Reversible dispersion and aggregation of Ag2S nanoparticles capped with azobenzene-derivatized alkanethiols

Article abstract of DOI:10.1166/jnn.2012.5396

Ag₂S nanoparticles (NPs) of 3.1±0.6, 8.1±1.0, and 10.2±1.6 nm in diameter capped with a longchain amidoamine derivative (C18AA) were synthesized by a modified Brust method. Ag₂S NPs capped with two types of azobenzene-derivatized alkanethiol differing in chain length (2AM10SH and 8AM5SH) were obtained by the ligand exchange method. The trans to cis photoisomerization conversion of 2AM10SH and 8AM5SH on Ag₂S NPs dispersed in toluene was above 95%. 2AM10SH-capped Ag₂S NPs of 8.1 nm or more in toluene were found to show reversible dispersion-aggregation behavior under alternating irradiation with UV and visible lights, i.e., Ag ₂S NPs capped with trans- and cis-2AM10SH were in dispersed and aggregated states, respectively. However, Ag₂S NPs of 3.1 nm capped with 2AM10SH and Ag₂S NPs of 8.1 nm capped with 8AM5SH were always in a dispersed state regardless of whether 2AM10SH and 8AM5SH were in the trans or cis conformation. Copyright

Full text of DOI:10.1166/jnn.2012.5396

Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 648–655, 2012
Reversible Dispersion and Aggregation of
Ag2S Nanoparticles Capped with
Azobenzene-Derivatized Alkanethiols
Takahiro Ishisone, Hiroshi Endo, and Takeshi Kawai∗
Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-Ku, Tokyo 162-8601, Japan
Ag2S nanoparticles (NPs) of 3ꢀ1 0ꢀ6, 8ꢀ1 1ꢀ0, and 10ꢀ2 1ꢀ6 nm in diameter capped with a long-
chain amidoamine derivative (C18AA) were synthesized by a modified Brust method. Ag2S NPs
capped with two types of azobenzene-derivatized alkanethiol differing in chain length (2AM10SH
and 8AM5SH) were obtained by the ligand exchange method. The trans to cis photoisomer-
ization conversion of 2AM10SH and 8AM5SH on Ag2S NPs dispersed in toluene was above
95%. 2AM10SH-capped Ag2S NPs of 8.1 nm or more in toluene were found to show reversible
dispersion–aggregation behavior under alternating irradiation with UV and visible lights, i.e., Ag2S
NPs capped with trans- and cis-2AM10SH were in dispersed and aggregated states, respectively.
However, Ag2S NPs of 3.1 nm capped with 2AM10SH and Ag2S NPs of 8.1 nm capped with
8AM5SH were always in a dispersed state regardless of whether 2AM10SH and 8AM5SH were in
the trans or cis conformation.
Keywords: Reversible, Nanoparticle, Azobenzene, Photoisomerization, Dispersion–Aggregation,
Delivered by Publishing Technology to: Chinese University of Hong Kong
Ag S.
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2
Copyright: American Scientific Publishers
1. INTRODUCTION
Surface phenomena,
converted by photoisomerization, it would offer the poten-
tial to control the dispersibility of NPs in a solvent; that is,
a light-controlled dispersion–aggregation system of NPs
in a solvent would be fabricated. In a previous report,9
we demonstrated the dispersion–aggregation behavior of
Au NPs capped with an azobenzene-derivatized alkanesul-
fide in an apolar solvent under UV and visible lights. The
azobenzene sulfide-capped Au NPs were aggregated via
photoisomerization of the capping molecule from the trans
to the cis isomer, but did not change from the aggregated
to the dispersed state.
