Towards highly efficient photocatalysts using semiconductor
nanoarchitectures1-3
In stage I, only photons with energy above the bandgap of the photocatalyst can
be absorbed; thus, a match between the bandgap and solar spectrum must be
considered to maximize the total light absorption. Beyond the width of the bandgap,
the levels of the conduction and valence bands should also be considered for overall
solar water splitting reaction. Overall, the bandgap of the semiconductor
photocatalyst needs to be engineered to satisfy both a high absorption of solar energy
as well as the sufficient built-in potential for a direct water splitting reaction. There
is generally a tradeoff between these two requirements, as a smaller bandgap is
usually more desired for efficient solar absorption and a larger bandgap is more
desired for sufficient built-in potential for the water splitting reaction.
Stages II and III take place simultaneously. Charge recombination and
separation/migration processes are two key competing processes that largely
determine the internal quantum efficiency of a photocatalyst. For higher
photocatalytic efficiency, the electron–hole pairs should be efficiently separated and
rapidly transported to the active redox sites for the desired chemistry. The
recombination is often facilitated by crystalline defects. Therefore, a better
crystallinity with few defects can usually minimize the trapping states and
recombination sites, resulting in an increased efficiency in the usage of the
photo-generated carriers for desired photoreactions. From these perspectives,
photocatalysts with smaller sizes (or in porous and hollow structures) are also often
desired, since the distance between the photoinduced charges and the reaction sites
becomes shorter, which can consequently reduce the probability for recombination.
Stage IV, the redox chemistry, is greatly affected by the surface properties, such
as active sites and surface area. Even if the photo-generated electrons and holes
possess thermodynamically sufficient potential for water splitting, they may not be
effectively used for proton reduction or water oxidation reactions if there are no
proper catalysts to reduce the activation energy (over-potential) to facilitate hydrogen
and oxygen evolution. Inorganic or biomimetic cocatalysts, such as Pt, Au, and many
enzymes are usually used to introduce active sites and reduce the redoxoverpotential
for H2 and O2 evolution. Lastly, the free electrons and holes are highly reductive and
oxidative. If these free carriers cannot be effectively consumed for desired
photochemistry, they can often reduce or oxidize the semiconductor photocatalyst
itself, leading to a complete deactivation and disintegration of the photocatalyst.
1.
Efficient solar energy absorption
(the bandgap engineering of the photocatalysts to satisfy the requirements for both
efficient solar energy absorption and sufficient built-in potential for redox reactions)
Although TiO2 based photocatalysts can function as effective photocatalysts, they
cannot be used for effective solar energy harvesting and conversions. Owing to its
large bandgap ( Eg>3eV), TiO2 can only absorb photons with light wavelengths
shorter than about 400 nm in the UV or near-UV wavelength regime, which accounts
for less than 5% of the total solar energy irradiation through the atmosphere to the sea
level. Significant challenges remain in developing a visible light driven photocatalyst
with high efficiency and chemical stability.
To this end, considerable effort has been placed on improving the solar
absorption of the photocatalysts. Overall, three typical approaches have been explored
for the enhanced light absorption in the visible light region, which include: (1)
chemical doping of TiO2, such as incorporation of additional metal or nonmetal
species ( e.g. , nickel, vanadium, chromium, platinum, nitrogen, tungsten, fluorine,
sulfur, etc. ) (2) the exploration of alternative narrow bandgap semiconductor
photocatalysts, such as Si, InP, CdSe, and GaZnON; and (3) the loading of the visible
light sensitizer, such as organic dye, metal nanoparticles, or a narrow bandgap
semiconductor.
Recently, Chen et al. developed an interesting approach to significantly enhance
the visible light absorption (up to 1200 nm) by intentionally creating a disordered
shell of several atomic layers of nanophase TiO2 using a hydrogenation method to
create a so-called ‘‘black TiO2’’. The disorder-engi-neered black TiO2 exhibited a
higher efficiency of the visible-light-driven photocatalysis compared to traditional
white TiO2. Soon after, Y. Li and co-workers obtained a solar-to-hydrogen efficiency
of 1.63% by using hydrogen treated TiO2 nanowires for photoelectrochemical water
splitting, which is the best value for a TiO2 photoanode.
