Mechanistic Understanding of Efficient Photocatalytic H2 Evolution on Two-Dimensional Layered Lead Iodide Hybrid Perovskites

Article abstract of DOI:10.1002/anie.202014623

Three-dimensional (3D) organic–inorganic hybrid perovskites have demonstrated excellent capability in solar fuel production, while the two-dimensional (2D) counterparts are generally considered inferior candidates due to the high exciton binding energy an

Full text of DOI:10.1002/anie.202014623

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 7376–7381
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Energy Materials
Mechanistic Understanding of Efficient Photocatalytic H Evolution on
2
Two-Dimensional Layered Lead Iodide Hybrid Perovskites
Hong Wang, Hefeng Zhang, Junhui Wang, Yuying Gao, Fengtao Fan, Kaifeng Wu, Xu Zong,*
and Can Li
Abstract: Three-dimensional (3D) organic–inorganic hybrid
perovskites have demonstrated excellent capability in solar fuel
production, while the two-dimensional (2D) counterparts are
generally considered inferior candidates due to the high exciton
binding energy and weak light absorption. Herein, contrary to
our common understanding, we find that 2D perovskites can
achieved remarkable progress with a record power conver-
[
4]
sion efficiency of up to 25.5% in ten years. Moreover, the
attractive features of hybrid perovskites enable them to be
excellent platform for solar fuel production via photocatalytic
[
5]
hydrogen production and CO reduction reactions. How-
2
ever, due to the notorious instability of hybrid perovskites in
aqueous solution, the most challenging issue that perovskites
encounter is how to keep stable during photocatalytic
reactions. Moreover, developing strategies that can improve
the charge separation & transportation efficiency and surface
catalysis is important towards their potential deployment.
Nam etc. first reported the stabilization of three-dimen-
sional (3D) hybrid perovskite methylammonium lead iodide
perform photocatalytic H production from HI splitting more
2
efficiently than their 3D counterparts. We observed sharp
difference between 2D perovskites crystals with organic
phenylalkylammonium cations of different lengths and the
3
D counterparts in their stabilization behavior in aqueous
solution. Moreover, we show that the organic cations length of
the 2D perovskites affects the nanostructures, optoelectronic
properties, and the charge transfer process significantly, which
determines the photocatalytic activity of the 2D perovskites.
Among the 2D perovskites under investigation, phenylmethy-
lammonium lead iodide with the shortest organic cations
achieved the best solar-to-chemical conversion efficiency of ca.
(MAPbI ) crystals in aqueous solution and the application of
3
MAPbI for photocatalytic HI splitting reaction under light
3
[5a]
irradiation.
The authors showed that MAPbI3 can be
stabilized in concentrated hydriodic acid solution via estab-
lishing dissolution-reprecipitation equilibrium and the as-
stabilized MAPbI3 crystals can act as photocatalyst to
perform photocatalytic HI splitting reactions. Following this
pioneer work, different hybrid perovskites including
1
.57%, which is the highest value ever reported for hybrid
perovskites.
Introduction
MAPbBr3 I , MA Bi I , MAPbBr , etc. were stabilized with
Àx x 3 2 9 3
similar strategy and realized photocatalytic HI or HBr
[
6]
Producing solar fuels via artificial photosynthesis offers
a promising strategy for the conversion and storage of
splitting reactions. Moreover, the solar-to-chemical (STC)
conversion efficiency achieved in these systems increased
rapidly through developing strategies that can promote
charge separation & transportation efficiency and surface
catalysis. For example, Wang and Huang etc. found that the in
[
1]
sunlight. Exploring semiconductor materials with excellent
optoelectronic properties represents one of the key scientific
[2]
and technical challenges in this field. In recent years,
organic-inorganic hybrid perovskites have emerged as ex-
cellent candidates for fabricating photovoltaic due to their
impressive optoelectronic properties, including high optical
absorption, long carrier lifetime and large carrier mobility.
