Probing titanate nanowire surface acidity through methylene blue adsorption in colloidal suspension and on thin films

• The interaction of methylene blue with titanate nanowires was investigated.
• Dye cations form monomers, dimers and larger aggregates on the TiONW films.
• The aggregation was observed at acidic, neutral and basic pH.
• At pH ⩽ 4.0 the TiONWs’ surface Brøntsted acid sites were protonating the dye.
• Reversible indicator role of the dye to probe the surface acidity was suggested.

The interaction of the cationic dye methylene blue (MB) with titanate nanowires (TiONWs) was investigated in different pH environments using visible spectroscopy and electrophoresis on thin films as well as in aqueous suspension. The surface charge of the TiONWs depends on the pH and ionic strength leading to positive charge under acidic and negative under alkaline conditions. The TiONWs have the same adsorption capacity on films and in suspensions at neutral pH while under alkaline conditions they are able to adsorb significantly more MB in suspension due to their higher surface area. Detailed adsorption studies in water revealed that dye cations form monomers, dimers and larger aggregates of H-type (face-to-face) on the TiONW films. The results indicate that below pH = 4.0 the TiONWs’ external surface consists of Brøntsted acid sites capable of protonating MB. It was suggested that reversible indicator role of MB molecule dimers probes the TiONW surface acidity (Brøntsted sites).

Figure optionsDownload high-quality image (110 K)Download as PowerPoint slide