DOI: 10.1002/chem.201303391
Full Paper
& Electronic Devices
Composite PET Membrane with Nanostructured Ag/AgTCNQ Schottky Junctions: Electrochemical Nanofabrication and ChargeTransfer Properties** Li Huang,[a] Yong Chen,[b] Shujuan Bian,[b] Yi-Fan Huang,[a] Zhong-Qun Tian,[a] and Dongping Zhan*[a]
metallic Ag, a bipolar mechanism is proposed to explain the successive growth of AgTCNQ nanorods and Ag film on each side of the PET membrane. Due to the well-formed nanostructure of Ag/AgTCNQ Schottky junctions, the direct electrochemical behavior is observed, which is essential to explain the physicochemical mechanism of its electric performance. Moreover, the composite PET membrane with nanostructured Ag/AgTCNQ Schottky junctions is tailorable and can be assembled directly into electric devices without any pretreatment.
Abstract: Large-area nanostructured Ag/Ag-tetracyanoquinodimethane (TCNQ) Schottky junctions are fabricated electrochemically on a mesoporous polyethylene terephthalate (PET) membrane-supported water/1, 2-dichloroethane (DCE) interface. When the interface is polarized, Ag + ions transfer across the PET membrane from the aqueous phase and are reduced to form metallic Ag on the PET membrane, which reacts further with tetracyanoquinodimethane (TCNQ) in the DCE phase to form nanostructured Ag/AgTCNQ Schottky junctions. Once the mesoporous membrane is blocked by
Introduction
tion.[5] Electrochemistry also plays an important role in the preparation of AgTCNQ materials, in which the precursors are generated through a reduction reaction on the electrode.[6] Because both Ag + and TCNQ are redox active, the electrochemical synthesis can undergo through Equation (1), Equation (2), or a mixed mechanism by tuning the applied potential.[6c]
Silver-tetracyanoquinodimethane (AgTCNQ), a semiconductive organic salt, has been extensively studied for sensors, reversible hysteretic switches, large capacity memory, and microelectronic devices because of its excellent conductivity, electronic and photonic bistability.[1, 2] In principle, AgTCNQ can be synthesized through either of the following reactions:
Ag þ TCNQ ! AgTCNQ
ð1Þ
Agþ þ TCNQC ! AgTCNQ
ð2Þ
When two immiscible liquids meet together, a liquid/liquid interface is formed, usually known as a water/organic interface.[7] Liquid/liquid interface has been employed as a medium to generate nanocrystals or nanocrystalline films of metals, semiconductors, and oxides through the diffusion of precursors in each phase followed by an interfacial reaction.[8] The processes can be performed better through electrochemistry. When a potential is applied to the liquid/liquid interface, metal ions are transferred from the water phase into the organic phase and reduced therein to form metallic nanoparticles or film at the interface.[9–11] However, the size, morphology, and homogeneity of nanocrystalline materials are hard to control. To our knowledge, the synthesis of AgTCNQ nanomaterials has not been reported yet although TCNQ is used as a classic redox couple in the electrochemistry of a liquid/liquid interface. On the other hand, scientists are never satisfied to simply discover a nanomaterial, but also strive to understand how to assemble them in the meso- and macroscale for practical utilities.[12] With the help of lithography, AgTCNQ nanowire patterns were obtained on silicon wafer, glass and quartz slides, and flexible plastic sheets by using patterned Ag film as precursor.[3, 4c] Unfortunately, these substrates are insulators and cannot be used directly in thin-film electronic devices. Based
In experiments, AgTCNQ nanomaterials have been obtained through vacuum vapor deposition,[3] vapor-solid chemical reaction,[4] and homogeneous or heterogeneous solution reac[a] L. Huang, Dr. Y.-F. Huang, Prof. Z.-Q. Tian, Prof. D. Zhan State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry College of Chemistry and Chemical Engineering Xiamen University, Xiamen 361005 (P.R. China) Tel: (+ 86) 592-2185797 E-mail: [email protected] [b] Prof. Y. Chen, S. Bian School of Chemical and Environmental Engineering Shanghai Institute of Technology Shanghai 201418 (China) [**] TCNQ = Tetracyanoquinodimethane; PET = polyethylene terephthalate. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303391. Chem. Eur. J. 2014, 20, 724 – 728
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Full Paper 100 mV s1. That means the transferred Ag + ions are consumed completely and predicts the further reaction between Ag and TCNQ (Curve 3 in Figure 1 b). Figure 1 c shows the photos of the two sides of the membrane: on one side of the PET membrane the blue color shows the formation of AgTCNQ; on the other side the grey color shows the formation of the metallic Ag film. From the SEM images in Figure 1 d, the AgTCNQ are actually nanorods with an average diameter of 120 nm and length of 800 nm.
on the mesoporous PET membrane-supported water/1,2-dichloroethane (W/DCE) interface, an electrochemical method is proposed for the large-area nanofabrication of orderly Ag/ AgTCNQ Schottky junctions, which have excellent electrochemical and electric properties.