The solar cells fabricated with hybrid perovskite have
situ hybridization of MAPbI with reduced graphene oxide
3
(rGO) can significantly accelerate the transport of electrons
from MAPbI3 to rGO, thus significantly improving the
[3]
[7]
photocatalytic activity of MAPbI3. Li etc. showed that the
hybridization of TiO nanoparticles with MAPbI can en-
2
3
hance the photocatalytic activity of MAPbI by 88 times due
3
to the formation of dynamic heterojunction that can lead to
improved charge separation. Furthermore, the simultaneous
hybridization of electron and hole transporting motifs with
[
*] H. Wang, H. Zhang, Y. Gao, Prof. F. Fan, Prof. X. Zong, Prof. C. Li
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, Dalian National Laboratory
for Clean Energy, Zhongshan Road 457, Dalian 116023 (China)
E-mail: xzong@dicp.ac.cn
[8]
MAPbBr can realize dual charge modulation towards more
3
efficient charge separation/transportation, achieving more
[6c]
H. Wang, H. Zhang, Y. Gao
University of Chinese Academy of Sciences
Beijing 100049 (China)
challenging HBr splitting reaction. Besides hybridization
strategy, Wang etc. showed that the formation of a band gap
funnel structure in mixed-halide perovskite MAPbBr3 Ix can
Àx
Dr. J. Wang, Prof. K. Wu
facilitate the transportation of the photogenerated charges
from the interior to the surface and drastically enhanced the
photocatalytic activity, thus demonstrating the efficacy of
State Key Laboratory of Molecular Reaction Dynamics, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences
Dalian, Liaoning 116023 (China)
[6a]
electronic structure engineering in this field.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202014623.
In addition, the rapid development of 3D organic-
inorganic hybrid perovskites in optoelectronic and photonic
7
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devices has led to extended research on their two-dimensional
2D) counterparts that exhibit the significantly improved
environmental stability and distinct optoelectronic properties.
However, as far as we know, there is still no report on using
Herein, we report for the first time that a series of 2D
(
hybrid perovskites with phenylalkylammonium cations of
+
different lengths, including C H (CH ) NH (n = 1, 2, and 3),
6
5
2
n
3
can be stabilized in aqueous solution and drive photocatalytic
2
D layered organic-inorganic hybrid perovskites for solar fuel
H production under visible light irradiation. We find that the
2
[9]
production. A typical 2D network of perovskite consists of
inorganic layers of corner-sharing [MX6] octahedra con-
fined between interdigitating bilayers of intercalated bulky
alkylammonium cations. The inorganic and organic layers
are stacked together by a combination of Coulombic and
hydrophobic forces to maintain the structure integrity, and
the presence of the organic layer contributes to the enhanced
stability of the perovskite. The electronic structure of these
lengths of the organic cations can influence the dissolution
behavior, nanostructures as well as the optoelectronic proper-
ties (band gap & band edge positions) of these 2D perovskite
crystals significantly. Moreover, the charge transfer process
on the 2D perovskites crystals and interfacial charge transfer
(CT) between 2D perovskites and Pt cocatalyst increases with
decreasing the length of perovskites organic cations, which
has been testified with Kelvin probe force microscopy
(KPFM) and transient absorption spectroscopy (TAS).
Among the three 2D perovskites, phenylmethylammonium
lead iodide with the shortest organic cations realized the most
efficient charge separation and transportation at the nano-
4
À
[10]
2
D compounds can be regarded as quantum wells, in which
the semiconducting inorganic layers act as “wells” and the
insulating organic layers act as “barriers”.
[
10a]
This feature
enables 2D perovskites to show interesting characteristics
such as the unique quantum and dielectric confinement effect,
quantum size effect and anisotropic carrier transport proper-
ties. Furthermore, the optoelectronic properties of 2D
perovskites can be finely tuned by changing the chemical
compositions and quantum mechanical degrees of freedom,
which provides ample opportunities for optimization towards
scale domain and therefore achieved the highest activity of
À1
333 mmolh
for H2 evolution when loading Pt as the
cocatalyst. A solar-to-chemical conversion efficiency of ca.
1.57% is obtained, which is even higher than that obtained on
3D perovskites hybridized with charge transporting motif.
[11]
solar fuel production. On the other hand, compared with
their 3D counterpart, 2D perovskites are not considered to be Results and Discussion
good candidates for high-performance solar cells due to their
high exciton binding energy and relatively smaller light
2D organic-inorganic hybrid perovskite crystals with
[12]
absorption. Therefore, it is highly possible that 2D per-
ovskites are not good candidates for solar fuel production.
However, due to the different working mechanism of photo-
catalysis and solar cells, although the localized charge carriers
are unlikely to reach the electron/hole selective contacts in
a typical solar cell geometry, they could be utilized for
photocatalytic reactions that occur at localized domains
different organic cations were synthesized through a simple
one-step approach in HI solution. Three types of phenyl-
alkylammonium cations were utilized: benzylammonium,
phenethylamine and 3-phenyl-1-propylamine, that is, C H -
6
5
+
3
(CH ) NH with n = 1, 2, and 3. These organic cations are
2
n
abbreviated as PMA, PEA, and PPA, and the corresponding
PMA PbI , PEA PbI , and PPA Pb I perovskite crystals are
2
4
2
4
3
2 7
(
Scheme 1). Moreover, considering that there is energy loss
abbreviated as PMPI, PEPI, and PPPI, respectively. These 2D
perovskites are employed as the model due to their excellent
electronic properties, facile preparation and tunability. The
powder X-ray diffraction (XRD) patterns of the as-obtained
PMPI, PEPI and PPPI were shown in Figure 1a, in which all
the diffraction peaks can be well indexed to those of the
during the catalytic H2 evolution reaction and oxidation
reaction on the surface of the photocatalyst, the photo-
generated electrons and holes generated in wide band gap 2D
perovskite will have higher potential to overcome the energy
barrier toward catalytic reactions, which is beneficial for solar
fuel production.
