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Cite this: DOI: 10.1039/c7qm00020k
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Engineering the surface charge states of
nanostructures for enhanced catalytic performance
Yu Bai,ab Hao Huang,a Chengming Wang,a Ran Long*a and Yujie Xiong
*a
Published on 20 March 2017. Downloaded on 04/04/2017 01:45:59.
Charge transfer typically takes place between the catalyst surface and the reaction species, accompanied
by species adsorption and activation. The surface charge state of the catalyst thus becomes a major factor
for tuning catalytic performance in addition to surface active sites. By tailoring the surface charge states,
the reaction activity and selectivity can be tuned to optimize the performance of a specific catalytic
application. In this review, we focus on the recent progress in materials design concerned with the
modification of surface charge states toward enhanced catalytic performance. With the active sites
categorized into metal and semiconductor surfaces, we outline the strategies for tailoring the surface
Received 17th January 2017,
Accepted 17th March 2017
charge states of metal and semiconductor catalysts, respectively, which are mainly based on interfacial
DOI: 10.1039/c7qm00020k
implemented in a variety of model catalytic reactions including catalytic organic reactions, electrocatalysis,
photocatalysis and CO oxidation reaction. The fundamental mechanisms behind each case are elucidated
rsc.li/frontiers-materials
in this article. Finally, the major challenges and opportunities in this research field are discussed.
electronic effects with various contact matters. The surface charge engineering approach has been
1. Introduction
Catalysis is the process of increasing the rate of a chemical
reaction by lowering the required activation energy. The ultimate
goal of catalysis is to achieve a high-efficiency and product-specific
a
Hefei National Laboratory for Physical Sciences at the Microscale, iChEM
(Collaborative Innovation Center of Chemistry for Energy Materials), School of
Chemistry and Materials Science, and National Synchrotron Radiation Laboratory,
University of Science and Technology of China, Hefei, Anhui 230026, P. R. China.
E-mail: [email protected], [email protected]
b
Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and
Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences,
Hefei 230031, P. R. China
process in the reaction.1 The catalyst, an additional substance
participating in the reaction, is the key to achieving this goal.2
Heterogeneous catalysis, which employs solid materials as catalysts
enabling easy product separation, has been widely implemented in
chemical manufacturing. It is well known that a catalytic reaction
always occurs on the surface of heterogeneous catalysts. The
catalytic performance is thus related to the surface state of the
catalysts. In the past decade, great efforts have been made to
optimize the design of catalytic materials by tailoring their
shapes, sizes and structures.3–6 The central theme in this
research is to engineer the surface active sites where catalytic
reactions take place from the viewpoint of geometric effects. It is
worth mentioning that nanomaterials have received tremendous
Yu Bai received her PhD in inorganic
chemistry from the University of
Science and Technology of China
(USTC), under the tutelage of
Professor Yujie Xiong in 2016. She
is currently an assistant professor at
the Institute of Solid State Physics,
Chinese Academy of Sciences (CAS).
Her research interests focus on
controlled synthesis and catalytic
applications of inorganic nanostructures.
Yu Bai
This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2017
Hao Huang
Hao Huang was born in Jiangxi,
China, in 1994. He received his
BS in chemistry (Special Class for
the Gifted Young) in 2013 from
the University of Science and
Technology of China (USTC).
Since then he has studied as a
PhD candidate under the tutelage
of Professor Yujie Xiong at the
USTC. His research interests
focus on the controlled synthesis
and catalytic applications of
metal nanostructures.
Mater. Chem. Front.
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Review
Materials Chemistry Frontiers
attention simply because the number of surface active sites can be
dramatically boosted by shrinking particle sizes to the nanoscale.
In addition to active site engineering, electronic structure is
another factor to which we should pay sufficient attention.7–11
The charge transfer between the catalyst surface and reactants
is generally involved in various reaction models including
photocatalysis,12–14 electrocatalysis,15,16 catalytic organic reactions
and gaseous reactions.17,18 For this reason, the electronic structures
of catalysts would significantly affect their interactions with reaction
species particularly in the case of catalyst-reactant charge transfer.
The surface charge state is one of the critical parameters to depict
the electronic structures of catalysts. To this end, engineering
surface charge states would endow catalytic nanostructures with
unique functions in reactant adsorption and activation.
Upon identifying the importance of surface charge states to
catalysis, a question naturally arises: how can the surface
charge states be tailored? Interfacing the catalyst with another
material has been proven as a versatile approach to engineer
surface charge states. Metal nanocrystals are widely used in a
variety of catalytic reactions owing to their strong interactions
Chengming Wang received his PhD
in inorganic chemistry from the
University
of
Science
and
Technology of China (USTC), under
the tutelage of Professor Yujie Xiong
in 2013. He is currently a Senior
Research Engineer at the USTC. His
research interests focus on controlled
synthesis and catalytic applications
of metal nanocrystals.
with the reaction species.19–22 In order to enhance their catalytic
performance, various configurations for integrating different
components together have been developed,23,24 which exhibit
excellent performance that differs from bare metal catalysts. For
instance, noble metals Pd and Pt are generally considered as
active catalysts in various catalytic reactions for their high
capability in molecular activation.25,26 Integrating with a semiconductor to form a Schottky junction or with another metal for
interfacial charge polarization is an effective way to further
improve the catalytic activity of metal catalysts, in which the
static interfacial electronic effects originating from the difference
of work functions can tailor the surface charge states of metal
catalysts.15–17 Furthermore, light illumination may supply photoexcited electrons or plasmonic hot electrons to the metal surface,
increasing surface charge density for catalytic reactions as a
dynamic effect.27 In a specific case demonstrated by Park, Somorjai
and Nedrygailov, nonadiabatic electronic excitation can be
generated to supply a flow of energetic electrons, which are
not in thermal equilibrium and are called ‘‘hot electrons’’, for
exothermic catalytic reactions.28–30 The hot electrons would be
irreversibly transported across the metal–semiconductor interface toward metal catalysts for catalytic reactions, enabled by
the localized Schottky barrier.
Another class of heterogeneous catalysts with lower material
cost than noble metals are oxides and chalcogenides that possess
semiconductor characteristics. As heterogeneous catalysis usually
includes three steps – adsorption, activation and desorption –
surface electron density plays a similar role in tuning the
performance of semiconducting catalysts to the cases of metal
catalysts.31 For ultrathin two-dimensional semiconducting nanomaterials, surface electron density can be increased to promote
the adsorption and activation of reactants on account of their
disordered atomic structures.32 Moreover, introducing defects
into the nanomaterial surface can enhance the concentration of
carriers near the defects, which in turn reduces the catalytic
Chengming Wang
Ran Long
Mater. Chem. Front.
