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Functionality of reduced graphene oxide flakes at the growth of conducting zone in polyaniline-graphene composite films
Electrochimica Acta 228 (2017) 125–130
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Functionality of reduced graphene oxide flakes at the growth of conducting zone in polyaniline-graphene composite films Xiangdong Zenga , Koichi Jeremiah Aokib , Jingyuan Chena,* a b
Department of Applied Physics, University of Fukui, Fukui 910-8507 Japan Electrochemistry Museum, Fukui 910-0804 Japan
A R T I C L E I N F O
Article history: Received 30 April 2016 Received in revised form 3 October 2016 Accepted 9 January 2017 Available online 10 January 2017 Keywords: composite of polyaniline and reduced graphene oxide growth of conducting zone electric percolation conductivity of films
A B S T R A C T
When reduced graphene oxide (RGO) flakes were included in polyaniline (PANI), the PANI-RGO composite film exhibited faster redox conversion than the PANI film did. A reason of the enhancement of the conversion rate was searched by examining the growth rates of the conducting zone which appeared at the boundary between the oxidized and the reduced PANI in the film when the reduced composite was oxidized electrochemically from one end of the film. The film was prepared by drying PANI-RGO colloidal suspensions, which were formed by coating flaky RGO particles with PANI. Higher anodic potentials and higher ratio of RGO fractions enhanced the growth rates. There are several possible reasons for the increase in the rates; electro-catalytic properties for the oxidation, enhancement of the conductivity by RGO, the electric percolation, and enhancement of specific surface area of RGO. The growth rate was modeled with the oxidation at the Tafel-typed rate, which was restrained by the IR-drop between the conducting front and the electrode. The growth length was expressed by a time-dependent non-linear differential equation. The numerical solution allowed us to analyze the experimental data of the time variation of the growth length. RGO functions as an increase in the rough surface area of the oxidation rather than electric percolation, enhancement of conductivity and catalytic effects. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction When conducting polymers are embodied with reduced graphene oxide particles (RGO), unpredicted properties can be disclosed. They result intuitively from interactions of the highly conjugated structure of RGO and delocalized electrons in conjugated polymers [1]. A number of the examples have been reported for polyaniline (PANI)-RGO composites. The PANI-RGO material exhibited a donor-acceptor interaction owing to the electron acceptability [2]. PANI-RGO composite particles embodied by poly(styrenesulfonic acid) showed the electroactivity of PANI even in alkaline solutions [3]. PANI-RGO nanocomposites including CdSe showed electrochemiluminescence used for sensitive immunosensors [4]. The layer-by-layer assembly of PANI and RGO was suitable for a counter electrode for a dye-sensitized solar cell in the points of high conductivity and electrocatalytic activity of triiodides [5–7]. Films composed of PANI, RGO and Co3O4 showed specific capacitance as high as 1 kF/g with negligible capacitance loss [8]. The network of silicon-RGO-PANI overcame
* Corresponding author. E-mail address: [email protected] (J. Chen). http://dx.doi.org/10.1016/j.electacta.2017.01.054 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
low conductivity and large volume changes of silicon during charge/discharge processes of Lithium ion batteries [9]. Multilayers of PANI and RGO showed the double layer capacitance over 500 Fg1 when a modifier required for the fabrication was removed [10]. The composite of PANI and RGO was able to be fabricated in solution of pH as high as 3.6 with the help of laccase [11]. Covalent bonding of PANI with RGO yielded stable supercapacitors [12]. The PANI-RGO composite increased the tensile strength to make the film freestanding [13]. Aligned nanowires of PANI were formed on RGO to exhibit high double layer capacitance [14]. These disclosed features seem to result from enhancement of catalytic activity at PANI|RGO interfaces [3,7,11,15–18], formation of covalent bonds between PANI and RGO [6,15,19–21], an increase in the conductivity with the help of RGO [22–25], usage of planar structure of RGO [14,26,27], specific enhancement of active surface area of RGO [8,10], and electrical packing of RGO particles with PANI [28–30]. However, each reason has not been specified yet from a quantitative viewpoint. Reasons may depend on compositions, structure of electrodes and fabrication techniques. These complications have been termed synergetic effects [2,14,19,25– 27,31]. Since the term of synergetic effects has been used for forcible elucidation of unexplained phenomena, the usage is not mature in science. In order to know reasons of the specific features,
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it is necessary to examine electrochemical mechanisms quantitatively in the composites. Kinetic mechanisms of the redox reaction of PANI films are different from those of conventional redox species immobilized on electrodes in that the oxidized state, being electronically conductive, works as an electrode. As a result, the oxidation is accompanied by growth of the conducting zone from the electrode to the boundary between the PANI film and the solution [32–34]. A moving boundary for the growth can be seen on the film when oxidation potential is applied to one end of the film. On the other hands, the reduction occurs almost instantaneously and stops at the volume fraction of the percolation threshold for the oxidized species because of cut-off of the conduction paths [35–37]. These properties may be useful for analyzing the complicated effects of PANI-RGO composites, because the propagation rate depends in different manners on catalytic behavior, film conductance, and packing density of RGO. We consider possibilities of the growth of the conducting zone in the PANI-RGO film with electric percolation when the reduced film is electrochemically oxidized from the one end of the electrode. If a fraction of RGO flakes is low enough for nonpercolation, the conducting zone grows only through PANI conduction. On the other hands, when the percolation is fulfilled by high fraction of RGO, the zone grows through the conducting domain of the RGO. Then, the growth rate is higher than the former. This prediction is examined here by carrying out the growth experiments for some fractions of RGO of the composites and some applied potentials. The observation of the front will provide mechanisms of the oxidation relevant to conductance, catalytic effects, and percolation. The theory of the growth rate is presented, which is delayed by the film resistance.