Silver sulfide (Ag2S) is a direct and narrow band-gap
semiconductor with good chemical stability and excel-
lent optical properties.31–35 Ag2S nanostructures,36–39 are
promising materials for developing nano- and atomic-
scale switching devices with applications in random access
memory (RAM), cross bar electronic circuits, and fuel
cells or batteries based on ionic conductors and H2S sens-
ing. In the present work, Ag2S NPs capped with SAMs
of azobenzene-derivatized alkanethiols (Fig. 1) were pre-
pared, and the dispersion–aggregation behavior of the
resulting Ag2S NPs in toluene was investigated by observ-
ing the trans-to-cis isomerization of the thiols under
irradiation with UV or visible light.
including
liquid
crystal
alignment,1ꢀ2 wettability,3 molecular recognition,4–8 and
dispersibility,9ꢀ10 are governed by the atomic constitution
of the outermost surface of a material. Therefore, control
of the chemical structure of the outermost monomolecular
layer of a substrate by light or other external stimuli may
allow access to a broad range of new applications. Self-
assembled monolayers (SAMs) of azobenzene derivatives
are particularly promising candidates for creating films
that allow control over their outermost surface. The SAM
technique should allow control of molecular order and
orientation in films, and photoisomerization of the azoben-
zene chromophore is reversible, with little degradation
even after numerous switching cycles.
There are numerous reports on nanoparticle (NP) dis-
persion systems and the aggregation behavior of the
NPs.11–23 However, there are very few on the reversible
dispersion–aggregation of NPs driven by changes in pH24
or temperature,25 molecular recognition,26 and photoiso-
merization of capping molecules.27–30 If the conformation
of the outermost capping molecules on a NP could be
∗Author to whom correspondence should be addressed.
648
J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 1
1533-4880/2012/12/648/008
doi:10.1166/jnn.2012.5396

Ishisone et al.
Reversible Dispersion and Aggregation of Ag2S Nanoparticles Capped with Azobenzene-Derivatized Alkanethiols
2.1.3. 5-[2-methyl-4-(4-ethyl-phenylazo)-phenoxy]-
Decane-1-Thiol (2AM10SH)
A mixture of 3.93 g (8.3 mmol) of 2AM10Br, 1.25 g
(17 mmol) of thiourea, and 100 mL of dehydrate ethanol
was refluxed for 12 h at 90 ꢀC. After the solvent was
evaporated, ethyl acetate was added the residue. The insol-
uble products were filtered out, and a thiouronium salt was
obtained. Next, 3.63 g (8 mmol) of the thiouronium salt
and 1.6 g (40 mmol) were dissolved in 100 mL of ethanol
ꢀ
and the mixture was refluxed for 12 h at 90 C. The reac-
tion mixture was cooled to room temperature. After the
Fig. 1. Molecular structures and synthetic routes of azobenzene-
derivatized alkanethiols (2AM10SH: n = 2 and m = 10, 8AM5SH: n = 8
and m = 5).
pH of the reaction mixture was adjusted to 5 by adding
1 M HCl, the precipitate was filtered. The crude filtrate
was purified by column chromatography using a mixed
solvent of acetone and hexane (1:4). After extract evapora-
tion, 2AM10SH was obtained followed by recrystallization
using methanol.
2. EXPERIMENTAL DETAILS
2.1. Synthesis of Azobenzene Derivatives
1H NMR (400 MHz, CDCl3, TMS) ꢁ: 7.81–7.79 (d,
2 H, ArH); 7.78–7.77 (d, 1 H, ArH); 7.75 (s, 1 H, ArH);
7.32–7.3.1 (d, 2 H, ArH); 6.92–6.90 (d, 1 H, ArH); 4.06–
4.03 (t, 2 H, ArOCH2ꢂ; 2.73–2.70 (q, 2 H, CH2SH); 2.55–
2.49 (q, 2 H, ArCH2CH3ꢂ; 2.29 (1 H, ArCH3ꢂ; 1.85–1.81
(q, 2 H, ArOCH2CH2ꢂ; 1.63–1.59 (t, 3 H, ArCH2CH3ꢂ;
1.38–1.35 (q, 2 H, CH2CH2SH); 1.49–1.26 (m, 14 H,
−CH2CH2−), MS(FAB+ꢂ m/z: 413.
