Universal Oxide Shell Growth Enables in Situ Structural Studies of Perovskite Nanocrystals during the Anion Exchange Reaction

Article abstract of DOI:10.1021/jacs.9b02061

The ability to tune thin oxide coatings by wetchemistry is desirable for many applications, yet it remains a key synthetic challenge. In this work, we introduce a general colloidal atomic layer deposition (c-ALD) synthesis to grow an alumina shell with tunable thickness around nanocrystalline cores of various compositions spanning from ionic semiconductors (i.e., CsPbX₃, with X = Br, I, Cl) to metal oxides and metals (i.e., CeO₂ and Ag). The distinctive characteristics of each core (i.e., emission, facile surface functionalization, stability) allowed us to optimize and to elucidate the chemistry of the c-ALD process. Compared to gas-phase ALD, this newly developed synthesis has the advantage of preserving the colloidal stability of the nanocrystalline core while controlling the shell thickness from 1 to 6 nm. As one example of the opportunities offered by the growth of a thin oxide shell, we study the anion exchange reaction in the CsPbX₃ perovskites nanocrystals by in situ X-ray diffraction, which had been impeded so far by the instability of this class of materials and by the fast exchange kinetics.

Full text of DOI:10.1021/jacs.9b02061

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Article
Universal Oxide Shell Growth Enables In-situ Structural Studies
of Perovskite Nanocrystals during the Anion Exchange Reaction
Anna Loiudice, Michal Strach, Seryio Saris, Dmitry Chernyshov, and Raffaella Buonsanti
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02061 • Publication Date (Web): 02 May 2019
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Universal Oxide Shell Growth Enables In-situ Structural Studies
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of Perovskite Nanocrystals during the Anion Exchange Reaction
Anna Loiudice,† Michal Strach,† Seryio Saris,† Dmitry Chernyshov,⊥ Raffaella Buonsanti*†
Laboratory of Nanochemistry for Energy Research, Institute of Chemical Sciences and
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†
Engineering, Ecole Politechnique Fédérale de Lausanne, Sion, CH-1950, Switzerland.
⊥ The Swiss-Norvegian Beamline (SNBL) - ESRF CS40220, 38043 Grenoble CEDEX 9, France.
*Correspondence to: raffaella.buonsanti@epfl.ch
ABSTRACT
The ability to tune core@shell nanoparticles is desirable for many applications, yet it
remains a key synthetic challenge especially when the shell is a metal oxide. In this work, we
introduce a general colloidal atomic layer deposition (c-ALD) synthesis to grow an alumina
shell with tunable thickness around a nanocrystalline core of various compositions spanning
from ionic semiconductors (i.e., CsPbX3, with X=Br, I, Cl) to metal oxides and metals (i.e.,
CeO2 and Ag). The distinctive properties of each core (i.e., emission, surface chemistry,
stability) allowed to optimize and to elucidate the chemistry of the c-ALD process. Compared
to gas phase ALD, this newly developed synthesis has the advantage of preserving the
colloidal stability of the nanocystalline core while controlling the shell thickness from 1 nm
to 6 nm. As one example of the opportunities offered by the growth of a thin oxide shell, we
study the anion exchange reaction in the CsPbX3 perovskites nanocrystals by in-situ X-ray
diffraction, a study impeded so far by the instability of these class of materials and by the fast
exchange kinetics.
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INTRODUCTION
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Metal oxide core@shell nanoparticles are pivotal for many areas of science, including
medicine, biology, optics, electronics, energy storage and catalysis.1-4 Most oxide shells are
prepared via hydrolysis/condensation reactions, with one representative example being the
so-called Stober method where tetraethylortosilicate is used as the precursor for the growth of
silica shells.5,6 The main drawbacks of the available wet-chemistry approaches are the
encapsulation of multiple nanoparticles within the same shell, which results in micron-size
powders as the final product, and the poor control over the shell thickness below 5 nm.7-9 Yet,
thinner oxide layers are interesting when the aim is to manipulate the communication of the
core with the external environment rather than to completely isolate it (i.e., charge transfer or
diffusion of atomic species across the shell). One must turn to atomic layer deposition (ALD)
in gas phase to achieve control and tunability at the atomic scale over conformal oxide
layers.10-12 However, gas-phase ALD suffers from some limitations such as the difficult
control over the amount of precursor, which is dictated by the pressure range covered in the
reactor, and the loss of colloidal stability in the case of powder reactors. Colloidal ALD (c-
ALD) allows to overcome these issues, however its potential has been mostly limited to the
synthesis of chalcogenide core@shell nanoparticles.13-17 Pioneering work in the field has
been carried out by Itthuria and co-workers with other follow-ups reported in the literature.14-
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17
In these studies, the ALD process starts with the functionalization of the nanocrystalline
4-
core surface with inorganic ligands (e.g. S2-, Se2-, Sn2S6 , etc.) followed by the addition of
the chalcogenide precursor.14-17
Here, we propose a general c-ALD synthesis to grow a metal oxide shell, exemplified by
aluminum oxide (AlOx), around a nanocrystalline core of different composition, including
semiconductors (i.e. CsPbX3, with X=Br, I, Cl), metals (i.e. Ag) and metal oxides (i.e. CeO2).
