Control of Surface and ζ Potentials on Nanoporous TiO₂ Films by Potential-Determining and Specifically Adsorbed Ions

Abstract

The effect of a specifically adsorbed ion, phosphate, on the electrochemical response and adsorption properties of nanocrystalline TiO2 is examined. Phosphate is known to affect the ζ potential, as measured by electrophoretic mobility, by changing the charge of the oxide surface. The adsorption of a cationic probe molecule, thionine, onto TiO2 was monitored with an in-situ cell using UV−vis spectroscopy. The adsorption of the cationic dye molecule was found to be governed by changes in the ζ potential, whether the ζ potential was modified by pH or by changes in phosphate concentration. Onset potential measurements were used to estimate the flat-band potential of a Ti/TiO2 electrode. The flat-band potential results for the electrode showed a nearly Nernstian response to changes in the pH for a broad pH range. The addition of phosphate had no effect on the onset potential or on the shape of the photocurrent/potential curve. Flat-band potentials determined by Mott−Schottky analysis in the absence of phosphate were Nernstian only for pH 3−7, matching the pH dependence of the electrophoretic mobility results. With the addition of phosphate, impedance spectroscopy results showed additional space charge capacitance, peaking at potentials 150 mV positive of the flat-band potential. UV irradiation also resulted in an additional space charge capacitance. For both cases, the additional space charge capacitance was accompanied by a decrease in the resistance of the electrode, as shown in Nyquist plots. The change in film conductivity is believed to affect the space charge layer capacitance. Similarly, a decrease in film resistance was also seen with lower pH values. Currently, this change in TiO2 film conductivity with surface acidity is being investigated in our laboratory for application in fuel cell electrolytes.

Photocatalytical Properties of TiO₂ Nanotubes

“…Actually, the defect structure of TNTs formed by oxygen vacancies favors the adsorption of water on the surface and then dissociates water into hydroxyl groups and protons [61][62][63]. This dissociation behavior leads to the charged surface of TNTs due to the loss or gain of protons and complexation reactions of surface hydroxyl groups [64,65] Electrophoretic mobility is the most common technique for investigating the surface charges of the metal oxide/electrolyte solutions [66,67]. …”

Absolute Determination Method for Trace Quantities of Enantiomer of Glufosinate by 7-Cyclodextrin Modified Capillary Zone Electrophoresis Combined with Solid-Phase Extraction and On-capillary Concentration

“…The isoelectric point of titania is reported to be around 5. 12,13 Since the pKa of phosphinate group of D,L-GLUF is 2.37, 14 phosphinate can be bonded to cationic Ti at the titania surface at the acidic region, as reported for the phosphate anion. 15 The adsorption of D,L-GLUF decreased with increase in pH beyond pH 3.…”

Phosphate Modification of Nano Titania and Its Effect on the Photodegradation of EDTA

“…It has been already reported that the point of zero zeta potential of the nano titania particles are shifted to lower pH value in the presence of phosphate anion which indicates the accumulation of negative charges on the surface of nano titania. 37 It seems, a negative electrostatic field could be formed in the surface layer of the nano titania due to the existence of negatively charged phosphate anions. During the photocatalysis process, the separation of electrons and holes could be promoted by this negative electrostatic field and as a result the charge recombination on phosphate modified nano titania is suppressed compared with the pure nano titania.…”