Programmable Electrochemical Rectifier Based on a Thin-Layer Cell

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Programmable Electrochemical Rectifier Based on a Thin-Layer Cell Seungjin Park, Jun Hui Park, Seongpil Hwang, and Juhyoun Kwak ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 27, 2017

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ACS Applied Materials & Interfaces

Programmable Electrochemical Rectifier Based on a Thin-Layer Cell

Seungjin Park†,┴, Jun Hui Park‡,┴, Seongpil Hwang*,§, Juhyoun Kwak*,†

†

Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 34141,

Korea ‡

Department of Chemistry Education and Institute of Fusion Science, Chonbuk National

University, Jeonju 54896, Korea §

Department of Advanced Materials Chemistry, Korea University, Sejong 30019, Korea

Keywords: Thin-gap electrode, Self-assembled monolayer, Unidirectional Charge Transfers, Redox cycling, Rectification

ACS Paragon Plus Environment


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Abstract A programmable electrochemical rectifier based on thin-layer electrochemistry is described here. Both the rectification ratio and the response time of the device are programmable by controlling the gap distance of the thin-layer electrochemical cell, which is easily controlled using commercially available beads. One of the electrodes was modified using a ferrocene-terminated self-assembled monolayer (SAM) to offer unidirectional charge transfers via soluble redox species. The thin-layer configuration provided enhanced mass transport, which was determined by the gap thickness. The device with the smallest gap thickness (~ 4 µm) showed an unprecedented, high rectification ratio (up to 160) with a fast response time in a two-terminal configuration using conventional electronics.

1. Introduction Wetware, which is analogous to hardware and software in the water systems of organisms, has drawn attention to the need to bridge a gap between abiotic electronics and biotic systems for various applications, including implantable machines and robots.1,2 Because the interface within a wet aqueous phase is the key to connecting electronics with living creatures, electrochemical devices in aqueous phases that mimic the elements in electronics are a potential solution for wetware.3 The diode is a simple but crucial element in electronics, and it is based on a junction between two heterogeneous semiconductors. Recently, a molecular diode composed of donor/acceptor groups4 was investigated as a molecular and flexible electronic.5,6 As an analog in a wet environment, electrochemical rectifiers based on self-assembled monolayers (SAMs),2,7 heterogeneous polymers,8 and asymmetric nanopores have been reported.9,10 The electrochemical


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rectifier based on a redox-active SAM and polymers relies on unidirectional charge transfers between the surface-tethered redox molecules (R1) and the different redox moieties in the outer region (R2) according to the different standard reduction potentials of R1 and R2. The rectification performance (the ratio of the forward current to the reverse current at a specific applied voltage; we defined Vforward as 0.5 V and Vreverse as −0.5 V) depends on the direct charge transfer of R2 through defects on chemically modified electrodes, which is called leakage, in a reverse bias and on the current flux from the redox reaction between R1 and R2 in a forward bias. To minimize the leakage current through defects, a thick polymeric layer coating was applied. The thick polymer layer slows devices by slowing the rate of electron hopping between the redox moieties within the polymers. The charge transfer rates between both the electrode/R1 and R1/R2 also affect the response speed.11 As an alternative, thin films, such as SAM, provide advantages in terms of speed and ease of preparation, but they suffer from a low rectification ratio caused by leakage or breakdown12,13 due to an incomplete and fragile layer. Thus, thick polymeric layers have been widely applied to electrochemical rectifiers despite the slower charge transfer between the redox moieties in the polymer.14 SAM-based tunneling junctions, however, have been intensively investigated,15 inspiring the possibility of high performance under electrochemical conditions. In a given operating voltage range, the SAM was stable unless a high reverse bias (ca. − 1 V) was applied. SAM-based rectifier systems, however, have a comparatively low rectification ratio. Is there an approach that can enhance the rectification performance of SAMbased electrochemical rectifiers in a wet environment? Thin-layer electrochemical cells might be a solution. Thin-layer electrochemical cells are composed of two parallel electrodes separated by a small gap, and the cells have been used in the field of electrochemistry.16-18 Redox cycling19,20



