SCIENCE JOURNAL
Ubon Ratchathani University
Sci. J. UBU, Vol. 1, No. 1 (January-June, 2010) 1-8
http://scjubu.sci.ubu.ac.th
Research Article
Synthesis of Nanocrystalline N-Doped TiO2 and Its Application
on High Efficiency of Dye-Sensitized Solar Cells
C. Kusumawardani1*, K. Indriana2, Narsito2
1
Chemistry Department, Yogyakarta State University, Yogyakarta 55281, Indonesia.
2
Chemistry Department, Gadjah Mada University, Yogyakarta 55283, Indonesia.
Received 22/12/09; Accepted 12/03/10
Abstract
Nanocrystalline of Nitrogen-doped TiO2 (N-doped TiO2) has been synthesized through the hydrolysis
of N-substituted titanium isopropoxide precursors. The XRD result showed that the structure of
annealed N-doped TiO2 was anatase. The pore properties were investigated from nitrogen gas sorption
analyzer, showing mesoporous structure of its N-doped TiO2 with a high specific surface area (167
m2/g) and a sharp pore radius distribution. The substitution of oxygen sites with nitrogen atoms in the
titania structure was confirmed by X-ray photoemission spectroscopy (XPS) with 3.6% nitrogen
contain. A clear decrease in the band gap on N-doped TiO2 (compared to Degussa P25) is deduced
from the optical absorption spectroscopy results. Application of synthesized nanocrystalline N-doped
TiO2 on dye-sensitized solar cells resulted 4.8% overall conversion efficiency, which was higher than
the Degussa P25.
Keywords: N-doped TiO2, Hydrolysis, Dye-sensitized solar cells.
1. Introduction
Highly energy demand of our world onto
fossil fuels has posed several drawbacks
since fossil fuels are nonrenewable, causing
many environmental problems and are likely
not to continue to remain abundant for the
next generations. Therefore, the search for
alternative renewable energy technologies is
of crucial importance for the sustenance and
development of modern society [1]. One of
the possible solutions to the energy challenge
*Corresponding author.
E-mail address: irienuny@yahoo.com
cahyorini.k@uny.ac.id
is to make efficient use of solar energy,
which is abundant, long lasting and clean.
Solar cells are being the most area of interest
in solar energy utilization because it can
directly convert the solar energy to electricity.
A new type of solar cells developed by
O’Regan and Grätzel in 1991, dye-sensitized
solar cells (DSSCs) have been attracting
much attention over last decade as potential
low-cost alternative to commercial solar cells
based on silicon due to their ease of fabrication and high photo-conversion efficiencies
[2-5]. Despite low-cost, their system had a
Copyright  2010 Faculty of Science, Ubon Ratchathani University. All Rights Reserved.
2
Synthesis of Nanocrystalline of Dye-Sensitized Solar Cells
light-to-electricity conversion efficiency of
around 12%, a level that is not easily
obtained since the first concept firstly noticed
in early 1970 [6]. It is necessary to further
improve the energy conversion efficiency in
order to commercialize DSSC successfully.
Many methods for improving the conversion
efficiency of the DSSC have been attempted
[7-10]. Some consider-able efforts have been
devoted to find the most efficient dyes to
increase the efficiency but effort to modify
semi-conductor TiO2 as the most efficient
support for the dye have been overlooked.
Nanoparticles TiO2 and thin films TiO2 are
easily produced, inexpensive and shown good
stability under illumination in most environments [11-13]. However, the intrinsic wide
band-gap nature (around 3.2 eV) of TiO2
impairs TiO2 from playing such an important
role, because it only allows TiO2 to adsorb
UV light, which accounts for merely 5 % of
the incoming solar energy on the earth’s
surface [1]. To improve the photoactivity of
TiO2, it is desirable to red-shift the absorption
onset to also include the less energetic but
more intense visible part of the solar
spectrum [14]. Traditionally, this has been
achieved by anchoring organic dyes as
sensitizers, which are usually Ru(II) complexes, to harvest the visible light [2,9].
