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Song Bai, Jing Ge, Lili Wang, Ming Gong, Mingsen Deng, Qiao Kong, Li Song,
Jun Jiang,* Qun Zhang,* Yi Luo, Yi Xie, and Yujie Xiong*
Photoexcited electrons need to be efficiently separated from
the holes in a semiconductor, allowing their delivery to specific
sites for various applications including photovoltaics and photocatalysis.[1,2] For photocatalysis, reaction molecules receive the
charges (i.e., electrons and holes) from the surface sites for further redox reactions. In competition with this charge-transfer
process is electron–hole (e–h) recombination, which is a key
factor that limits the quantum efficiency of semiconductors
in related applications.[3] In order to suppress e–h recombination, a widely used approach is to decorate metal nanoparticles
on an n-type semiconductor,[1,4] whereby the photoexcited electrons migrate to the metal across a junction and become trapped
due to the Schottky barrier whereas the holes freely diffuse to
the semiconductor surface (Figure 1a). In comparison, when a
p-type semiconductor is used to form an interface with the metal
particles, the carriers will flow in the opposite direction, whereby
the holes transfer to the metal (also driven by the Schottky barrier) and the photoexcited electrons are left on the semiconductor
(Figure 1b).[3] In both cases, the efficacy of such a strategy for
reducing e–h recombination is often subjected to two factors: 1)
the emergence of defects through the formation of an interface
in the hybrid structures, as recombination tends to occur in bulk
defects or on surface defects;[1,5–8] 2) the lack of bulk-to-surface
channels for the electrons and/or holes, as the formed interface
reduces the exposed surfaces of the metal and the semiconductor
and thus makes it difficult for the reaction molecules to pick up
the photogenerated charges for redox reactions on the surface.[9]
In this communication, we demonstrate a novel semiconductor–metal–graphene stack design, which can perfectly overcome these two limitations and allows the harnessing of the
photo-induced charge flow for efficient e–h separation. Our
proof-of-concept demonstration is based on the use of cuprous
S. Bai,[+] J. Ge,[+] L. Wang, Prof. M. Gong, Dr. M. Deng,
Q. Kong, Prof. L. Song, Prof. J. Jiang, Prof. Q. Zhang,
Prof. Y. Luo, Prof. Y. Xie, Prof. Y. Xiong
Hefei National Laboratory for Physical
Sciences at the Microscale
Collaborative Innovation Center of Chemistry
for Energy Materials
School of Chemistry and Materials Science
Laboratory of Engineering and Material Science
National Synchrotron Radiation Laboratory
University of Science and Technology of China
Hefei, Anhui 230026, P.R. China
E-mail: [email protected]; [email protected]; [email protected]
[+]These
authors contributed equally to this work.
DOI: 10.1002/adma.201401817
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A Unique Semiconductor–Metal–Graphene Stack Design to
Harness Charge Flow for Photocatalysis
oxide (Cu2O) as a semiconductor in the stack structure. In the
literature, a study of a Au–Cu2O core–shell system has shown
that Cu2O is present in its single-crystal form in the hybrid
structure.[10,11] We thus employed Cu2O as the semiconductor
in our stack design, as the single-crystal nature of Cu2O would
reduce the number of interfacial defects. In addition, Cu2O, a
p-type semiconductor with a direct bandgap of around 2 eV, is
known to be capable of harvesting solar energy in a very broad
visible spectral region and has been widely employed as a prototypical photocatalyst in various reaction systems.[12–16] With
such a p-type semiconductor implemented in the metal–semiconductor hybrid structures the holes are efficiently extracted
by the Schottky barrier (see the difference in Figure 1a and 1b).
In fact, the low mobility of hole carriers in bare semiconductors
constitutes one of the bottlenecks for their efficiency in photocatalysis.[1,17] The charge flow in the p-type semiconductor–
metal hybrid structures may expedite the migration of holes
from the semiconductor to the metal, thus better balancing the
rates of photogenerated electrons and holes that reach the surface so that more efficient redox reactions can be accomplished.
