Rectification and Amplification of Ionic Current in Planar Graphene

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Rectification and Amplification of Ionic Current in Planar Graphene/ Graphene-Oxide Junctions: an Electrochemical Diode and Transistor Sourav Kanti Jana, Sangam Banerjee, Sayan Bayan, Harish Reddy Inta, and Venkataramanan Mahalingam J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01717 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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The Journal of Physical Chemistry

Rectification and Amplification of Ionic Current in Planar Graphene / Graphene-Oxide Junctions: An Electrochemical Diode and Transistor Sourav Kanti Jana1,2*, Sangam Banerjee2, Sayan Bayan2, Harish Reddy Inta1, Venkataramanan Mahalingam1* 1. Nanomaterials Lab, Department of Chemical Sciences Indian Institute of Science Education and Research Kolkata Mohanpur – 741246 West Bengal, India E-mail: [email protected], [email protected] 2. Surface Physics and Materials Science Division Saha Institute of Nuclear Physics 1/AF, Bidhannagar, Saltlake Kolkata-700064, India

ABSTRACT This manuscript describes the fabrication of a two dimensional planar junction, formed with graphene oxide (GO) and selective electrochemical reduced graphene oxides (ERGO), which exhibits rectification of ionic current in presence of electrolyte. Moreover, amplification of the ionic current has also been demonstrated in planar transistor configuration constituted with two back to back planar GO-ERGO junctions. Structural modification induce change in electronic property of ERGO samples compared to GO is observed and Mott-Schottky analysis confirms that the GO and ERGO are of n-type and p-type conductivity respectively, which determine interfacial charge transfer from either electrode to electrolyte or vice versa, correspondingly. Thus the ionic current is controlled by the modulation of the interfacial charge concentration by external voltage applied across the junction sample. Hence, this device exhibits bias dependent unidirectional ion current, presumably through electrochemical oxidation of OH- ions on ERGO (p side) and reduction of H+ ions on GO (n side) interface, which confirms the formation of an electrochemical p-n junction diode.

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INTRODUCTION Silicon based conventional semiconductor diodes and bipolar junction transistors have already laid a solid foundation in modern electronic devices industry. The popular Si based p-n junction diode fabricated with two electronically dissimilar semiconductors (n-type and p-type) sandwiched with each other allows the flow of electric current in one direction and blocks in the reverse direction. Owing to the doping (both n and p-type) of the two semiconductors, a depletion region (comprised of positive charges in the n side and negative charges in p side of the junction) is formed at the junction which controls the flow of ionic current and display rectification of the ionic current depending upon the polarity of the applied electric field.1 Recently, graphene has been identified as an alternative to Si based technology and has a huge potential for the development of new generation faster electronic devices due to its honeycomb structure of the carbon. The transport of charge carriers in graphene is similar to the mass less Dirac Fermions, Klein tunneling or ballistic transport at room temperature.2, 3 The remarkable mobility of charge carriers,1 and a significant mechanical strength4 make graphene a promising material for developing ultrafast and flexible electronics.5-7 However, the devices made from the zero-bandgap graphene are difficult to switch off which makes them less suitable for digital logic applications.8 Therefore for the development of graphene-based electronic devices, modification of the band structure of graphene is essential. Unlike in semiconductors, doping is very difficult to achieve in graphene to tune or modify its band structure. Consequently, a lot of research has been dedicated to find the way to control the doping of graphene with both type of charge carries (i.e. electrons and holes). There are few reviews on the research progress of graphene doping which are mostly categorized by mainly three processes: chemical modification,9 electric field tuning10-13 and hetero atom doping.14-17 Until now, electric field tuning finds to be one of the methods to achieve reliable control on the type and density of the charge carriers in graphene based electronic devices. Using this field effect method, Fermi level of pristine graphene can be finely tuned from conduction band to valence band by changing the gate voltage from negative to positive, resulting in the p and n-type graphene.18 Besides these doping processes, another strategy named surface transfer doping has been developed which draws much attention.19-21 This doping method finds to be efficient to tune the surface electronic properties of the active electrode material through interfacial charge transfer process between electrode and electrolyte. Electron transfer from electrode to electrolyte and electrolyte to electrode makes the electrode n2 Environment ACS Paragon Plus

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type or p-type, respectively. This electron transfer process at the electrode and electrolyte depends upon the relative position of the Fermi level with respect to the chemical potential of the electrolyte. In fact, the Fermi level can be tuned by modulating the work function of the graphene by tuning the concentration of the functional groups and by creating defects in the basal plane of the graphene.22 Our objective is to take advantage of the surface transfer electrochemical doping processes occurring at the electrode/electrolyte interface to develop a planar p-n junction diode by taking advantage of the electrochemical processes occurring at the semiconductor–electrolyte interface. Recently, a theoretical model by T. Yamato et al

