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ZnO composite modified electrode
Sensors and Actuators B 166–167 (2012) 372–377
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A novel nonenzymatic hydrogen peroxide sensor based on reduced graphene oxide/ZnO composite modified electrode Selvakumar Palanisamy a , Shen-Ming Chen a,∗ , Ramiah Sarawathi b a b
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan, ROC Department of Materials Science, School of Chemistry, Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 3 January 2012 Received in revised form 17 February 2012 Accepted 24 February 2012 Available online 4 March 2012 Keywords: Reduced graphene oxide Zinc oxide Electrocatalysis Hydrogen peroxide
a b s t r a c t A novel nonenzymatic, amperometric sensor for hydrogen peroxide (H2 O2 ) was developed based on an electrochemically prepared reduced graphene oxide (RGO)/zinc oxide (ZnO) composite using a simple and cost effective approach. RGO/ZnO composite was fabricated on a glassy carbon electrode (GCE) by a green route based on simultaneous electrodeposition of ZnO and electrochemical reduction of graphene oxide (GO). The morphology of the as-prepared RGO/ZnO composite was investigated by scanning electron microscopy (SEM). Attenuated total reflectance (ATR) spectroscopy has also been performed to confirm the ample reduction of oxygen functionalities located at graphene oxide (GO). The electrochemical performance of the RGO/ZnO composite modified GCE was studied by amperometric technique, and the resulting electrode displays excellent performance towards hydrogen peroxide (H2 O2 ) at −0.38 V in the linear response range from 0.02 to 22.48 M, with a correlation coefficient of 0.9951 and short response time (<5 s). The proposed sensor also has good operational and storage stability with appreciable anti-interferring ability. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Graphene is the hottest material in the electrochemical field because of its unique properties and potential applications towards the sensors [1], drug delivery [2], and solar cells [3]. Nowadays, graphene can be prepared in large quantities by thermal [4], chemical [5] and solvothermal reduction [6] of graphene oxide (GO). These methods are multipurpose, scalable, and are adaptable to a wide variety of applications. Hydrogen peroxide (H2 O2 ) is the vital intermediate in the food industry and also involved in our life process. In the past decades, the determination of H2 O2 has been more active because of its importance in the daily life [7]. Owing to the importance of H2 O2 , the sensitive and precise determination of H2 O2 is mandatory and highly appreciable. Though many enzyme-based H2 O2 assays possess good selectivity and sensitivity, they are environmentally instable [8] and high cost [9]. On the other hand, compared with enzyme-based assays, inorganic metal oxides and their composites are less cost effective, very easy to prepare and are more stable even at high temperatures. In particular, zinc oxide (ZnO) is an imperative wide band gap semiconductor material and has
∗ Corresponding author. Tel.: +886 2270 17147; fax: +886 2270 25238. E-mail address: [email protected] (S.-M. Chen). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.02.075
its great potential applications in H2 O2 sensing [10–16]. To synthesize ZnO, various physical [17,18] and chemical routes [19,20] have been developed in the past. Their morphological properties and possible applications in biosensors have been studied widely. Recently, there has been great interest revealed towards the synthesis of ZnO structures with different morphologies using versatile strategies for potential H2 O2 sensing applications [21]. Among various approaches, the electrochemical deposition has attracted much interest because of its simplicity, low temperature operation, involving less hazardous and reliable strategies which can help to prepare ZnO with novel, fascinating nano and micro-structures. To date, only few reports are available for the electrodeposition of ZnO micro structures at low temperature [22–25] and to the best of our knowledge, there are no reports available in literature for the room temperature synthesis of ZnO microstructures on RGO surface. For the first time, we report the one-step electrodeposition of ZnO flower-like microstructures and RGO on the glassy carbon electrode (GCE) surface at ambient conditions. The major advantages of the present approach are: it does not involve any hazardous chemicals, expensive capping agents or tedious hydrothermal treatments. The highly conductive graphene layers serve as a good platform for the deposition of flower-like ZnO microstructures. The as-synthesized RGO/ZnO composite exhibits high electrocatalytic activity towards H2 O2 , which is ascribed to their large surface area as well their synergistic effect.
