Graphene oxide-palladium modified Ag-AgBr: A visible-light-responsive photocatalyst for the Suzuki coupling reaction

Article abstract of DOI:10.1039/c4ra05988c

The efficient utilization of the abundant and economical solar energy has received tremendous attention due to increased environmental and energy concerns. In this report, a plasmonic Ag-AgBr photocatalyst functionalized with graphene oxide-Pd was successfully fabricated and used as an efficient visible-light-responsive photocatalyst for the Suzuki coupling reaction. The integration of plasmonic Ag-AgBr with Pd nanoparticles supported on graphene oxide enabled efficient visible light harvesting for the catalytic reaction on the heterojunction structure. A possible catalytic mechanism for the highly efficient photocatalyst is proposed. This work provides a convenient means for the design of efficient catalysts that are composed of a photocatalyst and a catalytically active transition metal for various organic transformations driven by visible light.

Full text of DOI:10.1039/c4ra05988c

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Graphene oxide–palladium modified Ag–AgBr: a
visible-light-responsive photocatalyst for the
Suzuki coupling reaction†
Cite this: RSC Adv., 2014, 4, 39242
*
Shutao Gao, Ningzhao Shang, Cheng Feng, Chun Wang and Zhi Wang
The efficient utilization of the abundant and economical solar energy has received tremendous attention
due to increased environmental and energy concerns. In this report, a plasmonic Ag–AgBr photocatalyst
functionalized with graphene oxide–Pd was successfully fabricated and used as an efficient visible-light-
responsive photocatalyst for the Suzuki coupling reaction. The integration of plasmonic Ag–AgBr with
Pd nanoparticles supported on graphene oxide enabled efficient visible light harvesting for the catalytic
reaction on the heterojunction structure. A possible catalytic mechanism for the highly efficient
photocatalyst is proposed. This work provides a convenient means for the design of efficient catalysts
that are composed of a photocatalyst and a catalytically active transition metal for various organic
transformations driven by visible light.
Received 19th June 2014
Accepted 18th August 2014
DOI: 10.1039/c4ra05988c
www.rsc.org/advances
including metal oxide, polymeric graphitic carbon nitride,
plasmonic and dye sensitized photocatalysts. However, the
Introduction
available visible light photocatalysts suitable for selective
organic transformations are still very few.4b The scope and
efficiency of the reactions catalyzed by visible light photo-
catalysts need to be enlarged and enhanced signicantly. Great
challenges still exist and need to be resolved before sunlight
becomes a viable source of energy for organic synthesis.
With increased consumption of energy and growing environ-
mental contamination, the development of clean and sustain-
able energy utilization and environmentally friendly
technologies has become a research focus for world-wide
scientic researchers. Solar energy from sunlight is widely
believed to be the key to solving the future problems of the
environment and energy resource because of its clean,
sustainable, abundant and economical characteristics.1 To
efficient harvest of solar energy, various kinds of photocatalysts
have been explored for applications in splitting water into
hydrogen, photocatalytic degradation of organic pollutants,
preparing dye sensitized solar cells and so on.2
Recently, the use of sunlight to drive chemical reactions has
received much attention and has been studied as an important
way to pursue environmentally benign and green synthetic
process.3 However, most of the photocatalysts were used only
under irradiation by ultraviolet (UV) light. Consequently, only
3–5% of the solar energy reaching the earth can be utilized. So
the development of an efficient system for visible light energy
conversion into chemical energy is one of the urgent subjects
for research because sunlight is mostly composed of visible
light.4 To date, visible light-driven chemical reactions, such as
photocatalytic oxidation,5 reduction,6 C–H bond activation, C–C
and C–N bond formation,7 dechlorination,8 and so on, have
been realized by using different visible light photocatalysts
The palladium-catalyzed C–C cross-coupling reaction plays a
key role in the synthesis of many important chemicals including
pharmaceuticals, herbicides, and polymers. Up to now, only a
few visible light photocatalysts have been developed for the C–C
cross-coupling.4b More recently, Au–Pd/ZrO2 was prepared by
Zhu et al. and used as photocatalyst for Suzuki coupling of
iodobenzenes and aromatic boronic acids under visible light
irradiation.7b The results indicated that the Au/Pd molar ratio of
the alloy nanoparticles has an important impact on the
performance of the catalysts. Huang et al. developed an efficient
one-pot strategy for the fabrication of Au–Pd core–shell nano-
wheels, which were successfully used as photocatalyst for the
Suzuki coupling reaction of iodobenzene and phenylboronic
acid at 50 ꢀC.9 The unique structure of the AuPd nanowheels
signicantly enhanced not only their catalytic activity but also
their ability to exploit the light. Plasmonic Au–Pd nano-
structures could harvest visible-to near-infrared light for Suzuki
coupling of bromobenzenes and aromatic boronic acids under
the irradiation of 809 nm laser or sunlight.10 The plasmonic Au
nanorods absorb visible light while the Pd shell acts as the
direct catalyst for the coupling reaction. Differently sized Au–Pd
nanostructures were prepared and mixed together to make the
mixture light-responsive over the visible to near infrared region.