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was

developed on ionic current rectification using two oppositely charged polyelectrolyte gel sandwiched between two symmetric electrodes and the potential dependent redistribution of counter ions control the rectification of ion current. Based on the above model, we report a new strategy to fabricate planar electrochemical diode using aqueous electrolyte in this work. This is achieved using two dissimilar electrodes like graphene oxide (GO) and electrochemically reduced graphene oxide (ERGO). The GO behaves as n-type whereas ERGO shows p-type nature when exposed to the electrolyte. Appropriate deposition of these two materials on a stainless steel (SS) led to the fabrication of planer p-n junction. Such junction displays diode like characteristics when placed in an electrolyte, while the same device shows explicitly different non-linear behaviour in the solid state mode (without electrolyte). This observation was further confirmed by the successful fabrication of an n-p-n type (GO-ERGO-GO) transistor. The observed input and output characteristics of the bipolar transistor strongly confirms our hypothesis. RESULTS AND DISCUSSION Prior to fabrication and electrochemical performance of GO-ERGO junction sample, detail synthesis protocol of GO, ERGO, and GO-ERGO junction samples are discussed in the experimental section as shown in scheme S1 of Supporting Information. Moreover, surface morphology of both GO and ERGO samples was characterized by scanning electron microscopy (SEM, images as shown in Figure S1) and variation of elemental oxygen along with carbon content of both GO and ERGO samples were recorded by EDX (spectra shown in Figure S2). With increase of reduction time, the amount of oxygen is decreasing in ERGO samples compared to GO sample. Structural characterizations of the samples were analyzed by secondary



ion mass spectrometry (SIMS, spectra shown in Figure S3 and S4) and Raman spectroscopy (shown in Figure S5). However, in our experiment, we have synthesized GO and ERGO samples separately, and where three ERGO samples (ERGO1, ERGO2, and ERGO3) were prepared based on the different electrochemical reduction time for optimization of their electrochemical performance. The variation of functional groups of both GO and ERGO samples can be estimated by secondary ion mass spectroscopy (SIMS) technique and the finger print mass spectra of all the samples are shown in Figure S3a and S3b. From the SIMS analysis the gradual decrease of Oemission (Figure S4a) in the reduced samples is ascribed to the conversion of SP3 domain to continuous SP2 domains in the basal plane. On the contrary, in the case of reduced samples the emission of C- ions tends to increase under increased reduction time, although ERGO2 and ERGO3 samples almost have same intensity compared to ERGO1 as seen in Figure S4b. This could be attributed to the tendency of C-C bonds breaking upon increasing reduction time of the ERGO samples. However, the observed initial hump in all the samples is likely due to bond breaking phenomena occurring on the surface or beneath the surface of the samples. This effect is more prominent in the case of C2- emission (see Figure S4c). Furthermore, change in H+ ions in both GO and ERGO samples shown in Figure S4d). The observed increasing C2- ion intensity of ERGO samples with increasing reduction times is attributed to the breaking of graphene symmetry of the basal plane into small domains (shown in scheme S2). These small domains have higher chemically reactive edges and therefore ERGO3 shows highest C 2- emission intensity compared to other samples. This indicates that ERGO3 sample might have large number of defects compared to pristine GO. The observed dominant C2- ion intensity in the mass spectra is likely due to higher electronic charges associated with C=C having delocalized π– electrons. It is established that the breaking of C-C and C=C bonds during reduction leads to the formation of large number of defects as schematically shown in scheme s2. Thus prolonged electric field creates defects on the basal plane. Similar kind of defects in reduced graphene oxide via thermal energy is observed by Bagri et al.

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This observation is further supported by

Raman spectroscopy analysis as ID/IG ratio increases with increase in the reduction of the GO indicating the removal of oxygen functional groups from the basal plane of ERGO samples (as seen in Figure S5). Moreover, prolonged reduction of GO (i.e. ERGO3) may increase the defects density by breaking of sp2 carbons and the sp2 domain size varies inversely with the ID/IG ratio.


