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 TiO ₂ 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.