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Polyacrylamide hydrogel as an electrolyte for oxidation-based organic rectifiers.

INTRODUCTION

Anodic treatment of aluminum has considerable scientific and technological interest due to its diverse applications. For example, it has been used to improve corrosion resistance of metal surface [1], applied in the electronic industry to form dielectric film for use in electrolytic capacitors [2], used in decorative layers by adding organic or metallic pigments [3], and used to obtain self-organized nanopore structures during the anodization of aluminum in acidic electrolytes for synthesis of highly ordered nanostructures [4-6].

The oxide films formed on the surfaces of the valve metals such as Al, Nb, Ta, Zr, Hf, introduced by Gunterschultze and Betz [7, 8] have the rectification property, defined as the current passing through in one direction but not in the reverse one. Stable solid oxide films of these metals have good ionic but poor electronic conductivity [9].

Anodic oxidation (anodization) is an electrochemical process to form an oxide film ([M.sub.x][O.sub.y]) on a metal surface by applying an electrical potential, using the metal as anode in a suitable electrolyte system. The nature of the electrolyte that is used for anodization thought as the primary one which affects the type of the oxide layer, barrier or porous oxide layer [9-12], In addition to the electrolyte, the anodization time is also an important parameter for the type of the oxide layer [13].

Despite there are different considerations for the reason of the rectification no complete theory has been presented up to date. Some of the considerations are discussed in Refs. 14-16. The difficulties for modeling the rectification of the current for this system is probably due to the amorphous nature of the oxide layer that include high impurity and lower mobility of charge carriers. The fact that the oxide layer is very thin (100-5000 [Angstrom]) makes the evolution of the charge transfer phenomena difficult [17]. Wang and Chang [18] modeled the oxidation region similar to a p-n junction where the middle region of the oxide film is considered as depletion region.

A hydrogel can carry huge amount of water in it. It can keep its shape even if the volume is increased to 100-fold of its initial value [19], Hydrogels are electrical insulators in their dried state. When the gels swell in an appropriate solution, ions can move in the gel thus they become conductive. The conductivity of the hydrogels can be increased by doping with metal particles [20, 21] or salt molecules. If the hydrogels were doped with some salt molecules, the charge carriers would be the counter ions [22, 23] of these doped molecules in addition to [H.sup.+] and O[H.sup.-] ions.

Polyacrylamide (PAAm) gels have a heterogeneous nature when they are synthesized in neutral form. Some high density regions so-called the "blobs" in the literature are created through the gel due to the difference in the reaction rates for AAm-AAm and for AAm-BIS (crosslinker). Addition of charged groups during the polymerization may result in a considerably more homogeneous internal structure of the gels. This behavior in the PAAm gels results in a porous structure of which the average size will change depending on the polymer contents [24-26].

The fact that the gels can keep their shapes with huge amount of water makes them soft and a good candidate for filler. Due to the conductivity properties and compatibility to biological systems, hydrogels provide us a strong motivation in the field of design in organic electronic devices.

Although there are plenty of works that dealt with the formation of the oxide layers, very few studies examining the effect of the hydrogel electrolyte on the oxide film formation has been published so far in the literature [27, 28], In this study, the effects of the composition thus the morphology of PAAm hydrogel electrolyte (PHE) on the formation of the oxide layers and the rectification properties were investigated in detail.

We studied the rectification of the ionic current due to the oxidation formed on the surface of aluminum anode. For the anodic oxidation of the aluminum surface, PHE was used as filler instead of traditional electrolytes like ammonium hydrogen tetra borate solution [14], ammonium tartrate solution [29], sulfuric acid solution [27], aqueous electrolytes [1], electrolyte consisted of ethylene glycol, water, and oxalic acid [30], The PHE was sandwiched between the aluminum and the platinum electrodes (Al/PHE/Pt), and the voltage was applied to bias the system. In addition to this configuration, Pt/PHE/Pt and A1/PHE/A1 configurations were also examined for comparison.

