Photoinduced Anion Exchange in Cesium Lead Halide Perovskite Nanocrystals

Article abstract of DOI:10.1021/jacs.7b01480

Cesium lead halide (CsPbX₃) perovskite nanocrystals (NCs) possess the unique capability of post-synthesis anion exchange providing facile tunability of the optical properties, which is usually achieved by mixing NCs with reactive anion precursors. In this work, we show that the controllable anion exchange can be achieved in a dihalomethane solution of CsPbX₃ NC in the absence of any spontaneously reacting anion source using photoexcitation of CsPbX₃ NCs as the triggering mechanism for the halide ion exchange. The reaction begins with the photoinduced electron transfer from CsPbX₃ NCs to dihalomethane solvent molecules producing halide ions via reductive dissociation, which is followed by anion exchange. The reaction proceeds only in the presence of excitation light and the rate and extent of reaction can be controlled by varying the light intensity. Furthermore, the asymptotic extent of reaction under continuous excitation can be controlled by varying the wavelength of light that self-limits the reaction when light becomes off-resonance with the absorption of NCs. The light-controlled anion exchange demonstrated here can be utilized to pattern the post-synthesis chemical transformation of CsPbX₃ NCs, not readily achievable using typical methods of anion exchange.

Full text of DOI:10.1021/jacs.7b01480

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Communication
Photoinduced Anion Exchange in Cesium
Lead Halide Perovskite Nanocrystals
David Parobek, Yitong Dong, Tian Qiao, Daniel Rossi, and Dong Hee Son
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01480 • Publication Date (Web): 14 Mar 2017
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Photoinduced Anion Exchange in Cesium Lead Halide Perov-
skite Nanocrystals
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David Parobek, Yitong Dong, Tian Qiao, Daniel Rossi, Dong Hee Son
Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
Supporting Information Placeholder
tunability of the photophysical properties of perovskite NCs, ani-
on exchange has been extensively explored using various sources
of anions for different applications. 12,13 In most of these cases, the
precursors possessed high reactivity to undergo spontaneous anion
exchange reaction upon contact with the NCs and the extent of
reaction was usually controlled by stoichiometrically limiting the
reactant.
ABSTRACT: Cesium lead halide (CsPbX3) perovskite nano-
crystals (NCs) possess the unique capability of post-synthesis
anion exchange providing facile tunability of the optical proper-
ties, which is usually achieved by mixing NCs with reactive anion
precursors. In this work, we show that the controllable anion ex-
change can be achieved in a dihalomethane solution of CsPbX3
NC in the absence of any spontaneously reacting anion source
using photoexcitation of CsPbX3 NCs as the triggering mecha-
nism for the halide ion exchange. The reaction begins with the
photoinduced electron transfer from CsPbX3 NCs to dihalome-
thane solvent molecules producing halide ions via reductive dis-
sociation, which is followed by anion exchange. The reaction
proceeds only in the presence of excitation light and the rate and
extent of reaction can be controlled by varying the light intensity.
Furthermore, the asymptotic extent of reaction under continuous
excitation can be controlled by varying the wavelength of light
that self-limits the reaction when light becomes off-resonance
with the absorption of NCs. The light-controlled anion exchange
demonstrated here can be utilized to pattern the post-synthesis
chemical transformation of CsPbX3 NCs, not readily achievable
using typical methods of anion exchange.
In this work, we report the photoinduced anion exchange in
solution of CsPbX3 (X=Cl, Br) NCs suspended in dihalomethane,
where the extent of the anion exchange reaction and its rate are
controlled by the intensity or wavelength of light without using
any reactive sources of halide. In our study, dihalomethane mole-
cules that normally function as the unreactive solvent produce
halide ions via photoinduced electron transfer from CsPbX3 NCs
resulting in reductive dissociation of the solvent molecules. The
halide ions generated in-situ near the surface of NCs drive the
anion exchange reaction efficiently, as long as the CsPbX3 NCs
are photoexcited above the bandgap. The proposed mechanism of
the photoinduced anion exchange in CsPbX3 NCs is illustrated in
Scheme 1. The photoinduced anion exchange in CsPbX3 NCs can
give a unique capability not available from the earlier anion ex-
change reactions, such as spatial patterning with controllable stoi-
chiometry.