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AlOx was chosen because it is among the most studied metal oxides obtained by gas phase
ALD and for direct comparison with our previous work on gas phase ALD for CsPbX3.10-12
During the c-ALD, trimethylaluminum (TMA) and pure oxygen are sequentially injected at
room temperature in the octane solution containing the nanocrystals (NCs). Infrared
spectroscopy reveals that the nucleation of AlOx on the NC surface occurs on an oxygen site.
By combining different techniques, including electron microscopy and dynamic light
scattering, we observe a layer-by-layer growth of the alumina shell which is tailored from 1
nm up to 6 nm.
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Then, we demonstrate the advantage of the as-synthesized alumina shell to study
nanoscale chemical transformations by focusing on the anion exchange reaction in CsPbX3
NCs. To date, remarkable studies on ion-exchange in NCs have been performed, mostly
focusing on cation exchange (i.e., CdSe to Ag2Se, CdS to PbS NCs, PbS to PbS core-shell
CdS and Cu2-xS to CuInS2).18-22 Only recently, with the booming of perovskites in solar cells
and lighting applications, anion exchange reactions have attracted increasing interest in the
chemistry community.23-30 The facile and fast anion exchange occurring in both NCs and
bulk perovskites (i.e. CsPbBr3 to CsPb(BryCl1-y)3 or CsPb(BryI1-y)3) plays an important role in
photostability, anomalous hysteresis and light induced segregation.31-35 Despite the
tremendous interest in understanding such phenomena, the ion diffusion mechanism in
perovskite is still largely unknown. Because of the fast anion exchange kinetics (completed at
the time scale of seconds), the few studies conducted so far have used optical probes, which
provide only indirect proofs of structural changes.36-38 In this article, thanks to the AlOx shell
around the CsPbX3 NCs, the anion exchange reaction is slowed down by an order of
magnitude and in-situ X-ray diffraction (XRD) studies could be performed. The temporal
evolution of the NC structure during the reaction reveals anomalous alloy properties of the
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halide perovskites compared to conventional semiconductors, in agreement with a recent
theoretical study.39
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EXPERIMENTAL SECTION
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Chemicals. All chemicals were purchased from Sigma Aldrich and used without
purification, unless specified. Cesium carbonate (Cs2CO3, 99.9%), lead (II) bromide (PbBr2,
99,99%, Alfa Aesar), lead (II) iodide (PbI2 99.9985%, Alfa Aesar), lead (II) chloride (PbCl2,
99.99%), oleic acid (OLAC, technical grade 90%), oleylamine (OLAM, technical grade
70%), 1-octadecene (ODE, technical grade 90%), octane (anhydrous, ≥99%), toluene
(anhydrous, ≥99%), N,N-dimethylformamide, (DMF, anhydrous, ≥99%), methyl acetate
(MeOAc, anhydrous 99.5%), Cerium (III) nitrate hexahydrate (Ce(NO3)3•6H2O (99%)),
Silver
nitrate
(AgNO3),
trimethyloxoniumtetrafluoroborate
(Me3OBF4)
and
Trimethylaluminium (98%, Strem).
Material Synthesis. Synthesis of Cs-oleate (Cs-OLA) precursor: 0.8 g of Cs2CO3, 2.5 mL
OLAC and 80 mL ODE were added to a 100 mL 3-necked round bottom flask and stirred
under vacuum for 1h at 120 °C. The flask was purged with N2 for 10 min and then placed
back under vacuum. This process of alternately applying vacuum and N2 was repeated for a
total of 3 times to remove moisture and O2. The reaction temperature was increased to 150ºC
and kept at this temperature until the solution was clear, indicating that the Cs2CO3 has
completely reacted with the OLAC. The Cs-OLA solution in ODE was stored in N2 until
needed for the NC synthesis. This Cs-OLA was used for the synthesis of CsPbBr3 and
CsPbCl3 NCs. Instead for the synthesis of CsPbI3 NCs the amount of reagents was slightly
different: 0.25 g of Cs2CO3, 1 mL OLAC and 25 mL ODE, while the procedure was kept the
same.
