Heterojunction-Type Photocatalytic System Based on Inorganic Halide Perovskite CsPbBr3?

Article abstract of DOI:10.1002/cjoc.202000333

The heterojunction-type structure has shown significant merits in tuning the optical properties and carrier physics. Inspired by the excellent absorption capability of halide perovskite materials for visible light, a series of CsPbBr₃/TiO₂

Full text of DOI:10.1002/cjoc.202000333

Chinese Journal
of Chemistry
CJC
中 国 化 学 - An International Journal
Accepted Article
Title: Heterojunction-Type Photocatalytic System Based on Inorganic Halide
Perovskite CsPbBr3
Authors: Enbo Zhu, Yizhou Zhao, Yi Dai, Qiuhe Wang, Yuanyuan Dong, Qi Chen, and
Yujing Li*
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DOI: 10.1002/cjoc.201800215
Breaking Report
Heterojunction-Type Photocatalytic System Based on Inorganic
Halide Perovskite CsPbBr3
Enbo Zhua,‡, Yizhou Zhao a,b,‡, Yi Dai a, Qiuhe Wang a, Yuanyuan Dong a, Qi Chen a and Yujing Li *, a, b
aBeijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, Experimental Center of Advanced Materials,
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, P. R. China.
bBeijing Institute of Technology Chongqing Innovation Center, Chongqing, 401120, China.
‡ These authors contributed equally to this work.
Cite this paper: Chin. J. Chem. 2019, 37, XXX—XXX. DOI: 10.1002/cjoc.201900XXX
Summary of main observation and conclusion The heterojunction-type structure has shown significant merits in tuning the optical properties and
carrier physics. Inspired by the excellent absorption capability of halide perovskite materials for visible light, a series of CsPbBr3/TiO2 heterojunction-type
photocatalyst with various all-inorganic-perovskite/TiO2 ratios are successfully fabricated by the ligand-assisted reprecipitation (LARP) method. The
heterojunction structure extends the light absorption of TiO2 with good chemical and structural stability. A Pb-O interaction at the CsPbBr3/TiO2 interface
is observed by the XPS and confirmed by the TRPL showing improved interface carrier transport and stability. The heterojunction-type catalyst shows
drastically enhanced photocatalytic activity towards the direct oxidation of toluene with O2 molecule, as an important reaction for organic chemical
synthesis. The optimized sample shows an activity of 2356 μmol g-1h-1 at 75 °C, 4 times with the naked CsPbBr3 nanocrystals, and 3 times the bare TiO2.
Based on the investigation with trapping agents, a reaction mechanism is hence proposed suggesting that the photo-generated hole may be involved in
the rate-limiting step. This work demonstrates the potential of inorganic halide perovskite-based heterojunction structure for photocatalytic
applications.
The halide perovskite has been studied for the oxidation of
Background and Originality Content
The halide perovskite materials have gained considerable
attention for solar energy conversion since the first report of the
benzylic alcohols[26-27] and C(sp3)-H bond[28]. However, due to the
limited studies on perovskite-based photocatalysis, the successful
oxidation reactions are quite limited with limited understanding of
methylammonium lead iodide (MAPbI3) -sensitized solar cell in
reaction mechanisms such as the carrier transport and molecular
2009.[1] Due to their unique optoelectronic properties such as long
conversion process in the photocatalytic system. Therefore,
charge carrier lifetime[2], high absorption coefficient[3], high charge
improving the carrier separation efficiency and catalytic efficiency
is of high importance to the insightful understanding towards the
photocatalysis of perovskite material.
carrier mobility[4-5], and flexible band structure engineering[6-7]
along with their low processing cost[8], the halide perovskite
,
materials are promising candidates for highly efficient and cost-
competitive solar conversion devices.[9] More recently, it has been
reported that the halide perovskite materials show unique
activities in the photocatalytic conversion of multiple
chemicals.[10]
Herein, inspired by the excellent absorption of halide
perovskite materials, we report a series of CsPbBr3/TiO2 (CPB/Ti)
heterojunction-type catalysts with various ratios synthesized via
the ligand-assisted reprecipitation (LARP) method, and the
demonstrates their activity towards the photocatalytic oxidation
reaction of toluene. The CPB/Ti heterojunction-type structure is
confirmed by transmission electron microscopy (TEM) and X-ray
diffraction (XRD). The interaction at the interface of the two
components is studied by X-ray photoelectron spectroscopy (XPS).
