Stable and Highly Efficient Photocatalysis with Lead-Free Double-Perovskite of Cs2AgBiBr6

Article abstract of DOI:10.1002/anie.201900658

Composition engineering of halide perovskite allows the tunability of the band gap over a wide range so that photons can be effectively harvested, an aspect that is of critical importance for increasing the efficiency of photocatalysis under sunlight. However, the poor stability and the low photocatalytic activity of halide perovskites prevent use of these defect-tolerant materials in wide applications involving photocatalysis. Here, an alcohol-based photocatalytic system for dye degradation demonstrated high stability through the use of double perovskite of Cs₂AgBiBr₆. The reaction rate on Cs₂AgBiBr₆ is comparable to that on CdS, a model inorganic semiconductor photocatalyst. The fact of fast reaction between free radicals and dye molecules indicates the unique catalytic properties of the Cs₂AgBiBr₆ surface. Deposition of metal clusters onto Cs₂AgBiBr₆ effectively enhances the photocatalytic activity. Although the stability (five consecutive photocatalytic cycles without obvious decrease of efficiency) requires further improvements, the results indicate the significant potential of Cs₂AgBiBr₆-based photocatalysis.

Full text of DOI:10.1002/anie.201900658

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Accepted Article
Title: Stable and Highly Efficient Photocatalysis with Lead-Free
Double-Perovskite of Cs2AgBiBr6
Authors: Zhenzhen Zhang, Yongqi Liang, Hanlin Huang, Xingyi Liu, Qi
Li, Langxing Chen, and Dongsheng Xu
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201900658
Angew. Chem. 10.1002/ange.201900658
Link to VoR: http://dx.doi.org/10.1002/anie.201900658
http://dx.doi.org/10.1002/ange.201900658

10.1002/anie.201900658
Angewandte Chemie International Edition
COMMUNICATION
Stable and Highly Efficient Photocatalysis with Lead-Free Double-
Perovskite of Cs2AgBiBr6
Zhenzhen Zhanga, b, Yongqi Liangb, Hanlin Huangb, Xingyi Liub, Qi Lib, Langxing Chena*, Dongsheng
Xub*
Abstract: Composition engineering of halide perovskite allows the
tunability of the bandgap over a wide range so that photons can be
effectively harvested, which is of critical importance for increasing the
efficiency of photocatalysis under sunlight. However, the poor stability
and the low photocatalytic activity of halide perovskites are preventing
these defect-tolerant materials from wide applications in
photocatalysis. Here, an alcohol based photocatalytic system for dye
degradation with high stability is demonstrated for double perovskite
of Cs2AgBiBr6. The reaction rate on Cs2AgBiBr6 is comparable to that
on CdS, a model inorganic semiconductor photocatalyst. The fact of
fast reaction between free radical and dye molecules indicates the
unique catalytic properties of Cs2AgBiBr6 surface. Deposition of metal
clusters onto Cs2AgBiBr6 effectively further enhances the
photocatalytic activity. Although the stability (five consecutive
photocatalytic cycles without obvious decrease of efficiency) requires
further improvements, our results indicate the significant potential of
Cs2AgBiBr6 based photocatalysis.
these problems, lead-free perovskite materials with high ambient
stability are turned to[13]. Bi-based Cs2AgBiBr6 double perovskite
was found to exhibit reasonably high optical absorption coefficient
and stability under suitable conditions.[14] Recently, photocatalytic
reduction of CO2 with Cs2AgBiBr6 was reported though only low
efficiency was shown for Cs2AgBiBr6.[15]
Herein, we successfully develop an alcohol-based
photocatalytic system to achieve dye degradation by using
Cs2AgBiBr6 to harvest the visible light. It is found that Cs2AgBiBr6
preserves its high chemical stability in ethanol during
photocatalytic processes. At the same time, no obvious
photocatalytic oxidation of the ethanol solvent is observed in the
presence of Cs2AgBiBr6. The photocatalytic degradation process
of four different dyes reveals that the dye molecules undergo
complete mineralization process and the main active specie is
-
superoxide radical (·O2 ). In contrast to the pseudo-first-order
reaction kinetics found for CdS, the degradation process exhibits
a pseudo-zeroth-order reaction kinetics for Cs2AgBiBr6. This
implies that the photo-initiation and surface catalysis processes
on Cs2AgBiBr6 play dominant roles in the photocatalytic
degradation of dyes. Furthermore, the deposition of Pt or Au
facilitates the transfer of photogenerated electrons from
Cs2AgBiBr6 to metal centers, and the photocatalytic degradation
rate increases about two times in the case of Rhodamine B
degradation, which exceeds the degradation rate of Rhodamine
B on the photocatalyst of CdS under the same condition. Finally,
Cs2AgBiBr6 shows good stability with no obvious decrease of
photodegradation efficiency in five runs of photocatalytic reaction.
