rGO Composites

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ScienceDirect Materials Today: Proceedings 4 (2017) 5138–5145

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ISMAI 2016

Morphological and Electrochemical Properties of Hybridized PPy/rGO Composites Nur Atikah Md Jania, Muhammad Aidil Ibrahima, Tunku Ishak Tunku Kudina, Ab Malik Marwan Alib, Hazwanee Osmanc, Oskar Hasdinor Hassand,* Ionics Materials & Devices Research Laboratory (iMADE), Institute of Science, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia bFaculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia cCentre of Foundation Studies, Universiti Teknologi MARA, 43800 Dengkil Campus, Selangor, Malaysia dFaculty of Art and Design, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia

a

Abstract Polypyrrole (PPy) was synthesized via microemulsion method from pyrrole (Py) monomer, while reduced graphene oxide (rGO) was produced from graphite powder. PPy/rGO composites harvested by ultrasonication with various weight ratio of PPy:rGO to 4:1, 3:1, 2:1 and 1:1, respectively. The composites successfully characterized via scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The characterizations revealed the composites morphology was well developed. The rGO was successfully embedded with PPy forming an excellent network structure. The electrochemical of the PPy/rGO composite was determined via electrical conductivity and cyclic voltammetry. The highest electrical conductivity obtained by PPy/rGO composites with ratio 1:1 of 2.504 Ω-1cm-1 with excellent reversible redox reaction. © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Conference Committee Members of International Symposium on Materials and Asset Integrity (ISMAI 2016). Keywords: Polypyrrole; Reduced graphene oxide; Morphological; Electrochemical; Hybridized composites

* Corresponding author. Tel.: +6-035-544-4099; fax: +6-035-544-4011. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Conference Committee Members of International Symposium on Materials and Asset Integrity (ISMAI 2016).

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1. Introduction Conducting polymer is a conjugated π-electron materials with a combination of various electrical, optical and other semiconductor properties which could be used for vary application [1]. These conducting polymers have a unique characteristic that makes them suitable to act as a transducer that converts the biological signal to an electrical signal. It also provides an excellent platform for the immobilization of biomolecules onto the electrodes. In addition to that, these conductive polymers also provide a better signal transduction in biological biosensors where it enhances the sensitivity, selectivity, durability, biocompatibility, direct electrochemical synthesis and flexibility for immobilization of biomolecules onto the solid surface [2]. Existing conducting polymers are polynapthalens, polyparaphenylene, polypyrrole, polyaniline, polycarbazole, polythiophene, polyazulene and several others. However, among those entire conducting polymers, only polyaniline, polypyrrole and polythiophene were commonly used in biosensor due to their characteristic of biocompatibility which results in the minimal and reversible disturbance to the working environment and protect electrodes from fouling [2]. Among those commonly used polymer, polypyrrole (PPy) has become an interest as potential transducer in biosensor due to its excellent properties as compared to polyaniline and polythiophene [3]. In practice, PPy was obtained from the pyrrole (C4H5N) monomer that consists of a 5-membered ring, containing a nitrogen (N) heteroatom [4]. As other organic molecules, pyrrole polymerization occurs upon oxidation of the monomer that resulted through the formation of a conjugated polymer chain with overlapping π-orbitals and a positive charge along the polymer backbone. Due to this polymerization process, it will coupled with the loss of hydrogen from the polymer chain and grows until termination [4]. This conjugation gives a great potential for the PPy to become biocompatible to various type of biomolecules in biosensor [5]. However the pure PPy as conducting polymer provides low conductivity in electronic devices, therefore to increase the electrical performance of PPy, a nanometer-sized filler with conductive path structure and high surface area could be used to enhance the conductivity PPy in the form of composite. Reduced graphene oxide (rGO) could be considered as a nanosized-filler for PPy according to its high surface area and excellent conductivity properties [6]. Many work on enhancing the electrical performance of PPy has been carried out such as intercalation it with graphene via in-situ polymerization technique and chemical reduction using hydrazine monohydrate [6,7]. Both techniques did increase the PPy conductivity but the techniques employed hazardous chemicals. In this work, the hybridization technique was employed to increase the PPy conductivity in the form of composite materials. The PPy was synthesized from pyrrole (Py) monomer and yielded rGO was obtained from graphite powder. Then the PPy and rGO was intercalated via hybridization through dispersion, ultrasonication, filtration and drying to obtain the PPy/rGO composite. The different weight ratio of PPy/rGO composites was investigated with scanning electron microscopy (SEM) and Raman spectroscopy. Finally the electrical conductivity and cyclic voltammetry (CV) was determined. 2. Experimental 2.1. Preparation of Polypyrrole (PPy)/rGO Composites The composite consist of two components namely PPy and rGO. The PPy was synthesized via microemulsion technique as reported in [8]. The technique involves the mixing of 0.08 M sodium dodecylsulfate (Sigma Aldrich, 98.5%), 0.08 M pyrrole (Sigma Aldrich, 98%), 0.08 M n-amyl alcohol (R & M Chemicals, 99.7%) and 0.08 M ammonium persulphate (Sigma Aldrich, 98%). The mixture was stirred vigorously for 24 hours at 4 C by the stirrer (model HP-3000, Lab. Companion, USA). The mixture then washed several times with excessive methanol, filtered and dried at 60 C [3]. While the rGO was prepared by using improved Hummer’s method according to the method described in [9]. The obtained rGO was mixed with water and ethanol with ratio of 50 to 1 (50:1), respectively to form rGO suspension as referred to the [10]. Subsequently the as-prepared PPy was dispersed into rGO suspension to compose the PPy/rGO solution with the ratio of PPy to rGO of 4:1, 3:1, 2:1, and 1:1, respectively. Then the solution was ultra-sonicated by ultrasonication machine (model Newpower Ultrasonic Electronic, China) for 1 hour

