Electrochemical and Electronic Properties of Transparent Coating from

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Article Cite This: ACS Omega 2018, 3, 7106−7116

Electrochemical and Electronic Properties of Transparent Coating from Highly Solution Processable Graphene Using Block Copolymer Supramolecular Assembly: Application toward Metal Ion Sensing and Resistive Switching Memory Koomkoom Khawas,† Soumili Daripa,‡ Pallavi Kumari,† and Biplab Kumar Kuila*,‡ †

Center for Applied Chemistry, Central University of Jharkhand, Brambe, Ranchi 835205, Jharkhand, India Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India

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ACS Omega 2018.3:7106-7116. Downloaded from pubs.acs.org by 80.82.77.83 on 06/30/18. For personal use only.

S Supporting Information *

ABSTRACT: Here, we have discussed the preparation of a highly solution processable graphene from a novel supramolecular assembly consisting of block copolymer polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and pyrenebutyric acid (PBA)-modified reduced graphene oxide (RGO). The PBA molecules anchored on the graphene surface form supramolecules with PS-b-P4VP through H-bonding between the carboxylic acid group of 1pyrenebutyric acid and the pyridine ring of P4VP. The formation of a supramolecular assembly results in a highly stable solution of reduced graphene oxide in common organic solvents, such as 1,4-dioxane and chloroform. Highly transparent and mechanically stable thin films can be deposited from these supramolecular assemblies on a relatively smooth surface of different substrates such as silicon wafer, glass, indium tin oxide, and flexible polymer substrates like poly(ethylene terephthalate). The graphene surface modifier (PBA) can be selectively removed from the thin film of the hybrid material by simple dissolution, resulting in a porous structure. Hybrid thin films of around 50 nm thickness exhibit interesting electrochemical properties with an areal capacitance value of 17.73 μF/cm2 at a current density of 2.66 μA/cm2 and good electrochemical stability. The pendent P4VP chains present in the composite thin film were further exploited for electrochemical detection of metal ions. The electrical measurement of the thin film sandwich structure of the composite shows a bipolar resistive switching memory with hysteresis-like current−voltage characteristics and electrical bistability. The OFF state shows ohmic conduction at a lower voltage and trap-free space-charge-limited current (SCLC) conduction at high voltage, whereas the ON state conduction is controlled by ohmic at low bias voltage, trap-free SCLC at moderate voltage, and tarpassisted SCLC at high voltage.

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INTRODUCTION Carbon-based materials have attained considerable research attention as electrode materials for different applications such as energy storage devices (supercapacitors, batteries), electrochemical sensors, corrosion inhibition materials, actuators, flexible electronic devices, etc., because of their unique properties such as high surface area, excellent electrical conductivity, compatibility with other materials, and good mechanical stability.1−8 Graphene, owing to its large in-plane conductivities, is one of the most promising materials for the development of electrodes for a wide range of potential applications.9 Graphene and its derivatives have been blended with different materials such as metals, metal oxides, and polymers with an aim of fabricating graphene-based composite materials with superior properties.7,8,10−13 However, achieving graphene−polymer nanocomposites with good dispersion is an ongoing challenge because of strong agglomeration and settling of graphene especially at a higher loading.11−13 One smart approach is to assemble graphene into the polymer matrix in a © 2018 American Chemical Society

controllable manner by supramolecular chemistry through specific noncovalent interactions like hydrogen bonding. Block copolymers (BCPs) self-assemble into well-ordered periodic structures (spherical, cylindrical, gyroidal, lamellar) depending on the ratio of block lengths and segment−segment interaction parameters.14−16 Block copolymers have been widely used as a host matrix/template to manipulate the guest additive in hybrid systems with physical and chemical stability.17 There are few reports of block copolymer graphene nanocomposite materials where graphene oxide (GO) dispersion in a polymer matrix is improved by hydrogen bonding interactions between the oxygen-containing functionality on the graphene oxide sheet and one of the blocks of the block copolymer.18 The weak interaction force with block copolymers which mainly relies on the extent of oxygen-containing functionalities Received: May 2, 2018 Accepted: June 18, 2018 Published: June 29, 2018 7106

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Figure 1. (a) Schematic structure of the supramolecular assembly (SMA) formed between PBA-RGO and block copolymer (BCP) PS-b-P4VP. (b) Photograph of the supramolecular assembly solution in 1,4-dioxane.