Photochromic5-[2-methyl-4-(4-ethyl-phenylazo)-phenoxy]-
decane-1-thiol (2AM10SH) was synthesized according
to the scheme in Figure 1.9 5-[2-methyl-4-(4-octyl-
phenylazo)-phenoxy]-pentane-1-thiol (8AM5SH) was syn-
thesized according to a previous paper.40
2.1.1. 4-(4-Ethyl-phenylazo)-2-methyl-phenol (2AMOH)
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2.2. Synthesis of C18AA Capping Ligand
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An aqueous solution of 4.13 g (59.9 mmol) of sodium
nitrite in 15 mL of water was dropped into 5.41 g
Copyright: American Scientific Publishers
C18AA was synthesized according to a previous paper.41–43
Briefly, 10.22 g (0.12 mol) of methyl acrylate was added
to 2.0 g (7.12 mmol) of octadecylamine in 15 mL
of methanol. The solution was stirred at 40 ꢀC for 3
days and then the solvents and excess methyl acrylate
were removed from solution by rotary evaporation. 3-[(2-
methoxycarbonyl-ethyl)-octadecyl-amino]-propionic acid
methyl ester (C18ME) was obtained as a viscous liquid.
Yield: 95%. Next, C18ME (3.2 g) and ethylenediamine
17.8 g (0.30 mol) were dissolved in 15 mL of methanol
and the mixture was stirred for 1 week at room tempera-
ture. Upon removal of the solvent and ethylenediamine by
evaporation and freeze-drying, C18AA was obtained as a
light yellow solid. The crude solid was recrystallized from
a mixed solvent of toluene and methanol. Yield: 90%.
1H NMR(400 MHz, CDCl3, TMS): ꢁ 0.88 (t, 3 H,
(44.6 mmol) of 4-n-Ethylaniline in 100 mL of water con-
ꢀ
taining 11 mL (0.13 mol) of 12 N HCl at 0 C (in an ice
bath) for 30 min. The reaction solution was dropped into
14.98 g of o-cresol in 150 mL water containing 2.20 g
(55.0 mmol) sodium hydroxide at 0 ꢀC for 1 h. After
the reaction mixture was neutralized with 1 N HCl, the
precipitate was filtered and recrystallized using methanol
and water. 2AMOH was obtained as a brownish yellow
solid.
2.1.2. [4-(10-Bromo-dodecyloxy)-3-methyl-phenyl]-
(4-ethoxy-phenyl)-Diazene (2AM10Br)
Twenty grams (66.7 mmol) of 1,10-Dibromodecane was
dropped into 5.00 g (20.8 mmol) of 2AMOH and 10.0 g
(72.4 mmol) of potassium carbonate were dissolved in
150 mL of dehydrate acetone and refluxed for 12 h
in a nitrogen atmosphere at 60 ꢀC. After the mix-
ture was extracted to chloroform, the chloroform solu-
tion was washed with water and evaporated to obtain
crude 2AM10Br as orange-colored solid. The crude
2AM10Br was purified by column chromatography using
a mixed solvent of chloroform and hexane (1:1). After
extract evaporation, 2AM10Br was obtained followed by
recrystallization using methanol.
CH3ꢂ, 1.25 (br, 28 H, CH2ꢂ, 1.45 (br,
4 H,
CH2CH3, CH2CH2CH2N), 2.36 (t, 4 H, CH2CH2O), 2.42
(t, 2 H, CH2N), 2.73 (t, 4 H, NCH2CH2CO), 2.82
(t, 4 H, CH2NH2ꢂ, and 3.29 (q, 4 H, NHCH2ꢂ ppm.
HRMS: calcd for C18AA (M+H+) 498.47, found 497.48.
