Round-the-Clock Photocatalytic Hydrogen Production with High Efficiency by a Long-Afterglow Material

Article abstract of DOI:10.1002/anie.201810544

Long afterglow materials can store and release light energy after illumination. A brick-like, micrometer-sized Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ long-afterglow material is used for hydrogen production by the photo

Full text of DOI:10.1002/anie.201810544

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Accepted Article
Title: Round-the-clock Photocatalytic Hydrogen Production with High
Efficiency by a Long Afterglow Material
Authors: Guanwei Cui, Xiuli Yang, Yujia Zhang, Yaqi Fan, Ping Chen,
Hongyu Cui, Yan Liu, Xifeng Shi, Qiaoyan Shang, and Bo
Tang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810544
Angew. Chem. 10.1002/ange.201810544
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10.1002/anie.201810544
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COMMUNICATION
Round-the-clock Photocatalytic Hydrogen Production with High
Efficiency by a Long Afterglow Material
Guanwei Cui, Xiuli Yang, Yujia Zhang, Yaqi Fan, Ping Chen, Hongyu Cui, Yan Liu, Xifeng Shi,
Qiaoyan Shang and Bo Tang*
Abstract: Long afterglow materials can store and release light
energy after illumination. Here, brick-like, micron-sized
industrial requirement of 10%.[6] In addition to a low solar energy
conversion efficiency, another limitation of the practical use of
photocatalytic technology is that optoelectronic devices and
materials cannot maintain operation in the dark without energy
storage. To overcome these restrictions, new kinds of materials
or methods are needed.
Long afterglow materials, also named long lasting phosphors,
persistent luminescence materials or energy storage materials,
can maintain an afterglow for minutes to hours after excitation.[7]
Under light irradiation, the excited electrons are trapped and
stored; when subjected to peripheral thermal perturbations, the
stored electrons are released and combine with the holes,
accompanied by light emission. Considering the whole
photoluminescence process, the carriers can be regarded as
having an extremely long lifetime, which is beneficial to
photocatalysis. We proposed that if long-lived carriers could be
applied in photocatalysis, they would significantly improve the
a
Sr2MgSi2O7:Eu2+,Dy3+ long afterglow material is used for hydrogen
production via the photocatalytic reforming of methanol under round-
the-clock conditions for the first time, achieving a solar-to-hydrogen
(STH) conversion efficiency of 5.18%. This material is one of the
most efficient photocatalysts and provides the possibility of practical
use on a large scale. Its remarkable photocatalytic activity is
attributed to its unique carrier migration path and large number of
lattice defects. Our findings expand the application scope of long
afterglow materials and provide a new strategy to design efficient
photocatalysts by constructing trap levels that can prolong carriers’
lifetimes.
Hydrogen is the cleanest energy with a high energy density and
is widely used as a raw material for chemical production.[1]
Especially in applications related to traffic and transportation,
hydrogen can be utilized as fuel for hydrogen fuel cells to power
locomotives and provide clean kinetic energy.[2] However,
hydrogen is flammable and explosive. Moreover, hydrogen
molecules can seep into metallic crystal lattices, causing
"hydrogen embrittlement".[3] Therefore, the safe storage and
transportation of hydrogen is difficult. On-site hydrogen
production is an effective way to solve this problem. However,
hydrogen is mainly produced at a large scale by the thermal
cracking of hydrocarbons or water-gas shift (WGS) reactions
under high temperatures. Recently, Lin et al. realized hydrogen
production at a low temperature of 150°C by reforming methanol
and water using Pt/MoC catalysts,[4] but this process requires a
considerable amount of external thermal energy. On-site
hydrogen production still remains difficult. Solar energy is clean
energy with almost unlimited reserves and has attracted
considerable attention in recent decades.[5] Photocatalytic
hydrogen production can solve not only the on-site hydrogen
production problem but also the issues related to the energy
economy. However, to date, the highest solar-to-hydrogen
(STH) conversion efficiency is no more than 5% for a particulate
photocatalyst system, which is still far below the estimated
photocatalytic efficiency and could realize
a continuous
photocatalytic reaction in the dark. This type of material could
solve the abovementioned issues. To the best of our knowledge,
although long afterglow materials have been used in many fields,
such as safety displays, biological imaging and light sources,[8-13]
there are no reports on such materials being directly applied in
photocatalytic hydrogen production.