Although this method broadens the range of
the visible light response effectively, the
problem appear with organic dyes that they
can detach from the surface when employed
in aqueous solution and the long term
stability of many dyes can be questioned. The
use of pure TiO2 semiconductors, which there
is some oxygen deficiency in the crystal
structure can create electron-hole pairs [1518] and that the oxidizing holes can either
react with the dye and destroy it and/or is
scavenged by iodide ions [20], may lead to
shorten the lifetime of the dye-sensitized
solar cells. Therefore, there is a need to
increase the DSSC efficiency and stability by
other approaches. To solve these problems,
we introduce nitrogen-doped TiO2 into the
DSSC system to enhance the efficiency due
to the replacement of pure TiO2 by visible
light active nitrogen-doped TiO2.
Nitrogen-doped TiO2 have been produced
through various techniques, such as hydrolytic process [15,19], mechanochemical technique [21], reactive DC magnetron sputtering
[22,23], solvothermal process [24], high
temperature treatment of TiO2 under N atomsphere [25,26] and sol gel method [1,12,14]
whereby the approach involves the hydrolysis of Ti precursor, in the presence of Nprecursor such as ammonia and organic
amines. Among these methods, the N-doped
TiO2 nanoparticles are mainly obtained
through the last two methods, but more
commonly through sol gel approach because
it affords simplicity in controlling the
nitrogen doping level and particle size by
simple variations in experimental condition,
such as hydrolysis rate, solution pH and
solvent systems. Burda et al. added triethylamine to the colloidal nanoparticle solution
and heated the samples to obtain ~4%
nitrogen-doped TiO2 with an aver-age grain
size of 6-10 nm, which could absorb well into
the visible region up to 600 nm, but the
samples had to be prepared under low pH
condition and temperature as low as 2oC
through the sol gel process [18].
In this article, we report a sol gel method for
synthesizing of visible light active nitrogendoped (N-doped) TiO2 and its application
into the DSSC system.
2. Theory
Improvement of TiO2 photoactivity through
nitrogen doping could be determined by light
absorption shift to visible region. It is
quantified by bandgap energy which can be
calculated by the equation [2]
Eg=1239.8/λ
(1)
where Eg is the band gap (eV) and λ (nm) is
the wavelength of the absorption edges in the
spectrum.
Dye-sentisized solar cell efficiency has been
determined by current-voltage measurement.
The I-V measurements were performed using
Copyright  2010 Faculty of Science, Ubon Ratchathani University. All Rights Reserved.
C. Kusumawardani et al., Sci. J. UBU, Vol. 1, No. 1 (January-June, 2010) 1-8
computerized control of Keithley instrument
with a 450 W Xenon lamp, which was
focusing to provide 1000 W/m2, equivalent to
one sun at AM 1.5 at the surface of the cells.
The instrument was calibrated using silicon
diode. The spectral output of the lamp was
matched in the region 350-800 nm with the
aid of a Schott KG-5 sunlight filter so as to
reduce the mismatch between the simulated
and the true solar spectrum to less than 2%.
The current-voltage characteristics of the
cells were determined by biasing the cells
externally and measuring the generated
photocurrents. The overall photo-conversion
efficiency  is calculated from the integral
photocurrent density (Isc), the open circuit
photocurrent (Voc), the fill factor of the cell
(ff), and the intensity of incident light (Is)
using the formula
  I scVoc ff / I s
3
mL of Ti(OPr)4, 10 mL dodecylamine, and
80 mL of ethanol absolute solution was
refluxed for 4 hours at 70 oC to provide a
clear solution. This precursor solution was
then cooled to room temperature and 5 mL of
CH3COOH was added to neutralize the
excess of dodecylamine. Hydrolysis process
was then achieved by adding 20 mL of
distilled water dropwise into the solution
under vigorous stirring, the solution continues stirred for 24 hours. The resulting
yellowish precipitate was centrifuged and
washed subsequently with distilled water and
ethanol. Finally, the N-doped TiO2 were
vacuum-dried for 12 hours. The surfactant
was removed from the as-made N-doped
TiO2 powders by calcined at a heating rate of
2 oC/min in air atmosphere for 4 hours at 450
o
C.
