富山大学研究推進機構水素同位体科学研究センター研究報告 36：1－19，2016.

総説

不均一系光触媒反応における光生成電荷の分離機構
萩原
富山大学

英久

研究推進機構 水素同位体科学研究センター〒930-8555 富山市五福 3190

Separation Mechanism for Photogenerated Charge Carriersin Heterogeneous Photocatalysts
Hidehisa Hagiwara
Hydrogen Isotope Research Center, Organization for Promotion of Research,
University of Toyama, Gofuku 3190, Toyama, 930-8555, Japan(Received June 30, 2017; accepted July 31, 2017)
Abstract
In heterogeneous photocatalysts, it is very important to move photoexcited charges in the semiconductor to
the reaction site on the surface without recombination. In this review, we describe the process of thephotocatalytic reaction from photoexcitation of semiconductor photocatalyst to surface redox reaction, andsummarized various researches on the charge separation of photogenerated electrons and holes insemiconductors reported up to the present.

1. Introduction

Heterogeneous photocatalysts have been widely used for various applications such as
antibacterial coating,1,2 deodorizing,3 antifogging treatment,4,5 and water purification,6,7 andbecome more important in the future. Furthermore, a photocatalytic solar energy conversion,

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Fig. 1 Elementary reactions in TiO2 photocatalysis with corresponding timescales.

so-called artificial photosynthesis, has actively studied to solve global energy problems.8,9 To
improve the activity and the functionality of photocatalysts, it is necessary to understand a seriesof photocatalytic processes in detail. Figure 1 shows elementary reactions of TiO2 photocatalyst
with corresponding timescales. Under light irradiation with a higher energy than the bandgap
energy of the semiconductor photocatalyst, electrons in the valence band (VB) make transition
to the conduction band (CB) with leaving holes in the valence band. While the photogenerated
electrons transfer to the bottom of the CB, the holes transfer to the top of the VB, via vibrational
relaxation process. These charge generation and relaxation processes occur in the order of2不均一系光触媒反応における光生成電荷の分離機構

femtoseconds. The photogenerated electrons and holes diffuse in the CB and the VB,
respectively, and part of them are trapped at the energy level formed by atomic vacancy,
impurity, or functional groups on the photocatalyst surface. In the charge recombination process,the photogenerated charge carriers annihilate each other releasing the energy as a photon orphonons. These trapping and recombination processes take place in the order of nanoseconds
or microseconds. On the other hand, the photogenerated electrons and holes without
recombination react with adsorbed chemical species, or transfer to metal or metal oxides on thephotocatalyst surface. The redox reactions on the photocatalyst surface occur in the order of the
microseconds or milliseconds, thus it is considered that the photocatalytic activity is improvedby suppressing the charge recombination at trap site or photocatalyst surface. Therefore, thisreview covered up the several approaches of the photogenerated carrier separation inheterogeneous photocatalysts, for development of the highly active photocatalysts.

2. Charge separation in semiconductor photocatalysts
Photogenerated electrons and holes in optical semiconductors diffuse in conduction andvalence bands along electric fields. In silicon type solar cells, the charge separation takes placeat the interface between p-type semiconductors (e.g. boron-doped silicon) and n-type

Fig. 2 Energy band diagrams of (a) p-n junction and (b) semiconductor-electrolyte interface.

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semiconductors (e.g. phosphorus-doped silicon), so-called p-n junction, because of a potential
gradient in a space charge region (Fig. 2a). The formation of space charge region insemiconductor photoelectrodes of photoelectrochemical cells are explained by a band-bending
model, which is based on an interdiffusion of electrons and holes between semiconductors andredox species in electrolyte (Fig. 2b). In the case of semiconductor photocatalyst powders, thespace charge region forms in a similar manner to that of the photoelectrodes. The length of thespace charge region depends on the carrier density of the semiconductor photocatalysts, and it
is estimated within 20-100 nm.4,10 Therefore, the reduced effect of the band bending is one of
the reasons for increasing charge recombination in photocatalyst particles with size within 100
nm. Although the space charge properties are determined by surface/interface electronic states,
the photogenerated charges can also be controlled by electric fields induced dielectric structures
or difference in the surface electronic band structures of crystal facets, as described below.

2.1. Ferroelectric materials
Barium titanate (BaTiO3), which is one of the most popular ferroelectric materials, has been
investigated to reveal the charge separation effect of internal electric fields.11-17 BaTiO3 shows
the spontaneous polarization between 278 K and 393 K, because Ti and Ba atoms shifted to<100> direction and O atoms shifted negatively fromcubic perovskite structure, as shown in Fig.3.18 To
confirm the effect of the spontaneous polarization on the
charge separation and the photocatalytic reactivity, Ag+,
Pb2+, and Mn2+ ions were employed as reaction marker.
These ions were deposited on the photocatalyst surface
as Ag, PbO2, and MnO2 by photocatalytic reduction oroxidation reactions, and the migration behavior of

4

Fig. 3 Crystal structure of BaTiO3.