2.3. Preparation and Characterization of Ag2S NPs
Ag2S NPs capped with C18AA were prepared using
a method modified from Brust et al.44 Ten milliliters
of a 7.5 mM aqueous solution of Na3Ag(S2O3ꢂ2 were
J. Nanosci. Nanotechnol. 12, 648–655, 2012
649

Reversible Dispersion and Aggregation of Ag2S Nanoparticles Capped with Azobenzene-Derivatized Alkanethiols
Ishisone et al.
added to 0.15 mmol of the phase-transfer reagent,
(C8H17ꢂ4 NBr, dissolved in toluene (15 mL), and the
mixture was stirred for 15 min. The toluene phase
was subsequently collected and 15 mL of the desired
concentration of C18AA in toluene were added. After
the solution was stirred for 2 h, 0.075 mmol of sodium
borohydride were added. The reaction mixture was vig-
orously stirred for 24 h. The particle size was controlled
by changing the molar ratio of Na3Ag(S2O3ꢂ2 to C18AA
([Na3Ag(S2O3ꢂ2]/[C18AA] = 0ꢃ33–3.1).
3. RESULTS AND DISCUSSION
3.1. Characterization of Ag2S NPs
Figure
2
shows the representative XRD pattern
of the product prepared under the condition of
[Na3Ag(S2O3ꢂ2]/[C18AA] = 1. Similar XRD patterns
were obtained from other molar ratios of C18AA to
Na3Ag(S2O3ꢂ2. All peaks except for 2ꢅ = 39ꢃ06 and
41.62ꢀ can be assigned to monoclinic ꢇ-Ag2S with lattice
constants of a = 0.42 nm, b = 0ꢃ69 nm, and c = 0.79 nm—
values in agreement with those in JCPDS card 14-72. The
peaks at 2ꢅ = 39ꢃ06 and 41.62ꢀ were assigned to (111)
and (200) of metallic Ag, respectively. Thus, the product
contained small amounts of metallic silver as impurities.
Figure 3 shows the typical TEM micrographs of the
resulting Ag2S NPs. The particle size was strongly affected
by the molar ratio of Na3Ag(S2O3ꢂ2 to C18AA and
increased with increasing molar ratio. Namely, Ag2S
NPs of 3.1 0.6 (Fig. 3(a)), 8ꢃ1 1ꢃ0 (Fig. 3(b)),
and 10ꢃ2 1ꢃ6 nm (Fig. 3(c)) were obtained for
[Na3Ag(S2O3ꢂ2]/[C18AA] = 0ꢃ33, 1.5, and 1.0, respec-
tively. However, we could not obtain Ag2S particles larger
than 10 nm because the particle size was unchanged at
ratios of [Na3Ag(S2O3ꢂ2]/[C18AA] > 1. Ag2S NPs of
20 nm (Fig. 3(d))ꢀ were then obtained by heating 10 nm
Ag2S NPs at 150 C for 15 min.
Ligand exchange of Ag2S NPs from C18AA to
2AM10SH or 8AM5SH was conducted in the follow-
ing manner. 2AM10SH or 8AM5SH (0.15 mmol) was
added to the Ag2S NP solution (3.1 mL), and the mixture
was stirred for 24 h. The resulting particles were washed
twice with a solvent mixture of toluene and ethanol (vol-
ume fraction 1:4) to remove the phase transfer catalyst,
excess azobenzene derivatives, and reaction byproducts.
After washing the NP dispersion twice by centrifuga-
tion, the supernatant was colorless, indicating that excess
azobenzene derivatives were completely removed.
2.4. Isomerization of Azobenzene Derivatives and
UV-Visible Measurements
The effect of changing the ligand from C18AA to
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Trans-to-cis isomerization of 2AM10SH and 8AM5SH
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2AM10SH (or 8AM5SH) on the particle size was exam-
was carried out by irradiation of either UV or visible light.
