Synthesis of Monodispersed TiO2 Nanosheets by Wet Chemical Method and their Applications in Dye-Sensitized Solar Cells

Abstract

Anatase TiO2 nanosheets were prepared by wet chemical method in the presence of Tripropylamine as capping ligand. The obtained samples were characterized by Powder X-ray diffraction, Raman spectroscopy, Ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, Field emission scanning electron microscopy, Transmission electron microscopy and high-resolution transmission electron microscopy. The growth mechanism for the formation of TiO2 nanosheet has been investigated. The photovoltaic properties of anatase TiO2 nanosheets were investigated by assembling the TiO2 nanosheets as photoanodes in dye-sensitized solar cells (DSSCs). Experimentally, it shows that the anatase phase of TiO2 nanosheet films have conversion efficiency of 3.25%.

Keywords

Nanosheets, Nanomaterials, Semiconductors

Introduction

Titanium dioxide (TiO₂) is a widely studied metal oxide. When serving as the anode material for Dye sensitized solar cell (DSSCs), TiO₂ delivers higher conversion efficiency when compared to that of other semiconducting materials [1-5]. It’s most significant advantage is the ability to adsorb more number of dye molecules with less dye aggregation [6-13]. Even though conversion efficiency of TiO₂, is not enough. So proper nanostructure engineering is required to achieve the high conversion efficiency in DSSCs. Moreover, such a strategy also appears to be effective in improving the DSSCs of TiO₂ [14-19]. Yang et al. synthesized anatase single crystals of TiO₂ with 47% of the highly reactive {0 0 1} facets by using hydrofluoric acid (HF) as a capping agent under hydrothermal conditions [20]. Yang et al. reported a new solvothermal method using 2-propanol as a synergistic capping agent and reaction medium together with HF to synthesize high-quality anatase TiO₂ single-crystal nanosheets (SCNSs) with 64% of the {0 0 1} facets [21]. Guo et al. synthesized anatase TiO₂ nanosheets by using hydrofluoric acid (HF) as a capping agent under hydrothermal conditions [22]. Wang et al. have represented hierarchically structured spheres consisting of ultrathin nanosheets by using hydrothermal method and achieved the power conversion efficiency of 4.66% without any TiCl4 treatment [23]. Fortunately, the recently discovered anatase TiO₂ nanosheets (NSs) demonstrate the conversion efficiency upto 8% [24]. With the aim of obtaining enhanced photoanode performance, many studies on different modifications of TiO₂ have been performed [20]. Moreover, during the last few years, an increasingly great number of TiO₂ nanosheet structure has been synthesized and tested as possible alternatives to other TiO₂ nanostructures to improve its light to electric conversion efficiency in DSSCs with templates as shape controllers. In our previous reports, TEA was used as capping ligand to obtain size confined monodispersed nanoparticles of CdS and PbS [25,26]. Hexamethylenetetramine was used as capping ligand to obtain ZnO nanosheets for dye sensitized solar cell characteristics [27].

In the present work, TiO₂ nanosheets were synthesized by simple wet chemical method using tripropylamine (TPA) as a capping ligand at low temperature without any templates. Further, photo anodes were successfully fabricated using TiO₂ nanosheets and the cell performances were characterized.

Materials and Methods

Synthesis of TiO₂ nanosheets

All chemicals were purchased from Wako chemicals (Japan) and used without further purification. 0.5 mL of tripropylamine (TPA) was added slowly to 40 mL ethanol under vigorous stirring. 4 mL titanium isopropoxide (TIP) was then added drop wise to the parent solution under vigorous stirring. A white precipitate was obtained after few minutes. The ratios of titanium isopropoxide (TIP), tripropylamine (TPA) and ethanol in the above solution were 4:0.5:40. The reaction was continued for 12 h at room temperature. Finally, the precipitates were washed with water for several times and dried at 100°C for 12 h and it was labelled as S2. The same preparation procedure was followed for the preparation of uncapped sample without TPA and it was labelled as S1.

Dye sensitized Solar cell fabrication

Firstly, 2 g of TPA capped TiO₂ nanosheets was dispersed in 50 ml of ethanol and this mixture was ground in a mortar for a few minutes to form colloidal suspensions. Then, 5 drops of triton-X were added to the solution as an organic binder. Fluorine doped tin oxide (FTO) substrates were cleaned ultrasonically using a mixture of acetone and ethanol for 30 minutes. The TiO₂ colloidal solutions were sprayed over the FTO substrate at a substrate temperature of 150°C by spray deposition method. TPA capped TiO₂ nanosheets coated FTO substrates (photoanodes) were successively sintered at 530°C for 2 h. Sintered FTO photoanodes were sensitized with ruthenium [0.03 M di-tetrabutylammoniumcis–bis (isothiocyanato) bis (2,2″-bipyridyl-4,4′-dicarboxylato) ruthenium (II) (N-719)] solution in ethanol. The dye-sensitized photoanode and Pt-coated counter electrode were clamped using clips. Finally, an iodide based redox electrolyte was filled between the electrodes via capillary action.

