Preparation of TiO2 nanoparticles by using tripodal tetraamine ligands as complexing agent via two-step sol-gel method and their application in dye-sensitized solar cells

Article abstract of DOI:10.1016/j.materresbull.2013.01.006

The effect of different tripodal tetraamine ligands was investigated on the particle size, agglomeration level, optical and photovoltaic properties of TiO₂ nanoparticle prepared via a two-step sol-gel method. The products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron spectroscopy (TEM), scanning electron microscopy (SEM), and UV-vis spectroscopy. The results showed that the symmetry of ligands has a crucial effect on the size and agglomeration level of the products. The optical and photovoltaic properties of the products were studied, as well. The reflectivity property of the samples due to different agglomeration sizes is shown to be very important factor in increasing conversion efficiency of DSSC.

Full text of DOI:10.1016/j.materresbull.2013.01.006

Materials Research Bulletin 48 (2013) 1660–1667
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Materials Research Bulletin
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Preparation of TiO2 nanoparticles by using tripodal tetraamine ligands as
complexing agent via two-step sol–gel method and their application in
dye-sensitized solar cells
Noshin Mir c, Masoud Salavati-Niasari a,b,
*
a Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran
b Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran
c Department of Chemistry, University of Zabol, Zabol, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
The effect of different tripodal tetraamine ligands was investigated on the particle size, agglomeration
level, optical and photovoltaic properties of TiO2 nanoparticle prepared via a two-step sol–gel method.
The products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy
(FT-IR), transmission electron spectroscopy (TEM), scanning electron microscopy (SEM), and UV–vis
spectroscopy. The results showed that the symmetry of ligands has a crucial effect on the size and
agglomeration level of the products. The optical and photovoltaic properties of the products were
studied, as well. The reflectivity property of the samples due to different agglomeration sizes is shown to
be very important factor in increasing conversion efficiency of DSSC.
Received 12 June 2012
Received in revised form 6 January 2013
Accepted 8 January 2013
Available online 14 January 2013
Keywords:
A. Nanostructures
A. Semiconductors, B. Sol–gel chemistry, D.
Optical properties, D. Energy storage
ß 2013 Elsevier Ltd. All rights reserved.
1. Introduction
The important role of crystalline size of nano TiO2 lattice on the
excited carrier (electron or hole) transport and its effect on
TiO2 nanoparticles (TNPs) can be used in
a
variety of
photovoltaic properties due to the critical influence of particle size
on the band-gap energy has been individually mentioned in many
reports [16–18]. Recently, a great number of reports have focused
on the key role of organic surfactants in determining the size and
shape of the products [19–22]. The bulky structures of amines can
provide great steric hindrance to control the size of nanoparticles
[23–26]. Sugimoto examined some amines as a shape controller for
TiO2 nanostructure synthesis and observed the specific adsorption
behaviour of some typical primary amine. For primary amines,
elongated ellipsoidal particles with a similar particle volume were
obtained. On the other hand, secondary and tertiary amines gave
very large ellipsoids with small aspect ratios, suggesting their
significant role as a complexing agent to promote the particle
growth rather than as a shape controller [27].
A complexing agent-assisted sol–gel process using an organic
ligand as a modulator can tailor the crystal structure and optical
properties of thin films such as TiO2 [28,29]. Of course, not every
molecule is an equally good ligand. Basically, ligands that fulfil the
task of adsorbent and stabilizer have a functional head group and
one or more hydrocarbon large groups, and both elements play a
role in the control of nucleation and growth [30].
applications for photovoltaic, photocatalytic, lithium ion batteries
and gas sensing systems [1–4]. Among the different applications of
TNPs, solar energy conversion using dye-sensitized solar cells
(DSSCs) has recently received great attention due to its cost-
effectiveness and high conversion efficiency [5].
Recent investigations on different synthetic methods show a
variation of possible routes for obtaining nanocrystalline titanium
dioxide [6], including sol–gel [7], hydrothermal [8], micelle
methods [9,10], sonochemical [11], and microwave [12]. One of
the widely utilized methods to obtain pure anatase TiO2 is a two-
step gel–sol process developed by Sugimoto et al. [13] which gives
rise to a better control on the size, crystallinity, morphology, and
surface chemistry of TiO2 compared with other synthetic
processes. In contrary to the conventional sol–gel method, the
consequences of the reaction are contrived in a way that continues
gel is formed first and then it transforms to a homogenous sol. This
method was reported to have an advantage for production of
monodispersed particles with controlled mean size in order to
improve different properties of TNPs such as solar energy
conversion efficiency [14,15].
In this paper, TiO2 nanoparticles were synthesized using three
different tripodal tetraamine ligands via a two-step sol–gel
method. Tripodal tetraamine ligands, also known as podands,
have had an important role in coordination chemistry since many
* Corresponding author. Tel.: +98 361 5555 333; fax: +98 361 555 29 30.
E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).
0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2013.01.006

