Identifying reactive organo-selenium precursors in the synthesis of CdSe nanoplatelets

Article abstract of DOI:10.1039/c8cc06326e

In the synthesis of CdSe nanoplatelets, the selenium-to-selenide reduction pathway is unknown. We study solvent-free growth of CdSe nanoplatelets and identify bis(acyl) selenides as key reactive intermediates. Based on our findings, we prepare a series of bis(acyl) selenides that provide useful precursors with tailored reactivity for liquid-phase syntheses of nanoplatelets.

Full text of DOI:10.1039/c8cc06326e

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Identifying reactive organo-selenium precursors in
the synthesis of CdSe nanoplatelets†
Cite this: Chem. Commun., 2018,
4, 11789
5
ab
a
a
a
Andreas Riedinger, a
Aniket S. Mule, Philippe N. Knusel,
¨
Florian D. Ott,
a
Received 2nd August 2018,
Accepted 14th September 2018
Aurelio A. Rossinelli and David J. Norris
*
DOI: 10.1039/c8cc06326e
rsc.li/chemcomm
In the synthesis of CdSe nanoplatelets, the selenium-to-selenide in situ thermogravimetric analysis (TGA), differential scanning
reduction pathway is unknown. We study solvent-free growth of calorimetry (DSC), and electron ionization mass spectrometry
CdSe nanoplatelets and identify bis(acyl) selenides as key reactive (EI-MS). Based on our results, we then synthesize a series of
intermediates. Based on our findings, we prepare a series of bis(acyl) bis(acyl) selenide molecules at the multi-gram scale. The reactivity
selenides that provide useful precursors with tailored reactivity for of these selenide precursors towards cadmium carboxylates can
liquid-phase syntheses of nanoplatelets.
be tuned by molecular design. Thus, useful molecular precursors
are obtained for the growth of CdSe NPLs.
CdSe nanoplatelets (NPLs) are quasi-two-dimensional semi-
conductor nanocrystals with precise atomic-scale thickness.
When solid mixtures of cadmium carboxylate [e.g.
1
–3
Cd(propionate) ] and Se powder are heated to 200 1C under
2
4
–6
Due to their spectrally pure fluorescence,
large absorption N , CdSe NPLs are obtained along with CdO (Fig. 1). The same
2
7
8
cross-sections, and enhanced energy-transfer rates, they have reaction with S powder yields analogous results for CdS NPLs.
8
–12
been heavily studied for applications.
To obtain these The presence of CdO is providing an important clue about the
materials, the standard synthesis involves heating long-chain reaction mechanism. Our goal is to use this and other clues to
cadmium carboxylates (e.g. cadmium myristate or cadmium identify the chalcogen reduction pathway in this reaction.
oleate) with elemental selenium in a non-coordinating solvent Because Cd is already in its highest oxidation state, Se and S
(
1-octadecene, ODE). At a certain temperature, cadmium acetate must be reduced by the only other reactant, the propionate
1,13,14
powder is introduced, triggering zincblende NPL formation.
(or its decomposition products). To determine the intermediates
We have recently exploited a simple solvent-free route to unravel involved, we collected TGA and DSC data in situ while measuring
the growth mechanism that leads to the highly anisotropic shape EI-MS of volatile species (Fig. 2). We analyzed reactions with
15
2
of NPLs. However, the critical selenium reduction pathway in Cd(propionate) alone and mixed with equimolar amounts of
this process remains unclear. Understanding this chemistry can S or Se. In all three cases, weight loss below 180 1C was
lead to further control of NPL growth and improved production of negligible (Fig. 2a). Above this temperature, differences
high-quality materials.
2
between Cd(propionate) alone or with Se or S were observed
Our solvent-free route to CdSe NPLs involves heating a solid in DSC (Fig. 2b). In addition to the melting of Cd(propionate)
2
blend of cadmium propionate and elemental selenium. CdSe at 180 1C, the samples with chalcogen showed melting of
NPLs grow in the melt from cadmium propionate coordination elemental Se (220 1C) and S (115 1C), respectively. These
1
5
polymers. We stress that the utility of this approach is not to findings can be combined with the simultaneously recorded
obtain high-quality NPLs for applications but rather to study the EI-MS data. In general, EI-MS of transition-metal propionates
underlying chemistry under simplified conditions. Here, we take shows two distinct decomposition products: propionyl radicals,
advantage of this solvent-free reaction and investigate the reactive observed as propionyl cations with a molecular ion peak of
+
+
17,18
organo-chalcogenide intermediates that form by performing M = 57, and 3-pentanone with M = 86.
