Scholarly article on topic 'Elaboration of modified poly(NiII-DHS) films as electrodes by the electropolymerization of Ni(II)-[5,5′-dihydroxysalen] onto indium tin oxide surface and study of their electrocatalytic behavior toward aliphatic alcohols'

Elaboration of modified poly(NiII-DHS) films as electrodes by the electropolymerization of Ni(II)-[5,5′-dihydroxysalen] onto indium tin oxide surface and study of their electrocatalytic behavior toward aliphatic alcohols Academic research paper on "Chemical sciences"

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{"Nickel-Schiff base complexes" / "Modified electrodes" / Electrocatalysis / "Methanol oxidation" / "Methanol sensor"}

Abstract of research paper on Chemical sciences, author of scientific article — Ali Ourari, Bouzid Ketfi, Larbi Zerroual

Abstract Nickel(II)-DHS complex was obtained from N,N′-bis(2,5-dihydroxybenzylidene)-1,2-diaminoethane (H2DHS) ligand and nickel acetate tetrahydrated in ethanolic solution with stirring under reflux. This complex, dissolved in an alkaline solution, was oxidized to form electroactive films strongly adhered on the ITO (indium tin oxide) electrode surface. In this alkaline solution, the poly-[NiII-DHS]/ITO films showed the typical voltammetric response of (Ni2+/Ni3+) redox couple centers which are immobilized in the polymer-film. The modified electrodes (MEs) obtained were also characterized by several techniques such as scanning electronic microscopy, atomic force microscopy and electrochemical methods. The electrocatalytic behavior of these MEs toward the oxidation reaction of some aliphatic alcohols such as methanol, ethanol, 2-Methyl-1-propanol and isopropanol was investigated. The voltammograms recorded with these alcohols showed good electrocatalytic efficiency. The electrocatalytic currents were at least 80 times higher than those obtained for the oxidation of methanol on electrodes modified with nickel hydroxide films in alkaline solutions. We noticed that these electrocatalytic currents are proportional to the concentration of methanol (0.050–0.30μM). In contrast, those recorded for the oxidation of other aliphatic short chain alcohols such as ethanol, 2-methyl-1-propanol and isopropanol are rather moderately weaker. In all cases the electrocatalytic currents presented a linear dependence with the concentration of alcohol. These modified electrodes could be applied as alcohol sensors.

Academic research paper on topic "Elaboration of modified poly(NiII-DHS) films as electrodes by the electropolymerization of Ni(II)-[5,5′-dihydroxysalen] onto indium tin oxide surface and study of their electrocatalytic behavior toward aliphatic alcohols"

Accepted Manuscript

Elaboration of modified poly(Ni n-DHS) films as electrodes by the electropo-lymerization of Ni(II)-[5, 5'-dihydroxySalen] onto indium tin oxide surface and study of their electrocatalytic behavior towards aliphatic alcohols

Arabian Journal of Chemistry

PII: DOI:

Reference:

Ali Ourari, Bouzid Ketfi, Larbi Zerroual

S1878-5352(14)00245-7 http://dx.doi.org/10.1016/j.arabjc.2014.10.033 ARABJC 1429

To appear in:

Arabian Journal of Chemistry

Received Date: 27 July 2013

Accepted Date: 14 October 2014

Please cite this article as: A. Ourari, B. Ketfi, L. Zerroual, Elaboration of modified poly(Ni n-DHS) films as electrodes by the electropolymerization of Ni(II)-[5, 5'-dihydroxySalen] onto indium tin oxide surface and study of their electrocatalytic behavior towards aliphatic alcohols, Arabian Journal of Chemistry (2014), doi: http:// dx.doi.org/10.1016/j.arabjc.2014.10.033

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Elaboration of modified poly(Ni II-DHS) films as electrodes by the electropolymerization of Ni(II)-[5, 5'-dihydroxySalen] onto indium tin oxide surface and study of their electrocatalytic behavior towards aliphatic alcohols.