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for hybridization process to form PPy/rGO composite. Later the hybridized PPy/rGO composite was filtered, purified and dried in vacuum oven (model RV3, Edwards, United Kingdom) at 25 C for 24 hours [10]. 2.2. Materials Characterization The microstructural analysis of as-prepared PPy, rGO and PPy/rGO composites was carried out by scanning electron microscopy, SEM (TM3030Plus, Hitachi, Japan) with 15 kV power for each samples. Typical back scattered electron (BSE) analysis was carried out for all samples. Samples prepared was characterized for the Fourier transform infrared spectroscopy, FTIR (Model Spectrum 400 spectrophotometer, Perkin Elmer, USA). The spectrophotometer records the spectra in the frequency range of 600 cm-1 to 2000 cm-1 with a resolution of 2 cm-1. The Raman spectroscopy (UniRAM-3500, UniRAM, South Korea) was performed for all samples. The samples were illuminated with laser power of 50 mW and a wavelength of 532 nm. The laser was focused with object lens of 100x into 1 µm size. Data obtained was recorded in the range wavenumber (Raman Shift) of 600 cm-1 to 2400 cm-1. The microstructural analysis of as-prepared PPy, rGO and PPy/rGO composites was carried out by scanning electron microscopy, SEM (TM3030Plus, Hitachi, Japan) with 15 kV power for each samples. Typical back scattered electron (BSE) analysis was carried out for all samples. The Raman spectroscopy (UniRAM-3500, UniRAM, South Korea) was performed for all samples. The samples were illuminated with laser power of 50 mW and a wavelength of 532 nm. The laser was focused with object lens of 100x into 1µm size. Data obtained was recorded in the range wavenumber (Raman Shift) of 600 cm-1 to 2400 cm-1. 2.3. Electrical Measurements The electrical conductivity of the samples were measured via 4–point probes direct current (DC) at room temperature. The probe was directly placed onto the filter membrane with the range current between 0.01 A to 0.05 A. The data were collected by WEIS510 multichannel (model WEIS 510, WonATech, Korea). While the cyclic voltammetry (CV) measurement was carried out by the potentiostat (model Autolab, Eco-Chemie, Netherland) with NOVA software. Each samples were placed on glassy carbon electrode (GCE). The CV was carried out using 3electrode system namely Ag/AgCl, platinum, GCE as reference, counter and working electrodes, respectively. All electrodes were immersed in solution of 0.1 M potassium chloride and 5 mM potassium ferricynide as a redox probe. CV was performed between -0.4 V to 0.8 V at a scan rate of 100 mV s-1. 3. Results and discussion 3.1. Scanning Electron Microscopy (SEM) The morphology of the PPy, rGO, PPy/rGO composites was observed by SEM. The synthesized PPy, shows a large protruding sharp rocky shape and typical rough surface of PPy as depicted in Fig. 1.(a) which similar finding was reported in [11]. While the rGO morphology shown in Fig. 1.(b), it was flake-like shape with a wrinkled surface which alike as reported in [12]. Microstructure of PPy/rGO composites in Fig. 1(c-f) shows a clear layer structure with dense stacking of PPy together with the flake-like look of rGO. All of these effects are due to the ultrasonication of PPy and rGO that form fully exfoliated PPy/rGO solution. From the SEM images in Fig. 1(c-e), it was revealed that the PPy grown on the surface of the rGO and a little agglomeration of PPy is found, which usually occurred during the synthesis process. This agglomeration indicates that the polymerization of pyrrole occurred on the surface of rGO owing to the 𝝅-𝝅 interaction and hydrogen bond between pyrrole and rGO which also reported in [13]. Wang et al. [14] also reported similar morphology of the PPy/rGO composite. They also explain that their rGO has a large 𝝅-electron system and forms the 𝝅-𝝅 noncovalent interactions as the predominant force between the adsorbent and the adsorbate. Thus, in this work the rGO adsorbent should show a high binding capacity for aromatic molecules and the interactions of electrostatics and induction also contribute to the further stabilization of the 𝝅-𝝅 interaction between rGO and aromatic molecules. While the long chains, mesoporous surface and delocalized 𝝅electron system in PPy also undergo attractive interactions with analyte molecules through enhanced van der Waals and dipole-induced dipole interactions. Therefore, the PPy/rGO could achieve high absorption [14]. When the mass

