VIETNAM JOURNAL OF CHEMISTRY DOL 10 15625/0866-7144.2014-0029
VOL. 52(5) 548-552 OCTOBER 2014
PYROLYSIS SPRAY COATING FOR TiOz PREPARATION - TOWARD THE DYE SENSITIZED SOLAR CELLS
Vuong Son', Doan Tuan Anh', Tran Quang Thinh', Tran Anh Phuong^ Luong Thi Thu Thuy\
Nguyen Due Chien', Mai Anh Tuan'*
'international Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology, No. I Dai Co Viet Road, Hanoi, Vietnam
^Faculty of Chemistry, Hanoi University of Education, 136 Xuan Thuy Road, Hanoi, Vietnam Received 17 July 2014; Accepted for Publication 15 October 2014
Abstract
This paper reports the preparation of the TiOjthin film by using pyrolysis spray technique toward the application in dye sensitized solar cell. The morphology and microstructure of the Ti02 film were characterized by seanning/transmission electron microscope. X-ray dif&action and Raman spectroscopy. The obtained average size of Ti02 thin film prepared was found at 30 nm, the optical band gap of the material is near 3.5 eV; and theTiO^ film allows transmission of the light in visible range.
Keywords: Pyrolysis spray coating, Ti02, DSSC.
1. INTRODUCTION
The Ti02, especially in anatase phase form, is a photocatalyst under ultraviolet (UV) light. TiOi can oxidize directly the organic materials meaning that it can accept the electron from another organic reagent. In dye-sensitized solar cells, when irradiated, the dye donates its electron to TiOi and creates the electron-hole pairs. Since Akira Fujishima reported the photocatalytic properties in 1970s [1], many works have been devoted to the titanium dioxide in order to find out more interesting features of such advanced material.
A dye sensitized solar cells (DSSC), introduced for the first time in 1990s by Professor Michael
Gratzel and Dr Brian O'Regan at Ecole Polytechnique Federale de Lausanne, Switzerland (can also be referred to as the Graetzel cell), is a low cost solar cell belonging to the group of thin film solar cells [2]. It is based on a semiconductor formed between a photo sensitized anode and an electrolyte, The basic structure of DSSC has three primary parts as schematically shown in figure 1; the transparent anode covered by a thin layer of nanocrystalline Ti02; the photon adsorbed dye; a counter electrode and an electrolyte [3, 4]. In DSSC, the Ti02 nanoparticles act as a roadway for the elecfrons created by the light exciting from the dye sensitized complex.
J TO TIO. Dye CBl>io<Je Glass
Figure 1: The basic structure and operation mechanism of DSSC
Since the first invention by Gratzel [1] the semiconductor material and dye [5-7] or scientists have kept developing the DSS(5 from opfimizafion of the inter-contacts between the layers
VJC, Vol. 52(5), 2014
of the DSSC [8, 9]. In this paper, T1O2 was prepared by pyrolysis spray coatmg technique. The membrane should satisfy the matching requirements for DSS development such as wide optical band gap, acceptmg the exited electrons from dye, transparent to visible light.
2. EXPERIMENTAL 2.1. Chemical and apparatus
Teh-apropylorthotitanat (C!2H2g04Ti) and acetylaceton (C5Hg02) were purchased from Merck, Germany. Isopropyl alcohol (C3H7OH), ethanol (C2H5OH) and nih-ic acid 65% (HNO3) are from China.
The Spray Pyrolysis System TST1303, developed by BIOMAT research group, can heat up
Mai Anh Tuan, et al.
the substrate to 600 "C with ±5°C accuracy. The rampmg rate can be set to 10 "C per minute. The effective coating area can be up to 100 x 100 mm . In our case, the substrate area is 25 x 25 mm^.
2.2. Preparation of TiOi solution by sol-gel technique
The Ti02, in this work, was synthesized using sol-gel technique as the described in a typical procedure, figure 2. The mixture Ti(C3H70)4:i- C3H70H:C5H802 = 1:2:0.01 in volume was magnetically stirred for 30 minutes. Then a solution of HNO3 O.I M was added to the mixture and kept stirring at 70 "C for 60 minutes. The precursor solution of Ti02 was then filtered to remove possible impurities in the solution.
Ti(C,H,0), 2-C,H,0H
1:2:0.01 i n v o l u m
Stirring 30 min / RT
e C,H,0,
Stirring and heating 60 m i n / 7 0 ' C
HNO.O.IM
S O L T i O ,
Fig. 2: Synthesis Ti02 by sol-gel method
The glass substrate was cleaned by sonicating using distilled water and ethanol followed by rinsing in water and air-dried. The obtained sol of Ti02 in ethanol was deposited on the surface of subsfrate by spray coating technique. The heated glass subsfrate temperature was at 150 "C in open atmosphere. The nozzle was fixed 15 cm from the surface and the carrier gas pressured was 0.5 MPa. After spraying, the samples were heated up 400 "C then annealed in air for 30 minutes. The temperature ramping is adjustable.