A great number of new photocatalytic materials have been proposed as potential
substitutes of TiO2; many of them follow two possible ways. One is searching for
native semiconductors with a bandgap covering the visible light region; a -Fe2O3
(hematite), CdS, BiVO4, and Ag3PO4 are well known native visible-light-driven
photocatalysts. The other is the solid solutions of photocatalysts with the same
crystal structure. Since2005, Domen’s group has systematically studied the GaN–
ZnO solid solutions for photocatalytic purposes, which have been demonstrated split
water into hydrogen and oxygen under visible light irradiation.
Narrower bandgap semiconductors, dye molecules, and/or metal
nanoparticles are widely used as the sensitizers on the surface of photocatalysts to
capture additional visible or infrared light and consequently enhance the
photocatalytic efficiency. Only the narrow bandgap sensitizer can be excited to
generate electrons in their conduction band under the visible light irradiation. For an
efficient electron transfer between the sensitizer and photocatalyst, the energy level of
the conduction band of the photocatalyst must be lower than that of the sensitizers.
Thus, the electrons created in sensitizers are subsequently injected into the
photocatalyst conduction band to perform a reduction reaction. If the energy level of
the valence band of the sensitizer is higher than that of the photocatalyst, the
photoinduced holes are localized inside the sensitizer. In this heterostructure,
several advantages can be obtained: (1) an enhanced visible light absorption; (2)
a more effective charge separation; (3) a rapid charge transfer to catalyst; and (4)
a longer lifetime of the charge carriers. Several semiconductor sensitizers with a
narrow bandgap were shown in which have been proven to enhance the solar energy
harvesting efficiency in the visible light region. Organic dye molecules have also
been employed as sensitizers for semiconductor photocatalysts due to their active
light absorption in the visible wavelength regime. The dye-sensi-tizers used for solar
cell and photocatalytic reactions have attracted more and more attention because of
their stability, low cost, and high efficiency. Lastly, some metal nanoparticles, with
surface plasmon induced absorption in the visible light region, could also be
loaded on the surface of wide bandgap photocatalysts ( e.g. ,TiO2) to enhance their
light harvesting efficiency. The surface plasmon resonance is created by the coherent
oscillation of the metal conduction band electrons upon excitation by irradiating light.
A dipolar oscillation of all the electrons induces strong absorption of the
electromagnetic energy when its frequency becomes in resonance with the electron
oscillation. The frequency of the surface plasmon absorption highly depends on the
size and morphology of the metal nanoparticles, as well as their dielectric
environment. For most metals such as In, Sn, Pb, and Cd, the plasmon frequency lies
in the UV region with a broad and poor absorption band, while noble metals such as
Cu, Ag, and Au exhibit a strong visible-light plasmon resonance. The tunable
plasmonic absorption bands of various Au and Ag nanostructures throughout the
visible range make them ideal candidates for enhancing photocatalytic absorption and
efficiency.
2. Separation and transportation of the photogenerated charge carriers
(heterostructured junctions with internal built-in electrical potential to effectively
direct electron–hole separation and transportation)
The formation of semiconductor heterostructure (formed by the direct contact
of semiconductors A and B) is an effective approach to promote charge separation for
improved photo-catalytic activity. Considering a type-II band alignment, the energy
gradient existing at the interfaces tends to spatially separate electrons and holes on
different sides of the hetero-junction, where electrons may be confined to one side and
holes to the other. The spatially localized charges in the type-II nanostructures should
make these materials more suitable for photocatalytic applications.