N. Mir, M. Salavati-Niasari / Materials Research Bulletin 48 (2013) 1660–1667
1661
years ago [31,32]. Tetradentate tripodal ligands consist of a central
donor atom attached to three arms, each of which also contains at
least one methylene group and a donor atom. The binding sites in
these podands are hard (mostly N and O), and therefore stabilize
transition metal centres very well [33]. They have drawn much
attention in recent years, mainly due to their possession of
spheroidal cavities which enable them to efficiently sequester
metal ions. These multidentate ligands have shown coordination
ability to different metallic centres as well as titanium [34–36].
However, to our knowledge there is not any report on the effect of
these ligands on the size and photovoltaic properties of
nanoparticles. Therefore, in this study the effect of three tripodal
amines with different symmetries were investigated on the size of
the TiO2 nanocrystals and its effect on the performance of dye-
sensitized solar cells.
2. Materials and methods
The chemical reagents including titanium(IV) ethoxide and
triethanolamine (TEOA), used in our experiments were purchased
from Merck. Butylmethylimidazolium iodide (BMII), 4-tert-butyl-
pyridine, and guanidinium thiocyanate were purchased from
Sigma–Aldrich. All the mentioned chemicals were used as received
without further purification. Three tripodal tetraamine ligands
were synthesized as follows: Three individual methanol (20 ml)
solution of 20-Hydroxyacetophenone, 20,60-Dihydroxyacetophe-
none, and 3-indolcarbaldehyd (3 mmol) were added to three
separate 20 ml methanol solution of tris(2-aminoethyl) amine
(1 mmol) with constant stirring to obtain three Schiff bases
labelled as L1, L2 and L3, respectively. The resulting reaction
mixture was refluxed for 1 h. The methanol was allowed to
evaporate. The resulting solid of Schiff bases L1, L2 and L3 as shown
in Scheme 1 were obtained. They were recrystallized in methanol
and dried in vacuum [37].
For characterization of the products XRD patterns were
recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-
filtered Cu Ka radiation. SEM images were obtained on Philips XL-
30ESEM. TEM image was obtained on a Philips EM208 transmis-
sion electron microscope with an accelerating voltage of 200 kV.
Fourier transform infrared (FT-IR) spectra were recorded on
Shimadzu Varian 4300 spectrophotometer in KBr pellets. Optical
analyses were performed using a V-670 UV-VIS-NIR Spectropho-
tometer (Jasco).
Scheme 1. Three different tripodal tetraamine ligands used in this work.
Photocurrent–voltage (I–V) curve was measured by using
computerized digital multimeters (Ivium-n-Stat Multichannel
potentiostat) and a variable load. A 300 W metal xenon lamp
(Luzchem) served as a simulated sun light source, and its light
intensity (or radiant power) was adjusted to simulated AM 1.5
radiation at 100 mW/cm2 with filters for this purpose.
nucleate and grow TiO2 nanocrystals. The gel network formed in
the first ageing played a decisive role in the second ageing to
produce the uniform particles of titania, as a reservoir of the
metal ions to lower the super saturation for preventing
extensive nucleation and as an anticoagulant fixing the growing
particles in the matrix [13]. The resulting sol from each solution
synthesized using L1, L2 and L3 (hereafter denoted as S1, S2, and
S3, respectively) was washed with and centrifuged from NaOH
(six times), HNO3 (two times), and distilled water (four times) to
remove residual organic compounds from the surface of the
nanoparticles.
3. Experimental
3.1. Synthetic method
The standard procedure for the preparation of TiO2 nano-
particles was conducted according to the literature [14,27].
Three stock solution of Ti4+ (0.5 M) which are stable against
hydrolysis at room temperature were prepared by mixing
titanium(IV) ethoxide (TEO) and TEOA with a molar ratio of
TEO:TEOA = 1:2, followed by the addition of distilled water. The
final pH of solutions was 9.6 and was fixed at this pH by mixing
with HClO4 or NaOH solution after addition of complexing
agents. Three different amines including L1–L3 (0.4 M) were
further added to each solution. Each solution was placed in a
Teflon-lined autoclave and aged at 100 8C for 24 h for gelation;
then, the temperature was increased to 140 8C for 72 h to
3.2. Fabrication of DSSC
According to the procedure reported by Gratzel et al. [38] the
TiO2 paste was prepared. The slurry was produced by mixing and
grinding 6.0 g of the nanometer sized TiO2 with ethanol and water
in several steps. Then, the grinded slurry was sonicated with
ultrasonic horn (Sonicator 3000; Bandeline, MS 72, Germany) and
then mixed with terpineol and ethyl cellulose as binders. After
removing the ethanol and water with a rotary-evaporator, the final
paste was prepared. The prepared uniform TiO2 paste was coated

