Experimental Study of TiO 2 Nanoparticles Fabrication by Sol-gel and Co-precipitation Methods for TiO 2 / SnO 2 Composite Thin Film as Photoanode

Sol-gel and coprecipitation methods successfully prepared titanium dioxide (TiO2) powders with anatase structure. The TiO2 powders are then used to fabricate pure TiO2 thin-film or mixed with SnO2 powders for the TiO2/SnO2 composite thin film. Furthermore, the structural, morphological, as well as the optical properties of films were also investigated. The results showed that the synthesized thin-film of TiO2 powders by sol-gel method obtained better crystallinity and microstructure compared to the synthesized thin film by co-precipitation method. In the DSSC system, these features are needed to increase the electron mobility that responsibility for transport and recombination of photoexcited electrons. SEM images exhibited the smooth surface and uniform in particle size obtained by the addition of SnO2 powders in composite films. The composite thin film also indicated a higher transmittance value.

The microstructure and morphology of film photoanodes have a significant influence on solar-to-electrical energy conversion process in DSSC system (Wali et al., 2016).Tsai et al. explained that a typical nanoporous structure of the film is required to increase dye absorption as well as providing sufficient light absorption due to its high surface area.They also declared that the smooth and homogeneous surface morphology of film photoanodes improve the electron transport between nanoparticles (Lee et al., 2011).Significant efforts have been given to fulfill these prerequisites, for example by varying the synthesis method of semiconductor even by modifying two or more the materials used for photoanode fabrication (Liu et al., 201;Surya et al., 2017).
Semiconducting materials that have been most utilized for photoanode DSSC are Titanium dioxide (TiO2) (Valencia et al., 2010) and Tin dioxide (SnO2) (Surya et al., 2017;Di Paola et al., 2013).TiO2 and SnO2 had been extensively studied due to its high surface area (Su'ait et al., 2015;Essalhi et al., 2016), safety and matched energy band structure (Behnajady et al., 2011).The optical band gap of TiO2 is reported at ~3.0, ~3.1 and ~3.2 eV for rutile, brookite and anatase structure, respectively.Whilst, SnO2 has band gap at ~3.6 eV (Hamadanian et al., 2014;Muniz et al., 2011;Su'ait et al., 2015;Behnajady et al., 2011;Yeh et al., 2014;Yang et al., 2006) with tetragonal cassiterite and orthorhombic phase (Valencia et al., 2010;Vijayalakshmi and Rajendran, 2012).Therefore, combining these two materials to build a photoanode is expected could improve the microstructural and morphological characteristics of film photoanodes.(Endarko, dkk) Various methods have been employed to synthesize TiO2 nanopowders.Choi and Sohn reported that Sol-gel methods are used very often because it does not require high temperatures treatment (Choi and Sohn, 2012), but also Co-precipitation methods can be done because of the easy and simple process (Xu et al., 2014).
The main purpose of the study was to compare the crystalline phases, crystallinity, uniformity morphology and optical properties of TiO2 nanoparticles which obtained by solgel and co-precipitation methods.The effect of the addition of SnO2 on characteristics of film photoanodes will also be investigated.The crystalline phases, crystallinity were characterized by XRD, the morphology of film was observed using SEM imaging.Meanwhile, the opticalproperties of filmswas measured by UV-Vis spectrometer.

Preparation of TiO2 nanoparticles by Sol-gel method
Titanium Isopropoxide (TTIP, Sigma-Aldrich) was slowly dissolved in the Isopropanol (IPA, Sigma-Aldrich) and was then stirred for 30 min.Few drops of HNO3 were added and stirred for 24 h.Subsequently, TiO2 powders can be achieved by calcination for 3 h at 400°C.

Preparation of TiO2 nanoparticles by Coprecipitation method
TiO2 powders were prepared from Titanium (III) chloride (TiCl3, Sigma-Aldrich) as a titanium precursor.Initially, TiCl3 was mixed with distilled water and stirred for an hour.NH4OH solution was added dropwise until pH of the mixture reached to 9. Subsequently, the mixture was stirred until resulting white precipitate.The obtained precipitate was filtered and then washed with distilled water, reaching the pH equal to 7. The precipitate was finally calcined at 450°C for 3 h to get the TiO2 powders.

Preparation of TiO2 paste
The synthesized TiO2 powders were blended with distilled water and stirred for 10 min, subsequently added PEG, acetylacetone, acetic acid, and Triton-X to the mixture.Preparation of SnO2 paste Commercial SnO2 powder was procured from Sigma-Aldrich.The paste solution was made by dissolving ethylcellulose and isopropanol and stirred for 90 min.The paste was obtained by mixing SnO2 powders with the solution.

Preparation of TiO2/SnO2 composite paste
The oxide paste was made from a mixture of SnO2 and TiO2 powders that ground using a mortar and then heated at 450°C for 30 min with the solution that has been synthesized by mixing distilled water and ethanol as a solvent with ethylcellulose and terpineol as a binder and kept stirring for 10 min.

Fabrication of Photoanodes
Indium Tin Oxide glass (ITO) was purchased from MianyangProchema Commercial Co., Ltd., China.Removal process of organic impurities such as fat and oils was carried out by washing the ITO glass with a size 1 × 1 cm 2 with alcohol for 60 min in an ultrasonic cleaner.Subsequently, the paste was deposited onto the glass using doctor blade technique.

Immersion of Photoanodes
The dye solution was made by mixing 1.1 mg of N-749 dye powders into 20 mL of ethanol and was then stirred using a magnetic stirrer for 10 min.Immersing the photoanode in the dye solution was done for 24 h, and this process aims to enhance the photosensitivity of the photoanodes.

