Molecular dynamics simulation of the effect of pH on the adsorption of rhodamine laser dyes on TiO₂ hydroxylated surfaces

Abstract

We have carried out a study of adsorption, on the (1 0 1) surface of anatase TiO₂, of two industrially relevant rhodamine molecules [rhodamine 6G (R6G) and rhodamine 800 (R800)] employing molecular dynamics. These theoretical studies have shown that R6G must adsorb on surfaces under basic conditions. Moreover, the adsorption of this molecule shows a strong dependence upon the pH of the system, i.e. under basic conditions the adsorption energy is quite high, under neutral conditions the adsorption energy is lower and under acidic conditions an even lower adsorption energy indicates that there must be very little adsorption under such conditions. By contrast, for R800, there is little dependence of the adsorption energy upon the pH, suggesting that the amount of adsorption of these molecules is little affected by this parameter. These theoretical results are in qualitative agreement with the experimental results consisting of the incorporation of these dye molecules into porous thin films.

Keywords:

• rhodamine
• adsorption
• TiO₂
• anatase
• (1 0 1) surface
• molecular dynamics
• PZC

Acknowledgements

We dedicate this work to the memory of José Antonio Mejías, an excellent friend and a good scientist, who put the first stones of this research line in our group. We will always remember him kindly. We thank the Ministry of Science and Education of Spain (project MAT 2007-65764/NAN2004-09317 and the CONSOLIDER INGENIO 2010-CSD2008-00023) and the Junta de Andalucía (projects TEP2275 and P07-FQM-03298) for the financial support. We would also like to thank the EU for the financial support provided by the EU contract no. 033793.