However, the above-mentioned photocatalysts are expensive.
College of Science, Agricultural University of Hebei, Baoding 071001, Hebei, China.
E-mail: chunwang69@126.com; Fax: +86-312-7528292; Tel: +86-312-7528291
† Electronic supplementary information (ESI) available: Characterization data of
all products. See DOI: 10.1039/c4ra05988c
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Mesoporous polymeric graphitic carbon nitride (mpg-C3N4) is a diffraction intensity of GO, which is consistent with the repor-
graphene-like layered material and can function as a photo- ted works about other GO-involved nanocomposites.16 For the
catalyst for selective organic reactions.2d In 2013, Antonietti Ag–AgBr and GO–Pd@Ag–AgBr composites, the peak at 38.1ꢀ
et al. reported that Pd nanoparticles supported on mpg-C3N4 (marked with “A”) corresponding to the (111) crystal plane of
could catalyze the Suzuki coupling reaction at room tempera- Ag clearly represents the formation of Ag (JCPDS no. 04-0783),
ture,11 in which mpg-C3N4 acts as the photocatalyst whilst Pd which could be ascribed to partial conversion of AgBr to
acts as the coupling catalyst. However, this reaction was driven elemental Ag through the reduction with hot glycerol during the
by the direct irradiation with a 150 W Xe lamp which contains high-temperature incubation process.
UV light rather than visible light only. By ltering off the UV part
X-ray photoelectron spectroscopy was used to obtain infor-
of the xenon lamp (<420 nm), the yield of the product decreased mation on surface of GO–Pd@Ag–AgBr sample. Ag 3d, Br 3d, Pd
to 49% aer reaction for 12 hours. So exploring more efficient 3d and C 1s XPS are spectra were shown in Fig. S2.† Two specic
visible light phototcatalysts to facilitate the coupling reaction peaks of Ag 3d located at ca. 368 and 374 eV could be attributed
under mild conditions is still a challenge.
to Ag 3d5/2 and Ag 3d3/2 binding energies, respectively.17 The XPS
Silver halides decorated with silver (Ag@AgX), which exhibit of Ag 3d5/2 showed that the tted peaks at 367.60 and 368.2 eV
high absorption in the visible region due to the surface plasmon corresponded to Ag(I) and Ag(0), respectively, while the Ag 3d3/2
resonance (SPR) of silver nanoparticles, represent a class of peaks at 373.6 and 374.3 eV corresponded to Ag(I) and Ag(0),
highly efficient visible-light-driven photocatalyst.12 Current respectively. Fig. S2b† shows the peaks of Br 3d5/2 and Br 3d3/2
applications of Ag@AgX photocatalysts are mainly focused on with the binding energy of 68.2 eV and 69.4 eV, which could be
the decomposition of organic pollutants and the evolution of assigned to Br(I). The XPS of Pd 3d5/2 showed that the tted
H2,13 whilst catalytic organic transformations (including Suzuki peaks at 337.6 and 335.3 eV corresponded to Pd(II) and Pd(0),
coupling reaction) involving Ag@AgX under light irradiations respectively, while the Pd 3d3/2 peaks at 342.8 and 340.5 eV
stay essentially untouched.