CsPbX3 NCs synthesis. CsPbX3 NCs were synthesized with slight modification from the
one previously reported by Protesescu et al.40 For the synthesis of CsPbBr3 and CsPbCl3 NCs:
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PbBr2 (0.21 g) or PbCl2 (0.26 g) and ODE (15 mL) were stirred in a 50 mL round bottom
flask and degassed under vacuum at 120 °C for 1 h. The flask was then filled with N2 and
kept under constant N2 flow. OLAC and OLAM (1.5 mL each) were injected and the mixture
was kept at this temperature until all the PbBr2 was dissolved. The temperature was then
increased to 165 °C. The Cs-OLA (1.2 mL) precursor, pre-heated to 100 °C under N2
atmosphere, was swiftly injected into the reaction mixture. The reaction mixture turned
yellow or white and the reaction was quenched by immediate immersion of the flask into an
ice bath (~5 s after injection). The synthesized NCs were precipitated by centrifugation at
6000 rpm for 30 min, the supernatant was removed and the NCs were re-dissolved in 1.5 ml
of hexane. A second wash was carried out by adding ethyl acetate in a ratio 1:1 with hexane,
the mixture was centrifuged and the precipitate was dissolved in octane obtaining a final
concentration of ~10 mg/ml. The synthesis of CsPbI3 NCS was carried in the same way with
some modification in the amount of reagent used: PbI2 (0.70 g), 35ml ODE, OLAC and
OLAM 3.5 each and 5.6ml of Cs-OLA prepared as described previously was injected. Methyl
acetate was used for the second washing as reported by A. Swarnkar et al..41
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CeO2 NCs synthesis. CeO2 NCs were synthesized by following a previously reported
procedure.42 1 mmol of Ce(NO3)3•6H2O was mixed with 1 mL of OLAM in 6.3 mL of ODE
at room temperature and dissolved at 80ºC for 30 min. CeO2 NCs were grown by heating the
mixture at 260ºC for 2h. The as-prepared NCs were purified by washing, precipitation and
centrifugation cycles, using ethanol, acetone and hexane at least 4 cycles to remove any
unreacted cerium precursor, surfactants, and excess ODE. The resultant dark brown
precipitate was re-dispersed in octane in a concentration of around 10 mg/ml.
Ag NCs synthesis. Ag NCs were synthesized by following a previously reported
procedure.43 1 mmol AgNO3 was mixed with 20 ml OLAM at room temperature followed by
heating up to 60°C, which was maintained until the granular AgNO3 crystals were completely
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dissolved. The solution was then quickly heated up (≥10°C/min) to 180°C and the
temperature was maintained for 1hr before the reaction system was cooled down to room
temperature. The resulting dark-brown solution was washed with acetone and re-dispersed in
octane. Size-selective precipitation was used to narrow their size distribution.
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Ligand Exchange on Ag NCs. Ag NCs with OLAC as ligand were obtained by using a
reported ligand-stripping procedure.44 1ml of acetonitrile containing MeO3BF4 was added to
1ml of Ag NCs with native OLAM ligands (Ag-OLAM NCs) dispersed in hexane (with a
concentration of 10 mg/ml by ICP-OES); after 1 min stirring, 1 ml of toluene was added and
the mixture was centrifuged to allow NCs precipitation. The obtained NCs were dispersed in
1 ml DMF. Successively 1ml of hexane containing OLAC was added to the NCs dispersed in
DMF and after 1 min stirring, the NCs were precipitated and re-dispersed in octane. Ag NCs
with oxidized surface (Ag-AgOx NCs) were obtained by flowing for 1hr pure O2 gas into the
Ag-OLAM NC solution.
c-ALD synthesis. In a 3-necked flask a solution of NCs (typically 150 l of NCs solution
with a concentration of 5 mM) dissolved in octane as the solvent was gently stirred under N2
flow. An extra amount of octane (typically 2 ml) was added so to ensure good NCs separation
and to guarantee an homogeneous shell growth. Figure S1 in the supporting information (SI)
sketches the synthesis set-up. One c-ALD cycle consists of: 1) drop-wise addition of TMA
diluted in octane to the NC solution (the optimized rate of the syringe pump was fixed at
1ml/h); 2) 5 min waiting time so to ensure that the reaction in step 1) was completed; 3)
addition of O2 gas by mean of a mass flow controller; 4) 5 min waiting time. This cycle is
repeated n-times to reach the desired thickness. The optimal amount of TMA (80 l from a
diluted TMA solution in octane with a concentration of 0.4*10-3 M) to add each cycle was
obtained by monitoring the change of the CsPbX3 NCs’ optical properties upon addition of
increasing amount of TMA (Figure S2). The optimal amount could slightly change from
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batch to batch. The full process was automated by a home-made Lab-View software where
precursor amount, injection speed, and waiting time are independently defined. Different c-
ALD conditions were tested to optimize the process (Figure S3). In order to retain the
colloidal stability in the organic solvent of the as-synthesized core@shell NCs, OLAC was
introduced in place of TMA after 8-10 cycles. This step functionalizes the surface of the shell
which can continue to grow without any precipitation of the NCs. A gas tight syringe was
used for the TMA injection to ensure that the TMA concentration stayed unchanged during
the full process. The TMA is used in very diluted amount to avoid any safety issue.
