Multicomponent hybrids with surfactant-encapsulated europium polyoxometalate covalently bonded ZnO and tunable luminescence

Article abstract of DOI:10.1039/c3ra43635g

In this paper, europium polyoxometalate (Na₉EuW ₁₀O₃₆·32H₂O, abbreviated as POM) is encapsulated into the surfactant dodecyl(11-(methacryloyloxy)undecyl) dimethylammonium bromide (DMDA) to form surfactant-encapsulated polyoxometalate (SEP). Methacrylic group-modified ZnO nanoparticles (designated ZnO-MAA) are also prepared through a sol-gel reaction between zinc methacrylate and LiOH. Finally, the synthesized SEP, ZnO-MAA and three different ester units (ethyl methacrylate (EMA), 2-hydroxyethyl methacrylate (HEMA) and 2,2,3,4,4,4- hexafluorobutyl methacrylate (HFMA)) are covalently bonded through a copolymerization reaction in the presence of benzoyl peroxide (BPO). A detailed physical characterization and especially the photoluminescence of these hybrid materials are studied in detail. The results provide a method to assemble multicomponent hybrid materials with europium polyoxometalate, a semiconductor compound and polymer unit, whose luminescence can be tuned and integrated.

Full text of DOI:10.1039/c3ra43635g

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Multicomponent hybrids with surfactant-
encapsulated europium polyoxometalate
Cite this: RSC Adv., 2014, 4, 3318
covalently bonded ZnO and tunable luminescence
Bing Yan* and Yan-Fei Shao
9 2
In this paper, europium polyoxometalate (Na EuW10O36$32H O, abbreviated as POM) is encapsulated into
the surfactant dodecyl(11-(methacryloyloxy)undecyl) dimethylammonium bromide (DMDA) to form
surfactant-encapsulated polyoxometalate (SEP). Methacrylic group-modified ZnO nanoparticles
(
designated ZnO-MAA) are also prepared through a sol–gel reaction between zinc methacrylate and
LiOH. Finally, the synthesized SEP, ZnO-MAA and three different ester units (ethyl methacrylate (EMA),
-hydroxyethyl methacrylate (HEMA) and 2,2,3,4,4,4-hexafluorobutyl methacrylate (HFMA)) are
2
covalently bonded through a copolymerization reaction in the presence of benzoyl peroxide (BPO). A
detailed physical characterization and especially the photoluminescence of these hybrid materials are
studied in detail. The results provide a method to assemble multicomponent hybrid materials with
europium polyoxometalate, a semiconductor compound and polymer unit, whose luminescence can be
tuned and integrated.
Received 15th July 2013
Accepted 22nd October 2013
DOI: 10.1039/c3ra43635g
www.rsc.org/advances
So, it is necessary to modify POMs to realize the assembly of
POMs and other building blocks, which depends on the elec-
Introduction
Polyoxometalates are inorganic metal oxide clusters with several trostatic interaction between the anions and cations of the
8
nanometers dimension. It is fascinating to use them as building POMs themselves. In recent research, polyoxoanions can be
blocks to fabricate large supramolecular structures, because phase transferred into organic media with a necessary func-
9
they possess not only a well-dened molecular weight and nano- tionalization of surfactants. Therefore, many water-soluble
structure but also chemical, structural, and electronic versa- materials such as POM can be encapsulated into organic
1
tility. Their unique properties allow them to play an important ingredients with surfactants, to obtain a surfactant-encapsu-
role in many elds such as catalysis, electrochemistry, semi- lated POM (abbreviated as SEP). The countercations on the
2
,3
10,11
conductors and magnets.
The decatungstoeuropate(9ꢀ) POM surface are substituted by cationic surfactants.