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2. Experimental 2.1. Materials Graphite powder was purchased from Sigma–Aldrich. H2 O2 (30%) was obtained from Wako pure chemical Industries. Zinc nitrate hexa hydrate (Zn (NO3 )2 ·6H2 O, 99% pure) and potassium nitrate (KNO3 ) was purchased from Sigma–Aldrich. Phosphate buffer solution (PBS) with pH 7 and pH 5 were prepared using 0.05 M Na2 HPO4 and NaH2 PO4 . All the chemicals used in this work were of analytical grade and all the solutions were prepared using doubly distilled water. Electrochemical experiments were carried out using a single compartment, three-electrode cell apparatus under nitrogen atmosphere. 2.2. Apparatus Cyclic voltammetry (CV) experiments were carried out using CHI 750a work station. A conventional three electrode cell containing freshly prepared 0.05 M PBS (pH 5 and pH 7) was used for the electrochemical studies. GCE (surface area = 0.079 cm−2 ) was used as a working electrode. Pt wire was used as a counter electrode and standard Ag/AgCl was used as a reference electrode. Surface morphology studies were carried out using field emission scanning electron microscope (FESEM), JSM-6500F and Hitachi S-3000 H scanning electron microscope (SEM). Attenuated total reflectance (ATR) spectroscopy has been performed by using Perkin-Elmer IR spectrometer. Amperometric (i–t curve) measurements were performed using an analytical rotator AFMSRX (PINE instruments, USA) and rotating disc electrode with an area of 0.24 cm−2 . 3. Results and discussions 3.1. Electrochemical preparation and the formation mechanism of RGO/ZnO composite Graphite oxide was prepared by Hummers method [26]. As prepared graphite oxide was dispersed in water, bath sonicated for 2 h, finally exfoliated to form graphene oxide (GO). As purified GO (0.5 g/ml) was dispersed well in water. About 8 l of GO was drop casted on the pre-cleaned GCE and dried at room temperature. The GO modified GCE was transferred to an electrochemical cell containing 0.05 M (ZnNO3 )2 ·6H2 O and KNO3 solution. 30 successive cyclic voltammograms were performed in the potential range between 0 and −1.5 V at the scan rate of 50 mV s−1 (see Fig. 1). During the first cathodic potential scan, a large cathodic peak appears at −1.14 V with an onset potential of −0.70 V. After several cycles, this cathodic peak disappeared completely, attributed to the reduction of oxygen moieties at the GO basal plane [27]. Besides, during the first anodic potential scan, with an onset potential of −0.83 V, a small anodic hump appears at −0.66 V, indicating the dissociation of zinc nitrate and the analogous formation of Zn2+ ions. However, no anodic peak (−0.66 V) was observed when GO alone was electrochemically reduced in the same potential window (figure not shown). This provides further evidence for the Zn2+ ions formation in the solution during the electrochemical deposition of ZnO on GO modified GCE. As reported previously, ZnO electrochemical deposition will possibly occur by any one of the following reversible mechanistic pathways [28,29]. Zn2+ + NO3 − + 2e− → ZnO + NO2 − 2+
Zn
−
+ ½O2 + 2e → ZnO
(1) (2)
In our case, as we maintained the N2 atmosphere during the entire RGO/ZnO electrochemical deposition process, we believe that the ZnO deposition may not have proceeded via step (2).
Fig. 1. 30 consecutive cyclic voltammograms recorded at a GO modified GCE in 0.5 mM (ZnNO3 )2 ·6H2 O and 0.5 mM KNO3 containing N2 saturated aqueous solution at the scan rate of 50 mV s−1 .
Instead, ZnO formation should have occurred via step (1), where Zn2+ ions will associate with the oxygen atoms produced by the reduction of NO3 − ions. As it is well known that, GO sheets are mostly decorated with epoxy and hydroxyl groups on the basal plane, while carbonyl and carboxyl groups are located at the edges [30]. During the electrochemical deposition process, the positively charged Zn2+ ions from the bath solution will be attracted towards the negatively charged oxygen functionalities of GO by electrostatic driving force, and thus got anchored at the GO surface. During the second cathodic potential scan, at more negative potential (∼1 V), the as dissociated NO3 − ions will undergo further reduction forming both NO2 − ions and oxygen. Meanwhile, the released oxygen should be captured immediately by the anchored Zn2+ ions, resulting in the formation of RGO/ZnO composite (step 1). Here, it should be noted that, as the Zn2+ ions are anchored at GO only via electrostatic force, and as the oxygen present in the functional groups are covalently bonded with the carbon or hydrogen, they may not be involved in the ZnO formation. 3.2. ATR studies As shown in the ATR spectra of GO (Fig. 2, curve a), the bands at 1076, 3332, 1402 and 1731 cm−1 corresponds to the C O, C OH stretching vibrations, C O H deformation peak and C O stretching of COOH groups, respectively. On the other hand, all these bands related to the oxygen-containing functional groups almost gone astray in the ATR spectra of RGO/ZnO composite (Fig. 2, curve b), revealing that these oxygen functionalities were partially removed during the electrochemical reduction process. No notable shift in absorption peaks were observed in the ATR spectra of RGO/ZnO, confirming that association of ZnO with RGO does not cause any drastic change to the morphology of RGO. 3.3. Morphological study of RGO/ZnO composite Fig. 3A shows the FESEM image of electrochemically synthesized RGO, displaying their well exfoliated, typical wrinkled surface morphology. Fig. 3B shows the top view of as-grown ZnO micro-forest with numerous closely assembled tiny flower-like
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retained, revealing the eco-friendly approach used in this work. The SEM results indicated that flower-like ZnO microstructures have been grown successfully on the RGO surface. We also attempt to explore the effect of post thermal treatment on the morphology of the as grown RGO/ZnO composite. Fig. 3D shows the SEM image of RGO/ZnO composite taken after subjecting it to post heat treatment at 80 ◦ C. It is clear that the prepared composite is thermally more stable as no noteworthy cracking was observed on its surface. 3.4. Electrocatalytic activity of H2 O2 at RGO/ZnO composite
Fig. 2. ATR transmittance spectra of exfoliated GO (a), and RGO/ZnO (b).