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ratio of PPy to rGO decreasing from 4:1 to 3:1 to 2:1 and 1:1, the physical appearances of individual material changes when they were hybridized into a composite, the lesser the amount of PPy in the composite, the flatter surface of the image observed. This is due to the PPy itself, because it tends to grow on the rGO to form thick-layer structure, where the dense packed of PPy still could be observed in Fig. 1(c), 1(d) and 1(e), that represent the PPy/rGO (4:1), PPy/rGO (3:1) and PPy/rGO (2:1), respectively. However, when the mass ratio of PPy to rGO is 1:1, the rGO is completely covered by PPy thus it forms a well-designed PPy/rGO structure with great network of bonding which does not tend to form a thick-layer structure and prevent the restacking of both rGO and PPy (see Fig. 1.(f)). Thus it shows that the PPy and rGO are hybridized to each other. Zhang et al. [15] also discovered similar results with their hybridization process.

Fig. 1. SEM of the composites a) PPy at 300 X magnification; b) rGO; c) PPy/rGO (4:1); d) PPy/rGO (3:1); e) PPy/rGO (2:1); f)PPy/rGO (1:1) at 500 X magnification.

3.2. Fourier Transform Infrared Spectroscopy (FTIR) The FTIR spectra of the PPy, rGO, PPy/rGO (4:1, 3:1, 2:1 and 1:1) are presented in Fig. 2. PPy peaks were located at 1579 cm-1 and 1489 cm-1 are associated with C=C to the asymmetric and C-N symmetric ring-stretching vibration, respectively as reported in [6]. While at 1047 cm-1 corresponding to C-H and N-H in-plane deformation vibrations and 1219.5 cm-1 is attributed to C-N stretching as a breathing vibration of the pyrrole ring as referred to [15]. While rGO is entirely composed of carbon (peak at 1588 cm-1) which represent the C=C stretching groups of rGO. The PPy/rGO (4:1, 3:1, 2:1 and 1:1) composites, also shows peak at around ~1582 cm-1 to ~1588 cm-1 that is corresponding to the C=C stretching of pyrrole ring and peaks at around ~1189.5 cm-1 to ~1201.5 cm-1 represent CC stretching, which were in consistent with the PPy characteristics, indicating the successful functionalization of rGO by PPy.

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922.5

1048.5

1582

1195.5

PPy/rGO 1:1 913.5

1039

901.5

1051

1585

1195.5

PPy/rGO 3:1

Intensity (a.u.)