It is verified through the time-resolved photoluminescence (TRPL)
that the formation of Ti-O-Pb at the interface improves the photo-
generated carrier transport and hence the photocatalytic
performance. By tuning the perovskite/TiO2 ratio, the
The all-inorganic CsPbBr3 perovskite quantum dots/graphene
oxide composite for photocatalytic CO2 reduction reaction was
reported by Kuang et al, as the first demonstration of inorganic
halide perovskite for photocatalytic application.[11] So far, in
addition to the CO2 reduction[12-13], some other reactions have
been achieved by photocatalysis with inorganic halide perovskite,
such as the degradation of organics [14-18], polymerization[19-20]
,
organic synthesis[21-24]. Selective oxidation reaction plays an
essential role in the chemical industry, whereby the photocatalytic
oxidation of organic molecules has drawn intensive interest.[25]
photocatalytic reaction rate of the direct conversion of toluene
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into benzaldehyde can be optimized with the highest activity 2356
μmol g-1h-1 achieved by the 6%CPB/Ti, more than 4 times that of
the CsPbBr3 nanocrystals (NCs) and 3 times the TiO2. The active
intermediate species and the reaction mechanism are studied by
using trapping agents. This work demonstrates the potential
application of CsPbBr3/TiO2 heterojunction for photocatalytic
organic synthesis.
Results and Discussion
Material structure and morphology
The LARP method has been widely adopted for the synthesis
of Pb-based perovskite NCs[29], we manage to fabricate CsPbBr3
perovskite NCs loaded on TiO2 by LARP method. By dissolving the
precursors in dimethylformamide (DMF) with the hexanoic acid
and octylamine as ligands, followed with the re-precipitation in
toluene, the obtained heterojunctions are protected with ligands.
As the perovskite NCs is precipitated with the existence of TiO2,
which involves heterogeneous nucleation on the TiO2 surface, the
obtained perovskite NCs show irregular shapes with sub-10 nm
size. The morphology and size distribution of 6%CsPbBr3/TiO2 and
other catalysts are characterized by TEM, as shown in Figure 1(a)
and S1 (SI). Figure S1(a) shows that P25-type TiO2 (Degussa)
possesses a size distribution in the range of 10-50 nm. In Figure
1(a), the CsPbBr3 NCs can be observed as black dots distributed on
the TiO2 surface. The average size of the monodispersed CsPbBr3
is 8.3 nm by measuring 50 randomly selected crystals. All CsPbBr3
NCs are attached on the surface of TiO2. The density of CsPbBr3
NCs increases with the CsPbBr3/TiO2 mass ratio (Figure S1(b-e))
but without an obvious increase in the size of CsPbBr3, enabling a
fair comparison that screens the size effect towards the
Figure 1 (a) HRTEM image of 6%CPB/Ti. The CsPbBr3 and TiO2 are
marked by cycles (dark blue) and squares (light blue). The FFT images with
lattice information for TiO2 and CsPbBr3 are also displayed in inserts. (b)
XRD patterns of 11%CPB/Ti and pure CsPbBr3, Rutile TiO2, Anatase TiO2. (c)
EDX elements mapping in the HADDF mode of 6%CPB/Ti.