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As ID/IG of ERGO3 is larger than that of GO, it is expected that large number of small sp 2 domains present in the plane and thus ERGO1 and ERGO2 should have less defects as compared to ERGO3. Impedance spectroscopy measurements for estimation of electronic property of GO and ERGO samples To understand the impact associated with the change in both functional groups and structural defects on the electronic property of electrolyte interfaced ERGO3 sample, electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M-S) measurements of both GO and ERGO3 were performed. EIS results of both the samples are depicted in Figure S6. The results suggest that charge transfer resistance of ERGO3 is lower than GO and this validates the higher conductivity of ERGO3 compared to GO. M-S plots of both GO and ERGO3 are shown in Figure 1a.

(a)

(b)

GO

ERGO3

Figure 1: (a) Comparison of Mott-Schottky plot between GO and ERGO3 and red arrows indicate the Fermi level position after band bending at the electrolyte interface of the respective samples. (b) Band diagram of both GO and ERGO3 at the electrolyte interface with respect to standard hydrogen electrode potential, where  is work function,  is electron affinity, Vf is the flat band voltage, and Vsc is the space charge potential of the electrode material.

Detailed calculation and analysis of M-S plot are shown in Figure S7 and Figure S8. The slope of the linear profile of 1/Csc2 vs. V attributes to the n-type or p-type characteristics of the respective GO and ERGO3 samples. The straight line corresponding to the positive slope justifies the n-



type conductivity of the GO. However, the appearance of negative slopes along with the positive slopes confirms the existence of both p and n-type charge carriers in ERGO3 sample as observed in the same Figure. Both positive and negative slopes variations of the M-S profile of GO and ERGO3 samples suggest the change in the hole (p) and electron (n) concentration in both samples (see also in Figure S8a for other samples). Figure S8b shows the variation in the holeelectron concentration ratio of both GO and ERGO samples individually. The calculation and analysis of hole-electron concentration ratio has been discussed elaborately in the Supporting Information (see in page no. S10-S11). We can estimate from Figure S8b that hole is the dominating charge carrier in ERGO3 while it is electron in GO. Moreover, the ratio (N +ERGO3/NGO)

between majority charge carriers of ERGO3 (i.e. hole, N+ERGO3) and GO (i.e. electron, N-GO)

sample is calculated and the detailed calculation are explained in the SI (page number S11). From the calculation it is noted that the hole concentration for the ERGO3 is higher than the electron concentration of GO sample. The vertex of parabolic nature of Mott-Schottky (M-S) spectra was used to calculate the flat band voltage of the respective samples at the electrodeelectrolyte interface, as shown in Figure 1a. A significant positive shift of the Fermi level (-0.25 to +0.03V as seen in Figure 1a) is noted for ERGO3 sample compared to GO sample. Also at +0.03 V, we observed an abrupt increase in the negative slope of M-S plot for ERGO3 due to formation of an inversion charge layer at the electrode-electrolyte interface. This might be because of change in electronic property due to change in functional groups on the ERGO samples compared to the pristine GO sample. Change in the functional groups of the samples is characterized by the SIMS technique and results are analyzed. In SIMS survey scan analysis, it is observed that GO has both electron donating (hydroxyl, epoxy, carbonyl etc.) and electron withdrawing oxygen functional groups (carboxyl groups)

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. SIMS survey scan of GO sample

confirms O- (electron donating group) emission intensity is higher than the COOH (electron withdrawing group) intensity which signifies its n-type conductivity. Therefore, as soon as GO comes in contact with an electrolyte, the electron can easily transfer from the GO to the electrolyte. This resulted in the formation of a space charge region at the electrode/electrolyte interface (positive charge in the GO side and negative charge in the electrolyte side), and this is an upward bending of the band edges towards the electrolyte as shown in Figure 1b. This makes GO an n-type semiconductor. On other hand, ERGO samples are formed by removal of oxygen containing functional groups yielding the sp2 carbons. Moreover, prolonged application of


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electric field during electrochemical reduction may create “hole” like defects which might have the zigzag edges or armchairs on the basal plane in the ERGO surface.

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Thus the density of

these defects may increase upon electrochemical reduction time (ERGO3 sample). These defects might be associated with either breaking of C-C bonds or some smaller sp2 domains in the basal plane. With increasing electrochemical reduction time, there is higher probability to form large number of sp2 domains in the sample. Therefore, highest reduced sample (ERGO3) might have higher density of lattice defects in the basal plane. These defects may create some uniaxial strain inside the material having dominating effect in shifting the Fermi level of ERGO3 sample

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.

