Synthesis Of Hierarchical TiO₂Honeycomb Nanostructures And Their Application In Dye-sensitized Solar Cells

Recently, the assembly of hierarchical array nanostructures has attra cted wideconcern owing to their special properties such as superior specific surface area,abundant and tunable multi-level interior structures, and multiphase anisotropicinterfaces. However, due to the structural complexity, developing the hierarchicalTiO₂nanostructure with the controllable morphologies and high surface area stillremains challenging. Therefore, it is desirable to find a simple, convenient, and lowcost approach to fabricate novel TiO₂nanostructures in a facile reaction system.In this study, highly-ordered hierarchical TiO₂nanostructures involving primaryhoneycomb and secondary nanoparticles and nanowires are prepared via a two-stepfacile process. The TiO₂nanotube arrays grow firstly on Ti foil through anodization.After the wet-chemical reaction of the TiO₂nanotube arrays with alkaline and acidsolution in turn, the hierarchical nanostructures wire-in-honeycomb and poroushoneycomb are obtained via a dissolution-coagulation and dissolution-adsorptionmechanism, respectively. The effect of different acid, time of acid treatment, diameterof nanotube and crystallization condition on the morphology of hierarchical TiO₂nanostructures was studied. Current–Voltage characteristics of hierarchical TiO₂nanostructures DSSC was measured under AM1.5illumination. Furthermore, westudy methods of detachment and transfer of hierarchical TiO₂arrays on FTO tofabricate front-illuminated dye-sensitized solar cells （DSSCs）. The performance ofTiO₂/FTO as an electrode of DSSC with different thickness was investigated.The results show that （1） The acid treatment plays an important role in formingdifferent hierarchical TiO₂nanostructures. The strong acid could impel thenanoclusters forming nanowires, while the week acid could enhance the dissociationof TiO₂nanoclusters and forming porous honeycomb nanostructure.（2） Theformation process of the hierarchical TiO₂honeycomb nanostructures was furtherinvestigated by time-dependent acid treatment. As the prolonging of the HCltreatment time, the diameter of nanowires decreases and even disappears. The optimalphotovoltaic conversion effciency of DSSC achieved3.48%after reacting with HClfor30min. As the prolonging of the CH3COOH treatment time, the thickness ofhoneycomb increases. The optimal photovoltaic conversion effciency of DSSC achieved5.73%after reacting with CH₃COOH for2h.（3） The free-standing TiO₂nanotubes could be prepared by HF treatment and voltage pluses. The one-end openmembrane was obtained by HF treatment, while the through-hole membrane obtainedby voltage pluses.（4） The photovoltaic conversion effciency firstly increased withthickness of the membrane, and then decreased. The optimal photovoltaic conversioneffciency achieved2.89%.