Pd nanoparticles in hollow magnetic mesoporous spheres: High activity, and magnetic recyclability

Article abstract of DOI:10.1016/j.molcata.2012.10.023

The nanoreactor of hollow magnetic mesoporous silica spheres (Pd/HMMS), with Pd and Fe₃O₄ nanoparticles embedded in the mesoporous silica shell, were successfully prepared by using the colloidal carbon spheres of glucose, Pd and Fe₃O₄ heteroaggregates as the hard template together with a coating of tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB) mixture. The synthesized Pd/HMMS shows excellent catalytic activity in the Suzuki cross-coupling reaction of iodobenzene with phenylboronic acid with over 99% yield in 3 min and can be recycled multiple times without any significant loss in catalytic activity.

Full text of DOI:10.1016/j.molcata.2012.10.023

Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Pd nanoparticles in hollow magnetic mesoporous spheres: High activity, and
magnetic recyclability
Jian Sun, Zhengping Dong, Xun Sun, Ping Li, Fengwei Zhang, Wuquan Hu, Haidong Yang,
∗
Haibo Wang, Rong Li
The Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering Lanzhou University, Lanzhou 730000,
China
a r t i c l e i n f o
a b s t r a c t
Article history:
The nanoreactor of hollow magnetic mesoporous silica spheres (Pd/HMMS), with Pd and Fe O4 nanopar-
3
Received 2 February 2012
Received in revised form 20 October 2012
Accepted 22 October 2012
Available online 30 October 2012
ticles embedded in the mesoporous silica shell, were successfully prepared by using the colloidal
carbon spheres of glucose, Pd and Fe3O4 heteroaggregates as the hard template together with a coat-
ing of tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB) mixture. The synthesized
Pd/HMMS shows excellent catalytic activity in the Suzuki cross-coupling reaction of iodobenzene with
phenylboronic acid with over 99% yield in 3 min and can be recycled multiple times without any signifi-
cant loss in catalytic activity.
Keywords:
Suzuki coupling reaction
Palladium
©
2012 Elsevier B.V. All rights reserved.
Hollow silica sphere
Mesoporous
Magnetic properties
1
. Introduction
[26–31], and rare work succeeded in designing HMMS with
metal nanoparticles and well-defined nanostructures for advanced
catalysis [32–35].
Hollow mesoporous silica spheres (HMS) have attracted con-
siderable attention because their nanopore channels with large
surface area and pore volume, high porosity, ordered and tun-
able size. They can be used as nanoreactors for applications in
adsorption, separation, catalysis [1–9], and so forth. Compared
to the conventional mesoporous silica materials such as MCM-
In this paper, we developed a facile route to synthesize HMMS
nanoreactors with Pd/Fe O nanoparticles resided inside the meso-
porous spheres by using the colloidal carbon spheres as the
templates. The fabrication procedure involved four steps as shown
in Fig. 1. The first step involved the preparation of the col-
3
4
4
1 [10–14], SBA-15 [15–18], FSM-16 [19] and MCF [20], HMS
loidal carbon spheres with attached Fe O4 nanoparticles. In the
3
exhibited good catalyst loading properties for confined coop-
erative catalysis, as they could prevent aggregation of metallic
nanoparticles and promote the mass diffusion and transport of
reactants.
As is well known, magnetic nanoparticles have been widely
applied in heterogeneous catalyst systems [21–25]. Generally,
magnetic microspheres consisting of an iron oxide core and
functional shell have gained much attention due to their
unique separable feature. Therefore, to enhance their perfor-
mance in practical applications, many research efforts have
recently been devoted to HMMS to combine the high sur-
face area and magnetic properties. Meanwhile, considerable
efforts focused on the adsorption or drug delivery application
next step, deposited highly dispersed Pd nanoparticles onto the
surface of carbon nanospheres with Fe O4 nanoparticles. Then,
3
the organosilicate incorporated silica shells were deposited on
the colloidal carbon spheres through the simultaneous sol–gel
polymerization of tetraethoxysilane (TEOS) and cetyltrimethylam-
monium bromide (CTAB). Finally, the HMMS nanoreactors were
obtained after the calcination to remove the carbon templates and
the organic groups of CTAB. The designed Pd/HMMS nanoreactors
possess large magnetization, highly open and ordered mesopores,
and stably confined but exposed Pd nanoparticles. The multi-
component nanostructured materials showed excellent activity
for Suzuki cross-coupling reaction and could be easily recycled
multiple times without visible decrease in the catalytic perfor-
mance. Furthermore, the method is also extended to introduce
other noble metal nanoparticles including Au, Ag, Pt, and their
bimetallic nanoparticles inside the HMMS to prepare other new
catalysts.
