International Journal of PhotoenergyVolume 2011 (2011), Article ID 373210, 4 pagesdoi:10.1155/2011/373210

Research Article

A Convenient Method for Manufacturing TiO₂ Electrodes on Titanium Substrates

Wei Qin,¹ Songtao Lu,² Xiaohong Wu,² and Song Wang²

¹School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China²Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China

Received 11 June 2011; Revised 15 September 2011; Accepted 15 September 2011

Academic Editor: Leonardo Palmisano

Copyright © 2011 Wei Qin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Thin titanium dioxide films were successfully prepared on titanium plates in ammonium sulfate solution with the micro-plasma oxidation method. The thin TiO₂ films were sensitized with a cis-RuL₂(SCN)₂·2H₂O (L = cis-2, -bipyridine-4, -dicarboxlic acid) ruthenium complex, and implemented into a dye sensitized solar cell configuration. The influence of current density on the surface structure and photoelectric performance of the TiO₂ films was investigated. The results show that the thin TiO₂ films are porous, and the dye-sensitized solar cell based on the film prepared at 14 A/dm² has exhibited higher overall light-to-electricity conversion efficiencies of 0.095% under the illumination at 40 mW/cm².

1. Introduction

The dye-sensitized solar cell (DSSC) has attracted much attention as the next generation solar cell during the last decade [1, 2]. Remarkably, high enough efficiency and low cost of manufacturing are important characteristics to make the DSSC as a substitute of the conventional silicon and thin film photovoltaic devices. Various methods of preparing dye-sensitized solar cells have therefore been developed [3–5]. At present, the TiO2 photoelectrode of DSSC is usually prepared by depositing a suspension or paste-containing TiO₂ nanoparticles with organic additives onto conductive glass substrates or polymer substrate [6, 7]. The deposited film is then subject to a posttreatment with the purpose of forming a continuous nanoparticle network with sufficient adherence and electrical contact to the substrate and between the nanoparticles. Although the conventional preparation method of using conductive glass substrate can achieve good interconnection between particles, a batch process must include heat treatments, which is not fast enough to produce the necessary devices. In addition, the use of glass substrates with frangibility limits the manufacture process and the practical application of DSSCs. The use of plastic substrates is another choice of the TiO₂ suspension [8, 9]. However, these methods present a weak adherence of the films to the substrates, and obtain very thin films.

In this paper we, therefore, looked into the possibility of developing microplasma oxidation (MPO) method to prepare TiO₂ thin films on the thin light titanium substrates. This method is based on the anodic oxidation, which occurs at potentials above the breakdown voltage of the oxide film growing on the anode surface, such as Al, Mg, Ti, Nb, and Zr. As the process combines electrochemical oxidation with a high voltage spark treatment in an electrolyte bath, metal oxides are synthesized inside high voltage breakthrough channels across the former oxide layer. So the prepared thin oxide films have good adherence with substrate metal and can endure strong impact [10, 11]. In addition, the process of preparing thin films by MPO need very short time. In this paper, MPO in the ammonium sulfate solution with different current densities was used to prepare TiO₂ films on the surface of Ti substrate. The objective of this research was to investigate the structure and surface morphology of the films and measure their photoelectricity performance as photo anode of the DSSC.

2. Experimental

2.1. Preparation of Films

Plate samples of a titanium sheet (99.9% in purity) with a reaction dimension of 2 mm² were washed in 40% HF and 65% HNO₃ (1 : 1 in volume) aqueous solution. A home made-electrical source with the power at 5 kW was used for microplasma oxidation of the samples in a water-cooled electrolyte, and a copper sheet serves as the counterelectrode. The reaction temperature was controlled to below 30°C by adjusting the cooling water flow. The electrolyte used in the experiment is ammonium sulfate solution (0.5 mol/L). The set-up scheme is shown in the past report [12]. The whole MPO process was carried out under different current densities (12, 14, 16, and 18 A/dm²) for 10 min. After the treatment, the coated samples were flushed with water and dried in air. Then, these dried TiO₂ films were sensitized by cis-RuL₂(SCN)₂·2H₂O in the anhydrous ethanol (3 mmol/L) at 40°C for 12 h.

2.2. Characterization of Films

The photoelectrochemical experiments were performed in two sandwich-type electrode cells. The dye-coated TiO₂ film was used as working electrode, and transparent conducting glass (<20/Ω) as counter electrodes. A drop of electrolyte was then placed between the electrodes, and allowed to wet the surfaces of the electrodes by capillary action. The electrolyte was a solution of 0.5 M potassium iodide and 0.05 M iodine in a mixture of Ca. 80% acetonitrile and 20% glycol. For the photocurrent-photovoltage measurements, the dye-sensitized TiO₂ films were illuminated through the conductive glass using a 500-W high pressure Xe lamp with a water IR filter, and a 420 nm long pass UV filter served as a light source as the simulating sunlight.