So, SEP
anion, as a potassium salt, was rstly prepared by Peacock and possesses the structure of a hydrophobic surfactant shell and an
Weakley in 1971 and its luminescence spectrum was rst encapsulated hydrophilic POM core with a certain stoichio-
reported by Stillman and Thomson. Later, Blasse et al. studied metric ratio in an organic medium. Then, the surface properties
4
5
the luminescence spectrum of solid samples of Na
9
Eu- of the POM are changed and the SEP becomes soluble in
W O $18H O and K EuW O $18H O. Among the known organic media such as chloroform, benzene or toluene. This
6
1
0
36
2
9
10 36
2
luminescent polyoxometalates, Na EuW O $32H O (POM) offers an effective approach to obtain some so materials based
9
10 36
2
7
10,11
possesses the highest luminescent quantum yield. The pho- on POMs.
Some surfactants possess unsaturated double
toluminescent property of such polyoxometaloeuropates can bonds, which can behave as a linkage for a copolymerization
10–12
help us to understand the charge transfer transition state reaction, to form a copolymer with other monomers.
(
LMCT) from oxygen to the metal ion. In this poly- Subsequently, POMs can be introduced into polymer matrices,
6
ꢀ
oxometalloeuropate, the inorganic W
5
O
18
ion affects the which can be expected to have promising properties, since
coordination geometry of Eu and results in a change in the polymers are processed easily and have good stability.
3
+
3
+
emission properties of Eu . However, POM is only soluble in
In fact, POMs themselves are used as the luminescent center
water generally and the processability of pure POMs is inferior to assemble photofunctional hybrid materials and have been
whether they are crystalline or powdered materials, which limits reported for their favorable luminescent performance and
the applications of POMs.
crystalline framework. The main method is to compose POMs
7,13
through an ion exchange reaction. On the other hand, other
luminescent units can be introduced into the hybrid system,
such as inorganic semiconductor oxides or suldes. These
photoactive units can be expected to integrate with the POMs to
Department of Chemistry, Tongji University, State Key Lab of Water Pollution and
Resource Reuse, Siping Road 1239, Shanghai 200092, China. E-mail: byan@tongji.
edu.cn; Fax: +86-21-65981097; Tel: +86-21-65984663
3318 | RSC Adv., 2014, 4, 3318–3325
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realize the tunable luminescence and even, white lumines-
Synthesis of surfactant dodecyl(11-(methacryloyloxy)-
cence. Among these, ZnO nanoparticles are a popular choice for undecyl)dimethylammonium bromide (denoted as DMDA). The
15
their cheap, non-toxic and chemically stable virtues. ZnO synthesis of DMDA was completed according to the literature.
nanoparticles possess visible emission bands, covering the Firstly, 11-bromo-1-undecanol (2.01 g, 8.0 mmol) and triethyl-
14
blue-green region, whose emission colors depend on the amine (0.87 g, 8.8 mmol) were dissolved in 30 mL of dry tetra-
15
synthetic methods. Generally, the visible emission is related to hydrofuran. Then, a dry tetrahydrofuran solution (10 mL)
16
surface defects, such as oxygen vacancies or zinc interstitials.
containing methacryloyl chloride (0.92 g, 8.8 mmol) was added
Among these, a long band green and yellow emission is ascribed dropwise to the above-mentioned solution, while stirring. The
ꢁ
to electron transition from a shallow donor state to a deep- mixture was stirred for about 2.5 h at 60 C. Aer cooling, the
trapped defect state, while a short band blue emission is due to mixture was ltered and the ltrate was obtained. Then,
surface defects. Aer the composition of ZnO nanoparticles the ltrate was concentrated by removing the solvent tetrahy-
ꢁ
with europium components, the whole hybrid systems oen drofuran using a rotary vacuum evaporator at 35 C and puried
show both the emission of ZnO nanoparticles in the blue-green by column chromatography (silica gel, CH Cl ). A colorless
2
2
region and the europium component in the red region. More- liquid was obtained. Aerwards, the obtained product (1.90 g,
over, ZnO can be easily modied with a polymer shell by the sol– 6 mmol) was dissolved in 30 mL chloroform with N,N-dime-
17
gel route. Thus, ZnO nanoparticles can be connected with the thyldodecylamine (1.28 g, 6 mmol) and the mixture was heated
ꢁ
SEP and polymer through polymerization.
