Synthesis of InP/ZnS Nanocrystals and Phase Transfer by Hydrolysis of Ester

Article abstract of DOI:10.1515/zpch-2018-1167

The synthesis of highly luminescent non-toxic nanocrystals (NCs) and the subsequent phase transfer to aqueous solution by hydrolysis of the crystal-bound ester are presented. Therefore, the synthesis of the spherical semiconductor system InP/ZnS was modified by changing the sulfur precursor in the synthesis from 1-dodecanethiol to dodecyl 3-mercaptopropionate (D3MP). By employing D3MP both as sulfur precursor for the ZnS shell growth and as stabilizing ligand, the phase transfer from organic to aqueous solution can be performed easily. Instead of the usually employed ligand exchange with mercaptopropionic acid, the NCs are only shaken with a sodium borate buffer in order to obtain aqueous soluble NCs by hydrolysis of the ester. In future work, the NCs must be protected against aggregation and the long term stability has to be increased. The optical properties of the samples are investigated by UV/Vis and photoluminescence spectroscopy, and the morphology of the nanoparticles (NPs) before and after phase transfer is determined by transmission electron microscopy.

Full text of DOI:10.1515/zpch-2018-1167

Z. Phys. Chem. 2018; aop
Franziska Lübkemann, Timo C. Gusenburger, Dominik Hinrichs,
Rasmus Himstedt, Dirk Dorfs and Nadja C. Bigall*
Synthesis of InP/ZnS Nanocrystals and
Phase Transfer by Hydrolysis of Ester
https://doi.org/10.1515/zpch-2018-1167
Received February 23, 2018; accepted May 9, 2018
Abstract: The synthesis of highly luminescent non-toxic nanocrystals (NCs) and
the subsequent phase transfer to aqueous solution by hydrolysis of the crystal-
bound ester are presented. Therefore, the synthesis of the spherical semiconduc-
tor system InP/ZnS was modified by changing the sulfur precursor in the synthesis
from 1-dodecanethiol to dodecyl 3-mercaptopropionate (D3MP). By employing
D3MP both as sulfur precursor for the ZnS shell growth and as stabilizing ligand,
the phase transfer from organic to aqueous solution can be performed easily.
Instead of the usually employed ligand exchange with mercaptopropionic acid,
the NCs are only shaken with a sodium borate buffer in order to obtain aqueous
soluble NCs by hydrolysis of the ester. In future work, the NCs must be protected
against aggregation and the long term stability has to be increased. The optical
properties of the samples are investigated by UV/Vis and photoluminescence
spectroscopy, and the morphology of the nanoparticles (NPs) before and after
phase transfer is determined by transmission electron microscopy.
Keywords: hydrolysis of ester; ligands; nanocrystal; nanoparticles; phase transfer;
saponification; water transfer.
1 Introduction
In past decades the interest in the synthesis of highly photoluminescent colloi-
dal semiconductor nanoparticles with different shapes, sizes and compositions is
severely increased [1–5]. Due to their versatile properties, there is a broad field of
*Corresponding author: Nadja C. Bigall, Institute for Physical Chemistry and Electrochemistry,
Leibniz Universität Hannover, Callinstraße 3A, 30167 Hannover, Germany; and Laboratory
for Nano- and Quantum Engineering, Leibniz Universität Hannover, Schneiderberg 39,
30167 Hannover, Germany, e-mail: nadja.bigall@pci.uni-hannover.de
Franziska Lübkemann, Timo C. Gusenburger, Dominik Hinrichs, Rasmus Himstedt and
Dirk Dorfs: Institute for Physical Chemistry and Electrochemistry, Leibniz Universität
Hannover, Callinstraße 3A, 30167 Hannover, Germany; and Laboratory for Nano- and Quantum
Engineering, Leibniz Universität Hannover, Schneiderberg 39, 30167 Hannover, Germany
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applications e.g. in biochemistry, biological analysis, or electronic devices. In the
field of biology and medicine water soluble fluorescent non-toxic nanoparticles
are used as cell marker to enable the visibility of biological process in the human
body [6–8]. Nowadays, there is a broad range of methodologies for nanoparticle
synthesis. Generally, they can be divided in wet chemical preparation (in organic
solvent e.g. hot-injection [9–11] or heating-up [12], in aqueous solvent e.g. by
chemical precipitation [13–16]), solid-state preparation (e.g. ball milling [17, 18]),
gas-phase preparation (e.g. furnace flow reactors [19, 20]) or deposition methods
(e.g. or molecular beam epitaxy [21, 22]). The hot-injection method is mainly used
for colloidal nanoparticle synthesis due to the high degree of control over shape
and size [9, 10]. By controlling the shape and size of nanocrystals (NCs) it is pos-
sible to tune their optical properties [9, 10]. In addition, the wide range of acces-
sible II–VI (e.g. CdSe [9, 23], CdTe [14], ZnSe and ZnS) and III-V semiconductors
(e.g. GaP, GaAs and InP [24, 25]) gives rise to a broad range of optical properties.
