Nghiên cứu tổng hợp vật liệu quang xúc tác nano: Heterojunction Photocatalysts

Review

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Heterojunction Photocatalysts
Jingxiang Low, Jiaguo Yu,* Mietek Jaroniec, Swelm Wageh, and Ahmed A. Al-Ghamdi
applications.[8–10] However, the practical
applications of photocatalysis are still limited by its low photocatalytic activity.[11–14]
In simple terms, a photocatalytic reaction on a semiconductor includes at least
five main steps: i) light absorption by the
semiconductor, ii) formation of photogenerated electron–hole pairs, iii) migration
and recombination of the photogenerated
electron–hole pairs, iv) adsorption of reactants and desorption of products, and
v) occurrence of redox reactions on the
semiconductor surface (see Figure 1).
Among them, the recombination of electron–hole pairs plays a negative role in
the photocatalytic processes.[15–17] During
photocatalytic reaction, the photogenerated
electron–hole pairs can either transfer to
the photocatalyst surface and initiate redox
reactions, or recombine and create useless
heat. To better understand this process, let us illustrate it by a
somewhat similar situation: the effect of the gravitational force
on a man jumping off the ground (see Figure 2a,b). The expressions for both the gravitational force acting on a man jumping
off the ground and the Coulomb force acting between the electron and the hole have similar forms:

Semiconductor-based photocatalysis attracts wide attention because of its
ability to directly utilize solar energy for production of solar fuels, such as
hydrogen and hydrocarbon fuels and for degradation of various pollutants.

However, the efficiency of photocatalytic reactions remains low due to the
fast electron–hole recombination and low light utilization. Therefore, enormous efforts have been undertaken to solve these problems. Particularly,
properly engineered heterojunction photocatalysts are shown to be able to
possess higher photocatalytic activity because of spatial separation of photo­
generated electron–hole pairs. Here, the basic principles of various heterojunction photocatalysts are systematically discussed. Recent efforts toward
the development of heterojunction photocatalysts for various photocatalytic
applications are also presented and appraised. Finally, a brief summary and
perspectives on the challenges and future directions in the area of heterojunction photocatalysts are also provided.

1. Introduction
A fast-growing industry and the rising global population in
recent years are the key factors contributing to the energy
shortage and environmental pollution. Thus, to assure a longterm and sustainable development of human society, there
is an urgent need for the development of environmentally
friendly and renewable technologies for green energy production and environmental remediation. Among the various proposed technologies, semiconductor-based photocatalysis has
great potential because it directly utilizes solar energy both for
the production of valuable chemical fuels, such as hydrogen
and hydrocarbon fuels, and for the degradation of harmful
pollutants.[1–6] Since the pioneering work on photocatalysis by
Honda and Fujishima in 1972,[7] many semiconductors have
been investigated and developed for various photocatalytic
J. X. Low, Prof. J. G. Yu
State Key Laboratory of Advanced Technology for
Materials Synthesis and Processing
Wuhan University of Technology
122 Luoshi Road, Wuhan 430070, P. R. China
E-mail: ;
Prof. J. G. Yu, Prof. S. Wageh, Prof. A. A. Al-Ghamdi
Department of Physics
Faculty of Science

King Abdulaziz University
Jeddah 21589, Saudi Arabia
Prof. M. Jaroniec
Department of Chemistry and Biochemistry
Kent State University
Kent, Ohio 44242, USA

DOI: 10.1002/adma.201601694
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Mm
R2 

(1)

qe q h
r2 

(2)

Fg = G
Fc = k

In Equation (1) and (2), Fg is the gravitational force, G is the
gravitational constant, M is the Earth’s mass, m is the mass of
the man, R is the distance between the center of the Earth and
the man, Fc is the Coulombic force, k is the Coulomb constant,
qe and qh are the charge magnitudes of the electron and the

hole, respectively, and r is the distance between the centers of
these charges. When the man (cf. the electron) jumps from the
ground (cf. from the valence band) into the air (cf. the conduction band), he will fall back to the ground rapidly (cf. recombine with the hole) because of the action of gravitational force
(cf. the Coulombic force between the electron and the hole).
In order to keep the man off the ground (cf. to separate the
photogenerated electron–hole pairs), a stool (cf. the conduction band of the semiconductor B) is provided (see Figure 2c,d);
then, the aforementioned man will land again on the stool and
be kept off the ground (cf. the electron and hole pairs can be
separated). Moreover, it should be noted that the value of the
Coulomb constant (8.99 × 109 N m2 C−2) is much larger than
that of the gravitational constant (6.67 ì 1011 N m2 kg2). Thus,

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Figure 1.  Schematic illustration of the typical photocatalytic processes
on a semiconductor.

the aforementioned man will fall back to the ground on the
timescale of seconds, while electron–hole pairs will recombine
much faster, i.e., in the range of nanoseconds. Therefore, the
separation of photogenerated electron–hole pairs on the surface of a photocatalyst is very difficult to achieve. Although preventing electron–hole recombination is a very challenging task,
it can be accomplished by the proper design of a photocatalyst.
Various strategies have been proposed to efficiently separate the photogenerated electron–hole pairs in semiconductor

photocatalysts, for instance by doping,[18,19] metal loading,[20,21]
and/or introducing heterojunctions.[22,23] Among the proposed
strategies, engineering heterojunctions in photocatalysts has
been proved to be one of the most promising ways for the
preparation of advanced photocatalysts because of its feasibility
and effectiveness for the spatial separation of electron–hole
pairs (see Figure 2d). Basically, five different heterojunctions
have been mainly investigated by our group at Wuhan University of Technology (WUT) as well as by others and proved to
be efficient for enhancing the activity of photocatalysts; these
are conventional type-II heterojunctions,[24–27] p–n heterojunctions,[28–30] surface heterojunctions,[31–34] direct Z-scheme heterojunctions,[35,36] and semiconductor–graphene (SC–graphene)
heterojunctions.[37,38]
In the past 15 years, many studies have been published by
our group on the development of advanced heterojunction photocatalysts for various applications. As shown in Table 1, the
conventional type-II heterojunction mechanism was studied by
our group in 2001 to explain the high photocatalytic activity of
anatase–brookite TiO2.[24,25] Then, the p–n heterojunction was
found to be more effective for enhancing the photocatalytic
activity than the aforementioned conventional type-II heterojunction.[39] Thereafter, two new heterojunction concepts were
firstly proposed by our group, which were the direct Z-scheme
heterojunction in 2013[35] and the surface heterojunction in
2014,[31] to further improve the photocatalytic activity. Recently,
two-dimensional (2D) graphene nanosheets have been widely
used by our group to prepare advanced sc–graphene heterojunction photocatalysts with enhanced activity due to the fascinating properties of graphene, such as its high conductivity,
large specific surface area, and high photostability.

Adv. Mater. 2017, 29, 1601694

Jiaguo Yu received his B.S.
and M.S. in chemistry
from Central China Normal

University and Xi’an Jiaotong
University, respectively, and
his Ph.D. in materials science in 2000 from Wuhan
University of Technology. In
2000, he became a Professor
at Wuhan University of
Technology. He was a postdoctoral fellow at the Chinese
University of Hong Kong from 2001 to 2004, a visiting scientist from 2005 to 2006 at the University of Bristol, and a
visiting scholar from 2007 to 2008 at the University of Texas
at Austin. His current research interests include semiconductor photocatalysis, photocatalytic hydrogen production,
CO2 reduction to hydrocarbon fuels, and so on.
Mietek Jaroniec received his
M.S. and Ph.D. from
M. Curie-Sklodowska
University (Poland) in 1972
and 1976; afterward, he was
appointed as a faculty at
the same University. Since
1991, he has been Professor
of Chemistry at Kent State
University, Kent, Ohio (USA).
His research interests include
interfacial chemistry and
the chemistry of materials, especially adsorption at the
gas/solid and liquid/solid interfaces and nanoporous
materials. At Kent State, he has established a vigorous
research program in the area of nanomaterials, such as
ordered mesoporous silicas, organosilicas, inorganic
oxides, carbon nanostructures, and nanostructured catalysts/photocatalysts, focusing on their synthesis, characterization, and environmental and energy-related applications.

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Review

Jingxiang Low obtained
his B.Eng. (Hons) from
Multimedia University,
Malaysia in 2011 and his
M.S. in materials science
from Wuhan University of
Technology. He is a now a
Ph.D. candidate under the
supervision of Prof. Jiaguo Yu
at the State Key Laboratory
of Advanced Technology
for Materials Synthesis and
Processing, Wuhan University of Technology. His current
research includes photocatalytic H2 production and CO2
reduction.


