Adsorption of N719 dye on anatase TiO2 nanoparticles and nanosheets with exposed (001) facets: equilibrium, kinetic, and thermodynamic studies

Abstract

Anatase TiO(2) nanosheets (TiO(2) NS) with dominant (001) facets and TiO(2) nanoparticles (TiO(2) NP) with dominant (101) facets are fabricated by hydrothermal hydrolysis of Ti(OC(4)H(9))(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 TiO(2) NS at various temperatures, determined using the Langmuir equation, are 65.2 (30 °C), 68.2 (40 °C), and 76.6 (50 °C) mg g(-1), which are smaller than those on TiO(2) NP, 92.4 (30 °C), 100.0 (40 °C), and 108.2 (50 °C) mg g(-1), respectively. The larger adsorption capacities of N719 for TiO(2) 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 °C), 1.6 (40 °C), and 1.7 (50 °C) mg m(-2) for TiO(2) NS, which are otherwise higher than those for NP, 0.9 (30 °C), 1.0 (40 °C), and 1.1 (50 °C) mg m(-2), respectively. The larger specific adsorption capacities of N719 for TiO(2) 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 TiO(2) samples increase with increasing temperature, suggesting that adsorption of N719 on the TiO(2) 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 TiO(2) NS and NP, and this should be of great importance for enhancing the performance of dye-sensitized solar cells.