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Characterizations. Electron Microscopy. Transmission electron microscopy (TEM)
images were acquired on a FEI Tecnai-Spirit at 120 kV. High-angle annular dark-field
scanning TEM (HAADF-STEM) images and X-ray energy dispersive (EDX) elemental maps
were acquired on a FEI Tecnai-Osiris at 200 kV. This microscope is equipped with a high
brightness X-FEG gun, silicon drift Super-X EDX detectors and a Bruker Esprit acquisition
software. Samples were prepared by dropping hexane or octane solution containing the
nanoparticles on the surface of ultrathin carbon-coated copper grids (Ted Pella, Inc.).
Optical Spectroscopy. All optical measurements including photoluminescence (PL)
emission spectra, time-resolved fluorescence lifetimes (TRPL) and quantum yield (QY)
measurements were collected on a Horiba Jobin Yvon Fluorolog-3 with a PMT as detector.
The crude CsPbX3 NC solutions were diluted in octane to reach an optical density of about
0.1-0.2 at the excitation wavelength. For the TRPL the excitation source is a Horiba
nanoLED-370 with an excitation wavelength of 369 nm, a pulse duration of 1.3 ns and a
repetition rate of 100 kHz. The absolute QY measurements were performed in the integrating
sphere Fluorolog-3 accessory where a quartz cuvette containing the sample was placed. The
sample was excited with a monochromated xenon lamp at 470 nm. The emitted light was
collected at the exit of the sphere by a PMT detector. For each sample, four measurements
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were performed: (i) the sample emission (Sem) that collect the photons emitted by the sample;
(ii) the blank emission (Bem) that collects the emission from the cuvette containing only the
solvent; (iii) the sample excitation (Sexc), that records the photons at the pumping wavelength
that are not absorbed by the sample; (iv) the blank excitation (Bexc), that records the photons
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at the pumping wavelength through the blank. The QY was then calculated as:
푆푒푚 ― 퐵푒푚
푄푌(%) =
× 100
퐵푒푥푐 ― 푆푒푥푐
Any reabsorption correction factor was neglected in our calculation of QY, since the
solutions investigated were diluted to the point that reabsorption of the PL could be
neglected. The final reported QY are an average over 3 measurements.
UV−vis absorption measurements were performed in transmission mode using a
PerkinElmer Lambda 950 spectrophotometer equipped with a deuterium lamp as a light
source for the ultraviolet range and a tungsten halide lamp as a light source for the visible and
infrared range, and a PMT with a Peltier-controlled PbS detector.
Dynamic Light Scattering (DLS). DLS measurements were carried out using a Zetasizer
Nano ZS (Malvern) instrument. The Nano ZS system is equipped with a 4 mW red laser (633
nm) and a detection angle of 173°. The samples were prepared in a quartz cuvette in octane
solvent. For each sample 3 measurements were performed with a fixed run time of 10s. The
Malvern DTS 5.10 software was applied to process and analyze the data.
Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR).
ATR-FTIR was performed using a Perkin Elmer instrument Spectrum 100 by drop-casting 30
l of the NC solution on the ATR window, and measured with a resolution of 4 cm-1.
Inductively Coupled Plasma - Optical Emission Spectrometry (ICP-OES). ICP-OES was
performed on Agilent 5100 model to determine the CsPbX3 NCs, CeO2 NCs and Ag NPs
concentration. 5 standard solutions of Pb, Ce and Ag were prepared to obtain the calibration
curve used to determine the concentrations of the digested nanocrystal solution. The sample
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solution was prepared by dissolving the nanocrystals in 70% HNO3 followed with the
addition of a specific amount of ultrapure water until the solution reaches the 2% acid content
needed for the analysis.
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Synchrotron set-up. The series of in-situ XRD experiments were carried out at Swiss-
Norwegian beamline BM01 at ESRF in Grenoble (France). We used a custom-made capillary
flow cell and two syringe pumps to inject the different NCs into the probed volume within a
glass capillary (Figure S4). The XRD patterns were recorded with a PILATUS 2M detector.
We started acquiring XRD data 10 s before the injection took place, one pattern each 2.5
seconds. Details of the beamline and equipment have been described elsewhere.45 A lambda
of 0.64066 Å was used for all the experiments. The spot size was adjusted to the capillary
size (1.5 mm external diameter, 0.01 mm wall thickness). The diffraction patterns were
analyzed using TOPAS 5.46 The calibration of diffractometer parameters was done with LaB6
NIST standard and PyFai software.47 The raw data were processed into powder patterns with
Bubble.45 To monitor the evolution of the lattice parameters, all patterns were fitted using the
Le Bail method. The utilized crystallographic data were taken from the ICSD database:
CsPbBr3 (Pm-3m, collection code: 97852), CsPbI3 (Pm-3m, collection code: 161481), and
CsPbCl3 (Pm-3m, collection code: 29067).