Transmittance (%)

PPy/rGO 2:1 1585

1201.5

1584 922.5

1036

1189.5

PPy/rGO 4:1

600

1333.79

1552.85

2321.19

1370.48

1568.87

2318.00

PPy/rGO 3:1

1348.49

1558.19

1554.63 1341.15

PPy/rGO 4:1 rGO

1579 937.5

800

970.5

1047

1000

1219.5

Graphite oxide

1489

1400

-1

1600

1800

2000

600

1561.75

2316.40

1341.15

1581.30

2313.21

1348.49

1561.75

1348.49

PPy

1200

2316.40 2318.00

1588 rGO

PPy

PPy/rGO 1:1 PPy/rGO 2:1

800

1000

D

1200

G 1400

1600

1800

2319.59

2000

2200

2D 2400

-1

Wavenumbers (cm )

Raman Shift (cm )

Fig. 2. FTIR of PPy; rGO; PPy/rGO (4:1, 3:1, 2:1, 1:1).

Fig. 3. Raman Spectroscopy of PPy; GO; rGO; PPy/rGO (4:1, 3:1, 2:1, 1:1).

3.3. Raman Spectroscopy The Raman spectra of PPy, GO, rGO and PPy/rGO composites are depicted in Fig. 3. The structural difference of the samples was investigated based on three bands in the range from 1200 cm-1 to 2400 cm-1. From these results, all samples show a significant shift at wave length of 1350 cm-1, 1560 cm-1 and 2320 cm-1 denoted as D, G and 2D bands, respectively. The D band is attributed to the vibration of K-point phonons of A1g symmetry, arises from the breathing vibration of aromatic rings, which are related to the edges, structural defects that correspond to the conversion of sp2-hybridized carbon to a sp3-hybridized carbon. While the G band originates from the zone center E2g mode, which corresponding to ordered sp2-bonded carbon atoms which alike as reported in [16]. Typically peak of PPy appeared at 1348.49 cm-1 and 1561.75 cm-1 which were due to the C=C backbone stretching and the ringstretching mode, respectively as referred to [15]. The Raman shifts of PPy/rGO composites shows the presence of PPy and rGO, however the G and D bands of the composites were significantly broadened and their intensity increased with introduction of rGO. G band is directly proportional to the graphitic in-plane crystallite size, thus it showed increasing of G band intensity revealing that the polymer thickness increased after covering the rGO by PPy. Similar finding reported in [17-18]. 3.4. Electrical Conductivity Measurements The electrical conductivity measurements were performed on PPy, rGO and PPy/rGO composites by using 4point probes direct current (DC). Electrical conductivity of all samples were calculated using the equations (1), (2) and (3) below. The sheet resistance, Rs was calculated by employing set of equations and the electrical conductivity can be determined. Thus, Resistance sheet (Ω, Ohm), 𝑅𝑠 =

𝜌 𝑡

=

= 4.35

( )( ) 𝜋𝑡 ln 2

𝑉 𝐼

( )Ohm –Centimeter for s ≫ t 𝑉 𝐼

(1)

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The electrical resistivity ρ (Ω. cm) was determined using the formula, 𝜌 = 𝑅𝑠 × 𝑡

(2)

and hence, the electrical conductivity σ (Ω-1cm-1) was obtained by using the equation, 1

(3)

𝜎=𝜌 where, t is the thickness of the sample, V is the measured voltage and I is the current [18].

The results of calculated electrical conductivity is represented in Table 1. The electrical conductivity of PPy/rGO composites is strongly dependent upon the weight ratio of PPy and rGO. The conductivity of PPy/rGO composites increases as rGO increased. The presence of rGO in PPy/rGO composites does increased the conductivity which also reported in [17]. Table 1. Conductivity Measurements (4-point probes) Sample

Thickness (t, cm)

Resistance (ohm, Ω)

Resistivity (ρ, Ω.cm)

Conductivity (σ, Ω-1cm-1)

rGO PPy PPy/rGO (4:1) PPy/rGO (3:1) PPy/rGO (2:1) PPy/rGO (1:1)

0.005 0.004 0.007 0.006 0.009 0.010

365.439 233.179 120.011 109.492 56.124 39.934

1.827 0.933 0.840 0.657 0.505 0.399

0.547 1.072 1.190 1.522 1.980 2.504

2.6

2.2

-1

-1

Conductivity (  cm )

2.4

2.0 1.8 1.6 1.4 1.2 1.0 0

4:1

3:1

2:1

1:1

PPy weight ratio to rGO (PPy:rGO)

Fig. 4. The conductivity of PPy; PPy/rGO (4:1, 3:1, 2:1, 1:1).

