A dendritic palladium electrode for a hydrogen peroxide sensor fabricated by electrodeposition on a dynamic hydrogen-bubble template and dealloying

Sensors and Actuators B 213 (2015) 329–333

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

A dendritic palladium electrode for a hydrogen peroxide sensor fabricated by electrodeposition on a dynamic hydrogen-bubble template and dealloying Chang Yong An a , Kai Zhuo a , Woo-Jae Kim b , Chan-Hwa Chung a,∗ a b

School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Department of Chemical and Environment Engineering, Gachon University, Sungnam 461-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 2 December 2014 Received in revised form 11 February 2015 Accepted 16 February 2015 Available online 25 February 2015 Keywords: Dendritic Pd electrode Dynamic hydrogen template Dealloying Electrochemical sensor Hydrogen peroxide

a b s t r a c t A dendritic Pd–Cu electrode has been prepared by co-electrodeposition of palladium and copper with high cathodic over-potentials, in which hydrogen evolution occurred. The dendritic Pd–Cu was electrochemically de-alloyed to selectively remove copper. This de-alloyed Pd–(Cu) electrode has a dramatically increased surface-area that is appropriate for electrochemical sensor applications. Electrode characteristics for use as a non-enzymatic sensor were monitored by cyclic voltammetry and amperometry, in which detection signals for hydrogen peroxide (H2 O2 ) were precisely measured. The prepared dendritic Pd–(Cu) electrode demonstrated outstanding electrocatalytic activity for H2 O2 reduction with a detection limit of 1.78 × 10−8 M, a wide dynamic range of 50 ␮M to 10 mM, and high sensitivity of 915.25 ␮A cm−2 mM−1 . © 2015 Elsevier B.V. All rights reserved.

1. Introduction The analysis of hydrogen peroxide (H2 O2 ) is extremely important, because of its presence in various areas such as foods, industrial manufacturing, pharmaceuticals, clinical and environmental analysis, antiseptics, and biochemistry [1–3]. The sensitive, accurate, rapid response, and low cost detection of hydrogen peroxide has been especially important and widely surveyed. Hydrogen peroxide can be analyzed by titrimetry [4], spectrometry [5], chemiluminescence [6], electrochemical luminescence [7], and electrochemistry [8]. Among these methods, electrochemical sensors are particularly fascinating because they exhibit a fast response, high sensitivity, and simplicity of use, and they utilize low cost equipment. Among the several electrode materials, the palladium electrodes exhibit excellent electrocatalytic activity towards H2 O2 reduction [9,10] and show both enhanced electron transfer and reduced overpotentials [11]. Moreover, the abundance of Pd over other novel metals makes it a cheaper substitute for utilization in various fields [12]. Recently, many researchers fabricated palladium

∗ Corresponding author. Tel.: +82 31 290 7260; fax: +82 31 290 7272. E-mail address: [email protected] (C.-H. Chung). http://dx.doi.org/10.1016/j.snb.2015.02.085 0925-4005/© 2015 Elsevier B.V. All rights reserved.

electrodes into various morphologies such as spherical [16,17], dendritic [14,15,18], and hollow structured [19]. In particular, the dendritic morphology of palladium shows excellent characteristics of high surface area and remarkable electrocatalytic activity. Several processes have been proposed for preparation of palladium electrodes such as sol–gel processes [13], colloidal reduction [20], galvanic displacement [21,22], as well as electroless [23] and electro-deposition [24]. Among these methods, electro-deposition has advantages because it is much simpler and faster than other methods. For sensor applications of palladium electrodes, Zhou et al. [15] reported that the sensing performance for hydrogen peroxide using dendritic Pd particles was 70.8 ␮A cm−2 mM−1 . There have been other efforts to increase sensitivity by loading palladium on high surface area substrates. Bo et al. [16] reported their results of 228.5 ␮A cm−2 mM−1 sensitivity with Pt–Pd alloy nanoparticles on large mesoporous carbon vesicles, and You et al. [17] achieved 325.55 ␮A cm−2 mM−1 sensitivity using multi-walled carbon nanotubes (MWCNT). In this work, a high surface area Pd electrode with dendritic morphology was fabricated by electrodeposition of dendritic Pd–Cu on a dynamic hydrogen-bubble template [25,26] followed by a Cu dealloying process. The H2 O2 electrochemical sensing performance of the prepared Pd electrode was characterized by cyclic voltammetry and amperometry.