Light from a 300 W Xe lamp was passed through either
a Toshiba UV-360 or an L-42 optical glass filter to pro-
duce UV of a selected wavelength or visible light, respec-
tively. UV-visible spectra were obtained using a V-500
spectrophotometer (JASCO). The trans-to-cis isomeriza-
tion conversion was estimated from the decrease in the
intensity of the ꢄ–ꢄ* band of 2AM10SH and 8AM5SH
at 360 nm. UV-visible spectra were measured immediately
in the dark after exposure to UV or visible light.
Copyright: American Scientific Publishers
ined by TEM observations. We could not detect any differ-
ence in the particle size or shape between before and after
ligand exchange, however, a new peak at 350 nm, which is
assigned to the ꢄ–ꢄ* band of trans-azobenzene, appeared
in the UV-vis spectra after the ligand exchange (Fig. 4).
Therefore, we concluded that the exchange of ligands on
Ag2S NPs from C18AA to 2AM10SH or 8AM5SH was
entirely successful.
3.2. Photoisomerization of 2AM10SH on Ag2S NPs
Figure 5 shows the UV-vis spectra of 2AM10SH on 8.1 nm
Ag2S NPs dispersed in toluene under irradiation with
2.5. TEM and XRD Measurements
Transmission electron microscopy (TEM) samples of Ag2S
NPs capped with C18AA were prepared by placing a drop
of the Ag2S NP dispersion on a 100-mesh carbon-covered
copper TEM grid. However, in order to clarify the coagu-
lation behavior of Ag2S NPs capped with 2AM10SH and
8AM5SH, the copper TEM grid was immersed in a dis-
persion of the Ag2S NPs for a few minutes, after which
the grid was gently rinsed with toluene. The samples were
examined using an H-9500 transmission electron micro-
scope (HITACHI). X-ray powder diffraction (XRD) pat-
terns were recorded at a scanning rate of 4ꢀ/min in the 2ꢅ
range of 20–60ꢀ using a Rigaku UltimaIV diffractometer
with CuKꢆ radiation.
Fig. 2. X-ray powder diffraction pattern of the product.
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Ishisone et al.
Reversible Dispersion and Aggregation of Ag2S Nanoparticles Capped with Azobenzene-Derivatized Alkanethiols
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Fig. 3. TEM images of Ag2S NPs obtained under the condition of [Na3Ag(S2O3ꢂ2]/[C18AA] = (a) 0.33, (b) 0.5 and (c) 1.0. (d) TEM image of Ag2S
NPs prepared by heating the sample (c) at 150 ꢀC for 15 min.
visible and UV lights. The peaks at 350 and 450 nm are
assigned to the ꢄ–ꢄ* band of trans-azobenzene and the
n–ꢄ* band of characteristic cis-azobenzene, respectively.
Note that the peak position of the former band was almost
the same as that in the toluene solution, indicating that the
2AM10SH molecules on the Ag2S NPs do not form H- and
J-aggregates. Irradiation with UV light resulted in a signif-
icant decrease in the ꢄ–ꢄ* band of the trans isomer, and
the appearance of n–ꢄ* band of cis isomer. When visible
light was irradiated, the intensity of the ꢄ–ꢄ* band recov-
ered and the n–ꢄ* band disappeared. The dependence of
the intensity of the ꢄ–ꢄ* band on the irradiation time with
alternating UV and visible lights is shown in Figure 6. It is
apparent that trans-to-cis isomerization of 2AM10SH on
Ag2S NPs is completely reversible for numerous cycles.