by studying the variation in the effective excluded volume in inorganic/poly(4-vinylpyridine)-b-poly(ε-caprolactone) hybrids. The combined properties of the electrical double layer formed from graphene and the binding affinity of the free 4VP moiety in the P4VP chain with metal ion can be exploited for electrochemical applications like detection of metal ions. Interestingly, such a composite electrode shows selective, quantitative, and more sensitive detection toward toxic Hg2+ ions. However, development of new graphene−polymer semiconducting composite materials is one of the highest priority areas because of their wide range of potential applications in organic field-effect transistors, photovoltaics, sensing devices, memory devices, etc.32−34,18 For fabricating such composite materials, different organic semiconducting polymers have been mainly employed where the nature of interaction of the polymers with graphene has been exploited to produce working electronic devices. Introduction of a percolating graphene network within an insulating polymer matrix to develop semiconducting materials was originally reported by Eda and Chhowalla,35 who managed to fabricate working transistors via blending reduced graphene oxide with an insulating polymer. Here, we have shown that the composite film in a sandwich structure shows nonlinear I−V characteristics typically of the semiconductor and exhibits bipolar memristive switching with hysteresis-like current−voltage characteristics and electrical bistability. We have also studied the conduction and switching mechanisms of the composite film.

(−OH, COOH group) on the graphene oxide surface, strong stacking force, and its hydrophilic nature make it still difficult for graphene oxide to produce homogeneous fully organic solvent processable composites for fabrication of a highly transparent and conducting coating. However, the noncovalent modification19 of graphene oxide is an elegant way to introduce desired functionalities without creating defect sites and can be exploited for the preparation of block copolymer supramolecular assembly (SMA). In this article, we have reported a novel approach for preparing block copolymer-reduced graphene oxide (RGO) supramolecular assembly consisting of block copolymer polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and pyrenebutyric acid (PBA)-modified reduced graphene oxide (RGO). The supramolecular assembly is formed through hydrogen bonding between the anchored PBA molecules of the graphene surface and the pyridine ring of P4VP. The formation of supramolecular assembly results in a highly stable solution of reduced graphene oxide in common organic solvents such as 1,4-dioxane and chloroform. The graphene surface modifier (PBA) can be selectively removed from the thin film of the hybrid material by simple dissolution, resulting in a porous structure. Graphene-based ultrathin films have attracted significant research attention because of their superior electrochemical properties especially their use in the fabrication of supercapacitor electrodes.20 So far, different strategies such as layerby-layer assembly, coating from a dispersion of reduced graphene oxide-conducting polymer hybrid, and spray coating are commonly used to fabricate such thin films.21−26 In our case, we have fabricated graphene-based porous thin films simply by spin-coating of the reduced graphene oxide-block copolymer composite solution. The study of the electrochemical properties of these hybrid thin films shows an areal capacitance value of 17.73 μF/cm2 at a current density of 2.66 μA/cm2 with good electrochemical stability. We are also aware of the extensive previous investigation of the transition metal complexes of P4VP by Agnew.27 There are several reports where the metal ion−P4VP interaction is extensively used for the preparation of metal nanostructures or patterns and gels.28−30 Ho et al.31 made a detailed investigation on the association strength of different metal ions with P4VP

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RESULTS AND DISCUSSION The schematic structure of the supramolecular assembly formation between block copolymer (BCP) PS-b-P4VP and PBA-RGO and the photograph of its solution in 1,4-dioxane are depicted in Figure 1. The formation of a highly homogeneous stable colloidal solution stable up to 1 month clearly indicates sufficient interaction between PBA-modified reduced graphene oxide and the block copolymer. The percentage of reduction in the oxygen functionality in RGO of PBA-RGO from GO was calculated from thermogravimetric analysis (TGA) (Figure S1, Supporting Information), and it is around 71%. Fourier transform infrared (FTIR) spectra were 7107

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Figure 2. FTIR spectra of BCP, BCP-PBA-RGO, and PBA-RGO in the following regions: (a) 1030−980 cm−1 and (b) 1800−1610 cm−1.

