Triplet–Triplet Annihilation Upconversion for Photocatalytic Hydrogen Evolution

Article abstract of DOI:10.1002/chem.201904025

The solar generation of hydrogen by water splitting provides a promising path for renewable hydrogen production and solar energy storage. Upconversion of low-energy photons into high-energy photons constitutes a promising strategy to enhance the light harvesting efficiency of artificial hydrogen production systems. In the present study, upconversion micelles are integrated with Cd_0.5Zn_0.5S to construct solar energy conversion systems. The upconversion micelle is employed to upconvert red photons to cyan photons. Cd_0.5Zn_0.5S is sensitized by upconverted cyan light to produce hydrogen, but not by incident red light without triplet–triplet annihilation upconversion (TTA-UC). The performance of the upconversion photocatalytic system was dramatically affected by the concentration of Cd_0.5Zn_0.5S and the irradiation intensity. This novel system was able to produce about 2.3 μL hydrogen after 5 h of red light (629 nm) irradiation (2.4 mW cm^?2). The present study provides a candidate for applications using low-energy photons for solar hydrogen generation.

Full text of DOI:10.1002/chem.201904025

A Journal of
Accepted Article
Title: Triplet-triplet annihilation upconversion for photocatalytic
hydrogen evolution
Authors: Tianjun Yu, Yanpeng Liu, Yi Zeng, Jinping Chen, Guoqiang
Yang, and Yi Li
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201904025
Link to VoR: http://dx.doi.org/10.1002/chem.201904025
Supported by

10.1002/chem.201904025
Chemistry - A European Journal
RESEARCH ARTICLE
Triplet-triplet annihilation upconversion for photocatalytic
hydrogen evolution
Tianjun Yu*[a], Yanpeng Liu[a, c], Yi Zeng[a, c], Jinping Chen[a], Guoqiang Yang[b, c], Yi Li*[a, c]
Abstract: The solar generation of hydrogen by water splitting
provides a promising path for renewable hydrogen production and
solar energy storage. Upconversion of low-energy photons into high-
energy photons constitutes a promising strategy to enhance the light
harvesting efficiency of artificial hydrogen production systems. In the
present study, we integrate upconversion micelles with Cd0.5Zn0.5S for
constructing solar energy conversion system. The upconversion
micelle is employed to upconvert red photons to cyan photons.
Cd0.5Zn0.5S is sensitized by upconverted cyan light to produce
hydrogen, but not by incident red light without TTA-UC. The
performance of the upconversion photocatalytic system was
dramatically affected by the concentration of Cd0.5Zn0.5S and the
irradiation intensity. This novel system was able to produce about 2.3
L hydrogen after 5 h red light (629 nm) irradiation (2.4 mW/cm2). The
present study provides a candidate for the application of using low-
energy photons for solar hydrogen generation.
photocatalytic water splitting using solar energy has been
considered as one of the most important candidates for hydrogen
evolution,[4] because of their stability during long-time irradiation.
One of the limitations for increasing efficiency of solar energy
utilization is the spectral mismatch between the distribution of the
solar radiation and the absorption spectra of semicoductors.[5]
High catalytic efficiencies can only be obtained under UV, violet
and blue light irradiation, because the energy of long-wavelength
photon is either lower than the bandgap of semiconductors or not
sufficient to overcome the thermodynamic driving force (1.23 V,
standard conditions) and the practical overpotential.[6] Hence,
creating energy-efficient and cost-effective strategies for
hydrogen production using low energy photons remains a
formidable challenge.
