REACTIONS OF METHYL RADICALS WITH CU°
NANOPARTICLES IN AQUEOUS SUSPENSIONS
T. Zidki1, A. Elisseev1,2, H. Cohen1,3 and D. Meyerstein1,3
1
Department of Biological Chemistry, Ariel University, Ariel,
Israel 40700
2
Chemistry Institute, The Hebrew University of Jerusalem,
Jerusalem 91904, Israel
3
Department of Chemistry, Ben-Gurion University, P.O. Box 653,
Beer Sheva 84105, Israel
tomerzi@ariel.ac.il
Abstract
Radical reactions at surfaces have been suggested to be
involved in various chemical processes. It is of interest to study
these processes in order to understand the mechanisms of
homogeneous or heterogeneous catalytic reactions. It was shown
that the powders of iron, chromium, manganese, cobalt, nickel, and
zinc reduce the methyl radicals to produce methane, while copper
powder oxidizes the methyl radicals to produce methanol. Another
study has shown that gold and silver nanoparticles (NPs) catalyze
the radicals' dimerization to produce ethane. It was decided to
explore whether the behavior of copper NPs (the same chemical
family of gold and silver) would behave similarly to that of gold
and silver NPs and catalyze the dimerization of the methyl radicals
or it will oxidize the radicals as the copper powder does.
In this study the reaction between methyl radicals, ·CH3 and
copper NPs was investigated. As it is essential to produce the NPs
without any organic stabilizers (that might interfere with the
experimental results) the Cu°-NPs synthesis was carried out via
reduction of cupric ions with NaBH4 resulting in production of 3-4
nm relatively stable copper NPs.
Radiolytic production of ·CH3 ((CH3)2SO solutions saturated
with nitrous oxide) in the presence of Cu°-NPs produced methane
as the main product, i.e., the radicals oxidize Cu°-NPs. It is
proposed the observation that methane and not methanol is formed
(oxidation of Cu°-NPs vs. reduction of copper powder) might be
due to the difference in the redox potential of copper from that of
silver and gold and of Cu°-NPs from the metallic copper.
259
Introduction
Radical reactions at surfaces have been suggested to be
involved in various chemical processes, e.g. heterogeneous radical
induced catalytic processes; electrochemical processes;
photochemical processes; environmental processes. Therefore it is
of interest to study these processes in order to understand the
mechanisms of homogeneous or heterogeneous catalytic reactions.
For many years, the properties of metal nanoparticles (NPs) as
catalysts have attracted the attention of many scientists. This is
mainly due to the fact that their surface-to-volume ratio is
considerably larger than that of the traditional catalysts. Metal NPs
react with strong single electron reducing radicals in aqueous
solutions.1-5 There are several plausible mechanisms for the
reactions between these radicals and metal NPs.
When the NP acts as a catalyst for the reduction of water the
radical, R·, is oxidized and ROH is formed via the following
mechanism1, 6, 7:
R + (NP)
R+ + (NP)-
(1)
Reaction (1) may be repeated n times to yield (NP)n-.
(NP)n- + H3O+/H2O
(NP)―H(n-1)- + H2O/OH-
(2)
Reaction (2) may be repeated m times to yield (NP) ―Hm(n-m)-.
(NP)―Hm(n-m)- → (NP)―Hm-2(n-m)- + H2
R+ + H2O → ROH + H+
(3)
(4)
However the radical might also oxidize the NP via:
R + (NP) → R- + (NP)+
R- + H3O+/H2O → RH + H2O/OH-
(5)
(6)
If reaction (5) is repeated m times forming (NP)m+, it will be
followed by:
(NP)m+ → Mn+ + (NP)(m-n)+
260
(7)
Where M is the metal of which the NP is composed and n is
the lowest common oxidation state of this metal. It should be noted
that Mn+ can hydrolyze water to yield MlOk or MlOi(OH)j.
There is another possibility, i.e., the catalytic formation of
R2, or disproportionataion of ·R on the surface of the metal NP, i.e.
a process that does not involve a redox process of the NP. R2 might
be formed via one of two plausible mechanisms, which were
proposed by Henglein:7
I.