collected (Figure 2a,b) to elucidate the successful supramolecular assembly formation through hydrogen bonding between P4VP and PBA-RGO. As seen from Figure 2a, the decrease in the peak intensity of the free pyridine ring at 1002 cm−1 and the appearance of a new broad peak at 1014 cm−1 due to the hydrogen-bonded pyridine ring from BCP to BCPPBA-RGO clearly evidence the interaction of PS-b-P4VP and PBA-RGO through hydrogen bonding between the pyridine ring of P4VP and the carboxylic group of PBA anchored on the GO surface.36,37 The formation of hydrogen bonds is further confirmed by the bifurcation of the band at 1694 cm−1 (corresponding to the stretching frequency of the CO bond from the carboxylic acid group of PBA) to 1696 and 1656 cm−1 in BCP-PBA-RGO (Figure 2b). This bifurcation of the CO stretching frequency in BCP-PBA-RGO may be due to the different types of arrangements of the pyrenebutyric acid group inside the block copolymer microdomain in BCP-PBARGO, which create different types of chemical environments around the CO bond compared to those in PBARGO.16,36,37 The increase of the CO bond length of PBA in PBA-RGO after hydrogen bonding with the block copolymer also contributes to the lowering of the CO stretching frequency. A similar type of bifurcation or trifurcation of the CO bond stretching frequency of PBA in block copolymer supramolecular assembly was also observed in our previous study.16 Figure 3 shows the optical image of the BCP-PBA-RGO composite thin film on different substrates. From the images, it is clear that the composite thin films of thickness around 180 nm are highly transparent. The transparency values of the films on the glass substrate were measured by a UV−vis spectrometer to be 95 and 92% for 50 and 180 nm thick films, respectively. The transparency values of the poly(ethylene terephthalate) (PET) and indium tin oxide (ITO) substrates after coating with the PBA-RGO-BCP thin film are around 76 and 83%, respectively, almost equal to the transparency of the original substrate. Thin films from BCP-PBA-RGO on a glass or an ITO substrate were further washed in ethanol to remove PBA and make porous thin films of BCP-RGO. The formation of a porous structure through removal of PBA molecules through simple dissolution is already reported in our previous work.10,37 The complete removal of PBA is evidenced by UV−vis studies (Figure S2, Supporting Information). The porous structure developed from both block copolymer microphase separation and removal of the PBA molecule (through creation of void space) will increase the percolation

Figure 3. Plot of the film thickness of the device vs the transmittance at 550 nm (the inset shows the image of the composite transparent film on the glass substrate and flexible transparent substrate by depositing BCP-PBA-RGO on an ITO-coated PET film).

of the electrolyte and should enhance the electrochemical performance of the composite film. The surface morphology of the composite thin film on the ITO surface was studied by atomic force microscopy (AFM) (Figure 4). The pristine block copolymer thin film deposited from 1,4-dioxane shows a hexagonally packed ordered cylindrical morphology consisting of a P4VP cylinder and PS matrix36,37 (Figure 4a). The BCPPBA-RGO composite films deposited on the ITO substrate exhibit (Figure 4b) good dispersion of graphene oxide layers of varying aggregated sizes in the block copolymer matrix. The line scan of the height profile (Figure S3, Supporting Information) data clearly indicates that graphene oxide layers are present ranging from nearly single layer to aggregates having multiple numbers of layers. These aggregates may be attributed to supramolecular assembly formation between PBA-modified RGO and block copolymer through hydrogen bonding. The block copolymers may act as a binder to bind multiple graphene sheets, which form aggregates of different heights, as observed from AFM images. The highermagnification image (Figure 4c) clearly indicates the microphase separation of the block copolymer in the composite material. The microphase separation is not as regular as in the pristine block copolymer because of ill-defined microphase separation of the block copolymer guided by large graphene sheets. The morphology of BCP-PBA-RGO was further characterized by transmission electron microscopy (TEM) 7108

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Figure 4. AFM height images of BCP and BCP-PBA-RGO thin film: (a) pristine BCP, (b) BCP-PBA-RGO thin film at a lower magnification, and (c) BCP-PBA-RGO thin film at a higher magnification.

Figure 5. TEM micrograph of BCP-PBA-RGO composites at a (a) lower magnification and (b) higher magnification.