Measurement techniques

Phase identification was done by the powder X-ray diffraction, using Rigaku X-ray diffractometer with Cu Ka radiation at 0.02°/s step interval. Raman spectra were obtained using a JASCO NR 1800 Raman spectrophotometer equipped with Nd:YAG laser. UV–visible absorption analyses were performed using a Shimadzu (Japan) 3100 PC spectrophotometer with ethanol as dispersing medium. Fourier transform infrared (FTIR) spectra were obtained from JEOL JIR-WINSPEC 50 spectrometer. The morphology of the samples was observed through transmission electron microscopy (JEOL JEM 2100F, Japan) and field emission scanning electron microscopy (FESEM, JEOL JSM 6320F, Japan). Photocurrent–voltage measurements were performed under AM1.5 100 mW/cm² simulated light irradiation (YSS-80A, Japan) using a Xe lamp as the light source equipped with an infrared (IR) filter.

Results and Discussion

Figure 1 shows the XRD patterns of uncapped and TPA capped TiO₂ powder samples S1 and S2 respectively. XRD patterns of uncapped S1 sample show the amorphous nature of TiO₂ powder. XRD patterns of TPA capped S2 sample show the anatase phase of TiO₂ powder (JCPDS Card no. 78-2486). The width of XRD diffraction peaks of anatase becomes broad, indicating the formation of greater TiO₂ crystallites and enhancement of crystallization. The TEM images of S2 sample are shown in below figures. It can be clearly shown that the growth of crystalline TiO₂ nanocrystals is in concurrence with the experiment results of XRD (Figure 1).

Figure 1: XRD patterns of uncapped TiO₂nanoparticles (S1) and TPA capped TiO₂ nanosheets (S2).

To investigate the crystalline phase of TiO₂ and quality of sample, Raman spectroscopy was applied as a powerful tool to detect the significant structural changes in TiO₂ during the wet chemical reaction process. The Raman spectra (Figure 2) show remarkable difference due to amorphous (uncapped) and crystalline material (TPA capped) of TiO₂. Uncapped (S1) TiO₂ nanoparticles do not show any mode of vibration because the sample was in amorphous nature. Four representative modes of Eg (151 cm⁻¹), B1g (404 cm⁻¹), A1g (517 cm⁻¹) and Eg (640 cm⁻¹) are the same as the modes of anatase TiO₂ TPA capped (S2) nanosheets. The results confirm that the phase of TiO₂ sheets is in agreement with XRD analysis [28].

Figure 2: Raman spectra of uncapped TiO₂nanoparticles (S1) and TPA capped TiO₂ nanosheets (S2).

The absorption spectra (Figure 3) reveal that the absorption edges of TPA capped (S2) sample shifted towards blue region compared to uncapped (S1) TiO₂ sample. The absorption edge of uncapped TiO₂ observed is at 360 nm and that for TPA capped observed 340 nm. These differences in UV spectra reveal the possibility of the structural change such as reduction in the size.

Figure 3: UV spectra of uncapped TiO₂nanoparticles (S1) and TPA capped TiO₂ nanosheets (S2).

FTIR spectrum of TPA capped (S1) and uncapped (S2) samples of TiO₂ are presented in Figure 4. Absorption in the low wavenumber region between 1000–900 cm⁻¹ is typically assigned to O–Ti–O bond in crystalline TiO₂ [29]. FTIR band at 2365 cm- 1can be assigned to C-O vibration. The bands in the 3000–3500 cm⁻¹ range are attributed to OH vibration on the surface of TiO₂ from the environment [29]. FTIR bands in the 1393 cm⁻¹ and 1623 cm⁻¹ are due to N-H vibration of TPA [30]. It clearly demonstrates the surface passivation of TPA molecule on the surface of TiO₂ nanoparticles.

Figure 4: FTIR spectra of uncapped TiO₂nanoparticles (S1) and TPA capped TiO₂ nanosheets (S2).