The ability of colloidal CsPbX3 perovskite NCs to undergo
rapid anion exchange reactions in solution phase has opened the
doors to many interesting applications of these materials. A broad
range of tunable photoluminescence (PL) from CsPbX3 NCs
could be obtained via a simple post-synthesis procedure, where
mixing the solution of NCs with the reactive anion precursors
results in the formation of the anion-exchanged NCs with tunable
bandgap1-3 for perovskite nanostructures of various morpholo-
gies.4-6 The labile nature of the halide anions in the perovskite
system is generally accepted responsible for their facile exchange
in perovskite NCs and other processes involving ion transport,
such as long-range anion diffusion under weak perturbation7,8 and
the phase segregation in the methylammonium lead mixed halide
system.9 Several recent studies investigated methylammonium
lead iodide/bromide films and CsPbX3 colloidal solutions to un-
derstand the nature of anion transport and exchange of halide ions
in perovskites of various phases.10,11 Due to the simple and wide
Scheme 1. Illustration of the proposed mechanism for pho-
toinduced anion exchange of perovskite nanocrystals.
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Figure 1. Photoinduced anion exchange of CsPbX3 NCs in dihalomethane solvent. (a,b,c) Time dependence of absorption spectrum (a),
PL emission spectrum (b), and PL emission peak (c) for CsPbBr3 → CsPbCl3 in DCM/hexane. (d,e,f) Time dependence of absorption
spectrum (d), PL emission spectrum (e), and PL emission peak (f) for CsPbCl3 → CsPbBr3 in DBM/hexane.
We demonstrate the photoinduced anion exchange using di-
chloromethane (DCM) and dibromomethane (DBM) as the inert
solvent that also functions as the locally photoactivatable source
of halide anions in the immediate vicinity of the perovskite NCs.
CsPbBr3 and CsPbCl3 NCs used in this study were synthesized
following a previously reported method1 with minor modifica-
tions. The detailed procedures of CsPbX3 NC synthesis and pho-
toinduced anion exchange reaction, TEM images and X-ray dif-
fraction patterns of the NCs before and after the anion exchange
are in the Supporting Information. Figure 1 summarizes the con-
tinuous changes of the absorption and emission spectra during the
process of photoinduced anion exchange in CsPbBr3 NCs dis-
persed in mixed solvent of DCM/hexane (Figure 1 a,b) and CsP-
bCl3 NCs in DBM/hexane (Figure 1 d,e). The time-trace of the PL
emission peak and the PL quantum yield correlated with PL emis-
sion peak are also shown for both reactions (Figure 1 c,f). The
mixture of hexane and dihalomethane was used as the solvent
instead of pure dihalomethane for better solubility of the NCs
facilitating more accurate measurement of the optical spectra,
while the same photoinduced chemistry can be performed in pure
dihalomethane. The entire volume of the solution sample con-
tained in a 1 cm-thick cuvette was illuminated with weak 405 nm
light (~20 mW/cm2) to excite the NCs without stirring the solu-
tion.
In order to confirm that the exchange between the two different
halide ions is the direct result of light absorption by perovskite
NCs, the progress of the reaction was monitored by changing both
the intensity and wavelength of the excitation light. Figure 2a
shows the shift of the PL emission peak with alternating ‘on’ and
‘off’ periods of the photoexcitation for the anion exchange reac-
tion of CsPbBr3 NCs in DCM/hexane. The shift of the PL emis-
sion peak occurs only during the ‘on’ period, indicating that the
extent of reaction can be precisely controlled with photon dose. In
addition, the rate of reaction could also be controlled by varying
the intensity of light (See Supporting Information), indicating that
the rate of production of halide ions via photoexcitation is the
main rate limiting step for the overall exchange reaction.
The essential role of photoexcitation of the perovskite NCs in
anion exchange is further demonstrated in the ‘self-limiting’ be-
havior of the reaction when the excitation light becomes off-
resonance with the absorption of the perovskite NCs during the
exchange process. Figure 2b compares the PL emission peak shift
from the anion exchange of CsPbBr3 in DCM/hexane under exci-
tation at 405 and 445 nm. When 445 nm light is used, the reaction
stops near the PL emission peak of 440 nm due to the inability of
the NCs to absorb light, in contrast to the case of 405 nm excita-
tion that results in a nearly complete anion exchange. This
demonstrates that the extent of reaction can be controlled also
with the wavelength of the excitation light in addition to the pho-
ton dose. Furthermore, two-photon excitation of the perovskite
NCs at 800 nm, where there is no linear absorption in solution,
could induce the anion exchange (See Supporting Information),
suggesting an interesting possibility of photo patterning even in 3-
dimensions, with an appropriate medium that limits the diffusion
of the NCs and solvent. The above observations demonstrate the
unique benefits of the photoinduced anion exchange reaction in
gaining precise post-synthesis control of the perovskite NCs
bandgap.