BCP-RGO thin film on the ITO substrate were carried out at different current densities (Figure 6c). All of the curves exhibit an equatorial triangle shape, indicating high reversibility of the ultrathin hybrid electrode during the charge−discharge process. The charging/discharging process took a longer time at a lower current density, attributed to the sufficient insertion or release of ions during the charging/discharging process. If we compare the charge−discharge plot of BCPRGO with blank ITO (Figure S5, Supporting Information) or pristine BCP thin film, it can be easily concluded that the BCPRGO composite electrode shows a dramatic improvement in the electrochemical capacitance performance compared to that of blank ITO or BCP. The BCP-modified electrode does not show any stable charge−discharge curve, whereas blank ITO shows very small discharge time of 3.0 s at a current density of 1 μA/cm2. The areal capacitance (C) of the electrode at different current densities was calculated according to the following equation: C = (iΔt)/(AΔV), where “i” is the charge−discharge current, Δt is the discharge time, ΔV is the potential window, and A is the active area of the electrode. The areal capacitance values for the BCP-RGO-modified electrode are 17.73, 16.25, and 16.66 μF/cm2 for current densities of 2.66, 1.33, and 1 μA/cm2, respectively, which agree well with the reported value for the graphene-based thin film of a similar thickness in earlier literature.38−40 This better electrochemical capacitance performance of BCP-RGO can be attributed to the

(Figure 5), which clearly shows the good dispersion of the graphene layers in the block copolymer matrix without very large agglomerates and the block copolymer shows microphase separation, which is not ordered as a pure block copolymer. For the electrochemical study, the thin films deposited from BCP-PBA-RGO on the ITO substrate were washed in ethanol to remove PBA molecules and denoted BCP-RGO. Cyclic voltammograms (CVs) of blank ITO, BCP, and BCP-RGO thin film (∼50 nm thickness) on the ITO surface taken in 0.2 M Na2SO4 solution at a scan rate 0.1 V/s are shown in Figure 6a. The voltammetry curves of BCP-RGO show a considerably rectangular shape, characteristic of capacitive behavior of carbon materials. The area under the CV curve of BCP-RGO is increased drastically compared to that of blank ITO or pristine BCP thin film, clearly indicating its better capacitance performance at this scan rate. The better capacitance performance of BCP-RGO can be attributed to the extra double layer capacitance contribution from the graphene material dispersed in the block copolymer matrix. The porous structure of the BCP-RGO layer with an interconnected network of graphene will further improve the capacitance value. The CV curves of the BCP-RGO thin film deposited on the ITO substrate at different scan rates are displayed in Figure 6b. The subsequent increase of the redox current with the increasing scan rate clearly indicates its good rate ability. The galvanostatic charge−discharge (GCD) experiments of the 7109

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Figure 6. (a) Cyclic voltammetry (CV) curve of the blank ITO electrode, pristine BCP thin film deposited on ITO and BCP-RGO thin film deposited on ITO, (b) BCP-RGO thin film deposited on ITO at different scan rates, (c) galvanostatic charge−discharge plot of the BCP-RGO thin film deposited on ITO at different current densities, and (d) cyclic stability study of 100 cycles for the BCP-RGO thin film deposited on ITO.

However, the P4VP homopolymer or its block copolymerbased hybrid materials have been explored very little for electrochemical sensing of metal ions. Figure 7a shows the CV curves of different electrodes (bare ITO and BCP-RGOdeposited ITO) in the presence of the Hg2+ ion (1 mmol solution of HgCl2 in water). CV from bare ITO produces an oxidative peak at 0.32 V and a reductive peak at 0.08 V, corresponding to the oxidation/reduction process of the Hg/ Hg2+ species.4 In the case of the BCP-RGO electrode, the oxidation and reduction peaks dramatically decreased compared to those in bare ITO, clearly indicating its sensitivity toward the detection of metal ions. Now, the selective detection of a particular metal ion is another essential criterion for a detection technique to be useful for practical application. We have chosen the potential window 0.7 to −0.3 V and tried with other commonly used transition metal ions, such as Cr3+ (CrCl3·6H2O, E0 = −0.74 V), Cd2+ (CdCl2, E0 = −0.4 V), Fe3+ (Fe(NO3)·9H2O, E0 = −0.04 V), Fe2+ (FeCl2·4H2O, E0 = −0.44 V), Cu 2+ (CuSO 4 ·5H 2 O, E 0 = +0.34 V), Zn 2+ (Zn(NO3)2·6H2O, E0 = −0.7618 V), Pb2+ (Pb(CH3COO)2, E0 = −0.13 V), and Ag+ (AgNO3, E0 = 0.80 V), where E0 is the standard electrode potential with respect to the hydrogen electrode. As expected from their standard electrode potentials, Hg/Hg2+ and Ag/Ag+ should show complete oxidative/ reductive peaks in the scanned potential window (from −0.3 to +0.7 V). The electrochemical study of both bare ITO and the composite-modified ITO electrode was also performed in the presence of the Ag+ ion (AgNO3 solution, 1 mmol in water; Figure S6, Supporting Information). A similar reduction