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2. Experimental PdCl2 (Kojima Chemicals, Japan) CuSO4 ·5H2 O, and H2 SO4 (95%, Duksan Chemicals, Korea) were purchased and combined with aqueous electrolyte for electrodeposition. H2 O2 (38%, Duksan Chemicals) and phosphate buffer solution (PBS, Sigma–Aldrich, USA; 0.1 M, pH 7) were also prepared for analysis of electrochemical performance of the fabricated electrode. A Pt/Ti/Si substrate with an area of 0.1256 cm2 was prepared and used as the working electrode for Pd–Cu electrodeposition. Ag/AgCl (3 M NaCl) and platinum electrodes were used as a reference electrode and counter electrode, respectively. The distance between working electrode and counter electrode is about 2 cm, and the reference electrode is placed in the middle of them. First, the Pd–Cu electrode was fabricated by applying a high cathodic overpotential of −4 V for 1 min to the substrate in an electrolyte of 10 mM PdCl2 , 2 mM CuSO4 ·5H2 O, and 1 M H2 SO4 , which induced electrodeposition of a dendritic Pd–Cu layer accompanied by hydrogen bubble evolution. During this electrodeposition, the 450 mA of current was monitored and resulted the 19 ␮m-thick dendritic Pd–Cu layer. Next, to enhance the surface area of the Pd electrode, selective electrochemical dealloying of copper on the as-prepared dendritic Pd–Cu layer was performed. The cyclic potential sweep between 0 and +1 V in 1 M H2 SO4 electrolyte selectively de-alloyed copper from the dendritic Pd–Cu. The morphological structure of the electrode was monitored by field-emission scanning electron microscopy (FESEM, JEOL JSM7000F), and the energy dispersive X-ray (EDX) in FESEM was also used to analyze the chemical composition of the prepared electrodes. All of the electrochemical experiments were performed using an electrochemical workstation (Zahner Elecktrik IM6ex, Germany) at room temperature with three electrodes. Oxygen in the electrolyte was removed by purging with N2 for 10 min before electrochemical measurements. The H2 O2 sensing performance of the dendritic dealloyed Pd-(Cu) electrode was characterized by cyclic voltammetry with a 20 mV s−1 scan rate in an electrolyte

Table 1 Chemical compositions of the prepared electrodes characterized by EDX analysis. Electrode

Pd (At%)

Cu (At%)

Dendritic Pd–Cu De-alloyed Pd–(Cu)

80.13 90.92

19.87 9.08

of 0.1 M PBS. The voltage was scanned between −0.6 and +0.8 V vs. Ag/AgCl. The sensitivity was also evaluated by amperometry measurements of the Pd–(Cu) electrode. 3. Results and discussion The morphologies of electrodeposited Pd–Cu and dealloyed Pd–(Cu) electrodes were characterized by FESEM, as shown in Fig. 1. The dendritic backbone structure was evident in the electrode and was maintained even after 20 cycles of the Cu dealloying process in 1 M H2 SO4 electrolyte, which resulted in an increase in surface area. There was a small difference in the dendritic features before and after dealloying the Pd–(Cu) electrode. The morphology in Fig. 1c looks sharper and less dense, which is caused by removal of Cu from Pd–Cu during de-alloying. The EDX data in Table 1 presents changes in the chemical compositions of the dendritic Pd–Cu and dealloyed electrodes, confirming that copper had been selectively removed by dealloying. The electrochemical surface area of each dendritic-structured electrode was monitored by cyclic voltammetry obtained in an electrolyte of 1 M H2 SO4 . The small anodic current peak at 0.33 V (vs. Ag/AgCl) was due to oxidation of copper on the electrode. More importantly, the peak intensity of the cathodic current at 0.5 V (vs. Ag/AgCl) represents the amount of active sites for H2 O2 sensing. As noted in Fig. 2, the Cu-dealloyed Pd–(Cu) electrode exhibited its most intense cathodic current peak at 0.5 V (vs. Ag/AgCl), which is caused by the increase in surface area after removal of copper from the Pd–Cu layer.

Fig. 1. (a) The cross-sectional SEM images of electrodeposited dendritic Pd–Cu, and the planer SEM images of (b) dendritic Pd–Cu and (c) the dendritic dealloyed Pd–(Cu).

