DOI : 10.2240/azojomo0248

Preparation and Heat Treatment Effects of TiO2 Nanotubes Used in Dye-Sensitized Solar Cells

Dec 7 2007

Hong Lin, Ning Wang, Luozheng Zhang, Chunfu Lin and Jianbao Li

This is an AZo Open Access Rewards System (AZo-OARS) article distributed under the terms of the AZo–OARS https://www.azom.com/oars.asp which permits unrestricted use provided the original work is properly cited but is limited to non-commercial distribution and reproduction.

AZojomo (ISSN 1833-122X) Volume 3 December 2007

Topics Covered

Abstract
Keywords
Introduction
Results and Discussion
Conclusions
Acknowledgement
References
Contact Details

Abstract

The dye-sensitized solar cell (DSC) is highlighted by its low cost in recent years. In the present study, TiO₂ nanotubes (TNTs) with high surface area were prepared, which can be used in the anodic electrode of the dye-sensitized solar cell (DSC). The preparation methods, heat-treatment effects and photovoltaic properties of the electrodes were discussed. Firstly, titanate nanotubes were synthesized by hydrothermal method with commercial antase-type TiO₂ powder as the raw material. TNTs can be obtained by heat-treating the as-prepared titanate nanotubes at 450°C. Secondly, the electrode with 30% TiO₂ nanotubes for the DSC was designed and prepared successfully. By using the nano-electrode as the photoanode of the DSC, the light-to-energy conversion efficiency of 5.42% was obtained.

Keywords

Dye-sensitized solar cell (DSC), electrode, TiO₂, nanotube, hydrothermal.

Introduction

With the decrease of energy resources and the improvement of the environmentalism, sustainable energy becomes more and more necessary and important. Experts estimated that the market demand for solar cells would keep on with an increasing rate of 25-30% annually. The DSC comes into the world from the last decades [1], with the advantages of low cost and simple processing. The schematic diagram of the DSC is shown in Figure 1. It is composed of transparent conductive glass (TCG), nanostructured TiO₂ electrode with absorbed dye, an electrolyte including redox of I^-/I₃^-, and counter electrode with evaporated Pt on TCG.

Figure 1. Schematic diagram and the working mechanism of DSC

It has been calculated that the theoretical conversion efficiency of the DSC is about 33%. The current highest efficiency, however, is only about 11% [2]. This is usually ascribed to the low transfer rate of the photo-induced electrons in TiO₂(see arrow 4 in Figure 1), which is about 10²~10⁰ s^-1, much lower than the recombination rate of the electrons and holes.

On the other hand, the TiO₂electrode is generally made up of nanocrystalline particles with a porous structure, which provides a rather large surface area. As shown in Figure 2, the surface area of the TiO₂ electrode decides the adsorption amount of the dye, which in turn directly affects the absorption of incident light.

Figure 2. Illustrated diagram of TiO₂ electrode with adsorbed dye.

In the present study, TiO₂ nanotubes (TNTs) were utilized in the electrode. Comparing with the nanocrystalline particles, TNTs may offer larger surface area and may increase charge transfer rate by the increase of diffusion length. The preparation methods, heat-treatment effects and photovoltaic properties of the nano-electrodes are now described and discussed.

Experimental

All chemicals were analytical grade and used without further purification. Commercial anatase titania powder (3.75 g, diameter: ~100nm) was dispersed in an aqueous solution of NaOH (10 M, 30 ml) and moved into a Teflon-lined autoclave [3]. The autoclave was heated at 130°C for 20 h. After hydrothermal treatment, the precipitate was repeatedly centrifugated and washed with distilled water until the pH value was near to 7~8. Subsequently, the above solution was filtrated and air-dried at 80°C to get an as-prepared sample. Finally, the as-prepared sample was annealed at 450°, 550° and 650°C for 1 h.

An aqueous paste was obtained by mixing 30% the above as-prepared sample with 70% nanocrystalline TiO₂ (P25, Degussa AG, Germany) powder. TiO₂ electrode was obtained by depositing a film (thickness: ~5µm) from the paste on a TCG (ITO), and heat-treating at 450°C for 1 h. A DSC was assembled using the above electrode and other traditional materials [4]. The electrodes were immersed for 8 h in a 3×10^-4M solution of the sensitizer dye, RuL₂(SCN)₃ (Solaronix, L = 4, 4’-dicarboxy-2, 2’-bipyridine) in pure ethanol. Pt sputtered ITO glass was used as a counter electrode. In the photochemical cell configuration, RuL₂(SCN)₃ /TiO₂ films on the ITO glass were employed in a sandwich-type cell incorporating Pt sputtered ITO glass and a non-aqueous electrolyte consisting of 0.04 M LiI, 0.02 M iodine in acetonitrile. Tertbutylpyridine (TBP) was used or not used in the electrolyte. The cell, whose active area was 0.123 cm², was tested under 30 mW irradiation with a 500 W xenon lamp. The photovoltaic properties of the DSC were measured using a Source Meter (Keithley-4200, Keithley Co. Ltd., USA).

Phase identification of the samples was carried out by X-ray diffraction analysis (XRD, RIGAKU, D/Max-RB, Japan). The morphology of the samples were observed by a transmission electron microscope (TEM, JEOL JEM-200CX, 200 kV, Japan). The crystallographic characteristics of the products were evaluated by selected-area electron diffraction (SAED), an accessory of TEM. Nitrogen adsorption-desorption measurements were carried out at 77 K using a Micromeritics ASAP 2010 to determine the Brunauer-Emmett-Teller (BET) surface area.