EXPERIMENTAL

Preparation of Hydrogels

The monomer (acrylamide, AAm), the initiator (ammonium per sulfate, APS), and the multifunctional crosslinker (N, N' methylene bisacrylamide, BIS) were supplied by Merck (Darmstadt, Germany). All chemicals were used as received. Distilled water was used in the gelation and swelling experiments of the hydrogels. Here the hydrogel was swollen in pure water with the conductivity of approximately 5.5 x [10.sup.-5] mS [cm.sup.-1]. The gels were synthesized via free radical crosslinking copolymerization of AAm and BIS. The pregel samples were deoxygenated by bubbling nitrogen for 15 min, and then the gelation was performed at 60[degrees]C in a heat bath. The procedure for the synthesis of the PAAm hydrogels is given in detail in Refs. 23, 24, 31.

Compositions of the gels are given in Table 1. The crosslinker concentrations were changed for each set while the monomer and the initiator concentrations were kept fixed for all sets. By this way we aimed to investigate the effect of the internal morphology of the gel on the oxidation of the metal, and thus on the rectification.

Electrical Measurements

The gels in cylindrical form of radius around 3 mm were cut into thin slices, and then they were dried in room temperature for 1 week. Then they were smoothed by sandpaper till they come to approximately the same width of 1 mm. After these gels were swollen to different swelling degrees, they were placed between the electrodes and the current was measured using Keithley 6487 Picoammeter/Voltage Source. Platinum--platinum, platinum-aluminum, and aluminum-aluminum parallel plate configurations were used as electrodes for investigating the effect of metal. 1050 quality aluminum alloy which contains 99.5% pure aluminum was used in the experiments. Schematic representation of the experimental setup for the electrical measurements is shown in Fig. 1. All measurements were performed between -10 V and +10 V range at room temperature, about 22[degrees]C. In the sweep experiments, the voltage sweep rate was chosen as 0.125 V [s.sup.-1] for all experiments. After the electrical measurements, we observed a slight decrease in the initial mass of the gels.

RESULTS AND DISCUSSION

In Fig. 2, the current-voltage (I-V) plots are sampled for two different electrolytes; (i) water impregnated tissue (WIT) in Fig. 2a and (ii) PHE in Fig. 2b between platinum electrodes. For all measurements, a 2 k[ohm] resistance was connected in series to the circuit. Each symbol in the figures denotes a new sweep of voltage; first from 0 to 10 V and then from 0 to -10 V.

As seen from Fig. 2, the same behavior was observed for both WIT and PHE; the current increases almost linearly after a threshold value of voltage both in forward and reverse bias. The main difference between WIT and PHE is the magnitude of the current for corresponding voltages. There are two reasons for Pt/PHE/Pt configurations that affect the intensity of the current: one is the swelling degree; the ratio of the masses of the swollen gel to the dried gel, m/[m.sub.0], and the other one is the heterogeneous nature of the PAAm gels as discussed in Refs. 24-26.

The symmetrical behavior of the current under the applied external potential in Fig. 2 shows that Pt/WIT/Pt and Pt/PHE/Pt configurations have ohmic character. For these configurations, no interaction between the water and the Pt electrodes or no physical deformation on the surfaces of the platinum electrodes was observed, as expected, before and after the measurements.

The fact that the plots for each sweep do not overlap each other is probably due to the drying of the hydrogels and WIT to some extent during the experiments where a small decrease was observed in the mass of the gels after the electrical measurements as mentioned in Experimental section.

Here, only the result of 2 M PHE of Set 2 at a specific swelling degree was given as an example. I-V characteristics of the other PHE fillers with different compositions synthesized for this work at different swelling degrees have similar behaviors. The ohmic behavior was observed for all PHE configurations. Only the maximum values of the currents changed depending on the composition and the swelling degree. The current was maximum for PHE with low polymer and low crosslinker concentrations at high swelling degrees.