to 60 C for 3 days. Aer cooling, the mixture was concentrated
In this paper, we rstly synthesize surfactant DMDA and by a rotary vacuum evaporator to 3–5 mL. Cold diethyl ether
POMs and then prepare DMDA-encapsulated poly- (30 mL) was added dropwise and a white precipitate was
ꢁ
oxometalloeuropate (SEP). At the same time, methacrylic group- observed. Three days later by cold-storage at 0 C, the formed
1
modied ZnO nanoparticles (ZnO-MAA) are prepared. Then, white precipitate was ltered and washed with diethyl ether. H
three different ester units with polymerizable groups are chosen NMR: (CDC1 , Me Si): d (ppm) ¼ 6.10–5.56 (d, 2H, CH ¼ C),
3
4
2
to connect with the SEP and ZnO-MAA through an addition 4.14 (t, 2H, –CH –O), 3.52 (m, 4H, –(CH ) N), 3.41 (s, 6H,
2
2 2
polymerization. We assemble the hybrid system including the (CH ) –), 1.95 (s, 3H, CH –C), 1.71 (m, 4H, –(CH CH ) N), 1.57
3
2
3
2
2 2
polyoxometalate, nanoparticles and polymer. The detailed (2H, –CH –CH –O), 1.29 (32H, –(CH ) – and –(CH ) –), 0.89
2
2
2 9
2 7
characterizations of the obtained materials are performed, and (t, 3H, CH –).
3
especially the photophysical properties are discussed in depth.
Synthesis of surfactant-encapsulated POM clusters (denoted
as SEP). The polyoxometalate POM, freshly prepared, was dis-
solved in water and a chloroform solution of surfactant DMDA
was added while stirring, according to the literature. The
initial molar ratio of DMDA to POM was controlled at 9 : 1. The
polyoxometalate was extracted and transferred into a chloro-
form solution. The organic phase was separated, and SEP was
obtained by evaporating the chloroform to dryness. The prod-
ucts were washed with deionized water three times, in order to
removed the salt NaBr. Aer re-crystallization with ethanol, the
Experimental section
Materials
13
Eu(NO) $6H O was obtained by dissolving Eu O in concen-
3
2
2 3
trated nitric acid. Methacrylic acid (MAA) and lithium hydroxide
monohydrate (LiOH$H O) were purchased from the Shanghai
2
Yaohua chemical plant. Na
ride were purchased from Aldrich. Three different esters
ethyl methacrylate, 2-hydroxyethyl methacrylate and
,2,3,4,4,4-hexauorobutyl methacrylate) were purchased from
2 4 2
WO $2H O and methacryloyl chlo-

nal product was obtained as a light yellow powder. The
(
2
elemental analysis data suggested the formula with
EuW (DMDA) : W 27.68, C 47.20, H 7.92. Found (%) W 27.90, C
Aldrich and used as received. 11-Bromo-1-undecanol and
N,N-dimethyldodecylamine were purchased from TCI. The
solvent tetrahydrofuran (THF) was used, aer desiccation with
anhydrous magnesium sulfate. All the other reagents were
analytically pure and used as received.
10
9
47.55, H 7.69.
Synthesis of SEP covalently bonded with ZnO-MAA nano-
particles and ester polymers L (denoted as SEP–ZnO-MAA–L, L
PEMA, PHEMA and PHFMA). Methacrylic group-modied
ZnO nanoparticles (designated ZnO-MAA) were synthesized
¼
17,19
based on the previously reported procedures.
Then, SEP
Synthetic procedures
(2.5 wt%) and ZnO-MAA (2.5 wt%) were, respectively, dissolved
Preparation of Na EuW O $32H O (denoted as POM). in ethyl methacrylate (EMA), 2-hydroxyethyl methacrylate
9
10 36
2
Na
the published literature. Firstly, Na
dissolved in 20 mL of deionized water. The solution pH was benzoyl peroxide (BPO) as the initiator. The mixture was
9 2
EuW10O36$32H O (POM) was freshly synthesized in light of (HEMA) and 2,2,3,4,4,4-hexauorobutyl methacrylate (HFMA)
18
2
4 2
WO $2H O (4.15 g) was via an addition polymerization reaction in the presence of
adjusted to 7.0–7.5 with CH
aqueous solution of Eu(NO
3
COOH. Next, about 2 mL of an agitated magnetically and a thermal treatment was performed
ꢁ
3
)
3
$6H
2
O (0.55 g) was added to the at 75 C for about 5 h. The synthetic protocol of the nal
above-mentioned solution dropwise, while stirring at about 85 material SEP–ZnO-MAA–PEMA is shown in Fig. 1.
