Novel Bifunctional Electrocatalyst for ORR Activity ... - ACS Publications

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Novel Bifunctional Electrocatalyst for ORR Activity and Methyl Parathion Detection Based on Reduced Graphene Oxide/Palladium Tetraphenylporphyrin Nanocomposite Subramanian Sakthinathan,† Subbiramaniyan Kubendhiran,† Shen-Ming Chen,*,† Chelladurai Karuppiah,*,‡ and Te-Wei Chiu§ †

Electroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan, R.O.C. ‡ Department of Chemistry, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 106, Taiwan, R.O.C. § Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan, R.O.C. S Supporting Information *

ABSTRACT: We introduce a facile, eco-friendly reduced graphene oxide/palladium tetraphenylporphyrin (RGO/Pd− TPP) nanocomposite for oxygen reduction reaction (ORR) and environmental pollutant detection. The catalytic properties of RGO/Pd−TPP toward ORR and methyl parathion (MP) reduction showed excellent performance due to the synergism between RGO and Pd−TPP. The other properties of RGO/Pd−TPP nanocomposite were characterized by various microscopic and spectroscopic techniques. The asprepared RGO/Pd−TPP electrocatalyst was further fabricated on electrode to study the ORR activity and MP sensing. The RGO/Pd−TPP modified electrode shows higher current density and less methanol tolerance than the Pt/C electrode, which revealed that the proposed material can be used as an excellent alternative to commercial Pt/C electrode. In addition, the electrocatalytic reduction of MP on RGO/Pd−TPP/GCE was briefly investigated and achieved a good analytical performance with excellent linear range (0.1−125 μM), low detection limit (7.4 nM), and higher sensitivity (4.1084 μA μM−1 cm−2). Besides, the modified electrode has successfully applied for the determination of MP insecticide in various water and food samples. Therefore, the finding of RGO/Pd−TPP nanocomposite can be regarded as an effective catalyst to enhance the electrocatalytic activity toward ORR and MP reduction.

1. INTRODUCTION In recent years, the oxygen reduction reaction (ORR) generates the electricity in fuel cells due to the cathodic reduction of molecular oxygen. It has been great interest in the field of clean energy technology such as catalytic converters, metal-air batteries, solid oxide electrolysis cells and solid oxide fuel cells (SOFCs).1−3 The direct methanol fuel cells (DMFCs) as a class of membrane fuel cells which has received a considerable attention in the field of energy storage and conversion due to their unique properties such as low emission, high power density and simple processing. Nevertheless, there are some disadvantages that preventing the commercialization of DMFCs including the slow kinetics, methanol crossover problem through the membrane and high cost of catalysts using as the cathode electrode.4−6 In recent years, Pt metal has been used as the electrocatalyst in fuel cell to avoid high over potential and provide a tremendous ORR activity. On the other hand, the high quantity of catalyst loading is essential for the electrode fabrication that limits the Pt usage in practical application, because, it is more expensive and less abundance.7,8 In addition, Pt related catalysts are highly sensitive for CO and methanol. © 2017 American Chemical Society

Hence, there is an urgent need to search novel alternative catalyst for Pt with lower cost, high efficiency and excellent methanol-tolerance effect.9,10 Consequently, recent efforts have focused on the discovery of various Pd-related catalysts for ORR; they delivered a high electrocatalytic activity and are considered to be a low cost material when compared with the commercial Pt/C catalyst. However, Pd-based catalysts have poorer durability than Pt under acid conditions.11 Hence, we need to improve the synthesis method to get the highly active palladium-based catalyst for the ORR reaction.12 Methyl parathion (MP), is an organophosphorus organic compound, has been widely used for pest control in crops, cereals, coffee, potato, fruits and sugar cane.13,14 Because of the widespread usage of MP, it can be easily entered into the human body through food and water. They causes several health illness to animal and human particularly muscular paralysis, convulsions, bronchial constriction and death. ThereReceived: March 2, 2017 Revised: June 12, 2017 Published: June 19, 2017 14096

DOI: 10.1021/acs.jpcc.7b01941 J. Phys. Chem. C 2017, 121, 14096−14107

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The Journal of Physical Chemistry C Scheme 1. Synthesis of Palladium Tetraphenylporphyrin (Pd−TPP)