Ran Long was born in Anhui,
China, in 1987. She received her
BS in chemistry in 2009, and her
PhD in inorganic chemistry under
the tutelage of Professor Yujie
Xiong in 2014, both from the
University of Science and
Technology of China (USTC).
After two years of postdoctoral
training with Professors Yujie
Xiong and Li Song, she is now a
Research Associate Professor at
the USTC. Her research interests
focus on the controlled synthesis
and catalytic applications of
metal nanocrystals.
Yujie Xiong received his BS in
chemical physics in 2000 and
PhD in inorganic chemistry in
2004 (with Professor Yi Xie),
both from the University of
Science and Technology of China
(USTC). After four-year training
with Professors Younan Xia and
John A. Rogers, he joined the NSFNNIN at Washington University
in St. Louis as the Principal
Scientist and Lab Manager.
Since 2011, he has been a
Yujie Xiong
Professor of Chemistry at the
USTC. He has published more than 130 papers with over 10 000
citations (H-index 48). His research interests include the synthesis,
fabrication and assembly of inorganic materials for energy and
environmental applications.
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Materials Chemistry Frontiers
energy barrier.33 Another approach to surface charge engineering
is the interfacial charge polarization between the metal and the
semiconductor that enriches electrons on the surface of the
semiconductor, which can facilitate molecular activation.18 Overall,
the increased charge density on the semiconductor surface can
accelerate reactant diffusion, reduce reaction activation energy and
facilitate molecular activation in heterogeneous catalysis.
Taken together, tailoring surface charge states should be a
versatile approach to tuning the catalytic performance of both
metal and semiconductor catalysts. In this review article, we will
outline the recent progress in engineering the surface charge
states of catalytic nanostructures toward enhanced catalysis,
together with the fundamental mechanisms behind each case.
The implemented catalytic reaction systems include organic
catalytic reactions, electrocatalysis, photocatalysis and CO oxidation
reaction. From the working mechanisms, the readers can recognize
how surface charge states can be maneuvered and in turn alter
catalytic performance, and be able to design catalytic nanostructures
for specific applications through electronic effects.
To elucidate the fundamental mechanisms, we categorize
the catalytic materials by their electronic structures into metals
and semiconductors. We first summarize the methods for
tuning the surface charge states of metal catalysts mainly based
on interfacial electronic effects with various contact matters –
semiconductors, metals and organic ligands. Then the approaches
to tunable surface charge densities in semiconductor catalysts –
creating defects in ultrathin two-dimensional nanostructures, and
interfacing with metals – will be outlined. In order to have a
comprehensive overview on this topic, both static (i.e., interfacial
charge polarization) and dynamic (e.g., Schottky junction and
surface plasmon under light illumination) effects will be discussed
in this article. Finally, we point out the existing challenges in terms
of engineering the charge state in the current research and the
opportunities open for this research field. Organized succinctly,
this review article will deepen the readers’ understanding of the
structure–property relationship in various catalytic systems and
provide a bridge between material design and catalytic applications
from the viewpoint of electronic effects, which hence puts forward a
new direction in terms of optimizing catalytic properties.
Review
2.1
Interfacing with semiconductors
As a metal catalyst is in contact with a semiconductor, their
different electronic structures will drive an electron diffusion
process to build up an equilibrium state. The electronic structures
of both metal and semiconductor materials can be depicted with
work functions (W), which represent the minimum required
energy to elevate an electron from the Fermi level (Ef) to the
vacuum level (Vac).39 For noble metals Pt and Pd, their work
functions are typically larger than those of n-type semiconductors,
but smaller than those of p-type semiconductors.
The Fermi level of the n-type semiconductor (Efs), which
stands near the conduction band edge (Ec), is higher than the
Fermi level (Efm) of the metal before their contact. Thus an
equilibrium state between the Efs and Efm should be established
when they are put in contact. As a result, the electrons of the
semiconductor will diffuse to the metal at the lower energy level,
leading to the accumulation of negative charges (Fig. 1a). This
electron diffusion process will deplete the free electrons in the
semiconductor near the interface, forming a space charge
region. As such, the energy band of the n-type semiconductor
will bend upward to establish a Schottky barrier.40 Apparently,
the process of forming the Schottky barrier with the n-type
semiconductor will increase the electron density of the metal
2. Engineering the surface charge
states of metal catalysts
The charge state on the surface of metal nanocrystals plays
a regulatory role in the state of adsorbed molecules, which in
turn affects the adsorption and activation of reactants,17
thereby leading to tunable catalytic properties. Unfortunately,
the charge state of a specific metal surface is largely restricted
in a certain range. For this reason, one has to tailor the surface
charge states by introducing electrons to the surface of metal
catalysts.34 As a matter of fact, integrating with another nanomaterial is an effective means of regulating the surface electron
densities of metals.35–38 This section mainly describes how the
catalytic activity can be tuned by tailoring the surface charge
states of metal catalysts.
Fig. 1 Schematics illustrating the processes of forming the Schottky
junction between a metal and (a) an n-type semiconductor or (b) a p-type
semiconductor. Reproduced with permission from ref. 12. Copyright 2015
Royal Society of Chemistry.
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component as a static effect. In the case of a p-type semiconductor, the band alignment and electron diffusion fall into
a different situation (Fig. 1b). The Efs of the p-type semiconductor
is close to the Ev, and Efs is lower than Efm. For this reason, the
free electrons on the metal will flow to the semiconductor upon
their contact. This process can bend the energy band of the p-type
semiconductor downward to form a space charge region, and
accumulate positive charges on the metal component.39
Given this static interfacial electronic effect, it is anticipated
that semiconductor supports may play a role in tailoring the
surface charge states of metal catalysts. As a matter of fact, n-type
semiconductors have been widely used as supports for metal
catalysts in various organic reactions.41–43 Most recently, Zheng
et al. developed a stable atomically dispersed Pd–TiO2 catalyst
(namely, Pd1/TiO2) in which single Pd atoms are stabilized with
ethylene glycolate (EG) on ultrathin TiO2 nanosheets (Fig. 2a
and b).44 This catalyst exhibits 9 times higher catalytic activity
in hydrogenation of CQC bonds than commercial Pd catalysts
in terms of surface Pd atoms (Fig. 2c). It is believed that the
static electronic effect plays an important role in the catalytic
performance enhancement. As suggested by first-principles
simulations (Fig. 2d), the Pd atoms supported on TiO2 can
efficiently split the adsorbed H2 into H atoms. While one H atom
migrates to the EG nearby, the other H atom would form Pd–Hd
which can in turn transfer to CQC for efficient hydrogenation.
