Highly stable CuInS2@ZnS:Al core@shell quantum dots: The role of aluminium self-passivation

Article abstract of DOI:10.1039/c5cc01137j

A simple approach was introduced to enhance the photostability of CuInS₂@ZnS core@shell quantum dots (QDs) by doping aluminium into the ZnS shell. Aluminium in the as-prepared QDs was oxidized to Al₂O₃, which formed a passivation oxide layer that effectively prevents photo-degradation of QDs during long-term light irradiation. This journal is

Full text of DOI:10.1039/c5cc01137j

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Highly stable CuInS2@ZnS:Al core@shell quantum
dots: the role of aluminium self-passivation†
Pinhua Rao,‡a Wei Yao,‡a Zhichun Li,b Long Kong,b Wenqi Zhanga and Liang Li*b
Cite this: Chem. Commun., 2015,
51, 8757
Received 6th February 2015,
Accepted 17th April 2015
DOI: 10.1039/c5cc01137j
www.rsc.org/chemcomm
A simple approach was introduced to enhance the photostability of of moisture and air, and thus enhanced the stability of CIS@ZnS
CuInS2@ZnS core@shell quantum dots (QDs) by doping aluminium QDs,19 but such an approach usually results in the decrease of
into the ZnS shell. Aluminium in the as-prepared QDs was oxidized to QY of QDs. Song et al. found that CIS@ZnS QDs overcoated with
Al2O3, which formed a passivation oxide layer that effectively prevents ZnGa2O4 showed strong resistance against photo-stimulated
photo-degradation of QDs during long-term light irradiation.
degradation.20 However, the preparation process was very com-
plicated. Therefore, it is important to find a simple and efficient
Quantum dots (QDs) with electrical and optical properties are of method to enhance the stability of CIS QDs without altering their
great interest for fundamental studies1,2 and technology applica- optical properties and their size related characters.
tions such as light emitting diodes (LEDs),3,4 solar cells,5,6 sensors,7,8
Self-passivation of material surface is a common phenom-
and biomedical labeling.9,10 During the past few decades, inorganic enon and generally employed to protect the exposed surface of
metal chalcogenide QDs including CdS, CdTe, PbS and CdSe have metals against oxidation and other chemical reactions.21 For
been intensively studied. However, the applications of these QDs example, aluminium (Al) is an active metal but very stable under
in practical devices are limited because of the intrinsic toxicity of ambient conditions because it usually forms a compact Al2O3
cadmium and lead.
protective layer covering the bulk material by the self-passivation
CuInS2 (CIS) QDs with a band gap of 1.5 eV, which are process. Similar self-passivation was observed for copper alloy
cadmium-free and lead-free, have been considered as emerging and metal Zn in air.22,23 This phenomenon is also existing on
alternatives for the toxic conventional QDs.11–14 However, CIS nano-scale materials. It was reported that Al nanoparticles could
core-only QDs tend to be unstable in air and provide poor fluores- be stabilized by forming an Al2O3 passivation layer.24
cence quantum yield (QY).15 Various methods were developed
However, rare reports have been found to use the self-passivation
to enhance the stability of CIS QDs. Kruszynska et al. indicated technology for improving the photostability of QDs. In this com-
that the organic ligand had an important effect on the stability of munication, we reported a simple and efficient method to enhance
CIS QDs.16 Zhang et al. found that the Cu/In molar ratio affected the photostability of QDs by using the self-passivation behavior of
the stability of the CIS QDs.17 Another common method to Al. The Al element was introduced into the shell of ZnS on the CIS
enhance the stability of CIS QDs is by growing a shell of a higher core, which was oxidized to Al oxides and then formed a passivation
band gap material to form core@shell structured QDs.18 ZnS is layer against further oxidation when the surface ZnS was degraded
typically chosen as a shell material for CIS QD passivation. under ambient conditions or under irradiation (Scheme 1).
