Plasmon-free SERS self-monitoring of catalysis reaction on Au nanoclusters/TiO2 photonic microarray

Article abstract of DOI:10.1039/c5cc02468d

Here, using the (Au/TiO₂)-catalyzed reduction of 4-nitrothiophenol as a probe reaction, a catalysis/SERS bifunctional composite fabricated by uniformly dispersing catalysis-active Au nanoclusters (～2 nm) on a SERS-active TiO₂ photonic microarray was successfully applied to the non-plasmonic SERS self-monitoring of a catalytic reaction for the first time, which is superiorly sensitive and interference-free.

Full text of DOI:10.1039/c5cc02468d

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Plasmon-Free SERS Self-monitoring of Catalysis Reaction on Au
Nanoclusters/TiO2 Photonic Microarray
Dianyu Qi,a Xuefeng Yan,a Lingzhi Wang* a and Jinlong Zhang* a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Here, using (Au/TiO2)-catalyzed reduction of 4-nitrothiophenol as substrate.6 This kind of interference can be reduced by
a probe reaction, catalysis/SERS bifunctional composite fabricated decreasing the power density, using the wavelength far from
by uniformly dispersing catalysis-active Au nanoclusters (~2 nm) the LSPR region or introducing a thin inert shell between SERS-
on SERS-active TiO2 photonic microarray was sucessfully applied active substrate and catalysis-active nanoparticles (NPs).5a, 7
to the non-plasmonic SERS self-monitoring of catalytic reaction for Different from the electro-magnetic enhancement by coinage
the first time, which is superiorly sensitive and interference-free.
metals, SERS signals observed from semiconductors such as
TiO2 and ZnO is due to chemical enhancement,8 which is
expected to be more effective on avoiding the interference
caused by plasmon. It is known that reactions catalyzed by
semiconductor-supported metals have improved catalysis
efficiency through the coordination between metal and
semiconductor. 9 Therefore, it is desirable that the reaction on
metal/semiconductor can be SERS self-monitored. However,
SERS from semiconductor has been not successfully applied to
Surface-enhanced Raman scattering (SERS) is a label-free and
non-destructive analysis technique with fingerprint spectrum.
When applied on monitoring a catalyzed reaction process, it
can identify abundant surface/interface information including
the surface structure, molecule bonding, configuration and
orientation.1 Exploring the molecular activation and chemical
transformation processes on the surface of different catalysts
is necessary to figure out the catalysis mechanism and finally
guide the design of highly efficient catalyst system surface.2
Coinage metals are well-known SERS-active substrate, so it is
desirable that Au or Ag-catalyzed reactions can be self-
monitored by SERS spectra. However, this idea has been
unfortunately restricted by the incompatibility between SERS
(particle size>20 nm) and catalysis (particle size <10 nm)
activities.3
monitor
a catalysis reaction due to its low sensitivity.
Fortunately, we have recently revealed that TiO2 in the form of
ordered photonic array shows highly SERS activity by
improving the light-matter coupling effect,10 which triggered
us to study whether catalytic reactions on TiO2 supported
metal can be SERS self-monitored.