By comparing the standard XRD patterns of the anatase phase
and the rutile phase TiO2, it can be concluded that the peaks (☆)
at 2θ = 25.3°, 27.1°, 37.8° and 48.0° in Figure 1(b) are
characteristic peaks of TiO2. The P25 TiO2 employed in this study
contains a 75% anatase phase and a 25% rutile phase by
integrating the corresponding XRD peaks. The peaks at 2θ = 15.2°,
21.2°, 30.7°, and 43.8°, correspond to the (001), (110), (002) and
(220) planes (*) of CsPbBr3, respectively, implying the monoclinic
phase of the CsPbBr3 synthesized. When increasing the amount of
CsPbBr3, e.g. 11%CPB/Ti, the diffraction peaks of both TiO2 and
CsPbBr3 can be observed clearly in the XRD pattern. The HRTEM
image indicates that the CsPbBr3 nanocrystals are strongly
attached to the surface of TiO2. The XRD patterns of 4%CPB/Ti,
8%CPB/Ti, 11%CPB/Ti are displayed in Figure S3. The peaks of
CsPbBr3 could barely be observed in 4%CPB/Ti due to the low
loading. The XRD patterns, along with the HRTEM, indicate that
CsPbBr3/TiO2 heterojunction-type catalyst has been successfully
obtained using the LARP method.
photocatalysis. The TEM image of pure CsPbBr3 nanocrystals is
shown in Figure S2 as a comparison.
The lattice of the CsPbBr3 NCs is further observed by high-
resolution transmission electron microscopy (HRTEM) as shown in
Figure 1(a). Two different lattice fringes, measured from the fast
Fourier transform (FFT) as insets in Figure 1(a), can be found for
the 6%CPB/TiO2 catalyst. The lattice spacing of 0.353 nm can be
ascribed to the (101) crystallographic plane of the anatase TiO2
that is the most thermodynamically stable phase.[30] The lattice
spacing of 0.294 nm corresponds to the (200) plane of CsPbBr3.
They form a physical contact at the interface, which enables an
efficient transfer of photo-generated carriers from CsPbBr3 to
TiO2. The lattice fringes of CsPbBr3 for other catalysts, e.g. the
11%CPB-Ti as shown in Figure S1(f) with better resolution, which
also agrees with the values calculated from XRD in Figure 1(b).
Besides, the high angle annular dark field (HAADF) image and
energy-dispersion X-ray (EDX) mapping images of different
elements of 6%CPB/Ti in selected areas (Figure 1c) indicate the
overlapping distribution of cesium, lead, and bromine on the TiO2
surface, and hence confirm the uniform loading of CsPbBr3 NCs on
TiO2.
Interface interaction and carrier physics
The X-ray photoelectron spectroscopy (XPS) is performed to
investigate the interactions between the CsPbBr3 NCs and TiO2.
The core-level XPS spectra of the Cs, Pb, and Br elements in
CsPbBr3, along with the Ti and O in TiO2 are measured. The
spectra of Cs 3d with binding energy peaks at 738.2 eV and 724.2
eV, Br 3d at 68.2 eV and 69.2 eV in CsPbBr3, Ti 2p at 464.4 eV and
458.5 eV in TiO2 for 6%CPB/Ti are shown in Figure S4-S6. The
binding energies of the above three elements do not show an
obvious shift upon the formation of CsPbBr3/TiO2 junction
compared with pure CsPbBr3 and TiO2. It is worth noting that the
Pb 4f peak shows a 0.4 eV shift to the higher binding energy after
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Chin. J. Chem.
loading to TiO2 compared to the pristine CsPbBr3 (Figure 2(a)).
Meanwhile, the O1s peak in TiO2 can be deconvoluted into two
sub-peaks possibly associating with the Ti-O and Ti-O-H
bonding.[31] Although the binding energy of the Ti-O bond (529.7
eV) remains unchanged, the Ti-O-H bond displays a shift of 0.2 eV
(from 531.18 eV to 530.98 eV) to lower binding energy (Figure
2(c)). It suggests the possible chemical interaction between the
perovskite and the top surface of TiO2: 1) the Cs+, Br-, and Ti4+ may
not be intensively involved in the interaction; 2) the chemical
interaction may originate mainly from the coordination between
Pb(II) and the -OH on the TiO2 surface. The halide vacancy on the
surface of perovskite exposes the Pb(II) that may form bonds to
the O2- in the -OH on TiO2, creating the Pb-O-Ti motif and hence
anchoring the perovskite NCs on the TiO2. The bonding between
perovskite and TiO2 may contribute to the improved charge
transfer at the interface.