Characterization. The N-doped TiO2 powder
was analyzed by thermogravimetric analysis
(TGA) with a Q500TGA Instrument using a
Is = 1000 W/m2 at air mass (AM) 1.5 or under 10 oC/min ramp up to 800 oC. The structure
full sunlight. Fill factor (ff) is given by
of the powder was examined with X-Ray
powder diffractometer (XRD, Shimadzu)
with Cu K radiation ( = 0.15406 nm),
V I
ff  pp pp
(3) X-ray photo-electron spectroscopy (XPS,
Voc I sc
PHI-5300) and N2 adsorption-desorption
measurements at 77 K (NOVA Quantawhere Vpp is a maximum voltage and Ipp is a chrome). UV-visible diffuse reflectance
spectra were obtained for N-doped TiO2
maximum current.
using a UV-visible spectrophotometer (UV2550, Shimadzu).
3. Materials and Methods
(2)
Materials. Titanium Tetra Iso-propoxide,
Ti(OPr)4, 97% and acetylacetone were
purchased from Aldrich. Dodecylamine 98 %
and TritonX-100 were purchased from Fluka.
Ethanol absolute and CH3COOH were
obtained from Merck. All materials were
used as received. Di-tetrabutylammonium
cis-di(isothio-cyanato)bis (2,2’-bipyridyl-4,4’
-dicarboxylato) Ru(II) (N719 dye), electrolyte EL-HSE, TEC 15 electrode glass plate
and Pt-coated counter electrode are commercial products of Dyesol (Australia).
DSSC Fabrication. To fabricate the DSSCs,
the N-doped TiO2 electrodes were immersed
in 0.3mM solution of N719 dye in acetoneitrile overnight. Cells were assembled by
placing the Pt-counter electrode (CE) over
the active area of N-doped TiO2 working
electrode. The electrolyte was introduced
through drilled hole on CE by capillary
action, the hole was then sealed. For
comparison, DSSCs based on Degussa P25
thin film working electrode was also fabricated under the same conditions.
Synthesis of N-doped TiO2. For synthesis of
the N-doped TiO2 powders, a mixture of 3
Copyright  2010 Faculty of Science, Ubon Ratchathani University. All Rights Reserved.
4
Synthesis of Nanocrystalline of Dye-Sensitized Solar Cells
N-doped TiO2 can be prepared in several
ways. In this research we synthesized Ndoped TiO2 through sol gel method by
hydrolysis of N-substituted titanium isopropoxide precursors in alcohol solution. We
found that complexation of organic amines
on the Ti metal center creates highly efficient
precursors N-doped TiO2 nanoparticles.
Advantages of this route are simple method,
high doping levels, controllable structure and
high surface area at low cost.
The crystal structures of the synthesized Ndoped TiO2 were studied by X-ray powder
diffraction (XRD), as shown in Figure 1. The
peaks in the spectra indicated that the crystal
phase of prepared N-doped TiO2 was anatase
and that no crystal phase of the rutile was
observed after annealing at 450 oC. The
crystallite sizes of N-doped TiO2 is determined to be 8 from the half-width of the
(101) peak using the Scherrer formula.
is about 20% weight loss of The N-doped
TiO2 sample. From 420 to 450 oC there is
much slower (about 2 %) weight loss of the
N-doped TiO2 sample in association with the
crystallization of these nanoparticles. Above
450 °C, there is very little, about 3% weight
loss of the N-doped TiO2 sample in
association with the crystallization of these
nanoparticles. Annealing temperature of 450
o
C was chosen based on TGA data that the
surfactant will be released by calcinations
above 400 oC, but there still nitrogen left in
the TiO2.
100
95
Weight (%)
4. Results and Discussion
90
85
80
200
600
800
Temperature (oC)
500
001
400
Intensity
400
Figure 2. TGA graph of N-doped TiO2.
300
200
200
105 211
004
100
0
20
30
40
2 Theta (deg)
50
60
Figure 1. XRD Spectra of N-doped TiO2
after calcination at 450 oC.