In this study, Au nanoclusters (NCs, ~2 nm) supported on
TiO2 photonic array (Au NCs/TiO2 photonic array) has been
fabricated and applied to the catalytic reduction of 4-NTP in
NaBH4 solution. A quick and direct reaction process from 4-
NTP to 4-ATP was successfully self-monitored by Au NCs/TiO2
photonic array with slow light region matching with the
incident laser wavelength (785 nm). This reaction has been
intensively studied by SERS but there still remain some
ambiguous and hardly resolvable points. 4-NTP can be
accordingly reduced to 4-ATP in a direct way or through the
formation of intermediate 4, 4’-DMAB.4 However, a side
reaction from 4-ATP to 4, 4’-DMAB caused by laser irradiation
has also been commonly observed.3a, 4-5, 5c, 11 The situation may
become more entangled in a plasmonic system with LSPR-
induced heating effect and widened plasmonic resonance peak,
significantly interfering the understanding about the
mechanism of original reaction. Here, the side reaction is
completely eliminated on Au NCs/TiO2 photonic array by using
785 nm laser far from the intrinsic absorption band of Au NCs
The combination of SERS-active plasmonic metal substrate
and catalysis-active metal nanoparticles has been commonly
used to achieve the SERS monitoring of a catalysis reaction.3a, 4
However, the photo- or thermo-induced side reaction caused
by laser irradiation in plasmonic system has posed a big
obstacle to the monitoring of a real reaction process,5 which
may become more complicated in “hot spot” areas with strong
local surface plasmon resonance (LSPR) effect. The side
reaction can be avoided on very few plasmonic metal SERS
a.Key Laboratory for Advanced Materials and Institute of Fine Chemicals
East China University of Science and Technology
130 Meilong Road, Shanghai 200237, P. R. China.
E-mail: jlzhang@ecust.edu.cn
b.Department of Chemistry
Tsinghua University
Beijing 100084, P. R. China.
† Electronic Supplementary Information (ESI) available: [Experimental details and
additional characterization of the substrates]. See DOI: 10.1039/x0xx00000x
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not show any peak (Fig. 2a), indicating the negligible plasmon
resonance effect of Au NCs. Some signals can be observed
from P25 but the intensity is extremely low. In comparison, 4-
NTP on TiO2 photonic array presents well-resolved Raman
signals with a high peak at 1342 cm-1, which ascribed to the
stretching mode of -NO2.3a SEM image indicates TiO2
microarray has
a highly interconnected and ordered
macroporous structure with the pore size of ~350 nm (Fig. 2b).
Based on our previous report, this pore size is chosen to
achieve the highest detection sensitivity by maximizing the
light-matter coupling effect.10 As verified by UV-Vis reflectance
spectrum, the macroporous microarray actually possesses a
slow-light region near 785 nm (Fig. 2c). Similar to that on TiO2,
the Raman signals observed from Au NCs/TiO2 photonic array
still preserve the high intensity, indicating the deposition of Au
does not decrease the SERS-enhancing effect of ordered array.
Moreover, it is noted that the peak attributed to –NO2 is split
into two peaks at 1342 cm-1 and 1332 cm-1, where the latter
new peak at 1332 cm-1 should be ascribed to the -NO2
stretching of 4-NTP adsorbed on Au.
The reduction of 4-NTP adopts NaBH4 as the reducing agent.
4-NTP was first adsorbed on Au NCs/TiO2 photonic array and
then immersed into NaBH4 solution for different times. The
reaction process was monitored by SERS under the irradiation
of 785 nm laser, where the characteristic peaks of 4-NTP and
4-ATP are marked by black square and pink triangle (Fig. 3c),
respectively. The small peak at 1332 cm-1 quickly disappears in
the first one minute, indicating 4-NTP molecules adsorbed on
Au surface are firstly reduced. In the next six minutes, the
peaks around 1342 cm-1 and 1114 cm-1 gradually fade away,
along with a shift of the peak at 1575 cm-1 to 1585 cm-1,
indicating 4-NTP molecules are completely reduced to 4-ATP.3a
Fig. 1 TEM (a and b) and HRTEM (c) images of Au NCs/TiO2 photonic array. The inset is
the magnification of a Au NC. (d) Schematic diagram of Au NCs/TiO2 photonic
microarray.
and TiO2, resulting in the successful plasmon-free SERS self-
monitoring of the real reduction process. In contrast, the side
reaction cannot be avoided by Au NCs deposited on plasmonic
Au NPs (Au NCs/Au NPs) under the same laser irradiation,
leaving an ambiguous reduction process of 4-NTP on Au
NCs/Au NPs.