In general, the enhancement of catalytic activities by semiconductor supports is not very significant as the static interfacial electronic effect can only accumulate a limited number of
electrons on metal catalysts. To make full use of the Schottky
Fig. 2 (a) TEM and (b) HAADF-STEM images of a Pd1–TiO2 catalyst.
(c) Catalytic performance of Pd1–TiO2 and reference catalysts in styrene
hydrogenation. (d) Energies and models of intermediates and transition
states in the heterolytic H2 activation process for Pd1–TiO2. Reproduced
with permission from ref. 44. Copyright 2016 American Association for the
Advancement of Science.
Mater. Chem. Front.
junction for enhanced catalysis, one can employ light illumination
to excite the n-type semiconductor and transfer a large number of
electrons to metal catalysts as a dynamic interfacial electronic
effect. Under light illumination, the electrons in n-type semiconductors are elevated to the conduction band while the holes
are left in the valence band. Once the electron–hole pairs enter
the space charge region, the photogenerated electrons in the
conduction band and the holes in the valence band will migrate
to the bulk semiconductor and the metal under the effect of an
internal electric field, respectively. Under the successive photoexcitation of the semiconductor, a large number of electrons will
be accumulated in the semiconductor, making them energetic
enough to transfer to the metal.12 The Schottky barrier will in
turn prevent the electron from drifting back to the semiconductor.
As such, the electron density of the metal surface can be increased.
Similarly, photoexcitation can substantially reduce the electron
density of the metal surface when a p-type semiconductor is used
as a support.12
Here we employ oxygen activation on a Pd surface as a
model reaction to demonstrate the mechanisms. For most
oxidation reactions, the activation of molecular oxygen is an
important process determining catalytic performance.45–49 The
molecular adsorption is a process that highly depends on the
atomic arrangement and charge state of the catalyst surface.50–52
In 2013, we found that molecular oxygen can be better activated
at the Pd{100} surface than Pd{111}.17 As shown in Fig. 3a and b,
the molecular O2 is better activated on Pd{100} according to the
larger O–O bond length (1.402 Å on {100} versus 1.324 Å on {111}),
which has been verified by synchrotron radiation-based nearedge X-ray absorption fine structure spectroscopy (NEXAFS).
During the activation process, an electron transfer takes place
from Pd to O2 molecules. The transferred electrons occupy the
antibonding orbital of oxygen so as to activate the molecule. In
principle, the more electrons are transferred, the better oxygen is
activated. The spin charge density analyses (Fig. 3c and d)
revealed that about 0.7 electrons can be transferred from the
Pd{100} surface to the adsorbed O2, while the charge transfer in
the case of Pd{111} was onlyB0.4 electron charge. Thus the
different amounts of electrons transferred into the anti-bonding
p* orbital of O2 should be the fundamental reason for the
differentiated O2 activation behavior. For this reason, the catalytic
performance of Pd{100} in O2 activation and oxidation reactions
can be further enhanced by increasing its electron density. As
displayed in Fig. 3e, higher electron density of the Pd surface
enables the stretching of O–O bonds, leading to better O2
activation.27
To increase the electron density of the Pd surface, we
developed Pd–TiO2 hybrid structures (Fig. 4a and b) in which
TiO2 can be used as the electron donor to modulate the charge
state of the Pd surface under light illumination owing to the
metal–semiconductor Schottky junction.27 We have employed
2,2,6,6-tetramethyl-4-piperidone hydrochloride (4-oxo-TMP) as
a probe molecule to examine the activated oxygen species, which
can produce nitroxide radicals (4-oxo-TEMPO) upon trapping the
oxygen species. The electron spin resonance (ESR) spectra in
Fig. 4c show that the signals for 4-oxo-TEMPO can be enhanced
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Fig. 4 (a) Schematic illustrating the working mechanism for the enhancement
of O2 activation using a Pd–TiO2 hybrid structure. (b) TEM image of the Pd–TiO2
hybrid structure. (c) ESR spectra of the samples formed by mixing a solution of
4-oxo-TMP with Pd nanocubes, the Pd–TiO2 hybrid structure and bare TiO2.
(d) Catalytic activity of Pd nanocubes, the Pd–TiO2 hybrid structure and bare
TiO2 for glucose oxidation under incident UV light with different power
intensities. Reproduced with permission from ref. 27. Copyright 2014 John Wiley.
Fig. 3 The most favorable adsorption configurations of O2 on (a) Pd{100}
facets and (b) Pd{111} facets. The spin charge densities of O2 on (c) Pd{100}
facets and (d) Pd{111} facets. Reproduced with permission from ref. 17.
Copyright 2013, American Chemical Society. (e) Curves showing the
changes in O–O distance and O2 magnetic moment by introducing
additional electrons or holes into the Pd{100} system. Reproduced with
permission from ref. 27. Copyright 2014 John Wiley.
with the Pd–TiO2 hybrid structures compared with bare Pd. This
suggests that integrating the Pd{100} catalyst with an n-type
semiconductor surely promotes the ability of Pd in activating
O2. When implemented for catalytic oxidation of glucose, the
Pd–TiO2 catalyst exhibits excellent catalytic activity in glucose
oxidation under illumination, especially at the light intensity
of 5.6 mW cm 2 (Fig. 4d). Certainly a strong dependence of
performance on light intensity has also been observed. The complex
charge kinetics in this system has been revealed by our ultrafast
absorption spectroscopy characterizations.27 It indicates that too
strong light illumination may induce the generation of plasmonic
hot electrons on Pd although it is a metal with weak plasmonic
properties.53,54 The generated plasmonic hot electrons are injected
into the conduction band of TiO2, which lowers the electron density
of the Pd surface and reduces the efficacy of the Schottky
junction in enhancing O2 activation.12,14,27 Nevertheless, this
case well demonstrates that the dynamic interfacial electronic
effect by the metal–semiconductor interface under light illumination
can provide a powerful approach to tailor the surface charge states of
metal catalysts.