double layer contribution from the interconnecting graphene layer network in the composite materials as well as its porous structure, which maximizes the percolation of electrolyte throughout the materials. Figure 6d demonstrates the charge− discharge cycles for the BCP-RGO-modified electrode for consecutive first 100 cycles at 2.66 A/cm2. The reproducible charge−discharge curve for a larger number of cycles clearly implies its utilization in real applications. As the graphene block copolymer composite films contain free 4-vinyl moieties that can interact with different metal ions, they may be used for electrochemical detection of metal ions. The introduction of reduced graphene oxide has improved the redox current as discussed earlier and will increase the detection limit. For electrochemical detection, glassy carbon electrodes are often used.41,42 However, the applications of glassy carbon electrodes are limited due to their high price and the small size of the electrode tip. In contrast, in our case, the electrode material with large surface area, in square centimeters, can be easily fabricated by spin-coating. Previously, graphene−polymer hybrid materials have been successfully exploited for the detection of heavy metal ions such as Hg2+, Zn2+, Cd2+, and Pb2+.43−47 Li et al.46,47 developed a Nafion−graphene composite film for the detection of metal ions Pb2+ and Cd2+, where the synergistic effect of graphene and Nafion improved the sensitivity compared to that form the individual component. He et al.43 demonstrated a novel DNA-grafted graphene biosensor based on the hybridization of a probe DNA with a target DNA having four thymine−thymine (T− T) mismatches through T−Hg2+−T coordination chemistry. 7110

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Figure 7. (a) CV curves of the blank ITO electrode and BCP-RGO-coated ITO electrode in the presence of Hg2+ ions (HgCl2, 1 mM in aqueous solution), (b) EIS curves of the ITO electrode and the BCP-RGO electrode in the presence of Hg2+ ions (HgCl2, 1 mM in aqueous solution), (c) change in the intensity of the reductive peak current as a function of Hg2+ concentration, and (d) relative (%) decrease of the reductive current at a potential of 0.092 V for different ions.

(EIS) is a very useful technique to investigate the ability of the material to transfer and exchange charges with the surrounding molecules/electrode surface.48 Figure 7b shows the EIS spectra of blank ITO and BCP-RGO-modified graphene in the presence of the Hg2+ ion. The drastic increase in charge transfer resistance (Rct) from the ITO to the BCP-RGO electrode may be due to the strong coordination of Hg2+ to P4VP chains, resulting in resistance to charge transfer to the electrode surface and increase in charge transfer resistance. We further investigate the quantitative detection performance of the BCP-RGO-modified electrode, which is essential for its practical applications. It is observed that the peak current increases with an increase in the concentration of both Hg2+ and Ag+ (Figure S7, Supporting Information). The variation of the reductive peak current versus concentration of Hg2+ is shown in Figure 7c. The linear nature of the plot clearly indicates its real applicability for quantitative detection of metal ions in an unknown solution. The relative decrease (%) of the reductive current with respect to the blank electrode at a potential of 0.092 V is shown in Figure 7d. Interestingly, Hg2+ and Ag+ (for other transition metal ions, the current also decreases depending on the ion) show a drastic reduction in the relative current, whereas alkali or alkaline earth metal ions show a significant increase in the relative current in the potential range studied. The increase in the CV current in the case of alkali or alkaline earth metal ions, such as K+, Ba2+, and

in the peak current in the case of the composite-modified electrode was observed, but Hg2+ produced more reduction in the peak current compared with the blank ITO electrode (99%) and Ag+ (85%). Furthermore, as the oxidation and reduction potentials of both the ions are different, one can easily differentiate between the two ions. As the current electrochemical detection technique employs both the standard electrode potential of a metal ion and its interaction with the electrode surface, the employed electrode will have marked advantage for selective detection. The more quenching of the reductive peak current for Hg2+ may be attributed to the strong interaction of the Hg2+ ion with P4VP chains compared to that of Ag+ ions31 (UV−vis spectra; Figure S7, Supporting Information). The relatively strong coordination of Hg2+ to the P4VP chains anchored to the graphene surface was also evidenced from the FTIR study (Figure S8, Supporting Information). The characteristic absorption peak at 1602 cm−1 corresponding to the CN stretching vibration of pyridine of P4VP chains31 shifted to 1614 cm−1 upon coordination with Hg2+, whereas the peak at 1602 cm−1 remained unchanged after coordination with the Ag+ ion. In the case of both ions, the dramatic decrease in the peak current may be explained by strong binding of the metal ions with the P4VP polymer chains located at the outer surface of the electrode, resulting in a substantial reduction in charge transfer at the electrode− solution interface. Electrochemical impedance spectroscopy 7111