However, similar to other types of core@shell QDs, CIS@ZnS
Preparation processes of CIS, CIS@ZnS and Al doped CIS@ZnS
QDs are also susceptible to photo-induced degradation. Besides, (CIS@ZnS:Al) QDs as well as characterization methods of
CIS@ZnS QDs are easily hydrolyzed in moisture, which is a general as-prepared QDs were given in the ESI.† Fig. 1 shows the corre-
issue for nano-scale and micro-scale sulfides. It was reported sponding evolution of the photoluminescence (PL) and UV-vis
that silica-embedded CIS@ZnS QDs could stop the penetration absorption spectra of the CIS QDs with the overcoating process
of Al doped ZnS (ZnS:Al) shell. The PL and UV-vis absorption
a School of Chemistry and Chemical Engineering, Shanghai University of
spectra both blue shift to a short wavelength with overcoating,
resulting from the diffusion of Zn into the CIS core.14 No signi-
ficant differences in PL and UV-vis absorption spectra were
observed between the QDs with/without doping of Al (Fig. 1 and
Fig. S1, ESI†). Fig. 2a–d shows TEM images of CIS@ZnS:Al QDs
synthesized at the same overcoating time (10 h) but with
Engineering Science, Shanghai 201620, P. R. China
b School of Environmental Science and Engineering, Shanghai Jiao Tong University,
Shanghai 200240, P. R. China. E-mail: liangli117@sjtu.edu.cn;
Tel: +86-21-54747567
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cc01137j
‡ These authors contributed equally to this work.
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Scheme 1 Schematic illustration for enhancing the photostability of CIS@ZnS
QDs by doping aluminium.
Fig. 2 TEM images of CIS@ZnS:Al QDs with the Al/Zn molar ratios of (a) 0,
(b) 0.5, (c) 1, and (d) 2; (e) effects of the Al/Zn molar ratio on the photostability
of CIS@ZnS:Al QDs.
measured for comparison purposes. The four samples were
synthesized by overcoating of ZnS:Al for 10 h, and they have
similar sizes that can avoid the effects from shell thickness on
photostability. As seen in Fig. 2e, the PL intensities of all
samples decrease with strong irradiation of blue light, and the
bare CIS QDs show the worst photostability among them. It is
obvious that the photostability of CIS@ZnS QDs is dramatically
enhanced by Al doping, and the sample synthesized with the Al/Zn
molar ratio of 0.5 has the best photostability. The possible reason
is that too much of Al would worsen the intrinsic chemical
stability of ZnS:Al and subsequently decrease the photostability of
CIS@ZnS:Al QDs due to the unstable Al–S bond. The improvement
of the photostability of CIS@ZnS:Al QDs can also be reflected
from the change in the fluorescence peak with irradiation time
(Table S3, ESI†). During the first 30 min of irradiation, the
fluorescence peaks of both CIS@ZnS QDs and CIS@ZnS:Al QDs
showed a blue shift (B2 nm), which could result from the
Fig. 1 (a) Crystal growth process and color change of CIS@ZnS:Al QDs;
(b) photoluminescence (PL) and (c) UV-vis absorption spectra of CIS@ZnS
QDs; (d) PL and (e) UV-vis absorption spectra and CIS@ZnS:Al QDs with a
Al/Zn molar ratio of 0.5.
different Al/Zn molar ratios. The four samples show similar quenching of the big QDs with more red emission during irradia-
sizes at around 3.0 nm, which indicates that Al doping had no tion (Fig. S3, ESI†). After 30 min irradiation, the fluorescence peak
significant effect on the sizes of CIS@ZnS:Al QDs. Elemental of CIS@ZnS QDs showed a red shift with the increase of irradiation
analysis results showed that the Al/Zn molar ratios of three doped time, possibly due to the further corrosion of the gradient alloyed
samples were 0.075, 0.188 and 0.218 (Table S1, ESI†), respectively, buffer layer between ZnS and CIS after the spalling of ZnS, which
lower than nominal Al/Zn molar ratios (0.5, 1, 2). With the can be confirmed by the red-shift of UV-vis absorption spectra
increase of the overcoating time, however, the crystal size tended (band gap becomes smaller) of the CIS@ZnS QDs during irradia-
to increase (Fig. S2, ESI†) and the (Cu + In)/(Al + Zn) molar ratio of tion (Fig. S4 and Scheme S2, ESI†). However, the fluorescence peak
QDs decreased (Table S2, ESI†), resulting from the increase of of CIS@ZnS QDs with Al doping, i.e., CIS@ZnS:Al QDs, maintained
ZnS:Al shell thickness. When the overcoating time was set at 20 h, a stable value, possibly resulting from the formation of Al2O3 which
the crystal size was about 4.0 nm.
prevented the photo-corrosion of ZnS.
