Enhanced photoredox activity of CsPbBr3
nanocrystals by quantitative colloidal
ligand exchange
Cite as: J. Chem. Phys. 151, 204305 (2019); https://doi.org/10.1063/1.5129261
Submitted: 27 September 2019 . Accepted: 07 November 2019 . Published Online: 27 November 2019
Haipeng Lu
, Xiaolin Zhu, Collin Miller, Jovan San Martin, Xihan Chen
, Elisa M. Miller
, Yong Yan
, and Matthew C. Beard
J. Chem. Phys. 151, 204305 (2019); https://doi.org/10.1063/1.5129261
© 2019 Author(s).
151, 204305
The Journal
of Chemical Physics
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Enhanced photoredox activity of CsPbBr3
nanocrystals by quantitative colloidal
ligand exchange
Cite as: J. Chem. Phys. 151, 204305 (2019); doi: 10.1063/1.5129261
Submitted: 27 September 2019 • Accepted: 7 November 2019 •
Published Online: 27 November 2019
Haipeng Lu,1
Yong Yan,2,a)
Xiaolin Zhu,2 Collin Miller,2 Jovan San Martin,2 Xihan Chen,1
and Matthew C. Beard1,a)
Elisa M. Miller,1
AFFILIATIONS
1
Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, USA
2
Department of Chemistry and Biochemistry, San Diego State University, San Diego, California 92182, USA
Note: This paper is part of the JCP Special Topic on Colloidal Quantum Dots.
a)
Authors to whom correspondence should be addressed: yong.yan@sdsu.edu and Matt.Beard@nrel.gov
ABSTRACT
Quantitative colloidal ligand exchange on lead-halide perovskite nanocrystals (NCs) has remained a challenge due to the dynamic passivation
of amines and carboxylic acids and the instability of core lead-halide perovskite systems. Here, we present a facile colloidal ligand exchange
process using cinnamate acid ligands to quantitatively displace native oleate ligands on CsPbBr3 NCs. The short cinnamate ligands lead to a
23-fold enhancement of the electron-donating ability of the CsPbBr3 NCs when benzoquinone is used as an electron acceptor. A significantly
increased photoredox activity is also observed in a complete photocatalytic reaction: the α-alkylation of aldehydes. Our results provide a new
strategy to tune the photoredox activity of halide perovskite NCs as well as the exploration of NC-ligand interactions.
Published under license by AIP Publishing. https://doi.org/10.1063/1.5129261., s
Lead halide perovskite nanocrystals (NCs) have gained tremendous interest in optoelectronic technologies, for example, as photon emitters, solar cell absorber layers, and in photocatalytic applications, because of their exceptional light harvesting and emitting
properties.1–6 While most investigations focus on manipulating the
rich, tunable chemistry within the inorganic core (i.e., cation/anion
exchange7,8 and impurity doping9 ), controlling and understanding the surface chemistry of these perovskite NCs is critical for
their technological development. Postsynthetic modification of the
surface chemistry of semiconductor NCs influences their optoelectronic properties, such as modifying band edge positions,10 increasing or decreasing optical absorption and/or emission efficiency,11
and affecting exciton lifetimes.12 However, colloidal ligand exchange
reactions of Pb-halide perovskite NCs remains particularly challenging. Unlike typical semiconductor NCs, which undergo facile
ligand exchange using X-, L-, or Z-type schemes,13 colloidal ligand
exchange for perovskite NCs is largely underexplored. For instance,
Pb-halide perovskite NCs often require a two-step, solid-state ligand
exchange procedure when fabricating electrically coupled perovskite
J. Chem. Phys. 151, 204305 (2019); doi: 10.1063/1.5129261
Published under license by AIP Publishing
NC films.14 This arises due to the coupled surface passivation of both
amines and carboxylic acids during the colloidal synthesis,15 where
a complete ligand exchange, therefore, requires another ion-pair of
ligands. Additionally, the more ionic perovskite NCs often suffer
from significant instabilities (i.e., phase transformation or dissolution)2,16,17 during the ligand exchange. For instance, surface ligand
shells18 and solvents19 have been shown to play critical roles in stabilizing the crystalline phase of halide perovskite NCs. This is due
to the significant surface distortion or reconfiguration in halide perovskite NCs induced by different ligand shells or solvents. Therefore,
extra care needs to be taken in developing colloidal ligand exchange
methods for perovskite NCs.