Long afterglow materials mainly include rare earth-doped
aluminate, silicate, stannite, phosphate, gallate and
germanate.[14] Among them, silicate long afterglow materials
have the advantages of a high luminescence intensity, long
afterglow time, excellent chemical stability and low cost, making
them
suitable
for
use
as
photocatalysts.[15,16]
Sr2MgSi2O7:Eu2+,Dy3+ is a commonly used silicate long afterglow
material with excellent luminescence performance. Due to the
similar radii of Eu2+ (198 pm), Dy3+ (192 pm) and Sr2+ (195 pm),
the former two ions can perfectly replace Sr2+ while maintaining
its unit cell size during synthesis.[17] Meanwhile, oxygen
vacancies, which can store electrons, will be formed around the
doped rare earth ions.[18] Therefore, Sr2MgSi2O7:Eu2+,Dy3+ is
expected to undergo round-the-clock photocatalytic hydrogen
evolution.
Based on the above considerations, we synthesized a brick-
like, micron-sized, long afterglow material, Sr2MgSi2O7:Eu2+,Dy3+,
by the sol-gel method.[19] This material showed good
photoluminescence and photoelectrical response characteristics.
Photogenerated carriers with long lifetime were successfully
applied in the photocatalytic reforming reaction of methanol and
water to produce hydrogen under light and dark conditions, with
an STH conversion efficiency of 5.18%, which is one of the
highest efficiencies reported.[6]
[a]
Title(s), Initial(s), Surname(s) of Author(s) including Corresponding
Author(s)
College of Chemistry, Chemical Engineering and Materials Science
Collaborative Innovation Center of Functionalized Probes for
Chemical Imaging in Universities of Shandong
Key Laboratory of Molecular and Nano Probes, Ministry of
Education
Shandong Provincial Key Laboratory of Clean Production of Fine
Chemicals, Shandong Normal University, Jinan, 250014 (P. R.
China)
E-mail: tangb@sdnu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201810544
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COMMUNICATION
respectively.[13, 21, 22] In addition, the doping of rare earth ions can
cause lattice distortion of the Sr2MgSi2O7 matrix and create
surface defects which can be observed in the HRTEM images
(Figure S2). It is ascribed to the presence of large number of
oxygen vacancies,[23-25] which is further confirmed by EPR
spectra. As shown in figure S3, the low-field signal with g-
factor=1.97, close to the free-electron value (g=2.0023) is
generally attributed to an unpaired electron trapped on an
oxygen vacancy site.[26] It is generally believed that the existence
of a large number of lattice defects is beneficial to the
enhancement of photocatalytic activity.[27]
Figure 1. Morphology and Structure characterizations of Sr2MgSi2O7:Eu2+,Dy3+
.
a-b, SEM of Sr2MgSi2O7:Eu2+,Dy3+
.
c, XRD of Sr2MgSi2O7 and
Sr2MgSi2O7:Eu2+,Dy3+. d-i, EDS-mapping of Sr2MgSi2O7:Eu2+,Dy3+
.
The morphology and structure of the as-prepared
Sr2MgSi2O7:Eu2+,Dy3+ were determined by scanning electron
microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron
spectroscopy (XPS), energy-dispersive X-ray spectroscopy
(EDS) and high-resolution transmission electron microscopy
(HRTEM). The material has an irregular morphology with large
particles composed of regular brick-like polyhedral particles with
sizes of 1-2 microns (Figure 1, a, b). The XRD peaks of
Sr2MgSi2O7:Eu2+,Dy3+ (Figure 1, c) approximately agree with the
those of the Sr2MgSi2O7 host material, which has a tetragonal
phase structure (PDF No. 75-1736).[17,19,20] This agreement
indicates that the crystal structure of the host material is not
fundamentally changed by the doped Eu2+ and Dy3+. However,
the intensity of the diffraction peak centred at 2θ=32.02°and
32.78°, corresponding to the (220) and (130) crystal plane of
Sr2MgSi2O7, decreased significantly after doping with Eu2+ and
Dy3+, indicating that the doped Eu2+ and Dy3+ were indeed
Figure 2. Optical properties and photoelectricity response characterization of
Sr2MgSi2O7:Eu2+,Dy3+
.
