Article
pubs.acs.org/accounts
Effective Charge Carrier Utilization in Photocatalytic Conversions
Peng Zhang, Tuo Wang, Xiaoxia Chang, and Jinlong Gong*
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin
University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
CONSPECTUS: Continuous efforts have been devoted to searching for sustainable energy
resources to alleviate the upcoming energy crises. Among various types of new energy
resources, solar energy has been considered as one of the most promising choices, since it is
clean, sustainable, and safe. Moreover, solar energy is the most abundant renewable energy,
with a total power of 173 000 terawatts striking Earth continuously. Conversion of solar
energy into chemical energy, which could potentially provide continuous and flexible energy
supplies, has been investigated extensively. However, the conversion efficiency is still relatively
low since complicated physical, electrical, and chemical processes are involved. Therefore,
carefully designed photocatalysts with a wide absorption range of solar illumination, a high
conductivity for charge carriers, a small number of recombination centers, and fast surface
reaction kinetics are required to achieve a high activity.
This Account describes our recent efforts to enhance the utilization of charge carriers for
semiconductor photocatalysts toward efficient solar-to-chemical energy conversion. During photocatalytic reactions,
photogenerated electrons and holes are involved in complex processes to convert solar energy into chemical energy. The
initial step is the generation of charge carriers in semiconductor photocatalysts, which could be enhanced by extending the light
absorption range. Integration of plasmonic materials and introduction of self-dopants have been proved to be effective methods
to improve the light absorption ability of photocatalysts to produce larger amounts of photogenerated charge carriers.
Subsequently, the photogenerated electrons and holes migrate to the surface. Therefore, acceleration of the transport process can
result in enhanced solar energy conversion efficiency. Different strategies such as morphology control and conductivity
improvement have been demonstrated to achieve this goal. Fine-tuning of the morphology of nanostructured photocatalysts can
reduce the migration distance of charge carriers. Improving the conductivity of photocatalysts by using graphitic materials can
also improve the transport of charge carriers. Upon charge carrier migration, electrons and holes also tend to recombine. The
suppression of recombination can be achieved by constructing heterojunctions that enhance charge separation in the
photocatalysts. Surface states acting as recombination centers should also be removed to improve the photocatalytic efficiency.
Moreover, surface reactions, which are the core chemical processes during the solar energy conversion, can be enhanced by
applying cocatalysts as well as suppressing side reactions. All of these strategies have been proved to be essential for enhancing
the activities of semiconductor photocatalysts. It is hoped that delicate manipulation of photogenerated charge carriers in
semiconductor photocatalysts will hold the key to effective solar-to-chemical energy conversion.
1. INTRODUCTION
Developing sustainable energy resources is one of the most
urgent tasks for human beings since the increasing energy
demand is in drastic conflict with the limited global fossil fuel
storage. Among various types of sustainable energy resources,
solar energy is considered to be promising because of its
inexhaustibility, universality, high capacity, and environmental
benignancy.1,2 However, the solar irradiation in nature is
decentralized, fluctuant, and intermittent. Therefore, effective
utilization of solar energy in a clean, economical, and
convenient way remains a grand challege.1
Solar energy could be utilized primarily through photothermal, photovoltaic, and photocatalytic approaches. The
photocatalytic conversion of solar energy into chemical energy,
by means of artificial photosynthesis, photodegradation of dyes,
and photocatalytic chemical synthesis, could realize the
application of solar energy in a variety of fields.3−6 During
such photocatalytic processes, different photocatalysts are
usually applied to achieve high efficiencies, among which
© 2016 American Chemical Society
semiconductor materials have been investigated extensively
since they possess outstanding chemical, physical, and electrical
properties.