R + (NP) → R―(NP)
R―(NP) + R → (NP) + R―R / R+ + R-
(8)
(9)
II. When several radicals react with the NP followed by
radical migration on the surface to produce R2 via dimerization:
R―(NP) + (n-1) R → (NP)―Rn
(NP)―Rn → (NP)―Rn-2 + R―R / R+ + R-
(10)
(11)
When the methyl-radicals are formed, at a low dose rate in
the presence of (CH3)2S=O, methane and ethane are formed via:8, 9
CH3 + (CH3)2S=O → CH4 + CH2S(O)CH3
k129 = 100 M-1s-1
CH3 + CH3 → C2H6
k138 = 1.6 x 109 M-1s-1
(12)
)31(
Previous studies show that powders of iron, chromium,
manganese, cobalt, nickel, and zinc reduce the methyl radicals to
produce methane, while copper powder oxidizes the methyl
radicals to produce methanol.9, 10 Another study has shown that
gold and silver nanoparticles (NPs) catalyze the radicals'
dimerization to produce ethane.6 In addition, recent studies have
also shown that the lifetime of the transients M°-NP-(CH3)n is long
and that Pt°-NPs catalyze different reaction at different active sites
on the NP.11, 12 It was decided to explore whether the behavior of
copper NPs (the same chemical family of gold and silver) behave
similarly to gold and silver NPs and catalyze the dimerization of
the methyl radicals or oxidize the radicals as the copper powder
does.
261
Cu°-NPs are not stable in aerated solutions and they have to be
kept under an inert atmosphere.13
In this study the reaction between methyl radicals, ·CH3 and
copper NPs was investigated. As it is essential to produce the NPs
without any organic stabilizers (that might react with the methyl
radicals) the Cu°-NPs synthesis was carried out via reduction of
cupric ions with NaBH4 resulting in the formation of 3-4 nm
relatively stable copper NPs. The reactions of these Cu°-NPs with
methyl-radicals are reported.
Experimental
Materials. All chemicals were of A.R. grade and were used
without further purification. The water used was deionized and
was further purified by a Millipore Milli-Q setup with a final
resistivity of > 10 MΩ cm.
Instrumentation. UV-Vis measurements were carried out
using an Agilent Diode Array spectrophotometer model 8453,
which enables measurements in the range of 190–1100 nm and
resolution of < 2 nm. TEM analyses were performed using a
Tecnai F20 G2 high resolution TEM (FEI). The NP solutions were
dried AGAR carbon films on 400 mesh nickel from BELGAR.
Irradiations were performed in a 137Cs gamma source (Radiation
Machinery Corporation Parsippany, NJ), which emits  rays of
0.662 Mev. The dose rate delivered to the sample by the 137Cs
source, as determined by the Fricke dosimetry, was 6.9 Gy min-1.14
The product gases were analyzed using a gas-chromatograph
(Varian model 3800) equipped with flame ionization and thermal
conductivity detectors connected in series. The gases were
separated on a carbosieve B 1/8′′, 9′ stainless steel column using
He as the carrier gas. The gaseous atmosphere was sampled (1 mL
samples) after the irradiation, with gas-tight syringes (Precision
Syringes, model A2).
Cu°-NPs syntheses. The copper NPs (Cu°-NPs) suspensions
are -irradiated and are thus in contact with the highly reactive
hydroxyl and methyl radicals. Therefore the Cu°-NPs suspensions
should be free from stabilizers (and in particular organic
stabilizers) which can react with the radicals and affect the results.
For that reason the Cu°-NPs suspensions were prepared according
to the modified Creighton’s procedure,15 by reducing the cupric
262
ions with NaBH4 in aqueous solution. This procedure does not
necessitate the additions of stabilizers since the resultant borate
ions stabilize the NPs.16 The reduction was done by adding 10 mL
of 82.5 mM NaBH4 at once to 110 mL CuSO4 (1.08 mM, pH 5.5 ±
0.2) under vigorous stirring. The almost colorless (very light blue)
solution immediately turned brown and then slight color changes
occurred. After ca. 30 min the color stabilized as reddish-brown
and the pH was 10 ± 0.2. The resultant Cu°-NPs suspension was
stable up to ca. 4 h. After this time gap, the suspensions turned
blue (different and darker blue than the blue color of the cupric
ions) and at later times, the copper precipitated as black particles.