TiO2 inverse opal photonic microarray was prepared through
impregnation-calcination procedure using closely packed
polystyrene (PS) microarray as the hard template. Au NCs
were first prepared using L-glutathione containing amide and
mercapto groups as the ligand, and then loaded on TiO2
microarray through impregnation. Transmission electronic
microscopy (TEM) images indicate TiO2 inverse opal has a 3D-
interconnected macroporous structure (Fig. 1a) and high-
density Au NCs are deposited on the macroporous framework
(Fig. 1b). High-resolution TEM (HRTEM) image shows Au NCs
are highly dispersed and have uniform size of about 2 nm (Fig.
1c). Two kinds of lattice fringes with spacing of 0.35 nm and
0.20 nm are observed, corresponding to the (101) plane of
anatase TiO2 and (200) plane of Au (inset in Fig. 1c),
respectively. Fig. 1d presents a schematic model for this
Au/TiO2 composite to further illustrate the uniform
combination of Au NCs and TiO2 photonic microarray, which
are responsible for the roles of catalysis and SERS detection,
respectively.
Raman spectra of 4-NTP adsorbed on different substrates
were first analyzed under the irradiation of 785 nm laser,
which is far from the absorption edge of both Au NCs (~500
nm) and TiO2 (~380 nm) to avoid the possible photocatalytic
effect (Fig. S2). Two control experiments were carried out by
using Au NCs and a commercial TiO2 nanoparticle (P25) as the
SERS substrates. Raman spectrum collected on Au NCs does
Fig. 2 (a) Raman spectra of 4-NTP adsorbed on different substrates, (b) SEM image of
TiO2 photonic array. The scale bar is 500 nm and (c) UV-Vis reflectance spectrum of TiO2
photonic array.
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To rule out the possible catalysis by TiO2, TiO2 photonic array
without Au NCs was also used to monitor the reaction (Fig.S3),
and the result indicates TiO2 has no catalytic activity to the
reaction. Given that Au NCs are the only active sites, and 4-
NTP molecules are adsorbed on both Au NCs and TiO2 surface.
We consider that 4-NTP molecules on TiO2 surface can diffuse
to the neighbor Au NCs and are reduced there.6b Because of
the ultrahigh dispersion of Au NCs, the complete reduction is
finally achieved. Moreover, Au NCs dispersed on TiO2
microarray shows good stability during catalysis since no
particle growth is observed from the TEM image, while strong
agglomeration occurs between non-loaded Au NCs (Fig. S4). To
further reveal the reliability of the self-monitoring
performance, Au NCs/Au NPs (Fig. 3b) fabricated by depositing
Au NCs (~2 nm) on Au NPs (~20 nm) is also used for the
plasmonic SERS monitoring of 4-NTP reduction, which however
shows characteristic peaks of 4, 4’-DMAB (blue sphere) at 1145,
1391 and 1438 cm-1 besides the signal of 4-ATP (Fig. 3d).11a
The generally accepted reduction mechanism of the
nitroaromatic compounds is Haber scheme, in which two
routes (direct route and condensation route) are proposed. In
direct route, nitro group of 4-NTP is reduced into nitroso,
hydroxylamine, and amino groups successively. In
condensation route, an azobenzene compound is generated as
an intermediate from the condensation of a nitroso compound
and a hydroxylamine compound, and it is finally reduced into
two amino compounds.9b, 12 Therefore, 4, 4’-DMAB may be an
intermediate product from the reduction of 4-NTP to 4-ATP,
and it is possible that the absence of 4, 4’-DMAB signals on Au
NCs/TiO2 is attributed to the complete reduction of 4-NTP on
Au NCs/TiO2 photonic array. The concentration of 4, 4’-DMAB
Fig. 4 Raman spectra collected from 4-ATP adsorbed on different substrates. Inset
above is the scheme for laser-induced formation of 4, 4’-DMAB from 4-ATP on Au NPs.
can increase with the decrease of the reductant amount if 4-
NTP is reduced to 4-ATP through the intermediate of 4, 4’-
DMAB. However, no signal of 4, 4’-DMAB is observed with the
decrease of the NaBH4 amount (Fig. S7), indicating 4, 4’-DMAB
is not an intermediate product catalytically formed by Au
NCs/TiO2.
