Effect of different synthesis routes on the

Journal of Molecular Liquids 225 (2017) 919–925

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Effect of different synthesis routes on the electrocatalytic properties of NiOX nanoparticles A.S. Danial a, Mohamed I. Awad a,c,⁎, Faisal A. Al-Odail b, M.M. Saleh a,b,⁎ a b c

Department of Chemistry, Faculty of Science, Cairo University, Cairo, Egypt Chemistry Department, College of Science, King Faisal University, Al-Hassa, Saudi Arabia Chemistry Department, Faculty of Applied Sciences, Umm Al-Qura University, Makkah Al Mukarramah 13401, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 25 April 2016 Received in revised form 20 June 2016 Accepted 10 November 2016 Available online 12 November 2016 Keywords: Nickel oxide Nanoparticles Electrocatalysis Glucose Sol-gel

a b s t r a c t Nanoparticles of nickel oxide (nano-NiOx) have been synthesized using electrochemical and sol-gel routes. Impacts of the synthesis route on the structural and electrochemical properties of NiOx nanoparticles modified glassy carbon electrode (nano-NiOx/GC) are studied. Cyclic voltammetry (CV), field emission scanning electron microscopy (FE-SEM), X-ray diffraction and transmission electron microscopy (TEM) have been used for the characterization of nano-NiOx/GC. The size of the particles using the sol-gel technique (NiOx-SG) is uniform and have smaller particle size (~20 nm) than that obtained using the electrochemical route (NiOx-El, ~80 nm). The specific activity (given in mA per mg of the NiOx) of the Ni(OH)2/NiOOH redox couple of the NiOx-SG is higher with higher reversibility compared with the NiOx-EL. Impacts of the synthesis route on the electrocatalytic properties of the thus prepared NiOx-SG and NiOx-El are tested by the glucose oxidation in alkaline solution as a probing reaction for the electrocatalytic activity. Enhancement of glucose electrocatalytic oxidation is discussed in the light of the difference in structural and electrochemical characteristics of both catalysts. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Synthesis route is a crucial factor that considerably affects the structural, electrochemical and electrocatalytic properties of the synthesized catalysts [1–3]. Nickel and nickel oxide nanoparticles represent an important category of electrocatalysts since they have been used in many technological applications. For instance they have been used in supercapacitors [4,5], alkaline rechargeable batteries [6,7] and biosensors [8–12]. Nanostructured modified electrodes can be fabricated using dry and wet techniques [13–15]. Of those techniques, sol-gel route is widely used in preparation of metal oxide nanoparticles as it is simple and cost effective [16–18]. Metal oxides are usually prepared at different calcination temperatures and it has been reported that the annealing temperature has dramatic effects on the particle size and on the specific surface area of the prepared powder [19]. Different methods have been used for the synthesis of nano-NiOx. These methods include (but not limited to): electrochemical synthesis [20], sol-gel method [21–23], surfactant-mediated synthesis [24], thermal decomposition [25] and polymer matrix assisted synthesis [26]. Electrochemical methods are

⁎ Corresponding authors at: Department of Chemistry, Faculty of Science, Cairo University, Cairo, Egypt. E-mail addresses: [email protected], [email protected] (M.I. Awad), [email protected], [email protected] (M.M. Saleh).

http://dx.doi.org/10.1016/j.molliq.2016.11.018 0167-7322/© 2016 Elsevier B.V. All rights reserved.

quite easy but suffer from lack of controlling the particle size [20,26– 30]. Possible mass production of nanoparticles of metal oxide powder with relatively high specific surface area is considered to be major advantage of the sol-gel method [31–33]. Sol-gel technique, in the other hand, suffers from contamination by anions and cations from the used precursor solutions of different salts [34]. Electrooxidation of glucose is of prime importance as it is of interest in many applications such as the development of blood sugar sensor, bio-processing and in the development of renewable, sustainable fuel cells [35]. Glucose electrooxidation depends strictly on the electrode material and its crystalline state as well as the pH of the electrolyte solution. In the present work, glucose electrooxidation has been used as a probing reaction for comparing the electrocatalytic activity of nano-NiOx/GC prepared via electrochemical and sol-gel routes. Morphological as well as voltammetric differences of the two electrodes will be compared. NiOx-El and NiOx-SG will be used hereafter as abbreviations for electrochemically and so-gel prepared nickel oxide, respectively.