The photostability of CIS@ZnS:Al QDs with different Al/Zn
The effects of overcoating time, i.e., the thickness of the ZnS:Al
molar ratios is shown in Fig. 2e. A bare CIS QD sample was also shell, on the photostability of CIS@ZnS:Al QDs were also studied,
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and the results indicated that the thickness of the ZnS:Al shell
significantly affected the photostability of CIS@ZnS:Al QDs
(Fig. S5, ESI†). The highest photostability of CIS@ZnS:Al QDs
was found when the overcoating time was designed at 10 h.
Longer overcoating time did not cause further improvement of
the photostability of QDs.
The QY of CIS@ZnS:Al QDs was measured using a standard
method.18 It can be seen that the QY of CIS QDs is dramatically
improved by the overcoating of ZnS, and the doping of Al into
ZnS can further improve the QY of CIS@ZnS QDs (Fig. S6, ESI†).
The best sample with the highest QY is that synthesized by
overcoating ZnS:Al for 10 h (Fig. S7, ESI†), and longer over-
coating often worsens their QY. The XRD characterization of QDs
with different Al doping levels was conducted and the results
indicated that the diffraction peaks of CIS@ZnS:Al QDs shifted
to a smaller angle with the increase of Al doping concentration
compared to the undoped QDs even though they are of similar
size, possibly due to Al alloyed into the lattice of CIS@ZnS QDs
(Fig. S8, ESI†). No mixture of different phases was observed,
which indicated that Al precursors did not self-nucleate and
formed some Al related crystals such as Al2O3 crystals.
In order to investigate the mechanism of photostability improve-
ment of CIS@ZnS QDs by Al doping, the changes in the chemical
states of the elements (Al, Cu, In, S, and Zn) of CIS@ZnS QDs
and/or CIS@ZnS:Al QDs before and after irradiation were measured
by XPS. As shown in Fig. 3a and b, the peaks corresponding to Al 2p
and Al 2s binding energy before irradiation are centered at 74.5 eV
and 121.8 eV. After CIS@ZnS:Al QDs were irradiated, the peaks
corresponding to Al 2p and Al 2s binding energy shifted
towards to higher energy, which generally indicated that Al
existed mainly in the form of Al–O (Al–OH or Al2O3).25 During
irradiation, CIS@ZnS:Al QD solutions were heated to 60–70 1C
because of the high temperature of the LED module, which may
accelerate the formation of Al oxides on the surface of CIS@ZnS:Al
QDs. Nascent Al2O3 could act as a passivation layer and inhibit the
change of the chemical states of Cu, In, Zn in CIS@ZnS:Al QDs.
This speculation was echoed by the XPS spectra of Cu (Fig. 3c
and d), In (Fig. 3e and f) and Zn (Fig. S9 and S10, ESI†). After
CIS@ZnS QDs had been irradiated, the chemical states of the
elements Cu, In and Zn were changed and the peaks corresponding
to Cu 2p, In 3d and Zn 2p shifted towards higher energy compared
to that before irradiation. However, when CIS@ZnS QDs were
doped by Al, the irradiation to as-prepared QDs (i.e., CIS@ZnS:Al
QDs) hardly changed the chemical states of the elements Cu, In
and Zn, indicating stronger stability of CIS@ZnS:Al QDs which was
confirmed by the results of photostability experiments (Fig. 2e). The
XPS spectra of S further supported the mechanism of Al2O3 as the
passivation layer. As observed in Fig. 3g, chemical states of S were
Fig. 3 XPS spectra of (a) Al 2p, (b) Al 2s, (d) Cu 2p, (f) In 3d and (h) S 2p of
CIS@ZnS:Al QDs (Al/Zn molar ratio is 0.5) before and after five-hour
irradiation; XPS spectra of (c) Cu 2p, (e) In 3d and (g) S 2p of CIS@ZnS QDs
before and after five-hour irradiation.