have been used into various fields such as in electrocatalysis, sensors, environmental remediation, energy storage and conversion applications. Conversely, GO has lower activity than graphene since it has more oxygen functionalities on the edge and basal planes which can be decreased the GO activity. Therefore, the removal of oxygen functionalities on the graphene oxide surface is more important one to restore the sp2 carbon network and improves the electrical conductivity.35−39 So far, the GO reduction was carried out in three different way such as chemical, thermal and electrochemical method. The chemical reduction method produce better RGO from GO, but this reduction method involves toxic chemicals as a reducing agents such as hydrazine, NaBH4, hydroquinone, hydrogen sulfide, and aluminum powder. Also, this method has some drawbacks such as impurity formation, affect the environmental condition and long time process. Therefore, we have chosen an eco-friendly green reduction method which is the best alternative method for GO reduction. Because, it is a simple, inexpensive, efficient, low-cost and environmentally friendly method. In particular, nontoxic caffeic acid (CA) has used as a green-reducing agent for GO reduction due to the excellent adsorption, stabilization and autoxidation properties. Such kind of autoxidation properties of CA is the main strategy for the reduction of GO.40−42 In the present work, we have demonstrated that the Pd−TPP macromolecules are noncovalently interacted with the eco-friendly reduced GO surface to form a RGO/Pd−TPP nanocomposite. The RGO/Pd−TPP nanocomposite modified electrode shows the higher electrocatalytic performance toward ORR and the environmental pollutant detection of methyl parathion (MP) pesticide. Interestingly, the fabricated electrode exhibited the excellent ORR activity and low-level detection, long linear response range and excellent practicality toward the detection of MP. To the best of our knowledge, first time, we have reported the RGO/Pd−TPP/GCE used for both ORR activity and MP sensor.

fore, a sensitive and selective detection of MP is more important for human health and environmental safety.15,16 Several analytical methods have been used to determine the level of MP in water samples such as, gas chromatography mass spectrometry,17,18 chemiluminescence,19 high performance liquid chromatography,20 and colorimetric detection.21 These methods are performed well with an accurate results, however, those methods are long time processes and expensive for practical applications.22 Therefore, research is still looking for less time consumable and low cost method for MP determination. On the contrary, the electrochemical technique is an alternative and cost-effective method to determine the MP compared with other traditional analytical methods.23 However, it has attained some limitations along with high reduction overpotential and low sensitivity when using unmodified electrodes toward MP detection. Therefore, the electrode modification is more preferable and is an important one to improve the detection sensitivity of MP. In recent years, carbon-based nanomaterials and metal porphyrin modified electrodes have been widely employed in MP sensors.24 Porphyrin is the macrocyclic organic molecules which composed of tetrapyrrolic ring with 18 π- electrons conjugated system. They have coordination with various transition metals to form stable metalloporphyrin at different oxidation states. Moreover, metal coordinated tetraphenylporphyrin (m-TPP) has widely been used in electroanalysis due to the excellent electron transport properties. In particular, the Pd−TPP have important role in optoelectronics, nonlinear optics, solar cell, catalysis, sensor and photodynamic therapies, because of its strong excited-state absorption and delocalized electron density.25−27 In addition, the metalated porphyrin has been devoted to the development of highly active ORR catalyst due to the low molecular weight, higher stability and high pH tolerance. However, the direct application of Pd−TPP in aqueous solution is challenging because of their catalytic inactive dimers in the oxidizing reaction media. Therefore, improving the metalated porphyrin is more important for ORR and sensor related application.28,29 In this regards, the metalated porphyrin can be noncovalently functionalized with reduced graphene oxide surface via π−π interaction. Graphene oxide (GO) based nanocomposites have emerged as a new star materials, because of their extraordinary physical and chemical properties.30−32 On the other hand, graphene is a two-dimensional (2D) materials which consist of sp2 hybridized single layer carbon network with a hexagonal lattice structure and it has specific surface area around 2620 m2 g−1.33,34 The attractive layer-by-layer structure and π−π stacking interactions of graphene provide highly stable surface and well electrical conductive properties. Hence, the graphene based materials

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Graphite powder (1−2 μm), pyrrole, propionic acid, benzaldehyde, palladium(II) chloride, caffeic acid, methanol, potassium ferrocyanide, potassium ferricyanide, and platinum on carbon electrode (Pt/C) and all other chemicals were obtained from Sigma-Aldrich. The phosphate buffer (PB) solution was prepared by NaH2PO4 and Na2HPO4. The prepared buffer solution pH was adjusted by either 0.1 M H2SO4 or NaOH and the other solution was prepared using double distilled water. The electrochemical measurement was carried out using CHI 410 and CHI 750 electrochemical workstation (Shanghai Chen Hua. Co), which 14097


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The Journal of Physical Chemistry C

Scheme 2. Graphical Representation for the Preparation and Fabrication of RGO/Pd−TPP Nanocomposite Electrode for ORR and MP Sensor

2.3. Preparation of Eco-Friendly Reduced Graphene Oxide/Palladium Tetraphenylporphyrin (RGO/Pd−TPP) Nanocomposite. Graphene oxide (GO) was prepared from the graphite by modified Hummer’s method.46 The RGO was prepared by environmental friendly method, which means the hazardous and strong reducing agent has not been used.47 In brief, the GO (20 mg) was dispersed in deionized (200 mL) water and sonicated for 15 min. In addition, 5 mg of caffeic acid was added into the prepared GO dispersion. The mixture was heated at 90 °C for 24 h with the assistance of stirring. The prepared suspension was collected and washed with deionized water and ethanol. Finally, the prepared RGO was dried at 60 °C under in vacuum condition. The RGO/Pd−TPP nanocomposite was prepared through π−π stacking interaction between the RGO and Pd−TPP. The prepared Pd−TPP/DMF solution (5 μL) was dropped into the RGO (1 mL) suspension and then allowed sonication for 15 min to form a stable RGO/ Pd−TPP nanocomposite. Finally, 8 μL (optimized, Figure S3) of RGO/Pd−TPP nanocomposite was coated on the well cleaned GCE surface and dried at room temperature, then it is transferred into electrochemical cell and used to study the ORR activity and MP detection (Scheme 2).