RESULTS AND DISCUSSION
In a typical experiment, the optimal amount of TMA diluted in octane is added drop-wise
to a NC solution; after a reaction time of 5 minute, O2 gas is introduced followed by other 5
min waiting time (see Experimental Section for details). These steps constitute one cycle,
which is then repeated multiple times. We observed that after a certain number of cycles
(typically 8-10 cycles) the as-synthesized core@shell NCs would lose their colloidal stability.
In order to prevent this issue, when this stage is reached, OLAC is added in place of the O2 to
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prevent the formation of any precipitate. The whole c-ALD synthesis is illustrated in Figure
1. The proposed procedure mimics the gas-phase ALD process by translating it into solution.
Similarly, to the gas-phase ALD, the repetition of sequential steps multiple times is adopted
to precisely control the thickness of the metal oxide shell resulting in a core@shell NC
structure. The generality of the proposed approach is demonstrated by growing the AlOx shell
on various NC cores in terms of composition and intrinsic properties. As we discuss later,
these NC cores have distinctive characteristics that combined all together helped to elucidate
different aspects of the developed c-ALD synthesis.
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Figure 1. A) Schematic of the c-ALD synthesis and B) sketch of the obtained core@shell
NCs.
Figure 2 gives an overview on the synthesized core@shell NCs with different
compositions (i.e., CsPbBr3@AlOx, CeO2@AlOx, Ag@AlOx NCs) characterized by electron
microscopy and DLS. The HAADF-STEM images with the corresponding EDX maps in
Figure 2A-E and the low magnification TEM images in Figure S5 evidence the preservation
of the size and shape of the inorganic core which is surrounded by a lower density alumina
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Figure 2. HAADF-STEM images and corresponding EDX coloured maps of A,B)
CeO2@AlOx and C,D) Ag@AlOx NCs; E) EDX maps of Ag@AlOx NCs obtained at
different c-ALD cycles. F,G,H) HAAFD-STEM images of CsPbBr3@AlOx NCs without any
shell, with thin (8 cycles) and thick (17 cycles) AlOx shell respectively; I) DLS size
distribution of CsPbBr3@AlOx NCs obtained by performing 0, 5, 10, 14, and 17 c-ALD
cycles; L) The calculated shell thickness estimated by DLS measurements as a function of c-
ALD cycles fitted with a linear curve (black line). From the slope of the fitted curve a growth
rate of 0.36 nm/cycle is estimated.
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shell whose thickness increases with the number of cycles (Figure 2E). The presence of the
AlOx shell around each single NC core is further evidenced by the strong contrast between
the cubic assemblies of the as-synthesized CsPbBr3 NCs on the TEM grid and the highly
disordered one for the CsPbBr3@AlOx core@shell NCs (Figure 2F-H). For the CsPbBr3
NCs, characterization EDX mapping cannot provide clear information regarding the presence
of the Al in the shell because of the overlap between Br and Al peak in the spectrum (Figure
S6). Thanks to the preservation of colloidal stability of the core@shell NCs, DLS was used to
estimate the AlOx shell thickness vs. the number of cycles (Figure 2I,L) and allowed to
determine the overall growth rate on the NCs. The results show a fine-tuning of the shell
thickness ranging from 1 nm to 6 nm and an estimated growth rate during the c-ALD of
around 0.36 nm/cycle. The average bond length Al-O-CH3 is around 0.30±0.11 nm (min =
0.29 nm and max = 0.38 nm based on a CSD search comprising 2892 structures) thus
approximately corresponds to the growth rate we have measured in our system. The
similarity between the measured AlOx growth rate and the estimated bond length suggests
that each cycle correspond to the growth of a single layer around the NC core.
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The optical properties of the CsPbBr3 NC cores were studied to assess the preservation of
their structure and composition (Figure S7). The AlOx was found to even improve the
stability and the photoluminescence quantum yield (QY) of the final CsPbBr3@AlOx NCs
which is three times higher compared to the uncoated counterpart, suggesting that surface
shallow trap passivation occurs during the first c-ALD cycle, also evident by the increased
lifetime (Figures S8, S9 and Tables S1,S2). The changes of QY with the amount of TMA
added in the solution were used to calibrate the optimal amount of TMA to be added in each
c-ALD cycle (Figure S2, S3 for details regarding the optimization process by tuning
injection time, waiting time, temperature). FTIR analysis was performed on the core@shell
NCs to characterize the AlOx structure (Figure S10). C-H vibrational peaks corresponding to
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Figure 3. A) TEM images of the Ag NCs with different surface chemistry along with a
representative sketch: Ag-OLAM (blue), Ag-OLAC (red) and Ag-AgO (black); B) FTIR
spectra of the samples in A); C) TEM images of the obtained Ag-OLAM@AlOx (left) and
Ag-AgOx@AlOx (right) NCs.
the NC native ligands are still present after the c-ALD, suggesting that the ligands are not
removed during the c-ALD process but are rather trapped in the AlOx shell. Instead, the new
features at lower wavenumbers were associated with the presence of an amorphous AlOx
shell.