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Figure 8. (a) Current−voltage (I−V) characteristics of different ITO/BCP-PBA-RGO/Al devices, (b) electrical switching characteristics of the device made from BCP-PBA-RGO containing 15 wt % PBA-RGO and with a thickness of ∼180 nm, (c) schematics of the BCP-PBA-RGO devices, and (d) energy level alignment of ITO, PBA-RGO, and Al in the fabricated devices.

Figure 9. (a) Retention test of HRS and LRS of the ITO/ BCP-PBA-RGO-C/Al device in the negative voltage region. The current was read at a voltage of −4 V. (b) Experimental and fitted I−V characteristics of the device in the OFF state and ON state.

Mg2+, may be attributed to their weak association with P4VP chains compared to that of transition metal ions, resulting in relatively low charge transfer resistance and their extra contribution toward double layer formation from the graphene layers present in the composite materials. The preliminary results clearly indicate that the proposed composite material may be useful for electrochemical detection of metal ions. The sensitivity and specificity can be further improved by functionalizing the P4VP chains, i.e., with metal/metal oxide nanoparticles. Figure 8a depicts the I−V characteristics of ITO/BCP-PBA-RGO/Al devices at room temperature (for I− V characteristics of a flexible device from ITO-coated PET, see Figure S9). The work function of ITO ranges from 4.2 to 4.7 eV, and for RGO, it varies between 4.7 and 4.9 eV depending upon the material processing,49 whereas Al is known to have a work function of 4.3 eV.50 The device made from an insulating

block copolymer shows a very low current because of its insulating nature. As expected, devices of composites containing a small amount of PBA-RGO (2 and 5%) do not show any electric transition in the voltage sweep because of the high percolation threshold. However, devices made from a block copolymer composite thin film with a higher content of PBA-RGO, 10% (BCP-PBA-RGO-A, thickness ∼60 nm) and 15% (BCP-PBA-RGO-B, thickness ∼50 nm and BCP-PBARGO-C, thickness ∼180 nm), result in a high current, indicating a low electrical percolation threshold value. The thin film devices of BCP-PBA-RGO-B and BCP-PBA-RGO-C are highly symmetrical in nature, indicating the absence of the Schottky barrier. However, the device made from the block copolymer composite having 10% PBA-RGO (BCP-PBARGO-A) shows a rectifying characteristic (inset of Figure 8a). Resistive memory exhibits two distinct resistive states, which 7112

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charge hopping can occur with a coexisting charge trapping environment. It is reported that graphene oxide can serve as a charge trapping center from the structural defect.55,56 At a lower voltage, charge transport is limited because of the insulating barrier provided by BCP and PBA molecules anchored on the graphene surface, and RGO may capture the injected charge from the electrode. At the switching voltage, interplane hopping occurs between the RGO sheets, traps are filled, and the overall electrical conductivity increases through the trap-free environment. In this case, the ON/OFF ratio is lower compared to that in other polymer graphenebased devices reported in the literature18 because of the less amount of structural defects in PBA-RGO compared to those in GO, resulting in less trapping of charges.