corresponded to Pd(II) and Pd(0), respectively.8 Fig. S2d†
Graphene, a novel carbon nanomaterial, has attracted showed C 1s XPS spectra of GO–Pd@Ag–AgBr. The deconvo-
extensive attention in recent years due to its excellent electronic luted peak at a binding energy of 284.6 eV is attributed to C–C,
and optical properties, great mechanical strength, and extreme C]C, and C–H bonds. The peaks at the binding energy of 286.7,
large specic surface area, which make graphene-related and 288.9 eV were attributed to the C–O bands (expoxy and
materials attractive for many potential applications including hydroxyl) and carboxylate C]O from carboxylic acid,
energy storage, catalysis, biosensors, molecular imaging and respectively.18
drug delivery.14 Especially, graphene-based photocatalysts have
The EDX spectrum of GO–Pd@Ag–AgBr was shown in
attracted a lot of attention due to their high electron and hole Fig. S3.† Oxygen and carbon element were detected distinctly,
transport ability, good electron conductivity, large specic which suggests the existence of GO species in the sample.
surface area and high adsorption ability for organic Moreover, quantitative analysis shows that the atomic ratio
molecules.13b,15
between Ag and Br is about 1.33 : 1. This value is larger than the
In this report, a plasmonic Ag–AgBr photocatalyst function- theoretic stoichiometric atomic ratio between Ag and Br in
alized with graphene oxide–Pd (GO–Pd@Ag–AgBr) was AgBr, which should be 1 : 1. The results suggests the possible
successfully fabricated and used as an efficient visible-light- generation of metallic Ag during the synthesis process. The
responsive photocatalyst for the Suzuki coupling reaction. The quantitative amount of components in the sample can be
integration of plasmonic Ag–AgBr with catalytic Pd nano- approximately estimated from the elemental composition
particles in one nanostructure signicantly enhanced not only obtained from the EDX analysis. The weight percentage of Ag0
their ability to exploit the light, but also the intrinsic catalytic in the GO–Pd@Ag–AgBr was about 12.1%.
activity of Pd.
The above facts strongly conrm the existence of metallic Ag
species in the photocatalyst, which could arouse surface plas-
mon resonance absorptions in the visible region.
Fig. 1 showed the typical scanning electron microscopy
(SEM) images of the as-prepared Ag–AgBr and GO–Pd@Ag–AgBr
Results and discussions
Material characterizations
The components of the obtained GO–Pd@Ag–AgBr (for the
preparation details, see ESI†) were investigated by X-ray
diffraction spectroscopy (XRD), X-ray photoelectron spectros-
copy (XPS) and energy dispersive X-ray spectroscopy (EDX).
Fig. S1† showed the XRD patterns of (a) Ag–AgBr, (b) AgBr,
and (c) GO–Pd@Ag–AgBr. All the samples of AgBr, Ag–AgBr, and
GO–Pd@Ag–AgBr exhibited characteristic diffraction peaks of
AgBr (JCPDS le: 06-438). The peaks at 26.6ꢀ, 31.0ꢀ, 44.2ꢀ, 52.4ꢀ,
55.0ꢀ, 64.4ꢀ and 73.1ꢀ (marked with“ꢁ”) are attributed to (111),
(200), (220), (311), (222), (400) and (420) crystal planes of AgBr,
respectively. No diffraction peak attributed to GO was observed
in the XRD pattern due to the relatively low amount and low Fig. 1 SEM image of (a) Ag–AgBr and (b) GO–Pd@Ag–AgBr.