∗ Corresponding author. Tel.: +86 0931 891 3597; fax: +86 0931 891 2582.
E-mail address: liyirong@lzu.edu.cn (R. Li).
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.10.023

J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
47
Fig. 1. Scheme of the synthetic procedure for the preparation of Pd/hollow magnetic mesoporous spheres.
2
. Experimental
environment for adsorbing metal ion-nanoparticles by electrostatic
attraction in acidic solution. 0.6 g Fe O4 nanoparicles and 1.0 g
3
2
.1. Materials and reagents
negatively charged carbon nanospheres were dispersed in 80 mL
and 60 mL hydrochloric acid solution (pH ≈ 2.3) with ultrasonic for
20 min, respectively. Then the latter suspension was added drop-
wise into the former under vigorous stirring at room temperature.
Glucose, ethanol, sodium formate, tetraethoxysilane (TEOS),
ammonia solution (28 wt%), SnCl ·2H O, PdCl , K CO , FeCl ·6H O
2
2
2
2
3
3
2
(
99%), FeCl ·4H O (99%), were purchased from Tianjing Chemical
After 6 h, Fe O4 nanoparticles attaching on the surface of carbon
2
2
3
Reagent Co. Iodobenzene, phenylboronic acid and cetyltrimethy-
lammonium bromide (CTAB) were bought from Alfa Aesar. All
chemicals were of analytical grade and were used as received with-
out further purification.
nanospheres, were separated from the solution by an external mag-
net and washed several times with distilled water until the pH value
of the solution was close to 7. Finally, the products were oven-dried
◦
at 50 C for further use.
2
2
.2. Preparation of catalysts
2
.2.4. Loading Pd nanoparticles onto the carbon nanosphere with
.2.1. Synthesis of carbon nanospheres [36]
Glucose (6.0 g) was dissolved in 40 mL distilled water to form
Fe3O4 nanoparticles (Pd/Fe3O4/C) [37]
200 mg Fe3O4/C was dispersed in 50 mL distilled water and
stirred for 20 min as part A. 0.2 g SnCl2 was dissolved in 40 mL
0.02 M HCl solution as part B. Parts A and B were mixed together
under stirring for 30 min. The suspension was separated from the
solution by an external magnet, washed several times with distilled
water and dispersed in 80 mL distilled water. Then 50 mL 10 mg
PdCl2 was added into it. 10 min later, 20 mL of 0.15 M sodium for-
mate solution was added, following stirring for 8 h. The suspension
was separated from the solution by an external magnet, washed
a clear solution and then transferred into a 100 mL Teflon-sealed
autoclave. The autoclave was maintained at 180 C for 4.5 h. The
products were separated by centrifugation, followed by washing
three times with distilled water and ethanol. Finally, the products
were oven-dried at 50 C for the following work.
◦
◦
2.2.2. Synthesis of iron oxide nanoparticles (Fe O )
3
4
Magnetic nanoparticles were prepared using simple chemi-
cal co-precipitation. Typically, 4.8 g of FeCl ·6H O and 2.0 g of
◦
3
2
with distilled water and ethanol for several times and dried at 50 C
for 12 h.
FeCl ·4H O were dissolved in 40 mL of distilled water in a three-
2
2
necked bottom (100 mL). The obtained transparent solution was
degassed with nitrogen for 1 h. After being rapidly magnetic stirred
◦
2.2.5. Production of Pd/Fe O @mesoporous SiO nanoreactor
3 4 2
for 30 min, the transparent solution was heated to 90 C. Thereafter,
1
0
at 90 C. After cooling to room temperature, the obtained magnetic
dispersion was subjected to magnetic separation with a magnet,
and dried at 60 C for 6 h.