ꢁ
C. Colorless crystals were obtained aer cooling the solution to
Synthesis of SEP covalently bonded with ester polymer PEMA
room temperature. The nal crystals were collected aer ltra- (denoted as SEP–PEMA-1, SEP–PEMA-2). The method of SEP
tion and dried in the air. Elemental analysis data: W 54.88, H covalently bonded with ester polymer PEMA is the same as the
1.92. Found (%) W 45.25, H 1.74.
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MAA modied ZnO nanoparticles are constructed in situ from
the reaction of LiOH and the Zn(MAA) complex, from Zn(OH)2
2
and MAA. ZnO colloids are produced via the sol–gel process
have methacrylic groups, which originate from the polymer
18,20
precursor adsorbed on the surface of the ZnO.
Then, the ZnO
polymer nanocomposites are assembled with MAA modied
ZnO nanoparticles and monomer EMA (HEMA, HFBMA)
through the copolymerization reaction between the MAA group
and EMA (HEMA, HFBMA). Finally, the nal multicomponent
hybrid systems are assembled through the copolymerization
reaction between the modied ZnO unit and SEP through the
saturated bonds among them.
The FTIR spectra of the POM, SEP and SEP–ZnO-MAA–
PHFMA are shown in Fig. 2. In the spectrum of SEP, the bands
ꢀ1
at 2924, 2850 cm
symmetric CH stretching modes of the DMDA alkyl chains,
respectively. A C–H bending vibration appears at 1466 cm
are attributed to the asymmetric and
Fig. 1 Scheme of the synthesis process of SEP–ZnO-MAA–PEMA.
2
ꢀ1
.
The existence of the unsaturated group at the terminal of one
synthesis of SEP–ZnO-MAA–L. The differences are that ZnO-
MAA is not added and the ratio of SEP is 2.5 wt% (SEP–PEMA-1)
and 5 wt% (SEP–PEMA-2), respectively.
chain can be proved by the characteristic band located at
ꢀ1
1
636 cm (n , C]C). In addition, the stretching vibration
as
ꢀ1
absorption bands n (C]O, 1731 cm ) and n (C–O, 1255 and
ꢀ1
1
159 cm ) are characteristic of a carboxyl group. These bands
Physical measurements
suggest that the DMDA surfactants have successfully replaced
ꢀ
1
Fourier transform infrared (FTIR) spectra were measured within the sodium ions. The band centered at 3452 cm corresponds
ꢀ
1
the 4000–400 cm region on a Nexus 912 AO446 spectropho- to the stretching vibration of O–H, which indicates the presence
tometer with the KBr pellet technique. The elemental analyses of water molecules and it is a common phenomenon that the
(
C, H) were measured with a CARIO-ERBA 1106 elemental SEP enwraps some water in its long alkyl chains. Compared with
ꢀ1
analyzer and the contents of the metal element (W) were SEP, the band at 1636 cm for the C]C stretching vibration in
determined on a Perkin Optima 2100DV Inductively Coupled the spectrum of SEP–ZnO-MAA–PHFMA cannot be observed, as
Plasma Optical Emission Spectrometer (ICP-OES). The UV-vis a result of the polymerization reaction.