Results and Discussion

The XRD result of the samples annealed at different temperature is shown in Figure 3. The as-prepared sample was mainly composed of monoclinic H₂Ti₃O₇. At 450ºC, the sample was mainly composed of anatase TiO₂, with a trace of monoclinic H₂Ti₃O₇ and triclinic Ti₅O₉. When the temperature was increased, more anatase and triclinic Ti₅O₉were obtained in increasing amounts. With the increase of the heat-treating temperature to 650ºC, well-crystallized triclinic Ti₅O₉and anatase TiO₂ formed without other phase.

Figure 3: XRD patterns of the as-prepared sample (a) and samples heat-treated at 450°C (b), 550°C (c) and 650°C (d). (M: H₂Ti₃O₇; A: anatase TiO₂; T: Ti₅O₉).

Figure 4 shows TEM images of the as-prepared sample and the samples heat-treated at 450°, 550° and 650°C for 1 h. From Figure 4 (a) and its bottom right inset (HRTEM image), it can be clearly seen that all the raw TiO₂ particles changed to nanotubes at the present experimental condition, and all the nanotubes are open-ended with multiwall. Their inner diameter and outer diameter are approximately 3-5 nm and 8-12 nm, respectively. From Figure 3 we know that these as-prepared nanotubes are not TiO₂ but H₂Ti₃O₇ (titanate). After heat-treated at 450°C for 1 h, the titanate became to TiO₂, and the outer diameter of the nanotubes clearly increased (Figure 4 (b)) to about 20 nm. When the temperature was increased to 550° and 650°C, tubular crystals were hardly found in the sample and a great number of rod-shaped and granular crystals formed, and the size of these rod-shaped and granular crystals increased with the increase of the temperature.

Figure 4. TEM images of the as-prepared sample (a) and samples heat-treated at 450°C (b), 550°C (c) and 650°C (d).

The BET surface area of the as-prepared sample was up to 375.6 m²·g^-1. When the sample was heat-treated, its BET surface area decreased. The higher the temperature, the smaller the BET surface area of the sample. The BET surface area was 182.5 m²·g^-1 after heat-treated at 450°C, which is the highest BET surface area among the annealed samples. The surface area was reduced to 71.2 m²·g^-1 after heat-treated at 650°C.

As shown in Figure 3, the as-prepared sample is titanate (H₂Ti₃O₇) but not TiO₂, and the TiO₂ can be obtained by heat-treating the as-prepared sample at more than 450°C. Moreover, the sample heat-treated at 450°C has the largest surface area. Therefore, the electrode of our DSC was obtained by using 30% as-prepared samples, and heat-treating at 450°C for 1 h. This means that the electrode we used contains 30% TNTs with high surface area.

Figure 5 shows the typical current-voltage curves of the DSC with the TNTs electrode with and without the TBPmodification. It can be found that the photovoltage increases from 0.49 V to 0.63 V after TBP modification, which can be explained from Eqs. (1):

(1)

Where V_oc, q, (E_Fe_r_mi)_TiO2and E_R/R^-are the open circuit voltage, the electric charge transferred in a redox cycle, the quasi-Fermi energy level of TiO₂ and the Nernst potential of the redox couple (R/R^-), respectively. TBP and TNTs react to form a new complex, which increases (E_Fe_r_mi)_TiO2 and then V_oc also increased.

Figure 5. Current-voltage curves of the DSC with the TNTs electrode with and without the TBPmodification (30 mW irradiation).

It is also found that the photocurrent density is decreased from 5.8 mA·cm^-2 to 4.5 mA·cm^-2 after TBP modification, which can be ascribed to that TBP molecules is less conductive.

The light-to-energy conversion efficiency for the DSC with 30% TNTs as the electrode is up to 5.42% with TBP modification. In fact, when 100% P25 (without TNTs) was used as the electrode with TBP modification, the photocurrent density, photovoltage and conversion efficiency were 5.5 mA·cm^-2, 0.66 V and 7.68%. The conversion efficiency of DSC with 30% TNTs was lower than that without TNTs, which is ascribed to that the inner surface of the TNTs may not be fully used to adsorb the dye. Therefore, further investigation would focus on the adsorption process of the dye on TiO₂.

Conclusions

In this paper, the preparation method and heat-treatment effect of TNTs has been demonstrated. TNTs with relative high surface area were synthesized by a hydrothermal method at 130°C and then by heat-treatment at 450°C. The surface area and the outer diameters of the TNTs (the sample heat-treated at 450°C) are 182.5 m²·g^-1 and 20 nm, respectively. The TNTs are open-ended with multiwall. The light-to-energy conversion efficiency for the DSC with 30% TNTs is 5.42%.

Acknowledgement

The authors would like to express their gratitude to the support from The Project-sponsored by SRF for ROCS, SEM and the support from Tsinghua Basic Research Foundation (JCpy2005055).

References

B. O’Regan and M. Grätzel, “A Low-cost, High-efficiency Solar Cell Based on Dye-sensitized Colloidal TiO₂ Films”, Nature, 353 (24) (1991) 737-739.
M. Grätzel, “Conversion of Sunlight to Electric Power by Nanocrystalline Dye-sensitized Solar Cells”, J. of Photochem. and Photobiol. A: Chem., 164 (2004) 3–14.
T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, and K. Niihara, “Formation of Titanium Oxide Nanotube”, Langmuir, 14 (1998) 3160-3163.
C. J. Barbé, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover and M. Grätzel, “Nanocrystalline Titanium Oxide Electrodes for Photovoltaic Applications”, J. Am. Ceram. Soc., 80 (12) (1997) 3157-3171.