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Review

This review represents an appraisal of the
recent efforts in engineering various heterojunction photocatalysts and highlights
their application in photocatalysis, including
hydrogen production, CO2 reduction, and
pollutant degradation. The above-mentioned
five important types of heterojunctions in
photocatalysts are reviewed and discussed.
Finally, the current status, opportunities,
and future directions of these heterojunction
photocatalysts are presented. We hope that
this review can provide some useful guidelines and shed some light toward the development of highly efficient heterojunction
photocatalysts for different applications.

2. Conventional Heterojunctions

Figure 2.  Schematic illustration of: a) the effect of gravitational force on a man who jumps
off the ground, b) electron–hole recombination on a single photocatalyst, c) use of a
stool to keep a man off the ground, and d) electron–hole separation on a heterojunction
photocatalyst.

A heterojunction, in general, is defined as
the interface between two different semiconductors with unequal band structure, which
can result in band alignments.[47,48] Typically,
there are three types of conventional heterojunction photocatalysts, those with a straddling gap (type-I), those with a staggered
gap (type-II), and those with a broken gap
(type-III) (see Figure 3). For the type-I heterojunction photocatalyst (see Figure 3a), the
conduction band (CB) and the valence band
(VB) of semiconductor A are respectively
higher and lower than the corresponding

Table 1.  List of heterojunction photocatalysts studied by our group at WUT for various photocatalytic applications.
Sample

Heterojunction type

Application

Year

Ref.

Anatase–brookite TiO2

Conventional type-II

Acetone degradation

2001

[24]

Anatase–brookite TiO2

Conventional type-II

Acetone degradation

2003

[25]

Ag–multiphase TiO2

Conventional type-II

Methyl orange (MO) degradation

2005

[40]

SnO–TiO2

Conventional type-II

Rhodamine B (RhB) degradation

2008

[41]

NiO–TiO2

p–n

p-Chlorophenol degradation

2010

[42]

BIOI–TiO2

p–n

MO degradation

2011

[28]

NiS–CdS

p–n

Hydrogen production

2013

[29]

{001} and {101} TiO2

Surface

CO2 reduction

2014

[31]

N-doped {001} and {101}TiO2

Surface

CO2 reduction

2015

[32]

g-C3N4–TiO2

Direct Z-scheme

HCHO decomposition

2013

[35]

Anatase–rutile TiO2

Direct Z-scheme

Hydrogen production

2014

[43]

Ag2CrO4–graphene oxide

Direct Z-scheme

MB degradation

2015

[36]

CdS–WO3

Direct Z-scheme

CO2 reduction

2015

[44]

SC–graphene

Hydrogen production

2011

[37]

Reduced graphene oxide–CdS
g-C3N4–graphene

SC–graphene

Hydrogen production

2011

[45]

Graphene–TiO2 nanosheets

SC–graphene

Hydrogen production

2011

[38]

CdS–graphene

SC–graphene

CO2 reduction

2014

[46]

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Figure 3.  Schematic illustration of the three different types of separation of electron–hole pairs in the case of conventional light-responsive heterojunction photocatalysts: a) type-I, b) type-II, and c) type-III heterojunctions.

bands of semiconductor B.[49] Therefore, under light irradiation, the electrons and holes will accumulate at the CB and the
VB levels of semiconductor B, respectively. Since both electrons
and holes accumulate on the same semiconductor, the electron–hole pairs cannot be effectively separated for the type-I
heterojunction photocatalyst. Moreover, a redox reaction takes
place on the semiconductor with the lower redox potential,
thereby significantly reducing the redox ability of the heterojunction photocatalyst. For the type-II heterojunction photocatalyst (see Figure 3b), the CB and the VB levels of semiconductor
A are higher than the corresponding levels of the semiconductor B. Thus, the photogenerated electrons will transfer to
semiconductor B, while the photogenerated holes will migrate
to semiconductor A under light irradiation, resulting in a spatial separation of electron–hole pairs.[50–52] Similar to the type-I
heterojunction, the redox ability of the type-II heterojunction
photocatalyst will be also reduced because the reduction reaction and the oxidation reaction take place on semiconductor B
with lower reduction potential and on semiconductor A with
lower oxidation potential, respectively. As shown in Figure 3c,

the architecture of the type-III heterojunction photocatalyst is
similar to that of the type-II heterojunction photocatalyst except
that the staggered gap becomes so extreme that the bandgaps
do not overlap.[53,54] Therefore, the electron–hole migration and
separation between the two semiconductors cannot occur for the
type-III heterojunction, making it unsuitable for enhancing the
separation of electron–hole pairs. Among the aforementioned
conventional heterojunctions, it is obvious that the type-II heterojunction is the most effective conventional heterojunction
to be used for improving photocatalytic activity because of its
suitable structure for spatial separation of electron–hole pairs.
In the past several decades, enormous efforts have been made

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to prepare different type-II heterojunction photocatalysts, such
as TiO2/g-C3N4,[55] BiVO4/WO3,[56] g-C3N4–WO3,[57] g-C3N4–
BiPO4,[58] and so on, for enhancing the photocatalytic activity.
Generally, type-II heterojunction photocatalysts exhibit good
electron–hole separation efficiency, wide light-absorption range,
and fast mass transfer.[59]
For example, Zhou et al. prepared a SnO2/TiO2 type-II heterojunction photocatalyst by an electrophoretic-deposition (EPD)–
calcination method for photocatalytic RhB degradation.[41] Specifically, a commercial TiO2 was first electrophoretically deposited on F-doped SnO2-coated glass, followed by calcination at
200, 300, 400, 500 and 600 °C to obtain crystallized SnO2/TiO2
type-II heterojunction photocatalyst films. It was found that all
the prepared samples show good photocatalytic activities due
to the fast electron–hole separation through the type-II heterojunction between the TiO2 and the SnO2. Particularly, the
sample prepared at 400 °C exhibited the highest photocatalytic
activity among all the samples studied. This high activity could
be attributed to the optimal crystallinity and specific surface
area, which can reduce the number of recombination centers

on the sample and provide larger surface area with active sites
for photocatalytic reaction.
Meanwhile, Wetchakun et al. reported a hydrothermal synthesis of BiVO4/CeO2 type-II heterojunction photocatalysts for
the photocatalytic degradation of methylene blue (MB), methyl
orange (MO), and a mixture of MB and MO.[60] It was found
that the the BiVO4 and the CeO2 possessed different isoelectric
points, located at pH values of 4.56 and 7.33 respectively. The
isoelectric-point difference for these two semiconductors was
shown to be beneficial for adsorbing both cationic and anionic
dyes simultaneously. Specifically, the BiVO4 and the CeO2 can
preferentially respectively adsorb cationic MB and anionic MO

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Figure 4. Schematic illustration of the photocatalytic-activity enhancement mechanism of: (a) Ag/AgCl/pCN type-I, and b) Ag/AgBr/pCN type-II
heterojunction photocatalysts under light irradiation. Reproduced with permission.[61] Copyright 2014, Elsevier.

Moreover, the morphology tuning of heterojunction phoduring degradation reactions. As a result, the BiVO4/CeO2
tocatalysts is also critical for optimizing the performance of

composite exhibited higher photocatalytic-degradation activity
type-II heterojunction photocatalysts. For example, Shen and
toward the mixture of MB and MO as compared with the single
co-workers reported visible-light-active CdS/ZnO core/shell
BiVO4 or CeO2 photocatalysts, which were not active toward the
nanofibers with a type-II heterojunction (see Figure 5a,b), with
photocatalytic degradation of the anionic MO and the cationic
good photocatalytic activity toward hydrogen production.[10] The
MB, respectively, due to the electrostatic repulsion between
their surface charges and the charges of the dye molecules. This
type-II heterojunction significantly facilitated the electron–hole
remarkable activity of the composite photocatalyst is attributed
separation efficiency between the ZnO and the CdS. Meanto the enhanced electron–hole separation efficiency and strong
while, the core–shell structure of the CdS/ZnO featured a large
electrostatic attraction between the composite and the dye
contact interface, which can further enhance the separation
molecules. This study suggests that the proper coupling of two
efficiency of electron–hole pairs. As a result, the photocatalytic
different semiconductors can not only enhance the electron–
hydrogen-production efficiency of ZnO/CdS was higher than
hole separation efficiency but can also afford photocatalysts with
that of the single ZnO and CdS compounds.
good adsorption ability toward both anionic and cationic dyes.
Furthermore, type-II heterojunction photocatalysts can
Ong et al. systematically investigated the photocatalytic
be also created between two different phases of a semiconactivity of type-I Ag/AgCl/g-C3N4 and type-II Ag/AgBr/gductor.[62–64] For instance, the mixed phase of TiO2, i.e., P25
C3N4 heterojunction photocatalysts for photocatalytic CO2
(consisting of anatase and rutile TiO2), has been commercialreduction.[61] It was found that both Ag/AgCl/g-C3N4 and Ag/
ized and has attracted wide attention for various photocatalytic
applications due to its efficient charge-carrier separation. In