We first examined the charge behavior in Cu2O nanocubes
decorated with Pd, a widely used semiconductor–metal hybrid
configuration. Pd has been demonstrated as a metal with a
perfect work function for forming a Schottky junction with
the {100} facets of the Cu2O nanocubes.[18] Figure 1c and 1d
show the morphologies of the bare and Pd-decorated Cu2O
nanocubes used in our study, respectively (also see Figure S1,
Supporting Information). The Pd nanoparticles have irregular
shapes and facets that normally cause tremendous interfacial
defects in the hybrid structures. To examine the charge-separation behavior, we employed photocurrent measurements
to characterize the samples under illumination of <550 nm
(which corresponds to the bandgap of Cu2O) (Figure 1e). The
photocurrents for photoelectrodes made of the Pd-decorated
Cu2O are roughly twice as high as those of the bare Cu2O nanocubes. Given that the samples have a comparable capability for
light absorption below 550 nm (Figure S2, Supporting Information), the photocurrent measurements are believed to offer an
informative evaluation for the efficiency of e–h separation.
With this acquired information, the question remained why
our metal-decorated semiconductor structure did not induce
a significant improvement in the charge separation.In order
to reveal this we employed ultrafast transient absorption (TA)
spectroscopy, a robust tool for real-time tracking of the carrier
dynamics that are involved in nanostructured systems,[19–21] to
characterize the samples. Using a time-resolved, visible pump/
white-light continuum (WLC) probe scheme spanning several
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Figure 1. a,b) Schematic band diagrams showing the migration of photoexcited carriers to the metal driven by the Schottky junction between metal
and a) n-type semiconductor or b) p-type semiconductor. c,d) TEM images of the synthesized c) bare Cu2O nanocubes and d) Pd-decorated Cu2O
nanocubes. e) Photocurrent vs. potential responses of photoelectrodes made of bare Cu2O nanocubes and Pd-decorated Cu2O nanocubes at the same
Cu2O loading weight, measured in a 0.5 M Na2SO4 electrolyte under chopped irradiation (λ < 550 nm). f) Ultrafast transient absorption signal (probed
at 600 nm) as a function of probe delay for bare Cu2O nanocubes and Pd-decorated Cu2O nanocubes, recorded with a 480-nm pump. The inset of (f)
shows the PL spectra of the corresponding samples excited at 275 nm.
nanoseconds with femtosecond resolution, we investigated the
bare Cu2O nanocubes and the Cu2O nanocubes decorated with
Pd nanoparticles to evaluate their performance for e–h separation. The 500–700 nm WLC probe yielded essentially the same
TA features when pumped with a center wavelength at 480 nm,
and a set of representative data taken at 600 nm is represented
in Figure 1f. From the temporally resolved TA spectra shown in
Figure 1f, one can see that bare Cu2O exhibits a decay profile
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of the positively valued excited-state absorption (ESA) transients
within the initial time window of around 100 ps after photoexcitation, followed by the negatively valued TA transients. The
negatively valued transients can be assigned to ground-state
bleaching (GSB) and stimulated-emission (SE). Basically, the
carrier dynamics of interest can be retrieved from the ESA,
GSB, and SE transients: 1) the ESA tracks the evolution of
energized electrons in their excited states; 2) the GSB reflects
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Figure 2. a) TEM image of the synthesized Pd−Cu2O core−shell structures. b) Schematic illustration for the limitation in metal–semiconductor core–
shell structures. c) Ultrafast transient absorption signal (probed at 600 nm) as a function of probe delay for Pd−Cu2O core−shell structures in reference
to bare Cu2O nanocubes, recorded with a 480-nm pump. The inset of (c) shows the PL spectra of the corresponding samples excited at 275 nm. d)
Photocurrent vs. potential responses of photoelectrodes made of Pd−Cu2O core−shell structures in reference to bare Cu2O nanocubes at the same
Cu2O loading weight, measured in a 0.5 M Na2SO4 electrolyte under chopped irradiation (λ < 550 nm).
the population of holes in the ground state or electrons around
the conduction band edge; 3) the SE results from radiative
recombination of energized electrons with holes. It is expected
that for a well-designed photocatalyst structure, the TA signals
arising from SE and GSB can be greatly suppressed. Evidently,
the TA spectra of bare Cu2O show distinct SE and GSB signals indicating a low efficiency of charge separation. To verify
this finding, we collected the photoluminescence (PL) emission
spectra of both samples (inset of Figure 1f; see also the excitation spectrum in Figure S3, Supporting Information) which
may provide insight into the e–h recombination. The welldefined PL peaks suggest remarkable charge recombination
in the bare Cu2O. In comparison, this situation is not significantly improved when the Cu2O nanocubes are decorated with
Pd nanoparticles, as observed from both the TA and PL spectra.