Elaboration of modified poly(Ni II-DHS) films as electrodes by the electropolymerization of Ni(II)-[5, 5'-dihydroxysalen] onto indium tin oxide surface and study of their electrocatalytic behavior towards aliphatic alcohols.

Ali OURARIa, Bouzid KETFIa, Larbi ZERROUALb

Laboratoire d'électrochimie, d'Ingénierie Moléculaire et de Catalyse Rédox (LEIMCR), Faculté de Technologie, Université Sétif-1, 19000 Sétif - Algeria.

talyse Réa

Laboratoire d'Energétique et d'Electrochimie des Solides (LEES), Faculté de Technologie,

Solides

alc( The

Université Sétif-1, 19000 Sétif - Algeria. Abstract

Nickel (II)-DHS Complex was obtained from N,N'-bis(2,5-dihydroxybenzylidene)-1,2-diaminoethane (H2DHS) ligand and nickel acetate tetrahydrated in ethanolic solution with stirring under reflux. This complex, dissolved in an alkaline solution, was oxidized to form electroactive films strongly adhered on the ITO (Indium tin oxide) electrode surface. In this alkaline solution, the poly-[NiII-DHS]/ITO films showed the typical voltammetric response of (Ni2+/Ni3+) redox couple centers which are immobilized in the polymer-film. The modified electrodes (MEs) obtained were also characterized by several techniques such as scanning electronic microscopy, atomic force microscopy and electrochemical methods. The electrocatalytic behavior of these MEs towards the oxidation reaction of some aliphatic alcohols such as methanol, ethanol, 2-Methyl-1-propanol and isopropanol was investigated.

e voltammograms recorded with these alcohols showed a good electrocatalytic efficiency. The electrocatalytic currents were at least 80 times higher than those obtained for the oxidation of methanol on electrodes modified with nickel hydroxide films in alkaline solutions. We noticed that these electrocatalytic currents are proportional to the concentration of methanol (0.050 to 0.30 ^M). In contrast, those recorded for the oxidation of other aliphatic short chain alcohols such as ethanol, 2-Methyl-1-propanol and isopropanol are rather

moderately weaker. In all cases the electrocatalytic currents presented a linear dependence with the concentration of alcohol. These modified electrodes could be applied as alcohol sensors.

Keywords: Nickel-Schiff base complexes, Modified electrodes, Electrocatalysis, Methanol oxidation, Methanol sensor.

• Corresponding author. Tel: +213.669.46.58.31, Fax: +213.36.92.51. E-mail address: zerroual@yahoo.fr (L. Zerroual).

1. Introduction

The electrochemical methods as cyclic voltammetry, chronoamperometry, differential pulse voltammetry and impedancemetry are revealed as efficient techniques for the determination of several biomolecules with high accuracy and low detection limits. Thus, some references may be mentioned: for instance, the determination of captopril, thioguanine and levodopa (Beitollahi et al., 2014, 2011a, 2011b), L-cysteine with ascorbic acid, (Raoof et al., 2007, 2006a, 2006b), hydroquinone derivatives (Taleat et al., 2008) and simultaneous determinations of carbidapa (Tajik et al., 2013).

The direct oxidation of alcohols presenting many advantage such as increasing its efficiency, higher selection of possible electrode materials and minimizing the interference arising from the oxidation of other organic fuels reported in the literature (Ganesh et al., 2011). Compared with ethanol, methanol has the significant advantage of high selections to CO2 formation in the electrochemical oxidation process (Arico et al., 2001; Wasmus and Kuver, 1999). Nickel complexes modified electrodes have been widely used for electrocatalytic oxidation of alcohols especially methanol (Golikand et al., 2006a). The nickel complexes have been successfully performed to fabricate new catalyst systems for alcohols oxidation (Wang et al., 11).