[13]
standard samples. Moreover, PMPI and PEPI with short
Scheme 1. Schematic illustration of the charge separation & trans-
portation pathway for 2D perovskites in a) solar cells and b) photo-
catalyst powder. In solar cells geometry, the photogenerated electrons
and holes have to migrate for a long distance to reach the electron/
hole selective contacts. In contrast, for photocatalyst powder, photo-
generated electrons and holes only need to migrate at the nanoscale
range for redox reactions, which is much easy to be realized.
Figure 1. a) XRD patterns, b) UV/Vis diffuse reflectance spectra, and
c–e) SEM images of PMPI, PEPI, and PPPI powder.
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organic cations consist of inorganic layers of corner-sharing
4À
[
PbI6] octahedron separated by organic cations, whereas for
PPPI with long organic cations the inorganic layers contain
4
À
both corner- and face-sharing [PbI6] octahedron. By calcu-
lating the full width at half maximum (FWHM) value of the
(
001) diffraction peak using the Scherrer equation, the
distance of perovskites along the c axis was determined to
be 1.45 nm, 1.64 nm and 1.70 nm for PMPI, PEPI and PPPI,
respectively. Therefore, the interlayer distance of 2D per-
ovskite crystals increases with the increase in the length of the
organic cations.
The absorption spectra of PMPI, PEPI, and PPPI crystals
were then measured with UV/Vis diffuse reflectance spec-
troscopy. As shown in Figure 1b, PMPI crystals exhibit an
absorption edge at ca. 570 nm, corresponding to a direct
optical band gap energy of 2.18 eV according to the Kubelka-
Munk equation (Figure S1). As for PEPI, the absorption edge
is blue-shifted by ca.20 nm when compared with that of PMPI
and the optical band gap is ca. 2.23 eV. Further increase in the
length of the organic cation leads to additionally increased
Figure 2. The stabilization process of a) PMPI, b) PEPI, and c) PPPI
powder in aqueous solution containing different concentrations of
corresponding iodine salts. d) Schematic illustration of the 2D perov-
skites powder stabilized in iodine salt aqueous solutions.
found to decrease with the increase of the length of the
organic cations, which could be related with the different
hydrophobicity of different organic cations. Furthermore, we
analyzed the concentration of the dissolved Pb species in
aqueous solution by inductively coupled plasma atomic
emission spectrometry (ICP-AES) (Table S1). We find that
the concentration of dissolved Pb species is negligible for the
three 2D perovskites, indicating that these 2D perovskites are
almost intact in the stabilized aqueous solution (Figure 2d).
In a sharp contrast, for the typical 3D perovskites like
MAPbI and MAPbBr , the concentration of the dissolved Pt
[13]
band gap of ca. 2.42 eV for the PPPI crystals. Therefore,
with the increase of the organic cations, the band gap of the
2
D perovskite gradually increases. The morphological fea-
tures of the as-prepared 2D perovskites crystals were then
investigated with scanning electron microscopy (SEM). As
shown in Figure 1c–e, both the PMPI and PPPI crystals show
a rectangle-like morphology. However, the length and width
of PPPI are ca. 30 and 5 mm, respectively, which is much
smaller than those of PMPI. As for PEPI crystals, they exhibit
a distinct plate-like morphology with a width of ca. 200 mm.
All the 2D perovskites exhibit typical layered structures from
the cross-sectional SEM images (Figure S2).
3
3
À1 [5a,6c]
species is ca. 0.645 and 0.24 molL .
These facts reflect
the distinct dissolution behavior of the 2D and 3D perovskites
in aqueous solution, which could potentially affects their
photocatalytic performance.