micro-structures with rough and porous architectures. Fig. 3C shows the ZnO micro flowers closely anchored at the surface of RGO sheets. The as-grown ZnO structures were between 0.5 and 1 m in size. The magnified view of RGO/ZnO has also been shown in the inset. Here, thin RGO sheets have formed branched networks that interconnect the micro flowers. It is notable that porous morphology of ZnO and the wrinkled structure of RGO structures has been
Electrochemical experiments were carried out to investigate the catalytic activity of H2 O2 at RGO/ZnO composite. Compared with bare GCE (curve a, Fig. 4), RGO/ZnO composite modified GCE (curve d–h) shows enhanced activity towards reduction of H2 O2 . Whereas under same experimental conditions RGO alone (curve b) displays less catalytic activity. It is notable that, with an onset potential of (−0.1 V) the catalytic current increased slightly at RGO/ZnO composite modified electrode for 0.02 M H2 O2 . Upon increasing the concentration of H2 O2 (up to 8.8 M H2 O2 ), as shown in curves (d–h), the cathodic current increased linearly while the anodic current decreased. Where, both the increase in peak current and decrease in over potential are considered as electrocatalysis [31]. Particularly, a small cathodic hump was observed at (−0.38 V) for 4.48–8.8 M H2 O2 with a trivial shift in peak potential. A maximum reduction peak was observed for 8.8 M H2 O2 . However, in the presence of same H2 O2 concentration (8.8 M), no significant catalytic peak was observed at bare GCE. This result confirms that the composite film modified GCE exhibits enhanced electrocatalytic activity towards H2 O2 and it considerably lowers the overpotential. The electrocatalytic response towards the H2 O2 in linear enslavement could be witnessed between the current response and concentration of H2 O2 in the range of 0.2–8.8 M with a correlation coefficient of 0.9402 and with a sensitivity of 8.50 A M−1 cm−2 . The linear regression equation was obtained as, I (A) = 0.6718 C
Fig. 3. FESEM image of RGO (A), SEM images of electrochemically deposited ZnO (B), RGO/ZnO composite prepared at room temperature (C) and with 80 ◦ C post heat treatment (D). Inset in (C) is the RGO/ZnO composite at higher magnification.
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Fig. 4. Cyclic voltammograms obtained at bare/GCE (a) and RGO/GCE (b) in the presence of 8.8 M H2 O2 at the scan rate of 50 mV s−1 . Cyclic voltammograms obtained at RGO/ZnO/GCE in the (c) absence and presence of (d–h) 0.02, (e) 2.19, (f) 4.48, (g) 6.24 (h) 8.8 M H2 O2 under similar conditions. Supporting electrolyte: N2 saturated 0.05 M PBS (pH 7). Inset is the plot of cathodic peak current vs. [H2 O2 ].
(M) + 19.91. The high electrocatalytic activity of RGO/ZnO composite can be ascribed to the large surface area and edge-plane like defective sites on RGO. The porous nature and the typical flower like morphology of ZnO microstructures can offer more catalytic sites for the efficient diffusion of analyte. The other plausible reason for the enhanced catalytic activity can be due to the synergistic effect of the RGO and ZnO. 3.5. Amperometric determination of H2 O2 at RGO/ZnO composite modified electrode Amperometric i–t curve is the most often used method to evaluate the electrocatalytic activity of electrochemical sensors and enzyme-based biosensors. In the present study, we have used amperometric technique to evaluate the performance of the developed RGO/ZnO film. During the amperometric i–t measurements the electrode potential was hold at −0.38 V (from the CV electrocatalysis) and the N2 saturated PBS (pH 7) was constantly stirred at 1200 rpm. For every 50 s, aliquots of H2 O2 were successively injected into the supporting electrolyte solution. Fig. 5A shows the amperometric i–t response obtained at RGO/ZnO composite rotating disc GCE upon various H2 O2 concentration additions. These results clearly show that, composite film exhibits fast and
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Fig. 5. Amperometric i–t response at RGO/ZnO film modified rotating disc GCE upon successive additions of 1–22.24 M H2 O2 into continuously stirred N2 saturated 0.05 M PBS (pH 7). Applied potential: −0.38 V; Rotation rate: 1200 rpm. Inset (above) is the enlarged view of the amperometric i–t response for 1–4 M H2 O2 . Inset (below) is the plot of response current vs. [H2 O2 ].