(
Pd/HMMS)
The Pd/Fe O /C composite obtained from last step was first dis-
2 mL of ammonia hydroxide was added to the reaction mixture in
.1 min time using a syringe, and the mixture was stirred for 2 h
3
4
◦
persed in the solution containing 80 mL H O, 60 mL ethanol, 0.3 g
2
CTAB and 1.2 mL NH ·H O with ultrasonic for 20 min. The 300 ␮L
3
2
◦
TEOS was added into the mixture, and vigorously stirred for 8 h.
The suspension was separated from the solution by an external
magnet, washed several times with distilled water and ethanol,
2
.2.3. Preparation of carbon nanospheres with attached Fe O
3 4
◦
nanoparticles (Fe O /C) [31]
respectively, then oven-dried at 50 C. Then the product was cal-
cined at 400 C in air atmosphere for 7 h to remove carbon sphere,
CTAB template and other organic species. The amount of Pd in
3
4
◦
These hydroxyl carbon nanospheres possess functional groups
OH) on the surface, which offer an important chemical
(

4
8
J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
that the Pd particles have been coated by hollow mesoporous SiO2.
Transmission electron microscopy (TEM) experiments have been
carried out to observe a possible structure of the metallic Pd par-
ticles (Fig. 3). Apparently, Fe O4 and Pd nanoparticles are located
3
at the inside pore mouths of the mesopores. This is consistent with
the XRD patterns (Fig. 2).
The morphologies and structures of the products at different
synthetic steps were observed by TEM. Fig. 3a shows the TEM
image of carbon nanospheres, in which well-dispersed particles
with average diameter about 200 nm could be observed clearly.
These carbon nanospheres possess functional groups ( OH, C O)
on the surface, which offer an important chemical environment
for adsorbing metal ion-nanoparticles by electrostatic attraction in
acidic solution. Fig. 3b is a TEM image of the Fe O /C composite.
3
4
The Fe O4 particles with an average size of about 9 nm are uni-
3
formly distributed on the surfaces of the carbon nanospheres. Using
TEOS as the silica source and CTAB as a soft template, a thin layer
of mesoporous silica is coated onto the Pd/Fe O /C spheres. Then,
3
4
the Pd/HMMS nanoreactors were obtained after the calcination to
remove the carbon templates and the organic groups of CTAB. As
Fig. 2. XRD patterns of (a) HMMS and (b) Pd/HMMS.
shown in Fig. 3c and d, the TEM images show that Pd, Fe O4 par-
3
ticles were evenly coated with a layer of mesoporous shells about
4
0 nm thick, while the Fe O particles remained initial their cluster
3 4
the Pd/HMMS nanoreactor was found to be 2.0 wt% based on ICP
analysis.
morphologies in the Pd/HMMS. To further probe the components
of the mesoporous shell, we used EDX measurement to analyze the
Pd/HMMS after calcination at 400 C. The EDX spectra confirmed
that the nanoreactor consisted of Fe, Si, Sn, Pd and O elements (Cu
and C came from the TEM copper grid of the sample holder). EDX
analysis (Fig. 3e) further confirms the existence of Pd. According
to the ICP analysis, the mass percent of Sn for the composites was
◦
2.3. Measurement of catalytic performance
For Suzuki cross-coupling reaction, 20 mg Pd/HMMS nanore-
actor catalyst, 0.5 mmol iodobenzene, 1 mmol phenylboronic acid,
mmol K CO were added to 4 mL ethanol under magnetic stir-
1
2
3
0
.9 wt%, and the presence of Sn was not significantly affected the
◦
ring. The reaction was carried out at reflux (ca. 80 C) for definite
time. Then, the catalyst was collected by an external magnet, and
the reaction system was analyzed by gas chromatography. For
recycling, the recovered catalyst was washed with ethanol and
distilled water several times, respectively, and then dried under
catalytic activity of the nanoreactor [8].