a, Ultraviolet–visible absorbance spectrum of
Sr2MgSi2O7:Eu2+,Dy3+ and Sr2MgSi2O7. b, Excitation and emission spectra of
Sr2MgSi2O7:Eu2+,Dy3+
.
c,
Luminescence
and
Decay
curves
of
Sr2MgSi2O7:Eu2+,Dy3+ in the dark. d, Photocurrent density of
Sr2MgSi2O7:Eu2+,Dy3+ irradiated by 300 W Xe lamp.
The colour changes of a material directly correspond to the
changes in its light absorbance properties. As shown in Figure
2a, after doping with Eu2+ and Dy3+, the colour of the Sr2MgSi2O7
host material changed from white to yellow. Ultraviolet-visible
diffuse reflectance spectra (UV-vis DRS) showed that the
maximum absorption wavelength significantly expanded from
425 nm to 480 nm after doping with rare earth ions. As
estimated from the absorption tail,[28] the absorption energy gap
of Sr2MgSi2O7:Eu2+,Dy3+ was approximately 2.34 eV, which is
significantly lower than the forbidden band width of 7.9 eV of
Sr2MgSi2O7 reported previously.[29] This lower gap is attributed to
the appearance of a doped energy band in the forbidden band of
Sr2MgSi2O7 afforded by Eu2+ and Dy3+.[18,29] As shown in Figure
4a, according to the ultraviolet photoelectron spectroscopy (UPS,
Figure S4) and valence bond X-ray photoelectron spectroscopy
(VBXPS, Figure S5) data and the previously reported
results,[30,31] the Sr2MgSi2O7:Eu2+,Dy3+ valence band top and
conduction band bottom are located at -1.44 eV, and the
intermediate state level formed by rare-earth Eu2+, which acts as
the valence band, is located at 1.58 eV (the intrinsic valence
band of the Sr2MgSi2O7 host material is located at 6.46 eV).
Therefore, the energy band of Sr2MgSi2O7:Eu2+,Dy3+ meets the
incorporated into the Sr2MgSi2O7 material and occupied the Sr2+
sites.[17] No crystal phase peaks ascribed to rare earth oxides
were observed in the XRD peaks of Sr2MgSi2O7:Eu2+,Dy3+,
showing that the doped Eu2+ and Dy3+ were evenly distributed in
the long afterglow materials, which was further confirmed by the
EDS-mapping images (Figure 1, d-i). The contents of doped
Eu2+ and Dy3+ were 0.48% and 1.04%, respectively. The
chemical states of Sr, Mg, Si, O, Eu and Dy in
Sr2MgSi2O7:Eu2+,Dy3+ were determined from the XPS spectra
(Figure S1). Four peaks centred at 1164.66 eV, 1154.19 eV,
1135.06 eV, and 1124.94 eV in the Eu 3d binding energy region
were ascribed to Eu3+ 3d3/2, Eu2+ 3d3/2, Eu3+ 3d5/2, and Eu2+
3d5/2 (Figure 4, b); two peaks centred at 1297.43 eV and
152.88 eV were assigned to Dy3+ 3d and Dy3+ 4d (Figure 4, c),
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10.1002/anie.201810544
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requirements for a hydrogen evolution redox potential. The as-
prepared long afterglow material showed long afterglow
emission with a maximum emission peak at 465 nm under
ultraviolet excitation and the emission persisted for more than
20 hours (Figure 2, b-c；Figure S6). To determine whether the
photoexcited carriers can be transferred to the interface of the
achieved good hydrogen evolution performance in the dark after
illumination. As shown in Figure 3b, after being irradiated by UV
light for 15 min, H2 production gradually increased over time in
the dark, and the amount of hydrogen reached 264 μmol/g after
8 hours with an STH conversion efficiency of 0.39%. Due to its
chemical stability, Sr2MgSi2O7:Eu2+,Dy3+ exhibited a long lifetime,
maintaining the same photoactivity after 6 cycles (Figure 3c). In
contrast, the host material Sr2MgSi2O7 exhibits photocatalytic
hydrogen production under only UV light conditions, not under
visible light and dark conditions (Figure S7). This indicates that
the presence of Eu2+ and Dy3+ is necessary for hydrogen
production under visible light and dark conditions.