We then tentatively employ the as-prepared 2D perov-
After successfully stabilizing 2D perovskites in aqueous
solution, we tentatively employed these 2D perovskites for
hydrogen production in aqueous solution containing the
corresponding iodine salts as the sacrificial reagents. As
shown in Figure 3a and S3, the rate of hydrogen evolution on
skite crystals as photocatalysts for photocatalytic H produc-
2
tion reaction in aqueous solution. However, we find that all
the 2D perovskites crystals decompose into PbI precipitate,
2
À
I
and the corresponding organic cations once immersed in
À1
water. This is similar to what have been observed for 3D
perovskites. Therefore, although 2D perovskites have showed
improved stability than the 3D counterparts in humid
conditions, they still suffer from poor stability in aqueous
solution. Previous studies show that 3D hybrid perovskites
like MAPbI3 or MAPbBr3 can be stabilized in saturated
halide acid solution due to the establishment of dynamic
bare PMPI, PEPI and PPPI is 17, 4.5, and 0.27 mmolh under
visible light irradiation, respectively. To the best of our
knowledge, this is the first time that 2D perovskites are
demonstrated to be capable of driving photocatalytic H2
production in aqueous solution. After loading 0.5 wt% of Pt
as the hydrogen evolution reaction (HER) cocatalyst, the rate
of hydrogen evolution on the PPPI particles was increased to
[5a,6c]
À1
dissolution-reprecipitation equilibrium.
In these cases,
ca. 2.7 mmolh (Figure 3a). As for PEPI and PMPI, the rates
+
the presence of high concentration of halide ions and H are
found to be crucial for the dynamic stabilization of 3D
perovskites. Therefore, we tentatively tune the concentration
of iodine salts in aqueous solution to realize the stabilization
of the 2D perovskites. We found that the PMPI crystals
of hydrogen evolution are further increased to ca. 128 and
À1
259 mmolh , respectively, under the same conditions, which is
around 47 and 81 times of that obtained on Pt (0.5 wt%)/
PPPI. Therefore, we conclude that the photocatalytic activ-
ities of the 2D perovskites decrease with the increase of the
length of the organic cations.
decomposed to yellow PbI precipitate when the concentra-
2
tion of PMAI is low. However, when the concentration of
PMAI is higher than 1.8 molL , PMPI crystals can be well
However, due to the necessity of stabilizing the 2D
perovskites in aqueous solution, the concentrations of the
iodine salts employed for photocatalytic reactions were
different. Therefore, the different photocatalytic activities
observed on these 2D perovskites could be ascribed to this
difference. We then tested the photocatalytic activity of PEAI
in aqueous solution containing different amounts of PEAI to
À1
preserved (Figure 2a). In a similar manner, we realized the
stabilization of PEPI (Figure 2b) and PPPI (Figure 2c)
crystals in aqueous solution that contains PEAI or PPAI
À1
iodine salts with concentrations higher than 0.3 molL or
À1
0
.2 molL , respectively. Therefore, the concentration of the
iodine salts required for the stabilization of 2D perovskites is
clarify this point. As show in Figure 3b, H is found to evolve
2
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perovskites (Table S2). We then compared the activities of
PMPI and 3D perovskite MAPbI at their optimum reaction
3
[
8]
conditions (Figure S5, S6). It is interesting to find that
although MAPbI is a more suitable candidate than PMPI in
3
solar cells field due to the wider light absorption and more
excellent charge transport properties, it demonstrated much
inferior photocatalytic activity than PMPI when loaded with
Pt catalyst. Moreover, even the photocatalytic activities of
MAPbI is greatly improved by hybridization with charge
3
transporting motif like Pt/TiO , it is still lower than that of Pt/
2
PMPI at the optimum conditions. Therefore, it is reasonable
to conclude that electrons and holes can be efficiently
generated and separated on Pt/PMPI alone in the nanoscale
domain without the necessity of external charge transporting
motif. Moreover, high resolution scanning electron micros-
copy (HR-SEM) images show that Pt cocatalyst is loaded on
the surface of PMPI, PEPI, and PPPI intimately, which is
desirable for the efficient charge transfer between the 2D
perovskites and Pt cocatalyst towards subsequent catalytic
Figure 3. a) Time courses of photocatalytic H evolution over Pt/PMPI,
2
Pt/PEPI, and Pt/PPPI. PMAI (2 M), PEAI (0.5 M), and PPAI (0.5 M)
were used in the reaction solution, respectively. b) Influence of the
concentrations of PEAI on the photocatalytic activity of Pt/PEPI. The
photocatalytic activity of Pt/PMPI is also shown as a comparison.
c) Dependence of the photocatalytic activity of Pt/PMPI on the amount
of Pt cocatalyst. d) Cycling tests of photocatalytic hydrogen evolution
on Pt (2.0 wt%)/PMPI under visible light irradiation. Reaction con-
ditions: 27 mL deionized water and 3 mL H PO with precursor iodine
reactions (Figure S7). In a sharp contrast, 3D MAPbI crystals
3
exist in a more dynamic manner in aqueous solution, which is
unfavorable for the stable anchoring of Pt cocatalyst on the
surface and thus leads to inefficient charge transfer between
these two components (Scheme S1). As a consequence, much
3
2
salt, 150 mg of perovskite photocatalyst, 300 W xenon lamp
lower activity is obtained on Pt/MAPbI if no external charge
3
(l>420 nm), reaction cell: top-irradiation cell with a Pyrex window.
transporting motif for immobilizing Pt cocatalyst (TiO etc.) is
2
used.