thickness was 0.04 mm. From the volume and the weight, we estimated the density to be 0.54 g cm3. The density was used for determination of the thickness 2 mm for the film by the single castdry process, which was used for the growth experiment. The cut film into 0.8 5 mm2 was placed on a glass plate with a carbon tape. It was soaked in 10 wt% hydrazine solution for one hour to reduce PANI. One end of the film was connected to a platinum plate for electric lead, and put in 1 M (=mol dm3) HCl solution for potential step experiment. A film 0.5 mm thick was formed by the iterative cast-dry processes. Fig. 1 shows morphology of the surface of the composite film (A) and the PANI film (B). The former is rougher than the latter by the grain size of RGO. The cross section of the film by AFM is shown in Fig. 2. Laminar structure was found in which the layers were parallel to the substrate. This morphology implies that RGO flakes should be arranged parallel to the filter during the drying step. The surface roughness with more than 1 mm was found. Cyclic voltammetry and chronoamperometry were performed with a potentiostat, Compactstat (Ivium Tech., Netherlands). The working, the reference and the electrodes were a platinum plate, Ag|AgCl and a platinum wire, respectively. The optical microscope VH (Keyence, Osaka) with a video was used to record dynamic color change of the film.
2. Experiment All the chemicals were of analytical grade. Water was distilled and then ion-exchanged by the ultrapure water system, CPW-100 (Advantec, Tokyo). Graphite powder (98%, 7 mm in average diameter) was purchased from Ito Koken (Mie, Japan). Aniline hydrochloride (Kanto) and ammonium peroxydisulfate (Wako) were used as received. The PANI-RGO suspension was prepared as previously reported [38]. The suspension of 156 mg with the concentration of 1.27 mg solid g1 was casted on polytetrafluoroethylene filter (JGWP04700, Omnipore Membrane Filter: f 47 mm, pore size: 0.2 mm), which worked as absorption of solution from the suspension. The suspension on the filter was dried for 3 h at room temperature to yield a film. The cast-dry processes were iterated until the
Fig. 2. Morphology of the cross section of the PANI-RGO film observed by AFM.
Fig. 1. SEM images of (A) the composite film with fraction 0.30 of RGO and (B) PANI surfaces.
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3. Results and Discussion 3.1. Growth of conducting zone Cyclic voltammograms of the PANI-RGO film in ca. 2 mm thick cast on the Pt electrode were obtained in hydrochloric acid for several values of the fraction of RGO, ð1Þ
where wRGO and wPANI are weights of RGO and PANI in the composite, respectively. Not only their waveforms but also the anodic peak potentials were invariant to the fraction, as shown in the inset of set of Fig. 3. Therefore, RGO does not contribute to the catalytic activation of the oxidation of the reduced PANI. Difference in the peak currents is due to that in film thickness. Since the anodic peak currents were proportional to the scan rates, they should be controlled by surface reactions. It is difficult to obtain a dynamic distribution of redox concentrations within the cross section of a thin film or in direction perpendicular to the electrode. A distribution in the cross section might be observed in a film more than a few millimeters thickness. If a self-standing film is set perpendicularly on the electrode, a distribution may readily be measured. The PANI-RGO film supported with the filter was mounted on a glass plate, and was reduced with hydrazine. A platinum plate was mounted on one end of the reduced film for an electric lead. The film was inserted in 1 M hydrochloric acid into which the reference and the counter electrodes were equipped. The reduction potential, 0.1 V, was applied immediately after the insertion of the film into the solution in order to prohibit the oxidation of the PANI by dissolved oxygen. When some oxidation voltages were applied to the electrode, the dark blue zone grew from the electrode with the oxidation time, invading the reduced, brown zone, as shown in Fig. 4. The boundary was clear, moved away from the electrode. Since the front was observed only by applying the oxidation voltage, the front should be a boundary between the conducting (oxidized) zone and the insulating (reduced) zones. The length, x, of the growing conducting zone, i.e., the distance between the front and the electrode, was plotted against the time, t, of applying the oxidation potential, as shown in Fig. 5. It was proportional to t at a time shorter than 5 s, and hence the front rate is a constant. The constant rate means that a controlling step is not ascribed to diffusion but probably to the charge transfer rate. The PANI-RGO films for f = 0.39 exhibited a uniform color change without any boundary, just like a color change of thin elecropolymerized PANI films on Pt. PANI-RGO composites for f > 0.4 were so brittle that they did not generate self-standing films.
Fig. 3. Cyclic voltammograms of the PANI-RGO film in ca. 2 mm thick cast on the Pt electrode in 1.0 M hydrochloric acid at scan rate 10 mV s1 for f = (a) 0, (b) 0.21, (c) 0.25 and (d) 0.30, where the ordinate is normalized with the weight of PANI. The inset is the variation of the anodic peak potential with f.
Fig. 4. Photographs of PANI-RGO films taken at indicated times after 0.4 V was applied to the bottom of the film in hydrochloric acid. The brown and the dark blue regions are the reduced and the oxidized PANI, respectively.