Specifically, the PL spectrum of the Pd-decorated Cu2O sample
shows an enhancement of photon emission at >600 nm (where
the energy is lower than the Cu2O energy gap), which can be
ascribed to the e–h recombination of the shallow surface states
coming from interfacial defects.[21] Given that the Schottky junction aids in facilitating the charge separation, this finding suggests that certain amounts of defects serving as recombination
centers may be involved in the Cu2O–Pd interface in the decorated configuration.
Intuitively, the metal–semiconductor core–shell structures
could be candidates to reduce the number of interfacial defects,
as long as the interface is built based on single-crystal cores and
shells. In this hybrid structure the charge transfer occurring
Adv. Mater. 2014, 26, 5689–5695
at the metal–semiconductor interface can also benefit from
a larger interfacial area. Moreover, the metal–semiconductor
interface could be protected against corrosion in the reaction
medium. For this reason, we synthesized Pd–Cu2O core–shell
structures as shown in Figure 2a. Unfortunately, the core–shell
structure bears an intrinsic drawback: it is hard for the holes
created in the cores to be transferred beyond the shells,[9] as
illustrated in Figure 2b. The holes accumulated at the metal–
semiconductor interface should be regularly depleted; if not,
they can recombine with the photoexcited electrons in the semiconductor shells. Electron-donating molecules in photocatalysis
are known to consume holes but they require a charge-transfer
“vehicle” with high mobility that obviously cannot be offered by
the core–shell structure. This feature has been evidenced by the
ultrafast TA spectra recorded for the core–shell structures. Predominant SE and GSB transients were observed in the Pd–Cu2O
core–shell sample (Figure 2c), which is consistent with the PL
observations (inset of Figure 2c). Nevertheless, the unchanged
photon emissions of the shallow surface states (>600 nm) imply
the success in reducing the number of interfacial defects.[22]
These observations suggest that the inevitable trapping of holes
in the cores still causes a high recombination rate with the energized electrons in the shells across the interface. Consequently,
similarly to the decorated configuration the core–shell system
does not exhibit a significant improvement of the charge separation, as verified by the photocurrent tests (Figure 2d).
We then decided to adopt a different approach to circumvent
this undesirable situation. As illustrated in Figure 3a, this new
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Figure 3. a) Schematic illustration for the charge flow in semiconductor–metal–graphene stack design. b) Schematic illustration of the synthetic
approach. c) TEM and d) HRTEM images of the synthesized Cu2O–Pd–rGO stack structures. e) Ultrafast transient absorption signal (probed at
600 nm) as a function of probe delay for Cu2O–Pd–rGO stack structures in reference to bare Cu2O nanocubes, recorded with a 480-nm pump. The
inset of (e) shows the PL spectra of the corresponding samples excited at 275 nm. f) Photocurrent vs. potential responses of photoelectrodes made of
Cu2O–Pd–rGO stack structures in reference to bare Cu2O nanocubes at the same Cu2O loading weight, measured in a 0.5 M Na2SO4 electrolyte under
chopped irradiation (λ < 550 nm).
design perfectly integrates a graphene sheet, which is a wellknown two-dimensional material with high in-plane conductivity,[23] into our metal–p-type semiconductor core–shell structure. As the graphene sheet is in intimate contact with the metal
component in our design, we expect that the holes accumulated
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on the metal can migrate to the graphene through a metal-tographene pathway and be consumed by oxidized molecules in
due course, whereas the energized electrons stay on the semiconductor for reduction reactions. Graphene-based materials
have been extensively used in solar-conversion applications
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This ternary stack was proven to substantially suppress
undesired e–h recombination, which is readily identified
from its ultrafast TA spectra (Figure 3e). The SE and GSB
transients associated with charge recombination are shown
to nearly vanish in the TA spectrum of our ternary stack,
which also conforms well with the PL measurements (inset of
Figure 3e). These observations indicate that the holes in the
ground state have been substantially dispersed throughout the
graphene sheet, and at the same time the defect density on the
metal–semiconductor interface remained low. Markedly, such
improvement has led to a roughly 7-fold photocurrent enhancement as compared to the bare Cu2O at –0.5 V (Figure 3f).