The synthesized Cs2AgBiBr6 particles are of several
micrometers in size (Figure S1A) and cubic double perovskite
structure (Figure S1B) [14, 16]. Powdered samples show a weak
absorption region between 600 nm to 700 nm and a sharp
increase between 500 nm to 600 nm in UV−Vis absorption
spectrum (Figure S1C). The Tauc plot (Figure S1D) assuming an
indirect transition shows a bandgap of 1.89 eV and 2.08 eV
occurring with absorption and emission of a phonon, respectively
Lead halide perovskites have demonstrated great potential for
optoelectronic applications such as solar cells[1], light emitting
diodes (LEDs)[2], lasers[3], photoelectrodes[4], and optical
sensors[5] due to their high extinction coefficients, optimal band
gaps, high photoluminescence quantum yields and long
electron/hole diffusion lengths[6]. Specifically, the power
conversion efficiency of lead-based halide perovskite based solar
cells has steadily risen from 3.8%[7] to 23.3%[8]. The electronic
band structure (conduction band edge and valence band edge
positions) of halide perovskite materials also promises for efficient
photocatalytic applications. Recently, photocatalytic hydrogen
evolution[9], photocatalytic reduction of CO2[10] and photocatalytic
[11]
organic synthesis
have been reported for lead halide
perovskites. However, due to the instability in aqueous solution,
the hydrogen evolution reaction must be conducted inside
oversaturated MAPbI3 solution to suppress the decomposition of
perovskite.[9]. Besides, the proverbial toxicity of lead is a serious
concern of this material for large-scale applications.[12] To address
[14]
.
The as-prepared Cs2AgBiBr6 was studied as photocatalyst for
the degradation of organic dyes including Rhodamine B (RhB),
Rhodamine 110 (Rh110), Methyl red (MR) and Methyl orange
(MO) under visible light irradiation (λ > 420 nm). As shown in
Figure 1 and FigureS2, the characteristic absorption peaks of RhB
(λmax = 552 nm, Figure 1A), Rh110 (λmax = 509 nm, Figure S2A),
MR (λmax = 493 nm, Figure S2B) and MO (λmax = 415 nm, Figure
S2C) decrease gradually with increasing the irradiation time with
the presence of Cs2AgBiBr6 photocatalyst. For RhB, the
degradation is almost complete (~98%) upon irradiation over a
period of continuous 120 minutes. For the other dyes of Rh110,
MR and MO, the degradation processes are slower than that of
[a]
[b]
Z. Zhang, Prof. L. Chen
Research Center for Analytical Sciences, College of Chemistry,
Nankai University
Tianjin 300071, China
E-mail: lxchen@nankai.edu.cn
Y. Liang, H. Huang, X. Liu, Q. Li, Prof. D. Xu
Beijing National Laboratory for Molecular Sciences, State Key
Laboratory for Structural Chemistry of Unstable and Stable Species,
College of Chemistry and Molecular Engineering, Peking University
Beijing 100871, China
E-mail:dsxu@pku.edu.cn
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10.1002/anie.201900658
Angewandte Chemie International Edition
COMMUNICATION
RhB. Interestingly, the C/C0 shows a linear dependence on the
irradiation time, which indicates a zeroth-order dynamic for
degradation. As shown in Figure 1 B, the data fit well with pseudo-
zeroth-order reaction model (R2 larger than 0.99), which is deviant
from the reported pseudo-first-order reaction model.[17] Previous
literatures attributed the pseudo-zeroth-order kinetics to the large
adsorption equilibrium constant of dyes onto the catalyst,
Figure S3A). As shown in the inset of Figure 1C, the reaction rate
is linearly dependent on the light intensity. For comparison, CdS
was adopted for photocatalytic degradation of RhB. In
accordance with previous reports,[19] pseudo-first-order reaction
model fits well with the data on CdS (Figure S3 B and C). The
difference of reaction kinetics between Cs2AgBiBr6 and CdS may
originate from the difference of surface catalytic sites between the
two semi-conductors. It is possible that the unique surface
structure related to Ag or Bi sites on Cs2AgBiBr6 can facilitate the
activation of dye or O2 molecules and accelerate the reaction rate
according to the photodegradation kinetic equation below[18]
:
C
ln( 0 )  Ka (C0 C)  Kapp
t
（1）
C
between dye molecules and active species[20]
.