2.2
Interfacing with metals
The electron density of a metal surface can also be tailored by
interfacing with another metal material. The mechanism is
quite similar to the principle of a galvanic cell (also called
voltaic cell) – an electrochemical cell that derives electrical
energy from spontaneous redox reactions taking place within
the cell. The voltaic pile, invented by Alessandro Volta in the
1800s, consists of a pile of cells with different metals (e.g., Cu
and Zn). The potential difference of metals is utilized to induce
electron accumulation at the metal side with a high work
function, eventually generating electricity in the devices. We
call this effect interfacial charge polarization. For instance, the
work functions of Pd{100} and Pt{100} are 5.14 and 5.68 eV,
respectively. This work function difference causes an electron
to transfer from Pd to Pt so as to equilibrate the electron Fermi
distribution at their interface. As indicated by the simulated
differential charge density (Fig. 5), distinct interfacial polarization
occurs when the Pt atoms are of 1–2 atomic layers, resulting in a
substantial accumulation of negative charges on the Pt surface.
However, as the Pt thickness reaches 4 atomic layers, the surface
polarization effect becomes substantially weaker. In other words,
the thinner the thickness, the more the charges accumulated on
the Pt surface. This increased electron density on the Pt surface
would facilitate a number of catalytic applications.
A typical catalytic application facilitated by the interfacial
charge polarization is the electrocatalytic hydrogen evolution
reaction (HER, 2H+ + 2e - H2) – a highly important process
for hydrogen generation constituting reversible hydrogen fuel
cell technology.55–61 Pt is the most efficient electrocatalyst for
the HER due to its low overpotential and high current density;
however, the usage of Pt is yet to be significantly reduced to
minimize material costs. Enabled by the interfacial charge
polarization, we have developed an approach to reduce Pt usage
and boost HER performance at the same time. As along as the
Pt layers on Pd substrates are thin enough, the interfacial
charge polarization can accumulate negative charges on the
Pt surface to facilitate hydrogen evolution. The developed
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Fig. 5 Differential charge density by first-principles simulations illustrating
the alterations of electron distributions with the Pt thickness: (a) 1 layer,
(b) 2 layers, (c) 3 layers and (d) 4 layers. The olive and cyan colors represent
increase and decrease in electron density, respectively. Reproduced with
permission from ref. 16. Copyright 2014 John Wiley.
design is based on a Pt–Pd-reduced graphene oxide (rGO) stack
structure with controllable Pt thickness ranging from 0.8 nm to
3.2 nm (Fig. 6a–c).16 As anticipated, the Pt–Pd–rGO I sample
with the thinnest Pt shells (0.8 nm) exhibits a current density of
791 mA cm 2 at a potential of 300 mV, dramatically larger
than that of the other two samples (Fig. 6d). This suggests
that the low thickness of Pt shells is more favorable for
HER performance. In addition, the calculated Tafel slope of
10 mV decade 1 for Pt—Pd–rGO I is the lowest among those
reported for HER electrocatalysts (Fig. 6e). This further demonstrates that the HER performance has a strong correlation with
the Pt thickness, in accordance with the proposed interfacial
charge polarization mechanism. Accordingly, the requirement for
Fig. 6 TEM images for (a) Pd–Pt–rGO I with 0.8 nm Pt shell thickness,
(b) Pd–Pt–rGO II with 1.8 nm Pt shell thickness and (c) Pd–Pt–rGO III with
3.2 nm Pt shell thickness. (e) Polarization curves and (f) corresponding
Tafel plots of Pd–Pt–rGO I, II and III, respectively. Reproduced with
permission from ref. 16. Copyright 2014 John Wiley.
Mater. Chem. Front.
ultrathin Pt shells by this working mechanism also meets our
demand for reducing Pt usage.
Another example sharing a similar working mechanism is
the cocatalyst design for photocatalytic water splitting – a
promising approach to transform solar energy into hydrogen
fuel.62–64 A typical process of photocatalysis involves three
steps – light absorption, electron–hole separation, and surface
reactions. To improve the efficiencies of the latter two steps,
metal cocatalysts are widely used for integration with semiconductor photocatalysts. The cocatalyst not only promotes
electron–hole separation via the Schottky junction, but also
provides active sites for H2O adsorption and activation, opening the
possibility of improving the overall photocatalytic efficiency.12,40,65
There is no doubt that Pt is the most active cocatalyst for
hydrogen evolution in water splitting but still suffers from high
material cost. To overcome this limitation, the interfacial charge
polarization is implemented in the cocatalyst design for this
application, similarly to the HER system.
In this case, Pt is deposited on the Pd nanocubes, which are
loaded on TiO2 nanosheets to form the TiO2–Pd@Pt quasi-core–
shell structure with controllable Pt thickness (Fig. 7a).15 As
illustrated in Fig. 7b, this design utilizes the Schottky junction
between TiO2 and Pd to trap the photoexcited electrons on Pd,
and then induces the electron accumulation at the outer surface
of Pt through interfacial charge polarization. As indicated by
Table 1, the high electron density on the Pt surface can enhance
the adsorption of H2O molecules and thus facilitate the water
splitting process.
The effect of interfacial charge polarization can make a
significant contribution to the electron accumulation on the Pt
surface only when the Pt shell thickness is less than 4 atomic layers.
Fig. 7 (a) TEM image of a TiO2–Pd@Pt3L sample with the Pt shell thickness
of 3 atomic layers. (b) Schematic illustrating the working mechanism for
interfacial charge polarization in photocatalytic water splitting. (c) CO
chemisorption DRIFTS spectra of TiO2–Pd@Pt3L and TiO2–Pd@Pt10L in
reference to TiO2–Pt and TiO2–Pd hybrid structures. (d) Photocatalytic
H2 evolution from water with TiO2, TiO2–Pd, TiO2–Pt, TiO2–Pd@Pt3L and
TiO2–Pd@Pt10L, respectively. Reproduced with permission from ref. 15.
Copyright 2015 John Wiley.
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Table 1 Adsorption energies of H2O molecules to various Pt(100) surfaces
from first-principles simulations by altering the Pt shell thickness and substrate
material. The simulations are performed in the neutral state or in the presence
of 1 additional electron (1e ). Reproduced with permission from ref. 15.