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nanocomposites. It clearly showed the AgBr as a polyhedron exhibits a higher adsorption capacity for the organic mole-
(Fig. 1a). As shown in Fig. 1b, the Ag–AgBr nanoparticles were cules,16b which is benecial for the enhancement of the photo-
well dispersed and encapsulated by the GO nanosheets, indi- catalytic activity of GO–Pd@Ag–AgBr. The synergistic effects of
cating the well coupling between the Ag–AgBr nanoparticles and the coupling of Ag–AgBr and GO enhanced the electrons
the GO nanosheets.
transfer from Ag–AgBr to GO, which could suppress the elec-
The UV-visible diffuse reectance spectra (DRS) of AgBr, Ag– tron–hole recombination and thus lead to the enhancement of
AgBr, and GO–Pd@Ag–AgBr are shown in Fig. 2. Compared with the photocatalytic performance of the GO–Pd@Ag–AgBr
AgBr, the much stronger absorption of Ag–AgBr in the visible- photocatalyst.
light region can be attributed to the surface plasmon reso-
When the reaction of phenyl bromide and phenyl boronic
ꢀ
nance of silver nanoparticles. Aer modied with GO–Pd, GO– acid was carried out with visible light irradiation at 25 C for
Pd@Ag–AgBr has a large number of different shapes and 20 min and then reacted in the dark (light off) for 40 min, the
diameter, the surrounding environment of Ag–AgBr is also yield of the reaction is 67%. While the yields of the coupled
changed, so that the plasmon oscillations cover a wide range of product with visible light irradiation at 25 ꢀC for 20 min and 1 h
frequencies. Hence GO–Pd@Ag–AgBr can absorb a wide range were 64% and 99%, respectively. The results indicate that light
of visible light, which makes the photocatalyst use the visible is indeed the driving force in this system.
light more efficiently.
The dependence of the photocatalytic activity on light
Results of SEM observation, absorption, EDX, and XPS wavelength is a characteristic of plasmon resonance photo-
revealed that Ag–AgBr nanoparticles were successfully covered catalysts, which differs from semiconductor photocatalysts.3b,19
with GO–Pd, indicating that the Ag–AgBr plasmonic photo- For semiconductor photocatalyst, the wavelength of absorption
catalyst was facilely functionalized with Pd.
is xed, or limited to a certain wavelength range in which the
The photo catalytic activity of GO–Pd@Ag–AgBr was investi- photons have an energy larger than the semiconductor band
gated using the Suzuki reaction of phenyl bromide and phenyl gap. The photocatalytic activity of semiconductor photocatalyst
boronic acid as the model reaction. Considering that photo- does not vary with the change of irradiation wavelength.19 The
thermal effect could contribute to the conversion reaction in effects of light wavelengths on the photocatalytic activity of GO–
principle, the reactor was carefully thermostatted to exclude the Pd@Ag–AgBr were investigated. The wavelength range was
possibility of thermal reaction and thus investigate the photo- tuned by using various glass lters to cut off the irradiation
catalytic nature of the process. As shown in Table S1,† GO– below a certain value of wavelength. As shown in Fig. 3, when
Pd@Ag–AgBr gives an excellent yield (99%) of the coupled the reactor was illuminated with different wavelength ranges of
product with visible light irradiation at 25 ꢀC for 1 h. For light, 400–800, 510–800, 570–800 and 620–800 nm, the yields of
comparison, we also conducted a similar catalytic reaction biphenyl decreased signicantly. From the results obtained
under the same conditions but without visible light irradiations from the irradiation of different wavelengths, the contribution
(reacted in the dark). As a result, only a yield of 9% was obtained of the light in a narrow wavelength range could be estimated.
(Table S1,† entry 2). These results clearly demonstrated that Since the reaction temperature was held constant, the yields of
light irradiation can smoothly trigger the coupling reaction the thermal reaction (reacted in the dark) remain unchanged
under mild conditions. However, no product was generated regardless of the lter used, which is 9% in this case (gray
with either Ag–AgBr or GO–Pd under light irradiation, which column in Fig. 3). So the total light irradiation enhancement
veried the importance of combining the photocatalyst with the (the contribution of light irradiation in the wavelength range of
intrinsic palladium catalyst for driving the Suzuki coupling 400–800 nm) is 90% (¼ 99% ꢂ 9%). This gure also shows that
reaction. To demonstrate the structural advantage of the GO– light irradiation in the wavelength range of 510–620 nm
Pd@Ag–AgBr, we also used Pd@Ag–AgBr as the catalyst for
comparison, and only a yield of 88% was obtained. GO usually
Fig. 3 Influence of cut-off threshold wavelength on the Suzuki
Fig. 2 UV-vis DRS spectra of (a) GO–Pd@Ag–AgBr, (b) Ag–AgBr, and coupling reaction. Gray column: the yields of the thermal reaction
(c) AgBr.