By combining two semiconductor systems in one NC, so-called nanoheterostruc-
tures can be synthesized. They can be, e.g. dot-in-rod shaped CdSe/CdS [1], CdSe/
ZnS [26] or spherical core-shell NCs like CdSe/ZnS [4], InP/ZnS [5, 27, 28], Cu-Zn-
In-S/ZnS [11]. This enables the possibility to protect the particle cores from e.g.
oxidative processes and improves the optical properties like the photolumines-
cence quantum yield (PLQY). To make the semiconductor suitable for biological
applications, however, the NCs must be soluble in aqueous solutions. There-
fore, different techniques have been developed in recent years to receive water
stable NCs, including ligand exchange reactions with thiol-based molecules
[29–31] as well as encapsulation into hydrophilic or amphiphilic (e.g. silica [32,
33] and amphiphilic polymers [34–36]) materials. Recently, a new approach by
using hydrolysis of crystal-bound esters on the nanocrystal surface for the phase
transfer was introduced by the working group Macdonald [37, 38]. This approach
associated the nanoparticle synthesis part, in which dodecyl 3-mercaptopropion-
ate (D3MP) is used as sulfur precursor as well as crystal-bound ligand, with the
subsequent phase transfer from organic to aqueous solution by hydrolysis of the
crystal-bound ester. The previous works of the working group Macdonald show
the synthesis of Cu2S and CdSe/ZnS NCs by using D3MP as sulfur precursor and
crystal-bound ligand. Via a basic treatment the organic soluble NCs were trans-
ferred to aqueous solution by the hydrolysis of the ester. This method has the
advantage, that the surface of the NCs are not affected by the hydrolysis of the
ester in comparison to the usually used ligand exchange procedures for the phase
transfer. For fluorescent NCs, this approach seems promising, since the interface
between the semiconductor and the ligand has a high influence on the optoelec-
tronic properties, and it is worth investigating the suitability of this method for
highly luminescent non-toxic nanocrystal systems.
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Synthesis of InP/ZnS Nanocrystals and Phase Transferꢂ ꢂ3
In the present work, we investigated the synthesis of organic soluble
InP/ZnS NCs by the hot-injection method. Instead of the recently employed
1-dodecanethiol, here, we used D3MP both as the sulfur precursor and as capping
ligand in the synthesis. We investigated the synthesis of highly luminescent
organic soluble InP/ZnS NCs by using D3MP and the subsequent water transfer by
saponification of the ester by using an alkaline solution. Due to the incorporation
of the capping ligand in the ZnS shell of the InP/ZnS core shell particle surface, a
significant decrease of luminescence through the creation of surface trap states
by the phase transfer procedure could be excluded for large particles. Caused
by insufficient stabilization due to crystal-bound mercaptopropionic acid of the
phase transferred InP/ZnS NCs, they start to aggregate after some time in aqueous
solution which has to be overcome in future work. The applied technique allows
an easy transfer of the InP/ZnS NCs from organic to aqueous solution without a
significant alteration of the optoelectronic properties like e.g. a decrease of the
photoluminescent quantum yield (PLQY) in case of large particle sizes.
2 Experimental section
InP/ZnS NCs were synthesized by following the previous report from Reiss et al.
with some modifications [5]. Instead of the reported one-pot one step synthesis,
here the core/shell NCs were synthesized in a one-pot but two step synthesis.