Besides optimizing the photocatalytic activity of PMPI,
we also tested the stability of PMPI for photocatalytic
hydrogen production reactions. As shown in Figure 3d, no
decrease in the photocatalytic activity of Pt/PMPI is observed
in cycling tests. The similar XRD patterns (Figure S8–S10),
SEM images (Figure S11) and UV/Vis spectra (Figure S12–
S14) of PMPI before and after hydrogen production reactions
give a further proof on its good photochemical stability. The
excellent activity and good stability could enable 2D per-
À1
at 128 mmolh on Pt (0.5 wt%)/PEPI when the concentration
of PEAI is 0.5 M. When 1.0 M PEAI iodine salt was
employed, the rate of hydrogen evolution is increased to ca.
33 mmolh . Further increasing the concentration of PEAI
to 2.0 M only led to a slight increase in the activity, which is ca.
À1
1
À1
1
40 mmolh . Therefore, increasing the concentration of the
iodine salts is beneficial for improving the photocatalytic
activity of 2D perovskites due to the improved oxidation half
reaction. However, the photocatalytic activity of Pt/PEPI is
still much inferior than that of Pt/PMPI when the same
concentration of iodine salts is employed. Therefore, it is
reasonable to conclude that the different photocatalytic
activities observed on the three 2D perovskites is mainly
associated with their optoelectronic properties instead of the
salt concentrations.
We also investigated the influence of the amount of Pt
cocatalyst on the photocatalytic performance using the most
active PMPI as an example. As shown in Figure 3c, the rate of
hydrogen evolution on bare PMPI is increased from ca. 17 to
69 mmolh after depositing 0.25 wt% Pt as the cocatalyst.
By increasing the amount of Pt, the rate of hydrogen
evolution on Pt/PMPI increased substantially as can be
observed from the vigorous H bubbles evolved (Figure S4).
An optimum value of 333 mmolh was obtained when
.0 wt% of Pt is used. Considering that the specific surface
area of PMPI is only increased from 0.434 to 0.661 m g after
loading 2.0 wt% Pt cocatalyst, the drastically increased
photocatalytic activity should be ascribed to the promotion
effect of Pt cocatalyst. A solar-to-chemical conversion
efficiency (STC) of ca. 1.57% is obtained, which is as far as
we know, the highest value ever reported for hybrid
ovskites to be good candidates for solar H production from
2
HI splitting.
In order to develop more efficient 2D perovskites for
photocatalytic hydrogen production, we then moved to
investigate the factors that influence their intrinsic activities.
Generally, the intrinsic activity of a photocatalyst is influ-
enced by its electronic structures. Therefore, we first analyzed
the band structures of PMPI, PEPI and PPPI with ultraviolet
photoelectron spectroscopy (UPS) together with the results
obtained with UV/Vis spectroscopy. According to the UPS
valence spectra shown in Figure 4a, the valence band (VB)
energy levels of PMPI, PEPI and PPPI are estimated to be ca.
1.62, 1.19, and 1.45 eV below the Fermi level (Details are
provided in the Supporting Information). Moreover, the
Fermi level of PMPI, PEPI and PPPI are estimated to be ca.
À0.39, 0.19, and 0.16 V (vs. normal hydrogen electrode
(NHE)). As the band gap of PMPI, PEPI and PPPI is 2.18,
2.23 and 2.42 eV, respectively, the conduction band (CB) and
VB energy levels of these 2D perovskites can be inferred to
be at ca. À0.95 and 1.23 V, À0.85 and 1.38 V, and À0.81 and
1.61 V vs. NHE, respectively. The schematic diagram of the
energy levels for the three 2D perovskites was presented in
Figure 4b. It is evident that although both the valence band
À1
1
2
À1
2
2
À1
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photocatalytic activities observed on 2D perovskite with
shorter organic cations.
To further interpret the different photocatalytic activities
observed on the above 2D perovskite with different organic
cations, we then investigated the electron transfer kinetics in
detail using transient absorption (TA) spectroscopy. Details
on the TA set-ups can be found in the SI and Figure S15. The
pump pulse was tuned to 350 nm to selectively excite the
perovskite films. Figure S16 show the TA spectra of various
perovskite and Pt/perovskite heterojunction films excited
with a low pump fluences to avoid the multi-exciton
generation, which are all dominated by an exciton bleach
(XB) feature of perovskite component due to the state-filling
signals of band edge electrons and holes. The XB features for
free perovskite films (PMPI, PEPI and PPPI) all show
ultrafast decay, with nearly 95%, 94% and 78% of its
amplitude decaying within 8000 ps for PMPI, PEPI and PPPI
samples, respectively (Figure S17), presumably due to carrier
trapping, radiative and/or nonradiative electron-hole recom-
bination. However, the XB kinetics for Pt/perovskite hetero-
junction films are all much faster than that of free perovskite.