Photon upconversion is a promising photon-management
technique to overcome the wavelength limitation of
photochemical conversion systems.[7] The application limitation of
long-wavelength light can be remedied by converting low-energy
photons into high-energy photons. Second harmonic generation
and multistep excitation of lanthanides are two kinds of methods
for photon upconversion, and have been widely used in optical
devices. But the harsh requirement for coherent or high power
excitation laser source, and low conversion efficiencies detract
from their appeal.[8] In contrast to these methods, triplet-triplet
annihilation (TTA) upconversion is particularly interesting due to
its tunable excitation and emission wavelength, noncoherent low-
power excitation, strong absorption of visible light as
photoexcitation and high upconversion quantum yields.[9]
Converting and harvesting low-energy photons by TTA
upconversion is highly relevant for some application fields, such
as solar energy conversion and photocatalysis.[10]
Introduction
In nature, photosynthesis is a process by which plants and other
organisms convert solar energy into chemical energy, which is
later released to fuel their activities. Inspired by natural
photosynthesis, much effort has been devoted to developing
efficient photocatalytic systems that convert solar energy into
chemical energy.[1] Hydrogen is an ideal chemical for energy
storage because of its high specific enthalpy of combustion and
contaminant-free combustion product. Photocatalytic water
splitting is an alternative energy conversion pathway for hydrogen
evolution, and has received much attention.[2] Our research group
has also done a lot of work in this field by simulated
photosynthesis in recent years.[3] Among the photocatalytic
Until now, there are only a few cases in the literature where
TTA upconversion has been used to construct hydrogen
production system.[11] The design of these literatures is to
combine the TTA upconversion system with the photoelectron
chemical (PEC) device. In 2012, Castellano and co-workers used
a WO3 photoanode and a green-to-blue TTA upconversion
system based on a palladium(II) octaethylporphyrin (PdOEP) and
9,10-diphenylanthracene (DPA) TTA couple, demonstrated that
TTA upconversion was responsible for the generated
hydrogen
production
systems,
semiconductor-based
[a]
Assoc. Prof. T. J. Yu, Y. P. Liu, Prof. Y. Zeng, Prof. J. P. Chen, Prof.
L. Yi
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
Beijing, 100190, P.R. China
photocurrent.[12] Meinardi et al. employed
a
broadband
E-mail: tianjun_yu@mail.ipc.ac.cn, yili@mail.ipc.ac.cn
Prof. G. Q. Yang
Beijing National Laboratory for Molecular Sciences (BNLMS), Key
Laboratory of Photochemistry, Institute of Chemistry
Chinese Academy of Sciences
upconversion elastomer coupled to a classical Fujishima−Honda
photocatalytic water splitting cell.[13] The unused long-wavelength
light passing through the photoanode was upconverted to blue
photons by a TTA upconversion layer, and then reabsorbed by
the photoanode. These work showed the possibility of hydrogen
generation with low-energy photons, even if they did not report
any detectable enhancement of hydrogen generation
performance, except for the change of current. The TTA
upconversion device used in photocatalytic system, in most cases,
was isolated by glass, because TTA upconversion device was
[b]
[c]
Beijing, 100190, P.R. China
Y. P. Liu, Prof. Y. Zeng, Prof. G. Q. Yang, Prof. L. Yi
University of Chinese Academy of Sciences
Beijing, 100039, P.R. China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201904025
Chemistry - A European Journal
RESEARCH ARTICLE
very easy to be quenched by other components. The realization
of TTA upconversion devices for direct catalytic hydrogen
generation in one system remains challenges.
Structures of the sensitizer PdTPTBP and the annihilator (also
called acceptor) NBPEA are depicted in Figure 1. PdTPTBP was
synthesized
by
treating
free
base
meso-
Herein, an upconversion micelle,
a
semiconductor
tetraphenyltetrabenzoporphyrin (H2TPTBP) with Pd(OAc)2
according to the procedure reported in the literature.[15] NBPEA
was synthesized by a Pd/Cu-catalyzed Sonogashira reaction of
9,10-dibromoanthracene with a quaternary ammonium modified
alkyne derivative and was characterized by 1H NMR and IR
spectroscopy as well as high-resolution ESI-TOF mass
spectrometry. The details of the synthesis and the
characterization of NBPEA are described in the Supporting
Information.
Cd0.5Zn0.5S and sacrificial agents were integrated together to
construct an upconversion system for hydrogen production. Since
the pair of PdTPTBP and 9,10-bis(phenylethynyl)anthracene
(BPEA) has a good upconversion luminescence performance,[14]
we try to use it in the upconversion micelle. Unfortunately, it is
very difficult for BPEA to disperse in water even if Tween-80 is
present, because of the high concentration requirement of BPEA
in the upconversion system. Therefore, BPEA is replaced by a
cationic derivative with quaternary ammonium, NBPEA, which
has good solubility in water. We employed the micelle to
upconvert red photons to cyan photons. Cd0.5Zn0.5S is sensitized
by upconverted cyan light to produce electron-hole pairs, which
subsequently reduce protons to produce hydrogen. About 2.3 L
hydrogen was achieved after 5 h red light irradiation at 2.4
mW/cm2 intensity. Control experiments validated that the
upconversion micelle is essential for the utilization of red light. No
hydrogen was detected when it was absent. This work
demonstrated that TTA-UC method could be successfully applied
in hydrogen production system for increasing light harvesting
capability of low energy photons. Furthermore, this novel catalytic
system allow one to envisage it as real candidate for the next
generation of photochemical devices for solar hydrogen
generation.
