Selective reduction of nitroaromatic compounds on silver nanoparticles by visible light

Article abstract of DOI:10.1002/jrs.3143

For the first time, we report experimentally and theoretically that nitroaromatic compounds, 2,4-dinitrobenzenethiol and 4-chloro-2- nitrobenzenethiol, on silver sols can be selectively reduced to 2-amino-4-nitrobenzenethiol and 2-amino-4-chlorobenzenethiol simply by irradiating with a visible light in ambient conditions, and the selective photoreduction is a very facile process. The results of quantum chemical calculations are in good agreement with our experimental data. Copyright

Full text of DOI:10.1002/jrs.3143

Research Article
Received: 8 May 2011
Revised: 8 November 2011
Accepted: 12 November 2011
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/jrs.3143
Selective reduction of nitroaromatic compounds
on silver nanoparticles by visible light
Lixin Xia,a* Xinhu Hu,b Mengtao Sun,c* Jushi Li,a Donghui Yang,a
Xiaofang Wanga and Hongxing Xuc,d
For the first time, we report experimentally and theoretically that nitroaromatic compounds, 2,4-dinitrobenzenethiol and 4-chloro-2-
nitrobenzenethiol, on silver sols can be selectively reduced to 2-amino-4-nitrobenzenethiol and 2-amino-4-chlorobenzenethiol
simply by irradiating with a visible light in ambient conditions, and the selective photoreduction is a very facile process. The results
of quantum chemical calculations are in good agreement with our experimental data. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: 2,4-dinitrobenzenethiol; 4-chloro-2-nitrobenzenethiol; silver colloids; surface-enhanced Raman scattering spectroscopy;
selective photoreduction
Since the discovery of the photodecomposition of aromatic
sulfides, pyridine, pyrazine, and benzaldehyde adsorbed on silver
substrates using visible or near-ultraviolet radiation,[1,2] the
photoreactions of adsorbates on metal substrates have attracted
considerable interest because of potential applications in many
areas of science and technology, including optoelectronics,
chemical and biosensors, drug delivery, microfabrication, and
catalysis.[3] Various types of photoreactions on metal substrates
have subsequently appeared, including the photodecomposition
of rhodamine 6 G,[4] dimethyl cadmium,[5] triazine,[6] phthalazine,[7]
fluorobenzene,[8] and methyl orange dye[9]; the photodimerization
of pyridyl-substituted ethylenes,[10] substituted stilbenes,[11]
rhodanine,[12] and 4-aminobenzenethiol (4-ABT)[13–16]; and the
photoreduction of N-methylpyridinium and methyl viologen.[17–19]
Particularly, with the advent of more sensitive detection systems,
nitrobenzene derivatives have also been proved to undergo photo-
chemical reactions on rough silver surfaces, which used to be not
recognized to be able to readily undergo such reactions. For
instance, 4-nitrobenzoic acid (4-NBA)[3,20] and 4-nitrobenzenethiol
(4-NBT)[21–23] adsorbed on rough silver were converted in ambient
conditions to 4-aminobenzoic acid (4-ABA) and 4-ABT, respectively,
by radiation with a visible laser. As another example of 4-NBA, azo
dibenzoate has also been considered as the photoproduct.[20,24,25]
This is plausible because 4-ABA adsorbed on silver surface can also
undergo hydrolysis and dimerization to form azo dibenzoate by an
oxidative route when exposed to atmospheric moisture and laser
light.[26] In addition, which nitroaromatic compounds adsorbed silver
substrates were reduced by other methods have been reported in
sequence.[27–30] Thus, it would be very interesting to determine
whether a selective reduction for nitroaromatic compounds could
also take place on a metal surface under visible light radiation and
how such reaction proceeds if it could occur. To reach such a goal,
we synthesize first 2,4-DNBT and 4-C-2-NBT, and then attempt to
obtain the SERS spectra of 2,4-DNBT and 4-C-2-NBT on silver sols to
investigate if a selective photoreduction could take place in a
manner similar to that for 4-NBA or 4-NBT; on the basis of the
experiment data, we further see how such reaction proceeds and
expect to get a deeper insight into the photoreaction mechanisms,
which is still a matter of conjecture to this day. The synthesis strategy
of 2,4-DNBT is shown in Scheme 1.
Silver sols were prepared using the Lee–Meisel method.[31] (see
supporting information). The final Ag particle concentration in the
solution was estimated to be 3.5 Â 10À11 mol L–1. 2,4-DNBT and
4-C-2-NBT were prepared as low-concentration solutes in ethanol
and added to the silver sols resulting in final concentrations of 2,4-
DNBT and 4-C-2-NBT being around 1 Â 10À6 mol L–1. The mixture
was incubated for 3 h to allow for efficient adsorption. Prior to the
observation of photochemical reaction, field emission scanning
electron microscopy (FESEM) was used to analyze the surface mor-
phology and size distribution of the silver nanoparticles. According
to measurements made with FESEM, most of the silver particles were
approximately spherical in shape, with sizes ranging from 60 to
100 nm, and there was a portion of nanorods, which exhibited a mean
diameter of about 60 nm and lengths ranging from 90 to 170 nm.
Direct observation of a surface-enhanced photochemical reac-
tion has been investigated by means of surface-enhanced Raman
scattering (SERS) spectroscopy.[24–26] Here, we investigated the
feasibility of a selective reduction by monitoring the SERS spec-
trum of 2,4-DNBT adsorbed on the silver surface under the visible
*
Correspondence to: Lixin Xia, College of Chemistry, Liaoning University, 66
Chongshan Middle Road, Huanggu District, Shenyang 110036, China. E-mail:
lixinxia@lnu.edu.cn
Mengtao Sun, Beijing National Laboratory for Condensed Matter Physics, Insti-
tute of Physics, Chinese Academy of Sciences, Beijing 100190, China. E-mail:
mtsun@aphy.iphy.ac.cn
a College of Chemistry, Liaoning University, Shenyang 110036, China
b Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian
116023, China
c
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics,
Chinese Academy of Sciences, Beijing 100190, China
d Division of Solid State Physics, Lund University, Lund 22100, Sweden
J. Raman Spectrosc. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