4
x / mm
f ¼ wRGO = ðwRGO þ wPANI Þ
2
0 0
10
20
t/s Fig. 5. Variations of the growth length of the conducting zone with the time of applying potentials of (circles) 0.6, (triangles) 0.5 and (squares) 0.4 V vs. Ag|AgCl for the film at f = 0.30.
Logarithm of the initial growth rates, v = (dx/dt)t=0, of the fronts varied linearly with the applied potentials, Eap, for several fractions of RGO, as seen in the supporting information. Since the amount of the oxidation charge is proportional to x, the rate is proportional to the oxidation current. Thus the linearity reminds us of the Tafel equation, ln v = aFEap/RT + b. The slope, i.e. a value of a is ca. 0.17, almost independent of f, which is close to the previously obtained value [32]. The common value of a indicates that the activation energy of the oxidation of PANI on RGO should be close to that on the conducting (oxidized) PANI that works as the electrode. Therefore, no catalytic effect of RGO on the oxidation rate of PANI can be recognized. The rate becomes slow with the lapse of the oxidation (Fig. 5), regardless of the fractions of RGO and applied potentials. The slowdown may be caused by the film resistance which decreases the anodic potential at the front. In order to demonstrate the participation in the film resistance, we measured the resistance of dried composite films by a two terminal resistor, where PANI was under the oxidized state. Fig. 6 shows the variation of the conductance with f. The conductance for f < 0.15 keeps the value of only PANI (f = 0), and increases suddenly at f = 0.15. This variation is characterized with electric percolation for conductance [39]. Of interest is to consider the effect of the film resistance on the growth rate. In order to analyze the time-dependence of the growth rates in Fig. 5, it is necessary to obtain a theoretical relation of the growth rates involving the film resistance
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6
(b)
(a)
2
3.2. Theory of growth complicated by film resistance When the potential Eap at which the reduced PANI is oxidized is applied to one end of the PANI-RGO film, the oxidized PANI being electronically conductive works as an electrochemical electrode for the oxidation. The oxidation rate is assumed to be proportional to exp(aFEap/RT), according to a Tafel equation. Then we can write the current density as j = j0 exp[aFEap(Eap E00)], where E00 is standard potential and j0 is the current density at E00. This equation is valid immediately after the potential application or at a short time, as is shown by the proportionality in Fig. 5 as well as the linearity in the Tafel-like plot. When the conducting zone grows by x, the potential at the conducting front decreases by the film resistance, rxj, where r is the resistivity of the film. Then the net oxidation potential is given by h i j ¼ j0 exp ðaF=RT Þ Eap E0 rxj ð2Þ
(a) (d)
(b)
0 0
0 0 Fig. 6. Dependence of conductance of dried films on the fraction of RGO. The distance of two terminals was 5 mm.
0.5
y
y
4
(c)
u
10
u
0.5
20
Fig. 7. Variation of the dimensionless growth length (a) with the dimensionless time, numerically computed from Eq. (6), where (b) is the tangent line at u = 0, (c) is the curve of y = ln(1 + u + 0.06u3), (d) is by Eq. (7).
y = ln(1 + u + 0.06u3) for u < 14 within 3% errors, as shown as the dotted curve in Fig. 7(c). The approximate equation for small values u is further reduced to (Fig. 7 (d)) y u u2/2
(7)
or j V M ez ar j20 V M e2z t 1 t x 0 F 2RT
! ð8Þ
3.3. Functionality of RGO in the growth
Since j is the oxidation current density of PANI without any capacitive component, it should be proportional to x. Letting the molar volume of PANI of the PANI-RGO composite be VM and the electric charge density for the oxidation during t be q, we obtain (q/ F)VM = x. Combining it with the j = dq/dt, we obtain
The experimental values of v increased largely with an increase in the fraction, while the theory predicts that j0VMez/F (=v) in Eq. (8) does not seem to include strongly f-dependent terms. Since VM decreases slightly with an increase in f, it is opposite to the experimental result. If the oxidation occurs at the projected area, j0 should be independent of f. According to Fig. 2, the reaction occurs at rough surface area rather than the projected area. An increase in f necessarily increases the net surface area to enhance j0. We estimate the surface roughness quantitatively. Taking the logarithm of v in Eq. (8), i.e.
j ¼ ðF=V M Þðdx=dtÞ
ln(v) = (aF/RT) Eap (aF/RT) Eo + ln(j0VM/F)
Eliminating j from Eqs. (2) and (3) yields ! F dx aF 2 r dx ¼ exp x V M j0 ez dt RTV M dt
ð3Þ
ð4Þ
Where z = (aF/RT)(Eap Eo). This is non-linear equation for x with respective to t including two conjugated parameters. When we use the following change of variables: y ¼ ðaF r j0 ez =RTÞx; u ¼ ðar j20 V M e2z =RTÞ t Eq. (4) is reduced to dy 1dy2 ¼ exp du 2 du
(9)
we can express the intercepts of the lines in the Tafel-like plot as –(aF/RT) Eo + ln(j0VM/F). From the values of the intercepts and Eo = 0.19 V by the CV in Fig. 3, we evaluated ln(j0VM/F). The plot of ln (j0VM/F). against ln f exhibits a line with a slope 1.6, as shown in Fig. 8, suggesting proportionality of j0 to f1.6. The power of f1.0 means the equality of the specific surface area with the projected area. The power more than 1 implies that the specific area is larger than the projected area. The enhancement of the net surface area
ð5Þ
ð6Þ
Some limiting values for t ! 0 or u ! 0 are x ! 0 or y ! 0 and dy/ du ! 0. Eq. (6) has too strongly non-linear to be solved analytically. We applied the forward difference method to Eq. (6). The non-linearity was solved by the Newton method. The details are described in Appendix A. Fig. 7 shows the variation of y with u. With a lapse of the time t or u, values of y increase proportionally with u at a short time (Fig. 7(a)), and then deviate downward from the proportional line (b) for a long time. The variation is similar to the experimental results in Fig. 5. The relation of y with u can be approximated as
Fig. 8. Logarithmic plots of joVMF1 vs. the fraction of RGO, obtained from the variations in the Tafel-like plot through Eq. (9).