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N. Mir, M. Salavati-Niasari / Materials Research Bulletin 48 (2013) 1660–1667
on fluorinated tin oxide glass (FTO glass, Pilkington Glass, TEC7) by
a doctor blade technique. The active area of the electrodes was
0.25 cm2. After natural drying at room temperature, the electrodes
coated with the TiO2 pastes were gradually heated under an air
flow at 325 8C for 5 min, at 375 8C for 5 min, at 450 8C for 15 min
and 500 8C for 15 min. The coating process and further sintering
were performed two times. At 80 8C in the cooling, the TiO2
electrodes were immersed into a 0.5 mM cis-di(thiocyanato)-N,N0-
bis(2,20-bipyridyl-4-carboxylic acid-40-tetrabutylammonium car-
boxylate) ruthenium (II) (N-719) (Dyesol) dye solution in ethanol
and kept at room temperature for 24 h to complete the sensitizer
uptake. The dye-adsorbed TiO2 electrodes were then rinsed with
ethanol and dried under a nitrogen stream. A Pt coated FTO glass
electrode was prepared as a counter electrode. The Pt electrode
was placed over the dye-adsorbed TiO2 electrode and the edges of
A more detailed attention on the XRD patterns shows that the
degree of crystallinity of TiO2 treated with L3 (Fig. 1c) is higher than
those processed with L1 and L2 which is represented by the higher
diffraction intensity of S3 sample. Generally, the crystalline
domain size decreases with increasing line broadening. The line
broadening of the peak of the A (1 0 1) index is related to the size of
the hexagonal crystalline phase. The data for the full width at half
maximum (FWHM) of all samples at around 2u = 25.308 was
estimated to be 0.4133. The crystallite size diameter (D) of the TiO2
products has been calculated by Debye–Scherrer equation,
the cell were sealed with 50
mm thick sealing sheet (Surlyn150,
Dyesol). Sealing was accomplished by pressing the two electrodes
together on a double hot-plate at a temperature of about 110 8C.
The electrolyte solution was prepared according to the literature
[38]:
a solution 0.6 M BMII, 0.03 M I2, 0.1 M guanidinium
thiocyanate and 0.5 M 4-tert- butylpyridine. The electrolyte was
introduced into the cell through one of two small holes drilled in
the counter electrode. Finally, these two holes were sealed by a
small square of sealing sheet.
4. Results and discussion
4.1. Characterization of two-step sol–gel process-based TiO2
nanoparticles.
4.1.1. X-ray diffraction patterns
Fig. 1 shows the X-ray diffraction patterns of as-prepared TiO2
nanoparticles treated with different ligands. All the nanoparticles
show anatase as a unique phase. All diffraction peaks can be well
indexed to pure anatase structural titanium oxide. No excess peaks
were detected, and all the peaks are labelled and can be indexed to
JCPDS Card No. 04-0477.
Fig. 1. XRD patterns of the three types of TiO2 prepared by different ligands using
Fig. 2. SEM images of the three types of TiO2 prepared by different ligands using
two-step sol–gel method: (a) S1 (b) S2 (c) S3.
two-step sol–gel method: (a) S1 (b) S2 (c) S3.