The same experiments were repeated for Al/PHE/Pt configuration where one side of the sandwich is aluminum the other is platinum. The results were compared in Fig. 3 as an example for WIT and PHE fillers.

Note that for this configuration the first sweep has an ohmic-like character, but it turns into the rectifying junction due to the formation of a highly resistive oxide layer as the sweep number is increased. The aluminum oxide layer blocks the current in one direction and allows the current to flow in the other direction. Here, as the number of sweep is increased the current in one direction, where the aluminum side is set to higher potential, decreases. The current is rectified by both of the systems, WIT and PHE. The rectification becomes more pronounced after each sweep for both WIT and PHE, which indicates that the oxidation on the aluminum increases more and more after each step. However, the rectification becomes more rapid and more effective for PHE as can be seen from Fig. 3b where the gel accelerates the oxidation. The effect of the morphology of the PHE on the oxidation will be discussed below in detail.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

We investigated the effect of the polymer and the crosslinker concentration on the current for reverse bias. It was observed that the decreasing rate of the current becomes more pronounced when both the concentrations of the polymer and the crosslinker are increased. The variation in the current intensity is presented in Fig. 4a for 4 M polymer with increasing crosslinker concentration. Figure 4b shows the variation in the current intensity for varying polymer concentration at a fixed crosslinker concentration (Set 3). It is seen that as the sweep number is increased, the reverse current decreases exponentially to a constant value. This final value becomes almost zero for higher crosslinker and higher polymer concentrations. As no trapping effect occurs in WIT to decrease the mobility of the ions, the difference in the initial and the final values of the reverse current takes the lowest value as compared with PHE fillers, as expected. These results indicate that the morphology of the gel affects the reaction rate between the aluminum and the oxygen containing ions to form the oxide layer on the A1 surface.

[FIGURE 3 OMITTED]

In Fig. 5, the effects of the polymer concentration, the swelling degree, and the crosslinker concentration on the rectification ratio (the ratio of the current in forward bias to reversed bias, [I.sub.f]/[I.sub.r]) are represented. The observations and possible interpretations deduced from Fig. 5 are as follows:

(i) When the crosslinker concentration is increased, the rectification ratio increases. This is a pronounced effect especially for higher swelling degrees. For example, when the swelling degree is about 3.00 for 5 M polymer concentration, the rectification ratio increases to 850 from 10 upon increasing the crosslinker concentration to fourfold.

The fact that the rectification ratio increases with crosslinker concentration (compare Fig. 5a and b) is due to a considerable change in the morphology of the gels. When the crosslinker concentration is increased, the internal lower cutoff between the crosslinked points (like the lattice constant of a regular cubic lattice) becomes smaller. The ions trapped in the network can interact well with [Al.sup.+3] ions due to lowered mobility of these ions. In addition to the trapping effect, the number of the ions that are candidate to react with [Al.sup.+3] ions is another important parameter. Therefore, up to a certain swelling degree, at which the water can be assumed still trapped, the oxidation efficiency will increase due to the increase in the number of ions. Thus the rectification ratio will increase. Above this swelling degree, of course the number of ions still will increase, but their contribution to the oxidation will decrease since they cannot be trapped anymore.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

(ii). The rectification ratio passes through a peak when the swelling degree is increased. This observation becomes more evident for the PHE fillers with low crosslinker and comparatively high polymer content (see Fig. 5a).

In this work, two parameters which affect the oxidation yield and thus the rectification ratio are important. These are the mobility and the number of the ions at the PHE-A1 interface.

[FIGURE 6 OMITTED]

For the PHE with low crosslinker-low polymer content, the blobs are more or less open. Therefore, the trapping effect for the ions may not be considerable. Thus, the efficiency of the reaction will be small due to the high mobility of the ions. As the gel swells, the rectification ratio increases due to the increasing number of the ions but not due to the trapping effect. As a result, as the sole effect on the rectification ratio is the number of ions, it is not expected that the rectification ratio will pass through a peak even if the swelling degree is increased to some higher values.

