Water driven photoluminescence enhancement and recovery of CH3NH3PbBr3/Silicon oil/PDMS-urea composite

Article abstract of DOI:10.1016/j.jallcom.2020.155088

Lead halide perovskites (LHPs) are very promising for new optoelectronic devices due to their excellent optical and electronic properties. However, the poor stability against water, heat and UV light greatly restricts their practical applications. In this work, a CH₃NH₃PbBr₃/silicon oil/PDMS-urea composite is prepared by encapsulating CH₃NH₃PbBr₃ nanocrystals and silicon oil into the PDMS-urea polymer matrix. This composite shows characteristics of water-enhanced photoluminescence (PL) and water-driven PL recovery. After immersed in water, the photoluminescence quantum yield of the composite increases from initial 26.0%–52.2%, which is ascribed to the passivation of surface defects by the capping of polymer chains, H₃O⁺ and OH^?. The composite possesses superior stability against UV light illumination in water. Furthermore, the composite film exhibits a capability of water-driven PL recovery after UV light irradiation. By applying alternate water/UV treatments, the PL can be reversibly switched “on and off”. The “on/off” ratio value of the PL intensity reached more than 4. These advantages make this composite highly promising in applications utilising luminescent switching and for under water lighting.

Full text of DOI:10.1016/j.jallcom.2020.155088

Journal of Alloys and Compounds 834 (2020) 155088
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Water driven photoluminescence enhancement and recovery of
CH NH PbBr /Silicon oil/PDMS-urea composite
3
3
3
a
a
a
a
a
b
Chunhua Luo , Sihui Huang , Ting Zhang , Chunli Jiang , Ruijuan Qi , Mengqin Liu ,
Hechun Lin , Jadranka Travas-Sejdic , Hui Peng a
Key Laboratory of Polar Materials and Devices (MOE), Department of Electronic, School of Physics and Electronic Science, East China Normal University,
Shanghai, 200241, China
Key Laboratory of Functional Metal-Organic Compounds of Hunan Province, Hengyang Normal University, Hengyang, 421008, China
Polymer Electronics Research Centre (PERC), School of Chemical Sciences, The University of Auckland, New Zealand
a
c
,
d
, e, *
a
b
c
d
MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand
Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi, 030006, China
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 November 2019
Received in revised form
Lead halide perovskites (LHPs) are very promising for new optoelectronic devices due to their excellent
optical and electronic properties. However, the poor stability against water, heat and UV light greatly
restricts their practical applications. In this work, a CH
3 3 3
NH PbBr /silicon oil/PDMS-urea composite is
6
March 2020
prepared by encapsulating CH NH PbBr nanocrystals and silicon oil into the PDMS-urea polymer ma-
3
3
3
Accepted 6 April 2020
Available online 10 April 2020
trix. This composite shows characteristics of water-enhanced photoluminescence (PL) and water-driven
PL recovery. After immersed in water, the photoluminescence quantum yield of the composite increases
from initial 26.0%e52.2%, which is ascribed to the passivation of surface defects by the capping of
Keywords:
Lead halide perovskites
Nanocrystals
Photoluminescence recovery
Polymer composite
Water induced
þ ꢀ
polymer chains, H O and OH . The composite possesses superior stability against UV light illumination
3
in water. Furthermore, the composite film exhibits a capability of water-driven PL recovery after UV light
irradiation. By applying alternate water/UV treatments, the PL can be reversibly switched “on and off”.
The “on/off” ratio value of the PL intensity reached more than 4. These advantages make this composite
highly promising in applications utilising luminescent switching and for under water lighting.
©
2020 Elsevier B.V. All rights reserved.
1
. Introduction
specific surface area. The instability is directly related to the strong
ionic nature and low formation energy of LHPs [8,9]. It has been
theoretically and experimentally shown that moisture is the prin-
cipal factor leading to materials degradation and devices failure
[10e13]. In addition, the moisture induced decomposition can be
accelerated by light irradiation and heat treatment.