To this extent, we developed an alternative synthetic protocol, which utilizes an amine-free synthesis20 that results in colloidal CsPbBr3 NCs with only oleate-terminated surfaces. The assynthesized CsPbBr3 NCs are found to crystallize in the expected
orthorhombic phase21 , with a cuboid morphology and an average
diameter of 11.2 ± 1.6 nm (supplementary material, Fig. S1). Photoluminescence quantum yield (PLQY) of the oleate-terminated NCs
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FIG. 1. X-type ligand exchange of CsPbBr3 NCs where surface bound OA− is
replaced by functionalized cinnamic acid molecules.
is determined to be ∼70%, suggesting high quality NCs that are relatively well-passivated. The 1 H NMR spectrum indicates that oleate
ligands (OA− ) dominate the postsynthesized NC surfaces (Fig. S2).
Compared to the traditional hot-injection method which affords
CsPbBr3 NCs with an amine-oleate dual passivation (NCs precipitate within a week), our synthetic strategy gives CsPbBr3 NCs with
improved colloidal stability, i.e., no precipitates are observed from
the crude solution over at least 2 months.
With these oleate only terminated CsPbBr3 NCs, we can examine colloidal ligand exchange reactions of the native oleate ligands with other carboxylic ligands, such as cinnamic acids. We
have shown previously that cinnamic acid molecules undergo a
quantitative X-type ligand exchange reaction with oleate on PbSoleate terminated QDs.22 Cinnamic acid derivatives can tune the
QD/ligand optical absorbance11 as well as their band offsets10 due
to the electronic interaction between the QDs and ligands. Similar
to the oleate-terminated PbS QDs, we find that the native oleate
ligands on the CsPbBr3 NCs can be displaced with functionalized cinnamic acid molecules quantitatively via an X-type ligand
exchange reaction (Fig. 1). We explored several functionalized cinnamic acid ligands and found that trans-cinnamate (CA) and trans3,5-difluorocinnamate (3,5-F-CA) ligands gave the best colloidal stability after the ligand exchange reaction. Notably, these cinnamate
passivated CsPbBr3 NCs can now be suspended in polar solvents
such as methyl acetate (3,5-F-CA− /CsPbBr3 ) or dichloromethane
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(CA− /CsPbBr3 ) with reasonable stability, whereas nonpolar solvents
such as hexanes work as an antisolvent to precipitate the NCs. Our
approach thus provides a new capability to process halide perovskite
NCs in different solvents, potentially enabling the preparation of
new heterogeneous structures.
The integrity of the exchanged CsPbBr3 NCs is assessed by
UV-Vis absorption, PLQY, powder X-ray diffraction (XRD), and
transmission electron microscopy (TEM). Absorption spectra show
that the exchanged NCs display the same absorption onset indicating no significant surface etching [Fig. 2(a)]. However, cinnamate passivated CsPbBr3 NCs generally exhibit a lower PLQY as
compared to oleate-passivated NCs, likely due to unintended surface defects induced during the ligand exchange reaction. Importantly, the crystalline phase of the exchanged NCs remains the same
based on XRD data [Fig. 2(b)]. No significant change in the diffraction peaks suggests that the morphology of NCs remains intact.
This is further confirmed by TEM images (Fig. S1), which show
no obvious etching or particle aggregation. Ligand exchange efficacy is further probed by FT-IR spectra. As shown in Fig. 2(c), the
ν (C–H) stretching intensity between 3000 and 2800 cm−1 is significantly reduced, suggesting a quantitative removal of the native
oleate ligands. In situ 19 F NMR experiments shows a broad signal
for 3,5-F-CA/CsPbBr3 NCs, confirming that the new cinnamate ligands are bound on the NCs surface (Fig. S3). This is further corroborated with high-resolution X-ray photoelectron spectroscopy
(XPS) data, which shows the appearance of F signals in 3,5-F-CA
exchanged NCs (Fig. S4). The stoichiometry of the as-synthesized
CsPbBr3 NCs is revealed to be Cs:Pb:Br = 1:0.93:2.45, and thus,
excess Cs+ and Pb2+ are present on the surface, which is consistent
with the carboxylate ligands as the ligand shell to establish the charge
balance.
Since CsPbBr3 NCs have been demonstrated as efficient photoredox catalysts for a variety of organic synthesis,4,5 here, we sought
to investigate if the new ligand shell affected their redox activity. We
first performed photoinduced electron transfer (PET) experiments
with a standard electron acceptor, benzoquinone (BQ). Previous
studies have shown that a rapid PET occurs at the CsPbBr3 /BQ interface (∼65 ps).23 The PET process relies on the reduction potential of
NCs and the permeability of the ligand shell.24 In order to probe
the PET process, steady-state PL and time-resolved PL of CsPbBr3
FIG. 2. Characterization of surface ligands exchange process. UV-vis absorption and PL (a), XRD (b), and FT-IR (c) spectra of CsPbBr3 NCs capped with native oleate (OA,
black), trans-cinnamate (CA), and trans-3,5-difluorocinnamate (3,5-F-CA) ligands.