contains the catalyst coated glassy carbon electrode (GCE; area =0.0793 cm2) as a working electrode, Ag/AgCl (saturated KCl) as a reference electrode and platinum wire as a counter electrode. The electrolyte solution was deoxygenated by purified nitrogen for 15 min after specified. In addition, the H1 NMR spectra were recorded in JEOL 500 MHz NMR instrument and UV−visible spectra were recorded in the JASCO V770 instrument. Scanning electron microscope (SEM) was performed using Hitachi S-3000H scanning electron microscope. Transmission electron microscope (TEM) investigations were performed using a JEOL 2000 transmission electron microscope. The thermogravimetric analysis (TGA) was determined by PL thermal science instrument (model PL-STA) using a heating rate of 10 °C min−1 from room temperature to 800 °C. Electrochemical impedance spectra (EIS) were carried out by ZAHNER (Kroanch, Germany) 0.1 Hz to 1 MHz using for the impedance analysis. X-ray diffraction (XRD) study was investigated by XPERT-PRO diffractometer. The X-ray photoelectron spectroscopy (XPS) was studied by using PerkinElmer PHI-5702. Elemental analysis was carried out from HORIBA EMAX -ACT (model 51-ADD0009). 2.2. Synthesis of Palladium Tetraphenylporphyrin (Pd−TPP). The Pd−TPP was prepared (Scheme 1) by Adler et al. method with slight modification.43−45 Here, the tetraphenylporphyrin (TPP) was synthesized by adding pyrrole (50 mL/0.8 M) and benzaldehyde (80 mL/0.8 M) into 3 L of reflux flask and further the propionic acid was added into the above mixture. The whole reaction solution was refluxed for 45 min and the refluxed solution was cooled down to room temperature, then filtered and washed with methanol. Further, the Pd−TPP was prepared from the as-synthesized TPP. The TPP (50 mg) was dissolved in chloroform (20 mL) and the calculated amounts of palladium chloride and methanol (5 mL) were added, then the whole mixture was refluxed for 1 h. Finally, the reaction mixture was further washed with water twice to remove unreacted materials. The prepared Pd−TPP was confirmed by the NMR (Figures S1 and S2) and stored in desiccator.

3. RESULTS AND DISCUSSION 3.1. Characterization of RGO/Pd−TPP Nanocomposite. The morphology of RGO/Pd−TPP nanocomposite was examined by SEM and TEM technique. Figure 1 shows the SEM images of (A) GO, (B) RGO, (C) Pd−TPP, (D) GO/ Pd−TPP, and (E) RGO/Pd−TPP. The GO reveals that a crumpled structure and RGO shows the exfoliated layer folding structure. These well-exfoliated RGO sheets could provide more space for the interaction of Pd−TPP molecule. Moreover, the GO/Pd−TPP composite exhibited the granules spherical shape structure which causes Pd−TPP (Figure 1D) decoration on GO surface. Besides, the RGO/Pd−TPP nanocomposite shows the curtain-like and/or aggregated sheet like structure, which indicates the Pd−TPP molecule uniformly decorated on the RGO sheets. Therefore, the surface area of nanocomposite increased due to the combination of RGO and Pd−TPP molecule. This is clearly states that the RGO/Pd−TPP 14098