AgBr/g-C3N4 exhibited good photocatalytic CO2-reduction perdetail, the CB and VB levels of anatase TiO2 are higher than
formance toward CH4 production due to the presence of the
heterojunction, which may improve the charge-carrier sepathe corresponding levels of rutile TiO2. Therefore, a type-II
ration in these photocatalysts. Notably, the
photocatalytic CO2 reduction activity of Ag/
AgBr/g-C3N4 toward CH4 production was
much higher than that of Ag/AgCl/g-C3N4.
This was ascribed to the formation of the
type II-heterojunction instead of the type-I
heterojunction in the case of Ag/AgBr/gC3N4, which can lead to the spatial separation of electrons and holes by accumulating
them in the Ag/AgBr and the g-C3N4, respectively (see Figure 4). This study proved that
the type-II heterojunction in photocatalysts
is more effective than the type-I heterojunc- Figure 5.  a,b) Scanning electron microscopy (SEM) (a) and transmission electron microscopy
tion for improving their photocatalytic CO2- (TEM) (b) images of CdS/ZnO core/shell nanofibers with a type-II heterojunction. Reproduced
reduction activity.
with permission.[10] Copyright 2013, The Royal Society of Chemistry.
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Figure 7.  Schematic illustration for the transfer and separation of charge
carriers on an anatase/rutile/Ag heterojunction photocatalyst under light
irradiation.

Figure 6.  Comparison of the rate constant of anatase–brookite composites: with 80% anatase and 20% brookite (A), with 92% anatase and 8%
brookite (B), with 100% anatase (C); as well as P25, in the photocatalytic
degradation of acetone in air. Reproduced with permission.[24] Copyright
2001, The Royal Society of Chemistry.

heterojunction photocatalyst can be formed by combining the
anatase and rutile TiO2 in P25, which results in an enhancement of the electron–hole separation. Also, in 2001 our group
reported an anatase–brookite dual-phase type-II heterojunction
photocatalyst by hydrolysis of titanium tetraisopropoxide in
water or 1:1 ethanol–H2O solution.[24] It was found that the content of brookite in the composite was reduced in the presence
of ethanol because the latter can suppress the hydrolysis of the
titanium alkoxide and inhibit the rapid crystallization of the
TiO2 nanoparticles into brookite by adsorbing on the surface of
the TiO2. Notably, the photocatalytic activity of the sample consisting of anatase and brookite toward degradation of acetone
was higher than that of one consisting of pure anatase (see
Figure 6). This is due to the formation of a type-II heterojunction between the brookite and the anatase, which can greatly
enhance the separation efficiency of the electron–hole pairs.
In order to further enhance the photocatalytic activity of the
multi-phase type-II heterojunction photocatalysts, an Ag–TiO2
multiphase composite was designed and synthesized by Yu
et al.[40] It was found that the silver precursor, AgNO3, has a
great influence on the crystallization of the TiO2. The phase
composition of the TiO2 changed with changing AgNO3 concentration. Particularly, brookite TiO2 started to appear and
steadily grow, starting from a AgNO3 concentration of 0.02 M,
because the AgNO3 facilitates and catalyzes the formation of
brookite TiO2. Meanwhile, the rutile TiO2 began to be formed

at a AgNO3 concentration of 0.03 M. Notably, the phase-transformation temperature of TiO2 from anatase to rutile was significantly reduced from 700 °C to 500 °C due to the presence
of AgNO3. Thus, AgNO3 can suppress the growth of anatase
domains and thus increase the total boundary energy of TiO2
by promoting the phase transformation of anatase to rutile. It

Adv. Mater. 2017, 29, 1601694

was found that the optimal Ag–TiO2 (0.05 M of AgNO3) sample
exhibited very high photocatalytic activity for MO degradation.
This is due to the formation of multiphase type-II heterojunctions, such as anatase/rutile, anatase/brookite and rutile/
brookite, which can greatly enhance the electron–hole separation efficiency. Moreover, Ag loading can also further enhance
the charge-carrier separation in the Ag–TiO2 composite. Taking
anatase/rutile/Ag as an example (see Figure 7), the photogenerated electrons and holes can be spatially separated in rutile
and anatase TiO2, respectively, through the type-II heterojunction. Meanwhile, the electrons on the rutile TiO2 can further
migrate to Ag nanoparticles (NPs) through the Schottky junctions between the Ag and the rutile TiO2, thus achieving high
electron–hole separation efficiency.
Although type-II heterojunction photocatalysts possess good
electron–hole separation efficiency, there are still some problems that limit their practical applications. For example, the
reduction and oxidation reactions on the type-II heterojunction
photocatalysts take place on the semiconductors with the lower
reduction and oxidation potentials, respectively, thereby greatly
suppressing their redox ability (see Figure 3b). Moreover, the
migration of electrons from semiconductor A to the electronrich CB of semiconductor B or the corresponding migration
of holes from semiconductor B to the hole-rich VB of semiconductor A are physically unfavorable because of the electrostatic repulsion between electron and electron or hole and hole,
respectively. Therefore, the development of more effective heterojunctions in photocatalysts is urgently needed.

3. p–n Heterojunctions
Although the type-II heterojunction can ideally separate electron–hole pairs in space, the achieved enhancement in the
electron–hole separation across a type-II heterojunction is not
sufficient to overcome the ultrafast electron–hole recombination

on the semiconductor. Thus, a p–n heterojunction photo­catalyst
concept was proposed, which is able to accelerate the electron–
hole migration across the heterojunction for improving the
photocatalytic performance by providing an additional electric
field.[65–67] Specifically, an effective p–n heterojunction photocatalyst can be obtained by combining p-type and n-type semiconductors. Before light irradiation, the electrons on the n-type

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Figure 8. Schematic illustration of the electron–hole separation under
the influence of the internal electric field of a p–n heterojunction photocatalyst under light irradiation.

semiconductor near the p–n interface tend to diffuse into the
p-type semiconductor, leaving a positively charged species (see
Figure 8).[68–71] Meanwhile, the holes on the p-type semiconductor near the p–n interface tend to diffuse into the n-type
semiconductor, leaving a negatively charged species. The electron–hole diffusion will continue until the Fermi level equilibrium of the system is achieved. As a result, the region close to
the p–n interface is charged, creating a “charged” space or the
so-called internal electric field.[72–74] When the p-type and n-type
semiconductors are irradiated by incident light with an energy

equal to or higher than their bandgap value, both p-type and
n-type semiconductors can be excited, generating electron–hole
pairs. The photogenerated electrons and holes in the p-type and
n-type semiconductors will migrate under the influence of the
internal electric field to the CB of the n-type semiconductor and
the VB of the p-type semiconductor, respectively, which results
in the spatial separation of the electron–hole pairs. It should
be noted that this electron–hole separation process is also thermodynamically feasible because both the CB and the VB of the
p-type semiconductor are normally located higher than those of
the n-type semiconductor in a p–n heterojunction photocatalyst.[75,76] As a result, the electron–hole separation efficiency in
p–n heterojunction photocatalysts is faster than that of type-II
heterojunction photocatalysts due to the synergy between the
internal electric field and the band alignment.[42]
For example, our group has demonstrated that the design
of NiS/CdS nanorods with p–n heterojunctions can greatly
improve the photocatalytic hydrogen-production efficiency of
the CdS.[29] NiS NPs with sizes of 10–30 nm were uniformly
dispersed on the surface of CdS nanorods, thereby providing
larger contact interface for creating p–n heterojunctions
between the NiS and the CdS (see Figure 9a). The formation of
p–n heterojunctions can facilitate charge transfer between the
NiS and the CdS and suppress the charge-carrier recombination
(see Figure 9b,c). Meanwhile, the CdS nanorods can enhance
electron transport because of their one-dimensional (1D) structure. As a result, the photocatalytic hydrogen-production rate
on the NiS/CdS nanorods with p–n heterojunctions, having
5 wt% NiS (56.6 µmol h−1) is much higher than that of the pure
CdS (2.8 µmol h−1) and 1 wt% Pt-loaded CdS (36.3 µmol h−1)
(see Figure 9d). However, a further increase in the NiS loading
on the CdS causes a decrease in the photocatalytic activity

Figure 9.  a) SEM image of 5 wt% NiS-loaded CdS. b,c) Schematic illustration of the charge-carrier separation on the NiS/CdS nanorods with p–n
heterojunctions (b) and across the NiS/CdS p–n heterojunction (c). d) Comparison of the photocatalytic activity of CdS with different NiS loadings:
Ni0 (0 wt% NiS), Ni0.5 (0.5 wt% NiS), Ni1 (1 wt% NiS), Ni3 (3 wt% NiS), Ni5 (5 wt% NiS), Ni10 (10 wt% NiS), 1 wt% Pt–CdS, and pure NiS under
visible-light irradiation. Reproduced with permission.[29] Copyright 2013, The Royal Society of Chemistry.