Fig. 4 shows the XPS spectrum of the synthesized Pd/HMMS
nanoreactor. Peaks corresponding to oxygen, silicon, carbon, palla-
dium, tin and iron are observed. However, typical peaks of Fe, Sn and
Pd elements are not obviously found. Fig. 6 shows the XPS spectrum
◦
vacuum at 60 C.
of the elemental survey scan of Pd/Fe O /C. Meanwhile, typical
3
4
peaks of Fe, Sn and Pd elements are clearly found. This indicates that
2.4. Characterization of catalysts
Sn, Fe O4 and Pd particles have been coated by mesoporous SiO2,
3
since the analysis depth of XPS is only several nanometers. In Fig. 4,
both the peaks of Si 2s (153.5 eV) and Si 2p (102.6 eV) are quite clear
and this means that SiO2 is the main component of the outer sur-
face. To ascertain the oxidation state of the Pd, X-ray photoelectron
spectroscopy (XPS) studies were carried out. In Fig. 5, the binding
energy of Pd exhibits two strong peaks centered at 342.6 and
Powder X-ray diffraction (XRD) patterns were obtained on a
Rigaku D/max-2400 diffractometer using Cu-K␣ radiation as the X-
◦
ray source in the 2Â range of 10–80 . The conversion was estimated
by GC (P.E. AutoSystem XL) or GC–MS (Agilent 6890N/5973N).
Pd content of the catalyst was measured by inductively coupled
plasma (ICP) on IRIS Advantage analyzer. The morphology of the
catalyst was observed by a Tecnai G2 F30 transmission electron
microscopy and samples were obtained by placing a drop of a col-
loidal solution onto a copper grid and evaporating the solvent in
air at room temperature. X-ray photoelectron spectroscopy (XPS)
was recorded on a PHI-5702 instrument and the C1s line at 297.8 eV
was used as the binding energy reference. Magnetic measurement
3
37.2 eV, which are assigned to Pd 3d3/2 and Pd 3d5/2, respectively.
On the basis of above analysis, it can be concluded that the XPS data
further identified the hollow mesoporous structure of the synthe-
sized Pd/HMMS (Fig. 6).
Magnetization curves revealed the superparamagnetic behav-
ior of the magnetic nanoreactor, the hysteresis loop of Pd/HMMS
is shown in Fig. 7. The magnetization saturation (Ms) of the meso-
of Pd/Fe O @mesoporous SiO nanoreactor was investigated with
−1
3
4
2
porous spheres reaches 21.0 emu g , which suggests that Fe O4
3
a Quantum Design vibrating sample magnetometer (VSM) at room
particles become encapsulated in hollow mesoporous spheres. This
is consistent with the TEM patterns (Fig. 3). The catalyst can be
efficiently separated easily from the solution with the help of an
external magnetic force.
temperature in an applied magnetic field sweeping from −15 to
1
5 kOe.
3
. Results and discussion
3.2. Suzuki reaction catalyzed by Pd/HMMS nanoreactor
3.1. Characterization of catalysts
The catalytic activity of Pd/HMMS nanoreactor was tested in the
The wide angle XRD patterns of the samples are presented in
Suzuki reaction of aryl halides, namely the reactivity of aryl iodides
and aryl bromides with phenylboronic acid in ethanol solvent using
Fig. 2. An almost identical XRD pattern was obtained for Pd/HMMS
comparing with HMMS (Fig. 2b), indicating the crystalline struc-
ture of the support was well maintained in the Pd samples. No
obviously peaks due to metallic Pd were detected, owing to the fact
K CO as a base. The reaction results are summarized in Table 1. A
2
3
range of activities were observed from 25% to >99% yields depend-
ing on the nature of aryl bromo and iodo derivatives. Iodobenzene

J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
49
Fig. 3. (a) TEM of carbon nanospheres, (b) TEM of Fe3O4/C nanospheres, (c) TEM of final composite nanoreactor (Pd/HMMS) in low magnification, (d) TEM of final composite
nanoreactor in high magnification, (e) energy-dispersive X-ray absorption spectroscopy (EDX) of the Pd/HMMS nanoreactor.
Fig. 4. XPS spectrum of the elemental survey scan of Pd/Fe3O4@mesoporous SiO2.
Fig. 5. XPS spectrum of the Pd/HMMS showing Pd 3d5/2 and Pd 3d3/2 binding ener-
gies.