For Cd(propionate)
2
alone, both species were observed below 250 1C. With Se or S,
these were absent (see Fig. 2c and Fig. S1 in the ESI†).
These data lead to our proposed reaction mechanism for
CdSe (or CdS) NPLs (Fig. 2e). The absence of propionyl radicals
in the presence of Se (S) strongly suggests the formation of
bis(propionyl) selenide (sulfide). These molecules can then
a
b
Optical Materials Engineering Laboratory, Department of Mechanical and Process
Engineering, ETH Zurich, 8092 Zurich, Switzerland. E-mail: dnorris@ethz.ch
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz,
Germany
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cc06326e
2
react with Cd(propionate) to yield CdSe (CdS), releasing
This journal is ©The Royal Society of Chemistry 2018
Chem. Commun., 2018, 54, 11789--11792 | 11789

View Article Online
Communication
ChemComm
selenide (or sulfide) appears to be the reactive chalcogenide
intermediate in NPL growth. Note that CdO, which was present
in Fig. 1, is an expected by-product of the in situ formation of
bis(propionyl) selenide (or sulfide) (Fig. 2e).
Fig. 2 also presents results for Te. However, the equimolar
mixture of Cd(propionate)
Cd(propionate) alone. This is consistent with the observation
that when blends of Te powder and Cd(propionate) are heated
2
and Te behaves similarly to
2
2
to 200 1C, CdTe NPLs do not form. This may be due to the
higher melting point of Te (450 1C). Unfortunately, the reaction
temperature cannot simply be increased; Cd(propionate) starts
2
to decompose above 200 1C (Fig. 2c). Thus, this synthetic
approach cannot yield CdTe, because reactive Te compounds do
not form. This is reflected in the mass trace of Cd(propionate)
with Te, which closely resembles the trace of Cd(propionate)
alone at useful reaction temperatures.
2
2
However, for Se and S, our work suggests that bis(acyl)
selenides and sulfides are involved. More generally, they could
be exploited as precursors in nanocrystal syntheses, where
elemental Se or S is often combined with metal carboxylates.
Fig. 1 Heating cadmium propionate and Se powder to 200 1C for 18 h
yields zincblende (zb) CdSe NPLs (inset: transmission electron micrograph,
1
00 nm scale bar). The powder X-ray diffraction pattern of the product is
plotted (black). The reflections for CdSe NPLs are shifted to lower angles Recently, substituted thio and selenourea precursors with
16
compared to bulk zb CdSe (blue, PDF: 01-088-2346 19-191). Other tailored reactivity were used to improve PbS and PbSe quantum
observed reflections are consistent with CdO (red, PDF: 01-073-2245
-640) and Se (green, PDF: 01-083-2437 6-362). Several additional weak
reflections are present presumably due to unreacted Cd(propionate) or Se
19,20
dots.
Libraries of bis(acyl) selenide or sulfide precursors could
5
similarly allow for tailored reactivity. In contrast to substituted
thio or selenourea, bis(acyl) chalcogenides yield only carbonyl
byproducts after reaction with cadmium carboxylates. It is well
known that the by-products of nanocrystal syntheses with
thio or selenoureas (amines and most likely carbamides) can
2
in various modifications. Such phases can be present in the crude reaction
product of the simple solvent-free reaction examined here.
2
1,22
propionate anions and propionyl cations. The latter can decom- significantly influence growth.
Hence, bis(acyl) selenides or
pose to carbon monoxide, ethene, and protons (Fig. 2e). These sulfides should be better for syntheses where amines and
protons can then be captured by free propionate to yield carbamides have a negative impact. This is expected for CdSe
propionic acid. Indeed, traces of this molecule were detected, and CdS NPL syntheses, which rely on phase separation of
+
15
but only with Se and S (M = 74, Fig. 2d). Thus, bis(propionyl) cadmium carboxylate coordination polymers. Amines and
Fig. 2 In situ analysis of the reactive chalcogenide species formed in melts of Cd(propionate)
scanning calorimetry (DSC) data for Cd(propionate) alone or with one equivalent of S, Se, or Te. Cd(propionate)
220 1C) or S (115 1C). The melting of Cd(propionate) occurred in all samples (180 1C). (c and d) During TGA/DSC measurements, volatile species were
analyzed by electron-ionization mass spectrometry (EI-MS). Below 250 1C, propionyl radicals (M , mass-to-charge ratio of 57) were detected only for
2
. (a) Thermogravimetric analysis (TGA) and (b) differential
2
2
with Se or S showed melting of Se
(
2
+
+
Cd(propionate)
TGA/DSC/EI-MS data. Decomposition of Cd(propionate)
sulfide) reacts with Cd(propionate) to produce CdSe (CdS) NPLs, releasing propionate anions and propionyl cations. The latter decompose to ethene,
2
alone or with Te (c). Release of propionic acid (M = 74) was observed only with Se or S (d). (e) The reaction mechanism consistent with
2
yields propionyl radicals, which are captured by Se (or S). The resulting bis(propionyl) selenide
(
2
carbon monoxide, and protons. Free propionate can capture these protons. Indeed, propionic acid was detected only in samples containing Se (or S). See
also Fig. S1 in the ESI.†
11790 | Chem. Commun., 2018, 54, 11789--11792
This journal is ©The Royal Society of Chemistry 2018

View Article Online
ChemComm
Communication
The resulting QD size (Fig. S2, ESI†) increased from 1 to 2 to 3,
2
0
as expected from the predicted reactivity.