well defined response towards each H2 O2 additions. The response time of the RGO/ZnO composite film towards H2 O2 is less then 5 s, validating the rapid catalytic reduction process occurring at the composite film surface. The response current increases linearly between 1.0 and 22.48 M of H2 O2 with a sensitivity of 13.49 A M−1 cm−2 . As shown in Table 1 the performance characteristics of the proposed sensor such as applied potential, LOD and the response time have been compared with that of the other H2 O2 sensors reported previously [32–39]. It is evident that the RGO/ZnO electrode showed good performance than the previously reported enzymatic and nonezymatic H2 O2 sensors. In order to investigate the stability of RGO/ZnO film, the modified electrode was stored in PBS (pH 7) at 4 ◦ C, and the background current was monitored periodically. The proposed sensor exhibited no noticeable decrease in current response in the first two weeks and maintained about 95% of its initial value after one month, indicating its excellent storage stability. The background current was also 95% stable even after 200 consecutive cycles, which indicates the excellent stability of the composite film. In addition, we also attempted to evaluate the stability of the RGO/ZnO film by monitoring the response current with respect to time by using CV studies. The other experimental conditions are same as discussed
Table 1 Comparison of electroanalytical data of various modified electrodes towards H2 O2 reduction. Electrode
Applied potential (V)
a
ZnO/Au/Nafion/HRP/GCE b HRP/GCE Nano Ag/GCE Ag/c MWCNT/GCE Fe3 O4 /Ag/GCE d PoPD/Ag/GCE RGO/Fe3 O4 /GCE Ag/ZnO/GCE RGO/ZnO/GCE
−0.30 −0.38 −0.5 −0.45 −0.5 −0.5 −0.3 −0.25 −0.38
15 0.05 1.2 1.6 1.2 1.5 3.2 0.42 0.02
a b c d
LOD = limit of detection. HRP = horseradish peroxidase. MWCNT: multiwalled carbon nanotubes. PoPD = poly(o-phenylenediamine).
LOD (M)
Response time (s)
Ref.
5 – 3.5 3 3 2 >5 >10 3
[32] [33] [34] [35] [36] [37] [38] [39] This work
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Acknowledgement This work was supported by the National Science Council and the Ministry of Education of Taiwan (Republic of China). I am very much thankful to Dr. Arun Prakash Periasamy for his timely help and valuable suggestions.
References
Fig. 6. Amperometric i–t response at RGO/ZnO modified rotating disc GCE for the successive additions of 1 M H2 O2 , 1 mM AA, 1 mM DA, and 1 mM glucose solutions into the continuously stirred N2 saturated 0.05 M PBS (pH 7). Applied potential: −0.38 V; rotation rate: 1200 rpm.
in Section 3.4. The response current observed at the RGO/ZnO fim modified GCE towards 4.5 M H2 O2 (within working range) was monitored every 2 h (until 13 h). The modified electrode exhibits stable response towards H2 O2 and 89.3% of response current was retained even after 13 h, validating the good stability of the composite film. The good storage and operational stability of the RGO/ZnO composite can be attributed to the well anchored ZnO microstructures on the RGO sheets.
3.6. Anti-interference study of the developed H2 O2 sensor As it is well known that, some co-existing electroactive species in nature will affect the sensor response. So, we have monitored the response of the sensor towards each 1 mM of ascorbic acid (AA), glucose and dopamine (DA) additions. The working potential was hold at −0.38 V. Each addition of the electroactive interfering species brought out hardly discernible current response, whereas notable response was observed for 1 M H2 O2 (Fig. 6). These results suggest that the interfering effect caused by these electroactive species is quite negligible, validating the highly selective detection of H2 O2 at the composite film.
4. Conclusions In summary, we report a facile green approach for the room temperature, electrochemical deposition of flower-like ZnO microstructures on the GO surface. The SEM images of RGO/ZnO composite films revealed that ZnO micro flowers were well formed and closely anchored at the surface of RGO sheets. The as-prepared RGO/ZnO composite showed good catalytic activity towards H2 O2 . Besides, RGO/ZnO sensor was highly sensitive and selective for H2 O2 and it holds excellent storage and operational stability. As a future perspective, we believe that RGO/ZnO composite material could be a promising electrode material for the fabrication of enzyme based biosensors, super capacitors and solar cells at low cost.
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Biographies
Selvakumar Palanisamy received his B.S. degree in Chemistry from Madurai Kamaraj University, Tamilnadu, India, in 2006. He received his M.S. degree in chemistry from Madurai Kamaraj University, Tamil Nadu, India, in 2009. He received his Master of philosophy degree in chemistry from Madurai Kamaraj University, Tamil Nadu, India, in 2010. Now, he is a second year Ph.D. student in Chemical Engineering and Biotechnology at National Taipei University of Technology. His research interest mainly focuses on the nanomaterial synthesis for enzyme immobilization related to biosensor and biofuel cell applications. Dr. Shen-Ming Chen received his B.S. degree in Chemistry in 1980 from National Kaohsiung Normal University, Taiwan. He received his M.S. degree (1983) and Ph.D. degree (1991) in Chemistry from National Taiwan University, Taiwan. He is currently a professor at the Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taiwan. His current research interests include electroanalytical chemistry, bioelectrochemistry, fabrication of energy conservation and storage devices and nanomaterial synthesis for electrochemical applications. He has published more than 230 research articles in SCI journals. Dr. R. Saraswathi received her Ph.D. degree in Chemistry in 1989 from the Indian Institute of Technology, Madras, Tamil Nadu, India. She joined the Madurai Kamaraj University in 1988, where she is now the Head of the Department of Materials Science. She is a recipient of the Commonwealth Visiting Scientist Fellowship during 1997–1998 at the University of Leicester, England. She was a visiting professor twice at the National Taipei University of Technology, Taiwan. Her research interest focuses on the electrochemical aspects of conducting polymers, nanomaterials and nanocomposites.