N. Mir, M. Salavati-Niasari / Materials Research Bulletin 48 (2013) 1660–1667
1663
D = 0.9
l
/
˚
b
cos
u
[39], where
(1.5406 A for Cu K ), the full width at half maximum height in
radians, and the diffraction angle. Calculated crystalline domain
l
is the wavelength of incident X-rays
absorbance of bulky tripodal ligands on the surface of TiO2 nuclei,
the nucleation of TiO2 must have been arrested resulting in larger
agglomeration size after the complete conversion of Ti(OH)4 to
TiO2. Very crucial in this study is the symmetry of each ligand
because it determines the availability of active head groups to
absorb on the surface of initial nuclei. The 3D structures of ligands
are illustrated in Scheme 2. It can be clearly observed that all three
ligands have completely different symmetries and group orienta-
tions. The TiO2 nuclei shown in Scheme 2 by brown colour are
considered as the very initial nuclei of TiO2 which are surrounded
by tripodal amines. It is proposed that the bulky pods which are
more or less parallel to xy plane (denoted with red arrows) have an
influence on the agglomeration size of the particles. Interestingly,
the L1 ligand which has all the three pods in this direction results
in significantly smaller agglomeration size. The L3 and L2 ligands
with one and no pod in this direction give rise to the second and
third small agglomeration sizes, respectively. Basically, both
special structure and adsorbent functional head groups in a ligand
determines the final particle size and morphology.
a
b
u
sizes have been found to be 20.29 nm for all three samples. It
shows that the crystalline domain size is same in each case and
therefore any difference in optical or photovoltaic properties
would be attributed to either agglomeration or aggregation size of
the particles.
4.1.2. SEM images
The agglomeration size of the products was evaluated by
scanning electron microscopy. Fig. 2 shows the surface morphol-
ogy of the synthesized nanostructures. In all three obtained
products, uniform nearly spherical TiO2 nanoparticles were
obtained when L1, L2 or L3 ligand was used. These spheres are
the agglomerations of smaller particles. As is shown in Fig. 2a–c,
the approximate agglomeration size of nanoparticles in S1, S2 and
S3 ranged 60–80, 80–130, and 55–100 nm, respectively. Accord-
ing to the literature [27], tertiary amines give ellipsoids with small
aspect ratios, suggesting their significant role as a complexing
agent to promote the particle growth rather than as a shape
controller. Therefore, the nearly spherical shape of resulting
nanoparticles is anticipated. On the other hand, formation and
stability of Ti(IV)–L complex at the first step of the reaction has a
critical role on the nucleation and growth rate of the particles. It is
accepted that an effective complexing agent, as a growth
controller, has a low stability constants [27]. The tripodal amines
used in this study have a high steric hindrance giving rise to
unstable Ti(IV)–L complexes. We propose that due to the
4.1.3. FT-IR spectroscopy
The characteristics bands as well as the FT-IR spectra of three
different ligands are presented in Table 1 and Fig. 3, respectively.
All three products are pure since the stretching vibrations of –CHO
aldehyde group of initiators which usually appear at around 2750
and 2850 cmꢀ1 are not present the spectra. The L3 is already
synthesized and its FT-IR characteristics are reported elsewhere
[40,41] which the values reported here are in a good agreement
with previous results. The stretching vibration at 3394 is assigned
Scheme 2. Effect of symmetry of different ligands on the agglomeration size of nanoparticles.