(reacted in the dark).
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contributed the main part of driving the Suzuki reaction, photocatalysis simultaneously proceeded because AgBr can be
accounting for 83.3% of the total light irradiation enhance- directly photo-excited under light illumination, to generate
ment, of which, 44.4% came from the light irradiation with electrons/holes pairs (eꢂ/h+) in AgBr. For the coupling reaction
wavelengths between 510 nm and 570 nm and 38.9% from the between bromobenzene and boronic acid, the rate-determining
light between 570 nm and 620 nm. The remaining 16.7% step is the breaking of the C–Br bond,21 in which the electron
contribution of the total light irradiation enhancement resulted transfer from the Pd atom to the Br and C atoms exists. These
from the light with wavelengths between 400–510 nm and 620– photo-generated electrons in the conduction band, together with
800 nm account for. Given that the strong plasmon resonance the injected SPR electrons from Ag NPs, could transfer to Pd
absorption of Ag–AgBr is in the wavelength range of about 500 nanoparticles located on the surface of GO nanosheets and then
nm to 650 nm (Fig. 2), the above results indicated that the silver initiate the catalytic reaction. The energetic electrons at the
nanostructures' SPR plays an important role in enhancing the surface Pd sites increase the electron density and thus could
reaction yield.
enhance the intrinsic catalytic activity of Pd in promoting the
Additional control experiments were conducted to further coupling reaction.
investigate the catalytic mechanism of GO–Pd@Ag–AgBr for the
To examine the efficacy of the visible light responsive pho-
coupling reaction. Under visible light irradiation, both AgBr and tocatalyst, the coupling reactions of a series of substituted aryl
Ag nanoparticles can produce photo-generated electrons and halides and phenylboronic acids were carried out under visible
holes. In principle, the energetic electrons and holes could light irradiations (Table 1). It is known that the yields of the
activate the electron-decient and electron-rich intermediates Suzuki coupling reactions depend on the type of halide
respectively, to facilitate various organocatalytic reactions, element, the positions of the substitutional groups and their
including C–C coupling reactions.11 The role of the photo- electron withdrawing/donating capabilities. When the active
generated holes in promoting the Suzuki coupling reaction substrates, iodobenzenes, were used, the coupling reaction can
was investigated by adding hole scavenger. As a result, aer the be completed within 30 min with yields of 96–97% (Table 1,
addition of hole scavenger, i.e. triethanolamine, no product was entries 1 and 2). For the less reactive bromobenzene, the pho-
obtained under light irradiation (Table S1,† entry 6), However, tocatalytic coupling reactions could also proceeded well and the
in the presence of triethanolamine, the reaction could proceed reactions could be completed in 1–2 hours with a good to
at 80 ꢀC without light irradiations (Table S1,† entry 7). The excellent yields. Bromobenzenes with electron-donating groups
above results clearly demonstrated that the light induced (CH3, and OCH3) and electron-withdraw groups (CHO and
Suzuki coupling reaction can only be realized with the CH3CO) were examined to investigate the electronic inuence
involvement of photo-generated holes, which could be ascribed on the catalytic performance of the photocatalyst. It was
to that the holes could activate arylboronic acids by cleaving the observed that the electron-donating groups had a negative effect
C–B bonds, and this is in agreement with the results of litera- on the conversions of the corresponding coupled products.
ture.11 According to the activation mechanism of Pd-containing Moreover, yields of 78% and 90% were obtained even when
catalysts, the activity of Pd-based catalysts would be enhanced inactive substrates, such as o-tolylboronic acid, were supplied
by increasing their electron density.20 The photo-generated (Table 1, entries 9 and 10). In contrast, only a yield of 6% was
electrons transferred to the Pd sites might enhance the cata- obtained for the coupling reaction of inactive chlorobenzene
lytic activity of Pd.