diffuse reectance was acquired by a BWS003 spectrophotom-
The UV-vis diffuse reection absorption spectra of the
eter. X-ray powder diffraction patterns (XRD) were recorded on a obtained materials are given in Fig. 3. From the gure, we can
Rigaku D/max-rB diffractometer equipped with a Cu anode in see that all the spectra exhibit a similar broad absorption band
ꢁ
the 2q range from 10–70 . The uorescence excitation and in the UV-vis range. This can be attributed to the absorption of
6
ꢀ
emission spectra were obtained on a RF-5301 spectrophotom- the inorganic ligand W O18 and the inuence of the ZnO-MAA
5
eter. Luminescence lifetime measurements were carried out on and the polymer L (L ¼ PEMA, PHEMA and PHFMA). At the
an Edinburgh FLS920 phosphorimeter using a 450 W xenon same time, a large broad absorption band from 320 to 490 nm
lamp as the excitation source. Thermogravimetric analysis (TG) exists in each curve. These partially overlap with the absorption
was performed on a Netzsch STA 409 under a nitrogen atmo- bands of the uorescent excitation spectra in Fig. 6(a). The two
ꢁ
ꢀ1
sphere in Al
3
2
ꢁ
O
3
crucibles, at a heating rate of 15 C min from peaks at about 320 and 480 nm in the wide bands originated
1
0 to 1000 C. The H NMR spectra were recorded on a Bruker from the ZnO and POMs, respectively. Furthermore, it can be
Avance-500 spectrometer with tetramethylsilane (TMS) as the obviously seen that an inverse absorption peak locates at about
6
internal reference, using DMSO-d as the solvent.
Results and discussion
As shown in Fig. 1, the key to understanding the multicompo-
nent hybrid systems is the chemical bonding among the
different units within them. Dodecyl(11-methacryloyloxy-
undecyl)dimethylammonium bromide (DMDA) is a kind of
polymerizable surfactant, composed of two equal length alkyl
chains and containing an unsaturated group at the terminal of
one chain. Therefore, the surfactant-encapsulated POM can
further be copolymerized to another substrate, such as polymer
chains and nanocomposites. The SEP is prepared from the ion
exchange between the surfactant DMDA and the POM using the
electrostatic attraction force between their different charges.
Fig. 2 Selected FTIR spectra of POM, SEP and SEP–ZnO-MAA–
PHFMA.
13
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Fig. 3 Selected UV-vis diffuse reflection absorption spectra of SEP–
ZnO-MAA–L (L ¼ PEMA, PHEMA and PHFMA).
Fig. 5 Derivative thermogravimetric (DTG) and thermogravimetric
(TG) curves of SEP–ZnO-MAA–PEMA (a) and SEP–PEMA (b).
Fig. 4 Wide-angle X-ray diffraction patterns of POM, SEP–PEMA-1
and SEP–ZnO-MAA–PEMA.
PEMA, three main steps in weight loss can be observed. The rst
ꢁ
weight loss is about 5.0% from 50 to 256.9 C, which can be
6
14 nm. This is because of the characteristic emission peaks of attributed to the evaporation of adsorbed water and residual
3
+
Eu , excited by the UV component in the incident ray during solvent, without any decomposition of the chemical bonds. The
the measurement.
second step of weight loss (about 24.0%) between 256.9 and
ꢁ
The room temperature X-ray diffraction (XRD) patterns 374.5 C is due to the rst decomposition of the organic
ꢁ
from 10 to 70 of the POM, SEP–PEMA-1 and SEP–ZnO-MAA– ingredient PEMA. This stage has the greatest speed of weight
ꢁ
PEMA are shown in Fig. 4. From the gure, we can see that the loss, when the temperature reached 349.6 C. The third mass
XRD pattern of SEP–PEMA-1 shows similar peaks to POM, but loss (about 35.2%), beyond 374.5 C, is assigned to the further
ꢁ
a little different. That should be inuenced by the surfactant decomposition of the surfactant DMDA and organic ingredient
DMDA and the polymer PEMA in the periphery of the POM. MAA. Finally, the material retains a mass of about 35.8%, which
Moreover, compared with SEP–PEMA-1, SEP–ZnO-MAA–PEMA represents the weight of ZnO and POM. The TG curve of SEP–
also shows differences because of the introduction of ZnO- PEMA-1 shows a similar three steps of weight loss in compar-
MAA. The obvious peak of ZnO-MAA is located at approxi- ison with SEP–ZnO-MAA–PEMA. The mass loss (about 2.8%) at
ꢁ
16
ꢁ
mately 35 , and in the diffraction peak of SEP–ZnO-MAA– about 247 C is less than the rst weight loss (about 5.0%) of
ꢁ
PEMA, a peak located at 34 can also be observed. Additionally, SEP–ZnO-MAA–PEMA because of the lack of ZnO-MAA. It may
some small peaks, dependent on ZnO-MAA can also be found be inferred that ZnO is more inclined to absorb moisture from
in the SEP–ZnO-MAA–PEMA pattern, but not in SEP–PEMA-1. the air. Another mass loss of 24.0% is also because of the
So, the characteristic ZnO-MAA diffraction peaks appear as a decomposition of organic ingredients PEMA. The third step of
result of the presence of ZnO-MAA in the nal hybrid materials mass loss, of 62.9%, is due to the decomposition of DMDA, but
and we can predict that the intensity of the ZnO-MAA diffrac- not MAA. There is no ZnO-MAA, so the residual mass is rela-
tion peaks may increase with the increase in the ZnO-MAA tively low, only 10.3%.