Nowadays, methanol is commonly known as a promising candidate for fuel cells application. Compared to other cells, the direct methanol fuel cell (DMFC) presents several advantages such as high efficiency, very low polluting emissions, a potentially renewable fuel source, fast and convenient refueling (Ganesh et al., 2011; Jacobson et al., 2005; kordech and Simader, 1995; Binachini and Shen, 2009; Service, 2002; Shukla and Raman, 2003). In addition, its

low operating temperature allows easy start up and rapid response to changes in load or operating conditions (Oliveira, 2006). Thus, fuel cells can be considered as one alternative for energy conversion in spite of several decades of concerted attempts. Among these, numerous studies carried out essentially with unmodified or chemically modified electrodes such as platinum (Pt) and its various alloys were performed (Iwasita, 2002; Nonaka and Matsumura, 2002; Fleischmann et al., 2004; Heli et al., 2004; Bang et al., 2007; Rivera et al., 2004; Kabbabi et al., 1998; Tsuji et al., 2007; Schmidt et al., 1999). The electrode materials investigated have not yet reached satisfactory results expected from each metal in terms of electrocatalytic properties without complementary effects. The electrochemical oxidation of methanol is a complicated process that affects the performance of the cell due to its poisoning of the Pt active sites (Jarvis et al., 1998). Concerning the mechanistic studies and kinetics of methanol oxidation, several investigations were also performed with Pt or Ni on modified electrodes (Samant and Femandes, 1999; Golikand et al., 2006a; Golikand et al., 2005), Pt-Ru or Ni-Cu alloys (Schmidt et al., 1999; Jafarian et al., 2006), nickel or cobalt hydroxides modified glassy carbon electrodes (El-Shafei, 1999; Jafarian et al., 2003). Moreover, different complexes of nickel such as NiII-salen ( Trevin, 1997), NiII-tetraazamacrocyclic complexes (Roslonek and Taraszewska, 1992), NiII-curcumin(Ciszewski, 1995), NiII-tetrakis(3-methoxy-4-hydroxyphenyl)porphyrin (Ciszewski and Milczarek, 1996) or Nin-hematoporphyrin IX (Golabi and Golikand, 2004) have been studied as modifying agents in alkaline media yielding polymeric films at the electrode surface of glassy carbon. These electrodes have shown interesting catalytic properties towards the electro-oxidation of methanol. The main objective of this work is the preparation, characterization and electrochemical study of the electrocatalytic behavior of indium tin oxide electrodes (ITO), modified with films derived from the electropolymerization of NiII-(N,N'-bis(2,5-dihydroxybenzylidene)-1,2- diaminoethane) (NiII-DHS) in an alkaline solution. To our knowledge, this complex was not studied in the elaboration of modified electrodes. The tivity and stability of poly-(NiII-DHS) films will also be studied in alkaline solutions to orate new sensors for a quantitative determination of methanol or its analogs of other short chain aliphatic alcohols (El-Shafei, 1999).

elab shor

2. Experimental

2.1. Reagents and apparatus

1,2-Diaminoethane and 2,5-dihydroxybenzaldehyde were supplied from Aldrich Chemical Co. and were used as received. Nickel acetate Ni(OAc)2,4H2O and absolute ethanol were obtained from Prolabo. All other chemicals used in this work were of reagent quality. N,N'-bis(2,5-dihydroxybenzylidene)-1,2-diaminoethane (H2DHS) was prepared as previously described (Revenga-Parra et al., 2005) Stock solutions of [Ni-DHS)]2+ (typically 1.0 mM) were prepared just prior to use. The IR spectra were recorded on KBr pressed pellets from 5000 to 500 cm-1 using a Shimadzu FTIR spectrometer. UV-vis spectra were recorded in Unicam 300 and software vision 32, operating from 200 to 800 nm in 1.0 cm quartz cells.1H NMR analysis was conducted in a Brucker (250 MHz). Mass spectrum (electrospray) was recorded on a Jeol JMS 70 spectrometer. Water was bi-distilled and electrochemical measurements were carried out in a conventional three-compartment cell using a Voltalab PGZ301 Potensiostat-galvanostat controlled with voltamaster 4 software. Indium tin oxide (ITO) electrodes (surface, 0.25 cm2) were used as working electrodes. A coiled platinum wire served as auxiliary electrode. The potentials were measured using a saturated calomel (SCE) reference electrode. All experiments were performed using 0.1 M sodium hydroxide as the background electrolyte.