2. Experimental All chemicals used in this work were of analytical grade. They were purchased from Fisher and Sigma Aldrich and were used as received without further purification. All solutions were prepared using double distilled water.

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2.1. Preparation of nickel oxide nanoparticles Two routes had been used for the synthesis of the NiOx nanoparticles. The first route is the sol-gel technique and the details of the process can be found elsewhere [36]. A solution of 5.8 g of nickel nitrate (Ni(NO3)2·6H2O) in 100 mL water and a solution of 4.2 g of citric acid (C6H8O7·H2O) in 100 mL were prepared. Then, the nickel nitrate solution was dripped into the above citric acid solution with continuous stirring. The formed mixed solution was then heated at 70 °C with mechanical stirring for about 12 h. After the removal of water through evaporation, a green gel was formed. Next, the obtained gel was aged and dried at 110 °C for 24 h and then the sample was calcined at 400 °C for 4 h. A suspension of nano-NiOx, for being anchored on the glassy carbon (GC) electrode, was prepared by adding the proper mass of the above prepared nano-NiOx powder in a test tube containing 5 mL ethanol + 50 μL of Nafion solution (5% in water). The above mixture was sonicated for 30 min in an ice bath. Glassy carbon, GC electrode as an underlying substrate for nickel oxide nanoparticles is cleaned by mechanical polishing with aqueous slurries of successively finer alumina powder (down to 0.06 μm), then washed thoroughly with double distilled water and then ethanol. A volume of 50 μL of a freshly prepared nano-NiOx suspension is casted onto the thus cleaned GC electrode and left overnight to dry in air. The second route for synthesis of nano-NiOx is the electrochemical method; the potentiostatic deposition of metallic nickel on the working electrode (i.e., GC) from an aqueous solution of 0.1 M acetate buffer solution (ABS, pH = 4.0) containing 1 mM Ni(NO3)2·6H2O by applying a constant potential of −1.0 V. The time period for the electrodeposition is 6 min. The formed metallic Ni was passivated in 0.1 M phosphate buffer solution (PBS, pH = 7) by potential cycling between −0.5 and 1 V for 10 cycles at a scan rate of 200 mV s−1. Prior to each of the above steps (deposition and passivation), the electrode was rinsed in water to get rid of any contaminates from a previous step. The electrode was then activated for 20 cycles in 0.3 M KOH solution in the potential range −0.2 to 0.6 V. The CVs were repeated twice to confirm the reproducibility of the results. Electrochemical measurements were performed using an EG&G potentiostat (model 273A) operated with E-chem 270 software. All electrochemical measurements were performed at room temperature (25 °C). An electrochemical cell with a three-electrode configuration was used in this study. A platinum spiral wire and an Ag/AgCl/KCl (sat.) were used as counter and reference electrodes, respectively. All potentials are given with respect to this reference electrode. The working electrode was glassy carbon (d = 3.0 mm). A field emission scanning electron microscope, FE-SEM, (QUANTA FEG 250) was used to identify the structure of the nano-NiOx. High resolution transmission electron micrograph (TEM, JEOL-JEM-1230), and X-ray diffraction, XRD (PANalytical, X'Pert PRO) operated with Cu target (λ = 1.54 Å) were used to identify the structure of the nano-NiOx.