Results and Discussion
A photocatalytic hydrogen evolution system using sub-band gap
photons was constructed by integrating a TTA upconversion light-
harvesting system and a photocatalytic water splitting system.
The design of this system is based on the following
considerations: 1) efficient TTA upconverion Light-harvesting
system; 2) photocatalyst with high activities for H2 evolution from
water; 3) Spectral overlap between the upconversion emission
and the catalyst absorption. Moreover, other factors such as
solubility were also considered in the practical preparation of TTA
upconversion hydrogen evolution systems.
Figure 1. Structures of PdTPTBP and NBPEA.
Nanocrystals Cd0.5Zn0.5S with nano-twin structures were
chosen as the photocatalyst because of their superior
photocatalytic activity for H2 evolution from water upon visible light
irradiation in the absence of noble metal co-catalysts.
Nanocrystals of Cd0.5Zn0.5S were prepared by a precipitate-
hydrothermal method reported in the literature,[16] and the
experimental details are described in the Supporting Information.
Transmission electron microscopy (Figure S1a) reveals that the
morphology of product is granular with size around 40–100 nm.
The regular parallel dislocation structures can be observed on the
surface of many particles, which are clarified to be coherent twin
boundaries of the {111}/[112] type (Figure S1b). The crystal lattice
spacing of 0.32 nm was indexed to the {111} planes of the zinc-
blende lattice. Elemental analysis was carried out by scanning
TEM (STEM) and energy dispersive X-ray spectroscopy (EDS)
(Figure S1c-f). The homogeneous distribution of Cd, Zn and S
indicates the solid solution formation evidently and no phase
segregation between CdS and ZnS in Cd0.5Zn0.5S particles. The
bandgap of Cd0.5Zn0.5S is estimated to be 2.40 eV (Figure S2),
Properties of TTA upconversion systems and the
photocatalyst
PdTPTBP and 9,10-bis(phenylethynyl)anthracene (BPEA) are a
TTA upconversion pair, which performs well in organic phase,[14]
but cannot be used in aqueous phase because of their inherent
insolubility in water. For applying the PdTPTBP-BPEA TTA
upconversion system in aqueous phase, a non-ionic dispersant
Tween-80 was used to improve the solubility of PdTPTBP and
BPEA in water. Tween-80 solubilizes PdTPTBP well up to 10 M
in water, but it cannot disperse BPEA to the concentration (250
M) required for the good TTA upconversion performance. To
obtain enough concentration of annihilator, BPEA was modified
with two quaternary ammonium groups, giving a cationic
derivative NBPEA, which demonstrates good solubility in water.
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10.1002/chem.201904025
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RESEARCH ARTICLE
indicating that photons above 517 nm cannot be absorbed by
Cd0.5Zn0.5S and initiate any photocatalytic process.
The TTA upconversion system was constructed in deaerated
aqueous phase by using PdTPTBP as the photosensitizer,
NBPEA as the annihilator and Tween-80 as the solubilizer. The
aqueous dispersion of PdTPTBP (1  10-5 M) and NBPEA (2.5 
10-4 M) in the presence of Tween-80 (15 mg/mL) is transparent.
According to the critical micelle concentration (CMC) of Tween-
80 (0.016 g/L)[17] and the DLS measurement, micelles with an
average diameter ~8 nm are formed in the system (Figure S3).
Measurement on the TTA upconversion system shows a positive
zeta potential (Figure S4). Tween-80 is a non-ionic surfactant and
PdTPTBP is an electric neutral molecule, therefore, the positive
charge must come from NBPEA and keeps separation of micelles
from each other. Considering the hydrophobic property of
PdTPTBP and the zeta potential result, PdTPTBP and NBPEA
are encapsulated into micelles.
Figure 3. (a) Photoluminescence spectra of the upconversion micelle at
different excitation power densities. (b) Double logarithmic plots of upconversion
emission intensity at 515 nm in aqueous media as a function of the 640 nm
incident laser power density. Inset is the photo obtained from the micelle
excitation with a 629 nm LED. [PdTPTBP] = 1  10-5 M, [NBPEA] = 2.5  10-4 M,
[Tween-80] = 15 mg/mL.
