Abstract
We measured the adsorption characteristics of two organic dyes; triphenylamine-cyanoacrylic acid (TPA-C) and phenoxazine (MP13), on TiO2 , directly in a solution based on quartz crystal microbalance (QCM). Monitoring the adsorbed amount as a function of dye concentration and during rinsing allows determination of the equilibrium constant and distinction between chemisorbed and physisorbed dye. The measured equilibrium constants are 0.8 mM-1 for TPA-C and 2.4 mM-1 for MP13. X-ray photoelectron spectroscopy was used to compare dried chemisorbed layers of TPA-C prepared in solution with TPA-C layers prepared via vacuum sublimation; the two preparation methods render similar spectra except a small contribution of water residues (OH) on the solution prepared samples. Quantitative Nanomechanical Mapping Atomic Force Microscopy (QNM-AFM) shows that physisorbed TPA-C layers are easily removed by scanning the tip across the surface. Although not obvious in height images, adhesion images clearly demonstrate removal of the dye.
Highlights
In contrast to the classical methods for measuring dye adsorption and their equilibrium constant via absorption spectroscopy, we use a fast and easy method to evaluate adsorption characteristics for on TiO2 including the absorbed amount, equilibrium constant and presence of physisorbed aggregates or multilayers directly in solution, based on quartz crystal microbalance (QCM-D) in a liquid flow cell
Two different dyes Triphenylamincyanoacrylic acid (TPA-C) and Phenoxazine (MP13), which has been used in other research group [11, 12] are used
MP13 adsorbs in larger amounts than TPA-C, which could be due to a better packing structure for MP13 than TPA-C dye
Summary
In contrast to the classical methods for measuring dye adsorption and their equilibrium constant via absorption spectroscopy, we use a fast and easy method to evaluate adsorption characteristics for on TiO2 including the absorbed amount, equilibrium constant and presence of physisorbed aggregates or multilayers directly in solution, based on quartz crystal microbalance (QCM-D) in a liquid flow cell. To support the evaluation we performed X-ray photoelectron spectroscopy (XPS) on dried liquid phase prepared dye layers and on vacuum sublimated monolayer films on a TiO2 single crystal. The mechanical properties of the dye film were investigated by atomic force microcopy (AFM).
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