Fig. 4 Effects of five-hour irradiation on FTIR spectra of CIS@ZnS QDs
and CIS@ZnS:Al QDs with a Al/Zn molar ratio of 0.5.
changed after CIS@ZnS QDs has been irradiated. A new broad peak improvement during irradiation of the as-prepared QDs. As
2À
occurred near 170.4 eV, corresponding to S–O binding energy of shown in Fig. 4, typical SO4 peaks at around 1115 cmÀ1 and
sulfate.26 However, chemical states of S in CIS@ZnS:Al QDs almost 1065 cmÀ1 occurred, after CIS@ZnS QDs have been irradiated,27
did not change (Fig. 3h), indicating that the anti-oxidation of S was which indicated that CIS@ZnS QDs were unstable during irradia-
greatly enhanced, resulting from the protection of the Al2O3 tion. However, the FTIR spectra of CIS@ZnS:Al QDs before and
passivation layer on the CIS@ZnS:Al QDs.
after irradiation just caused a small change, and those two
The FTIR spectra of CIS@ZnS and CIS@ZnS:Al QDs were peaks obtained from oxidation of sulfur were not observed in
recorded to further elucidate the mechanism of the photostability the irradiated sample. This further confirmed the stability
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improvement of CIS@ZnS QDs by doping Al. The peak at about Notes and references
1380 cmÀ1 was reported to be assigned to both the vibration
1 N. Zheng, X. Bu, H. Lu, Q. Zhang and P. Feng, J. Am. Chem. Soc.,
2005, 127, 11963.
of Al–O28,29 or the carboxyl group30,31 from little zinc oleate
adsorbed on QDs while the peak of CIS@ZnS:Al QDs at about
1380 cmÀ1 was broad compared to that of CIS@ZnS QDs, which
was attributed to the overlapping of the vibrations of Al–O and the
carboxyl group, and it became stronger after irradiation possibly
because of the formation of more Al oxides. Unfortunately, no
other direct evidence of Al2O3 formation was found on the FTIR
spectrum of the doped sample after irradiation, so further inves-
tigations are needed to be done in the future.
`
2 P. Reiss, M. Protiere and L. Li, Small, 2009, 5, 154.
3 M. Roushan, X. Zhang and J. Li, Angew. Chem., Int. Ed., 2012, 51, 436.
4 F. Wang, M. Kreiter, B. He, S. Pang and C. Liu, Chem. Commun.,
2010, 46, 3309.
´
´
5 V. M. Blas-Ferrando, J. Ortiz, V. Gonzalez-Pedro, R. S. Sanchez,
´
´
´
´
I. Mora-Sero, F. Fernandez-Lazaro and A. Sastre-Santos, Chem.
Commun., 2015, 51, 1732.
6 O. E. Semonin, J. M. Luther, S. Choi, H. Y. Chen, J. Gao, A. J. Nozik
and M. C. Beard, Science, 2011, 334, 1530.
7 L. Shi, V. D. Paoli, N. Rosenzwig and Z. Rosenzweig, J. Am. Chem.
Soc., 2006, 128, 10378.
In order to further investigate the applicability of Al doping
in enhancing the photostability of QDs, we doped Al into the
CdS shell of CIS@CdS QDs using the same approach as above.
The photostability test demonstrated that the photostability of
CIS@CdS QDs with Al doping was enhanced significantly
compared to CIS@CdS QDs without Al doping (Fig. S11, ESI†),
indicating that the approach to enhancing the photostability of
QDs by Al doping is not limited to the ZnS shell, and can be
extended to other shell materials.
A simple approach was developed to enhance the photo-
stability of CIS@ZnS core@shell QDs by doping Al into the ZnS
shell. Compared to the undoped ones, the photostability of
CIS@ZnS:Al QDs was drastically improved without significantly
altering their optical properties. According to XPS results, the Al in
the as-prepared QDs was confirmed to be oxidized to Al–O, which
formed a passivation oxide layer that could effectively prevent
photo-degradation of QDs during long-term light irradiation. The
Al/Zn molar ratio and thickness of the doped ZnS shell had
significant effects on the photostability of QDs. Such an approach
was also confirmed to be applied in enhancing the stability of
CIS@CdS QDs and thus would be a very promising technology.