Results and Discussion The electrochemical cell used for the nanofabrication of Ag/ AgTCNQ Schottky junctions on mesoporous PET membrane is formulated as follows:[13] Cell 1: Ag j x mmol Ag2SO4 + 100 mmol Na2SO4 (W) j j 5 mmol BTPPATPBCl + y mmol TCNQ (DCE) j AgTPBCl j Ag in which, the symbol j j is the mesoporous PET membrane-supported W/DCE interface. When there is no Ag2SO4 in the aqueous phase and TCNQ in the DCE phase, only the potential window is obtained (Curve 1 in Figure 1 b). With the presence of Ag2SO4 in aqueous phase, a pair of Faraday current peaks appears in the potential window, which corresponds to the Ag + transfer across the interface (Curve 2 in Figure 1 b). However, the cathodic peak current will be much smaller than the anodic current at a scanning rate of 10 mV s1 (inset in Figure 1 b). The result indicates that Ag + can be reduced by the supporting anion in the DCE phase (TPBCl) to form metallic Ag on the PET film with a slow kinetic rate.[14] If TCNQ is added into DCE phase, the cathodic current disappears and the Ag + transfer behaves totally irreversible even at a scanning rate of
Figure 2. Raman spectra of a) pure TCNQ and b) AgTCNQ nanorods synthesized at the PET membrane-supported W/1,2-DCE interface. c) XRD data of the Ag/AgTCNQ nanorods/PET composite membrane. d) Background of the PET membrane.
Figure 1. a) Schematic diagram of the electrochemical cell: 1 is the DCE phase; 2 the aqueous phase; 3 the reference electrode in DCE phase; 4 the reference electrode in aqueous phase; 5 the counter electrode; 6 the PET membrane supported W/DCE interface. b) Cyclic voltammograms of ion transfer obtained from Cell 1 with a scanning rate of 100 mV s1, curve 1: x = 0, y = 0; curve 2: x = 1, y = 0; curve 3: x = 1, y = 5; the insert: x = 1, y = 0, scanning rate 10 mV s1. c) Photograph of the PET membrane after the electrochemical synthesis of AgTCNQ nanorods. d) SEM images of AgTCNQ nanorods obtained by cyclic voltammetry at scanning rate of 50 mV s1 for 25 cycles. The inset shows its high resolution SEM images. Chem. Eur. J. 2014, 20, 724 – 728
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Figure 2 shows the Raman and XRD spectra of the AgTCNQ nanorods composite PET membrane obtained by a scanning rate of 50 mV s1 for 25 cycles. The Raman bonds at 1208, 1456, 1603, and 2228 cm1 correspond to the C=CH bending mode, CCN wing stretching mode, C=C ring stretching mode, and CN stretching mode of neutral TCNQ molecules. The Raman bonds of neutral TCNQ molecules at 1456 and 2228 cm1 shift to 1386 and 2212 cm1, which confirm the full charge-transfer between Ag and TCNQ to form AgTCNQ. No peak appears at 1456 cm1, which indicates that the AgTCNQ nanorods are of high purity.[1d, 2a] In XRD spectra, the small peaks at 10.4, 14.7, 19.1 8 are indexed to the orthorhombic structure of AgTCNQ with lattice constants of a = 6.95, b = 16.69, c = 17.45 , which accord harmoniously to the previous reported results.[15] Note that the peaks attribute to the reduced metallic Ag at 38.2 8, and the background of the PET membrane at 25.9 8 (Figure 2 d). Combining with the results shown in Figure 1 c, it is evident that a composite PET membrane with large-area nanostructured Ag/AgTCNQ Schottky junctions is obtained. Actually, the morphology of the AgTCNQ nanorods is influenced by the electrochemical modulations and the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper concentration of the precursors (see Figure S1 in the Supporting Information). Based on the above experimental results, the formation mechanism of the composite PET membrane with large-area nanostructured Ag/AgTCNQ Schottky junctions can be summarized as follows. Initially, Ag + ions transfer across the PET membrane and enter into the DCE phase in which they meet and react with TPBCl anions to produce metallic Ag. The produced Ag reacts with TCNQ in DCE phase to form AgTCNQ: Agþ ðWÞ ! Agþ ðDCEÞ
ð3Þ
Agþ ðDCEÞ þ TPBCl ðDCEÞ ! Ag ðPETÞ þ TPBClC
ð4Þ
Ag ðPETÞ þ TCNQ ðDCEÞ ! AgTCNQ ðPETÞ
ð5Þ Figure 3. a) Schematic diagram of the all-in-solid-state electrolytic cell, in which Cu foil and Al film are adopted as the current collectors. b) All-insolid-state cyclic voltammetric behaviors of the Schottky junctions of Ag/ AgTCNQ nanorods at different scanning rates, the inset shows the linear relationship between peak currents and scanning rates. c) All-in-solid-state Tafel behavior of the Schottky junctions of Ag/AgTCNQ nanorods, the inset shows the linear Tafel relationship between the current and overpotential of the cathodic branch. d) Multiple write–read–erase–read (WRER) cycles of the testing device as a memory element, the writing, reading, erase and reading voltages are 4, 0, 4, and 4 V, respectively.