Therefore, we observed a positive shift of minima of the M-S curve as seen in Figure 1a [from 0.25V (GO) to +0.03V (ERGO3)] of ERGO3 sample. The band diagram of ERGO3 at the electrolyte interface is shown schematically in Figure 1b. Shifting of the Fermi level of ERGO3 with respect to that of GO sample confirms the electron transfer from electrolyte to the empty conduction band of ERGO3 (or holes transfer from ERGO3 to the electrolyte). This generates a negative charge in the ERGO3 side of the space charge region, which causes a downward bending in the band edges

27, 28

. The charge transfer proceeds until thermodynamic equilibrium

between the electrochemical potential of the electrons involved in the red-ox reaction and the Fermi level of ERGO3 is reached. Henceforth, we can consider GO and ERGO3 samples as n and p-type materials, respectively. Electrical measurements of p-n junction based electrochemical diode The details of the fabrication process have been described in the experimental section. Before making the junction we measured the I-V characteristics (as shown in Figure S9) of individual GO and ERGO samples in the presence and absence of aqueous electrolyte. The non linear I-V response of GO-ERGO3 junction sample attributes to the Schottky junction formed between them (shown in Figure S10). However, in presence of electrolyte this sample shows a different but salient behavior of I-V characteristic as shown in Figure 2a. The schematic of the measurement is shown in Figure 2b. The results imply hardly any current conduction in the reverse bias (RB), however it conducts in the forward bias (FB) when the potential applied to the same sample. This behavior is similar to that observed for a conventional p-n junction diode. Electric field induced charge accumulation and depletion at the electrolyte interface of GOERGO3 junction is explained with the variation of interfacial capacitance (shown in Figure 2c) across the p-n junction at the electrolyte interface. The formation of the interfacial capacitance in



the two sides of the junctions is shown in Figure 2d. The whole electrochemical phenomena along with unidirectional ionic current through planar junction are described schematically in Figure S11.

(a)

, CEQ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(c)

(b)

(d)

Figure 2: (a) I-V characteristic of GO-ERGO3 junction sample in the presence of electrolyte, (b) schematic representation of I-V measurements on planar GO-ERGO3 junction in the presence of an electrolyte and inset shows the actual photograph of the I-V measurement of the junction device, (c) Potential dependent charge layer or capacitance variation at the electrolyte interfaced GO-ERGO3 junction and (d) Scheme for the interfacial capacitance measurement.

For further verification, as a control experiment, we have performed I-V measurement of planar GO and ERGO3 sample without a junction, (i.e. separated by 3 mm on the same substrate). A comparative study between the data obtained from I-V measurements of the electrodes with and


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without p-n junction was done and the results is shown in Figure S12. We observed a distinct rectification feature of the junction sample compared to other. Electrochemical bipolar junction transistor characteristics We then extended our idea to fabricate an in-plane bipolar junction transistor to further validate the results. The details of the fabrication steps are shown in scheme S1b. The n-p-n transistor (GO-ERGO3-GO) was designed with two diodes Din & Dout (GO-ERGO3 and ERGO3-GO junctions) connected with back to back configuration and the digital image of the device is shown in scheme S1c (see experimental section in Supporting Information). The principle of the transistor is to control the flow of current in one diode terminal by varying the voltage and current of another diode1. The characteristics performance (input and output characteristics) of this n-p-n transistor at the electrolyte interface is shown in Figure 3a and 3b. Among the two junctions, one (Din) can be considered as input junction formed between electrodes E1 (here, GO) and E2 (ERGO3), while other (Dout) is output terminal formed within electrodes E2 (ERGO3) to E3 (GO) of the device. The main function of the input characteristics is to understand the ionic current dynamics through the input junction (Din) at the electrolyte interface with the influence of an external electric field applied to the Dout junction. In other words, this is the process, to know how ionic current can be modulated through input Din, is influenced by the double layer charge width controlled by the externally applied electric field (V21) to the Din junction as shown schematically in Figure 3c. While Figure 3d demonstrates the output characteristics of the electron transport feature from the electrode E3 to E1 (through the Din & Dout junction) by controlling the double layer charge width at the E2 electrode by external current (I2) applied to it. With gradual increase of V32 raises the DLC width which reflects on the Vinv of Din junction. In each case Vinv has been pointed out as shown by the „orange‟ color arrow indicated in Figure 3a. In this case, Vinv is also shifting to the higher potential region (as Vinv4Vinv-3Vinv-2Vinv-1)

and at the same time the input current (I2) is also decreasing due to

increase of the resistive path because of the increase in double layer width. On the other hand, external current (I2) applied to the E2 reduces the double layer width in the same region as shown in the output characteristics of Figure 3b. Therefore, gradual decrease of the DLC layer width due to increase of the I2 causes the inversion band bending at the interface of electrode E2 (here, ERGO). As a result, inversion of charge on electrode E2 plays a crucial role for external bias dependent electrochemical oxidation of OH- on electrode E1 (GO) and reduction of H+ ions



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on electrode E3 (GO), respectively. Therefore, the output current I3 increases linearly upto certain potential applied between E1 and E3 and beyond a certain potential (~0.25V) I3 saturates.