This work was financially supported by the National Natural
Science Foundation of China (NSFC 21271179), the Program for
New Century Excellent Talents (NCET-13-0364), the State Key
Program of National Natural Science Foundation of China
8 Z. Y. Ma, J. B. Pan, C. Y. Lu, W. W. Zhao, J. J. Xu and H. Y. Chen,
Chem. Commun., 2014, 50, 12088.
9 R. Gill, M. Zayats and I. Willner, Angew. Chem., Int. Ed., 2008, 47, 7602.
10 X. Tan, Y. Li, X. Li, S. Zhou, L. Fan and S. Yang, Chem. Commun.,
2015, 51, 2544.
11 L. Li, N. Coates and D. Moses, J. Am. Chem. Soc., 2010, 132, 22.
12 R. Xie, M. Rutherford and X. Peng, J. Am. Chem. Soc., 2009, 131, 5691.
13 L. Li, A. Pandey, D. J. Werder, B. P. Khanal, J. M. Pietryga and V. I.
Klimov, J. Am. Chem. Soc., 2011, 133, 1176.
14 B. Chen, H. Zhong, W. Zhang, Z. Tan, Y. Li, C. Yu, T. Zhai, Y. Bando,
S. Yang and B. Zou, Adv. Funct. Mater., 2012, 22, 2081.
15 T. Pons, E. Pic, N. Lequeux, E. Cassette, L. Bezdetnaya, F. Guillemin,
F. Marchal and B. Dubertret, ACS Nano, 2010, 4, 2531.
16 M. Kruszynska, H. Borchert, J. Parisi and J. K. Olesiak, J. Am. Chem.
Soc., 2010, 132, 15976.
17 R. Zhang, P. Yang and Y. Wang, J. Nanopart. Res., 2013, 15, 1910.
18 L. Li, T. J. Daou, I. Texier, T. T. K. Chi, N. Q. Liem and P. Reiss,
Chem. Mater., 2009, 21, 2422.
19 W. S. Song, J. H. Kim and H. Yang, Mater. Lett., 2013, 111, 104.
20 W. S. Song, E. P. Jang, J. H. Kim, H. S. Jang and H. Yang, J. Nanopart.
Res., 2013, 15, 1462.
21 J. Koike and M. Wada, Appl. Phys. Lett., 2005, 87, 041911.
22 W. H. Lee, B. S. Cho, B. J. Kang, H. J. Yang, J. G. Lee, I. K. Woo, S. W. Lee,
J. Jang, G. S. Chae and H. S. Soh, Appl. Phys. Lett., 2001, 79, 3962.
23 H. Wang, Rev. Adv. Mater. Sci., 2013, 33, 383.
24 Y. Zhou, H. Wang, F. Xiang, H. Zhang, K. Yu and L. Chen, Appl. Phys.
Lett., 2011, 98, 182906.
25 E. Loginova, F. Cosandey and T. E. Madey, Surf. Sci., 2007, 601, L11.
26 I. Yu, T. Isobe and M. Senna, J. Phys. Chem. Solids, 1996, 57, 373.
27 D. Peak, R. G. Ford and D. L. Sparks, J. Colloid Interface Sci., 1999,
218, 289.
28 F. J. Gracia, S. Guerrero, E. E. Wolf, J. T. Miller and A. J. Kropf,
J. Catal., 2005, 233, 372.
(NSFC 21436007). Innovation Projects from Shanghai Univer- 29 D. H. Lee and R. A. Condrate Sr., Mater. Lett., 1995, 23, 241.
30 V. Pilla, S. R. De Lima, A. A. Andrade, A. C. A. Silva and N. O. Dantas,
Chem. Phys. Lett., 2013, 580, 130.
31 M. S. A. EI-sadek, J. R. Kumar and S. M. Babu, Curr. Appl. Phys.,
sity of Engineering Science (No. 2013gp09), and the State Key
Laboratory of Pollution Control and Resource Reuse Founda-
tion (No. PCRRF12019).
2010, 10, 317.
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