To understand the loss of organic resi-dues
on the surface of the N-doped TiO2 under
calcinations, we used the thermo-gravimetric
analysis (TGA) to study the weight loss of
the N-doped TiO2. The TGA results of the Ndoped TiO2 in Figure 2 show that there is less
weight loss, about 20% from room
temperature up to 220 oC, which is mostly
due to the loss of physically adsorbed or
embedded water and solvent on the surface of
the N-doped TiO2. From 220 to 420 oC, there
The substitution of the oxygen sites with
nitrogen atoms in the titania structure was
confirmed by X-ray photoemission spectroscopy (XPS), as shown in Figure 3. The
nitrogen 1s core level of N-TiO2 shows three
peaks at 396.0, 398.8 and 400.5 eV, which is
consistent with the reported results for NTiO2. The two peaks at higher binding
energies may attribute to molecularly
adsorbed nitrogen species, whereas the peak
at 396 eV was assigned to the substitutionally
bound N- species in the TiO2 lattice [22].
Thereafter, Irie et al. and Diwald et al.
reported that the peak at 396 eV in the XPS
spectra was attributed to a chemically bound
N- species within the crystalline TiO2 lattice
[26-29]. Most N species in the N-TiO2
existed in the form of nitrides such as N in
the O-Ti-N linkage, corresponding to the
binding energy (BE) of 399.4 eV, while only
small amounts of N were present in form of
Copyright  2010 Faculty of Science, Ubon Ratchathani University. All Rights Reserved.
5
C. Kusumawardani et al., Sci. J. UBU, Vol. 1, No. 1 (January-June, 2010) 1-8
140
(a)
120
Volume (cc/g)
surface adsorbed ammonia, with the BE
located at 396.0 eV. In the Ti2p3/2 XPS
spectra (Figure 3), the major titanium peak is
centered at 459.0 eV. There is an intermediate peak centered at ca. 457.5 eV which
correspond to Ti-N bound. The XPS elemental analysis was resulted 3.6% nitrogen doped
on TiO2.
100
80
60
Adsorption
Desorption
40
20
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure, P/Po
70
N1s
1.4
(b)
1.2
1.0
50
Dv (log d) [cc/g]
Intensity
60
40
392
394
396
398
400
402
404
0.8
0.6
0.4
0.2
Binding energy (eV)
0.0
1000
0
2p3/2
Ti2p
10
20
30
40
50
pore radius (nm)
Intensity
800
Figure 4. Isotherm curve of N-doped TiO2 (a)
and its corresponding pore distribution (b).
600
2p1/2
400
200
0
454
456
458
460
462
464
466
468
Binding Energy (eV)
Figure 3. XPS spectra of N1s (up) and Ti2p
(down) of N-doped TiO2 calcined at 450 oC.
Figure 4 shows the typical nitrogen isotherm
of N-doped TiO2 and its corresponding pore
radius distribution. The adsorption desorption
curve exhibits a type-IV isotherm curve with
an H2 hysteresis loop according to IUPAC
classification [30], which means the material
has mesoporous structure. The N-doped TiO2
shows high BET surface areas 167 m2/g
because of the mesoporous structure and the
large amount of nanometer crystallites. The
pore radius distribution of N-doped TiO2
shows a sharp pore radius peak at 5.2 nm.
The optical absorbance and reflectance was
used to study the capability to photo-sensitize
the TiO2 nanoparticles. The absorbance shift
of the N-doped TiO2 NPs can be observed
from the reflectance spectra of undoped
(Degussa P25) and N-doped TiO2 NPs in
Figure 5. The yellowish N-doped TiO2 sphere
powders show good absorbance of visible
light. It can be seen from Figure 5 that the
visible light absorption is high and extended
up to 550 nm in the case of N-doped TiO2
calcined at 450 oC compared to that of pure
TiO2 which could only absorb light in the UV
range. It may be due to nitrogen species
occupy some of the oxygen positions in the
lattice. This also rules out the occupancy of N
in any other positions such as interstitial sites,
which should give rise to a mid gap
band/level between valence and conduction
bands.
Copyright  2010 Faculty of Science, Ubon Ratchathani University. All Rights Reserved.