The participation of TPBCl is very important to form the metallic Ag (see the Supporting Information, Figure S2), which is the precursor for the formation of AgTCNQ nanorods.[14] Because the PET membrane is hydrophilic and filled with the aqueous solution, AgTCNQ nanorods are formed on the DCE side of the PET membrane. If some pores on PET membrane are blocked by metallic Ag, a bipolar mechanism works because the polarizing potential is actually applied on the metallic Ag composite PET membrane.[15] On the water side of PET membrane, Ag + ions accept electrons to produce metallic Ag:
Agþ ðWÞ þ e ! Ag ðPETÞ
AgTCNQ þ e Ð Ag þ TCNQ
The faradic peak current is in proportion to the scanning rate and the integrate charge of the anodic peak is equal to that of the cathodic peak, which shows the typical characteristics of solid-state electrochemistry. A Tafel experiment is performed to evaluate the kinetic parameters of the nanostructured Ag/AgTCNQ Schottky junctions (Figure 3 c). From the Tafel Equation:
ð6Þ
On the DCE side of the PET membrane, metallic Ag dissolves into the DCE phase as Ag + ions and donates electrons to TCNQ, consequently, AgTCNQ is obtained.
Ag ðPETÞ ! Agþ ðDCEÞ þ e
ð7Þ
TCNQ ðDCEÞ þ e ! TCNQ ðDCEÞ
ð8Þ
þ
Ag ðDCEÞ þ TCNQ ðDCEÞ ! AgTCNQ ðPETÞ
h¼
ð9Þ
Because the nanostructured Ag/AgTCNQ Schottky junctions are well-formed during the preparation, the contact effect between the current collector and AgTCNQ nanorods can be avoided, which might predict their excellent electric properties.[13] The composite PET membrane with orderly nanostructured Ag/AgTCNQ Schottky junctions was tailored with a diameter of 6 mm and assembled between a copper and aluminum foils to construct an all-in-solid-state electrolytic cell (Figure 3 a). Figure 3 b shows the obtained cyclic voltammograms. The charge-transfer reaction occurred at the nanostructured Ag/AgTCNQ Schottky junctions can be expressed as follows: www.chemeurj.org
2:303RT 2:303RT log i0 þ log i anF anF
ð11Þ
in which, h is the overpotential, n the transferred charge number, F the Faraday constant, R the gas constant, T the absolute temperature, a the charge-transfer coefficient, i0 is the exchanging current density, i is the experimental current density, the values of a and i0 are obtained as 0.533 and 2.21 106 A cm2, which indicate the excellent chemical reversibility and stability. The pulse potential response of the system is shown in Figure 3 d. It can be observed that the steady-state current is in harmonious accordance with that of the cyclic voltammetry. The results explain the physicochemical mechanism for the low-potential switch effects (LPSE) of AgTCNQ reported previously.[18] To our knowledge, this is the first case of electrochemical evidence for the charge-transfer mechanism of Ag/ AgTCNQ Schottky junctions although it has been hypothesized previously.[1d, 19] The electron transfer across the Schottky junctions predicts the composite PET membrane with nanostructured Ag/AgTCNQ Schottky junctions has an excellent electronic properties at low potential.
The biopolar mechanism is crucial to explain the growth of AgTCNQ and metallic Ag film on each side of the PET membrane. The whole effect involves Ag + ion transfer coupled by an electron transfer, which is similar to the ion-transfer-based growth mechanism for the preparation of CuTCNQ nanowire in an alumina template.[17]
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ð10Þ
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Full Paper To construct a practical electronic device, a series load resister (4.7 kW) is added to the all-in-solid-state electrolytic cell mentioned above (Figure 3 a). The I/V performance of the composite PET membrane with nanostructured Ag/AgTCNQ Schottky junctions is shown in Figure 4 a. When the potential is
electronic devices without any further pretreatments. Because the Schottky junctions between Ag and AgTCNQ nanorods are well-formed during the process of nanofabrication, the reversible electron-transfer across the Ag/AgTCNQ Schottky junctions results in an excellent all-in-solid electronic performances. Most importantly, the chemical principle of the electric properties of Ag/AgTCNQ Schottky junctions is convinced experimentally.
