Preparation of anatase TiO2 nanoparticles using low hydrothermal temperature for dye-sensitized solar cell

One device being developed as an alternative source of renewable energy by utilizing solar energy source is dye-sensitized solar cells (DSSC). This device works using simple photosynthetic-electrochemical principle in the molecular level. In this device, the inorganic oxide semiconductor of titanium dioxide (TiO2) has a great potential for the absorption of the photon energy from the solar energy source, especially in the form of TiO2 nanoparticle structure. This nanoparticle structure is expected to improve the performance of DSSC because the surface area to weight ratio of this nanostructures is very large. In this study, the synthesis of TiO2 nanoparticle from its precursors has been performed along with the fabrication of the DSSC device. Effort to improve the size of nanocrystalline anatase TiO2 was accomplished by low hydrothermal treatment at various temperatures whereas the crystallinity of the anatase phase in the structure was performed by calcination process. Characterization of the materials was performed using X-ray Diffraction (XRD) and scanning electron microscope (SEM), while the DSSC performance was examined through a high precision current versus voltage (I-V) curve analyzer. The results showed that pure anatase TiO2 nanoparticles could be obtained at low hydrothermal of 100, 125, and 150 °C followed by calcination at 450 °C. The best performance of photocurrent-voltage characteristic was given by TiO2 hydrothermally synthesized at 150 °C with power conversion efficiency (PCE) of 4.40 %, whereas the standard TiO2 nanoparticles has PCE only 4.02 %. This result is very promising in terms low temperature and thus low cost of anatase TiO2 semiconductor preparation for DSSC application.


Introduction
Titanium dioxide (TiO 2 ) has long been the subject of in-depth research because of its wide range applications such as sensors [1], optical emission, photonic crystals, catalysts, photocatalysts [2], purification of environment [3], photovoltaic cells, batteries and electrochemical devices [4][5][6]. This material is one of the inorganic oxide metals known as transitional metal oxides (TMO) [7]. The discovery that crystalline TiO 2 can be used as electrode in the dye-sensitized solar cell (DSSC) has driven even more intensive research due to its high efficiency and low fabrication cost [6].
The presence of oxide semiconductor layer plays a very important role in the DSSC for converting light energy into electrical energy. Moreover, the interaction between the dye and semiconductor oxide particles will be very instrumental in determining the process of converting light energy into electrical energy [8].
In the field of nanomaterials, it has been well known that with the small size down to the nanometer scale, the ratio between the surface area to volume ratio (surface-area-to-volume-ratio) of the material will be greater [9]. In this aspect, the possibility of surface interactions with the environment will be higher, and hence, the use of TiO 2 nanomaterial in the form of nanoparticles, nanotubes and or mesoporous as semiconductor oxide layers in a DSSC is expected to improve the system performance [10].
There are many routes for synthesizing TiO 2 nanopartcile, one of which through the hydrolysis of titanium alkoxide followed by a sol-gel reaction in acidic environments and hydrothermal growth [11]. However, the hydrothermal temperature treatment for this method is usually above 190 °C, which is still quite high. Because of that, in this paper, preparation of TiO 2 nanoparticles was optimized at relatively low hydrothermal temperature that can be applied for DSSC using titanium tetra isopropoxide precursor by the sol-gel technique in a tetra methyl ammonium hydroxide basic environment. Monodisperse TiO 2 nanoparticles of homogeneous morphology were obtained by optimizing the synthesis method. The results obtained are reported and discussed.

Preparation of TiO 2
The precursors were titanium tetra isopropoxide (TTIP) with chemical formula Ti{OCH(CH 3 ) 2 } 4 and tetra methyl ammonium hydroxide (TMAH) with chemical formula N(CH 3 ) 4 + OH − , both were purchased from Sigma Aldrich. These precursors were used during the sol-gel followed by hydrothermal process. The preparation of TiO 2 is in according to the literature [11] with a slight modification to lower the hydrothermal temperature process and the method is explained as the following: A total of 40.5 mL TTIP was added into 112.5 ml of distilled water with a ratio of 1: 50 at 15 °C under a magnetic stirrer for 1 hour. White precipitates resulting from the mixing process were filtered and washed three times using 50 ml of distilled water. The precipitates were transferred into a solution of TMAH and were refluxed at 100 °C for 4 hours. TiO 2 content was set at 10 % by mass. The colloids produced from peptizing were then treated hydrothermally at three different low temperature variations 100, 125, and 150 °C for 12 hours in a Teflon autoclave. The colloidal suspension was then centrifuged at 5,000 rpm and the obtained precipitates were washed with dry ethanol (Merck KGaA, Germany). At the last stage, TiO 2 nanoparticles were calcined at 450 °C for two hours before being characterized using X-ray diffraction (XRD, ARL OPTX-2050) and scanning electron microscope (SEM, FEI Quanta-650).