A careful comparison of the XB kinetics measured for various
free perovskite and Pt/perovskite heterojunction films reveal
faster decay in the latter, which can be better visualized by
normalizing them to the long-lived tail (Figure S17). Note
that the only difference for kinetics between free perovskite
and Pt/perovskite heterojunction is the electron transfer (ET)
process from perovskite to the Pt particle. Thus, by normal-
izing the XB kinetics of free perovskite and Pt/perovskite
sample correspondingly to their slowly decaying component
(as in Figure S17) and performing a subtraction between
them, we can exclude the contribution from the perovskite
film self and obtain the kinetics for the ET from perovskite to
Figure 4. a) UPS spectra of PMPI, PEPI and PPPI. The UPS spectra of
the secondary electron edge and the valence bands are measured with
He-I radiation (21.2 eV). b) The schematic energy diagrams of PMPI,
PEPI, and PPPI with respect to the redox potentials of HI splitting
reaction. c) Surface potentials of the PMPI, PEPI and PPPI single
crystal under chopped 450 nm illumination. d) Interfacial ET kinetics
of the PMPI and PEPI and PPPI samples obtained by performing
a subtraction between XB kinetics with 2D perovskites and Pt/2D
perovskites and its single-exponential fit (green solid line).
and conduction bands of the 2D perovskites shift to a more
negative potential with the increase of the length of the
organic cations, they are all highly favourable for both proton
À
reduction and I oxidation reactions. Therefore, it is reason-
able to infer that the difference in the energy band positions
of the three 2D perovskites is not the main reason for the
different activities, which inspires us to probe other factors
that could influence the photocatalytic activity.
[15]
Pt nanoparticle for Pt/perovskite films, which is plotted in
Figure 4d. It should be noted that the interfacial charge
transfer (CT) between semiconductors and cocatalyst Pt are
almost universally multi-exponentials due to heterogeneities
in binding numbers and geometries. For Pt/PMPI and Pt/
PEPI samples, bi-exponentials are required to describe the
ET process, while only a single-exponential is needed to fit
the subtracted kinetics for Pt/PPPI sample. And the obtained
average ET time constants eventually are 70.7 Æ 2.5 ps,
116.0 Æ 3.0 ps and 562.2 Æ 41.6 ps for Pt/PMPI, Pt/PEPI and
Pt/PPPI sample, respectively (Table S3). Evidently, the inter-
facial ET rate of the 2D perovskites are much faster as
decreasing the length of perovskites organic cation, which
may be attributed to the strong electronic coupling between
2D perovskites and cocatalyst Pt for shorter organic cation at
the closer range, and is well consistent with the photocatalytic
hydrogen-evolution activity under light irradiation for the Pt/
perovskite sample in sequence, that is, PMPI > PEPI > PPPI.
Such remarkable ultrafast interfacial electron transfer from
PMPI after decreasing the cation length further indicates that
2D perovskite is a promising catalyst for charge transfer,
enhancing photocatalytic hydrogen-evolution activity under
light irradiation finally.
Kelvin probe force microscopy (KPFM) was then con-
ducted to measure the surface photovoltage on PMPI, PEPI
and PPPI under light irradiation. This can provide informa-
tion that can directly reflect the photoinduced charge
separation and migration on the surface of the photocata-
[14]
lyst.
In order to adapt to the sensitivity of KPFM, 2D
perovskites single crystals were used (method for crystal
growth is provided in the Supporting Information). Moreover,
the measurements were performed under chopped irradiation
with a wavelength of ca. 450 nm. As shown in Figure 4c, PPPI
single crystal exhibits a negligible surface potential changes
before and after light irradiation. This indicates that the
photogenerated electrons hardly arrive at the photocatalyst
surface. Compared with that of PPPI, the surface potential of
PEPI decreased by 59 mV upon light irradiation, indicating
that more photogenerated electrons migrate to the surface of
PEPI under the same conditions. Among the three samples,
PMPI exhibits the highest decrease in the surface potential
(
71 mV), indicating the most efficient charge separation or
migration on PMPI. Therefore, we conclude that the length of
the organic cations can significantly affect the charge
separation/migration efficiency of the 2D perovskite crystals
at the nanoscale domain. By decreasing the length of the
organic cations, more efficient charge separation/migration
will occur, which is the main reason for the increased
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380
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Angewandte
Research Articles
Chemie
Conclusion
[3] a) A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem.