5
0
J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
Table 2
Recyclability of catalyst .
a
Entry
Run
Yieldb (%)
>99
>99
99
99
99
1
2
3
4
5
Fresh
First
Second
Third
Fourth
a
Reaction conditions: iodobenzene (0.5 mmol), phenylboronic acid (1 mmol),
◦
K2CO3 (2 mmol), ethanol (4 mL), 80 C, catalyst (20 mg, with Pd loading 2.0 wt%).
b
Yield based on GC analysis.
can be easily recovered in a facile manner from the reaction solution
by using a permanent magnet. Furthermore, the catalyst revealed
a remarkable activity and was reused up to four consecutive cycles
without any significant loss in catalytic activity (Table 2).
4. Conclusions
In summary, we have developed a facile method for fabri-
cation of uniform Pd/HMMS nanoreactor with desired hollow
core/mesoporous shell structures, high catalytic activity and mag-
netic recyclability properties. The present method is versatile, and
many transition noble metals and their bimetallic nanoparticles can
be resided inside the HMMS via the sol–gel process. The nanore-
actor shows superior catalytic activity in the Suzuki cross-coupling
reaction of iodobenzene with phenylboronic acid with over 99%
yield in 3 min and can be recycled multiple times without any sig-
nificant loss in catalytic activity.
Fig. 6. XPS spectrum of the elemental survey scan of Pd/Fe3O4/C.
Acknowledgment
This work was supported by the Fundamental Research Funds
for the Central Universities (lzujbky-2012-68).
References
[
[
[
1] Q. Zhang, T. Zhang, J. Ge, Y. Yin, Nano Lett. 8 (2008) 2867–2871.
2] X. Fang, C. Chen, Z. Liu, P. Liu, N. Zheng, Nanoscale 3 (2011) 1632–1639.
3] L. Du, S. Liao, H.A. Khatib, J.F. Stoddart, J.I. Zink, J. Am. Chem. Soc. 131 (2009)
15136–15142.
[
[
4] J. Lee, J.C. Park, U. Bang, H. Song, Chem. Mater. 20 (2008) 5839–5844.
5] S.-H. Wu, C.-T. Tseng, Y.-S. Lin, C.-H. Lin, Y. Hung, C.-Y. Mou, J. Mater. Chem. 21
(2011) 789–794.
Fig. 7. Room temperature magnetization curves of Pd/HMMS.
[
[
[
6] L. Tan, D. Chen, H. Liu, F. Tang, Adv. Mater. 22 (2010) 4885–4889.
7] X. Yang, L. Du, S. Liao, Y. Li, H. Song, Catal. Commun. 17 (2012) 29–33.
8] Z. Chen, Z.-M. Cui, F. Niu, L. Jiang, W.-G. Song, Chem. Commun. 46 (2010)
derivatives were found to give a good yield in 5 min reaction time
Table 1, entries 2 and 3). It was observed that with the increase in
6524–6526.
(
[
9] X. Yang, S. Liao, J. Zeng, Z. Liang, Appl. Surf. Sci. 257 (2011) 4472–4477.
reaction time the conversion of bromobenzene increases (Table 1,
entry 4). Bromobenzene derivatives were found to give a moder-
ate yield in 30 min reaction time (Table 1, entries 5–7). The catalyst
[10] K. Bachari, J.M.M. Millet, P. Bonville, O. Cherifi, F. Figueras, J. Catal. 249 (2007)
2–58.
5
[11] A.B. Bourlinos, M.A. Karakassides, D. Petridis, J. Phys. Chem. B 104 (2000) 4375.
12] M.E. Domine, M.C. Hernández-Soto, M.T. Navarro, Y. Pérez, Catal. Today 172
[
(
2011) 13–20.
[
13] P. Selvam, S.K. Mohapatra, S.U. Sonavane, R.V. Jayaram, Appl. Catal. B: Environ.
49 (2004) 251–255.
Table 1
Suzuki reactions catalyzed by the Pd/HMMS.a
[14] P. Krawiec, E. Kockrick, P. Simon, G. Auffermann, S. Kaskel, Chem. Mater. 18
2006) 2663–2669.
15] P. Han, X. Wang, X. Qiu, X. Ji, L. Guo, J. Mol. Catal. A: Chem. 272 (2007) 136–141.