Complicated processes are involved in semiconductor-based
photocatalytic reactions.7 Electrons and holes are generated in
the bulk of the semiconductor photocatalyst under irradiation
with solar light (step (i) in Scheme 1). Then the electrons and
holes migrate to the surface of the photocatalyst (step (ii) in
Scheme 1). Meanwhile, electrons and holes tend to recombine
and release the energy in the form of light or heat (step (iii) in
Scheme 1). Electrons and holes that reach the surface would
take part in reduction and oxidation reactions to complete the
energy conversion process (step (iv) in Scheme 1). Since the
semiconductor-based photocatalytic conversion of solar energy
is realized by such complex photo-, electro-, and chemical
processes, simultaneous promotion of the generation, migraReceived: January 20, 2016
Published: April 14, 2016
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2. ENHANCING THE GENERATION OF CHARGE
CARRIERS
The photogenerated charge carriers play a crucial role in
photocatalytic reactions. Enhancing the generation of charge
carriers in semiconductor photocatalysts is essential to improve
their photocatalytic performance. Extending the light absorption range of some outstanding semiconductor photocatalysts
with large band gaps (e.g., TiO2 and ZnO) is an important
approach to convert more photons into excited electrons and
holes.
Integration of plasmonic metals such as Au and Ag, which
exhibit visible-light plasmonic excitations, is an effective method
to extend the light absorption range of semiconductor
photocatalysts with large band gaps. The plasmonic enhancement mechanism mainly includes (i) light absorption and
scattering, (ii) hot electron injection, and (iii) plasmon-induced
resonance energy transfer.9,10 Upon loading with plasmonic
nanoparticles, semiconductor photocatalysts can absorb and
utilize more light (especially in the visible range), extract hot
electrons from the plasmonic metals, and generate electron−
hole pairs much more readily. Consequently, the generation of
charge carriers is enhanced, leading to a higher solar energy
conversion efficiency. For example, we loaded Au nanoparticles
onto the tips of one-dimensional (1-D) ZnO nanopencil arrays
(Figure 1a,b), which showed a much higher photocurrent
density compared with the bare ZnO nanopencil arrays,
especially under visible-light irradiation.11 From the UV−vis
absorption spectra, an additional absorption peak located at
∼530 nm was observed for the Au-loaded ZnO nanopencils,
indicating that the surface plasmon resonance of Au nanoparticles could extend the light absorption range of the ZnO
nanopencils. Moreover, the plasmon energy transfer process of
Scheme 1. Schematic Illustration of Different Steps during
Semiconductor-Based Photocatalytic Solar Energy
Conversion: (i) Generation of Electron−Hole Pairs under
Solar Irradiation; (ii) Migration of Electrons and Holes to
the Surface Active Sites; (iii) Recombination of Unreacted
Electrons and Holes; (iv) Surface Reduction and Oxidation
Reactions
tion, and reaction of charge carriers is highly desirable to
enhance the activity of semiconductor photocatalysts. Moreover, suppressing the unfavorable recombination of electrons
and holes is also of great importance.8 Therefore, taking full
advantage of photogenerated charge carriers is the key for
improving the activity of photocatalysts. Different strategies,
including extending the light absorption range, controlling the
morphology, enhancing the transport of charge carriers,
constructing heterojunctions, and promoting surface reaction
kinetics, have been developed to obtain highly active photocatalysts for effective solar-to-chemical energy conversion.
Figure 1. (a) Schematic illustration and (b) transmission electron microscopy (TEM) image of Au-loaded 1-D ZnO nanopencils. (c) Schematic
illustration and (d) TEM image of Au-loaded 1-D branched TiO2 nanorods. Adapted with permission from refs 11 and 12. Copyright 2014 and 2013
Royal Society of Chemistry, respectively.
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Figure 2. (a) Schematic illustration and (b−d) corresponding TEM images of the synthesis procedures for the Au nanorod@TiO2 yolk−shell
photocatalysts. The yellow, blue, and red colors in (a) represent Au, SiO2, and TiO2 materials, respectively. The aspect ratios of the Au nanorods in
(b), (c), and (d) are 2.4, 4.2, and 5.4, respectively. Reproduced with permission from ref 13. Copyright 2015 Wiley.
