of ligand on thesurface of the silver(0) nanoparticles.

Table 1. Structural parameters from EXAFS analysis of themetallosurfactant of Ag(dod-en) microemulsion systems.
samplebond

C.N.

r/Å

Ag(oct-en)

Ag-N

3.5

2.28

Ag(dod-en)

Ag-N

4.2

2.27

Ag(hexd-en)

Ag-N

2.3

2.21

Ag(dod-en)/heptane/H2O

Ag-N

2.4

2.20

Ag(dod-en)/heptane/H2O + NaBH4

Ag-Ag

2.6

2.86

Ag-N

1.6

2.20

Ag(dod-en)/heptane/H2O+NaBH4(1day)Ag(dod-en)/heptane/H2O+NaBH4(1month)Experimental
The amphiphilic bis(octylethylenediamine (= octen))silver nitrate, bis(dodecylethylenediamine (= doden))silver nitrate, and bis(hexadecylethylenediamine (=hexd-en))silver nitrate complexes were prepared by thereactions of oct-en, dod-en, and hexd-en ligands withsilver nitrate in methanol, respectively. The
microemulsion
waspreparedasthesilvercomplex/heptane/water (water/metal complex molar ratio
is in the range of 4-50) ternary system. The reduction of
the metallosurfactants was performed by adding NaBH4
aqueous solution to the microemulsion system and thenby shaking the water/heptane phase.
EXAFS measurements of Ag-K edge of these
metallosurfactant microemulsions were performed using
polyethylene bag with 1 mm path length at room
temperature in a transmission mode at BL-10B. The
above obtained samples were sealed into polyethylenebags for measurements, and the metal concentration of[Ag] in these microemulsion systems is in the range of
0.1-0.8 (mol/L). In order to extract the backscattering
amplitude and phase shift functions for the curve-fitting,the reference compounds (Ag foil and Ag(NH3)2+ solidcompound) were used in this experiment.
Results and Discussion
Table1 shows the coordination numbers (C.N.) andbond distances (r) of the metallosurfactants of Ag(dod-en)
microemulsion before and after the chemical reduction
with NaBH4 as well as Ag(oct-en) and Ag(hexd-en)complexes.

Ag-Ag

6.3

2.87

Ag-N

0.8

2.19

Ag-Ag

5.9

2.87

Ag-N

0.5

2.14

In the Fourier transforms of all the samples (not shownin this report), the peak around 1.8 nm assigned to thebond of Ag-N appears with the same height. The peakassigned to the bond of Ag-Ag appears immediately afterthe reduction, and their peak heights increase withincreasing the duration of left under air. The particle
diameters of the Ag nanoparticles (approximately 1.0-1.5nm) estimated by the C.N. were significantly larger thanthose by the TEM observation and the difference isgenerally larger for the Ag(0) particles. It is characteristicthat the Ag-Ag coordination number appreciably
increases accompanying with the decrease in the Ag-N
coordination number within 1 day after the reduction. The
increase in the coordination number of Ag-Ag will mean
the growth of the Ag(0) particles and the decrease in thatof Ag-N will be the elimination of the Ag(0) coating
ligands. The distances between the Ag-Ag and Ag-Nbonds are nearly constant over 1 month, resulting in thestability of Ag nanoparticles encapsulated by the capping
ligands of alkylethylenediamine.
References[1] A. Manna, T. Imae, M. Iida and N. Hisamatsu,
Langmuir, 17, 6000 (2001).
[2] M. Iida. S. Ohkawa, H. Er, N. Asaoka and H.
Yoshikawa, Chem. Lett., 2002, 1050.
*harada@cc.nara-wu.ac.jp

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