of nickel from the used bath (1 mM nickel nitrate dissolved in acetate buffer (pH = 4)) results in the formation of metallic nickel. Therefore, cycling of potential of the thus deposited nickel in phosphate buffer (pH 7, as mentioned in the Experimental section) is essential for converting metallic nickel to oxide. On passivation using potential cycling, the current decreases to a constant level which proves that metallic nickel (high currents) is converted (passivated) to the oxide (lower currents) upon potential cycling in pH = 7. Activation process converts the NiOx to Ni(OH)2 (as evident for both cases (NiOx-SG and NiOx-El) from the peaks in Fig. 3). ii. Direct electrodeposition of Ni(OH)2, as given in Corrigan Approach [38,39]. The electrodeposition takes place in a higher concentration of Ni(NO3)2 (at least 0.01 M) and without using an acidic buffer (pH = 4) which allows for the reactions below to take place; [38,39] − − − NO− 3 þ H2 O þ 2e →NO2 þ 2OH

ð1Þ

Ni2þ þ 2OH− →NiðOHÞ2

ð2Þ

In the other synthesis route, the sol-gel process, the thus synthesized nano-NiOx (as discussed in the Experimental section) is activated in 0.3 M KOH by potential cycling (results are not shown). The current increases and the anodic and cathodic peaks of the NiOOH/Ni(OH)2 couple becoming sharpener with the potential cycling until it reaches to constant levels. The above prepared nano-NiOx was characterized by taking SEM microimages as shown in Fig. 1. This figure shows SEM images of NiOx-El (image A) and NiOx-SG (image B). In the case of NiOx-El, the image

A

B

3. Results and discussion 3.1. Characterization of the nano-NiOx In this study, nano-NiOx was prepared via two routes: electrochemical and sol-gel routes. In both cases nano-NiOx is firstly prepared and then activated in 0.3 M KOH in order to activate the oxide and achieve enrichment in the couple NiOOH and Ni(OH)2. In the electrochemical synthesis route, one can discuss the background of the synthesis procedure. In this context, many approaches of synthesis of Ni(OH)2 were found in literatures. These include (but not limited to): i. Electrodeposition of metallic nickel followed by two steps: passivation and activation [37]. Passivation is essential as the electrodeposition

Fig. 1. SEM images of GC/NiOx-El (A) and GC/NiOx-SG (B).


3.2. Electrochemical characterization Fig. 3 depicts CV responses for GC/NiOx-El (A) and GC/NiOx-SG (B) in 0.3 M KOH solution at a scan rate of 100 mV s− 1. The loading of the 120

A

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NiO

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2-Theta Fig. 2. X-ray patterns for GC/NiOx-El (A) and GC/NiOx-SG (B).

40

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i /mA mg-1

reveals semi-spherical nanoparticles with uniform distributions of the particles with an average particles size of ~ 80 nm. In the other hand, the NiOx-SG (image B) shows spherical nanoparticles with average particles size of ~25 nm. The structures of the NiOx samples are evaluated based on the measured X-ray diffraction (XRD) patterns. Fig. 2 shows such patterns for NiOx-El (pattern A) and NiOx-SG (pattern B). In the case of NiOx-El, the XRD pattern consists of a set of peaks that assigned for Ni and NiO. The peaks appeared at 2θ = 44.2° and 52.1° (assigned for (111) and (200) planes, respectively) are indexed for cubic Ni (JCPDS No. 01-071-4655). The peaks appeared at 2θ = 37.0° and 43.0° (assigned for (111) and (200) planes, respectively) are indexed for cubic NiO (JCPDS No. 01-089-5881). In the NiOx-SG pattern (B), the peaks appeared at 44.0°, 52.3° and 76.5° (assigned for (111), (200) and (220) planes, respectively) are indexed for face-centered cubic (FCC) Ni phase (JCPDS No. 87-0712). The characteristic peaks of NiO appeared at 2θ = 37.20°, 43.0°, 62.87° and 75.20° and can be indexed to the face-centered cubic (FCC) crystalline structure of NiO as (101), (012), (110), (110) and (113) planes, respectively (JCPDS No. 040835). It may be concluded that higher crystallinity of NiOx-SG than NiOx-El is evident from intensity and sharper peaks appeared in the XRD patterns of the NiOx-SG. However, in both cases; we have nickel oxide along with contribution of nickel in the matrix structure. Fig. S1 (see Supplementary materials) shows TEM micrograph of NiOx particles of NiOx-SG. The size of the nanoparticles of the NiOx-SG as given from the TEM image is 17.3 nm. This may confirm the size of the NiOx-SG derived from the SEM image in Fig. 1B.