the Au nanoparticles prolonged the lifetime of the charge
carriers, which also accounted for the enhanced activity. We
also integrated Au nanoparticles onto the tips of 1-D branched
TiO2 nanorod arrays (Figure 1b,c), which exhibited a
photoconversion efficiency of ∼1.27% for water oxidation at
the low bias of 0.50 V vs reversible hydrogen electron (RHE)
under 100 mW cm−2 irradiation (AM 1.5G).12 Upon loading of
Au nanoparticles, the branched TiO2 nanorod arrays showed
improved activity under visible-light irradiation. The carrier
density of the Au-loaded branched TiO2 nanorods was nearly 6
times higher than that of the pristine branched TiO2 nanorods
according to the Mott−Schottky plots, which proved that the
integration of plasmonic metals can effectively increase the
charge carrier density of semiconductor photocatalysts. By
tuning of the morphology of the plasmonic materials, the light
absorption range of the composite photocatalysts could be
further extended to the infrared region. We conducted a
multitemplating method to synthesize Au nanorod@TiO2
yolk−shell photocatalysts (Figure 2), which showed additional
light absorption peaks at larger wavelengths as a result of the
plasmonic oscillations along the longitudinal direction of the
Au nanorods (Figure 3 inset).13 Moreover, the light absorption
peaks could be tuned by adjusting the aspect ratio of the Au
nanorods. We found that the Au nanorod@TiO2 yolk−shell
photocatalyst with a medium aspect ratio exhibited the highest
activity for the photooxidation of benzyl alcohol to
benzaldehyde (1130 μmol gcat−1 benzaldehyde was generated
over 16 h, as shown in Figure 3). A balance between the
absorption of high-energy irradiation and the exposure of highly
active surface was achieved by the medium-aspect-ratio Au
nanorods, which resulted in the optimized activity.
As another important approach to extend the light
absorption range of semiconductor photocatalysts, doping
Figure 3. (a−c) Activities for photooxidation of benzyl alcohol into
benzaldehyde over different Au nanorod@TiO2 yolk−shell photocatalysts. The aspect ratios of the Au nanorods in (a), (b), and (c) are
4.2, 2.4, and 5.4, respectively. (d, e) Control experiments with Au
nanoparticle@TiO2 yolk−shell and TiO2 hollow-sphere photocatalysts. (f) Control experiment without photocatalyst. The experiments in (a−f) were carried out under visible−infrared irradiation (λ >
420 nm). (g) Control experiment without photocatalyst under dark
conditions. The inset shows the light absorption spectra of different
photocatalysts. Adapted with permission from ref 13. Copyright 2015
Wiley.
with external elements to introduce inter-band-gap energy
levels has been shown to be effective.14 However, external
dopants would also become recombination centers, which
might lower the overall efficiency of charge carrier utilization.
Recently, self-doping has been demonstrated to extend the light
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Figure 4. Photocatalytic H2 production over platinized TiO2 photocatalysts (sub-10 nm Ti3+-doped rutile TiO2 (red), Ti3+-doped rutile TiO2 (blue),
and pure rutile TiO2 (green)) under (a) UV light (320 nm < λ < 400 nm, ca. 83 mW cm−2), (b) visible light (400 nm < λ < 780 nm, ca. 80 mW
cm−2), and (c) sunlight (AM 1.5 irradiation, ca. 100 mW cm−2). (d) Cycling tests of sub-10 nm Ti3+-doped rutile TiO2 photocatalyst under visible
light. (e) Visible-light photocatalytic activity of sub-10 nm Ti3+-doped rutile TiO2 photocatalyst after long-term storage. Pt (1 wt %) was
photodeposited in situ on the surface of the samples as the cocatalyst, and 10 mL of methanol was used as the sacrificial reagent for water splitting.
Adapted with permission from ref 17. Copyright 2015 Nature Publishing Group.
Figure 5. (a) Diffuse-reflectance UV−vis spectra of sub-10 nm Ti3+-doped rutile TiO2, Ti3+-doped rutile TiO2, and pure rutile TiO2. (b) Band energy
diagram of pure rutile TiO2 and sub-10 nm Ti3+-doped rutile TiO2 demonstrated by DFT calculations. Adapted with permission from ref 17.
Copyright 2015 Nature Publishing Group.