Experimental Section Preparation of the PET membrane Anhydrous ethanol (7.6792 g), Tetraethylorthosilicate (11.5774 g), and a solution of HCl (1 mL, 2.8 mmol L1) were added into a three-necked flask and mixed evenly. The flask was heated and refluxed in an oil bath for 90 min at a temperature of 60 8C. Then, the mixture was cooled down to room temperature. Next, cetrimonium bromide (1.5246 g), anhydrous ethanol (15.0022 g), and a solution of HCl (4 mL, 55 mmol L1) were mixed and added to the three-necked flask for 30 min to obtain the precursor solution. After this, at room temperature, the obtained precursor solution (2 mL) was filtered through the blank PET membrane by suction filtration for 5 min until the surface of the PET membrane was substantially dry. Next, the PET membrane was placed in the mixed solution of concentrated HCl acid and ethanol (volume ratio: 1:5), demolded through ultrasonic extraction for 15 min, and filtered under a vacuum. The PET membrane was then washed with the concentrated HCl acid and ethanol mixture (volume ratio: 1:5) at 80 8C, water and ethanol in turn. Finally, the PET membrane was dried in an oven at the temperature of 60 8C and was then ready for use in the following experiments.
Figure 4. a) Switching effect of the testing device between high and low resistance states at a potential scanning rate of 1 V s1. b) Multiple write-readerase-read (WRER) cycles of the testing device as a memory element, the writing, reading, erase and reading voltages are 4.5,0, 6, and 4.5 V, respectively.
scanning cathodically from zero volts, the composite PET membrane is in high resistance state (Process 1). The resistance decreases dramatically when the applied potential arrives at 5.8 V (Process 2). The system remains in a stable low-resistance state when the potential is scanning anodically (Processes 3 and 4). The resistance increases dramatically when the applied potential is at + 4.5 V (Process 5) and switches into the high-resistance state. The ON/OFF ratio of this composite membrane is higher than 103. Once the composite PET membrane is stimulated into the low-resistance state, the resistance remains almost constant in the potential region. This result is valuable for its further application as memory devices. As shown in Figure 4 b, three states exist (+ 1, 0 and 1) when changing the polarity of applied potentials (+ 4.5 V, 0 V and 6 V). It can be concluded from the results that the writing and erasing processes are very reproducible, that is, excellent high-potential switch effects (HPSE). The reversible hysteretic electrical switching behavior may be caused by an electricfield-induced redox reaction at the interface, which involved electrochemical formation and dissolution of Ag filaments at the Ag/AgTCNQ interface from the high impedance AgTCNQ salt, which is coupled by the oxidation and reduction of the Al current collector.[20] The difference between LPSE and HPSE lies in that in the case of the LPSE reaction, Equation (10) occurs in the vicinity of Ag/AgTCNQ junction, whereas in the case of HPSE reaction, Equation (10) propagates from the Ag/AgTCNQ junction into the buck phase of AgTCNQ. It can be concluded that Equation (10) is the basic chemical principle for the electric performances of Ag/AgTCNQ junction.
Electrosynthesis and characterization of nanostructured Ag/ AgTCNQ Schottky junction composite PET membrane A three-electrode system was employed for the nanofabrication (Figure 1 a). The orifice of a glass tube was sealed with the prepared PET membrane. The aqueous solution was put into the PET membrane sealed glass tube, whereas the DCE solution was contained in the electrolyte cell. The Ag/Ag2SO4 wire was used as the quasi-referenced electrode in aqueous phase whereas the Ag/ AgTPBCl wire was used as the electrode in the DCE phase. A platinum wire was employed as the counter electrode. Both cyclic voltammetry and potential step method were performed to synthesize the Ag/AgTCNQ nanorods composite membrane. The obtained Ag/AgTCNQ nanorods composite PET membrane was removed from the glass tube, rinsed with water and DCE to remove the resident supporting electrolytes, and dried at room temperature. The membrane was then characterized by using XRD (PANalytical B.V. Co., Holland), SEM (Hitachi High-Technologies Co., Japan), and Raman (Renishaw Co., British) spectroscopy. The electrochemical and electronic measurements were performed by a CHI920C electrochemical workstation (CHI Instrument Co., USA) through assembling Ag/AgTCNQ nanorods composite membrane directly into all-in-solid sandwich cells.