In the first reaction step InP core particles were synthesized by heating-up the
reaction mixture under inert atmosphere. In the second step dodecyl 3-marcap-
topropionate (D3MP) dissolved in octadecene was injected at 230ꢀ°C to grow a
ZnS shell around the InP core. D3MP is sulfur precursor and stabilizing ligand at
the same time. By using different amounts of myristic acid (MA) in the synthesis
of the indium myristate precursor, different sizes of InP cores can be obtained.
For the sake of clarity, we introduce the following abbreviations: InP/ZnS (S) for
small InP core, InP/ZnS (M) for medium sized InP core and InP/ZnS (L) for large
InP core. The amount of zinc and sulfur precursor was constant in all syntheses.
2.1 Chemicals
Indium(III) acetate (99.99%), zinc stearate (12.5–14%) purchased from Alfa
Aesar. 1-dodecanol (98%), n-hexane (99%), 3-mercaptopropionic acid (99%),
myristic acid (MA, 99%), 1-octadecene (ODE, 90%), potassium hydroxide (KOH,
>85%), sulfuric acid (95–97%), tetrahydrofuran (THF, 99.9%) were purchased
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from Sigma Aldrich. Tris(trimethylsilyl)phosphine (98%) was purchased from
ABCR. Dichloromethane (>99.9%) 2-propanol (99.5%) purchased from Carl Roth.
Magnesium sulfate (MgSO4, 62–70%) was purchased from Fisher Chemical.
2.2 Synthesis of indium myristate In(MA)x stock solution
The indium myristate precursors were synthesized according to the procedure
of Li and Reiss [5]. Two millimolar indium acetate, a varying quantity (2; 3;
4.3 mmol) of MA and 20 mL ODE were mixed in a three-neck flask equipped with
condenser under inert atmosphere. The mixture was heated up to 120ꢀ°C for 2 h
under vacuum, cooled down to room temperature, refilled with argon, and stored
under inert atmosphere.
2.3 Synthesis of dodecyl 3-merctopropionate (D3MP)
D3MP was synthesized according to the procedure of Byrne et al. with some mod-
ifications [39]. Therefore, 3.5 mL 3-mercaptopropionic acid (20 mmol), 26.9 mL
1-dodecanol (60 mmol) and 10 drops of conc. H2SO4 were mixed in a screw cap
vial and stirred over night at room temperature. The reaction mixture was diluted
with dichloromethane and washed with distilled water. The organic phase was
dried with MgSO4 and evaporated. D3MP was obtained as colorless oil (24.03 g,
90.6%) and stored under inert atmosphere.
2.4 Synthesis of InP/ZnS core/shell NCs
One milliliter of In(MA)x (2 mmol, 3 mmol, 4.3 mmol) stock solution, 0.063 g zinc
stearate, 7 mL ODE and 29 μL tris(trimethylsilyl)phosphine were mixed in a three-
neck flask under inert atmosphere. For the InP core growth, the reaction mixture
was kept under inert atmosphere for 30 min at 150ꢀ°C, subsequently for 1 h at
230ꢀ°C. 1 mL of the sample was taken with a glass syringe. Then, 40 μL of D3MP
diluted in 500 μL ODE were injected, and the reaction mixture was heated 2 h at
230ꢀ°C followed by cooling to room temperature. All samples were precipitated
with 10–15 mL 2-propanol, centrifuged (8000 RCF) and subsequently dissolved
in 2 mL n-hexane. After that, 500 μL chloroform were added to the samples, fol-
lowed by centrifugation to remove excess of organics. The samples were precipi-
tated twice with 2-propanol, centrifuged and stored in n-hexane.
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Synthesis of InP/ZnS Nanocrystals and Phase Transferꢂ ꢂ5
2.5 Preparation of water-soluble InP/ZnS NCs
One hundred microliters InP/ZnS NCs were precipitated with 1 mL 2-propanol,
centrifuged 5 min (14.100 RCF), and redispersed in 2 mL THF. To decompose
D3MP at the nanoparticle surface 100 μL of Na2B4O7ꢀ·ꢀ10 H2O buffer solution (pH
9.3, 8.9 mL 0.1 M NaOH, 0.05 M sodium tetraborate decahydrate in 100 mL volu-
metric flask) were added. The reaction mixture was kept 2 h at 50ꢀ°C in an ultra-
sonic bath. The NCs were precipitated with 4 mL n-hexane and centrifuged for
5 min (6.000 RCF). Afterwards, the InP/ZnS NCs were washed with 4 mL ethanol
and centrifuged again for 5 min (6.000 RCF). The cleaned NCs were dispersed and
stored in water.