absorption range of semiconductor photocatalysts with fewer
induced recombination centers.15 Exfoliation of layered tungstic
acid was conducted to prepare WO3 nanosheets.16 With the
subsequent introduction of oxygen vacancies by post-treatment
under vacuum or a hydrogen atmosphere, the as-prepared WO3
nanosheets became substoichiometric and showed enhanced
performance in both photocurrent response and photocatalytic
water oxidation compared with pristine WO3. The oxygen
vacancies, acting as self-dopants, could also induce surface
plasmon resonance, which extended the light harvesting range
to the near-infrared region and also promoted the light
harvesting in the UV and visible regions. We also synthesized
sub-10 nm rutile TiO2 with a large number of surface/
subsurface defects (in favor of Ti3+ self-dopant), which showed
the state-of-the-art activity among TiO2-based photocatalysts
for visible-light-driven water splitting (932 mmol h−1 g−1 H2
with 1 wt % Pt cocatalyst under 80 mW cm−2 irradiation at 400
nm < λ < 780 nm) with methanol as a sacrificial reagent (Figure
4).17 The existence of Ti3+ self-dopant greatly improved the
visible-light absorption of TiO2 (Figure 5a) by shifting the top
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of the valence band (VB) upward for band-gap narrowing,
which was confirmed by VB X-ray photoelectron spectroscopy
(XPS) experiments and density functional theory (DFT)
calculations (Figure 5b).
nanorods reduced the migration distance to the surface for the
photogenerated holes. Meanwhile, electrons were rapidly
transferred to the counter electrode because of the high
conductivities of both the well-crystallized branches and the
nanorods. In addition to nanorod structures, nanotubes with a
thin wall, which further decreases the migration distance of
charge carriers, are promising 1-D structures. We developed a
facile and general template-free approach to synthesize
graphitic C3N4 (g-C3N4) nanotubes (Figure 6c) by direct
heating of melamine powder.22 The obtained g-C3N4 showed
enhanced photocatalytic activities compared with bulk g-C3N4,
resulting from the fast transfer of charge carriers to the surface.
We also found that TiO2 photocatalysts with hollow (hemi)spherical structures (Figure 6d) exhibit superior performance
for solar energy conversion compared with spherical TiO2
photocatalysts.23,24 The thin shell of the hollow (hemi)spheres
reduces the migration distance of charge carriers. Moreover, the
high surface area of hollow (hemi)spheres provides abundant
reaction sites to promote the photocatalytic activity.25
In addition to reducing the migration distance of charge
carriers, improving the conductivity of semiconductor photocatalysts can also accelerate the transport of charge carriers. In
this scenario, the charge carriers can migrate to the surface
reaction sites within a shorter time and are more likely to take
part in surface reactions rather than recombination. To enhance
the transport of charge carriers in semiconductor photocatalysts, integration of graphitic materials has been proved to
be effective. We constructed a reduced graphene oxide (rGO)/
BiVO4 composite photocatalyst through an evaporationinduced self-assembly process (Figure 7a−c).26 The obtained
rGO/BiVO4 photocatalyst showed enhanced activity, with a
reaction rate constant for photocatalytic degradation of
methylene blue that was 2.14 times higher than that of
BiVO4. The formation of well-defined rGO/BiVO4 interfaces
and the high conductivity of rGO accounted for the improved
photocatalytic activity. Moreover, the ultrathin nature of the
3. ACCELARATING THE TRANSPORT OF CHARGE
CARRIERS
The efficiency of solar energy utilization can be enhanced by
accelerating the transport of charge carriers in semiconductor
photocatalysts, such as TiO2, Fe2O3, BiVO4, and Ta3N5, which
have low conductivities. Different strategies have been applied
to promote this process, among which reducing the size of the
photocatalysts to nanoscale to decrease the required diffusion
distance for the charge carriers is an effective approach.18
Among various nanostructures of semiconductor photocatalysts, the 1-D morphology provides large surface area,
sufficient length to absorb incident light, and a shortened
diffusion distance for minority carriers, which are all desired
properties for accelerating the transport and the utilization of
charge carriers.19
We demonstrated that 1-D ZnO nanorod arrays (Figure 6a)
showed enhanced performance toward solar water splitting
Figure 6. Scanning electron microscopy (SEM) images of (a) ZnO
nanopencil arrays, (b) branched TiO2 nanorod arrays, (c) g-C3N4
nanotubes, and (d) TiO2 nanocups. The inset of (b) shows a TEM
image of a single branched TiO2 nanorod. Adapted with permission
from refs 20−23. Copyright 2014 Elsevier, 2013 The PCCP Owner
Societies, and 2014 and 2013 Royal Society of Chemistry, respectively.
