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一种制备用于高效SnO2-TiO2核壳染料敏化太阳能电池的SnO2纳米管的简便方法
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用于染料敏化太阳能电池(DSSC)的高效光电极应兼具快速电子传输,慢速界面电子复合和大比表面积的优点。 但是,在当前的最新研究中,通常无法同时实现这三个要求。 在这里我们报告了一个简单的程序,通过使用多Kong的SnO2纳米管-TiO2(SnO2 NT-TiO2)核壳结构的DSSCs阳极来组合这三个相互矛盾的要求。 SnO2纳米管是通过电纺聚乙烯吡咯烷酮(PVP)/二氯化锡二水合物(SnCl2中心点2H(2)O)溶液,然后将初纺纳米纤维直接烧结而制备的。 提出了一种可能的进化机制。 SnO2 NT-TiO2核壳结构DSSC的功率转换效率(PCE)值(约5.11%)比SnO2纳米管(SnO2 NT)DSSC的功率转换效率(约0.99%)高五倍以上。 该PCE值也高于TiO2纳米颗粒(P25)DSSC(约4.82%),即使吸附到SnO2 NT-TiO2光电阳极上的染料分子数量少于P25膜的一半。 这个简单的过程提供了一种同时满足三个冲突要求的新方法,这已被证明是获得高效DSSC的有前途的策略。
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A facile method to prepare SnO
2
nanotubes for use in efficient SnO
2
–TiO
2
core–shell dye-sensitized solar cells†
Caitian Gao,
*
a
Xiaodong Li,
a
Bingan Lu,
b
Lulu Chen,
a
Youqing Wang,
a
Feng Teng,
a
Jiangtao Wang,
a
Zhenxing Zhang,
a
Xiaojun Pan
a
and Erqing Xie
*
a
Received 14th February 2012, Accepted 16th April 2012
DOI: 10.1039/c2nr30349c
A high-efficiency photoelectrode for dye-sensitized solar cells (DSSCs) should combine the
advantageous features of fast electron transport, slow interfacial electron recombination and large
specific surface area. However, these three requirements usually cannot be achieved simultaneously in
the present state-of-the-art research. Here we report a simple procedure to combine the three conflicting
requirements by using porous SnO
2
nanotube–TiO
2
(SnO
2
NT–TiO
2
) core–shell structured
photoanodes for DSSCs. The SnO
2
nanotubes are prepared by electrospinning of polyvinyl
pyrrolidone (PVP)/tin dichloride dihydrate (SnCl
2
$2H
2
O) solution followed by direct sintering of the
as-spun nanofibers. A possible evolution mechanism is proposed. The power conversion efficiency
(PCE) value of the SnO
2
NT–TiO
2
core–shell structured DSSCs (5.11%) is above five times higher
than that of SnO
2
nanotube (SnO
2
NT) DSSCs (0.99%). This PCE value is also higher than that of
TiO
2
nanoparticles (P25) DSSCs (4.82%), even though the amount of dye molecules adsorbed to the
SnO
2
NT–TiO
2
photoanode is less than half of that in the P25 film. This simple procedure provides
a new approach to achieve the three conflicting requirements simultaneously, which has been
demonstrated as a promising strategy to obtain high-efficiency DSSCs.
Introduction
Dye-sensitized solar cells (DSSCs) have attracted a great deal of
attention as a promising candidate for future green energy due to
their facile, low-cost, and environmentally-friendly fabrication
process.
1–3
Recently, Yella et al.
4
reported a new record power
conversion efficiency (PCE) of >12% by using a porphyrin-
sensitized nanocrystalline TiO
2
photoanode together with
cobalt(
II/III)-based redox electrolyte. However, the nanocrystal-
line TiO
2
photoanode is still a limiting component that needs to
be further improved before the technology can be commercial-
ized. The overall sunlight-to-electric power conversion process of
a DSSC can be summarized as the combination of photo-
generation, charge-carrier transport, and collection.
1,5
In other
words, a high-efficiency photoanode for DSSCs should combine
the advantageous features of fast electron transport, slow inter-
facial electron recombination, and high specific surface area.
Considerable attention has been focused on 1D core–shell
structures for the purpose of speeding up electron transport
and slowing recombination to achieve a high charge collection
efficiency.