Lixin Xia et al.
NO2
NO2
NO2
OH
OCSN(CH3)2
SCON(CH3)2
K2CO3/DMF
RT, N2, 24 h
(CH3)2NCSCl
+
+
O2N
O2N
O2N
NO2
SCON(CH3)2
pyrolysis
170 oC, N2, 1 h
H3O+
O2N
KOH (10%)
SH
Methanol, N2
O2N
NO2
Scheme 1. Synthesis of 2,4-DNBT.
light radiation at 632.8 nm in ambient air. The data acquisition
time was 20 s. As shown in Fig. 1, the complete absence of the
S–H stretching peak in the SERS spectrum, observable at
2546 cm–1 for pure 2,4-DNBT, indicates that the 2,4-DNBT is
chemisorbed onto the silver surface as thiolate after the S–H bond
cleavage. It is noteworthy that a new set of peaks appears, for in-
nitro group[21–23] still is prominent. These results indicates that only
the two-site nitro group of 2,4-DNBT was subjected to photoreac-
tion while the four-site nitro group was not subjected to change.
Although other molecules such as nitroso-, hydroxylamine-,
and azo compounds would also be produced by reduction
of nitroaromatic molecules,[32] no peak because of any such com-
pound is identified at all in our SERS spectrum. The procedure for
the selective photoreduction is depicted in Scheme 2.
[24–26]
stance, at 1042, and 875 cm–1. Referring to previous work,
the band at 1042 cm–1 is due to the in-plane C–H bending plus
the C–S stretching modes plus N–H deformation vibrations; the
band 875 cm–1 results from the C–H bending plus N–H deforma-
tion modes. Moreover, the asymmetric and symmetric stretching
vibration peaks of the original two-site nitro group at 1524 and
1306 cm–1 disappear compared with the Raman spectrum of pure
2,4-DNBT. On the other hand, the SERS peak at 1346 cm–1 that can
be assigned to the symmetric stretching vibration of the four-site
To obtain insight into experimental SERS spectra shown above,
the quantum chemical calculations (see supporting information)
were performed with GAUSSIAN 09 suite.[33] The assignment of
Raman peaks at 1524, 1346, 1306, 1042, and 875 cm–1 according
to the calculations can be seen in Fig. 2. Our qualitatively calcu-
lated results support strongly the experimental observations.
To confirm the fact that the results described above is caused
by the selective photoreduction of 2,4-DNBT or only by its incom-
plete photoreduction, the sample was radiated with the same
laser line for various lengths of time from 10 to 50 s (Fig. 3). The
changing spectra as a function of laser exposure time clearly
reveal the appearance of a new molecule as described above.
We assume that the SERS intensity is proportional to the quantity
of the species adsorbed on the surface. Thus, the intensity of the
SERS bands near 1453 and 1346 cm–1 can be used to represent
the amino and nitro group quantity of the photoproduct, respec-
tively, and thus their changes in intensity reflect that a certain
photoreaction occurs at the nitro groups of 2,4-DNBT. As
indicated by the dot lines in Fig. 3, the intensity rates of
I(1453 cm–1)/I(1346 cm–1) are gradually heightened with the
increase in the exposure time up to the 30 s point and reach a
constant value after 30 s with the radiation, suggesting that the
new band at 1453 cm–1 results from the selective photoreduction
of 2,4-DNBT rather than its incomplete photoreduction.
To examine the universality of the selective photoreduction, we
also synthesized 4-C-2-NBT, and then obtained its SERS spectra
under the same conditions as in the case of 2,4-DNBT. Concretely,
Figure 1. (a) The normal Raman spectrum of pure 2,4-DNBT in its neat
solid state and (b) the SERS spectrum of the 2,4-DNBT adsorbed on silver
sols, which was irradiated with 632.8 nm laser light of 10 mW for 20 s.
Scheme 2. Schematic representation of the procedure for the selective photoreduction of 2,4-DNBT on a silver surface under 632.8 nm laser radiation.
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J. Raman Spectrosc. (2012)