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within a thin-layer electrochemical cell is one application in which oxidation of the redox active molecules occurs at one electrode and reduction of the corresponding redox reaction proceeds at the other. In other words, one electrode generates the product, and the other converts the product to the reactant. Both the regeneration of the reactant and the cell configuration (i.e., small gap) enhance the mass transportation and generate higher currents. To apply the concept of thin-layer electrochemistry to a rectifier, two heterogeneous electrodes, in terms of charge flow, must be designed: an electrode for the unidirectional charge transfer, such as a redox-active, SAMmodified electrode similar to a previously reported electrochemical rectifier, and a bare electrode to regenerate the redox species. While the leakage current from defects at the SAM-modified electrode is a kinetically controlled current that depends only on the concentration of the redox molecules in the electrolyte, the current from the electrochemical catalytic reaction (EC' mechanism) in the forward bias is a diffusion controlled current and linearly increases in proportion to the reciprocal of the gap thickness and the concentration. Therefore, the rectification performance can be systematically programed by controlling the gap thickness because the current in the forward bias dramatically increases regardless of the constant leakage current due to thin-layer electrochemistry. In addition, a conventional diode with two terminals can be easily fabricated21 in a thin-layer configuration in the absence of a complex electrochemical setup, such as a three-electrode system with reference electrodes. Thus, the thinlayer configuration combined with an SAM-based electrochemical rectifier may create a new method to produce high-performance electrochemical rectifiers. Herein, we report on a highly rectifying, thin-layer electrochemical device in an aqueous phase. A mixed SAM of 11-(ferrocenyl)undecanethiols (Fc-UDT) and 1-hexadecanethiols (HDTs) served as the rectifying interface. A thin-layer electrochemical cell was fabricated. One side was


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an SAM-modified electrode, and the other was a bare, gold electrode. The ferrocyanide (Fe(CN)64-) in the gap electrolyte donates electrons to reduce and immobilize the ferrocenium cation produced by the charge transfer of the electrode because the standard electrode potential of ferrocyanide/ferricyanide is lower than that of ferrocene/ferrocenium. The reverse process (i.e., electron flow from ferrocene to ferricyanide (Fe(CN)63-)) is thermodynamically prohibited, consequently forming a rectifying interface.7 This behavior was confirmed via electrochemical characterization using a typical, three-electrode system. The enhancement in the rectifying behavior was systematically investigated using thin-layer electrochemical cells with various gap thicknesses in either a four- or two-electrode configuration. Redox cycling via a thin-layer electrochemical cell allows for two terminal operations within conventional electronics, a controlled rectification ratio and a response time dependent on the gap distance. The combination of thin-layer electrochemistry and SAM-based rectification provides a design strategy for highly rectifying devices based on electrochemistry.

2. Experimental

2.1. Chemicals All chemicals (obtained from Sigma-Aldrich, US) were analytical or better grade. Dynabead (M450-epoxy, 4.5 µm, Invitrogen) and magnetic beads (6.2 µm, 8.18 µm, UMC3N, UMC4N, respectively, Bangs Lab) served as the spacers. Epoxy was purchased from the Henkel



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Corporation. The buffer solution for all the experiments was composed of 0.1 M CH3COOH and 0.18 M CH3COONa and contained 0.1 M NaClO4 as the electrolyte (pH 5).

2.2. Electrochemical measurements Electrochemical measurements were performed using a CHI 900B bipotentiostat with a conventional, four-electrode system. Pt wire and Ag/AgCl (3 M KCl) served as the counter electrode and reference electrode, respectively. In the I-V curve experiment, a two-terminal electrode system was introduced; the counter and reference connectors were connected to bare gold (which served as the common ground), and the working connector was connected to the modified Au electrode. The values of rectification ratio were calculated from measuring both forward current at 0.5 V and reverse current at ─ 0.5 V in a two terminal electrode system.

2.3. Fabrication of the Thin-Layer Electrochemical Cell A pair of Au electrodes (100 nm Au, 5 nm Ti on Si) were prepared by submerging them in a piranha solution (3:1=H2SO4:H2O2; WARNING: this solution is highly dangerous) for 2 mins, rinsing them with water and isopropyl alcohol, and drying them with N2. One of the Au electrodes (the upper, the generator) was dipped into a particular ratio of a mixed SAM solvent for 18 h to characterize the densely packed SAM. The other (the lower, the collector) electrode was treated with O2 plasma to ensure the surface of the electrode was hydrophilic. Then, the diluted bead solution (in ethanol solvent) was dropped onto the lower electrode. Both the complete wetting on the hydrophilic surface and the low surface tension of the ethanol block multilayer formation. After evaporation, a tiny amount of beads remained on the lower electrode.


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The upper electrode was carefully aligned in face-to-face orientation on the beads (an electroactive area of 10 mm × 10 mm). A temporary assembly using Kapton tape (3 M, US, 3 mm × 30 mm, to block the penetration of epoxy into the gap) was sealed using epoxy at the sides with pressure from a 10 g weight and was followed by thermal curing (35 °C for 30 min). After removing the weight, additional epoxy was applied to the uncovered top surface and followed with the same thermal treatment. Smallest gap thickness (4 µm) was measured by SEM. (see the Supporting Information, Figure S1)