The FTIR shows that the organic ligands are still present after the c-ALD. At the same
time, oxygen sites (or more rarely other chemical groups) are required to initiate the ALD by
reacting with the TMA. To elucidate the role played by the ligands functionalizing the NC
core during the nucleation and growth of the AlOx shell, the Ag@AlOx NCs were chosen as
the model system because of their robustness and versatile surface chemistry compared to the
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CsPbX3 and the CeO2 NCs. Moreover, the metallic nature of the Ag NCs allows to study the
role of surface oxidation considering that Ag is relatively easy to oxidize.48,49 Starting from
the as-synthesized Ag NCs that have OLAM as ligands (Ag-OLAM NCs), Ag NCs
functionalized with OLAC (Ag-OLAC NCs) were obtained by performing a mild ligand
exchange. Instead, Ag NCs with an oxidized surface (Ag-AgOx NCs) were obtained by
flowing pure O2 gas in the Ag-OLAM NC solution.48,49 The UV-Vis absorption spectra
(Figure S11) evidenced a slight red shift in the plasmon peak consistent with the formation
of a thin oxide shell around the Ag-OLAM NCs. Figure 3A,B shows the corresponding TEM
images and FTIR spectra of the Ag NCs with the three different surface chemistry. The peaks
at 1714 cm-1, which is characteristic of the C=O stretching, and 1405 cm-1, which
corresponds to the C=C stretching, evidence the monodentate binding of OLAC on the Ag
NC surface. Ag-OLAM NCs and Ag-AgOx NCs possess the IR signature typical of OLAM
functionalization. An equal number of c-ALD cycles (8 cycles) was performed on these Ag
NCs. Figure 3C shows that when c-ALD is performed on Ag-OLAM NCs the AlOx
nucleates in solution heterogeneously, thus separated from the Ag NCs. On the contrary, the
AlOx shell grows on both Ag-OLAC and Ag-AgO NCs surface. These results demonstrate
that oxygen, whether on the surface of the NCs or in a monodentate ligand like OLAC,
initiates the c-ALD by reacting with the TMA during the first cycle (Figure S12).
Furthermore, Figure S10 shows that the C=O stretch almost disappears in the Ag-
OLAC@AlOx NCs, which points to the breaking of the double bond following the reaction
with the TMA.
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Having demonstrated that we can successfully grow a tunable metal oxide shell on NCs
with different composition as long as oxygen atoms are present on their surface, it is
interesting to explore the new opportunities open by this general synthesis to study the
communication of the inner core with its surroundings. Solid-state transformations via
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diffusion at the nanoscale are difficult to investigate especially when the kinetics are too fast
to allow structural characterization. The anion exchange reaction in CsPbX3 NCs is of
particular interest for the present work. As aforementioned, the understanding of this
phenomena is very relevant to device implementation and performance, in addition to
fundamental science.31-35 Yet, because of the ion exchange occurring within seconds, only
optical probes have been used so far.
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To closely monitor the structural change occurring during the anion-exchange, CsPbX3
NCs should mix rapidly and reproducibly to ensure a homogeneous environment during the
reaction while allowing the X-rays to capture structural information. These requirements
were fulfilled by building a home-made set-up where two NC solutions of different
composition react in a capillary (see Experimental Section and Figure S4 for details). In our
experiments, the solution of the CsPbX3@AlOx NCs with different shell thicknesses and
composition, all in anhydrous octane, were simultaneously injected through a mixing frit
placed at the entrance of the reaction chamber (i.e., the capillary) in which the two CsPbX3
NCs have a few sub-milliseconds to mix. This time is shorter than the time needed for the
exchange to come to completion, which varies from seconds to tens of minutes depending on
the investigated NCs (composition and w/o shell) as discussed later. After the injection, the
structural changes occurring during the exchange were monitored in real time by acquiring
XRD patterns every 2.5 s. The combination of sub-millisecond mixing time, the millisecond
acquisition time permitted by synchrotron radiation, the fact that the AlOx shell slows down
the reaction kinetics and the high resolution data with a high signal-to-noise ratio, enabled us
to follow the structural evolution of the CsPbX3 NCs during the anion exchange reaction. To
the best of our knowledge this is the first time that such kind of studies are reported.
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Figure 4. A) Schematics of the anion exchange reactions investigated during the in-situ X-
ray diffraction experiments; B) XRD spectra of the as-synthesized CsPbX3 NCs used in the
synchrotron experiments together with the XRD spectra of the final products: CsPb(Br0.5I0.5)3
and CsPb(Br0.5Cl0.5)3 NCs.