Soc. 2009, 131, 6050 – 6051; b) L. N. Quan, B. P. Rand, R. H.
Friend, S. G. Mhaisalkar, T.-W. Lee, E. H. Sargent, Chem. Rev.
In summary, we realized the stabilization of 2D hybrid
perovskites in aqueous solution towards efficient and stable
photocatalytic H production under visible light irradiation
2
019, 119, 7444 – 7477.
[
4] a) H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A.
Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E.
Moser, M. Grätzel, N.-G. Park, Sci. Rep. 2012, 2, 591; b) M. M.
Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith,
Science 2012, 338, 643 – 647; c) M. Saliba, T. Matsui, K.
Domanski, J. Y. Seo, A. Ummadisingu, S. M. Zakeeruddin, J. P.
Correa-Baena, W. R. Tress, A. Abate, A. Hagfeldt, M. Gratzel,
Science 2016, 354, 206 – 209; d) N. J. Jeon, H. Na, E. H. Jung,
T. Y. Yang, Y. G. Lee, G. Kim, H. W. Shin, S. I. Seok, J. Lee, J.
Seo, Nat. Energy 2018, 3, 682 – 689; e) Q. Jiang, Y. Zhao, X.
Zhang, X. Yang, Y. Chen, Z. Chu, Q. Ye, X. Li, Z. Yin, J. You,
Nat. Photonics 2019, 13, 460 – 466; f) NREL, https://www.nrel.
gov/pv/assets/pdfs/cell-pv-eff-emergingpv.pdf, 2020.
2
for the first time. We find that the lengths of the organic
cations can influence the dissolution behavior, nanostructures
as well as the optoelectronic properties (band gap & band
edge positions) of these 2D perovskite crystals significantly.
More importantly, we demonstrated that the charge transfer
process on the 2D perovskites crystals and interfacial charge
transfer (CT) between 2D perovskites and Pt cocatalyst
increase with decreasing the length of perovskites organic
cations, which is the determining factor that influences the
photocatalytic activity of the 2D perovskites. As a conse-
quence, among the three 2D perovskites, phenylmethylam-
monium lead iodide with the shortest organic cations
exhibited superb charge separation and transportation effi-
[
[
5] a) S. Park, W. J. Chang, C. W. Lee, S. Park, H. Y. Ahn, K. T. Nam,
Nat. Energy 2017, 2, 16185; b) Y.-F. Xu, M.-Z. Yang, B.-X. Chen,
X.-D. Wang, H.-Y. Chen, D.-B. Kuang, C.-Y. Su, J. Am. Chem.
Soc. 2017, 139, 5660 – 5663.
6] a) Y. Wu, P. Wang, Z. Guan, J. Liu, Z. Wang, Z. Zheng, S. Jin, Y.
Dai, M.-H. Whangbo, B. Huang, ACS Catal. 2018, 8, 10349 –
10357; b) Y. Guo, G. Liu, Z. Li, Y. Lou, J. Chen, Y. Zhao, ACS
Sustainable Chem. Eng. 2019, 7, 15080 – 15085; c) H. Wang, X.
Wang, R. Chen, H. Zhang, X. Wang, J. Wang, J. Zhang, L. Mu, K.
Wu, F. Fan, X. Zong, C. Li, ACS Energy Lett. 2019, 4, 40 – 47.
7] Y. Q. Wu, P. Wang, X. L. Zhu, Q. Q. Zhang, Z. Y. Wang, Y. Y.
Liu, G. Z. Zou, Y. Dai, M. H. Whangbo, B. B. Huang, Adv.
Mater. 2018, 30, 1704342.
ciency at the nanoscale domain and demonstrated the highest
activity of 333 mmolh for H evolution when loading Pt as
the cocatalyst. This value is even higher than that achieved on
À1
2
3
D perovskites decorating with charge transfer motif, dem-
onstrating the great potential of using 2D perovskites as
photocatalysts for efficient and robust solar fuel production.
[
Acknowledgements
[8] X. Wang, H. Wang, H. Zhang, W. Yu, X. Wang, Y. Zhao, X.
Zong, C. Li, ACS Energy Lett. 2018, 3, 1159 – 1164.
[
9] a) H. Wang, X. Wang, H. Zhang, W. Ma, L. Wang, X. Zong,
Nano Energy 2020, 71, 104647; b) J. Chen, C. Dong, H. Idriss,
O. F. Mohammed, O. M. Bakr, Adv. Energy Mater. 2020, 10,
This work was financially supported by the National Natural
Science Foundation of China (No. 21872142), LiaoNing
Revitalization Talents Program (XLYC1807196), Strategic
Priority Research Program of Chinese Academy of Sciences
1
902433; c) W. Wang, M. O. TadØ, Z. Shao, Chem. Soc. Rev. 2015,
4, 5371 – 5408.