C.Y. An et al. / Sensors and Actuators B 213 (2015) 329–333

8

10

2

Current density [mA/cm ]

dendritic Pd dendritic Pd-Cu dealloyed Pd-(Cu)

4 Current [mA]

331

0 -4 -8 -12 0.0

0.2

0.4

0.6

0.8

1.0

5 0 -5 -10

0 mM

-15

10 mM

-0.8

-0.4

Potential [V] vs. Ag/AgCl Fig. 2. Cyclic voltammograms obtained from the prepared electrodes in an electrolyte of 1 M H2 SO4 at a scan rate of 20 mV s−1 .

0.0

0.4

0.8

Potential [V] vs. Ag/AgCl Fig. 4. Cyclic voltammograms obtained from the dealloyed Pd–(Cu) electrode in an electrolyte of 0.1 M PBS (pH 7) containing different concentrations of H2 O2 . The scan rate was 20 mV s−1 .

2

Current density [mA/cm ]

20 10 0 5mVs-1 10mVs-1 20mVs-1 40mVs-1 60mVs-1 80mVs-1 100mVs-1

-10 -20 -30 -0.8

-0.4

0.0

0.4

0.8

Potential [V] vs. Ag/AgCl Fig. 3. Cyclic voltammograms obtained from (a) Pt substrate, (b) porous Pd electrode, (c) dendritic Pd–Cu electrode, and (d) dealloyed Pd–(Cu) electrode in an electrolyte of 0.1 M PBS (pH 7) at a scan rate of 20 mV s−1 .

Fig. 5. Cyclic voltammograms obtained from the dealloyed Pd(-Cu) electrode in an electrolyte of 0.1 M PBS (pH 7) containing 5 mM H2 O2 with different scan rates.

2

Current density [mA/cm ]

-5 The electrochemically active sites on the various electrodes were also evaluated in an electrolyte of 0.1 M PBS (pH 7) solution with a scan rate of 20 mV s−1 in the potential range between −0.6 and 0.8 V (vs. Ag/AgCl). As shown in Fig. 3, the Pt/Ti/Si substrate was nearly electrochemically inactive, and the dealloyed Pd–(Cu) electrode gave a larger cathodic current. The peak position of the cathodic reduction current for the de-alloyed electrode also shifted to −0.3 V (vs. Ag/AgCl), which may result from an improvement of electrochemical activity from the dealloying process. With the addition of H2 O2 in 0.1 M PBS electrolyte, the cathodic-current intensity at −0.4 V (vs. Ag/AgCl) increased from −12.5 mA/cm2 for 0 mM H2 O2 to −15.0 mA/cm2 for 10 mM H2 O2 , as shown in Fig. 4. On the other hand, the cathodic current also increased as the scan rate was lengthened from 5 to 100 mV s−1 , as shown in the cyclic voltammograms obtained with 5 mM H2 O2 (cf. Fig. 5). As presented in Fig. 6, the current density due to H2 O2 reduction was inversely proportional to the square-root of the scan rate, which exhibited diffusion-controlled kinetics in electron transfer for the H2 O2 reduction reaction. It is also evident that the peaks were shifted to more negative potential due to irreversibility of the H2 O2 reduction reaction on the Pd electrode. In Fig. 7, the amperometric I–t curve of the dendritic dealloyed Pd–(Cu) electrode with successive additions of 50, 100 ␮M, and 1 mM H2 O2 in the stirred electrolyte of 0.1 M PBS (pH 7) at the

-10

-15

-20

-25

-30 2

4

6

8

10

-1 1/2

(υ / mVs )

Fig. 6. Plot of H2 O2 reduction current density on the dendritic dealloyed Pd–(Cu) electrode with different square-roots of scan rate.

applied potential of −0.4 V revealed a step-wise response as the H2 O2 concentration was increased. The dynamic range with a linear response was from 50 ␮M to 12 mM as observed in Fig. 8. The sensitivity of our Pd–(Cu) electrode was calculated to be 915.25 ␮A cm−2 mM−1 with a detection limit of 1.78 × 10−8 M at

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Table 2 Performance comparison of the hydrogen-peroxide sensor developed in this work with previously developed sensors. Electrode

Dynamic range [M] −6

1.0 × 10 1.0 × 10−7 1.0 × 10−6 2.5 × 10−6 5.0 × 10−5

Pd particles/GCE Pd particle/MCV/GCE Nafion/MWCNT/Pd/GCE PtPd/MWCNTs/GCE Dendritic dealloyed Pd–(Cu)

