Adsorption of N719 Dye on Anatase TiO2 Nanoparticles and Nanosheets with Exposed (001) Facets: Equilibrium, Kinetic, and Thermodynamic Studies

Anatase TiO2 nanosheets (TiO2 NS) with dominant (001) facets and TiO2 nanoparticles (TiO2 NP) with dominant (101) facets are fabricated by hydrothermal hydrolysis of Ti(OC4H9)(4) in the presence and absence of hydrogen fluoride (HF), respectively. Adsorption of N719 onto the as-prepared samples from ethanol solutions is investigated and discussed. The adsorption kinetic data are modeled using the pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetics equations, and indicate that the pseudo-second-order kinetic equation and intraparticle diffusion model can better describe the adsorption kinetics. Furthermore, adsorption equilibrium data of N719 on the as-prepared samples are analyzed by Langmuir and Freundlich models; this suggests that the Langmuir model provides a better correlation of the experimental data. The adsorption capacities (q(max)) of N719 on TiO2 NS at various temperatures, determined using the Langmuir equation, are 65.2 (30 degrees C), 68.2 (40 degrees C), and 76.6 (50 degrees C) mg g(-1), which are smaller than those on TiO2 NP, 92.4 (30 degrees C), 100.0 (40 degrees C), and 108.2 (50 degrees C) mg g(-1), respectively. The larger adsorption capacities of N719 for TiO2 NP versus NS are attributed to its higher specific surface areas. However, the specific adsorption capacities (q(max)/S-BET) at various temperatures are 1.5 (30 degrees C), 1.6 (40 degrees C), and 1.7 (50 degrees C) mg m(-2) for TiO2 NS, which are otherwise higher than those for NP, 0.9 (30 degrees C), 1.0 (40 degrees C), and 1.1 (50 degrees C) mg m(-2), respectively. The larger specific adsorption capacities of N719 for TiO2 NS versus NP are because the (001) surface is more reactive for dissociative adsorption of reactant molecules compared with (101) facets. Notably, the q(max) and q(max)/S-BET for both TiO2 samples increase with increasing temperature, suggesting that adsorption of N719 on the TiO2 surface is an endothermic process, which is further confirmed by the calculated thermodynamic parameters including free energy, enthalpy, and entropy of adsorption process. The present work will provide a new understanding on the adsorption process and mechanism of N719 molecules onto TiO2 NS and NP, and this should be of great importance for enhancing the performance of dye-sensitized solar cells.