Angewandte
Chemie
DOI: 10.1002/anie.201309482
Photochemistry
Copper Nanoparticles on Graphene Support: An Efficient
Photocatalyst for Coupling of Nitroaromatics in Visible Light**
Xiaoning Guo, Caihong Hao, Guoqiang Jin, Huai-Yong Zhu,* and Xiang-Yun Guo*
Abstract: Copper is a low-cost plasmonic metal. Efficient
photocatalysts of copper nanoparticles on graphene support
are successfully developed for controllably catalyzing the
coupling reactions of aromatic nitro compounds to the
corresponding azoxy or azo compounds under visible-light
irradiation. The coupling of nitrobenzene produces azoxyben-
zene with a yield of 90% at 608C, but azobenzene with a yield
of 96% at 908C. When irradiated with natural sunlight (mean
light intensity of 0.044 WcmÀ2) at about 358C, 70% of the
nitrobenzene is converted and 57% of the product is azoben-
zene. The electrons of the copper nanoparticles gain the energy
of the incident light through a localized surface plasmon
resonance effect and photoexcitation of the bound electrons.
The excited energetic electrons at the surface of the copper
performance of Cu nanoparticles have been reported even
though Cu nanoparticles exhibit strong LSPR absorption in
the visible light range[1,3,5,6] and are catalytically active for
many reactions such as the catalytic hydrogenation of
dimethyl oxalate.[7] The primary challenge for Cu nano-
particles to be used as photocatalyst is their chemical stability
under catalytic reaction conditions. Cu nanoparticles are
easily oxidized to Cu2O or CuO in air or in the presence of
traces of molecular oxygen.[1,3,8] Recently we found that Cu2O
nanoparticles could exist stably when dispersed on graphene
sheets.[9] The Cu2O/graphene composite exhibits a remarkable
electrocatalytic activity for the oxygen reduction reaction and
the Cu2O nanoparticles on graphene cannot be oxidized to
CuO. Graphene is a two-dimensional network of sp2-bonded
carbon atoms;[10,11] and the delocalized electrons in graphene
can move freely in the network with a low resistance.[12] The
carbon vacancies or dangling bonds in graphene can influence
the electronic structure of Cu atoms on graphene and improve
their chemical stability.[13–15] Therefore, it is expected to use
graphene as the support to stabilize Cu nanoparticles. Here,
we prepared metallic Cu nanoparticles on graphene sheets
(Cu/graphene) by reducing Cu2O/graphene composites in
a mixture of H2 (5 vol%) and Ar at 5008C (see the Supporting
Information for details). The Cu/graphene samples were used
as photocatalysts for the synthesis of aromatic azoxy and azo
compounds from their corresponding aromatic nitro com-
pounds under visible-light irradiation.
Azo compounds are widely used as organic dyes, indica-
tors, food additives, and therapeutic agents.[16–18] They are
currently produced by the reduction of aromatic nitro
compounds using transition metals as the reductant,[19] or by
the reaction of diazonium salts with electron-rich aromatic
compounds.[20] The former consumes stoichiometric transition
metals, and the latter uses considerable nitrite salts. Both
reactions are noncatalytic, and generate a large amount of
inorganic waste.[21] Recently, Grirrane et al. reported that
a catalyst of gold nanoparticles supported on titania could
catalyze the production of azobenzene from nitrobenzene
through a two-step and one-pot reaction at 1008C or above.[22]
The aromatic nitro compounds are reduced to corresponding
amines under 9 bars of H2 first. Then, the amines are oxidized
to aromatic azo compounds under 5 bars of O2. Zhu et al.
found that Au/ZrO2 can catalyze the coupling of aromatic
nitro compounds to corresponding azo compounds at 408C
with high yields when illuminated with incandescent light or
ultraviolet (UV) light.[2] The goal of the present study is to
develop efficient photocatalysts using copper, the low-cost
plasmonic metal, for this important type of transformations.
Figure 1A,B show the TEM images of a Cu/graphene
catalyst with a Cu loading of 5 wt% (5 wt% Cu/graphene).
À
nanoparticles facilitate the cleavage of the N O bonds in the
aromatic nitro compounds. Hence, the catalyzed coupling
reaction can proceed under light irradiation and moderate
conditions. This study provides a green photocatalytic route for
the production of azo compounds and highlights a potential
application for graphene.
T
he localized surface plasmon resonance (LSPR) effect is
a collective oscillation of conduction electrons in metallic
nanoparticles, which resonate with the electromagnetic field
of incident light in the visible light range.[1–4] The conduction
electrons of the nanoparticles of gold (Au), silver (Ag), and
copper (Cu) can gain visible light energy through the LSPR
effect. Thus, one may use the major part of the sun spectrum
to improve the yield of chemical synthesis at ambient
temperature and pressure. Studies on light-driven reactions
catalyzed by Au or Ag nanoparticles have formed the basis of
a new and fast-expanding field in green photocatalysis.
Recently, Sarina et al. found that the Au–Pd alloy nano-
particles can strongly absorb light and efficiently enhance the
conversion of some reactions such as the Suzuki–Miyaura
cross coupling.[4] However, few studies on the photocatalytic
[*] Dr. X. N. Guo, C. H. Hao, Prof. G. Q. Jin, Prof. Dr. X. Y. Guo
State Key Laboratory of Coal Conversion
Institute of Coal Chemistry, Taiyuan 030001 (China)
E-mail: xyguo@sxicc.ac.cn
Prof. Dr. H. Y. Zhu
Chemistry Discipline, Queensland University of Technology
Brisbane, QLD 4001 (Australia)
E-mail: hy.zhu@qut.edu.au
[**] The work was financially supported by SKLCC (grant numbers
2013BWZ006 and 2014BWZ006). Thanks are due to S. Sarina for her
work on action spectroscopy. We acknowledge Sinocarbon Materi-
als Technology Co., Ltd. for providing graphene materials.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2014, 53, 1973 –1977
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1973