Selective reduction of nitroaromatic compounds on silver nanoparticles by visible light
Figure 2. The calculated vibrational modes of 2,4-DNBT–Ag complex and pure 2,4-DNBT.
the purpose is to investigate that 4-C-2-NBT would undergo a
selective reduction of the two-site nitro group similar to 2,4-DNBT
or dehalogenation of the four-site chloro group in a manner similar
to that for 4-chloro-benzenethiol and 4-bromo-benzenethiol.[34]
The synthesis strategy of 4-C-2-NBT is shown in Scheme 3.
Figure 4 shows the SERS spectrum of 4-C-2-NBT on silver sols
and the normal Raman spectrum of pure 4-C-2-NBT using the
632.8 nm as the excitation. It was also observed that the S–H
stretch at 2544 cm–1 in the normal Raman spectrum was missing
in the SERS spectrum. This indicates that the 4-C-2-NBT is chemi-
sorbed onto the silver surface through its sulfur atom, as likely
the 2,4-DNBT. It is very clear that several new peaks appear in Fig. 4
(b) compared with the Raman spectrum of pure 4-C-2-NBT, for
instance, at 1448 and 878 cm–1. The band at 1448 cm–1 can be
attributed to the C–H bending plus the N–H deformation modes,
and the peak at 878 cm–1 results from the N–H deformation vibra-
tions. Moreover, the asymmetric stretching vibration peak of the
Figure 4. (a) The normal Raman spectrum of pure 4-C-2-NBT in its neat
solid state and (b)–(d) the time dependence of SERS spectra for 4-C-2-
NBT adsorbed on silver sols, which was irradiated with 632.8 nm laser
light for various lengths of time from 20 to 50 s.
Figure 3. The time dependence of SERS spectra for 2,4-DNBT adsorbed
on silver sols, which was irradiated with 632.8 nm laser light for various
lengths of time from 10 to 50 s.
NO2
(CH3)2NCSCl
TEDA/DMF
Cl
OH
NO2
Bi(NO3)3 5H2O
OCSN(CH3)2
acetone
RT, N2, 24 h
HO
Cl
RT, N2, 24 h
Cl
SCON(CH3)2
NO2
pyrolysis
SH
H3O+
KOH (10%)
Methanol, N2
Cl
Cl
NO2
Scheme 3. Synthesis of 4-C-2-NBT.
J. Raman Spectrosc. (2012)
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Lixin Xia et al.
Figure 5. The calculated vibrational modes of 4-C-2-NBT–Ag complex and pure 4-C-2-NBT.
a
a
b
b
Figure 7. UV–Vis adsorption spectra of (a) 2,4-DNBT and (b) 4-C-2-NBT.
Figure 6. The normal Raman spectrum of pure simple in its neat solid
state and the time dependence of SERS spectra for simple adsorbed on
silver sols using 514.5 nm laser light of 10 mW for various lengths of time
from 10 to 50 s. (a) 2,4-DNBT and (b) 4-C-2-NBT.
the Raman spectrum of pure 4-C-2-NBT. These results indicate
that the two-site nitro group of 4-C-2-NBT was subjected to
photoreaction, although the extent of the photoreaction cannot
be precisely evaluated. On the other hand, the SERS peak at
331 cm–1 that can be assigned to the in-plane C–Cl deformation
vibration still is prominent, and its relative intensity keeps a
two-site nitro group at 1552 cm–1 disappears, and the relative
intensity of the symmetric stretching vibration peak of the nitro
group at 1333 cm–1 are dramatically weakened compared with
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J. Raman Spectrosc. (2012)