6–11
However, in the present state-of-the-art research,
these 1D core–shell structures usually increase the charge
collection efficiency at the expense of specific surface area,
resulting in a low PCE.
One promising approach to achieving the three conflicting
requirements simultaneously is to use electrospun SnO
2
nano-
tubes in SnO
2
nanotube–TiO
2
(SnO
2
NT–TiO
2
) core–shell
structured photoanodes for dye-sensitized solar cells. Firstly, we
fabricated SnO
2
nanofibers and nanotubes as photoanodes
for DSSCs by electrospinning followed by direct sintering the
as-spun nanofibers. Electrospinning generates 1D polycrystalline
SnO
2
nanomaterials with extremely high aspect ratios and
specific surface areas, and thus, is different from most other
methods used to prepare SnO
2
nanomaterials, such as template
methods,
12
and the hydro-thermal method.
13
Therefore, the
electrospun 1D structures, such as nanofibers and nanotubes,
can provide a direct pathway for electron transport and large
surface area for dye loading. On the other hand, SnO
2
is an
excellent metal oxide semiconductor with higher electron
mobility (100 to 200 cm
2
V
1
S
1
) and larger band gap (3.6 eV)
than TiO
2
, indicating a faster transport of photogenerated elec-
trons and more excellent long-term stability compared to TiO
2
for DSSCs applications.
7,14,15
However, SnO
2
-based DSSCs were
developed with less success due to at least two weak points: (1)
a
School of Physical Science and Technology, Lanzhou University, Lanzhou
730000, Gansu, People’s Republic of China. E-mail: xieeq@lzu.edu.cn;
caitiangao10@163.com
b
Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of
Education, State Key Laboratory for Chemo/Biosensing and
Chemometrics, Hunan University, Hunan, People’s Republic of China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2nr30349c
This journal is ª The Royal Society of Chemistry 2012 Nanoscale, 2012, 4, 3475–3481 | 3475
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a 300 mV positive shift of the conduction-band edge of SnO
2
with respect to that of nanocrystalline TiO
2
, leading to a faster
interfacial electron recombination and lower trapping density,
16
and (2) less adsorption of the dye with acidic carboxyl groups due
to a lower isoelectric point. To solve these problems, SnO
2
–TiO
2
core–shell structures have been adopted to combine the advan-
tageous features of the two materials.
7,14,15,17,18
The SnO
2
–TiO
2
core–shell structured DSSCs show higher short-circuit photo-
current density (J
sc
) than that of TiO
2
nanoparticle (P25) DSSCs.
The contribution of one mole of dye molecules to J
sc
for SnO
2
–
TiO
2
core–shell DSSC is almost three times larger than that of
P25 DSSC, indicating an extremely fast and efficient charge
collection in SnO
2
–TiO
2
core–shell DSSCs.
Experimental
Preparation of 1D SnO
2
nanostructure by electrospinning
To obtain nanofibers and nanotubes, two kinds of precursor
solutions were prepared. Firstly, certain amounts of tin
dichloride dihydrate (SnCl
2
$2H
2
O, Tianjin Chemical Corp.,
China) were dissolved in a mixture of 2.2 g ethanol and 2.2 g
N,N-dimethyl formamide by magnetic stirring for 1 h at room
temperature. Secondly, 0.4 g polyvinyl pyrrolidone (PVP, Sigma
Aldrich, M
w
z 1 300 000) was added to the resulting solution
and vigorously stirred for 3 h at room temperature. The weight
ratios of PVP to SnCl
2
$2H
2
O were 1.33 (solution A) and 1.78
(solution B). The applied voltage and the distance between the tip
and the collector were 13.5 kV and 15 cm, respectively. After
electrospinning, the non-woven membranes of the fibers were
annealed at 500
C for 2 h with a heating rate of 3.5
C min
1
in air.
Fabrication of the SnO
2
–TiO
2
core–shell photoanodes for
DSSCs
To obtain the SnO
2
–TiO
2
core–shell photoanodes, the annealed
nanofibers and nanotubes (0.08 g) were ultrasonically dispersed
in a mixture of 1.5 ml acetic acid, 0.4 ml deionized water and 0.1 g
ethanol for durations of 10 min. Then 0.02 g polyethylene glycol
(PEG, M
w
¼ 20 000) was added to the above solutions and
stirred for 30 min. The pastes were then coated on F-doped tin
oxide (FTO) glass substrates (2.2 mm in thickness, >90% trans-
mittance, 14 U per square, Nippon, Japan) by drop-drying
method. The thickness of the films was fixed at 13 mm. The films
were then sintered at 500
C for 30 min. Coating of TiO
2
on SnO
2
nanofibers and nanotubes was performed by dipping the SnO
2
nanofibers and nanotubes electrodes into a 0.08 M TiCl
4
aqueous
solution at 85
C for 2 h, and then again sintering at 500
C in air
for 30 min.