J. Chem. Phys. 151, 204305 (2019); doi: 10.1063/1.5129261
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FIG. 3. (a) Steady-state PL of CsPbBr3 (CA) NCs with various concentrations of BQ molecules (0–1.6 mM). (b) Time-resolved PL spectra. The green solid lines are the fit
with a biexponential function. (c) IPL0 /IPL vs [BQ] of CsPbBr3 NCs with different surface ligands (black: OA; blonde: 3,5-F-CA; and blue: CA). IPL0 represents the steady-state
PL intensity of CsPbBr3 NCs without BQ.
NCs with different concentrations of BQ molecules are measured.
Figures 3(a) and 3(b) show the typical spectra for CsPbBr3 NCs with
a CA ligand shell, and other samples are shown in Fig. S5. Steadystate PL measurements indicate a sequential quenching upon the
addition of BQ molecules. Time-correlated single-photon counting
(TCSPC) experiments reveal that CsPbBr3 NCs possess two decay
components, one fast decay (4–8 ns) and one slow decay (20–30 ns),
similar to literature reported values.1 We find that CsPbBr3 NCs are
dynamically quenched by BQ, with both CA-terminated NCs being
more effective PET donors than the OA-terminated NCs. This is
supported by the linear relation between IPL0 /IPL and BQ concentration in Fig. 3(c) based on the Stern-Volmer equation [Eq. (1)],
where K SV is the steady-state Stern-Volmer quenching constant, IPL0
is the PL intensity without BQ, IPL is the PL intensity with various concentrations of BQ. The linear relation also suggests that the
PET process is a diffusion-limited process. The bimolecular quenching constant, kq , can be extracted from the TCSPC measurements
[Eq. (1), τ̄0 is the intensity-averaged lifetime from a biexponential
fit to the CsPbBr3 NCs without any electron scavengers]. All of the
quenching constants are summarized in Table I. Notably, both the
CA and 3,5-F-CA ligand shells give one order of magnitude larger
K SV, as compared to native oleate-capped NCs. The bimolecular
quenching constant, kq , is 14 and 23 times larger in 3,5-F-CA and CA
ligand-terminated NCs, respectively, when comparing with native
oleate-capped NCs.
TABLE I. Key parameters of photoinduced electron transfer from CsPbBr3 NCs to
benzoquinone, BQ, molecules.
Ligands
OA
3,5-F-CA
CA
K SV (mM−1 )a
τ̄0 (ns)b
kq (M−1 ns−1 )
0.52
4.09
9.15
21.4
12.1
16.5
24.3
338
554
a
K SV is the Stern-Volmer quenching constants calculated from Eq. (1) (steady-state
PL).
b
τ̄0 is the intensity-averaged lifetime from a biexponential fit to the CsPbBr3 NCs
without any BQ molecules.
J. Chem. Phys. 151, 204305 (2019); doi: 10.1063/1.5129261
Published under license by AIP Publishing
IPL0 /IPL = 1 + KSV [BQ] = 1 + kq τ̄0 [BQ].
(1)
The larger K SV for the CA ligand shell is likely ascribed to the
enhanced ligand shell permeability of the exchanged NCs or the shift
of band edge positions of NCs, which impacts their redox potential. Different solvents can also impact the PET rate through the
reorganization energy. In order to distinguish the solvent effect, we
performed the PET measurements for OA-capped CsPbBr3 NCs in
different solvents and compared the results. We find that KSV only
varies by a factor of 2 (see Fig. S6). Therefore, we conclude that the
impact of the solvent alone cannot account for the observed difference in the electron transfer rate, which is an order of magnitude
faster with cinnamate ligands. We estimated the shift of band edge
positions by the ligand cooperative model: Δϕ = μρ cos(90 − γ)/ε,25
where μ is the dipole moment (1D = 3.34 × 10−30 C m), ρ is the surface ligand density (m−2 ), γ is the ligand tilting angle with respect
to the surface, and ε is the dielectric constant. If we assume that all
ligands are perpendicular to the surface (γ = 90○ ), then the equation
is simplified to Δϕ = μρ/ε. Using the constants we reported previously10,26 and an estimated surface ligand density of 3 l/nm2 , we can
calculate an upper bound to the shift of band energy of 1.38 eV and
0.31 eV for CA and 3,5-F-CA ligands, respectively. Therefore, we
expect that these functional cinnamate ligands can clearly impact the
band edge positions of CsPbBr3 NCs,
We explored how these functional ligands can impact the
NC photocatalytic reactivity in a complete photoinduced organic
reaction—here, the α-alkylation of aldehydes [Fig. 4(a)] that has
been previously investigated.4,5 In this prototypical reaction, the
CsPbBr3 NCs serve as a heterogeneous photocatalysts for efficient
C–C bond formation.4 Here, we systematically studied the photocatalytic reaction activity (i.e., reaction yield) as a function of the
different surface ligands terminating the CsPbBr3 NCs. Figure 4(b)
shows the plot of reaction yield as indicated by in situ NMR monitoring (see Figs. S7 and S8) as the photocatalytic reaction proceeds. Interestingly, we find that the surface passivating ligands do
not seem to significantly affect the photocatalytic reaction rate in
the early stages. In fact, within the first 20 min of the reaction,
there appears to be a slightly slower reaction rate [as indicated by
the slope of the reaction plot in Fig. 4(b)]. After the initial time
(∼20 min), the reaction system exhibits a higher reaction activity
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In conclusion, we present a facile ligand exchange protocol
for oleate-terminated CsPbBr3 NCs with functionalized cinnamic
acid molecules. The ligand exchange reaction proceeds through a
quantitative X-type exchange, resulting in CsPbBr3 NCs dispersed
in polar solvents. The short cinnamate ligands enhance the electrondonating ability of the CsPbBr3 NCs by up to ∼23x. An enhanced
photocatalytic activity for α-alkylation of aldehydes is also observed
with cinnamate-exchanged CsPbBr3 NCs as compared to native
oleate-capped NCs. Our work highlights the chemical tunability of
the photoredox activity for CsPbBr3 NCs through controlling the
surface ligands shells, providing new perspectives for future design
and parameter optimization for halide perovskite nanomaterials.
