Electrochemical Tuning the Activity of Nickel ... - ACS Publications

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Electrochemical Tuning the Activity of Nickel Nanoparticle and Application in Sensitive Detection of Chemical Oxygen Demand Qin Cheng,† Can Wu,† Jianwei Chen,‡ Yikai Zhou,‡ and Kangbing Wu*,† † ‡

School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China MOE Key Lab of Environment and Health, School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430030, China ABSTRACT: Nickel nanoparticles were in situ prepared on the surface of glassy carbon electrode via electrochemical reduction using NiSO4 as the precursor. The shape of nickel nanoparticles was successfully controlled by variation of reduction potential, deposition time, pH value, and concentration of Ni2+. The resulting nickel nanoparticles exhibited high activity to the electrochemical oxidation of glycine, and greatly increased the oxidation current of glycine. As a result, the electrochemical activity of nickel nanoparticles toward the oxidation of glycine was easily tuned. The application of nickel nanoparticles in the sensitive and rapid detection of chemical oxygen demand (COD) was studied. Based on the greatly enhanced oxidation current signal of glycine on the surface of nickel nanoparticles, a novel electrochemical method was developed for the detection of COD. The linearity is from 0.24 to 480 mg L 1, and the limit of detection is as low as 0.14 mg L 1. Finally, nickel nanoparticle-modified electrode was used to detect COD values of different water samples, and the results were tested using conventional dichromate method.

’ INTRODUCTION Metal nanoparticles have been widely studied because of their extraordinary physical and chemical properties that largely differ from those of their bulk materials.1 Nickel is a commercially important metal that is extensively used as catalyst,2 electrode materials,3 soft magnetic materials,4 and chemically protective coating.5 Recently, studies have shown that the catalytic activity and selectivity of metal nanoparticles are strongly dependent on their size and shape.6 Therefore, the size-controlled synthesis of nickel nanoparticles is still worthy of investigation. Electrochemical deposition is a convenient and feasible technology for in situ preparation of metal nanoparticles. For example, nickel nanoparticles were prepared by galvanic deposition on the surface of TiO2/Ti electrode and then used for methanol electrooxidation.7 In addition, potentiostatic deposition was also used for the preparation of nickel nanoparticles on electrode surface.8 10 In this work, nickel nanoparticles were successfully prepared on the surface of glassy carbon electrode (GCE) using constant potential reduction. It was found that the resulting nickel nanoparticles showed high electrochemical activity to the oxidation of glycine. On the surface of nickel nanoparticles-modified GCE (nano-Ni/GCE), the oxidation current signal of glycine remarkably enhanced. Via altering the preparation parameters such as reduction potential, deposition time, pH value, and concentration of Ni2+, the shape of nickel nanoparticles was easily controlled. Therefore, the electrochemical activity of nanoNi/GCE toward the oxidation of glycine was conveniently tuned. Moreover, the prepared nickel nanoparticles showed great potential in the sensitive detection of chemical oxygen demand (COD) based on the greatly increased oxidation current signal of glycine. r 2011 American Chemical Society

COD is defined as the number of oxygen equivalents consumed in the oxidation of organic compounds using strong oxidizing agents. It is quite important to detect COD value because it indicates the organic pollution in water. Until now, various electrochemical methods were reported for the detection of COD because of their high sensitivity, short analysis time, low cost, and handling convenience. To enhance the response signal, developing novel electrode material is paramount for the electrochemical detection of COD. Up to now, different materials were successfully used to construct sensing electrodes for COD, including PbO2,11 13 doped PbO2,14 TiO2,15 17 Rh2O3,18 AgO and CuO,19 Cu20 23 and boron-doped diamond.24,25 However, electrochemical detection of COD using nickel nanoparticles is still missing. On the surface of nickel nanoparticles, the oxidation of glycine, a commonly used standard substance for evaluating COD value, was facilitated; then, a very sensitive response signal was observed. Undoubtedly, the sensitivity of electrochemical detection of COD was greatly improved by nickel nanoparticles.