CsPbBr3; τave is the overall average of τ1 and τ2; A1, A2 are the
contributing ratio of two different recombination pathway.[32]
The calculated carrier lifetimes are shown in Table 1 (CsPbBr3
NCs and 6%CPB/Ti) and S1 (4%CPB/Ti, 8%CPB/Ti, 11%CPB/Ti).
Compared with naked CsPbBr3 NCs, the τave of 6%CPB/Ti shows a
tremendous decrease from 13.46 to 2.64 ns. Besides, the fast time
decay component (τ1) shows a slight decrease from 1.95 to 1.11 ns
upon the contact with TiO2. This is possibly associated with the PL
quenching of CsPbBr3 by TiO2 through the electron transfer as a
result of the considerable interaction at the interface (also see
Figure S7 and Table S1, with data for other comparison catalysts in
SI). These data, along with the XPS results, confirm the unique
merits of TiO2 towards the formation of heterojunction with
CsPbBr3 NCs.
Table 1 The decay traces and the parameters of the CsPbBr3/TiO2
samples.
Sample
CsPbBr3 NCs
6%CPB/Ti
τ1 (ns)
1.95
A1 (%)
78.7
τ2 (ns)
18.06
4.09
A2 (%)
26.5
τave (ns)
13.46
2.64
1.11
77.7
22.3
The optical property and photocatalytic performance
The conduction band of TiO2 (~-4.0 eV vs.Vacuum) is slightly
lower than that of CsPbBr3 (~-3.4 eV vs.Vacuum), which may lead
to the formation of the Type II heterojunction.[33] As shown in
Figure 3(a), the ultraviolet-visible (UV-Vis) absorption of pure TiO2
starts at 420 nm, meaning that the light with a wavelength higher
than 420 nm cannot be absorbed. However, the heterojunction
structure greatly expands the light absorption range of the
catalysts. Upon the loading of perovskite NCs, the light absorption
edge is extended from 420 nm to 530 nm. Meanwhile, with the
increasing loading of CsPbBr3, the light absorption within 300-530
nm also dramatically increases (Figure S8), indicating that the
CsPbBr3 NCs serve as the main light absorption material in
heterogeneous junction. Figure S9 shows the converted (αhʋ)2 - Eg
plot where the α is the light absorption coefficient and Eg is the
photon energy. The absorption edge of the 6%CBP/Ti indicates an
optical band gap of 2.32 eV, compared with that of TiO2 at 3.2 eV.
Figure 2 (a) High-resolution XPS spectra of Pb 4f in CsPbBr3 and
6%CPB/Ti. (b) XPS spectra of O 1s in TiO2 and 6%CPB/Ti. (c) Enlarged XPS of
O 1s in OH from Figure. (d) TRPL of CsPbBr3 and 6%CPB/Ti.
The carrier dynamics is studied by the TRPL spectroscopy.
Carrier lifetime indicates the speed of recombination or quenching
of photo-generated free electrons/holes or the unseparated
electron-hole pairs (excitons) in the lattice. The TRPL is measured
with an excitation laser of wavelength 375 nm. The carrier
lifetimes are calculated by fitting the data with the following
equation based on a bi-exponential model:
For the PL measurement, the intensity of the PL peaks
represents the radiative recombination process in the materials.
For the Type II heterojunction, the photo-induced electrons can
be effectively injected into the material with a lower conduction
band edge. Hence, the radiative recombination would be
suppressed, leading to the reduced PL intensity. Specifically, the
photo-excited electrons of CsPbBr3 NCs will be transferred into
TiO2 that will greatly suppress the radiative recombination and the
fluorescence intensity. In Figure 3(b), compared with pristine
CsPbBr3, the PL intensity of 6%CPB/Ti is drastically weakened,
suggesting that the photo-excited electrons are efficiently
transferred to TiO2 instead of recombination in the bulk of
perovskite NCs (Also in Figure S10). The PL peak of CsPbBr3 NCs is
centered at 518 nm, consistent with the results in the
y=A1*exp(-t/τ1) + A2*exp(-t/τ2)
(1)
(2)
2
2
τ
ave=(A1*τ1 +A2*τ2 )/(A1*τ1+A2*τ2)
wherein the τ1 represents the attenuation lifetime of
excitations in CsPbBr3 NCs induced by carrier transfer to TiO2 (for
CsPbBr3/TiO2 hetero-junction) and the surface recombination on
CsPbBr3 NCs; τ2 represents the recombination within the bulk of
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literature.[34] Upon the loading on TiO2, the peak shows a slight
blue shift from 518 nm to 514 nm, probably due to the quantum
confinement effect triggered by the different sizes of the pure
CsPbBr3 (50 nm) and the CsPbBr3 attached on the TiO2 surface (10
nm).