can be written at a specific voltage and nondestructively read with another voltage, resulting in hysteresis in its current− voltage (I−V) characteristics. The device can be reversibly switched between a high-resistance state (HRS, OFF) and lowresistance state (LRS, ON) in direct current (dc) sweep or pulse mode.51,52 Figure 8b demonstrates the memristive switching of composite thin film device BCP-PBA-RGO-C by showing typical pinch hysteresis-like current−voltage characteristics and electrical bistability on a linear scale obtained by sweeping the dc bias in four steps (−6 → 0 → +6 → 0 → −6). Indeed, bipolar memristive switching characteristics are observed. Multiple switching I−V curves (where each curve almost follows the same path of its predecessor) from the same device (Figure S10, Supporting Information) clearly demonstrate its high degree of repeatability. The current after the application of a V-sweep of positive and negative polarity demonstrated that a positive V-sweep (−6 to 0 V) induces a low-resistance state (LRS, OFF) in the negative voltage region by the repeated read-out at −4 V (Figure 8b and Supporting Information) and a subsequent application of a large negative V-sweep (0 to −6) induces a high-resistance state (HRS, ON) in the negative voltage region by the repeated read-out at −4 V. Note that the resistance switching states can also be established in the positive voltage region. Therefore, application of a large positive V-sweep (0 to +6 V) induces the HRS and a large negative V-sweep induces the LRS. The characteristics are reproducible in all memory cells of the composite device. Both HRS and LRS are stable at a read-out voltage of −4 V (Figure 9a). Space-charge-limited conduction (SCLC) takes place in low-mobility semiconductors, when the injected charge density exceeds the intrinsic free carrier density of the materials. It is the dominant transport mechanism if at least one contact is able to inject relatively higher carrier densities than those the material has in thermal equilibrium. In the devices under investigation, ITO is expected to form an ohmic contact with PBA-RGO by making the conduction space-charge-limited. Therefore, the measured I−V data were analyzed in light of SCLC to get insight into the conduction mechanism of BCP-PBA-RGO. At moderate voltage, the current−voltage relationship becomes nonlinear as the SCLC dominates. When the trap states are absent or do not affect the charge transport, the current density is defined by the Mott−Gurney law: J = (9/8)εε0μV2/L3, where J is the current density, ε is the dielectric constant of the polymer, ε0 is the permittivity of the free space, V is the voltage drop across the device, and L is the thickness of the layer. To find the SCLC region, we fitted measured data with the relation J ∝ V, where the symbols are experimental data points and solid lines represent fit to them. Under the low voltage of the OFF state (−5 to 0 V), the curve shows a linear relationship with m ≈ 1, signifying ohmic conduction that might be controlled by the thermally generated free carriers. At a higher voltage, the slope of the I−V curve in the log−log scale increases to 2 (m ≈ 2) at the high voltage bias that reveals predominated trap-free SCLC conduction. However, the ON state (0 to −5) shows ohmic conduction in the lower-voltage region and trap-free SCLC conduction in the intermediate-voltage region. Above a voltage of 1.5 V, m > 2, indicating that the conduction is governed by traps that are exponentially distributed in energy.53,54 Such conduction is known as exponentially distributed trap-limited SCLC. The bistable hysteresis properties correspond to the amount of traps present in the composite thin film layer, and

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CONCLUSIONS A novel strategy for preparing highly solution processable graphene through a supramolecular assembly consisting of block copolymer polystyrene-b-poly(4-vinylpyridine) (PS-bP4VP) and pyrenebutyric acid (PBA)-modified reduced graphene oxide (RGO) is reported here. The PBA molecules anchored on the graphene surface form supramolecules with PS-b-P4VP through H-bonding between the carboxylic acid group of 1-pyrenebutyric acid and the pyridine ring of P4VP. The formation of a supramolecular assembly results in a highly stable solution of reduced graphene in common organic solvents. Transparent and mechanically stable thin films on versatile substrates, such as silicon, glass, ITO, and flexible polymer substrates like PET, can be fabricated from this supramolecular assembly solution. The reduced graphene oxide surface modifier can be selectively removed from the thin film to create a porous electrode material with free pyridine moieties. The electrochemical properties of this ultrathin electrode show an areal capacitance value of 17.73 μF/cm2 at a current density of 2.66 μA/cm2 with good electrochemical stability. The pendent free P4VP chains that can coordinate with metal ions were further exploited for their possible use in the electrochemical detection of metal ions. Interestingly, such a composite electrode exhibits selective, quantitative, and more sensitive detection of toxic Hg2+ ions. The electrical measurement of the thin film sandwich structure of the composite clearly indicates a bipolar resistive switching memory with hysteresis-like current−voltage characteristics and electrical bistability. The conduction mechanism of the sandwich device was investigated. The OFF state shows ohmic conduction at a lower voltage and trap-free SCLC conduction at high voltage. The ON state conduction is controlled by ohmic at low bias voltage, trap-free SCLC at moderate voltage, and trap-assisted SCLC at high voltage. We believe that these types of functional thin films will find application for thin film capacitors, metal ion sensors, and electronic and memory devices.