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In 2014, our group proposed the concept of
the surface heterojunction to explain the
unique electron–hole separation phenomenon observed on the crystal facets of a single
semiconductor.[31] It is well-known that the different crystal facets on a single semiconductor
can have different band structures.[31,78] Since
a heterojunction is formed by combining
two semiconducting materials with different
band structures, it is possible to create a hetFigure 10.  a,b) SEM images of flower-like NiO (a), and a flower-like NiO/TiO2 p–n heterojuncerojunction between two crystal facets of a
tion photocatalyst (b) with the inset showing high resolution. Reproduced with permission.[42]
single semiconductor, namely a surface hetCopyright 2010, Wiley-VCH.
erojunction.[31,79,80] In our work, a series of
anatase TiO2 samples with different ratios of the exposed {001}

because the shielding effect of the NiS may reduce the number
of active sites on the surface and thus the light-absorption
and {101} facets was reported. These samples were prepared
ability of the CdS.
by a F−-ions-assisted hydrothermal method. The photocatalytic
Furthermore, Feng and co-workers reported a MoS2/CdS
activity of the sample with 55:45 ratio of the exposed {001} and
{101} facets was much higher than that of the samples domip–n heterojunction photocatalyst for enhancing photocatanated either by exposed {001} or {101} facets. According to denlytic hydrogen-production efficiency,[77] which was simply presity functional theory (DFT) calculations (see Figure 11a), the
pared by a one-pot solvothermal method. It was shown that a
observed enhancement in the photocatalytic activity was due to
“V-shaped” Mott–Schottky plot can be obtained by performing
the formation of surface heterojunctions between the {001} and
electrochemical testing, which suggests the formation of p–n
{101} facets. In fact, the basic principle of the surface heteroheterojunctions by the combination of the p-type MoS2 and the
junction is similar to that of the type-II heterojunction, in which
n-type CdS. It was found that the n-type CdS is uniformly disthe CB and VB levels of the {001} facets are higher than the corpersed on the surface of nanosheets of the p-type MoS2, which
responding levels of the {101} facets of anatase TiO2. Thus, elecresults in a larger contact interface between the CdS and the
MoS2, being beneficial for accelerating charge transfer and septrons and holes can be spatially separated on the {101} facets
for reduction reactions and on the {001} facets for oxidation
aration. Therefore, the MoS2/CdS p–n heterojunction photocatreactions, respectively (see Figure 11b). This finding enables the
alyst exhibited an excellent electron–hole separation efficiency
design of heterojunction systems into the surface of single nandue to its large contact interface and rapid electron–hole sepaoparticle. In addition, the fabrication cost of the resulting hetration through the formed p–n heterojunctions. Consequently,
erojunction photocatalysts can be greatly reduced because only
the 2D MoS2/CdS p–n heterojunction photocatalyst exhibited a
one semiconductor is used. Notably, the redox potential loss of
hydrogen-production rate of 137 µmol h−1, which is 10 times
a type-II heterojunction photocatalyst can be also minimized
higher than that obtained for pure CdS. This work shows
by creating a surface heterojunction, because the difference in
that an enlarged contact interface on the p–n heterojunction

the band structures between the {001} and {101} facets of TiO2
photocatalysts is beneficial for enhancing their photo­catalytic
performance.
is small. Therefore, the photocatalytic CO2 reduction activity of
The morphology of the p–n heterojunction photocatalysts
anatase TiO2 with an optimal ratio of exposed {001} and {101}
can be tuned for achieving large specific surface area and an
facets was 3.5 times higher than that of commercial TiO2, i.e.,
abundant number of surface active sites, which has been
P25 (Figure 11c). Furthermore, it was observed that the overproved to be effective for further improvement of the photocataexposed {101} or {001} facets on the TiO2 caused an overflow
lytic activity of p–n heterojunction photocatalysts. Recently, our
effect of holes and electrons (see Figure 12a,c), respectively, and
group showed that a flower-like NiO/TiO2 p–n heterojunction
consequently reduced the photogenerated electron–hole separation efficiency. Apparently, the fabrication of anatase TiO2 with
photocatalyst exhibited an exceptional photocatalytic activity
toward degradation of p-chlorophenol.[42] The flower-like NiO/
the optimal ratio of the exposed {001} and {101} facets is crucial
for enhancing the photocatalytic activity of the anatase TiO2 (see
TiO2 p–n heterojunction was afforded by a simple hydrothermal
method. Specifically, TiO2 NPs were uniformly deposited on
Figure 12b).[31,81–84]
the surface of flower-like NiO particles, which resulted in the
Selective deposition of oxidation and reduction co-catalysts
formation of a NiO/TiO2 p–n heterojunction photocatalyst (see
on a semiconductor has been widely applied to confirm the type
of oxidation and reduction sites thereon.[85] Recently, Liu et al.
Figure 10a,b). The resulting photocatalyst featured a large specific surface, which assured the abundance of surface active
reported that the spatial separation of electrons and holes on the
sites for photocatalytic reactions and enhanced the adsorption
different facets of a single anatase TiO2 crystal can be confirmed

ability for dye molecules. In addition, it was shown that the forby photo-deposition of Pt NPs. Specifically, Pt NPs were selecmation of p–n heterojunctions in NiO/TiO2 can improve the
tively loaded on the surface of the electron-rich {101} facets due
to the spatial separation of electrons and holes on the anatase
electron–hole separation rate. As a result, the NiO/TiO2 photo­
TiO2.[86] Moreover, they found that an optimal ratio of the {001}
catalyst exhibited a superior photocatalytic activity toward the
degradation of p-chlorophenol.
and {101} facets is also an important issue for photocatalytic

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4. Surface Heterojunctions


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photocatalytic activity. Specifically, 1, 2, and
4 mL of H2PtCl6 solution were used to prepare a series of TiO2/Pt photocatalysts, named

as TP1, TP2, and TP4, respectively. Interestingly, Pt NPs were selectively deposited on
the surface of the {101} and {010} facets of
anatase, which was due to the formation of
surface heterojunctions. Particularly, {001}
and {110} have a higher CB value, while {101}
and {010} have a lower VB value. The electrons and holes tend to migrate to the {101}
and {010} facets for reduction reactions and
to {001} and {110} facets for oxidation reactions, respectively. Therefore, the Pt ions are
accumulated on the {101} and {010} facets
and reduced into Pt NPs by photogenerated
electrons during the photoreduction process.
Due to the synergistic effect of surface heterojunctions and Schottky junctions, the photocatalytic activity of the resulting TP1 toward
the degradation of phenol was much higher
than that of pure TiO2. However, a further
increase in the loading of Pt NPs (e.g., samples TP2 and TP4) led to the light-shielding
effect, which may significantly reduce the
photocatalytic activity of these samples.
Furthermore, Li et al. reported BiVO4 with
co-exposed {010} and {110} facets for the spatial separation of photogenerated electrons
and holes.[87] It was found that the {010} and
{110} facets exhibited different band structures, in which the CB of the {110} facets is
higher than that of the {010} facets, while
Figure 11.  a) Density of states (DOS) plots of the {101} and {001} facets of anatase TiO2,
where O 2P, Ti 3d, and TDOS are the partial DOS of O2P, partial DOS of Ti 3d, and total DOS, the VB of the {010} facets is lower than that
of the {110} facets. Therefore, electrons and
respectively. b) Electron–hole separation on the surface heterojunction of anatase TiO2 with an
optimal ratio of the exposed {001} and {101} facets (55:45). c) Comparison of the photocata- holes tend to migrate to the {010} and {110}
lytic CH4 production activity of P25, HF0 (anatase TiO2 with a ratio of the exposed {001} and facets, respectively, resulting in their spa{101} facets equal to 11:89), HF3 (anatase TiO2 with the ratio of 49:51), HF4.5 (anatase TiO2
tial separation. In order to further improve
the ratio of 55:45), HF6 (anatase TiO2 with the ratio of 72:28), and HF9 (anatase TiO2 with the