Figure 4A summarizes schematically the in-situ experiments that were performed.
Specifically, CsPbBr3@AlOx NCs with two different AlOx shell thickness (Figure 2F-H)
constitute one of the reagents and they will be referred to as Br/no-shell, Br/shell1 and
Br/shell2, respectively. As the second reagent, CsPbCl3 NCs, CsPbI3 NCs and CsPbI3@AlOx
shell1 NCs (named Cl/no-shell, I/no-shell and I/shell1) were selected. The XRD patterns of
the uncoated CsPbX3 NCs used in this work together with the XRD pattern obtained for the
final exchanged product (CsPb(X1-yBry)3 NCs with X=I or Cl) are reported in Figure 4B,
which evidence that the different composition CsPbX3 NCs are characterized by
distinguished diffraction features. The corresponding core@shell NCs have similar XRD
patterns (data not shown). The initial concentrations of the NCs in the mixture were adjusted
so that the final product will result in a mixed halide with 1:1 ratio. Figure S13 shows that all
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the NCs used during the in-situ experiment have an average edge size of 7.5 nm, so to
exclude that any broadening of the XRD peaks is due to inhomogeneity in the NC size.
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Figure 5. A) Representative 2D maps from the in-situ XRD experiments illustrating the
changes observed in the selected 2Theta range during the reaction from CsPbBr3@AlOx to
CsPb(Br0.5I0.5)3 with increasing AlOx shell thickness, specifically: Br-no shell/ (left), Br-
shell2/Br-no shell (centre), and CsPbBr3@AlOx shell2/CsPbI3@AlOx shell1 (right); B)
Snapshots at time=0 min and time=1 min from the maps in A).
A representative data set of the temporal evolution of the XRD patterns during the reaction
from CsPbBr3@AlOx NCs to CsPb(Br0.5I0.5)3 NCs with various AlOx shell thicknesses is
shown in Figure 5A. The maps report the time evolution of the two reagents in a selected
region of the XRD spectrum between 14.5 and 15.7 degrees which allows one to monitor the
(211) diffraction peak from [CsPbBr3@AlOx NCs + CsPbI3@AlOx NCs] at time 0 to the final
CsPb(Br0.5I0.5)3 composition. For clarity, the corresponding XRD patterns acquired at time 0
and 1 min for these three mixtures are shown in Figure 5B. It is immediately evident that the
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thicker the AlOx shell, the slower the anion exchange reaction, because no change in the
XRD pattern is observed after 1 minute for the thicker shell.
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Figure 6. A) Time evolution of the lattice parameters for the CsPbBr3 NCs (Lp Br-
component) with increasing shell thickness when reacting with CsPbI3 NCs, specifically: Br-
no shell/ I-no shell (black), Br-shell1/I-no shell (green), Br-shell2/I-no shell (red), and Br-
shell2/I-shell1 (blue); B) Time-dependent change of the Lp for the Br-shell2 in presence of
uncoated CsPbCl3 NCs.
The good separation between the XRD peaks allows for Le Bail refinement. The temporal
evolution of the lattice parameters (Lp) obtained from the refinement for all the different
mixtures are reported in Figures 6A,B and S14. In particular, for the mixture Br/shell2 and
I/shell1 the final product composition is obtained after more than 12 min compared to only 1
min in the case of uncoated CsPbBr3 and CsPbI3 NC mixtures (Figure 6A). Similar
experiments were conducted for the mixture with CsPbCl3 NCs (Figure 6B, S14). In the
latter case, the anion exchange reaction between Cl- and I- is much slower in agreement with
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the data obtained by optical probing.36-38 In particular, for the mixture Br/shell2 and Cl/no
shell in Figure 6B, the reaction reaches completion after 70 min.