where C0 is the initial concentration of degradable dye; C is the
concentration of degradable dye at different reaction time t; Ka
and Kapp are the adsorption equilibrium constant and apparent
rate constant, respectively.
When Ka (C0 – C) ≥ 10, the equation can be simplified to Ka (C0
– C) = Kappt, resulting in pseudo-zeroth-order kinetics.
The RhB degradation rate under monochromatic light with
different wavelength was also investigated. Different degradation
rates are detected for different wavelengths and are plotted
together with the UV-Vis absorption spectrum of the Cs2AgBiBr6
particles (Figure 1D). The change of k0 at various wavelengths
corresponds well to the UV-Vis absorption spectrum of
Cs2AgBiBr6. This suggests that the dye degradation reaction is
mainly caused by the carriers generated inside Cs2AgBiBr6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(B)
(A)
RhB
Rh110
MR
100
80
60
40
20
0
MO
0 min
instead of by a dye-sensitized process[21]
.
(B) -0.20
(A)
100
120 min
283 K
301 K
313.5 K
-0.25
80
60
40
20
0
-0.30
300
400
500
600
700
800
0
100
200
300
400
500
-0.35
Wavelength (nm)
Time (min)
0.4
(C)
1.0
(D)
100
80
60
40
20
0
-0.40
0.8
0.6
0.3
0.2
0.1
0.0
-0.45
0
20
40
60
80
100 120
3.2
3.3
3.4
3.5
0.4
Time (min)
1/T (1000)
0.8
0.6
0.4
0.2
0.2
(C)
(D)
4 min
1.0
0.8
0.6
0.4
0.2
0.0
0.0
R2=0.993
TEMPO
-0.2
-0.4
40
60
80
100
Light intensity (mW cm-2
)
2 min
0 min
dark
-20
0
20
40
60
80 100 120
300
400
500
600
700
800
Time (min)
Wavelength (nm)
Figure 1. (A) UV-vis spectra of RhB at different irradiation time (20, 40, 60, 80,
100, 120 min) in the presence of Cs2AgBiBr6 (inset: digital photographs of RhB
at different irradiation time). (B) Kinetic curves of the RhB (pink curve), Rh110
(green curve), MR (red curve) and MO (orange curve) degradation fitted using
pseudo-zeroth-order reaction model. (C) Plots of C/C0 versus the irradiation
time for the photodegradation of RhB on Cs2AgBiBr6 under different light
intensities (black: 33 mW cm-2; red: 58 mW cm-2; blue: 79 mW cm-2; pink: 93
mW cm-2; orange : 110 mW cm-2) (inset: linearly fitting between reaction rate
and light intensity). (D) Absorbance spectrum of Cs2AgBiBr6 powder and the
normalized k0 at different monochromatic incident light.
0
20
40
60
80
100
317.3
317.4
317.5
317.6
Time (min)
Magnetic field (G)
Figure 2. (A) Kinetic curves of the RhB degradation at different
temperatures. (B) Arrhenius plot of ln (k0) versus (1/T) for the photocatalytic
degradation of RhB. The apparent activation energy Ea is estimated to be
5.0 kJ mol-1. (C) Photocatalytic behavior with the adding of TEMPO during
the illumination. (D) The DMPO spin-trapping ESR spectra of Cs2AgBiBr6 for
·O 2- under different visible light irradiation time.