2.2. Synthesis of H2DHS ligand

d using

H2DHS ligand was synthesized using the following procedure, 60 mg (1 mmol) of ethylenediamine were dissolved in 10 ml of absolute ethanol. To this we add an ethanolic solution containing 276 mg (2 mmol) of 2,5-dihydroxybenzaldehyde. The mixture was then refluxed for about one hour. The crude yellow solid obtained was crystallized from ethanol and dried under vacuum to give rise to 219 mg (yield 73%) of pure H2DHS. The main physical characteristics of this compound are as following:

IR (KBr): vch=n 1632 cm-1; vc-o 1294 cm-1; vc=c 1450 cm-1; vo-h 3450 cm-1; UV-vis in " lethylsulfoxide (DMSO), ^ 264 nm (Do = 1,935, e = 19350 l cm-1mole-1); Amax2 =352

dimethy

(Do = 1,402, e = 14020 l cm-1mole-1).

2.3. Synthesis of Ni(II)-DHS complex

150 mg (0.5 mmol) of (H2DHS) ligand were dissolved in 20 ml of ethanol. To this solution 120 mg (0.5 mmol) of tetrahydrayed nickel acetate (Ni(OAc)2,4H2O) were added. The resulting mixture was then refluxed for 3h. After cooling, the brown precipitate was collected

by filtration, washed and dried under vacuum. Thus, 118 mg of the crude product were obtained (yield of 65%). Its main physical characteristics are reported below:

IR (KBr): Vc=n 1623 cm-1. UV-vis in dimethylsulfoxide (DMSO), Amax: 270 nm (D0=2.937, emax=29370); 336 nm (D0=1.028, emax=19280); 362 nm (D0=0.911, emax=9110); 438 nm D0=0.949, emax=9490). 1H NMR (DMSO): 5 = 8.7 (CH=N, s, 2H). 5 = 12.9 (OH, broad: s, 2H); 5 = 3.4 (CH2CH2, s, 4H). MS: The molecular peak found is m/z, M+ =357 (100%), accompanied with a deprotonated fragment corresponding to [M-2H+]+=355 (8.8%).

2.4. Electrode modification with [Ni11-DHS]2+complex

(8.8%). <j

Prior to each experiment, the surface of indium tin oxide was activated by an aqueous solution of 45% nitric acid and washed by ethanol to eliminate any trace of grease then copiously rinsed with distilled water. In order to prepare a modified ITO surface with [NiII-DHS]2+ complex, the electrode was immersed in 1 mmol aqueous solution of the complex containing 0.1 M NaOH. The electrode was cycled between 0.0 and +1.0 V/SCE at a scan rate of 100 mV/s. The anodic Qa or cathodic Qc charges were determined from the integration of their corresponding waves at low scan

2.5. Surface characterization of the modified electrode

ological as

The structural and morphological aspects of these modified ITO-electrodes were analyzed using surface characterization techniques, namely scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The atomic force microscopy (AFM) studies were carried out using a nanoscope III (Digital Instrument) electronics. The tapping mode was employed to observe the film topography.

2.6. Film 1 stability

The time

NiII-DHS film was prepared as previously described, the ITO electrode was cycled 100

times in the same solution and conditioned (50 scans) in 0.1 M NaOH at 100 mV/s. The electrochemical stability of the polymeric film was checked each 05 cycles by measuring the anodic and cathodic peak currents (ipa and ipc) of the electrode.

2.7. Electrocatalytic testing of the modified electrode

The electrochemical behavior of the modified electrode (Nin-DHS film) was studied in 0.1 M NaOH solutions containing respectively 25 mmol of methanol or 2-methyl-1-prapanol. In each solution, the electrode was cycled between 0.0 and 0.7 V/SCE at a scan rate of 20 mV/s.