compared with planar ZnO films. When conical tips with high
aspect ratios were further grown on the top of these nanorods,
forming ZnO nanopencil arrays, a nearly doubled photocurrent
density was obtained at 1.6 V vs RHE under 100 mW cm−2
irradiation (AM 1.5G).20 Moreover, the activity of 1-D
photoelectrodes could be further improved by constructing
hierarchical nanostructures. Branched TiO2 nanorod arrays
with a hierarchical 1-D configuration (Figure 6b) showed
higher activity than TiO2 nanorod arrays.21 The tiny branches
(with diameters ranging from 10 to 20 nm) on the TiO2
Figure 7. SEM images of (a) as-synthesized BiVO4 with a smooth
surface, (b) rGO/BiVO4 with the rGO layer uniformly attached on
BiVO4 and forming a wrinkled surface, as illustrated by the inset
scheme. (c, d) TEM images of rGO/BiVO4 exhibiting a tight contact
between BiVO4 and the rGO layer of about 2 nm. Reproduced from
ref 26. Copyright 2014 American Chemical Society.
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Figure 8. (a) SEM image, (b) TEM image, and (c) schematic diagram of energy band structures and the charge separation mechanism of the
Ag3PO4/BiVO4 heterojunction photocatalyst. (d) SEM image, (e) TEM image, and (f) schematic diagram of energy band structures and the charge
separation mechanism of the g-C3N4/BiVO4 heterojunction photocatalyst. Adapted with permission from refs 31 and 32. Copyright 2013 and 2014
Wiley, respectively.
of electron−hole pairs and enhanced activities.30 However,
recombination centers could also be induced at the
heterojunction. Thus, a finely controlled interface between
the different components of the heterojunction is desirable. We
selectively deposited Ag3PO4 on the highly active (040) facets
of truncated bipyramidal monoclinic BiVO4 by an in situ
deposition method (Figure 8a,b).31 A homotype heterojunction, considered as a junction composed of semiconductors
with similar electronic band structures, was built between
visible-light-responsive Ag3PO4 and BiVO4 photocatalysts. This
heterojunction photocatalyst showed high catalytic activity for
the photodegradation of methylene blue as a result of the
enhanced separation of charge carriers and the effective
utilization of visible light (Figure 8c). A simple annealing
approach was employed to load g-C3N4 nanoislands onto the
surface of coralline BiVO4, forming heterojunction photocatalysts with intimate contact (Figure 8d,e).32 Control
experiments using metal salts as the electron detector showed
that upon irradiation with solar light, the photogenerated
electrons in the g-C3N4 were transferred to the surface of
BiVO4, while holes traveled in the reverse direction (Figure 8f).
Therefore, the charge separation efficiency was greatly
enhanced to reach high photocatalytic activity.
In addition to traditional heterojunction photocatalysts, other
types of junctions, such as p−n junctions, phase junctions, and
facet junctions, also show promotive effects on solar energy
conversion by semiconductor photocatalysts. Atomic layer
deposition (ALD) was utilized by Wang’s group to grow a thin
layer of p-type hematite (α-Fe2O3) on the surface of n-type
hematite.33 The photogenerated charge carriers could be
separated effectively by the built-in electric field created by
the p−n junction. This electric field resulted in an enhanced
photocurrent density (especially at low applied bias) and a 200
mV cathodic shift of the onset potential (Figure 9a,b). For
TiO2 photocatalysts, the surface contents of rutile and anatase
rGO (<5 nm) prevented it from competing with BiVO4 for
light absorption (Figure 7d).
In photoelectrochemical systems, improving the charge
transfer between the semiconductor photocatalyst and the
substrate is essential to enhance the activity. Domen and coworkers employed a through-mask anodization method to
synthesize vertically aligned parallel Ta3N5 nanorod arrays.27
The strong connection between the 1-D Ta3N5 nanorods and
the substrate as well as the formation of highly conductive
Ta5N6 and Ta2N phases at the interface, which facilated the
transport of electrons, played very important roles in enhancing
their photoelectrochemical properties. We used a two-step
anodization method to obtain tightly adhered Ta3N5 nanotube
arrays on Ta substrate, which showed a high activity toward
solar water oxidation as a result of the bridged transport
pathway for charge carriers.28 This two-step anodization
approach was also carried out by Wang et al. to synthesize
well-ordered Ta3N5 nanotubes with an improved activity for
solar water splitting.29 It was demostrated that the formation of
subnitride (TaNx) at the nitride−metal interface accounted for
the high photocatalytic activity.