The semiconductor p–n junction is another effective architec-ture for the
highly efficient charge separation and transportation. In general, when the p- and
n-type semiconductor materials are in contact, they form a p–n junction with a
space-charge region at the interfaces due to the diffusion of electrons and holes, and
thus create a built-in electrical potential that can direct the electrons and holes to
travel in the opposite direction (Fig. 11). When the p–n heterojunction is irradiated
with photon energy higher or equal to the bandgaps of the photocatalysts, the
photogenerated electron–hole pairs can be quickly separated by the built-in electric
field within the space charge region. Driven by the electric field, the electrons are
transferred to the conduction band of the n-type semiconductors and the holes to the
valence band of the p-type semiconductors.
Metal–semiconductor contacts with a potential offset have also been explored to
facilitate the separation and transportation of the photogenerated electron–hole pairs.
In a way much similar to the semiconductor junctions, the potential offset at a metal–
semiconductor contact can create a space charge region and a built-in potential to
drive the separation and transportation of photogenerated electron–hole pairs. The
eventual impact of metal–semiconductor contacts is however much more complicated
due to other possible contributions of metal clusters such as co-catalyst effect,
plasmonic resonance effect and charge trapping effect.
3. Redox cocatalysts for efficient charge utilization
(the integration with a proper cocatalyst to reduce the redox overpotential)
The redox cocatalyst is an important factor determining the eventual efficiency
of the photocatalytic process because it dictates the efficiency in the utilization of
photogenerated charges for actual redox reactions. Due to the limitation of the
activation energy (overpotential), the photo-generated and separated electrons and
holes may not be effectively utilized for the desired reduction or oxidation reaction
without proper cocatalysts to decrease the overpotential. Inorganic or biomimetic
cocatalysts, such as Pt, Au, and many enzymes are widely used to reduce the redox
overpotential. In general, the cocatalysts can be broadly classified into two groups:
reduction catalysts and oxidation catalysts, and more specifically for the water
splitting reaction: hydrogen evolution catalysts and oxygen evolution catalysts.
Although it is obvious that the effective integration of theredox cocatalysts can
play a rather important role in the efficiency and stability of photocatalytic systems,
little systematic effort has been made for this problem to date, largely due to the
difficulties in selectively integrating the reduction and oxidation catalysts with
the designed architecture to facilitate the desired charge transfer process. An
enhanced photocatalytic activity is expected by loading both reduction and oxidation
cocatalysts, compared to photocatalysts modified with only a single cocatalyst. Aided
by the cocatalysts, the charge carriers are eventually consumed on the interfaces of
cocatalysts/solution. The spatially separated redox active sites can effectively avoid
the back reaction, which is an essential issue for photocatalytic water splitting.
4. Reliability of the photocatalysts
(the electrochemical stability of the photocatalyst.)
Chemical stability is the most critical issue of semiconductor photocatalysts
dispersed in an aqueous solution under light irradiation. Photocorrosion, degradation
of the photocatalyst under light irradiation, is considered the main reason for the
instability of photocatalysts. Some of the oxide and most of the typical
semiconductors (e.g. ZnO, ZnS, CdS,etc. ) are susceptible to corrosion under these
conditions and therefore can hardly be explored as candidates for effective
photocatalysts. At a critical pH, thehydronium ion content will be enough to protonate
the catalyst itself, resulting in corrosion. For instance, ZnO under UV light irradiation
is known to dissolve readily. This susceptibility significantly decreased the
photocatalytic activity of ZnO in aqueous solutions and prevented the further
application of ZnO as an effective photocatalyst.
Tremendous efforts have been made to improve the photo-stability using various
methods. For example, the stability of some photocatalysts was found to increase in
the presence of certain inorganic sacrificial reagents, such as the S2-/SO32- system,
to avoid the etching process on the photocatalysts themselves. In addition, core–shell
structures with an inert oxide or polymer shell are developed to suppress the
photocorrosion .