Conclusion
Acknowledgements
In brief, a large-area nanofabrication method is proposed for the nanostructured Ag/AgTCNQ Schottky junctions composite PET membrane based on the electrochemistry of liquid/liquid interface, which can be tailored and assembled directly into Chem. Eur. J. 2014, 20, 724 – 728
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Financial support by the National Science Foundation of China (21061120456, 21005049, 21021002, 20973142), the National Basic Research Program of China (2012CB93290, 727
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Full Paper 2011CB933700), the Natural Science Foundation of Fujian Province of China (Grant 2012J06004), the Program for New Century Excellent Talents in University (NCET-12-0318) are appreciated. [9]
Keywords: composite materials · electrochemistry · electron transfer · nanostructures · silver [10]
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Received: August 29, 2013 Published online on December 11, 2013
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& Electronic Devices
Composite PET Membrane with Nanostructured Ag/AgTCNQ Schottky Junctions: Electrochemical Nanofabrication and ChargeTransfer Properties** Li Huang,[a] Yong Chen,[b] Shujuan Bian,[b] Yi-Fan Huang,[a] Zhong-Qun Tian,[a] and Dongping Zhan*[a]
metallic Ag, a bipolar mechanism is proposed to explain the successive growth of AgTCNQ nanorods and Ag film on each side of the PET membrane. Due to the well-formed nanostructure of Ag/AgTCNQ Schottky junctions, the direct electrochemical behavior is observed, which is essential to explain the physicochemical mechanism of its electric performance. Moreover, the composite PET membrane with nanostructured Ag/AgTCNQ Schottky junctions is tailorable and can be assembled directly into electric devices without any pretreatment.
Abstract: Large-area nanostructured Ag/Ag-tetracyanoquinodimethane (TCNQ) Schottky junctions are fabricated electrochemically on a mesoporous polyethylene terephthalate (PET) membrane-supported water/1, 2-dichloroethane (DCE) interface. When the interface is polarized, Ag + ions transfer across the PET membrane from the aqueous phase and are reduced to form metallic Ag on the PET membrane, which reacts further with tetracyanoquinodimethane (TCNQ) in the DCE phase to form nanostructured Ag/AgTCNQ Schottky junctions. Once the mesoporous membrane is blocked by
Introduction
tion.[5] Electrochemistry also plays an important role in the preparation of AgTCNQ materials, in which the precursors are generated through a reduction reaction on the electrode.[6] Because both Ag + and TCNQ are redox active, the electrochemical synthesis can undergo through Equation (1), Equation (2), or a mixed mechanism by tuning the applied potential.[6c]
Silver-tetracyanoquinodimethane (AgTCNQ), a semiconductive organic salt, has been extensively studied for sensors, reversible hysteretic switches, large capacity memory, and microelectronic devices because of its excellent conductivity, electronic and photonic bistability.[1, 2] In principle, AgTCNQ can be synthesized through either of the following reactions:
Ag þ TCNQ ! AgTCNQ
ð1Þ
Agþ þ TCNQC ! AgTCNQ
ð2Þ
When two immiscible liquids meet together, a liquid/liquid interface is formed, usually known as a water/organic interface.[7] Liquid/liquid interface has been employed as a medium to generate nanocrystals or nanocrystalline films of metals, semiconductors, and oxides through the diffusion of precursors in each phase followed by an interfacial reaction.[8] The processes can be performed better through electrochemistry. When a potential is applied to the liquid/liquid interface, metal ions are transferred from the water phase into the organic phase and reduced therein to form metallic nanoparticles or film at the interface.[9–11] However, the size, morphology, and homogeneity of nanocrystalline materials are hard to control. To our knowledge, the synthesis of AgTCNQ nanomaterials has not been reported yet although TCNQ is used as a classic redox couple in the electrochemistry of a liquid/liquid interface. On the other hand, scientists are never satisfied to simply discover a nanomaterial, but also strive to understand how to assemble them in the meso- and macroscale for practical utilities.[12] With the help of lithography, AgTCNQ nanowire patterns were obtained on silicon wafer, glass and quartz slides, and flexible plastic sheets by using patterned Ag film as precursor.[3, 4c] Unfortunately, these substrates are insulators and cannot be used directly in thin-film electronic devices. Based
In experiments, AgTCNQ nanomaterials have been obtained through vacuum vapor deposition,[3] vapor-solid chemical reaction,[4] and homogeneous or heterogeneous solution reac[a] L. Huang, Dr. Y.-F. Huang, Prof. Z.-Q. Tian, Prof. D. Zhan State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry College of Chemistry and Chemical Engineering Xiamen University, Xiamen 361005 (P.R. China) Tel: (+ 86) 592-2185797 E-mail: [email protected] [b] Prof. Y. Chen, S. Bian School of Chemical and Environmental Engineering Shanghai Institute of Technology Shanghai 201418 (China) [**] TCNQ = Tetracyanoquinodimethane; PET = polyethylene terephthalate. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303391. Chem. Eur. J. 2014, 20, 724 – 728
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Full Paper 100 mV s1. That means the transferred Ag + ions are consumed completely and predicts the further reaction between Ag and TCNQ (Curve 3 in Figure 1 b). Figure 1 c shows the photos of the two sides of the membrane: on one side of the PET membrane the blue color shows the formation of AgTCNQ; on the other side the grey color shows the formation of the metallic Ag film. From the SEM images in Figure 1 d, the AgTCNQ are actually nanorods with an average diameter of 120 nm and length of 800 nm.
on the mesoporous PET membrane-supported water/1,2-dichloroethane (W/DCE) interface, an electrochemical method is proposed for the large-area nanofabrication of orderly Ag/ AgTCNQ Schottky junctions, which have excellent electrochemical and electric properties.
