Low-Temperature and Surfactant-Free Synthesis of Mesoporous TiO2 Sub-Micron Spheres for Efficient Dye-Sensitized Solar Cells
Introduction
Dye sensitized solar cells exhibit characteristics of low cost, easy fabrication, and relatively high power conversion efficiency (PCE) and these advantages over other photovoltaic devices render it a competitive candidate as one of the next generation power sources[1], [2], [3], [4], [5], [6]. Typical DSSCs consist of three parts: dye-sensitized TiO2 photoanode, Pt counter electrode, and iodide/triiodide redox electrolyte. Great efforts have been made to improve the performance of the DSSCs by tailoring the photoanode structures, developing novel dye molecules and Pt-free counter electrode, and optimizing the content of the electrolyte[7], [8], [9], [10], [11], [12], [13], [14], [15]. Among the factors affecting the PCE value, the structure and geometry of the photoanode is of great importance since it not only determines the dye loading and thus light harvesting ability but also provides an electron transport pathway which significantly influences the charge transport and collection efficiency.
Efficient DSSCs typically utilize TiO2 nanoparticles to form a nanoporous structure with a high surface area for dye loading. Recently, much attention has been devoted to one-dimensional nanostructures such as nanowires and nanotubes exhibiting direct electrical pathways for photogenerated electrons[16], [17], [18], [19], [20]. Nevertheless, the efficiency is limited by the insufficient surface area for dye loading. Meanwhile, the most widely used ruthenium dyes show much reduced light absorption in the wavelength range of 600–800 nm and nanosized TiO2 particles are poor scattering centers for light in the visible and near-infrared region, so that a considerable part of light can transmit directly through the thin film without being absorbed by dye molecules. By adding an upper scattering layer, the light harvesting ability of the photoanode can be largely enhanced. According to the Mie theory, nanostructures with a size comparable to the wavelength of visible light are suggested and scattering layers composed of different structures and morphologies have been widely investigated[21], [22]. However, introduction of large particles with low surface areas degrades the dye loading capacity of the photoanode and thus makes limited contribution to the photoelectron generation.
The incompatibility of the dye loading and light scattering efficiency can be resolved by using mesoporous sub-micron spheres with dual functions of satisfactory dye loading and scattering effect. A synthetic method of mesoporous beads by combining sol-gel and solvothermal process has been successfully developed by Chen, resulting in a high PCE of 8.84%[23]. Similar results have also been reported by Lee et al.[24] and Dai et al.[25]. Other synthetic strategies such as template assisted growth and electrochemical synthesis have also been reported to obtain the hierarchical TiO2 structures[26], [27], [28].
Unfortunately, above mentioned multi-step approaches are usually performed with participation of organic additives as chelating agents, concentrated alkaline solution or oxidizing precursors; thus, most of which are not environmentally friendly. Meanwhile, special conditions including solvothermal process, high-temperature calcination as well as a long reaction time are indispensible to complete the reaction and mainly responsible for an increase in the cost. So it remains a great challenge to develop a facile and green method to synthesize the hierarchical TiO2 structure. In traditional DSSCs configurations, anatase TiO2 has been mostly used for its excellent electrochemical performance. There are, as far as we are concerned, rare reports about mesoporous rutile nanostructures and their applications in DSSCs[29], [30]. Rutile TiO2, in fact, has some unique advantages over anatase, such as better chemical stability, higher refractive index, and lower cost[6].
Herein, we report a one-step, low-temperature synthesis of rutile TiO2 mesoporous submicrometer-sized spheres without surfactants and subsequent hydrothermal treatment. We also investigate their application in DSSCs as light scattering centers on top of a dense TiO2 nanoparticle layer.
Section snippets
Material synthesis
Transparent TiCl4 aqueous solution (0.2 mol/L) was prepared by adding dropwise 2.2 mL pre-cooled TiCl4 into 100 mL ice water mixture under vigorous magnetic stirring. In the synthesis process, 50 mL TiCl4 aqueous solution (0.2 mol/L) was transferred into a beaker and treated in an electric oven at 70 °C for 75 min. Then the beaker was taken outside the oven promptly and the white precipitants were collected by centrifugation at 6000 r min–1 for 5 min at room temperature and washed with ethanol
Results and Discussion
The morphology of the as-obtained products was characterized by SEM and TEM. Fig. 1(a) shows the SEM image of the as-synthesized monodisperse TiO2 spheres with an average diameter of 600 nm. The surfaces of the spheres are rough and composed of needle-like structures, whose formation is mainly due to the presence of Cl– ions in the precursor solution which can preferably adsorb and retard the growth of (110) surfaces[31]. X-ray diffraction peaks can be well indexed to the TiO2 rutile phase
Conclusion
In summary, mesoporous rutile TiO2 spheres with sub-micron size were synthesized through the hydrolysis of TiCl4 aqueous solution at ambient temperature of 70 °C without adding surfactants. Scattering layer composed of such spheres on top of TiO2 nanoparticle layer enables the bilayered nanostructure to possess pronounced light scattering efficiency and excess surface area for dye loading, leading to an improved conversion efficiency of 8.43%, compared to 7.15% for the reference nanoparticle
Acknowledgments
We acknowledge the support from the National Natural Science Foundation (Nos. 51422206, 51372159, 11304217), the 1000 Youth Talents Plan, the National Program on Key Basic Research Project of China (973 Program, No. 2015CB358600), the Jiangsu Shuangchuang Plan, the Project Supported by Jiangsu Science and Technology Committee for Distinguished Young Scholars (BK20140009) and funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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