5. Towards the rational design of freestanding photoelectrochemical
nanodevices as highly efficient photocatalysts
Considering the multiple-step nature of the photocatalytic process, it is
important to consider the entire process in order to develop a highly efficient and
stable photocatalyst. To this end, we have recently reported a rational strategy to
design a new generation of freestanding photoelectrochemical nanodevices as
highly efficient and stable photocatalysts. With the seamless integration of multiple
functional components in a single nanostructure and precise control of the
materials interfaces, this design of photocatalyst offers several key advantages
over conventional ones: (1) the internal built-in potential can facilitate efficient
electron–hole separation and transportation; (2) the one-dimensional morphology and
the insulating shell ensure that the electrons and holes are only directed towards the
redox nanocatalysts for the desired redox reactions, and therefore minimize
non-productive charge consumption by recombination or undesired side reactions on
semiconductor surface; (3) multiple photodiodes can be integrated in series to step-up
the electron energy and therefore simultaneously ensure sufficient electrochemical
potential for direct water splitting and good absorption overlap with the solar
spectrum (Fig. 14b), much like the natural photosynthesis that always uses two
photons to drive each electron reaction; and (4)the encapsulation of the semiconductor
portion in a protective insulating shell prevents direct electrochemical reactions on the
semiconductor surface to ensure the electrochemical stability, and therefore allows
flexible selection of semiconductor materials that better match the solar spectrum for
efficient solar energy harvesting and conversion.
Together, these initial studies for the first time demonstrated the integration of
multiple distinct functional components into a single nanostructure to form a
standalone photoelectrochemical nanodevice, to enable a photocatalyst that is both
efficient and stable throughout theentire solar spectrum. It can thus open a rational
avenue to the design and synthesis of a new generation of photoelectrochemical
nanosystems with unprecedented efficiency and stability, and will have a broad
impact in areas including environmental remediation, artificial photosynthesis and
solar fuel production. However, it should be noted that the built in potential (0.3 eV)
of this initial system is not enough for direct water splitting or CO2 reduction
( >1.23–1.34 eV); alternative larger bandgap materials or multijunction must be
explored to address this critical challenge. Additionally, it would also be important to
explore various redox catalysts to facilitate the electron transfer process, reduce
overpotential and increase the efficiency of the photocatalytic redox reactions.
6. Conclusion
In general, photocatalysis involves a complicated sequence of multiple
synergistic or competing steps, including light harvesting, charge generation,
separation, transportation, recombination and utilization. Although significant effort
has been devoted to optimize each one of these processes, which has led to the
development of a wide range of materials and architectures for effective
photocatalytic systems, few studies have treated these problems coherently. Very
often, the optimization of one parameter could lead to the degradation of the other. It
remains a significant challenge to distinguish which factor(s) dominates the
overall photocatalytic activity, and it is important to address these problems
together. New insights are required to design the light absorption, charge separation
and transfer processes and the integration with redox cocatalysts. Rational design of
the photocatalyst with the consideration of the whole process could facilitate the
creation of highly effective photocatalysts. Semiconductor nanoarchitectures that
integrate multiple functional components in the nanoscale could allow efficient
charge generation, separation, transportation and utilization to enable a new
generation of highly efficient photocatalysts. In a specific example, we have
discussed a freestanding photoelectrochemical nanodevice that consisted of a p–n
junction nanowire with a protecting shell and two exposed redox cocatalysts, which
represents an interesting design for a highly efficient and stable photocatalyst.
附录：通俗来说，欧姆接触和肖特基接触都属于金属和半导体接触的情况，
理想的欧姆接触就像是导线连通，无论正反向测试，电阻都是零，而肖特基接触
在接正向电压时，电阻为零，接反向电压时，电阻无穷大，即：具有单向导电性。
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Heterointegration of Pt/Si/Ag Nanowire Photodiodes and Their Photocatalytic
Properties. Adv Funct Mater 2010, 20, 3005-3011.
3. Qu, Y.; Liao, L.; Cheng, R.; Wang, Y.; Lin, Y.-C.; Huang, Y.; Duan, X., Rational
Design and Synthesis of Freestanding Photoelectric Nanodevices as Highly Efficient
Photocatalysts. Nano letters 2010, 10, 1941-1949.