4. SUPPRESSING THE RECOMBINATION OF CHARGE
CARRIERS
Recombination is one of the most unfavorable processes that
takes place commonly in semiconductor photocatalysts
(especially in materials such as Fe2O3 and BiVO4). Thus,
reducing the possibility of recombination is an important aspect
of enhancing the activity of photocatalysts toward effective solar
energy conversion. Constructing heterojunctions is an effective
approach to achieve this goal. In a typical heterojunction
photocatalyst, the different band-edge energetics of semiconductor photocatalysts can lead to enhanced separation of
photogenerated charge carriers, achieving prolonged lifetimes
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Figure 9. (a) Photocurrent density−potential (J−V) curves under 100 mW cm−2 irradiation (AM 1.5G), with dark currents shown in dashed lines,
and (b) incident photon to charge conversion efficiencies (IPCEs) of n-type Fe2O3 with and without p-type coating. (c) SEM image of BiVO4 with
Pt and MnOx photodeposited on {010} and {110} facets, respectively. The inset shows a schematic illustration of the structure. (d) Photocatalytic
water oxidation performance of BiVO4 with cocatalysts prepared with different methods (imp, impregnation method; P.D., photodeposition method;
the contents of the deposited cocatalysts are all 0.1 wt %). Adapted from ref 33 and with permission from ref 35. Copyright 2012 American Chemical
Society and 2013 Nature Publishing Group, respectively.
5. PROMOTING THE REACTION KINETICS OF CHARGE
CARRIERS
Compared with the rapid generation and transport of charge
carriers in semiconductor photocatalysts (within femtoseconds
to picoseconds), surface reduction and oxidation reactions
normally take place on a longer time scale (several hundred
picoseconds to microseconds).39 The energy barrier makes the
surface reaction the rate-determining step of the whole
photocatalytic process. Slow surface reactions are the
prominent problem for materials such as Fe2O3, BiVO4, and
ZnO. Therefore, acceleration of the surface reactions is
essential to improve the solar energy utilization efficiency.
Integration of cocatalysts has been widely investigated as an
effective approach to enhance the surface reaction kinetics of
semiconductor photocatalysts. Cocatalysts can promote the
separation of charge carriers by providing trapping sites that are
selective for either electrons or holes, improve the stability of
photocatalysts by timely consumption of charge carriers, and
most importantly, lower the activation energy of surface redox
reactions.40 Among various cocatalysts, cobalt-based materials
have shown a great potential to improve the activities of
oxidation reactions on semiconductor photocatalysts. We
integrated discrete nanoisland p-type Co3O4 cocatalysts onto
n-type BiVO4 photoanodes (Figure 10a,b).41 The Co3O4
cocatalysts effectively enhanced the surface reaction kinetics
and the photocatalytic activities (Figure 10c,d). Moreover, the
phases could be tuned simply by changing the annealing
temperature.34 The phase junction formed between the surface
anatase nanoparticles and rutile particles, which promotes the
spatial separation of charge carriers, can greatly enhance the
activity for solar water splitting. Li et al.35 demonstrated that
efficient charge separation can be achieved on different crystal
facets of BiVO4 (i.e., {010} and {110} facets; Figure 9c).
Moreover, selective deposition of reduction and oxidation
cocatalysts onto electron- and hole-enriched facets, respectively,
led to dramatically improved activities, with a water oxidation
rate as high as ∼650 μmol h−1 g−1 (Figure 9d).
Recombination of charge carriers at surface defect states is
apparently an undesired process for solar energy conversion
that can be partly overcome by surface passivation.36 Applying
oxide passivating overlayers by a chemical bath deposition
method has been proved by Grätzel’s group to be effective in
decreasing the density of surface defect states on an α-Fe2O3
photoelectrode.37 After the loading of overlayers, the onset
potential of the α-Fe2O3 photoanode showed a 200 mV
cathodic shift, indicating the removal of surface defects.
