Flower-shaped ZnO nanocrystallite aggregates synthesized
through a template-free aqueous solution method for
dye-sensitized solar cells
Synthesis of aggregates
To synthesize ZnO nanocrystallite aggregates, 100 ml of 0.05 M Zn(NO3)2•6H2O
and 20 ml of 1.0 M NaOH aqueous solutions were brought to 80°C, thoroughly mixed,
and allowed to age at the same temperature for 2-8 h. The resulting white precipitate
was centrifugally collected, washed with pure water, and dried.
Fabrication and photovoltaic characterization of cells
To fabricate photonaode films, ZnO pastes, consisting of ZnO nanomaterials
dispersed in an aqueous solution of tert-butanol (volume ratio of tert-butanol to water
was 2:1), were applied onto the fluorine-doped tin oxide (FTO) substrates (Nippon
Sheet Glass, 8-10 Ω/□, 2.2 mm-thick) by the doctor-blade technique using adhesive
tape as the frame and spacer. The electrodes were then heat treated at 150°C for 1 h to
remove organic materials in the pastes before dye loading and solar cell assembly.
The ZnO films, with an active area of 0.25 cm2 and a thickness of approximately 32
m, were sensitized with a dye solution containing 0.5 mM D149 (see Fig. 4(a) for
chemical structure; Mitsubishi Paper Mills Limited) and 1 mM chenodeoxycholic acid
(Sigma-Aldrich) in equal volume of acetonitrile and tert-butanol. The dyed
photoanode and the Pt counter electrode were sandwiched together with a 60
μm-thick hot-melting spacer in between, and the space between the electrodes was
filled
with
an
acetonitrile-based
electrolyte
containing
0.6
M
1,
2-
dimethyl-3-propylimidazolium iodide (PMII, Merk), 0.05 M I2 (Sigma-Aldrich), and
0.5 M tert-butylpyridine (TBP, Sigma-Aldrich). The photovoltaic performance of the
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resulting devices was investigated using a class AAA solar simulator under one sun
(AM 1.5) using a white light source (Yamashita Denso, YSS-100A), with the
irradiance calibrated using a silicon photodiode (BS-520, Bunko Keiki).
Characterization of ZnO nanomaterials
The morphologies of the prepared electrode films were characterized by using an
FEI Nova230 field emission scanning electron microscope (FE-SEM). SEM images
of are given and Fig. 1 and Fig. S1. The as-synthesized ZnONFs were also examined
using a transmission electron microscope (H-7100, Hitachi, Japan). The TEM image
in Fig. S2(a) displays a star-like morphology, consistent with the flower-like structure
observed in Fig. 1(a). The enlarged TEM image in Fig. S2(b) confirms ZnONFs are
assemblies of crystallites ca. 20 nm in size. The inset in Fig. S2(b) presents a
representative high-resolution TEM image (HRTEM) of a region near the edge of a
petal and shows clear lattice fringes, indicating ZnONFs are not amorphous, but
highly crystalline. The lattice fringes observed have an inter-planar distance of 0.252
nm and can be assigned to the (101) plane fringe spacing of the hexagonal wurtzite
ZnO.
We also subjected the synthesized aggregates to powder X-ray diffraction (XRD)
characterization using a PANalytical X’Pert PRO diffractometer with Cu Kα radiation,
and further confirmed all the aggregates were composed of pure ZnO nanocrystals.
Fig. S3 presents powder XRD data of four samples, i.e., the commercially available
ZnONPs and the as-synthesized ZnONFs, aggregate-2 h and aggregate-8 h. The
diffractograms show similar patterns and can be readily indexed to the wurtzite
hexagonal phase of pure ZnO (JCPDS Card No. 36-1451). All samples exhibit
preferred orientation of (101). The average crystallite sizes of ZnONPs and ZnONFs,
estimated from major reflections using the Scherrer's equation, were 20.8 nm and 19.4
2
nm, respectively. The calculated crystallite sizes are consistent to those observed in
Figs. S1 and S2.
LHE measurement
The LHE of ZnONF-based and ZnONP-based photoanodes were determined
using the method developed by Arakawa and coworkers1. A spectrophotometer (V-570,
Jasco) fitted with an integrating sphere was used to measure the total transmission and
reflection of bare and sensitized ZnO films, from which the total absorption of the
films was calculated. The LHE of the adsorbed dye was determined as the absorption
difference between the dyed and undyed films. Three films were prepared for each
type of photoanode.
Dye loading measurement
Dye loading was determined by desorbing the dye from the photoanode in a
constant volume of dimethylformamide aqueous solution and measuring its
absorption spectrum. For this purpose, films of the same thickness and area are used.
Because a constant volume of desorbing solution was used, the absorbance directly
reflects the amount of adsorbed dye in each photoanode film. To determine the
concentration of the desorbed dye, the solution’s absorbance at 528 nm was compared
to a calibration curve, which was generated by preparing a series of dye solutions of
known concentration, measuring their absorbance at 528 nm, and then plotting the
absorbance vs. dye concentration. A straight line resulted. The amount of dye per unit
film area (μmole/cm2 of film) was then calculated as follows:
Dye loading 
volume of desorbed dye solution  concentrat ion of desorbed dye
film area
The reported dye loadings are the average of two measurements.
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References
1. Y. Tachibana, K. Hara, K. Sayama, and H. Arakawa, Chem. Mater. 14, 2527
(2002).
(a)
(b)
Fig. S1 (a) Low magnification SEM image of ZnONF film; (b) SEM image of
ZnONF film.
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(a)
(b)
Fig. S2
(a) Low magnification TEM image of ZnONF and (b) high magnification
TEM image of ZnONFs, with HRTEM image in the inset.
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Fig. S3
Powder XRD patterns of commercial ZnONPs and the as-synthesized ZnO
nanocrystallite aggregates.
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