We then tested our bis(acyl) selenide precursors in the
liquid-phase reaction of CdSe NPLs. The goal was not to
improve the quality of CdSe NPLs obtained, but to test if the
reactivity of the selenide precursor influenced the nucleation
and growth. Following classical nucleation theory, control over
the nucleation process can be gained through changes in
surface energy (g), temperature (T), and supersaturation (S).
In NPL syntheses, g is given by the carboxylate surface passiva-
Scheme 1 One-pot reaction for multi-gram synthesis of bis(acyl) selenides.
2
3
carbamides can break down these polymers and should be tion and largely fixed. Current NPL syntheses primarily control
avoided. nucleation via T (i.e. changing the temperature at which the
To investigate the utility of bis(acyl) selenides as precursors, cadmium acetate is added). Here, we demonstrate that the
24,25
we adapted a known synthesis
grams of bis(acyl) selenides from a one-pot reaction. First, lithium the nucleation process in NPL syntheses.
aluminum hydride is reacted at À10 1C with elemental Se in For this, Cd(myristate) in ODE was degassed at room
tetrahydrofuran (THF) to form LiAlHSeH, a greyish solid. Without temperature and heated under N to 240 1C. Acetic anhydride
(Scheme 1) to obtain multiple precursor reactivity (via changes in S) also allows control over
2
2
further purification, addition of acyl chlorides to this selenating (1 eq.) was injected, and the mixture was allowed to stand for
agent yielded bis(acyl) selenides. We synthesized three different 1 min at 240 1C. Acetic acid, which forms in situ, reacts
examples: bis(stearoyl) selenide (1), bis(benzoyl) selenide (2), and with Cd(myristate)2 to yield mixed coordination polymers
1
5
bis(4-chloro-benzoyl) selenide (3). After purification by recrystalliza- Cd(myristate)2 (acetate) , which phase separate from ODE.
Àx
x
1
13
77
tion, we characterized the products by H, C, and Se nuclear Then, precursor 1, 2, or 3 was injected and the reaction was
magnetic resonance spectroscopy (NMR, see ESI†).
As the reaction between bis(acyl) selenide and Cd(carboxylate)
to yield CdSe is expected to be of the Lewis acid/base type,
allowed to continue at 240 1C for 9 min, followed by quick
cooling to room temperature. Fig. 3a plots the resulting absorp-
the tion spectra of the unpurified reaction products. Fig. 3b and c
2
26,27
Lewis basicity of the selenide precursor determines its reactivity. In show the corresponding transmission electron microscopy
aliphatic compound 1, only inductive electron pushing depolarizes (TEM) images for 1 and 2, respectively. We intentionally
the C–Se bond. In aromatic compounds 2 and 3, mesomeric effects avoided any size-selective purification to obtain a full picture
cause further depolarization of this bond. Moreover, in compound of the reaction products.
3
, the lone-pair electrons of Cl are conjugated with the aromatic
system resulting in a lower Lewis basicity compared to 2. Thus, the The most reactive precursor 1 yielded mostly thin 3-monolayer
reactivity towards Cd(carboxylate) should decrease from 1 to 2 to 3. (ML) thick NPLs [heavy-hole optical transition (hh) at
We confirmed this trend with a liquid-phase synthesis of quasi- B460 nm]. The less-reactive precursor 2 gave primarily 4 ML
The product obtained correlates with the precursor reactivity.
2
1
6
1
6
spherical CdSe quantum dots (QDs). 1, 2, or 3 was combined CdSe NPLs (hh at B510 nm). Precursor 3 was apparently not
with Cd(oleate) in ODE at 180 1C, as described in the ESI.† sufficiently reactive, yielding polydisperse CdSe QDs.