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A novel nonenzymatic hydrogen peroxide sensor based on reduced graphene oxide/ZnO composite modified electrode Selvakumar Palanisamy a , Shen-Ming Chen a,∗ , Ramiah Sarawathi b a b
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan, ROC Department of Materials Science, School of Chemistry, Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 3 January 2012 Received in revised form 17 February 2012 Accepted 24 February 2012 Available online 4 March 2012 Keywords: Reduced graphene oxide Zinc oxide Electrocatalysis Hydrogen peroxide
a b s t r a c t A novel nonenzymatic, amperometric sensor for hydrogen peroxide (H2 O2 ) was developed based on an electrochemically prepared reduced graphene oxide (RGO)/zinc oxide (ZnO) composite using a simple and cost effective approach. RGO/ZnO composite was fabricated on a glassy carbon electrode (GCE) by a green route based on simultaneous electrodeposition of ZnO and electrochemical reduction of graphene oxide (GO). The morphology of the as-prepared RGO/ZnO composite was investigated by scanning electron microscopy (SEM). Attenuated total reflectance (ATR) spectroscopy has also been performed to confirm the ample reduction of oxygen functionalities located at graphene oxide (GO). The electrochemical performance of the RGO/ZnO composite modified GCE was studied by amperometric technique, and the resulting electrode displays excellent performance towards hydrogen peroxide (H2 O2 ) at −0.38 V in the linear response range from 0.02 to 22.48 M, with a correlation coefficient of 0.9951 and short response time (<5 s). The proposed sensor also has good operational and storage stability with appreciable anti-interferring ability. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Graphene is the hottest material in the electrochemical field because of its unique properties and potential applications towards the sensors [1], drug delivery [2], and solar cells [3]. Nowadays, graphene can be prepared in large quantities by thermal [4], chemical [5] and solvothermal reduction [6] of graphene oxide (GO). These methods are multipurpose, scalable, and are adaptable to a wide variety of applications. Hydrogen peroxide (H2 O2 ) is the vital intermediate in the food industry and also involved in our life process. In the past decades, the determination of H2 O2 has been more active because of its importance in the daily life [7]. Owing to the importance of H2 O2 , the sensitive and precise determination of H2 O2 is mandatory and highly appreciable. Though many enzyme-based H2 O2 assays possess good selectivity and sensitivity, they are environmentally instable [8] and high cost [9]. On the other hand, compared with enzyme-based assays, inorganic metal oxides and their composites are less cost effective, very easy to prepare and are more stable even at high temperatures. In particular, zinc oxide (ZnO) is an imperative wide band gap semiconductor material and has
∗ Corresponding author. Tel.: +886 2270 17147; fax: +886 2270 25238. E-mail address: [email protected] (S.-M. Chen). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.02.075
its great potential applications in H2 O2 sensing [10–16]. To synthesize ZnO, various physical [17,18] and chemical routes [19,20] have been developed in the past. Their morphological properties and possible applications in biosensors have been studied widely. Recently, there has been great interest revealed towards the synthesis of ZnO structures with different morphologies using versatile strategies for potential H2 O2 sensing applications [21]. Among various approaches, the electrochemical deposition has attracted much interest because of its simplicity, low temperature operation, involving less hazardous and reliable strategies which can help to prepare ZnO with novel, fascinating nano and micro-structures. To date, only few reports are available for the electrodeposition of ZnO micro structures at low temperature [22–25] and to the best of our knowledge, there are no reports available in literature for the room temperature synthesis of ZnO microstructures on RGO surface. For the first time, we report the one-step electrodeposition of ZnO flower-like microstructures and RGO on the glassy carbon electrode (GCE) surface at ambient conditions. The major advantages of the present approach are: it does not involve any hazardous chemicals, expensive capping agents or tedious hydrothermal treatments. The highly conductive graphene layers serve as a good platform for the deposition of flower-like ZnO microstructures. The as-synthesized RGO/ZnO composite exhibits high electrocatalytic activity towards H2 O2 , which is ascribed to their large surface area as well their synergistic effect.