The morphology of Ti02 film was studied using a Scanning Elecfron Microscope (FESEM, S4800- Hitachi). The XRD measurements (Bruker D8 Advance Diffractometer) and R a m ^ spectroscopy was used to evaluate the crystal structure of Ti02
film. The thickness and roughness of TiOi film was measured by a surface profilometer (Alpha step IQ).
The optical absorption spectra were recorded between 200-800 nm using Jasco V670 spectrometer, allowing the calculus band gap of TiOjfilm.
3. RESULTS AND DISCUSSION 3.1. Characterization of TiOj film
As presented in figure 3, all the X-ray vibration peaks, plotted usuig raw data, of the synthesized material are well matched with the standards tags of Ti02 in anatase phase. This kind of microstructure Ti02 is the desired one that is suitable ui dye
VJC, Vol. 52(5), 2014 sensitized solar cell.
Pyrolysis spray coatingjor TiOipreparatlon..
w
-Anatase ri02 ICSD#98S2
JLJL
2eft
Figure 3: XRD of nanoporous Ti02 film obtained by spray coating technique The Raman spectra of the Ti02 thin film deposited on glass subsfrate is illustrated in figure 4.
The high intensity of the Raman is shift to the higher frequencies, which are corresponding to the anatase form of Ti02.
Figure 4: Raman spectra of the Ti02 film obtained by spray coating technique
In lower frequencies that identify the presence of other forms (rutile at 447 and 616 cm"' and of brookite at 128 and 247 cm"') of such material, the intense peaks are very weak or negligible. Thus the presence of rutile or brookite phases can be ruled out.
Table I: Raman intensity of Ti02 obtained by spray coating technique.
The subsfrate was heated to 150 °C in air. The nozzle was fixed 15 cm from the surface; N2 carrier gas pressured was 0.5 MPa. After spraying, the material was annealed at 400 °C in air for 30 minutes Raman
frequency of anatase (cm"')
144 197 400 507 519 640
Symmetry species of anatase
Eg Eg B,g A,g Big Eg
Raman frequency of rutile (cm"')
143 247 447
616 826
Symmetry species of rutile
Big Big + Big
A|g
A|g B2g
Raman frequency of brookite (cm"')
128 153 247 366
636
Symmetry species of brookite
Ag Ag A | B2g
A,
The SEM micrograph shown in figure 5 indicates a porous sfructure in the nano-ctystalline Ti02 film. The average diameter of the anatase Ti02 was about 20 nm.
Since Ti02 nanoparticles act as a roadway for the electrons that created by the light exciting, travel from the dye sensitized complex. The nano-size and well disfribution of the nanoparticles were recognized to facilitate the elecfron jumping in filling elecfrolyte.
3.2. Optical characterization of TiOi film Using the optical transmittance specfra, the absorption coefficient and the band gap of TiOj film were evaluated. The absorption coefficient was calculated using the equafion:
a = ln(l/r)//
where T is the fransmittance and / is the thickness of the film. The thickness of the film was evaluated by surface profiler Alpha Step IQ and the average value corresponds to 200 nm for nanoporous TiOi film, figure 6.
VJC, Vol. 52(5), 2014 Mai Anh Tuan, et al.
Fig. J: SEM of nanoporous TiOi film obtained by spray coating technique
I
Figure 6: The thickness and the roughness of Ti02 film investigated by using surface profilometer Alpha Step
The band gap of energy was obtained by plotting the optical absorption, (ahu)^ vs. the photon energy, (hu), and extrapolating the linear portion of the curve to (ahu)^ - 0, figure 7.
Ti02 film iibsorbs appreciably wavelengths at less than 360 nm (which corresponds to band gap
energy of 3.5 eV).
On the glass substrate, Ti02 film allows ttansmission of wavelength in visibility range as figure 8. Tliat makes TiOj suitable photo-anode electrode in DSSC.
VJC, Vol. 52(5), 2014 4. CONCLUSION
Ti02 film, in this work, was prepared by pyrolysis spray coating technique and thermal treatment with thickness about 200 nm. The X-ray diffraction and SEM show that the Ti02 film is anantase crystalline (average size is about 20 nm).
The nanoparticles are homogeneous and well distributed on the substrate. The optical characterization of Ti02 film also shows that film can be used as photo-anode for DSSC.
Figure 7: The optical band gap energy of nanoporous Ti02 film was extrapolated from the thickness and the transmission of the
material
30D 350 40a 450 500 5St) GOO 650 700 T50 B Wave ten gtii (nm)
Figure 8: The absorption spectra of Ti02 film
Pyrolysis spray coatingfor Ti02preparation...
Acknowledgement. This -^ork was financially supported by the Vietnamese National Foundation for Science and Technology Development
(NAFOSTED) for a basic research project (104.99- 2011.44 code).
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Corresponding author. Mai Anh Tuan
International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology,
1 Dai Co Viet St., Hai Ba Trung Dist., Hanoi, Vietnam E-mail: [email protected].