Hamann’s group demonstrated that treatment of an α-Fe2O3
photoanode at a high temperature (800 °C) can successfully
reduce the number of surface defect states with an energy
slightly below the flat band potential, which act as electron−
hole recombination centers.38
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Figure 10. (a) Schematic of the p−n Co3O4/BiVO4 photoanode. (b) SEM image of 4-Co/BV. (c, d) J−V curves for (c) water oxidation and (d)
sulfite oxidation with different photoanodes measured with 100 mW cm−2 irradiation (AM 1.5G). (e, f) Charge separation efficiency (e) in the bulk
(ηbulk) and (f) on the surface (ηsurface) of photoanodes. 2-Co/BV, 4-Co/BV, and 8-Co/BV represent Co3O4/BiVO4 photoanodes with the Co3O4
mass ratios of 2%, 4%, and 8%, respectively. Adapted from ref 41. Copyright 2015 American Chemical Society.
electron one-step reaction, was proposed. Specifically, the
conversion of H2O into H2O2 first took place on the surface of
C3N4, followed by CDot-catalyzed H2O2 decomposition into
H2O and O2.
Side reactions, such as the photocorrosion reaction, would
compete with the water splitting reaction for photogenerated
charge carriers and subsequently jeopardize the stability of the
photocatalyst. For instance, ZnO is an earth-abundant photocatalyst with high electrical conductivity and relatively high
activity. However, it suffers from photocorrosion caused by
photogenerated holes under UV-light irradiation, especially
under extreme pH conditions. To solve this problem, an
ultrathin (1.5 nm) amorphous Ta2O5 film was coated on the
surface of ZnO nanorods by an ALD approach (Figure 11a).45
The uniform Ta2O5 layer prevented the corrosion of ZnO and
reduced the surface recombination, leading to stable solar water
splitting activity for over 5 h. Additionally, an enhanced
photocurrent density and a nearly 2-fold increase in the
photoconversion efficiency were also obtained (Figure 11b−d).
The chemical inertness and optical transparency of Ta2O5
toward sunlight were the main reasons for the improved
stability. Moreover, the passivation effect of Ta2O5 accounted
for the enhanced activity by suppressing surface recombination.
p−n junction formed between Co3O4 and BiVO4 promoted the
separation of charge carriers simultaneously. Because of the
synergistic effect of the cocatalyst and the p−n junction, charge
separation efficiencies of up to 77% in the bulk and 47% on the
surface of the catalysts were obtained, with a photoconversion
efficiency of 0.659% (Figure 10e,f). A cobalt phosphate (Co-Pi)
oxygen evolution reaction cocatalyst was first reported by
Kanan and Nocera in 2008.42 Since then, Co-Pi systems for
solar water splitting operated at neutral pH with a moderate
overpotential have become an intensively investigated topic
because of their superior performance. Dual-cocatalyst loading
was performed by Kim and Choi43 to obtain a BiVO4/
FeOOH/NiOOH photoelectrode for solar water splitting.
They indicated that serially applying FeOOH and NiOOH
cocatalysts could reduce the possibility of interface recombination at the BiVO4−cocatalyst interface. At the same time, this
BiVO4/FeOOH/NiOOH photoelectrode could create a more
favorable Helmholtz layer potential drop at the solid−
electrolyte junction.
More recently, nonmetal-based cocatalysts have been
investigated by Kang and co-workers to synthesize a metalfree carbon nanodots−C3N4 (CDots−C3N4) visible-lightresponsive photocatalyst, which showed an unassisted overall
solar energy conversion efficiency of 2.0%.44 A stepwise twoelectron two-step pathway, in contrast to the conventional four918
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carrier utilization. Moreover, the innovative design and delicate
fabrication of active photocatalysts and solar energy conversion
systems are also important. Optimizing the performance of
semiconductor photocatalysts by simultaneously enhancing the
processes involving charge carriers can be achieved by
combining different strategies. Loading of highly effective
cocatalysts on the active facets of semiconductor photocatalysts
to form an intimate interface (i.e., with less mismatch and fewer
recombination centers) holds promise to boost the solar energy
conversion efficiency. Investigation of bifunctional cocatalysts,
which could not only enhance the surface reactions but also
have plasmonic or upconversion effects to extend the light
absorption range, is also possible to improve the activity. We
believe that future efforts are surely not limited to what is
mentioned above. The fast development in this field will finally
lead to the delicate manipulation of photogenerated charge
carriers in semiconductor photocatalysts, which is the key to
effective photocatalytic conversions.