3. Results and Discussion

For the electrochemical rectification, a mixed SAM of HDT and Fc-UDT was formed on Au. A cyclic voltammogram (CV) in the electrolyte showed the symmetric behavior for the redox reaction of the surface-tethered Fc, which indicated the successful immobilization, fast charge transfer and small quantity of immobilized Fc (2.62×10-11 mol/cm2) (Figure 1a). Only a small amount of the Fc-SAM was immobilized due to the preferred adsorption/displacement of the longer alkyl thiol (HDT) owing to lateral van der Waals interactions between the neighboring chains.22,23 The ferrocene moiety is buried under the HDT, causing a positive shift in the formal potential (see the Supporting Information, Figure S4). On the other hand, the SAM24,25 dipole might change the electrical double layer structure (EDL), which is called the Frumkin effect,26,27 but this effect was not observed in our experiments. The mixed SAM contains a smaller number of defects that generate the leakage current when it is used in a rectifier (See the Supporting Information, Figure S5). Although a high-quality SAM on a template-stripped metal substrate



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has been reported,15,28,29 an evaporated Au film on silicon was used in our experiments. When the electrolyte contains Fe(CN)64-, the CVs of the mixed-SAM Au film showed the well-known electrochemical rectification (Figure 1b) caused by the charge transfer from the electrochemically oxidized Fc to the dissolved Fe(CN)64- in solution. In particular, the redox potential of Fe(CN)63-/Fe(CN)64- (0.15 V vs. Ag/AgCl) is more cathodic than that of the surfacetethered ferrocene (0.6 V vs. Ag/AgCl), which makes the electron transfer from Fe(CN)64- to ferrocenium thermodynamically favorable and blocks the electron transfers from Fe(CN)63- to Fc or Au. Figure 1b agrees with the previously reported electrochemical rectifier.7,11 From an engineering point of view, however, the electrochemical rectifier does not satisfy several requirements for diodes. First, the diffusion controlled current in the forward bias deviates from the ideal I-V curve of diodes. For example, the anodic current decreases over time to 0.7 V vs. Ag/AgCl, as seen in Figure 1b. Therefore, the electrochemical rectifier causes a non-ideal, timedependent behavior at a low frequency. Second, the rectification ratio, which is defined as the ratio between the anodic current and cathodic current, is low. The rectification ratio is typically less than 20, even in molecular electronics30, and only a few exceptional results have been reported in terms of the rectification ratio31,32 and GHz response time.33 Recently, a reported measurement platform using a eutectic of gallium-indium allowed for the analysis of a statistically large number of data for a specific junction.31,34-36 In our case, the acquisition of a large number of tests is difficult due to the device fabrication. Nonetheless, the electrochemical cells showed good reproducibility because the penetration of the relatively larger, redox-active molecules on the SAM-modified electrode decreased the sensitivity to defects and the large electroactive area (1 cm2 in our system) helped average the effect of the defects. Third, a threeelectrode system with a reference electrode and a potentiostat has been investigated instead of


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two terminals in conventional electronics. For practical applications, the electrochemical rectifier must solve these problems to attain a steady-state current, high rectification ratio, and twoterminal operation. Fortunately, thin-layer electrochemistry can provide (1) steady-state current via enhanced mass transport, (2) a high rectification ratio from the amplified current in the forward bias with a constant leakage current in the reverse bias, and (3) operation under two terminals via redox cycling. Thin-layer electrochemical cells a few µm in distance were fabricated using a bead-based method.21 A mixed-SAM Au and bare Au electrode acted as the generator electrode (GE) and collector electrode (CE), respectively. Namely, the GE offers the unidirectional charge transfer shown in Figure 1b, and the CE is close to the GE to convert the product into the reactant, which enhances the mass transport. The steady-state current at the GE can be predicted using a previously reported equation, and it is inversely proportional to the gap thickness.21,37-39 The reported relationship is based on a Nernstian (reversible) reaction on the electrode. The reaction on the SAM-modified Au electrode, however, is not a Nernstian reaction and depends on the ferrocene coverage at the electrode surface. We derived a modified equation for the steady-state current using the Allemen-Weber-Creager approach.11,40 (see the Supporting Information) I=

,

Γ =

Γ

Γ Γ +

(Eq. 1)

!"

#

Here, n is the number of electrons involved in the reaction, DFe2+ is the diffusion coefficient of ferrocyanide, F is the Faraday constant, A is the electroactive area, CFe2+,bulk is the