DSSC Fabrication
The TiO 2 nanoparticle paste was prepared by adding an amount of methanol into TiO 2 nanoparticle powder along with two drops of TRITON X-100 (Sigma Aldrich). The blend was mixed until a homogeneous phase was obtained. Two fluorine-doped tin oxide (FTO, Solaronix) conductive glass substrates were prepared. One of the glass substrates was drilled to create two tiny perforations at one side and dry cleaned by using methanol. The other FTO glass was also dry cleaned by using methanol and attached to a flat layer. This substrate was tape masked and coated with TiO 2 nanoparticle paste using a doctor-blade method under an area of about 1 cm 2 . The paste was dried at 450 °C for an hour and allowed to cool down before soaking it in the commercial organic dye solution (RK1, Solaronix) and air dried. As a comparison, the commercial TiO 2 nanoparticle (Degussa P25) was also prepared under the same condition. The other perforated FTO glass substrate was coated with a platinum paste (Platisol, Solaronix) and dried at 400 °C for an hour. Both the coated glass substrates were then attached to one another separated by a spacer and sealed to avoid electrolyte leakage. Once dried, the electrolyte (Iodolyte, Solaronix) was then injected through the two tiny holes drilled at one side of the substrate. The holes were then sealed and the cell was ready for characterization. Cell activity was measured by The power conversion efficiency (PCE) of the device was calculated in accordance with the following equation [12]: where J sc is the short-circuit photocurrent density (mA cm -2 ), V oc is the open-circuit voltage (volts), I in is the intensity of the incident light (mW cm -2 ) and FF is the fill factor defined as: where i max is the maximum photocurrent, V max is the maximum voltage, respectively, and i oc is the opencircuit current (mA). The values of i max and V max can be extracted from the maximum power of the I-V characteristics.

Results and Discussion
The samples prepared from three different hydrothermal temperature variations and calcined at 450 °C were characterized by using X-ray diffraction. As a comparison, diffraction pattern from the commercially available TiO 2 nanoparticle (Degussa P25) was also taken. The diffraction patterns are shown in figure 1.   [13]. No other characteristic peaks of rutile phase are detected in the diffractograms of the as-synthesized TiO 2 . Other peaks of rutile, however, are observed in the commercial TiO 2 nanoparticles (Degussa P25) as indicated by the arrows in figure 1(b).
Based on the diffractograms, the crystallite size of the as-synthesized TiO 2 nanoparticles was calculated from the Debye-Scherrer's equation using the XRD line broadening [14]: where c s is the crystallite size, λ is the wavelength of the X-ray radiation with Cu K = 0.15406 nm, k is a constant taken as 0.94, θ is the diffraction angle, and B is the full width at half maximum (FWHM) peak in radians. All of the diffraction peaks were taken into account from which the average crystallite size of each temperature variation was obtained, i.e. 5.9 nm, 6.3 nm, and 8.6 nm at temperature variation 100 °C, 125 °C, and 150 °C, respectively. It seems that with the increase of hydrothermal temperature resulted in increase of crystallite size. This can be understood since increasing the hydrothermal temperature will result in more energy for the crystal to grow.
Scanning electron microscope was used to study the surface morphological features of the TiO 2 nanoparticle and the results are given in figure 2. Figure 2(a) is the secondary electron image of the commercial TiO 2 nanoparticle (Degussa P25), whereas the hydrothermally synthesized TiO 2 at 100, 125, and 150 °C is given in figure 2(b), 2(c), and 2(d), respectively. As seen in figure 2, the as-synthesized TiO 2 nanoparticles are still quite diverse in size and some of them are still clumped and agglomerated as compared to those of commercial nanoparticles TiO 2 . Nonetheless, image processing shows that the average particle size is 450, 500, and 590 nm for temperature variation 100, 125, and 150 °C, respectively. This result is in agreement with that of the crystallite size confirmed in the X-ray diffraction analysis mentioned previously. Compared to the commercial TiO 2 nanoparticles with an average particle size < 80 nm, the obtained particle sizes from the as-synthesized TiO 2 nanoparticles are still quite large. Nonetheless, since all of the as-synthesized TiO 2 nanoparticles have shown the characteristics of pure anatase, the material thus could be used as semiconductor photoactive layer in dye-sensitized solar cell (DSSC).
Photocurrent-voltage characteristics of the DSSC from commercial TiO 2 (Degussa P25) and assynthesized TiO 2 at various hydrothermal temperature are given in figure 3. As seen in figure 3, photocurrent-voltage characteristic of both as-synthesized TiO 2 nanoparticle hydrothermally synthesized at various temperatures and commercial TiO 2 nanoparticle show a current density of around 10 mA/cm 2 . Power conversion efficiency (PCE) of the DSSC are given in table 1. As seen in table 1, the PCE from all of the samples are above 4 %. For the as-synthesized TiO 2 nanoparticle, it seems that there is an anomaly on the PCE of hydrothermal at 125 °C. In according to the crystallite and particle size, the increase in hydrothermal should also increase the PCE, which is not the case. This could result from particle inhomogeneity and or agglomeration; however, the exact reason has yet to be confirmed. The interesting point is that PCE from the as-synthesized TiO 2 hydrothermally treated at various temperatures is larger than that of commercial TiO 2 . This can be understood since the commercial TiO 2 nanoparticle contains rutile phase. This rutile phase can reduce the power efficiency due to different interparticle connectivity associated with the particle packing density results in a slow electron transport [15,16].

Conclusion
In this work, low hydrothermal temperature method for synthesizing TiO 2 nanoparticles has been successfully carried out at 100, 125, and 150 °C followed by a calcination at 450 °C. The method was able to produce a pure anatase crystalline structure. The smallest crystallite size was achieved at hydrothermal temperature of 125 °C, however, the best performance for DSSC application was achieved at hydrothermal temperature of 150 °C with power conversion efficiency 4.40 %. This result is very promising in terms low temperature and thus low cost of anatase TiO 2 semiconductor preparation for DSSC application