To acquire the accurate reason for the observation of
different reduction products from Au NCs/TiO2 and Au NCs/Au
NPs, the Raman spectra of 4-ATP are respectively recorded on
Au NCs/TiO2 photonic array (Fig. 4, blue line), Au NCs/Au NPs
(Fig. 4, black line) and Au NPs (Fig. 4, red line), assuming that 4-
NTP molecules are completely reduced into 4-ATP. Signals
attributed to 4, 4’-DMAB are clearly observed on Au NPs and
Au NCs/Au NPs but are absent on Au NCs/TiO2 photonic array.
The results demonstrate 4-ATP can be transformed to 4, 4’-
DMAB under laser illumination in the presence of Au NPs, and
the transformation may be caused by plasmon-induced charge
transfer under the laser irradiation 3a, because 4-ATP/Au NPs
composite shows absorption at 785 nm (Fig. S8). In
comparison, no photo-chemical side reaction is caused by Au
NCs/TiO2 photonic array, indicating its reliability on self-
revealing the catalytic kinetics. Therefore, our plasmon-free
SERS substrate provides a good solution to the problem caused
by plasmon-involved photochemical reaction, where the actual
reduction process of 4-NTP is difficult to be distinguished in
the presence of interference reaction from 4-ATP to 4, 4’-
DMAB. To the best of our knowledge, this is the first successful
demonstration of the plasmon-free SERS on TiO2 substrate
used for the self-monitoring of catalytic reaction process.
In conclusion, plasmon-free substrate with Au NCs uniformly
distributed on TiO2 photonic microarray was used for SERS
self-monitored reduction of 4-NTP. The photonic microarray
with macroporous size of 350 nm possesses enhanced
coupling effect with incident laser of 785 nm, thus providing
strong and well-resolved Raman signals for the reaction
monitoring. Compared with plasmonic Au NCs/Au NPs, a
higher catalytic activity and 100% selectivity for 4-ATP can be
real-time revealed on Au NCs/TiO2 photonic array substrate
Fig. 3 TEM images and schemes of Au NCs/TiO2 photonic array (a) and Au clusters/ Au
NPs (b). Raman spectra for monitoring the reduction process of 4-NTP on Au NCs/ TiO2
photonic array (c) and Au NCs/NPs (d). The characteristic peaks of 4-NTP, 4-ATP and 4,
4’-DMAB are respectively labelled by◆, ▼and●.
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due to the effective screening of plasmon-involved
photochemical side reaction from 4-ATP to 4, 4’-DMAB. We
hope this study offer
a useful reference for reaction
monitoring by SERS on different semiconductor carriers.
Notes and references
The authors declare no competing financial interest.
1
(a)M. Fleischmann, P. J. Hendra and A. J. McQuillan, Chem
Phys Lett, 1974, 26, 163-166; (b)R. Huber, F. Tauser, A.
Brodschelm, M. Bichler, G. Abstreiter and A. Leitenstorfer,
Nature, 2001, 414, 286-289; (c)I. Yoon, T. Kang, W. Choi, J.
Kim, Y. Yoo, S.-W. Joo, Q. H. Park, H. Ihee and B. Kim, J Am
Chem Soc, 2008, 131, 758-762; (d)S. Nie and S. R. Emory,
Science, 1997, 275, 1102-1106; (e)E. C. Le Ru, P. G. Etchegoin
and M. Meyer, J Chem Phys, 2006, 125, -; (f)J. P. Camden, J.
A. Dieringer, Y. Wang, D. J. Masiello, L. D. Marks, G. C. Schatz
and R. P. Van Duyne, J Am Chem Soc, 2008, 130, 12616-
12617; (g)G. Chen, Y. Wang, M. Yang, J. Xu, S. J. Goh, M. Pan
and H. Chen, J Am Chem Soc, 2010, 132, 3644-3645; (h)X. Li,
G. Chen, L. Yang, Z. Jin and J. Liu, Adv Funct Mater, 2010, 20
,
2815-2824.