Figure 2. Absorption and normalized emission spectra of PdTPTBP (red, 1 
10-5 M) and NBPEA (blue, 5  10-5 M) in Tween-80 (4 mg/mL) aqueous
despersion in nitrogen atmosphere. The emission spectra were obtained with
excitation at 629 nm for PdTPTBP and 444 nm for NBPEA, respectively.
Selective excitation of the Q-band of PdTPTBP in the TTA
upconversion system with a 640 nm continuous-wave laser
results in an anti-Stokes emission characteristic of NBPEA with a
maximum at 516 nm (Figure 3a), indicative of the occurrence of a
TTA upconversion process. The range of upconversion spectra is
similar to that of the NBPEA fluorescence upon direct excitation
at 444 nm except the attenuated short-wavelength emission,
which is caused by the inner filter effect. The dependence of the
upconversion emission spectra on the excitation power was
studied. With increasing the excitation power density from from
22.0 to 48.6 mW/cm2, an enhancement of the upconversion
To examine the photocatalytic hydrogen evolution of
Cd0.5Zn0.5S initiated by TTA upconversion photons, the basic
photophysical properties of the TTA upconversion light-harvest
system were investigated. Figure 2 illustrates the absorption and
normalized emission spectra of PdTPTBP (1  10-5 M) and
NBPEA (5  10-5 M) in Tween-80 (4 mg/mL) aqueous phase.
PdTPTBP exhibits typical Soret-band at 441 nm and Q-bands at
582 and 628 nm. All the absorption bands of NBPEA lie below
500 nm, indicating that the Q-bands of PdTPTBP can be
selectively excited in the TTA-UC systems. An intense
phosphorescence band with maximum at 802 nm for PdTPTBP
was detected in deaerated Tween-80 aqueous dispersion upon
excitation with 629 nm light. The 0 – 0 band of the fluorescence
spectrum of NBPEA is located at 489 nm, which is higher than the
bandgap of Cd0.5Zn0.5S. Therefore, photons emitted from the
excited NBPEA can be absorbed by Cd0.5Zn0.5S.
emission was observed.
A double logarithm plot of the
upconversion emission intensity emanating from NBPEA at 515
nm as a function of the incident light power density is shown in
Figure 3b. Two linear fitting regions are obtained with slopes of
2.0 below 34.8 mW/cm2 and 1.2 above 34.8 mW/cm2, which
strengthens the upconversion emission deriving from the TTA
mechanism.[18] The TTA-UC quantum yield of the micelle system
under the same condition was found to be 0.3% upon the
excitation power density of 41.0 mW/cm2, which was lower than
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that observed in other supramolecular assembly upconversion
affected by the concentration of Cd0.5Zn0.5S, and the optimal
concentration is 2 mg/mL. The poor photocatalytic performance
at the lower concentration (1 mg/mL) of Cd0.5Zn0.5S can be
ascribed to the insufficient amount of catalyst, therefore, the
produced upconversion photons cannot be captured completely.
The increased amount of Cd0.5Zn0.5S should be in favor of
catalysis, but the enhanced scattering effect severely weakens
the excitation light, and consequently, reduces the upconversion
hydrogen production (Figure S6). Therefore, when the
concentration of Cd0.5Zn0.5S further increases to 3 mg/mL, the
efficiency of hydrogen production decreases steeply, indicating
that the scattering effect plays a dominant role.
systems.[19]
Upconversion photocatalytic production of hydrogen
The photocatalytic hydrogen evolution initiated by upconverted
photons was examined by integrating the photocatalyst
Cd0.5Zn0.5S with the upconversion system PdTPTBP/NBPEA in
Tween-80 aqueous phase. Na2S and Na2SO3 are used as the
sacrificial electron donors. All the components were dispersed by
ultrasound treatment for 30 min before red light irradiation. Control
experiments in the absence of either the photosensitizer
PdTPTBP or the accepter NBPEA showed no hydrogen
production with red LED irradiation (Figure S5), validating that the
PdTPTBP/NBPEA upconversion pair is essential for the
upconversion photocatalytic evolution of hydrogen. These results
collectively confirm the successful sensitization activation of
Cd0.5Zn0.5S via TTA-UC process for hydrogen evolution. Since the
reaction condition plays an important role in the photocatalytic
system, it is necessary to optimize the reaction conditions to
improve the efficiency of the upconversion photochemical
production of hydrogen.