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A

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200

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600

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E /mV vs Ag/AgCl/KCl(sat) Fig. 3. CVs obtained for GC/NiOx-El (A) and GC/NiOx-SG (B) in 0.3 M KOH solution at a scan rate of 100 mV s−1.

NiOx-SG and NiOx-El is 0.10 mg cm2 and 0.065 mg cm−2, respectively. For the GC/NiOx-SG, the loading of nano-NiOx is controlled by the casting volume of the NiOx suspension (see the Experimental section). For the GC/NiOx-El, the loading is controlled by the amount of passed charge, Q during the electrodeposition method. For instance, Q was 15 mC and from which the above loading level was estimated using Faraday's law assuming 100% efficiency. In both cases the CVs reveal a couple of redox species; NiOOH/Ni(OH)2 appears between 0.35 and 0.45 V. Note that the current is presented in mA per mg of the nano-NiOx in order to normalize the current with the loading level. The anodic and cathodic peak currents depend on the method of synthesis. For instance, they are higher in the case of the GC/NiOx-SG than in the case of the GC/NiOx-El. This may be attributed to the difference in the structural difference of the two nanoparticles. The SEM images in Fig. 1 showed smaller particle size of the NiOx-SG which may be the reason for the obtained difference in electrochemical activity. Accidently the difference in the particle size is around 3 folds which coincident with the 3 folds increase in the currents of both anodic and cathodic peaks. The onset potential of the NiOOH/Ni(OH)2 is almost the same for both electrodes. The peak separation, ΔEp is 120.0 and 61.0 mV for the GC/NiOx-SG and GC/NiOx-El, respectively, denoting faster kinetics at the former electrode. CVs at different scan rates were measured for both GC/NiOx-El and GC/NiOx-SG in 0.3 M KOH. Fig. 4A shows the CV responses of GC/NiOxSG. As the scan rate increases the anodic and cathodic peak currents increase. The peak separation, ΔEp increases with the scan rate indicating a quasi-reversible process. Similar CVs (data are not shown) were measured for the GC/NiOx-El and the same features were revealed. Fig. 4B depicts the change of the peak current, Ip with the scan rate for both anodic and cathodic sweeps for the GC/NiOx-SG (b-1 and b-2) and GC/NiOx-El (a-1 and a-2). In both cases, a straight line is obtained. The values of Ip and the slope of the straight line depend on the electrode type. The GC/NiOx-SG electrode demonstrates higher currents for both anodic and cathodic branches and also has higher slopes than those obtained at the GC/NiOx-El. For instance, the equations corresponding to GC/ NiOx-El and GC/NiOx-SG, are Ip = 0.22 + 0.0056 ν and Ip = 0.45 + 0.017 ν (R2 = 0.997), respectively. Relative activity of the GC/NiOx-El and GC/NiOx-SG may be investigated by estimating the surface concentration of the active nickel sites, Γ. It can be done using the relation; Γ = Q/nF, where Q is the amount of charge consumed in the Ni(OH)2 → NiOOH conversion. Note that Q can be estimated from the area under the CV curve for the two electrodes. In using n = 1 and F = 96,500 C mol−1, the values of Γ were

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consistent with the surface nature of the Ni(OH)2 → NiOOH transformation process.

