224
T. Torimoto and T. Kameyama
quantum confinement in QDs have been reported. For example, the relaxation times
of hot carriers in highly excited states to band-edge states were remarkably increased
by the discrete energy levels of QDs, because of the phonon bottleneck effect [3].
This increased the probability of hot carrier transfer [4, 5]. Furthermore, multiple
exciton generation (MEG) in a single QD is a nonlinear phenomenon caused by the
absorption of one photon having an energy of more than twice the E g [6–8]. The
MEG efficiency was reported to become much larger in QDs than in bulk materials
[6]. Thus, although strategies to efficiently extract photogenerated carriers from QDs
without energy loss have not been developed yet, these nonlinear phenomena have
attracted much attention as a means for increasing the efficiency of solar light energy
conversion.
So far, binary QDs, such as PbS, PbSe, CdS, and CdSe, have been utilized for
developing novel photocatalysts and photovoltaics because synthetic methods for
obtaining high-quality QDs have been established. Although the use of these conventional binary QDs has enabled fabrication of systems having relatively high efficiencies, their use in practical applications has been severely restricted due to the large
content of highly toxic elements, such as Cd and Pb. On the other hand, group
I-III-VI-based multinary semiconductors consisting of elements with less toxicity,
such as AgInS 2 , CuInS 2 , and their solid solutions with ZnS, have been more and
more intensively investigated to develop thin-film solar cells or visible light-driven
photocatalysts. Since the colloidal syntheses were reported to produce highly photoluminescent QDs of these materials, these multinary QDs have also been receiving
much attention as alternatives to highly toxic binary ones [9–12]. In this chapter, we
introduce solution-phase synthetic methods to obtain high-quality QDs consisting
of Ag-III-VI semiconductors and their solid solutions with Zn-VI semiconductors.
The prepared QDs had optical properties that were tunable by changing the chemical
composition and morphology of particles. Furthermore, their photoelectrochemical
responses were investigated with irradiation of visible light.
13.2 Tunable Optical Properties of Solid Solution QDs
13.2.1 Photoluminescence Properties of AgInS 2 -Based QDs
High-quality multinary QDs could be colloidally synthesized by methods similar to
these used for the preparation of binary QDs, though the difference in reactivities
of two or more metal salts used as precursors often resulted in non-stoichiometry
of QDs. We prepared highly photoluminescent multinary QDs composed of a ZnSAgInS 2 solid solution, (AgIn) x Zn 2(1-x) S 2 (ZAIS), by a heating up method [13]. The
reaction of corresponding metal acetates with thiourea was carried out at 250 °C in
oleylamine (OLA) containing 1-decanethiol (DDT), in which the ratio of Ag
+ : In
3+ :
Zn
2+ was fixed to x: x: 2(1−x). A decrease of the x value in the preparation resulted in
an increase of the Zn fraction in the obtained spherical ZAIS QDs. Furthermore, the
T. Torimoto and T. Kameyama
quantum confinement in QDs have been reported. For example, the relaxation times
of hot carriers in highly excited states to band-edge states were remarkably increased
by the discrete energy levels of QDs, because of the phonon bottleneck effect [3].
This increased the probability of hot carrier transfer [4, 5]. Furthermore, multiple
exciton generation (MEG) in a single QD is a nonlinear phenomenon caused by the
absorption of one photon having an energy of more than twice the E g [6–8]. The
MEG efficiency was reported to become much larger in QDs than in bulk materials
[6]. Thus, although strategies to efficiently extract photogenerated carriers from QDs
without energy loss have not been developed yet, these nonlinear phenomena have
attracted much attention as a means for increasing the efficiency of solar light energy
conversion.
So far, binary QDs, such as PbS, PbSe, CdS, and CdSe, have been utilized for
developing novel photocatalysts and photovoltaics because synthetic methods for
obtaining high-quality QDs have been established. Although the use of these conventional binary QDs has enabled fabrication of systems having relatively high efficiencies, their use in practical applications has been severely restricted due to the large
content of highly toxic elements, such as Cd and Pb. On the other hand, group
I-III-VI-based multinary semiconductors consisting of elements with less toxicity,
such as AgInS 2 , CuInS 2 , and their solid solutions with ZnS, have been more and
more intensively investigated to develop thin-film solar cells or visible light-driven
photocatalysts. Since the colloidal syntheses were reported to produce highly photoluminescent QDs of these materials, these multinary QDs have also been receiving
much attention as alternatives to highly toxic binary ones [9–12]. In this chapter, we
introduce solution-phase synthetic methods to obtain high-quality QDs consisting
of Ag-III-VI semiconductors and their solid solutions with Zn-VI semiconductors.
The prepared QDs had optical properties that were tunable by changing the chemical
composition and morphology of particles. Furthermore, their photoelectrochemical
responses were investigated with irradiation of visible light.
13.2 Tunable Optical Properties of Solid Solution QDs
13.2.1 Photoluminescence Properties of AgInS 2 -Based QDs
High-quality multinary QDs could be colloidally synthesized by methods similar to
these used for the preparation of binary QDs, though the difference in reactivities
of two or more metal salts used as precursors often resulted in non-stoichiometry
of QDs. We prepared highly photoluminescent multinary QDs composed of a ZnSAgInS 2 solid solution, (AgIn) x Zn 2(1-x) S 2 (ZAIS), by a heating up method [13]. The
reaction of corresponding metal acetates with thiourea was carried out at 250 °C in
oleylamine (OLA) containing 1-decanethiol (DDT), in which the ratio of Ag
+ : In
3+ :
Zn
2+ was fixed to x: x: 2(1−x). A decrease of the x value in the preparation resulted in
an increase of the Zn fraction in the obtained spherical ZAIS QDs. Furthermore, the