Copyright 2015 John Wiley
Table 2 Adsorption energies of H to Pd{100}, Pd{730} surfaces, and Pd13
clusters from first-principles simulations. The simulations are performed in the
neutral state or in the presence of 1 additional electron (1e ). Reproduced with
permission from ref. 20. Copyright 2015 John Wiley
Structural model
Structural model
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Ead (eV) neutral
Ead (eV) with 1e
Pd5L@
Pt1L
Pd5L@
Pt2L
Pd5L@
Pt3L
Pd5L@
Pt4L
0.346
4.317
0.252
4.627
0.263
4.892
0.664
4.495
Pt5L@
Pt4L
Ead (eV) neutral
Ead (eV) with 1e
{100}
2.090
2.735
{730}
2.095
2.739
Pd13-a
2.929
2.957
Pd13-b
2.864
2.877
0.101
4.161
The increased electron density has been proven by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using CO as
a probe molecule. During the chemisorption, the CO molecules can
accept electrons from the Pt surface to enter their antibonding
orbitals. At higher electron density, the Pt surface would donate
more electrons to the antibonding orbitals of CO molecules, which
reduces the vibrational frequency of CO.66 Fig. 7c shows that the
vibrational frequency for CO chemisorption has been lowered by the
TiO2–Pd@Pt3L with 3 atomic Pt layers. This suggests the higher
electron density of the Pt surface with respect to TiO2–Pt and
TiO2–Pd@Pt10L. The increased electron density can also be
confirmed by the reduced binding energies in X-ray photoelectron
spectroscopy (XPS).15 As a result, the accumulated electrons on the
Pt surface enable a dramatically higher hydrogen production rate
by TiO2–Pd@Pt3L (Fig. 7d).
Basically, the interfacial charge polarization is a static electronic
effect. Under light illumination, the surface plasmon of noble
metal nanostructures will generate plasmonic hot electrons,
which may induce a dynamic effect at the metal–metal interfaces.
The surface plasmon can be defined as the resonant collective
oscillations of the electrons excited by incident photons at the
surface of noble metal Au and Ag nanoparticles,19,67 which decays
through radiative photon re-emission and non-radiative Landau
damping. The non-radiative decay generates energetic electrons and
holes, which are commonly referred to as hot electrons and hot
holes, respectively. The majority of energetic electrons are dissipated
by electron–electron scattering within about 100 femtoseconds.
However, a small number of energetic electrons can be transferred to adjacent species, which may alter the catalytic process
by maneuvering surface charge densities.
For instance, Pd is a widely used catalyst for organic hydrogenation reactions – a large group of addition reactions of
hydrogen at unsaturated bonds, which are among the simplest
transformations in organic chemistry and are extensively
involved in various chemical processes.68–74 The hydrogenation
reactions usually take place through the dissociation of molecular
H2 and desorption of Had atoms from the surface.69 As molecular
H2 can be easily dissociated at the Pd surface,75 the desorption
of Had atoms from the Pd surface becomes the rate-limiting step
in the process of hydrogenation reactions. Our simulation
(Table 2) indicates that the binding of dissociated H atoms to
the Pd surface is enhanced by the addition of one electron,
regardless of the locations of H atoms on Pd nanocrystals.20
Thus the high electron densities on the Pd surface (e.g., through
plasmonic hot-electron injection) would disfavor hydrogenation
reactions.
One of our recent works proved this hypothesis, in which
ultrathin layers of catalytic metal Pd are grown on plasmonic
gold nanorods (Fig. 8a and b).76 The Pd thickness can be
controlled from 2 atomic layers to tens of atomic layers. Once
the plasmonic hot electrons are generated in Au, they have to
travel through the Pd layers before reaching the reaction
species (Fig. 8c). As such, the lifetime of hot electrons can be
modulated through Pd layer thickness. We have acquired the
time constant of hot electrons using ultrafast absorption spectroscopy, which can be well correlated with catalytic performance
(Fig. 8d). The longer the lifetime the plasmonic hot electrons
exhibit, the lower the conversion yield the catalytic reaction
achieves. This suggests that plasmonic hot electrons indeed
make a negative contribution to hydrogenation reactions.
The correlation of hot electron lifetime with the Pd thickness
(Fig. 8d) reveals that the plasmonic hot electrons tend to interact
more thoroughly with the Pd lattice by increasing Pd shell
thickness. This interaction will reduce the number of hot
electrons reaching the Pd surface. Nevertheless, if the Pd shells
are too thick, hot electrons would be generated by the Pd shells
on their own, which brings more electrons to the Pd surface.
Thus the charge densities of the Pd surface can be tailored by
Fig. 8 (a) TEM images and (b) EDS mapping profiles of Au–Pd2L core–
shell nanostructures with 2 atomic Pd layers. (c) Schematic illustrating the
plasmonic effects on styrene hydrogenation by Au–Pd2L core–shell
nanostructures. (d) Plasmonic hot-electron effect on styrene hydrogenation
under irradiation of l 4 700 nm in comparison with time constants
responsible for charge recombination. Reproduced with permission from
ref. 76. Copyright 2016 American Chemical Society.
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controlling the Pd thickness over plasmonic nanostructures.
Although plasmonic hot electrons suppress the hydrogenation
reactions, this working mechanism should be implemented to
promote the reactions that require high surface electron densities
(e.g., oxygen activation and oxidation reactions). It is anticipated
that future works will validate this assumption.
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2.3
Interfacing with organic ligands
Organic ligands are key components in homogeneous catalysis
which can induce significant steric and electronic effects for
catalytically active metal sites to tune catalytic selectivity.77–79
In terms of heterogeneous catalysis, the organic ligands are
mainly employed as the agents for improving catalytic selectivity
through steric effects or the stabilizers for loading atomically
dispersed metal atoms.80–82 In principle, the organic ligands
should be capable of creating electronic effects on catalytic sites
when the catalytic materials are shrunk to the atomic level. In
recent years, it has been demonstrated that such electronic
effects can indeed tune catalytic activities.83,84
Most recently, Zheng et al. reported that electronic effects
can serve as a knob for tuning the selectivity of catalytic reactions
in the partial hydrogenation of nitroaromatics to N-hydroxylaniline
derivatives (an important class of intermediates for many highvalue products85–87).88 The demonstration is based on the
ethylenediamine (EDA)-chelated ultrathin Pt nanowires. The
nanowires have an ultralow diameter of 1.1 nm (Fig. 9a), which
allows the significant electronic effects of EDA on Pt nanowires.
As indicated by Bader charge analysis (Fig. 9b), the EDA ligands
can donate electrons to Pt nanowires and make the Pt surface
highly electron rich. Such an increase in the electron density on
the Pt surface favors the adsorption of electron-deficient reactants
(i.e., nitroaromatics) and the desorption of electron-rich substrates
(i.e., N-hydroxylaniline derivatives) (see Fig. 9c). As a result, full
hydrogenation can be effectively prevented to achieve high
selectivity in the production of N-hydroxylanilines by EDA-chelated
Pt nanowires. As a matter of fact, the electronic effects by EDA
ligands can be readily implemented in various Pt catalysts. For
instance, when commercial Pt black is treated with Pt(EDA)2(acac)2,
the Pt-EDA chelating units can be deposited on the surface. As a
result, the EDA-modified Pt black catalyst exhibits a dramatically
enhanced selectivity to N-hydroxylaniline derivatives (Fig. 9d and e),
which are applicable to nitroaromatics with various substitutions.