6
Synthesis of Nanocrystalline of Dye-Sensitized Solar Cells
The band gap for N-doped TiO2 and Degussa
P25 were calculated by Eq. (1) for the
absorption edge in the visible region was
found to be 2.25 eV and 3.09 eV, respectively. It is evident that the conduction band
edge has changed by nitrogen doping.
It assumed that visible light absorption of
nitrogen-doped TiO2 supports intrinsically
increases the efficiency value due to the
photoresponse of N-doped TiO2 in the visible
light region, which is also supported by the
results reported by Lindgren et al. [22]. They
have demonstrated that the photoinduced
current due to the visible light activity of the
best N-doped TiO2 electrode prepared by
reactive DC magnetron sputtering can increase significantly by approximately 200
times over those of the undoped TiO2
electrodes. On the basis of these results, it
can be expected that the optimization of the
amount of nitrogen doping in titania nanoparticles and electrode can further improve
the performance of the DSSCs.
1.2
Figure 5. DRUV Spectra of N-doped TiO2
compared to Degussa P25.
N719
N-doped TiO2
1.0
P25
0.8
Abs.
C urren t d ensity (m A/cm 2 )
14
N-doped TiO2
P25
12
0.6
10
0.4
8
0.2
6
0.0
400
4
500
600
700
800
nm.
2
0
0.0
0.2
0.4
Voltage (V)
0.6
0.8
Figure 7. Absorption spectra of desorbed dye
after leaching process.
Figure 6. Current-voltage curves of the dye- It also should be pointed out that a higher
sensitized cell based on the N-doped TiO2 Ruthenium dye uptake was observed on the
N-doped TiO2 film, compared to that for P25.
and Degussa P25.
It has been known that a larger surface area
Figure 6 shows the current-voltage curves of of the N-doped TiO2 film can increase the
the open cells based on the N-doped TiO2 and amount of dye uptake and further lead to an
Degussa P25 photoelectrodes. A pronounced increase in the conversion efficiency of the
increase in the photo-current for the DSSC dye-sensitized solar cell. The adsorption of
based on the nitrogen-doped titania was the dye on both materials was characterized
observed. The performance parameters of the by desorbing the dye into KOH (1:1 etanolDSSCs are summarized in Table 1. A high- /H2O) and measuring their light absorption.
energy conversion efficiency of 4.8% was The UV Visible absorption spectra (Figure 7)
achieved, which was higher than that of the clearly showed higher dye absorption on the
N-doped TiO2.
P25 (4.5 %), respectively.
Copyright  2010 Faculty of Science, Ubon Ratchathani University. All Rights Reserved.
7
C. Kusumawardani et al., Sci. J. UBU, Vol. 1, No. 1 (January-June, 2010) 1-8
Table 1. Performance parameters of DSSCs based on N-doped TiO2 and Degussa P25.
Character
N-doped TiO2 DSSC
Degussa P25 DSSC
Isc (mA/cm2)
Voc (V)
Ipp (mA/cm2)
Vpp(V)
Ff (%)
 (%)
12.86
0.68
9.48
0.51
0.55
4.80
13.44
0.63
10.14
0.44
0.58
4.50
5. Conclusions
The N-doped TiO2 nanocrystalline materials
were synthesized successfully by a novel sol
gel method. Three binding energy peaks were
observed at 396.0, 399.4 and 400.5 eV in the
N 1s region of the XPS. The first signal
(around 396 eV) was assigned to the molecularly adsorbed nitrogen species. Whereas
two signals on higher energy binding were
attributed to a chemically bound N- species
and the O-Ti-N linkages within the crystalline TiO2 lattice, respectively. A new absorption light was observed for the UV-vis
spectrum of the nitrogen-doped TiO2 in the
visible light region. The action spectrum of
the DSSC based on the N-doped TiO2 was in
agreement with the corresponding optical
spectrum. The high energy conversion efficiency was achieved successfully for the DSSC
based on the nanocrystalline N-doped TiO2
electrode.
Acknowledgements
Financial support from Indonesian Government by Directorate General of Higher
Education through “Hibah Penelitian untuk
Mahasiswa Doktor” project is gratefully
acknowledged.
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