Results and Discussion The electrochemical cell used for the nanofabrication of Ag/ AgTCNQ Schottky junctions on mesoporous PET membrane is formulated as follows:[13] Cell 1: Ag j x mmol Ag2SO4 + 100 mmol Na2SO4 (W) j j 5 mmol BTPPATPBCl + y mmol TCNQ (DCE) j AgTPBCl j Ag in which, the symbol j j is the mesoporous PET membrane-supported W/DCE interface. When there is no Ag2SO4 in the aqueous phase and TCNQ in the DCE phase, only the potential window is obtained (Curve 1 in Figure 1 b). With the presence of Ag2SO4 in aqueous phase, a pair of Faraday current peaks appears in the potential window, which corresponds to the Ag + transfer across the interface (Curve 2 in Figure 1 b). However, the cathodic peak current will be much smaller than the anodic current at a scanning rate of 10 mV s1 (inset in Figure 1 b). The result indicates that Ag + can be reduced by the supporting anion in the DCE phase (TPBCl) to form metallic Ag on the PET film with a slow kinetic rate.[14] If TCNQ is added into DCE phase, the cathodic current disappears and the Ag + transfer behaves totally irreversible even at a scanning rate of
Figure 2. Raman spectra of a) pure TCNQ and b) AgTCNQ nanorods synthesized at the PET membrane-supported W/1,2-DCE interface. c) XRD data of the Ag/AgTCNQ nanorods/PET composite membrane. d) Background of the PET membrane.
Figure 1. a) Schematic diagram of the electrochemical cell: 1 is the DCE phase; 2 the aqueous phase; 3 the reference electrode in DCE phase; 4 the reference electrode in aqueous phase; 5 the counter electrode; 6 the PET membrane supported W/DCE interface. b) Cyclic voltammograms of ion transfer obtained from Cell 1 with a scanning rate of 100 mV s1, curve 1: x = 0, y = 0; curve 2: x = 1, y = 0; curve 3: x = 1, y = 5; the insert: x = 1, y = 0, scanning rate 10 mV s1. c) Photograph of the PET membrane after the electrochemical synthesis of AgTCNQ nanorods. d) SEM images of AgTCNQ nanorods obtained by cyclic voltammetry at scanning rate of 50 mV s1 for 25 cycles. The inset shows its high resolution SEM images. Chem. Eur. J. 2014, 20, 724 – 728
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Figure 2 shows the Raman and XRD spectra of the AgTCNQ nanorods composite PET membrane obtained by a scanning rate of 50 mV s1 for 25 cycles. The Raman bonds at 1208, 1456, 1603, and 2228 cm1 correspond to the C=CH bending mode, CCN wing stretching mode, C=C ring stretching mode, and CN stretching mode of neutral TCNQ molecules. The Raman bonds of neutral TCNQ molecules at 1456 and 2228 cm1 shift to 1386 and 2212 cm1, which confirm the full charge-transfer between Ag and TCNQ to form AgTCNQ. No peak appears at 1456 cm1, which indicates that the AgTCNQ nanorods are of high purity.[1d, 2a] In XRD spectra, the small peaks at 10.4, 14.7, 19.1 8 are indexed to the orthorhombic structure of AgTCNQ with lattice constants of a = 6.95, b = 16.69, c = 17.45 , which accord harmoniously to the previous reported results.[15] Note that the peaks attribute to the reduced metallic Ag at 38.2 8, and the background of the PET membrane at 25.9 8 (Figure 2 d). Combining with the results shown in Figure 1 c, it is evident that a composite PET membrane with large-area nanostructured Ag/AgTCNQ Schottky junctions is obtained. Actually, the morphology of the AgTCNQ nanorods is influenced by the electrochemical modulations and the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper concentration of the precursors (see Figure S1 in the Supporting Information). Based on the above experimental results, the formation mechanism of the composite PET membrane with large-area nanostructured Ag/AgTCNQ Schottky junctions can be summarized as follows. Initially, Ag + ions transfer across the PET membrane and enter into the DCE phase in which they meet and react with TPBCl anions to produce metallic Ag. The produced Ag reacts with TCNQ in DCE phase to form AgTCNQ: Agþ ðWÞ ! Agþ ðDCEÞ
ð3Þ
Agþ ðDCEÞ þ TPBCl ðDCEÞ ! Ag ðPETÞ þ TPBClC
ð4Þ
Ag ðPETÞ þ TCNQ ðDCEÞ ! AgTCNQ ðPETÞ
ð5Þ Figure 3. a) Schematic diagram of the all-in-solid-state electrolytic cell, in which Cu foil and Al film are adopted as the current collectors. b) All-insolid-state cyclic voltammetric behaviors of the Schottky junctions of Ag/ AgTCNQ nanorods at different scanning rates, the inset shows the linear relationship between peak currents and scanning rates. c) All-in-solid-state Tafel behavior of the Schottky junctions of Ag/AgTCNQ nanorods, the inset shows the linear Tafel relationship between the current and overpotential of the cathodic branch. d) Multiple write–read–erase–read (WRER) cycles of the testing device as a memory element, the writing, reading, erase and reading voltages are 4, 0, 4, and 4 V, respectively.