In order to prevent the color change and the precipitation, the
suspensions were de-aerated with argon 30 min after preparation
(copper particles precipitated when the reduction of Cu2+ was
performed under argon atmosphere). All the irradiation
experiments were done one day after preparation to ensure full
decomposition of the excessive NaBH4. In order to study the
reaction of between the radicals and oxidized NPs, the Arsaturated Cu°-NPs suspension was opened to the atmosphere one
day after preparation for 30 min and then de-aerated using N2O.
The color of this suspension changed from reddish-brown to
greenish-blue. This suspension is marked as ox-Cu°-NPs.
Irradiation. 6 mL of the Cu°-NPs and ox-Cu°-NPs
suspensions (at the original concentration or diluted with water)
containing 0.1 M (CH3)2S=O were transferred to small sealable
glass bulbs (15 mL). These bulbs were sealed with rubber septa
and were deaerated by bubbling N2O for 15 min. The blank
solution was an aqueous solution of borate (the reduction byproduct) containing 0.1 M(CH3)2S=O without the NPs. These
bulbs were irradiated using the -source. After the irradiation, the
gas phase was analyzed using the GC. The results from the GC
analysis (Tables 2-3) are converted to G-value which is defined as
the number of molecules of each product per 100 eV of radiation
absorbed by the solution.
Results and Discussion
Cu°-NPs characterization. The UV-Vis spectrum of the Cu°NPs and ox-Cu°-NPs are shown in Fig. 1. The spectrum in Fig. 1a
shows a typical plasmon band at 565± 2 nm,17 which indicates
263
Cu°-NPs formation.17 Furthermore, this spectrum is stable for at
least one day, thus the Cu°-NPs are stable during that period of
time. This time gap is required for NaBH4 to decompose before the
suspension is irradiated and the reaction between the Cu°-NPs and
the radicals takes place. Fig. 1b shows the spectrum of the oxidized
Cu°-NPs which differs from Fig. 1a due to the oxidation of the
particles and the corresponding color change. The plasmon peak is
red-shifted to 660 ± 10 nm. According to Mie theory,18 this shift
can be due to larger aggregates of the copper or due to less
negative charge on the NPs as a result of the oxidation.
Fig. 1. UV-Vis spectrum of. a) Cu°-NPs, the spectrum taken immediately
after preparation; b) ox-Cu°-NPs spectrum taken after exposure of
30 min air, one day after preparation.
Fig. 2 shows a high resolution TEM micrograph of the
synthesized Cu°-NPs. It can be seen that the Cu°-NPs are small, 3-
264
4 nm. From the measured size of the NPs, one can calculate their
molar concentration, [Cu°-NPs], using Equations (14)-(15):
n
3
M NP 4RNP
N

 A
m Atom
3
MW
[ NP] 
C
n
(14)
(15)
Where: n is the number of metal atoms in a NP; M NP is the
mass of the NP; mAtom is the mass of a Cu atom; RNP is the average
metal NPs’ radius; ρ is the density of the metal, and equals
8.96 g cm-3;19 NA is Avogadro's number; MW is the molar weight of
the Cu; C is the molar concentration of the cupric ion precursor
and equals 0.99 mM. The calculated average number of Cu atoms
in one particle is n = 1900 Cu atoms/NP. From this number, the
[Cu°-NPs] was calculated to be 5.2x10-7 M.
The copper NPs have fringes and twin-lines which indicate
the poly-crystalline structure of the NPs. The Cu°-NPs are
spherical and grow as individual particles.
Fig. 2. HR-TEM micrograph of freshly prepared Ar-saturated Cu°-NPs.
The Cu°-NPs suspensions have to be N2O-saturated prior to
the -irradiation in order to convert the hydrated electrons into
265
hydroxyl radicals, ∙OH.20 When the Cu°-NPs suspensions are
saturated with N2O, slow color changes are observed (within two
days): the reddish-brown color changes to greenish-blue. In order
to explore the effect of N2O on the particles, TEM micrographs
were taken as well as Energy Dispersive Spectroscopy (EDS) and
Selected Area Diffraction (SAD) analyses. Fig. 2 and 3 compare
the morphology between freshly prepared Cu°-NPs (Fig. 2), N2Osaturated suspension of Cu°-NPs (Fig. 3a) and ox-Cu°-NPs (Fig.