Fig. 4 shows the electrical conductivity of PPy/rGO composites. The conductivity of pure rGO and pure PPy has the electrical conductivity as stated in Table 1 is almost as the same value as reported in [17]. However, the PPy/rGO composite with different weight ratio of PPy and rGO produced a better electrical conductivity compared to the pure PPy and pure rGO. The increasing of electrical conductivity of PPy/rGO composite is almost linear with the presence of rGO. This is due to the presence of synergetic effect between rGO and PPy in forming interpenetrating conductive networks which also reported in [17]. The electrical conductivity for PPy/rGO (4:1), PPy/rGO (3:1), PPy/rGO (2:1) and PPy/rGO (1:1) as stated in Table 1 shows that the conductivities of PPy/rGO composites decrease with the increment of content of PPy in the composites. This is the results of conductive

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network theory, the effective conductive network was gradually formed where the composite conductivity increased before the local maximum reached. The increasing feeding ratio of PPy in the composites has the excessive PPy formed aggregation outsite the rGO, which prevent the transport of effective charge along the conductive networks that resulting the decreased conductivity as reported in [17]. The PPy/rGO (1:1) has the highest conductivity compared to the other ratios because hybridization between the PPy with rGO in this ratio resulting a complete exfoliated for satisfactory intercalation of PPy with rGO, as observed from the morphology image shown in Fig. 1(f). This figure shows that the PPy/rGO composites present homogenous intercalation with plush PPy wrapping around rGO, as it allow the transport of effective charge along the conductive networks as to form better conductivity which alike as reported in [17]. Increasing in magnitude of conductivity as compared to the pure PPy and pure rGO may also be attributed to the 𝜋-𝜋 stacking between the rGO and PPy which alike as reported in [6]. 3.5. Cyclic Voltammetry (CV) Fig. 5 depicts a comparative study of cyclic voltammetry (CV) for PPy, rGO and PPy/rGO composites fabricated onto glassy carbon electrode (GCE) as working electrodes in a three-electrode system which include Ag/AgCl electrode as reference electrode and platinum electrode act as counter electrode. This CV procedure was constructed in the presence of combination of 0.1 M potassium chloride (KCl) and 5 mM potassium ferricyanide [Fe (CN)6]3-/4as a redox probe, at the scan rate of 100 mV s-1 in the potential range from -0.4 V to 0.8 V. All CV curves demonstrating that all samples have an ideal capacitive nature with ion response, where it illustrates symmetric current-potential characteristics, indicative of good conductivity and high reversibility of the electrode materials. Obviously, the PPy/rGO (1:1) composites has a higher Cs than the PPy, rGO and other PPy/rGO composites at the same scan rate, which due to its larger enclosed area in the CV curve. PPy/rGO(1:1) composites exhibits better electrochemical capacitive characteristic, high degree of electroactivity, faster ion diffusion rate and superior reversible redox reaction than PPy due to the contribution of both layer and the pseudo-capacitance which alike as reported in [16]. Meanwhile, by comparing the PPy/rGO (1:1) composites with the rGO as shown in the CV curve is leveled current separation between leveled anodic and cathodic current for rGO was much smaller due to the poor supercapacitive activity of stacking of the samples which also been reported in [13,15,17,19]. -5

1.5x10

-5

1.0x10

-6

5.0x10

Current (A)

0.0 -6

-5.0x10

-5

-1.0x10

GCE GCE/PPy GCE/rGO GCE/PPyrGO (4:1) GCE/PPyrGO (3:1) GCE/PPyrGO (2:1) GCE/PPyrGO (1:1)

-5

-1.5x10

-5

-2.0x10

-5

-2.5x10

-5

-3.0x10

-5

-3.5x10

-0.4

-0.2

0.0

0.2 Potential (v)

0.4

0.6

0.8

Fig. 5. Cyclic Voltammetry curves of PPy, rGO and PPy/rGO (4:1, 3:1, 2:1, 1:1) composites fabricated onto the glassy carbon electrode (GCE).