Figure 5. Influence of the illumination intensity on the photochemical production
of hydrogen under irradiation with red LED. Other experimental conditions were
as follows: [PdTPTBP] = 1  10-5 M, [NBPEA] = 2.5  10-4 M, [Tween-80] = 15
mg/mL, [Cd0.5Zn0.5S] = 2 mg/mL, [Na2S] = 0.175 M, [Na2SO3] = 0.125 M.
The effect of irradiation intensity on the efficiency of
upconversion photocatatlyic production of hydrogen was further
investigated. In principle, the photocatalytic performance is
significantly dependent on the upconversion efficiency in this
system. High irradiation intensity (more than 34.8 mW/cm2) is
more conducive to high upconversion efficiency, which is
advantageous for enhancing the photocatatlyic efficiency.
However, no hydrogen was detected at 41 mW/cm2 after 2 h
irradiation (Figure 5). High light intensity accelerates the
photodegradation process of PdTPTBP seriously, which is
inferred from the obvious fading phenomenon within half an hour
irradiation. In order to slow down the photodegradation, the
experiments of hydrogen production upon irradiation with 1, 2.4
and 10 mW/cm2 red LED (629 nm) were carried out by keeping
all other conditions constant ([Cd0.5Zn0.5S] = 2 mg/mL, [PdTPTBP]
= 10 M, [NBPEA] = 0.25 mM, [Na2S] = 0.175 M, [Na2SO3] = 0.125
Figure 4. Influence of the concentrations of Cd0.5Zn0.5S on the photochemical
production of hydrogen under irradiation with red LED (2.4 mW/cm2). The
amounts of hydrogen were normalized by the concentrations of Cd0.5Zn0.5S. The
concentrations of Cd0.5Zn0.5S for A-C are 1.0 mg/mL, 2.0 mg/mL and 3.0 mg/mL,
respectively. Other experimental conditions were as follows: [PdTPTBP] = 1 
10-5 M, [NBPEA] = 2.5  10-4 M, [Tween-80] = 15 mg/mL, [Na2S] = 0.175 M,
[Na2SO3] = 0.125 M.
The upconversion photon-induced hydrogen production were
firstly carried out by varying the concentration of Cd0.5Zn0.5S while
keeping the other experimental conditions fixed. The
concentrations of Tween-80, PdTPTBP, NBPEA, Na2S and
Na2SO3 were set to be ca. 15 mg/mL, 1  10-5 M, 2.5  10-4 M,
0.175 M and 0.125 M, respectively, and the red LED (629 nm, 2.4
mW/cm2) was used as the light source. The results of hydrogen
production normalized to the concentration of Cd0.5Zn0.5S are
shown in Figure 4. The efficiency of hydrogen production is
M, [Tween-80]
= 15 mg/mL). The performance of the
upconversion hydrogen evolution system increase at first and
then decline as the irradiation intensity increases. As shown in
Figure 5, increasing the excitation intensity from 1 to 2.4 mW/cm2
increased the initial rate and the total amount of hydrogen evolved
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10.1002/chem.201904025
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RESEARCH ARTICLE
for the system. The higher irradiation intensity accelerates the
upconversion process and then produces more upconversion
photons for the semiconductor catalyst, resulting in a higher
hydrogen production efficiency for the catalytic system. However,
when the irradiation intensity was increased from 2.4 to 41
mW/cm2, enhancing the irradiation intensity does not benefit the
catalytic output, resulting in lower amount of hydrogen compared
with that upon 2.4 mW/cm2 illumination. This abnormal trend is
also presumably caused by the accelerated degradation of
PdTPTBP under the higher irradiation intensity. The apparent
quantum yield (AQY) of hydrogen production is less than 0.1%,
which was estimated based on the amount of hydrogen produced
in the first hour under the light intensity of 2.4 mW/cm2.
between PdTPTBP and NBPEA, and the quenching efficiencies
were different. The shorter lifetime can be assigned to the
PdTPTBP located near the NBPEA, and the longer lifetime can
be assigned to the part of PdTPTBP located a little far away from
the NBPEA. The average quenching efficiency, estimated from
the average lifetime of PdTPTBP, is 63%, which is consistent with
the steady-state measurement.