To improve the stability of LHP nanocrystals, various strategies
have been developed including compositional engineering, surface
engineering and matrix encapsulation [9,14]. Among them, poly-
mer matrices with compact molecular chain can protect the LHP
NCs from surrounding environment, therefore, the stability of the
encapsulated NCs, especially the resistance against water is
significantly improved. Besides, by choosing suitable polymer ma-
trix, the luminescent properties can also be enhanced [15,16]. Up to
now, great progress has been made on the preparation of both bulk
perovskite/polymer composite and monodisperse nanocomposites
Colloidal nanocrystals (NCs) of lead halide perovskite (LHP) have
attracted a great deal of attentions due to their excellent opto-
electronic properties, such as tunable emission from the visible to
near infrared range, narrow band widths, high photoluminescence
quantum yield (PL QY) and low cost solution processability [1e3].
These advantages make them superior candidates for fabricating
optoelectronic devices with high performance, such as solar cell,
light-emitting diode (LED), laser and photodetector [4e6]. How-
ever, instability of the LHPs against moisture, oxygen, light and
temperature is a great obstacle to their practical applications [7].
This is more significant for perovskite based nanocrystals with large
*
Corresponding author. Key Laboratory of Polar Materials and Devices (MOE),
[
9,17]. For instance, Sun et al. introduced a crosslinkable and pol-
ymerizable ligand V18 for the preparation of colloidal CH NH PbBr
(MAPbBr ) NCs. After polymerization, the resulted bulk composites
Department of Electronic, School of Physics and Electronic Science, East China
Normal University, Shanghai, 200241, China.
3
3
3
E-mail address: hpeng@ee.ecnu.edu.cn (H. Peng).
3
https://doi.org/10.1016/j.jallcom.2020.155088
0925-8388/© 2020 Elsevier B.V. All rights reserved.

2
C. Luo et al. / Journal of Alloys and Compounds 834 (2020) 155088
presented superior water resistance with negligible difference in
the obtained PL spectra when the composites were immersed in
water over 90 days [18]. Zhong et al. reported an in situ preparation
purification.
3 3
2.2. Synthesis of CH NH Br
3
of the MAPbBr /PVDF composite film, which greatly improved the
PL QY of MAPbBr
3
up to 94.6% due to the strong interactions be-
To a 50 mL flask containing 10 mL methylamine, 10.9 mL HBr
(48% in water) was added dropwise into the flask under magnetic
þ
tween the eNH
3
group in the MAPbBr and the eCF ‒ group in the
3
2
ꢂ
PVDF [16]. Victoria et al. synthesized a core/shell monodisperse
CsPbBr NCs encapsulated with the APTES polymer [17]. Compared
stirring at 0 C for 2 h. Solvent was removed from the reaction
ꢂ
3
mixture by rotary evaporation at 60 C to yield a white solid which
with the uncoated counterparts, the emission efficiency was 2-fold
improved [17].
Water does have negative effect on the application of LHPs, but a
coin has two sides. For example, previous works revealed the
was filtrated and washed with diethyl ether for three times. The
crude product was recrystallized from ethanol and diethyl ether.
ꢂ
The obtained product was vacuum dried at 60 C overnight.
mechanism of CH
7,19e23]. Monohydrate intermediate CH
formed after exposure to moisture, and further hydration of
CH NH PbI O resulted in dihydrate (CH NH PbI ꢁ2H O, fol-
ꢁH
lowed by final degradation to PbI , CH NH and HI [7,19e23]. The
monohydrating process could be reversed by dry treatment to
produce dehydrated CH NH PbI due to water loss and this char-
3
NH
3
PbI
3
decomposition caused by moisture
2.3. Synthesis of PDMS-urea copolymer
[
3
NH PbI was
3
3
ꢁH
2
O
To
diisocyanate (0.17 g, 1.0 mmol) in dried toluene (5.0 mL), PDMS-
NH (2.6 mL, 1.0 mmol) was added and stirred for 24 h under ni-
a deoxygenated, anhydrous solution of tolylene-2,4-
3
3
3
2
3
3
)
4
6
2
2
3
2
2
trogen atmosphere. The resulting viscous solution was stored un-
der nitrogen atmosphere for further use.
