Nanostructure Properties And Dye-sensitized-solar-cell open-circuit Voltage of A TiO2 Aerogel and Pre-Hydrothermally Treated Xerogels

Dye-sensitized solar cells (DSSCs) are third-generation photovoltaic devices, which are considered to be a very promising renewable-energy source that offers an alternative to fossil fuels due to their low cost, ease of production, and eco-friendliness. One of the most important components in DSSCs is the TiO₂ layer, which serves as an active inorganic semiconductor oxide for photoelectron activity. Herein, TiO₂ nanoparticles were synthesized via a sol-gel process using titanium tetra-n-butoxide, ethanol, hydrochloric acid, and deionized water at molar ratios of 0.4:0.83:1:1.39 upon sol preparation, followed by hydrothermal processes at three different temperatures (i.e., 100°C, 120°C, and 150°C); ambient drying; and multi-step calcination. For comparison, TiO₂ aerogel nanoparticles were also prepared via supercritical extraction followed by multi-step calcination. The samples were analyzed by X-ray diffraction, Brunauer–Emmett–Teller surface-area measurements, UV–Vis spectroscopy in the diffuse-reflectance-) mode, and scanning electron microscopy. The results showed that the pre-hydrothermally treated samples exhibited band-gap energies of 3.34, 3.29, and 3.32 eV after treatment at 100°C, 120°C, 150°C, respectively, whereas the aerogel sample had a band-gap energy of 3.33 eV. Open-circuit-voltage measurements revealed that the DSSCs fabricated by pre-hydrothermal treatment at 120°C generated a higher voltage (320 mV) than aerogel cells (21 mV).

Aerogel; Multi-step calcination; Open circuit voltage; Pre-hydrothermal treatment; Supercritical extraction

In addition, the use of TiO₂ is excellent from the viewpoint of its optical properties because it has a band-gap energy of ~3.2 eV with a wavelength of <380 nm (Langlet et al., 2002). Furthermore, the cystallinity and high-surface-area properties of the TiO₂ anatase nanostructure are preferred due to the high photoactivity of the material. Many studies are focusing on the methods to prepare TiO₂ to meet the required properties for photoanodes, e.g., by solid-state, sol-gel routes with various thermal treatments. The sol-gel method is one of the most useful techniques for synthesizing nanoparticles with high purity of precursor to obtain a high surface area, large pore volume, and uniform pore-size distribution.

A hydrothermal treatment was applied here to prepare TiO₂ anatase [instead of the supercritical extraction (SCE) method] because it offers several advantages, including high purities, a good distribution of the material, and increased photoactivity and crystallite size (Dai et al., 2010). Earlier studies on hydrothermal synthesis have shown good results for titanium-tetra-isopropoxide, which exhibited an open circuit voltage of 69.33 mV and a crystallite size of 12.46 nm (Yuwono et al., 2010). In contrast, the SCE technique involves CO₂ as the extraction solvent, which has the potential to augment the surface area of the nanoparticles but may also lead to a low degree of crystallinity. Thus, to overcome the crystallinity issue, both of them must be calcinated at various temperatures and different conditions. A modification of the simple nanostructure-synthesis process used in this study to obtain TiO₂ is the bottom-up method (sol-gel), as described above, which includes supercritical and hydrothermal treatments followed by gradual calcination. Synthesizing nanoparticles using the sol-gel method can improve the properties of TiO₂ structures regarding the degree of crystallinity, surface area, and photoactivity. Therefore, this study aims at designing a better preparation technique for optimum results, perform both treatments to improve the crystallite structure, and reduce the size of the band-gap-energy (Kim et al., 2007), coupled with a gradual calcination in some condition.

In our previous study, TiO₂ nanoparticles were successfully synthesized from titanium tetra-n-butoxide using the sol-gel method, followed by supercritical extraction and multi-step calcination, to obtain a highly crystalline aerogel with a vast surface area and increased photoelectrochemical sensitivity (Priyono et al., 2013). In the present study, the preparation technique has been modified to improve the nanostructure properties for use in DSSC devices. The method uses a xerogel combined with a hydrothermal process and followed by multi-step calcination to improve the crystallinity of all the samples while maintaining a minimum loss in surface area due to crystal growth. The results were then compared to those obtained for a TiO₂ aerogel prepared by the SCE method with the same multi-step calcination process as that demonstrated by (Brodsky & Ko, 1994).

The correlation between TiO₂ nanostructure properties such as crystallite size, surface area, and band-gap energy and the performance of the material as a semiconductor for better and more efficient DSSCs was investigated.

All the samples resulting from this research had a nanocrystalline structure and were successfully assembled into DSSCs to study their ability to convert light into electrical energy, as shown by their Voc values.

The properties of the TiO₂ aerogel reported herein exceeded those of materials presented in previous research, exhibiting a larger surface area and achieving a high enough crystallinity. Furthermore, compared to samples submitted to pre-hydrothermal treatment, the aerogel had the highest surface area (110.31 m²/g) and a crystallite size of 8.07 nm.