The trans-to-cis isomerization conversion of 2AM10SH
on Ag2S NPs was estimated from the decrease in the
ꢄ–ꢄ* band intensity to be more than 95%. In gen-
eral, the conversion rates of azobenzene derivatives with
long chains in highly organized films such as self-
assembled monolayers (SAMs), LB films, or Langmuir
monolayers are small. In previous papers,9 the con-
version of 8AM5SH and di[10-(4-ethoxyphenylazo)-2-
methyl-phenoxydecanyl]sulfide, (2AM10)2SH] SAM on
flat Au plates were 20–40%, whereas those of SAM on Au
NPs were 70–80%. This is due to the large curvature of the
NPs, which would ensure sufficient distance between adja-
cent azobenzene moieties. Thus, the photoisomerization of
2AM10SH on Ag2S was more efficient than analogous
azobenzene derivatives on Au NPs. This improvement in
conversion is thought to be due to the lower adsorp-
tion density of 2AM10SH on Ag2S, because 2AM10SH
molecules adsorb only on the Ag sites of Ag2S surface and
not on the S sites.
The photo-response behavior of 2AM10SH adsorbed
on Ag2S NPs of different sizes was similar to that on
3.1 nm Ag2S NPs, as shown in Figure 7. In the case
of 20 nm Ag2S, although the ꢄ–ꢄ* band intensity was
quite low, regular changes in intensity were observed
under alternating irradiation of UV and visible lights.
Thus, the reversible trans-to-cis isomerization behavior of
2AM10SH on Ag2S NPs was found to be independent
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Reversible Dispersion and Aggregation of Ag2S Nanoparticles Capped with Azobenzene-Derivatized Alkanethiols
Ishisone et al.
(a)
Fig. 6. Changes in the intensity of the ꢄ–ꢄ* band at 350 nm for
8.1 nm Ag2S NPs with alternating irradiation by UV and visible lights.
Irradiation times were 20 and 40 min for UV and visible lights,
respectively.
(b)
due to the decrease in the total surface area of the NPs,
and (ii) a rising of the baseline in a shorter wavelength
region of UV-vis spectrum due to a large scattering cross
section of light.
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Fig. 4. UV-vis spectra of 8.1 nm Ag S NPs (a) before and (b) after
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2
ligand exchange from C18AA to 2AM10SH dispersed in toluene.
of particle size. The low intensity of the ꢄ–ꢄ* band for
20 nm Ag2S NPs is probably caused by the following
effects. The increase of the particle size brings about (i) a
decrease in the total amount of 2AM10SH on Ag2S NPs
Fig. 5. UV-vis spectra of 8.1 nm Ag2S NPs capped with 2AM10SH
dispersed in toluene. The initial state (1) was first irradiated with UV
light for 20 min (2). It was subsequently irradiated with visible light for
40 min (3).
Fig. 7. UV-vis spectra of Ag2S NPs capped with 2AM10SH dispersed
in toluene under irradiation with UV (dashed line) and visible (solid line)
lights. Particle size: (a) 3.1 nm, (b) 10 nm, and (c) 20 nm.
652
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Ishisone et al.
Reversible Dispersion and Aggregation of Ag2S Nanoparticles Capped with Azobenzene-Derivatized Alkanethiols
Delivered by Publishing Technology to: Chinese University of Hong Kong
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Fig. 8. TEM images of 8.1 nm Ag S NPs capped with 2AM10SH under alternating irradiation with (a and c) visible light for 40 min and (b and d)
Copyright: American Scientific Publishers
2
UV light for 20 min.
3.3. Dispersion–Aggregation Behavior of Ag2S
easily be controlled for many cycles by changing the con-
formation of 2AM10SH, i.e., by switching between the
trans and cis forms of 2AM10SH.
The stability of the Ag2S NP dispersion is to some
extent determined by the balance of the following fac-
tors: the steric interaction between 2AM10SH molecules,
van der Waals interaction between the NPs, and affin-
ity between the solvent and 2AM10SH. The aggregation
of Ag2S NPs capped with cis-2AM10SH is presumably
Figure 8 shows the TEM images of 8.1 nm Ag2S NPs
capped with 2AM10SH under alternating irradiation with
visible and UV lights. Irradiation with UV light resulted in
the formation of many aggregates of Ag2S NPs, whereas
the aggregates reverted to the dispersed state by irradia-
tion with visible light. Thus, this result indicated that the
dispersion–aggregation states of Ag2S NPs in toluene can
Fig. 9. TEM images of 8.1 nm Ag2S NPs capped with 8AM5SH under irradiations with (a) UV light for 20 min and (b) visible light for 40 min.