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molecule was randomly arranged on the RGO surface. In addition, the thermal stability study was confirmed that the RGO/Pd−TPP nanocomposite retained about 70% of the actual mass up to 800 °C heating (Figure S4). Hence, the TGA studies performed, where the stability of the Pd−TPP molecule is significantly enhanced after the interaction with RGO as a composite. The atomic structure and interlayer spacing of the prepared nanocomposite were characterized by the XRD. Figure 3A shows the XRD spectrum of (a) RGO, (b) Pd−TPP, and (c) RGO/Pd−TPP nanocomposite. The diffraction peaks of (a) RGO shows at 21.02° and indexed to the (002) plane, which is indicated that the d-spacing of RGO was greatly decreased due to the removal of oxygen containing functional groups on the GO surface. The diffraction peak of Pd−TPP shows 41.0°, 46.5°, 64.18°, and 82.3° are described to (111), (200), (220) and (311) planes, respectively. Moreover, the diffraction peaks of RGO/Pd−TPP nanocomposite was slightly shifted from the peaks of RGO and Pd−TPP.48,49 The XRD studies confirms that the non covalent formation of RGO/Pd−TPP nanocomposite via the π−π interaction. Figure 3B shows the FT-IR spectra of (a) RGO, (b) Pd− TPP, and (c) RGO/Pd−TPP nanocomposite. The stretching band of RGO was observed at 1632, 1050, and 3390 cm−1 which is due to the CC, C−O and O−H stretching vibration of reduced GO surface. The FT-IR spectrum of Pd−TPP exhibited a vibrational frequency at 3400 and 2925 cm−1, which is attributed to the N−H and C−H stretching vibration of TPP molecule. Moreover, the bands at 1496 and 1334 cm−1 were assigned to CC and CN stretching vibrations of TPP. In addition, the vibration band of CC observed at 1628 cm−1 and CN observed at 1250 cm−1 for RGO/Pd−TPP nanocomposite. When the Pd−TPP molecule was immobilized on the RGO, the CC and CN vibration bands of RGO/ Pd−TPP nanocomposite were positively shifted.49,50 Hence, the positive shift changes confirmed the noncovalent interaction between the RGO sheets and Pd−TPP molecule. Figure 4A shows the UV−visible absorbance spectra of (a) TPP, (b) Pd−TPP, (c) RGO/Pd−TPP, (d) RGO, and (e) GO. The GO and RGO shows absorbance peak at 278 and 255 nm, respectively. The peak intensity of RGO is lower than GO due to the eco-friendly reduction of GO. Moreover, the TPP absorption peak appeared at 417 nm for soret band and the weaker absorption at 509, 543, 586, and 643 nm for Q-band. In addition, the Pd−TPP shows the absorption peak of soret and Q-band at 415 nm and 532, 610 nm, respectively. The Pd−TPP absorbance peak intensity also lower than the TPP due to the interaction with Pd metal. When the Pd metal ion was inserted into the TPP ring, the intensity of Q bands were decreased and the soret band was shifted. The reason might be that the increase in molecular symmetry from D2h to D4h and the energy gap decreased in Pd−TPP. Furthermore, the RGO/Pd−TPP shows the absorption peak at 414 nm for soret band and 533 nm for Q-band, which is indicating that the interaction between the RGO and Pd−TPP.51 The electrochemical impedance spectroscopy (EIS) was used to investigate the electrical properties and resistance of the electrode materials. Figure 4B displayed the EIS spectrum of (a) bare GCE, (b) GCE/GO, (c) GCE/RGO, (d) GCE/GO/ Pd−TPP, (e) GCE/Pd−TPP, and (f) GCE/RGO/Pd−TPP in 0.1 M KCl solution containing 5 mM [Fe(CN)6]3‑/4‑. The EIS spectrum of bare GCE and GO shows the large semicircle with the charge transfer resistance (Rct) of 364 and 239 Ω,

Figure 1. SEM images of (A) GO, (B) RGO, (C) Pd−TPP, (D) GO/ Pd−TPP, (E) RGO/Pd−TPP and (F) EDX spectrum of RGO/Pd− TPP nanocomposite.

modified electrode could be effectively used for the ORR and MP sensor. The elemental analysis provides the both qualitative and quantitative information about the prepared nanocomposite. The Figure 1F exhibits the EDX spectrum of RGO/Pd−TPP nanocomposite. The result confirms that the Pd metals are present on the RGO/Pd−TPP nanocomposite. In addition, the decreasing oxygen content indicates the ecofriendly reduction of GO. Moreover, Figure 2 shows the TEM images of (A) RGO, (B) Pd−TPP and (C, D) RGO/Pd−TPP. The RGO shows wrinkled and folding sheet like structure and the Pd−TPP exhibits the aggregated small cubes structure. Besides, the TEM image of RGO/Pd−TPP shows the Pd−TPP

Figure 2. TEM images of (A) RGO, (B) Pd−TPP, (C and D) RGO/ Pd−TPP and, (inset of D) higher magnification image of Pd−TPP. 14099


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Figure 3. (A) XRD pattern of (a) RGO, (b) Pd−TPP, and (c) RGO/Pd−TPP nanocomposite. (B) FT-IR spectra of (a) RGO, (b) Pd−TPP, and (c) RGO/Pd−TPP nanocomposite.

Figure 4. (A) UV−vis spectroscopy analysis of (a) TPP, (b) Pd−TPP, (c) RGO/Pd−TPP, (d) RGO, and (e) GO. (B) EIS spectra of (a) bare GCE, (b) GCE/GO, (c) GCE/RGO, (d) GCE/GO/Pd−TPP, (e) GCE/Pd−TPP, and (f) GCE/RGO/Pd−TPP in 0.1 M KCl containing 5 mM [Fe (CN) 6]3‑/4‑.

Figure 5. (A) XPS survey spectra of (a) RGO and (b) RGO/Pd−TPP nanocomposite. (B-E) XPS core level spectra of C 1s, O 1s, N 1s, and Pd 3d of RGO/Pd−TPP nanocomposite.

very small semicircle with Rct of 51 Ω than all other aforementioned modified electrodes, which is revealed that the excellent electron transfer capability of RGO/Pd−TPP nanocomposite. The morphological structure and chemical properties of the prepared nanocomposite was characterized by the XPS. Figure 5A shows the XPS survey spectra of RGO and RGO/Pd−TPP nanocomposite. As an illustration, the XPS spectrum of RGO (Figure S5) is observed at 287 eV for C and 536.6 eV for O. Moreover, the XPS spectrum of the RGO/Pd−TPP nano-

respectively, confirms the high resistance behavior of bare GCE and GO modified electrode. Moreover, the GCE/Pd−TPP exhibits the lower semicircle with Rct value of 158 Ω than bare and GCE/GO. In addition, the RGO modified electrode shows the small semicircle with Rct of 153 Ω than that of the bare GCE, GO and Pd−TPP modified GCE due to the high electron transfer properties of RGO. Furthermore, the GCE/ GO/Pd−TPP acquires the small semicircle with Rct of 77 Ω due to the GO interaction with the Pd−TPP molecule. However, the GCE/RGO/Pd−TPP nanocomposite exhibits a 14100