and phenylboronic acid, which indicated that the photocatalyst
Without inert atmosphere, the yield of the product decreased can not activate the chlorobenzene substrates.
to 87% (Table S1,† entry 8). The result could be explained as
The recyclability and reusability of a heterogeneous catalyst
follows: The photo-generated electrons coming from AgBr and is a very important aspect for its practical applications. Aer the
Ag nanoparticles can be transferred to GO, and subsequently photocatalytic reaction, GO–Pd@Ag–AgBr can be recycled by
combined with the dissolved oxygen to produce superoxide centrifugation and washing with water and ethanol, respec-
anion radicals (cO2ꢂ). The active species, cO2ꢂ, can decompose tively. The XRD pattern of the used photocatalyst (see Fig. S5†)
the adsorbed organic molecules and induce the formation of demonstrated that the structure has not been changed except
undesired products. And the consumption of photo-generated for the amount of metallic Ag increased slightly, which could be
electrons by oxygen is unfavorable for increasing the electron ascribed to the photoreduction of AgBr when it endured pro-
density of Pd.
longed light irradiation. The photocatalytic activity of the used
Although a more precise reaction mechanism for the photo- GO–Pd@Ag–AgBr did not have a signicant loss aer three
induced coupling reaction is still under investigation, a plau- runs. However, the catalyst was gradually deactivated from the
sible reaction mechanism can be depicted as Fig. S4† on the basis fourth run on. The cause for that might be attributed to the
of the above investigations and previous reports. Under light leaching of Pd. In fact 18% of Pd was leached aer four runs.
illumination, the metal Ag could effectively act as miniature Further experiments are required to tackle the issue of the Pd
antennae to absorb light. The SPR-excited electrons would be leaching problem to maintain the stability of the catalyst. It is
generated and enriched at the surface of Ag nanoparticles. Since worth noting that bare Pd nanostructures or even supported Pd
the Fermi energy level of AgBr is lower than that of Ag, so the NPs in conventional Suzuki coupling reactions also usually
transfer of SPR electrons from Ag to the conduction band of AgBr suffer from signicant loss in the activity due to Pd leaching.22
is energetically favorable. Apart from the plasmonic photo-
In conclusion, a plasmonic Ag–AgBr photocatalyst func-
catalysis process, the conventional AgBr-based semiconductor tionalized with graphene oxide–Pd was successfully fabricated
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Table
1 Suzuki–Miyaura coupling reactions catalyzed by GO–
structure of the GO–Pd@Ag–AgBr could signicantly enhance
not only their catalytic activity but also their ability to exploit the
light. This ability was vital to the development of a new catalyst
system that could effectively drive chemical conversion under
mild conditions. The ndings in the current study revealed that
the combination of a photocatalyst and a metal with an intrinsic
catalytic activity could be a class of potential photocatalysts for
driving chemical reactions. The ndings in the current work
could provide a general guiding line for designing new catalyst
structures to catalyze specic reactions where the light, instead
of heat is used as the energy source. The knowledge acquired in
this study may inspire further studies in searching for the new
efficient visible-light-responsive photocatalysts for a wider
range of organic synthesis.
Pd@Ag–AgBra
Entry
Aryl halides
Arylboronic acids
Time/h
Yield/%
1
2
0.5
0.5
97
96
3
4
5
6
7
1
1
1
2
2
99
99
99
99
84
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (no. 31171698, 31471643) and
Natural Science Foundation of Hebei Province (B2011204051).
Notes and references
8
2
1
2
1
82
78
90
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Reaction conditions: 2 mL of water, 2 mL of ethanol, 1.5 mmol K2CO3,
0.6 mmol benzeneboronic acid, 0.5 mmol bromobenzene, 25 mg of
catalyst, N2 atmosphere, 300
W Xe lamp with UV cutoff lter
(providing visible light with >400 nm), 25 ꢀC.
and used as an efficient visible-light-responsive photocatalyst
for the Suzuki coupling reactions. Pd-free Ag–AgBr and Ag–
AgBr-free GO–Pd were inactive, whereas the GO–Pd@Ag–AgBr
sample was active in the photocatalytic coupling reaction. These
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