一种制备用于高效SnO2-TiO2核壳染料敏化太阳能电池的SnO2纳米管的简便方法资源-CSDN文库

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A facile method to prepare SnO

nanotubes for use in efﬁcient SnO

–TiO

core–shell dye-sensitized solar cells†

Caitian Gao,

Xiaodong Li,

Bingan Lu,

Lulu Chen,

Youqing Wang,

Feng Teng,

Jiangtao Wang,

Zhenxing Zhang,

Xiaojun Pan

and Erqing Xie

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

nanotube–TiO

(SnO

NT–TiO

) core–shell structured

photoanodes for DSSCs. The SnO

nanotubes are prepared by electrospinning of polyvinyl

pyrrolidone (PVP)/tin dichloride dihydrate (SnCl

$2H

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

NT–TiO

core–shell structured DSSCs (5.11%) is above ﬁve times higher

than that of SnO

nanotube (SnO

NT) DSSCs (0.99%). This PCE value is also higher than that of

TiO

nanoparticles (P25) DSSCs (4.82%), even though the amount of dye molecules adsorbed to the

SnO

NT–TiO

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.

reported a new record power

conversion efﬁciency (PCE) of >12% by using a porphyrin-

sensitized nanocrystalline TiO

photoanode together with

cobalt(

II/III)-based redox electrolyte. However, the nanocrystal-

line TiO

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

nano-

tubes in SnO

nanotube–TiO

(SnO

NT–TiO

) core–shell

structured photoanodes for dye-sensitized solar cells. Firstly, we

fabricated SnO

nanoﬁbers and nanotubes as photoanodes

for DSSCs by electrospinning followed by direct sintering the

as-spun nanoﬁbers. Electrospinning generates 1D polycrystalline

SnO

nanomaterials with extremely high aspect ratios and

speciﬁc surface areas, and thus, is different from most other

methods used to prepare SnO

nanomaterials, such as template

methods,

and the hydro-thermal method.

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

is an

excellent metal oxide semiconductor with higher electron

mobility (100 to 200 cm

1

) and larger band gap (3.6 eV)

than TiO

, indicating a faster transport of photogenerated elec-

trons and more excellent long-term stability compared to TiO

for DSSCs applications.

7,14,15

However, SnO

-based DSSCs were

developed with less success due to at least two weak points: (1)

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

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

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a 300 mV positive shift of the conduction-band edge of SnO

with respect to that of nanocrystalline TiO

, leading to a faster

interfacial electron recombination and lower trapping density,

and (2) less adsorption of the dye with acidic carboxyl groups due

to a lower isoelectric point. To solve these problems, SnO

–TiO

core–shell structures have been adopted to combine the advan-

tageous features of the two materials.

7,14,15,17,18

The SnO

–TiO

core–shell structured DSSCs show higher short-circuit photo-

current density (J

) than that of TiO

nanoparticle (P25) DSSCs.

The contribution of one mole of dye molecules to J

for SnO

–

TiO

core–shell DSSC is almost three times larger than that of

P25 DSSC, indicating an extremely fast and efﬁcient charge

collection in SnO

–TiO

core–shell DSSCs.

Experimental

Preparation of 1D SnO

nanostructure by electrospinning

To obtain nanoﬁbers and nanotubes, two kinds of precursor

solutions were prepared. Firstly, certain amounts of tin

dichloride dihydrate (SnCl

$2H

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

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

$2H

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

–TiO

core–shell photoanodes for

DSSCs

To obtain the SnO

–TiO

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

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

on SnO

nanoﬁbers and nanotubes was performed by dipping the SnO

nanoﬁbers and nanotubes electrodes into a 0.08 M TiCl

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.

In brief, after cooling to 80



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

PtCl

$6H

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

and 0.5 M 4-tert-butylpyridine in acetonitrile. The

effective area was ﬁxed at 0.18 cm

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

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

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

(JCPDS no. 41-1445).

Formation mechanism of SnO

nanotubes

To investigate the formation mechanism of the porous SnO

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.

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