3b).
a
b
Oxide
Layer
Fig. 3. TEM micrographs. a) N2O-saturated suspension of Cu°-NPs–
brown suspension; b) ox-Cu°-NPs - blue suspension.
The results point out that the N2O affects the NPs
morphology. The particles in Fig. 3a are partially aggregated and
are covered with a thin white layer which indicates oxidation of the
copper. When this solution is oxidized by air (oxygen) its color
changes to greenish-blue and larger aggregates are produced with a
significantly thicker oxide-layer (white layer and white rectangles
marked with arrows in Fig. 3b). The EDS (Table 1) and SAD (fig.
5) analyses support the copper-oxidation assumption.
The three suspensions (freshly prepared Cu°-NPs, N2Osaturated Cu°-NPs for several hours and ox-Cu°-NPs) were
analyzed using the EDS and SAD techniques, Fig. 5. The EDS
results in Table 1 point out that the Cu°-NPs are indeed oxidized
upon exposure to N2O and oxygen as the atomic % of oxygen
increases under these exposures. The atomic % of oxygen
266
increases from 19.4 to 24.4 % for Cu°-NPs and N2O-saturated
Cu°-NPs, respectively and to 38.7 % for ox-Cu°-NPs. Also, Table
1 shows the ratios between Cu° atoms and oxidized Cu atoms (in
CuO or in Cu2O) from the data collected from the areas marked in
Fig. 4. These results indicate that a significant portion of the
copper atoms on the particles surfaces are oxidized upon exposure
to mild oxidants such as N2O and atmospheric air. It should be
pointed out that the oxidation of the Cu°-NPs results in a color
change of the suspension and, at longer periods, precipitation of
the particles. This is in contrast to the finding that Cu°-NPs are
dissolved upon oxidation with air.21 In the latter case, a ligand was
present that creates a soluble copper complex, thus shifting the
redox potential of the copper, and favors its dissolution.
Table 1. Numeric data from EDS analyses of freshly prepared Arsaturated Cu°-NPs, N2O-saturated suspension of Cu°-NPs and ox-Cu°NPs.
Freshly prepared Ar-saturated Cu°-NPs – brown suspension
Element
Weight %
Atomic %
O(K)
5.8
19.4
Cu(K)
92.9
79.0
Cu:O
Cu:O
(CuO)*
Cu:O
(Cu2O)**
4:1
3:1
2:1
N2O-saturated suspension of Cu°-NPs – brown suspension
O(K)
7.5
24.4
Cu(K)
92.5
75.6
3:1
2:1
1:1
ox-Cu°-NPs – blue suspension
All of Cu
are
3:2
1:2
oxidized
Cu(K)
86.3
61.3
as Cu2O
*
The theoretical ratio between Cu° atoms and oxidized Cu atoms in CuO.
**
The theoretical ratio between Cu° atoms and oxidized Cu atoms in
Cu2O.
O(K)
13.7
38.7
267
Fig. 4. TEM micrographs, the marked areas represent the regions from
which the EDS analyses were taken. a) freshly prepared Arsaturated Cu°-NPs; b) N2O-saturated suspension of Cu°-NPs; c)
ox-Cu°-NPs.
In order to find which copper oxide species is in the
suspension, SAD analyses were performed, Fig. 5. The SAD
analysis provides a tool to distinguish between crystal structures.
Crystal structures have data-cards in the database of the
International Centre for Diffraction Data (ICDD). The conclusions
from Fig. 5 are that the d-spacing of freshly prepared Ar-saturated
Cu°-NPs, N2O-saturated suspension of Cu°-NPs and ox-Cu°-NPs
fit to copper,22 CuO23 and Cu2O,24 respectively. Clearly, these are
the dominant structures observed and probably on all the NPs
mixtures of the oxides/hydroxides are present.
268
Fig. 5. SAD analyses of: a) freshly prepared Ar-saturated Cu°-NPs; b)
N2O-saturated suspension of Cu°-NPs; c) ox-Cu°-NPs.