3
3
3
acteristic has been utilized to enhance the performance and sta-
bility of perovskite based optoelectronic devices [9,24,25]. The
higher power conversion efficiency and better cell stability were
achieved through controlling the process humidity [26,27] or water
post-treatment [24,28], due to deactivation of the nonradiative
recombination centers and healing of the pinholes. In terms of
luminescent LHP NCs, it is reported that the postsynthetic water
3 3 3 3
2.4. Synthesis of CH NH PbBr (MAPbBr ) nanocrystals (NCs)
For the synthesis of MAPbBr
(0.2 mmol), PbBr (0.2 mmol), oleic acid (1 mL) and octylamine
(40 L) were dissolved in 10 mL DMF under magnetic stirring to
3
3 3
nanocrystals, CH NH Br
2
m
form a precursor solution. Then 1.8 mL of the prepared precursor
solution was injected into 30 mL of toluene under vigorous stirring
for 10 s to give a colloidal solution. The resulted solution was
centrifuged (10 min, 8000 rpm) and the solid settled on the bottom
of the tube was collected for further use. UVeVis, PL and TEM
treatment of luminescent CsPbCl
resulted in the improvement of their PL QYs from 5%, 69.3%, 68.1%e
8.5%, 95.5%, 91%, respectively [29]. Water assisted synthesis of
CsPbBr nanocrystals with higher PL QY and better crystallization
3 3 3
, CsPbBr and CsPbI nanocrystals
7
3
was also reported [30]. However, the related work on MAPbBr
not been reported yet.
3
has
characterization of the MAPbBr
the obtained nanoparticles in 5 mL toluene.
3
NCs were carried out by dispersing
Previously, we synthesized PDMS-urea polymer encapsulated
MAPbBr NCs and the obtained composite exhibited excellent self-
healing properties and improved luminescent stability [31]. How-
ever, the PL intensity of the composite in water still decreased with
immersed time [31]. In this work, a modified composite was pre-
3
2.5. Preparation of MAPbBr
3
/silicon oil/PDMS-urea composite
NCs and the
Silicon oil (20 L) was blended with MAPbBr
m
3
blended mixture was added into 1 mL solution of PDMS-urea
copolymer in toluene under continuous stirring until a uniform
solution was obtained. The solution was casted onto a Teflon model
and vacuum dried at room temperature for 30 min to give the
pared by combining silicon oil encapsulated MAPbBr
PDMS-urea copolymer. A gradual enhancement of the PL intensity
of the as-prepared MAPbBr /silicon oil/PDMS-urea composite film
3
NCs with
3
was observed after immersed in water, due to the encapsulation of
the perovskite NCs by the hydrophobic silicon oil. More interest-
ingly, the PL intensity of the composite film could be recovered by
water after the treatment of the composite film under UV light
MAPbBr
the MAPbBr
3
/silicon oil/PDMS-urea composite gel. The loading ratio of
NCs was 2.4% by weight. The composite gels used for
3
UVevis and PL tests have a thickness of 0.8 mm and width and
length of 15 mm.
ꢂ
illumination or at 70 C. FTIR and time resolve PL characterization
indicated that the recovery of the photoluminescence intensity was
due to the uptake of water molecules into the composite and the
passivation of the NCs surface defects by water molecules. To the
2.6. Characterization
The UVevis absorption spectra were recorded on a TU 1901
spectrometer and the photoluminescence (PL) spectra were
measured on a PerkinElmer LS 55 Fluorescence Spectrometer.
Time-resolved photoluminescence measurements were performed
on time-correlated single photon counting system (TCSPC, Pico-
Harp 300) by exciting the samples with a pulsed laser of wave-
length of 400 nm. The absolute PL quantum yield was determined
using a fluorescence spectrometer with an integrated sphere on a
HORIBA FluroMax-4 spectrofluorimeter excited at a wavelength of
3
best of our knowledge, this is the first report on MAPbBr based
polymer composites with water driven PL recovery performance,
which provides a new insight to the development of LHP NCs based
composite with recoverable PL properties.