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2. Experimental 2.1. Materials Graphite powder was purchased from Sigma–Aldrich. H2 O2 (30%) was obtained from Wako pure chemical Industries. Zinc nitrate hexa hydrate (Zn (NO3 )2 ·6H2 O, 99% pure) and potassium nitrate (KNO3 ) was purchased from Sigma–Aldrich. Phosphate buffer solution (PBS) with pH 7 and pH 5 were prepared using 0.05 M Na2 HPO4 and NaH2 PO4 . All the chemicals used in this work were of analytical grade and all the solutions were prepared using doubly distilled water. Electrochemical experiments were carried out using a single compartment, three-electrode cell apparatus under nitrogen atmosphere. 2.2. Apparatus Cyclic voltammetry (CV) experiments were carried out using CHI 750a work station. A conventional three electrode cell containing freshly prepared 0.05 M PBS (pH 5 and pH 7) was used for the electrochemical studies. GCE (surface area = 0.079 cm−2 ) was used as a working electrode. Pt wire was used as a counter electrode and standard Ag/AgCl was used as a reference electrode. Surface morphology studies were carried out using field emission scanning electron microscope (FESEM), JSM-6500F and Hitachi S-3000 H scanning electron microscope (SEM). Attenuated total reflectance (ATR) spectroscopy has been performed by using Perkin-Elmer IR spectrometer. Amperometric (i–t curve) measurements were performed using an analytical rotator AFMSRX (PINE instruments, USA) and rotating disc electrode with an area of 0.24 cm−2 . 3. Results and discussions 3.1. Electrochemical preparation and the formation mechanism of RGO/ZnO composite Graphite oxide was prepared by Hummers method [26]. As prepared graphite oxide was dispersed in water, bath sonicated for 2 h, finally exfoliated to form graphene oxide (GO). As purified GO (0.5 g/ml) was dispersed well in water. About 8 l of GO was drop casted on the pre-cleaned GCE and dried at room temperature. The GO modified GCE was transferred to an electrochemical cell containing 0.05 M (ZnNO3 )2 ·6H2 O and KNO3 solution. 30 successive cyclic voltammograms were performed in the potential range between 0 and −1.5 V at the scan rate of 50 mV s−1 (see Fig. 1). During the first cathodic potential scan, a large cathodic peak appears at −1.14 V with an onset potential of −0.70 V. After several cycles, this cathodic peak disappeared completely, attributed to the reduction of oxygen moieties at the GO basal plane [27]. Besides, during the first anodic potential scan, with an onset potential of −0.83 V, a small anodic hump appears at −0.66 V, indicating the dissociation of zinc nitrate and the analogous formation of Zn2+ ions. However, no anodic peak (−0.66 V) was observed when GO alone was electrochemically reduced in the same potential window (figure not shown). This provides further evidence for the Zn2+ ions formation in the solution during the electrochemical deposition of ZnO on GO modified GCE. As reported previously, ZnO electrochemical deposition will possibly occur by any one of the following reversible mechanistic pathways [28,29]. Zn2+ + NO3 − + 2e− → ZnO + NO2 − 2+
Zn
−
+ ½O2 + 2e → ZnO
(1) (2)
In our case, as we maintained the N2 atmosphere during the entire RGO/ZnO electrochemical deposition process, we believe that the ZnO deposition may not have proceeded via step (2).
Fig. 1. 30 consecutive cyclic voltammograms recorded at a GO modified GCE in 0.5 mM (ZnNO3 )2 ·6H2 O and 0.5 mM KNO3 containing N2 saturated aqueous solution at the scan rate of 50 mV s−1 .
Instead, ZnO formation should have occurred via step (1), where Zn2+ ions will associate with the oxygen atoms produced by the reduction of NO3 − ions. As it is well known that, GO sheets are mostly decorated with epoxy and hydroxyl groups on the basal plane, while carbonyl and carboxyl groups are located at the edges [30]. During the electrochemical deposition process, the positively charged Zn2+ ions from the bath solution will be attracted towards the negatively charged oxygen functionalities of GO by electrostatic driving force, and thus got anchored at the GO surface. During the second cathodic potential scan, at more negative potential (∼1 V), the as dissociated NO3 − ions will undergo further reduction forming both NO2 − ions and oxygen. Meanwhile, the released oxygen should be captured immediately by the anchored Zn2+ ions, resulting in the formation of RGO/ZnO composite (step 1). Here, it should be noted that, as the Zn2+ ions are anchored at GO only via electrostatic force, and as the oxygen present in the functional groups are covalently bonded with the carbon or hydrogen, they may not be involved in the ZnO formation. 3.2. ATR studies As shown in the ATR spectra of GO (Fig. 2, curve a), the bands at 1076, 3332, 1402 and 1731 cm−1 corresponds to the C O, C OH stretching vibrations, C O H deformation peak and C O stretching of COOH groups, respectively. On the other hand, all these bands related to the oxygen-containing functional groups almost gone astray in the ATR spectra of RGO/ZnO composite (Fig. 2, curve b), revealing that these oxygen functionalities were partially removed during the electrochemical reduction process. No notable shift in absorption peaks were observed in the ATR spectra of RGO/ZnO, confirming that association of ZnO with RGO does not cause any drastic change to the morphology of RGO. 3.3. Morphological study of RGO/ZnO composite Fig. 3A shows the FESEM image of electrochemically synthesized RGO, displaying their well exfoliated, typical wrinkled surface morphology. Fig. 3B shows the top view of as-grown ZnO micro-forest with numerous closely assembled tiny flower-like
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retained, revealing the eco-friendly approach used in this work. The SEM results indicated that flower-like ZnO microstructures have been grown successfully on the RGO surface. We also attempt to explore the effect of post thermal treatment on the morphology of the as grown RGO/ZnO composite. Fig. 3D shows the SEM image of RGO/ZnO composite taken after subjecting it to post heat treatment at 80 ◦ C. It is clear that the prepared composite is thermally more stable as no noteworthy cracking was observed on its surface. 3.4. Electrocatalytic activity of H2 O2 at RGO/ZnO composite
Fig. 2. ATR transmittance spectra of exfoliated GO (a), and RGO/ZnO (b).