The surface morphology of the films was observed on an S-570 scanning electron microscope (SEM) from Hitachi. The X-ray diffraction (XRD) with a Cu K_source (D/max-r B from Ricoh) was applied to study the crystalline structure of the films with an accelerating voltage and an applied current of 40 kV and 30 mA, respectively. Surface roughness of the TiO₂ films were examined with a digital Instruments Nanoscope III atomic force microscope. The thickness of the films are measured by CTG-10.

3. Results and Discussion

3.1. Photoelectricity Properties of the Films

The distinct structure of TiO₂ films lead to dissimilar photoelectricity properties. A I–V curves between the TiO₂ films prepared with different current density are given in Figure 1. Table 1 shows the averaged data extracted from I–V curve measurement on dye-sensitized nanostructured TiO₂ electrode.

Table 1: Results from I–V characteristics of TiO₂ electrodes prepared at different current density.

Type (A/dm²)
(mV)
(μA/cm²)

η (%)

12
619
112
0.35
0.061

14
652
149
0.39
0.095

16
581
95
0.32
0.044

18
542
76
0.33
0.034

Figure 1: Results from I–V characteristics of TiO₂ electrodes prepared at different current density.

As is shown in Figure 1 and Table 1, the overall efficiency (), open circuit voltage (), and short circuit current () of the dye-sensitized solar cells firstly increase and then decrease with current density of MPO. The increases from 619 to 652 mV, while the increases from 112 to 149 μA/cm² when the current density increase from 12 A/dm² to 14 A/dm². The and the reach maximum at 14 A/dm2 and then decrease at the 16 and 18 A/dm². The highest conversion efficiency of 0.095% has been achieved for the cell, employing the film prepared at 14 A/dm².

3.2. Morphology of the Films

The TiO2 films prepared at different current densities have different surface images (see Figure 2). It can be seen that the surface of prepared films are mesoporous and the microporous size increase with the current density. The mean roughness values of the TiO₂ films prepared at 12, 14, 16, and 18 A/dm2 are 110.25, 138.65, 131.36, and 128.49 nm, respectively. When the current density is 14 A/dm², the TiO2 film obtains the largest roughness and then decreases with the increasing of the current density. The rough surface is propitious to absorb the sensitizer. The thickness of the films prepared at 12, 14, 16, and 18 A/dm² are 3.7 μm, 5.6 μm, 7.9 μm, 12.5 μm, respectively.

(a)

(b)

(c)

Figure 2: SEM images under different current densities: (a) 12 A/dm², (b) 14 A/dm², and (c) 16 A/dm².

3.3. Structural Analysis of the Films

Figure 3 shows crystalline structures of the TiO₂ films. It can be noticed that the films consist of much rutile phase and less Ti substrates when current density is below 14 A/dm². The content of rutile TiO₂ reaches almost 100% at the current density of 16 and 18 A/dm². And the disappearance of Ti substrate peak could result from the increase of film thickness.

Figure 3: XRD of films prepared under different current densities.

From I–V curve and SEM photographs, it can be seen that TiO₂ crystallite and pores are formed on the surface of the Ti substrate, and this kind of films have photoelectricity properties. From SEM photographs, the surface grain size and the density of the pores reach the maximum at 14 A/dm². These changes could improve the photoelectricity properties of the films because more mesopores can absorb more OH⁻ to absorb the cis-RuL₂(SCN)₂·2H₂O, which can increase the utilization ratio of visible light. So, the overall efficiency, short circuit current and open circuit voltage of the TiO₂ film prepared at 14 A/dm² are higher than that of the films prepared at 12 A/dm². When the current density of MPO is up to 16 A/dm², the efficiency of the fabricated cell decline. The reason is that the reaction temperature increases simultaneity with the increasing of the current density, and many bigger blocks TiO₂ are formed around the pores (see Figure 2). The sample showed inferior performance owing to the decreased surface area, the reduction in the pore size, which is related to the amount of dye adsorption.

4. Conclusion

In conclusion, uniform and porous thin TiO₂ films have been successfully prepared with the microplasma oxidation method in the (NH₄)₂SO₄ electrolyte solution. This method can prepare TiO2 electrode conveniently. A higher photoelectric performance of TiO₂ electrode is obtained when the electrode is prepared by MPO under the current density of 14 A/dm². The dye-sensitized solar cell using this TiO₂ photoanode exhibited the overall conversion efficiency of 0.095% (AM-1.5, 40 mW/cm²).

Acknowledgments

The authors thank the National Natural Science Foundation of China (nos. 51173033, 51078101) the Program for New Century Excellent Talents in University (NCET-09-0064) and Heilongjiang Natural Science Foundation (no. B2007-04) for the financial support for this work.

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