To evaluate the interaction between Cs2AgBiBr6 and the four
kinds of ionic dyes (RhB and Rh110: cationic dye; MO: anionic
dye; MR: neutral dye), zeta potential of Cs2AgBiBr6 was measured.
The zeta potential value of -27 mV suggests that Cs2AgBiBr6
surface is negatively charged, which will favor the adsorption of
cationic dyes and anionic or neutral molecules like MR and MO
should only be weakly adsorbed onto Cs2AgBiBr6. However, the
pseudo-zeroth-order kinetic data are also observed for both MR
and MO degradation. This indicates that the large adsorption
equilibrium constant of dyes onto Cs2AgBiBr6 could not be the
main factor accounting for the zeroth-order kinetics.
To estimate the activation energy (Ea) of the RhB degradation,
reactions under different temperature were carried out in the
temperature range of 10-40.5 °C using a water bath. The obtained
data are presented in Figure 2A. As the temperature increases,
k0 increases slowly. From Fig 2B, it can be seen that ln (k0)
depends linearly with 1/T, suggesting that the temperature
dependence of the reaction follows Arrhenius equation:
Ea
(2)
ln k  ln A
RT
where A is the pre-exponential factor, Ea is the activation energy,
and R is the molar gas constant. Ea is estimated to be 5.0 kJ mol-
1. The relative small value of Ea implies the photocatalytic
degradation of RhB may be a free radical based reaction, which
The photocatalytic reactions under various light intensities
were carried out. For Cs2AgBiBr6, variation of light intensity does
not change the pseudo-zeroth-order kinetics (Figure 1C and
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10.1002/anie.201900658
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COMMUNICATION
(B)
(A)
Cs2AgBiBr6
14
12
10
8
1.0
0.8
0.6
0.4
0.2
0.0
is smaller than a typical activation energy needed for a reaction
when direct breaking of chemical bonds is involved.
Cs2AgBiBr6-Pt
Cs2AgBiBr6-Au
Different kinds of active species including electrons (e-), holes
Cs2AgBiBr6
-
(h+), hydroxyl radicals (·OH) and superoxide (·O2 ) might be
Cs2AgBiBr6-Pt
Cs2AgBiBr6-Au
6
involved during the photocatalytic process.[19] Experiments were
carried out to identify the active species that are accounting for
the photocatalytic process for dye degradation. As shown in
Figure 2C, the photocatalytic degradation of dyes gets almost
completely blocked after 2, 2’, 6, 6’-tetramethylpiperidine-1-oxyl
(TEMPO) was added to the solution. As TEMPO is a well-
documented free radical scavenger[22], this indicates that the
photocatalytic degradation of dyes is a free radical-based process.
The free radical species accounting for the photocatalytic
degradation were further identified through ESR technique. By
using 5, 5’-dimethyl-1-pyroline-N-oxide (DMPO) as a spin trap
during ESR measurements, six characteristic peaks upon visible
light irradiation with the existence of Cs2AgBiBr6 proves the
existence of ·O2− (Figure 2D).