[16] C. Li, Q. Zhang, Y. Wang, H. Wan, Catal. Lett. 120 (2008) 126–136.
17] H. Wang, C.-j. Liu, Appl. Catal. B: Environ. 106 (2011) 672–680.
[18] P. Wang, X. Zheng, Powder Technol. 210 (2011) 115–121.
(
[
.
[
Entry
X
R
Time/min
Yieldb (%)
[
[
[
19] A. Fukuoka, H. Araki, Y. Sakamoto, S. Inagaki, Y. Fukushima, M. Ichikawa, Inorg.
1
2
3
4
I
I
I
Br
H
3
5
5
3
>99
Chim. Acta 350 (2003) 371–378.
20] S. Park, D.R. Park, J.H. Choi, T.J. Kim, Y.-M. Chung, S.-H. Oh, I.K. Song, J. Mol. Catal.
A: Chem. 332 (2010) 76–83.
2-NH2
4-Me
H
86
66
25
46
57
84
49
21] D.-H. Zhang, G.-D. Li, J.-X. Lia, J.-S. Chen, Chem. Commun. (2008) 3414–3416.
30
[22] S. Shylesh, V. Schünemann, W.R. Thiel, Angew. Chem. Int. Ed. 49 (2010)
3428–3459.
[23] B. Lv, Y. Xu, H. Tian, D. Wu, Y. Sun, J. Solid State Chem. 183 (2010) 2968–2973.
[24] Y. Deng, Y. Cai, Z. Sun, J. Liu, C. Liu, J. Wei, W. Li, C. Liu, Y. Wang, D. Zhao, J. Am.
Chem. Soc. 132 (2010) 8466–8473.
5
6
7
Br
Br
Br
4-COMe
4-NH2
4-Me
30
30
30
a
Reaction conditions: iodobenzene (0.5 mmol), phenylboronic acid (1 mmol),
[
25] F. Zhang, J. Jin, X. Zhong, S. Li, J. Niu, R. Li, J. Ma, Green Chem. 13 (2011) 1238.
◦
K2CO3 (2 mmol), ethanol (4 mL), 80 C, catalyst (20 mg, with Pd loading 2.0 wt%).
[26] Y. Zhu, E. Kockrick, T. Ikoma, N. Hanagata, S. Kaskel, Chem. Mater. 21 (2009)
2547–2553.
b
Determined by GC or GC–MS.

J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
51
[
[
[
27] Y. Zhu, Y. Fang, S. Kaskel, J. Phys. Chem. C 114 (2010) 16382–16388.
28] Y. Zhu, T. Ikoma, N. Hanagata, S. Kaskel, Small 6 (2010) 471–478.
29] J. Zhou, W. Wu, D. Caruntu, M.H. Yu, A. Martin, J.F. Chen, C.J. O’Connor, W.L.
Zhou, J. Phys. Chem. C 111 (2007) 17473–17477.
[33] Y. Zhang, J. Li, D. Han, H. Zhang, P. Liu, C. Li, Biochem. Biophys. Res. Commun.
365 (2008) 609–613.
[34] J. Li, Y. Zhang, D. Han, Q. Gao, C. Li, J. Mol. Catal. A: Chem. 298 (2009) 31–35.
[35] C. Tu, J. Du, L. Yao, C. Yang, M. Ge, C. Xu, M. Gao, J. Mater. Chem. 19 (2009)
1245.
[
30] W. Zhao, H. Chen, Y. Li, L. Li, M. Lang, J. Shi, Adv. Funct. Mater. 18 (2008)
2
780–2788.
[36] X. Sun, Y. Li, Angew. Chem. Int. Ed. 43 (2004) 597–601.
[37] L.-S. Zhong, J.-S. Hu, Z.-M. Cui, L.-J. Wan, W.-G. Song, Chem. Mater. 19 (2007)
4557–4562.
[
[
31] L.Y. Xia, M.Q. Zhang, C.E. Yuan, M.Z. Rong, J. Mater. Chem. 21 (2011) 9020.
32] P. Jin, Q. Chen, L. Hao, R. Tian, L. Zhang, L. Wang, J. Phys. Chem. B 108 (2004)
6
311–6314.