Stable copper NPs were synthesized using sodium
borohydride without adding any stabilizer. The environment in
which the Cu°-NPs are synthesized or kept is crucial for
stabilization. On one hand the synthesis has to be performed under
air (the NPs precipitated when the synthesis is performed under
Ar) while on the other hand the NPs suspensions have to be kept
under Ar. It is suggested that the formation of a partial oxide layer
(CuO or Cu2O) is important for the stabilization of the Cu°-NPs.
The oxide layer probably prevents the aggregation of the copper
NPs to big aggregates that are thermodynamically more stable.
269
However, when the NPs are fully-oxidized the stabilization of the
suspension decreases resulting in precipitation.
Irradiation of Cu°-NPs: N2O-saturated Cu°-NPs suspensions
containing (CH3)2S=O were irradiated in the -source, within one
hour of N2O-saturation, with a total dose of 69 and 207 Gy. Under
these conditions, in the absence of the NPs, methane and ethane
are formed via reactions (12) & (13).
Table 2 summarizes the irradiation results. It can be seen that
the total yields of carbonaceous materials (G-value) is 6 within the
error-limit (15%), which indicates that methane and ethane are the
only significant products, so one can assume that no other product
is formed.
Table 2. GC analysis of the gases formed by the -irradiation of the Cu°NPs suspensions.
69
2.1
1.5
5.1
G(methane)/
G(ethane)
1.4
69
4.0
1.2
6.4
3.3
69
2.9
1.5
5.9
1.9
207
2.3
1.6
5.5
1.4
207
4.2
1.1
6.4
3.8
Description* Dose, Gy G(methane) G(ethane) G(total)†
Blank‡
¥
Cu°-NPs
Cu°-NPs/2
Blank
¥
Cu°-NPs
§
Cu°-NPs/2§ 207
2.2
1.7
5.6
1.3
The results’ error-limit is ± 15%. This error-limit is high as there are
several sources for error, e.g.: homogeneity of the -source, GC analysis
error, gas phase volume, etc.
The results are averages of at least three runs of each experiment.
*
All the solutions contained 0.1 M (CH3)2S=O and were N2O-saturated,
pH 10.
†
G(total) = G(CH4) + 2G(C2H6).
‡
A NaBH4 solution that was kept overnight so that all the BH4decomposed to yield borate.
¥
[Cu°-NPs] = 5.2x10-7 M
§
[Cu°-NPs]/2 – two times diluted Cu°-NPs suspension – 2.6x10-7 M.
The results presented in Table 2 clearly point out that the
Cu°-NPs react with the methyl radicals. As a result of this reaction
270
the CH4 yield increases whereas the C2H6 yield decreases. The
proposed mechanism for methane formation involves two steps:
(Cu°-NP) + nCH3.  (Cu°-NP)-(CH3)n
(16)
followed by:
(Cu°-NP)-(CH3)n+mH2O(Cu°-NP)m+-(CH3)n-m+mCH4+mOH-
(17)
The (Cu°-NP)m+─(CH3)n-m probably reacts with water to
form more oxides/hydroxides on the surface of the NP. This
mechanism includes two major stages: in the first stage methyl
radicals bind to the partially oxidized Cu°-NP to yield the transient
(Cu°-NP)-(CH3)n, an analogous reaction was reported for the
reaction of methyl radicals with other M°-NPs.6, 10-12 In the second
stage the M°-C bonds heterolyse, in contrast to observations for
other (M°-NP)-(CH3)n.6, 10-12
The ratio G(methane)/G(ethane) increases when the [Cu°NP] increases, see Table 2. The results indicate that most of the
methane is produced via reactions (16) & (17) and that reaction
(12) almost does not contribute to the methane yield, clearly in the
presence of the Cu°-NPs the yield of methane via reaction (12) is
(CH4) < 1. This means that in the presence of the Cu°-NPs the
steady-state concentration of ∙CH3 is lower than half the steadystate concentration in the blank solutions. Therefore, assuming that
all the ethane is formed via reaction (13), G(C2H6) should
decreases by a factor of 4. As G(C2H6) decreases only by ca. 30%,
one has to conclude that most of the ethane is formed at the surface
of the NPs. The ethane formation mechanism is illustrated in
Scheme 1:
Scheme 1. The proposed mechanism of ethane formation on the surface
of Cu°-NPs.