The participation of TPBCl is very important to form the metallic Ag (see the Supporting Information, Figure S2), which is the precursor for the formation of AgTCNQ nanorods.[14] Because the PET membrane is hydrophilic and filled with the aqueous solution, AgTCNQ nanorods are formed on the DCE side of the PET membrane. If some pores on PET membrane are blocked by metallic Ag, a bipolar mechanism works because the polarizing potential is actually applied on the metallic Ag composite PET membrane.[15] On the water side of PET membrane, Ag + ions accept electrons to produce metallic Ag:
Agþ ðWÞ þ e ! Ag ðPETÞ
AgTCNQ þ e Ð Ag þ TCNQ
The faradic peak current is in proportion to the scanning rate and the integrate charge of the anodic peak is equal to that of the cathodic peak, which shows the typical characteristics of solid-state electrochemistry. A Tafel experiment is performed to evaluate the kinetic parameters of the nanostructured Ag/AgTCNQ Schottky junctions (Figure 3 c). From the Tafel Equation:
ð6Þ
On the DCE side of the PET membrane, metallic Ag dissolves into the DCE phase as Ag + ions and donates electrons to TCNQ, consequently, AgTCNQ is obtained.
Ag ðPETÞ ! Agþ ðDCEÞ þ e
ð7Þ
TCNQ ðDCEÞ þ e ! TCNQ ðDCEÞ
ð8Þ
þ
Ag ðDCEÞ þ TCNQ ðDCEÞ ! AgTCNQ ðPETÞ
h¼
ð9Þ
Because the nanostructured Ag/AgTCNQ Schottky junctions are well-formed during the preparation, the contact effect between the current collector and AgTCNQ nanorods can be avoided, which might predict their excellent electric properties.[13] The composite PET membrane with orderly nanostructured Ag/AgTCNQ Schottky junctions was tailored with a diameter of 6 mm and assembled between a copper and aluminum foils to construct an all-in-solid-state electrolytic cell (Figure 3 a). Figure 3 b shows the obtained cyclic voltammograms. The charge-transfer reaction occurred at the nanostructured Ag/AgTCNQ Schottky junctions can be expressed as follows: www.chemeurj.org
2:303RT 2:303RT log i0 þ log i anF anF
ð11Þ
in which, h is the overpotential, n the transferred charge number, F the Faraday constant, R the gas constant, T the absolute temperature, a the charge-transfer coefficient, i0 is the exchanging current density, i is the experimental current density, the values of a and i0 are obtained as 0.533 and 2.21 106 A cm2, which indicate the excellent chemical reversibility and stability. The pulse potential response of the system is shown in Figure 3 d. It can be observed that the steady-state current is in harmonious accordance with that of the cyclic voltammetry. The results explain the physicochemical mechanism for the low-potential switch effects (LPSE) of AgTCNQ reported previously.[18] To our knowledge, this is the first case of electrochemical evidence for the charge-transfer mechanism of Ag/ AgTCNQ Schottky junctions although it has been hypothesized previously.[1d, 19] The electron transfer across the Schottky junctions predicts the composite PET membrane with nanostructured Ag/AgTCNQ Schottky junctions has an excellent electronic properties at low potential.
The biopolar mechanism is crucial to explain the growth of AgTCNQ and metallic Ag film on each side of the PET membrane. The whole effect involves Ag + ion transfer coupled by an electron transfer, which is similar to the ion-transfer-based growth mechanism for the preparation of CuTCNQ nanowire in an alumina template.[17]
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ð10Þ
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Full Paper To construct a practical electronic device, a series load resister (4.7 kW) is added to the all-in-solid-state electrolytic cell mentioned above (Figure 3 a). The I/V performance of the composite PET membrane with nanostructured Ag/AgTCNQ Schottky junctions is shown in Figure 4 a. When the potential is
electronic devices without any further pretreatments. Because the Schottky junctions between Ag and AgTCNQ nanorods are well-formed during the process of nanofabrication, the reversible electron-transfer across the Ag/AgTCNQ Schottky junctions results in an excellent all-in-solid electronic performances. Most importantly, the chemical principle of the electric properties of Ag/AgTCNQ Schottky junctions is convinced experimentally.