3 Characterization
3.1 Transmission electron microscopy
The morphology of the InP/ZnS NCs in solution was investigated by transmis-
sion electron microscopy (TEM) using a FEI Tecnai G2 F20 TMP (CSꢀ=ꢀ2 mm,
CCꢀ=ꢀ2 mm) transmission electron microscope, equipped with a 200 kV field emis-
sion gun. Micrographs were taken in bright field mode. For the grid preparation,
10 μL of the purified and diluted sample were drop casted on a 300 mesh carbon
coated copper grid, purchased from Quantifoil. The intensities of the selected
area electron diffraction (SAED) ring patterns were azimuthal integrated utiliz-
ing the profile analysis of SAD (PASAD) plugin for Digital Micrograph [40, 41].
The raw profiles were exported without any background corrections.
3.2 Spectroscopic characterization
UV/Vis absorption spectra were recorded using a Cary 5000 absorption spectro-
photometer equipped with an integrating sphere DRA-2500 from Agilent. The
spectra were measured in hexane and dist. water and the concentration was
adjusted to an optical density of 0.1 at the first absorption maximum from InP.
Photoluminescence emission spectra were measured using a Horiba Fluoro-
max-4. PLQYs (measured in absolute mode by dividing the number of emitted
photons by the number of absorbed photons) of the samples were recorded using
a Dual-FL spectrophotometer equipped with a Quanta-φ integrating sphere pur-
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chased from Horiba. All optical measurements were performed in fluorescence
quartz cuvettes purchased von Hellma Analytics.
3.3 Powder X-ray diffraction
Powder X-ray diffraction (XRD) pattern were recorded using a Bruker D8 Advance
in reflection mode with CuK source operating at 400 kV and 30 mA. For sample
preparation 10 μL of the NP αsolution were drop casted several times on a crystal-
line silicon sample holder.
4 Results and discussion
In this work, we want to demonstrate the synthesis of InP/ZnS NCs by a modified
synthesis route and the direct phase transfer after the synthesis due to hydrolysis
of esters. The scheme in Figure 1 shows the synthetic route for the here presented
InP/ZnS NP.
4.1 InP/ZnS nanoparticles
The transmission electron microscopic (TEM) images and size histograms from
the as synthesized InP cores and InP/ZnS NCs are shown in Figure 2. The InP core
NCs and the InP/ZnS NCs have a spherical shape and a narrow size distribution.
For the InP core NCs the particle size varied from 2.1ꢀ ꢀ0.5 nm for small cores over
2.5ꢀ ꢀ0.4 nm for medium sized cores to 3.2ꢀ ꢀ0.7 nm for large InP cores. After ZnS
Fig. 1:ꢀScheme of synthesis route. In first reaction step InP core NPs were formed. Due to the
addition of D3MP as sulfur precursor and ligand, a ZnS shell was grown on the InP core. The last
reaction step is the hydrolysis of the D3MP ester to phase transfer the particles from organic to
aqueous solution.
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Synthesis of InP/ZnS Nanocrystals and Phase Transferꢂ
Fig. 2:ꢀTEM images with size histograms (inset) of InP core NCs and InP/ZnS NCs with different
InP core sizes. (a) Small InP cores, (c) medium sized InP cores, (e) large InP cores, (b) InP/ZnS
(S) with small InP cores, (d) InP/ZnS (M) NCs with medium sized InP core, (f) InP/ZnS (L) with
large InP core.
shell growth the particle size increased and is within the range of 2.8ꢀ ꢀ0.5 nm for
InP/ZnS (S) with a small InP core, 3.1ꢀ ꢀ0.6 nm for InP/ZnS (M) NCs with medium
sized InP core and 3.6ꢀ ꢀ0.6 nm for InP/ZnS (L) NCs with a large InP core. Due to
the low contrast, the difference between the InP core and the ZnS shell cannot be
seen in the TEM images.