As the electronic effects of organic ligands have been extensively
investigated in homogeneous catalysis – the main branch in organic
chemistry studied for centuries – we would envision that many
concepts and findings can be borrowed from traditional organic
chemistry in the future. Moreover, despite rare attempts in dynamic
electronic effects, it is anticipated that some photosensitizers can be
employed as ligands to integrate with heterogeneous catalysts for
light-enhanced catalysis.
3. Engineering the surface charge
states of semiconductor catalysts
In addition to noble metals, metal oxides and chalcogenides
are a class of catalysts at lower material costs. For instance,
metal oxides have been widely employed as catalysts for CO
oxidation – a typical reaction well studied in heterogeneous
catalysis.18,32,33,89–91 Many metal oxides and chalcogenides possess
semiconductor characteristics, which provides an opportunity for
tuning their catalytic performance through electronic effects.
3.1 Creating defects in ultrathin two-dimensional
nanostructures
Fig. 9 (a) TEM image of EDA-chelated ultrathin Pt nanowires. (b) Structural
models and Bader charge analysis for bare Pt nanowires and EDA-chelated
Pt nanowires. (c) Free energies (DGpo) for the adsorption of N-containing
aromatics over EDA-chelated Pt nanowires. Black, nitrobenzene; red,
nitrosobenzene; green, N-hydroxylaniline; blue, aniline. Catalytic performance
comparison for the hydrogenation of substituted nitrobenzene over Pt
black (d) before and (e) after Pt(EDA)2(acac)2 modification. Reproduced
with permission from ref. 88. Copyright 2016 Nature Publishing Group.
Mater. Chem. Front.
As the surface charge density of catalysts plays a key role in
catalysis, ultrathin two-dimensional nanosheets that offer high
charge density provide an opportunity for achieving high catalytic
performance. Taking CO oxidation as an example, the high charge
density can promote the activation of oxygen and accelerate CO
diffusion, facilitating the CO oxidation process.
The high catalytic performance of ultrathin two-dimensional
nanosheets is related to their large surface area, abundant
active sites and high carrier concentration. For instance, SnO2
nanosheets with 5 atomic layers at the thickness of about
0.66 nm can be obtained by EDA-assisted synthesis.32 The
unique electronic structures of the 0.66 nm SnO2 nanosheets
enable their excellent activities in CO oxidation, with lower
conversion temperature and lower activation energy, as compared
with 1.9 nm SnO2 nanosheets, SnO2 nanoparticles and bulk SnO2
(Fig. 10a and b). As metal oxides such as SnO2 serve as catalysts,
CO oxidation generally follows the Mars van Krevelen mechanism.92
In this case, CO diffusion is one of the important factors that affect
the catalytic activities. The density of states (DOS) and charge
density contour plots show the increased charge density at the
valence band of ultrathin SnO2 nanosheets (Fig. 10c–f). This feature
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Fig. 11 (a) Top-view structural schematic for CeO2 nanosheets with
3 atomic layers and with numerous pits on the surface. CO molecules
prefer to adsorb at the P2 site, while O2 molecules tend to adsorb and
dissociate at the adjacent P1 sites. (b) Calculated densities of states (DOS)
for the ultrathin CeO2 nanosheets with numerous pits, ultrathin CeO2
nanosheets and bulk CeO2. (c) Catalytic activities for CO oxidation vs.
reaction temperature and (d) the corresponding Arrhenius plots for the
three catalysts. Reproduced with permission from ref. 33. Copyright 2013
Nature publishing Group.
Fig. 10 (a) Catalytic activity for CO oxidation vs. reaction temperature for
0.66 nm SnO2 nanosheets, 1.9 nm SnO2 nanosheets, SnO2 nanoparticles
and bulk SnO2 (from left to right). (b) The corresponding Arrhenius plots.
Calculated density of state (DOS) for (c) 0.66 nm SnO2 nanosheets and
(d) bulk SnO2. Charge density contour plots projected along the (100)
plane for the valance band maximum of (e) 0.66 nm SnO2 nanosheets and
(f) bulk SnO2. Reproduced with permission from ref. 32. Copyright 2013
John Wiley.
would result in fast electron transport to facilitate CO diffusion
so that CO can more efficiently react with the activated O atom,
improving the overall performance of CO oxidation.
The atomically thin nanosheets set up a platform for further
engineering the subtle features toward tunable charge densities.
The surface defect is such a structural feature in ultrathin
nanosheets that can further boost CO oxidation activity. Taking
ultrathin CeO2 nanosheets with surface pits as an example, two
different kinds of coordinated Ce atoms are generated on
account of plentiful surface pits: the 4-coordinated Ce atom at
which CO is more likely to adsorb, and the 5-coordinated Ce
atom at which O2 prefers to adsorb and activate (Fig. 11a).33
During the formation of CO2, CO should overcome a diffusion
barrier to react with the adjacent O atom. The pit-surrounding
Ce sites help to increase the hole carrier density by increasing
the DOS near the Fermi level (Fig. 11b), thereby lowering the
CO diffusion barrier. In this case, the CO adsorbed at the
4-coordinated Ce site can easily diffuse to the O atom absorbed
at the neighboring 5-coordinated Ce site to produce CO2. Although
O2 activation is the rate-determining step in CO oxidation, the
reduced CO diffusion barrier caused by the increased carrier
density also contributes to lowering the overall activation energy.
As a result, the ultrathin CeO2 nanosheets with surface pits can
efficiently improve the CO oxidation rate and lower the activation
energy (Fig. 11c and d). It is anticipated that lattice engineering
on ultrathin two-dimensional nanostructures with semiconductor
characteristics can create more efficient catalysts for other catalytic
applications from the angle of electronic effects.
However, it is worth mentioning that the defects are often
the centers for charge recombination particularly in the case of
light illumination (i.e., dynamic charge systems).12,93–97 As ultrathin two-dimensional nanostructures are further interfaced with
other materials (e.g., metals), charge recombination would become
more severe at the defects existing at the interfaces (i.e., interfacial
defects).13,93–97 Thus creating defects toward tunable charge states
requires systematic study in which the material models and their
interlinked effects should be carefully analyzed.