4
2
0
0
20
40
60
80
100 120
0
40
80
120
160
200
Time (min)
Time (s)
103
102
101
(D)
(C)
Cs2AgBiBr6
Cs2AgBiBr6-Pt
Cs2AgBiBr6-Au
Cs2AgBiBr6
Cs2AgBiBr6-Pt
Cs2AgBiBr6-Au
450 500 550 600 650 700 750 800
Wavelength (nm)
200
400
600
800
Time (ns)
1H NMR, HPLC and GC analyses were carried out to examine
the products after the photocatalytic process. As shown in Figure
S4, the characteristic peaks of the H in benzene rings of the dye
molecules (RhB and Rh110) between 6.5 ppm and 8.5 ppm
disappear after photocatalytic reaction, indicating the
decomposition of benzene rings. HPLC analysis also confirms the
decomposition of benzene ring. As can be seen in Figure S5A,
characteristic HPLC peak of RhB at tR= 5.8 min disappears after
illumination and no obvious new HPLC peaks are observed. To
further confirm the final products, we detected the gas products in
O2 atmosphere by GC. From Figure S5B, it can be seen that after
photocatalysis, new peaks which can be attributed to CO2 and CO
appear. All these results prove that RhB and Rh110 are
completely mineralized after the photocatalytic process. Thus, we
propose a free radical reaction process as follows:
Figure 3. (A) Plots of C/C0 versus the irradiation time for the photodegradation
of RhB in the presence of Cs2AgBiBr6, Cs2AgBiBr6-Pt and Cs2AgBiBr6-Au; (B)
Photocurrent response of Cs2AgBiBr6, Cs2AgBiBr6-Pt and Cs2AgBiBr6-Au; (C)
PL emission spectra of Cs2AgBiBr6, Cs2AgBiBr6-Pt and Cs2AgBiBr6-Au; (D)
Time-resolved PL of Cs2AgBiBr6, Cs2AgBiBr6-Pt and Cs2AgBiBr6-Au and their
fitting curves using three-exponential decay kinetics
To further increase photocatalytic efficiency, deposition of
noble metals on Cs2AgBiBr6 was done to promote
photogenerated carrier separation. Figure S6 shows the EDX
spectra of Cs2AgBiBr6-Pt and Cs2AgBiBr6-Au and the existence of
Pt or Au proves the successful loading of noble metals. Powder
XRD patterns of Cs2AgBiBr6-Pt and Cs2AgBiBr6-Au (Figure S7)
show no obvious new peaks, which may be ascribed to the small
sizes of Pt and Au particles and their relatively low amount of
loading. For RhB degradation, both Cs2AgBiBr6-Pt and
Cs2AgBiBr6-Au show faster degradation rates than pure
Cs2AgBiBr6 (Figure 3A). This implies that the separation of
photogenerated electron-hole pairs can be facilitated through
either Pt or Au deposition. To verify this hypothesis, photocurrent
and PL experiments were done. In Figure 3B, it can be seen that
both the Cs2AgBiBr6 and metal deposited Cs2AgBiBr6 electrodes
exhibit repeatable photocurrent response to light on−off cycles
under a bias of 0 V v.s. Ag/AgCl. Cs2AgBiBr6-Pt based electrode
exhibits the highest photocurrent response, which is consistent
with the photocatalytic efficiency on RhB degradation. From
Figure 3C, it can be seen that the PL emission of Cs2AgBiBr6 is
almost completely quenched after deposition of Pt and Au. To
understand the charge carrier recombination processes, time-
resolved PL decay traces were collected (Figure 3D). The PL
decay of Cs2AgBiBr6 can be fitted with a three-exponential decay
kinetics, presenting an apparent fast initial drop from multiple
recombination centers in the Cs2AgBiBr6 powder and a slow
decay (Table S1). The average lifetime of Cs2AgBiBr6 is 3.18 ns.
After deposition of Pt and Au, the average lifetime decreases to
0.71 ns and 1.52 ns and τ3 decreases from 234.3 ns to 72.4 ns
and 92.0 ns, respectively. These observations indicate the
appearance of non-radiative pathway, as well as an interfacial
charge transfer process. All these experiments demonstrate that
the deposition of Pt and Au promotes the carrier separation.
Cs2AgBiBr6 + dyes → *dyes
(3)
Cs2AgBiBr6+ hν → h++ e-
(4)
(5)
(6)
O2 + e- → ·O−2
·O−2 + *dyes → mineralization products
Where *dyes means activated dye molecules by Cs2AgBiBr6.
First, Cs2AgBiBr6 activates dyes during adsorption-desorption
equilibrium process. When Cs2AgBiBr6 is irradiated under visible
light (the energy of which was higher than the band gap of
Cs2AgBiBr6), the absorption of photos leads to the excitation of
elecrons from the valence band (VB) to the conduction band (CB),
producing photogenerated electrons (e-) and holes (h+). The
-
electrons are captured fast by O2 molecules, generating ·O2
-
radicals. Then, ·O2 react with activated dyes to mineralization
products. As the dye molecules are activated, degradation
process in equation (6) becomes fast and makes photo-initiation
process in equations (4) and (5) relatively slow. Thus, photo-
initiation process acts as rate-controlling step in the whole
photocatalytic degradation reaction and the reaction rate is
independent on the dye concentration. As a result, the whole
reaction should exhibit a pseudo-zeroth-order kinetics.