micro-structures with rough and porous architectures. Fig. 3C shows the ZnO micro flowers closely anchored at the surface of RGO sheets. The as-grown ZnO structures were between 0.5 and 1 m in size. The magnified view of RGO/ZnO has also been shown in the inset. Here, thin RGO sheets have formed branched networks that interconnect the micro flowers. It is notable that porous morphology of ZnO and the wrinkled structure of RGO structures has been
Electrochemical experiments were carried out to investigate the catalytic activity of H2 O2 at RGO/ZnO composite. Compared with bare GCE (curve a, Fig. 4), RGO/ZnO composite modified GCE (curve d–h) shows enhanced activity towards reduction of H2 O2 . Whereas under same experimental conditions RGO alone (curve b) displays less catalytic activity. It is notable that, with an onset potential of (−0.1 V) the catalytic current increased slightly at RGO/ZnO composite modified electrode for 0.02 M H2 O2 . Upon increasing the concentration of H2 O2 (up to 8.8 M H2 O2 ), as shown in curves (d–h), the cathodic current increased linearly while the anodic current decreased. Where, both the increase in peak current and decrease in over potential are considered as electrocatalysis [31]. Particularly, a small cathodic hump was observed at (−0.38 V) for 4.48–8.8 M H2 O2 with a trivial shift in peak potential. A maximum reduction peak was observed for 8.8 M H2 O2 . However, in the presence of same H2 O2 concentration (8.8 M), no significant catalytic peak was observed at bare GCE. This result confirms that the composite film modified GCE exhibits enhanced electrocatalytic activity towards H2 O2 and it considerably lowers the overpotential. The electrocatalytic response towards the H2 O2 in linear enslavement could be witnessed between the current response and concentration of H2 O2 in the range of 0.2–8.8 M with a correlation coefficient of 0.9402 and with a sensitivity of 8.50 A M−1 cm−2 . The linear regression equation was obtained as, I (A) = 0.6718 C
Fig. 3. FESEM image of RGO (A), SEM images of electrochemically deposited ZnO (B), RGO/ZnO composite prepared at room temperature (C) and with 80 ◦ C post heat treatment (D). Inset in (C) is the RGO/ZnO composite at higher magnification.
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Fig. 4. Cyclic voltammograms obtained at bare/GCE (a) and RGO/GCE (b) in the presence of 8.8 M H2 O2 at the scan rate of 50 mV s−1 . Cyclic voltammograms obtained at RGO/ZnO/GCE in the (c) absence and presence of (d–h) 0.02, (e) 2.19, (f) 4.48, (g) 6.24 (h) 8.8 M H2 O2 under similar conditions. Supporting electrolyte: N2 saturated 0.05 M PBS (pH 7). Inset is the plot of cathodic peak current vs. [H2 O2 ].
(M) + 19.91. The high electrocatalytic activity of RGO/ZnO composite can be ascribed to the large surface area and edge-plane like defective sites on RGO. The porous nature and the typical flower like morphology of ZnO microstructures can offer more catalytic sites for the efficient diffusion of analyte. The other plausible reason for the enhanced catalytic activity can be due to the synergistic effect of the RGO and ZnO. 3.5. Amperometric determination of H2 O2 at RGO/ZnO composite modified electrode Amperometric i–t curve is the most often used method to evaluate the electrocatalytic activity of electrochemical sensors and enzyme-based biosensors. In the present study, we have used amperometric technique to evaluate the performance of the developed RGO/ZnO film. During the amperometric i–t measurements the electrode potential was hold at −0.38 V (from the CV electrocatalysis) and the N2 saturated PBS (pH 7) was constantly stirred at 1200 rpm. For every 50 s, aliquots of H2 O2 were successively injected into the supporting electrolyte solution. Fig. 5A shows the amperometric i–t response obtained at RGO/ZnO composite rotating disc GCE upon various H2 O2 concentration additions. These results clearly show that, composite film exhibits fast and
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Fig. 5. Amperometric i–t response at RGO/ZnO film modified rotating disc GCE upon successive additions of 1–22.24 M H2 O2 into continuously stirred N2 saturated 0.05 M PBS (pH 7). Applied potential: −0.38 V; Rotation rate: 1200 rpm. Inset (above) is the enlarged view of the amperometric i–t response for 1–4 M H2 O2 . Inset (below) is the plot of response current vs. [H2 O2 ].