2
Fig. 3 (a) Absorption spectra of CdSe NPLs synthesized by combining 1, 2, or 3 with Cd(myristate)2Àx(acetate)
measured without further purification). Precursor 1 yields 3 monolayer (ML) NPLs [heavy-hole optical transition (hh) at B460 nm], 2 yields 4 ML NPLs
hh at B510 nm), and 3 yields mostly quantum dots. (b and c) Transmission electron micrographs of the unpurified reaction product for NPLs synthesized
x
in ODE at 240 1C (aliquots after 10 min,
(
with (b) 1 and (c) 2. Scale bars are 100 nm. We intentionally avoided any size-selective purification before analysis to see all products. XRD patterns for the
products are shown in the ESI.†
This journal is ©The Royal Society of Chemistry 2018
Chem. Commun., 2018, 54, 11789--11792 | 11791

View Article Online
Communication
ChemComm
The spectra at early time points further clarify these observations Notes and references
(
Fig. S3, ESI†). In the synthesis with 1, just traces of 4 ML NPLs are
1
S. Ithurria and B. Dubertret, J. Am. Chem. Soc., 2008, 130,
1
present after 2 min, while with 2 and 3, they are much more evident.
This is seen from their absorption features at B510 nm. With 3
however, larger quantities of QDs are also observed early. As we have
6504–16505.
2 J. Joo, J. S. Son, S. G. Kwon, J. H. Yu and T. Hyeon, J. Am. Chem. Soc.,
006, 128, 5632–5633.
2
3
Y.-H. Liu, F. Wang, Y. Wang, P. C. Gibbons and W. E. Buhro, J. Am.
Chem. Soc., 2011, 133, 17005–17013.
15
recently shown, thin NPLs grow faster than thicker NPLs but are
less stable. Indeed, the thermodynamically most stable shape for
CdSe nanocrystals are quasi-spherical QDs. Hence, in all three
syntheses, 3 ML NPLs are expected to appear early but disappear
4 A. W. Achtstein, A. Schliwa, A. Prudnikau, M. Hardzei,
M. V. Artemyev, C. Thomsen and U. Woggon, Nano Lett., 2012, 12,
3151–3157.
5
6
7
8
9
S. Ithurria, M. D. Tessier, B. Mahler, R. Lobo, B. Dubertret and
A. Efros, Nat. Mater., 2011, 10, 936–941.
M. Olutas, B. Guzelturk, Y. Kelestemur, A. Yeltik, S. Delikanli and
H. V. Demir, ACS Nano, 2015, 9, 5041–5050.
A. Yeltik, S. Delikanli, M. Olutas, Y. Kelestemur, B. Guzelturk and
H. V. Demir, J. Phys. Chem. C, 2015, 119, 26768–26775.
C. E. Rowland, I. Fedin, H. Zhang, S. K. Gray, A. O. Govorov,
D. V. Talapin and R. D. Schaller, Nat. Mater., 2015, 14, 484–489.
J. Q. Grim, S. Christodoulou, F. Di Stasio, R. Krahne, R. Cingolani,
L. Manna and I. Moreels, Nat. Nanotechnol., 2014, 9, 891–895.
28
due to Ostwald ripening as the reaction proceeds. The released
CdSe] monomers can redeposit either on other 3 ML NPLs (Ostwald
[
ripening within a thickness population) or on thermodynamically
more stable 4 ML NPL seeds or QDs. In the synthesis with 1, in
which very little 4 ML NPLs or QDs are available, mainly lateral
28
Ostwald ripening within a thickness population is observed.
However, in the synthesis with 2, 4 ML NPL seeds are present in
sufficient quantities that the monomers mainly deposit on them,
leading after 9 min predominantly to 4 ML NPLs. In the synthesis
with 3, even though 4 ML NPL seeds are present, QDs are the most
abundant species. Over time, the material transferred from NPLs to
QDs only increases, driven by the high initial QD concentration.
1
0 C. She, I. Fedin, D. S. Dolzhnikov, A. Demorti `e re, R. D. Schaller,
M. Pelton and D. V. Talapin, Nano Lett., 2014, 14, 2772–2777.
1 E. Lhuillier, S. Pedetti, S. Ithurria, H. Heuclin, B. Nadal, A. Robin,
G. Patriarche, N. Lequeux and B. Dubertret, ACS Nano, 2014, 8,
1
3
813–3820.