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10.1002/anie.201900658
Angewandte Chemie International Edition
COMMUNICATION
(A)
(B)
Cs2AgBiBr6-Pt before photocatalysis
1st run
2nd run 3rd run 4th run 5th run
1.0
0.8
0.6
0.4
0.2
0.0
China (grant No. 21821004).We are grateful to Dr. Yan Guan
(college of chemistry and molecular engineering, Peking
University) for her assistance on PL measurement.
Cs2AgBiBr6-Pt after 1 run of photocatalysis
Cs2AgBiBr6-Pt after 5 runs of photocatalysis
Keywords: Lead-free perovskite • Highly efficient photocatalysis

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0
60
120
180
240
300
10
15
20
25
30
35
40
45
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Time (min)
2 Theta (degree)
Figure 4. (A) Recycling test of Cs2AgBiBr6-Pt under visible light irradiation in
photocatalytic degradation of RhB. (B) Powder XRD patterns of Cs2AgBiBr6-Pt
before and after photocatalytic degradation of RhB.
Cycling experiments of the RhB photodegradation using
Cs2AgBiBr6 and Cs2AgBiBr6-Pt as the photocatalysts were carried
out to test their stability. As shown in Figure 4A and Figure S8A,
no obvious decline of photocatalytic efficiency for Cs2AgBiBr6 and
Cs2AgBiBr6-Pt is observed after five cycles of reaction. From
Figure S9, it can be seen that the optical absorption properties of
Cs2AgBiBr6 are similar before and after five cycles of
photocatalysis for RhB degradation. We also note the four-hour
heating of the Cs2AgBiBr6 in the air up to 240°C does not
deteriorate the performance during the following cycle for
photocatalytic dye degradation (Figure S8C), which clearly
indicates the robustness of the perovskite as the photocatalyst.
Only after five photocatalytic cycles, small peaks at 30.9° and
44.2° in the XRD spectra (Figure 4B and Figure S8B), which are
assigned to AgBr, start to appear. A slight shift in the positive
direction is observed for the binding energy of Bi 4f in the XPS
spectra (Figure S10C). The slight shift may also originate from the
existence of AgBr, which might lead to the change of chemical
environment of Bi. We conclude that the reaction products such
as water molecules might lead to partial dissolution of Cs2AgBiBr6.
Though the amount of Cs2AgBiBr6 dissolved increases as the
photocatalysts undergo further cycling, which results in the
decrease of photocatalytic efficiency, stability of five cycles of
photocatalysis represents the best performance ever reported for
Cs2AgBiBr6.
In conclusion, an alcohol based photocatalytic system was
successfully developed for halide perovskites, Cs2AgBiBr6 for this
specific case. Different from the classical pseudo-first-order
reaction kinetics, our results of pseudo-zeroth-order kinetics
indicate that the surface structure of Cs2AgBiBr6 also function as
the reaction sites for photocatalysis. Moreover, deposition of
noble metals on Cs2AgBiBr6 further improves the reaction rate
significantly. The perovskite of Cs2AgBiBr6 has huge potentials for
thermocatalysis and photocatalysis and it can be expected that
the utilization of Cs2AgBiBr6 for photocatalysis will provide new
insights for catalytic behavior at the surface of perovskites.
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Acknowledgements
This work was supported by the Ministry of Science and
Technology of China (973 project, grant Nos. 2013CB932601 and
2014CB239303) and the National Natural Science Foundation of
X. Lang, J. Zhao, Chem. Asian J. 2018, 13, 599-613.
This article is protected by copyright. All rights reserved.

10.1002/anie.201900658
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A novel alcohol-based photocatalytic
system using lead-free double
perovskite of Cs2AgBiBr6 as the
photocatalyst was developed. Stable
and highly efficient photocatlaytic
degradation of dyes has been
demonstrated.
Zhenzhen Zhanga, b, Yongqi Liangb,
Hanlin Huangb, Xingyi Liub, Qi Lib,
Langxing Chena*, Dongsheng Xub*
Page No. – Page No.
Stable and Highly Efficient
Photocatalysis with Lead-Free
Double-Perovskite of Cs2AgBiBr6
This article is protected by copyright. All rights reserved.