Regarding the crystallite size, the largest structures were obtained by pre-hydrothermal treatment at 150°C (9.73 nm), with a surface area of 82.17 m²/g. The size of the samples treated at 120°C was 7.79 nm (surface area: 85.43 m²/g) and that of the materials treated at 100°C was 7.45 nm (in this case the surface area was only 71.31 m²/g). Pre-hydrothermal treatment was performed to suppress the fast development of stiff Ti–OH structures, which lead to largely amorphous particles. This treatment transforms the Ti–OH network into a flexible Ti–O–Ti arrangement. Subsequent multi-step calcination enhances the crystallinity of the resulting TiO₂ particles further, thus confirming the findings of previous researchers.

Voc measurements revealed that DSSCs fabricated using the aerogel provided 21 mV. The best results were obtained for the pre-hydrothermally treated sample prepared at 120°C (320 mV), followed by the samples treated at 150° and 100°C, respectively.

The authors would like to acknowledge financial support from the Directorate of Research and Community Services - Universitas Indonesia (DRPM-UI) through Hibah Riset Madya Universitas Indonesia under contract no. DRPM/RII/175/RM-UI/2013.

Titania Aerogels. Journal of Materials Chemistry, Volume 4(4), pp. 651–652

Dai, S., Wu, Y., Sakai, T., Du, Z., Sakai H., Abe, M., 2010. Preparation of Highly Crystalline TiO₂ Nanostructures by Acid-assisted Hydrothermal Treatment of Hexagonal-structured Nanocrystalline Titania/Cetyltrimethyammonium Bromide Nanoskeleton. Nanoscale Research Letters, Volume 5(11), pp. 1829–1835

Ge, L., Xu, M., Fang, H., Sun, M., 2006. Preparation of TiO₂ Thin Films from Autoclaved Sol Containing Needle-like Anatase Crystals. Applied Surface Science, Volume 253(2), pp. 720–725

Khan, M.Z.H., Al-Mamun, M.R., Halder, P.K., Aziz, M.A., 2017. Performance Improvement of Modified Dye-sensitized Solar Cells. Renewable and Sustainable Energy Reviews, Volume 71, pp. 602–617

Kim, D.S., Han, S.J., Kwak, S.Y., 2007. Synthesis and Photocatalytic Activity of Mesoporous TiO₂ with the Surface Area, Crystallite Size, and Pore Size. Journal of Colloid and Interface Science, Volume 316(1), pp. 85–91

Langlet, M., Kim, A., Audier, M., Herrmann, J.M., 2002. Sol-Gel Preparation of Photocatalytic TiO₂ Films on Polymer Substrates. Journal of Sol-Gel Science and Technology, Volume 25(3), pp. 223–234

Naghibi, S., Sani, M.A.F., Hosseini, H.R.M., 2014. Application of the Statistical Taguchi Method to Optimize TiO₂ Nanoparticles Synthesis by the Hydrothermal Assisted Sol-Gel Technique. Ceramics International, Volume 40(3), pp. 4193–4201

Nursama, N.M., Muliani, L., 2012. Investigation of Photoelectrode Materials Influences in Titania-Based-Dye-Sensitized Solar Cell. International Journal of Technology, Volume 3(2), pp. 129–139

O’Regan, B., Grätzel, M., 1991. A Low-cost, High-efficiency Solar-Cell based on Dye-sensitized Colloidal TiO₂ Films. Nature, Volume 353, pp. 737–740

Priyono, B., Yuwono, A.H., Munir, B., Rahman, A., Maulana, A., Abimanyu, H., 2013. Synthesis of Highly-ordered TiO₂ through CO₂ Supercritical Extraction for Dye-sensitized Solar Cell Application. Advanced Materials Research, Volume 789, pp. 28–32

Schneider, M., Baiker, A., 1997. Titania-based Aerogels. Catalysis Today, Volume 35(3), pp. 339–365

Slamet, Nasution, H.W., Purnama, E., Kosela, S., Gunlazuardi, J., 2005. Photocatalytic Reduction of CO₂ on Copper-doped Titania Catalysts Prepared by Improved-impregnation Method. Catalyst Communications, Volume 6(5), pp. 313–319

Sofyan, N., Ridhova, A., Yuwono, A.H., Udhiarto, A., 2017. Fabrication of Solar Cells with TiO₂ Nanoparticles Sensitized using Natural Dye Extracted from Mangosteen Pericarps. International Journal of Technology, Volume 8(7), pp. 1229–1238

Sugathan, V., John, E., Sudhakar, K., 2015. Recent Improvements in Dye Sensitized Solar Cells: A Review. Renewable and Sustainable Energy Reviews, Volume 52, pp. 54–64

Wang, C.-C., Ying, J.Y., 1999. Sol-Gel Synthesis and Hydrothermal Processing of Anatase and Rutile Titania Nanocrystals. Chemical Materials, Volume 11(11), pp. 3113–3120

Yuwono, A.H., Munir, B., Ferdiansyah, A., Rahman, A., Handini, W., 2010. Dye Sensitized Solar Cell with Conventionally Annealed and Post Hydrothermally Treated Nanocrystalline Semiconductor Oxide TiO₂ Derived from Sol-Gel Process. Makara Journal of Technology, Volume 14(2), pp. 53–60

Yuwono, A.H., Xue, J., Wang, J., Elim, H.I., Ji, W., 2006. Titania-PMMA Nanohybrids of Enhanced Nanocrystallinity. Journal of Electroceramics, Volume 16(4), pp. 431–439