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P. Wang et al. / Inorganica Chimica Acta 467 (2017) 251–255
To overcome the disadvantages of the halide precursors men-
with 5 mL of ODE in a 3-neck flask and kept under vacuum at
100 °C for 20 min. Certain amount of surfactants (Table 1) were
injected at 100 °C under Ar flow. After complete dissolution of
the anion source, the temperature was lowered to room tempera-
ture and CsPbX3 NCs dispersed in cyclohexane were injected to ini-
tiate the anion-exchange reactions. After reaction, the NCs were
isolated by centrifugation at 7000 rpm for 5mins. After centrifuga-
tion, the supernatant was discarded and the precipitate was dis-
persed in cyclohexane and centrifuged again. The obtained
precipitate was re-dispersed in cyclohexane forming colloidally
solutions for further analysis.
tioned above, here in this report, for the first time, we introduce
a facile anion exchange approach using Sn-based halide precursors
to achieve the purpose of green chemistry. In our experiments, we
found that the preparation of the Sn-based halide precursors does
not need to use toxic chemicals such as PbX2 salts, TOP and HX.
Anion exchange reactions were observed to be very fast and could
be completed in several seconds at room temperature. The PL spec-
tra of the resulting NCs could be controlled to cover the entire vis-
ible spectra region (410–681 nm) by changing the amounts of
added Sn-based halide precursors. The anion exchange reactions
do not change the cubic perovskite structure, even the size and
shape of the original CsPbX3 NCs are preserved. In addition, the
exchanged CsPbX3 (CsPbCl3 and CsPbI3) NCs still hold the ability
to be used as anion-exchange sources, by simply mixing them with
CsPbBr3 NCs together. The synthesis routes of the previous
researchers are totally optimized to conform to the environmen-
tally-friendly concept with a product of similar quality to that of
the original synthesis. Based on our research, we can predict that
some other metal halogenide such as MgX2, ZnX2, MnX2, etc. can
also be used to prepare lead-free halide precursors for the anion
exchange reactions in CsPbX3 NCs.
2.5. Characterization
X-ray diffraction (XRD) patterns were measured using a BRU-
KER D8 ADVANCE X-ray diffractometer with a CuKa source. Trans-
mission electron microscope (TEM) images were obtained on a
JEOL-JEM 1200 TEM at an accelerating voltage of 100 kV. High-res-
olution TEM (HRTEM) images were taken with a FEI Tecnai TEM at
an accelerating voltage of 200 kV. Photoluminescence (PL) spectra
were collected using an Edinburgh FLS 980 fluorescence spec-
trophotometer. Ultraviolet and visible absorption (UV–vis) spectra
were performed by a Shimadzu UV-2550 ultraviolet–visible spec-
trometer with an integrating sphere.
2. Experimental section
2.1. Materials
3. Results and discussion
Cesium carbonate (Cs2CO3, 99%), lead bromide (PbBr2), 1-octa-
decene (ODE, ꢀ90%), oleic acid (OA, 90%), oleylamine (OAm, 80–
90%) and cyclohexane (CYH, ꢀ99%) were purchased from Aladdin.
SnCl2 (97.5%, anhydrous), SnBr2 (99%) and SnI2 (99%) were pur-
chased from Stream Chemicals. All chemical reagents were used
directly without any further purification.
Fig. 1 shows the XRD patterns of the parent CsPbBr3 NCs and the
anion-exchanged products. By direct synthesis at 170 °C, CsPbBr3
NCs are formed in the cubic phase (space group Pm3m, ICSD
29073). The patterns of the almost fully exchanged NCs correspond
to those recorded on directly synthesized CsPbCl3 (space group
Pm3m, ICSD 29072) and CsPbI3 (space group Pm3m, ICSD
181288) NCs. The anion exchanges do not affect the cubic per-
ovskite crystal structure as a result of the rigidity of the cationic
framework. So the XRD patterns of the partially exchanged NCs
still show the cubic phase. We can also observe that there is only
a slight shift in the XRD patterns after the anion exchange reac-
tions. Because, the combination of ClÀ ions leads to the shrink of
the cell and all the peaks move to higher angles, while the combi-
nation of I- ions expands the cell and all the peaks move to lower
angles. The results are consistent with the anion exchange reac-
tions using Pb-based halide precursors or oleylammonium halide
precursors.
2.2. Preparation of Cs-oleate
0.2666 g of Cs2CO3 was mixed with 10 mL of ODE and 0.835 mL
of OA in a 100 mL 3-neck flask. The mixture was degassed and
dried under vacuum at 100 °C for an hour to remove the water,
and then heated to 150 °C under Ar flow for half an hour until all
Cs2CO3 reacted with OA.
2.3. Synthesis of CsPbBr3 NCs
As shown in Fig. 2(a), CsPbBr3 NC has a cubic structure, which is
based on the corner-shared PbBr46À octahedra locating in the center
of Cs+ formed framework. In the CsPbBr3 structure, the PbBr64À
octahedra are binding together by sharing their BrÀ ions and the
Cs+ ion is owned by eight Cs+ formed frameworks at the same time.
This kind of structure leads to the rigidity of the cubic framework
and the basis of the anion exchange nature. The lattice distance
between the Cs+ ions is 5.82 Å, which is verified by the HRTEM
image of a single CsPbBr3 NC (Fig. 2b). This lattice distance corre-
sponds to the (1 0 0) plane set in the XRD pattern. Based on the
results of the analysis of the XRD patterns, this plane set should
10 mL of octadecene (ODE), 1 mL of OAm, 1 mL of OA, and
0.1321 g of PbBr2 were loaded into a 100 mL three-neck flask and
dried under vacuum at 120 °C for 30 min. After complete solubi-
lization of PbBr2 salt, the temperature was increased to 170 °C
under Ar flow and 1 mL of Cs-oleate solution (prepared as
described above) was injected. After 10 s, the reaction mixture
was cooled to room temperature using a water bath. The NCs were
isolated by centrifugation at 7000 rpm for 5 min. After centrifuga-
tion, the supernatant was discarded and the precipitate was dis-
persed in cyclohexane and centrifuged again. The obtained
precipitate was re-dispersed in cyclohexane forming colloidally
solutions for further use in anion-exchange reactions. Considering
cyclohexane is less toxic than the hexane and toluene used by
other researchers, we choose it as the solvent for the perovskite
NCs in our search. We also find that the cyclohexane is a good sol-
vent for the perovskite NCs.
Table 1
Reaction parameters of the anion-exchange reaction. All anion-exchange reactions
were conducted at room temperature for several seconds. X = Cl, Br, I.
Starting NCs (mmol)
Halide Precursor (mmol)
OA (mL)
OAm (mL)
0.02
0.02
0.02
0.02
0.02
SnX2 – 0.006
SnX2 – 0.012
SnX2 – 0.018
SnX2 – 0.024
SnX2 – 0.030
0.2
0.4
0.6
0.8
0.8
0.2
0.4
0.6
0.8
0.8
2.4. Anion-exchange reactions
Exchange reactions were performed under air-free conditions.
SnX2 (SnCl2, SnBr2 or SnI2) as the anion exchange source was mixed