Moreover, the stack structure shows much smaller radii of the
semicircular Nyquist plots as compared to the other samples
in electrochemical impedance spectroscopy (EIS) (Figure S11,
Supporting Information), indicating a lower charge-transfer
resistance. These data clearly demonstrate that the harmful e–h
recombination has been greatly suppressed by the fast chargetransfer processes in the ternary stack.
To better understand the interfaces involved, we analyzed the
electronic structures of the Cu2O–Pd and Pd–graphene interfaces by first-principles simulations.[34] It was found that the
partial projected density of states (PDOS) of Pd(100) were significantly altered upon interfacing with Cu2O(100) or graphene
(Figure S12, Supporting Information), suggesting a strong
hybridization of their electronic states. This is indicative of the
high probability of charge transfer at the interfaces of Cu2O–Pd
and Pd–graphene. In comparison, the hybridization of a direct
Cu2O–graphene interface was relatively weak, not favoring the
charge transfer between Cu2O and graphene. This feature was
experimentally confirmed by the distinct PL and improved photocurrents for the Cu2O–rGO structure (Figure S13, Supporting
Information). It has been shown that this deficiency limits the
applications of reported semiconductor–graphene hybrid structures decorated with metal nanoparticles.[25–28] The PL and photocurrent measurements for our Pd-decorated Cu2O–rGO structures also confirmed this (Figure S14, Supporting Information).
In addition, the potential line-up diagrams show that
the interfacial barrier of the Cu2O–Pd junction is 0.414 eV
(Figure S15, Supporting Information), indicating the formation of a Schottky junction in our p-type semiconductor–metal
hybrid structure.[3] Such a Schottky junction ensures the electron flow from Pd to Cu2O. On the other hand, Pd is known
to have a relatively high work function and can easily wet the
carbon surface,[35,36] which is favorable to the formation of an
intimate interface with the graphene sheet. As there is no barrier at the Pd–graphene interface, the holes accumulated on the
Pd can freely migrate to the graphene.
The information of charge flow gleaned above validated
our concept. We were in a position to further implement our
designed materials for photocatalysis, using hydrogen production from water as a model reaction.[37] In this reaction, methanol was used as a sacrificial agent to consume the holes, and
the light source was confined to λ < 550 nm which corresponds
to the bandgap of Cu2O. It was anticipated that the improved
e–h separation by our stack design (whereby its light-absorption
capability at λ < 550 nm was retained) should result in a better
photocatalysis performance. In a proof-of-concept demonstration, the hydrogen production rates (per catalyst weight) were
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as it is a good material for charge transport (including electrons and holes).[24] Note that our stacked structure is clearly
different from the hybrid structures composed of a semiconductor, graphene, and metal that have been reported in the literature. The reported structures are semiconductor–graphene
hybrids decorated with metal nanoparticles, in which the metal
is randomly deposited on the surface of either graphene or
semiconductor.[25–28]
Our stack structure was fabricated by embedding {100}-faceted Pd nanocrystals between a single-crystal Cu2O semiconductor and a reduced graphene oxide (rGO), a system termed
Cu2O–Pd–rGO for short, as illustrated in Figure 3b. In this
synthesis, the Pd nanocrystals are firstly formed on the rGO
sheets by reducing K2PdCl4 with ascorbic acid on the graphene
oxide (GO). From the transmission electron microscopy (TEM)
images (Figure S4a, Supporting Information) it can be seen
that the Pd nanocrystals deposited on the rGO sheets have a
well-defined cubic profile with an edge length of around 10 nm.
The high-resolution TEM (HRTEM) images (Figure S4b, Supporting Information) further confirmed that the cubic nanocrystals are single crystals enclosed by Pd {100} facets.
