Ionic Diffusion‐Driven Ionovoltaic Transducer for Probing Ion‐Molecular Interactions at Solid–Liquid Interface

Abstract Ion–solid surface interactions are one of the fundamental principles in liquid‐interfacing devices ranging from various electrochemical systems to electrolyte‐driven energy conversion devices. The interplays between these two phases, especially containing charge carriers in the solid layer, work as a pivotal role in the operation of these devices, but corresponding details of those effects remain as unrevealed issues in academic fields. Herein, an ion–charge carrier interaction at an electrolyte–semiconductor interface is interrogated with an ion‐dynamics‐induced (ionovoltaic) energy 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, originated from a dipole potential effect of the self‐assembled molecular layer (SAM). In addition, this effect is found to be modulated via chemical functionalization of the interfacial molecular layer and transition metal ion complexation therein. With the aiding 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, 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.

S-3 water bath. Hence, as shown in Figure 1b, the electric output could have an apex at the first charging (C 1 ) and then be irregularly attenuated as time goes by. Hence, as shown in Figure 1b, the electric output could have an apex at the first charging (C 1 ) and then be decayed as time goes by. Herein, the apparent peak of electric outputs is considered as a major signal, and a corresponding simplified equivalent circuit is displayed at Figure S6b. In the circuit of Figure S6b, a relation between each current flow can be expressed by = − (S1) Voltage measured in external voltammeter (V) can be presented as follows; , where is a resistance in external circuit including contact resistances and assumed to be much smaller than . by the solution injection can be expressed with ⁄ , where Q is an amount of ions (electrons) adsorbed (accumulated) at the electrolyte-SAM-semiconductor interface. Hence, , and ∆ is a potential difference between electrolyte and semiconductor, which are in the vicinity of the SAM interface. This is equivalent to a potential difference between underneath the ion-adsorbed interface (denoted with subscript's') and un-adsorbed (DI water) interface (denoted with subscript'd') in the semiconducting layer. Hence, the equation (S1) can be denoted with both the equation (S2) and the equation (S3) as follows; , and , denote a potential of semiconductor near the SAM interface under the ion-adsorbed region and un-adsorbed region, respectively. Due to the diffusion-induced capacitive charging ( Figure   1a), dC 1 /dt can be expressed with an adsorption speed in electric double layer ( ⁄ , where and are a diffusion coefficient of adsorbed ion and Debye length, respectively) as follows [4] ; S-4 Herein, 0 , , and are vacuum permittivity, the relative permittivity of SAM, and a thickness of SAM, respectively. is assumed as DI water condition (equivalent to an initial state in the experiment). Thus, the equation (S4) can be expressed as follows; In a short-circuited condition, V=0, and then In an open-circuited condition, I=0, and then , which is equivalent to = × . The equation (S8) can be expressed with a sheet resistance of semiconductor (R sq ) by considering the extrinsic factors as follows.

Supporting Note 2. Interpretation of the ESS interface
In the equation (S7), , − , is a driving force of charge carrier flows near the SAMsemiconductor interface. As assumed in Supporting Note 1, the electrolyte-SAM-semiconductor interface can be regarded as the capacitor, of which electrodes have different potential screening capabilities. As shown in Figure S7a,b, an equal quantity of charges (ions and electrons) can be accumulated at the SAM (PFOTS) interface, which can have a interfacial charge density of .
Hence, in both high (subscript 's') and low (subscript 'd') concentration regions, an adsorbed ion density at Stern layer ( , and , , respectively) can be equivalent to the charge density (per unit area) at the SAM-semiconductor interface ( , and , , respectively) ( = ). As can be expressed as , where q, N, and W are an electron charge, a dopant density, and a width of space charge region, S-5 respectively. [5] qN is constant throughout the semiconducting layer. From the zeta potential ( ) measurement, < indicated that , > , ( Figure S1a). Hence, in the high concentration region, larger , than , can induce a relatively wider than ( Figure S7a,b). can be shown with and as follows; [5] = 2 2 0 = − 2 0 (S11) is a dielectric constant of the semiconductor. Hence, as shown in Figure S7c,d, , can be much larger than , and spanned to deeper region in the semiconductor. Therefore, in the PFOTS case,   ) and an internal resistor ( ) near the SAM interface, respectively); = 1 + 2 + ⋯ + . After the first interfacial charging at 1 , electrolyte can be diffused in aqueous phase (switch on), and therefore, drive both sequential interfacial charging ( +1 ) and S-9 dissipation towards bulk phase, simultaneously. ′ +1 is an resistance in each ionic diffusion.
(b) A simplified equivalent circuit that is considering the first interfacial charging event.  Figure S12. Device resistance as a function of exposed FeCl 3 concentrations. Resistances in each condition was obtained from a slope of current-voltage curves, which were measured at both terminals.

S-13
Figure S13. Cl 2p XPS spectrum of Fe 3+ -adsorbed CA surface when exposed to 100 μM  Table S1. Stability constant (log K) of mono-complex formation of metal ion and catechol ligand, and pKa value for metal ion acidity in aqueous state.