as-prepared
long
afterglow
material,
photocurrent
measurements were performed. As shown in Figure 2d, the long
afterglow material showed good photoelectric response
characteristics. Interestingly, the photoelectric response curve
exhibits a unique hill peak shape, which may be due to the
cumulative storage and slow release characteristics of the
photogenerated carriers. This is consistent with the
characteristics of the long afterglow. The abovementioned
results indicate that this material has the capacity for continuous
round-the-clock photocatalytic hydrogen evolution.
Figure 4. Proposed mechanism involved in the photocatalytic process. a,
Energy-level and photogenerated electrons transfer process diagrams. b, XPS
spectra of Eu3d before and after photocatalytic reaction. c, XPS spectra of
Dy3d and Dy4d before photocatalytic reaction. d, XPS spectra of Dy3d and
Dy4d after photocatalytic reaction.
Figure 3. Photocatalytic performance of Sr2MgSi2O7:Eu2+,Dy3+. a, H2 evolution
in different pH reactive mediums. b, Time course of H2 evolution under dark for
Sr2MgSi2O7:Eu2+,Dy3+ and Sr2MgSi2O7 after irradiation by 500 W high-pressure
Hg lamp for 15 min. c, Stability test of H2 evolution (evacuation every 5 h) for
Sr2MgSi2O7:Eu2+,Dy3+ under UV light irradiation. d, H2 evolution under different
temperature in the dark for Sr2MgSi2O7:Eu2+,Dy3+ after irradiation by UV light.
The excellent photocatalytic hydrogen evolution performance
of Sr2MgSi2O7:Eu2+,Dy3+ was suggested to be due to the unique
carrier migration pathway that causes its long afterglow.
According to the previous reported results,[29,32] although the
mechanism of persistent luminescence is still open to question,
herein, the emission mechanism of the long afterglow material is
proposed as follows (Figure 4a): When the electrons in the
ground-state 4f of Eu2+ are excited to the energy level of 5d, they
are further excited to the conduction band of the Sr2MgSi2O7
host material by thermal energy, and Eu2+ becomes Eu3+. The
excited electrons in the conduction band will be trapped by Dy3+,
with the formation of Dy2+ (Path I denoted by the red arrows).
Then, when the trapped electrons in the ground state of Dy2+ are
released to the conduction band, they will recombine with Eu3+
to reform Eu2+ with the emission of a long afterglow (Path II
denoted by the blue arrows). During the photocatalytic reaction
under light irradiation, most of the excited electrons in the
conduction band originating from Eu2+ can directly interact with
the reaction substrates, while the electrons that are not
consumed can continue to be transferred; these electrons are
trapped by Dy3+ ions and are therefore stored temporarily. As a
The photocatalytic properties of the as-prepared materials
were investigated for photocatalytic hydrogen evolution from the
reforming reaction of methanol and water under ultraviolet light,
visible light and dark conditions. Sr2MgSi2O7:Eu2+,Dy3+ showed
high reactivity over a wide pH range of 5-10, even in the pure
water reaction medium without any buffer (Figure 3a). Under the
optimal photocatalytic reaction conditions, the maximum H2
production rate achieved was 14 mmol/g•h under irradiation by a
500 W high-pressure mercury lamp with a maximum absorption
wavelength of 365 nm. Correspondingly, under the assumption
that all incident light was absorbed by the photocatalyst, the
STH conversion efficiency was 5.18% per gram of
Sr2MgSi2O7:Eu2+,Dy3+, which is one of the highest conversion
efficiencies of light energy reported in the field of photocatalytic
H2 evolution. Moreover, under visible light irradiation, H2 evolved
at a rate of 60 μmol/g•h with an STH conversion efficiency of
0.18%. Most notably, the as-prepared long afterglow material
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10.1002/anie.201810544
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result, the carrier lifetime is improved, and thus, the catalytic
efficiency is ultimately improved in light conditions. Under dark
conditions after irradiation, the excited electrons in the