3.2
Interfacing with metals
Similarly to the case of metal catalysts, the surface charge states
of semiconductor catalysts can also be tailored through interfacial electronic effects. Here we first name an example using
CO oxidation as a model reaction. Cu2O/CuO is a catalyst that
has been intensively investigated for CO oxidation owing to its
relatively high catalytic activity.89 Recently we have revealed
that interfacial charge polarization can enhance the catalytic
activity of Cu2O/CuO through integration with Ag.18 In the
synthesis, Cu2O is grown on Ag nanoplates in the solution
phase, in which the coverage of Cu2O on Ag can be well
controlled to form tunable interfacial lengths. As a result, three
Ag–Cu2O samples with various partial coverage and full coverage
sample can be obtained, while Cu2O frames are prepared by
removing Ag as a reference sample (Fig. 12a–e). Prior to the
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catalytic assessment, all the samples have been treated in a
cycle of CO oxidation to form a stable CuO surface. Our catalytic
assessment (Table 3) indicates that the apparent activation
energies can be dramatically reduced to 36.7–42.7 kJ mol 1
with partial coverage, which should be ascribed to the exposed
Ag–CuO interfaces. Moreover, the CO conversion rates turn out
to be dependent on the interfacial lengths (Fig. 12f), suggesting
that the interfacial lines are the locations of active sites.
The high catalytic activity enabled by the exposed Ag–CuO
interfaces has been well elucidated by our first-principles
simulations. Ag and CuO possess different work functions
(4.45 eV vs. 5.97 eV) to induce an interfacial charge polarization
effect (Fig. 12g and h), which increases the electron density of
the CuO surface near the Ag–CuO interface and dramatically
decays with the distance to the interface. Consequently, the
increased electron density on the CuO surface can lower the
transition energy barrier from 0.60 to 0.24 eV in CO activation
as indicated by our simulations. This feature designates the
exposed Ag–CuO interface as the location of active sites so that
the number of active sites can be readily tailored by controlling
the interfacial lengths.
The interfacial charge polarization is also applicable to other
materials and reaction systems. For example, MoS2 is a material
with a layered structure to replace expensive Pt for electrocatalytic
HER application.98–101 As compared with metallic 1T-MoS2, semiconducting 2H-MoS2 is more chemically stable but exhibits
relatively low HER activity. The limitation of the 2H-MoS2 catalyst
Fig. 12 SEM images of (a) partial coverage I, (b) partial coverage II,
(c) partial coverage III and (d) full coverage Ag–Cu2O hybrid structures, and
(e) Cu2O frames. (f) Catalytic performance of Ag–CuO/Cu2O partial coverage I,
II and III samples in Cycle 2 of CO oxidation. (g) Schematic illustration for
Ag–CuO/Cu2O hybrid structures with interfacial charge polarization for CO
oxidation (side view for the cross-section). (h) Differential charge density by firstprinciples simulations illustrating the increase (olive color) and decrease (cyan
color) of electron distributions in the atomic model for the Ag(111) substrate
interfacing with the extension part of the side CuO layer. Reproduced with
permission from ref. 18. Copyright 2014 American Chemical Society.
Mater. Chem. Front.
Table 3 Apparent activation energies (Ea) of CO oxidation catalyzed by
various CuO/Cu2O-based samples calculated from the Arrhenius plots
employing the steady-state CO conversion data measured in the second
cycle of catalytic CO oxidation. Reproduced with permission from ref. 18.
Copyright 2014 American Chemical Society
Partial
cover. I
Sample
Partial
cover. II
Partial
cover. III
Full cover. Frames
1
Ea (kJ mol ) 42.7 1.5 37.0 1.8 36.7 1.3 62.8 2.1 66.4 7.1
originates from the limited number of active sites – the unsaturated
sulfur atoms located along the edges of the MoS2 layers and the low
mobility for charge transport.98,100 To overcome this limitation, we
have developed a hybrid structure of 2H-MoS2 and Pd nanowires
(Fig. 13a).102 As a result, the hybrid structure exhibits a current
density of 122 mA cm 2 at 300 mV versus RHE and a Tafel slope of
44 mV decade 1 with excellent durability (Fig. 13b and c), which
well exceeds the performance of bare 2H-MoS2 and 1T-MoS2.
Fig. 13 (a) Schematic illustration for the synthesis of a Pd NWs@2H-MoS2
hybrid structure. (b) Polarization curves and (c) the corresponding Tafel
plots for the Pd NWs@2H-MoS2 hybrid structure in reference to bare
2H-MoS2, Pd NWs and Pt/C. (d) Raman spectra and (d) S L2,3-edge NEXAFS
spectra of the Pd NWs@2H-MoS2 hybrid structure and bare 2H-MoS2.
(f) Schematic illustrating the working mechanisms involved in the Pd
NWs@2H-MoS2 HER electrocatalyst. Reproduced with permission from
ref. 97. Copyright 2016 Springer.
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Fig. 14 Conversion of ethylene hydrogenation catalyzed by Ni supported
on TiO2, Ta2O3, CaTiO3, BaTiO3 and Al2O3, respectively. Reproduced with
permission from ref. 103. Copyright 2000 American Chemical Society.
In addition to the improved charge transport by Pd nanowires, the interfacial charge polarization between 2H-MoS2 and
Pd is believed to play a key role in the performance enhancement.
The difference in their work functions (5.12 eV for Pd vs. 6.53 eV
for MoS2) induces the accumulation of negative charges on MoS2
through charge polarization at the Pd–MoS2 interface. XPS
characterization indicates that the increased negative charges
are donated to the S sites of 2H-MoS2. This argument has been
further supported by Raman spectroscopy and NEXAFS spectroscopy. The increased electron density stiffens the out-of-plane
vibration of the S atom, which has been resolved by the blueshifted Raman peak (Fig. 13d). Moreover, the partial occupancy
of upper energy levels in S by the donated electrons reduces the
probability of the transitions of S 2p electrons to the S s-like
state (peak A) and to the empty S 3d state (peak B), which is
reflected in the reduced NEXAFS peak intensities (Fig. 13e). As
such, the polarized electrons on the S sites can promote the
Heyrovsky step – a key step in the HER – enhancing the HER
performance. Overall, interfacial electronic effects can serve as a
versatile tool for tailoring the electron densities of active sites and
maneuvering the catalytic activities in various reaction systems.