A 400 mV s-1

3.3. Electrocatalytic study

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scan rate, ν / mV s-1 Fig. 4. (A) CV responses for GC/NiOx-SG in 0.3 M KOH at different scan rates; 20, 50, 100, 200 and 400 mV s−1. (B) Relation of Ip with the scan rate for GC/NiOx-El (a) and GC/ NiOx-SG (b) in 0.3 M KOH (“1” is for anodic and “2” is for cathodic branch).

found to be 6.5 and 16.5 nmol cm−2 for GC/NiOx-El and GC/NiOx-SG, respectively. The values of Γ indicate that the surface concentration of NiOx species for GC/NiOx-SG is larger than that at GC/NiOx-El. The values of Γ are consistent with the trends that found in the above recorded CVs. That is to say, at lower particle size (in case of GC/NiOx-SG), there is an increase in the corners, edges and defects on the NiOx surface which results in higher electrochemical activity of NiOx species [40]. The above values of Γ were used to estimate the loading of Ni(OH)2 at the different electrodes using the relation; [loading (mg cm−2) = Γ × Molar mass of Ni(OH)2 × 103]. The estimated loading values using the above method were found to be 6.5 × 10−4 and 1.56 × 10−3 mg cm−2 for GC/NiOx-El and GC/NiOx-SG, respectively. Further, the utilization percentage, UP can be found according to the relation; UP ¼

Estimated loading 100 real loading

Glucose oxidation in alkaline solution is used here as a probing reaction for testing and differentiating the electrocatalytic properties of GC/NiOx-El and GC/NiOx-SG electrodes. Fig. 5 shows CV responses for (a) GC, (b) GC/NiOx-El and (c) GC/NiOx-SG in 0.3 M KOH containing 10 mM glucose at a scan rate of 100 mV s−1. Electrooxidation of glucose takes place on both electrodes as evident from the well-defined anodic peak with high currents owing to glucose oxidation. The peak current and peak potential depend on the electrode type. It can be seen that the cathodic peak of the reduction of NiOOH to Ni(OH)2 is disappeared on both electrodes pointing to the electrocatalytic nature of the glucose oxidation process. The onset potential and peak potential of glucose oxidation on GC/NiOx-SG are shifted to more negative values as compared with that of GC/NiOx-El. The peak current on GC/NiOx-SG is three folds higher than that on GC/NiOx-El electrode. The above observations point to the enhancement of glucose oxidation on GC/NiOx-SG compared to GC/NiOx-El. This may be attributed to the differences in structural and electrochemical characteristics of the two electrodes. In the GC/NiOxSG electrode, the smaller particle size give rise to higher enhancement of the activity of the NiOOH/Ni(OH)2 redox couple and higher surface utilization of the GC/NiOx-SG than GC/NiOx-El. Electronic and geometric characteristics of nanocatalysts with small particle size offer enhancement of electrochemical oxidation of organic molecules [41,42]. It is noteworthy to mention that the oxygen evolution reaction, a kinetically controlled reaction, can take off on nickel oxide modified electrodes at higher overpotential compared with the glucose electrooxidation. For instance, in Fig. 3 in which CVs obtained in glucose-free electrolyte, the onset of the OER is ~ 0.6 V (see arrow in Fig. 3) with low currents. Also, the onset potential of the OER in the presence of glucose is obtained at ~ 0.68 V (see the small arrows in Fig. 5). In this case, it can be noticed that the peak currents in curves “b” and “c” (in the presence of glucose, Fig. 5) are obtained at 520 and 550 mV at GC/NiOx-SG and GC/NiOx-El, respectively. These currents are large enough compared to curve “A” in Fig. 3, i.e., in the absence of glucose. Accordingly, there is no interference from the OER with the glucose oxidation at the present conditions. This is in agreement with the literature [43,44]. The catalytic mechanism of small organic molecules has been documented in literatures [45,46]. The results indicated that nano-NiOx