X. Zeng et al. / Electrochimica Acta 228 (2017) 125–130
may be caused by orientation of the stacked RGO flakes in laminate. The variation in Fig. 6 is not smooth at f = 0.15, whereas that in Fig. 8 is smooth. The non-smooth variation results from the critical change in geometrical connections of RGO flakes, called the electric percolation, which is specifically observed under solid states as in Fig. 6. When conducting particles are in solution, Brownian motion of the particles relax the critical variation to yield the smooth variation as in Fig. 8. The resistivity is included in the coefficient of t2 in Eq. (8), and hence it may participate in the long term growth. When Eq. (8) is divided by t, i.e. x j0 V M ez ar j0 V 2M e3z t t F 2RTF 3
ð10Þ
the coefficient of t2 in Eq. (8) appears as a slope of the plot of x/t against t in Eq. (10). Fig. 9 shows plots of x/t against t for x > 1 mm at the PANI-RGO film at f = 0.21 for four values of potentials. Values of x/t vs. t at a given potential fall on a line approximately, as can be justified by Eq. (10). The slope of the line means the acceleration of the growth rate. The values are negative, and are smaller as the potential is more anodic. The logarithmic plot of the values of the slopes against the potential exhibits a linear relation (shown in supporting information), as predicted from exp(3z) in Eq. (10). If r were independent of the potential, the slope of the line should be 3aF/RT. The value of a determined from the slope is ca. 0.1, smaller than the value (0.17) obtained from the Tafel-like plot. The difference can be explained in terms of getting large values of r at potentials closer to Eo. An advantage of the PANI-RGO comp1 osite appears in enhancement of the redox conversion rate. When the oxidation and the reduction potentials were applied to the film 2 mm thick cast on the Pt electrode in 1 M HCl solution, the charge-response for the PANI-RGO was faster than that of the PANI film, as shown in Fig. 10. The amounts of PANI are common to the both films in Fig. 10. Although the orientation of the RGO flakes are parallel to the electrode, the enhancement was still observed probably because of perturbation of the parallel directions. 4. Conclusions When PANI films are mixed with RGO flakes, the growth rate of the conducting zone in the films is increased more than ten times than that without RGO. The quantitative measurements and analysis of the growth rate can provide the following mechanisms of the enhancement.
x t -1 / mm s -1
0.4 0.3
(a)
Fig. 10. Charge-time curves at (a) the PANI-RGO coated Pt and (b) the PANI coated Pt for (c) the iterative potential steps between 0.2 V and 0.5 V.
(a) The rate is determined mainly by the a-times of the electrode potential, the exchange current density and the area of the reacting front. (b) RGO flakes increase the net surface area owing to parallel arrangement of flat structure of the flakes rather than the projected area. (c) The increase in the conductivity by percolation does not enhance directly the growth rate. (d) Any electrocatalytic activity causing a potential shift has no effect on the growth rate. Synergetic effects for PANI-RGO composites have thought to cause complicated behavior of electrocatalytic activity, enhancement of electric conductivity, modification of double layer capacitance by covalent bonds, and specific reaction conditions. However, the enhancement of the rate is mainly ascribed to the increase in fractal-like surface areas of RGO by the quantitative analysis. Appendix A. The method of solving Eq. (6) is shown here. The logarithm of Eq. (6) is 2ln(dy/du) = dy2/du, to which application of the forward difference, dy/du = (yn+1–yn)/Du for u = nDu, yields ðA1Þ y2nþ1 þ 2Duln ðynþ1 yn Þ=Du y2n ¼ 0 Since y0 = 0, y1 can be obtained by solving the non-linear equation of Eq. (A1) for n = 0. Eq. (A1) can be rewritten as f (z) z2 + a ln(z b) c = 0, where z = y1, a = 2Du, b = y0, c = y02 + 2Du lnDu. A value of z was determined by the Newton-Raphson method, i.e. a crude value zi was improved through the replacement, zi+1 = zi f(zi)/f’(zi), where the first and the second derivatives are f ’(z) = 2z + a/(z b) and f ”(z) = 2 a(z b)2. An initial value is either z0 = 0 for y0 > b + (a/2)1/2 or a very large for y0 < b + (a/2)1/2., according the signs of f ”(z0). Values of zi did not vary with i for i > 5, and hence the convergent value should be y1. Next, y2 was determined in the above process by regarding y1 as an initial value. It was confirmed that values of Du did not vary with y. References
0.2 (b)
0.1 (c) (d)
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20
t/s
40
Fig. 9. Variations of xt1 with t for the PANI-RGO composite for f = 0.21 at Eap = (a) 0.6, (b) 0.5, (c) 0.4 and (d) 0.3 V.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Functionality of reduced graphene oxide flakes at the growth of conducting zone in polyaniline-graphene composite films Xiangdong Zenga , Koichi Jeremiah Aokib , Jingyuan Chena,* a b
Department of Applied Physics, University of Fukui, Fukui 910-8507 Japan Electrochemistry Museum, Fukui 910-0804 Japan
A R T I C L E I N F O
Article history: Received 30 April 2016 Received in revised form 3 October 2016 Accepted 9 January 2017 Available online 10 January 2017 Keywords: composite of polyaniline and reduced graphene oxide growth of conducting zone electric percolation conductivity of films
A B S T R A C T