the photocatalytic activity of such samples,
[31]
ratio of 83:17). Reproduced with permission. Copyright 2014, American Chemical Society.
reduction (Pt) and oxidation (MnOx) cocatalysts were loaded onto BiVO4 by the
photo-deposition method. It was found that Pt and MnOx cohydrogen production by water splitting. Namely, the photocatalytic hydrogen-production activity of the sample with an
catalysts are preferably loaded on the {010} and {110} facets
optimal ratio of the exposed {001} and {101} facets and 0.5%
(see Figure 13a,b), respectively. This is because Pt and MnOx
loading of Pt was ca. 9 times higher than
that of the corresponding sample dominated
by {101} facets. Furthermore, Gao et al. prepared anatase TiO2 with exposed {101}, {010},
{001}, and {110} facets by using a simple
hydrothermal method to create surface heterojunctions on the anatase TiO2 for photocatalytic phenol degradation.[33] Transmission
electron microscopy (TEM) images of these
samples suggested that the anatase TiO2
with exposed {101}, {010}, {001}, and {110}
Figure 12.  Schematic illustrations of: a) overflow effect of holes on the anatase TiO2 dominated
facets was successfully fabricated. Then, difby {101} facets, b) spatial separation of electrons and holes on anatase TiO2 with an optimal
ferent amounts of Pt NPs were loaded on the ratio of the exposed {001} and {101} facets, and c) overflow effect of electrons on anatase
as-prepared anatase TiO2 to form Schottky TiO2 dominated by {001} facets. Reproduced with permission.[31] Copyright 2014, American
junctions for further improvement of the Chemical Society.
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Figure 13.  a,b) SEM images of BiVO4 with deposited Pt (a) and MnOx (b). Reproduced with permission.[87] Copyright 2013, Nature Publishing Group.

can be easily formed on the photogenerated electron-rich surface of the {010} facets and the photogenerated hole-rich surface of the {110} facets, respectively. By adjusting the loadings
of the reduction and oxidation co-catalysts, the electrons on the
{010} facets and the holes on the {110} facets of the BiVO4 can
further migrate to Pt and MnOx, respectively, which results in
accelerating the electron–hole separation. Due to the spatial
separation of the electron–hole pairs and the proper loading of
reduction and oxidation co-catalysts, the resulting photocatalyst
exhibited a high photocatalytic activity for water oxidation.

5. Direct Z-Scheme Heterojunctions
Although all the above-mentioned heterojunction photocatalysts are efficient for enhancing electron–hole separation, the
redox ability of the photocatalyst is sacrificed because the reduction and oxidation processes occur on the semiconductor with
the lower reduction and oxidation potentials, respectively.[88–91]
In order to overcome this problem, the Z-scheme photocatalytic concept was proposed by Bard et al. in 1979 to maximize
the redox potential of the heterojunction systems.[92] A conventional Z-scheme photocatalytic system is composed of two different semiconductors, photocatalyst I (PS I) and photocatalyst
II (PS II), and an acceptor/donor (A/D) pair (see Figure 14). PS
I and PS II are not in physical contact. During the photocatalytic reaction, photogenerated electrons migrate from the CB of
the PS II to the VB of the PS I through an A/D pair via following redox reactions:
A + e− → D 

(3)

D + h+ → A 

(4)

Specifically, A is reduced into D by reacting with the photogenerated electrons from the CB of the PS II. After that, the
D is oxidized into A by the photogenerated holes from the VB.
Since electrons accumulate on the PS I, with the higher reduction potential, and holes accumulate on the PS II, with the
higher oxidation potential, a spatial separation of electron–hole
pairs and an optimal redox ability can be achieved. However,
conventional Z-scheme photocatalysts can only be constructed

Adv. Mater. 2017, 29, 1601694

in the liquid phase, thereby limiting their wide application in
photocatalysis.
In 2006, Tada et al. proposed the concept of an all-solidstate Z-scheme photocatalyst, which consisted of two different
semiconductors (PS I and PS II) and a solid electron mediator
between them.[93] As shown in Figure 15, electrons on the VB
of the PS II are firstly excited to the CB under light irradiation,
leaving holes on the VB. Then, the photogenerated electrons on
the PS II migrate to the VB of the PS I via an electron mediator
(such as Pt, Ag, and Au), and are further excited to the CB of
PS I. As a result, photogenerated holes and electrons are accumulated in the PS II, with a higher oxidation potential, and in
the PS I, with a higher reduction potential, respectively, which
results in the spatial electron–hole separation and optimization of the redox potential. Moreover, all-solid-state Z-scheme
photocatalysts can be used in solution, gas, and solid media,
thereby extending their photocatalytic applications.[94–96] However, electron mediators required for improving the migration
path for electrons in the all-solid-state Z-scheme photocatalysts
are expensive and rare, which limits large-scale applications of
these photocatalysts.
In 2013, our group proposed a direct Z-scheme heterojunction photocatalyst concept. A direct Z-scheme photocatalyst

was prepared by combining two different semiconductors
without an electron mediator.[35] As shown in Figure 16, the
construction of this direct Z-scheme heterojunction photocatalyst is identical to that of conventional all-solid-state Z-scheme

Figure 14.  Schematic illustration of electron–hole separation on the conventional Z-scheme photocatalytic system under light irradiation.

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Figure 15.  Schematic illustration of the electron–hole separation on allsolid-state Z-scheme photocatalysts under light irradiation.

heterojunction photocatalysts, except that the rare and expensive electron mediators are not required in this system.[35,97–99]
Similarly, electrons and holes are spatially separated on the
semiconductor with the higher reduction potential and oxidation potential of the direct Z-scheme heterojunction photocatalyst, respectively.[100–102] Furthermore, the fabrication cost of
this direct Z-scheme heterojunction photocatalyst is low and
comparable to that of conventional type-II heterojunction systems.[103] Also, it shows other advantages; for instance, its redox
potential can be optimized for specific photocatalytic reactions.
Moreover, the charge transfer on the direct Z-scheme heterojunction photocatalyst is physically more favorable than that on
the type-II heterojunction photocatalyst because of the electrostatic attraction between electrons and holes. In particular, in
the case of the direct Z-scheme photocatalysts, the migration of

photogenerated electrons from the CB of the PS II to the photogenerated hole-rich VB of the PS I is easier, due to the electrostatic attraction between the electrons and the holes. In contrast, for conventional type-II heterojunction photocatalysts, the
migration of photogenerated electrons from the CB of semiconductor A to the photogenerated electron-rich CB of semiconductor B is apparently harder due to the electrostatic repulsion
between electrons (see Figure 3b). Due to the aforementioned
advantages, direct Z-scheme heterojunction photocatalysts have
recently attracted a lot of attention.
For instance, our group reported the photocatalytic decomposition of the major indoor pollutant formaldehyde on a g-C3N4–
TiO2 direct Z-scheme heterojunction photocatalyst.[35] Specifically, a series of the samples was prepared by calcining mixtures of P25 titania with different amounts of urea. As can be
seen from Figure 17a, the sizes of the TiO2 NPs were ca. 30 nm.
More interestingly, the TiO2 NPs were covered by g-C3N4.
Therefore, an intimate contact between the TiO2 and the g-C3N4
was created, enabling a rapid transport of charge carriers across
the contact interface. The actual loading of g-C3N4 on TiO2 was
further confirmed by TGA analysis. The pure TiO2 exhibited no
weight loss on the TG curve, while the g-C3N4 was fully decomposed at 600 °C. Furthermore, the U100 sample exhibited
about 12% loss at 600 °C, indicating that the actual loading of
the g-C3N4 on the TiO2 was ca. 12%. Then, the presence of the
direct Z-scheme heterojunction was also confirmed by radicaltrapping experiments. Specifically, an increase in the photoluminescence intensity of 2-hydroxy-terephthalic acid (•OH-TA)
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Figure 16.  Schematic illustration of electron–hole separation on a direct
Z-scheme heterojunction photocatalyst under light irradiation.

at about 425 nm with increasing irradiation time is visible (see
Figure 17b), indicating the generation of the •OH radical. If a
type-II heterojunction is formed by coupling the g-C3N4 and the
TiO2, instead of a direct Z-scheme heterojunction, no increase
in the PL intensity should be observed because the photogenerated holes are accumulated on the valence band of g-C3N4,
which does not have sufficient oxidation power for producing