Based on the morphological studies, the formation mechanism of TPA capped TiO₂ nanosheet is explained as follows: lone pair of electrons in the nitrogen atom of TPA was attached to the surface of the TiO₂ nanoparticles due to the affinity of the nitrogen with Ti4+. The nitrogen atom forms a co-ordinate bond with the surface of TiO₂ because of its chemisorption properties. In addition to that, the longer growth time and alcoholysis of TPA in the presence of ethanol resulting the formation of nanosheets. The addition of TPA molecules to the titanium hydroxyl ions restricts further growth of the c-axis owing to the chemisorption or adsorption of amine molecules on the surface. Finally, annealing at 100°C leads to the formation of TiO₂ nanosheets.

The morphology analyses of uncapped and TPA capped TiO₂ samples were carried out using FESEM and TEM as shown in Figure 5. Uncapped sample have shown that spherical morphology with more agglomeration. Nanosheets like morphology was exhibited by TPA capped samples with crystalline quality. Wang et al. [31] synthesized TiO₂ nanosheets with a typical thickness of 97 nm with average diameter of about 493 nm and length 520 nm. TiO₂ nanosheets with a typical thickness of 260 nm average diameter about 1.09 μm were synthesized by Yang et al. [21]. Guo et al. [22] synthesized TiO₂ nanosheets with a typical thickness of 40-60 nm and length 500-800 nm. In the present work, TiO₂ nanosheets of average diameter and length are 150 nm and 250 nm, respectively. Moreover, TiO₂ nanosheets were synthesized by using TPA as a capping ligand with high monodispersity. From this, one can conclude that TPA acted as effective capping ligand for TiO₂ nanosheets synthesis by the simple chemical method. High resolution transmission electron microscopy (HRTEM) showed lattice fringes and indicates that TPA capped TiO₂ nanosheets were crystalline. The schematic representation for synthesizing TiO₂ nanosheets is shown in Figure 6.

Figure 5: FESEM and TEM images of uncapped (a) TiO₂ nanoparticles and TPA capped TiO₂ nanosheets (b-d).

Figure 6: Formation mechanism of TiO₂ nanostructures.

Figure 7 shows the I-V characteristics for dye sensitized solar cell which was fabricated by the TiO₂ films prepared with nanosheets for sample S2. The current-voltage curves reveal that the DSSC prepared based on the synthesized TiO₂ nanosheets presents less short-circuit current (Jsc) over that of the reports already published [32]. The DSSC which was fabricated based on the as synthesized TiO₂ nanosheets shows Jsc=6.55 mA/cm² and Voc=0.79 V with the power conversion efficiency (PCE) of 3.25%, respectively, whereas the DSSC based on anatase TiO₂ nanosheets based microspheres Jsc=15.2 mA/cm² and Voc=0.65 V with the power conversion efficiency (PCE) of 6.64%, respectively, indicating 50% decrease in Jsc and PCE compared to the reported results [32]. It is well-known that an increase in the light-scattering ability plays an important role in the light-harvesting efficiency and Dye adsorption ability. Compared to the monodispersed anatase TiO₂ nanosheets, nanosheets-based microspheres have more ability to absorb more amounts of dye molecules due to the porous nature of the microsphere. Light-scattering ability and amount of dye adsorption, these are the two important factors for the less power conversion efficiency for as synthesized TiO₂ nanosheets. However, as synthesized TiO₂ nanosheets preparation procedure is very simple compared to the published report [32].

Figure 7: I-V curve of TPA capped TiO₂ nanosheets (S2).

There have been, over the past decades, a number of synthetic routes to the preparation of TiO₂ nanostructures. For example, conventional hydrolytic sol-gel process and emulsion precipitation [33-36] performed at relatively low temperature yield amorphous products with polydisperse particles, and subsequent calcination is needed to induce crystallization. We presented a simple growth process for fabrication of TiO₂ nanosheets in ethanol solution at low temperature. The results indicate that monodispersed crystalline TiO₂ nanosheets in large scale have been successfully synthesized by chemical method. Meanwhile, the morphology of crystalline TiO₂ nanosheets easily controlled using TPA as a capping ligand without further calcination.

Conclusions

Nanocrystalline anatase TiO₂ nanosheets were employed as a photoanode material in dye-sensitizedsolar cells. XRD and Raman spectra confirms the formation of anatase phase TiO₂ nanosheets. UV spectra confirm the TPA capped sample shifted towards blue region compare to that of uncapped sample. FTIR spectra confirm the passivation of TPA on surface of the TiO₂. FESEM and TEM results confirm the TPA capped sample with monodispersity and crystalline quality. The efficiency of the cell using TiO₂ nanosheets was about 3.25%.

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