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Figure 6. (A) Cyclic voltammetric response of (a) RGO/Pd−TPP in N2 saturated 0.1 M KOH and (b) bare GCE, (c) GO, (d) Pd−TPP, (e) RGO, (f) GO/Pd−TPP, and (g) RGO/Pd−TPP in O2 saturated 0.1 M KOH. (B) LSVs of the ORR at (a) GCE, (b) GO, (c) RGO, (d) Pd−TPP, (e) GO/Pd−TPP, (f) RGO/TPP, and (g) RGO/Pd−TPP in O2 saturated 0.1 M KOH with the rotation speed of 2000 rpm. (C) LSVs of the ORR on RGO/Pd−TPP in O2-saturated 0.1 M KOH under various rotation speeds (100 to 2500 rpm) at the scan rate of 50 mV s−1. (D) K−L plots measured at −0.4, −0.5, and −0.6 V.

composite shows the C, O and N peak appeared at 289, 538.8, and 405.2 eV, respectively (Figure 5B−D). In addition, the double spectral peaks observed at 347.3 and 352.6 eV, which corresponds to the Pd 3d5/2 and Pd 3d3/2 (Figure 5 E). However, the intensity of all signals is almost same with RGO but new Pd and N peaks appeared at the RGO/Pd−TPP nanocomposite due to the Pd−TPP interaction with the RGO surface. Hence, the above results are strongly confirmed the successful formation of RGO/Pd−TPP nanocomposite. 3.2. Electrochemical Investigation of ORR Activity. The ORR activity of different modified electrodes such as (a) RGO/Pd−TPP (N2 saturated), (b) bare GCE, (c) GO, (d) Pd−TPP, (e) RGO, (f) GO/Pd−TPP and (g) RGO/Pd−TPP nanocomposite modified GCEs were investigated using CV in 0.1 M KOH solution saturated with O2 at a scan rate of 50 mVs−1 (Figure 6A). There is no noteworthy peak appearing for bare GCE toward ORR. On the other hand, the GO, Pd−TPP, RGO, and GO/Pd−TPP modified electrodes exhibited the ORR activity at higher reduction potential. However, the RGO/Pd−TPP nanocomposite obtained strong cathodic peak at −0.12 V (vs Ag/AgCl) with higher peak current density than the aforementioned modified electrodes. The result exposed the high ORR activity on the RGO/Pd−TPP nanocomposite, which is due to the high redox active site of Pd as well as more active surface area of RGO. Moreover, the TPP is a delocalized and/or conjugated π bond molecular structure which also enhanced the stability and redox properties of Pd (II).35,38

Hence, the RGO/Pd−TPP nanocomposite acts as a well electrocatalyst toward the reduction of oxygen. Figure 6B shows the RDE measurements of (a) bare GCE, (b) GO, (c) RGO, (d) Pd−TPP, (e) GO/Pd−TPP, (f) RGO/ TPP and (g) RGO/Pd−TPP modified electrode in O2 saturated 0.1 M KOH with the rotation speed of 2000 rpm. The onset potential of all the modified electrodes were of ∼−0.1 to +0.1 V and the peak current was shifted toward the positive side. Among these, however, the peak current of RGO/ Pd−TPP nanocomposite was increased sharply and the onset potential is very lower than the other. Therefore, these results confirmed that the RGO/Pd−TPP nanocomposite is the best electrocatalyst material for ORR. In order to gain better insight into the ORR performance, the influence of different rotation speed from 100 to 2500 rpm were evaluated by LSV on RGO/ Pd−TPP/RDE (Figure 6C). Here, the peak current was increased significantly while changing the rotation speed of the RGO/Pd−TPP/RDE. Moreover, Figure 6D shows the corresponding K−L plots and the number of electrons transferred in this ORR process can be calculated by the Koutecky−Levich (K−L) equation, eq 1.36 1 1 1 1 1 = + + + J JK JL nFK CO Bω1/2

(1)

Here J is current density, JK and JL are the kinetic and diffusion current densities, n is the electron transfer number, F is the Faraday constant, DO is the diffusion coefficient, CO is the concentration of O2 dissolved in the electrolyte, ν is the 14101


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Figure 7. (A) CV responses of Pt/C modified electrode modified electrode at the scan rate of 50 mV s−1 in O2-saturated 0.1 M KOH solution without (a) and with (b) 3 M methanol addition. (B) RGO/Pd−TPP modified electrode at the scan rate of 50 mV s−1 in O2-saturated 0.1 M KOH solution with 3 M methanol addition (a) and without methanol addition (b). (C) Chronoamperometric responses of (a) RGO/Pd−TPP and (b) Pt/C at 0.1 V in O2-saturated 0.1 M KOH with a rotation speed of 1000 rpm.