200

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-2

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100 b 50

a 0

ð3Þ

The real loadings as given above were 0.065 and 0.1 mg cm−2 for GC/NiOx-El and GC/NiOx-SG, respectively. In this case, UP was estimated to be 1.1 and 5.5% for GC/NiOx-El and GC/NiOx-SG, respectively which is

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E /mV vs Ag/AgCl/KCl(sat) Fig. 5. CVs obtained at a) GC, b) GC/NiOx-El and c) GC/NiOx-SG electrodes in 0.3 M KOH solution containing 10 mM glucose at a scan rate = 100 mV s−1.


modified electrodes can catalyze the electrooxidation of glucose to gluconolactone [45,46] due to the existence of Ni (ΙΙ) ions according to the following reactions: 2NiðΙΙÞ→2NiðΙΙΙÞ þ 2e−

ð4Þ

2NiðΙΙΙÞ þ glucose→2NiðΙΙÞ þ gluconolactone

ð5Þ

The redox transition of nickel species from Ni (ΙΙ) to Ni (ΙΙΙ) occurs, and then in a next step glucose is oxidized on the modified surface. NiOðOHÞ þ glucose→NiðOHÞ2 þ gluconolactone

ð6Þ

Fig. 6A shows LSV responses of GC/NiOx-SG in 0.3 M KOH containing 20 mM glucose at different scan rates. The peak potential for the catalytic oxidation of glucose shifts to more positive values with increasing the scan rate as a characteristic feature of irreversible voltammetric behavior. This result suggests a kinetic limitation in the reaction between redox sites of the GC/NiOx modified electrode and glucose. In the present study, note that a strong supporting electrolyte with relatively

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high concentration (0.3 M KOH) was employed. Thus, the possibility of potential shift to more positive values due to higher ohmic drop is expected to be negligible. Fig. 6B depicts the relationship between the function (Ipv−0.5) and the scan rate of glucose oxidation on GC/NiOx-SG and GC/NiOx-El. As can be seen, (Ipv−0.5) decreases with increasing the scan rate, which is a characteristic feature of catalytic reactions [47]. The above conclusion is in agreement with the above mechanism. Fig. 7A depicts LSV responses of GC/NiOx-SG in 0.3 M KOH containing different concentrations of glucose at a scan rate of 100 mV s−1. As the glucose concentration increases, the peak current, Ip of glucose oxidation increases and the peak potential shifts to more positive values. Similar results were obtained at GC/NiOx-El electrode and the Ip of glucose oxidation is plotted against the [glucose] as shown in Fig. 7B. At [glucose] ≤ 20 mM, the Ip increases linearly with the [glucose] at the two electrodes. It can be concluded that the glucose oxidation is diffusion-controlled at [glucose] ≤ 20 mM. Note that the sensitivity of Ip-[glucose] relation (slope) decreases from GC/NiOx-SG (curve b) to GC/NiOx-El (curve a) which points to better electrocatalytic properties of GC/NiOx-SG compared to GC/NiOx-El. At both electrodes and at [glucose] N 20 mM, an inflection in the straight line is found and the 25

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scan rate, ν / mV s-1 Fig. 6. (A) LSV responses for GC/NiOx-SG in 0.3 M KOH containing 5 mM glucose at different scan rates; 20, 50, 100, 200 and 400 mV s−1. (B) The variation of Ip/ν0.5 with scan rate obtained at a) GC/NiOx-El and b) GC/NiOx-SG electrodes.