不均一系光触媒反応における光生成電荷の分離機構

Fig. 4 SEM images of faceted BaTiO3 crystals after reaction in AgNO3 aqueous
solution Reproduced with permission from Ref. 17, copyright Springer
Science+Business Media, LLC 2008.

photogenerated carriers in semiconductor crystals was determined by surface observation.
Figure 4 shows SEM images of BaTiO3 crystals prepared after the photoirradiation in AgNO3
aqueous solution.17 In this study, BaTiO3 crystals were prepared by KCl flux method, and thedeposition of Ag particles was observed on (001) plane of BaTiO3 crystals. This result suggeststhat Ag is reduced by photogenerated electrons migrated along the direction of ferroelectric
polarization. In addition to the research on BaTiO3, various ferroelectric materials, such as
Pb(Zr, Ti)O3,19-26 BiFeO3,27 LiNbO3,28 and Sr2Nb2O7,17 have been reported as active
photocatalysts for water splitting, hydrogen production, and water purification.
The photocatalytic properties of pyroelectric materials have been also studied forunderstanding the efficient charge separation. Huang et al. reported that bismuth-based
compounds, BiOIO3,29 Bi2O2[BO2(OH)],30 and Bi4V2O1131 showed high photocatalytic activity
for organic compound oxidation or oxygen formation from AgNO3 aqueous solution. As shownin Fig. 5, BiOIO3 is composed of central Bi2O4 layer shares O corners with IO3 units, thus the
direction of the pyroelectric polarization is along c-axis. The DFT calculations of BiOIO3
revealed that Bi 6p and I 5p contributes to the CB while O 2p strongly contributes to the VB.
Therefore, the photoexcited electrons in the CV migrate to Bi2O4 layer along c-axis, and the

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Fig. 5 A perspective view of the BiOIO3 slab. Reproduced with permission from
Ref. 29, copyright (2013) Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

photogenerated holes located at the O 2p. This charge separation process is important for thephotocatalytic property of bismuth-based photocatalysts. Related study of the pyroelectric
polarization effect on the photogenerated charge separation in bismuth-based oxyhalides, suchas carbon-doped Bi3O4Cl,32 Bi12O17Cl2,33 and BiOX (X=Cl, Br, I),34-36 have been also reported
from different research groups. Furthermore, Huang et al., also reported silver silicates,
Ag6Si2O7,37 Ag9(SiO4)2NO3,38 Ag10Si4O13,39,40 as the pyroelectric photocatalysts. As describedabove, the internal polar electric fields in semiconductor photocatalyst enhance the charge
separation of the photogenerated carriers and improve the photocatalytic activity.

2.2. Crystal facets
The surface electronic band structures depend on surface facets because of the surface
atomic arrangements and adsorption species. Redox abilities of photogenerated charge carriers
tuned by the surface electronic band structures of different facets, and accumulations ofphotogenerated electrons or holes on different facets.41 Ye et al. reported the photoinduced6不均一系光触媒反応における光生成電荷の分離機構

charge transfer properties of anatase TiO2 crystal
between (101) facets and (001) facets.42 They
prepared TiO2 crystals by using a hydrothermal
synthesis

methodhexafluorotitanate

withammonium((NH4)2TiF6),

tetrabutyl

titanate, and hydrochloric acid (HCl), andcontrolled the ratio of different facets bychanging the hydrothermal temperature and thevolume ratio of HCl and water. As shown in Fig.
6, the photogenerated electrons and holes
efficiently separated to (101) facet and (001)
facet, and thus reduction and oxidation reactions
took place on each facet. The spatial charge
separation between different facets of TiO2
crystal, containing other crystal structure,brookite43 and rutile,44 has been reported by

Fig. 6 Electronic band structures and charge
distribution of (101)-(001) facets. Reproducedwith permission from Ref. 42, copyright(2013) Elsevier.

several research groups.45-51 To date, the effectof spatial charge separation between different facets on the photocatalytic reactivity have beenstudied by using various photocatalytic semiconductors, such as metal oxides (CeO252 andCu2O53-55), complex oxides (BiVO4,41,56-58 SrTiO3,59,60 La2Ti2O7,61 and BaLa4Ti4O1562,63), metal
oxyhalides,64-66 and metal sulfides.67,68

3. Charge trapping
Diffusion and trapping of the photogenerated carriers after the charge separation is also
important factor for the activity of photocatalyst materials. In addition to the conduction and