’ EXPERIMENTAL SECTION Reagents. All chemicals were of analytical reagent grade and used as received. NiSO4, glycine, H2SO4, HCl, HAc, NaAc, Na2HPO4, NaH2PO4, NaOH, K2Cr2O7, Ag2SO4, HgSO4, and (NH4)2Fe(SO4)2 were purchased from Sinopharm Group Received: August 4, 2011 Revised: September 30, 2011 Published: October 11, 2011 22845

dx.doi.org/10.1021/jp207442u | J. Phys. Chem. C 2011, 115, 22845–22850

The Journal of Physical Chemistry C

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Figure 1. Three-dimensional AFM images of GCE (A) and the prepared nano-Ni/GCE at 1 (B), 1.1 (C), 1.2 (D), 1.3 (E), 1.4 (F), and 1.5 V (G).

Chemical Reagent (Shanghai, China). The stock solution of glycine (7.5 g L 1) was prepared using doubly distilled water. Instruments. Electrochemical measurements were performed on a CHI 660D electrochemical workstation (Chenhua Instrument, Shanghai, China) with a conventional three-electrode system. The working electrode is a nano-Ni/GCE or a bare GCE, the reference electrode is a saturated calomel electrode (SCE), and the counter electrode is a platinum wire. Atomic force microscopy (AFM) was performed with a SPA 400 microscope SEIKO, Chiba, Japan). Preparation of Nano-Ni/GCE. Before electrodeposition, the GCE with diameter of 3 mm was polished with 0.05 μm alumina

slurry and then sonicated in doubly distilled water to give a clean surface. After that, nickel nanoparticles were coated on the surface of GCE under 1.3 V for 30 s in 0.1 M, pH 6.5 phosphate buffer solution containing 5 mM NiSO4. Finally, the resulting electrode was rinsed with doubly distilled water to remove any adsorbed species. Detection of COD using Conventional Dichromate Method. The conventional dichromate method was used to measure COD value according to the National Standard of China (GB 11914-89). Sample solution (10 mL) and 5 mL of K2Cr2O7 solution (0.025 M) were added to a cuvette and then refluxed for 2 h in a thermostat at 433 K. After that, the excess of dichromate 22846

dx.doi.org/10.1021/jp207442u |J. Phys. Chem. C 2011, 115, 22845–22850


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was determined by titration using 0.005 M (NH4)2Fe(SO4)2 as the titrant. Finally, the value of COD was calculated after subtracting the blank value of doubly distilled water.

’ RESULTS AND DISCUSSION Potential Tuning the Morphology of Nano-Ni. The reduction potential is very important for preparing the metal nanoparticles because it largely affects the shape of prepared nanoparticles. In the solution of 0.1 M, pH 6.5 phosphate buffer, different nickel nanoparticles were prepared on the GCE surface under constant potentials using 5 mM NiSO4 as the precursor. The reduction time was 30 s, and the studied potentials were 1, 1.1, 1.2, 1.3, 1.4, and 1.5 V. The morphology of resulting nano-Ni was characterized using AFM, and the 3D AFM images were shown in Figure 1. If the reduction potential was higher than 1 V, then no nano-Ni was formed on the GCE surface because the AFM image (Figure 1B) is very similar to that of bare GCE (Figure 1A). Change of the reduction potential to 1.1 (Figure 1C) and 1.2 V (Figure 1D) resulted in a few and irregular particles appearance on the GCE surface. At the potential of 1.3 V (Figure 1E), it was found that the number of nano-Ni on GCE surface greatly increased, and the arrangement tended to be regular. When the reduction potential was further lowered to 1.4 (Figure 1F) and 1.5 V (Figure 1G), the size of nano-Ni gradually increased, and the surface coating became thicker. Apparently, the shape of nickel nanoparticles can be easily controlled by variation of the reduction potential. To elucidate the relationship between the electrochemical activity of nano-Ni and its shape, we investigated the electrochemical oxidation of glycine on the surface of nano-Ni that prepared at different potentials. The amperometric detection at 0.4 V was employed to study the oxidation of glycine. After the current of nano-Ni/GCE was allowed to reach a steady state in 0.5 M NaOH, glycine standard was then added, and the net increase in current was measured as the response signal. Figure 2A displays the amperometric curve of 37.5 mg L 1 glycine on the surface of different nickel nanoparticles, and Figure 2B shows the variation of response signal with the reduction potential. On the bare GCE (curve a), no response signal was observed for glycine, suggesting that the oxidation activity of glycine is very poor on glassy carbon. However, glycine was easily oxidized on the surface of nano-Ni, and obvious oxidation current appeared, indicating that Ni exhibits high activity to the oxidation of glycine. When the reduction potential shifts from 1 to 1.3 V (curves b e), the current of glycine remarkably increases. With further lowering the potential to 1.4 (curve f) and 1.5 V (curve g), the current of glycine decreases slightly. Clearly, the nano-Ni that prepared at 1.3 V exhibits the highest response activity to the electrochemical oxidation of glycine. From the AFM images, it is clear that the particle size of nano-Ni that was prepared at 1.3 V is smaller, and the arrangement is regular on the GCE surface. Without doubt, the oxidation signal of glycine on the surface of nano-Ni that formed at 1.3 V is much higher. In addition, the oxidation response of glycine at Ni disk electrode with diameter of 3 mm was also examined (curve h). Compared with the prepared nanoNi, bulk Ni possesses much lower electrochemical activity to the oxidation of glycine. This phenomenon reveals that nano-Ni has unique properties that largely differ from those of bulk nickel. Effects of pH Value and Deposition Time on the Activity of Nano-Ni. Various nickel nanoparticles were prepared using