conduction band released from Dy2+ by the thermal disturbance
can continue to interact with the substrates to result in
continuous hydrogen production. The catalytic reaction in this
dark environment will break the electron transport pathway and
quench the long afterglow. This quenching phenomenon was
observed during the catalytic reaction. Under dark conditions,
the intensity of the long afterglow luminescence obviously
decreased with the addition of methanol (Figure S8). This
decrease preliminarily demonstrates that the active electrons for
the photocatalytic reaction originate from the photogenerated
carriers to produce the long afterglow. In addition, as shown in
Figure 3d, the H2 production rates were related to the reaction
temperature in the dark environment, showing the maximum
hydrogen production rate at 77 °C. This is consistent with the
thermoluminescence phenomenon of Sr2MgSi2O7:Eu2+,Dy3+,[33]
further indicating that the carriers utilized during hydrogen
production by the material are derived from active electrons
trapped during the long afterglow luminescence processes. The
XPS results show that after the photocatalytic reaction, the
content ratio between Eu2+ and Eu3+ increased from 0.52:1 to
0.84:1(Figure 4 b); for Dy3+, although no reduced chemical state
Dy2+ was detected, a slight bonding energy shift of 0.17 and 0.66
eV for the Dy 3d and Dy 4d electrons, respectively, was
observed before and after the photocatalytic reaction (Figure 4 c,
d). It is preliminary suggested that the Eu2+ and Dy3+ doped in
Sr2MgSi2O7 may be the key active sites of the photocatalytic
reaction. Detailed photocatalytic reaction mechanisms will be
studied in our future work.
This work was supported by the National Natural Science
Foundation of China (21535004, 91753111, 21575082,
21507074, 21390411) and the Key Research and Development
Program of Shandong Province (2018YFJH0502), Development
plan of science and technology for Shandong Province of China
(2013GGX10706), Shandong Province Natural Science
Foundation (ZR2015BM023), and
A Project of Shandong
Province Higher Educational Science and Technology Program
(J13LD06).
Keywords: round-the-clock •Long afterglow • photocatalytic
hydrogen evolution• methanol • water
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In summary, a long afterglow material, Sr2MgSi2O7:Eu2+,Dy3+,
was applied to hydrogen production by the photocatalytic
reforming of methanol and water under round-the-clock
conditions for the first time. An STH conversion efficiency of 5.18%
was obtained, which is one of the highest efficiencies obtained
to date. This exceptional photocatalytic activity is attributed to
the unique carrier transport path and large number of lattice
defects. Compared with thermodynamic hydrogen production,
the photocatalytic process has the advantages of being clean,
safe and economical. We believe that through energy level
renovation or assembly with other materials, it is possible to
effectively expand the spectral utilization range of such long
afterglow materials and further improve their light energy
conversion efficiency. Our findings widen the application scope
of long afterglow materials and provide a strategy to design
efficient photocatalysts by constructing trap levels that can
prolong carriers’ lifetimes.
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Entry for the Table of Contents
Layout 1:
COMMUNICATION
A brick-like, micron-sized
Guanwei Cui, Xiuli Yang, Yujia
Sr2MgSi2O7:Eu2+,Dy3+ long
afterglow material is used for
hydrogen production via the
photocatalytic reforming of
methanol under round-the-clock
conditions for the first time,
achieving a solar-to-hydrogen
(STH) conversion efficiency of
5.18%. This material is one of the
most efficient photocatalysts and
provides the possibility of practical
use on a large scale.
Zhang, Yaqi Fan, Ping Chen,
Hongyu Cui, Yan Liu, Xifeng Shi,
Qiaoyan Shang and Bo Tang*
Page No. – Page No.
Round-the-clock Photocatalytic
Hydrogen Production with High
Efficiency by a Long Afterglow
Material
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