Photocatalytic Mechanism Based on TTA
According to the spectral features of the upconversion micelle and
the catalyst, the photocatalytic hydrogen production and the
control experiments, proposed mechanism for the upconversion
hydrogen evolution system is illustrated in Figure 6. In the micelle,
following red light excitation of the sensitizer and intersystem
crossing (ISC), two sensitizers transfer their triplet energy to the
annihilators, generating the triplet state annihilators. The
subsequent diffusion and collision of two triplet state annihilators
generate an excited singlet annihilator of higher energy through
triplet-triplet annihilation process, resulting in a higher energy
photon emission. The micelle act as the antenna to trap the red
light and upconverts it into the cyan TTA-UC emission, and then,
the TTA-UC emission is absorbed by the photocatalyst
Cd0.5Zn0.5S via the emission-reabsorption process, which uses
the light energy to drive the hydrogen evolution reaction.
Cd0.5Zn0.5S that loses electrons can recover from Na2S and
Na2SO3 and continue to play photocatalytic role in the next
catalytic cycle.
Our assumed mechanism in the upconversion catalytic
system is further evidenced by the steady-state and time-resolved
spectroscopy experiments. The emission spectra of PdTPTBP in
micelle with or without NBPEA are depicted in Figure 7a. The
emission around 800 nm is ascribed to the phosphorescence of
PdTPTBP.[15] The phosphorescence intensity of PdTPTBP shows
about 70% decrease in the present of NBPEA, which is attributed
to the high efficient triplet-triplet energy transfer from PdTPTBP to
NBPEA. The energy transfer process was further strengthened by
time-resolved spectroscopy experiments. The phosphorescence
lifetimes for PdTPTBP in micelle with or without NBPEA were
determined under the excitation of 635 nm, and the
phosphorescence decays were monitored at 800 nm. The
phosphorescence decay of PdTPTBP in micelle exhibit
monoexponential profile, but the decay curve obtained from the
micelle in the present of NBPEA cannot be fitted well by the
monoexponential mode. The decay at 800 nm was treated with a
double exponential process, showing two lifetimes of 6.6 s (52%)
and 148 s (48%). The results suggest that PdTPTBP and
NBPEA in micelle were uneven distribution, thus, the distance
Figure 6. (a) Schematic diagram of the red to cyan TTA-UC process; (b)
Schematic illustration showing the mechanism of the photocatalytic hydrogen
evolution.
The photon energy harvested by the upconversion micelle is
delivered to the photocatalyst Cd0.5Zn0.5S through radiative or
non-radiative energy transfer. Steady-state spectroscopy
measurements were not suitable for studying the energy transfer
between the micelle and Cd0.5Zn0.5S due to the obvious light
scattering in the Cd0.5Zn0.5S suspension. To understand the
energy transfer process, we measured the fluorescence lifetimes
for NBPEA in the absence and presence of Cd0.5Zn0.5S by using
a 405 nm laser as the excitation source. The kinetic trace at 515
nm in the absence of Cd0.5Zn0.5S can be fitted well with single
exponential process, giving a lifetime of 5.5 ns. In the presence of
Cd0.5Zn0.5S, the lifetime of NBPEA is estimated to be 5.2 ns, which
is not substantially shorten compared to that in the absence of the
photocatalyst. The energy transfer between NBPEA and
Cd0.5Zn0.5S is more likely to undergo radiative energy transfer.
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Figure 7. (a) Emission spectra of PdTPTBP in micelles with or without NBPEA,
After absorbing the upconversion light, Cd0.5Zn0.5S served as the
reaction center for hydrogen evolution, and then, reductively
quenched by the hole scavengers Na2S/Na2SO3, which have
been confirmed in the previous literatures.[16] Radiative energy
transfer is highly dependent on the absorber on the optical path,
as well as the spectral overlap between the emitter and the
absorber. In the present upconversion catalytic system, scattering
seriously affects the absorption of Cd0.5Zn0.5S and intense
upconversion emission leaks out the catalytic system, there is still
plenty room for improving the upconversion hydrogen evolution
system.
λ
ex = 629 nm; (b) Photoluminescence decays at 800 nm of PdTPTBP in micelles
with (red line) or without NBPEA (black line) under pulsed excitation at 635 nm;
(c) Photoluminescence decays at 515 nm of NBPEA in micelles in the absence
(black line) and present of Cd0.5Zn0.5S (red line) under pulsed excitation at 405
nm. The cyan lines are the fitting curves. Concentrations of each component:
[PdTPTBP] = 1  10-5 M, [NBPEA] = 2.5  10-4 M, [Tween-80] = 15 mg/mL,
[Cd0.5Zn0.5S] = 2 mg/mL.