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MATERIALS AND METHODS Materials. Graphite powder, pyrenebutyric acid (PBA), 1,4-dioxane, and hydrazine monohydrate were purchased from Sigma-Aldrich. Other chemicals such as HgCl2, AgNO3, H2O2, KMnO4, NaNO3, sulfuric acid (98%), and NaOH were purchased from commercial sources in analytical grade. ITO (resistance 70−100 Ω/sq.) was purchased from Sigma-Aldrich and cleaned before use in thin film fabrication and electro7113

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chemical measurements. PS (32 900)-b-P4VP (8000) (PDI = 1.06) was purchased from Polymer Source Inc. Synthesis of the Materials. Graphene oxide was synthesized following the modified Hummers and Offeman method.57,58 The preparation of PBA-functionalized reduced graphene oxide (PBA-RGO) was carried out according to the procedure reported in the earlier literature with some modification.10 The block copolymer (BCP) supramolecular assembly with different compositions of PBA-functionalized graphene oxide (BCP-PBA-RGO) was prepared by dissolving PS-b-P4VP and PBA-RGO in 1,4-dioxane by sonication for 1 h. PBA-RGO was taken as 15% by weight for all measurements unless otherwise mentioned because this composition shows the best results. The resulting solution in 1,4-dioxane (the total concentration of PS-b-P4VP and PBA-RGO, 1−2% (w/v)) was further heated at 60 °C for 3 h and kept overnight at room temperature to complete the hydrogen-bond formation. The solution is further centrifuged at 5000 rpm for 10 min to remove any undissolved large coagulation. The resulting dark colloidal solution of the reduced graphene oxide and block copolymer after centrifugation is quite stable even for 1 month and more. Thin films of the composite solution on the ITO/ glass substrate were deposited by spin-coating from the filtered solutions. Using varying spin rates and concentrations, films of different thicknesses were deposited and the thicknesses were measured by AFM. Before electrochemical measurements, the composite films were annealed in the 1,4-dioxane vapor for 20 min to obtain the equilibrium structure. The solvent-annealed thin films were immersed into ethanol for 10 min (a good solvent for P4VP and a nonsolvent for PS). The dipping or washing process in ethanol will selectively remove PBA, and surface reconstruction will occur with a porous and fine structure.16,36 Characterization. FTIR measurements in the region of 4000−500 cm−1 were performed at room temperature using a KBr pellet of the samples in a PerkinElmer FTIR spectrometer (Spectrum Two). UV−vis absorption spectra were recorded on Shimadzu UV-3600 plus (UV−vis−NIR spectrophotometer). AFM imaging was performed on spin-coated thin films (NT-MDT, Russia; Solver Pro-4, atomic force microscope) in the tapping mode. All electrochemical experiments, such as cyclic voltammetry (CV), galvanostatic charge−discharge (GCD) methods, and electrochemical impedance spectroscopy (EIS), were performed using a CHI6087E electrochemical workstation (CHI) by a conventional three-electrode system. In the present study, Ag/AgCl as the reference electrode, a Pt wire electrode as the counter electrode, blank ITO or block copolymer graphene thin film deposited on an ITO-coated glass substrate as a working electrode, and 0.5 M Na2SO4 as the supporting electrolyte were used. For the electrochemical sensing study, different metal salts of prerequisite concentrations were dissolved in 0.5 M Na2SO4 solution and studied. I−V measurements were carried out from the sandwich structures on the ITO-coated glass substrate. Thin films were spin-casted from the solution of BCP-PBA-RGO on a thoroughly cleaned ITO-coated glass substrate. The samples were dried at 60 °C for an additional 3 h in a vacuum oven for the complete removal of the solvent. A layer of Al was deposited on the thin film to act as the top electrode. I−V characteristics of the samples were measured using a Keithley SourceMeter, model 2450 SMU, under vacuum.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00883. TGA data of PBA, PBA-RGO, and GO; UV−vis spectra of BCP-PBA-RGO; AFM image with height profile; GCD of ITO; CV curves for Ag+; I−V characteristics of BCP-PBA-RGO; and FTIR spectra of BCP-PBA-RGO after coordinating with metal ions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Biplab Kumar Kuila: 0000-0002-1646-2870 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.K.K. is grateful to the DST for a Ramanujan Fellowship (SR/ S2/RJN81/2010). B.K.K. is also grateful to DST (SR/FT/CS169/2011) and CSIR [02(0117)/13/EMR-II] for financial assistance under the projects. P.K. and K.K. acknowledge DST, Govt. of India, for awarding the fellowship.

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REFERENCES

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