Synthesis of TiO2 hollow spheres by selective etching of Au@TiO2 core-shell nanoparticles for dye sensitized solar cell applications

Heterostructured Au@TiO2 core-shell nanoparticles (NPs) were synthesized by a microwave assisted hydrothermal method. A colloidal method was used to synthesize 40 +/- 5 nm Au NPs, whereas a microwave-assisted hydrothermal method was used to deposit a TiO2 shell layer with 60 +/- 10 nm shell thickness on Au NPs. The average size of TiO2 NPs was 17 +/- 2 nm and size was increased with increasing reaction temperature without considerable change in shell thickness. The stability of Au@TiO2 core-shell NPs in iodide electrolyte solution was examined. It was found that the Au NPs are unstable in the iodide electrolyte and lost their surface plasmon resonance (SPR) characteristics. Hollow TiO2 NPs (150-200 nm in diameter) were produced by selective etching of as-prepared Au@TiO2 core-shell NPs in KCN solution. The final hollow TiO2 spheres were applied as a scattering layer on top of a nanocrystalline TiO2 film, serving as the photoanode of dye sensitized solar cells (DSCs). A high efficiency of 7.40% was achieved with TiO2 hollow spheres, compared with 5.21% for the electrode with commercial TiO2. It was also found that the efficiency increased with increasing crystallinity of TiO2 NPs. The increment in efficiency was related to efficient light scattering, electrolyte diffusing feasibility for better electron transport, and a high surface area for higher dye loading.