Characterization and Reaction Mechanisms of Gold-Doped Anatase TiO2 Nanotube Arrays Using Anodic Oxidation for Dye-Sensitized Solar Cells

1. Introduction

Dye-sensitized solar cells (DSSCs) are considered as a new generation of solar cells due to their low cost and high performance [1-4]. DSSCs based on a TiO₂ nanoparticle (TiO₂ NP) network with air mass (AM) 1.5 solar efficiencies of more than 10% have been demonstrated. However, further enhancement in power conversion efficiency (PCE) has been difficult to achieve partly due to charge recombination and a reduced electron transport rate through the nanocrystalline photoanodes. Metal nanoparticles can contribute to the effective light absorption of solar cells, both by local field enhancement through the localized surface plasmon resonance and by light scattering leading to prolonged optical-path lengths [5]. Many researchers demonstrated that the size of the metal nanoparticles is a key factor for determining the plasmonic phenomena, and especially, light scattering occurs more dominantly than light absorption as the size of the metal nanoparticles increases [6]. In this work, TiO₂ nanotubes (TNs) array synthesized by anodization method had a much organized standing nanotubes and tubes with uniform length, thickness, and diameter. The anodic oxidation process was carried out at 30 V, 25°C, 0.3 wt% NH₄F + 3 vol% H₂O as electrolyte and an anodization time of 1 h. The DSSC with TNs array as working electrode had fill-factor of 77.94%, V_oc of 0.69 V, I_sc of 6.85 mA/cm², and 3.68% of efficiency. Since TNs array had roughly 1.2% higher efficiency than randomly stacked TNs, I have decided to further enhance the TNs array with gold nanoparticles. The DSSC with gold-lined TNs array as working electrode had fill-factor of 81.09%, V_oc of 0.72 V, I_sc of 7.54 mA/cm², and finally an efficiency of 4.40%.

2. Experimental

Once the Ti plate was cleaned thoroughly, apply Teflon^® tape at the back of the plate to avoid oxidation on the back of the Ti plate. Clamp the plate with positive (anode) charge. The buffer solution used for the anodization is a mixture of 0.25 wt% NH₄F plus 3 vol% H₂O in ethyl glycol, stirred for one hour. The optimized voltage, setting temperature, and oxidation time are 30 V, 25°C, and one hour, respectively. After anodization, the Ti plate is immersed in ethanol and place in an ultrasonic bath to remove any excess contaminations, and then dried in room temperature overnight. Synthesize TNs array using optimized parameter setting, after annealing process, dipped the Ti foil into 1 mM of AuCl₄ solution for 5 min, rinsed with deionized water, then dipped the foil into 0.15 M NaBH₄ solution for another 5 min.

To fabricate DSSCs, TiO₂ nanotube arrays with or without Au modifications were soaked with N719 dye was added tert-butanol and acetonitrile solution, such mixing ratio 1:1 to prepare a concentration of 5 × 10^-4 M dye solution. The DSSCs were finally packed by sealing the dye-coated electrode with a thermally platinized FTO counter electrode through a thin thermal plastic film (Surlyn, 25 ƒÝm, Solaronix). An electrolyte composed of 0.1 M N-methyl benzimidiazoium, 0.6 M 1-propyl-3-methylimidiazolium iodide, 0.05 M I₂, 0.1 M guanidinium thiocyanate, and 0.2 M NaI in 3-methoxypropionitrile was introduced into the cell through a pre-drilled hole in the counter electrode. The hole was subsequently sealed with a microscope slip using Surlyn film.

3. Results and Discussion

Fig. 1 Gold nanoparticles within TNs array.

Fig. 2           I-V characteristic curves of DSSCs with Au nanoparticles by anodic oxidation at different voltage.

4. Conclusions

In conclusion, anatase TiO₂ nanotube arrays with Au nanoparticles was synthesized by anodic oxidation. At the deposition voltage of 30 V and Au-doped TiO₂ nanotube arrays, the η, Jsc, and FF all reached the maximum. Compared with the conventional DSSCs Jsc, Voc, and η are 7.54 mA/cm², 0.72 V, and 4.4%. Au-doped DSSCs has evident adsorption peak in visible range, indicating that Au-doped TiO₂nanotubes are hopeful to become visible light photocatalyst.

 

References

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[2]K. S. Lin et al., Adsorption, 16, pp.47–56, 2010.

[3] Mor G.K et al., Solar Energy Mater. Solar Cells, 90, (14), pp.2011-2075, 2006.

[4]H. E. Wang et al., Appl. Phys. Lett., 96, 263104, 2010.

[5]H. A. Atwater et al., Nature Mater., 9, pp.205-213, 2010.

[6]D. D. Evanoff et al., J. Phys. Chem. B., 108, pp.13957-13962, 2014.