In the absence of photoexcitation in the dark, the absorption
and emission spectra were constant over time, indicating that no
anion exchange occurs in the solution of CsPbCl3 and CsPbBr3
NCs in their respective dihalomethane/hexane solvent (See Sup-
porting Information). Upon illumination with 405 nm light, CsP-
bBr3 NCs in DCM/hexane showed a blueshift of both absorption
and emission spectra indicating the exchange of bromide with
chloride. Similarly, CsPbCl3 NCs in DBM/hexane showed a red-
shift of the spectra from the exchange of chloride with bromide.
This indicates that the exchange of anions can occur in both direc-
tions and that the direction of the reaction can be varied by chang-
ing the solvent composition. The PL quantum yield, shown as
insets in Figure 1 c,f, generally decreases with increasing Cl con-
tent in CsPb(Cl/Br)3 mixed halide NCs for the anion exchange
reaction in both directions, consistent with earlier observations.2,3
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and PL emission spectra before and after excitation of CsPbBr3
NCs in DBM/hexane.
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For the two anion exchange reactions shown in Figure 1, the
absorbance of CsPbX3 NCs at the excitation wavelength and light
intensities are comparable. However, the rate of reaction is an
order of magnitude faster for CsPbCl3 to CsPbBr3 in DBM/hexane
than CsPbBr3 to CsPbCl3 in DCM/hexane. This may be explained
from the combination of the differences in the reduction potential
of the solvent molecules and conduction band edge of CsPbX3
NCs rather than photon absorption rate per each NC dictated by
the absorption cross section. For alkyl halide molecules, the re-
duction potential of bromide is less negative than chloride.16 Fur-
thermore, the conduction band edge of CsPbCl3 is higher than that
of CsPbBr3. Therefore, one would expect more facile electron
transfer from CsPbCl3 to DBM than from CsPbBr3 to DCM, re-
sulting in a large difference in the rate of halide ion production. In
the case of chloromethane series, teterachloromethane showed a
faster photoinduced anion exchange reaction than DCM. This may
be partially due to the lower reduction potential promoting the
faster electron transfer. It is worth to mention that photoinduced
anion exchange is less controllable in trihalomethane because of
the potentially higher concentration of the reactive contaminants
that can be produced under light exposure, resulting in the partial
anion exchange even in the absence of light.
Figure 2. PL emission peak shift for CsPbBr3 in DCM/hexane (a)
under ‘on’ and ‘off’ period of excitation light (b) under two dif-
ferent excitation wavelengths, 405 and 445 nm.
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The possible mechanism for the photoinduced anion exchange
reaction is the dissociation of the reduced solvent molecules via
interfacial electron transfer from the photoexcited perovskite NCs.
Most alkyl halides are known to undergo concerted reductive
dissociation into halide ion and alkyl radical under both heteroge-
neous (e.g., electrochemical) and homogeneous reduction condi-
tions.14,15 The reduction potential of various alkyl halides are less
negative than -1V vs SCE (saturated calomel electrode) in com-
mon organic solvents.16 Therefore, the reduction can be achieved
via interfacial electron transfer from the conduction band of
CsPbX3 NCs that are energetically located above the reduction
potential. For instance, the conduction band edge of CsPbBr3
measured electrochemically17 and from ultraviolet photoelectron
spectroscopy,18 is 3.35 eV below vacuum level, which is suffi-
cient to reduce DCM that has a reduction potential of -1.04 V vs
SCE corresponding to 3.7 eV below vacuum level (See Support-
ing Information). From the interfacial electron transfer to solvent
molecules, halide ions should be produced in the immediate vicin-
ity of the NCs creating a relatively high local concentration which
should facilitate the exchange reaction. This scenario is also cor-
roborated by the observation of the slower reaction rate when the
solution is rapidly stirred, which can be attributed to the reduced
local concentration of the photogenerated halide ions near the
surface of NCs. The holes remaining on NCs should ultimately be
removed to continue the reaction and this may involve back elec-
tron transfer from remaining halide ions. The detailed understand-
ing of the fate of holes and other organic species is beyond the
scope of this study and will be addressed in a later work.