4
(
(
XDB17000000) and National Key R&D Program of China
No. 2017YFA0204804).
[10] a) L. Mao, C. C. Stoumpos, M. G. Kanatzidis, J. Am. Chem. Soc.
2019, 141, 1171 – 1190; b) Y. Fu, H. Zhu, J. Chen, M. P.
Hautzinger, X. Y. Zhu, S. Jin, Nat. Rev. Mater. 2019, 4, 169 – 188.
[
11] J. C. Blancon, H. Tsai, W. Nie, C. C. Stoumpos, L. Pedesseau, C.
Katan, M. Kepenekian, C. M. M. Soe, K. Appavoo, M. Y. Sfeir,
S. Tretiak, P. M. Ajayan, M. G. Kanatzidis, J. Even, J. J. Crochet,
A. D. Mohite, Science 2017, 355, 1288 – 1291.
Conflict of interest
The authors declare no conflict of interest.
[
12] a) H. Tsai, W. Nie, J.-C. Blancon, C. C. Stoumpos, R. Asadpour,
B. Harutyunyan, A. J. Neukirch, R. Verduzco, J. J. Crochet, S.
Tretiak, L. Pedesseau, J. Even, M. A. Alam, G. Gupta, J. Lou,
P. M. Ajayan, M. J. Bedzyk, M. G. Kanatzidis, A. D. Mohite,
Nature 2016, 536, 312; b) X. Zhang, X. Ren, B. Liu, R. Munir, X.
Zhu, D. Yang, J. Li, Y. Liu, D.-M. Smilgies, R. Li, Z. Yang, T. Niu,
X. Wang, A. Amassian, K. Zhao, S. Liu, Energy Environ. Sci.
Keywords: organic–inorganic hybrid perovskite ·
photocatalysis · solar energy · two-dimensional materials
[
[
1] a) T. Takata, J. Jiang, Y. Sakata, M. Nakabayashi, N. Shibata, V.
2
017, 10, 2095 – 2102; c) I. C. Smith, E. T. Hoke, D. Solis-Ibarra,
M. D. McGehee, H. I. Karunadasa, Angew. Chem. Int. Ed. 2014,
3, 11232 – 11235; Angew. Chem. 2014, 126, 11414 – 11417.
Nandal, K. Seki, T. Hisatomi, K. Domen, Nature 2020, 581, 411 –
4
14; b) C. M. Wolff, P. D. Frischmann, M. Schulze, B. J. Bohn, R.
5
Wein, P. Livadas, M. T. Carlson, F. Jäckel, J. Feldmann, F.
Würthner, J. K. Stolarczyk, Nat. Energy 2018, 3, 862 – 869;
c) M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q.
Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 2010, 110, 6446 – 6473;
d) A. J. Bard, M. A. Fox, Acc. Chem. Res. 1995, 28, 141 – 145.
2] a) Q. Wang, M. Nakabayashi, T. Hisatomi, S. Sun, S. Akiyama, Z.
Wang, Z. Pan, X. Xiao, T. Watanabe, T. Yamada, N. Shibata, T.
Takata, K. Domen, Nat. Mater. 2019, 18, 827 – 832; b) S. Ghosh,
A. Nakada, M. A. Springer, T. Kawaguchi, K. Suzuki, H. Kaji, I.
Baburin, A. Kuc, T. Heine, H. Suzuki, R. Abe, S. Seki, J. Am.
Chem. Soc. 2020, 142, 9752 – 9762; c) X. C. Wang, K. Maeda, A.
Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M.
Antonietti, Nat. Mater. 2009, 8, 76 – 80.
[13] M. E. Kamminga, H.-H. Fang, M. R. Filip, F. Giustino, J. Baas,
G. R. Blake, M. A. Loi, T. T. M. Palstra, Chem. Mater. 2016, 28,
4
554 – 4562.
[14] a) R. T. Chen, F. T. Fan, T. Dittrich, C. Li, Chem. Soc. Rev. 2018,
7, 8238 – 8262; b) R. Chen, S. Pang, H. An, J. Zhu, S. Ye, Y. Gao,
4
F. Fan, C. Li, Nat. Energy 2018, 3, 655 – 663.
[15] a) J. Wang, T. Ding, J. Leng, S. Jin, K. Wu, J. Phys. Chem. Lett.
2
018, 9, 3372 – 3377; b) X. Luo, G. J. Liang, J. H. Wang, X. Liu,
K. F. Wu, Chem. Sci. 2019, 10, 2459 – 2464.
Manuscript received: November 2, 2020
Version of record online: February 15, 2021
Angew. Chem. Int. Ed. 2021, 60, 7376 – 7381
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www.angewandte.org
7381