When reduced graphene oxide (RGO) flakes were included in polyaniline (PANI), the PANI-RGO composite film exhibited faster redox conversion than the PANI film did. A reason of the enhancement of the conversion rate was searched by examining the growth rates of the conducting zone which appeared at the boundary between the oxidized and the reduced PANI in the film when the reduced composite was oxidized electrochemically from one end of the film. The film was prepared by drying PANI-RGO colloidal suspensions, which were formed by coating flaky RGO particles with PANI. Higher anodic potentials and higher ratio of RGO fractions enhanced the growth rates. There are several possible reasons for the increase in the rates; electro-catalytic properties for the oxidation, enhancement of the conductivity by RGO, the electric percolation, and enhancement of specific surface area of RGO. The growth rate was modeled with the oxidation at the Tafel-typed rate, which was restrained by the IR-drop between the conducting front and the electrode. The growth length was expressed by a time-dependent non-linear differential equation. The numerical solution allowed us to analyze the experimental data of the time variation of the growth length. RGO functions as an increase in the rough surface area of the oxidation rather than electric percolation, enhancement of conductivity and catalytic effects. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction When conducting polymers are embodied with reduced graphene oxide particles (RGO), unpredicted properties can be disclosed. They result intuitively from interactions of the highly conjugated structure of RGO and delocalized electrons in conjugated polymers [1]. A number of the examples have been reported for polyaniline (PANI)-RGO composites. The PANI-RGO material exhibited a donor-acceptor interaction owing to the electron acceptability [2]. PANI-RGO composite particles embodied by poly(styrenesulfonic acid) showed the electroactivity of PANI even in alkaline solutions [3]. PANI-RGO nanocomposites including CdSe showed electrochemiluminescence used for sensitive immunosensors [4]. The layer-by-layer assembly of PANI and RGO was suitable for a counter electrode for a dye-sensitized solar cell in the points of high conductivity and electrocatalytic activity of triiodides [5–7]. Films composed of PANI, RGO and Co3O4 showed specific capacitance as high as 1 kF/g with negligible capacitance loss [8]. The network of silicon-RGO-PANI overcame
* Corresponding author. E-mail address: [email protected] (J. Chen). http://dx.doi.org/10.1016/j.electacta.2017.01.054 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
low conductivity and large volume changes of silicon during charge/discharge processes of Lithium ion batteries [9]. Multilayers of PANI and RGO showed the double layer capacitance over 500 Fg1 when a modifier required for the fabrication was removed [10]. The composite of PANI and RGO was able to be fabricated in solution of pH as high as 3.6 with the help of laccase [11]. Covalent bonding of PANI with RGO yielded stable supercapacitors [12]. The PANI-RGO composite increased the tensile strength to make the film freestanding [13]. Aligned nanowires of PANI were formed on RGO to exhibit high double layer capacitance [14]. These disclosed features seem to result from enhancement of catalytic activity at PANI|RGO interfaces [3,7,11,15–18], formation of covalent bonds between PANI and RGO [6,15,19–21], an increase in the conductivity with the help of RGO [22–25], usage of planar structure of RGO [14,26,27], specific enhancement of active surface area of RGO [8,10], and electrical packing of RGO particles with PANI [28–30]. However, each reason has not been specified yet from a quantitative viewpoint. Reasons may depend on compositions, structure of electrodes and fabrication techniques. These complications have been termed synergetic effects [2,14,19,25– 27,31]. Since the term of synergetic effects has been used for forcible elucidation of unexplained phenomena, the usage is not mature in science. In order to know reasons of the specific features,
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it is necessary to examine electrochemical mechanisms quantitatively in the composites. Kinetic mechanisms of the redox reaction of PANI films are different from those of conventional redox species immobilized on electrodes in that the oxidized state, being electronically conductive, works as an electrode. As a result, the oxidation is accompanied by growth of the conducting zone from the electrode to the boundary between the PANI film and the solution [32–34]. A moving boundary for the growth can be seen on the film when oxidation potential is applied to one end of the film. On the other hands, the reduction occurs almost instantaneously and stops at the volume fraction of the percolation threshold for the oxidized species because of cut-off of the conduction paths [35–37]. These properties may be useful for analyzing the complicated effects of PANI-RGO composites, because the propagation rate depends in different manners on catalytic behavior, film conductance, and packing density of RGO. We consider possibilities of the growth of the conducting zone in the PANI-RGO film with electric percolation when the reduced film is electrochemically oxidized from the one end of the electrode. If a fraction of RGO flakes is low enough for nonpercolation, the conducting zone grows only through PANI conduction. On the other hands, when the percolation is fulfilled by high fraction of RGO, the zone grows through the conducting domain of the RGO. Then, the growth rate is higher than the former. This prediction is examined here by carrying out the growth experiments for some fractions of RGO of the composites and some applied potentials. The observation of the front will provide mechanisms of the oxidation relevant to conductance, catalytic effects, and percolation. The theory of the growth rate is presented, which is delayed by the film resistance.