2
. Experimental details
2.1. Materials
4
50 nm. The diffuse reflectance spectra (DRS) were investigated by
PerkinElmer Lambda 950 UVevis spectrometer with an integrating
sphere of 150 mm. The morphologies of the MAPbBr NCs and the
Methylamine (30e33 wt % ethanol solution), ether ethanol (AR),
hydrobromic acid (HBr, 48 wt % in water), ethanol (AR), N,N-
dimethylformamide (DMF, AR), lead bromide (PbBr 98%),
toluene (AR), octylamine (OLA, 99%) and oleic acid (OA, 90%) were
purchased from Aladdin Co., Ltd. Tolylene-2, 4-diisocyanate was
3
2
,
composite gels were analyzed by transmission electron microscope
(TEM, JEM-2100F, Japan) and field emission scanning electron mi-
croscopy (SEM, JEOL-JSM-6700F) equipped with energy dispersive
x-ray spectroscopy (EDS), respectively. Powder X-ray diffraction
(XRD) patterns were recorded on a Brucker AXS D8 powder
diffractometer by using Cu Ka radiation (l ¼ 0.154 nm). FTIR
bought from TCL. PDMS-NH
Sigma-Aldrich. Toluene was dried on sodium/benzophenone before
use. The other reagents were used as received without further
2
(Mn ¼ 2500) was obtained from

C. Luo et al. / Journal of Alloys and Compounds 834 (2020) 155088
3
Fig. 1. (a) UVevis absorption and PL spectra of the MAPbBr
3 3
NCs dispersed in toluene (up) and MAPbBr /silicon oil/PDMS-urea composite gel (down). (b) The TEM image of
MAPbBr NCs (Inset: HTEM pattern of nanocrystal). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3
measurements were performed on Nicolet iS50 FTIR spectrometer.
. Results and discussion
MAPbBr NCs were synthesized by using the solvent-induced
FWHM value of 27 nm, which was red-shifted of 9 nm compared
with that of colloidal MAPbBr NCs (Fig. 1a). This red-shift of
emission wavelength is attributed to the NCs aggregation as the
solvent was pumped out of the composite. The PL QY of the
3
3
3
MAPbBr /silicon oil/PDMS-urea composite was 26.0%, lower than
3
the colloidal solution.
When the MAPbBr
reprecipitation method with some modification [15]. Fig. 1a pre-
sents the UVeVis absorption and PL spectra of the colloidal
3
/silicon oil/PDMS-urea composite film was
immersed in water, a steady increase in the PL intensity of the
composite against the water soaking time was observed (Fig. 2a),
although some initial PL intensity fluctuations was detected. The PL
QY increases from 26.0% to 52.2% after 288 h of water treatment,
twice of its initial value. The FWHMs and peak wavelengths were
extracted from the PL spectra and depicted in Fig. 2b. It can be seen
that the emission wavelength shifted from 538 nm to the lower
wavelength during the first 24 h and then kept constant at 536 nm
afterwards. The FWHM changed from 27 to 26 nm within the first
MAPbBr
teristic absorption peak at 523 nm. The emission peak was at
29 nm with a full-width at half maximum (FWHM) of 26 nm. The
absolute PL QY was 78% for the obtained colloidal nanocrystals,
measured with an excitation wavelength of 450 nm. The TEM image
revealed that the obtained MAPbBr NCs were nanoparticles with
3
an average diameter of about 4 nm. The clear interference fringes
from the lattice plane presented in the high-resolution TEM
3
NCs dispersed in toluene. The NCs exhibited a charac-
5
(
HRTEM) image (the inset in Fig. 1b) illustrated the good crystalline
structure of the prepared MAPbBr NCs.
Scheme 1 shows the preparation procedure of the MAPbBr
silicon oil/PDMS-urea. In brief, silicon oil was added to the collected
MAPbBr NCs, and the blended mixture was added into the solution
24 h water treatment and kept the same afterwards. These results
3
not only illustrate the excellent PL stability of the prepared com-
posite, but also show the capability of water to improve the optical
3
/
properties of the MAPbBr
The scanning electron microscopy (SEM) images of the
MAPbBr /silicon oil/PDMS-urea composite film were taken before
and after the water treatment of 288 h and shown in Fig. 3a and b.
The initial surface of MAPbBr /silicon oil/PDMS-urea composite
3
/silicon oil/PDMS-urea composite.
3
of PDMS-urea copolymer in toluene under continuous stirring until
a uniform solution was obtained. After the solvent was evaporated
3
under vacuum, the MAPbBr
produced. The absorption and PL spectra of MAPbBr
PDMS-urea composite are given in Fig. 1a. The absorption spec-
trum of MAPbBr /silicon oil/PDMS-urea showed the characteristic
absorption peak of colloidal MAPbBr NCs at 523 nm. The PL
3
/silicon oil/PDMS-urea composite was
3
/silicon oil/
3
was relatively smooth with uniformly distributed tiny sand-like
domains. After the water treatment, the surface became corru-
gated and wrinkled, which was caused by the swelling and
shrinking of the composite. To further verify the infiltration of
3
3
spectrum showed the emission peak centered at 538 nm with a
3
Scheme 1. The synthetic route for the MAPbBr /silicon oil/PDMS-urea composite.