3. Results and discussion

3.1 Identification of the [Niu-DHS]2+complex

The formula of the Nickel complex was elucidated using spectroscopic methods such as FTIR, UV-Vis and mass spectrometry. The data of these analyses (Table 1) corroborate with the molecular structure proposed in Figure 1.

3.2. Elaboration of the ITO modified electrode

Fig. 2A shows the electroactive film growth pattern obtained under continuous potential cycling. During the first scan no electrochemical response ascribed to the oxidation of the DHS quinone functional groups is observed. This result is in contrast with recent reports indicating that DHS can be electropolymerized onto ITO surfaces, in neutral solutions, giving rise to films with a moderate electroactivity (Revenga-Parra et al., 2005).

From the second scan onwards, we note the appearance of two peaks respectively at 0.442 and 0.306 V/SCE which correspond to the Ni2+/Ni3+ redox system. The growth of the film is accompanied by a gradual increase in the current values of these two peaks. This clearly indicates the formation of an electroactive film on the surface of the indium tin oxide electrode. The oxidation of Ni2+ to Ni3+ in aqueous media has been reported to be very difficult due to the strong grade of hydration of the nickel ions. When the nickel is coordinated such as in the DHS hydrophobic polymeric film, the oxidation conditions of the metal changed significantly (Junior et al., 2004; Cataldi et al., 1996; Yang et al., 2006; Bard and Faulkner, 2002; Vermillion and Pearl, 1964). The mechanism of the film formation is ly the same as in the case of the free ligand but the incorporation of Ni2+ into the cture makes the film conductive and favorites the growth of a multilayer system. This organization of multilayer film, as will be discussed below, could explain the strong adherence of poly-[NiII-DHS]+2 to the electrode surface (Vasil'eva et al., 1993; Malinski et

stru orga

al., 1991; Bukowska et al., 1996; Cataldi et al., 1995; Vukovic, 1994; Pasquini and Tissot, 2005). As for the electropolymerization process, a linear increasing of the anodic peak current was observed during the successive cycling namely the first 100 cycles. Afterwards, the current growth decreases gradually until 150 cycles after which it becomes practically

constant. For the cathodic peak current, it shows only a linear increase during the first 40 scans (Fig. 2B) while the next sixty scans appeared as broad showing a significant decrease in intensity. This behavior, observed during electropolymerization process, suggests that either charge or mass transfer present limitations associated to the electrochemical reduction of

Ni3+ to Ni2+ in the heart of the polymer film.

3.3. Surface characterization of the modified electrode

con Ni2+

Atomic force microscopy was used to provide information concerning the polymer morphology. The values of average roughness of ITO-free and the modified electrodes are presented in Table 2. These results show a decrease in roughness, a homogeneous and uniform polymer film was obtained. Similar results are reported in the literature (Junior et al., 2004). Thus, the difference of average roughness shown in Figure 3 (A and B images) indicates obviously that a thicker film of poly-(NiII-DHS) was effectively formed by electropolymerization of Ni(II)-DHS monomer. SEM and EDS techniques were also carried out for the characterization of poly-[NiII-DHS]2+/ITO modified electrodes. The electronic image (Fig. 4) of the electrode, modified by electropolymerization of nickel(II)-Schiff base complex, shows a high homogeneity of the film surface electrodeposited from [NiII-DHS]+2 complex (Cataldi et al., 1996; Yang et al., 2006). These results were consistent with those previously obtained by AFM technique. Thus, the presence of the nickel was confirmed by EDS method (Fig. 5) which proceeds by computing the percentage of different elements constituting the surface of material such as nickel (Ni), tin (Sn), carbon (C) and others elements as shown in Table 3 (Peng et al., 2013; Ganesh et al., 2011; Yang et al., 2006; Bard and Faulkner, 2002; Cataldi et al., 1996; Scheer and Lewerenz, 1994).