See the supplementary material for the synthesis and ligand
exchange of CsPbBr3 NCs, TEM, 1 H NMR, XPS, PL quenching
experiments, and photocatalytic reaction systems.
FIG. 4. Reaction scheme (a) of α-alkylation of 3-phenylpropionaldehyde and reaction yield (b) determined by in situ 1 H NMR as a function of time for CsPbBr3 NCs
passivated with oleate or cinnamate ligands.
for the CA-capped CsPbBr3 NCs. The yield of α-alkylation product with CA-capped NCs increases from 10% to 70% from 20 min
to 1 h, while the reaction yield for the OA-capped CsPbBr3 NCs
increases from ∼20% to 60% within the same timeframe. Thus, the
reaction rate [as indicated by yield slope in Fig. 4(b)] increases by
∼50%. This enhanced photocatalytic activity of CA-capped CsPbBr3
NCs agrees with the increased PET rate in the ligand-exchanged
NCs observed in the BQ quenching experiments. Furthermore, the
overall yield is higher for the CA-capped NCs, indicating that the
catalytic activity is maintained for a longer time. That is, the catalyst is degraded quicker for the OA-capped NCs than it is for the
CA-capped NCs.
These results clearly indicate that the ligand can be used to tune
the reactivity of the NCs within a photocatalytic reaction. However,
simply increasing the PET rate may not directly yield a higher photocatalytic activity. There are at least 3 ways in which the ligands
can impact the total photocatalytic reaction cycle. (1) Changes to the
photoredox potentials, where the CA ligands increase the band edge
positions resulting in a faster quenching of the QD PL. However, in
this photocatalytic reaction, the QDs serve to both reduce one of the
substrates and photooxidize either the other substrate or an intermediate species in order to close the photocatalytic cycle. Hence, an
increase in the PET may also reduce the hole transfer rate, resulting
either in a slower overall reaction or in a reduction in the photocatalytic activity. (2) Changes in the QD/ligand permeability such
that substrates can approach the QDs more or less readily. If the ligands are bound too tightly, they may not allow for the substrates
to approach the QD surface. Again, for the complete photoredox
reaction, both halves of the reaction should be considered. (3) The
ligands also passivate surface defects which can either increase the
lifetime of photocarriers or reduce surface catalytic sites. Therefore,
these results suggest that the catalytic mechanism needs to be understood in order to tailor their performance in a full photocatalytic
reaction.
J. Chem. Phys. 151, 204305 (2019); doi: 10.1063/1.5129261
Published under license by AIP Publishing
We gratefully acknowledge support for nanocrystal synthesis,
ligand exchange, and characterization of energy transfer from the
Center for Hybrid Organic Inorganic Semiconductors for Energy
(CHOISE), an Energy Frontier Research Center funded by the Office
of Basic Energy Sciences, Office of Science within the U.S. Department of Energy. XPS data collection was funded by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences,
Division of Chemical Sciences, Geosciences and Biosciences. All
DOE work at NREL was funded through Contract No. DE-AC3608G028308. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The
U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains
a nonexclusive, paid-up, irrevocable, worldwide license to publish or
reproduce the published form of this work, or allow others to do so,
for U.S. Government purposes. Y.Y. acknowledges partial support
from NSF under Grant No. CHE-1851747 for photocatalytic C–C
bond formation reactions.
The authors declare no competing financial interest.
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