0

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[glucose] / mM Fig. 7. (A) LSV responses obtained at GC/NiOx-SG electrode in 0.3 M KOH solution containing different concentrations of glucose at a scan rate of 100 mV s−1: a) 2, b) 5, c) 7.5, d) 10, d) 20 and e) 40 mM. (B) Inset shows the relation between the peak current, Ip, and glucose concentration for a) GC/NiOx-El and b) GC/NiOx-SG electrodes.


peak current, Ip does not increase with the same regime and hence lower currents are obtained than expected. This may be attributed to the higher extent of adsorption of glucose molecules on the electrode surface at high glucose concentration and hence hinder the accessibility of the OH− ions to the electrode surface. Another possible reason is the higher concentrations of poisoning products resulting from glucose oxidation which may poison the electrode surface. Fig. 8 depicts Tafel plots derived from a current–potential curve obtained at GC/NiOx-SG and GC/NiOx-El in the presence of 10 mM glucose using scan rate of 10 mV s− 1 electrode. The plots demonstrate the enhancement of the glucose oxidation on GC/NiOx-SG compared to that at GC/NiOx-El. Tafel slopes estimated from the plots are 131 and 125 mV dec−1 for the GC/NiOx-SG and GC/NiOx-El, respectively. This indicates that the mechanism of glucose oxidation is the same on the different studied electrodes and it is one-electron controlled process (see Eqs. (4) and (5)). The above values of Tafel slope is consistent with those found in literatures at similar conditions [47]. In the present case the value of 128 mV (±3 mV dec−1) decade indicates that a one-electron transfer would be the rate-limiting step. This may confirm the above suggested mechanism which is similar at both electrodes. Stability of the two electrodes towards electrocatalytic oxidation of glucose was tested by measuring current-time (I-t) relations at the peak current of glucose oxidation. For achieving the above purpose, I-t curves were recorded for glucose oxidation at the GC/NiOx-El and GC/ NiOx-SG at a constant potential of 0.5 V and 0.45 V, respectively. These curves are shown in Fig. 9. This figure depicts that, GC/NiOx-SG electrode supports higher oxidation currents than that obtained at GC/NiOx-El electrode. This level of enhancement could still be observed after 2 h of continuous operation. This indicates good mechanical and catalytic stability of the existing oxides. The present study may lead us to some findings. The present technology of nanocatalyst especially in fuel cells and biosensors rely on preparation of ink catalyst of specific metal and/or metal oxide. The ink catalyst is usually consists of a mixture of the catalyst itself (metal or metal oxide powder) with for an example carbon black as a support material. The latter supports extremely high surface area. Hence, the ink catalyst is a mixture of two powders; catalyst + carbon black. Accordingly, synthesis of metal oxide in a powder shape (using for an example a sol-gel method) can allow for mixing the catalyst (NiOx in our case) with a support material of high surface area. Synthesis of the oxide by an electrochemical route, in the other hand, does not allow such a technological approach. Also, the particle size can be controlled in the so-gel method e.g., by controlling the calcination temperature. In electrochemical procedure, if the size of the particle is controlled,

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time / s Fig. 9. Current-time relations obtained during glucose oxidation at a) GC/NiOx-El and b) GC/NiOx-SG electrodes in 0.3 M KOH containing 10 mM glucose.

the loading level will be changed since on increasing the particle size by increasing the amount of charge passed, it will also cause a concurrent increase in the loading level. Smaller size can always be guaranteed simultaneously with a controlled loading level in a sol-gel route, a property that supports high electrocatalytic activity as it was found in the present work. 4. Conclusions Nickel oxide nanoparticles (nano-NiOx) were synthesized using solgel (NiOx-SG) and electrochemical (NiOx-El) techniques. The obtained NiOx-SG nanoparticles showed smaller particle size with higher electrochemical activity, compared with GC/NiOx-EL, towards the NiOOH/ Ni(OH)2 redox transform. The results showed that NiOx-SG had a superior catalytic activity compared with NiOx-El. The GC/NiOx-SG showed higher electrocatalytic activity towards glucose oxidation from alkaline solution and this was correlated to the surface, structural and electrochemical properties of the synthesized NiOx nanoparticles. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2016.11.018. References

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-3.0

log(i / A cm-2) Fig. 8. Tafel plots for A) GC/NiOx-El and B) GC/NiOx-SG electrodes in 0.3 M KOH containing 10 mM glucose at a scan rate 10 mV s−1.

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