thickness was 0.04 mm. From the volume and the weight, we estimated the density to be 0.54 g cm3. The density was used for determination of the thickness 2 mm for the film by the single castdry process, which was used for the growth experiment. The cut film into 0.8 5 mm2 was placed on a glass plate with a carbon tape. It was soaked in 10 wt% hydrazine solution for one hour to reduce PANI. One end of the film was connected to a platinum plate for electric lead, and put in 1 M (=mol dm3) HCl solution for potential step experiment. A film 0.5 mm thick was formed by the iterative cast-dry processes. Fig. 1 shows morphology of the surface of the composite film (A) and the PANI film (B). The former is rougher than the latter by the grain size of RGO. The cross section of the film by AFM is shown in Fig. 2. Laminar structure was found in which the layers were parallel to the substrate. This morphology implies that RGO flakes should be arranged parallel to the filter during the drying step. The surface roughness with more than 1 mm was found. Cyclic voltammetry and chronoamperometry were performed with a potentiostat, Compactstat (Ivium Tech., Netherlands). The working, the reference and the electrodes were a platinum plate, Ag|AgCl and a platinum wire, respectively. The optical microscope VH (Keyence, Osaka) with a video was used to record dynamic color change of the film.
2. Experiment All the chemicals were of analytical grade. Water was distilled and then ion-exchanged by the ultrapure water system, CPW-100 (Advantec, Tokyo). Graphite powder (98%, 7 mm in average diameter) was purchased from Ito Koken (Mie, Japan). Aniline hydrochloride (Kanto) and ammonium peroxydisulfate (Wako) were used as received. The PANI-RGO suspension was prepared as previously reported [38]. The suspension of 156 mg with the concentration of 1.27 mg solid g1 was casted on polytetrafluoroethylene filter (JGWP04700, Omnipore Membrane Filter: f 47 mm, pore size: 0.2 mm), which worked as absorption of solution from the suspension. The suspension on the filter was dried for 3 h at room temperature to yield a film. The cast-dry processes were iterated until the
Fig. 2. Morphology of the cross section of the PANI-RGO film observed by AFM.
Fig. 1. SEM images of (A) the composite film with fraction 0.30 of RGO and (B) PANI surfaces.
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3. Results and Discussion 3.1. Growth of conducting zone Cyclic voltammograms of the PANI-RGO film in ca. 2 mm thick cast on the Pt electrode were obtained in hydrochloric acid for several values of the fraction of RGO, ð1Þ
where wRGO and wPANI are weights of RGO and PANI in the composite, respectively. Not only their waveforms but also the anodic peak potentials were invariant to the fraction, as shown in the inset of set of Fig. 3. Therefore, RGO does not contribute to the catalytic activation of the oxidation of the reduced PANI. Difference in the peak currents is due to that in film thickness. Since the anodic peak currents were proportional to the scan rates, they should be controlled by surface reactions. It is difficult to obtain a dynamic distribution of redox concentrations within the cross section of a thin film or in direction perpendicular to the electrode. A distribution in the cross section might be observed in a film more than a few millimeters thickness. If a self-standing film is set perpendicularly on the electrode, a distribution may readily be measured. The PANI-RGO film supported with the filter was mounted on a glass plate, and was reduced with hydrazine. A platinum plate was mounted on one end of the reduced film for an electric lead. The film was inserted in 1 M hydrochloric acid into which the reference and the counter electrodes were equipped. The reduction potential, 0.1 V, was applied immediately after the insertion of the film into the solution in order to prohibit the oxidation of the PANI by dissolved oxygen. When some oxidation voltages were applied to the electrode, the dark blue zone grew from the electrode with the oxidation time, invading the reduced, brown zone, as shown in Fig. 4. The boundary was clear, moved away from the electrode. Since the front was observed only by applying the oxidation voltage, the front should be a boundary between the conducting (oxidized) zone and the insulating (reduced) zones. The length, x, of the growing conducting zone, i.e., the distance between the front and the electrode, was plotted against the time, t, of applying the oxidation potential, as shown in Fig. 5. It was proportional to t at a time shorter than 5 s, and hence the front rate is a constant. The constant rate means that a controlling step is not ascribed to diffusion but probably to the charge transfer rate. The PANI-RGO films for f = 0.39 exhibited a uniform color change without any boundary, just like a color change of thin elecropolymerized PANI films on Pt. PANI-RGO composites for f > 0.4 were so brittle that they did not generate self-standing films.
Fig. 3. Cyclic voltammograms of the PANI-RGO film in ca. 2 mm thick cast on the Pt electrode in 1.0 M hydrochloric acid at scan rate 10 mV s1 for f = (a) 0, (b) 0.21, (c) 0.25 and (d) 0.30, where the ordinate is normalized with the weight of PANI. The inset is the variation of the anodic peak potential with f.
Fig. 4. Photographs of PANI-RGO films taken at indicated times after 0.4 V was applied to the bottom of the film in hydrochloric acid. The brown and the dark blue regions are the reduced and the oxidized PANI, respectively.