Figure 2. (A) Amperometric curves of 37.5 mg L 1 glycine on GCE (a), Ni disk electrode (h), and nano-Ni/GCE prepared at 1 (b), 1.1 (c), 1.2 (d), 1.3 (e), 1.4 (f), and 1.5 V (g). (B) Variation of the oxidation current of glycine as the reduction potential. Error bar represents the standard deviation of triple measurements. Deposition time: 30 s, NiSO4 concentration: 5 mM, detection potential: 0.4 V.

Figure 3. Oxidation current of 37.5 mg L 1 glycine on nano-Ni/GCEs that were prepared in different pH-valued solution. Error bar represents the standard deviation of triple measurements. Reduction potential: 1.3 V, deposition time: 30 s, NiSO4 concentration: 5 mM, detection potential: 0.4 V.

5 mM NiSO4 in different solution including 0.01 M HCl (pH 2); 0.1 M acetate buffer solution with pH of 3, 4 and 5; and 0.1 M phosphate buffer solution with pH of 5.7, 6, 6.5, 7, and 7.5. The reduction potential was 1.3 V, and the time was 30 s. After that, the electrochemical activity of resulting nano-Ni was analyzed based on the amperometric oxidation current of 37.5 mg L 1 glycine that measured under 0.4 V and in 0.5 M NaOH. As shown in Figure 3, the oxidation current of glycine on the surface of nano-Ni greatly increases with pH value from 2 to 6. When the pH value increases from 6 to 7, the oxidation current of glycine almost keeps constant. However, the oxidation current of glycine starts to decrease when the pH value is >7. The pH value of 22847



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Figure 6. Effect of concentration of NaOH on the oxidation current of 37.5 mg L 1 glycine on nano-Ni surface. Error bar represents the standard deviation of triple measurements. Reduction potential: 1.3 V, deposition time: 30 s, NiSO4 concentration: 5 mM, detection potential: 0.4 V.

Figure 4. (A) Amperometric curves of 37.5 mg L 1 glycine on nanoNi/GCEs that deposited at 1.3 V for 0 (a), 10 (b), 30 (c), 60 (d), 90 (e), 120 (f), and 240 s (g). (B) Variation of the oxidation current of glycine as the deposition time. Error bar represents the standard deviation of triple measurements. NiSO4 concentration: 5 mM, detection potential: 0.4 V.