3.4. Stability of the ITO modified electrode

As mentioned above, the polymeric film obtained shows a high adherence to the indium tin oxide. When a poly-[Ni"-DHS]+2/ITO electrode is transferred to a 0.1 M NaOH solution, ntaining no monomer, the cyclic voltammograms obtained show the typical response of the Ni2+/Ni3+ redox couple (Fig. 6A). As can be seen in Fig. 6C, peak potentials as well as the formal potential corresponding to the Ni2+/Ni3+ couple reached stable values after about 10 cycles. Fig. 6B depicts the anodic and cathodic surface coverage as a function of the number of potential cycles. As can be seen, after 50 conditioning cycles the decrease of coverage is less than 10% and probably this loss of material corresponds to both ligands and monomeric complexes weakly adsorbed. After the conditioning step, the electrochemical properties of the

electropolymerized films remain stable for several weeks if the modified electrodes are stored under dry conditions.

3.5. Methanol oxidation

As can be seen in Fig. 7A (curve 1), methanol oxidation at bare glassy carbon electrodes ii alkaline solution is very poor and it is not possible to obtain oxidation prior to the discharge of the supporting electrolyte. In contrast, Indium tin oxide electrode modified with DHS]2+ presents an important electrocatalytic activity towards the oxidation of this alcohol as illustrated in Fig. 7A (curve 2, 3, 4 and 5). The electrooxidation process takes place in two different regions of potential. The first one corresponds to the oxidation of Ni2+ to Ni3+ and appears as a sharp peak at +0.442 V (a). For the second region of potential, a new catalytic wave with a peak potential of +0.700 V and a significantly large peak current appears. This result clearly suggests an interaction between the methanol and the film redox centers confined at the electrode surface. At the reverse scan, no reduction peak is observed and methanol is still oxidized. As a result a new peak at +0.682 V is observed. The results of different sizes are summarized in Table 4.

Fig. 7A shows the cyclic voltammograms obtained with a poly-[Ni -DHS]/ITO (r = 6.3x10 mol cm-2) electrode placed in a 0.1 M NaOH solution with increasing concentrations of methanol ranging from 0.25 to 1.00 M. It is clearly seen that the first anodic wave (a) practically disappears at methanol concentrations higher than 0.25 M. Moreover, the catalytic peak at +0.700 V shows a moderate anodic shift in the peak potential associated to a gradual increase in the peak current. The cathodic back wave (a') shows a similar behavior as that described above for (a). At higher concentrations, probably all the Ni (III) catalytic centers present in the film, generated by the previous electrochemical oxidation of Ni (II) (Revenga-Parra et al., 2008), are interacting with methanol and the limiting step of the process is the rate such interaction. This fact can explain the total disappearance of the (a) and (a') waves at igh methanol concentrations ( Golikand et al., 2009). The dependence of peak currents response on the concentration of methanol was linear in the range 0.25-1.00 M, as shown in the Figure 7B. Linear regression statistical analysis (Y = a + bX) yielded a slope (sensitivity) of 3.696 mA.cm-2.M-1, an intercept of 1.435 mA.cm-2. In order to obtain the analytical

of such high m respon

properties of the methanol sensor described above, chronoamperometric experiments were carried out by poising the modified electrode at a potential of +0.650 V vs. SCE. According to

the results (see Fig. 7C), the amperometric sensor response is moderately rapid, since it takes less than 2 min to obtain a steady-state current after the addition of methanol.

The electro-oxidation of methanol takes place always at potentials more positive than the Ni2+/Ni3+ oxidation. On the basis of these results, and taking into account the literature reported data, a possible mechanism for the methanol oxidation can be proposed according to the following steps: (a) formation of poly-[NiO(DHS)] in the presence of OH- ions; (b) oxidation of metal as a consequence of the potential scanning to gives rise poly-[NiIIIOOH(DHS)]; (c) oxidation of methanol on the film to give an intermediate product and poly-[NiO(DHS)] and (d) reaction of the intermediate with oxygenated molecules present in the film, such OH- ions, to gives rise the final product of oxidation (Golabi and Golikand, 2004; Kim et al., 1997). Although we have no conclusive results, this sequence of reaction mechanisms may interpret at least qualitatively, all the facts experimentally observed.

