12 Plasmon-Induced Carrier Transfer for Infrared Light Energy …
213
acceptors [9]. These disadvantages have restricted the use of plasmonic materials
as light energy conversion materials. Thus, a better understanding of this hotcarrier dynamics at the interface is essential to achieve highly efficient IR-responsive
photocatalyst.
A heterostructure based on CdS/Cu 7 S 4 nanocrystals (HNCs) is an ideal system for
spectroscopic tracing of the LSPR-induced carrier transfer from the plasmonic Cu 7 S 4
phase to the CdS phase. Furthermore, the LSPR band of Cu 7 S 4 NCs covers almost
all the IR region of the solar spectrum. In the present work, we elucidated the LSPRinduced behavior of hot carriers in plasmonic Cu 7 S 4 NCs and CdS/Cu 7 S 4 HNCs by
time-resolved transient absorption spectroscopy. Transient absorption spectroscopy
enabled us the directly observe carriers in photo-excited NCs.
Here, we investigated the LSPR-induced hot-carrier dynamics of CdS/Cu 7 S 4
HNCs with the use of femtosecond transient absorption measurement [13]. Through
detailed investigations of the carrier dynamic of CdS/Cu 7 S 4 HNCs we identified efficient electron transfer from the Cu 7 S 4 phase to the CdS phase. Notably, CdS/Cu 7 S 4
has long-lived charge separation (>273 μs), which has not been observed in plasmoninduced carrier-injection systems. On the basis of the ultralong lifetime of the charge
separation, the CdS/Cu 7 S 4 HNCs achieved an efficient photocatalytic H 2 evolution reaction (HER). The CdS/Cu 7 S 4 HNCs exhibited an exceedingly high apparent
quantum yield (AQY) of 3.8% at 1100 nm, which exceeds the current record for
photocatalytic HER under IR-light irradiation. Furthermore, we demonstrated that
our novel system can convert energy from the solar spectrum up to long wavelengths
(i.e., 2000–2500 nm).
We revealed that the high catalytic activity is related to efficient hot-electron
injection and long-lived charge separation at the plasmonic p–n heterojunction of
the CdS/Cu 7 S 4 HNCs. This behavior is unlike a conventional Schottky junction at
the heterointerface of plasmonic metal/semiconductor NCs. Because ultrafast charge
recombination is a major drawback of all plasmonic energy conversion systems, the
observed mechanism proposed here will change the consensus on LSPR-induced
energy conversion and highlight the great advantages of high hot-carrier transfer
efficiency and long-lived charge separation.
12.2 Synthesis and Characterization of IR-Responsive
Photocatalyst
The CdS/Cu 7 S 4 HNCs were synthesized through a seeded growth reaction of diskshaped Cu 7 S 4 NCs and subsequent partial cation exchange of the Cu 7 S 4 NCs with
Cd
2+ cations (Fig. 12.2). Figure 12.2a shows transmission electron microscope
(TEM) images of monodisperse disk-shaped Cu 7 S 4 NCs (size: 16.2 ± 0.9 nm,
thickness: 3.5 ± 0.4 nm). After the cation exchange reaction, CdS phases were
formed on the Cu 7 S 4 NCs to give dimer structures (Fig. 12.2b). The X-ray diffraction (XRD) patterns in Fig. 12.1c clearly show that the CdS/Cu 7 S 4 HNCs comprised
213
acceptors [9]. These disadvantages have restricted the use of plasmonic materials
as light energy conversion materials. Thus, a better understanding of this hotcarrier dynamics at the interface is essential to achieve highly efficient IR-responsive
photocatalyst.
A heterostructure based on CdS/Cu 7 S 4 nanocrystals (HNCs) is an ideal system for
spectroscopic tracing of the LSPR-induced carrier transfer from the plasmonic Cu 7 S 4
phase to the CdS phase. Furthermore, the LSPR band of Cu 7 S 4 NCs covers almost
all the IR region of the solar spectrum. In the present work, we elucidated the LSPRinduced behavior of hot carriers in plasmonic Cu 7 S 4 NCs and CdS/Cu 7 S 4 HNCs by
time-resolved transient absorption spectroscopy. Transient absorption spectroscopy
enabled us the directly observe carriers in photo-excited NCs.
Here, we investigated the LSPR-induced hot-carrier dynamics of CdS/Cu 7 S 4
HNCs with the use of femtosecond transient absorption measurement [13]. Through
detailed investigations of the carrier dynamic of CdS/Cu 7 S 4 HNCs we identified efficient electron transfer from the Cu 7 S 4 phase to the CdS phase. Notably, CdS/Cu 7 S 4
has long-lived charge separation (>273 μs), which has not been observed in plasmoninduced carrier-injection systems. On the basis of the ultralong lifetime of the charge
separation, the CdS/Cu 7 S 4 HNCs achieved an efficient photocatalytic H 2 evolution reaction (HER). The CdS/Cu 7 S 4 HNCs exhibited an exceedingly high apparent
quantum yield (AQY) of 3.8% at 1100 nm, which exceeds the current record for
photocatalytic HER under IR-light irradiation. Furthermore, we demonstrated that
our novel system can convert energy from the solar spectrum up to long wavelengths
(i.e., 2000–2500 nm).
We revealed that the high catalytic activity is related to efficient hot-electron
injection and long-lived charge separation at the plasmonic p–n heterojunction of
the CdS/Cu 7 S 4 HNCs. This behavior is unlike a conventional Schottky junction at
the heterointerface of plasmonic metal/semiconductor NCs. Because ultrafast charge
recombination is a major drawback of all plasmonic energy conversion systems, the
observed mechanism proposed here will change the consensus on LSPR-induced
energy conversion and highlight the great advantages of high hot-carrier transfer
efficiency and long-lived charge separation.
12.2 Synthesis and Characterization of IR-Responsive
Photocatalyst
The CdS/Cu 7 S 4 HNCs were synthesized through a seeded growth reaction of diskshaped Cu 7 S 4 NCs and subsequent partial cation exchange of the Cu 7 S 4 NCs with
Cd
2+ cations (Fig. 12.2). Figure 12.2a shows transmission electron microscope
(TEM) images of monodisperse disk-shaped Cu 7 S 4 NCs (size: 16.2 ± 0.9 nm,
thickness: 3.5 ± 0.4 nm). After the cation exchange reaction, CdS phases were
formed on the Cu 7 S 4 NCs to give dimer structures (Fig. 12.2b). The X-ray diffraction (XRD) patterns in Fig. 12.1c clearly show that the CdS/Cu 7 S 4 HNCs comprised