In the next step, we conformally coated Cu2O single-crystal
shells on the Pd nanocrystals. This step remained the biggest
challenge because of the large (ca. 9.8%) lattice mismatch
between Cu2O and Pd [refer to the lattice constants: 4.27 Å
for Cu2O (JCPDS No. 78-2076) versus 3.89 Å for Pd (JCPDS
No. 65-6174)]. Previous research has indicated that to attain a
single-crystal heterostructure via conventional epitaxial growth
only a moderate lattice mismatch (<2%) between the two materials is allowed.[29] To accomplish the single-crystal growth of
our Cu2O shells, we developed a non-epitaxial growth approach
to circumvent the large lattice mismatch between Pd and Cu2O.
In this approach, the Pd–rGO samples are coated with Cu2O
shells by reducing CuSO4 with sodium ascorbate, during which
hydroxyl intermediates Cu(OH)42− appear,[30] followed by a
three-stage nucleation and crystallinization process (Figure S5–
S7, Supporting Information).
Electron-microscopy characterization demonstrates that it is
possible to grow a single-crystal Cu2O shell on the Pd core via
this method. As shown in Figure 3c, the Pd nanocubes on the
rGO sheets are conformally coated with a 15-nm thick layer of
Cu2O. The HRTEM image (Figure 3d) identifies that the core is
the original Pd nanocube and the shell is a piece of {100} facetenclosed Cu2O single crystal. As the core–shell structure is wellsustained on the rGO sheets, the components form an intimate
interface. The single-crystal nature of both Pd and Cu2O in our
stack structure ensures a high efficiency of the charge transport
in each component as compared to that of their polycrystalline
counterparts.[31] Inherited from the Pd–rGO precursors, the
Pd cores remain in direct contact with the rGO sheets in the
ternary stack. Energy-dispersive spectroscopy (EDS) mapping
analyses, together with corresponding scanning TEM (STEM)
images (Figure S8, Supporting Information), further confirmed the compositions of the Pd cores and the Cu2O shells.
It is worth noting that most O-containing functional groups
were removed from the rGO sheets during their reduction by
ascorbic acid and ascorbate (Figure S9, S10, Supporting Information),[32,33] enabling the rGO sheets to transport the charges
with a relatively high mobility.
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Acknowledgements
This work was financially supported by the 973 Program (No.
2014CB848900, 2010CB923300), the NSFC (No. 21101145, 91127042,
21173205, 91221104, 20925311), the Recruitment Program of Global
Experts, the CAS Hundred Talent Program, the CAS Strategic Priority
Research Program B (No. XDB01020000), the Fundamental Research
Funds for the Central Universities (No. WK2060190025, WK2310000035),
and the Construction Project for Guizhou Provincial Key Laboratories
(ZJ[2011]4007).
Received: April 22, 2014
Revised: May 21, 2014
Published online: June 11, 2014
Figure 4. Photocatalytic average rates of hydrogen production under irradiation (λ < 550 nm). 25 mg of Cu2O-based photocatalysts were added to
water with 25 vol % methanol for the measurements.
found to be in the order of Cu2O–Pd–rGO stack > Pd–Cu2O
core–shell ≅ Pd-decorated Cu2O > bare Cu2O (Figure 4), where
the ternary stack came out as the best as was expected (the performance of our stack was also superior to that of Cu2O–rGO,
see Figure S13d, Supporting Information). The stack structures
turned out to have an excellent photocatalytic stability, as the
morphology and composition of the stack sample was well
maintained after photocatalysis (Figure S16, Supporting Information). Note that the Pd–rGO sample did not show any photocatalytic activity in the absence of the Cu2O semiconductor.
In summary, the improved photocatalytic performance demonstrates a niche solution from our designed stack structure
for efficient charge transfer and e–h separation. As a matter
of fact, the separation of e–h pairs is a critical step to nearly
all applications involving semiconductor materials. The interfacial defect density and charge-transfer channels represent
two critical factors to these processes, both of which were well
modulated by designing our stack structure. Apparently, this
strategy is not only limited to the case of p-type semiconductors but may also be applicable to hybrid structures of metal
and n-type semiconductors, as all the limitations and solutions
presented in this work are universal; thus this work represents
an important advancement towards designing semiconductorbased hybrid materials. We envision that it will open new
windows to rationally designing hybrid materials for photoinduced applications.
Experimental Section
See the Supporting Information for materials synthesis, sample
characterizations, ultrafast spectroscopy characterizations, photocurrent
and photocatalytic measurements, and computational methods.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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