3.6. Oxidation of other short chain aliphatic alcohols

ele< pro

It was found that indium tin oxide ITO-electrodes, modified with poly-[NiII-DHS] films, can act as a catalyst for the electro-oxidation of other short chain aliphatic alcohols, such as ethanol, 2-Methyl-1-propanol. Fig. 8A shows three voltammograms, obtained with poly-[Nin-DHS]/ITO electrode which is immersed in a solution containing 0.1 M of the used alcohol. The potential was swept at 0.02 V/s and the figure depicts only the anodic peak current since the cathodic one disappears suggesting an electrocatalytic effect for an oxidation reaction. This behavior is observed for both ethanol and 2-Methyl-1-propanol; whereas a residual cathodic current was observed with methanol exhibiting a significant enhancement of the anodic current characterizing the electrocatalytic process. This result indicates that the oxidation of Ni2+ to Ni3+ in the poly-[NiII-DHS] film is independent of the alcohol nature. In addition, the electrocatalytic peak currents in all cases decrease as the aliphatic chain length increases. In Fig. 8B, the chronoamperometric method at a potential 0.650 V indicates that ilectrocatalytic currents of various alcohols such as methanol, ethanol and 2-Methyl-1-propanol decrease as the aliphatic chain increases. This result could probably be due to the increase of the hydrophobic character affecting the diffusion of the alcohol molecules into the bulk of the poly-[NiII-DHS] film and to the decrease of the species number involved in the

oxidation processes (Golikand et al., 2006b).

5. Conclusion

The new electrode materials, elaborated in this work as poly-[NiII-DHS] films, contain nickel catalytic sites uniformly dispersed on the polymer films. This modification of ITO surface was performed by anodic oxidation of tetradentate nickel (II)-Schiff base. These new modified electrodes showed that the electrooxidation of methanol is more efficient than other alcohols such as ethanol and 2-Methyl-1-propanol. These results were supported by the total disappearance of the peak current ipc of the redox system Ni+2/Ni+3 to the benefit of its ipa suggesting the presence of higher reactivity for methanol oxidation. The oxidation of ethanol and 2-Methyl-1-propanol show a lower reactivity. This could be due to the increase of the hydrophobic character in the short chain of the aliphatic alcohols.

and his Lai

Aknowledgements

The authors would like to thank Lahcene OUAHAB and his Laboratory of Chemical Sciences of Rennes1-University, France for his helping.

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ta 56, 5754-575

Figures captions:

Table 1 Spectroscopic data for the complex.

Table 2 Average roughness of poly-[Ni(II)-DHS]+2 films electrodeposited on ITO-Electrode.

Table 3 Percentage of different elements constituting the poly-[NiII-DHS]2+ films.

Table 4 Electrochemical characteristics of poly-[NiII-DHSaien]a and poly-[Nin-DHSalophen]B films in the electrocatalytic conditions for oxidations of methanol.

Figure 1 Molecular structure of [Ni(II)-DHS]+2 complex.

inset) co

Figure 2 Cyclic voltammograms (A) of 0.1 mM of NiII-DHS (structure inset) complex at ITO electrode in 0.1 M NaOH during modification of the electrode surface. The inset (B) shows the plot of the cathodic (open circles) and anodic (open squares) peak current vs. the number of electropolymerizing scans.

Figure 3 AFM image of the surface morphology of a ITO-free (A) and poly-[NiII-DHS]+2 film deposited in ITO (B). 3-D view elaborated by 50 voltammetric scans between 0.0 and 1.0 V vs. SCE.

Figure 4 Scanning electron micrograph of poly-[NiII-DHS]2+ films deposited electrochemically on a (ITO) indium tin oxide by cycling from 0.0 to +1.0 V vs. SCE.

Figure 5 EDS spectra of poly-[NiII-DHS]2+ films, deposited electrochemically on a (ITO) indium tin oxide by cycling from 0.0 to +1 V vs. SCE.