•OH radicals (see Figure 17c,d). The observed production of
•OH radicals confirms the accumulation of photogenerated
holes on the valence band of the TiO2, suggesting the formation of the g-C3N4–TiO2 direct Z-scheme heterojunction. As a
result, the photocatalytic formaldehyde-decomposition activity
of the optimized g-C3N4–TiO2 photocatalyst (12% g-C3N4-loaded
TiO2) is 2.1 times higher than that of commercial TiO2, i.e., P25
(see Figure 18a,b). Moreover, it was shown that the photocatalytic activity of the g-C3N4-TiO2 direct Z-scheme heterojunction
is greatly dependent on the amount of g-C3N4 loaded on the
TiO2. An overloading of g-C3N4 on the TiO2 led to a decrease
in the photocatalytic activity (see Figure 18c,d). This is due to
the shielding effect of the g-C3N4 on the TiO2, which causes a
reduction in the light-absorption ability of the TiO2 and inhibits
the reaction between the photogenerated holes in the TiO2 and
reactants.
In 2014, Katsumata et al. fabricated a WO3/g-C3N4 Z-scheme
heterojunction photocatalyst for hydrogen production under
visible-light irradiation by a simple calcination method.[104]
In this direct Z-scheme heterojunction system, the photogenerated electrons migrated from the WO3 to the g-C3N4, while
the photogenerated holes were kept on the WO3 during the
photocatalytic reaction. Thus, the reduction and oxidation
reactions took place on the g-C3N4, having higher reduction
potential, and on the WO3, having higher oxidation potential,
respectively, thereby optimizing the redox ability of the composite photocatalyst. As a result, the photocatalytic hydrogenproduction rate of the WO3/g-C3N4 Z-scheme heterojunction
photocatalyst is much higher than that of the pure WO3 and
g-C3N4 components.
Very recently, our group reported the synthesis of a hierarchical CdS–WO3 direct Z-scheme heterojunction photocatalyst
in the form of hollow spheres (CdS–WO3 HSs) for photocatalytic CO2 reduction.[44] According to the photocurrent characterization, the electron–hole separation efficiency of CdS–WO3
HSs is higher than that of the pure CdS and WO3 HSs. This

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Figure 17.  a) TEM image of g-C3N4–TiO2 direct Z-scheme heterojunction photocatalyst. b) •OH-TA PL spectral changes of the same photocatalyst in a
0.002 M NaOH solution in the presence of 0.0005 M terephthalic acid under UV irradiation. c) Schematic illustration of the band structures of g-C3N4
and TiO2 together with the OH−/•OH potential. d) The •OH radicals cannot be formed on the conventional g-C3N4–TiO2 heterojunction photocatalyst.
Reproduced with permission.[35] Copyright 2013, The Royal Society of Chemistry.

Figure 18.  a) Comparison of the photocatalytic HCHO decomposition activity of U0 (pure TiO2), U20 (TiO2 calcined with 20 wt% urea), U100 (TiO2 calcined with 100 wt% urea), U200 (TiO2 calcined with 200 wt% urea), U500 (TiO2 calcined with 500 wt% urea), and pure g-C3N4 samples. b–d) Schematic
illustrations of the electron–hole separation on the g-C3N4-TiO2 direct Z-scheme heterojunction photocatalysts (b), g-C3N4-TiO2 with optimal loading
of g-C3N4 (c), g-C3N4-TiO2 with overloaded amount of g-C3N4 (d). Reproduced with permission.[35] Copyright 2013, The Royal Society of Chemistry.

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Figure 19.  Schematic illustration of the charge-transfer pathway of the
photogenerated electron–hole pairs on CdS–WO3 direct Z-scheme heterojunction photocatalyst under visible-light irradiation.

is due to the spatial separation of the electrons and holes on
the CB of the CdS and the VB of the g-C3N4, respectively (see
Figure 19). Moreover, the formation of the CdS–WO3 HS direct
Z-scheme heterojunction can also promote electron accumulation on the surface of the CdS, which is beneficial for the
multielectron CO2-reduction reaction. Therefore, the main
product of the photocatalytic CO2 reduction reaction on the
CdS–WO3 HS Z-scheme heterojunction photocatalyst was CH4
(see Figure 20a). As a result, the photocatalytic CO2 reduction
activity of the optimized CdS–WO3 HSs (0.1 µmol h−1 of CO2
were reduced to CH4 on the catalyst with 5 mol% CdS) was 100
and 10 times higher than the corresponding values obtained
for the pure WO3 HS and CdS (see Figure 20b), respectively.
However, an overloading of CdS led to a decrease in the photocatalytic CO2 activity, which can be also related to the observed
decrease in the number of surface active sites on the WO3 HSs
and the light-shielding effect.
Furthermore, Wong and co-workers reported that a low-cost
natural mineral can be used for constructing a direct Z-scheme
heterojunction photocatalyst.[105] Specifically, natural magnetic
pyrrhotite (NP) mineral, after thermal treatment at 600 °C,

was used as a photocatalyst. It was shown that the thermally
treated NP (TNP) was composed of hematite (Fe2O3) and pyrite
(FeS2). Notably, the resulting TNP exhibited good photocatalytic
activity for Escherichia coli disinfection in comparison to that

of the initial NP. This activity was attributed to the formation
of the direct Z-scheme heterojunction between the Fe2O3 and
the FeS2, greatly suppressing electron–hole recombination and
optimizing the redox ability of the system. In addition, during
thermal treatment, oxygen or sulfur vacancies can be generated and utilized as electron traps prohibiting the electron–hole
recombination. More interestingly, the presence of Fe2O3 in the
TNP direct Z-scheme heterojunction photocatalyst resulted in
its strong magnetic behavior, thereby enabling the simple magnetic recovery of the photocatalyst, which is beneficial for longterm application.
More recently, Xu et al. showed that the direct Z-scheme
heterojunction can also be constructed between two different
semiconductor phases. They successfully prepared an anatase/
rutile biphase electrospun TiO2 as a direct Z-scheme heterojunction photocatalyst through the rapid cooling of electrospun
TiO2 calcined at 500 °C (see Figure 21a,b).[43] As can be seen
from Figure 21b,c, TiO2 nanofibers with both rutile and anatase
phases were successfully prepared. Normally, the prepared TiO2
nanofibers are dominated by anatase due to the fast transformation rate of rutile into anatase during slow cooling. Notably,
the proposed rapid-cooling method was shown to be effective
for suppressing the transformation rate of rutile to anatase, and
thus the rutile TiO2 phase can be preserved after the cooling
process. Moreover, it was shown that during the photo-deposition process of Pt NPs, the Pt NPs were favorably deposited on
the surface of rutile instead of anatase (see Figure 21b), suggesting that the photogenerated electrons tend to migrate to
the CB of the rutile phase due to the formation of the direct
Z-scheme heterojunction. Furthermore, the electron–hole
separation efficiency of the sample prepared by a rapid cooling
method was better than that of the TiO2 nanofiber prepared by
slow cooling because, in the former case, the direct Z-scheme
heterojunctions on the TiO2 nanofibers can greatly reduce the
electron–hole-recombination rate. Since the reduction and oxidation reactions occur on the semiconductors with the higher
reduction and oxidation potentials, respectively, the redox ability

Figure 20.  a) Gas-chromatography spectra for the CO2 reduction on CdS–WO3 hollow spheres (HS) at different irradiation times. b) Photocatalytic CO2
reduction activity of C0 (pure WO3 HS), C1 (1 mol% of CdS loaded on WO3 HS), C2 (2 mol% of CdS loaded on WO3 HS), C5 (5 mol% of CdS loaded
on WO3 HS), C10 (10 mol% of CdS loaded on WO3 HS), C20 (20 mol% of CdS loaded on WO3 HS), C100 (pure CdS), and N5 (5 mol% CdS–WO3
nanoparticle composite) for CH4 production. Reproduced with permission.[44] Copyright 2015, Wiley-VCH.

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Figure 21.  a,b) TEM (a) and HRTEM (b) images of the mixed-phase composite of anatase and rutile TiO2 with photo-deposited Pt NPs. c) X-ray diffraction (XRD) patterns of anatase–rutile composites: I (pure anatase), II (with 72% of anatase and 28% of rutile), III (with 55% of anatase and 45%
of rutile), and IV (pure rutile). d) Comparison of the photocatalytic-hydrogen-production activity of the anatase–rutile composites I, II, III, and IV.
Reproduced with permission.[43] Copyright 2014, Elsevier.

of the composite can be also optimized. As a result, the photocatalytic hydrogen-production efficiency of TiO2 nanofibers
prepared by the rapid-cooling method (324 µmol h−1) is higher
than that of the TiO2 nanofibers prepared under slow-cooling
conditions (188 µmol h−1) (see Figure 21d).

