Probing Ion-Molecular Interactions at Solid-Liquid Interface Through Ionic Diffusion-Driven Ionovoltaic Transducer

Recently, electrolyte-interfacing systems ranging from electrochemical devices such as batteries and capacitors to electronic devices like electrolyte-gated neuromorphic devices have been actively developed, and solid-liquid interactions are one of the fundamental principles in their systems. These interactions also played a critical role in water motion-driven electricity-generating phenomena which originated from ion dynamics at the interface. Specifically, ion adsorption at electrolyte-insulator-semiconductor (EIS) structure accumulates or depletes the charge carrier of the semiconductor by means of coulomb interactions, and then changes its density near the interface, which was demonstrated as ionovoltaic phenomena. However, more details about various specific interactions such as chemical adsorption of ions, ion specificity, and coulombic pairing remain unrevealed issues in academic fields. Herein, an ion–charge carrier interaction at EIS structure is interrogated with an ionovoltaic transducer, controlled by interfacial self-assembled molecules. An electricity-generating mechanism from interfacial ionic diffusion is elucidated in terms of the ion–charge carrier interaction, originating from a dipole potential effect of the self-assembled molecular layer (SAM). While the ion diffuses through the distilled water (DI water) at the EIS structure, the potential difference between the semiconductor under the DI water and the ionic solution evolves. The ionovoltaic signal inversion occurs by applying the surficial molecules with opposite dipoles which are trichloro(1H,1H,2H,2H-perfluoro-n-octyl)silane (PFOTS) and (3-Aminopropyl)triethoxysilane (APTES) and induce cation and anion adsorption, respectively. In addition, this effect is found to be modulated via chemical functionalization of the interfacial molecular layer and transition metal ion complexation therein. The catechol group (CA), which can form a coordination complex with ferric ions (Fe³⁺) was chemically modified on the APTES-treated device, and the variation of ionovoltaic signal was observed when the device was exposed to the Fe³⁺ solution and CA formed complexation with Fe³⁺. In the case of Al³⁺and Cu²⁺ which have a low binding affinity with CA, there was no evident change in generated signal. Those results show the Fe³⁺ concentration dependence and selectivity. With the aid of surface analytic techniques and a liquid-interfacing Hall measurement, electrical behaviors of the device depending on the magnitude of the ion-ligand complexation are interrogated. Specific chemical adsorption of Fe³⁺ on CA affected the interfacial molecular structure and therefore the change in carrier density of the semiconductor was observed, thereby demonstrating the ion–charge carrier interplays spanning at electrolyte–SAM-semiconductor interface. Hence, this system can be applied to study molecular interactions, including chemical and physical influences, occurring at the solid–liquid interfacial region.