Figure 6 (A) Cyclic voltammogram of NiII-DHS modified electrode prepared by 100 electropolymerizing scans and 50 conditionating scans in 0.1 M NaOH at 100 mV/s. (B) Anodic (open squares) and cathodic (open circle) surface coverage obtained at different conditionating scans for a film derived from NiII-DHS. (C) Anodic (open squares) and cathodic (open circle) potentials obtained at different conditionating scans for a film derived

from N

iII-DHS.

igure 7 (A) Cyclic voltammogram of bare ITO electrode in the supporting electrolyte in the presence of 1.00 M of methanol (1). Cyclic voltammograms of ITO electrode modified with film derived from NiII-DHS in the supporting electrolyte (2) and in the presence of 0.25, 0.50 and 1.00 M of methanol (3, 4 and 5). (B) The variation of anodic peak currents vs. methanol concentrations. Scan rate 20 mV/s. (C) Amperometric response of the modified electrode kept in 650 mV (vs. SCE) in 0.1 M NaOH solution containing different

concentration of methanol. The numbers 1-5 correspond to 0.00, 0.25, 0.50, 0.75 and 1.00 M, respectively.

Figure 8 Cyclic voltammograms (A) of a ITO electrode modified with NiII-DHS in 0.1 M NaOH in the presence of 0.025 M of (a) methanol; (b) ethanol; (c) 2-Methyl-1-propanol (Inset B) Plots of the alcohol chronoamperometric currents normalized to the electr coverage at t = 120 s against alcohol concentration. Scan rate 20 mV/s.

Figure 1

Figure 2

Figure 4

ll. S 1

1 sn J S. ■

i) D5 1 15 i is 3 )J 4 4S S 5i S ti T TS 1 85 3 kev

Binding Energy / eV

Figure 5

Figure 8

Formula Infrared ' KBr Ü. V.-Vis,/DM SO Mass spectrometry

Bonds (cm'1) (cm-1) e (1 cm" M*1) M~ [Ba<sepeak](iii z)

C-H 2928 270 29370

CidHijOjNjNi CH=N C-0 Ni-0 Ni-N 1623 1446 668 469 336 362 438 19280 9110 9490 357 (138]

Table 1

Roughness

[TO-free

ITO/poly-[NpàHS]

Total Area Average roughness Room mean square Ten Points Height Sz.

2491 urn2 391.450 ftm 60.7379 um 65536 ran

2491 ¡jirr 50.403 um 104.1002 nm 1339.57 nm

Table 2

Table 3

Standards Elements %Mass %Atomic

C QC03 CK 4.40 13.11

0 Si02 OK. 27,89 62.38

Si Si02 Si K 3.19 4.06

Ni Ni Ni K 3.23 1.97

Sn Sn Sn L 61.30 18.48

Tutaux 100.00 100.00

Modified electrodes EM i'mA cm':

Epa Eje AEP Sis ipc ty/íjc

Ni"-DHsdn ■ UCr 442 306 136 374 1.00 0,65 1.54 5,23

frmw cv 480 316 165 398 0,50 0.18 2,77 4,04

№M-DH^/nOc 427 327 1ÛÛ 377 0.55 0.2 2.75 1.32

Nilt)Hs„u^/CVd 461 344 117 402 0,13 0.07 1.85 2,52

ydroxyb(

aNin-(N,N'-bis(2,5-dihydroxybenzylidene)-1,2-diaminoethane),bNin-(N,N'-bis(2,5- dihydroxybenzylidene)-1,2-diaminobenzene), cAll voltammograms were recorded in H2O solutions containing 0.1M NaOH; v = 20 mV s-1, dSimilar results obtained from the literature in the same experimental conditions (Revenga-Parra et al., 2008),eratios expressing the electrocatalytic currents for the oxidation of methanol [ipa(MeOH)/ ip;

Epa-Epo E1/2= (Epa+Epc)/2.

Table 4

ation of met

ipa(without MeOH), ^Ep-