6. Semiconductor/Graphene Heterojunctions
Graphene, a 2D single layer sheet of sp2-hybridized carbon

atoms with hexagonally packed structure, has attracted great
attention due to its extraordinary physical properties including
superior charge transport, unique optical properties, high
thermal conductivity, large theoretical specific surface area, and
good mechanical strength.[106–108] Since the discovery of singlelayer graphene nanosheets by Geim and Novoselov in 2004,[109]
graphene has been known as a perfect candidate for various
applications, including solar cells,[110–112] supercapacitors,[113,114]
batteries,[115,116] photocatalysis,[117–119] etc. Particularly, tremendous efforts have been made for coupling graphene with
other semiconductors to fabricate heterojunction photocatalysts with improved photocatalytic activity. The ultrahigh electron conductivity of graphene allows the flow of electrons
from the semiconductor to its surface, assuring efficient electron–hole separation. Moreover, the potential of graphene/
graphene− (−0.08 V vs standard hydrogen electrode (SHE), pH
= 0) is normally lower than the conduction-band potential of

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the photocatalyst, thereby enabling the fast electron migration
from the photocatalyst to the graphene.[38] The use of graphene
in photocatalytic applications was firstly proposed by Zhang
et al. in 2010.[120] It was shown that the photocatalytic activity
of P25 titania toward MB degradation was greatly enhanced
by adding graphene, which was attributed to the rapid separation of charge carriers across the P25–graphene heterojunction.
Furthermore, the large specific surface area of graphene is also
beneficial for providing a greater number of surface active sites
for photocatalytic reactions.
In 2011, our group firstly reported the CdS–graphene heterojunction system for photocatalytic hydrogen production.[37]
Specifically, a CdS/reduced graphene oxide (CdS/RGO) composite was prepared by the simple solvothermal method. Prior
to the reaction, graphene oxide (GO) and cadmium acetate
were mixed in dimethyl sulfoxide (DMSO). Since GO has a
negatively charged surface at pH = 7, the Cd+ ions from the
cadmium acetate in a DMSO solution can be adhered to the

surface of GO because of the electrostatic attraction between
the Cd+ ions and the GO. Then, during the solvothermal process, H2S is produced from the DMSO and reacts in situ with
the Cd+ ions to create CdS NPs on the GO surface. Simultaneously, in the presence of DMSO, GO can be reduced to
RGO during the solvothermal process. The in situ growth of
CdS nano­particles on RGO can not only create an intimate
contact between the CdS and the RGO, assuring rapid migration of electrons from the CdS to the RGO, but also limits the

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Figure 22.  a) TEM images of the graphene/TiO2 NS at low resolution and high resolution (inset). b) Light-absorption spectra of G0 (pure TiO2 NS),
G0.2 (0.2 wt% graphene on TiO2 NS), G0.5 (0.5 wt% graphene on TiO2 NS), G1.0 (1.0 wt% graphene on TiO2 NS), G2.0 (2.0 wt% graphene on TiO2
NS), and G5.0 (5.0 wt% graphene on TiO2 NS). c) Comparison of the photocatalytic-hydrogen-production activity of the G0, G0.2, G0.5, G1.0, G2.0,
G5.0, and P1.0 (1.0 wt% Pt on TiO2 NS). d) Schematic illustration of the photocatalytic mechanism on graphene/TiO2 NS. Reproduced with permission.[38] Copyright 2013, The Royal Society of Chemistry.

growth of CdS and consequently enlarges the specific surface
area of the CdS/RGO composite. As a result, the photocatalytic
hydrogen-production activity of the CdS/RGO composite was
4.87 times higher than that of CdS with added Pt.
After that, our group further investigated the effect of graphene on the photocatalytic activity of TiO2 by preparing graphene/TiO2 nanosheet composites (graphene/TiO2 NS).[38] As

shown in the TEM images of the sample (see Figure 22a), the
TiO2 NSs were face-to-face dispersed on the surface of the graphene, enabling the intimate contact between the TiO2 NSs
and the graphene. Therefore, electrons can rapidly migrate
from the TiO2 NSs to the surface of the graphene. The lightabsorption ability of the TiO2 NSs was also greatly enhanced by
adding graphene. The graphene/TiO2 NS composite exhibited
a broad background absorption in the visible-light region (see
Figure 22b), which can be attributed to the 0 eV bandgap of graphene. Although this enhanced light absorption by graphene
cannot generate any active charge carriers for redox reaction,
the absorbed light can be used for producing heat and creating
a unique photothermal effect around the photocatalyst surface.
This effect is favorable for accelerating charge transport.[118] A
similar photothermal effect can be also observed in the case
of the ZnxCd1−xS/graphene,[121] Cu2O/graphene,[122,123] g-C3N4/
graphene,[124] Bi2WO6/graphene,[125] and other composites.
Therefore, the transient photocurrent density of grapheneloaded TiO2 NSs is higher than that of pure TiO2 NSs, indicating high electron–hole separation efficiency on the former
NSs. As a result, the 1.0 wt% graphene-loaded TiO2 NSs exhibited the highest photocatalytic activity (36.8 µmol h−1) among
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all the sample studied, due to the rapid migration of electrons from the TiO2 NSs to the graphene (see Figure 22c,d).
However, if the graphene content is higher than 1.0 wt% in
the composite, the photocatalytic activity of such composites
is drastically reduced, because the graphene overloading leads
to the light-shielding effect, which greatly suppresses the light
absorption of TiO2 NS. Therefore, it should be noted that
finding the optimal content of graphene on the semiconductor
is crucial for enhancing its photocatalytic activity.
In addition, a large π–π conjugation on the graphene surface can be also utilized for the adsorption of different reactants during the photocatalytic reaction. For example, our
group prepared a Bi2WO6/graphene/Ag (Bi2WO6/G/Ag)

composite for photocatalytic RhB degradation by a hydrothermal–photoreduction method.[20] It was shown that the
addition of graphene can significantly enhance adsorption of
the aforementioned dye pollutant. In particular, the adsorption ability of the prepared samples was tested by analyzing
the concentration changes of the RhB solution in the presence of the different samples, including Bi2WO6, Bi2WO6/Ag,
Bi2WO6/G, and Bi2WO6/G/Ag, under dark conditions. It was
shown that the samples with added graphene exhibited higher
RhB adsorption ability (see Figure 23a) due to a large π–π conjugation on the graphene surface, which could easily adsorb
RhB molecules through the strong π–π interactions between
the graphene and the RhB molecules (see Figure 23b). Dye
pollutants other than RhB, such as methylene blue, methyl
violet, methyl green, and so on, can be adsorbed on graphene
as well.[126–128]

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Figure 23.  a) RhB concentration changes with time in the presence of Bi2WO6, Bi2WO6/Ag, Bi2WO6/G, and Bi2WO6/G/Ag under dark conditions, where
C0 and C represent the initial and actual (at a given time) concentrations of the RhB solution. b) Schematic illustration of the π–π interactions between
graphene and RhB molecules. Reproduced with permission.[20] Copyright 2014, The Royal Society of Chemistry.

Meanwhile, the adsorption ability of the graphene toward
CO2 molecules was also confirmed by Yu et al.[46] Particularly, a
CdS-nanorods/RGO composite was prepared for photocatalytic

CO2 reduction. The CdS nanorods were uniformly dispersed on
the surface of RGO nanosheets, which was beneficial for creating a greater number of surface active sites (see Figure 24a).
As shown in Figure 24b, G0.5 exhibited a smaller semicircle
in the Nyquist plot than pure CdS, indicating faster interfacial electron transfer on the G0.5 sample. Moreover, according
to the nitrogen adsorption–desorption measurements, no

significant changes in the specific surface area were observed
for the CdS-nanorods/RGO composites in comparison with
pure CdS nanorods. However, the CO2-adsorption ability of
the CdS-nanorods/RGO was shown to be higher than that of
the pure CdS nanorods. This was attributed to the π–π conjugation interaction between the CO2 molecules and the RGO.
Moreover, the π–π conjugation interaction between RGO and
CO2 can also lead to the destabilization and activation of CO2
molecules to facilitate their reduction. Thus, the photocatalytic CO2-reduction activity of CdS-nanorods/RGO toward CH4

Figure 24.  a) TEM image of optimized CdS-nanorods/RGO (G0.5). (Note: the weight percentage of RGO to CdS nanorods was varied from 0, 0.1, 0.25,
0.5, 1.0 and 2.0 wt%, the corresponding samples were labeled as G0, G0.1, G0.25, G0.5, G1.0 and G2.0, respectively.). b) Nyquist plots for G0 and G0.5.
c) Comparison of photocatalytic CO2 reduction activity of the G0, G0.1, G0.25, G0.5, G1.0, G2.0, N0.5 (0.5 wt% graphene on CdS nanoparticles), P0.5
(0.5 wt% Pt on CdS nanorods), and RGO for CH4 production. d) Schematic illustration of the mechanism for the enhancement of the photocatalytic
activity of the CdS-nanorods/RGO. Reproduced with permission.[46] Copyright 2014, The Royal Society of Chemistry.