content.
The luminescence performances of the lanthanide hybrid
To investigate the thermal stabilities of the obtained mate- materials were investigated at room temperature. Fig. 6 and 7
rials, the thermogravimetric (TG) and differential thermog- show the luminescent excitation and emission spectra of
ravimetry (DTG) analyses were carried out. Fig. 5 (a) and (b) hybrids with the ZnO unit SEP–ZnO-MAA–L (L ¼ PEMA, PHEMA
present the TG and DTG curves of SEP–ZnO-MAA–PEMA and and PHFMA) and the hybrids without the ZnO host (SEP–PEMA-
SEP–PEMA, respectively. From the TG curve of SEP–ZnO-MAA– 1, SEP–PEMA-2), respectively. The excitation spectra are
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Fig. 6 The excitation spectra of hybrid materials SEP–ZnO-MAA–
PEMA (A), SEP–ZnO-MAA–PHEMA (B) and SEP–ZnO-MAA–PHFMA (C)
Fig. 7 The emission spectra of hybrid materials SEP–ZnO-MAA–L (L ¼
PEMA, PHEMA and PHFMA) (lex ¼ 270 nm) (a) and SEP–PEMA-1, SEP–
PEMA-2 (lex ¼ 278 nm) (b).
by monitoring emission of 614 nm (lem) (a) and SEP–PEMA-1 (A lem
14 nm, B 594 nm), and C SEP–PEMA-2 (lem ¼ 614 nm) (b).
¼
6
3
+
obtained by monitoring the emission of Eu at 614 nm for the
Fig. 7(a) and (b) show the emission spectra for the excitation
SEP–ZnO-MAA–L hybrids and 591 nm for SEP–PEMA-1(2), and emission spectra of hybrids with the ZnO unit SEP–ZnO-
respectively (Fig. 6). For the hybrid materials containing ZnO- MAA–L (L ¼ PEMA, PHEMA and PHFMA) and the hybrids
MAA, the broad excitation bands from 240 to 320 nm centered without the ZnO host (SEP–PEMA-1, SEP–PEMA-2), respectively.
at about 260 nm in the ultraviolet region can be seen, which is From Fig. 7(a) for ZnO-MAA–L hybrids with the polymer func-
attributed to the Eu–O charge transfer transition state of the tionalized ZnO unit, it can be seen that there are ve main
SEP host. The sensitizers absorb energy and transfer it to the emission peaks at around 581, 592, 614, 653 and 702 nm,
5
7
accepting energy level of the lanthanide ions, depending on the respectively, corresponding to the D / F transition (J ¼ 0, 1,
0
J
energy gap between the triplet state energy of the ligands and 2, 3, 4). Furthermore, there exists one weak broad emission peak
3
+
the resonance energy level of the central RE ions through the in the range 400 to 550 nm, centered at 470 nm, corresponding
intramolecular energy transfer process. Besides, there exist to the emission of ZnO in the resulting system. This suggests
3
+
apparent sharp lines in the 350–500 nm range, ascribed to intra- that the luminescence of the host is quenched by Eu and the
3
+
congurational narrow 4f–4f transitions of Eu in the host energy transfer occurred between them. So, the hybrid system is
3
+
lattice, and especially the strongest excitation peaks are at effective for the luminescence of Eu . From Fig. 7(b) for the
7
5
7
5
3
96 nm ( F
0 6 0 2
/ L ) and 466 nm ( F / D ), respectively. It can SEP–PEMA-1, SEP–PEMA-2 hybrids without the ZnO unit, a
be observed in the gure that the intensities of the 4f–4f tran- strong orange luminescence is obtained in the emission
20
sitions are comparable to the charge transfer band.