Assembling of DSSCs
The details of the assembling process of the DSSCs have been
described in our previous work.
19
In brief, after cooling to 80
C,
the sintered electrodes were immersed into a 0.3 mM solution of
N-719 dye in a mixture of tert-butyl alcohol and acetonitrile
(volume ratio of 1 : 1) and kept in the dark at room temperature
for 12 h. Sensitized electrodes were rinsed with ethanol for 30 min
to remove the physisorbed dye molecules, and then assembled
with the platinum counter electrodes. The counter electrodes
were prepared by spin-coating a 4.5 mM isopropanol solution of
H
2
PtCl
6
$6H
2
O followed by sintering at 400
C for 20 min. The
interelectrode space was filled with a liquid electrolyte consisting
of 0.1 M LiI, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide,
0.05 M I
2
and 0.5 M 4-tert-butylpyridine in acetonitrile. The
effective area was fixed at 0.18 cm
2
.
Characterizations
The morphologies of the samples were characterized by field
emission scanning electron microscopy (FE-SEM, Hitachi
S-4800) and transmission electron microscopy (TEM, FEI Tec-
nai F30). The thermal gravimetry analysis was performed on
a Thermo-Gravimetric Analysis (TGA, PerkinElmer, Diamond
TG/DTA) in air with a heating rate of 3.5
C min
1
. X-Ray
diffraction (XRD, Philips, X’pert pro, Cu Ka, 0.154056 nm) was
employed to characterize the structural properties of the samples.
The specific surface area was measured by Brunauer–Emmett–
Teller (BET, ASAP 2010) method. The amount of adsorbed dye
was measured by chemical desorption with a 0.1 M NaOH
solution in 1 : 1 EtOH/H
2
O. The dye concentration of the
resultant solution was determined by UV-VIS absorption
measurement (TU-1901) of the solution using the absorption
peak intensity of N719 at 515 nm. Current–voltage (I–V) curves
were obtained by applying an external bias to the cell and
measuring the generated photocurrent with an Electrochemical
Workstation (RST5200, Zhengzhou Shiruisi Technology Co.,
Ltd, China). Electrochemical impedance spectroscopy (EIS)
measurements were carried out in the frequency range of 0.01 Hz
to 100 kHz at open-circuit voltage with a potential pulse of
10 mV in amplitude.
Results and discussion
Characterization of electrospun SnO
2
nanofibers and nanotubes
Fig. 1 shows the typical SEM and TEM images of the samples
electrospun from solution A and solution B followed by
annealing at 500
C for 2 h with a heating rate of 3.5
C min
1
.
The electrospun nanofibers from solution A and B yielded
nanofibers and nanotubes after annealing at 500
C for 2 h, as
shown in Fig. 1a and c, and Fig. 1b and d, respectively. The
average diameter and particle size of the nanofibers is 130 and
13 nm respectively. While the nanotubes exhibit an average
outer diameter of 110 nm, a wall thickness of 10 nm, and an
average particle size of 14 nm. These 1D structures consisting
of closely packed nanoparticles have a large surface-to-volume
ratio which is in favor of dye loading. The high resolution TEM
image and selected area electron diffraction pattern are shown in
Fig. 1e and f, respectively, revealing the polycrystalline nature of
the nanotubes, and the patterns could be completely indexed to
the rutile SnO
2
(JCPDS no. 41-1445).
Formation mechanism of SnO
2
nanotubes
To investigate the formation mechanism of the porous SnO
2
nanotubes, the annealing process was carried out under
temperatures of 300, 350 and 425
C, and the morphology
evolution of the two samples mentioned above were observed by
3476 | Nanoscale, 2012, 4, 3475–3481 This journal is ª The Royal Society of Chemistry 2012
Published on 18 April 2012. Downloaded by Lanzhou University on 04/01/2016 08:52:30.
View Article Online
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