(a)W. Cai, X. Tang, B. Sun and L. Yang, Nanoscale, 2014, 6,
2
3
7954-7958; (b)X. Tang, W. Cai, L. Yang and J. Liu, Nanoscale,
2014,
2014,
6, 8612-8616; (c)L. Yang, P. Li and J. Liu, RSC Advances,
4, 49635-49646.
(a)W. Xie, B. Walkenfort and S. Schlücker, J Am Chem Soc,
2012, 135, 1657-1660; (b)J. T. Krug, G. D. Wang, S. R. Emory
and S. Nie, J Am Chem Soc, 1999, 121, 9208-9214; (c)C. T.
Campbell, Science, 2004, 306, 234-235; (d)M. Valden, X. Lai
and D. W. Goodman, Science, 1998, 281, 1647-1650.
4
5
W. Xie, C. Herrmann, K. Kömpe, M. Haase and S. Schlücker, J
Am Chem Soc, 2011, 133, 19302-19305.
(a)Y.-F. Huang, H.-P. Zhu, G.-K. Liu, D.-Y. Wu, B. Ren and Z.-
Q. Tian, J Am Chem Soc, 2010, 132, 9244-9246; (b)V. Joseph,
C. Engelbrekt, J. Zhang, U. Gernert, J. Ulstrup and J. Kneipp,
Angew Chem Int Ed, 2012, 51, 7592-7596; (c)Q. Zhou, X. Li,
Q. Fan, X. Zhang and J. Zheng, Angew Chem Int Ed, 2006, 45
3970-3973.
,
6
7
(a)J. Huang, Y. Zhu, M. Lin, Q. Wang, L. Zhao, Y. Yang, K. X.
Yao and Y. Han, J Am Chem Soc, 2013, 135, 8552-8561; (b)R.
Liu, J.-F. Liu, Z.-M. Zhang, L.-Q. Zhang, J.-F. Sun, M.-T. Sun
and G.-B. Jiang, J Phys Chem lett, 2014, 5, 969-975.
(a)Z. Sun, C. Wang, J. Yang, B. Zhao and J. R. Lombardi, J Phys
Chem C, 2008, 112, 6093-6098; (b)J. F. Li, Y. F. Huang, Y.
Ding, Z. L. Yang, S. B. Li, X. S. Zhou, F. R. Fan, W. Zhang, Z. Y.
Zhou, Y. WuDe, B. Ren, Z. L. Wang and Z. Q. Tian, Nature,
2010, 464, 392-395.
8
9
(a)A. Musumeci, D. Gosztola, T. Schiller, N. M. Dimitrijevic,
V. Mujica, D. Martin and T. Rajh, J Am Chem Soc, 2009, 131,
6040-6041; (b)X. Wang, W. Shi, G. She and L. Mu, Phys Chem
Chem Phys, 2012, 14, 5891-5901; (c)Y. Wang, W. Ruan, J.
Zhang, B. Yang, W. Xu, B. Zhao and J. R. Lombardi, J Raman
Spectrosc, 2009, 40, 1072-1077.
(a)A. Corma and P. Serna, Science, 2006, 313, 332-334; (b)A.
Grirrane, A. Corma and H. García, Science, 2008, 322, 1661-
1664.
10 D. Qi, L. Lu, L. Wang and J. Zhang, J Am Chem Soc, 2014, 136
9886-9889.
,
11 (a)K. Kim, D. Shin, K. L. Kim and K. S. Shin, Phys Chem Chem
Phys, 2012, 14, 4095-4100; (b)Y.-F. Huang, D.-Y. Wu, H.-P.
Zhu, L.-B. Zhao, G.-K. Liu, B. Ren and Z.-Q. Tian, Phys Chem
Chem Phys, 2012, 14, 8485-8497.
12 A. Corma, P. Concepción and P. Serna, Angew Chem Int Ed,
2007, 46, 7266-7269.
4 | J. Name., 2012, 00, 1-3
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