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total concentration of the redox species, d is the gap thickness, kcross is the homogeneous secondorder rate constant for the reaction of Fe(CN)64- with Fc+, and ΓFCSAM is the concentration of ferrocene at the electrode surface. To investigate the electrochemical behavior of the fabricated devices, a four-electrode system using bipotentiostat was applied. Two closely spaced working electrodes (W1, W2), a Pt wire (auxiliary electrode) and an Ag/AgCl (reference electrode, 3 M KCl) electrode were set up. For redox cycling, a cathodic potential of 0.15 V was applied at the CE to reduce Fe(CN)63- to Fe(CN)64-. Figure 2 shows the CVs of a thin-layer electrochemical rectifier with various gap thicknesses in a four-electrode system. In the anodic region, the anodic current from the oxidation of ferrocyanide and the steady-state current from the redox cycling were observed. In the cathodic region, the cathodic current from the reduction of ferricyanide was small because of blocking by the SAM. Thus, the rectification ratio, which is defined as the anodic current over the cathodic current, is larger. Moreover, the anodic steady-state currents were inversely proportional to the gap thickness, as predicted by Eq. 1. In fact, the steady-state currents at 4 µm and 6 µm agreed with the values from Eq. 1 (within a 10% error, see the Supporting Information). Two facts are clear at this stage. The thin-layer electrochemical rectifier demonstrated steady-state behavior instead of a diffusion controlled current, and the programmable rectification ratio is dependent on the gap thickness. Thus, the thin-layer configuration offers unique advantages for SAM-based electrochemical rectification in terms of the I-V behavior and rectification ratio. Thin-layer electrochemistry provides the additional advantage of two terminal operations for an SAM-based rectifier. Compared with a complex electrochemical system, such as a three-electrode system and special electrochemical instruments, thin-layer electrochemistry enables simple working conditions with conventional electronic circuits composed of two


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terminal configurations. The theory of generation-collection in thin-layer electrochemistry using a two-electrode configuration without the reference and auxiliary electrodes has been discussed in previous reports.37,39,41 Moreover, the two-electrode operation of thin-layer electrochemistry has been widely applied to electrochemical energy generation/storage devices, such as dyesensitized solar cells (DSSCs) and supercapacitors. Thus, our thin-layer electrochemical rectifier may offer the ability for two-electrode operation following the established electrochemical theory. To investigate the two-electrode configuration, the auxiliary and reference connectors were connected to bare gold (which served as the common ground or CE in a four electrode system), and the working connector was connected to the Fc-SAM-modified Au (which served as the signal or GE in a four electrode system). Figure 3a shows a representative I-V curve demonstrating the rectifying behavior of a thin-layer electrochemical rectifier with two terminal operations. The I-V behaviors approach ideal behavior, i.e., low leakage current, timeindependent current in the forward bias, and a higher current in the forward bias due to the thinlayer configuration. In theory, the electrochemical behavior of a thin-layer cell in a two-terminal mode relies on a simple rule. The anodic current at one electrode must be the same as the cathodic current at the other electrode. Namely, a slower redox reaction with smaller currents deeply influences on the I-V curve and the distribution of the external voltage between two electrodes in the absence of a reference electrode. In our rectifying device, the redox reaction at the mixed-SAM Au electrode, rather than the bare Au electrode, might determine the current of the system. When an external voltage is applied to our device in a two-terminal configuration, the rectifying I-V curve can be categorized into two regions. In the forward bias (positive electrode: mixed-SAM Au, negative electrode: bare Au), the Fe(CN)64- in the electrolyte donates electrons via the immobilized Fc to the Au electrode, generating anodic currents, which govern



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the current of the device. In Figure 3a, an anodic steady-state current, similar to that seen in Figure 2, was observed under a forward bias and was enhanced by redox cycling, which increased linearly based on 1/d in the following equation.39 I forward bias =

! $

Γ

(Eq. 2)

Here, CR is the concentration of Fe(CN)64-, d is the gap thickness. Thus, the current in the forward bias was amplified at a smaller gap distance. On the other hand, in the reverse bias, the Fc and HDT blocked the cathodic process on the mixed-SAM Au, and only a small cathodic current flowed from the reduction at the defects within the SAM. To minimize this leakage, we used an Fc/HDT-mixed SAM, as shown in Figure 1. In this case, the cathodic current at the mixed-SAM was the kinetically controlled current caused by the slow charge transfer through the defects within the SAM. From Butler-Volmer model at sufficient overpotential in cathode,42 current on SAM-modified Au can be written as follow.

,

I reverse bias = I leakage = nFA % & '()*+'+ -.

(Eq. 3)

Here, CO is the concentration of the oxidant, k0 is the standard rate constant, α is the transfer coefficient, and E0 is the standard reduction potential of redox couples. We assumed that oxidation process is negligible at sufficient overpotential for reduction, because of the very sluggish kinetics due to the presence of the SAM.43 The current under a 4-electrode configuration is determined by the total concentrations of the oxidant and reductant, as shown in Eq. 1, and only the specific concentration of either the oxidant or reductant controls the current in the twoelectrode configuration (Eq. 3). The theoretical rectification ratio from a thin-layer electrochemical rectifier can be written as the following:


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R theory ≡

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Programmable Electrochemical Rectifier Based on a Thin-Layer Cell

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