production rate of benzyl alcohol at 372 μmol g-1h-1. The
selectivity of benzaldehyde of 6%CPB/Ti is calculated to be 82.9%.
For the CsPbBr3 loading less than 6%, the poor light absorption of
NCs limits the number of carriers, resulting in lower catalytic
efficiency. When the CsPbBr3 content is higher than 6%, the
catalytic efficiency starts to descend again. A possible explanation
is that the more DMF used to dissolve the precursors during the
synthesis will lead to a higher concentration of polar solvent in the
toluene and hence reduce the quality of as-synthesized NCs,
resulting in a lower carrier separation efficiency. The product
selectivities of the CPB/Ti catalysts are all close to 82% with no
significant change, indicating that heterojunction-type catalysts
could effectively oxidize toluene to benzaldehyde with high
selectivity.
As can be seen from Figure 3(d) (for 6%CPB/Ti), with the
elevated reaction temperature, the production rate of
benzaldehyde gradually increased from 1089 μmol g-1h-1 at 45 °C
to 2356 μmol g-1h-1 at 75 °C, with the production rate of benzyl
alcohol also increasing more than 2 times from 205 to 578 μmol g-
1h-1. The selectivity of benzaldehyde slightly decreases from 84.2%
at 45 °C to 80.3% at 75 °C. The 11%CPB/Ti shows a similar
temperature dependence with benzaldehyde production rate
rising from 620 μmol g-1h-1 at 45 °C to 2026 μmol g-1h-1 at 75 °C
(Figure S11). The production rate of benzyl alcohol also rises from
128 to 591 μmol g-1h-1, within the same temperature span. The
selectivity of benzaldehyde decreases from 82.9 % at 45 °C to
77.4% at 75 °C. In summary, with the increase in temperature, the
production rates of benzyl alcohol and benzaldehyde also
increase. The comparison experiments with 4 other conditions are
carried out to confirm the photocatalytic effect of CPB/Ti (Table
S2): (a) without O2; (b) without light radiation; (c) without
photocatalyst; (d) using only PbBr2. The results verify that the
reactions are catalyzed by the target materials under visible light.
Figure 3 (a) The UV-Vis absorption spectra of TiO2 and 6%CPB/Ti. (b) The
PL spectra of pristine CsPbBr3 NCs and 6%CPB/Ti. (c) The photocatalytic
efficiencies of different catalysts at 60 °C. (d) The photocatalytic
efficiencies of 6%CPB/Ti at varied temperatures.
In a short conclusion, with the formation of Type II
heterojunction between the CsPbBr3 NCs and TiO2, the perovskite
NCs can improve the light absorption of TiO2. The decreasing
steady-state fluorescence intensity reflects the efficient
separation of photo-generated carrier pairs and the charge
transfer by heterojunction structure.
Photocatalytic mechanism
The light-induced electrons and holes are known to possess
reduction and oxidation capabilities, respectively. They can
capture oxygen and water molecules in the environment and
The photocatalytic activities of the catalysts towards the
oxidation of toluene are evaluated under the visible light
irradiation (in a multichannel photochemical reaction device by
Beijing Perfectlight Technology). A total of 15 mg catalyst is
dispersed in 5 mL toluene which is then saturated by O2 and
sealed for reactions carried out at 60 °C (see SI for details). Figure
3(c) shows the production rates of benzaldehyde and benzyl
alcohol from the oxidation of toluene with different catalysts. The
pristine P25 TiO2 and CsPbBr3 NCs show much lower catalytic
efficiencies than heterojunction structure. Due to the wide
bandgap, the benzaldehyde production rate from TiO2 is only 653
μmol g -1h-1. As for the CsPbBr3 NCs, although they possess a
smaller bandgap and more efficient carrier generation, there is a
severe problem with the carrier separation due to the quantum
confinement and hence the strong radiative recombination.