Figure 8. (A) Cyclic voltammetry response of (a) bare GCE, (b) RGO, (c) Pd−TPP, (d) RGO/Pd−TPP in 0.05 M PB solution containing 100 μM MP and (e) RGO/Pd−TPP without 100 μM MP at the scan rate 50 mV s−1. (B) Cyclic voltammetry response at RGO/Pd−TPP/GCE for different concentration addition of MP in 0.05 M PB solution at the scan rate 50 mV s−1. (C) Cyclic voltammetry response of RGO/Pd−TPP/GCE for different pH (5−11) analysis in 0.05 M PB solution containing 200 μM MP at the scan rate of 50 mV s−1 and inset shows the calibration plot for pH vs Ip. (D) Different scan rate analysis for MP reduction at RGO/Pd−TPP/GCE in 0.05 M PB solution containing 100 μM of MP. Inset shows the calibration plot between the scan rate vs peak current.

O2 + H 2O + 2e− → HO2− + OH− (two electron process)

kinematic viscosity, K is the electron transfer rate constant, and ω is the angular velocity of the ring disk. The electron transfer number calculated from the K−L plots. The ORR undergoes by multielectron transfer reaction that has involving two main possible way; one is two-electron pathway and the other is fourelectron pathway. The corresponding electron transfer mechanism in alkaline medium was described in eq 2, 3 and 4.

(2)

H 2O + 2HO2− + 2e− → 3OH− (two electron process) (3)

O2 + 2H 2O + 4e− → 4OH− (four electron process) 14102

(4)


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Figure 9. (A) DPV response for the addition of different concentration MP in 0.05 M PB solution. B) The calibration plot of peak current vs MP concentration.

which is attributed to the conversion of a nitro group into hydroxylamine formation (R 1 ) and reversible (O1/R 2 ) conversion of hydroxylamine to nitroso compound formation (Figure S6).58 On one hand, there is no responsible peak observed at RGO/Pd−TPP/GCE (e) in the absence of 100 μM MP. On the other hand, very poor catalytic activity and lower current efficiency were obtained when using the other electrodes such as bare GCE, RGO, and Pd−TPP modified GCEs. The results indicates that the RGO/Pd−TPP nanocomposite has the excellent electrocatalytic activity toward MP reduction than the other electrodes, which is due to the large specific surface area and high electron transport properties of RGO and Pd−TPP, respectively. To further confirm the electrocatalytic properties of RGO/Pd−TPP nanocomposite, the CVs were performed using RGO/Pd−TPP/GCE with successive addition of different concentration of MP ranges from 50 to 450 μM (Figure 8B). The cathodic peak current of MP increases with increasing the concentration of MP, resulting the linear relationship between the concentration of MP and cathodic peak current (R1), with the correlation coefficient of R2 = 0.9984 (Figure 9B inset). The results reveals that the excellent electrocatalytic behavior of RGO/Pd−TPP nanocomposite toward MP reduction owing to its large active area and high electron transfer communication. The influence of pH on the peak current and peak potential of MP reduction was further evaluated using the RGO/Pd−TPP/GCE over the range of pH 5.0−pH 11.0 (Figure 8C). The RGO/Pd−TPP nanocomposite modified electrode exhibits the strong and high reduction peak current at pH 7.0 (Figure 8C, inset). Hence, we have chosen the pH 7.0 for all the electrochemical experiment. Besides, the electrocatalytic reaction of MP at the RGO/Pd− TPP/GCE observed as an equal number of electron and proton transfer reaction. Moreover, the CV was also performed for the effect of scan rate on the electrocatalytic behavior of MP reduction at RGO/Pd−TPP nanocomposite modified electrode, which was investigated by varying the scan rate from 20 to 140 mVs−1 (Figure 8D). The reduction peak current was increased linearly with increasing the scan rates. The inset shows the linear relation between the cathodic peak current and scan rates with the corresponding linear regression equation was expressed as Ic (μA) = 0.098 ν (mVs−1) + 11.71 (R2 = 0.9975). Hence, this phenomenon indicates that the MP reduction at RGO/Pd−TPP/GCE is a surface-controlled reaction process.59 3.3.1. Differential Pulse Voltammetry. Differential pulse voltammetry (DPV) is more sensitive method than that of the