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Focusing on the change of Lp over time for the two cases of Br to BrI and of Br to BrCl,
some key information were extracted. First of all, the experiments conducted on the uncoated
CsPbX3 NC mixtures evidence a very fast structural evolution resulting in an exponential
increase of the Lp with time for Br to BrI and in an exponential decrease of the Lp for Br to
BrCl (black curves in Figure 6A and Figure S14). Second, the temporal evolution of the Lp
in the mixture containing CsPbX3@AlOx NCs reveal a more complex behaviour for both
iodine and chlorine exchange (Figure 6A,B). In particular, after an initial steady change of
the Lp, a deviation from the exponential trend is noticed and a bump, corresponding to the Lp
remaining constant for a certain time, is observed. For the iodine exchange, the Lp stays
constant at a value of 5.867 Å for a short time (around 10 seconds), to then start to change
again with a slower rate of exchange (Figure 6A). While the AlOx shell thickness determines
the time at which the deviation from the exponential trend occurs (i.e. thicker shell, longer
time), the bump is observed always at the same Lp value (highlighted as a grey area in
Figure 6A). A similar behaviour is observed also for the chlorine exchange wherein after
around 20 minutes the lattice parameter stabilizes at a value of 5.768 Å and then starts to
change with a slower rate of exchange (Figure 6B). Noteworthy, the change in the rate of
exchange occurs in both cases at a specific CsPb(BryI1-y)3 and CsPb(BryCl1-y)3 composition,
where y = 0.33 and 0.39 for iodine and chlorine exchange respectively. These results suggest
that the two systems undergo a similar anion exchange mechanism, where at a certain mixed
halide ratio a more stable structure is formed and this structure is kept until new ions are
incorporated into the structure, and the Lp starts to change with a slower rate until the final
composition is obtained. These results can be explained by referring to a recent theoretical
study by W.-J. Yin et al..39 The authors used first-principle calculations together with cluster-
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expansion methods to study the structural properties of mixed halide perovskites. They find
that halide perovskites exhibit anomalous alloy properties compared to conventional
semiconductors; specifically, they find that a particular stable alloy structure exist for specific
composition with formation energies lower than other structures. They calculated a minimum
of formation energy for y=1/3 that is very close to the composition (y= 0.33 and 0.39 for
iodine and chlorine exchange) where we observe that the lattice parameters have the
anomalous change in the rate of ion exchange. The reason for this is that halide perovskite
have a strong ionic characters, so Coulomb interactions plays a major role in this case, which
lowers the formation energy and favours certain structures. It is interesting to compare our
results from the in-situ structural analysis with those obtained from the in-situ optical
monitoring of the anion exchange reaction reported by Kosher et al.36 Here, the authors study
the exchange kinetics for uncoated CsPbBr3 NCs mixed with I-oleate and Cl-oleate solutions
by following the band gap change. They observe a very similar trend for the Br-to-Cl
transformation, with distinct intervals of constant bandgap which closely resembles our
structural data and which are attributed to a transient structure. Instead, for the Br-to-I
exchange, the faster exchange kinetics prevents to catch this intermediate transformation and
the authors conclude that two different exchange mechanisms are taking place in the two
systems. These contrasting results compared to our data reinforce the need to slow down the
reaction in order to correctly interpret the behaviour of the perovskite and to elucidate the
anion exchange mechanisms.
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In conclusion, we have developed a new versatile synthetic approach to grow a metal
oxide shell around various NC cores. In particular CsPbX3@AlOx, CeO2@AlOx and
Ag@AlOx NCs were synthesized with tunable AlOx shell thickness from 1 nm to 6 nm. The
combination of different NC cores helped to elucidate the nucleation and growth of the
alumina shell during the c-ALD process. In particular, FTIR highlights the need of oxygen,
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whether on the surface or in the ligands, to initiate the c-ALD. Future engineering of the NC
ligands with oxygen-rich groups would be interesting to further advance or generalize the
developed c-ALD process. Finally, the CsPbX3@AlOx NCs were exploited to get insights
into the anion exchange reaction by in-situ XRD studies. Our results show that a transient
structure is formed during the alloy process in both iodine and chlorine exchange reactions.
The formation of this transient structure during the exchange is attributed to the existence of a
certain mixed halide composition for which the alloy formation energy is minimized due to
the strong ionic characters of halide perovskite materials. The anion exchange study
performed in this work exemplifies the new opportunities offered by nm-thick metal oxide
shell around active NC cores. The proposed systems could also act as materials platform to
study solid-state ion diffusion across metal oxides, which is extremely interesting for
perovskites solar cell, wherein an oxide layer acts both as electron or hole transporting layer
to improve the charge transport and to reduce the instability issue due to diffusion of metal
contact into the active perovskite layer.50,51 In a more general prospective, the oxide shell
could enable distance-dependent energy transfer studies between various donor and acceptor
NCs (e.g. metal/semiconductor, semiconductor/semiconductor) as an alternative to using
ligands of different lengths.52,53
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ASSOCIATED CONTENT
Supporting Information. Information related to synthesis, additional EDX results and
discussion, optical properties, FTIR data and discussion and additional XRD in-situ results.
This material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGEMENT
A.L. and M. S. acknowledge a H2020-Marie Curie Individual Fellowship with grant numbers
701745 and 753124, respectively, for financial support. S.S. acknowledges the Swiss
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National Science Foundation (AP Energy Grant, project number PYAPP2_166897/1). The
XRD experiments were performed on the Swiss-Norwegian beamline BM01 at the European
Synchrotron Radiation Facility (ESRF), Grenoble, France (proposal number: 01-02 1192).
We are grateful to Dr. Diadkin Vadimat at ESRF for helpful discussions on the XRD data
analysis. A. L. acknowledges Seyedebehnaz Varandili and Valerie A. Niemann for providing
some of the nanocrystalline cores.
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