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N. Mir, M. Salavati-Niasari / Materials Research Bulletin 48 (2013) 1660–1667
Table 1
The relevant characteristic IR bands (cmꢀ1).
Compound
y(C5N)
y
(N–H)
y
(O–H)
y(C–H)al
y(C–H)ar
y(C5C)ar
y(C–N)
L1
1617
–
–
–
–
–
3429
2812
2851
2926
2853
2923
2836
2920
–
3053
–
1449
1600
–
1154
–
–
–
–
–
–
–
L2
L3
1589
–
3380
–
3052
–
1449
–
1267
–
1634
–
3339
3394
–
3053
3085
3162
1446
1578
–
1240
–
–
–
–
–
–
to
n
(N–H). Five characteristic peaks for C–H stretching modes of
4.1.4. Optical properties
benzylic as well as aliphatic and pyrrolic C–H bonds are expected
in FT-IR spectrum of L3. Therefore, three stretching bands at 3053,
3085 and 3162 cmꢀ1 and those observed in the two lower
frequencies correspond to the benzylic and pyrrolic C–H stretch-
ing, respectively [41]. The vibration frequencies of C55N for L1 and
L2 correspond with the similar Schiff bases reported elsewhere
[42]. The absorption from 3000 to 3600 cmꢀ1 for L1 and L2 can be
assigned to the stretching vibration of the hydrogen-bonded OH
groups of the adsorbed water.
The absorption spectra of calcined three different TiO2-based
photoelectrodes before the dye up-taking process are shown in
Fig. 5. The fundamental absorption edge in most semiconductors
follows the exponential law. Using the absorption data the band
gap was estimated by Tauc’s relationship:
n
a0ðh
y
h
ꢀ EgÞ
a
¼
(1)
y
where
a
is absorption coefficient, h
y
is the photon energy,
a0 and h
In order to illuminate the surface conditioning of as-prepared
TiO2 products, FT-IR spectra of TiO2 nanoparticles were recorded,
as well. Fig. 4 shows a comparison of FT-IR spectra of different TiO2
samples. The absorption around 1630 cmꢀ1 is due to the bending
vibration of water molecules. A sharp peak appeared at 1383 cmꢀ1
is due to stretching vibration of nitrate ion that remained from
washing the samples with HNO3. This amount of nitrate ion is
decomposed after calcination procedure of TiO2 electrode. This is
consistent with a previous study by Warrier et al. [43,44], in which
the decomposition of nitrate ions occurred at around 230–400 8C.
As shown in Fig. 4, the broad band below 950 cmꢀ1 in the FT-IR
spectra of all the TiO2 samples belongs to the characteristic
vibrations of the inorganic Ti–O–Ti network [45]. The bands at
around 2920 and 2850 cmꢀ1 are assigned to the antisymmetric and
symmetric C–H stretching vibrations of hydrocarbon moiety
remained on the surface of TiO2 nanoparticles [46].
are the constants, Eg is the optical band gap of the material, and n
depends on the type of electronic transition and can be any value
between ½ and 3 [47]. The energy gap of the film has been
determined by extrapolating the linear portion of the plots of
(ah ) against hy to the energy axis (inset in Fig. 5). The Eg values
y 1/2
are calculated 3.14, 3.06, and 3.02 eV for the S3, S2 and S1 samples,
respectively. For the TiO2 anatase phase, values from 2.86 to 3.34 eV
have been reported in the literature and the differences were
attributed to variations in the stoichiometric of the synthesis, the
impurities content, the crystalline size and the type of electronic
transition. The observed trend for band gap in this work is
accounted for on the basis of the quantum confinement effect
provoked by the small particle size the variations in the
stoichiometric of the synthesis as well as the impurities content
[48,49]. From the band gap calculations, the particle size order is as
Fig. 4. FT-IR spectra of the three types of TiO2 prepared by different ligands using
Fig. 3. FT-IR spectra of the three different ligands.
two-step sol–gel method.

N. Mir, M. Salavati-Niasari / Materials Research Bulletin 48 (2013) 1660–1667
1665
Fig. 5. UV–vis diffuse absorption spectra of the synthesized TiO2 samples before dye up-taking process (a) S1 (b) S2 (c) S3. The inset shows plots of (ahy) y for
1/2 versus h
corresponding samples.
follows: S1 > S2 > S3. It is mentioned that the comparison for
particle size from UV–vis results should not be mixed up with the
reported agglomeration size from Fig. 2. The demonstrated values
from SEM results show the size of the spheres containing smaller
particles.
The agglomeration size is an important factor which influences
on the photovoltaic properties of the material. The scattering
properties of different samples reflect the size of aggregates in the
products. Light-scattering properties of the various TiO2 films
before dye up-taking are investigated and showed in Fig. 6. If the
incident light is reflected back into the cell, it enhances the
absorption of light by dye in the cell [50]. It is shown that the S3
film exhibits an improved reflectance over the S1 and S2 samples.
According to the Mie scattering effect, the effective scattering
particles are those with the size comparable to the wavelength of
light and the light scattering occurred due to the higher light
trapping in the device [51]. By comparing SEM images, UV–vis
absorption and reflectivity, it seems that S1 sample contains the
nanoparticles with the largest size but the smallest agglomeration
level resulting in the high surface area but the lowest trapping
capability. From the SEM micrographs S2 and S3 samples have
rather comparable sizes and agglomeration levels; however from
Fig. 6. UV–vis diffused reflectance spectra of the synthesized TiO2 samples before
Fig. 7. UV–vis diffused reflectance spectra of the synthesized TiO2 samples after dye
dye up-taking process.
up-taking.