The maximum energy density can be obtained only at fourelectron reaction pathway. The K−L plot at different electrode potentials (from −0.4 to −0.6 V) reveals the good linear range with approximately constant slope value (Figure 6D). It exhibit that the same number of electrons were transferred at different potentials. The obtained linearity of K−L plots indicated that the same reaction mechanism happened for ORR process. The “n” value was calculated as 3.7, which suggests that the 4electron transfer process is involved in the present ORR study while using RGO/Pd−TPP as an electrocatalyst. The proposed RGO/Pd−TPP nanocomposite shows higher catalytic activities toward ORR in alkaline medium. The crossover effect is a main problem in the fuel cell application therefore, the investigation of methanol tolerance in ORR activity on as-fabricated electrode is more important. The CV was performed to study the crossover effect on RGO/Pd−TPP and Pt/C in 3 M of methanol containing O2-saturated 0.1 M KOH. Figure 7A shows the crossover effect of methanol on Pt/C electrode, there clear ORR peak was appeared at ∼ −0.19 V in absence of 3 M methanol (Figure 7A(a)), whereas it is disappeared when adding 3 M methanol into the electrolyte. Instead, a new peak appeared at −0.19 V, which is the peak responsible for the reverse oxidation of methanol (Figure 7A(b)). From this result, a strong cross over effect was observed at Pt/C electrode. On the other hand, at RGO/Pd−TPP-modified GCE, there was no notable peak appeared while adding 3 M methanol, which confirms the no crossover effect on our proposed catalyst electrode (Figure 7B). These results revealed the excellent ORR activity of RGO/Pd−TPP catalyst electrode and also proved its high selectivity toward ORR. In addition, the stability of the RGO/Pd−TPP modified electrode was examined using chronoamperometry technique by applying constant potential of 0.1 V in O2 saturated 0.1 M KOH (Figure 7C). From this, RGO/Pd−TPP modified electrode shows higher stability than Pt/C electrode, which losses only 7% of steady state response from its initial current. These results confirmed that the RGO/ Pd−TPP catalyst electrode provides stable performance toward ORR activity. 3.3. Electrochemical Reduction of MP at Different Modified Electrodes. Figure 8A shows the typical CV responses (second cycle) of MP reduction at (a) bare GCE, (b) RGO, (c) Pd−TPP, and (d) RGO/Pd−TPP-modified GCE in 0.05 M PB solution (pH= 7.0) containing 100 μM MP at a scan rate 50 mVs−1. From the results, the RGO/Pd−TPP modified GCE (curve d) showed a strong reduction peak at −0.63 V and a well redox peak was appeared at 0.08 and 0.02 V, 14103


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The Journal of Physical Chemistry C Table 1. Comparison of Analytical Performance of the RGO/Pd−TPP Nanocomposite Modified Electrode electrodes b

c

RGO /Pd−TPP OMCd/GCEe Db-Silicaf/CPEg stearic acid/GCE MWCNTh/MIPi CoTCPPj/Co3O4k/GOl/GCE gemini surfactant/GCE SCTABm/GCE nano Aun/SDBSo/GCE Au/Nafion/GCE

LODa (nM)

linear range (μM)

references

7.4 7.6 13 950 67 11 70 10.4 86 100

0.1−125 0.09−61 0.125−2.56 2.07−12.42 0.2−10 0.4−20 0.4−8.5 0.10−00 0.50−00 0.50−20

this work 14 17 13 52 53 54 55 56 57

a

Limit of detection. bReduced graphene oxide. cPalladium tetraphenylporphyrin. dOrdered mesoporous carbon. eGlassy carbon electrode. Diazoniabicyclosilica. gCarbon paste electrode. hMultiwalled carbon naotubes. iMolecularly imprinted polymer. jmeso-tetrakis(4-carboxyphenyl)cobalt porphyrin. kCobalt oxide. lGraphene oxide. mSilicate−cetyltrimethylammonium bromide. nNano gold. oSodium dodecylbenzenesulfonate. f

other voltammetric methods. Figure 9A shows DPV responses of MP reduction on RGO/Pd−TPP/GCE in 0.05 M PB solution containing different concentration addition of MP (0.1−450 μM). The reduction peak current was increased with increasing the concentration of MP. Figure 9B shows the linear calibration plot between the peak current (Ip) and MP concentration. Besides, the linear concentration range of modified electrode is 0.1−125 μM and the detection limit (LOD) is 7.4 nM with signal-to-noise ratio of 3. Moreover, the sensitivity of the RGO/Pd−TPP/GCE is calculated about 4.1084 μAμM−1 cm−2, which is higher than that of the other reported graphene based modified electrodes.17,59−62 Hence, the results proved that the modified electrode shows the better performance for the determination of MP due to the higher electron transfer rate and well catalytic properties of the Pd− TPP. Moreover, the high surface area of RGO provides a strong interaction between the RGO and Pd−TPP. Therefore, the RGO/Pd−TPP nanocomposite modified electrode used as an excellent electrode material for the electrochemical detection of MP. The obtained results are compared with other previously reported MP sensor as mentioned in Table 1. 3.3.2. Repeatability, Reproducibility, and Stability Studies of the RGO/Pd−TPP Electrode. Repeatability, reproducibility and stability studies of the RGO/Pd−TPP nanocomposite modified electrodes were carried out by CV in 0.05 M PB solution containing 100 μM concentration of MP at the scan rate 50 mVs−1. A single RGO/Pd−TPP/GCE showed an acceptable repeatability with the RSD of 2.36% for 20 repetitive measurements. In addition, the RGO/Pd−TPP modified electrode exhibited well appreciable reproducibility with the RSD of 2.38% for the five independent measurements that was examined by five independent modified electrodes. Moreover, the storage stability of the RGO/Pd−TPP/GCE toward the detection of MP was monitored by every day. After that, the electrode was stored in PB solution at 4 °C when not in use. Furthermore, during the storage period, the modified electrode exhibit the well catalytic response toward the detection of MP without any peak current and potential changes. Interestingly, 97.62% of the response was observed for MP reduction even after one month continuous monitoring, which evidence the good storage stability of the proposed sensor. 3.3.3. Selectivity and Real Sample Analysis. The selectivity of the RGO/Pd−TPP nanocomposite modified GCE was investigated using DPV method. Figure 10 shows the DPV response of RGO/Pd−TPP nanocomposite modified GCE in 0.05 M PB solution containing 10 μM addition of (a) MP and