well defined response towards each H2 O2 additions. The response time of the RGO/ZnO composite film towards H2 O2 is less then 5 s, validating the rapid catalytic reduction process occurring at the composite film surface. The response current increases linearly between 1.0 and 22.48 M of H2 O2 with a sensitivity of 13.49 A M−1 cm−2 . As shown in Table 1 the performance characteristics of the proposed sensor such as applied potential, LOD and the response time have been compared with that of the other H2 O2 sensors reported previously [32–39]. It is evident that the RGO/ZnO electrode showed good performance than the previously reported enzymatic and nonezymatic H2 O2 sensors. In order to investigate the stability of RGO/ZnO film, the modified electrode was stored in PBS (pH 7) at 4 ◦ C, and the background current was monitored periodically. The proposed sensor exhibited no noticeable decrease in current response in the first two weeks and maintained about 95% of its initial value after one month, indicating its excellent storage stability. The background current was also 95% stable even after 200 consecutive cycles, which indicates the excellent stability of the composite film. In addition, we also attempted to evaluate the stability of the RGO/ZnO film by monitoring the response current with respect to time by using CV studies. The other experimental conditions are same as discussed
Table 1 Comparison of electroanalytical data of various modified electrodes towards H2 O2 reduction. Electrode
Applied potential (V)
a
ZnO/Au/Nafion/HRP/GCE b HRP/GCE Nano Ag/GCE Ag/c MWCNT/GCE Fe3 O4 /Ag/GCE d PoPD/Ag/GCE RGO/Fe3 O4 /GCE Ag/ZnO/GCE RGO/ZnO/GCE
−0.30 −0.38 −0.5 −0.45 −0.5 −0.5 −0.3 −0.25 −0.38
15 0.05 1.2 1.6 1.2 1.5 3.2 0.42 0.02
a b c d
LOD = limit of detection. HRP = horseradish peroxidase. MWCNT: multiwalled carbon nanotubes. PoPD = poly(o-phenylenediamine).
LOD (M)
Response time (s)
Ref.
5 – 3.5 3 3 2 >5 >10 3
[32] [33] [34] [35] [36] [37] [38] [39] This work
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Acknowledgement This work was supported by the National Science Council and the Ministry of Education of Taiwan (Republic of China). I am very much thankful to Dr. Arun Prakash Periasamy for his timely help and valuable suggestions.
References
Fig. 6. Amperometric i–t response at RGO/ZnO modified rotating disc GCE for the successive additions of 1 M H2 O2 , 1 mM AA, 1 mM DA, and 1 mM glucose solutions into the continuously stirred N2 saturated 0.05 M PBS (pH 7). Applied potential: −0.38 V; rotation rate: 1200 rpm.
in Section 3.4. The response current observed at the RGO/ZnO fim modified GCE towards 4.5 M H2 O2 (within working range) was monitored every 2 h (until 13 h). The modified electrode exhibits stable response towards H2 O2 and 89.3% of response current was retained even after 13 h, validating the good stability of the composite film. The good storage and operational stability of the RGO/ZnO composite can be attributed to the well anchored ZnO microstructures on the RGO sheets.
3.6. Anti-interference study of the developed H2 O2 sensor As it is well known that, some co-existing electroactive species in nature will affect the sensor response. So, we have monitored the response of the sensor towards each 1 mM of ascorbic acid (AA), glucose and dopamine (DA) additions. The working potential was hold at −0.38 V. Each addition of the electroactive interfering species brought out hardly discernible current response, whereas notable response was observed for 1 M H2 O2 (Fig. 6). These results suggest that the interfering effect caused by these electroactive species is quite negligible, validating the highly selective detection of H2 O2 at the composite film.
4. Conclusions In summary, we report a facile green approach for the room temperature, electrochemical deposition of flower-like ZnO microstructures on the GO surface. The SEM images of RGO/ZnO composite films revealed that ZnO micro flowers were well formed and closely anchored at the surface of RGO sheets. The as-prepared RGO/ZnO composite showed good catalytic activity towards H2 O2 . Besides, RGO/ZnO sensor was highly sensitive and selective for H2 O2 and it holds excellent storage and operational stability. As a future perspective, we believe that RGO/ZnO composite material could be a promising electrode material for the fabrication of enzyme based biosensors, super capacitors and solar cells at low cost.
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Biographies
Selvakumar Palanisamy received his B.S. degree in Chemistry from Madurai Kamaraj University, Tamilnadu, India, in 2006. He received his M.S. degree in chemistry from Madurai Kamaraj University, Tamil Nadu, India, in 2009. He received his Master of philosophy degree in chemistry from Madurai Kamaraj University, Tamil Nadu, India, in 2010. Now, he is a second year Ph.D. student in Chemical Engineering and Biotechnology at National Taipei University of Technology. His research interest mainly focuses on the nanomaterial synthesis for enzyme immobilization related to biosensor and biofuel cell applications. Dr. Shen-Ming Chen received his B.S. degree in Chemistry in 1980 from National Kaohsiung Normal University, Taiwan. He received his M.S. degree (1983) and Ph.D. degree (1991) in Chemistry from National Taiwan University, Taiwan. He is currently a professor at the Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taiwan. His current research interests include electroanalytical chemistry, bioelectrochemistry, fabrication of energy conservation and storage devices and nanomaterial synthesis for electrochemical applications. He has published more than 230 research articles in SCI journals. Dr. R. Saraswathi received her Ph.D. degree in Chemistry in 1989 from the Indian Institute of Technology, Madras, Tamil Nadu, India. She joined the Madurai Kamaraj University in 1988, where she is now the Head of the Department of Materials Science. She is a recipient of the Commonwealth Visiting Scientist Fellowship during 1997–1998 at the University of Leicester, England. She was a visiting professor twice at the National Taipei University of Technology, Taiwan. Her research interest focuses on the electrochemical aspects of conducting polymers, nanomaterials and nanocomposites.