(a)

(b)

(c)

(d)

Figure 3: (a) transistor input characteristics obtained by varying potential of input junction Din (electrode E2 to E1) keeping the fixed potential of output junction Dout (electrode E3 to E2), (b) transistor output characteristics obtained by varying potential (V31) between E1 and E3 keeping the constant current to electrode E2, and schematic representation of electric field induced double layer charge variation during the measurement of (c) input characteristics, and (d) output characteristics of planar junction transistor. This trend exists for all the values of current applied to electrode E2. The only difference between these current profiles is that increasing tendency of I3 with the increase of I2 which increases the inversion charge layer on E2. The schematic representation of charge layer formed during output characteristics measurements of n-p-n junction device is shown in Figure 3d. The above measurements clearly suggest that the electron transport characteristics of both Din and Dout diode of the device can be modulated through variation of the external bias of the Dout and 10 Environment ACS Paragon Plus

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Din diode, respectively. Hence, both input and output characteristics of this new bipolar junction device validate the conventional transistor principle. Furthermore, we have measured I-V measurements in 0.5 M of H2SO4 (pH~2), Na2SO4 (pH~7) and NaOH (pH ~12) and the spectra is shown in Figure S13a in the Supporting Information. It is observed that ionic current decreases with the increase of the pH of the electrolyte, however, Vinv increases with the increase of pH of the electrolyte. In addition, we have also performed I-V measurement (spectra is shown in Figure S13b) of the device with transistor configuration; and we observed the amplification of ionic current in one junction (V21) by keeping constant potential in the second junction (V32) in acidic medium compared to neutral electrolyte.

CONCLUSIONS In summary, we demonstrate both GO and ERGO at the electrolyte interface shows electronically n and p-type characteristics, respectively. On the basis of this feature, we have fabricated for the first time both planar electrochemical junction diode and bipolar junction transistor with both GO and ERGO using a simple, scalable and reproducible procedure. The typical planar single p-n junction shows a non linear unipolar ion current by controlling the double layer charge formed at the electrolyte interface of the junction device which actually demonstrates bias dependent simultaneous electrochemical oxidation and reduction process at the electrolyte interface. This feature of the device confirms the p-n junction diode characteristics. Moreover, two back to back planar junction shows external electrical field induced conventional bipolar junction transistor characteristics due to the controlling of the double layer charge at the electrolyte interface. We strongly believe this idea can find practical application if the aqueous electrolyte is replaced by solid electrolyte.

ASSOCIATED CONETENT Supporting Information The whole experimental section including material synthesis and device fabrication, detail material characterizations including SEM images, Raman and SIMS spectra with detail analysis, detail Impedance Spectroscopy with Mott-Schottky analysis to study the change in electronic property of GO and ERGO samples, electrical I-V measurements of the GO and ERGO samples with and without electrolyte, I-V measurements of junction and without junction sample,



mechanism of ion current rectification, and the pH effect on rectification and amplification of ion current

ACKNOWLEDGEMENTS We are thankful to Prof. P. Chakraborty, Surface Physics and Materials Science, Saha Institute of Nuclear Physics, India for fruitful discussion of SIMS analysis. S. K. Jana and H. R. Inta acknowledge IISER-Kolkata and UGC for fellowships. MV thanks DST, India & IISER Kolkata for funding. AUTHORS’S CONTRIBUTIONS S. Bayan did SIMS experiments and analysis the data. H. R. Inta was involved during fabrication of the sample. S. Banerjee and V. Mahalingam had scientific discussion and analysis of the results during execution of the project. S. K. Jana did overall plan, design, experiments and analysis of the project. AUTHOR INFORMATION Corresponding Author Dr. Sourav Kanti Jana *Email: [email protected] ORCID: 0000-0001-5772-7022 Present address DST Unit of Nanoscience and Thematic Unit of Excellence Pradeep Research Group Department of Chemistry IIT Madras, Chennai-600036 India Dr. Venkataramanan Mahalingam *Email: [email protected] ORCID: 0000-0003-1414-805X


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(28) Maier, F.; Riedel, M.; Mantel, B.; Ristein, J.; Ley, L. Origin of Surface Conductivity in Diamond. Physical Review Letters 2000, 85, 3472-3475.



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Rectification and Amplification of Ionic Current in Planar Graphene

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