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4. Conclusions In this work, the PPy/rGO composites were successfully prepared via hybridization between PPy and rGO as shown by the presence of sharp rocky shape and flake like morphology in the microstructure analysis. While the intercalation and interaction between PPy and rGO was clearly shown by Raman shift at G band of 1560 cm-1. The electrical conductivity is increasing as the increment weight of rGO increases. The highest conductivity was obtained by PPy/rGO composite of 1:1 ratio with the highest value of 2.504 -1 cm-1. Subsequently, the Py/rGO composite with 1:1 ratio also shows high reversibility of redox reaction as obtained from cyclic voltammetry (CV). Based on this work, this material could be a potential candidate to be applied as supercapasitors and transducers due to its great performance. Acknowledgements The authors would like to express our gratitude to Ministry of Higher Education for financial support under the funding of Research Acculturation Grant Scheme (RAGS) and Fundamental Research Grant Scheme (FRGS). We would also like to thank Universiti Teknologi MARA for the facilities involved in making this research a success. 5. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

A. Ehsani, B. Jaleh, and M. Nasrollahzadeh, J. Power Sources., 257, (2014) 300–307. K. Arshak, V. Velusamy, O. Korostynska, K. Oliwa-Stasiak, and C. Adley, IEEE Sens. J., 9, (2009) 1942–1951. B. Batra, S. Kumari, and C. S. Pundir, Enzyme Microb. Technol., 57, (2014) 69–77. G. W. Chun, C. C. Sheng, H. W. Chiung, American Chem. Soc., 18 (2012), 7473-7481. A. Sassolas, L. J. Blum, and B. D. Leca-Bouvier, Biotechnol. Adv., 30, (2012) 489–511. S. Bose, T. Kuila, M. E. Uddin, N. H. Kim, A. K. T. Lau, and J. H. Lee, Polymer (Guildf)., 51, (2010) 5921–5928. B. Liang, Z. Qin, T. Li, Z. Dou, F. Zeng, Y. Cai, M. Zhu, and Z. Zhou, Electrochim. Acta., 177, (2015) 335–342. S. Lata, B. Batra, N. Singala, and C. S. Pundir, Sensors Actuators, B Chem., 188, (2013) 1080–1088. A. I. Kamisan, A. S. Kamisan, R. Md Ali, T. I. Tunku Kudin, O. H. Hassan, N. Abdul Halim, and M. Z. A. Yahya, Adv. Mater. Res., 1107, (2015) 542–546. J. W. Park, C. Lee, and J. Jang, Sensors Actuators B Chem., 208, (2015) 532–537. Y. S. Lim, Y. P. Tan, H. N. Lim, W. T. Tan, M. a. Mahnaz, Z. a. Talib, N. M. Huang, a. Kassim, and M. a. Yarmo, J. Appl. Polym.Sci., 128, (2013) 224–229. F. Wang, L. Zhu, and J. Zhang, Sensors Actuators B Chem., 192, (2014) 642–647. L. Q. Fan, G. J. Liu, J. H. Wu, L. Liu, J. M. Lin, and Y. L. Wei, Electrochim. Acta., 137, (2014) 26–33. L. Wang, M. Wang, H. Yan, Y. Yuan, and J. Tian, J. Chromatogr. A., 1368, (2014) 37– 43. F. Zhang, F. Xiao, Z. H. Dong, and W. Shi, Electrochim. Acta., 114, (2013) 125–132. S. Khamlich, F. Barzegar, Z. Y. Nuru, J. K. Dangbegnon, a. Bello, B. D. Ngom, N. Manyala, and M. Maaza, Synth. Met., 198, (2014) 101–106. Y. Liu, H. Wang, J. Zhou, L. Bian, E. Zhu, J. Hai, J. Tang, and W. Tang, Electrochim. Acta., 112, (2013) 44–52. A. A. Daniyan, L. E. Umoru, A. Y. Fasasi, J. O. Borode, K. M. Oluwasegun, S. Oloruntoba, and O. Olusunle, J. Miner. Mater. Charact. Eng., 2014, (2014) 15–20. X. Yan, X. Zhang, H. Liu, Y. Liu, J. Ding, Y. Liu, Q. Cai, and J. Zhang, Synth. Met., 196, (2014) 1–7.