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valence band levels, there are several
energy levels in semiconductors, as
shown in Fig. 7. The depth of the charge
trap levels depends on how the defects
stabilize the trapped charges throughpolaron formation.69-71 To understandthe relationship between the trap sites
and the photocatalytic activity, timeresolved photoabsorption spectroscopy
is generally employed for the lifetime

Fig. 7 Schematic image of electronic band
structure of a semiconductor with various
energy levels.

measurement of the photogenerated electrons and holes.72-75 Yamakata et al. reported that thephotocatalytic activity of SrTiO3 was improved by SrCl2 flux treatment because of the
prolongation of photogenerated charge lifetime with decreasing the number of deep trap sites.76
They also investigated the relationship between the surface defects and the photocatalytic
activity by using single- and poly- crystals of SrTiO3.77 Although the free and shallow trappedelectrons were dominant in single crystal SrTiO3, they recombined within 50 ns. On the other
hand, defect-rich polycrystalline SrTiO3 showed the long-time charge separation longer than 1
ms and the higher photocatalytic activity for hydrogen production from methanol aqueous
solution. This positive effect of surface defects on the photocatalytic activity have been reported
by using other semiconductor materials. Hoch et al. reported that the activity of photocatalytic
CO2 reduction on In2O3-x(OH)y strongly enhanced by introducing surface defects as oxygen
vacancies and hydroxyl groups.78 Furthermore, the positive effect of surface vacancy has beenreported on the photodecomposition activity of BiOCl for organic compounds.79 Whereas thedeep trap sites in semiconductors work as recombination center of photogenerated charges, thesurface defects prolonged charge separation lifetime and enhanced the photocatalytic activity.

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These results are important for the design of highly active photocatalysts.
4. Interfacial charge transfer
In multi-component photocatalysts, junctions are formed through the combination of asemiconductor with other materials to enhance charge separation. In this section, the topics ofsemiconductor-semiconductor, semiconductor-metal complex, and semiconductor-metal (ormetal oxide) co-catalyst photocatalysts are introduced.
4.1. Semiconductor-semiconductor heterojunction
Until now, various photocatalysts with semiconductor junction have been developed, andmainly used TiO2-based composites (e.g. Fe2O3,80-85 Cu2O,86-89 ZnO,90,91 WO3,92,93 Ag3PO4,94,95
In2O3,96-99 CeO2.100-102 Furthermore, semiconductor composites with different crystal phasewere investigated, especially anatase-rutile TiO2 composites.103-105 Wang et al. reported thecomposite effect of -Ga2O3 and -Ga2O3 on the photocatalytic water splitting activity.106 The

Fig. 8 (a) HR-TEM image of Ga2O3 calcined at 863 K. (b) Illustration of charge transfer
cross the a–b phase junction. Reproduced with permission from Ref. 106, copyright(2012) Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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composite of -Ga2O3 and -Ga2O3 was prepared by adjusting calcination temperature, andeffective charge separation was achieved at well-developed phase junction (Fig. 8). A similareffect was observed in the composite of α-Bi2O3 and β-Bi2O3.107 In this system, thephotogenerated electrons moved to the CB of β-Bi2O3 and holes moved to the VB of α-Bi2O3.
The photocatalytic activity for organic dye decomposition was improved by the formation ofheterojunction. In recent years, composite photocatalysts of inorganic semiconductors andmetal sulfides,108-113 metal halides114-116 and organic semiconductors117,118 have also beenreported in addition to metal oxides. The formation of heterojunction at semiconductor interface
is one of the effective way to separate photogenerated charges in photocatalysts.

4.2. Semiconductor-metal complex heterojunction
The effects of surface modification withmetal complexes for semiconductor materials
were investigated in the research area ofphotoelectrochemicalcells,

anddye-sensitized solar cells are one of the typicalresearch. In the dye-sensitized system, photoexcited electrons in dyes inject to the CB of
metal oxide semiconductors along thepotential gradient formed by band-bending,
and

the

photogenerated

chargesFig. 9 Schematic image of charge transfer
mechanism of dye-sensitized photocatalysts.