RESULTS AND DISCUSSION
Fig. 1 shows the XRD spectra of TiO2 powders.The spectra revealed that both patterns are nearly identical and had a polycrystalline structure with a dominant peak (101) appearing around 26 degrees.
It can also be seen in Fig. 1; there are a few differences in peak intensity of both patterns.The spectrum of TiO2 prepared via the sol-gel process (Sol-gel TiO2) has sharper peaks and can be distinguished easily.The peaks around obtained by coprecipitation method (Coprecipitation TiO2) almost look like a single peak due to it has weak peak intensities.The higher and stronger the intensity of diffraction peaks indicate she improved in the degree of crystallinity (Vidyasagar and Arthoba Naik, 2016).
The degree of crystallinity of two powders was summarized in Table 1.The data reveals that the TiO2 powders synthesized by the solgel process have better crystallinity compared to the powders obtained by coprecipitation method (Jian et al., 2005) These results also implied the less distinct peaks of TiO2 that had prepared via coprecipitation process due to the formation of the anatase phase had not been completely formed.This indicates that the chemical bonds are formed less stable so that the interconnection and poor continuity between titania nanoparticles, which in turn decreases the efficiency of electron transfer in photoanode (Adawiyah and Endarko, 2017).Table 1 also showed both TiO2 powders have crystallite size and lattice constants which are not much different.Fig. 2 represents the XRD spectra of thin film photoanodes after calcination at 450°C for 90 min.

Figure 2. XRD patterns of thin-film photoanodes
The pattern confirms that phase transformations did not occur during the heating process hence the anatase phase is still observed in the pure TiO2 photoanodes.Meanwhile, the composite spectra showed that mixed anatase-rutile phases were formed.It was identified that the presence of rutile phase is a contribution of tetragonal cassiterite structure of SnO2.The tetragonal SnO2 peaks at ~26. 04, 33.49, 37.45, 51.03 and 54.16 degrees belong to ( 110), ( 101), ( 200), and ( 211) planes (PDF no 00-077-0447, ICDD).Furthermore, it can also be analyzed that the addition of SnO2 could reduce the ITO peak (222) in composite photoanodes (Adawiyah and Endarko, 2017).It is intended that only the semiconductor phase will be formed.Morphological Characteristics SEM images in Fig. 3 reveals that all samples have spherically shaped particles.It can be seen that the co-precipitation TiO2 film (a) displays many cracks on its surface.Meanwhile, the sol-gel TiO2 film (b) exhibits large and intense agglomerations although it also shows free from cracks.A more uniform distribution of particle size with porous structure was found in both composite images in Fig. 4, which may be due to the addition of SnO2 (Adawiyah and Endarko, 2017).It can be seen in Fig. 3, a microstructure of SnO2 film which has a smooth surface with very uniform particle size and its porous structure was exposed in Fig. 4. As also shown by Camacho-López M. A et al. in their SEM pictures (Sociedad Mexicana de Ciencia Superficies y Vacio. et al., 2013).The pore structure will allow more dye molecules to be absorbed in the semiconductor, thus will increase the photosensitivity of the photoanode to solar radiation, which means that the dye will inject more electrons.In addition, the homogeneous and crack-free surface of the film will also facilitate the electron flow in the photoanode material thus enhancing the transport and recombination processes in the DSSC system (Ye et al., 2015).Sol-gel TiO2/SnO2 (e).A Sno2 film with 5000× magnitude The SEM analysis results also provide information on the particle size of pure TiO2 films had value in the range of 1560 nm whereas the composite is smaller in size, which is 1030 nm.The SnO2 film shows the surface looks very smooth, flat and rather dense the particles are not visible and the particle size determination is difficult to do.

Optical Characteristics
Table 2 gave the optical band gap values (Eg) of the film photoanodes calculated using the Tauc plot method.It was found that the band gap of TiO2/SnO2 composites is smaller than the pure TiO2 film.The result corresponds to the result reported by Essalhi et al. that the addition of SnO2 may decrease the value of the energy band gap (Essalhi et al., 2016).The narrowing of the band gap would increase the probability of electrons being injected into the conduction band of semiconductor (Nguyen, Tran,and Bach, 2014).The higher concentration of electrons in the conduction band will trigger an increase in photocurrent (JSC) value.This condition will undoubtedly lead to an increase in efficiency of DSSC.It appears that the composite bandgap energy value is between the ranges of TiO2 and SnO2 band gap energy values.

CONCLUSION
In summary, we fabricated TiO2/SnO2 composite DSSC using TiO2 powders synthesized by sol-gel and co-precipitation methods.The higher crystallinity was obtained through the sol-gel method.The films also showed smooth and crack-free morphology on the surface.Additionally, the mixing TiO2 and SnO2 powders result in a composite film with homogeneous surface and porous structure.It explains that SnO2 addition could be improved the microstructure of the films.Asa consequence, utilization titanate powders obtained by the sol-gel method as a photoanode material followed by SnO2 addition an effective strategy for enhancing the overall photovoltaic parameters in the future application.

Figure 4 .
Figure 4. SEM micrographs of (a).Coprecipitation TiO2 film, (b).Solgel TiO2 film (c).Co-precipitation TiO2/SnO2, (d).Sol-gel TiO2/SnO2 (e).A Sno2 film with 5000× magnitude The SEM analysis results also provide information on the particle size of pure TiO2 films had value in the range of 1560 nm whereas the composite is smaller in size, which is 1030 nm.The SnO2 film shows the surface

Table 2 .
The energy gap of photoanode immersed in dye N749 for 24 h