4
x / mm
f ¼ wRGO = ðwRGO þ wPANI Þ
2
0 0
10
20
t/s Fig. 5. Variations of the growth length of the conducting zone with the time of applying potentials of (circles) 0.6, (triangles) 0.5 and (squares) 0.4 V vs. Ag|AgCl for the film at f = 0.30.
Logarithm of the initial growth rates, v = (dx/dt)t=0, of the fronts varied linearly with the applied potentials, Eap, for several fractions of RGO, as seen in the supporting information. Since the amount of the oxidation charge is proportional to x, the rate is proportional to the oxidation current. Thus the linearity reminds us of the Tafel equation, ln v = aFEap/RT + b. The slope, i.e. a value of a is ca. 0.17, almost independent of f, which is close to the previously obtained value [32]. The common value of a indicates that the activation energy of the oxidation of PANI on RGO should be close to that on the conducting (oxidized) PANI that works as the electrode. Therefore, no catalytic effect of RGO on the oxidation rate of PANI can be recognized. The rate becomes slow with the lapse of the oxidation (Fig. 5), regardless of the fractions of RGO and applied potentials. The slowdown may be caused by the film resistance which decreases the anodic potential at the front. In order to demonstrate the participation in the film resistance, we measured the resistance of dried composite films by a two terminal resistor, where PANI was under the oxidized state. Fig. 6 shows the variation of the conductance with f. The conductance for f < 0.15 keeps the value of only PANI (f = 0), and increases suddenly at f = 0.15. This variation is characterized with electric percolation for conductance [39]. Of interest is to consider the effect of the film resistance on the growth rate. In order to analyze the time-dependence of the growth rates in Fig. 5, it is necessary to obtain a theoretical relation of the growth rates involving the film resistance
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6
(b)
(a)
2
3.2. Theory of growth complicated by film resistance When the potential Eap at which the reduced PANI is oxidized is applied to one end of the PANI-RGO film, the oxidized PANI being electronically conductive works as an electrochemical electrode for the oxidation. The oxidation rate is assumed to be proportional to exp(aFEap/RT), according to a Tafel equation. Then we can write the current density as j = j0 exp[aFEap(Eap E00)], where E00 is standard potential and j0 is the current density at E00. This equation is valid immediately after the potential application or at a short time, as is shown by the proportionality in Fig. 5 as well as the linearity in the Tafel-like plot. When the conducting zone grows by x, the potential at the conducting front decreases by the film resistance, rxj, where r is the resistivity of the film. Then the net oxidation potential is given by h i j ¼ j0 exp ðaF=RT Þ Eap E0 rxj ð2Þ
(a) (d)
(b)
0 0
0 0 Fig. 6. Dependence of conductance of dried films on the fraction of RGO. The distance of two terminals was 5 mm.
0.5
y
y
4
(c)
u
10
u
0.5
20
Fig. 7. Variation of the dimensionless growth length (a) with the dimensionless time, numerically computed from Eq. (6), where (b) is the tangent line at u = 0, (c) is the curve of y = ln(1 + u + 0.06u3), (d) is by Eq. (7).
y = ln(1 + u + 0.06u3) for u < 14 within 3% errors, as shown as the dotted curve in Fig. 7(c). The approximate equation for small values u is further reduced to (Fig. 7 (d)) y u u2/2
(7)
or j V M ez ar j20 V M e2z t 1 t x 0 F 2RT
! ð8Þ
3.3. Functionality of RGO in the growth
Since j is the oxidation current density of PANI without any capacitive component, it should be proportional to x. Letting the molar volume of PANI of the PANI-RGO composite be VM and the electric charge density for the oxidation during t be q, we obtain (q/ F)VM = x. Combining it with the j = dq/dt, we obtain
The experimental values of v increased largely with an increase in the fraction, while the theory predicts that j0VMez/F (=v) in Eq. (8) does not seem to include strongly f-dependent terms. Since VM decreases slightly with an increase in f, it is opposite to the experimental result. If the oxidation occurs at the projected area, j0 should be independent of f. According to Fig. 2, the reaction occurs at rough surface area rather than the projected area. An increase in f necessarily increases the net surface area to enhance j0. We estimate the surface roughness quantitatively. Taking the logarithm of v in Eq. (8), i.e.
j ¼ ðF=V M Þðdx=dtÞ
ln(v) = (aF/RT) Eap (aF/RT) Eo + ln(j0VM/F)
Eliminating j from Eqs. (2) and (3) yields ! F dx aF 2 r dx ¼ exp x V M j0 ez dt RTV M dt
ð3Þ
ð4Þ
Where z = (aF/RT)(Eap Eo). This is non-linear equation for x with respective to t including two conjugated parameters. When we use the following change of variables: y ¼ ðaF r j0 ez =RTÞx; u ¼ ðar j20 V M e2z =RTÞ t Eq. (4) is reduced to dy 1dy2 ¼ exp du 2 du
(9)
we can express the intercepts of the lines in the Tafel-like plot as –(aF/RT) Eo + ln(j0VM/F). From the values of the intercepts and Eo = 0.19 V by the CV in Fig. 3, we evaluated ln(j0VM/F). The plot of ln (j0VM/F). against ln f exhibits a line with a slope 1.6, as shown in Fig. 8, suggesting proportionality of j0 to f1.6. The power of f1.0 means the equality of the specific surface area with the projected area. The power more than 1 implies that the specific area is larger than the projected area. The enhancement of the net surface area
ð5Þ
ð6Þ
Some limiting values for t ! 0 or u ! 0 are x ! 0 or y ! 0 and dy/ du ! 0. Eq. (6) has too strongly non-linear to be solved analytically. We applied the forward difference method to Eq. (6). The non-linearity was solved by the Newton method. The details are described in Appendix A. Fig. 7 shows the variation of y with u. With a lapse of the time t or u, values of y increase proportionally with u at a short time (Fig. 7(a)), and then deviate downward from the proportional line (b) for a long time. The variation is similar to the experimental results in Fig. 5. The relation of y with u can be approximated as
Fig. 8. Logarithmic plots of joVMF1 vs. the fraction of RGO, obtained from the variations in the Tafel-like plot through Eq. (9).