■
Figure 11. (a) TEM image of a ZnO nanorod coated with a Ta2O5
layer. The inset shows the selected-area electron diffraction pattern of
the ZnO/Ta2O5 nanorod. (b) Stability test at 1.23 V vs RHE under
100 mW cm−2 irradiation (AM 1.5G), (c) I−V curves under 100 mW
cm−2 irradiation (AM 1.5G), and (d) photoconversion efficiency as a
function of applied potential vs RHE for ZnO nanorods and ZnO/
Ta2O5 nanorods. Adapted with permission from ref 45. Copyright
2015 Royal Society of Chemistry.
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Fax: +86-22-87401818.
Notes
The authors declare no competing financial interest.
Biographies
6. SUMMARY AND OUTLOOK
Taking full advantage of photogenerated charge carriers in
semiconductor photocatalysts is the key to realizing efficient
solar energy utilization. In particular, enhancing the generation,
accelerating the transport, suppressing the recombination, and
promoting the reaction kinetics of charge carriers are urgently
desired. Different strategies, including extension of the light
absorption range, morphological control, charge transfer
enhancement, heterojunction construction, and surface reaction
promotion, have been proved to be of great significance,
leading to improved efficiency of charge carrier utilization.
However, precise manipulation of charge carriers with greater
controllability and directionality is still a challenging mission,
requiring further efforts and cooperation of researchers from
different fields.
The grand challenges in fine manipulation of electrons and
holes in semiconductor photocatalysts provide many opportunities for breakthroughs. For instance, the exact band-gap
configuration and alignment between the semiconductor
photocatalyst and the cocatalyst are rarely discussed in the
literature. Moreover, there is no widely accepted answer to the
question of specific reaction pathways what are electrons and
holes at the surface of photocatalysts or cocatalysts. To better
understand the mechanism and realize elaborate manipulation
of charge carriers in photocatalysts, innovative utilization of
modern characterization techniques is needed. X-ray absorption
techniques could provide detailed information about the surface
and structural properties of semiconductor photocatalysts.
Transient absorption spectra can be used to obtain information
about the lifetime of photogenerated charge carriers. Scanning
electrochemical microscopy is useful for learning about the
local electrochemical behavior of interfaces. Furthermore, the
development of in situ characterization techniques is very
important for the study of photocatalytic mechanisms. Such
techniques can provide insights into the behaviors of electrons
and holes under real reaction conditions, which can help with
the design of photocatalysts with high efficiencies for charge
Peng Zhang received his B.S. and Ph.D. degrees in chemical
engineering from Tianjin University in 2010 and 2015, respectively,
under the tutelage of Professor Jinlong Gong. He is currently a
postodoctoral fellow at Nanyang Technological University with
Professor Xiongwen Lou. His research focuses on the synthesis of
multifunctional materials and advanced utilization of solar energy.
Tuo Wang received his B.S. degree from Tianjin University and his
Ph.D. degree from the University of Texas at Austin, both in chemical
engineering. After gaining another year of research experience as a
postdoctoral associate, he joined Novellus Systems, Inc., in Tualatin,
OR, as a process development engineer. Since August 2012 he has
been an associate professor of chemical engineering in Tianjin
University. His research focuses on nanostructured materials for
energy conversion and storage systems.
Xiaoxia Chang received his B.S. degree in chemical engineering from
Tianjin University and is currently a Ph.D. candidate under the
supervision of Professor Jinlong Gong. His research focuses on fuel
production by solar water splitting and photocatalytic CO2 reduction.
Jinlong Gong studied chemical engineering and obtained his B.S. and
M.S. degrees from Tianjin University and his Ph.D. degree from the
University of Texas at Austin under the guidance of C. B. Mullins.
After a stint with Professor George M. Whitesides as a postdoctoral
fellow at Harvard University, he joined the faculty of Tianjin
University, where he currently holds a Pei Yang Professorship in
chemical engineering. His research interests in surface science and
catalysis include catalytic conversions of green energy, novel
utilizations of carbon oxides, and synthesis and applications of
nanostructured materials. He is an elected Fellow of the Royal Society
of Chemistry.
■
ACKNOWLEDGMENTS
We acknowledge the National Natural Science Foundation of
China (21525626, U1463205, 21222604, and 51302185), the
Program for New Century Excellent Talents in University
(NCET-10-0611), the Scientific Research Foundation for the
919
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