spectra. Five main emission peaks in the emission spectra are
Comparing the two kinds of excitation spectra of (a) and (b), the observed, located at 580, 594, 614, 652 and 701 nm, corre-
5
7
introduction of ZnO decreases the excitation intensity of SEP sponding to the D
0
/ F
J
transition (J ¼ 0, 1, 2, 3, 4), respec-
host containing of Eu–O CTS, which is due to the fact that the tively. The strongest orange luminescence for them is similar to
content of SEP is decreased. For the hybrid materials SEP– the pure POM, suggesting that the modication of the surfac-
PEMA-1 and SEP–PEMA-2, their excitation bands from 220 nm tant DMDA and polymer cannot change the luminescent
to 350 nm, centered at about 280 nm in the ultraviolet region performance of the POM. Besides, there is no emission of ZnO.
can be seen. It is a little different from the materials containing So, it can be inferred that the coordination behavior and envi-
3
+
ZnO-MAA, as the excitation bands are broader. Besides, it can be ronment surrounding Eu in SEP–ZnO-MAA–L hybrids have
seen that the excitation spectra are identical for SEP–PEMA-1 been changed by the introduction of the ZnO-MAA.
using different emission wavelengths (orange emission of 594
nm and red emission of 614 nm).
Furthermore, based on the emission both in the blue
region from the ZnO unit and the red region for Eu from the
3
+
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SEP, it was expected to realize a tunable color of the lumi-
nescence for different excitation wavelengths. Here, we
selected SEP–ZnO-MAA–PHFMA as an example. Firstly, we
checked the emission spectra monitored using different
wavelengths, 270, 320, 350, 396 and 465 nm, respectively
(
see Fig. 8a). It was found the emission color can be changed
to oyster white, light blue and yellow colors (Fig. 8b). Among
these, the color is close to white using 270 nm and 396 nm as
the excitation wavelength. Considering that the excitation
wavelengths at 395 nm and 465 nm are in the visible region
(
violet light and blue light), the excitation light will integrate
with the emission light.
Fig. 9(a) and (b) show the color coordinate diagrams based
on both the excitation (395 nm, 465 nm) and emission spectra
Fig. 9 The luminescent color coordinate diagrams for SEP–PEMA-1
(b), SEP–PEMA-2 (b).
(614 nm), which are close to a white color.
For a further study of the uorescence properties, we
compared the luminescent relative intensities and lifetimes of
the ve obtained hybrid materials. The typical decay curves that
On the basis of the emission spectra of these obtained hybrid
5
can be described as a single exponential (ln[S(t)/S ] ¼ ꢀk t ¼ materials and the lifetimes of the D0 emitting level (s), the
0
1
ꢀ
t/s) were measured at room temperature, indicating that all emission quantum efficiency (h) can be determined. Assuming
the Eu ions occupy the same average coordination environ- that only non-radiative and radiative processes are essentially
3
+
5
ment. For SEP–ZnO-MAA–PEMA, SEP–ZnO-MAA–PHFMA and involved in the depopulation of the D excited state, h can be
SEP–ZnO-MAA–PHEMA, their luminescence lifetimes are 0.40, dened as follows:
0
21
0
1
.13 and 0.10 ms, respectively, while the lifetimes of SEP–PEMA-
(0.01 ms) and SEP–PEMA-2 (0.01 ms) are relatively much
h ¼ A
r
/(A
r
+ Anr
)
(1)
shorter. These results demonstrate that ZnO-MAA plays a role in
protecting the Eu ions from quenching.