Conclusion
In summary, an upconversion photocatalytic system has been
constructed by incorporating the upconversion micelle with a
photocatalyst Cd0.5Zn0.5S. The micelle effectively converted red
photons into cyan photons, which can be absorbed by Cd0.5Zn0.5
S
to initiate hydrogen evolution. The micelle complement the
shortage of Cd0.5Zn0.5S absorption and extend the spectral
utilization of hydrogen photoproduction systems, thus advancing
photocatalysis. The experiment design shown here provides a
novel method to construct promising photosynthesis systems for
the conversion of long-wavelength solar energy into hydrogen.
Experimental Section
Steady-state and time-resolved spectroscopic measurements
Steady-state and time-resolved spectra were performed in a 1 cm pass
length quartz cuvette at ambient temperature. Samples were deaerated by
purging with nitrogen for 30 minutes prior to the measurements of
phosphorescence spectrum and lifetime.
Preparation of triplet-triplet annihilation (TTA) upconversion micelles
The triplet-triplet annihilation upconversion system consists of palladium
(II)
meso-tetraphenyltetrabenzoporphyrin
(PdTPTBP)
as
the
photosensitizer and
a
9,10-bis(phenylethynyl)anthracene derivative
(NBPEA) as the acceptor. The TTA upconversion micelle was prepared by
the following procedure by using Tween-80 as the surfactant. Tween-80
(750 mg) and deionized water (45 mL) were added to a weighing bottle
and stirred at ambient temperature until Tween-80 was fully dissolved. A
THF solution (250 L) containing PdTPTBP (0.5 mol) and NBPEA (12.5
mol) was added to the prepared Tween-80 solution and the mixture was
stirred vigorously until it became homogeneous and transparent. The
mixture was placed in a vacuum at ambient temperature for about 1 h to
remove THF, and then the mixture volume was set to 50 mL with deionized
water.
TTA upconversion measurements
The TTA upconversion emission measurements were carried out in
aqueous phase at ambient temperature. A laser (λ = 640 nm) was used as
the excitation source for the upconversion emission measurements. An
Ophir Nova II powermeter with PD300-3W photodetector was used to
measure the intensity of the excitation light. The TTA upconversion
emission spectra were recorded in an integrating sphere (Labsphere)
equiped with a Princeton Instrument Acton SP2500 spectrograph and
SPEC-10:400B/LN CCD. Samples were deaerated by purging with
nitrogen for 30 minutes and then placed to equilibrate for another 30
minutes prior to measurements.
Hydrogen photoproduction
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10.1002/chem.201904025
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RESEARCH ARTICLE
All hydrogen photoproduction experiments were performed in oxygen-free
conditions. Na2S and Na2SO3 were used as the sacrificial electron donor.
Samples were kept in a glass reactor with 10 mL solution and a magnetic
stir. Before measurement, the reaction mixture was purged with nitrogen
for 0.5 h to remove any dissolved air, and then 5 mL methane was injected
as an internal standard followed by sonicated for 30 minutes to disperse
Cd0.5Zn0.5S. A 10 W LED light (λ = 629 nm) was used as the red light
source. The incident light intensity was measured using an Ophir Nova II
power meter with PD300-12W photodetector. The amount of
photogenerated hydrogen at various time intervals was analyzed by gas
chromatography (GC) using a nitrogen carrier gas, molecular sieve 5 Å
columns and a TCD detector.
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21472201), and the Strategic Priority Research Program of
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Keywords: triplet-triplet annihilation upconversion • hydrogen
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10.1002/chem.201904025
Chemistry - A European Journal
RESEARCH ARTICLE
Entry for the Table of Contents (Please choose one layout)
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RESEARCH ARTICLE
Photocatalytic hydrogen
evolution via TTA
Tianjun Yu*, Yanpeng Liu, Yi Zeng,
Jinping Chen, Guoqiang Yang, Yi
Li*
upconversion: An upconversion
photocatalytic system has been
constructed by incorporating the
upconversion micelle with a
semiconductor photocatalyst,
giving a novel strategy for using
low-energy photons for hydrogen
production.
Page No. – Page No.
Triplet-triplet annihilation
upconversion for photocatalytic
hydrogen evolution
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