For high crosslinker-high polymer content, the rectification ratio increases with the water uptake, similar to low polymer-low crosslinker case. But, the main difference here is that both of the magnitude and the rate of increase of the rectification ratio are comparably bigger. Here, the trapping effect plays a crucial role, that is, for high crosslinker-high polymer content the trapping effect becomes more pronounced compared to low crosslinker-low polymer case.

In the case of low crosslinker-high polymer content, a peak is observed with increasing swelling degree. Here two parameters, the number of ions and the trapping, affect the rectification ratio oppositely. As the gel swells, the number of ions will still increase, but the trapping effect will be diminished. Therefore, a peak appears in the rectification ratio with increasing swelling degree.

(iii). For low crosslinker concentration and low swelling degrees, the rectification ratio increases with increasing polymer concentration, but it does not change for higher swelling degrees.

For low crosslinker concentration and low swelling degrees, the trapping effect of the ions at the interface increases upon increasing polymer concentration. As discussed above, the trapping effect leads to a decrease in the mobility of the ions and an increase in the efficiency of the oxidation, which results in increase in the rectification ratio.

For high swelling degrees, as the network becomes more open, the system behaves just like a medium of free water. That is, at this scale the effect of the polymer becomes negligible, thus the effect of the morphology of the gel on the rectification ratio disappears.

Results in Fig. 6 clearly indicate, both for WIT and PHE fillers, how the rectification ratio varies for each sweep. It shows also the effect of the crosslinker concentration on the rectification ratio. Here the rectification ratio increases for both systems with increasing number of sweep. This is an expected result because at each sweep the oxidation will increase. The increase in the rectification ratio for WIT is very small compared with PHE. Comparing the rectification ratios for the PHE fillers with different crosslinker concentrations, it can be clearly seen that it increases when the crosslinker concentration is increased. The effect of the increased crosslinker concentration is discussed above. The results are presented in this figure as an example for 10 V.

Here we performed some experiments to carry out the effect of the applied voltage on the formation of the oxide layer, thus on the rectification ability of the rectifier designed in this study. The oxide layer was formed on the Al surface for Al/WIT/Pt and Al/PHE/Pt configurations under the applied voltages fixed at 5 V, 10 V, and 30 V, respectively. Each voltage was applied during 15 min and for each voltage a new Al electrode was used. Then the rectification ratio was determined in the voltage range of -10 V and +10 V using data given in Fig. 7 for Al/ PHE/Pt configuration.

As seen from Fig. 7 when the applied voltage is increased from 5 V to 30 V, the reverse biased current decreases considerably. This indicates that the thickness of the oxide layer on the Al surface increases upon increasing the applied voltage. These observations are consistent with the literature [9, 13, 32]. For 5 V, 10 V, and 30 V, the rectification ratios were calculated as 1.19, 1.39, 2.37 for Al/WIT/Pt configuration, and as 1.47, 2.32, 10.75 for Al/PHE/Pt configuration. Comparison of the rectification ratios for WIT and PHE clearly shows the predominant effect of the gel on the oxidation yield.

This can be interpreted as follows: The drag force created by the applied potential leads the ions to diffuse into deeper regions of Al surface, where the interaction takes place between the oxygen containing ions and [Al.sup.+3]. Therefore, the thickness of the oxide layer increases. As the voltage, so thus the thickness of the oxide layer, is increased the rectification ratio increases as seen from Fig. 7.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

The oxidation on the aluminum electrode is presented by the photographs given in Fig. 8. Here, the pattern of the oxidation on the contact surface becomes clearly visible after the voltage-sweep experiments.

Here we would like to discuss the case where the two electrodes were replaced with the same metal, aluminum. For AL/PHE/ Al configuration, I-V plots are given in Fig. 9. Here we observed a remarkable difference between Al/PHE/Al and the configurations Al/PHE/Pt and Pt/PHE/Pt. The difference of Al/PFIE/Al configuration from others is that the maximum current for both forward and reverse bias decreases with the sweep number, and finally it goes to zero after a certain sweep number. This observation indicates that oxidation occurs on both of the electrodes.