The crystal phase of the InP/ZnS NCs is identified by means of powder X-ray
diffraction (XRD) measurements, which are shown in Figure 3. By comparing the
measured XRD patterns (Figure 3a–c) to literature diffraction pattern (Figure 3d),
the cubic phases from InP and ZnS can be assigned to the synthesized samples.
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Fig. 3:ꢀPowder X-ray diffractograms of (a) InP/ZnS (S), (b) InP/ZnS (M), and (c) InP/ZnS (L) NCs.
Diffraction pattern for comparison (d) of cubic InP (red) (PDF card No. 03-065-2889) and cubic
ZnS (blue) (PDF card No. 03-065-1691). Electron diffraction (SAED) of InP and InP/ZnS (L) NCs (e).
Due to the similarity of the cubic InP and ZnS phase, electron diffraction (SAED)
of the largest InP/ZnS NCs was measured to further proof the ZnS shell growth on
the InP core. Therefore, electron diffraction was recorded from the InP core NPs
and the InP/ZnS NPs. Both are shown in Figure 3e. It can be seen that the reflec-
tions (111), (200) and (311) of the InP/ZnS sample are shifted to shorter lattice dis-
tances in comparison to the pure InP sample, which was taken before the sulfur
precursor was injected. This behavior is in line with a ZnS shell growth around
the InP core NCs.
Additional, optical characterization (absorption and photoluminescence
measurements) of the InP cores and the related InP/ZnS NCs were conducted and
shown in Figure 4. In all obtained absorption spectra, a slight shift of the InP
absorption band to higher wavelengths after the ZnS shell growth compared to
the pure InP core absorption spectra can be seen. The InP absorption band shifted
from 430 nm for small InP cores to 470 nm for InP/ZnS (S) NCs, from 480 nm for
medium sized InP cores to 490 nm for InP/ZnS (M) NCs and from 530 nm for the
large InP cores to 550 nm for InP/ZnS (L) NCs. This red shift with shell growth is
an indicator that the charge carrier probability density function of the excited
charge carriers spreads significantly into the shell materials despite of the high
band gap of ZnS. Furthermore, in all cases a second absorption band and shorter
wavelengths arises. This additional band is located at 420 nm for small InP/ZnS
(S) NCs and medium sized InP/ZnS (M) NCs and at 430 nm for the large InP/ZnS
(L) NCs. It appears after the shell growth and can be assigned to the ZnS. The
emission band of the particles is located at at 526 nm for the InP/ZnS (S) NCs, at
542 nm for the InP/ZnS (M) NCs and at 616 nm for the InP/ZnS (L) NCs. This shift
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Synthesis of InP/ZnS Nanocrystals and Phase Transferꢂ
Fig. 4:ꢀAbsorption spectra (solid lines) and emission spectra (dashed lines) of (a) small InP
cores (blue) and InP/ZnS (S) NCs (black), (b) medium sized InP cores (blue) and InP/ZnS (M) NCs
(black) and (c) large InP cores (blue) and InP/ZnS (L) NCs (black).
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is simply explained by size quantization. The fwhm of the emission band of the
larger particles is slightly broader than the fwhm of the smaller particles. Com-
pared to the InP cores all emission maxima are shifted to higher wavelengths after
ZnS shell growth. The photoluminescent quantum yield (29.4%, 24.4% and 17.3%)
is decreasing with increasing particle size but always much higher for the core-
shell particles than for the pure InP particles, since the ZnS effectively prevents
charge carriers from being trapped at the particle surface.
4.2 Phase transferred InP/ZnS NCs
The optical characterization of the aqueous soluble InP/ZnS NCs (red lines) and
the comparison to the organic solution (black lines) is shown in Figure 5. As can
be seen, the slope of the absorption spectra and the absorption band position of
the InP/ZnS core-shell particles is in all cases the same in organic and aqueous
solution. This indicates, that the ZnS shell and the InP core are not strongly
Fig. 5:ꢀAbsorption- and emission spectra of organic solution (black line) and aqueous solution (red
line) of (a) small InP/ZnS NCs (S), (b) medium sized InP/ZnS NCs (M) and (c) large InP/ZnS NCs (L).