271
As the reaction of methyl radicals with ox-Cu°-NPs yields
ethane, see below, it is tempting to propose that the ethane is
formed on the oxidized sites of the partially oxidized Cu°-NPs. An
oxidized site may contain CuO, Cu2O or both. However, one
cannot rule out the option that the surface of the Cu°-NPs contains
different non-oxidized sites and that some of them catalyze the
ethane formation as Ag°-NPs and Au°-NPs do.6, 10 Since the Cu°NPs are poly-crystalline (Fig. 2), the methyl radicals might react
differently on each site (crystal structure) yielding different
products. This was also reported for the reaction of ∙CH3 radicals
with Pt°-NPs that yields CH4, C2H4 and C2H6.11
Copper powder oxidize methyl radicals to produce
methanol.9 The fact that Cu°-NPs reduce the radicals is attributed
to the redox properties of the copper that are size-dependent, i.e. to
the fact that Cu°-NPs are stronger reducing agents than bulk
copper metal. The CH3 has sufficient potential to oxidize copper
and to reduce water. Therefore, the reaction mechanism is
determined by the activation energy of the different reactions.
Irradiation of ox-Cu°-NPs: The Ar-saturated synthetized
Cu°-NPs were exposed to the atmosphere one day after preparation
for 30 min and then were de-aerated with N2O. The color of these
suspensions changed to greenish-blue as a result of oxidation of
the copper. The irradiation procedure was repeated using the same
method of the Cu°-NPs. The irradiation results are summarized in
Table 3.
The results presented in Table 3 clearly demonstrate that the
fully oxidized ox-Cu°-NPs (greenish-blue suspension) catalyze the
formation of ethane: the ratio G(methane)/G(ethane) decreases, i.e.
more ethane is produced while the methane yield is decreased
considerably. The suggested mechanism of the ethane formation is
analogous to that outlined in Scheme 1. This reaction is similar to
the reaction of TiO2-NPs with ∙CH3.25 Thus, the properties of oxCu°-NPs resemble the properties of the TiO2-NPs. As the copper
oxide is an insulator, the methyl radicals cannot oxidize the copper
and the fate of the radicals can only lead to dimerization yielding
ethane.
272
Table 3: GC analysis of the gases formed by the -irradiation of the oxCu°-NPs.
69
2.4
1.6
5.6
G(methane)/
G(ethane)
1.5
Ox-Cu°-NPs
69
0.49
2.5
5.49
0.20
Blank
207
2.6
1.7
6.0
1.5
Description* Dose, Gy G(methane) G(ethane) G(total)†
Blank‡
Ox-Cu°-NPs 207
0.46
2.4
5.26
0.19
The results’ error-limit is ± 15%. This error-limit is high as there are
several sources for error, e.g.: homogeneity of the -source, GC analysis
error, gas phase volume, etc. The results are averages of at least three
runs of each experiment.
*
All of the solutions contained 0.1 M (CH3)2S=O and were N2Osaturated, pH 10.
†
G(total) = G(CH4) + 2G(C2H6).
‡
A NaBH4 solution that was kept overnight so that all the BH 4decomposed to yield borate.
Concluding Remarks
1. Stable copper NPs were synthesized using sodium
borohydride without adding any stabilizer. The environment in
which the Cu°-NPs are synthesized or stored is crucial for
stabilization.
2. Oxygen does not dissolve Cu°-NPs unless a good ligand of
CuII is present.
3. Cu°-NPs reduce ∙CH3. This result is attributed to the size
dependence of the redox properties of copper.
4. Oxidized Cu°-NPs catalyze the dimerization of the methyl
radicals to yield ethane in a similar way that TiO2-NPs do.25
Acknowledgments:
This study was supported by a grant from the Ministry of
Science & Technology, Israel and the Russian Foundation Basic
Research. We are indebted to Professors Vladimir Shevchenko,
Joseph Rabani and Sara Goldstein for helpful discussions.
273
1.
2.
3.
4.
5.
6.
7.
8.
4.
11.
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