1
2 Z. Chen, B. Nadal, B. Mahler, H. Aubin and B. Dubertret, Adv. Funct.
Mater., 2014, 24, 295–302.
These results demonstrate effects on nucleation by tailoring the 13 S. Ithurria, G. Bousquet and B. Dubertret, J. Am. Chem. Soc., 2011,
1
33, 3070–3077.
precursor reactivity via their molecular design.
1
4 B. Mahler, B. Nadal, C. Bouet, G. Patriarche and B. Dubertret, J. Am.
Chem. Soc., 2012, 134, 18591–18598.
In summary, we have identified bis(acyl) selenides (and
sulfides) as reactive organo-selenium precursors for nanocrystal 15 A. Riedinger, F. D. Ott, A. Mule, S. Mazzotti, P. N. Kn u¨ sel, S. J. Kress,
F. Prins, S. C. Erwin and D. J. Norris, Nat. Mater., 2017, 16, 743–748.
6 D. Chen, Y. Gao, Y. Chen, Y. Ren and X. Peng, Nano Lett., 2015, 15,
growth. We uncovered such species as intermediates during a
detailed analysis of the solvent-free synthesis of CdSe (CdS)
1
4477–4482.
NPLs. While we focused here on the selenides, we note that 17 M. Nasui, R. B. Mos, T. Petrisor Jr, M. S. Gabor, R. A. Varga,
L. Ciontea and T. Petrisor, J. Anal. Appl. Pyrolysis, 2011, 92, 439–444.
8 J. P. Kercher, E. A. Fogleman, H. Koizumi, B. Szt ´a ray and T. Baer,
J. Phys. Chem. A, 2005, 109, 939–946.
some of the bis(acyl) sulfides are commercially available. In
addition to the utility of these chalcogenide precursors in
1
liquid-phase synthesis of semiconductor nanocrystals, such pre- 19 M. P. Campos, M. P. Hendricks, A. N. Beecher, W. Walravens,
R. A. Swain, G. T. Cleveland, Z. Hens, M. Y. Sfeir and J. S. Owen,
cursors can provide tailored reactivity for studying the funda-
J. Am. Chem. Soc., 2017, 139, 2296–2305.
0 M. P. Hendricks, M. P. Campos, G. T. Cleveland, I. Jen-La Plante and
J. S. Owen, Science, 2015, 348, 1226–1230.
1 T. Qi, H.-Q. Yang, D. M. Whitfield, K. Yu and C.-W. Hu, J. Phys.
Chem. A, 2016, 120, 918–931.
2 Z. H. Sun, H. Oyanagi, H. Nakamura, Y. Jiang, L. Zhang, M. Uehara,
K. Yamashita, A. Fukano and H. Maeda, J. Phys. Chem. C, 2010, 114,
mentals of nucleation and growth of these materials.
2
2
2
This work was supported by ETH Research Grant ETH-38 14-
1
and by the Swiss National Science Foundation under Grant
No. 200021-165559. We are grateful to H. Sch o¨ nberg and
H. Gr u¨ tzmacher for technical assistance and discussion of
the TGA/DSC/EI-MS data. We thank V. Lin, K. McNeill, O.
1
0126–10131.
3 R. Garcia-Rodriguez and H. Liu, Chem. Commun., 2013, 49,
857–7859.
NMR and XRD measurements and K. Boldt for stimulating 24 H. Ishihara, M. Koketsu, Y. Fukuta and F. Nada, J. Am. Chem. Soc.,
2
7
7
Waser, F. Starsich, and S. Pratsinis for help with the Se
7
2001, 123, 8408–8409.
discussions. We utilized facilities at the Scientific Center for
Optical and Electron Microscopy (ScopeM) at ETH Zurich.
2
2
2
2
5 M. Koketsu, F. Nada, S. Hiramatsu and H. Ishihara, J. Chem. Soc.,
Perkin Trans. 1, 2002, 737–740.
6 J. S. Owen, E. M. Chan, H. T. Liu and A. P. Alivisatos, J. Am. Chem.
Soc., 2010, 132, 18206–18213.
7 H. T. Liu, J. S. Owen and A. P. Alivisatos, J. Am. Chem. Soc., 2007, 129,
Conflicts of interest
305–312.
8 F. D. Ott, A. Riedinger, D. R. Ochsenbein, P. N. Kn u¨ sel, S. C. Erwin,
The authors declare no competing financial interests.
M. Mazzotti and D. J. Norris, Nano Lett., 2017, 17, 6870–6877.
11792 | Chem. Commun., 2018, 54, 11789--11792
This journal is ©The Royal Society of Chemistry 2018