Figure 10. Interference analysis performed using RGO/Pd−TPP nanocomposite modified GCE by DPV in 0.05 M PB solution. The interference analysis was performed the solution containing 10 μM MP (a) in the presence of other potential interfering ions namely 100 μM addition of (b) nitrobenzene, (c) p-nitrophenol, (d) nitroaniline and 500 μM addition of (e) K+, (f) Na+, (g) Ca2+, (h) Cu2+, (i) Fe3+, (j) Ni2+, (k) PO43−, (l) CO32−, (m) Mg2+, (n) NO3−, and (o) Cl−.

100 μM addition of (b) nitrobenzene, (c) p-nitrophenol, and (d) nitroaniline and 500 μM of K+, Na+, Ca2+, Cu2+, Fe3+, Ni2+, PO43−, CO32−, Mg2+, NO3−, and Cl− was evaluated (e−o). The RGO/Pd−TPP modified GCE was exhibited good response for MP. However, there is no response that appeared for excess addition of other interfering compounds and ions, but the response of MP reduction was changed by less than 5% (95.6− 104.9%), which is suggesting that the RGO/Pd−TPP nanocomposite has an excellent selectivity for the detection of MP in the presence of many interfering compounds. In order to study the practicability, the fabricated electrode was used to determine MP in various water and food samples (Figure S7 and S8). The corresponding recovery results are summarized in Table 2, and this reveals the acceptable recoveries of 101.2%, 101.3%, 100.5%, 101.4%, and 100% for the different real samples. From this result, the RGO/Pd−TPP nanocompositemodified electrode exposed the excellent practicality for the determination of MP in water and food samples.

4. CONCLUSIONS In summary, we have successfully prepared the RGO/Pd−TPP nanocomposite via an eco-friendly route and applied for ORR and sensor application. The prepared nanocomposite was characterized and confirmed by suitable physical and chemical characterization technique. The electrocatalytic ORR activity was carried out using CV and LSV techniques. The obtained 14104



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Table 2. Determination of MP in Water and Food Samples Using RGO/Pd−TPP Nanocomposite Modified Electrode samples

added (μM)

found (μM)

recovery (%)

RSDa (%)

river water

100 200 100 200 100 200 100 200 100 200

103.2 198.3 101.5 202.4 101.5 199.2 102.2 201.2 100.1 200.3

103.2 99.1 101.5 101.2 101.5 99.6 102.2 100.6 99.9 100.1

3.4 3.7 2.8 2.6 3.2 2.9 3.7 3.5 3.0 3.3

sea water tap water tomato cucumber a

electrochemical studies were proposed that the RGO/Pd−TPP nanocomposite act as a remarkable electrocatalytic materials toward the ORR and the sensitive detection of MP. The electrocatalytic performance of the RGO/Pd−TPP-modified electrode exhibited appreciable reduction peak current with lowest over potential for ORR and it shows good stability compared with commercial Pt/C electrode. On the other hand, the RGO/Pd−TPP-modified electrode exhibited high electrochemical activity toward the detection of MP with lowest detection limit (LOD) about 7.4 nM. Furthermore, the sensor was applied for the determination of MP in different water and food samples with satisfactory recovery results. Hence, the RGO/Pd−TPP nanocomposite used as an excellent electrode material for the ORR and MP detection. In addition, this superior electrode material can be used for various electrochemical sensors and energy device applications.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01941. NMR spectra of TPP and Pd−TPP, optimization of loading of RGO/Pd−TPP nanocomposite on GCE surface, thermogravimetric analysis of RGO, Pd−TPP, and RGO/Pd−TPP nanocomposites, XPS survey and core level spectra of RGO, electrocatalytic reduction mechanism of MP, and electrochemical detection of MP from different real samples (PDF)

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REFERENCES

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Relative standard deviation of three individual measurements.

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Article

AUTHOR INFORMATION

Corresponding Authors

*(S.M.C.) Fax: +886 2270 25238. Telephone: +886 2270 17147. E-mail: [email protected]. *E-mail: [email protected] (C.K.). ORCID

Shen-Ming Chen: 0000-0002-8605-643X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the ministry of science and technology (MOST) and Ministry of Education (MOE), Taiwan (ROC). C.K. gratefully acknowledges the Department of Chemistry, National Taiwan University, for a postdoctoral fellowship during the years 2015−2016. 14105


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