pH-Dependence of Binding Constants and Desorption Rates of Phosphonate- and Hydroxamate-Anchored [Ru(bpy)₃]²⁺ on TiO₂ and WO₃

Abstract

The binding constants and rate constants for desorption of the modified molecular dye [Ru(bpy)] anchored by either phosphonate or hydroxamate on the bipyridine ligand to anatase TiO and WO have been measured. In aqueous media at pH 1-10, repulsive electrostatic interactions between the negatively charged anchor and the negatively charged surface govern phosphonate desorption under neutral and basic conditions for TiO anatase due to the high acidity of phosphonic acid (p K = 5.1). In contrast, the lower acidity of hydroxamate (p K = 6.5, p K = 9.1) leads to little change in adsorption/desorption properties as a function of pH from 1 to 7. The binding constant for hydroxamate is 10 in water, independent of pH in this range. These results are true for WO as well, but are not reported at pH > 4 due to its Arrhenius acidity. Kinetics for desorption as a function of pH are reported, with a proposed mechanism for phosphonate desorption at high pH being the electrostatic repulsion of negative charges between the surface and the anionic anchor. Further, the hydroxamic acid anchor itself is likely the site of quasi-reversible redox activity in [Ru(bpy)(2,2'-bpy-4,4'-(C(O)N(OH)))], which does not lead to any measurable deterioration of the complex within 2 h of dark cyclic voltammogram scans in aqueous media. These results posit phosphonate as the preferred anchoring group under acidic conditions and hydroxamate for neutral/basic conditions.

First-Principles Modeling of Dye Anchoring on (001) γ-Monoclinic WO3 Surfaces: The Role of Oxygen Vacancies

“…It is worthwhile to stress that the possibility to realize a bidentate anchoring between the dye and the metal oxide surface has been shown to be important for maximizing the efficiency of the interfacial electron transfer processes and for the overall device stability and performance. The present results, providing for the first time atomic-scale details on the dye anchoring mechanism on monoclinic WO 3 substrates, nicely rationalize the experimental evidence of a lower binding strength and surface coverage of carboxylic and phosphonic acids groups onto WO 3 substrates with respect to TiO 2 surfaces . Our findings thus suggest novel design rules for the optimization and the selection of alternative anchoring functionalities, better matching the surface W–W patterns, for optimized WO 3 -based DSSCs and hybrid photocatalysts.…”

“…WO 3 functionalization with polydopamine (PDA) has been also reported in the context of photocatalysis and immunosensor devices, , indicating the formation of a stable interface between PDA and WO 3 via catechol functionalities. Also, the binding constants of the [Ru­(bpy) 3 ] 2+ dye anchored by either a phosphonate or a hydroxamate group to TiO 2 and WO 3 surfaces are reported in ref , revealing, again, a stronger dye binding and a higher surface coverage on TiO 2 than on WO 3 . The lower isoelectric point of WO 3 and the differences in the acidity of Lewis sites between the two metal ions (Ti 4+ serves as stronger Lewis’s acid site compared to W 6+ ) were individuated as the main reasons of the different binding constants.…”

“…On the other hand, phosphonic acid provides excellent anchoring stability, with an adsorption strength estimated to be approximately 80 times higher than that of the carboxylic acid, and negligible desorption in the presence of water. , Many alternative anchoring groups have been proposed and been tested, such as hydroxyl groups . Contrarily to the deep knowledge and characterization that has been achieved on the grafting mechanism of dyes and catalysts on TiO 2 nanoparticles, only a few experimental studies, addressing adsorption on WO 3 substrates, are available in the literature. − Bishop et al studied the binding of citric acid to different metal oxide nanoparticles (ZnO, TiO 2 , Fe 2 O 3 , and WO 3 ), finding a weak adsorption onto the WO 3 surface, that was attributed to its extremely low isoelectric point. WO 3 functionalization with polydopamine (PDA) has been also reported in the context of photocatalysis and immunosensor devices, , indicating the formation of a stable interface between PDA and WO 3 via catechol functionalities.…”

“…i v i d u a l n o t d i r e c t l y a s s o c i a t e d w i t h s c i t e i s p r o h i b i t e d .onto WO 3 substrates with respect to TiO 2 surfaces 68. Our findings thus suggest novel design rules for the optimization and the selection of alternative anchoring functionalities, better matching the surface W−W patterns, for optimized WO 3based DSSCs and hybrid photocatalysts.…”

Ruthenium(ii) complexes with phosphonate-substituted phenanthroline ligands: synthesis, characterization and use in organic photocatalysis

“…Ru(II) complexes bearing phosphonate-substituted bpy ligands were prepared for photovoltaic applications [28][29][30][31][32][33][34][35] and recently used in organic photocatalysis. [36][37][38] We were interested in attaching this group to the phen ligand, which is known for inertness in ligand exchange reactions.…”