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Figure 25.  Schematic illustration of the advantageous contact between
both components in 2D–2D composite photocatalysts in comparison to
0D–2D and 0D-1D composites. Reproduced with permission.[12] Copyright 2014, The Royal Society of Chemistry.

production was higher than that of pure CdS nanorods (see
Figure 24c,d) due to the high electron–hole separation efficiency, strong CO2 adsorption, and unique CO2 destabilization.
Taking into account the 2D structure of graphene, its potential can be fully utilized in constructing graphene–semiconductor 2D–2D heterojunction photocatalysts. Namely, the
contact interface in a 2D–2D (face contact) heterojunction
photocatalyst is larger than that in the zero-dimensional–2D
(0D–2D) (point contact) and 1D–2D (line contact) heterojunction photocatalysts, which facilitates the migration of charge
carriers across the the graphene–semiconductor heterojunction (see Figure 25).[12] Recently, our group reported a low-cost
2D–2D graphene–g-C3N4 composite prepared by an impregnation–chemical-reduction method for photocatalytic hydrogen
production.[45] In the aforementioned composite, 2D layered
g-C3N4 was intimately coupled with graphene nanosheets to
form a 2D–2D layered composite (see Figure 26a,b). Moreover,
based on the nitrogen adsorption–desorption isotherms and
UV–vis absorption spectra, the specific surface area and lightabsorption ability of g-C3N4 were enhanced by adding graphene. Meanwhile, the electron–hole recombination on g-C3N4
was also greatly suppressed after graphene addition due to the
fast electron–hole separation across the graphene–g-C3N4 heterojunction and a large contact interface of the 2D–2D graphene–
g-C3N4 structure. Therefore, the photocatalytic activity of g-C3N4
increases with increasing graphene loading. The photo­catalytic
activity of an optimized graphene–g-C3N4 composite (1.0 wt%
graphene) toward hydrogen production in methanol aqueous
solution was 3 times higher than that of g-C3N4 with Pt as a

co-catalyst (see Figure 26c). However, a further increase in graphene loading to 2.0 wt% and 5.0 wt% resulted in a smaller
and smaller photocatalytic activity due to the overloading of
graphene on the g-C3N4, which can cause shielding effects and
greatly inhibits the light absorption ability of the g-C3N4.
Meanwhile, Liang et al. carefully investigated the photocatalytic activity of 1D–2D single-wall carbon nanotubes (SWCNT)/
TiO2 nanosheets and 2D–2D graphene/TiO2 nanosheets toward
CO2 reduction.[129] It was shown that both SWCNT and graphene can significantly enhance the photocatalytic CO2-reduction activity of the TiO2 nanosheets because of the improved
electron–hole separation efficiency and enlarged specific surface area. However, the photocatalytic activity of graphene/TiO2
nanosheets was higher than that of SWCNTs/TiO2 because, in
the former, a 2D–2D structure was formed with a large contact
interface for electron migration.
In addition to the above-presented simple coupling of graphene nanosheets with a single semiconductor, graphene can
be utilized to further improve the photocatalytic activity of
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Figure 26.  a,b) TEM images of graphene nanosheets (a) and graphene–
g-C3N4 composite (b). c) Comparison of the photocatalytic activity of
GC0 (pure g-C3N4), GC0.25 (0.25 wt% graphene added to g-C3N4), GC0.5
(0.5 wt% graphene added to g-C3N4), GC1.0 (1.0 wt% graphene added to
g-C3N4), GC2.0 (2.0 wt% graphene added to g-C3N4), GC5.0 (5.0 wt% graphene added to g-C3N4), and N-doped TiO2 for photocatalytic hydrogen
production. Reproduced with permission.[45] Copyright 2011, American
Chemical Society.

heterojunction photocatalysts. For example, Babu et al. reported
a Cu2O/TiO2 p–n heterojunction photocatalyst with incorporated graphene oxide (GO) for enhancing its photocatalytic
activity toward hydrogen production.[130] The electron–hole
separation rate of Cu2O–TiO2 was higher than that of TiO2 due
to presence of the p–n heterojunction. The aforementioned

electron–hole separation in this Cu2O–TiO2 p–n heterojunction
photocatalyst was further improved by adding GO to facilitate
the migration of electrons from the space-charge region of the
Cu2O/TiO2 p–n heterojunction to GO. Incorporation of GO
to this p–n heterojunction photocatalyst enhanced its photocatalytic hydrogen efficiency under light irradiation, which was
14 times higher than that of a pure TiO2. Moreover, Zhao et al.
showed that the photocatalytic hydrogen-production activity of
the WO3/g-C3N4 direct Z-scheme heterojunction photocatalyst
can be further improved by the loading of RGO as a metal-free
electron mediator.[131] Zhao’s group demonstrated that RGO
can improve the flow of electrons from WO3 to g-C3N4 and
suppress reverse migration of electrons to WO3. Therefore, the
photocatalytic activity of the WO3/g-C3N4 Z-scheme heterojunction photocatalyst was greatly enhanced after the incorporation
of RGO. Furthermore, Kuai et al. reported a low-cost direct
Z-scheme heterojunction CdS/RGO/TiO2 photocatalyst[132] and

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7. Conclusions and Future Perspectives
In the past several decades, many studies have been reported for
the preparation of various heterojunction photocatalysts. Here,
a concise appraisal of the current achievements in the field of
heterojunction photocatalysts is presented, including the fundamental aspects in their design, synthesis, characterization, and

applications, demonstrating that this research field is important,
exciting, and highly rewarding. However, the practical applications and commercialization of heterojunction photocatalysts
require further substantial progress in the engineering of highly
efficient heterojunction photocatalysts. Future research directions in this field should be focused on the following aspects.
i) Significant challenges still remain in the development of
facile, efficient, and economic methods for preparing highquality heterojunction photocatalysts at the large scale for
practical applications. Moreover, a further advancement is
needed in controlling their morphology, contact interface,
crystallization, and hierarchical assembly. Note that further
progress in the preparation of heterojunction photocatalysts
is possible in conjunction with substantial advancements in
nanoscience and nanotechnology.
ii) The migration pathway of the photogenerated electrons and
holes in the heterojunction photocatalysts requires further
systematic studies. It was shown that the photogenerated
electrons and holes can be spatially separated on the heterojunction photocatalysts; however, up to now, there has been
no direct evidence to show the actual migration pathway
of electron–hole pairs at the heterojunction interface. This
issue is important for confirming the formation of different
types of heterojunction photocatalysts and should be further
investigated by more-powerful characterization tools.
iii)Studies regarding theoretical calculations and modeling
methods should attract much more attention. To achieve a
deeper understanding of the mechanism and charge-migration kinetics in the heterojunction photocatalysts, further
advancements in theoretical calculations are highly desirable to shed some light on the true picture of the photocatalytic processes in the heterojunction photocatalysts.
iv)Further development of new photocatalyst materials for
the design and fabrication of heterojunction photocatalysts is one of the key research goals. The existing photocatalytic materials feature various drawbacks such as high
cost, large bandgaps, low active surface area, etc. It is of

Adv. Mater. 2017, 29, 1601694

great significance to find cost-effective and advanced materials to prepare heterojunction photocatalysts for practical
applications. A perfect material for engineering a heterojunction photocatalyst should fulfill several requirements,
such as visible-light activity, high solar-conversion efficiency, proper bandgap structure for redox reactions, high
photostability for long-term applications, and scalability for
commercialization.
In conclusion, we have summarized recent work related to
heterojunction photocatalysts and their application in photo­
catalysis. The preparation and investigation of heterojunction
photocatalysts provides a meritorious platform for accelerating
their practical applications. We hope that this review can stimulate further exploration of the heterojunction systems in photo­
catalysis, solar cells, batteries, and other important research areas.

Acknowledgments
This study was partially supported by the 973 program (2013CB632402),
NSFC (21433007, 51320105001, 21573170, 51372190 and 51272199),
the Fundamental Research Funds for the Central Universities (2015-III034), the Self-determined and Innovative Research Funds of SKLWUT
(2015-ZD-1), and the Natural Science Foundation of Hubei Province of
China (No. 2015CFA001)
Received: March 29, 2016
Revised: November 4, 2016
Published online: February 21, 2017

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