areeffectively separated spatially between metal oxides and dye molecules (Fig. 9). Most of dyesensitized photocatalyst systems have been fabricated using TiO2 and metal complexes, such as
ruthenium complexes119-121 and porphyrins.122-125 Hirano et al. studied the ligand effects of Rucomplexes on the photocatalytic activity of dye-sensitized Pt/TiO2 for hydrogen production10不均一系光触媒反応における光生成電荷の分離機構

from EDTA aqueous solution.120 They used Ru(bpy)32+, Ru (dcbpy)34-, and Ru(bpym)32+ as
sensitization dye, and Ru(bpym)32+-Pt/TiO2 showed the highest photocatalytic activity.
Ru(bpym)32+ contains more nitrogen in the molecular structure, which showed higher affinity
with the TiO2 surface than other Ru-complexes, resulted in enhanced electron transfer to TiO2.
The same tendency was reported in various Ru-complexes,126-128 thus the strong affinity
between metal oxides and dyes are important for the charge separation and the photocatalytic
activity of the dye-sensitized photocatalysts.
On the other hand, the electron injection from the CB of semiconductor to metal complex
molecules also reported for the dye-semiconductor composite photocatalysts. We developed
chromium tetraphenylporphyrin chloride/Zr-doped KTaO3 (Cr-TPPCl/KTa(Zr)O3) composite
photocatalysts for water splitting reaction.129-131 In this system, the water splitting takes placewith two-step excitation process in both semiconductors and dyes, as shown in Fig. 10. Thischarge transfer mechanism is similar to that of light-dependent reaction of photosynthesis, socalled Z-scheme. The half-life of the charge separation lifetime of KTa(Zr)O3 was prolonged

Fig. 10 Schematic image of charge transfer mechanism of Cr-TPPCl/KTa(Zr)O3 composite photocatalyst.

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from 11.5 s to 100 s by Cr-TPPCl modification, and thus the photocatalytic activity was
improved.129 Sekizawa et al. also reported Ag/TaON-Ru binuclear complex as Z-scheme typephotocatalyst for photocatalytic CO2 fixation.132 Although the Z-scheme photocatalysts need
two photons for the photocatalytic reaction, efficient charge separation can be achieved.

4.3. Semiconductor-metal (metal oxide) co-catalysts
The co-catalysts act as acceptors of the photogenerated carriers for separating the electrons
and the holes from the CB and the VB of the semiconductor, and as more efficient active sites
for the photocatalytic reaction. The most commonly used metals for co-catalysts are platinum(Pt), rhodium (Rh), or palladium (Pd) nanoparticles due to their noble and catalytic
properties.133–135 Besides noble metal nanoparticle, metal oxide (NiO, RuO2), metal sulfides,

Fig. 11 Decay of transient absorption of CoOx-loaded LaTiO2N photocatalysts in a
vacuum. The transient absorption was measured at 2000 cm−1. The sample was excitedby 355 nm UV laser pulses (6 ns duration, 0.5 mJ cm−2, 1 Hz). The inset shows the
decay in the second region (pump pulse repetition rate at 0.01 Hz). Reproduced withpermission from Ref. 143, copyright (2014) American Chemical Society.

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and CoPi have been investigated as co-catalysts.136-142 Yamakata et al. investigated the loading
effect of Pt and CoOX co-catalysts on LaTiO2N by using time-resolved absorption
spectroscopy.143 In the case of LaTiO2N, photogenerated holes in the VB of LaTiO2 were
captured rapidly by CoOX in a few picoseconds, and the lifetime of electrons were prolonged
to the second region (Fig. 11). Loading co-catalysts enhance the charge separation ofsemiconductor photocatalysts and decrease the activation energy of surface reaction. Therefore,co-catalysts are important especially for hydrogen, oxygen, or carbon monoxide formation inwater splitting or CO2 fixation as artificial photosynthesis.

5. Summary
In this paper, various researches of the charge separation in heterogeneous photocatalysts
have been reviewed. In order to develop a highly active photocatalyst, it is necessary toefficiently separate photogenerated charges and suppress the annihilation due to the charge
recombination. The charge separation is promoted along the electric field in the crystal for
dielectrics and pyroelectric materials with polarized electric fields inside the crystal. To
suppress the charge recombination, it is also effective to expose a well-developed crystal facets.
Since the electronic states of the semiconductor surface varies depending on the atomic
arrangements and the adsorption properties of the crystal planes, the photogenerated charges
are separated between different crystal facets. Furthermore, the formation of heterojunction isan effective method for promoting charge separation. Although charge recombination at theinterface should be carefully avoided, the junction of semiconductor-semiconductor or metal
complex contributes to improved charge separation efficiency. Co-catalyst loading is a powerful
technique for improving photocatalytic activity. Co-catalysts are widely used especially in
artificial photosynthesis type photocatalysts, because it works as a catalyst for surface chemical
reaction as well as an acceptor for photogenerated electrons and holes.

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Heterogeneous photocatalysts have been widely used for various applications and become
more important in the future. Especially in the research field of light energy conversion, highly
active photocatalysts are intensively studied for solar energy conversion and storage. To solve
the global energy issue, it is expected that a new highly efficient photocatalyst will be developedby combining the charge separation technics introduced in this review.
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