3
+
Here, A and A are the radiative and non-radiative transi-
r
nr
tion rates, respectively. A can be calculated by summarizing
r
5
7
over the radiative rates A for each D / F (J ¼ 0–4) transi-
0
J
0
J
3
+
tions of Eu as follows:
P
A
r
¼
A
0J ¼ A00 + A01 + A02 + A03 + A04
(2)
5
7
5
7
Because the branching ratio for the D / F and D / F
0
5
0
6
transitions are both not detected experimentally, their inuence
5
can be ignored in the depopulation of the D
0
excited state.
belongs to the isolated magnetic dipole tran-
sition and is nearly independent of the chemical environments
5
7
0 1
Since D / F
3
+
around Eu , it can be considered as an internal reference for
the whole spectrum. A0J, the experimental coefficients of
spontaneous emission, can be calculated according to the
22
following equation: a
A
0J ¼ A01 (I0J/I01)(n01/n0J
)
(3)
Here, A0J are the experimental coefficients of spontaneous
emission. A01 is the Einstein coefficient of spontaneous emis-
5
7
sion between the D and F energy levels. In a vacuum, the
0
ꢀ
1
1
value of A01 is 14.65 s , when an average index of refraction n
equal to 1.506 is considered, the value of A01 can be determined
to be about 50 s (A01 ¼ n A01(vac)) in an air atmosphere. The
emission intensity I can be seen as the integrated intensity of
ꢀ
1
3
23
5
7
the D
0
/ F
J
(J ¼ 0–4) emission bands with n01 and n0J (n0J ¼
1
/l ) energy centers, respectively. n directs at the energy barrier
J
0J
Fig. 8 The luminescent emission spectra (a) A for 320 nm, B for
5
7
and can be determined by the D / F emission transitions
0
J
270 nm, C for 350 nm, D for 396 nm and E for 465 nm and (b) the color
3
+
peaks of Eu .
coordinate diagram for SEP–ZnO-MAA–PHFMA, the crosses from the
bottom to the top are for different excitation wavelengths, 320, 270,
r
The lifetime (s), radiative (A ), and non-radiative (Anr) tran-
3
50, 396 and 465 nm, respectively.
sition rates are related according to the following equation:
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ꢀ
nr
1
sexp ¼ (A + A )
(4)
r
Acknowledgements
According to above discussion, we can nd that the value of h
mainly depends on the values of the lifetimes and the I02/I01
ratio. The quantum efficiency must be high when the lifetimes
and red-orange ratio are large. As clearly shown in Table 1, the
luminescent efficiencies of three ZnO-containing hybrid
This work was supported by the National Natural Science
Foundation of China (20971100, 91122003), and the Program
for New Century Excellent Talents in University (NCET 2008-08-
0398).
-
materials have been calculated in the following order SEP–ZnO-
Notes and references
MAA–PEMA
>
SEP–ZnO-MAA–PHFMA
>
SEP–ZnO-MAA–
PHEMA. We can nd that the material SEP–ZnO-MAA–PEMA
has the largest I02/I01 ratio and the longest lifetime, so its
quantum efficiency is the highest. Furthermore, the absolute
quantum yields are also measured using an integrating sphere
attachment, showing a similar order to the calculated data,
except for the higher value.
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s (ms)
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SEP–ZnO-MAA–PEMA
SEP–ZnO-MAA–PHFMA
SEP–ZnO-MAA–PHEMA
SEP–PEMA-1
5.44
5.05
4.66
0.64
0.25
0.70
0.63
0.60
1.20
1.11
24.6 (14.1)
20.9 (13.0)
20.3 (12.5)
35.3
1
SEP–PEMA-2
31.0
a
5
7
0 2
The luminescent lifetimes of D / F transition for SEP–ZnO-MAA–
5 7
PEMA (PHFMA, PHEMA) and of
1
0 1
D / F transition for SEP–PEMA-
b
(2). The luminescent quantum yields for absolute value using
integrating sphere and calculated data from lifetime and spectrum in
bracket.
3324 | RSC Adv., 2014, 4, 3318–3325
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