Selective reduction of nitroaromatic compounds on silver nanoparticles by visible light
constant value after a prolonged irradiation (50 s). These results
suggest that no dehalogenation occurs, and further 4-C-2-NBT
takes place a selective photoreduction. The calculated vibrational
modes of 4-C-2-NBT–Ag cluster and pure 4-C-2-NBT are shown
in Fig. 5, which are in good agreement with our experimental
data.
10874234, 90923003 and 20703064), the National Basic Research
Project of China (Grant 2009CB930701).
Supporting Information
Supporting information may be found in the online version of
this article.
The reason why two-site nitro groups of 2,4-DNBT and 4-C-2-
NBT was subjected to photoreaction might be attributed to the
close distance from the silver surface. The closer the distance of
nitro groups from the silver surface was, the more possible it
was for photoreaction to take place that was induced by a surface
plasmon resonance effect. In other words, the oscillating
electrons interact easily with the two-site nitro groups of
2,4-DNBT and 4-C-2-NBT because of closer distance. On the other
hand, as can be seen in Figs 1, 3, and 4, the C–H vibrations of
benzene ring were still retained after the photoreduction. This
implies that water present under ambient conditions is the
H-atom source for the selective photoreduction rather than
the benzene ring hydrogen atoms of 2,4-DNBT or 4-C-2-NBT.
It is believed that water was catalytically decomposed on the
silver surface to provide H-atom,[9] thus the two-site nitro
group of 2,4-DNBT or 4-C-2-NBT is in a more favorable position
to react with the H-atom than the four-site nitro or chloro
group that is far from the Ag surface. That is not to say that
the hydrogen atoms of the 2,4-DNBT or 4-C-2-NBT itself cannot
be used as the H-atom source for the selective photoreduction
but rather that water present under ambient conditions should be
the primary H-atom source in our experiments in terms of the C–H
vibrations of benzene ring reservation after photoreduction as
shown in Figs 1, 3, and 4.[3]
Our further interest was to investigate the selective photore-
action mechanisms. Therefore, we tried to record SERS spectra
of the adsorbed 2,4-DNBT or 4-C-2-NBT with 514.5 nm excitation.
It can be seen in Fig. 6 that the SERS spectra recorded with the
514.5 nm line are nearly the same as those obtained with
632.8 nm excitation relative to the normal Raman spectra. In
other words, no frequency dependence of the selective photo-
reaction was observed for both 2,4-DNBT and 4-C-2-NBT. This
is the usual understanding that a photoelectron is ejected more
readily using a shorter-wavelength than a longer-wavelength
laser, the present contradiction can be rationalized by assuming
that the enhanced photoreaction at longer wavelength is deeply
associated with charge transfer from the metal to the absorbed
molecule, which showed via UV–Vis spectra ( Fig. 7).
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In summary, we have discovered the 2,4-DNBT and 4-C-2-NBT on
silver sols were selectively reduced to 2-amino-4-nitro-benzenethiol
and 2-amino-4-chlorobenzenethiol simply by irradiating with
632.8 or 514.5 nm laser in ambient conditions. After only 30 or
20 s of continuous laser illumination, the reactions are finished,
suggesting that the selective photoreduction is a very facile
process. To the best of our knowledge, this is the first report of
visible light-induced selection reduction for nitroaromatic
compounds on silver sols.
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Acknowledgements
This work was supported by the Liaoning Natural Science
Foundation, China (20092016), the Shenyang Natural Science
Foundation, China (F10-230-4-00 and 090082), and the Liaoning
University 211-Projects of the third period. Mengtao Sun thanks
the National Natural Science Foundation of China (Grants
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J. Raman Spectrosc. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.
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