Eventually, the efficiency of the heterojunction-type catalyst is
higher than both components. With the increment of CsPbBr3, the
photocatalytic efficiency first rises and then descends, wherein
the highest catalytic efficiency is obtained from the 6%CPB/Ti
catalyst. The production rate of benzaldehyde is measured to be
1849 μmol g-1h-1, almost 3 times that of TiO2, along with the
-
generate superoxide radicals (∙O2 ) and hydroxyl radicals (∙OH),
which may both take part in the photocatalytic oxidation reaction.
Therefore, 4 different chemicals ammonium oxalate (AO),
potassium persulfate (PP), 1, 4-benzoquinone (BQ), and tert-
butanol (TPA) are selected as trapping agents to capture the
-
possible photo-generated holes, electrons, ∙O2 and ∙OH,
respectively.
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Conclusions
In summary, the CsPbBr3/TiO2 heterojunctions with different
ratios are successfully synthesized via the LARP method. Under
light illumination, the CsPbBr3/TiO2 materials show higher
photocatalytic activity for the oxidation of toluene into
benzaldehyde than the naked CsPbBr3 and TiO2 due to the
improved optical absorption and the reduced recombination of
photo-generated electron-hole pairs, along with high selectivity.
The optimal 6%CPB/Ti sample shows the activity of 2356 μmol g-
1h-1 at 75 °C, more than 4 times that of the CsPbBr3 NCs, and 3
times that of the TiO2. The XPS result suggests that the Ti-O-Pb
coupling at the interface of the two components improves the
transport of charge carriers. Furthermore, this study presents a
strategy to construct a highly efficient heterojunction-type
photocatalytic system based on inorganic halide perovskite
materials.
Figure 4 (a) The photocatalytic efficiency of the different trappers. (b)
The schematic plot of the heterojunction and the reaction mechanism.
As shown in Figure 4(a), all the catalytic efficiencies reduce
after the addition of four sacrificial agents. Upon the addition of
AO, the production rate drastically decreases, indicating the
participation of the holes in the rate-limiting step in the whole
reaction. For the PP and BQ, they also decrease the production
-
rates but to a less extent, indicating that the electrons and ∙O2
also participate in the photocatalytic reaction. However, as the
TBA is added, the efficiency only slightly decreases, meaning that
the ∙OH is not involved in the reaction. Based on the above
experiments, it can be implied that electrons, holes, and
superoxide free radicals participate in the reaction among which
the holes may play a crucial role. Therefore, the reaction
mechanism is speculated as follows:
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
Acknowledgement
CsPbBr3/TiO2+hʋ→CsPbBr3/TiO2+h++e-
(3)
This work is financially supported by the National Natural
Science Foundation of China (21975028, 51673025), National Key
Research and Development Program of China Grant
O2 +e-→∙O2
(4)
(5)
-
(2016YFB0700700), and the start-up funding of BIT.
C6H5CH3 + h+ → C6H5CH2∙ + H+
-
C6H5CH2∙+ ∙O2 → C6H5CHO + OH-
(6)
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C6H5CH2∙+ O2 → C6H5CH2OO∙
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(7)
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(8)
(9)
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Manuscript accepted: XXXX, 2019
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Chin. J. Chem. 2019, 37, XXX－XXX
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Heterojunction-Type Photocatalytic System
Based on Inorganic Halide Perovskite CsPbBr3
Enbo Zhu, Yizhou Zhao, Yi Dai, Qiuhe Wang,
Yuanyuan Dong, Qi Chen and Yujing Li*
CsPbBr3/TiO2 heterojunctions photocatalyst with activity 4 times that of the CsPbBr3 NCs, and 3
times that of the TiO2
www.cjc.wiley-vch.de
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2019, 37, XXX－XXX
This article is protected by copyright. All rights reserved.