It seems that the current through one direction both for WIT and PHE, especially for PHE system, goes rapidly to zero than that of the other direction. This happens since the first time the potential was applied in this direction which results in a rapid growing of oxide layer on this side. In Fig. 9, only the results of the PHE with 2 M PAAm gel of Set 2 were given as an example. It is expected that the results would be similar for all PHE fillers with different compositions sandwiched between Al electrodes. It is also expected that the morphology of the PHE filler would affect the maximum of the current and the time interval in which the current goes to zero.

The results given in the above paragraphs clearly show that when one of the electrodes is aluminum and the other is platinum, the current is rectified. This is due to the formation of oxide layer on the aluminum electrode. This oxide layer blocks the current in one direction and allows the current to flow in the other direction.

For an example, we showed that the current-voltage plot in Fig. 10a for Al/PHE/Pt configuration in which 5 M PAAm gel of Set 3 with mass ratio, m/m0 = 3.014 [+ or -] 0.002, was used. The data were taken after 21 sweeps where the current reached the steady state. We observed very high values for the rectification ratio for this composition, around 850 at an applied potential of [+ or -] 7 V, where the gel was swollen in water. Figure 10b shows the increased rectification ratio with increasing voltage. To be able to get a stable rectification and a long-life usage of this organic rectifier the stability of the gel-based electrolyte used in this system plays a crucial role [26].

In this part, we would like to introduce the work performed on the stability of the gels used as electrolyte. To understand the stability, swelling kinetics of the gel was examined as function of the number of usage: first 30 V was applied to Pt/PHE/ Pt configuration for 1 h where PHE includes swollen gel. Then the gel was brought to its initial swollen state and then this process was repeated for n times (n = 5, 15, 45). For each number of usages (n), the mass of the gel swollen in the water was measured as a function of time.

The experimental results for stability measurements for 2 M gel of Set 1 are summarized in Fig. 11, where no considerable change is observed in the swelling behavior of the gel compared with the gel to which no voltage was applied before. We observed that the gels were shown to remain stable regardless of their molarities. This indicates that these gels are able to fulfill their function independent of the number of usage.

CONCLUSIONS

The method introduced in this article is a novel method that uses the PAAm hydrogel as an electrolyte between the metal electrodes instead of conventional electrolytes. We observed that when the hydrogel was used as an electrolyte, the oxidation yield (thus the rectification ratio) on the metal increases considerably, and the time to reach the maximum oxidation stage becomes shorter. This is likely due to the trapping of the water in the gel. This trapping increases the efficiency of the chemical reactions--the reactions between the aluminum and the ions carrying oxygen--for oxidation.

In this work, the maximum rectification ratio was measured to be around 850 at [+ or -] 7 V for Al/PHE/Pt configuration. This value is just for a certain composition of the gel. It can also be increased to some higher values by varying the composition of the gel, increasing the number of sweep, and increasing the applied voltage. Stability of the hydrogel was studied via swelling and drying experiments and showed that no considerable change in the performance of the electrolyte occurs upon multiple usages.

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Sevcan Tabanli, Ali Gelir, Yasar Yilmaz

Department of Physics Engineering, Faculty of Science and Letters, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey

Correspondence to: Sevcan Tabanli; e-mail: tabanlisevcan@itu.edu.tr

DOI 10.1002/pen.23906

Published online in Wiley Online Library (wileyonlinelibrary.com).
TABLE 1. Compositions of the pre-gel solutions.

AAm (mol/L)    Fixed for all sets    1      2      3       4       5

APS (mmol/L)   Fixed for all sets   7.1    14.1   21.2   28.3    35.5
BIS (mmol/L)         Set 1          7.3    14.7   21.9   29.3    36.5
                     Set 2          14.6   29.3   43.9   58.6    73.0
                     Set 3          29.2   58.6   87.8   117.2   146.0
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