4
C. Luo et al. / Journal of Alloys and Compounds 834 (2020) 155088
Fig. 2. (a) Time dependent PL spectra of the MAPbBr
3
/silicon oil/PDMS-urea composite immersed in water and b) The variation of peak wavelength and FWHM extracted from the
PL spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3 3
Fig. 3. The SEM images of MAPbBr /silicon oil/PDMS-urea system (a) before and (b) after 288 h’ water treatment. (c) FTIR spectra and (d) XRD patterns of the MAPbBr /silicon oil/
PDMS-urea composite before (black line) and after (red line) water immersion for 288 h. (For interpretation of the references to color in this figure legend, the reader is referred to
the Web version of this article.)
water into the composite, Fourier transform infrared (FTIR) spectra
of the MAPbBr /silicon oil/PDMS-urea composite before and after
the water treatment were recorded, as depicted in Fig. 3c. The
examine the crystalline structure of the composite. As shown in
Fig. 3d, the XRD patterns of as-prepared composite exhibited the
3
ꢂ
ꢂ
ꢂ
characteristic diffraction peaks of MAPbBr
3
NCs at 14.8 , 21.1 , 30.1 ,
33.7 , 37.1 , 43.1 , 45.8 and 48.5 , which agree well with the
standard cubic phase for MAPbBr [29]. After the water treatment,
the characteristic diffraction peaks of MAPbBr NCs were also
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
obtained FTIR spectra profiles are quite similar. However, the ab-
ꢀ1
sorption band centered at around 3400 cm , became broader and
more intensive, which undoubtably confirmed the presence of
water molecules in the composite. Additionally, the disappearance
3
3
detected. This result demonstrates that the penetration of water
into the composite did not destruct the crystal structure of the
MAPbBr NCs, which is attributed to the protection of polymer
3
ꢀ
1
of the peak located at 2274 cm , which corresponds to the typical
CO asymmetric stretching vibration, implies that water molecules
2
permeate into the composite and push the air outside of the
composite. X-ray diffraction (XRD) measurement was carried out to
network and the silicon oil to the NCs.
UV light and high temperature commonly cause the decrease of

C. Luo et al. / Journal of Alloys and Compounds 834 (2020) 155088
5
ꢂ
3
/silicon oil/PDMS-urea composite (a) irradiated with 365 nm UV light follow by (b) water-driven recovery and (c) treated at 70 C follow by (d)
Fig. 4. The PL spectra of the MAPbBr
water-driven recovery. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
PL intensity of MAPbBr
cess is generally irreversible [32,33]. Interestingly, the MAPbBr
3
NCs due to the degradation and this pro-
II) and 24 h water-driven recovery (stage III). The results are pre-
sented in Fig. 5a. The PL decay curves can be well fitted by a biex-
ponential equation and the obtained parameters were listed in
3
/
silicon oil/PDMS-urea composite not only showed water-enhanced
photoluminescence, but also exhibited ability to recover the pho-
toluminescence performance after illuminated by UV light and high
temperature heating. The water induced PL intensity recovery
measurements were performed by irradiating with 365 nm UV
Table 1. The decay times (
¼ 113.10, 103.49, 118.06 ns for the composite at the stage I, II and
III, respectively. The corresponding average decay times are 97.30,
87.99 and 103.46 ns, respectively. It is clear that the value of
t) are
t
1
¼ 21.33, 24.80, 18.60 ns and
t
2
2
t ,
ꢂ
light or heating at 70 C of the water infiltrated composites and
which corresponds to long-lived radiative recombination process,
decreased from 113.10 ns to 103.49 ns after UV light irradiation, and
increased from 103.49 ns to 118.06 ns after the water-driven re-
then the composites were immersed in water for photo-
luminescence recovery. The evolution of PL spectra during UV light
irradiation, heat and recovery processes are shown in Fig. 4aed. As
can be seen, the PL intensity decreased sharply during the first few
hours’ UV light illumination and then slowly to 19.5% of its initial
value after 24 h’ UV light irradiation (Fig. 4a). In contrast, the PL
intensity of the composite reduced to 13.0% of its initial values after
covery. On the contrary,
1
t presented an opposite change, illus-
trating that the nonradiative recombination was greatly suppressed
after the water treatment. It is known that fewer surface traps lead
to longer lifetime [34,35]. Thus, the changes of
t
1
and
2
t indicate
the surface defects of MAPbBr
water treatment process.