Detection limit [M] −3

−7

to 3.0 × 10 to 6.1 × 10−3 to 1.0 × 10−2 to 1.25 × 10−4 to 1.2 × 10−3

3.2 × 10 7.9 × 10−8 3.0 × 10−7 1.2 × 10−6 1.78 × 10−8

Sensitivity [␮A cm−2 mM−1 ]

References

70.8 228.5 325.55 414.8 915.25

[15] [16] [17] [27] This work

were evaluated by cyclic voltammetry, and the peak position of H2 O2 reduction current with the de-alloyed Pt–(Cu) electrode shifted to −0.3 V (vs. Ag/AgCl), which represents an improvement in electrochemical activity from the dealloying process. The amperometric I–t curve indicates that our dendritic dealloyed Pd–(Cu) electrode exhibited outstanding performance for H2 O2 sensing with a high sensitivity of 915.25 ␮A cm−2 mM−1 in the wide linear range between 50 ␮M and 12 mM. Acknowledgements

Fig. 7. Amperometric I–t curves for the dealloyed Pd–(Cu) electrode with successive additions of 50, 100 ␮M, and 1 mM H2 O2 in 0.1 M PBS electrolyte (pH 7) at an applied potential of −0.4 V.

0

References 2

R = 0.99962

-400 Current [μA]

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (NRF-2013R1A2A2A01014102) and the Materials & Components Technology Development Program (10047681, Development of Low Cost Conductive Paste Capable of Fine Pattern for Touch Panel and High Conductivity for Solar Cell Using Metal Composite with Core-Shell Structure prepared by Highly Productive Wet Process) funded by the Ministry of Trade, Industry & Energy (MI, Korea).

-800

-1200

0

2

4

6

8

10

12

Mole concentration [mM] Fig. 8. The amperometric calibration curve with H2 O2 concentrations between 50 ␮M and 12 mM detected with the dendritic dealloyed Pd–(Cu) electrode.

a signal-to-noise ratio (S/N) of 3. Compared with other Pd-based H2 O2 sensors reported in the literature (cf. Table 2), the sensitivity of our dendritic dealloyed Pd–(Cu) electrode was much higher in a similar dynamic range. During electrodeposition on the dynamic hydrogen-bubble template and the Cu dealloying process, the surface area of the prepared electrode increased dramatically to generate a higher sensitivity sensor. As noted in this work, the process for fabricating our H2 O2 sensor electrode of dendritic dealloyed Pd–(Cu) was very simple, being based entirely on an electrochemical method in an aqueous electrolyte. 4. Conclusions The high surface area Pd–(Cu) electrode with dendritic morphology was fabricated by electrodeposition of dendritic Pd–Cu on a dynamic hydrogen-bubble template followed by a Cu dealloying process. Electrochemically active sites on the prepared electrodes

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Biographies Chang Yong is currently doing the PhD course of Chemical Engineering at Sungkyunkwan University, Korea. He received his BS from Hongik University and MS from Sungkyunkwan University in South Korea, both in Chemical Engineering. Zhuo Kai received his BS in chemistry from Liaocheng University, China, and completed the MS and PhD from Sungkyunkwan University in South Korea, both in Chemical Engineering. Woo-Jae Kim is currently an assistant professor of Department of Chemical and Biological Engineering at Gachon University, Korea. He received his BS and PhD from Seoul National University in South Korea, both in chemical engineering. He was a postdoctoral research associate under the guidance of Michael S. Strano at Massachusetts Institute of Technology, Department of Chemical Engineering from 2007 to 2009. His research focuses on surface chemistry of low-dimensional systems, carbon nano-material separations, and their applications for future energy and nano-electronic devices. Chan-Hwa Chung is a professor of chemical engineering at Sungkyunkwan University, South Korea. He received his BS from Yonsei University, MS and PhD from Seoul National University in South Korea, both in Chemical Engineering. He was a postdoctoral research associate at University of California at Santa Barbara, Department of Chemical Engineering from 1995 to 1997, and adjunct associate professor at University of New Mexico from 2005 to 2007. He has explored advanced electrochemical systems, including synthesis of nanoporous metals using electrodeposition, and his current research focuses on their applications for energy devices and metal pastes.