Figure 5. Oxidation current of glycine on nano-Ni/GCEs that prepared using NiSO4 solution with different concentrations. Error bar represents the standard deviation of triple measurements. Reduction potential: 1.3 V, deposition time: 30 s, detection potential: 0.4 V.

solution not only affects the mass transport of Ni2+ but also influences its electron transfer rate on GCE surface. As a result, the shape of nano-Ni shows difference in different pH-valued solution. Herein, nano-Ni was prepared in pH 6.5 phosphate buffer to obtain high electrochemical activity. The influence of deposition time on the electrochemical activity of nano-Ni was also examined. The amperometric curves of glycine on different nano-Ni/GCEs are given in Figure 4A, and the oxidation current as a function of deposition time is displayed in Figure 4B. When the deposition time extends from 0 to 30 s,

the activity of prepared nano-Ni toward the electrochemical oxidation of glycine remarkably increases, accompanied by notable oxidation current enhancement. However, the oxidation current of glycine on the surface of nano-Ni gradually decreases with further extending the deposition time from 30 to 240 s. This is because that the particle size of nano-Ni obviously increases when the deposition time exceeds 30 s. Consequently, the electrochemical activity of nano-Ni to the oxidation of glycine also decreases. Influence of Ni2+ Concentration on the Activity of NanoNi. Nickel nanoparticles were coated on the GCE surface using different-concentrated NiSO4 solution such as 1, 2.5, 5, 7.5, 10, 15, and 20 mM. Subsequently, the activity of obtained nano-Ni was tested using the oxidation signal of glycine. As displayed in Figure 5, the amperometric oxidation current of 37.5 mg L 1 glycine significantly increases with Ni2+ concentration from 1 to 2.5 mM and then changes slightly from 2.5 to 7.5 mM. If further improving Ni2+ concentration to 20 mM, the oxidation current signal of glycine conversely decreases. When the concentration of Ni2+ is too high, the reduced nickel particles have larger size, fading the property of nanomaterials. So the electrochemical activity of metal nanoparticles also can be tuned by the concentration of precursor. Detection of COD. It is very convenient to select some substances as the standard reagent for electrochemical detection of COD. Herein, glycine was used as the standard compound, and its oxidation current on nano-Ni/GCE surface was applied to evaluate the value of COD. The COD value of glycine standard solution was detected using conventional dichromate method. For 75 mg L 1 glycine solution, the value of COD was measured to be 49.3 mg L 1 of O2, which is consistent with the theoretic value of 48 mg L 1 of O2.26 To choose a suitable medium for the electrochemical detection of COD, we studied the oxidation responses of glycine in NaOH solution with different concentrations using amperometric detection. Nano-Ni was prepared at 1.3 V for 30 s in 0.1 M, pH 6.5 phosphate buffer containing 5 mM NiSO4. Figure 6 illustrates the effect of concentration of NaOH on the oxidation current of glycine. It is found that the oxidation current of glycine greatly increases with NaOH concentration over the range from 0.1 to 0.75 M and then increases slightly from 0.75 to 1 M. When the concentration of NaOH is higher than 1 M, the oxidation signal of glycine begins to decrease. Similarly, the influence of detection potential was also 22848



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Figure 7. Amperometric response of nano-Ni/GCE to different COD values in 1 M NaOH. Reduction potential: 1.3 V, deposition time: 30 s, NiSO4 concentration: 5 mM, detection potential: 0.4 V.

Figure 9. (A) One nano-Ni/GCE to successive response of 2.4 mg L 1 COD. (B) Eleven nano-Ni/GCEs to response of 2.4 mg L 1 COD. Reduction potential: 1.3 V, deposition time: 30 s, NiSO4 concentration: 5 mM, detection potential: 0.4 V, detection medium: 1 M NaOH. Figure 8. Calibration curve for COD on nano-Ni/GCE. Error bar represents the standard deviation of triple measurements. Reduction potential: 1.3 V, deposition time: 30 s, NiSO4 concentration: 5 mM, detection potential: 0.4 V, detection medium: 1 M NaOH.