Experimental Section Preparation of the PET membrane Anhydrous ethanol (7.6792 g), Tetraethylorthosilicate (11.5774 g), and a solution of HCl (1 mL, 2.8 mmol L1) were added into a three-necked flask and mixed evenly. The flask was heated and refluxed in an oil bath for 90 min at a temperature of 60 8C. Then, the mixture was cooled down to room temperature. Next, cetrimonium bromide (1.5246 g), anhydrous ethanol (15.0022 g), and a solution of HCl (4 mL, 55 mmol L1) were mixed and added to the three-necked flask for 30 min to obtain the precursor solution. After this, at room temperature, the obtained precursor solution (2 mL) was filtered through the blank PET membrane by suction filtration for 5 min until the surface of the PET membrane was substantially dry. Next, the PET membrane was placed in the mixed solution of concentrated HCl acid and ethanol (volume ratio: 1:5), demolded through ultrasonic extraction for 15 min, and filtered under a vacuum. The PET membrane was then washed with the concentrated HCl acid and ethanol mixture (volume ratio: 1:5) at 80 8C, water and ethanol in turn. Finally, the PET membrane was dried in an oven at the temperature of 60 8C and was then ready for use in the following experiments.
Figure 4. a) Switching effect of the testing device between high and low resistance states at a potential scanning rate of 1 V s1. b) Multiple write-readerase-read (WRER) cycles of the testing device as a memory element, the writing, reading, erase and reading voltages are 4.5,0, 6, and 4.5 V, respectively.
scanning cathodically from zero volts, the composite PET membrane is in high resistance state (Process 1). The resistance decreases dramatically when the applied potential arrives at 5.8 V (Process 2). The system remains in a stable low-resistance state when the potential is scanning anodically (Processes 3 and 4). The resistance increases dramatically when the applied potential is at + 4.5 V (Process 5) and switches into the high-resistance state. The ON/OFF ratio of this composite membrane is higher than 103. Once the composite PET membrane is stimulated into the low-resistance state, the resistance remains almost constant in the potential region. This result is valuable for its further application as memory devices. As shown in Figure 4 b, three states exist (+ 1, 0 and 1) when changing the polarity of applied potentials (+ 4.5 V, 0 V and 6 V). It can be concluded from the results that the writing and erasing processes are very reproducible, that is, excellent high-potential switch effects (HPSE). The reversible hysteretic electrical switching behavior may be caused by an electricfield-induced redox reaction at the interface, which involved electrochemical formation and dissolution of Ag filaments at the Ag/AgTCNQ interface from the high impedance AgTCNQ salt, which is coupled by the oxidation and reduction of the Al current collector.[20] The difference between LPSE and HPSE lies in that in the case of the LPSE reaction, Equation (10) occurs in the vicinity of Ag/AgTCNQ junction, whereas in the case of HPSE reaction, Equation (10) propagates from the Ag/AgTCNQ junction into the buck phase of AgTCNQ. It can be concluded that Equation (10) is the basic chemical principle for the electric performances of Ag/AgTCNQ junction.
Electrosynthesis and characterization of nanostructured Ag/ AgTCNQ Schottky junction composite PET membrane A three-electrode system was employed for the nanofabrication (Figure 1 a). The orifice of a glass tube was sealed with the prepared PET membrane. The aqueous solution was put into the PET membrane sealed glass tube, whereas the DCE solution was contained in the electrolyte cell. The Ag/Ag2SO4 wire was used as the quasi-referenced electrode in aqueous phase whereas the Ag/ AgTPBCl wire was used as the electrode in the DCE phase. A platinum wire was employed as the counter electrode. Both cyclic voltammetry and potential step method were performed to synthesize the Ag/AgTCNQ nanorods composite membrane. The obtained Ag/AgTCNQ nanorods composite PET membrane was removed from the glass tube, rinsed with water and DCE to remove the resident supporting electrolytes, and dried at room temperature. The membrane was then characterized by using XRD (PANalytical B.V. Co., Holland), SEM (Hitachi High-Technologies Co., Japan), and Raman (Renishaw Co., British) spectroscopy. The electrochemical and electronic measurements were performed by a CHI920C electrochemical workstation (CHI Instrument Co., USA) through assembling Ag/AgTCNQ nanorods composite membrane directly into all-in-solid sandwich cells.
Conclusion
Acknowledgements
In brief, a large-area nanofabrication method is proposed for the nanostructured Ag/AgTCNQ Schottky junctions composite PET membrane based on the electrochemistry of liquid/liquid interface, which can be tailored and assembled directly into Chem. Eur. J. 2014, 20, 724 – 728
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Financial support by the National Science Foundation of China (21061120456, 21005049, 21021002, 20973142), the National Basic Research Program of China (2012CB93290, 727
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Full Paper 2011CB933700), the Natural Science Foundation of Fujian Province of China (Grant 2012J06004), the Program for New Century Excellent Talents in University (NCET-12-0318) are appreciated. [9]
Keywords: composite materials · electrochemistry · electron transfer · nanostructures · silver [10]
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Received: August 29, 2013 Published online on December 11, 2013
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