3
NCs were passivated during the
ꢂ
the treatment at 70 C for 30 min (Fig. 4c) and prolonged heat
treatment for 24 h only resulted in the slight red-shift of the PL
peak position without obvious decrease of the PL intensity. After
immersed in water, the UV light and heat treated samples exhibited
a fast increase in the PL intensity within the first 24 h, followed by
subsequent slow recovery of the emission intensity (Fig. 4b and d),
indicating the PL recovery primarily happens in the first 24 h’ water
immersion. These experiments demonstrate that the MAPbBr /
3
silicon oil/PDMS-urea composite possesses reversible water-driven
PL recovery property.
In order to understand the mechanism of observed phenome-
non, the time-resolved PL spectra (TRPL) of the composite at
different stages were recorded, namely, as-synthesized (stage I), 2 h
UV light illuminated sample that is subsequent water treated (stage
3
The emission wavelength of the MAPbBr /silicon oil/PDMS-urea
composite red shifted after UV light irradiation and this shift could
be recovered by the water treatment (Fig. 4a and b). Thus, we
measured the diffuse reflectance spectra (DRS) of the composite at
the three different stages mentioned above. Fig. 5b reveals that the
obtained diffuse reflectance profiles were quite similar with a sharp
absorption edge ascribed to the electron transition from the top of
the valence band to the bottom of conduction band. The as-
synthesized composite (stage I) presented an absorption edge at
about 552 nm. The absorption edge of the composite at the stage II
red shifted to 573 nm. After the water-driven recovery (stage III),
the absorption edge shifted back to 558 nm. The corresponding
optical band gaps were estimated from the Tauc plot (Fig. S1) in the

6
C. Luo et al. / Journal of Alloys and Compounds 834 (2020) 155088
Fig. 5. (a) TRPL decay curves and (b) Diffuse reflectance spectra for the as-synthesized (stage I), water treated followed by 2h UV light illuminated (stage II) and 24 h water-assisted
recovery (stage III) MAPbBr
article.)
3
/silicon oil/PDMS-urea composite. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this
Table 1
TRPL data for the MAPbBr
stages.
3
/silicon oil/PDMS-urea composite at three different
MAPbBr
3
NCs/silicon oil/PDMS-urea
A
1
t
1
A
2
t
2
t
avg
Stage I
Stage II
Stage III
0.43 21.33 0.39 113.10 97.30
0.44 24.80 0.43 103.49 87.99
0.47 18.60 0.43 118.06 103.46
form of the transformed Kubelka-Mumk equation to be 2.32, 2.22
and 2.29 eV. The changes of optical band gap well explained the
observed emission shifts of the composite at the different stages.
The diffuse reflectance analysis indicated that the UV light irradi-
Scheme 2. Systematic diagram of the PDMS-urea polymer anchoring on the MAPbBr3
NCs after water treatment.
ation produced more defects on the surface of MAPbBr
the water treatment eliminated these surface defects.
3
NCs and
the nanocrystal structure, this composite holds a great potential for
use in luminescent switching systems and in under water lighting.
To inspect the composite applications, the water treated composite
was irradiated with 365 nm UV light for 2 h, and then immersed in
water for 24 h. This procedure was repeated for 5 cycles. The PL
intensity was monitored and displayed in Fig. 6a and Fig. S2. The PL
of the composite could be switched “on and off” by exposure to
water and UV light irradiation. In addition, the switched “on/off”
ratio of the PL intensity first increased and then became nearly
constant. The ratio was higher than 4 after three cycles. The
emission wavelength showed a slight blue-shift and the FWHM
remained almost unchanged (Fig. 6b). These results revealed the
good reversibility of the water-driven PL recovery. Fig. 6c presents
optical images of the “ECNU” made by the composite taken under
ambient light and 365 nm UV light during these treatments. After
the UV light irradiation, the color of the “ECNU” changed from
yellow to brown and the emission faded significantly. After the
water-driven recovery, the color came back to yellow and the
emission intensity also recovered.