J. Nanosci. Nanotechnol. 12, 648–655, 2012
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Reversible Dispersion and Aggregation of Ag2S Nanoparticles Capped with Azobenzene-Derivatized Alkanethiols
Ishisone et al.
caused by the difference in the functional group of the
outermost surface of 2AM10SH-capped Ag2S NPs. The
outermost surfaces of Ag2S NPs in the trans and cis forms
of 2AM10SH are probably the tail part of the hydrocarbon
chain and the azobenzene moiety of 2AM10SH, respec-
tively. Given that the presence of a hydrocarbon chain
is considered preferable in terms of Ag2S NP dispersion
in toluene, whereas the azobenzene moiety is likely to
have a low affinity for toluene, the Ag2S NPs capped
with cis-2AM10SH aggregate. This was consistent with
the fact that 8.1 nm Ag2S NPs capped with cis-8AM5SH,
which are almost the same size as the Ag2S NPs capped
with 2AM10SH in Figure 8, did not aggregate in toluene
(Fig. 9). The reason for this is that the outermost group,
even for Ag2S NPs capped with cis-8AM5SH, is thought
to be a hydrocarbon chain rather than the azobenzene moi-
ety because 8AM5SH has a longer terminal hydrocarbon
chain than 2AM10SH.
Figure 10 shows the effect of particle size on the aggre-
gation behavior of Ag2S NPs under irradiation with UV
and visible lights. A similar dispersion–aggregation reg-
ulation of Ag2S NPs was effectively accomplished using
particle sizes of 10.2 and 20 nm. However, NPs with
a smaller size of 3.1 nm always maintained a dispersed
state in toluene regardless of whether 2AM10SH was in
the trans or cis conformation. As mentioned above, the
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Fig. 10. TEM images of Ag2S NPs capped with 2AM10SH under irradiations with (b, d and f) UV light for 20 min and (a, c and e) visible light for
40 min. Particle size: (a and b) 3.1 nm, (c and d) 10 nm, and (e and f) 20 nm.
654
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Ishisone et al.
Reversible Dispersion and Aggregation of Ag2S Nanoparticles Capped with Azobenzene-Derivatized Alkanethiols
stability of the Ag2S NP dispersion depends in part on
the van der Waals interaction between the NPs. Thus, the
strength of the van der Waals interaction between 3.1 nm
Ag2S NPs capped with cis-2AM10SH is insufficient to
aggregate them, even though the cis-2AM10SH molecules
have a conformation with a lower affinity for toluene.
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4. CONCLUSION
We prepared Ag2S NPs capped with two kinds
of azobenzene-derivatized alkanethiol, 2AM10SH and
8AM5SH, and investigated the dispersion–aggregation
behavior of the Ag2S NPs in toluene. We successfully con-
trolled the reversible aggregation–dispersion behavior for
Ag2S NPs of 8.1 nm or more capped with 2AM10SH
by alternating irradiation with UV and visible lights. On
the other hand, the dispersions of 8.1 nm Ag2S NPs
capped with 8AM5SH and 3.1 nm Ag2S NPs capped with
2AM10SH were stable, regardless of whether 8AM5SH
was in the trans or cis conformation.
Acknowledgments: This work was partially supported
by a Grant-Aid for Scientific Research B from the Ministry
of Education, Culture, Sports, Science, and Technology of
Japan.
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Received: 2 November 2010. Accepted: 1 February 2011.
J. Nanosci. Nanotechnol. 12, 648–655, 2012
655