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N. Mir, M. Salavati-Niasari / Materials Research Bulletin 48 (2013) 1660–1667
reflectivity results (Fig. 6), it is obvious that the scattering effect of
S3 sample is higher than that of S2 which can be attributed to a
higher number of traps and pores in S3 sample resulting in
improved scattering properties.
absorption of the dye molecules [52]. The higher scattering effect
of S3 sample brings about the higher diffused reflectance of the
corresponding dye-sensitized photoanodes. The order for the two
other samples is a repeated trend of previous result.
The trend in the diffused reflectance properties of photoanodes
after dye up-taking process is the same (Fig. 7). There is a valley in
the spectra in the wavelength range of 500–600 nm due to the light
The results for absorption and reflectance indicate that light
scattering is significantly increased in S3 sample and solar light
reflects back into the cell in S3- more than those of S1- and S2-
based DSSCs. Therefore, we choose the S3 sample for TEM study to
estimate the exact size of the sample.
4.1.5. TEM image
The TEM micrographs of the S3 sample is illustrated by three
differentmagnificationsinFig. 8. Itshowssmallellipsoidswithsmall
aspect ratios. The average size of ellipsoids is about 30 nm in length
and 20 nm in diameter which is in agreement with the XRD result.
From the TEM image it can be observed that the size distribution is
narrow and the morphology is homogenous. It also reveals that the
sizes estimated from SEM image depict the agglomerations
containing a bunch of smaller particles. Making a comparison of
different amines as complexing agent (Table 2) shows that by
employing a tripodal tetraamine ligand such as L3, not only the
agglomeration size but also the particle size is better controlled.
4.2. DSSCs performance
For having a comparison between performances of DSSCs
fabricated from different samples Fig. 9 shows the current–voltage
(I–V) curves of the assembled DSSCs of different TiO2 samples with
the same thickness of around 3
the highest current density is for S3 sample. The overall conversion
efficiency ( ) of the S3-based DSSC is 1.67% which is higher than
mm. From Fig. 9, it is observed that
h
those estimated for S1 (1.33%) and S2 (0.75%). By using L3 ligand
containing indole group, the conversion efficiency has increased
20% relative to L1 with phenolic group. It is noteworthy that
changing the complexing agent from L2 to L1 with only different
numbers of hydroxyl groups, the conversion efficiency has
enhanced by 43%. The maximum open circuit voltage (Voc) was
calculated to be 0.81 V for S3-based DSSC which represents the
reduced interface charge recombination and mass transport
limitations in the films [51,53]. The low value of Jsc in all samples
is attributed to low thickness of the prepared films. On the other
hand, the photoelectrodes were fabricated without the compact
TiO2 layer that is usually employed between the porous TiO2 and
Fig. 8. TEM images of the S3 sample prepared via two-step sol–gel method in
Fig. 9. I–V characteristics of the DSSC from the prepared samples.
different magnifications.

N. Mir, M. Salavati-Niasari / Materials Research Bulletin 48 (2013) 1660–1667
1667
Table 2
Influence of different amines on the size and shape of TiO2 products.
Amine t
Amine type
Products
Shape of particles
Approximate size of particles (nm)
Ref.
Diethylenetriamine (DETA)
Diethylamine (DEA)
Primary
Secondary
Tertiary
Primary
Tertiary
Rod
200 ꢁ 20
70 ꢁ 40
[27]
[27]
[27]
[54]
Ellipsoid
Ellipsoid
Tube
Trimethylamine (TMA))
Dodecanediamine (DDA)
Tripodal tetraamine (L3)
200 ꢁ 100
30–60
Ellipsoid
30 ꢁ 20
This work
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5. Conclusions
In summary, TiO2 nanoparticles of different sizes and agglom-
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tetraamines with different symmetries. It was shown that the
3D structure and the symmetry of the complexing agent have a
crucial role on the morphology of products. The ligands with lower
steric hindrance result in smaller agglomeration size of the
nanoparticles. It was shown that for better photovoltaic properties
of DSSC it is beneficial to have small particle size on one hand and
high agglomeration level on the other hand in order to benefit both
high surface area and enhanced scattering ability. In this study,
employing the L3 ligand containing indole group gave rise to the
higher scattering properties of prepared TiO2 sample and the
conversion efficiency has increased 20% relative to nanoparticles
prepared from L1 ligand with phenolic group. Interestingly, by
changing the complexing agent from L2 to L1 with only different
numbers of hydroxyl groups, the conversion efficiency of TiO2
nanoparticles was enhanced by 43%. It is noteworthy that in order
to determine the exact chemical and spacial role of various head
groups on controlling the particle size of TiO2 products, further
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Acknowledgements
The authors are grateful to University of Kashan for supporting
this work by Grant No (159271/16).
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