studied. The oxidation current of glycine on nano-Ni/GCE remarkably increases with the detection potential from 0.1 to 0.4 V and then levels off. This is due to the fact that higher potential leads to faster electron transfer. However, the background current of nano-Ni/GCE obviously increases when the detection potential is >0.4 V. Therefore, the detection potential is controlled at 0.4 V, and the medium is 1 M NaOH. Figure 7 depicts the typical amperometric response of different-valued COD on nano-Ni/GCE surface in 1 M NaOH. When the oxidation potential is controlled at 0.4 V, the background current of nano-Ni/GCE decreases dramatically during the first 20 s and then falls slightly. After addition of doubly distilled water at ∼50 s, no response is observed (curve a). With the addition of 0.375 mg L 1 of glycine (theoretic COD value of 0.24 mg L 1), an obvious oxidation current step is observed (curve b). With gradual increase in COD value to 0.48 (curve c), 0.96 (curve d), 1.44 (curve e), and 1.92 mg L 1 (curve f), it is found that the oxidation response current increases linearly, suggesting good linear response. Further studies show that the oxidation current signal (i, μA) is proportional to the concentration of COD (C, mg L 1 of O2) over the range from 0.24 to 480 mg L 1, as shown in Figure 8. The linear regression equation is i = 1.1433 C, and the correlation coefficient is 0.999. Otherwise, the limit of detection (LOD) was evaluated to be 0.14 mg L 1 according to IUPAC regulations (S/N = 3).

Figure 10. Amperometric response of lake water samples on nano-Ni/ GCE that was prepared at 1.3 V for 30 s. NiSO4 concentration: 5 mM, detection potential: 0.4 V, detection medium: 1 M NaOH.

The reproducibility of one nano-Ni/GCE for successive detections was evaluated by measuring the current signal of 2.4 mg L 1 COD. As shown in Figure 9A, the relative standard deviation (RSD) is 19.8% for 11 determinations, suggesting poor reproducibility. Therefore, the reproducibility between multiple nano-Ni/GCEs was tested based on the response signal of 2.4 mg L 1 COD. For 11 nano-Ni/GCEs, the value of RSD is just 4.9% (Figure 9B), indicating the excellent reproducibility. The stability of nickel nanoparticles was studied by checking the current response at a fixed COD concentration of 24 mg L 1 over a long period. The nano-Ni/GCE was stored in air, and the oxidation response signal decreases only 4.7% after 8 days, suggesting good stability. 22849



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’ ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (973 Program, No. 2009CB320300) and the National Natural Science Foundation of China (No. 61071052). ’ REFERENCES

Figure 11. Relationship between COD values obtained by nano-Ni/ GCE and dichromate method.

The influence of chloride ion on the amperometric detection of COD was investigated. In the presence of 0.1 M chloride ion, the oxidation current signal of 24 mg L 1 COD remains unchanged on the surface of nano-Ni, revealing high tolerance level to chloride ion. Application in Water Sample. The prepared nano-Ni was used to detect the COD value of different lake water samples that were collected from Wuhan city. Figure 10 demonstrates the amperometric response of six water samples on the nano-Ni/ GCE surface. The COD concentration of water samples was detected using dichromate method, and the values are 8.3, 13.8, 20.1, 27.7, 35.2, and 41.5 mg L 1. It is found that the current response of water sample increases linearly with the COD value, suggesting promising application. Subsequently, the nano-Ni/ GCE was then used in a large number of water samples. For testing the accuracy of nano-Ni, the COD values of water samples were also measured by the conventional dichromate method. Figure 11 shows the relationship between the results obtained by nano-Ni and dichromate method. It is found that the COD values detected by dichromate method (indicated as CODCr) and nano-Ni/GCE (denoted as CODnano‑Ni) obey the following equation: CODCr = 1.609CODnano‑Ni + 4.649. The correlation coefficient is 0.98, revealing that this new electrochemical method using nano-Ni is feasible for the detection of COD.

’ CONCLUSIONS A convenient and efficient method was developed for in situ preparation of nickel nanoparticles on electrode surface. Moreover, the morphology and electrochemical activity of nickel nanoparticles were easily tuned by controlling the reduction potential, pH value, deposition time, and concentration of Ni2+. In pH 6.5 phosphate buffer containing 5 mM NiSO4, the nickel nanoparticles that reduced at 1.3 V for 30 s exhibited high activity to the electrochemical oxidation of glycine. The nano-Nimodified electrode showed promising application in the electrochemical detection of COD. Compared with the reported electrochemical methods for COD detection, this new one displayed much higher sensitivity.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. 22850