When CsPbBr3 and CsPbCl3 NCs are photoexcited in solvent
containing their respective halide, i.e., DBM and DCM, there is
no net exchange of halides. However, a significant enhancement
of PL quantum yield was observed especially in CsPbCl3, typical-
ly exhibiting low PL quantum yield (~1%) when freshly synthe-
sized.3 The absorption and PL emission spectra before and after
the photoinduced exchange of the same anion are shown in Figure
3. In the case of CsPbCl3 NCs, the PL quantum yield increases by
an order of magnitude (~1%→~10%) accompanied by a substan-
tial increase of the absorption intensity (Figure 3 a,b). In the case
of CsPbBr3 NCs that already exhibit 60-80% PL quantum yield
when freshly synthesized, the increase of PL quantum yield is
~10% with negligible change in the absorption intensity (Figure 3
c,d). This observation suggests one possible origin of particularly
low PL quantum yield in CsPbCl3 NCs and the potential role of
photoinduced ‘self-anion exchange’. The lower PL quantum yield
of CsPbCl3 compared to CsPbBr3 could occur from the relatively
high concentration of halide vacancies which could be removed
through photoinduced self-anion exchange. The increase of the
absorption intensity may reflect the filling of vacancies that in-
creases the density of states involved in the optical absorption.
Further confirmation of the above hypothesis will require addi-
tional work on the measurement of the optical absorption cross
section and accurate elemental analysis.
In a separate experiment, we tested whether the anion ex-
change reaction can be performed without directly exciting the
perovskite NCs by using a separate species that can transfer the
electron to the solvent molecules. For this purpose, we examined
the anion exchange reaction in the mixture of CsPbCl3 NCs and
CdSSe/ZnS core/shell quantum dots dispersed in DBM/hexane
under 445 nm excitation, where only CdSSe/ZnS quantum dots
absorb light. Figure 4 compares the result from this measurement
with that from the reaction of CsPbCl3 NCs under 405 nm excita-
tion, where the average photon absorption rates in the two sample
solutions were kept similar. As expected, the mixture of CsPbCl3
NCs and CdSSe/ZnS can also undergo anion exchange reaction,
although the rate is slower likely due to the lower local anion
concentration near the surface of the perovskite NCs.
Figure 3. (a,b) Absorption and PL emission spectra before and
after excitation of CsPbCl3 NCs in DCM/hexane. (c,d) Absorption
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Figure 4. Comparison of the photoinduced anion exchange reac-
tions occurring from the excitation of CsPbCl3 NCs (orange
curve) and excitation of CdSSe/ZnS quantum dots (blue curve) in
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In summary, we have shown a highly controllable method of
anion exchange using light as the triggering mechanism in the
solution of CsPbX3 NCs dispersed in dihalomethane. In-situ pro-
duction of halide anions via reductive dissociation of the solvent
molecules following the interfacial electron transfer from the
photoexcited CsPbX3 NCs is considered as the mechanism for the
photoinduced anion exchange. We were able to control the extent
of anion exchange reaction precisely by controlling either photon
dose or wavelength of the excitation light. Multiphoton excitation
was also able to induce the anion exchange, creating the potential
use for photo patterning in a viscous medium. The self-exchange
of the same anions, particularly in CsPbCl3 NCs, resulted in an
order of magnitude increase of the PL quantum yield, which may
have resulted from the removal of the anion vacancy. The high
level of control of the rate and extent of anion exchange reaction
and the potential to spatially pattern the chemical transformation
with light will be valuable for the post-synthesis modification of
perovskite NCs for various applications.
ASSOCIATED CONTENT
Supporting Information
The synthetic procedure, experimental details, TEM and XRD
patterns of the nanocrystals, PL shift from anion exchange under
varying excitation intensity and under two photon excitation con-
ditions are provided in the supporting information and can be
found free of charge on the ACS publication website.
AUTHOR INFORMATION
Corresponding Author
dhson@chem.tamu.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by the Robert A. Welch Foundation (A-
1639) and the NSF GRFP (DGE-1252521) for D.P. The authors
would also like to thank Forrest Martinez for assistance in the thin
film patterning.
REFERENCES
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2015, 15, 3692.
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