4. Summary and outlook
The regulation of surface charge states for catalytic nanostructures
exerts an important and far-reaching influence on various catalytic
reaction systems. In this article, we summarize the efforts made
by our research group and other researchers to tailor the
compositions and structures of catalysts through lattice and
interface engineering. Interfacial electronic effects have been
proven as a powerful tool for tailoring the surface charge states
of catalytic materials. Beyond the cases that have been discussed
in this article, there would be a number of directions worth
exploring in this research field.
(1) We mainly focus on the interfaces between metals and/or
semiconductors in this review; however, the interfacial electronic
effects can generally work for various material types. For instance,
the integration of metal catalysts with a ferroelectric substrate
Review
also provides an opportunity for tuning catalytic performance
with electronic effects.103 The catalytic activity of Ni in ethylene
hydrogenation can be maneuvered by altering the substrates
(Fig. 14).44
When supported on g-Al2O3, the conversion yield is raised
along with the increase of temperature. Nevertheless, as the Ni
supported on ferroelectric materials catalyzes the hydrogenation
of ethylene, the reaction exhibits parabola-like rules around the
substrate’s Curie temperature (e.g., the Curie temperature of
BaTiO3 is 120–130 1C). It is well known that the chemical
structures and charge densities of ferroelectric materials will
dramatically change at the Curie temperature owing to internal
electronic polarization. As a consequence, the polarization of
ferroelectric materials promotes electronic perturbation especially
at the Curie temperature. This promotion in turn changes the
chemical structure and charge density of the metal catalyst, and
hence influences the catalytic activity. This case highlights that
future efforts should be made for substrate engineering toward
catalytic performance tuning. As a matter of fact, substrate
engineering has been widely investigated in organic synthesis;
however, more attention should be paid to electronic effects in
future studies.
(2) The utilization of interfacial electronic effects with
organic ligands is another direction in this field, which has
been briefly discussed in Section 2.3. Organic ligands have been
widely used in homogeneous catalysis, so many concepts of
traditional organic chemistry can be borrowed to maneuver the
surface charge states of heterogeneous catalysts. Furthermore,
this strategy should be able to extend to other soft matters such
as polymers with coordination atoms. Our recent work indicates
that polymeric materials can play a similar role to organic ligands
in tuning electronic structures.104
(3) In addition to the material design alone, external field is
another possible approach to tune catalytic performance
through electronic effects. In solid-state electrochemistry, the
activity and selectivity of metal catalysts can be modified by the
non-Faradaic electrochemical modification of catalytic activity
(NEMCA) effect.7 However, only when the interface of the metal
and solid–electrolyte is fully polarized can the NEMCA effect
happen. Fig. 15a shows the principle of NEMCA in a solid–
electrolyte cell composed of gaseous reactants, metal catalyst
|ZrO2–Y2O3|M and O2. The concentration of O2 at the polarized
metal–electrolyte interface can be changed by applying an
external voltage, which induces a variation in work function
and thus adjusts the catalytic reaction rate. As the polarizationinduced NEMCA effect is used in Pt-catalyzed ethylene oxidation
reaction, the catalytic reaction rate and activation energy exhibit
exponential and linear relationships with the work function,
respectively (Fig. 15b). Consequently, the polarization can serve
as an effective electron regulatory tool for varying the ion
concentration and work function, further optimizing the catalytic
properties. Thus the use of an external magnetic or electric field
would become a promising strategy for tuning catalytic performance
complementarily to material design.
(4) As discussed in Sections 2.1 and 2.2, light illumination
on reaction systems would induce dynamic interfacial electronic
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Fig. 15 (a) Principle of NEMCA. The right (left) side of panel a shows the supply (removal) of oxide anions (O2 ) to (from) the polarized solid-electrolyte/
catalyst (W) interface. The electrochemically induced spillover of oxygen anions O produced (consumed) at the solid-electrolyte/catalyst/gas threephase boundaries causes an increase (decrease) in the catalyst work function (eDF). (b) Impact of catalyst work function on the activation energy (E) and
catalytic rate enhancement ratio (r/ro) for C2H4 oxidation on Pt. Reproduced with permission from ref. 7. Copyright 1990 Nature Publishing Group.
effects, which opens up the possibility of harvesting solar energy to
drive catalytic reactions. The dynamic process is rather complex but
offers a broader range for modulating surface charge states as
compared with static electronic effects. This research would be
performed at the intersection with photocatalysis and plasmonics.12
In this case, charge separation is a key process for the enhancement
of catalytic performance.12
Plenty of opportunities are usually accompanied with grand
challenges. In spite of the considerable progress, research into
novel hybrid catalytic systems designed to regulate surface
charge states still has a long way to go. The challenges mainly
come from two aspects. (1) This research calls for future efforts
on lattice and interface engineering at atomic precision for
catalyst design, in efforts to rationally control surface charge
states. Large-scale production techniques for catalysts have to
be developed before moving forward to the next application
stage. (2) The fundamental understanding in this research field
has yet to be deepened, and more information should be
collected to bridge the catalyst structures, surface charge states
and catalytic performance. In this case, we have to develop
characterization techniques for surface and interface structures at
atomic resolution, surface charge states at high spatial resolution,
dynamic charge transfer processes at high temporal resolution,
and in situ chemical reactions. Synchrotron radiation-based
spectroscopy and ultrafast spectroscopy are the currently used
techniques with relatively high spatial and temporal resolutions.
In particular, it is imperative to develop operando surface
characterization techniques for directly probing the surface
charge states. For instance, Salmeron et al. has employed ambientpressure XPS (APXPS) to measure the charge state of Au nanoparticles supported on TiO2 under O2 pressure, which provides
valuable information for decoding the mechanism for high
catalytic activities in various reactions such as CO oxidation.105
Overall, it is anticipated that further research on the catalyst
design from the angle of surface charge states will definitely
Mater. Chem. Front.
strengthen our capability of controlling catalytic activity and
selectivity. Moreover, the modification on surface charge states
will provide a new train of thought for industrial production of
advanced materials to realize the efficient utilization of energy
and the maximization of atom economy. Apart from the surface
charge states, other factors such as reaction dynamics may make
contributions to the enhanced catalytic performance. Taken
together, the factors tightly controlled in a well-designed system
would boost the overall catalytic performance.
Acknowledgements
This work was financially supported in part by NSFC (no.
21471141, U1532135, 21601173, 21573212), CAS Key Research
Program of Frontier Sciences (QYZDB-SSW-SLH018), Recruitment
Program of Global Experts, CAS Hundred Talent Program, and
Anhui Provincial Natural Science Foundation (1608085QB24,
1508085MB24).
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