The insets in a and b show the magnification of the emission spectra of InP/ZnS NCs in aqueous
solution. Transmission electron microscope image (d) of large aqueous soluble InP/ZnS NCs.
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affected by the hydrolysis reagent. The emission maxima of the aqueous InP/ZnS
NCs solutions shift in all cases to larger wavelengths and the emission intensities
decreases. This shift becomes smaller with increasing particles size. One reason
for the red shift can be changes in the coordination of Zn from stearate to hydrox-
ide due to the base addition. The hydroxide anions are known to charge balance
the surface of NCs. This new electron traps are localized surface defects and can
change the electron wavefunction [42]. Furthermore, the addition of base can
induce defects e.g. trap-states on the NC surface. This trap-states can also be a
reason for a red-shift of the emission maxima. In all cases the emission intensity
is decreasing from organic to aqueous solution. This decrease can also cause by
trap-states on the NC surface. Another explanation for the decrease in emission
intensity after the phase transfer can be the poor passivation against the environ-
ment due to the short length of the crystal-bound MPA. The PLQYs decrease after
phase transfer from organic to aqueous solution. The quenching effect is higher
with decreasing particle sizes. In case of the smallest InP/ZnS NCs it drops from
29.4% to 0.8%, at the medium sized NCs from 24.4% to 3.3% and at the larges
InP/ZnS NCs from 17.2% to 10.6%. This observed tendency could be explained by
the different magnitude of the probability density function of the charge carriers
at the particle surface for the different InP core sizes. The smallest InP core will
obviously show the strongest size quantization and hence the probability density
function will leak strongly into the ZnS shell. Therefore, at the new surface of
the core shell particle, the probability density function for the charge carriers
will have a larger value for the smaller InP cores, resulting in a larger trapping
probability. Hence, in future experiments, possibly thicker ZnS shells should be
invoked when very small InP particles serve as core particles. The TEM image of
the largest phase transferred InP/ZnS NCs are shown exemplarily in Figure 5d.
Due to the strong aggregation on the TEM grid it was difficult to visualize the NCs.
It can be seen, that the crystallinity due to the visible lattice planes and size of the
large InP/ZnS (L) are not affected by the hydrolysis of the ester.
5 Conclusion
In this work we presented the synthesis of spherical InP/ZnS NCs with different
sizes by using D3MP as sulfur precursor and ligand. The sizes are varied from
2.8 to 3.6 nm by the amount of the used indium myristate precursor and have
a narrow size distribution. It was shown, that the variation in size also leads to
variation of the optical properties. The obtained organic soluble InP/ZnS NCs
show PLQY up to 29.4% for NCs with 2.8 nm diameter and 17.3% for NCs with
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3.6 nm diameter. The InP/ZnS NCs were easily transferred to aqueous solution
by hydrolysis of the surface bound D3MP esters by using a sodium borate buffer.
The water soluble NCs show a decrease in the PLQY. Insufficient incorporation of
crystal-bound D3MP in the ZnS shell could be avoided by reducing the reaction
temperature and the reaction time of the shell growth. Even though future work
has to be conducted in order to apply this strategy for obtaining optimum col-
loidal and optical properties, our work demonstrates a first step towards an easy
synthetic route for phase transfer of highly luminescent non-toxic NCs direct after
synthesis by hydrolysis of the crystal-bound ester.
Acknowledgement: The authors (N.B. and F.L.) are grateful for financial support
from the German Federal Ministry of Education and Research (BMBF) within the
framework of the program NanoMatFutur, support code 03X5525. Furthermore,
the project leading to these results has in part received funding from the Euro-
pean Research Council (ERC) under the European Union’s Horizon 2020 research
and innovation program (grant agreement No. 714429). The authors (D.D., F.L.
and D.H.) are grateful for the financial support from Volkswagen foundation
(lower Saxony/Israel cooperation, Grant ZN2916). The author D.D. thanks the DFG
(research Grant 1580/5-1). R.H. is grateful to Hannover School for Nanotechnol-
ogy (HSN) for funding. The authors thank Prof. Caro and Prof. Armin Feldhoff for
access to powder X-ray diffraction measurements. The authors thank the Labora-
torium of Nano- and Quantum Engineering of the Leibniz Universität Hannover
for support.
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