Structural and morphological characterization of TiO2-SnO2 thin film prepared by combining doctor-blade and sol-gel techniques

The TiO2-SnO2 thin film has successfully been synthesized using the co-precipitation method and was coated on ITO (Indium Tin Oxide) substrate by doctor-blade technique. The structure and morphology of the film were investigated by XRD and SEM, respectively. The results showed that the film with SnO2 has a stronger formation of anatase phase compared to TiO2 film. The morphological study is also revealed that the TiO2-SnO2 film has a more porous nature and uniform particle aggregates, and the presence of SnO2 has been confirmed with EDX spectra.


Introduction
Titanium dioxide (TiO2) is one of the most selected and studied extensively as photoanodes materials in DSSCs [1][2]. TiO2 can be found in its three polymorphs in nature: anatase, brookite, and rutile. However, anatase is mostly used due to its excellent stability and photoactivity. As the main material of the photoanode, characteristics of TiO2 such as structure and morphology will certainly affect the whole characteristics of DSSC.
The photoanode in DSSC should possess stable and porous structure, enormous surface area, high molecules dye-loading capacity and solid structure of the surface film [2][3][4]. The physical and chemical properties of TiO2 can be modified by incorporating metals and nonmetals element to tailor the properties of TiO2 for enhancing the photovoltaic application especially in DSSCs [1]. Doping with SnO2 can improve the stability of crystallinity of TiO2 because it has porous nature, fast rate of electron transport, higher electric mobility, long-term stability under UV-illumination and excellent performance in an acidic condition [2,5,6].
Many methods have been employed to synthesize TiO2 nanoparticles, such as Stӧber [7], hydrothermal [8] and sol-gel [9]. Recently, the co-precipitation method has been extensively investigated to synthesized TiO2 nanoparticles due to low processing temperature, ease of composition control, high purity and good chemical homogeneity [10]. In this work, we synthesized TiO2 and TiO2-SnO2 nanocomposites by a co-precipitation method, then coated on ITO film. The structure and morphology of TiO2-SnO2 film were characterized and compared to TiO2 film.

TiO2 synthesis
TiO2 powders were synthesized by mixing 20 mL of Titanium (III) chloride (TiCl3) with 100 mL of aquades and was stirred for 1 hour. To this mixture, NH4OH solution was added dropwise until pH reached to 9. The resultant solution was stirred until resulting white precipitate. The precipitate was filtered and was then washed several times with distilled water, reaching a value of pH equal to 7. Removal process of residual organics and the stabilization of the materials were carried out by calcination for three hours at 450°C.

TiO2-SnO2 synthesis
SnO2 nanopowders was purchased from Sigma Aldrich. SnO2 powders were mixed with TiO2 nanopowders. The powders mixture was grinded with a grinding bowl and then annealed at 450°C for 30 min. To the mixture aquades, ethylcellulose, terpineol and ethanol were added and kept stirring for 10 min. The mixed oxide pastes were obtained from 0.7 mL of acetic acid, 0.3 g of PEG, and 0.7 mL of Triton X-100. The TiO2 pastes were prepared in the same way.

TiO2-SnO2 thin films
ITO glass substrates were purchased from Mianyang Prochema Commercial Co., Ltd., China. ITO with a size of 1 × 1 cm 2 were thoroughly rinsed with deionized water and anhydrous ethanol and dried on a hot plate. TiO2 and TiO2-SnO2 pastes were deposited onto a conductive glass substrates-ITO glass using doctor blade technique. The films were heated at 450°C for one hour and cooled naturally to obtain a nanoporous film. The structure and morphology of the films were characterized by XRD and SEM-EDX.

Structural study
The crystalline phase of photoanodes films was evaluated by XRD analyses, and the result is shown in figure 1. It can be seen that the films are polycrystalline, and the diffraction peaks observed around 26 and 49 degrees correspond to the (101) and (200) of the anatase phase of TiO2 with the tetragonal crystal structure (PDF no. 00-078-2486, ICDD) [11]. The TiO2 film exhibits a new diffraction peak (222) plane around 31 degree, which belongs to the ITO peak. These results agree with the analysis of the microstructure of pure TiO2 film by Arunachalam et al. [5].
Furthermore, the diffraction peaks from rutile phase appear in the X-ray patterns due to the addition of the SnO2 in the film. The (110), (101) and (211) planes at 2θ values 27, 34.1 and 53 degrees were observed as the characteristic peaks of SnO2 in the doped TiO2 film. The similar results have also been reported in the previous studies [11][12][13].
Nevertheless, the intensity of (101) plane is higher than the intensity of TiO2 film because of the electronegativity, and the ionic radius of Sn 4+ ions are larger than Ti 4+ . It allowed easily for Sn 4+ ions to replace and occupy the oxygen position in the TiO2 lattice [3,5] Thus, the spectrum shows the intensity of (101) plane is increased with the decrease ITO peak (222) plane, which indicated the better crystallinity than the TiO2 film was obtained. This feature gives a more stable chemical bond and permits excellent interconnection and continuity between titania nanoparticle, which in turn enhances electron transfer efficiency in photoanode [3][4]. This phenomenon showed that the inclusion of SnO2 in the TiO2 may stabilize the anatase as the main and strongest phase [5,13]. The smaller radius of Ti 4+ (0.68 Å) as compared to Sn 4+ (0.69 Å) [5] also made the crystallite size of doped TiO2 film is bigger than the undoped one. The average crystallite size, which is calculated from XRD data using Rietveld method, is 34.2 and 10.3 nm for the doped TiO2 and the undoped TiO2 films respectively.   Figure 2 represents the top-view SEM images of the TiO2-SnO2 and TiO2 films. These images confirm that the microstructure of both samples exhibits spherical shaped particle with irregular morphology due to the agglomeration of primary particles during the anneal treatment. It can be seen that smaller particles with an average diameter of 10-11 nm were observed for TiO2 film and around 35 nm for SnO2 doped TiO2 film. The enlarging particle size of the doped TiO2 film results in the larger surface area of the film [9], thus enabling a high dye loading capacity [14] as well as enhancing the photosensitivity to solar radiation [15]. The porous nature was observed in both films and this structure also plays a role in enhancing the surface area of the film photoanode [5,9,16]. A bit rough, large and intense inhomogeneous agglomerations were formed in the pure TiO2 film. On the other hand, TiO2-SnO2 film exhibits smooth and rather well-distinguished uniform aggregates, although also there are few voids and cracks which may be due to losing of the binder during the annealing process [16]. These results indicate that the presence of SnO2 can effectively suppress the grain growth of anatase compare with the pure TiO2 [5].   The EDX spectra represents the element analysis of TiO2 film and SnO2 doped TiO2 film. The EDX analysis of the TiO2 film (figure 3a) confirms the presence of Ti and O. Figure 3b shows the existence of the SnO2 dopants into TiO2 lattice. From table 1 can be deduced that the Sn was presented around 1.28% in the TiO2-SnO2 film, and the percentage of O in the composites was decreased compared to TiO2 film. The condition proved that the doping of SnO2 has been successful.

Conclusions
In summary, the proposed work was a comparison the structural and morphological of the TiO2-SnO2 nanocomposite and the synthesized of TiO2. The XRD pattern of TiO2 anatase with SnO2 besides showing the better crystallinity is also showing the stronger formation when compared to the anatase TiO2. Additionally, the EDX analysis reveals the existence of the SnO2 dopants into the TiO2 lattice. The surface of the doped TiO2 film showed smooth and rather homogeneous aggregate.