To further explore the potential application of the composite for
under-water lighting, the water treated composite was immersed
in water and irradiated with 365 nm UV light for long-term PL
stability examination. The PL intensity dropped relatively fast
within the first 24 h followed by a slow decline afterwards. As
shown in Fig. 6d, after UV light irradiation for 11 day, the PL in-
tensity decreased to 50% of its initial value and kept unchanged
after that, indicating the superior under-water stability of the
composite against UV light.
Based on the above discussion, we propose a mechanism of
water-driven PL recovery of MAPbBr /silicon oil/PDMS-urea com-
3
posite. It was reported that the capping ligands oleic acid (OA) and
octylamine (OLA) are not tightly bound to the perovskite NCs and
could be easily removed during purification process [36]. The
encapsulation of the NCs into the polymer matrix generally lead to
the aggregation of NCs and loss of the capping ligands after the
evaporation of solvent, which results in low PL QY of the formed
composite. When water penetrates the composite, the water mol-
þ
ꢀ
‒
ecules can ionize to form H
3
O
and OH with the help of OA and
þ
OLA , which can function like capping ligands and passivate the
surface defects [37,38]. This leads to the decrease of the surface
defects. Furthermore, the composite swells with the presence of
water. during this swelling process, the polymer chains get freedom
3
to move and can anchor on the MAPbBr NCs surface via coordi-
nation bonding between the amine group of the polymer chain and
2þ
Pb of the perovskite and hydrogen bonding between the pro-
tonated amine of the polymer chain and the dangled bromide anion
of the MAPbBr
MAPbBr NCs (Scheme 2) and enhance the crosslinking density of
the composite. The swelling of the polymer and anchoring of
polymer chains can also weaken the aggregation of MAPbBr NCs
3
NCs [38], which reduce the surface defects of
3
3
due to the better confinement in the polymer network. All these
factors contribute to the enhancement of the PL QYand PL intensity
for the immersed composite.
3
Since the MAPbBr /silicon oil/PDMS-urea composite possessed
reversible water-driven PL recovery property without degrading

C. Luo et al. / Journal of Alloys and Compounds 834 (2020) 155088
7
Fig. 6. (a) The variation of PL intensity, (b) peak wavelengths and fwhm values and (c) optical images of the MAPbBr
3
/silicon oil/PDMS-urea composite suffered from UV light
irradiation and water-driven recovery; (d) Under-water PL stability against 365 nm UV light for the composite. (For interpretation of the references to color in this figure legend, the
reader is referred to the Web version of this article.)
4
. Conclusions
Investigation, Writing - original draft. Ting Zhang: Investigation.
Chunli Jiang: Investigation. Ruijuan Qi: Investigation. Mengqin
Liu: Investigation. Hechun Lin: Investigation. Jadranka Travas-
Sejdic: Writing - review & editing. Hui Peng: Conceptualization,
Supervision, Writing - review & editing.
3
In conclusion, a MAPbBr /silicon oil/PDMS-urea composite was
prepared. This composite exhibits a water-enhanced PL property.
After immersing in water, the PL QY of the composite increased
from initial 26.0%e52.2%. The observed PL enhancement was
ascribed to the passivation of surface defects by the capping of
Acknowledgements
þ ꢀ
and OH , which was confirmed by FTIR
polymer chains, H
3
O
analysis and time resolved PL measurements. Additionally, the
swelling of the polymer and hydrogen bonding between the pro-
tonated amine and the dangled bromide anions weakened the
This work was supported by Shanghai Science and Technology
Innovation Action Plan (No. 19JC141670, 17JC1402500), Natural
Science Foundation of Shanghai (No. 18ZR1410900) and China
Postdoctoral Science Foundation (No. 2017M621540), National
Natural Science Foundation of China (No. 61671206).
3
aggregation of MAPbBr , which also contributed to the PL
enhancement. By applying alternate water/UV treatments, the PL
could be reversibly switched “on and off”. The “on/off” ratio of the
PL intensity reached value higher than 4. The composite in water
has superior stability against UV light. After UV light irradiation for
Appendix A. Supplementary data
1
1 days, the composite still retained 50% of its initial value. These
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jallcom.2020.155088.
advantages make this composite a potential candidate for lumi-
nescent switchers and under-water lighting.
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