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may be caused by orientation of the stacked RGO flakes in laminate. The variation in Fig. 6 is not smooth at f = 0.15, whereas that in Fig. 8 is smooth. The non-smooth variation results from the critical change in geometrical connections of RGO flakes, called the electric percolation, which is specifically observed under solid states as in Fig. 6. When conducting particles are in solution, Brownian motion of the particles relax the critical variation to yield the smooth variation as in Fig. 8. The resistivity is included in the coefficient of t2 in Eq. (8), and hence it may participate in the long term growth. When Eq. (8) is divided by t, i.e. x j0 V M ez ar j0 V 2M e3z t t F 2RTF 3
ð10Þ
the coefficient of t2 in Eq. (8) appears as a slope of the plot of x/t against t in Eq. (10). Fig. 9 shows plots of x/t against t for x > 1 mm at the PANI-RGO film at f = 0.21 for four values of potentials. Values of x/t vs. t at a given potential fall on a line approximately, as can be justified by Eq. (10). The slope of the line means the acceleration of the growth rate. The values are negative, and are smaller as the potential is more anodic. The logarithmic plot of the values of the slopes against the potential exhibits a linear relation (shown in supporting information), as predicted from exp(3z) in Eq. (10). If r were independent of the potential, the slope of the line should be 3aF/RT. The value of a determined from the slope is ca. 0.1, smaller than the value (0.17) obtained from the Tafel-like plot. The difference can be explained in terms of getting large values of r at potentials closer to Eo. An advantage of the PANI-RGO comp1 osite appears in enhancement of the redox conversion rate. When the oxidation and the reduction potentials were applied to the film 2 mm thick cast on the Pt electrode in 1 M HCl solution, the charge-response for the PANI-RGO was faster than that of the PANI film, as shown in Fig. 10. The amounts of PANI are common to the both films in Fig. 10. Although the orientation of the RGO flakes are parallel to the electrode, the enhancement was still observed probably because of perturbation of the parallel directions. 4. Conclusions When PANI films are mixed with RGO flakes, the growth rate of the conducting zone in the films is increased more than ten times than that without RGO. The quantitative measurements and analysis of the growth rate can provide the following mechanisms of the enhancement.
x t -1 / mm s -1
0.4 0.3
(a)
Fig. 10. Charge-time curves at (a) the PANI-RGO coated Pt and (b) the PANI coated Pt for (c) the iterative potential steps between 0.2 V and 0.5 V.
(a) The rate is determined mainly by the a-times of the electrode potential, the exchange current density and the area of the reacting front. (b) RGO flakes increase the net surface area owing to parallel arrangement of flat structure of the flakes rather than the projected area. (c) The increase in the conductivity by percolation does not enhance directly the growth rate. (d) Any electrocatalytic activity causing a potential shift has no effect on the growth rate. Synergetic effects for PANI-RGO composites have thought to cause complicated behavior of electrocatalytic activity, enhancement of electric conductivity, modification of double layer capacitance by covalent bonds, and specific reaction conditions. However, the enhancement of the rate is mainly ascribed to the increase in fractal-like surface areas of RGO by the quantitative analysis. Appendix A. The method of solving Eq. (6) is shown here. The logarithm of Eq. (6) is 2ln(dy/du) = dy2/du, to which application of the forward difference, dy/du = (yn+1–yn)/Du for u = nDu, yields ðA1Þ y2nþ1 þ 2Duln ðynþ1 yn Þ=Du y2n ¼ 0 Since y0 = 0, y1 can be obtained by solving the non-linear equation of Eq. (A1) for n = 0. Eq. (A1) can be rewritten as f (z) z2 + a ln(z b) c = 0, where z = y1, a = 2Du, b = y0, c = y02 + 2Du lnDu. A value of z was determined by the Newton-Raphson method, i.e. a crude value zi was improved through the replacement, zi+1 = zi f(zi)/f’(zi), where the first and the second derivatives are f ’(z) = 2z + a/(z b) and f ”(z) = 2 a(z b)2. An initial value is either z0 = 0 for y0 > b + (a/2)1/2 or a very large for y0 < b + (a/2)1/2., according the signs of f ”(z0). Values of zi did not vary with i for i > 5, and hence the convergent value should be y1. Next, y2 was determined in the above process by regarding y1 as an initial value. It was confirmed that values of Du did not vary with y. References
0.2 (b)
0.1 (c) (d)
0 0
129
20
t/s
40
Fig. 9. Variations of xt1 with t for the PANI-RGO composite for f = 0.21 at Eap = (a) 0.6, (b) 0.5, (c) 0.4 and (d) 0.3 V.
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